U.S. patent application number 14/796669 was filed with the patent office on 2016-06-30 for imaging apparatus and methods.
This patent application is currently assigned to UCL Business PLC. The applicant listed for this patent is UCL Business PLC. Invention is credited to Paul Anthony Kirkby, K. M. naga Srinivas Nadella, Robin Angus Silver.
Application Number | 20160187761 14/796669 |
Document ID | / |
Family ID | 37232800 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160187761 |
Kind Code |
A1 |
Kirkby; Paul Anthony ; et
al. |
June 30, 2016 |
IMAGING APPARATUS AND METHODS
Abstract
Methods, systems and apparatus for manipulating electromagnetic
radiation such as laser beams include a first acousto-optic
deflector optimised for efficient transmission at the input beam
angle and a second acousto-optic deflector of lower peak efficiency
than the first acousto-optic deflector but which accepts beams from
a wider range of angles at better transmission efficiency than the
first acousto-optic deflector by passing a beam optimised for
efficient transmission through the first acousto-optic deflector at
the input beam angle, deflecting the beam using the first
acousto-optic deflector, passing the deflected beam through the
second acousto-optic deflector, which has a lower peak efficiency
than the first acousto-optic deflector and accepts beams from a
wider range of angles at better transmission efficiency than the
first acousto-optic deflector, and deflecting the beam using the
second acousto-optic deflector.
Inventors: |
Kirkby; Paul Anthony;
(London, GB) ; Nadella; K. M. naga Srinivas;
(London, GB) ; Silver; Robin Angus; (London,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UCL Business PLC |
London |
|
GB |
|
|
Assignee: |
UCL Business PLC
London
GB
|
Family ID: |
37232800 |
Appl. No.: |
14/796669 |
Filed: |
July 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14176729 |
Feb 10, 2014 |
9104087 |
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14796669 |
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13616680 |
Sep 14, 2012 |
8687268 |
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14176729 |
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12440809 |
Nov 25, 2009 |
8294977 |
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PCT/GB2007/003455 |
Sep 12, 2007 |
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13616680 |
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Current U.S.
Class: |
359/310 |
Current CPC
Class: |
G02F 1/332 20130101;
G01N 21/6458 20130101; G02B 21/0024 20130101; G02F 2201/16
20130101; G02B 21/0036 20130101; G02F 2203/28 20130101; G02F 1/0072
20130101; G02F 1/33 20130101 |
International
Class: |
G02F 1/33 20060101
G02F001/33; G02F 1/00 20060101 G02F001/00; G02B 21/00 20060101
G02B021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 2006 |
GB |
GB0617945.1 |
Claims
1. Apparatus for deflecting a beam of electromagnetic radiation,
said apparatus comprising: a first acousto-optic deflector
optimised for efficient transmission at the input beam angle; and a
second acousto-optic deflector of lower peak efficiency than said
first acousto-optic deflector but which accepts beams from a wider
range of angles at better transmission efficiency than said first
acousto-optic deflector.
2. Apparatus according to claim 1, wherein said first acousto-optic
deflector comprises a first crystal transducer and said second
acousto-optic deflector comprises a second crystal transducer,
wherein said second crystal transducer is arranged to create a more
diverging acoustic wave in the second acousto-optic deflector than
the acoustic wave created by said first crystal transducer in said
first acousto-optic deflector.
3. Apparatus according to claim 1, wherein said first crystal
transducer is wider in the direction of light propagation than said
second crystal transducer.
4. Apparatus according to claim 1, wherein said second crystal
transducer has a width parallel to the direction of light
propagation of less than 1 mm.
5. Apparatus according to claim 1, further comprising: a third
acousto-optic deflector optimised for efficient transmission at the
input beam angle; and a fourth acousto-optic deflector of lower
peak efficiency than the third acousto-optic deflector but which
accepts beams from a wider range of angles at better transmission
efficiency than said third acousto-optic deflector.
6. Apparatus according to claim 5, wherein said acousto-optic
deflectors are arranged in this order along the path of the beam:
first, second, third, fourth; or wherein said acousto-optic
deflectors are arranged in this order along the path of the beam:
first, third, second, fourth.
7. Apparatus according to claim 1, wherein said second
acousto-optic deflector comprises a crystal cut with less than
three degrees deliberate misorientation of the optic axis from the
direction of propagation of the acoustic wave; and/or wherein the
second acousto-optic deflector is oriented such that acoustic waves
propagating therethrough will have approximately twenty degrees
between their wave vector and their Poynting vector.
8. Apparatus according to claim 1, wherein said acousto-optic
deflectors are made from a high efficiency anisotropic
acousto-optic crystal, preferably TeO.sub.2 crystals.
9. A method of deflecting a beam of electromagnetic radiation, said
method comprising: passing a beam through a first acousto-optic
deflector that has been optimised for efficient transmission at the
input beam angle; deflecting said beam using said first
acousto-optic deflector; passing said deflected beam through a
second acousto-optic deflector that has a lower peak efficiency
than said first acousto-optic deflector but which accepts beams
from a wider range of angles at better transmission efficiency that
said first acousto-optic deflector; and deflecting said beam using
said second acousto-optic deflector.
10. A method according to claim 9, wherein said step of deflecting
said beam using said first acousto-optic deflector comprises
creating an acoustic wave in said first acousto-optic deflector,
wherein said step of deflecting said beam using said second
acousto-optic deflector comprises creating an acoustic wave in said
second acousto-optic deflector, and wherein a more diverging
acoustic wave is created in the second acousto-optic deflector than
in the first acousto-optic deflector.
11. A method according to claim 9, wherein said first acousto-optic
deflector comprises a first crystal transducer, said second
acousto-optic deflector comprises a second crystal transducer, and
wherein said first crystal transducer is wider in the direction of
light propagation than said second crystal transducer; and/or
wherein said second crystal transducer has a width parallel to the
direction of light propagation of less than 1 mm.
12. A method according to claim 9, further comprising: passing said
beam through a third acousto-optic deflector that has been
optimised for efficient transmission at the input beam angle;
deflecting said beam using said third acousto-optic deflector;
passing said deflected beam through a fourth acousto-optic
deflector that has a lower peak efficiency than said third
acousto-optic deflector but which accepts beams from a wider range
of angles at better transmission efficiency that said third
acousto-optic deflector; and deflecting said beam using said fourth
acousto-optic deflector.
13. A method according to claim 12, wherein said steps of passing
said beam through said third acousto-optic deflector and deflecting
said beam using said third acousto-optic deflector are carried out
prior to passing the beam through said second acousto-optic
deflector.
14. A method according to claim 9, wherein said second
acousto-optic deflector comprises a crystal cut with less than
three degrees deliberate misorientation of the optic axis from the
direction of propagation of the acoustic wave; and/or wherein the
second acousto-optic deflector is oriented such that acoustic waves
propagating therethrough will have approximately twenty degrees
between their wave vector and their Poynting vector.
15. A method according to claim 9, wherein said acousto-optic
deflectors are made from a high efficiency anisotropic
acousto-optic crystal, preferably TeO.sub.2 crystals.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/176,729 filed on Feb. 10, 2014, which is a
continuation of Ser. No. 13/616,680 filed on Sep. 14, 2012, which
is a continuation of Ser. No. 12/440,809 filed Nov. 25, 2009, which
is a 371 U.S. National Stage of International Application No.
PCT/GB2007/003455, filed Sep. 12, 2007 which claims the benefit of
United Kingdom Patent Application No. GB 0617945.1, filed Sep. 12,
2006. The disclosures of the above applications are incorporated
herein by reference.
[0002] The present invention relates to apparatus and methods
involving the manipulation of a beam of electromagnetic radiation,
such as a laser beam. More particularly, the invention relates to
apparatus and methods which use a laser beam to image a target
space, such as by selectively focussing the laser beam in the
target space, which may be a 2D plane or a 3D volume. Several
independent improvements to the state of the art are disclosed.
[0003] The ability to steer and focus electromagnetic radiation,
such as a laser beam, rapidly in three-dimensions is very
attractive for several applications in biology, microfabrication
and data storage.
[0004] Laser scanning confocal imaging is an important and widely
used tool in biology because it allows high contrast visualization
of subcellular structures and monitoring of physiological processes
with fluorescence indicators within living tissue by excluding
contaminating of out-of focus light. Conventional confocal methods
work best at relatively shallow depths where light penetration is
good and scattering is minimal. Unfortunately, conventional
confocal imaging cannot be used to image biological activity deep
(>100 .mu.m) within the living tissue. However, more recently, a
new type of laser scanning confocal microscopy has been developed
that relies on non-linear multiphoton excitation to selectively
activate fluorophores where the light intensity exceeds the
2-photon threshold at the centre of the focal volume. Fluorescent
light is emitted in all directions by these fluorophores and is
picked up by a high numerical aperture lens system and
photomultipliers. As the focal spot is scanned through the tissue
the light intensity emitted by the fluorophores varies according to
the intensity of staining by the fluorescence indicators in that
part of the tissue. Combining the photomultiplier signal with the
known position of the 2-photon focal volume enables a 2D or 3D
image of the fluorescence intensity within the tissue to be
reconstructed. This technique, known as two-photon microscopy,
allows imaging at much greater depth because of the longer
excitation wavelengths used for multiphoton excitation (wavelengths
of 700-1000 nm), which scatter less than those used in conventional
confocal imaging, and because confocality arises intrinsically from
the excitation volume allowing all emitted photons to be used to
construct the image. These properties together with the low levels
of photodamage achievable have made 2-photon imaging an extremely
powerful method for examining physiological processes at the
cellular and subcellular levels both in vitro and in vivo.
[0005] Two-photon imaging has been particularly popular in
neuroscience, as it has allowed the dynamic properties of neuronal
network activity to be imaged in intact brain tissue using calcium
indicators. The spatial resolution of 2-photon microscopy is well
suited to this task even allowing the small synaptic connections
between neurons to be resolved. Multiphoton excitation has also
begun to be used to photolyse "caged compounds" that release
neurotransmitters, allowing synaptic inputs onto a cell to be
mimicked. This technique is potentially very important for
understanding synaptic integration and thus determining how
individual neurons carry out low-level computations.
[0006] Conventional laser scanning microscopes have traditionally
used galvanometer mirrors to scan a laser beam. Such galvanometer
mirrors are configured to scan in the X-Y plane only. Focussing in
the Z direction is achieved by moving the apparatus relative to the
sample (for example by moving the objective lens closer to or
further away from the sample). The use of galvanometer mirrors has
an inherent disadvantage in that the mirrors necessarily have a
mass and the speed at which the mirror can be moved from one
position to another is limited by inertia. In practical terms, this
means that it takes of the order of 200-300 .mu.s to move a mirror
from one selected position to another selected position. In turn,
this limits the number of spots upon which a laser beam can be
focussed during a given time frame.
[0007] The temporal resolution of the present state of the art
galvanometer-based two-photon imaging systems is one or two orders
of magnitude too slow to accurately image signalling in a network
of neurons. In such neurons, the elementary signal event (action
potentials) occurs on the millisecond time scale. Moreover, the
signals are spatially distributed in three-dimensions as they flow
through the neural networks and building a 3-D stack of images
using galvanometer-based technology takes minutes. Furthermore,
galvanometers are too slow for studying synaptic integration in
individual neurons using photolysis because the excitation beam
needs to be moved to many (for example 30) sites within a
millisecond in order to stimulate synapses distributed over the
dendritic tree. For example, assuming that it takes 300 .mu.s to
move from one spot to another using a galvanometer mirror and
assuming a dwell time at each spot of 5 .mu.s, it would take 9.15
ms to image 30 sites. This is approximately 10 times too slow for
current needs.
[0008] One approach suggested in the prior art to overcome some of
the disadvantages is to use rapid acousto-optic deflectors (AODs)
instead of galvanometers to steer the two-photon laser beam. The
advantage of using AODs is that they allow the laser beam to be
moved much more rapidly from point-to-point than in a
galvanometer-based system (compare a movement time of 5-25 .mu.s
with AODs to 200-400 .mu.s with galvanometers). This has several
potential advantages. Firstly, images can be scanned rapidly.
Secondly, multiple point measurements can be made with long dwell
times at very high temporal resolution (e.g. using an AOD system
with 15 .mu.s movement time, 33 points can be simultaneously
sampled at 1 KHz sample rate with a 15 .mu.s dwell time or in other
words 33 points can be monitored 1000 times per second). The use of
AODs therefore allows more of the time to be devoted to collecting
photons from the regions of interest rather than being taken up in
moving the laser beam between sites.
[0009] As well as deflecting the laser beam in the X,Y plane, the
use of two AODs per axis can, in principle, also be used to focus
the laser beam in the Z dimension. For example, Kaplan et al
describe in "Acousto-Optic Lens with Very Fast Focus Scanning"
Optics Letters, Vol. 26, No. 14, Jul. 15 2001, pp 1078-1080, the
use of two or four AODs to focus a laser beam in the X and Z plane
or anywhere in an X,Y and Z volume. To achieve focussing in a 3D
volume, two AODs for focussing in the X-Z plane are followed by two
AODs for focussing in the Y-Z plane.
[0010] One particular problem associated with multi-photon AOD
scanning is spatial and temporal dispersion. Multi-photon
applications typically require ultra-short laser pulses, for
example of the order of 100 fs. However, the shorter the pulse, the
larger the spread of wavelengths that exist in the pulse. The
limiting example is an infinitely short pulse which has a
completely flat frequency spectrum (i.e. a white spectrum). A pulse
of 100 fs typically has a full-width, half-maximum spectral width
of approximately 10 nm. The angle by which an AOD deflects a laser
beam is related to the wavelength of the laser beam. Longer
wavelengths are deflected more than shorter wavelengths. Thus, a
form of spatial dispersion, also known as chromatic aberration,
occurs when an ultra-short pulse is deflected by an AOD. When a
laser pulse is diffracted through an AOD and then brought to a
focus, the low frequency (long wavelength) parts of the pulse are
focussed to a different position than the high frequency (short
wavelength) parts of the pulse. This causes the pulse to be
focussed to a line rather than a spot, the length of the line being
related to the spectral width of the pulse. Further, different
wavelengths of light travel at different speeds through the AOD
which causes temporal dispersion, i.e. elongation of the pulse in
time. These problems are described in "Compensation of Spatial and
Temporal Dispersion for Acousto-Optic Multiphoton Laser-Scanning
Microscopy" by Iyer et al, Journal of Biomedical Optics, 8(3), July
2003, pp 460-471.
[0011] These dispersion problems provide two system
limitations--(i) they worsen the spatial resolution of the system,
and (ii) they limit the excitation energy density that is achieved
at the focus, thus reducing excitation efficiency.
[0012] Iyer et al propose the use of a diffraction grating matched
to the central acoustic wavelength in order to relieve some of the
spatial dispersion. However, this solution only corrects the
chromatic aberration for a single spot in the image plane with the
chromatic aberration steadily increasing as you move away from this
spot in the image plane. This effect is known as magnification
chromatic aberration because it can also be described by saying
that the longer wavelength component of the image has greater
spatial magnification than the shorter wavelength component.
[0013] Reddy & Saggau ("Fast Three-Dimensional Laser Scanning
Scheme Using Acousto-Optic Deflectors", Journal of Biomedical
Optics, 10(6), November/December 2005) and Salome et al ("Ultrafast
Random-Access Scanning in Two-Photon Microscopy using Acousto-Optic
Deflectors", Journal of Neuroscience Methods, 154 (2006), pp
161-174) disclose a similar result in which two AODs are used to
correct for chromatic aberration at one line in the image field but
with chromatic aberration increasing as the deflection diverges
from the compensation line. This is illustrated in FIG. 4c of Reddy
& Saggau.
[0014] Thus, even with the best compensation systems disclosed in
the prior art, chromatic aberration can be eliminated at one single
point only in the field of view. The consequent focussing of a
laser pulse to a line, rather than a diffraction limited spot,
reduces the number of resolvable spots in the image plane. In a 3D
image space, the effect of chromatic aberration can be quantified
by considering the Number of Resolvable Detection Volumes (NRDV) in
the image field. The detection volume of a 2-photon system is the
volume of tissue or other excitable material that has sufficient
intensity to be above its fluorescence activation threshold. As
2-photon processes are dependent on the square of optical intensity
this volume is always smaller than the conventional focal volume of
the point spread function of the focused laser beam. The prior
compensation systems show increasing chromatic aberration as one
moves away from the compensation point and thus the NRDV is much
less than if the chromatic aberration were corrected for
substantially the whole image space. Such prior compensation
systems therefore do not give the desired spatial resolution to be
used effectively for many applications, such as most 3D random
access applications (e.g. neuroscience applications).
[0015] For such applications it is highly desirable to be able to
randomly address any location in a volume of approximately
250.times.250.times.250 .mu.m with a spatial resolution of
1.times.1.times.2 .mu.m at a rate of 20-30 .mu.s per point. This
corresponds to a number of resolvable detection volumes (NRDV) of
7.8 million. Detailed modelling of the designs proposed to date has
shown that the prior art methods struggle to achieve an NRDV in
excess of 200,000. This is a factor of 40 below what is a desirable
target for neuroscience applications.
[0016] Reddy & Saggau disclose coupling two adjacent AODs using
telecentric relay optics (for example see FIG. 1c of their paper,
supra). When four AODs are provided in a system capable of scanning
a spot within a 3D volume, this necessitates at least three sets of
telecentric relay optics. A typical length of a prior art
telecentric relay, in the direction of the laser beam, is 400 mm.
Thus, the requirement to utilise three telecentric relays adds 1.2
m to the length of the system in the path of the electromagnetic
radiation. Accordingly, it is difficult to construct a device with
a compact configuration and with minimal losses. It would be
desirable to implement a shorter configuration, with less
loss-introducing components, but maintaining the functionality of
the AODs in focussing the laser beam.
[0017] When an AOD deflects a laser beam, it is the first order
component that is usually of interest. The AOD will typically also
pass an undeflected zeroth order component that can interfere with
the signal. Kaplan et al solves this problem by ensuring that the
two AODs deflect the laser beam in the same direction such that the
undeflected zeroth order component does not reach the image field.
An alternative configuration proposed by Reddy & Saggau of two
parallel AODs suffers the potential problem that undeflected zeroth
order components can reach the image field. Note that for the
highest efficiency, the anisotropic crystals of the deflectors are
used in the shear acoustic mode. These AODs have the property that
the polarisation of the diffracted first order beam has its
polarisation rotated through 90 degrees compared to the incoming
laser beam and the zero order undeflected transmitted beam.
[0018] Another problem lies in the physical design of the AODs.
Usually, the AOD devices are designed to have a good transmission
efficiency (e.g. approximately 80%), but only for a very narrow
range of input acceptance angles, (typically .+-.1.5 mrad). As the
input acceptance angle varies, efficiency typically reduces. Thus,
when two AODs are used in series, the second AOD will receive light
at an angle defined by the deflection angle of the first AOD. Where
the first AOD deflects the beam by a relatively large angle (e.g.
greater than 1.5 mrad), this can cause the diffraction efficiency
of the second AOD to be very low. It is therefore be desirable to
design an AOD having higher efficiencies at a larger range of input
acceptance angles.
[0019] There also exists a problem in that, for some applications,
obtaining efficient transmission of the laser power is paramount,
whereas for others, obtaining high spatial accuracy is paramount.
For example, the use of a two-photon system for photolysis requires
a much higher laser power than when used for imaging. Further, a
reduced NRDV can be tolerated in photolysis applications. It would
therefore be desirable to have a system in which the NRDV/power
trade-off can be varied in accordance with the application to which
the system is put.
[0020] It is convenient to be able to perform two-photon microscopy
or photolysis at a selected wavelength. Typically, the useful
wavelength range is 700 nm to 1000 nm. However, there is a problem
in that diffractive optics inherently deflect by different amounts
at different wavelengths (due to the fact that the diffraction
angle increases as the wavelength increases). Providing a system
that can operate under a range of different wavelengths is
therefore difficult and would be desirable.
[0021] Furthermore, it would be desirable to provide a system that
can provide selectable chromatic aberration correction. For
example, it would be desirable to provide a system that can be
configured easily to correct all the chromatic aberration in one
X-Y plane. Such a system should preferably also be capable of being
configured to correct all chromatic aberration in the Z plane.
Preferably, such configuration should be via simple means such as
moving lens systems, rather than by replacing components.
[0022] It is also desirable to provide a system that can operate in
more than one mode. For example, a pointing mode in which a series
of predetermined points can be visited sequentially is useful.
Also, a scanning mode, in which the laser beam focus moves smoothly
over the target is also useful. A system which can be easily
switched between these modes is therefore very desirable.
[0023] It is furthermore desirable to provide a system which can
perform scanning in a smooth fashion even though there are limits
on the minimum and maximum frequencies that can be put through an
acousto-optic deflector. These limits traditionally mean that
scanning has to be stopped when the limit is reached. A system that
can overcome this problem would be extremely desirable.
[0024] These and other problems are addressed by embodiments of the
present invention.
[0025] In a first aspect, the present invention provides a system
for selectively focussing a laser beam, said system comprising:
diffractive optics for focussing the laser beam in an image field,
said diffractive optics being such that, when said laser beam has
spectral width, said diffractive optics will cause, in use,
magnification chromatic aberration in said image field; and at
least one optical element for at least partially correcting said
magnification chromatic aberration, which at least one optical
element is arranged to modify said image field such that the longer
wavelength components are magnified less than the shorter
wavelength components.
[0026] The diffractive optics for deflecting and/or focussing the
laser beam are preferably one or more acousto-optic deflectors.
[0027] The spectral width of the laser beam (for example 10 nm for
a 100 fs pulsed laser beam) causes chromatic aberration in the
image field. Further there are different amounts of chromatic
aberration at different positions in the image field. In the usual
case, there is one point in the image field, known as the
compensation point, where chromatic aberration is at a minimum. The
chromatic aberration generally increases at positions away from
this compensation point in the image field. If one takes a slice
through the image field intersecting the compensation point, a
graph of the chromatic aberration would cross the zero line at the
compensation point as the aberration changed from negative to
positive (or vice versa) and would thus increase in magnitude
either side of the compensation point, usually in a straight line.
The gradient of this line is known as the magnification chromatic
aberration and the first aspect of the invention diminishes the
magnification chromatic aberration by reducing the gradient of this
line. Thus, the magnitude of the chromatic aberration existing at
every point in the image field is reduced (except at the
compensation point where the magnitude is already zero).
[0028] The system preferably comprises a laser for supplying the
laser beam, which laser is preferably a pulsed laser having laser
pulses of 2 ps or less, preferably 500 fs or less, more preferably
still about 100 fs.
[0029] The centre frequency of the laser beam is typically in the
range 600 to 1000 nm, preferably 700 to 900 nm, more preferably 800
to 875 nm, and more preferably still approximately 850 nm.
[0030] The correction can be carried out in a 2D image plane in
which case it is possible to use either a modified microscope
objective lens as the optical element for correcting the
magnification chromatic aberration or an additional dispersive lens
prior to the objective, for instance at the usual tube lens
position or at some similar position earlier in the optical relay
chain.
[0031] In one embodiment, a telecentric relay is used to provide
the necessary correction. Preferably, the telecentric relay has
first and second lenses and the rates of change of focal length
with wavelength for the first and second lenses are of opposite
sign. It is preferable that the rates of change of focal length
with wavelength for the first and second lenses are of
substantially the same magnitude. Preferably, the first lens (i.e.
the one the laser beam encounters first) has a shorter focal length
for longer wavelengths than for shorter wavelengths and the second
lens has a longer focal length for longer wavelengths than for
shorter wavelengths. This means that the longer wavelengths are
magnified less by the telecentric relay and this provides the
necessary correction to the magnification chromatic aberration.
[0032] The first and/or second lenses are preferably dispersive
lenses and can be made from combinations of crown glass, flint
glass and diffractive optical elements.
[0033] The use of a telecentric relay allows the magnification
chromatic aberration to be at least partially corrected for all
points in a 3D image field and not just in a 2D plane.
[0034] In one preferable embodiment, a compensation factor C can be
defined, a value of C=1 providing perfect compensation for all of
the chromatic aberration in the Z-direction, a value of C=2
providing perfect compensation for all of the chromatic aberration
in the X and Y directions, and wherein C is selected to be less
than 2.
[0035] Preferably, C is selected to be around 1.3.
[0036] In connection with the first aspect of the invention, there
is also provided a method for at least partially correcting
magnification chromatic aberration introduced into a laser beam by
diffractive optics, said method comprising: passing said laser beam
through at least one optical element so as to at least partially
correct the magnification chromatic aberration.
[0037] The diffractive optics can be one or more AODs or any
alternative dynamically controlled system for deflecting and
focussing a laser beam. Such alternative dynamic diffractive
systems might for example be based on liquid crystal holographic
optical elements, magneto-optic arrays, digital micromirror arrays
or any other spatial light modulator device.
[0038] In connection with the first aspect of the invention, there
is also provided a magnification chromatic aberration correcting
telecentric relay comprising: a first lens; second lens; wherein
the rates of change of focal length with wavelength for the first
and second lenses are of opposite sign in the wavelength range of
interest.
[0039] Preferably, the rates of change of focal length of
wavelengths for the first and second lenses are of substantially
the same magnitude. Also, the first and second lenses can be
separated by a distance approximately equal to the sum of their
focal length for all wavelengths in the wavelength range of
interest.
[0040] The lenses can be made of combinations of crown and flint
glass. More preferably, the lenses comprise diffractive elements
attached to conventional lenses.
[0041] A second aspect of the invention provides apparatus for
selectively deflecting a laser beam, said apparatus comprising: a
first acousto-optic deflector that is arranged to modulate a laser
beam into at least (i) a zeroth order component of identical
polarisation to the input laser beam and (ii) a first order
component having a polarisation rotated by 90.degree. compared to
the input laser beam; a first half-wave plate that is arranged to
rotate the polarisation of the output of said first acousto-optic
deflector by 90.degree.; a first polariser that is arranged to pass
said polarisation-rotated first order component and to block said
polarisation-rotated zeroth order component; a second acousto-optic
deflector that is arranged to modulate said passed first order
component to produce at least (i) a second zeroth order component
of identical polarisation to said passed first order component and
(ii) a second first order component of polarisation rotated by
90.degree. compared to said passed first order component; a second
polariser that is arranged to pass said second first order
component and to block said second zeroth order component.
[0042] The use of the half-wave plate and two polarisers allows the
undesirable zeroth order components to be blocked effectively
without substantially reducing the power of the desirable first
order components.
[0043] The first and second acousto-optic deflectors can be used to
deflect and focus the laser beam in the X-Z plane. Additional third
and fourth acousto-optic deflectors may be provided to provide
additional focussing in the Y-Z plane. An additional half-wave
plate and a further two polarisers can be used to block the zeroth
order components that may be created in the third and fourth
acousto-optic deflectors.
[0044] This construction allows the wanted output beam (that is the
first order diffracted beam from each AOD) to be diffracted first
in one direction, then in the opposite direction by the counter
propagating acoustic wave in the second crystal of each pair. Thus
the final net beam deflection at the centre of the image field is
zero. This makes the arrangement naturally self compensating for
chromatic dispersion at the centre of the image field. The use of
the polarisers has eliminated the potentially interfering zero
order undeflected beams from each AOD.
[0045] In connection with the second aspect, there is also provided
apparatus for selectively deflecting a laser beam, said apparatus
comprising: a first acousto-optic deflector; a second acousto-optic
deflector; a first polariser; and a second polariser.
[0046] The first polariser is preferably located between the first
and second acousto-optic deflectors. This allows it to cut out the
unwanted zero diffraction order and transmit the useful first
diffraction order.
[0047] The first and second acousto-optic deflectors are
conveniently arranged to deflect and focus a laser beam in a first
plane, such as in the X-Z plane, and the first and second
polarisers are conveniently arranged to pass only the first order
components of diffraction (and block the zeroth order components of
diffraction).
[0048] The first polariser preferably follows the first
acousto-optic deflector and the second polariser preferably follows
the second acousto-optic deflector.
[0049] To achieve additional deflection and focussing in the Y-Z
plane, third and fourth acousto-optic deflectors can be provided
together with third and fourth polarisers.
[0050] A particularly preferred configuration arranges the
acousto-optic deflectors in the order first, third, second, fourth.
This dispenses with the need for half-wave plates. Alternatively,
the acousto-optic deflectors can be arranged in the order first,
second, third, fourth and two half-wave plates can be arranged
between the first and second and the third and fourth acousto-optic
deflectors respectively.
[0051] In accordance with the second aspect, there is also provided
a method for selectively deflecting a laser beam, said method
comprising: using first and second acousto-optic deflectors to
focus a laser beam in a first plane; and using first and second
polarisers to pass the first order components of diffraction and to
block any zeroth order components of diffraction.
[0052] Third and fourth acousto-optic deflectors are preferably
used to focus the laser beam in the second plane and third and
fourth polarisers are preferably used to pass the first order
components of diffraction and block any zeroth order components of
diffraction.
[0053] The polariser of the laser beam is preferably rotated by
90.degree. subsequent to the laser beam exiting the first and third
deflectors but prior to the laser beam entering the second and
fourth polarisers respectively.
[0054] In accordance with a third aspect of the invention, there is
provided apparatus for deflecting a laser beam, said apparatus
comprising: a first acousto-optic deflector optimised for efficient
transmission at the input laser beam angle; and a second
acousto-optic deflector of lower peak efficiency than the first
acousto-optic deflector but which accepts laser beams from a wider
range of angles at better transmission efficiency than said first
acousto-optic deflector.
[0055] This aspect of the invention addresses the problem that, in
the prior art, the range of angles at which a laser beam could
enter the second acousto-optic deflector of each X-Z or Y-Z pair is
large which means that transmission efficiency is very poor at some
input angles. In accordance with this aspect of the invention, the
second acousto-optic deflector is designed so as to have an
efficiency versus input angle curve having a flatter and broader
peak than the first acousto-optic deflector. Thus, although
efficiency is reduced at the optimum input angle, efficiency is
increased at variations from the optimum angle such as to provide
acceptable transmission throughout the whole range of possible
input angles. To maintain sufficient output power from the system
for 2-photon imaging great care needs to be taken to minimise other
system losses. The loss of power can also be compensated by
increasing the power of the source laser.
[0056] There thus exists an optimum designed acceptance angle for
the second of each pair of AODs (i.e. the second and fourth AODs).
As their acceptance angle increases so the NRDV increases as the
scannable volume increases, but suddenly if the acceptance angle is
increased beyond the optimum, the diffraction efficiency drops too
low and the intensity of the laser spot reduces below the threshold
required for 2-photon fluorescence and the NRDV drops rapidly (note
the NRDV only counts detection volumes where the laser intensity is
above the two-photon threshold). Examples of such optimisation
curves can be seen in FIGS. 23 and 24.
[0057] The third aspect of the invention also provides a method of
deflecting a laser beam, said method comprising: passing a beam
through a first acousto-optic deflector that has been optimised for
efficient transmission at the input laser beam angle; deflecting
said beam using said first acousto-optic deflector; passing said
deflected beam through a second acousto-optic deflector that has a
lower peak efficiency than said first acousto-optic deflector but
which accepts laser beams from a wider range of angles at better
transmission efficiency that said first acousto-optic deflector;
and deflecting said beam using said second acousto-optic
deflector.
[0058] In a fourth aspect of the invention, there is provided
apparatus for deflecting a laser beam, said apparatus comprising:
first and second acousto-optic deflectors for focusing a laser beam
in a first direction; and third and fourth acousto-optic deflectors
for focusing a laser beam in a second direction; wherein said
acousto-optic deflectors are arranged in this order along the path
of the laser beam: first, third, second, fourth.
[0059] This particular order of acousto-optic deflectors dispenses
with the need for half-wave plates to rotate the polarisation of
the light. This first acousto-optic deflector will transmit a first
order component of diffraction that is rotated by 90.degree.
compared to the input laser beam. The third acousto-optic deflector
is well-suited for receiving this first order component of
diffraction and will transmit a further first order component that
is rotated by a further 90.degree.. The second acousto-optic
deflector is well-suited for receiving this laser beam and will
again rotate the polarisation by a further 90.degree., making it
suitable for reception by the fourth acousto-optic deflector. The
inherent polarisation rotation introduced to the first order
components of diffraction by the AODs are, when this order of AODs
is used, compatible with the polarisation acceptance of the next
AOD in the sequence.
[0060] In accordance with the fourth aspect, there is also provided
a method of deflecting a laser beam, said method comprising: using
first and second acousto-optic deflectors to focus a laser beam in
a first direction; and using second and third acousto-optic
deflectors to focus a laser beam in a second direction; wherein
said acousto-optic deflectors are arranged in this order along the
path of the laser beam: first, third, second fourth.
[0061] A fifth aspect of the invention provides an acousto-optic
deflector comprising: a crystal for propagating an acoustic wave
that will diffract an input laser beam; a first crystal transducer
for supplying acoustic vibrations to the crystal; and a second
crystal transducer for supplying acoustic vibrations to the
crystal; wherein said first and second crystal transducers are
located on the same side of the crystal.
[0062] The first crystal transducer is preferably arranged to
create a more diverging acoustic wave in the crystal than said
second crystal transducer.
[0063] The effect of the more diverging acoustic wave is preferably
to allow the efficient diffraction of laser beams coming from a
wider range of angles.
[0064] A more diverging acoustic wave can be created by adjusting
the width of the first crystal transducer to be smaller in the
direction parallel to the direction of light propagation. For
example, a crystal transducer width of less than 1 mm can create an
appropriate diverging acoustic wave.
[0065] The second crystal transducer is preferably wider in the
direction of light propagation than the first crystal transducer.
This allows the first crystal transducer to be one which supplies a
more diverging acoustic wave and the second crystal transducer to
be one which supplies a less diverging acoustic wave. The two
transducers therefore are preferably designed to create acoustic
waves having different properties giving added flexibility to the
system.
[0066] Preferably, each crystal transducer can be independently
selectable such that one or both may be excited to modulate the
divergence of the acoustic wave in the crystal.
[0067] There can be provided any number of crystals, such as one,
two, three, four or more.
[0068] A switch mechanism can be provided to selectively allow for
only one transducer to be excited, two transducers to be excited
together or three transducers to be excited together. The
transducers are preferably adjacent to one another.
[0069] In a preferred embodiment, the width of each transducer
increases in a geometric progression in a direction parallel to the
direction of light propagation.
[0070] In accordance with a sixth aspect of the invention, and a
selection switch is provided to allow either the first or second
crystal transducer to be excited. In more particularity, the sixth
aspect of the invention provides an acousto-optic deflector
comprising: a crystal for propagating an acoustic wave that will
diffract an input laser beam; a first crystal transducer for
supplying acoustic vibrations to the crystal; a second crystal
transducer for supplying acoustic vibrations to the crystal; and a
selection switch for selecting whether the first or second crystal
transducer is excited.
[0071] The sixth aspect of the invention provides a method of
deflecting a laser beam, said method comprising: selecting one of a
first or second crystal transducer arranged to supply acoustic
vibrations to a crystal; exciting said selected crystal transducer
so as to propagate an acoustic wave in said crystal; and
diffracting said laser beam with said acoustic wave.
[0072] In accordance with a seventh aspect of the invention, the
crystal of the acousto-optic deflector has a particular
orientation. More specifically, the seventh aspect of the invention
provides an acousto-optic deflector comprising: a crystal having a
laser input direction defined by the negative Z-axis; wherein said
crystal structure is rotated by approximately 2.degree. about the
X-axis and approximately 3.degree. about the Y-axis.
[0073] Also in accordance with the seventh aspect of the invention,
there is provided an acousto-optic deflector comprising: a crystal
oriented such that acoustic waves propagating therethrough will
have approximately 20.degree. between their wave vector and their
Poynting vector.
[0074] Preferably, the second of a pair of acousto-optic deflectors
has a construction in accordance with the second aspect of the
invention. In such a case, it is useful that the first deflector in
the pair also has the same construction.
[0075] According to an eighth aspect of the invention, there are
provided systems and methods which can account for a non-zero
effective optical separation between adjacent AODs.
[0076] This can preferably be achieved by providing a system for
manipulating a beam of electromagnetic radiation, said system
comprising: a first acousto-optic deflector; a second acousto-optic
deflector positioned downstream of said first acousto-optic
deflector and being separated from said first acousto-optic
deflector by an effective optical separation; a driver for
providing acoustic waves in said first and second acousto-optic
deflectors, said acoustic waves being chirped at different ramp
rates to account for said effective optical separation between said
first and second acousto-optic deflectors.
[0077] Preferably, the driver is arranged to provide acoustic waves
that cause the electromagnetic radiation to be focused to a
stationary line in space.
[0078] Preferably, there is provided a system wherein said driver
provides an acoustic wave with a ramp rate a.sub.1 to said first
acousto-optic deflector and provides an acoustic wave with a ramp
rate a.sub.2 to said second acousto-optic deflector, and wherein
said ramp rates are related by:
a 1 a 2 = 2 d 2 ' 2 d 2 ' + d 1 ##EQU00001##
where d.sub.1 is the effective optical separation between said
first and second acousto-optic deflectors and d.sub.2' is the
effective optical distance to the focal line from the second
acousto-optic deflector.
[0079] In preferred embodiments, the system further comprises a
third acousto-optic deflector; a fourth acousto-optic deflector
positioned downstream of said third acousto-optic deflector and
being separated from said third acousto-optic deflector by an
effective optical separation; wherein said driver is arranged to
provide acoustic waves in said third and fourth acousto-optic
deflectors, said acoustic waves being chirped at different ramp
rates to account for said effective optical separation between said
third and fourth acousto-optic deflectors.
[0080] The driver is preferably arranged to select frequencies of
the acoustic waves that scan a target in the X and/or Y
direction.
[0081] The driver is preferably arranged to select frequencies for
said first and second acousto-optic deflectors such as to achieve
an angular scan rate of .delta..theta./.delta.t by adjusting the
ramp rate a.sub.1 of the first acousto-optic deflector to be:
a 1 = V .lamda. ( V d 2 ' - .delta..theta. .delta. t ) 2 + d 1 d 2
' - d 1 V .delta..theta. .delta. t ##EQU00002##
[0082] and by adjusting the ramp rate a.sub.2 of the second
acousto-optic deflector to be:
a 2 = V 2 2 .lamda. d 2 ' + V 2 .lamda. .delta..theta. .delta. t
##EQU00003##
[0083] where V is the speed of sound in the first and second
acousto-optic deflectors, .lamda. is the wavelength of the laser
beam to be deflected, d.sub.2' is the distance to the focal
line/point from the second acousto-optic deflector and d.sub.1 is
the effective optical separation between said first and second
acousto-optic deflectors.
[0084] The driver preferably provides acoustic waves such as to
scan a target in the Z and/or Y direction, said scan being composed
of a series of mini-scans, with a non-active scan period between
each active scan time of each mini-scan.
[0085] This non-active period can be used to adjust the value of
the frequencies without moving the focus position and preferably
consists of a frequency resetting time and a AOD fill time.
[0086] The non-active time starts at the end of the active scan
time of one mini-scan and ends at the beginning of the active scan
time of the subsequent mini-scan.
[0087] The active scan time is preferably that time for which
measurements are taken and, generally, measurements are not taken
during the non-active time duration.
[0088] Also included in the eighth aspect of the invention is a
method of manipulating a beam of electromagnetic radiation, said
method comprising passing said electromagnetic radiation through a
first acousto-optic deflector and a second acousto-optic deflector
downstream of said first acousto-optic deflector, the deflectors
containing first and second acoustic waves respectively; wherein
said first and second acoustic waves are chirped at different ramp
rates to account for the effective optical separation between said
first and second acousto-optic deflectors.
[0089] In accordance with the ninth aspect of the invention, there
is provided a method of scanning a target volume with a beam of
electromagnetic radiation, said method comprising passing said
electromagnetic radiation through a first acousto-optic deflector
and a second acousto-optic deflector downstream of said first
acousto-optic deflector, the deflectors containing first and second
acoustic waves respectively so as to move a focus position of said
beam along a scan path in said target volume; wherein said first
and second acoustic waves are chirped to have a constantly
increasing or decreasing frequency; and when one of said acoustic
waves is at a predetermined maximum or minimum frequency value,
offsetting the frequency of each acoustic wave such that the
acoustic waves may continue to be chirped while having frequencies
lower than said predetermined maximum frequency and higher than
said predetermined minimum frequency.
[0090] The frequency offsetting can be carried out whether or not
the acousto-optic deflectors are telecentrically coupled or there
is a real optical separation between the deflectors. Preferably,
said first and second acousto-optic deflectors are separated by an
effective optical separation d.sub.1 and said focal position is an
effective optical distance d.sub.2' from said second acousto-optic
deflector, said offsets satisfying:
.DELTA. f 1 .DELTA. f 2 = 2 d 2 ' 2 d 2 ' + d 1 ##EQU00004## [0091]
where .DELTA.f.sub.1 is the frequency offset for the first acoustic
wave and .DELTA.f.sub.2 is the frequency offset for the second
acoustic wave.
[0092] Preferably, the target volume is scanned as a series of
mini-scans, the active scan time of each mini-scan terminating
approximately at the point where the frequency of each acoustic
wave is offset and the active scan time of subsequent mini-scans
beginning after a non-active period from said termination point of
the previous mini-scan.
[0093] Also in accordance with the ninth aspect of the invention
there is provided a system for scanning a target volume, said
system comprising a first acousto-optic deflector; a second
acousto-optic deflector positioned downstream of said first
acousto-optic deflector; a driver for providing acoustic waves in
said first and second acousto-optic deflectors; wherein said driver
is arranged to offset the frequency of each acoustic wave when one
of said acoustic waves reaches a predetermined maximum or minimum
frequency value so as to maintain a predetermined chirp rate for
said acoustic weaves while keeping the absolute frequency value for
said first and second acoustic waves between said predetermined
minimum and maximum frequency values.
[0094] In accordance with the tenth aspect of the invention, there
is provided a system for selectively focussing a beam of
electromagnetic radiation, said system comprising diffractive
optics for focussing the beam in an image field, said diffractive
optics causing chromatic aberration in said image field; corrective
optics for at least partially correcting said chromatic aberration,
said corrective optics being adjustable such that said chromatic
aberration can be at least partially corrected when the
electromagnetic radiation has a wavelength falling within a range
of wavelengths of interest.
[0095] Preferably, the corrective optics is capable of ensuring
that the beam of electromagnetic radiation fills the same design
system aperture for substantially all wavelengths falling within
the range of the wavelength of interest. For example, the radiation
can be made to fill the aperture of the system objective lens to
provide the maximum focussing resolution.
[0096] The corrective optics preferably comprises first and second
diffractive correction plates.
[0097] These correction plates may also be comprised of
conventional lenses.
[0098] Preferably, the first correction plate has a first
diffractive element having positive power attached to a negative
focal length real lens and the second correction plate has a second
diffractive element having negative power, attached to a positive
focal length real lens. The attachment is preferably intimate, for
example by optical gluing.
[0099] The diffractive optical element and real lens in each
correction plate is preferably balanced such that at a
predetermined wavelength in the mid-operating range (for example
800-850 nm), both correction plates are of close to zero effective
power.
[0100] Further lenses can be placed either side of the correction
plate.
[0101] These further lenses can be implemented by a pair of zoom
lenses.
[0102] In accordance with the tenth aspect, there is provided a
method of selectively focussing a beam of electromagnetic
radiation, said method comprising passing said beam of
electromagnetic radiation through diffractive optics to focus the
beam in an image field, said diffractive optics causing chromatic
aberration in said image field; adjusting corrective optics when
the wavelength of said electromagnetic radiation is changed within
a range of wavelengths of interest; passing said electromagnetic
radiation through said adjusted corrective optics to at least
partially correct said chromatic aberration.
[0103] In accordance with an eleventh aspect of the invention,
there is provided a system for selectively focussing a beam of
electromagnetic radiation, said system comprising diffractive
optics for focussing the beam in an image field, said diffractive
optics causing chromatic aberration in said image field; corrective
optics for at least partially correcting said chromatic aberration,
said corrective optics being capable of substantially fully
correcting chromatic aberration in an X-Y plane when a compensation
factor C is equal to 2 and being capable of substantially fully
correcting chromatic aberration in a Z plane when said compensation
factor C is equal to 1, said compensation factor being user
selectable.
[0104] The compensation factor C is preferably set by moving
diffractive elements.
[0105] The system is preferably arranged to receive from a user a
desired compensation factor and a desired electromagnetic radiation
wavelength.
[0106] Preferably, upon receiving such input, the system moves
elements of the corrective optics so as to provide a chromatic
aberration correction in accordance with the compensation factor at
the desired wavelength.
[0107] In accordance with the eleventh aspect, there is provided a
method of selectively focussing a beam of electromagnetic
radiation, said method comprising passing said electromagnetic
radiation through diffractive optics, said diffractive optics
causing chromatic aberration in an image field; selecting a
compensation factor C; configuring corrective optics in accordance
with said selected compensation factor C; passing said
electromagnetic radiation through said corrective optics to at
least partially correct said chromatic aberration, wherein said
corrective optics are capable of substantially fully correcting
chromatic aberration in an X-Y plane when said compensation factor
C is equal to 2 and are capable of substantially fully correcting
chromatic aberration in a Z plane when said compensation factor C
is equal to 1.
[0108] In accordance with the twelfth aspect of the invention,
there is provided a system for manipulating a beam of
electromagnetic radiation, said apparatus comprising a first
acousto-optic deflector; a second acousto-optic deflector; a driver
for providing first and second acoustic waves to said first and
second acousto-optic deflectors respectively; a user operated
switch for selecting between a random access mode and a scanning
mode.
[0109] Preferably, when said random access mode is selected, a
series of points in a target volume can be programmed into the
system and the acousto-optic deflectors are thereafter used to
focus a beam of electromagnetic radiation to each of said plurality
of points in the target volume for a predetermined dwell time.
[0110] Preferably, when the scan mode is selected the system is
arranged to scan a focal position along a predetermined path using
said acousto-optic deflectors.
[0111] Preferably, the scan is made up of a plurality mini-scans
having a duration determined in part by the Z-position at which
scanning takes place.
[0112] Preferably, the apparatus is configured to perform the
following method: scanning a target in three dimensions; presenting
to a user images of the target; receiving inputs from the user
identifying a plurality of points within the target; calculating
the signals to provide to said driver for causing the beam of
electromagnetic radiation to sequentially point to said selected
plurality of points in the target; and pointing the beam of
electromagnetic radiation sequentially to said selected points.
[0113] This aspect of the invention also provides a method of
manipulating a beam of electromagnetic radiation, said method
comprising determining a selection from a user; passing said beam
of electromagnetic radiation through first and second acousto-optic
deflectors, said first and second acousto-optic deflectors
respectively containing first and second acoustic waves; wherein
when a user has selected a random access mode, said waves are
configured to cause said beam of electromagnetic radiation to
sequentially point to a series of points within a three-dimensional
volume for a predetermined respective dwell time; and when said
user has selected a scanning mode, said waves are configured to
cause said beam of electromagnetic radiation to scan a path in said
three-dimensional volume at a predetermined scan rate.
[0114] In addition, this aspect can provide a method of
manipulating a beam of electromagnetic radiation, said method
comprising: scanning a beam of electromagnetic radiation around a
path in a three-dimensional volume so as to provide an image of
said volume; receiving an identification of a plurality of points
within said target volume from a user; sequentially pointing said
beam of electromagnetic radiation to said plurality of identified
points.
[0115] In accordance with a thirteenth aspect of the invention,
there is provided a method of scanning a target volume with a beam
of electromagnetic radiation, said method comprising passing said
electromagnetic radiation through a first acousto-optic deflector
and a second acousto-optic deflector downstream of said first
acousto-optic deflector, the deflectors containing first and second
acoustic waves respectively so as to move a focus position of said
beam along a scan path in said target volume at an angular scan
rate given by .delta..theta./.delta.t; wherein said first and
second acoustic waves are chirped to have a constantly increasing
or decreasing frequency; and wherein the ramp rates of said chirped
acoustic waves are selected in accordance with:
a 1 = V .lamda. ( V d 2 ' - .delta..theta. .delta. t ) 2 + d 1 d 2
' - d 1 V .delta..theta. .delta. t ##EQU00005## a 2 = V 2 2 .lamda.
d 2 ' + V 2 .lamda. .delta..theta. .delta. t ##EQU00005.2##
where a.sub.1 is the ramp rate in the first acousto-optic
deflector, a.sub.2 is the ramp rate in the second acousto-optic
deflector, V is the speed of sound in the first and second
acousto-optic deflectors, .lamda. is the wavelength of the
electromagnetic radiation beam, d.sub.1 is the effective optical
separation between the first and second acousto-optic deflectors
and d.sub.2' is the distance to the focus position from the second
acousto-optic deflector.
[0116] The value d.sub.1 can be made zero, in which case the
acousto-optic deflectors can be coupled together by a telecentric
relay.
[0117] Alternatively, the value of d.sub.1 can be made non-zero and
the values of the chirp rates can be found in accordance with the
above equations (taking into account the small corrections to these
equations that may be needed to account for small errors in the
alignment of components).
[0118] Preferably, the ramp rate a.sub.2 of the acoustic wave in
the second acousto-optic deflector is determined such that the
additional curvature provided to the wavefront of said
electromagnetic radiation by said second acousto-optic deflector is
a predetermined amount more or less than the curvature of the
wavefront as it arrives at said second acousto-optic deflector from
said first acousto-optic deflector, such as to provide for the
scanning of said focal position.
[0119] In accordance with this aspect there is provided a system
for scanning a target volume with a beam of electromagnetic
radiation, said system comprising: a first acousto-optic deflector;
a second acousto-optic deflector positioned downstream of said
first acousto-optic deflector and being separated from said first
acousto-optic deflector by an effective optical separation; a
driver for providing respective first and second acoustic waves in
said first and second acousto-optic deflectors, said first acoustic
wave having a ramp rate given by;
a 1 = V .lamda. ( V d 2 ' - .delta..theta. .delta. t ) 2 + d 1 d 2
' - d 1 V .delta..theta. .delta. t ##EQU00006##
[0120] and said second acoustic wave having a ramp rate given
by:
a 2 = V 2 2 .lamda. d 2 ' + V 2 .lamda. .delta..theta. .delta. t
##EQU00007##
where a.sub.1 is the ramp rate in the first acousto-optic
deflector, a.sub.2 is the ramp rate in the second acousto-optic
deflector, V is the speed of sound in the first and second
acousto-optic deflectors, .lamda. is the wavelength of the
electromagnetic radiation beam, d.sub.1 is the effective optical
separation between the first and second acousto-optic deflectors
and d.sub.2' is the distance to the focus position from the second
acousto-optic deflector.
[0121] In any of the aspects of the present invention, the
following are preferable features.
[0122] The electromagnetic radiation is selectively focussed to a
line and/or to a point.
[0123] The electromagnetic radiation passes through a system
comprising microscope optics, for example a system including a
microscope objective lens.
[0124] The method, apparatus and system of the present invention is
particularly useful for implementing non-linear optical processes,
such as multi-photon processes or two-photon processes.
[0125] In all embodiments, chromatic aberration is preferably
substantially corrected over a 3D image field.
[0126] Any of the acousto-optic deflectors of the present invention
are preferably made from a higher frequency anisotropic
acousto-optic crystal of which TeO.sub.2 is one example.
[0127] The present invention will now be further described, by way
of non-limitative example only, with reference to the accompanying
schematic drawings, in which: --
[0128] FIG. 1 shows an acousto-optic deflector (AOD) and the
principle of diffraction of a laser beam using an ultrasonic
acoustic wave;
[0129] FIG. 2 shows an AOD focussing a laser beam;
[0130] FIG. 3 shows the moving focal spot obtainable with a single
AOD;
[0131] FIG. 4a shows a graph of the frequency of the acoustic wave
as it varies with time;
[0132] FIG. 4b shows a graph of the frequency of the acoustic wave
as it varies with distance across the AOD;
[0133] FIG. 5 shows a configuration comprising two AODs which allow
a laser beam to be focussed to a fixed spot in the X-Z plane;
[0134] FIG. 6 is a similar view to FIG. 5 but additionally shows
the undiffracted zeroth order component of diffraction;
[0135] FIGS. 7a-7c show how a lens 70 can be used to focus the AOD
output to a real position in a target;
[0136] FIG. 8 shows a configuration of two parallel AODs in
accordance with the present invention;
[0137] FIG. 9 is similar to FIG. 8 and shows the chromatic
aberration that can occur when the input laser has a spectral
width;
[0138] FIG. 10 shows an overview of the components of a two-photon
system according to the present invention;
[0139] FIGS. 11a, 11b and 11c show graphs of the chromatic
aberration versus the distance X in the image field. FIG. 11a shows
the completely uncorrected graph, FIG. 11b shows a graph corrected
at a single point in the image field and FIG. 11c shows a graph in
which magnification chromatic aberration has also been
corrected;
[0140] FIG. 12 is a graph showing the chromatic aberration at
various points in the image field;
[0141] FIG. 13 is a graph similar to FIG. 12 showing the chromatic
aberration at various points in the image field;
[0142] FIG. 14 shows a system with a modified tube lens or
microscope objective lens in accordance with the present
invention;
[0143] FIG. 15 shows a dispersive lens in accordance with the
present invention;
[0144] FIG. 16 shows a telecentric relay in accordance with the
present invention;
[0145] FIG. 17 shows the telecentric relay in accordance with the
present invention together with an objective lens;
[0146] FIG. 18 shows a telecentric relay and objective lens in
accordance with another configuration of the present invention;
[0147] FIG. 19 shows a telecentric relay and objective lens in
accordance with a further configuration of the present
invention;
[0148] FIG. 20 is a graph showing the improvement in magnification
chromatic aberration obtainable with the present invention;
[0149] FIG. 21 is a graph showing the number of resolvable
detection volumes (NRDV) and how this varies with a correction
factor C related to the amount of magnification that the shorter
wavelengths are subjected to compared to the longer
wavelengths;
[0150] FIG. 22 is a graph similar to FIG. 20 in which the chromatic
aberration has been corrected perfectly in the X-direction;
[0151] FIG. 23 is a graph similar to FIG. 20, in which the
chromatic aberration has been corrected perfectly in the
Z-direction;
[0152] FIG. 24 is a graph showing the number of resolvable
detection volumes (NRDV) and how this varies with design acceptance
angle range of the second AOD in an AOD pair according to the prior
art;
[0153] FIG. 25 is a graph showing the number of resolvable
detection volumes (NRDV) and how this varies with design acceptance
angle range of the second AOD in an AOD pair according to the
present invention;
[0154] FIG. 26 is an arrangement of two AODs according to the
present invention;
[0155] FIG. 27 shows two orthogonal views of an arrangement of four
AODs in accordance with the present invention;
[0156] FIG. 28 is a three-dimensional plot of the diffraction
efficiency of a known AOD;
[0157] FIG. 29 is the same as FIG. 28, but viewed from a different
direction;
[0158] FIG. 30 shows a plot of the diffraction efficiency of an AOD
in accordance with the present invention;
[0159] FIG. 31 shows an AOD crystal having a wide ultrasonic
transducer;
[0160] FIG. 32 shows an AOD crystal having a narrow ultrasonic
transducer;
[0161] FIG. 33 shows an AOD crystal having a selectable pair of
transducers;
[0162] FIG. 34 shows an AOD crystal having three independently
selectable transducers;
[0163] FIGS. 35a and 35b show a practical arrangement of AODs and
telecentric relays;
[0164] FIG. 36 shows a laser beam being focussed by two AODs;
[0165] FIG. 37 shows the AOD drive frequencies utilised in the
arrangement of FIG. 36;
[0166] FIG. 38 shows distances used to derive equations to explain
the ramp rates used in the AOD configuration of FIG. 36;
[0167] FIGS. 39a and 39b show a configuration of four AODs;
[0168] FIG. 40 shows a scan pattern in the X-Y plane;
[0169] FIG. 41 shows raster scanning with four AODs;
[0170] FIG. 42 shows the frequency applied to two AODs in order to
focus to a stationary spot;
[0171] FIG. 43 shows how these frequencies need to be changed in
order to provide a scanning X-deflection;
[0172] FIG. 44 shows how the maximum and minimum drive frequencies
in the AODs limit the scan time;
[0173] FIG. 45 shows a series of mini scans in X;
[0174] FIG. 46 shows frequency offsets being applied between mini
scans;
[0175] FIG. 47 shows a geometric derivation useful in determining
the frequency offsets;
[0176] FIG. 48 shows a four AOD system for providing a constant
change of scan angle .theta. in the X-Y plane with time;
[0177] FIG. 49 shows the four AOD system, in which a constant
change of scan angle .phi. is provided in the Y-Z plane.
[0178] FIG. 50 shows an embodiment of apparatus for correcting
chromatic aberration;
[0179] FIGS. 51a to 51c show how diffractive optical elements
modulate a beam width in accordance with the wavelength of the
beam;
[0180] FIG. 52 shows the embodiment of FIG. 50 with the diffractive
optical elements axially moved;
[0181] FIG. 53 shows the spectra of laser pulse trains at three
different wavelengths;
[0182] FIG. 54 shows the effect of beam wavelength on the
embodiment of FIG. 50;
[0183] FIG. 55 shows an embodiment of a chromatic aberration
correcting system using zoom lenses and diffractive optical
elements;
[0184] FIG. 56 shows the embodiment of FIG. 55 and the focal length
of the zoom lenses;
[0185] FIG. 57 is a graph useful for explaining the embodiment of
FIG. 55;
[0186] FIG. 58 is another graph useful for explaining the
embodiment of FIG. 55;
[0187] FIG. 59 shows the embodiment of FIG. 55 when a wavelength of
900 nm is used;
[0188] FIG. 60 shows the embodiment of FIG. 55 when a short
wavelength is used; and
[0189] FIG. 61 shows the total length of the chromatic aberration
correcting system.
TECHNICAL BACKGROUND
[0190] In order to fully understand the invention, it is useful to
explain the technical effects relevant to the invention. FIG. 1
illustrates the principle of Bragg diffraction in an acousto-optic
deflector.
[0191] The acousto-optic deflector comprises a crystal 10 and a
crystal transducer 12. The crystal is preferably a high-efficiency
anisotropic acousto-optic crystal such as a TeO.sub.2 crystal. The
crystal transducer 12 is attached to one side of the crystal and is
arranged to propagate an ultrasonic acoustic wave 14 through the
crystal, preferably using the slow shear mode of propagation.
[0192] An incoming laser beam 16 entering the crystal at an angle
.PHI..sub.1 will be diffracted by the acoustic wave and the first
order component of diffraction will have an angle .PHI..sub.2 as
shown in FIG. 1. The first order component of diffraction is
labelled 18 in FIG. 1. There will also be a zeroth order component
of diffraction which is simply a continuation of the input laser
beam 16, i.e. the zeroth order of diffraction is an undeflected
laser beam.
[0193] The laser beam 16 typically has a width of 10 to 15 mm and
the plural beams illustrated in FIG. 1 are merely illustrative of a
single wide laser beam.
[0194] The equation governing the angle of diffraction is:
.phi. 2 - .phi. 1 = .lamda. 0 f ac V ac ( 1 ) ##EQU00008##
[0195] where .PHI..sub.2-.PHI..sub.1 is the angle of diffraction,
.lamda..sub.0 is the wavelength of the laser beam, f.sub.ac is the
frequency of the acoustic wave propagating in the crystal and
V.sub.ac is the velocity of the acoustic wave propagating in the
crystal. In FIG. 1, the acoustic wave has a constant frequency
f.sub.ac.
[0196] It is apparent from this equation that the amount of
deflection that the laser beam undergoes is directly proportional
to the wavelength of the laser beam. Thus, higher wavelength
components of light will be deflected by more than lower wavelength
components.
[0197] By manipulating the acoustic wave propagating in the
crystal, special effects can be achieved.
[0198] For example, the acoustic wave can be "chirped" such that
its frequency linearly increases or decreases with time, for
example by giving it the form:
f.sub.ac(t)=f.sub.ac(0)+at (2)
[0199] In this equation the constant a is known as the "chirp rate"
and is measured in MHz per second. It is clear from this equation
that the frequency of the ultrasonic wave is a linear function of
time. FIG. 2 shows the situation where the chirp rate a is
negative, i.e. the frequency of the acoustic wave linearly
decreases with time. As the angle of diffraction is proportional to
the frequency of the acoustic wave, those parts of the laser beam
that are deflected by the high-frequency portion of the acoustic
wave will be deflected more than those parts which are diffracted
by the low frequency portion. This is illustrated in FIG. 2 and it
can be seen that the effect is to focus the laser beam at a
position in the general direction of the dotted arrow 20 in FIG. 2.
The distance D to the focal position in the vertical direction is
given by the following equation:
D = V ac 2 .lamda. 0 a ( 3 ) ##EQU00009##
[0200] As illustrated in FIG. 3, the acoustic wave moves in the
direction of arrow 24 at the acoustic wave velocity V.sub.ac. The
focal position 22 created by the converging laser beam will thus
also move in the direction of arrow 26 at the acoustic velocity.
Accordingly, one AOD can be used to focus a laser to a position
that moves at the acoustic velocity V.sub.ac.
[0201] It is also pertinent to point out that the range of acoustic
frequencies that may be propagated through the crystal 10 is
limited because the diffraction efficiency drops rapidly outside
the design range of the AOD. FIG. 4a shows the frequency of the
acoustic wave as it varies with time and FIG. 4b shows the
frequency of the acoustic wave as it varies with distance.
[0202] As can be see from FIG. 4a, it is necessary to keep the
frequency of the acoustic wave between the limits f.sub.min and
f.sub.max. It is therefore not possible to indefinitely chirp the
frequency of the acoustic wave and, once the frequency reaches
f.sub.min it is necessary to very quickly change the frequency to
f.sub.max such that the chirping can continue. This creates a
"saw-tooth" graph in FIG. 4a. This same saw-tooth pattern occurs in
FIG. 4b, but it is reversed because the frequencies present in the
acoustic wave on the right-hand side of the crystal represent
frequencies at an earlier time point than the frequencies present
in the acoustic wave at the left-hand side of the crystal.
[0203] For one design of AOD, typical values for f.sub.min are
50-60 MHz and typical values for f.sub.max are 90-100 MHz. However,
a special design of AOD may be provided that is more efficient at
lower frequencies, for example 20-50 MHz, more preferably 25-45
MHz, more preferably still 30-40 MHz and more preferably still
32-37 MHz. f.sub.min and f.sub.max may thus be chosen in accordance
with these lower and upper limits. A low range of acoustic
frequencies are useful because they minimise the deflection
provided by any one AOD and reduce the need to provide AODs that
have large acceptance angles. This allows the efficiency to be kept
high.
[0204] For those points in time where the "fly-back" portion of the
graph is present in the central region of FIG. 4b or, in other
words, for those points in time where the discontinuity between the
highest and lowest frequency exists in the crystal of the AOD,
proper focussing cannot be achieved. There are therefore certain
periods of time for which the AOD cannot be used for focussing. In
two-photon applications, it is therefore important to measure
signals induced by the laser pulses only at points in time where
there is minimal discontinuity in chirped frequency across the AOD.
There is therefore a "duty cycle" limitation on the AOD which duty
cycle is the amount of time, expressed as a percentage, that the
AOD may be used for useful focussing. It is apparent that this duty
cycle will be reduced by increasing the gradient of frequency
increase/decrease in FIGS. 4a and 4b.
[0205] The focal spot 22 can be made stationary by utilising a
second AOD, as described by Kaplan et al (supra) and as illustrated
in FIG. 5.
[0206] In this configuration, a second AOD crystal 10 and
ultrasonic transducer 12 is utilised and the ultrasonic waves in
the AODs are made to propagate in substantially opposite
directions. In FIG. 5, the first (upstream) AOD has an ultrasonic
wave propagating from the right to the left and the second AOD has
an ultrasonic wave propagating from the left to the right. The
first AOD modifies the input laser beam 16 to be a focussed laser
beam 18 with the focal spot moving substantially from the right to
the left and the second AOD modifies the laser beam 18 to be a
stationary focussed laser beam 28. As illustrated in FIG. 5,
resultant focal spot 22 does not move.
[0207] FIG. 6 shows the same set up as FIG. 5 but additionally
shows the undiffracted beam (known as the "zeroth order component
of diffraction") that is transmitted through the first AOD. Due to
the offset positioning of the AODs, the undiffracted beam passes
well to the right of the focal spot 22 and so does not interfere
with the light reaching the focal spot 22. Baffles or other
mechanisms may be used to cut the undiffracted beam out of the
system altogether.
[0208] For the sound wave direction and diffraction order
illustrated, utilising a chirp rate of zero (as shown in FIG. 1)
provides a parallel laser beam. Utilising a negative chirp rate (as
shown in FIG. 2) provides a converging laser beam. Utilising a
positive chirp rate provides a diverging laser beam. These three
possibilities are illustrated in FIGS. 7a, 7b and 7c. In any
practical system the AODs will be followed by one or more lenses 70
which serve to provide further focussing. Thus, whether the laser
beam leaving the AOD system is converging (FIG. 7a), parallel (FIG.
7b) or diverging (FIG. 7c) the subsequent lens system brings the
laser beam to a real focus. The system is preferably calibrated
such that when the laser beam leaving the AOD system is parallel
(FIG. 7b) the point at which the subsequent lens system 70 focuses
the beam is designated the Z=0 point. Then, for this configuration,
applying a positive chirp rate moves the resultant focal point
upwards (see FIG. 7a) and applying a negative chirp rate moves the
focal point downwards (see FIG. 7c). In practice, the laser beam
passes through several lenses before reaching the physical
target.
[0209] It will be apparent from FIG. 6 that a problem can arise
when the first and second AODs are aligned so as to be parallel. In
this case, the undiffracted beam 16 can interfere with the beams
reaching the focal point 22. This problem is alleviated in
accordance with the second aspect of the invention (please see
later).
[0210] FIG. 8 illustrates how the focal spot 22 can be moved within
the target volume. The following and subsequent explanations ignore
the effect of subsequent lens systems (such as the lens 70 in FIG.
7) in order to provide clarity. In any practical embodiment, such a
lens system will be present and the principles below apply equally
to the case when the AODs themselves provide a diverging laser beam
(in which case there is a virtual focus above the laser beams that
is relayed by the subsequent lens optics to a negative Z position).
In order to assist in understanding this aspect of the invention
the following Figures take the example when the chirp rate is
positive which in this configuration produces a converging laser
beam.
[0211] As explained above, the distance to the focal position is
inversely proportional to the chirp rate a. Increasing the chirp
rate therefore brings the focal position upward in the Z direction
and decreasing the chirp rate brings the focal position downward in
the Z direction. As explained in FIG. 8, varying the slope of the
frequency time graph (i.e. modifying the chirp rate a) serves to
move the focal position 22 in the Z direction. As also illustrated
in FIG. 8, the focal position 22 may be moved in the X direction by
varying the separation between the two ramps in the frequency-time
graph. When the two AODs are excited with acoustic waves that are
identical and without any chirp, the resultant focal position is
defined as the X=0, Z=0 position. When a chirp is introduced, this
moves the focal position in the Z direction. When the absolute
frequency of the waves applied to the two AODs differs, this causes
the focal position 22 to be moved in the X direction.
[0212] FIG. 9 illustrates the problem of chromatic aberration and
how it causes a laser beam having any sort of spectral width to be
focussed to a blurred area, rather than to a distinct position in
the X-Z plane.
[0213] In FIG. 9, the input laser beam 16 has a certain spectral
width. The input laser beam might be a continuous laser beam having
several spectral components or might be a pulsed laser beam of a
single frequency. When a laser beam is pulsed (that is to say
time-windowed by mode locking the laser) this introduces a spectral
width to the laser beam. The longest wavelength component of the
pulse is shown by the arrows 16, 18, 28 (displayed in grey in FIG.
9) and the shortest wavelength component of the pulse is shown by
the arrows 16a, 18a, 28a (drawn darker in FIG. 9).
[0214] It can be seen from FIG. 9 that the longer wavelengths are
diffracted through a larger angle.
[0215] As illustrated in FIG. 9, the focal point 22 for the long
wavelength component does not coincide with the focal point 22a for
the shorter wavelength component. Wavelengths in between the two
illustrated will be focussed to a point somewhere on the line
linking focal spot 22 with focal spot 22a. The effect of the AODs
is therefore to not properly focus a laser beam having spectral
width to a unique point.
[0216] This problem can be alleviated by using longer laser pulses
(which can have a narrower spectral width). However, making the
pulses longer makes them less suitable for two-photon microscopy
applications as the two-photon microscopy effect is predicated on
being able to supply a large number of photons in a very short
space of time.
Two-Photon Microscopy System
[0217] FIG. 10 shows a two-photon microscopy system in accordance
with the present invention.
[0218] An input laser beam 16 is passed through four acousto-optic
deflectors 30, 40, 50, 60 and a lens 70. The laser beams forms a
focal spot 22 in the first image field which has Cartesian axes
Xi1, Yi1, Zi1. This image is projected through other relay optics
(not shown for clarity) which can create a second image field Xi2,
Yi2, Zi2. This is projected by a tube lens 80 through a microscope
objective lens 90 to form a focal spot 32 in the third image field
Xi3, Yi3, Zi3. This third image field is the target field and, in
two-photon applications, the target is placed in this field. Such a
target might be a slice of brain tissue or other biological
material with a fluorescent dye that requires imaging.
[0219] The input laser beam 16 in two-photon applications takes the
form of an ultra-short femtosecond or picosecond pulse in order to
get sufficiently intense electric fields at the focal point. The
pulses are typically spaced in time by a duration very much larger
than the pulse length. Typical pulse lengths are 2 ps or less,
preferably 500 fs or less, even more preferably 50 to 200 fs. The
pulses are typically repeated at a frequency of 50 to 200 MHz (e.g.
80 MHz).
[0220] Two distinct experiments can be carried out with a
two-photon microscopy system. The first experiment is to image
fluorescent materials and such experiments typically require powers
of 10 mW to be focussed to an area of just over 1 .mu.m.sup.2
(corresponding to a power density of around 600,000 W/cm.sup.2).
Typical laser wavelengths of 800-1000 nm (e.g. 850 nm) are
utilised. The second experiment is photolysis in which the laser is
used to uncage biologically active compounds. Lasers having a
wavelength of 720 nm are often used and the power requirement is
much higher, there being a need for in excess of 100 mW of power
per micron squared.
[0221] In a preferred embodiment of the invention, the laser is
supplied by a mode locked Ti sapphire laser tuneable in the near
infrared region having an average power of 1 to 10 W and supplying
100 fs pulses at 80 MHz.
[0222] Sensitive collection photomultipliers are utilised near to
the target area to pick up any fluorescence from the two-photon
excitation of fluorophores in the target. This enables a 3D image
to be constructed in imaging applications and further enables any
sequence of spots in 3D space to be interrogated by the laser beam
for repeatedly monitoring the state of tissue at each spot during
dynamic biological processes.
[0223] The AODs used in the present invention are preferably
shear-mode anisotropic AODs. Suitable materials for the AOD crystal
are TeO.sub.2 crystals. Such AODs rotate the polarisation of
incoming laser light by 90.degree.. The AODs 30, 40, 50, 60 are
schematically illustrated in FIG. 10 (and in other Figures of the
present application) with no intervening components between them.
However, in practice, such components will be present. Typically,
these components may include half-wave plates and polarisers (the
reason will be explained later). Furthermore, a telecentric relay
can be used between each AOD (as disclosed by Reddy & Saggau)
to properly couple the AODs together. If such a telecentric relay
were not used, then it would be difficult to achieve a stationary
focal position, without taking other measures.
[0224] The light emitted by the fluorophores is picked up by a
photomultiplier (not shown) coupled to the system by a dichroic
mirror in the standard fashion.
[0225] FIGS. 11a and 11b graphically exemplify how chromatic
aberration can be corrected for a single point in the image field
according to the prior art. FIG. 11a shows the situation prior to
correction. The chromatic aberration has a positive magnitude for
all points in the image field and, as can clearly be seen from FIG.
11a, the magnitude of the chromatic aberration varies across the
image field in a generally linear fashion. In FIG. 11a, the
chromatic aberration at the right-hand side of the imaging field is
larger than the chromatic aberration at the left-hand side of the
imaging field. Using the best compensation methods known in the
art, the chromatic aberration can be corrected for a single point
in the image field, as shown in FIG. 11b. The single point is here
selected to be the centre of the image field such that the
magnitude of the chromatic aberration at the extremities of the
image field is equal and opposite. This provides the least overall
chromatic aberration. However, it is apparent from FIGS. 11a and
11b that the slope of the line defining the chromatic aberration
has not at all been changed. The present invention discloses
apparatus and methods for modifying this slope so that a chromatic
aberration graph similar to FIG. 11c can be obtained. Modifying
this slope is referred to herein as at least partially correcting
the magnification chromatic aberration.
Chromatic Aberration Correction
[0226] FIG. 12 shows the effect of chromatic aberration (as
explained in FIG. 9) for points in the first image field Xi1, Yi1,
Zi1 of FIG. 10.
[0227] In this embodiment, the lens 70 has a focal length of 0.3 m
and this focal point is allocated the zero point along the Z-axis.
The zero point along the X-axis is the point of symmetry (i.e. the
centre line) of the lens 70. The dots in FIG. 12 show positions in
the image field that the shortest wavelength components of a 1 ps
laser pulse at 850 nm can be focussed to by varying the chirp and
frequency difference between the first and second AODs used to
focus in the X-Z plane. The lines emanating from the dots show the
points where the other frequency components of the 1 ps pulse will
be focussed. Thus, the end of the line furthest from the dot
represents where the longest wavelength components of the pulse
will be focussed.
[0228] Some observations can be made about the nature of the
chromatic aberration. Firstly, in common with the results of
Kaplan, Salome and Reddy & Saggau (supra), there is no
chromatic aberration at the point Xi1=Zi1=0. The reason for this is
that, at this point, there is no net X deflection and the AODs are
being operated with acoustic waves having a single frequency and
there is no chirp to produce Z focussing. All frequency components
are therefore focussed to the same spot. As one moves along the
X-axis from this "compensation point" the amount of chromatic
aberration increases accordingly. Similarly, as one moves along the
Z-axis, the amount of chromatic aberration increases. Looking at
FIG. 12 as a whole, for positions in the image plane at Z<0.15
m, the chromatic aberration seems to have the effect of magnifying
the longer wavelength components in the image plane more than the
shorter wavelength components. In other words, if FIG. 12 were
re-drawn such that the longer wavelength components were shown as
dots, this graph would look like a magnified version of the dots
representing the shorter wavelength components. This magnifying
effect of the chromatic aberration is referred to herein as
magnification chromatic aberration.
[0229] FIG. 12 is illustrated for a 1 ps pulse. For even shorter
pulses, such as 100 fs, even more chromatic aberration is
apparent.
[0230] Another observation from FIG. 12 is that the X dispersion
(i.e. the chromatic aberration in the direction of the X-axis)
reduces to zero for the value of Z=0.15. Analysis shows that, for
the case when the imaging lens 70 is very close to the final AOD,
this will occur generally for values of Z at approximately half the
focal length of the lens 70.
[0231] FIG. 13 shows a view similar to FIG. 12 although here the
image is that obtainable under an objective lens having 40.times.
magnification. As with FIG. 12, the tails indicate the direction
and relative size of chromatic aberration. In this case the system
lenses have been placed in telecentric positions so that the image
field is rectangular rather than trapezoidal. As in FIG. 12,
chromatic aberration has been reduced to zero for X=Z=0 but has
increasing values further away from this compensation point.
[0232] The present invention teaches to at least partially correct
the magnification chromatic aberration by utilising at least one
optical element.
[0233] In a first embodiment, this at least one optical element can
be a specially manufactured tube lens 80 or microscope objective
lens 90.
[0234] For a particular X-Y plane at a certain value of Z, all of
the chromatic aberration will be in a direction that is directed
radially away from the objective lens 90. This fact can be taken
advantage of by manufacturing the objective lens 90 so as to have a
dispersive quality. That is to say, the objective lens 90 is
manufactured from a material which magnifies the longer wavelengths
less than the shorter wavelengths. Such lenses can be made from
combinations of conventional crown and flint glasses or from
diffractive elements. If the correct amount of dispersion is
introduced into the objective lens 90 (or alternatively the tube
lens 80) the chromatic aberration in the whole of the selected X-Y
plane can be substantially corrected. This can increase the NRDV in
that X-Y plane by a factor of 50 or more.
[0235] FIG. 14 shows a slice through the X-Z plane and also shows
first and second AODs 30, 40 designed to allow focussing in this
plane. Naturally, a preferred embodiment also includes third and
fourth AODs for focussing in the Y-Z plane and the compensation
element 80 or 90 can equally correct the magnification chromatic
aberration in the Y direction.
[0236] The provision of a dispersive lens to correct the
magnification chromatic aberration is thus a significant advance in
the art as the chromatic aberration can be corrected not just at a
single point X=Y=Z=0 but for a whole plane in the image field.
[0237] A more preferred embodiment, representing the best mode of
operating the invention, provides for significant correction of the
magnification chromatic aberration not just in a 2D plane but
throughout the majority of the 3D image field. This can be achieved
in the present embodiment by utilising a telecentric relay to
correct the magnification chromatic aberration.
[0238] The telecentric relay advantageously comprises two lenses
both having dispersive qualities. The first lens is preferably one
in which the focal length decreases with increasing wavelength. The
second lens is preferably one in which the focal length increases
with increasing wavelength. Accordingly, the first lens will tend
to project the longer wavelength components to a point nearer to
the first lens than the shorter wavelength components. This is
illustrated in FIG. 15. An image 120 (here of a semi-circle with a
dot at the centre of the curvature of the semi-circle) is projected
through a dispersive lens 110 which has a quality of having a
reduced focal length with increasing wavelength. Assuming that the
light making up the image 120 has some spectral width, the long
wavelength components will be projected to form the image 130 and
the short wavelength components will be projected to form the image
140. As can be seen in FIG. 15, the long wavelength components are
projected to a point closer to the lens 110 than the short
wavelength components 140. As a result of this, the long wavelength
components 130 are magnified less than the short wavelength
components 140. Another way to explain the qualities of the
dispersive lens 110 is to state that it has a negative dF/d.lamda.
wherein F is the focal length and .lamda. is the wavelength of
light being transmitted through the lens 110.
[0239] FIG. 16 shows a telecentric relay having first lens 110 and
second lens 150.
[0240] The second lens 150 has the quality of positive dF/d.lamda..
In other words, the focal length for longer wavelengths is greater
than the focal length for shorter wavelengths. As the longer
wavelength components 130 of the projected image 120 are further
away from the lens 150 than the shorter wavelength components 140,
projection through the lens 150 will tend to realign the centre
points of the images 130, 140 to form projected images 160, 170
respectively (see FIG. 16). Furthermore, because the longer
wavelength components 130 are further away from the lens 150 than
the shorter wavelength components 140, they will be magnified less
than the shorter wavelength components. Thus, what results is an
image comprising long wavelength components 160 and short
wavelength components 170 which are centred on one another but at
which the long wavelength components are magnified less than the
short wavelength components.
[0241] Such a telecentric relay can be utilised in the system of
FIG. 10 to project the first image (in the Xi1, Yi1, Zi1
coordinates) to the second image (not shown) or to project the
second image to the third image (in the Xi3, Yi3, Zi3 coordinates).
As will be apparent from a consideration of FIG. 12, the effect of
the relay in reducing the magnification of the longer wavelength
components will be to substantially correct the magnification
chromatic aberration that exists in the first image.
[0242] The lenses of the telecentric relay can be made of any
dispersive material such as combinations of conventional crown and
flint glass lenses and diffractive elements. Furthermore, the
invention is not limited to utilising two lenses and more or less
may be used.
[0243] FIG. 17 shows a view similar to FIG. 16 but also including
the microscope objective lens 90. As in FIG. 16, the first lens 110
has a negative dF/d.lamda. whereas the second lens 150 has a
positive dF/d.lamda.. The objective lens 90 has a dF/d.lamda. of
zero. The dotted line of FIG. 17 shows light of a longer wavelength
than the solid line. At the final image, the fact that the longer
wavelength is focussed on the image from a larger numerical
aperture shows that it has a smaller magnification.
[0244] FIG. 18 shows an alternative embodiment in which first lens
110 has a negative dF/d.lamda., second lens 150 has zero
dF/d.lamda. and objective lens 90 has a negative dF/d.lamda.. It is
apparent from this diagram that, yet again, the longer wavelengths
are magnified less than the smaller wavelengths.
[0245] FIG. 19 shows a further alternative embodiment. Here, first
lens 110 has zero dF/d.lamda., second lens 150 has a negative
dF/d.lamda. and objective lens 90 has a positive dF/d.lamda.. As in
the other embodiments, the longer wavelengths are magnified less
than the shorter wavelengths.
[0246] It will be apparent to one of ordinary skill in the art that
various other combinations of lenses can be utilised to achieve the
technical effect of magnifying the longer wavelengths less. The
Figures presented herein are just some examples from a multitude of
possibilities.
[0247] FIG. 20 shows the image after correction and it can
immediately be seen that the lines representing the chromatic
aberration are much shorter. This translates into an increase in
NRDV of over 30 times.
[0248] Using the system of the present invention, the magnification
achievable is not isotropic in the X, Y and Z volume. In general
the magnification in the Z-direction is equal to the square of the
magnification in the X and Y-directions. Thus, if the X and Y
coordinates are magnified by two times, the Z coordinates will be
magnified by four times. Similarly, if the X and Y coordinates are
magnified by 0.5, the Z components will be magnified by 0.25.
[0249] FIG. 21 shows a graph of how the NRDV varies with a
"compensation factor" C. A compensation factor C=1 is selected to
coincide with the amount of chromatic dispersion in the compensator
that gives perfect compensation for all the chromatic aberration in
the Z-direction. A value of twice this chromatic dispersion (C=2)
gives perfect compensation in the X and Y-directions. The
compensation factor can be selected in accordance with the
application to which the apparatus is put. For example, if the
apparatus is being applied in a 2D imaging scenario, where
focussing to different points in different Z-positions is not
required, the compensation factor C can be set equal to 2 so as to
achieve perfect chromatic aberration correction in the whole X-Y
plane. This compensation factor also gives the highest NRDV and is
suitable for imaging 3D spaces where the depth of interest remains
within the high resolution Z range. If greater resolution imaging
is required over the largest possible Z range then a compensation
factor of near 1 is better albeit at the expense of some loss of
resolution at the extremes of X and Y range (see FIG. 21).
[0250] The parameter C in FIG. 21 can be further defined with
reference to FIG. 17. In this symmetrical case, at the design mid
wavelength, the rate of change of focal length of the first lens
110 is equal to the positive rate of change of the focal length of
the second lens 150, and the input and output beams are parallel
and of equal diameter,
C = - 4 .lamda. f 1 .differential. f 1 .differential. .lamda. = 4
.lamda. f 2 .differential. f 2 .differential. .lamda.
##EQU00010##
where f.sub.1=focal length of lens 110 f.sub.2=focal length of lens
150 .lamda.=operating wavelength
[0251] FIG. 20 shows that the longer wavelength components of the
original image have been magnified by less than the shorter
wavelength components of the original image. The compensation is
such as to slightly overcompensate in the Z-direction and slightly
undercompensate in the X-direction (C=1.3). Depending on the
application, it is possible to select or position lenses that
perfectly compensate in the X-direction (but not perfectly in the
Z-direction) (C=2, see FIG. 22) or which perfectly compensate in
the Z-direction (but not perfectly in the X-direction) (C=1, see
FIG. 23). The example of FIG. 20 is a compromise solution
(C=1.3).
[0252] FIGS. 24 and 25 illustrate the effect of the invention in
another way.
[0253] FIG. 24 shows the NRDV for a prior art system in which the
second AOD of the AOD pair has an acceptance angle range of .+-.1.5
mrad. As can be seen from FIG. 23, this leads to a maximum NRDV of
approximately 200,000. The NRDV is calculated as the number of
distinguishable points in the image field where enough power can be
supplied to achieve the two-photon effect. The threshold density
selected for achievement of the two-photon effect is 600,000
W/cm.sup.2 and FIG. 24 takes account of losses in each optical
component.
[0254] FIG. 24 also shows notional graphs for laser powers of 6 W,
12 W and 24 W. The best currently commercially available lasers
have powers of 3 W. Thus, FIG. 24 graphically illustrates that,
even if a laser having a power of 24 W was available, the target
NRDV of 7.8 million could never be reached using the prior art
systems. Indeed, FIG. 24 shows that, using prior art AOD input
acceptance angles of .+-.1.5 mrad leads to a system having an NRDV
of approximately 200,000.
[0255] FIG. 25 shows a graph similar to FIG. 24, but taking into
account the magnification chromatic aberration correction provided
by the present invention. It is firstly apparent from FIG. 25 that,
even when an input acceptance angle range for the second AOD in the
pair is selected at .+-.1.5 mrad, the NRDV is larger than in the
prior art. Furthermore, the magnification chromatic aberration
correction has moved the graphs such that it is now possible to
obtain an NRDV of 2.4 million using a 3 W laser. This was simply
impossible in the prior art. This represents a 12 times improvement
in NRDV compared to the prior art. The present inventors also
believe that further optimisation can be carried out to achieve the
target NRDV of 7.8 million. For example the threshold 600,000
W/cm.sup.2 was determined experimentally using laser pulses
estimated to be 400 fs long (to account for temporal dispersion in
the microscope). Using an optical pre-chirper (as suggested by Iyer
et al) to pre-compensate the laser pulses entering the microscope
would enable the 100 fs pulses to be delivered from the objective
and would thus reduce this threshold considerably and enable wider
acceptance angle range AODs to be used. This would easily enable
the 7.8 million MRDV target to be achieved.
Zeroth Order Component Blocking
[0256] A comparison of FIGS. 6 and 8 above reveals that any zeroth
order components of diffraction occurring in the first AOD of FIG.
8 will be transmitted through the second AOD and can interfere with
the image field. The reason for this is that FIG. 8, unlike FIG. 6,
has the AODs mounted in a parallel configuration such that the
undiffracted beam passes in a very similar direction to the
diffracted beam. This problem is alleviated by the second aspect of
the invention which involves the use of polarisers and optional
half-wave plates to prevent the zeroth order components of
diffraction being transmitted.
[0257] In order to be accepted and successfully diffracted by an
AOD, the light must have the correct polarisation. In particular,
for high efficiency slow acoustic wave AODs (using for example
anisotropic tellurium dioxide crystals), the optical input
polarisation needs to be aligned with the direction of propagation
of the acoustic wave. Thus, where the acoustic wave is such as to
cause focussing of an input laser beam in the X-Z plane, the input
laser beam needs to be X polarised. Any first order components of
diffraction transmitted by the AOD will have had their polarisation
rotated by 90.degree. such that they are Y polarised. Such light is
not compatible with the second AOD shown in FIG. 7 for example.
Thus, according to this aspect of the present invention, a
half-wave plate and a pair of polarisers are used, as shown in FIG.
26. Input laser beam 16 having X polarisation is provided to the
first AOD 30. The first order components of diffraction 18 leave
the first AOD 30 in a Y polarised state. The undeflected zeroth
order components of diffraction remain in the X polarised state. A
half-wave plate 200 is disposed after the first AOD 30 in order to
rotate the polarisation by 90.degree.. Thus, the Y polarised first
order components of diffraction are now X polarised and the X
polarised zeroth order components of diffraction are now Y
polarised. An X polariser 210 is disposed after the half-wave plate
and has a function of only allowing X polarised light to pass.
Thus, the X polarised first order components of diffraction will
pass and the zeroth order components of diffraction will be blocked
(because they are Y polarised following rotation by the half-wave
plate). These X polarised first order components of diffraction are
suitable for input into the second AOD 40 where they will by
rotated by 90.degree. to become Y polarised first order components
of diffraction. Any undiffracted light leaving the second AOD 40
will be X polarised and so will be blocked by the Y polariser 220
situated downstream of the second AOD. Thus, light reaching the
focal spot 22 will solely consist of the first order components of
diffraction with any zeroth order components of diffraction being
effectively blocked by the polarisers.
[0258] With the configuration shown in FIG. 26, the focal spot 22
is actually a line perpendicular to the page because there is no
focussing in the Y direction. If, as is preferred, focussing is
also required in the Y direction, then an identical configuration
to FIG. 26 can be utilised, it being merely rotated by 90.degree.
about the Z axis. In this configuration, the first and second AODs
30, 40 perform the focussing in the X-Z plane and the third and
fourth AODs 50, 60 perform the focussing in the Y-Z plane.
[0259] In this configuration, all of the AODs are mounted in
parallel, that is to say the acoustic waves travelling through the
AODs travel in parallel planes (parallel to the X-Y plane). Also,
in this configuration, the components are mounted in the following
order (in the direction of laser propagation): First AOD, half wave
plate, X polariser, second AOD, Y polarizer, third AOD, half wave
plate, Y polariser, fourth AOD, X polarizer.
[0260] There exists an even more preferred configuration of AODs
and this is shown in FIGS. 10 and 27.
[0261] FIG. 27 shows two orthogonal views of the AOD configuration.
The first AOD 30 and second AOD 40 are used to provide focussing in
the X-Z plane. The third AOD 50 and fourth AOD 60 are used to
provide focussing in the Y-Z plane. As is apparent from FIG. 27,
the AODs are configured in the order first, third, second, fourth
starting from the laser beam entry end and finishing at the laser
beam exit end. This configuration is preferred because it avoids
the need to utilise half-wave plates. Not shown in FIG. 27, but
preferably present in a practical embodiment, are first to fourth
polarisers. A polariser is located subsequent to each AOD. Laser
light 16 entering the first AOD 30 will be converted into a zeroth
order component of X polarisation and a first order component of Y
polarisation. It is desirable to only transmit the first order
component. A Y polariser is therefore located after the first AOD
to block the zeroth order component. This Y polarised light is
suitable for input into the third AOD 50 in which a zeroth order
component of Y polarisation and a first order component of X
polarisation is produced. A X polariser is therefore located after
the third AOD 50. Such X polarised light is suitable for input into
the second AOD 40 which produces a zeroth order component having X
polarisation and a first order component having Y polarisation. A Y
polariser is therefore located after the second AOD 40. This serves
to block the zeroth order component. Such Y polarised light is
suitable for acceptance by the fourth AOD 60 which produces a
zeroth order component having Y polarisation and a first order
component having X polarisation. An X polariser is therefore
located after the AOD 60 to block the Y polarised zeroth order
component. As a result, all light reaching focal spot 22 is the
result of properly diffracted first order components and no
undiffracted zeroth order components can filter through the system.
Furthermore, this configuration does not require a half-wave plate
to adapt the polarisation at various stages.
[0262] As is well known to those skilled in the art of AODs, the
precise degree of polarisation of the first order diffracted wave,
although close to linear and at 90 degrees to the direction
propagation of the acoustic wave, is not exact. Particularly if the
AOD crystal is cut with less than 2 or 3 degrees deliberate
misorientation of the optic axis from the direction of propagation
of the acoustic wave, the optimised input beam and the diffracted
and zero order output beams of light can be slightly elliptically
polarised so the configurations described here, which use linear
polarisers would not maximally transmit the diffracted wave nor
perfectly suppress the undesired undiffracted zero order components
of the light. In such cases, to further improve performance, small
rotations of inserted half wave plates or insertion of appropriate
phase plates with small fractions of a wave correction (e.g. 1/4 or
1/20 wave) may fine tune the performance of the configuration
concerned. The key point is for the polariser after each AOD to
maximally transmit the wanted diffracted first order beams and
maximally suppress the unwanted zero order beam. If the polariser
is before another AOD, then there may be more polarisation state
adjustment before the next AOD to optimise its performance.
Improved Acceptance Angle Crystals
[0263] Anisotropic acousto-optic crystals utilised to manufacture
AODs typically have a quoted acceptance angle for the laser light.
The crystals themselves are optimised for maximum transmission
efficiency at this acceptance angle. For the first and third AODs
in the system which receive laser light at a constant acceptance
angle, such crystals are highly suitable. However, a problem arises
when such crystals are utilised in the second and fourth AODs
because the acceptance angle will vary across a range defined by
the range of deflection angles capable of being carried out in the
first and third AODs respectively. These known devices are capable
of deflecting an 800 nm laser beam having 3 W of acoustic power
over .+-.20 mrad (17.43 mrad=1.degree.). The efficiency of
transmission is over 80%.
[0264] FIG. 28 is a graph of the efficiency of the known AOD
crystals. FIG. 29 is the same graph viewed from a slightly
different angle. Both of the graphs show the diffraction efficiency
for various frequencies of acoustic wave and for various incident
light angles. It can be seen from the graph that maximum efficiency
is obtained with a centre frequency of acoustic wave of about 95
MHz and an instant angle of about 0.121 rad. FIG. 29 shows that
acceptable diffraction efficiencies can be obtained in this crystal
for a range of incident angles of approximately .+-.1.5 mrad. If
the incident angle presented to the crystals strays outside this
range, then quite low diffraction efficiencies will be present
which in turn limit the energy being provided to the focal spot and
thus limits the possibility of performing the two-photon
interactions necessary in two-photon microscopy. It has been found
that the diffraction efficiency of an AOD reduces approximately in
inverse proportion to its design input acceptance angle. This means
that as the overall deflection angle of the four AOD system
increases from the .+-.3 mrad (=2x.+-.1.5 mrad) possible with the
standard device pairs, the efficiency falls in proportion to the
inverse square of the designed deflection angle.
[0265] The third aspect of the invention alleviates this problem by
providing an acousto-optic deflector crystal which has a reasonable
diffraction efficiency across a larger range of acceptance angles.
A graph similar to that shown in FIGS. 28 and 29 for the new
crystal is shown in FIG. 30. As can be seen, a crystal configured
in this manner maintains a diffraction efficiency of at least 80%
of its peak across an incident angle range of 10 mrad. However, the
peak diffraction efficiency obtainable is not as high as with the
conventional AOD. Thus, the AOD of the invention has a lower peak
efficiency than a conventional AOD but accepts laser beams from a
wider range of angles at better transmission efficiencies than the
conventional AOD. The method by which this effect is achieved will
be explained with reference to FIGS. 31 and 32. FIG. 31 shows a
conventional AOD crystal 10 with an ultrasound transducer 12
attached to one side thereof. The ultrasound transducer 12 has a
width W parallel to the direction of light propagation of
approximately 3 mm. This causes the acoustic wave 14 formed in the
crystal 10 to be not very diverging. As a result, an input laser
beam 16 can be deflected to become laser beam 18 but only if the
laser beam 16 is input within a narrow incidence angle range.
[0266] FIG. 32 shows an AOD in accordance with this aspect of the
invention in which the ultrasound transducer 12 is made much more
narrow in the direction of light propagation. In this embodiment,
the width W of the ultrasound transducer 12 is 1 mm or less. As
shown in FIG. 32, this causes the propagated ultrasound wave 14 to
take on a more diverging configuration. This in turn means that a
greater range of angles of laser beam 16 can be accepted and
successfully diffracted into laser beams 18. Thus, the narrow
crystal creates a more diverging acoustic wave which allows the
efficient diffraction of laser beams coming from a wider range of
angles than if the acoustic wave was less diverging (as in FIG.
31).
[0267] Appropriate crystal transducer widths are less than 1 mm,
more preferably less than 0.5 mm, more preferably approximately
0.25 mm or less.
Dual Transducer AODs
[0268] This aspect of the invention provides an AOD having two
crystal transducers. This is shown in FIG. 33. The first crystal
transducer 12a is configured to have a narrow width in the
direction of light propagation and the second transducer 12b is
configured to be wider in the direction of light propagation. In
this embodiment, the transducer 12a has a width of 0.25 mm and the
transducer 12b has a width of 3 mm. An excitation source 300 is
provided to supply power to the transducers and a switch 310 allows
an operator to select whether then first or second transducer is
excited.
[0269] The provision of this switch allows the AOD to be operated
in one of two modes. In the first mode, the wider transducer 12b
can be utilised and this optimises efficiency for a narrower range
of acceptance angles. This is useful in applications in which it is
desirable to deliver a lot of power to a small target volume, such
as uncaging (photolysis) applications. The second transducer can be
selected where it is important to achieve reasonable transmission
across a greater range of acceptance angles, for example when a
larger target volume is desired to be imaged with a larger
NRDV.
[0270] The AODs designed with two crystal transducers, as explained
above, are highly suitable for use in the second and/or fourth AODs
of the invention.
Multiple Transducer AODs
[0271] This aspect of the invention is illustrated in FIG. 34. A
single AOD crystal may be provided with two or more crystal
transducers. Each crystal transducer may be selectively utilised to
help propagate the acoustic wave. In the example of FIG. 34, three
crystal transducers 12a, 12b and 12c are shown. The width of the
transducers preferably increases in a geometric series, for example
by a factor of 2 each time. The crystal transducers preferably have
the property that each subsequent transducer is twice as wide as
its predecessor. For example, transducer 12a can be 0.25 mm wide,
transducer 12b can be 0.5 mm wide and transducer 12c can be 1 mm
wide. By appropriate selection of the switches 310a, 310b or 310c,
an effective transducer width in the range between 0.25 mm and 1.75
mm can be obtained. This allows the AOD to be utilised in the
manner most appropriate to the application for which it is used. It
thus helps to provide a general purpose apparatus that can be used
for a variety of different experiments. More transducers can be
provided if desired.
[0272] As shown in FIG. 34, when switch 310a is in the "on"
position and all other switches are in the "off" position, the
driver 300 excites the crystal 12a only. As this crystal is quite
narrow, it provides an acoustic wave W1 that has a high divergence
angle. When switches 310a and 310b are activated, this produces
acoustic wave W2 which diverges less. When switches 310a, 310b and
310c are activated, this produces acoustic wave W3 which diverges
still less and which has the least amount of divergence. When the
widest effective transducer is used, this produces an AOD with the
highest efficiency but with the narrowest acceptance angle for the
incoming laser beam. When the narrowest transducer is used, this
produces an AOD with a lower efficiency but a better range of
acceptable angles for the incoming laser beam. Accordingly, the
width of the transducer can be selected in accordance with the
desired trade-off between the efficiency of the AOD and the range
of acceptance angles. As an example, the first or third AODs in a
four AOD system (i.e. the first AOD for focussing in the X-Z plane
and the first AOD for focussing in the Y-Z plane) can be provided
with wide transducers to give good efficiency and a low range of
acceptance angles whereas the second AOD in each focussing pair can
be provided with narrower transducers so as to give a better range
of acceptance angles at the expense of lower efficiency.
Improved Crystal Orientation
[0273] AOD crystals are usually rotated by about 6.degree. about
the X-axis and 0.degree. about the Y-axis. This enables the centre
frequency to be increased to maximise deflection angle range and
avoids the degenerate re-diffraction of power out of the diffracted
beam. Because the soundwave propagation is highly anisotropic, the
6.degree. crystal rotation results in the soundwave power
propagating at an angle of about 50.degree. to the Y-axis.
[0274] The crystal orientation is measured with respect to the
crystal axes and the crystal axes can be determined using an X-ray
diffraction technique, as described by Young et al, "Optically
Rotated Long Time Aperture TeO.sub.2 Bragg Cell", Advances in
Optical Information Processing, IV, 1990, SPIE Vol. 1296, pp
304-316.
[0275] FIGS. 32 and 33 are also representative of another aspect of
the invention in which the crystal of the acousto-optic deflector
has a particular orientation. In this orientation, the input laser
beam is defined as being the negative Z axis ([001] direction) and
the crystal structure is rotated by 2.degree. about the X axis
([110] direction) and 3.degree. about the Y axis ([110] direction).
With this crystal orientation, the soundwave power propagates at an
angle of about 20.degree. to the Y-axis and it has been
mathematically modelled that this reduces aberration in the image.
The 3.degree. tilt about the Y-axis is necessary to avoid loss from
degenerate mode.
[0276] It has also been found that reducing the centre frequency of
the acoustic waves from the range of 50 to 90 MHz to the range 30
to 50 MHz improves the diffraction efficiency with this design.
[0277] In accordance with this aspect, the crystal is oriented such
that acoustic waves propagating through it have approximately
20.degree. between their wave vector and their Poynting vector. In
order to achieve proper focussing, the speed of propagation of the
sound waves across the AOD must be identical whether or not the
first transducer 12a or second transducer 12b is being used.
[0278] This improved crystal orientation can be utilised with the
second AOD in one of the focussing pairs (i.e. the AODs labelled 40
and 60). Additionally, it may also be used with a first AOD in each
of the pairs (i.e. the AODs labelled 30 and 50). It is preferable
that all of the AODs in the system have this improved crystal
orientation.
[0279] Any of the embodiments and aspects described herein can be
provided with AODs according to this orientation.
Compact AOD Configuration
[0280] FIGS. 35a and 35b show a typical practical configuration for
the four AOD system shown in FIG. 27. As can be seen, each of the
AODs 30, 50, 40, 60 is coupled to the subsequent AOD by a
telecentric relay 400. Such telecentric relays typically have
lengths along the laser path beam of 400 mm or more. As can be seen
from FIG. 35b, each telecentric relay has a total length of 4f,
where f is the focal length of one relay lens. Typically f=100 mm.
Accordingly, the requirement to utilise at least three telecentric
relays to couple the AODs together adds 1.2 m to the total beam
length of the system. As explained above, different wavelengths of
light are diffracted by different amounts. Accordingly, when the
laser wavelength is changed, the AODs and telecentric relays have
to be repositioned. FIG. 35b shows two displacements H.sub.1 and
H.sub.2. These are the displacements of the output beam centre line
compared to the input beam centre line. This displacement varies
with the wavelength of light. With a wavelength .lamda.=700 nm,
this displacement is approximately 32 mm. With a wavelength of
.lamda.=900 nm, this displacement is approximately 40 mm.
Consequently, when changing the laser wavelength from 700 nm to 900
nm, the optical components have to be realigned by 8 mm. Such
realignment is a necessary consequence of utilising telecentric
relays. Accordingly, telecentric relays are not ideal in a system
for which it is intended to change the laser wavelength frequently.
This aspect of the invention thus provides a means for dispensing
with the telecentric relays and thus allows a more compact and
configurable system to be provided.
[0281] The telecentric relays provided in the prior art are
necessary to couple together the AODs appropriately. As shown in
FIG. 8, the first AOD modulates the input laser beam 16 to be a
laser beam 18 having a curved wavefront. This wavefront is moving
at the speed of sound in the X direction. The second AOD modulates
the incoming laser beam 18 to be a laser beam 28 with a curved
wavefront. The curvature here will be equal to the sum of the
curvature brought about by the first AOD added to the curvature
brought about by the second AOD. The resulting focal position 22
will only be stationary if the curvature endowed on the laser beam
by the second AOD equals that of the wavefront as it enters the
second AOD. In the absence of the second AOD, it is apparent from
FIG. 8 that the curvature of the laser beam 18 increases as you
move further away from the first AOD. When the AODs are set up to
endow an incoming laser beam with the same curvature (i.e. the AODs
are set up with the same ramp rates), it is thus necessary to
either place the AODs extremely close together or to
telecentrically relay the output of one AOD to the input of the
next AOD.
[0282] This aspect of the present invention is based on the
realisation that the AODs 30, 40 can be excited with different
acoustic waves so as to allow realistic practical separations
between the AODs without the requirement of a telecentric relay.
The acoustic waves can be modified either to allow the generation
of a completely stationary focal position 22 or precisely
controlled scanning.
[0283] In FIG. 36, d.sub.1 is the separation between the first AOD
30 and the second AOD 40 and d.sub.2' is the distance from the
second AOD to the focal point 22.
[0284] This aspect of the invention is based on the appreciation
that the curvature of the wavefront arriving at the second AOD 40
must exactly match the additional curvature induced by the second
AOD 40. As is apparent from FIG. 36, as the distance d.sub.1
increases, the curvature of the arriving wavefront increases
because the light is converging downwards towards a focus. This is
compensated for in the present invention by providing a less rapid
ramp (chirp) on the first AOD 30 than on the second AOD 40. This is
illustrated in FIG. 37 where it can be seen that the ramp rate
a.sub.1 for the first AOD 30 is lower than the ramp rate a.sub.2
for the second AOD 40 (a.sub.1 is equal to the gradient of the line
31 and a.sub.2 is equal to the gradient of the line 41). This
serves to produce a focal position 22 which is stationary in the X
direction, as shown in FIG. 37.
[0285] Referring to FIG. 38, the first AOD 30 is excited with an
acoustic wave having a chirp rate of a.sub.1. Accordingly, an
incoming laser beam 16 is converted to converging laser beam 18
that is focussed at the point 23 a distance d.sub.1' from the first
AOD 30. As is well known, this distance d.sub.1' is given by:
d 1 ' = V 2 .lamda. a 1 ( 4 ) ##EQU00011##
wherein: V=speed of sound in the AODs (m/s) a.sub.1=ramp rate of
first AOD drive (Hz/s) .lamda.=wavelength of light (m)
[0286] It follows from this that the radius of the curvature of the
wavefront of the laser beam 18 at the point where it meets the
second AOD 40 is given by:
d.sub.1'-d.sub.1 (5)
[0287] In order that the resulting focal position 22 is stationary,
the curvature added to the laser beam 18 by the second AOD 40 must
equal the curvature of the laser beam 18 as it arrives at the
second AOD 40. Accordingly:
d 2 ' = d 1 ' - d 1 2 ( 6 ) ##EQU00012##
[0288] The factor of 2 appears in this equation because the
curvature added by the second AOD 40 is identical to the curvature
that already exists at the laser beam 18 as it enters the second
AOD 40. The resulting curvature of the laser beam 28 is thus twice
the curvature of the laser beam 18. From these equations, it can be
deduced that:
a 2 = V 2 2 .lamda. d 2 ' ( 7 ) a 1 = V 2 .lamda. ( 2 d 2 ' + d 1 )
( 8 ) a 1 a 2 = 2 d 2 ' 2 d 2 ' + d 1 ( 9 ) ##EQU00013##
[0289] In these equations, d.sub.1 is always a positive value. The
values d.sub.2', a.sub.1 and a.sub.2 are positive for converging
rays as shown in FIG. 7a and negative for diverging rays as shown
in FIG. 7c. As explained earlier, even when the rays are diverging,
a real focal position is achieved using subsequent optics, such as
the lens 70.
[0290] When equation (9) is studied, it is apparent that if d.sub.1
is made to be zero then a.sub.1 equals a.sub.2. This is the
assumption utilised in the prior art because coupling two AODs
together with a telecentric relay exactly couples the output of the
first AOD onto the input of the second AOD and thus gives an
effective separation of the AODs of zero. Up until now, it has
always been thought that the frequency chirp across the two AODs
ought to be the same and that the effective separation between the
AODs should be zero (by virtue of utilising a telecentric relay).
The equations derived by the present inventors show that the chirp
rate across the two AODs can be made slightly different, in
accordance with equation (9), to account for a real separation of
d.sub.1 between the two AODs, to provide a system which provides a
stationary focal position 22 without a telecentric relay between
the AODs.
[0291] This is achieved by adjusting the ramp rate a.sub.1 of the
first AOD 30, in accordance with equation (8), to allow for the
change in wavefront curvature between the first AOD 30 and the
second AOD 40. Preferably, the wavefront curvature arriving at the
second AOD 40 equals the additional curvature that is added by the
second AOD 40. This "matching of curvature" provides for a
stationary focal position.
[0292] In the equations and analysis above, the distances are
apparent optical thicknesses. If further optical components are
interposed between the AODs, such as half wave plates and
polarisers, then the apparent optical separation needs to be
calculated by taking into account the refractive index of such
additional components. Also, the refractive index of the AODs
themselves needs to be taken into account. This can be done by
assuming that the acoustic wave enters and leaves the AOD at its
thickness-midpoint such that the apparent optical distance d.sub.1
is equal to the distance in air between the AODs plus half the
thickness of the first AOD 30 divided by its refractive index plus
half the thickness of the second AOD 40 divided by its refractive
index. When the two AODs are identical, then the value d.sub.1
equals the distance in air plus the thickness of the AOD divided by
its refractive index.
[0293] These principles can be extended to a system which utilises
four AODs to focus in more dimensions. As discussed above, when two
AODs are used, as shown in FIG. 38, the focal position 22 is a line
extending perpendicularly out of the page. Four AODs can be
utilised to focus in both X and Y to produce a point focal position
22.
[0294] FIGS. 39a and 39b show two orthogonal views of a preferred
four AOD system. As in FIG. 38, the first AOD 30 is separated from
the second AOD 40 by a distance d.sub.1 and the second AOD 40 is a
distance d.sub.2' from the focal point 22. In addition, third and
fourth AODs 50, 60 are provided, the distance between the third AOD
50 and the fourth AOD 60 being d.sub.3 and the distance from the
fourth AOD 60 to the focal point 22 being d.sub.4'. The ramp rates
for the third and fourth AODs can be calculated in a similar way as
for the first and second AODs. Very similar equations apply: --
a 4 = V 2 2 .lamda. d 4 ' ( 10 ) a 3 = V 2 .lamda. ( 2 d 4 ' + d 3
) ( 11 ) a 3 a 4 = 2 d 4 ' 2 d 4 ' + d 3 ( 12 ) ##EQU00014##
[0295] Accordingly, in the four AOD system, the first and second
AODs are stimulated in the same way as the first and second AODs of
the two-AOD embodiment. This provides the necessary focussing in
the X-Z plane. In addition, third and fourth AODs are stimulated
such that the curvature of the wavefront arriving at the fourth AOD
equals the additional curvature added by the fourth AOD, hence
doubling the curvature of the wavefront as it leaves the fourth
AOD. This provides the necessary focussing in the Y-Z plane. The
distances d.sub.2' and d.sub.4' are selected to ensure that the
final focal spot position 22 is as desired. As will be apparent
from FIGS. 39a and 39b, the actual distances between the AODs and
the optical thickness of any intervening components, as well as the
AODs themselves, needs to be taken into account when determining
d.sub.1, d.sub.3, d.sub.2' and d.sub.4'.
[0296] Depending on the exact configuration used, further fine
tuning may be applied to achieve an exactly stationary spot. The
equations above are based on the simplified assumption of AOD
crystals having surfaces that are approximately perpendicular to
the direction of propagation of the light. It is possible to
manufacture the AODs with slightly angled faces (and there are
practical reasons to do exactly this) and this can cause errors in
the separations used in the equations that can result in a small
residual movement of the focal position. These residual movements
can be corrected by small adjustments to the ratio of ramp rates
a.sub.1/a.sub.2, a.sub.2/a.sub.4. These corrections can either be
found experimentally or by building an accurate optical model using
a commercial programme like Zemax. When such angled faces are used,
typical corrections are much less than +/-2% to the ramp rate of
each AOD. Similarly, small corrections may be applied to the ratio
of the X ramp rate to the Y ramp rate to fine tune the astigmatism
of the focal position 22. This is equivalent to adjusting the ratio
of d.sub.2' to d.sub.4' so that the Z value of the focal position
in the X-Z and X-Y planes is the same. These fine tuning
corrections are a function of the Z position of the focal spot and
can readily be built into the algorithms that compute the ramp rate
of the AODs before each scan.
[0297] As will be understood from the above, it is possible with
the present invention to utilise two or four AODs to achieve a
completely stationary focal line or focal point inside or on a
target. This can be achieved without lengthy telecentric relays
between the AODs by appropriate manipulation of the ramp rates of
the acoustic waves applied to the AODs. The resulting system can
thus be used to achieve random access focussing at very fast
speeds. For example, it is possible to repetitively focus to 30
different positions within or on the target at a frequency of 1000
Hz. In other words, in one second, the laser beam can be focussed
to 30 points one thousand times. To achieve this, the laser beam
focal point is repositioned 30,000 times in one second. This is
simply not achievable with prior art galvanometer mirrors.
Scanning a Target
[0298] To build up a three-dimensional image of a target, it is
useful to be able to follow a raster scan with the focal point
along a predetermined path through the target. One potential raster
scan is to move the focal point in the X direction, keeping the Y
and Z values constant, to then increment the Y position by some
small amount, to perform another scan in the X direction and so on
until a two-dimensional grid of scans is achieved. Thereafter, the
Z direction is incremented and another two-dimensional grid is
scanned until a three dimensional volume has been built up. This
can be done quite quickly with the system of the present invention
such that a three-dimensional image can be provided.
[0299] One problem encountered when implementing a raster scan
using the system of the present invention is that there are minimum
and maximum limits on the frequency of acoustic waves that can be
put through the AODs. This is illustrated as f.sub.max and
f.sub.min in FIG. 4a, for example. Typical values are 30 Mhz for
the minimum frequency and 40 Mhz for the maximum frequency. As
shown in FIG. 4a, there will be a "flyback" portion whereby the
frequency is suddenly switched from the maximum frequency to a
lower frequency (in the case of applying a positive chirp rate) or
a sudden switch from the minimum frequency to a higher frequency
(in the case when applying a negative chirp rate). The X, Y and Z
positions depend on the chirp rate and the difference in absolute
frequencies between the first and second AODs (see FIG. 8).
Accordingly, this sudden changing of the absolute values of both
frequencies will not cause a movement in the X Y and Z position of
the focal spot if it is implemented properly.
[0300] If desired, the X and Y scans can be carried out
simultaneously with the Y scan being much slower than the X scan.
This leads to the two-dimensional scanning pattern shown in FIG.
40.
[0301] FIG. 41 shows how a raster scan performed in the X-Y plane
can be achieved. Movements in X or Y correspond to changes in the
angle of the output laser beam from the AOD system.
[0302] As is apparent from the above equations and from FIG. 42, to
focus a stationary spot at X=0 requires a.sub.2 to be slightly
larger than a.sub.1 in accordance with equation (9). The situation
shown in FIGS. 41 and 42 is where the focal position has a positive
value of Z such that the laser beam converges upon exiting the AOD
system. In order to provide an X deflection that moves at some
constant linear velocity, it becomes necessary to vary a.sub.1 and
a.sub.2 such that there is a linearly increasing difference between
the absolute frequency of the acoustic wave in the first AOD and
the absolute value of the frequency in the second AOD. This is
illustrated in FIG. 43 where the dotted lines show a.sub.1 being
reduced and a.sub.2 being increased. This provides a scanning X
deflection whereby the X value of the focal point decreases
linearly with time as shown in the lower part of FIG. 43.
[0303] As shown in FIG. 8, varying the slope a.sub.1, a.sub.2 of
the ramps varies the Z position. For Z=0, a.sub.1 and a.sub.2=0 and
X scanning can easily be achieved by making a.sub.1 slightly
negative and a.sub.2 slightly positive. For other values of Z,
higher magnitudes of a.sub.1 and a.sub.2 are required and the
limits on the minimum and maximum drive frequencies of the AODs
mean that it is possible to hit one of the limits very quickly. For
non-zero values of Z, it is possible to hit either the minimum or
maximum drive frequency before one has completed an X scan across
the target. In such cases, it is convenient to perform a series of
X "mini-scans" in which the X scan as a whole is interrupted at
various points in time to allow the frequency to be reset in a
"flyback" period. This is illustrated in FIG. 44, which is a
cropped version of FIG. 42 showing how the maximum and minimum
drive frequencies limit the amount of X deflection that can take
place.
[0304] FIG. 45 illustrates a series of mini-scans in X. The
frequency of the acoustic wave in the first AOD 30 is designated by
line 31 and the frequency of the acoustic wave in the second AOD 40
is designated by the line 41. In this example, line 31 has a
shallower gradient than line 41 which mans that a.sub.1 is less
than a.sub.2, which from equation (9) means that d.sub.2' is
positive, which in turn means that this situation is for some
positive value of Z. As shown in FIG. 45, when the absolute value
of the frequency of the acoustic wave in the first AOD 30 reaches
the maximum value f.sub.max it becomes necessary to reset the
frequencies. As is apparent from FIG. 45, it will not be possible
to change the frequency in the first AOD 30 to be f.sub.min because
the frequency in the second AOD 40 must be less than the frequency
in the first AOD 30 to ensure that the difference in frequencies
between the two AODs continues to give the correct value of X.
Accordingly, the frequency in the second AOD 40 is reduced to
f.sub.min and the frequency in the first AOD 30 is reduced by the
same amount. This frequency resetting takes place in a period of
non-active time when the laser is switched off (or at least when
measurements are not recorded or are ignored). The non-active time
generally has two components. The first component, known as the
"reset time" is the time it takes to reset the frequency from the
maximum value to the new value or from the minimum value to the new
value. This is typically 4 .mu.s. The second component, known as
the "AOD fill time" is the time it takes to fill the AOD with
appropriate acoustic waves. This is typically equal to the width of
the AOD divided by the speed of the acoustic wave in the AOD. For
example, if the AOD is 15 mm wide and the acoustic waves travel at
600 m/s, then the AOD fill time will be 25 microseconds. The total
non-active time is thus typically around 30 .mu.s.
[0305] After the AOD fill time has elapsed, the frequencies in the
AODs should be different by an amount equal to the difference at
the end of the previous mini-scan. As the frequencies are different
by the same amount, it might be expected that the X position would
be the same at the beginning of the second mini-scan as it was at
the end of the first mini-scan. This is true when the AODs are
telecentrically relayed. However, it has been found that this is
not the case when there is some separation between the AODs.
Instead, the X position is different to that which is expected as
shown in FIG. 45, bottom graph. There it can be seen that the X
position is different at the start of the second mini-scan than at
the end of the first mini-scan.
[0306] The present inventors have found that the reason for this
lies in the assumption that the separation of the ramps alone
causes the variation in X. This assumption is only true if there is
no physical separation between the AODs, or alternatively, if the
AODs are coupled by telecentric relays. In the case where there is
an actual separation between the AODs, a more complicated algorithm
needs to be utilised to calculate the frequency offsets necessary
to maintain the position in X between the end of one mini-scan and
the start of the next mini-scan. In FIG. 45, the frequency offsets
are calculated by making the frequency in the second AOD zero and
reducing the frequency in the first AOD by some amount that causes
the frequency difference to be the same at the start of the next
mini-scan. In fact, different offsets will be needed for each AOD
as illustrated in FIG. 46. Here, the frequency in the first AOD is
reduced by .DELTA.f.sub.1 and the frequency in the second AOD is
reduced by .DELTA.f.sub.2. This second frequency reduction,
.DELTA.f.sub.2 is calculated as the offset needed to reduce the
frequency in the second AOD to f.sub.min. It has been found that
.DELTA.f.sub.1 and .DELTA.f.sub.2 are related by the following
equation:
.DELTA. f 2 .DELTA. f 1 .apprxeq. d 1 ' d 1 ' - d 1 ( 13 )
##EQU00015##
[0307] This equation can be proved by referring to FIG. 47. The
centre line of the first AOD is referenced 30 and the centre line
of the second AOD is referenced 40. The ultimate focal point is
shown at 22. If a frequency offset is introduced into the acoustic
wave of the first AOD this will produce an angular deflection of
.DELTA..theta..sub.1 at the central position (X=0) of the AOD. This
is graphically illustrated in FIG. 47 by the angle
.DELTA..theta..sub.1 between the line of the laser beam before
deflection (vertically downward in FIG. 47) and the line 35 of the
laser beam after deflection. However, at the second AOD, at the
position X=0 the ray will apparently be deflected by a different
angle .DELTA..theta..sub.2. As is apparent from FIG. 47,
.DELTA..theta..sub.2 is larger than .DELTA..theta..sub.1. A
geometric deduction leads to the equation (which is valid for all
small angles of .DELTA..theta..sub.1 and .DELTA..theta..sub.2):
.DELTA. .theta. 2 .DELTA. .theta. 1 .apprxeq. d 1 ' d 1 ' - d 1 (
14 ) ##EQU00016##
[0308] Equation (14) is derived once it is realised that the change
in angles of the beam is directly proportional to the change in
frequency. As shown in the lower part of FIG. 46, applying offsets
that have the relationship of equation (13) means that the X
deflection is not changed between the end of one mini-scan and the
beginning of the next mini-scan. Please note that FIG. 46 does not
show any AOD fill time for clarity although there will in reality
be an AOD fill time as shown in FIG. 45. In practice,
.DELTA.f.sub.1 and .DELTA.f.sub.2 are calculated to be correct at
the end of the AOD fill time, when the data collection restarts.
Converting equation (13) to the nomenclature of FIGS. 39a and 39b,
we find:
.DELTA. f 1 .DELTA. f 2 = 2 d 2 ' 2 d 2 ' + d 1 ( 15 )
##EQU00017##
[0309] Similarly, when there are four AODs present:
.DELTA. f 3 .DELTA. f 4 = 2 d 4 ' 2 d 4 ' + d 3 ( 16 )
##EQU00018##
[0310] The scan rate .delta..theta./.delta.t can be calculated as
follows:
[0311] For the simple case where the AODs are coupled by
telecentric relays (i.e. d.sub.1 is considered to be zero) the scan
rate is proportional to the difference in slopes of the chirp
signal provided to each AOD. In fact, the scan rate is given
by:
.delta..theta. .delta. t = .lamda. V ( a 1 - a 2 ) ( 17 )
##EQU00019##
[0312] In the simple case of d.sub.1=0, the following equations
result for the ramp rates a.sub.2 and a.sub.1.
a 2 = V 2 2 .lamda. d 2 ' + V 2 .lamda. .delta..theta. .delta. t (
18 ) a 1 = V 2 2 .lamda. d 2 ' - V 2 .lamda. .delta..theta. .delta.
t ( 19 ) ##EQU00020##
[0313] It can be seen from these equations that the ramp rate
a.sub.2 is increased by the same amount that the ramp rate a.sub.1
is decreased. This, however, only applies when d.sub.1 is
considered to be zero (i.e. when the AODs are telecentrically
coupled). In the more complicated case when d.sub.1 is non-zero.
The values for a.sub.1 and a.sub.2 are instead given by:
a 1 = V .lamda. ( V d 2 ' - .delta..theta. .delta. t ) ( 2 + d 1 d
2 ' - d 1 V .delta..theta. .delta. t ) ( 20 ) a 2 = V 2 2 .lamda. d
2 ' + V 2 .lamda. .delta..theta. .delta. t ( 21 ) ##EQU00021##
[0314] These equations apply where there are two AODs for focussing
in the X-Z plane or, as shown in FIG. 48, when there are four AODs.
In this case, the angular scan rate .delta..theta./.delta.t is that
measured about the second AOD 40. The apparent rate as measured
about the last AOD 60 can be obtained y multiplying this scan rate
by d.sub.2'/d.sub.4'.
[0315] FIG. 49 shows the appropriate equations for the Y-Z plane.
Here, .PHI. is the angle as measured from the fourth AOD 60.
a 3 = V .lamda. ( V d 4 ' - .delta..phi. .delta. t ) ( 2 + d 3 d 4
' + V 2 .lamda. .delta..theta. .delta. t ) ( 22 ) a 4 = V 2 2
.lamda. d 4 ' + V 2 .lamda. .delta..theta. .delta. t ( 23 )
##EQU00022##
[0316] As will be appreciated from FIGS. 39a and 39b, the spacing
between each adjacent AOD is related to the distances d.sub.1,
d.sub.3, d.sub.2' and d.sub.4' by the following equations:
[0317] The effective separation between the first AOD 30 and the
third AOD 50 is:
d.sub.1+d.sub.2'-d.sub.3-d.sub.4'
[0318] The effective separation between the third AOD 50 and the
second AOD 40 is:
d.sub.3+d.sub.4'-d.sub.2'
[0319] The effective separation between the second AOD 40 and the
fourth AOD 60 is:
d.sub.2'-d.sub.4'
Multi-Wavelength System
[0320] FIGS. 20 to 23 above show the possibility of varying the
compensation factor C to either fully compensate for chromatic
aberration in the Z-plane (FIG. 23), to fully compensate for
chromatic aberration in the Z and Y planes (FIG. 22) or to achieve
some intermediate compensation in which chromatic aberration is
compensated in all planes, but not for the maximum extent (FIG.
20). In order to vary this compensation factor C, it is necessary
to vary the strength of the lenses used in the telecentric relay.
For example, the lenses 110 and 150 in FIG. 17 can be replaced with
more strongly or less strongly dispersive lenses in order to vary
the compensation factor C. In any practical embodiment of a system,
it would be beneficial to design into the system a method for
varying the compensation factor C, which does not involve having to
physically replace lenses.
[0321] Another problem lies in the fact that it is desirable for
most neuroscience applications to be able to select the wavelength
of electromagnetic radiation that is used. Typically, wavelengths
of 690 to 1000 nm are used in neuroscience applications. Changing
the wavelength of a laser beam passing through diffractive optics
automatically changes the deflection angles introduced by such
diffractive optics (because the deflection angle is proportional to
the wavelength). Accordingly, while it is straightforward to design
a system that can operate at a single laser wavelength, it is more
difficult to design a system that can operate across a range of
wavelengths.
[0322] One possible approach is to design the system to operate at
the maximum wavelength (e.g. 1000 nm). In such a system, the
maximum diffraction angles would be catered for and it can be
ensured that no light is lost by light being diffracted through a
greater angle than designed. The serious drawback of such a system
is that the system inherently works non-optimally for any
wavelength less than 1000 nm. In particular, due to the smaller
deflection angles, the aperture of the objective lens will not be
filled when wavelengths of less than 1000 nm are used and this
seriously reduces the amount of power that can be introduced to the
target at the focal position.
[0323] It would be desirable to design a system in which the
electromagnetic radiation wavelength can be varied without
influencing the amount of power delivered to the target or the
intensity of the focus provided.
[0324] FIG. 50 shows one embodiment of the system shown in FIG. 17.
Here, the first optical element 110 is provided by a lens 112
having a positive focal power and a compensation plate 111. The
second optical element 150 is provided by a lens 152 having a
positive focal power and a second compensation plate 151.
Compensation plate 111 is comprised of a positive focal length
diffractive optical element 115 intimately attached (e.g. glued)
onto the plane surface of a negative focal length conventional
lens. Compensation plate 151 is comprised of a negative focal
length diffractive optical element 155 intimately attached (e.g.
glued) onto the plane surface of a positive focal length
conventional lens 156. Accordingly, the combined effect of
compensation plate 111 and lens 112 is to have dF/d.lamda. negative
and the combined effect of compensation plate 151 and lens 152 is
to have dF/d.lamda. positive.
[0325] The lenses 112 and 152 have approximately the same effect on
all wavelengths of light. Accordingly, virtually all of the
chromatic aberration correction is achieved by the compensation
plates 111 and 151. FIGS. 51a to 51c show the effect of changing
the laser wavelength on the chromatic aberration correction of the
compensation plates 111 and 151.
[0326] At the design centre wavelength, as shown in FIG. 51a, each
compensation plate 111 and 151 provides no overall lensing effect.
This is because at this wavelength, the focal length of the
diffractive optical elements 115, 155 is exactly equal and opposite
to the focal length of the attached lens 116, 156. Accordingly,
light at the design wavelength passes through the compensation
plates like a flat glass plate.
[0327] As the wavelength of laser light is increased from the
design wavelength, the diffractive optical elements 115, 155 become
stronger (the diffraction angle is proportional to the wavelength)
so as to produce a lensing effect as shown in FIG. 51b.
[0328] At wavelengths less than the design wavelength, the
diffractive optical elements become weaker, producing the lensing
effect shown in FIG. 51c. It is apparent from FIGS. 51a-51c that
the diameter of the output laser beam will be influenced by the
centre wavelength of the laser, due to the varying lensing effect
of the diffractive optical elements 115, 155.
[0329] One advantage of the arrangement shown in FIG. 50 is that
the positions of the compensation plates 111, 151 can be altered to
alter the degree of chromatic aberration correction. This applies
only at the design wavelength because moving the diffractive
optical elements closer together or further apart at the design
wavelength does not introduce any additional lensing (the
diffractive optical elements act like a flat glass plate, as shown
in FIG. 51a at the design wavelength). Accordingly, the degree of
chromatic aberration correction, C, can be varied by simply moving
the axial positions of the compensation plates 111, 151 at the
design wavelength. There is thus no need to replace any lenses when
altering the compensation factor C. In FIG. 52, a value of C less
than 1 is shown. In FIG. 50, a value of C=2 is shown. Accordingly,
slowly moving the correction plates 111, 151 from the position
shown in FIG. 50 to the position shown in FIG. 52 will slowly vary
the degree of chromatic aberration correction from a correction of
C=2 (which will provide the correction shown in FIG. 23) to a value
of C=1 (which will provide the correction shown in FIG. 22) or
less.
[0330] As explained above with regard to FIGS. 51a-51c, the
compensation plates 111, 151 can be moved to independently adjust
the chromatic aberration correction, C, only at the design
wavelength. At other wavelengths, the plates will introduce some
amount of lensing and thus moving them will change the output beam
diameter. FIG. 53 shows a typical range of wavelengths over which
it is desirable to operate a system. It shows three potential laser
spectra, a first spectrum 81 having a width of 10 nm centred on 850
nm, a second spectrum 82 having a width of 10 nm centred around 980
nm and a third spectrum 83, also having a width of 10 nm centred
around 720 nm. These laser pulse streams having spectral widths of
10 nm are typical for pulses having 100 fs duration in the time
domain.
[0331] The problem is therefore how to design a system which
provides sufficient differential magnification to correct the
chromatic aberration due to the spectral width of a single laser
pulse stream but which yields a constant output beam size whatever
the centre wavelength of the laser pulse stream.
[0332] FIG. 54 illustrates the problem diagrammatically. At the 850
nm design wavelength, the objective lens 90 is more or less fully
filled. This means that the spot 22 will be of minimal size (or, in
other words, the spot will be maximally focussed). At longer
wavelengths (e.g. 1000 nm) the chromatic aberration correction
mechanism causes the laser beams to be deflected more and some
light leaving the compensation plate 151 is not captured by the
subsequent optics. As light is lost, the power provided to the
focal position 22 will be reduced. At shorter wavelengths, e.g. 700
nm as shown in FIG. 54, the laser beams will be deflected through a
smaller angle and the focal position 22 will be created from a
smaller diameter beam at the objective lens 90. As the objective
lens is under-filled, the diameter of the diffraction limited focal
spot 22 will be increased compared to the minimum it could be at
this wavelength and the intensity of the spot will therefore be
less than it could be.
[0333] For neuroscience applications, one important use of 2-photon
microscopes is for uncaging neurotransmitter chemicals to
selectively excite particular neurons. This generally requires much
higher light intensity at the focal position than 2-photon imaging.
It has been found that the most effective molecules for 2-photon
uncaging are sensitive around the short wavelength end of a
Ti-sapphire operating range (around 600-750 nm). It is therefore
highly desirable to be able to fill the aperture of the objective
lens at short wavelengths.
[0334] If the compensator is designed to fill the aperture at 1000
nm (such that no light is lost at this wavelength, see FIG. 54) the
diameter of the beam at 700 nm will be only about 70% of that at
1000 nm. This would produce a focal spot of 1.4 times the diameter
which corresponds to a focal spot having twice the area of that
produced by a properly filled objective lens. The light at the
focal position would therefore have half the intensity and one
quarter of the 2-photon excitation rate compared to the situation
where the aperture is filled. It is therefore desirable to fill the
aperture at the lower end of the wavelength range and at the same
time not lose any light at the upper end of the wavelength range.
It would be preferable to design a system in which this can be
achieved while at the same time allowing the degree of chromatic
aberration compensation to be adjusted at least over the range
C=1-2 and which does not require any lenses to be removed or
replaced during use. Preferably, lenses would be simply moved
within the system to provide the correct configuration.
[0335] One solution for achieving this is shown in FIG. 55. In this
system, the lens 112 in FIG. 54 is replaced by a pair of zoom
lenses 113, 114. Similarly, the lens 152 in FIG. 54 is replaced by
a pair of zoom lenses 153, 154. The system of FIG. 55 thus includes
a first pair of positive focal length zoom lenses 113, 114, a pair
of compensation plates 111, 151 and a second pair of positive focal
length zoom lenses 153, 154. The positions of the zoom lenses 113,
114, 153, 154 are adjustable to alter their effective focal length.
Further, the separation between the two compensation plates 111,
151 is adjustable to vary the compensation factor C and also to
take account of different wavelengths of light being utilised in
the device. FIG. 55 shows the nomenclature used in the following
description to describe the lenses L and the diffractive optical
elements D.
[0336] A method for designing a zoom compensator will now be
described. Firstly, the aperture diameter d3 of the diffractive
optical elements 115, 155 can be selected to be a fraction R of the
diameter of the input beam d1. It has been found that a fraction
R=0.5 allows a system to be designed that covers a 700-900 nm range
of wavelengths. A smaller fraction can be selected to cover a wider
proportionate range.
[0337] The maximum separation of s3 of the compensation plates 111,
151 can then be selected to be greater than the minimum working f
number (the ratio of focal length to diameter of a lens) of the
diffractive optical element (and other lenses) multiplied by the
diameter d3.
[0338] For a maximum compensation factor (C=2) the focal length of
the lenses L3 and L4 can be made equal to the maximum separation s3
of the compensation plates. As described above, L3 has negative
focussing power and L4 has positive focussing power, and the focal
lengths of both L3 and L4 are equal to s3.
[0339] The focal length of the diffractive optical elements (F(D3)
and F(D4)) at the mid-point wavelength can be made equal to the
maximum separation s3. F(D3) will be positive and F(D4) will be
negative. This ensures that at the mid-point wavelength, the
compensator plates have zero lensing power.
[0340] In use, the zoom lens spacings s1 and s2 are adjusted for
each operating wavelength so that parallel input light is focussed
to the mid-point between L3 and L4. The separations s4 and s5 are
then adjusted to give the required constant parallel output beam
diameter. The spacings s1 and s2 can be calculated by calculating
the effective focal lens of the zoom lens needed to transform
between the correct diameter parallel external beams and the
central focal point, given the strength of the compensator plates
at the wavelength concerned.
[0341] For example, at the mid-point wavelength where the
compensator plates have zero power, if R=0.5 then the effective
focal length of the input and output zoom lens pairs is equal to
the separation s3. This follows simply because the distance from
the centre point to the compensator is s3/2 and as R=0.5 the light
cone diameter is doubled before becoming parallel. This is shown in
FIG. 56.
[0342] The effective focal length of the zoom lenses can be
calculated for different operating wavelengths by taking into
account the extra required divergence or convergence of the light
cone outside of the compensating plates. At a shorter operating
wavelength (e.g. 700 nm), s3 needs to be made longer because the
beam will not be diffracted through such a large angle with shorter
wavelengths. Similarly, for longer wavelengths, the distance s3
needs to be made shorter because the beams will be diffracted more
at longer wavelengths. Once s3 has been selected, it is possible to
calculate the focal length of the first zoom pair, FT1 and the
focal length of the second zoom pair, FT2. In general, a longer s3
will require a longer FT2 and a smaller s3 will require a smaller
FT2. The positions of the zoom lenses to meet this condition can
then be determined using standard textbook geometric optic
equations for zoom lenses.
[0343] FIGS. 57 and 58 show actual values for an example when the
chosen separation s3 is 75 mm at a centre wavelength of 800 nm. It
can be seen that, at the centre wavelength, FT1=FT2=s3. For longer
wavelengths, all of these values are reduced and for shorter
wavelengths all of these values are increased.
[0344] FIG. 58 shows how the values of s2 or s4 and the values of
FT1 and FT2 vary as s1 or s5 vary utilising two 125 mm focal length
lenses. Accordingly, it is possible to set FT1 and FT2 and
thereafter derive s1, s2, s4 and s5.
[0345] FIG. 59 shows the positions of the lenses for a longer laser
wavelength of 900 nm. FIG. 60 shows the lens positions of a
wavelength for a shorter wavelength of 700. It will be observed by
comparing FIGS. 59 and 60 that the diameter of the output beam is
substantially the same whether a long or short wavelength is used.
Furthermore, due to the positioning of the compensation plates 111,
151, the same amount of selectable chromatic aberration correction
C is achieved in both cases.
[0346] It will be appreciated that the zoom compensator of the
present invention enables an aperture within the system (for
example the objective lens aperture) to be fully filled even when
the centre wavelength of the laser beam is changed. In general, the
compensator can be modified to fill any aperture in the system. The
system aperture is defined here as the diameter of the maximum
diameter optical beam at the entrance of the system (or subsystem
concerned) that can get through the complete system to the final
image without any of the rays being cut off by any intermediate
apertures internal to the optical system. Accordingly, the
corrective optics of this zoom compensator is capable of ensuring
that the beam fills the same design system aperture for
substantially all wavelengths falling within the wavelength range
of interest. This is illustrated in FIGS. 59 and 60 by showing the
beam filling the objective lens aperture, but it may be applied to
any system aperture.
[0347] With the solution described above, the values FT1 and FT2
vary with the wavelength of the electromagnetic radiation used.
This causes the overall length of the telecentric relay zoom
compensator to vary, as shown in FIG. 61. In FIG. 61, the length
from the output of the last AOD grating to the entrance to the
microscope is illustrated. As will be apparent from FIG. 61, the
overall length will be equal to 2.times.FT1 plus 2.times.FT2.
[0348] Accordingly, to implement this system it would be necessary
to move the objective lens relative to the AODs each time the laser
wavelength is changed. While this is feasible, it would be
convenient to provide a telecentric relay that is of constant
length whatever the wavelength of light used.
[0349] One solution is to use four mirrors in a standard "optical
trombone" arrangement before, or preferably after, the zoom
compensator in order to make the overall path length constant.
[0350] Another possibility is to design a system such that the sum
of FT1 and FT2 is constant. This can be done by varying R with the
wavelength. Since varying R will vary C (the compensation factor),
s3 can be varied in order to balance this to keep C on target. C
will be constant if the product of R and s3 is constant.
[0351] The above description of this zoom compensator has several
specific aspects which should not be taken as limitations to our
claims. Firstly, the example shows that the diameter of the input
aperture (FIG. 61) is equal to the diameter of the output aperture.
This could simply be changed by adjusting the focal lengths of FT1
and FT2. Secondly, the focal lengths of the sub components 115,116,
155 and 166 of each compensator plate 111, 151 have been taken as
equal to one another in magnitude at the design mid wavelength. The
system could however be designed with different focal lengths here,
the key point being that the resulting overall system balances the
positive and negative chromatic compensation in the overall
telecentric relay so that the magnification chromatic aberration is
reduced throughout the working field, preferably with the zero
aberration point still in the middle so that the edge of field
aberrations are minimised.
[0352] Accordingly, the present invention proposes the use of zoom
lens systems to adjust the overall magnification of the chromatic
aberration correction system so as to achieve a full objective lens
aperture even at different wavelengths.
* * * * *