U.S. patent application number 14/591489 was filed with the patent office on 2015-07-09 for tunable mid-ir fiber laser for non-linear imaging applications.
The applicant listed for this patent is THORLABS, INC.. Invention is credited to Alex Cable, Peter Fendel, Reza Salem.
Application Number | 20150192768 14/591489 |
Document ID | / |
Family ID | 53495033 |
Filed Date | 2015-07-09 |
United States Patent
Application |
20150192768 |
Kind Code |
A1 |
Salem; Reza ; et
al. |
July 9, 2015 |
TUNABLE MID-IR FIBER LASER FOR NON-LINEAR IMAGING APPLICATIONS
Abstract
A microscopy system, including: a mode-locked fiber laser
configured to output a pulse having a center wavelength; a
nonlinear waveguide configured to shift the wavelength of the pulse
from the mode-locked fiber laser; a fiber amplifier configured to
amplify the output from the first nonlinear waveguide; a
second-harmonic generator configured to generate femtosecond pulses
at twice the optical frequency from the output of the fiber
amplifier; and an imaging system.
Inventors: |
Salem; Reza; (Columbia,
MD) ; Fendel; Peter; (Sparta, NJ) ; Cable;
Alex; (Newton, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THORLABS, INC. |
Newton |
NJ |
US |
|
|
Family ID: |
53495033 |
Appl. No.: |
14/591489 |
Filed: |
January 7, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61924629 |
Jan 7, 2014 |
|
|
|
Current U.S.
Class: |
250/227.2 |
Current CPC
Class: |
G02B 21/0032 20130101;
H01S 3/0092 20130101; G01N 21/255 20130101; G01N 21/59 20130101;
G02B 21/361 20130101; G02B 21/0076 20130101; G01N 21/47 20130101;
G01N 21/45 20130101; G01N 21/35 20130101; H01S 3/11 20130101; H01S
3/302 20130101; G01N 2021/3595 20130101; G02B 2207/114
20130101 |
International
Class: |
G02B 21/36 20060101
G02B021/36 |
Claims
1. A two-photon microscopy system, comprising: a mode-locked fiber
laser configured to output a pulse having a center wavelength; a
nonlinear waveguide configured to shift the wavelength of the pulse
from the mode-locked fiber laser; a fiber amplifier configured to
amplify the output from the first nonlinear waveguide; a
second-harmonic generator configured to generate femtosecond pulses
at twice the optical frequency from the output of the fiber
amplifier; and a microscopy imaging system.
2. The system of claim 1, wherein the mode-locked fiber laser
outputs pulse that supports a transform-limited pulse width shorter
than 1 ps and has a center wavelength between 1500 nm and 1650
nm.
3. The system of claim 1, wherein the first nonlinear waveguide
shifts the output wavelength from the mode-locked fiber laser to a
wavelength longer than 1700 nm and shorter than 2800 nm.
4. The system of claim 1, wherein the first fiber amplifier
operates in the wavelength region between 1700 nm and 2800 nm.
5. The system of claim 1, further comprising a second fiber
amplifier configured to boost the power from the mode-locked fiber
laser and to control the amount of wavelength shift.
6. The system of claim 1, further comprising a first polarization
controller for controlling an amount of wavelength shift through a
Raman soliton self-frequency shifting process.
7. The system of claim 1, further comprising a first dispersive
element configured to create a desired amount of chirp on the pulse
entering the first fiber amplifier.
8. The system of claim 1, further comprising a second polarization
controller configured to adjust the polarization state of the
pulses entering the first fiber amplifier.
9. The system of claim 1, further comprising a second dispersive
element configured to adjust the amount of chirp on the pulse
entering the second-harmonic generator.
10. The system of claim 1, further comprising a third polarization
controller configured to adjust the polarization state of the
pulses entering the second-harmonic generator.
11. A microscopy system comprising a mode-locked fiber laser, a
splitter after the mode-locked fiber laser for splitting the output
of the fiber laser into a first path and a second path, the first
path further comprising: a first nonlinear waveguide; a first fiber
amplifier; a first second-harmonic generator nonlinear medium; and
the second path comprising: a second second-harmonic generator
nonlinear medium; and the system further comprising a microscope
that receives one or two outputs from the first path or the second
path.
12. The system of claim 11 including a second fiber amplifier
before the first nonlinear waveguide.
13. The system of claim 12. Where the splitter is placed after the
second fiber amplifier and before the first nonlinear
waveguide.
14. The system of claim 11, further comprising a variable delay
line on the first path or on the second path.
15. The system of claim 11, further comprising a third fiber
amplifier on the second path.
16. A method for operating a multi-photon microscopy system that
comprises a fiber laser configured to output a pulse having a
center wavelength; a first nonlinear waveguide configured to shift
the wavelength of the pulse from the fiber laser; a fiber amplifier
with at least one stage configured to amplify the output from the
first nonlinear waveguide; and a nonlinear medium configured to
frequency-double the output from the first fiber amplifier, the
method comprising: receiving a feedback from, the output of the
first nonlinear waveguide, the output of the first fiber amplifier
or the image generated by the microscope; and adjusting peak power,
energy, wavelength or polarization of the pulse entering the
nonlinear medium.
17. A three-photon microscopy system, comprising: a mode-locked
fiber laser configured to output a pulse having a center
wavelength; a nonlinear waveguide configured to shift the
wavelength of the pulse from the mode-locked fiber laser; a fiber
amplifier configured to amplify the output from the first nonlinear
waveguide; and a microscopy imaging system.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/924,629, filed on Jan. 7, 2014, the contents of
which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of non-linear
imagining, in particular, microscopy applications.
BACKGROUND
[0003] Non-Linear imaging and in particular two-photon imaging
systems are a workhouse in today's medical and life science
labs.
[0004] A non-linear imaging system consists of one or multiple
excitation and detection beam paths and a processing unit. The
excitation beam path is comprised of a laser, beam forming optics,
namely a beam expander, a two dimensional scan unit, a set of
optics to relay the beam onto the back aperture of an objective. A
microscope objective focuses the beam onto the sample. The scan
unit is used to create a 2D scan pattern on the sample to
illuminate the region of interest by focus volume. Light scattered
backward or forward from the sample is collected by a high NA
objective, separated from the excitation light by means of a
wavelength selective beam splitter or filters. The light is then
detected by one or multiple light detectors. A processing unit
reconstructs the image from the individually recorded pixels.
[0005] The non-linear excitation commands high peak intensity,
which limits the excitation volume to focus of the microscope
objective. This allows for depth-resolved measurements. Another
advantage of 2 p-microscopy over standard fluorescence or confocal
microscopy is that the excitation wavelengths are about twice as
long. Long wavelength excitation has two advantages. 1. It allows
to image deeper into the sample as longer wavelengths scatter less
in dense media like human tissue. 2. Excitation with NIR light
reduces photo toxicity and photo bleaching of the specimen.
[0006] A great area of interest is to image samples tagged with
fluorescence proteins like Green and Yellow florescence's proteins
(GFP and YFP, respectively) or mCherry. Often the proteins are
genetically encoded in the sample. These fluorophores exhibit
strong excitation cross sections in the wavelength region above 950
nm.
[0007] Wavelength of up to 1050 nm can be produced using
mode-locked Titanium Sapphire Lasers. These lasers, however, are
complex and expensive and often present a high barrier of entry
into the field. In addition the gain maximum of TiSa is at 800 nm
and the gain curve drops quickly when approaching 950 nm limiting
the power available at 950 nm and above.
[0008] Therefore, there is a need for a cost effective laser system
which can produce high output power with short pulses at wavelength
above 950 nm.
[0009] A fairly new emerging imaging technique deploys three-photon
excitation. All the advantages of long wavelength and non-linear
excitation mentioned above apply also for three photon imaging. The
excitation wavelengths between 1500 nm and 2000 nm are used. The
advantage is even less scattering of the excitation light than in
the case of two photon excitation. The reduced scattering permits
even deeper imaging in highly scattering tissue. The disadvantage
of going to higher and higher order non-linear excitation is a
drastically reduced excitation cross section. Hence which technique
to deploy needs to be carefully decided upon the objectives of the
experiment.
[0010] Another important technique used in live cell studies is
photo activation. One example of photo activation is to release
certain substances into the cell upon exposing the cell or part of
the cell to intense light. Another term used for this application
might be uncaging. A second example of photo activation are
light-gated ion channels. Channelrhodopsins are often used to
enable light to control electrical excitability, intracellular
acidity, calcium influx, and other cellular processes.
Channelrhodopsins can be activated with green light (540 nm) in a
single photon step or with light above 1 um in a two photon step in
case one wants to activate deep in the tissue.
[0011] It is often desirable to observe the sample through a 2 p
microscope and record certain cell functions time resolved after
the photo activation took place. Precise synchronization (<100
ps) between the photo activation and the images taken thereafter is
of the essence. Besides it is important to be able to take images
at a wavelength different from the activation wavelength
immediately after the photo activation without any downtime e.g.
caused by tuning of a laser source.
[0012] It can therefore be advantageous to have a laser source
which emits two wavelengths simultaneously.
[0013] Yet another use of the described laser system with a
synchronized two wavelength output would be for Stimulated Coherent
Raman Imaging or Coherent Anti-Stokes Raman Imaging. Raman imaging
provides specificity without the necessity to label the specimens
with fluorophores or dyes. Raman imaging in general makes use of
the unique rotational and vibrational level structure of molecules
to provide specificity in analyzing a sample. Spontaneous Raman
Scattering, however, is a low probability event and hence the
signal strength is typically low. The Raman signal, however, can be
enhanced by several orders of magnitude in the presence of two,
intense driving light fields typically provided by two mode-locked
lasers. The wavelength difference between the two light fields
needs to be tuned to a transition frequency of the inner molecule
level structure to get the signal enhancement. In addition it is
imperative that both laser pulses overlap in space and time on the
sample. It can therefore be advantageous to have a laser source
with two tightly synchronized outputs and which allows for a
tunable wavelength difference between the two outputs.
SUMMARY
[0014] An embodiment of the invention provides a femtosecond fiber
laser at the telecommunications band around 1550 nm and a tunable
wavelength shifting method that converts the pulse wavelength to
the amplification band of Thulium or Holmium doped optical fibers
(around 2000 nm). This approach offers two advantages: (a) the
femtosecond fiber lasers at 1550 nm have been developed into
reliable and stable systems in the recent years and are
commercially available from several companies, and (b) the amount
of wavelength shift in the system can be tuned, offering the
capability to adjust the ultimate output wavelength of the source.
The output average power can be scaled up using a fiber amplifier
in the 1800 nm to 2100 nm wavelength range. The output from said
amplifier is then frequency-doubled in a non-linear medium to cover
the biological interesting wavelength range from 950 nm to 1050
nm.
[0015] Another embodiment of the present invention provides a
method for operating the imaging system that includes a mode-locked
fiber laser configured to output a pulse having a center
wavelength; a first nonlinear waveguide configured to shift the
wavelength of the pulse from the mode-locked fiber laser; a first
fiber amplifier configured to amplify the output from the first
mode-locked fiber laser; a first fiber amplifier configured to
amplify the output from the first nonlinear waveguide; and a
nonlinear medium configured to frequency-double the output from the
first fiber amplifier, the method including: receiving a feedback
from the output of the first nonlinear waveguide, the output of the
first fiber amplifier or the output of the nonlinear medium; and
adjusting a gain of the first fiber amplifier, the light
polarization, or the amount of wavelength shift in the first
nonlinear waveguide to optimize the image brightness and
quality.
[0016] In another embodiment the light from the 1550 mode locked
laser is split into two arms. In one arm the light is shifted to
the amplification band of Thulium or Holmium doped optical fibers
(around 2000 nm) and then frequency doubled. The other arm is
amplified in an Erbium doped fiber amplifier (EDFA) before
frequency doubling. This embodiment produces two precisely synced
laser pluses at two different wavelengths in the two photon
excitation window from 760 nm to 1050 nm.
[0017] Another embodiment of the present invention provides a
three-photon microscopy system, including: a mode-locked fiber
laser configured to output a pulse having a center wavelength; a
nonlinear waveguide configured to shift the wavelength of the pulse
from the mode-locked fiber laser; a fiber amplifier configured to
amplify the output from the first nonlinear waveguide; and a
microscopy imaging system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a block diagram of an imaging system in accordance
with an embodiment of the invention.
[0019] FIG. 2 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0020] FIG. 3 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0021] FIG. 4 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0022] FIG. 5 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0023] FIG. 6 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0024] FIG. 7 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
[0025] FIG. 8 is a block diagram of an imaging system in accordance
with another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The description of illustrative embodiments according to
principles of the present invention is intended to be read in
connection with the accompanying drawings, which are to be
considered part of the entire written description. In the
description of embodiments of the invention disclosed herein, any
reference to direction or orientation is merely intended for
convenience of description and is not intended in any way to limit
the scope of the present invention. Relative terms such as "lower,"
"upper," "horizontal," "vertical," "above," "below," "up," "down,"
"top" and "bottom" as well as derivative thereof (e.g.,
"horizontally," "downwardly," "upwardly," etc.) should be construed
to refer to the orientation as then described or as shown in the
drawing under discussion. These relative terms are for convenience
of description only and do not require that the apparatus be
constructed or operated in a particular orientation unless
explicitly indicated as such. Terms such as "attached," "affixed,"
"connected," "coupled," "interconnected," and similar refer to a
relationship wherein structures are secured or attached to one
another either directly or indirectly through intervening
structures, as well as both movable or rigid attachments or
relationships, unless expressly described otherwise. Moreover, the
features and benefits of the invention are illustrated by reference
to the exemplified embodiments. Accordingly, the invention
expressly should not be limited to such exemplary embodiments
illustrating some possible non-limiting combination of features
that may exist alone or in other combinations of features; the
scope of the invention being defined by the claims appended
hereto.
[0027] This disclosure describes the best mode or modes of
practicing the invention as presently contemplated. This
description is not intended to be understood in a limiting sense,
but provides an example of the invention presented solely for
illustrative purposes by reference to the accompanying drawings to
advise one of ordinary skill in the art of the advantages and
construction of the invention. In the various views of the
drawings, like reference characters designate like or similar
parts.
[0028] Multi-Photon Imaging
[0029] An embodiment of the invention is a system that comprises
four key components, as shown in FIG. 1. The first component is a
mode-locked fiber laser (MLFL) (110) supporting a transform-limited
pulse width shorter than 1 ps and a center wavelength between 1500
nm and 1650 nm. The MLFL (110) is built based on a doped optical
fiber as the gain medium and a mode-locking mechanism. The output
from the fiber laser is coupled into Nonlinear Waveguide 1 (120),
which shifts its wavelength to a wavelength longer than 1700 nm and
shorter than 2800 nm by the process known as Raman soliton
self-frequency shifting. In one embodiment, Nonlinear Waveguide 1
(120) has an anomalous dispersion at the input pulse wavelength and
a nonlinear coefficient larger than 1 W.sup.-1km.sup.-1. The third
stage, Fiber Amplifier 1 (130), is a fiber amplifier operating in
the wavelength region between 1700 nm and 2800 nm, for example, an
amplifier system based on Thulium and/or Holmium doped fiber. In
some embodiments, Fiber Amplifier 1 (130) is a dual or multi-stage
amplifier. In some embodiments, Fiber Amplifier 1 (130) adds
additional spectral bandwidth by nonlinear processes like Self
Phase modulation and/or compresses the pulses in addition to
amplifying their energy. The amplifier output is coupled into a
nonlinear medium (140). The medium is designed to change the output
frequency of the input pulse through a non-linear process such as
Second Harmonic Generation (frequency doubling) or Third Harmonic
Generation.
[0030] In one embodiment the nonlinear medium could be a bulk
nonlinear crystal like BBO.
[0031] In another embodiment the nonlinear medium could be a
periodically poled nonlinear crystal.
[0032] The generated pulses have center wavelengths between 900 nm
and 1350 nm and can be used to excite e.g. fluorescence markers or
dyes with excitation wavelengths within this range. The pulses are
sent into a microscopy system (150)
[0033] In other embodiments of the invention, one or more of the
following components can be added to the system to improve its
performance, as shown in FIG. 2.
[0034] Fiber amplifier 2 (260): A fiber amplifier can be included
between the MLFL (210) and Nonlinear Waveguide 1 (220). The
amplifier has a gain in the wavelength region from 1500 nm to 1650
nm, for example, an Er-doped fiber amplifier. The amplifier has
three functions. First, it boosts the power from a low-power MLFL
to the level needed for the Raman self-frequency shifting process.
Second, it compresses the pulses from the mode-locked oscillator,
which improves the efficiency of the frequency-shifting process,
leading to a pulse energy increase or a pulse width decrease for
the frequency-shifted pulses. Third, by adjusting the amplifier
gain, it provides means for adjusting the amount of wavelength
shift. The wavelength adjustment is used to tune the output of the
frequency doubled light.
[0035] Polarization controller 1 (250): This device is a manual or
an automated polarization controller inserted between the MLFL
(210) and Nonlinear Waveguide 1 (220). The polarization controller
is used as a second adjustment mechanism for controlling the amount
of wavelength shift through the self-frequency shifting process. An
automated controller can be used to dynamically tune the wavelength
to a desired point in the spectrum for added stability.
[0036] In some embodiments, the MLFL (210) and Fiber Amplifier 2
(260) are built using polarization maintaining fibers. In these
cases, the wavelength shift is adjusted only using the gain of
Fiber Amplifier 2 (260).
[0037] Note that in one embodiment, polarization controller 1 (250)
can be placed directly after the Mode-Locked Fiber Laser (210) or
in between Fiber Amplifier 2 (260) and Nonlinear Waveguide 1
(220).
[0038] Dispersive Element 1 (270): This component is included after
Nonlinear Waveguide 1 (220) in order to create a desired amount of
chirp on the pulse entering Fiber Amplifier 1 (230). The component
comprises a dispersive device, including but not limited to optical
waveguides, chirped Bragg gratings, prism pairs, and diffraction
grating pairs. In some embodiments, the dispersion value is
designed to compress the output pulse from Fiber Amplifier 1 (230)
to the shortest duration through the interplay between the
dispersion and the nonlinearity in the amplifier. In other
embodiments, Dispersive Element 1 is designed to increase the pulse
duration in order to reduce the nonlinear effects in the amplifier.
In such cases, the pulses are re-compressed using the Dispersive
Element 2 (see below).
[0039] Polarization controller 2 (290): This component adjusts the
polarization state of the pulses before entering Fiber Amplifier 1.
By controlling this polarization state, the effective nonlinearity
in Fiber Amplifier 1 can be adjusted, which is used to optimize the
nonlinear pulse compression in Fiber Amplifier 1.
[0040] In some embodiments, Fiber Amplifier 1 (230) is built using
polarization maintaining fibers. In these cases, the nonlinearity
in Fiber Amplifier 1 is adjusted using the gain of Fiber Amplifier
1 (230).
[0041] Note that in one embodiment, polarization controller 2 (290)
can be placed directly after Nonlinear Waveguide 1 (220) or in
between Dispersive Element 1 (270) and Fiber Amplifier 1 (230).
[0042] Dispersive Element 2 (280): This component is included
before the Nonlinear medium as means to adjust the amount of chirp
on the pulse entering the nonlinear medium (240). The component
comprises a dispersive device, including but not limited to optical
waveguides, chirped Bragg gratings, prism pairs, and diffraction
grating pairs.
[0043] Polarization controller 3 (291): This component is included
before the nonlinear medium (240) as means to adjust the state of
polarization of the pulse entering the nonlinear medium (240) to
optimize the efficiency of the frequency doubling process.
[0044] In some embodiments, Fiber Amplifier 1 (230) is built using
polarization maintaining fibers and the light polarization entering
the nonlinear medium (240) is linear. In such cases, the frequency
doubling efficiency can be simply adjusted by rotating the
orientation of the output fiber from Fiber Amplifier 1 (230).
[0045] An embodiment of the invention provides a system and method
for stabilizing and tuning the pump wavelength and pulse shape and
consequently optimizing the parameters of the two-photon imaging by
adjusting the gains of Fiber Amplifiers 1 or 2 (330 or 360), or the
polarization controllers 1 or 2 or 3 (350, 390, or 391), as shown
in FIG. 3. As discussed above, in addition to the MLFL (310),
Nonlinear Waveguide 1 (320), Fiber Amplifier 1 (330) and Nonlinear
Medium (340), one or more of the components: Polarization
controller 1 (350), Fiber amplifier 2 (360), Dispersive Element 1
(370), Polarization controller 2 (390), Dispersive Element 2 (380),
and Polarization Controller 3 (391) are optionally included. By
receiving feedback via a Feedback loop filter (392) from the image
generated by the microscope (393), the output from the nonlinear
medium (340), the output from Nonlinear Waveguide 1 (320), or the
output from Fiber Amplifier 1 (330), the variables (gain or
polarization) are dynamically adjusted to optimize and stabilize
the system to a desired state. The parameters are tuned in order to
optimize the output image brightness and quality.
[0046] Three-Photon Fluorescence Microscopy
[0047] In another embodiment, the output from Amplifier 1 (530) can
be sent directly into a microscopy system (550) for three-photon
imaging, as shown in FIG. 5. As discussed above, the output from
MLFL (510) is coupled to Nonlinear Waveguide 1 (520), and amplified
by Fiber Amplifier 1 (530). The fluorophore excitation wavelength
should be between 600 nm and 900 nm.
[0048] The various embodiments discussed in above section also
apply to this embodiment as well.
[0049] Furthermore, there are various possible applications of some
of the embodiments discussed in this document, such as photo
activation combined with three photon imaging. FIGS. 6-8 illustrate
some possible combinations of the components disclosed in
accordance with some embodiments.
[0050] Dual-wavelength microscopy
[0051] In another embodiment, the light from the 1550 mode locked
laser (410) is split into two arms using a splitter (440), as shown
in FIG. 4. In one arm the light is wavelength-shifted using a
Nonlinear Waveguide 1 (420) to a center wavelength between 1700 nm
and 2800 nm, passed through an optional delay (460), amplified in
Fiber Amplifier 1 (430), and is frequency-doubled by passing
through Nonlinear Medium 1 (450). In the other arm, the light is
passed through an optional delay (470), amplified in an optional
Fiber Amplifier 3 (480) and is frequency-doubled in Nonlinear
Medium 2 (490). This embodiment produces two precisely synced laser
pluses at two different wavelengths. The pulses generated from both
arms are separately or simultaneously coupled into the microscope
(491). One or both of the delay components (460 or 470) can be
adjustable delay lines that are used to adjust the temporal
alignment between the pulses at the two wavelengths. The
dual-wavelength system can be used for two-color two-photon
imaging, two-color three-photon imaging, or a combination of
photo-activation and two-photon imaging. Additionally, the
dual-wavelength system can be used for coherent Raman imaging.
[0052] While the present invention has been described at some
length and with some particularity with respect to the several
described embodiments, it is not intended that it should be limited
to any such particulars or embodiments or any particular
embodiment, but it is to be construed with references to the
appended claims so as to provide the broadest possible
interpretation of such claims in view of the prior art and,
therefore, to effectively encompass the intended scope of the
invention. Furthermore, the foregoing describes the invention in
terms of embodiments foreseen by the inventor for which an enabling
description was available, notwithstanding that insubstantial
modifications of the invention, not presently foreseen, may
nonetheless represent equivalents thereto.
* * * * *