U.S. patent application number 14/860611 was filed with the patent office on 2017-03-23 for thermally compensated optical probe.
The applicant listed for this patent is Ondax, Inc.. Invention is credited to James Carriere, Frank Havermeyer, Randy Heyler, Lawrence Ho, Eric Maye, Christophe Moser.
Application Number | 20170082489 14/860611 |
Document ID | / |
Family ID | 58162203 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170082489 |
Kind Code |
A1 |
Ho; Lawrence ; et
al. |
March 23, 2017 |
THERMALLY COMPENSATED OPTICAL PROBE
Abstract
Systems and methods are provided herein. An exemplary system may
include a laser source, the laser source having a laser center
wavelength; at least one narrowband optical element receiving light
emitted by the laser, the narrowband optical element having a
filter center wavelength, the narrowband optical element being
arranged such that the filter center wavelength is initially
spectrally aligned with the laser center wavelength, the filter
center wavelength changing in response to a temperature change such
that the filter center wavelength is not substantially aligned with
the laser center wavelength; and a passive adjustment mechanism
coupled to the narrowband optical element, the passive adjustment
mechanism including an actuator, the actuator moving in response to
the temperature change, the actuator motion rotating the narrowband
optical element, the rotation compensating for the temperature
change such that the filter center wavelength and laser center
wavelength remain spectrally aligned.
Inventors: |
Ho; Lawrence; (Arcadia,
CA) ; Havermeyer; Frank; (La Verne, CA) ;
Moser; Christophe; (Pasadena, CA) ; Carriere;
James; (La Crescenta, CA) ; Maye; Eric;
(Torrance, CA) ; Heyler; Randy; (Newport Beach,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ondax, Inc. |
Monrovia |
CA |
US |
|
|
Family ID: |
58162203 |
Appl. No.: |
14/860611 |
Filed: |
September 21, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J 3/0286 20130101;
G01J 3/44 20130101; G02B 7/008 20130101; G03H 2001/186 20130101;
G01J 3/0237 20130101; G01J 3/0227 20130101; G01J 3/06 20130101;
G03H 2227/03 20130101; G02B 5/203 20130101; G03H 1/0248 20130101;
G02B 27/4283 20130101; G03H 2001/2289 20130101 |
International
Class: |
G01J 3/02 20060101
G01J003/02; G02B 7/00 20060101 G02B007/00; G03H 1/02 20060101
G03H001/02; G02B 5/32 20060101 G02B005/32; G01J 3/44 20060101
G01J003/44; G01J 3/06 20060101 G01J003/06 |
Claims
1. An optical probe comprising: a laser source, the laser source
having a laser center wavelength; at least one narrowband optical
element receiving light emitted by the laser source, the narrowband
optical element having a filter center wavelength, the narrowband
optical element being arranged such that the filter center
wavelength is initially spectrally aligned with the laser center
wavelength, the filter center wavelength changing in response to a
temperature change such that the filter center wavelength is not
substantially aligned with the laser center wavelength; and a
passive adjustment mechanism coupled to the narrowband optical
element, the passive adjustment mechanism including an actuator,
the actuator moving in response to the temperature change, the
actuator motion rotating the narrowband optical element, the
rotation compensating for the temperature change such that the
filter center wavelength and laser center wavelength remain
spectrally aligned.
2. The optical probe of claim 1 wherein the filter center
wavelength and the laser center wavelength are spectrally aligned
when the filter center wavelength and the laser center wavelength
are separated from each other by less than half a difference
between a transition width of the narrowband optical element and a
linewidth of the laser source.
3. The optical probe of claim 1 wherein the passive adjustment
mechanism further comprises a rotating element, the rotating
element being coupled to the actuator, the narrowband optical
element being disposed on the rotating element.
4. The optical probe of claim 3 wherein the narrowband optical
element is disposed on the rotating element such that a midpoint of
the narrowband optical element is disposed approximately at a
center of rotation of the rotating element.
5. The optical probe of claim 3 wherein the actuator comprises at
least one of a bimetallic or memory metal component, the bimetallic
or memory metal component contorting in a predefined manner in
response to the temperature change, the contortion effecting a
rotation of the narrowband optical element turning the rotating
element.
6. The optical probe of claim 3 wherein the actuator comprises an
arm, the arm comprising a substantially homogeneous material having
a coefficient of thermal expansion such that the arm expands or
contracts in response to the temperature change, the expansion or
contraction turning the rotating element.
7. The optical probe of claim 3 wherein the actuator has a spiral
or curved shape and a known movement path with respect to
temperature changes, such that the actuator moves in response to
the temperature change, the movement turning the rotating
element.
8. The optical probe of claim 1 wherein an optical beam path angle
is maintained by using at least one pair of rotating optical
elements.
9. The optical probe of claim 1 wherein at least one pair of
rotating optical elements is mounted to a common rotating
element.
10. The optical probe of claim 1 wherein the narrowband optical
element is a narrowband diffractive volume holographic grating
(VHG) comprising at least one of photosensitive glass,
photosensitive polymers, and other photosensitive optical
materials.
11. The optical probe of claim 1 wherein the light from the laser
source has a wavelength approximately in the range of 300-2,000
nanometers (nm).
12. The optical probe of claim 1 wherein optical feedback from at
least one of the narrowband optical elements within the optical
probe is optically coupled back to the laser source in order to
stabilize and synchronize the laser center wavelength to the filter
center wavelength of the narrowband optical element.
13. The optical probe of claim 1 wherein optical feedback from the
at least one narrowband optical element is optically coupled to a
detector external to the optical probe, the optical feedback
including parameters for stabilizing and synchronizing the laser
center wavelength to the filter center wavelength of the narrowband
optical element.
14. An optical system comprising: the optical probe of claim 1; a
narrowband laser; and a detection system, the narrowband laser and
detection system being optically coupled via a fiber-optic cable to
the optical probe.
15. The optical system of claim 14 further comprising: a computer,
the computer analyzing an optical output of the detection
system.
16. An optical system comprising: a laser source, the laser source
having a laser center wavelength; a first narrowband optical
element receiving light transmitted by the laser source, the first
narrowband optical element having a first filter center wavelength,
the first filter center wavelength being initially spectrally
aligned with the laser center wavelength, the first filter center
wavelength changing in response to a temperature change such that
the first filter center wavelength is not spectrally aligned with
the laser center wavelength; a first passive adjustment mechanism
coupled to the first narrowband optical element, the first passive
adjustment mechanism including a first actuator, the first actuator
moving in response to the temperature change, the first actuator
motion rotating the first narrowband optical element, the rotation
compensating for the temperature change such that the first filter
center wavelength and laser center wavelength remain spectrally
aligned; at least one additional narrowband optical element having
a respective filter center wavelength, the respective filter center
wavelength being initially spectrally aligned with the laser center
wavelength, the respective filter center wavelength changing in
response to the temperature change such that the respective filter
center wavelength is not spectrally aligned with the laser center
wavelength; an additional passive adjustment mechanism coupled to
each additional narrowband optical element, each passive adjustment
mechanism including a separate actuator, each separate actuator
moving in response to the temperature change, each separate
actuator motion rotating an additional narrowband optical element,
the rotation compensating for the temperature change such that all
the filter center wavelengths and laser center wavelength remain
spectrally aligned; and an optically coupled detection system and
computational means for analyzing the filtered optical signals.
17. The optical system of claim 16 wherein an optical beam path
angle is maintained by using at least one pair of rotating optical
elements.
18. The optical system of claim 16 wherein a second narrowband
optical element and the first narrowband optical element are
initially positioned such that their reflective surfaces are
approximately parallel to each other; and respective rotations of
the first and second narrowband optical elements are such that the
reflective surfaces of the first and second narrowband optical
elements remain approximately substantially parallel to each
other.
19. The optical system of claim 16 wherein at least one pair of
rotating optical elements is mounted to a common rotating
element.
20. The optical system of claim 16 wherein the first filter center
wavelength, a second filter center wavelength, and the laser center
wavelength are spectrally aligned when the filter center
wavelengths and the laser center wavelength are separated from each
other by less than half a difference of a transition width of the
narrowband optical elements and a linewidth of the laser
source.
21. The optical system of claim 16 wherein at least one of the
first and second passive adjustment mechanisms each further
comprises a rotating element, the rotating element being coupled to
the actuator, the narrowband optical element being disposed on the
rotating element.
22. The optical system of claim 21 wherein at least one of the
first and second narrowband optical elements is disposed on the
rotating element such that a respective midpoint of at least one of
the first and second narrowband optical elements is disposed
approximately at a center of rotation of the rotating element.
23. The optical system of claim 21 wherein at least one of the
first and second actuators comprises a bimetallic component, the
bimetallic component comprising two metals having different
coefficients of thermal expansion such that the bimetallic
component contorts in response to the temperature change, the
contortion turning the rotating element.
24. The optical system of claim 21 wherein at least one of the
first and second actuators comprises an arm, the arm comprising a
homogeneous material having a known coefficient of thermal
expansion such that the arm expands or contracts in response to the
temperature change, the expansion or contraction turning the
rotating element.
25. The optical system of claim 21 wherein at least one of the
first and second actuators has a spiral shape and a known movement
path with respect to temperature changes, such that the actuator
moves in response to the temperature change, the movement turning
the rotating element.
26. The optical system of claim 16 wherein at least one of the
first and second narrowband optical elements is a narrowband
diffractive volume holographic grating (VHG) comprising at least
one of photosensitive glass, photosensitive polymers, or other
photosensitive materials.
27. The optical system of claim 16 wherein a bandwidth of at least
one of the narrowband optical elements in an excitation beam path
is less than a bandwidth of the narrowband optical elements
disposed in a path of reflected light.
28. A method for temperature compensated spectroscopy comprising:
initially arranging a narrowband optical element such that a filter
center wavelength of the narrowband optical element is initially
spectrally aligned with a laser center wavelength of a laser
source; receiving light emitted by the laser source; filtering the
light by the narrowband optical element; undergoing a temperature
change, the filter center wavelength changing in response to the
temperature change such that the filter center wavelength is not
substantially aligned with the laser center wavelength; and
passively rotating the narrowband optical element by an actuator in
response to the temperature change, the rotation compensating for
the temperature change such that the filter center wavelength and
laser center wavelength remain spectrally aligned.
29. The method of claim 28 wherein the filter center wavelength and
the laser center wavelength are spectrally aligned when the filter
center wavelength and the laser center wavelength are separated
from each other by less than half a difference between a transition
width of the narrowband optical element and a linewidth of the
laser source.
30. The method of claim 28 wherein the rotating the narrowband
optical element is rotating a rotating element, the actuator being
coupled to the rotating element, and the narrowband optical element
being disposed on the rotating element.
31. The method of claim 30 wherein the narrowband optical element
is disposed on the rotating element such that a midpoint of the
narrowband optical element is disposed approximately at a center of
rotation of the rotating element.
32. The method of claim 30 wherein the actuator comprises at least
one of a bimetallic or memory metal component, the bimetallic or
memory metal component contorting in a predefined manner in
response to the temperature change, the contortion effecting a
rotation of the narrowband optical element turning on the rotating
element.
33. The method of claim 30 wherein the actuator comprises an arm,
the arm comprising a substantially homogeneous material having a
coefficient of thermal expansion such that the arm expands or
contracts in response to the temperature change, the expansion or
contraction turning the rotating element.
34. The method of claim 30 wherein the actuator has a spiral or
curved shape and a known movement path with respect to temperature
changes, such that the actuator moves in response to the
temperature change, the movement turning the rotating element.
35. The method of claim 28 further comprising: maintaining an
optical beam path angle using at least one pair of the rotating
optical elements.
36. The method of claim 28 further comprising: providing optical
feedback from the narrowband optical element to a detector, the
optical feedback including parameters for stabilizing and
synchronizing the laser center wavelength to the filter center
wavelength of the narrowband optical element.
Description
TECHNICAL FIELD
[0001] The present technology pertains generally to optical and
spectroscopic measurement, and more specifically to Raman
spectroscopy.
BACKGROUND OF THE INVENTION
[0002] Thermal effects often lead to changes optical system
performance due to shifts in mechanical position of the elements or
spectral shifts in optics or light sources such as lasers. Many
effects need to be taken into consideration, including mechanical
shifts of optical mount positions, thermal expansion and spectral
shifts of optical materials, components and coatings, and drifts in
emission wavelength of any optical sources in the system.
SUMMARY
[0003] This summary is provided to introduce a selection of
concepts in a simplified form that are further described in the
Detailed Description below. This summary is not intended to
identify key features or essential features of the claimed subject
matter, nor is it intended to be used as an aid in determining the
scope of the claimed subject matter.
[0004] The present disclosure is related to approaches for
compensation of thermal effects in an optical system that uses
narrowband optical elements, in particular volume holographic
gratings, in optical systems such as a Raman spectroscopy system.
In some embodiments, the invention enables an optical probe to
simultaneously and continuously capture low frequency and
anti-Stokes Raman scattering signals to within very close proximity
(for example <10 cm.sup.-1) of the laser excitation wavelength
in extreme environments and over large temperature ranges.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Embodiments are illustrated by way of example, and not by
limitation, in the figures of the accompanying drawings, in which
like references indicate similar elements and in which:
[0006] FIG. 1 shows a simplified block diagram of a low frequency
Raman spectrometer system, including an optical probe, according to
some embodiments.
[0007] FIG. 2 illustrates the transition width (TW) of a narrowband
notch filter to a typical (narrowband) edge filter.
[0008] FIG. 2A shows an example transmission spectrum of a
narrowband optical filter 200a, and FIG. 2B shows an example
emission spectrum of a narrowband optical source (laser) 200b,
highlighting the spectral center wavelengths .lamda..sub.C and
.lamda..sub.L as the midpoints of the full-width half maximum
(FWHM) of the respective curves. FIG. 2C shows an overlay 200c of
the two curves of FIGS. 2A and 2B, demonstrating the constraints
for required spectral alignment. FIGS. 2D and 2E show the
transmission curves of a narrowband notch filter and a bandpass
filter (e.g., beamsplitter), respectively. FIG. 2F shows an example
comparison of spectra from a well calibrated and spectrally aligned
system, and from a spectrally misaligned system. FIG. 2G shows
example measured and calculated results of filter performance with
and without temperature compensation.
[0009] FIG. 3 shows an example shift of center wavelength
.lamda..sub.C (or in the case of a volume holographic grating, or
VHG, the Bragg wavelength) of a narrowband optical element over
temperature.
[0010] FIG. 4 is an example plot of change of a narrowband VHG
filter wavelength .lamda..sub.C as a function of angle .theta..
[0011] FIG. 5 depicts embodiments of temperature compensating
mechanisms.
[0012] FIG. 6 illustrates operation of embodiments of a temperature
compensating mechanism.
[0013] FIG. 7 shows temperature compensating mechanisms according
to various embodiments.
[0014] FIGS. 8A and 8B illustrate example shifts in angle and
displacements of beam path when optical elements are rotated
according to some embodiments.
[0015] FIG. 9 is a simplified flow diagram for a method in
accordance with various embodiments.
DETAILED DESCRIPTION
[0016] The following detailed description includes references to
the accompanying drawings, which form a part of the detailed
description. The drawings show illustrations in accordance with
exemplary embodiments. These exemplary embodiments, which are also
referred to herein as "examples," are described in enough detail to
enable those skilled in the art to practice the present subject
matter. The embodiments can be combined, other embodiments can be
utilized, or structural, logical, and electrical changes can be
made without departing from the scope of what is claimed. The
following detailed description is therefore not to be taken in a
limiting sense, and the scope is defined by the appended claims and
their equivalents. In this document, the terms "a" and "an" are
used, as is common in patent documents, to include one or more than
one. In this document, the term "or" is used to refer to a
nonexclusive "or," such that "A or B" includes "A but not B," "B
but not A," and "A and B," unless otherwise indicated.
[0017] In the case of spectroscopic instruments, an optical source
(e.g., a laser) is used to excite a response by a sample material
of interest via either absorption or scattering, and the resulting
changes in the optical signal can be used to determine the chemical
composition and/or the molecular structure of the sample. Multiple
optical elements, including at least one of lasers, filters, beam
splitters and gratings, are utilized in such systems (FIG. 1), and
a precise spectral calibration reference between each of the
elements and as a system overall is used to achieve accurate
results. When subjected to changes in temperature, the spectral
changes of each element of the system are often different in
magnitude, leading to changes in system performance and/or
requiring additional recalibration.
[0018] Raman spectroscopy systems can use edge and/or notch filters
to remove the laser (e.g., Rayleigh) light from the Raman signal,
and these typically have relatively large transition widths that
allow for substantial spectral shifts of either or both laser and
filters to occur without impairing the ability of the filters to
block the Rayleigh light (FIG. 2). However, "low frequency" or "low
wavenumber" Raman spectroscopy systems, which offer significant
advantages in overall system performance and easy access to
important low-frequency and in some embodiments also anti-Stokes
signals, utilize designs where one or more of the optical elements
is an extremely narrowband thin film edge or notch filter or volume
holographic grating (VHG) notch filter. A "narrowband" filter is
defined herein as having a 50% transition width of <50
cm.sup.-1. These systems are much more susceptible to spectral
shifts related to changes in temperature, which can impair the
ability of the filters to block the Rayleigh signal. They may need
the center wavelengths .lamda..sub.C1, .lamda..sub.C2,
.lamda..sub.C3, . . . of the narrowband optical elements (also
known as the Bragg wavelength for a VHG) to stay spectrally aligned
with each other, as well as with the center wavelength
.lamda..sub.L of the narrowband optical source (normally a
narrowband or single frequency laser), to ensure adequate
attenuation of the laser excitation and acceptable performance of
the overall optical system (FIGS. 2A-2C, and 2F).
[0019] Low-frequency spectroscopy systems typically place all the
narrowband optical elements within an enclosure or onto one
platform, enabling the elements to remain at the same temperature
during operation and thereby maintaining stable spectral alignment,
which is critical for system performance. Alternatively (or
additionally), the system may be designed so that the center
wavelength for all elements (including the laser) changes at the
same rate with respect to temperature, so that the center
wavelength(s) of all optical elements remain spectrally aligned
with respect to one another (or "synchronized"), similarly
preserving system performance.
[0020] However, many applications require that at least one of the
narrowband optical elements be placed in close proximity to, or in
an environment of, significantly higher or lower temperatures than
other system elements, and/or subjected to changing temperatures,
or may require a separation of the laser excitation source from the
rest of the system. Examples include when the system is to be used
in or near a vessel, crucible, pipeline, or other location that is
unsuitable for the entire system or for the laser and/or other
electronic components. Such applications typically require one or
more of the narrowband optical elements to be housed in an "optical
probe" configuration and remotely connected via fiber optic cable
or free space connections to the laser source and other system
components such as a spectrometer (FIG. 1).
[0021] Alternatively (or additionally), the system may be comprised
of components with different wavelength shift rates. In these
situations, changes in the temperature of the probe during
operation (e.g., relative to initial conditions), as well as
differential temperatures between the probe, laser, and/or the rest
of the spectrometer system, may lead to spectral misalignments that
can drastically reduce or entirely disable system performance.
[0022] These spectral shifts may be detected and compensated for
using various methods. In some embodiments, using grating or
interference-based (e.g. thin-film) filters, spectral alignment can
be achieved by tuning the filter(s), for example, via physical
movement (e.g. rotation), or by changing the temperature.
Measurement of the laser power through the filters as a function of
angle or temperature generates a curve that allows for
determination of a local maximum (or minimum point), which
corresponds to the point at which the "center wavelengths" of both
the laser (.lamda..sub.L) and filter (.lamda..sub.C) are spectrally
aligned. In various embodiments, the laser wavelength may be tuned
(e.g., via temperature or physical movement of a mirror, grating,
prism, etc.), while the filters remain stable in both position and
temperature, similarly enabling determination of a center
wavelength where the laser and filters are spectrally aligned.
[0023] The position (or temperature) of the filters and/or the
wavelength of the laser, can be set to their respective optimum
values, and spectral alignment is achieved. Spectral alignment is
the condition where the center wavelengths of all elements are
within an application-dependent range from each other. This range
is typically less than the transition width of the individual
optical elements (FIG. 2C).
[0024] In most cases, "active" control of (e.g., manual or
electronic intervention in) one or more of the laser wavelength and
filter positions is done to perform these operations for initial
calibration, and may be repeated as necessary when temperatures
and/or other conditions cause enough shift in center wavelengths of
one or more of the elements so as to impact system performance.
This type of active intervention may be acceptable in a laboratory
instrument used in a controlled environment. However, in many
applications (and especially where a fiber-optically coupled probe
is required), it is either impractical or entirely unacceptable to
allow manual intervention during operation, or to have electrical
connections and/or components within the probe itself. Accordingly,
some embodiments of the present invention offer several solutions
for passively maintaining spectral alignment and synchronization
between multiple narrowband optical elements, between the optical
probe and the laser, and across the entire optical system, ensuring
optimal performance across a wide range of temperature and
operating environments.
[0025] Embodiments of the present invention described below
advantageously maintain a parallel beam path during temperature
excursions and do not change the beam angle and limit any beam
displacement, which is preferable for many optical systems and in
particular a confocal Raman spectroscopy probe geometry.
[0026] FIG. 1 is a block diagram of a low-frequency Raman
spectroscopy system, according to some embodiments. The system
includes a laser source (101), optical probe (103), detection
system (117), computer (118), and optical fiber coupling (102 and
116) between elements. Laser 101 is optically coupled to
fiber-optic cable 102, which is optically coupled to optical probe
103. The laser may be a narrowband or single frequency laser with a
wavelength in the UV, visible, or infrared light spectrums (e.g.,
.about.200 nm to .about.2,000 nm). By way of non-limiting example,
the output power is between 1 mW and 5 W. A plurality of optical
elements 104 are disposed in optical probe 103. Optical elements
may include at least one of collimating lens 105 (e.g., to create a
free space collimated excitation beam (119)), an amplified
spontaneous emission (ASE) filter assembly 106 to remove additional
spectral noise from the excitation beam, a mirror or narrowband
filter 107 to redirect the excitation beam onto the beamsplitter
108, at least one narrowband edge or notch filter (or filters) 114,
and a focusing lens 115.
[0027] In some embodiments, ASE filter assembly 106, beamsplitter
108 and/or filter 107 may additionally reduce spectral noise from
the laser and/or generated within fiber 102 (e.g., typical broad
band fluorescence and/or Brillouin scattered light that may be
generated in the fiber). Beamsplitter 108 may redirect the
excitation beam through exit port 109. Excitation beam 119 may pass
through an optical enclosure or tube 110 and may be focused via
focusing lens 111 onto a sample 112. In various embodiments,
backscattered light 113 is reflected back through focusing lens
111, tube 110, and beamsplitter 108, where reflected excitation
(e.g., Rayleigh) light may be filtered by the beamsplitter 108.
[0028] The reflected/backscattered light (Raman signal plus
residual Rayleigh light) optionally passes through one or more
additional filtering elements 114, which may include narrowband
optical elements such as volume holographic gratings. One or more
of elements 114 each is a narrowband optical element or volume
holographic grating filter. In exemplary embodiments, the bandwidth
of narrowband optical elements 114 is larger than the bandwidth of
the narrowband optical elements (106, 107, 108) disposed in the
path of the excitation beam.
[0029] Each of the narrowband optical elements or assemblies (e.g.,
106, 107, 108, and 114) is attached or mounted upon an adjustment
mechanism 120 that will induce a change in the angular position of
the optical element with any change in temperature according to a
predefined relationship.
[0030] The reflected light may be refocused via refocusing lens 115
into collection fiber 116 and optically coupled to a detection
system 117. In one exemplary embodiment, the detection system is a
spectrometer. It can be advantageous to remove sufficient Rayleigh
light from the optical beam so that additional fluorescence,
Brillouin, or other scattering is not generated in the collection
fiber 116. The spectrometer may use a volume phase grating,
reflective grating, micromirror array, or other optical element to
disperse or redirect the signal onto a CCD detector or comparable
electronic sensor to resolve the optical signal. The optical signal
may then be sent to computer 118. Computer 118 may analyse the
resolved optical signal by comparing to established spectral
libraries or providing data for offline chemometric or numerical
analysis.
[0031] FIG. 2 shows a comparison of an example transmission curve
of a narrowband notch filter 201 and a narrowband edge filter 202.
The transition width (TW) is also shown as the spectral width
measured between the maximum and 50% transmission points.
Narrowband filters herein are those with transition widths of
<50 cm.sup.-1. In some embodiments, example transmission curve
201 is descriptive of at least one of ASE filter assembly 106,
mirror or narrowband filter 107, beamsplitter 108, and elements
114.
[0032] FIG. 2A shows an example transmission curve 203 of a
narrowband optical element, where the spectral centerpoint or
filter center wavelength .lamda..sub.C is the midpoint of the
full-width half maximum (FWHM) of the filter, also known as
bandwidth (BW.sub.filter). .lamda..sub.C may also be referred to as
the Bragg wavelength .lamda..sub.B for VHGs. FIG. 2B shows an
example emission curve 204 of a narrowband laser source, where the
laser linewidth (LW.sub.laser) is the full-width half maximum
(FWHM) with laser center wavelength .lamda..sub.L. In various
embodiments, emission curve 204 is descriptive of laser 101 (FIG.
1).
[0033] FIG. 2C shows examples of spectrally aligned and misaligned
conditions between the narrowband optical element(s) and laser
source. In the aligned condition, .lamda..sub.C=.lamda..sub.L,
resulting in optimal operation of a low-wavenumber Raman
spectroscopy system. In the misaligned condition
.lamda..sub.C.noteq..lamda..sub.L and Rayleigh scattered light will
block some low wavenumber signals and potentially saturate the
detector system, preventing reading of additional Raman signals
(FIG. 2F). Spectral alignment can be achieved when the following
conditions are satisfied: the laser linewidth LW.sub.laser is less
than the filter transition width (TW.sub.filter); the spectral
misalignment (or wavelength difference) between the center
wavelengths of the laser and filter(s)
|.lamda..sub.C-.lamda..sub.L| is less than half the difference
between the filter transition width TW and the laser linewidth LW
(condition expressed by equation (1) below); the center wavelengths
of all components (laser .lamda..sub.L and narrowband optical
elements .lamda..sub.C1, .lamda..sub.C2, .lamda..sub.C3 . . . )
fulfil the condition expressed by equation (1), and suppression of
the amplified spontaneous emission (ASE) of the laser is sufficient
to prevent interference with the low frequency Raman signals. These
ideal conditions may be expressed as:
|.lamda..sub.C-.lamda..sub.L|<1/2(TW.sub.Filter-LW.sub.Laser)
(1)
for any narrowband optical element disposed in the beam path with
center wavelengths .lamda..sub.C1, .lamda..sub.C2, .lamda..sub.C3 .
. . and transition widths TW.sub.1, TW.sub.2, TW.sub.3; and
ASE suppression>.about.60 dB (2)
[0034] In the case where a narrowband filter element is an edge
filter and not a notch filter, the center wavelength .lamda..sub.C
is defined as the wavelength where the optical density (OD) is at
least OD 4.
[0035] In low frequency Raman spectroscopy systems, the tolerance
for shifts of either the center wavelength of a filter or the laser
is extremely small compared to traditional Raman systems. Typical
tolerance levels for allowable relative shifts between components
are on the order of <<10 cm.sup.-1 and can occur over
relatively small temperature ranges, for example less than 10
degrees Celsius in uncompensated systems. It has been shown that
temperature shifts of less than 10 degrees Celsius can dramatically
affect system performance (FIG. 2G). When spectral shifts occur
beyond these limits, the Rayleigh scattered light intensity will
grow and eventually may greatly exceed the low frequency Raman
signals, and possibly saturate the sensor, preventing
identification of low frequency peaks (FIG. 2F). From a practical
perspective, the tolerance for center wavelength shift can depend
on both the end user application and the function of the narrowband
optical element. For example, notch filter(s) or narrowband optical
elements 114 reject residual Rayleigh (excitation) wavelength from
the backscattered Raman signal with high efficiency, while
maintaining high transmission for all other wavelengths. Optical
Density, or OD, is often used to describe the performance of an
optical filter for Rayleigh rejection. In Raman spectroscopy system
100 (FIG. 1), the cumulative OD (over one or more filters) should
be sufficiently high in order to remove enough Rayleigh signal such
that the Raman signal may be clearly seen. An OD of greater than 6
is typically required; however the cumulative OD may be achieved
via a combination of narrowband filters 114 within the probe and
additional notch or narrowband filters disposed in the path of the
Raman signal 116 prior to entering the detection system 117. As an
example, FIG. 2D shows transmission curve 200d of a single
narrowband VHG element, noting the full width where the OD=4.
Typically, two or more such filters are used in Raman spectroscopy
system 100 for Rayleigh rejection, totalling an OD=8. This "OD4
bandwidth" represents the effective bandwidth of the element for
the Raman system, also defining the boundaries within which the
excitation laser emission profile should remain for optimal
operation over temperature shifts for such filters. The laser
emission profile is also shown for comparison on the diagram.
[0036] A bandpass filter (e.g., beam splitter 108 in FIG. 1)
preferably reflects a high proportion of the Rayleigh light toward
the sample, as well as rejecting it from the backscattered signal.
This proportion is commonly referred to as Diffraction Efficiency
(DE) for a narrowband VHG filter, and a typical value for a Raman
system would be DE=90%. FIG. 2E shows bandwidth 200e at the DE=90%
level for both a single and multiple bandpass filter example.
Again, the laser excitation (also shown) must remain within this
bandwidth for optimal operation during temperature shifts.
[0037] FIG. 2F shows an example comparison of spectra 200f from a
well calibrated and spectrally aligned system, and from a
misaligned system where Rayleigh light is leaking through,
preventing the low frequency signals from being seen.
[0038] FIG. 2G compares compensated and uncompensated system
performance 200g, measured as optical throughput (also referred to
as diffraction efficiency for a narrowband VHG pass band filter).
The narrow (uncompensated) curve 210g shows a dramatic diffraction
efficiency change over relatively small temperature changes, with
filter performance dropping from 90% to less than 70% over just 10
or 20 deg. C. However, when passive compensation is implemented,
simulations curve 220g shows the temperature over which the filter
efficiency remains over 80% is expanded to nearly 200 deg. C.
Actual test results 230g show diffraction efficiency can be kept
above 80% over a 100 deg. C. span.
[0039] For embodiments including narrowband elements (e.g., ASE
filter assembly 106, narrowband filter 107, beam splitter 108, and
notch filters 114 in FIG. 1), temperature changes can shift the
center wavelength (and for VHG filters, the Bragg wavelength)
resulting in a change or loss of diffraction efficiency relative to
a fixed laser wavelength (FIG. 3). Subsequent shifting of the laser
wavelength and/or filter center wavelength can restore any loss.
Filter wavelength shifts may be instigated by temperature changes
and/or changing the Angle of Incidence (AOI) onto the element. In
the case of an external temperature change, the filter may be
rotated or tilted, thus changing the AOI to compensate for the
temperature induced shift. The filter center wavelength as a
function of temperature and angular position (e.g., relative to the
optical beam angle of incidence) can be predetermined by either
mathematical modelling or empirical measurement, and this
relationship may be used to establish a predefined rotational
actuation as a function of temperature for system design.
[0040] FIGS. 3 and 4 show example typical relationships for these
variables in some exemplary embodiments of VHG filters. For a given
change in temperature .DELTA.T, one may predetermine or predict the
change in filter center wavelength .DELTA..lamda..sub.C and, using
the angle/wavelength relationship, may determine the required
change in filter angle .THETA..sub.F to compensate for (or offset)
temperature change .DELTA.T. Accordingly, an adjustment mechanism
is provided to deliver the desired rotation/temperature
relationship. Example adjustment mechanisms according to various
embodiments are described further in relation to FIGS. 5-7
below.
[0041] FIG. 5 illustrates one example of an adjustment mechanism
500 for rotating elements (e.g., in low-frequency Raman
spectroscopy system 100) in response to temperature changes. For
example, narrowband optical element 501 is beam splitter 108 (FIG.
1). In some embodiments, the narrowband optical element 501 is
mounted on a rotating element 502 with pivot (center of rotation)
point 503. The midpoint of the filter may be placed over the center
of rotation 503. Arm 504 is affixed to mount 505 at
attachment/mounting point 506 and acts as a lever to actuate the
rotating element. In some embodiments, arm 504 is a bi-metallic
component. Arm 504 is physically coupled to the rotating element so
that the contact point (e.g., with screw 507) is at distance
L.sub.1 from the mounting point 506.
[0042] For example, the two metals comprising arm 504 have
different coefficients of thermal expansion, resulting in a bending
motion with temperature changes (according to a known coefficient
of curvature) that will effect a rotation of element 502. By way of
further non-limiting example, arm 504 comprises at least one of a
high-expansion material or "memory" material such as Nytenol or a
Cu--Ni--Al alloy that changes position or shape with temperature in
a known and repeatable manner. Adjustment screw 507 is disposed in
rotating element 502 at a distance L2 from center of rotation 503
and sets an initial angle of incidence .THETA..sub.0. Spring 508
maintains/ensures continuous contact/coupling between screw 507 and
component 504 and may act as a preload to inhibit hysteresis
effects. As an ambient temperature of system 100 and/or optical
probe 103 changes by .DELTA.T, arm 504 curves, effecting a change
of angle of incidence .THETA. onto the optical element. Varying the
ratio L1/L2 provides one exemplary means for adjusting the angle
change to match the predetermined grating rotational position
change required to maintain spectral alignment.
[0043] Lever arm 504 may be affixed to mount 505 at attachment
point 506 and acts as a lever to actuate the rotating element.
Lever arm 504 is coupled to the rotating element so that the
contact point is an adjustable distance L.sub.1 from the mounting
point. Adjustment screw 507 is positioned in rotating element 502
at a distance L2 from the center of rotation 503, which allows for
setting of initial angle of incidence .THETA..sub.0. As the
temperature changes by .DELTA.T, the lever arm curves/bows,
effecting a change of angle of incidence .THETA. onto the optical
element. In various embodiments, the change in angle occurs
according to the relationship:
.THETA..apprxeq..THETA..sub.0+(a/s)(L1.sup.2/L2).DELTA.T (3)
Where a is the specific deflection (also referred to as DIN 1715
for a bimetallic component) and s is the thickness of the
bimetallic component. Varying the ratio L1/L2 allows for adjusting
the angle change to match the required grating change with a fixed
coefficient of curvature .alpha. of bimetallic component 504.
Alternatively or additionally, the values of a and s may be varied
to achieve the desired angle change characteristics.
[0044] FIG. 6 illustrates an example 600 of a path traversed by arm
504 in response to changes in temperature and the associated
rotation of the optical mount. With temperature change .DELTA.T
from T.sub.0 to T.sub.1, arm 504 will bend/curve from position
A.sub.0 to position A.sub.1, resulting in a shift of rotational
position from .THETA..sub.0 to .THETA..sub.1. With temperature
changes in the opposite direction (e.g., from T.sub.1 to T.sub.0),
a commensurate shift in position will occur.
[0045] FIG. 7 illustrates another embodiment, apparatus 700, where
arm 504 (FIG. 5) is a simple beam, or rod that expands and
contracts with temperature. In some embodiments the beam or rod
(e.g., arm 504) may be made of a material with a known thermal
coefficient of expansion (.alpha.). As the temperature changes
(e.g., from T.sub.0 to T.sub.1), the beam expands or contracts in a
predetermined manner, effecting a rotation of the filter:
.THETA..apprxeq..THETA..sub.0+(.alpha.-.alpha..sub.substrate)(L1/L2).DEL-
TA.T (4)
where .alpha. is the linear coefficient of expansion of beam 504
and .alpha..sub.substrate is the averaged (effective) linear
coefficient of expansion of the material bridge between elements
506 and 503. In various embodiments, the rotating mount or optical
element itself may be mounted onto a spiral or curved element. The
spiral or curved element may be made of a bi-metallic or memory
material. Upon heating or cooling, the spiral element will rotate
the mount or the optical element in a predetermined and calculable
manner.
[0046] When an optical element is rotated (e.g., as in the
embodiments of FIGS. 5 and 7) in order to maintain spectral and/or
angular relationships, it is also desirable to minimize angular
and/or displacement changes in the alignment of the overall beam
path. For example, beam displacements advantageously remain within
the clear apertures of all optical elements and angular
relationships along the path maintained.
[0047] FIGS. 8A and 8B illustrate example beam displacements
occurring in response to a number n of optical elements being
rotated (e.g., as in the embodiments of FIGS. 5 and 7).
[0048] In FIG. 8A, entrance beam 1a hits mirrors M1 and M2, which
have been aligned so that exiting beam 1b is parallel to entrance
beam 1a and exits through the center of an optical element having
aperture A. In some embodiments, M1 and/or M2 are narrowband
optical elements, wavelength selective optics, or VHGs. In response
to an angular shift .THETA.1 in mirror M1, mirror M1 shifts to
position M1a, reflecting the beam at a different angle before
hitting mirror M2. If Mirror M2 then shifts .THETA.2 substantially
concurrently with Mirror M1 (e.g., the Mirror M2 shift being equal
in magnitude and simultaneously with Mirror M1, thereby maintaining
a "fixed angular relationship" between them such that
.THETA.2=.THETA.1), then exiting beam 2b will remain in a fixed
angular relationship to the entrance beam 2a at displacement d. In
response to displacement d being sufficiently small (e.g., less
than 1/2 the aperture A diameter) the beam will exit through
aperture A without loss and at an unchanged angle with respect to
the optical axis. This can be critical in an optical system where
many optical elements are aligned along the beam axis. However, if
mirror M2 undergoes no rotation, or a rotation
.THETA.2.noteq..THETA.1, then it will exit Mirror M2 at angle
.THETA.3 relative to the original exit beam 1b, and optical
elements will no longer be encountered at the correct angle of
incidence, resulting in compromised performance of angle-sensitive
optical elements, clipping of the beam, and/or other problems in
the downstream optical path.
[0049] In some embodiments (as depicted in FIG. 8A), the entrance
beam and exit beam are parallel to one another. In various
embodiments, Mirrors M1 and M2 are disposed on a common platform
such that their angular relationship remains substantially fixed
and constant at all times, and rotational adjustments of both
elements may be implemented by rotation of only the common
platform. The center of rotation of the common platform may be
located between the optical elements or underneath either of the
elements in order to optimize system parameters and performance,
including beam displacements and overall mechanical layout and
complexity.
[0050] When at least one of M1 and M2 are VHG narrowband optical
elements, a change in angle will result in a shift of diffraction
efficiency and center (e.g., Bragg) wavelength according to the
relationship in FIG. 4. Angular changes are matched to compensate
for calculated temperature-induced shifts in center wavelength of
the VHG, as described in relation to FIG. 3. When both M1 and M2
are VHGs, it is preferable that they have equal grating spacings
and center (or Bragg) wavelengths so as to allow a common mounting
platform and synchronous angular rotations.
[0051] In some embodiments, the excitation laser energy passing
through optical fiber cable 102 may generate a spurious
fluorescence or scattering signal that interferes with the low
frequency Raman signals collected from the sample. In such cases,
the input beam is further filtered using ASE assembly 106, or one
or more narrowband filters (e.g., mirror or narrow band filters 107
and beam splitter 108) prior to illuminating or exciting the
sample. In embodiments advantageously having multiple narrowband
filters, the fixed mirror and/or beam angular relationships are
preferably maintained as described above, in order to preserve both
spectral alignment/synchronization and beam path
clearance/relationships throughout the optical system. In some
embodiments, maintaining the fixed mirror and/or beam angular
relationships is accomplished when an even number (2n where n>0)
of reflections are used in the optical path towards the sample. In
various embodiments, a multi-pass (2n reflection) assembly with two
or more narrowband elements may be used (for example ASE filter
assembly 106), which may also be mounted onto a temperature
compensating mechanism.
[0052] In some embodiments, laser 101 (FIG. 1) is a fixed
wavelength source and narrowband elements in the optical path
(e.g., ASE filter assembly 106, mirror or narrowband filter 107,
beamsplitter 108, and filtering elements 114) are shifted or
rotated by an appropriate amount in order to preserve spectral
alignment. For example, for optical probe 103 (FIG. 1), narrowband
notch filter elements 114 are mounted on temperature compensating
mechanisms, such as those shown in FIGS. 5 and 6, and set such that
they will track or maintain the same center wavelength.
[0053] In various embodiments, the diffracted or reflected beam
(e.g., excitation beam 119 in FIG. 1) from one of the narrowband
optical elements (e.g., ASE filter assembly 106) in optical probe
103 may also be used as a feedback source to stabilize the
narrowband laser source 101. In some embodiments, a small
percentage (e.g., .about.10%) of the beam (e.g., excitation beam
119) is reflected directly back through input optical fiber 102 to
laser 101. The reflected beam may be directed into a detector to
provide feedback for active control of the laser temperature or
current, or may be fed directly back into the laser for passive
optical stabilization purposes. This has the further advantage of
keeping the laser stabilized and synchronized to the center
wavelength of the narrowband optical elements while they undergo
shifts with temperature.
[0054] FIG. 9 describes a method for consistent optical performance
of a low-frequency Raman spectroscopy probe with narrowband optical
elements over a wide range of ambient temperatures, according to
some embodiments.
* * * * *