U.S. patent application number 16/743742 was filed with the patent office on 2021-07-15 for apparatus, a handheld electronic device, and a method for carrying out raman spectroscopy.
The applicant listed for this patent is OSRAM Opto Semiconductors GmbH. Invention is credited to Christoph Goeltner, Hubert Halbritter, Ann Russell.
Application Number | 20210215609 16/743742 |
Document ID | / |
Family ID | 1000004642735 |
Filed Date | 2021-07-15 |
United States Patent
Application |
20210215609 |
Kind Code |
A1 |
Russell; Ann ; et
al. |
July 15, 2021 |
Apparatus, a Handheld Electronic Device, and a Method for Carrying
Out Raman Spectroscopy
Abstract
An apparatus, a handheld electronic device and a method for
carrying out Raman Spectroscopy are disclosed. In an embodiment an
apparatus includes at least one optoelectronic laser configured to
provide excitation radiation to a sample, the excitation radiation
being generated by an electric current flowing through the at least
one optoelectronic laser during operation of the apparatus and a
transistor configured to modulate the electric current flowing
through the at least one optoelectronic laser, to thereby switch on
and off generation of the excitation radiation.
Inventors: |
Russell; Ann; (San Jose,
CA) ; Halbritter; Hubert; (Dietfurt, DE) ;
Goeltner; Christoph; (Cupertino, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OSRAM Opto Semiconductors GmbH |
Regensburg |
|
DE |
|
|
Family ID: |
1000004642735 |
Appl. No.: |
16/743742 |
Filed: |
January 15, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 21/65 20130101;
G01N 2201/0697 20130101; G01N 2021/653 20130101; G01N 2201/0612
20130101 |
International
Class: |
G01N 21/65 20060101
G01N021/65 |
Claims
1. An apparatus for carrying out Raman spectroscopy on a sample,
the apparatus comprising: at least one optoelectronic laser
configured to provide excitation radiation to the sample, the
excitation radiation being generated by an electric current flowing
through the at least one optoelectronic laser during operation of
the apparatus; and a transistor configured to modulate the electric
current flowing through the at least one optoelectronic laser, to
thereby switch on and off generation of the excitation
radiation.
2. The apparatus of claim 1, wherein the at least one
optoelectronic laser is a DFB laser diode or DBR laser diode.
3. The apparatus of claim 1, wherein the transistor comprises an
electric contact and the optoelectronic laser comprises an electric
contact, and wherein the electric contact of the transistor is
directly coupled electrically to the electric contact of the
optoelectronic laser.
4. The apparatus of claim 1, wherein the transistor is a GaN FET
configured to operate the at least one optoelectronic laser such as
to generate pulsed excitation radiation.
5. The apparatus of claim 1, wherein the at least one
optoelectronic laser is configured to generate pulses of excitation
light, wherein each pulse has a time duration of less than 200
ps.
6. The apparatus of claim 1, further comprising a temperature
sensor configured to monitor a temperature of the at least one
optoelectronic laser.
7. The apparatus of claim 1, further comprising a Bragg
grating.
8. The apparatus of claim 1, further comprising a spectrometer
configured to analyse Raman light scattered from the sample in
response to exposing the sample to the excitation radiation,
wherein Raman light comprises one or more spectral components, and
wherein the spectrometer comprises diffraction element configured
to split the Raman light into its spectral components.
9. The apparatus of claim 8, wherein the diffraction element
comprises at least one of the following: a diffraction grating, a
photonic crystal, or a plasmonic Fabry Perot filter.
10. The apparatus of claim 1, further comprising a driver for the
transistor, wherein the driver is configured to provide a control
signal for controlling operation of the transistor.
11. The apparatus of claim 10, further comprising a shutter between
a spectrometer and a detector configured to detect spectral lines
in a Raman signal, wherein the spectral lines are spatially split
by the spectrometer from the Raman signal, and wherein the shutter
is operated based on the control signal.
12. The apparatus of claim 10, further comprising a scanning mirror
between a spectrometer and a detector for detecting spectral lines
in a Raman signal, wherein the spectral lines are spatially split
by the spectrometer from the Raman signal, and wherein the scanning
mirror is operated based on the control signal.
13. A handheld electronic device comprising: a housing; and an
apparatus with at least one optoelectronic laser configured to
provide excitation radiation to a sample, the excitation radiation
being generated by an electric current flowing through the at least
one optoelectronic laser during operation of the apparatus, wherein
the apparatus further comprises a transistor configured to modulate
the electric current flowing through the at least one
optoelectronic laser, to thereby switch on and off generation of
the excitation radiation, and wherein the apparatus is arranged in
the housing of the handheld electronic device.
14. The handheld electronic device of claim 13, wherein the
handheld electronic device is a smartphone or a tablet.
15. A method of carrying out Raman spectroscopy on a sample, the
method comprising: providing, by at least one optoelectronic laser
of an apparatus, excitation radiation to the sample, the excitation
radiation being generated by an electric current flowing through
the at least one optoelectronic laser during operation of the
apparatus, wherein the apparatus further comprises a transistor for
modulating the electric current flowing through the at least one
optoelectronic laser, to thereby switch on and off generation of
the excitation radiation; and operating the transistor such as to
cause the at least one optoelectronic laser to generate pulses of
excitation radiation.
16. An apparatus for carrying out Raman spectroscopy on a sample,
the apparatus comprising: a GaN FET; and at least one DFB or DBR
laser, wherein the GaN FET is configured to directly modulate the
DFB or DBR laser for generating at least one laser pulse which fast
enough to capture Raman scatter prior to fluorescence.
17. The apparatus of claim 16, further comprising tandem slits
and/or MEMS mirrors configured to image while a laser modulation
signal is used to as a frame sync.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to an apparatus for carrying
out Raman spectroscopy on a sample. The present disclosure relates
to a handheld electronic device and a method for carrying out Raman
spectroscopy on a sample.
BACKGROUND
[0002] The chemistry of samples, such as molecules, can be probed
by exposing a sample to laser light and by collecting the
inelastically backscattered light. Light at a wavelength of the
laser light in the backscattered light, also referred to as
Rayleigh scattering, can be filtered out using, for example, a high
pass filter. The remaining red shifted light, also called Raman
scattered light, can be imaged onto a detector. This method of
probing matter is a common way to obtain a unique Raman spectrum
with greater accuracy than broadband spectroscopy and it can be
used to reliably identify the chemical makeup and structure of the
matter in question. When examining, for example, biological media,
a problem can be the occurrence of fluorescence light which can be
several orders of magnitude greater than the Raman signal, and the
fluorescence light can obfuscate the identifying information.
[0003] A repeatable, portable and affordable apparatus and method
is sought to carry out Raman spectroscopy on a sample, for example
to identify pesticides on food, drugs in urine and contamination in
liquid milk.
SUMMARY
[0004] Embodiments of the invention seek to provide an apparatus
for carrying out Raman spectroscopy, such as time-gated Raman
spectroscopy, on a sample. The apparatus comprises at least one
optoelectronic laser for providing excitation radiation to the
sample, the excitation radiation being generated by an electric
current which flows through the at least one optoelectronic laser
during operation of the apparatus, and a transistor, such as a GaN
FET, for modulating the electric current flowing through the at
least one optoelectronic laser, to thereby switch on and off the
generation of the excitation radiation.
[0005] The at least one optoelectronic laser can be one or more
optoelectronic lasers, and the electric currents that flow through
the optoelectronic lasers can be different from each other.
However, the transistor can be employed to control each of the
electric currents simultaneously. The electric current through an
optoelectronic laser causes the generation of coherent laser light.
A GaN FET (FET=field effect transistor) allows rapid control of the
electric current, and thus, it can be employed to switch on and off
the generation of excitation radiation. Time gated Raman
spectroscopy can therefore be carried out.
[0006] For example, the transistor can modulate the electric
current such as to switch intermittently an optoelectronic laser
between on operational mode and a non-operational mode. The laser
can emit pulses of excitation radiation in the operational mode.
The electric current is then at a level at which the pulses of
excitation radiation can be generated. In the non-operational mode,
the electric current is at a level where light emission does not
occur.
[0007] Raman measurements on a sample will lead to fluorescence
when trying to collect a Raman signal. The concept of time-gated
Raman scattering is related to the aspect of collecting Raman
scattering prior to fluorescence. For example, laser pulses with a
time duration in the pico-second range can be used to stimulate the
immediate Raman scattering prior to the fluorescing portion
saturating a detector, which is used to detect the Raman light.
Raman scattering of a sample is proportional to
1/.lamda.{circumflex over ( )}4, wherein .lamda. denotes the
wavelength of the excitation radiation. It can for example be
possible to use direct transition 520 nm picosecond laser light to
stimulate intense Raman scattering without damaging a sample.
[0008] In some embodiments, the at least one optoelectronic laser
is configured to provide single mode operation which is wavelength
stabilized, and/or to maintain a wavelength in a robust way and for
a long time, and/or to provide excitation radiation with a long
coherence length.
[0009] In some embodiments, the at least one optoelectronic laser
comprises a semiconductor based lasing device, in particular a
laser diode. In some embodiments, the laser diode emits green
light, for example at a wavelength of 520 nm.
[0010] In some embodiments, the semiconductor based lasing device
can have a DBR (=Distributed Bragg Reflector) or DFB (=Distributed
feedback) or VCSEL (=vertical-cavity surface-emitting laser)
architecture. Thus, for example, the optoelectronic laser can be a
DBR or DFB laser diode or a VCSEL.
[0011] A distributed Bragg reflector is a periodic structure, which
is formed of alternating dielectric layers having different indices
of refraction. A DBR might be used to achieve nearly total
reflection within a range of frequencies, wherein the range of
frequencies includes the frequencies of the excitation radiation of
the laser. The DBR can be formed by use of dielectric layers that
are included in the layer structure of the laser.
[0012] A distributed feedback laser diode can be a type of laser
diode, quantum cascade laser or optical fiber laser where the
active region of the device contains a periodically structured
element or diffraction grating.
[0013] A VCSEL is a type of semiconductor laser diode that emits
laser light in a direction, which is perpendicular to the top
surface of the laser diode.
[0014] In some embodiments, the at least one optoelectronic laser
is a DFB or DBR laser diode.
[0015] In some embodiments, the at least one optoelectronic laser
comprises a laser diode and at least one of an external cavity and
a wavelength multiplexer.
[0016] In some embodiments, the at least one optoelectronic laser
can be two, three or more lasers. For example, each laser can be a
laser diode that provides laser light at a defined wavelength. The
light from the laser diodes can be coupled, for example by use of a
combiner, into a single fiber or into a single, preferably
collimated, beam. For example, using three laser diodes, one
emitting red light, one emitting green light, and one emitting blue
light, a laser beam including the red, green and blue light can be
obtained. Thus, an RGB laser beam could be obtained.
[0017] In some embodiments, the transistor is a Gallium Nitride
field effect transistor (GaN FET). The GaN FET can be configured as
a high-power GaN FET. The GaN FET can be configured to allow
picosecond rise times, thereby enabling the optoelectronic laser to
generate pulses with a time duration in the pico-second range.
[0018] In some embodiments, the transistor is embedded in a
substrate, wherein an optoelectronic laser is arranged on the
substrate. The optoelectronic laser and the transistor can be
accommodated in a single package. As the transistor can be placed
directly underneath the optoelectronic laser, basically none or
only very short wiring is required to connect electrically the
transistor to the optoelectronic laser. Thus, the transistor can be
electrically coupled to the optoelectronic laser at least in
substance without using bond wires. This allows short switching
times of a voltage provided by use of the transistor to the
optoelectronic laser. For example, a change in voltage (V) over
time (t), dV/dt, can be larger than 100 V/s. For an optoelectronic
laser with approximately 8V forward bias, this can for example
result in a turn on and off time of 160 ps.
[0019] In some embodiments, the at least one optoelectronic laser
and optionally the transistor can be arranged in a package. A
connecting pad can be placed under the at least one optoelectronic
laser, and the transistor can be placed on or beyond the connecting
pad. The at least one optoelectronic laser in the package is
preferably a DFB or DBR laser diode.
[0020] In some embodiments, the GaN FET has an electric contact and
an optoelectronic laser has an electric contact, and the electric
contact of the transistor is directly coupled electrically to the
electric contact of the optoelectronic laser. Basically no bond
wire is used to connect the electric contacts of the transistor and
the optoelectronic laser.
[0021] The transistor can be an FET transistor which has a drain
electrode. The optoelectronic laser can be a laser diode which has
a cathode and an anode. In some embodiments, the drain of the FET
transistor can be directly coupled to the cathode of the laser
diode. This allows for short switching times.
[0022] In some embodiments, a driver is configured to operate the
transistor such as to cause the optoelectronic laser to cause the
generation of pulsed excitation radiation. The driver can provide a
control signal to the transistor. The control signal can also be
called frame sync signal.
[0023] For example, if the transistor is a FET transistor, the
driver can provide the control signal to the gate of the
transistor. The cathode of the laser diode can be connected to the
drain of the FET transistor. The application of a voltage to the
gate by use of the control signal allows modulating the electric
current through the laser diode. Thereby, the laser diode can be
rapidly switched between an on state and an off state. The control
signal can for example be a square-wave signal.
[0024] In some embodiments, the at least one optoelectronic laser
can be configured to generate pulsed excitation radiation, with
pulses having a time duration of less than 100 picosecond. The
pulse duration can be measured at FWHM (=full width have maximum).
The pulse duration can therefore correspond to the full width at
half of the maximum of the time signal of a pulse.
[0025] In some embodiments, the at least one optoelectronic laser
can be operated to generate pulses of excitation radiation, wherein
each pulse has for example a time duration of less than 500 ps or
less than 500 fs.
[0026] In some embodiments, the apparatus comprises a temperature
sensor configured to monitor a temperature of an optoelectronic
laser. A change in temperature can cause a wavelength shift in the
emitted excitation radiation from the optoelectronic laser. Such a
wavelength shift can thus be accounted for by monitoring the
temperature.
[0027] In some embodiments, the apparatus comprises a Bragg
grating.
[0028] The Bragg grating can be helpful in producing laser light
with a long coherence length. The Bragg grating can be integrated
into a package, which further includes the optoelectronic light
source and the transistor. Such a package may have, but does not
require a non-hermetic facet coating, which is preferably used in
conjunction with DFB or DBR laser diodes.
[0029] In some embodiments, the apparatus comprises a spectrometer
for analyzing Raman light scattered from the sample in response to
exposing the sample to the excitation radiation, the Raman light
comprising one or more spectral components, and wherein the
spectrometer comprises a diffraction element configured to split
the Raman light into its spectral components.
[0030] Thus, the diffraction element can divide the Raman light
into its spectral components and thereby spread the Raman light
into an optical spectrum of spatially separated wavelength
components.
[0031] The spectrometer can further comprise a focusing lens system
for directing at least a portion of the spectrum to a detector,
such as a one- or two-dimensional array detector.
[0032] The spectrometer can comprise an entrance slit. The slit can
help to tighten the window of observation for Raman scattering
prior to fluorescence, thereby eliminating fluorescence, which will
prevent collection of the Raman signal.
[0033] In some embodiments, the diffraction element comprises at
least one of the following: a diffraction grating, a photonic
crystal, and a plasmonic Fabry Perot filter.
[0034] In some embodiments, the apparatus includes a scanning
mirror in a light path of the Raman scattered light, in particular
between a spectrometer and a detector, wherein the transistor is
operated based on a control signal, and wherein the scanning mirror
is also operated based on the control signal.
[0035] Embodiments of the invention relate to a handheld electronic
device which comprises a housing, and an apparatus with at least
one optoelectronic laser for providing excitation radiation to the
sample, the excitation radiation being generated by an electric
current which flows through the at least one optoelectronic laser
during operation of the apparatus, and the apparatus further
comprising a transistor for modulating the electric current flowing
through the at least one optoelectronic laser, to thereby switch on
and off the generation of the excitation radiation and the
apparatus being arranged in the housing of the handheld electronic
device.
[0036] In some embodiments, the handheld electronic device is a
smartphone or a tablet.
[0037] Embodiments of the invention also relate to a method of
carrying out Raman spectroscopy on a sample, wherein the method
comprises providing an apparatus in accordance with at least some
of the embodiments as described herein, and operating the
transistor, for example a GaN FET, such as to cause the
optoelectronic laser to generate pulses of excitation
radiation.
[0038] A feature mentioned in conjunction with an embodiment can
also be present in another embodiment, even if not explicitly
mentioned in conjunction with this embodiment.
[0039] The sample is not a part of the claimed apparatus, handheld
electronic device or method. Rather, the sample is the piece of
matter or a volume of gas or liquid on which Raman spectroscopy is
carried out.
[0040] The electric current flows through the optoelectronic laser
during operation of the laser and thus during the intended use of
the apparatus or handheld electronic device.
[0041] In some embodiments, a handheld electronic device can
comprise an apparatus for carrying out Raman spectroscopy on a
sample using time gated Raman spectroscopy via direct modulation of
a laser diode and MEMS mirrors plus slits to image Raman scattering
prior to fluorescence as well as various instantiations using Bragg
gratings (single wavelength laser) versus multiwavelengths (surface
relief gratings). MEMS mirrors plus a double slits and various
types of detectors (filter array, electrostatically charged deep
well large pixels, Chromation device, etc.) can all provide various
ways to detect separated Raman scattering. The use of double slits
with a MEMS mirror prevents saturation of the Raman signal by the
following fluorescence.
[0042] In some embodiments, an apparatus for carrying out Raman
spectroscopy comprises a GaN FET, and a directly modulated (by the
GaN FET), visible DFB or DBR laser which is used to generate laser
pulses fast enough (for example <200 ps) to capture Raman
scattering prior to fluorescence. Tandem slits and/or MEMS mirrors
can be used to image while a laser modulation signal is used to as
a frame sync. The laser modulation signal can be provided by a
driver which drives the transistor based on the laser modulation
signal. The use of small laser diodes, MEMS mirrors, and linear
arrays as detectors can mean that the apparatus can fit into a
handheld device, such as a cell phone, smart phone, tablet,
etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] One or more examples will hereinafter be described in
conjunction with the following drawing figures, where like numerals
denote like elements.
[0044] FIG. 1 shows a block diagram of an exemplary embodiment of
an apparatus;
[0045] FIG. 2 shows schematically an exemplary embodiment of a
handheld electronic device;
[0046] FIG. 3 illustrates schematically a functional approach for
carrying out time-gated Raman spectroscopy;
[0047] FIG. 4 illustrates schematically a further functional
approach for carrying out time-gated Raman spectroscopy;
[0048] FIG. 5 shows schematically a further exemplary embodiment of
an apparatus;
[0049] FIG. 6 shows schematically a further exemplary embodiment of
an apparatus;
[0050] FIG. 7 shows schematically a further exemplary embodiment of
an apparatus; and
[0051] FIG. 8 shows schematically yet a further exemplary
embodiment of an apparatus.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0052] The apparatus 101 as shown in FIG. 1 can be used for
carrying out Raman spectroscopy, such as time-gated Raman
spectroscopy, on a sample 103, which is not part of the apparatus
101. The apparatus 101 comprises an optoelectronic laser 105 for
providing excitation radiation to the sample 103. The sample 103
can be arranged, for example by a user of the apparatus, such that
it can be exposed to the excitation radiation, which usually
consists of or comprises laser light.
[0053] The apparatus 101 further comprises a transistor 107, for
example a Gallium Nitride field effect transistor, for modulating
an electric current, which flows during operation of the apparatus
101 through the optoelectronic laser 105 and which causes the
generation of the excitation radiation.
[0054] At least some embodiments of the apparatus 101 can be
incorporated into a handheld electronic device, such as a cell
phone, a smartphone or a tablet computer. For example, the handheld
electronic device 201 of FIG. 2 comprises such an apparatus having
an optoelectronic laser 203, for example a DFB or DBR laser diode,
for providing excitation radiation 207 to sample 205, which is
arranged outside a housing 209 of the handheld electronic device
201.
[0055] The excitation radiation 207 can have an average power of
more than wo mW. The excitation radiation can comprise green laser
light, and the excitation radiation can include one or more
wavelengths. For example, two wavelengths, one in the visible and
one in the infrared, can help to obtain a better confirmation of
the Raman signal.
[0056] The apparatus 201 also comprises a transistor 211, such as a
GaN FET, for modulating an electric current, which can flow through
the optoelectronic laser 203 to cause the generation of the
excitation radiation 207.
[0057] The apparatus 201 also includes an objective 213, for
example in form of a focusing lens system which can comprise a
plano convex lens. The objective 213 can focus the excitation
radiation 207 to a spot 215 outside the housing 209. The sample 207
is placed such that the spot 215 is located on the surface of the
sample 205. The objective 213 also serves to collect light
scattered from the sample 205. The scattered light includes Raman
scattered light with wavelengths that are different from the
wavelengths of the excitation radiation 207.
[0058] A high pass filter 217 is configured to reflect the
excitation radiation 207 from the optoelectronic laser 203 and to
guide the excitation radiation 207 to the objective 213. The high
pass filter 217 is furthermore transparent for light with
wavelengths, which are longer than the wavelengths of the
excitation radiation 207. Thus, the red shifted portion of the
Raman scattered light can pass the high pass filter 217 and it can
be focused through a slit 219 of a spectrometer 221.
[0059] The spectrometer 221 comprises a diffraction element 223,
such as a diffraction grating, a photonic crystal, or a plasmonic
Fabry Perot filter, which spatially splits the Raman light into its
spectral components. A focusing lens system (not shown) images the
spectral components on an array detector, such as a CCD array
detector (CCD=charged coupled device).
[0060] The diagram 15 as shown in FIG. 3 illustrates a time-gated
Raman spectroscopy process. The diagram 15 is also related to the
setup of FIG. 5, which will be described further below in more
detail.
[0061] As illustrated in 301 of FIG. 3, an optoelectronic laser is
operated such as to provide laser pulses. The laser can be a low
power (for example with an average power of 100 mW) DFB or DBR
laser diode. A laser diode 9 is shown in FIG. 5.
[0062] Alternatively, a VCSEL and a green laser, such as a direct
transition green laser, could be used to provide laser light. The
VCSEL can for example be configured to emit light in the
infrared.
[0063] A control signal (frame sync signal) 11 is generated in 303,
which is used for controlling the operation of the laser 9. The
signal 11 can also trigger the flashing of a detector array, for
example, a linear array, from existing charges according to
305.
[0064] Regarding the generated laser pulse, it is reflected off the
filter 2 (see FIG. 5) according to 307. In 309, the pulse travels
through objective 1 and hits sample 215 (see also FIG. 5). Light
which includes Raman light is back scattered and collected by the
objective 1 according to 311 of FIG. 3. The red-shifted Raman light
passes the high pass filter 2, and lens 3 focuses the light through
slit 4 into the spectrometer according to 313 of FIG. 3.
[0065] The Raman light further passes through collimation and
aberration correction optics 5 and diffracting element 6 which
spatially splits the Raman light into its spectral lines. At least
some of the spectral lines in the Raman light are imaged by use of
imaging lens 7 on the detector 8. A shutter 14 is placed in front
of the detector 8 and the shutter 14 is operated based on the frame
sync signal 11 by use of which the laser 9 is operated. As
indicated in 315, the frame sync signal 11 allows the detector to
collect the spectral lines of the Raman light by causing the
opening of the shutter 14.
[0066] Fluorescence is generated by the sample according to 317
with a time delay with regard to the Raman light. Fluorescence
light can arrive at the detector 8 according to 319, but data about
the spectral lines in the Raman signal have already been collected
due to the use of the frame sync signal as shown in 315 which has
in the meantime closed the shutter 14. Thus, the detector 8 will
not collect fluorescence light.
[0067] The data collected in 315 will be further processed in 312,
for example by use of an artificial intelligence (AI) system or the
like, in order to identify the spectral lines and/or the sample. A
result is output in 323.
[0068] The diagram 25 as shown in FIG. 4 differs from the diagram
15 in FIG. 3 by block 401. Instead of using a shutter 14 (see FIG.
5), double slits 46 and 48 and a scanning mirror 47, for example a
MEMS scanning mirror (MEMS=Micro-Electro-Mechanical System) are
used, as illustrated in FIG. 6. The use of a MEMS scanning mirror
47 allows reduction of exposure time and prevents saturation of the
linear array 8, trimming out the undesirable fluorescence in a
different manner than shutter 14.
[0069] In some embodiments, this can be similar to tandem slit
scanning microscopy. A tandem scanning slit microscope is for
example described in the scientific publication by Stephen C. Baer
"Tandem Scanning Slit Microscope", Proc. SPIE 1139, Optical Storage
and Scanning Technology, (28 Sep. 1989);
https://doi.org/10.1117/12.961780.
[0070] For example, in order to confine illumination to just the
plane of focus, a tandem scanning mirror can be used similar to an
epi-illumination tandem scanning pinhole microscope using slits
instead. Epi-illumination is an operational mode used in microscopy
in which illumination and detection occurs from the same side of
the sample. The mirror image of one field aperture is coincident
with the other, where an opaque mirror is used at the edge of the
plane defined by the viewing slit and the center of the objective
aperture. The mirror can then reflect light from the illuminated
slit onto just one semicircle of the objective aperture. The
remaining semicircle can be used for projecting light from the
specimen to the viewing slit. Scanning can be accomplished by
reciprocally rotating the two slits and the mirror. MEMS can be
used to achieve rotating movements.
[0071] As further shown in diagram 35 of FIG. 5, the operation of
optoelectronic laser 9 is controlled via GaN FET 10. The laser 9
comprises an anode A and a cathode C. The anode A is connected to a
voltage supply provided by a voltage source (not shown). The
cathode C is electrically connected to the drain D of transistor
10. The source S of transistor 10 is connected to ground gnd. The
control signal (frame sync signal) 11 is applied to the gate of
transistor 10. The control signal 10 can for example be a
square-wave signal, and it can be configured to rapidly switch the
electric current that drives the laser 9 between a level at which
lasing occurs and a level at which the laser 9 is not emitting
light.
[0072] The control signal 10 is provided to the shutter 14 to open
and close the shutter 14 in dependence on the control signal 14. In
the setup 45 of FIG. 6, the control signal 10 can be used to
control the operation of the scanning mirror 47.
[0073] The laser 9 is turned on and off using the GaN FET 10, which
can for example switch at dV/dt>100V/s. For example, the laser 9
can have an approximate 8V forward bias, a turn on and off time of
160 ps is then possible. As the laser 9 can be a low power laser,
it will not require a high voltage rail. The GaN FET 10 is well
suited for these types of fast switching applications.
[0074] The generated pulses of the excitation radiation provided by
the laser diode 9 is collimated using a lens 12, which can produce
a Gaussian beam which is desirable for accurate Raman scattering
analysis. The pulses of the excitation radiation are then condensed
via objective 1, also referred to as probe optics, for example by
using a common low f-number optics.
[0075] The pulses of laser light can be focused down to a spot size
of approximately 20 microns to stimulate Raman scattering on the
sample 215.
[0076] The backscattered light is mostly rejected at high pass
filter 2, which can be a dichroic mirror, except the red-shifted
component of the Raman scattered light. Thus, only the Stokes
shifted light of the Raman light is further processed. The high
pass filter 2 can start at the laser wavelength of the pulses as
provided by laser 9, for example corresponding to a wavelength of
520 nm, 785 nm, 850 nm, or 940 nm.
[0077] Condensing lens 3 focuses the pulses of Raman light through
slit 4 which determines the resolution of the system and optical
throughput. For example, a 10-50 micron slit 4 is used to filter
the signal. The pulses of Raman light pass through collimation and
aberration correction optics 5, such as anachromat. The expanded
and somewhat collimated pulses pass through diffracting element 6
which can be a 2D photonic crystal or a volume Bragg grating, and
it acts as a wavelength separator.
[0078] Imaging lens 7 directs the first order of the spatially
separated lines of the Raman light towards detector 8 while
avoiding the zero order. The shutter 14 is used to prevent
fluorescence light from saturating the detector 8 and the shutter
14 is operated based on the frame sync signal 11.
[0079] The now wavelength separated Raman light is imaged on
detector 8, for example a linear array 8 such as a SiPM, SPAD,
InGaAS detector, or cut filtered silicon with bias voltage
applied.
[0080] The frame sync 11 can also be used to clear excess charges
prior to Raman scattering being imaged on the detector 8.
[0081] The linear array 8 can be a deep well, large pixel (for
example 8 um.times.8 um) linear array, and it can display an
extremely tight form factor (8 mm.times.1 mm).
[0082] A temperature sensor or TEC 13 can be used to monitor the
laser diode temperature to account for wavelength shift of laser
diode 9.
[0083] As shown in FIG. 7, the setup 55 provides multiple
wavelengths of excitation radiation to sample 215. This can be
realized by use of three lasers 9, 58, and 59, each of which
provides laser pulses at a particular wavelength. Each laser 9, 58,
and 59 is a laser diode and the cathode C of each laser diode is
connected to the drain D of transistor 10.
[0084] The control signal 11 is applied to gate G of transistor 10
to control the electric current through the laser diodes 9, 58, and
59 and, thus, to switch the lasers 9, 58, and 59 on and off. The
control signal 11 is also used to control the operation of the
shutter 14.
[0085] A blazed diffraction grating 56 is further used to diffract
any wavelength. For example, consider a 520 nm laser 9, a 785 nm
laser 58, and an infrared laser 59 providing pulses at 1064 nm. The
control signal 11 is again used along with a shutter 14 and linear
array 8.
[0086] As an alternative to the diffraction grating 56, the Raman
scattered light from the sample 215 under investigation can be
split into its spectral lines by means of a prism or optical
grating to fall onto a linear detector grid. The respective
spectrum can be derived from the light intensity on each of the
linearly aligned detector elements of detector 8.
[0087] In some alternative embodiments, the Raman light is directed
to a sensor array, where each sensitive element or pixel is using a
unique filter that only allows a specified narrow waveband to reach
the sensor element. In this way, a diffraction element is not
required. The number of pixels and the bandwidth of each
corresponding filter in front of each pixel determine the spatial
resolution of the detected spectrum.
[0088] The setup 65 as shown in FIG. 8 includes double slits 46 and
48 as well as scanner 47 instead of the shutter 14 as used in setup
55 of FIG. 7. The control signal 11 is used to control operation of
the scanner 47, which can be a scanning mirror or a MEMS scanning
mirror. A multiplexing waveguide can also be utilized. The
waveguide can be used for compactly combining multiple wavelengths
such as those used, for example, in communication servers.
[0089] The foregoing description, for purpose of explanation, has
been described with reference to specific embodiments. However, the
illustrative discussions above are not intended to be exhaustive or
to limit the invention to the precise forms disclosed. Many
modifications and variations are possible in view of the above
teachings. The embodiments were chosen and described in order to
best explain the principles of the techniques and their practical
applications. Others skilled in the art are thereby enabled to best
utilize the techniques and various embodiments with various
modifications as are suited to the particular use contemplated.
* * * * *
References