CN114994013A - High-sensitivity stimulated Raman spectrometer - Google Patents
High-sensitivity stimulated Raman spectrometer Download PDFInfo
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- CN114994013A CN114994013A CN202210584589.9A CN202210584589A CN114994013A CN 114994013 A CN114994013 A CN 114994013A CN 202210584589 A CN202210584589 A CN 202210584589A CN 114994013 A CN114994013 A CN 114994013A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/65—Raman scattering
- G01N21/658—Raman scattering enhancement Raman, e.g. surface plasmons
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/01—Arrangements or apparatus for facilitating the optical investigation
Abstract
The invention discloses a high-sensitivity stimulated Raman spectrometer, which comprises: a first laser source; a second laser source; the beam combining mirror is used for combining the laser and passing through the sample; the laser passing through the sample passes through a filter to filter the wavelength of the pump light, and is detected by a first photoelectric detector; one of light paths from the first laser source and the second laser source to the beam combiner is modulated by the chopper, the other light path sequentially passes through the half-wave plate and the first polarization beam splitter, and extra light paths are divided and detected by the second photoelectric detector; the first photoelectric detector and the second photoelectric detector are connected in parallel in the same direction and are connected with the transimpedance amplifier, the transimpedance amplifier outputs a voltage signal to enter the phase-locked amplifier, the chopper driving power supply drives the chopper, and the pulse generator triggers the phase-locked amplifier and the chopper driving power supply. The invention can obtain the stimulated Raman spectrum with the same signal-to-noise ratio by using weaker light, reduces the requirement on the stability of the output light of the laser and avoids the damage of the laser to a sample.
Description
Technical Field
The invention relates to the technical field of spectrometers, in particular to a high-sensitivity stimulated Raman spectrometer.
Background
Stimulated raman spectroscopy is a spectroscopic technique that is widely used in the fields of physics, chemistry, biology, and the like. Stimulated raman scattering is a third-order nonlinear optical phenomenon. Two high intensity lasers interact with a substance, one of which (called stokes light) is stimulated to amplify and the other (called pump) when the difference between the photon energies of the two beams (called raman shift) coincides with the difference between the energy levels of the substanceLight) will be reduced. Stimulated raman spectroscopy is widely used to measure the structure of a substance in different states, and to obtain other properties of the substance from spectral data, such as the temperature of the combustion state, microscopically stimulated raman can be used to study phenomena in organisms by cellular imaging, etc. In a conventional stimulated Raman spectrometer, two beams of light adopt laser output by a high-stability laser, the collection time is 1 second, and the sensitivity can reach 10 -7 . However, in fast scanning, such as imaging, the sensitivity limits the imaging speed, and thus the data acquisition efficiency.
Disclosure of Invention
The invention aims to: in order to overcome the defects of the background art, the invention discloses a high-sensitivity stimulated Raman spectrometer.
The technical scheme is as follows: the invention discloses a high-sensitivity stimulated Raman spectrometer, which comprises:
the first laser source emits pulse laser as pump light in the stimulated Raman spectrum;
a second laser source that emits pulsed laser as stokes light in the stimulated raman spectrum;
the laser of the first laser source and the laser of the second laser source are combined by the beam combining mirror and pass through the sample;
the laser passing through the sample passes through the filter to filter the wavelength of the pump light, and is detected by the first photoelectric detector;
one of the light paths from the first laser source and the second laser source to the beam combiner is modulated by a chopper, a modulated laser pulse sequence is output, the other light path sequentially passes through a half-wave plate and a first polarization beam splitter, an extra light path is separated and detected by a second photoelectric detector, and the light intensity ratio of the sample entering the sample and the second photoelectric detector is adjusted by rotating the half-wave plate;
the photoelectric detector comprises a transimpedance amplifier, a phase-locked amplifier, a pulse generator and a chopper driving power supply, wherein a first photoelectric detector and a second photoelectric detector are connected in parallel in the same direction and are connected with the transimpedance amplifier, a voltage signal output by the transimpedance amplifier enters the phase-locked amplifier, the chopper driving power supply drives the chopper, and the pulse generator triggers the phase-locked amplifier and the chopper driving power supply.
Furthermore, in the light paths from the first laser source and the second laser source to the beam combiner, an extra light path separated by the half-wave plate and the first polarization beam splitter is combined with the original light beam by the second polarization beam splitter, and the extra light path and the original light beam pass through the beam combiner in tandem, are combined with the light beam of the other laser source, pass through the sample, and are detected by the second photodetector after passing through the third polarization beam splitter.
Furthermore, a first focusing lens and a second focusing lens are respectively arranged on two sides of the sample, and the laser beams of the first laser source and the laser beams of the second laser source sequentially pass through the first focusing lens, the sample and the second focusing lens after being combined.
Further, the first photodetector and the second photodetector are photodiodes and avalanche photodiodes with low equivalent current noise.
Further, the bandwidth of the trans-impedance amplifier is matched with the laser modulation frequency output by the chopper.
Furthermore, during measurement, the laser source on the light path of the chopper is blocked, the half-wave plate is adjusted, the photoelectric signals received by the first photoelectric detector and the second photoelectric detector are balanced, the signals obtained by the phase-locked amplifier are enabled to be as small as possible, and a balanced detection structure is formed.
Has the advantages that: compared with the prior art, the invention has the advantages that: the stimulated Raman spectrum with the same signal-to-noise ratio can be obtained by using weaker light, so that the damage of laser to a sample is avoided; or with the same intensity of light, the spectrum with the same good signal-to-noise ratio can be obtained in a shorter time, so that the signal acquisition efficiency is improved, and in imaging application, the imaging can be obtained more quickly.
Drawings
FIG. 1 is an enhanced optical diagram of the invention for measuring Stokes light in stimulated Raman spectroscopy;
FIG. 2 is a diagram of the attenuation path of pump light in the stimulated Raman spectrum according to the present invention;
FIG. 3 is a schematic diagram of the same optical path balanced detection scheme in the stimulated Raman spectrum of the present invention.
Detailed Description
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
Example 1
As shown in fig. 1, a high sensitivity stimulated raman spectrometer includes:
a first laser source 1 that emits pulsed laser as pump light in a stimulated raman spectrum; the pulsed laser light emitted by the second laser light source 2 serves as stokes light in the stimulated raman spectrum. (typically, to achieve high sensitivity, both laser sources employ highly stable mode-locked laser sources, e.g., the first laser source is an ultrafast laser pumped optical parametric oscillator and the second laser source is a ultrafast laser oscillator directly outputting laser light.)
The laser light emitted from the first laser light source 1 is modulated by the chopper 7 (driven by the chopper drive power source 14), and a modulated laser light pulse train is output at a frequency f. The laser light emitted from the second laser source 2 is split into two beams of light by the half-wave plate 8 and the first polarization beam splitter 9. The ratio of the light intensity entering the sample 4 and entering the second photodetector 10 can be adjusted by rotating the half-wave plate 8. A beam of light passing through the first polarization beam splitter 9 passes through a reflector and is detected by the second photodetector 10; the other beam of light passes through the beam combining mirror 3, is combined with the light emitted by the first laser source 1, passes through the first focusing mirror 15, is collimated by the sample 11 through the second focusing mirror 16, is filtered by the optical filter 5 to remove the wavelength of the pump light without filtering out the stokes light, and is detected by the first photodetector 6. After the pump light and the Stokes light pass through the sample, a stimulated Raman effect can occur, and the Stokes light can be amplified. The optical signal detected by the first photodetector 6 is the sum of the input stokes light signal plus the stimulated raman enhanced signal.
The first photodetector 6 and the second photodetector 10 may be photodiodes, avalanche photodiodes, or the like, having low equivalent current noise. The first photodetector 6 and the second photodetector 10 are connected in parallel in the same direction and then connected to the transimpedance amplifier 11, and the bandwidth of the transimpedance amplifier 11 is matched with the laser modulation frequency f output by the first laser source 1 through the chopper 7. This constitutes a structure for balanced detection. The voltage signal output by the transimpedance amplifier 11 enters the phase-locked amplifier 12. The lock-in amplifier 12 detects an ac signal at frequency f. The pulse generator 13 triggers the lock-in amplifier 12 and the chopper drive power supply 14.
In this embodiment, the measured stokes light enhancement signal. The enhanced signal is proportional to the incident light intensity of the pump light and the Stokes light, as follows
ΔI s ∝I p I s
Wherein I p Is the intensity of the pump light, I s Is the intensity of the stokes light.
In implementation, the half-wave plate 8 needs to be adjusted under the condition of blocking the first laser source 1, and photoelectric signals received by the two photodetectors are balanced, so that signals obtained by the lock-in amplifier are as small as possible, which is the key of high-sensitivity detection. This balance needs to be achieved at any combination of wavelengths emitted by the first laser source 1 and the second laser source 2 to ensure that a high sensitivity is achieved at any raman shift of the acquired stimulated raman spectrum.
The conventional stimulated raman spectroscopy does not adopt a balanced detection mode, but only uses one photoelectric detector without a signal of reference light. In the absence of the reference optical signal, the current at the input of the transimpedance amplifier 11 is the sum of the stokes light and the stimulated raman enhanced photocurrent. In the balanced detection structure, the current at the input end of the transimpedance amplifier 11 is only the stimulated raman enhanced photocurrent. The equivalent current noise in the former conventional method is given by:
wherein, I PN Is the equivalent current noise of the photodetector, e.g. typical value 10fA/Hz 1/2 ；I TN Is equivalent current noise of a transimpedance amplifier, e.g.Typical value of 1MHz Bandwidth 270fA/Hz 1/ 2；I LN The equivalent current noise of Stokes light can be obtained by calculating the responsivity of the diode, and the following formula
I LN ＝P LN ·R(λ)
Wherein P is LN Is the equivalent noise of light, R λ Is the response coefficient of the photodiode at the wavelength lambda, e.g. 0.5A/W at 800 nm. The mean Stokes light power through the sample is 40mW, 0.5nJ per pulse, repetition frequency is 80MHz, then the equivalent current intensity (I) of the probe light L ) Is 20 mA. The Stokes light adopts laser directly output by an ultrafast laser, and the typical value of equivalent power noise at 1MHz is I L 10 (c) -7 Then I LN (1MHz)＝2nA/Hz 1/2 This is I N Is the main source of (1). If the measurement bandwidth of the lock-in amplifier is 1Hz, the sensitivity (corresponding to the case where the signal-to-noise ratio is 1: 1) is I N /I L ＝1×10 -7 . Under low power stokes light conditions, the signal-to-noise ratio gradually decreases.
In this example, the equivalent current noise under balanced probing conditions is given by:
wherein, I P1N And I P2N Respectively, the equivalent current noise of the two photodetectors, and CMRR is the common mode rejection ratio of the two photodiodes. The CMRR can be typically made to 40-60dB (corresponding to 100-. Also estimated using the above conditions, the optical noise is greatly reduced due to the balanced detection. Finally can calculate I N /I L ＝1×10 -9 Or 1X 10 -10 (for CMRR of 40 and 60) the signal-to-noise ratio is improved by 2-3 orders of magnitude. Thus, in this example, if our sensitivity requirement is 1 × 10 -7 Our stability requirements for the first laser source 1 can be reduced to 1 x 10 -4 And 1X 10 -5 . This laser stabilization allows the use of supercontinuum white light or optical parametric amplifiersExpensive lasers such as optical parametric oscillators are not required.
The first focusing lens 15 and the second focusing lens 16 can realize an imaging function if both objective lenses are adopted.
Example 2
As shown in fig. 2, the structure of the present embodiment substantially corresponds to that of embodiment 1. Example 1 measures the enhancement of the stokes light, while example 2 measures the attenuation of the pump light. Only the differences will be described in detail below.
In this embodiment, the purpose is to achieve the sensitivity of the conventional stimulated raman spectrometer, but the first laser source 1 only needs to use a cheap and simple-structured super-continuous white light source or an optical parametric amplifier. The light emitted by the first laser source 1 is split into a part of light by the half-wave plate 8 and the polarization beam splitter 9, and the part of light is detected by the first photodetector 6. The laser emitted by the second laser source 2 is modulated by the chopper 7, and the modulated laser pulse sequence is output, wherein the frequency is f, and the optical filter 5 filters out the wavelength of the Stokes light without filtering out the pump light. The rest is the same as in example 1. According to the analysis of embodiment 1, the most dominant noise source is optical noise. In embodiment 2, by the balanced detection, the optical noise of the laser source 1 can be reduced according to the value of the CMRR, so that the equivalent current noise of the optical noise can be reduced by 100-times and 1000-times, and the sensitivity of 1 × 10 can be realized -6 To 1X 10 -7 The cost of the whole set of the instrument is greatly reduced.
Example 3
As shown in fig. 3, the structure of the present embodiment is substantially the same as that of embodiment 1. Only the differences will be described in detail below.
The difference between this embodiment and embodiment 1 is mainly the difference between the two optical paths measured in the balanced detection structure. The light emitted by the second laser source 2 is split into a part of light by the half-wave plate 8 and the first polarization beam splitter 9, and the part of light passes through an optical path and then is combined with the other beam by the second polarization beam splitter 17. The combined light beams are spatially overlapped, but temporally one before and one after, and then are combined with the light beams emitted from the first laser source 1 through the beam combining mirror 3. The combined light passes through a first focusing lens 15, a sample 11 and a second focusing lens 16, and the light emitted by the first laser source 1 on the sample 11 is only time-overlapped with one of the two beams of light emitted by the second laser source 2, so as to generate a stimulated Raman effect. Through the third polarizing beam splitter 18, the stokes light generating the stimulated raman effect is detected by the first photodetector 6, and the other light not subjected to the stokes light enhancement is detected by the second photodetector 10. The first photodetector 6 and the second photodetector 10 need to ensure that the detected optical distances are the same or close to each other, so as to ensure a good balanced detection effect. The biggest advantage of this embodiment is that only the light emitted from the first laser source 1 needs to be blocked, and the light intensity entering the first photodetector 6 and the second photodetector 10 is the same by adjusting the half-wave plate 8, so as to realize balanced detection. This technique is particularly suitable for stimulated raman microscopy imaging. No matter whether the laser power emitted by the second laser source 2 is stable or not, the scattering, transmission and absorption of the sample to the light can be changed due to the position change of the laser irradiating the sample, and the balance can be always ensured because the light paths through which the two balanced paths of light pass are always consistent.
Claims (6)
1. A high sensitivity stimulated raman spectrometer, comprising:
a first laser source (1) that emits pulsed laser light as pump light in a stimulated raman spectrum;
a second laser source (2) that emits pulsed laser light as stokes light in the stimulated raman spectrum;
the laser of the first laser source (1) and the laser of the second laser source (2) are combined through the beam combining mirror (3) and pass through the sample (4);
the laser light passing through the sample (4) passes through the filter (5) to filter the wavelength of the pump light, and is detected by the first photoelectric detector (6);
one of the light paths from the first laser source (1) and the second laser source (2) to the beam combiner (3) is modulated by a chopper (7) to output a modulated laser pulse sequence, the other light path sequentially passes through a half-wave plate (8) and a first polarization beam splitter (9), an extra light path is separated and detected by a second photoelectric detector (10), and the light intensity ratio of the extra light path entering the sample (4) and the extra light path entering the second photoelectric detector (10) is adjusted by rotating the half-wave plate (8);
transimpedance amplifier (11), lock-in amplifier (12), impulse generator (13) and chopper drive power supply (14), first photoelectric detector (6) and second photoelectric detector (10) syntropy are parallelly connected, are connected with transimpedance amplifier (11), transimpedance amplifier (11) output voltage signal gets into lock-in amplifier (12), chopper drive power supply (14) drive chopper (7), impulse generator (13) trigger lock-in amplifier (12) and chopper drive power supply (14).
2. The highly sensitive stimulated raman spectrometer of claim 1, wherein: in the light paths from the first laser source (1) to the beam combiner (3) and the second laser source (2), an extra light path separated by the half-wave plate (8) and the first polarization beam splitter (9) is combined with the original light beam by the second polarization beam splitter (17), and the extra light path and the original light beam pass through the beam combiner (3) in time in tandem, are combined with the light beam of the other laser source and pass through the sample (4), and are detected by the second photoelectric detector (10) after passing through the third polarization beam splitter (18).
3. The highly sensitive stimulated raman spectrometer of claim 1 or 2, wherein: the sample (4) both sides are equipped with first focusing mirror (15) and second focusing mirror (16) respectively, loop through first focusing mirror (15), sample (4) and second focusing mirror (16) after the laser of first laser source (1) and second laser source (2) closes.
4. The highly sensitive stimulated raman spectrometer of claim 1 or 2, wherein: the first photodetector (6) and the second photodetector (10) are photodiodes and avalanche photodiodes with low equivalent current noise.
5. The highly sensitive stimulated raman spectrometer of claim 1 or 2, wherein: the bandwidth of the trans-impedance amplifier (11) is matched with the laser modulation frequency output by the chopper (7).
6. The highly sensitive stimulated raman spectrometer of claim 1 or 2, wherein: during measurement, the laser source of the light path where the chopper (7) is located is blocked, the half-wave plate (8) is adjusted, photoelectric signals received by the first photoelectric detector (6) and the second photoelectric detector (10) are balanced, signals obtained by the phase-locked amplifier (12) are enabled to be as small as possible, and a balanced detection structure is formed.
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