CN114839174A - Time-resolved Raman spectrometer based on super-continuum spectrum laser - Google Patents

Abstract

The invention discloses a time-resolved Raman spectrometer based on a supercontinuum laser, which comprises a pulse laser system, an optical light splitting system, a signal acquisition system and a data processing system. The method takes a super-continuum spectrum laser as a laser source, takes an off-chip digital delay unit as time gating, takes a Single Photon Avalanche Diode (SPAD) array with time resolution capability as a Raman signal detector, records time domain information of Raman spectrum through an on-chip time-to-digital converter (TDC) integrated on a CMOS SPAD line sensor, a Field Programmable Gate Array (FPGA) control circuit acquires and processes signals of the CMOS SPAD line sensor, and finally performs data processing such as dark counting removal and the like on a computer to obtain the time-resolved Raman spectrum with high fluorescence inhibition. The invention adopts a mode of combining the supercontinuum laser and the monochromator, and utilizes the pulse lasers with different frequencies to irradiate the sample to be measured, so that richer Raman spectrums and more accurate sample component information can be obtained, and the method has good use value and application prospect.

Description

Time-resolved Raman spectrometer based on super-continuum spectrum laser

Technical Field

The invention relates to the technical field of Raman spectrometers, in particular to a time-resolved Raman spectrometer which selects multi-frequency excitation light by utilizing a supercontinuum laser and a monochromator.

Background

Raman spectroscopy is widely used in the food and petroleum industries, mining, medical diagnostics, pharmaceuticals, forensic medicine, and archaeology. In many applications, the biggest challenge facing raman spectroscopy is fluorescence. Time resolved pulsed raman spectroscopy is a desirable choice for fluorescence suppression because the raman signal is collected simultaneously with the laser pulse and photons emitted after the laser pulse can be suppressed. Pulsed excitation and time-gated measurements are used to reduce the number of detected fluorescence photons, resulting in efficient fluorescence and background suppression. Time-gated single photon avalanche diodes (CMOS SPADs) fabricated based on CMOS processes have established time-gated raman spectrometers. In order to achieve effective fluorescence suppression by time-gating, the pulse width of the excitation laser should be as short as possible compared to the fluorescence lifetime of the sample to be tested. The best signal-to-noise ratio is achieved when the gate width is 1-2 times the width of the raman photon distribution [ full width at half maximum (FWHM) ]. The exact optimal gate width depends on the properties of the sample (fluorescence level and lifetime) and the sensor [ Dark Count Rate (DCR), timing deviation ]. The time-resolved raman spectrometers on the market mostly use single-frequency lasers, cannot meet the raman spectrum measurement requirements of different samples, if the excitation wavelength can be selected, the selection can be used to set the raman peak to a wavelength range with the least fluorescence, and simultaneously, the pulse lasers with different frequencies are used to irradiate the same sample to obtain a more complete raman spectrum of the sample.

Disclosure of Invention

The invention discloses a time-resolved Raman spectrometer based on a supercontinuum laser, which is characterized in that a Raman peak can be set to be a wavelength range with minimum fluorescence by selecting different excitation wavelengths through the combination of the supercontinuum laser and a monochromator, pulsed laser with various frequencies is used for irradiating a sample, and a multiband excitation time-resolved Raman spectrum with high fluorescence inhibition and rich information is obtained by matching with the time gating capacity of a CMOS SPAD line sensor and the data processing of a computer.

The invention is realized by the following technical scheme: the time-resolved Raman spectrometer comprises a supercontinuum laser (1), a monochromator (2), a neutral density filter (3), a beam splitter (4), a photoelectric detector (10), a collimating lens (5), a dichroic mirror (6), a microscope objective (11), a sample to be detected (12), a long-pass filter (7), a focusing lens (8), a diffraction grating (9), an off-chip digital delay unit (13), a time-resolved line sensor (14) of a Complementary Metal Oxide Semiconductor (CMOS) avalanche single photon diode (SPAD) of an integrated on-chip time-to-digital converter (TDC), a Field Programmable Gate Array (FPGA) control circuit (15) and a computer (16); the method is characterized in that: the laser frequency of the supercontinuum laser (1) is adjusted by a monochromator (2), proper excitation light is selected to enter a beam splitter (4) after being subjected to light intensity attenuation through a neutral density filter (3), a small part of laser pulse energy is guided to a photoelectric detector (10) through the beam splitter (4), the photoelectric detector (10) generates a logic level trigger signal and transmits the logic level trigger signal to a CMOS SPAD line sensor (14) through a digital delay unit (13), most of the laser pulse energy is transmitted to a sample through a collimating lens (5), a dichroic mirror (6) and a microscope objective lens (11), Rayleigh scattering photons from the sample reach a diffraction grating (9) through a dichroic mirror (6) and a long-pass filter (7), the diffraction grating (9) disperses the scattering photons, and a Field Programmable Gate Array (FPGA) control circuit performs signal acquisition and processing on the CMOS SPAD line sensor, and the computer performs treatment such as dark mark removal and the like on the received data to obtain the time-resolved Raman spectrometer with high fluorescence inhibition. The problem that Raman information obtained by using one Raman spectrometer to measure a sample is not rich enough is solved.

The supercontinuum laser (1) can obtain a spectral range (0.4-2.4 um) wider than that of an adjustable laser, namely a visible region to a near infrared region, and a monochromator (2) is added behind the supercontinuum laser to perform Raman detection on a sample to be detected by using pulse laser with different frequencies, so that richer sample component information can be obtained.

The neutral density filter (3) attenuates the main laser beam to a power level suitable for the sample to reduce the damage of the exciting light to the sample.

The photodetector (10) will generate a logic level trigger signal that is passed through a digital delay unit (13) to the CMOS SPAD line sensor (14) to bias the SPADs into the geiger mode where they can detect single photons.

A time-to-digital converter (TDC) integrated on the CMOS SPAD line sensor (14) is used to measure the time of arrival of backscattered raman photons (reflection configuration with zero source-to-detector distance).

The digital delay unit (13) is used to compensate for the difference between the optical and electrical delays of the system, so that the SPAD is biased, i.e. time gated, only upon arrival of photons from the sample to collect all backscattered raman photons from the sample, but only a small fraction of the total fluorescence emission and other background radiation due to ambient light, resulting in efficient fluorescence and background suppression;

the dichroic mirror (6) is used for transmitting stokes, raman and fluorescence and reflecting most of the excitation light.

The microscope objective (11) is used for collecting Raman and fluorescence scattered at 180 degrees by the sample (12) and Rayleigh scattering excitation light.

The long-pass filter (7) is used for removing the exciting light left after dichroism.

The purpose of the diffraction grating (9) to disperse the scattered photons is to make the position of each SPAD in the line sensor correspond to a certain wavenumber range.

The Field Programmable Gate Array (FPGA) control circuit (14) is used for controlling the CMOS SPAD and reading data from the CMOS SPAD, and communication between a computer and the CMOS SPAD is realized.

A time-resolved raman spectrometer based on a supercontinuum laser,

compared with the prior art, the invention has the advantages that: the combination of the super-continuum spectrum laser and the monochromator can select multi-frequency excitation light, pulse laser with different frequencies can be selected for irradiating the same sample to be detected or Raman information detection requirements of different samples can be met, and based on the time gating capacity and good uniformity among channels of the CMOS SPAD line sensor, the data processing of a computer is combined, and finally, the multi-band excitation time-resolved Raman spectrum with high fluorescence suppression and rich information can be obtained. Has strong innovation and practical value and good application prospect.

Drawings

FIG. 1 is a schematic diagram of a structure of a supercontinuum laser based time resolved Raman spectrometer.

FIG. 2 is an optical path diagram of the principle of operation of a monochromator.

Detailed Description

As shown in FIG. 1, the time-resolved Raman spectrometer based on the supercontinuum laser adjusts the laser frequency of the supercontinuum laser (1) through the monochromator (2) to select proper excitation light to perform light intensity attenuation through the neutral density filter (3), so that the damage of the excitation light to a sample is reduced. A small fraction of the laser pulse energy is directed through a beam splitter (4) to a photodetector (10), the photodetector (10) generates a logic level trigger signal that is passed through a digital delay unit (13) to a SPAD array consisting of 16 columns and 256 rows (spectral points) to synchronize the electrical and optical signals of the system to achieve photon collection only during the time that the raman echo pulse from the sample reaches the detector. Most of the laser pulse energy passes through the collimator lens (5), dichroic mirror (6) and microscope objective lens (11) onto the sample, the same dichroic mirror (6) can transmit stokes, raman and fluorescence in the backscattered light from the sample and reflect most of the excitation light. Collected light reaches a diffraction grating through a long-pass filter (7) after dichroism is removed, residual exciting light (the filter only transmits the wavelength longer than the wavelength of laser light) reaches the diffraction grating, the diffraction grating (9) disperses scattered photons, the position of each SPAD in the line sensor corresponds to a certain wave number range, a 16 x 256CMOS SPAD line sensor (14) is provided with an integrated TDC (one channel for each spectral point) on a 256-channel 3-bit sheet and is used for measuring the arrival time of the backscattered Raman photons, a Field Programmable Gate Array (FPGA) control circuit (15) collects and processes signals of the 16 x 256CMOS SPAD line sensor (14), and a computer performs treatments such as dark memory number removal and the like on the received data to obtain a high-fluorescence-inhibition multiband-excited time-resolved Raman spectrum.

As shown in FIG. 2, the principle of operation of the plane grating monochromator is that the light from the supercontinuum laser is uniformly illuminated at the entrance slit S1, S1 being located at the focal plane of the off-axis parabolic mirror. The light irradiates the grating in parallel through a collimating mirror M1, is diffracted back to M1 through the grating, is converged to an S2 emergent slit through a reflecting mirror M2 after being reflected by M1, and finally irradiates a Single Photon Avalanche Diode (SPAD) array with time resolution capability. Due to the diffraction effect of the grating, the light coming out of the exit slit is monochromatic light. When the grating rotates, light coming out of the emergent slit sequentially appears from short wave to long wave.

Claims (5)

1. A time-resolved Raman spectrometer based on a super-continuum spectrum laser is composed of a pulse laser system, an optical light splitting system, a signal acquisition system and a data processing system. Wherein, pulsed laser system includes: the system comprises a supercontinuum laser (1), a monochromator (2) and a neutral density filter (3); the optical splitting system includes: a beam splitter (4) and a collimating lens (5); the signal acquisition system includes: the device comprises a dichroic mirror (6), a microscope objective lens (11), a sample to be detected (12), a long-pass filter (7), a focusing lens (8), a diffraction grating (9), an off-chip digital delay unit (13), and a time resolution line sensor (14) of a Complementary Metal Oxide Semiconductor (CMOS) Single Photon Avalanche Diode (SPAD) integrated with an on-chip time-to-digital converter (TDC); the data processing system includes: a Field Programmable Gate Array (FPGA) control circuit (15) and a computer (16).

2. The pulsed laser system of claim 1, wherein: the laser frequency of the supercontinuum laser (1) can be adjusted by the monochromator (2) according to different Raman spectrum measurement requirements, and the laser is incident to the beam splitter (4) after being subjected to light intensity attenuation by the neutral density filter (3).

3. The optical splitting system according to claim 1, wherein: a small portion of the laser pulse energy is directed to a photodetector (10) through a beam splitter, and the other large portion of the laser pulse energy is directed to a sample (12) through a collimating lens (5).

4. The signal acquisition system of claim 1, wherein: most of laser pulse energy reaches a sample (12) through a collimating lens (5), a dichroic mirror (6) and a microscope objective lens (11), Rayleigh scattered light from the sample (12) reaches a diffraction grating (9) through the dichroic mirror (6) and a long-pass filter (7), and the diffraction grating (9) disperses scattered photons and then reaches a CMOS SPAD line sensor (14) of a TDC on an integrated chip to acquire Raman signals.

5. The data processing system of claim 1, wherein: a Field Programmable Gate Array (FPGA) control circuit (15) collects and processes signals of the CMOS SPAD line sensor (14), and a computer (16) receives data and performs dark counting removal and other processing to obtain a multiband excitation time-resolved Raman spectrum with high fluorescence inhibition and rich information.

Images