Rotating mirror type Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling

Abstract

The invention discloses a rotating mirror type Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling. The rotating mirror type Fourier transform ultraviolet Raman spectrometer is mainly composed of a laser self-reflection optical system, a signal shaping circuit and a signal gating circuit. By designing a laser self-reflection optical system, excitation laser is reflected into a Michelson interferometer to serve as reference laser to realize equal optical path difference interval sampling; a signal shaping circuit and a signal gating circuit are designed, so that the frequency of a trigger signal is doubled, the Nyquist sampling theorem is met, and finally, an accurate recovered Raman spectrum is obtained. The rotating mirror type Fourier transform Raman spectrometer suitable for the ultraviolet band is simple in structure, small in size, low in cost and suitable for the ultraviolet band.

Description

Technical Field

[0001] The invention relates to the technical field of rotating mirror type Fourier transform spectrometers, and in particular to a rotating mirror type Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling. Background Art

[0002] Compared with the linear Fourier transform spectrometer and the swinging Fourier transform spectrometer, the rotating mirror Fourier transform spectrometer (Turbo FTS) has the advantage of high stability. However, the optical path difference generated by the rotating mirror of Turbo FTS during rotation is nonlinearly related to the angle of rotation. The uniform time sampling obtains the non-equal optical path difference interval sampling interference signal, and the spectral signal cannot be accurately restored directly through the fast Fourier transform (FFT). The reference laser method can achieve accurate equal optical path difference interval sampling, but in order to meet the Nyquist sampling theorem, the wavelength of the reference laser should be less than half of the detection wavelength. For the ultraviolet band (near 355 nm), it is difficult to find such a short wavelength laser (the shortest wavelength with mature technology is 190 nm). Even if there is such a laser, the volume will be very large and the price will be very high, which is not suitable as a reference laser.

[0003] Therefore, it is necessary to study a low-cost, small-volume reference laser sampling method suitable for the ultraviolet band, so that the rotating mirror Fourier transform spectrometer can achieve equal optical path difference interval (linear) sampling and obtain accurate sample Raman spectra. Summary of the invention

[0004] The invention provides a rotating mirror Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling. By designing a laser self-reflection optical system, an excitation laser is reflected into a Michelson interferometer as a reference laser. By designing a signal shaping circuit and a signal gating circuit, the rising edge and the falling edge of the voltage waveform of the interference signal of the reflected laser are detected and a trigger sampling signal is output, thereby realizing equal optical path difference interval sampling of the interference signal of the Raman scattered light and satisfying the Nyquist sampling theorem, thereby accurately restoring the Raman spectrum of the sample.

[0005] The specific technical solution adopted by the present invention is as follows: A rotating mirror Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling is mainly composed of a laser self-reflection optical system, a signal shaping circuit and a signal gating circuit. The laser self-reflection optical system generates and detects the interference signal of the self-reflected laser, and uses a plane mirror to reflect the laser transmitted from the Raman filter, which is reflected by the same Raman filter into the Michelson interferometer, and the interference signal is reflected by another Raman filter into the photomultiplier tube for detection; The signal shaping circuit converts the interference signal voltage waveform output by the photomultiplier tube into a square wave voltage waveform for subsequent detection of rising and falling edges; if the interference signal voltage value is lower than the detection limit, no square wave voltage waveform is output; The signal selection circuit detects the square wave voltage waveform; when the square wave voltage waveform is detected, its rising edge and falling edge are detected, and a narrow rectangular pulse is output when the rising edge and falling edge arrive to trigger the acquisition card to sample; when the square wave voltage waveform is not detected, a low-frequency narrow rectangular pulse is output.

[0006] Optionally, the laser self-reflection optical system includes a laser, a Raman filter A, a collimating lens, a reflector C, a cubic beam splitter, a reflector A, an end mirror A, a reflector B, an end mirror B, a rotating mirror, a Raman filter B, a photomultiplier tube B, a long-wave pass edge filter, a focusing lens and a photomultiplier tube A.

[0007] Beneficial effects of the present invention: The present invention designs a laser self-reflection optical system to use part of the excitation laser as a reference laser to generate a trigger signal, thereby realizing equal optical path difference interval sampling; designs a signal shaping circuit and a signal gating circuit to double the trigger signal frequency, satisfy the Nyquist sampling theorem, and finally obtain an accurate restored Raman spectrum. The present invention provides a rotating mirror Fourier transform Raman spectrometer suitable for ultraviolet bands with a simple structure, small size, and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A schematic diagram of a laser self-reflection optical system according to an embodiment of the present invention; Figure 2 A diagram showing the working process of a signal gating circuit in one embodiment of the present invention; Figure 3 is an interference signal diagram of CCL4 in one embodiment of the present invention; Figure 4 FIG. 4 is a Raman spectrum of CCL44 restored in one embodiment of the present invention. DETAILED DESCRIPTION

[0009] The present invention will be described in detail below with reference to the embodiments and the accompanying drawings, but the present invention is not limited thereto.

[0010] The schematic diagram of a rotating mirror Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling in this embodiment is mainly composed of a laser self-reflection optical system, a signal shaping circuit and a signal gating circuit.

[0011] The laser self-reflection optical system generates and detects the interference signal of the self-reflected laser. A plane mirror is used to reflect the laser transmitted from the Raman filter, which is reflected by the same Raman filter into the Michelson interferometer. The interference signal is reflected by another Raman filter into the photomultiplier tube for detection.

[0012] Laser self-reflective optical system Figure 1 As shown, it includes a laser, a Raman filter A, a collimating lens, a reflector C, a cubic beam splitter, a reflector A, an end mirror A, a reflector B, an end mirror B, a rotating mirror, a Raman filter B, a photomultiplier tube B, a long-wave pass edge filter, a focusing lens and a photomultiplier tube A. According to the direction of the self-reflected laser, the order of its optical path structure composition and spatial positioning is as follows: laser → Raman filter A → reflector C → Raman filter A → cubic beam splitter, and the reflected laser is divided into two paths by the cubic beam splitter. The direction of one laser path is: cubic beam splitter → reflector A → rotating mirror → end mirror A → rotating mirror → reflector A → cubic beam splitter. The direction of the other laser path is: cubic beam splitter → reflector B → rotating mirror → end mirror B → rotating mirror → reflector B → cubic beam splitter. The two laser beams interfere at the cubic beam splitter, and the interference signal enters the photomultiplier tube B after being reflected by the Raman filter B. According to the direction of the sample Raman scattered light, the order of its optical path structure and spatial positioning is as follows: laser → Raman filter A → collimating lens → sample → collimating lens → Raman filter A → cubic beam splitter. The Raman scattered light is divided into two paths by the cubic beam splitter, and the direction of one laser beam is: cubic beam splitter → reflector A → rotating mirror → end mirror A → rotating mirror → reflector A → cubic beam splitter. The direction of the other laser beam is: cubic beam splitter → reflector B → rotating mirror → end mirror B → rotating mirror → reflector B → cubic beam splitter. The two laser beams interfere at the cubic beam splitter, and the interference signal passes through the Raman filter B and the long-wave pass edge filter in turn, and then converges to the photomultiplier tube A by the focusing lens.

[0013] The signal shaping circuit converts the interference signal voltage waveform output by the photomultiplier tube into a square wave voltage waveform for subsequent detection of the rising and falling edges; if the interference signal voltage value is lower than the detection limit, no square wave voltage waveform is output.

[0014] The voltage waveform output by the photomultiplier tube is similar to a sine curve. A threshold value (±0.3 V) is set. When the interference signal voltage exceeds 0.3 V, a 5 V voltage is output. When the interference signal voltage is lower than -0.3 V, a -5 V voltage is output.

[0015] The signal gating circuit detects the square wave voltage waveform. When the square wave voltage waveform is detected, its rising edge and falling edge are detected, and a narrow rectangular pulse is output when the rising edge and falling edge arrive to trigger the acquisition card to sample; when the square wave voltage waveform is not detected, a low-frequency narrow rectangular pulse is output.

[0016] In a Fourier transform infrared absorption spectrometer, the wavelength of the detection light (near infrared to far infrared) is usually greater than twice that of the reference laser (532 nm or 632.8 nm). The spectrum can be restored by simply detecting the rising edge or falling edge of the reference laser interference signal and triggering sampling when it arrives. In this case, the reference laser originates from the reflected partial excitation laser, and its wavelength is slightly smaller than the wavelength of Raman scattered light. Therefore, it is necessary to simultaneously detect the rising edge and falling edge of the reference laser interference signal and trigger sampling when it arrives.

[0017] The reason why a low-frequency narrow rectangular pulse is output when no square wave voltage waveform is detected is that when the acquisition card works in the external control sampling mode, once the control signal is interrupted, the sampling process stops.

[0018] The core of the signal gating circuit is an FPGA chip. The principle of detecting the rising and falling edges of the square wave voltage waveform is as follows: if more than 6 voltage values ​​are detected to increase or decrease continuously, it is considered that the rising and falling edges have arrived. When the rising or falling edge is detected, a narrow rectangular pulse is output when the rising and falling edges arrive to trigger the acquisition card to sample; when the square wave voltage waveform is not detected, a low-frequency narrow rectangular pulse is output. Figure 2 The working process diagram of the signal gating circuit is given. The Raman scattered light interference signal of CCL4 collected with a narrow rectangular pulse as the trigger signal is shown in Figure 3 As shown, the Raman spectrum of the restored CCL4 is as follows Figure 4 shown.

[0019] The above description is only an example of a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A rotating mirror Fourier transform ultraviolet Raman spectrometer based on self-reflection laser sampling, mainly composed of a laser self-reflection optical system, a signal shaping circuit and a signal gating circuit, characterized in that: The laser self-reflection optical system generates and detects the interference signal of the self-reflected laser, and uses a plane mirror to reflect the laser transmitted from the Raman filter, which is reflected by the same Raman filter into the Michelson interferometer, and the interference signal is reflected by another Raman filter into the photomultiplier tube for detection; The signal shaping circuit converts the interference signal voltage waveform output by the photomultiplier tube into a square wave voltage waveform for subsequent detection of rising and falling edges; if the interference signal voltage value is lower than the detection limit, no square wave voltage waveform is output; The signal selection circuit detects the square wave voltage waveform; when the square wave voltage waveform is detected, its rising edge and falling edge are detected, and a narrow rectangular pulse is output when the rising edge and falling edge arrive to trigger the acquisition card to sample; when the square wave voltage waveform is not detected, a low-frequency narrow rectangular pulse is output.

2. The spectrometer according to claim 1, characterized in that The laser self-reflection optical system comprises a laser, a Raman filter A, a collimating lens, a reflector C, a cubic beam splitter, a reflector A, an end mirror A, a reflector B, an end mirror B, a rotating mirror, a Raman filter B, a photomultiplier tube B, a long-wave pass edge filter, a focusing lens and a photomultiplier tube A.

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