Method for rapidly turning off pulsed light cavity ring-down system light source based on frequency shift keying

Abstract

The invention relates to a method for rapidly turning off a pulsed light cavity ring-down system light source based on frequency shift keying, belongs to the technical field of cavity ring-down spectroscopy, and aims to couple laser generated by a laser light source into a ring-down cavity and realize the function of rapidly and conveniently turning off an optical signal coupled into the ring-down cavity by utilizing the frequency selectivity of the ring-down cavity and combining the frequency shift keying technology. An oscilloscope is used for observing and recording a ring-down signal, when a detection signal reaches a set trigger threshold value, a positive pulse trigger signal is output to a signal generator to start frequency shift keying, and the frequency of incident light is modulated, so that a light source coupled into a ring-down cavity is cut off. According to the invention, an extra circuit control plate is not needed to turn off the current of the laser driver, and a solution is provided for rapidly turning off the laser source in the pulsed cavity ring-down technology.

Description

[0001] Technology Neighborhood

[0002] The invention relates to the technical field of cavity ring-down spectroscopy, and in particular to a method for quickly shutting down a light source of a pulsed optical cavity ring-down system based on frequency shift keying. Background Art

[0003] Cavity ring-down technology is a laser direct absorption spectroscopy technology with ultra-high sensitivity, high resolution, strong selectivity and fast response. In recent years, cavity ring-down technology has benefited from the rapid development of laser light sources and has been widely used in atmospheric environment monitoring, chemical gas analysis, atmospheric sensing, trace material detection and isotope detection. The cavity ring-down system is mainly composed of a laser source, a ring-down cavity, a detector and other parts, which need to be subjected to a series of modulation, signal acquisition and data processing analysis. The optical ring-down cavity of the traditional spatial cavity ring-down system is composed of two or more high-reflection mirrors (generally, the reflectivity of the high-reflection mirror is R ≥ 0.9999). The laser light source is coupled to the optical ring-down cavity through one of the high-reflection mirrors by mode matching to achieve coherent growth, which increases the absorption optical path. When the detected light signal reaches a certain trigger threshold, the laser source must be quickly shut down to prevent subsequent light pulse signals from continuously entering the ring-down cavity and affecting the formation and acquisition of the ring-down signal. In addition, there is also a fiber loop ring-down system with fiber waveguide as the medium, which uses fiber optic devices such as fiber couplers and fiber gratings with high splitting ratios to replace high reflective mirrors to form a ring-down cavity, and couples light into the fiber waveguide for resonance. Compared with the spatial cavity ring-down system, the fiber loop ring-down system with optical waveguide as the medium is more suitable for the sensing and detection of liquid and gas concentrations in complex environments due to its strong anti-interference ability, safety and reliability, corrosion resistance, and simple structure.

[0004] Whether it is the traditional spatial optical cavity ring-down technology or the fiber loop cavity ring-down technology, both involve the need to quickly shut down the optical signal that occasionally enters the resonant cavity to obtain the accurate ring-down time in order to fit the precise ring-down time. At present, the main methods for quickly shutting down the optical signal are electrical shutdown, that is, the laser driver current is quickly shut down to zero or reduced to below the laser current threshold through internal circuit board modulation, so that the laser cannot generate laser light; the invention patent of Chinese patent application No. 202410350736.5 "An optical cavity ring-down control system based on electrical shutdown technology of constant current circuit" uses an electrical shutdown control unit of a triode constant current circuit to shut down the laser beam and obtain the ring-down event; but the electrical shutdown method requires the design of the corresponding circuit control module, and the switching time is easily limited by the response speed of the electronic device. The invention patents of Chinese patent application No. 200610165082.0 “Method for measuring reflectivity of high reflective mirror” and the invention patents of Chinese patent application No. 200710098755.X “Method for measuring high reflectivity based on self-mixing effect of semiconductor laser” couple continuous laser light into a spatial ring-down cavity composed of high reflective mirrors along the optical axis of the ring-down cavity, modulate the excitation voltage of the semiconductor laser by a square wave, and use the falling edge of the square wave to shut down the laser output. This requires fine adjustment of the ring-down cavity mirrors and places high demands on the collimation of the system. The high requirements of the spatial optical cavity system on precision and collimation are not conducive to the application and promotion of cavity ring-down technology in complex natural environments or underwater environments. Summary of the invention

[0005] The present invention provides a method for quickly shutting off the light source of a pulsed optical cavity ring-down system based on frequency shift keying, which can achieve the effect of a convenient, simple and high-speed optical switch in the pulsed optical cavity ring-down system. The structure is simple and no additional circuit control panel is required to shut off the optical path.

[0006] A method for quickly shutting down a light source of a pulsed optical cavity ring-down system based on frequency shift keying, wherein the optical cavity ring-down system comprises a light source, an electro-optical intensity modulator, a signal generator, an optical fiber loop ring-down cavity unit, a photodetector, and an oscilloscope;

[0007] The output end of the electro-optical intensity modulator enters the optical fiber loop ring-down unit. The photodetector converts the optical signal in the optical fiber loop ring-down unit into an electrical signal and transmits it to the oscilloscope. The oscilloscope records the detected ring-down signal in real time. When the trigger threshold is reached, the oscilloscope outputs a trigger signal to the signal generator. The signal generator starts frequency shift keying as an external signal source to quickly shut down the light source.

[0008] Furthermore, the light source is emitted by a narrow linewidth laser source, which includes a laser diode and a laser driver. The laser driver includes a driving current control module and an operating temperature control module, which respectively control the driving current and adjust the operating temperature. The laser driver is used for the laser diode to output laser of suitable wavelength. The laser emitted by the narrow linewidth laser source is transmitted to the polarization controller through a single-mode optical fiber.

[0009] Furthermore, the polarization controller is used to adjust the polarization state of the polarization state incident light, and the polarization state incident light can be changed into any polarization state after being adjusted by the polarization controller.

[0010] Furthermore, it also includes an electro-optical intensity modulator, which modulates a continuous light laser source into a pulsed laser source by adjusting the bias voltage and the carrier waveform, and the signal generator provides a carrier signal for the electro-optical intensity modulator.

[0011] Furthermore, the fiber loop ring-down cavity unit uses two fiber couplers, which are connected to each other to form a ring-down cavity, and utilizes its frequency selectivity, that is, only light of a specific frequency will form a stable coherent superposition in the cavity. The light energy enhanced by resonance can pass through a high-reflection mirror to be detected by a photodetector as a signal.

[0012] Furthermore, the signal generator is provided with frequency shift keying, and a carrier frequency and a frequency hopping frequency are preset, wherein the carrier frequency is the normally-on frequency of the modulation signal, and the frequency hopping frequency is the frequency at which the signal generator starts the frequency shift keying after receiving an external signal. The carrier frequency can only enter the resonance of the ring-down cavity when it matches the characteristic frequency of the ring-down cavity, and the frequency hopping frequency cannot enter the resonance of the ring-down cavity if it does not match the characteristic frequency of the ring-down cavity.

[0013] Furthermore, the output end of the electro-optic modulator couples the modulated light into the optical fiber loop ring-down cavity through a single-mode optical fiber, and then transmits the ring-down optical signal to the photodetector at the other end of the ring-down cavity.

[0014] Furthermore, the oscilloscope supports a trigger output function, and when a single trigger reaches a set threshold, a positive pulse trigger signal is output to the signal generator to start frequency shift keying modulation, thereby realizing rapid shutdown of the light source.

[0015] The beneficial effects of the present invention are:

[0016] The present invention provides a method for quickly shutting off the light source of a pulsed optical cavity ring-down system based on frequency shift keying, which can achieve the effect of a convenient, simple and high-speed optical switch in the pulsed optical cavity ring-down system. The structure is simple and no additional circuit control panel is required to shut off the optical path. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1It is a structural schematic diagram of a light source of a pulse optical cavity ring-down system based on frequency shift keying rapid shutdown in an embodiment of the present invention;

[0018] Figure 2 This is a schematic diagram of the related pins of the electro-optical intensity modulator;

[0019] Figure 3 This is a signal schematic diagram of a pulse light incidentally entering a ring-down cavity and the light source is not turned off;

[0020] Figure 4 It is a signal schematic diagram of quickly shutting down a light source based on frequency shift keying according to the present invention. DETAILED DESCRIPTION

[0021] The following is a detailed analysis of a specific implementation case of the present invention, which includes detailed operating steps:

[0022] The laser light source needs to be a narrow linewidth laser to meet the requirements of resonant cavity frequency selection. The half-width of the characteristic frequency of the ring-down cavity built in this example is 6kHz, and the linewidth of the narrow linewidth laser used is ≤5kHz. The butterfly laser diode is installed on the laser driver, and then the laser driver is powered on. The laser driver includes a drive current module and a temperature control module. To turn on the laser, the temperature control module must be set and started first, and then the laser drive current module is set; the current drive range is 0-600mA, and the temperature control range is 5-50℃; adjust the drive current size and operating temperature of the laser driver so that the laser diode outputs a laser of a suitable wavelength, which depends on the absorption wavelength of the target gas or liquid to be detected or sensed, and adjust the operating wavelength to the absorption range of the target substance.

[0023] Subsequently, the light source is connected to the polarization controller through a single-mode optical fiber. A three-ring polarization controller is used in the embodiment of the present invention. By winding a single-mode optical fiber around three circular paddles and applying pressure to induce stress birefringence, the controller can be equivalent to an independent wave plate, which is equivalent to λ / 4, λ / 2, and λ / 4 wave plates, respectively. By adjusting the three circular paddles, the polarization state of the incident light can be flexibly converted to any desired polarization state. When adjusting the polarization controller, an optical power meter is connected to the light output port of the electro-optical intensity modulator, and the power meter reading is observed in real time until the power of the transmitted light is adjusted to the maximum, thus completing the adjustment of the polarization controller.

[0024] The pin diagram of the electro-optic modulator is as follows: Figure 2As shown, pins 1 and 2 are the cathode and anode of the photodiode, pin 3 is connected to a DC bias voltage, and pin 4 is grounded; pin 3 is powered by a DC regulated power supply, and pin 5 is a radio frequency signal interface, which is connected to a signal generator through an SMP-BNC data connection line to receive a carrier signal from the signal generator. When looking for the half-wave voltage of the electro-optic modulator, the output signal of the electro-optic modulator is connected to an oscilloscope to observe the modulated signal on the oscilloscope, and the carrier waveform of the signal generator is set to a pulse wave, and the output voltage of the DC regulated power supply is slowly adjusted from small to large until the waveform set by the signal generator is observed on the oscilloscope.

[0025] The signal generator has a sampling rate of 250MHz / s, a waveform repetition rate of 125MH, a resolution of 14 bits, and a memory depth of 16k points. It can realize the output of pulse waves and supports frequency shift keying function. The specific setting steps of the signal generator are as follows: first, the carrier waveform is set to a pulse wave, and then the carrier frequency is set to the characteristic frequency of the ring-down cavity, which can be obtained through theoretical calculation, and then the pulse width is set to 10ns. This value is the lower limit of the pulse width produced by the oscilloscope used in this example. The pulse width setting principle is: pulse width < single round-trip time of the ring-down cavity; then the modulation mode is set to frequency shift keying, and a frequency hopping frequency is set, and then the trigger source of the frequency shift keying is selected as an external source trigger, and the external trigger port of the signal generator is connected to the trigger output port of the oscilloscope through a BNC-BNC data connection line to receive the trigger pulse from the oscilloscope; the output port of the electro-optical modulator is connected to the optical fiber ring-down cavity.

[0026] Two 1×2 couplers with a splitting ratio of 99.9%:0.1% are connected to form a ring-down cavity. The specific connection steps of the fiber ring-down cavity are as follows: Figure 1 As shown in the figure, the output end of the electro-optic modulator is connected to the Port3 port of coupler 1, the Port2 port of coupler 1 is connected to the Port2 port of coupler 2, and the Port1 port of coupler 1 is connected to the Port1 port of coupler 2. Finally, the ring-down optical signal is transmitted to the photodetector through the Port3 port of coupler 2. The photodetector used in this example has a bandwidth of 125 MHz, a response speed of 8 ns, and a photoelectric conversion gain of 2.4×10 4 V / W, the lower the bandwidth of the photodetector, the higher the responsiveness, that is, it can better detect weak signals; then it is connected to an oscilloscope to display and record the detected signal in real time.

[0027] The bandwidth of the oscilloscope used in this example is 3GHz, the maximum real-time sampling rate is 10GSa / s, and the vertical resolution is dynamically adjustable from 9 to 12 bits. The specific setting steps of the oscilloscope are as follows: adjust the horizontal time base knob of the oscilloscope to set the appropriate time window, and then set the corresponding trigger signal source. Rotate the trigger level knob to adjust the trigger threshold. Set the trigger type to rising edge trigger, and set the trigger mode to single trigger. In this trigger mode, the oscilloscope triggers and collects once only when the specified trigger condition is searched, and then stops. This trigger mode is suitable for situations where only a single acquisition of a specific event is required and the acquisition results are analyzed. It is especially suitable for the acquisition of ring-down events. For the setting of the trigger output connector, select "System" in "Auxiliary Function" and then select "AUX Output". Select "Pass Fail". In the pass / fail test, when a test pass or fail event is detected, that is, a ring-down event, a pulse signal will be output from the rear panel [TRIG OUT] connector. This pulse signal is used as an external signal source of the signal generator to start frequency shift keying and complete the rapid shutdown of the pulse light source.

[0028] After all components are connected, adjust the carrier frequency of the signal generator slightly to find the frequency that can resonate in the ring-down cavity, such as Figure 3 As shown in the figure, it shows the resonant signal detected by the photodetector. At this time, the frequency shift keying is not turned on, that is, the light source is not turned off; then the frequency shift keying mode of the signal generator is turned on, and the following is obtained: Figure 4 The ring-down signal after a fast light source is shown.

[0029] In summary, the above implementation examples prove that the method of quickly shutting down the light source in the cavity ring-down technology based on frequency shift keying can quickly and conveniently cut off the optical signal accidentally entering the ring-down cavity by combining the frequency shift keying technology.

[0030] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for quickly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying, characterized in that: The optical cavity ring-down system includes a light source, an electro-optical intensity modulator, a signal generator, an optical fiber loop ring-down cavity unit, a photodetector, and an oscilloscope; The output end of the electro-optical intensity modulator enters the optical fiber loop ring-down unit. The photodetector converts the optical signal in the optical fiber loop ring-down unit into an electrical signal and transmits it to the oscilloscope. The oscilloscope records the detected ring-down signal in real time. When the trigger threshold is reached, the oscilloscope outputs a trigger signal to the signal generator. The signal generator starts frequency shift keying as an external signal source to quickly shut down the light source.

2. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 1, characterized in that: The light source is emitted by a narrow linewidth laser source, which includes a laser diode and a laser driver. The laser driver includes a driving current control module and an operating temperature control module, which respectively control the driving current and adjust the operating temperature. The laser driver is used for the laser diode to output laser light of a suitable wavelength. The laser emitted by the narrow linewidth laser source is transmitted to the polarization controller through a single-mode optical fiber.

3. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 2, characterized in that: The polarization controller is used to adjust the polarization state of the polarization state incident light. The polarization state incident light can be changed into any polarization state after being adjusted by the polarization controller.

4. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 1, characterized in that: It also includes an electro-optical intensity modulator, which modulates a continuous light laser source into a pulsed laser source by adjusting the bias voltage and the carrier waveform, and the signal generator provides a carrier signal for the electro-optical intensity modulator.

5. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 1, characterized in that: The fiber loop ring-down cavity unit uses two fiber couplers, which are connected to each other to form a ring-down cavity. The frequency selectivity is utilized, that is, only light of a specific frequency will form a stable coherent superposition in the cavity. The light energy enhanced by resonance can pass through a high-reflection mirror and be used to be detected by a photodetector as a signal.

6. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 4, characterized in that: The signal generator is set with frequency shift keying, and a carrier frequency and a frequency hopping frequency are preset, wherein the carrier frequency is the normally-on frequency of the modulation signal, and the frequency hopping frequency is the frequency at which the signal generator starts the frequency shift keying after receiving an external signal. The carrier frequency can only enter the resonance of the ring-down cavity when it matches the characteristic frequency of the ring-down cavity, while the frequency hopping frequency does not match the characteristic frequency of the ring-down cavity and cannot enter the resonance of the ring-down cavity.

7. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 1, characterized in that: The output end of the electro-optic modulator couples the modulated light into the optical fiber loop ring-down cavity through a single-mode optical fiber, and then transmits the ring-down optical signal to the photoelectric detector at the other end of the ring-down cavity.

8. The method for rapidly shutting down the light source of a pulsed optical cavity ring-down system based on frequency shift keying according to claim 1, characterized in that: The oscilloscope supports a trigger output function. When a single trigger reaches a set threshold, a positive pulse trigger signal is output to the signal generator to start frequency shift keying modulation, thereby realizing rapid shutdown of the light source.

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