CN101126676B - Method for time sequence optimization of switching laser beam for high reflectivity measurement - Google Patents

Abstract

The invention discloses a time sequence optimization method of a switching laser beam for high reflectivity measurement, and belongs to the technical field of measurement of parameters of optical elements. The optimization of the switching moment of the laser beam is realized by two ways: optimizing the square wave modulation frequency and the duty ratio to enable the average amplitude of the optical cavity output signal of the square wave falling edge to be maximum; when the amplitude of the output signal of the optical cavity is larger than the set threshold value, the laser beam is turned off by the threshold value trigger circuit, and the laser beam is turned on again at the rising edge of the next square wave period. The invention improves the signal-to-noise ratio of ring-down signals by optimizing the switching time of the laser beam, thereby improving the measurement precision of high reflectivity.

Description

Timing Optimization Method for Switching Laser Beams for High Reflectivity Measurements

technical field

The invention relates to a method for measuring optical element parameters, in particular to a timing optimization method for switching laser beams for high reflectivity measurement.

Background technique

The wide use of high reflectivity optical components in laser systems urgently requires accurate measurement of high reflectivity, and traditional methods can no longer meet the measurement accuracy requirements of high reflectivity. Chinese Patent Application No. 98114152.8, Publication No. CN1242516A, the patent of invention published on January 26, 2000 discloses "a method for measuring the high reflectivity of mirrors", using a pulsed laser system as the light source, incident on two high-reflective mirrors The formed optical resonant cavity receives the exponential ring-down signal of the optical cavity, respectively determines the ring-down time τ- of the straight cavity and the ring-down time τ< of the folded cavity, and calculates the reflectivity R of the mirror to be tested. The disadvantages of this method are: due to the poor quality of the pulsed laser beam and the mode competition in the ring-down cavity, the measurement accuracy is limited; moreover, the high cost of the used pulsed laser system is not conducive to popularization and use. Chinese Patent Application No. 200610011254.9, Publication No. CN1804572A, Publication Date July 19, 2006 The purpose of the invention patent application provides "a method for measuring the reflectivity of a high mirror"; "China Laser" published in September 2006, Gong Yuan, Li Bincheng, Volume 33, No. 9, pages 1247-1250, discloses a method of "accurate measurement of high reflectance by continuous laser cavity ring-down method". The rate measurement method uses a square wave to modulate a continuous laser, and uses a phase-locked method to detect the amplitude ringing and phase delay of the output signal, so as to obtain the optical cavity ringing time and high mirror reflectivity. In this method, the coupling efficiency of the laser power to the ring-down cavity is not high. Chinese Patent Application No. 200610165082.0, Publication No. CN1963435A, Invention Patent Publication Date May 16, 2007 "High Mirror Reflectivity Measurement Method" and Chinese Patent Application No. 200710098755.X Invention Patent Application "Based on Semiconductor Laser Self-mixing The high reflectivity measurement method of the effect "uses simple mechanical devices or optical components to control the intensity of the backward feedback light reflected from the first cavity mirror back to the laser, so that the output spectral characteristics of the semiconductor laser are changed, and the laser beam is greatly improved. The efficiency of coupling into the ring-down optical cavity causes a peak signal with a large amplitude to appear in the output signal of the optical cavity. This peak signal is used to obtain the exponential ring-down signal, and is fitted to obtain the reflectivity of the cavity mirror and the test mirror. The method can measure high reflectance with high precision, and has low cost and simple device. However, although the position of the spike in a cycle is relatively stable, the specific position of the peak has certain random characteristics, and the amplitude fluctuates greatly, which reduces the measurement accuracy of the signal-to-noise ratio and high reflectivity of the ring-down signal.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a timing optimization method for switching on and off laser beams for high reflectivity measurement. The laser beam is interrupted and the exponential decay signal is recorded, thereby improving the signal-to-noise ratio of the ring-down signal and further improving the measurement accuracy of high reflectivity.

Technical problem solving of the present invention: the method for timing optimization of switching laser beams for high reflectivity measurement is characterized in that it is realized by one of the following schemes:

在此基础上，优化方波调制频率f和/或占空比P使光腔输出信号最大振幅处于方波下降沿，从而关断激光束，同时触发采集指数衰减信号，经过时间Δt₂后，由下一个周期的方波上升沿重新开启激光束，重复上述过程实现衰荡信号和高反射率的重复测量。(1) Use the square wave output by the function generator or function generator card to modulate the excitation current or voltage of the semiconductor laser, and the rising and falling edges of the square wave correspond to turning on or off the semiconductor laser, respectively. Determine the time interval between the moment when the maximum amplitude of the optical cavity output signal appears and the rising edge of the square wave

On this basis, optimize the square wave modulation frequency f and/or duty cycle P so that the maximum amplitude of the output signal of the optical cavity is at the falling edge of the square wave, thereby turning off the laser beam and triggering the acquisition of the exponential decay signal at the same time. After time Δt ₂ , The laser beam is turned on again from the rising edge of the square wave in the next period, and the above process is repeated to realize the repeated measurement of the ring-down signal and high reflectivity.

(2) The laser beam is modulated by the square wave output by the threshold trigger circuit, and the rising and falling edges of the square wave correspond to turning on or off the laser beam, respectively. When the output signal amplitude of the optical cavity is greater than the preset threshold, the threshold trigger circuit quickly shuts off its output current to form a falling edge to turn off the laser beam, and at the same _time triggers the collection of exponentially decaying signals. The laser beam is turned on again at the rising edge of the wave, and the above process is repeated to realize the repeated measurement of the ring-down signal and high reflectivity;

The scheme (1) and scheme (2) switch the laser beam through one of the following ways:

(1) Use a square wave to modulate the excitation current or voltage of the semiconductor laser to turn on or off the laser beam;

(2) The laser itself is not modulated, and the laser beam is continuously output. An electro-optic or acousto-optic switch is inserted between the laser and the ring-down cavity, and the excitation current or voltage of the optical switch is modulated with a square wave to turn on or off the laser beam.

The initial value range of the optimized front wave modulation frequency f in the scheme (1) and scheme (2) is 1 Hz-1 MHz.

The initial value of the duty ratio P before optimization in the scheme (1) and scheme (2) ranges from 10000:1 to 1:100.

为多个Δt₁的统计平均值，Δt₁表示一个周期内最大振幅出现的时刻与本周期方波上升沿之间的时间间隔。In the scheme (1)

It is the statistical average value of multiple Δt ₁ , and Δt ₁ represents the time interval between the moment when the maximum amplitude appears in a cycle and the rising edge of the square wave in this cycle.

The scheme (1) realizes the timing optimization of switching the laser beam by controlling the modulation frequency f and/or the duty cycle P, and there are three forms to realize the optimization of the modulation frequency and the duty cycle:

(1) The modulation frequency f remains unchanged, and the duty cycle P becomes $p=\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2,$ in $\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2=T-\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1},$ T=1/f;

(2) The modulation frequency f becomes $f=P/\left[(1+P)\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}\right],$ The duty cycle P remains unchanged, and the initial value of the duty cycle satisfies $\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/10000\mathrm{\&tau;}<P<\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/5\mathrm{\&tau;};$

(3) The modulation frequency f becomes $f=1/(\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}+\mathrm{\&Delta;}{t}_2),$ The duty cycle P becomes $P=\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2,$ Where Δt ₂ is an integer multiple of the ring-down time τ, that is, Δt ₂ =kτ, τ is roughly determined by the ring-down signal without timing optimization, and the value of k depends on the specific situation, and the range is 5<k<10000.

The threshold in the scheme (2) is set by the threshold trigger circuit, which is adjustable between 5mV and 5V. The initial value is the maximum peak value of the optical cavity output signal. If the set threshold cannot trigger the laser beam to be turned off within a certain period of time, then The threshold is automatically adjusted down slightly until it can be triggered.

The square wave in the scheme (2) is generated by the threshold trigger circuit itself, or generated by a function generator or a function generating card, and then input to the threshold trigger circuit and output after being converted by the threshold trigger circuit.

_Δt in the scheme (2) is determined in the following two ways:

(1) The square wave modulation frequency remains unchanged, and Δt ₃ of each cycle is determined by the time interval between the trigger moment and the rising edge of the next square wave cycle, and Δt ₃ is changed by the change of Δt ₁ when the laser beam is triggered off Passive change, ie Δt ₃ =1/f-Δt ₁ .

(2) Δt ₃ remains unchanged, and its value is an integer multiple of the ring-down time τ, that is, Δt ₃ =kτ, τ is roughly determined by the ring-down signal without timing optimization, and the value of k depends on the specific situation, and the range is 5 <k<10000. At this time, the modulation frequency of the square wave is passively changed due to the change of the time Δt ₁ when the laser beam is triggered to be turned off, that is, f=1/(Δt ₃ +Δt ₁ ).

Compared with the prior art, the present invention has the following advantages:

(1) Turn off the laser beam and record the exponential decay signal at the moment when the maximum peak frequency of the output signal of the optical cavity is the highest, which improves the signal-to-noise ratio of the ring-down signal and is conducive to further improving the measurement accuracy of high reflectivity.

(2) Method (1) does not require a trigger circuit, and the device is simpler than method (2), which can effectively reduce system cost.

Description of drawings

1 is a schematic diagram of an embodiment of a straight cavity high reflectivity measuring device based on a function generation card in the present invention;

Fig. 2 is the input square wave (a) and optical cavity output signal (b) without timing optimization of the present invention and the input square wave (c) and optical cavity output signal (d) after timing optimization according to scheme (1);

Fig. 3 is a schematic diagram of the configuration of the folding cavity of the present invention;

Fig. 4 is the optical cavity ring-down signal and fitting curve detected after timing optimization of the present invention;

5 is a schematic diagram of a straight cavity high reflectivity measuring device based on a threshold trigger circuit in the present invention;

Fig. 6 is the input square wave (a) without timing optimization of the present invention and the input square wave (b) and optical cavity output signal (c) after timing optimization according to scheme (2);

Detailed ways

Example 1:

Embodiment 1 illustrates in detail the timing optimization scheme (1) for turning on and turning off the laser beam of the present invention. FIG. 1 is a schematic diagram of a straight-cavity high-reflectivity measurement device based on a function generation card according to the present invention. As shown in Figure 1, the measurement device consists of a function generator card 1, a light source 2, a spatial filter and telescopic system 3, a plano-concave mirror 4, 5, a lens 6, a detector 7, a data acquisition card 8 and a computer 9. The thick line in the figure indicates the optical path, and the thin line indicates the connection of the signal line.

The function generator card 1 outputs two channels of square wave signals. One channel of square wave signals is used to modulate the excitation current or voltage of the continuous semiconductor laser 2, so that the laser output power is modulated by a square wave. The rising edge and falling edge of the square wave correspond to turning on and off the laser beam respectively. . The other square wave signal is used to trigger the data acquisition card 8 to record the exponential decay signal after cutting off the laser beam at the falling edge of the square wave. The spatial filtering and telescopic system 3 is composed of two lenses and a pinhole, and is used to shape the laser beam output by the light source 2 into a fundamental mode and match it with the optical cavity mode. The spatial filtering and telescopic system 3 may not be used, that is, without The laser beam is directly coupled into the optical cavity through mode matching. Two identical plano-concave high-reflection mirrors 4 and 5 are coated with high-reflection film on their concave surfaces, the reflectivity is greater than 99%, and the concave surfaces relatively form a straight cavity. The laser beam is output after multiple reflections in the cavity, and is received by the detector 7 after being converged by the lens 6 . The detector 7 converts the optical signal into an electrical signal. At the same time as the falling edge of the square wave turns off the laser beam, the trigger signal triggers the data acquisition card 8 to record the exponential decay signal, and the computer 9 performs data processing and storage. Turn off the laser beam and turn on the laser output again after the interval time Δt ₂ . Repeat the above steps to achieve multiple repeated measurements of ringing signals and high reflectivity. The function generation card 1 and the data acquisition card 8 are directly controlled by the computer 9 through the PCI bus.

As shown in Figure 2(a), before timing optimization, the function generator card 1 outputs a square wave signal with a modulation frequency of f=1kHz, a duty ratio of 1:1, and a period of 1/f=1ms. The corresponding optical cavity output signal is shown in Fig. 2(b). The value range of the modulation frequency before timing optimization is 1Hz~1MHz, and the value range of the duty cycle P is 10000:1~1:100. The values of both are set at the same time so that the duration of the square wave low level is greater than 5 times the ringdown time . Although the ring-down signal can be recorded after the laser is turned off at the falling edge of the square wave, the amplitude of the ring-down signal at the falling edge is generally small and the signal-to-noise ratio is low due to the lack of timing optimization, which is not conducive to high-precision measurement of high reflectivity.

表示多个Δt₁的统计平均值，也是最大振幅出现频率最高的位置，用来优化调制频率和占空比。The position of the maximum amplitude in each period of the optical cavity ring-down signal is relatively stable, but it also has certain random characteristics. Use Δt ₁ to represent the time interval between the moment when the maximum amplitude appears in a cycle and the rising edge of the square wave in this cycle, and Δt ₁ in each cycle is slightly different. Figure 2(b) marks the position Δt ₁ where the maximum amplitude of one period occurs.

Indicates the statistical average value of multiple Δt _1s , which is also the position with the highest frequency of occurrence of the maximum amplitude, and is used to optimize the modulation frequency and duty cycle.

The timing optimization mode (1) of opening and closing the laser beam of the present invention realizes the timing optimization of switching the laser beam by controlling the modulation frequency f and the duty ratio P, and has the following three forms to realize the optimization of the modulation frequency and the duty ratio:

(1) The modulation frequency f remains unchanged, and the duty cycle P becomes $P=\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2,$ in $\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2=T-\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1},$ T=1/f;

(2) The modulation frequency f becomes $f=P/\left[(1+P)\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}\right],$ The duty cycle P remains unchanged, and the initial value of the duty cycle satisfies $\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/10000\mathrm{\&tau;}<P<\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/5\mathrm{\&tau;};$

(3) The modulation frequency f becomes $f=1/(\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}+\mathrm{\&Delta;}{t}_2),$ The duty cycle P becomes $P=\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}/\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2,$ Where Δt ₂ is an integer multiple of the ring-down time τ, that is, Δt ₂ =kτ, τ is roughly determined by the ring-down signal without timing optimization, and the value of k depends on the specific situation, and the range is 5<k<10000.

控制函数发生卡输出方波的调制频率使其变为 $f=1/2\stackrel{\&OverBar;}{\mathrm{\&Delta;}{t}_1}.$ 在此调制频率时，方波下降沿触发数据采集卡记录的衰荡信号平均振幅最大。由数据采集卡8记录的衰荡信号输入计算机9，拟合得到直腔衰荡时间τ₁，如图3所示。由R＝1-L/cτ₁计算得到腔镜反射率。插入待测平面镜10，形成折叠腔，如图4所示。重复上述过程，拟合得到折叠腔衰荡时间τ₂，由R_x＝exp(L/cτ₁-L/cτ₂)计算得到待测平面镜反射率。Embodiment 1 adopts the above-mentioned second optimization method of modulation frequency and duty ratio, and the duty ratio is 1:1, that is $\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00021.png"\; state="\{\{state\}\}">t</figure-callout>}}_2=\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1},$ As shown in Figure 2(c)(d). First detect the Δt ₁ of multiple cycles, and calculate its statistical average

The modulation frequency of the output square wave of the control function generation card makes it become $f=1/2\stackrel{\&OverBar;}{\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="S2007101224086E00011.png,S2007101224086E00031.png"\; state="\{\{state\}\}">t</figure-callout>}}_1}.$ At this modulation frequency, the average amplitude of the ring down signal recorded by the data acquisition card triggered by the falling edge of the square wave is the largest. The ring-down signal recorded by the data acquisition card 8 is input to the computer 9, and the ring-down time τ ₁ of the straight cavity is obtained by fitting, as shown in FIG. 3 . The cavity mirror reflectivity is calculated by R=1-L/cτ ₁ . Insert the plane mirror 10 to be tested to form a folding cavity, as shown in FIG. 4 . Repeating the above process, the ring-down time τ ₂ of the folded cavity is obtained by fitting, and the reflectivity of the plane mirror to be measured is calculated by R _x =exp(L/cτ ₁ -L/cτ ₂ ).

Example 2:

Embodiment 2 illustrates in detail the timing optimization scheme (2) for turning on and turning off the laser beam of the present invention. FIG. 5 is a schematic diagram of a straight cavity high reflectivity measuring device based on a threshold trigger circuit according to the present invention. As shown in Figure 5, the measurement device consists of a light source 2, a spatial filtering and telescopic system 3, a plano-concave mirror 4, 5, a lens 6, a detector 7, a data acquisition card 8, a computer 9 and a threshold trigger circuit 11. The thick line in the figure indicates the optical path, and the thin line indicates the connection of the signal line.

Embodiment 2 measuring device and Chinese patent application number 200610165082.0, publication number CN1963435A, the embodiment measuring device of " high mirror reflectance measuring method " disclosed in the invention patent on May 16th, 2007 of the publication date is identical, but the implementation of the present invention Example 2 focuses on illustrating the timing optimization method of switching laser beams based on threshold trigger circuits. The light source 2 is a continuous semiconductor laser. The spatial filtering and telescopic system 3 is composed of two lenses and a pinhole, and is used to shape the laser beam output by the light source 2 into a fundamental mode and match it with the optical cavity mode. The spatial filtering and telescopic system 3 may not be used, that is, without The laser beam is directly coupled into the optical cavity through mode matching. Two identical flat-concave high-reflection mirrors 4 and 5 are coated with a high-reflection film on their concave surfaces, and the reflectivity is greater than 99%. The concave surfaces face each other to form a straight cavity resonator. The laser beam is output after multiple reflections in the resonant cavity, and is received by the detector 7 after being converged by the lens 6 . The detector 7 converts the optical signal into an electrical signal, and outputs it to the data acquisition card 8 and the threshold trigger circuit 11 at the same time. The threshold trigger circuit 11 is used to compare the set threshold with the magnitude of the output signal amplitude of the optical cavity. When the output signal amplitude of the optical cavity is greater than the set threshold, the threshold trigger circuit 11 turns off the laser beam and at the same time triggers the data acquisition card 8 to record the exponential decay signal after turning off the laser beam. The data acquisition card 8 records the exponential decay signal and sends it to the computer 9 for processing. After turning off the laser beam for an interval of Δt ₃ , turn on the laser output again. Repeat the above steps to achieve multiple repeated measurements of ringing signals and high reflectivity.

In Embodiment 2, the threshold value trigger circuit 11 quickly shuts down the laser output by changing the excitation current or voltage of the semiconductor laser, thereby obtaining an exponential decay signal. An electro-optic or acousto-optic switch can also be inserted between the laser and the ring-down cavity, and the threshold trigger circuit controls the excitation current or voltage of the optical switch to turn on or off the laser beam.

In Embodiment 2, the square wave before the timing optimization is directly generated by the threshold trigger circuit, which is used to modulate the excitation current of the semiconductor laser, and the excitation current of the semiconductor laser is turned off when the output signal amplitude of the optical cavity is greater than the threshold. The square wave can also be generated by a function generator (or a function generator card) and input to the threshold trigger circuit, and then output by the threshold trigger circuit as the excitation current modulation signal of the semiconductor laser. The value range of the square wave modulation frequency is 1Hz～1MHz, and the value range of the duty cycle P is 10000:1～1:100. The values of both are set at the same time so that the duration of the square wave low level is greater than 5 times the ringdown time.

As shown in Figure 6(a), before timing optimization, the threshold trigger circuit outputs a square wave signal with a modulation frequency of f=1kHz, a duty ratio of 1:1, and a period of 1/f=1ms. A threshold trigger circuit sets the threshold to 153mV. When the amplitude of the output signal of the optical cavity exceeds the threshold when it is about 0.29 ms away from the rising edge, the threshold trigger circuit quickly shuts off the excitation current of the semiconductor laser, forming a falling edge of a square wave, as shown in Figure 6(b). After the falling edge turns off the laser beam, the cavity exponential decay signal is obtained, as shown in Figure 6(c). The dotted line in Figure 6(c) represents the set threshold. At the same time as the falling edge turns off the laser beam, the threshold trigger circuit triggers the data acquisition card to record the exponential decay signal, and sends it to the computer for fitting to obtain the ring-down time and reflectivity results. The processing of the attenuation signal and the calculation process of the reflectivity of the cavity mirror and the plane mirror to be measured are the same as those in Embodiment 1. After a period of time Δt ₃ , the rising edge of the next square wave period turns on the laser beam, and the above steps are repeated to realize repeated measurements of the ring-down signal and high reflectivity.

In Embodiment 2, the modulation frequency of the square wave remains unchanged, f=1kHz. Since the specific position where the peak appears in each period has certain random characteristics, the triggering time Δt ₁ of each period is different. Δt ₃ is determined by the time interval between the triggering moment and the rising edge of the next square wave cycle, so Δt ₃ changes passively with the change of Δt ₁ when the laser beam is triggered and turned off, that is, Δt ₃ =1/f-Δt ₁ . The following method can also be used: Δt ₃ remains unchanged, and the value is an integer multiple of the ring-down time τ, that is, Δt ₃ =kτ, τ is roughly determined by the ring-down signal without timing optimization, and the value of k depends on the specific situation. The range is 5<k<10000. At this time, the modulation frequency of the square wave is passively changed due to the change of the time Δt ₁ when the laser beam is triggered to be turned off, that is, f=1/(Δt ₃ +Δt ₁ ).

Claims (9)

1. the timing optimization method that is used for the laser beam switching of high reflection rate measurement, it is characterized in that: adopt function generator or function that the square-wave frequency modulation semiconductor laser of card output takes place, square wave rising edge and negative edge are corresponding respectively to be opened or the shutoff semiconductor laser, determines the moment of optical cavity output signal peak swing appearance and the time interval between the square wave rising edge

On this basis, optimize square-wave frequency modulation frequency f and dutycycle P or optimization square-wave frequency modulation frequency f or dutycycle P and make optical cavity output signal peak swing be in the square wave negative edge, thereby turn-off laser beam, while triggering collection exponential damping signal, elapsed time Δ t ₂After, open laser beam again by the square wave rising edge of next cycle, repeat said process and realize declining swinging the duplicate measurements of signal and high reflectance.

2. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 1 is characterized in that: described employing square-wave frequency modulation semiconductor laser is realized by following approach: exciting current or voltage with the square-wave frequency modulation semiconductor laser realize opening or turn-offing laser beam.

3. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 1, it is characterized in that: the initial value scope of described square-wave frequency modulation frequency f is 1Hz～1MHz, and the initial value scope of described dutycycle P is 10000: 1～1: 100.

4. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 1 is characterized in that: the moment that described optical cavity output signal peak swing occurs and the time interval between the square wave rising edge

Be a plurality of Δ t ₁Assembly average, Δ t ₁The moment that peak swing occurs in the expression one-period and the time interval between this cycle square wave rising edge.

5. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 1 is characterized in that: described optimization square-wave frequency modulation frequency f and dutycycle P or optimization square-wave frequency modulation frequency f or dutycycle P have one of following three kinds of forms to realize:

(1) modulating frequency f remains unchanged, and dutycycle P becomes

Wherein

T=1/f;

(2) modulating frequency f becomes

Dutycycle P remains unchanged, and wherein the initial value of dutycycle satisfies

(3) modulating frequency f becomes

Dutycycle P becomes

Δ t wherein ₂Be the integral multiple of ring-down time τ, i.e. Δ t ₂=k τ, τ does not swing signal and determines that roughly the k value is determined on a case-by-case basis by carrying out declining of timing optimization, and scope is 5＜k＜10000.

6. the timing optimization method that is used for the laser beam switching of high reflection rate measurement, it is characterized in that: by the square-wave frequency modulation laser beam of threshold triggers circuit output, the rising edge of square wave and negative edge are corresponding respectively to be opened or the shutoff laser beam, when optical cavity output amplitude during greater than pre-set threshold, the threshold triggers circuit turn-offs its output current fast and forms negative edge shutoff laser beam, while triggering collection exponential damping signal, elapsed time Δ t ₃After, open laser beam again by the square wave rising edge of next cycle, repeat said process and realize declining swinging the duplicate measurements of signal and high reflectance;

Described Δ t ₃Determine by following dual mode:

(1) the square-wave frequency modulation frequency remains unchanged, the Δ t in each cycle ₃The time interval that is carved into when triggering between the rising edge of next square-wave cycle is determined Δ t ₃Turn-off laser beam Δ t constantly because of triggering ₁Change and passive change, i.e. Δ t ₃=1/f-Δ t ₁

(2) Δ t ₃Remain unchanged, value is the integral multiple of ring-down time τ, i.e. Δ t ₃=k τ, τ does not swing signal and determines that roughly k value is determined on a case-by-case basis by carrying out declining of timing optimization, and scope is 5＜k＜10000, and this moment, the square-wave frequency modulation frequency was turn-offed laser beam moment Δ t because of triggering ₁Change and passive change, i.e. f=1/ (Δ t ₃+ Δ t ₁).

7. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 6 is characterized in that: the initial value scope of described square-wave frequency modulation frequency f is 1Hz～1MHz.

8. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 6, it is characterized in that: described threshold value is set by the threshold triggers circuit, adjustable between 5mV～5V, initial value is an optical cavity output signal peak-peak, can not trigger within a certain period of time as if the threshold value that is provided with and turn-off then automatic small size the downward modulation till triggering of threshold value of laser beam.

9. the timing optimization method that is used for the laser beam switching of high reflection rate measurement according to claim 6 is characterized in that: described square wave is generated by threshold triggers circuit itself or takes place to input to the threshold triggers circuit after card generates and by exporting after the threshold triggers circuit conversion by function generator or function.

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