CN101865727A - Single picosecond pulse signal-to-noise ratio measuring instrument - Google Patents

Abstract

A single picosecond pulse signal-to-noise ratio measuring instrument is based on the cross-correlation measurement scheme of an optical imaging system, and comprises a frequency doubling crystal, a harmonic separation reflector, a fundamental frequency reflector, a frequency doubling reflector, a sum frequency crystal, an optical imaging system, a high dynamic range detector, and a data acquisition and processing system. The present invention can achieve high dynamic range signal-to-noise ratio measurement of a single picosecond pulse, with a time measurement range of 150ps, a time resolution of 3ps, and a dynamic range of 10 ⁸ . High dynamic range signal-to-noise ratio measurement within a range of 150ps can be achieved.

Description

Single picosecond pulse signal-to-noise ratio measuring instrument

technical field

The invention relates to laser parameter diagnosis, and is a single picosecond pulse signal-to-noise ratio measuring instrument.

Background technique

The signal-to-noise ratio of ultrashort pulses is an important parameter in ultrashort and ultraintense laser devices. With the development of laser technology, the focusable power density of the laser focal spot can reach 10 ²⁰ W/cm ² or higher, and its noise floor also increases. When the focused spot is used in related physical experiments such as plasma physics, laser-matter interaction, etc., it is hoped that the focused power density of the background noise is less than 10 ¹² W/cm ² , so as to avoid generating plasma by noise pulses and affecting the subsequent main laser. Interaction of pulses with matter. Therefore, physical experiments require that the ratio of the power density (intensity) of the main pulse of the ultrashort and ultra-intense laser to the noise pulse 10 ps before it is greater than 10 ⁸ or higher. This parameter is defined as the signal-to-noise ratio of ultrashort pulses.

At present, there is no suitable means to realize the precise measurement of this parameter. In 1995, RAGaneev proposed the autocorrelation method to realize the ultra-short pulse time waveform measurement of 0.2-50 ps (Optics Communications, Vol.114, 1995, 432-434), and M.Raghuramaiah perfected the autocorrelation method in 2001 to measure Theoretical Analysis Method of Short Pulse Time Waveform (SADHANA-ACADEMYPROCEEDINGS IN ENGINEERING SCIENCES, Vol.26, 2001, 603~611). J. Collier proposed a signal-to-noise ratio measurement scheme based on the cross-correlation principle in 2001 (LASER AND PARTICLEBEAMS, Vol.19, 2001, 231-235). The angle between the cross-correlation beams is 20°, and the dynamic range of the measurement results is 10 ⁵ , the time range is 30ps. Dongfang Zhang proposed a signal-to-noise ratio measurement scheme based on the cross-correlation method in 2008 (Optics Letters, Vol.33, 2008, 1969-1971), in which a photomultiplier tube was used as a detection device to make the dynamic range of the measurement results ~10 ⁷ can be achieved. Since the length of the crystal used in this scheme is 6 mm, the angle between the two beams where cross-correlation occurs is 3°, so the time measurement range is only ±1 ps. In order to realize the SNR measurement before the main pulse of the ultrashort pulse and beyond 10 ps, the present invention proposes a cross-correlation measurement scheme based on an optical imaging system to meet the high dynamic range SNR measurement requirements of a single picosecond pulse .

Contents of the invention

The problem to be solved by the present invention is to provide a single picosecond pulse signal-to-noise ratio measuring instrument, the maximum measurable pulse time range of the signal-to-noise ratio measuring instrument is 150ps, the resolution is 0.2ps, and the dynamic range is ~10 ⁸ .

The single picosecond pulse signal-to-noise ratio measuring instrument of the present invention is based on the principle of cross-correlation. Its characteristic is to obtain large-diameter cross-correlation signals by increasing the size of the measured pulse and double frequency crystals, sum frequency crystals, and reflectors. Then, the cross-correlation signal is precisely imaged onto a small-aperture high dynamic range detector through an optical imaging system to realize the collection and processing of the cross-correlation signal.

Technical solution of the present invention is:

A single picosecond pulse signal-to-noise ratio measuring instrument is characterized in that its composition includes a double frequency crystal, a harmonic separation mirror, a fundamental frequency mirror, a double frequency mirror, a sum frequency crystal, an optical imaging system, a high dynamic The detectors and data acquisition and processing system of the scope, the positional relationship of each component is as follows:

The incident picosecond laser pulse to be measured first generates a double frequency pulse through the double frequency crystal, and the double frequency pulse and the remaining fundamental frequency pulse of the laser pulse are incident on the harmonic separation reflection On the mirror, the harmonic separation mirror separates the fundamental frequency pulse and the double frequency pulse: after the fundamental frequency pulse is reflected by the harmonic separation mirror, it is guided by the fundamental frequency mirror to the sum frequency crystal; after the double frequency pulse passes through the harmonic separation reflector, it is directed to the sum frequency crystal by the double frequency reflector, and the fundamental frequency pulse and the double frequency pulse are A cross-correlation signal of a single pulse is generated on the sum-frequency crystal, and the cross-correlation signal is imaged on the high dynamic range detector by the optical imaging system, and finally processed by the data acquisition and processing system Data acquisition and processing to achieve high dynamic range signal-to-noise ratio measurement of a single picosecond pulse.

The magnification of the optical imaging system is ≤1/10 times.

The high dynamic range detector is composed of two sets of optical fiber array receivers and photomultiplier tubes, and the number of optical fibers in each set of optical fiber array receivers is ≥ 61, so the total number of optical fibers in the two sets of optical fiber array receivers is ≥ 122.

Technical effect of the present invention is:

The invention adopts the optical imaging method and solves the contradiction between the large-diameter cross-correlation signal and the small-sized high dynamic range detector. An optical imaging system is used in the measurement device to image the spatial distribution of large-aperture cross-correlation signals onto the receiving surface of a small-sized high-dynamic-range detector, thereby achieving a single picosecond pulse high-dynamic range of ~150ps Signal-to-noise ratio measurements of the range with a temporal resolution of 3 ps and a dynamic range of 10 ⁵ .

In the scheme of realizing ultrashort pulse signal-to-noise ratio measurement based on the cross-correlation method, the time measurement range is proportional to the beam width in the cross-correlation process. In order to increase the time measurement range, a large-aperture beam must be used, as well as double frequency crystals, sum frequency crystals, harmonic separation mirrors, fundamental frequency mirrors, and double frequency mirrors of corresponding sizes. The measured beam width corresponding to the time measurement range of 150ps in the present invention is 127mm, the diameter of the double frequency crystal and the sum frequency crystal used in the measurement optical path should be 150mm, and the diameter of the fundamental frequency reflector and the double frequency reflector should be It is 210mm. The width of the cross-correlation signal obtained after passing through the sum frequency crystal 8 is 129mm, and there is no suitable high dynamic range photodetector capable of receiving the signal at present.

The signal-to-noise ratio measurement device proposed by Dongfang Zhang in 2008 uses photomultiplier tubes and fiber array receivers as photodetectors. The end face length of the fiber array receiver is 7.75mm, and its dynamic range can reach ~10 ⁷ The range is only ±1ps. Photomultiplier tubes have high sensitivity. In 2009, we reported the signal-to-noise ratio measurement results of ultrashort pulses with a repetition rate of 10 ⁸ dynamic range in "China Laser". The cross-correlation measuring instrument Sequoia of French Amplitude Company can realize the dynamic range measuring result of 10 ¹⁰ . Therefore, the use of photomultiplier tubes in signal-to-noise ratio measurement can currently achieve a dynamic range measurement result of up to 10 ¹⁰ .

After we pass the cross-correlation signal through the optical imaging system with a magnification of 1/10 times, the width of the image of the cross-correlation signal is 12.9 mm. The sum of the end face lengths of the two groups of optical fiber array receivers is 15.5 mm, which is greater than 12.9 mm. Therefore, the data acquisition and processing system (19) composed of two sets of optical fiber array receivers and photomultiplier tubes can be used for receiving and processing. The number of optical fibers of each group of optical fiber array receivers is 61, so two groups of optical fiber array receivers have 122 optical fibers, and the corresponding number of optical fibers for the 12.9mm related signal is 122×12.9mm/15.5mm=102 (rounded), each The time range corresponding to the optical fiber is 150ps/102=1.47ps. According to the Nyquist sampling law, the corresponding time resolution is 1.47ps×2=3ps (rounded up).

To sum up, the present invention can realize the high dynamic range signal-to-noise ratio measurement of a single picosecond pulse, the time measurement range is 150 ps, the time resolution is 3 ps, and the dynamic range is 10 ⁸ .

Description of drawings

Fig. 1 is the structural diagram of single picosecond pulse signal-to-noise ratio measuring instrument of the present invention;

Figure 2 is an analysis of the generation process of cross-correlation signals.

Detailed ways

The present invention will be further described below in conjunction with the embodiments and accompanying drawings, but the protection scope of the present invention should not be limited thereby.

Please refer to FIG. 1 first. FIG. 1 is a schematic structural diagram of a single picosecond pulse signal-to-noise ratio measuring instrument of the present invention. As can be seen from the figure, the single picosecond pulse signal-to-noise ratio measuring instrument of the present invention comprises a double frequency crystal 2, a harmonic separation reflector 3, a fundamental frequency reflector 5, a double frequency reflector 7, a sum frequency crystal 8, Optical imaging system 10, high dynamic range detector 11 and data acquisition and processing system 19, the positional relationship of each component is as follows:

The incident picosecond level laser pulse 1 to be measured first generates a double frequency pulse 6 through the double frequency crystal 2, and the double frequency pulse 6 and the remaining fundamental frequency pulse 4 of the laser pulse 1 are incident on the On the harmonic separation reflector 3 described above, the harmonic separation reflector 3 separates the fundamental frequency pulse 4 and the double frequency pulse 6: after the described fundamental frequency pulse 4 is reflected by the harmonic separation reflector 3, the The base frequency reflector 5 guides the sum frequency crystal 8; after the double frequency pulse 6 passes through the harmonic separation reflector 3, it is guided by the double frequency reflector 7 to the The sum-frequency crystal 8, the base-frequency pulse 4 and the double-frequency pulse 6 generate a single-pulse cross-correlation signal 9 on the sum-frequency crystal 8, and the cross-correlation signal 9 passes through the optical imaging system 10 is imaged on the high dynamic range detector 11, and finally the data collection and processing system 19 performs data collection and processing to realize the high dynamic range signal-to-noise ratio measurement of a single picosecond pulse.

For the time-domain signals I(t) and I′(t) of two ultrashort pulses, the mathematical description of the cross-correlation process is

X(τ)=∫I(t)I'(t-τ)dt

where τ is the time delay between two pulses.

In the physical process of cross-correlation measurement of ultrashort pulses, there is only one incident pulse I(t), namely the fundamental frequency pulse. Another pulse I'(t) is generated by I(t) through the double frequency crystal, that is, the double frequency pulse, expressed as

I'(t)=I _2ω (t)

Therefore, the cross-correlation signal can be expressed as:

X(τ)＝∫I(t)I _2ω (t-τ)dt

In the cross-correlation method measurement of a single pulse, the time delay τ is realized by using the oblique intersection of two wide beams to generate an optical path delay.

In the present invention, when the time at which the center position 13 of the fundamental frequency pulse reaches the sum frequency crystal 8 is the same as that at the center position 16 of the double frequency pulse, the time at which the upper edge 15 of the double frequency pulse arrives at the sum frequency crystal 8 will lag behind the base frequency pulse The upper edge 12 and the lower edge 17 of the double frequency pulse will reach the sum frequency crystal 8 earlier than the lower edge 14 of the fundamental frequency pulse.

On the surface of the sum frequency crystal 8, the distance between the upper edge 12 of the fundamental frequency pulse and the central position 13 of the fundamental frequency is:

Δ _{12, 13} = D _beam /2cosΦ

Among them, D _beam is the beam width, and 2Φ is the angle between the fundamental frequency pulse and the double frequency pulse.

The time advance of the base frequency pulse upper edge 12 arriving at the sum frequency crystal 8 relative to the base frequency pulse center position 13 is:

δ _12,13 =Δ _12,13 sinΦ/c=D _beam tanΦ/2c

Similarly, the time delay of the upper edge 15 of the double frequency pulse reaching the sum frequency crystal 8 relative to the center position 16 of the double frequency pulse is:

δ _{15, 16} = D _beam tanΦ/2c

Therefore, the double frequency is used as a scanning pulse, and the time delay between its upper edge 15 and the upper edge 12 of the fundamental frequency pulse is:

δ _{15, 12} ＝+D _beam tanΦ/c

Similarly, the time delay between the lower edge 17 of the double frequency pulse and the lower edge 14 of the fundamental frequency pulse is:

δ _{17, 14} = -D _beam tanΦ/c

Therefore, in the present invention, the value range of the time delay τ of the autocorrelation process is: (-D _beam tanΦ/c, +D _beam tanΦ/c).

Considering that the frequency doubling efficiency is less than 10%, the full width at half maximum (FWHM) of the double frequency pulse is smaller than the full width at half maximum (FWHM) of the fundamental frequency pulse. Therefore, the time waveform of the cross-correlation signal is the time waveform of the measured pulse, and the measurable time range of the present invention is 2D _beam tanΦ/c.

In order to measure a laser pulse with a time range of τ, the corresponding beam width of the measured pulse is:

${\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; beam</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&tau;c</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; the\; tan</span>}\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}<span\; class="notranslate">\; ,</span>$

Wherein, c is the speed of light, 2Φ is the angle between the fundamental frequency pulse and the double frequency pulse, and in the present invention, Φ=15°. When τ=150ps, the corresponding measured pulse width is D _beam =127mm. Further, the spatial width D _X of the cross-correlation signal 9 generated after the sum frequency crystal 8 is: D _beam /cosΦ=129mm. The relationship between the spatial width D _X of the cross-correlation signal and the time range τ is

${\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&tau;c</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}$

After passing through the optical imaging system 10 with M=1/10 in this embodiment, an image 18 of the cross-correlation signal generated on the sum frequency crystal 8 can be obtained. The spatial width D′ _X of the image 18 of the cross-correlation signal is 12.9 mm, which is still equivalent to a time delay range of 150 ps. In the image 18 of the cross-correlation signal, the relationship between the spatial width D′ _X and the time measurement range τ is:

${{\mathrm{<span\; class="notranslate">\; D.</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; M\&tau;c</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}$

In this embodiment, the image width of the cross-correlation signal corresponding to 150 ps is 12.9 mm, which is received and processed by the high dynamic range detector 11 composed of two groups of optical fiber array receivers and photomultiplier tubes, and the receiving end face is 15.5 mm. The unit is 122 fibers. The number of optical fibers corresponding to the 12.9mm cross-correlation signal is 102 (rounded up), and the time range corresponding to each optical fiber is 150ps/102=1.47ps. According to the Nyquist sampling law, the corresponding time resolution is 1.47ps×2=3ps (rounded up). The dynamic range is 10 ⁸ .

Claims (3)

1. single picosecond pulse signal-to-noise ratio measuring instrument, be characterised in that its formation comprises the detector (11) and the data acquisition processing system (19) of two frequency-doubling crystals (2) harmonic separation catoptrons (3), fundamental frequency catoptron (5), two frequency multiplication catoptrons (7) and frequency crystal (8), optical imaging system (10), high dynamic range, the position relation of each element is as follows:

The picosecond magnitude laser pulse (1) to be measured of incident, at first produce two double frequency pulses (6) by described two frequency-doubling crystals (2), the remaining fundamental frequency pulse (4) of this two double frequency pulse (6) and laser pulse (1) is incided on the described harmonic separation catoptron (3), this harmonic separation catoptron (3) separates fundamental frequency pulse (4) with two double frequency pulses (6): described fundamental frequency pulse (4) is led described and frequency crystal (8) by described fundamental frequency catoptron (5) after harmonic separation catoptron (3) reflection; Described two double frequency pulses (6) see through described harmonic separation catoptron (3) afterwards, described and the frequency crystal (8) by described two frequency multiplication catoptrons (7) guiding, described fundamental frequency pulse (4) and two double frequency pulses (6) are gone up the cross-correlated signal (9) that produces single pulse at described and frequency crystal (8), this cross-correlated signal (9) is imaged on the detector (11) of described high dynamic range through described optical imaging system (10), carry out data acquisition process by described data acquisition processing system (19) at last, realize the high dynamic range snr measurement of single picosecond pulse.

2. single picosecond pulse signal-to-noise ratio measuring instrument according to claim 1 is characterized in that magnification≤1/10 times of described optical imaging system (10).

3. single picosecond pulse signal-to-noise ratio measuring instrument according to claim 1, the detector (11) that it is characterized in that described high dynamic range is made up of two groups of fiber array receivers and photomultiplier, therefore the number of fibers of every group of fiber array receiver 〉=61, two groups of total fiber count of fiber array receiver 〉=122.

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