CN103900724B - Precise calibrating method used for time resolution of single correlation measuring instrument - Google Patents

Abstract

A precision calibration method for the time resolution of a single-shot correlation measuring instrument. First, install the reticle into the first correlation beam or the second correlation beam so that it is perpendicular to the beam transmission direction; then measure the correlation beam through an angle ruler The included angle; finally, the projection of the reticle on the relevant signal is detected by the photodetector, and the time resolution is calculated. The method of the present invention is not sensitive to the pulse width of the calibration pulse, and can obtain a pulse with less error and higher accuracy when the pulse width of the calibration pulse is close to or greater than that of the autocorrelator, and the top changes gently, which makes it difficult to identify the pulse peak value of the correlation signal. time resolution.

Description

A Precise Calibration Method for Time Resolution of Single Shot Correlation Meter

technical field

The invention relates to a single autocorrelation and single cross-correlation instrument, in particular to a precise calibration method for the time resolution of a single correlation measuring instrument.

Background technique

There are two traditional methods for measuring the pulse width of laser pulses: an oscilloscope and a streak camera. When using a 16GHz high-speed oscilloscope for pulse width measurement, considering the bandwidth of the fast-response photocell, shielded cable, and corresponding interface circuit components, the rising edge response is 71.5ps, that is, the time resolution is greater than 71.5ps. When using a streak camera, its temporal resolution is 2.85ps. In order to be able to measure <10ps or even femtosecond-level pulse width, scientists have developed related measurement principles.

In 1995, R.A.Ganeev proposed the original scheme of autocorrelation method to realize ultrashort pulse time waveform measurement (see Optics Communications, Vol.114, 1995, 432-434). In 2001, M. Raghuramaiah perfected the theoretical analysis method of autocorrelation method to measure the pulse width of ultrashort pulse (see SADHANA-ACADEMYPROCEEDINGS IN ENGINEERING SCIENCES, Vol.26, 2001, 603-611). In 1995, L. Zheng realized the time waveform of the pulse through the cross-correlation method, including the pulse width and its front and rear edge resolution (see Optics Letters, Vol.20, 1995, 407-409). Compared with the autocorrelation process, the advantage of the cross-correlation process is that it can distinguish the front and rear edges of the pulse, but the disadvantage is that the cross-correlation conversion efficiency in the working medium is very low. In conventional pulse width measurement requirements, the autocorrelation method has been widely used.

Based on the above research basis, Coherent Inc. of the United States has developed a commercial femtosecond-level pulse width measuring instrument SSA-P, with a measurement range of 30-300 fs and a nominal resolution of 5 fs. Shanghai Institute of Optics and Fine Mechanics has developed a picosecond-level pulse width measuring instrument PsWidth20 with a measurement range of 1-18ps and a nominal resolution of 0.1ps.

In order to calibrate and verify the time resolution of the autocorrelation measuring instrument, I carried out the calibration of the picosecond-level pulse width measuring instrument based on the self-calibration method in 2012 (see "China Laser", Vol.39, 2012, 0408003-1～ 0408003-4).

The disadvantage of the self-calibration method is that due to the limitation of the pulse width of the pulse in the calibration experiment (hereinafter referred to as the calibration pulse), the obtained time resolution cannot be smaller than the pulse width of the calibration pulse. Moreover, when the pulse width of the calibration pulse is close to or greater than the measurement range of the autocorrelator, the top is relatively gentle, and the movement of the peak cannot be accurately observed, and accurate time resolution cannot be obtained. In order to make the time resolution smaller than the pulse width of the calibration pulse and achieve more precise and accurate pulse width measurement, it is necessary to consider a technical solution that is not sensitive to the pulse width of the calibration pulse.

Contents of the invention

The problem to be solved by the present invention is to provide a precise calibration method for the time resolution of a single correlation measuring instrument, to solve the deficiencies of the prior art, improve the time resolution in the correlation measurement principle, thereby improving the reliability of the measurement results and accuracy.

Technical solution of the present invention is:

A precise calibration method for time resolution of a single-shot correlation measuring instrument is characterized in that the method comprises the following steps:

①Install the reticle into the first correlation beam or the second correlation beam so that it is perpendicular to the beam transmission direction, and the size of the reticle is expressed as x;

② Make the detection surface of the photodetector parallel to the relevant working surface of the first correlation beam and the second correlation beam, and detect the projection y of the reticle on the correlation signal;

③ The angle between the first correlation beam and the second correlation beam is φ;

④ Calculate the time resolution ρ of a single autocorrelation or cross-correlation measuring instrument according to the following formula:

ρ=2z/cy=2x×tan(φ/2)/cy

where c is the speed of light.

The reticle is a transparent material with scales, a transparent material or an opaque material with fixed spacing and patterns.

The first related light beam and the second related light beam are: the reflected light and transmitted light of any percentage of the beam splitter, the signal and transmitted light generated by the reflected light of any percentage of the beam splitter passing through the nonlinear crystal, and the reflection of any percentage of the beam splitter Signals generated by light and transmitted light passing through a nonlinear crystal.

The related process working surface is the front surface of the nonlinear crystal, and the nonlinear crystal includes but not limited to BBO, LBO, KDP, PPLN.

The photodetector is CCD, photodiode or fast response photoelectric tube.

The gist of the above steps is:

(1) The reticle should be as perpendicular to the beam transmission direction as possible to reduce the length deviation of the projection of the reticle on the relevant signal caused by the angle deviation.

(2) Use high-precision angle rulers as much as possible to measure the angle of the relevant beams, such as combined angle rulers and universal angle rulers;

(3) In order to improve the detection accuracy of the photodetector, an optical imaging system is required to satisfy the object-image relationship between the working surface of the relevant process and the photodetector.

Technical effect of the present invention is as follows:

The sensitivity of the calibration process of the time resolution of the autocorrelator of the present invention to the pulse width of the calibration pulse is greatly reduced, that is, the time resolution can be obtained in real time under the continuous change of the pulse width, even in the pulse width When it is close to or greater than the measurement range of the autocorrelator.

The invention has an ideal application prospect in a chirped pulse laser system with functions of stretching and compressing. For example, for the pulse width adjustment range of 1-50 ps in a large picosecond petawatt laser system, the calibration experiment of time resolution can be completed under the condition of 20 ps or wider pulse. Therefore, reliable results of pulse width measurement can be provided in real time during the pulse width compression process, especially when the pulse width is in the range of 1-10 ps, it also has small errors and high reliability. It also has the same effect on femtosecond-level pulses with an adjustment range of 30fs-300fs.

Description of drawings

Fig. 1 is a schematic structure diagram of Embodiment 1 of the precise calibration method for time resolution of a single-shot correlation measuring instrument according to the present invention.

detailed description

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 Embodiment 1 of the precise calibration method for time resolution of a single-shot correlation measuring instrument according to the present invention. The first correlation beam 1 is represented by three thick solid lines in Figure 1, and the second correlation beam 2 is represented by three thick dashed lines in Figure 1, forming an angle φ between them, which is the correlation beam angle 3, and the correlation process The working surface 4 is the plane where the solid line AO is located. The intersection point of the centerline of the first correlation beam 1 and the centerline of the second correlation beam 2 on the correlation process working surface 4 is point O. The equi-phase plane 5 where the center of the first correlated beam 1 is located is perpendicular to its own propagation direction, and the equi-phase plane 6 where the center of the second correlated beam 2 is located is perpendicular to its own propagation direction. The reticle 7 used in the calibration experiment is located in the first correlation beam 1 . The projection 8 of the reticle 7 in the correlation signal, the correlation signal 9 and the photodetector 10 indicated by the dotted line are located behind the correlation process working surface 4 .

The first correlation beam 1 and the second correlation beam 2 used in the correlation process both have a certain width in the plane where the paper is located, and are a collection of countless thin beams. When the center of the first correlation beam 1 and the center of the second correlation beam 2 reach the correlation working surface 4 (the plane where AO is located) at the same time, the correlation action time at point O is the longest, which is the entire pulse width, so the best strong correlation signal. That is, the peak value of the correlation signal is located at point O.

Due to the existence of the angle 3 of the relevant beam, on the straight line AO, along the direction from point O to point A, the optical path L _1A of the thin beam in the first relevant beam 1 reaching the working surface 4 of the relevant process is gradually smaller than that at the center The optical path L _1O of the beamlets, the optical path L _2A of the beamlets in the second correlated beam 2 to the working surface 4 of the relevant process, is gradually greater than the optical path L _2O of the beamlets at the center. Taking point A as an example, the thin beam passing through point A in the first relevant light beam 1 has an optical path length -z shorter than the equiphase plane 5 where the thin beam passing through point O is located. Similarly, the thin beam passing through point A in the second correlated light beam 2 has an optical path length +z longer than that of the equiphase plane 6 where the thin beam passing through point O is located. That is, the following equation holds:

L _1O -z=L _1A (1)

L _2O +z=L _2A (2)

L _1O =L _2O (3)

In the self-calibration method, if the peak value of the correlation signal is to be moved from point O to point A, and the moving distance is denoted as y, it is necessary to make the thin beam in the first correlation beam 1 at point A be the same as that in the second correlation beam 2 The optical path lengths of the thin beams are equal. In the self-calibration method, the optical path L _1A of the first correlation beam 1 is changed to L' _1A , that is, L' _1A =L _1A -2z, so that L' _1A =L _2A . The time change corresponding to the optical path change 2z is Δt=2z/c, c is the speed of light, so the relationship between the movement y of the peak value of the autocorrelation signal and the time change Δt can be expressed as

ρ=Δt/y=2z/cy (4)

ρ is the time resolution required for the calibration experiment.

In the technical solution of the present invention, a static method is used to identify the relationship between the optical path 2z and the movement amount y of the peak value of the autocorrelation signal. The specific operation steps are:

① Install the reticle 7 into the first correlation beam 1 or the second correlation beam 2 so that it is perpendicular to the beam transmission direction;

② Detection of the projection 8 of the reticle on the relevant signal by the photodetector 10;

③Measure the angle 3 of the relevant beam through the angle ruler;

④ Calculate the time resolution ρ according to the measured value φ of the angle 3 of the relevant beam, the measured value x of the reticle 7 , and the measured value y of the photodetector 10 .

In Embodiment 1, the size of the reticle is represented by x. According to the trigonometric function relationship associated with the correlation beam angle 3 in FIG. 1, it can be known that the autocorrelation signal peak value movement y, the optical path change 2z, and the dimension x in the first correlation beam 1 form a right triangle. The projection of the dimension x in the first correlation beam 1 onto the correlation process working surface 4 is

y=x/cos(φ/2) (5)

The projection of the dimension x in the beam propagation direction of the first correlation beam 1 is:

z=x×tan(φ/2) (6)

Therefore, according to formulas (4), (5), (6), (7), the temporal resolution ρ can be expressed as

ρ=2z/cy=2x×tan(φ/2)/cy (7)

In the formula (7), the size x of the reticle and the angle φ of the relevant beams are both fixed quantities, which can be measured by tools such as vernier calipers and combined angle rulers. The speed of light c is constant. The projection y of the dimension x on the reticle 7 on the relevant working surface 4 needs to be measured by a photodetector, such as a CCD, in the calibration experiment. Therefore, the time resolution ρ can be obtained from only one measurement result of one pulse. However, in the self-calibration method, the measurement of the relative signal movement amount y needs at least two measurement results of two pulses to be subtracted.

Calibration errors for this invention are discussed below.

The space size on the reticle 7 is guaranteed by mechanical processing, and the absolute error is ±5um; when the picosecond-level autocorrelator is selected with a spacing of 1mm, the relative error is ±0.5%;

The relative beam angle 3 is measured by a combination angle ruler, and the absolute error of the angle φ is dφ<±1°. In the picosecond autocorrelator, φ=55°, the relative error is tan(55°±1°/2)/tan(55°/2)=±2%;

The amount of movement of the correlation signal 9 on the photodetector is measured by the CCD. The CCD pixel size in the picosecond autocorrelator is 20um, and the absolute error is ±20um. The corresponding optical path delay when the pulse width is 18ps is

2z=18×10 ^-12 s×3×10 ⁸ m/s=5400um

The relevant signal range is obtained by formulas (5) and (6):

y=z/sin(φ/2)=5.85×10 ³ um

The relative error is ±20um/5.85×10 ³ um=±0.3%;

In summary, in the calibration method of the present invention, the calibration error of the picosecond-level autocorrelator is

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&PlusMinus;</span>\sqrt{{\mathrm{<span\; class="notranslate">\; 0.5</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 0.3</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&PlusMinus;</span>\mathrm{<span\; class="notranslate">\; 2.1</span>}<span\; class="notranslate">\; \%</span>$

In embodiment 2, the basic structure and calibration steps of the optical path are the same as in embodiment 1. The difference lies in the size of the reticle, the angle of the relevant beams, and the pixels of the CCD.

The relative error is ±5% when the femtosecond-level autocorrelator selects the size on the reticle with a spacing of 100um.

In the femtosecond autocorrelator, φ=10°, the relative error is tan(55°±1°/2)/tan(55°/2)=±10%;

The CCD pixel size in the femtosecond autocorrelator is 8um, and the absolute error is ±8um. The corresponding optical path delay when the pulse width is 300fs is

2z=300×10 ^-15 s×3×10 ⁸ m/s=90um

The relevant signal range is obtained by formulas (6) and (7)

y=z/sin(φ/2)=516um

The relative error is ±8um/516um=±1.6%.

In summary, the calibration error of the femtosecond autocorrelator is

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&PlusMinus;</span>\sqrt{{\mathrm{<span\; class="notranslate">\; 0.5</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 1.6</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&PlusMinus;</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; \%</span>$

Therefore, with the calibration of this invention, even if the pulse width of the calibration pulse is close to or greater than the measurement range of the autocorrelator, and the top changes slowly, high reliability and high-precision time resolution can still be obtained, and only one pulse is required. 1 measurement result. This is very useful for large picosecond-level petawatt laser systems with a 2-hour interval between shots, limited debugging shots, and a wide range of pulse width variations.

Claims (2)

1. a kind of precision calibration method for single correlation-measuring instrument temporal resolution it is characterised in that the method include following Step:

1. graticle (7) is installed in the first associated beam (1) or the second associated beam (2) so as to pass perpendicular to light beam Defeated direction, the size Expressing of graticle is x；

2. by the test surface of photodetector (10) parallel to the first associated beam (1) and the second associated beam (2) related work Make face (ao), projection y on related work face for detection graticle (7)；

3. measure the first related associated beam and the angle of the second associated beam isDescribed the first associated beam, the second phase Closing light beam is: arbitrarily the spectroscopical reflected light of percentage ratio and transmitted light, the spectroscopical reflected light of any percentage ratio are through non-linear Signal produced by crystal and transmitted light, the spectroscopical reflected light of any percentage ratio and transmitted light are produced through nonlinear crystal Signal；

4. by temporal resolution ρ of following equation calculating single auto-correlation or cross correlation measurement instrument:

In formula, c is the light velocity.

2. scaling method according to claim 1 is it is characterised in that described photoelectric detector is ccd, photodiode Or fast-response photocell.