CN2927012Y - Simple femtosecond pulse real-time measuring instrument - Google Patents

Abstract

The utility model provides a simple and easy real-time measuring apparatu of femto second pulse, is an utilize the direct electric field of ultrashort pulse spectrum phase coherence to rebuild the method measuring device, comprises half-wave plate, quartz crystal, beam expanding device, wide-angle fresnel biprism, thick nonlinear crystal, diaphragm, spectrum appearance, CCD detector and computer, the utility model has the advantages of get rid of femto second pulse grating widening device, real-time measurement, auto-collimation and light path are adjusted conveniently, compact structure, measurement accuracy height and with low costs.

Description

Femtosecond pulse simple real-time measuring instrument

technical field

The utility model relates to a femtosecond laser pulse, in particular to a simple real-time measuring instrument for a femtosecond pulse.

Background technique

With the rapid development of ultrashort laser pulse technology, it is now possible to generate ultrashort pulses of several femtoseconds. The ultra-short and ultra-strong characteristics of femtosecond laser pulses make it widely used in physics, chemistry, biology, medicine, and industry. Femtosecond laser pulses are powerful tools for studying ultrafast phenomena and various nonlinear phenomena ; At the same time, accurate measurement of the phase and intensity of femtosecond pulses can promote the improvement of pulse generation devices and more reliable femtosecond pulse applications.

The pulse width of the femtosecond laser is on the order of 10 ^-15 seconds, which exceeds the response time range of the electrical method measurement, so the indirect measurement method is generally used to measure the light intensity distribution of the second harmonic of the interference light through the interference of two beams of light pulses. And the pulse width of the measured femtosecond laser pulse is calculated from the light intensity distribution of the second harmonic. There are many femtosecond laser pulse measurement methods, currently there are two main methods in the world: spectral phase interferometry for direct electric-field reconstruction (spectral phase interferometry for direct electric-field reconstruction, referred to as SPIDER method) [see prior art 1 "Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses”C.Iaconis, A.Walmsley, Optics Letters, Vol.23 Issue 10 1998] and frequency-resolved optical switching method (frequency-resolved optical gating, hereinafter referred to as FROG method) [see in Prior Art 2 "Measuring ultrashort laser pulses in the time-frequency domain using frequency-Resolved optical Gating" Rick Trebino, Kenneth W. DeLong et al., Review Science Instrument, Vol.68 Issue9 1997].

Fig. 1 is the measurement device of the existing femtosecond pulse spectrum phase coherent direct electric field reconstruction method. The basic structure of the femtosecond pulse spectrum phase coherent direct electric field reconstruction method measurement device is to divide the femtosecond pulse into two beams through a semi-transparent and semi-reflective beam splitter, one of which is used as the probe light, and the other beam is widened by a widening device such as a grating. And the beam used as the probe is divided into two beams again through the beam splitter, and a time delay τ is introduced by adjusting the micro-motion stage, and then the two beams are synchronized with the broadened beam to generate frequency conversion on the nonlinear crystal, and the converted secondary The harmonic beam forms an interferogram, records the interference image through a spectrometer and a CCD detector, and introduces the image data into a well-written computer program. are three different parts separated by pulse delay τ. The intermediate DC part does not contain phase information, so the t=+τ AC part containing phase information can be filtered by a Gaussian filter function above the fourth order. The filtered AC component is Fourier transformed to the frequency domain again, and the phase difference is obtained by taking the amplitude angle, and the phase difference of the two beams is obtained by subtracting the linear term of the phase difference. Through the phase concatenation, the femtosecond can be obtained The phase of the pulse. If the time delay τ is zero, the interference image has no frequency clipping at this time, and the intensity of the incident femtosecond laser pulse can be obtained directly. The spectral phase coherent direct electric field reconstruction method needs to ensure that a pulse is stretched by a complex stretcher, so the device is complicated and the cost is high. At the same time, it can be seen from Figure 1 that there are many mirrors, requiring time synchronization, which increases the difficulty of optical path adjustment. Since the phase matching bandwidth requires the use of a very thin second harmonic crystal, the processing is more complicated and the price is high. Therefore, the common SPIDER method is not only complicated to operate, but also expensive.

Contents of the invention

The purpose of this utility model is to make up for the shortcomings of the above-mentioned existing spectral phase coherent direct electric field reconstruction method, and provide a simple real-time measuring instrument for femtosecond pulses. , high measurement accuracy and low cost advantages.

The technical solution of the utility model is as follows:

A simple real-time measurement instrument for femtosecond pulses, which consists of half-wave plates, quartz crystals, beam splitters, beam expanders, large-angle Fresnel double prisms, thick nonlinear crystals, An aperture, a spectrometer, a CCD detector and a computer are formed, and the positional relationship is: when a horizontally polarized ultrashort pulse beam to be measured is vertically incident on the half-wave plate, the half-wave plate divides the ultrashort pulse beam into With horizontally polarized e-light and vertically polarized o-light, the two polarized lights with different polarization directions will produce dispersion delay when passing through the quartz crystal whose slow axis is in the horizontal direction, and are incident on the said On the beam splitter, the beam is divided into two beams of light with an angle of 90°: the reflected beam directly enters the beam expander, and the transmitted beam enters the beam expander parallel to the reflected beam after being reflected by the second mirror , the two outgoing beams of the beam expander have different optical paths, and after being delayed, they exit parallel to the incident two parallel beams, and the parallel outgoing beams are incident on the large-angle Fresnel double prism to form two small-angle intersections The two intersecting beams are converted by frequency doubling at the central position of the thick nonlinear crystal, and the two beams of second harmonic outgoing light with delay are generated, pass through the diaphragm, and finally enter the spectrometer to form interference Images, recorded by the CCD detector, are then sent to a computer for data processing.

The ultrashort laser pulse is the pulse output by the Ti:Sapphire oscillator, and its width is required to be greater than 70 femtoseconds.

The beam expander is composed of two reflecting plane mirrors perpendicular to each other.

The apex angle of the large-angle Fresnel biprism is between 179° and 179.6°, the thickness is less than 3mm, and the material is fused silica.

The thick nonlinear crystal is potassium dihydrogen phosphate crystal.

The quartz crystal pre-delays the group velocity of light with different polarization directions. The time delay of the two beams of light separated by the beam splitter is due to the time delay caused by the difference in the optical path of the two beams of light after being split and passing through the beam expander. The beam passes through the large-angle Fresnel double prism to form two beams of converging light. According to the selection of the apex angle of the prism, the angle between the two beams of light is 0.2° to 0.5°. It is frequency-multiplied and converted on the thick nonlinear crystal, and passed through the diaphragm. Spectrometer and CCD detector imaging.

The working process is as follows:

When a beam of horizontally polarized ultrashort pulse laser to be measured is incident on the half-wave plate, the half-wave plate forms the light beam with horizontal and vertical polarization. After the two beams of polarized light are dispersed by the quartz crystal, they form polarized light separation. , e light is delayed relative to o light. After the reflection mirror is incident on the beam splitter, it is separated into two equal beams of light with an included angle of 90°. After the two beams enter the beam expander in parallel and expand the beam, they emerge as parallel lights with a relative delay. Then incident on the large-angle Fresnel double prism, the large-angle Fresnel double prism converges the beam at a small angle, and the two intersecting beams are frequency-multiplied and converted on the thick nonlinear crystal by the second-type phase matching method, and the converted two There is frequency clipping between subharmonics. The interference image of the second harmonic spectrum is obtained through the diaphragm, and the spectrometer and CCD detector record and collect it, and then send it to the computer for processing.

The utility model is a simple real-time measurement instrument for femtosecond pulses, which adopts the spectral phase coherent direct electric field reconstruction method, and its working principle is explained as follows:

This instrument uses a nonlinear frequency-doubling crystal with a large thickness. After the light beam passes through the thick nonlinear crystal, the group velocity delay of the o-light and e-light will occur. Through the pre-delay effect of the quartz crystal, the delay of the two polarized lights can be compensated, so that the o-light and e-light can perform frequency multiplication conversion efficiently . There are two polarization directions of light beams, due to the phase matching function, the thick nonlinear crystal allows the converted o light to have a matching bandwidth of more than 10nm, while the e light has a matching bandwidth of less than 1nm, which can considered monochromatic light. Therefore, the two beams of light expanded by the beam expander, the o-light and the monochromatic e-light in each beam are frequency-multiplied and converted in a second-type phase-matching manner to generate a second harmonic. When the two beams of light incident on the nonlinear crystal have a certain angle, the phase matching angle of the beams is slightly different, and the frequency of the two beams of second harmonic light after frequency doubling is sheared, that is, the frequency conversion The center frequency of the second harmonic light is different. At the same time, the two beams of second harmonic light also have a time delay, so an interference image will be formed. The interference image is recorded by the spectrometer and CCD detector, and sent to the computer for analysis and calculation, so as to obtain the intensity and phase of the incident beam.

This instrument adopts the above-mentioned large-angle Fresnel double prism, selects different double prism apex angles, can automatically make the two beams of light incident on the crystal have a cross angle of 0.2° to 0.5°, and the second harmonic after frequency doubling conversion The difference in wave frequency centers results in frequency shearing.

The technical effect of the utility model is as follows:

1. The utility model uses thick nonlinear crystals to match different bandwidths of o-light and e-light frequency multiplication conversion with different polarization states, and replaces the beam broadening and dispersion device of the grating pair. The optical path is simple, the components are few, and the cost is low.

2. The core of the utility model is to adopt a thick nonlinear crystal, which can easily realize the generation of the second harmonic. Since the thick nonlinear crystal is used instead of the thin crystal, the requirements for the intensity of the incident pulse can be reduced, and it has the advantages of high measurement accuracy and low cost.

3. The utility model adopts a large-angle Fresnel double prism to automatically generate the deviation angle of the incident direction of the beam and the nonlinear crystal phase matching cutting angle to form the frequency shearing of the second harmonic, and it is easy to realize the equal optical path of the pulse. It is easy to produce stable interference fringes, and its thickness has less than 0.5% influence on the measurement results. The instrument achieves self-collimation effect and can measure in real time. It has the advantages of high measurement accuracy, compact structure and convenient optical path adjustment.

Description of drawings

Fig. 1 is a schematic structural diagram of a measurement device of an existing spectral phase coherent direct electric field reconstruction method.

Fig. 2 is a structural schematic diagram of an embodiment of the utility model femtosecond pulse simple real-time measuring instrument.

Detailed ways

Below in conjunction with accompanying drawing and embodiment the utility model is further described.

Please refer to FIG. 2 first. FIG. 2 is a schematic structural diagram of an embodiment of the utility model femtosecond pulse simple real-time measuring instrument. As can be seen from the figure, the composition of the utility model femtosecond pulse simple real-time measuring instrument is: along the advancing direction of the light beam to be measured, it sequentially includes a half-wave plate 9, a quartz crystal 10, a beam splitter 1, a beam expander 13, and a large-angle Fresnel Er double prism 14, thick nonlinear crystal 15, aperture 16, spectrometer 7, CCD detector 8 and computer 17, its positional relationship is: when the ultrashort pulse light beam to be measured of a horizontal polarization vertically incident on described half-wave On the plate 9, the half-wave plate 9 divides the ultrashort pulse beam into e light with horizontal polarization and o light with vertical polarization, and the light beams with different polarization directions pass through the quartz crystal 10 with the slow axis in the horizontal direction. The light is dispersed and delayed, and is incident on the beam splitter 1 after being reflected by the first reflector 11, and the beam is split into two beams of light with an included angle of 90°: the reflected beam directly enters the beam expander 13, and transmits After the light beam is reflected by the second reflector 12, it enters the beam expander 13 parallel to the reflected beam. The two outgoing beams of the beam expander 13 have different optical paths, and after being delayed, they exit parallel to the incident two beams of parallel light. The parallel outgoing light beams are incident on the large-angle Fresnel biprism 14 to form two small-angle intersecting beams, and the two intersecting beams are frequency-multiplied and converted at the central position of the thick nonlinear crystal 15, resulting in delayed The two beams of second harmonic outgoing light pass through the diaphragm 16 and finally enter the spectrometer 7 to form an interference image, which is recorded by the CCD detector 8 and then sent to the computer 17 for data processing. In this example:

The ultrashort laser pulse is the pulse output by the Ti:Sapphire oscillator, and its width is required to be greater than 70 femtoseconds.

The quartz crystal 10 is 14 mm in length, and the slow axis of the crystal is placed horizontally.

The beam splitter 1 and the second reflector 12 make the light split into two beams of light with an included angle of 90° through the beam splitter. The distance between the two mirrors makes the two beams of light have a time delay effect. After adjustment, the delay is selected. 3 picoseconds.

The beam expander 13 is composed of two vertical plane mirrors.

The included angle of the large-angle Fresnel biprism 14 is 179°20", the thickness is 3 mm, and the material is fused silica.

The thick nonlinear crystal 15 is a potassium dihydrogen phosphate crystal, that is, a KDP (potassium dihydrogen phosphate KH ₂ PO ₄ , KDP for short) crystal with a thickness of 2 cm.

Consider the situation that the ultrashort pulse output by the Ti:Sapphire oscillator has a central wavelength of 830nm and a time domain width of 70 femtoseconds. The measured horizontally polarized femtosecond pulse is divided into horizontally and vertically polarized polarized light (i.e. e-light and o-light) by the half-wave plate 9, and the light with different polarization directions passes through the quartz crystal 10 whose slow axis is in the horizontal direction for dispersion, resulting in The light group velocity of the light beam in the two polarization directions is different, and the e light is delayed by 450 femtoseconds relative to the o light. The light beam is split by the beam splitter 1, and the two parallel beams of light emitted after passing through the beam expander 13 have a different optical path and have a time delay, which is determined by the distance between the beam splitter 1 and the second reflector 12, and the adjustment delay is 3 pico. Second. The two beams of light converge at a small angle through the large-angle Fresnel double prism 14, and the angle between the two converging cross beams is 20 ". The o-light and e-light contained in each beam can be seen due to the difference in the matching bandwidth of the nonlinear crystal 15 Perform frequency multiplication conversion of o light and monochromatic e light to generate a beam of second harmonic light. The conversion adopts the second type of phase matching method, and the phase matching angle of the nonlinear crystal is cut at 68°. Since the beam is incident on the nonlinear The angle of the crystal is different, and the phase matching angle of the nonlinear crystal is slightly different, so that the frequency centers of the converted two beams of second harmonic light are different to form a frequency shear. Two second harmonics with frequency shear and delay time The wave beam passes through the diaphragm 16 to generate an interference image, and the adjustment effect can be observed in real time through the spectrometer 7 and the CCD detector 8; according to the frequency shearing of the converted second harmonic beam and the phase matching requirements of the nonlinear crystal, the selected Fresnel The vertex angle of the double prism is 179°20″, the thickness is 3mm, and the fused silica material with low dispersion is selected. The KDP crystal (potassium dihydrogen phosphate KH ₂ PO ₄ , referred to as KDP) with a nonlinear crystal thickness of 2cm can meet the matching bandwidth requirements of e-light and o-light; at the same time, it has a high second harmonic conversion efficiency and can To measure weak femtosecond pulsed light, crystal 15 uses KDP crystal as the frequency conversion crystal.

The utility model realizes ultra-short laser pulse light measurement by using a thick nonlinear crystal. The crystal requires different matching bandwidths for light of different polarization states, and the grating stretcher can be removed, so that the light beam with a certain bandwidth and the approximate monochromatic The frequency doubling conversion requirements of the light beam; the thick nonlinear crystal eliminates the influence of the traditional crystal thickness on the incident intensity requirement and the second harmonic generation efficiency, and reduces the intensity requirement of the measurement pulse. Moreover, since the large-angle Fresnel double prism does not require multiple adjustments, the angle between the incident beam and the crystal phase matching cutting angle can be realized, so that the frequency shearing of the second harmonic is easy to generate, and it can be self-collimated and realized in real time. measurement, and the influence of the thickness of the double prism on the measurement result is less than 0.5%, so the instrument has the advantages of simple structure, easy adjustment, high precision and low cost.

Claims (5)

1, a kind of flying second pulse simple real-time measuring instrument, it is characterized in that comprising half-wave plate (9) successively along the working direction of light beam to be measured, quartz crystal (10), beam splitter (1), beam expander (13), wide-angle Fresnel double prism (14), the linear crystal (15) of being altogether unjustifiable, diaphragm (16), spectrometer (7), ccd detector (8) and computing machine (17), its position relation is: when the ultrashort pulsed beam to be measured of a horizontal polarization impinges perpendicularly on the described half-wave plate (9), this half-wave plate (9) is divided into the e light with horizontal polarization and the o light of vertical polarization with ultrashort pulsed beam, the described slow axis of light beam process with different polarization direction is that quartz crystal (10) two polarized lights of horizontal direction postpone chromatic dispersion, after first catoptron (11) reflection, incide on the described beam splitter (1), beam splitting is that angle is 90 ° a two-beam line: folded light beam directly enters described beam expander (13), transmitted light beam enters described beam expander (13) abreast with folded light beam after second catoptron (12) reflection, the two outgoing beam light path differences of this beam expander (13), produce delay and restraint the parallel outgoing of directional lights with two of incident, this parallel outgoing beam incides described wide-angle Fresnel double prism (14) and goes up the light beam that forms two low-angles intersection, this two intersect light beam in the described linear crystal of being altogether unjustifiable (15) center through frequency-doubled conversion, the two bundle second harmonic emergent lights that produce with delay, by diaphragm (16), incide described spectrometer (7) at last and go up the formation interference image, by described ccd detector (8) record, deliver to computing machine (17) then and carry out data processing.

2, flying second pulse simple real-time measuring instrument according to claim 1 is characterized in that described ultrashort laser pulse is the pulse of titanium jewel oscillator output, and its width requirement is greater than 70 femtoseconds.

3, flying second pulse simple real-time measuring instrument according to claim 1 is characterized in that described beam expander (13) is made of two orthogonal plane of reflection mirrors.

4, flying second pulse simple real-time measuring instrument according to claim 1, the drift angle that it is characterized in that described wide-angle Fresnel double prism (14) is between 179 ° to 179.6 °, and thickness is lower than 3mm, and material is a fused quartz.

5, flying second pulse simple real-time measuring instrument according to claim 1 is characterized in that the described linear crystal of being altogether unjustifiable (15) is a potassium dihydrogen phosphate crystal.