CN104121994A - Transient grating effect based femtosecond laser pulse measuring device - Google Patents

Abstract

A femtosecond laser pulse measurement device based on transient grating effect, which essentially divides the incident laser pulse to be measured into four beams, three of which pass through the transient grating effect of the third-order nonlinear medium to generate self-reference light, and The fourth beam attenuated with a certain delay, that is, the light to be measured, is spatially coincident and focused into the spectrometer to obtain a self-reference interference spectrum. By measuring the interference spectrum and using the self-reference spectrum coherence method to calculate the femtosecond laser pulse shape, laser spectrum and spectrum Phase; adopt concave reflector and convex reflector to form a focus system or planar reflector and concave reflector to form a focus system in the present invention, this makes device structure more compact, stable, and whole device is miniaturized; No need for optical polarization element , can be applied to the femtosecond laser optical system with spectrum and pulse width within 200-3000nm and 10-300fs respectively.

Description

Femtosecond laser pulse measurement device based on transient grating effect

technical field

The invention relates to femtosecond laser pulse measurement, in particular to a femtosecond laser pulse measurement device based on transient grating effect.

Background technique

Femtosecond laser has been developed extremely rapidly due to its needs in scientific research, biology, medical treatment, processing, communication, national defense and other fields. As the important optical parameters of femtosecond laser pulses, the shape and pulse width of femtosecond laser pulses are very necessary for their measurement or real-time monitoring. Therefore, finding a simple, convenient, fast and effective method and device for measurement and real-time monitoring plays a very important role in promoting the development of femtosecond laser itself and expanding its application in various fields.

At present, the most widely used femtosecond laser pulse measurement techniques are: 1. Frequency Resolved Optical Gating (FROG); 2. Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER). Both techniques are based on self-referencing and achieve single-shot measurements of femtosecond laser pulses through non-collinear harmonic generation methods. However, these two techniques are not straightforward for reconstructing the algorithm of femtosecond laser pulses and require a long time. In SPIDER technology, nonlinear crystals are usually required to convert and generate measurement signals. Due to the phase matching conditions of nonlinear optical crystals, each measuring instrument can only be adapted to a specific spectral range, which limits the use of these methods in a wide spectral range. Applications within the system, and these systems and measurement processes are relatively complex. Self-referenced spectral interferometry (SRSI) based on cross-polarized waves (XPW) has also been recently used to measure laser pulses. In this method, the spectrum and phase of the laser to be measured can be quickly obtained with only 3 simple iterative calculations. This is the most simple and convenient method so far, and can be used for single-shot measurement of pulse width. However, this method requires optical polarizing elements. Since the optical polarization element is only effective for a specific laser wavelength and has a certain spectral width, this limits the application of this method and instrument to a specific spectral range. The dispersion of polarized optical components also limits the measurement of short pulses below 10 fs. On a similar SRSI method, we have recently proposed two methods based on the self-diffraction effect [see literature 1: J.Liu, Y.L.Jiang, T.Kobayashi, R.X.Li, and Z.Z.Xu, "Self-referenced spectral interferometry based on self-diffraction effect," J.Opt.Soc.Am.B29(1):29-34(2012); see patent: application number: 201210267065.3] and transient grating effect [see Document 2: J.Liu, F.J.Li, Y.L.Jiang, C.Li, Y.X.Leng, T.Kobayashi, R.X.Li, and Z.Z.Xu, “Transient-grating self-referenced spectral interferometry for infrared femtosecond pulsecharacterization,” Opt. Lett. 37, 4829 (2012); see Patent: Application No.: 201210079324.X] femtosecond laser pulse measurement method, the two methods are not limited by dispersion, but the two methods are still relatively complex in structure.

Contents of the invention

The invention provides a femtosecond laser pulse measurement device based on the transient grating effect. The invention adopts the optical path structure of telescope or microscope objective lens, the device structure is simple, small and exquisite, easy to adjust, data acquisition and data processing are fast, and can be applied to the measurement and measurement of femtosecond laser pulse width and pulse shape in different pulse widths and different wavelength ranges. Real-time monitoring, the obtained spectral phase can be fed back to the relevant phase compensation device to optimize the output of femtosecond laser pulses.

Technical solution of the present invention is as follows:

A femtosecond laser pulse measurement device based on the transient grating effect, characterized in that its composition includes: a first concave reflector, a convex reflector, a four-hole aperture plate, a variable neutral attenuation delay film, a third-order Linear optical medium, aperture plate, second concave mirror and spectrometer with high spectral accuracy. The positional relationship of the components is as follows:

The center of the first concave reflector has a small hole, and the four-hole aperture plate has four small holes distributed in a square, and the first concave reflector and the convex reflector form a focusing system . The femtosecond laser beam to be measured passes through the small hole of the first concave reflector and is focused by the focusing system, and the four small hole light beams are placed at an appropriate position between the convex reflector and the focal point of the focusing system The aperture plate divides the focused light beam into four beams called the first beam, the second beam, the third beam and the fourth beam, and the four-hole aperture plate and the focal plane are placed between the The above-mentioned variable neutral attenuation delay film and attenuating the fourth light beam and introducing a delay as the light to be measured, the first light beam, the second light beam and the third light beam are freely propagating in the air, and In the third-order nonlinear optical medium at the focal plane, a transient grating effect is generated to form a transient grating signal light wave as a reference light, and the light to be measured is combined with the third-order nonlinear optical medium after passing through the third-order nonlinear optical medium The above-mentioned reference light overlaps in space, and the two overlapped laser pulses pass through the aperture diaphragm plate and the second concave mirror in turn and then enter the spectrometer with high spectral precision for measurement, and obtain the Measurement of self-referencing interference spectra of femtosecond lasers.

A femtosecond laser pulse measurement device based on the transient grating effect, characterized in that its composition includes: four small aperture diaphragm plates, variable neutral attenuation delay film, plane reflector, first concave reflector, third-order non-conductor Linear optical medium, aperture plate, second concave mirror and spectrometer with high spectral accuracy. The positional relationship of the components is as follows:

The four-aperture aperture plate has four small holes distributed in a square shape, the first concave reflector has a small hole in the center, and the plane reflector and the first concave reflector form a focusing system. The femtosecond laser beam to be measured is divided into four beams after passing through the four-hole aperture plate, which are respectively called the first beam, the second beam, the third beam and the fourth beam. The first beam, the second beam and the The third light beam is focused by the focusing system into the third-order nonlinear optical medium placed at the focal plane of the focusing system, and a transient grating effect is generated in the third-order nonlinear optical medium to form a transient state The light wave of the grating signal is used as the reference light, and the fourth light beam is attenuated by the variable neutral attenuation delay film and introduced into the delay as the light to be measured, passes through the focusing system, and then passes through the third-order non- The linear optical medium is spatially coincident with the reference light. The two overlapping laser pulses pass through the small hole aperture plate and the second concave mirror in sequence and then enter the spectrometer with high spectral precision for measurement, so as to obtain the self-referencing interference of the femtosecond laser to be measured spectrum.

The present invention has following salient features:

1, the present invention has used the first concave reflector and the convex reflector to form a focus system in the first kind of typical device, has used the plane reflector and the first concave reflector to form a focus system in the second typical device, This makes the structure of the device more compact and stable, and the whole device is more miniaturized;

2. The variable neutral attenuation time delay film used in the present invention introduces time delay to the light to be measured, and attenuates its intensity, so as to adjust the distance between the obtained reference light and the delayed light to be measured in real time. Energy ratio, so as to distinguish the reference light from the light to be measured and ensure the modulation of the interference spectrum is accurate.

3. The present invention is applicable to femtosecond laser optical systems with spectra and pulse widths within 200-3000nm and 10-300fs respectively. In addition, the present invention is applicable to the measurement of the pulse width and pulse shape of the megahertz and single femtosecond laser pulses at the same time, and can be used for real-time monitoring of the femtosecond laser pulses, and the obtained spectral phase can be fed back to the relevant phase compensation device to optimize the output of femtosecond laser pulses.

Description of drawings

Fig. 1 and Fig. 2 are respectively the optical path diagrams of two typical examples of the present invention.

Fig. 3 is to utilize the example device of the present invention Fig. 1 to measure 800nm central wavelength 40fs laser pulse experimental result figure, have provided respectively the spectrum (dotted line) of pulse to be measured, the transient grating signal spectrum (dash line) that produces, incident pulse and Coherence spectrum of the transient grating signal (solid line).

Detailed ways

First, the interference spectrum signal is obtained by using the optical module based on the transient grating effect of the transparent medium and the spectrometer with high spectral precision.

An embodiment device of the present invention is shown in FIG. 1 . The optical path mainly includes: 1 is the incident femtosecond laser beam; 2 is the first concave mirror; 3 is the convex mirror; 4 is the four-hole aperture plate; 5 is the variable neutral attenuation delay film; 6 is the third order Non-linear optical medium, used to generate transient grating effect; 7 is a small hole diaphragm plate, used to transmit signal light and block stray light; 8 is a second concave mirror; 9 is a spectrometer with high spectral accuracy, used to Measure laser spectra and interference spectra.

In the optical path shown in Fig. 1, the first concave reflector 2 and the convex reflector 3 form a focusing system; the four-aperture diaphragm plate 4 has four apertures arranged in a square at the center The equal small holes are used to select four parts on the incident laser spot to form four laser beams with equal apertures.

The optical path trend of the embodiment device in Fig. 1 is as follows: a sufficiently large incident laser beam 1 passes through the focusing system (the trend of the optical path in the focusing system is: the incident laser beam 1 passes through the After the central aperture of the first concave reflector 2, after the reflection of the convex reflector 3, and then the reflection of the concave reflector 2), through the four small hole diaphragm plates 4, the The four apertures of the four-aperture diaphragm plate 4 pass through four laser beams of equal diameter, which are respectively called the first, second, third, and fourth beams. The four beams of laser light are located at the four corners of the square. The four light beams pass through the variable neutral attenuation delay film 5 with a suitable thickness to obtain the light to be measured after the attenuation delay, and the first, second, and third light beams propagate freely in the air, so There is a certain time delay between the light to be measured and the first, second and third light beams freely propagating in the air. The first, second, and third light beams generate transient grating signal light waves as reference light due to the transient grating effect in the third-order nonlinear optical medium 6 placed at the focal plane of the focusing system , the generated reference light just coincides with the light to be measured in space. The reference light and the light to be measured are selected by the aperture diaphragm plate 7 and the stray light is blocked, and focused into the high-precision spectrometer 9 by the second concave mirror 8 to measure the interference spectrum signal.

In the embodiment device in FIG. 1 , the size of the small hole in the center of the first concave reflector 2 is determined according to the size of the incident laser spot, and usually the incident spot is slightly larger than the small hole. The size and spacing of the four apertures of the four-aperture diaphragm plate 4 are selected according to the size of the light spots that shoot to it, and the principle is that the four light spots do not affect each other and the passing energy is the largest. The films of the first concave reflector 2, the convex reflector 3 and the second concave reflector 8 can be plated with corresponding silver films, gold films and dielectric high-reflection films according to the different center wavelengths of the incident laser light. The material of the third-order nonlinear medium 6 needs to be transparent to the incident laser light, and the third-order nonlinear coefficient should be high. 9 is a spectrometer with high spectral precision, which can improve the measurement accuracy.

Finally, the obtained femtosecond laser pulse shape, laser spectrum and spectral phase are obtained by performing self-reference spectral coherence calculation on the obtained interference spectral signal.

The femtosecond laser pulse was measured by using the device shown in FIG. 1 and in combination with the above-mentioned specific embodiments. When the incident laser pulse 1 has a repetition frequency of 1KHz, a center wavelength of 800nm, and a pulse energy of 1.7uJ, the results shown in Figure 3 are obtained. Of course, the incident laser pulse conditions should not limit the protection scope of the present invention.

Claims (2)

1. the femto-second laser pulse measurement mechanism based on transient grating effect, be characterised in that its formation comprises: be the first concave mirror (2) successively along light beam input direction, convex reflecting mirror (3), four aperture plates (4), variable neutral decay time delay sheet (5), third-order nonlinear optical medium (6), aperture plate (7), the spectrometer (9) of the second concave mirror (8) and high spectral accuracy, described the first concave mirror (2) center has an aperture, four described aperture plates (4) have four apertures that are square profile, focusing system of the relative composition of reflecting surface of described the first concave mirror (2) and described convex reflecting mirror (3), femtosecond laser light beam to be measured (1) passes through the aperture of described the first concave mirror (2) successively, convex reflecting mirror (3), the first concave mirror (2) is also focused on by described focusing system, between described convex reflecting mirror and the focal plane of described focusing system, place four described aperture plates (4), described focused beam is divided into four bundles and is called the first light beam, the second light beam, the 3rd light beam and the 4th light beam, between four described aperture plates (4) and described focal plane, in the light path of the 4th light beam, place described variable neutral decay time delay sheet (5), the 4th described light beam is decayed to time delay as treating photometry, the first described light beam, the second light beam and the 3rd light beam Free propagation in air, and in the described third-order nonlinear optical medium (6) at described focal plane place, produce transient grating effect and form transient grating signal as with reference to light, the spectrometer (9) of described high spectral accuracy described in entering after described aperture plate (7) and described the second concave mirror reflection (8) successively together with space coincidence with described reference light after described third-order nonlinear optical medium (6) until photometry is measured, obtain the self-reference interference spectrum of femtosecond laser to be measured.

2. the femto-second laser pulse measurement mechanism based on transient grating effect, be characterised in that its formation comprises: be four aperture plates (2) successively along light beam input direction, variable neutral decay time delay sheet (3), plane mirror (4), the first concave mirror (5), third-order nonlinear optical medium (6), aperture plate (7), the spectrometer (9) of the second concave mirror (8) and high spectral accuracy, four described aperture plates (2) have four apertures that are square profile, described the first concave mirror (5) center has an aperture, focusing system of the relative composition of reflecting surface of described plane mirror (4) and the first concave mirror (5), femtosecond laser light beam to be measured (1) is divided into four bundles after described four aperture plates (2), be called the first light beam, the second light beam, the 3rd light beam and the 4th light beam, four described bundle light are successively through the first concave mirror (5), plane mirror (4) also passes the aperture at the first concave mirror (5) center, be focused into the described third-order nonlinear optical medium (6) of the focal plane place placement of described focusing system by described focusing system, and in described third-order nonlinear optical medium (6), the first described light beam, the second light beam and the 3rd light beam produce transient grating effect and form transient grating signal light-wave as with reference to light, the 4th described light beam is measured as the spectrometer (9) of the high spectral accuracy described in entering after described aperture plate (7) and described the second concave mirror (8) reflection successively together with space coincidence until photometry and described reference light after being placed in described variable neutral decay time delay sheet (3) the decay time delay between described four aperture plates (2) and described plane mirror (4), obtain the self-reference interference spectrum of femtosecond laser to be measured.