阅读说明：本技术 一种啁啾方波脉冲放大激光系统 (chirped square wave pulse amplification laser system ) 是由 曹祥东 于 2019-08-29 设计创作，主要内容包括：本发明实施例提供了一种啁啾方波脉冲放大激光系统,包括：依次连接的激光振荡器、啁啾方波脉冲发生器、放大链和频谱整形脉冲压缩单元；所述激光振荡器发射种子激光脉冲至所述啁啾方波脉冲发生器,在所述啁啾方波脉冲发生器中将所述种子激光脉冲转换为啁啾方波脉冲；所述放大链将接收的所述啁啾方波脉冲进行功率放大后传递给所述频谱整形脉冲压缩单元；所述频谱整形脉冲压缩单元对功率放大后的所述啁啾方波脉冲进行脉冲整形。本发明实施例提供的一种啁啾方波脉冲放大激光系统,通过脉冲整形产生啁啾方波降低脉冲放大过程中的非线性畸变,使得放大后的脉冲峰值功率相比传统的啁啾放大系统提高数个数量级。(the embodiment of the invention provides a chirped square wave pulse amplification laser system, which comprises: the system comprises a laser oscillator, a chirped square wave pulse generator, an amplification chain and a spectrum shaping pulse compression unit which are connected in sequence; the laser oscillator emits seed laser pulses to the chirped square wave pulse generator, and the seed laser pulses are converted into chirped square wave pulses in the chirped square wave pulse generator; the amplification chain amplifies the power of the received chirped square wave pulse and transmits the chirped square wave pulse to the spectrum shaping pulse compression unit; and the frequency spectrum shaping pulse compression unit is used for carrying out pulse shaping on the chirped square wave pulse after power amplification. According to the chirped square wave pulse amplification laser system provided by the embodiment of the invention, the chirped square wave is generated through pulse shaping, so that the nonlinear distortion in the pulse amplification process is reduced, and the amplified pulse peak power is improved by several orders of magnitude compared with that of the traditional chirped amplification system.)

1. a chirped square wave pulse amplification laser system, comprising:

the system comprises a laser oscillator, a chirped square wave pulse generator, an amplification chain and a spectrum shaping pulse compression unit which are connected in sequence;

The laser oscillator emits seed laser pulses to the chirped square wave pulse generator, and the seed laser pulses are converted into chirped square wave pulses in the chirped square wave pulse generator;

The amplification chain amplifies the power of the received chirped square wave pulse and transmits the chirped square wave pulse to the spectrum shaping pulse compression unit;

and the frequency spectrum shaping pulse compression unit is used for carrying out pulse shaping on the chirped square wave pulse after power amplification.

2. The chirped square wave pulse amplification laser system according to claim 1, further comprising:

and the input end of the repetition frequency adjusting unit is connected with the chirped square wave pulse generator, and the output end of the repetition frequency adjusting unit is connected with the amplification chain.

3. the chirped square wave pulse amplification laser system according to claim 2, wherein the repetition frequency adjustment unit comprises:

The device comprises a preamplifier, an acousto-optic modulator, a post-amplifier and an optical isolator which are sequentially connected;

the input end of the preamplifier is connected with the output end of the chirped square wave pulse generator;

The output end of the optical isolator is connected with the input end of the amplification chain.

4. The chirped square wave pulse amplification laser system according to claim 1, wherein the spectral shaping pulse compression unit comprises:

a band pass filter and a pulse compressor;

the input end of the band-pass filter is connected with the output end of the amplification chain, and the output end of the band-pass filter is connected with the input end of the pulse compressor.

5. The chirped square wave pulse amplification laser system according to claim 4, wherein the pulse compressor comprises:

grating pairs or chirped volume bragg gratings.

6. the chirped square wave pulse amplification laser system according to claim 1, wherein an amplifier used in the amplification chain comprises one or more of a laser amplifier, an optical parametric amplifier, and a raman amplifier.

7. the chirped square wave pulse amplification laser system according to claim 1, wherein the chirped square wave pulse amplification laser system is composed of an optical fiber material and is doped with a rare earth material.

8. The chirped square wave pulse amplification laser system according to claim 1, further comprising:

an optical system component for protecting the chirped square wave pulse amplification laser system from deleterious nonlinear effects produced by external feedback.

9. The chirped square wave pulse amplification laser system according to claim 1, wherein the laser oscillator comprises:

a broadband mode-locked fiber laser, a mode-locked titanium or short pulse laser.

Technical Field

the invention relates to the technical field of laser, in particular to a chirped square wave pulse amplification laser system.

Background

Ultrafast laser sources with high power and high pulse energy and pulse widths in the nanosecond, picosecond, femtosecond and attosecond range have important applications in many fields, for example: laser material processing, lidar, imaging technology, laser fusion, and medical research, among others. Currently, these applications mainly use ultrashort pulse lasers, especially femtosecond lasers based on Chirped Pulse Amplification (CPA), whose main challenge is nonlinear pulse distortion caused by the high peak power of the ultrashort pulse, its several nonlinear optical effects, mainly Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and self-phase modulation (SPM), and bulk optical impairments. For wideband pulses, SPM is the most important limitation of ultrashort pulse amplification. CPA uses dispersion to broaden the input pulses, thereby reducing the peak power before amplification to reduce nonlinear pulse distortion. The chirped pulses may be compressed to the initial short pulse width after amplification. Larger grating pairs are typically used in CPA systems, resulting in high cost, alignment difficulty and large size. On the other hand, there is still a residual non-linear limit in the CPA system, and the cumulative SPM needs to be less than pi. Although CPA has become the most widely used method of increasing pulse energy and peak power, the pulse energy and peak power obtained are still insufficient for many applications and scientific research.

disclosure of Invention

in order to solve the above problems, embodiments of the present invention provide a chirped square wave pulse amplification laser system that overcomes, or at least partially solves, the above problems.

in a first aspect, an embodiment of the present invention provides a chirped square wave pulse amplification laser system, including:

The system comprises a laser oscillator, a chirped square wave pulse generator, an amplification chain and a spectrum shaping pulse compression unit which are connected in sequence;

the laser oscillator emits seed laser pulses to the chirped square wave pulse generator, and the seed laser pulses are converted into chirped square wave pulses in the chirped square wave pulse generator;

the amplification chain amplifies the power of the received chirped square wave pulse and transmits the chirped square wave pulse to the spectrum shaping pulse compression unit;

and the frequency spectrum shaping pulse compression unit is used for carrying out pulse shaping on the chirped square wave pulse after power amplification.

Wherein the chirped square wave pulse amplification laser system further comprises:

And the input end of the repetition frequency adjusting unit is connected with the chirped square wave pulse generator, and the output end of the repetition frequency adjusting unit is connected with the amplification chain.

Wherein the repetition frequency adjustment unit includes:

The device comprises a preamplifier, an acousto-optic modulator, a post-amplifier and an optical isolator which are sequentially connected;

The input end of the preamplifier is connected with the output end of the chirped square wave pulse generator;

the output end of the optical isolator is connected with the input end of the amplification chain.

Wherein the spectral shaping pulse compression unit comprises:

a band pass filter and a pulse compressor;

The input end of the band-pass filter is connected with the output end of the amplification chain, and the output end of the band-pass filter is connected with the input end of the pulse compressor.

Wherein the pulse compressor comprises:

Grating pairs or chirped volume bragg gratings.

Wherein the amplifier used in the amplification chain comprises one or more of a laser amplifier, an optical parametric amplifier, and a raman amplifier.

the chirped square wave pulse amplification laser system is made of an optical fiber material and is doped with a rare earth material.

wherein the chirped square wave pulse amplification laser system further comprises:

An optical system component for protecting the chirped square wave pulse amplification laser system from deleterious nonlinear effects produced by external feedback.

Wherein the laser oscillator includes:

a broadband mode-locked fiber laser, a mode-locked titanium or short pulse laser.

According to the chirped square wave pulse amplification laser system provided by the embodiment of the invention, the chirped square wave is generated through pulse shaping, so that the nonlinear distortion in the pulse amplification process is reduced, and the amplified pulse peak power is improved by several orders of magnitude compared with that of the traditional chirped amplification system.

drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.

fig. 1 is a schematic structural diagram of a chirped square wave pulse amplification laser system according to an embodiment of the present invention;

FIG. 2a is a schematic diagram of pulse distortion caused by SPM provided by an embodiment of the present invention;

FIG. 2b is a schematic diagram of the present invention utilizing innovative pulse shapes to minimize SPM induced pulse distortion;

fig. 3a is a schematic diagram of pulse intensity and instantaneous chirp reflecting the recovery capability of square wave pulse amplification to SPM in the frequency domain provided by an embodiment of the present invention;

FIG. 3b is a schematic diagram of a pulse spectrum reflecting the recovery capability of square wave pulse amplification to SPM in the frequency domain according to an embodiment of the present invention;

fig. 4 is a schematic diagram of a recovery capability of square wave pulse amplification to SPM in the time domain according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating tolerance of SPM by CSPA according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating a comparison of CSPA and CPA pulse distortions provided by an embodiment of the present invention;

FIG. 7 is a schematic diagram of a design of a DFT-based chirped square-wave pulse generator according to an embodiment of the present invention;

FIG. 8 is a schematic diagram of a chirped square wave generated by DFT according to an embodiment of the present invention;

Fig. 9 is a diagram illustrating experimental results of a chirped pulse generator according to an embodiment of the present invention;

FIG. 10a is a graph of experimental results comparing pulse widths of CSPA and CPA compressed pulses provided by an embodiment of the present invention;

FIG. 10b is a graph of the results of an experiment comparing the peak power of CSPA and CPA compressed pulses provided by an embodiment of the present invention;

fig. 11 is a schematic structural diagram of a preferred CSPA system according to an embodiment of the present invention;

FIG. 12a is a schematic diagram of the pulse characteristics provided by an embodiment of the present invention without the use of a filter;

FIG. 12b is a schematic diagram of the pulse characteristic with a filter bandwidth of 6nm according to an embodiment of the present invention;

FIG. 12c is a schematic diagram of the pulse characteristic with a filter bandwidth of 4nm according to an embodiment of the present invention;

FIG. 13a is a diagram illustrating the effect of spectral filtering of amplified square pulses to improve the contrast of compressed pulses according to an embodiment of the present invention;

FIG. 13b is a graph of the efficiency of spectral filtering of amplified square wave pulses provided by an embodiment of the present invention;

figure 13c is a graph of the results of the spectral filtering of the amplified square wave pulses provided by embodiments of the present invention on the compressed pulse width.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments, but not all embodiments, of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Fig. 1 is a schematic structural diagram of a chirped square-wave pulse amplification laser system according to an embodiment of the present invention, as shown in fig. 1, including:

The laser oscillator 1, the chirped square wave pulse generator 2, the amplification chain 3 and the spectrum shaping pulse compression unit 4 are connected in sequence;

the laser oscillator 1 emits a seed laser pulse to the chirped square wave pulse generator 2, and the seed laser pulse is converted into a chirped square wave pulse in the chirped square wave pulse generator 2;

The amplification chain 3 amplifies the power of the received chirped square wave pulse and transmits the chirped square wave pulse to the spectrum shaping pulse compression unit 4;

the spectrum shaping pulse compression unit 4 performs pulse shaping on the chirped square wave pulse after power amplification.

It should be noted that the conventional laser system employs a chirped Pulse amplification system, i.e. a CPA system, while the present embodiment employs a chirped Square Pulse amplification system, which may also be referred to as a cspa (chirped Square Pulse generator) system. Aiming at the problem, the embodiment of the invention adopts different modes to process the nonlinearity. It is understood that the most important nonlinear effect of making ultrashort pulse laser systems is self-phase modulation (SPM). Self-phase modulation (SPM) is an effect in nonlinear optics and refers to the nonlinear phenomenon caused by medium nonlinearity in the propagation of an optical pulse in a kerr medium (e.g., an optical fiber). The phase modulation of the medium is proportional to its own intensity distribution. This time-dependent phase change is related to a spectral change that depends on the initial frequency modulation of the pulsed electric field, SPM causing spectral broadening if the initial pulse is fourier transform limited or up-chirped, while the down-chirped initial pulse is spectrally compressed by the influence of SPM. For strong SPM, the spectrum may exhibit strong oscillations.

aiming at the principle of this effect, the embodiment of the present invention designs a new CSPA system, as shown in fig. 1, the output end of the laser oscillator 1 is connected to the input end of the chirped square-wave pulse generator 2, the output end of the chirped square-wave pulse generator 2 is connected to the input end of the amplification chain 3, the output end of the amplification chain 3 is connected to the input end of the spectrum shaping pulse compression unit 4, and the output end of the spectrum shaping pulse compression unit 4 serves as the output of the entire CSPA system.

The laser oscillator 1 may generally adopt a seed laser providing an initial pulse, the initial laser pulse signal emitted is a seed laser pulse in the embodiment of the present invention, the seed pulse laser is emitted into the chirped square wave pulse generator 2, the laser is converted into the chirped square wave pulse laser under the action of the chirped square wave pulse generator 2, and then the chirped square wave pulse laser is subjected to the action of the amplification chain 3 with high power amplification and the action of the spectral shaping pulse compression unit 4 which improves the pulse quality and performs pulse shaping, and then the ultrafast laser with high pulse energy, high average power and high repetition frequency required by the embodiment of the present invention is output.

It can be seen from the above that the SPM effect causes pulse distortion, and the CSPA system provided by the embodiment of the present invention utilizes the fact that the constant SPM does not generate pulse distortion. Fig. 2a is a schematic diagram of pulse distortion caused by SPM according to an embodiment of the present invention, and as shown in fig. 2a, the SPM alone does not necessarily cause pulse deformity, but the pulse deformity is caused by the nonlinear frequency chirp caused by the SPM. The nonlinear frequency chirp is related to a change in nonlinear phase according to the physical characteristics of the SPM. Thus, embodiments of the present invention minimize the impact of SPM on pulse malformations by altering the pulse shape, thereby minimizing the negative impact of SPM. Fig. 2b is a schematic diagram of the inventive pulse shape for minimizing SPM-induced pulse distortion, and fig. 2b uses the same non-linear frequency chirped pulse as fig. 2a except for a flat-topped pulse or a square pulse. As shown in fig. 2b, the main part of the pulse has no frequency distortion except for the rising and falling edges of the square pulse. Therefore, the embodiment of the present invention utilizes an innovative pulse shape design to minimize the pulse malformation caused by SPM instead of minimizing the SPM itself, thereby achieving an improvement in the order of magnitude of SPM tolerance, i.e., the design principle of the chirped square wave pulse generator in the embodiment of the present invention. The chirped square pulse generated by the chirped square wave pulse generator is chirped and has a flat top, and most of the pulse energy has the same amount of SPM, thus resulting in zero frequency offset, which results in zero pulse malformation. Only the pulse edges will be affected by the SPM gradient, which translates into a frequency shift, resulting in pulse malformation.

Fig. 3a and 3b reflect the recovery capability of square wave pulse amplification for SPM in the frequency domain, fig. 3a is a schematic diagram of the pulse intensity and the instantaneous chirp reflecting the recovery capability of square wave pulse amplification for SPM in the frequency domain, and fig. 3b is a schematic diagram of the pulse spectrum reflecting the recovery capability of square wave pulse amplification for SPM in the frequency domain. It can be seen from the contents of fig. 3a and 3b that the center spectrum remains unaffected SPM, while the frequency fluctuations outside the main spectral part are caused by the interference of the non-linear frequency chirp at the edges of the pulse.

fig. 4 is a schematic diagram of the recovery capability of square-wave pulse amplification in time domain to SPM according to the embodiment of the present invention, as shown in fig. 4, the dashed line in fig. 4 represents the shape of the compressed pulse after amplification of the conventional gaussian pulse, and the solid line represents the compressed pulse after amplification of the chirped square pulse. Typical gaussian pulses suffer heavily from pulse malformations caused by SPM, while chirped square pulses are hardly affected. Fig. 5 is a diagram illustrating the tolerance of the CSPA to SPM according to the embodiment of the present invention, and fig. 6 is a diagram illustrating the comparison between the CSPA and the CPA pulse distortion according to the embodiment of the present invention, as shown in fig. 5 and 6, the CSPA system has higher peak power and also means less pulse malformation. The SPM tolerance of CSPA systems can be greater than 200 π, whereas conventional CPA systems are limited to 1 π SPM, a two-order improvement.

it can be seen from the above that the core of the embodiments of the present invention is to provide a chirped square pulse generator instead of the original CPA generator. It should be noted that any pulse shaping technique capable of providing the functions required by the CSPA is applicable to the embodiments of the present invention. Such as spatial light modulators based on 4f pulse shapers. Fig. 7 is a schematic design diagram of a DFT-based chirped square-wave pulse generator according to an embodiment of the present invention, which is implemented by using one or more optical fibers, specifically, a nonlinear fiber 71 for generating spectral broadening, and a dispersive fiber 72 for performing a Dispersive Fourier Transform (DFT). The filter 73 in combination with the dispersive fiber 72 shapes the spectrum and the pulse shape. Specifically, the gaussian pulse from the seed laser is directly connected to the nonlinear fiber 71, and the dispersive fiber 72 generates a linear frequency chirp and broadens the input pulse from the nonlinear fiber. The DFT transform is because it can produce a time-frequency correspondence, i.e., the shape of the spectrum is the same as the waveform in the time domain. Thus, if the filter 73 has a flat-top transfer function, it can be used to generate square pulses. Fig. 8 is a schematic diagram of a chirped square wave generated by DFT, which is fully feasible and provided by the embodiment of the present invention, as shown in fig. 8. And experimental verification is performed in the embodiment of the present invention, and fig. 9 is a schematic diagram of an experimental result of the chirped square pulse generator provided in the embodiment of the present invention.

further, a simulation comparison experiment is performed on the CSPA system and the CPA system according to the embodiment of the present invention, and fig. 10a and 10b are schematic diagrams illustrating experimental results of the CSPA system according to the embodiment of the present invention, where fig. 10a is a graph illustrating experimental results of pulse width comparison between CSPA and CPA compressed pulses according to the embodiment of the present invention, and fig. 10b is a graph illustrating experimental results of peak power comparison between CSPA and CPA compressed pulses according to the embodiment of the present invention, and as shown in fig. 10a and 10b, the CSPA system according to the embodiment of the present invention is better than the conventional CPA system.

the chirped square wave pulse amplification laser system provided by the embodiment of the invention can generate ultrashort pulses with higher order of magnitude pulse energy by minimizing pulse distortion caused by optical nonlinear effect generated in the pulse propagation and amplification processes.

On the basis of the above embodiment, the chirped square wave pulse amplification laser system further includes:

and the input end of the repetition frequency adjusting unit is connected with the chirped square wave pulse generator, and the output end of the repetition frequency adjusting unit is connected with the amplification chain.

fig. 11 is a schematic structural diagram of a preferred CSPA system according to an embodiment of the present invention, and as shown in fig. 11, the CSPA system according to the embodiment of the present invention may further include a repetition frequency adjustment unit, where the repetition frequency adjustment unit is configured to reduce a pulse repetition frequency. It should be noted that if the output power of the laser oscillator is insufficient, a preamplifier may be added in front of the chirped square pulse generator to adjust the input power.

Preferably, the repetition frequency adjustment unit includes:

the device comprises a preamplifier, an acousto-optic modulator, a post-amplifier and an optical isolator which are sequentially connected;

The input end of the preamplifier is connected with the output end of the chirped square wave pulse generator;

The output end of the optical isolator is connected with the input end of the amplification chain.

Specifically, referring to fig. 11, in the CSPA system according to the embodiment of the present invention, the seed oscillator 110 operating in a pulse mode is connected to the chirped square pulse generator 120, the repetition frequency adjusting unit 130 composed of the preamplifier 131, the acousto-optic modulator 132, the post-amplifier 133 and the first optical isolator 134 is respectively connected to the chirped square pulse generator 120 and the amplifying chain 140, the amplifying chain 140 is connected to the final tunable or fixed bandpass filter 151 and the matched dispersion compensator 152, wherein the filter 151 and the dispersion compensator 152 constitute the spectrum shaping pulse compression unit 150. It should be noted that a second optical isolator 141 can be added at the end of the amplification chain to achieve better results.

on the basis of the above embodiment, the spectral shaping pulse compression unit includes:

A band pass filter and a pulse compressor;

the input end of the band-pass filter is connected with the output end of the amplification chain, and the output end of the band-pass filter is connected with the input end of the pulse compressor.

As can be seen from the contents of fig. 3, a small amount of non-linear frequency chirp is generated at the edges of the chirped square pulse. Therefore, a filter is used to eliminate the influence of the pulse characteristic in the embodiment of the present invention, fig. 12a, 12b, and 12c are schematic diagrams comparing the influence of the filter bandwidth on the pulse characteristic provided in the embodiment of the present invention, where fig. 12a is a schematic diagram of the pulse characteristic when the filter is not used, fig. 12b is a schematic diagram of the pulse characteristic when the filter bandwidth is 6nm, fig. 12c is a schematic diagram of the pulse characteristic when the filter bandwidth is 4nm, fig. 13a, 13b, and 13c are schematic diagrams illustrating the spectrum filtering effect of the amplified square wave pulse provided in the embodiment of the present invention, where fig. 13a is a schematic diagram illustrating the effect of the spectrum filtering of the amplified square wave pulse provided in the embodiment of the present invention to improve the contrast of the compressed pulse; FIG. 13b is a graph of the efficiency of spectral filtering of amplified square wave pulses provided by an embodiment of the present invention; figure 13c is a graph of the results of the spectral filtering of the amplified square wave pulses provided by embodiments of the present invention on the compressed pulse width. Since typical CSPA systems use flat-topped pulses, whose compressed pulses will always have some ripple, as shown in fig. 12a, 12b and 12c, an optimizable filter can eliminate non-linear frequency chirp and pulse ripple, the results in conjunction with fig. 13a, 13b and 13c show the possibility of optimizing between pulse contrast, transmission efficiency and pulse width, where pulse width is relatively insensitive to filter bandwidth.

on the basis of the above embodiment, the pulse compressor includes:

Grating pairs or chirped volume bragg gratings.

on the basis of the above embodiments, the amplifier used in the amplification chain includes one or more of a laser amplifier, an optical parametric amplifier, and a raman amplifier.

the high-power ultrashort pulse laser provided in the embodiment of the present invention is formed by using a hybrid design of an optical fiber element and a solid-state amplifier, wherein the solid-state amplifier used may preferably be any one of a laser amplifier, an optical parametric amplifier, or a raman amplifier, or a combination of a plurality of the laser amplifiers, the optical parametric amplifier, or the raman amplifier.

on the basis of the above embodiment, the chirped square wave pulse amplification laser system is composed of an optical fiber material and doped with a rare earth material.

preferably, the chirped square wave pulse amplification laser system provided by the embodiment of the present invention is made of a fiber material, in which several proportions of rare earth materials are mixed, such as: erbium, ytterbium, thulium, etc., so that the operating wavelength of the laser system can be controlled to be around 1550nm, 1064nm and 2000 nm.

On the basis of the above embodiment, the chirped square wave pulse amplification laser system further includes:

an optical system component for protecting the chirped square wave pulse amplification laser system from deleterious nonlinear effects produced by external feedback.

Preferably, the chirped square wave pulse amplification laser system provided by the embodiment of the invention further comprises various types of optical system components, wherein the optical system components are connected with the laser oscillator and various parts for time shaping and spectrum shaping of the laser pulse, and can play the roles of spatial mode matching and optical isolation, and simultaneously can be used for protecting the amplification part from the harmful nonlinear effect generated by other parts or external feedback of optical elements of the laser system. For example: an optical isolator is placed between the amplification chain and the spectrally shaped pulse compression unit.

On the basis of the above embodiment, the laser oscillator includes:

A broadband mode-locked fiber laser, a mode-locked titanium or short pulse laser.

preferably, the laser oscillator provided by the embodiment of the present invention may be a broadband mode-locked fiber laser, a mode-locked titanium: sapphire or other short pulse lasers such as gain switched mode locked semiconductor lasers. In the embodiment of the invention, the best proposal is to use an ultra-short fiber laser which has compact structure, low cost and better stability. It will be appreciated that in one particular embodiment of a nonlinear fiber based chirped square pulse generator, an ultra-short fiber laser can be directly coupled with minimal loss. The pulses generated in a gain switched semiconductor laser can be as short as a few picoseconds, with strong adiabatic and dynamic chirps, and can be further compressed to femtoseconds by using appropriate pulse shaping.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.