Identification method of continuous light components of pulsed laser based on supercontinuum technology

Abstract

The present invention proposes a method for identifying continuous light components in a pulsed laser, which uses a supercontinuum broadening laser technology that combines a high-energy nanosecond pulse laser and a photonic crystal fiber, and belongs to the field of laser technology and nonlinear optics. The output signal of a pulsed laser is mostly responded to by a photodetector of an oscilloscope, and data distortion is prone to occur in the process of photoelectric signal conversion. In addition, pulsed lasers are also prone to produce continuous light components during the amplification process. Starting from the supercontinuum laser broadening technology, the present invention compares the supercontinuum optical properties under constant pulse energy, indirectly realizes the qualitative analysis of the continuous light component in the pulse signal, and reversely deduces the generalized nonlinear Schrödinger equation to obtain the relative content of the continuous light component. The present invention is rigorous and universal, and provides a way of thinking for determining the existence of continuous light components in pulsed lasers, which is of great significance to the design and practical application of high-energy pulsed lasers.

Description

Technical Field

[0001] The invention belongs to the field of laser technology and nonlinear optics, and in particular relates to a method for identifying continuous light components of a pulsed laser based on a supercontinuum spectrum. Background Art

[0002] In recent years, high-energy nanosecond pulse lasers with high peak power and narrow pulse width have unique performance advantages and broad application prospects in industrial material processing, laser radar, nonlinear frequency conversion, etc. due to their high photoelectric conversion efficiency, good beam quality and high reliability. Determining whether the output signal of a pulsed laser contains continuous light components is of great significance for the design and performance evaluation of high-energy nanosecond pulse lasers.

[0003] At present, oscilloscopes and energy meters are used as the main detection means to judge the continuous light component in the output signal of the pulsed laser. The National University of Defense Technology used an oscilloscope method to determine that the output signal of the all-fiber pulsed laser contained a continuous light component, and pointed out that the pulse ripple of the oscilloscope was generated by the detector response and lacked authenticity. Tianjin Modern Laser Research Institute used an energy meter to successfully determine the presence of DC components in the laser output signal, but the repetition frequency of the laser was only 10kHz. The existing method for determining the continuous light component in the laser output signal has certain limitations due to the distortion of the signal conversion of the detection instrument. The method for determining the continuous light component of high-energy, high-repetition-frequency pulsed lasers is still in a blank stage. Therefore, it is imperative to develop a method for determining the continuous light component in the output signal of a laser based on high energy and high repetition frequency.

[0004] Supercontinuum technology is a technology that generates a wide range of continuous spectrum with a spectrum width much larger than the incident spectrum width, by injecting ultrashort pulse laser with high peak power into nonlinear dielectric materials, and then through the combined effects of self-phase modulation, cross-phase modulation, stimulated Raman scattering, four-wave mixing, soliton self-frequency shift and group velocity dispersion, the spectrum produces many new frequency components. Starting from Maxwell's equations, this technology theoretically analyzes the transmission process of pulses in nonlinear dielectric materials, and describes the influence of dispersion and nonlinear effects on supercontinuum by deriving the generalized nonlinear Schrödinger equation and solving the process, which has a rigorous theoretical basis. When using optical fiber as a nonlinear medium for research, it was found that the broadening mechanism of the supercontinuum spectrum varies depending on different pumping conditions: when pumping in the anomalous dispersion region of the optical fiber, the spectral broadening caused by femtosecond pulse pumping the optical fiber is mainly determined by soliton dynamics; the spectral broadening of the optical fiber pumped by picosecond and nanosecond pulses is mainly controlled by modulation instability at the beginning, followed by stimulated Raman redshift and dispersion waves; and when pumping in the normal dispersion region of the optical fiber, the mechanism of spectral broadening caused by femtosecond pulses is mainly self-phase modulation, and the spectral broadening of the optical fiber pumped by picosecond and nanosecond pulses is mainly dominated by four-wave mixing and stimulated Raman scattering. The physical mechanism of supercontinuum generation in optical fiber has been basically perfected by researchers during its development, which is sufficient to meet the supercontinuum broadening of pulsed lasers under different pumping conditions. Therefore, high-energy, high-repetition-rate nanosecond pulse lasers can also be used as pump sources for supercontinuum broadening laser technology. Summary of the invention

[0005] In order to overcome the disadvantage of the existing photoelectric signal conversion being easily distorted and to improve the determination range of the continuous light component of the pulsed laser, the present invention proposes a method for detecting the continuous light component of the pulsed laser based on the supercontinuum spectrum technology. The method adopts the high nonlinear optical fiber of the supercontinuum spectrum output system to control the variable control idea, the energy and pulse width of the pulsed laser are kept constant, and the supercontinuum spectrum broadening degree and the residual pump light content under different repetition frequencies are compared to achieve a qualitative analysis of the continuous light component in the pulsed laser. On the premise that it is clear that there is a continuous light component in the output signal of the pulsed laser, it is assumed that the continuous light content is 0 when the high repetition frequency pulsed laser is working, and the broadening of the supercontinuum spectrum with a low repetition frequency is adjusted to be similar to the broadening of the high repetition frequency. The relative content of the continuous light component in the pulsed laser is calculated based on the peak power inversion nonlinear Schrödinger equation.

[0006] The output laser of a pulsed laser is mainly composed of pulsed light and continuous light. The output laser is affected by the nonlinear effect of the optical fiber material, and together with the dispersion effect, a broadened supercontinuum spectrum is formed. Considering the transmission problem of continuous waves in optical fibers, the normalized nonlinear Schrödinger equation can be expressed in the following form:

[0007]

[0008] Where s = sgn(β2) = ±1, and its positive or negative value is related to the fiber dispersion. N is a parameter related to the fiber and pulse parameters, specifically expressed as:

[0009]

[0010] Considering the specific parameters of the optical fiber and the pulse, we can obtain:

[0011]

[0012] Where P0 is the peak power of the pulsed laser, and T0 is the pulse width of the incident pulse. Therefore, the basic parameters of the nonlinear fiber material, the pulse width of the pulsed laser, and the peak power are the main factors affecting the shape of the supercontinuum spectrum.

[0013] Since the peak power of the pulsed light in the output signal of the laser is much greater than the power of the continuous light component, the pulsed light plays a major role in the supercontinuum broadening process, and the continuous light component plays almost no role. The pulse width and peak power of the pulsed laser are controlled to remain consistent to pump the same highly nonlinear optical fiber, that is, when the single pulse energy of the output laser is the same, the broadening degree of the supercontinuum spectrum remains the same, and the residual pump light content is similar. The single pulse energy used in the actual test is obtained by calculating the ratio of the average power and the pulse repetition frequency. If the pulsed laser output signal contains a continuous light component, the actual single pulse energy value is less than the theoretical single pulse energy value, which is manifested as a reduction in the supercontinuum spectrum width and an increase in residual pump light.

[0014] The technical solution of the present invention:

[0015] Based on laser technology and nonlinear optical principles, the present invention proposes a method for identifying the continuous light component in a pulsed laser. It mainly includes qualitative analysis and quantitative evaluation of the continuous light component when the pulsed laser is working, and the specific steps are as follows:

[0016] Phase 1: Qualitative analysis of the CW light component during pulsed laser operation;

[0017] Step 1: construct a supercontinuum spectrum output system, including: a pulse laser and a highly nonlinear optical fiber; the output end of the pulse laser is connected to the highly nonlinear optical fiber by fusion splicing, and the pulse laser generates a supercontinuum spectrum after being output through the highly nonlinear optical fiber;

[0018] Step 2: setting the calculated pulse energy of the pulse laser to a fixed value to ensure that the ratio of average power to pulse repetition frequency remains unchanged;

[0019] Assume that the high pulse repetition frequency f of the pulsed laser is a Under the working condition, the output supercontinuum spectrum is broadened by c;

[0020] Assume that the low pulse repetition frequency f of the pulsed laser is b Under the working condition, and f a >>f b , the output low supercontinuum spectrum is broadened d;

[0021] Step 3: Compare the supercontinuum width values ​​at different pulse repetition frequencies of the pulsed laser:

[0022] If c≈d, it is determined that there is no continuous light component in the pulsed laser output signal, and the process ends;

[0023] If c>d, it is determined that the pulse laser is at a low repetition frequency f b There is a continuous light component when working and it enters the second stage;

[0024] Phase II: Quantitative evaluation of the CW light component in pulsed laser operation;

[0025] Step 4: Assume that the pulsed laser operates at a high repetition rate f a There is no continuous light when the state is working, and the repetition frequency f b There is a continuous light component when the state is working;

[0026] Step 5: Record the pulsed laser at a high repetition rate f a The supercontinuum broadening shape and single pulse signal energy E of the state fa ;

[0027] Step 6: Adjust and record the pulsed laser at a low repetition rate f b The average pump light power P 低平均 , so that the pulsed laser at a low repetition rate f b The supercontinuum broadening shape and single pulse signal energy E of the state fb With high repetition frequency f a The same as the state, that is, E fb =E fa ;

[0028] Step 7: Calculate the pulsed laser at low repetition rate f b Low continuous optical power P b , the formula is as follows:

[0029] P b＝ E fa *f b

[0030] Step 8: Calculate the low repetition frequency f b The relative content of continuous light in the state is η, and the formula is as follows:

[0031] η=Pb / P 低平均 *100%.

[0032] The pulse laser outputs a pulse signal with a pulse width of 0.1ns to 1μs.

[0033] The highly nonlinear optical fiber is a photonic crystal optical fiber structure or a step-index optical fiber structure; the longitudinal direction is a uniform optical fiber or an uneven tapered optical fiber, and the matrix material is quartz, fluoride, tellurate, phosphate or germanate.

[0034] The supercontinuum spectrum refers to the spectrum broadening caused by various nonlinear effects such as self-phase modulation, cross-phase modulation, stimulated Raman scattering, four-wave mixing, and optical solitons and dispersive waves.

[0035] The diameter of each fiber core in the photonic crystal fiber is 2-6 μm, and the air hole duty ratio is 0.4-0.9; the beveled tail end of the photonic crystal fiber is used to prevent light from being reflected back to the pump laser, thereby protecting the pulse fiber laser from damage.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The supercontinuum technology is used to determine the continuous light component of the pulsed laser by adjusting the repetition frequency of the pulsed laser and comparing the supercontinuum spectrum width and residual pump light content. This avoids signal distortion caused by the photoelectric signal conversion attenuation process, and the determination result of the continuous light component of the pulsed laser is reliable.

[0038] 2. The method of controlled variable analysis can theoretically realize the determination of the continuous light component in the output laser of a pulsed laser that produces a supercontinuum spectrum under different pumping conditions. The determination range is much larger than the coverage of the existing technology, and it is effective for the determination of the continuous light component in the output laser of a high-energy nanosecond pulse laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a diagram of supercontinuum broadening in an embodiment of the present invention;

[0040] Figure 2 for Figure 1 A partial enlarged schematic diagram of the 1000-1400nm band;

[0041] Figure 3 This is a supercontinuum spectrum broadening diagram for adjusting a low repetition frequency to be close to a high repetition frequency in an embodiment of the present invention; the inset in the figure is a partial enlarged schematic diagram of the dotted box area in the supercontinuum spectrum broadening diagram. DETAILED DESCRIPTION

[0042] The present invention "a method for identifying continuous light components of pulsed lasers based on supercontinuum spectrum" is a method for building a supercontinuum spectrum output system, which is mainly composed of a pulsed laser and a uniform photonic crystal fiber. The output end of the pulsed laser is connected to the photonic crystal fiber using a special coupling technology, and the pulsed laser generates a supercontinuum spectrum output through the photonic crystal fiber.

[0043] In this embodiment, the pulse laser is a nanosecond pulse laser with an operating wavelength of 1064 nm, a pulse width of 5 ns, a pulse repetition frequency of 1500 kHz, and an average power of 30.2 W.

[0044] In this embodiment, the photonic crystal fiber is a uniform quartz photonic crystal fiber, and the air holes are arranged periodically in a hexagon, wherein the diameter d of the air holes is 3 μm, the spacing Λ between any two adjacent air holes is 4 μm, the air hole duty ratio d / Λ is 0.75, the fiber cladding diameter is 125 μm, and the fiber length is 3 m.

[0045] In this embodiment, the same photonic crystal fiber is used to carry out experiments.

[0046] The nanosecond pulse laser is set to have the same calculated pulse energy value, that is, the ratio of the average power of the laser to the pulse repetition frequency remains the same, which is 20μJ after energy conversion. If there is no continuous light component in the pulse laser output signal, when the repetition frequency is 1500kHz and 700kHz, the supercontinuum laser finally generated has the same spectral width and similar residual pump light content. According to the actual measurement results, when the repetition frequency of the pulse laser is 700kHz, the spectral width of the supercontinuum obtained is slightly smaller than the spectral width when the repetition frequency is 1500kHz, and the residual pump light content is more, thus proving that the pulse laser has a continuous light component when operating at a repetition frequency of 700kHz.

[0047] Under the premise of confirming the existence of continuous light component in the output signal of pulsed laser, it is assumed that there is no continuous light component in the output signal of high repetition frequency 1500kHz pulsed laser, and the supercontinuum spectrum generated by nonlinear action is all pulsed light. Since there is continuous light in the low repetition frequency 700kHz laser signal, the continuous light component hardly participates in the spectrum broadening, so there will be a lot of residual pump light in the generated supercontinuum spectrum, which is shown as the dotted box part in the supercontinuum spectrum diagram. Adjust and record the average power P of the pump light with low repetition frequency 700kHz 低平均 , so that its supercontinuum spectrum has the same spectral shape as the supercontinuum spectrum with a high repetition rate of 1500kHz. At this time, the energy of the single pulse signal of 1500kHz is equivalent to the energy of the single pulse signal of 700kHz, that is, E f高 =E f低脉冲The low repetition frequency pulse power is the product of the high repetition frequency single pulse signal energy and the low repetition frequency, that is, P 低脉冲 =E f高 *f 低重频 Calculate the average power P at 700kHz 低平均 With pulse power P 低脉冲 The difference between the values ​​of P and y is the continuous optical power P at low repetition frequency. 低连续光 The relative content of continuous light at low repetition frequency is η = P 低连续光 / P 低平均 *100%. The relative content of the continuous light component in a pulsed laser operating at a repetition frequency of 700kHz is calculated to be 3.05%.

[0048] The experiment shows that the method of the present invention adopts the idea of ​​controlling variables, comparing the broadening trend of the supercontinuum spectrum and the residual pump light content, which fills the blank area of ​​the existing technology for the determination of the continuous light component of the pulsed laser, and also avoids the systematic error generated by the signal conversion process, providing a new method for the determination of the continuous light component of the pulsed laser. Compared with the existing discrimination technology, it has the characteristics of wide judgment range, high accuracy and strong rigor.

Claims

1. A method for identifying continuous light components of a pulsed laser based on a supercontinuum spectrum, characterized in that: The steps include: Phase 1: Qualitative analysis of the CW light component during pulsed laser operation; Step 1: construct a supercontinuum spectrum output system, including: a pulse laser and a highly nonlinear optical fiber; the output end of the pulse laser is connected to the highly nonlinear optical fiber by fusion splicing, and the pulse laser generates a supercontinuum spectrum after being output through the highly nonlinear optical fiber; Step 2: setting the calculated pulse energy of the pulse laser to a fixed value to ensure that the ratio of average power to pulse repetition frequency remains unchanged; Assume that the high pulse repetition frequency f of the pulsed laser is a Under the working condition, the output supercontinuum spectrum is broadened by c; Assume that the low pulse repetition frequency f of the pulsed laser is b Under the working condition, and f a >>f b , the output low supercontinuum spectrum is broadened d; Step 3: Compare the supercontinuum width values ​​at different pulse repetition frequencies of the pulsed laser: If c≈d, it is determined that there is no continuous light component in the pulsed laser output signal, and the process ends; If c>d, it is determined that the pulse laser is at a low repetition frequency f b There is a continuous light component when working and it enters the second stage; Phase II: Quantitative evaluation of the CW light component in pulsed laser operation; Step 4: Assume that the pulsed laser operates at a high repetition rate f a There is no continuous light when the state is working, and the repetition frequency f b There is a continuous light component when the state is working; Step 5: Record the pulsed laser at a high repetition rate f a The supercontinuum broadening shape and single pulse signal energy E of the state fa ; Step 6: Adjust and record the pulsed laser at a low repetition rate f b The average pump light power P 低平均 , so that the pulsed laser at a low repetition rate f b The supercontinuum broadening shape and single pulse signal energy E of the state fb With high repetition frequency f a The same as the state, that is, E fb =E fa ; Step 7: Calculate the pulsed laser at low repetition rate f b Low continuous optical power P b , the formula is as follows: P b =E fa *f b Step 8: Calculate the low repetition frequency f b The relative content of continuous light in the state is η, and the formula is as follows: η6P b / P 低平均 *100%。 2. The method for identifying continuous light components of a pulsed laser based on a supercontinuum spectrum as claimed in claim 1, characterized in that: The pulse laser has a pigtail output, and the pulse width of the output pulse signal is 0.1ns-1μs.

3. The method for identifying continuous light components of a pulsed laser based on a supercontinuum spectrum as claimed in claim 1, characterized in that: The highly nonlinear optical fiber is a photonic crystal optical fiber structure or a step-index optical fiber structure; the longitudinal direction is a uniform optical fiber or an uneven tapered optical fiber, and the matrix material is quartz, fluoride, tellurate, phosphate or germanate.

4. The method for identifying continuous light components of a pulsed laser based on a supercontinuum spectrum as claimed in claim 1, characterized in that: The supercontinuum spectrum refers to the spectrum broadening produced by self-phase modulation, cross-phase modulation, stimulated Raman scattering, four-wave mixing, and optical solitons and dispersion waves.

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