Research progress of high-power narrow-linewidth lasers based on spectral broadening

1 Introduction
All-fiber high-power narrow-linewidth CW lasers have the advantages of high beam quality, good reliability, high wall-plug-optic conversion efficiency, small size, and convenient thermal management [1], and have important applications in scientific research, industrial processing, and military offensive and defensive fields such as coherent synthesis and nonlinear frequency conversion [2-4]. 1 μm
High-power narrow linewidth lasers can be used for welding thick plates of metals such as aluminum alloys to increase the penetration depth [5], and can also be converted into green light through frequency-doubling crystals, because the absorption rate of green light by metal materials is much greater than that of medium red
In addition, its high output energy density can effectively improve the processing effect of laser cutting, welding and other processes [6]. Industrialized countries, led by the United States, Ben, and Germany, have included fiber laser processing in their national development plans [7]. In the military field, high-power CW fiber lasers, as the core part of laser attack and defense, have superior performance such as high concentration of energy, fast and flexible launch, anti-electromagnetic interference, and strong continuous work ability, and play a unique role in optoelectronic countermeasures, air defense and strategic defense. August 2021
For the first time, the US Army developed and demonstrated a 50 kW high-power laser weapon, which is an important part of the Army's strategy to modernize its air defense and missile defense. In the future, laser weapons will be used as strategic equipment to attack strategic missiles and
Satellites are an effective means of territorial defense and the struggle for space power. To meet the needs of these applications, multiple fiber lasers are required for coherent or spectral synthesis to output lasers with very high power and thus for a single laser
Performance puts forward higher requirements, that is, to continuously increase the emission power under the premise of ensuring the quality of the output beam, which is also the main research goal of current high-power lasers.

The narrow linewidth CW laser with direct oscillator structure has better laser mode distribution, but the resonator increases the laser power density and requires a high damage threshold for the active fiber, resulting in limited output power. Take one
The fixed-wavelength laser output is used as a seed source, and its power is further amplified in the Master Oscillator Power Amplification (MOPA) structure, which can form a higher power output [8]. MOPA is considered to be the preferred junction for high-power CW fiber lasers due to its easy control and significant power enhancement performance
[9-10], it can also be used for nanosecond pulsed light amplification to improve the quality of laser engraving
volume and efficiency [11]. Ideally, the laser output power has a near-linear relationship with the pump power or gain coefficient, however, when the output power continues to increase, the output power curve enters the nonlinear region and exhibits gain compression due to a variety of nonlinear effects in the fiber, such as cross-phase modulation, stimulated Raman scattering, stimulated Brillouin scattering (SBS) and four-wave mixing, which limits the maximum power output and reduces the power conversion efficiency. Among them, the SBS effect threshold in the fiber is the lowest, which becomes a bottleneck problem that limits the maximum output power of the laser.

There are four traditional methods of SBS effect suppression, including using fibers with a large effective mode area, controlling temperature or stress gradients to alter the Brillouin gain spectrum, reducing the effective length of the fiber, or increasing the seed line width. Among them, the scheme of using phase modulation to broaden the bandwidth of the seed source has the characteristics of simple and controllable operation and high power improvement efficiency, which has become a research hotspot for suppressing the SBS effect in high-power fiber lasers. In recent years, researchers
The effects of relative intensity noise evolution [12], pump mode [13-14] and modulation type [15] were analyzed to improve the laser performance. In particular, the control of the drive signal for external phase modulation is one of the key steps to ensure the laser quality and maximum output power. In addition, recent studies have found that when the laser power is further increased, the forward output will produce a time-domain self
Pulsing phenomenon [16]. Saturation absorption, stimulated Raman scattering, and stimulated Brillouin scattering effects can all cause the self-pulsing effect in high-power ytterbium-doped fiber lasers, and its characteristics are related to the mode field characteristics of the gain fiber. Among them, the SBS effect has a low threshold, which is easy to produce relaxation during power amplification
Yu oscillation, forming a series of nanosecond pulse trains, which in turn induce a self-pulsing effect [17]. With the increase of laser power, the forward random pulse increases sharply, which has the characteristics of high peak power, short pulse width, and strong randomness, and the ultra-high instantaneous energy will seriously threaten the safety of high-power laser systems [18-19].

In this paper, starting from the suppression methods of SBS effect in high-power narrow linewidth lasers, the effects of different suppression schemes are compared, focusing on high-power lasers in the 1 μm band, the influence of different designs and optimization of phase modulation signals outside the seed source on the laser power threshold is introduced, and the future development trend is discussed, so as to provide reference for the related research of high-power lasers.

2 Method for suppressing stimulated Brillouin scattering effect in high-power lasers

The all-fiber high-power laser based on MOPA structure is a laser with the required frequency generated by the seed source, and after multi-stage power pre-amplification, the ytterbium-doped fiber is used for the main amplification process, which can increase the seed optical power from tens of watts to kilowatts, and the SBS effect is very easy to produce in this process. According to the coupling equation of the SBS effect generation process, the formula for the threshold of SBS effect in optical fiber [20] can be obtained

where: Aeff is the effective mode field area of the optical fiber; Δvs is the spectral bandwidth; ΔvB is the gain spectrum bandwidth of SBS; ΩB is the frequency of the sound wave, and gSBS (ΩB) is the peak value of the SBS gain spectrum. Leff is the effective fiber length; G is the linear gain coefficient of the fiber amplifier. Visible, for narrow linewidth high power fiber lasers
The smaller the SBS threshold, the more likely it is to have the SBS effect and reduce the effective output power of the laser. From Eq. (1), the following four methods can be summarized to increase the threshold of SBS effect:

1) Increase the effective mode field area of the optical fiber. In 2004, Alegria et al. [21] used a 30/400 μm large-mode field erbium-ytterbium co-doped fiber to amplify the output power of a distributed feedback laser seed source with a linewidth of 13 kHz to 83 W
The threshold at which it should be. In 2007, Gray et al. [22] adopted a MOPA structure and utilized
The seed light with a linewidth of 3 kHz and an output power of 100 mW achieves a single-frequency laser output of 100 W in an 8.5 m long Al-Ge gradient doped fiber with a core diameter of 39 μm. In 2020, Valero et al. [23] adopted a non-darkening fiber with a core diameter of 35 μm, based on a MOPA junction
The laser output is achieved with a center wavelength of 976 nm and a maximum power of 39 W. Although the SBS effect can be effectively suppressed by using fibers with large mode field areas, they are limited by the type of fiber and laser, and are also darkened by photons
effect, the laser output power is limited, which in turn limits its use in the industrial field. The application of the domain..

2) Change the Brillouin gain spectrum. By applying temperature, stress, etc. to the hand segment, which can effectively reduce the peak value of Brillouin gain, thereby improving the SBS effect Threshold. In 2009, Liu [24] analyzed the inclusion of gain fibers and energy transfer light
The effect of temperature gradient on the SBS effect threshold at 0 °C
Obtain a higher SBS threshold. 2013, Shanghai Optics, Chinese Academy of Sciences and Zhang et al. [25] of the Institute of Precision Mechanics, theoretical analysis and experimental testing It is demonstrated that 20 stress gradients are applied to the gain fiber, supplemented by the corresponding temperature
distribution, the threshold for the SBS effect was increased by a factor of 7. But for the sake of getting better The threshold lifting effect requires a greater range of stress and temperature control, increasing .The burden on the system, while being limited by the capacity of the fiber, is the most SBS effect. The large threshold boost is limited and is not suitable for the output of fiber lasers on the military
Power boost.

3) Reduce the effective length of the fiber. Doping the medium in the gain fiber can reduce the effective length of the gain fiber, thereby increasing the threshold of the SBS effect. In 2010, Shi et al. [26] obtained a near-diffraction-limited pulsed laser output with a peak power of 1.2 kW using a polarization-maintaining double-clad large-mode field erbium-ytterbium co-doped phosphate glass fiber (LC-EYPF) with a length of only 15 cm and a core diameter of 25 μm. In terms of CW fiber lasers, Wang et al. [27] used 1. A 5 m long high-ytterbium-doped large-mode field fiber was used to build an all-fiber MOPA system, and finally a 1064 nm wavelength single-frequency laser output with a beam diffraction magnification of 1.3 M2 and a power of 31 W was finally realized. Compared with pulse output, high-power continuous laser output requires a higher gain coefficient, and the length of gain fibers in practical applications is usually in the meter range, and the continuous high energy accumulation puts forward higher requirements for the radiation resistance of gain fibers, etc., and the laser power threshold improvement effect obtained by reducing the effective length alone is limited, and it is difficult to meet the application requirements of the kilowatt range.

3 Compared with the seed-source spectral broadening method based on phase modulation,

The seed-source phase modulation spectral broadening scheme has a significant effect on improving the SBS effect threshold in the MOPA structure fiber laser amplification system. Traditional modulation schemes include sinusoidal signal modulation, white noise source (WNS) modulation, and pseudo-random binary sequence (PRBS) modulation, and the modulated output spectral amplitude envelope is developed according to the Bessel function. In recent years, new ones have been introduced, such as P-modulation sequences, multi-frequency signals, and chirp modulation
In order to improve the flexibility of seed source spectra, optimize the spectral pattern and increase the laser power threshold. This section will introduce and compare the above phase modulation schemes in detail.

3. 1 Sinusoidal signal modulation
In 2011, Fibertek [30] used two-tone signal phase modulation to broaden the spectrum of seed light, as shown in Figure 1, with a phase modulation bandwidth of only 450 MHz and a large 35 μm core doped with ytterbium doped in the main amplifier stage
The fiber with a higher content achieves a laser output of 1 kW and a near-diffraction-limited beam mass of M2<1.4 with an internal quantum efficiency of >83%, as shown in Figure 2, where the SBS threshold has not yet been reached. Although spectrum broadening using sinusoidal signals is easy to achieve, the SBS threshold is limited by the modulation depth of the phase modulator and the microwave power that can withstand it, and the broadening bandwidth is limited and the spectrum is completely separated after broadening Limited.

3.2 White Noise Modulation
The output power spectrum of white noise sources (WNS) is continuous across the entire frequency band, making them an excellent choice for spectral broadening [31]. In 2007, the Olam Company of the United States applied for a special paper on the use of white noise as a phase modulation drive signal to suppress the SBS effect [32], and then WNS modulation was widely studied and applied in industrial production. In 2017, Su et al. [33-34] of the National University of Defense Technology broadened the single-frequency laser after phase modulation.
In order to optimize the polarization state in the process of power amplification, the output signal of a single-frequency linearly polarized laser with a linewidth of 20 kHz and a central wavelength of 1064 nm is broadened by white noise phase modulation, combined with a polarization controller composed of four electro-optical crystals for polarization state control, and the power is amplified by a three-stage Yb-doped fiber amplifier, and the overall experimental structure is shown in Figure 3(a)

show. The first two stages of fiber amplifiers are double-clad Yb fibers with core/inner cladding diameters of 6/125 μm and 10/125 μm, respectively, to amplify the laser power to 18 W. The main amplifier is a 15 m long double-clad Yb-doped fiber with a core/inner cladding diameter of 20/400 μm, which is pumped in one direction. A part of the laser output signal is received by the photodetector (PD), and the polarization state is optimized by the random parallel gradient descent method (SPGD), and the digital-to-analog conversion of the corresponding algorithm is realized by combining the field programmable gate array (FPGA) to control the polarization controller. Finally, the output signal linewidth of the laser is about 0.17 nm, the maximum output power is 1437 W, the pump power is 1764 W, the polarization extinction ratio is greater than 11.1 dB, and the corresponding light-to-light conversion efficiency is 81.0%. Single-frequency laser output after phase modulation and laser output after power amplification
The signal spectrum is shown in Figure 3(b), and Figure 3(c) is the far-field intensity distribution of the output line-polarized laser beam after power amplification, and the beam quality is maintained very well during the power supply.

In 2020, Wang et al. [19] from the Institute of Applied Electronics, China Academy of Engineering Physics, used a white noise phase modulation method to study the suppression method of SBS effect and self-pulse effect during the power amplification process of MOPA-structured lasers. By experimentally comparing the power amplification effects of different WNS phase modulation modes, compared with single-stage WNS modulation, the cascaded WNS modulation with similar laser output linewidth shows a better self-pulse suppression effect than the single-stage WNS modulation, so the spectral spikes of the time-modulated signal are suppressed to a certain extent. The experiment used a plum-shaped curved mold
The mode of stabilizing the high-power narrow linewidth fiber amplifier system is shown in Figure 4(a). With cascaded WNS, the laser amplification power threshold for the 32 GHz linewidth is 2.5 kW, the polarization extinction ratio (PER) is greater than 14 dB, and the beam figure of merit M2 is less than 1.3, as shown in Figure 4(b), (c). However, as random noise, WNS has low spectral and bandwidth controllability, and bandwidth adjustment is achieved by adding low-pass filters with different bandwidths.

3. 3 Pseudo-random sequence modulation
A pseudorandom binary sequence (PRBS) is a classical sequence in which "0" and "1" bits occur with equal and random probabilities, and its spectral distribution is a frequency comb with a Gaussian-like envelope, with an equally spaced discrete optical power spectral density, and the spectral line spacing is a function of modulation frequency and mode length. Researchers at Charles de Gaulle University in France and the University of California in the United States were the first to study the use of PRBS in high-power laser seed-source modulated signals [35-36]. Subsequently, Zeringue et al. [37-39] at the U.S. Air Force Base set up an experimental platform to verify the effect of PRBS signals in suppressing the SBS effect in high-power lasers at the output line
At a width of 6 GHz, the maximum output power reaches 1 kW. The following year, the researchers improved the PRBS sequence, reducing the linewidth to 3 GHz and increasing the output power to 1.17 kW. Anderson, 2015
et al. [40] found that for a given fiber length and signal linewidth, PRBS modulation can provide better spectrum control and SBS effect threshold enhancement factor than WNS.

In 2020, Liu et al. [41] from the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, reported that based on PRBS phase modulation, the PRBS spectral line spacing was adjusted through theoretical simulation and experimental verification to improve the amplification of narrow linewidth fibers
The feasibility of the output power of the generator, and the optimized spectrum of the PRBS modulated signal are shown in Figure 5. The experimental structure of a four-stage doped Yb fiber amplifier as shown in Figure 6 is used, and a 2.2 GHz low-pass radio frequency is used when the sequence length is 9
The filter, the frequency interval of the modulated signal is optimized to 12.7 MHz, and the power of the Bragg reflection (DBR) and distributed feedback (DFB) diode laser output signals is amplified to 1.2 kW and 1.27 kW, respectively, and the system shows a good SBS effect suppression effect. WNS and PRBS phase modulation techniques suppress the SBS effect to a certain extent and increase the output power of high-power narrow linewidth fiber lasers. But the WNS and PRBS institutes
Experimental studies have shown that the near-rectangular spectrum has a better power threshold enhancement effect [42-43], which provides ideas for subsequent spectral optimization schemes.

3. 4 Modulation of P sequences
Since 2020, our research group has been exploring new solutions for increasing laser power based on high-order phase modulation and seed source broadening spectrum optimization. By increasing the phase modulation depth, more higher-order sidebands can be excited, and a single phase modulator can be used to obtain a broadened signal with a wide bandwidth. When the PM is driven by a high-power single-frequency electrical signal, a series of higher-order modulation sidebands are generated, as shown in Figure 7(a), in this case, the resulting signal spectrum
is completely discrete. When a high-power electrical signal with a Gaussian-like envelope is used to drive the PM, the seed source linewidth is broadened and has a Gaussian-like envelope, as shown in Figure 7(b). Theoretically, using the phase modulation method, only the phase information of the optical signal is changed, while the amplitude information remains the same. After multi-stage amplification based on MOPA structure, the power of the seed source can be greatly increased. SBS effect as a nonlinear effect with a minimum threshold, as long as:. The signal power of a single frequency point in the bandwidth is higher than that of other signals, and it is preferentially excited during amplification. As a result, seed source broadening spectra with flat in-band spectrum and steep out-of-band roll-off curves have higher SBS effect threshold, while reducing wasted out-of-band power. Ideally, if the electrically driven signal has an approximately rectangular spectrum, the higher-order phase modulated spectrum of the seed source will also be close to a rectangle. But due to the signal happens. The nonlinear response of the instrument, the electric amplifier (EA), and the PM at different frequencies reduces the power of the electrical signal at high frequencies, which in turn affects the flatness of the spectrum after the PM, as shown in Figure 7(c). The drive signal with a near-triangle envelope can compensate for the low responsivity at high frequencies and achieve near-rectangular broadening, as shown in Figure 7(d) [44].

In this study, a programmable electric drive signal was designed to flexibly control the shape and in-band of the broadened spectrum by using the high-precision controllability of an arbitrary waveform generator (AWG).
Power distribution. By changing the sequence distribution of the binary sequences "0" and "1" in the time domain, a variety of signals can be formed consisting of the specific distribution of "0" and "1" in the time and frequency domains. The probability of inversion between adjacent bits in a sequence
(from "0" to "1" or from "1" to "0") is defined as P, and this sequence type is defined as a modulated P sequence with a tunable probability of reversal. For a completely randomly distributed sequence, the probability of reversal is 0.5. For a sequence with a fixed distribution of "010101...", the probability of reversal is 1. In a binary sequence, the frequency domain distribution of the signal is determined by the distribution of "0" and "1" in the time domain, signal
The relationship between the power spectral distribution and the rollover probability [45] can be expressed as

where: B is the signal rate; f is the frequency. The amplitude envelope simulation of the modulated P sequence at different P values is shown in Figure 8 [44]. The higher the P value, the higher the amplitude of the high-frequency component and the lower the amplitude of the low-frequency component. By changing the signal rate and amplitude, the power distribution of the high and low frequency components in the specified bandwidth can be modified, which can effectively improve the flexibility of bandwidth and spectral adjustment of the seed source. High-power narrow linewidth using seed source spectral broadening and multi-stage amplification
The experimental structure of the laser is shown in Figure 9 [44], when the electric drive signal rate is 2.5 Gb/s, and the flip probability P value is 5/8, 11/16, 6/8, 13/16, and 7/8, the output signal of the arbitrary waveform generator is used as the phase modulator to drive the electrical signal, and the broadened seed source spectrum is shown in Figure 10(a) [44], and the signal bandwidth is about 27 GHz. with cascading white noise
Figure 10(b) [44] shows the relationship between the laser output signal and the reverse return signal at the same bandwidth. The laser power obtained by cascaded white noise modulation is 1150 W, and the corresponding reverse power is 57.6 mW. When the P value is 5/8, the laser output power is 1243 W, and the reverse power is 81.6 mW, and with the increase of the P value, the laser output power gradually increases, and when the P value is 7/8, the highest output power is 1748 W, and the self-pulse effect is observed, and the corresponding reverse power is 182 mW. Under the same bandwidth, the range of laser output power changes under multiple tests is 2%, which verifies the laser power stability of the scheme.

Furthermore, in this study, we propose to use FPGAs and DACs instead of AWGs, as shown in Fig. 11(a) [46], and a single-stage phase modulator to form a compact and low-cost spectral broadening module, which can realize the real-time generation of P-modulated sequences, and realize real-time control of the bandwidth and spectral pattern of the seed source broadening spectrum by changing the P-value, signal rate, and amplitude. The cascaded WNS and the real-time P-tuning sequence generated by the module are used to drive the telecommunications
to measure the output power of the MOPA laser system at the same widened bandwidth. The relationship between the forward output power and the reverse power of the laser at different bandwidths is shown in Figure 11(b). The inset shows an oscilloscope
(OSC) observed self-pulsation phenomenon. When the spread bandwidth is 10 GHz, using the cascaded WNS as the modulated signal, the forward output power of the laser is 1068 W and the reverse when the self-pulsing effect is observed through the oscilloscope

The power is only 18.4 mW. In contrast, the output power of the real-time P-modulation sequence modulation scheme is 1410 W, and the corresponding reverse power is 95 mW. When the seed source spreads the bandwidth to 20 GHz, the real-time modulation of the P sequence modulator
The maximum output power of 2060 W was achieved, which is an increase of 356 W compared to the cascaded WNS modulation scheme with the same spread bandwidth. The increase of the maximum power of the laser fully proves the effectiveness and practicability of the scheme, and lays the foundation for the realization of a compact, flexible and standardized spectral broadening module
Foundation.
3. 5 Chirp signal modulation
In 2017, White et al. [47] of the U.S. Army Research Laboratory used chirp seed sources to disrupt the coherence between lasers and Stokes waves to raise the threshold for SBS effects in high-power fiber amplifiers. A vertical cavity surface-emitting diode laser with the outer mirror removed is used to generate a chirp signal, and a photoelectric feedback loop is introduced to stabilize the chirp frequency while maintaining the output power. Excitation via a chirp-based diode as shown in Figure 12(a).
The SBS effect suppression experimental structure of the optical device (ChDL) is measured, and the reverse power and forward output power of the laser at different chirp frequencies are measured as shown in Figure 12(b). Under the chirp frequency of 5×1017 Hz/s and the transmission fiber of 19 m, the SBS effect threshold of 1.6 kW was obtained, and the results showed that the threshold power was independent of the fiber length. In contrast, using only a seed source with a bandwidth of 40 GHz is more likely to cause a spike in reverse power
The resulting power threshold is about 1.3 kW. It is further proposed that for the coherent synthesis of multiple power amplifiers, a phase shifter can be used to compensate for the difference in static and dynamic path lengths to stabilize the linear jitter of the phase over time.

The above scheme is suitable for the suppression of SBS effect in the amplification process of seed optical power amplification with a spectral bandwidth of 1~100 nm under the modulation of chirped signals with a period of μs~ms [48-49]. For the sub-nanometer spectra required for chirp modulation with a period of 10~100 ns and spectral beaming, in 2019, White et al. [43] further studied the chirp modulation scheme of piecewise parabolic phase, Fig. 13(a) is the sawtooth and triangular chirp signal, Fig. 13(b)

The above scheme is suitable for the suppression of SBS effect in the amplification process of seed optical power amplification with a spectral bandwidth of 1~100 nm under the modulation of chirped signals with a period of μs~ms [48-49]. In 2019, White et al. [43] further investigated the chirp modulation scheme of the piecewise parabolic phase, Fig. 13(a) is the sawtooth and triangular chirp signal, and Fig. 13(b) is the simulation of the laser power and the backward Stokes wave power under different modulation formats. In the simulation, when the bandwidth is defined as a frequency range containing 85% of the total power, the seed light threshold of parabolic phase chirp modulation with a period of 23 ns is higher than that of pseudorandom sequence modulation at 1.5 GHz modulation bandwidth when the bandwidth is defined as a frequency range containing 85% of the total power. 4 times, the maximum phase shift required is about 30 rad. The results show that sawtooth or triangular linear frequency chirp modulation can increase the SBS effect threshold and achieve a compact broadened spectrum. However, the swept period, phase, and The fiber length of the scale-up process needs to be matched by precise calculations.

3. 6 Miscellaneous
At present, the broadened spectral amplitude envelope of most seed sources is limited to Gaussian or flat-topped, and the spectrum types of seed sources with the highest laser power threshold are not explored enough, and the driving signal still needs to be optimized. Custom multi-frequency signals have multi-dimensional design capabilities including amplitude, frequency, frequency interval, bandwidth, and phase, which can achieve high-precision control of broaded spectrum and in-band power distribution. The amplitude envelope simulation of multi-frequency drive signals with different bandwidths and shapes is shown in Figure 14, which enables a bandwidth-adjustable moment
Shaped, triangular, arc spectral envelopes. In order to compensate for the weak responsiveness of optoelectronic devices at high frequencies, the high-frequency component of the driving signal should have a higher amplitude than the low-frequency component. The scheme has more dimensions and higher precision spectral pattern adjustment space, and can be extended to any spectral pattern signal design.

Multi-frequency signals offer multi-dimensional control flexibility, but the drive signal bandwidth is typically in the GHz range, with frequency intervals of kHz or Hz, which greatly increases the complexity of the design. In this study, we further propose binary multi-frequency signals to maintain configurable tuning while reducing design complexity [50]. The time- and frequency-domain simulations of a binary multi-frequency signal are shown in Figure 15. Firstly, the multi-band drive signal is preset according to the target spectrum type and bandwidth. Then, set it up .Amplitude threshold, the frequency component with amplitude above the threshold is set to 1, otherwise set to 0, after binarization, the in-band flatness and spectral continuity

be improved; Finally, a digital bandpass filter is used to filter out harmonics. By adjusting the threshold in the time domain, the signal amplitude of different frequency components can be changed, and the shape of the spectrum can be controlled. When the bandwidth of the binary multi-band drive signal increases from 312.5 MHz to 1.25 GHz, the seed-source measurement spectra with a spread bandwidth from 10 GHz to 30 GHz are shown in Figure 16(a). Due to the limited measurement accuracy of commercial spectrometers, fine spectral shapes cannot be observed. This study is further 1 Vol. 60 No. 15 / August 2023 / Advances in Lasers and Optoelectronics uses a self-made high-precision spectral measurement scheme to measure the spectrum accurately
The increase from GHz to MHz provides a basis for high-precision spectrum optimization. By tuning the amplitude, frequency, phase and other information of the binary multi-frequency drive signal, a diversified broadening spectrum is realized. Figure 16(b) shows
The broadened spectra with triangular, Gaussian and ultra-Gaussian shapes fully demonstrate the flexibility of binary multi-frequency signal modulation and lay the foundation for spectral optimization in different scenarios.

In addition to the above modulation signal design scheme, the driving signal based on the optimization algorithm is also applied to the study of seed spectrum broadening. In 2018, Harish et al. [15] from the Optoelectronics Research Center of the University of Southampton conducted a theoretical study
The method of nonlinear optimization of periodic phase modulation to suppress the SBS effect in single-mode fiber is studied. The simulation adopts the nonlinear multi-objective Pareto optimization method, and uses the finite difference distribution abyss solver in the time amplitude domain with noise to find the optimal phase modulation mode. The modulation depth of 10 phase samples obtained by Pareto optimization algorithm at ±π, 1 m, 1. 5 m、2. The relationship between SBS effect threshold power and laser RMS linewidth at 5 m and 5 m fiber lengths is shown in Figure 17(a), and the relationship between the product value of SBS threshold and length Pth ×L and laser RMS linewidth is shown in Figure 17(b). The simulation results show that for the same laser linewidth,
Smaller line spacing can increase the SBS effect threshold. For a given lineweight, the maximum modulation depth and modulation frequency have less influence on the threshold of the SBS effect, and shorter fiber lengths have a higher threshold. This study provides an optimization idea for spectral broadening modulation signals, but does not take into account the threshold effect in the laser amplification mechanism, and related research is still ongoing.

3. 7 Comparison of different signal modulation schemes
A high-power narrow-linewidth laser in the 1 μm band with different modulation signals
The performance comparisons are shown in Table 2 and include single-frequency signals, white noise, PRBS, P-modulated sequences, multi-frequency signals, and chirped signal modulation. Among them, the single-frequency drive signal is the easiest to generate, however, its completely separated spectrum leads to a higher average power of the seed source spectrum and the worst inhibition performance of the SBS effect.
White noise has a continuous spectrum, but the power spectrum distribution is limited to the Gaussian shape, and the cascaded phase modulation scheme is usually used to achieve the Gaussian shaped seed source spectrum with a large bandwidth, which brings higher cost to a certain extent. In addition, the randomness of its time-domain sequence can lead to random spikes, limiting the maximum laser output power. PRBS is a digital signal with a split line with an envelope distribution similar to that of WNS, and by controlling the frequency spacing, the spectrum of the RRBS can achieve an output power equivalent to that of WNS modulation, which provides a digital solution for the spectral broadening of the seed source.

In contrast, in addition to adjusting the amplitude, frequency, and bandwidth of the driving signal, the spectrum design of the broadened spectrum can also be realized by adjusting the spectrum type of the driving signal. The tone of the P sequence proposed in this paper
The modulation spectrum type can be simply modified by varying the inversion probability of adjacent bits, thus showing better nonlinear rejection performance. As the reversal probability P increases, so does the energy of the high-frequency component. The P modulation sequence is easy to implement and adjust, and the spectral bandwidth can be adjusted by changing the bit rate and amplitude. Single-stage high-order phase modulation with low bandwidth can be used to achieve a wide bandwidth spectrum, which can effectively reduce the complexity and ruler of the structure
Inch. Approximate rectangular spectral broadening with a tunable bandwidth of up to 30 GHz is achieved, while a single driving bandwidth is less than 1.5 GHz. This flexibility of spectrum control is difficult to achieve with the first three options. In addition, the time series is pre-designed to have far fewer random spikes in the broadened spectrum compared to the high time-domain randomness of WNS, with a higher power threshold and signal stability, thus improving the stability of the laser output power. The multi-frequency drive signal and chirp signal have more spectral adjustment dimensions, which are expected to improve the bandwidth, spectral type, roll-off and other characteristics of seed source broadening spectrum to optimize the high-power narrow-bandwidth fiber laser system based on MOPA structure, and the related spectral optimization is being further studied. Future development requires high-power lasers with small size, low power consumption and low cost It has high laser performance, which promotes the development of spectral broadening modules with real-time controllable and compact structure to meet the needs of different scenarios demand.

4 Concluding remarks
The development of cutting-edge scientific research, industrial processing, national defense and other important fields has continuously promoted the performance improvement, technology maturity and industrialization transformation of high-power narrow-linewidth CW fiber lasers. In this paper, we focus on the performance of the seed source using external phase modulation for spectral broadening, in the MOPA structure laser power amplification system, and the modulation signal types range from traditional white noise, PRBS, and single-frequency signals to P-modulated sequences, multi-frequency signals, and chirps
Chirp signals, etc., have more dimensional spectrum adjustment flexibility. In addition, various algorithms such as gradient descent method and multi-objective function optimization have been gradually applied to the optimization process of spectral types to analyze the laser light with different modulation spectra
The influence of power threshold, taking into account the high beam quality and high stability, and exploring the power limit of high-power lasers. With the development of artificial intelligence, it is expected that real-time laser power amplification models will be established in the future to achieve intelligence
The design and optimization of seed source spectrum for different application scenarios and needs Provide spectral design ideas.