Fiber laser has the advantages of high conversion efficiency, compact structure, and easy maintenance, and it has been widely used in industrial processing, material treatment, radar detection, and other fields. In some specific fields, requirements for the output power of fiber lasers have been further proposed, such as using high-power femtosecond laser filament principle to realize atmospheric multi-component detection, using narrow linewidth fiber lasers to achieve long-distance Doppler wind speed measurement, and using 532 nm green laser for copper material processing and 3D printing applications. By increasing the output power of fiber lasers, the detection distance and processing efficiency can be improved. However, due to the presence of nonlinear effects such as stimulated Brillouin scattering and mode instability in fibers, the output power of a single fiber is limited. Coherent beam combining (CBC) with multiple lasers is one of the effective ways to obtain high average output power fiber lasers. In a pulse fiber laser CBC system, a shorter pulse indicates wider spectral width. In a high-power continuous fiber laser CBC system, it is often necessary to apply phase modulation, so as to appropriately broaden the seed linewidth and suppress nonlinear effects in the fiber amplifier. A wider spectral width poses higher requirements on the optical path control of each laser in a CBC system. Meanwhile, there exists a coupling between the phase and the optical path. Therefore, simultaneous control of the optical path and phase in the CBC system is needed.

We investigated CBC of broadband light sources based on spectral filtering. Numerical simulations of phase and optical path simultaneous control in broadband laser interferometry were performed using a stochastic parallel gradient descent algorithm. A rectangular spectral light source with a 3 dB spectral width of 10 nm was obtained by using an amplification of spontaneous emission (ASE) light source and a fiber bandpass filter with a center wavelength of 1064 nm. Phase modulation, static optical path compensation, and real-time optical path control were achieved using phase modulators, fiber delay lines, and fiber stretchers, respectively. Two narrowband bandpass fiber bandpass filters were used to filter out laser light with center wavelengths of 1061 nm and 1066 nm and spectral widths of 2 nm, so as to achieve phase and optical path control across the entire spectrum range.

The numerical simulation results of phase and optical path simultaneous control are shown in (Fig. 5). The phase error is random, and the optical path difference is set to 50 μm. Figure 5(a) shows the light intensity change when the wavelength of (1066±1) nm laser is used as the phase control signal. Figure 5(b) shows the light intensity change when the wavelength of (1061±1) nm laser is used as the optical path control signal. Figure 5(c) shows the total light intensity variation. Before the 2100th iteration step, only phase control is performed without optical path control. After 2100 iterations, optical path control is performed once every 100 iteration steps to achieve simultaneous control of phase and optical path. After about five optical path iterations, the total light intensity can reach a normalized intensity of 1. At this time, the light intensities in Fig. 5(a) and Fig. 5(b) are also close to the corresponding normalized intensities of 1, realizing effective optical path control. In addition, an experimental system is set up to validate the results, as shown in Fig. 3. By separately enabling phase and optical path control, the interference light intensity curves from the open loop to the closed loop (Fig. 8) are obtained. Figure 8(a) shows the interference light intensity change after filtering at 1066 nm. Figure 8(b) shows the interference light intensity change after filtering at 1061 nm, and Fig. 8(c) shows the total light intensity change. From Fig. 8, it can be seen that when the system is in an open loop, the interference light intensity fluctuates randomly under the influence of noise due to the lack of phase and optical path control. After phase control is performed, similar to that in Fig. 5, the phase-controlled interference light intensity in Fig. 8(a) can reach a higher value, and the optical path-controlled interference light intensity and the total interference light intensity are stabilized around a fixed value. After optical path control is enabled, the optical path control cycle is about 1 s, and all the interference light intensities are effectively improved, indicating that effective compensation for the optical path has been performed. The conservative estimate of the optical path control range is over 0.1 ps, the phase residue error is less than λ/16, and the combined efficiency is about 90.3%.

We conduct research on the phase and optical path simultaneous control in CBC of broadband light sources based on spectral filtering. The combined laser is appropriately filtered to obtain different spectral components, and simultaneous control of phase and optical path across the entire spectrum range is achieved by separately controlling the phase of different spectral components. The feasibility of this method is verified theoretically and experimentally, and a detailed control process is provided. This method has the advantages of a clear theoretical model, high reliability, and good scalability, and it has important application value in the CBC of femtosecond pulse lasers and high-power continuous broadband lasers. Future work will focus on optimizing the control system, improving the control accuracy, using fiber gratings to achieve different bandwidths filtering, and clarifying the influence of filtering wavelength on the combining effect. This method will also be applied to ultra-short pulse CBC and the realization of phase and delay locking between different spectra, achieving more channels and wider spectral width in ultra-short pulse CBC.