The power and energy enhancement of fiber lasers are mainly restricted by four factors: nonlinear effects, thermal effects, optical damage, and pumping limits. Therefore, the average power and pulse energy output from a single fiber have limits. Coherent synthesis technology is an effective means to break through this limit. Figure 1 shows the main research content in this field.
Incoherent synthesis does not guarantee the coherence of the synthesized beam, but only realizes the superposition of lasers in space. The device is relatively simple, and the application scenario is mainly laser weapons. Incoherent synthesis is mainly divided into three types: parallel synthesis, passive device synthesis and spectral synthesis. In parallel synthesis, the laser output ends are arranged side by side, and the output beam reaches a larger average power in a smaller area at a distance. Passive device synthesis synthesizes multiple lasers into one through devices such as polarization beam splitters and beam combiners. Spectral synthesis refers to the synthesis of multiple narrow-bandwidth continuous light into one, which is mostly completed by volume Bragg gratings, dichroic mirrors, filters, diffraction prisms or prisms.
In coherent synthesis, it is necessary to ensure that each laser has the same phase, optical path, power, polarization, beam diameter and spatial direction. Figure 2 is a schematic diagram of the coherent synthesis system, which can be mainly divided into four parts: beam splitter/beam combiner, seed/amplifier, phase locking and delay locking.
Coherent combining can be measured by four parameters: beam quality, Strehl ratio, combining efficiency and brightness. Beam quality refers to the similarity between the combined light and the Gaussian beam, which is expressed by the beam quality factor M2. The closer M2 is to 1, the higher the beam quality. Strehl ratio refers to the ratio of the peak power of the combined light to the ideal peak power with perfect phase matching. It is related to the phase locking situation and the aperture filling factor. The aperture filling factor refers to the ratio of the beam aperture area to the total area of the array to be combined.
The smaller the phase mismatch, the higher the aperture filling factor, the higher the Strehl ratio, and the closer the coherent combining is to the ideal state. The combining efficiency is the ratio of the combined light power to the total power of each channel before combining. The closer the ratio is to 1, the more ideal it is. Brightness is related to output power, wavelength and beam quality, as shown in formula 1, where C is a coefficient related to the beam shape, and C corresponding to the Gaussian beam is 1. The brightness of the combined beam is the product of the combining efficiency, the number of combined channels and the brightness of a single channel.
Based on the type of beam splitter/combiner, coherent synthesis can be divided into two types: tiled aperture and filled aperture. The aperture filling factor of tiled aperture synthesis is less than 1, which can be achieved through four types of devices: collimator array, microlens array, fiber bundle and multi-core fiber. Figure 3 shows the simulation results of light intensity distribution at different propagation distances when using collimator array for synthesis. The more compact the collimator arrangement, the closer the aperture filling factor is to 1, the better the synthesis effect, and the theoretical limit efficiency is 76% [2]. The device of tiled aperture synthesis is simpler, but the synthesis efficiency is lower.
The filling factor of the filled aperture synthesis is 1, and the synthesis efficiency is relatively high. It can be divided into four types: polarization synthesis, intensity synthesis, diffraction synthesis, and reflection synthesis, as shown in Figure 4. Polarization synthesis refers to the use of a polarization beam splitter or a thin film polarizer to synthesize two orthogonally polarized light beams into one, and the number of synthesis paths can be increased through a cascade structure. Intensity synthesis refers to the method of using an intensity beam splitter to synthesize two paths of light with the same power into one path, and the interference of the idler light port is achieved through phase locking, and multi-path synthesis can also be achieved through a cascade structure.
Compared with polarization synthesis, intensity synthesis is suitable for occasions with higher average power. Diffraction synthesis uses diffraction optical devices, such as gratings and prisms, to synthesize light incident at angles corresponding to different diffraction orders into one beam. A two-stage series structure can be used to expand the synthesis dimension from one dimension to two dimensions to achieve N×N synthesis. The power of diffraction synthesis is limited by thermal effects. Reflection synthesis is achieved through a petal mirror. Different areas of the petal mirror have different reflectivities and transmittances. Coherent synthesis is achieved through destructive interference between the incident light and the reflected light in the direction of the reflected light. The reflectivity of each part has a specific value. Two-dimensional synthesis can also be achieved through a secondary structure.
In addition, there is hybrid aperture synthesis based on microlens arrays. The light beam is split and synthesized through two microlens arrays and a lens. The position of the synthesized beam can be adjusted by controlling the phase of each beam [3].
Under the influence of thermal effects and environmental disturbances, each signal has a certain phase noise, which affects the quality of the synthesized beam and the synthesis efficiency. Figure 5 shows the synthesized light spot when the phase lock is turned on and off when the collimator array is used for synthesis. It can be seen that when the phase lock is turned off, the synthesis effect is very poor.
Phase locking can be classified into active phase locking and passive phase locking. Passive phase locking mainly includes four types: co-resonant cavity phase locking[4], phase conjugation[5], self-organization[6] and evanescent wave coupling. In co-resonant cavity phase locking, the output ends of multiple gain fibers are fed back to each other, which is equivalent to sharing the same resonant cavity, thereby achieving phase locking. In phase conjugation phase locking, based on phase conjugation mirrors, the phase is reversed in time through nonlinear effects such as stimulated Brillouin scattering, thereby compensating for the phase noise in the main amplifier. In self-organizing mode locking, a fiber Bragg grating and a beam splitter are used to form a Michelson interferometer to achieve coupling between the amplifiers, thereby locking the phase. Evanescent wave coupling couples the multi-channel amplifiers into a super mode, thereby achieving coherence between the channels, and is often used in multi-core optical fibers.