Coherent beam combining of 60 fiber lasers by using the stochastic parallel gradient descent algorithm has been demonstrated. The functions of pinhole(s) on the power distributions in the far-field have been systematically simulated on both in-phase and out-of-phase modes. Only one photoelectric detector was used to detect the combined power in the far-field central lobe of the in-phase mode state. When the phase controller was in a closed loop, the contrast of the far-field intensity pattern was as high as ～97% with residual phase error of ～λ/30, and ～34.7% of the total power was contained in the central lobe.

We study modulation properties of two-element phased-array semiconductor lasers that can be described by coupled mode theory. We consider four different waveguide structures and modulate the array either in phase or out of phase within the phase-locked regions, guided by stability diagrams obtained from direct numerical simulations. Specifically, we find that out-of-phase modulation allows for bandwidth enhancement if the waveguide structure is properly chosen; for example, for a combination of index antiguiding and gain-guiding, the achievable modulation bandwidth in the case of out-of-phase modulation could be much higher than the one when they are modulated in phase. Proper array design of the coupling, controllable in terms of the laser separation and the frequency offset between the two lasers, is shown to be beneficial to slightly improve the bandwidth but not the resonance frequency, while the inclusion of the frequency offset leads to the appearance of double peak response curves. For comparison, we explore the case of modulating only one element of the phased array and find that double peak response curves are found. To improve the resonance frequency and the modulation bandwidth, we introduce simultaneous external injection into the phased array and modulate the phased array or its master light within the injection locking region. We observe a significant improvement of the modulation properties, and in some cases, by modulating the amplitude of the master light before injection, the resulting 3 dB bandwidths could be enhanced up to 160 GHz. Such a record bandwidth for phased-array modulation could pave the way for various applications, notably optical communications that require high-speed integrated photonic devices.

We proposed a novel wavelength-spread compression technique for spectral beam combining of a diode laser array. A reflector, which is parallel to the grating, is introduced to achieve a double pass with a single grating. This facilitated the reduction of the wavelength spread by half and doubled the number of combined elements in the gain range of the diode laser. We achieved a power of 26.1 W under continuous wave operation using a 19 element single bar with a wavelength spread of 6.3 nm, which is nearly half of the original wavelength spread of 14.2 nm, demonstrating the double-compressed spectrum capability of this structure. The spectral beam combining efficiency was 63.7%. The grating efficiency and reflector reflectance were both over 95%; hence, the efficiency loss of the double-pass grating with a reflector is acceptable. In contrast to double-grating methods, the proposed method introduces a reflector that efficiently uses the single grating and shows significant potential for a more efficient spectral beam combining of diode laser arrays.

A multi-wavelength sampled Bragg grating (SBG) quantum cascade laser array operating between 7.32 and 7.85 μm is reported. The sampling grating structure, which can be analyzed as a conventional grating multiplied by a sampling function, is fabricated by holographic exposure combined with optical photolithography. The sampling grating period was varied from 8 to 32 μm, and different sampling order ( 1st, 2nd, and 3rd order) modes were achieved. We propose that higher-order modes with optimized duty cycles can be used to take full advantage of the gain curve and improve the wavelength coverage of the SBG array, which will be beneficial to many applications.

We review the recent work of distributed-feedback (DFB) multi-wavelength semiconductor laser arrays (MWLAs) based on the reconstruction equivalent chirp (REC) technique. The experimental results show that the proposed MWLA has very high wavelength precision (<±0.1 nm), while the fabrication cost is low. Only one step of holographic exposure and another step of photolithography are required for grating fabrication. The packaging technique for a high-bandwidth analog DFB laser and laser array was developed. A directly modulated MWLA transmitter module was achieved. In addition, an improved MWLA with an integrated reflector was proposed and successfully applied in a radio-over-fiber system.

We propose a nonparallel double-grating structure in a spectral-beam combining technique, where two gratings are placed nonparallel satisfying the Littrow mount in the focal region of the convergent lens. The most attractive advantage of this approach is that it will compress the spectral span into half of its original spectrum, which means the number of combined elements can be doubled in the gain range of diode lasers. Experimental results demonstrate that the CW output power of the combined beam is 30.9 W with a spectral span of 7.0 nm, compared with its original spectrum span of 13.6 nm, and the spectral beam combining efficiency is 70.5%. In consideration that a single grating could have a high efficiency of >97% in a bandwidth of over ten nanometers, the efficiency loss of the grating pair should be less than 6%, which is acceptable for most applications, so this method of using double gratings should be highly interesting for practical applications when a nearly doubled number of diode lasers could be combined into one single laser compared with the previous single-grating methods.