External-to-cavity lateral-mode harnessing devices for high-brightness broad-area laser diodes: concept, realizations, and perspectives

Abstract

An external linear optical system (with no feedback to the laser) converts the laser's multimode emission into a multilongitudinal but spatially single-mode (diffraction-limited) spot. Recent progress in laser diode mode analysis demonstrating the high stability of the mode pattern of broad-area laser diodes and the development of the low-loss highspectral-resolution phase modulator are the key enablers of such a technique. These advancements led to the demonstration of 60% conversion efficiency of multimode 980 nm diode-laser emissions into a single-mode fiber. INTRODUCTIONBroad-area (BA) laser diodes are the most efficient coherent light sources and are widely used today. The extraordinary efficiency, modulation easiness, and compactness are the three principal drives stimulating the development of light sources based on laser diodes. Despite the fact that kilowatts of multimode power can be easily extracted from laser diode arrays, the reliable single-mode single-element power from the laser diode is always limited by a value approximately equal to 1W due, essentially, to the planar nature of p-n junction injection mechanism. The potential substitution of YAG and fiber lasers [1,2] by a compact direct single-mode laser diode source would bring significant advancements in a number of applications requiring laser beam modulation and unusual wavelengths such as high-range high-resolution high-speed LIDAR/LADAR's [3] , broad-band satellite communication systems, laser HDTV projection systems, cutting/dicing/marking material processing, and many more.The problem is not new. During several decades, numerous experiments were conducted to force a large-emission-area laser diode to laze in a single-spatial-mode regime. Multiple solutions have been proposed. [4][5][6][7][8] Mechanical cavity instabilities, non-linear effects of all kinds, inherently low mode-gain profile overlap, and fabrication difficulties were the major reasons why until today all these experimental demonstrations were not converted into commercial products.This article is attempting to review an unusual approach for the extraction of spatially coherent light from a BA semiconductor laser. The laser is allowed to run in his natural high-efficiency, spatially-multi-mode regime. An external linear optical system (with no feedback to the laser) converts the laser's multimode emission into a multi-longitudinal, but spatially-single-mode (diffraction-limited) spot. The key enablers of such a technique are the recent progress in laser diode mode analysis clearly demonstrating the rock-solid stability of the mode pattern of BA laser diodes and the development of the low-loss spectrally-resolved phase modulator. Accordingly, the article summarizes the basis of mode structure BA laser diode cavities, shows the spatially-and angularly-resolved spectra for BA lasers of several types at high power, briefly summarizes the mode harnessing experiments, and, finally, proposes the potential future development scope for the lateral-mode harnessing approach. SPATIO-SPE...

Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers

“…Many studies of the lateral mode characteristics of broad area lasers have previously been presented, of various degrees of sophistication. Where separated modal profiles were available, a series of guided lateral modes are typically observed, whose profiles and wavelength spacing are similar to those expected based on the simple 'box' model of equation ( 1) [3,9,11,19,20]. In the very earliest studies, the reason was clear-very deep (sawed) mesa structures were used, which generate a large refractive index step at the edges of the stripes [3,19].…”

“…Although vertically single moded, these devices operate in multiple lateral modes, limiting how effectively they can be laterally collimated and fibre coupled [3][4][5][6][7][8][9][10][11][12][13]. The techniques necessary to significantly reduce this far field angle (and to improve the beam quality) continue to be intensively studied [1,2,10,11,[14][15][16][17].…”

“…Although vertically single moded, these devices operate in multiple lateral modes, limiting how effectively they can be laterally collimated and fibre coupled [3][4][5][6][7][8][9][10][11][12][13]. The techniques necessary to significantly reduce this far field angle (and to improve the beam quality) continue to be intensively studied [1,2,10,11,[14][15][16][17]. In the vertical axis, the guided optical modes are predominantly determined by the refractive index profile of the epitaxial layer structure, and the structure is limited by design to one vertical mode (although vertical mode profile changes have been observed at very high injection levels [18]).…”

“…It should be noted that it is possible for the lateral modes corresponding to different longitudinal mode orders to have the same wavelength. In this case, the modes can interact with each other coherently [11].…”

Experimental Demonstration of Central-Lobe Energy Enhancement Based on Amplitude Modulation of Beamlets in 19 Elements Fiber Laser Phased Array

“…The amplitude modulation is employed to adjust the characteristics of the central-lobe in farfield in phased array technique and the amplitude coefficients of array wavelets can be obtained indirectly by solving the radiation field of the equivalent single large-aperture electromagnetic wave [14,15]. According to the feature that the array is formed by splicing multiple wavelets, the amplitude coefficients of the array wavelets can be obtained by sampling the amplitude of the equivalent single large-aperture wave-front.…”