Broad-area laser diodes are the most efficient coherent light sources and are widely used today. The extraordinary efficiency, modulation easiness, availability to virtually any wavelength and compactness are the principle drives stimulating the development of light sources based on laser diodes. However, due to fundamental limitations of laser diode gain medium, the emission from laser diodes has a major drawback — the emission is not spatially coherent. In other words laser diode light is often seen as a light bulb emission that cannot be focused in a diffraction limited spot of λ/2 or easily transmitted as a narrow beam. Despite the fact that kilowatts of multimode power can be easily extracted from a laser diode array, the resulting single-lobe single-element power from a laser diode is always limited by a value of several watts. Many applications are waiting for a spatially coherent laser diode source offering power from 1 to 100W. The potential substitution of YAG and fiber lasers by a compact, direct single- mode laser diode source would bring significant advancements in a number of applications such as LIDAR/LADAR (Light Intensity Detection and Ranging/Light Amplification Detection and Ranging) systems; high-bit-rate, long-haul free space communication systems; industrial processing applications; and many more.

Figure 1. Near-field (left) and far-field (right) spectrograms of a broad-area laser diode at 2W output power
The problem is not new. Over several decades, multiple research groups conducted experiments to force a large-stripe laser diode to laze in a single-mode regime. Multiple solutions were proposed. However, mechanical instabilities and associated coupled cavity problems, difficulties to realize sufficiently low and long-term reliable AR coating, and strong energy coupling between lateral modes in single-mode operation were the major reasons why, until today, all these intra-cavity experiments were not converted into commercial products.

Figure 2. Experimental set-up
Our concept is based on the “no-feed-back combining” technology, which is a profoundly different architectural concept for a broad-area laser diode source. An external linear optical system (with no feedback to the laser) converts the laser’s multimode emission into a spatially-coherent (diffraction-limited) spot.

The key enabler of this method is the “rock-solid” stability of the far-field mode patterns of high-power broad-area lasers. The most common laser diode testing procedure is the observation of the spectrally resolved near-field. Typically, messy clouds of overlapping longitudinal and lateral modes are present. This very common and simple observation led many laser diode specialists to a hopeless conclusion of the impossibility of linear combining of broad-area laser diode modes.

The observation of the same pattern in the far-field plane, however, provides a completely different picture; instead of a messy cloud of modes, a well defined regular pattern is observed. These “parabolic” (Fig. 1) pictures of far-field stay valid up to the power levels of catastrophic degradation (up to 10 watts for 100μm wide stripe). All modes of broad-area laser diode emission follow the rectangular box-model description, and are perfectly stable (as a whole) with respect to current and temperature, and are perfectly distinguishable in wavelength or angle. Therefore, all the modes are convertible into one spatially-coherent spot by means of a simple linear optical device. Fig. 1 illustrates this dramatic change in mode identification as a measurement method switches from near-field to the far-field observations.

The experimental demonstration capitalizing these far-field properties of laser diode radiation and realizing the spatial mode multiplexing is done with the set-up shown in Figure 2.

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