Tapered Laser Diodes
Single-mode laser diodes can produce laser light with high beam quality, but only with a modest optical power of e.g. some tens of milliwatts. For much higher output powers, one can use broad area laser diodes, having a broad emitting region; these, however, typically have a much reduced beam quality and therefore only a moderate amount of radiance (brightness). For achieving the combination of high output power and high beam quality, tapered laser diodes have been developed. They may be called high brightness laser diodes. While their output powers are similar to those of broad area laser diodes (tentatively somewhat lower), their M2 factor is typically of the order of 2, i.e., not very far from diffraction-limited.
A tapered laser diode has a semiconductor chip design which is similar to that of a tapered amplifier diode. The most essential difference is that while laser operation is suppressed in a tapered amplifier by minimizing reflections from the end faces, a tapered laser diode exhibits such end reflections, so that it can lase without external optical feedback. The back side is typically equipped with a high reflectivity, while the reflectivity of the front face is relatively small. One may also integrate a Bragg grating into the small waveguide region for selecting the emission wavelength and reducing the emission linewidth.
In contrast to an ordinary broad area laser diode, where the width of the active region is constant along the whole length, a tapered laser diode exhibits substantial additional power losses for light getting back from the output side into the small waveguide. This is associated with the spatial filtering action which improves the beam quality. Some of the returning light might cause optical pumping of regions outside the small waveguide, which is unwanted; in order to prevent that, one may make that region absorbing with additional means (e.g. by applying a reverse bias voltage in that region), or include some additional trenches for eliminating such light. The mentioned additional filter loss does not necessarily have a strong impact on the power conversion efficiency, particularly if the front face reflectivity is small. The details of laser operation with mode filtering are complicated, and one requires sophisticated simulation models to analyze the details and optimize the device design.
Tapered Diode Bars
Some diode bars are made from tapered laser diodes instead of ordinary broad area laser diodes. That results in substantially improved beam quality and an accordingly higher radiance (brightness). While the emission of different emitters is still normally mutually incoherent, the beam divergence in the horizontal direction is substantially reduced.
Further substantial increases of radiance are possible with techniques of beam combining – for example, spectral beam combining, where the different emitters operate at slightly different wavelengths.
External-cavity Diode Lasers with Tapered Amplifiers
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|||Y. Li et al., “The output power and beam divergence behaviors of tapered terahertz quantum cascade lasers”, Opt. Express 21 (13), 15998 (2013), DOI:10.1364/OE.21.015998|
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