Editing Laser diode (section)

== Reliability ==
{{Technical|section|date=July 2011}}
Laser diodes have the same [[Reliability engineering|reliability]] and [[List of LED failure modes|failure issues]] as [[light-emitting diode]]s. In addition, they are subject to ''[[catastrophic optical damage]]'' COD, when operated at higher power.

Many of the advances in reliability of diode lasers in the last 20 years{{When|date=November 2024}} remain proprietary to their developers. ''[[Reverse engineering]]'' is not always able to reveal the differences between more-reliable and less-reliable diode laser products.

[[Semiconductor]] lasers can be surface-emitting lasers such as [[Vertical-cavity surface-emitting laser|VCSELs]], or in-plane edge-emitting lasers.  For edge-emitting lasers, the edge facet mirror is often formed by [[Cleavage (crystal)|cleaving]] the semiconductor wafer to form a [[Specular reflection|specularly reflecting]] plane.<ref name="ColdrenCorzine2012"/>{{rp|24}} This approach is facilitated by the weakness of the [110] [[crystallographic plane]] in III-V semiconductor crystals, such as [[GaAs]], [[Indium(III) phosphide|InP]], [[Gallium(II) antimonide|GaSb]], etc. compared to the other planes.

The atomic states at the cleavage plane are altered compared to their bulk properties within the crystal by the termination of the perfectly periodic lattice at that plane. [[Surface states]] at the cleaved plane have energy levels within the otherwise forbidden bandgap of the semiconductor. Thus, when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal a fraction of the light energy is absorbed by the surface states, where it is converted to the heat by [[phonon]]-[[electron]] interactions. This heats the cleaved mirror. In addition, the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-than-perfect contact with the mount that provides a path for heat removal. 

The heating of the mirror causes the bandgap of the semiconductor to shrink in the warmer areas. The bandgap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy, causing yet more absorption. This is [[thermal runaway]], a form of the [[positive feedback]], and the result can be melting of the facet, known as ''[[catastrophic optical damage]]'' - COD.

In the 1970s, this problem, which is particularly nettlesome for [[GaAs]]-based lasers emitting between 0.630&nbsp;μm and 1&nbsp;μm (less so for InP-based lasers used for long-haul [[telecommunications]], which emit between 1.3&nbsp;μm and 2&nbsp;μm), was identified. 

Michael Ettenberg, a researcher and later Vice President at [[RCA]] Laboratories' [[David Sarnoff Research Center]] in [[Princeton, New Jersey]] devised a solution. A thin layer of [[aluminum oxide]] was deposited on the facet. If the aluminum oxide thickness is chosen correctly, it functions as an [[anti-reflective coating]], reducing reflection at the surface. This alleviated the heating and [[Catastrophic optical damage|COD]] at the facet.<!--Why?-->

Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10&nbsp;μm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of the interest.

In the very early 1990s, SDL Inc. began supplying high-power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., [[The International Society for Optical Engineering|SPIE]] Photonics West conferences of the era. The methods used by SDL to defeat [[Catastrophic optical damage|COD]] were considered to be highly proprietary and were still undisclosed publicly as June of 2006.

In the mid-1990s, IBM Research - Ruschlikon, [[Switzerland]] announced that it had devised its so-called ''E2 process'', which conferred extraordinary resistance to the [[Catastrophic optical damage|COD]] in [[GaAs]]-based lasers. This process also was undisclosed as of June 2006.

Reliability of high-power diode laser pump bars (used to pump solid-state lasers) remains difficult problem in the variety of applications, in spite of these proprietary advances. Indeed, the physics of diode laser failure is still{{When|date=November 2024}} being worked out, and research on this subject remains active, if proprietary.

Extension of the lifetime of laser diodes is critical to their continued adaptation to a wide variety of applications.