Optically pumped external cavity semiconductor lasers (OPS-lasers) are finding favor for diverse applications such as forensic science, video displays, optical inspection, and optically pumping fiber lasers. One advantage of such a laser is that the emitting wavelength thereof is arbitrarily selectable over a broad range of wavelengths through the visible portion of the electromagnetic spectrum into the infrared portion of the electromagnetic spectrum. Another advantage of such a laser is that it is relatively straightforward to operate in a single longitudinal mode to provide a very high quality output beam.
A fundamental component of an OPS-laser is what is commonly termed an OPS-chip or OPS-structure. One preferred OPS-structure includes an epitaxially-grown multilayer mirror-structure surmounted by an epitaxially-grown semiconductor gain-structure. After the mirror-structure and gain-structure are grown, the growth substrate is etched away and the chip is bonded mirror-side down to heat-sink substrate, usually a relatively massive copper block. A diamond-heat-spreader is typically located between the mirror-structure and the copper block.
An OPS-laser-resonator is usually formed between the mirror-structure of the OPS-chip and a separate conventional mirror axially spaced-apart from the chip. The power output of the resonator is typically limited by the ability of the diamond spreader and copper block to remove heat from the chip. This heat is generated by power absorbed in the gain-structure that is not extracted as laser radiation. The mirror-structure impedes the extraction of that heat. As pump power is increased, output-power of the resonator rises until heat can no longer be effectively removed at which point power output drops dramatically due to free-carrier absorption by the gain-structure. This is called “thermal roll-off” by practitioners of the art.
Regarding epitaxially grown mirror-structures, structures formed from alternating layers of gallium arsenide (GaAs) and aluminum arsenide (AlAs), fortunately, can provide high reflectivity and reasonable thermal conductivity at wavelengths between about 800 nm and 1100 nm. Such structures, of course are grown on a GaAs substrate. No other semiconductor system, for example indium phosphide InP and gallium antimonide (GaSb), which would be used for longer wavelength operation offers such a fortunate combination. Coupled with this the problem presented by the mirror-structure impeding heat extraction, is the fact that the heat impedance of a mirror structure increases with increasing wavelength. This is because quarter-wave optical thickness layers of the mirror-structure become physically thicker with increasing wavelength. Further, the efficiency of OPS-gain-structures decreases with increasing wavelength.
In theory at least, the power limitations of a single OPS-chip can be overcome by including two or more OPS-chips in a resonator either in a folded standing-wave resonator or in a traveling wave ring-resonator. One impediment to effecting this in practice is that an OPS-chip can deform during operation due to differential coefficients of expansion of the chip materials and heat-removal components. Deformation of one OPS-chip can deflect the lasing mode in the resonator such that the mode becomes misaligned with the pump-radiation on a second OPS-chip. Because of this it will usually be found that the maximum power available using N identical OPS-chips in a resonator is somewhat less that N times the power obtainable with one such OPS-chip in a resonator.
Another factor limiting the power multiplication is that in a typical resonator arrangement for accommodating multiple OPS-chips, no more than two such chips may be used with circulating fundamental incident thereon at normal incidence. An OPS-chip used at non-normal incidence can be somewhat less efficient than a chip used at non normal incidence due to laterally distributed interference effects in the gain-structure. There is a need to overcome these disadvantages of current OPS-chips.