12 June 2013
Researchers at the State University of New York have used inductively coupled plasma (ICP) reactive ion etching (RIE) to manufacture a ridge waveguide based diode laser. The result was record-breaking 37 mW continuous-wave operation at room temperature for micrometre wavelengths. Additionally, their manufacturing technique is efficient and reduces demand on equipment, saving costs and time.
Modern diode lasers are based on semiconductor heterostructures containing wide band-gap contact layers that also serve as waveguide claddings, and a lower band-gap waveguide core layer. Inside the waveguide core there are several quantum wells that provide optical gain when electrically pumped, which achieves population inversion.
Takashi Hosoda, one of the authors behind the research explained that “the introduction of a quantum well active region in these lasers led to the dramatic reduction of the injected carrier concentration necessary to generate enough optical gain to overcome optical losses – the so called threshold carrier concentration.” He went on to say that “this is especially critical for mid-wave infrared lasers where Auger recombination processes reduce carrier lifetime and thus increase threshold current density.”
The optical cavity for the laser is generally prepared by defining the ridge waveguide in the lateral dimension and cleaving with laser mirrors in the longitudinal direction. The ridge waveguide has been previously fabricated using a wet etching technique. However, as Hosoda explained, there are several difficulties in achieving the desired characteristics: “firstly, the wet etching processes rely on different etching selectivity on different materials, thus it was hard to stop the etching in the middle of a layer with specific etching depth.”
In the work presented in this issue of Electronics Letters, the etching was, said Hosoda, “stopped in AlGaAsSb cladding layer to minimise internal optical loss due to overlap of optical field with metal layers, which was not possible with wet etching techniques.” Additionally, the ridges were suffering from severe undercutting by the isotropic etching property of the wet etching techniques, so it had proved impossible to obtain the desired ridge width with vertical sidewalls, and the dry etching fabrication eliminates these issues. “The recipe used in the fabrication,” said Hosoda, “gives the reliable etching rate of ~ 230 nm/min for both GaSb (p-cap layer) and AlGaAsSb (p-cladding layer) yielding nearly vertical side walls.” Finally, the device yield was improved by introducing the dry etching process. The benefits of the proposed technique were summed up by Hosoda when he said “we have gained the freedom to fabricate quality spatial single mode devices with a specific beam property and well controlled optical losses.”
Given the relatively high and stable output of the team’s laser, it is already possible to implement it in tuneable diode laser spectroscopic systems, or for development of a high power master oscillator – power amplifier sources. However, Hosoda explained that the major challenge is to extend the operating range of the GaSb-based diode lasers to 3.5 µm range and maintain the same output power level.
“We are developing the GaSb-based devices with novel laser heterostructure and quantum well designs to improve the threshold current, temperature stability and output power levels in spectral range from 3 to 3.5 µm,” said Hosoda, “and we are developing single spectral mode distributed feedback diode lasers based on the same low loss efficient lateral ridge waveguide design approach.”
The group is now working on the design and development of infrared Sb-based emitters and detectors, including linear and two dimensional arrays. Working towards these systems, the group have “recently developed a metamorphic growth technique that allows the lattice constant to be treated as a design parameter, and demonstrated novel efficient midwave and longwave infrared diode lasers, light emitting diodes and barrier photodetectors.”
This article is based on the Letter: Room temperature operated diffraction limited λ ≈ 3 μm diode lasers with 37 mW of continuous wave output power (new window).
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