FBI raises the bar for distributed feedback lasers
The team claims that this performnace equates to the smallest ever efficiency difference between a DFB laser and comparable Fabry-Perot (FP) laser, which had an efficiency of 67 percent.
The difference is attributed to the lower slope efficiency of the DFB laser. However, extrapolation of differential quantum efficiency calculations suggests that the power conversion difference between the DFB and FP lasers may be further reduced by using a smaller grating coupling coefficient.
Enhanced power and efficiency in the new design are attributed to the InGaAs double quantum well (DQW) active wavelength, the 2.1mm thick Al15Ga85As waveguide and a two-step grating fabrication process. This addition promises to make the DFB far more attractive to the tuneable,pumping fibre and solid-state laser markets.
Although DFB lasers possess distinct advantages over their Fabry-Perot counterparts, such as extremely narrow line widths and superior spectral stability, they tend to be hampered by comparitively low power and power conversion efficiency. Typically this class of laser is limited to an output of 5.5W and effciiencies in the range 36-57 percent . In comparison, typical FP lasers deliver more than 8W and realize efficiencies in excess of 73 percent.
The GaAs-based lasers investigated in this study were grown using low pressure MOVPE and the InGaAs active region in the DFB laser - which is periodically structured as a diffraction grating - has GaAsP barriers. The grating in such devices is designed to reflect a narrow band of wavelengths, which leads to a narrow laser linewidth.
Waveguide composition in this DFB laser was optimised for low voltage and leakage current, high carrier mobility and reduced oxidation. A novel vertical design with a far-field emission angle of 450was targeted by optimizing the refractive index profile and adjusting the waveguide thickness and asymmetry.
The design includes a 20nm InGaP etch-stop layer 630nm above the DQW enabling the wafer to be patterned using holography and lithography, after which a 2nd order grating is formed. Subsequent overgrowth of the waveguide creates a high refractive index contrast within the thin single layer grating, which has just two interfaces.
A high slope efiiciency stems from a relatively low grating coupling coefficient of k ~ 3cm-1. The DFB laser, the same material without the grating and a reference FP laser were heated under continuous-wave operation by 20 to 30 K.
Results showed that the presence of the grating layer and the epitaxy processused in making the DFB laser only resulted in a very small increase in resistance as compared to the FP laser.
The researchers reported their results in the journal Electronic Letters (volume 8, p.580)
