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Breakthrough in power and efficiency puts DFB laser in competition with the FP

Researchers from the Ferdinand-Braun-Institute in Berlin have developed a 976nm DFB laser emitting 11W and having a power conversion efficiency of 58%. This is reported to be the smallest ever difference between a DFB laser and comparable FP laser which had an efficiency of 67%.

The difference is attributed to be due to the relatively lower slope efficiency of the DFB. 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 k. 

The enhanced power and efficiency in the new design are attributed to the InGaAs DQW active wavelength, the 2.1mm thick Al15Ga85As waveguide and a two-step grating fabrication process. This improvement would make the DFB far more attractive to the tuneable, pumping fibre and solid-state laser markets.

Although they possess distinct advantages over their Fabry-Perot counterparts, having extremely narrow line widths and superior spectral stability, the conventional DFB laser suffers from comparitively low power and power conversion efficiency, typically < 5.5W and 36-57%  respectively. Typical values for FP lasers are > 8W and > 73% efficiency.

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 constructed so as to reflect only a narrow band of wavelengths, and thus produce a narrow linewidth of laser output.

The 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 450 was 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. After the subsequent overgrowth of the waveguide, a high refractive index contrast within the thin single layer grating which has just two interfaces, results. 

In order to enable a high slope efiiciency, a relatively low grating coupling coefficient of k  ~ 3cm-1 was employed. The DFB laser, the same material without the grating and a reference FP laser were heated under continuous-wave operation by 20~30K. 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.

These recordresults show that with further modifications, the DFB is one to watch.

Source: Electronics Letters

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