Simpler fabrication promises cheaper tunable DFB lasers
A simpler way of making tunable indium gallium arsenide phosphide-indium phosphide (InGaAsP-InP) external-cavity lasers does not require epitaxial overgrowth and shows potential for low-cost manufacturing and high process reproducibility of these lasers for WDM sources. Rupert S. Schreiner and others at the University of Stuttgart (Germany) and Alcatel (Marcoussis, France) reported making lasers with an active Bragg reflector integrated with an unpatterned separately pumped gain region, which could be electrically tuned from 1590.8 to 1595.2 nm.1 Wavelength-division multiplexing systems with a large number of channels are likely to require compact and relatively inexpensive tunable laser sources such as these.
The research team made an InGaAsP-InP ridge-waveguide laser consisting of two 500-µm-long segments in series: an uncorrugated section and a section with a lateral grating. The segments are galvanically separated to allow for distinct control of each section by different currents for the active Bragg section and for the uncorrugated section (see figure).
Usually, making InGaAsP-InP distributed-feedback (DFB) lasers requires first etching the grating in the waveguide, preparing the sample, and then growing an epitaxial layer over top of the etched waveguide. Preparation includes removing the grating etch mask, and cleaning the area.
The researchers eliminated the overgrowth step, and the sample preparation that goes along with it. The definition of the grating and the separation of the segments are completely separated from the epitaxy step.
They first fabricated the contact stripes, which serve directly as the mesa etch mask. This defines both the ridges and the segment separation, and acts as a protective coating during the succeeding process steps.
The separate current-injection areas allow tuning of the laser wavelength. While the laser can work if only one section is supplied with current (assuming enough current is supplied to increase the photon density above the threshold value), the researchers chose to drive the uncorrugated gain section at 31 mA, and tune the Bragg section by supplying from 0 to 16 mA. This changes the effective refractive index of the Bragg grating from 3.19 to 3.18, resulting in 10 wavelength channels with an average spacing of 5 nm.
Additional gain in the Bragg grating results in more power output, but this can be compensated by decreasing the current to the uncorrugated section. Temperature changes can be used to fine-tune the wavelength. The laser can be electronically tuned over a 5-nm wavelength range, providing up to 11 channels with an average spacing of 0.5 nm and a constant optical output power of 0.5 mW.
For more information contact Rupert S. Schreiner at email@example.com.
- R. S. Schreiner et al., IEEE Photon. Tech. Lett. 13(12), 1277 (December 2001).