Unique fabrication technique for DFB lasers promises cheaper optical components
Bookham Technology (Oxford, UK) presented a paper at ECOC last month describing a novel method to fabricate ridge-waveguide distributed feedback (DFB) lasers with only a single growth stage. Decreasing the number of growth stages and associated processes promises significant cost reductions and higher yields, especially for chips integrating multiple lasing wavelengths, as in DFB laser arrays, or several optical functions, as in integrated laser modulators.
The fabricated corrugated-ridge DFB lasers show "very good" singlemode behaviour, says Bookham, with a side-mode suppression ratio (SMSR) of over 55 dB and linewidths of <1 MHz up to injection currents of 700 mA. They offer high-power operation of >100-mW optical power at 500-mA injection current, the highest power demonstrated to date for a single-growth DFB laser.
"Most single-frequency lasers, like distributed feedback lasers or multisection distributed Bragg reflector lasers, used in transmitters require complex multiple processing steps for fabrication," explains the paper's lead author, Benoit Reid. "There is now pressure to decrease the cost of optical components so ideally you want to minimise the number of growth and processing steps. For chips with multiple single-frequency laser sources or with more than one optical function, we want simpler fabrication technology."
Single-growth DFB lasers have been demonstrated before, but most designs are based on e-beam lithography, which can be a slow and expensive technique. Also, many proposals require very different approaches, using metallic gratings or very deep grating etches, which are not supported by extensive historic data and therefore raise issues of performance and reliability.
The paper presents Bookham's initial fabrication of DFB lasers with no overgrowth. The design goal was to develop a process flow close to that of a simple ridge-waveguide process using stepper photolithography. "To achieve this, we have designed DFB lasers with third-order gratings written on the ridge sidewalls, forming a corrugated-ridge DFB laser," adds Reid. "Defining the gratings on the sidewalls gives a lot of flexibility in the laser design; one can easily vary the grating pitch and strength along the cavity to achieve a given performance." The devices use preexisting epiwafers for 14xy pump lasers, so their structure was not optimised for the design. The structure is based on a nominally undoped active region of four 55-Å-thick and 1.3% compressively strained InGaAsP quantum wells separated by 100-Å-thick InGaAsP barriers. The grating strength was targeted to minimise spatial hole burning.
For corrugated-ridge fabrication, an I-line stepper was used for photolithography and patterns were transferred to an SiO2 hard-mask layer. The corrugated-ridge waveguide was formed by etching the semiconductor in an induction-coupled plasma. HBr and Cl2/N2 etch chemistries were explored. Conformal p-contact metallization and full wafer backend processing completed device fabrication.
The paper also describes devices fabricated with two different grating pitches on the ridge sidewalls. The lasers emit light at one of two wavelengths, depending on the drive current and the temperature. This behaviour could be used to achieve wider wavelength tunability.