Bookham reveals corrugated ridge DFB laser
Most single-frequency DFB lasers, or multi-section DBR lasers, used in transmitters require complicated multiple growths and processing steps for fabrication. This usually drives down yields and lengthens cycle times, which means higher costs. 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.
Single-growth DFB lasers have already been demonstrated, but most of the designs are based on e-beam lithography, which can be a slow and expensive technique. Also, many of the previously demonstrated devices used metallic gratings or very deep grating etches, which are not tried-and-tested laser technologies and therefore raise reliability concerns.
Bookham’s 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,” said Benoit Reid, the paper’s lead author. “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 described in the paper use pre-existing epiwafers for 14xx nm pump lasers and so their structure was not optimized for the design. The structure is based on a nominally undoped active region of four 5.5 nm-thick and 1.3% compressively strained InGaAsP quantum wells separated by 10 nm-thick InGaAsP barriers. The grating strength was targeted to minimize 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 inductively-coupled plasma. Conformal p-contact metallization and full wafer back end processing completed device fabrication.
The fabricated corrugated-ridge DFB lasers show very good single-mode behavior, with a side-mode suppression ratio (SMSR) of over 55dB and linewidths of less than 1MHz up to injection currents of 700mA. They offer high-power operation of more than 100mW of optical power at 500mA injection current. This is the highest power demonstrated to date for a single-growth DFB laser.
To demonstrate the potential of the approach, 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 behavior could be used to achieve wider wavelength tunability than that of a conventional temperature-tuned DFB laser.
“With conventional approaches it would be challenging, and perhaps impossible, to produce such effects,” said Reid. “Having the gratings on the sidewalls, and also the associated flexibility, can open up the way to new types of devices or new functionality on a single chip.”
