By Yvonne Carts–Powell
Erik Skogen and others at University of California, Santa Barbara (UCSB) have developed a novel quantum–well intermixing technique that can be used to create wavelength–agile InP–based photonic integrated circuits (PICs). They described using the method to produce a widely tunable multisection laser.1 Skogen and coworkers Jonathon Barton, Steven DenBaars, and Larry Coldren demonstrated a device with an optimized active region, providing high modal gain and multiple QWI tuning regions, providing good tuning efficiency.
Quantum–well intermixing (QWI) is a process in which the bandgap of a quantum well (QW) structure is raised by smearing the boundaries between the wells with the barriers. This allows the absorption edge of the QW structures to be controllably tuned with relatively high spatial selectivity. Using QWI, areas with different bandgap energies can be made without epitaxial regrowth.
The researchers achieved the bandgap variation across the device that they needed using impurity–free vacancy–enhanced QWI. This process involves implanting ions at an elevated temperature. The epitaxial layer structure included an implant buffer layer designed to capture the ion implant, creating vacancies in the material. A rapid thermal anneal diffused the mobile vacancies through the structure, which smears the well/barrier interfaces, and increases the quantized energy level in the well. After the mixing, the buffer layer was removed, leaving a thin planar InP surface onto which the gratings were formed.
The group's QWI method allows simultaneous optimization of the active region design for high modal gain, while achieving good tuning efficiencies in QWI material. The process can be applied to a variety of optoelectronic components.
The group used this method to create a four–section sample–grating distributed Bragg reflector (SGDBR) laser that can be quickly wavelength tuned. The design integrates a backside absorber so that there are multiple active sections and multiple tuning sections. The device consists of three active/passive interfaces, requiring intermixed QWs in the phase, front mirror, and back mirror sections (see figure). The active region provides gain, and the mirrors and phase sections are nonabsorbing regions used for tuning via current injection.
Each mirror is capable of continuously tuning more than 6 nm, which translates to a group modal index of 1.5%. By differentially tuning the front and back mirrors, the laser could tune over a range of 37 nm. The sidemode suppression ratio was measured to be 40 dB or more for all channels.
Previous designs used offset quantum wells, rather than QWI material, to alter the bandgap. The new design's optimized active region structure increases the confinement factor by 50% and yields a higher modal gain than the previous designs.
The QWI material is used for tuning via current injection. The threshold current was 15.4 mA, with a slope efficiency of 23% and a threshold voltage of 0.9 V. This is the lowest reported threshold current for a SGDBR laser, say the researchers. The laser emitted 14–mW output power when driven at a current of 100 mA.
For more information, contact Erik Skogen at email@example.com.
1. E. J. Skogen et al, OFC 2002, Postdeadline Paper FB8.