Wanted: applications for tunable VCSELs
A European project called Subtune has produced VCSELs with unprecedented tunability by making exotic hybrid GaAs/InP structures.
The €2.8 million ($4.4 million) project started in April 2008, aiming to combine the materials and make different emitters for wavelengths from 750 nm to 2100 nm.
“At 1.5 µm we have already achieved a 60 nm tunability range, and it looks like that will be achievable down at 850 nm,” said Peter Meissner, the program's co-ordinator.
Subtune s broadly tunable VCSELs are currently intended to address a number of applications that include gas sensing at 2 µm and optical communications at 1.5 µm. Of the eight institutions included in the project laser manufacturers Vertilas and IR Microsystems are providing commercial input concerning markets like these.
However Meissner is also looking to hear from anyone else who has novel end uses that are not currently included in the project.
“Are there any other applications that can use highly tunable VCSELs in this range?” he asked.
A different tune
Meissner points out that although existing lasers offer maximum tunable ranges of around 80 nm, either as external-cavity or edge-emitting devices, VCSELs offer benefits over both.
External-cavity lasers are complex and therefore expensive in comparison to VCSELs, whose lower power consumption can also reduce energy costs. VCSELs also offer tunability without mode-hopping "“ or sometimes erratic step-changes in wavelength "“ and longitudinal single-mode light output.
Within Subtune, Meissner's group at Technical University Darmstadt are teaming with their German compatriots at Technical University Munich and Chalmers University, Sweden, to produce InP “half-VCSELs”. These devices uses InAlGaAs for its active region, and are similar to standard VCSELs without their upper mirror layers.
Putting a movable, concave, GaAs/AlGaAs distributed Bragg reflector mirror on top of the half-VCSEL allows the device to be tuned by ohmic heating. When a small tuning current is injected using via-hole contacts through the substrate the material expands and increases the air gap underneath the mirror, changing the resonance wavelength.
At the moment the two halves of the device are fabricated separately and bonded using a UV-curable adhesive. However, the project is also investigating mirrors made from machined silicon nitride/silica dielectrics, which can be deposited directly on top of the VCSELs using inductively-coupled, plasma-enhanced chemical vapor deposition (IC-PECVD).
Further refinement of the device design offers potential to improve the response speed of tunable VCSELs from 1 kHz currently, up to 100 kHz in the future. However here, as well, Subtune is looking for input from potential users of the resulting devices.
Meissner would like to know: “Would it be useful to go to higher tuning speeds?”