An Australia/US collaboration has lead to the fabrication of a vertical-cavity surface-emitting laser (VCSEL) with output that can be steered by as much as 20°. Output direction is controlled by changing the input current. Researchers from Australian National University (ANU; Canberra, Australia) and Sandia National Laboratory (Albuquerque, NM) have demonstrated that the laser can switch between two detectors with low crosstalk and only a small variation in received power. In addition, they have shown that—given certain restrictions in frequency and signal type—current modulation can be also be used to encode optical signals.
The device is based on the fact that, at lower currents, VCSELs tend to lase in so-called "daisy" modes. For a symmetrical device, this means that four sub-beams are formed in the shape of a square. The angle between beamlets is based on two parameters: current and confinement. The first step was to produce a VCSEL with very tight confinement in the y-direction and much looser confinement in the x-direction. With constant current, the result was that the four beamlets had essentially turned into two.1
The next step was to vary the angle of the two sub-beams. Because of the relatively loose confinement horizontally, there are lots of different modes between which energy can be transferred. As the current varied from 11 to 25 mA, the direction of the beam from normal went from about 30° to about 10°.Thermal lensing, say the researchers, is the phenomenon behind this beam steering: not changes in the cavity due to heating.
To test how well the device worked for switching applications, the team set up an experiment with two detectors, both in a plane 6 cm from the VCSEL and 12 mm away from each other. They found that when the light level was maximized for the first detector, the power was 18.2 dB higher than that falling on the second detector (the crosstalk). For the second detector, the crosstalk was even lower. The variation between the maximized power levels was 1.7 dB.
The ANU/Sandia team varied modulation frequency and depth and found that for high-frequency signals (over 50 kHz) the crosstalk would remain low, and the device can transmit with a crosstalk of -18 dB.
REFERENCE
- M. I. Cohen et al., IEEE Phot. Tech. Lett. 13(6), 544 (June 2001).