Special Report: Optical Patterns

Wavelength tuning Continuously tunable lasers have a huge number of potential applications. These range from free-space optical interconnects and wavelength-division multiplexing for communications to holographic data storage and spectroscopy. The microlasers can be tuned in the same way as for edge emitting lasers by varying the drive current applied to them. Recently a new method of wavelength tuning based on a deformable membrane mirror has been demonstrated [38]. As shown in Fig. 12.7, the mirror is fabricated at the end of a light emitting cavity, with a small air gap separating the two. As the mirror moves back and forth, the length of the cavity changes, resulting in a change in the resonant wavelength. A tuning range of 15 nm has been demonstrated using the method. Currently, the device requires extremely high threshold current, 37 mA to 64 mA and thus had to be operated in pulsed mode, rather than continuous wave.

Fig. 12.7. Wavelength tuning of a microlaser by a deformable membrane mirror. (Reprinted with permission from Ref. [38] by Wu et al, © 1991 IEE.)

12.2.8 Efficiency High electrical-to-optical power conversion (wall-plug) efficiency is one of the most important parameters for estimating power consumption. Microlasers suffer from poor wall-plug efficiencies because of excessive voltage drops across the DBR's. However, recent overall cw power efficiencies have approached 20% [39]. Also, dramatic improvements in microlaser power conversion efficiency of up to 50% at low currents have been realized [40] with an index-guided top-emitting structure based on selective oxidation. The efficiencies demonstrated are at least comparable with those of edge emitting lasers.

12.2.9 Modulation speed The injection current modulation bandwidth of small microlasers has been predicted to be very high (>100 GHz) for the following reasons [41] : For a give injection current, the small volume of a microlaser leads to a large photon density and hence a short stimulated lifetime. Cavity quantum electrodynamic effects have been expected to increase the differential gain, because of an enhancement of the emission rate into the lasing mode. Recent measurements indicate a fast intrinsic response, with a 3-dB modulation bandwidth of more than 50 GHz [42]. Recent analyses indicate that a microlaser, under the constraint of nonlinear gain or current density limitations, has the same intrinsic modulation bandwidth as conventional edge emitting lasers with the same cavity losses and photon density [43]

12.2.10 High power output

In spite of the small size of the microlaser, it can be operated at reasonably high power levels. A cw output of more than 100 mW with a wall plug efficiency of 20% has been demonstrated by a group at the University of California at Santa Barbara; they tailored the laser's operation to increased temperatures [44]. Also, 1-W pulsed operation has been demonstrated with a top-surface emitting 100mm X 100 mm broad-area laser with a grid contact segregating the laser into a 10 X 10 array of 8 mm X 8 mm emission windows [45].

12.3 Optical correlators with microlasers

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