Special Report: Optical Patterns

The microlaser has many good features that make it an excellent candidate for optical information processing. Some of the important features include: Individual microlasers can be as small as a few micrometers. In addition, the surface emitting features allow 2-D integration of many lasers. Therefore approximately 106 lasers can be arranged in a 1 cm2 chip area. Because of the small volume of the active medium, the threshold current can be very low (up to a few microamperes), greatly reducing the power requirements of a system (compared with edge emitters, whose threshold currents are 5 mA to 20 mA). The spectral linewidth of a microlaser is very narrow, typically less than 0.01 nm, for reasons that are explained below. Therefore a microlaser has a longer coherence length than a conventional Fabry Perot laser diode. The beam profile of a microlaser is naturally circular. Therefore the beam can be directly used to illuminate optical components or spatial light modulators (SLM's) without requiring anamorphic prisms or lenses as in edge emitting lasers. In this way, a conventional gas laser and a pinhole spatial filter can be replaced by a simple microlaser, making a compact and robust system. The manufacturing and test process of a microlaser is simple, once the crystal growth step is completed. Millions of microlasers can be obtained simultaneously. Moreover, microlasers can be easily tested in wafer form, whereas edge emitters must be cleaved before testing. Therefore microlasers have a potential for low-cost mass production.

Table 12.1 lists some of the differences between edge emitters and surface emitters.

However, current microlasers have the following limitations:

The wavelengths of the microlasers are mostly limited to 0.75 mm - 1.0 mm. Therefore a holographic recording with a microlaser is currently difficult because most of the holographic recording materials are sensitive to visible wavelengths. Polarization of a microlaser is random or uncontrollable. This problem can be solved in a controllable manner by use of various methods, as explained in subsection 12.2.5. The output power of a microlaser is not as high as that of an edge emitting laser diode. The microlaser is still in its infancy; therefore extensive studies of packaging, addressing, and reliability are few compared with those of edge emitters.

Nonetheless, the microlaser is certainly one of the most important photonic devices developed in this decade and will play an increasingly important role in the future of optical information processing. Currently, active research is being pursued over the world to overcome these problems.

12.2 Status of microlasers

Below, the current status of microlasers related to optical information processing is summarized. 12.2.1 Low threshold current An important parameter used to characterize the power requirements of any diode laser is its threshold current, the minimum current needed for lasing. A low threshold current of 8.7 mA single quantum well [8] and a low threshold voltage 1.33V VCSEL grown by metal-organic chemical vapor deposition (MOCVD) were demonstrated [9]. An extremely large power conversion efficiency of over 50% was also achieved [10]. Also, a microlaser with a threshold current density of 80 A/cm2 was demonstrated.[11]. An ultralow low threshold operation can be expected with reduction in the active region volume until we meet the limitation that is due to nonradiative recombination, and optical and electrical confinement [11,12]. A thresholdless microlaser that does not have a threshold current like a light emitting diode (LED) but still possesses narrow linewidths by photon recycling has also been reported [13].

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