Special Report: Optical Patterns

Optical pattern recognition with microlasers

Optical pattern recognition has existed for approximately 45 years. The computational power of optics to perform 2-D (two-dimensional) Fourier transform, convolution and correlation at the speed of light is certainly unique over other technologies. Many different system architectures have been demonstrated to utilize the merits of optics. However, the evolution of optical pattern recognition has been rather slow and its performance often has been challenged by its counterpart, digital computers. This was mainly due to the lack of suitable input /output devices such as SLMs (spatial light modulators) and detectors available in the market. Most of the input/output devices were developed for other consumer applications and their speeds were usually limited to conventional video rates (30 frames per second). With the rapid advances in digital computers that can perform near-video-rate 2-D Fourier transformation, higher speed devices are desperately needed in optical information processing. New device possibilities exist in optical pattern recognition and information processing. Recent advances in optoelectronic device technology include surface emitting microlaser arrays, among others, which have a great potential for ultra-high speed (more than two orders of magnitude faster than video rate) optical pattern recognition and ATM header recognition due to their parallel computing capability. The current status of the microlaser technology will be briefly reviewed and possible applications of new devices for optical pattern recognition and information processing will be described. Advances in these technologies would contribute to the underlying device infrastructure needed for advanced communications services, such as those sketched by the National Information Initiative or "information superhighway".

12.1 Introduction: microlasers, surface emitting laser diode arrays, or vertical-cavity surface emitting lasers

12.1.1 What is a surface emitting laser ? Fig. 12.1 compares the surface emitting laser (SEL) diode [Fig. 12.1(a)] with the conventional edge emitting semiconductor diode laser in a compact-disc player [Fig. 12.1(b)]. In a conventional edge emitting laser diode, the laser beam emerges parallel to the active layer. Therefore a two-dimensional (2-D) arrangement of lasers is difficult. In contrast, the light from a SEL diode emerges perpendicular to the active layer, allowing a 2-D stack of many lasers on a planar wafer substrate. Iga and his co-workers first proposed and demonstrated the feasibility of such SEL diodes in the late 1970's [1,2].

Fig. 12.1. (a) Vertical cavity surface emitting microlaser, (b) edge emitting diode laser. (Reprinted from Ref. [4] by Jewell et al.)

12.1.2. What is a microlaser? The original SEL's had a relatively large active volume (typically 10 mm3), requiring a large driving current. In May 1989, a miniaturized surface emitting microlaser with a very small active volume (typically 0.05 mm3) than that of previous SEL's was developed [3-7]. As can be seen in Fig. 12.2, a small hairlike structure with a diameter of only 1.5 mm is an independent laser. The hairlike structure consists of an active layer surrounded by a pair of high-reflectivity mirrors on both sides. The laser cavity is formed along the direction normal to the wafer surface and laser light is emitted in either the top or the bottom direction. Therefore such lasers are often called vertical cavity surface emitting lasers (VCSEL's) to differentiate them from other surface emitting lasers that have the cavity along the wafer surface. The small size of the active layer of the device, which consists of very thin (approximately 100 A0) quantum well layers, requires a very low driving current to turn on the laser. To compensate for the low gain through the thin active region, the resonators at both ends of the active medium should have extremely high reflectivity to minimize losses, typically 99.9% or a finesse of 3000. This high-reflectivity mirror and the efficient confinement of both electrons and photons within a waveguide define the two major difficulties in fabricating microlasers. We call such an array of microlasers a surface emitting laser diode array (SELDA), or microlasers, in addition to the more commonly used term, VCSEL.

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