Special Report: Optical Patterns

Two-dimensional addressing schemes In a SELDA, more than 106 lasers are available in a 1 cm X 1 cm area. A major issue in operating such devices is their addressing scheme. Three different approaches have been developed for 2-D addressing: individual addressing, matrix addressing, and optical addressing. Individual addressing is the most straightforward method and uses an independent wire for each microlaser. This requires a tremendous amount (N2, where N is the number of lasers along one direction) of wires and pads and becomes impractical as N becomes large. Several individual addressing schemes have been demonstrated [27]. Matrix addressing is the scheme most commonly used to address 2-D devices such as liquid crystal displays or dynamic random access memories (DRAM's). All the SEL's in the same row (or column) are connected together with their common external wire pad. These rows and columns are on the opposite side of the SEL's. To turn on a particular SEL at (i, j), a voltage is applied across the i-th row and the j-th column pad. This point-by-point addressing can be easily extended to line-by-line addressing by the simultaneous application of voltages across all the columns and a row. Such a matrix-addressing scheme requires only 2N electrodes and is easy to fabricate. A monolithic 32 X 32 matrix addressable SELDA has been demonstrated by Orenstein et al., as shown in Fig. 12.4 [28]. Recently a high-density array of microlasers with a spacing of 30 mm between neighboring lasers was demonstrated [29].

Fig. 12.4. Matrix addressing of a microlaser array. (Reprinted with permission from Ref. [28] by Orenstein et al., © 1991 IEE.)

Finally, an optical addressing scheme of a SELDA is shown in Fig. 12.5. In this device, a 2-D image illuminating one side of a SELDA is detected by an array of heterojunction photo-transistors (HPT's), and the current generated by each HPT turns on the corresponding laser. This method allows a complete parallel load of an image, without the need for electrical connections. A monolithic array of such an optically addressable SELDA has been demonstrated [30]. A similar concept had been developed for LED's by various groups and was used for optical information processing [31-33]. The output-light versus input-light relationship is described below in Subsection 12.5.2.

Fig. 12.5. Optically addressable integrated SELDA: (a) structure of the device, (b) light-output versus light-input relationship. (Reprinted with permission from Ref. [30] by CHAN ET AL, APPLIED PHYSICS LETTERS, 58, 2342-2344, 1991. © 1991 American Institute of Physics.)

12.2.5 Polarization control The output light from a microlaser is not linearly polarized. Therefore polarization control is one of the most important subjects to be resolved, especially for polarization-sensitive applications such as magneto-optic disks and coherent detection systems. Recently it was theoretically and experimentally predicted that the polarization of a microlaser grown on an (n11)-oriented substrate could be simply controlled by use of its intrinsic in-plane anisotropic gain distribution characteristics [34]. Stable polarization characteristics based on the prediction were realized for a conventional microlaser structure [35].

12.2.6 Multiple wavelengths Multiple wavelengths and tuning are highly desirable in optical information processing and communications. A 2-D multicolor SELDA (MC-SELDA) is a 2-D array of microlasers, each with its own wavelength. Fig. 12.6(a) shows a monolithic 2-D MC-SELDA originally demonstrated by Chang-Hasnain et al. [36]. The 7 X 20 array has a total of 140 microlasers, and each laser has a unique wavelength that is uniformly separated from its neighbors by 0.3 nm, spanning a total wavelength of 43 nm. Such a wavelength variation was obtained by varying the laser cavity lengths when growing the wafers as shown in Fig. 12.6 (b). More recently, the same group demonstrated a record wavelength span of 62.7 nm by using a modified patterned-substrate growth technique in a molecular beam epitaxy system [37]. The authors claim highly uniform threshold currents with an average of approximately 2 mA with a high repeatability of wavelength spacing and a sharp wavelength-shift rate of 117.14 nm/mm.

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