Functional meso-optic devices and applications

Interconnect is an important factor which limits electronic system design. The interconnect distance, the number of signal lines per unit length (or form-factor) and the maximum signaling rate adversely constrain large electronic system designs to greater volumes, increased power consumption, and less than optimal bisection bandwidth [1]. In recent years there has been a concerted effort to develop optoelectronic technology to replace copper-based approaches for interconnect lengths greater than 1 m. Arrays of efficient, low-threshold current laser diodes and arrays of pin detectors have been used to provide an effective very high-performance replacement of copper-based interconnects. However, within the constraints of laser diodes and pin detectors there is a limit to which such components can effectively compete with electrical solutions. To insert optics deeper into the core of systems with interconnect lengths less than 1 m it is necessary to develop new devices and concepts which enhance functionality well beyond that of a laser diode or a pin detector and simultaneously ensure scalability to larger numbers of devices. This is both the challenge and application envisioned for meso-optics.

The development of the Vertical Cavity Surface Emitting Laser (VCSEL) has enabled the reduction in physical size (scaling) of laser diodes [2-4]. Active region volumes can approach 10-14 cm-3 and optical resonator volumes can approach 10-12 cm-3. Significantly, scaling experiments performed at USC have demonstrated that laser threshold currents can be reduced to the µA range [4]. On the other hand, scaling of VCSELs has not resulted in any increase in functionality. The device remains merely a diode. From an overall system perspective, without demonstrable additional functionality, diodes are of limited use. They may be used to establish dense arrays of point-to-point interconnects but, for example, they cannot be used to route or process optical signals. The limited functionality of VCSEL and pin-based components requires electronic circuits to perform routing and signal processing functions. A central challenge for researchers in the optoelectronic community is to develop new technologies for future, highly functional, integrated optical circuits that will be capable of replacing significant portions of otherwise all-electronic systems.

The ability to digitally switch lasing wavelength is key to developing optoelectronic components with the needed functionality since it is anticipated that future logic or switching devices will use wavelength coding. To achieve this functionality requires investigation of active coupled-cavity devices. Recently this has been explored using hybrid active and passive components and sequential wavelength switching controlled by intracavity loss has been reported [5]. Applying knowledge gained from these experiments it has been possible to demonstrate the equivalent of an electronic SR flip-flop [6]. The hybrid wavelength-selective electro-optic flip-flop is able to toggle between two Bragg grating (BG) defined lasing wavelengths by application of electrical set and reset signals.

The fact is that functional optoelectronic devices can be made. It is also worth mentioning that all-optical photonic logic devices are also possible to create. However, meso-optic research is needed to scale such devices so that they may be integrated to provide the needed level of complex functionality in future photonic enhancements of otherwise all-electronic digital systems. Key to integration of devices is controlling coupling between active elements in the meso-optic domain. This may require research into coupling between microdisk lasers or other novel devices [7-8].


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