Overview of Optics Research

Research Supervisor - Dr. Azeemuddin Syed

Optics research at CVEST spans over the areas of optical sensors, ring lasers, modulators and all-optical signal processing. The optics group complements the electronics by using on-chip photonic integrated circuits (PICs) to improve the performance and speed of electronic systems. The major areas of focus in optics and photonics are:

Optical Gyroscopes

Optical gyroscopes are inertial rotation sensors working on the principle of Sagnac effect. They are used in aircrafts, missiles and satellites to monitor its orientation and rotation. Along with accelerometers, they form an essential part of any Inertial Navigation System (INS). The conventional rotation sensors used in today's navigation systems are the He-Ne gyros, containing gaseous laser gain medium. Although highly accurate and sensitive, they suffer from high cost, large size, high power requirements and short operating lifetime. We, at CVEST, strive to overcome these drawbacks by using semiconductor gain medium, fiber cavity and optical signal processing to build a low cost, compact and low power optical gyro. One of our ring laser gyro design is shown in Fig. 1

Fig1. - Proposed semiconductor ring laser gyro design using fiber cavity and polarization rotators.

The configuration of Fig. 1 uses two non-reciprocal Faraday Rotators (nFR) to change the polarization states of counter-traveling light waves. This is done so that they are orthogonally polarized and do not couple inside the gain medium. This overcomes the problem of lock-in in the gyro and improves its sensitivity.

Fig2. - The polarization states of counter-traveling waves and their interaction inside the gain medium.

An on-chip integrated version of semiconductor gyroscopes is also proposed by us, which uses an external compound cavity semiconductor ring laser as the primary sensing element as shown in Fig. 2. This design uses conservative coupling at the active-passive junction to suppress the effect of lock-in caused by nonlinear coupling inside the active region.

Fig3. - An external compound- cavity semiconductor ring laser gyroscope proposed to overcome lock-in problem.

All-Optical Signal Processing

Signal processing in optical domain enhances the capacity of optical networks by eliminating the need of optical-electronic-optical (OEO) conversion. Also, since operating at the speed of light, all-optical signal processing is much faster than electronic signal processing. The optical devices required for signal processing include multiplexers, delta sigma modulators, filters, wavelength convertors, flip-flops etc. We have implemented a binary delta sigma modulator using the integrated semiconductor ring lasers. The proposed configuration is as shown in Fig. 4

Fig4. - A binary delta sigma modulator implemented using unidirectional semiconductor ring lasers.

Another step towards achieving all-optical signal processing has been the development of optical logic gates. While a single semicondutcor ring laser can act as an optical inverter, implementing other logic functions require interconnection of ring lasers. An XOR gate implementation using 4 ring lasers has been proposed as shown in Fig. 5

Fig4. - Implementation of a XOR gate using 4 coupled semiconductor ring lasers.