Monolithically integrated Sagnac interferometer for all-optical wavelength conversion

摘要

All-optical wavelength conversion in the C-band is demonstrated using a regrowth free monolithically integrated Sagnac interferometer. A single semiconductor optical amplifier (SOA) placed asymmetrically in the loop acts as the non-linear element in the interferometer. Most integrated wavelength converters use the Mach-Zehnder geometry, which requires two SOAs, hence balancing the interferometer becomes an arduous task. The Sagnac geometry, on the other hand, uses a single SOA and the clockwise and the counter clockwise signals traverse the same optical path, thereby negating the effects of optical path length differences. A schematic diagram of the device along with an electron microscope image is shown in fig. 1. The passive elements of the device are comprised of etched waveguides and multimode-interference (MMI) couplers. Planar integration of the SOA onto the passive section is achieved using adiabatic tapered mode transformers. The principle of operation of the wavelength converter lies along the lines of the terahertz optical asymmetric demultiplexer (TOAD), or the semiconductor laser amplifier in a loop mirror (SLALOM). The SOA which is placed asymmetrically in the loop introduces a switching window. A continuous wave signal is fed into port 1 of the interferometer and is split equally by the 3dB MMI coupler. The two signals that are split by the MMI propagate in the clockwise and counter clockwise direction, finally interfering at the input MMI and dropping out of port 1 (reflective mode). The introduction of a time varying control signal (data) in port 3 makes the optical properties of the SOA also time dependent. When a wave front in the loop experiences an index difference relative to the counter propagating complement, the loop is no longer reflective and the signal is dropped out of port 2 for the duration corresponding to the switching window. The maximum repetition rate at which the wavelength converter can operate is limited by the recovery time of the SOA and hence is on the order of 10GHz.