Optical multi-stable operations of coupled lasers

Optical memories are optical bi(multi-)stable systems whose states can be switched all optically. Acting as a fundamental building block for digital optical signal processing, they have received considerable attention. Many types of optical memories have been explored, which all have in common that they are optical storage elements with two states. Multi-stable optical logic building blocks are interesting for applications in telecommunication systems, since they have potential to process a large number of wavelength channels in parallel. In this thesis, we present two types of multi-stable operation of coupled lasers. The first one is based on coupled ring lasers, which share a single active element and a feedback arm. A single ring laser with feedback can be regarded as an oscillator, since the intensity of the lasing light in the lasing cavity is periodically oscillating. When two such oscillators are coupled together, sharing the same active element and the same feedback arm, they synchronize in a common oscillation frequency if their individual oscillation periodicities are close to each other; otherwise they show bistability between the two oscillators. Switching between different stable states can be realized by injecting external light, in this sense, the system act as an optical memory. Moreover, this concept can easily realize multi-state operation, since only one active element is required. An eight-state optical memory is demonstrated. The second type of multi-stable operation of coupled lasers is based on serially interconnected lasers using the principle of gain quenching. The light from the dominant laser suppresses its neighboring lasers through gain saturation, but still receives amplification by the active element of the suppressed lasers, compensating for coupling losses. This light passes through each of the successive lasers, simultaneously suppressing and being amplified. By this mechanism all the other lasers are suppressed. Only one of the lasers can lase at a time, thus the state of the optical memory is determined by the wavelength of the dominant laser, as same as the first type. A five-state optical memory based on this concept is experimentally demonstrated. Moreover, we use the optical memories as a fundamental logic unit to realize sophisticated optical logic. We present an optical shift register that consists of two serially connected optical memories driven by common clock pulses. The concept is demonstrated at an operation speed of 20 kHz, which is limited by the laser cavities implemented by 10 meter long fiber pigtailed components. Furthermore, we cascade the optical shift register and an optical XOR gate to realize an optical pseudorandom number generator based on optical memories