Slow and fast light using nonlinear processes in semiconductor optical amplifiers

Ability to control the velocity of light is usually referred to as slow or fast light depending on whether the group velocity of light is reduced or increased. The slowing of light as it passes through the glass to 2/3rd its original value is a well known phenomenon. This slowing down happens due to the interaction of light with the electrons in the medium. As a general principle, stronger the interaction, larger is the reduction in velocity. Recently, a fascinating field has emerged with the objective of not only slowing down the velocity of light but also speeding it up as it goes through the medium by enhancing light-matter interaction. This unprecedented control opens up several exciting applications in various scientific disciplines ranging from nonlinear science, RF photonics to all-optical networks. Initial experiments succeeded in reducing the velocity of light more than a million times to a very impressive 17 m/s. This speed reduction is extremely useful to enhance various nonlinear processes. For RF photonic applications including phased array antennas and tunable filters, control of phase velocity of light is required while control of group velocity serves various functionalities including packet synchronization and contention resolution in an optical buffer. Within the last 10 years, several material systems have been proposed and investigated for this purpose. Schemes based on semiconductor systems for achieving slow and fast light has the advantage of extremely high speed and electrical control. In addition, they are compact, operate at room temperature and can be easily integrated with other optical subsystems.In this work, we propose to use nonlinear processes in semiconductor optical amplifiers (SOAs) for the purpose of controlling the velocity of light. The versatility of the physical processes present in SOAs enables the control of optical signals ranging from 1GHz to larger than 1000 GHz (1 THz). First, we experimentally demonstrate both optical and electrical control of phase shifts near 1 GHz using four wave mixing (FWM) process. By operating the SOA in the gain region, the refractive index change achievable using four wave mixing processes can be greatly enhanced. Using this, we demonstrate both slowing down (by a factor of 150) and speeding up of light (by a factor of 40). For RF applications, we show electrically controllable phase shift of 1.8n at 0.5 GHz bandwidth. Control of RF phase using FWM has the advantage of achieving true time delays (TTDs) which not only avoids the problem of squinting but also allows steering of beam at faster speeds in a phased array antenna system. The same scheme can be used to construct tunable RE filters.For all-optical networking applications, control of velocity of light at bandwidths greater than 100 Gb/sec is required. The second half of this work investigates utilizing ultra-fast nonlinear processes (including spectral hole burning and carrier heating) in semiconductor optical amplifiers to control optical signals at THz bandwidth. An ultra-short light pulse travelling through the SOA experiences a change in refractive index due to these nonlinear processes. This refractive index change is dependent on the gain of the amplifier. Hence, the group velocity can be controlled electrically by changing the applied current to the device. Quantifying the refractive index change induced by the nonlinear processes involves solving the density-matrix equations for a semiconductor which is extremely complicated and requires the knowledge of the detailed band structure. One usually resorts to the numerical methods for these reasons. We develop simplified analytical frame work to understand the contributions of various nonlinear process and present analytical expressions for the refractive index change induced by spectral hole burning and carried heating. Further, we experimentally demonstrate temporal control of the light pulses to more than 3.7 times the input pulse width (or equivalently a delay-bandwidth product, DBP of 3.7) at extremely high bandwidths (>1 Tb/sec). Finally, an increase in performance is achieved by using a novel scheme based on chirped light pulses. By combining delay and advance results, we demonstrate a continuously tunable DBP of 8.7. We predict that a five-fold increase in DBP to 55 can be achieved by properly designing the semiconductor optical amplifier and the chirper.