Superluminal And Slow Light Propagation In The Active Media Based On Coherent Population Oscillations

The effects of slow and fast light can be used to tailor the time delay of optical signal based on different physics mechanisms. It is one of the key technology for all optical networking and all optical signal processing, which can be employed in a controllable time delay for accurate synchronization of the high bit rate signals in the optical domain to overcome the redundant O/E/O (optical-electrical- optical) conversion. Many approaches have been proposed to realize slow/fast light, where coherent population oscillation (CPO) is one of the most attractive techniques. CPO is particularly interesting because the system is simple and the slow/fast light is tunable in solid-state materials at the room temperature. CPO occurs when the ground state population of a saturable medium oscillates at the beat frequency between two applied optical fields. The oscillation will lead to the effect of slow/fast light due to the hole-burning in the absorption/gain spectrum with a line width on the order of the inverse of the metastable-state lifetime. This paper mainly studied the slow/fast light of the periodic signal propagating in the Erbium-doped fiber (EDF) and semiconductor optical amplifier (SOA) based on CPO, and then we proposed a scheme to break the bandwidth limitation due to CPO and realize the tunable time delay of a single pulse and optical sequence in the SOA at much higher operation frequency.The primary innovative works are as follows.(1) The arbitrary waveform periodically modulated signals propagating in the active media (EDF and SOA) are investigated to obtain the time delay or advance based on the CPO for the first time. A new evaluation method of slow/fast light based on the fundamental harmonic fractional delay (FHFD) and the total harmonic distortion (THD) are proposed and employed to evaluate the slow/fast light in EDF, without the dependence on the reference time as the present evaluation method. The experiment setups of slow light in the EDF and SOA for high bit rate and low bit rate signal are demonstrated. The slow/fast light at 1 KHz and 1 GHz are experimentally studied with the dependence on the incident power, pump configuration, and optical gain.(2) The slow light of the rectangle signal propagating in the active media is studied for the first time, and the relationship of the fractional delay between fundamental harmonic and high order harmonic is also discussed. When the small power signal injects into the EDF, the fractional delays of fundamental and high order harmonic vary with the modulation frequency. The experiments also indicate that the maximum fundamental and higher order fractional delay are almost same and they will be obtained at the same frequency. A factor Q is simply defined to evaluate the trade-off between the FHFD and THD for optical telecommunications and signal processing.(3) To our knowledge, it is the first time that the time delay is tailored over 145.6 ps by the slow light effect for a optical sequence RZ-PRBS at 2 Gbps with the period of 27-1 and the ratio cycle of 20%. It demonstrates that the time delay/advance heavily depends on the injection current, the wavelength (or the pass band of the BPF), and the input power. For the shorter wavelength, the blueshifted sideband is blocked by the BPF before detection, and corresponds to an additional phase advance. For longer wavelength, the redshifted sideband is blocked by the BPF before detection, and corresponds to an additional phase delay. Therefore, a tunable time delay for optical sequence can be flexibly achieved by tuning the wavelengths.(4) A tunable time delay for a 100 ps pulse is achieved via a system of cascaded SOA and band-pass filter (BPF). The time delay of the pulse can be tailored by tuning the filter or changing the SOA injection current. When the 100 ps-duration optical pulse propagates through the cascaded system, a delay of 99.6 ps and an advance of 42.6 ps can be achieved by altering the injection current. For the same pulse, the time delay can be tailored in the range of 165 ps by tuning the filtering configuration (tuning the filter, or tuning the input wavelength) with 500 mA injection to the SOA.