Investigation Of Tunable Fiber-optic Delay Line Based On Stimulated Brillouin Scattering

Tunable fiber-optic delay line is a key component for future all-optical network. Furthermore, it is also very useful in applications such as optical signal processing and microwave photonics. Tunable fiber-optic delay line based on stimulated Brillouin scattering is a novel tunable optical delay technology originated in recent years. It has already been one of the hottest topics in tunable optical delay technology owing to its prominent advantages such as a low control power requirement, flexibility of wavelength, room-temperature operation, and seamless integration with fiber-optic systems. In this dissertation, Brilllouin slow light and optical buffer based on acoustic storage are researched, where special attentions are paid to the problems demanding prompt solution, such as reducing signal distortion and enhancing delay-bandwidth product. The main contents of the dissertation are given as follows:1. Various tunable fiber-optic delay line technologies are summarized and compared. Current research status and application prospect of the tunable fiber-optic delay line based on Brillouin slow light are also summarized and analyzed.2. Physical image and mathematical model of the Brillouin scattering in an optical fiber are given. A set of improved coupled amplitude equations involving optical and acoustic waves for stimulated Brillouin scattering is presented by considering the difference of the transverse distribution between the optical wave and the acoustic wave in an optical fiber. Moreover, the principle of the Brillouin slow light is introduced, and the delay performance both in the undepleted pump regime and in the depleted pump regime are also analyzed when the pump light is monochromatic. The results show that the compensation and overcompensation for the normal dispersion through the anomalous dispersion introduced by the hole-burning effect in the gain specrum is responsible for the delay saturation, reduction, and even pulse advance.3. Stimulated-Brillouin-scattering characteristic of a pump light involving two spectral lines is discussed. Special focus is put on the stimulated-Brillouin-scattering characteristic of a broadband pump light, where transmission equation for the complex amplitude of the Stokes light in the frequency domain is deduced. The delay performance of the Brillouin slow light using a broadband pump light with a Gaussian spectrum is discussed. The results show that the second-order gain nonuniformity is mainly responsible for the signal distortion, and it confines the enhancement of the delay-bandwidth product.4. The method of using broadband Brillouin slow light with a flat-top gain spectrum to reduce signal distortion induced by the second-order gain nonuniformity is proposed. Transmission relationship for the complex amplitude of the Stokes light in the frequency domain is deduced. Furthermore, analytical expressions of the delay, the gain, the pulsewidth broadening ratio, and the delayed Gaussian pulse amplitude are derived. The delay performance of both single-stage and cascaded Brillouin slow-light systems are also theoretically analyzed. The results show that signal distortion induced by the second-order gain nonuniformity can be greatly reduced in the broadband Brillouin slow light with a flat-top gain spectrum and the further enhancement of the delay-bandwidth product is constrained due to the signal distortion introduced by the positive third-order dipersion. The calculating results are then compared with the experimental ones for a single-stage Brillouin slow-light system with a flat-top gain spectrum, where the results show a good agreement between them. Finally, the simulated delay performance for the RZ-OOK, NRZ-OOK and RZ-DPSK bit streams in the broadband Brillouin slow light with a flat-top gain spectrum and a Gaussian gain spetrum are compared. The results indicate that broadband Brillouin slow light with a flat-top gain spectrum is favourable for reducing signal distortion.5. Transparent broadband Brillouin slow light utilizing two absorption peak is studied. Analytical expressions of the delay, the gain, the pulsewidth broadening ratio, and the delayed Gaussian pulse amplitude are derived, which can be used as a reference for designing transparent broadband Brillouin slow-light system. The limiting factor for the transparent broadband Brillouin slow light is also discussed. The results show that there is no gain saturation problem in the transparent Brillouin slow light, but the enhancement of the delay is confined by the Brillouin threshold. Moreover, strong anomalous dispersion in the two absorption peak put a limit to the signal bandwidth, which can also reduce the delay and introduce serious signal distortion.6. The operation principle of the optical buffer based on acoustic storage is introduced, and the mathematical model is also established. The relationship between the system performances, such as maximum storage time, bandwidth,"Write"/"Read"efficiency, and output signal-to-noise ratio, and the system parameters, such as fiber parameters and control pulse parameters, is discussed. Then, the method of using chirped control pulses and a single-mode As2Se3 fiber to improve the buffer performance is proposed. Buffer performance for a single signal pulse is simulated. Furthermore, the buffer capability is analyzed, and the buffer performance for 2.5 Gb/s NRZ-OOK and RZ-DPSK bit streams are simulated. The results show that the output signal-to-noise ratio and the readout efficiency are larger than 15 dB and 30%, respectively, when the control pulse energy is about 0.34μJ and the storage time is smaller than 15 ns.