The Study On All-Optical Fiber Acousto-optic Devices Based On Optical Fiber Fused Taper And Coupler

The study on all-optical fiber acousto-optic (AO) devices based on optical fiber taper and coupler is one of AO field with exploratory significance. In this dissertation, we investigate this type of all-fiber AO devices technologically and experimentally.The physical elements of the all-fiber AO devices are formulated in theory, including the excitation and propagation of an acoustic wave in optical fiber, the optical fiber tapers or couplers that can support two or even more optical modes, the mechanism of interaction among the optical modes and an acoustic wave in optical fiber taper or null couplers. This new class of all-fiber devices can function as modulator, frequency shifter, tunable splitter, tunable attenuator, switch, and tunable filter, and the design rules of these devices are depicted.Emphasis is put on the key techniques of fabrication of the optical fiber taper, couplers, especially on the null coupler and acoustic transducers, which determine the final performance of the devices. Firstly, the proper design of ultrasonic transducers is demonstrated, the constituents of them are analyzed in detail, and low-loss, high efficient and wideband transducers are fabricated for the experiments. Secondly, The equipment for optical fiber taper and coupler is improved significantly, including the size of brush flame, the control of the traveling and temperature, and the control of the velocity of the translator. This yields a general recipe for the fabrication of any shaped taper or coupler, and the conditions required for the fabrication of an optimal adiabatic coupler (including null coupler) in a standard single-mode optical fiber are described in detail. The applicability of these techniques for the shape of couplers made in real fabrication systems is considered, and a practical fabrication procedure is also considered. Finally, the experimental systems are built for the study of AO devices.A set of tapers, common couplers and null couplers are fabricated, and AO experiments on them are conducted. Firstly, A single-mode fiber was heated and stretched to form a taper with a uniform waist of 16 mm length, and 14 (m in diameter, with short taper transition of 10 mm long. The insertion loss is 0.2 dB. The AO experiment on the taper is performed with an acoustic transducer. For operation wavelength of 1310 nm, AO resonance is centered at 1.33242 MHz with coupled efficiency of 95.6%, and for wavelength of 1350 nm, acoustic frequency of 1.36183 MHz, coupled efficiency is 97.7%, and the extinction ratio is 16.4 dB when it acts as switch and the electrical drive power is about 13 mW. FWHM bandwidth is about 200 nm, and the center wavelength can be tuned at the range of 500 nm. Also, at theacoustic frequency of 0.50028 MHz, the throughput light of any wavelength is nearly zero, and the extinction ratio can be more than 40dB, which does not agrees with the theory of AO in context, and the possible reason has been given. The experiments show that the device can function as tunable attenuator, switch and tunable filter, and the device is polarization dependent. Secondly, A coupler is made with a long waist of 20 mm, 16 (m in diameter, two transactions of 18 mm length, and insertion loss is about 0.15 dB. Modulation experiment on it shown that for optical wavelength of 1310 nm, and acoustic frequency of 169 kHz, nearly 73% intensity modulation can be obtained, the output signals with double acoustic frequency (338 kHz), and the RF drive power is about 10 mW. Also in the same experiment, 64% and 61% of modulations are achieved at the frequency of 367 kHz and 521 kHz, respectively, but the output signal frequency is equal to the acoustic frequency due to partial resonance and polarization dependence. The analysis about the AO modulation results is given. Finally, The technique of fabrication of null coupler is demonstrated in detail, and null coupler can be formed successfully after lots of experiments, the maximum coupling ratios as small as 1:10000 and the insertion loss less than 0.2 dB can be obtain...