Abstract-An all-fiber acousto-optic tunable bandpass filter based on a 1.185-mm long coreless core mode blocker is reported. Experimental results demonstrate a minimal insertion loss of 1.2 dB at the optical resonant wavelength of 1527.7 nm with 3-dB optical bandwidth of 0.83 nm. The optimization of the device takes into account the attenuation of the acoustic wave and leads to an asymmetric configuration in which the coupling section is shorter than the recoupling part. Under the effect of a standing flexural wave the device can be operated as a bandpass modulator. The device exhibits a maximum modulation depth of 28 %, 4 dB of insertion loss and 0.97 nm of optical bandwidth at 4.774 MHz.Index Terms-Acousto-optic filters, acousto-optic modulation, bandpass filters.

Active Q-switching of an all-fiber ring laser by utilizing a novel in-fiber acousto-optic tunable bandpass filter (AOTBF) is reported. Transmission characteristics of the AOTBF are controlled by amplitude modulation of the acoustic wave, it exhibits a 3-dB power insertion loss, 0.91 nm of optical bandwidth, and 28 dB of nonresonant light suppression. Cavity loss modulation is achieved by full acousto-optic mode recoupling cycle induced by traveling flexural acoustic waves. When the acoustical signal is switched-on, cavity losses are reduced, and then laser emission is generated. Additionally, by changing the acoustic wave frequency, a wide wavelength tuning range of 30.7 nm is achieved from 1542 to 1572.7 nm. Best Q-switched pulses were obtained at 1.1 kHz repetition rate, with a pump power of 242 mW, at the optical wavelength of 1569.4 nm. A maximum pulse energy of 8.3 J at an average output power of 9.3 mW was achieved, corresponding to optical pulses of 7.8 W peak power and 1 μs temporal width.

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The design and implementation of in-fiber acousto-optic (AO) devices based on acoustic flexural waves are presented. The AO interaction is demonstrated to be an efficient mechanism for the development of AO tunable filters and modulators. The implementation of tapered optical fibers is proposed to shape the spectral response of in-fiber AO devices. Experimental results demonstrate that the geometry of the tapered fiber can be regarded as an extra degree of freedom for the design of AO tunable attenuation filters (AOTAFs). In addition, with the objective of expanding the application of AOTAFs to operate as an amplitude modulator, acoustic reflection was intentionally induced. Hence, a standing acoustic wave is generated which produces an amplitude modulation at twice the acoustic frequency. As a particular case, an in-fiber AO modulator composed of a double-ended tapered fiber was reported. The fiber taper was prepared using a standard fusion and pulling technique, and it was tapered down to a fiber diameter of 70 μm. The device exhibits an amplitude modulation at 2.313 MHz, which is two times the acoustic frequency used (1.1565 MHz); a maximum modulation depth of 60%, 1.3 dB of insertion loss, and 40 nm of modulation bandwidth were obtained. These results are within the best results reported in the framework of in-fiber AO modulators.