Pedram Z. Dashti

Coherent acousto-optic mode coupling in dispersion- compensating fiber by two acoustic gratings with orthogonal vibration directions

The acousto-optic mode coupling from core to LP11(cl) mode of a dispersion-compensating fiber excited by two acoustic waves with orthogonal perturbations is studied by use of a novel composite piezoelectric transducer. Highly polarization-dependent mode-selective spectra are observed that are due to coherent acousto-optic coupling between the core and various constituent cladding modes. Potential applications for implementing an all-fiber wavelength-tunable polarization analyzer are also discussed.

Compensation of polarization-dependent loss in transmission fiber gratings by use of a Sagnac loop interferometer

We analyze the transmission characteristics of a Sagnac loop interferometer containing a polarization-dependent loss element and lossless polarization-converting elements by use of Jones matrices. We show that polarization independence in the transmission mode can be achieved in such a configuration and that maximum transmittance occurs when a half-wave plate is used for the polarization-converting element. The result is verified experimentally for a fiber acousto-optic tunable filter and cascaded long-period fiber gratings with either intrinsic or process-induced polarization-dependent filtering characteristics.

All-fiber narrowband polarization controller based on coherent acousto-optic mode coupling in single-mode fiber

An all-fiber, narrowband, tunable polarization controller is proposed and demonstrated. The device is based on coherent acousto-optic mode coupling induced by two orthogonal acoustic waves on a dispersion-compensating fiber. The cooperative coupling between the two polarizations of the core mode and the TE01 cladding mode through the two gratings permits indirect coupling between the two polarizations of the core mode with nearly 100% efficiency, which makes the polarization-controlling function possible. Experimental results verify the operation of the polarization controller with an insertion loss of <1 dB.

Demonstration of a novel all-fiber bandpass acousto-optic tunable filter

An all-fiber tunable bandpass filter is demonstrated. A dual acousto-optic grating inside a Sagnac loop changes the polarization of the light over a narrow bandwidth which redirects the light from reflection to the transmission port.

Measurement of acoustic wavelength in optical fiber via acousto-optic interaction

The wavelength of the lowest-order propagating flexural acoustic wave in a single-mode fiber is measured by creating an acoustic cavity with variable length. The amplitude of the acoustic standing wave is probed by means of acousto-optic coupling between the core and cladding modes in the optical fiber. By monitoring the periodic modulation of the core–mode transmittance versus the cavity length, the acoustic wavelength can be measured to a high degree of accuracy. Excellent agreement (<1%) between the measured values and theoretical calculations for three types of 125 μm diameter single-mode fibers is obtained in the frequency range from 1.2 to 3.5 MHz.

Measurement of modal dispersion in optical fiber by means of acousto-optic coupling

We show that frequency-wavelength tuning characteristics of acousto-optic coupling can be used for measuring the difference of effective index, group index, and chromatic dispersion between core and cladding modes in single-mode fibers. Chromatic dispersion measurements of a 30-cm-long conventional single-mode fiber, a nonzero dispersion-shifted fiber, and a dispersion-compensating fiber with this new method are presented for the wavelength range 1500-1600 nm. Qualitative agreement with independently measured data is obtained.

All-fiber acousto-optic polarization monitor.

We describe the operational principle of and experimentally demonstrate a narrowband, wavelength-tunable polarization monitor based on a fiber acousto-optic tunable filter. Two orthogonally vibrating acoustic waves are used to create a variable polarizer that can be used to measure the state of polarization of an incident narrowband light source. The accuracy of the polarization monitor is measured at two different wavelengths in reference to a commercial polarimeter and is shown to be within 5%.

Transfer of orbital angular momentum between acoustic and optical vortices in optical fiber

Transfer of orbital angular momentum from an electronically controlled acoustic vortex to an optical vortex in optical fiber is experimentally demonstrated. Potential applications include quantum key encryption and decoding and optical trapping

Sensing of dynamic strain via acousto-optic coupling in single-mode fiber

We demonstrate dynamic strain sensing using a fiber acousto-optic tunable filter (AOTF). The sensing mechanism is based on the shift of resonant wavelength of the AOTF filter in the presence of mechanical strain on the fiber. An input laser signal can thus be modulated by a dynamic strain. The modulation due to a low-frequency (~10 Hz) dynamic strain applied to the fiber AOTF was measured

A compact all-fiber PDL-compensated acoustooptic wavelength monitor

A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL) compensation is demonstrated. A Sagnac loop interferometer is used to compensate the intrinsic PDL, and hence, the polarization-dependent wavelength shift of the fiber acoustooptic tunable filter (AOTF). This increases the extrinsic wavelength accuracy of the device to 0.05 nm, a three-fold improvement from a previously reported uncompensated device. The relaxation in the PDL specifications for the AOTF also results in a reduction of fiber length from 38 to 14 cm.