Su-shan Li

Fiber-Optic Pressure Sensor Based on Tunable Liquid Crystal Technology

We propose a photoelastic pressure sensor using a liquid crystal (LC) tunable fiber polarizer. The polarizer contains an etched single-mode fiber sealed in an LC cell. A low-index LC is used as a tunable birefringent cladding of the fiber. Any electric-field-induced LC director reorientation may thus affect the fibers light-guiding characteristics. Different polarization states experience different LC cladding indices. The transmittance contrast between the x- and y-polarized modes reaches 31.9 dB, which is sensitive enough to monitor any polarization state change in the fiber. A pressure sensor with the sensitivity of 0.25 rad/N is thus demonstrated. The corresponding stress-optic coefficient of the fiber is measured at 1550-nm telecomm band. The advantages and applications of the LC tuning and sensing technologies are discussed.

Dispersion Enhancement and Linearization in a Dynamic DWDM Channel Blocker

A dynamic dense-wavelength-division-multiplexing (DWDM) channel blocker and equalizer is developed based on liquid crystal (LC) and dispersion control technology. A multipixel LC array is adopted to regulate the power level of each DWDM channel, while a reflective grating diffracts the input signals spatially to corresponding LC pixels. A dispersion control unit is proposed and employed to enhance the dispersion and compensate the intrinsic nonlinear dispersion of the grating. Therefore, all LC pixels could handle corresponding lights centered at the International Telecommunications Union Grids. A 40-channel, 100-GHz channel-spacing dynamic wavelength blocker/equalizer is thus demonstrated with -5 dB insertion loss and over 40 dB extinction ratio. The maximum center frequency shift of all 40 channels is ~ ±2 GHz, which means our dispersion control technology works very well for grating-based DWDM devices.

Measurement of Surface Plasmon Polariton Enhanced Goos–Hanchen Shift Based on Grating and Liquid Crystal Technologies

We measured the surface plasmon polariton (SPP) enhanced Goos-Hanchen (GH) shift based on liquid crystal (LC) and grating technologies. An optical setup is used to convert the spatial displacement to incidence angle variation to a Littrow mounted diffraction grating. As a consequence, the GH shift information could be obtained from the back-reflected center wavelength that fulfills the Littrow condition. An LC cell is used to adjust the polarization state of the incident light without mechanical movement. About 10- μm GH shift difference between transverse-electric (TE) and transverse-magnetic (TM) mode lights were measured associated with the SPP excitation. The corresponding center wavelength shift of the returned beam is 404 pm. The relationship between energy conversion and GH shift is also investigated.

Liquid Crystal Based Tunable Fiber Polarizer for Pressure Sensing

We propose a photoelastic pressure sensor using a liquid crystal (LC) tunable fiber polarizer. The polarizer contains an etched single mode fiber sealed in a LC cell. A low index LC is used as a tunable birefringent cladding of the fiber. Any electric-field induced LC director reorientation thus may affect the fibers light-guiding characteristics. Different polarization states experience different LC cladding indices. The transmittance contrast between x- and y-polarized modes reaches 31.9 dB, which is sensitive enough to monitor any polarization state change in the fiber. A pressure sensor with the sensitivity of 0.25 rad/N is thus demonstrated. The corresponding stress-optic coefficient of the fiber is measured at 1550 nm telecomm band. Advantages and applications of the LC tuning and sensing technologies are discussed.

A Wavelength Selective Bidirectional Isolator

A wavelength selective optical isolator is proposed. Being different from conventional isolators, a well-designed wave plate is employed and works together with the Faraday rotator. Different wavelengths thus experiences different phase retardation so that wavelength-dependant polarization states are obtained for bidirectional beams. A (1.49μm, 1.31μm) wavelength selective isolator is demonstrated, which means 1.49μm light only may propagate along one-direction why the opposite wave is only for 1.31μm light. Over 60 dB optical isolation is obtained, which has promising applications in access optical networks.

A Liquid Crystal Tunable Wavelength-Interleaved Isolator With Flat Spectral Response

A liquid crystal tunable bidirectional isolator is proposed. A wave plate is employed to realize a wavelength interleaving function. If it works as a full wave plate, the corresponding light only may pass along a specific direction while the opposite way is isolated. However, for wavelengths that the light experiences a half wave plate, the isolators passing direction is reversed. A liquid crystal cell is utilized to tune the isolator so that both the isolated wavelength and direction could be reconfigured. A 100-GHz channel-interleaved tunable bidirectional isolator is demonstrated with 30 dB isolation in both directions. The spectral response at ITU grids are further flattened by using a wave plate stacking technique.