Zhang-di Huang

A fast response variable optical attenuator based on blue phase liquid crystal

Blue phase liquid crystals (BPLCs) are promising candidates for next generation display thanks to their fast response and quasi-isotropic optical properties. By taking these advantages, we propose to introduce the material into fiber-optic applications. As an example, a BPLC based variable optical attenuator (VOA) is demonstrated with a polarization independent design. The device shows normally-off feature when no field is applied. Response time down to submillisecond scale is achieved in switching between two arbitrary attenuation states. The attenuation range is also measured from 1480 to 1550 nm, which cover the whole telecomm S-band and part of the C-band. The overall performances reach the requirements for practical use; while still have room for further improvement. Through this example, the applicability of BPLC in fiber-optic devices is presented, which may impel the development of many other photonic applications from infrared to even microwave regions.

Liquid crystal modulator with ultra-wide dynamic range and adjustable driving voltage

We demonstrated a reflective-type liquid crystal (LC) intensity modulator in 1550 nm telecomm band. An effective way to compensate the residual phase of a LC cell is proposed. With the adjustment of a true zero-order quarter wave plate and enhanced by total internal reflection induced birefringence, over 53 dB dynamic range was achieved, which is much desired for some high-end optical communication, infrared scene projection applications. In addition, the driving voltages were decreased and adjustable. Mechanical and spectral tolerance measurements show that our LC modulator is quite stable. Further applications of our experimental setup were discussed including bio-sensors and high speed modulators.

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.

Aberration analysis and efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirec- tional optical subassembly BOSA modules is proposed and experimen- tally demonstrated. We analyzed the wavefront aberration coefficients of a typical BOSA. It was found that the 45-deg wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode to fiber coupling. We measured the BOSA efficiencies based on a series of different filters. For a typical 0.5-mm filter, 25% coupling efficiency im- provement was achieved by optimizing the filter parameters.

D-Shaped Optical Fiber Microwire Devices

D-shaped optical fiber microwire (OFM) devices are investigated theoretically. They can be fabricated by wrapping the OFM on a removable rod or substrate. Optical wave guiding properties of these devices are discussed with numerical simulations. Dispersion properties of OFMs with different radius are studied as well. A grating based D-shaped OFM is proposed for sensing applications. The corresponding sensitive could reach 1100 nm/RIU (Refractive Index Unit) due to its large outside evanescent field.

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.

The efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirectional optical sub-assembly (BOSA) modules is proposed and experimentally demonstrated. We analyzed the wave-front aberration coefficients of a typical BOSA. It was found that the 45° wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode (LD) to fiber coupling. We measured the BOSA efficiencies based on a serial of different filters. For a typical 0.5 mm filter, 25% coupling efficiency improvement was achieved by optimizing the filter parameters.