Highly sensitive optical temperature sensor based on a SiN micro-ring resonator with liquid crystal cladding.

This work develops a sensitivity-enhanced optical temperature sensor that is based on a silicon nitride (SiN) micro-ring resonator that incorporates nematic liquid crystal (NLC) cladding. As the ambient temperature changes, the refractive index of the NLCs, which have a large thermal-optical coefficient, dramatically varies. The change in the refractive index of the NLC cladding that is caused by the temperature shift can alter the effective refractive index of the micro-ring resonator and make the resonance wavelength very sensitive to the ambient temperature. The temperature-sensitivity of the device with 5CB cladding for TM-polarized light was measured to be as high as 1nm/°C between 25 and 33 °C and over 2nm/°C at temperatures close to clearing temperature of the 5CB cladding. The temperature-sensitivity of the proposed device is at least 55 times that of the micro-ring resonator with air cladding, whose temperature-dependent wavelength shift for TM-polarized light is 18pm/ °C.

Improved technique for the characterization of micro-ring resonator using low coherence measurement.

Low-coherence interferometric measurement has been used to investigate optical waveguide devices with high accuracy. By utilizing an incoherent light source, one can generate separate interferogram features for each optical path. The distance between adjacent features of a ring resonator is related to ring length. With small ring radius, the interferogram spectrum exhibits severe cross-interference between adjacent features that hinders one to analyze the optical path individually. We propose a novel technique to overcome the light-source bandwidth limitation by signal-processing technique, which allows one to characterize small radius micro-ring resonator. This technique has been applied to both numerical simulations and experimental data with significant improvement of the extracted ring parameters. The improvements allow one to better understand the wavelength dependency properties of small radius micro-ring resonators.

Electrically tunable high Q-factor micro-ring resonator based on blue phase liquid crystal cladding.

This work demonstrates an electrically tunable silicon nitride (SiN) micro-ring resonator with polymer-stabilized blue phase liquid crystals (PSBPLCs) cladding. An external vertical electric field is applied to modulate the refractive index of the PSBPLCs by exploiting its fast-response Kerr effect-induced birefringence. The consequent change in the refractive index of the cladding can vary the effective refractive index of the micro-ring resonator and shift the resonant wavelength. Crystalline structures of PSBPLCs with a scale of the order of hundreds of nanometers ensure that the resonator has a very low optical loss. The measured tuning range is 0.45 nm for TM polarized light under an applied voltage of 150V and the corresponding response time is in the sub-millisecond range with a Q-factor of greater than 20,000.

Study of coupling loss on strongly-coupled, ultra compact microring resonators.

Small-radius microring resonators with large free spectral range (FSR) are of great interest for optical communication and optical interconnect applications. The resonator loss of a waveguide-coupled ring resonator, if the gap width between the microring and the bus waveguide is extremely small, can be significantly influenced by the coupling loss which corresponds to the microring operated in a strong coupling regime. This effect is particularly prominent for small radius microrings. We have studied the coupling loss with respect to the gap width on a waveguide-coupled microring both experimentally and theoretically, using two-dimensional (2D) finite difference time domain (FDTD) and effective index method (EIM).The coupling loss was confirmed by measuring transmission spectra of Si microring filters fabricated on silicon-on-insulator (SOI) wafers. Our experimental data show that the ring loss increases rapidly as the coupling gap decreases to less than 200 nm. The measured results show that the ring loss of a silicon microring with a radius of 2.75 µm is around 0.01382 dB/circumference as the gap width is greater than 325 nm, referred to as the intrinsic ring loss. However, for a smaller gap of 150 nm, the loss of the microring increases to 0.07084dB/circumference. The added ring loss is attributed to the coupling loss at small coupling gap for small radius ring.

Optical bistability in a silicon nitride microring resonator with azo dye-doped liquid crystal as cladding material.

This investigation reports observations of optical bistability in a silicon nitride (SiN) micro-ring resonator with azo dye-doped liquid crystal cladding. The refractive index of the cladding can be changed by switching the liquid crystal between nematic (NLC) and photo-induced isotropic (PHI) states by. Both the NLC and the PHI states can be maintained for many hours, and can be rapidly switched from one state to the other by photo-induced isomerization using 532 nm and 408 nm addressing light, respectively. The proposed device exhibits optical bistable switching of the resonance wavelength without sustained use of a power source. It has a 1.9 nm maximum spectral shift with a Q-factor of over 10000. The hybrid SiN- LC micro-ring resonator possesses easy switching, long memory, and low power consumption. It therefore has the potential to be used in signal processing elements and switching elements in optically integrated circuits.

Side-mode transmission diagnosis of a multichannel selectable injection-locked Fabry-Perot Laser Diode with anti-reflection coated front facet.

Theory and experiments on the side-mode-suppression-ratio (SMSR) enhancement and the linewidth reduction of a Fabry-Perot laser diode (FPLD) side-mode-injection-locked by using another FPLD are demonstrated to realize its potential application as a DWDM transmitter source. The SMSR, the spectral linewidth and the linewidth enhancement factor are simulated to realize the limitation of the FPLD-FPLD link under side-mode injection-locking condition. A degradation of the linewidth enhancement factor from 1.5 to 2.1 is observed due to the slave FPLD injection-locked at principle- and side-mode conditions. Up to 22-channel selectability of the 2.5 Gbit/s directly modulated FPLD based transmitter under side-mode injection-locking is demonstrated with a SMSR >35 dB, a Q-factor 6.8-9.2, a locking range of 24 nm, a power penalty of -0.7 dB, and a BER of 10(-10) at -17 dBm. The side-mode injection-locked FPLD shows high-quality transmission performance and meet the demand for cost-effective and high-capability 2.5 Gbit/s WDM systems.

Evaluating the performance improvement of differential phase-shift keying signals by amplitude regeneration and phase-noise suppression.

Theoretical bit error rates of the differential phase-shift keying format with various regeneration schemes is presented. A comparison with ideal regeneration reveals that averaging the phase noise of adjacent bits is found to eliminate most penalties that are induced by phase noise.