Wanguo Liang

Optical Transceivers for Fiber-to-the-Premises Applications: System Requirements and Enabling Technologies

A comprehensive review of the optical transceivers for fiber-to-the-premises applications is presented, with emphasis on system requirements and enabling technologies. Recent progress in photonic integration to realize low-cost, high-performance, and compact optical transceivers for the optical network terminal is reviewed

Label-Free Fiber Optic Biosensors With Enhanced Sensitivity

A sensitivity-enhanced long-period grating in-fiber Michelson interferometer with diameter-reduced cladding was proposed for the first time, and was also demonstrated as a label-free biosensor based on refractive index measurement. The cladding structure dependence of the sensitivity for antibody-antigen immuno-sensing was studied experimentally. The proposed biosensor has shown an 8.5-fold sensitivity enhancement as compared to sensors without cladding reduction.

Wavelength and temperature dependence of photorefractive effect in quasi-phase-matched LiNbO3 waveguides

The photorefractive effect (PRE) in quasi-phase-matched LiNbO3 waveguides is studied systematically by employing the pump-probe technique. It is found that the PRE is gradually eased with an increase of the device operation temperature. With increasing light irradiation power, the PRE becomes severe, the buildup (relaxation) time constant decreases (increases), while the second harmonic generation (SHG) efficiency is enhanced for the devices used in studies. The PRE under a shorter wavelength (980nm) light irradiation is much stronger than that at a longer wavelength one (1550nm), and the PRE for the latter is mainly caused by the SHG of the pump.

Ultrabroadband tunable continuous-wave difference-frequency generation in periodically poled lithium niobate waveguides

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.

Electro-optic beam deflection based on a lithium niobate waveguide with microstructured serrated electrodes.

We report on electro-optic beam deflection using an annealed proton exchange waveguide in lithium niobate (LN) with microstructured serrated array electrodes. Due to the electro-optic effect of the LN material, the experimental results show that the beam deflection and modulation of the LN waveguide can be realized with relatively low voltages. The total length of the serrated prism array electrodes structure is ∼5  mm. With 20 V applied to the electrodes of 50, 100, and 150 μm wide waveguides, ∼1.28, ∼0.96, and ∼0.64  μm beam deflections were obtained, respectively, which are in accordance with theoretical simulation. This configuration can be potentially applied in optical beam scanning, high-speed switches, and optical beam smoothing technology.

Crosstalk analysis in in-line transceiver

An in-line diplexer optical transceiver chip based on an integrated DFB laser and photodiode has been demonstrated and analyzed. The transceiver transmits at 1310nm for upstream signal and receives at 1490nm for downstream signal. For the in-line integrated device, crosstalk between the transmitting and receiving channels become a key issue of system performance. Such crosstalk includes optical-optical, electrical-electrical, and electrical-optical crosstalk. The optical-optical crosstalk is caused by the limited optical isolation between the DFB laser and the photodiode. The electrical-electrical crosstalk is the result of electrical interference between transmitter driver and receiver amplifier. In the in-line transceiver design, the 1490nm downstream signal goes through the DFB laser cavity. When the laser is modulated, the 1490nm signal will experience modulation through the gain and refractive index change in the cavity. This electrical-optical crosstalk has been confirmed by numerical simulation and experimental measurement. The results show that the electrical-optical crosstalk power is proportional to the received 1490nm power and dependent on modulation depth of the DFB laser. A bit error rate model is also presented to describe the impact of the modulation crosstalk from the system performance point of view.

Simulation of crosstalk in integrated distributed feedback laser-photodiode optical transceivers

In this paper, modulation crosstalk from 1310nm Distributed Feedback laser (DFB) upstream signals to 1490nm downstream signals in the monolithic integrated DFB-PD (Distributed Feedback laser-Photodiode) optical transceiver structure was simulated by the time-domain traveling wave (TDTW) rate equations numerically. It was found that high differential gain coefficient at 1490nm can contribute to the modulation crosstalk at about -25 to -30 dB. It was also found that the modulation crosstalk is modulation frequency dependent which will be enhanced about 3 dB at the relaxation oscillation frequency compared with low modulation frequency. Increasing modulation extinction ratio of the 1310nm DFB section from 10 to 18 dB, the crosstalk increases 3 dB. Such a higher modulation crosstalk (larger than -30 dB) should be considered since such crosstalk will degrade strongly the sensitivity of the 1490nm photo-detector.

Annealed proton-exchanged LiNbO3 ridge waveguide for photonics application

In this paper, we report a novel LiNbO3 ridge waveguide fabrication technique based on the combination of Annealed Proton-Exchanging (APE) and precise diamond blade dicing. The process is ultra compact and compatible with periodically polled LiNbO3 (PPLN). By selecting optimized fabrication conditions, ridge waveguide with low propagation loss and single transmission mode can be formed at 1064nm and 1500nm wavelength, respectively. Such APE ridge waveguides have potential applications in optical communication, biomedical detection, and especially in nonlinear wavelength conversion.

Second Harmonic Generation Using an All-Fiber Q-Switched Yb-Doped Fiber Laser and MgO:c-PPLN

We have experimentally demonstrated an efficient all-fiber passively Q-switched Yb-doped fiber laser with Samarium doped fiber as a saturable absorber. Average output power of 3.4 W at a repetition rate of 250 kHz and a pulse width of 1.1 microseconds was obtained at a pump power of 9.0 W. By using this fiber laser system and an MgO-doped congruent periodically poled lithium niobate (MgO:c-PPLN), second harmonic generation (SHG) output at 532 nm was achieved at room temperature. The conversion efficiency is around 4.2% which agrees well with the theoretical simulation.

Characteristics of femtosecond second-harmonic generation in periodically poled lithium niobate

The temporal and spectral characteristics of femtosecond second-harmonic-generation (SHG) in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) are investigated experimentally and theoretically. In the experiments, we studied SHG in PPLN waveguides with different device design and fabrication conditions by adopting ~100-fs optical pulses with different peak powers in the 1.55-μm band. Large phase modulation and spectral broadening of the fundamental wave, and a shift of the centre frequency of the second-harmonic wave were observed when the peak power of the fundamental wave was high enough. Saturation of the conversion efficiency was also observed, which depends on fundamental pulse width and peak power. These phenomena could be attributed to nonlinear phase accumulation under large group-velocity mismatch. In the theory, evolutions of pulse waveforms and optical spectra were studied for both the fundamental and second-harmonic pulses by solving the coupled-mode equations. The simulation results are in good agreement with the experimental data.