Mode characteristics of equilateral triangle resonators (ETRs) are analyzed based on the symmetry operation of the point group C/sub 3v/. The results show that doubly degenerate eigenstates can be reduced to the A/sub 1/ and A/sub 2/ representations of C/sub 3v/, if the longitudinal mode number is a multiple of 6; otherwise, they form the E irreducible representation of C/sub 3v/. And the one-period length for the mode light ray is half of the perimeter of the ETR. Mode Q-factors are calculated by the finite-difference time-domain (FDTD) technique and compared with those calculated from far-field emission based on the analytical near-field pattern for TE and TM modes. The results show that the far-field emission based on the analytical field distribution can be used to estimate the mode Q-factor, especially for TM modes. FDTD numerical results also show that Q-factor of TE modes reaches maximum value as the longitudinal mode number is a multiple of 7. In addition, photoluminescence spectra and measured Q-factors are presented for fabricated ETR with side lengths of 20 and 30 /spl mu/m, and the mode wavelength intervals are compared with the analytical results.

A novel design of grating-assisted microcylinder surface-emitting laser (GAMSEL) is presented. GAMSEL has two gratings: top grating and side grating. Both gratings have the number of periods close to the azimuthal mode number of the whispering gallery mode (WGM) supported by the microcylinder cavity, therefore, they behave like second-order gratings of a straight waveguide. The top grating scatters the WGM vertically so as to form the surface emission while the side grating selects the mode having the proper emission pattern to lase. Through the cooperation of the two gratings, GAMSEL becomes a single-mode surface-emitting laser. Due to its small volume, GAMSEL is predicted through simulation to have a threshold current about 1 mA and a direct modulation bandwidth higher than 40 GHz. Besides, relying on the lateral microcylinder cavity, GAMSEL does not require highly reflective mirrors as vertical-cavity surface-emitting lasers do, which makes this design easily apply to different wavelengths such as the near infrared 1310- and 1550-nm wavelength bands.

Characterization of the thermo-optically tuned multi -channel interference widely tunable semiconductor laser for quasi-continuous tuning is demonstrated for the first time. The characterization strategy is carried out by simultaneously controlling seven arm phase sections to shift the reflection peak and adjusting the common phase section to fine-tune the longitudinal mode. Output wavelengths at 50 GHz frequency spacing are characterized, and a quasi-continuous tuning range beyond 60 nm is achieved at two temperatures. In the entire tuning range: side-mode suppression ratios are higher than 40 dB; wavelength deviations are less than ± 10 pm from international telecommunication union grid; total thermal tuning powers are lower than 35 mW, and Lorentzian linewidths are less than 100 kHz. The tuning information near any output wavelength is obtained and the characterization speed is greatly improved. This strategy provides a new route for characterizing tunable lasers with multiple tuning currents.

High performance InP-based polarization beam splitters (PBSs) with a symmetrical Mach-Zehnder interferometer (MZI) are theoretically and experimentally demonstrated. The p-i-n waveguide is exploited to employ the phase shifters in the two arms of the MZI. Injection current and reverse bias inducing electro-optic effects in each of the MZI arms are analysed in detail when the waveguides are aligned along both the [011] and ${[01\bar1]}$ directions. Results indicate that the birefringence from reverse bias help realize the PBS combining with the polarization-independent plasma effect from the injection current. Theoretical prediction shows that the polarization extinction ratio (PER) is over 35 dB at 1550 nm for the PBSs with the waveguides aligned along both directions. Furthermore, the PBS with the waveguides aligned along the [011] direction can be adjusted more easily to work at the best operating point and get the higher birefringence which requires lower reverse bias exhibiting better performance across the C-band compared with the ${[01\bar1]}$ direction. The fabricated PBSs exhibit the PER over 19 dB (14 dB) in the wavelength range from 1525 to 1570 nm with one arm injected of 2.88 mA (0.2 mA) current and the other arm reversed biased at 5.63 V (7.89 V) when the waveguides aligned along the [011] ${[01\bar1]}$ ) direction. The PER of both PBSs is expected to be 20 dB across the C-band with the help of birefringence contribution from the inserting waveguide arms of different widths.

We present a linear frequency-swept laser for long-range frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR), utilizing negative voltage tuning with a high output power distributed-Bragg-reflector (DBR) laser integrated with a spot size converter (SSC) and semiconductor optical amplifier (SOA). Compared to positive voltage tuning, negative voltage tuning offers key advantages including a linear tuning response and reduced spectral broadening. To enhance the coupling efficiency to the lensed fiber, an SSC with a buried waveguide structure is introduced at the output port. The laser features a free spectral range of 0.3 nm and a Lorentzian linewidth of 140 kHz. Negative voltage tuning in the phase region enables linear frequency sweeping, while phase noise compensation allows for long-range measurements.

In the next generation coherent passive optical network (PON), the local oscillation (LO) laser and the upstream laser in optical network unit (ONU) need to maintain a stable frequency difference with the downstream laser at the optical line termination (OLT). This work designs and packages distributed feedback (DFB) laser chips into transistor outline to provide low-cost light sources. High performance laser control print circuit board makes the wavelength jitter less than ± 0.3 pm, output power greater than 40 mW and Lorentzian linewidth less than 270 kHz, which is suitable for advanced modulation formats. A novel direct frequency locking system based on high-speed analog-to-digital convertor and an indirect frequency locking system based on asymmetric Mach-Zehnder interferometer (AMZI) are demonstrated. Both systems can realize frequency difference stability less than ± 25 MHz between the 1556 nm downstream laser in OLT and LO laser in ONU, which is much more precise than actual system requirement (< ± 100 MHz). By utilizing the periodic transmission frequency characteristics of the AMZI, the 1526 nm upstream laser in ONU can realize a 30 nm ± 25 MHz frequency difference stability with the downstream laser, which is demonstrated for the first time and realize the highest DFB lasers frequency difference locking accuracy of wide wavelength interval (> = 30 nm). This low-cost and high-precision frequency locking system demonstrates excellent potential for application in the next generation coherent PON.