Kok Wai Chang

Frequency domain analysis of an optical FM discriminator

Measurement of optical phase and frequency deviations using a frequency-domain network analysis approach is described. The frequency domain transfer functions are given which relate the conversion of optical phase, frequency, and intensity modulation into photodetector current after passing through a quadrature biased Mach-Zehnder interferometer. It is shown that the error term in the phase and frequency transfer functions depends only to second order on any accompanying intensity modulation. Experimental data are given illustrating the analytical results. >

Simultaneous blue and green upconversion lasing in a laser‐diode‐pumped Pr3+/Yb3+ doped fluoride fiber laser

An upconversion laser operating simultaneously at visible wavelengths of 520 and 490 nm under infrared semiconductor pumping is demonstrated. The Pr3+/Yb3+doped ZBLAN fluorozirconate fiber laser yielded 1.4 mW total power at green and blue wavelengths with approximately 350 mW of incident pump light at a wavelength of 856 nm. Threshold launched powers of 55 and 85 mW were obtained for the wavelengths of 520 and 490 nm, respectively.

High-performance single-mode fiber polarization-independent isolators

The design and performance of polarization-independent isolators that are substantially insensitive to variations of temperature and wavelength are presented. Three single-mode fiber isolators have been designed, constructed, and tested. The first isolator is a 1.3-microm two-stage isolator that has more than 40 dB of isolation and less than 2.0 dB of insertion loss over the temperature range of 15-70 degrees C. The second isolator is a two-stage dual-wavelength isolator with an insertion loss of less than 4 dB and an isolation of better than 35 dB at 1.3 and 1.52 microm over the temperature range of 15-75 degrees C. The third isolator is a three-stage isolator with an insertion loss of less than 2.5 dB and an isolation of better than 63 dB over the wavelength range 1275-1355 nm.

Magnetostatic surface wave straight-edge resonators

The performance of the magnetostatic surface wave straight-edge resonator (MSSW-SER) is presented. The resonator uses a rectangular YIG film to propagate MSSWs where the straight edge serves as a reflector. Problems arising from coupling to width mode resonances and their effect on the main resonance are investigated. Through a careful choice of YIG and transducer parameters, the interference effects of the width mode resonances with the main resonance are minimized. As a result, highQ tunable microwave resonators with a tuning range from 2–20 GHz, insertion loss less than 10 dB, and spurious rejection better than 10 dB could be designed and fabricated. This MSSW resonator could be used to construct a tunable low-phase-noise feedback oscillator. However, the tuning range of this MSW feedback oscillator is limited by the phase change of the external amplifier circuit.

MSW-SER Based Tunable Oscillators

Tunable Magnetostatic Wave Straight Edge Resonators (MSW-SERs) offer an alternative to YIG spheres and varactor diodes in microwave oscillators. Work on MSW-SERs is extended to coupling with active devices. Interactions increase insight into resonator characteristics. A brief review of resonator-based microwave oscillator topologies precedes a survey of available YIG/GGG materials and coupling structures. Circuit related properties of the MSW-SER are presented as a basis for design. Circuit topologies are compared and the negative resistance topology is selected. Several examples of tunable oscillators in the L, S, and K microwave frequency bands will be given with emphasis placed on tuning bandwidth, power handling, harmonic distortion and phase noise characteristics.

The Effect of Width Modes on the Performance of MSSW Resonators

Theoretical and experimental investigations were performed to study width mode resonances (WMR) observed in magnetostatic surface wave straight edge resonators (MSSW-SER). It was found that for each principal resonance (n=1,2,3, m=l) of the MSSW-SER, different width mode resonances (m=2,3 ,... ) can be excited. Even and odd width modes can be excited depending on the RF current distribution in the microstrip transducers. Furthermore, all high order width mode resonances (m=2,3,4 ... ) occur at the low frequency side of the principal resonances (n=1,2,3, m=l). At high operating frequencies (e.g. above 4 CHz for pure YIC devices) the frequency separation between the principal resonances decreases and as a result, the width modes corresponding to high order principal modes will interfere with the low order principal modes. Through a careful choice of YIG and transducer parameters, the interference effects of width modes were minimized. Tunable high Q MSSW resonators with low insertion loss ( lOdB) have been designed and fabricated.

Micro-size ball lenses for micro-optics: theory and experiment

Two modeling approaches for analyzing micro-size ball lenses will be described. Due to the low divergence angle of the light coming out of a single mode fiber (SMF), a Gaussian Optics analysis, integrated with ray tracing, is needed to design the optical subsystems such as fiber collimators. On the other hand, due to the large divergence angle of the light coming out of the laser diode (LD), an exact solution of Maxwells equation which can be obtained by spherical harmonic expansion, is needed in order to predict the coupling efficiency from a LD to a SMF accurately. These models were applied to the cases of forward coupling and back reflections with various arrangements of the optical elements. Excellent agreement was found between the predictions of these two models and the experimental results. These models are very important for assemblies using micro-machined micro-optical parts since they have little or no allowance for alignment adjustments.

Ball lens modeling for laser/fiber coupling. A direct solution of Maxwell's equations

We present optical designs for coupling between a commercial laser and a single mode fiber using one or two ball lenses. Our approach exploits the fact that the scattering of an arbitrary electromagnetic beam from a sphere is an exactly solvable problem. By solving Maxwells equations directly using spherical harmonic expansion of the input field, we can account for effects (spherical aberration, polarization, astigmatism) which are not easy to include by other approaches. We are able to implement this method to calculate reflection and transmission accurately with modest computational effort.