Chin-Hui Chen

Metalorganic chemical vapor deposition of high mobility AlGaN/GaN heterostructures

In this article, we discuss parameters influencing (a) the properties of thin AlxGa1−xN layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1−xN/GaN heterojunction. For xAl>0.3, the AlxGa1−xN layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained AlxGa1−xN layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the AlxGa1−xN/GaN interface strongly decreased with increasing aluminum mole fraction in the AlxGa1−xN layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to AlxGa1−xN deposition. In the case of modulation doped structures (GaN/AlxGa1−xN/AlxGa1−xN:Si/AlxGa1−xN), the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing sil...

Programmable optical buffering using fiber Bragg gratings combined with a widely-tunable wavelength converter

A 40 Gbps RZ all-optical buffering method, implemented by FBG and tunable wavelength converter, is presented. Preliminary results of time-delay up to 7 /spl mu/s and pulse broadening were measured. System measurements at 10 Gbps show the desired delay programmability.

Demonstration of a linear ultra-compact integrated coherent receiver

We demonstrate the operation of an ultra-compact coherent receiver for linear optical phase demodulation. The receiver, based on a broadband optical phase-locked loop (OPLL) has a bandwidth of 1.5 GHz. Physical delay in the feedback path is dramatically reduced by incorporating novel photonic and electronic components. Using the receiver in an analog link experiment, a spurious free dynamic range of 122dBHz2/3 is measured at 300 MHz. Additionally, the link loss is −2dB at low frequencies.

Compact beam splitters with deep gratings for miniature photonic integrated circuits: design and implementation aspects.

We present an extensive study of an ultracompact grating-based beam splitter suitable for photonic integrated circuits (PICs) that have stringent density requirements. The 10 microm long beam splitter exhibits equal splitting, low insertion loss, and also provides a high extinction ratio in an integrated coherent balanced receiver. We further present the design strategies for avoiding mode distortion in the beam splitter and discuss optimization of the widths of the detectors to improve insertion loss and extinction ratio of the coherent receiver circuit. In our study, we show that the grating-based beam splitter is a competitive technology having low fabrication complexity for ultracompact PICs.

InP/InGaAsP-Based Integrated 3-dB Trench Couplers for Ultra-Compact Coherent Receivers

We present the design, fabrication, and test results for ultra-compact 3-dB frustrated total internal reflection-based trench couplers in an InP/InGaAsP monolithic integration platform. The trench coupler is integrated with phase modulators and a balanced photodiode (BPD) pair to enable a 180° hybrid ultra-compact coherent receiver. Several trench splitter designs exhibit near 3-dB splitting with a loss of 3 dB. The BPD pair is used to characterize coherent mixing of two input optical signals into the trench splitter, and coherence efficiency of 75% is achieved.

Ultra-compact integrated coherent receiver for high linearity RF photonic links

We demonstrate a novel photonic integrated circuit(PIC) that combines an ultra compact trench beam splitter with monolithically integrated photodetectors and modulators. A coherent receiver is realized by flip chip bonding of this PIC with an electronic integrated circuit (EIC). Preliminary system results yield a third-order intermodulation distortion suppression of 46 dB at a signal frequency of 300 MHz.

Design and Implementation of Ultra-Compact Grating-Based 2×2 Beam Splitter for Miniature Photonic Integrated Circuits

We provide detailed experimental study of reflectivity, insertion loss, and interference extinction ratio for our recently proposed and demonstrated grating-based beam splitter. Both low- loss and high-extinction-ratio devices are demonstrated for reflectivities close to 50%.

Linear phase demodulation using an integrated coherent receiver with an ultra-compact grating beam splitter

With phase modulation, it is possible to realize high dynamic range analog optical links, provided the transmitted radio frequency (RF) signal can be linearly demodulated. The linearity of traditional interferometer-based phase demodulators, however, is often limited by their sinusoidal response. To achieve high linearity, negative feedback is introduced to suppress non-linearities arising from the phase demodulation process [1]. High feedback gain reduces the net phase swing across the demodulator such that it operates within the linear regime. Additionally, both the signal and the noise are reduced by the same feedback factor, so there is no penalty in signal-to-noise ratio (SNR) [1]. The challenge is to make a receiver incorporating feedback that is operable at high frequencies. Because high loop gain as well as a wide bandwidth is required for efficient phase tracking, the physical delay in the feedback path must be kept sufficiently short in order to prevent the loop from oscillating. Previously, we have demonstrated an ultra-compact grating-based beam splitter [2], which divides the incoming optical beams in a region, over 30 times shorter than a conventional surface-ridge Multimode Interference (MMI) beam splitter. This key feature leads to a significant reduction in loop delay.

Ultra-compact Grating-based 2×2 Beam Splitter for Miniature Photonic Integrated Circuits

A novel, InGaAsP/InP ultra-compact grating-based beam splitter that has less than 1.8 dB of loss has been fabricated. The beam splitter exhibits 50/50 splitting with a length of less than 25 mum for 1555 nm wavelength.

Wavelength-agile integrated optical transmitters for analog applications

A summary of current work involving the development of high performance, wavelength-tunable integrated optical transmitters for analog applications is given. The performance of sampled-grating DBR lasers integrated with an SOA and an electroabsorption or Mach-Zehnder modulator is evaluated in terms of E/O conversion efficiency, noise performance and dynamic range. Optimization options to maximize either gain, noise figure or spurious-free dynamic range in analog link applications are discussed. It is shown how the combination of chip-scale integration and the use of bulk waveguide Franz-Keldysh absorption allows coupling of a large optical power level into the electroabsorption modulator, and its effects on the modulation response and analog link performance. Link results on an integrated SGDBR-SOA-EAM device includes a sub-octave SFDR in the 125 to 127 dB/Hz4/5 range and a broadband SFDR of 103-107 dB/Hz2/3 limited by third order intermodulation products or 95-98 dB/Hz1/2, limited by second order intermodulation products, over a 1528 to 1573 nm wavelength range.