San-Liang Lee

High-performance InP/In/sub 0.53/Ga/sub 0.47/As/InP double HBTs on GaAs substrates

InP/In/sub 0.53/Ga/sub 0.47/As/InP double heterojunction bipolar transistors (HBTs) were grown on GaAs substrates. A 140 GHz power-gain cutoff frequency f/sub max/ and a 207 GHz current-gain cutoff frequency f/sub /spl tau// were obtained, presently the highest reported values for metamorphic HBTs. The breakdown voltage BV/sub CEO/ was 5.5 V, while the dc current gain /spl beta/ was 76. High-thermal-conductivity InP metamorphic buffer layers were employed in order to minimize the device-thermal resistance.

Directly modulated sampled grating DBR lasers for long-haul WDM communication systems

The limitations on high-speed data transmission, using widely tunable sampled-grating distributed Bragg reflector (SGDBR) lasers, are investigated. We demonstrate 1.244-Gb/s data transmission over a wavelength range of 45 nm using a single directly modulated tunable SGDBR laser diode. Data transmission was evaluated on four separate wavelength channels each spaced 15 nm apart. Less than 0.6 dB of dispersion penalty was measured on all four channels for transmission over 50 km of standard single-mode optical fiber.

Deep and tapered silicon photonic crystals for achieving anti-reflection and enhanced absorption

Tapered silicon photonic crystals (PhCs) with smooth sidewalls are realized using a novel single-step deep reactive ion etching. The PhCs can significantly reduce the surface reflection over the wavelength range between the ultra-violet and near-infrared regions. From the measurements using a spectrophotometer and an angle-variable spectroscopic ellipsometer, the sub-wavelength periodic structure can provide a broad and angular-independent antireflective window in the visible region for the TE-polarized light. The PhCs with tapered rods can further reduce the reflection due to a gradually changed effective index. On the other hand, strong optical resonances for TM-mode can be found in this structure, which is mainly due to the existence of full photonic bandgaps inside the material. Such resonance can enhance the optical absorption inside the silicon PhCs due to its increased optical paths. With the help of both antireflective and absorption-enhanced characteristics in this structure, the PhCs can be used for various applications.

Antireflective silicon surface with vertical-aligned silicon nanowires realized by simple wet chemical etching processes

Silicon antireflection is realized with vertical-aligned SiNWs by using improved metal-induced etching technique. The spectral responses of the transmission, reflection, and absorption characteristics for the SiNWs of different lengths are investigated. In order to realize short SiNWs to provide sufficiently low reflection, a post chemical etching process is developed to make the nanowires have a larger length fluctuation and/or tapered structure. The use of short SiNWs can allow a faster process time and avoid the sub-bandgap absorption that frequently occurs in long nanowires. Short SiNWs can also provide more compatible material structure and fabrication procedures than long ones can for applying to make optoelectronic devices. Taking the applications to solar cells as examples, the SiNWs fabricated by the proposed technique can provide 92% of solar weighted absorption with about 720 nm long wires because of the resultant effective graded index and enhanced multiple optical scattering from the random SiNW lengths and tapered wires after KOH etching.

On leaky mode approximations for modal expansion in multilayer open waveguides

We propose an analytic method to calculate the leaky mode functions to enable modal expansion without encountering the normalization and orthogonality problems of the unphysical mode shape. The wave functions of leaky modes are derived from the explicit formulas of normalized radiation modes. Using leaky modes to approximate a continuum of radiation modes greatly simplifies the analysis of excitation, transitions, propagation, and radiation of light waves in multilayer waveguide structures. Upon comparison with the beam propagation method and modal propagation method, we show that the leaky mode approximation while requiring a much simpler computation is as accurate. This method can also provide better physical insight to device operations. >

Transistor and circuit design for 100-200 GHz ICs

Compared to SiGe, InP HBTs offer superior electron transport properties but inferior scaling and parasitic reduction. Figures of merit for mixed-signal ICs are developed and HBT scaling laws introduced. Device and circuit results are summarized, including a simultaneous 450 GHz f/sub /spl tau// and 490 GHz f/sub max/ DHBT, 172-GHz amplifiers with 8.3-dBm output power and 4.5-dB associated power gain, and 150-GHz static frequency dividers (a digital circuit figure-of-merit for a device technology). To compete with advanced 100-nm SiGe processes, InP HBTs must be similarly scaled and high process yields are imperative. Described are several process modules in development: these include an emitter-base dielectric sidewall spacer for increased yield, a collector pedestal implant for reduced extrinsic C/sub cb/, and emitter junction regrowth for reduced base and emitter resistances.

Monolithically integrated multiwavelength sampled grating DBR lasers for dense WDM applications

For accurate control of the channel spacing in fabricating multiwavelength laser arrays or discrete multicolor lasers, we proposed a novel approach that exploits sampled grating distributed Bragg reflector (DBR) mirrors to vary the laser wave length across the wafer. This approach can realize a set of lasers with a wavelength spacing that meets the ITU recommendations for dense wavelength-division multiplexing systems and a wavelength range that can cover up to 40 nm or more. The wavelength variation across an array is achieved by changing the sampling periods of the DBR mirrors from laser to laser. The accuracy on the channel spacing of sampled grating DBR laser arrays was shown to be the same as that of conventional distributed feedback or DBR laser arrays, but their wavelengths can be better controlled for the gratings are fabricated with single holographic exposure. Arrays of 21 lasers have been successfully fabricated and have around 0.8-nm wavelength spacing with a simple tuning mechanism.

Using Superimposed ASK label in a 10-Gb/s multihop all-optical label swapping system

A novel optical-label-swapping technique is proposed, experimentally verified, and theoretically analyzed in this paper. The technique superimposes a low-speed amplitude-shift-keying (ASK) label on top of a high-speed dc-balanced-line-coded ASK payload. A multihop long-distance transmission experiment using a recirculating loop has been successfully demonstrated, and an experimental record is set when compared with other optical-label-swapping techniques.

Performance enhancement on SOA-based four-wave-mixing wavelength conversion using an assisted beam

We propose to use an additional injection beam of short wavelength to enhance the wavelength conversion that utilizes the four-wave-mixing effect in a semiconductor optical amplifier (SOA). The experiments demonstrated larger than 5-dB improvement on conversion efficiency, as the SOA is biased at the corresponding transparent condition for the assisted beam. At such a condition, an assisted beam can increase the saturation intensity without sacrificing the gain of an SOA, and this leads to an increase in conversion efficiency. The assisted beam also improves the signal-to-background-noise ratio in wavelength conversion. The effects of different assisted wavelengths and the measures for further improvement are discussed.

Ultra high frequency static dividers > 150 GHz in a narrow mesa InGaAs/InP DHBT technology

A static frequency divider with a maximum clock frequency >150 GHz was designed and fabricated in a narrow mesa InP/In/sub 0.53/Ga/sub 0.47/As/InP DHBT technology. The divider operation is fully static, operating from f/sub dk/ = 3 GHz to 152.0 GHz while dissipating 594.7 mW of power in the circuit core from a -4.07 V supply. The circuit employs single-buffered emitter coupled logic (ECL) and inductive peaking. The transistors have an emitter junction width of 0.5 /spl mu/m and a 3.0 collector-to-emitter area ratio. A microstrip wiring environment is employed for high interconnect density, and to minimize resonances and impedance mismatch at frequencies >100 GHz.