Joong-Seon Choe

Semiconductor microlenses fabricated by one-step wet etching

We fabricated refractive semiconductor microlenses using a diffusion-limited chemical etching technique based an Br/sub 2/ solution. The simple one-step wet etching process produced high-quality microlenses of GaAs and InP, the two most popular compound semiconductor materials used in optoelectronics. A spherical GaAs microlens with a nominal lens diameter of 30 /spl mu/m exhibited a radius of curvature and focal length of 91 and 36 /spl mu/m, respectively. The surface roughness, examined by atomic force microscopy (AFM), was measured to be below /spl plusmn/10 /spl Aring/. This microlens fabrication method should be readily applicable due to the simplicity in processing and the high-quality results.

Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols.

We propose and demonstrate the use of subcarrier/polarization-interleaved training symbols for channel estimation and synchronization in polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexed (CO-OFDM) transmission. The principle, the computational efficiency, and the frequency-offset tolerance of the proposed method are described. We show that the use of subcarrier/polarization interleaving doubles the tolerance to the frequency offset between the transmit laser and the receivers optical local oscillator as compared to conventional schemes. Using this method, we demonstrate 43-Gb/s PDM CO-OFDM transmission with 16-QAM subcarrier modulation over 5,200-km of ultra-large-area fiber.

Low bending loss metal waveguide embedded in a free-standing multilayered polymer film

Very low vertical bending loss is demonstrated in a flexible metal waveguide. The waveguide consists of an 8 nm-thick and 68 mm-long Ag strip embedded in a free-standing multilayered low-loss polymer film. The polymer film is composed of a 10 microm-thick inner cladding with a refractive index of 1.524, and a pair of 20 microm-thick outer claddings which both have a refractive index of 1.514, resulting in a total thickness of 50 microm. The measured vertical bending loss is lower than 0.3 dB/180 masculine at a wavelength of 1310 nm for the bending radii down to 2 mm.

Silver Stripe Optical Waveguide for Chip-to-Chip Optical Interconnections

Polymer-based silver stripe optical waveguides were developed for chip-to-chip optical interconnections, and their optical characteristics were investigated. The lowest propagation loss of 0.8 dB/cm was achieved with a 14-nm-thick 1.5-mum-wide stripe, at a wavelength of 1.3 mum . The 2.5-Gb/s optical signals were successfully transmitted via 10-cm-long silver stripe optical waveguides.

Characteristics of waveguide photodetectors at high-power optical input

High-power characteristics of waveguide photodetectors (PDs) with various absorbing layer thicknesses were analyzed. When large photocurrent was generated by high-power optical input, the saturation of radio frequency response was closely related with the optical absorption efficiency. The device with a single quantum well absorbing layer showed no saturation behavior until the device failure, while PDs with a thicker absorbing layer were saturated at photocurrent of a few milliamperes. It was also found that as optical confinement decreases, the maximum photocurrent the device can endure increases by more than three times.

An efficient and frequency-offset-tolerant channel estimation and synchronization method for PDM CO-OFDM transmission

We propose a computationally efficient and frequency-offset-tolerant channel estimation and synchronization method for polarization-division-multiplexed (PDM) coherent optical OFDM (CO-OFDM). Using this method, we demonstrate a 43-Gb/s PDM CO-OFDM transmission with 16-QAM subcarrier modulation over 5,200-km of ultra-large-area fiber.

Fabrication and Characteristics of 40-Gb/s Traveling-Wave Electroabsorption Modulator-Integrated DFB Laser Modules

We have developed 40-Gb/s traveling-wave electroabsorption-modulator-integrated distributed feedback laser (TW-EML) modules using several advanced technologies. First, we have adopted a selective area growth (SAG) method in the fabrication of the 40-Gb/s EML device to provide active layers for the laser and the electroabsorption modulators (EAMs) simultaneously. The fabricated device shows that the measured 3-dB bandwidth of electrical-to-optical (E/O) response reaches about 45 GHz and the return loss (S11) is kept below -10 dB up to 50 GHz. For the module design of the device, we mainly considered electrical and optical factors. The measured S11 of the fabricated 40 Gb/s TW-EML module is below -10 dB up to about 30 GHz and the 3-dB bandwidth of the E/O response reaches over 35 GHz. We also have developed two types of coplanar waveguide (CPW) for the application of the driver amplifier integrated 40 Gb/s TW-EML module, which is a system-on-package (SoP) composed of an EML device and a driver amplifier device in a module. The measured S11 of the two-step-bent CPW is below -10 dB up to 35 GHz and the measured S11 of the parallel type CPW is below -10 dB up to 39 GHz.

Development of Packaging Technologies for High-Speed (

We developed high-speed optoelectronics packaging technologies for a waveguide photodiode and a traveling wave electro-absorption modulator device for 40-Gb/s digital communication systems. The effects of the device and the packaging designs on the broadband performance were investigated to optimize broadband characteristics. For the receiver, inductive peaking was used for bandwidth control and an internal bias tee was implemented; in addition, two types of preamplifier devices were used to develop high-gain receiver and wide-bandwidth receiver. In the optical-to-electrical response, a 3-dB bandwidth of the high-gain module was about 32 GHz as compared to 42 GHz for the wide-bandwidth module. The clear 40-Gb/s nonreturn-to-zero (NRZ) eye diagrams showed a good system applicability of these modules. In addition, an optimized modulator module showed a 3-dB bandwidth of 38 GHz in the electrical-to-optical response, an electrical return loss of less than 10 dB at up to 26 GHz, an rms jitter of 1.832 ps, and an extinction ratio of 5.38 dB in a 40-Gb/s NRZ eye diagram

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We have oxidized AlAs layers laterally in a new type of oxidation system equipped with a closed chamber, in which the vapour pressure of water can be varied over a wide range as a function of the water temperature. Up to a fourfold increase in oxidation speed has been demonstrated with the new system. More strikingly, the saturation effect of the speed of oxidation, known to be caused by a dense As2O3-containing region at the oxidation front, appears to be eventually relieved and another oxidation evolution stage begins. This observation, which we believe has been made for the first time, was possible mainly owing to the much extended oxidation time that the closed-chamber system offers. Additionally, there is no active gas flow involved in this new oxidation method and therefore an improved spatial uniformity in oxidation is expected.

Gb/s) Optical Modules

Integration of a 40-Gb/s electroabsorption modulator integrated distributed feedback (DFB) laser (EML) module with a driver amplifier and bias tee was investigated. For the EML fabrication the selective area growth (SAG) technique was adopted for the first time. It is shown that, with the SAG technique, the 3-dB bandwidth of about 45 GHz was measured in the electrical to optical response, and the return loss (S11) of below -10 dB was achieved for up to 50 GHz . To integrate a bias tee within the module, a right-angle bent coplanar waveguide (CPW) was developed. The right-angle bent CPW was characterized with S11 of below - 10 dB for up to 35 GHz and insertion loss (S21) of about -1.4 dB for up to 40 GHz . The whole integrated module including the EML, a driver amplifier, and bias tee was characterized under the conditions of an operating temperature of 25degC, the modulator bias of 1.4 V, and the DFB laser current of 40 mA. S11 of below -10 dB was obtained for up to 14 GHz and the measured electrical-to-optical response has 3-dB bandwidth of about 20 GHz.