Wooram Lee

Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers

We propose a bidirectional wavelength-division-multiplexed passive optical network by employing gain-saturated reflective semiconductor optical amplifiers (RSOAs) for wavelength-independent optical network terminals. The fabricated RSOA module has the input saturation power less than -13 dBm and its saturated gain higher than 13 dB within the C-band for arbitrary polarization states. The upstream signals are generated by remodulating the downstream signals whose modulation amplitude is squeezed through gain-saturated RSOAs. We present experimental results on bidirectional transmission with 1.25 Gb/s for upstream and 2.5 Gb/s for downstream data rates over 20-km transmission distance.

Tunable external cavity laser based on polymer waveguide platform for WDM access network

We developed a wide-range wavelength tunable external cavity lasers based on a polymer waveguide platform for use in wavelength-division-multiplexing (WDM) access networks as a low-cost light source. The lasing wavelength is tuned over 13 nm by thermally changing the reflection spectrum of the polymer waveguide Bragg grating with a heater. With the sidemode suppression ratio higher than 30 dB, it can support 16 WDM channels spaced by 100 GHz. The directly modulated 1.25-Gb/s data streams were successfully transmitted over the single-mode optical fiber link of 20 km.

Over 26-nm Wavelength Tunable External Cavity Laser Based on Polymer Waveguide Platforms for WDM Access Networks

We demonstrated a widely tunable hybrid-integrated external cavity laser on a polymer-based waveguide platform. The lasing wavelength was tuned over 26 nm by locally heating the surface-relief-type polymer waveguide Bragg grating fabricated by using the phase mask. The phase-control heater section was inserted between the laser diode and the Bragg grating for the fine wavelength tuning and the stable single-mode lasing. In the 100-GHz-spaced wavelength-division-multiplexing systems, the 1.25-Gb/s directly modulated data streams at each tuned 32 channel were successfully transmitted

Distributed Parametric Resonator: A Passive CMOS Frequency Divider

We present an electrical distributed parametric oscillator to realize a passive CMOS frequency divider with low phase noise. Instead of using active devices, which are the main sources of noise and power consumption, an oscillation at half of the input frequency is sustained by the parametric process based on nonlinear interaction with the input signal. To show the feasibility of the proposed approach, we have implemented a 20-GHz frequency divider in a 0.13-μm CMOS process. Without any dc power consumption, 600-mV differential output amplitude is achieved for an input amplitude of 600 mV. The input frequency ranges from 18.5 to 23.5 GHz with varactor tuning. The output phase noise is almost 6 dB lower than that of the input signal for all offset frequencies up to 1 MHz. There is a good agreement among analysis, simulation, and 10-MHz measurement results. To the best of our knowledge, this is the first passive frequency divider in a CMOS process.

Optical Transceiver employing an RSOA with Feed-Forward Current Injection

We propose an optical transceiver employing an RSOA of which gain is controlled by feedforward current. With a low injection power below the gain saturation, the improved remodulation is clearly demonstrated at 1.25- Gb/s data rate.

Low-Noise Parametric Resonant Amplifier

We propose a resonant parametric amplifier with an enhanced noise performance by exploiting the noise-squeezing effect. Noise squeezing occurs through the phase-sensitive amplification process and suppresses one of the two quadrature components of the input noise. When the input signal is only in the direction of the nonsuppressed quadrature component, squeezing can lower that noise figure by almost 3 dB. The resonant structure of the proposed amplifier is inspired by a Fabry-Perot laser amplifier to achieve the squeezing effect using a low number of LC elements. We design and simulate the proposed noise-squeezing parametric amplifier in a conventional 65-nm CMOS process. A minimum noise-squeezing factor of -0.35 dB is achieved with a signal gain of 26 dB for one quadrature component of a 10-GHz narrow-band signal.

Harmonic Generation Using Nonlinear LC Lattices

Nonlinear LC lattices have shown promise for high-power high-frequency signal generation. Here we offer the first detailed study of the frequency response of these lattices, as well as a method designed to find input excitation frequencies that result in intense harmonic generation. The crux of the method is to locate regions in frequency space where the spectral norm of the lattice response matrix is large. When the fundamental excitation frequency (or one of its multiples) is located in these regions, the lattice harmonic response is intensified. These findings are supported by extensive numerical simulations and experimental measurements. We deal chiefly with a first-order dependency of capacitance (C) on voltage (V); however, it is also shown that lattices with higher order C-V dependencies achieve proportionally higher harmonic generation. Simulations using a 0.13-μm CMOS process indicate harmonic generation at 400 GHz (three times the cutoff frequency of the fastest active device in this process), suggesting potential applications of this lattice topology in terahertz range devices.

A Nonlinear Lattice for High-Amplitude Picosecond Pulse Generation in CMOS

In this paper, we study an electrical nonlinear medium consisting of voltage-dependent capacitors and inductors to generate sharp pulses from a lower frequency sinusoid. First, we analyze the optimum conditions for maximum harmonic generation in a uniform nonlinear line. Next, we extend the nonlinear line to a two-dimensional nonlinear lattice that is compatible with CMOS technology. Compared with the nonlinear line, the lattice relies on spatial power combining, higher cut-off frequency, and nonlinear wave interaction to enhance the pulse amplitude and sharpness. To show the feasibility of this method, we implement the first CMOS nonlinear lattice in a 0.13-μm CMOS process, and successfully demonstrate 2.7-Vpp, 6.3-ps pulses from a 22-GHz input signal.

Reduction of polarization-induced performance degradation in WDM PON utilizing MQW-SLD-based broadband source

We report on the reduction of polarization-induced performance degradation in WDM PON utilizing MQW-SLD-based ASE source for injection locking to FPLD. The results show that, to suppress the polarization-induced Q penalty sufficiently less than 0.5 dB, the MQW-SLD output should be depolarized within the locking range of the wavelength-locked FPLD.

Frequency detuning effects in a loop-back WDM-PON employing gain-saturated RSOAs

We report that, in a loop-back wavelength-division-multiplexed passive optical network link with gain-saturated reflective semiconductor optical amplifiers, the upstream performance is strongly affected not only by the gain saturation but also by the selective spectral filtering caused by the frequency difference between the optical downstream signal and the cascaded filters passband. We experimentally investigate these effects and propose to use an additional negatively detuned optical filter at an optical network terminal to improve the upstream transmission performance