Ka-Lun Lee

Fiber-Wireless Networks and Subsystem Technologies

Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks

The deployment rate of wavelength division multiplexed passive optical networks (WDM-PONs) is expected to accelerate with the availability of cost-efficient wavelength-specific transmitters. Fueled by this promise, we propose and experimentally demonstrate a novel scheme that facilitates the use of reflective semiconductor optical amplifiers (RSOAs) as colorless upstream transmitters. Central to the scheme is the use of a passive reflective path that is placed at the remote node (RN) to reflect a spectral slice of the broadband amplified spontaneous emission (ASE) light emitted from each RSOA. The reflected spectral slice, termed as a seeding light, establishes a self-seeding of the RSOA with measurements indicating the self-seeded output to be incoherent with a low relative intensity noise. The subsequent direct modulation of the self-seeding RSOA with nonreturn-to-zero data at 1.25 Gb/s for upstream transmission exhibits good transmission and crosstalk performance after traversing 21 km of single-mode fiber. Our proposed scheme eliminates the need for centralized broadband sources, external modulators, and active temperature control within the RN and between the RN and the optical network unit. Aside from the feasibility study of self-seeding RSOAs, we investigate the upstream performance dependence on the characteristics of the seeding light. Our investigations reveal that there exists a noise floor limit of the bit error rate (BER) of the self-seeded upstream signal. The noise floor is shown to vary with an initial optical seeding power that affects the level of ASE noise suppression of the self-seeded upstream signal. None the less, the RSOA self-seeds at a user-defined wavelength with a sufficient suppression of ASE noise to achieve a BER=10-9 with only -30.5 dBm of initial optical seeding power. Our characterization of the frequency response of the RSOA reveals a high-pass filter response that suppresses the modulation on the reflected seeding light, and thus stabilizing the self-seeded output. Collectively, these features highlight the potential of using the self-seeding RSOAs to realize a cost-efficient WDM-PON solution in the near future

Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment

In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two- and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.

Rational harmonic mode locking of an optically triggered fiber laser incorporating a nonlinear optical loop modulator

Rational harmonic mode locking is first demonstrated in an erbium-doped fiber laser using an all-optical approach. Both bright and dark pulse trains up to 4.58 GHz are obtained at 1.55 /spl mu/m by adjusting the initial reflection state of the intracavity loop mirror. The nonlinear loop modulator serves as a mode locker and an adjustable end reflector in the laser. The setup is useful for optional bit-rate multiplication while providing a means for the conversion of wavelength, as well as the conversion between bright and dark pulses. The output wavelength is also tunable upon the adjustment of a cavity fiber Fabry-Perot filter.

Waveband-switchable SOA ring laser constructed with a phase modulator loop mirror filter

An electrically tunable multiwavelength source has been developed using a birefringence-based optical comb filter in a semiconductor optical amplifier ring laser. The filter is constructed with an electrooptic phase modulator placed inside a fiber loop mirror. By controlling the birefringence of the modulator through the applied bias, the radio-frequency power, or the modulation frequency, we achieve a continuous shift of the spectral comb to access different interleaved wavebands. Electrical waveband switching has been successfully demonstrated for 21 wavelengths at 100-GHz grid spacing with an optical signal-to-noise ratio over 40 dB.

4 /spl times/ 2.5 GHz repetitive photonic sampler for high-speed analog to digital signal conversion

This letter presents a 10-gigasample/s (GS/s) photonic analog-to-digital converter (ADC) system constructed using a four-wavelength picosecond pulsed source. The lasing-to-nonlasing modes suppression ratio of the optical source is over 24 dB. By using the 10-GHz optical source, a 10-GS/s photonic ADC has been demonstrated and was used to sample an arbitrary radio-frequency signal. The system was further investigated by sampling a 2.4-GHz sinusoidal signal. Important parameters including the signal-to-noise and distortion ratio and the spurious-free dynamic range have been determined.

Optically controlled Sagnac loop comb filter

We propose a new type of dynamically tunable birefringence comb filter based on a semiconductor optical amplifier Sagnac loop interferometer. By optically modulating the birefringence of the amplifier, we demonstrate a tuning of the output wavelengths. The shift of wavelength increases monotonically with the power of the control light until saturation occurs. The tuning relation is independent of the comb spacing governed by the length and birefringence of the polarization maintaining fiber inside the loop. A tuning range up to 59% of the comb spacing has been achieved at 18.5 dBm input power.

Energy efficiency of optical transceivers in fiber access networks [invited]

The dramatic growth of Internet traffic is leading to a concern about the future power consumption of the Internet. Energy sustainability of communication networks is becoming a very important goal for the reduction of the global carbon footprint. As optical access networks gain more popularity, their share in the energy consumption of the data network will increase. Developing energy-efficient technologies for optical access networks is therefore crucial for the continuous scaling of the Internet. In this paper, we model the power consumption of different transceivers and demonstrate how various electronic and photonic technologies can help improve energy efficiency. We discuss the impact of different light sources and driver circuits on the transceiver power efficiency. We also show how energy efficiency is related to network topology.

Mitigation strategy for transmission impairments in millimeter-wave radio-over-fiber networks [Invited]

Hybrid fiber-wireless networks for fixed wireless access operating in the millimeter-wave (mm-wave) frequency region have been actively pursued to provide ultrahigh bandwidth for untethered connectivity. Moving the radio operating frequency into the mm-wave region overcomes the spectral congestion in the lower microwave region and is also capable of providing high-capacity broadband wireless services in a picocellular or microcellular architecture. Optical fiber backhaul provides the broadband interconnectivity between a centralized location and a large number of high-throughput antenna base stations necessary in such an architecture. The transportation of mm-wave wireless signals within the hybrid network is subject to numerous impairments ranging from low conversion efficiency to fiber chromatic dispersion and also to signal degradation due to nonlinearity along the link. One of the major technical challenges in implementing these networks lies in the mitigation of these impairments that the wireless signals experience while traversing the links. In this paper, we present an overview of the different techniques and schemes to overcome some of the impairments for transporting mm-wave signals over optical fibers.

Directly-Modulated Self-Seeding Reflective SOAs as Colorless Transmitters for WDM Passive Optical Networks

We present the first demonstration of directly-modulated self-seeding reflective semiconductor optical amplifiers as colorless transmitters in a WDM-PON, eliminating centralized broadband sources, active temperature control and external modulators.