Simple approaches of wavelength registration for monolithically integrated DWDM laser arrays

A simple approach is proposed to allocate the wavelengths of monolithically integrated sampled grating distributed Bragg reflector (SGDBR) laser arrays for applications in dense wavelength-division-multiplexing (DWDM) systems. The lasers in an array are thermally controlled altogether and each laser is operated with two electrodes. Arrays of accurate wavelengths can be realized by fine adjusting the bias current and phase current of each laser to output a uniform wavelength comb, and then varying the heat sink temperature to align all channels to the desired wavelength grid. Such procedures can provide arrays of high sidemode suppression ratio (SMSR) and no mode hopping over a wide range of operation condition. We demonstrated 50-GHz spaced arrays of accurate and stable wavelengths.     

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.     

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 evaluation of fault-branch detection in TDM passive optical networks by spectral analysis of interferometric signals

A new fault-branch detection scheme is proposed to troubleshoot the breaks of any distribution fibers in a time-division multiplexing (TDM) passive optical network. We employ a continuous optical frequency sweeper at the optical line terminal (OLT) and an interferometric (IF) device at each optical network unit (ONU). By analyzing the spectrum of the returned combined signals at the OLT, we can obtain the status of all branches. This detection method not only uses a small optical frequency band for surveillance monitoring, but is also simple to operate. Furthermore, a modified architecture is proposed to relax the specifications of IF devices. The tolerance of the IF device length was analyzed using the Monte–Carlo simulation method.     

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.     

On leaky mode approximations for modal expansion in multilayer openwaveguides

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     

Bidirectional Transmission Using Tunable Fiber Lasers and Injection-Locked Fabry–PÉrot Laser Diodes for WDM Access Networks

We propose and demonstrate the use of fiber ring lasers and Fabry-Perot laser diodes (FP-LDs) for wavelength-division-multiplexing access networks. The fiber ring laser not only generates downstream data traffic but also serves as the wavelength-selecting injection light source for the FP-LD located at the subscriber site. Moreover, it is wavelength tunable and can be applied to dynamic wavelength assignment networks. The ring laser has a tunable range of 30 nm in the C-band and a power fluctuation smaller than 0.6 dB. For 10-Gb/s downstream and 1.25-Gb/s upstream transmissions over 10-km single-mode fiber, power penalties less than 0.9 and 0.5 dB are demonstrated, respectively. A 40-dB sidemode suppression ratio is obtained for the FP-LD injection-locking at 1544.8 nm.     

Four-channel coarse-wavelength division multiplexing demultiplexer with a modified Mach-Zehnder interferometer configuration on a silicon-on-insulator waveguide

A coarse wavelength division multiplexer is designed on a silicon-on-insulator waveguide using the Mach-Zehnder interferometers with novel multimode interface-periodically segmented waveguide couplers and segmented waveguide arms. It is viable for metro and access applications, since it can be inexpensive and provide easy fabrication, compact size, and good output performance. As a design example, the channel spacing of the demultiplexer is chosen to be 24.5 nm for applications to the 10-Gigabit Ethernet. The simulation results show that the wide-passband demultiplexer can have insertion loss less than 2.3 dB and crosstalk better than 18 dB.     

Wavelength control of tunable dense wavelength-division multiplexing sources by use of a Fabry-Perot etalon and a semiconductor optoelectronic diode

A high-resolution tunable-wavelength controller is achieved by use of an etalon for control of wavelength drift and a semiconductor optical diode (SOD) for channel recognition. The etalon provides a stable wavelength reference, and the SOD can detect mode-hopping and incomplete-tuning problems in tuning a laser. With the help of a Fabry-Perot etalon as a precise wavelength reference, the usual concern with the temperature stability of a SOD can be relaxed at least tenfold compared with wavelength control with a single SOD. We demonstrate the feasibility of monitoring tunable lasers by using a Fabry-Perot laser diode (FPLD) or a semiconductor optical amplifier (SOA). The induced voltage of the FPLD and that of the SOA are modeled with analytic expressions that can help to optimize the operation of a SOD sensor.     

Dynamic responses of widely tunable sampled grating DBR lasers

The modulation bandwidth, dynamic mode suppression ratio, and wavelength chirp of directly modulated sampled grating DBR lasers have been measured. Although the tuning range can be up to an order of magnitude larger than in simple DBR lasers, the chirp is about the same or better over a wide range of operation parameters. The modulation bandwidth was in excess of 4 GHz and the dynamic MSR remained larger than 40 dB as long as the current did not swing below threshold. The linewidth enhancement factor was extracted from the measured chirp parameters and ranged from three to eight for different lasing wavelengths of the tunable lasers. The dispersion of the linewidth enhancement factor is consistent with published theoretical predictions.