S.L. Woodward

Reduction of optical-beat interference in subcarrier networks

We present a method for improving the performance of a subcarrier multiplexed passive optical network (SCM-PON) in the presence of optical beat interference (OBI). By introducing strong clipping tones (optical-modulation depth >1) on each of the lasers their optical spectrum can be broadened, thereby ameliorating the effects of OBI. By carefully selecting the RF band used to carry signals, and that used for clipping tones, OBI can be reduced without the clipping inducing errors. We have demonstrated the effectiveness of this technique by measuring the BER of a QPSK channel in the presence of OBI. A delayed self-homodyne technique was used to simulate worst-case conditions.

A DBR laser tunable by resistive heating

A distributed Bragg reflector (DBR) laser tuned by resistive heating is presented. It has a tuning range greater than 10 nm with only a 33% reduction in output power and a 10% increase in linewidth. Its behavior is easily modeled, agreeing well with simple theory.<<ETX>>

A control loop which ensures high side-mode-suppression ratio in a tunable DBR laser

The authors present a control loop which ensures single-mode operation of a tunable distributed-Bragg-reflector (DBR) laser. This control loop uses no external optics. The error signal is derived from an integrated detector which measures the light transmitted through the Bragg section. Using this signal to control the current to the Bragg section, the laser is tuned to where the side-mode-suppression ratio is high. This guarantees that the laser remains single-mode, even when modulated. This servo is capable of application in wavelength-division-multiplexed (WDM) systems.<<ETX>>

The onset of coherence collapse in DBR lasers

A study of the dependence of the onset of coherence collapse on laser output power is discussed. Three-section multiquantum-well-distributed-Bragg-reflector (MQW-DBR) lasers were used. The fraction of light reflected back into the lasing mode was varied, and the point at which the transition to coherence collapse occurred was measured. This feedback level varies approximately linearly with laser output power. For these lasers, when the output power is 1 mW, the transition to coherence collapse begins when the optical feedback into the lasing mode is below -40 dBm; when the feedback power is -35 dBm, the laser line is completely collapsed.<<ETX>>

The effects of aging on the Bragg section of a DBR laser

The effects of accelerated aging on the tuning section of seven distributed-Bragg-reflector (DBR) lasers were studied. These lasers tune by current injection. In all seven lasers the wavelength versus tuning curve changed significantly after aging. As the lasers aged, the tuning current became less effective, so that it would have to be increased to maintain a constant wavelength.<<ETX>>

A method for reducing multipath interference noise

One source of noise in optical communication systems is multipath interference (MPI)-intensity noise caused by multiple discrete reflections or double Rayleigh backscatter (DRB). MPI converts phase noise to intensity noise, which can cause significant impairments in analog lightwave systems transmitting CATV signals. We have found that by dithering the laser frequency by several gigahertz at kilohertz frequencies this noise can be reduced. We have demonstrated this technique using a DFB laser.<<ETX>>

The side-mode-suppression ratio of a tunable DBR laser

The authors investigate, both experimentally and theoretically, how the side-mode-suppression ratio (SMSR) of a multisection multiple-quantum-well distributed-Bragg-reflector (MQW-DBR) laser varies with tuning current. It is found that as the tuning current is adjusted the wavelength and the SMSR vary, with the peak SMSR=43 dB. The output power also varies with tuning current, and one can find when the laser is near a mode hop by examination of the light versus tuning current curve. This implies that the tuning current can be adjusted so that the laser frequency is near the center of the Bragg band without measuring the SMSR. The measurement are in good agreement with theory.<<ETX>>

Uncooled Fabry-Perot lasers for QPSK transmission

We investigate the use of uncooled lasers in an optical fiber transmission system employing quadrature-phase-shift keyed (QPSK) signals on subcarriers. The BER of a 2 Mb/s QPSK channel is measured in a system containing 60 subcarriers. Error-free performance (BER <10/sup -9/) is achieved over a wide range of modulation depths. This demonstrates that these inexpensive lasers can be used in multi-channel digital systems.<<ETX>>

Mini-fiber-node hybrid fiber coax networks for two-way broadband access

Summary form only given. In summary, by deploying low-cost mini fiber nodes (mFNs) deep into coax distribution networks, the mFN-based hybrid fiber coaxial (HFC) evolution strategy provides a novel solution to upstream limitations in traditional HFC networks with further enhanced system bandwidth even beyond the limits of coax amplifiers. This results in a transparent HFC upgrade for high capacity without incurring the cost of system re-design and re-engineering.

Wavelength stabilization of a DBR laser using an in-fiber Bragg filter

A control loop that stabilizes the wavelength of a tunable distributed-Bragg-reflector (DBR) laser is discussed. An in-fiber Bragg grating is used as the wavelength reference. Over a 25-hour period, this control loop held the laser wavelength constant to within 0.022 nm. This variation is traced to the temperature dependence of the in-fiber Bragg grating and can be reduced by controlling the gratings temperature.<<ETX>>