Bernard Villeneuve

WDM Performance and PMD Tolerance of a Coherent 40-Gbit/s Dual-Polarization QPSK Transceiver

We report the measured wavelength-division-multiplexing (WDM) performance and polarization-mode dispersion (PMD) tolerance of a coherent 40-Gbit/s dual-polarization quadrature phase shift keying (DP-QPSK) transceiver at 50-GHz minimum channel spacing in a 40-channel 40-span test bed comprised of 3200 km of uncompensated G.652 fiber. We also evaluate the impact of polarization-dependent loss (PDL) on system performance and present the measured tolerance to frequency misalignment between the transmitter and a multiplexer/demultiplexer filter pair.

High-stability wavelength-controlled DFB laser sources for dense WDM applications

Summary form only given. A compact, rugged, and commercially viable laser wavelength stabilization technique for dense WDM fibre communication applications was demonstrated. Production units were shown to be environmentally stable and exhibited very small wavelength shifts over extended aging periods. We also showed that the technique can be used for both single and multiple wavelengths applications.

Simultaneous absolute frequency control of laser transmitters in both 1.3 and 1.55 /spl mu/m bands for multiwavelength communication systems

Absolute frequency control will be an essential part of future dense WDM systems. In this paper, we demonstrate two promising techniques that allow the absolute frequency control of an ensemble of laser transmitters operating in both the 1.3 and 1.55 /spl mu/m bands. First, a Michelson interferometer is absolutely calibrated by means of a frequency-stabilized master DFB laser. This interferometer provides an ensemble of evenly-spaced absolute frequency references that covers both the 1.3 and 1.55 /spl mu/m regions. Lasers are frequency-locked to transmission nulls of this interferometer with a precision of a few hundred MHz. The second technique allows full flexibility in channel frequency assignment and relies on frequency offset control of an ensemble of laser sources relative to a master reference laser. The frequency comparator is based on a surface-emitting nonlinear semiconductor multilayer waveguide. This technique provides simultaneous frequency measurement and control of lasers in both bands with a precision of a few GHz.

A compact wavelength stabilization scheme for telecommunication transmitters

With the 100/200 GHz channel spacing of current commercial DWDM systems, severe restrictions are imposed on transmitter wavelengths in order to meet crosstalk specifications and ensure reliable operation of the system in all conditions of its 25 years lifetime. While very stable in the short-term, free-running DFB laser transmitters are expected to exhibit long-term wavelength drifts exceeding these requirements, leading to a need for practical wavelength monitoring and control. In this paper we describe a compact dither free wavelength stabilization method based on a Fabry-Perot (FP) filter. While the concept can be implemented in a stand-alone unit, its small footprint allows its incorporation into existing laser modules without increasing either real estate or dissipated power.

Measurement of inter-channel non-linear effects in a real-time, phase modulated, coherent transmission system

The performance of a coherent dual polarization quadrature phase key, DP-QPSK modem, with continuous real-time processing, is examined in the presence of cross phase modulation, XPM. The measured influence of optical compensation is reported.

Demonstration of a Cold Start Procedure for a Laser Source Frequency-Locked to Molecular Absorption Lines

We have recently proposed a referencing scale for multifrequency (WDM) optical communication systems with exact multiples of 100 GHz1. In such a system, the absolute frequency control of the sources is imperative since semiconductor lasers possess a nominal operating frequency which is difficult to control precisely upon fabrication and which fluctuates with injection current, junction temperature and aging. Locking the frequency of a laser source to an absorption line of an atomic or molecular gas improves the frequency stability by several orders of magnitude and sets its value precisely. Acetylene (C2H2), for example, provides lines whose absolute frequencies can be known to better than 30 MHz2, with drifts of less than 100 kHz/°C and 200 kHz/Torr3. Typical full width at half maximum (FWHM) of the lines are around 600 MHz at ambient temperatures. Once the laser is stabilized, a long term frequency stability of better than 20 kHz can be obtained. These figures meet the dense optical frequency multiplexing requirements.

Tightening and extending the grid for DWDM sources

We address the issue of stabilizing both fixed and tunable DWDM sources on grids spaced by less than 100 GHz, over a greater than 18 THz frequency range. We will discuss techniques that are compact, robust and cost-effective for actual transmitter cards. Fixed-frequency sources, generally DFBs, are the workhorse of existing DWDM systems.

Proposal for a multiwavelength coherent communication system using laser sources frequency-locked to atomic references

In recent years, much work has been done on the frequency stabilization of laser diodes through the electrical feedback loop, using for a frequency discriminator an optical resonator such as a fiber loop, a Fabry-Perot, or an atomic or molecular phenomenon. While allowing a reduction of frequency fluctuations, the lattar approach also gives the possibility of fixing precisely the absolute value of the laser frequency (wavelength) according to the quantum mechanics that govern the optical transition. Thus, if many resonances are observable in a close optical frequency domain, many sources can be set to precise distinguishable frequencies and become part of a muitiwavelength coherent communication system where the channel selectivity is done at the intermediate frequency level.1