Jason B. Stark

Nonlinear limits to the information capacity of optical fibre communications

The exponential growth in the rate at which information can be communicated through an optical fibre is a key element in the ‘information revolution’. However, as for all exponential growth laws, physical limits must be considered. The nonlinear nature of the propagation of light in optical fibre has made these limits difficult to elucidate. Here we use a key simplification to investigate the theoretical limits to the information capacity of an optical fibre arising from these nonlinearities. The success of our approach lies in relating the nonlinear channel to a linear channel with multiplicative noise, for which we are able to obtain analytical results. In fundamental distinction to linear channels with additive noise, the capacity of a nonlinear channel does not grow indefinitely with increasing signal power, but has a maximal value. The ideas presented here may have broader implications for other nonlinear information channels, such as those involved in sensory transduction in neurobiology. These have been often examined using additive noise linear channel models but, as we show here, nonlinearities can change the picture qualitatively.

Suppressed photocurrent multiple‐quantum‐well optical modulators by proton implantation

We investigate the reduction of power dissipation in absorptive semiconductor optical modulators via reduction in photocurrent. We present experimental results for proton implantation of p‐i‐n‐multiple‐quantum‐well modulators to levels of 1×1012, 1×1013, and 1×1014 cm−2. We find that photocurrent can be significantly reduced with moderate reduction in modulation performance relative to the unimplanted device. Application to asymmetric self‐electro‐optic devices is demonstrated.

Phased amplitude-shift signaling (PASS) codes: increasing the spectral efficiency of DWDM transmission

We propose and analyze a new class of transmission line codes, called PASS codes, capable of dramatically increasing the spectral efficiency of DWDM transmission. PASS codes generalize the spectral shaping characteristics of duobinary signaling to multi-bit data representations.

Line coding for dispersion tolerance and spectral efficiency: duobinary and beyond

We propose and analyze a new class of transmission line codes that dramatically increase the dispersion tolerance and spectral efficiency of dense wavelength-division multiplexing transmission. These codes generalize and optimize duobinary signaling, yielding new binary and multibit data representations.

103-channel chirped-pulse WDM transmitter

Chirped-pulse wavelength-division multiplexing (CPWDM) was recently proposed as a new technique for generating a comb of WDM channels. A key feature of this approach is the possibility to generate and encode data on a very large number of channels using a single short-pulse laser and a single time-division multiplexing modulator. This characteristic makes CPWDM attractive for applications where many channels with moderate bit rates per channels (10-622 Mbit/s) are required. In this paper, we report the generation of 103 independent WDM channels at a bit rate of 37 Mbit/s each from a single CPWDM transmitter. This represents more than a three-fold increase in the number of channels previously reported for this type of source and is, to our knowledge, the largest number of channels reported to date for any WDM transmitter.

206-channel chirped pulse wavelength-division-multiplexed transmitter

vices as well as with new international directions. Bit rates are also compatible with worldwide standards. Digital video, once a challenge in terms of both bit rate and cost, is becoming a commodity item thanks to markets rapidly developing around satellite TV and personal computers. Finally, it is now recognized that the firstcost premium of FTTH compared with other networks vanishes for certain early applications such as rural installations.’ As costs continue to fall, suburban then urban installations will subsequently reach parity. Surprisingly, though, even for suburban applications, Bellcore studies show that FTTH offers annual savings in operating ~os t s possibly large enough to offset today’s premium. The bottom line is that FTTH makes sense to install soon if not now, and manufacturers are responding with new FTTH product development. 1.

Light-emitting diode source for chirped wavelength division multiplexed local access network

for a chain of 100 ideal EDFAs where the amplifier gain is equal to the span loss (17 dB). Figure 3b shows the power spectral density (PSD) after 50 WADM stages. No significant power variation between the eight channels is observed. Irregularities of the ASE noise level are due to the specific add/drop scheme of Fig. 2. The above results show that a WDM network with more than SO WADM stages in cascade and with 900 dB of total interstage losses is theoretically feasible. This conclusion is exclusively based on the study ofthe optical SNR. The impact of other physical effects such as lasedfilter misalignment, chromatic dispersion, nonlinearities, crosstalk, polarizationdependent loss or polarization dispersion will be part of future simulator enhancements. This work was performed as a part of the MONET consortium under DARPA funding agreement MDA 972-95-3-0027. *Currently with the Center for Telecommunications Research, Columbia University 1. J. Lightwave Technol. 14, No. 6 (1996). 2. R. E. Wagner et al., J. Lightwave Technol. 14, No. 6, 1349-1355 (1996). 3. M. A. Scobey, D. E. Spock, “Passive DWDM components using Microplasma optical interference filters,” Technical Digest, OSA Optical Fiber Conference, San Jose, Calif, Feb. 1996. R. M. Jopson, A. A. M. Saleh, in Fiber Laser Sources and Amplifers III, vol. 1851

Femtosecond Chirped-Pulse Multifrequency Sources

We present a novel multifrequeney wavelength-division-multiplexed (WDM) source based on a femtosecond fiber laser. Pulses are linearly chirped and stretched in time by propagation through a fiber, and 32 WDM channels can be defined and modulated sequentially in time by a single modulator.

Chirped-pulse wavelength-division multiplexing at 155 Mbit/s with active feed-forward channel equalization

The broad bandwidth of femtosecond lasers make them potential sources for wavelength-division multiplexing (WDM) systems. The difficulty is in spectrally slicing out the channels and modulating them. A technique for doing both simultaneously was recently demonstrated. In this technique, known as chirped-pulse WDM (CPWDM), a femtosecond pulse is linearly chirped by passage through a dispersive delay line such as a single-mode fiber. Here we demonstrate the scaling of this concept to 155-MHz repetition rate and the use of a feed-forward technique to actively equalize the channels, increasing the number of channels by approximately 50%.