Robert W. Tkach

Capacity Trends and Limits of Optical Communication Networks

Since the first deployments of fiber-optic communication systems three decades ago, the capacity carried by a single-mode optical fiber has increased by a staggering 10 000 times. Most of the growth occurred in the first two decades with growth slowing to ten times in the last decade. Over the same three decades, network traffic has increased by a much smaller factor of 100, but with most of the growth occurring in the last few years, when data started dominating network traffic. At the current growth rate, the next factor of 100 in network traffic growth will occur within a decade. The large difference in growth rates between the delivered fiber capacity and the traffic demand is expected to create a capacity shortage within a decade. The first part of the paper recounts the history of traffic and capacity growth and extrapolations for the future. The second part looks into the technological challenges of growing the capacity of single-mode fibers by presenting a capacity limit estimate of standard and advanced single-mode optical fibers. The third part presents elementary capacity considerations for transmission over multiple transmission modes and how it compares to a single-mode transmission. Finally, the last part of the paper discusses fibers supporting multiple spatial modes, including multimode and multicore fibers, and the role of digital processing techniques. Spatial multiplexing in fibers is expected to enable system capacity growth to match traffic growth in the next decades.

Dependence of cross-phase modulation on channel number in fiber WDM systems

The phase term appearing in the expression for cross-phase modulation due to the optical Kerr effect depends on the sum of the powers carried by each wavelength channel. For this reason, one might expect that the amount of cross-phase modulation would increase with increasing channel number, causing increased interference among channels and hence limiting the total number of channels that a WDM system can support. However, computer simulations of multichannel systems have shown no change in signal distortion as the number of wavelength channels is increased from four to eight. In a simulated three-channel system, the signal distortion of the central channel approaches that of a single-channel system as the wavelength separation is increased to approximately 2 nm. Thus, even a moderate amount of dispersion tends to cancel out the influence of cross-phase modulation, so that beyond a certain wavelength spacing, additional channels do not interfere with the channel under consideration. From these observations, we conclude that cross-phase modulation does not limit the number of wavelength channels that a single optical fiber can support. However, self- and cross-phase modulation are not the only nonlinear effects influencing fiber lightwave systems. Stimulated Raman scattering tends to transfer optical power from short-wavelength channels to channels operating at longer wavelength, degrading their signal-to-noise ratio. The efficiency of this process increases with increasing wavelength spacing. Clearly, a compromise needs to be reached between the conflicting requirements imposed by the optical Kerr effect and by stimulated Raman scattering. >

Improving the Nonlinear Tolerance of Polarization-Division-Multiplexed CO-OFDM in Long-Haul Fiber Transmission

We present digital signal processing techniques and algorithms for improving the tolerance of polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexing (CO-OFDM) to fiber nonlinear effects, in both single-channel and wavelength-division multiplexed (WDM) transmission. For single-channel transmission, we discuss a self-phase modulation (SPM) compensation method that jointly processes two polarization components of a PDM CO-OFDM signal. For WDM transmission, we describe a novel OFDM channel estimation scheme that reduces the cross-phase-modulation (XPM) penalty among the WDM channels. The nonlinear tolerance improvements enabled by these signal processing techniques are quantified through numerical simulations for both dispersion-unmanaged and dispersion-managed long-haul optical fiber transmission with 112-Gb/s PDM CO-OFDM wavelength channels.

Experimental Investigation of Inter-Modal Four-Wave Mixing in Few-Mode Fibers

We experimentally demonstrate nondegenerate four-wave mixing (FWM) between waves belonging to different spatial modes of a 5-km-long few-mode fiber (FMF). Of the three inter-modal FWM (IM-FWM) processes possible, two have been experimentally observed. These IM-FWM processes are found to be phase-matched over very large frequency separations of several Terahertz between the waves. In contrast to FWM in single-mode fibers that require operating near the zero-dispersion wavelength to achieve phase matching, IM-FWM in a FMF can be fully phase matched in the presence of large chromatic dispersion in each spatial mode.

Observation of large quadratic electro‐optic effect in GaAs/AlGaAs multiple quantum wells

We simultaneously measure the intensity modulation level and the optical spectrum of the output of a multiple quantum well modulator, and use these data to deduce the electro‐optic coefficients. The effect is quadratic, with a measured ‖s33‖=4.6×10−13 cm2/V2 at a wavelength 12 meV below the band gap. This is approximately 800 times the coefficient measured further from the band gap. We are able to achieve a fractional change in the refractive index of 3.7%. Despite the size of this effect, when we operate the device as an intensity modulator, we observe a linewidth enhancement factor of α=1.0, which means the chirp induced in the device’s output will be small.

Demonstration of Record Sensitivities in Optically Preamplified Receivers by Combining PDM-QPSK and M-Ary Pulse-Position Modulation

We present the principle, implementation, and performance of a recently introduced high-sensitivity modulation format based on the combined use of polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK, or PQ) and m-ary pulse-position modulation (m-PPM). This novel modulation format, termed PQ-mPPM, offers high receiver sensitivity in optically preamplified receivers. We study the sensitivity of the PQ-mPPM format both analytically and experimentally, and compare it to common modulation formats such as PDM-QPSK, m-PPM, differential phase-shift keying, and polarization-switched QPSK. The bandwidth expansion factor of this format is also discussed. A record sensitivity of 3.5 photons per bit at BER = 10-3 is experimentally demonstrated at 2.5 Gb/s with a novel pilot-assisted digital coherent-detection scheme, outperforming PDM-QPSK by about 3 dB.

305-km combined wavelength and mode-multiplexed transmission over conventional graded-index multimode fibre

We present experimental results for mode-multiplexed WDM transmission over OM3 multimode fibres. We transmit 60 WDM channels and 3 spatial modes over a distance of 305 km. We use mode-selective photonic lanterns as mode couplers and for differential group delay compensation.

Experimental Observation of Inter-Modal Cross-Phase Modulation in Few-Mode Fibers

We experimentally demonstrate a broadband nonlinear interaction between a data-modulated pump and an unmodulated probe occupying different spatial modes of a few-mode fiber. This nonlinear interaction is attributed to inter-modal cross-phase modulation (IM-XPM) and found to be maximum when signal and probe have similar group velocities. This can correspond to a few THz of wavelength separation between waves. The magnitude of the measured IM-XPM is found to be comparable to the measured intra-modal XPM with similar nonlinear coefficients. A comparison to a semi-analytical model of XPM shows good agreement with the measurements.

Network traffic and system capacity: Scaling for the future

After the collapse of the market for optical communications equipment at the beginning of the millennium, there was diminished interest and investment in technologies for long-distance optical transmission. This was largely a result of the enormous capacities afforded by wavelength-division multiplexed systems. The systems deployed early in the decade had capacities comparable to the total network traffic. This situation is about to change. Those 2000-era systems are becoming full now, and growth in traffic, together with trends in the scaling of technologies for optical communications, indicates that capacity of systems is unlikely scale quickly enough to keep pace with demand. These trends will have profound impact on the market for equipment and the types of systems that will be needed. The tutorial will show evidence of these historical trends and the scaling they imply and discuss the implications.

Survey of atomic transitions for absolute frequency locking of lasers for lightwave systems

The authors survey 26 excited-state transitions of argon (Ar), krypton (Kr), and neon (Ne) around 1.3 and 1.5 mu m. A 1200-lines-per-mm grating mounted on a piezoelectric translator forms the adjustable reflective element of an external cavity semiconductor laser, reducing the laser linewidth to <100 kHz, and affording tens of nanometers of tunability. The laser may be tuned continuously over a range of approximately 2 GHz between mode hops by scanning the cavity length with the piezoelectric translator. Single-frequency oscillation is verified by the 750 MHz confocal Fabry-Perot, and the wavelength is measured by a wavemeter.<<ETX>>