Andrew R. Chraplyvy

Parametric amplifiers driven by two pump waves

Parametric amplifiers (PAs) are well-suited for optical communication systems. Not only can PAs provide high gain for arbitrary signal wavelengths, they can also conjugate the signals and convert their wavelengths. The four-sideband model of parametric amplifiers driven by two pump waves will be reviewed and used to describe the conditions required to produce broad-bandwidth gain. The flexibility of the two-pump architecture allows it to produce gain that is nearly independent of the signal polarization, and idlers whose spectral widths are comparable to that of the signal.

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. >

Roadmap of optical communications

Quantum physics allows one to produce truly random bits. Moreover, it allows one to distribute them in such a way that one can certify their privacy before eventually using them for cryptography applications. Quantum Random Number generators (QRNG) and Quantum Key Distribution (QKD) have found a few niche markets. Today, some commercial clients use QKD continuously 24×7 a week. In this workshop world specialists will talk about reliability tests in quantum networks; about quantum hacking, its importance and limitations, and its role in classical and quantum cryptography; about high rate and about low cost QKD systems; about free space quantum communication; and about future quantum repeaters for continental scale quantum communication.Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern societys needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.

Carrier-induced phase noise in angle-modulated optical-fiber systems

Phase noise in angle-modulated optical-fiber communication systems arising from optical power fluctuations is analyzed. The nonlinear refractive index of silica is the physical mechanism which converts power fluctuations into phase fluctuations. The effects of self-phase modulation (an optical wave acting on itself) and cross-phase modulation in wavelength-division multiplexed (WDM) systems (one optical wave modulating a channel at a different wavelength) have been calculated. The phase noise generated in single-channel systems is negligible for laser fluctuations less than 1-mW rms. In WDM systems containing as few as four channels the phase noise exceeds tolerable levels (0.15 rad) for power fluctuation of 1 mW in each channel.

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.

M-ary pulse-position modulation and frequency-shift keying with additional polarization/phase modulation for high-sensitivity optical transmission

We present a new class of optical modulation formats based on the combination of m-ary pulse-position modulation (m-PPM) or m-ary frequency-shift keying (FSK) with additional polarization and/or phase modulation, which is applied on the information carrying pulses in the case of m-PPM or on the information carrying frequency carriers in the case of m-FSK. We describe the principle and implementation of this class of optical modulation formats, and formulate their theoretical receiver sensitivities in optically pre-amplified receivers. Pilot-assisted frequency-domain equalization, similar to that used in coherent optical orthogonal frequency-division multiplexing (CO-OFDM), is used for reliable channel estimation and compensation. CO-OFDM also allows m-FSK to be implemented with high spectral efficiency. As a particular format in this class, m-PPM in combination with polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK), termed as PQ-mPPM, offers superior receiver sensitivity in optically pre-amplified receivers at bit error ratios (BERs) around the thresholds of common forward-error correction codes. Record receiver sensitivities of 3.5 photons per bit (ppb) at BER = 10(-3) and 2.7 ppb at BER = 1.5 × 10(-2) are experimentally demonstrated at 2.5 Gb/s and 6.23 Gb/s using PQ-16PPM and PQ-4PPM, respectively. We further demonstrate the transmission of a 6.23-Gb/s PQ-4PPM signal over a 370-km unrepeatered ultra-large-area-fiber span with 71.7-dB total loss budget.

Narrowband tunable optical filter for channel selection in densely packed WDM systems

One of the more vexing problems inhibiting implementation of densely packed wavelength division multiplexed (WDM) lightwave systems is channel selection or demultiplexing. Coherent detection in which a tunable local oscillator provides the channel selection has been proposed1 and demonstrated.2 However, this method of channel selection shares all the difficulties and additional complexities of coherent detection, i.e., the need for tunable narrow-linewidth lasers and more sophisticated receiver electronics than In direct detection receivers and the inability to operate at low bit rates. Here we demonstrate an alternate method based on stimulated Brillouin amplification tor channel selection, which does not have severe restrictions on laser linewidths and bit rates and uses present direct-detection receivers.

Scrambled coherent superposition for enhanced optical fiber communication in the nonlinear transmission regime

Coherent superposition of light waves has long been used in various fields of science, and recent advances in digital coherent detection and space-division multiplexing have enabled the coherent superposition of information-carrying optical signals to achieve better communication fidelity on amplified-spontaneous-noise limited communication links. However, fiber nonlinearity introduces highly correlated distortions on identical signals and diminishes the benefit of coherent superposition in nonlinear transmission regime. Here we experimentally demonstrate that through coordinated scrambling of signal constellations at the transmitter, together with appropriate unscrambling at the receiver, the full benefit of coherent superposition is retained in the nonlinear transmission regime of a space-diversity fiber link based on an innovatively engineered multi-core fiber. This scrambled coherent superposition may provide the flexibility of trading communication capacity for performance in future optical fiber networks, and may open new possibilities in high-performance and secure optical communications.

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.