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The benefit of Brillouin amplification for optical carrier recovery from pilot tones is evaluated in application to coherent signal detection without a conventional local oscillator (LO) laser. The focus being on the narrow gain bandwidth suppressing broadband spectral noise power relative to the pilot tone. Calculations indicate its use at the receiver with realistic 30 dB gain and 30 MHz gain bandwidth permit lowering the minimum pilot tone to signal power ratio (PSR) for low distortion signal demodulation by up to a24 dB in the case of a 10 GHz wide signal channel, and even larger for broader channels. The ultimate limit being its ratio to the narrowness of the Brillouin gain bandwidth. Low noise Brillouin gain is also accounted for from characterization by a coherent receiver and digital signal processing approach for an optical fiber used as the gain medium in experiments. Its use in optical carrier recovery from a 3 channel WDM 48 Gb/s 64-QAM signal with polarization multiplexed pilot tones demonstrates improvement consistent with predictions in reducing the impact of pilot tone noise to near that for a conventional LO. The estimated low PSR limit for a bit error rate below the hard decision FEC limit was extrapolated as a−38 dB. This rose to a−30 dB with inclusion of 80 km link transmission due to the restricted signal launch power for avoiding nonlinear distortion. The insight highlights the wide-ranging capabilities for enhancing performance in advanced coherent communications.

Brillouin amplification of a CW laser is evaluated by a coherent receiver and offline digital signal processing (DSP) to assess electric field distortions relevant to noise sensitive applications, and in particularly, optical communications. In this approach, the Brillouin amplifier output is heterodyne detected at the receiver so that the proportional electric field restored by DSP can be resampled into a virtual quadrature phase shift keyed (QPSK) signal. From this, an effective Q2 -factor distortion and amplitude and phase error are quantified. Characterization by the approach of Brillouin gain in a 4.46 km standard signal mode fiber shows amplitude error as the dominant noise feature while impact from phase error is generally negligible. Large gain a≥26 dB with minimal Q2 factor degradation

The limits of narrowband Brillouin gain for improving the carrier to noise ratio (CNR) of spectral lines is evaluated for 64-QAM communications. Calculations of the change in CNR for practical gain, noise, and bandwidth parameters indicate the maximum obtainable CNR enhancement factor and output CNR as being a25 dB and a40∼43 dB/0.1 nm, respectively, limited by the narrowness of the Brillouin gain bandwidth and Brillouin amplifier noise, respectively. These limits determined from the relative power suppression of out of band noise contained within a 10 GHz channel bandwidth are relevant to spectral line amplification either before data modulation at the transmitter, or for a local oscillator (LO) at the receiver for coherent detection. A calibration measurement of the bit error rate (BER) of a 48 Gb/s 64-QAM signal versus CNR of a LO loaded with ASE noise is used to extrapolate the BER from the calculated CNR for Brillouin gain. The results predict the BER being minimally degraded for an input CNR of a>20 dB/0.1 nm and falling below the hard decision FEC limit for an input CNR as low as a0 dB/0.1 nm. The estimates are found consistent with 64-QAM signal experiments of pilot tone carrier recovery by Brillouin gain for both single channel and WDM cases. The insight aids the design and estimation of achievable performance improvement for applying Brillouin amplifiers to coherent communications.

Enhanced Brillouin amplification with intensity noise suppressed by a gain saturated semiconductor optical amplifier (SOA) is demonstrated. Application to noisy input spectral-lines for 64-QAM coherent communications improves tolerance to detuned polarization and frequency from maximum gain.

Owing to advances in fabrication technology and device design, semiconductor optical amplifiers (SOAs) are evolving as a promising candidate for future optical coherent communication links. This review article focuses on the fundamentals and broad applications of SOAs, specifically for optical channels with advanced modulation formats, as an integrable broadband amplifier in commercial transponders and as a nonlinear medium for optical signal processing. We discuss the basic functioning of an SOA and distortions of coherent signals when SOAs are used as amplifiers. We first focus on the techniques used for low-distortion amplification of phase-modulated signals using SOAs. Then we discuss optical signal processing techniques enabled by SOAs with an emphasis on all-optical wavelength conversion, optical phase conjugation, and phase quantization of coherent optical signals.

To answer the question: “Is optical phase conjugation (OPC) capable of compensating nonlinear distortions caused by not only Kerr effect of optical fibre, but also the carrier dynamics of semiconductor optical amplifiers (SOAs)?", we investigate the effectiveness of OPC-based nonlinear compensation for SOAs amplifying a few-channel WDM signal modulated with m-QAM. We use a pair of SOAs with an OPC stage sandwiched between the two so that the combination works as a low-distortion amplifier. Symbol-period longer than the gain recovery time is chosen in our experiments to avoid bit-pattern effects introduced by the SOA. We amplify a 12Gbaud, 16QAM modulated three-channel WDM signal with this technique in the back-to-back configuration which remarkably outperforms a single SOA in the nonlinear regime of operation with an average Q2 improvement better than 4 dB for an output power of 4 dBm. We further demonstrate the practical advantage of the low distortion higher output power capability of the SOA shown in the back-to-back result by carrying out a transmission of the amplified signal through a 160-km fibre, where relatively high launch power is desirable. We also study the case of 64QAM signals and show that approximately a 3 dB Q2 factor improvement can be obtained over single SOA, while without nonlinear phase distortion compensation, the demodulation is nearly impracticable.

Amplification of single channel and three Wavelength division multiplexing (WDM) channels with 16 and 64QAM modulated signals is demonstrated using an SOA with holding beam in the non-transparency wavelength to minimize phase distortion. We demonstrate an optical gain of 10 dB for single channel and for each channel in WDM operation. We also show that, the performance with holding beam can provide with minimal phase distortions a better bit-error rate value when compared to the linear regime of SOA operation for the same output power level.

Modern fiber-optic coherent communications employ advanced, spectrally efficient modulation formats that require sophisticated narrow-linewidth local oscillators (LOs) and complex digital signal processing (DSP). Self-coherent optical orthogonal frequency-division multiplexing (self-CO-OFDM) is a modern technology that retrieves the frequency and phase information from the extracted carrier without employing a LO or additional DSP. However, a wide carrier guard is typically required to easily filter out the optical carrier at the receiver, thus discarding many OFDM middle subcarriers that limit the system data rate. Here, we establish an optical technique for carrier recovery, harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit·s−1 self-CO-OFDM signals, without requiring a separate LO. The narrow SBS linewidth allows for a record-breaking small carrier guard band of ∼265 MHz in self-CO-OFDM, resulting in higher capacity than benchmark self-coherent multi-carrier schemes. Chip-based SBS-self-coherent technology reveals comparable performance to state-of-the-art coherent optical receivers while relaxing the requirements of the DSP. In contrast to on-fiber SBS processing, our solution provides phase and polarization stability. Our demonstration develops a low-noise and frequency-tracking filter that synchronously regenerates a low-power narrowband optical tone, which could relax the requirements on very-high-order modulation signaling for future communication networks. The proposed hybrid carrier filtering-and-regeneration technique could be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.

An active, harmonically modelocked fiber laser with phase unstable output pulses from its long cavity is shown transformable into a low noise frequency comb suited for phase noise sensitive communications. Cross-phase modulation (XPM) of its output pulses with a continuous wave (CW) probe laser in a highly nonlinear fiber (HNLF) produces a frequency comb with noise at its minimum for low timing jitter pulses. Experiments with 2 ps pulses at 40 GHz repetition show that spectral lines obtained from within a bandwidth of 30 lines achieve low bit error rate (BER) after their individual modulation with 96 Gb/s dual polarization 64-QAM signals, reaching below the limit of hard-decision forward error correction (FEC), and close to that of a narrow linewidth CW laser. Its suitability as a seed for even broader comb generation is demonstrated by addition of a second stage HNLF, enabling a larger bandwidth of nearly 50-comb lines to reach a BER below the FEC limit for the same signal. The benefit of XPM for improving the phase noise of a modelocked semiconductor laser frequency comb is also shown, with limitation from larger pulse timing jitter. The results highlight the capability for enabling new applications of otherwise unusable pulse sources.

The dissemination of the digital coherent technology has enabled the recent growth of information networks, which has obviated signal processing in the optical domain such as optical dispersion compensation. However, the further scaling of the digital coherent technology will eventually suffer from the slowdown of the Moore's law and/or the energy crunch as a result of the longstanding relentless traffic increase. The use of all-optical signal processing, free from the electronic limitations, will then inevitably be reevaluated but in a slightly different way from how it was previously expected. In this paper, we highlight the unique features of all-optical signal processing that outperforms digital signal processing, and review the two latest results: one is upgrade-free, multi-channel wavelength conversion; and the other, all-optical back propagation for WDM channel nonlinearity compensation. Both of these functions are exploiting a versatile physical phenomenon of parametric processes in highly nonlinear fiber that is highly efficient in terms of cost and energy and is as cascadable as EDFAs. This paper discusses their practical aspects with emphasis on cascadability. In particular, the promising prospects for “cascaded phase-conjugating amplifier chains” to compensate nonlinear signal distortion will be highlighted after discussing the parameter tolerances for all-channel nonlinearity compensation.

With the evolution to ever more complex modulation formats for optical signals, all-optical wavelength converters offer great benefits to optical networks, thanks to the possibility of translating the wavelength of an optical signal regardless of its data rate or modulation format. To deliver its full advantages over conventional optical-electrical-optical conversion, an all-optical wavelength converter has to possess important features in terms of transparency, bandwidth, and cascadability for practical applications in real systems. In this paper, we review a recent development of an all-optical wavelength converter, which exhibits seamless conversion in the C-band with a low noise figure of 6.2 dB for dual-polarization (DP) phase-modulated signals. We then carry out an investigation by both experiment and calculation of the cascaded operation of the developed wavelength converter for high-order quadrature amplitude modulation (QAM) formats. A cascadability comparable to an erbium-doped fiber amplifier is achieved for DP-QPSK, DP-16QAM, and DP-64QAM signals, thanks to the exemplary characteristics of the wavelength converter.

Multiple phase conjugations within a transmission-optimized fiber link enable significant nonlinearity cancellation in the wavelength division multiplexed (WDM) systems. In particular, we demonstrate the fiber impairment mitigation of WDM $24\times 48$ Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signals using 12 optical phase conjugations (OPC). The power excursion and dispersion-tailored transmission line in which the phase conjugations acted periodically, enabled an 8-dB higher nonlinearity threshold for the WDM 1.1-Tb/s DP-QPSK signals. With the nearly perfect nonlinearity cancellation, large input powers per span became available. Under these circumstances, we also present a first approach to the analysis of the impact of multiple phase conjugations in a distributed Raman amplified link. The analysis considers the performance of the experimentally measured OPC subsystem within an optimized transmission line limited only by an amplified spontaneous emission noise. This raw estimate gives insight into the linear noise performance limit of the employed phase conjugator when inserted multiple times in such a transmission system.

Application of Brillouin amplification to 64-QAM communications is presented showing the benefit of narrowband gain for suppressing wideband noise from signal carriers before data modulation, and pilot tone based carrier recovery for coherent detection.

In this Letter, for the first time, to the best of our knowledge, we harness on-chip Brillouin scattering for narrowband amplification and spectral purification of frequency comb lines for coherent optical communications. A parametrically generated optical frequency comb with a low carrier-to-noise power ratio was filtered through narrowband Brillouin amplification utilizing the same comb as the optical pump. This was achieved on a photonic chip to enable successful transmission of an advanced modulation format signal: 64-level quadrature amplitude modulation. 96 Gb/s data were modulated on two polarizations on multiple comb lines across 1532.9-1557.5 nm, demonstrating the scalability of this concept for operation in wavelength division multiplexing applications. The small form factor of the photonic chip reduces the polarization drifts when compared to optical fibers and paves the way for photonic integration.

Integrated single-photon sources are a key component for photonic quantum technology but are generally limited to low single-photon rates. For sources based on photon pair generation by four-wave mixing, increasing the repetition rate of pump laser pulses is a straightforward way to enhance the single-photon rate, but the benefits and practical limitations have not yet been demonstrated and analyzed in a CMOS-compatible platform. In this work, we demonstrate correlated photon pair generation in integrated silicon nanowires and systematically analyze the count rate and coincidence to accidental ratio as the pump rate is varied between 156.25 MHz and 10 GHz. We show that the highest useful pump rate is limited by the timing resolution of the single-photon detection system, and that in this regime, the nonlinear loss of the silicon nanowire does not have a significant effect on the single-photon generation.

We present our recent results on harnessing the nonlinear stimulated Brillouin scattering effect on a photonic chip for coherent optical communications. At the transmitter-end, an optical frequency comb was regenerated through on-chip Brillouin scattering for 64-level QAM communications. At the receiver-end, low-latency self-homodyne reception was achieved through Brillouin-assisted carrier recovery.

Optical frequency comb lines with poor carrier to noise ratio (CNR) are significantly improved by Brillouin amplification using its extreme narrow bandwidth gain to suppress out of band noise, enabling higher quality signal modulation. Its application to spectral lines of narrow 10 GHz pitch and poor CNR is shown to suppress the otherwise strong phase distortion caused by poor CNR after encoding with 96 Gb/s DP-64-QAM signals and restore the bit error rate (BER) to below the limit for standard forward error correction (FEC). This is also achieved with the required frequency shifted optical pump for amplification obtained by seeding it from the comb itself, sparing the need for lasers and frequency locking. Simultaneous CNR improvement for 38 comb lines is also achieved with BER restored to below the FEC limit, enabled by a multi-line pump that is pre-dispersed to suppress its spectral distortion from the Kerr effect in the gain medium. Carrier performance at minimum BER shows minimal noise impact from the Brillouin amplifier itself. The results highlight the unique advantage of Brillouin gain for phase sensitive communications in transforming otherwise noisy spectral lines into useful high quality signal carriers.

We demonstrate the first all-optical carrier recovery of coherent signals using on-chip Brillouin gain, replacing digital techniques, with sub-nanosecond latency. This demonstration serves as a proof-of-concept for integrated photonic carrier recovery chips for coherent communications.

We for the first time, study the effect of signal power variations on the extinction ratio and bandwidth of on-chip Brillouin filters for applications in RF photonics and coherent optical communications. The passband extinction reduces from 35 dB to 25 dB on increasing the input signal power by 36 dB, while the bandwidth increased from 13 MHz to 30 MHz.

We demonstrate noise suppression for parametrically generated optical frequency comb-lines with low 10GHz-pitch by narrowband Brillouin amplification pumped self-seeded via single-sideband modulation (SSB). Comb-line carrier performance close to a reference laser is achieved for 96Gb/s-DP-64QAM.

A low noise, 40 GHz optical frequency comb is demonstrated using an actively mode-locked fiber laser with phase stabilization by cross-phase modulation in nonlinear fiber. Extracted comb-lines modulated with 96 Gb/s DP-64QAM signals achieve bit-error rates

We review waveform analysis and optical performance monitoring of ultrabroadband signals using a photonic-chip-based radio-frequency spectrum analyzer. This approach offers the potential for fast integrated monitoring and characterization of signals with bandwidths beyond 1 THz.

Progress in developing photonic chip based optical processing devices with ultrahigh-bandwidth is reviewed. The unprecedented processing bandwidth of our platform based on dispersion engineered chalcogenide waveguides is exploited to perform Tbaud optical switching and performance monitoring. Recent experiments and demonstrations of photonic logic operations are reviewed. Progress in developing photonic chip based optical processing devices with ultrahigh-bandwidth is reviewed. The unprecedented processing bandwidth of our platform based on dispersion engineered chalcogenide waveguides is exploited to perform Tbaud optical switching and performance monitoring. Recent experiments and demonstrations of photonic logic operations are reviewed

We investigate the waveform power spectrum (WPS) measurement of ultrashort pulses using the nonlinear Kerr effect in an optical waveguide. Our study focuses on a recent experiment reporting the WPS measurement of 260-fs pulses using a 6-cm-long, highly nonlinear chalcogenide (ChG) planar waveguide. By numerical simulation of the underpinning nonlinear propagation, we show the importance of low chromatic dispersion in the waveguide for avoiding measurement inaccuracy due to bandwidth narrowing and asymmetric distortion of the WPS. We also show the distortion effect of excessive input power on broadening the WPS bandwidth. In comparison to using conventional nonlinear fibers, highly nonlinear ChG waveguides are shown to generally enable a more accurate and broadband measurement of shorter pulses over a multiterahertz frequency range, and for a wider ranging signal wavelength. Furthermore, higher order dispersion effects are also avoided. Experiments with nonlinear fiber show its capability to measure the WPS of high-speed 640-Gb/s data signals, albeit for broader pulses and with less wavelength flexibility. Furthermore, the WPS is used to effectively retrieve the signal autocorrelation waveform. The factors impacting the measurement accuracy is compared to other recent experiments using ChG and silicon waveguides. Analysis shows that extending the technique to pulses shorter than 260 fs requires further optimizing the ChG waveguide dispersion, which would benefit broadband signal processing in general.

We demonstrate an aberration-free, all-optical, ultra-fast oscilloscope based on the concept of Fourier-transformation with an optical time-lens. By combining the four-wave mixing time-lens with a Fourier-domain optical processor as the dispersive element we avoid aberrations associated with the traditional method of using lengths of fibre for the dispersive elements. We investigate the impact of aberrations due to third-order dispersion and inaccuracies in matching the Fourier-transform condition and demonstrate how these are overcome using the optical processor. The resolution of the oscilloscope is 750 fs. © 2010 Elsevier B.V. All rights reserved.

We demonstrate bit error rate improvement, corroborated by eye diagram measurements, of a (10 MHz) amplitude distorted 10 Gbit/s data signal in silicon photonic crystal waveguides. This demonstration exploits a power clamping nonlinear transfer function, provided by slow light enhancement of nonlinear absorption in these waveguides.

We present a summary of our recent experiments showing how various nonlinear phenomena are enhanced due to slow light in silicon photonic crystal waveguides. These nonlinear processes include self-phase modulation (SPM), two-photon absorption (TPA), free-carrier related effects, and third-harmonic generation, the last effect being associated with the emission of green visible light, an unexpected phenomenon in silicon. These demonstrations exploit photonic crystal waveguides engineered to support slow modes with a range of group velocities as low as c/50 and, more crucially, with significantly reduced dispersion. We discuss the potential of slow light in photonic crystals for realizing compact nonlinear devices operating at low powers. In particular, we consider the application of SPM to all-optical regeneration, and experimentally investigate an original approach, where enhanced TPA and free-carrier absorption are used for partial regeneration of a high-bit rate data stream (10 Gb/s).

A system based on a highly nonlinear planar chalcogenide waveguide is demonstrated to be able to perform radio-frequency spectral measurements with a terahertz bandwidth. High bit-rate tests show that the chip-based system is potentially useful for ultrafast signal processing.

Ultrahigh nonlinear tapered fiber and planar rib Chalcogenide waveguides have been developed to enable highspeed all-optical signal processing in compact, low-loss optical devices through the use of four-wave mixing (FWM) and cross-phase modulation (XPM) via the ultra fast Kerr effect. Tapering a commercial As2Se3 fiber is shown to reduce its effective core area and enhance the Kerr nonlinearity thereby enabling XPM wavelength conversion of a 40 Gb/s signal in a shorter 16-cm length device that allows a broader wavelength tuning range due to its smaller net chromatic dispersion. Progress toward photonic chip-scale devices is shown by fabricating As2S3 planar rib waveguides exhibiting nonlinearity up to 2080 W-1ldr km-1 and losses as low as 0.05 dB/cm. The material's high refractive index, ensuring more robust confinement of the optical mode, permits a more compact serpentine-shaped rib waveguide of 22.5 cm length on a 7-cm- size chip, which is successfully applied to broadband wavelength conversion of 40-80 Gb/s signals by XPM. A shorter 5-cm length planar waveguide proves most effective for all-optical time-division demultiplexing of a 160 Gb/s signal by FWM and analysis shows its length is near optimum for maximizing FWM in consideration of its dispersion and loss.

Chalcogenide glasses offer many attractive properties for all-optical signal processing including large Kerr nonlinearity (up to 500 × silica glass), an intrinsic ultrafast response time and low to moderate two-photon absorption (TPA). These properties together with the convenience of a fibre format allow us to achieve all-optical signal processing at low peak power and in a very compact form. In this paper, several nonlinear processing functions will be demonstrated including: femto-second pedestal free, pulse compression; all-optic wavelength conversion; and all-optical regenerator. In addition, we show enhanced nonlinearity for more efficient signal processing by tapering the As2Se3 fibre. These applications show chalcogenide glass fibres are very promising candidate materials for nonlinear all-optic signal processing.