Phase sensitive amplification (PSA) is an attractive technology for integrated all-optical signal processing, due to it's potential for noiseless amplification, phase regeneration and generation of squeezed light. In this talk I will review our results on implementing four-wave-mixing based PSA inside integrated photonic devices. In particular I will discuss PSA in chalcogenide ridge waveguides and silicon slow-light photonic crystals. We achieve PSA in both pump- and signal-degenerate schemes with maximum extinction ratios of 11 (silicon) and 18 (chalcogenide) dB. I will further discuss the influence of two-photon absorption and free carrier effects on the performance of silicon-based PSAs.
Phase sensitive amplification (PSA) is an attractive technology for integrated all-optical signal processing, due to it's potential for noiseless amplification, phase regeneration and generation of squeezed light. In this talk I will review our results on implementing four-wave-mixing based PSA inside integrated photonic devices. In particular I will discuss PSA in chalcogenide ridge waveguides and silicon slow-light photonic crystals. We achieve PSA in both pump- and signal-degenerate schemes with maximum extinction ratios of 11 (silicon) and 18 (chalcogenide) dB. I will further discuss the influence of two-photon absorption and free carrier effects on the performance of silicon-based PSAs.
Phase-sensitive parametric interactions can selectively process the two complex quadratures of the optical field. We implement phase-sensitive amplification in a large band-gap semiconductor photonic crystal waveguide in order to avoid two-photon absorption and free-carrier-related effects. Experimentally, an extinction ratio of 15 dB is achieved in a 1.5-mm-long photonic crystal waveguide, at a peak pump power of about 600 mW. We show that cascaded parametric interaction has a strong impact on squeezing and phase-sensitive extinction ratio and that this depends on the dispersion profile of the waveguide.
Phase sensitive amplification (PSA) is a remarkably powerful tool if implemented correctly - it allows amplification with a noise figure lower than quantum mechanics normally dictates, and also allows the development of systems that coherently process the phase of their optical inputs. PSA at infrared wavelengths can be achieved using second or third order optical nonlinearities - indeed the first demonstrations of PSA utilised degenerate three photon mixing in bulk crystals, allowing the observation of effects such as quantum quadrature squeezing. This PhD research project was aimed at translating the fascinating science of PSAs into applications, based on Kerr nonlinearity in optical fibres, capable of being deployed in modern core optical networks runnning at 40 Gbit/s and higher. This objective was an integral part of the European Union FP7 project PHASORS. Studies, both theoretical and experimental, were carried out on wideband non-degenerate PSAs. The inline cascaded fiber optic parametric amplifier (FOPA), in which a first phase insensitive FOPA is used to generate phase locked signal-idler pairs, followed by a second FOPA in which PSA takes place, was used to experimentally demonstrate PSA gain characteristics in linear and saturated modes of operation, with PSA obtained over 20 nm in the telecom C-band. Focus was then re-directed towards applying a dual pump PSA to DPSK regeneration. The amplitude and phase limiting characteristics of these devices were experimentally studied, in particular revealing that amplitude saturation arises due to an interplay between input phase and nonlinear phase matching along the nonlinear fibre. This feature was used to identify a regime of operation for DPSK regeneration combining simultaneous phase and amplitude regeneration. A practical DPSK regenerator based on a degenerate dual pump phase sensitive FOPA was built. The device advanced the state of the art by incorporating a pump synthesis stage that allowed black-box operation. Detailed measurements using noise sources with varying frequency distributions in both amplitude and in phase, are presented, and the ability of the system to improve the phase and amplitude characteristics of signals at its input was verified. Also presented are results showing the regenerator installed in the middle of an 800 km dark fiber link. Finally, a novel scheme was proposed and demonstrated that utilised parametric mixing to perform arbitrary phase quantization. This relied on the coherent addition of an M-level PSK signal with a conjugated (M-1)th phase harmonic, scaled by a coefficient m, to achieve M-level phase quantization. The concept was successfully demonstrated with QPSK data signals at 56 Gbaud, as well as used to quantize lower bandwidth test signals from 2 to 6 phase levels.
Parametric processes in materials presenting a second- or third-order nonlinearity have been widely used to demonstrate a wide range of all-optical signal processing functionalities, including amplification, wavelength conversion, regeneration, sampling, switching, modulation format conversion, optical phase conjugation, etc.. The recent evolution of optical fiber communication systems towards advanced modulation formats making use of the phase dimension, as well as polarization and, more recently, space-multiplexing, has created new requirements, as well as new opportunities, for parametric all-optical signal processing. In this presentation, we will review our recent results on the demonstration of all-optical parametric signal processing using different nonlinear platforms, including highly nonlinear optical fibers (HNLFs), silicon nanowires, and periodically-poled lithium niobate (PPLN) waveguides. In particular, we will show how phase-sensitive processes can be engineered to demonstrate phase-quadrature separation, which we have recently demonstrated in HNLFs and PPLN waveguides. Silicon nanowires are particularly attractive for signal processing thanks to their compact size, CMOS compatible fabrication process, degrees of freedom in dispersion engineering, and high nonlinear coefficient. However, the detrimental effect of free-carrier absorption induced by two-photon absorption has so far prevented them from being used for the demonstration of phase-sensitive processing. Thanks to the introduction of p-i-n junctions across silicon waveguides, we have recently been able to demonstrate phase-sensitive extinction ratios as high as 20 dB, allowing the phase regeneration of phase-modulated signals under continuous wave pumping operation. One of the well-known limitations of planar waveguide devices for all-optical signal processing is their inherent polarization sensitivity. We will show how the introduction of polarization-diversity circuits relying on efficient and wideband polarization splitters and rotators can overcome this limitation. Finally, we will also discuss the introduction of signal processing functionalities that are compatible with the novel dimension of space multiplexing. More specifically, we will show how mode-selective wavelength conversion based on four-wave mixing can be realized in a multimode silicon waveguide.
The minimum resolvable signal in optical metrology and sensing applications is usually limited by the so-called standard quantum limit. One way to improve the signal-to-noise ratio is to use squeezed states of light. Squeezed light can be generated using different types of nonlinear interactions either in solid-state nonlinear crystals or in atomic systems. One way to generate squeezed light is to use a phase sensitive amplifier that will amplify one quadrature of the considered mode and deamplify the other one. We have recently shown that metastable helium vapor at room temperature can exhibit strong four-wave mixing effects and behave like a perfect phase sensitive amplifier when it is prepared in coherent population trapping situation. In this talk, we will present the application of this phase sensitive amplification to the generation of squeezed vacuum states of light and detail the performances and limitations of the system
Phase-sensitive amplifiers (PSAs) offer numerous advantages over phase-insensitive amplifiers in optical communications. Squeezing of optical phase through PSA can remove accumulated phase jitter, which is a critical functionality for an all- optical, phase-shift keyed network. In recent experiments, reviewed in this report, different implementations of PSA were used for phase regeneration of both return-to-zero differential phase-shift keying and nonreturn-to-zero differential phase-shift keying data. The first demonstration explored the properties and performance of PSA that occurs in nonlinear interferometers. Experiments confirmed that a PSA operating in the depleted pump regime provides simultaneous reduction of amplitude and phase noise (PN). Phase regeneration performance limit was reached as a consequence of pump-wave imperfections, which can be significantly reduced through proper design. PSA that occurs directly in fiber in a traveling-wave configuration through partially degenerate four-wave mixing was also studied. The latter implementation offers stronger phase-matched gain and suppression of amplitude-to-phase noise conversion. Technical issues that remain to be addressed are identified for each implementation. Results characterized using coherent detection offer direct measurements of the phase-regenerative behavior.
Summary form only given. When a signal propagates through a transmission line, it is deteriorated by amplitude and phase noise, which reduces its signal-to-noise ratio (SNR) [1]. According to Shannon's theory, the SNR determines the maximum spectral efficiency. It is therefore necessary to limit the noise [3]. For this purpose, one can use phase sensitive amplifiers (PSA) instead of phase insensitive amplifiers (PIA) like Erbium-doped fiber amplifiers. PSAs amplify only one phase quadrature of the signal while deamplifying the other, thanks to a required phase relationship between the pump and the signal. This unique property, known as phase-squeezing, allows to obtain noise figures (NF) as low as 0 dB whereas PIAs, based on stimulated emission, cannot be go below the 3 dB quantum limit [2]. Moreover, the phase-squeezing property of PSAs can be used for quantum communication or even in metrology [1]. PSA can be realized with χ(2)material through degenerate parametric amplification [2]. The large spectral separation between the pump and the signal makes it very easy to filter out the pump after amplification. A ± 11 dB amplification with a sub-quantum NF and a regeneration of a DPSK signal was experimentally performed in LiNbO3 [3]. A fiber based approach would however be preferable for telecommunication purposes. PSA's based on four wave-mixing have been demonstrated, but the long length and the difficulty to separate pump from the signal [1] hinders its practical implementation. Here we present the first demonstration of a PSA based on periodically poled silica fiber (PPSF). This design has the advantage of all-fiber devices as well as simple pump filtering. The experimental set-up is depicted in Figure 1. A 20 mW CW fiber laser at 1550 nm is sent through a 99:1 coupler to be divided into a signal and a pump. The pump is modulated into 1 ns wide pulses with a repetition rate of 50 MHz. It is then amplified and frequency doubled in a PPLN crystal. The peak power of the 775 nm pump pulses reaches 10W. The pump and CW signal are then combined in a WDM and sent through a 30 cm long PPSF [4] where the signal undergoes parametric amplification. A second WDM is used to separate the pump from the amplified signal. The wave plates and the polarization controller in the pump path, allow to simultaneously control the polarization of the pump and the power in the fundamental mode of the PPSF (at 775 nm the fibre is slightly multimode). The relative phase between the pump and signal is controlled by fine tuning the frequency of the laser via a built-in piezo.