Phase-sensitive Optical Amplifier Research Articles

Low-Noise Phase-sensitive Parametric Amplifiers Based on Integrated Silicon-Nitride-Waveguides for Optical Signal Processing

Low-noise optical amplification has been a long-lasting research topic in various fields including communication, metrology and quantum optics. Here we report optical signal processing at different wavelengths enabled by a low-noise optical phase-sensitive amplifier (PSA) using a single compact integrated silicon nitride photonic waveguide. For 10 Gbit/s non-return-to-zero optical signals with central wavelengths tuned in the telecommunication band, the waveguide-based PSA exhibits a gain of more than 10 dB and a noise figure of about 1.2 dB when the coupling losses are neglected. For a bit-error-rate level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> , penalties of less than 0.8 dB for all channels are observed after the signals propagating through the chip-based PSA, which may be due to the distortions in the few-mode waveguide. These experimental results generally agree with theoretical prediction and indicate that silicon-nitride-based PSAs are promising for the next generation of broadband optical signal processing and communication systems.

Power Efficient Communications Employing Phase Sensitive Pre-Amplified Receiver

The receiver sensitivity is a very important metric in optical communication links operating at low received signal powers. Phase sensitive optical amplifiers (PSAs) can amplify optical signals without excess noise, thus providing a fundamental sensitivity improvement of 3 dB when employed as a pre-amplifier compared to conventional erbium doped fiber amplifiers (EDFA). In this letter, we investigate, both theoretically and experimentally, the sensitivities achieved using power efficient multi-dimensional modulation formats such as M-ary pulse position modulation format (M-PPM) and M-PPM combined with quadrature phase shift keying (QPSK) along with a near-noiseless PSA pre-amplified coherent intradyne receiver. We find that at high signal to noise ratios (SNRs) corresponding to low bit-error-rates (BER), M-PPM+QPSK results in the best sensitivity, which is improved with the order M, while at low SNRs corresponding to high BER (~14% where 100% overhead forward error correction codes (FEC) would be needed to recover the data), QPSK is the most sensitive format, while at the same time providing the best spectral efficiency. We report experimental sensitivities of 2.1 photons per information bit (PPB) at a pre-FEC BER = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> using 64-PPM+QPSK and assuming 7% FEC, and 0.8 PPB at a pre-FEC BER = 0.14 using QPSK and assuming 100% FEC.

Optical phase‐sensitive amplification of higher‐order QAM signal with single Mach–Zehnder amplitude modulator

An optical phase-sensitive amplifier (PSA) has the potential of low-noise optical amplification. A frequency non-degenerate PSA (ND-PSA) can amplify arbitrary modulation formats including higher-order QAM. The ND-PSA requires a co-propagating phase conjugated light (idler light) that has been conventionally created with an optical phase conjugator at the transmitter side. We propose a transmitter configuration using simultaneous signal generation of both a signal and its idler by double-sideband modulation for transmission systems with ND-PSAs. The proposed scheme provides a simple configuration with a single Mach–Zehnder amplitude modulator without optically creating the idler light. We performed experiments using 16QAM, 32QAM, and probabilistically shaped 256QAM signals. As a result, a phase-sensitive amplification with each modulation format was successfully demonstrated using the proposed transmitter configuration.

One photon-per-bit receiver using near-noiseless phase-sensitive amplification

Space communication for deep-space missions, inter-satellite data transfer and Earth monitoring requires high-speed data connectivity. The reach is fundamentally dictated by the available transmission power, the aperture size, and the receiver sensitivity. A transition from radio-frequency links to optical links is now seriously being considered, as this greatly reduces the channel loss caused by diffraction. A widely studied approach uses power-efficient formats along with nanowire-based photon-counting receivers cooled to a few Kelvins operating at speeds below 1 Gb/s. However, to achieve the multi-Gb/s data rates that will be required in the future, systems relying on pre-amplified receivers together with advanced signal generation and processing techniques from fibre communications are also considered. The sensitivity of such systems is largely determined by the noise figure (NF) of the pre-amplifier, which is theoretically 3 dB for almost all amplifiers. Phase-sensitive optical amplifiers (PSAs) with their uniquely low NF of 0 dB promise to provide the best possible sensitivity for Gb/s-rate long-haul free-space links. Here, we demonstrate a novel approach using a PSA-based receiver in a free-space transmission experiment with an unprecedented bit-error-free, black-box sensitivity of 1 photon-per-information-bit (PPB) at an information rate of 10.5 Gb/s. The system adopts a simple modulation format (quadrature-phase-shift keying, QPSK), standard digital signal processing for signal recovery and forward-error correction and is straightforwardly scalable to higher data rates.

Fiber-based phase-sensitive optical amplifiers and their applications

Optical parametric amplifiers rely on second-order susceptibility (three-wave mixing) or third-order susceptibility (four-wave mixing) in a nonlinear process where the energy of incoming photons is not changed (elastic scattering). In the latter case, two pump photons are converted to a signal and to an idler photon. Under certain conditions, related to the phase evolution of the waves involved, this conversion can be very efficient, resulting in large amplification of an input signal. As the nonlinear process can be very fast, all-optical applications aside from pure amplification are also possible. If the amplifier is implemented in an optical input-phase-sensitive manner, it is possible to amplify a signal wave without excess noise, i.e., with a noise figure of 0 dB. In this paper, we will provide the fundamental concepts and theory of such amplifiers, with a focus on their implementation in highly nonlinear optical fibers relying on four-wave mixing. We will discuss the distinctions between phase-insensitive and phase-sensitive operation and include several experimental results to illustrate their capability. Different applications of parametric amplifiers are also discussed, including their use in optical communication links.

Signal advance and delay due to an optical phase-sensitive amplifier.

Fast and slow light media exploit a steep frequency dependence in their index of refraction in order to advance or delay a modulated signal. Here we observe a qualitatively similar advance and delay from an optical phase-sensitive amplifier (PSA). Unlike in the case of slow and fast light, this effect is due to a redistribution of power between imbalanced signal sidebands, and the advance or delay is dependent on the optical phase of the input. The PSA adds energy and also changes the frequency spectrum of the input. We show that the advances and delays observed in a PSA implemented using four-wave mixing in a warm rubidium vapor are consistent with the expected behavior of an ideal PSA.

Simultaneous measurement-device-independent continuous variable quantum key distribution with realistic detector compensation

We propose a novel scheme for measurement-device-independent (MDI) continuous-variable quantum key distribution (CVQKD) by simultaneously conducting classical communication and QKD, which is called “simultaneous MDI-CVQKD” protocol. In such protocol, each sender (Alice, Bob) can superimpose random numbers for QKD on classical information by taking advantage of the same weak coherent pulse and an untrusted third party (Charlie) decodes it by using the same coherent detectors, which could be appealing in practice due to that multiple purposes can be realized by employing only single communication system. What is more, the proposed protocol is MDI, which is immune to all possible side-channel attacks on practical detectors. Security results illustrate that the simultaneous MDI-CVQKD protocol can secure against arbitrary collective attacks. In addition, we employ phase-sensitive optical amplifiers to compensate the imperfection existing in practical detectors. With this technology, even common practical detectors can be used for detection through choosing a suitable optical amplifier gain. Furthermore, we also take the finite-size effect into consideration and show that the whole raw keys can be taken advantage of to generate the final secret key instead of sacrificing part of them for parameter estimation. Therefore, an enhanced performance of the simultaneous MDI-CVQKD protocol can be obtained in finite-size regime.

Improvement of unidimensional continuous-variable quantum key distribution systems by using a phase-sensitive amplifier

Inherent defects of the detectors in a practical system cause deterioration of the performance of the unidimensional coherent-state continuous-variable quantum key distribution protocol. Aiming to achieve optimal performance, we propose a method of using a phase-sensitive amplifier to overcome the limitation that the detectors introduce. The modified protocol, corresponding to a higher key rate, provides the practical system with more robustness. To conduct a thorough investigation into the effects that the phase-sensitive amplifier creates, the expressions of the secret key rates are derived, under the conditions of coherent states, homodyne detection against collective attacks, and a phase-sensitive optical amplifier added at the output of the channel. Then, we carry out simulations to observe the performance in the cases of different gains. The simulation results demonstrate that the method we propose can improve the performance of the unidimensional protocol using coherent states and make up for the imperfections of the detector in a practical system.

Simultaneous carrier extraction from wavelength-division-multiplexed phase-conjugated twin waves by using sum-frequency generation with second-order optical nonlinear medium for phase-sensitive optical amplifier repeaters

This paper proposes and numerically investigates simultaneous carrier extraction from wavelength-division multiplexed phase-conjugated twin waves (WDM-PCTWs) in optical phase-locked loop (OPLL) for phase-sensitive optical amplifier (PSA) repeaters on optical multi-relay transmission systems. First, it is mathematically derived that the proposed scheme contributes to a signal-to-noise ratio improvement of phase error signals between WDM-PCTWs and a pump wave for PSA. As a result, the quality of the pump wave is improved. Then, the influence of chromatic dispersion in a transmission fiber on the proposed scheme is numerically investigated. When a center optical frequency of WDM-PCTWs is equal to a zero-dispersion optical frequency of an 80 km transmission fiber, the extracted carrier power increases as the number of WDM channels increase. In contrast, an optical frequency difference between the center frequency of WDM-PCTWs and the zero-dispersion optical frequency of a transmission fiber severely impairs the extracted carrier power as the number of WDM channels increase.

Continuous-variable measurement-device-independent quantum key distribution using modulated squeezed states and optical amplifiers

The measurement-device-independent quantum key distribution (MDI-QKD) protocol is immune to imperfect measurement devices. However, its practicality and performance when implemented in continuous-variable (CV) mode are still not satisfactory. We propose a CV-MDI-QKD protocol based on modulated squeezed states, and analyze its security against two-mode Gaussian attack. The results show that the protocol can achieve a higher secret key rate and transmission distance than previous coherent-state and squeezed-state protocols. A method to compensate the imperfection of a practical homodyne detector using phase-sensitive optical amplifiers is also presented. By setting an appropriate optical amplification gain, realistic detectors with ordinary quantum efficiency can be employed. In addition, we discuss the impact of finite-size effects on the secret key rate of the protocol. The methods presented may aid in the practical application of the CV-MDI-QKD protocol.

Optical QPSK Signal Quality Degradation due to Phase Error of Pump Light in Optical Parametric Phase-Sensitive Amplifier Repeaters

The influence of pump phase error on phase-sensitive optical amplifier (PSA) repeaters and the waveform degradation due to chromatic dispersion and fiber nonlinearities in the optical multi-relay transmission of quadrature phase-shift keying phase-conjugated twin waves are considered theoretically. First, the influence of noise from the pump phase error, optical local oscillator, receiver, and the amplified spontaneous-emission (ASE) in PSA repeaters is investigated with the assumption that transmission fibers are linear lossy channels. The bit-error rate (BER) is estimated as a function of the signal-to-noise ratio, and the relationship between the number of transmission relays and the fiber launch power is clarified. Waveform degradation due to chromatic dispersion and the optical fiber nonlinearities in transmission fibers are investigated with the noiseless condition, and the maximum repeatable number as a function of the fiber launch power is calculated. Finally, we show the relationship among the maximum repeatable number, standard deviation of pump phase error in PSA repeaters, and the fiber launch power to clarify the optimum transmission condition with consideration of the noise and the waveform degradation.

Polarization Dependence of Carrier Extraction from Dual-Polarization Phase-Coujugated Twin Waves with Sum-Frequency Generation

This study aimed to build a numerical model of carrier extraction from dual-polarization phaseconjugated twin waves (DP-PCTWs) to investigate polarization dependence on the carrier extraction in a pump phase-locked loop for phase-sensitive optical amplifier repeaters. First, the numerical model was developed, and extracted carrier power of 40- Gbit/s DP-PCTWs with quadrature phase-shift keying format was calculated. Numerical results show the extracted carrier disappears when the PCTWs are circular polarization, and polarization dependence exists on the carrier extraction. Then, carrier extraction of a 10- Gbit/s DP-PCTW with binary phase-shift keying format was experimentally demonstrated. The extracted carrier power was found to be relatively low when the DP-PCTWs are circular polarization, and this trend agrees closely with the numerical results. These results show the reasonability of the numerical model and existence of polarization dependence on the carrier extraction from DP-PCTWs.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters.

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.

Effect of input phase modulation to a phase-sensitive optical amplifier.

Many optical applications depend on amplitude modulating optical beams using devices such as acousto-optical modulators (AOMs) or optical choppers. Methods to add amplitude modulation (AM) often inadvertently impart phase modulation (PM) onto the light as well. While this PM is of no consequence to many phase-insensitive applications, phase-sensitive processes can be affected. Here we study the effects of input phase and amplitude modulation on the output of a quantum-noise limited phase-sensitive optical amplifier (PSA) realized in hot 85Rb vapor. We investigate the dependence of PM on AOM alignment and demonstrate a novel approach to quantifying PM by using the PSA as a diagnostic tool. We then use this method to measure the alignment-dependent PM of an optical chopper which arises due to diffraction effects as the chopper blade passes through the optical beam.

Optical Signal Filtering Using Phase-Sensitive Amplification and De-Amplification

We utilize for the first time, to the best of our knowledge, the amplification and de-amplification properties of a fiber-based optical phase-sensitive amplifier (PSA) to implement a flexible, multiple notch filter with optical power extinction ratio given by the internal amplifier gain squared. Our PSA-based filter is completely realized in the optical domain, permitting further downstream optical signal processing, if desired. Direct detection leads to an additional gain-squared enhancement, resulting in electrical filter extinction that is a biquadratic function of the PSA gain. This suggests that the PSAs with even modest optical gains can be used to realize filters with extremely large electrical extinction ratios or notch depths. We characterize filter performance using bit error rates, and demonstrate highly effective isolation of a signal-of-interest from neighboring interferers. Finally, we measure filter extinctions of 20 and 40 dB in the optical and electrical domains, respectively.

Loss resilience for two-qubit state transmission using distributed phase sensitive amplification.

We transmit phase-encoded non-orthogonal quantum states through a 5-km long fibre-based distributed optical phase-sensitive amplifier (OPSA) using telecom-wavelength photonic qubit pairs. The gain is set to equal the transmission loss to probabilistically preserve input states during transmission. While neither state is optimally aligned to the OPSA, each input state is equally amplified with no measurable degradation in state quality. These results promise a new approach to reduce the effects of loss by encoding quantum information in a two-qubit Hilbert space which is designed to benefit from transmission through an OPSA.

Gaussian-only regenerative stations cannot act as quantum repeaters

Higher transmission loss diminishes the performance of optical communication---be it the rate at which classical or quantum data can be sent reliably, or the secure key generation rate of quantum key distribution (QKD). Loss compounds with distance---exponentially in an optical fiber, and inverse square with distance for a free-space channel. In order to boost classical communication rates over long distances, it is customary to introduce regenerative relays at intermediate points along the channel. It is therefore natural to speculate whether untended regenerative stations, such as phase-insensitive or phase-sensitive optical amplifiers, could serve as repeaters for long-distance QKD. The primary result of this paper rules out all bosonic Gaussian channels to be useful as QKD repeaters, which include phase-insensitive and phase-sensitive amplifiers as special cases, for any QKD protocol. We also delineate the conditions under which a Gaussian relay renders a lossy channel entanglement breaking, which in turn makes the channel useless for QKD.

Locking the phase of balanced homodyne detection system for squeezed light

The balanced homodyne detection technique is an important method to detect the quantum light field. The signal light injected into the phase sensitive optical parameter amplifier (OPA) is phase modulated. The output squeezed light of OPA is detected by the balanced homodyne detection system. When the phase sensitive OPA is operated at parametric amplification and deamplification respectively, we can judge which quadrature component of the quantum light field is measured by observing the modulation signal. We can demodulate the phase modulation signal to achieve the error signal, which is used to lock the relative phase of zero between the local and the quantum light fields (corresponding to the quadrature-amplitude of the unknown light field).

Probabilistic phase-insensitive optical squeezer in compliance with causality

A non trace-preserving map describing a probabilistic but heralded noiseless linear amplifier has recently been proposed and experimentally demonstrated. Here, we exhibit another remarkable feature of this peculiar transformation, namely its ability to serve as a universal single-mode squeezer regardless of the quadrature that is initially squeezed. Hence, it acts as an heralded phase-insensitive optical squeezer, conserving the signal-to-noise ratio just as a phase-sensitive optical amplifier but for all quadratures at the same time, which may offer new perspectives in quantum optical communications. Although this ability to squeeze all quadratures seemingly opens a way to instantaneous signaling by circumventing the quantum no-cloning theorem, we explain the subtle mechanism by which the probability for such a causality violation vanishes, even on an heralded basis.

Amplifying to perfection

The demonstration of a phase-sensitive optical amplifier with a noise figure of just 1.1 dB — three times lower than that of a conventional amplifier — could help significantly extend the reach of optical communications systems.