Raman Noise Research Articles

5.1 kW Tandem-Pumped Fiber Amplifier Seeded by Random Fiber Laser With High Suppression of Stimulated Raman Scattering

In this work, high-power tandem-pumped random fiber lasers are investigated in detail. A 6.5 W random fiber laser seed centered at 1070 nm with a half-open cavity structure was built. Boosted by two amplifier stages, the random fiber laser achieved a power of several kilowatts. Stimulated Raman scattering (SRS) mitigation is challenging in tandem-pumped fiber lasers. To suppress SRS, several methods were applied, including avoiding external feedback in the oscillator stage, incorporating a high core/cladding diameter ratio gain fiber, and using a band pass filter (BPF) to mitigate Raman noise. The latter proved to be the most effective method; therefore, SRS mitigation without and with a BPF was investigated at 3.5 kW, leading to suppression ratios rising from 14 to 30 dB. By increasing the pump power continuously to 5.6 kW, we realized a record output power of 5.1 kW, corresponding to an SRS suppression ratio of 27 dB. Owing to the high-brightness 1018 nm fiber lasers serving as the pump sources and the high gain coefficient in the second amplifier stage, the entire system demonstrated high efficiency and stability. At the maximum power level, the total optical-to-optical efficiency reached 89.0%. This work offers an economic and practical technique for realizing high-power and high-efficiency random fiber lasers with effective SRS mitigation, while the experimental results suggest the potential for further power scaling.

11.2 Tb/s Classical Channel Coexistence With DV-QKD Over a 7-Core Multicore Fiber

The feasibility of transmitting discrete-variable quantum key distribution channels with carrier-grade classical optical channels over multicore fibers is experimentally explored in terms of achievable quantum bit error rates, secret key rates as well as classical signal bit error rates. A coexistence transmission record of 11.2 Tb/s is achieved for the classical channels simultaneously with a DV-QKD channel over a 1 km-long 7-core multicore fiber. Coexistence over the same multicore fiber core is identified as a dominant factor for the performance of the quantum channel requiring optical bandpass filtering of 17 nm for the quantum channel to avoid the effect of Raman noise. Also, counter-propagation of classical channels and quantum channels probe more tolerance to noise proliferation than co-propagation. In addition, the performance of the quantum channel is maintained when more than three cores are used for the classical channels. Furthermore, by adding a second DV-QKD channel in the multicore fiber, the simultaneous transmission of classical channels as well as the generation of quantum-secured keys of two QKD channels is achieved with an operational range of 10 dB of launched power into the MCF.

Long-distance transmission of quantum key distribution coexisting with classical optical communication over a weakly-coupled few-mode fiber.

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

Ultrahigh-contrast cross-polarized entangled photon pairs from a strongly birefringent photonic-crystal fiber

Four-wave mixing with cross-polarized pump and cross-polarized sidebands is identified as a powerful resource of quantum entanglement, enabling creation of efficient fiber-optic sources of entangled photon pairs. Generation of cross-polarized photon pairs is demonstrated by coupling the laser pump into orthogonal polarization modes of a highly birefringent, highly nonlinear, anomalously dispersive photonic-crystal fiber. With the pump wavelength set to lie deeply within the anomalous dispersion region of the fiber, the frequency offset of the photon pairs relative to the pump exceeds 45 THz, enhancing the contrast of photon pairs against the Raman noise to yield photon pairs with a coincidence-to-accidental ratio as high as 250: 1.

Wavelength assignment in quantum access networks with hybrid wireless-fiber links

We propose a low-complexity near-optimal wavelength allocation technique for quantum key distribution access networks that rely on wavelength division multiple access. Such networks would allow users to send quantum and classical signals simultaneously on the same optical fiber infrastructure. Users can be connected to the access network via optical wireless or wired links. We account for the background noise present in the environment, as well as the Raman noise generated by classical channels, and calculate the secret key generation rate for quantum channels in the finite-key setting. This allows us to examine the feasibility of such systems in realistic scenarios when the secret key exchange needs to be achieved in a limited time scale. Our numerical results show that, by proper choice of system parameters for this noisy system, it is possible to exchange a secret key in tens of seconds. Moreover, our proposed algorithm can enhance the key rate of quantum channels, especially in high noise and/or high loss regimes of operation.

Modeling of Ultra-Long Span Bidirectional Raman Transmission Link Using Three-Segment Hybrid Fiber Core Structure

Ultra-long span unrepeatered systems using distributed Raman amplification are cost-effective solutions for bridging moderate transmission distances. However, there are two major limiting factors: nonlinear Kerr effect-induced nonlinear signal distortion and optical signal-to-noise ratio degradation due to spontaneous Raman noise. In this report, we proposed a model of three-segment hybrid fiber effective core area structure and developed a model covering: (1) generalized mathematical formulations, (2) analysis of three-segment Raman amplified link, and (3) simulation model of data transmission. The proposed model showed an improvement of the Raman gain profile, a reduction of the negative impact of the nonlinear Kerr effect, and an enhancement of the optical signal-to-noise ratio. A numerical simulation of the transmission performance of the three-segment hybrid structure was compared to conventional single-segment single fiber core structure on 80 Gb/s differential quadrature phase-shift keying (DQPSK) modulated data signals over a propagation distance of 390 km. The required optical signal-to-noise ratio was reduced by 2.71 dB to achieve the target error rate without using forward error correction. The numerical model and simulation of various data rates up to 100 Gb/s consistently showed that an improvement in transmission performance could be achieved by using three-segment hybrid fiber effective core area structure.

Optimized channel allocation scheme for jointly reducing four-wave mixing and Raman scattering in the DWDM-QKD system.

Conducting quantum key distribution (QKD) through existing optical fibers together with conventional communication signals is a viable way to expand its practical application, but weak quantum signals can be severely disrupted by co-propagating classical signals. In this paper, the suppression of four-wave mixing (FWM) noise and Raman noise is considered simultaneously for the first time, to the best of our knowledge, and the joint optimized channel allocation (JOCA) scheme is proposed. In the JOCA scheme, the quantum channels and classical channels are interleaved with each other to avoid FWM noise and optimal quantum channel positions are chosen in variable conditions according to the Raman scattering spectrum. Experimental measurements of the noise photons show that the JOCA scheme can effectively reduce the impairments on quantum signals compared with the single-target schemes. Additionally, simulation results verify that the JOCA scheme can increase the secure key generation rate and transmission distance, and that it also enables the DWDM-QKD system to tolerate higher-power classical signals and more classical channels, which improve the compatibility with a high-capacity communication system.

Experimental integration of quantum key distribution and gigabit-capable passive optical network

Quantum key distribution (QKD) ensures information-theoretic security for the distribution of random bits between two remote parties. To extend QKD applications to fiber-to-the-home optical communications, such as gigabit-capable passive optical networks (GPONs), an effective method is the use of wavelength-division multiplexing. However, the Raman scattering noise from intensive classical traffic and the huge loss introduced by the beam splitter in a GPON severely limits the performance of QKD. Here, we demonstrate the integration of QKD and a commercial GPON system with fiber lengths up to 14 km, in which the maximum splitting ratio of the beam splitter reaches 1:64. By placing the QKD transmitter on the optical line terminal side, we reduce the Raman noise collected at the QKD receiver. Using a bypass structure, the loss of the beam splitter is circumvented effectively. Our results pave the way to extending the applications of QKD to last-mile communications.

Fiber entangled photon pair source connecting telecom to quantum memories

A fiber optic based source of polarization entangled photon pairs is proposed to connect and entangle a telecom quantum memory to a solid state quantum memory based on single-qubits operating in the near infrared. To span this large wavelength difference while matching the narrow spectral bandwidth of the quantum memories, the photon pairs are generated by four wave mixing in a highly birefringent fiber. Raman noise is expected to be strongly suppressed under these conditions.

Phase matching as a gate for photon entanglement

Phase matching is shown to provide a tunable gate that helps discriminate entangled states of light generated by four-wave mixing (FWM) in optical fibers against uncorrelated photons originating from Raman scattering. Two types of such gates are discussed. Phase-matching gates of the first type are possible in the normal dispersion regime, where FWM sidebands can be widely tuned by high-order dispersion management, enhancing the ratio of the entangled-photon output to the Raman noise. The photon-entanglement gates of the second type are created by dual-pump cross-phase-modulation-induced FWM sideband generation and can be tuned by group-velocity mismatch of the pump fields.

Raman-noise enhanced stimulated Raman scattering in high-power continuous-wave fiber amplifier

The stimulated Raman scattering (SRS) in high-power continuous-wave (CW) fiber amplifier is investigated with the seeded Raman noise taken into account. It is revealed that the seeded Raman noise, as a portion of seed light, should enhance SRS in high-power CW fiber amplifier even when it is much weaker than the seeded signal light. The analytic formula for predicting the Raman threshold of bi-directional pumping fiber amplifier is given with the seeded Raman noise taken into account, and the predictions agree well with the numerical results. It is also revealed that the co-pumping amplifier suffer from the lowest Raman threshold, and the counter-pumping one is more beneficial to the SRS suppression than the bi-directional pumping one when the pump absorption of active fiber is larger than 8.7dB. These results will provide guidance for the design of high-power CW fiber amplifier.

Bright narrowband biphoton generation from a hot rubidium atomic vapor cell

We demonstrate the generation of high-quality narrowband biphotons from a Doppler-broadened hot rubidium atomic vapor cell. Choosing a double-Λ atomic energy level scheme for optimizing both spontaneous four-wave mixing nonlinear parametric interaction and electromagnetically induced transparency (EIT), we achieve a biphoton spectral brightness as high as 14 000 s−1 MHz−1. Meanwhile, we apply a spatially tailored optical pumping beam for reduction of the Raman noise and obtain a violation of the Cauchy-Schwarz inequality by a factor of 1023.

Degree of cure of epoxy/acrylic photopolymers: Characterization with raman spectroscopy and a modified phenomenological model

This study characterizes the degree of cure c, of a two‐part epoxy/acrylic photopolymer used in stereolithography, SLA. The use of solid free form fabrication has added a whole new list of thermosets with a chemical reaction—curing reaction—targeted for these specific manufacturing processes. The studied thermosetting system exhibits a fast curing reaction triggered by a UV‐light source. An innovative stepwise method to quantify the degree of cure using Raman spectroscopy is reported. This method abates the Raman noise and compensates for the weak signal of fast measurements. The Raman data processing algorithm BEADS was implemented. The technique presented here was capable of following the degree of cure of the individual components of the photopolymer. Based on the epoxy/acrylic monomers concentration (75%/25%), the degree of cure for the whole system was quantified. The experimental results were modeled using a modified phenomenological kinetics equation. This model includes the dependency of the rate of cure on the light intensity, and cure state. Based on the analysis reported here, the proposed model accurately fits the initiation, propagation, and termination phases of the cure cycle. The technique and methods presented here can potentially be integrated to in situ process monitoring with instant parameter feedback, and numerical modeling of residual stresses in additive manufacturing of thermosets. POLYM. ENG. SCI., 58:228–237, 2018. © 2017 Society of Plastics Engineers

Multimode quantum theory of nonlinear propagation in optical fibers

We theoretically reveal the potential of the parallelism of squeezed state generation by nonlinear pulse propagation in an optical fiber. Starting from a nonlinear Schr\odinger equation coupling with phonon modes that cause Raman noise, we develop a multimode quantum theory of nonlinear propagation in an optical fiber. Based on our proposed method, we numerically simulate fiber nonlinear propagation in two conditions: solitonlike and zero-group-delay-dispersion (zero-GVD) propagation. As a result, we find that zero-GVD propagation enables the large-scale parallel generation of squeezed states relative to solitonlike propagation owing to the broadband phase matching of the four-wave mixing process.

Analysis of Dual-Order Backward Pumping Schemes in Distributed Raman Amplification System

Abstract Backward pumping in fiber Raman amplifiers has been investigated in this paper in terms of on-off Raman gain, noise figure and optical signal-to-noise ratio. The results exhibit that with four first-order pumps and one second-order pump scheme can be employed to achieve 8.2 dB noise figure in 64 channel fiber optic communication system. It has also been reported that 2.65 dB gain ripple, 0.87 dB noise figure tilt and 2.02 dB OSNR tilt can be attained with the second-order pumping in fiber Raman amplifiers. The main advantage of the scheme is that only 50 mW second-order pump shows appreciable improvement in the system performance. It shows that further increase in first-order and second-order pump powers increase system noise implications.

Ultra-high bandwidth quantum secured data transmission.

Quantum key distribution (QKD) provides an attractive means for securing communications in optical fibre networks. However, deployment of the technology has been hampered by the frequent need for dedicated dark fibres to segregate the very weak quantum signals from conventional traffic. Up until now the coexistence of QKD with data has been limited to bandwidths that are orders of magnitude below those commonly employed in fibre optic communication networks. Using an optimised wavelength divisional multiplexing scheme, we transport QKD and the prevalent 100 Gb/s data format in the forward direction over the same fibre for the first time. We show a full quantum encryption system operating with a bandwidth of 200 Gb/s over a 100 km fibre. Exploring the ultimate limits of the technology by experimental measurements of the Raman noise, we demonstrate it is feasible to combine QKD with 10 Tb/s of data over a 50 km link. These results suggest it will be possible to integrate QKD and other quantum photonic technologies into high bandwidth data communication infrastructures, thereby allowing their widespread deployment.

Frequency conversion between UV and telecom wavelengths in a lithium niobate waveguide for quantum communication with Yb+ trapped ions

We study and demonstrate the frequency conversion of UV radiation, resonant with 369.5 nm transition in Yb+ ions to the C-band wavelength 1580.3 nm and vice-versa using a reverse proton-exchanged waveguide in periodically poled lithium niobate. Our integrated device can interface trapped Yb+ ions with a telecom infrastructure for the realization of an Yb+ based quantum repeater protocol and to efficiently distribute entanglement over long distances. We analyse the single photon frequency conversion efficiency from the 369.525 nm to the telecom wavelength and its dependence on pump power, device length and temperature. The single-photon noise generated by the spontaneous Raman scattering of the pump is also measured. From this analysis we estimate a single photon conversion efficiency of ∼9% is achievable with our technology with almost complete suppression of the Raman noise.

Circumventing spontaneous Raman noise in a correlated photon pair source

We propose a dual-pump third-order nonlinear scheme for producing pairs of correlated photons that is less susceptible to Raman noise than typical spontaneous four-wave mixing (SFWM) methods. Beginning with the full multimode Hamiltonian, we derive a general expression for the joint spectral amplitude, from which the probability of producing a pair of photons can be calculated. As an example, we demonstrate that a probability of 0.012 pairs per pulse can be achieved in an appropriately designed fused silica microfiber. As compared with single pump SFWM in standard fiber, we calculate that our process shows significant suppression of the spontaneous Raman scattering and an improvement in the signal to noise ratio.

Quantum secured gigabit optical access networks.

Optical access networks connect multiple endpoints to a common network node via shared fibre infrastructure. They will play a vital role to scale up the number of users in quantum key distribution (QKD) networks. However, the presence of power splitters in the commonly used passive network architecture makes successful transmission of weak quantum signals challenging. This is especially true if QKD and data signals are multiplexed in the passive network. The splitter introduces an imbalance between quantum signal and Raman noise, which can prevent the recovery of the quantum signal completely. Here we introduce a method to overcome this limitation and demonstrate coexistence of multi-user QKD and full power data traffic from a gigabit passive optical network (GPON) for the first time. The dual feeder implementation is compatible with standard GPON architectures and can support up to 128 users, highlighting that quantum protected GPON networks could be commonplace in the future.

Color controllable polarization entanglement generation in optical fiber at telecommunication wavelengths.

This article proposes a polarized entangled photon source in optical fiber with low Raman noise that features the controllable generation of specific signal and idler wavelengths (colors) by varying the pump power. The novel two color source can provide needed telecom entangled photon wavelengths for applications in quantum communications, quantum computing, and quantum imaging.