Wavefront‐Reversal, Low‐Threshold, and Enhanced Stimulated Brillouin Scattering for Arbitrary Structured Light

Stimulated Brillouin scattering (SBS) is one of important nonlinear effects in optical fiber communication systems. Due to its low threshold, it limits the optical power that couples into the fiber. In this paper, Stimulated Brillouin scattering and the effect of phonon loss to SBS in optical fiber communication systems are investigated. In experiments we measure first the power of backward Stokes wave and output light under various input powers of the systems with different lengths of fibers. Then for observing the effect of the phonon blocker to the SBS threshold, we repeat the same experiments but with a phonon blocker inserted at different locations of the fiber link. The experiments indicate that adding phonon blocker to the fiber is able to increase the SBS threshold, and the blocker at different locations makes different effects.

Recently, high power narrow linewidth fiber lasers are interested in wavelength beam combining. In narrow linewidth fiber laser, Stimulated Brillouin Scattering is the main power limiting factor. A Stimulated Brillouin Scattering suppressing method by modulating frequency distribution was proposed and examined both experimentally and theoretically. The Stimulated Brillouin Scattering threshold power increased a lot by increasing the frequency number and spacing. Finally, the linewidth of 8 GHz, power of 1kW and central wavelength of 1064.14nm, are achieved respectively in fiber laser. We analyzed the above results: the increasing of frequency number, controlling of frequency spacing all reduce power spectrum density. With the cascaded phase modulation technology, the Stimulated Brillouin Scattering threshold is increased, while the output power of fiber laser is increased a lot.Recently, high power narrow linewidth fiber lasers are interested in wavelength beam combining. In narrow linewidth fiber laser, Stimulated Brillouin Scattering is the main power limiting factor. A Stimulated Brillouin Scattering suppressing method by modulating frequency distribution was proposed and examined both experimentally and theoretically. The Stimulated Brillouin Scattering threshold power increased a lot by increasing the frequency number and spacing. Finally, the linewidth of 8 GHz, power of 1kW and central wavelength of 1064.14nm, are achieved respectively in fiber laser. We analyzed the above results: the increasing of frequency number, controlling of frequency spacing all reduce power spectrum density. With the cascaded phase modulation technology, the Stimulated Brillouin Scattering threshold is increased, while the output power of fiber laser is increased a lot.

Stimulated Brillouin Scattering(SBS) is one of important nonlinear optical effects in optical fiber transmission system, it limits the optical power that can be transmitted through fiber. If input optical power is over SBS threshold, part of power will be changed to backward Stokes wave, this process exacerbates the system transmission characteristic very much. In this paper, SBS effect was systemically studied by experiments. Firstly, backward Stokes wave was observed, we investigated its spectrum and frequency shift to pumping signal wave. Then, the relationship of SBS threshold characteristic with linewidth of light source and modulation formats was discussed, and approved by experiments. The impact on bite error rate characteristic of system due to SBS was also investigated experimentally.

We report on a high peak power pulsed fiber laser emitting in the L band limited by stimulated Brillouin scattering (SBS). On-going developments target at building fiber lasers for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensing from space such as ASCENDS emitting at 1572nm. Unfortunately the relevant CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> lines compatible with Erbium amplification are located in the L amplification band, where the cross sections are small. Consequently, those fiber sources have low non-linear threshold and low efficiency. The European Space Agency is funding a project dedicated to the design of a high power pulsed fiber laser for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> DIAL monitoring with emission lines located closed to 1579 nm. The system requires 150 ns pulse duration at 4 kHz pulse repetition frequency (PRF). The high precision on the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration measurement requires narrow linewidth and the laser is primarily limited by Stimulated Brillouin Scattering (SBS). SBS threshold can be improved by: reducing fiber length, reducing effective Brillouin gain or increasing effective area. Due to the small cross-sections of Erbium in L band, large doping concentration or large fiber length are required. Large mode area phosphate Erbium-Ytterbium codoped fibers have been used to generate 84 μJ for 1 μs pulses with 1% efficiency which corresponds to ~84 W peak power.

Stimulated Brillouin scattering (SBS) is a nonlinear optical process that generates backward scattered phase conjugate wave (Zel’dovich et al., 1972; Zel’dovich et al., 1985; Damzen et al., 2003; Brignon & Huignard, 2004). A device that generates the phase conjugate wave by the SBS process is called SBS phase conjugate mirror (PCM). An SBS-PCM can compensate wavefront distortion induced by a phase aberrator, such as a laser gain medium; hence, it is widely used in high-energy laser systems to obtain a high-quality beam. Efficient heat dissipation is a major issue in high-energy laser systems, particularly with regard to the high repetition rate. The combination of beams from small laser systems is a constructive approach to this issue. Of the various beam combined systems using SBS-PCMs, the crosstype beam combined system has many outstanding advantages, such as perfect isolation of leak beam, compensation of thermal birefringence, easy alignment and convenient maintenance (Kong et al., 1997, 2005a). Since the SBS wave generates from a thermal noise, it naturally has a random phase with respect to the incident beam. Therefore the phase controlling of the SBS wave is a key technology in the realization of a coherent beam combined system. For this reason, the self-phase control method was proposed and has been developed by Kong et al. (2005a, 2005b, 2005c), which can control the phase of the SBS wave with the simplest composition as well as ease of alignment, no limitations on the number of combined beams, and excellent phase conjugation. Furthermore, the active phase control with a piezoelectric translator (PZT) enables long-term phase stabilization (Kong et al., 2006, 2008). In addition to a random phase characteristic, the distortion that generally occurs in a pulse waveform of an SBS wave is another negative characteristic in terms of the beam combination. Kong et al. (2005d) has overcome this problem with the SBS waveform preservation technique, which is called the prepulse injection method. These works are expected to boost the development of laser systems in term of a high level of energy and

Using 2.4-nsec pulses at 350 nm, we have observed transverse stimulated Brillouin scattering (SBS) in fused-silica optical components. Transverse SBS sets in when the product of laser fluence and growth time for the scattered optical wave exceeds ∼2.3 J nsec/cm2. An increase in laser bandwidth to 8.3 GHz suppresses SBS losses up to approximately twice the SBS threshold. We review the theory of transverse, broadband SBS and its scaling with experimental parameters.

Based on pump-probe technique (PPT), we observed stimulated Brillouin scattering (SBS) in a 100-m-long silica graded-index multimode optical fiber with the same core diameter as that of a perfluorinated graded-index polymer optical fiber (PFGI-POF) (= 50 μm). The SBS threshold was measured to be -50 mW, which was much lower than 5.08 W, the SBS threshold obtained for the same fiber without PPT. This result indicates that PPT is useful even under multimode conditions and that the difficulty in observing SBS in PFGI-POFs consists not in their physical problems but in their material problems (high propagation loss, etc.).

Nonlinear distortion for single-sideband (SSB) signals will significantly reduce the performance of Kramers–Kronig (KK) receiver-based optical transmission. In this work, we present a proof-of-concept study of stimulated Brillouin scattering (SBS)-induced nonlinear distortion for 10 Gbaud and 28 Gbaud SSB QAM16 transmission over 80 km standard single mode fiber (SSMF) based on a KK receiver. Significantly reduced bit error rate (BER) has been experimentally observed due to the SBS and the threshold of SBS at about 7 dBm is detected for such an 80 km SSMF link. With left sideband (LSB) modulation of SSB, together with optical filtering, reduced SBS nonlinear distortion has been achieved with ~2 dB power tolerance improvement. The results reveal an important issue of SBS-induced nonlinear distortion, which would be of great significance for KK receiver-based optical transmission applications.

ABSTRACTStokes field demonstrated unusually narrow spectrum and non-Gaussian statistics for SBS in 300-rn single-mode opticalfiber with enhanced Rayleigh losses. We shown that SBS lasing takes place in the fiber. The effect is explained by dynamicdistributed feedback due to double Rayleigh scattering of the Stokes field. Results of numerical simulation of the cooperativeSBS-RS process in fiber are in good agreement with experimental results. 1. INTRODUCTION Stimulated Brillouin scattering (SBS) is the resonance interaction between pump wave backscattered optical Stokes wave,and hypersound wave induced in the medium. SBS is initiated by spontaneous scattering of the pump wave on occasionalhypersound waves caused by thermal noise. Being essentially nonlinear process, SBS in optical fibers demonstrates a complicateddynamic behavior'.The presence ofthe external feedback makes SBS dynamics even richer. Significant narrowing ofthe spectrumand decreasing ofthe SBS threshold are usually observed for SBS in fiber ring interferometers. Regularization ofthe Stokes fieldchaotic fluctuations and soliton mode generation are observed both in low fitness ring fiber resonators and in the long single-modeoptical fiber when small (—3%) parasitic reflections from the non-angled fiber ends creates low fitness Fabry-Perot interferometerin the fiber2._________________________________________________

We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5 kW output power with an FWHM linewidth of 9.63 GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400 µm. The reflectivity of the main stage is 0.049‰ at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.

Dynamic aspects of stimulated Brillouin scattering (SBS) and criteria for its occurrence in optical communications are studied numerically. SBS is shown to cause sharp amplitude modulations in phase-shift keying (PSK) modulated systems close to or above the SBS threshold. It is shown that the convolution method to high accuracy yields correct relative gains for different modulation sequences. The transient change of backscattered radiation caused by abrupt changes in the modulation format is studied and it is demonstrated that the transition can occur on different time scales. >

In this study, the authors presented a distributed optical fibre temperature sensor whose performance is improved using the optimisation techniques and Fourier wavelet regularised deconvolution (ForWaRD) method. As the input power launched into the sensing fibre is critical, the authors have considered both conventional optimisation technique and evolutionary optimisation techniques namely: genetic algorithm, differential evolution algorithm and particle swarm optimisation algorithm to increase the stimulated Brillouin scattering (SBS) threshold power. Using the optimised value of the parameters and employing evolutionary computing techniques, the proposed 50 km long temperature sensing system offers a 4.7 dB improvement in SBS threshold power over the design of experiment based system. It is being verified that power of the backscattered signals approximately are the convolution of the input pulsed power and corresponding backscatter optical fibre impulse responses. The Fourier wavelet regularised deconvolution (ForWaRD) method is employed to improve the spatial resolution of the proposed sensing system without reducing the pulse width of the input pulse. Employing ForWaRD technique a 10 m better spatial resolution observed as compared with the Fourier deconvolution technique. The proposed 50 km long temperature sensing system exhibits a temperature resolution of 1.85 K because of suppression of SBS threshold and noise reduction.

We introduce a new figure of merit (FOM) including the input pump power limit associated with stimulated Brillouin scattering (SBS) for evaluation of the Kerr nonlinearity efficiency of optical fibers. The new FOM is expressed as gammaL(eff)P(SBS) (gamma is a nonlinearity parameter, L(eff) is effective length, and P(SBS) is the SBS threshold), while the conventional FOM is given by gammaL(eff). Using the new FOM, we perform an efficiency comparison among four types of state-of-the-art nonlinear optical fiber: a Bi2O3-based nonlinear fiber, a silica-based holey fiber, a highly nonlinear dispersion-shifted fiber, and a conventional dispersion-shifted fiber. The Bi2O3-based nonlinear fiber is found to have the best Kerr nonlinearity efficiency owing to the superior nonlinear property of the Bi2O3 glass compared with that of the silica.

Summary form only given. For industrial applications like materials processing continuously pumped, repetitively Q-switched solid-state laser systems are attractive due to their and compactness and reduced financial efforts. Conventional laser systems suffer from thermally induced phase distortion, which reduces the beam quality significantly. Phase conjugation can overcome this problem by compensating phase distortion in double pass arrangements. Stimulated Brillouin scattering (SBS) facilitates self-pumped phase conjugation in multimode glass fibers. These solid-state devices are preferable because they are easy to handle and the SBS peak power threshold can be determined by the appropriate fiber core diameter. Due to the long interaction length inside the fiber the SBS threshold can be reduced down to the 200-W level. Continuously pumped, Q-switched all-solid-state MOPA systems with fiber phase conjugator are presented and different amplifier arrangements are discussed.

A bidirectional full duplex radio over fiber (RoF) system is proposed for multi-band signal generation and transmission. A multi-band signal is generated at the central station (CS), which includes 10 GHz baseband, 40, and 70 GHz millimeter-wave (mm-wave) by optical frequency multiplication technique of radio frequency (RF) local oscillator (LO) with frequency of 15 GHz. The simultaneous transmission of the 16-QAM OFDM-based multi-band downlink signal and 4-QAM uplink signal over a single fiber is demonstrated. The carrier for uplink transmission is extracted from the unmodulated downlink signal with tunable FBG which is modulated by OFDM-based 4-QAM uplink data obtained from the user end via an antenna. The frequency reuse from the downlink signal makes the base station (BS) free from the laser source. At the user end, the multi-band signals for multi-serve application are obtained by heterodyne beating of multiple bands. An increase in launch power induces fiber dispersion and non-linearities like self-phase modulation (SPM) and cross phase modulation (XPM), which depends on the nonlinear refractive index of the fiber. Different industrial standard fibers with common specifications and modulation format along with co-shared components are deployed, and system performance is analyzed. The stimulated Brillouin scattering (SBS) can be mitigated by optimizing the launch power within the SBS threshold. The nonlinear effects are minimized at the system level without the requirement of additional digital signal processor (DSP) . The theoretical and simulation analysis shows that the designed bidirectional RoF link exhibits an improved system performance for bidirectional transmission with negligible power penalty.