Transmission performance improvement using random DFB laser based Raman amplification and bidirectional second-order pumping

Review of Random Fiber Lasers for Optical Fiber Sensors

A random fiber laser does not need a traditional resonant cavity and only uses the multiple scattering of disordered media to provide feedback to achieve laser output. Therefore, it has the advantages of a simple structure, narrow linewidth, and low noise and is particularly suitable for fiber optic sensors. This paper provides an introduction to the categories and corresponding principles of random fiber lasers. The research progress of random fiber lasers in the sensing field in recent years, including various aspects of random fiber lasers as low-noise light sources or sensitive elements for fiber sensing systems, is the main focus. Finally, the future development trend of random fiber lasers for optical fiber sensors is explored.

Asymmetry Optimization for 10 THz OPC Transmission over the C + L Bands Using Distributed Raman Amplification

An optimized design for a broadband Raman optical amplifier in standard single-mode fiber covering the C and L bands is presented, to be used in combination with wideband optical phase conjugation (OPC) nonlinearity compensation. The use of two Raman pumps and fiber Bragg grating reflectors at different wavelengths for the transmitted (C band) and conjugated (L band) WDM channels is proposed to extend bandwidth beyond the limits imposed by single-wavelength pumping, for a total 10 THz. Optimization of pump and reflector wavelength, as well as pump powers, allows us to achieve low asymmetry across the whole transmission band for optimal nonlinearity compensation. System performance is simulated to estimate OSNR, gain flatness and nonlinear Kerr distortion.

Ultra-wideband discrete Raman amplifier optimization for single-span S-C-L-band coherent transmission systems

We experimentally compare the performance of two key ultra-wideband discrete Raman amplifier structures, a cascaded dual-stage structure and an in-parallel dual-band structure, in fully loaded S-C-L band coherent transmission systems over 70 km of single-mode fiber. Our results show that dual-band discrete Raman amplifier with minimized backreflections can effectively avoid unstable random distributed feedback lasing, reduce the noise figure, and therefore improve the transmission performance for signals at shorter wavelengths, versus the cascaded dual-stage structure. The average noise figure for S-band signals is 6.8 dB and 7.2 dB for the dual-band structure and cascaded dual-stage structure, respectively, while the average S-band Q² factor is similarly improved by 0.6 dB. Moreover, the cascaded dual-stage discrete Raman amplifier requires guard bands around the 1485-nm and 1508-nm pumps as the signal and pump wavelengths overlap, which results in a bandwidth loss of ∼10 nm and reduces the potential net data throughput to 28.6 Tb/s for 30-GBaud DP-16QAM signals. However, the dual-band structure can utilize the bandwidth more effectively, which leads to a higher estimated net data throughput of 31.2 Tb/s.

30-GBaud DP 16-QAM transmission in the E-band enabled by bismuth-doped fiber amplifiers

We report the transmission of five 30-GBaud dual polarization 16-QAM signals over 160 km of standard single-mode fiber in the E-band (1410-1460 nm). The transmission line consists of two 80-km spans and three independent bismuth-doped fiber amplifiers. The developed amplifiers feature a maximum gain of 27.3 dB, 33.8 dB, and 28.3 dB with a minimum noise figure of 4.8 dB, 4.7 dB, and 5.3 dB, respectively. The maximum signal Q² factor penalty is 4.5 dB, and the overall performance of the system is above the pre-forward-error-correction (FEC) threshold for a 10^-15 post-FEC bit error rate. To the best of our knowledge, this is the record experimentally demonstrated transmission length for a coherent detection signal in the E-band.

Towards optimal conversion efficiency of Brillouin random fiber lasers in a half-open linear cavity

We proposed and demonstrated an unprecedented high-efficiency Brillouin random fiber laser (BRFL) by fiber length optimization in a half-open linear cavity. In terms of the trade-off between Brillouin gain saturation and weak distributed Rayleigh feedback strength, optimal laser efficiency associated to proper fiber length in a BRFL was theoretically predicted. As a proof-of-concept, a unidirectional-pumped BRFL with a half-open linear cavity was experimentally conducted, in which a fiber Bragg grating at one end of gain fiber served as a high-reflection mirror while Rayleigh scattering enabled distributed feedback for random lasing resonance. Results show that the optimal fiber length of ∼3.4 km in the BRFL offers sufficient Rayleigh scattered random feedback whilst alleviating the Brillouin gain saturation to a large extent. Consequently, an optimal laser efficiency of 77.0% in the BRFL was experimentally demonstrated, which reaches the state-of-the-art high record. Laser characteristics, including the linewidth, statistics and frequency jitter were also systematically investigated. It is believed that such efficient BRFL could provide a promising platform for inspiring new explorations of laser physics as well as potentials in long-haul coherent communication and fiber-optic sensing.

Bandwidth Extension in a Mid-Link Optical Phase Conjugation

In this paper, we investigate various designs of distributed Raman amplifier (DRA) to extend amplification bandwidth in mid-link optical phase conjugation (OPC) systems and compare bands 191-197 THz and 192-198 THz giving a total bandwidth of 6 THz using a single wavelength pump. We demonstrate the use of highly reflective fiber Bragg grating (FBG) to minimize gain variation across a WDM grid by optimizing forward and backward pump powers as well as the wavelength of FBGs for original and conjugated channels. Finally, we also simulate OSNR and Kerr nonlinear reduction as a product of signals asymmetry and nonlinear phase shift (NPS) for all channels.

Impact of feedback bandwidth on Raman random fiber laser remote-sensing

In the ultra-long distance sensing domain, recently Raman random fiber laser (RRFL) demonstrated advantages of ultrawide sensing-bandwidth in dynamic sensing, compared with pulse-probing cases. However, such a scheme is still in the preliminary stage, and the key parameters such as sensitivity have not been characterized. In this work, a time-dependent spectrum-balanced model is proposed, which can accurately and quickly describe the spectral shape of RRFL and the evolution of the power and the spectrum. Based on this model, the relationship between the sensitivity and the feedback bandwidth is studied. The calculated results show that the sensitivity is inversely proportional to the feedback bandwidth. Then in the proof-of-concept experiment, by changing the bandwidth of sensing FBG, the results of sensitivity are well coincident with the simulation. This work provides an effective platform for studying the evolution of RRFL spectrum, as well as a novel way for further enhancing the performance of the dynamic sensing system based on ultra-long RRFL.

Low quantum defect random Raman fiber laser

The random Raman fiber laser (RRFL) has attracted great attention due to its wide applications in optical telecommunication, sensing, and imaging. The quantum defect (QD), as the main source of thermal load in fiber lasers, could threaten the stability and reliability of the RRFL. Conventional RRFLs generally adopt silica fiber to provide Raman gain, and the QD exceeds 4%. In this letter, we propose and demonstrate a phosphosilicate-fiber-based low-QD RRFL. There is a strong boson peak located at the frequency shift of 3.65 THz in the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain, we demonstrated an 11.71 W temporally stable random Raman laser at 1080 nm under a pump wavelength of 1066 nm. The corresponding QD is 1.3%, less than one third of the QD of the common silica-fiber-based RRFL. Compared with the full-cavity low-QD Raman fiber laser, this cavity-less low-QD RRFL has lower and flatter noise in the high frequency area (>100 kHz). This work provides a reference for suppressing thermal-induced effects, such as thermal-induced mode instability, thermal noise, and even fiber fusing in RRFLs.

Performance improvement in high speed optical networks with low cost amplifier configuration

In this paper, the application example of 8 × 100 Gbit/s over 463 km (76.9 dB) optical transport network is realized by using commercial bidirectional Raman amplifier and additional auxiliary laser without any remotely pump amplifier. The positions of the auxiliary laser in the commercial Raman amplifier are compared and optimized experimentally. The results show that 7.3 dB and 10.2 dB more power budget is obtained with the auxiliary laser in the backward direction and forward direction respectively comparing to the situation without any auxiliary laser. The results also show that forward second order Raman amplifier is a good choice in the condition without remotely pump amplifier.

Distributed Raman Amplification for Fiber Nonlinearity Compensation in a Mid-Link Optical Phase Conjugation System

In this paper, we review different designs of distributed Raman amplifiers which have been proposed to minimize the signal power profile asymmetry in mid-link optical phase conjugation systems. We demonstrate how the symmetrical signal power profiles along the fiber can be achieved using various distributed Raman amplification techniques in the single-span and more realistic multi-span circumstances. In addition, we show the theoretically predicted results of the Kerr nonlinear product reduction with different Raman techniques in mid-link optical phase conjugator systems, and then in-line/long-haul transmission performance using numerical simulations.

Raman Amplification Optimization in Short-Reach High Data Rate Coherent Transmission Systems

We compared the transmission performances of 600 Gbit/s PM-64QAM WDM signals over 75.6 km of single-mode fibre (SMF) using EDFA, discrete Raman, hybrid Raman/EDFA, and first-order or second-order (dual-order) distributed Raman amplifiers. Our numerical simulations and experimental results showed that the simple first-order distributed Raman scheme with backward pumping delivered the best transmission performance among all the schemes, notably better than the expected second-order Raman scheme, which gave a flatter signal power variation along the fibre. Using the first-order backward Raman pumping scheme demonstrated a better balance between the ASE noise and fibre nonlinearity and gave an optimal transmission performance over a relatively short distance of 75 km SMF.

Tunable narrowband cascaded random Raman fiber laser

Random Raman fiber lasers (RRFLs) with half-opened cavity have been used as a new platform for designing high performance, wavelength-agile laser sources in the infrared region due to their intrinsic modeless property and structural simplicity. To provide the point feedbacks for cascaded random Raman lasing at different wavelengths, wavelength-insensitive broadband reflectors are commonly used in cascaded RRFLs, resulting in the rather broad high-order random Raman lasing with several nanometers of typical spectral width. Here, we experimentally demonstrate a tunable narrowband cascaded RRFL with an air-spaced etalon assisted point reflector. To realize narrowband, single- or dual-wavelength emission for each order of random lasing, the etalon is specially designed to have broad operation wavelength range, narrowband transmission lines and large free spectral range (FSR) associated with the Raman frequency shift. As a result, 1st- to 3rd-order random Raman lasing with single-wavelength emission in 1.1-1.27 μm region are generated in a 15 km single mode fiber (SMF) with -3 dB bandwidths below 0.4 nm, which are approximately four times less than those of cascaded RRFL without etalon. The maximum output power of the 3rd-order random Raman lasing is 615 mW, with 10% of optical conversion efficiency. Moreover, a tunable cascaded RRFL is performed by tuning the wavelength of pump laser or tilting the etalon. Dual-wavelength emission for each order of random lasing can also be realized at specific pump wavelengths. We also verified, by employing shorter fiber (10 km), more than 1.5 W output power of high-order RRFL can be achieved with -3 dB bandwidths less than 0.6 nm. To the best of our knowledge, this is the first demonstration of tunable sub-1 nm narrowband cascaded RRFL with single- or dual-wavelength emission for each order of random lasing.

Tailoring the efficiency and spectrum of a green random laser generated by frequency doubling of random fiber lasers

Frequency doubling of random fiber lasers could provide an effective way to realize visible random lasing with the spectrum filled with random frequencies. In this paper, we make a comprehensive study on the efficiency and spectral manipulation of a green random laser generated by frequency doubling of an ytterbium-doped random fiber laser (YRFL). To tailor the efficiency of green random lasing generation, the ytterbium-doped random fiber lasing is filtered at different spectral positions, and then amplified to watt-level to serve as the fundamental laser source for frequency doubling in a periodically poled lithium niobate (PPLN) crystal. We found that by selecting different spectral components of ytterbium-doped random fiber lasing, the temporal intensity fluctuations of the filtered radiations vary dramatically, which plays an important role in enhancing the efficiency of frequency doubling. By fixing the filtering radiation wavelength at 1064.5 nm and tuning the central wavelength of YRFL, we experimentally demonstrate that, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. As a result, the conversion efficiency of the generated green random laser at 532.25 nm can be more than 11% when the input power of the polarized 1064.5 nm fundamental light is 2.85W. For spectral manipulation, we realize a spectral tunable green random laser in the range of 529.9 nm to 537.3 nm with >100 mW output power for the first time by tuning the wavelength of YRFL and the temperature of PPLN simultaneously. The system can be naturally modified to simultaneously realize the efficiency enhancement and wavelength tuning, thus providing a new route to generate high efficiency and tunable visible random laser via frequency doubling that are potentially useful for imaging, sensing and visible light communication applications.

Low-noise high-order Raman fiber laser pumped by random lasing

Raman fiber lasers (RFLs) have been widely utilized in long-haul optical transmission systems as pump sources for distributed Raman amplification (DRA) to increase transmission distance and capacity. However, RFLs with relatively large temporal intensity fluctuations would deteriorate signal quality due to the transfer of relative intensity noise (RIN). In this Letter, a low-noise high-order RFL common cavity pumped by an ytterbium-doped random fiber laser (YRFL) is proposed and demonstrated for the first time, to the best of our knowledge. Stable 4th-order random Raman lasing operating at 1365 nm is generated with 8.9 W of output power, without use of a multi-stage master oscillation power amplification system. Thanks to the YRFL common-cavity pumping where a wavelength division multiplexer (WDM)-assisted fiber-loop mirror is used to generate stable 1090 nm ytterbium-doped random lasing and cascaded random Raman lasing simultaneously, the RIN of the 1365 nm RFL is suppressed as low as -120dB/Hz without any peak over a 0-100 MHz span. Furthermore, the output power and lasing wavelength of this RFL can be flexibly tuned by adjusting the laser diode pump power, high-reflectivity fiber Bragg grating center wavelength, and single-mode fiber length. Hence, such a low-noise high-order RFL paves a way for the development of novel tunable RFLs with stable temporal output, leading to potential replacement of conventional RFLs for DRA in long-haul optical transmission systems to achieve better performances.

Spontaneous pump depolarization in ultralong cavity Raman fiber laser amplifiers

We present an experimental demonstration of polarization-independent performance in a forward and backward-pumped 2nd-order ultralong cavity Raman laser amplifiers with highly polarized pumps. Our findings show that the depolarization of the Stokes component due to gain saturation leads to polarization-insensitive performance in terms of output gain and relative intensity noise in the signal. These results pave the way for the use of individual highly polarized low-RIN semiconductor laser diodes in Raman-amplified optical communications.

Performance optimization in ultra-long Raman laser amplified 10×30 GBaud DP-QPSK transmission: balancing RIN and ASE noise

We experimentally evaluate the influence of RIN transfer from pump to signal on the transmission performance of a 10 × 30 Gbaud DP-QPSK transmission system using a 2nd-order ultra-long Raman fiber laser amplifier, considering the effect of cavity front-end reflectivity and forward pump power ratio. The evolution of the Q-factors with distance up to maximum reach is monitored for a 10 × 30 Gbaud DP-QPSK transmission system with WDM channels between 1542.94 nm to 1550.12 nm. A maximum transmission distance of 6479 km is found for configurations with low forward pump powers corresponding to the optimal balance between RIN and ASE impairments.

Impact of input FBG reflectivity and forward pump power on RIN transfer in ultralong Raman laser amplifiers

Relative intensity noise transfer from the pump to the signal in 2nd-order ultra-long Raman laser amplifiers for telecommunications is characterized numerically and experimentally. Our results showcase the need for careful adjustment of the front FBG reflectivity and the relative contribution of forward pump power, and their impact on performance. Finally, our analysis is verified through a 10 × 30 GBaud DP-QPSK transmission experiment, showing a large Q factor penalty associated with the combination of high forward pumping and high reflectivities.

Link optimization for DWDM transmission with an optical phase conjugation

We characterize in-span signal power asymmetry in random distributed feedback ultralong Raman laser-amplified WDM transmission and numerically optimize fiber span length and operating band to achieve the lowest inter-span signal power asymmetry between transmitted and optically conjugated channels in systems relying upon mid-link optical conjugation to combat fiber nonlinear impairments.