Ultra-long Raman Fibre Laser Research Articles

Corrigendum to “364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber” [Opt. Laser Technol. 161 (2023) 109094

Corrigendum to “364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber” [Opt. Laser Technol. 161 (2023) 109094

364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber

Raman fiber laser (RFL) based on distributed Raman gain along optical fiber is an ideal platform for realizing ultra-long-cavity laser, which is of great interest in fundamental laser science and multidisciplinary fundamental studies such as complex systems exhibiting turbulence-like characteristics. RFL based on standard single-mode fiber sets the record of the longest laser cavity reaching 270 km with a clearly resolvable mode structure. Here, we explore the effect of pumping scheme and key fiber parameters to further extend the ultimate length of RFL. We theoretically analyze and experimentally demonstrate that by adopting higher-order pumping and ultra-low-loss fiber (ULLF) with low transmission loss and low Rayleigh backscattering coefficient, the laser cavity length of RFL with resolvable mode structure can be significantly increased. As a result, a 364 km-long bi-directional pumped 6th-order ultra-long Raman fiber laser (URFL) based on ULLF is demonstrated with resolvable longitudinal mode, an extension of 35 % over the prior 270 km record. This work paves a way to lengthen cavity length of URFL and provides a new platform for fundamental laser physics study and the ultra-long fiber laser applications in communication and sensing.

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.

A self-pulsing ring cavity ultra-long Raman fiber laser

Ultra-long Raman fiber lasers are of significant interest due to their application in the telecommunications industry. In this work we propose a self-pulsing ultra-long Raman fiber laser with a ring cavity configuration. A TrueWave® REACH fiber is deployed as the gain medium for the Raman fiber laser. Three laser cavities with different total cavity lengths are investigated; these generate pulse repetition rates of approximately 25 MHz, 14 MHz and 10 MHz for TrueWave® REACH fibers of lengths 28 km, 56 km and 76 km, respectively. With a similar pump power of 2.48 W a broader spectral bandwidth is observed for the shorter laser cavity. The highest signal-to-noise ratios of 46 dB and 47 dB are attained for laser cavities with 56 km and 76 km TrueWave® REACH fibers at pump powers of 1.62 W and 1.97 W, respectively. This work differs from previous works which mostly employed a linear laser cavity configuration and other gain media to obtain a self-pulsing Raman fiber laser.

Transmission Span Optimization in Fiber Systems With Cavity and Random Distributed Feedback Ultralong Raman Laser Amplification

Optimal design of a Raman-amplified periodic cell depends strongly on the constraints set by overall system requirements. The best possible tradeoff in terms of cost-effectiveness, nonlinearity, pumping efficiency, optical signal-to-noise ratio (OSNR), and tolerance to relative intensity noise transfer varies depending on characteristics, such as total transmission length, desirable number of spans or power budget requirements. Here, we perform a thorough experimental and theoretical study of the best performing configurations for second-order amplification over a broad range of applications, with span lengths ranging from 10 to 150 km. Results highlight the presence of different regimes in which different contributions to noise become dominant. We find that ultralong Raman fiber laser cells with high forward pumping are to be preferred in unrepeatered transmission applications, whereas in long-haul transmission a shorter cell based on random distributed feedback lasing can be a better option as it enables low relative intensity noise transfer with a higher output OSNR at the cost of a reduced pumping efficiency.

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.

Distributed Vibration Sensing Over 125 km With Enhanced SNR Using Phi-OTDR Over a URFL Cavity

We describe the use of a phase-sensitive optical time domain reflectometer ( $\phi {\bf OTDR}$ ) over an ultra-long Raman fiber laser cavity allowing fully distributed detection of vibrations over 125 km. Compared to a first-order Raman-assisted $\phi {\bf OTDR}$ , this scheme shows an enhanced signal-to-noise ratio (SNR). This is due to the fact that the relative intensity noise introduced by the Raman amplification is mostly transferred to a lower frequency range, where the balanced detection implemented in the setup provides better suppression of the common-mode noise. The sensor was able to measure vibrations of up to 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and an average SNR of 8 dB with no postprocessing. This implies a >3 dB improvement in SNR over a first-order Raman-assisted setup with similar characteristics.

RIN transfer in second-order amplification with centrally-pumped random distributed feedback fiber lasers

The predicted relative intensity noise (RIN) transfer function for distributed amplification based on centrally-pumped random distributed feedback ultralong Raman fiber lasers (RDFLs) is found and studied over a broad range of possible lengths and signal powers, showing that the values of the RIN transfer function and the RIN cut-off frequencies are dependent upon both parameters. RIN transfer is shown to be generally higher than in a typical ultralong cavity Raman fiber laser (URFL) pumped from the extremes.

RIN transfer in random distributed feedback fiber lasers

We numerically investigate relative intensity-noise transfer from a noisy pump to the generated Stokes component in random distributed feedback ultralong Raman fiber lasers. Results show transfer levels comparable to those in distributed Raman amplification and cavity-based ultralong Raman fiber lasers, but with some unique spectral features.

Spatially cascaded cavities for power saving distributed Raman amplification

A novel distributed amplification scheme with cascaded (using distributed in-line cavities formed by notch filters based on fiber Bragg gratings) ultra-long Raman fiber lasers for transmission with suppressed noise and reduced pump power consumption is proposed and evaluated. Comprehensive numerical analysis shows the possibility of pump power reduction by 50% compared to similar scheme without notch filters while supporting acceptable optical signal-to-noise ratio over large transmission span.

L-band R-EDFA/Raman hybrid amplifier with enhanced higher-order pumping scheme utilizing stimulated Raman scattering

An enhanced pump delivery scheme for L-band remote erbium-doped fiber/Raman (R-EDFA/Raman) hybrid amplifier utilizing stimulated Raman scattering (SRS) is demonstrated. The technique benefits from the use of higher-order pumps that are realized by generating C-band SRS and ultra-long Raman fiber laser (ULRFL). A section of passive erbium-doped fiber is deployed at the end of the transmission span to exploit the ULRFL pump for additional amplification. Gain clamping effect is observed at the R-EDFA and passive EDF due to saturation from the higher-order pumps. Tuning the pump wavelengths to a region away from the amplification bandwidth could remove the gain-clamping effect and produce maximum gain performance.

Rayleigh-Based Raman Fiber Laser With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier

This work focuses on a secondary pumping scheme for remote L-band erbium-doped fiber amplifier (R-EDFA) generated by integrating Rayleigh-based ultralong Raman fiber laser (ULRFL) with a section of passive erbium-doped fiber (EDF). A selective wavelength reflector combined with Rayleigh feedback in the transmission span forms the ULRFL cavity while the passive EDF deployed at the end of the span converts the ULRFL into a secondary C-band pump. Optimum ULRFL wavelength is determined by tuning the selective wavelength reflector across the C-band region. Combination of R-EDFA gain, Raman amplification, and passive EDF provides gain improvement as high as 15.5 dB over conventionally pumped R-EDFA with minimal optical signal-to-noise ratio penalty.

300 km-ultralong Raman fiber lasers using a distributed mirror for sensing applications

Several configurations of ultralong Raman fiber lasers (URFL) based on a distributed mirror combined with Bragg gratings or fiber loop mirrors are studied. Two continuous-wave URFL configurations, with single and cascaded cavities using fiber Bragg gratings as mirrors are explored for a 300 km long fiber. For optical sensing, the cavity length was optimized for 250 km using one of the gratings an intensity sensor. Another URFL configuration based in a fiber loop mirror is also reported. For optical sensing using a 300 km long fiber it is shown that the best choice is a hybrid configuration. The sensitivity of the FBG laser sensor range was from (76 ± 2) × 10⁻⁶ με⁻¹ (for lower strain) to (9.0 ± 0.4) × 10⁻⁶ με⁻¹ (for higher strain).

An incoherent fibre laser

The demonstration of a 'mirrorless' ultralong Raman fibre laser that provides stable, spatially incoherent continuous-wave lasing may prove to be an important new light source for applications in nonlinear optics, sensing and telecommunications.

10.1007/s11490-008-2005-y

PACS numbers: A recent demonstration of lasing in ultralong Raman fiber lasers has attracted the attention to their specific features. In this paper, we analyze modifications of the mode structure, shape, and width of the generated spectrum, fluctuations of the intensity and phase upon the lengthening of such a laser, and increasing its power. The mode structure is shown to be resolvable in an RF spectra up to 84-km long (with ∼1 kHz spacing), and the RF peak width appears proportional to the intracavity power. At the same time, the optical spectra broaden with the power according to the square-root law and take the specific shape with exponential wings. The model based on the wave kinetic equations has been developed that describes the observed effects. It is shown that ultralong Raman lasers exhibit a new laser regime through the generation of multiple (up to ∼108) cavity modes interacting via FWM processes that lead to stochastic (turbulent) behavior of their phases and amplitudes.

Long-distance soliton transmission through ultralong fiber lasers

We present the first experimental demonstration (to our knowledge) of long-distance unperturbed fundamental optical soliton transmission in conventional single-mode optical fiber. The virtual transparency in the fiber required for soliton transmission, over 15 complete periods, was achieved by using an ultralong Raman fiber laser amplification scheme. Optical soliton pulse duration, pulse bandwidth, and peak intensity are shown to remain constant along the transmission length. Frequency-resolved optical gating spectrograms and numerical simulations confirm the observed optical soliton dynamics.

High efficiency supercontinuum generation using ultra-long Raman fiber cavities

Supercontinuum generation in a multi-fiber ultra-long Raman fiber laser cavity is experimentally investigated for the first time. We demonstrate significantly enhanced spectral flatness and supercontinuum generation efficiency using only conventional single mode silica fiber. With a pump power of only 1.63W a approximately 15 dB bandwidth >260 nm wide (from 1440 to >1700 nm) supercontinuum source is reported with a flatness of <1 dB over 180 nm using an optimised hybrid TW/HNLF cavity. We address the dependence of the supercontinuum spectrum on the input pump power and ultra-long Raman cavity.

Turbulent broadening of optical spectra in ultralong Raman fiber lasers

We study the properties of radiation generated in ultralong fiber lasers and find an interesting link between these optical systems and the theory of weak wave turbulence. Experimental observations strongly suggest that turbulentlike weak interactions between the multitude of laser cavity modes are responsible for practical characteristics of ultralong fiber lasers such as spectra of the output radiation.

New physical effects in ultralong Raman fiber lasers

PACS numbers: A recent demonstration of lasing in ultralong Raman fiber lasers has attracted the attention to their specific features. In this paper, we analyze modifications of the mode structure, shape, and width of the generated spectrum, fluctuations of the intensity and phase upon the lengthening of such a laser, and increasing its power. The mode structure is shown to be resolvable in an RF spectra up to 84-km long (with ∼1 kHz spacing), and the RF peak width appears proportional to the intracavity power. At the same time, the optical spectra broaden with the power according to the square-root law and take the specific shape with exponential wings. The model based on the wave kinetic equations has been developed that describes the observed effects. It is shown that ultralong Raman lasers exhibit a new laser regime through the generation of multiple (up to ∼108) cavity modes interacting via FWM processes that lead to stochastic (turbulent) behavior of their phases and amplitudes.

Experimental demonstration of mode structure in ultralong Raman fiber lasers

We present the first experimental demonstration of a resolvable mode structure with spacing c/2nL in the RF spectra of ultralong Raman fiber lasers. The longest ever demonstrated laser cavity (L=84 km), RF peaks of ~100 Hz width and spacing ~1 kHz have been observed at low intracavity powers. The width of the peaks increases linearly with growing intracavity power and is almost independent of fiber length.