Pumped Fiber Raman Amplifier Research Articles

High-power cladding pumped Raman fiber amplifier with a record beam quality.

In this Letter, a high-power, high-brightness all-fiberized Raman amplifier based on a cladding-pumping scheme is presented for the first time, to the best of our knowledge. The triple-clad passive fiber is employed as Raman gain fiber in the laser system. The maximum output power is 762.6 W emitting at 1130 nm. To the best of our knowledge, this is the highest power in the fields of cladding-pumped Raman amplifiers. Through a cladding-pumping process, the beam quality parameter ${{\rm M}^2}$M2 improves from 6.12 of seed laser to 2.24 at maximum output power of 762.6 W, while the best ${{\rm M}^2}$M2 is 1.9 at 267.2 W. It is also the best beam quality of Raman laser with brightness enhancement in any kind of configuration (graded-index fiber or multi-clad fiber, laser or amplifier, all-fiber or free-space configuration) with power of over 100 W.

Evaluation of gain spectrum of dual/triple pumped fiber Raman amplifier (FRA) by optimizing its pumping parameters in the scenario of dense wavelength division multiplexed (DWDM) systems

Fiber Raman Amplifier (FRA) is a fundamental amplifier that has the capability to operate in any communication band. No exceptional doping in the optical fiber is required in order to generate amplification using Raman scattering. In present work, we have investigated the gain spectrum evolutions of a dual/triple pumped FRA for 0.8 nm, 0.4 nm and 0.2 nm channel spacings by varying number of pumps, operating pump wavelengths and their respective pump powers. It has been observed in the case of triple pumped FRA that deployment of three continuous wave (CW) lasers at 1390 nm, 1410 nm and 1440 nm wavelengths yields enhanced gain >15 dB in the region 1489.4 nm–1578.20 nm (88.8 nm) at 0.2 nm channel spacing.

Sodium guide star laser pulsed at Larmor frequency.

589 nm lasers pulsed at Larmor frequency, several hundreds of kilohertz, can increase the brightness of a sodium guide star and are required in remote magnetometry with mesospheric sodium. By amplification of a continuous-wave single-frequency 1178 nm laser in a pulse-pumped Raman fiber amplifier and frequency doubling in an external cavity, high-power pulsed 589 nm laser at Larmor frequency is obtained for the first time, to the best of our knowledge. The pulse format is mainly determined by the 1120 nm Raman pump laser, whose pulse repetition rate and duty cycle are adjustable. Active pulse shaping is applied to minimize the relaxation spike at the leading edge of the pulses. A reduction in pulse width and conversion efficiency from 1120 to 1178 nm is observed in the backwardly pumped Raman fiber amplifier due to the pump pulse transition effect. A 589 nm laser pulsed at a 350 kHz repetition rate and 20% duty cycle with average power up to 17 W is demonstrated as an operation example intended for a geomagnetic field of 0.5 G.

Noise Gain Features of Fiber Raman Amplifier

The formation dynamics of the optical noise in a silica single mode fiber (SMF) as function of the pump power variation in the counter pumped fiber Raman amplifier (FRA) is experimentally studied. The ratio between the power of amplified spontaneous emission and the power of incoherent optical noise is quantitatively determined by detailed analysis of experimental data in the pump powers range of 100–300 mW within the full band of Stokes frequencies, including FRA working wavelengths over the C + L transparency windows. It is found out the maximum of Raman gain coefficient for optical noise does not exceed ~60% of corresponding peak at the gain profile maximum of coherent signal. It is shown that the real FRA noise figure may be considerably less than 3 dB over a wide wavelength range (100 nm) at a pump power of several hundreds of mW.

Experimental evaluation and theoretical investigations of fiber Raman amplifiers and its gain optimization based on single forward pump

In this paper, we perform the experiment evaluation and the optimization of forward pumped fiber Raman amplifiers. The pumping configuration characteristics and optimum single mode large-effective-area fiber lengths for the system are presented within the framework of the undepleted pump approximation. It is found that achievable optimal span length and Raman gain in optimized forward pumped is about 52 km and 3.92 dB (2.42 dB) using an efficient particle swarm optimization (PSO) method at signal wavelength 1500 nm, higher than that without using PSO in signal wavelength 1300 nm for 200 mW pumping power. The experimental results showed that the gain flatness of the forward Raman amplifier can be maintained over a wide wavelength with a gain ripple lower than 0.4 dB.

Optimization of fiber Raman amplifier by reduction of four wave mixing effect

We analyzed the four wave mixing (FWM) effect by propagating two channels through the same backward pumped fiber Raman amplifier (FRA) with variations in FRA parameters. It is observed that FWM increases with increase in signal input power as well as pump power. The FWM effect is also analyzed for Raman constant $$({f}_{\text{ r }})$$ ( f r ) to obtain the optimum value of $$({f}_{\text{ r }})$$ ( f r ) , and it is found that to have minimum FWM, the optimum value of Raman constant comes out to be 0.18. This Raman amplifier can be used for wavelength division multiplexed application with constant broadband gain.

An effective numerical method for gain profile optimizations of multi pumped fiber Raman amplifiers

In this paper, we have solved propagation equations of multi-pump fiber Raman amplifier using Runge–Kutta (RK 4th order) numerical method and pump power evolutions along with the fiber length. They are used to calculate the net gain and gain ripple by varying the input signals powers for different fiber lengths. The pump powers are optimized by genetic algorithm and resulting net gain and gain ripple are reported graphically as well as in tabular form. The optimum minimum gain ripple is 0.26dB for 1mW input signal powers for 50km fiber length. By increasing the fiber length gain ripple increases to 0.5dB for 0.1mW input signal power. In comparison to other methods reported in the literature, our method is simple to implement and efficient for numerical design of Raman amplification in optical communication systems.

Investigations on Multi Pumped Fiber Raman Amplifiers over WDM in Optical Communication System

Fiber Raman amplifiers are important component of wavelength division multiplexed fiber-optic communication systems. The number of pumps, determination of powers, wavelengths and pumping schemes are main design criteria to operate these amplifiers. This paper investigates the effect of counter propagating pumping in fiber Raman amplifier. Pumping options like single, two and seven counter propagating pumps are investigated and their effect on bandwidth and gain ripple and signal power conditions are explored. It is shown that optical bandwidth increases from 5.9 THz (45.3 nm) in the range 1535-1584 nm to 12.0 THz (97.6 nm) in the range 1511-1607 nm and gain ripple decreases to 0.47 from 0.82 by increasing the number from one to seven counter propagating pumps in the fiber Raman amplifier.

Theoretical analysis of single-frequency Raman fiber amplifier system operating at 1178nm

We analyze the scalability of amplifying the output from a single-frequency diode laser operating at 1178 nm through the utilization of a core pumped Raman fiber amplifier. A detailed model that accounts for stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in relation to the fiber mode field diameter, length, seed power, and available pump power in both co-pumped and counter-pumped configurations is developed. The backward travelling Stokes light is initiated from both spontaneous Brillouin and spontaneous Raman processes. It is found that when fiber length is optimized, the amplifier output scales linearly with available pump power. Although higher amplifier efficiency is obtained with higher seed power, the output power diminishes. In order to mitigate the SBS process for further power scaling, we employ and optimize a multi-step temperature distribution. Finally, we consider the feasibility of generating the D(2a) and D(2b) lines in a sodium guide star beacon from a single Raman amplifier by examining four-wave mixing (FWM).

在背向泵浦分布式光纤拉曼放大器中受激布里渊散射和瑞利散射的放大效应

The amplification effect on stimulated Brillouin scattering (SBS) and Rayleigh scattering in the backward pumped G652 fiber Raman amplifier (FRA) is studied. The pump source is a 1427.2-nm fiber Raman laser whose power is tunable between 0-1200 mW, and the signal source is a tunable narrow spectral bandwidth (l10 MHz) external cavity laser (ECL). The Rayleigh scattering lines are amplified by the FRA and Stokes SBS lines are amplified by the FRA and the fiber Brillouin amplifier. The total gain of SBS lines is the production of the gain of Raman amplifier and that of Brillouin amplifier. In experiment, the SBS gain is about 42 dB and the saturation gain of 25-km G652 backward FRA is about 25 dB, so the gain of fiber Brillouin amplifier is about 17 dB.

Simulation of multiwave pumped fiber Raman amplifiers

Simulation of multiwave pumped fiber Raman amplifiers

Threshold level and gain of forward stimulated Brillouin scattering in a forward pumped s-band discrete DCF fibers Raman amplifier

Amplification effect of forward stimulated Brillouin scattering (SBS) lines on the forward pumped s-band discrete DCF fiber Raman amplifier (FRA) has been studied. Pump threshold power of the forward first order Stokes SBS (FSB1-), second order Stokes SBS (FSB2-) and third order SBS (FSB3-) in the forward pumped FRA are 1.5 mW, 1.4 mW and 1.7 mW, respectively. The Stokes SBS lines are amplified by FRA and fiber Brillouin amplifier (FBA) at the same time. Gain of amplification is given as GA=GR · GB, where GR is Raman gain and GB is Brillouin gain. Saturation gain of FSB1-, FSB2- and FSB3- are about 52 dB, 65 dB and 65 dB, respectively. The saturation Raman gain of 10 km DCF forward FRA is about 14 dB, so Brillouin gain of FSB1-, FSB2- and FSB3- are about 38 dB, 51 dB and 51 dB, respectively. The forward cascaded SBS lines have been observed.

Experimental Study of the Performance of Bidirectionally Pumped Raman Fiber Amplifier

The noise characteristics, gain saturation and pump power conversation efficiency of bidirectionally pumped Raman fiber amplifier (RFA) are investigated in detail. The results show that the noise characteristics of bidirectionally pumped RFA lies between that of the co- and counter-pumped RFA, and can be improved by increasing the co-gain reasonably. The saturation power of bidirectionally pumped RFA is lower than both the co- and the counter-pumped RFA and the gain saturation phenomenon becomes more serious with the increment of the ratio of the co-gain to the counter gain. Higher pump power conversion efficiency (PCE) can be achieved by increasing the ratio of co-gain to counter-gain when the total input signal power is relatively low, and by decreasing the ratio of co-gain to counter-gain when the total input signal power is relatively high.

A Novel Efficient Optimal Design Method for Gain-Flattened Multiwavelength Pumped Fiber Raman Amplifier

In this letter, it is revealed that there exits an approximate linear relation between the changes in input pump powers and the resulting changes in the pump-power integrals in multiwavelength pumped fiber Raman amplifiers (MW-FRAs). Based on this fact, a novel efficient optimal design method for gain-flattened MW-FRAs is proposed. First, the optimal integral pump spectrum is found by genetic algorithm. Then, the matrix denoting the linear many-to-many relation is calculated. By this matrix, the optimal input pump powers resulting in the optimal pump-power integrals can be easily calculated. The optimal pump spectrum is obtained without time consuming trial-error adjustments or many times numerical simulations. Compared to other existing optimal design methods, our method has a substantial improvement in the efficiency without any expense of accuracy.

Statistics of gain fluctuations in a bidirectionally pumped fiber Raman amplifier

Due to the polarization sensitivity of Raman gain and the random nature of polarization mode dispersion (PMD) in the fiber, the gain of a fiber Raman amplifier (FRA) fluctuates over a wide range during a relatively long time. In this work, a numerical method of obtaining the probability distribution function of the random fluctuating Raman gain in a bidirectionally pumped FRA is presented for the first time, based on the vector theory of stimulated Raman scattering (SRS). After optimizing a bidirectionally pumped FRA to achieve the best tradeoff between an optical signal-to-noise ratio (OSNR) and nonlinear distortions, the proposed method is applied to study the impact of gain fluctuations. The results show that the fluctuations reach their peaks at two values of PMD parameters ( D p ), respectively, when D p is increased from zero. The heights of the two peaks are proportional to the power allocations for the forward and backward pumps, respectively. The results are useful for avoiding high gain fluctuations when bidirectional pumping is applied.

Flat-Gain Fiber Raman Amplifiers Using Equally Spaced Pumps

This paper analyzes the gain flatness of multiwavelength pumped fiber Raman amplifiers using equally spaced pumps with both a fixed and an optimized central pump wavelength. The signal gain ripple using equally spaced pumps is compared with the case in which the pump wavelengths are allowed to vary for two, four, and eight pumps with 20-, 40-, and 80-nm signal bandwidths. The paper shows that using an optimized central pump wavelength with equal pump spacing simplifies system design, while the gain ripple is no more than 0.4 dB larger than the ripple obtained when the pump wavelengths are optimized for the cases considered.

Optimal design for bidirectionally pumped Raman transmission systems

Optimal design is discussed for bidirectionally pumped Ra- man transmission systems. An optimal design algorithm is proposed for bidirectionally pumped fiber Raman amplifiers (BDP-FRAs), optimal pumping and gain schemes are analyzed for BDP-Raman systems, and the possibility of long-haul transmission is numerically verified for 7200-km 16-310.66-Gbits/s and 2400-km 16-342.7-Gbits/s BDP- Raman systems based on a common single-mode fiber (SMF) link. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1604400) Subject terms: optical fiber communication; fiber Raman amplifiers; optimization; pumping schemes; gain schemes.

Performance limitation due to statistical raman crosstalk in a wdm system with multiple-wavelength bidirectionally pumped raman amplification

A general theoretical model of statistical Raman crosstalk and its impact on system performance in a multiwavelength bidirectionally pumped Raman fiber amplifier (RFA) is developed for the first time, where we have taken modulation statistics, dispersion-induced pulse walk-off and signal-induced pump depletion into account. Two kinds of statistical Raman crosstalk, from signal-induced forward-pump depletion and from signal-signal Raman interaction, are included in one model. Formulas for normalized Raman crosstalk, Raman crosstalk-induced relative intensity noise spectral density, and its variance and system performance impact in terms of Q penalty are presented for both a single-span system and a dispersion-compensated multispan wavelength-division-multiplexed (WDM) link. Based on these formulas, we numerically investigate the impact of Raman crosstalk on system performance in a three-wavelengths bidirectionally pumped 40 × 40-Gb/s WDM system for various fiber types. In addition, Raman crosstalk in a four-wavelength bidirectionally pumped RFA was experimentally measured. The results agree well with our theory.

Study on Optimal Design Broadband and Flat-gain Multi-wavelength pumped Fiber Raman Amplifiers

Based on analysis of impact of pump scheme on gain performance, this paper develops a novel simple method to design the multi-wavelength pumped fiber Raman amplifier with optimal gain-flatness performance, can give the result of input pump distribution(λ i, p i ) at the condition of giving the parameter of signal bandwidth, pump number and limitation of gain ripple.

An efficient method to optimize gain spectra of fiber Raman amplifiers pumped by multiple laser diodes

AbstractWe propose a new and efficient method to optimize the output gain profile based on the characteristics of a WDM pumped Raman gain spectra and the fact that different wavelength pumps have different impacts on the gain tilt. As an application, we optimize a 10 wavelength pumped fiber Raman amplifier in a short time, and a 80 nm flat gain band with less than 0.4 dB ripple has been derived. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 381–383, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10614