Raman Gain Efficiency Research Articles

Raman/EDFA hybrid bidirectional amplifier for fiber-optic time and frequency synchronization.

Herein, we verify that a Raman/EDFA hybrid amplifier can improve the stability of fiber-optic time and frequency synchronization systems compared to the Er3+-doped fiber amplifier (EDFA), owing to its higher gain and lower noise figure (NF) performance. We studied the variation law of Raman gain efficiency for a fiber Raman amplifier (FRA) as a function of pump power and input signal power, designed a bidirectional Raman/EDFA hybrid amplifier, and proved that equivalent NF below 0 dB can be obtained. Finally, hybrid amplifiers were compared to EDFAs in a free-running frequency synchronization system. The transfer stabilities reached 1.9678 × 10-13/1 s and 2.0248 × 10-13/1 s when FRA + EDFA and EDFA + FRA configurations were used, respectively, both exhibiting better performance than the stability of 3.0905 × 10-13/1s obtained by EDFA.

Unrepeatered Transmission of 3.2-Tb/s (32$\,\times\,$120-Gb/s) Over 445-km Fiber Link

We demonstrated 3.2-Tb/s (32 × 120-Gb/s) unre-peatered transmission over 445-km fiber with A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> -managed-span. This was achieved by employing co-propagating second-order pumped Raman amplification and remote optically pumped amplifier, which was counter-propagating second-order Raman pumped. This letter also discusses design tradeoff on the nonlinear tolerance and Raman gain efficiency within A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> -managed-span for performance enhancement of unrepeatered transmission.

Novel Technique for Obtaining the Raman Gain Efficiency of Silica Fibers

We present a simple novel technique that determines with precision the Raman gain efficiency for telecom fibers. The method is supported by a mathematical relation, which is the solution for the coupled differential equations that govern the stimulated Raman scattering. This technique is valuable due to its simplicity and consists of measuring the residual pump and the Raman scattering signals at the stimulated Raman scattering threshold. Our technique was proven for different fibers; as a result, we found that the rate of pump power to stimulated Raman scattering at threshold is lower than 16, which is the historically used value.

Exact determination of scaling of the Raman gain efficiency in silica fibers

This paper presents a theoretical and experimental study on the behavior of the Raman gain efficiency for any Stokes and excitation wavelength. Analyzing the Raman threshold for two adjacent Stokes we find a mathematical relationship to the Raman gain efficiency. This equation scales inversely with wavelength and directly with the fiber attenuation, and also provides approximate quantities to numbers already published. From experimental data for a LEAF fiber, we construct two curves, one based on our equation and the other based on the approach normally used, and the comparison of the two curves reveals significant errors. Additionally, we show that the spectrum of the Raman gain efficiency has the same shape as the attenuation spectrum.

Impact of tellurite-based glass structure on Raman gain

Raman gain efficiency and structure of tellurite glasses in the TeO2–TaO5/2–ZnO system have been investigated by Raman and IR spectroscopies. It has been found that replacement of TaO5/2 by ZnO does not significantly modify the respective proportion of TeO4, TeO3 and TeO3+1 entities but induces an important decrease of the TeO4 structural units Raman cross section. Change of the glass structure at the middle range length scale is proposed to be at the origin of the large decrease of the Raman cross section of the TeO4 units.

Recent advances of distributed optical fiber Raman amplifiers in ultra wide wavelength division multiplexing telecommunication networks

Recently, many research works have been focused on the fiber optic devices for optical communication systems. One of the main interests is on the optical amplifiers to boost a weak signal in the communication systems. In order to overcome the limitations imposed by electrical regeneration, a means of optical amplification was sought. The competing technology emerged: the first was Raman amplification. One reason was that the optical pump powers required for Raman amplification were significantly higher than that for Erbium doped fiber amplifier (EDFA), and the pump laser technology could not reliably deliver the required powers. However, with the improvement of pump laser technology Raman amplification is now an important means of expanding span transmission reach and capacity. We have deeply studied an analytical model for optical distributed Raman amplifiers (ODRAs) in the transmission signal power and pump power within Raman amplification technique in co-pumped, counter-pumped, and bi-directional pumping direction configurations through different types of fiber cable media. The validity of this model was confirmed by using experimental data and numerical simulations. Key words: Distributed Raman amplifier, fiber link media, signal power, pump power, and Raman gain efficiency.

C band single pump photonic crystal fiber Raman amplifier

A novel twin-core photonic crystal fiber was proposed to reduce the complexity and cost of fiber Raman amplifiers. By means of a proper design, this fiber could acquire a higher and flatter Raman gain efficiency coefficient curve r R=g R/A eff over a specified band of wavelength than a conventional fiber. A Raman amplifier was designed with this novel twin-core photonic crystal fiber to operate in C band from 1530 nm to 1565 nm. A remarkable improvement over a conventional fiber Raman amplifier was obtained. It was numerically demonstrated that when pumped with a single source, an average gain of 8.7 dB with a fluctuation of less than 0.9 dB is achievable.

Experimental comparison of Raman gain efficiency of a dispersion compensating fiber in C and O‐bands

AbstractWe measured the Raman gain efficiency (RGE) of a dispersion compensating fiber in the C (1530–1565 nm) and O‐bands (1260–1360 nm) and the results revealed that the RGE reaches its maximum value inside the O‐band. The spectral analysis of the fiber loss and Raman efficiency shows that the increase in Raman gain efficiency toward short wavelengths is balanced by the increasing of the fiber attenuation which leads the RGE to be maximum around 1310 nm. In addition, we characterized the gain of a Raman amplifier in the O‐band with a moderate pump power and a few kilometers of fiber. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 151–154, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24865

Novel asymmetrical twin-core photonic crystal fiber for gain-flattened Raman amplifier

A novel asymmetrical twin-core photonic crystal fiber was proposed, whose effective overlap core area Aeff can be designed to synchronize the variation of Raman gain coefficient with respect to frequency. This fiber possesses a higher and flatter Raman gain efficiency coefficient curve rR=gR/Aeff over a specified band of wavelength than a conventional fiber. Therefore, it is a good candidate of gain medium for a flat, broad gain band fiber Raman amplifier. It was numerically demonstrated that for the Raman gain efficiency rR, relative fluctuations of less than 2.2% and 5.7% are achievable in the C (1530−1565 nm) band and L (1565−1625 nm) band, respectively.

Raman amplification characteristics of As_2Se_3 photonic crystal fibers

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.

A Measuring Technique of Raman Gain efficiency for an Optical Transmission Line Including Optical Fibers and Connectors by Using OTDR

The measuring technique of Raman gain efficiency along the transmission line composed of optical fibers and connectors based on the OTDR is proposed. It is a simple method of estimating Raman gain efficiency from the backscattered powers with and without pump power based on the OTDR. Experiments of Raman gain efficiency are carried out for the optical transmission line composed of four 3-km long single-mode fibers. By our present method, the Raman gain efficiency distribution is successfully estimated.

Photonic crystal fibre for use in fibre Raman amplifiers

A novel photonic crystal fibre is proposed and designed. For this fibre, the variation of its effective core area Aeff via wavelength can be designed to follow the track of its Raman gain coefficient gR, so that a flat Raman gain efficiency rR=gR/Aeff is resulted. A fluctuation of less than 5.5% is obtained for rR between 1565 and 1625 nm (L-band).

Analysis of a realistic and idealized dispersion compensating photonic crystal fiber Raman amplifier

In the present paper, the highly efficient Raman amplification properties of a realistic and idealized dispersion compensating photonic crystal fiber (DCPCF) are described numerically. We have used an accurate full-vectorial finite element modal solver with hybrid curvilinear edge/nodal elements and anisotropic perfectly matched layers for an accurate modal characterization of the realistic as well as idealized DCPCF. A good agreement is observed between numerically evaluated and experimentally [P.J. Roberts, B.J. Mangan, H. Sabert, F. Couny, et al., J. Opt. Fiber Commun. Rep. 2 (2005) 435–461] measured dispersion values. A high peak Raman gain efficiency of 10.5 W −1 km −1 is obtained at 13.1 THz frequency shift for a 1455 nm depolarized pump. A DCPCF module of 1-km length can compensate for the dispersion accumulated over 70-km of conventional single mode fiber link with a residual dispersion of ±50 ps/nm and can provide an average gain of 7.6 dB with ±0.9 dB gain ripples over C-band.

Measurement of chromatic dispersion and Raman gain efficiency of a hole-assisted fiber: Influence of bend

We have experimentally measured the Raman gain efficiency (RGE) and chromatic dispersion (CD) of a hole-assisted fiber (HAF). The RGE of a HAF was characterized using standard pump on/off technique while the CD of the fiber was measured using optical network analyzer. Theoretical simulations of the modal characteristics and the RGE of HAF were carried out using an accurate full-vectorial finite element method. Further, the bending effects on the CD and the RGE of a HAF with a smallest feasible bending radius are demonstrated. It was found that the CD increases while the RGE is decreased by bending HAF in a smallest bending radius of 5 mm. Numerical predictions from the theory are shown to be in good agreement with the experimental results.

Design and Performance of Low-Loss Submarine Optical Fiber Cable for Long-Distance Submarine Repeaterless Transmission System Employing Remotely Pumped EDF and Distributed Raman Amplification

This paper describes a low-loss submarine optical fiber cable for a long-distance submarine repeaterless transmission system that employs remote pumping. The features of this system are that it has an increased signal power budget and is cost effective and easy to maintain. First, we investigated the relationship between the signal and pump losses and the Raman gain efficiency of optical fiber needed to achieve a submarine repeaterless transmission system operating at 2.5 Gbps and over a distance exceeding 370 km. We manufactured a submarine optical fiber cable based on the results and confirmed that it had low-loss characteristics. Second, we evaluated the long-term loss stability of the optical fiber with a high-power continuous wave (CW) laser light as the pump source. We confirmed that the loss remained unchanged after 1900 hours of exposure to 8W CW laser light at a wavelength of 1.48μm. This submarine optical fiber cable is being employed in a commercial submarine repeaterless transmission system between Okinawa and Miyakojima.

Raman gain efficiency distribution measurement in single-mode optical fibers by using backscattering technique

This letter proposes a new method for measuring a longitudinal distribution of the Raman gain efficiency in concatenations of single-mode optical fibers by using a backscattering technique and a pulsed pump light. The advantages of this method is that it can provide us with an easy operation for measuring optical fiber networks because all the measurement instruments are installed together at one end of the concatenation of optical fibers, and that it also provides a high resolution of the measurement according to the pump pulsewidth. We perform an experiment employing this method and confirm that it can be used to measure Raman gain efficiency distribution easily.

Effect of a nonlinear photonic Crystal fiber on the noise characterization of a distributed Raman amplifier

The first experimental study of the effect of a nonlinear photonic crystal fiber (PCF) on the noise characteristics of a distributed backward-pumped Raman amplifier is reported. The PCF has a highly nonlinear Raman efficient, and a high Rayleigh scattering parameter. When an optical signal first passes through a 100-m nonlinear PCF followed by a 25-km single-mode fiber, the optical signal-to-noise ratios (OSNRs) of the amplified spontaneous emission and the double Rayleigh scattering (DRS) are improved because the high Raman gain efficiency of the PCF makes the Raman gain of the signal at the beginning of the link increase, and the signal power over the length of the transmission becomes near a constant. However, the improvement of the OSNR of DRS compared with the OSNR of amplifier spontaneous emission is limited by the large Rayleigh scattering in the PCF.

Optimum optical fiber design for a DRA-based DWDM transmission system

This paper describes the distributed Raman amplification (DRA) transmission characteristic in an optical fiber both numerically and experimentally. The paper clarifies the relationship between optical fiber parameters and DRA transmission performance with regard to the signal-to-noise ratio (SNR) characteristics and nonlinear impairments. These relationships can be successfully discussed, taking account of various fiber parameters, including Raman gain efficiency, the attenuation coefficient at signal and pump wavelengths, the Rayleigh scattering coefficient, and the nonlinear refractive index, as a function of the relative index difference in an optical fiber. An example of an optimum fiber design for a DRA-based dense-wavelength-division-multiplexing (DWDM) transmission system is also discussed.

Novel design of inherently gain-flattened discrete highly nonlinear photonic crystal fiber Raman amplifier and dispersion compensation using a single pump in C-band

In this paper, we report, for the first time, an inherently gain-flattened discrete highly nonlinear photonic crystal fiber (HNPCF) Raman amplifier (HNPCF-RA) design which shows 13.7 dB of net gain (with +/-0.85-dB gain ripple) over 28-nm bandwidth. The wavelength dependent leakage loss property of HNPCF is used to flatten the Raman gain of the amplifier module. The PCF structural design is based on W-shaped refractive index profile where the fiber parameters are well optimized by homely developed genetic algorithm optimization tool integrated with an efficient vectorial finite element method (V-FEM). The proposed fiber design has a high Raman gain efficiency of 4.88 W(-1) . km(-1) at a frequency shift of 13.1 THz, which is precisely evaluated through V-FEM. Additionally, the designed module, which shows ultra-wide single mode operation, has a slowly varying negative dispersion coefficient (-107.5 ps/nm/km at 1550 nm) over the operating range of wavelengths. Therefore, our proposed HNPCF-RA module acts as a composite amplifier with dispersion compensator functionality in a single component using a single pump.

Performance Prospects for Distributed Measurement of Raman Gain Characteristics in Optical Fibers

A new technique has already been proposed by the authors for measuring Raman gain characteristics in single-mode optical fibers. The advantage of this technique is that it enables the evaluation of the longitudinal distribution of the Raman gain efficiency in concatenated fibers. With such distributed measurements, it is important to investigate the measurable length, which is determined by the signal-to-noise ratio (SNR) of the detected signal. In this paper, the maximum measurable length available with the previous technique was clarified based on both theoretical and experimental investigation. The SNR of the detected signal has been calculated theoretically, and it has been found that calculated and experimental results were in good agreement. Based on the SNR calculation, the measurable lengths available with this technique for various spatial resolutions and signal wavelengths are discussed.