Gain Tilt Research Articles

Gain and saturation characteristics of dual-wavelength-pumped silica-fibre Raman amplifiers

Dual-wavelength-pumped silica-fibre Raman amplifiers operating at wavelengths around 1.55 /spl mu/m and with gain bandwidths around 60 nm have been constructed. An experimental comparison is made between dual and single stage configurations and the spectral gain tilt due to inhomogeneous saturation under large signal conditions has been characterised.

WDM transmission system performance: influence of non-Gaussian detected ASE noise and periodic DEMUX characteristic

This paper presents the first unified wavelength division multiplexing (WDM) transmission model for systems incorporating cascaded optical amplifiers and a realistic demultiplexing (DEMUX) characteristic with periodic transfer function. The bit error ratio (BER) is evaluated accounting in rigorous form for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the bandwidth of the electrical receiver noise filter, the gain tilt and effective noise figure of optical amplifiers (with as well as without optical ASE noise filtering), channel crosstalk, signal extinction ratio and a one-or two-stage DEMUX implementation. The model is compared to the Gaussian receiver model in realistic design cases providing important information as to the validity of the Gaussian model. Practical design results indicate the link budget dependence on the DEMUX design and the ASE noise filtering.

Low-Distortion and High-Output Optical Power Amplifier for Analog Transmission Employing Hybrid Erbium-Doped Fiber Configuration

We have proposed a novel configuration of erbium-doped fiber amplifier (EDFA) which realizes both a flat spectral gain and high output power for analog transmission. This EDFA uses hybrid EDF which consists of serially cascaded EDFs with different glass compositions, based on the fact that EDFs with certain glass compositions exhibit gain tilt of opposite signs in the wavelength range of interest. It is theoretically shown that, in the hybrid EDFA, the gain flattening under any required optical input/output conditions is possible without sacrificing output power by optimizing the constituent EDF lengths. By using thus designed hybrid EDFAs, 50-channel amplitude-modulated vestigial-sideband signals were successfully amplified in four stages with no significant degradation of signal distortions.

Analysis of AC gain tilt in erbium-doped fiber amplifiers

We analyze theoretically and experimentally the dependence of AC gain tilt variations on gain variations in homogeneously broadened erbium-doped fiber amplifiers. Theoretically, we find that this dependence can be evaluated from the fiber's absorption spectrum. This is experimentally verified for gain variations induced by a pump-power reduction. For AM CATV, AC gain tilt variations limit how much the gain can vary before composite second order distortions become too large. From this point of view, for an erbium-doped alumino-germanosilicate fiber, an operating wavelength between 1542-1555 nm is more suitable than other ones in the long wavelength range of the EDFA, normally considered for AM CATV.

High power Er-Yb linear optical fibre amplifier for CATV applications with gain tilt window over 11 nm and integrated dispersion pre-compensation

An Er-Yb fibre amplifier has been constructed with dispersion compensation and gain tilt correction. Gain tilt has been minimised within 0.2 dB/nm for the spectral width of 11 nm and input power range of 0–4 dBm. A noise figure of 5 dB and output power of 21 dBm has been obtained for this operating range. Tests using a 77 channel NTSC signal show its compatibility to CATV applications.

Performance analysis on transmission of M-QAM HDTV signals in FTTF network with EDFA

Transmission of 60 channels M-QAM HDTV signals in the CATV fiber-to-the-feeder (FTTF) network is investigated. The authors consider the 1550 nm system with the Er-doped fiber amplifiers (EDFA). They analyze the system performance by considering all noise terms, and nonlinear distortions from laser, fiber dispersion, EDFA gain tilt and chirping. The results show there exists the optimal optical modulation index for achieving the maximum fiber span under the required bit error rate. The 1550 nm system with EDFAs shows its benefits for the distribution of M-QAM HDTV signals in CATV networks. Using one DFB laser with 5 dBm output and one EDFA with 15 dBm saturation output power shared among 10 feeders, the maximum fiber span can be 32 km for 256-QAM signals and 58 km for 64-QAM signals. >

Second-order distortion of amplified intensity-modulated signals with chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFA's has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion.

Design of erbium doped fibre amplifiers with minimised gain tilt for CATV transmission systems

High output power and low gain tilt of erbium doped fibre amplifiers for analogue applications are often conflicting requirements. The strong dependence of the gain tilt on the absorption and emission cross-section gradients of erbium allows 980 and 1480 nm pumped amplifiers to be designed with high output power and zero gain tilt.

AC and DC gain tilt of erbium-doped fiber amplifiers

AC gain tilt and DC gain tilt are defined as the variable gain of an erbium-doped fiber amplifier (EDFA) with input laser wavelength chirp due to modulation, and with different static laser unmodulated wavelengths of no modulation. This paper proposes a novel experimental setup for measuring AC gain tilt. This paper also clarifies experimentally and theoretically the dependency of the AC gain tilt on the input light modulation frequency from near DC to high frequency. The measured AC gain tilt values at high frequency resemble the measured spontaneous gain tilt values for input wavelengths from 1530 to 1580 nm. The influence of the AC gain tilt effect on AM-SCM video transmission systems is evaluated. >

Second-order distortion improvements or degradations brought by erbium-doped fiber amplifiers in analog links using directly modulated lasers

Within lightwave analog amplitude-modulated (AM) CATV systems using directly modulated lasers, erbium-doped fiber amplifiers (EDFA's) act upon the signal distortion because of the interaction between the laser chirp and the EDFA wavelength-dependent gain. This interaction is theoretically investigated in order to predict the EDFA-induced distortion. The relevant gain tilt characteristic for analog applications and the way to measure it are described. Expected and measured distortions at the EDFA output are in excellent agreement. Fiber amplifiers are found to decrease the signal distortion level when the gain tilt is negative, i.e., for wavelength above the gain maximum. >

Gain tilt of erbium-doped fiber amplifiers due to signal-induced inversion locking

A theoretical analysis describing an interpretation of the fundamental processes, responsible for the second-order distortion of AM signals in erbium-doped fiber amplifiers (EDFAs) is presented. It is shown that the locked-inversion gain tilt rather than the widely used continuous wave gain tilt is responsible for the distortion. The theoretical analysis is confirmed by experiments showing that the two types of gain tilt may differ by factors of up to 60. In particular, it is demonstrated that the locked-inversion gain-tilt may be significant even at the CW gain peak. Experimental results indicate that the relevant gain tilt can be reduced to acceptable levels by proper design of the EDFA.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Second-order distortion generated by amplification of intensity-modulated signals with chirping in erbium-doped fiber

A rate equation analysis for second-order distortion generated in an erbium-doped fiber amplifier is reported. It is shown that the relevant mechanism of distortion is the interaction between the frequency chirping of input light and the local gain tilt in the erbium-doped fiber. It is found that the output light distortion is highly affected by the phase difference between the input light distortion and the chirping-induced distortion. This indicates that second-order distortion is not always degraded by erbium-doped fiber amplifiers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Using equalizers to offset the deterioration in SCM video transmission due to fiber dispersion and EDFA gain tilt

The carrier-to-noise ratio (CNR) and distortion of a 1.55 mu m amplitude-modulated subcarrier-multiplexed (AM-SCM) video transmission system are worsened by the dispersion of the 1.3 mu m zero-dispersion fiber. Distortion is also increased by the EDFA gain tilt. Both problems are shown to be corrected by the use of dispersion equalizers and gain tilt equalizers. The application of the equalizers to SCM video transmission systems is proposed. >

Distortion due to gain tilt of erbium-doped fiber amplifiers

It was found experimentally and theoretically that the second-order intermodulation product, namely composite second-order (CSO) distortion, is caused by coupling of the gain tilt of an erbium-doped fiber amplifier (EDFA) with laser chirp. When the signal light is not at the gain peak of the EDFA, gain tilt exists and EDFA cannot be used in an AM-FDM optical analog video transmission system because of CSO degradation. To counteract the gain tilt induced CSO, the authors demonstrated that codoping Al and setting the laser oscillation wavelength at the gain peak are useful. Er-Al-codoped fiber amplifiers have better linearity than Er-doped fiber amplifiers because the gain tilt of the former is smaller. >