Small-signal erbium-doped fibre amplifiers pumped at 980 nm: a design study

Abstact A model for the erbium-doped fibre amplifier (EDFA) is presented and discussed. The model is used in a detailed analysis of the waveguide design of 980-nm pumped EDFAs operated in the small-signal gain region. A comparison of results for signals at wavelengths 1532 and 1552 nm is given. The influence of the amplified spontaneous emission on the optimum cut-off wavelength for the LP₁₁-mode of step index fibres is shown to be significant for gains exceeding 40 dB. For lower gain levels, the optimum cut-off wavelength is 800 nm. An analysis of EDFAs used as preamplifiers indicates that significant improvements of the noise figures may be obtained by cut-back of the fibre to lengths that are 50% of that for maximum gain. Very low noise figures of 3.1 dB are only obtainable if a reduction of 10 dB in the gain is acceptable.     

Erbium fibre amplifiers and lasers

Erbium-doped fibre amplifiers and lasers have been the subject of intense research and development over the last five years. It is hoped that this review will serve as an introduction to the main device concepts and make accessible the rapidly expanding body of literature.     

A neural network model of erbium-doped photonic crystal fibre amplifiers

The evaluation of the general evolution equations that describe the longitudinal propagation of pump, signal, forward and backward amplified spontaneous emission in rare-earth-doped optical fibre amplifier could be computationally expensive. In this paper, to reduce the computational time, a neural network approach for the modeling of erbium-doped photonic crystal fibre amplifiers is proposed. A number of simulations have been performed to investigate the characteristics of the proposed approach. The numerical results show good agreement between the neural network approach and the conventional algorithm based on the solution of the power evolution equations. The proposed approach exhibits attractive performance in terms of flexibility, accuracy and computational time.     

Theoretical modelling of S-band thulium-doped silica fibre amplifiers

A comprehensive numerical model of a thulium-doped silica-based fibre amplifiers is presented. The model is spectrally and spatially resolved and is general in terms of pumping scheme used. The application of the model for predicting the S-band amplifier performance and for optimization of amplifier parameters is shown. For optimized Tm-doped fibre with ³H₄ level lifetime of 45 μs, which is the maximum value in the Tm-doped silica fibres prepared by the authors, above 20 dB of gain with 2000 mW pump power at the 1050 nm pump band can be expected according to the simulations.     

Optical fibre: Spatial transient and steady state

The concept of spatial transient is introduced to better appreciate the role of leaky modes on optical waveguides of circular cross section. Refracting leaky modes represent a relatively fast transient while tunnelling leaky modes represent a relatively slow transient. Bound modes provide the spatial steady-state solution. The greater the waveguide parameter V, the greater the duration of the transient. The relevance of leaky modes on fibres with loss and irregularities is discussed.     

High efficiency, dual-wavelength fibre Raman pump laser for U-band fibre Raman amplifiers

A dual-wavelength, continuous wave, high efficiency fibre Raman ring laser for pumping U-band (1625–1675 nm) fibre Raman amplifiers is demonstrated. Two fibre Bragg gratings were used as wavelength-selective elements resulting in lasing lines at 1529.7 and 1558.7 nm with respective maximum powers of 209.0 and 390.9 mW. A simple U-band Raman amplifier was constructed to test the dual-wavelength pump laser and on:off gain with a peak of 7.3 dB and a 3-dB bandwidth of 73 nm was obtained.     

Optical switching of semiconductor laser amplifiers

The underlying physics of optical switching in bistable diode-laser amplifiers is reviewed. The importance of minimizing the switching energy of optical bistable devices is emphasized. It is shown that conventional diode-laser amplifiers are switched with only thousands of photons in less than a nanosecond. Recent developments in single quantumwell lasers allow switching with total energy (including electronic) less than a picojoule.     

Tiled-aperture coherent beam combining of two high-power fibre amplifiers

We demonstrate coherent beam combining of two tiled-aperture single-frequency fibre amplifiers with a total output power of 29.65~W by using the multi-dithering technique. The two laser beams are packaged closely by using free-space mirrors side by side into a tiled-aperture with a near-field fill factor of 62\%. Active phase control of the amplifier is performed on commercially available digital lock-in amplifiers. Experimental results show that the power contained in the main-lobe in closed-loop is 1.72 times greater than that in open-loop, which is 86\% for the ideal case. The fringe contrast of the far-field fringe pattern is as high as 80\% when the system is in closed-loop. The beam quality of the combined beam is computed to be BQ = 1.48. The whole system in closed-loop performs well in a long-time observation.     

Analysis of the gain and noise characteristics of fibre Brillouin amplifiers

A theoretical analysis is performed of the transfer characteristics, gain, amplified spontaneous emission noise, on/off ratio and nonlinear phase shift of a fibre Brillouin amplifier, taking into account a possible signal detuning from the line centre and the nonlinear pump power depletion effect. Among other features, it is shown that the amplified spontaneous emission noise is reduced at higher signal levels and that detuning the signal reduces the amplifier gain in the linear regime and increases the spontaneous noise power in the saturation regime. As a consequence, the on/off ratio is significantly degraded by such an effect.     

Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation

This paper investigates the extension of optical fibre beam delivery to high-brightness applications, in particular laser percussion drilling, where both a good beam quality and high peak power are required. Beam quality preservation through a number of optical fibres is studied both experimentally and by using a ray propagation model. It is determined that in order to achieve the beam quality required for percussion drilling (M²<30) the largest fibre which can be used is 400 μm diameter. The laser-induced damage threshold is measured for a number of 400 μm fibres, and a CO₂ laser-annealing technique is shown to increase the damage threshold by a factor of 10, allowing 28 J, 1 ms pulses to be transmitted.     

Optical soliton perturbation with Raman scattering and saturable amplifiers

The method of multiple-scale perturbation is developed to study the propagation of solitons through an optical fiber described by the perturbed nonlinear Schrödinger's equation. We show that, by introducing a proper definition of the phase of the soliton, one can obtain the corrections to the pulse where the usual soliton perturbation approach fails. A comparison is made with results obtained by other methods as well as with numerical simulations.     

Optical fibre illuminators for holography

The paper describes an application of optical fibres to hologram recording and reconstruction. A number of advantages over the conventional optical arrangements are described. A specific application is the use of the layout for a portable “holography kit”. The type of fibre to be used is discussed with reference to the radiation characteristics. An experimental verification of the scheme is presented.     

Effect of dopant distribution on optimization of Pr-doped fibre amplifiers

The generalized Gaussian formalism is applied to praseodymium-doped fluoride fibre amplifiers to obtain accurate analytic expressions for the small signal gain. The effect of confining the dopant to a central portion of the core is investigated. We find that dopant confinement improves the efficiency of these fibre amplifiers. The optimum cutoff wavelength for fibres with confined dopant is longer than that for fibres with fully doped cores.     

Design of dual-core resonant leaky optical fibre based gain flattening filters for erbium doped fibre amplifiers

We propose optical fibre based filters employing dual-core resonant leaky structure for gain equalization of erbium doped fibre amplifier (EDFA). Spectral loss variation of the structure has been utilized to suppress gain peak and, thus, flatten overall gain profile in the C-band. We show 15.7 dB flat gain with ± 1.6 dB ripple in the wavelength range from 1525 nm to 1555 nm using a single filter and 18±0.7 dB gain using two cascaded filters.     

Four-wave mixing crosstalk in optical fibre parametric amplifiers with orthogonal pumps

We experimentally investigated the generation of new unwanted frequencies generated by four-wave-mixing in fibre optical parametric amplifiers with two orthogonally polarised pumps and 17 dB of small signal gain. At these gain levels, we simultaneously observe two distinct types of mixing interactions and demonstrate that they vary with changing output power and gain at different rates. We also demonstrate that a combination of high pump power and low nonlinearity fibre results in less power in the unwanted peaks than a combination of low pump power and highly nonlinear fibre. Because these new unwanted four-wave mixing peaks result in crosstalk between wavelength division multiplexed channels, building parametric amplifiers with a combination of high pump power and fibre of low nonlinearity is desirable.     

Optical fibre strength/fatigue experiments

New techniques for measuring and analysing the strength characteristics of optical waveguide fibres have evolved as the strength of the optical fibres improved. A recent life-fatigue experiment is described and the results analysed in terms of the most commonly used theoretical model which results in the power law V = AKⁿ. Ten m long fibre samples were tensilely stressed in a high humidity environment by both fixed (static fatigue) and constant rate (dynamic fatigue) loads. The respective test values for the power law exponent, the fatigue constant ‘n’, were 38 and 17. The results of the tests indicate the more conservative value, 17, for practical engineering design, and the need for incorporating an ageing term in the model.     

Analysis and design of electromagnetic crystals in microstrip technology using a fibre grating model

Electromagnetic crystals (EC) in microstrip technology have been recently proposed as efficient Bragg reflectors. They are implemented by etching the ground plane or by drilling the substrate following a periodic pattern. In this paper we propose a simple and fast fibre Bragg grating (FBG)-based model for the synthesis and analysis of these microstrip ECs with sinusoidal patterns. The method rests on the analogy found between the frequency responses of a microstrip EC and a FBG. Based on this analogy, an equivalence relationship between the physical parameters of a microstrip EC and those of an equivalent FBG has been established. The equivalence assures that the frequency response of the microstrip EC is a down-shifted replica of the one corresponding to its equivalent FBG. This model avoids the use of the time-consuming electromagnetic calculations involved in the analysis of the microstrip ECs as well as the trial method employed until now in the synthesis, since a FBG response arises immediately from coupled-mode theory. At the same time the theoretical difficulties encountered in the formal derivation of coupled-mode theory for microstrip ECs are also avoided by the equivalence relationship found.