A number of the fiber fabrication methods have evolved in response to the interest in active fiber devices based on rare earth doped core silicate fiber. Descriptions of fabrication methods along with associated host composition and waveguide design issues will be presented.
We review theoretical models of fiber amplifiers, resonant fiber lasers and superfluorescent fiber lasers made of a single mode laser fiber, for example a silica fiber doped with a rare earth laser ion. The quantities which are investigated are the optical gain, the threshold and conversion efficiency in fiber sources, and the spectral narrowing in superfluorescent sources. These models take into account the nature of the laser transition (i.e. 3- or 4-level), the potential presence of pump ESA, and the guided nature of the pump and signal waves interacting along the fiber. The emphasis is placed on the development of simple, closed-form expressions for these quantities.
This tutorial presents a discussion of the device requirements which must be satisfied to obtain optimal performance of fibre lasers and amplifiers. The constructional details of fibre lasers and amplifiers and the range of resonator types that have been explored for fibre lasers are addressed.
We present a review of the main non-linear effects for light generation and amplification in optical fibers. Stimulated Raman Scattering and its applications to optical amplication, Four-Photon Mixing, Brillouin Scattering and Harmonic Generation are described. We emphasize the advantages and drawbacks of using optical fibers for the development of non-linear effects.
In this paper we describe work aimed at identifying and measuring properties of rare earth doped fibres which may affect their use and application in oscillators and amplifiers. The main dopants of interest are erbium(1.55μm), and neodymium (0.9, 1.06, 1.3μm). We shall discuss the measurement, and effects, of parasitic absorption from the upper laser level (known as excited state absorption, or ESA), at both pump and signal wavelengths. We shall also consider how detailed measurements of fluorescence decay from the excited level can yield information about quenching, upconversion, and energy transfer processes.
The initial discovery of fluorozirconate glasses at the University of Rennes, France in 1974  followed an attempt to fabricate large pieces of ZrF4 crystals doped with Nd (NdZrF7) for laser applications. One of the samples prepared at the University turned out to be vitreous rather than crystalline and the fluorozirconate glasses had been discovered. Initial interest in these materials was as improved glasses for high energy fusion lasers and in 1978, in a joint publication with the University of Rennes, Weber concluded that the glasses should be considered as candidates for laser materials . A further significance of these new glasses was soon recognised when it was proposed that they might have applications for low loss IR fibres, with intrinsic losses below those in silica, and a considerable amount of work is now underway in an attempt to reach these targets . These two interests were amalgamated in 1987 when the first fluoride fibre laser was demonstrated by Brierley and France , doped with Nd and lasing at 1.05 um. Following this work many new dopants have been investigated and many different lasing lines have been reported. This paper reviews these recent developments in fluoride fibre lasers and amplifiers from several laboratories throughout the world and includes some of the latest results on semiconductor diode pumping close to 0.8 um.
Common techniques for the fabrication of optical waveguides (MCVD-Modified Chemical Vapor Deposition, OVD-Outside Vapor Deposition, VAD-Vapor Axial Deposition) depend upon the availability of high vapor pressure precursor compounds such as SiC14, GeC14, and POC13. Vapor delivery techniques can not be used to transport compounds with a low vapor pressure. To incorporate such elements into the glass structure, we are investigating aerosol doping for both MCVD and OVD. Low mass flow rate aerosol transport is being used for core doping of rare earth elements in MCVD, and a high mass flow aerosol transport may have application in overcladding in OVD, the fabrication of fiber boules of glasses with a high nonlinear refractive index, and for GRIN (Gradient Index Lenses) lenses.
Recent advances in the field of Er3+-doped optical fibre amplifiers and lasers operating around λ=1.54μm will be described, with particular reference to 'new' pump wavelengths and practical pump sources for these devices.
Despite the excellent performance that has been demonstrated for Er3+-doped fiber amplifiers at 1.5 µ m, they have several shortcomings that remain concerns in practical applications. The efficiency, gain, and signal/noise ratio of three-level devices are quite sensitive to the magnitude and spectrum of the cross sections. These, in turn, depend on the glass composition used as host. We report an investigation which determined the relevant cross sections for a variety of glasses and used them to predict performance trends for fiber amplifiers. Glasses have been identified which have advantages over silica, particularly for pumping with AlGaAs diode lasers at 800 nm.
The gain dynamics of erbium-doped fiber amplifiers are studied both experimentally and theoretically. It is shown that the transients associated with gain saturation and gain recovery during multichannel amplification have long characteristic times, i.e. in the 100 its-1 ms range. Such slow gain dynamics effectively prevent saturation-induced crosstalk and intermodulation distortion effects in the amplification of high-speed WDM and FDM signals.
A large signal model of an optically pumped Erbium-doped travelling wave fibre amplifier is presented, including the influence of excited state absorption. The maximum gain and optimal amplifier length are discussed, based on the intensity of pump, signal and amplified spontaneous emission. Special attention is given to the gain degradation depending on the mode distribution of the pump light, and noise figures of the amplifier is presented for co- and counter propagating pump light. Finally, the variation of calculated results towards changes in the values of measured constants is considered.
We report on a parametric study to optimize the Er3+-doped fiber amplifiers performances. A 24 dB efficient gain in Er3+- doped silica-based fibers has been achieved for small signal regime at 1.553 μm with a 40 mW pump power in the 1.48-1.49 μm range. The saturation output signal power was up to 20 mW for a gain of 17 dB. For the signal wavelength a 3dB-bandwidth of 30 nm has been obtained always with a pump around 1.48 4m. We achieved these results by optimizing the relevant parameters which are the pump and signal characteristics (wavelength and power), the Er3±-concentration and length for the doped fiber. We report on the main parameters which determine the amplifier efficiency (gain, saturation output power, saturation gain, bandwidth, amplified spontaneous emission,...). It is worth noticing that an outstanding amplification is obtained at 1.55 μm wavelength where the gain value is not optimal in order to demonstrate the capability of such amplifiers.
The gain of a highly efficient erbium-doped fiber amplifier was measured when pumped at wavelengths between 1.46 pm and 1.51 ,am. The optimal pump wavelength, Xrt, was determined to be 1.475 pm. At this pump wavelength the maximum gain coefficients for signals at 1.531 pm and 1.544 pm were measured to be 2.3 d13/mW and 2.6 dB/mW, respectively. At λ°pPt high gains were achieved, ranging from 32 dB at pump power Pp = 20 mW up to 40 dB at Pp = 80 mW. These modest pump powers are within the capabilities of currently available 1.48 pm diode lasers. The width about λ°IP for 3 dB gain variation exceeded 27 nm for Pp = 10 mW and 40 nm for pipn > 20 mW. With this weak dependence on pump wavelength, single longitudinal mode lasers do not have a significant advantage over practical Fabry-Perot multi-mode pump lasers.
The fabrication and characteristics of a variety of single-mode fibre Bragg reflection gratings are discussed. By simple modifications to the fabrication process fibre gratings can be made with very different characteristics ranging from very narrowband resonators with a 0.04nm bandwidth to broadband reflectors with a 17nm bandwidth.
We demonstrate lasing on the 1050nm and 1345nm 4-level transitions of neodymium in multimode fluoride glass fibers pumped by a commercial semiconductor diode laser. Low thresholds, high output powers, and high efficiencies are all demonstrated.
Techniques for the generation of second-order nonlinearities in optical fibres are described. Applications to nonlinear frequency-mixing and relevant phasematching techniques are discussed. Electra-optic modulation via the Pockels effect is demonstrated.
Germano-silicate optical fibres have been fabricated by MCVD with a refractive index difference between the core and the cladding glass of up to 0.05. It is demonstrated that by depositing within Vycor tubes during preform fabrication, fibre may be subsequently drawn at lower temperatures than are possible with conventional silica preforms and this reduces the germania-induced excess loss. The performance of such fibres as Raman fibre amplifiers is discussed with particular reference to their bandwidth for applications in 1.5 - 1.6 AM telecommunications systems.
Optical absorption strengths and spontaneous transition rates are presented for Er3+ ions in Ge-doped silica glass at concentrations from 0.5x1019 cm-3 to 4.0x1019 cm-3. Results for transitions in the 480 - 1700-nm range are extracted from measure-ments on Er-doped preform cores prepared by the solution-doping technique. A com-parison of the spontaneous lifetime of the 4113/2 level with the strength of the corresponding ground-state absorption reveals a conflict with the Einstein A-B relation. Measurements made on fibers exhibit a significant change in linewidth and fluorescence decay time as compared to the corresponding preforms. From these results the magnitude and spectral dependence of the cross sections for absorption as well as stimulated emission were determined for single-mode fibers.
The Stark levels of the 4115/2 ground state manifold have been determined for Er3+-doped fluoride, fluorophosphate, and silicate bulk glasses from fluorescence-line-narrowing measurements at 4.2 K. Splittings between adjacent Stark levels were observed to be 20-80 cm-1. The total energy spread of the manifold ranged from 335 to 400 cm-1. The energy of a given Stark level varied up to 60 cm-1 depending on the particular Er3+ sites excited. Using the 4.2-K results, homogeneous broadening is found to be a reasonable approximation for the 300-K luminescence band.
Neodymium and Erbium doped Silica-based optical fibres are a promising material in fibre laser and amplifier. The match-type with Si02 cladding and Ge0z-Si02 core single-mode fibres were fabricated by NCVD soot method. Rare-earth were doped by a soot imprenation technique. The fibres with various amounts of Nd3+·and Er3+codoping with Yb3+were also obtained. The quantity of dopants can be controlled by deposition temperature and doping concentrations in solution. The absorption spectrum and fluorescence characteristics were measured. The concentration of rare-earth ion in fibre can be in excess of 1 wt%. The minimum attenuation can be as low as 5-10 db/km at lasing wavelength. 1.555 um optical laser and amplifier had been made by Er3+/Yb3+doped optical fibre. The output of laser is >6 mw.
Two high power, broadband, 1.55 µm sources utilizing an erbium-doped silica-based single mode fiber pumped at 0.5145 p.m are presented. The first source is a non resonant superfluorescent fiber laser. With proper selection of the fiber length, it exhibits an output power of 5 mW and a bandwidth of 19 nm for an absorbed pump power of 460 mW. Spectral variations with pump power and/or fiber length, in particular spectral shape, center wavelength and spectral narrowing, are investigated. The second source is a resonant fiber laser with an intracavity acousto-optic modulator which, when frequency modulated at a rate lower than 1 kHz, produces modulation of the laser wavelength over 9 nm with an output power in excess of 5 mW.
The optical properties of trivalent Samarium doped silica glass fibres are described. This material has a narrow fluorescence of 2.2 nm f.w.h.m. at a wavelength of 650 nm. Visible laser emission is obtained at this wavelength when the fibre is pumped in a Fabry Perot cavity. The performance of the laser in continuous, Q-switched and self mode-locked operation is described.
We review recent investigation into the active mode-locking of rare earth doped silica, monomode fiber lasers. Dopants include Nd3+, Er3+ and Yb3+:Er3+ and mode-locking has been accomplished either by amplitude modulation or by phase modulation. Both bulk optic and integrated optic modulators have been used. Pulses of 4 psec duration have been observed in an Er3+-doped fiber laser with an integrated amplitude modulator. 20 psec pulses have been produced from a Nd3-doped fiber laser with a bulk phase modulator. In both these systems, the introduction of anomalous group velocity dispersion compensates for self-phase modulation, resulting in bandwidth-limited operation. The paper concentrates mainly on three experiments involving the mode-locking of fiber lasers by active phase modulation. A bulk LiNb03 phase modulator produces pulses of 20 psec and 37 psec respectively in Nd3-doped and Yb3:Er3-doped fiber lasers. The third system incorporates intra-fiber phase modulation. This allows considerable simplification of the laser cavity, resulting in a diode-pumpable source of 80 psec mode-locked pulses with a threshold pump power of 520 pW and a slope efficiency of 48%.
Laser fibers prepared from Nd- and Er-doped phosphate glass possessing a large stimulated emission cross section have been investigated both in a single fiber and in a fiber bundle. In the single fiber, continuous wave oscillations were successfully obtained at 1.054 p.m and 1.366 µm on a high Nd-doped single-mode fiber of 10 mm in length and also at 1.535 pm in a Er-doped single-mode fiber, sensitized by Nd, Yb. Especially, a low threshold of 1 mw and a high slope-efficiency of 50% were achieved in 1.054 pm laser oscillation on a Nd-doped fiber, end-pumped with a laser diode. A fiber bundle of phosphate glass doped with 8 wt% Nd2O3 yielded an average output power of 100 W at 50 pps where the bundle was 4.6 mm in diameter and was side-pumped with flash lamps.
The performance and charactersics of various Q-switched fibre lasers are examined. All results presented are for laser diode pumped systems, emphasizing the practicality of the Q-switched fibre laser. Operation at 0.94, 1.06, 1.09 and 1.57Am wavelengths are covered. The results presented show that Q-switching is currently limited by the performance of the modulator. Theoretical modelling of the process allows us to determine the required properties for a modulator and in addition to predict the expected performance of Q-switched fibre lasers under different conditions.
We describe the construction and operation of an FM mode-locked fiber laser using a large-diameter Nd3+ doped silica fiber with dielectric reflectors deposited directly onto the fiber ends and an integrated ZnO acousto-optic phase modulator clamped to the fiber. Optical pulses of less than 60 ps duration have been generated at 432.77 MHz with 400 mW of power to the transducer, and 300-400 ps pulses have been observed with only 4011,W of modulator power, correspond-ing to less than 130 wad of phase modulation.
The bandwidth, gain, saturation power and noise of the erbium-doped fiber-amplifier (EDFA) are reviewed in the context of high-speed optical communication systems. Recent experiments which have used EDFA postamplifiers, repeaters and preamplifiers to enhance the performance of fiber-optic systems are discussed.
Gain characteristics of an Er3+-doped fiber for high-power picosecond input pulses are studied with an InGaAsP laser diode pump source at 1.46-1.48 μm. The output energy and peak power of the amplified pulses reach as high as 7.9 pJ and 792 mW for a repetition rate of 100 MHz and a pulse width of 10 ps. The gain saturation is so slow that the gain in high speed pulse transmission systems is determined by a steady-state saturated gain. With the Er3±-doped fiber amplifier, it is shown that solitons can be amplified and transmitted over a long dispersion-shifted fiber by using the dynamic range of an N=1 soliton. Furthermore, optical solitons at wavelengths of 1.535 μm and 1.552 jim have been amplified and transmitted simultaneously over 30 km with an Er3±-doped fiber repeater for the first time. The collision experiments between these different wavelength solitons are described. It is shown that there is a saturation-induced cross talk between multi channel solitons, and the cross talk (the gain decrease) is determined by the average input power in high bit-rate transmission systems. Subpicosecond soliton and 20 GHz soliton pulse amplifications with Er3±-doped fiber are also described, which indicate that Er fibers are very advantageous for short pulse soliton communication. Finally, a gain coefficient as high as 2.4 dB/mW is reported using InGaAsP laser diodes.
Two fiber laser sources, a resonant fiber laser (RFL) and a superfluorescent fiber laser (SFL), have been given initial tests as gyro sources using a medium quality gyro test bed. The RFL reacted strongly to optical feedback from the gyro circuit resulting in very large unstable errors in the gyro output. These were suppressed substantially by an optical isolator which reduced feedback from the gyro, or by a phase modulator within the laser cavity. The SFL was found to be free of errors of this type when operated at output power levels below a threshold level (about 0.5 mw) for self oscillation due to external optical circuit reflections.
The applications of Raman amplifiers as repeater amplifiers or post transmitter amplifiers in a high density WDM system are theor-etically investigated. The limitation of amplifier gain and the existence of an optimum pump power which results in maximum amplif-ier gain are present. Based on the results, amplifier gains up to 50 dB and 20 dB are achievable for the uses as a repeater and a post transmitter amplifier respectively.