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The application of the recently developed erbium-doped optical-fibre amplifier to pre- and post-amplification of optical signals is described. Typical performance is >20dB gain over a 300GHz bandwidth for a 20mW pump.

Optical amplifiers are of interest as wideband in-line repeaters for telecommunications and as signal regenerators for a variety of sensor applications. Much current research has concentrated on semiconductor laser amplifiers which are difficult to splice to fiber systems. It is clear that an amplifier consisting of a special optical fiber which is compatible with telecommunication fiber would overcome this problem. This paper describes the first results of an optical fiber amplifier based on an Er3+-doped fiber which has a maximum gain at a wavelength of 1.536 µm. A single-pass gain of 26 dB and a maximum output of 13 dBm at 140 MHz has been demonstrated.

Fibre lasers present numerous advantages over other lasers not only in terms of low threshold, efficiency and low operating power but also with respect to their compatibility with standard single-mode fibres. Lasing action has been demonstrated in rare-earth doped silica single-mode fibres with both semiconductor laser and argon ion/dye laser pump sources. Semiconductor laser-diode pumping is highly convenient both in terms of cost and operational requirements. We describe here the characteristics of a single-mode Nd3+-doped fibre laser operating at 1.088µm. In particular results are presented for CW, Q-switched and mode-locked operation.

Fabrication and absorption/fluorescence properties of several rare-earth-doped fibres are reported. A 7.4 m long Er3+ fibre laser emitting at wavelength of 1.536 µm has been constructed

There is currently considerable interest in lasers based on the fiber waveguide geometry, and crystal fiber Nd:YAG lasers are now commercially available. We recently developed a new fiber laser technology based on doped glass which offers both a choice of dopant ions and the possibility of wavelength tunability. The Er3+-doped fiber laser is particularly interesting because its operational wavelength of 1.55µm coincides with the lowest loss region of silica optical fibres. Furthermore, the fiber can be used as a high-gain optical amplifier, which is compatible with quaternary (InGaAsP) devices.

A laser medium in the form of a single mode optical fibre offers a number of attractive features. For example, using a laser to end pump such a fibre provided with feedback mirrors, allows a very low oscillation threshold to be attained. In the case of Nd doped fibre a GaAs diode laser can be used as the pump, thus providing a very simple and compact laser device. The low threshold performance also suggests that other, weaker transitions may be capable of laser action eg. the 1.3µm 4F3/2 to 4I13/2 transition in Nd or transitions in other dopants which have not previously shown laser action in a glass host. Other possible applications include the use of fibres as amplifiers e.g. as an in-line amplifier in an optical communication system where it could play the role of a repeater. As a power amplifier the fibre device could have advantages over the bulk glass systems by reducing the problems of thermal distortion and thermal fracture. In this paper we report some results obtained with neodymium-doped single mode fibres.

One of the requirements of an optical fibre amplifier or laser is that it should have a high absorption at the pump wavelength and a very low loss at the laser wavelength. Conventionally, this has been achieved by using a high dopant concentration in a short length of neodymium-doped compound-glass or silica fibre. However, an attractive alternative approach is to use lower dopant levels and thus exploit the long interaction lengths and very low losses inherent in communications-grade optical fibres. We report here a novel extension of the MCVD fabrication process which allows the fabrication of both mono- and multimode optical fibres containing rare-earth ions at concentrations of up to 0.25 wt% in the core region. The technique is unique in that it allows the use of starting materials, e.g. rare-earth halides, which have a high melting point ( > 580°C) and hence have hitherto been unusable, since they exhibit a very low vapour pressure at the temperatures commonly encountered in reactant delivery systems for optical fibre fabrication. Initial work has concentrated on the lanthanide series as dopants, particularly neodymium and erbium, as these are of interest for both sensors and lasers. We have produced fibres with very high-loss absorption bands ( > 3000 dB/km) in the visible and near infra-red regions, while maintaining the characteristic low loss ( < 2 dB/km) of the MCVD fabrication process in the 'second window' for optical communications (1300nm). The technique may also be used to incorporate many other rare-earth and transition metals into optical fibres.

The rare-earth doped single-mode fibre laser has emerged as a new source of laser radiation in the near infrared region of the spectrum. Using an Ar+-ion, dye or semiconductor laser as a pump source, low threshold CW or pulsed operation is possible over a wide range of wavelengths. Low threshold CW laser emission is also possible on 3-level transitions at room temperature with no auxiliary cooling requirements. As an example, laser operation has been obtained at 1085nm on the 1G4 - 3H4 (groundstate) transition of Pr3+-ions in glass. The excitation source was a CW Rh6G dye laser operating at 590nm, although such was the absorption of the Pr3+-doped fibre that most of the Rh6G emission spectrum could be used for pumping. Using a cleaved fibre butted to two dielectric mirrors having high reflectivity at the lasing wavelength, CW threshold was found to occur for 10mW of absorbed power at 590m. Q-switching has been obtained using an intracavity acousto-optic modulator. In preliminary experiments using a non-optimised cavity, we have obtained pulses of 500ns duration and >1W peak power at a repetition rate of several kHz. Using a more efficient configuration, over an order of magnitude increase in peak power is anticipated. By replacing the output mirror with a diffraction grating blazed at 1µm, a 61m tuning range from 1048nm - 1109nm has been obtained. Using an alternative pump source, visible operation of this laser may be possible.

We report the first observation of optical bistability in a rare-earth doped single-mode fibre laser. Hysteresis has been obtained in the lasing characteristic of an Er3+-doped fibre laser operating at 1.54µm using the mechanism of saturable absorption

In many single-mode optical fibre sensors, in particular those based on fibre interferometers, a single-mode 4-port directional coupler is needed to transfer light from one fibre to another. Although the coupling can be achieved with discrete components, a more stable and robust system with a lower insertion loss can be obtained if a fibre coupler is used.

Single-mode fibres containing several different rare-earth dopants have been fabricated. Characteristics of the fibres, including absorption and fluorescence spectra, fluorescence lifetimes and uniformity of dopant incorporation have been measured and the results obtained and their influence upon the performance of these fibres as fibre-lasers and amplifiers will be discussed. Use of these fibres as tunable CW lasers centred an 1.088µm (Nd3+-doped fibres) and 1.54µm (Er3+-doped) with tuning ranges of 80nm and 25nm respectively are described. High peak-power output pulses have also been obtained by operating fibre-lasers in a Q-switched mode, with maximum peak-power of 2W in a 1µs pulse for a 200Hz pulse repetition rate.

A recent enhancement of the MCVD process has permitted the fabrication of long lengths of both mono- and multi-mode optical fibres containing low concentrations (1dB/m) whilst maintaining the low-losses (