Rare Earth Fiber Doped Laser

The first rare earth-doped fibres for use as optical amplifiers were developed as early as 1964, only four years after the invention of the laser, when light amplification in a neodymium-doped multimode fibre was reported. Little interest was shown in such devices, however, until the mid 1980s when two key events took place. These were the development of chemical vapour deposition techniques for the manufacture of high quality doped fibre preforms, and the recognition that erbium-doped fibre could form the heart of an optical amplifier for telecommunications systems whose capacity was being constrained by the limited speed of electronic repeaters. Since 1987 a huge amount of research has been carried out worldwide into the development and applications of rare earth-doped fibre amplifiers and lasers. Most of this has been concerned with erbium-doped silica fibre, which operates at around 1550 nm, the commercially important telecommunications window, but a number of other rare earths and hosts have also been considered.

Mode-locked fibre lasers

As well as simply providing gain to overcome loss in fibre transmission systems, rare earth-doped fibre can be used as the gain medium in all-fibre laser configurations. As erbium (Er) has a relatively broad gain bandwidth of ~50 nm at around 1550 nm, it has formed the basis of many mode-locked laser schemes for the generation of ultrashort pulses. The most successful of these have been passively mode-locked systems in which the nonlinear refractive index of the fibre provides the mode-locking mechanism (eg through nonlinear polarisation rotation or in a nonlinear loop mirror). In such systems fibre nonlinearity and dispersion can also act together to provide pulse shortening through soliton-like pulse compression, leading to the generation of extremely short pulses. We have carried out a substantial amount of research into the development, optimisation and characterisation of such lasers. Through our investigations we have shown, both theoretically and experimentally, that a limiting process in soliton fibre lasers (as in periodically amplified communications systems) is the shedding of energy from the pulses to a dispersive continuum that occurs when the laser cavity length (or amplifier spacing) becomes comparable to the characteristic soliton period of the pulses. In a laser this shed energy remains in the cavity and interference occurs between radiation shed on different round trips, leading to the build up of sidebands on the soliton spectrum. As soliton pulses in the laser compress, more and more energy is shed from the pulse spectrum, and this acts to limit the shortest pulses that can be generated from any given system.

Another rare earth of interest is praseodymium (Pr), as it is capable of providing gain at around 1300 nm, another low loss window in silica fibre and also the wavelength of minimum dispersion. Pr doped into a silica host has a very short upper-state lifetime, however, due to nonradiative decay processes. To overcome this problem heavier, fluoride-based glass hosts are generally used. Although this extends the upper-state lifetime by a factor of ~100 it introduces other problems associated with the material properties of the glass, including a low melting point which makes fusion splicing to silica fibres almost impossible. Despite this Pr doped fluoride fibre has attracted considerable attention and we have carried out some work on Pr-doped fibre lasers, including the first experimental report of femtosecond pulse generation from a mode-locked Pr-doped fibre laser.

High pulse energy fibre lasers

Although much of the work that has been carried out on fibre lasers has been done so in the name of telecommunications applications, particularly as pulse sources, it is now clear that some competing technologies may offer considerable advantages of simplicity, robustness and ease of use. Fibre lasers do represent compact, relatively efficient sources capable of average output powers of several watts, however, and it seems likely that they will find many applications as such. Recently we have been focussing our attentions on the development of systems for a variety of applications, with the emphasis being on the generation of high energy pulses and broad bandwidths.

We have discovered a new mechanism which produces passive Q-switching in fibre lasers, resulting from distributed backscattering from the fibre into the gain medium. This can occur in a linear configuration, but in our experiments is enhanced through the addition of an all-fibre loop interferometer, as shown in the schematic in figure 1. The resulting Q-switching behaviour is relatively stable with repetition rates in the range 1-20 kHz, depending on pump power. It can be made extremely stable, however, if the pump power to the laser is resonantly modulated.

Figure 1: Schematic of Q-switched fibre laser configuration.

This technique has been successfully applied both to erbium-doped and ytterbium-doped fibre lasers. In the erbium-doped case (operating around 1545 nm) pulses with a duration of 5-15 ns were generated with peak powers of ~100 W at repetition rates tunable in the range of 100s of Hz to 20 kHz. With the ytterbium-doped laser pulses shorter than 2 ns were generated with an average output power of 1 W at 1060 nm. The pulse energy was estimated to be ~0.05 mJ, resulting in peak powers of ~ 10 kW. Such high power is sufficient to generate a Raman supercontinuum in even a short length of optical fibre, and figure 2 shows the spectrum generated at the output of a diode-pumped Yb-doped fibre laser with an average output power of 450 mW. The fundamental lasing wavelength is ~1060 nm; however most of the energy is is converted into the supercontinuum covering the whole infrared range of transparency of the fibre up to 2.3 microns.

Figure 1: Output spectrum from Q-switched diode-pumped Yb fibre laser. The 'fingerprints' of most of the nonlinear phenomena that occur in optical fibres can be observed.

This combination of a broad spectrum and high peak power can also be exploited for frequency doubling to produce tunable radiation in the visible region. Using an unoptimised angle-tuned phase-matched LiIO₃ crystal we have generated light across the whole visible spectrum, as shown in figure 3.

Figure 3: Q-switched ytterbium fibre laser (front), second harmonic crystal and output projected on a screen.

Selected relevant publications

N. Pandit, D. U. Noske, S. M. J. Kelly and J. R. Taylor: 'Characteristic instability of fiber loop soliton lasers', Electronics Letters, 28, pp.455-457 (1992)

D. U. Noske, N. Pandit and J. R. Taylor: 'Source of spectral and temporal instability in soliton fiber lasers', Optics Letters, 17, pp.1515-1517 (1992)

D. U. Noske, N. Pandit and J. R. Taylor: 'Subpicosecond soliton pulse formation from self-mode-locked erbium fiber laser using intensity dependent polarization rotation', Electronics Letters, 28, pp.2185-2186 (1992)

M. J. Guy, D. U. Noske, A. Boskovic and J. R. Taylor: 'Femtosecond soliton generation in a praseodymium fluoride fiber laser', Optics Letters, 19, pp.828-830 (1994)

D. U. Noske, N. Pandit and J. R. Taylor: 'Time-resolved spectral development of ultrashort-pulse solitons in erbium fiber loop lasers', Optics Communications, 115, pp.105-109 (1995)

S. V. Chernikov, J. R. Taylor, D. V. Gapontsev, B. L. Davydov, I. E. Samartsev and V. L. Gapontsev: 'Q-switching of Er/Yb-doped fibre laser using backscattering from a fibre ring interferometer', Technical digest of CLEO '96, pp.529-530 (1996)

S. V. Chernikov, Y. Zhu, J. R. Taylor, N. S. Platanov, I. E. Samartsev and V. P. Gapontsev: '1.08-2.2 micron supercontinuum generation from Yb-doped fibre laser', Technical digest of CLEO '96, p. 210 (1996)

S. V. Chernikov, Y. Zhu, J. R. Taylor and V. P. Gapontsev: 'Supercontinuum self-Q-switched ytterbium fibre laser', to be published in Optics Letters

 

We would like to thank Ire Polus Group for the generous loan of some of the equipment used in this work.