Single frequency lasers are particularly useful in optical communication systems, fibre sensors, and spectroscopy as narrow linewidth sources. There has been increasing interest in developing single longitudinal mode fibre lasers.
One approach to obtain single longitudinal mode operation is to use a distributed feedback structure (DFB). As an example, reference is made to H Kogelnik and C V Shank, "Coupled-wave theory of distributed feedback lasers", Journal of Applied Physics vol. 43, no. 5, pp. 2327-2335, 1972. Recently, DFB fibre lasers have been demonstrated, using in fibre gratings directly written into rare earth doped fibres [J. T. Kringlebotn, J-L Archambault, L. Reekie, and D. N. Payne, "Er.sup.3+ :Yb.sup.3+ -codoped fibre distributed-feedback laser", Optics Letters., vol. 19, no. 24, pp. 2101-2103, 1994 and A. Asseh, H. Storoy, J. T. Kringlebotn, W. Margulis, B. Sahlgren, S. Sandgren, R. Stubbe, and G. Edwall, "10 cm Yb.sup.3+ DFB fibre laser with permanent phase shifted grating", Electron. Letters., vol. 31, no. 12, pp. 969-970, 1995]. Using fibre DFB structures permits all fibre devices and wavelength selectivity over a wide range. However, the use of short pieces of heavily doped fibre has a disadvantage of low slope efficiency and large linewidth. Another approach to obtain single longitudinal mode operation is to use the travelling wave operation of ring fibre lasers [G. J. Cowle, D. N. Payne, and D. Reid, "Single-frequency travelling-wave erbium-doped fibre loop laser", Electron. Letters., vol. 27, no. 3, pp. 229-230, 1991]. Long cavity lengths of ring lasers provide narrow linewidth operation. However, single longitudinal mode operation of ring lasers reported to date has been achieved mostly with pigtailed non-fibre intracavity elements.
Suppression of spatial hole burning by internal modulation of the laser cavity [T. Stolte and R. Ulrich, "Er-fibre lasers: suppression of spatial hole burning by internal modulation", Electron. Lett., vol. 29, no. 19, pp. 1686-1688, 1993] can be used to ensure single longitudinal mode operation. However, it also requires pigtailed non-fibre intracavity elements.
Further, single longitudinal mode narrow linewidth operation can be achieved using injection locking [J. D. C. Jones and P. Urquhart, "An injection-locked erbium fibre laser", Optical Communications., vol. 76 no. 1, pp. 42-46, 1990]. However, this approach requires an external single longitudinal mode narrow linewidth laser.
The disadvantage of using a coupled cavity fibre laser incorporating fibre Bragg gratings [S. V. Chernikov, J. R. Taylor, and R. Kashyap, "Coupled-cavity erbium fibre lasers incorporating fibre grating reflectors", Opt. Letters., vol. 18, no. 23, pp. 2023-2025, 1993] or Fox-Smith fibre lasers [P. Barnsley, P. Urquhart, C. Millar, and M. Brierley, "Fiber Fox-Smith resonators: application to single-longitudinal-mode operation of fibre lasers", Journal of the Optical Society of America vol. 5, no. 8, pp. 1339-1346, 1988] is that these approaches require several perfectly matched gratings, although the approaches permit all fibre devices.
A saturable absorber [M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, "Narrow-linewidth, singlemode erbium-doped fibre laser with intracavity wave mixing in saturable absorber", Electron. Letters., vol. 30, no. 8, pp. 648-649, 1994] can be used to achieve single longitudinal mode narrow linewidth operation. However, the absorber increases the lasing threshold and reduces slope efficiency of the laser.