Fibre Lasers

Abstract

INTRODUCTIONThe first fibre laser, described by Snitzer!, was also the first glass laser. The glass fibre, doped with trivalent neodymium ions (Nd3+) was transversely pumped by a flash lamp. The fibre geometry was chosen to exploit the brightness enhancement of pump light in the core after passing through the clear cladding, and also to provide a structure which was more forgiving of the optical inhomogeneities of the doped glass 2 . These early fibres clearly demonstrated the capability for very high gain, with 47dB measured 3 at 1.063~m, even if the transverse pumping geometry was of very low efficiency.The next major development was the demonstration that longitudinal pumping, using a laser as the pump source and end-launching the pump light into the doped core, gave greatly reduced threshold, and high efficiency4. In this way cw operation was easily obtained and it was recognised that pumping of Nd-doped fibre using laser diodes operating at -O. 8~m would provide a simple and practical form of fibre laser.The third major development, coming 12 years later, and which initiated an enormous growth of interest in fibre lasers and amplifiers, was the demonstration of lasing and amplification in rare-earth-doped mono mode silica fibres 5 ,6. The erbiumdoped fibre amplifier (EDFA) has been the main reason for this explositon of interest. Its success is based on the fact that erbium-doped silica fibre offers an amplifying transition 7 at -1.5~m, matching perfectly to the third telecommunications window, in a fibre which is compatible with telecom fibre, and which can be readily pumped by a diode laser to give high gain (in excess of 30dB, i.e. an intensity gain of 1000). These, and other attractive features of EDFAs have revolutionised the field of optical fibre communications.While the EDFA has received the greatest share of attention, there is also a growing appreci.ation that fibre lasers and amplifiers have a great deal more to offer. For example, as a result of developments in fibres based on heavy metal fluoride Solid State Lasers' New Developments and ApplicatIOnsEdtted by M IngusclO and R Wallen stem, Plenum Press, New York, 1993 231 glasses, there are now seen to be very exciting prospects for compact, visible, sources in the form of blue, green and red fibre lasers pumped by infrared diode lasers 8 -13 • There is also a growing realisation that while fibre lasers may have very low threshold pump powers, this does not imply that they are necessarily low power devices and indeed it has been demonstrated that powers in excess of 1 watt are achievable from diode-pumped fibre lasers l4 • The broadened lines of rare-earth ions in glasses means that significant tuning ranges are available, in some cases l5 broader than the range typically offered by a dye laser, and yet despite this broad linewidth it has been possible to demonstrate very narrow linewidth operation (a few kHz) in single-frequency fibre lasers16. By contrast, on the other hand, the high gain can allow "mirrorless lasing", resulting in a broad, mode...