IMPROVED OPTICAL FIBRE LASER

Abstract: An optical fibre laser includes an optical fibre having a core (14), an inner cladding (12) surrounding the core and an outer cladding (16) surrounding the inner cladding and core. The core (12) is doped with a first laser-active material, preferably, thulium. The inner cladding is doped with a second different laser-active material, for example, neodymium and is pumped by a multimode pump light source such as a diode array. Pumping of the inner cladding (12) causes laser emission in the inner cladding material which, in turn, serves as pump radiation for the laser-active dopant in the core (14).

Description: IMPROVED OPTICAL FIBRE LASER The present invention relates to monoirode optical fibre lasers and fibres for use therein, in particular, to those comprising a core, an inner cladding surrounding the core and an outer cladding surrounding the inner cladding and core. MDnαmode optical fibre lasers have a doped, active core of very small dimensions, typically, a few microns. The fact that the core only supports one mode because of its small size, is a major benefit and is a necessary feature in most applications of optical fibre lasers and amplifiers. Unfortunately, however, the small core diinension also means that the optical fibre laser can only be pumped effectively by a laser whose beam has essentially perfect spatial properties, that is, a single mode beam. Diode lasers offer a convenient pump source but the power available as a single mode beam from diode lasers is limited. Mich higher power, up to several watts, can be obtained from diode lasers in the form of arrays. However, in such arrays, the beam produced is highly multimode and not suitable for launching directly into the monαmode fibre core. One proposal for dealing with this difficulty is to use cladding puπping. The active doped monαmode core is surrounded by a much larger cladding, which is undoped and, hence, transparent. This cladding is then surrounded by an outer cladding so that any light launched into the inner cladding is guided within the inner cladding in the same way that light is guided in a conventional optical fibre. This arrangement permits a irπαltimode beam, from, say, a diode laser array, to be launched into the inner cladding and then, as it propagates, light from the π-__lti_ατode beam will leak into the monαrtode core. This enables the core to be puπped by a multimode beam, and eventually all of the absorbed multimode power will be converted into a monαmode laser emission within the core. Thus, this arrangement can provide the major benefit of increasing the brightness of a high-powered diode array beam and can, consequently, be used in a large number of applications. In accordance with the invention, the optical fibre laser and fibre for use therein are characterised in that an optical fibre laser having an optical fibre coπprising a core; an inner cladding surrounding the core and an outer cladding surrounding the inner cladding and the core; and a light source, characterised in that t core is doped with a first laser active material, the inner claddi is doped with a second different laser active material and the lig source is a irB-iltimode source arranged to pump the inner cladding s as to cause laser emission therein which, in txtrn, serves as pump radiation for the laser active dopant i the optical fibre core. Thus, the inner cladding can be pumped by ireans of a diode array or any other appropriate ιτ__-lt±-_rιode pump beam. The ends of the fibre are terminated in mirrors, in the usual optical fibre laser configuration, so that lasing occurs in the multimode inner claddi g. Laser light produced in the inner cladding then leaks into the mono ode core, where it serves as the pump radiation for the laser-active dopant in the core. This in turn lases, using th same end mirrors which are designed to have high reflectivity at t laser wavelength appropriate to the inner cladding dopant and the core dopant. This arrangement has a number of benefits over the earlier cladding pumping arrangement utilising a transparent inner claddin Firstly, it permits the pump radiation from the diode array t be absorbed in a much shorter length since it is absorbed directly by the inner cladding rather than by leakage into the core. This shorter length has several advantages, for example, it allows shor pulse operation when Q-switched and it avoids excessive losses. Secondly, the laser intensity in the lasing inner claddi g can be much higher than the intensity of the light from the pump source. The optical fibre laser of the invention is, consequently, particularly well suited to pumping a 3-level laser material in th core. 3-level laser materials need higher pump intensities than 4 levef lasers. Since many of the more potentially useful fibre lasers use three-level materials, this latter advantage is of grea importance. Whilst the benefits outlines above are the most important, there are other ways in which the use of the optical fibre laser o the invention could be beneficial. For example, incoherent lamps could be used to pump the inner core transversely, the inner core would then transfer power through rπultimode laser oscillation int the monαmode core. Ifcnamode output has reasonable efficency usin an incoherent lamp as pump source should, thus, be achievable. An optical fibre laser in accordance with the invention will now be described in detail, by way of example, with reference to drawing which is a schαtatic cross-section through the optical fi of the invention. The optical fibre 10 shown in Figure 1 has an inner cladding doped with neodymium and a mαnαmode core 14 doped with thulium. suitable outer cladding 16 surrounds the inner cladding 12 and co 14. The inner cladding is made to lase by pumping with a diode array (not shown) to provide laser light at l.Oβum. The laser li output by the neodymium pumps the core 14 which itself provides a laser output at 2um. Thus, the optical fibre may provide conversi from the multimode output of the diode array to monαmode output at 2um. There are a number of applications in which the availability a simple all solid state source with a monαmode power level around watt. Since diode lasers themselves are not capable of providing the required beam quality, use of the optical fibre laser of the invention offers a potentially significant breakt_hrough in these applications since the cheapness and reliability of diodes can be exploited but, at the same time, there beam properties can be enhanced without excessive loss of efficiency.

Claims: CLAIMS 1. An optical fibre laser having an optical fibre (10) comprising a core (14); an inner cladding (12) surroundi g the core and an outer cladding (16) surrounding the inner cladding and the core; an a light source, characterised in that the core (14) is doped with first laser active material, the inner cladding (12) is doped with second different laser active material and the liφt source is a _r_- t__mode source arranged to pump the inner cladding (12) so as to cause laser emission therein which, in turn, serves as pump radiation for the laser active dopant in the optical fibre core (14). 2. A laser according to claim 1 in which the multimode pump light source is a diode array. 3. An optical fibre (10) for use in an optical fibre laser, the fibre cσππprising a core (14); an inner cladding (12) surrounding th core, and an outer cladding (16) surrounding the inner cladding and core, the fibre being characterised in that the core (14) is doped with a first laser active material and the inner cladding (12) is doped with a second, different laser active material. 4. Apparatus according to claim 1, 2 or 3 in which the first lase active material is a 3-level laser material. 5. Apparatus according to claim 4 in which the first laser active material is thulium. 6. Apparatus to any preceding claim in which the second laser active material is neodymium.

Inventors: HANNA DAVID COLIN, HANNA, DAVID, COLIN
Applicants: UNIV SOUTHAMPTON, UNIVERSITY OF SOUTHAMPTON
Application year: 1990
Main classification: H01S3
IPC codes: H01S3