GLASS LASER

Abstract: A glass laser includes a glass optical gain medium containing samarium <3+> ions. The laser is capable of producing visible light as well as infrared emission.

Description: GLASS LASER Ihe present invention relates to a glass laser. Glass lasers were invented in 1961 (Snitzer E., "Optical maser action of Nd 3+ in a barium crown glass", Phys Rev., 1961,7,p444) and are now widely used. However all glass lasers to date have operated in the infrared region of the optical spectrum. The glass laser of the invention is characterised in that the glass gain medium oontans samarium Sm ions. The laser of the invention is a glass laser capable of producing visible red light, but which may also produce yellow, orange and infrared emission. Glass lasers are advantageous because they have high gain, long fluorescence life times and can be simply made in a variety of geometries. The samarium ion Sm is the active ion of the laser of the invention. Samarium doped glass has not previously been used to produce laser emission from a glass. There is only one reported observation of visible laser action from a samarium doped crystal (Kaza ov B.N. , OrlovM.S., Petrov M.V., StolovA.L., Tkachuk A.M.; 'Induced emission of Sm 3+ ions in the visible region 1 , Opt. Spectrosc (USSR) 1979, 47, p676-677.). Furthermore, laser emission due to the 4 G 5 , 2 to "9/2 transition in Sm ions has not previously been observed in any material. Preferably, a waveguide confines the optical field to a high intensity region which results in a lower threshold for laser action. Recent work (Mears R.J., Reekie L., Poole S.B., Payne D.N.; "Neodymium-doped silica single mode fibre lasers", Electon. Lett., 1986, 21, pp738-740.) has produced a single mode glass fibre laser with a threshold below lmW. Preferably, therefore, the glass laser of the invention is made from single mode silica fibre doped with samarium. An e±odiment of the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1(a) is an energy level diagram; and Figure 1(b) is an absorption spectrum for Samarium doped silica; Figure 2 shows the fluorescence spectrum of silica glass doped wth Sm ions; Figure 3 is a schematic diagram of a Fabry-Perot resonator forming the basis for an operative glass laser; Figure 4 shows the lasing characteristics of a glass laser based on the resonator of Figure 3; and Figure 5 shows an oscilloscope trace of a Q-switched pulse from the laser of Figure 3. As indicated above, the invention relates to an apparatus for producing visible lasing emission from a rare earth doped glass. A silica optical fibre preform is doped with 250 molar pp of Saπariu ions using a solution doping process (Stone, Burrus "Nd- doped SiC>2 lasers in end pumped fibre geometry", Acpl Phs Lett., 23 pp 388-389). The preform is pulled into an optical fibre with a numerical aperture of 0.18 arxi a single mode cut off wavelength of 620nm using well known technology. The optical properties of silica doped with Sm 3+ ions are illustrated by the energy level diagram Fig. 1(a) and absorption spectrum at Figure 1(b) . A fibre doped with approximately 250ppm Sm 3+ ions has a large absorption (lOOOdB/km) at the proposed pump wavelength (488πm) which corresponds to a transition from the 5 ^ ground state to the Ig/2 level. Other wavelengths may be used to pump the laser to levels above the eta-stable G 5 /2 level. It is interesting to note that the absorption band centred at 564πm has a line width of only 2.2πm, which is narrower than the line-width of any other trivalent rare-earths in silica. The atoms decay down to the meta-stable level which has a fluorescence lifetime of 1.68ms. Stimulated emission of 651nm light occurs between the meta-stable level and the ^9/2 level. This is a four level laser transition. Laser action may also occur from the meta-stable level to the other levels although will be less efficient than the main transition. The fibre has a low loss (50dB/km) at the lasing wavelength. Samarium 3+ doped silica is a particularly attractive lasing medium by virtue of the long meta- stable life-time combined with the high branching ratio and narrow line-width (3.3nm) of the radiative transition (see figure 2). These properties are more usually a characteristic of rare-earth ions in crystals rather than an amorphous glass host. An operative glass laser can be based on a Fabry-Perot resonator as shown in Figure 3. Dielectric mirrors 1 and 2 have reflectances at the laser wavelength of 651πm of 99% and 60% respectively. Both mirrors have reflectances less than 10% at the pump wavelength 488πm. The samarium doped fibre 3 is cleaved and butted up to the mirrors. The mirrors 1 and 2 provide optical feedback at the lasing wavelength and the fibre provides the gain medium. Optical pumping is achieved by coupling in up to 1 of optical power at 488nm from an argon-ion laser 4 using a microscope objective 5. Red laser light 6 is emitted frαn the output mirror 2 when threshold is reached at 20πW of pump power. The lasing characteristic of the fibre laser of Figure 3 is shown in Figure 4. Fibre losses reduce the slope efficiency of the device to 12%. The threshold is reduced to 2.3ιrW by increasing the reflectivity of the output mirror 2 to 93%. Pulsed Q-switched operation of the laser is achieved by inserting a lens and a rotating chopper into the laser cavity (Figure 3). The loss of the cavity is modulated with the chopper wheel or another type of modulator. In this way pulses of peak power 8W and width 144ns have been achieved (Figure 5) . It would also be possible to modulate the cavity loss at a frequency corresponding to the cavity round trip rate. This would produce short mode-locked pulses.

Claims: CT AIMS 1. A glass laser characterised in that the glass gain medium cont ins samarium S ions. 2. A laser as claimed in claim 1 wherein the glass gain iredium contains more than 80mol% silica. 3. A laser as claimed in claims 1 or 2 wherein the gain medium is an optical fibre waveguide containing samarium Sm 3+ ions in the core, cladding or both. 4. A laser as in claim 3 wherein the optical fibre waveguide is a single-mode design at a wavelength of 651nm. 5. A laser according to any preceding claim in which the optical gain medium is produced by the solution doping technique. 6. A laser according to any preceding claim which emits visible radiation.

Inventors: FARRIES MARK CUNNINGHAM, MORKEL PAUL ROOS, TOWNSEND JANET ELIZABETH, FARRIES, MARK, CUNNINGHAM, MORKEL, PAUL, ROOS, TOWNSEND, JANET, ELIZABETH
Applicants: UNIV SOUTHAMPTON, THE UNIVERSITY OF SOUTHAMPTON
Application year: 1989
Main classification: H01S3
IPC codes: C03C4, C03C13, H01S3