Abstract:
A laser is characterized by a glass gain medium which contains trivalent samarium Sm 3+  ions and which, when optically pumped on one or a number of absorption bands to achieve excitation at or above the metastable  4  G 5/2  level, radiates light at a wavelength substantially corresponding to transitions from the  4  G 5/2  level of samarium.

Description:
TECHNICAL FIELD 
     The present invention relates to the general subject of glass lasers, and, in particular to an apparatus for producing visible lasing emission from a rare earth doped optical fibre. 
     BACKGROUND OF THE INVENTION 
     Glass lasers were invented in 1961 (Snitzer E., &#34;Optical Maser Action of Nd 3+  in a Barium Crown Glass,&#34; Phys Rev., 1961, 7, p 444) and are now widely used. Glass lasers are advantageous because they have high gain, long fluorescence life times and can be simply made in a variety of geometries. 
     There is a reported observation of visible laser action from a samarium doped crystal (Kazakov B. N., Orlov M. S., Petrov M. V., Stolov A. L., Tkachuk A. M.; &#34;Induced Emission of Sm 3+  Ions in the Visible Region,&#34; Opt. Spectrosc (USSR) 1979, 47, p 676-677). However, laser emission due to the  4  G 5/2  to  6  H 9/2  transition in Sm 3+  ions has not previously been observed in any material, particularly glass. 
     SUMMARY OF THE INVENTION 
     The glass laser of the invention is characterised by a glass gain medium which contains trivalent samarium Sm 3+  ions and which, when optically pumped on one or a number of absorption bands to achieve excitation to the metastable  4  G 5/2  level, radiates light at a wavelength substantially corresponding to transitions from the  4  G 5/2  level of samarium. 
     The laser of the invention is a glass laser which is capable of producing visible red light, as well as yellow, orange and infrared light. According to a preferred embodiment of the invention, the glass gain medium is an optical fibre waveguide containing Sm 3+  ions in its core, the cladding, or both. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described in detail, by way of example, with reference to the drawings, in which: 
     FIG. 1(a) is an energy level diagram; 
     FIG. 1(b) is an absorption spectrum for samarium-doped silica; 
     FIG. 2 shows the fluorescence spectrum of silica glass doped with Sm 3+  ions; 
     FIG. 3 is a schematic diagram of a Fabry-Perot resonator forming the basis for an operative glass laser; 
     FIG. 4 shows the lasing characteristics of a glass laser based on the resonator of FIG. 3; and 
     FIG. 5 shows an oscilloscope trace of a Q-switched pulse from the laser of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The optical properties of silica doped with Sm 3+  ions are illustrated by the energy level diagram of FIG. 1(a) and by the absorption spectrum of FIG. 1(b). Samarium Sm 3+  doped silica has been found to be a particularly attractive lasing medium by virtue of the long meta-stable life-time combined with the high branching ratio and narrow line-width (3.3 nm) of the radiative transition (see FIG. 2). These properties are more usually a characteristic of rare-earth ions in crystals rather than an amorphous glass host. 
     An operative glass laser based on a Fabry-Perot resonator is shown in FIG. 3. Dielectric mirrors 1 and 2 have reflectances at the laser wavelength of 651 nm of 99% and 60% respectively. Both mirrors have reflectances less than 10% at the pump wavelenght 488 nm. A 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. 
     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.; &#34;Neodymium-doped silica single mode fibre lasers,&#34; Electron. Lett., 1986, 21, pp 738-740) has produced a single mode glass fibre laser with a threshold below 1 mW. Preferably, therefore, the glass laser of the invention is made from single mode silica fibre doped with samarium. 
     In particular, a silica optical fibre preform, containing more than 80 mole % silica (SiO 2 ), is doped with 250 molar ppm of Sm 3+  ions using a solution doping process (Stone, Burrus; &#34;Nd-doped SiO 2  Lasers in End Pumped Fibre Geometry,&#34; Appl Phys Lett., 23, pp 388-389). The preform is pulled into an optical fibre with a numerical aperture of 0.18 and a single mode cut off wavelength of 620 nm using well-known technology. 
     A fibre doped with approximately 250 ppm Sm 3+  ions has large absorption (1000 dB/km) at a pump wavelength of 488 nm, which corresponds to a transition from the  6  H 5/2  ground state to the  4  I 9/2  level. Other wavelengths may be used to pump the laser to levels above the meta-stable  4  G 5/2  level. Referring to FIG. 1(b), the absorption band centered at 564 nm has a line width of only 2.2 nm, which is narrower than the line-width of any other trivalent rare-earths in silica. 
     Referring to FIG. 1(a), the atoms decay down to the meta-stable level which has a fluorescence lifetime of 1.68 ms. Stimulated emission of 651 nm light occurs between the meta-stable level and the  6  H 9/2  level. This is a four level laser transition. Laser action may also occur from the meta-stable level to the other  6  H levels, although these should be less efficient than the main transition. The fibre has a low loss (50 dB/km) at the lasing wavelength. 
     Optical pumping is achieved by coupling in up to 1 W of optical power at 488 nm from an argon-ion laser 4 using a microscope objective 5. Red laser light 6 is emitted from the output mirror 2 when threhold is reached at 20 mW of pump power. 
     The lasing characteristic of the fibre laser of FIG. 3 is shown in FIG. 4. Fibre losses reduce the slope efficiency of the device to 12%. The threhold is reduced to 2.3 mW by increasing the reflectivity of the output mirror 2 to 93%. 
     From the foregoing description, it will be observed that numerous variations, alternatives and modifications will be apparent to those skilled in the art. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. Various changes may be made, materials substituted and features of the invention utilized. For example, pulse Q-switched operation of the laser can be achieved by inserting a lens and a rotating chopper into the laser cavity (See FIG. 3). The loss of the cavity is modulated with the chopper wheel or another type of modulator. In this way pulses of peak power 8 W and width 144 ns have been achieved (See FIG. 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. Thus, it will be appreciated that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is, of course, intended to cover by the appended claims all such modifications involved within the scope of the claims.