OPTICAL FIBRE AMPLIFIER AND LASER

Abstract: An optical fibre amplifier comprises a thulium-doped optical fibre (2) pumped at 790 nm by a semi-conductor diode laser (20) coupled to the fibre (2) via optical fibre coupler (12). The amplifier (2) is optically coupled in series to a pair of systems fibres (18 and 14) to provide amplification to optical signals.

Description: OPTICAL FIBRE AMPLIFIER AND LASER This invention relates to optical fibres amplifiers, lasers and optical communications systems incorporating them. It relates particularly to those based on 3+ transitions associated with the Tm ion in a fluorozirconate optical fibre host. A consequence of the reduced phonon energies in fluorozirconate glass hosts when compared with silica based glasses is that the lifetimes of many of the energy levels of rare earth ions doped into the matrix are significantly increased. This results in a greater number of metastable levels capable of providing an upper population reservoir for laser emission. Coupled with the confinement over a long length and good spatial overlap of the pump and signal modes in a waveguide geometry, this can lead to the realisation of many new laser sources in fibre form. Thulium is a particularly attractive dopant ion for use in a fibre laser since it has a strong absorption band centred at 790 nm, a wavelength which is readily available 3+ from AlGaAs laser diodes. Diode pumped Tm -doped fluorozirconate fibre lasers have already been reported at 1.9μm (Carter J. N. , Smart R. G.. , Hanna D. C. and Tropper A. C. : "C diode pumped operation of a 1.97μm thulium-doped fluorozirconate fibre laser", Electronics Letters, 1990, 26., pp 599-601 and Allen R. and Esterowitz L. : "CW diode pumped 2.3μm fiber laser", Appl. Lett., 1989, 15., pp 721- 722, respectively) and laser emission has also been reported at 820 nm (Allain J. Y. , Monerie M. and Poignant K. : Tunable cw lasing around 0.82, 1.48, 1.88 and 2.35μm in a thulium-doped fluorozirconate fibre", Electronics Letters, 1989, 2_5, pp 1660-1662 on the F 4 - K fi transition when pumped with a krypton ion laser at 676.4 nm. According to the present invention an optical fibre amplifier comprises a fluorozmconate optical fibre having 3+ its core doped with Tm and pump means for providing 3+ optical pump power to raise the Tm ions directly to the F 4 level. The transition exploited by the pumping scheme of the 3 3 present invention is the F 4 - H 6 transition which is of particular value as not only can it be pumped by diode lasers, but the gain also falls within the wavelength region of AlGaAs diode lasers. The prospect of high gain amplification and energy storage (hence Q-switching capability) available at AlGaAs diode laser wavelengths suggest that this transition may offer a means for greatly -, increasing the versatility of diode laser sources. The ~F 4 - 3 Hg transition is a particularly favourable transition in ZBLANP glass, as the non-radiative, multiphonon decay rate 3 out of the F 4 level is negligible compared to the radiative rate, and furthermore the branching ratio of this transition dominates the other radiative rates at 2.3μm, and 1.47μm. By contrast, in fused silica non-radiative 3 decay from the F 4 level is rapid (typically giving a lifetime of less than 20μs, whereas we have measured the lifetime in fluorozirconate to be 1.1 ms. Preferably the optical pump power raises the Tm 3+ ions 3 to the upper stark levels of the F 4 level. The pump means may be a diode laser, in particular an AlGaAs laser. According to a further aspect of the present invention a laser comprises a resonant cavity configured to resonate 3 3 at the wavelength of the F 4 - H 6 transition and an optical amplifier as claimed in any preceding claim within the resonant cavity. The resonant cavity may be defined by butting the ends of the fibre amplifier against dielectric mirrors, for example. According to a yet further aspect of the present invention an optical communication system comprises z. optical fibre communications network optically coupled :r.tc an optical amplifier or laser according to either of the first two asnects of the nresent invention. Embodiments of the present invention will now be described and their principle of operation explained with reference to the accompanying drawing of which Figure 1 is a graph of the absorption loss and fluorescence intensity of a Tm doped ZBLANP optical fibre -/; 3+ Figure 2 is a partial energy level diagram for Tm doped ZBLANP glass; Figure 3 is a graph of the output power versus incident pump power at 777 nm for 70% output coupling at 803 nm; Figure 4 is a schematic diagram of an optical communications system of the present invention incorporating an optical amplifier according to the present invention; and Figure 5 is a schematic diagram of a laser according to the present invention. Figure 1 shows both the absorption loss and fluorescence spectra over the range 740-850 nm 3 3 corresponding to transitions between the F 4 and H 6 levels 3+ where the absorption measurements were taken in a bulk Tm - doped ZBLANP glass sample. The fibre 50 (see Figure 5) used in these experiments was of the standard ZBLANP composition, fabricated by a known casting technique details of which are not included here. Reference can be made, however, to an article titled "Progress in fluoride fibres for optical communications" by PW France, SF Carter, MW Moore and CR Day, Electronics Letters, 1988, 2_i, pp 1222-1223. The fibre was doped with 1000 ppm by weight of thulium (Tm 3+) ions and had a core diameter of 6μm with a cut-off wavelength of 1.6μm pumped at 780 nm by a Ti: sapphire laser 52. The absorption loss at the 790 nm peak is calculated to be 24 Db/m for the concentration m the fibre. The fluorescence spectrum was obtained by pumping the fibre with light at 784 nm (into the upper Stark levels cf ~F , from a Ti: sapphire laser, and observed in side light so as 3 to avoid distortion by reabsorption. Ions in the F 4 level relax almost totally radiatively with calculated branching to the 3 H 5 , 3 H 4 and 3 H 6 levels of 3%, 9% and 89% respectively. The calculation is similar to that in Guery 3+ C. , Adam J. L. and Lucas J : "Optical properties of Tm ions in indium-based fluoride glasses", J Luminescence, 1988, 42. pp 181-188, but carried out here with parameters appropriate to ZBLANP glass. Decay giving rise to photons at around 800 nm is therefore the preferred route and the one exploited in this scheme. For laser operation a standard Fabry-Perot cavity was formed by butting the cleaved ends 46, 48 of a 0.7 m long fibre 50 up to two dielectric mirrors 54, 56 and flooding the contacts with index matching fluid 58. Pump light ' from a tunable Ti: sapphire laser 62 was coupled into the fibre using a xlO microscope objective lens 64. The input mirror 54 was highly reflecting (>99%) at wavelengths greater than 805 nm, had a maximum transmission of 90% at 777 nm and a steep transmission edge between 790 nm and 800 nm (where transmission was 80% and 5% respectively). These figures are quoted here for transmission in air and may be significantly degraded when the mirror/fibre butt is in index matching fluid. Continuous laser emission was observed at 806 nm corresponding to the 3F 4 -3H 6 .transition. Lowest threshold operation of 20 Mw of pump power incident on the launch optics (and therefore an estimated 6 mW launched into the single mode fibre) was achieved with a similar HR mirror 56, on the output and with the pump laser 62 tuned to 777 nm and the peak transmission of the input mirror. Excitation was therefore into the high energy side 3 of the F 4 level, and emission from this level to the ground Kg level, as shown in Figure 2. With this configuration of two highly reflecting mirrors 54, 56 at the signal wavelength, for an incident pump power of 650 mW, the maximum output power at 806 nm was 5 W. However, when the output mirror 56 was changed for a mirror with a shifted, but similarly sharp, transmission edge (so that the cavity now had feedback in air of 90%, 30% and 95% at the pump wavelength, 805 nm and 820 nm respectively) laser emission was observed continuous-wave at 803 nm and also pulsed (self terminating) at 823 nm, with threshold pump powers of 80 mW and 95 W respectively. The self-terminating nature of the 823 nm laser emission under these particular conditions was unexpected since this is a three level transition. It may be due to a number of processes, in particular the excited 3 1 state absorption of pump/signal photons on the H 5 - G 4 transition. The variation of output power at 803 nm with pump power is shown in figure 3. The slope efficiency is 15% with respect to incident pump power (45% w. r. t. launched power) and for a maximum power of 900 mW incident on the launch optics 125 mW of power at 803 nm was obtained. The scattering loss in the fibre was measured at Iμ and around 1.5μm to be a few dB/m while the exact loss at the signal wavelength could not be easily determined owing to the absorption at around 800 nm. The high threshold may be an indication of a loss in the cavity arising from either the intrinsic scattering loss of the fibre or high butt losses at the fibre/mirror interfaces. This laser scheme is of particular interest in that the excitation and output wavelengths are both within the AlGaAs laser diode range. Combined with the recently demonstrated lasing at 850 nm when pumped by excited state 3+ absorption of photons at 801 nm in Er -doped ZBLANP fibres (Millar C. A. , Brierley M. C. , Hunt M. H. and Carter S. F. "Efficient up-conversion pumping at 810 nm of an erbium- doped fluoride fibre laser operating at 850 nm" , submitted to Electronics Letters. ), amplification is now available over a wide range of AlGaAs diode laser wavelengths. The 3 3 F 4 - H fi transition in thulium can be expected to be highly efficient in a low loss fibre due to the small Stokes shift between pump and signal photons, the high pump quantum efficiency (since pumping is directly into the upper laser level) and the good spatial overlap between pump and signal modes. That, together with the expected high gain, suggest a number of applications providing a more versatile source at 800 nm than is currently available with diode lasers alone. Attractive schemes include exploiting the energy storage properties of the Fabry-Perot cavity to generate high power Q-switched pulses by the inclusion of an intracavity modulator, using an intracavity tuning element to provide wavelength selection over the range 800-825 nm, providing high gain amplification of a low powered pulsed diode sources or conversion of a low brightness, single transverse mode output. This last possibility could be realised either by using a double-clad fibre where pump light from the diode array is launched into the inner- cladding and is then coupled into the doped core (made to be single mode at the lasing wavelength) along the length of the fibre. A thulium doped fluoride fire pumped as described above can also form the basis of a fibre amplifier, an embodiment of which is shown in the schematic diagram Figure 4. 3+ In Figure 4 a Tm doped fluorozinconate fibre is coupled by splices 4 and 6 to a silica based optical fibre 8 and to a port 10 of a silica-based optical fibre coupler 12, respectively. The fibre 8 is fusion spliced to a silica based optical communications systems fibre 14. Port 16 of the coupler 12 is fusion spliced to a silica-based optical communication systems fibre 18. A route is thereby provided for an incoming signal which is to be amplified from fibre 18, though coupler 12 to the fibre amplifier 2 and to the systems fibre 14 for onward transmission of the amplified signal. The fibre amplifier 2 is pumped by a semiconductor diode 20 coupled to port 22 of the couplers 12 operating in the region of 780 nm.

Claims: 1. An optical amplifier comprising a fluorozinconate optical fibre having its core doped with Tm 3+ and pump means 3+ for providing optical pump power to raise the Tm ions 3 directly to the F 4 level. 2. An optical amplifier, as claimed in claim 1 in which the optical pump power raises the Tm 3+ ions to the upper stark levels of the F 4 level. 3. An optical amplifier as claimed in either of claims 1 and 2 in which the pump means is a diode laser. 4. An optical amplifier as claimed in claim 4 in which the diode laser in an AlGaAs laser. 5. An optical amplifier as claimed in any preceding claim in which the optical fibre is ZBLANP composition. 6. An optical amplifier as claimed in any preceding claim in which the optical fibre is doped with about 1000 ppm weight of Tm . 7. A laser comprising a resonant cavity defined by reflection means and configured to resonate at the 3 3 wavelength of the F 4 - H 6 transition and an optical amplifier as claimed in any preceding claim within the resonant cavity. 8. A laser as claimed in claim 7 in which the reflection means comprise dielectric mirrors. 9. A laser as claimed in claim 8 in which one of the mirrors has a reflectivity greater than 99% for wavelengths greater than 805 nm. 10. A laser as claimed in claim 9 in which said one of the mirrors has a steep transmission edge between 790 nm and 800 nm in air. 11. A laser as claimed in claim 9 and in which both mirrors have a reflectivity greater than 99% for wavelengths greater than 805 nm and a steep transmission edge between 790 nm and 800 nm in air. 12. A laser as claimed in claim 11 in which the optical pump power is at 777 nm. 13. A laser as claimed in claim 10 and in which the other mirror has a reflectivity in air of about 90%, 30% and 95% at 777 nm, 805 nm and 820 nm respectively 14. An optical communication system comprising an optical fibre communications network optically coupled into an optical amplifier or laser as claimed in any preceding claim.

Inventors: CARTER JEREMY NIGEL, HANNA DAVID COLIN, SMART RICHARD GORDON, TROPPER ANNE CHRISTINE, CARTER, JEREMY, NIGEL, HANNA, DAVID, COLIN, SMART, RICHARD, GORDON, TROPPER, ANNE, CHRISTINE
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: H01S3
IPC codes: H01S3, H01S5