Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Patent
1991-03-05
1992-10-20
Epps, Georgia Y.
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
372 23, 372 6, 372 70, 372 75, 372 97, H01S 3091, H01S 3094
Patent
active
051576830
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to laser systems, and particularly to laser systems comprising optical fibre amplifiers.
Single mode optical fibres doped with rare-earth ions are known to exhibit optical amplification in useful regions of the spectrum when longitudinally--pumped using light of a shorter wavelength than that at which the fibres are caused to lase. This pump wavelength corresponds to an atomic absorption of the dopant ion.
As is well known in the art, when a laser is pumped with light at the pump wavelength, the ions in the laser are excited by the pump, and the laser is caused to lase. Not all of the pump light is converted to the output light of the laser, the remainder being known as the remnant pump. The remnant pump is often unusable, and so reduces the efficiency of the system.
A silica or multi component glass fibre doped with a few hundred parts per million of erbium ions is known to show optical gain at approximately 1536 nm. Suitable absorption bands in which to pump the amplifier occur at 540 nm, 650 nm, 800 nm and 980 nm. Some of these pump bands are more efficient than others. This is due to the existance of parasitic excited state absorption (ESA) of pump photons at certain wavelengths for example, erbium in silica glass has no ESA at 650 nm and 980 nm, but has significant amounts at around 800 nm. Much more efficient performance results are achieved, therefore, using a pump wavelength of 650 nm or 980 nm, rather than a pump wavelength of 800 nm.
Unfortunately, there is a scarcity of pump lasers available which are capable of pumping in the 650 nm band. In addition, the 650 nm band is not as quantum efficient as the 980 nm, but there is also a shortage of radiation sources capable of producing an output at around 980 nm. As a result optical fibre amplifiers are generally pumped at around 800 nm by, for example, high-power GaAlAs laser diodes, even though this pump band does not give the most efficient performance results.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a laser system in which a laser may be pumped at a wavelength which has substantially no ESA.
According to the present invention, there is provided, a laser system comprising;
a first laser pumpable at a first and a second wavelength;
a second laser pumpable at the first wavelength whereby it is caused to lase at the second wavelength; the output of the second laser at the second wavelength, and the remnant pump at the first wavelength both being coupled to pump the first laser.
Preferably, the first laser comprises a single mode optical fibre doped with rare earth ions, for example, a silica based optical fibre doped with erbium ions. There will therefore be no ESA at pump wavelengths of around 650 nm and 980 nm. It has a further absorption band at 800 nm.
Preferably the second laser comprises a fluorozirconate fibre doped with Erbium ions and is pumped by a GaAlAs laser. The second laser may thus be pumped at around 800 nm and caused to lase at around 980 nm.
A further advantage of the invention is that the remnant pump is arranged to combine with the output of the second laser to pump the first laser. Thus the first laser may be pumped at both around 980 nm and around 800 nm, thus increasing the efficiency of the system.
Alternatively, the second laser may be a silica based fibre doped with at least two types of rare earth ions. The ions may be neodymium and ytterbium ions, and the laser may be pumped at 800 nm. Pump photons are absorbed by the neodymium ions and these excited ions then transfer their energy to the ytterbium ions via a non radiative relaxation. This may lead to a population inversion between the two ytterbium ion levels, the system lasing at 980 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example only with reference to the following drawings in which:
FIG. 1 is a schematic diagram of a laser system made in accordance with the invention;
FIG. 2 shows the energy levels of an erbium doped fluorozirconate fibre
REFERENCES:
patent: 3582820 (1971-06-01), Snitzer
patent: 4589118 (1986-05-01), Suzuki et al.
patent: 4956843 (1990-09-01), Akhavon-Leilabady et al.
patent: 4962995 (1990-10-01), Andrews et al.
Electronics Letters, vol. 23, No. 20, Sep. 24, 1987, (Stevenage, Herts., GB), L. Reekie et al: "Diode-laser-pumped operation of an Er.sup.3+ -doped single-mode fibre laser" pp. 1076-1078.
Electronics Letters, vol. 23, No. 17, Aug. 13, 1987, (London, GB), L. Reeke et al "Diode-laser-pumped Nd.sup.3+ -doped fibre laser operating at 938 nm", pp. 884-885.
Electronics Letters, vol. 23, No. 16, Jul. 30, 1987, (Stevenage, Herts., GB), M. C. Brierly et al: "Nedymium-doped fluoro-zirconate fibre laser", pp. 815-817.
ECOC 87, Technical Digest vol. III, 1987, D. N. Payne et al: "Bare-earth doped fibre lasers and amplifiers", pp. 89-94.
Armitage Jonathan R.
Millar Colin A.
British Telecommunications PLC
Epps Georgia Y.
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