Optical: systems and elements – Optical amplifier – Optical fiber
Patent
1993-01-12
1996-03-19
Moskowitz, Nelson
Optical: systems and elements
Optical amplifier
Optical fiber
359160, 372 40, 372 75, H01S 306, H01S 330
Patent
active
055007643
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
I. FIELD OF THE INVENTION
This invention relates to optical waveguide amplifiers and in particular to those comprising an optical waveguide doped with an active species providing a three-level laser scheme associated with a primary fluorescence peak at a first wavelength.
II. RELATED ART AND OTHER CONSIDERATION
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
A three-level laser scheme comprises a lower lasting level or band of levels (LLL), that is the ground state or an energy level so close to it that it has a significant thermal population, or band of such levels, an upper lasting level or band of levels (ULL) and one or more pump bands of higher energy than the bottom of the ULL. In such a scheme, absorption from the LLL band to the ULL band can take place in direct competition with the stimulated emission of photons obtained by transitions in the reverse direction.
Pumping such an amplifier with an optical pump of wavelength shorter than the fluorescence peak associated with the lasting transition and corresponding to a pump band will produce, in well known fashion, a depopulation of the LLL and an increase in population of the ULL. The fractional population of the ULL is dependent on the pump power and the relative gain coefficient of the amplifier for a given signal wavelength is proportional to N.sub.2 .sigma..sub.2 (.lambda.)-N.sub.1 .sigma..sub.1 (.lambda.) where N.sub.1 and N.sub.2 are the fractional population densities of the LLL and ULL respectively and .sigma..sub.1 and .sigma..sub.2 are the absorption and emission cross-sections.
Three-level laser scheme waveguide amplifiers can provide gain to signals at a wavelength in the long-wavelength tail of the fluorescence spectrum but there are problems connected with amplified spontaneous emission (ASE) at the gain-peak wavelength.
SUMMARY
According to the present invention an optical waveguide amplifier comprises an optical waveguide doped with an active species providing a three-level laser transition associated with a primary fluorescence peak at a first wavelength characterised in that the waveguide is coupled to at least a first optical pump source for providing optical pump power at a second wavelength which is longer than the first wavelength.
Pumping the amplifier at a wavelength longer than the wavelength of the primary fluorescence peak eliminates gain and therefore ASE, at this wavelength.
The invention is expected to find particular application in providing amplification of optical signals transmitted through optical fibre communications networks. A convenient class of waveguide amplifier for use with such silica-based optical fibre networks is the optical fibre doped with a rare-earth ion as it is readily incorporated in such a network. For example, a SiO.sub.2 --Al.sub.2 O.sub.3 --GeO.sub.2 optical fibre doped with Er.sup.3+ ions can provide gain in the 1.55 .mu.m telecommunications window. Such an optical amplifier has been found to provide useful gain around 1.60 .mu.m for a pump wavelength of 1.55 .mu.m achieving a maximum useful spectral bandwidth of 1.57 .mu.m to 1.61 .mu.m.
The present invention is not restricted in its application to this specific silica-based Er.sup.3+ doped lasing scheme. Other doped waveguides exhibiting three-level lasing schemes may be employed, for example planar silica or lithium niobate waveguides doped with a suitable dopant. Similarly other optical fibre hosts, such as fluoride fibres, or other appropriate dopants, for example ytterbium may be used with the pumping scheme of the present invention.
Preferably the amplifier includes a second optical pump source for providing optical pump power at a wavelength shorter than the first wavelength.
As will be explained in more detail belo
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Armitage Jonathan R.
Massicott Jennifer F.
Wyatt Richard
British Telecommunicaions Public Limited Company
Moskowitz Nelson
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