Coherent light generators – Particular active media – Amorphous
Reexamination Certificate
1998-10-26
2001-05-01
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular active media
Amorphous
C372S006000
Reexamination Certificate
active
06226308
ABSTRACT:
This invention relates to optical waveguides, optical waveguide amplifiers and lasers.
Optical waveguide amplifiers and lasers, and in particular optical fibre amplifiers, are used widely in the telecommunications industry.
An example of a known optical fibre amplifier is the erbium doped fibre amplifier (EDFA) which operates in the so-called third telecoms window (wavelengths around 1500 nanometres (nm)). These devices are characterized by their high gain (around 30 decibels (dB)) achieved with limited pump power (less than 50 milliwatts (mW)), which means that pumping can be performed using a single laser diode unit.
However, in the important second telecoms window (wavelengths around 1.3 micrometres (&mgr;m)), known optical fibre amplifiers operate with a much lower efficiency. The best performance reported so far has been obtained using trivalent praseodymium (Pr
3+
) doped fluoride fibre amplifiers (PDFFAs), which achieve a small signal gain of about 23 dB with about 120 mW of pump from a single semiconductor laser diode unit. Operating the pump diode at high drive current to achieve this pump power can severely curtail the lifetime of the diode. This problem has limited the application of this amplifier unit.
It has been proposed to use chalcogenide glass as a host for the rare earth dopants praseodymium or dysprosium to provide more efficient amplification at 1.3 &mgr;m. “Chalcogeride”, for the purposes of this description, means that the glass contains sulphur (S), tellurium or selenium, so examples of this type of glass can contain arsenic trisulphide, arsenic selenide, germanium sulphide, gallium sulphide, lanthanum sulphide and/or other elements. Chalcogenide glasses typically have properties which make them useful as a host glass for rare earth doped amplifiers for optical signals in the 1.3 &mgr;m wavelength range, in particular the glass typically having a maximum phonon energy of less than 450 cm
−1
. The low phonon energy decreases the probability of non-radiative decay of the rare earth dopant excited state, thus allowing more efficient use of pump power for signal amplification. Modelling has predicted that an optimised chalcogenide glass waveguide doped with praseodymium and pumped with about 50 mW at a suitable wavelength will provide a small signal gain in excess of 20 dB. A similar waveguide doped with dysprosium will again provide in excess of 20 dB of small signal gain for 50 mW of pump power.
However, a low-loss fibre from a suitable chalcogenide glass is extremely difficult to fabricate, as the glass is prone to crystallization which severely limits fibre transmission and strength. The difficulties in fibre drawing currently limit the exploitation of chalcogenide based optical fibres. Similar difficulties are experienced in fabricating planar optical waveguides.
In order to improve amplifier performance, it has been preferred that substantially all impurities be removed from the glass before waveguide fabrication. Of particular concern are oxide impurities in the from of hydroxides, sulphates and metallic oxides. Among these, metallic oxides are a known by-product of the formation of metallic sulphides used in chalcogenide glass preparation and are difficult to remove.
Knowledge to date regarding uncontrolled oxide impurities was that they decrease the device efficiency when an optical waveguide amplifier or laser is pumped in the conventional way. This loss of efficiency results from the decreased lifetime of the rare earth ion in the vicinity of an oxide impurity. The impurity locally raised the maximum phonon energy, thereby increasing the probability of non-radiative decay of the ion and preventing emission at the wavelength preferred. Accordingly, in previous chalcogenide glass amplifiers substantially all of the oxides have been removed from the glass before or during fabrication of the waveguide.
This invention provides an optical waveguide amplifier comprising:
a chalcogenide glass waveguide doped at least in part with
(a) a rare earth dopant; and
(b) oxygen in the form of a metallic oxide; wherein the total metal oxide content is about 0.01% to about 50% of the total weight of the glass; and
means for supplying pump radiation to the waveguide.
The invention recognises that the intentional doping of a chalcogenide glass with a metallic oxide which is then used to form an optical waveguide amplifier or laser can lead to several significant improvements over the prior art.
Doping with metal oxides in amplifier fabrication goes against conventional wisdom and teaching in this area. For example, U.S. Pat. No. 5,378,664 teaches extensive techniques using heat in the presence of flowing hydrogen sulphide for converting oxides into sulphides before or after using the materials as dopants. Previously, oxides have been regarded as impurities to be removed rather than as dopants with any beneficial effect (other than on glass stability), and so oxide-containing fibres have not been considered for use as amplifiers. In particular, a perceived disadvantage of the intentional doping of the chalcogenide glass with a metallic oxide has been that it results in a possible reduction in the small signal device efficiency (especially in the case of praseodymium-doped amplifiers). However, in embodiments of the invention this long-standing technical prejudice is overcome and oxide-doped chalcogenide waveguides are proposed for use in optical amplifiers.
The advantages of oxide doped chalcogenide glass in embodiments of the invention can include:
(i) a potential increase in glass stability by the reduction of the tendency of these glasses towards crystallization;
(ii) a modification of the glass viscosity during fibre drawing to ease the difficulties of fibre fabrication;
(iii) a broadening of the emission wavelength range around 1.3 &mgr;m from about 90 nm to greater than 120 nm;
(iv) a potential increase in efficiency when operated as a high output power amplifier through the decreased lifetime of the lower transition levels by reducing bottlenecking;
(v) an increase in the width of the ground state absorption spectrum allowing a greater range of pump wavelengths.
Embodiments of the invention show that the controlled addition of a predetermined level of oxide can improve the power amplifier device performance (e.g. when operating with output powers greater than about 1 mW) and that any detrimental effects on small signal amplifier performance (e.g. when operating with signal levels less than about −10 dBm (dB relative to 1 mW) and gain greater than about 15 dB) can be reduced through selection of suitable pump wavelengths.
It has also been found that the pump wavelength can have a strong bearing on the usefulness of the materials in amplification. Previously, pump wavelengths have tended to be selected at the peak absorption of the rare earth in the glass to be pumped. In the new oxide-doped glasses presented in this description, it is found that rare earth dopants can occupy “oxide” sites or “non oxide” sites in the glass. The non oxide sites can be excited by pump wavelengths at or near to the peak absorption to provide a small signal amplifier. However, if the pump wavelength is shifted away from the peak absorption and the device is pumped at shorter wavelengths to excite dopant ions at the oxide sites, a better performance can be obtained from the resulting device as a power amplifier.
This invention also provides a method of manufacture of an optical waveguide amplifier, the method comprising adding a metal oxide during formation of a doped a glass optical waveguide, the resulting waveguide having an oxide content of at least 0.01% by weight.
This invention also provides an optical waveguide formed at least in part of a chalcogenide glass doped with
(a) a rare earth dopant; and
(b) oxygen in the form of a metallic oxide;
wherein the total metal oxide content is about 5% to about 50% of the total weight of the glass.
Additional aspects and preferred features of the invention are defined in the appended claims. It will be appreciated that wh
Brocklesby William Simon
Hewak Daniel William
Ian Laming Richard
Payne David Neil
Samson Bryce Neilson
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jr. Leon Scott
Pirelli Cavi e Sistemi SpA
LandOfFree
Optical waveguide, waveguide amplifier and laser does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical waveguide, waveguide amplifier and laser, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical waveguide, waveguide amplifier and laser will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2561857