Optical waveguides – Integrated optical circuit
Reexamination Certificate
2001-07-06
2003-04-15
Ullah, Akm E. (Department: 2874)
Optical waveguides
Integrated optical circuit
C372S006000, C359S341430
Reexamination Certificate
active
06549688
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a monolithically-integrated optical device and in particular, to an integrated optical planar waveguide amplifier. The invention also relates to a method of fabricating such an optical device.
BACKGROUND OF THE INVENTION
Optical amplifiers are an important component in optical networks for distributing optical signals. In recent years, erbium-doped optical fibres have been developed which have the capability of amplifying an optical signal. In order to amplify an optical communications signal propagating in an erbium-doped fibre amplifier, light of a different wavelength is coupled into the fibre from a pumping laser. The pumping laser stimulates electronic transitions which amplify the communications signal as it passes through the erbium-doped optical fibre.
In applications where optical components need to be relatively small and device integration is desirable, it is advantageous to provide an optical amplifier in the form of a planar waveguide integrated on a single substrate. However, there are difficulties in integrating erbium-doped amplifiers. In particular, since the longitudinal dimensions of integrated amplifiers tend to be much smaller than the longitudinal dimensions of erbium-doped fibre amplifiers, it is necessary to increase the gain of the amplifier. Attempts have been made to increase the gain by increasing the percentage of erbium. However, the gain in erbium-doped fibre amplifiers has been found to decrease when the erbium doping concentration exceeds a critical level. For example, in silica-based amplifiers, maximum gain is achieved with an erbium concentration of around 0.01-0.02 atomic %. It is believed that at higher concentrations of erbium, the gain is reduced due to an increase in erbium-erbium interactions. One method of addressing this problem has been to increase the solubility of erbium in silica-based glass by incorporating various glass modifiers into the structure, such as sodium and calcium. However, this approach has had limited success, particularly when the core layer of the waveguide is deposited by sputter deposition.
Sputter deposition involves bombarding a target of source material in a manner which ejects electrons and target atoms from the target and deposits at least some of the ejected target atoms onto a substrate. In configurations where a magnet is positioned beneath the target so as to increase plasma densities closer to the target, the technique is referred to as magnetron sputtering. One of the characteristics of sputter film deposition is that different species of target atoms tend to have different deposition rates due to differences in gas scatter rates and substrate sticking coefficients. Thus, a film deposited from a composite target containing a number of different atomic species, (e.g. silica, erbium, sodium and calcium) can have a composition which is different to that of the composite target. There is therefore a need for an integrated optical planar waveguide amplifier which has an improved gain, and for an improved method of fabricating a planar optical device in which an amplifier is integrated with other optical devices.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention there is provided an integrated optical device comprising a metaloxide-based optical planar waveguide amplifier monolithically integrated on a common substrate with at least one additional planar waveguide selected from a group comprising:
(i) a planar waveguide signal-processing circuit arranged to process an optical communications signal; and
(ii) a planar waveguide pump-signal coupling circuit arranged to couple or decouple a pump wavelength to or from the amplifier;
wherein the amplifier has a metal-oxide-based core comprising an optically-transmissive metal oxide material doped with a gain medium and is arranged to amplify an optical communication signal when optically pumped with a source of pump radiation.
Preferably, the metal oxide comprises at least 50 mol % of the core of the amplifier, and more preferably at least 70 mol % of the core. The composition of the core of the amplifier may predominantly comprise aluminium oxide. In one embodiment, the metal oxide comprises at least 80 mol % of the core. The amplifier may be formed directly on the substrate. Alternatively, one or more additional layers, such as another planar waveguide, may be interposed between the amplifier and the substrate.
Preferably, the amplifier is integrated with and coupled to at least the planar waveguide signal-processing circuit. The planar waveguide communications-signal-processing circuit may comprise one or a combination of planar waveguide devices selected from a group comprising:
a communications-signal multiplexer, arranged to multiplex a plurality of communications wavelengths;
a communications-signal demultiplexer, arranged to demultiplex a plurality of optical communications wavelengths;
a channel gain equaliser;
an N×M optical switch matrix;
an optical modulator;
an optical attenuator;
a variable optical attenuator;
an add-drop multiplexer; and
a reconfigurable add-drop multiplexer.
The communications-signal multiplexer and the communications-signal demultiplexer may comprise an arrayed-waveguide grating. Any number of other optical components may be monolithically integrated on the common substrate with above-described planar waveguides. The pump-signal coupling circuit may comprise one or more planar waveguides selected from a group comprising:
a pump-signal multiplexer arranged to multiplex an optical communications wavelength and an optical pump wavelength; and
a pump-signal demultiplexer arranged to demultiplex an optical communications wavelength and an optical pump wavelength.
The pump-signal multiplexer and the pump-signal demultiplexer may each comprise an asymmetric Mach-Zehnder inferometer.
The pump-signal coupling circuit may incorporate a slab waveguide for collecting pump radiation from a plurality of sources and guiding the radiation towards the amplifier. In one embodiment, the pump-signal coupling circuit is arranged to be pumped by a laser diode bar array. The integrated optical device may firer comprise a laser diode bar array integrated on the common substrate and optically coupled to the planar waveguide amplifier. Pump radiation may be coupled into the amplifier using a cladding mode of the amplifier. In an alternative embodiment, the planar waveguide coupling circuit is arranged to be optically connected to an external pumping source not integrated in the optical device
The amplifier may be arranged to provide sufficient gain (communications sight amplification) to compensate for insertion losses of the integrated optical device. For example, the amplifier can be arranged to compensate for any optical losses in the signal-processing circuit and pump-signal coupling circuit or in optical connections between those circuits. In one embodiment, the signal-processing circuit comprises a communications-signal demultiplexer and the amplifier is arranged to compensate for any optical losses arising from the demultiplexer.
The amplifier may comprise one or more amplifiers, and may be arranged to amplify communications signals in one or more respective input channels of the communications-signal processing circuit. Alternatively, the amplifier may comprise one or more amplifiers arranged to amplify communications signals in one or more respective output channels of the signal-processing circuit.
The amplifier may be arranged on the substrate adjacent and substantially parallel to a side of the signal-processing circuit so as to conserve space on the substrate. The amplifier may include at least one mirror structure for reflecting and guiding optical signals. The mirror structure can be used to guide a signal around a sharp comer rather than a curved bend, and can thus enable the amplifier to be laid out over a smaller area than is possible if smooth bends are used. In one embodiment, the mirror structure operates by means of total internal reflection a
Redfern Integrated Optics Pty LTD
Ullah Akm E.
LandOfFree
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