Optical waveguides – With optical coupler – Particular coupling structure
Patent
1997-03-24
1999-06-29
Ullah, Akm E.
Optical waveguides
With optical coupler
Particular coupling structure
372 50, G02B 626
Patent
active
059179724
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an optical device and has particular but not exclusive application to integrated structures formed on a substrate, for use as an amplifier or a modulator.
BACKGROUND
It is well known that certain materials such as semiconductors are capable of acting as optical amplifiers. For example, when certain semiconductors, which exhibit a band gap, are subject to an injected electric current, an incident photon causes an electron to traverse the gap with the result that an additional photon is generated, thereby producing optical amplification. Semiconductor optical amplifiers and lasers which operate in this way are well known and reference is directed to "Long Wavelength Semiconductor Lasers" G. P. Agrawal and N. K. Dutta, Van Nostrand, Chapters 1 to 6.
The semiconductor material which is used as the active amplification region of the device suffers from a gain-saturation effect which imposes a limitation on the maximum power that can be obtained. In order to overcome this problem, proposals have been made in the past to provide the active amplification region as a tapered structure on the substrate, which widens along its length so that as amplification proceeds, a greater cross section of material is available for amplification, permitting increased amplification power to be achieved. Reference is directed to Mehuys et al: "525 W, CW Near Diffraction Limited Tapered Stripe Semiconductor Optical Amplifier", IEEE Phot Tech. Letts. 5 pp 1179-1182, 1993. In this arrangement, an expanding path through the active amplification region is achieved by means of an input lens arrangement and the resulting amplified light is collected by an output lens. Another arrangement with an expanding, tapered active region, but with an integrated laser source, is described in Parke et al "2.0 W CW Diffraction Limited Operation of a Monolithically Integrated Master Oscillator Power Amplifier", IEEE Phot Tech. Letts, 5 297-300 1993. Reference is also directed to a similar structure described in Bendelli et al "A New Structure for High Power TWSLA", IEEE Phot Tech Lets. 1, 1991, pp 42-44.
Reference is also directed to "Q-Switched Bow-Tie Lasers for High Energy Picosecond Pulse Generation," K. A. Williams et al, Elect Lett, February 1994, Vol 30 No. 4, pp 320-21 which illustrates outwardly tapering amplifier regions in a laser to avoid gain saturation. Reference is made to EP-A- 0 135 594 which discloses a semiconductor laser wherein tapering of an electrode is used to improve astigmatism.
In Koyama et al "Multiple Quantum Well GaInAs/GaInAsP Tapered Broad Area Amplifiers with Monolithically integrated Waveguide Lens for High Power Applications", IEEE Phot Tech Letts, 5 pp 916-919, 1993, there is described the use of an integrated lens to re-focus the output of the expanding, tapered active region in order to simplify coupling into a single mode fibre. However, in practice, difficulties arise in fabricating the integrated lens.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided an optical device comprising an elongate waveguide having a boundary that extends along the length thereof to an optical output at one end, an optical input at the other end of the waveguide region for introducing optical radiation to be amplified therein, and an active material for producing amplification of light travelling in the waveguide, the region boundary being configured to concentrate the amplified light laterally within the waveguide towards the output.
Thus, by means of the invention, the light can be concentrated within the waveguide region itself, towards the output, without the need for additional lens structures.
The waveguide region may have a width which tapers along the length thereof towards the output to achieve the concentration of the amplified light.
Optical amplification can be achieved by applying an electric current to the active material, and the current density that in use passes through the active material may be imparted with a non-uniform spati
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British Telecommunications public limited company
Ullah Akm E.
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