Coherent light generators – Particular active media – Semiconductor
Patent
1997-12-17
1999-11-02
Davie, James W.
Coherent light generators
Particular active media
Semiconductor
372 96, 372102, H02S 310
Patent
active
059784008
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF HE INVENTION
1. Field of the Invention
The present invention relates to a laser.
2. Related Art
There is a requirement, for example in optical telecommunications networks, for lasers having a narrow linewidth output. Such lasers can be used in, for example, wavelength division multiplex systems where closely spaced, different wavelengths are utilised to carry different communications channels, or in high bit rate systems which require narrow linewidth optical sources in order to avoid the effects of dispersion. DFB (distributed feedback) semiconductor lasers, having a diffraction grating within the semiconductor gain material, have been utilised for these applications. A disadvantage of these DFB lasers is that, due to the temperature dependence of the refractive index of the semiconductor material, their output wavelength varies as their temperature varies. This generally necessitates the use of a temperature controller, and associated electronics, to stabilise the temperature, and thus output wavelength, of DFB lasers.
This problem has been overcome, to some extent, in fibre grating lasers (FGL), in which a lensed fibre, comprising a fibre grating, is aligned with a semiconductor laser diode to form a short external cavity laser, see for example "Narrow Line Semiconductor Laser Using Fibre Grating", Bird et al, Electronics Letters, Jun. 20, 1991, Vol. 27, No. 13, pp1115-1116. In this device, the laser diode facet closest to the lensed fibre end is anti-reflection coated so that lasing occurs between the back facet of the laser diode and the grating within the fibre. Since the majority of the laser cavity, including the grating, is now within the fibre, rather than the semiconductor material, the change in output wavelength of the FGL, with temperature, is far lower than that of a DFB laser.
The present applicants have found that known FGLs are not suitable for use in practical optical telecommunication networks due lo instabilities in both their output wavelength, and their output power, as either the drive current, or the temperature, of the FGL is varied.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a laser comprising first and second feedback elements defining a laser cavity, and a gain medium within the laser cavity, the gain medium having first and second facets and an optical waveguide for guiding optical radiation between said first and second facets, wherein the second feedback element is wavelength selective, the optical waveguide comprises a curved section and is configured to direct optical radiation at an angle to the normal of the second facet, and wherein the optical waveguide is additionally configured to direct optical radiation substantially parallel to the normal of the first facet.
The present applicant has discovered that the instabilities of known FGLs are due to coupled cavities being formed between the back and front (i.e. closest to the fibre) facets of the semiconductor laser diode, and between the fibre grating and the front facet. The instabilities in the output of known FGLs are thought to be due to mode competition between these various coupled cavities. This mode competition renders known FGLs unusable in practical situations, despite anti-reflection coatings applied to the front facet of the semiconductor laser diode. A particular problem seen when prior art FGLs are employed in systems to transmit data is that severe mode hopping between the modes of the coupled cavities causes a degradation in the BER (bit error rate) performance of the system, due to consequent large changes in output power.
The use of angled facets is known from, inter alia, Electronics Letters, Vol24, 1988, No.23, pp 1439-1441. In that latter a diode amplifier of ridge waveguide configuration has its ridge positioned at an angle of 10.degree. with respect to the facet normal "to frustrate the internal resonances evident in cleaved facet semiconductor lasers", The device is used with two external reflectors, a mirror and a grating etched into an optica
REFERENCES:
patent: 4821277 (1989-04-01), Alphonse et al.
patent: 5715263 (1998-02-01), Trudo et al.
Electronics Letters, vol. 24, No. 23, Nov. 10, 1988, pp. 1439-1441, Burns et al "Active Mode-Locking of an External Cavity GalnAsP Laser Incorporating a Fibre-Grating Reflector".
Electronics Letters, vol. 25, No. 4, Feb. 16, 1989, pp. 270-272; Bachen et al "External Cavity Semiconductor Laser Comprising Brewster-Angled Rectangular Core Fibre".
Electronics Letters, vol. 30, No. 11, May 26, 1994, pp. 857-859, Lealman et al "1.56.mu.m InGaAsP/InP Tapered Active Layer Multiquantum Well Laser with Improved Coupling to Cleaved Singlemode Fiber".
Armitage Jonathan Richard
Brierley Michael Charles
Campbell Robert John
Payne Roger Alyn
Robertson Michael James
British Telecommunications public limited company
Davie James W.
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