Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-20
2004-04-06
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S046012
Reexamination Certificate
active
06717971
ABSTRACT:
FIELD OF INVENTION
The present invention relates to semiconductor laser devices, and in particular, though not exclusively, to a single mode index guided laser diode.
BACKGROUND OF INVENTION
In many applications there is a desire for semiconductor laser devices to operate with a single spatial mode output. This output is desirable, for example, for increased coupling to single mode fibres, and for generating small spot sizes with high light intensities. Typically laser diodes generating single mode outputs use index guided laser structures which have either a ridge or a buried heterostructure waveguide. Such devices comprise, for instance as disclosed in EP 0 475 330, a laser structure comprising a substrate, lower and upper charge carrier confining layers on said substrate, a ridge extending over a portion of said upper confining layer and laterally confining the optical mode of said laser, whereby a layer of active lasing material is sandwiched between said confining layers, said layer comprising a Quantum Well structure and being configured as an active region.
Though these devices provide a single spatial mode output, the total output power is limited due to the Catastrophic Optical Mirror Damage (COMD) level at ends (facets) of the device. Each laser facet is cleaved semiconductor, and as such contains a high density of vacancies and broken bonds which can lead to the absorption of generated light. Light or electrical current absorbed at the laser facets generates heat as excited carriers recombine non-radiactively. This heat reduces the semiconductor band-gap energy, leading to an increase in absorption inducing thermal runaway which results in COMD.
Other problems with these devices include the propagation of higher order modes at high drive currents. These higher order modes propagate due to high levels of injected carriers influencing the refractive index and optical gain in areas immediately adjacent the active region.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a laser device (such as a single mode index guided laser) which obviates or mitigates at least one of the aforementioned disadvantages of the prior art.
It is a further object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor laser device, which, by use of a diffractive region at an end of a laser region (such as a single mode index confined semiconductor laser region), provide as single mode output at increased output power levels.
It is a still further object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor laser device wherein, by incorporating a passive region formed by Quantum Well Intermixing using an impurity free technique in a gain region, beam steering characteristics of the laser device can be improved, ie by reducing the tendency to beam steer.
SUMMARY OF INVENTION
According to a first aspect of the present invention, there is provided a semiconductor laser device comprising:
an optical waveguide;
at least one electrical contact extending along part of a length of the waveguide; and wherein the at least one electrical contact is shorter than the optical waveguide.
Preferably, at least one end of the/each electrical contact is spaced from a respective end of the optical waveguide.
In one embodiment, the optical waveguide is a ridge waveguide, and the at least one electrical contact is provided on the ridge waveguide.
By this arrangement a part or parts of the ridge waveguide will not be electrically pumped, in use. It has been surprisingly found by this arrangement, that the semiconductor laser device may be operated as a mode control discriminator/stabiliser. Since the waveguide is single mode, the non pumped portion of the waveguide with no current injection should remain single mode, in use.
Preferably a length of the optical waveguide may be around 200 to 2000 &mgr;m, while a length/total length of the electrical contact(s) may be around 100 to 1900 &mgr;m.
In a modified embodiment there may be provided a first compositionally disordered or Quantum Well intermixed material bounding sides of the optical waveguide.
According to a second aspect of the present invention, there is provided a method of fabricating a semiconductor laser device comprising the steps of:
(i) forming an optical waveguide;
(ii) forming at least one electrical contact along part of a length of the waveguide, such that the at least one electrical contact is shorter than the optical waveguide.
According to a third aspect of the present invention there is provided a semiconductor laser device, comprising:
an optical active region including an optical waveguide and an optically passive region(s) provided at one or more ends of the optical waveguide; wherein
the at least one of the optically passive region(s) is broader than the optical waveguide so that, in use, an optical output of the optical waveguide diffracts upon traversing the at least one optically passive region.
In this way, the optical output may be expanded so that an intensity of optical radiation (light) impinging on an output facet of the device is reduced. Hence an output power of the device can be increased without reaching the COMD limit of the output facet.
Preferably the optically active and passive region(s) are provided within a core or guiding layer between first (lower) and second (upper) optical cladding/charge carrier confining layers, which guiding layer may comprise an active lasing material.
Preferably a ridge is formed in at least the second cladding layer and extends longitudinally from a first end of the device to a position between the first end and a second end of the device.
Additionally the active lasing material layer may include a Quantum Well (QW) structure.
Preferably the optically passive region(s) may comprise a first compositionally disordered or Quantum Well Intermixed (QWI) semiconductor (lasing) material region provided from or adjacent to the aforesaid position to the second end of the device.
In a modification of the device there may be provided second compositionally disordered (lasing) material regions laterally bounding the optical active region.
The first and second QWI materials may have a larger band-gap than the active region. The first and second compositionally disordered lasing materials may therefore have a lower optical absorption than the active region.
Preferably the device may be of a monolithic construction.
More preferably the device may include a substrate layer upon which may be provided the first cladding layer, core layer, and second cladding layer respectively.
Preferably the second end or facet may comprise an output of the semiconductor laser device. The first QWI material may therefore act as a diffractive region at the said output of the laser device. The diffractive region may act, in use, to reduce the intensity of optical radiation impinging on the said facet by spreading out the optical radiation.
More preferably the facet includes an anti-reflective coating on cleaved semiconductor. Preferably the anti-reflective coating may be around 1%-10% reflective. The combination of the first QWI diffractive region and the anti-reflective coating provides a Non-Absorbing Mirror (NAM) which further raises the COMD level of the facet and consequently the output power of the laser device can be raised.
Advantageously, the first and second compositionally disordered materials may be substantially the same.
The QWI washes out the Quantum Well confinement of the wells within the semiconductor laser material. More preferably, the QWI may be substantially impurity free. The QWI regions may be “blue-shifted”, that is, typically greater than 20-30 meV, and more typically, a 100 meV or more difference exists between the optical active region when pumped with carriers and the QWI passive regions. The first compositionally disordered lasing material therefore acts as a spatial mode filter as higher order modes will experience greater diffraction losses as they pr
Hamilton Craig James
Marsh John Haig
Ip Paul
Menefee James
Perkins Jefferson
Piper Rudnick LLP
The University Court of The University of Glasgow
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