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
C372S068000
Reexamination Certificate
active
06717970
ABSTRACT:
FIELD OF INVENTION
The present invention relates to laser devices, particularly though not exclusively, semiconductor laser devices such as laser diodes, eg a single mode index guided laser diode.
BACKGROUND OF INVENTION
For many applications semiconductor laser devices are desired to operate with a substantially 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 semiconductor laser diodes generating single mode outputs use index guided laser structures which have either a ridge or a buried heterostructure waveguide. Such laser diodes comprise, for instance as disclosed in JP-A-1 225 288, a device 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 Intermixing (QWI) structure and being configured as an active region. Further provided are regions of compositionally disordered lasing material laterally bounding said active region, said disordered regions having a larger band-gap energy than said active region.
The compositionally disordered lasing material can be provided by a technique known as Quantum Well Intermixing (QWI) Well Intermixing (QWI). Various QWI techniques exist such as Impurity Induced Layer Disordering, Ion Implantation, Impurity Free Vacancy Disordering, and a Damage Induced Technique.
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 the ends (facets) of the laser diode. The laser diode 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 absorbed at a laser diode facet generates heat as excited carriers recombine non-radioactively. This heat reduces the semiconductor band-gap energy leading to an increase in absorption so inducing thermal runaway which results in COMD.
Prior art techniques to improve COMD levels and consequently device lifetimes, disclose methods of fabricating Non Absorbing Mirrors (NAM) through the use of re-growth or Impurity Induced Disordering (IID) techniques and passivating the facets by evaporation of sulphur containing compounds or silicon. These methods have the disadvantage of being relatively complex.
An alternative prior art technique is to use a lossy confining waveguide which reduces the intensity on the facets by increasing the propagating optical mode size in the vertical direction or reducing wavelength confinement in the horizontal direction. Lossy confining waveguides have the disadvantage that they are susceptible to fabrication tolerances during manufacture.
It is an object of at least one aspect of the present invention to provide a laser device which obviates or mitigates at least one of the aforementioned disadvantages.
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 has a multiplied output power compared to prior art devices of similar length while retaining a substantially single lobed far field output beam profile.
SUMMARY OF INVENTION
According to a first aspect of the present invention there is provided a laser device comprising;
at least two lasing regions;
an interference region into which an output of each lasing region is coupled; and
an output region extending from the interference region to an output of the device.
In a preferred and advantageous implementation, the laser device is a semiconductor laser device such as a laser diode.
Preferably the semiconductor laser device is fabricated from a III-V semiconductor materials system such as a Gallium Arsenide (GaAs) based material systems operating in a wavelength range of substantially 600 to 1300 nm, or alternatively an Indium Phosphide (InP) based material system operating in a wavelength range of substantially 1200 to 1700 nm. For example AlGaAs or InGaAsP.
Preferably each lasing region may comprise an optically active waveguide.
Preferably also the lasing regions are arranged substantially parallel to each other.
Preferably at least an input of the output region may comprise an output waveguide, and the output waveguide may be positioned transversely between output ends of the at least two active waveguides. Preferably also the at least two active waveguides may be substantially the same, eg in construction and operation. Preferably there are provided two active waveguides.
Preferably the interference region is a multi-mode interference region, ie a multi-mode interference (MMI) coupler. This arrangement provides a laser device including a multi-mode interference (MMI) coupler.
In relation to the MMI couplers for a 3 dB MMI or 1 to 2 dB MMI, two regimes may operate:
(i) an optical signal injected down a single waveguide of the coupler may be split nominally 50/50 between two waveguides of the coupler with relative phases of a given optical mode in each of the two waveguides being zero; and
(ii) an optical signal injected down the two waveguides will be maximally coupled to the single waveguide when the two waveguides are substantially or effectively identical.
Thus for the present invention these features provide a laser device having an output which has a substantially single lobe in the far field.
Preferably the active waveguides may be current driven to provide optical gain in the laser device. The active waveguides may be ridge or buried heterostructure waveguides. Preferably the active waveguides may be Large Optical Cavity (LOC) waveguides, AntiResonant Reflecting Optical Waveguides (ARROW), Wide Optical Waveguides (WOW), or the like.
Preferably each active waveguide may be at least partly formed by a core layer of active lasing material sandwiched between first and second cladding/confining layers formed on a substrate. More preferably, the active lasing material may comprise or include a Quantum Well Intermixing (QWI) Well (QUANTUM WELL (QW) structure configured as an optically active region.
In a modification the active region may be laterally bounded by regions of compositionally intermixed or disordered lasing material. The disordered regions may have a larger band-gap energy and therefore a lower optical absorption than the active region.
Preferably, each active waveguide may comprise a ridge waveguide having a ridge formed in at least the second cladding layer distal the substrate.
More preferably the disordered regions may be formed by Quantum Well Intermixing (QWI) Well Intermixing (QWI). The QWI washes out the Quantum Well Intermixing (QWI) Well confinement of the wells within the core layer. The QWI may be impurity free. The QWI regions may be “blue-shifted”, that is, typically at least 20 meV or 30 meV and normally 100 meV or more difference exists between the active region which is electrically pumped, in use, and the QWI passive regions which are not electrically pumped.
The output waveguide may be optionally active or passive. The output waveguide may be a ridge or buried heterostructure waveguide. Preferably the output waveguide may be a Large Optical Cavity (LOC) waveguide, AntiResonant Reflecting Optical Waveguide (ARROW), Wide Optical Waveguide (WOW), or the like.
Preferably the output waveguide may comprise the core layer sandwiched between the first and second cladding layers.
In one arrangement, the interference region may be optically active. However, in a preferred arrangement, the interference region may be passive. The interference region may comprise a ridge or buried heterostructure.
Preferably the interference region may comprise the core layer sandwiched between the first and second cladding layers.
Preferably also the device further comprises attenuation means. The attenuation means may comprise a
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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