Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-20
2003-12-30
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010
Reexamination Certificate
active
06671300
ABSTRACT:
FIELD OF INVENTION
The present invention relates to optical devices and in particular, though not exclusively, to packaging or mounting of semiconductor optically active or optoelectronic devices such as lasers, modulators, amplifiers, switching structures, or the like.
BACKGROUND TO INVENTION
In semiconductor devices which include an active region current passing through the active region causes heating through non-radiative re-combination. To dissipate the heat the devices are typically attached/bonded to a heatsink. Typically the devices are operated junction side down to improve cooling by placing the active region close to the heatsink. For ease of coupling to the device, ends (facets) of the device overhang the heatsink. This arrangement has the disadvantage that heat builds up at the facets due to the lack of conductive pathways causing increased absorption at the facets resulting in a reduction of performance and potentially Catastrophic Optical Mirror Damage (COMD). Conversely, if the heatsink is made longer than the device the edge of the heatsink may cut-off some of the emitted light, and if as typically happens solder used to bond the device to the heatsink “balls” up at an output end of the device this also obstructs the emitted light.
A prior art arrangement used to seek to overcome these disadvantages uses a device in which the active region is arranged at an acute angle to sides of a heatsink. The device is then located on the heatsink which is effectively equal in length to the active length of the device. This arrangement reduces heat dissipation at the facets. Unfortunately, this arrangement requires high manufacturing tolerances, limits coupling to other optical components, eg optical fibres, and cannot be used for devices with two or more active regions arranged in parallel.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor optically active device which obviates or mitigates one or more of the aforementioned disadvantages.
SUMMARY OF INVENTION
According to a first aspect of the present invention there is provided an optically active device comprising:
a device body having an active region and an optically passive region(s) provided at one or more ends of the active region; and
a heatsink;
the device body and heatsink being retained in thermal association with one another such that a first end of the at least one of the optically passive region(s) adjacent an end of the active region is provided within an area of the heatsink, and a second end of the said at least one optically passive region(s) is provided outwith the area of the heatsink.
The active region may comprise an optically and optionally electically active region.
In a most preferred form, the optically active device is a semiconductor device, preferably fabricated in a III-V semiconductor materials system such as Gallium Arsenide (GaAs), eg working substantially in a wavelength range 600 to 1300 nm or Indium Phosphide (InP), eg working substantially in a wavelength range 1200 to 1700 nm. For example, the material may be AlGaAs or InGaAsP.
The device body may be selected from a laser device, eg laser diode, or optical modulator, an optical amplifier, an optical switch, or the like.
Preferably the/one of the optically passive region(s) is at an output(s) of the optically active device/device body.
An optically active device comprising a semiconductor laser device according to the present invention may have one optically passive region extending beyond an end/edge/side of the heatsink, while an optical amplifier according to the present invention may have two optically passive regions each extending beyond opposite ends/edges/sides of the heatsink.
Preferably, the semiconductor device may be of a monolithic construction. Preferably also the semiconductor device may be grown or otherwise formed on a substrate. More preferably the semiconductor device comprises an active core layer sandwiched between (or lower) optical cladding/charge carrier confining layer and a second (or upper) optical cladding/charge carrier confining layer. It will be appreciated that “upper” and “lower” are used herein for ease of reference, and not to imply any particular preferred disposition of the layers. Indeed, in use, the device may be caused to adopt an inverted disposition.
The semiconductor device may include a ridge (or rib) formed in at least the second cladding layer which ridge may act, in use, as an optical waveguide so as to laterally confine an optical mode in the semiconductor device.
Preferably the active core layer may comprise a lasing material which may comprise or include a Quantum Well (QW) structure being configured as the optically active region, the optically active region being confined by the ridge.
The/each at least one optically passive region may be as laterally extensive as the optically active region.
Preferably the optically passive region(s) may include a first compositionally disordered material within the core layer.
In a modification, the optically active region may be laterally bounded by lateral regions comprising a second compositionally disordered material within the core layer.
Advantageously the first and second compositionally lasing materials are substantially the same. Preferably the compositionally disordered materials may be formed by a Quantum Well Intermixing (QWI) technique. The QWI technique may wash out the quantum well confinement of the quantum wells within the active core layer.
More preferably, the QWI may be substantially impurity free. The QWI regions may be “blue-shifted”, that is, typically at least 20-30 meV, and likely around 100 meV or more difference exists between the band-gaps of the optically active region pumped with current, and the QWI optically passive region(s). The optically passive region(s) may have a higher band-gap energy and therefore a lower absorption than the optically active region.
Thus when the optically active region is electrically driven the optically passive region(s) limit heat dissipation at end(s) of the device body. The reduced heat dissipation allows the ends to be positioned over the ends of the heatsink, ie to overhang the heatsink. This leaves an input or output optical beam of the device free from obstruction, and gives clear access to the input/output beam at the ends of the structure to couple to or from other optical devices, eg fibre optic cable.
Typically the passive regions may be around 10 to 100 &mgr;m in length.
Preferably the device also comprises respective layers of electrical contact material contacting at least a portion of an upper surface of the second layer and second cladding layer and a (lower) surface of the first cladding layer, or more probably, a lower surface of the substrate. One of the contact material may be provided on an upper surface of the ridge.
Preferably the heatsink is made from a high thermal conductivity material, eg at least partly of Copper, Diamond, Silicon, Aluminium Nitride or the like.
Preferably also the heatsink is located against one of the contact material with a solder contact or thermal equivalent.
Preferably the second cladding layer is orientated to be closer to the heatsink than the first cladding layer. This configuration is termed as “junction side-down”, and by having the active region as close to the heatsink as possible provides an improved efficient cooling configuration.
According to a second aspect of the present invention, there is provided a method of forming an optically active device comprising the steps of:
(a) fabricating a device body having an active region and an optically passive region(s) provided at one or more ends of the active region; and
(b) positioning a heatsink and the device body in thermal association such that a first end of at least one of the optically passive regions adjacent an end of the active region is provided within an area of the heatsink and a second end of said at least one optically passive region is provided outwith the area of the heatsink.
Preferably step (a) compr
Hamilton Craig James
Marsh John Haig
Ip Paul
Jackson Cornelius H
Jefferson Perkins
Rudnick Piper
The University Court of the University of Glasgow
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