Patent Publication Number: US-6671300-B2

Title: Optical devices

Description:
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 μm 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) comprises: 
     (i) forming in order: 
     a first optical cladding/charge carrier confining layer; 
     an active layer which may comprise an optically and/or electrically active layer in which is optionally formed a Quantum Well (QW) structure; and 
     a second optical cladding/charge carrier confining layer; 
     (ii) forming the optically passive region(s) in the active layer; 
     (iii) forming a ridge from at least a portion of the second cladding layer to confine the active region and at least one of the optically passive region(s); 
     Step (i) may be performed by known growth techniques such as Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapour Deposition (MOCVD). 
     Preferably in step (ii) the passive region(s) may be formed by a Quantum Well Intermixing (QWI) technique which may preferably comprise generating vacancies in the passive regions, or alternatively implanting or diffusing ions into the passive region(s), and further annealing to create a compositionally disordered region(s) of the optically active layer (which may comprise a lasing material), having a larger band-gap than the Quantum Well structure. The passive region(s) may therefore be formed by Quantum Well Intermixing (QWI). 
     Preferably step (iii) may be achieved by known etching techniques, eg dry or wet etching. 
     Preferably the heatsink may be secured to a surface adjacent the second cladding layer. In this embodiment the heatsink may therefore be attached to an upper surface of the ridge in a “junction side-down” configuration. Preferably the first cladding layer may be formed on a substrate. In a modification the heatsink may be attached to a surface of the substrate. 
     Preferably, step (ii) may be performed by generating impurity free vacancies and more preferably may use a damage induced technique to achieve Quantum Well Intermixing. In a preferred implementation of such a technique, the method may include the steps of: 
     depositing by use of a diode sputterer and within a substantially Argon atmosphere a dielectric layer such as Silica (SiO 2 ) on at least part of a surface of the semiconductor laser device material so as to introduce point structural defects at least into a portion of the material adjacent the dielectric layer; 
     optionally depositing by a non-sputtering technique such as Plasma Enhanced Chemical Vapour Deposition (PECVD) a further dielectric layer on at least another part of the surface of the material; 
     annealing the material thereby transferring Gallium from the material into the dielectric layer. Such a technique is described in co-pending application entitled “Method of Manufacturing Optical Devices and Related Improvements” also by the present Applicant, and having the same filing date as the present application, the content of which is incorporated herein by reference. 
     Preferably, the method may include the step of applying first and second electrical contact layers on a surface of the first cladding layer, or more preferably an outer surface of the substrate, and an outer surface of the ridge. More preferably, the second electrical contact layer may be provided on a portion of the ridge within an area of the optically active region. 
     In a further embodiment of the present invention step (ii) may comprise the steps of, first selecting a first area and forming a first compositionally disordered material thereat, and second selecting a second area and forming a second compositionally disordered material thereat. The first and second compositionally disordered material may respectively provide first and second passive regions at first and second ends of the device body. 
     In a modification, the method may include, preferably in step (ii), forming regions of compositionally disordered material laterally bounding the active region. These may assist the ridge in confining the optical mode(s) of the device. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, which are: 
     FIG. 1 align a cross-sectional view of an optically active device according to a first embodiment of the present invention; 
     FIG. 2 a cross-sectional view through line X-X′ of the optically active device of FIG. 1; and 
     FIG. 3 a cross-sectional view of an optically active device according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     Referring to FIGS. 1 and 2 there is illustrated an optically active device, generally designated  10 , according to a first embodiment of the present invention. The device  10  comprises a device body  12 , which in this embodiment is fabricated from a semiconductor material defining an optically active or optical gain region  14  which is bounded at first and second ends  16 ,  18  thereof by optically passive regions  20 ,  22  respectively. Outer ends  24 ,  26  of the passive regions  20 ,  22  define ends or facets  23 , 24  of the device  10 . 
     A heatsink  28  is in thermal association with the device body  12  and arranged such that inner ends  16 ,  18  of the passive regions  20 ,  22  at ends  16 ,  18  of the gain region  14  are provided within an area or extent “A” of the heatsink  28 , while the second ends  24 ,  26  are provided outwith (outside) the area  28 , as viewed in the orientation of FIG.  1 . 
     This arrangement provides passive regions  20 ,  22  which “overhang” the heatsink  28 . Such an arrangement may be used for an optically active device  10  such as an optical amplifier, where access is required at both an input and output end of the. Thus the passive regions  20 ,  22  may be considered as input/output waveguides of the device  10 , which extend beyond opposite ends  24 ,  26  of an optically active region of the device  10 . 
     Referring now to FIG. 2, which shows the device  10  of FIG. 1 in cross-section along line X-X; as can be seen, the device  10   a  is monolithic, being grown on a substrate  30 . The device body  12  is a layered structure comprising substrate  30 , upon which is grown by known techniques such as Molecular Beam Epitaxy (MBE)or Molecular Organic Chemical Vapour Deposition (MOCVD), a first optical cladding/charge carrier confining layer  36 , and active/guiding layer  32 , eg of semiconductor lasing material, and a second cladding/charge carrier confining layer  34 , and also beneficially semiconductor contact layer  40 . 
     The device body  12  includes a waveguide  38  formed in the second cladding layer  34  by suitable etching techniques. The ridge  38  confines an optical beam within the optically active region  14  and the optically passive regions  20 ,  22  (not shown). The ridge  38  extends between the ends  24 ,  26  of the device  10 ,  10   a.    
     On an outer facing surface of the ridge  38  and below the substrate  30  are located respective electrical contact layers  41 , 42  used to electrically drive the device  10 . 
     In a preferred form of this embodiment, the active layer  32  comprises a Quantum Well structure  54  embedded in the active layer  32  and, by confinement of the ridge  38 , the gain region  14  is located in the active layer  32 . 
     In a modification of this embodiment the gain region  14  is laterally bounded by Quantum Well Intermixed (QWI) regions  50 ,  52  which aid confinement of an optical beam within the Quantum Well structure of the gain region  14 . 
     Returning again to FIG. 1, the passive regions  24 ,  26  within the active layer  32  comprise Quantum Well Intermixed regions providing a higher band-gap energy and therefore lower absorption than the Quantum Well structure of the gain region  14  within the active layer  32 . This has an advantage that when the active region  14  is pumped with current heat dissipation at ends  16 , 18  of the active region  14  are provided with thermal contact to an area of heatsink  28  giving cooling of the device  10 , while the passive regions  20 ,  22  overhang the heatsink  28  dissipating less heat. The ends  24 ,  26  of the passive regions  20 ,  22  are free from any obstruction of the heatsink  28 , thus giving clear access to the ends  24 ,  26  of the device  10  for input/output coupling, eg to other optical devices. As shown in FIG. 2, the device  10  is arranged in a “junction side-down” configuration so that the gain region  14  is as close to the heatsink  28  as possible. It will be appreciated that, in use, the heatsink  28  may be lowermost, and the (inverted) device body  12  uppermost. 
     In the device  10  the distance between the heatsink  28  and the gain region  14  is likely to be typically around 2 to 5 μm. 
     The heatsink  28  is chosen to be of a material having good thermal characteristics. In this embodiment the heatsink  28  is made substantially of Copper. However, other material such as Diamond, Silicon and Aluminium Nitride are appropriate also. The heatsink  28  is soldered or otherwise adhered to the contact  41 . 
     The QW Intermixed regions are the passive regions  20 ,  22 , in the active layer  32  shown in FIGS. 1 and 2. The ridge  38  is formed over the gain region  14 , and the passive regions  20 ,  22 . 
     Referring again to FIG. 2, a method will now be described for fabrication of an optically active device  10  according to an embodiment of the present invention made in a III-V semiconductor materials system such as GaAs or InP. 
     The method begins with providing substrate layer  30 . The substrate is, in this embodiment Gallium Arsenide (GaAs) and is highly n-type doped. Grown on the substrate layer  30  is the first cladding layer  36 . The first cladding layer  36  comprises Aluminium Gallium Arsenide (AlGaAs) and is n-type doped to a first concentration. This first cladding layer  36 , eg having a refractive of around 3.0 to 3.5, is typically 1 to 3 μm thick. Grown on the first cladding layer  36  is the semiconductor optically active layer  32 . The active layer  32  also comprises AlGaAs. Layer  32  is substantially intrinsic. The active layer  32 , eg having a refractive index of around 3.0 to 3.5, is grown to be a few hundred nm thick, typically. Within the active layer  32 , is provided a Quantum Well (QW) structure  54 . The QW structure  54  is typically embedded in the middle of the layer  32 . 
     On the active layer  32  is grown the second cladding layer  34 . The second cladding layer  34  is of p-type with a similar doping concentration to the first concentration. The second layer  34  is also made of AlGaAs with a thickness, composition, and refractive index similar to that of the first cladding layer  36 . Thus the Quantum Well (QW) structure  54  is sandwiched between n-type and p-type first and second cladding layers  36  and  34 , respectively. Active layer  32  has a lower Aluminium (Al) content than cladding layers  34 , 36 . The active layer  32  has a higher refractive index than the cladding layers  36 , 34 . 
     A selective QWI mask (not shown) is then placed at least over a portion of the device  10  where the ridge  38  will be defined, but leaving portions (coincident with the passive regions  20 , 22  to be formed) unmasked. The technique preferably used to create Quantum Well Intermixing (QWI) within the Quantum Well structure is a damage induced technique using vacancies. However, it will be understood that any other Quantum Well Intermixing technique which achieves a difference in the band-gap energy between the Quantum Well structure  54  and the QW intermixed passive regions  20 ,  22  could be used within this invention. The damage induced technique requires depositing by use of a diode sputterer and within a substantially Argon atmosphere a dielectric layer such as Silica (SiO 2 ) on at least part of a surface of the semiconductor laser device material so as to introduce point structural defects at least into a portion of the material adjacent the dielectric layer; 
     optionally depositing by a non-sputtering technique such as Plasma Enhanced Chemical Vapour Deposition (PECVD) a further dielectric layer on at least another part of the surface of the material; 
     annealing the material thereby transferring Gallium from the material into the dielectric layer. 
     Once the device body  12  has been annealed, portions of the second cladding layer  34  on either side of the ridge  38  is to be defined are etched away by known etching techniques once a suitable etch mask has been placed over an area defining the ridge  38 . 
     As can be seen from FIG. 2, at the time of growth of the device body  12 , a final layer  40  may be grown on the second cladding layer  34 , the final layer  40  being a highly doped p-type GaAs layer  40 . The final layer  40  acts as the upper contact for the device body  12 . 
     Contact metallisation  41 ,  42  are formed by known lithographic techniques on rib  38 , and on substrate  30  respectfully, so as to allow for electrical driving of the device body  12 . Finally the device body  12  is secured to the heatsink  28 . 
     Thus the device body  12  shown in the cross-section of FIG. 2 is a monolithic semiconductor active device. The active region  14  of the device  10  is within the active layer  32  and confined in the Quantum Well structure by the ridge  38  above. 
     Referring now to FIG. 3, there is shown an optically active device, generally designated  10   a,  according to a second embodiment of the present invention. The device  10   a  has similarities to the semiconductor device  10  of the first embodiment, and accordingly like parts have been given the same nomenclature but are suffixed “a”. 
     The device  10   a  comprises a device body  12  having an active region  14   a  which is bounded on one end  18   a  by an optically passive region  22   a , having a first end  25   a . Another end  16   a  of the gain region  14   a  and second end  26   a  of the passive region  22   a  define ends of the device  10   a . A heatsink  28   a  is in thermal association with the device body  12  and arranged such that the ends  16   a , 18   a  of the gain region  14   a  are provided within an area A of the heatsink  28   a  while the second end  26   a  of the passive region  22   a  is outwith (outside) the area A of the heatsink  28   a.    
     This embodiment provides one passive region  22   a  which “overhangs” the heatsink  28   a . Such an arrangement may be used for an optically active device such as a semiconductor laser diode where an output of the device  10   a  is provided it is at end  26   a.    
     The layered construction of the device  10   a  is as described hereinbefore for the first embodiment with reference to FIGS. 1 and 2. The passive region  26  is a Quantum Well Intermixed (QWI) region providing a higher band-gap energy, and therefore lower absorption than the QW structure of the gain region  14   a.    
     This arrangement has the advantage that the ends  16   a , 18   a  of the gain region  14   a  are provided in good thermal contact with the heatsink  28   a  while clear access is given to the output end  26   a  of the device  10   a , eg for output coupling to a fibre or other device. 
     The device  10   a  may be fabricated by a method similar to, or the same as, the method hereinbefore described for the device  10 . 
     It will be appreciated by those skilled in the art that modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. 
     It will also be appreciated that a principal advantage of the present invention is that, by the provision of passive regions at end(s) of an optical device overhanging a heatsink, problems associated with heating at facets of the device are improved as the regions are not active. A further advantage of the present invention is that coupling to the device is made easier by the protruding passive regions/waveguides which ensure the output/input beams are not impeded or influenced by the heatsink. 
     It will also be appreciated that in a modification a buried heterostructure waveguide could be used rather than a ridge waveguide. Further, other waveguides such as Large Optical Cavity (LOC), Antiresonant Reflecting Optical Waveguide (ARROW), Wide Optical Waveguide (WOW), or the like could be used.