Method for making a high power semiconductor laser diode

Abstract

Semiconductor laser diodes, particularly high power ridge waveguide laser diodes, are often used in opto-electronics as so-called pump laser diodes for fiber amplifiers in optical communication lines. To provide the desired high power output and stability of such a laser diode and avoid degradation during use, the present invention concerns an improved design of such a device, the improvement concerns a method of suppressing the undesired first and higher order modes of the laser which consume energy and do not contribute to the optical output of the laser, thus reducing it's efficiency. This novel effect is provided by a structure comprising CIG—for Complex Index Guiding—elements on top of the laser diode, said CIG being established by fabricating CIG elements consisting of one or a plurality of layers and containing at least one layer which provides the optical absorption of undesired modes of the lasing wavelength. This CIG preferably contains an insulating layer as a first contact layer to the semiconductor. The CIG elements are manufactured by a selected sequence of processing steps, in particular several masking steps, and are specifically shaped, both in thickness and coverage of the lasers semiconductor body, to provide desired suppression characteristics. Further, the CIG elements may be combined with the contact layer usually providing the electrical input power to the laser diode.

1 . A method for making a high power laser diode with a semiconductor body and a ridge waveguide as active region, 
 comprising the following steps    (a) providing said semiconductor body with said ridge waveguide by a first mask, in particular a photoresist mask,    (b) depositing an insulator layer over at least part of said semiconductor body including said first mask,    (c) depositing a photoresist on said insulator layer,    (d) removing part of said photoresist in a controlled way to provide a second mask,    (e) removing or thinning at least part of said insulator layer where it is uncovered by said second mask,    (f) removing both said first and said second masks, and    (g) depositing an absorption layer, in particular as part of a complex index guiding (CIG) element.    
   
   
       2 . The method according to  claim 1 , wherein 
 the second mask has a predetermined size wider than the ridge—waveguide.    
   
   
       3 . The method according to  claim 1 , wherein 
 the absorption layer serves as part of a complex index guiding (CIG) element and as contact layer.    
   
   
       4 . The method according to  claim 1 , wherein 
 the insulator is thinned to a predetermined thickness where it is uncovered by the second mask to provide insulator areas of a first thickness under said second mask and of a second thickness outside said second mask.    
   
   
       5 . The method according to  claim 1 , wherein 
 a third mask is applied as one of a plurality of process steps in generating multiple CIG elements to both sides of the optical axis of the waveguide.    
   
   
       6 . The method according to  claim 5 , wherein 
 the third mask provides for two or more longitudinally contiguous CIG sections, each said section having a different lateral extension, in particular at least one of said sections extending laterally to the border of the semiconductor body.    
   
   
       7 . The method according to  claim 1 , including 
 (f1) removing together with the first and the second masks at least part of a first insulator layer,    (f2) depositing a second, preferably thin, insulator layer over at least part of the semiconductor body including the ridge waveguide, and    (f3) before depositing the absorption layer, removing said second insulator layer at least partly in a contact region of said ridge waveguide,    (h) depositing a contact layer, in particular a P-contact layer.    
   
   
       8 . The method according to  claim 7 , including the steps of 
 (f2′) after deposition of the second insulator layer, depositing an absorption layer on said second insulator layer over at least part of the semiconductor body including the ridge waveguide, and    (f3′) at least partly removing said absorption layer and said second insulator layer in a contact region of said ridge.    
   
   
       9 . The method according to  claim 7 , including the steps of 
 (f2″) after deposition of the second insulator layer, depositing a stack of absorbing layers and insulator layers over at least part of the semiconductor body including the ridge waveguide, and    (f3″) removing said stack and said second insulator layer in a contact region of said ridge waveguide, essentially leaving said stack as CIG elements at both sides of said ridge waveguide.    
   
   
       10 . The method according to  claim 7 , wherein 
 the contact layer serves as part of the CIG element.    
   
   
       11 . The method according to  claim 1 , wherein 
 the insulator layer, especially Si 3 N 4 , is deposited over essentially the whole surface of the semiconductor body,    the photoresist is deposited over at least the center part of said semiconductor body,    said photoresist is removed, especially etched, to a desired distance from said ridge waveguide, thus providing the second mask,    said insulator layer is thinned or removed, especially etched, in particular etched down to the semiconductor surface so that only insulator areas covered by said second mask remains,    said masks are removed by lifting off,    at least one conductive layer is deposited as absorption layer of a complex index guiding (CIG) element, said conductive layer including at least one of Ti, Cr, Pt, Au, Si, or Ge.    
   
   
       12 . A method for making a laser diode with a semiconductor body having an active region, a lower cladding layer, an upper cladding layer with a ridge waveguide, and a top metallization for current injection, said laser diode further including an optically absorbing element for suppressing first and higher order modes of said laser diode, said absorbing element being part of one or more complex index guiding (CIG) elements, the method comprising: fabricating an insulation layer and an absorption layer, the insulation layer being provided on at least part of said upper cladding layer, separating at least part of said absorption layer from said laser semiconductor body, whereby said insulation layer is fabricated with a predetermined thickness having a maximum close to said ridge waveguide and a minimum, including zero, distant from said ridge waveguide.  
   
   
       13 . The method according to  claim 12 , wherein 
 at least one CIG element is fabricated to comprise or consist of two or more sections located along the optical axis of the waveguide, each said section having a predetermined extension along the optical axis of said waveguide    
   
   
       14 . The method according to  claim 12 , wherein 
 two sections each of substantially constant thickness, a first, greater thickness close to the ridge waveguide and a second, smaller thickness distant from said ridge waveguide are fabricated, said two sections forming the insulation layer separating the absorption layer from the semiconductor body.    
   
   
       15 . The method according to  claim 12 , wherein 
 the CIG element is fabricated as a plurality, or stack of, insulating and absorption layers.    
   
   
       16 . The method according to  claim 12 , wherein 
 at least two CIG elements are fabricated as layered structures, preferably located on both sides of the ridge waveguide and extending along part of or the full length of the semiconductor body.    
   
   
       17 . The method according to  claim 12 , wherein 
 the CIG element is shaped for maximizing the ratio of the suppression of first and higher order modes to the suppression of the fundamental mode.    
   
   
       18 . The method according to  claim 12 , wherein 
 the semiconductor body is made of a first material, including GaAs or InP based materials, and the complex index guiding element is made of a second material or a stack of second materials, in particular either a conductor or a semiconductor, including at least one of Ti, Cr, Pt, Au, Si, Ge, or an insulator, in particular at least one of TiO 2 , Si 3 N 4 , AlN, SiO 2 .    
   
   
       19 . The laser diode according to  claim 12 , wherein 
 the insulation layer is fabricated to separate the absorption layer from the laser semiconductor body in the vicinity of the ridge waveguide only, preferably covering at least part of the sides of said ridge and/or part of said semiconductor body.    
   
   
       20 . The method according to  claim 12 , wherein 
 the first greater thickness of the insulator close to the ridge waveguide is chosen to minimize absorption of the fundamental mode by the absorption layer, preferably to zero, and the second smaller thickness distant from the ridge waveguide is chosen to maximize absorption of the first and higher order mode, while keeping absorption of the fundamental mode at a minimum.    
   
   
       21 . The method according to  claim 15 , wherein 
 materials and/or thickness for at least one CIG element are selected to maximize the ratio of the suppression of first and higher order modes to the suppression of the fundamental mode, in particular maximizing suppression of first and higher order modes while minimizing the absorption of the fundamental mode.    
   
   
       22 . The method according to  claim 12 , wherein 
 the two sections of the absorption layer are fabricated to abut against a common shoulder which is self-aligned with the ridge waveguide.    
   
   
       23 . A method for making a high power diode with a semiconductor body and a ridge waveguide laser as active region, 
 comprising the following steps    (a) providing a first mask over said ridge waveguide,    (b) depositing a first insulator layer over at least part of said semiconductor body,    (c) depositing a photoresist over at least the center part of said semiconductor body,    (d) thinning or removing, especially etching, said photoresist to a desired distance from said ridge waveguide, thus providing a second mask, exposing part of said semiconductor body,    (e) thinning or removing at least part of said insulator layer where it is uncovered by said second mask,    (f) depositing at least one absorption layer as part of a complex index guiding (CIG) structure over at least part of said semiconductor body, and    (g) lifting off both said masks, thus exposing said ridge waveguide and said semiconductor body at least partly, whereby parts of said first insulator layer and of said absorption layer remain on said semiconductor body, and    (h) depositing a further layer as contact layer, especially a P-contact layer.    
   
   
       24 . The method according to  claim 23 , wherein 
 step (f) is replaced by step (f):    depositing, over at least part of the semiconductor body distant from the waveguide ridge, an insulating layer and an absorption layer or a stack of alternating insulating and absorption layers as part of a complex index guiding (CIG) structure.    
   
   
       25 . A method for making a high power diode with a semiconductor body and a ridge waveguide laser as active region, comprising the following steps 
 (a) providing a first mask over said ridge waveguide,    (b) depositing a photoresist over at least part of said semiconductor body,    (c) removing, especially etching, said photoresist to a desired, variable distance from said ridge waveguide, thus providing a second mask, exposing part of said semiconductor body,    (d) depositing an absorption layer as part of a complex index guiding (CIG) structure over at least part of said semiconductor body, said first mask and said second mask,    (e) lifting off both said first and said second masks, thus exposing at least part of said ridge waveguide and of said semiconductor body, whereby parts of said absorption layer remain on said semiconductor body,    (f) depositing an insulator layer over at least part of said semiconductor body,    (g) opening a contact area, especially on top of said ridge waveguide, and    (h) depositing a further layer as contact layer, especially a P-contact layer.    
   
   
       26 . The method according to  claim 25 , wherein 
 the second mask has a predetermined size wider than said ridge waveguide.    
   
   
       27 . The method according to  claim 25 , wherein 
 step (d) is replaced by step (d′):    depositing an absorption layer and an insulator layer or a stack of absorption and insulator layers over at least part of the semiconductor body as part of a complex index guiding (CIG) structure.    
   
   
       28 . The method according to  claim 25 , wherein 
 a third mask is applied as one of a plurality of process steps in generating multiple CIG elements to both sides of the optical axis of the waveguide.    
   
   
       29 . A high power laser diode fabricated according to a method defined in any of the claims  1 ,  4 ,  7 ,  10 ,  11 ,  22 , or  24 , said laser diode comprising a semiconductor body, a ridge waveguide, an active region, and a structure of optically absorbing elements for suppressing selected modes of said laser diode, said structure constituting part of one or more complex index guiding (CIG) elements, 
 said laser diode preferably having front and back facets and, extending between said facets, said ridge waveguide having: 
 a center segment with a substantially constant first cross section, preferably having a length of 40-70% of the diode length,  
 two tapered segments extending and widening from the center segment towards said facets in opposite direction, and  
 two end segments between said tapered segments and said facets, each said end segment having a substantially constant cross section larger than said first cross section,  
 in particular a first one of said tapered segments having a length of about 30-60% of the diode length and a second one of said tapered segments having a length of up to 10% of the diode length.  
   
   
   
       30 . A high power laser diode fabricated according to a method defined in  claim 25 , said laser diode comprising: 
 a semiconductor body, an active region, a lower cladding layer, an upper cladding layer with a ridge waveguide, a top metallization for current injection, said laser diode further including a structure of optically absorbing elements constituting part of one or more complex index guiding (CIG) elements for suppressing selected, especially first and higher, order modes of said laser diode, said structure extending along the length of said semiconductor body with a predetermined width across said semiconductor body, preferably having a variable width or having sections with at least two widths, preferably one wider width extending across a first part of the semiconductor body and one narrower width extending across only a fraction or a second part of said semiconductor body.