Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/923,370, filed Apr. 13, 2007 and U.S. Provisional Application Ser. No. 60/930,078, filed May 14, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. The Field of the Invention 
         [0003]    This application relates to distributed Bragg reflector lasers and, more particularly, to systems and methods for thermal tuning of a distributed Bragg reflector. 
         [0004]    2. The Relevant Technology 
         [0005]    In a DBR laser, a gain medium is in optical communication with one or more grating structures that define reflection peaks that control which wavelengths of light are reflected back into the gain section and amplified or output from the laser cavity. The grating structures therefore can be used to control the output spectrum of the laser. Where two grating structures are used having different free spectral ranges, the output spectrum of the laser is determined by the alignment of the reflective spectrum of the two grating structures. The alignment of the reflection spectrum may be shifted with respect to one another to accomplish a shift in the output frequency of the laser that is much larger than the frequency shift of the reflection spectrum due to the Vernier effect. 
         [0006]    In most DBR lasers current injection is used to tune the reflection peaks of the grating structures. However, current injection tends to degrade the materials of the DBR section over time, which limits the useful life of transmitters using current injection. 
         [0007]    In other DBR lasers the reflection spectrum is shifted by changing the temperature of the grating structures due to the thermo-optic effect. Temperature tuning does not shorten the useful life of a DBR laser to the same extent as current injection. However, prior temperature tuning systems and methods have high power requirements, slow frequency response, and narrow tuning bands. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the invention, a laser, such as a distributed Bragg reflector (DBR) laser, is formed on a base substrate comprising a semiconductor material such as InP. A gain medium is deposited on the base substrate. A wave guide is formed in optical communication with the gain section and having a substantial portion of the length thereof separated from the base substrate by a gap, which is preferably filled with air. The waveguide includes a grating structure such as a distributed Bragg reflector formed therein. A heating element is in thermal contact with the waveguide and a controller is electrically coupled to the heating element and configured to adjust optical properties of the waveguide by controlling power supplied to the heating element. 
         [0009]    In another aspect of the invention, the waveguide is formed in a raised substrate; the raised substrate has a lower surface, with the base substrate and lower surface defining the gap between the raised substrate and the base substrate. The raised substrate further includes exposed lateral surfaces perpendicular to the lower surface. 
         [0010]    In another aspect of the invention, the raised substrate is supported by pillars extending from the base substrate. 
         [0011]    In another aspect of the invention, a distributed Bragg reflector for a DBR laser is manufactured by forming a first layer of a first material, such as InP, forming a second layer of a second material, such as InGaAsP, and selectively etching the second layer to form at least two discrete areas defining a gap therebetween. 
         [0012]    Additional layers and a waveguide are then formed over the at least two discrete areas. An etching step is then performed through the additional layers to expose at least an edge of the at least two discrete areas. The at least two discrete areas are then exposed to an etchant that selectively removes the second material. 
         [0013]    These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0015]      FIG. 1  illustrates a laser transmitter suitable for use in accordance with embodiments of the present invention; 
           [0016]      FIG. 2  illustrates a distributed Bragg reflector (DBR) laser suitable for use in accordance with embodiments of the present invention; 
           [0017]      FIG. 3  illustrates a tunable twin guide sampled grating DBR laser suitable for use in accordance with embodiments of the present invention; 
           [0018]      FIG. 4  is an isometric view of a distributed Bragg reflector supported above a substrate by pillars in accordance with an embodiment of the present invention; 
           [0019]      FIGS. 5A through 5G  illustrate process steps for forming the distributed Bragg reflector of  FIG. 4 ; 
           [0020]      FIGS. 6A and 6B  illustrate alternative process steps for forming a distributed Bragg reflector supported by pillars comprising InGaAsP in accordance with an embodiment of the present invention; 
           [0021]      FIGS. 7A through 7C  illustrate process steps for protecting an InGaAsP contact layer during formation of pillars in accordance with an embodiment of the present invention; 
           [0022]      FIG. 8  is a cross sectional view of layers illustrating the formation of a protective SiO2 layer for shielding of an InGaAsP contact layer in accordance with an embodiment of the present invention; 
           [0023]      FIG. 9  is an isometric view of an alternative embodiment of a distributed Bragg reflector formed in a high-mesa structure in accordance with embodiments of the present invention; 
           [0024]      FIGS. 10A through 10C  illustrate process steps for forming the distributed Bragg reflector of  FIG. 9 ; 
           [0025]      FIG. 11  illustrates an alternative process for forming the distributed Bragg reflector of  FIG. 9 ; 
           [0026]      FIGS. 12A and 12B  illustrate another alternative process for forming the distributed Bragg reflector of  FIG. 9 ; and 
           [0027]      FIG. 13  illustrates another alternative process for forming the Distributed Bragg reflector of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Referring to  FIG. 1 , a transmitter system  10  may include a distributed laser  12  coupled to a data signal source  14  that supplies a modulation signal encoding binary data. The laser  12  may be a distributed Bragg reflector (DBR) laser, distributed feed back (DFB) laser, or other laser having one or more reflectors formed using a grating formed in or adjacent to a waveguide. The output of the laser  12  may be transmitted through an optical spectrum reshaper (OSR)  16 . The output of the OSR  16  may be transmitted through a fiber  18  to a receiver  20 . The OSR  16  converts a frequency modulated signal from the laser  12  to an amplitude modulated signal. In some embodiments, the output of the laser  12  is both frequency and amplitude modulated, such as adiabatically chirped pulses produced by a directly modulated DBR laser or distributed feedback (DFB) laser. The output of the OSR may also remain somewhat frequency modulated. 
         [0029]    The OSR  16  may be embodied as one or more filters, including, but not limited to, a coupled multi-cavity (CMC) filter, a periodic multi-cavity etalon, a fiber Bragg grating, a ring resonator filter or any other optical element having a wavelength-dependent loss. The OSR  16  may also comprise a fiber, a Gire-Tournois interferometer, or some other element with chromatic dispersion. 
         [0030]    In some methods of use the laser  12  is modulated between a peak and a base frequency in order to encode a data signal in the output of the laser  12 . In some embodiments the output of the laser  12  will also be modulated between peak and base amplitudes. The OSR  16  has a transmission function aligned with the base and peak frequencies such that the base frequency is attenuated more than the peak frequency in order to improve the extinction ratio of the output of the OSR  16 . 
         [0031]    Referring to  FIGS. 2 and 3 , various DBR lasers  12  may be used with the present invention. Although  FIGS. 2 and 3  illustrate two examples, they are not limiting of the type of DBR lasers that may benefit from embodiments of the present invention. 
         [0032]    Referring specifically to  FIG. 2 , a DBR section  22  receives light from a gain section  24 . The laser  12  may include other sections such as a phase control section  26  and/or electro-absorption section  28 . The gain section  24  and other sections such as the phase control section  26  and electro-absorption section  28  may be positioned between the DBR section  22  and a filter  30 . In some embodiments the filter  30  may be embodied as another DBR section. 
         [0033]    Referring to  FIG. 3 , another example of a DBR laser is a tunable twin guide sampled grating DBR (TTG-SG DBR), which includes a DBR section  22  embodied as two sampled gratings  22   a ,  22   b . The sampled gratings  22   a ,  22   b  are coupled to the gain section  24  by means of a multi-mode interface (MMI)  32 . The sampled gratings  22   a ,  22   b  preferably have reflection peaks having a different free spectral range such that the reflection peaks of the combined sampled gratings  22   a ,  22   b  may be tuned using the Vernier effect. 
         [0034]    In a DBR laser, such as those shown in  FIGS. 2 and 3 , a grating structure within the DBR section  22  defines reflection peaks that control which wavelengths of light are reflected back into the gain section  24 . The DBR section  22  therefore determines the output spectrum of the laser. The reflection peaks of the DBR section  22  may be shifted by means of current injection or heating due to the thermo-optic effect in order to control the output spectrum of the laser. 
         [0035]    Although current injection is a widely used means for tuning, it tends to degrade the materials of the DBR section over time, which limits the useful life of transmitters using current injection. Temperature tuning does not shorten the useful life of a DBR laser to the same extent as current injection. However, prior temperature tuning systems and methods have high power requirements, slow frequency response, and narrow tuning bands. 
         [0036]    Referring to  FIG. 4 , in some embodiments, a DBR section  22  may be formed in a waveguide  38  that is separated from a base substrate  40  by an air gap. In the illustrated embodiment, the waveguide  38  is formed in a raised substrate  42  supported above the base substrate by pillars  44 . The pillars have a height  46  that defines the height of the air gap between the raised substrate  42  and the base substrate  40 . The separation  48  between the pillars  44  is preferably much larger than the width  50  of the pillars  44  such that a majority of the length of the DBR section  22  is separated from the base substrate by an air gap. In a preferred embodiment, at least 90 percent of the length of the DBR section  22  parallel to the direction of propagation of light within the DBR section  22  is separated from the base substrate by an air gap. 
         [0037]    The material forming the pillars  44  may be the same as, or different from, the material forming the base substrate  40  and/or raised substrate  42 . For example, the pillars  44  may be formed of indium phosphide (InP), indium gallium arsenide phosphide (InGaAsP), or the like. In some embodiments 1.3 Q InGaAsP is used for the pillars  44  due to its highly insulative properties. 
         [0038]    The raised portion  42  of the substrate may include a heated portion  52  and a non-heated portion  54 . The DBR section  22  is preferably located in the heated portion whereas the gain section  24 , phase section  26 , and/or electro-absorption section are located in the un-heated portion  54 . 
         [0039]    In some embodiments, the DBR section  22  includes a sampled grating including gratings formed only at discrete areas  56  along the waveguide  38 . In such embodiments, heaters  60  may be formed only on the discrete areas  56 . The heaters  60  may be embodied as platinum stripe heaters. In such embodiments, metal layers  62 , such as gold, may be deposited between the discrete areas  56  to reduce heating of other portions of the waveguide  38 . In one embodiment, parallel to the optical axis of the waveguide  38 , the heaters  60  have a length of about 10 μm and the metal layers  62  have a length of 70 μm. In some embodiments, the pillars  44  are located at or near a mid point between discrete areas  56 , such as between 40 and 60 percent of a distance between the pillars. 
         [0040]    The air gap insulates the waveguide  38  from the base substrate  40  and reduces the power required to raise the temperature of the waveguide  38  in order to tune the response of the DBR section  22 . It also reduces the time required to raise the temperature of the waveguide  38 . 
         [0041]    Referring to  FIGS. 5A through 5G , an air gap may be created between the raised substrate  42  and the base substrate  40  by performing the illustrated steps. Referring specifically to  FIG. 5A , an n-InP substrate  70  is formed having an InGaAsP layer  72  and n-InP layer  74  formed thereon. The InGaAsP layer  72  may be about 0.1 μm thick and the n-InP layer is preferably 30 nm thick, however other thicknesses are also possible. The InGaAsP may have a bandgap wavelength of 1.3 μm. 
         [0042]    Referring to  FIG. 5B , silicon oxide (SiO 2 ) areas  76  may then be formed on the upper n-InP layer  74 . A gap  78  between adjacent areas  76  may have a width of 3 μm. As is apparent below, the width of the gap determines the width  50  of the pillars  44 . The areas  76  have a length  80  that defines the length of the air gap between the raised substrate  42  and base substrate  40 . Thus, the width of the gap  78  may be less than 90 percent of the length  80 . In the illustrated embodiment, the areas  76  have a width of about 10 μm perpendicular to the optical axis of the waveguide  38  formed in subsequent steps and a length of about 30 μm parallel to the optical axis of the waveguide  38 . In the illustrated example, the gap  78  is about equal to 3 μm in the direction parallel to the optical axis of the waveguide  38 . Other values may be used depending on the pillar size and air gap length desired. 
         [0043]    Referring to  FIG. 5C , the layers of the previous figures are then selectively etched to form the structure of  FIG. 5C , wherein portions of the n-InP layer  74  and InGaAsP layer  72  that are not covered by the SiO 2  areas  76  are etched away. 
         [0044]    Referring to  FIG. 5D , another n-InP layer  82  is grown over the remaining layers. In some embodiments the SiO 2  areas  76  are also removed. Referring to  FIG. 5E , layers for formation of the DBR laser  12  may then be formed on the n-InP layer  82 . Various layers may be grown as known in the art to form any of various types of lasers and grating structures known in the art. As an example, a multi-quantum well (MQW) layer  84  and p-InP layer  86  are grown as illustrated. In the illustrated example, the n-InP layer  86  has a thickness of about 3 μm. Referring to  FIG. 5F , an active MQW portion  88  and passive DBR portion  90  may then be formed coupled to one another by a butt joint according to known methods. Fe-InP blocking portions  92   a ,  92   b  may be formed along the MQW portions  88  and passive DBR portion  90  as known in the art. The passive DBR portion  90  may be embodied as a sampled grating DBR. However, other structures may be formed as known in the art to form other laser and/or grating types 
         [0045]    Referring to  FIG. 5G , the layers may then be selectively etched on either side of the DBR portion  90 . The etching may be performed using dry etching, deep reactive ion etching, or the like. The volume removed during the etching step preferably extends up to and including the InGaAsP layer  72 . The remaining InGaAsP layer  72  is then selectively removed in a wet etching step, such as by using an etchant that dissolves InGaAsP substantially faster than other materials forming other layers that are exposed to the etchant, such as InP. Upon removal of the InGaAsP layer, portions of the InP layer  82  between the remaining areas of the InGaAsP layers then become the pillars  44 . 
         [0046]    Referring to  FIG. 6A , in an alternative embodiment, the pillars  44  include InGaAsP, rather than only InP. Such embodiments provide the advantage of having improved insulative properties, which further reduce power consumption. In such embodiments, the SiO 2  areas  76  illustrated in  FIG. 5B  are replaced with areas  94   a ,  94   b  having an area  96  positioned therebetween. The area  96  is narrower than the areas  94   a ,  94   b  and is separated from the areas  94   a ,  94   b  by a small gap. 
         [0047]    For example, parallel to the optical axis of the waveguide  38 , the area  96  is separated from each area  94   a ,  94   b  by a gap of between 10 and 25 percent of the length of the area  96 . The length of the area  96  parallel to the optical axis of the waveguide  38  may be between five and ten percent of the lengths of the areas  94   a ,  94   b . Perpendicular to the optical axis of the waveguide  38 , the area  96  may have a width that is between 20 and 50 percent of the width of one of the areas  94   a ,  94   b . In the illustrated example, parallel to the optical axis of the waveguide  38 , the area  96  is separated from each area  94   a ,  94   b  by a gap of 0.5 μm and has a length of 3 μm. Perpendicular to the optical axis of the waveguide  38 , the area  96  may have a width of 3 μm whereas the areas  94   a ,  94   b  have widths of 10 μm. 
         [0048]    The other steps of  FIGS. 5C through 5F  may then be performed as described above. Referring to  FIG. 6B , when the dry etching step of  FIG. 5G  is performed up to the lines  98 , area  100  of InP remains and shields the portion of the InGaAsP layer  72  that was beneath area  96  from etching whereas the portion of the InGaAsP layer  72  that is beneath areas  94   a ,  94   b  is exposed and is etched away. Thus a pillar  44  having an InGaAsP center remains to support the raised substrate  42 . 
         [0049]    Referring to  FIGS. 7A through 7C , in some laser designs, an InGaAsP contact layer  102  is formed as part of the DBR laser  12  formed in step  5 F, or in another step prior to performing the steps of  FIG. 5G . In such embodiments, the wet etching step of  FIG. 5G  using an etchant that removes InGaAsP may damage the contact layer  102 . Accordingly, in such embodiments, an SiO 2  layer is formed to protect the contact layer prior to the etching step of  FIG. 5G , 
         [0050]    In one embodiment, the protective SiO 2  layer is formed by forming the structure illustrated in  FIG. 7A , having a thick SiO 2  etching mask  104  deposited on the contact layer up to the boundary where dry etching occurs in the dry etching step of  FIG. 5G . A slight undercut is formed in the contact layer  102 . The undercut may have, for example, a depth less than the thickness of the contact layer  102 . 
         [0051]    Referring to  FIG. 7B , an SiO 2  overcoat  106  is then formed over the SiO 2  etching mask  104  and surrounding exposed surfaces. Referring to  FIG. 8 , SiO 2  growth at the gap between the SiO 2  etching mask  104  and a layer  108  supporting the contact layer  102  projects beyond the mask  104  and  108 , such that a barrier spanning the gap is formed effective to protect the InGaAsP contact layer  102 . 
         [0052]    Referring to  FIG. 7C , the dry etching step of  FIG. 5G  progresses downwardly through the layers, removing some of the SiO 2  overcoat  106 , especially portions on horizontal surfaces. However, vertical portions of the SiO 2  overcoat  106  remain and protect the InGaAsP contact layer  102  whereas the lower InGaAsP layer  72  is exposed to wet etching. 
         [0053]    Referring to  FIG. 9 , in an alternative embodiment, a waveguide  38  having a distributed Bragg reflector formed therein is embedded within a high-mesa structure that isolates the waveguide  38  in order to improve thermal tuning efficiency. In the illustrated embodiment, the waveguide  38  is formed in an upper layer  120  of a multi layer structure. An insulative layer  122  is formed between the upper layer  120  and a lower layer  124 . In some embodiments, the upper layer  120  and lower layer  124  are formed of InP whereas the insulative layer  122  includes 1.3Q InGaAsP, which has much lower thermal conductivity than InP. In the illustrated embodiment, the insulative layer  122  has a height of 0.8 μm and a width of 3 μm, whereas the upper and lower layers  120 ,  124  have widths of 5 μm. The combined height of the layers  120 ,  122 ,  124  is 5 μm in the illustrated example. 
         [0054]    Areas  128  one either side of the waveguide  38  are etched, such as by dry etching to expose vertical faces of the upper layer  120  and lower layer  124 . In some embodiments, only layers  120  and  122  such that the lower layer  124  does not include exposed faces parallel to the exposed vertical faces of the upper layer  120 . The insulative layer  124  may be etched to form an undercut  129  between the upper layer  120  and lower layer  124  to further decrease the thermal conductivity therebetween. A heater  130 , such as a platinum stripe heater, may be deposited on the upper layer  120  to control the temperature of the waveguide  38 . 
         [0055]    Referring to  FIG. 10A , the high-mesa structure of  FIG. 9  may be formed by first forming a 1.3Q InGaAsP layer  132  on an InP substrate  134 . A second InP layer  136  is then formed on the layer  134 . Referring to  FIG. 10B , the structure of  FIG. 10A , is masked and etched to form parallel areas  138   a ,  138   b  of 1.3Q InGaAsP positioned in correspondence to the DBR reflectors of a DBR laser  12 . Referring to  FIG. 10C , an InP spacer layer  140  is then formed over the InP layer  134  and 1.3Q InGaAsP areas  138   a ,  138   b . One or more DBR sections  142 , a multi-mode interface (MMI)  144 , and a gain section  146  may then be formed on the InP spacer layer  140 . An additional InP layer  148  may be formed over the DBR sections  142  and MMI  144 . As is apparent in  FIG. 10C , the DBR  142  and MMI  144  are offset from one another due to the thickness of the InGaAsP areas  138   a ,  138   b , which may result in some coupling losses. However, the InP spacer layer  140  is preferably sufficiently thick to reduce losses to acceptable levels. 
         [0056]    Referring to  FIG. 11 , in an alternative embodiment, alignment between the DBR sections  142  and the MMI  144  may be improved by creating additional areas  150  and  152  of 1.3Q InGaAsP positioned under the MMI  144  and gain section  146 , respectively. Inasmuch as the area  152  under the gain section  146  is embedded within surrounding InP layer in the final product, heat is able to dissipate from the gain section not withstanding the presence of the 1.3Q InGaAsP area  152 . 
         [0057]    Referring to  FIGS. 12A and 12B , in an alternative embodiment, coupling between the DBR sections  142  and the MMI  144  is improved by performing a planarizing step prior to formation of the DBR sections  142  and MMI  144 . For example, the InGaAsP layer  132  and second InP layer  136 , such as are shown in  Figure 10A , may be selectively etched to leave areas  138   a ,  138   b  of the InGaAsP layer  132 . A mask layer  154  may be formed over the areas  152 . Alternatively the layer  154  may include portions of the second InP layer  136  that remain after selective etching. A third InP layer  156  is then selectively grown around the areas  138   a ,  138   b  and the upper surface of the layers is then planarized. The DBR sections  142 , MMI  144 , and gain section  146  are then formed having the DBR sections formed over the areas  138   a ,  138   b.    
         [0058]    Referring to  FIG. 13 , in another alternative embodiment, following the selective etching step of  FIG. 10B  that forms form parallel areas  138   a ,  138   b , areas  158  of a masking material, such as SiO 2 , are formed adjacent an area where the MMI  144  and gain section  146  are formed in subsequent steps. A third InP layer  160  is then grown over areas not covered by the areas  158  of masking material, including over the areas where the MMI  144  and gain section  146  are formed and over the parallel areas  138   a ,  138   b  of 1.3Q InGaAsP. The third InP layer  160  is then planarized and the DBR sections  142 , MMI  144 , and gain section  146  are formed. 
         [0059]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Technology Category: h