Abstract:
A beam expander for providing coupling between a semiconductor optical device and an optical fiber comprises a double layer structure that may be integrated with the optical device. The first, underlying layer of the expander comprises a relatively high refractive index material (e.g., 3.34), thus providing improved coupling efficiency between the optical device and the fiber. The second, covering layer of the expander comprises a relatively low refractive index material (e.g., 3.28), for providing the large mode size desired at the fiber input. The parameters of each layer can be adjusted independently, allowing for the two criteria (coupling efficiency and mode size) to be separately optimized.

Description:
TECHNICAL FIELD 
     The present invention relates to a beam expander for providing coupling between a semiconductor optical device and associated optical fiber and, more particularly, to a double layer beam expander integrated with the semiconductor optical device. 
     BACKGROUND OF THE INVENTION 
     Within recent years, the number of electronic applications that employ optical devices has been rapidly increasing. Typically, optical fibers are used to carry light signals from (or to) a variety of optical devices, such as lasers, amplifiers, modulators, splitters, multiplexers/demultiplexers, routers, switches and photodetectors. As is well known, the use of optical fibers and devices leads to higher data throughputs and increased communication channel bandwidths. These higher data rates result in an ever-increasing concern for maximizing the coupling efficiency between the optical device and associated fiber. In particular, low fiber coupling efficiency of laser diodes has been a major limitation for high power single mode fiber output. In a conventional laser diode, optical confinement in the semiconductor structure is asymmetric and the propagating mode profile is elliptical in shape. Also, the mode profile of these high power diode laser sources results in large beam divergence. The stronger divergence is normally in the vertical (i.e., transverse) direction due to the strong optical confinement in the vertical direction in the layered semiconductor laser structure, as opposed to the weaker optical confinement in the horizontal (i.e., lateral) direction. 
     This highly divergent, elliptical laser diode output beam profile presents a difficulty when attempting to couple the light from a high power laser diode source to a single mode optical fiber. This difficulty is primarily due to the large mode mismatch between the highly divergent semiconductor laser source and the small numerical aperture optical fiber. For example, the laser spot size is typically around 1 μm, while that of a fiber core is around 10 μm. This disparity limits the coupling efficiency between these two devices to about 10%, if perfectly aligned. Thus, a high power laser diode with a circular mode profile and a narrow far-field divergence is particularly desirable for efficient fiber coupling. 
     A number of techniques have been developed to increase the coupling efficiency between a semiconductor laser and an optical fiber. These include modifying the shape of the fiber end (e.g., tapering, lensing) so that the modal mismatch is reduced. However, the coupling efficiency is improved at the expense of very tight alignment tolerances. Untapered fibers have also shown to perform well, but there is a need for an additional lens between the laser and a modified end face fiber, increasing the difficulty of assembly. Still another approach is to modify the laser structure so that it has a tapered output section, thus increasing the spot size in the junction plane. As a variation of this approach, a tapered waveguide may be positioned between the laser and the fiber. The tapered waveguide comprises a layer with a low refractive index and is monolithically integrated with the laser. The composition of this layer is critical, since if the effective index mismatch between the laser and the tapered waveguide is high, a significant fraction of light is reflected or lost due to scattering. Alternatively, if the refractive index of the tapered waveguide is matched with the refractive index of the laser, lateral and/or transverse variation of its thickness is necessary to achieve mode expansion. Moreover, to obtain high coupling efficiency at the waveguide joint and large mode size at the tapered waveguide output, a variation of thickness in the tapered waveguide by a factor of approximately four is required. 
     Thus, a need remains in the art for an arrangement for improving the coupling efficiency between a semiconductor optical device and a fiber that is relatively easy to manufacture and include in an assembly, yet provides the desired high coupling efficiency. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to a beam expander for providing coupling between a semiconductor optical device and associated optical fiber and, more particularly, to a double layer beam expander integrated with the semiconductor optical device. 
     In accordance with the present invention, a semiconductor optical device structure is formed to include a double layer beam expander disposed at the output thereof. The beam expander comprises a first layer of high index material (e.g., InGaAsP) to provide high coupling efficiency and a second layer of low index material (e.g., In 1-x Ga x As y P 1-y ) to provide the required large mode size. In particular, the refractive index of the second layer will depend upon the values of x and y, 0&lt;x&lt;1 and 0&lt;y&lt;1, since in the extreme case InP cannot be used as a waveguiding layer. The first layer is deposited to taper away from the laser endface facet and the second layer is formed to cover the first layer. In an exemplary embodiment of the present invention for use with a laser having an emitting wavelength of 1.55 μm, the first (high birefringence) layer of the double layer beam expander may comprise InGaAsP with a refractive index of 3.34, and the second (low birefringence) layer of the double layer beam expander may comprise In 1-x Ga x As y P 1-y  with a refractive index of 3.28. 
     In one exemplary method of forming the double layer beam expander of the present invention, a first mask is used to cover the optical device region and a second mask is used to define the terminating endface of the first layer, with the first layer then deposited in the region between the first and second masks. The second layer is deposited after the second mask is removed. In an alternative method of forming the double layer beam expander, a first mask is used to cover the optical device and the first layer is blanket deposited on the remaining substrate surface. A second mask is then used to cover the predefined extent of the first layer and an etch step is used to remove the exposed layer. The second mask is then removed and the second layer is deposited. 
     Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, 
     FIG. 1 illustrates a conventional beam expander arrangement; 
     FIG. 2 illustrates a double layer beam expander formed in accordance with an embodiment of the present invention; 
     FIGS. 3-7 illustrate an exemplary process of forming the double layer beam expander of FIG. 2; and 
     FIGS. 8-12 illustrate an alternative process of forming a double layer beam expander in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     A conventional beam expander coupling arrangement  10  typical of the prior art is illustrated in FIG.  1 . As shown, a semiconductor optical device (in this example, a laser)  12  (nominally, an InP-based laser) is formed on a substrate  14 , where the substrate may also comprise InP. A tapered waveguide beam expander  16  is formed as shown to extend from an output facet endface  18  of laser  12  to a predetermined termination endface  20 . Although not shown in FIG. 1, a conventional optical fiber would then be coupled to terminating endface  20  of tapered waveguide beam expander  16 . A low refractive index material (such as In 1-x Ga x As y P 1-y ) is typically used to form beam expander  16 . The lateral variation in thickness of beam expander  16  from a maximum at laser endface  18  (denoted “A” in FIG. 1) to a minimum at terminating endface  20  (denoted “B” in FIG. 1) is necessary to achieve both sufficient coupling between the laser and the expander at the expander input and a large mode size at the expander output (that is, where the expander will couple into an optical fiber). In particular, the taper along the extent of beam expander  16  decreases its effective index (the effective index being determined by the refractive index of the material and the thickness of the waveguiding layer). To obtain a high coupling efficiency at laser endface  18  and large mode size at terminating endface  20 , a thickness ratio (A/B) of approximately four is required. Such a thickness ratio (also referred to hereinafter as a “thickness enhancement ratio”) is considered to be relatively large for current integrated optic applications and will become even more problematic as device sizes continue to shrink. 
     FIG. 2 illustrates an exemplary double layer beam expander  30  formed in accordance with the present invention. As will be discussed in detail below, beam expander  30  comprises a first layer  32  of relatively high refractive index material (e.g., InGaAsP, with, for example an index of 3.34) that serves to provide improved coupling efficiency with laser  12  at endface  18 . A second layer  34  of beam expander  30  comprises a relatively low refractive index material (e.g., In 1-x Ga x As y P 1-y  (0&lt;x&lt;1, 0&lt;y&lt;1) and comprising a different composition than first layer  32  with, for example, an index of 3.28) so as to provide improved mode coupling (i.e., large mode size) between terminating endface  36  of beam expander  30  and an optical fiber (not shown). An additional InP layer  38  may be formed between first layer  32  and second layer  34  to provide for lattice matching between the layers. 
     An advantage of the double layer expander of the present invention is that the characteristics of each layer can be individually tailored to accommodate the different expander requirements. For example, since high index layer  32  no longer needs to also provide large mode size, the thickness enhancement ratio (A/B) of this layer can be reduced to a factor of two or three, which is much easier to obtain and control in manufacture. The capability of using a relatively high index material without having to worry about the eventual mode size of the guided beam allows for increased coupling efficiency between the laser and the beam expander over conventional arrangements such as that illustrated in FIG.  1 . Additionally, the use of a separate layer to improve the mode coupling to the output fiber allows for a relatively low index material to be used (matching that of the fiber, for example) without needing to be concerned with the coupling efficiency at the laser output. 
     An exemplary process of forming a double layer beam expander will be now be described in detail. Referring to FIG. 3, a conventional laser structure  12  is first grown on substrate  14 , where laser  12  comprises successive layers of different InP-based materials and dopants so as to form separate cladding layers and an active region (not shown), where the active region is used to generate the light output from the device. After laser structure  12  is formed, a first mask  44  is disposed on top surface  46  of structure  12  so as to cover the extent of the final laser device and define endface  18  of laser  12 . FIG. 4 illustrates the placement of first mask  44  on top surface  46 , subsequent to the removal of the extraneous laser material using a conventional etchant (such as bromine ethanol, or a suitable dry etch). A top view of this arrangement is illustrated in FIG.  5 . Referring back to FIG. 4, once the extraneous laser material is removed by etching, a second mask  50  is disposed on surface  52  of substrate  14 , where second mask  50  is used to define the termination of the double layer expander upon formation. The dimensions of second mask  50  are roughly as shown in FIGS. 4 and 5, where second mask  50  is shown as relatively narrow and centered with respect to first mask  44 . The dimensions of opening  48  in first mask  44  are not considered critical. 
     Once second mask  50  is in place, a first layer  54  of an exemplary double expander is formed, as shown in FIG.  6 . First layer  54  comprises a material exhibiting a relatively high refractive index (for example, InGaAsP, refractive index of 3.34) and is formed using a selective area growth (SAG) technique so as to comprise a tapered profile as shown. In contrast to the prior art, the taper does not have to be extreme and exhibits a thickness enhancement factor (A/B) on the order of two or three. The SAG technique allows for first layer  54  to form only in the region between first mask  44  and second mask  50  and be tapered such that a first vertical sidewall  56  of first layer  54  will coincide with endface  18  of laser  12  and will extend upward to the junction of endface  18  and first mask  44 . A relatively thin index-matching layer  58  is then deposited to cover first layer  54 . FIG. 6 illustrates the expander structure at this point in the process. 
     Subsequent to the formation of index-matching layer  58 , second mask  50  is removed and second expander layer  60  is grown to conformally coat both underlying index-matching layer  58  and the exposed surface  52  of substrate  14 . As discussed above, second expander layer  60  comprises a relatively low refractive index material (e.g., index of 3.28), such as In 1-x Ga x As y P 1-y , (0&lt;x&lt;1, 0&lt;y&lt;1) and is used to provide mode matching (i.e., large mode size) between laser structure  12  and an optical fiber (not shown). 
     An alternative processing sequence of forming a double layer beam expander in accordance with the present invention is illustrated in FIGS. 8-12. This process begins with the same steps as discussed above in association with FIGS. 3 and 4, obtaining a structure as shown in FIG.  8 . However, instead of depositing a second mask layer on this structure, a selective area growth (SAG) process is used to form a first beam expander layer  70 , as shown in FIG. 9, where first beam expander layer  70  tapers away from endface  18  of laser  12  and extends in the lateral direction across top major surface  52  of substrate  14 . A relatively thin InP layer  72  is then deposited to cover first beam expander layer  70 . As shown in FIG. 9, first mask  44  remains in place to prevent first beam expander layer  70  from contacting the laser structure. In accordance with the present invention, first beam expander layer  70  comprises a material (e.g., InGaAsP) that exhibits a relatively high index of refraction (e.g., 1.3 μm), in order to achieve high coupling efficiency between laser  12  and the beam expander region. 
     Once layers  70  and  72  are formed, a second mask  74  is disposed as shown in FIG. 10 to cover the area that will be used to define the final beam expander terminations. That is, second mask  74  is disposed to extend from the termination of first mask  44  to a predetermined location  76  that is defined as the output of the double layer beam expander. With second mask  74  in place, the exposed portions of high refractive index layer  70  and InP layer  72  are removed, using an appropriate etchant, resulting in the structure as illustrated in FIG. 11, where edge  76  of second mask  74  now defines a terminating endface  78  of first beam expander layer  70 . After the layer material is removed by etching, second mask  74  is also removed, and a blanket deposition process is performed to cover the exposed structure with a layer  80  of relatively low refractive index material (e.g., In 1-x Ga x As y P 1-y , 0&lt;x&lt;1, 0&lt;y&lt;1, index of 3.28) as the second layer in the double layer beam expander. FIG. 12 illustrates this structure, which is essentially identical to that illustrated in FIG. 7, formed using the latter process described above in association with FIGS. 3-7. 
     In general, there exist a variety of different process sequences that may be used to form the double layer beam expander arrangement of the present invention. Moreover, other materials can be used to form each layer, as long as each layer separately achieves the purposes of: (1) high coupling efficiency and (2) mode matching to the fiber. 
     Although the above discussion describes the formation of a double layer beam expander in association with a semiconductor laser device, it is to be understood that the double layer beam expander of the present invention is suitable for use with any emitting or receiving semiconductor optical device including, but not limited to, modulators, amplifiers, photodetectors, and the like. Indeed, the subject matter of the present invention is considered to be limited only by the scope of claims appended hereto.