Abstract:
The output optical beam from an angled-facet semiconductor laser diode is made to propagate parallel to the optical axis of the laser package with low optical back-reflection. In this way, the angled-facet devices are made compatible with conventional semiconductor laser packages enabling them to be economically incorporated in a wide-range of external semiconductor lasers and amplified spontaneous emission sources. The parallel beam is achieved by tilting the laser diode with respect to the front and back surfaces of the package.

Description:
This application claims priority from provisional application No. 60/112,430 filed Dec. 16, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     (1). Field of the Invention 
     This invention relates to manufacturing of opto-electronic devices, and in particular angled-facet semiconductor laser packages and sub-assemblies for use in external cavity lasers and amplified spontaneous emission (ASE) sources. 
     (2). Description of the Related Art 
     The manufacture of external cavity lasers and ASE sources based on semiconductor laser diodes requires that anti-reflection coatings be applied to the semiconductor laser diode so that the optical reflection from one or more of the laser diode facets is substantially reduced. 
     By reducing the reflection from one facet, an external cavity semiconductor laser is formed by introducing spectrally controlled reflection from an external element such as a diffraction grating. A laser cavity is formed between the external reflecting element and the back facet of the laser diode. With appropriate design of the cavity, broadly tunable narrow-linewidth lasers can be implemented as described in U.S. Pat. No. 5,050,179. 
     In the case of an ASE source, anti-reflection coatings are typically applied to both semiconductor laser facets to eliminate the formation of a laser cavity. With appropriate design of the semiconductor laser device, broad spectral bandwidth ASE sources (super-luminescent laser diodes) can be implemented. 
     It has been demonstrated that orienting the semiconductor laser waveguide at an angle with respect to the laser facet is an effective technique to reduce the effective facet reflectivity without requiring anti-reflection coatings. This is desirable because extremely high-precision anti-reflection coatings with optical power reflectivity of less than 0.1% are required for most external cavity and ASE source applications. This is difficult to achieve in large-scale manufacturing and adds significantly to the cost and reliability of external cavity lasers and ASE sources based on semiconductor laser diodes. Angled-facet semiconductor lasers have been used to implement both external cavity lasers using a curved-waveguide single-angled-facet structure as explained by P. J. S. Heim et al in “Single-facet diode for widely tunable external cavity semiconductor lasers with high spectral purity”,  Electronics Letters , Jul. 31, 1997, Vol. 33, No. 16 and ASE sources (super-luminescent diodes) using an angled stripe structure as described in U.S. Pat. No. 4,856,014. 
     One difficulty with angled-facet devices is that the optical beam enters and exits the device at an angle relative to the facet surface normal. In most optical systems it is desirable to define a fixed optical axis that is perpendicular to an established reference plane, i.e. a surface of the laser diode package. The angled-facet device introduces an arbitrary oblique angle that complicates, and thereby discourages, its use in conventional optical systems as exemplified by Heim et al. One application where angled-facet devices have been successfully applied is tilted-stripe angled facet traveling wave semiconductor optical amplifiers, as described by J. V. Collins et al in “Passive alignment of second generation optoelectronic devices”,  Selected Topics in Quantum Electronics , Vol. 3, No. 6, 1997. However, in these applications custom sub-assemblies are developed to accommodate the angled facet in order to implement a self-contained optical fiber-coupled module. It would be highly desirable to supply the angled-facet semiconductor device so that it is compatible with standard laser diode packages as shown in U.S. Pat. No. 5,262,675 and can therefore be directly incorporated into existing optical systems without having to change the design of the optical system. 
     A conventional semiconductor laser package is shown in FIG.  1 . It comprises a semiconductor laser chip  30  that has been soldered to a sub-mount pedestal  20 . The submount pedestal is attached to the header base  10  and electrically connected to contact pin  12 . A photodiode  70  mounted on the front surface of the header base  18  detects the optical signal  42  emitted from the back facet  33  of the semiconductor laser. Electrical connections to the semiconductor laser chip  13  and photodiode  11  are provided via bond wires  50  and  51 , respectively. The semiconductor laser optical waveguide  31  is oriented perpendicular to the front facet  32  and back facet  33  of the semiconductor laser so that the front optical beam  40  and back optical beam  42  are emitted perpendicular to the respective facets. The semiconductor laser chip is attached to the sub-mount pedestal  20  with front facet  32  parallel to the front edge of the sub-mount pedestal  21 , which is also parallel with the front surface of the header base  18 , so that both front and back semiconductor laser optical beams propagate parallel to the sub-assembly optical axis  15 . The output light beam  40  from the sub-assembly emerges through a glass window  62  that has been anti-reflection coated with films  61  and  63  to reduce optical loss. The window  62  is attached to a cap structure  60  that is welded to the header base  10  in a hermetic sealing process. 
     The entire package shown in FIG. 1 comprised of the header  10 , with mounted semiconductor laser  30 , and attached window cap  60  is often called a “TO-can” package. This highly successful semiconductor laser package can be found, for example, in every manufactured compact disk player, laser pointer, and semiconductor laser bar-code scanner. The primary features of this package is that the optical beam is emitted parallel to the optical axis of the package which is in a well-defined direction perpendicular to the plane of the header base  10 . The position of the optical beam is axially centered on the header base to facilitate positioning and alignment of the beam. The window cap  60  also provides physical protection to the semiconductor laser and enables the entire assembly to be hermetically sealed. 
     However, the assembly shown in FIG. 1 has the drawback that the light beam propagates in a direction perpendicular to the window  60 , causing light reflecting from the window to couple back into the optical waveguide  31 . This is particularly undesirable for external cavity laser and ASE source application because the back-reflections set up a parasitic laser cavity which degrades the performance of the device. To avoid reflection, anti-reflection coatings  61  and  63  are used, but expensive and not efficient. When an angled-facet laser diode is used, it is difficult to have the optical signal emitted from the back facet  33  for the photo-sensor  70  to be parallel to the light emitting from the front window  62 . Also, for many external cavity laser applications it is advantageous to include a polarization plate in front of the semiconductor laser to improve laser performance as described by H. Lotem et al in “Tunable external cavity diode laser that incorporates a polarization half-wave plate”  Applied Optics , vol. 31, 1992. Even though the laser performance could be substantially improved, it is often difficult or expensive to insert a polarization plate in existing or planned external cavity laser optical systems since it require extra space. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to package an angle-facet laser diode to reduce the back reflection. Another object of this invention is to package an angled-facet laser diode in such a way that the optical beam enters and exits along the conventional optical axis of the sub-assembly. Another object of the present invention is to incorporate economically an angled-facet laser in a conventional semiconductor device package. Still another objective of this invention is to efficiently incorporate a polarization plate into a conventional semiconductor device package. 
     These objects are achieved by tilting an angled-facet laser in a conventional semiconductor device package, so that the optical beam enters and exits along the conventional optical axis of the package. These objects are further achieved by tilting the window of a conventional semiconductor device to eliminate back reflection and replacing the window with a polarization plate. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 shows a top view of a conventional semiconductor laser sub-assembly. 
     FIG. 2 shows a top view of a tilted-waveguide angled-facet semiconductor optical amplifier sub-assembly of the present invention. 
     FIG. 3 shows a top view of a curved-waveguide angled-facet semiconductor laser subassembly of the present invention. 
     FIG. 4 shows a top view of another embodiment of the curved-waveguide angled-facet semiconductor laser sub-assembly. 
     FIG. 5 shows a top view of a third embodiment of the curved-waveguide angle-facet semiconductor laser sub-assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first embodiment of the invention is shown FIG.  2 . It comprises a tilted-waveguide angled-facet semiconductor optical amplifier chip  38  that has been attached to a sub-mount pedestal  20 . The sub-mount pedestal is attached to the header base  10  and electrically connected to contact pin  12 . A photodiode  70  mounted on the front surface of the header base  18  detects the optical signal  42  emitted from the back facet  39  of the semiconductor laser. Electrical connection  13  to the semiconductor laser chip  38  and connection  11  to the photodiode are provided via bond wires  50  and  51 , respectively. The semiconductor optical amplifier waveguide  34  is tilted to intersect the front facet  37  and back facet  39  of the semiconductor device at an oblique angle so that the front optical beam  40  and back optical beam  42  are emitted at an angle θ 1  with respect to the facet surface normal  45 . The semiconductor laser chip  38  is attached to the sub-mount pedestal  20  with front facet  37  parallel to the beveled front face of the sub-mount pedestal  22 . Surface  22  has been beveled at an angle θ 1  with respect to the front surface of the header base  18  so that both front and back semiconductor laser optical beams  40  and  42  propagate parallel to the sub-assembly optical axis  15 . The beveled surface  22  provides clearance so that the path of the light beam  40  is not blocked or diffracted. The output light beam from the sub-assembly emerges through a transparent window  64  that has been anti-reflection coated with films  61  and  63  to reduce optical loss and back-reflection. The transparent window structure  64  can be optical glass or a polarization plate (half-wave, quarter-wave) or other polarizing element. The window is attached to a cap structure  60  with a wedge  65  inserted so that the window is oriented at an angle θ 2  with respect to the front surface of the header base  18 . The angle θ 2  is chosen to minimize the optical power reflected back to the semiconductor device. The cap  60  is welded to the header base  10  in a hermetic scaling process. 
     The second embodiment of the invention is shown FIG.  3 . It comprises a curved-waveguide angled-facet semiconductor laser chip  35  that has been attached to a sub-mount pedestal  20 . The sub-mount pedestal is attached to the header base  10  and electrically connected to contact pin  12 . A photodiode  71  mounted on the front surface of the header base  18  detects the optical signal  42  emitted from the back facet  39  of the semiconductor laser. Electrical connection  13  to the semiconductor laser chip and connection  11  to the photodiode are provided via bond wires;  50  and  51 , respectively. The semiconductor optical waveguide  36  is curved with a constant radius of curvature R given by R=L/sin(φ) where L is the length of the curved region and φ is the desired facet angle. Other sinusoidal or exponential curving functions can be used to define the functional form of the curved region. Arbitrary lengths at the end of the device can be straight or tilted corresponding to an infinite radius of curvature. The curved optical waveguide intersects the back facet  39  at normal incidence (perpendicular) but intersects the front facet  37  at an oblique angle so that the front optical beam  40  is emitted at an angle θ 1  with respect to the facet surface normal  45 . The back optical beam  42  is emitted parallel to  45 . The semiconductor laser chip is attached to the sub-mount pedestal with front facet  37  parallel to the beveled front face of the sub-mount pedestal  22 . Surface  22  has been beveled at an angle θ 1  with respect to the front surface of the header base  18  so that the front semiconductor laser optical beam  40  propagates parallel to the sub-assembly optical axis  15 . The beveled surface  22  provides clearance so that the path of the light beam  40  is not blocked or diffracted. The photodiode  71  is mounted on surface  18  to detect the emitted back facet optical beam  42 . This beam is incident on the photodiode at an oblique angle enabling the photodiode to be mounted flush against  18  without requiring any additional steps to reduce back-reflections. The output light beam from the sub-assembly  40  emerges through a transparent window  64  that has been anti-reflection coated with films  61  and  63  to reduce optical loss and back-reflection. The transparent window structure  64  can be optical glass or a polarization plate (half-wave, quarter-wave) or other polarizing element. The window is attached to a cap structure  60  with a wedge  65  inserted so that the window is oriented at an angle θ 2  with respect to the front surface of the header base  18 . The angle θ 2  is chosen to minimize the optical power reflected back to the semiconductor device  35 . The cap  60  is welded to the header base  10  in a hermetic sealing process. 
     The third embodiment of the invention is shown FIG.  4 . It consists of a curved-waveguide angled-facet semiconductor laser chip  35  that has been attached to a sub-mount pedestal  20 . The sub-mount pedestal is attached to the header base  10  and electrically connected to contact pin  12 . A photodiode  71  mounted on the front surface of the header base  18  detects the optical signal  42  emitted from the back facet  39  of the semiconductor laser  35 . Electrical connections to the semiconductor laser chip  13  and photodiode  11  are provided via bond wires  50  and  51 , respectively. The semiconductor optical waveguide  36  is curved with a constant radius of curvature R given by R=L/sin(φ) where L is the length of the curved region and φ is the desired facet angle. Other sinusoidal or exponential curving functions can be used to define the functional form of the curved region. Arbitrary lengths at the end of the device can be straight or tilted corresponding to an infinite radius of curvature. The curved optical waveguide intersects the back facet  39  at normal incidence (perpendicular) but intersects the front facet  37  at an oblique angle so that the front optical beam  40  is emitted at an angle θ 1  with respect to the facet surface normal  45 . The back optical beam  42  is emitted parallel to  45 . The semiconductor laser chip is attached to the sub-mount pedestal with front facet  37  parallel to edge  22 . Edge  22  is oriented at an angle θ 1  with respect to the front surface of the header base  18  so that the front semiconductor laser optical beam  40  propagates parallel to the sub-assembly optical axis  15 . Edge  22  is formed by a mechanical or chemical machining process that creates a recessed plane area  25  providing clearance so that the path of the light beam  40  is not blocked or diffracted. An optical element  80  is attached to the front surface  21  of the sub-mount pedestal  20  with attaching mechanisms  81  and  82 . The optical element  80  can be a refractive, diffractive, or holographic lens placed in close proximity to the angled facet to manipulate the beam shape and/or quality. The photodiode  71  is mounted on surface  18  to detect the emitted back facet optical beam  42 . This beam is incident on the photodiode at an oblique angle enabling the photodiode to be mounted flush against  18  without requiring any additional steps to reduce back-reflections. The output light beam from the sub-assembly  40  emerges through a transparent window  64  that has been anti-reflection coated with films  61  and  63  to reduce optical loss and back-reflection. The transparent window structure  64  can be optical glass or a polarization plate (half-wave, quarter-wave) or other polarizing element. The window is attached to a cap structure  60  with a wedge  65  inserted so that the window is oriented at an angle θ 2  with respect to the front surface of the header base  18 . The angle θ 2  is chosen to minimize the optical power reflected back to the semiconductor device. The cap  60  is welded to the header base  10  in a hermetic sealing process. 
     The fourth embodiment of the invention is shown FIG.  5 . It comprises a curved-waveguide angled-facet semiconductor laser chip  35  that has been attached to sub-mount  90 . The sub-mount  90  is attached to the sub-mount pedestal  20 . The sub-mount pedestal is attached to the header base  10  and electrically connected to contact pin  12 . A photodiode  71  mounted on the front surface of the header base  18  detects the optical signal  42  emitted from the back facet  39  of the semiconductor laser. Electrical connections to the semiconductor laser chip  13  and photodiode  11  are provided via bond wires  50  and  51 , respectively. The semiconductor optical waveguide  36  is curved with a constant radius of curvature R given by R=L/sin(φ) where L is the length of the curved region and φ is the desired facet angle. Other sinusoidal or exponential curving functions can be used to define the functional form of the curved region. Arbitrary lengths at the end of the device can be straight or tilted corresponding to an infinite radius of curvature. The curved optical waveguide intersects the back facet  39  at normal incidence (perpendicular) but intersects the front facet  37  at an oblique angle so that the front optical beam  40  is emitted at an angle θ 1  with respect to the facet surface normal  45 . The back optical beam  42  is emitted parallel to  45 . The semiconductor laser chip is attached to the sub-mount  90  with front facet  37  parallel to the front edge  22  of the sub-mount  90 . The sub-mount  90  is comprised of a material having desirable thermal and mechanical properties, such as aluminum nitride (AlN), diamond, or other material. This sub-mount can be either of the same material as the sub-mount pedestal  21 ) or of a different material. The sub-mount  90  is attached to the sub-mount pedestal  20  with the front edge  22  oriented at an angle θ 1  with respect to the front surface of the header base  18  so that the front semiconductor laser optical beam  40  propagates parallel to the sub-assembly optical axis  15 . The thickness of the sub-mount  90  is chosen to insure adequate clearance so that the path of the light beam  40  is not blocked or diffracted. An optical element  80  is attached to the front surface  21  of the sub-mount pedestal  20  with attaching mechanisms  81  and  82 . The optical element  80  can be a refractive, diffractive, or holographic lens placed in close proximity to the angled facet to manipulate the beam shape and/or quality. The photodiode  71  is mounted on surface  18  to detect the emitted back facet optical beam  42 . This beam is incident on the photodiode at an oblique angle enabling the photodiode to be mounted flush against  18  without requiring any additional steps to reduce back-reflections. The output light beam from the sub-assembly  40  emerges through a transparent window  64  that has been anti-reflection coated wish films  61  and  63  to reduce optical loss and back-reflection. The transparent window structure  64  can be optical glass or a polarization plate (half-wave, quarter-wave) or other polarizing element. The window is attached to a cap structure  60  with a wedge  65  inserted so that the window is oriented at an angle θ 2  with respect to the front surface of the header base  18 . The angle θ 2  is chosen to minimize the optical power reflected back to the semiconductor device. The cap  60  is welded to the header base  10  in a hermetic scaling process. 
     While the foregoing description refers to a conventional TO-can semiconductor package, other kinds of semiconductor packages may also be used. 
     While particular embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments without departing from the spirit of the present invention. Such modifications are all within the scope of this invention.