US 20030099273 A1
A surface emitting laser is coupled to an external modulator. The laser and the modulator aligned by photolithographically defined features. In a preferred embodiment, the electromagnetic output of the laser is reflected at a right angle from a mirror mounted on a substrate. The reflected output enters a modulator mounted on the same substrate as the mirror. A circuit coupled to the modulator controls the modulation undergone by the electromagnetic output. The modulated output is coupled to an optical fiber for transmission.
1. A system for coupling one or more surface-emitting lasers to one or more corresponding external optical devices, the system comprising:
(a) the one or more surface-emitting lasers, each laser adapted to produce an electromagnetic output; and
(b) an optical bench substrate having photolithographically defined therein, for each said surface-emitting laser, a coupling mirror positioned to receive the output of a corresponding surface-emitting laser and a corresponding optical device positioned to receive the output of said surface-emitting laser from the coupling mirror.
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said optical devices are electroabsorption optical modulators;
the optical bench substrate further comprises, for each said coupling mirror and corresponding optical modulator, an optical fiber coupled to the corresponding optical modulator;
the optical bench substrate has photolithographically defined therein, for each said coupling mirror and corresponding optical modulator, a V-shaped slot for positioning the optical fiber to receive the output of the corresponding optical device; and
the system is for use in one of data communications or telecommunications.
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one or more sources of single mode laser radiation comprising the one or more surface-emitting lasers, respectively; and
a detector for detecting the single mode laser radiation after passage thereof through a quantity of said measurement species.
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29. A system for coupling one or more surface-emitting lasers to one or more corresponding external optical devices, each laser adapted to produce an electromagnetic output, the system comprising an optical bench substrate having photolithographically defined therein, for each said surface-emitting laser, a coupling mirror positioned to receive the output of a corresponding surface-emitting laser and a corresponding optical device positioned to receive the output of said surface-emitting laser from the coupling mirror.
30. The system of
31. A method for coupling a surface-emitting laser to an external optical device, the method comprising the steps of:
(a) emitting an electromagnetic output from the laser; and
(b) reflecting the electromagnetic output from a photolithographically defined mirror in an optical bench substrate to the external optical device so as to couple the electromagnetic output into the external optical device.
32. The method of
 Other features and advantages of the invention will become apparent upon study of the following description, taken in conjunction with the drawings in which:
FIG. 1 is a cross-sectional view of a one-dimensional array of vertical cavity surface emitting lasers (VCSELs);
FIG. 2 is an isometric view of the one dimensional array of VCSELs of FIG. 1;
FIG. 3 is a cross-sectional view of one embodiment of a system for aligning and coupling the array of lasers of FIG. 2 to a corresponding array of external modulators;
FIG. 4 is a front view of the system of FIG. 3;
FIG. 5 is a cross-sectional view of an alternative embodiment of a system for aligning and coupling an array of lasers to an array of external modulators;
FIG. 6 is an isometric view of a two-dimensional array of VCSELs;
FIG. 7 is a cross-sectional view of a system for aligning and coupling the two-dimensional array of lasers of FIG. 6 to a corresponding array of external modulators; and
FIG. 8 is a block diagram of a spectroscopic measurement system employing one or more VCSELs of the present invention.
 Referring now to the drawings, the details of preferred embodiments of the invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.
 Referring now to FIG. 1, a one-dimensional array 10 of VCSELs is shown in cross-section, the cross-sectional view illustrating three VCSELs of VCSEL array 10. Array 10 is a one-dimensional array having a single row of N VCSELs. In alternative embodiments, a two-dimensional array having a plurality of rows and columns, or other two-dimensional arrangements of VCSELs, such as staggered rows of 2×N or 4×N, may be employed. As will be appreciated, a VCSEL is a semiconductor laser that emits its output perpendicular to its p-n junction. Each VCSEL is built on a substrate 12 of semiconductor material. Various semiconductor materials known to those in the art can be employed.
 Each VCSEL shares a first reflective plane 14. In one embodiment, the first reflective plane 14 comprises a DBR. A DBR consists of alternating layers of different semiconductors or different dielectrics. In one embodiment, forty alternating layers yield a reflectivity of 99.99%. In an alternative embodiment, the first reflective mirror or plane 14 can comprise a metal mirror rather than or in addition to a DBR.
 Each VCSEL also shares the common active region 16 of VCSEL array 10. The active region 16 is a p-n junction and the width of the active region 16 controls the wavelength of emitted light. The widths of the first reflective plane 14, active region 16, and second reflective plane 18 are very small compared to the width of the substrate 12. (The widths shown for the various layers in FIG. 1 are not to scale.)
 In the embodiment shown, the second reflective plane 18 is separate for each VCSEL. In another embodiment, the VCSELs can share a common second reflective plane 18. Like the first reflective plane 14, the second reflective plane 18 can comprise a DBR or a metal mirror. The second reflective plane 18 is highly reflective, but less reflective than the first reflective plane 14. In one embodiment, twenty-five alternating layers of a DBR yield a reflectivity of 99.9%.
 A cladding 20 protects the VCSEL structure. Adjacent the second reflective plane 18, the cladding 20 defines a gap 22 that allows emission of radiation from the active region 16. The profile of the emitted radiation can be controlled by modifying the geometry of the emission area. For example, changes in the shape and size of the gap 22 affect the spatial profile of the emitted radiation. The distance between the active region 16 and the gap 22 also affects the spatial profile of the emitted radiation. The use of photolithographic techniques in defining the features of the VCSEL array allow highly accurate placing of the VCSELs and highly accurate definition of VCSEL output profiles.
 Referring now to FIG. 2, an isometric view of a linear array 10 of VCSELs is depicted. Most of the structural aspects of the VCSELs are not visible from an outside view. The gaps 22 are visible and allow radiation to be emitted. As with FIG. 1, the dimensions have been rendered disproportional in order to make visible the various features. In various embodiments, the VCSEL separation and height are much reduced relative to the thickness of the substrate. FIG. 2 shows a linear array 10 of VCSELs manufactured such that radiation from each VCSEL is emitted along substantially parallel paths. While the array shown has three VCSELs, other embodiments include arrays having a large number of linearly arranged VCSELs.
 As noted above, there is a need to couple the output(s) of one or more SELs of an array of SELs to respective laser beam “sinks,” i.e. devices that receive the output of a given VCSEL. For example, it may be desired to couple the output of each VCSEL of VCSEL array 10 to a respective modulator. Referring now to FIG. 3, there is shown a cross-sectional view of an embodiment of a system 300 in which the VCSELs of VCSEL array 10 are aligned with and coupled to corresponding devices, namely modulators 54, for externally modulating the VCSELs. In an embodiment, modulators 54 are edge-absorbing and edge-emitting optical modulators of the type typically utilized for modulation of output from edge-emitting lasers.
FIG. 3 shows in cross-section VCSEL 22 of VCSEL array 10. In alternative embodiments, SELs other than VCSELs may be employed, such as a grating-coupled surface emitting laser (GCSEL). GCSELs are described in U.S. Pat. No. 5,867,521 (Macomber); and R. J. Noll & S. H. Macomber, “Analysis of Grating Surface Emitting Lasers,” IEEE J. Quantum Electronics, vol. 26, no. 3, March 1990, pp. 456-466, the entireties of which are incorporated herein by reference. In alternative embodiments, the VCSELs may be operatively (optically) coupled to devices other than modulators 54.
 As shown in FIG. 3, contacts 60, 62 are electrically coupled to different sides of the active region of VCSEL 22. Those of skill in the art are aware of several means by which contacts can be mounted on a surface-emitting laser. The laser can then be powered by current provided through the contacts 60, 62. The laser emits a coherent electromagnetic output, i.e. a laser beam, which is to be coupled to a device such as modulator 54. When the output laser beam of a given VCSEL is coupled to the input of a given device receiving the laser output, the VCSEL may be said to be coupled (or operationally coupled or optically coupled) to the device.
 In an embodiment, laser array 10 is mounted on a substrate 50. The substrate 50 includes a planar mirror 52 that is inclined at some desired angle relative to the surface of the substrate 50. In alternative embodiments, the mirror 52 is positioned at other angles. In a preferred embodiment, mirror 52 is inclined at 45° relative to the surface of substrate 50, because devices such as modulator 54, SOA 56, and/or fiber 58 are preferably mounted on substrate 50 in the plane of the surface of substrate 50 and the laser beam emitted by VCSEL 22, for example, is perpendicular to the surface of substrate 50, so that a 45° angle mirror will optimally couple laser output from VCSEL 22 to modulator 54.
 As will be appreciated, various fabrication techniques may be employed to form inclined mirror 52 having a desired angle, such as 45°. For example, KOH may be used as an etchant to form a 45° angle inclined mirror 52, when using a silicon optical support bench substrate 50. KOH etches into silicon at approximately 54.7° relative to the (001) crystal plane along the (111) plane, however. Therefore, in one embodiment, the silicon substrate is cut at an offset from the (001) crystal plane towards, e.g., the (110) plane, so that during etching, the mirror 52 is etched at the desired angle (e.g., 45°). In the case of a linear array of VCSELs, the silicon substrate may be cut such that the laser radiation is reflected into the input side of a device, such as modulator 54, mounted in a V-groove also etched into the substrate 50. However, for other types of VCSEL arrays, such as a 2×N array, the silicon may be cut at a rotating offset from the <011> crystal plane. This can cause the reflected laser radiation to be in the plane of the surface of substrate 50, but offset at an azimuthal angle from V-grooves. In this case, alignment structures other than V-grooves (e.g., mechanical stops and guides) may be employed to properly align coupling devices such as modulator 54, SOA 56, and/or fiber 58 with the reflected laser radiation.
 Alternatively, silicon substrate 50 may not be cut at an offset from the <011> crystal plane, so that KOH etching to form the inclined planar mirror surface 52 is at an angle of approximately 54.7° relative to the surface. In this case, if it is desired to reflect the VCSEL laser radiation into the plane of the surface of substrate 50, VCSEL array 10 may be mounted at an angle off of normal to the surface. For example, an end of VCSEL array 10 may be etched at an end (e.g., the left end shown in cross-section in FIG. 3) to have a mounting surface angled relative to its own surface, and a mounting structure formed on substrate 50 with a top surface parallel to substrate 50's surface (this mounting structure, e.g., may be formed to the left of mirror 52 in the cross-sectional view shown in FIG. 3). In such an embodiment, the angle of the emitted radiation from VCSEL 22 is such that it reflects off a 54.7° inclined mirror 52 into the plane of the surface of substrate 50 and thus is coupled into a device such as modulator 54. When other materials are used for substrate 50, other etchants may be employed, (e.g., H2SO4, H2O2, H2O for a GaAs substrate).
 Whether a Si, GaAs, or other substrate is used, a metal coating or dielectric coating may be employed to minimize mirror 52 losses. For example, metals such as Al, Au, or Ag may be employed to form a reflective surface for mirror 52. Mirror 52 may also be fabricated with a suitable DBR dielectric stack, e.g. Si/SiO2 or Al2O3/Si, or in combination with Al, Au, or Ag metals. [also CAN BE USED FOR Si???]
 In an embodiment, mirror 52 and V-shaped slot (V-groove) 72 (illustrated in FIG. 4) can be fabricated using lithographic alignment of a mask with the substrate 50 that has had a desired pattern imposed on the substrate by lithography. By aligning the substrate 50 in a preferred orientation prior to lithography, subsequent etching can take place preferentially along various crystal axes of the substrate, including the <111> axis or another axis that is oriented at a desired angle to the <100> axis. These and other alignment techniques are described in Hauffe, et al. “Methods of Passive Fiber Chip Coupling of Integrated Optical Devices,” Proc. 2000 Electronic Components and Technology Conference (May 2000), pp.238-243, and in Optoelectronic Packaging, eds. Mickelson et al. (New York: John Wiley & Sons, 1997), esp. ch. 9, “Array Device Packaging,” by Nagesh R. Basavanhally & Ronald A. Nordin (“Basavanhally & Nordin”). Chapter 9 (Basavanhally & Nordin) of the Optoelectronics Packaging text is incorporated herein by reference.
 In an embodiment, to precisely align the VCSELs of array 10 with corresponding optical devices, for effective optical coupling, array 10 is mounted on substrate 50, which forms a silicon (or other material) optical bench, using various photolithographically formed or etched alignment features, such as mechanical alignment stops, notches, pedestals, standoffs, and the like. These and other “silicon optical bench” (SiOB) alignment techniques are described in Basavanhally & Nordin, esp. pp. 143-148. By employing such alignment techniques, the VCSELs of a VCSEL array may be easily aligned with and thus coupled to corresponding optical devices mounted in precisely aligned positions on a support bench.
 For example, VCSELs of VCSEL array 10 are lithographically formed in precise positions in a common substrate 12 (FIG. 1). Similarly, various coupling structures (e.g., mirror 52) and external optical device alignment features (e.g., V-groove 72) are lithographically defined into optical support bench substrate 50. The latter alignment features permit devices such as modulator 54, SOA 56, and fiber 58 to be mounted on bench 50 in precise alignment with mirrors 52. The VCSEL substrate 12 and bench substrate 50 also include alignment features that permit the precise mounting of the VCSEL array 10 with corresponding external optical devices mounted in external optical device alignment positions of bench substrate 50. In this manner, an entire array 10 of VCSELs can be aligned together, with their corresponding optical devices (54), instead of having to align each VCSEL independently.
 In addition to alignment, to actually “flip-chip” mount laser array 10 on substrate 50, any suitable mounting technique may be utilized. For example, solder bump technology, bonding metal, or epoxy bonding may be employed.
 The laser output will diverge in profile as it travels. In another embodiment, therefore, a lens is used to shape the output between the laser and the mirror 52. In such embodiments, the lens can be placed in an etched groove between the mirror 52 and the modulator 54, or between the SOA 56 and the fiber 58. Alternatively, two lenses may be employed, with one being positioned in either of the aforementioned positions.
 An alternative embodiment of the invention involves the etching of a non-planar mirror in place of the plane mirror 52. For example, in the embodiment illustrated in FIG. 3, such a non-planar mirror would preferably be concave so as to focus the light output of the laser 22 onto the modulator 54 or other optical component (e.g., SOA, fiber, etc.). In this way, the lens envisioned in the previously described embodiment may be eliminated. Lenses may also be placed between other components in various other alternative embodiments of the present invention.
 The laser output reflects off the mirror 52 and a modulator 54 is positioned on the substrate 50 in the path of the output. In this manner, the laser output from VCSEL 22 is coupled to modulator 54, i.e. VCSEL 22 is optically coupled with modulator 54.
 In one embodiment, the optical modulator 54 is an electroabsorption modulator (EAM). Contacts 64, 68 are provided to control the modulation. The contacts 64, 68 can be provided by various methods known to persons of skill in the art. For example, the contacts 64, 68 can be flip-chip bonded. In another embodiment, the contacts 64, 68 are wirebonded from the modulator to an adjacent contact pad mounted on the substrate 50. When modulation is desired, a driver circuit can be used to provided electrical signals to one or both of the contacts 64, 68, as will be appreciated by those skilled in the art. The laser output is then modulated in accordance with the electrical signals.
 In an embodiment, the output of the modulator 54 is optically coupled to SOA 56, which increases the power of the modulated laser output. The SOA 56 includes contacts 66, 70 that can be mounted and placed as discussed with respect to the modulator 54. The contacts 66, 70 provide power to the SOA 56. For some applications, the SOA 56 is not necessary. In alternative embodiments, amplifiers or optical devices other than the SOA 56 may be used. For example, an optical fiber can be doped to produce an optical amplifier and used in place of the SOA 56. In one of many possible embodiments, the SOA 56 is mounted on the substrate 50. In alternative embodiments, the SOA 56 is placed between the mirror 52 and the modulator 56 in the path of the laser output.
 An optical fiber 58 is preferably provided and optically coupled to the SOA 56. The optical fiber 58 guides the modulated and amplified laser output to its destination. Persons of ordinary skill in the art are aware of techniques for coupling the optical fiber 58 to the SOA 56 (or in an alternative embodiment to the modulator 54), such as butt-coupling. The alignment of the VCSEL and modulator 54 may typically allow, in various embodiments, 3 dB of the power from the laser to enter the modulator 54 (i.e., 50% coupling efficiency). The use of an external modulator such as modulator 54 allows data to be encoded in the laser output at rates above 1 Gb/s.
 Referring now to FIG. 4, there is shown a front view of system 300. As illustrated in FIG. 4, the substrate 50 can continue to the left and right sides of FIG. 4 with additional laser-modulator combinations being mounted thereon. From the view of FIG. 4, some of the contacts 60, 62, and 66 are visible as well as a portion of the mirror 52. Additionally, the SOA 56 can be seen behind the fiber 58 to which it is coupled. A V-shaped slot or channel 72 in the substrate 50 is visible in FIG. 4. The V-shaped slot 72 precisely aligns the fiber 58 to receive the modulated and amplified laser output with minimum loss of power. In alternative embodiments, the fiber 58 may be positioned without a slot, or with a differently configured slot. As will be appreciated, the alignment of various components of embodiments of the present invention may employ SiOB techniques, as described above. In such SiOB techniques, a single crystal semiconductor material such as silicon is typically utilized as the support structure (optical bench) for various optical devices. This typically includes etching channels (e.g., the V-groove 72) on the surface of a silicon substrate in order to provide for mounting of optical components or to increase coupling efficiency between the components. Accordingly, the embodiment of the present invention shown in FIGS. 3-4 provides a method and system for accurately and efficiently aligning and coupling a SEL to an optical device such as a modulator.
 Referring now to FIG. 5, there is shown a cross-sectional view of an alternative embodiment of a system 500 for aligning and coupling an array of lasers to an array of external modulators. The view of FIG. 5 shows a single VCSEL 501 of the array, in cross-section. The embodiment of FIG. 5 differs from that of FIGS. 3-4 in that an optical fiber section 58 is placed between the mirror 52 and the modulator 54. Thus, in system 500, the VCSEL 501 is optically coupled to fiber 58, which is itself optically coupled to modulator 54. In this manner, VCSEL 501 is optically coupled (via fiber 58) to modulator 54. As will be understood, optical fiber 58 may, in some embodiments, be able to be placed closer to mirror 52 than modulator 54 can be, thus reducing the laser path distance from the mirror to the input of the device. This can reduce the divergence of the laser output. In an alternative embodiment, a lens is used in place of the fiber 58 to focus the laser output and reduce or counteract divergence. While VCSEL 501, optical fiber 58, modulator 54, SOA 56, and optical fiber 58 are shown mounted on a common substrate 50, in another embodiment the optical fiber 58 can be employed between the mirror 52 and modulator 54 where the modulator 54 is mounted on a separate substrate.
 Referring now to FIG. 6, an isometric view of a substrate mounted array 80 of VCSELs is illustrated. The array 80 extends in two dimensions having both rows and columns. While the 2-D array 80 is shown with two columns and two rows, other embodiments include arrays with more rows and/or columns, or with other 2-D arrangements (such as staggered columns). Arrays of VCSELs (both one dimensional and two dimensional) can be used to provided input sources for Wavelength Division Multiplexed (WDM) systems. In an embodiment, each VCSEL of array 80 generates a different wavelength laser beam. The different wavelength output beams are modulated and then combined in a single optical fiber. At the destination, the wavelengths are separately detected and demodulated. Combining several modulation systems such as those shown in FIGS. 3 and 5, a WDM system can be configured.
 Referring now to FIG. 7, there is shown a cross-sectional view of a system 700 for aligning and coupling the two-dimensional laser array 800 of FIG. 6 to a corresponding array of external modulators 54. The 2-D array 80 is shown having two columns. The number of rows is not shown. In various embodiments, there could be one to many rows. A single substrate 50 mounts various optical devices for aligning, modulating, and amplifying the VCSELs in each column. These devices are mounted for each column as discussed with reference to FIG. 3. The configuration of FIG. 7 allows for efficient manufacture of a plurality of VCSELs that can be accurately aligned with and coupled to a corresponding plurality of modulators for external modulation.
 The coupled VCSELs and systems of the present invention may be employed in a variety of applications, such as data communications, telecommunications, spectroscopy, and biosensing applications. For example, one or more VCSELs of the present invention may be used as radiation sources to perform molecular spectroscopy to determine the molecular composition of various measurement species. The measurement species can be a gas or liquid, for example (disposed in a measurement cell or an unconfined gas or liquid). The measurement species can also be human compounds, such as blood, in the case of biosensing spectroscopy applications. For biosensing, the measurement species could also be a human- or biological-related compound containing bacteria and/or viruses to be detected, in which case the measurement species may be regarded as a bacterial species or viral species, respectively. Thus, the VCSELs of the present invention may also be utilized as radiation sources for pollution monitoring and other applications that involve absorption measurements.
 These embodiments are illustrated in FIG. 8, which is a block diagram of a spectroscopic measurement system 800 employing one or more VCSELs of the present invention, such as VCSEL 300 of FIG. 3. The electromagnetic infrared radiation (laser beam) 811 is emitted from fiber 58 of a VCSEL of the VCSEL array of system 300 of FIG. 3, so that it passes through a given measurement species 812. The measurement species may be, for example, gas in cell, unconfined gas, or blood sample. The molecules in the species selectively absorb various wavelength radiation, and the exited radiation then impinges on a conventional detector 815. A computer 814 may then analyze the result to determine the presence of certain elements in the species 812.
 In an embodiment for spectroscopic absorption analysis of a gas in a cell, for example, one or more VCSELs of the present invention may be employed in a point sensing apparatus, as described, for example, in U.S. Pat. No. 5,901,168, the entirety of which is incorporated herein by reference. In such an embodiment, the (one or more) VCSEL(s) provides mid-IR radiation and is mounted on a temperature-controlled stage for coarse wavelength tuning. Mid-IR radiation from the laser passes through a conventional gas cell (optionally a multi-pass cell), with exited radiation impinging on a conventional detector. The gas cell, in this case, contains the measurement species. The electrical output of the detector is supplied to a lock-in amplifier (together with an appropriate modulation signal, e.g., a 1.2 kHz sine wave from a modulation signal generator), and the lock-in amplifier output is supplied to a computer for data analysis and formatting. The data is then displayed and/or stored in any suitable manner. The VCSEL is pumped with an appropriate electrical current. For instance, a low frequency current ramp (e.g., 250 ms period) from a ramp current generator, short bias pulses (e.g., 5 ns pulse width, 2 μs period) from a bias current generator, and a modulation signal from a modulation current generator are supplied to a combiner, and the resultant current ramp with superimposed current pulses and sine wave is applied to the laser. The current ramp serves to sweep the laser temperature over a predetermined range, and the pulses cause the emission of short laser pulses. The pulse wavelength is slowly swept over a range of wavelengths, and absorption as a function of wavelength is determined. Thus, the presence in the cell of a gas that has an absorption line in the range of wavelengths is readily detected, and the gas can be identified. In such an embodiment, the output of the VCSEL is coupled in accordance with the invention to the gas in the cell, e.g. by coupling the VCSEL to a fiber which is then used to pass the mid-IR radiation through the gas in the cell. In alternative embodiments, VCSEL(s) of the present invention produce radiation at other than mid-IR wavelengths, for spectroscopic or other applications.
 VCSELs of the present invention may also be employed in an embodiment in which the measurement species is an unconfined gas (e.g. for pollution monitoring). For example, one or more VCSELs of the present invention may be utilized in a remote-sensing system, wherein an emission source such as a factory emits a gaseous emission cloud (the unconfined gas or measurement species). One or more VCSELs of the present invention is positioned to emit mid-IR radiation which propagates through the emission cloud, and is reflected (e.g., by means of a corner reflector). The reflected radiation is then detected by means of a detector. The VCSEL can be pumped in any appropriate manner, e.g., as described above, and the detector output can be utilized in any appropriate manner, e.g., also as described above. A mirror or other appropriate reflector can be used instead of a corner reflector. The reflector can be on an aircraft or any elevated feature, including the smoke stack that is being monitored. The detector could also be on an aircraft, or be on an elevated feature. In general, any arrangement that results in a line-of-sight disposition of laser and detector is contemplated. In such an embodiment, the output of the VCSEL(s) is coupled in accordance with the invention to the unconfined gas. For example, multiple VCSELs of an array may each be coupled to a respective fiber, which are then combined into a single fiber, which directs the combined mid-IR radiation from all the VCSELs through the unconfined gas.
 In such applications and embodiments, the VCSEL of the present invention will generally be mounted in an appropriate housing for protection and control. The package will typically comprise cooling means (e.g., water cooling, thermoelectric cooling), temperature sensor means (e.g., a thermocouple) for use in a feedback loop for temperature control, and means for applying the pump current to the laser. The VCSEL is attached in conventional fashion to the cooling means. Optionally the housing may also contain detector means for controlling laser output power. The housing will typically have a window that is transparent for the laser radiation, and will typically be evacuated or filled with inert gas.
 In the case of biosensing applications, for example, the output radiation at wavelengths appropriate for absorption by the element or compound to be detected, is coupled in accordance with the present invention to a fiber, which directs the radiation into the measurement species (e.g., the human blood beneath the skin of a finger).
 As will be appreciated, multiple VCSELs may be employed in such applications. For example a 2×2 array of 4 VCSELs, each having a different wavelength, may be used to detect multiple gases, or to better detect one gas. In such an application, for example, each of the VCSELs may be coupled to a fiber, using the coupling technique and system of the present invention. These fibers may be used to direct the radiation of all four VCSELs through the gas species to be measured. Other variations may be employed, such as coupling the output of multiple VCSELs to a single fiber to emit the combined (superimposed) radiation through the gas species.
 Therefore, in such applications, one or more VCSELs are coupled in accordance with the present invention to a measurement species such as a gas disposed in a measurement cell or an unconfined gas. The VCSELs may be coupled to the gas via fibers to which they are respectively coupled, in accordance with the present invention, for example. In this application, the coupled VCSELs of the present invention are part of a system for measuring infrared radiation absorption by a measurement species, where the system has one or more sources of single mode infrared laser radiation (comprising the coupled VCSEL(s)), and a detector for detecting the single mode infrared laser radiation after passage thereof through a quantity of said measurement species.
 Coupled VCSELs of the present invention may also be utilized for communications applications, such as datacom and telecom. For example, an array of VCSELs may be fabricated, each of which has a narrow and closely-spaced wavelength in an appropriate range (e.g., around 850 or 1310 nm for datacom, or 1550 or 1310 nm for telecom), and each coupled in accordance with the invention to an external modulator. The modulated signals may be combined in a single fiber, and transmitted to subsequent destinations (e.g. shorter distances for datacom applications, larger distances for telecom applications). Alternatively, all of the VCSEL outputs may be combined together and modulated together for distribution on different wavelengths to various laser beam sinks.
 In embodiments of the present invention described above, the output of a VCSEL (of VCSEL array 10, for example) is coupled to some optical device utilizing various coupling structures (e.g., mirror 52) and external optical device alignment features (e.g., V-groove 72) which are lithographically defined into optical support bench substrate 50. In an alternative embodiment, one or more of the optical devices in substrate 50 form part of the working VCSEL. For example, optical support bench substrate 50 may include the top mirror on the other side of mirror 52 to complete the laser cavity. This top mirror may be formed, for example, in a fiber amplifier or fiber portion. Thus, for example, referring once more to FIG. 5, in an embodiment, fiber 58 has a modulated refractive index to form both a gain and mirror (DBR) portion. Thus, in this embodiment, VCSEL 501 is only a “partial” VCSEL, having some gain, where the overall VCSEL has gain portions both in VCSEL 501 and in fiber portion 58, having its top mirror portion in fiber 58, and having the laser cavity along the laser path from the bottom edge of VCSEL 501 and the right-most edge of fiber 58 (in accordance with the orientations shown in FIG. 5). Thus, in this embodiment, the VCSEL portion 501 comprising part of the gain and the “bottom” mirror is coupled, via mirror 52 and substrate 50, to gain/top mirror portion of fiber 58. Alternatively, fiber 58 may have only a DBR on the fiber end (right-most) to form the top mirror, with the gain region being located exclusively within VCSEL portion 501. As used herein, in such configurations, the VCSEL portion (e.g. 501) fabricated in its own substrate (e.g., substrate 12) may be referred to as a “surface-emitting laser” which is coupled to an external optical device, namely the “top” mirror (and possibly a gain portion) which is mounted in support bench substrate 50, where the coupling is by way of mirror 52 and the coupling completes the surface-emitting laser operation by providing the laser cavity and the top mirror.
 The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While the invention has been depicted and described and is defined by reference to particular preferred embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described preferred embodiments of the invention are exemplary only and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.
 1. Field of the Invention
 This invention relates generally to devices that emit electromagnetic radiation and, in particular, to methods and systems for coupling one or more surface-emitting lasers (SELs) to external optical devices such as modulators.
 2. Description of the Related Art
 The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
 Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., “Possibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,” Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
 There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (by a DC or AC current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide.
 Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation is output perpendicular to the wafer surface.
 Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced. Some of those photons travel in a direction perpendicular to the reflective planes of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times. When those photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflective planes tends to increase. A typical laser includes a small difference in reflectivity between its mirrors. The primary laser output is emitted through the reflective plane having lower reflectivity.
 The use of semiconductor diode lasers (both edge-emitting and surface-emitting) for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
 At present, conventional edge-emitting semiconductor lasers play a significant role in optical communication due to their high operating efficiency and modulation capabilities. However, edge-emitting semiconductor diode lasers have several shortcomings which make them difficult to use in many applications. For example, a conventional edge-emitting semiconductor laser typically has a large divergence angle and an elliptical, as opposed to circular, laser beam cross-section. This can require correction and collimating, which can be expensive or otherwise impracticable or undesirable.
 SELs typically have a circular laser beam and a smaller divergence angle, and are therefore more attractive than edge-emitting lasers in some applications. One type of SEL is the vertical cavity surface emitting laser (VCSEL). The VCSEL structure usually consists of an active (gain) region sandwiched between two distributed Bragg reflector (DBR) mirrors. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). Other types of VCSELs sandwich the active region between metal mirrors. The area between the reflective planes is often referred to as the resonator. Further background discussion of VCSELs and related matters are found in: U.S. Pat. No. 5,468,656 (1994), Shieh et al., “Method of making a VCSEL”; U.S. Pat. No. 5,985,686 (1999), Jayaraman, “Process for manufacturing vertical cavity surface emitting lasers using patterned wafer fusion and the device manufactured by the process”; MacDougal et al., “Electrically-Pumped Vertical-Cavity Lasers with AlO-GaAs Reflectors”, IEEE Photonics Letters, vol. 8, No. 3, March 1996. A variant on the standard VCSEL, the vertical-external-cavity surface-emitting laser (VECSEL), is also in use. VECSELs are described in J. Sandusky & S. Brueck, “A CW External-Cavity Surface-emitting Laser,” IEEE Photon. Techn. Lett. 8, 313-315 (1996).
 SELs such as VCSELs have other advantages, which has led to an increased interest in these devices. For example, the conventional VCSEL has several advantages, such as emitting light perpendicular to the surface of the die, and the possibility of fabrication of two dimensional arrays. Because VCSELs emit light perpendicular to the die surface, it is cheaper and easier to test them because they can be tested before dicing the wafer, unlike edge-emitting lasers. Also, as noted above, the output laser beam from an SEL has a much smaller divergence angle than a conventional edge-emitting laser. Light with a smaller divergence angle is easier to focus into a smaller spot size or collimate into a laser beam that can maintain a smaller spot size after the beam has traveled a long distance.
 Semiconductor lasers such as VCSELs are used in a variety of applications. In some applications, e.g., telecommunications and spectroscopy among others, the output laser light is modulated to achieve the objective of the system. Modulation consists of modifying a characteristic of the laser output, e.g., the amplitude, frequency, or phase. The modulation can be predetermined by response characteristics of the optical target. U.S. Pat. No. 5,981,957, for example, shows a fluorometric system that employs a predetermined modulation of a laser output used to detect the fluorescence response of a sample. In this case, a major concern is the accuracy of the modulation, because that accuracy limits the accuracy of the measurements. The use of VCSELs for applications such as spectroscopy is described in A. Garnache et al., “Application of a Diode-pumped Broadband Vertical-external-cavity Surface-emitting Semiconductor Laser to High-sensitivity Intracavity Absorption Spectroscopy,” forthcoming in Journal of Optical Society of America B (January 2000); A. Garnache et al., “High sensitivity Intra-Cavity Laser Absorption Spectroscopy with Vertical-External-Cavity Surface-Emitting semiconductor Lasers,” Optics Lett. 24 (1999): 826-828; and in U.S. Pat. No. 6,091,504.
 In the case of telecommunications, the modulations are patterned to correspond to information. When the radiation of the output laser beam is detected after it has traveled to another point, the modulations indicate the information that was encoded at the transmitter/modulator end. A typical telecommunications system uses optical fiber to guide the radiation from the modulation (or emission) point to the detection point.
 One major concern in such systems is ensuring that the modulated radiation reaches the detection point with sufficient power to be detected. Another major concern is choosing a modulation scheme that can keep up with the rate of information being provided, while ensuring that the receiver can decode the information. In digital systems that information is provided as bits. Current telecommunications systems can modulate laser radiation according to a pattern that represents a billion or more bits per second. Such rates of information transfer are referred to as 1+ Gb/s. The conventional equipment necessary to modulate laser radiation can be very expensive.
 VCSELs have a thin active (gain) region (measured in the direction of emission) and require high resonator efficiency. As a result, low currents can drive a VCSEL. VCSELs are typically amplitude modulated by varying the potential difference, and therefore current, applied across the active region. A drop in current reduces the rate at which photons are emitted. An increase in current increases the number of photons emitted. In this manner, the VCSEL can be “direct modulated” so as to (amplitude) modulate the output laser beam. There is a limit, however, to the speed at which the radiation output of the VCSEL will follow the change in driving current. That limit can impede the use of VCSELs in applications requiring a particular minimum rate of modulation.
 It is, therefore, sometimes desirable to externally modulate a VCSEL, or VCSELs of an array of VCSELs. To do this, the output of the VCSEL must be coupled to an external modulator. It is also desirable to use SELs in other applications, such as coupling the output laser beam into a fiber or semiconductor optical amplifier (SOA). In each such application or use of a VCSEL, it is necessary to couple the output laser beam from the SEL into some other optical device, such as a modulator, optical fiber, or SOA. The term “optical device” as used herein refers to any device, external to the SEL, to which the output of the SEL is coupled so that the optical device receives the output laser beam. The SEL is a laser beam source, and the optical device to which the SEL is optically coupled may be regarded as a laser beam sink.
 It can be difficult to accurately align the VCSEL and couple it with desired optical device. For example, VCSELs are currently “butt-coupled” with fibers, which can be expensive or difficult.
 The present invention is directed to a method and apparatus for coupling a surface-emitting laser to an external optical device such as an external modulator, fiber, or SOA. The laser and the external optical device to which it is coupled are aligned by photolithographically defined features.
 An advantage of the invention is that it enables the output of a surface emitting laser to be conveniently modulated.
 Another advantage of the invention is that it permits coupling a high percentage of power from a surface emitting laser to an external modulator or other device.
 Another advantage of the present invention is allowing data modulation rates higher than direct modulation limits.
 Another advantage is aligning arrays of surface emitting lasers with arrays of modulators.
 Still another advantage is decreasing the cost of manufacturing lasers with high modulation rate capacity.
 Another advantage is that it allows coupling to an SOA which amplifies the output of the modulator.
 Another advantage is separately optimizing the performance of the laser and modulator.
 Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings. Not all embodiments of the invention will include all the specified advantages. For example, one embodiment may only modulate the output of a surface emitting laser, while another only aligns an array of surface emitting lasers with an array of modulators.