Freeform collimator lenses for angled facet laser devices.

Abstract

The device includes a waveguide and a freeform collimating lens. The waveguide is characterized by a waveguide axis and a flat end having a normal axis tilted at an end angle greater than 0 degrees relative to the waveguide axis. The freeform collimating lens collimates light exiting the flat end of the waveguide into a collimated light beam characterized by a beam direction parallel to the waveguide axis. The device prevents reflections from the flat end of the waveguide from propagating back through the waveguide while providing a collimated light beam having a direction parallel to the waveguide axis.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 066,070, filed August 14, 2020, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] A quantum cascade laser device (QCL) typically consists of an elongated, roughly rectangular, box-shaped active semiconductor waveguide structure fixed to a metal submount that provides thermal and electrical contact. The divergent light emitted from the front facet of the waveguide in such devices is generally collimated using an aspherical, high numerical aperture lens, which is effective in correcting spherical aberrations.

[0003] QCL device waveguides are most commonly fabricated with normal-incidence output facets, i.e., the normal of the output facet is parallel to the waveguide long axis and the direction of light propagation. This is done for reasons of convenience and ease of manufacturing, because light is emitted straight along the device axis, thereby being squarely incident on the submount and can be collimated by a lens aligned on-axis with the waveguide, regardless of wavelength.

[0004] However, it has been demonstrated that introducing an angle (typically 7-10 degrees) to the output facet of a quantum cascade laser waveguide can enhance both its performance (maximum power, spatial mode quality) while simultaneously reducing or eliminating the need for high-complexity, high-cost multilayer anti-reflection coatings.

[0005] Unfortunately, introducing angled facets adds complexity to the broader optical design. The angled facets refract the emitted light from the waveguide at an angle relative to the long axis of the gain chip. In principle, the beam steering effect could be corrected by mounting the collimating lens at an angle relative to the gain chip mount, so that the light enters the collimating lens along its central axis. This would also require rotating the external cavity to accommodate the new beam direction. However, this solution introduces significant complexity into the mounting scheme for the gain chip and collimating lens. It also does not necessarily correct for refractive beam steering when the propagation angle from the waveguide is wavelength dependent, nor does it eliminate non-rotational, symmetric aberrations resulting from shifting object plane distances. Summary of the Invention [Means for solving the problem]

[0006] The present invention includes an apparatus having a waveguide and a freeform collimating lens. The waveguide is characterized by a waveguide axis and a flat end having a normal axis tilted at an end angle greater than 0 degrees relative to the waveguide axis. The freeform collimating lens collimates light emerging from the flat end of the waveguide into a collimated light beam characterized by a beam direction parallel to the waveguide axis.

[0007] In one embodiment, the end angle is greater than 7 degrees.

[0008] In another aspect, the device also includes an optical amplifier that amplifies light reflected from the flat end, the end angle being selected so that the intensity of the light reflected from the flat end is insufficient to cause lasing in a system including the device.

[0009] In another aspect, the optical amplifier includes a quantum cascade gain chip.

[0010] In another aspect, the optical amplifier comprises a doped optical fiber.

[0011] In another embodiment, the device also includes an external cavity reflector that returns the collimated beam of light to the freeform collimating lens, the returned light traveling in a direction parallel to the waveguide axis.

[0012] In another aspect, the external cavity comprises a wavelength selective filter.

[0013] In another aspect, the wavelength selective filter includes a diffraction grating.

[0014] In another aspect, the freeform collimating lens includes a freeform concave surface proximate the flat end and a spherical convex surface distal to the flat end.

[0015] In another aspect, the apparatus also includes a lens mount that positions the freeform collimating lens relative to the quantum cascade gain chip such that the optical axis of the freeform lens is parallel to the waveguide axis. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 shows a typical QCL with an EC for tuning the laser. [Figure 2] FIG. 1 shows a gain chip with facets cut at a slight angle. [Figure 3] FIG. 1 shows a gain chip with a rotated collimating lens. [Figure 4] FIG. 1 illustrates a collimating lens according to one embodiment of the present invention. [Figure 5] FIG. 1 illustrates an example of a master oscillation power amplifier (MOPA). [Figure 6] 6 shows a waveguide according to an embodiment of the present invention that can be used in the amplifier shown in FIG. 5. DETAILED DESCRIPTION OF THE INVENTION

[0017] The manner in which the present invention provides its advantages can be more easily understood with reference to a QCL that uses an external cavity (EC) to tune the output wavelength. Reference is now made to FIG. 1, which shows a typical QCL equipped with an EC for tuning the laser. Laser 40 includes a gain chip 41 attached to a mount 42. Light emitted from a front facet 43 of gain chip 41 is collimated by a lens 52 and reflected from a grating 46. The angle of grating 46 relative to the light beam from gain chip 41 is selected to lock the laser to a particular mode. The angle is set by actuator 45, which rotates the grating about a selected axis 53 such that the diffracted wavelength and cavity length are maintained to provide the desired wavelength. Lens 47 expands the output beam to a desired size and provides output light for use by a measurement system utilizing laser 40 as a light source. Lens 52 expands the light exiting front facet 43 of gain chip 41.

[0018] The above discussion assumes that no reflection occurs at facet 43. If facet 43 were to reflect light and be parallel to facet 48, the two facets would form a fixed-length optical cavity that "competes" with the desired optical cavity provided by facet 48 and grating 46. To avoid this problem, conventional external cavity lasers coat facet 43 with an anti-reflective coating, which increases the cost of the laser. Because these lasers are designed to be tunable over a large wavelength range, the cost of an anti-reflective coating that operates over the entire wavelength range can be significant.

[0019] In conventional systems, the gain chip is typically attached to a mount with wire bonds to electrically connect the chip to the mount. The collimator lens is typically rigidly mounted to a common structure with the chip mount. The collimator lens mount typically requires multiple degrees of freedom to properly align the lens relative to the gain chip.

[0020] One solution to this second cavity problem is to utilize a gain chip in which facet 43 is not parallel to facet 48. Refer now to FIG. 2, which shows a gain chip with facets cut at a slight angle. Gain chip 11 has facet 12 cut at an angle of 7° to 10° relative to facet 13. Because the two ends of the gain chip are no longer parallel, light reflected from the ends cannot form a resonant cavity. As a result of the angled surfaces, light exiting facet 12 is refracted away from the direction the light would travel in the case of conventional perpendicular facets. If collimating lens 14 remains in the same position as a corresponding collimating lens used with non-tilted facets, two problems are encountered.

[0021] To generalize the following discussion to other optical systems, the angle of the waveguide end is specified by the angle between the plane of the end and the light guide axis. In this nomenclature, the angle between the angled end and the waveguide axis is between 80 and 83 degrees.

[0022] First, the light emerging from facet 12 is not a point source, but extends over a small area on facet 12. This causes some of the light to be emitted at a point different from the focal point of collimating lens 14. This leads to a degradation in the collimation of the light.

[0023] Second, the lower part of the collimating lens 14 is less than optimally utilized because the beam from the angled facets illuminates the lens to the same extent as the lower part.

[0024] One method for reducing these problems involves rotating the collimating lens 14 to compensate for the change in emission angle introduced by the tilted facet. Reference is now made to FIG. 3, which shows a gain chip with a rotated collimating lens. In the configuration shown in FIG. 3, the collimating lens 14 is rotated so that the optical axis 15 of the collimating lens 14 is perpendicular to the surface of the facet 12 and is positioned at the center of the light-emission area. While this solution partially compensates for the optical aberrations mentioned above, it requires a complex mounting arrangement for the collimating lens 14. Furthermore, the axis of the collimated light beam is no longer collinear with the axis of the waveguide in the gain chip 13. This requires modifications to the mounting system of the diffraction grating, which also increases the cost of the laser.

[0025] The lasers of the present disclosure solve these problems by utilizing collimating lenses that can be placed in situ to compensate for the effects of tilted facets. As used in this disclosure, the term "collimated" refers to a beam that can propagate over laboratory-scale distances (centimeters to meters) without significantly changing size (beam divergence is reduced to the milliradian level). For mid-infrared beams (wavelengths of approximately 4 to 12 microns), the beam waist must be several millimeters. In one exemplary embodiment, the beam waist is 5 millimeters. In another exemplary embodiment, the beam waist is 7 millimeters. Similarly, the focal length of the collimating lens is 2 to 5 millimeters in one exemplary embodiment.

[0026] Reference is now made to Figure 4, which illustrates a collimating lens according to one embodiment of the present invention. Gain chip 11 includes angled facets 12 that emit light at locations centered on the optical axis of the waveguide in gain chip 11. The light exiting facets 12 is collimated by a lens called a "freeform lens." Freeform lens 60 has an outer convex spherical surface 61 and an inner concave surface 62 with a shape calculated to collimate the light rays emitted from facet 12. This inner surface is called the freeform surface.

[0027] Freeform lenses of the type shown in FIG. 4 are known in the art and therefore will not be described in detail here. For the purposes of this disclosure, it is sufficient to state that a freeform surface is defined by a function having a number of variable parameters determined so that the shape of the surface, together with the lens material and other surfaces of the lens, provides the desired collimation. In one embodiment, the freeform surface is represented as a three-dimensional polynomial with multiple free parameters determined so that light rays exiting facet 12 and passing through the surface region are diffracted into light rays exiting lens 60 in a direction parallel to optical axis 64. The function describing each small surface region is further constrained so that the overall surface constructed from the collection of surface regions is a continuous and smooth surface. The individual polynomial surfaces are preferably at least second order, or higher. Additionally, the parameters and the points where the surface regions join each other are selected so that the first derivative of the overall surface is continuous at the points where the two surface regions join.

[0028] The parameters of lens 60 are also selected so that lens 60 and gain chip mount 68 can be fixed to a common plane 67. In this arrangement, optical axis 64 is collinear with waveguide axis 65 of gain chip 11. This compensates for artifacts introduced by angled facets 12, thereby substantially reducing the need for anti-reflection coatings on facets 12 while preserving the existing geometry of the external cavity laser.

[0029] The above embodiments utilize gratings for wavelength-selective filters and reflectors for the external cavity. However, the collimation system of the present disclosure can be used with any external cavity quantum cascade laser to maintain the linear geometry of the laser cavity while correcting for distortions introduced by the sloped facet edges in the gain chip. In addition, this collimation system can be used with other wavelength-selective filters in the external laser cavity.

[0030] The above embodiments are directed to lasers. However, the disclosed system can be advantageously utilized in other optical systems where light reflected from the exit facet of a waveguide can obscure the system, since the exit facet provides one surface of a resonant cavity for light of the wavelength of interest. Reference is now made to FIG. 5 , which illustrates an example of a master oscillation power amplifier (MOPA). MOPA 80 utilizes a seed laser 81 to generate an optical signal that is coupled to an optical amplifier 82 by a coupler 83. To simplify the illustration, the power supply for the optical amplifier 82 has been omitted from the drawing. The output of the optical amplifier 82 is provided to an output optical fiber 84. The output optical fiber may be part of the optical amplifier 82. For example, the optical amplifier 82 can be a doped optical fiber that is pumped to provide amplification. Doped optical fibers for amplifying optical signals are known in the art and therefore will not be described in detail here. In this case, the output optical fiber 84 may be the end of a doped optical fiber. The output of the output optical fiber 84 is typically expanded into a collimated optical beam 87 by a collimating lens 86. Collimating lens 86 is rigidly connected to output optical fiber 84 by a mounting structure 88 similar to the mounting structure described in the previous embodiment of the laser.

[0031] One problem with the arrangement shown in FIG. 5 arises from reflections at surface 85. Typically, surface 85 is perpendicular to the axis of output fiber 84. If the reflectivity at surface 85 is sufficient, a laser cavity is formed by surface 85 and the reflective surfaces of seed laser 81 or coupler 83. As a result, optical amplifier 82 can become a separate laser with a spectral pattern different from that of seed laser 81. For this reason, surface 85 is often coated with an anti-reflective coating, which increases the cost of MOPA 80.

[0032] Reference is now made to FIG. 6, which illustrates a waveguide that can be used in place of the waveguide 84 shown in FIG. 5. The waveguide 95 has an end face 96 cut at an angle other than 90 degrees relative to the optical axis 97. The angle of the end face 96 relative to the axis 97 is selected so that reflections from the surface 96 do not return to the waveguide 95, thus avoiding the lasing problem described above. The freeform lens 90 provides collimation of the output light from the waveguide 95 in the direction of the axis 97. The optical axis 93 of the collimated beam coincides with the optical axis of the waveguide 95 in this example. The waveguide 95 is fixed on a mount 98, which is rigidly positioned relative to the freeform lens via attachment to a common surface 94.

[0033] Exemplary Embodiments Embodiment 1: An apparatus comprising: a waveguide characterized by a waveguide axis and a flat end having a normal axis tilted at an end angle greater than 0 degrees relative to the waveguide axis; and a freeform collimating lens that collimates light exiting the flat end of the waveguide into a collimated light beam characterized by a beam direction parallel to the waveguide axis.

[0034] Embodiment 2: The device of embodiment 1, wherein the end angle is greater than 7 degrees.

[0035] Embodiment 3: The device of embodiment 1 or 2, further comprising an optical amplifier that amplifies light reflected from the flat end, wherein the end angle prevents lasing in a system including the device.

[0036] Embodiment 4: The apparatus of embodiment 3, wherein the optical amplifier comprises a quantum cascade gain chip.

[0037] Embodiment 5: The apparatus of embodiment 3, wherein the optical amplifier comprises a doped optical fiber.

[0038] Embodiment 6: The device of any one of embodiments 1 to 4, further comprising an external cavity reflector that returns a beam of collimated light to the freeform collimating lens, the returned light traveling in a direction parallel to the waveguide axis.

[0039] Embodiment 7: The device of embodiment 6, including a cavity external to the quantum cascade gain chip, the cavity including a wavelength selective filter.

[0040] Embodiment 8: The device of embodiment 7, wherein the wavelength-selective filter comprises a diffraction grating.

[0041] Embodiment 9: The device of any one of embodiments 1 to 8, wherein the freeform collimating lens comprises a freeform concave surface near the flat end and a spherical convex surface distal to the flat end.

[0042] Embodiment 10: An apparatus described in any one of embodiments 1 to 9, further comprising a lens mount that positions the freeform collimating lens relative to the quantum cascade gain chip to maintain the beam direction parallel to the waveguide axis.

[0043] The above-described embodiments of the present invention are provided to illustrate various aspects of the invention. However, it should be understood that different aspects of the invention illustrated in different specific embodiments can be combined to provide other embodiments of the invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is limited only by the scope of the following claims. The claims as originally filed are as follows: Claim 1: a waveguide characterized by a waveguide axis and a flat end having a normal axis tilted at an end angle greater than 0 degrees relative to the waveguide axis; a freeform collimating lens that collimates the light exiting the planar end of the waveguide into a collimated light beam characterized by a beam direction parallel to the waveguide axis; An apparatus comprising: Claim 2: The device of claim 1 , wherein the end angle is greater than 7 degrees. Claim 3: 10. The apparatus of claim 1, further comprising an optical amplifier that amplifies light reflected from the flat end, the end angle preventing lasing in a system including the apparatus. Claim 4: The apparatus of claim 3 , wherein the optical amplifier comprises a quantum cascade gain chip. Claim 5: The apparatus of claim 3 , wherein the optical amplifier comprises a doped optical fiber. Claim 6: 5. The apparatus of claim 4, further comprising an external cavity reflector that returns a collimated beam of light to the freeform collimating lens, the returned light traveling in a direction parallel to the waveguide axis. Claim 7: 7. The apparatus of claim 6, further comprising a cavity external to the quantum cascade gain chip, the cavity including a wavelength selective filter. Claim 8: The apparatus of claim 7 , wherein the wavelength-selective filter comprises a diffraction grating. Claim 9: 10. The apparatus of claim 1, wherein the freeform collimating lens includes a freeform concave surface proximate the flat end and a spherical convex surface distal to the flat end. Claim 10: 5. The apparatus of claim 4, further comprising a lens mount that positions the freeform collimating lens relative to the quantum cascade gain chip to maintain the beam direction parallel to the waveguide axis.

Claims

1. a waveguide characterized by a waveguide axis and a flat end having a normal axis tilted at an end angle greater than 0 degrees relative to the waveguide axis; a freeform collimating lens having a freeform concave surface disposed near the flat end and a spherical convex surface disposed distal to the flat end, the freeform collimating lens collimating light emitted from the flat end of the waveguide to have a beam direction parallel to the waveguide axis; An apparatus comprising:

2. The device of claim 1 , wherein the end angle is greater than 7 degrees.

3. 10. The device of claim 1, further comprising an optical amplifier that amplifies light reflected from the flat end, the angle of the end being selected such that light reflection from a surface of the flat end does not propagate back up the waveguide so as to prevent lasing in a system including the device.

4. The apparatus of claim 3 , wherein the optical amplifier comprises a quantum cascade gain chip.

5. 4. The apparatus of claim 3, wherein the optical amplifier comprises a doped optical fiber.

6. 5. The apparatus of claim 4, further comprising an external cavity reflector that returns a collimated beam of light to the freeform collimating lens, the returned light traveling in a direction parallel to the waveguide axis.

7. The apparatus of claim 6 , further comprising a cavity external to the quantum cascade gain chip, the cavity including a wavelength selective filter.

8. The apparatus of claim 7 , wherein the wavelength-selective filter comprises a diffraction grating.

9. 5. The apparatus of claim 4, further comprising a lens mount that positions the freeform collimating lens relative to the quantum cascade gain chip to maintain the beam direction parallel to the waveguide axis.

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