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The present invention generally relates to structures for the alignment of the components of a mechanical assembly, such as an optical package. More specifically, some embodiments of the present invention relate to optical packages and the alignment of components thereof such that an output beam of a laser is positioned upon a waveguide input of a wavelength conversion device.
2. Technical Background
In many applications, there is a need for extremely accurate mechanical connection between components of an assembly. For example, accurate optical coupling is required in the assembly of component parts of a frequency doubled green laser apparatus or system. In such an application, a nonlinear optical crystal, such as a Mg—O doped periodically poled lithium niobate (PPLN) crystal, is used to convert the infrared light emission of a laser into visible green light. Both the diode laser and nonlinear optical crystal use single mode waveguide structures to confine and guide the light energy. In such a green laser application, there is a need for the components of the assembly to be maintained in rigid alignment such that the output beam of the laser is precisely aligned with the very small waveguide input that is located on an input face of the waveguide crystal. Waveguide optical mode field diameters of typical second harmonic generating (SHG) crystals, such PPLN crystals, can be in the range of a few microns. As a result, the present inventors have recognized that it can be very challenging to properly align and focus the output beam from the laser diode with the waveguide of the SHG crystal, particularly during assembly of the optical package.
Tolerances on the alignment of the laser and nonlinear crystal waveguides may be between 300 nm and 500 nm (for 5% degradation in coupling) in the plane perpendicular to the optical axis. The tolerance along the direction of the optical axis may be significantly looser, between about 3 μm and 4 μm. Therefore, the slightest misalignment between the laser output beam and the waveguide input may result in reduced coupling of the infrared energy and result in a loss of green output power.
Generally, there are two strategies to aligning the components in the green laser assembly: a passive alignment approach and an active alignment approach. In the passive alignment approach, a permanent attachment technique, such as laser welding or UV cured adhesive, is utilized to achieve a rigid, accurate attachment between components of the green laser. With regard to laser welding, due to weld heating and stresses, post-weld part shifts occur and it is difficult to achieve assembly accuracy better than about 1 μm. The requirements of the green laser assembly require an order of magnitude better accuracy (positional accuracy on the order of 0.1 μm is needed). UV cured adhesives make achieving assembly accuracy of approximately 0.1 μm possible, but such adhesives are susceptible to swelling due to heat and humidity. The stability of the components relative to one another must be maintained to a few tenths of a micron over the lifetime of the laser and a wide range of environmental conditions (e.g., +10° C. to +60° C., up to 85% relative humidity).
In an active alignment approach, an adjustable active component is used to insure that the infrared energy from the laser is accurately aligned with the small input of the crystal waveguide. Because of this adjustability, the requirements for alignment of the various component parts of the device can be relaxed by an additional order of magnitude or so, allowing the components to be assembled to much more relaxed positional tolerances, on the order of tens or hundreds of microns. The active component or components may also be used to accommodate alignment changes during the life and operation of the laser. The downside of the active alignment approach is the active component itself Typically, an active component is either a piezo-electric actuator or micro-electro-mechanical (MEMS) mirror device, which adds cost to the entire package, and reduces the overall reliability. Such devices can be susceptible to breakage during assembly, failures from environmental exposure, and shock induced damage.
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According to one embodiment, an optical package including a laser, a wavelength conversion device, a mirror and a connecting structure is provided. The laser is configured to emit a laser beam incident upon the mirror. The mirror is configured to reflect the laser beam such that the laser beam is incident upon an input facet of the wavelength conversion device. The connecting structure includes a structure base and three bipod flexures. Each of the bipod flexures includes first and second bipod legs extending from the structure base to the mirror and a heating element thermally coupled to the first and second bipod legs. Heat generated by the heating element changes the length of the bipod legs. The bipod flexures are arranged in a tripod configuration such that changes in the length of the bipod legs of the three bipod flexures alter the reflection of the laser beam from the mirror.
According to another embodiment, a connecting structure includes three bipod flexures, a first component and a second component. Each of the bipod flexures includes first and second bipod legs extending from the first component to the second component and a heating element thermally coupled to the first and second bipod legs such that heat generated by the heating element changes the length of the bipod legs. The bipod flexures are arranged in a tripod configuration such that changes in the length of the bipod legs of respective ones of the three bipod flexures alter the position of the first component with respect to the second component.
According to yet another embodiment, a connecting structure for rigidly connecting a first component to a second component is provided. The connecting structure couples the first component to the second component such that no freedom of motion exists between the first and second components. The connecting structure comprises a plurality of members having a selectively controllable length configured to produce a change in the position of the first component with respect to the second component upon the application of heat to the plurality of members.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
FIG. 1 is an illustration of an exemplary optical package according to one or more embodiments of the present disclosure;
FIG. 2 is an illustration of an exemplary structure base according to one or more embodiments of the present disclosure;
FIG. 3 is an illustration of an exemplary mirror and lens housing assembly and an exemplary connecting structure according to one or more embodiments of the present disclosure;
FIG. 4 is an illustration of an exemplary mirror and lens housing assembly and an exemplary connecting structure according to one or more embodiments of the present disclosure;
FIG. 5A is an illustration of an exemplary mirror and lens housing according to one or more embodiments of the present disclosure;
FIG. 5B is an illustration of an exemplary mirror and lens housing according to one or more embodiments of the present disclosure;
FIG. 6 is an illustration of an exemplary lens and an exemplary mirror according to one or more embodiments of the present disclosure;
FIG. 7 is an illustration of an exemplary lens according to one or more embodiments of the present disclosure;
FIG. 8 is an illustration of an exemplary optical path of a laser beam according to one or more embodiments of the present disclosure;
FIG. 9 is a schematic illustration of optical beam movement created by movement of exemplary bipod flexures arranged in a tripod configuration according to one or more embodiments of the present disclosure; and
FIG. 10 is an illustration of an exemplary connecting structure according to one or more embodiments of the present disclosure.
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Referring initially to FIG. 1, connecting structures comprising members configured as heated flexures may be utilized to rigidly and precisely couple a first component to a second component. Although embodiments described herein are described in the context of green laser optical packages, the connecting structures and heated flexures described herein may be utilized in other applications that require fine alignment between components. Although the general structure of the various types of optical packages in which the concepts of particular embodiments of the present invention can be incorporated is taught in readily available technical literature relating to the design and fabrication of frequency or wavelength-converted semiconductor laser sources, the concepts of particular embodiments of the present invention may be conveniently illustrated with general reference to an optical package 100 including, for example, a light source 40, such as a semiconductor laser, and a wavelength conversion device 30. The optical package 100 illustrated in FIG. 1 is particularly useful in generating a variety of shorter wavelength laser beams from a variety of longer wavelength semiconductor lasers and can be used, for example, as a visible laser source in a laser projection system.
Generally, a connecting structure of the present disclosure comprises a plurality of heated flexures 20 and a structure base 50. The heated flexures of the embodiment illustrated in FIGS. 1, 3 and 4 are configured as bipod flexures 20 having bipod legs 22a and 22b. It is understood that the heated flexures are not limited to this bipod configuration as other configurations are possible (e.g., the triangular flexure 120 illustrated in FIG. 10). The heated flexures 20 may be arranged in a tripod configuration about the aligned components. The heated flexures 20 may be arranged about the components to provide for rigid alignment such that no freedom of motion exists between the first and second components. The connecting structure may be used to precisely align the output beam of a laser 40 with a waveguide input positioned on an input facet of a wavelength conversion device 30. The heated flexures 20 comprise low-power heating elements that effect minute dimensional changes on the flexures connecting the optics (e.g., a mirror) to the laser 40 and wavelength conversion device 30. As will be described in more detail below, the motions of the heated flexures 20 may steer the infrared optical beam emitted by the laser 40, thereby allowing active control of its position upon the input facet of the wavelength conversion device 30 (and the waveguide input thereon). By controlling the heated flexures 20 with closed-loop feedback, the heated flexures 20 may be used to compensate for both assembly and environmentally (e.g., temperature and humidity) induced misalignment with low power consumption and high reliability.
Although some embodiments of the connecting structure disclosed herein are described and illustrated as a plurality of independent heated flexures, it is to be understood that embodiments of the present disclosure are not limited thereto. Embodiments of the present disclosure may comprise a unitary connecting structure wherein the heated flexures 20 are connected to one another rather than separate components.
The laser 40 may comprise one or more lasers or coherent light sources, such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, vertical cavity surface-emitting lasers (VCSEL), or vertical external cavity surface-emitting lasers (VECSEL), for example. The wavelength conversion device, such as a second harmonic generating (SHG) crystal or a higher harmonic generating crystal, may be used to frequency-double an output beam emitted by the laser 40 having a native wavelength in the infrared or near-infrared band. For example, a SHG crystal, such as a MgO-doped periodically poled lithium niobate (PPLN) crystal, may be used to generate green light by converting the wavelength of a 1060 nm DBR or DFB laser to 530 nm.
Referring to the embodiment illustrated in FIG. 1, the laser 40 and wavelength conversion device 30 may be rigidly coupled to a structure base 50. The structure base 50 may be rigidly attached to another mounting surface (not shown) such as a printed circuit board via through-hole mounting assembly 80 and pins 82. The connecting means used to connect the structure base 50 to the mounting surface is not limited to a through-hole configuration and may also include other connecting means, such as flexible or rigid circuit connections or surface mount technology, for example.
In the illustrated embodiment, the wavelength conversion device 30 is rigidly coupled to a first surface 52 of the structure base 50, while the laser 40 is rigidly coupled to a lower second surface 54 such that the wavelength conversion device 30 is positioned above the laser 40. The spatial modes of the laser 40 and the wavelength conversion device 30 of this embodiment are asymmetric with respect to the x and y directions (see FIG. 8).