SYSTEM AND METHOD OF ALIGNING TWO OPTICAL AXIS TOWARDS EACH OTHER
BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to a system for the precise positioning of an optical beam with respect to a reference location.
Technical Background
Fiber optic based circuits frequently require free-space optics such that light in an optical fiber must exit the fiber for filtering, coupling, or other functions after which it is subsequently reinjected into a second fiber. Such free-space optics require the use of collimators since light exiting the end of a fiber tends to divert spacially. CoUimators coupled to optical fibers, therefore, are employed to maintain a focused beam of light which is filtered or otherwise the subject of optical processing in free space. Subsequently, the processed signal is introduced into another fiber utilizing a mating collimator.
The use of collimators in such applications requires the alignment of the optical axis of the collimator to minimize insertion loss. Collimators typically have a mechanical axis which can be as much as two degrees or more offset from their optical axis, resulting in a pointing error for the collimator. Thus, it is not possible to merely align the mechanical axes of two collimators and expect signal communication between them. In order to align the optical axes of collimators, in the past, one of the collimators has been clamped on an optical bench and the second collimator spaced
anywhere from 3 to 100 mm from the first collimator, placed on a staging system which allows full axes of movement of translation and rotation such that a signal injected from a light source is coupled to the first collimator and the second collimator is coupled to a power meter. The second collimator is translated and rotated until the power meter indicates a peak. With such a system which is manually accomplished through an operator requiring a certain amount of skill, if not luck, the alignment process does not typically result in an optimal alignment of the optical axes inasmuch as several false peaks can be detected by the power meter. The process can take significantly longer than 5 minutes even for an operator with significant skill and experience. Unfortunately, the alignment utilizing such mechanical techniques is not particularly accurate and the rate of device rejections using such alignment techniques result in an unacceptable yield rate for optical circuits manufactured utilizing such mechanical alignment techniques.
In order to overcome these difficulties, a computer controlled stage for moving a second collimator with respect to the first collimator has been employed, which maps and stores all possible peaks detected by a power meter coupled to the second movable collimator. Although the most accurate alignment peak is detected and can be selected, the mapping process involving moving the stage throughout a plane in which the movable collimator is mounted can take up to one hour, thereby greatly increasing the cost of optical devices utilizing collimators aligned in such a manner.
There exists, therefore, a need for precisely positioning an optical light beam with respect to its surroundings and which eliminates the guess work required by mechanical alignment or the tedious and expensive utilization of computerized mapping.
SUMMARY OF THE INVENTION The system of the present invention provides an accurate, quick and relatively inexpensive system for precisely aligning a light beam with respect to its surroundings and, in one application, for the alignment of one collimator with respect to a second collimator. The concept of the system and method of the present invention is to first position the optical axis of a light beam in a known orientation with respect to a flat reflective surface of, for example, a precision gauge block by adjusting the angular
position of the optical device and subsequently substituting the gauge block with a precision reflective sphere and translating the optical device until the reflected beam is precisely positioned and detected. Two collimators can be aligned to a parallel opposed flat reflective surface, such as provided by a gauge block positioned in a location where, for example, a filter may then be inserted to form an optical component. Each collimator is initially aligned with emitted beams of optical energy aligned normally with each of the associated surfaces of the gauge block, such that the beams are then parallel to one another and perpendicular to the gauge block. Subsequently, the gauge block is replaced with a reflective precision sphere and the collimators translated until the beams intercept the center of the sphere and are not only parallel to one another but perfectly aligned with one another within arc seconds of the optical axes of the devices. The process of precisely aligning, for example, two collimators by an operator, once trained, can be accomplished in from one to two minutes and results in the alignment of the optical axes of the collimators within tenths of microns, thereby greatly reducing insertion loss in optical components having their optical axes aligned in such manner. The system and method of aligning, for example, two collimators utilizes relatively inexpensive commercially available gauge blocks and precision spheres and alignment techniques which can be easily learned and quickly applied to provide precise alignment of the optical axes of two devices or the precise positioning of one device with respect to its surrounding environment.
The systems and methods of the present invention can be employed not only for the manufacturing of optical circuits, it can also be used in the testing of optical components prior to their subsequent assembly with other components. Such testing, for example, may be used for determining the center wavelength of filters. Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are
incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side elevational schematic view of the apparatus employed for aligning the optical axes of two optical devices embodying the system of the present invention;
Fig. 2 is a schematic optical diagram of the equipment employed in connection with the apparatus shown in Fig. 1 for the practice of the present invention;
Fig. 3 is a schematic diagram illustrating a first step in a first stage in the alignment of the optical axes of two collimators embodying the system and method of the present invention;
Fig., 4 is a schematic diagram illustrating the completion of the first step of alignment of the pair of collimators embodying the system and method of the present invention;
Fig. 5 is a schematic view illustrating a first step in a second stage of the alignment of two collimators by the system and method of the present invention;
Fig. 6 illustrates a second step completing the second stage of the alignment of two collimators in the alignment system and method of the present invention; and
Fig. 7 is a schematic view of an optical component manufactured according to the system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to Fig. 1 , there is shown an apparatus 10 for the alignment of the optical axis of two optical components, such as a first collimator 12 and a second collimator 14 which are employed in a free-space optics application such as placing a filter between the two collimators (as seen in Fig. 7) which requires the precise alignment of the optical axes of the collimators. The collimators 12 and 14 may be spaced a distance D from one another which, depending upon the application and device, can range anywhere from 3 mm to 100 mm. The collimators are each held by a kinematic tip/tilt stage 13 and 15, respectively, which, in turn, are mounted to
translation stages 17 and 19, respectively. The coupled stages are then mounted to a stable base 20, such as an optical bench. The kinematic tip/tilt stages and the translation stages are commercially available from sources such as Newport Equipment and are in common use in the manufacture of optical components. Additionally, the apparatus 10 of Fig. 1 includes a substrate 30 which is employed for the final supporting of an optical component which includes collimators 12 and 14 and a filter positioned therebetween. Mounted to the substrate 30 is a holding clamp 16 which is interposed between and generally in alignment with the collimators 12 and 14 for holding a target device 18 which, as described below, is employed for alignment of the optical axes and subsequently is replaced by an optical component, such as a filter, to be positioned between the collimators.
The system and method of the present invention involves a two-stage alignment process with both stages employing the optical components for providing a source of light either visible or at signal wavelengths and detectors for detecting such light, as shown in Fig. 2. In Fig. 2, the alignment target device 18 is shown schematically positioned between collimator 12 and 14. Each of the collimators have an input optical fiber 22 and 24, respectively, coupled by a Y-splitter 26 and 28, respectively, to couple a light source 32 and 34, respectively, to input optical fiber 22 and 24 and, therefore, collimators 12 and 14. Power meters 36 and 38 receive optical signals received by collimators 12 and 14 through fibers 22 and 24, splitter 26, and detect the optical signal impinging upon the ends of collimators 12, 14.
In the schematic diagram of Fig. 3, there is schematically illustrated the collimators 12 and 14, each of which having a mechanical axis, such as axis 40 for collimator 12 and axis 42 for collimator 14. They also have an optical axis, such as axis 41 for collimator 12 and axis 43 for collimator 14, which are illustrated as being offset from the respective optical axis 40 and 42. The physical alignment, therefore, of devices 12 and 14 does not optically align the devices due to the pointing error which can be as much as two degrees or more for each component.
The first step in the first stage of alignment of collimators 12 and 14 is accomplished by injecting a visible optical signal from light source 32, such as by the use of a helium neon laser, to project a light beam represented by beam 50 in Fig. 2 onto a surface 52 of the alignment device 18 which, in this step, is a gauge block
having opposed, polished, parallel surfaces 52 and 53 which are flat and parallel to within as little as .03 microns. Such gauge blocks are relatively inexpensive commercially available devices which are typically used for the alignment of mechanical machinery. The visible light beam 50 is directed to surface 52 and reflected, as indicated by beam 54, back toward collimator 12. At this point in time, although the initial set up generally positions the mechanical axis perpendicular to surface 52, the reflected beam may be as much as an inch or more off of the end of the collimator. A temporary target, such as a card or the like, can be placed in front of collimator 12 and stage 13 adjusted to walk the beam onto the end of collimator 12 by adjustment of stage 13 which is visually confirmed by the operator due to the use of a visible light spectrum. Next, light source 32 is switched to a wavelength signal, such as a 1550 nm, for the final adjustment, and power meter 36 (Fig. 2) detects the signal wavelength while the tip/tilt stage 13 is adjusted by rotating the collimator 12 on two orthogonal axes (X, Y) until, as seen in Fig. 4, beams 50 and 54 are coincident and the beam is perpendicular to surface 52 of gauge block 18 for collimator 12.
The next step in the stage one alignment of collimators 12 and 14 is to repeat the process on the opposite side or surface 53 of gauge block 18 in the same sequence to align the optical axis of collimator 14 represented by incident and reflective beams 60 and 64 to surface 54 such that the optical axis represented by these beams 60, 64 is also coincident and aligned perpendicularly to the surface 53 of gauge block 18. As seen in Fig. 4, although the optical axis represented by beams 50, 54 and 60, 64 of collimators 12 and 14, respectively, are perpendicular to the opposed surfaces of gauge block 18 and, therefore, parallel to one another, they are offset from one another after the completion of the first stage of alignment. Accordingly, the system and method of the present invention provides for the subsequent translation of the parallel optical axes of devices 12 and 14 as represented by the steps shown in Figs. 5 and 6.
In Fig. 5, the gauge block 18 held by clamp 16 is replaced by a precision reflective sphere 18', which can be from VΛ" to %" in diameter, however, it is typically Α" in diameter and has a roundness of about .1 micrometers. Such spheres are commercially available and used in the coordinate positioning of mechanical machining equipment and are relatively inexpensive. Sphere 18' is positioned at the same location
as gauge block 18 and, as seen in Fig. 5, the beams of light 50, 60 from collimators 12, 14 are offset from one another and from the center 21 of sphere 18', resulting in reflected beams 54, 64.
The next step in the alignment process is to inject a helium neon signal from light source 32 to collimator 12 while again watching the beam 53 reflected off the spherical surface 55 of sphere 18'. Collimator 12 is then translated only along two orthogonal axes (X, Y) until, as represented in Fig. 6, the incident and reflective rays 50 and 54 coincide and correspond to the optical axes 40 of collimator 12. Thus, by translating the collimator 12 utilizing the translation stage 17 shown in Fig. 1, with respect to sphere 18 ', the optical axis is now aligned and its extension passes through the center 21 of sphere 18'.
Subsequent to the visual alignment, again the helium neon visible light source from source 32 is replaced by a signal frequency lightwave, and the power meter 36 (Fig. 2) is employed to assure the reflected beam 54 is at the peak detected power, resulting in the optical alignment of the optical axis 40 of collimator 12 with the center of sphere 18', as shown in Fig. 6. Subsequently and employing the same sequence of steps, collimator 14 is also aligned with the center 21 of sphere 18' utilizing first the visible light source for general positioning and subsequently the signal frequency light source for detecting the precise alignment of the optical axes with the center of sphere 18'. This, as seen in Fig. 6, perfectly aligns the optical axes 40 and 42 of the collimators 12 and 14 with respect to one another. The sphere 18' is subsequently removed and an optical component, such as a filter 62 including the newer smaller 1.4 mm filter is positioned by clamp 16 onto the substrate 30 and subsequently a bonding epoxy or other suitable material is employed for fixing the relative position of collimator 12, filter 62 and collimator 14 in precise optical alignment. It is noted that the distance D is preselected for the component to be manufactured and, therefore, the kinematic stage and translation stage 13 and 17, 15 and 19, respectively, need not move in a Z axis direction to move the collimators toward and away from one another nor rotate them on a roll axis Z. Although the preferred embodiment of the invention is employed for aligning collimators and/or providing an alignment process for an optical component resulting from the affixing of two collimators and a third optical component to a substrate, the
alignment system of the present invention, as best seen in Fig. 6, can also be employed for precisely aligning the optical axis, such as axis 40 of collimator 12, with respect to a fixed point in space, namely, the center 62 of sphere 18'. Thus, the system can be used for aligning the optical axis of a single component, such as a collimator, with respect to a fixed known reference position. The technique also can be employed for the precise testing of filters by assuring the optical pathway between a transmitting collimator 12 and a receiving collimator 14, for example, precisely aligned orthogonally to the opposed surfaces of the filter, such as element 60, either to be tested or to be inserted into the optical component. It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.