Optical focusing and steering apparatus

Abstract: A method and apparatus for focusing a free beam of light onto a target, which is often very small or angle restricted, provide a lens (20) surrounded by two parallel windows. The focusing of the free beam is achieved by tilting the first window (30) to orient the beam of light through the lens onto the target, adjusting the lens focus to focus the beam of light into a focused spot, and tilting the second window (40) to translate the focused spot across the target. The lens may also be tilted to provide a greater range of translation.

Description: The present invention relates to the focusing and aiming of free-space beams onto a target. The focusing and aiming of free-space beams onto a target is critical in such applications as fiber optics. It is often necessary to provide a long-range of travel for the focused beam while maintaining a small, undistorted spot and maintaining the average incidence angle within the angular tolerance of the target. Attempted solutions have failed to achieve all these objectives or do so in a manner that is inefficient or unwieldy. For example, single lens launchers do not provide enough degrees of freedom to compensate for common fabrication errors. For instance, if the incoming beam is not centered on the target to begin with, all the light may not enter the target no matter how well the lens is adjusted in a single lens launcher. Two-lens launchers provide more degrees of freedom, but the angle cannot be adjusted independent of the position. JP-A-01/277,710 discloses a distance detector having a system to shorten the length of a light path using relatively small parallel planar plates, in a distance detector for detecting a focus error, by arranging a concave lens between two parallel planar plates to change the convergent angle of converging light. US-A-5,172,268 discloses an image stabilizing optical system comprising a master lens, and a meniscus lens arranged between the master lens and the image surface thereof, and having a concave surface opposed to the exit pupil of the master lens, said meniscus lens having inclination variable relative to the optical axis of the master lens. US-A-5,592,331 discloses an adapter for an optical instrument to control the angle of the plane of focus of the instrument. The adapter has front and rear coupling faces, a secondary lens and a tilt-swing mechanism or a fixed tile. The adapter's rear coupling face is connected to the optical instrument, an interchangeable primary lens is connected to the front coupling face of the adapter and the tilt-swing mechanism tilts or swings the primary lens relative to the image plane. The apparatus disclosed in U.S. Patent No. 5,077,622, titled "Apparatus For Precision Focussing and Positioning of A Beam Waist On A Target" and issued December 31, 1991 to Lynch et al, achieves high-resolution positioning, but requires at least seven separate adjustment axes and is difficult to fit in a compact package. Finally, tapered fibers reduce the resolution required by making the target bigger at the input and tapering down to the final size. In addition to being expensive, these are also less efficient since light is lost in the taper. Indeed, the larger the taper the more light is lost. Tapered fibers are also not useful for polarization-maintaining applications, since the taper scrambles the polarization of the light. The present invention seeks to provide improved focusing and/or steering of a free-space beam. According to an aspect of the present invention there is provided apparatus for aligning a free-space beam of light and a target as specified in claim 1. According to another aspect of the present invention there is provided a method of aligning a free-space beam of light onto a target as specified in claim 5. The preferred apparatus comprises a lens surrounded by two parallel windows. These components may be available off the shelf. Even if they must be custom-made for a particular application, the windows are straightforward to fabricate. The first window is an orientation window that orients the beam through the lens and toward the target. The second window is a translation window that translates the focused spot across the target, correcting for any misalignment of the lens. The preferred adjustments of this apparatus are tilting of the two windows and longitudinal translation (focusing) of either the lens or the target. High efficiency and a small spot size can be achieved even if the alignment of the non-adjustable components is imperfect. The focused beam can be translated and tilted independently for the best alignment to the target. The apparatus provides a long range of travel while maintaining a small, undistorted spot. The apparatus can be made compact and rugged, for use in applications with limited space. Likewise, standard manipulators, such as flexure plates and screws, can be used to tilt the windows and focus the lens from the front of the apparatus. As compared to prior attempted solutions, the preferred embodiment requires a minimal number of adjustments to achieve precise alignment. In the preferred embodiment, the lens is also capable of tilting. Tilting the lens in addition to the translation window increases the distance the focused spot can be translated before coma significantly distorts the spot. This allows for a greater ability to correct the mis-alignment of the lens and the target. The preferred embodiment can provide a method and apparatus for precisely focusing and aiming onto a very small and/or angle restricted target. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagram of the preferred optical focusing and steering apparatus; Figure 2 is a diagram of the optical focusing and steering apparatus illustrating the tilting of an orientation window; Figure 3 is a diagram of the optical focusing and steering apparatus illustrating the tilting of a translation window; Figure 4 is a diagram of the optical focusing and steering apparatus illustrating the tilting of a lens and the translation window; and Figure 5 is a graph of correction for a 1mm beam and 5mm focal length aspheric lens. The optical focusing and steering apparatus 10 of the preferred embodiment allows for the precise alignment of a free-space beam of light to a small and/or angle-restricted target such as a single-mode fiber. The apparatus 10 comprises a lens 20 between two flat windows, as seen in Figure 1. Readily available optics may be used for the lens 20 and windows, minimizing costs and simplifying replacement of parts. Simple and rugged mechanisms including standard actuators and fasteners are used, likewise minimizing costs and simplifying replacement. The final package can be made compact allowing assembly and alignment in a confined space with access to only one side. Typically, the final package is a housing that contains the lens 20, the two windows 30, 40, tilting mechanisms, and a focusing mechanism. Preferably, the package provides access to these mechanisms in its front. The first window is the orientation window 30, as shown in Figure 1. The orientation window 30 generally orients the beam through the lens and toward the target (not shown). The orientation window 30 accomplishes this by tilting in the beam, as seen in Figure 2 and discussed below. The orientation window 30 is typically a low-aberration window designed according to the desired package size, the required angular aiming resolution and the centration tolerance stack-up. An angle-sensitive target will have acceptance angle range within which the converging beam average incidence angle must fall for best results. The acceptance angle range of the target determines the required angular aiming resolution of the orientation window 30. The centration tolerance is the target's tolerance for centering of the converging beam. The angular range and resolution achievable by the orientation window 30 is not only dependent on the thickness and refractive index of the orientation window 30. The focal length of the lens 20 and the range and resolution of the tilting mechanism must also be taken into consideration. The orientation window 30 translates the beam across the lens 20 and the lens 20 transforms that translation into a change in angle. First-order calculations determine the input beam translation "y" for a tilt angle "α" of an orientation window 30 having thickness "t" and refractive index "n" at the wavelength of interest: An equivalent thin lens of focal length "f", for small translations, will change the input translation "y" into an output angle "u": u ~ y f These calculations provide a starting point for choosing components, but a thorough treatment by an accurate ray-trace code is recommended before ordering the components. The orientation window 30 range will also be limited by the size of the lens 20 compared to the input beam, the aberration response of the lens 20 to decentration (it is advisable to choose the lens 20 that is forgiving of this type of displacement), and the amount of orientation window 30 tilt that can be allowed in the available space. The orientation window 30 resolution is largely determined by the angular resolution of its tilting mechanism. If an angle-sensitive target (not shown) is mounted at an angle, or if the input beam, lens 20 and target are not coaxial within the target's tolerances, tilting the orientation window 30 compensates for the error and couples the most light onto the target. As illustrated by Figure 2, tilting the orientation window 30 in the input beam can (1) center the beam on a slightly misplaced lens 20 to minimize optical aberration and/or (2) change the average incident angle at the target without moving the converging beam spot. This second effect of tilting the orientation window 30 is due to the fact that the converging beam pivots around its focus. Preferably, adjustment of the tilt of the orientation window 30 is accomplished from the front of the apparatus 10 with a tilting mechanism (not shown) including standard actuators, such as flexure plates and screws or piezos (piezodielectric positioners) that are connected to the orientation window 30. The lens 20 focuses the beam onto the target. The lens 20 is a high-quality lens 20 whose aberrations do not increase drastically if a window is placed in the converging beam. Many off the shelf lenses providing low aberration at short focal lengths are designed for use as microscope objectives, and were originally optimized with a window (i.e., the cover slip on a microscope slide) between the lens and the image plane. In the example shown in Figure 1, the lens 20 is an aspheric lens made from molded plastic or glass. The lens 20 can also be multi-spheric or anamorphic (for example, a lens that has a cylindrical or toroidal surface, and made from molded plastic or glass or blown glass). The size of the lens 20 depends on the application and the package size. If the lens' 20 optic axis and the target's optic are reasonably collinear (for example, a 5 mm focal length lens is centered within 0.025 mm), it only needs a focus adjustment to compensate for errors in the focal length or the placement of the target. If the target is fixably mounted tolerably within range of the beam waist, then a focus adjustment may not be needed even if the lens' 20 focal length varies within the manufacturer's specifications and/or efficiency is less critical. To focus the beam, the lens 20 may move, the entire apparatus 10 may move, and/or the target may move. The target may move if it will not be detached and replaced many times with highly repeatable beam couplings or if the lens' 20 focusing mechanism is so rugged that the stresses of removing and replacing the target will not cause a loss of alignment. Preferably, the focusing mechanism is actuated from the front of the apparatus 10 rather than requiring side access. The focusing mechanism includes standard actuators connected to the lens 20, such as flexure plates and screws or piezos. The translation window 40 is placed in the converging beam between the lens 20 and the target (not shown), as seen in Figure 1. The translation window 40 generally helps to avoid or correct any mismatch between the focused spot of the converging beam and the target, which has a finite size. Preferably, if the lens 20 was designed to image through a window, the translation window 40 will cause the same optical path differences as the design window the lens 20 was designed to image through. If the lens 20 was designed to image through a window (such as a microscope cover slip or the window in the housing of a diode laser) and is used at the design wavelength, then the translation window 40 will cause minimal aberrations if its characteristics are similar to the design window. However, if the design wavelength is different than the wavelength actually used then the translation window 40 should be chosen for the wavelength used. Tilting the translation window 40 in the converging beam, as seen in Figure 3, causes the focused spot of the converging beam to translate across the target without disturbing the incidence angle. This allows the focused spot to be "moved" so that it is within the centration tolerance of the target and on the optic axis of the target. This centering of the focused spot allows for the correction of the mis-alignment of the lens 20 and the target. However, too much tilt of the translation window 40 can cause the focused spot to spread out due to coma aberrations. More translation can occur for less tilt, and accordingly less aberration, if the translation window 40 is thicker or is made of a higher-index glass. The thickness of the translation window 40 is limited by the package size and polarization concerns. If polarization matters, such as in polarization-critical applications including coupling a free-space beam into a single-mode polarization-preserving fiber, the translation window 40 needs to be made of an isotropic material. Many popular high-index materials such as sapphire are birefringent and can degrade the polarization unless their crystalline axes are very precisely oriented. In addition, some nominally isotropic materials, such as BK7 glass, may become weakly birefringent if subjected to excessive mechanical stress while being molded, cut, ground, or mounted, or if the pieces are cut from low-grade bulk material that is not homogeneous. In highly polarization-critical applications, all translation window 40 glass should be Grade AA or better, and additional annealing is recommended if the translation window 40 is thicker than 2.5 mm. The translation window 40 shown in the Figures is a 2mm window, designed for use in a polarization-critical application. The allowable thickness of the translation window 40 will also be limited when the lens 20 has a very short back focal distance. Preferably, adjustment of the tilt of the translation window 40 is accomplished in a similar manner to that of the orientation window 30. Accordingly, adjusting the tilt of the translation window 40 is also accomplished from the front of the apparatus 10 using a tilting mechanism that includes standard actuators, such as flexure plates and screws or piezos, that are connected to the translation window 40. In order to compensate and correct even looser mechanical mounting tolerances on the lens 20 and target, the lens 20 can also have a tilt adjustment, as seen in Figure 4. Tilting the lens 20 along with the translation window 40 increases the range over which the focused spot can be translated without spreading out due to comatic aberration. Figure 5 is a graph showing the results of a computerized optical model for a 1 mm beam and the Smm-focal-length aspheric lens 20 shown in Figures 1 to 4. As seen from this graph of Figure 5, the greatest amount of translation occurs for the least amount of aberration when both the translation window 40 and the lens 20 are capable of tilting. Focusing the lens 20 when tilting the translation window 40 overcomes much of the aberration, but also tilting the lens 20 produces the best results. Preferably, adjustment of the tilt of the lens 20 is accomplished with a tilting mechanism including actuators, as with the orientation window 30 and translation window 40 described above. The tilting mechanism for the lens 20 may be coupled to the tilting mechanism for the translation window 40. Preferably the target (not shown) is mounted rigidly, independent of the adjustable components described above, so that if it is removed and replaced there is no risk of misaligning the rest of the assembly. This is ideal for fibers and other targets that removed and replaced by an end user with little or no optical expertise. For some applications, it may be desirable to flexibly mount the target to provide a greater degree of adjustment. This is true particularly for targets that do not undergo many remove-replace cycles during their lifetime, but are short-lived compared to the rest of the assembly and may exhibit differences in optical response from (original) part to (replacement) part. For example, instead of moving the lens 20 or the apparatus 10 to adjust the focus of the beam, the target may be moved to adjust the focus. The apparatus 10 may be modified for the particular application in which it is to be used. A variety of tradeoffs can be made between range and resolution through the choice of the windows and location of the tilting mechanism. For example, a thicker translation window 40 will provide greater translation for less tilt, but may be less homogeneous if the glass quality is not well-controlled, and will provide poorer resolution than a thinner window for the same tilting mechanism. The translation of the focused spot resulting from tilting the translation window 40 (alone) follows the same first-order equation (1) discussed above for the orientation window 30. For a given translation window 40, thickness "t" and refractive index "n" are constants and the only variable is the tilt angle α. If both the translation window 40 and the lens 20 are tilted, ray-trace modeling with a reliable code is recommended. The apparatus 10 shown in the Figures is designed for use in coupling a free-space beam into a single mode polarization-preserving fiber. The apparatus 10 may similarly be used in other applications with infinite conjugates and a collimated beam, such as diagnostics of beams and optical systems based on spatial analysis of the focused spot, focusing of beams through spatial filters, and injection of beams into integrated-optic systems or semiconducter-laser amplifiers. If the target is not angle-sensitive, no orientation window 30 is needed. While the invention has been disclosed with reference to the details of preferred embodiment, it is to be understood that the disclosure is intended in an illustrative rather than in a limiting sense, as it is contemplated that modifications will readily occur to those skilled in the art, within the scope of the appended claims.

Claims: Apparatus (10) for aligning a free-space beam of light onto a target, comprising: a plane parallel plate orientation window (30) to which a beam of light has an average incident angle at the target, means being provided to change angle of tilt of the orientation window (30); a lens (20) that focuses the beam after passage through the orientation window (30), to cause the beam to converge to a spot on the target; and a plane parallel plate translation window (40) located after the lens (20) in the direction of travel of the beam of light, means being provided to change the angle of tilt of the translation window (40) so as to translate the spot across the target without disturbing the average incidence angle; wherein the apparatus is operable to orient the beam of light onto the target by changing the average incident angle of the beam on the target; and wherein the lens (20) tilts to increase the range over which the focused spot can be translated without spreading out due to comatic aberration. Apparatus as in claim 1, wherein the lens is an aspheric lens. Apparatus as in any preceding claim, wherein, the tilting of the lens (20) is coupled to the tilting of the translation window (40) in order to translate the spot across the target. Apparatus as in any preceding claim, comprising an actuator connected to one or more of the orientation window, the lens and the translation window. A method of aligning a free-space beam of light onto a target, comprising: providing a plane parallel plate orientation window (30) in the path of the beam of light; adjusting the angle of tilt of the orientation window (30); providing a lens (20) in the path of the beam of light after the orientation window (30), focusing the beam of light into a converging beam onto the target; providing a plane parallel plate translation window (40) into the path of the converging beam; adjusting the angle of tilt of the translation window (40) so as to translate the converging beam across the target without disturbing the average incidence angle of the beam; and tilting the lens (20) to increase the range over which the focused spot can be translated without spreading out due to comatic aberration; the method being operable to orient the beam of light onto the target at a desired average incidence angle.

Inventors: NEVIS ELIZABETH ANNE, NEVIS, ELIZABETH ANNE
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 2000
Main classification: G02B7
IPC codes: G02B6, G02B27, G02B7, G02B26