Active Alignment System for Laser to Fiber Coupling

1. Technical Field

The invention relates to fiber optic communications.

2. Background Art

In fiber optic communications, an objective is to couple as much optical power as possible from a beam source into a fiber. A typical configuration of the prior art is illustrated in FIG. 3. A laser light source 31 has a window 32 through which a coherent beam of light 30 is outwardly directed, usually in a divergent beam. A lens or lens system 33 collects the diverging beam and focuses the beam into an optical polarization maintaining fiber 34. The fiber is aligned to the lens as well to a waveguide of an optical element and glued permanently in place on both ends. The waveguide may have an input surface 35, with index-matching glue which matches the characteristics of the fiber to the waveguide of the optical element.

The waveguide may be in an optical element such as an electro-optical modulator 36 which has a waveguide 45. An output fiber 47 is adhered within a fiber holder 40 which joins the fiber to the waveguide at the output surface 41 of the electro-optical modulator. The glue used at opposite ends of the electro-optical element to adhere the fibers to the input and output surfaces is an optical glue that helps match the index of refraction of the electro-optical element and the fibers in order to reduce reflections.

An electrical driver 42 feeds a bias voltage signal to the electro-optical element, such as a Mach Zender electro-optical modulator, to maintain the proper operating point for waveguide operation as well as to supply signal for data transmission. With fibers that are glued at opposite ends of the optical element there are problems of misalignment over system temperature changes, misalignment after assembly, such as during the curing process, and change of optical properties of the glue over time. The modulator 36 requires that the optical fiber 34 coupling laser 31 to modulator 36 be polarization maintaining in order to operate properly. Such fibers are more difficult to make and are consequently more expensive than ordinary optical fibers.

In U.S. Pat. No. 4,978,190 G. Veith teaches that to minimize coupling losses from a laser light source, the core of a fiber should be precisely aligned with the laser diode. A servo system is used to control the coupling of light between the fiber waveguide and the beam by measuring differences in reflection between the core region of the fiber and the cladding region of the end face of the waveguide. An electro-mechanical positioning device moves a lens in x and y directions in accord with an error signal derived from a segmented optical detector. The servo moves the lens so that all segments of the detector are evenly struck. With this alignment system, use of glue is eliminated. While this system of the prior art is useful in some applications, it is desirable to have a system where the light coupling efficiency for light actually used in a transmission medium is known, rather than merely at the input.

An object of the invention is to devise an active coupling between a source and a transmission medium where light coupling between a source and a transmission medium is measured beyond the system input. Another object of the invention is to devise an apparatus for simultaneously coupling light into the input and from the output of an electro-optical modulator to an optical fiber.

The above objects have been achieved in a laser alignment system for an optical element, especially a fiber, but also systems including other elements, where a means for sensing the beam amplitude is located at a place where at least a portion of the beam has traversed a waveguide in the system. In the preferred embodiment, a beam amplitude sensing means is associated with a fiber which is downstream from an optical element through which the beam passes and which has a servo controlled beam steering means at its input for maximizing light coupled into the optical element. Optionally a fiber steering means may be used to adjust the position of the fiber relative to the optical element using a signal derived from the sensing means. In the latter situation, two light path adjustments are used including an adjustment of the beam into the optical element and an adjustment of the fiber relative to the output of the optical element. A servo error signal is obtained through an optical coupler associated with the fiber. This signal is then used to drive actuators which move the beam steering means, serving as an input for the optical element, and also move the fiber steering means located near the output of the optical element. The actuators are piezoelectric transducers which may be very finely controlled. Even though the beam sensing means is located in the fiber, the efficiency of coupling light into the upstream optical element may be adjusted because sensing of light downstream provides information regarding coupling efficiency upstream.

FIG. 1 is a plan view of the apparatus of the present invention.

FIG. 2 is a detail of a fiber steering mechanism used in the apparatus of FIG. 1.

FIG. 3 is a plan view of a prior art approach in aligning a laser with a fiber through another optical element.

With reference to FIG. 1, a laser 11 directs a beam 10 outwardly toward a first mirror 13. Laser 11 could be either a semiconductor diode laser or a solid state laser pumped by a laser diode or laser diode array. Beam 10, emerging through a window 12, is a low divergence beam as emitted by the solid state laser or as collimated by a lens inside laser 11. The beam should be polarized in order to match the preferred polarization of the waveguide in the electro-optic modulator. From mirror 13, the beam is directed by reflection to a second mirror 17 where the beam is again reflected. The two mirrors 13 and 17 are mounted on piezoelectric flexure elements 14 and 16 respectively. These flexure elements act like bimetallic strips, and each has the capability of adjusting the beam in one dimension in response to an electrical signal, preferably a voltage. The flexure elements are termed "bending motors" and are manufactured by Piezo Systems, Inc. of Cambridge, Mass. For example, if each mirror is mounted with a pivot axis, and force is applied to cause bending of the mount, beam deflection will occur. Although such deflection is angular, it appears to be linear at a plane intercepting the beam. The two flexure elements are able to move beam 10 in two perpendicular directions, corresponding to approximate x-y motion in an image plane so that the beam can be focused into the input aperture of an optical element, such as electro-optical modulator 20 where the x-y plane is transverse to the beam path. The beam which is steered by the two mirrors is focused by a lens 18 onto an anti-reflection coated surface 19 where there is an input aperture for a waveguide within modulator 20. Sometimes a tilted surface is used as an input surface and/or an output surface to prevent back reflection from coupling back to the light source 11.

The electro-optical modulator 20 receives a modulating signal along line 31 from a driver 29 which places communications information onto the input beam via a Mach-Zender section of the electro-optical modulator. The beam is split into two beams 41 and 42 by a coupler within the modulator. Dual push-pull beams are preferred in multi-channel vestigal sideband CATV analog communications applications. Light from the two waveguides of the modulator 20 pass through an output surface 21 which has an anti-reflection coating or employs a tilted surface. The beams may be divergent and need to be focused prior to entry into a fiber. A coupling lens 22 receives the two output beams and focuses the beams onto a pair of fiberoptic fibers 43 and 44, which are steerable as described below. It is also possible to hold the fibers in proximity to the output waveguides, i.e. within 5 microns, using an index-matching fluid or adhesive.

Each of the fibers is connected to an electromechanical transducer having the ability to move the fiber tip in two dimensions. Fiber 43 is connected to transducer 23 while fiber 44 is connected to transducer 24. The two transducers are piezoelectric flexures which are capable of moving the input ends of the fibers, which receive the two beams from lens 22, in an x-y plane. Motion is controlled via a pair of wires 25 for one transducer 23 and another pair of wires, not shown, for transducer 24. The wires 25 carry a voltage to each of two flexure arms which comprise the transducer 23. Application of voltage to each of the two arms induces flexing along the length of each flexure arm. The greater the voltage, the proportionally greater amount of flexing. Transducer 24 is controlled in the same manner. Like the flexures 14 and 16 for adjusting the mirrors 13 and 17, flexures 23 and 24 may be those termed "bending motors" sold by Piezo Systems, Inc. of Cambridge, Mass.

The fibers 43 and 44 are preferably single mode fibers which pass through an optical coupler 36 which samples light in each fiber. Approximately two percent of the light in each fiber is coupled to the optical receivers 38 and 39. These receivers provide an indication of the amount of light being fed through the fibers 43 and 44.

The controller 27 receives the outputs from receivers 38 and 39 and applies corrective feedback to the input transducers 14 and 16 of the electro-optical modulator 20 as well as the output transducers 23 and 24 of the fiberoptic fibers 43 and 44. Controller 27 also monitors the optical power output of laser 11 and receives a measure of this power for reference from the laser 11 via a signal line 15. Various optimizing strategies may be adapted. For example, the transducers 14 and 16 may be jointly moved so that the beam traces a circular or spiral pattern. Upon maximizing the amplitude from this motion, these transducers are then fixed in position and the transducers 23 and 24 are moved to a circular or spiral pattern until light in each fiber is again maximized.

A cladding mounted light detector 37 is positioned adjacent to one of the fibers to detect light leakage from the cladding. At the initialization of the system, if light is not being received at the receivers 38 and 39, the detector 37 is positioned to report whether light is observed in the cladding of the single mode fiber 43. In such a situation, there would be almost no light in the core of the fiber, since the receiver reports none. Light leakage reported in the cladding detector 37 indicates that the input mirrors should be moved to get maximum light into the electro-optical element waveguide. The piezoelectric transducers 23 and 24 are then moved to maximize light from the electro-optical modulator waveguide into the cores within the fibers. Mirrors 13 and 17 can also be adjusted to maximize light into waveguides of the modulator 20. Iterative adjustments between the beam steering transducers 14 and 16 and the fiber steering transducers 23 and 24 are made until light through the system is maximized.

In FIG. 2, piezoelectric transducer 23, a fiber steering means, is seen. The transducers are made up of two flexure elements, including vertical element 52 and horizontal element 54. These elements flex in a manner similar to bimetal strips under thermal influence. The vertical flexure element 52 has a lower end which is constrained by a mounting surface 53. The upright flexure is able to flex along its length when voltage is applied, and is connected to horizontal flexure element 54 which is also able to flex along its length but in a plane perpendicular to the upright flexure. The horizontal flexure is in turn connected to fiber 43. A DC voltage along line 64 causes flexing of the piezoelectric element 52 into the plane of the paper for say positive voltage and out of the plane for negative voltage.

The horizontal flexure element 54, directly contacting fiber 43 is more constrained at a rearward end 66 than at a forward end 68. The flexure element 54 receives a DC voltage along line 74 for flexing in the plane of the paper similar to flexure element 52. The two dimensional motion of the two flexures is effective motion in the x-y plane for the tip of the fiber 43. The absolute amount of flexing is quite small, usually under plus or minus 20 microns in any particular direction. The size of each flexure element is approximately 10 millimeters in length, with approximate cross-sectional dimensions of 0.5 mm. by 4 mm. Each flexure is preferably made out of layers of piezoceramic, sometimes called bimorph piezoceramic. By applying DC voltages to each of the two piezoelectric elements the fiber 43 may be caused to scan a desired area in search for a maximum amount of light into the fiber core.