Abstract:
A method and apparatus for aligning an optical waveguide with a radiation source are provided. The waveguide has a longitudinal axis that defines a main optical propagation path. The optical waveguide is illuminated by the radiation source such that the waveguide generates light via photoluminescence, at least a portion of the light generated via photoluminescence being emitted from the waveguide along a direction generally transverse to the longitudinal axis. An output signal is generated at least in part on the basis of light emitted from the waveguide along a direction generally transverse to the longitudinal axis. The alignment of the radiation source and the waveguide is varied at least partly in dependence of the output signal.

Description:
FIELD OF THE INVENTION 
     This invention relates a method and an apparatus for aligning a waveguide with a radiation source, and more particularly, to a method an apparatus for aligning a waveguide with a radiation source using photoluminescence induced in the waveguide by the radiation source. 
     BACKGROUND 
     Many processes involving optical waveguides require a precise alignment between an optical waveguide and a radiation beam. For example, in Bragg grating writing by flood exposure, an ultraviolet (UV) laser light interference pattern is used to write the grating in a core of an optical fiber. The interference pattern, which is typically focussed, needs to be precisely aligned with the core. If the core receives light from a part of the interference pattern outside of the focus, the intensity of the interference pattern will not be maximal and an exposure time to the beam required to write a given Bragg grating will be increased with respect to the exposure time that would be required if the core was at the focus of the interference pattern. In addition, if no monitoring of the Bragg grating writing process can be performed during the writing phase, the absence of a well-controlled interference pattern intensity may lead to a Bragg grating into which index of refraction variations are not large enough to provide a required grating performance. 
     The photoluminescence of several materials used to manufacture optical fibers and other optical waveguides can be used to align the optical fiber with a laser beam in preparation for Bragg grating writing. Once the optical fiber is properly aligned, the laser beam is replaced by the interference pattern and the Bragg grating writing process can be performed. 
     Typically, an ultraviolet (UV) laser is held immobile and produces the laser beam. A supporting member supports the optical fiber, the longitudinal axis of the optical fiber being perpendicular to the longitudinal axis of the laser beam. The supporting member is mobile in a direction perpendicular to the longitudinal axis of the optical fiber and perpendicular to the longitudinal axis of the laser beam. The supporting member can be displaced either manually or with a motorized actuator. 
     When a portion of the laser beam illuminates the photoluminescent core of the optical fiber, the light produced by photoluminescence is propagated through the optical fiber to its extremities. A power meter located at one extremity of the fiber can then measure the intensity of the photoluminescence light, which depends on the power carried by the portion of the laser beam illuminating the core of the optical fiber. Accordingly, when the focus of the laser beam is centered on the core of the optical fiber, the intensity of the photoluminescence measured at the power meter is maximal. Therefore, to center the optical fiber on the laser beam, the supporting member is displaced to achieve a maximal value of the intensity of the photoluminescence detected at the extremity of the optical fiber. 
     The method described above requires that the power meter block one extremity of the optical fiber. In some instances, this is undesirable as it could be advantageous to have other equipment, such as Bragg grating writing monitoring equipment, connected to the extremities of the optical fiber. 
     Against this background, there exists a need to provide novel methods and devices for aligning a waveguide with a radiation source. 
     SUMMARY 
     In accordance with a broad aspect, the invention provides and apparatus for aligning an optical waveguide with a radiation source. The waveguide has a longitudinal axis that defines a main optical propagation path and the radiation source illuminates the waveguide such that the waveguide generates light via photoluminescence. At least a portion of the light generated via photoluminescence is emitted from the waveguide along a direction generally transverse to the longitudinal axis. The apparatus includes a sensor responsive to the light emitted from the waveguide along a direction generally transverse to the longitudinal axis for producing an output signal. Alignment means then vary the alignment of the radiation source and the waveguide at least partly in dependence of the output signal. 
     Advantageously, the invention allows aligning an optical waveguide with a radiation source, such as a laser, by using light generated through photoluminescence and emitted along a direction generally transverse to the longitudinal axis of the waveguide. By using photoluminescence emitted transversely to the longitudinal axis of the waveguide rather than detecting photoluminescence emitted at an extremity of the optical waveguide, the extremities of the optical waveguide remain free and can therefore by used for other useful purposes such as signal analysis. 
     In a specific example of implementation, the output signal generated by the sensor is an intensity signal indicative of an intensity of light. In a specific example of implementation, the alignment means includes a controller module responsive to the intensity signal for causing the alignment of the radiation source and the waveguide to be varied. 
     In a first non-limiting implementation, the controller module causes the waveguide to be displaced in order to cause the alignment of the radiation source and the waveguide to be varied. 
     In a second non-limiting implementation, the controller module causes the radiation source to be displaced in order to cause the alignment of the radiation source and the waveguide to be varied. 
     In a non-limiting implementation, the alignment means further comprise a light reflecting member positioned such as to redirect a radiation beam emitted by the radiation source. The reflecting member may be any suitable component adapted to reflect a radiation beam. A specific example of a light reflecting member is a mirror. In a specific non-limiting implementation, the light reflecting member is in the form of a mirror. The controller module is operative to cause the light reflecting member to be displaced in order to cause the alignment of the radiation source and the waveguide to be varied. 
     The controller module generates a control signal at least in part on the basis of the intensity signal. An actuator, responsive to the control signal generated by the controller module, displaces the light reflecting member such as to vary the alignment of the radiation source and the waveguide at least in part on the basis of the control signal. The displacing of the light reflecting member may be effected by means of rotation, by means of translation or by a combination of the translation and rotation of the light reflecting member. 
     In accordance with another broad aspect, the invention provides a method for aligning an optical waveguide with a radiation source, the waveguide having a longitudinal axis that defines a main optical propagation path. The method includes illuminating the waveguide such that the waveguide generates light via photoluminescence, at least a portion of the light generated via photoluminescence being emitted from the waveguide along a direction generally transverse to the longitudinal axis. An output signal is generated at least in part on the basis of light emitted from the waveguide along a direction generally transverse to the longitudinal axis. The alignment of the radiation source and the waveguide is then varied at least partly in dependence of the output signal. 
     In accordance with another broad aspect, the invention provides an apparatus for aligning an optical waveguide with a radiation source. The apparatus includes a waveguide support member, a sensor and a controller module. The waveguide support member is for holding an optical waveguide, the waveguide having a longitudinal axis that defines a main optical propagation path. The radiation source illuminates the waveguide such that the waveguide generates light via photoluminescence, at least a portion of the light generated via photoluminescence being emitted from the waveguide along a direction generally transverse to the longitudinal axis. The sensor is positioned in proximity to the optical waveguide and is responsive to the light emitted from the waveguide along a direction generally transverse to the longitudinal axis to produce an intensity signal indicative of a measure of the light detected. The controller module is responsive to the intensity signal for causing the alignment of the radiation source and the waveguide to be varied at least partly in dependence of the intensity signal. 
     In a first specific example of implementation, the waveguide support member is moveable and the controller module is responsive to the intensity signal for causing the waveguide support member to be displaced such as to cause the alignment of the radiation source and the waveguide to be varied. 
     In a first specific example of implementation, the controller module is responsive to the intensity signal for causing the direction of the radiation beam emitted by the radiation source to be altered such that the alignment of the radiation source and the waveguide to be varied. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of examples of implementation of the present invention is provided herein below with reference to the following drawings, in which: 
     FIG. 1 shows an apparatus for aligning a laser beam with an optical fiber in accordance with a specific example of implementation of the invention; 
     FIG. 2 shows an enlarged view of a portion of the apparatus of FIG. 1; 
     FIG. 3 a  shows a light reflecting member in the form of a mirror position to reflect a radiation beam in a first direction in accordance with a specific example of implementation of the invention; 
     FIG. 3 b  shows a light reflecting member in the form of a mirror position to reflect a radiation beam in a second direction in accordance with a specific example of implementation of the invention; 
     FIG. 3 c  shows a light reflecting member in the form of a mirror position to reflect a radiation beam in a third direction in accordance with a specific example of implementation of the invention. 
    
    
     In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention. 
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an apparatus  100  for aligning an optical waveguide in the form of an optical fiber  110  with a radiation beam  120 . While an optical fiber  110  is aligned with a radiation beam  120  in the apparatus  100 , a similar apparatus could be used to align any other type of waveguide, such as optical fibers pre-assembled on a module or waveguides manufactured through integrated optics processes, with a radiation beam. 
     The apparatus  100  includes a waveguide support member in the form of a fiber support  130 , an actuator  140 , a controller  150 , a mirror  160  mounted on an axle  155 , a laser  170  and a sensor  180 . In operation, the laser  170  emits the radiation beam  120  towards the mirror  160 . The mirror  160  redirects the radiation beam  120  in the general direction of the optical fiber  110 , which is held by the fiber support  130 . During the alignment procedure, the actuator  140  rotates the mirror  160  through the axle  155  and under the control of the controller  150 , thereby changing the direction of the radiation beam  120  reflected by the mirror  160 . 
     When the mirror  160  is oriented such that the beam  120  illuminates the optical fiber  110 , the optical fiber  110  emits photoluminescence in the form of visible light, which is propagated through the optical fiber  110 . The sensor  180  then detects the visible light propagated in the optical fiber  110  at a location remote from the point at which the beam  120  intersects the optical fiber  110 . As shown on FIG. 1, the sensor  180  detects visible light emitted radially from the optical fiber  110 . The sensor  180  then produces an intensity signal related to an intensity of the light propagated by the optical fiber  110 . The intensity signal is fed to the controller  150 , which uses the intensity signal to control the actuator  140  in order to align the radiation beam  120  with the optical fiber  110 . 
     As shown on FIG. 2, the optical fiber  110  includes a core  205 , a cladding  210  and, optionally, a coating  215 . The core  205  includes a photoluminescent material. In the specific example of implementation presented on FIGS. 1 and 2, the core emits visible light when illuminated with UV radiation. However, the reader skilled in the art will readily appreciate that a core  205  having any other type of photoluminescence properties can be used without detracting from the spirit of the invention. Typically, the photoluminescence of the core  205  shows a reduction in intensity as a function of time when the UV radiation illuminates steadily the core  205 . The cladding  210  is composed of a material having optical properties suitable for allowing the propagation of light in the core  205  through total internal reflection. The optional coating  215  protects the cladding  210 . Such coatings  215  are well known in the art and will not be described in further details. In the specific example of implementation shown on FIGS. 1 and 2, the coating  215  is opaque to UV radiation and a portion of the coating  215  is removed from the optical fiber  110  prior to the alignment process. It will be appreciated that if an UV transparent coating  215  is used, removal of the portion of the coating  215  may be omitted. 
     Only a small portion of the optical fiber  110  is shown on FIGS. 1 and 2. The person skilled in the art will appreciate that the alignment of the optical fiber  110  with the radiation beam  120  can be performed as described herein irrespective of the total length of the optical fiber  110 . 
     When the radiation beam  120  illuminates the core  205 , photoluminescence is produced and the visible light thereby generated is propagated through the optical fiber  110 . A portion of the visible light is emitted radially from the optical fiber  110  at a location remote from the point at which it is produced. 
     The fiber support  130  holds a portion of the optical fiber  110  which is to be aligned with the radiation beam  120 . The exact shape and material of the fiber support  130  are not critical to the present invention. In a specific example of implementation, the fiber support  130  is immobile. In another specific example of implementation, the fiber support can be displaced manually to facilitate the access to the optical fiber  110 . In a further specific example of implementation, the fiber support  130  is mounted on a mobile platform which allows a coarse alignment of the optical fiber  110  with the radiation beam  120 . 
     The laser  170  produces the radiation beam  120 . While a laser  170  producing an UV radiation beam  120  is used in the specific example of implementation shown on FIGS. 1 and 2, the reader skilled in the art will readily appreciate that any other suitable source of radiation could be used with the present invention as long as it has the capability to produce a radiation beam  120  that causes photoluminescence in the optical fiber  110 . In a specific example of implementation, the laser  170  includes optical components for focussing and collimating the radiation beam  120 . 
     The radiation beam  120  coming from the laser  170  is redirected in the general direction of the optical fiber  110  by the mirror  160 . The mirror  160  is mounted on the axle  155  which allows the mirror  160  to rotate around the axis of the axle  155 . In the specific example of implementation shown on FIG. 1, the radiation beam  120  exits the laser  170  in a direction generally parallel with the optical fiber  110 . The mirror  160  is mounted at a 45 degrees angle with respect to the optical fiber  110 . The axis of the axle  155  is also substantially parallel to the optical fiber  110 . The mirror  160  is adapted to sweep the radiation beam  120  in a plane generally perpendicular to the optical fiber  110 . The skilled person in the art will appreciate that the radiation beam  120  may exit the laser  170  in any suitable direction and does not need to be parallel with the optical fiber  110 . In such a case, the mirror  160  is mounted at an angle that allows the mirror to sweep the radiation beam  120  originating from the laser  170  in a plane generally perpendicular to the optical fiber  110 . 
     The actuator  140 , which is controlled by the controller  150 , rotates the mirror  160  around the axis of the axle  155 . The method used by the controller  150  to control the rotation of the mirror  160  is described in further details below. 
     The rotation of the mirror  160  changes an amount of power carried by the radiation beam  120  to the core  205 . Therefore, the photoluminescence produced in the core  205  varies in intensity. The sensor  180  measures the intensity of the photoluminescent visible light which exits the optical fiber  110  radially. The sensor  180  is located at a position remote from the location at which the radiation beam  120  induces the photoluminescence. In a specific example of implementation, the sensor  180  is located approximately 2 cm from the source of the photoluminescence and within 50 to 200 micrometers from the surface of the optical fiber  110 . However, depending on the exact type of sensor  180  used in the apparatus  100 , the sensor  180  could be located within a few millimeters of the source of the photoluminescence or a few kilometers away from the source of the photoluminescence without detracting from the spirit of the invention. Preferably, the sensor  180  is affixed to the apparatus  100  so that the position of the sensor  180  relatively to the optical fiber  110  does not vary while the alignment method is performed. 
     In a specific example of implementation, the coating  215  of the optical fiber is removed form the fiber at the location at which the sensor  180  is located. Alternatively, if the coating is transparent to the visible light emitted by photoluminescence, the sensor  180  can be located at location wherein the coating  215  is intact. In this alternative, the sensor can be in contact with the coating  215 . 
     In a variant not shown in the drawings, the sensor  180  includes a multimode optical fiber connected to a remote power meter. The multimode optical fiber collects a portion of the photoluminescence visible light emitted radially from the optical fiber  110  and carries this portion of the visible light to the remote power meter, which generates a measurement of the intensity of the visible light. 
     The sensor  180  issues an intensity signal to the controller  150  through a sensor output  182 . The intensity signal includes information regarding the intensity of the visible light received by the sensor  180 . The controller  150  receives the intensity signal at a controller input  152  and is adapted to store corresponding intensity values in a memory. 
     In addition, the controller  150  is operative to issue control signals to an actuator input  142  of the actuator  140  through a controller output  154 . The control signals instruct the actuator  140  to rotate the mirror  160  at a desired angle through the axle  155 . In a specific example of implementation, the controller  150  is adapted to angle the mirror  160  at an angle that maximizes the power of the radiation beam  120  illuminating the core  205 . As the reader skilled in the art will appreciate, other alignment criteria are possible without detracting form the spirit of the invention. 
     In a specific example of implementation, the alignment of the radiation beam  120  with the optical fiber  110  is performed in accordance with the following method. First, the optical fiber is coarsely aligned with the radiation beam while the mirror  160  is kept immobile. Then, the controller  150  sends control signals instructing the actuator  140  to rotate the mirror  160  in an oscillating manner while storing in the memory the intensity signals from the sensor  180 . A value of an angle at which the mirror  160  is positioned is stored in the memory each time an intensity value is stored. After a predetermined number of oscillations, the controller  150  uses the intensity values and the mirror angle values stored in the memory to determine an optimal angle that the mirror  160  should assume so that the radiation beam  120  illuminates the optical fiber  110  in an optimal manner. As mentioned previously, in a specific example of implementation, the illumination is optimal when the intensity of the photoluminescence produced by the radiation beam  120  is maximal. If the radiation beam  120  is focussed, this corresponds to having the focal region of the radiation beam centered on the core  205 . 
     For the purpose of illustration only, FIG. 3 a  shows a simplified diagram of the mirror  160  rotated to direct a radiation beam in a first direction such that the beam illuminates a first portion of the waveguide. FIG. 3 b  shows a simplified diagram of the mirror  160  rotated to direct a radiation beam in a second direction such that the beam illuminates a second portion of the waveguide. FIG. 3 c  shows a simplified diagram of the mirror  160  rotated to direct a radiation beam in a third direction such that the beam illuminates a third portion of the waveguide. In a non-limiting implementation, FIG. 3 c  shows the tilting mirror at an optimized position for alignment. 
     The coarse alignment of the optical fiber with the radiation beam is the first step performed. During this coarse alignment, the mirror  160  is kept immobile and the fiber support  130  is displaced. As mentioned previously, the coarse alignment is optional and can either be performed manually by an operator or automatically, for example using cameras and image processing software. The coarse alignment serves to locate the optical fiber  110  within a range of positions accessible by the radiation beam  120  under the rotation of the mirror  160 . 
     After the coarse alignment is performed, the angle of the mirror around the axle  155  is changed in an oscillating manner by the actuator  140  under the control of the controller  150 . This causes the radiation beam  120  to be swept from one side of the optical fiber to the other, In a specific example of implementation, the radiation beam  120  is swept at a frequency of approximately 5 to 50 Hz, but other suitable sweep frequencies can be used, depending on the exact type of fiber used without detracting from the spirit of the invention. The radiation beam  120  is swept continually in order to illuminate only briefly the core  205  at each sweep. This brief illumination is preferable because most currently available core materials present photoluminescence which reduces in intensity when the radiation causing the photoluminescence illuminates constantly a given portion of the core  205 . 
     While the mirror  160  oscillates, the controller  150  stores in the memory intensity values for the photoluminescence conveyed by the intensity signal. The controller also stores in the memory a value of the angle at which the mirror  160  is positioned each time an intensity value is stored. In a first example of implementation, the value of the angle is determined by the controller  150  according to the control signals sent to the actuator  140 . Therefore, in this example of implementation, there is an implicit assumption that the actuator  140  positions the mirror  160  at angle values contained in the control signals. Alternatively, the angle values can be measured independently and fed to the controller  150 . 
     After a variable number of sweeps, which depends on the required precision in the alignment and on the uncertainties present in the stored angle values and intensity signal values, the controller  150  uses the angle values and the intensity signal values stored in the memory to find the optimal angle for the mirror  160 . In a specific example of implementation, the optimal angle is an angle for which the measured intensity value is maximal. Methods to determine the optimal angle are well known in the art and will therefore not be described in further details. Finally, the mirror is angled at the optimal angle. 
     In a variant, the mirror is not rotatably mounted on the axle  155  but is instead translatably mounted on a suitable actuator. In this variant, the translation of the mirror sweeps the beam  120  back and forth across the optical fiber  110 . Alternatively, the optical fiber  110  can be supported by a mobile fiber support  130 . Then, the fiber support  130  is swept back and forth across an immobile laser beam. The reader skilled in the art will readily recognize other possible implementations that do not depart from the spirit of the invention. 
     In a further variant, the laser beam  120  is not swept rapidly enough across the optical fiber for the natural decay of the photoluminescence in time to be negligible. However, the decay of the photoluminescence in the core  205  can be modeled by the controller  205  to correct the stored intensity values by generating adapted intensity values, thereby allowing the optimal angle to be determined as described above. The adapted intensity values take into account a natural decay of photoluminescence in time. 
     While the alignment procedure described above has been presented in the context of an initial alignment prior to performing a process on the optical fiber  110 , the reader skilled in the art will appreciate that the method could also be used periodically while the process is performed to maintain the alignment of the optical fiber  110  with the radiation beam  120 . 
     Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention, which is defined more particularly by the attached claims.