Abstract:
A method and an apparatus for tuning a laser includes creating a laser with a path between a cavity end element and a tuning element of an external cavity, both being a high reflective or semitransparent mirror, selecting at least one longitudinal mode of the laser by introducing a dispersion element in the path of the laser, rotating the tuning element about a pivot axis theoretically defined by the intersection of the surface planes of the cavity end element, the dispersion element and the tuning element to tune the laser. The method and apparatus also include provisions for moving the dispersion element along such a predetermined path to at least partly compensate a shift between the real position of the pivot axis and the theoretically defined position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to laser tuning. 
     In the optical communication industry there is a need for testing optical components and amplifiers with lasers that can be tuned in wavelength continuously without mode hopping. To perform these tests Littman cavities can be used as external cavities to allow single-mode tuning of the laser. The geometry of these cavities is known, see e.g.: Liu and Littman, “Novel geometry for single-mode scanning of tunable lasers”, Optical Society of America, 1981, which article is incorporated herein by reference. The advantage of the Littman cavity is that it is possible to tune the wavelength and the optical length of the cavity at the same time by changing only one parameter of the geometry, i.e. the tuning element. 
     Examples of tunable lasers, in particular based on the Littman geometry, can be found e.g. in U.S. Pat. No. 5,867,512, DE-A-19509922, Wenz H. et al: “Continuously Tunable Diode Laser” in ‘Laser und Optoelekronik’ (Fachverlag GmbH, Stuttgart, DE, Vol.28 No.1, p.58-62, Feb. 1, 1996, XP000775842, ISSN: 0722-9003), Wandt D. et al: “Continuously Tunable External-Cavity Diode Laser with a Double-Grating Arrangement” (Optics Letter, Optical Society of America, Washington, US, vol.22, no.6, Mar. 15, 1997, pages 390-392, XP000690335, ISSN: 0146-9592), DE-A19832750, EP-A-938171, JP-A-05 267768, or U.S. Pat. No. 5,319,668. 
     The Littman geometry, however, is extremely sensible to deviations of the real geometry with respect to the perfect Littman configuration. This does impose severe requirements on the rotation mount for the tuning element of the Littman cavity. Smallest errors in the positioning of the pivot axis of the tuning element reduce the mode hop free tuning range of the cavity heavily. This requires costly precision when manufacturing and maintaining such tunable lasers. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide improved tuning of a laser. The object is solved by the independent claims. 
     An advantage of the present invention is the provision of a tunable laser that autonomously and easily compensates for deviations, e.g. a shift of the real position of the pivot axis of the tuning element with respect to the theoretical perfect position of the pivot axis. This compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element. The compensation is done by moving the dispersion element, preferably along a predetermined path, corresponding with the rotation of the tuning element. Therefore, method and an apparatus of the present invention for tuning of lasers avoid the aforementioned problems of the prior art and provide a tunable laser with a wide mode hop free tuning range without heavy duties to the precision when manufacturing and maintaining such laser. 
     In a preferred embodiment of the present invention the moving of the dispersion element is done simultaneously with the rotation of the tuning element. This achieves an online correction, so that always the correct position of the dispersion element for the full feedback of the tuning element is guaranteed. 
     In another preferred embodiment of the invention the correction is done by moving the dispersion element by rotating it by a predetermined rotating angle about a predetermined rotating axis. This way of correction can be easily implemented in the inventive apparatus, e.g. by using a piezo-electric driven rotation stage that can precisely move the respective tuning element of the laser. 
     In yet another preferred embodiment of the invention the correction is done by a dispersion element which is a diffraction grating and in which the rotating axis is at least not perpendicular, more preferred parallel, to the axes of the rules of the grating. This positioning serves for maximum efficiency of the inventive method and apparatus. 
     In another preferred example of the invention the method further comprises steps for at least approximately evaluating a function which determines the rotating angle of the dispersion element for generating mode or wavelength hop free rotating of the tuning element within a predetermined tuning range of the tuning element per rotation angle of the tuning element. This evaluation is done by the following steps: step a: substantially detecting mode or wavelength hops during rotation of the tuning element, step b: rotating the tuning element about a predetermined angle until at least one mode or wavelength hop substantially has occurred, step c: rotating the dispersion element about an arbitrary angle, step d: rotating back the tuning element about the predetermined angle of step a, and repeating steps a to d with increasing or decreasing rotating angle of step c until substantially no mode or wavelength hops during rotation of the tuning element are detected in step b, and using the overall rotating angle of step c per rotating angle of step b to evaluate an approximation of the function which determines the rotating angle of the dispersion element per rotating angle of the tuning element. This can be done fast and easy so that a quick adjustment of the apparatus for full feedback of the tuning element is achieved. 
     After performing the above described determination it is preferred to move the dispersion element according to the approximation function before or while rotating the tuning element. 
     Other preferred embodiments are shown by the dependent claims. 
     It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s). 
     FIG. 1 shows a schematic view of a first embodiment of the apparatus of the present invention; and 
     FIG. 2 shows a schematic view of a second embodiment of the apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now in greater detail to the drawings, FIG. 1 shows a schematic view of a first embodiment 1 uof the apparatus of the present invention. The apparatus  1  of FIG. 1 comprises an external cavity  2  in which laser light provided by an active medium (not shown), e.g. a laser diode, can resonate to provide a laser beam  4 . The beam  4  travels in the cavity  2  along a path between a cavity end element  6  and a tuning element  8  of the external cavity  2 . The cavity end element  6  and the tuning element  8  both providing a high reflective mirror. The apparatus  1  further comprises a dispersion element  10  introduced in the path of the beam  4  for selecting at least one longitudinal mode of the laser. The dispersion element comprises a grating  11  having rules  11   a.    
     The tuning element  8  can be rotated by an actuator (not shown) according to arrow  12  about a pivot axis  14  to tune the laser. The pivot axis  14  is theoretically defined by the intersection of the surface plane (indicated by line  6   a ) of the cavity end element  6 , the surface plane (indicated by line  10   b ) of the dispersion element  10  and the surface plane (indicated by line  8   a ) of the tuning element  8 . 
     The dispersion element  10  is mounted on one end of an electrically driven bimorph type piezo-electric element (not shown) which serves as the moving element of the invention. One end of the bimorph type piezo-electric element is freely slewable whereas the other end of the bimorph type piezo-electric element is fixed relative to the cavity  2 . 
     The bimorph type piezo-electric element allows moving the dispersion element  10  corresponding, preferably simultaneously with the rotation of the tuning element  8  to compensate a shift between the real position of the pivot axis  14  and the theoretically defined position. This is done preferably by moving the dispersion element  10  along such a predetermined path that the compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element  8 . Moving the dispersion element  10  in this embodiment means rotating the dispersion element  10  according to arrow  16  by a predetermined rotation angle (for the predetermination of the rotation angle see below) about the rotating axis  10   a  which is substantially parallel to the rules  11  a and substantially lies in the plane of the grating  11 . 
     The apparatus of FIG. 1 preferably further comprises as a measuring device a wire strain gauge (not shown) for measuring the rotating angle  16  of the rotating of the dispersion element  10  and to output a measured value of the rotating angle  16 , a comparator (not shown) connected with the wire strain gauge for comparing the measured value of the rotating angle  16  with the predetermined value of the rotating angle  16  and to output a signal indicating a difference between the measured value and the predetermined value of the rotating angle  16 , a controller (not shown) connected with the output of the comparator and with the moving element for adjusting the rotating angle  16  when the comparator has detected a difference between the measured value and the predetermined value. 
     For the above mentioned predetermination of the predetermined value of the rotating angle  16  of the dispersion element  10  the following steps can be performed: step a: substantially detecting mode or wavelength hops during rotation of the tuning element  8 , step b: rotating the tuning element  8  about a predetermined angle  12  until at least one mode or wavelength hop substantially has occurred, step c: rotating the dispersion element  10  about an arbitrary angle  16 , step d: rotating back the tuning element  8  about the predetermined angle  12  of step a, and repeating steps a to d with increasing or decreasing rotating angle  16  of step c until substantially no mode or wavelength hops during rotation of the tuning element  8  are detected in step b, and using the overall rotating angle  16  of step c per rotating angle  12  of step b to evaluate an approximation of the function which determines the rotating angle  16  of the dispersion element  10  per rotating angle  12  of the tuning element  8 . The approximation can be done by known approximation methods. The more repeats of steps a to d are performed the more exact is the predetermination. 
     FIG. 2 shows a schematic view of a second embodiment  100  of the apparatus of the present invention. Basically the embodiment of FIG. 2 works the same way the embodiment of FIG. 1 does. However, in embodiment  100  there is no rotation axis  10   a  to rotate the grating  11 . Instead the grating  11  is linearly moved along the axis  10   c  which is perpendicular to the grating  11  and lies in the plane of the axis  10   b  and  8   a . The linear move of the grating  11  serves also to compensate a shift between the real position of the pivot axis  14  and the theoretically defined position. The linear moving of the dispersion element  10  has to be done along such a predetermined length of the path that the compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element  8  (for the predetermination of the rotation angle see below). 
     The apparatus of FIG. 2 also comprises as a measuring device a wire strain gauge (not shown) for measuring the length of the move of the dispersion element  10  along axis  10   c  and to output a measured value of the length, a comparator (not shown) connected with the wire strain gauge for comparing the measured value of the length with the predetermined value of the length and to output a signal indicating a difference between the measured value and the predetermined value of the length, a controller (not shown) connected with the output of the comparator and with the moving element for adjusting the length of the move when the comparator has detected a difference between the measured value and the predetermined value. 
     Similarly to the embodiment of FIG. 1, for the above mentioned predetermination of the predetermined value of the length of the move of the dispersion element  10  the following steps are performed: step a: substantially detecting mode or wavelength hops during rotation of the tuning element  8  about axis  14 , step b: rotating the tuning element  8  about a predetermined angle  12  about axis  14  until at least one mode or wavelength hop substantially has occurred, step c: moving the dispersion element  10  along an arbitrary length along axis  10   c , step d: rotating back the tuning element  8  about axis  14  about the predetermined angle  12  of step a, and repeating steps a to d with increasing or decreasing moving length along axis  10   c  of step c until substantially no mode or wavelength hops during rotation of the tuning element  8  are detected in step b, and using the overall moving length of step c per rotating angle  12  of step b to evaluate an approximation of the function which determines the moving length along axis  10   c  of the dispersion element  10  per rotating angle  12  of the tuning element  8 . The approximation can be done by known approximation methods. The more repeats of steps a to d are performed the more exact is the predetermination. It is clear that the positioning of the axes  14 ,  6   a ,  8   a ,  10   a ,  10   b ,  10   c  according to the FIGS. 1 and 2 only show the ideal case of the positioning of the axes  14 ,  6   a ,  8   a ,  10   a ,  10   b ,  10   c . The axes  14 ,  6   a ,  8   a ,  10   a ,  10   b ,  10   c  however can be positioned in another way, i.e. in other angles or positions as shown in FIGS. 1 and 2, e.g. in other angles relative to the cavity end element  6 , the tuning element  8  and/or the dispersion element  10 . Moreover, the axes  14 ,  6   a ,  8   a ,  10   a ,  10   b ,  10   c  can be combined with each other. 
     Additionally, it is possible to have variations in the position of the axes  14 ,  6   a ,  8   a ,  10   a ,  10   b ,  10   c  during the rotation about the axes  14  or  10   a  respectively the movement along axis  10   c . E.g. these variations can be caused by the piezo-electric element or rotation stage for rotating the cavity end element  6 , the tuning element  8  or the dispersion element  10  respectively the movement of the dispersion element  10 . However, these variations can be corrected by calibrating the inventive apparatus  1  or  2 .