Patent Publication Number: US-2007104231-A1

Title: Wavelength tunable resonator with a prism

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a wavelength tunable resonator.  
      Wavelength tunable resonators are acquiring an increasing importance, in industry, particularly in optical communication measurement device industry. Typical designs of wavelength tunable cavities are, e.g., provided by Liu, K. &amp; Littmann, G. in “Novel Geometry for Single-Mode Scanning of Tunable Lasers”, Optics Letters 6 (3), p. 117-p. 118 (1981), or by Xu, G., Fujii, K.-I. &amp; Nakayama, S. in “Experimental Study on a Prism—External—Resonator Semiconductor Laser with Dispersion Feedback”, Review of Laser Engineering, Vol. 25, No. 6, p. 431-p. 433, (1997).  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide an improved wavelength tunable resonator. The object is solved by the independent claims. Preferred embodiments are provided by the dependent claims.  
      According to preferred embodiments of the invention a wavelength tunable resonator comprises at least two reflectors, which define the optical path length of a resonator. Both mirrors reflect an incident beam of electromagnetic radiation towards the respective other mirror. As a result of the resonator such defined, resonance modes form out of the electromagnetic radiation of the beam reflected between both mirrors. The wavelength of the respective resonance modes depends on the optical path length of the resonator.  
      A gain medium is placed within the resonator for amplifying or generating and emitting a beam of electromagnetic radiation into the resonator. The gain medium can be any light amplifying medium, in particular a semiconductor laser chip.  
      A prism is arranged within the optical path of the beam reflected between the mirrors. The prism refracts an incident beam depending on an incident angle between the direction of the beam and a normal direction applied to a first surface of the prism. The prism also comprises a second surface, which is inclined with respect to the first surface. The optical path of said beam exits the prism through said second surface towards the second reflector.  
      It is to be understood, that according to the present invention any substantially transparent device composed of dispersive material and having a first and a second surface, both surfaces being inclined with respect to each other for separating portions of an incident beam into beams of different refraction angles depending on their wavelength, is considered by the term “prism”. The first and/or the second surface of the prism may even be curved.  
      The prism comprises dispersive material, such that the beam exiting the prism, e.g., towards the second reflector has a particular wavelength range, which fulfills the condition of being reflected directly back towards the prism and the first reflector. Portions of the beam having different wavelengths refracted or redirected by the prism not fulfilling this condition will thus leave the resonator. Accordingly, the prism acts as a wavelength filter selecting a comparatively narrow wavelength range.  
      In order for the resonator to be tunable by wavelength, at least one of the reflectors is provided being moveable for increasing or decreasing the optical path length of the resonator. As a result the wavelengths of the resonance modes forming within the resonator are shifted.  
      The prism is provided with the feature of rotation, such that the incident angle with respect to the first surface of the prism can be varied. The rotation can be performed about an axis of rotation, which may, e.g. according to an embodiment of the present invention, advantageously extend along a line across the first surface of the prism, whereby said incident beam emitted from the gain medium intersects with the axis of rotation. Thus, the incident beam intersects the surface at a predetermined position irrespective of the incident angle of the beam with respect to the surface, or the actual rotation angle of the prism within the resonator, equivalently.  
      The effect of rotating the prism with respect to the other optical elements, i.e. the first and second reflectors, etc., is, that the beam exiting the prism through the second surface into the direction of the second reflector experiences a shift of the filter function defining said selected wavelength range. The filtered wavelength range depends on an angle defined by the incident beam reflected by the first reflector and/or emitted by the gain medium towards the prism and a line connecting the prism and the second reflector. The direction of movement of the second reflector is preferably arranged along this line. The advantage arises from the fact, that the only angle, which changes due to a rotation, is the incident angle. Accordingly, the rotation of the prism leads to a shift of the filtered wavelength range, while a movement of the second reflector leads to a tuning of the resonator resonance mode wavelength.  
      In a preferred, advantageous embodiment of the present invention the prism is connected to a means for rotating the prism, while the second reflector is connected to a means for moving the mirror, both means being connected to a common support. E.g., the means for rotating the prism can be a lever being rigidly connected to the prism and being connected to the support by means of a joint. The means for moving the second reflector can for example as well be mounted to the support. Moving the support then leads to a time coincidence of a rotation of the prism and a movement of the second reflector. Applying suitable dimensions to the lever, the prism can be rotated, such that a resonance mode selected by the filtered wavelength range is actually followed by the shifted filter function when tuning the resonator. In accordance with the present invention a mode-hop free tuning on a large wavelength range scale becomes feasible using a prism as a wavelength filter. Costs can be saved, since just one drive is needed in order to actuate the prism and the second reflector. Moreover, irregularities during the movement of the support act in the same way on the prism and on the second reflector, thus leading to an improved tuning result 
    
    
     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. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
       FIG. 1  displays a wavelength tunable resonator according to the present invention in a tuning position selecting a short wavelength resonance mode,  
       FIG. 2  displays the same resonator as in  FIG. 1 , but in a position selecting a long wavelength resonance mode,  
       FIG. 3  displays a diagram representing the beam deviation for a prism with strong dispersion,  
       FIG. 4  displays an embodiment being arranged as a ring resonator. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
       FIG. 1  displays a wavelength tunable resonator having a semiconductor laser diode as a gain medium  20 , a first reflector  10  and a second reflector  50  according to the present invention. A beam  5  of electromagnetic radiation is generated and emitted by the gain medium  20  and reflected between the first and second reflectors  10 ,  50 . A resonator lens  30  serves for collimating the beam  5  emitted from the gain medium  20  towards the second reflector  50 .  
      A prism being composed of a photonic crystal is arranged within the optical path of beam  5  refracting the beam transmitted from the resonator lens  30  towards the second reflector  50 . The material of the prism  40  has refractive indices in the range 1.5-1.7, thus being highly dispersive.  
      In the plane of the second reflector  50  a spectrum of the light beam emitted from prism  40  evolves, but only those portions of beam  5  are reflected towards the prism  40 , which orthogonally fall towards the surface of the mirror  50 . Other portions having different wavelength are not reflected to the prism  40  and thus leave the resonator. In  FIG. 1 a  wavelength is selected, which is comparatively short.  
      The prism  40  can be rotated around an axis of rotation  110  in order to shift the wavelength range that fulfills the condition of orthogonality upon the surface of the second reflector  50 . It is to be understood that the same effect arises, when a retroreflector or other mirror-like reflectors are used instead of a plane mirror as the second reflector  50 . In particular, the second reflector can be provided with a curved surface.  
      A lever  100  is connected with the prism  40  for rotating it about the axis of rotation  110 . By means of a joint the lever  100  is connected with one end of a piezo actuator. The piezo actuator  70  is designed to fine-adjust the incident angle of beam  5  with respect to the surface of prism  40 , which is orientated towards the resonator lens  30 .  
      The other end of the piezo actuator  70  is connected to a movable support bar  80 , which can be shifted and controlled by means of gas cylinder  85 . A plate  90  is rigidly connected with support bar  80 . The plate  90  mounted with one end of a second piezo actuator  60 . The second piezo actuator  60  has another end, which is connected with the second reflector  50  for providing a fine-adjustment of the position of that mirror with respect to the plate position.  
      The direction of movement of the support bar  80  is parallel to the optical path of the beam  5  between the prism  40  and the second reflector  50 . When the support bar  80  is shifted, as can be seen by the arrows in  FIGS. 1 and 2 , a rotation of the prism  40  and a movement of the second reflector  50  are effected. For example, the support bar  80  shift displayed in  FIG. 2  increases the optical path length of the resonator resulting in a larger wavelength of a resonance mode under consideration. At the same time the prism  40  rotates such that the incident angle of the beam  5  with respect to the first surface of the prism becomes large.  
      The length of the different elements in the resonator and the motion of the prism are chosen, such that the resonator resonance mode is always close to the filter function curve of the prism  40 . Using the resonator displayed in  FIG. 1 a  tuning in the wavelength range 1250 nm-1600 nm is fostered. In this embodiment the distance between the first reflector  10  and the prism amounts to 1 cm. The optical path within the prism amounts to 0.5 cm. The distance between the prism and the second reflector is variable and amounts to 0.5 cm at a tuned wavelength of 1250 nm and to 1.2 cm at a wavelength of 1600 nm. Accordingly a path difference is realized by a linear motion of the second reflector that amounts to 0.7 cm. The length of the lever  100  is 1.2 cm. A range of 28° of the incident angle is covered by the rotation motion enabled by a shift of the support bar  80 . This results in an almost continuous tuning that can be adjusted for continuous tuning by piezo actuators.  
      Further embodiments of the present invention relate to one or more of the reflectors being semitransparent for outcoupling of beam portions. In case of the second reflector being semitransparent, a low source spontaneous emission (LSSE) outcoupling of light becomes possible.  
       FIG. 3  illustrates the behavior of prisms  40  having different refractive indices n. The diagram shows an angle β that the output beam of the prism towards the second reflector  60  has with an incident beam  5 , i.e., the prism deflection angle, as a function of angle α that the incident beam  5  has with a normal of the prism surface. In this embodiment angle β is held constant as is obvious from the embodiments shown  FIGS. 1 and 2 . A value of 50 degrees for β is indicated by a horizontal line in  FIG. 3 .  
      As incident angle α is, e.g., increased by the rotation movement according to the preferred embodiments of the invention, the refractive index of a prism  40 , which provides the combination of angle α and angle β, approaches a maximum value first and decreases thereafter. A refractive index n of a prism is not easily changed once a prism is manufactured, however, it becomes clear that the varying refractive index n can be interpreted in terms of the varying tuning wavelength.  
      According to a further embodiment of the present invention the reflectors and the prism are arranged within a ring resonator as shown in  FIG. 4 , wherein the second reflector reflects the beam of electromagnetic radiation towards the first reflector via a second optical path, that does not transmit back through the prism  40 . Rather, a third semitransparent reflector  11  is provided to admit beam  5  to be transmitted towards the first reflector. The third semitransparent reflector  11  serves for further outcoupling a portion of beam  5 .  
      The resonator with a prism according to the present invention increases the quality of a resonator tuning process and/or simplifies the designs of wavelength tunable cavities thereby reducing costs and fostering further miniaturization.