Abstract:
A ring laser arrangement adapted for providing an optical beam travelling on an optical path representing a closed loop includes a laser gain medium coupled into the optical path for amplifying the optical beam by stimulated emission, and a wavelength filter coupled into the optical path for providing a wavelength selection to the optical beam travelling along the optical path.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to wavelength tunable lasers.  
         [0002]     In the optical communication industry there is a need for testing e.g. optical components and amplifiers with lasers at different wavelengths. For this purpose, various types of laser cavities are known.  
         [0003]     Tunable lasers are described e.g. as the so-called Littman geometry in “Liu and Littman, Novel geometry for single-mode scanning of tunable lasers, Optical .Society of America, 1981”, or as the so-called Littrow geometry in EP 0 952 643 A2. Bragg-reflector type cavities are shown e.g. in “A. Nahata et al., Widely Tunable Semiconductor Laser Using Dynamic Holographically-Defined Distributed Bragg Reflector, 2000 IEEE”. The teaching of those documents shall be incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the present invention to provide an alternative solution for wavelength tunable lasers. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.  
         [0005]     According to the present invention a ring laser arrangement is adapted for providing an optical beam travelling on an optical path representing a closed loop. A laser gain medium is coupled into the optical path for amplifying the optical beam by stimulated emission. Collimating devices might be provided in case the beam provided by the laser medium is not already collimated. The ring laser further comprises a wavelength filter coupled into the optical path for providing a wavelength selection to an optical beam travelling along the optical path.  
         [0006]     Resonance condition in the ring resonator is that the optical path length for one roundtrip along the closed loop must me an integer multiple of the resonance wavelength (e.g. in contrast to the resonance condition for end-mirror-type laser resonators, wherein the optical path length between two end mirrors must be an integer multiple of half of the wavelength). The fact that there are travelling waves in the ring resonator avoids the effect of spatial hole burning present in some laser gain media (e.g. in contrast to the standing wave in the end-mirror-type laser resonators).  
         [0007]     In a preferred embodiment, the ring laser further comprises a length modification allowing to modify the optical path length (determined by the geometrical path length and refractive index of each element in the path) of the optical path. The length modification is preferably coupled with the wavelength filter in order to adjust and/or synchronize variations in the wavelength selection provided by the wavelength filter with the optical path length. Thus, e.g. mode hops occurring when tuning the optical beam in wavelength can be reduced or even be avoided.  
         [0008]     Coupling between the wavelength filter and the length modification can be provided e.g. by designing the geometry of the ring laser following the principles of one of the aforementioned geometries (preferably Littman or Littrow), by applying a control unit controlling operation of the wavelength filter as well as of the length modification (preferably for providing a synchronization between both), or by combinations thereof. Such control unit might be based on a predefined parameter setting derived e.g. from previous runs of the ring laser for example to achieve substantially mode hop free tuning. Alternatively or in addition thereto, the control unit might be embodied as disclosed in the International Patent Application PCT/EP02/04736 by the same applicant; the teaching thereof shall be incorporated herein by reference.  
         [0009]     Alternatively or in addition thereto, the control unit might directly monitor the optical beam in order to detect indications of mode hops likely to occur and to initiate appropriate counteractions in order to avoid such mode hops occurring. Such counteractions can be at least one of to adjust the optical path length to the present wavelength filter property or to adjust the wavelength filter to the present optical path length. In a preferred embodiment discussed further below, the optical path length is modulated resulting in a wavelength modulation of the corresponding optical beam in order to derive an error signal representative for a deviation of a wavelength of the optical beam (preferably the wavelength of a dominant mode) from a wavelength of the filter characteristic of the wavelength filter (preferably the wavelength of a filter extreme value e.g. a wavelength where the filter has local maximum transmission).  
         [0010]     Without influencing the sense of direction for the optical beam travelling along the loop of the optical path, one direction might become more dominant e.g. under the influence of mode competition. For the sake of easier understanding, the more dominant direction shall be referred to in the following as the forward direction, while the less dominant direction shall be referred to in the following as the reverse direction. Accordingly, forward beam shall mean an optical beam travelling into the forward direction, while reverse beam shall mean an optical beam travelling into the reverse direction.  
         [0011]     In case of a mode selection provided within the ring resonator and assuming same optical path lengths in the forward and reverse direction, a forward mode (i.e. a dominant mode travelling into the forward direction) and a reverse mode (i.e. a dominant mode travelling into the reverse direction) may occur, both having the same wavelength but opposite propagation directions. Under the influence of mode competition, usually one of the modes will become dominant and exhibit a significantly higher intensity than the other. Following the above naming convention, the dominant mode shall be referred to in the following as the forward mode, and the less dominant mode shall be referred to in the following as the reverse mode.  
         [0012]     In one embodiment, a direction controller is provided into the optical path in order to have the forward mode always into the same direction (or, in other words, to define the forward direction to be in one defined direction). This can be achieved e.g. by attenuating the reverse beam with respect to the forward beam.  
         [0013]     In one preferred embodiment, the reverse beam (preferably the reverse mode) is utilized for controlling at least one property of the forward beam. Since there is known relation between the properties of the reverse and forward beam, the reverse beam can be used for monitoring the characteristic of the forward beam, or vice versa, e.g. in order to reduce, avoid, or even provoke mode hops. Preferably, the beam less in optical power is employed for controlling purposes. For the sake of explanation here, it shall be assumed that the reverse beam is less in optical power and employed for controlling.  
         [0014]     In one embodiment, the optical path length of the reverse mode is modulated without modulating the optical path length of the forward mode resulting in a wavelength modulation of the reverse mode only, and a control signal can be derived from resulting variations in intensity of the reverse mode. Such differences in intensity of the reverse mode result from the characteristic of the filter curve of the wavelength filter generally having at least one transmission maximum at one wavelength value and ranges over the wavelength with reducing transmission behavior. When modulating the wavelength of the reverse mode, its intensity will be modulated correspondingly by the wavelength characteristic of the wavelength filter. Using known controlling mechanisms (e.g. deriving a control or error signal from the determined deviation) thus allows controlling deviations of the wavelength of the forward mode, e.g. from the maximum in the filter characteristics of the wavelength filter. Such determined deviation might then be used e.g. to modify the optical path length in order to adjust the wavelength of a selected dominant mode (e.g. the forward mode) with the present setting of the wavelength characteristics of the wavelength filter (for example in order to avoid or reduce mode hops) or might be used e.g. to modify the maximum transmission wavelength of the wavelength filter for the same purpose.  
         [0015]     In another embodiment, the wavelength of the reverse mode is offset with. respect to the wavelength of the forward mode. This can be achieved e.g. by modifying the optical path length in the reverse direction with respect to the optical path length in the forward direction. This can be accomplished, for example, by at least partly separating the optical beams in forward and reverse direction and modifying the optical path length for at least one of the separated beams. The separation can be done spatially and the modification can be done by at least partly changing the refractive index or the geometrical path length. Additionally or alternatively, the separation can be done by using different polarizations of light at least partly but maintaining the same geometrical path for the reverse and forward beams/modes. The modification of the optical path length can then be achieved e.g. by a birefringent element in that part of the path where both modes use different polarizations. The difference in the optical path length is then given by the difference in the index of refraction for the polarizations used by the reverse and forward beams/modes.  
         [0016]     Such wavelength offset between the forward and reverse beam/mode can then be utilized for detecting a direction of a change in the deviation of the dominant mode from a filter maximum. Preferably, the offset between the wavelength of the forward and reverse modes is selected in a way that in case the wavelength of the forward mode substantially coincides with the wavelength of the filter maximum, the (offset) wavelength of the reverse mode is selected in a range wherein the filter characteristic shows a falling or rising edge (and thus a stronger dependency on the wavelength). Thus, small variations in wavelength will be “amplified” by the falling or rising edge into the modulated or unmodulated intensity of the reverse mode, and the direction of deviation from the filter maximum can be unambiguously determined from the course of the variation in the reverse mode intensity or from the change of the ratio of the intensity of the forward and reverse mode  
         [0017]     In one preferred embodiment, the ring resonator comprises at least one parallel-reflecting device. Each parallel-reflecting device receives an incident beam and reflects a beam, whereby the reflected beam is substantially parallel to the incident beam but spatially separated therefrom. Each parallel-reflecting device can be provided by adequately arranging reflecting surfaces (e.g. mirrors) or combinations of lenses and mirrors, e.g. by at least two assembled plane mirrors or at least one circular or cylindrical lens and a mirror or as solid device as known in the art. For easier adjustment, a dihedral or trihedral element might be used having the advantage of providing substantially fixed surfaces with respect to each other thus allowing reducing effort for aligning the reflecting surfaces in angle with respect to each other and with respect to the beams. The dihedral or trihedral element might be a prism in which the reflection mechanism is total internal reflection at an interface where the index of refraction changes from a high to a low value and the angle of incidence is above the angle for total internal reflection. Alternatively, the dihedral or trihedral element might apply metallic or dielectric reflection as reflection mechanism to avoid that a part of the optical path has to be inside a material with a high index of refraction. In another preferred embodiment, the parallel-reflecting device can be provided as a retro-reflecting device as disclosed in the International Application No. PCT/EP02/01433 by the same applicant, the teaching thereof shall be incorporated herein by reference.  
         [0018]     The reflecting surfaces of each parallel-reflecting device are preferably provided to be large in area with respect to incident area of the optical beam in order to ensure spatial separation between input and output beams.  
         [0019]     In one embodiment, two parallel-reflecting devices are arranged that a beam launched from a first one of the parallel-reflecting devices is received by the second one of the parallel-reflecting devices and returned spatially separated back to the first one of the parallel-reflecting devices, thus providing the closed loop of the ring resonator. In other words, the two parallel-reflecting devices can be provided to span up the closed loop. It goes without saying that other components can be arranged within such provided loop.  
         [0020]     The wavelength filter can be arranged within the ring resonator so that the optical beam travelling one loop within the ring resonator will pass the wavelength filter only once. Preferably, however, the wavelength filter is arranged so that the beam passes the wavelength filter twice each loop. This can be achieved e.g. by providing both beams received on opposite sides of the wavelength filter to be substantially parallel to each other and by providing sufficiently large surfaces at the wavelength filter to receive both beams. This can be accomplished in particular when using the aforedescribed arrangement with the two parallel-reflecting devices for spanning up the loop.  
         [0021]     One or more outputs for coupling out a portion of the laser beam within the ring resonator can be provided substantially at any place within the optical path. Preferably, one or more beam splitters can be introduced into the optical path for coupling out a portion of the laser beam. In a preferred embodiment, a portion of the forward beam is coupled out after passing the wavelength filter at least once and before returning to the laser medium. Controlling the direction of the forward beam and suppressing the reverse beam (if not provided automatically, e.g. due to mode competition) thus allows a wavelength purified beam of the wavelength filter without weakening the optical beam due to unwanted coupling out at the beam splitter in the reverse direction.  
         [0022]     Another option for coupling out is to use a beam zeroth order provided by a diffraction grating used as wavelength filter. A further option is to provide at least one reflecting surface to be a least partly transmittive. Preferably, at least one reflecting surface of at least one of the parallel-reflecting devices might be applied for coupling out. However, preferably unmoved components are applied for coupling out in order to reduce mechanical effort.  
         [0023]     In another preferred embodiment, the design of the ring resonator follows the principle of the aforementioned Littman architecture. In that embodiment, the wavelength filter comprises a diffraction grating. The diffracted beam from the diffraction grating is directed to an input of a first parallel-reflecting device, which will return the beam towards the diffraction grating substantially parallel to the beam from the diffraction grating but spatially separated therefrom.  
         [0024]     The beam from the parallel-reflecting device, which is parallel to the beam received by the first parallel-reflecting device but spatially offset thereto, is directed again towards the diffraction grating diffracting the beam parallel to the beam initially received by the diffraction grating, however, with opposite propagation direction and spatially separated therefrom. A second parallel-reflecting device is arranged to receive the thus twice diffracted beam from the diffraction grating and will provide a beam parallel thereto but spatially separated therefrom towards the diffraction grating, thus closing the loop of the ring resonator.  
         [0025]     Following the Littman geometry, rotating at least one element of the two parallel-reflecting devices and the diffraction grating around a pivot point (theoretically defined by the intersection of the optical planes provided by the two parallel-reflecting devices and the diffraction grating) thus theoretically allows to continuously tune the ring resonator in wavelength without mode hops occurring. Deviations of an actual pivot point from the theoretically defined pivot point might be compensated e.g. by modifying the filter curve for selecting modes. Such variation in the filter curve can be achieved e.g. by at least one of the following: moving a dispersion element for selecting at least one laser mode (e.g. as disclosed in the aforementioned European Patent application 01113371.7), modifying the dispersive characteristic of a dispersive element e.g. by modifying the periodicity of the dispersive element, modifying the direction of the beam incident to the dispersive element, by moving a retro-reflective dispersive element, etc.  
         [0026]     Additionally or alternatively, deviations of an actual pivot point from the theoretically defined pivot point might be compensated e.g. by modifying the optical path length of ring resonator. This can be provided e.g. by at least one of: moving one of the cavity elements to change the geometrical length (for example by moving at least one of the parallel-reflecting devices), moving an optical element such as a wedge substantially perpendicular to the optical beam, controlling the optical path length of at least one of the cavity elements by an external parameter (such as applied electrical or magnetic field, temperature, uniaxial or hydrostatic pressure), controlling the orientation of the optical active axis by an external parameter, etc.  
         [0027]     In another preferred embodiment, the design of the ring resonator follows the principle of the aforementioned Littrow architecture. In that embodiment, the dispersive element acts as device that reflects the incident beam into itself. In order to spatially offset the reflected beam, a preferred embodiment provides a triangular loop geometry using a. slightly tilted reflection grating as dispersive element. In another embodiment, the beams are separated spatially after being reflected by the dispersive element. Alternatively, the two separated beams can be rotated (preferably Faraday rotated) by substantially π/2 with respect to each other and combined by a polarization beam combiner. After both separated beams are combined, the polarization is rotated back by substantially −π/4, and the dispersive reflective element in the Littrow geometry is applied.  
         [0028]     The invention can be partly or entirely 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. Software programs or routines are preferably applied to calculate the deviation of the optical path length and/or selected wavelength of the wavelength selective element from the values that support mode hop free tuning and to calculate the correction values for one or more tuning element to readjust the optical path length and/or the selected wavelength of the wavelength selective element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     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).  
         [0030]     FIGS.  1  illustrate preferred embodiments according to the present invention.  FIG. 1A  shows a principal top view, and  FIG. 1B  shows the three-dimensional arrangement from a side view, both following a preferred Littman geometry.  FIG. 1C  shows a preferred parallel reflecting arrangement that might be applied when using a Littrow geometry  
         [0031]      FIGS. 2A and 2B  depict preferred embodiments for direction controllers.  
         [0032]      FIG. 3  illustrates mode control using modulation or wavelength shifting. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     In  FIGS. 1 , a laser medium  10  emits a laser beam on each side of its end facets. The two laser beams will be collimated by collimating devices (such as lenses)  20  and  30 , if the beam is not already collimated. The laser medium  10  is preferably a semiconductor laser chip but other types such as ion-doped crystals or dye cells might be applied accordingly.  
         [0034]     The features in FIGS.  1  shall now be introduced with respect to a (forward) laser beam travelling (in  FIG. 1A ) anti-clockwise (referred to as forward direction) starting from the laser medium  10 . After being collimated by the collimating device  20 , the beam passes an optional direction controller  40 , which function will be illustrated later. After the direction controller  40 , the beam impinges on a wavelength filter  50 , which is embodied In FIGS.  1  by a diffraction grating. However, other type of wavelength filters  50  such as prisms, one-dimensional filters like etalons or birefringent filters (wherein output and input beams are substantially in one linear line), or any other dispersive element can be applied accordingly, but might require a different geometry in particular for providing continuous tunability.  
         [0035]     The beam from the direction controller  40  is diffracted by the diffraction grating  50  and launched to a first parallel-reflecting device  60 . The first parallel-reflecting device  60  receives the beam from the grating  50  and reflects a beam back toward the grating  50 , whereby the reflected beam is substantially parallel to the incident beam but spatially separated therefrom. The parallel-reflecting device  60  can be embodied by various kind of arrangements of reflecting surfaces, such as a dihedral element as shown in FIGS.  1  or a set of plane mirrors arranged orthogonal with respect to each other.  
         [0036]     The beam returning from the parallel-reflecting device  60  is diffracted again by the diffraction grating  50 . Since both beams to and from the parallel-reflecting device  60  are substantially parallel to each other, the angles of diffraction at the grating  50  are also substantially equal, as illustrated in  FIG. 1B .  
         [0037]     After being diffracted twice by the diffraction grating  50 , the beam is launched to a second parallel-reflecting device  70 , which can have substantially the same properties as the first parallel-reflecting device  60 . The second parallel-reflecting device  70  will reflect a beam towards the laser medium  10 , whereby the reflected beam is substantially parallel to the twice diffracted beam received by the second parallel-reflecting device  70  but having opposite propagation direction and being spatially separated therefrom. Thus, a closed optical loop is provided representing an optical ring resonator.  
         [0038]     In order to couple out light from the ring resonator, various options are possible and some shall be illustrated in  FIGS. 1 . A first option is to provide an optical beam splitter  80  substantially anywhere into the optical path. Preferably in order to provide an output beam  90  having a high spectral purity, the beam splitter  80  is arranged, as shown in  FIG. 1A , after the beam having twice passed the wavelength filter  50  and before entering the laser medium  10  again. The portion of the output beam  90  with respect, to the incident forward beam will be determined by the coupling ratio of the beam splitter  80  that designed as appropriate for the respective requirements.  
         [0039]     In accordance with the above said, a (reverse) beam travelling clockwise (herein after referred to as reverse direction) is emitted by the laser medium  10  towards the (optional) collimating device  30 , is reflected spatially separated by the second parallel-reflecting device  70 , passes for a first time the wavelength filter  50 , is reflected spatially separated by the first parallel-reflecting device  60 , passes for a second time the wavelength filter  50 , passes the direction controller  40 , and is eventually focussed by the collimating device  20  back into the laser medium  10 .  
         [0040]     Accordingly, a portion of the reverse beam passing the beam splitter  80  will also be coupled out as an output beam  100  with substantially the same coupling ratio as for the forward beam.  
         [0041]     In case the beam splitter  80  is arranged as shown in  FIG. 1A , the forward beam impinges the beam splitter  80  after having twice passed the wavelength filter  50  and before entering the laser medium  10  again. In contrast thereto, the reverse beam impinges the beam splitter  80  straight from the laser medium  10  before being wavelength-filtered. Thus, the forward beam can be coupled out at the beam splitter  80  having higher spectral purity but being reduced in power (with respect to power of the beam emitted by the laser medium), while the reverse beam can be coupled out at the beam splitter  80  having lower spectral purity but being not being reduced in power (with respect to power of the beam emitted by the laser medium).  
         [0042]     Another option for coupling out light from the ring laser arrangement is to provide at least one of the surfaces of at least one of the parallel-reflecting devices  60  or  70  to be partly transmittive, as shown for one surface of the second parallel-reflecting device  70 . The forward beam will lead to an output  110 , while the reverse beam will lead to an output  120 . It goes without saying that any other reflecting surface of the parallel-reflecting devices  60  and  70  can be provided alternatively or in addition thereto to be partly transmitted in order to couple out light from the ring resonator. However, in order to limit further effort it is preferred to couple out at components within the optical path, which are provided to be non-movable or are at least not actively operated or moved during coupling out.  
         [0043]     Monitoring devices  100   a  and/or  120   a  can be provided in order to monitor the coupled out portion of the reverse beam.  
         [0044]     In most cavity arrangements, normally either one of the forward or the reverse beam will become more dominant in intensity than the other. In order to achieve the dominant beam always in one propagation direction, the direction controller  40  might be introduced into the optical path. The direction controller  40  provides a different attenuation for the beams travelling in forward and reverse direction. Thus, it can be achieved that the less attenuated direction becomes more dominant. In the example of FIGS.  1  it shall be assumed that the forward beam will become significantly dominant over the reverse beam. Having a thus defined dominant direction in the ring resonator in particular allows to defined coupling out light with desired properties. Specifically a dominant forward beam will lead to the output  90  with high-spectral purity while the more or less “unwanted” output beam  100  at the beam splitter  80  will be significantly less than the output  90 .  
         [0045]     In another preferred embodiment the direction control is utilized to control optical properties of the dominant forward beam using the less dominant reverse beam as a control beam. Thus, properties of the reverse beam can be modified for controlling the forward beam, however, without adversely affecting optical properties of the forward beam. In one embodiment (not shown in  FIGS. 1 ), the reverse beam will be modulated in wavelength in order to control a deviation of a dominant mode wavelength from a maximum wavelength of the wavelength filter  50 . With such control, mode hops can be reduced or even be avoided. Known control mechanisms for deriving a deviation control signal and feeding back the control signal to synchronize mode and filter wavelength can be applied.  
         [0046]      FIG. 1C  illustrates an arrangement to support the Littrow geometry. Instead of the elements  50  and  60  of  FIGS. 1A and 1B , the elements of  FIG. 1C  are used, while the remaining features of  FIGS. 1A and 1B  might be applied accordingly. In this: embodiment, the spatially separated beams (incident from the left of  FIG. 1C ) are combined to a single beam but with different polarizations. Assuming the forward beam/mode is coming from the left in the lower beam, it will be spatially shifted by a polarization beam combiner  140 . Then the polarization of the forward beam/mode will be rotated by substantially π/4 by a Faraday Rotator  150 . After being reflected by the wavelength selective element  50 , the beam will be rotated by substantially π/4 by the Faraday Rotator  150  again. Therefore, the polarization of the beam is rotated by substantially π/2 with respect to the incident polarization. Because of that the beam will pass the polarization beam combiner  140  without offset and the polarization will be rotated back substantially −π/2 by a Faraday Rotator  160 . Thus, the forward beam has passed the elements without undergoing a rotation in polarization with respect to its initial state of polarization (when coming from the left in the lower beam) but is spatially shifted thereto.  
         [0047]     Assuming now the reverse beam coming from the left but in the upper beam, it will be rotated by substantially −π/2 by the element  160  and will pass the element  140  without spatial offset. The element  150  will rotate the polarization by substantially π/4 back again before the beam hits the wavelength selective element  50 . After the beam has passed the element  150  for the second time, the polarization is again in the initial state and therefore the beam will be spatially shifted by the element  140 . Thus, the reverse beam has passed the elements without undergoing a rotation in polarization with respect to its initial state of polarization (when coming from the left in the upper beam) but is spatially shifted thereto.  
         [0048]     In one embodiment, the element  50  is provided having a polarization dependency for passing the beam in either direction. Since the state of polarization is different for the forward and reverse beam when passing the element  50 , such polarization dependency can be utilized to provide attenuation different for both beams. This might be applied to provide direction control in addition or alternatively to the direction control of the direction controller  40  as illustrated below. In case of a grating used as element  50 , the rulings of the grating  50  and the orientation of the element  140  can be chosen e.g. to provide an attenuation of the reverse mode.  
         [0049]      FIG. 2A  illustrates a first preferred embodiment for the direction controller  40 . Considering first the forward beam (incident from the left of the direction controller  40 ), a π/4 Faraday rotator  200  rotates the polarization of the forward beam that the forward beam is substantially (fully) transmitted (denoted by arrow  240 ) through a polarization beam splitter  220  without leading to a partial beam (travelling in the upper path of  FIG. 2A  into the direction opposite to the arrow  250 ) being spatially offset.  
         [0050]     As can be seen in the representation in  FIG. 2A , the polarization beam splitter  220  comprises a polarization beam splitting element (here: the lower diagonal line) for splitting up an incident beam (from the left side) in portions according to its state of polarization, and a reflecting element (here: the upper diagonal line) for directing in parallel but spatially offset with respect to each other both (partial) beams provided by the polarization beam splitting element. Beams incident from the right side will be reflected and split up accordingly.  
         [0051]     The forward beam then might travel through a non-absorbing component  260 , which might be applied for compensating the optical path length of an absorbing element  270  (in the reverse path). A next element  280 , e.g. a phase modulator, might be used to slightly adjust the optical path length of the forward path in case a continuous tuning is accomplished by adjusting the optical path length. Alternatively, the element  280  might be used to fine-tune the wavelength of the forward mode in case a coarse tuning element lacks in resolution. Alternatively, it might also be applied to compensate for the optical path length of an element  290  (in the reverse path).  
         [0052]     The forward beam (denoted by arrow  240 ) is then passed to a second polarization beam splitter  230  (corresponding in function to the first polarization beam splitter  220  but applied here in opposite direction) leading to a spatial offset for the forward beam travelling now in the upper path of  FIG. 2A  with opposite direction to arrow  250 . The polarization of the forward beam is rotated back by a second −π/4 Faraday rotator  210  to maintain the polarization before passing the direction controller  40 . The orientation of the polarization beam splitters  220  and  230  are designed to establish the above-described path.  
         [0053]     The reverse beam (incident from the right of the direction controller) is transmitted through the −π/4 Faraday rotator  210 . The polarization of forward and reverse beam are orthogonal between element  210  and  230 , therefor the reverse beam passes the polarization beam splitter  230  in the upper path with the direction of arrow  250  (thus not leading to a spatial offset for the reverse beam).  
         [0054]     A phase shifter  290  might be inserted for modulating the optical path length of the reverse mode (denoted by arrow  250 ). In addition or alternatively, an attenuation element  270  might be applied to attenuate the reverse mode in such a way the mode competition will prefer the forward mode but the reverse mode will still exist but with substantially lower optical power. It is clear that the phase modulators  290  and  280  might be omitted in case the optical path length of the forward and reverse mode only have to be different but no modulation is applied. For that purpose, the optical path lengths of elements  270  and  260  might be chosen in a way that a preferred path difference is obtained. This might be accomplished e.g. by simply omitting the element  260 .  
         [0055]      FIG. 2B  illustrates a second preferred embodiment for the direction controller  40 , preferably applied for different optical path lengths for the reverse and forward mode. Again, the forward beam (coming from the left side) is transmitted through the π/4 Faraday rotator  200  in such a way that the output polarization is oriented along the slow or fast axis (depending on whether the forward mode should have an optical path length longer or shorter than the reverse mode) of a birefringent element  310 . After passing the birefringent element  310 , a polarizer  300  follows, which in oriented such that the forward mode is passed through substantially without attenuation. The −π/4 Faraday rotator  210  is utilized to again rotate back the polarization into the original orientation.  
         [0056]     The reverse beam (coming from the right side) will be transmitted through the −π/4 Faraday rotator  210  in such a way that the orientation of the polarization between elements  210  and  300  is now oriented in the fast axis (if the forward beam was oriented in the slow axis of the birefringent element  310 ), or vice versa. But before travelling through the element  310 , the polarizer  300  will attenuate the reverse beam because the orientation of the transmitted polarization is substantially orthogonal to the polarization of the reverse beam. The reverse beam will then be transmitted through the birefringent element  310  and the polarization will be rotated back to the incident polarization by the π/4 Faraday rotator  200 . The optical path difference of the direction controller  40  can be denoted as the geometrical length of the element  310  times the difference of the indices of refraction for the slow and fast axis. Instead of the polarizer  300 , any other type of dichroic element might be applied accordingly. The term dichroism shall refer to the selective absorption of one of the two orthogonal P-state components of an incident beam (see e.g. Hecht, Optics, 3 rd  Edition, p.327).  
         [0057]     In one embodiment, the optical path length of one of the forward or the reverse path is varied with respect to the other. Thus, an offset in wavelength between modes in the forward and the reverse beam can be achieved. In a preferred embodiment, such wavelength offset is designed in a way that it can be modulated. This shall now be illustrated as an example in  FIG. 3 . The wavelength selective element  50  shall have a characteristic shape  400  with its maximum at the wavelength λ 0 . The effect of the respective position of a mode (with respect to the characteristic shape  400 ) on the modulation shall be depicted for three different forward modes  410 ,  420  and  430  of the ring resonator. The mode  410 , in this example here, shall be exactly centered with respect to the maximum transmission at λ 0  of the wavelength selective element  50 , while the modes  420  and  430  are offset with respect to λ 0 .  
         [0058]     Modulation of the reverse modes ( 410 A,  420 A and  430 A corresponding to the forward modes  410 ,  420  and  430 ) over time between wavelength positions  411  and  412 ,  421  and  422 , and  431  and  432 , respectively, leads accordingly to a modulation of the output power of each of the reverse modes over the time (as depicted by graphs  410 B,  420 B and  430 B).  
         [0059]     The lower diagram in  FIG. 3  illustrates an example of an amplitude of a demodulated output signal  440  of the reverse mode. The demodulated signal  440  vanishes if the center position of the modulation exactly matches the center wavelength λ 0  of the filter curve  400 . Furthermore, the sign of the amplitude changes at exactly that position, and the amplitude increases monotonically with the deviation from the center wavelength. Therefore, this signal  440  can be used to control a modification of at least one of the center wavelength of the wavelength filter or the wavelength of both the reverse and forward mode e.g. in order to reduce or avoid mode-hops during operation.  
         [0060]     In another embodiment, the wavelength of the reverse mode is shifted with respect to the wavelength of its corresponding forward mode, so that e.g. the wavelength of the reverse mode corresponding to the forward mode  410  is slightly shifted towards the position  411  or  412  (and/or the wavelength of the reverse mode corresponding to the forward mode  420  is slightly shifted towards the position  421  or  422 , and/or the wavelength of the reverse mode corresponding to the forward mode  430  is slightly shifted towards the position  431  or  432 ). The difference between the (preferably normalized) output power (with respect to the maximum) of the corresponding forward reverse modes will yield to a shape similar to  440  and can be utilized e.g. to detect whether the forward mode is shifted to the right or left of the center wavelength λ 0 . Again, the corresponding correction can be made e.g. in order to reduce or avoid mode-hops during operation. It is clear that such wavelength shift between reverse and forward mode might be applied alternatively or in addition to the aforedescribed modulation.  
         [0061]     Returning again to  FIG. 1B , a preferred operation mode is illustrated. Following the aforementioned Littman architecture, a theoretically mode-hop free continuous tuning can be achieved when rotating at least either one of the parallel-reflecting devices  60  and  70  and the grating  50  around a pivot point  130  theoretically defined by the intersection of an optical plane  50   a  of the grating  50 , an optical plane  60   a  of the first parallel-reflecting device  60 , and an optical plane  70   a  of the second parallel-reflecting device  70 . As explained in detail in some of the aforementioned documents, deviations of a real pivot point from the theoretically pivot point  130  can lead to mode hops during wavelength sweeps. However, adequate correction is also explained in detail in some of the aforementioned documents can be applied accordingly in particular to adjust the optical path-length or the maximum transmission wavelength of the wavelength selective element. However, considering an ideal geometry, turning at least one of the first and second parallel-reflecting devices  60  and  70  or the grating  50  around the pivot point  130  allows to continuously tune the wavelength of the optical beam without having mode hops.  
         [0062]     Instead of following the Littman geometry, other geometries can be applied accordingly such as e.g. the afore-illustrated Littrow geometry. In case that mode hops can be tolerated, and/or the overall tuning range is limited and/or a synchronization between the characteristic of the wavelength filter  50  with the mode wavelength determined by the effective optical path length is achieved otherwise, any other architecture or geometry can be applied accordingly for tuning the wavelength of the ring resonator.