Abstract:
An apparatus and a method of determination of at least one optical parameter of an optical signal includes providing a beam of the optical signal having a diameter, manipulating the beam, the manipulation having polarization properties, the properties being dependent of the position in the beam laterally with respect to a direction of propagation of the beam during manipulation, detecting in intensities at least three parts of the beam in their dependence of the position in the beam laterally with respect to a direction of propagation of the beam during detection.

Description:
BACKGROUND OF THE INVENTION 
   The determination of properties of optical components has become quite important in the last few years. However, polarization effects as polarization mode dispersion (PMD) or polarization dependent loss (PDL) influence the quality of the signal transmission because the transmission rates of optical communication links were continuously enhanced. Therefore, the measurement of PMD and of PDL are well-known when characterizing optical components (see L. E. Nelson, R. M. Jopson, H. Kogelnik, “Influence of Measurement Parameters on Polarization Mode Dispersion Measurements using the Signal Delay Method”, Proc. ECOC 2000, Munich, Germany Vol. I (3.4.4), pp. 143-144 (2000); and A. Galtarossa, L. Palmieri, M. Schiano, T. Tambosso, “Improving the Accuracy of the Wavelength-Scanning Technique for PMD Measurements”, IEEE Photonics Technology Letters 12(2), pp. 184-186 (2000)). Moreover the precise measurement of polarization and the degree of polarization (DOP) are useful for compensation of PMD (see N. Kikuchi, S. Sasaki, “Polarization-Mode Dispersion (PMD) Detection Sensitivity of Degree of Polarization Method for PMD Compensation”, Proc. ECOC&#39;99, Nice, France Vol. II (WeA1.3) pp. 8-9 (1999); and H. Rosenfeldt, R. Ulrich, U. Feiste, R. Ludwig, H. G. Weber, A. Ehrhardt, “PMD compensation in 10 Gbit-s NRZ field experiment using polarimetric error signal”, Electronics Letters 36(5), pp. 448-449 (2000)). 
   However, polarimeters of the prior art require a lot of beam splitters, polarizers and detectors, thereby negatively influencing the production costs of such polarimeters. In addition it is necessary to precisely adjust such components with respect to each other. An example of a polarimeter of the prior art is shown in Dennis Derickson “Fiber optic, Test and Measurement”, Prentice Hall PTR, Upper Saddle River, N.J. 07458, pp. 231-232 (1998). The polarimeter shown in this publication uses one four-way beam splitter, three linear polarizers, one quarter-wave plate and four detectors to evaluate the Stokes parameters. 
   EP-A-439127 discloses a fast polarization meter for measuring the polarization of a light beam by splitting it up into four beams, individually analyzing each beam, and calculating the Stokes parameter from the results. Other optical analysis devices are known e.g. from U.S. Pat. No. 5,841,536 and U.S. Pat. No. 3,927,945 (wavemeters), U.S. Pat. No. 6,052,188 (spectroscopic ellipsometer), U.S. Pat. No. 6,043,887 (polarimeter), U.S. Pat. No. 4,042,302 (broadband wavelength discriminator), or U.S. Pat. No. 4,822,169 (analyzer). 
   U.S. Pat. No. 4,158,506 discloses an electro-optical system for determining the polarization state of optical pulses of nanosecond durations. The output from six optical polarizers each with a different polarization is detected with six detectors, whereby the polarizations of the polarizers is selected in a way that the Stokes parameter can be directly received from the detected output signals. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the invention to provide improved determination of an optical parameter of an optical signal. The object is solved by the independent claims. 
   One of the main features of the present invention is the combination of two steps, both dependent on the position in the beam. The first position dependent step is the manipulation of the beam, wherein the manipulation having polarization properties, the properties being dependent of the position in the beam laterally with respect to a direction of propagation of the beam during this manipulation. This is combined with the second step of detecting intensities in at least three, preferably four, parts of the beam in their dependency of the position in the beam laterally with respect to a direction of propagation of the beam during this detection. Based on this combination it is possible to derive an optical parameter, e.g. the wavelength or the state of polarization of the signal, on the basis of the detected intensities. For this evaluation the invention uses the perception that the detected intensities are correlated to a vector, preferably the Stokes vector, describing the optical parameter, e.g. the state of polarization, by a certain matrix. Since it is possible to built up this matrix for a certain position dependency of the polarization properties of the manipulation step and since it is possible to invert this matrix it is also possible to derive the vector, preferably the Stokes vector from the detected intensities. For this purpose it is preferred to detect intensities in four different parts of the beam to be able to derive all four vector elements of the vector, preferably the Stokes vector, unambiguously at once. If only three different parts are detected then the vector, preferably the Stokes vector, cannot be fully determined and a priori information, for example about the DOP can help to derive the complete vector, preferably the Stokes vector. 
   In a preferred embodiment of the invention the manipulation of the beam is done by retarding the beam, the retardation being dependent of the position in the beam laterally with respect to a direction of propagation of the beam during retardation, and polarizing the beam using a known polarization. With this combination the inventive manipulation step can be realized very easily. Moreover, if the beam is provided with a known wavelength, it is possible in a preferred embodiment to evaluate as an optical parameter the state of polarization of the signal. 
   Preferably, the polarization properties of the manipulation step are a linear function of the position in the beam. Moreover, it is preferred that the retardation is a linear function of the position in the beam. Additionally, it is preferred that the detection being a non-linear function of the position in the beam. In other preferred embodiments the spatially linear variation (and even nonlinear variations) of the retardation may be caused by other means, for example by using liquid or ferroelectric crystals and by applying an electric field to these elements. 
   A separate aspect of the invention for which aspect is claimed independent protection, also, is an element for manipulating an optical signal which comprises at least two sub-elements, each of these sub-elements having a variation in their polarization manipulation property along one of its axes and these axes have some angle with respect to each other. Preferably each of the sub-elements is a retardation element. More preferred, each of the sub-elements has the shape of a wedge. 
   In a further embodiment of the above-mentioned invention the manipulation element can comprise such an element, also. 
   If the beam is provided with a known state of polarization it is possible within a preferred embodiment to evaluate as an optical parameter the wavelength of the signal, i.e. to provide a wavelength sensor or wavemeter. 
   In another preferred embodiment of the invention the wedge-shaped retardation plates are positioned in the path of the beam such that their axes of the extraordinary index of refraction have some angle with respect to each other, preferably an angle not equal to 45°, 135°, 225°, 315°. This choice of angles as well as the choice of the appropriate retardation guarantees that all Stokes parameters can be determined unambiguously. Other arrangements with crystals having different birefringent properties may also ensure that all Stokes parameters can be determined unambiguously. 
   An advantage of the present invention is therefore the provision of a method and an apparatus which provide high precision measurement of the optical parameter with an apparatus having a small number of parts, in particular of a low cost, small form factor and high precision wavelength sensor and a low cost and high precision polarimeter without the need for using a large number of different parts which have to be adjusted with respect to each other. 
   In a preferred embodiment of the invention the wavelength sensor is realized by splitting the beam into a first and a second beam, providing the first beam with a known first polarization, providing the second beam with a known second polarization, detecting the intensities of the first and the second beam in their dependence on the position in the beam laterally with respect to a direction of propagation of the beam, and evaluating as an optical parameter the wavelength of the initial beam. Therefore, this embodiment makes it possible to evaluate the wavelength of a beam just by using the inventive manipulation and detection facilities that are dependent on the position in the beam. 
   Another preferred embodiment of the invention combines the wavelength sensor with the polarization sensor. The incident light of unknown wavelength and unknown state of polarization is split into three beams. Two of these beams are used to prepare well-defined states of polarization for a wavelength measurement. The third beam is used to determine the state of polarization. The parallel measurement of wavelength and state of polarization, which is made possible by this part of the invention, enables to “compensate” for the wavelength dependence of an arrangement for measuring the state of polarization. This presents a considerable advantage. Many of the possible expansions described below also apply here, especially the fact that all three beams may be steered such that they pass through or directed to the same optical devices such as retarders and polarizers. 
   In another example of the invention the evaluating of the optical parameter is done by describing the effect of the manipulation step with the help of a matrix, by inverting the matrix and by multiplying the vector of the detected intensities with the inverted matrix to get the Stokes parameters of the optical signal. In this respect it is preferred to find the matrix by performing a calibration procedure for determination of each matrix element. This calibration procedure can be done by using several defined states of polarization for the initial beam so that by the detected intensities the effect of the manipulating step can be evaluated and therefore the matrix elements can be evaluated. Another possibility for calibration is to use unknown states of polarization of the initial beam but fully polarized (DOP=1) initial beams, using a first approximation for the matrix and then using known methods for searching the gradient of the real matrix of the manipulation step. 
   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). 
       FIGS. 1-3  show preferred embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now in greater detail to the drawings,  FIG. 1  shows a first embodiment  1  of the present invention. Embodiment  1  realizes a polarimeter and uses two optical retardation plates  2  and  4  and a linear polarizer  6  to achieve a position-dependent retardation and thus a polarization-sensitive intensity variation across the diameter of an expanded preferably parallel beam  8  originating from a laser source  10 . This variation is detected in the different segments of a four-quadrant detector  12 . From the different intensities measured, it is possible to determine the complete state of polarization (e.g. the four Stokes parameters) of the incident light, assuming that its wavelength is known. This determination is done with the help of the above-referenced inverted matrix. 
   In embodiment  1  there are used birefringent optical retardation plates  2  and  4  which have nonparallel planar surfaces. For this purpose plates  2  and  4  are shaped like a wedge. This causes a linear variation of the retardation across the plates  2 ,  4 . To detect all states of polarization unambiguously with the single four-quadrant detector  12 , the two retarders  2 ,  4  are positioned in the path of the beam  8  such that their axes of the extraordinary index of refraction (uniaxial direction, in the plane of the plates) have some angle with respect to each other. 
     FIG. 2  shows a second embodiment  100  of the present invention. Embodiment  100  realizes a wavelength sensor. In embodiment  100  the incident light originating from a laser source  102  is expanded into a preferably parallel beam  104  and then separated into two beams  106  and  108  by means of a polarizing beam splitter  110  and deflecting means  112 , e.g. a mirror or a beam steering prism. Subsequently, polarization means  114  and  116  are used to create two well-defined states of polarization, e.g. horizontal (H) and vertical (V). Alternatively (not shown), a non-polarizing beam splitter may be used and the two different states of polarization are solely created by the polarization means. In some cases, even the polarization created by the polarizing beam splitter may be sufficient. Here are used two different but well-defined states of polarization to provide a sufficiently strong signal on one channel in those situations where the signal of the other channel vanishes, respectively, due to polarizer extinction. Also, favorable states of polarization are chosen such that the largest possible signal variations (dynamic range) further down the beam path (at the detectors, see below) are obtained. 
   After the polarization means  114  and  116 , the two beams  106  and  108  pass an arrangement of two retardation plates  118  and  120 . As in the embodiment of  FIG. 1 , these plates  118  and  120  have planar but nonparallel surfaces. Therefore, they exhibit a linearly varying retardation across their diameter. The optical axes (of birefringence) of the plates  118  and  120  are in the plane of the retardation plates  118  and  120  and are aligned under some angle. The purpose of this arrangement is to manipulate the states of polarization of the two (expanded) beams  106  and  108  in a controlled and spatially varying fashion. Both parallel beams  106 ,  108  pass through the same retardation plates  118 ,  120  in different areas, thus using them in an economic fashion. 5   
   After the retardation plates  118 ,  120 , the light of the beams  106 ,  108  passes through a linear polarizer  122  and hits the different segments of four-quadrant detectors  124  and  126 . A single polarizer  122  is used for both beams  106 ,  108 . The wavelength information is then derived from the signal intensities registered in the different segments of both detectors  124 ,  126  with the help of the above mentioned inverted matrix. 
   With embodiment  100  light of unknown wavelength and state of polarization is prepared with defined states of polarization in order to obtain the wavelength information alone. In embodiment  1  this is not possible with an unknown wavelength, because the phase shift in the retarder plates  2 ,  4  depends on both, wavelength and state of polarization. In embodiment  100  it is achieved a separation of these two effects and therefore it is possible to obtain information on a single quantity, i.e. the wavelength. 
   Similar information may also be obtained with a single four-quadrant detector (not shown) when using a switching or other appropriate beam steering means, that allows one to subsequently hit the detector area with one or the other of the two beams  106 ,  108 , possibly e.g. in a periodically oscillating fashion. Other position-sensitive detectors, like arrays, may also be used in order to detect the signals from the two beams  106 ,  108 , either in parallel or sequentially. The use of four quadrants is driven by the minimum number of independent information channels that give the complete unambiguous information about all four parameters describing a polarization state. More detectors may be used as well. 
   In addition, the wavelength sensor of the second embodiment  100  can be combined with the polarization sensor of the first embodiment  1  in order to measure both, the wavelength and the complete state of polarization of the incident light beam in one instrument. This instrument is shown in the third embodiment  200  according to FIG.  3 . The incident light  202  of a laser source  204  of unknown wavelength and state of polarization is split into three beams  206 ,  208 ,  210 . Beams  206  and  208  are used to prepare well-defined states of polarization for a wavelength measurement. The third beam  210  is used to determine the state of polarization. In embodiment  200  all three beams  206 ,  208 ,  210  are steered such that they pass through the same retarders  212 ,  214  and polarizer  216 . At the end the beams  206 ,  208 ,  210  hit three four-quadrant detectors  218 ,  220  and  222 . The evaluation of wavelength and state of polarization is done with the help of the above-mentioned inverted matrix, similarly to the determination as in the other embodiments  1 ,  100 .