Device and method for reducing polarization dependent loss in an optical monitor device

A device and method for monitoring characteristics of optical signals in an optical telecommunications system are disclosed. The device includes a wedge of birefringent material disposed in an optical path of the device. The thickness of the wedge of birefringent material varies in at least one dimension along the direction of propagation of an optical beam passing through the device. The variation in wedge thickness causes the polarization state of the optical beam to be a function of position along the wedge. The composite energy in the optical beam appears to be formed of two orthogonal polarization components with no phase relationship to each other. If the two orthogonal polarization components of the optical beam are disposed at a predetermined angle relative to the principal efficiency axes of the device, such as approximately 45 degrees, the polarization dependent loss of the device is substantially reduced.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Embodiments of the present invention reduces or otherwise eliminate PDL in an OPM by transforming an input optical wavefront of substantially uniform polarization state into an optical wavefront whose polarization varies along a single spatial dimension. Embodiments of the present invention reduce PDL in an OPM by employment of a wedge of birefringent material therein. Referring to FIG. 2 , there is shown a wedge 202 of birefringent material showing principles of the present invention utilized in reducing PDL in an OPM. A quasi-collimated optical input beam 201 has an electric field oriented along the y-axis, travels along the z-axis and is incident on a wedge 202 of birefringent material. It is understood that input beam 201 being polarized along the y-axis is shown for exemplary purposes only, and that input beam 201 may be polarized in other polarization states. The ordinary axis n o and the extraordinary axis n e of wedge 202 are rotated approximately 45 degrees with respect to the x-y plane. The surface 206 of wedge 202 is angled or oriented relative to the y-axis. Light entering the wedge 202 may be decomposed into polarization states along the ordinary and extraordinary axes of the wedge 202 . The thickness of the wedge 202 along the direction of propagation of input beam 201 determines the relative phase shift between two principal polarization states and thus the output polarization state. Since the wedge surface 206 is angled, the thickness of the wedge 202 of birefringent material is a function of position along the y axis. As a result, the polarization state of the output beam 203 will be a function of position along the y′-axis of beam 203 , which itself depends upon the wedge y-axis and the angle of wedge 202 . It should be noted that more than one surface of the wedge 202 may be angled. If an analyzer 204 , whose axis of transmission is rotated with respect to either of the wedge's principal planes, is placed on the output side of wedge 202 in the path of output beam 203 , one would see a sinusoidally varying intensity pattern 205 . The polarization of output beam 203 periodically varies along the y′-axis of beam 203 . If the angle of surface 206 of wedge 202 is such that one or more periods of the polarization state variation occurred, then if spatially integrated, the output beam 203 approximates the input beam 201 broken down into the two principal polarization (ordinary and extraordinary) axes of wedge 202 with no average phase relationship with respect to each other. It is understood that the spatial mode of the beam 201 determines how many, or few, periods of the spatially varying polarization are required per unit length for the energy to spatially average to achieve an approximately zero phase relationship. It is also understood that a similar effect can be achieved using circular birefringence rather than linear birefringence. In this case, birefringence will be treated as linear birefringence for exemplary purposes only, with the understanding that the principles of the present invention also include use of a circular birefringence. For maximum reduction in PDL, the principal axes of the wedge 202 may be oriented or rotated by approximately 45 degrees with respect to the principal efficiency axes of the corresponding OPM. Consequently, an input beam 201 whose polarization is along one of the principal polarization axes of wedge 202 will be transmitted with substantially no change in polarization state. For input beams 201 whose polarization are not along one of the principal polarization axes of the wedge 202 , the polarization state of the output beam averages to a substantially equal split between the two principal efficiency axes due to the rotation of the principal axes of wedge 202 with respect with the principal efficiency axes of the OPM in which wedge 202 may be disposed. The above-described spatial averaging of the phase of the output beam 203 is further illustrated in FIGS. 3 A- 3 C. The input beam 201 may be polarized along the y-axis as shown FIG. 3A . It is understood that the polarization along the y-axis is for illustrative purposes only, and that input beam 201 may be polarized in other directions. Upon entering the birefringent wedge 202 , the polarization state of the beam may be broken down into two components along the principal polarization axes of wedge 202 as shown in FIG. 3B . Upon traversing the wedge 202 , the polarization of output beam 203 is shown in FIG. 3C . The polarization of output beam 203 includes exponential phase factors defined along the ordinary axis n o and extraordinary axis n e of wedge 202 that are unequal due to the difference in index of refraction between the ordinary and extraordinary axes. Integrating output beam 203 over the spatial dimensions of wedge 202 , in order to measure power, causes the phase relationship between the two principal polarization states to have a sinusoidal beat term. The sinusoidal beat term averages to approximately zero, thereby resulting in substantially no phase relationship between the principal polarization axes of wedge 202 . The angular tilt created due to the refraction from the wedge 202 may be undesirable. Referring to FIG. 4 A, there is shown a wedge plate 400 a having a wedge 102 of birefringent material and a glass compensating wedge 402 . As with wedge 202 of FIG. 2 , the birefringent material of wedge 102 may be crystalline quartz, but it is understood that the material of wedge 102 may be any linear birefringent or circularly birefringent material. Similarly, the compensating wedge 402 may be any glass having an index of refraction that substantially matches the index of refraction of the birefringent material of wedge 102 , such as N-BAK2 manufactured by Schott Glas or a subsidiary thereof. Alternatively, the compensating glass of wedge 402 could be a birefringent material whose principal axes are rotated with respect to the principal axes of wedge 102 by approximately 45 degrees. This forms a Halle depolarizer. It is understood that the angle of the surface 403 may be common between wedges 102 and 402 . Alternatively, wedges 102 and 402 may be spaced from each other. In this alternative case, the angle of wedge surface of wedges 102 and 402 may be substantially equal. The wedge plate 400 a formed by wedges 102 and 402 performs substantially the same function as the wedge 202 of FIG. 2 but advantageously has little if any angular tilt of the output beam relative to the input beam due to refraction. In some high dynamic range OPMs, employment of a wedge or wedge plate having a flat surface relative to the direction of propagation of a light beam may be less desirable because beam reflections may propagate to the detector of the OPM. Another alternative to wedge plate 400 a is shown in FIG. 4B . The wedge plate 400 c includes input & output surfaces 407 that are angled by substantially equal amounts so as to cause substantially no angular deviations to the main beam but send reflections off at an angle. Note that in this case, the choice of the angle of surface 403 will depend on the desired thickness variation of the wedges 102 and 402 as well as the choice of angles of surfaces 407 . Referring to FIG. 5 , there is shown an OPM 500 according to an exemplary embodiment of the present invention. A source optical spot 501 , which is assumed Gaussian but more generally may have any mode shape, is collimated by lens system 502 . The collimated beam makes a first pass through the wedge plate 400 a. Wedge plate 400 a may include birefringent wedge 102 whose ordinary and extraordinary axes are rotated at approximately 45 degrees with respect to the principal efficiency axes of the diffraction grating 503 and/or OPM 500 . The approximately 45 degree rotation may be either positive or negative. As discussed above, after passing through the birefringent wedge 102 , the principal polarization components of the beam may have accumulated spatially varying phase factors, as shown in FIG. 3C . Wedge plate 400 a may include compensating glass wedge 402 which minimizes the linear tilt from the wedge 102 , as discussed above. The spatially varying phase shift between the two principal polarization states of the beam exiting wedge 102 ensure that when the energy is integrated on a detector 505 , the two polarization states appear independent of one another. The beam diffracts off the diffraction grating 503 with an efficiency equal to the average of the high and low efficiencies of diffraction grating 503 due to the approximately 45 degree rotation or phase offset between the principal efficiency axes of diffraction grating 503 and that of the two polarization states of the incident beam. Following the optical path from diffraction grating 503 , the diffracted light passes through the wedge plate 400 a a second time and is imaged onto the detector 505 . It is understood that OPM 500 may have the ability to measure wavelength, OSNR and power. The detector 505 may be an array of pixilated detectors in the case of a spectrometer having no moving parts. In the case of a moving diffraction grating, such as in a scanning-based optical spectrum analyzer (OSA), a slit would be placed in front of a single element detector to give spectral selectivity for sequentially measuring wavelength and power of the input beam. The thickness variation of the birefringent wedge 102 may depend on a number of factors including the effective focal length (EFL) of the lens system 502 , the size of detector 505 , optical signal wavelength, the desired reduction in PDL, and the indices of refraction of the components in OPM 500 . Two main effects leading to spot spreading as a function of input polarization state are diffraction and refraction. Both effects spread the spot along the axis of the wedge angle. Diffraction comes about due to the spatially varying polarization state being induced. This variation of polarization state causes a sinusoidal intensity modulation for any two orthogonal polarization states into which the beam can be decomposed (i.e. basis states). The sinusoidal modulation of polarization state diffracts similarly to a diffraction grating. Scanning and diffraction grating based optical spectrum analyzers (OSAs) may not be concerned with this diffraction effect if the detector element is large. Such OSAs would only need to orient the axis of wedge 102 such that it is aligned with the slit aperture's longitudinal dimension. OPM devices which employ linear focal plane arrays tend to have smaller pixel dimensions. Thus the wedge thickness and angle selection are a more important consideration for proper operation of OPMs having linear focal plane detector arrays. The extent of the diffraction is relatively easy to predict. Referring to FIGS. 6 A- 6 C, the Fourier transform relationship between the electric field amplitude at the focal plane 603 and that in collimated space 601 , and assuming a lens system 602 with effective focal length F, may be represented as E ( x f , y f ) &equals;∫∫E ( x,y )*e −j(2&pgr;/&lgr;F)*(x*x f &plus;y*y f ) dxdy where x f and y f are the focal plane spatial coordinates. As can be seen, if the electric field amplitude's spatial distribution is a sinusoid in collimated space as shown in FIG. 6 B, the field amplitude at the focal plane would be given by two delta functions at positions &plus;/−&lgr;F/T as shown in FIG. 6C . Since the polarization state modulation is sinusoidal, due to the wedge plate 400 a, the field amplitude in collimated space would be the product of the Fourier Transform of the input spot (assumed Gaussian) and the sinusoidal modulation. Thus the spatial field distribution in the focal plane would be a Gaussian convolved with the two delta functions, i.e., two Gaussians separated by 4*&lgr;F/T. Note that the extra factor of two is due to the fact that in the reflection based OPM 500 of FIG. 5, a second pass of the beam occurs through the wedge plate 400 a and thus the effective sinusoidal period is doubled. The period T of the sinusoidal modulation will be a function of the input polarization state of the input beam. If the input polarization corresponds with one of the principal axes of wedge 102 , then T&equals;° and the two Gaussians collapse into a single Gaussian at the origin of the focal plane 603 . If the input polarization is circular, then there will be an equal amount of power in each polarization state and, for a wedge angle &thgr;, T &equals;&lgr;/&lsqb;( n c −n o )*sin(&thgr;)&rsqb;. Therefore, as the input polarization state changes, the focal plane power distribution is that of two Gaussians whose separation varies. As a result, there are competing goals of providing a large wedge angle to minimize PDL and providing a smaller wedge angle to prevent overfilling detector pixels of fixed dimension in the non-dispersion direction. One method to reduce the spot spreading is to start with an astigmatic spot. If spot 501 in FIG. 5 is astigmatic such that the spot was smaller in the non-dispersion direction, the diffractive spreading would be reduced as compared to the case of a symmetric spot, due to a reduced wedge angle being required for the same PDL reduction. Of course this assumes that both cases hold the spot size in the dispersion direction the same. An operation of OPM 500 will be described with reference to FIG. 10 . Initially, an optical beam is received at spot 501 at 1010 . Next, lens system 502 collimates the optical beam at 1020 . The collimated optical beam is modified by wedge plate 400 a at 1030 . The optical beam exiting wedge plate 400 a has a spatially varying polarization state, as described above, with a component of linear polarization appearing along the ordinary axis n o of wedge 102 and a component of linear polarization appearing along the extraordinary axis n e thereof. The linear polarization components, averaged spatially, have substantially no phase relation to each other, and are each rotated approximately 45 degrees relative to the principal efficiency axes of diffraction grating 503 . The modified optical beam is diffracted from diffraction grating 503 at time 1040 . The diffracted light passes through wedge plate 400 a at 1050 and is collimated by lens system 502 at 1060 so as to be focused onto detector 505 at 1070 . Referring to FIG. 7 , there is shown an OPM 700 according to another exemplary embodiment of the present invention. OPM 700 is a diffraction grating-based spectrometer in a transmission mode. In this case, the object spot 701 is collimated by a lens system 702 . A beam collimated by lens system 702 passes through wedge plate 400 a only one time. The polarization state of the beam spatially varies, as discussed above. The beam is diffracted by transmissive diffraction grating 703 to provide channel dispersion. A lens system 704 is disposed in the optical path to receive and re-image the diffracted light towards the focal plane 705 . A detector and/or detector array 706 is disposed at the focal plane 705 . It is understood that although OPM 700 may be physically larger than OPM 500 , light passes through wedge plate 400 a only once. Consequently, the diffractive spreading of the spot along the direction of wedge 102 will be cut approximately in half, relative to the diffractive spreading of the spot in OPM 500 . A third exemplary embodiment of the present invention is shown in FIG. 8 . OPM 810 is a reflection based diffractive spectrometer. In this case, the input spot 807 , such as a spot appearing at the end of an input fiber, is collimated by a lens system 800 . The wedge plate 400 a is disposed to receive a beam collimated by lens system 800 . The output of the wedge plate 400 a has a spatially varying polarization, as discussed above with respect to FIGS. 2 - 4 , and is focused to intermediate spot 802 via lens system 801 . The intermediate spot 802 is collimated by lens system 803 and diffracted by reflective diffraction grating 804 . The diffracted light passes through lens system 803 and is imaged at spot 805 . A detector and/or detector array 805 is placed in the focal plane at spot 805 to detect the diffracted, collimated optical signal. Design considerations of OPM 810 are similar to that of OPM 500 of FIG. 5 with the exception that only a single pass is made through the wedge plate 400 a, so that the diffractive spreading of the spot along the wedge axis of the wedge plate 400 a is reduced relative to the diffractive spreading in OPM 500 . Referring to FIG. 9 , there is shown an OPM 900 according to a fourth exemplary embodiment of the present invention. The OPM 900 is similar to the OPM 500 of FIG. 5 with the exception that an optical system 904 with astigmatic optics (i.e., having power in one dimension only) is inserted in the optical path between lens system 902 and detector 906 . The astigmatic optical system 904 may be made of a small positive-negative cylindrical lens pair. Alternatively, astigmatic optical system 904 may employ an anamorphic prism pair. The function of the astigmatic optical system 904 is to change the magnification of the spectrometer so as to reduce the optical spot size in the non-dispersion dimension. Since the non-dispersion dimension is the direction along which the wedge axis is oriented, a reduction of spot size in this dimension will reduce the diffractive spreading induced by the wedge plate 400 a. Another exemplary embodiment of the present invention is based upon the OPM 810 of FIG. 8 and OPM 700 of FIG. 7 . With reference to FIG. 11 , OPM 1100 includes a lens system 1102 , wedge plate 400 a and lens system 1103 which function the same as lens system 800 , wedge plate 400 a and lens system 801 of OPM 810 , respectively. The intermediate spot 1104 is collimated by lens system 1105 and diffracted by transmission diffraction grating 1106 . The diffracted light is received by lens system 1107 and reimaged towards focal plane 1108 . A detector and/or detector array 1109 is disposed at the focal plane 1108 . As can be seen, lens system 1105 , diffraction grating 1106 , lens system 1107 and detector 1109 function the same as lens system 702 , diffraction grating 703 , lens system 704 and detector 706 of OPM 700 of FIG. 7 . OPM 1100 provides a similar result as OPM 700 of FIG. 7 . Components of OPM 1100 , such as lens system 1102 , wedge plate 400 a and lense system 1103 , may be constructed as a single microoptic subassembly so that OPM 1100 may be reduced in size. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.