Abstract:
A waveguide under test can be exposed to a light signal whose polarization rotates between the vertical and horizontal polarizations. The intensity detected at a photodetector can be separated into AC and DC components. The AC components may be utilized to derive a characteristics which is indicative of birefringence of the waveguide. If the light signal is scanned over the waveguide under test, a measure of the birefringence at each position along the waveguide may be determined.

Description:
BACKGROUND  
       [0001]     This invention relates generally to monitoring waveguide birefringence.  
         [0002]     Birefringence is the difference in refractive indexes along the X and Y axes of a waveguide. In order to characterize a waveguide, such as a planar waveguide, it is desirable to know its birefringence. Currently there is no test equipment known to the inventors that can directly monitor waveguide birefringence.  
         [0003]     Birefringence, for example, causes polarization-mode dispersion. Polarization-mode dispersion is pulse spreading caused by a change of waveguide polarization properties. This is a random dispersion that is difficult to compensate for. In order to describe the polarization-mode dispersion, it is necessary to determine the birefringence.  
         [0004]     Generally, any type of unintended dispersion is undesirable since it changes the characteristics of a light pulse. Thus, to some degree, it is desirable to either avoid or compensate for such dispersion. In the case of polarization-mode dispersion, in order to compensate or avoid the dispersion, it is first desirable to characterize that dispersion.  
         [0005]     A birefringence contribution may add to polarization dependent loss (PDL) in planar light wave circuits based on optical interference. This may become an issue, for example, in arrayed waveguides and Mach-Zehnder interferometers.  
         [0006]     Thus, there is a need for a way to directly characterize the birefringence of a waveguide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic depiction of one embodiment of the present invention; and  
         [0008]      FIG. 2  is a schematic depiction of the device  20  shown in  FIG. 1  for measuring refractive index profile. 
     
    
     DETAILED DESCRIPTION  
       [0009]     Referring to  FIG. 1 , a laser diode  32  may generate a laser beam “L.” In one embodiment, the laser beam L may have a wavelength of 1550 nanometers. The beam L is passed through a polarizer  36 . The polarizer  36  extracts one fixed polarization, either the vertical or horizontal polarization. The fixed polarization is passed to an electrooptic modulator ir phase  38 , for example at one megahertz. The electrooptic modulator  38  may be an electrooptic crystal in one embodiment.  
         [0010]     The modulator  38  continuously rotates the fixed polarization it receives from the polarizer  36  between the vertical and horizontal polarizations under control of the voltage from the generator  40 . The rate at which the polarization is rotated is determined by the frequency of the driver  40 , which in one embodiment may be 1 megaHertz. The light beam L, with its polarization rotating continuously, is then passed through a high numerical aperture lens  14  before exposing the refractive index profiler  20 .  
         [0011]     The refractive index profiler  20  operates, in one embodiment, using a refracted near-field approach. The device  20  may be utilized to enable planar lightwave circuit designers to accurately control and optimize such devices, as one example. The device  20  is commercially available from EXFO in Vanier, Canada GIM 3G7.  
         [0012]     Referring to  FIG. 2 , the refractive index profiler  20  is shown in more detail. A sectorial stop  12  may be utilized to stop a portion of the laser beam L as indicated. The high numerical aperture lens  14  focuses the rotating polarization, collimated light beam on the end face  24  of a waveguide  22  under test. The lens  14  may be an immersion objective lens in one embodiment. The waveguide under test  22  may be placed vertically in the test cell.  
         [0013]     The test cell may include a diopter  18  supported on positioning stages  16 . Reference blocks  30   a  and  30   b  may be positioned over the diopter  18  on one side thereof. A photodetector  10  may be positioned along side of the waveguide under test end face  24  at substantially right angles thereto. A leaky mode cache  26  may be positioned under the detector  28  adjacent the end face  24 .  
         [0014]     The test cell may be scanned in steps in the X and Y directions across the laser beam L, focused by the high numerical aperture lens  14 . For example, 0.1 micro scan steps may be used. The Z direction allows the laser beam L to be focused accurately on the waveguide under test end face  24 . The photodetector  10 , placed above the sample end face  24 , collects a portion of the beam refracted out of the waveguide  22  under test.  
         [0015]     The detected signal is inversely proportional to the changes in the index of refraction encountered at the waveguide under test end face  24  during a scan across the focus of the beam L. From the known refractive index values of the two reference blocks  30   a  and  30   b , a linear interpolation in the module  42  provides a sample refractive index profile.  
         [0016]     In one embodiment, the electrooptic modulator  38 , working at 1 to 10 megaHertz, introduces TE/TM polarization alternately into the waveguide  22  under test. The waveguide  22  under test stress birefringence introduces light intensity modulation at the photodetector  10  at 1 to 10 megaHertz.  
         [0017]     An AC signal at 1 to 10 megahertz, for example, corresponds to the difference between the intensity detected by the photodiode  10  at each polarization. The capacitor  46  isolates the modulator  42  and receives the AC component at each X,Y point on the waveguide  22  from the photodetector  10 . The photodetector  10 , in one embodiment, may have a response time that is the inverse of 10 megahertz and may have an impedance of 1-2 megaohms in one embodiment. The difference signal can be scaled to a DC signal, which is inversely proportional to the refractive index of the waveguide  22  under test and may be analyzed by the module  44 . Since the waveguide  22  under test is translated in the X and Y directions, the birefringence profile at each position in the X,Y plane of the waveguide  22  under test can be obtained with submicron accuracy spatial resolution in some embodiments.  
         [0018]     The spatial profile analysis device  44  provides the refractive index profile from the DC signal from the photodetector  10 . The spatial profile analysis module  42  receives a synchronization signal from the generator  40  and develops a birefringence profile using the AC information from the photodiode  10 . The synchronization signal syncs the module  42  to the polarization rotation supplied by the modulator  38 .  
         [0019]     AC measurements at 1 megahertz lock-in can pick up relatively minute difference signals in two polarizations at each spatial position. Those signals can be calibrated against a DC refractive index, resulting in a birefringence measurement that, in some embodiments, is better than 10 −3 , which is the average refractive index measurement.  
         [0020]     In one embodiment, the lens  14  may match the resolution of the profiler  20 , which may be approximately 0.2 microns in one embodiment.  
         [0021]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.