Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to the interferometric measurement of optical devices parameters including the determination of the “S”-parameters of optical devices with one or more ports, in transmission and/or reflection.  
         [0002]     “S”-Parameters are concepts widely used in the microwave engineering practice, which facilitate the analysis of the signal transfer between the ports of a multi-port device, therefore, its application is also feasible in optical device techniques. However, while based on similar principles, optical “S”-parameters differ substantially from microwave “S”-parameters due to the fact that the polarization characteristics of the light transmitted through the DUT (Device Under Test) must be taken into account. In the case of microwave “S”-parameters, each “S xy ” is a complex number that represents the characteristics of transmission and/or reflection from port Y to port X of the DUT. In the case of optical “S”-parameters, each “S xy ” it is represented using the Jones&#39; formalism (Jones matrix) and/or the Müller&#39;s formalism (Müller matrix). From each “Sxy” it is possible to deduct all the usual optical properties for the characterization of photonic devices, such as: bandwidth, phase, time delay, chromatic dispersion, 2 nd  order chromatic dispersion, reflectance, reflection coefficient, transmittance from port “y” to port “x” and vice-versa, transmission coefficient from port “y” to port “x” and vice-versa, insertion loss, polarization dependent loss, polarization mode dispersion DGD/PMD), 2 nd  order DGD, etc.  
       DESCRIPTION OF THE PREVIOUS ART  
       [0003]     Optical components have become increasingly important in WDM systems (Wavelength Division Multiplexing), high capacity optical systems, all-optic communications systems, dispersion compensation, fiber sensing and other technologies. In the last twenty years, a significant amount of research has been focused on the development of optical devices equivalent to electronic components, in order to allow the development of all-optical networks and of the photonics field in general. The full utilization of the benefits of such devices, requires the accurate measurement of their optical characteristics, such as: bandwidth, phase, time delay, dispersion, reflectance, transmittance, insertion loss, polarization dependent loss, polarization mode dispersion etc. The optical characteristics of the DUT are generally defined for specific wavelengths, therefore, to extend these characteristics over a certain bandwidth, as it is normally the case, the characterization process should be repeated for a finite number of wavelengths, Several equipments, systems and methods have been proposed to avoid the need of conducting a great number of measurements in several wavelengths. One well-known process is the so-called “RF Phase Shift” technique. Such method of characterization of optical devices demands a set of expensive equipments and entails a trade-off between precision and resolution of wavelength.  
         [0004]     Due to the above mentioned shortcoming, current solutions use interferometric techniques which have become more efficient, more accurate and less costly  
         [0005]     One known system that employs an interferometric optical technique, is described in document EP 1182805. In this arrangement, a laser generator is swept in wavelength with a constant sweep speed, its signal being split into two arms, of necessarily different lengths, whith the DUT inserted in one of them. The signal transmitted through the “known” arm (called reference arm) and the one which traveled through the arm with the DUT (Device Under Test) are mixed in a photodetector, giving rise to an electric signal from the beating of the different frequencies of optical signals, the displacement between said frequencies being due to the propagation delay in the different signal paths. The resulting heterodyne (or quasi-homodyne) signal, ranging in frequency from some KHz to a few MHz, is directed to a signal processing system that determines the desired optical characteristics of the device. This procedure allows the translation of the information regarding the optical characteristics of the DUT from the optical to the electrical domain. For example, the instantaneous-wavelength-dependent coefficient of transmission is given by the instantaneous amplitude of the heterodyne electrical signal. A considerable disadvantage of this technique, called SWI (Swept Wavelength Interferometry), is the need to use only “swept” lasers, which aft continuously swept in wavelength. Another shortcoming is the fact that the lambda noise (wavelength) of the laser is amplified, due to the required large length imbalance of the interferometer arms.  
       OBJECTS OF THE INVENTION  
       [0006]     In view of the above, the first aim of the invention is to provide a system that allows the complete characterization of multi-port passive optical devices in a speedy manner, with the feature of being able to operate both in the continuous sweep swept mode or in the stepped swept modes of the tunable laser source.  
         [0007]     It constitutes another purpose of the invention to furnish a system that provides great precision in the measurements of transmission coefficient, reflection coefficient, transmitance, reflectance, intrinsic loss, bandwidth, phase, time delay, chromatic dispersion, 2 nd  order chromatic dispersion, differential group delay (DGD)/polarization mode dispersion, 2 nd  order DGD, polarization dependent loss of optical devices, as well as providing high resolution in wavelength.  
         [0008]     Yet another object is to provide a system where the effect of the mechanical vibrations is minimized.  
         [0009]     Another additional object is to provide a system where the effect of the variations of ambient temperature is minimized.  
         [0010]     Another object is to furnish a system and a method that allows the simultaneous determination of all the above mentioned optical characteristics in all the transmission directions of a multi-port DUT, with a single wavelength sweep of the tunable laser source.  
       SUMMARY OF THE INVENTION  
       [0011]     The above mentioned aims are attained by means of an interferometric optical arrangement in which the paths of the test signals (or DUT signals) and the reference signals has approximately equal lengths, without requiring any length imbalance in the arms of the interferometer.  
         [0012]     According to another feature of the invention, the optical signal of at least one of the arms of the interferomneter is phase- or frequency-modulated.  
         [0013]     In accordance with another feature of the invention, the optical phase or frequency modulator can be constructed by any known optical technologies.  
         [0014]     In accordance with another feature of the invention, the optical arms of the interferometer can be constructed using different physical paths for propagation and conduction of the optical signal, such as: optical waveguides, planar waveguides, free space (FSO) etc., 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Additional advantages and features of the invention will be more easily understood through the description of some exemplary embodiments which exemplify the arrangements used in the diverse kinds of measurements as well as the operating principles of the system, together with the related figures, in which:  
         [0016]      FIG. 1  shows the arrangement used in the measurement of the reflection parameters of a passive component with only one port, according to the invention.  
         [0017]      FIG. 2  shows the arrangement used in the measurement of the transmission parameters of a passive component with two ports, according to the invention.  
         [0018]      FIG. 3  illustrates an arrangement used for the partial characterization of a two-port DUT, simultaneously in transmission and reflection.  
         [0019]      FIG. 4  shows an arrangement used in the simultaneous characterization of all ports, in transmission and reflection, of a two-port device.  
         [0020]     FIGS.  5  to  8  illustrate the paths of the optical signals in the characterization of optical “S”-parameters, using the arrangement shown in the previous figure.  
         [0021]      FIG. 9  illustrates a block diagram showing the operating principle for suppressing the effects of vibration and temperature changes,  
         [0022]      FIG. 10  illustrates the arrangement used for the above mentioned suppression being applied to the optical circuitry shown in  FIG. 2 .  
         [0023]      FIG. 11  illustrates the arrangement used for simultaneous measurement of the polarization characteristics in transmission and reflection of a 2-port DUT. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     The invention now will be detailed through specific examples related to some typical applications. The first embodiment refers to an arrangement used for the characterization of the reflection parameters of a DUT.  FIG. 1  illustrates relative positions of the elements used in the test, to wit; 
        a tunable laser signal source  11  (TLS—Laser Tunable Source), that is controlled by the control system  30 ;     an optical coupler  14 ;     a device under test  17  (DUT);     an optical modulator  21 ;     a signal generator  22 ;     an optical fiber mirror  24 ;     optical detection system  26      electronic system for data acquisition  27         
 
         [0033]     The system shown in  FIG. 1 , whose optical part forms a Michelson interferometer, operates in the following way: the control system  30 , which manages the optical characterization process, issues a command to TLS  11  to generate an optical signal  12 . This signal is directed by the optical fiber  13  to the optical coupler  14 , where it is split in two signals  12 ′ and  12 ″ that are directed, through optical fibers  15  and  20 , to DUT  17  and optical modulator  21 , respectively. The signal  12 ′ that impinges on the DUT can be transmitted or be reflected, depending of its wavelength and the specific optic characteristics of the DUT. The transmitted signal is absorbed at output device  10 . The reflected signal  18  returns by the optical fiber  15  to coupler  14 , where it is split again: part of it returns through optical fiber  13  and another part  18 ′, is transmitted by optical fiber  19 . In turn, the signal  12 ″ passes though modulator  21 , where it is modulated in phase or frequency by the modulating signal  23  provided by the signal generator  22 . The modulated optical signal  25  is reflected by mirror  24  and passes again though the modulator  21 , returning to optical fiber  20  and going to the coupler  14 , where it is split. The portion  25 ′ of this modulated signal enters optical fiber  19 , that also transmits signal  18 ′ to the optical detection system  26 .  
         [0034]     The optical detection system  26  produces the heterodyning between the two signals  18 ′ and  25 ′, translating information from the optical domain to the electrical domain, giving at its output, in addition to the original signals, the products of the heterodyning, particularly the difference signal. This is an electrical signal whose spectrum contains frequency components whose amplitude and phases depend on the modulating signal  23  and on the optical characteristics of the DUT. The data acquisition circuit  27  extracts information about the optical characteristics of the DUT from the electrical signal. This process of extraction of the information contained in the electric signal can be carried through using different techniques, such as filtering and direct detection, Lock-in, FFT (Fast Fourier Transform) etc, which can be implemented using analog techniques (analogic processing of signals), digital (digital processing of signals) and/or through software. The amplitude information extracted from the electric signal is proportional to the characteristic called “reflection coefficient” of DUT  17 . This amplitude information enables the extraction of other information about the DUT, such as: reflectance, insertion loss, bandpass etc. The phase information extracted from the electric signal refers to the phase deviation introduced by the DUT in the reflected signal, allowing the acquisition of other information, such as: group delay, chromatic dispersion etc.  
         [0035]     Besides registering the data about the reflection coefficient and phase deviation of the DUT, the control system manages the process, selecting the series of wavelengths, which must be sufficiently close so as to provide a good resolution in the determination of the DUT characterstics.  
         [0036]     As already mentioned, the optical phase/frequency modulation uses any know technique of modulation, such as for example, changing the refraction index of an optical element, changes in the signal propagation length, electric-optic effects, etc. Amongst these, one exemplary embodiment uses a piezoelectric ceramic cylinder over which the optical fiber is wrapped. Applying the modulating signal to this cylinder, its dimensions change in accordance with this signal, stretching the optical fiber which changes its length as well as its refractive index, producing the phase modulation in the phase of the optical signal that traverses the fiber.  
         [0037]     The optical modulator  21  doesn&#39;t have to be located in the reference arm of the interferometer. It can alternatively be located in the DUT arm or in both arms.  
         [0038]     The system is not limited to the use of a saw-tooth modulating signal; other waveshapes can be used, such as square wave, sine wave, waves composed of linear segments etc.  
         [0039]     One of the advantageous features of the invention is the fact that the system can work with laser sources in which the wavelength is continuously changed or where this wavelength is changed by steps (“Swept” and “Stepped” Lasers).  
         [0040]      FIG. 2  illustrates the arrangement used in the measurement of the transmission characteristics of a DUT  17 . For clearness sake, control lines  31  that connect the control system to TLS  11  and to the electronic acquisition circuitry  27  had been omitted in this figure, however such control exists in the same way as in the previous arrangement. In the arrangement of  FIG. 2 , whose optical part forms an Mach-Zehnder interferometer, the signal  12  generated by the laser  11  is conveyed by the optical fiber  13  to the coupler  14 , where it splits into the signals  12 ′ and  12 ″. The first one of these is transmitted by optical fiber  15  to the DUT  17 , where it can be reflected, spread, absorbed or even transmitted as signal  41 , depending on the specific optical characteristics of the DUT. The signal  12 ″ is directed to modulator  21 , where it is modulated by the signal provided by the signal generator  22 , resulting in the phase- or frequency-modulated signal  25 , that it is directed by the optical fiber  33  to a second coupler  34 , where it is added to signal  41  transmitted through DUT  17 . Part of these added signals,  25 ′ and  41 ′, is directed to the optical detection system  37 , where the heterodyning between this signals occurs. In a similar way to that shown in the arrangement of  FIG. 1 , the signal difference is introduced in one of the inputs of the acquisition circuit  27 , which receives in its other input the reference signal from the signal generator, that is used to determine the transmission characteristics of DUT  17 . Devices  10  and  10 ′ are terminations that do not reflect the signal.  
         [0041]     The  FIG. 3   a  illustrates one of the arrangements that can be used for simultaneous characterization of the DUT in transmission and reflection. Signal  12  of laser  11  is introduced in the optical coupler  14 , which splits it in two components  12 ′ and  12 ″, directed respectively, to DUT  17  and modulator  21 , in which occurs the modulation in phase or frequency by the modulating signal generated by the signal generator  22 . The modulated optical signal  25  is directed to the optical coupler  44 , where it divides in two components  25 ′ and  25 ″, the first one being transmitted to the optical coupler  16  where it is added to the transmitted signal  41  through said DUT. This sum of signals is detected by the optical detection system  43  where the heterodyning between these signals occurs producing several other signals, that are directed to the first input of the acquisition circuit  47 , including the difference signal ( 25 ′− 41 ). This signal has a frequency spectrum that contains phase and amplitude information of the DUT for a determined wavelength. The second input of the acquisition circuit  47  receives the modulating signal proceeding from generator  22  to provide a phase and amplitude reference for the circuit operation. In the output  47 , it is possible to get the information concerning the S 21  transmission parameter (transmission of port  1  to the port  2 ) of the DUT.  
         [0042]     The second component  25 ″ of the modulated signal is reflected by mirror  45  and returns through coupler  44 , modulator  21  and coupler  14 , where it is added to signal  18  reflected by the DUT. These signals are directed to the optical detection system  42  whose output produces, among others, the difference signal ( 25 ′″″− 18 ) that is inputted to the acquisition circuit  27  whose output has the information of amplitude and phase of the reflected signal, providing the characterization of the reflection parameter of the DUT (S 11 ).  
         [0043]     This arrangement illustrated in the  FIG. 3   a  can be interpreted as being equivalent to the overlapping of two optical interferometers, that can be better seen in  FIGS. 3   b  and  3   c . In the first one, the optical part forms an Michelson interferometer, composted by the segments of optical fiber  13 ,  15 ,  19 ,  20 ,  32  and  34 , the mirror  45 , couplers  44  and  14  and the optical modulator  21 .  FIG. 3   c  shows that the optical elements used in the measurement of the transmission characteristics of the DUT forms a Mach-Zehnder interferometer, composted by the optical fiber segments  13 ,  15 ,  20 ,  32 ,  33 ,  41 ,  35 ,  36  as well as couplers  14 ,  44 ,  16  and the optical modulator  21 . It is seen that many elements of said interferometers are part of both devices. Such is the case of the optical fiber segments  13 ,  15 ,  20  and  32 , as well as the couplers  14  and  44  and optical modulator  21 . This overlapping—that is meant to provide the simultaneous measurement of two parameters of the DUT—is possible by using the optical modulation in phase or frequency of the reference signal, entailing the advantage of making the operation of the interferometers totally independent of the physical lengths of its interferometer arms.  
         [0044]     For characterization of the two other parameters S 12  and S 22  with the arrangement of the  FIG. 3 , it is necessary to invert the position of the DUT. For the concurrent of both ports of a two port device, simultaneously in transmission and reflection, the arrangement illustrated in  FIG. 4  must be used. This simultaneous characterization refers to the determination of the reflection and transmission parameters of the two-port DUT in all directions of propagation (S 11 , S 21 , S 22  and S 12 ), in a single wavelength sweep. In this arrangement, two different modulating signals, whose frequencies ω m1  and ω m2 , generated by generator  49  cannot be multiple or have coincident harmonics. In this figure, the electronic circuit that performs the treatment of the signals detected by the detection system  42  and  43  are grouped in blocks  50  and  50 ′, which are responsible for the acquisition of the parameters “S 11  and S 12 ” and “S 22  and S 21 ”, respectively.  
         [0045]     The  FIG. 5  shows the paths of the optical signals in the characterization of the reflection parameters of port  1  (S 11 ). In this measurement, the signal generated by the laser is split by coupler  14  in two components, the first one being directed, through the optical fiber  15  and the coupler  54 , to the modulator  21  where it is modulated in phase or frequency with the modulating signal with frequency ω m1  and going from there to the P 1  port of DUT  17 . The second component traverses optical fiber  20  to coupler  52 , which forwards part of this component through fiber  53  to coupler  54 , where is added to the reflected signal from the DUT that returned through modulator  21 . These added signals traverse optical fiber  55  to the optical detection system  42 , the resulting electric signal of this detection being processed by block  50 , which includes the acquisition circuitry to allows the characterization of the S 11  parameter.  
         [0046]     The  FIG. 6  shows the paths of the optical signals for the characterization of the S 21  parameter. In this case, the first component of the signal produced by the laser is directed through the optical fiber  15  to coupler  54 , where it is split: part of this signal goes to the phase or frequency modulator  21 , where is modulated by the modulating signal with frequency ω m1  and traverses DUT  17 , in the direction from the P 1  port to the P 2  port, as well as to modulator  51  where it is modulated by the modulating signal with frequency ω m2  and forwarded to coupler  52 , where it is added to the unmodulated signal that arrives from optical fiber  53 . The detection, by the optical detection system  43 , of these added signals produces the difference signal that will be treated by the electronics circuitry  50 ′, enabling the determination of the S 21  parameter associated with the transmittance of DUT  17 , in the direction of port P 1  port to port P 2 .  
         [0047]     The paths of the optical signals in the characterization of the reflection parameters in port  2  (S 22 ) are illustrated in the  FIG. 7 . In this measurement, the optical signal generated by the laser is split by the coupler  14  in two components, the second one being directed, through the optical fiber  20  and coupler  52 , to the modulator  51  where is modulated in phase or frequency by the modulating signal with frequency ω m2  and from there to the P 2  port of DUT  17 . The first component leaves coupler  14 , traverses optical fiber  15  to coupler  54 , that sends part of this component through fiber  53  to the coupler  52 , where is added to the signal reflected by the DUT returned thorough modulator  51 . These summed signals traverse optical fiber  56  to the optical detection system  43 , the resultant electric signal of this detection being processed by the block  50 ′ that supplies the data for the characterization of the S 22  parameter.  
         [0048]     The  FIG. 8  depicts the paths of the optical signals for the characterization of S 12 . In this case, the second signal component produced by the laser is transmitted through optical fiber  20  to coupler  52 , where it is split. One part of this signal is modulated in phase or frequency by the optical modulator  51  with frequency ω m2  then traverses the DUT  17 , in the direction of port P 2  to port P 1 , further traversing modulator  21  where this signal is modulated by the frequency ω m1  being directed from there to coupler  54 , where it is added to the unmodulated signal from the optical fiber  53 . The detection of the summed signals by the optical detection system  42  produces the signal difference that will be processed by block  50 , enabling the determination of the S 12  parameter associated with the transmittance of DUT  17  in the direction of port  2  to port  1 .  
         [0049]     As occurs in the arrangement of the  FIG. 3 , the present disposition also is equivalent to the overlapping of diverse optical interferometers, that share the same segments of optical fibers. Thus, in  FIGS. 5 and 7 , both Michelson interferometers have in common the ring formed by the segments of optical fibers  15 ,  20  and  53 , as well as couplers  14 ,  52  and  54 . In the arrangements of  FIGS. 6 and 8 , the Mach-Zehnder interferometers share the optical fibers segments  53 , as well as the path that goes from coupler  54 , passing by the modulator  21 , the DUT  17  and the modulator  51  to the coupler  52 .  
         [0050]     The arrangement shown uses only two optical detection systems— 42  and  43 —each one receiving the signals related to two parameters: the signals that allow the determination of the parameters S 11  and S 12  are received simultaneously by system  42 , and the ones referring to the parameters S 21  and S 22  are received simultaneously by the optical detection system  43 . The discrimination between signals that arrive at the same detection system is possible by the different modulations applied to these signals. Thus, the signal used for determination of S 11  is modulated by the frequency ω m1  (as shown in  FIG. 5 ) while the signal that allows the determination of S 12  is modulated by the frequencies ω m2  (as shown in  FIG. 8 ). In general, the electronic acquisition circuits select information in the frequencies of interest, allowing the discrimination of the different S xy  parameters, even when they are received by the same optical detection system, because these information are individualized by the modulating signals.  
         [0051]     According to the invention, the measurements of the characteristics of the DUT&#39;s are reached by optical interferometry, in which the light signals propagate between two different paths or arms and are later recombined. The results of these measurements are influenced by any changes occurring in these paths, such as, for example, the refractive index of the fiber, the physical distance covered by the light etc. Thermal variations and mechanical vibrations can stretch the optical fiber or modify its refraction index, affecting differently the two arms of the interferometer and, consequently, introducing detrimental variations in the output signals of the interferometer.  
         [0052]     The changes in the properties of the optical paths are neutralized in the present invention by means of an active control of the changes in the optical system, which compensates the errors due to thermal variations and/or mechanical vibrations. This device consists of the virtual duplication of the interferometer, making it to operate in two distinct wavelengths. A first group of wavelengths is used to characterize the DUT. A second and fixed wavelength allows the evaluation of the variations that occurring in the interferometer due to variation of temperature and/or mechanical vibrations and feeding back the system with a correction signal that is applied to the interferometer that characterizes the DUT.  
         [0053]     The block diagram that shows the working principle of the temperature compensation is depicted in  FIG. 9 . As illustrated, two sources of laser light are used, the first one  81  generating the signal in variable wavelengths λ S  for DUT test, and the second  82  generating a ted wavelength signal λ T  for the control and compensation of vibrations and temperature changes. Both signals are introduced in interferometer  83 . At the interferometer output there are two optical detection systems, the first one  84  being the optical detection system for characterization of the DUT and the second,  85 , for the monitoring signal λ T . This second optical detection system feeds a comparator and error signal generator block  86 . The interferometer receives a negative error signal feedback through the optical modulators. If a variation in the system produced by thermal variation or mechanical vibration occurs, this will be compensated by the feedback link  87 , and it will not affect the measurement results.  
         [0054]      FIG. 10  illustrates the system of temperature compensation in a more detailed form. In this diagram, two laser generators are used, the first  11  producing the test signal (variable wavelength) and the second  11 ′ producing the compensation signal (fixed wavelength λ T  falling outside the test signal wavelength). These signals are added in coupler  14 , being split in two components that are transmitted by the optical fibers  15  and  20 . Signal  41  that traversed the DUT is split again by coupler  34  and arrives through the fibers  35  and  36  at the two optical reception systems  37  and  38 . The signal  12 ″ traverses modulator  21  and is also split by coupler  34  following by fibers  35  and  36  to the optical reception systems  37  and  38 . The optical reception system  38  has a selective filter  39  tuned to the control wavelength. Therefore, the signal produced by photo detection system  38  is only related to the control wavelength. The temperature compensation signal is directed to the block  27 ′, which consists of an electronic circuit similar to that used in the treatment of the measurement signals. As the optical paths are fixed for λ T  and the control light source also operates in a fixed wavelength, the photodetected signal should not suffer a phase change. In case that some change of phase occurs, this will have been caused by thermal or mechanical disturbances, and can be compensated in the modulators. As the response of the optic system λ T  is almost identical for the control and measure wavelengths, the compensation also occurs in the wavelength band of the test device. Thus, the optical interferometer setup formed by the acquisition circuit associated to the optical detection systems  38  allows to obtain the error signal that will be negatively fed back to the interferometer through the existing optical phase modulators. On the other hand, the elements associated with the optical detection systems  37 , the selective filter  39 ′ for test wavelengths and the acquisition circuit  27  operate in the characterization of the DUT like the previously detailed arrangement of  FIG. 2 .  
         [0055]      FIG. 11  shows the device configuration that allows the simultaneous determination of the polarization characteristics of the DUT for two orthogonally polarized light waves. The test signal generated by the tunable laser  11  is split by coupler  14  in two components and directed by the optical fibers  110  and  111  to couplers  112  and  113  where they are split again. The sub-components derived from coupler  112  are modulated in phase or frequency by the modulators  114  and  116  with modulating signals ω s  and ωp. The modulated signals are processed by the polarization controllers (PC)  115  and  117 , which maximize the orthogonal polarization components of light s and p, respectively. These signals are summed in the polarization combiner (PBC—Polarization Beam Combiner)  118 , that guarantees the orthogonality between both and then directed to coupler  119 , where the sum of the signals is split in two components, directed through couplers  121  and  122  to DUT  125 . In this path, each component of the sum of the signals is modulated by the modulating signals ω 1  and ω 2 . Part of these components traverse DUT  125  and part are reflected by it. Each one of these parts undergo then a second modulation by the modulating signals ω 1  or ω 2 , as the case be. The resultant signals are then diverted by couplers  121  and  122  and directed to the Polarization Beam Splitter (PBS)  126  and  127  and from there to the optical detection systems  128 ,  132 ,  133  and  135 , followed by the processing and acquisition systems. The modulations suffered by the optical signal during its passage through the modulators allow to identify the individual polarization components in quadrature, allowing the determination of the DUT polarization characteristics. For example, the optical signal that arrives at the optical detection system  128  is modulated by the following frequencies, related to the transmission through the DUT: 
        ω s +ω 2 +ω 1       ω p +ω 2 +ω 1       ω s −ω p +ω 2 +ω 1            
         [0059]     As concerns the reflected signal, the optical signals that arrive at the optical detection system  128  are modulated by the following frequencies: 
        ω s +2 ω 1       ω p +2 ω 1       ω s −ω p +ω 1          
 
         [0063]     These 6 signals can be electronically separated and can be individually analyzed by the electronic circuits.  
         [0064]     The electronic circuit  129 , the optical detection system  128 , the circuit  131  associated to the optical detection system  132  form a polarization diversity receiver, capable of extracting the amplitude and phase information of the components and allowing the selective optical characterization of the S 11  and S 12  parameters The other optical detection systems and the associated circuitry operate in a similar way, providing the selective polarization characterization of all parameters of the DUT, namely S 11 , S 12 , S 22  and S 21 . Dedicated computational algorithms correlate the information acquired by the electronic circuits  129 ,  131 ,  134  and  136  and allow the complete characterization of the DUT, as well as the polarization characteristics of the device, the whole process being carried out simultaneously in a single wavelength sweep of the Tunable Laser Source.  
         [0065]     The measurement technique described previously exemplifies the characterization of two-port optical devices, generating  4  optical “S”-parameters (two of reflection and two of transmission). This concept may be extended, without any loss of generality, to the characterization of N-ports devices. In this case, taking the most complete version ( FIG. 11 ) the setup “DUT+modulators” ( 123 ,  124  and  125 ) is substituted by a DUT of N ports (N=3, 4, 5 . . . ) where in each port is inserted an optical modulator whose frequency is distinct and not multiple of the remaining ones. Optical couplers sum all these signals proceeding from the diverse ports of the DUT forwarding these to the couplers  121  and  122 , which transmit said summed test signals as well as the reference signal to the optical detection system, where the heterodyning occurs. In this way, a plurality of electrical signals is generated in the optical detection system that contains information of amplitude and phase of the combination of all the DUT ports, each one centered in a specific modulating frequency.

Technology Category: g