the gist of the present invention is using a polarization - selective electro - optic modulator and an optical channel recognizing unit for osnr monitoring and channel recognition in dwdm networks . fig3 shows a block diagram of the apparatus for osnr monitoring and channel recognition of this invention . the apparatus comprises a polarization controller 301 , a dithering signal 302 , a polarization - selective e - o modulator 303 , a beam splitter 304 , a photodetector 305 and an optical element 306 . the polarization controller 301 , the e - o modulator 303 , the beam splitter 304 and the photodetector 305 are configured as an osnr measurement unit . the beam splitter 304 and the optical element 306 are configured as an optical channel recognizing unit . the dithering signal 302 is provided and fed into the e - o modulator 303 and then applied to a selected channel in order to improve the detection sensitivity and dynamic range for both the photodetector 305 and optical element 306 . the optical element is used for recognizing the channel wavelength . according to this invention , the optical element 306 may be a led sensor , soa or optical filter plus photodiode . the polarization - selective e - o modulator may be a linear polarizer in serial with an e - o modulator or an e - o modulator with a structure that one of the polarizations is leaky as it propagates in the modulator it is also notable that all the elements in the apparatus of the invention can be packaged into a compact module and integrated on a chip . referring to fig3 the optical signal 307 is first tapped out from a selected channel and is fed into the polarization controller 301 then the e - o modulator 302 . the sequentially adjusting the polarization controller 301 is performed with the optical signal 307 fed into the polarization controller 301 . a dithering signal 302 of the e - o modulator 302 is provided to improve the detection sensitivity . the beam splitter 304 then splits the dithered optical signal 308 , the output of the e - o modulator 303 , into two clusters . one is applied to the photodetector 305 for measuring the osnr value . the other is applied to the optical element 306 for recognizing the optical channel . [ 0029 ] fig4 shows an embodiment of fig3 in which a led sensor is used as optical element and the dithered optical signal 308 is split into two clusters with a ratio of 50 : 50 . as can be seen from the fig4 the signal and noise are passed through a tunable optical filter 401 to simulate the channel selection or dropping function . the optical signal 307 is tapped out from the selected channel and is fed into the polarization controller 301 . sequentially adjusting the polarization controller 301 then is performed with the optical signal 307 . to enable the needed function , the e - o modulator 303 is very polarization - selective , i . e ., it allows one of the two othogonal polarizations to pass through , like a substantially linear polarizer . with such an e - o modulator and the polarization controller , the channel osnr can be measured in a similar way as the polariztion nulling approach . on the same time , the e - o modulator applies the dithering signal 302 to the selected channel to improve the detection sensitivity and dynamic range for both the photodetector 305 and led sensor 406 . the dithered optical signal 308 is split into two clusters by a coupler 404 with a ratio of 50 : 50 in the embodiment . these two clusters are respectively provided for the photodetector 305 and the led sensor 406 . the osnr and channel number are measured with the photodetector 305 and the led 406 sensor , respectively . the led sensor 406 is used for recognizing the channel wavelength from the induced junction voltage across the led diode . it is the same as the wavelength detection approach that utilizes the wavelength - dependent transparent properties in a fpld or a soa . like a laser diode or soa , the led has a gain material of which the transparent current decreases as the wavelength moves to the long - wavelength side . the junction volatage across the electrodes of the led is almost linearly proportional to the difference between the transparent current and the bias current . it is clear that the induced voltage varies with the wavelength change and can be used for detecting the wavelength . the led sensor can monitor multiple channels and its monitoring wavelength window can be easily tuned to cover different wavelength bands by adjusting its bias current . for instance , the induced voltage of an edge - emitting led ( pd - ld model pld e15 - 506 ) is shown in fig5 . the induced junction voltage in response to a dithered input light can be detected with a narrow electric bandpass filter to eliminate the noise associated with the detection process . the output signal is very sensitive to a wavelength change . the output voltage shown in fig5 was measured with using a phase sensitive detection ( psd ) circuit , so the negative magnitude of induced voltage is displayed at the loss regime . when a psd circuit , e . g ., a lock - in amplifier , is used to sense the voltage , a monotonic relationship between the detected voltage and the input wavelength can be obtained . this invention allows a larger range of wavelengths to be detected by a led . the led sensor can provide many advantages over the previously demonstrated channel monitoring approaches , including the use of an optical filter , double detectors , and interferometers , etc . it is also superior to the soa or fpld sensor that exploits the same detection principle as a led . the led sensor is very low - cost and can monitor a wide range of wavelengths at a given bias current . the monitoring wavelength window can be easily adjusted to achieve the best performance by changing its bias current . though a low cost fpld was demonstrated to monitoring dwdm channels with satisfactory performance , it is subject to the voltage resonance induced from the facet reflection , which requires choosing an appropriate ld length and careful adjustment of the bias current . the only concern for using a led sensor is that it may require temperature control . since the led needs to simply distinguish the channels from each other rather than to accurately detect the channel wavelength , the detection window is relatively large , so it does not need tight temperature control in the invention . according to this invention , the maximal voltage measured by the photodiode indicates the signal power plus the half noise power , while the minimal voltage corresponds to half of the noise power if the signal is completely suppressed by the polarized modulator . thus , the osnr monitoring is performed with the combination of the polarization controller 301 and the substantially linear polarized e - o modulator 303 . the e - o modulator 303 also applied a single - tone sinusoidal signal to the input signal and noise to improve the detected quality for both monitoring paths . the frequency of the dithering signal 302 is 10 khz in the experiments the channel wavelength is then detected by the led sensor 406 which is followed by a narrow bandpass filter to reject the detection noise . [ 0037 ] fig6 shows the induced junction voltage of a led versus wavelength drift for eight dwdm channels , ch 1 ˜ ch 8 , of 100 ghz spacing . fig6 also shows the variation of the voltage with the wavelength drift . typically , the wavelength locker can assure the wavelength drift of a dwdm channel to be within +/− 0 . 02 nm . for such an amount of possible wavelength drift , the induced voltage between the adjacent channels can be clearly distinguished . moreover , due to the large separation in the detected voltage for different channels , the tolerance on temperature control of the sensor can be relaxed to 0 . 24 ° c . the reliability and stability of this type of channel monitoring technique have been verified for the fpld sensor . the following further describes the operating principle of the invention for simultaneous channel and osnr monitoring . the sequentially adjusting the polarization controller 301 is performed with the optical signal 307 fed into the polarization controller 301 . then the polarization controller 301 switches the polarization states to where the maximal or minimal value occurs at the output of the e - o modulator 303 . by such sequentially adjusting the polarization controller , the photodetector 305 can detect the measured minimum optic power p min and maximum optic power p max . rig . 7 shows the measured optical spectra at the output of the e - o modulator for two orthogonal polarization states at the 2 . 5 gbps and 10 gbps bit rates , respectively . the outputs for two orthogonal polarization states are compared in the fig7 . the results indicate that the polarization extinction ratio is 37 . 9 db and 36 . 7 db for a single 2 . 5 gbps and 10 gbps channel , respectively . the osnr value can be calculated from the detected maximal and minimal voltages by switching the two orthogonal polarization states . fig8 compares the osnr measured by the apparatus of the present invention and a high - performance optical spectrum analyzer for the bit rate of 2 . 5 gbps and 10 gbps , respectively . the calibrated osnr refers to the measured value but correct the factors that are different for the two measurement techniques . the factors include the 3 db difference in the measured noise level since only half of the ase noise is measured by the polarized modulator . they also include the difference ( 4 . 2 db ) in the filter bandwidth between the tunable optical filter and the monochrometer in the osa . the results indicate that the measured osnr is very consistent with the data measured by the osk our module can measure the osnr up to 31 db while the error is kept within 05 db . from the above , the key components of the present invention are the e - o modulator and led sensor for osnr monitoring and channel recognition . in an experiment , this invention chooses a packaged mach - zehnder amplitude modulator made by the at & amp ; t ( model m2122aa ). the e - o modulator is pigtailed with standard single - mode fiber ( smf ) on both ends . it is very polarization - selective and has a polarization extinction ratio larger than 30 db . the led sensor is a 1 . 55 μm edge - emitting led . generally speaking , the apparatus and method for simultaneous channel and osnr monitoring by using a polarization - selective e - o modulator and a low - cost optical element have demonstrated the following advantages : ( 1 ) low cost ; ( 2 ) compact ; it can be packaged into a compact module and integrated on a chip ; ( 3 ) the channel monitoring can cover a wide wavelength range and is tunable ; and ( 4 ) the osnr monitoring can be accurate over a wide dynamic range . although the present invention has been described with reference to the preferred embodiments , it will be understood that the invention is not limited to the details described thereof . various substitutions and modifications have been suggested in the foregoing description , and others will occur to those of ordinary skill in the art . therefore , all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims .