Coherent optical receiver testing

Abstract

An heterodyne apparatus and method for measuring performance parameters of a coherent optical receiver at RF frequencies is disclosed. Two coherent lights are launched into signal and LO ports of the receiver with an optical frequency offset f. One of the lights is modulated in amplitude at two phase-locked modulation frequencies F1 and F2. COR performance parameters are determined by comparing two frequency components of the COR output. The group delay variation (GDV) information is obtained by comparing phases of two time-domain traces corresponding to frequency components of the COR output signal at the two modulation frequencies shifted by the optical frequency offset f.

1 - 20 . (canceled) 
     
     
         21 . A method for measuring a group delay variation (GDV) for a coherent optical receiver (COR), the method comprising:
 providing first light and second light into the COR, wherein the first light is modulated at a first modulation frequency F 1  and a second modulation frequency F 2 >F 1 , and wherein the second light is shifted in optical frequency from the first light by a frequency shift f; and,   determining a phase shift between frequency components of an output signal of the COR at the first and second modulation frequencies F 1  and F 2  shifted by the frequency shift f.   
     
     
         22 . The method of  claim 21  comprising computing the GDV based on the phase shift. 
     
     
         23 . The method of  claim 21  wherein the frequency shift f is smaller than the first modulation frequency F 1 . 
     
     
         24 . The method of  claim 21  wherein the first light is generally coherent with the second light. 
     
     
         25 . The method of  claim 21  comprising:
 obtaining, from an output COR signal received from the one or more output ports of the COR, a first time-domain trace corresponding to a frequency component of the output signal at a first shifted modulation frequency (F 1 +f) or (F 1 −f), and a second time-domain trace corresponding to a frequency component of the output signal at a second shifted modulation frequency (F 2 +f) or (F 2 −f); 
 wherein determining the phase shift comprises determining a relative phase between the first and second time-domain traces. 
 
     
     
         26 . The method of  claim 21  including:
 splitting light from a coherent light source to obtain the first and second lights, 
 frequency shifting one of the first and second lights by the frequency shift f, and 
 modulating the first light in amplitude at the first and second modulation frequencies F 1  and F 2 . 
 
     
     
         27 . The method of  claim 21  wherein the second modulation frequency F 2  is a harmonic of the first modulation frequency F 1 , comprising:
 passing the first light through an optical modulator, and 
 applying to the optical modulator a periodic electrical modulation signal at the first modulation frequency F 1  so as to modulate the first light at the first modulating frequency F 1  and the harmonic thereof F 2 . 
 
     
     
         28 . The method of  claim 27  comprising using the optical modulator which transmission characteristic is a non-linear function of an applied electrical signal, and wherein the periodic electrical modulation signal is characterized by a substantially sinusoidal waveform of an amplitude sufficient to cause the first light to be modulated at the first modulation frequency F 1  and the harmonic thereof F 2 . 
     
     
         29 . The method of  claim 21  wherein the second modulation frequency F 2  is a first harmonic of the first modulation frequency F 1 , so that F 2 =2F 1 . 
     
     
         30 . The method of  claim 29  comprising using a Mach-Zehnder modulator that is driven with a substantially sinusoidal electrical modulating signal of an amplitude sufficient to modulate the first light in amplitude at the first modulation frequency F 1  and the second modulation frequency F 2 =2F 1 . 
     
     
         31 . The method of  claim 29  wherein (c) comprises:
 recording a duration of the output signal of the COR in memory; 
 frequency-shifting the recorded duration of the output signal of the COR by the frequency shift f to obtain a frequency-shifted COR signal; and, 
 filtering the frequency-shifted COR signal at the first modulation frequency F 1  and the harmonic thereof F 2  to obtain two time-domain traces. 
 
     
     
         32 . The method of  claim 29  further comprising determining the phase shift for a plurality of values of the first modulation frequency F 1  to obtain a phase response of the COR as a function of frequency, and computing the GDV as a function of frequency. 
     
     
         33 . The method of  claim 21  wherein the first light is provided in a first input optical port of the COR, and the second light is provided in a second input optical port of the COR. 
     
     
         34 . The method of  claim 33  wherein the COR comprises four output ports configured to output in-phase (Ix) and quadrature (Qx) signals of a first polarization, and in-phase (Iy) and quadrature (Qy) signals of a second polarization, the method comprising:
 summing squares of the Ix and Qx signals to obtain a power signal Ax of the first polarization plane ‘X’ of the COR; 
 summing squares of the Iy and Qy signals to obtain a second squared signal Ay of the second polarization plane ‘Y’ of the COR; 
 estimating a time delay Δτ XY  between frequency components of the power signals Ay and Ax at the first modulation frequency F 1  or the second modulation frequency F 2  to determine a polarization skew characteristic of the COR. 
 
     
     
         35 . The method of  claim 33  comprising rotating the polarization of the one of the first and second light so as to measure the GDV of the COR at two orthogonal polarizations.