Improvement of group delay measurement accuracy using a two-frequency modulation phase-shift method

New Modulation Zero-Shift Method to Characterize Fast Group Delay Ripple of Dispersion-Compensating Fiber Bragg Gratings

“…We have obtained a ripple of about 3-pm period and 20-ps amplitude. When comparing these results to those in [4], we see that we have found similar values of GDR amplitude but we have been able to resolve an almost four times smaller period.…”

“…The phase-shift differences between the respective two upper sidebands and that for the lower sidebands (see Fig. 2) are given by (4) Corresponding group delays at , and at , come from (5) Care has been taken when measuring the because as the modulating frequency is increased, the DC-FBG-induced phase shifts may impact the detected photocurrent causing it to drop below required levels for correct phase determination if the bias voltage is kept at the quadrature point. In our experiments, the algorithm that obtains the voltage bias for minimum photocurrent also finds those best suited for phase measurements, i.e., the ones providing maximum detected photocurrent at .…”

“…The modulation phase-shift method (MPSM) is well established as an all-purpose dispersion measurement and it gives the basis of compact laboratory equipment of increasingly extended use [4]. It produces excellent results in the characterization of long, broadband optical systems such as optical fibers.…”

“…For characterizing fast variations in the group delay response, the sidebands should be close to the carrier, limited by the device-induced phase shift dropping below the measurement phase noise. The choice of is then subject to a trade-off between wavelength accuracy and amplitude uncertainty limiting the application of MPSM to the characterization of fast GDR [4], [6]. A further limitation comes from the stability and accuracy of the tunable laser used to scan the optical spectrum [7].…”

Phase and Amplitude Responses of Narrowband Optical Filter Measured by Microwave Network Analyzer

“…Low modulation frequency gives better frequency accuracy but limits the phase accuracy. Dual-frequency phaseshift method may obtain better accuracy [4].…”