Simple and Precise Characterization of Full-Field Frequency Response for Coherent TOSA

The performance of high baud-rate fiber optical transmission with the use of advanced modulation formats is sensitive to the imperfection arising in transmitter optical sub-assembly (TOSA), and thus the effective calibration of the TOSA full-field response is essential. However, traditional TOSA frequency response characterization solutions are either inconvenient for the on-field measurement or unable to obtain the phase frequency response (PFR). Here, we report a simple and precise scheme to characterize the full-field frequency response of broadband TOSA, including both amplitude frequency response (AFR) and PFR, when only a single photodetector (PD) with narrow bandwidth is employed. A multiple dual-tone signals with specially designed frequency positions are implemented for electrical-to-optical modulation. Then, a few beat signals after direct detection occur at unique frequencies without the spectral overlapping. Thus, it is feasible to extract the full-field frequency response of broadband TOSA, under the single-shot measurement. Moreover, the in-phase and quadrature (IQ) skew can be obtained by comparing PFRs between I and Q tributaries. In the experiment, we successfully characterize a coherent TOSA with a 10 dB bandwidth of 40 GHz, when a single PD with a 3 dB bandwidth of 2 GHz is used. In comparison with the optical spectrum analyzer (OSA) based scheme, the measured AFR deviation is less than 1 dB over the 10 dB bandwidth. When multiple single-shot measurements are implemented within 1 minute, the fluctuation of measured PFR and IQ skew are less than 0.2 rad and 1 ps, respectively. With the help of the measured full-field frequency response, the coherent TOSA can be calibrated to realize 64 GBaud 16-quadrature amplitude modulation (QAM) back-to-back transmission, when the threshold of 7% hard-decision forward error correction (HD-FEC) is set.