Abstract
We report a scheme to measure the broadband frequency response of photodetectors (PDs) with the capacity of self-calibration. The optical carrier at ${f_0}$ is modulated through one modulator, and this modulator is driven by a microwave signal with a fixed frequency ${f_1}$ to produce the carrier-suppressed double sideband optical signals ${f_0} - {f_1}$ and ${f_0} + {f_1}$. The frequency interval of the optical signals is ${2}{f_1}$. Subsequently, the two optical signals are sent to another modulator driven by a swept microwave signal ${f_m}$. Two pairs of carrier-suppressed double sideband signals with a high signal-to-noise ratio are generated. The frequencies of these signals are ${f_0} - {f_1} - {f_m},\;{f_0} - {f_1} + {f_m}$, ${f_0} + {f_1} - {f_m}$, and ${f_0} + {f_1} + {f_m}$. After the PD under test, the frequency response can be extracted from the amplitude of the microwave signals at ${2}{f_1}$, ${2}{f_1} - {2}\;{f_m}$, and ${2}{f_1} + {2}{f_m}$. Two PDs in our laboratory are experimentally characterized from 0.1 to 40 GHz with a resolution of 100 MHz. Compared with the traditional vector network analyzer swept method, the proposed method extends the measurement range from ${f_m}({\max})$ to ${2}{f_1} + {2}{f_m}({\max})$ and has the capacity of self-calibration to eliminate the influence caused by the modulator frequency response on the measurement results.
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