Photonic scheme for broadband frequency response measurement of PDs

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 2f 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 2f ₁, 2f ₁-2f _m, and 2f ₁+2f _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 2f ₁+2f _m(max) and has the capacity of self-calibration to eliminate the influence caused by the modulator frequency response on the measurement results.