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.
In this paper, we theoretically and experimentally analyze the influence of the round-trip phase of distributed feedback (DFB) lasers on the harmonic-phase of photocurrent, caused by the external injection current. The results indicate that the harmonicphase of photocurrent introduced by the round-trip phase (HPIRP) reduces with the increase of modulation depth at a fixed frequency. Then, based on the HPIRP of the DFB laser, a simple photonic generation method of optical microwave waveforms is proposed and verified. In this scheme, three lasers with different wavelengths are mainly used. By adjusting the tunable optical delay lines and attenuators, the amplitude and phase of the photocurrent of the three branches converted by the photodetector (PD) can be controlled individually. The photocurrent of the superposition of the three branches can be controlled properly to form the target waveform. Dispersive elements and complex microwave photonic filtering are not required. Through experiments, rectangular and triangular waveforms with a repetition frequency of 3.75 GHz are generated.
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