An accurate frequency response characterization method for photoreceivers with optical heterodyne technique is presented in this paper. The characterization is implemented with two single-mode tunable lasers operating near the wavelength of 1.55 m. The errors introduced by extra fixtures as well as laser output fluctuations are considered and calibrated simultaneously. Compared with previous works, the proposed calibration procedures are more complete. Experimental results indicate that the significant improvement in measurement precision has been achieved with the proposed method in the frequency range from 0 to 30 GHz, which proves the proposed frequency response characterization method to be feasible and reliable.Frequency response characterization is very important for evaluating the performance of photoreceivers [1] . In recent two decades, a number of characterization techniques have been developed, such as the swept frequency method [2] , the pulse spectrum analysis [3] , the interferometric FM sideband method [4] , and the white optical noise method [5] . However, for the above mentioned methods, it is limited by either relatively narrow measurable frequency band or complicated configurations [6] . Compared with other frequency response characterization schemes, the optical heterodyne technique [6][7][8][9][10][11][12] , which utilizes the coherence property of optical sources, is more attractive because of its relatively simple configuration, easy operation and possibility of continuous tuning of the beat frequency over a very wide range. But two error sources limit the measurement accuracy: First, the extra connection fixtures (such as microwave probe, bias tee, coaxial cable, etc) will degrade the measured power of the beat signal; Second, for many tunable lasers, the linewidth and power fluctuations with tuning wavelength will lead to the power fluctuations of the measured beat signal. In previous work, the errors introduced by laser output fluctuations have been removed [6] , but the connection fixtures have not been taken into consideration. To obtain accurate characterization of photoreceiver frequency response, more complete calibrations are necessary.This paper proposes precise calibration procedures for photoreceiver frequency response characterization with the optical heterodyne technique. Besides calibrating fluctuations of laser output beams, by eliminating the errors introduced by extra fixtures, the characterization of photoreceiver frequency response can be more accurate. The proposed calibration procedures are more complete than calibration of only one single error source. Experimental results indicate that the significant improvement in measurement accuracy has been achieved in the frequency range from 0 to 30 GHz with the proposed calibration method, which proves the proposed method to be feasible and reliable.The schematic diagram of the measurement system is shown in Fig.
