Abstract
We demonstrate the use of two dual-output Mach-Zehnder modulators (DO-MZMs) in a direct comparison between a femtosecond (fs) pulse train and a microwave signal. Through balanced detection, the amplitude-to-phase modulation (AM-PM) conversion effect is suppressed by more than 40 dB. A cross-spectrum technique enables us to achieve a high-sensitivity phase noise measurement (−186 dBc/Hz above 10-kHz offset), which corresponds to the thermal noise of a +9 dBm carrier. This method is applied to compare a 1-GHz fs monolithic laser to a 1-GHz microwave signal generated from photodetection of a free-running 500 MHz mode-locked laser. The measured phase noise is −160 dBc/Hz at 4-kHz, −167 dBc/Hz at 10-kHz, and −180 dBc/Hz at offset frequencies above 100-kHz. The measurement is limited by the free-running 500-MHz laser’s noise, the flicker noise of the modified uni-traveling carrier photodiode and the thermal noise floor, not by the method itself. This method also has the potential to achieve a similar noise floor even at higher carrier frequencies.
Highlights
Great progress in the generation of ultra-low phase noise optical pulse trains and associated optical frequency-division (OFD)-based microwave generation[1,2,3,4,5] has enabled novel approaches to radar applications[6], and very-long baseline interferometry[7], high-speed analog-to-digital conversion[8] and timing distribution at large-scale scientific facilities such as next-generation X-ray free-electron lasers[9,10,11]
For a single dual-output Mach-Zehnder modulators (DO-MZMs), the 1/f slope from 2 kHz to 10 kHz is caused by the flicker noise of the DO-MZM and the balanced photodiode[24], and the noise floor above 10 kHz is due to the shot noise
Note that carrier scattering in the modified uni-traveling carrier photodiode (MUTC) with long pulse duration may limit the noise floor of the OFD25
Summary
The output voltage of the balanced photoreceiverV(∆φ) is proportional to ∆φ and crosses zero as shown in (iv), which makes the phase detector insensitive to AM on the microwave signal and the reference femtosecond laser to first order. That is, when the discrimination signal is locked to the zero-crossing point (Fig. 1(b) (ii)), the output voltage of the balanced detector does not depend on the amplitude of the microwave or the reference laser. When the peak-to-peak value of the signal from the balanced detector is 10 mV around 0 V with the full scale of the discriminator of 1 V, the AM suppression can be roughly estimated to be −40 dB The sensitivity of this phase detector depends on the optical and microwave powers. For the DO-MZM method, an ultra-low noise monolithic fs laser with a fundamental repetition rate of 1 GHz20 was used as the timing reference. The phase noise PSD was measured by a calibrated, commercial signal analyzer (SA: Keysight, PXA N9030A) to make sure our calibration was correct
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