Ultralow-noise microwave extraction from optical frequency combs using photocurrent pulse shaping with balanced photodetection

Abstract

The phase noise of microwaves extracted from optical frequency combs is fundamentally limited by thermal and shot noise, which is inherent in photodetection. Saturation of a photodiode due to the high peak power of ultrashort optical pulses, however, prohibits further scaling of white phase noise by increasing incident optical power. Here we demonstrate that the photocurrent pulse shaping via balanced photodetection, which is accomplished by replacing a single photodiode with a balanced photodetector (BPD) and delaying one of the optical pulses, provides a simple and efficient optical-to-electrical interface to increase achievable microwave power and reduces the corresponding thermal noise-limited phase noise by 6-dB. By analysing contributing noise sources, we also show that the thermal noise floor can reach − 166 dBc/Hz even at a low photocurrent of 2-mA (4-mW optical input per photodiode) when using a p-i-n BPD. This finding may be useful for on-chip microwave generation, which consists of standard p-i-n structure photodiodes with relatively low saturation optical power.

The Photodetection of Ultrashort Optical Pulse Trains for Low Noise Microwave Signal Generation

“…[71,72] The 10 GHz power curves of Figure 7b show the improvement in microwave power after five cascaded interleaver stages (four doublers and one 1/f μ stage), where >25 dB increase in the saturation power is realized. For best power efficiency, the two outputs of the final interleaver stage can illuminate separate detectors and be combined electronically, [73,74] polarization multiplexing can be used in the final interleaver stage, [20,75] or ring interferometer designs can be implemented. [69] The full transfer function of an optical pulse interleaver (defined as the spectrum of the output optical power profile divided by the spectrum of the input optical power profile) is easily derived when the interleaver is recognized as a delay line filter.…”

“…[ 71,72 ] The 10 GHz power curves of Figure 7b show the improvement in microwave power after five cascaded interleaver stages (four doublers and one 1/ f µ stage), where >25 dB increase in the saturation power is realized. For best power efficiency, the two outputs of the final interleaver stage can illuminate separate detectors and be combined electronically, [ 73,74 ] polarization multiplexing can be used in the final interleaver stage, [ 20,75 ] or ring interferometer designs can be implemented. [ 69 ]…”

Integrated Optical Frequency Comb for 5G NR Xhauls

“…Different techniques have been used for OFC generation, including MLL 22 , multiple four-wave mixing 23 , and electro-optics modulators 24 . It is worth highlighting that microwaves and mm-waves generation with attractive performance in terms of phase noise have been achieved by taking advantage of OFCs [25][26][27][28][29] .…”