[Paper Review] Nanophotonic soliton-based microwave synthesizers

Junqiu Liu, Erwan Lucas+8 more|arXiv (Cornell University)|Jan 29, 2019

Advanced Fiber Laser Technologies57 references58 citations

TL;DR

The paper demonstrates integrated Si3N4 soliton microcombs that operate in the X- and K-band microwaves, enabling low-noise microwave synthesis and injection-locked repetition rates on a CMOS-compatible platform.

ABSTRACT

Microwave photonic technologies, which upshift the carrier into the optical domain to facilitate the generation and processing of ultrawide-band electronic signals at vastly reduced fractional bandwidths, have the potential to achieve superior performance compared to conventional electronics for targeted functions. For microwave photonic applications such as filters, coherent radars, subnoise detection, optical communications and low-noise microwave generation, frequency combs are key building blocks. By virtue of soliton microcombs, frequency combs can now be built using CMOS compatible photonic integrated circuits, operated with low power and noise, and have already been employed in system-level demonstrations. Yet, currently developed photonic integrated microcombs all operate with repetition rates significantly beyond those that conventional electronics can detect and process, compounding their use in microwave photonics. Here we demonstrate integrated soliton microcombs operating in two widely employed microwave bands, X- and K-band. These devices can produce more than 300 comb lines within the 3-dB-bandwidth, and generate microwave signals featuring phase noise levels below 105 dBc/Hz (140 dBc/Hz) at 10 kHz (1 MHz) offset frequency, comparable to modern electronic microwave synthesizers. In addition, the soliton pulse stream can be injection-locked to a microwave signal, enabling actuator-free repetition rate stabilization, tuning and microwave spectral purification, at power levels compatible with silicon-based lasers (<150 mW). Our results establish photonic integrated soliton microcombs as viable integrated low-noise microwave synthesizers. Further, the low repetition rates are critical for future dense WDM channel generation schemes, and can significantly reduce the system complexity of photonic integrated frequency synthesizers and atomic clocks.

Motivation & Objective

• Motivate the need for low-noise, integrated microwave sources for radar, communications, and timing applications.
• Show that soliton microcombs on a Si3N4 platform can reach microwave repetition rates with practical pump powers.
• Overcome material and fabrication limits (low Q, thermal effects) to enable on-chip soliton generation at f_rep < 20 GHz.
• Characterize phase noise and stabilization strategies to approach or surpass electronic microwave sources.
• Demonstrate soliton injection-locking to external microwaves to improve long-term stability.

Proposed method

• Fabricate high-Q Si3N4 microresonators with the Damascene reflow process to achieve ultralow loss.
• Pattern with deep-UV stepper lithography to reduce stitching errors andCrack-preventing stress-release patterns.
• Operate resonators to generate single solitons at X- and K-band repetition rates and measure spectra (3-dB bandwidth, comb lines).
• Characterize dispersion and linewidth via frequency-comb-assisted diode laser spectroscopy.
• Measure soliton phase noise with a fast photodetector and phase-noise analyzer; explore cavity-pump detuning stabilization and quiet-point operation.
• Demonstrate soliton injection-locking to an external microwave source to assess spectral purification and coherence.

Experimental results

Research questions

• RQ1 Can integrated Si3N4 soliton microcombs operate at microwave X- and K-band repetition rates with low pump powers compatible with on-chip lasers?
• RQ2 What are the phase-noise performance and stabilization strategies achievable for soliton-based microwave carriers on a photonic chip?
• RQ3 Can soliton repetition rates be stabilized or disciplined by injection-locking to external microwave signals to improve long-term stability?
• RQ4 How do fabrication choices (Q, coupling, dispersion) impact soliton generation, threshold, and spectral properties on chip-scale resonators?
• RQ5 What is the potential for dense WDM channel generation and system-level integration using low-repetition-rate solitons?

Key findings

• Single solitons at 19.6 GHz repetition rate achieved with 38 mW on-chip power (76 mW in input fiber); spectra show 11.0 nm 3-dB bandwidth (red sample A).
• Single solitons at 19.6 GHz with 210 mW input power yield 26.9 nm 3-dB bandwidth and 170 comb lines (sample B).
• Single solitons at 9.78 GHz repetition rate achieved with 125 mW (sample C) and 340 mW (sample D); 3-dB bandwidths of 17.4 nm (139 lines) and 25.8 nm (327 lines).
• Measured SSB phase noise of the microwave carrier: about −80 dBc/Hz at 1 kHz, −110 dBc/Hz at 10 kHz, and −130 dBc/Hz at 100 kHz offset (case with stabilized detuning).
• Demonstrated cavity-pump detuning stabilization (offset PDH) and power stabilization to reduce low-frequency drifts; a quiet point around 439 MHz detuning yields best phase noise.
• Soliton injection-locking to an external microwave source achieved a lock range of roughly <40 kHz, enabling spectral purification above ~10 kHz offset and synchronization of f_rep to f_inj.