Ultrafast laser frequency microcombs provide equidistant coherent frequency markers over a broad spectrum, enabling new frontiers in chip-scale frequency metrology, laser spectroscopy, dense communications, precision metrology. Measuring and understanding the fundamental noise parameters in these high-clock-rate frequency microcombs are essential to advance the underlying physics and the precision microwave-optical clockwork. In this talk we describe the noise characteristics and timing jitter in adiabatic laser frequency microcombs. We compare and contrast the fundamental noise and fluctuation parameters for a series of laser frequency microcomb states, from multiple soliton to soliton crystals and single-soliton regimes. Each of the noise families and their noise coupling mechanisms are examined with our theoretical models. This aids the understanding of frequency, intensity and phase noise characteristics of frequency microcombs towards the precision limits.
In this talk, first, we describe chip-scale coherent mode-locking in microresonator frequency combs, verified by interferometric femtosecond timing jitter measurements and phase-resolved ultrafast spectroscopy. Normal dispersion sub-100-fs mode-locking is also observed, supporting by nonlinear modeling and analytics. Second we describe the noise limits in full microcomb stabilization, locking down both repetition rate and one comb line against a reference. Active stabilization improves the long-term stability to an instrument-limited residual instability of 3.6 mHz per root tau and a tooth-to-tooth relative frequency uncertainty down to 50 mHz and 2.7×10−16. Third we describe graphene-silicon nitride hybrid microresonators for tunable frequency modes, variants of soliton mode-locked states and crystals, and controllable Cerenkov radiation. Our studies provide a platform towards precision spectroscopy, frequency metrology, timing clocks, and coherent communications.
Absolute distance measurement (ADM) with high precision is required for various fields of precision engineering, which has long been implemented by means of time-of-flight measurement of a pulsed laser, intensity or frequency modulation of a continuous-wave laser, and cross-correlation of pseudo-random micro-wave signals. Recently, in response to increasing demands on the measurement precision and range beyond conventional limits, femtosecond pulse lasers began to draw attention as a new light source that permits realizing various advanced ADM principles such as synthetic radiofrequency wavelength generation, Fourier-transform-based dispersive analysis and multi-wavelength interferometry. In this talk, we present the state-of-the-art measurement principles and performance demonstrated by exploiting the unique temporal and spectral characteristics of femtosecond laser pulses for high-precision ADM applications.
We measure absolute distances by performing multi-wavelength interferometry (MWI) using four different wavelengths generated simultaneously from the frequency comb of a femtosecond laser. The measurement precision is estimated to be less than 63 nm in peak-to-valley over a distance of 1 m as compared to an incremental HeNe laser interferometer. We also evaluate the operational stability and robustness of the interferometer hardware system over a time period of 12 hours. Finally, it is concluded that the proposed frequency-comb-referenced multi-wavelength interferometry is capable of providing fast, precise and high stable absolute distance measurements, being well suited for industrial precisionengineering applications and near-future space missions.
We revisit the method of synthetic wavelength interferometry (SWI) for absolute measurement of long distances using the radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser. Our intention here is to extend the nonambiguity range (NAR) of the SWI method using a coarse virtual wavelength synthesized by shifting the pulse repetition rate. The proposed concept of NAR extension is experimentally verified by measuring a ∼13-m distance with repeatability of 9.5 μm (root-mean-square). The measurement precision is estimated to be 31.2 μm in comparison with an incremental He–Ne laser interferometer. This extended SWI method is found to be well suited for long-distance measurements demanded in the fields of large-scale precision engineering, geodetic survey, and future space missions.