A dual-comb nonlinear asynchronous optical sampling method is proposed to simplify determination of the time interval and extend the non-ambiguity range in absolute length measurements. Type II second harmonic generation facilitates curve fitting in determining the time interval between adjacent pulses. Meanwhile, the non-ambiguity range is extended by adjusting the repetition rate of the signal laser. The performance of the proposed method is compared with a heterodyne interferometer. Results show that the system achieves a maximum residual of 100.6 nm and an uncertainty of 1.48 μm in a 0.5 ms acquisition time. With longer acquisition time, the uncertainty can be reduced to 166.6 nm for 50 ms and 82.9 nm for 500 ms. Moreover, the extension of the non-ambiguity range is demonstrated by measuring an absolute distance beyond the inherent range determined by the fixed repetition rate.

We present a frequency-sweeping heterodyne interferometer to measure an absolute distance based on a frequency-tunable diode laser calibrated by an optical frequency comb (OFC) and an interferometric phase measurement system. The laser frequency-sweeping process is calibrated by the OFC within a range of 200 GHz and an accuracy of 1.3 kHz, which brings about a precise temporal synthetic wavelength of 1.499 mm. The interferometric phase measurement system consisting of the analog signal processing circuit and the digital phase meter achieves a phase difference resolution better than 0.1 deg. As the laser frequency is sweeping, the absolute distance can be determined by measuring the phase difference variation of the interference signals. In the laboratory condition, our experimental scheme realizes micrometer accuracy over meter distance.

In this paper, we propose an improved subtraction algorithm for rapid recovery of Raman spectra that can substantially reduce the computation time. This algorithm is based on an improved Savitzky-Golay (SG) iterative smoothing method, which involves two key novel approaches: (a) the use of the Gauss-Seidel method and (b) the introduction of a relaxation factor into the iterative procedure. By applying a novel successive relaxation (SG-SR) iterative method to the relaxation factor, additional improvement in the convergence speed over the standard Savitzky-Golay procedure is realized. The proposed improved algorithm (the RIA-SG-SR algorithm), which uses SG-SR-based iteration instead of Savitzky-Golay iteration, has been optimized and validated with a mathematically simulated Raman spectrum, as well as experimentally measured Raman spectra from non-biological and biological samples. The method results in a significant reduction in computing cost while yielding consistent rejection of fluorescence and noise for spectra with low signal-to-fluorescence ratios and varied baselines. In the simulation, RIA-SG-SR achieved 1 order of magnitude improvement in iteration number and 2 orders of magnitude improvement in computation time compared with the range-independent background-subtraction algorithm (RIA). Furthermore the computation time of the experimentally measured raw Raman spectrum processing from skin tissue decreased from 6.72 to 0.094 s. In general, the processing of the SG-SR method can be conducted within dozens of milliseconds, which can provide a real-time procedure in practical situations.

Coherent Raman microscopy provide label-free imaging by interrogating the intrinsic vibration of biomolecules.Nevertheless, trade-off between high chemical-specificity and high imaging-speed currently exists in transition from spectroscopy to spectroscopic imaging when capturing dynamics in complex living systems. Here, we present a novel concept in dual-comb scheme to substantially beat this trade-off and facilitate high-resolution broadband coherent antiStokes Raman spectroscopic imaging based on down-converted, automatically varying delay-time in spectral focusing excitation. A rapid measurement of vibrational microspectroscopy on sub-microsecond scale over a spectral span ~700 cm -1 with solid signal-to-noise ratio provides access to well-resolved molecular signatures within the fingerprint region. We demonstrate this high-performance spectroscopic imaging of spatially inhomogeneous distributions of chemical substances as well as the carotenoids accumulation in plant cells. This technique offers an unprecedented route for broadband CARS imaging with hundreds of kHz frame rate using available 1-GHz oscillators.

Silicon carbide (SiC), which is the leading representative of the third-generation of semiconductors, possesses many excellent physical properties. However, its advantages also incur difficulties in processing, which calls for special processing techniques, such as femtosecond laser machining. In addition, SiC has shown unprecedented potential for optoelectronic applications. Knowledge of the nonlinear absorption coefficient and the nonlinear refractive index coefficient of SiC is required in both of the fields. In this work, the nonlinear absorption coefficient and the nonlinear refractive index coefficient of semi-insulating (SI) 6H-SiC and SI 4H-SiC, the most pervasive SiC polytypes, are measured in an extensive spectral range from 400 nm to 1000 nm with the Z-scan technique. Besides, the spectral dependence of the nonlinear optical properties is analyzed, facilitated by linear absorption spectrum. Especially, two-photon absorption (2PA) and three-photon absorption (3PA) coefficients of SI 6H-SiC and SI 4H-SiC are characterized in the respective spectral ranges. From the characterization of SiC, we can observe self-focusing phenomenon for nonlinear refraction. In the end, we unravel the potential of SiC for ultrafast all-optical switching based on the measured nonlinear optical properties.