We proposed an optical-fiber-type broadband cavity ring-down spectroscopy system using wavelength-tunable ultrashort pulsed light. The absorbance of glucose in various concentrations in water was derived from the ring-down plots of intensities of the interference waveforms generated using a Mach–Zehnder interferometer with different optical delay path lengths, which were shifted by an automatic optical switching module. The absorption spectrum of glucose was obtained in the wavelength region from 1620 to 1690 nm by varying the wavelength using wavelength-tunable ultrashort pulsed light, which was generated from a femtosecond pulsed laser and polarization-maintaining fiber. The measurement error of concentration was improved using multiple linear regression analysis of absorption spectra. The results demonstrate that the optical-fiber-type cavity ring-down spectroscopy system has the potential to measure broadband absorption spectra with high sensitivity.
We have measured the temperature of a Si substrate using an optical low-coherence interferometer employing supercontinuum light (SC). The accuracy of temperature measurement and the minimum measurable thickness of a layer are determined by the maximum resolving power of the optical path length of the medium in low-coherence interferometry, which depends on the coherent length defined by the spectrum profile and the wavelength of the light source. Low-noise, ultraflat, and highly coherent SC, generated using ultrashort laser pulses and optical fibers, was used as a light source. The wavelength dispersion of SC on the Si substrate was compensated by using a silicon mirror as a reference mirror, resulting in shaper interference waveforms of SC at the front and back surfaces of Si substrate than those of the superluminescent diode (SLD) light used as a conventional low-coherence light source. The measurement accuracy of the temperature using SC was improved to be ±0.4 °C from ±1.0 °C for the case of using the SLD. The temperatures of the Si substrate and SiO2 thin film were simultaneously measured using SC on an 800-µm-thick Si substrate with an 8.55-µm-thick SiO2 film. The temperature of the thin film, the thickness of which is several micrometers, was measured using SC and a compensation technique of wavelength dispersion using the silicon reference mirror.
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