In the last three decades, the terahertz science has achieved great development, possibility to be utilized in a wide variety of fields such as biology, quality control, and security. Especially terahertz time-domain spectroscopy is very useful for nondestructive measurement of materials. Many excellent researchers have approached the commercialization of terahertz technology. However, in the ambient atmosphere, not in the laboratory environment, the signal is distorted due to absorption of water vapor. As a result, unintentional noise is generated in the absorption line, resulting in a great difficulty in spectroscopy. We propose an algorithm that selects a frequency band with no signal distortion and performs spectroscopy. It has an advantage that it can apply algorithms without complex calculation and additional optical components. We measure terahertz time-domain signals of several samples within nitrogen filled chamber, and then the nitrogen was removed to measure the signal in a high relativity humidity environment. We extract optical parameters from the obtained signals. The algorithm is verified by comparison of experimental results and literature value. The values of absorption peaks of samples from our algorithm show good agreement with a literature value. We compared these values and conclude that the thickness was measured and we can check the peak of the absorption line. We find that the proposed algorithm can extract optical parameters even in a high relativity humidity environment.
Fiber Bragg grating is used in a variety of applications. In this study, we suggest compact, cost-effective coherent anti- Stokes Raman spectroscopy which is based on the pulse shaping methods via commercialized fiber Bragg grating. The experiment is performed incorporating a commercialized femtosecond pulse laser system (MICRA, Coherent) with a 100 mm length of 780-HP fiber which is inscribed 50 mm of Bragg grating. The pump laser for coherent anti-Stokes Raman spectroscopy has a bandwidth of 90 nm and central wavelength of 815 nm with a notch shaped at 785 nm. The positive chirped pulse is compensated by chirped mirror set. We compensate almost 14000 fs2 of positive group delay dispersion for the transform-limited pulse at the sample position. The pulse duration was 15 fs with average power of 50 mW, and showed an adequate notch shape. Finally, coherent anti-Stokes Raman signals are observed using a spectrometer (Jobin Yvon Triax320 and TE-cooled Andor Newton EMCCD). We obtained coherent anti-Stokes Raman signal of acetone sample which have Raman peak at the spectral finger-print region. In conclusion, the proposed method is more simple and cost-effective than the methods of previous research which use grating pairs and resonant photonic crystal slab. Furthermore, the proposed method can be used as endoscope application.
Single-pulse coherently controlled nonlinear Raman spectroscopy is the simplest method among the coherent anti-Stokes Raman spectroscopy systems. In recent research, it has been proven that notch-shaped femtosecond pulse laser can be used to collect the coherent anti-Stokes Raman signals. In this study, we applied a fiber Bragg grating to the notch filtering component on the femtosecond pulse lasers. The experiment was performed incorporating a titanium sapphire femtosecond pulse laser source with a 100 mm length of 780-HP fiber which is inscribed 30 mm of Bragg grating. The fiber Bragg grating has 785 nm Bragg wavelength with 0.9 nm bandwidth. We proved that if the pulse lasers have above a certain level of positive group delay dispersion, it is sufficient to propagate in the fiber Bragg grating without any spectral distortion. After passing through the fiber Bragg grating, the pulse laser is reflected on the chirped mirror for 40 times to make the transform-limited pulse. Finally, the pulse time duration was 37 fs, average power was 50mW, and showed an adequate notch shape. Furthermore, the simulation of third order polarization signal is performed using MATLAB tools and the simulation result shows that spectral characteristic and time duration of the pulse is sufficient to use as an excitation source for single-pulse coherent anti-Stokes Raman spectroscopy. In conclusion, the proposed method is more simple and cost-effective than the methods of previous research which use grating pairs and resonant photonic crystal slab.
This paper presents a method to obtain spectral ring-down information simultaneously via double-looped Mach-Zehnder interferometry. The symmetric split-step Fourier method was used to simulate the pulse propagation through the fiber loop to check fiber-induced effects on the pulse. The loop was built with a segment of fiber, output coupler, and fiber Bragg grating. The fiber Bragg grating was used as an artificial sample. We found that the pulse spectrum is not distorted as the pulse propagates the fiber loop. We have also demonstrated the possibility to extract the ring-down signals from spectrum variation via simulation. This approach can be applied to sample detection which requires high sensitivity and high spectral resolution.
The concentration of trace sample can be determined by, and therefore an indicator of, the absorption property in fiber-loop ring-down spectroscopy. The aim of this study was to investigate the use of an ultrashort-pulse laser as a source of the fiber-loop ring-down spectroscopy and its applicability for loss-compensated fiber loops. A composite-type carbon-nanotube saturable absorber mode-locked fiber laser was built to be used as a source of experimental setup. The simulation has been done to check dispersion effects on ring-down signals, and it was found that the initial pulse duration affected ring-down information. To avoid this limitation, we just used the chirped pulses without any compensation process in the experiment. A loss-compensated fiber loop was constructed to measure the concentration of water in water/acetone mixture samples. The measurements were shown to lead to an accurate determination of the ring-down time. The result indicates that it is possible to measure the sample-induced absorption with the ultrashort-pulse laser source and loss-compensated fiber loop to determine the concentration of the sample.
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