43 research outputs found
Dual terahertz comb spectroscopy with a single free-running fibre laser
Dual THz comb spectroscopy has the potential to be used as universal THz
spectroscopy with high spectral resolution, high spectral accuracy, and broad
spectral coverage; however, the requirement for dual stabilized femtosecond
lasers hampers its versatility due to the bulky size, high complexity, and high
cost. We here report the first demonstration of dual THz comb spectroscopy
using a single free-running fibre laser. By tuning the cavity-loss-dependent
gain profile with an intracavity Lyot filter together with precise management
of the cavity length and dispersion, dual-wavelength pulsed light beams with
slightly detuned repetition frequencies are generated in a single laser cavity.
Due to sharing of the same cavity, such pulsed light beams suffer from
common-mode fluctuation of the repetition frequency, and hence the
corresponding frequency difference between them is passively stable around a
few hundred hertz within millihertz fluctuation. This considerably stable
frequency difference enables dual THz comb spectroscopy with a single
free-running fibre laser. While greatly reducing the size, complexity, and cost
of the laser source by use of a single free-running fibre laser, the dual THz
comb spectroscopy system maintains a spectral bandwidth and dynamic range of
spectral power comparable to a system equipped with dual stabilized fibre
lasers, and can be effectively applied to high-precision spectroscopy of
acetonitrile gas at atmospheric pressure. The demonstrated results indicate
that this system is an attractive solution for practical applications of not
only THz spectroscopy but also THz-pulse-based measurements.Comment: 29 pages, 7 figure
Scan-less full-field fluorescence-lifetime dual-comb microscopy using two-dimensional spectral mapping and frequency multiplexing of dual-optical-comb beats
Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool for
quantitative fluorescence imaging because fluorescence lifetime is independent
of concentration of fluorescent molecules or excitation/detection efficiency
and is robust to photobleaching. However, since FLIM is based on point-to-point
measurements, mechanical scanning of a focal spot is needed for forming an
image, which hampers rapid imaging. In this article, we demonstrate scan-less
full-field FLIM based on a one-to-one correspondence between two-dimensional
(2D) image pixels and frequency-multiplexed RF signals. A vast number of
dual-optical-comb beats between dual optical frequency combs is effectively
adopted for 2D spectral mapping and high-density frequency multiplexing in
radio-frequency region. Bimodal images of fluorescence amplitude and lifetime
are obtained with high quantitativeness from amplitude and phase spectra of
fluorescence RF comb modes without the need for mechanical scanning. The
proposed method will be useful for rapid quantitative fluorescence imaging in
life science.Comment: 38 pages, 8 figures, 1 tabl
Multicascade-linked synthetic wavelength digital holography using an optical-comb-referenced frequency synthesizer
Digital holography (DH) is a promising method for non-contact surface
topography because the reconstructed phase image can visualize the nanometer
unevenness in a sample. However, the axial range of this method is limited to
the range of the optical wavelength due to the phase wrapping ambiguity.
Although the use of two different wavelengths of light and the resulting
synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range
up to a few tens of microns, this method is still insufficient for practical
applications. In this article, a tunable external cavity laser diode
phase-locked to an optical frequency comb, namely, an optical-comb-referenced
frequency synthesizer, is effectively used for multiple synthetic wavelengths
within the range of 32 um to 1.20 m. A multiple cascade link of the phase
images among an optical wavelength (= 1.520 um) and 5 different synthetic
wavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) enables the
shape measurement of a reflective millimeter-sized stepped surface with the
axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the
maximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves
1.7*10^8, which is much larger than that of previous synthetic wavelength DH.
Such a wide axial dynamic range capability will further expand the application
field of DH for large objects with meter dimensions.Comment: 19 pages, 7 figure
Optical image amplification in dualcomb microscopy
Dual-comb microscopy (DCM), based on a combination of dual-comb spectroscopy (DCS) with two-dimensional spectral encoding (2D-SE), is a promising method for scan-less confocal laser microscopy giving an amplitude and phase image contrast with the confocality. However, signal loss in a 2D-SE optical system hampers increase in image acquisition rate due to decreased signal-to-noise ratio. In this article, we demonstrated optical image amplification in DCM with an erbium-doped fiber amplifier (EDFA). Combined use of the image-encoded DCS interferogram and the EDFA benefits from not only the batch amplification of amplitude and phase images but also significant rejection of amplified spontaneous emission (ASE) background. Effectiveness of the optical-image-amplified DCM is highlighted in the single-shot quantitative nanometer-order surface topography and the real-time movie of polystyrene beads dynamics under water convection. The proposed method will be a powerful tool for real-time observation of surface topography and fast dynamic phenomena
Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography
We construct a full-field phase-shifting terahertz digital holography (PS-THz-DH) system by use of a THz quantum cascade laser and an uncooled, 2D micro-bolometer array. The PS-THz-DH enables us to separate the necessary diffraction-order image from unnecessary diffraction-order images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-μm axial resolution. Also, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. Furthermore, the internal stress distribution of an externally compressed object is visualized from the phase image. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials
Improvement of dynamic range and repeatability in refractive-index-sensing optical comb by combination of saturable-absorber-mirror mode-locking with intracavity multi-mode interference fiber sensor
Mode-locked fiber comb equipped with multi-mode-interference fiber sensor functions as high-precision refractive-index (RI) sensor benefitting from precise radio-frequency measurement. However, its dynamic range and repeatability are hampered by inherent characteristics in nonlinear-polarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with wide dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness to cavity disturbance, self-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10×10-8 every mode-locking activation. Improved dynamic range and repeatability will be useful for enhanced performance of RI sensing
Dual terahertz comb spectroscopy with a single free-running fibre laser
Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications