Wideband wavelength swept lasers (WSLs) are widely used as light sources for dynamic fiber optic sensors. In this study, we implemented an ultra-wideband wavelength-swept laser (WSL) that achieved a 10 dB bandwidth over 430 nm using a single polygonal scanning mirror-based wavelength tunable filter. The wavelength scanning range with a 1.8kHz scanning frequency is 1136.0~1567.2nm. Comparing the WSL output signal in the temporal and spectral domains resulted in an error of 0.7 nm in the mid-crossing region of the two gain media.
To confirm WSL performance, the transmission band was measured by changing the electric field applied to the cholesteric liquid crystal cell, and it was confirmed that the transmitted beam according to the applied electric field matched each other in the spectral and temporal domains.
In this study, the feasibility of a volatile organic compounds (VOC) gas sensor was confirmed through a porous cholesteric liquid crystal film (CLCF) coated on the cross-section of an optical fiber ferrule. The device was fabricated by injecting the CLCF mixture between two ferrules and then UV cured. After separating the two ferrules, porous CLCF was prepared by immersing the CLCF-coated ferrule in acetone. To measure the change in the reflection spectrum of the device for each VOC gas, a broadband wavelength swept laser with a 10dB bandwidth of ~430nm was used. In conclusion, it was found that the reflection band was continuously red-shifted for acetone gas and THF gas.
Many optical devices that can be used in THz systems are continuously being researched and developed. Among them, liquid crystal (LC)-based polarizers and phase shifting devices have also been developed. The LC devices used in the THz system are fabricated in the form of a cell. The substrate of the cell should be transparent with low absorption in the THz frequency range. In the THz frequency band, research and development using various glass materials such as BK7 glass, slide glass, and quartz as substrates have been reported.
In this paper, we report the measurement results of the refractive indices and birefringence of glass materials in the THz band and the possibility of using them as substrates for LC cells. In addition, based on these results, we report the results of measuring refractive indices, absorption coefficient, and birefringence in the THz band after fabricating a LC cell.
Recently, with the development of fabrication technology, sub-nm patterning has been developed, and it has become sufficiently possible to make a nanostructure having a sub-wavelength period. For this reason, metasurface has been a topic of increasing interest in the field of nanophotonics. In this paper, we report the results of realizing mode locking by inserting a metasurface in the form of a saturable absorber into an Yb-doped fiber laser. The metasurface-based saturable absorber has the advantage of reducing the loss due to the deterioration of the existing saturable absorber.
In this study, we report the results of confirming the possibility of optical fiber temperature sensors by fabricating cholesteric liquid crystal (CLC) cells combined with optical fibers. The CLC cell was fabricated with a Fabry-Perot etalon using the cross-sections of two optical fiber ferrules as substrates. A 1.2 um wide bandwidth wavelength swept laser was used to measure the spectrum change of the CLC cell according to the applied temperature. The reflection spectra were measured by changing the temperature of the CLC cell at intervals of 2o from 23o to 45o, and it was confirmed that the reflection band shifted discontinuously to a shorter wavelength as the temperature increased.
We propose and demonstrate a wide-bandwidth unidirectional ring cavity wavelength-swept laser using a polygonal scanning wavelength filter and two semiconductor optical amplifiers (SOAs). The center wavelengths of the gain media of the two SOAs are 1020 nm and 1140 nm, respectively. At a scanning speed of 1.8 kHz of the WSL, a wide scanning band of about 223 nm with a 10-dB bandwidth from 959 nm to 1182 nm and an average optical power of 58.28 mW were obtained. It can be used as a broadband optical source for dynamic optical fiber sensor or optical coherence tomography.
We present an investigation into the characteristics of a normal dispersion mode-locked Yb-doped fiber laser using a birefringent plate as a spectral filter. Before implementing mode locking using a birefringent plate, the transmission characteristics of the birefringent plate were analyzed. It was observed that the center wavelength and the full width at half maximum (FWHM) of the transmitted light varied according to the azimuthal angle and optical axis of the birefringent plate. The total dispersion value of the laser cavity was 0.04673 ps2 at a center wavelength of 1.03 μm. The mode locking was therefore achieved in the normal-dispersion region. The total cavity length of the Yb-doped fiber laser was about 1.9 m including free space, and the repetition rate of the mode-locked pulse was ∼104.2 MHz. The FWHM of the optical spectrum was ∼18.09 nm, and the pulse width measured by an autocorrelator was ∼1.71 ps assuming a Gaussian pulse shape. To obtain the transform limited pulse, a pulse was compressed using a pair of diffraction gratings outside the cavity. We achieved a compressed pulse of ∼148 fs assuming a Gaussian pulse shape. The output characteristics of the mode-locked Yb-doped fiber laser were observed while rotating the optical axis of the birefringent plate in the laser cavity. It was found that the center wavelength and FWHM of the optical spectrum could be varied by rotating the optical axis of the birefringent plate. The center wavelength changed by about 18.7 nm from 1026.9 to 1045.6 nm during rotation of the optical axis of the birefringent plate.
We propose a compact and easy to use photoacoustic imaging (PAI) probe structure using a single strand of optical fiber and a beam combiner doubly reflecting acoustic waves for convenient detection of lymph nodes and cancers. Conventional PAI probes have difficulty detecting lymph nodes just beneath the skin or simultaneously investigating lymph nodes located in shallow as well as deep regions from skin without any supplementary material because the light and acoustic beams are intersecting obliquely in the probe. To overcome the limitations and improve their convenience, we propose a probe structure in which the illuminated light beam axis coincides with the axis of the ultrasound. The developed PAI probe was able to simultaneously achieve a wide range of images positioned from shallow to deep regions without the use of any supplementary material. Moreover, the proposed probe had low transmission losses for the light and acoustic beams. Therefore, the proposed PAI probe will be useful to easily detect lymph nodes and cancers in real clinical fields.
An Nd:YAG laser is generally used as the optical source for photoacoustic imaging (PAI) systems, which support a high-power pulsed laser. However, PAI systems based on Nd:YAG lasers have several disadvantages, such as instability against impact and vibration, poor mobility, and large size. To overcome the limitations, we demonstrate a PAI system using a master oscillator power amplifier (MOPA) fiber laser and an arrayed ultrasound transducer. The fabricated MOPA fiber laser has a variable repetition rate in the range of 1–50 kHz, a pulse width of 10–70 ns, and an output power of 160 μJ. Furthermore, it is stable to impact and vibration, mobile, and compact with a size of 427×250×170 mm3. We achieved PAI 150 times faster using MOPA fiber laser than using Nd:YAG laser with the arrayed ultrasound transducer composed of 128 elements. Therefore, we believe that the PAI system based on the MOPA fiber laser has significant potential for use as a clinical ultrasound imaging system.
We present a terahertz (THz) radiation pumped by a passively mode-locked Yb-doped fiber laser using two fiberpigtailed log-spiral-based low-temperature-grown (LTG) InGaAs photoconductive antenna (PCA) modules. The modelocked fiber laser produces over 220 mW of the average optical power with positively chirped of 1.49 ps pulses. In order to generate THz radiation using the fiber-pigtailed PCA modules, the mode-locked optical pulses are pre-chirped with 538 fs using two diffraction gratings. We successfully achieved THz radiation over 2.0 THz using the pre-chirped pulses. We successfully observed the various absorption lines of water vapor dips in the free space of 120 mm.
We measure the pitch variation of cholesteric liquid crystals (CLCs) according to the applied electric field using a wavelength-swept laser. While the electric field is applied to the CLC, the pitch of the CLC is elongated normal to the direction of electric field. Therefore, the reflection band is shifted to the longer wavelength. When the applied electric field to the CLC cell was over 1.52 V/μm, the reflection band was changed to the longer wavelength of about 75.1 nm. We believe that the dynamic behavior of the CLC can be analyzed if a high-speed wavelength-swept laser is used as an optical source.
KEYWORDS: Acoustics, Single mode fibers, Signal processing, Signal to noise ratio, Data acquisition, Signal detection, Reflectometry, Semiconductor lasers, Semiconductor optical amplifiers, Fiber Bragg gratings
We analyze the relations of parameters in moving average method to enhance the event detectability of phase sensitive optical time domain reflectometer (OTDR). If the external events have unique frequency of vibration, then the control parameters of moving average method should be optimized in order to detect these events efficiently. A phase sensitive OTDR was implemented by a pulsed light source, which is composed of a laser diode, a semiconductor optical amplifier, an erbium-doped fiber amplifier, a fiber Bragg grating filter, and a light receiving part, which has a photo-detector and high speed data acquisition system. The moving average method is operated with the control parameters: total number of raw traces, M, number of averaged traces, N, and step size of moving, n. The raw traces are obtained by the phase sensitive OTDR with sound signals generated by a speaker. Using these trace data, the relation of the control parameters is analyzed. In the result, if the event signal has one frequency, then the optimal values of N, n are existed to detect the event efficiently.
We successfully demonstrate a THz generation using an ytterbium (Yb)-doped mode-locked femtosecond fiber laser and a home-made low-temperature grown (LTG) InGaAs Photoconductive antenna (PCA) module for THz Time-domain spectroscopy (TDS) systems. The Yb-doped fiber ring laser consists of a pump laser diode (PLD), a wavelength division multiplexer (WDM) coupler, a single-mode fiber (SMF), a 25 cm-long highly Yb-doped fiber, two collimators, two quarter wave plates (QWPs), a half-wave plate (HWP), a 10 nm broadband band pass filter, an isolator, and a polarizing beam splitter (PBS). In order to achieve the passively mode-locked optical short pulse, the nonlinear polarization rotation (NPR) effect is used. The achieved center wavelength and the 3 dB bandwidth of the modelocked fiber laser are 1.03 μm and ~ 15.6 nm, respectively. It has 175 fs duration after pulse compression with 66.2 MHz repetition rate. The average output power of mode-locked laser has more than 275 mW. The LTG-InGaAs PCA modules are used as the emitter and receiver in order to achieve the THz radiation. The PCA modules comprise a hyper-hemispherical Si lens and a log-spiral antenna-integrated LTG-InGaAs PCA chip electronically contacted on a printed circuit board (PCB). An excitation optical average pumping and probing power were ~ 6.3 mW and 5 mW, respectively. The free-space distance between the emitter and the receiver in the THz-TDS system was 70 mm. The spectrum of the THz radiation is achieved higher than 1.5 THz.
We propose an electric field sensor using a cholesteric liquid crystal (CLC) Fabry-Perot etalon and a broadband optical source. The CLC cell consists of glass substrates, polyimide layers, electrodes, and CLC layer. There is a threshold behavior for CLC cell and no change in the transmitted wavelength occurs until a threshold value. The threshold value is 0.8 V/μm for fabricated CLC cell in this experiment. The transmitted or reflected wavelength from the CLC Fabry-Perot etalon depends on the applied electric field. The valley wavelengths of the transmitted light from the CLC device are linearly increased from 1303 nm to 1317 nm as the applied electric field to the CLC device is increased from 0.8 V/μm to 1.9 V/μm.
The wavelength swept laser (WSL) is a promising optical source in optical coherence
tomography, optical fiber sensor, and optical beat source generation. It is demonstrated by employing a
narrowband wavelength-scanning filter, such as a fast rotating polygonal-scanner-filter, a diffraction
grating on a galvo-scan mirror, or a fiber Fabry-Perot tunable filter (FFP-TF). In this manuscript, we
present our researches on the dynamic fiber-optic sensors. Two kinds of WSLs are used to demonstrate
the dynamic measurement in the fiber-optic sensors. One is the WSL using a polygon-scanner-based
wavelength filter and the other is the Fourier domain mode-locked (FDML) WSL using a FFP-TF. The
dynamic fiber Bragg grating (FBG) sensor interrogation up to 2 kHz by using the WSL with a polygonscanner-
based wavelength filter is reported. And by using the FDML WSL with a FFP-TF, we
demonstrate a resonance FBG sensor interrogation. As another application of the WSL, we successfully
measure a dynamic modulation frequency of the applied electric field using a nematic liquid crystal
Fabry-Perot etalon.
We propose a fiber-optic dynamic electric field sensor using a nematic liquid crystal (NLC) Fabry-Perot etalon and a
wavelength-swept laser. The transmission wavelength of the NLC Fabry-Perot etalon depends on the applied electric
field intensity. The change in the effective refractive index of the NLC is measured while changing the applied electric
field intensity. It decreases from 1.67 to 1.51 as the applied the electric field intensity is increased. Additionally, we
successfully measure the dynamic variation of the electric field using the high-speed wavelength-swept laser. By
measuring the modulation frequency of the transmission peaks in the temporal domain, the frequency of the modulated
electric field can be estimated.
A dual-wavelength tunable fiber laser is demonstrated using two polymer waveguide Bragg gratings (PWBGs) as a wavelength tunable filter. The wavelength tuning in the laser could be achieved by changing the applied electrical power to the PWBG. The interval of dual wavelength can be tuned by 18.9 nm for the maximum applied electrical power of 100 mW. The corresponding frequency range is 2.375 THz. The variation of the lasing wavelength is linearly proportional to the applied electrical power. The side mode suppression ratio of the dual wavelength laser has more than 35 dB.
We demonstrate the tunable continuous-wave (CW) terahertz generator based on the λ/4 phase-shifted 1.3 μm dual-mode laser diode (DML) and travelling-wave photodiode (TWPD). The DML and TWPD operate as an optical beat source and terahertz photomixer, respectively. The laser diodes (LDs) operating at the 1.3 μm have more suitable characteristics as optical beat sources than the LDs operating at 1.55 μm because of their high efficiency and better thermal stability. The micro-heaters are integrated on top of each DFB LD for mode beat frequency tuning. The fabricated DML was continuously tuned from 230 GHz to 1485 GHz by increasing the temperature of each DFB section independently via integrated micro-heaters. The high-speed TWPD with an InGaAs absorber was designed and fabricated to efficiently generate the photomixing terahertz CW. A complementary log-periodic antenna was integrated with the TWPD to radiate the generated terahertz wave with minimum reflection in the wide frequency range. The terahertz characteristics of the tunable CW terahertz generator based on the DML and TWPD were measured in a fiber-coupled, homodyne terahertz photomixing system. Our results of the tunable CW terahertz generator show the feasibility of a compact and highly efficient CW terahertz spectrometer and imager.
We report the development of a combined dual-modal photoacoustic and optical coherence tomography (PA-OCT) system using a single near-infrared (NIR) supercontinuum laser source which can provide both optical absorption and scattering contrasts simultaneously. By using a small sized pulsed Nd:YAG microchip laser and a photonic crystal fiber, we fabricated a pulsed broadband supercontinuum source from 600 to 1700 nm. Under the same optical hardware system, intrinsically registered PA and OCT images are acquired in a single scanning. In order to demonstrate feasibility of our system, we successfully acquired the PA and OCT images of black and white hairs images at the same time. The black hair was detected in both PA and OCT images, while the white hair appeared only in the OCT image. This result suggests the potential of compact, cost-effective, and simple dual-modal PA-OCT system. Moreover, we believe that this approach will be a key point for commercialization and clinical translation.
We report a performance comparison of fiber Bragg gratings (FBGs) sensor interrogation using two kinds of
wavelength swept lasers. In a wavelength swept laser with a fiber Fabry-Perot tunable filter (FFP-TF), the reflected
signal from the FBGs in temporal domain has a nonlinear response with respect to the sinusoidal modulation signal. The
output of the time delay for the strained FBGs sensor array in the wavelength swept laser with FFP-TF has different
slope coefficient for each FBGs, while the reflected output of the strained FBGs in the wavelength swept laser with
polygon scanning filter has same slope coefficient for each FBGs.
We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated,
low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection
are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The
dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently
tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as
THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based
communication technologies because the well-developed InP-based optoelectronic technologies are already expected to
enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers
(SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled
THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the
CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize
compact, cost-effective, and portable THz spectrometer.
We propose a new method for k-domain linearization using fiber Bragg gratings (FBGs) in a wavelength-swept
source for optical coherence tomography (OCT). A wavelength-swept source with a scanning fiber
Fabry-Perot tunable filter is constructed using a conventional ring laser cavity. Five FBGs are used to
recalibrate the nonlinear response from the wavelength-swept source. We achieved good quality sample
imaging using the k-domain linearization algorithm based on FBGs. The sensitivity at 2 mm is improved by
more than 10 dB after k-domain linearization.
We demonstrate a high speed fiber Bragg grating sensor array interrogation using 1.3 μm Fourier-domain modelocked
(FDML) wavelength-swept laser. The slope coefficients for measured relative wavelength difference and relative
time delay from the static strain are 1.02 pm/μstrain and 0.16 ns/μstrain, respectively. We demonstrate the dynamic
response of the FBG sensor array with 100 Hz modulation strain based on the FDML wavelength swept laser at 25.3 kHz
scanning rate. The FBG sensor interrogation system using FDML wavelength swept laser can be realized for the high
speed and high sensitivity monitoring systems.
Ultra-fast 31.3 kHz interrogation is newly demonstrated for fiber Bragg grating (FBG) sensor system using Fourierdomain
mode-locked (FDML) wavelength-swept laser. Higher speed interrogations are useful for the real-time
monitoring of dynamic sloshing pressure and strain.
A high-speed, broadband Fourier domain mode-locked (FDML) wavelength swept source at center wavelength of 1300 nm for high-resolution and high-speed Fourier domain optical coherence tomography was demonstrated. With two semiconductor optical amplifiers as gain media, the laser is capable of FWHM tuning range of more than 135 nm and the edge-to-edge scanning range of more than 160 nm at 45.6 kHz sweeping rate. The peak power is 11.4 mW for both the forward and backward scans. With the built swept source, a FDOCT system was developed which can achieve 6.6 μm axial resolution in air.
Reflectively tapped fiber-optic delay-line matched filters with metal film reflectors evaporated on cleaved fiber ends have been fabricated. This novel device can provide ultra-high speed signal processing capability, small size, and low loss, and be used as an optical packet address detector for all-optical wavelength division multiplexing switching networks. Using the filters, detection of 2.5 Gb/s, 6-bit packet addresses for optical ring networks has been successfully demonstrated. Experimental and calculation results on correlation outputs of the filters are reported.
A mode locked fiber laser gyroscope (MLFLG) [1] has a laser cavity formed with a fiber amplifier with a planar mirror at one end and a Sagnac loop interferometer at the other end, as shown in figure 1. The Sagnac interferometer functions as a loop mirror, and a phase modulator in the interferometer is used to modulate the reflectivity of the loop mirror.
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