A highly sensitive, reversible, and linear sensor, exhibiting excellent stability in response to temperature and humidity, has been successfully proposed and demonstrated for the first time. This sensor is achieved by wrapping a polyvinyl alcohol/graphene nanofiber film onto a chiral long-period fiber grating (CLPG), which is fabricated by periodically twisting single mode fiber. In the experiment, the CLPG sensor demonstrates a temperature sensitivity of 74 pm/°C, which is approximately twice as high as that of conventional fiber grating sensors. Note that, by wrapping the graphene nanofiber film on CLPG, the temperature sensitivity of the sensor is up to 115.23 pm/°C in the range of 30°C to 75°C. In addition, CLPG using for humidity sensing is first demonstrated. The humidity sensitivity measures −9.92 pm/%RH with linearity of 0.995 during a change from 40%RH to 80%RH. In comparison to other humidity sensors, the sensitivity of the CLPG is comparable, whereas its sensing linearity stands out notably above the rest. The results show that CLPG has the characteristics of simple fabrication, easy combination with materials, stable performance, and high sensitivity and holds significant development potential in optical fiber sensing application fields.
In this paper, we proposed a highly sensitive temperature and humidity sensors based on two types of physically deformed long-period fiber gratings (LPFGs) which was fabricated by periodically twisting and micro-tapering single- mode fiber, also known as chiral long-period fiber grating (CLPG) and microtapered long-period fiber grating (MTLPFG), respectively. Meanwhile, the superior sensing characteristics of graphene-enhanced CLPG sensor for temperature and polyvinyl alcohol (PVA) coated MTLPFG sensor for humidity are successfully demonstrated. In the temperature experiment, due to the excellent thermal conductivity of graphene, the CLPG transmission spectrum has excellent responsiveness and linearity, and the measured temperature sensitivity is 115 pm/℃. Compared with the traditional fiber gratings, the temperature sensitivity has been significantly improved. Unfortunately, as a humidity measure, fiber grating is insensitive to humidity. Therefore, the MTLPG coated with PVA nanofilms is proposed to realize humidity sensing measurement. Due to the perfect combination of the special micro-nano structure and humidity sensitive film, this humidity sensor obtained excellent sensing sensitivity and linearity. Note that the humidity sensitivity of the PVA-coated MTLPFG has a humidity sensitivity of up to 13.27 pm/%RH. This physically deformed LPFGs are non-degradable, stable and higher sensitive, we believe that it will providing a key role in high-precision temperature and humidity sensing fields.
Spectral confocal technology is widely used in the field of object contour scanning with non-contact measurement. For high-speed collection of spectral confocal signal, the collection speed is not only related to the integration time of the photodetector but limited by the efficiency of reading out the spectral signal from the detector. In order to solve this problem, a spectral confocal signal collection method based on acquisition and tracking algorithm with variable window width is proposed to improve the data collection efficiency. The algorithm improves collection efficiency by only collecting the useful signals in the spectrum. The simulation results show that the signal collection efficiency with the proposed algorithm for the CMOS sensor is improved significantly compared to the conventional method. For smooth object surfaces, the data collection efficiency is improved above 44.5 times. It is proved that the proposed method in this paper providing a novel approach for implementing high-speed collection of spectral confocal signal.
Spectral confocal technology uses chromatic aberration which is generated by a dispersion lens to detect surface shape. The axial dispersion generated by the dispersion lens will affect the measurement range of the whole spectral confocal displacement sensor. The refractive index of an axial GRIN (gradient index, GRIN) lens varies non-homogeneous along the axial direction and is constantly perpendicular to an optical axis of the plane. The paper explored the design of a dispersive objective lens for a spectral confocal displacement sensor based on the GRIN lens. Firstly, the optical power and axial dispersion models of the GRIN lens are established. The axial dispersion can be realized by focusing the light of different wavelengths at different positions of the optical axis. Secondly, based on the optic power and dispersion function of the GRIN lens, the refractive index distribution of the GRIN lens and the simulation design of the dispersive objective lens is obtained by using MATLAB and ZEMAX software respectively. Finally, the GRIN dispersive objective lens is optimized by setting different merit function operands. The experimental results show that the axial GRIN lens can achieve a focal shift of 1130μm within the wavelength range from 486nm to 656nm. Moreover, the linearity of the lens behaves well. All the blur spot is much smaller than the airy spot. The lens has well-focused as well as high-precision. The research results provide a reference and theoretical basis for the application of the GRIN lens in spectral confocal technology.
The chromatic confocal technology (CCT) uses the dispersion principle to establish an accurate encoding relationship between the spatial position and the axial focus point of each wavelength to achieve non-contact measurement. The accuracy of the measurement results is affected by the peak wavelength extraction accuracy. The flexible and adaptable characteristics of machine learning techniques are used to model the spectral wavelength and light intensity nonlinearly, establish the response relationship between input wavelength and output normalized light intensity, and refit the spectral curve distribution. In this paper, we apply the network of regression aspect of machine learning, Extreme Learning Machine (ELM), Back Propagation Neural Network (BPNN), and Genetic Algorithm optimized Back Propagation Neural Network(GA-BPNN) to fit the spectral response of the system to accurately locate the peak wavelength and compare it with the traditional peak extraction methods of Gaussian fitting, polynomial fitting, and center of the mass method to verify that the machine learning method used is significantly better than the traditional peak extraction methods in terms of peak extraction accuracy. The ELM network is the best among the three networks, with a peak extraction error of only 0.04μm and a Root Mean Square Error(RMSE) of only 6.8×10-4. The analysis of calibration experiments, resolution, and stability experiments found that the ELM algorithm was found to have the shortest calculation time, and the system measurement resolution was explored through the ELM algorithm to be about 2μm. The research results of this paper have contributed to the improvement of the system measurement accuracy and measurement efficiency.
The chromatic confocal technology (CCT) has ultra-high distance measurement resolution and the characteristics of multi-surface tomography. It is merely widely used to measure the thickness of uniform materials currently. As a typical inhomogeneous material, the iso-refractive index surface of the radial GRIN lens is a cylindrical surface with central axis symmetry. The radial GRIN lens is an important optical element in the field of micro integrated optical instruments, such as optical fiber sensing, optical communication, etc. High precision thickness measurement parameters help to guide the accurate application of the GRIN lens and control the performance of related ultra-precision optical instruments. To measure the thickness of the radial GRIN lens with a single probe by the advantage of the CCT technology in measuring tiny distances. In addition, the placement tilt of the GRIN lens during the measurement would change the incident position of the probe light entering the lens to change the propagation path of the light and inevitably affect the accuracy of thickness measurement. The influence of the GRIN lens placement tilt on thickness measurement is studied theoretically. The thickness measurement error caused by the inclination of the GRIN lens and its axial measurement position is simulated and analyzed. The research results have significance for optimizing the system structure and further improving the system performance for the application of the CCT in measuring non-homogeneous materials or optical thin film.
The chromatic confocal technology (CCT) has ultra-high distance measurement resolution and the characteristics of multi-surface tomography. Based on the principle of optical dispersion and confocal, the technology achieves accurate axial position or micro-displacement measurement. The radial gradient index (GRIN) lens is a typical important inhomogeneous material. Its refractive index gradually decreases from the center to the edge along the radial direction. We propose to measure the thickness of the radial GRIN lens based on CCT. The thickness measurement model is established by the ray-tracing method, the optical Lagrange function, and the ray arc differential in the Cartesian coordinate system. The refractive index distribution makes the eccentric tilt of the lens change the propagation of the probe light and affects the extraction of the peak of the spectral response causing the thickness measurement error. The influence of eccentricity and tilt of the radial GRIN lens on its thickness measurement is studied. The precision shift table drives the radial GRIN lens movement to simulate the eccentric tilt state of the lens under the dispersion probe. According to the comparative analysis between the experimental and simulation results, it is concluded that the greater the eccentric distance and inclination degree of the radial GRIN lens is, the greater the influence on the thickness measurement is. The larger the lens thickness is, the greater the measurement error is. For the radial GRIN lens with a thickness of 2.36 mm, the measurement error is about 20 μm when the eccentricity is 0.1 mm and the tilt is 6 deg. The research results have significance for optimizing the system structure and further improving the system performance for the application of the CCT in measuring non-homogeneous materials or optical thin film. And the research will encourage the development of GRIN lens instrument preparation and application technology by improving the thickness measurement accuracy and precision of the GRIN lens.
Using rolling shutter effect of the CMOS camera can increase the data rate of camera-based visible light communication (VLC) system. However, when fewer pixel columns represent one bit, the grayscale value fluctuation becomes serious, which leads to data logic not being correctly defined by the threshold, and therefore degrades the demodulation performance. In this paper, a thresholding scheme based on adjacent pixel grayscale value to reduce the influence of grayscale value fluctuation is proposed, which sets threshold using the grayscale values of the forward and backward pixels adjacent to the current pixel. And combining with segment downsampling, experimental results show that the proposed demodulation algorithm can achieve a bit-error-rate (BER) performance well below the forward error correction (FEC) limit of 3.8×10-3 with the bit resolution of 2.962 pixel/bit, and the net data rate can reach 10.56 kbit/s.
In this paper, for the first time, we propose a large-broadband orbital angular momentum (OAM) mode converter based on helical long period fiber grating (HLPG) working at turning point (TP). Owing to the combination of dual-resonance peaks at TP, an OAM mode converter with 3-dB bandwidth of ~287 nm is readily obtained, and a high coupling efficiency of ~100% is achieved. We believe that this OAM mode converter will play a key role in fiber communication and other fields.
As the demand for higher capacity and longer reach of optical access networks is garnering momentum in recent years,
coherent access technology attracts renewed interests in high-capacity optical networking. In this paper, designs of
various coherent access networks are investigated and compared. From our results, it may be the most promising solution with high capacity at optimized cost. When coherent detection is employed in access network, the laser linewidth is a key design issue for optimized performance and cost as laser phase noise is larger with lower bit rate. One of our objectives is to examine the feasibility of employing a conventional DFB as carrier sources. We evaluated the
performance of ultra-dense WDM access networking system. Experimental results show that 2.5GB/s QPSK data can be
transmitted over 35-km SMF-28 with 6.5GHz channel spacing using DFB lasers.
An efficient bit-to-symbol mapping method for star 16-quadrature amplitude modulation (16-QAM) is proposed in this
paper, which is similar to Gray coding for square 16-QAM. With the consideration of optimal ring ratio of constellation,
our simulation results show that a linewidth of 3MHz per laser can be tolerated for star 16-QAM optical coherent
systems with SNR penalty of 1dB at BER of 10-3, which is much better than that of square 16-QAM. Further
optimization possibility with variable number of points on each of the rings for star 16-QAM constellation is also
discussed.
Ultracompact wave plate (UWP) will be one of the key elements in future all-optical photonic integrated circuits (PICs).
In this paper, we propose UWPs based on periodic dielectric waveguides (PDWs) with air holes in conventional
dielectric waveguides. The mode characteristics (for both TE and TM) and birefringence of PDWs are calculated by
plane wave method (PWM). The transmission efficiencies and phase changing of TE and TM waves in PDW are
obtained by finite-difference time-domain (FDTD) scheme. Based on the PDWs, the quarter-wave plates (QWPs) and
half-wave plates (HWPs) are designed. Calculating results show that the proposed PDW has large birefringence (Δn>1)
and can introduce 2π phase difference with a short length being less than λ. The size of low order UWPs are compact.
The transmission efficiency of PDW is improved by taper structure. Based on the taper PDW, ultra-wide band (>100 nm)
achromatic QWP is designed. Profiting from the waveguide guiding, the UWPs have low beam divergence and can be
easily integrated with other photonic components. The UWPs have many potential applications in future PIC systems
such as optical communications, optical measurements and sensors.
In order to achieve higher output powers, double-clad fibers (DCF's) are widely used by optical fiber lasers and amplifiers. In this paper, we present a comprehensive mathematical model for the novel multi-mode (MM) double-clad (DC) Er: Yb co-doped hexagonal fiber, Based on the rate and propagation equations, the pump light and forward and backward-amplified spontaneous emissions (ASE±) light transmission in the fiber are analyzed numerically and measured. The simulative and experimental results show that pump power was absorbed almost completely when the length of the fiber is about 3.5~4 m, the suitable length of the fiber in optical fiber lasers is 2~2.5 m. and the 2 m long fiber emits at 1535 and 1543 nm simultaneously, and the peak-value wavelength of ASE+ changes to the long wavelength with the increase of the pump power. The results investigated are useful for the design of optical fiber lasers.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.