In this paper, alcohol solution is added into the center hole of PCF-SPR sensor to realize the dual parameter sensor of temperature and refractive index (RI) simultaneously. Firstly, the external temperature is fixed at 20 °C, and the RI of the alcohol solution is changed. The resonance wavelength sensitivity is -6015.4 nm/RIU and the linearity is 0.99985 when the RI of alcohol solution is from 1.34 to 1.3654 with a resonance-loss sensitivity of -2640 dB/RIU and a linearity of 0.99475, respectively. When the concentration of alcohol solution is fixed at 70.4% and the external temperature of PCF is changed, the resonance loss sensitivity is 1.0375 dB/°C and the linearity is 0.976 when the external temperature is changed between 20 °C and 80 °C, its resonance loss sensitivity is 2.4219 nm/°C and the linearity is 0.9997. Finally, the sensor matrix of temperature and refractive index with respect to the shift of resonant wavelength and the change of resonant loss is obtained.
In this paper, we propose an optical fiber pH sensor with homemade poly (acrylic acid-co-acrylamide) hydrogel coating based on Mach-Zehnder interferometer (MZI), which is designed via concatenating no-core fiber, few-mode fiber and no-core fiber in sequence. The fiber surface of the sensing region is silanizated to enhance the adhesion of the hydrogel onto the fiber. When the pH value of the solution varied, the amount of free carboxylic acid ions in the hydrogel changes. This change causes the hydrogel to expand or contract, resulting in a change in refractive index. Such a refractive-index change of the hydrogel alters the effective refractive index of the cladding layer of the NFN MZI, leading to a spectral shift of the MZI. The sensor can be employed to measure a wide pH range of 2 to 12. According to the relationship between the pH values and spectral dips, the corresponding sensitivities are -0.346 nm/pH and -64 pm/pH at the pH ranges of 2-5 and 5-12, respectively. Meanwhile, the coefficients of determination of the fitting lines are both large than 0.973, which indicated that the sensor possesses an excellent linearity. Moreover, the response time of is 21.78 s. The results suggest that the proposed sensor exhibits a good repeatability and stability and holds considerable potential in fields such as environmental monitoring and chemical analysis.
KEYWORDS: Networks, X-rays, Anatomy, Education and training, Diseases and disorders, Optical engineering, Medical imaging, Lithium, Integrated circuits, Head
Dental oral disease is one of the most prevalent diseases worldwide, as of a medical analysis in The Lancet 2022[1]. The most common oral diseases worldwide are dental caries (cavities), periodontal disease, tooth loss, and overdevelopment of the jaw caused by excessive unilateral chewing. Dental radiography plays a very important role in clinical diagnosis, treatment and surgery. Automatic segmentation of medical lesions is a prerequisite for efficient clinical analysis. Therefore, accurate positioning of anatomical landmarks is a crucial technique for clinical diagnosis and treatment planning. In this paper, we propose a novel deep network to detect anatomical landmarks. Our proposed network consists of a multi-scale feature aggregation module for channel attention and a deep network for feature refinement. To demonstrate the superiority of our network, training comparisons with several popular networks are performed on the same dataset. The end result is that our network outperforms several popular networks today in both mean radial error (MRE) and successful detection rate (SDR).
Combining the respective advantages of Nd:YAG and Nd:YVO4 crystals in achieving the 1.06-μm band laser, we proposed and demonstrated a highly efficient acousto-optically (AO) Q-switched dual-crystal hybrid gain laser. It not only had excellent power performance but also exhibited satisfactory polarization characteristics across a broad pulse repetition frequencies (PRFs) range. Under 42.00-W incident pump power, the maximum average output powers were 12.10 W and 19.64 W at the PRFs of 10 kHz and 200 kHz, respectively. The corresponding optical-to-optical efficiency rose from 28.81% to 46.76%. The results were significantly better than those of the conventional single-crystal laser in our control experiments. The laser polarization ratios at maximum average output power were ~7.6:1 and ~11:1 when the PRFs were 10 kHz and 200 kHz, respectively.
An all-fiber sensor for heart rate monitoring is proposed and demonstrated based on the disturbance of the evanescent field in the no-core fiber (NCF). The sensing structure is realized through splicing a piece of single mode fiber (SMF) at the ends of the NCF, respectively. When a broad-band light is injected into the structure, the vibration of the pulse signal applied to the sensing structure will lead to the disturbance of the evanescent field in the NCF and modulate the intensity of the optical output power. Therefore, when the sensing structure is placed at the wrist of a human, it can be used to monitor the heart rate. It is demonstrated that a standard electrocardiogram (ECG) signal can be obtained when 30-mm long NCF is used in monitoring the heart rate. According to the measured ECG signals, the proposed sensor can have a response to the heart pulse at different rates ranging from 60 beats per minute (bpm) to 120 bpm.
An all-fiber optic high-sensitivity displacement sensor based on 45°-spliced PM Lyot filter is proposed and its sensing performance is investigated experimentally. According to the relationships between the dips and the displacements, the sensor has a good linearity in passive mode, whose R square is larger than 0.998, and the highest sensitivity of 132.55 pm/μm is obtained in the range of 200 μm displacement variation. Moreover, it can be compatible to an intracavity displacement sensing system, achieving narrow linewidth, high signal-to-noise ratio (SNR), and high resolution. It can be found that the sensitivity of the intracavity displacement sensor can be 60 pm/μm when the PMF fiber length is about 20 cm with a linewidth narrower than 0.05 nm and a SNR higher than 55 dB.
A multiband tunable metamaterial terahertz absorber based on VO2 material is introduced and studied. The absorber consists of gold planes and periodic metallic materials. The first layer is gold plane, the second layer is polyimide, the third layer is silicon film, and the fourth layer is super surface. From the simulation results, we can see that the phase transition occurs when the temperature of VO2 is greater than or equal to 68°C. After the phase transition, there are three absorption peaks, and the absorption rates are more than 99% at 2.53, 5.7, and 8.67 THz, realizing perfect absorption. When the temperature of VO2 is less than 68°C, the maximum absorption rate is 30.4%, which fails to meet the absorption requirements. By increasing the thickness of polyimide, the absorption spectrum moves slightly to the low frequency band to achieve redshift. Compared with most of the proposed multiband absorbers, the metamaterial absorber with adjustable absorption spectrum has better application prospects. VO2 is a phase change material with insulator metal phase transition characteristics and reversible process. By using the phase transition characteristics of VO2, the absorber has a switching function and is conducive to the flexible regulation of the absorption spectrum.
A multiplexed gas sensing network based on hollow-core photonic crystal fiber (HC-PCF) and active intra-cavity absorption spectrometry is designed and demonstrated experimentally. Sensing channels are extended to eight by using hybrid dense wavelength division multiplexing (DWDM) and time division multiplexing (TDM). What’s more, wavelength scanning technique combined with voltage gradient method are adopted in the designed sensing network, which improves the sensing efficiency at least five times when comparing with the whole scan. In experiment, by recording and analyzing the laser output intensity at acetylene absorption peaks of 1528.01 nm and 1530.37 nm, the minimum detection limit (MDL) of 30.16 ppmv and 26.28 ppmv are achieved, respectively. Therefore, the designed gas sensing network can realize detection of low-concentration gas with high capacity and efficiency.
A metamaterial- and graphene-based broadband terahertz (THz) electromagnetic wave absorber that is composed of a stack of patterned gold film layer, patterned graphene layer, polyimide layer, and silicon layers was studied in detail. Our study is based on numerical simulations and electromagnetic absorption level higher than 90% over 2.09 THz band was demonstrated. The absorption frequency band significantly depends on the thickness of the polyimide layer and red shifts in parallel with its thickness. The major role of the added patterned graphene layer is to introduce frequency tunability property to the almost perfect absorption. As the Fermi level of graphene changed from 0 to 0.6 eV, the center frequency of the absorption band blueshifts from 6.19 to 8.15 THz. We also investigated incoming beam angle of incidence dependence and polarization sensitivity of the absorption. Metamaterial-based tunable absorbers prospected to be utilized in high-performance THz devices as a performance boosting critical element.
In this paper, we propose and demonstrate a high-integration intracavity displacement-sensor through inserting U-shape single-mode fiber interferometer (U-SMFI) into an Er-doped fiber ring laser. Considering that the U-SMFI can be realized only through bending a SMF, its characteristic of easy fabrication can reduce the cost of manufacturing process in contrast with those wavelength-modulated displacement sensors based on fiber Bragg grating, long-period grating and surface plasmon resonance. When the U-SMFI is inserted into the laser cavity, the variation of the bending radii can modulate the cavity loss so as to have an effect on the spectrum of the output laser. The proposed sensor has a higher signal to noise ratio and a narrower full width at half maximum. Through measuring the change of the spectrum, a high-resolution displacement sensor can be realized. The experimental results indicate that the sensitivities are 39 pm/μm when the bending radius is 6.5 mm.
A localized surface plasmon resonance (LSPR) temperature sensor based on photonic crystal fiber (PCF) filled with liquid and silver nanowires is demonstrated both theoretically and experimentally. Simulation results show that a blueshift is appeared along with temperature increasing. The resonance wavelength and resonance intensity can be tuned effectively by adjusting the volume ratios of the liquid constituents. To investigate the sensor’s performance, a large temperature range from 25°C to 60°C is detected in experiment and the sensitivity of -2.08 nm/°C with figure of merit (FOM) 0.1572 is obtained. The all-fiber device with strong mechanical stability is easy to realize remote sensing by changing the downlead fiber length, also promising for developing a high sensitive, real-time and distribute fiber sensor in temperature sensing applications.
A linear cavity all normal dispersion Yb-doped fiber laser based on the reflection volume grating and the SESAM(Semiconductor Saturable Absorber Mirror) has been demonstrated. Stable wavelength continuous tuning passively mode-locked laser pulse is obtained at room temperature with the repetition frequency of 16.52MHz. The spectral bandwidth of passively mode-locked pulse is 0.33nm at the central wavelength of 1032nm with the maximum average output power of 10.3mW and the monopulse energy of 0.64nJ. The central wavelength of the mode-locked pulse is tuned in the range of 1011.8~1050.7nm with the tuning range of 38.9nm by rotating the volume grating in taking advantage of its dispersion and wavelength selection characteristic. The fiber laser can be used as the optical source in DWDM/OTDM communication system or OCT system due to its wavelength tuning characteristic.
We obtain a switchable and tunable dual-wavelength single-frequency Er-doped ring fiber laser. In order to realize single-longitudinal output, two saturable-absorber-based tracking narrow-band filters are formed in 3- meter-long unpumped Er-doped fiber to narrow the linewidth via using the PM-FBG as a reflection filter. The maximum output power is 2.11 mW centered at 1550.16 nm and 1550.54 nm when the fiber laser operates in dual-wavelength mode. The corresponding linewidths of those two wavelengths are measured to be 769 Hz and 673 Hz, respectively. When the temperature around the PM-FBG is changed from 15 °C to 55 °C, the dual-wavelength single-frequency fiber laser can be tuned from 1550.12 nm to 1550.52 nm and from 1550.49 nm to 1550.82 nm, respectively.
Based on the 1550 nm single-frequency Erbium-Ytterbium co-doped fiber amplifier, the output linewidth with different seed power and gain fiber temperature was experimental investigated. The results demonstrated that, to obtain the same output power, the increment of the seed power was benefit to improve the output signal to noise ratio (SNR), and to reduce the linewidth broadening. The increment of gain fiber temperature can improve slope efficiency and enhance the ASE intensity but broaden linewidth. Meanwhile, the ASE was considered as one of the reason of linewidth variation.
The 7×1 end-pumped pump combiners employing 105/125 μm multimode fibers as pump fibers are investigated. Based on the results of our theoretical analysis, sufficient taper length (TL) and low refractive index (RI) of the capillary have been adopted to fabricate high transmission efficiency combiners. A 7×1 end-pumped pump combiner with an average transmission efficiency of 98.9% and a total return loss of 1.1‰ is fabricated in experiments, which could find its application in high-power fiber laser systems.
We propose a sensitivity-enhanced intracavity-absorption gas sensor based on the phenomenon of mode competition in the dual-wavelength ring fiber laser. The laser configuration possesses the sensing and reference wavelengths as 1530.372 nm and 1532.168 nm, respectively. When the hollow-core photonic crystal fiber (HC-PCF) is filled with 1000-ppmv acetylene, a sudden change on absorption intensity of more than 30 dB can be achieved by adjusting the optical loss in the laser cavities, resulting from the mode competition between the sensing and reference wavelengths. The minimum detectable acetylene concentration (MDAC) of 29.53 ppmv is obtained in experiment, one order of magnitude higher than former works.
We achieve a dual-wavelength single-frequency Erbium-doped ring fiber laser by using umpumped Erbium-doped fiber and polarization-maintained fiber Bragg grating. The maximum output power is 2.11 mW when the pump power is about 225 mW, corresponding to a slope efficiency of 1%. And the SNR is larger than 60 dB. The two lasing wavelengths of the dual-wavelength ring fiber laser are 1550.16 nm and 1550.54 nm, corresponding to the linewidths of 769 Hz and 673 Hz, respectively. Meanwhile, these two lasing wavelengths can be tuned from 1550.12 nm to 1550.52 nm and from 1550.49 nm to 1550.82 nm, respectively, when the temperature is verified from 15 °C to 55 °C. It can be used as a temperature sensor with a sensitivity about 0.01 nm/°C, which possesses a resolution about ∼4×10-6 °C through using optical heterodyne method. In addition, it can be used to realize high-resolution strain sensor by employing heterodyne method to measure the wavelength separation at the same time.
Single-frequency fiber lasers have been attracting extensive interest for the applications on fiber sensing over the past few years, because the high signal-to-noise ratio and narrow-linewidth facilitate the realization of long-distance and high-resolution sensing. In this paper, we reviewed the research progress on single-frequency fiber lasers for fiber sensing applications. Performance improvement in laser noise and linewidth has been addressed with the newly developed physical mechanisms.
1018nm short wavelength Yb3+-doped fiber laser can be widely used for tandem-pumped fiber laser system in 1 μm regime because of its high brightness and low quantum defect (QD). In order to achieve 1018nm short wavelength Yb3+-doped fiber laser with high output power, a steady-state rate equations considering the amplified spontaneous emission (ASE) and Stimulated Raman Scattering (SRS) has been established. We theoretically analyzed the ASE and SRS effects in 1018nm short wavelength Yb3+-doped fiber laser and the simulation results show that the ASE is the main restriction rather than SRS for high power 1018nm short wavelength Yb3+-doped fiber laser, besides the high temperature of fiber is also the restriction for high output power. We use numerical solution of steady-state rate equations to discuss how to suppress ASE in 1018nm short wavelength fiber laser and how to achieve high power 1018nm short-wavelength fiber laser.
We demonstrate an automatic channel-switched intracavity- absorption acetylene sensor via Sagnac loop filter based on the mode-competition in a ring fiber laser. When the photonic crystal fiber gas cell is filled with 1% acetylene, the corresponding absorption intensity can be ~14.0 dB and ~7.2 dB at 1532.83 nm and 1534.01 nm, respectively. Compared with the single transmission pass method, the sensitivity can be improved up to more than 10 times. It spends 50 seconds in scanning the absorption spectra through applying gradient voltage to the tunable F-P filter.
A 2-μm linear-polarized single-frequency Brillouin-Thulium fiber laser (BTFL) has been experimentally investigated for linewidth narrowing. The threshold for the Brillouin pump is around 200 mW, and more than 205 mW single-frequency Stokes laser was achieved with the 793 nm pump power of 8.5 W. The linewidth of the fiber laser has been narrowed for ~8 times, from 34 to 4.6 kHz. The measured RIN of the BTFL is <-150 dB/Hz for frequency above 2 MHz, which approaches the shot noise limit.
We propose a principle to achieve a high-resolution temperature sensor through measuring the central frequency shift in the single-frequency Erbium-doped fiber ring laser induced by the thermal drift via the optical heterodyne spectroscopy method. We achieve a temperature sensor with a sensitivity about 9.7 pm/°C and verify the detection accuracy through an experiment. Due to the narrow linewidth of the output singlefrequency signal and the high accuracy of the optical heterodyne spectroscopy method in measuring the frequency shift in the single-frequency ring laser, the temperature sensor can be employed to resolve a temperature drift up to ~5.5×10-6 °C theoretically when the single-frequency ring laser has a linewidth of 1 kHz and 10-kHz frequency shift is achieved from the heterodyne spectra.
The 7×1 end-pumped combiner employing 105/125 μm multimode fibers as pump fibers is investigated. The theoretical analysis reveals that sufficient taper length and low refractive index of the capillary should be adopted to fabricate high transmission efficiency combiners. Based on the simulation results, we fabricate a 7×1 end-pumped pump combiner with an average transmission efficiency of 98.9% and a total return loss of 1.1‰. The measured internal operating temperature of this combiner indicates it can endure pump power of the order of kilowatts.
We report herein an all-fiber single-frequency master oscillator power amplifier (MOPA) at 1550 nm with Er/Yb-codoped active fiber and wavelength-stabilized 976-nm LD pump source. A pump-limited maximum continuous-wave output power of 56.4 W was achieved under the pump power of 150 W, with corresponding slope efficiency being 37.0%. Via the self-heterodyne method, the evolution of spectrum linewidth during the amplification was investigated for the high-power MOPA-based single-frequency fiber laser. The linewidth and relative intensity noise at the maximum output power are 4.21 kHz and −110 dBm/Hz, respectively.
Single-frequency fiber laser operating at 1950 nm has been demonstrated in an all-fiber distributed Bragg reflection (DBR) laser cavity by using a 1.9-cm commercial available Thulium-doped silica fiber, for the first time. The laser was pumped by a 793-nm single-mode diode laser and had a threshold pump power of 75 mW. The maximum output power of the single longitudinal mode laser was 18 mW and the slope efficiency with respect to the launched pump power was 11%. Moreover, the linewidth and relative intensity noise (RIN) at different pump power has been measured and analyzed. The successful demonstration with the Thulium-doped silica fiber used here is considered to further promote the commercialization of single frequency fiber laser at 2 μm.
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