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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Photothermal Gas Detection of CO <sub/>2</sub> in an Intracavity Solid-State Laser Configuration
摘要： Laser spectroscopy has proven to be a powerful tool for quantitative and selective measurements of gas samples. Numerous techniques have been developed throughout the recent decades. This progress has been driven mostly by the growing demand on sensitive sensors, also for out-of-lab applications. Here we describe a new gas detection technique, which relies on photo-thermal (PT) effects. In PT spectroscopy the gas sample concentration can be derived by measuring a change in the refractive index (RI) of the excited gas particles. This is usually achieved by exciting the gas particles using a radiation source and by measuring the small variations of the RI (usually in the range of 10-5), e.g. in an interferometric sensor configuration, which is complicated and prone to mechanical and acoustic noise. Moreover, appropriate construction of a PT sensor enables encoding the gas concentration into frequency variations (similarly to dispersion spectroscopy), which provides a baseline-free measurement. Here we propose a novel, patent-pending configuration, which is non-complex, cost effective, miniaturized and offers detection limits at least comparable with other methods, having similar complexity, e.g. quartz enhanced photo-acoustic spectroscopy. Base of the sensor is a standard configuration of a monolithic, solid-state laser based on Nd:YVO4 active crystal and a YVO4 birefringent crystal ensuring single polarization operation of the laser at 1064 nm. To enable implementing PT gas detection an intentional air-gap has been obtained inside the resonator by separating the output mirror with a 2x2x2mm3 quartz crystal. In this particular experiment the air gap was filled with CO2 under ambient pressure and excited with an auxiliary fiber laser targeting a strong absorption line localized at 2003,5 nm. Due to the PT effect the density of the excited gas changes, which results in a change of its RI. A change of RI inside the solid-state laser resonator results in a slight variation of the optical pathlength and hence is directly translated to frequency changes of emission of the solid-state laser. The optical frequency changes are detected in a heterodyne configuration (the reference laser emission was obtained in the same crystal structure to minimise noise; the gas is excited in the path of only one of the emissions). The 2 μm excitation laser was modulated with a f0=1 kHz sinewave function and tuned across the absorption line. The induced PT effect was conveniently filtered-out at 2xf0 using a lock-in amplifier. The schematic of the sensor configuration, along with 2f signal registered for a 1000 ppmv and 200 ppmv CO2 sample is depicted in figure 1.
关键词： photothermal gas detection,refractive index,intracavity solid-state laser,CO2,heterodyne configuration
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Slow and Stopped Light in a Time-Dependent Moiré Grating
摘要： The reduction in the group velocity of a light pulse due to a resonance is termed slow light. A velocity reduction to 17 m/s was demonstrated using electromagnetically induced transparency (EIT) [1]. It was later demonstrated that light could be stopped completely [2-3]. Whilst slow light EIT experiments have been ground breaking, they require a vapour cloud cooled close to 0 K. Here we present a possible alternative using gratings and a time-dependent refractive index. Slow light can be induced by a Bragg grating by propagating a pulse with a carrier frequency close to the rejection band created by the Bragg resonance. This is accompanied by a substantial increase in group velocity dispersion (GVD) causing significant pulse broadening and limiting practical uses. A Moiré grating, which is a superposition of two grating periods has been suggested as a potential solution to reduce GVD [4]. The two grating periods produce two rejection bands separated by a transmission band. Propagating a pulse through the transmission band induces slow light, but the GVD generated by one rejection band is compensated by the other. We have developed a parametrisation for the Moiré grating periods such that they produce a transmission band centred on a given carrier frequency. Further using the parametrisation, we have derived a set of modified coupled mode equations, describing the coupling of the amplitudes A and B of the forward and backward propagating light for gratings of strength (cid:78) and with (cid:47)s the Moiré period and (cid:39)b the detuning. Our numerical simulations of a frequency-resolved version of Eq. (1) confirm that Moiré gratings produce slow light with minimal pulse broadening. The bandwidth of the rejection bands is dependent on the strength of the grating which is controlled by the grating modulation ?n. As the grating strength is increased, the rejection bands broaden reducing the transmission gap. If the grating strength is increased sufficiently the transmission band will close. Figures 1(a) and (b) show simulations of pulse propagation through a Moiré grating using finite difference time domain (FDTD) methods. The group velocity is given by the gradient of propagation distance versus time. Figure 1(a) shows propagation for a time-independent ?n. In Figure 1(b), ?n is increased by a time-dependent refractive index [5]; at 190 ps ?n is increased so that the transmission gap closes and is decreased to its original value at 440 ps, reopening the transmission gap. Closing the transmission band whilst a pulse is propagating through the grating induces stopped light. The pulse becomes trapped and remains stationary within the grating until the grating strength is reduced and the pulse can continue to propagate, analogous to results achieved in EIT [2-3]. Figure 1(c) shows how the transmission band closes as a function of the grating modulation. While the realisation of a time-dependent Moiré grating remains challenging, it presents a versatile alternative to the storage and release of light pulses in a solid-state platform, which would provide an essential element for, e.g., quantum information processing.
关键词： time-dependent refractive index,stopped light,slow light,group velocity dispersion,Moiré grating
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Analysis of Various Approaches for the Fabrication of Diffractive Optical Elements
摘要： Diffractive Optical Elements (DOEs) exist for a large range of applications. Usually DOEs are obtained by combining lithographic techniques and etching in order to achieve pre-determined phase patterns. That way, it is possible to shape or split laser beams with very high efficiencies. Indeed for binary element which consist of two phase levels (0 and π) the diffraction efficiency is around 75% and this efficiency can approach 100% for multilevel elements. Various illustrations of DOEs can be found in the literature for example using organic materials like photoresists or inorganic materials such as silica or chalcogenide layers. Recent work has shown that DOEs can also be recorded in the volume of photosensitive chalcogenide layers, but no precise comparison of this class of DOEs has ever been made with classical DOEs. In this paper we propose to study binary DOEs fabricated with conventional lithography (based on local control of the thickness) and similar one fabricated using photo-induced refractive index change in As2S3 layers.
关键词： etching,phase patterns,silica,laser beams,photo-induced refractive index change,photoresists,lithography,As2S3 layers,chalcogenide layers,Diffractive Optical Elements,diffraction efficiency
更新于2025-09-16 10:30:52
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The Enhanced Sensitivity of a Porous Silicon Microcavity Biosensor Based on an Angular Spectrum Using CdSe/ZnS Quantum Dots
摘要： To improve the detection sensitivity of porous silicon microcavity biosensors, CdSe/ZnS quantum dots are used to label complementary DNA molecules for the refractive index amplification and angular spectrum method for detection. In this method, the TE mode laser is used as the detection light and the light source is changed into a parallel beam by collimating and expanding the beam, which illuminates the PSM surface and receives the reflected light from the PSM surface through the detector. The angle corresponding to the weakest reflected light intensity before and after the biological reaction between probe DNA and complementary DNA of different concentrations labeled by quantum dots was measured by the detector, and the relationship between the angle change before and after the biological reaction and the complementary DNA concentration labeled by quantum dots was obtained. The experimental results show that the angle change increases linearly with increasing complementary DNA concentration. The detection limit of the experiment, as determined by fitting, is approximately 36 pM. The detection limit of this method is approximately 1/300 of that without quantum dot labeling. Our method has a low cost because it does not require the use of a reflectance spectrometer, and it also demonstrates high sensitivity.
关键词： refractive index amplification,angular spectrum,quantum dot,porous silicon microcavity
更新于2025-09-16 10:30:52
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Wavelength and pump-power dependent nonlinear refraction and absorption in a semiconductor disk laser
摘要： We characterize both nonlinear refraction and absorption in a vertical-external-cavity surface-emitting laser (VECSEL) as a function of pump irradiance over the whole range of lasing wavelengths. We observe an approximately linear decrease in magnitude for both nonlinear refraction and nonlinear absorption with optical pumping when the thermally induced shift of the lasing wavelength is considered. Our results are of particular significance for understanding self-modelocking of VECSELs which might be driven by nonlinear lensing.
关键词： mode-locked semiconductor lasers,Z-scan,Nonlinear refractive index,VECSELs
更新于2025-09-16 10:30:52
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - High-Sensitive All-Fiber Fabry-Perot Interferometer Gas Refractive Index Sensor Based on Lateral offset Splicing and Vernier Effect
摘要： Abstract—A high-sensitivity all-fiber Fabry-Perot interferometer (FPI) gas refractive index (RI) sensor based on lateral offset splicing and Vernier Effect is proposed. The sensing head consist of a reference FPI and a sensing FPI. A reference FPI and the sensing FPI is connected by two circulators. The sensing FPI is an open cavity made by lateral offset splicing of single mode fibers (SMFs), and the reference FPI is spliced by SMF and fiber tube. The Cascaded double-cavity produces a Vernier Effect and improves the sensitivity of gas RI. Experimental results show that the sensor has a high gas RI sensitivity of 16335.96 nm/RIU. The sensor has broad application prospects in gas composition detection, environmental monitoring, biosensors and other fields.
关键词： Fabry-Perot Interferometer,Fiber Optics Sensors,Vernier Effect,Gas Refractive Index Measurement
更新于2025-09-16 10:30:52
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Sensitivity-enhanced refractive index sensor based on series structure of two microfiber knot resonators with Vernier effect
摘要： This paper proposes and experimentally demonstrates a sensor for sensitivity-enhanced refractive index (RI) sensing based on Vernier effect. The Vernier effect achieved by connecting two microfiber knot resonators (MKRs) in series. The sensing principles and spectrum of the structure are studied theoretically and then verified experimentally. The results exhibit that the RI sensor has high sensitivity up to 3301.76 nm/RIU and it is detection resolution is 6.057×10?6 RIU. That is because the microfiber has high proportional evanescent field and Vernier effect multiplies the spectrum of individual MKR. The proposed series structure of two MKRs with high sensitivity, easy fabrication and low cost can be extensively applied to physical, chemical and biological signal detection.
关键词： series structure,microfiber knot resonators,Vernier effect,sensitivity-enhanced,refractive index sensing,easy fabrication
更新于2025-09-16 10:30:52
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Theoretical analysis of a wide detection range refractive index sensor based on exposed core tellurite micro-structured optical fiber
摘要： A novel refractive index (RI) sensor based on the multimode interference in an exposed-core tellurite microstructure optical fiber (TMOF) has been designed and implemented in this paper. With different liquid analytes covered onto TMOF core surface, the interference between any two specific modes experiences a fringe shift (or phase change). The exposed-core optical fiber design provides direct access to the evanescent field, allowing RI measurement with a sensitivity of up to 2919.4 nm/RIU. The proposed sensor offers a higher sensitivity while maintaining the fiber robustness and a much wider RI detection range (reaching to 1.58) than its existing counterparts based on multimode interference effect of traditional silica optical fiber. It is also reliable, cost-effective and offers a possibility for label-free biosensing.
关键词： mode interference,tellurite,refractive index sensor,microstructure optical fiber
更新于2025-09-16 10:30:52
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Ultrasensitive Microfiber Refractive Index Sensor Based on Mach-Zehnder Interference of Core Offset Structure
摘要： An ultrasensitive microfiber refractive index (RI) sensor was proposed and demonstrated based on the core offset Mach-Zehnder interference (MZI) fiber structure. It consisted of a core offset single-mode fiber (SMF) fusion spliced between two SMFs, where the center SMF was tapered to diameter of few micrometers. The experimental results showed that the sensor exhibits an ultrahigh RI sensitivity of 16485 nm /RIU (RI unit) within the RI range between 1.410 and 1.413 where the core offset and fiber diameter were 7.5 μm and 9 μm, tapered respectively. This proposed fiber sensor has the great potential as a base for biosensor in applications of air pollution monitoring, chemical elements detection and medical diagnostics.
关键词： core offset,refractive index sensing,microfiber
更新于2025-09-16 10:30:52
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Sensitivity Enhancement of a Concave Shaped Optical Fiber Refractive Index Sensor Covered with Multiple Au Nanowires
摘要： In the present paper, a new kind of concave shaped refractive index sensor (CSRIS) exploiting localized surface plasmon resonance (LSPR) is proposed and numerically optimized. The LSPR effect between polaritons and the core guided mode of designed CSRIS is used to enhance the sensing performance. The sensor is characterized for two types of sensing structures coated with gold (Au) film and Au nanowires (AuNWs), respectively. The influence of structural parameters such as the distance (D) of the concave shaped channel (CSC) from the core, the diameter of the nanowire (dn) and the size (s) of the CSC are investigated here. In comparison to Au film, the AuNWs are shown to significantly enhance the sensitivity and the performance of the designed sensor. An enhanced sensitivity of 4471 nm/RIU (refractive index unit) is obtained with AuNWs, for a wide range of analytes refractive index (na) varying between 1.33 to 1.38. However, for conventional Au film; the sensitivity of 808.57 nm/RIU is obtained for the same range of analytes.
关键词： nanowires,microfluidic channel,surface plasmon resonance,sensitivity,refractive index
更新于2025-09-16 10:30:52