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Photonics, Volume 7, Issue 3 (September 2020) – 34 articles

Cover Story (view full-size image): The interband cascade laser (ICL) has recently become the go-to coherent optical source for mid-wave infrared (especially 3–6 μm) applications that require high efficiency, low drive power, and small system footprint. The ICL combines the long upper-level lifetime characteristic of interband transitions with the voltage-efficient cascading of the quantum cascade laser (which employs intersubband transitions). This paper reviews the history, development, design principles, operating characteristics, and specialized architectures of ICLs. The present understanding of mechanisms that limit performance is discussed, along with potential future improvements. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time. View this paper.
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16 pages, 2218 KiB  
Article
Properties of Bilayer Graphene Quantum Dots for Integrated Optics: An Ab Initio Study
by Majid Ghandchi, Ghafar Darvish and Mohammad Kazem Moravvej-Farshi
Photonics 2020, 7(3), 78; https://doi.org/10.3390/photonics7030078 - 22 Sep 2020
Cited by 8 | Viewed by 3472
Abstract
Due to their bandgap engineering capabilities for optoelectronics applications, the study of nano-graphene has been a topic of interest to researchers in recent years. Using a first-principles study based on density functional theory (DFT) and thermal DFT, we investigated the electronic structures and [...] Read more.
Due to their bandgap engineering capabilities for optoelectronics applications, the study of nano-graphene has been a topic of interest to researchers in recent years. Using a first-principles study based on density functional theory (DFT) and thermal DFT, we investigated the electronic structures and optical properties of bilayer graphene quantum dots (GQDs). The dielectric tensors, absorption spectra, and the refractive indexes of the bilayer GQDs were obtained for both in-plane and out-of-plane polarization. In addition, we calculated the absorption spectra via time-dependent DFT (TD-DFT) in the linear response regime. The TDDFT results show that a blue shift occurs in the absorption spectrum, which is consistent with the experimental results. In this investigation, we consider triangular and hexagonal GQDs of various sizes with zigzag and armchair edges. Our simulations show that unlike monolayer GQDs, for which light absorption for out-of-plane polarization occurs in the ultraviolet wavelength range of 85–250 nm, the out-of-plane polarization light absorption peaks in the bilayer GQDs appear in the near-infrared range of 500–1600 nm, similar to those in bilayer graphene sheets. The out-of-plane polarization light absorption peaks in the near-infrared range make bilayer GQDs suitable for integrated optics and optical communication applications. Full article
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12 pages, 22068 KiB  
Communication
Salinity Sensing Characteristics Based on Optical Microfiber Coupler Interferometer
by Lingjun Zhou, Yang Yu, Huimin Huang, Yuyu Tao, Kui Wen, Guofeng Li, Junbo Yang and Zhenrong Zhang
Photonics 2020, 7(3), 77; https://doi.org/10.3390/photonics7030077 - 21 Sep 2020
Cited by 17 | Viewed by 2421
Abstract
In this paper, we report a novel and compact sensor based on an optic microfiber coupler interferometer (OMCI) for seawater salinity application. The OMCI device is fabricated by connecting Faraday rotating mirrors to the two out-ports of the microfiber coupler, respectively. The sensor [...] Read more.
In this paper, we report a novel and compact sensor based on an optic microfiber coupler interferometer (OMCI) for seawater salinity application. The OMCI device is fabricated by connecting Faraday rotating mirrors to the two out-ports of the microfiber coupler, respectively. The sensor signal processing is based on a wavelength demodulation technique. We theoretically analyze the sensing characteristics with different device structure parameters. Besides, the results show that the date reading error decreases with the thinner waist region and longer arm difference. Through the experiment, the reflection spectra red-shifted as the sea water salinity increased; the highest response sensitivity of the OMCI salinity sensor reached 303.7 pm/‰ for a range of 16.6–23.8‰, and the resolution was less than 0.03‰. This study provides a new technical solution for the development of practical optical fiber seawater salinity sensors. Full article
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14 pages, 5446 KiB  
Article
Structural, Optical and Electrical Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice Barrier Photodetector
by U. Zavala-Moran, M. Bouschet, J. P. Perez, R. Alchaar, S. Bernhardt, I. Ribet-Mohamed, F. de Anda-Salazar and P. Christol
Photonics 2020, 7(3), 76; https://doi.org/10.3390/photonics7030076 - 18 Sep 2020
Cited by 14 | Viewed by 4115
Abstract
In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime [...] Read more.
In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime value equal to 1 µs at 80 K, carried out on dedicated structure showing photoluminescence peak position at 4.9 µm, is extracted from a time resolved photoluminescence measurement. Dark current density as low as 3.2 × 10−5 A/cm2 at 150 K is reported on the corresponding device exhibiting a 50% cut-off wavelength around 5 µm. A performance analysis through normalized spectral response and dark current density-voltage characteristics was performed to determine both the operating bias and the different dark current regimes. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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58 pages, 8781 KiB  
Review
The Interband Cascade Laser
by Jerry R. Meyer, William W. Bewley, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, Charles D. Merritt and Igor Vurgaftman
Photonics 2020, 7(3), 75; https://doi.org/10.3390/photonics7030075 - 15 Sep 2020
Cited by 89 | Viewed by 11423
Abstract
We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for [...] Read more.
We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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12 pages, 3786 KiB  
Article
Cavity Ring-Down Spectroscopy for Molecular Trace Gas Detection Using A Pulsed DFB QCL Emitting at 6.8 µm
by Komlan S. Gadedjisso-Tossou, Lyubomir I. Stoychev, Messanh A. Mohou, Humberto Cabrera, Joseph Niemela, Miltcho B. Danailov and Andrea Vacchi
Photonics 2020, 7(3), 74; https://doi.org/10.3390/photonics7030074 - 14 Sep 2020
Cited by 16 | Viewed by 3523
Abstract
A trace gas sensor based on pulsed cavity ring-down spectroscopy (CRDS) was developed for measurement of the ν4 fundamental vibrational band of ammonia (NH3) centered at 1468.898 cm−1. A pulsed distributed feedback quantum cascade laser (DFB-QCL) operating at 6.8 [...] Read more.
A trace gas sensor based on pulsed cavity ring-down spectroscopy (CRDS) was developed for measurement of the ν4 fundamental vibrational band of ammonia (NH3) centered at 1468.898 cm−1. A pulsed distributed feedback quantum cascade laser (DFB-QCL) operating at 6.8 µm (1470.58 cm−1) quite well covered the absorption band of the ammonia and strong fundamental vibrational absorption bands of different molecular gases in this unexplored region. The cavity was partially evacuated down to 0.4 Atm by a turbo-molecular pump to reduce the partial interference between the NH3 spectra and water near the absorption peak of ammonia. A sensitivity of nine parts per billion was reached for a measurement time of 120 s as well as an optical path length of 226 m. The device demonstrated high spectral performance and versatility due to its wide tuning range, narrow linewidth, and comparatively high-energy mid-IR radiation in the relatively unexplored 6.8 µm region, which is very important for high-resolution spectroscopy of a variety of gases. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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11 pages, 3980 KiB  
Article
Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3
by Hao Jiang, Chen Gong, Tatsuhiko Nishimura, Hironaru Murakami, Iwao Kawayama, Hidetoshi Nakanishi and Masayoshi Tonouchi
Photonics 2020, 7(3), 73; https://doi.org/10.3390/photonics7030073 - 14 Sep 2020
Cited by 15 | Viewed by 3762
Abstract
Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser [...] Read more.
Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped β-Ga2O3 wafers. Femtosecond (fs) laser illumination generated THz waves based on the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga2O3. TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and diffusion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them. Full article
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16 pages, 9927 KiB  
Article
Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System
by Elena A. Anashkina, Maria P. Marisova, Alexey V. Andrianov, Rinat A. Akhmedzhanov, Rihards Murnieks, Mikhail D. Tokman, Laura Skladova, Ivan V. Oladyshkin, Toms Salgals, Ilya Lyashuk, Arseniy Sorokin, Sandis Spolitis, Gerd Leuchs and Vjaceslavs Bobrovs
Photonics 2020, 7(3), 72; https://doi.org/10.3390/photonics7030072 - 11 Sep 2020
Cited by 16 | Viewed by 4328
Abstract
Optical frequency comb (OFC) generators based on whispering gallery mode (WGM) microresonators have a massive potential to ensure spectral and energy efficiency in wavelength-division multiplexing (WDM) telecommunication systems. The use of silica microspheres for telecommunication applications has hardly been studied but could be [...] Read more.
Optical frequency comb (OFC) generators based on whispering gallery mode (WGM) microresonators have a massive potential to ensure spectral and energy efficiency in wavelength-division multiplexing (WDM) telecommunication systems. The use of silica microspheres for telecommunication applications has hardly been studied but could be promising. We propose, investigate, and optimize numerically a simple design of a silica microsphere-based OFC generator in the C-band with a free spectral range of 200 GHz and simulate its implementation to provide 4-channel 200 GHz spaced WDM data transmission system. We calculate microsphere characteristics such as WGM eigenfrequencies, dispersion, nonlinear Kerr coefficient with allowance for thermo-optical effects, and simulate OFC generation in the regime of a stable dissipative Kerr soliton. We show that by employing generated OFC lines as optical carriers for WDM data transmission, it is possible to ensure error-free data transmission with a bit error rate (BER) of 4.5 × 10−30, providing a total of 40 Gbit/s of transmission speed on four channels. Full article
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12 pages, 1656 KiB  
Article
Simplified Aberration Analysis Method of Holographic Waveguide Combiner
by Wei-Chia Su, Shao-Kui Zhou, Bor-Shyh Lin and Wen-Kai Lin
Photonics 2020, 7(3), 71; https://doi.org/10.3390/photonics7030071 - 10 Sep 2020
Cited by 4 | Viewed by 2224
Abstract
Generally, the diffractive waveguide combiner and computer-generated hologram (CGH) technique have the potential to achieve compact head-mounted display (HMD) with a natural 3D display function. However, the diffractive waveguide combiner will degrade the image quality because of aberration. In order to resolve this [...] Read more.
Generally, the diffractive waveguide combiner and computer-generated hologram (CGH) technique have the potential to achieve compact head-mounted display (HMD) with a natural 3D display function. However, the diffractive waveguide combiner will degrade the image quality because of aberration. In order to resolve this issue, the complex analysis based on the ray-tracing method is necessary. Since the major aberration of the waveguide combiners is only astigmatism and anamorphic distortion, only these two aberrations were discussed in this paper. Furthermore, two common waveguide structures were discussed here. In total, four formulas were summarized to analyze aberration and anamorphic distortion in these two structures. Finally, the simplified formulas were verified with the commercial ray-tracing software Zemax. The calculated results of the proposed method match the simulation of Zemax software well. Therefore, the aberration of an arbitrary similar diffractive waveguide can be analyzed by the proposed method. This will make the designing process simpler and faster. Full article
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16 pages, 412 KiB  
Article
Geodesic Length Measurement in Medical Images: Effect of the Discretization by the Camera Chip and Quantitative Assessment of Error Reduction Methods
by Ady Naber, Daniel Berwanger and Werner Nahm
Photonics 2020, 7(3), 70; https://doi.org/10.3390/photonics7030070 - 5 Sep 2020
Cited by 2 | Viewed by 2141
Abstract
After interventions such as bypass surgeries the vascular function is checked qualitatively and remotely by observing the blood dynamics inside the vessel via Fluorescence Angiography. This state-of-the-art method has to be improved by introducing a quantitatively measured blood flow. Previous approaches show that [...] Read more.
After interventions such as bypass surgeries the vascular function is checked qualitatively and remotely by observing the blood dynamics inside the vessel via Fluorescence Angiography. This state-of-the-art method has to be improved by introducing a quantitatively measured blood flow. Previous approaches show that the measured blood flow cannot be easily calibrated against a gold standard reference. In order to systematically address the possible sources of error, we investigated the error in geodesic length measurement caused by spatial discretization on the camera chip. We used an in-silico vessel segmentation model based on mathematical functions as a ground truth for the length of vessel-like anatomical structures in the continuous space. Discretization errors for the chosen models were determined in a typical magnitude of 6%. Since this length error would propagate to an unacceptable error in blood flow measurement, counteractions need to be developed. Therefore, different methods for the centerline extraction and spatial interpolation have been tested and compared against their performance in reducing the discretization error in length measurement by re-continualization. In conclusion, the discretization error is reduced by the re-continualization of the centerline to an acceptable range. The discretization error is dependent on the complexity of the centerline and this dependency is also reduced. Thereby the centerline extraction by erosion in combination with the piecewise Bézier curve fitting performs best by reducing the error to 2.7% with an acceptable computational time. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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12 pages, 931 KiB  
Article
A Programmable Mode-Locked Fiber Laser Using Phase-Only Pulse Shaping and the Genetic Algorithm
by Abdullah S. Karar, Raymond Ghandour, Ibrahim Mahariq, Shadi A. Alboon, Issam Maaz, Bilel Neji and Julien Moussa H. Barakat
Photonics 2020, 7(3), 69; https://doi.org/10.3390/photonics7030069 - 4 Sep 2020
Cited by 7 | Viewed by 2567
Abstract
A novel, programmable, mode-locked fiber laser design is presented and numerically demonstrated. The laser programmability is enabled by an intracavity optical phase-only pulse shaper, which utilizes the same linearly chirped fiber Bragg grating (LC-FBG) from its two opposite ends to perform real-time optical [...] Read more.
A novel, programmable, mode-locked fiber laser design is presented and numerically demonstrated. The laser programmability is enabled by an intracavity optical phase-only pulse shaper, which utilizes the same linearly chirped fiber Bragg grating (LC-FBG) from its two opposite ends to perform real-time optical Fourier transformation. A binary bit-pattern generator (BPG) operating at 20-Gb/s and producing a periodic sequence of 32 bits every 1.6 ns, is subsequently used to drive an optical phase modulator inside the laser cavity. Simulation results indicate stable programmable intensity profiles for each optimized user defined 32 code words. The laser operated in the self-similar mode-locking regime, enabling wave-breaking free operation. The programmable 32 bit code word targeting a specific intensity profile was determined using 100 generations of the genetic algorithm. The control of ultrashort pulse intensity profiles on the picosecond and femtosecond time scales is difficult. The process of stretching and compressing the pulse in the time domain allows for a slower BPG to impose a predefined phase modulation prior to pulse compression. This results in control over the fine features of the intensity profile of the compressed pulse on a picosecond or femtosecond time scale inside the laser cavity. The stability of the proposed scheme depends on the consistency and accuracy of the BPG rise and fall times in practice. Full article
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8 pages, 2409 KiB  
Letter
Demonstration of Planar Type-II Superlattice-Based Photodetectors Using Silicon Ion-Implantation
by Arash Dehzangi, Donghai Wu, Ryan McClintock, Jiakai Li, Alexander Jaud and Manijeh Razeghi
Photonics 2020, 7(3), 68; https://doi.org/10.3390/photonics7030068 - 3 Sep 2020
Cited by 7 | Viewed by 3447
Abstract
In this letter, we report the demonstration of a pBn planar mid-wavelength infrared photodetectors based on type-II InAs/InAs1−xSbx superlattices, using silicon ion-implantation to isolate the devices. At 77 K the photodetectors exhibited peak responsivity of 0.76 A/W at 3.8 µm, [...] Read more.
In this letter, we report the demonstration of a pBn planar mid-wavelength infrared photodetectors based on type-II InAs/InAs1−xSbx superlattices, using silicon ion-implantation to isolate the devices. At 77 K the photodetectors exhibited peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency, without anti-reflection coating, of 21.5% under an applied bias of +40 mV with a 100% cut-off wavelength of 4.6 µm. With a dark current density of 5.21 × 10−6 A/cm2, under +40 mV applied bias and at 77 K, the photodetector exhibited a specific detectivity of 4.95 × 1011 cm·Hz1/2/W. Full article
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14 pages, 1801 KiB  
Article
Coupling of Integrated Waveguide and Optomechanic Cavity for Microwave Phonon Excitation in Si Nanobeams
by Abdellatif Gueddida, Bahram Djafari Rouhani, Yan Pennec, Andrea Di Donato, Luca Pierantoni, Alexander Korovin and Davide Mencarelli
Photonics 2020, 7(3), 67; https://doi.org/10.3390/photonics7030067 - 31 Aug 2020
Cited by 2 | Viewed by 2391
Abstract
The availability of high quality manufacturing for optical micro/nano patterned cavities paves the way to the development of scalable circuits and devices based on optomechanical (OM) interaction of sound and light in extremely small volumes. In this contribution, we propose a new study [...] Read more.
The availability of high quality manufacturing for optical micro/nano patterned cavities paves the way to the development of scalable circuits and devices based on optomechanical (OM) interaction of sound and light in extremely small volumes. In this contribution, we propose a new study on OM cavities that can lead to precise control of their coupling with closely integrated waveguides, a necessary condition to enhance mode excitation and wave energy trapping, opening the possibility for many potential applications in wave guiding, filtering, confinement, and sensing. Moreover, in this way the need for bulky experimental setups and/or optical fiber coupling/excitation is avoided. At the same time, quality factors of mechanical and optical modes resonating in the cavity are optimized, together with their OM coupling coefficients: high confinement of both excitations is a prerequisite to enable their acousto-optic (AO) interaction. To this aim, the transversal size of the cavity has been parabolically tapered, with the additional benefit of separating the cavity and the integrated waveguide far from the coupling region. The finite-element method has been used to perform full-wave analysis, and an accurate discussion about the simulation setup needed to properly describe optical scattering and radiation has been provided. Full article
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13 pages, 1224 KiB  
Article
Application of 150 kHz Laser for High-Order Harmonic Generation in Different Plasmas
by Ganjaboy S. Boltaev, Vyacheslav V. Kim, Mazhar Iqbal, Naveed A. Abbasi, Vadim S. Yalishev, Rashid A. Ganeev and Ali S. Alnaser
Photonics 2020, 7(3), 66; https://doi.org/10.3390/photonics7030066 - 31 Aug 2020
Cited by 10 | Viewed by 2421
Abstract
Application of high pulse repetition rate lasers opens the way for increasing the average flux of the high-order harmonics generating in the ions- and nanoparticles-containing plasmas ablated on the surfaces of various metal targets. We demonstrate the harmonic generation of 37 fs, 150 [...] Read more.
Application of high pulse repetition rate lasers opens the way for increasing the average flux of the high-order harmonics generating in the ions- and nanoparticles-containing plasmas ablated on the surfaces of various metal targets. We demonstrate the harmonic generation of 37 fs, 150 kHz, 1030 nm, 0.5 mJ pulses in different plasmas. The formation of plasma plumes on the surfaces of carbon, titanium, boron, zinc, and manganese targets was performed during laser ablation, using 250 fs pulses from the same laser. The ablation of the mixed powder of boron nanoparticles and silver microparticles was used for generation of harmonics with high yield. Harmonics up to the fortieth orders from the carbon plasma were obtained. The estimated conversion efficiencies in laser-produced plasmas were ≤10−5. The photon flux for a single harmonic generating in carbon plasma was estimated to be 8 × 1013 photons/s. Full article
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11 pages, 8226 KiB  
Article
A Novel Data-Aided Frame Synchronization Method Based on Hough Transform for Optical Communications
by Huiwen Yin, Sida Li, Zhiping Huang and Jie Chen
Photonics 2020, 7(3), 65; https://doi.org/10.3390/photonics7030065 - 27 Aug 2020
Cited by 3 | Viewed by 2863
Abstract
In optical communication systems, frame synchronization is essential for subsequent operations, such as error correction and payload extraction. Various methods, so far, have been proposed in the published literature, but the performance is unsatisfactory under high bit error rate (BER) conditions. We present, [...] Read more.
In optical communication systems, frame synchronization is essential for subsequent operations, such as error correction and payload extraction. Various methods, so far, have been proposed in the published literature, but the performance is unsatisfactory under high bit error rate (BER) conditions. We present, in this work, a novel data-aided frame synchronization technique for optical packet transmission systems, in which the transmitter sends a sequence of packets with a specific synchronization word periodically inserted, and the receiver blindly recognizes the synchronization word to attain frame synchronization. The proposed algorithm detects the synchronization word based on Hough transform (HT), a classic method for line detection in digital image processing. The core principle of the algorithm is to exploit the periodicity of the frame synchronization word, which appears as black-and-white spaced stripes on a binary image when the frames are all aligned. Simulations are conducted over a 56Gbps optical QPSK transmission system, and the results show that our algorithm is still effective in attaining frame synchronization at a BER of 0.3. A comparison is also made between our algorithm and existed autocorrelation (AC)-based algorithm. The results demonstrate that our algorithm has a better error resilience performance. When the BER is higher than 0.03, our algorithm outperforms an AC-based algorithm significantly. Full article
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18 pages, 9468 KiB  
Article
Dynamic Scattering Approach for Solving the Radar Cross-Section of the Warship under Complex Motion Conditions
by Zeyang Zhou and Jun Huang
Photonics 2020, 7(3), 64; https://doi.org/10.3390/photonics7030064 - 26 Aug 2020
Cited by 2 | Viewed by 3260
Abstract
To obtain the electromagnetic scattering characteristics of the warship under complex motion conditions, a dynamic scattering approach (DSA) based on physical optics and physical theory of diffraction is presented. The observation angles, turret rotation, hull attitude changes and sea wave models are carefully [...] Read more.
To obtain the electromagnetic scattering characteristics of the warship under complex motion conditions, a dynamic scattering approach (DSA) based on physical optics and physical theory of diffraction is presented. The observation angles, turret rotation, hull attitude changes and sea wave models are carefully studied and discussed. The research results show that the pitching and rolling angles have a large effect on the radar cross-section (RCS) of the warship. Turret movement has a greater impact on its own RCS but less impact on the warship. The RCS of the warship varies greatly at various azimuths and elevations. Different sea surface models have a greater impact on the lateral RCS of the warship. The DSA is effective and efficient to study the dynamic RCS of the warship under complex motion conditions. Full article
(This article belongs to the Section Optical Communication and Network)
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7 pages, 4730 KiB  
Article
Output Power Monitoring of Ultraviolet Light-Emitting Diode via Sapphire Substrate
by Ching-Hua Chen, Jia-Jun Zhang, Chang-Han Wang, Tzu-Chieh Chou, Rui-Xiang Chan and Pinghui S. Yeh
Photonics 2020, 7(3), 63; https://doi.org/10.3390/photonics7030063 - 25 Aug 2020
Cited by 7 | Viewed by 2320
Abstract
Ultraviolet (UV) light plays an important role in air/water/surface sterilization now. Maintaining a certain light intensity is often required to attain the targeted effect. In this paper, on-chip power monitoring of a UV-A light-emitting diode (LED) via sapphire substrate is reported. A p–i–n [...] Read more.
Ultraviolet (UV) light plays an important role in air/water/surface sterilization now. Maintaining a certain light intensity is often required to attain the targeted effect. In this paper, on-chip power monitoring of a UV-A light-emitting diode (LED) via sapphire substrate is reported. A p–i–n photodiode loop that surrounds the UV-A LED was designed and fabricated to monitor the output power by detecting the scattered light of the LED propagating through the sapphire substrate. No particular waveguide structure or processing parameter control was needed. The monitoring responsivities per unit of surface-emitting power obtained were approximately 21 and 25 mA/W at photodiode biases of 0 and 3 V, respectively. When combined with a transimpedance amplifier, a monitoring responsivity of 1.87 V/mW at zero bias was measured, and a different monitoring responsivity could be customized by adjusting the gain of the transimpedance amplifier. The operation principle of this device might be applicable to UV-B or UV-C LEDs. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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5 pages, 188 KiB  
Editorial
Special Issue “Neurophotonics—Optics for the Brain”
by Sergio Fantini
Photonics 2020, 7(3), 62; https://doi.org/10.3390/photonics7030062 - 25 Aug 2020
Viewed by 2353
Abstract
Light-tissue interactions allow for a multitude of possibilities to sense; image; and impact the brain at molecular, cellular, and tissue levels. The application of optical techniques to neuronal tissue is the essence of neurophotonics, to which this Special Issue is dedicated. The eleven [...] Read more.
Light-tissue interactions allow for a multitude of possibilities to sense; image; and impact the brain at molecular, cellular, and tissue levels. The application of optical techniques to neuronal tissue is the essence of neurophotonics, to which this Special Issue is dedicated. The eleven articles in this Special Issue are representative of the broad scope of this field and of the wide range of optical techniques that it encompasses. In more detail, the articles cover basic neuroscience, neuroanatomy, functional imaging, cerebral hemodynamics, brain development and aging, brain–computer interfaces, and therapeutic treatments. The optical techniques considered include various types of optical microscopy, optical coherence tomography, photoacoustic imaging, diffuse optical spectroscopy and imaging, photobiomodulation, and optogenetics. Full article
(This article belongs to the Special Issue Neurophotonics – Optics for the Brain)
13 pages, 3239 KiB  
Article
VOC Monitoring and Ozone Generation Potential Analysis Based on a Single-Photon Ionization Time-of-Flight Mass Spectrometer
by Yuefeng Zhao, Yurong Zhang, Jing Gao, Xu Wang, Hui Li, Yanqi Wang, Mengjun Duan, Kaifa Cao, Yangjian Cai and Jie Pan
Photonics 2020, 7(3), 61; https://doi.org/10.3390/photonics7030061 - 24 Aug 2020
Cited by 3 | Viewed by 2463
Abstract
The single-photon ionization time-of-flight mass spectrometer (SPI-TOFMS), which has high sensitivity, high accuracy, and a short response time, is effective for the real-time monitoring of volatile organic compounds (VOCs). In this study, the theory and structural composition of the SPI-TOFMS are described. Its [...] Read more.
The single-photon ionization time-of-flight mass spectrometer (SPI-TOFMS), which has high sensitivity, high accuracy, and a short response time, is effective for the real-time monitoring of volatile organic compounds (VOCs). In this study, the theory and structural composition of the SPI-TOFMS are described. Its detection limit can reach parts per billion by volume (ppbv), the dynamic range is better than three orders of magnitude, and the response speed can reach milliseconds. The distribution of VOCs and the ozone generation contribution rate in the Hefei Economic Development Zone are analyzed using this instrument with a vehicle platform for online navigation detection. The experimental results showed that aromatics were the primary components of the total volatile organic compounds (TVOCs), and aromatics and alkenes contributed more to ozone formation in an industrial manufacturing area. This research indicates that the SPI-TOFMS can rapidly and accurately conduct online monitoring of VOCs in industrial development zones. In addition, it has been applied in the fields of atmospheric composition observation, environmental monitoring, and industrial VOC leakage monitoring. Full article
(This article belongs to the Special Issue Recent Advances in the Study of Light Propagation in Optical Fibers)
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21 pages, 4301 KiB  
Article
Off-Axis Vortex Beam Propagation through Classical Optical System in Terms of Kummer Confluent Hypergeometric Function
by Ireneusz Augustyniak, Weronika Lamperska, Jan Masajada, Łukasz Płociniczak and Agnieszka Popiołek-Masajada
Photonics 2020, 7(3), 60; https://doi.org/10.3390/photonics7030060 - 15 Aug 2020
Cited by 12 | Viewed by 2661
Abstract
The analytical solution for the propagation of the laser beam with optical vortex through the system of lenses is presented. The optical vortex is introduced into the laser beam (described as Gaussian beam) by spiral phase plate. The solution is general as it [...] Read more.
The analytical solution for the propagation of the laser beam with optical vortex through the system of lenses is presented. The optical vortex is introduced into the laser beam (described as Gaussian beam) by spiral phase plate. The solution is general as it holds for the optical vortex of any integer topological charge, the off-axis position of the spiral phase plate and any number of lenses. Some intriguing conclusions are discussed. The higher order vortices are unstable and split under small phase or amplitude disturbance. Nevertheless, we have shown that off-axis higher order vortices are stable during the propagation through the set of lenses described in paraxial approximation, which is untypical behavior. The vortex trajectory registered at image plane due to spiral phase plate shift behaves like a rigid body. We have introduced a new factor which in our beam plays the same role as Gouy phase in pure Gaussian beam. Full article
(This article belongs to the Section Classical Optics)
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10 pages, 2575 KiB  
Letter
The Significance of Carrier Leakage for Stable Lasing in Split-Well Direct Phonon Terahertz Quantum Cascade Lasers
by Nathalie Lander Gower, Silvia Piperno and Asaf Albo
Photonics 2020, 7(3), 59; https://doi.org/10.3390/photonics7030059 - 14 Aug 2020
Cited by 10 | Viewed by 2572
Abstract
We studied the temperature performance of split-well direct phonon terahertz quantum cascade lasers and found that it is limited by a lasing instability that becomes significant as the temperature increases. When the hot electrons of the upper laser level cannot scatter effectively to [...] Read more.
We studied the temperature performance of split-well direct phonon terahertz quantum cascade lasers and found that it is limited by a lasing instability that becomes significant as the temperature increases. When the hot electrons of the upper laser level cannot scatter effectively to excited states due to the high radiative barriers of the structures, a lasing instability occurs, which limits the temperature performance. Full article
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18 pages, 6015 KiB  
Article
Reflectivity of Cholesteric Liquid Crystals with an Anisotropic Defect Layer Inside
by Dariusz Grzelczyk and Jan Awrejcewicz
Photonics 2020, 7(3), 58; https://doi.org/10.3390/photonics7030058 - 10 Aug 2020
Cited by 2 | Viewed by 2940
Abstract
In this study, first, we numerically investigated the reflectivity of a cholesteric liquid crystal with an anisotropic defect layer inside. To model optical phenomena in the examined system, a 4 × 4 matrix method was employed. The tests were carried out for different [...] Read more.
In this study, first, we numerically investigated the reflectivity of a cholesteric liquid crystal with an anisotropic defect layer inside. To model optical phenomena in the examined system, a 4 × 4 matrix method was employed. The tests were carried out for different thicknesses of the whole system, different thicknesses of the defect layer, as well as different defect layer locations inside the cell. Next, a cholesteric liquid crystal comprising a defect layer and held between two parallel electrical conductors was also considered. In this case, the optical properties of the system could also be adjusted by an external applied electric field. Some interesting simulation results of the reflection coefficient (i.e., the fraction of electromagnetic energy reflected) were obtained, illustrated, and discussed. The simulation results showed a significant influence of both the defect and the external electric field on the selective reflection phenomenon, and the possibility of controlling the shape of the reflection spectrum. Finally, some potential applications of the analyzed optical system were discussed. Full article
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11 pages, 3110 KiB  
Communication
Robust Conformal Perfect Absorber Involving Lossy Ultrathin Film
by Lei Zhang, Kun Wang, Hui Chen and Yanpeng Zhang
Photonics 2020, 7(3), 57; https://doi.org/10.3390/photonics7030057 - 6 Aug 2020
Cited by 1 | Viewed by 2308
Abstract
Perfect absorbers have been extensively investigated due to their significant value in solar cell, photodetection, and stealth technologies. Various subwavelength structures have been proposed to improve the absorption performances, such as high absorptance, broad band, and wide absorption angle. However, excellent performances usually [...] Read more.
Perfect absorbers have been extensively investigated due to their significant value in solar cell, photodetection, and stealth technologies. Various subwavelength structures have been proposed to improve the absorption performances, such as high absorptance, broad band, and wide absorption angle. However, excellent performances usually put forward higher requirements on structural designs, such as varying the geometry sizes or shapes to fit different center wavelengths, which inevitably increases the fabrication burden. Here, a planar sandwich structure involving a layer of highly lossy material is proposed to achieve a robust perfect absorption with 95% absorptance ranging from the visible to near infrared range. Such an excellent absorption performance is also polarization-independent and applicable to a wide incident angle. Furthermore, the proposed design can also be applied to conformal surfaces with a 90% fluctuation over a steep surface. We believe that the proposed perfect absorber with distinguished performances can find wide application. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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9 pages, 2191 KiB  
Communication
Wide-Angle Beam-Steering Using an Optical Phased Array with Non-Uniform-Width Waveguide Radiators
by Youngin Kim, Hyeonho Yoon, Jong-Bum You, Minchul Kim and Hyo-Hoon Park
Photonics 2020, 7(3), 56; https://doi.org/10.3390/photonics7030056 - 3 Aug 2020
Cited by 10 | Viewed by 4051
Abstract
We demonstrate wide-angle beam-steering using an optical phased array (OPA) with waveguide radiators designed with non-uniform widths to reduce the crosstalk between waveguides. The OPA consists of a silicon based 1 × 16 array of electro-optic phase shifters and end-fire radiators. The 16 [...] Read more.
We demonstrate wide-angle beam-steering using an optical phased array (OPA) with waveguide radiators designed with non-uniform widths to reduce the crosstalk between waveguides. The OPA consists of a silicon based 1 × 16 array of electro-optic phase shifters and end-fire radiators. The 16 radiators were configured with four different widths and a half-wavelength spacing, which can remove the higher-order diffraction patterns in free space. The waveguides showed a low crosstalk of −10.2 dB at a wavelength of 1540 nm. With phase control, the OPA achieved wide beam-steering of over ±80° with a side-lobe suppression of 7.4 dB. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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14 pages, 3258 KiB  
Article
Characterization and Direct Modulation of a Multi-Section PIC Suited for Short Reach Optical Communication Systems
by Mohab N. Hammad, Aleksandra Kaszubowska-Anandarajah, M. Deseada Gutierrez Pascual, Pascal Landais, Prajwal D. Lakshmijayasimha, Gaurav Jain and Prince M. Anandarajah
Photonics 2020, 7(3), 55; https://doi.org/10.3390/photonics7030055 - 31 Jul 2020
Cited by 3 | Viewed by 3815
Abstract
A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking [...] Read more.
A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking (OIL) of the slave laser. The beneficial impact of the injection is characterized by static and dynamic measurements. The results show a reduction of the optical linewidth from 8 MHz to 2 MHz, a relative intensity noise (RIN) value as low as −154.3 dB/Hz and a 45% improvement of the slave laser modulation bandwidth from 9.5 GHz to 14 GHz. This frequency response enhancement allows the direct modulation of the slave gain section at a data rate of 10.7 Gb/s and an error-free transmission over 25 km of standard single-mode fiber (SSMF). Transmission performance of the injected case shows a 2 dB improvement in the minimum optical power required to achieve a bit error rate of 3.8×103 (hard decision forward error correction limit). These results demonstrate that the multi-section PIC can serve as an attractive cost-efficient transmitter in a wide variety of low-cost short-reach data communication applications. Full article
(This article belongs to the Section Optical Communication and Network)
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15 pages, 3485 KiB  
Article
Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures
by Samuel Morales-Bonilla, Cecilia Mercado-Zúñiga, Juan Pablo Campos-López, César Carrillo-Delgado, Claudia Lizbeth Martínez-González and Carlos Torres-Torres
Photonics 2020, 7(3), 54; https://doi.org/10.3390/photonics7030054 - 30 Jul 2020
Cited by 2 | Viewed by 2732
Abstract
A strong contrast in the third-order nonlinear optical effects exhibited by hierarchical nanostructures explored in a bidirectional optical circuit is reported. The samples were integrated by multiwall carbon nanotubes and platinum-decorated carbon nanotubes synthetized by an aerosol pyrolysis technique and followed by a [...] Read more.
A strong contrast in the third-order nonlinear optical effects exhibited by hierarchical nanostructures explored in a bidirectional optical circuit is reported. The samples were integrated by multiwall carbon nanotubes and platinum-decorated carbon nanotubes synthetized by an aerosol pyrolysis technique and followed by a chemical vapor deposition method. Coupled and decoupled third-order nonlinear optical properties of the nanocomposites were studied. A nanosecond two-wave mixing experiment at 532 nm wavelength was conducted to analyze the optical Kerr effect in the samples. Multi-photonic interactions were evaluated by a single-beam transmittance as a function of input irradiance and volume fraction of the nanoparticles integrated in the nanohybrids. A two-photon absorption process was identified as the main physical mechanism responsible for the anisotropy in the observed optical nonlinearities. Random carbon nanotube networks in film form were put on top of platinum-decorated carbon nanotubes in order to build up a bilayer sample featuring optical selectivity. The switching of optical signals in propagation through the samples was obtained by an orientation-selectable optical transmittance. Unidirectional optically controlled laser pulses dependent on irradiance and polarization in a two-wave mixing was proposed with potential nanophotonic and nanoelectronic applications. The design of signal processing functions driven by nanohybrid platforms can be contemplated. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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17 pages, 3872 KiB  
Review
Nanoplasmonics in High Pressure Environment
by Grégory Barbillon
Photonics 2020, 7(3), 53; https://doi.org/10.3390/photonics7030053 - 28 Jul 2020
Cited by 6 | Viewed by 3800
Abstract
An explosion in the interest for nanoplasmonics has occurred in order to realize optical devices, biosensors, and photovoltaic devices. The plasmonic nanostructures are used for enhancing and confining the electric field. In the specific case of biosensing, this electric field confinement can induce [...] Read more.
An explosion in the interest for nanoplasmonics has occurred in order to realize optical devices, biosensors, and photovoltaic devices. The plasmonic nanostructures are used for enhancing and confining the electric field. In the specific case of biosensing, this electric field confinement can induce the enhancement of the Raman signal of different molecules, or the localized surface plasmon resonance shift after the detection of analytes on plasmonic nanostructures. A major part of studies concerning to plasmonic modes and their application to sensing of analytes is realized in ambient environment. However, over the past decade, an emerging subject of nanoplasmonics has appeared, which is nanoplasmonics in high pressure environment. In last five years (2015–2020), the latest advances in this emerging field and its application to sensing were carried out. This short review is focused on the pressure effect on localized surface plasmon resonance of gold nanosystems, the supercrystal formation of plasmonic nanoparticles stimulated by high pressure, and the detection of molecules and phase transitions with plasmonic nanostructures in high pressure environment. Full article
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12 pages, 586 KiB  
Article
Cloaking Using the Anisotropic Multilayer Sphere
by Sidra Batool, Mehwish Nisar, Fabrizio Frezza and Fabio Mangini
Photonics 2020, 7(3), 52; https://doi.org/10.3390/photonics7030052 - 26 Jul 2020
Cited by 11 | Viewed by 3348
Abstract
We studied a Spherically Radially Anisotropic (SRA) multilayer sphere with an arbitrary number of layers. Within each layer permittivity components are different from each other in radial and tangential directions. Under the quasi-static approximation, we developed a more generalized mathematical model that can [...] Read more.
We studied a Spherically Radially Anisotropic (SRA) multilayer sphere with an arbitrary number of layers. Within each layer permittivity components are different from each other in radial and tangential directions. Under the quasi-static approximation, we developed a more generalized mathematical model that can be used to calculate polarizability of the SRA multilayer sphere with any arbitrary number of layers. Moreover, the functionality of the SRA multilayer sphere as a cloak has been investigated. It has been shown that by choosing a suitable contrast between components of the permittivity, the SRA multilayer sphere can achieve threshold required for invisibility cloaking. Full article
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12 pages, 11616 KiB  
Article
Microstructured Fibers Based on Tellurite Glass for Nonlinear Conversion of Mid-IR Ultrashort Optical Pulses
by Elena A. Anashkina, Vitaly V. Dorofeev, Sergey A. Skobelev, Alexey A. Balakin, Sergei E. Motorin, Alexey F. Kosolapov and Alexey V. Andrianov
Photonics 2020, 7(3), 51; https://doi.org/10.3390/photonics7030051 - 25 Jul 2020
Cited by 10 | Viewed by 2467
Abstract
Compact fiber-based sources generating optical pulses with a broadband spectrum in the mid-IR range are in demand for basic science and many applications. Laser systems producing tunable Raman solitons in special soft-glass fibers are of great interest. Here, we report experimental microstructured tellurite [...] Read more.
Compact fiber-based sources generating optical pulses with a broadband spectrum in the mid-IR range are in demand for basic science and many applications. Laser systems producing tunable Raman solitons in special soft-glass fibers are of great interest. Here, we report experimental microstructured tellurite fibers and demonstrate by numerical simulation their applicability for nonlinear soliton conversion in the mid-infrared (-IR) range via soliton self-frequency shift. The fiber dispersion and nonlinearity are calculated for experimental geometry. It is shown numerically that there are two zero dispersion wavelengths for the core size of 2 μm and less. In such fibers, efficient Raman soliton tuning is attained up to a central wavelength of 4.8 μm using pump pulses at 2.8 μm. Full article
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11 pages, 1023 KiB  
Article
Photonic Nanojets and Whispering Gallery Modes in Smooth and Corrugated Micro-Cylinders under Point-Source Illumination
by Ibrahim Mahariq, Thabet Abdeljawad, Abdullah S. Karar, Shadi A. Alboon, Hamza Kurt and Alexey V. Maslov
Photonics 2020, 7(3), 50; https://doi.org/10.3390/photonics7030050 - 21 Jul 2020
Cited by 17 | Viewed by 3372
Abstract
We numerically investigate the generation of photonic nanojets (PNJs) and the excitation of whispering gallery modes (WGMs) supported by both smooth and corrugated dielectric micro-cylinders under point-source illumination. Results show that the location of the point-source defines the location and properties of PNJs, [...] Read more.
We numerically investigate the generation of photonic nanojets (PNJs) and the excitation of whispering gallery modes (WGMs) supported by both smooth and corrugated dielectric micro-cylinders under point-source illumination. Results show that the location of the point-source defines the location and properties of PNJs, whereas stability of WGMs exists in smooth micro-cylinders but vanishes in corrugated ones. It is shown that the location of the point-source acts as an additional degree of freedom for controlling the characteristics of the generated PNJs for both smooth and corrugated dielectric micro-cylinders. Furthermore, the influence of the point-source location on the stability of the excited WGMs was diminished for the smooth micro-cylinders, while being fully pronounced for their corrugated counterparts. Full article
(This article belongs to the Special Issue Photonic Devices and Systems)
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18 pages, 2344 KiB  
Article
Novel High-Resolution Lateral Dual-Axis Quad-Beam Optical MEMS Accelerometer Using Waveguide Bragg Gratings
by Balasubramanian Malayappan, Narayan Krishnaswamy and Prasant Kumar Pattnaik
Photonics 2020, 7(3), 49; https://doi.org/10.3390/photonics7030049 - 18 Jul 2020
Cited by 14 | Viewed by 3776
Abstract
A novel lateral dual-axis a-Si/SiO2 waveguide Bragg grating based quad-beam accelerometer with high-resolution and large linear range has been presented in this paper. The sensor consists of silicon bulk micromachined proof mass suspended by silica beams. Three ridge gratings are positioned on [...] Read more.
A novel lateral dual-axis a-Si/SiO2 waveguide Bragg grating based quad-beam accelerometer with high-resolution and large linear range has been presented in this paper. The sensor consists of silicon bulk micromachined proof mass suspended by silica beams. Three ridge gratings are positioned on the suspending beam and proof mass to maximize sensitivity and reduce noise. Impact of external acceleration in the sensing direction on the Bragg wavelength of gratings and MEMS structure has been modelled including the effects of strain, stress and temperature variation. Acceleration induces stress in the beam thus modifying the grating period and introducing chirp. The differential wavelength shift with respect to reference grating on the proof mass is the measure of acceleration. To compensate for the effect of the weight of the proof mass and increase the sensitivity of the sensor, electrostatic force of repulsion is applied to the proof mass. For the chosen parameters, the designed sensor has a linear response over a large range and a sensitivity of 30 pm/g. The temperature of surroundings, which acts as noise in sensor performance is compensated by taking differential wavelength shift with respect to reference grating. By design and choice of material, low cross-axis sensitivity is achieved. The proposed design enables a high-resolution well below 1 μ g/ Hz and is suitable for inertial navigation and seismometry applications. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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