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10 pages, 5839 KiB

Open AccessCommunication

Broadband Thermo-Optic Photonic Switch for TE and TM Modes with Adiabatic Design

by Babak Hashemi, Maurizio Casalino, Teresa Crisci, Mohamed Mammeri and Francesco G. Della Corte

Photonics 2024, 11(12), 1177; https://doi.org/10.3390/photonics11121177 - 14 Dec 2024

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Abstract

Optical power switches are essential components in fiber optic communication systems, requiring minimal losses, a broad operating wavelength range, and high tolerance to fabrication errors for optimal performance. Adiabatic optical power switches inherently meet these criteria and are well suited for manufacturing processes [...] Read more.

Optical power switches are essential components in fiber optic communication systems, requiring minimal losses, a broad operating wavelength range, and high tolerance to fabrication errors for optimal performance. Adiabatic optical power switches inherently meet these criteria and are well suited for manufacturing processes which support large-scale production at low costs. This paper presents the design and simulation of an adiabatic switch with a flat response in the whole 1525–1625 nm wavelength range (C band and L band) for both TE and TM polarizations. The switch is based on the thermo-optic effect induced by local variations in temperature. The impacts of the design parameters, such as the device length and dissipated heat, are analyzed. The simulation results indicate that the switch achieved high efficiency and low insertion losses, highlighting the potential of adiabatic switches for reliable and scalable integration into advanced optical circuits. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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15 pages, 9179 KiB

Open AccessArticle

Underwater Dynamic Polarization-Difference Imaging with Greater Applicability

by Jinxin Deng, Jingping Zhu, Haoxiang Li, Yucai Kuang, Angze Li and Xiaofang Liu

Photonics 2024, 11(11), 1069; https://doi.org/10.3390/photonics11111069 - 14 Nov 2024

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Abstract

Available polarization-difference imaging techniques face serious challenges in imaging speed and application range. To address these issues, this paper proposes an underwater dynamic polarization-difference imaging method with greater applicability. First, the intensity distribution of backscattered light is estimated via the Stokes vector. Afterward, [...] Read more.

Available polarization-difference imaging techniques face serious challenges in imaging speed and application range. To address these issues, this paper proposes an underwater dynamic polarization-difference imaging method with greater applicability. First, the intensity distribution of backscattered light is estimated via the Stokes vector. Afterward, the differential operation between the total intensity of light and the amplified estimation result of backscattered light makes clear imaging immediately accessible. Regardless of the movement states and polarization characteristics of the target, experimental results consistently demonstrate that the backscattered light can be eliminated to a great extent, and imaging quality and applicability are significantly enhanced. Meanwhile, the proposed method is immune to unexpected factors such as uneven illumination and has good stability. More importantly, there are also apparent advantages in terms of imaging time. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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9 pages, 8246 KiB

Open AccessArticle

Passively Mode-Locked Erbium-Doped Fiber Laser and Application in Laser Thrombolysis

by Xiaoli Zhao, Linyu Cong, Congyu Zhang, Chenxi Zhang, Ijaz Ahmad and Bo Fu

Photonics 2024, 11(11), 1006; https://doi.org/10.3390/photonics11111006 - 25 Oct 2024

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Abstract

Fiber lasers have been widely used in surgery with the development of fiber photonics. Since the human body is prone to myocardial infarction caused by blood clots, laser thrombolysis was proposed as a safe and efficient treatment. Mode-locked fiber lasers have high peak [...] Read more.

Fiber lasers have been widely used in surgery with the development of fiber photonics. Since the human body is prone to myocardial infarction caused by blood clots, laser thrombolysis was proposed as a safe and efficient treatment. Mode-locked fiber lasers have high peak power and narrow pulse width. In order to observe the effect of laser thrombolysis with mode-locked fiber lasers, a 1.5 µm mode-locked fiber laser based on carbon nanotubes was built, showing a pulse width of 1.46 ps, a 3 dB bandwidth of 1.65 nm, and a repetition rate of 29.5 MHz. The output pulses were amplified by an erbium-doped fiber amplifier to the hundred-milliwatt level and were applied to the surface of a self-made thrombus. The influences of lasing power and time on the damage diameter of the thrombus surface were evaluated. A low threshold damage power of 45 mW was observed, which resulted from the high peak power of the mode-locked pulses. These results demonstrate that high ablation efficiency can be achieved by using mode-locked pulses with a narrow pulse width and high peak power. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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11 pages, 3729 KiB

Open AccessArticle

One-Second Touchscreen Disinfection with Internal UVC-LEDs

by Kilian Noller, Ben Sicks and Martin Hessling

Photonics 2024, 11(11), 1001; https://doi.org/10.3390/photonics11111001 - 24 Oct 2024

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Abstract

The transmission of pathogens via touchscreens is a concern both in medical environments regarding nosocomial infections and in public spaces. This study presents the development of a self-disinfecting touchscreen system and its implementation into an existing medical device. The disinfection was achieved by [...] Read more.

The transmission of pathogens via touchscreens is a concern both in medical environments regarding nosocomial infections and in public spaces. This study presents the development of a self-disinfecting touchscreen system and its implementation into an existing medical device. The disinfection was achieved by irradiating UVC light laterally into the quartz front glass of the touchscreen. The system also included a two-stage safety shutdown feature to protect users from UVC exposure. To detect the presence of the user’s hand, a proximity sensor behind the front glass of the touch screen was installed in combination with touch input registration. A rapid disinfection of staphylococci was achieved without compromising usability and user safety. The prototype demonstrated a bacterial reduction of 99.96% or 3.4 (±0.36) log levels in less than 1 s. The results suggest that the technology has the potential to make a significant contribution to reducing the transmission of infections in healthcare settings and beyond. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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9 pages, 1617 KiB

Open AccessArticle

Nd:YVO₄ Random Laser with Preferential Emission at 1340 nm over 1064 nm

by Jessica Dipold, Luciana R. P. Kassab and Niklaus U. Wetter

Photonics 2024, 11(10), 898; https://doi.org/10.3390/photonics11100898 - 25 Sep 2024

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Abstract

Neodymium-doped yttrium vanadate random lasers have presented exceptional efficiency and output power at the 1064 nm emission wavelength. However, emission at 1340 nm has not yet been observed for these random lasers, even though regular bulk lasers have presented many impressive properties in [...] Read more.

Neodymium-doped yttrium vanadate random lasers have presented exceptional efficiency and output power at the 1064 nm emission wavelength. However, emission at 1340 nm has not yet been observed for these random lasers, even though regular bulk lasers have presented many impressive properties in this infrared region. Here, we present a dual-emission Nd³⁺:YVO₄ pellet random laser, which emits at both 1064 nm and 1340 nm using a 585 nm pump wavelength, showing a new property corresponding to a much lower laser threshold at 1340 nm than with 1064 nm. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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14 pages, 309 KiB

Open AccessArticle

Theoretical Investigation of the Influence of Correlated Electric Fields on Wavefront Shaping

by Niklas Fritzsche, Felix Ott, David Hevisov, Dominik Reitzle and Alwin Kienle

Photonics 2024, 11(9), 797; https://doi.org/10.3390/photonics11090797 - 27 Aug 2024

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Abstract

Wavefront shaping is a well-known method of restoring a focus deep within scattering media by manipulating the incident light. However, the achievable focus enhancement depends on and is limited by the optical and geometrical properties of the medium. These properties contribute to the [...] Read more.

Wavefront shaping is a well-known method of restoring a focus deep within scattering media by manipulating the incident light. However, the achievable focus enhancement depends on and is limited by the optical and geometrical properties of the medium. These properties contribute to the number of linearly independent transmission channels for light propagating through the turbid medium. Correlations occur when the number of incident waves coupled into the scattering medium exceeds this finite number of transmission channels. This paper investigates the wavefront shaping of such correlated electric fields. The influence of the observed correlations persists even though the average electric field distribution at positions in the focal plane follows a circular complex Gaussian. We show that correlations of the transmitted electric fields reduce the achievable intensity enhancement, even deep in the turbid medium. The investigations are carried out using a Monte Carlo algorithm. It is based on the speckle statistics of independent waves and introduces correlations of neighbouring electric fields via a Cholesky decomposition of the covariance matrix. Additional investigations include scenarios where the electric fields are not completely randomized, such as for ballistic or insufficiently scattered light. Significant contributions from such little-scattered light are observed to reduce the intensity enhancement further. Data from simulations solving Maxwell’s equations are compared with the results obtained from the Monte Carlo simulations for validation throughout this paper. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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14 pages, 863 KiB

Open AccessArticle

Interferometrically Enhanced Intensity and Wavelength Modulation in Tunable Diode Laser Spectroscopy

by Sander Vervoort and Marcus Wolff

Photonics 2024, 11(8), 740; https://doi.org/10.3390/photonics11080740 - 8 Aug 2024

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Abstract

Tunable diode laser spectroscopy (TDLS) is a measurement technique with high spectral resolution. It is based on tuning the emission wavelength of a semiconductor laser by altering its current and/or its temperature. However, adjusting the wavelength leads to a change in emission intensity. [...] Read more.

Tunable diode laser spectroscopy (TDLS) is a measurement technique with high spectral resolution. It is based on tuning the emission wavelength of a semiconductor laser by altering its current and/or its temperature. However, adjusting the wavelength leads to a change in emission intensity. For applications that rely on modulated radiation, the challenge is to isolate the true spectrum from the influence of extraneous instrumental contributions, particularly residual intensity and wavelength modulation. We present a novel approach combining TDLS with interferometric techniques, exemplified by the use of a Mach–Zehnder interferometer, to enable the separation of intensity and wavelength modulation. With interferometrically enhanced intensity modulation, we reduced the residual wavelength modulation by 83%, and with interferometrically enhanced wavelength modulation, we almost completely removed the residual derivative of the signal. A reduction in residual wavelength modulation enhances the spectral resolution of intensity-modulated measurements, whereas a reduction in residual intensity modulation improves the signal-to-noise ratio and the sensitivity of wavelength-modulated measurements. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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8 pages, 7013 KiB

Open AccessArticle

Bessel-Beam Single-Photon High-Resolution Imaging in Time and Space

by Huiyu Qi, Zhaohui Li, Yurong Wang, Xiuliang Chen, Haifeng Pan, E Wu and Guang Wu

Photonics 2024, 11(8), 704; https://doi.org/10.3390/photonics11080704 - 29 Jul 2024

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Abstract

Synchronous laser beam scanning is a common technique used in single-photon imaging where the spatial resolution is primarily determined by the beam divergence angle. In this context, Bessel beams have been investigated as they can overcome the diffraction limit associated with traditional Gaussian [...] Read more.

Synchronous laser beam scanning is a common technique used in single-photon imaging where the spatial resolution is primarily determined by the beam divergence angle. In this context, Bessel beams have been investigated as they can overcome the diffraction limit associated with traditional Gaussian beams. Notably, the central spot of a Bessel beam retains its size almost unchanged within a non-diffractive distance. However, the presence of sidelobes in the Bessel beam can negatively impact spatial resolution. To address this challenge, we have developed a single-photon imaging system with high-depth resolution, which allows for the suppression of echo photons from the sidelobe light in the depth image, particularly when their flight time differs from that of the central spot. In our LiDAR setup, we successfully achieved high-resolution scanning imaging with a spatial resolution of approximately 0.5 mm while also demonstrating a high-depth resolution of 12 mm. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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15 pages, 23783 KiB

Open AccessArticle

Evaluation of Fluorescence Contrast for the Differentiation of Ex Vivo Tissue Slides from Collagen-Related Degenerative Skin Diseases

by Tsanislava Genova, Petya Pavlova, Lidia Zaharieva, Petranka Troyanova and Ivan Terziev

Photonics 2024, 11(8), 687; https://doi.org/10.3390/photonics11080687 - 24 Jul 2024

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Abstract

Histopathological analysis is one of the primary pillars in clinical diagnostics. The efforts to implement optical techniques aim at alleviating the burden of delivering timely and accurate diagnoses. We have explored the potential application of unstained tissue slides’ autofluorescence to differentiate collagen-related skin [...] Read more.

Histopathological analysis is one of the primary pillars in clinical diagnostics. The efforts to implement optical techniques aim at alleviating the burden of delivering timely and accurate diagnoses. We have explored the potential application of unstained tissue slides’ autofluorescence to differentiate collagen-related skin degenerative diseases, such as psoriasis, lupus erythematosus, scleroderma, and Syndrome of Raynaud. This exploration involved two techniques: fluorescence microscopy combined with colorimetric analysis and synchronous fluorescence spectroscopy. We addressed the main characteristic peculiarities of the examined samples and discussed the evaluation of potential classification parameters along with their diagnostic values. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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17 pages, 5806 KiB

Open AccessArticle

Subset-Optimized Eight-Dimensional Trellis-Coded Modulation Scheme in High-Speed Optical Communication

by Jiexin Chen, Qi Zhang, Qihan Zhao, Xiangjun Xin, Ran Gao, Haipeng Yao, Feng Tian, Yongjun Wang, Qinghua Tian, Leijing Yang, Lan Rao, Fu Wang and Sitong Zhou

Photonics 2024, 11(7), 584; https://doi.org/10.3390/photonics11070584 - 21 Jun 2024

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Abstract

In this paper, a subset-optimized eight-dimensional trellis-coded quadrature amplitude modulation (SO-8DTCM-16QAM) format for higher-order constellations in high-speed optical communications is proposed. This scheme increases the number of subsets of base 2D constellation divisions. On this basis, it is further optimized by using 2D [...] Read more.

In this paper, a subset-optimized eight-dimensional trellis-coded quadrature amplitude modulation (SO-8DTCM-16QAM) format for higher-order constellations in high-speed optical communications is proposed. This scheme increases the number of subsets of base 2D constellation divisions. On this basis, it is further optimized by using 2D subsets for Cartesian product combinations to obtain 4D subsets and eliminate the combinations with small Euclidean distances. Finally, the 4D subsets are utilized to construct interrelated 8D subsets for trellis coding modulation and signal transmission. The proposed scheme can effectively reduce the decoding complexity and outperforms the conventional scheme at a high signal-to-noise ratio (SNR). Simulation verification of the proposed scheme is carried out, and the results show that the SO-8DTCM-16QAM achieves signal-to-noise ratio (SNR) gains of 1.60 dB, 1.56 dB, 1.51 dB, and 1.33 dB, respectively, compared with the conventional 8D-16QAM signals when BTB and 5/20/30 km optical signal transmission are performed. The SO-8DTCM-16QAM also achieves an SNR gain of 1.86 dB, 1.75 dB, and 1.22 dB at a net transmission rate of 14/21/28 GBaud. In addition, the SO-8DTCM-16/32/64QAM achieves an SNR gain of 1.27 dB, 0.80 dB, and 1.24 dB, respectively, when compared with the unoptimized 8DTCM-16/32/64QAM. Meanwhile, the proposed eight-subset SO-8DTCM-QAM scheme reduces the complexity of the decoding computation in the subset selection part and the constellation point selection part by 93.75% and 50%, respectively, compared with the unoptimized eight-subset and four-subset 8DTCM-QAM schemes. It can be seen that the proposed scheme simultaneously optimizes the transmission performance and complexity of high-speed optical communication systems and has practical application value. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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17 pages, 4176 KiB

Open AccessArticle

High-Dynamic-Range Absorption Spectroscopy by Generating a Wide Path-Length Distribution with Scatterers

by Ayaka Mori, Kyohei Yamashita and Eiji Tokunaga

Photonics 2024, 11(6), 556; https://doi.org/10.3390/photonics11060556 - 13 Jun 2024

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Abstract

In absorption spectroscopy, it is challenging to detect absorption peaks with significant differences in their intensity in a single measurement. We enable high-dynamic-range measurements by dispersing scatterers within a sample to create a broad distribution of path lengths (PLs). The sample is placed [...] Read more.

In absorption spectroscopy, it is challenging to detect absorption peaks with significant differences in their intensity in a single measurement. We enable high-dynamic-range measurements by dispersing scatterers within a sample to create a broad distribution of path lengths (PLs). The sample is placed within an integrating sphere (IS) to capture all scattered light of various PLs. To address the complexities of PLs inside the IS and the sample, we performed a ray-tracing simulation using the Monte Carlo (MC) method, which estimates the measured absorbance

A

and PL distribution from the sample’s absorption coefficient µ_a and scattering properties at each wavelength λ. This method was validated using dye solutions with two absorption peaks whose intensity ratio is 95:1, employing polystyrene microspheres (PSs) as scatterers. The results confirmed that both peak shapes were delineated in a single measurement without flattening the high absorption peak. Although the measured peak shapes A(λ) did not align with the actual peak shapes µ_a(λ), MC enabled the reproduction of µ_a(λ) from A(λ). Furthermore, the analysis of the PL distribution by MC shows that adding scatterers broadens the distribution and shifts it toward shorter PLs as absorption increases, effectively adjusting it to µ_a. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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13 pages, 9090 KiB

Open AccessArticle

Discriminating Glioblastoma from Normal Brain Tissue In Vivo Using Optical Coherence Tomography and Angiography: A Texture and Microvascular Analysis Approach

by Trung Nguyễn-Hoàng, Tai-Ang Wang, Chia-Heng Wu and Meng-Tsan Tsai

Photonics 2024, 11(5), 435; https://doi.org/10.3390/photonics11050435 - 8 May 2024

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Abstract

Brain tumors arise from abnormal cell growth in the brain. Glioblastoma, the most common and aggressive type, poses significant challenges for identification during surgery. The primary goal of this study is to identify and differentiate normal brain tissue from glioblastoma tissue using optical [...] Read more.

Brain tumors arise from abnormal cell growth in the brain. Glioblastoma, the most common and aggressive type, poses significant challenges for identification during surgery. The primary goal of this study is to identify and differentiate normal brain tissue from glioblastoma tissue using optical coherence tomography (OCT) and OCT angiography (OCTA). These techniques offer a non-invasive way to analyze the morphological and microvascular alternations associated with glioblastoma in an animal model. To monitor the changes in morphology and vascular distribution of brain tissue as glioblastoma tumors grow, time-series OCT and OCTA results were collected for comparison. Texture analysis of OCT images was proposed using the gray-level co-occurrence matrix (GLCM), from which homogeneity and variance were calculated as discriminative parameters. Additionally, OCTA was used to assess microvascular characteristics, including vessel diameter, density, and fractal dimension. The findings demonstrate that the proposed methods can effectively distinguish between normal and cancerous brain tissue in vivo. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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11 pages, 2059 KiB

Open AccessArticle

The Diffraction Efficiency of Acrylate-Based Holographically Photopolymerized Gratings Enhanced by the Dark Reaction

by Ziyan Bai, Wenfeng Cai, Ming Cheng, Shun Lan, Delai Kong, Jian Shen, Mengjia Cen, Dan Luo, Yuan Chen and Yan Jun Liu

Photonics 2024, 11(4), 320; https://doi.org/10.3390/photonics11040320 - 29 Mar 2024

Viewed by 1108

Abstract

Photopolymers, especially acrylate-based ones with low cost and simple preparation, are promising materials for high-efficiency holographic gratings. However, it is still challenging to achieve high-performance gratings, due to the influences of many factors. In this work, we found that the dark reaction plays [...] Read more.

Photopolymers, especially acrylate-based ones with low cost and simple preparation, are promising materials for high-efficiency holographic gratings. However, it is still challenging to achieve high-performance gratings, due to the influences of many factors. In this work, we found that the dark reaction plays a critical role. The effect of the dark reaction on the optical properties of holographic gratings was investigated. Experimental results reveal that the diffraction efficiency of the gratings can be improved by a factor of three by involving the dark reaction process, and the highest diffraction efficiency for gratings can reach 97.8% after optimization. Therefore, the dark reaction can greatly enhance the optical performance of acrylate-based holographic gratings and other optical elements, thus holding great potential for many applications. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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Review

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37 pages, 22844 KiB

Open AccessReview

A Linear, Direct Far-Field Subwavelength Imaging Method: Microparticle-Assisted Nanoscopy

by Constantin Simovski

Photonics 2024, 11(11), 1005; https://doi.org/10.3390/photonics11111005 - 25 Oct 2024

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Abstract

Microparticle-assisted nanoscopy (MAN) is a novel emerging technique of direct far-field deeply subwavelength imaging, which has been developed since 2011 as a set of experimental techniques. For a decade, the capability of a simple glass microsphere without fluorescent labels or plasmonic elements to [...] Read more.

Microparticle-assisted nanoscopy (MAN) is a novel emerging technique of direct far-field deeply subwavelength imaging, which has been developed since 2011 as a set of experimental techniques. For a decade, the capability of a simple glass microsphere without fluorescent labels or plasmonic elements to grant a direct, broadband, deeply subwavelength image of a nanostructured object was unexplained. Four years ago, the explanation of MAN via the suppression of diffraction was suggested by the author of the present overview. This explanation was confirmed by extensive full-wave simulations, which agreed with available experimental data and revealed new opportunities for MAN. Although the main goal of the present paper is to review recent works, state-of-the-art concepts in MAN are also reviewed. Moreover, so that the peculiarities of MAN are better outlined, its uniqueness compared to other practically important methods of far-field subwavelength imaging is also discussed. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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