Photonics

Research

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

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, 4307 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

Viewed by 919

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 780

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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