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Reduced resolution of polarized images makes it difficult to distinguish detailed polarization information and limits the ability to identify small targets and weak signals. A possible way to handle this problem is the polarization super-resolution (SR), which aims to obtain a high-resolution polarized image from a low-resolution one. However, compared with the traditional intensity-mode image SR, the polarization SR is more challenging because more channels and their nonlinear cross-links need to be considered as well as the polarization and intensity information need to be reconstructed simultaneously. This paper analyzes the polarized image degradation and proposes a deep convolutional neural network for polarization SR reconstruction based on two degradation models. The network structure and the well-designed loss function have been verified to effectively balance the restoration of intensity and polarization information, and can realize the SR with a maximum scaling factor of four. Experimental results show that the proposed method outperforms other SR methods in terms of both quantitative evaluation and visual effect evaluation for two degradation models with different scaling factors.
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In this Letter, we present a self-supervised method, polarization to polarization (Pol2Pol), for polarimetric image denoising with only one-shot noisy images. First, a polarization generator is proposed to generate training image pairs, which are synthesized from one-shot noisy images by exploiting polarization relationships. Second, the Pol2Pol method is extensible and compatible, and any network that performs well in supervised image denoising tasks can be deployed to Pol2Pol after proper modifications. Experimental results show Pol2Pol outperforms other self-supervised methods and achieves comparable performance to supervised methods.
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In this work, we experimentally demonstrate a passband filter for the 2-µm wave band on the silicon-on-insulator platform. The device consists of a strip waveguide and an apodized subwavelength-structured waveguide. Fabricated on a 340-nm-thick silicon membrane, the proposed passband filter shows a 3-dB bandwidth of 16-33â nm, a high sidelobe suppression ratio (SLSR) of 24â dB, and a low insertion loss (IL) of 0.4â dB.
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In this paper, fiber sensor based on Vernier effect for simultaneous measurement of relative humidity (RH) and temperature is proposed. The sensor is fabricated by coating two kinds of ultraviolet (UV) glue with different refractive indexes (RI) and thicknesses on the end face of a fiber patch cord. The thicknesses of two films are controlled to generate the Vernier effect. The inner film is formed by a cured lower-RI UV glue. The exterior film is formed by a cured higher-RI UV glue, of which thickness is much thinner than the inner film. Through the analysis of the Fast Fourier Transform (FFT) of the reflective spectrum, the Vernier effect is formed by the inner lower-RI polymer cavity and the cavity composed of both polymer films. By calibrating the RH and temperature response of two peaks on the envelope of the reflection spectrum, simultaneous measurements of RH and temperature are realized by solving a set of quadratic equations. Experimental results show that the highest RH and temperature sensitivities of the sensor are 387.3 pm/%RH (in 20%RH to 90%RH) and -533.0 pm/°C (in 15 °C to 40 °C), respectively. The sensor has merits of low cost, simple fabrication, and high sensitivity, which makes it very attractive for applications that need to simultaneously monitor these two parameters.
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Mid-infrared (mid-IR) silicon photonics has been attracting great attention due to its tremendous potential applications in nonlinear optics, ranging, sensing, and spectroscopy. To date, mid-IR silicon devices have usually been developed based on silicon wafers with top-layer silicon thicknesses of hundreds of nanometers. Compared with the thick silicon devices, tens-of-nanometers thin silicon devices can provide giant evanescent-field energy proportions and optical mode areas, being significant for many biochemical sensing and nonlinear optics applications. However, ultra-thin mid-IR silicon devices have seldom been studied due to the difficulty of light coupling. Here, we demonstrated an ultra-thin focusing subwavelength-grating coupler for mid-IR ultra-thin suspended subwavelength-grating-cladding waveguide coupling. The results show that the grating has a maximum coupling efficiency of -7.1 dB at a center wavelength of 2200 nm with a 1-dB bandwidth of â¼115 nm and back reflection of -19.9 dB. We also measured the fiber alignment tolerance of 12 µm for 3-dB coupling efficiency reduction and bending optical loss of 0.25 dB/90°. Our results pave the way to developing mid-IR ultra-thin photonic integrated circuits.
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Mid-infrared (Mid-IR) (2-20â µm) silicon photonics has attracted much attention in the past few years due to its application potential in free-space optical communications, light detection and ranging, and molecular analysis. The grating coupler technology is one of the most widely employed approaches for light coupling between optical fibers and waveguides. In the mid-IR spectral region, due to the lack of reliable chalcogenide-fiber or ZBLAN-fiber polarization controllers, grating couplers usually suffer from huge insertion losses induced by the arbitrary polarization states of light coupled out of mid-IR fibers. As a result, it is significant to explore polarization-insensitive grating coupling techniques in mid-IR wavelengths. However, the study is currently still in its infancy. Here, we demonstrate an ultra-thin mid-IR polarization-insensitive grating coupler. The grating coupler has a maximum coupling efficiency of -11.5â dB at a center wavelength of â¼2200â nm with a 1-dB bandwidth of â¼148â nm. Compared with conventional subwavelength grating couplers, the polarization-dependent loss was improved from 9.6â dB to 2.1â dB. Moreover, we demonstrated a polarization-insensitive grating coupler at 2700-nm wavelength with a maximum coupling efficiency of -12.0â dB. Our results pave the way for the development of mid-IR photonic integrated circuits.
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In this Letter, we propose an attention-based neural network specially designed for the challenging task of polarimetric image denoising. In particular, the channel attention mechanism is used to effectively extract the features underlying the polarimetric images by rescaling the contributions of channels in the network. In addition, we also design the adaptive polarization loss to make the network focus on the polarization information. Experiments show that our method can well restore the details flooded by serious noise and outperforms previous methods. Moreover, the underlying mechanism of channel attention is revealed visually.
Assuntos
Redes Neurais de Computação , Razão Sinal-Ruído , Análise EspectralRESUMO
In the myopia correction surgery by femtosecond laser, such as Laser in Situ Keratomileusis (LASIK) and Small Incision Lenticule Extraction (SMILE), the nonlinear refractive index of the cornea may cause the deviation of cutting depth. In order to improve the cornea cutting's accuracy and reduce the possibility of undercorrection, the nonlinear refractive index coefficient n2 of the human cornea must be measured with high accuracy. The spectral domain Z-scan technique can measure n2 of the highly scattering biological tissues with much better signal to noise ratio and thus better accuracy than the conventional methods. In this paper, the n2 coefficient of one ex-vivo human corneal sample was measured by the spectral domain Z-scan technique. Experimental results show that as this corneal sample gradually dehydrates, its n2 coefficients are 1.1 ± 0.1×10-19 m2/W, 1.4 ± 0.2×10-19 m2/W and 1.6 ± 0.2 ×10-19 m2/W respectively for the corneal sample with water contents of 89%, 82%, and 78%. The increase of the water content reduces the value of n2, which is reasonable since the nonlinear refractive index coefficient of water is one order of magnitude smaller than that of the cornea.
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Córnea/fisiologia , Miopia/cirurgia , Refração Ocular/fisiologia , Água Corporal , Córnea/cirurgia , Humanos , Ceratomileuse Assistida por Excimer Laser In Situ/métodos , Masculino , Melhoria de Qualidade , Espalhamento de Radiação , Manejo de Espécimes/métodos , Fatores de Tempo , Adulto JovemRESUMO
Previous polarization underwater imaging methods based on the physical scattering model usually require background region included in the image and the prior knowledge, which hinders its practical application. In this paper, we analyze and optimize the physically feasible region and propose an improved method by degenerating intermediate variables, which can realize automatic underwater image recovery without background region or any prior. The proposed method does not need to estimate the intermediate variables in the traditional underwater imaging model and is adaptable to the underwater image with non-uniform illumination, which avoids the poor and unstable image recovery performance caused by inaccurate estimation of intermediate parameters due to the improper identification of the background region. Meanwhile, our method is effective for both images without background region and images in which the background region is hard to be identified. In addition, our method solves the significant variation in recovery results caused by the different selection of background regions and the inconsistency of parameter adjustment. The experimental results of different underwater scenes show that the proposed method can enhance image contrast while preserving image details without introducing considerable noise, and the proposed method is effective for the dense turbid medium.
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Stimulus-triggered drug delivery systems (DDSs) based on lanthanide-doped upconversion nanoparticles (UCNPs) have attracted significant attention for treating cancers due to their merits of high drug availability, precisely controlled drug release, and low side-effects. However, such DDSs usually exhibit a single stimulus-response, which may limit the efficiency of cancer treatment. To extend response types in a single DDS, we construct NaYF4:Yb/Tm@SiO2-doxorubicin (Dox)/curcumin (Cur)-chitosan (CS)/2-Octen-1-ylsuccinic anhydride (OSA) nanoparticles with core-shell structures. Our method is based on the exploration of the synergistic effect of UCNPs and multiple drugs. In particular, the NaYF4:Yb/Tm is used to convert near-infrared light to visible light, activating Cur photosensitizers to produce singlet oxygen for photodynamic therapy, while CS/OSA responds to a low pH environment to release cancer drugs, including Dox and Cur for chemotherapy through breaking a free carboxyl group. The results show that the UCNPs with a 40 nm diameter, 23 nm thick mesoporous SiO2, and 19/1 mol% Yb3+/Tm3+concentrations could continuously release Dox and Cur at a pH value of 6.5 within 6 h after the excitation of a 980 nm-wavelength CW laser. Our study provides a promising approach for developing efficient DDSs for cancer treatment.
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The variation of environment temperature is a crucial problem for optical magnetic field sensors based on the magneto-optical crystal. In this paper, we propose a novel temperature compensation method for optical alternating magnetic field measuring by analyzing the demodulation principle and establishing the temperature compensation model, which can implement the functions of temperature compensation and on-line measuring simultaneously. Both the temperature and the alternating magnetic field flux density can be obtained only by adding two magnet rings on the magnetic field sensor. The experimental phenomenon agrees well with the temperature characteristics of the magneto-optical crystal and the theoretical compensation model. The experimental results demonstrate that this sensor has excellent stability whose max relative fluctuation is only 0.7402% in the range of 0-4 mT under a constant temperature. In the temperature compensation experiment of 0 °C, 20 °C and 40 °C, the sensor shows strong temperature robustness that the max absolute and relative errors are 0.07 mT and 3.50%, respectively. Meanwhile, compensation efficiency reaches 83.968%, which can effectively avoid temperature crosstalk to a large extent. Additionally, it has a better compensation performance whose max absolute and relative errors are 0.15 mT and 1.66% in the broader range of 0-16 mT when the actual temperature is accurately known.
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Mode-division multiplexing (MDM) has attracted broad attention as it could effectively boost up transmission capability by utilizing optical modes as a spatial dimension in optical interconnects. In such a technique, different data channels are usually modulated to the respective carriers over different spatial modes by using individual parallel electro-optic modulators. Each modulated channel is then multiplexed to a multi-mode waveguide. However, the method inevitably suffers from a high cost, large device footprint and high insertion loss. Here, we design intensity and phase dual-mode modulators, enabling simultaneous modulations over two channels via a graphene-on-silicon waveguide. Our method is based on the exploration of co-planar interactions between structured graphene nanoribbons (GNs) and spatial modes in a silicon waveguide. Specifically, the zeroth-order transverse electric (TE0) and first-order transverse electric (TE1) modes are modulated separately and simultaneously by applying independent driving electrodes to different GNs in an identical modulator. Our study is expected to open an avenue to develop high-density MDM photonics integrated circuits for tera-scale optical interconnects.
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Based on measuring the polarimetric parameters which contain specific physical information, polarimetric imaging has been widely applied to various fields. However, in practice, the noise during image acquisition could lead to the output of noisy polarimetric images. In this paper, we propose, for the first time to our knowledge, a learning-based method for polarimetric image denoising. This method is based on the residual dense network and can significantly suppress the noise in polarimetric images. The experimental results show that the proposed method has an evident performance on the noise suppression and outperforms other existing methods. Especially for the images of the degree of polarization and the angle of polarization, which are quite sensitive to the noise, the proposed learning-based method can well reconstruct the details flooded in strong noise.
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On-chip mode-division multiplexing (MDM), as a promising method of scaling communication bandwidth, has attracted tremendous attention due to potential applications in optical interconnects ranging from intra-chip to board-to-board communications. However, the MDM technique usually suffers from signal degradation due to crosstalk between spatial modes in multimode waveguide devices. Here we design waveguide-integrated graphene spatial mode filters for on-chip MDM to overcome this limitation. Specifically, TE1-mode-pass and TE2-mode-pass filters are designed based on different waveguide architectures. For the TE1-mode-pass filters, we have obtained a maximum TE1-to-TE0 modal extinction ratio (ER) of 9.19 dB in a 200-µm-long waveguide. While, for the TE2-mode-pass filters, we have achieved a maximum TE2-to-TE1 modal ER of 5.37 dB and TE2-to-TE0 modal ER of 6.44 dB in a 200-µm-long waveguide. Our study could help improve the signal-to-noise ratio for on-chip MDM optical interconnects.
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Broadband polarimetric imaging consists of forming an image under spectrally wide illumination after having optimized the polarization state analyzer (PSA) to maximize the target/background discriminability. In previous works, the image sensor was monochrome, and only the intensity contrast was optimized. However, due to its spectrally varying response, the PSA not only changes the light's intensity, but also its color. This color information can serve as a further parameter to improve discrimination. In this paper, we employ a color camera in a broadband Stokes (passive) polarimetric imaging system and take into color difference's contribution to discrimination ability in optimizing the PSA setting. We show through experiments that a significant improvement of discrimination ability over monochrome imaging is obtained, especially when there are multiple objects in the scene.
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Optical activity is an effect of prominent importance in stereochemistry, analytical chemistry, metamaterials, spin photonics, and astrobiology, but is naturally minuscule. Metallic nanostructures are commonly exploited as basic elements for artificially producing large optical activity by virtue of surface plasmon resonance (SPR) on the nanostructures. However, their intrinsic high ohmic loss amplified by the SPR results in low energy efficiency and large photothermal heat generation, severely limiting their performance and practical utility. Giant optical activity by inducing magnetic resonance in an all-dielectric spiral nanoflower (spiral-flower-shaped nanostructure) is demonstrated here. Specifically, a large circular-intensity difference of ≈35% is theoretically predicted and experimentally demonstrated by optimizing the magnetic quadrupole contribution of the nanoflower to scattered light. The nanoflower overcomes the bottleneck of the traditional metallic platforms and enables the development of diverse chiroptical devices and applications.
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The two ellipsometric parameters of an isotropic sample can be measured with simplified polarimetry setups that acquire at least four intensity measurements. However, these measurements are perturbed by noise and the measurement strategy has to be optimized, in order to limit noise propagation. We determine two different measurement strategies that are optimal for both white Gaussian additive noise and Poisson shot noise. The first one involves a polarization state generator (PSG) with a single state of polarization and a polarization state analyzer (PSA) with four states. The second one involves both PSG and PSA having two states. The total estimation variances obtained with both strategies are demonstrated to be minimal, of equal values, and independent of the ellipsometric parameters to be measured. They are based on simple optical elements and could simplify and accelerate ellipsometric measurements.
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A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with coupling efficiencies of (18.3%, 20.1%) and (14.5%, 17.5%), 3-dB bandwidths of (55.0, 30.5) nm and (44.0, >39.5) nm, respectively. A DWB FSWGC working at (1480, 1830) nm with a wavelength separation of 350 nm and coupling efficiencies of (38.0%, 33.2%) is also numerically predicted.
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Germanium is a promising material for mid-infrared (MIR) integrated photonics due to its CMOS compatibility and wide transparency window covering the fingerprint spectral region (2-15 µm). However, due to the limited quality and structural configurations of conventional germanium-based integration platforms, the realization of high-Q on-chip germanium resonators in the MIR spectral range remains challenging to date. Here we experimentally demonstrate an air-cladding MIR germanium microring resonator with, to the best of our knowledge, the highest loaded Q-factor of â¼57,000 across all germanium-based integration platforms to date. A propagation loss of 5.4 dB/cm and a high extinction ratio of 22 dB approaching the critical coupling condition are experimentally realized. These are enabled by our smart-cut methods for developing high-quality germanium-on-insulator wafers and by implementing our suspended-membrane structure. Our high-Q germanium microring resonator is a promising step towards a number of on-chip applications in the MIR spectral range.
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A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9 dB/cm at the wavelength of 2.25 µm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is limited only by the absorption of silicon. Numerical simulation shows that its sensitivity, defined as the ratio between the change of the effective index and the ambient refractive index, can reach 1.123, which is 9.7% higher than the maximal sensitivity of conventional SOI slot waveguides on BOX.