Yongji Liu

Design for a Single-Polarization Photonic Crystal Fiber Wavelength Splitter Based on Hybrid-Surface Plasmon Resonance

A novel single-polarization photonic crystal fiber wavelength splitter based on hybrid-surface plasmon resonance is proposed. A full-vector finite-element method is applied to analyze the guiding properties. Numerical simulations show that the proposed splitter, which is only several hundred microns in length, gives single polarization in the 1.31-μm and 1.55-μm bands. The loss of the unwanted polarized mode is 102.6 and 245.0 dB/cm in the two aforementioned communication windows, respectively, and the corresponding insertion loss is as low as 3.5 and 1.7 dB/cm, respectively. Moreover, the dependence of the bandwidth on the fiber length is given, and according to that function, the bandwidth can reach 40 nm (1.31-μm band) and 100 nm (1.55-μm band) when the fiber length is up to 1 mm. Additionally, the tolerances for a realistic fabrication are analyzed. In the last part, we discuss other methods to deal with an anticrossing phenomenon in detail.

Simultaneous strain and temperature measurement by cascading few-mode fiber and single-mode fiber long-period fiber gratings

A hybrid optical fiber structure of two cascading long-period fiber gratings (LPFGs), respectively, inscribed on a segment of few-mode fiber (FMF) and single-mode fiber (SMF), is proposed and experimentally demonstrated. This structure could be used for simultaneous measurement of strain and temperature. The FMF-LPFG exhibits an opposite temperature response and higher strain sensitivity compared to that of the SMF-LPFG, which can improve the measurement resolutions. Experimentally, the strain and temperature sensitivities of the proposed sensor are -2.9  pm/με and -17.6  pm/°C, respectively, for the FMF-LPFG; for the SMF-LPFG, these are -1.47  pm/με and 46.4  pm/°C, respectively. The maximum errors are ±7.98  με and ±0.54°C for strain and temperature, respectively.

Simultaneous directional bending and temperature measurement with overlapping long period grating and fiber Bragg grating structure

A simple and compact device for simultaneous directional bending and temperature sensing is proposed and demonstrated. The device is constructed by overlapping a long period grating (LPG) on a fiber Bragg grating (FBG), and is capable of measuring the directional bending and the temperature at the same position. The LPG written with CO2 laser irradiation has a cross asymmetrical refractive modulation and the FBG is formed by UV laser exposure in the fiber core. The cross-sensitivity between measurement of the directional bending and of the temperature of an LPG is effectively eliminated by solving a matrix equation, by considering the temperature response characteristics of FBG. Experimental results show that the bending and temperature sensitivities are − 6.819 nm m−1 in a range from −2 to 2 m−1 and 10.25 pm °C−1, respectively.

Real time and simultaneous measurement of displacement and temperature using fiber loop with polymer coating and fiber Bragg grating

An all-fiber sensor scheme for real time and simultaneous displacement and temperature measurement is presented and demonstrated. The sensor head is formed by cascading a fiber loop with polymer coating with a fiber Bragg grating. The compatibility of the two components is fully utilized. A sensor resolution of 0.14314 V/μm in displacement and 0.00795 nm/°C in temperature are experimentally achieved within a displacement range of 0-50 μm and a temperature range of 20 °C-75 °C, respectively. The fiber loop with the protection of polymer coating is mechanically reliable, which means the sensor head also suits measuring dynamic displacement. A 500 Hz mechanical micro-vibration is successfully measured by the proposed sensor experimentally. In the last part, we perform a test making the sensor reach its maximum deformation and find the surviving sensor still possesses the same responsiveness as before.

Study on chromatic aberration in a population of Chinese myopic eyes by means of optical design

Two kinds of individual eye models, involving and without involving the angle between visual axis and optical axis, are established by means of optical design. We use them to study the properties of the transverse chromatic aberration (TCA) and longitudinal chromatic aberration (LCA) over the visible spectrum. Then the effects of the LCA and TCA on the visual quality of human eyes are evaluated. The statistical averages of TCA and LCA over the visible spectrum for Chinese myopic eyes are obtained. Results show that both TCA and LCA restrict the visual performance, and LCA is more detrimental than TCA.

Simultaneous measurement of strain and temperature using a long period fiber grating based on waist-enlarged fusion bitapers

A new long period fiber grating (LPFG) fabrication method based on periodically arranging a series of waist-enlarged fusion bitapers (WFBs) along the axis of a standard single mode fiber has been proposed and experimentally demonstrated. A strong grating modulation induced by the WFBs excites the mode coupling between the core and cladding modes, resulting in the formation of a grating resonance band. The mode coupling strength is directly proportional to the number of WFBs. A compact WFB-LPFG with a length of 3.28 mm has been achieved by five periods and displays a contrast of over 30 dB. By measuring the transmission intensity and resonant wavelength shift of the transmission spectrum, strain and temperature can be simultaneously determined.

Polarization Rotator Based on Hybrid Plasmonic Photonic Crystal Fiber

A photonic crystal fiber polarization rotator (PR) is proposed based on a novel mechanism-mode interference by two hybrid waveguide modes, rather than adjusting optical property in the light-guided part. The proposed PR is achieved by introducing two defects and an ancillary Au nanowire into the background lattice, and under the optimized parameters, the PR can offer almost a 100% polarization conversion ratio in the wavelength of 1.55-μm band (~100-nm bandwidth), compact length ~162.7 μm, low modal insertion loss of ~1.2 dB, and high coupling efficiency over 0.92 for both TE and TM mode based on the numerical simulation result of the full-vector finite-element method.

Aspheric spectacles for correcting presbyopia with myopia and astigmatism

The aim of this paper is to propose a method for designing aspheric spectacles capable of realizing good optical performance at both far and near vision for presbyopia. We have experimentally measured wavefront aberrations and axial lengths for eight myopic eyes. In consideration of the field of view (FOV), the rotation of the eyeball, and a small amount of accommodation retained for presbyopia, we constructed individual eye models and optimized all the spectacle-eye systems for visible wavelengths by Zemaxs simulation. Finally, we evaluated the image quality by the modulation transfer function (MTF) and visual acuity (VA) with different pupil sizes (2, 2.8, 4 mm). Results show that when the pupil size is 2 mm or 2.8 mm, the spectacles designed for the full FOV (0° and ±4° FOV) provide a strong ability to transfer low contrast at both far and near vision, and the VA for all reaches 0.8, and up to 1.1 for eyes NO. 1, NO. 5, NO. 6, and NO. 8 for the 0° FOV. As the pupil size increases to 4 mm, the VA for all comes to 0.6 for the full FOV, indicating that presbyopia is able to acquire a good visual resolution at both far and near vision by the designed aspheric spectacles. Furthermore, we verified the visual continuity of the spectacle-eye systems by studying intermediate vision, which demonstrates that the method used in our design is accurate and practicable.

Investigation of aberration characteristics of eyes at a peripheral visual field by individual eye model

We propose a method of constructing an individual eye model with a large visual field, and then investigate aberration characteristics of eyes in peripheral fields with constructed models. Twelve eyes of different aberrations are selected from 89 myopic eyes. It is shown that astigmatism increases as visual field in a quadratic manner. The variation tendency of defocus can be expressed by the cubic curve for 50% of eyes. For most of the eyes, the variation of spherical aberration shows a quadratic rule within ±24° visual field. Coma exhibits obvious individual differences. The impact of high-order aberrations on vision is mainly at a smaller visual field, and it becomes negligible beyond 24° visual field.

Statistical characteristics of aberrations of human eyes after small incision lenticule extraction surgery and analysis of visual performance with individual eye model

Preoperative and postoperative wavefront aberrations of 73 myopic eyes with small incision lenticule extraction surgery are analyzed in this paper. Twenty-eight postoperative individual eye models are constructed to investigate the visual acuity (VA) of human eyes. Results show that in photopic condition, residual defocus, residual astigmatism, and higher-order aberrations are relatively small. 100% of eyes reach a VA of 0.8 or better, and 89.3% of eyes reach a VA of 1.0 or better. In scotopic condition, the residual defocus and the higher-order aberrations are, respectively, 1.9 and 8.5 times the amount of that in photopic condition, and the defocus becomes the main factor attenuating visual performance.