With the application of freeform surface, beam shaping can achieve greater flexibility and precision. Ray mapping is an efficient geometric method for freeform surface design. However, the mapping may not always be integrable, which will result in actual performance falling short of expectations. Additionally, this method often relies on fitting the surface through the calculated coordinates of discrete points, and the fitting accuracy can also affect actual performance. In this study, adaptive weighted particle swarm optimization (AWPSO) is employed to optimize freeform polynomial coefficients to improve beam shaping performance. The initial freeform structure is obtained by non-integrable ray mapping method and fitted into a surface polynomial. Then, the process of optical design is transformed into a multi-dimensional optimization problem within mathematical models. Therefore, performance improvements do not depend on time-consuming ray tracing. A challenging beam shaping example of circle to regular hexagon is given to prove the effectiveness of the method. The results show a significant improvement in the uniformity of the ray spot on the target surface, with the contour becoming sharper and smoother after optimization. From the numerical results, the potential of this method is shown to be used in more irregular beam shaping conditions.
Flexible polymer optical waveguides demonstrate outstanding advantages over those on rigid substrates, including flexibility, bendability, lightweight, versatility, and ease of integration. However, flexible polymer optical waveguides face significant challenges in achieving efficient connection and integration with optical fibers. In this study, we propose a simple and reliable method for fabricating flexible polymer optical waveguides on polyimide (PI) using an ultraviolet (UV) lithography process. Additionally, MT connectors are fabricated at each end of the flexible waveguide with passive alignment features. This integration facilitates pluggable and efficient connections between waveguides and MT fiber arrays, significantly improving the ease of assembly and the reliability of optical connections. This integrated connector not only enhances the flexibility and reliability of applications in the field of optical interconnects but also lays the foundation for the development of future high-performance optoelectronic systems.
We present a low-loss Er 3+ -doped silicate microsphere laser, exploring its temperature sensing characteristics. We measure the threshold power of the Er 3+ -doped silicate glass microsphere laser under 980 nm pumping, achieving an impressively low value of 3.14 mW. Subsequently, in order to study the effect of temperature on the laser, the microsphere laser is placed into a temperature controller, and high-temperature sensitivity of 17.56 pm/℃ is obtained in the experiment. The proposed microsphere is significant for applications in lasing and temperature sensing, offering substantial advantages in terms of lower threshold power and heightened temperature sensitivity.
We demonstrate a high-sensitivity curvature sensor based on the negative curvature hollow core fiber (NCHCF). The graded index fiber (GIF) is introduced between single mode fiber (SMF) and NCHCF for the sensor of cascaded structure of SMF-GIF-NCHCF-GIF-SMF. The coupling efficiency can be improved by cascading the GIF. The experimental results show that there are two coexisted mechanisms in the cascaded structure, where multi-mode interference (MMI) is sensitive to curvature change and anti-resonance (AR) is insensitive to curvature change. The obtained curvature sensitivity based on MMI is up to -4.97 nm/m-1 . With advantages of compactness, flexibility, high sensitivity and so on, the curvature sensor shows prospects in structural health monitoring, medical applications, energy exploration and other fields.
A novel parallel Fabry-Perot interferometer (FPI) assisted by a concave cavity structure and an external reflector, which can be employed for large-range displacement and temperature sensing, is proposed and experimentally demonstrated. Therein, the concave cavity consists of a single mode fiber (SMF), a graded index fiber (GIF) and a hollow core fiber (HCF), which are fusion spliced to realize flat and concave surfaces of SMF-GIF and GIF-HCF, respectively. The certain length of GIF is utilized to serve as an collimator, facilitating the reduction of the divergence angle and the expansion of the mode field radius of the propagating beam. Thus the propagating beam can split into the concave cavity structure’s sidewall and inner air area, when the concave cavity size is smaller than the mode field diameter of the beam from GIF. The external Fabry- Perot interferometer (EFPI), formed by GIF endface and the external reflector, is utilized to detect displacement. Meanwhile, the intrinsic Fabry-Perot interferometer (IFPI), formed by the endfaces of GIF and HCF, serves as a temperature sensor. The length of the silica cavity changes with temperature due to the thermal expansion and thermo-optical effect of silica. In consequence, the corresponding wavelength shift can be accurately observed in response to temperature variations. The experimental results indicate the displacement sensing range of EFPI is up to 30 mm, which is greatly increased compared to other FPI displacement sensors. Moreover, the proposed sensor has a temperature sensitivity of 11.52 pm/◦C in the range of 30◦C 100 ◦C.
Polymer optical waveguide vertical optical coupler and mode division (de)multiplexer on optical printed circuit boards have garnered considerable attention. We have studied a stepped laser-ablation method for the fabrication of concave micromirrors in rectangular optical waveguides. The vertical coupling loss can be reduced to 1.83 dB experimentally. Further, based on gray-tone optical lithography technology combined with the overlay alignment method, a spherical concave micro-mirror has been fabricated, with the vertical coupling loss of 1.39 dB. Furthermore, a polymer three-mode (de)multiplexer with two cascaded waveguide directional couplers is proposed. The device can ensure that the E11 mode of the two narrower waveguides are highly coupled into the E21 and E31 modes of the central waveguide. The fabricated device exhibits coupling ratios of 98.07% and 95.43%. Moreover, a two-mode (de)multiplexer has been studied. The coupling ratio and extinction ratio of the fabricated (de)multi
We propose a generation method of higher-order orbital angular momentum (OAM) based on the cascade of two fabricated lower-mode selective couplers (MSCs) with a home-made four-mode fiber (4MF) and a ring-core fiber (RCF). This all-fiber OAM (also called optical vortex beam, which has a spiral phase wavefront) generation technology has the advantages of simple structure and low cost, which is suitable for the field of vortex-optics fiber communication.
A tapered hollow annular core fiber (HACF) coupler for excitation of whispering-gallery modes (WGMs) of an embedded microsphere resonator is proposed and demonstrated. Using HACF as the coupling medium can avoid the complicated fabrication process such as chemical corrosion and improve the robustness and stability of the device. The coupling efficiency from the SMF to HACF is enhanced after tapering the joint section to excite various WGMs. In both theory and experiment, we observe symmetrical Lorentzian and asymmetric Fano line shapes by varying the microsphere resonator size and location. Finally, the temperature sensing sensitivity and stability of the device are tested. The sensitivity of the device reaches 10.8 pm/°C. This novel tapered HACF based microsphere resonator is expected to promote the environmental adaptability in the practical application.
The antiresonance spectral characteristics based on a silica capillary sandwiched between two single-mode fibers are investigated on the aspects of both transmission and reflection. Basic theory of the antiresonance reflecting optical waveguide model is presented and analyzed for this structure. During the fabrication, suitable parameters are adopted in in the manual welding process to keep the cross section of the capillary fiber away from the discharge electrode, which ensures the smoothness between the welding surfaces. Subsequently, three experimental samples are fabricated with the same inner diameter and various lengths of 650 μm, 837 μm, and 1070 μm, respectively. It can be observed that devices with different lengths have the same resonance wavelength in the wavelength range of 1500-1700 nm. However, the transmission depth at the resonance wavelength increases with the increase of the capillary length. We also discuss the relationship between inner diameter and transmission spectra through three samples with inner diameters of 25 μm, 50 μm, and 75 μm. The experimental results show that the free spectral range is 22.9 nm, 29.8 nm, and 44.1 nm, respectively. It also exists antiresonance in the reflection, which shows a novel mechanism for possible sensing applications.
Femtosecond laser is used to inscribe the reflectivity-changed mirrors in the single mode fiber that formed the device named cascaded Fabry-Perot micro-cavities, with details of fabrication given. The cascaded Fabry-Perot micro-cavities are made up of three reflected mirrors with different lengths of microcavities. By establishing the theoretical model of the device with the transfer matrix method, the effect of the cascaded Fabry-Perot micro-cavities can be seen as a superposition of the two Fabry-Perot cavities. Its reflection fringe contrast is experimentally up to 10dB. The proposed structure is regarded as an effective way to strengthen the contrast in order to improve the sensing resolution.
This paper investigates the transmission spectral properties of tilted long period fiber gratings (TLPFGs) written in fiber tapers with different radiuses. We found that the tilted angle, the radius of fiber and the external refractive index both have influences on the transmission spectral by simulations. With the decrease of the radius of the fiber taper and the increase of refractive index, the resonant wavelength of the tilted LPFG was found to shift towards shorter wavelength. Experimental results show the agreement with the simulation predictions. The TLPFG inscribed in fiber taper also brings the high sensitivity of external refractive index as 566.7 nm/RIU.
In this paper, effects of diameter on characteristics of a LPFG with numerical simulations adopting a three-layer model and experimental demonstrations are presented, including the normalized coupling coefficient, the effective refractive index, the resonant wavelength, the shape of the attenuation dip and the refractive index sensitivity. The couplings between the core mode and the EH cladding modes increase faster than that of the HE cladding modes as the fiber diameter decreases. Moreover, the depth and bandwidth of the attenuation dip vary faster in thinner fiber due to the increase of the coupling coefficient, comparing with LPFGs in common fiber. The resonant wavelengths of lower order cladding modes move mainly toward the shorter wavelength, while that of higher order cladding modes move mainly toward the longer wavelength as a function of the exponential shape as the fiber radius reducing and the external refractive index increasing. The refractive index sensitivity is greatly enhanced when reducing the fiber diameter and using the taper structure. Therefore, we can utilize the higher sensitivity of different cladding mode at a suitable wavelength by controlling the diameter of the fiber taper waist. Furthermore, the sensing resolution and the cladding mode can also be selected by controlling the fiber diameter, which can be greatly used in many fields.
A long-distance Optical Printed Circuit Boards (OPCB) was fabricated to realize high-speed optical interconnects. The OPCB was made up of polymer optical waveguides fabricated by using ultraviolet (UV) photolithography technique. The length of OPCB is 30cm. The minimal transmission loss of the optical waveguide is approximate 5.36dB at 850 nm wavelength. Two MT-RJ optical connecters were assembled on the OPCB to realize optical coupling between optical fibers and waveguides. The dependence of coupling loss on misalignment was obtained with different offset value theoretically and experimentally. The minimal total insertion loss is only 8.06dB. With 10 gigabits-per-second (Gbps) optical transceivers as the input/output module, we tested the performance of the OPCB. The date rate of 10Gbps can be transmitted successfully.
A series-cascaded fractal topological structure of microring arrays is proposed, by introducing fractal topological
structures to series-cascaded all-pass microring resonators. Its analytical model is established by the transfer matrix
method. Then the characteristics of this structure are investigated by the effects of coupling coefficients and loss.
Generally, there are dips with the resonator number in the transmission spectra. As the loss increases, the average
intensity decreases. However, the delay becomes interesting positive and negative delays. With the ring-bus coupling
coefficient decreases, the dips of the spectrum become degenerated. However, there are several delay peaks with some
positive delays and the other negative ones. These kinds of multiple positive and negative delays can be developed for
applications in all-optical delay lines.
A multimode Y-branch interconnect optical waveguide was designed and fabricated. The transmitting properties of the Y-branch splitter were designed and simulated by using commercial waveguide simulation software. For a 1×2 Y-branch splitter with 50μm×50μm channel waveguide, the relationship between insertion loss and splitting angle was calculated. According to the theoretical result, a multimode Y-branch waveguide was fabricated based on a UV-lithography method on the printed circuit board. To balance the trade-off between the insertion loss and the component size, the Y-branch waveguide was designed of two sections with different splitting angles. An insertion loss of 0.36dB was obtained theoretically with a 5cm Y-branch waveguide.
A sensing configuration of the fiber taper intrinsic Fabry-Perot interferometer (FTIFPI) was presented and developed.
Direct inscription method of intrinsic Fabry-Perot interferometers in optical fiber tapers with a femtosecond laser has
been fabricated experimentally. Fabry-Perot interferometers are firstly realized in the waist of fiber tapers. The
experimental results indicate that the change in wavelength has a good linear relationship with temperature, while the
fringe visibility keeps the same approximately. In the range of 0°C~100°C, the interference spectrum sensitivity is
12pm/°C. The fringe visibility decreases linearly with the liquid refractive index increasing, while extremes keep the
same approximately.
In this work, we report a sensing configuration of the fiber taper intrinsic Fabry-Perot interferometer directly inscribed in
single-mode optical fiber tapers with different waist diameters from 14 to 80 μm using a femtosecond laser
micromachining system. By controlling the inscribing depth and intensity of the fs laser pulse, the fringe visibility can
exceed 9.0 dB when the fiber taper waist diameter is around 15 μm, which is sufficient for most sensing applications.
The sensor sensitivity depends on the fiber taper waist diameter, while a smaller diameter corresponds to a large
sensitivity. Different free spectral ranges can be achieved for various cavity lengths. Such a structure can combine the
high sensitivity properties of fiber taper sensors with the high resolution features of Fabry-Perot interferometer sensors.
Meanwhile, this structure can have a number of outstanding advantages, such as its small size, unique geometry, easy
fabrication, low cost and capability for mass production. These fiber taper intrinsic Fabry-Perot interferometer sensors
have high potential in fast detection and high precision measurement while maintaining superior reliability for chemical
and biological sensing.
We present two configurations of novel dual coupled microring resonators with different coupling points between dual
coupled microring resonators. Analytical expressions of transmission spectra and delays of the two configurations of
dual microring resonators are developed using the transfer matrix method. Then the effects of loss and different coupling
points on transmission spectra and normalized delays of novel dual coupled microring resonators are investigated. Under
the case of considering loss effects, different transmission spectra and delays are found out for the two configurations.
Both the transmission spectra and delays of the two configurations have different characteristics as the loss changes. As
the loss increases, amplitude of the transmission spectra decreases at different rates. Multiple transmission spectra peaks
and delay peaks exist, which can degenerate into a single peak under certain loss condition. Besides, the resonance
frequencies of transmission spectra and delays are analyzed as the loss changes. As the loss changes, corresponding
resonance frequencies of transmission spectra peaks are partially coincident with the resonance frequencies of delay
peaks on both sides of the center frequency, while the delay at the center frequency has significant changes.
The microring resonator is a high-performance and low-cost optical waveguide device, suitable for integration with large
dimensions. This paper presents the design of laterally coupled racetrack microring resonators working around 1.55μm,
using combinations of the 3D full vectorial film mode matching method, the coupled mode theory and the parameter
model. This combined simulation technique shows convenience to perform the design process.
A fiber-optic intrinsic Fabry-Perot (F-P) interferometric sensor was inscribed in a standard single mode fiber by inducing
refractive index change in the core. It was written directly by using a femtosecond laser. The reflection spectrum of the
fiber-optic F-P interferometric sensor was measured and was investigated as a temperature sensor. The sensitivity of the
sensor is 9.2 pm/°C in the temperature range of -20~80°C.
This paper theoretically investigates spectral characteristics of the 3×2 microring resonator array, with its analytical
model developed firstly. Simulation results show that the case of the ring-bus coupling coefficient smaller than the ringring
coupling coefficient is suitable for box-like filters. After design principles are given, the optimization process are
carried out by evaluating the side lobe rejection ratio, the passband ripple rejection ratio and the roll-off coefficient of the
passband edge. The FWHM of the designed box-like filter can be adjustable in a flexible range.
We theoretically studied a two cascaded stages of microring coupled-resonator optical waveguides with corresponding
analytical model developed. Simulations show that multiple electromagnetically induced transparency-like spectra
appear in this system, which can be ascribed to the combined effects of mode splittings and instructive interferences.
Applications of such multiple electromagnetically induced transparency-like spectra are promising to be tailored for
multi-channel delaying lines.
Dual microring resonator array coupled via 3×3 couplers are studied using matrix formalisms. Transmission spectra are
investigated as functions of column numbers and coupling coefficients in detail. Multiple transparent peaks appear when
the column number increases for arrays of the first two types. The spectra for arrays of the other two types show
complicated characteristics as functions of coupling coefficients.
This paper presents a design for rectangular ring resonators based on low-index polymers operating at an 850 nm
wavelength. Multimode interference couplers are used to enable signal splitting and combining and air trench assisted
90° mirrors are used as beam turning elements. 3D full-vector, 2D FDTD and parameter models are combined to
simulate the ring resonator design. This design, suitable for spectral shaping, achieves a FSR of 0.29 nm, a FWHM of
0.066 nm and an on-off ratio of approximately 20 dB.
A 2×2 microring resonator array is investigated analytically and numerically in this paper. Based on the transfer matrix formalisms, its analytical model is developed for characterizing the transmission spectrum. When the coupling coefficients are in a medium range, there are two zero transmissions symmetrically locating between four unit peaks. The envelope of the transmission spectrum varies greatly as coupling coefficients change, which is called the mode evolution. It is found that mode evolutions are highly sensitive to the inter-resonator coupling coefficient when it is small and the waveguide-resonator coupling coefficient when it is large. Rigorous finite-difference time-domain simulations show agreement with the analytical model. Mode Evolutions are expected to be used for wavelength selectors and sensors.
In this paper, we propose new types of dual microring resonators coupled via 3×3 couplers. By employing the transfer
matrix method, a model for these four types is developed and analytical expressions for characterizing their
transmissions are derived. The first two types show a coupled-resonator-induced-transparency-like transmission
spectrum at the through port. The third type holds the same transmission spectrum, while the last type simultaneously
realizes a first-order and a second-order filters at two drop ports. The effects of coupling coefficients on their
transmission spectra are analyzed in more detail. These proposed types can be found applications in fields such as
sensors and filters.
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