Thermal stress simulation analysis of aerospace optical fibers and connectors and related extensions to high-speed railway area

Aerospace optical cables and fiber-optic connectors have numerous advantages(e.g.,low loss,wide transmission frequency band,large capacity,light weight,and excellent resistance to electromagnetic interference).They can achieve optical communication interconnections and high-speed bidirectional data transmission between optical terminals and photodetectors in space,ensuring the stability and reliability of data transmission during spacecraft operations in orbit.They have become essential components in high-speed networking and optically interconnected communications for spacecrafts.Thermal stress simulation analysis is important for evaluating the temperature stress concentration phenomenon resulting from temperature fluctuations,temperature gradients,and other factors in aerospace optical cables and connectors under the combined effects of extreme temperatures and vacuum environments.Considering this,advanced optical communication technology has been widely used in high-speed railway communication networks to transmit safe,stable and reliable signals,as high-speed railway optical communication in special areas with extreme climates,such as cold and high-temperature regions,requires high-reliability optical cables and connectors.Therefore,based on the finite element method,comprehensive comparisons were made between the thermal distributions of aerospace optical cables and J599III fiber optic connectors under different conditions,providing a theoretical basis for evaluating the performance of aerospace optical cables and connectors in space environments and meanwhile building a technical foundation for potential optical communication applications in the field of high-speed railways.

Influence of Waveguide Properties on Wave Prototypes Likely to Accompany the Dynamics of Four-Wave Mixing in Optical Fibers

In this article, we study the impacts of nonlinearity and dispersion on signals likely to propagate in the context of the dynamics of four-wave mixing. Thus, we use an indirect resolution technique based on the use of the iB-function to first decouple the nonlinear partial differential equations that govern the propagation dynamics in this case, and subsequently solve them to propose some prototype solutions. These analytical solutions have been obtained;we check the impact of nonlinearity and dispersion. The interest of this work lies not only in the resolution of the partial differential equations that govern the dynamics of wave propagation in this case since these equations not at all easy to integrate analytically and their analytical solutions are very rare, in other words, we propose analytically the solutions of the nonlinear coupled partial differential equations which govern the dynamics of four-wave mixing in optical fibers. Beyond the physical interest of this work, there is also an appreciable mathematical interest.

Specialty optical fibers for advanced sensing applications

Optical fiber technology has changed the world by enabling extraordinary growth in world-wide communications and sensing.The rapid development and wide deployment of optical fiber sensors are driven by their excellent sensing performance with outstanding flexibility,functionality,and versatility.Notably,the research on specialty optical fibers is playing a critical role in enabling and proliferating the optical fiber sensing applications.This paper overviews recent developments in specialty optical fibers and their sensing applications.The specialty optical fibers are reviewed based on their innovations in special structures,special materials,and technologies to realize lab in/on a fiber.An overview of sensing applications in various fields is presented.The prospects and emerging research areas of specialty optical fibers are also discussed.

Modulation instabilities in randomly birefringent two-mode optical fibers

Modulation instabilities in the randomly birefringent two-mode optical fibers （RB-TMFs） are analyzed in detail by accounting the effects of the differential mode group delay （DMGD） and group velocity dispersion （GVD） ratio between the two modes, both of which are absent in the randomly birefringent single-mode optical fibers （RB-SMFs）. New MI characteristics are found in both normal and anomalous dispersion regimes. For the normal dispersion, without DMGD, no MI exists. With DMGD, a completely new MI band is generated as long as the total power is smaller than a critical total power value, named by Per, which increases significantly with the increment of DMGD, and reduces dramatically as GVD ratio and power ratio between the two modes increases. For the anomalous dispersion, there is one MI band without DMGD. In the presence of DMGD, the MI gain is reduced generally. On the other hand, there also exists a critical total power （Per）, which increases （decreases） distinctly with the increment of DMGD （GVD ratio of the two modes） but varies complicatedly with the power ratio between the two modes. Two MI bands are present for total power smaller than Per, and the dominant band can be switched between the low and high frequency bands by adjusting the power ratio between the two modes. The M1 analysis in this paper is verified by numerical simulation.

Theoretical analysis of optical force density distribution inside subwavelength-diameter optical fibers

We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter（SD）fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain（FDTD） method.Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.

New Pyrometer with Double Y-type Optical Fibers

Abstract: A new pyrometer, which can solve the affection of emissivity change in temperature measuring, has been developed by double Y - type optical fibers. The mathematical model of the new pyrometer is presented and the errors of the new pyrometer are analyzed.

Development of Glass Optical Fibers 1970-2020,Providing Us the Digitalized Communication World

New types of communication cables were found to be needed already during the 1960-decade,because the copper cables had,and still would have,too high attenuation and especially limited bandwidth,due to extremely high dispersion at communication signals above 2 Mbit/s.Already the first commercially available multimode optical fibers(1979),developed from pure silica glass with a Ge-doped core,had much lower attenuation at signal frequencies of the order of 2-9 Mbit/s and above it.However,fiber core,cladding and coating materials,cable structures and materials,as well as manufacturing-,measurements-and test methods have been needed to be developed much further to get the reliable fiber cable communication networks.The important development stages and solutions to the most significant childhood problems of the optical fibers and cables are described in this paper.Now over 500 million km of optical fibers are manufactured and installed worldwide for the communication networks.The understanding of how to make the fibers with the very good transmission,mechanical and reliability properties exists at the manufacturers of the fibers and cables.

Effect of radiation-induced mean wavelength shift in optical fibers on the scale factor of an interferometric fiber optic gyroscope at a wavelength of 1300 nm

In order to analyze the effect of wavelength-dependent radiation-induced attenuation （RIA） on the mean trans- mission wavelength in optical fiber and the scale factor of interferometric fiber optic gyroscopes （IFOGs）, three types of polarization-maintaining （PM） fibers are tested by using a 60Co γ-radiation source. The observed different mean wave- length shift （MWS） behaviors for different fibers are interpreted by color-center theory involving dose rate-dependent absorption bands in ultraviolet and visible ranges and total dose-dependent near-infrared absorption bands. To evaluate the mean wavelength variation in a fiber coil and the induced scale factor change for space-borne IFOGs under low radiation doses in a space environment, the influence of dose rate on the mean wavelength is investigated by testing four germanium （Ge） doped fibers and two germanium-phosphorus （Ge-P） codoped fibers irradiated at different dose rates. Experimental results indicate that the Ge-doped fibers show the least mean wavelength shift during irradiation and their mean wavelength of optical signal transmission in fibers will shift to a shorter wavelength in a low-dose-rate radiation environment. Finally, the change in the scale factor of IFOG resulting from the mean wavelength shift is estimated and tested, and it is found that the significant radiation-induced scale factor variation must be considered during the design of space-borne IFOGs.

Influence of Refractive Index Distribution on Brillouin Gain Spectrum in GeO_(2)-Doped Optical Fibers

GeO_(2) is commonly used as dopant to adjust the refractive index profile(RIP)and the acoustic velocity profile(AVP)in the fiber,thereby forming different Brillouin gain spectrum(BGS)characteristics such as Brillouin gain,acoustic mode number and peak intensity difference.When an optical fiber is used in optical fiber sensing or communication system,its BGS characteristics may play an important role in determining the performance of the system.In this paper,finite element analysis(FEA)method is used to study the influence of refractive index distribution and its corresponding AVP on the BGS in step-index,graded-index,and complex-index optical fibers.A new method has also been proposed to efficiently discriminate acoustic mode solution and obtain the new and full images of total Brillouin gain and acoustic modes number of the fiber as a function of the refractive index distribution,considering the influence of changing the refractive index difference and the geometric size simultaneously.For each type of optical fiber,the recommended parameter range is provided for optical fiber sensing and optical fiber communication.Moreover,the suitable optical fiber with close peak intensity in its multi-peak BGS is explored and achieved,which can be used in Brillouin beat spectrum detection systems to improve sensing accuracy.

Effect of radiation-induced color centers absorption in optical fibers on fiber optic gyroscope for space application

The effects of color centers＇ absorption on fibers and interferometric fiber optical gyroscopes（IFOGs） are studied in the paper. The irradiation induced attenuation（RIA） spectra of three types of polarization-maintaining fibers（PMFs）, i.e.,P-doped, Ge-doped, and pure silica, irradiated at 100 Gy and 1000 Gy are measured in a wavelength range from 1100 nm to1600 nm and decomposed according to the Gaussian model. The relationship of the color centers absorption intensity with radiation dose is investigated based on a power model. Furthermore, the effects of all color centers＇ absorption on RIA and mean wavelength shifts（MWS） at 1300 nm and 1550 nm are discussed respectively. Finally, the random walk coefficient（RWC） degradation induced from RIA and the scale factor error induced by MWS of the IFOG are simulated and tested at a wavelength of 1300 nm. This research will contribute to the applications of the fibers in radiation environments.

Stress and modal birefringence of single-mode specialty optical fibers with three-stress regions

Stress-induced birefringence and modal birefringence of single-mode specialty optical fibers with three stress regions are numerically analyzed by the vector finite element method. Stress distribution and stress-induced birefringence distribution of three kinds of optical fibers with different cross structures are presented and compared, and the influence on the stress- induced birefringence by temperature change are analyzed as well. The results show that the fibers with three-stress regions have a lower linear birefringence, which is very important for the fabrication of the circular polaxization-maintalning fiber with high performance drawn from the same fiber preform by using the spinning method.

Rare Earth Doped Optical Fibers for Temperature Sensing Utilizing Ratio-Based Technology

A new and practical fluorescence temperature detecting system based on fluorescence intensity ratio was proposed . The background theory of fluorescence intensity-ratio method was presented simply. And the characters of rare earth doped samples were detailed. The erbium-doped fiber was chosen as the sensing element. The energy levels of 2H11/2 and 4S3/2 are responsible for the emission of radiation at approximately 530 and 555 nm. The erbium-doped (960 ppm) fiber of length 20 cm and core diameter 3.2μm was used as the sensing part. A silica photodiode transfers the fluorescence signal to electric signal, then the ratio of the average of the two different signals was calculated by the computer and the temperature was obtained. The ratio R of the intensity resulting from the transition between the two levels varies proportionly with temperature interval from 293 K to 373 K. The sensitivity of the sensor is approximately 0.05 K-1.

A New Method for In-Situ Measurement of Internal Solitary Waves Based on the Stimulated Raman Scattering in Optical Fibers

In-situ measurement of internal solitary waves(ISWs)is complicated in the ocean due to their randomness.At present,the ISWs are mainly detected by the chain structure of conductivity-temperature-depth systems(CTDs)or temperature sensors.The high cost limits the spatial resolution,which ultimately affects the measuring accuracy of the ISW amplitude.In this paper,we developed an experimental measurement system for detecting ISWs based on the stimulated Raman scattering in distributed optical fibers.This system has the advantages of high precision,low cost,and easy operation.The experimental results show that the system is consistent with CTDs in the measurement of vertical ocean temperature variation.The spatial resolution of the system can reach 1.0 m and the measuring accuracy of temperature is 0.2℃.We successfully detected 3 ISWs by the system in the South China Sea and two optical remote sensing images collected on May 18,2021,the same day of two detected ISWs,verify the occurrence of the measured ISWs.We used the image pairs method to calculate the phase velocity of ISW and the result is 1.71 ms^(-1).By extracting the distances between wave packets,it can be found that the semi-diurnal tide generates the detected ISWs.The impact of the tidal current velocity on the ISW in amplitude is undeniable.Undoubtedly,the system has a great application prospect for detecting ISWs and other dynamic phenomena in the ocean.

Temperature dependence of the refractive index of optical fibers

Many experimental investigations on the temperature dependence of the refractive index of optical fibers have been reported previously, however a satisfying theoretical explanation for it is still absent. In this paper, a theoretical model about the temperature dependence of the refractive index of optical fibers is presented and it is in agreement with the previous experimental results. This work is a significant reference for the research and development of temperature sensors based on optical fiber delay lines.

High resolution angular-displacement sensor based on whispering gallery mode resonance in bent optical fibers

A simple fiber sensor to measure angular displacement with high resolution, which is based on whispering gallery mode （WGM） resonance in bent optical fibers,is proposed. The sensor is composed of a single loop formed by loosely tying a knot using single mode fiber. To measure the transmission spectra, a tunable laser and an optic power meter are connected to the two ends of fi- ber loop, respectively. Significant WGM resonances occur over the investigated wavelength range for all the sensors with different bend radius. The angular-displacement sensitivity is studied in the range from -0. 1°to 0. 1°. The detection limit of 1.49 × 10 ^-7 rad can be achieved for the detecting system with the resolution of lpm. The simple loop-structure fiber sensor has potential application prospect in the field of architecture or bridge building with low detection limit and low cost.

Grating Writing in Structured Optical Fibers

Grating writing in structured optical fibers is reviewed. Various laser sources have been used including UV and near IR nanosecond and femtosecond lasers, each enabling different material processing regimes. The issue of scattering is modeled through simulation and compared with experiment. Good agreement has been established.

Radiation-induced effects in polarization-maintaining optical fibers for interferometric gyroscope

Radiation-induced attenuation （RIA） in four types of polarization-maintaining optical fibers for interfer-ometric fiberoptic gyroscope （IFOG） at 1310 nm is measured. The measurements are conducted during and after steady-state γ-ray irradiation using a 60 Co source in order to observe significantly different RIA behavior and recovery kinetics. Mechanisms involving dopants and manufacturing process are introduced to analyze the RIA discrepancy as well as to guide the choice and hardening of optical fibers during the design of IFOG. Medium-accuracy IFOG using Ge–F-codoped fiber and pure silica core fiber can survive in the space radiation environment.

Method to improve the performance of reflectance diffuse optical imaging based on polygonal optical fibers arrangement

In order to improve the performance of reflectance diffuse optical imaging （rDOI）, a novel polynomial geometry （PG） of optical fibers arrangement is proposed. Polynomial geometry is based on the hexagonal geometry （HG） and multicentered double-density （MD） mode. The overlapping sensitivity matrix, area ratio （AR）, reconstruction image, two-absorber model, and contrast-to-noise ratio （CNR） in different depths are used to evaluate the performance of PG. The other three geometries including HG, rectangular geometry （RG）, and MD mode are also compared with PG. The deformation of the reconstruction images is evaluted by circular ratio （CR）. The results prove that the proposed PG has high performance and minimum deformation in quality of reconstruction image in rDOI.

High-precision group-delay dispersion measurements of optical fibers via fingerprint-spectral wavelength-to-time mapping

The group-delay dispersion of an optical fiber was measured with the time-of-flight method, using fingerprint-like characteristic spectra from a mode-locked fiber laser source. To determine the group-delay dispersion up to the fourth order, least-squares fitting was applied to the overall time waveform mapped on the time axis for the fingerprint-spectral broadband pulses through a long optical fiber. The analysis of all 4003 data points reduced statistical uncertainty, and provided second-, third-, and fourth-order dispersion with uncertainties of 0.02%, 0.4%, and 4%,respectively.

Temperature dependence of radiation-induced attenuation of optical fibers

We investigate the temperature dependence of radiation-induced attenuation （RIA） at 1310 nm for a Ge/P co-doped fiber after a steady-state γ-ray irradiation. A V irradiation facility 6^60Co source is used to irradiate the fiber at a dose rate of 0.5 Gy/min, satisfying a total dose of 100 Gy. The test temperature ranges from -40 to 60 ℃ by 20 ℃, and the RIA of the fiber is obtained using a power measuring device. The experimental result demonstrates that RIA exhibits a steady, monotonic, and remarkable temperature dependence after approximately 48 h of accelerated annealing at 70 ℃. The optical fiber irradiated with a high dose and annealed sufficiently can be used as a temperature sensor.