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A graded-index (GI)-type photonic crystal fiber (PCF) that has an α-power effective refractive index profile operates as a multimode optical fiber and can be applied to a collimator for single-mode PCFs. The α-power effective index distribution is realized by an α-power air hole diameter distribution with uniform air hole pitch. However, a pressure controlling in fiber drawing is difficult due to the difference of air hole diameters in fiber cross section. In this paper, we propose a GI-PCF with uniform air hole diameter while the air hole pitch varies in radial direction. Light propagation characteristics in the Gaussian beam excited GI-PCF with uniform air hole diameter were theoretically studied. The period and amplitude of mode field diameter variation along the propagation direction for the innermost air hole pitch, air hole diameter, and radial distribution profile of the pitch were clarified. The collimator application of GI-PCF with uniform air hole diameter for a conventional single-mode PCF was investigated. The suitable GI-PCF structure to obtain high optical coupling efficiency and good tolerance for GI-PCF length error was clarified. Wavelength characteristics of the GI-PCF collimator were also studied. It was clarified that the appropriate GI-PCF length was longer for shorter operating wavelength in collimator application. [ABSTRACT FROM AUTHOR]

It is difficult for sapphire fiber Fabry-Perot (FP) sensors to generate high quality interference fringes because their highly multimode property. In this paper, we report a sapphire fiber FP strain sensing system with high fringe visibility, realized by the use of a self-made fiber collimator. The collimator is made by a quarter-period graded index optical fiber (GIOF) fused to a single mode fiber (SMF). In the contrast experiment, no effective signal was collected from a sensing system without the fiber collimator. While adding a fiber collimator into the sensing system, high visibility interference fringes were collected. The fringe visibility is 49.37% when the FP cavity length is 341.1 μm, and 20.09% when the cavity length is 1343.3 μm. The effective range of the cavity is over 1000 μm. Strain experiment at room temperature shows the measuring range is 10037 μϵ with relative error of 0.37% while the gauge length is 100 mm. High temperature experiment was also carried out and the result shows that the sensor can work at 1300 °C. This method is expected to be helpful for decreasing the difficulty of fabricating high fringe visibility sapphire fiber FP sensors, and increasing the measuring range.

It is difficult for sapphire fiber Fabry-Perot (FP) sensors to generate high quality interference fringes because their highly multimode property. In this paper, we report a sapphire fiber FP strain sensing system with high fringe visibility, realized by the use of a self-made fiber collimator. The collimator is made by a quarter-period graded index optical fiber (GIOF) fused to a single mode fiber (SMF). In the contrast experiment, no effective signal was collected from a sensing system without the fiber collimator. While adding a fiber collimator into the sensing system, high visibility interference fringes were collected. The fringe visibility is 49.37% when the FP cavity length is 341.1 μm, and 20.09% when the cavity length is 1343.3 μm. The effective range of the cavity is over 1000 μm. Strain experiment at room temperature shows the measuring range is 10037 μϵ with relative error of 0.37% while the gauge length is 100 mm. High temperature experiment was also carried out and the result shows that the sensor can work at 1300 °C. This method is expected to be helpful for decreasing the difficulty of fabricating high fringe visibility sapphire fiber FP sensors, and increasing the measuring range. [ABSTRACT FROM AUTHOR]

• In this paper, we design and fabricate a twist sensor MZI with an MSM fiber structure, and combines it with a fiber collimator to achieve control of the number of cladding modes in SMF. • After cascading with an FPI, twist sensing enhancement based on optical vernier effect has been achieved. • The experimental results indicate that the sensing MZI has a torsional sensitivity of −0.011 nm/(rad/m) within a large detection range of 0–84 rad/m. • After amplification by optical vernier effect, it reaches −0.218 nm/(rad/m) with an amplification factor of 19.8 times. • The sensor has a relatively small response to other physical quantities, with a strain sensitivity of −1.1 pm/με, a temperature sensitivity of 60.4 pm/℃, and a refractive index sensitivity of 1.1543 nm/RIU, exhibiting excellent anti crosstalk performance. • In addition, this article discusses a phase demodulation scheme for interferometric sensors based on optical vernier effect sensitization for the first time, which can filter out noise caused by dispersion or loss, eliminate the influence of initial phase of the sensing system, and point out that improving the basic sensitivity of the sensing interferometer is a further direction for sensitization. • This article uses a graded index fiber collimator to successfully modulate the optical mode transmitted in an interferometric sensor. • The design concept of integrating functional optical devices into interferometric sensing systems will be the future development direction in the field. • The new torsion sensor has a simple processing method, is easy to promote, and has practicality, which can play an important role in engineering detection, human activity recognition, and other fields. Fiber interferometric sensor can achieve sensing enhancement of various physical quantities based on the optical vernier effect, but there are currently no proposed torsion detection enhancement schemes. In this paper, we control the number of cladding modes in the sensing single-mode fiber by graded index fiber collimator and form a dual beam interference with the core mode, resulting in a transmission spectrum with regular waveform. The experimental results indicate that the sensing Mach-Zehnder interferometer (MZI) has a torsional sensitivity of −0.014 nm/(rad/m) within a large detection range of 0–60 rad/m. After amplification by optical cursor effect, it reaches −0.361 nm/(rad/m) with an amplification factor of 25.8 times, the detection limit for envelope valley is 0.345 rad/m. The sensor has a relatively small response to other physical quantities, with a strain sensitivity of 20.4 pm/με, a temperature sensitivity of 1.792 nm/℃, and a refractive index sensitivity of 77.885 nm/RIU, exhibiting excellent anti refractive index crosstalk performance. In addition, we discuss the phase demodulation scheme for interferometric sensor based on optical vernier effect sensitivity enhancement for the first time, which can filter out noise caused by dispersion or loss, eliminate the influence of the initial phase of the sensing system, and evaluate the direction of improving the detection performance of the phase demodulation system. [ABSTRACT FROM AUTHOR]

A new collimator based on a homemade concentric multilayer-core fiber (CMCF) is proposed and experimentally demonstrated. This collimator was fabricated using a tail fiber with large mode area and single-mode operation. By exploiting the optical transmission matrix, the propagation characteristic and coupling mechanism of this CMCF-based collimator was introduced meticulously. The coupling losses of the laser beam using this collimator in the off-axis, angular, and axial deviations were analyzed separately. In order to determine the relationship between the geometric redundancy of this collimator and the effective mode field area of the tail fiber, the corresponding mathematical model was established. Through model calculation and experiment measurement, the coupling properties of the collimator were improved effectively. Compared with the common SMF-based collimator, the declination redundancy of the CMCF-based one improved by 20%, which could make the coupling of the optical fiber collimator easier. Therefore, this collimator has potential application value in the laser diode coupling unit and high-speed optical communication system. [ABSTRACT FROM AUTHOR]

A new collimator based on a homemade concentric multilayer-core fiber (CMCF) is proposed and experimentally demonstrated. This collimator was fabricated using a tail fiber with large mode area and single-mode operation. By exploiting the optical transmission matrix, the propagation characteristic and coupling mechanism of this CMCF-based collimator was introduced meticulously. The coupling losses of the laser beam using this collimator in the off-axis, angular, and axial deviations were analyzed separately. In order to determine the relationship between the geometric redundancy of this collimator and the effective mode field area of the tail fiber, the corresponding mathematical model was established. Through model calculation and experiment measurement, the coupling properties of the collimator were improved effectively. Compared with the common SMF-based collimator, the declination redundancy of the CMCF-based one improved by 20%, which could make the coupling of the optical fiber collimator easier. Therefore, this collimator has potential application value in the laser diode coupling unit and high-speed optical communication system.

A graded-index type photonic crystal fiber (GI-PCF) has a graded effective index distribution that can be realized by an α-power air hole diameter distribution in radial direction. Since a beam diameter varies periodically along the propagation direction in the GI-PCF with Gaussian beam excitation, an appropriate length of GI-PCF should be spliced to a single-mode PCF for collimator application. It is desired that GI-PCF structures to obtain high optical coupling efficiency and good tolerance for fiber length error are designed. In this paper, we theoretically studied coupling characteristics of GI-PCF collimator for conventional single-mode PCF depending on air hole diameter ratio to pitch, where the GI-PCF had parabolic air hole diameter distribution in radial direction, and had the air hole pitch that was equal to that in the conventional single-mode PCF. It was found that the optical coupling efficiency could be higher for larger air hole diameter ratio to pitch, where the coupling efficiency almost equal to that in conventional GI-type multimode fiber (GI-MMF) collimator could be obtained. On the other hand, it was clarified that GI-PCF collimators were more tolerant for fiber length error compared with GI-MMF collimator and that the GI-PCF with smaller air hole diameter ratio to pitch exhibited a better tolerance in fiber length error. We clarified that the GI-PCF collimator had an ability to obtain better wavelength characteristics with fiber length error compared with the GI-MMF collimator. [ABSTRACT FROM AUTHOR]

A graded-index type photonic crystal fiber (GI-PCF) that has α-power air hole diameter distribution in radial direction has a characteristic that the beam diameter varies periodically along the propagation direction in the Gaussian beam excited GI-PCF, and it is similar to a GI type multimode fiber (GI-MMF). Since GI-MMFs are used as fiber collimators, it is considered that GI-PCFs are also possible to apply for fiber collimators. However, it is difficult to draw fiber with air hole diameter control in considering higher coupling efficiency at 1 ≤ α ≤ 2 . In this paper, we theoretically analyzed light propagation characteristics in Gaussian beam excited GI-PCF having α-power air hole diameter distribution with α < 1 and investigated the coupling efficiency with conventional PCF and its wavelength characteristics. It was found that the coupling efficiency became larger as the air hole diameter d was smaller at α < 1. In particular, a better tolerance for fiber length error and wavelength characteristics were obtained at α = 1/2. From these results and the difficulty of GI-PCF drawing, we designed the suitable structure of GI-PCF for fiber collimator application. [ABSTRACT FROM AUTHOR]

Stability, robustness, sensitivity, and reproducibility are important parameters in the interferometry. We propose a wavefront splitting fiber Mach–Zehnder interferometer (MZI) to achieve such performances. A polyethylene terephthalate (PET) film splits the wavefront of a Gaussian beam emitted from the first fiber collimator. Then, the Gaussian beam is focused into a single mode fiber by another collimator, constituting a stable and robust fiber MZI. The effective spectral width is larger than 390 nm, which is wide enough for constructing a sensitive fiber MZI. The COMSOL multiphysics simulation and the experimental results demonstrate that this MZI is based on the multi-beam interference. Thanks to the low insertion loss of the fiber MZI, two types of stable and repeatable interference fringes are obtained when the sampling accuracy of the optical spectrum analyzer is as high as 0.1 nm. It ensures that the fiber sensor based on this MZI has a high resolution. For the first type of interference fringe, the two adjacent resonant wavelengths, respectively, shift –0.78 and –0.81 nm when salinity increases one part per thousand, showing high sensitivities. The low order interference fringe has a larger sensitivity. However, its visibility may be small due to the modulation of the second type of interference fringe that has small sensitivity to the refractive index of the sample. Since the PET film has uniform thickness, the fiber MZI can ensure good reproducibility. It can be widely utilized in detecting physical, chemical, or biological quantities. [ABSTRACT FROM AUTHOR]

An 8 m/9.6 Gbps underwater wireless optical communication (UWOC) system based on a two-stage injection-locked 405 nm blue light laser diode (LD) transmitter with a 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal is proposed and experimentally demonstrated. To the best of our knowledge, it is the first one that employs a two-stage injection-locked technique to improve the transmission performance of UWOC systems. With the assistance of an equalizer at the receiver site, acceptable bit error rate (BER) performance and a constellation map are obtained over an 8-m underwater link. Such a two-stage injection-locked 405 nm blue light LD transmitter-based UWOC system is shown to be a promising alternative, presenting its feasibility for long-range and high-speed underwater links.

An innovative free-space optical (FSO) link using laser light propagation to achieve transmission rate of 40 Gb/s at a wavelength of 1550 nm is proposed and experimentally demonstrated. Over a 20-m free-space link, brilliant bit-error-rate performance and clear eye diagram are obtained in the proposed 1550-nm distributed feedback (DFB) laser diode (LD)-based FSO links. As far as we know, it is the first time that a 1550-nm externally modulated laser transmitter cascaded with an erbium-doped fiber amplifier and a pair of fiber collimators to successfully set up a 20-m/40-Gb/s FSO link has been employed. Compared with the 680-nm vertical-cavity surface-emitting laser-based FSO link, this proposed 1550-nm DFB LD-based FSO link is attractive not only because it has longer free-space transmission distance but because it supplies higher bandwidth operation as well. Such a 1550-nm DFB LD-based FSO link provides the benefits of optical wireless communications for longer transmission distance and higher transmission rate, which is thoroughly helpful for optical wireless network applications.

In this paper, a high-radial-tolerance fiber collimator (HRTFC) consisting of beam expanding fiber (BEF) and thick-lens (T-lens) is designed. The HRTFC uses BEF to reduce coupling loss, while T-lens are used to increase the outgoing spot to enhance the tolerance of radial mismatch. Firstly, the Gaussian beam ABCD optical transmission matrix is used to analyze transmission properties of light in the T-lens. Then, a beam transmission coupling model is constructed to explore the coupling losses of radial, angular, and axial mismatches under different spots. Finally, a static experimental platform is built to verify the influence of spot size on the coupling mismatch, and a dynamic experimental platform is built to monitor the temperature without contact. The experimental results show that the coupling loss decreases from 1.808 dB to 0.368 dB in the HRTFC, with the fiber mode field diameter (MFD) increasing from 10 μ m to 28 μ m. Increasing the spot of the HRTFC to a certain extent can reduce the tolerance of angular mismatch but can significantly increase the tolerance of radial mismatch. • We designed a high-radial-tolerance fiber collimator (HRTFC) to solve the problem of low radial mismatch tolerance. • Increasing the spot of the HRTFC to a certain extent can significantly improve the radial mismatch tolerance with a small loss of angular mismatch tolerance. • In the rotating state, the HRTFC enables contactless temperature monitoring. [ABSTRACT FROM AUTHOR]

An innovative free-space optical (FSO) link using laser light propagation to achieve transmission rate of 40 Gb/s at a wavelength of 1550 nm is proposed and experimentally demonstrated. Over a 20-m free-space link, brilliant bit-error-rate performance and clear eye diagram are obtained in the proposed 1550-nm distributed feedback (DFB) laser diode (LD)-based FSO links. As far as we know, it is the first time that a 1550-nm externally modulated laser transmitter cascaded with an erbium-doped fiber amplifier and a pair of fiber collimators to successfully set up a 20-m/40-Gb/s FSO link has been employed. Compared with the 680-nm vertical-cavity surface-emitting laser-based FSO link, this proposed 1550-nm DFB LD-based FSO link is attractive not only because it has longer free-space transmission distance but because it supplies higher bandwidth operation as well. Such a 1550-nm DFB LD-based FSO link provides the benefits of optical wireless communications for longer transmission distance and higher transmission rate, which is thoroughly helpful for optical wireless network applications. [ABSTRACT FROM PUBLISHER]

Abstract: An effective one-axis passive packaging method for a D lens fiber collimator is presented and compared with the traditional five-axis active packaging method. The dependence of collimator insertion loss on the gap between fiber pigtail and lens is shown experimentally to verify the packaging tolerance. In case of a 20mm working distance, a 10μm gap tolerance is obtained for a D lens collimator to obtain an insertion loss of <0.3dB. In addition, a 2000h damp heat test was carried out to demonstrate the reliability of this effective packaging method. [Copyright &y& Elsevier]