Fiber Collimator Research Articles

Twist detection Mach-Zehnder interferometer based on optical vernier effect and its phase demodulation system

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.

Analysis of the Sapphire Fiber Fabry-Perot Interferometer Fringe Visibility under Different Excitation Conditions

In this paper, a theoretical analysis of how the excitation conditions affect the sapphire fiber Fabry-Perot interferometer (SFPI) visibility was performed. The conditions were considered, in which an SFPI was excited by a single-mode fiber (SMF), a multimode fiber (MMF), and a fiber collimator. The finite difference method (FDM) was used to realize the numerical solution of the modal electric fields, and then, the modal excited distributions in the sapphire fiber and the SFPI visibility were calculated. The results showed that different numbers of modes were excited in sapphire fibers under different excitation conditions and finally affected the fringe visibility of the SFPI. The fiber collimator excited the fewest modes and the visibility remained at the highest level. Finally, an experiment was performed, and the experimental results agreed well with the theoretical results.

Optical transmission characteristics of Large-tolerance Fiber Collimator

A Large-tolerance Fiber Collimator (LTFC) consisting of a Thermally Expanded Core Fiber (TECF) and an aspherical lens is designed to solve the problems of low beam coupling efficiency and small coupling tolerance of conventional collimators. According to the Gaussian beam coupling theory, the beam deviation coupling model of the LTFC is established to analyze the effects of coupling deviation and fiber Mode Field Diameter (MFD) on the collimator coupling efficiency. Beam coupling simulations are carried out by ZEMAX, which analyzes the influence of the interaction of the three coupling deviations on the beam coupling efficiency. And the final experimental platform is built to test the performance of the designed LTFC in combination with simulation. The results show that the LTFC can achieve a beam coupling efficiency more than 80% with an angular deviation less than 0.06°, a radial deviation less than 0.06 mm, and an axial deviation less than 2 mm. The coupling efficiency can reach 97.24% when the collimator fiber MFD is 28μm and all the deviations are extremely small(close to 0). The LTFC has a large coupling tolerance while obtaining a high coupling efficiency, which reduces the difficulty of beam coupling.

Multi-channel synthetic aperture infrared imaging and experimental research.

The synthetic aperture infrared radio imaging method based on laser local oscillator coherent detection has potential application value for astronomical observations. This paper studies the multi-channel synthetic aperture infrared imaging method and conducts experimental verification using a principle prototype. In the short-wave infrared band, five beam-expanding fiber collimators are used to build an observation structure of five laser local oscillator coherent detection channels at a near-field distance of 5m to carry out physical experiments. The laser local oscillator wavelength is 1.55µm, and the AD sampling rate is 4GHz. For the infrared radiation source signal, the phase relationship of the infrared signals between channels acquired by the prototype principle is stable, and the five-channel synthetic aperture imaging results are consistent with the computer simulated results. The experiment verified the effectiveness of the laser local oscillator comprehensive aperture infrared radio imaging method.

Multiplex fluorescence detection of single-cell droplet microfluidics and its application in quantifying protein expression levels.

This study presented a platform of multiplex fluorescence detection of single-cell droplet microfluidics with demonstrative applications in quantifying protein expression levels. The platform of multiplex fluorescence detection mainly included optical paths adopted from conventional microscopy enabling the generation of three optical spots from three laser sources for multiple fluorescence excitation and capture of multiple fluorescence signals by four photomultiplier tubes. As to platform characterization, microscopic images of three optical spots were obtained where clear Gaussian distributions of intensities without skewness confirmed the functionality of the scanning lens, while the controllable distances among three optical spots validated the functionality of fiber collimators and the reflector lens. As to demonstration, this platform was used to quantify single-cell protein expression within droplets where four-type protein expression of α-tubulin, Ras, c-Myc, and β-tubulin of CAL 27 (Ncell = 1921) vs WSU-HN6 (Ncell = 1881) were quantitatively estimated, which were (2.85 ± 0.72) × 105 vs (4.83 ± 1.58) × 105, (3.69 ± 1.41) × 104 vs (5.07 ± 2.13) × 104, (5.90 ± 1.45) × 104 vs (9.57 ± 2.85) × 104, and (3.84 ± 1.28) × 105 vs (3.30 ± 1.10) × 105, respectively. Neural pattern recognition was utilized for the classification of cell types, achieving successful rates of 69.0% (α-tubulin), 75.4% (Ras), 89.1% (c-Myc), 65.8% (β-tubulin), and 99.1% in combination, validating the capability of this platform of multiplex fluorescence detection to quantify various types of single-cell proteins, which could provide comprehensive evaluations on cell status.

Wide-angle high-precision electronically controlled continuous beam scanning implementation based on MLAS cascaded AFOC

Microlens array scanner (MLAS), as a rapidly growing branch of the semisolid micro-mechanical beam scanning system, can achieve large-scale beam scanning through small-scale (μm) physical motion, which is an effective beam scanning scheme that balances aperture, field of view (FOV), inertia, and other vital indicators. However, the discrete addressing caused by the periodic structure of microlens array (MLA) will affect the scanning accuracy in applications. We design and develop a continuous laser beam scanning device with an effective aperture of 20 mm based on MLAS and adaptive fiber collimator (AFOC), and the scanning beam quality is close to the diffraction limit. In addition, the electrical control mechanism of the device is explained, which achieves a scan FOV of 20°, a scan angular velocity of 10,000°/s, and two-dimensional (2D) scan. This approach effectively realizes a beam scanning system with excellent comprehensive performance, including large aperture, large scan angle, high precision, high speed, tunable wavelength, and high power.

Study on optical coupling characteristics of a high-radial-tolerance thick-lens beam expanding fiber collimator

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 10μm to 28μm. 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.

Trace Gas Detection System Based on Photoacoustic and Photothermal Spectroscopy Using Ring Fiber Laser and Quartz Tuning Fork

In this paper, a dual-spectroscopy gas sensing system based on ring cavity laser and quartz tuning fork (QTF) is designed. 15 m erbium-doped fiber with 1500 ppm doping concentration is used as gain medium, piezoelectric transducer (PZT) and acousto-optic modulator (AOM) are used as detection method. The first fiber collimator (FC) emits laser beam, laser is received by the second FC after passing through an off-beam acoustic micro-resonator (AmR) for quartz-enhanced photoacoustic spectroscopy (QEPAS) signal excitation and enhancement. The received laser is transmitted through the optical fiber and then incident on the QTF prongs by the third FC, which excites and quartz-enhan-ced photothermal spectroscopy (QEPTS) signal. Combined with the ring intra-cavity fiber laser and wavelength scanning and intensity modulation technology, dual-spectroscopy gas detection is realized. Compared with the single detection of QEPAS or QEPTS signals, the detection limit is enhanced. Under the conditions of 400mW pump power and 2500 ppmv acetylene (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas concentration, the signal to noise ratio (SNR) of QEPAS, QEPTS and dual-spectroscopy signals are 838.07, 1117.47 and 2096.26, respectively, and the minimum detection limits (MDL) of 2.98 ppmv, 2.24 ppmv, 1.19 ppmv are achieved, respectively.

All-fiber-device-coupled compact, transportable ultra-stable laser.

In response to the demand for operation in non-laboratory environments, there has been a trend toward the development of compact, transportable ultra-stable lasers. This paper reports on this sort of laser system assembled in a cabinet. The whole optical part utilizes fiber-coupled devices to simplify the integration. In addition, spatial beam collimation and alignment into the high-finesse cavity are realized by a five-axis positioner and a focus-adjustable fiber collimator, which significantly relax the alignment and adjustment. A theoretical analysis is performed on how the collimator adjusts the beam profile and coupling efficiency. The support structure of the system is specially designed as well so that it features robustness and transportation without performance degradation. The observed linewidth is 1.4Hz within a duration of 1s. After subtracting the linear drift of 70 mHz/s, the fractional frequency instability is better than 4 × 10-15, for the averaging time ranging from 1 to 100s, which is close to the thermal noise limit of the high-finesse cavity.

Fiber Laser Methane Detection Device based on TDLAS and Its Application

Methane is a flammable and explosive gas, which is the main component of natural gas, mashgas, and biogas. Using the technology of tunable diode laser absorption spectroscopy (TDLAS) to detect methane, which does not require oxygen, no poisoning, no calibrating, has gas selectivity, strong environmental adaptability, and is stable and reliable. The laser transmission with the optical fiber has the characteristics of low optical loss, long distance and flexible networking. This manuscript demonstrated an optical fiber laser methane detection device based on TDLAS and its application. The laser was transmitted to the methane detection position by optical fiber, and the laser interacted with methane in the optical fiber gas box. The optical fiber collimator coupled the surplus laser into the optical fiber and sent it back to the monitoring device.The photoelectric detector converted the surplus laser to an electrical signal. The monitoring device operation was based on the principle of TDLAS and analyzed the returned laser strength to obtain the concentration of methane. This manuscript described the principle, composition and monitoring spot network of the fiber laser methane monitoring device and shown a practical application.

Bidirectional transmission of a collimator with double-combined collimating lenses and thermally expanded core fibers.

The collimator is an essential part of the fiber optic rotary joint design. This study proposes the Large-Beam Fiber Collimator (LBFC) with a double collimating lens and a Thermally Expanded Core (TEC) fiber structure. The transmission model is constructed based on the defocusing telescope structure. The effects of TEC fiber's mode field diameter (MFD) on the coupling loss are investigated by deriving the loss function for the influence of collimator mismatch error and implementing it on a fiber Bragg grating temperature sensing system. The experimental results show that the coupling loss decreases with the increase of the MFD of TEC fiber, while the coupling loss is less than 1dB when the mode field diameter is greater than 14µm. TEC fibers can reduce the effect of angular deviation. Considering the coupling efficiency and deviation, the preferred mode field diameter for the collimator is 20µm. The proposed LBFC enables bidirectional transmission of optical signals for temperature measurement.

Analysis of Collimation Errors in Off-axis Parabolic Reflective Fiber Collimators

Analysis of Collimation Errors in Off-axis Parabolic Reflective Fiber Collimators

Achromatic Flat Metasurface Fiber Couplers within Telecom Bands

We proposed a single metalens for fiber coupling within telecom bands. This proposed fiber coupler combined a single layer metalens and a Polydimethylsiloxane (PDMS) layer. Instead of traditional fiber collimators, which are bulky and require complex calibration processes, we used a metalens for the focusing of incident and outgoing lasers and achieve achromatic aberration over a certain wavelength band. The focal length was kept as 514.9 μm with a 6.92-μm tolerance. The average coupling efficiency of an achromatic lens was calculated as 0.43. The different phases were produced with the nanopillar element structures. The aim is to provide an idea for creating a more convenient, integrated and efficient way of coupling fiber optics. This approach can also be applied to the design of achromatic lenses in other wavelength regions.

Intracavity ranging enabled by a single-frequency self-sweeping fiber laser with a few-longitudinal-mode range.

Self-sweeping fiber lasers have carved out numerous applications such as spectral detection, fiber sensor, etc. In this work, we propose a single-frequency self-sweeping fiber laser with a few-longitudinal-mode range by employing a length of space path to achieve the function of intracavity ranging. Different from the previous design, a fiber collimator and mirror are utilized to act as the reflector, and the distance between them can be adjusted flexibly. Based on this design, we achieve a few-longitudinal-mode self-sweeping operation containing seven longitudinal modes. When the distance is set as a fixed value, the behaviors of fiber laser containing central wavelength, quasi-continuous wave pulse, as well as radio frequency spectrum at different pump power are measured. The intracavity ranging systems are also demonstrated at different distances between collimator and mirror, showing a promising accuracy. This work provides a new laser ranging tool and opens up the applied scenario of self-sweeping fiber laser.

Sapphire Fiber Fabry-Perot Sensors With High Fringe Visibility

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.

Cantilevered adaptive fiber-optics collimator based on piezoelectric bimorph actuators.

A novel, to the best of our knowledge, cantilever construction design of an adaptive fiber-optics collimator (AFOC) based on piezoelectric bimorph actuators for tip/tilt control is introduced. With this new cantilever structure, an AFOC with a diameter of only 6 mm was developed, and the output laser beam deviation angle and resonance frequency of the device were measured. The experimental results show that this new AFOC can provide more than 1 mrad deflection angle at a 20 V driving voltage, and the first resonance frequency is about 500 Hz. Further, in order to verify whether the cantilever structure can be used in a high-power fiber collimator, a high-power X-Y positioner with an 8 mm diameter fiber end cap was developed. The experimental results show that the high-power X-Y positioner can output more than 2 kW laser power and provide about 330 µm displacement of the fiber end cap in the X direction and about 770 µm in the Y direction at a 150 V driving voltage.

Long-distance in-situ near-infrared gas sensor system using a fabricated fiber-coupled Herriott cell (FC-HC) operating within 1.5–2.3 μm

Long-distance field gas sensing in hazardous environment is highly desirable in industry but remains challenging due to ultralow concentration levels of the target gas species. A fiber-coupled Herriott cell (FC-HC) was fabricated for long-distance gas detection, which is composed of a sealed Herriott cell with a dimensional size of 8.3 cm × 9.0 cm × 47.2 cm, a single-mode fiber collimator for input light coupling and a multi-mode optical fiber for output light coupling. The experimentally measured absorption path length of the FC-HC is ∼24.6 m through acetylene (C2H2) detection. Also, this FC-HC is compatible with distributed feedback lasers operating from ∼1.5 μm to ∼2.3 μm. C2H2 and methane (CH4) measurements were carried out using the fabricated FC-HC. The measurement precision of C2H2 is 0.38 parts-per-million in volume (ppmv) with a 0.5 s averaging time, which can be further decreased to 27.20 parts-per-billion in volume (ppbv) with a 70 s averaging time. The limit of detection (LoD) of CH4 is 1.11 ppmv with an averaging time of 0.5 s and can be improved to 0.04 ppmv with an averaging time of 122 s. Field measurement of the sensor system for in-situ CH4 leakage was performed to validate the normal operation of the system. The proposed FC-HC shows potential applications in the field of long-distance gas sensing based on laser absorption spectroscopy.

Core Selective Switch With Low Insertion Loss Over Ultra-Wide Wavelength Range for Spatial Channel Networks

The core selective switch (CSS) is an optical spatial switch that has been recently proposed as a key building block to achieve a scalable and low-insertion-loss spatial cross-connects for use in future spatial channel networks. In this paper, we report on a novel CSS design employing a two dimensionally arranged microlens-based multicore fiber (MCF) collimator array and a micro-electromechanical systems (MEMS) mirror array. The former enables precise alignment between MCFs and collimator lenses, and the latter yields polarization-independent high reflection over a wide wavelength range and a large tilt angle. Based on the design, a compact (∼50 mm) five-core <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times 8$</tex-math></inline-formula> CSS prototype is fabricated. We experimentally show that the CSS prototype exhibits low insertion loss (1.2∼2.7 dB), low polarization dependent loss (< 0.25 dB), and low crosstalk (< <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 30$</tex-math></inline-formula> dB) characteristics over an ultra-wide wavelength range from 1500 nm to 1630 nm. Bit-error-rate measurements using optical signals in the C-band, S-band, and L-band show that the CSS prototype incurs no optical signal-to-noise ratio penalty in spatial channel routing over an ultra-wide wavelength band.

Sensitivity enhanced fiber optic hydrophone based on an extrinsic Fabry-Perot interferometer for low-frequency underwater acoustic sensing.

A miniaturized fiber optic hydrophone (FOH) based on a composite metal diaphragm with an air back cavity and a high finesse extrinsic Fabry-Perot interferometric (EFPI) scheme for low-frequency underwater acoustic sensing is proposed and experimentally demonstrated in this paper. A composite metal diaphragm is used to improve the stability of the hydrophone. A balance channel is used to equilibrate the hydrostatic pressure and maintain an air cavity, which improves the mechanical sensitivity. In addition, a white light interferometry (WLI) phase demodulation is used to demodulate the high finesse interferometer consisted of the fiber collimator end face and the diaphragm, which improves the phase sensitivity. Experimental results show that the enhanced phase sensitivity of the hydrophone is about -122.5 dB re 1 rad/µPa @ 200 Hz and the sensitivity fluctuation is below 2.5 dB between 3 Hz and 400 Hz, while the minimal detectable pressure (MDP) is 63.7 µPa/Hz1/2 @ 400 Hz. Due to its miniaturized structure and high sensitivity, the FOH may have an enormous potential in underwater target detection.

Characteristics of Collimators Based on the Large-Mode-Area CMCF for Coupling Laser Beam

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.