Optical fiber spectroscopy technology is widely used in astronomical surveys. Due to the flexibility and long-distance transmission characteristics of the fiber, astronomical observation can gain larger scale and higher precision spectral data. Nowadays, a lot of representative technologies have been presented to enhance spectral resolution, including fiber integral field spectroscopy, fiber positioning technology in the sky survey, adaptive optics, and photonic lantern technology. Fiber spectroscopy technology plays a crucial role in astronomy. The long-distance transmission characteristics of fibers separate the telescope from the spectrograph. The intrinsic flexibility of fibers lends itself readily to reconfigurable sampling of the field. The method to improve the spectral resolution has been gradually proposed. Fiber integral field spectroscopy is one of the most typical techniques to enhance the spectrum resolution. The flexible combination of fiber bundle and microlens is used to improve the sampling rate of target stars and fitting factor. In the observation of target stars by a single fiber, the alignment accuracy between the fiber and the star image determines the spectrum resolution. In the multi-object telescope, the position of a large number of multi-mode fibers needs to be detected. As a kind of optical waveguide device with multi-mode and single-mode conversion, photonic lantern can convert the energy collected by multi-mode fiber into the output of single-mode fiber. This review introduces optical fiber technology on astronomical observation.
Fiber spectroscopy technology is important in many areas of astronomical surveys. The fiber is used to transfer light from the telescope to the spectrograph. On the detector of the spectrograph, the image of fiber ends after dispersing can be obtained. In multi-mode fiber, multiple modes propagating in the fiber form a granular speckle pattern on the fiber end. In high-resolution spectral measurement, the speckle disturbs the energy distribution of the spot and reduces spectral resolution. The influence of fibers with different parameters on the centroid shift, signal-noise ratio, and radial power spectrum under artificial and mechanical disturbance is explored in this paper. The experimental results show that when the number of modes propagating in the fiber is higher, the precision of the centroid offset of the speckle is higher. Under the same disturbance condition, the speckle suppression effect is better with more mode numbers. This will be a reference for the parameter selection of optical fiber in the new instrument.
The accuracy of fiber positioning is crucial for the observation of multi-target fiber spectral telescopes such as LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope). Most of the methods used for fiber positioning are openloop control or semi-closed-loop control, the efficiency cannot meet the requirements of next-generation spectral telescopes. Considering that, this paper proposes a fiber positioning method that can achieve a completely closed loop without blind spots based on SMART (Special-shaped Micro-lens Aimer for Real-time Targeting) and a dual-rotary positioning mechanism. The entire correction process can be described as follows: first, the light intensity obtained by the 6-channel photodetector is stored in a buffer. Then the pulses required for correction are calculated based on the data in the buffer and the correction algorithm by the computer. The running command is then sent to the stepper motor controller using serial communication to drive the stepper motor. After the movement, the program will run again to verify if the correction is done. After selecting a position in the first quadrant, 8 directions were selected to conduct automatic correction experiments after the optical fiber position offset. The experimental results show that the average farthest distance that the method could correct is 600μm, and 75.9% positioning accuracy under our closed control method can reach 10μm, 94.8% positioning accuracy can reach 20μm, 100% positioning accuracy can reach 30μm. For corrections within the 500μm offset distance, 97.5 % of the correction time is within one minute.
High precision alignment between the fiber core in the focal plane and the image of the target star is of great significance for the observation of multi-target telescopes. In this work, we propose and demonstrate a Special-shaped Micro-lens Aimer for Real-time Targeting, namely SMART, combining a special-shaped microlens and a fiber bundle to realize online alignment and improve the coupling efficiency of fibers. The platform in the center of the microlens transmits the starlight to the science fiber of the fiber bundle without changes in focal ratio. Six side micro-lenses couple leakage light to six feedback fibers and return misalignment signals. The structural parameters of SMART are well designed. Fresnel diffraction theory is applied to build a model for simulating the performance of SMART. In the SMART measurement, a pinhole with a diameter of 200 μm is used to imitate the effect of atmospheric turbulence during astronomical observations. Experimental results indicate that when the image spot is offset relative to the science fiber, the misaligned direction and displacement distance are identified by the signal of feedback fibers in SMART with a resolution of 0.02 mm and a detection range of 0.08 mm to 0.26 mm.
A fiber IFU with 8064 fibers is designed and manufactured for the Fiber Arrayed Solar Optical Telescope. 8064 fibers are divided to two 2D arrays for different polarization states and 12 pseudo fiber slits for 12 spectrometers. There are many relative techniques have been developed during this process. The hexagon microlens array fits the 100% filling factor. The quartz micropores plate guarantee the positioning accuracy among different temperatures. The 18m fiber cables with special designs transfer the signal with low focal ratio degradation. The quartz V-grooves are used to control the positions of the fibers to form those pseudo slits. Besides, a six-dimensional alignment system and a fast alignment and detection system are built to align the microlens array with micropores and measure the focal ratio, transmission efficiency and alignment accuracy of the IFU, respectively.
A multiwavelength Q-switched pulse operation obtained directly using a Tm-doped fiber laser and employing gold nanoparticles (GNPs) as a saturable absorber (SA material) is reported. The GNPs SA exhibits a modulation depth of 5.4% and a nonsaturable loss of 21.3%. Due to the saturable absorption and the high nonlinearity, dual-wavelength and triple-wavelength Q-switched pulse operations are obtained at pump power levels of 200 and 500 mW. To the best of our knowledge, this is the first ever reported use of GNPs as SA in multiwavelength Q-switched lasing generation.
In this work we investigated the fabrication of a singlemode-multimode-singlemode (SMS) fibre structure based on a chalcogenide (As2S3 and AsxS1-x) multimode fibre (MMF) sandwiched between two standard silica singlemode fibres (SMFs) using a commercial fibre fusion splicer. The temperature dependence of this hybrid fibre structure was also investigated. A first proof of concept showed that the hybrid SMS fibre structure has an average experimental temperature sensitivity of 50.63 pm/°C over a temperature range of 20 °C~100°C at wavelengths around 1.55 μm. The measured results show a general agreement with numerical simulations based on a guided-mode propagation analysis method. Our result provides a potential platform for the development of compact, high-optical-quality and robust sensing devices operating at the mid-infrared wavelength range.
In this research, we present a packaged add–drop filter composed of a silica microsphere resonator and a strongly coupled optical microfiber coupler. A one-step fabrication process using UV curable epoxy is shown to stabilize the microsphere resonator coupled to the microfiber coupler, which is used as add and drop ports. A high Q-factor of 3×107 is obtained at around 780 nm from the packaged microspheres coupled with the microfiber coupler in the add–drop configuration.
A linear 5-core fiber was sandwiched in between two single mode fibers (SMF) to construct an all fiber Mach-Zehnder interferometer (MZI). The interferences between the fundamental supermodes, between the fundamental supermodes and the high order core modes, and between the fundamental supermodes and the low order cladding mode are investigated. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain.
A long period grating (LPG) is inscribed by a femtosecond laser in the multimode region of a singlemode-multimode-singlemode fibre device to provide a compact refractive index sensor. An average sensitivity of 39 nm/RIU and a resolvable index change of 2.56×10-4 are obtained experimentally with a 44.4 mm long multimode fibre over a measured refractive index range of 1.33-1.35. Because of its compactness, ease of fabrication, linear response, high sensitivity, easy connectivity to other fiberized optical components and low cost, this refractometer could find various applications in chemical and biological sensing.
A high sensitivity refractive index (RI) sensor based on a tapered small core singlemode fibre (SCSMF) structure is proposed and developed. By tapering the SCSMF section, this sensor has very high sensitivity around the RI of water. Experimentally we have demonstrated a sensitivity of 1988.5 nm/RIU (RI unit) in the RI range from 1.3325 to 1.3407 by tapering a SCSMF section from a diameter of 125 μm to a diameter of 16 μm. The feasibility of using this type of sensor for biosensing application has also been verified by experiments. This was undertaken using a suitable Fibrinogen antigen surface functionalisation, which successfully showed that the sensor can in principle detect differences in the concentration of a Fibrinogen antibody.
A compact fiber tip refractive index sensor using FIB-milled gold-coated singlemode-multimode-singlemode structure is demonstrated. Focused ion beam (FIB) is exploited to cut the fiber tip to obtain a flat end-face and then a layer of gold is coated on the tip surface, to increase its reflection. An average sensitivity of 265 nm/RIU is obtained experimentally with a ~2.94 μm diameter singlemode-multimode-singlemode fiber tip (SMST). Because of several advantages, including compactness, ease of fabrication, linear response, high sensitivity, easy connection with other fiberized optical components and low cost, this refractive index sensor could find various applications in chemical and biological area.
A compact singlemode-photonic crystal fibre-singlemode fibre tip (SPST) refractive index sensor is demonstrated in this paper. A CO2 laser cleaving is exploited to provide a clean-cut fibre tip which is then coated by a layer of gold to increase reflection. An average sensitivity of 39.1 nm/RIU and a resolvable index change of 2.56×10-4 are obtained experimentally with a ~3.2 µm diameter SPST. Because of its compactness, ease of fabrication, linear response, high sensitivity, easy connectivity to other fibreized optical components and low cost, this refractometer could find various applications in chemical and biological sensing.
We studied the effect of a star-shaped gold nanoparticles layer coated on the surface of the microfiber coupler (MFC) on the sensitivity of the embedded MFC biosensor. It is shown that deposition of the layer of star-shaped gold nanoparticles on the MFC sensor surface results in a significantly increased spectral shift (on average 3.05 nm shift compared to a 1.08 nm shift per layer of electrolyte for the sample without the nanoparticles layer). In addition, introducing the nanoparticle layer results in the decrease of the transmission power; measurement of the changes in transmission also could be used as a means for the sensor interrogation.
A focused CO2 laser beam has been previously used to successfully fabricate both symmetric and asymmetric long period fiber gratings which have been used for a variety of sensing applications. However fabrication by a CO2 laser beam demands a time consuming laser scanning process which increases the difficulty and cost of fabrication. In this paper a fibre sensor based on a fibre heterostructure with a simple configuration consisting of a series of periodical tapers in a photonic crystal fibre (PCF) sandwiched between two singlemode fibres is proposed and investigated experimentally. The tapers are periodically fabricated along the PCF section using a CO2 laser beam. The proposed fibre heterostructure can be used for strain sensing by measuring the wavelength blueshift of the multimode interference dip of the transmission spectrum as a function of strain. An average stain sensitivity of -68.4 pm/μ ε has been experimentally achieved over a microstrain range from 0 to 100 μ ε. Assuming in practice that the sensor is interrrogated with a ratiometric power measurement system, then the strain resolution is estimated to be better than 1.18×10-2 microstrain. The mechanisms for refractive index modulation periodically tapered PCF under tensile strain measurements are complex but may be regarded as a combination of stress-relaxation and refractive index perturbations over the length of the tapered PCF induced by strain and by tapering. The proposed fibre strain sensor has the advantage of low temperature sensitivity (average 8.4 pm/°C) and an experimental demonstration of this reduced sensitivity is also presented. The proposed strain sensor benefits from simplicity of fabrication and achieves a competitive sensitivity compared with other existing fibre-optic sensors.
KEYWORDS: Liquid crystals, Microsoft Foundation Class Library, Polymers, Temperature metrology, Sensors, Optical fibers, Telecommunications, Overlay metrology, Tunable filters, Ultraviolet radiation
The paper presents the results of experimental studies of the temperature dependence of a microfibre coupler (MFC) with a waist diameter of ~4 μm covered with a layer of liquid crystal (LC) material. The microfiber coupler is fabricated by fusing together and tapering of two standard telecom fibers using a microheater brushing technique, followed by partially embedding the structure in a low-refractive index UV curable polymer (Efiron PC-363) for stability and later by placing a thin heated LC layer over the polymer-free uniform taper waist region. The temperature dependence of the embedded in polymer MFC sensor before the application of the LC layer demonstrates a redshift of the coupler’s spectrum with an average sensitivity of ~0.5 nm/°C in the temperature range of 14-70 °C. The application of the LC overlay increases the average temperature sensitivity to ~0.7 nm/°C. The demonstrated device offers several advantages such as ease of fabrication and light coupling, the potential for better stability and the possibility of electric field tuning for realizing temperature, electric field, bio-, chemical sensors and tunable add-drop filters for fiber communication systems. Further work is ongoing to explore various tuning mechanisms of the MFC spectrum.
KEYWORDS: Temperature metrology, Temperature sensors, Optical fibers, Fiber Bragg gratings, Microsoft Foundation Class Library, Optical testing, Reflection, Silica, Fabrication, Electronics engineering
A compact fast temperature sensor based on a broadband microfiber coupler tip is demonstrated. The thermometer
dynamic range spans from room temperature to 1511 °C with a response time of tens of ms. This is the highest
temperature measured with an optical fiber device. The resolution of 0.66 °C was achieved for a coupler tip diameter of
~12.56 μm.
An enhanced refractive index (RI) sensor with combination of long period fiber grating (LPG) and a small core
singlemode fiber (SCSMF) structure is proposed and developed. Since the LPG and SCSMF transmission spectra
experience a blue and red shift respectively as the surrounding RI (SRI) increases, the sensitivity is improved by
measuring the separation between the resonant wavelengths of the LPG and SCSMF structures. Experimental results
show that the sensor has a sensitivity of 1028 nm/SRI unit in the SRI range from 1.422 to 1.429, which is higher than
individual sensitivities of either structure alone used in the experiment.
KEYWORDS: Liquid crystals, Microsoft Foundation Class Library, Temperature metrology, Optical fibers, Fabrication, Silica, Waveguides, Liquids, Motion controllers, Control systems
This paper demonstrates temperature-induced tuning of the optical spectrum of a microfiber coupler covered with a low-refractive
index liquid crystal (LC) layer. The microfiber coupler with a minimum waist diameter of ~6 μm is fabricated
by fusing together and tapering of two standard telecom fibers using a microheater brushing technique and by placing a
thin heated LC layer over the uniform taper waist region. Repeatable and reversible tuning of the optical spectrum
(~4 nm) of the microfiber coupler is demonstrated for the temperature range of 50-78 °C.
KEYWORDS: Polarization, Temperature metrology, Temperature sensors, Error analysis, Sensors, Fiber optics sensors, Microsoft Foundation Class Library, Fiber couplers, Photonic devices, Fiber lasers
The dependence on polarization of the performance of a microfiber coupler based temperature sensor is
experimentally investigated. The optical microfiber coupler based temperature sensor has a diameter circa 2 μm and can
sense temperature in the range from 100°C to 1000°C, with an average sensitivity of 18.9 pm/°C. It is shown that
different polarization states of the input signal have a significant influence on the proposed temperature sensing
accuracy, with an estimated peak error of 63°C at 1000°C.
An all-fibre refractive index sensor with a simple periodical tapers configuration is proposed and investigated
experimentally. The proposed fibre refractive index sensor consists of a small core singlemode fibre with tapers
periodically fabricated along the fibre using a focused CO2 laser beam, and sandwiched between two standard
singlemode fibres. Such a structure can be used for sensing of refractive index by measuring the dip wavelength shift of
the multimode interference within the small core fibre cladding. A minimum sensitivity of 125 nm/RIU is measured for a
refractive index of 1.33 and a maximum sensitivity of 383 nm/RIU for a refractive index of 1.38. The proposed
refractive index sensor benefits from simplicity and low-cost and achieves a competitive sensitivity compared with other
fibre-optic sensors.
We theoretically and experimentally investigate a singlemode-multimode-singlemode (SMS) structure based on
chalcogenide (As2S3) multimode fiber and conventional silica singlemode fibers. The experimental results show a general
agreement with the numerical simulation results based on a wide angle-beam propagation method (WA-BPM). The
chalcogenide fiber and silica fibers were mechanically spliced and packaged using a UV cured polymer with a low
refractive index on a microscope slide. Multimode interference variation was observed by photo-induced refractive index
changes resulting from both a localized laser irradiation at a wavelength of 405 nm and a UV lamp. Our result provides a
platform for the development of compact, high-optical-quality, and robust photonic nonlinear devices.
A review of passive devices and sensors manufactured from optical fiber microwires at the Optoelectronics Research
Centre (University of Southampton) is presented.
The well known beam propagation method (BPM) has become one of the most useful, robust and effective numerical
simulation tools for the investigation of guided-wave optics, for example integrated optical waveguides and fiber optic
devices. In this paper we examine the use of the 2D and 3D wide angle-beam propagation method (WA-BPM) combined
with the well known perfectly matched layer (PML) boundary conditions as a tool to analyze TIR based optical switches,
in particular the relationship between light propagation and the geometrical parameters of a TIR based optical switch. To
analyze the influence of the length and the width of the region in which the refractive index can be externally controlled,
the 3D structure of a 2x2 TIR optical switch is firstly considered in 2D using the effective index method (EIM). Then
the influence of the etching depth and the tilt angle of the reflection facet on the switch performance are investigated
with a 3D model.
A bent singlemode-multimode-singlemode (SMS) fiber structure based vibration sensor is proposed and developed.
This sensor configuration is very simple and employs a bent SMS fiber structure and a narrow band optical source such
as a laser. The vibration applied to the bent SMS fiber structure will change the bend radius and hence the intensity of the
transmitted optical power will also vary. Experimental results show that the sensor can detect vibration frequencies over
a broad range with good sensitivity, from hertz to a few kHz.
An improved ratiometric wavelength measurement system incorporating two fibre comb filters is presented, which
performs both rough and fine wavelength measurements simultaneously. The resolution of the system is significantly
improved, compared to a single edge filter system, to better than 5 pm while maintaining the potential for high
measurement speed and wide measurable wavelength range.
In this paper an edge filter based on multimode interference in an integrated waveguide is optimized for a wavelength
monitoring application. This can also be used as a demodulation element in a fibre Bragg grating sensing system. A
global optimization algorithm is presented for the optimum design of the multimode interference device, including a
range of parameters of the multimode waveguide, such as length, width and position of the input and output waveguides.
The designed structure demonstrates the desired spectral response for wavelength measurements. Fabrication tolerance is
also analysed numerically for this structure.
We report an experimental study of a SMS fiber sensor for simultaneously measuring both displacement and
temperature. By measuring both central wavelength spectral shifts and peak power variations, displacement and
temperature can be independently determined with a demonstrated sensitivity to displacement of 0.62 μm and a
temperature sensitivity of 1 °C by using a typical OSA which has a wavelength resolution of 10 pm and power
measurement resolution of 0.01 dB.
This paper presents a simultaneous measurement technique for strain and temperature with
dynamic temperature compensation using two SMS structures with one acting as a temperature compensating
element and another SMS structure acting as a strain sensor. Experimental results show that this technique
offers a resolution of better than 2 με for strain measurements in the range from 0 to 1000 με and the
temperature induced error is as low as 7.5 με in the temperature range from 15 to 45 °C. The temperature can be
measured simultaneously with an experimentally demonstrated resolution better than 0.84 °C.
An all-fiber vibration sensor based on a bare macrobending singlemode fiber is proposed and developed. The fiber
sensor consists of a half-loop bending fiber structure and utilizes the well-known Whispering Gallery mode (WGM)
effect. A measurement system involving the proposed fiber vibration sensor is presented and investigated. By using this
system, the vibration can be characterized by measuring the fluctuations in the macrobending fiber loss corresponding to
the variation of the bending diameter of the fiber. The proposed vibration sensor is capable of measuring vibrations up to
2 kHz; and further investigations are ongoing to improve the frequency range.
An analysis and performance evaluation of several macrobending high-bend loss fiber based filters utilizing the
well-known Whispering Gallery mode (WGM) effect is investigated and presented. Experimental results indicate that the
WGM spectra of the macrobend fibre can be utilized for different types of optical filters by adjusting the bending
diameter and the length of the fiber loop.
Copper ion exchange technique was used to fabricate soda-lime glass planar waveguides. Prism coupling method was
applied to measure the effective indices, and the refractive index profiles were reconstructed through Inverse WKB
(IWKB) method. Optical absorption and photoluminescence analysis were carried out as well. The emission spectra
centered at 520nm are attributed to Cu+ located in distorted octahedral sites. It was found that the ion exchange time and
temperature both play an important role in the waveguides luminescence properties. The emission spectra intensities
decrease with the ion exchange time increasing. The emission peak wavelength slightly blue shifts as the ion-exchange
time increasing as well. The emission band intensity nearly increases consistently with the ion-exchange temperature
increasing within proper ion-exchange time. Different excitation wavelengths were tested as well in order to study the
site effect on photoluminescence properties.
The influence of the spectral response of the optical source, the signal-to-noise ratio (SNR) of the
FBG and the slope of the edge filter used within a ratiometric wavelength measurement system was investigated
based on three system configuration cases: (1) the reflected FBG signal passes through both the reference arm
and the edge filter arm, (2) the reflected FBG signal is connected directly to the edge filter arm and does not pass
through the reference arm, (3) the edge filter sits in-line with the FBG and thus the source power is filtered prior
to reaching the FBG. Both numerical simulations and experimental results show that cases 1 and 2 have a
similar system performance whilst case 3 is the configuration which is shown to offer the highest wavelength
resolution.
An all-fiber micron displacement sensor with a simple configuration is proposed and investigated
experimentally. The proposed fiber displacement sensor consists of a half-loop structure of bare high-bend loss
singlemode fiber-1060XP, employing the well-known whispering gallery mode (WGM) effect. A ratiometric power
measurement system for interrogating the proposed sensor is also presented. By measuring the change in ratio of bend
loss in the ratiometric system, a change in displacement can be measured assuming the ratiometric system is calibrated.
The proposed macrobending fiber based displacement sensor achieves a higher resolution (less than 100 nm of
displacement) than other conventional fiber-optic sensors and also benefits from simplicity.
Copper-doped planar glass waveguides were prepared by the thermal ion-exchanged of commercial soda-lime glass
wafers in molten mixture of CuSO4 with sodium sulfates at temperature of about 600°C. The effective indices were
measured by means of prism-coupled technique, and the guided modes were not monotonic increasing with the ionexchange
times. The properties of photoluminescence (PL) spectra of the waveguides were studied. The broad emission
bands centered at around 520 nm (at the excitation wavelength of 310 nm, at the room temperature) were measured on
the samples, which were also strongly influenced by the ion-exchange times and the intensity of the PL emission is not
monotonic increasing with the ion-exchange times as well.
For an all-fiber edge filter used in a rapid wavelength measurement system for optical sensing, a
low polarization dependent loss (PDL) is required to ensure high measurement accuracy. The
calculation of the bend loss for the TE and TM modes based on scalar approximations results in a
discrepancy between the calculated PDLs and measured results. Here a full vectorial finite difference
beam propagation method (FV FD-BPM) is used to compute the complex propagation constant and the
field distributions of the TE and TM modes in the bending fiber, allowing the accurate calculation of
the PDL of bending fiber.
Many fiber Bragg grating interrogation systems uses ratiometric wavelength measurement based on an edge filter. In any
ratiometric scheme a 3dB coupler is a vital component which splits the signal and makes the system ratiometric. All
commercial 3dB couplers exhibit a wavelength dependency and polarization dependent loss. In this paper the effects of
the wavelength dependency and polarization dependent loss of the 3dB coupler in a ratiometric wavelength measurement
system are investigated using both simulation and experimental techniques. The ratio response of the system is simulated
considering the wavelength dependency of the coupler and is compared with that of a response with a wavelength
independent coupler. A comparison study of the polarization induced ratio fluctuation and corresponding errors in
wavelength with a polarization insensitive 3 dB coupler (very low PDL) and an ordinary 3 dB coupler is also presented.
The results show that the 3 dB coupler has a significant influence on the ratio response and accuracy of a ratiometric
wavelength measurement system.
Theoretical analysis and experimental investigations are presented on the resolution of a ratiometric wavelength measurement system. Theoretical modelling indicates that the resolution of a ratiometric wavelength measurement system is determined by the signal-to-noise ratio of the input signal and the noise of the photodetectors associated with optical-to-electronic conversion. For the experimental verification, a ratiometric system employing a macrobending standard singlemode fiber is developed and corresponding results are in a good agreement with the theoretical prediction.
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