KEYWORDS: Sensors, Temperature metrology, Temperature sensors, Ring lasers, Polydimethylsiloxane, Fiber lasers, Interferometers, Signal to noise ratio, Single mode fibers, Photonic crystal fibers
A mode interferometer coated with polydimethylsiloxane (PDMS) film based on photonic crystal fiber (PCF) is embedded into the fiber ring laser (FRL) sensing system for temperature and relative humidity measurement. High optical signal to noise ratio (OSNR) of ~54 dB, narrow 3 dB bandwidth of ~0.26 nm, and high-quality factor Q of 6×103 were obtained, which indicate that the designed FRL system has ability to realize the measurement of environmental parameters with high resolution and accuracy and has potential application in remote monitoring of temperature. As the temperature increased from 35 °C to 115 °C, the spectral response has wavelength blue-shifts trend. The average temperature sensitivity of temperature sensor based on PCF is 90 pm/°C with small error bar and excellent linear correlation coefficient of 0.992. The average temperature sensitivity of temperature sensor based on PCF is 89 pm/℃ during the cooling process, revealing the proposed sensor has excellent reversibility. In addition, the sensor is insensitive to the change of relative humidity. Furthermore, the stability of temperature sensor based on PCF was experimentally demonstrated and analyzed. The maximal dip wavelength shifts of the peaks at 45 °C and 95 °C were 0.074 and 0.075 nm, respectively, within 3 hours. Besides, the FRL temperature sensor based on PCF has simple structure, excellent reversibility, repeatability, and stability. Therefore, the proposed FRL temperature sensor based on PCF has a great potential in practical applications where the high measurement precision and accuracy is required such as chemical reaction.
KEYWORDS: Ultrasonics, Signal detection, Sensors, Aluminum, Single mode fibers, Signal to noise ratio, Fiber lasers, Ferroelectric materials, Signal attenuation, Fiber Bragg gratings
We proposed and analyzed a fiber-laser ultrasonic sensor based on remote coupling. Utilizing the unique anti-environmental disturbance ability of the remote coupling, together with the transient response characteristics of the fiber laser, a good detection of the ultrasonic signal can be achieved under configurations with different coupling parameters. The detection effect of continuous and pulsed ultrasonic signals under different distances among the ultrasonic signal source, the fiber sticking position and the ultrasonic receiving unit was experimentally demonstrated. The ultrasonic sensor has high value in practical engineering applications.
Silicon photonics optical phased arrays have been studied intensively in recent years. As the divergence angle of the light beam is inversely proportional to the size of optical antennas, developing long grating emitters is crucial to the implementation of large aperture optical phased arrays. However, because of the high refractive index contrast between silicon and cladding materials on silicon-on-insulator (SOI) platform and fabrication limitation, the grating strength of a conventional grating is so strong that the light can only propagate a short distance within the grating. Because of the capability to engineer the macro optical properties of materials, subwavelength structures have become important building blocks in integrated photonics. In this paper, we propose subwavelength silicon segments as a promising approach to form long grating emitters. Subwavelength segments are placed a distanced away from a conventional waveguide to assure that they only interact with the evanescent wave of the guided mode. The grating strength can be tailored to any values of interest by optimizing the dimensions and positions of subwavelength segments. As a proof-ofconcept, a millimeter-long, through-etched grating and an apodized grating are designed and fabricated, which shows a divergence angle of 0.081 ° and 0.079°.
Circular polarization shift-keying (CPolSK) modulation maps binary digital signals to the two circular polarization states of the optical carrier, which can be applied to the modulation-demodulation of free space optical communication systems. This paper constructs a new optical communication system that uses vortex beams with different orbital angular momentum (OAM) modes as the optical carriers to multiplex CPolSK modulated signals. And with limited OAM modes, we combine amplitude shift-keying (ASK) modulation and CPolSK modulation to construct a modulation and demodulation system which is similar to quadrature amplitude modulation. In the receiver, a designed reference beam is used to interfere with the signal beam. Then the hybrid modulation signals can be easily converted into the intensity changes that can be directly detected for demodulation. The numerical simulation results show that this modulation system can effectively expand the communication capacity of CPolSK modulation system. And after combining ASK and CPolSK, the system can get better communication performance. It shows a wide prospect in the field of free space optical communication.
An all-fiber Fabry-Perot interferometer (FPI) strain sensor based on offset splicing fibers and Vernier effect is proposed and experimentally demonstrated. The proposed sensor consists of two separated air-cavity FPIs connected by a long section of single mode fiber (SMF) in a fiber link. The two separated FPIs have approximately equal optical paths, so that the Vernier effect can be generated. One FPI is used as the sensing FPI (SFPI) for strain measurement, which is formed by splicing a section of microfiber between two SMFs with large lateral offset. The other FPI is used as the reference FPI (RFPI) to employ the Vernier effect and amplify the sensitivity, which is formed by splicing a section of silica tube between two SMFs. Compared to a single FPI based sensor, the strain sensitivity of the proposed sensor with Vernier effect can be improved by tens of times. The strain sensitivity of our proposed sensor reaches 1.3 nm/με, which is the highest strain sensitivity of fiber sensor based on FPI and wavelength demodulation mechanism. Since the aircavity SFPI is insensitive to temperature, the proposed sensor also exhibits low temperature sensitivity of 50.2 pm/℃. With the advantages of high strain sensitivity, low temperature cross-sensitivity, compact size and easy fabrication, the proposed sensor has great applications in many fields.
Ultrasonic nondestructive testing plays a more and more important role in modern production and construction. In this paper, a novel ultrasonic energy transfer technique based on peanut shape structure is proposed and demonstrated. The ultrasonic device is able to effectively couple the laser energy in the fiber core to the cladding of the single-mode fiber (SMF) by using the micro machined fiber structure. The laser energy coupled to the cladding can be absorbed by the ultrasonic conversion material coated on the surface of the SMF. Due to the ultrasonic conversion material has high thermal elastic coefficient, it can produce mechanical expansion and contraction process after absorbing the laser, resulting in the generation of the ultrasonic signal. The ultrasonic transducer based on peanut shape taper only needs to be fused by SMF and fiber splicer. The proposed laser-ultrasonic transducer has many advantages, such as simple preparation, cost-effective, and high energy conversion efficiency. Such characteristics makes the proposed laser-ultrasonic transducer attractive for practical applications.
We investigate the conversion of mode locking of bright pulses, dark pulses and dark-bright pulses in an Erbium-doped fiber laser (EDFL) based on a Molybdenum disulfide (MoS2) films saturable absorber (SA) experimentally. When the pump power is 150 mW, by changing the polarization controller (PC) to change the polarization state in the laser cavity, we observed the bright pulse trains, the dark pulse trains and the dark-bright pulses trains in turn. All pulse interval are about 58 ns, and the repetition frequency of pulse is 1.7 MHz. The study provides a method for the conversion among bright pulses, dark pulses and dark-bright pulses.
An all-fiber Fabry-Perot (FP) interferometer (FPI) sensor which can simultaneously measure strain and temperature is proposed and experimentally demonstrated. The FPI sensor is composed of a cascaded-cavity structure with a hollow-core tube fiber cavity and a standard single-mode fiber (SMF) cavity. Different cross section areas and thermal-optic coefficients of the two cavities make them differently sensitive to temperature and strain. The individual interference spectrum based on the air cavity or silica cavity can be extracted by band-pass filtering the spectrum of the sensor in frequency domain. The temperature and strain sensitivity of each cavity are obtained by tracing the wavelength shifts of the interference spectra. Furthermore, the temperature-strain cross-talk is eliminated by solving a cofficient matrix equation composed by the temperature and strain sensitivities of the two FP cavities. The strain sensitivities of two FP cavities achieved are 1.173 nm/με and 1.86 nm/με. The temperature sensitivities of two FP cavities are 0.00574 nm/°C and 0.00549 nm/°C. The advantages of the proposed sensor including simultaneous measurement of temperature and strain, no temperature-strain cross-talk, simple fabrication, and all-fiber structure make it appropriate for the real sensing applications.
KEYWORDS: Fiber lasers, Linear filtering, Signal to noise ratio, Laser systems engineering, Mach-Zehnder interferometers, Optical filters, Polarization, Optical isolators, Switching
Serving as a frequency selective device, comb filter is a very important component to the multiwavelength erbium-doped
fiber laser. There are all kinds of comb filters, among which the Mach–Zehnder interferometer (MZI) is the commonly
used one. Since each types of Mach–Zehnder interferometers have its output features, then the study of its influence to
the output characteristics of multiwavelength erbium-doped fiber laser is necessary. In this paper, the filtering properties
of three basic Mach–Zehnder interferometer are discussed, including single-pass MZI, dual-pass MZI, dual-pass MZI
with a optical isolator embedded into the second loop. Furthermore, the working principle and tunable operation of the
tunable modified dual-pass MZI are analyzed. Above all, the influence of the four types of MZI to the output
characteristics of multiwavelength erbium-doped fiber laser is experimentally studied. The results of this paper are
helpful to understand the working principle of these comb filters and improve the output performance of
multiwavelength erbium-doped fiber laser.
In this paper, the performance study on the tunable multiwavelength erbium-doped fiber laser is conducted, which is
based on combination of Lyot birefringence fiber filter and nonlinear fiber loop mirror. The method to achieve its
optimal working is introduced in detail. The parameters choice based on the best tunability is analyzed from three
different perspectives: gain medium, nonlinear effect and comb filter, respectively. All the experimental results will be
given through several panels of comparison experiments. The optimal performance will finally be achieved by setting
appropriate parameters. The study in this paper will further improve the function of tunable fiber filter.
In this paper, we present a temperature-insensitive refractive index sensor based on π-phase-shifted Bragg gratings fabricated on side-hole fibers processed by wet chemical etching technique. The reflection spectrum of the π-phase shifted gratings on etched side-hole fiber features two notches with large spectral separation, which was used for refractive index (RI) detection in our application. The relative spectral notch separation exhibited a RI sensitivity of −278.5 pm/RIU (RIU: RI unit). Theoretical simulation obtained the temperature sensitivity of −0.00241 pm/°C, and experimental results also showed little sensitivity to temperature of our RI sensor.
Fiber-optic ultrasonic transducers are an important component of an active ultrasonic testing system for structural health monitoring. Fiber-optic transducers have several advantages such as small size, light weight, and immunity to electromagnetic interference that make them much more attractive than the current available piezoelectric transducers, especially as embedded and permanent transducers in active ultrasonic testing for structural health monitoring. In this paper, a distributed fiber-optic laser-ultrasound generation based on the ghost-mode of tilted fiber Bragg gratings is studied. The influences of the laser power and laser pulse duration on the laser-ultrasound generation are investigated. The results of this paper are helpful to understand the working principle of this laser-ultrasound method and improve the ultrasonic generation efficiency.
A method based on the quantum-behaved particle swarm optimization algorithm is presented to design a bandpass filter
of the fibre Bragg gratings. In contrast to the other optimization algorithms such as the genetic algorithm and particle
swarm optimization algorithm, this method is simpler and easier to implement. To demonstrate the effectiveness of the
QPSO algorithm, we consider a bandpass filter. With the parameters the half the bandwidth of the filter 0.05 nm, the
Bragg wavelength 1550 nm, the grating length with 2cm is divided into 40 uniform sections and its index modulation
is what should be optimized and whole feasible solution space is searched for the index modulation. After the index
modulation profile is known for all the sections, the transfer matrix method is used to verify the final optimal index
modulation by calculating the refection spectrum. The results show the group delay is less than 12ps in band and the
calculated dispersion is relatively flat inside the passband. It is further found that the reflective spectrum has sidelobes
around -30dB and the worst in-band dispersion value is less than 200ps/nm . In addition, for this design, it takes
approximately several minutes to find the acceptable index modulation values with a notebook computer.
A fiber Bragg grating (FBG) sensor demodulation scheme based on a multi-wavelength erbium-doped fiber laser (EDFL)
with linear cavity configuration is presented and demonstrated. The scheme is one linear fiber laser cavity with two FBG
sensors as its filters. One is for strain sensing, and the other one is for temperature compensation. A power-symmetric
nonlinear optical loop mirror (NOLM) is utilized in the laser in order to suppress the mode competition and hole-burning
effect to lase two wavelengths output that correspond with two FBG sensors. The sensing quantity, which is demodulated
by spectrometer, is represented by the output wavelength shift of the EDFL with temperature and strain applying on FBG
sensors. In the experiment, strain measurement with a minimize resolution of 0.746με, i.e. 0.9pm and adjustable linear sensitivity are achieved. Due to utilizing the linear cavity multi-wavelength EDFL with a NOLM as the light source, the
scheme also exhibits important advantages including obviously high signal and noise ratio (SNR) of 40.467dB and low
power consuming comparing with common FBG sensors with broadband light as the light source.
The influence of pump power on the output characteristics of multiwavelength erbium-doped fiber laser, which employs
the effect of the intensity-dependent loss induced by a power-symmetric nonlinear optical loop mirror, is investigated.
The results of studies show that the multiwavelength output characteristics including the number, the flatness and the
spectral region of output wavelength can be controlled by adjusting the pump power. To optimis the multiwavelength
operation, a suitable pump power must be chosen for this kind of laser.
The surface defects (indentations, humps, scores) of semiconductor wafers are the major factor to affect the performance
of the semiconductor devices. Moreover, these defects make the manufacturing process difficult or yield of the
semiconductor devices decrease. Consequently, it is necessary to do an on-line wafers detection in the benefit of the
performance and yield of the devices. Thus, developing a way suited for practical detection is the urgent affair faced to
people. The optical methods are the most suited ways for surface defects detection because of their non-contact
operation. We discuss a relatively unknown method of the light reflex surface defects detection technology (Makyoh (or
magic-mirror) topography (MT)). The optical system that we designed achieves the detection mode of the parallel light
vertical incidence for the first time. It corrects the deviation between the tradition light path and arithmetic, promotes the
detection precision. A lot of experiments based on this optical system are taken to get the difference between the
detection modes of vertical incidence and the oblique incidence. The experiments prove that the vertical incidence mode
excels the oblique incidence mode. The MT images are also analyzed and disposed. The quantitative measurement can
be realized by digital image processing technology.
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