Center Wavelength Shift Research Articles

Smart Photonic Carbon Brush: FBG Length as Sensing Parameter

This article deals with problem of carbon brush’s length measurements. There are many applications where regular inspection is not feasible because of a number of factors including, for example, time, labor, cost and disruptions due to down time. Thus, there is a need for a system that can monitor the brush’s length to calculate it’s wear rate, while the component is in operation or without removing of the component from its operational position. We propose a novel method for characterization of carbon brush’s length. This method based on the usage of advantages of the multiplicative response of FBGs and FBG arrays: spectral parameters depend on several aspects, such as grating’s period, refractive index, it’s physical length and so on. We are the first, in our point of view, who proposed to use third parameter for sensing application and prospectively all three parameters for complex measurement: the change of FBG’s length is used to measure length of the brush and it’s wear rate, grating’s central wavelength shift for temperature (due to refractive index change) and mechanical stress (due to grating’s period variations) measurements. The results of modelling and experiments are presented.

A chiral sensor based on weak measurement for the determination of Proline enantiomers in diverse measuring circumstances

A new chiral sensor based on weak measurement to accurately measure the optical rotation (OR) has been developed for the estimation of a trace amount of chiral molecule. With the principle of optical weak measurement in frequency domain, the central wavelength shift of output spectra is quantitatively relative to the angle of preselected polarization. Hence, a chiral molecule (e.g., L-amino acid, or D-amino acid) can be enantioselectively determined by modifying the preselection angle with the OR, which will cause the rotation of a polarization plane. The concentration of the chiral sample, corresponding to its optical activity, is quantitatively analyzed with the central wavelength shift of output spectra, which can be collected in real time. Immune to the refractive index change, the proposed chiral sensor is valid in complicated measuring circumstance. The detections of Proline enantiomer concentration in different solvents were implemented. The results demonstrated that weak measurement acted as a reliable method to chiral recognition of Proline enantiomers in diverse circumstance with the merits of high precision and good robustness. In addition, this real-time monitoring approach plays a crucial part in asymmetric synthesis and biological systems.

Temperature Compensation Fiber Bragg Grating Pressure Sensor Based on Plane Diaphragm

Pressure sensors are the essential equipments in the field of pressure measurement. In this work, we propose a temperature compensation fiber Bragg grating (FBG) pressure sensor based on the plane diaphragm. The plane diaphragm and pressure sensitivity FBG (PS FBG) are used as the pressure sensitive components, and the temperature compensation FBG (TC FBG) is used to improve the temperature cross-sensitivity. Mechanical deformation model and deformation characteristics simulation analysis of the diaphragm are presented. The measurement principle and theoretical analysis of the mathematical relationship between the FBG central wavelength shift and pressure of the sensor are introduced. The sensitivity and measure range can be adjusted by utilizing the different materials and sizes of the diaphragm to accommodate different measure environments. The performance experiments are carried out, and the results indicate that the pressure sensitivity of the sensor is 35.7 pm/MPa in a range from 0 MPa to 50 MPa and has good linearity with a linear fitting correlation coefficient of 99.95%. In addition, the sensor has the advantages of low frequency chirp and high stability, which can be used to measure pressure in mining engineering, civil engineering, or other complex environment.

Deformation control method of composite structural elements by fiber-optic acoustic emission sensor

Subject of Research.The paper presents the study of a graphite-epoxy composite plate strain detection possibility by the fiber-optic acoustic emission sensor mounted on its surface. Method. The proposed method consisted in additional low-frequency phase-generated carrier implementation in the impulse Fabry-Perot interferometer and its amplitude evaluation. The phase-generated carrier amplitude depends on the interferometer optical path difference, therefore, its value change can be used for the studied composite strain estimation. VCSEL with a wavelength of 1550 nm was used as a light source. The phase carrier was generated by current modulation of the light source that caused center wavelength shift of the VCSEL. Main Results. The low-frequency phase-generated carrier signal amplitude dependence on the interferometer optical path difference and wavelength shift of the light source were obtained. According to simulation results the sensitivity of the proposed method is 1.6 urad×ppm, 5.3 urad×ppm and 13.3 urad×ppm at different values of the coefficient Kd 30, 100 and 250 pm, respectively. Experimental study of the proposed method and results analysis were performed. According to experimental results, the accuracy of the proposed method was about 1´10–3% that corresponds to the sensor relative stretch of 10 me, while the accuracy of the market available fiber optic systems based on fiber Bragg grating sensors equals to 4 ppm. Practical Relevance.The proposed method can be used for strain detection of the graphite-epoxy composite constructions along with its acoustic emission control by one fiber-optic sensor.

Reconfigurable and tunable compact comb filter and (de)interleaver on silicon platform

We propose and demonstrate a reconfigurable and tunable chip-scale comb filter and (de)interleaver on a silicon platform. The silicon-based photonic integrated device is formed by Sagnac loop mirrors (SLMs) with directional couplers replaced by multi-mode interference (MMI) assisted tunable Mach-Zehnder interferometer (MZI) couplers. The device can be regarded as a large SLM incorporating two small SLMs which form a Fabry-Perot (FP) cavity. By appropriately adjusting the micro-heaters in tunable MZI couplers and cavity, switchable operation between comb filter and (de)interleaver and extinction ratio and wavelength tunable operations of comb filter and (de)interleaver are achievable by thermo-optic tuning. Reconfigurable comb filter and (de)interleaver is demonstrated in the experiment. The central wavelength shifts of comb filter and (de)interleaver are demonstrated with wavelength tuning efficiencies of ~0.0224 nm/mW and ~0.0193 nm/mW, respectively. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm. The obtained results indicate that the designed and fabricated device provides switchable comb filtering and interleaving functions together with extinction ratio and wavelength tunabilities. Reconfigurable and tunable silicon-based comb filter and (de)interleaver may find potential applications in robust wavelength-division multiplexing (WDM) optical communication systems.

Non-contact temperature-independent random-displacement sensor using two fiber Bragg gratings.

We present a full-range displacement sensor system using two fiber Bragg gratings (FBGs). The magnetic-scale-combined FBGs allow the exploration of random position. The sinusoidal function variations are displayed by two detectors with a phase difference of 90deg, and the optimal magnetic gap is explored through numerical simulations. The feasibility of the method is demonstrated in experiments that show the sinusoidal relation between center wavelength shifts with the linear displacement. Results showed that the amplitudes of the tensile-compressive load were 446.1μϵ and 434.7μϵ, respectively, with linearity of 0.998 and 0.999 at 1.5mm between the detector and the magnetic scale. These results demonstrate that the sensors can realize non-contact, temperature-independent and full-range measurement.

Design and Testing of a 2D Optical Fiber Sensor for Building Tilt Monitoring Based on Fiber Bragg Gratings

In this paper, a new type of optical fiber tilt sensor based on fiber Bragg grating (FBG) is presented for 2D dual-axis tilt angle sensing. The tilt sensor is composed of two cylindrical floats suspended in water, connected with FBG. When the external environment causes the tilting of the sensor, the surface of the liquid within the container will form a new balanced liquid surface plane due to the gravity and change the height of the liquid at different locations in the container. So the buoyancy force of the cylindrical floats of the sensor will vary with the depth of liquid, thus the changed FBG strain will cause the FBG reflection central wavelength shift. According to the measured central wavelength shift by the optical spectral analyzer (OSA), we can obtain the two-axis tilt angle of the sensor. The proposed sensor can detect a tilt angle range of −5 degrees to +5 degrees and achieve a sensitivity of 0.1° with optical spectrum analyzer resolution of 0.01 nm. Due to its good sensing linearity, the proposed sensor can be applied in building tilt measurement.

A fiber bragg grating pressure sensor with temperature compensation based on diaphragm-cantilever structure

Pressure sensors are essential devices for pressure measurement and safety evaluation in modern industrial production and engineering application. According to the FBG sensing technology, this paper presents and studies a FBG pressure sensor that uses a diaphragm and a cantilever as a sensing unit. Two FBGs of this sensor are bonded on the top and bottom surfaces of the cantilever, respectively, and the center wavelength shift difference between the two FBGs is used as a measuring signal, thus improving the pressure measuring sensitivity and avoiding any effect of temperature cross-sensitivity. The measuring principle of this sensor is introduced, and the diaphragm structure of the sensor is analyzed through theoretical calculation and finite element analysis. Experimental results indicate that this sensor has a sensitivity of 339.956pm/MPa and a linear fitting coefficient of 99.997% in the pressure range from 0 to 10MPa, and the maximum pressure measuring error caused in the temperature range from 5 to 70°C is 0.93%, so the temperature compensation effects are remarkable. Such a FBG pressure sensor can be extensively used for quasi-distributed long-term online monitoring of the static or dynamic pressure of liquid or gas in industrial production.

Highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator.

Mid-infrared of the 3 - 5 μm spectral region has attracted considerable attention in various fields. We report a highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator. Two hybrid etalon resonators are serially stacked to make a single peak resonance with a well-suppressed sideband. Each etalon resonator, which exploits a pair of cavities, includes not only a cavity featuring a resonance but also a functional cavity lowering the sideband of transmission spectrum. By taking advantage of double cavities, each etalon features a resonant characteristic with suppressed sideband. Consequently, the filter shows a peak transmission of 92% at resonance and well-suppressed sideband within the 3 - 5 μm spectral region. Transparent dielectric materials make the device highly efficient for both transmission and reflection. Furthermore, the filter utilizing a high-index cavity leads to an enhanced angular tolerance regardless of incident light polarization. The transmission of the filter exhibits center wavelength shifts of around 0.1 μm at an angle of 50° for both transverse electric and transverse magnetic modes, compared to normal incidence.

Nondisturbing transverse acoustic sensor based on weak measurement in Mach–Zehnder interferometer

We proposed an optical sensing scheme, based on the mechanism of weak measurement, to detect the sonic pressure transversally without any disturbance. According to the theory of weak measurement in frequency domain, a tiny phase difference between the two eigenstates of the system could be detected, depending on the shift of central wavelength of output spectra. The optical path difference between two paths of a Mach–Zehnder interferometer is induced by the varied acoustic waves propagating in liquid. The propagating direction of the testing light beam is orthogonal to that of the acoustic wave. Thus, the phase shift corresponding to the changing acoustic pressure could be monitored transversally. The experiment with the hydrophone is implemented as a reference to demonstrate the feasibility and authenticity of this weak measurement scheme. With this method, an optical phase resolution of 1.12×10−5 rad was acquired in such a weak measurement system.

Molecular imprinting sensor based on quantum weak measurement

A new type of sensing protocol, based on a high precision metrology of quantum weak measurement, was first proposed for molecularly imprinted polymers (MIP) sensor. The feasibility, sensitivity and selectivity of weak measurement based MIP (WMMIP) sensor were experimentally demonstrated with bovine serum albumin (BSA). Weak measurement system exhibits high sensitivity to the optical phase shift corresponding to the refractive index change, which is induced by the specific capture of target protein molecules with its recognition sites. The recognition process can be finally characterized by the central wavelength shift of output spectra through weak value amplification. In our experiment, we prepared BSA@MIP with modified reversed-phase microemulsion method, and coated it on the internal surface of measuring channels assembled into the Mach-Zehnder (MZ) interferometer based optical weak measurement system. The design of this home-built optical system makes it possible to detect analyte in real time. The dynamic process of the specific adsorption and concentration response to BSA from 5×10−4 to 5×10−1μg/L was achieved with a limit of detection (LOD) of 8.01×10−12g/L. This WMMIP shows superiority in accuracy, fast response and low cost. Furthermore, real-time monitoring system can creatively promote the performance of MIP in molecular analysis.

Influences of quadratic spectral phase on characteristics of two crystal cross-polarized generation with femtosecond pulses

The rapid developments of ultra-intense and ultra-short laser offer the possibility to study laser driven ion acceleration with using solid density target. However, the prepulse and amplified spontaneous emission generated in the amplification can create preplasma at the target front by heating, melting and evaporating a portion of a solid density. The main pulse then interacts with the preplasma, which would be harmful to laser ion acceleration. Therefore, many methods have been developed to enhance the temporal contrast of high power laser system, such as saturable absorber, cross polarized wave generation (XPW) and plasma mirror. With many advantages, such as high conversion efficiency, introducing neither spatial nor spectral distortions, and easy setup compared with other mechanisms, XPW has been used to clean the femtosecond laser system. Besides that, the spectrum of the XPW pulse could be broadened by 3 times under the best condition compared with the initial spectrum. It can solve the spectrum narrowing problem during the laser amplification to obtain ultra-short femtosecond laser pulse. Here, we experimentally investigate the output power, spectrum bandwidth and center wavelength shift of the generated cross-polarized wave according to the input pulse quadratic spectral phase. The femtosecond laser pulse in compact laser plasma accelerator system at Peking University is used to investigate the role of quadratic spectral phase in characterizing the two crystal cross-polarized generation. The Ti:Sapphire-based laser system has a central wavelength of 798 nm and bandwidth of 35.5 nm which allows the pulse to be compressed down to 40 fs duration (FWHM). Typical the input pulse energy of XPW is 150 upJ and the laser system operates well at 1 kHz repetition rate. The quadratic spectral phase can be increased by changing the position of compressor grating. The conversion efficiency, spectrum bandwidth and the central wavelength shift by changing the quadratic spectral phase are measured. The conversion efficiency is 17% when quadratic spectral phase 2=0, and decreases as quadratic spectral phase increases. The rapid decrease is caused by negative quadratic spectral phase. The spectrum bandwidth is 62 nm under the optimum condition, and the broadening effect exists when quadratic spectral phase is in a range of -280 fs2 2 1400 fs2. It is slowly blue-shifted when 20 and stays at 772 nm when 21000 fs2. It starts to be red-shifted when 20 and stays at 806 nm finally. In conclusion, with the increase of quadratic spectral phase, we observe the effects of conversion efficiency and spectrum bandwidth and the shift of central wavelength. Moreover, the influences of positive and negative quadratic spectral phase on characteristics of XPW are different. Our result shows that the negative quadratic spectral phaseis more effective at reducing the conversion efficiency and spectrum bandwidth than the positive one.

Optical fiber sensing technologies based on femtosecond laser micromachining and sensitive films

Integration of novel functional material with fiber optic components is one of the new trends in the field of novel sensing technologies. The combination of fiber optics with functional materials offers great potential for realizing the novel sensors. Typically in optical fibre sensing technology, fibre itself acts as sensing element and also transmitting element, such as fiber Bragg grating (FBG), Brillouin or Raman optical time domain reflectometer. However such sensing components can only detect limited physical parameters such as temperature or strain based on the principle of characteristic wavelength drifts. While the idea of optical fiber sensing technology with functional materials is quite different from that of the traditional technology, functional materials can be employed as sensing components, therefore many parameters, including chemical or biological parameters, can be detected, depending on the designs of different sensing films. When compared with the common fiber sensing technologies such as FBG and optical time domain reflectometer, fiber optic sensors based on functional materials show advantages in the diversity of measurement parameters. However, functional materials can be realized by many techniques including e-beam evaporation, magnetron sputtering, spin-coating, electro-chemical plating, etc. The mechanical stability of tiny optical fibers is still problematic, which could be a challenge to industrial applications. In this work, a femtosecond laser fabricated fiber inline micro Mach-Zehnder interferometer with deposited palladium film for hydrogen sensing is presented. Simulation results show that the transmission spectrum of the interferometer is critically dependent on the microcavity length and the refractive index of Pd film, and a short microcavity length corresponds to a high sensitivity. The experimental results obtained in a wavelength region of 1200-1400 nm, and in a hydrogen concentration range of 0-16%, accord well with those of the simulations. The developed system has high potential in hydrogen sensing with high sensitivity. Three-dimensional multitrench microstructures, femtosecond laser ablated in fiber Bragg grating cladding, TbDyFe sputtering are proposed and demonstrated for magnetic field sensing probe. Parameters such as the number of straight microtrenches, translation speed (feed rate), and laser pulse power of laser beam have been systematically varied and optimized. A 5-m-thick giant Terfenol-D magnetostrictive film is sputtered onto FBG microtrenches, and acts as a magnetic sensing transducer. Eight microtrench samples produce the highest central wavelength shift of 120 pm, nearly fivefold more sensitive than nonmicrostructured standard FBG. An increase in laser pulse power to 20 mW generates a magnetic sensitivity of 0.58 pm/mT. Interestingly, reduction in translational speed contributes dramatically to the rise in the magnetic sensitivity of the sample. These sensor samples show magnetic response reversibility and have great potential in the magnetic field sensing domain. Furthermore hydrogen sensors based on fiber Bragg gratings micro-machined by femtosecond laser to form microgrooves and sputtered with Pd/Ag composite film are proposed and demonstrated. The atomic ratio of the two metals is controlled at Pd:Ag=3:1. At room temperature, the hydrogen sensitivity of the sensor probe micro-machined by 75 mW laser power and sputtered with 520 nm of Pd/Ag film is 16.5 pm/%H. Comparably, the standard FBG hydrogen sensitivity becomes 2.5~pm/%H for the same 4% hydrogen concentration. At an ambient temperature of 35℃, the processed sensor head has a dramatic rise in hydrogen sensitivity. Besides, the sensor shows good response and repeatability during hydrogen concentration test.

Noise-like pulse generation from a thulium-doped fiber laser using nonlinear polarization rotation with different net anomalous dispersion

A mode-locked thulium-doped fiber laser (TDFL) based on nonlinear polarization rotation (NPR) with different net anomalous dispersion is demonstrated. When the cavity dispersion is −1.425 ps2, the noise-like (NL) pulse with coherence spike width of 406 fs and pulse energy of 12.342 nJ is generated at a center wavelength of 2003.2 nm with 3 dB spectral bandwidth of 23.20 nm. In the experimental period of 400 min, the 3 dB spectral bandwidth variation, the output power fluctuation, and the central wavelength shift are less than 0.06 nm, 0.04 dB, and 0.4 nm, respectively, indicating that the NPR-based TDFL operating in the NL regime holds good long-term stability.

A design method based on photonic crystal theory for Bragg concave diffraction grating

A design method based on one-dimensional photonic crystal theory (1-D PC theory) is presented to design Bragg concave diffraction grating (Bragg-CDG) for the demultiplexer. With this design method, the reflection condition calculated by the 1-D PC theory can be matched perfectly with the diffraction condition. As a result, the shift of central wavelength of diffraction spectra can be improved, while keeping high diffraction efficiency. Performances of Bragg-CDG for TE and TM-mode are investigated, and the simulation results are consistent with the 1-D PC theory. This design method is expected to be applied to improve the accuracy and efficiency of Bragg-CDG after further research.

Bidirectional 50 Gb/s/λ WDM-PON based on optical intensity modulation and direct detection

We experimentally demonstrate an optical intensity modulation transmitter with a half-cycle 32 QAM Nyquist subcarrier modulation and direct detection receiver-based channel-reuse wavelength-division-multiplexing passive optical network. Chromatic dispersion-induced power fading in the direct detection system is avoided with fiber Bragg grating filter-based vestigial-sideband filtering. The inter-symbol interference is mitigated with a digital signal processing-based overlap frequency-domain equalizer and iterative distortion-cancellation technique. A frequency division multiplexing-based wavelength management method is employed for the tunable laser used in a colorless optical network unit to achieve a bidirectional 50 Gb/s/λ transmission over a 40 km-long standard single-mode-fiber on a 100 GHz WDM grid. The transmission performance is also measured with different central wavelength shifts between upstream and downstream in the channel-reuse system.

Method for predicting junction temperature distribution in a high-power laser diode bar.

A hybrid experimental/numerical method is proposed for predicting the junction temperature distribution in a high-power laser diode (LD) bar with multiple emitters. A commercial water-cooled LD bar with multiple emitters is used to illustrate and validate the proposed method. A unique experimental setup is developed and implemented first to measure the average junction temperatures of the LD bar emitters. After measuring the heat dissipation of the LD bar, the effective heat transfer coefficient of the cooling system is determined inversely from the numerical simulation using the measured average junction temperature and the heat dissipation. The characterized heat dissipation and effective heat transfer coefficient are used to predict the junction temperature distribution over the LD bar numerically under high operating currents. The results are presented in conjunction with the wall-plug efficiency and the center wavelength shift.

Progressive redshifts in the late-time spectra of Type Ia supernovae

We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000-6000 A in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 A feature shows a progressive central wavelength shift from ~4600 A to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature's red-ward shift slowing or halting at an [Fe III] blend centroid of 4701 A as has been proposed. The width of the feature also steadily increases with a FWHM ~170 A at day +100 growing to 200 A or more by day +350. Two weaker adjacent features around 4850 and 5000 A exhibit very similar red shifts to that of the 4700 A feature but show no change in width until very late times. We discuss possible causes for the observed red-ward shifting of these late-time optical features including contribution from [Co II] emission at early nebular epochs and the emergence of additional features at later times. We conclude that the ubiquitous red shift of these common late-time, nebular SN Ia spectral features is not mainly due to a decrease in a blueshift of forbidden Fe lines but the result, in part, of decreasing velocity and/or optical depth of permitted Fe lines.

Nitride superluminescent diodes with broadened emission spectrum fabricated using laterally patterned substrate.

We demonstrate InGaN/GaN superluminescent diodes with broadened emission spectra fabricated on surface-shaped bulk GaN (0001) substrates. The patterning changes the local vicinal angle linearly along the device waveguide, which results in an indium incorporation profile in InGaN quantum wells. The structure was investigated by microphotoluminescence mapping, showing a shift of central emission wavelength from 413 nm to 430 nm. Spectral full width at half maximum of processed superluminescent diodes is equal to 6.1 nm, while the reference chips show 3.4 nm. This approach may open the path for using nitride devices in applications requiring broad emission spectrum and high beam quality, such as optical coherence tomography.

IMDD-based bidirectional 20 Gb/s/λ WDM-PON with Nyquist 4 PAM employing Rayleigh backscattering noise detection-based self wavelength management

We experimentally demonstrate an optical intensity modulation transmitter with a Nyquist 4 level pulse amplitude modulation and direct detection (DD) receiver-based channel-reuse wavelength-division-multiplexing passive optical network (WDM-PON). A Rayleigh backscattering noise detection-based wavelength management method is employed for the tunable optical transmitter used in colorless optical network unit to achieve bidirectional 20 Gb/s/λ transmission over 45-km standard single-mode-fiber on a 100-GHz WDM grid. Chromatic dispersion induced power fading in the DD system is avoided with fiber Bragg grating filter-based vestigial-sideband filtering. The inter-symbol interference is cancelled with digital signal processing-based overlap frequency-domain equalizer. The transmission performance is also measured with different optical signal to Rayleigh backscattering noise ratio and different central wavelength shift between upstream and downstream in the channel-reuse system.