Random Bragg-grating-based wavelength-tunable random fiber laser with a full-open cavity

A full-open-cavity wavelength-tunable random fiber laser(WT-RFL) with compact structure and hundreds of picometers tuning range is proposed and demonstrated. A π fiber Bragg grating(FBG) is used in the WT-RFL as a filter to select lasing wavelengths. The two random Bragg grating arrays(RBGAs) and a section of high gain erbium-doped fiber result in a low lasing threshold and high stability. A numerical model to analyze the tunable characteristics is developed. The results show that the laser threshold is 22 m W, and the maximum peak-power fluctuation is 0.55 d B. To the best of our knowledge, it is the first time that a compact and full-open-cavity WT-RFL with two RBGAs and a π-FBG is proposed.     

A novel fiber-laser-based fiber Bragg grating strain sensor with high-birefringence Sagnac fiber loop mirror

A novel fiber-laser-based strain sensor is proposed and experimentally demonstrated. The laser cavity is composed of a high-birefringence Sagnac fiber loop mirror （HiBi-SFLM） and a fiber Bragg grating （FBG） which also acts as a strain-sensing element. In the linear region of the HiBi-SFLM reflection spectrum, when the strain applied on the FBG makes the Bragg grating wavelength shift, the laser output power changes due to refleetivity variation of the HiBi-SFLM. Experimental results show that the laser output power varies almost linearly with the applied strain. The measurement of the output power can be performed by a conventional photo-detector.     

Symmetry restoring dynamics in a two-species bosonic Josephson junction by using occasional coupling

In this paper, by employing an occasionally coupling scheme in a two-species bosonic Josephson junction, it is found that for nonlocal measure synchronized states appearing in the two dynamic modes, known as 0-phase mode and π phase mode, their broken-symmetry can be restored.Nevertheless, there are dramatic differences for the results. For 0-phase mode, we can restore the broken symmetry by turning the nonlocal MS state into a conventional quasiperiodic MS state.However, for the π-phase mode, the broken symmetry is restored accompanied by the appearance of chaotic MS states.     

An analysis of As_2S_3 chirped fiber grating formed by two-photon absorption effect

When femtosecond laser pulses interfere with chirped femtosecond laser pulses in As2S3 fiber, a chirped fiber grating is formed. An analytical expression is given to describe the chirped grating, and its Bragg reflectivity is calculated. Because of the high photosensitive effect of As2S3 material, the chirped fiber grating has a wide Bragg reflective spectrum and high reflectivity by choosing proper parameters. This indicates that the chirped fiber grating can be used as a stretcher in the femtosecond chirped pulse amplification (CPA) system.     

All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating

A few-mode erbium-doped fiber(FM-EDF) is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm,respectively. The refractive index difference is 0.98%, numerical aperture(NA) is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating(LPFG) is fabricated, which can convert LP01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum(OAM) is also generated by combining the LPFG and polarization controller(PC). Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 d B at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme,which can be used in the optical communication fields.     

A plating method for metal coating of fiber Bragg grating

We present a method for metal coating optical fiber and in-fiber Bragg grating. The technology process which is based on electroless plating and electroplating method is described in detail. The fiber is firstly coated with a thin copper or nickel plate with electroless plating method. Then, a thicker nickel plate is coated on the surface of the conductive layer. Under the optimum conditions, the surfaces of chemical plating and electroplating coatings are all smooth and compact. There is no visible defect found in the cross-section. Using this two-step metallization method, the in-fiber Bragg grating can be well protected and its thermal sensitivity can be enhanced. After the metallization process, the fiber sensor is successfully embedded in the 42CrMo steel by brazing method. Thus a smart metal structure is achieved. The embedding results show that the plating method for metallization protection of in-fiber Bragg grating is effective.     

Impact of spontaneous Raman scattering on quantum channel wavelength-multiplexed with classical channel in time domain

In the quantum key distribution system, quantum channel is always affected by spontaneous Raman scattering noise when it transmits with classical channels that act as synchronization and data channels on a shared fiber. To study the effect of the noise exactly, the temporal distribution characteristics of the Raman scattering noise are analyzed theoretically and measured by a single-photon detector. On the basis of this, a scheme to decrease the noise is proposed.     

Multi-wavelength fiber ring laser based on a chirped moiré fiber grating and a semiconductor optical amplifier

A simple and cost-effective multi-wavelength fiber ring laser based on a chirped Moiré fiber grating (CMFG) and a semiconductor optical amplifier (SOA) is proposed.Stable triple-wavelength lasing oscillations at room temperature are experimentally demonstrated.The measured optical signal-to-noise ratio (SNR) reaches the highest value of 50 dB and the power fluctuation of each wavelength is less than 0.2 dB within a 1-h period.To serve as a wavelength selective element,the CMFG possesses excellent comb-like ...     

Magneto-Optic Fiber Gratings Useful for Dynamic Dispersion Management and Tunable Comb Filtering

Intelligent control of dispersion management and tunable comb filtering in optical network appfications can be performed by using magneto-optic fiber Bragg gratings （MFBGs）. When a nonuniform magnetic field is applied to the MFBG with a constant grating period, the resulting grating response is equivalent to that of a conventional chirped grating. Under a linearly nonuniform magnetic field along the grating, a linear dispersion is achieved in the grating bandgap and the maximal dispersion slope can come to 1260ps/nm2 for a l0-mm-long fiber grating at 1550nm window. Similarly, a Gaussian-apodizing sarnpled MFBG is also useful for magnetically tunable comb filtering, with potential application to clock recovery from return-to-zero optical signals and optical carrier tracking.     

Quantitative coherence analysis of dual phase grating x-ray interferometry with source grating

Dual phase grating x-ray interferometry is compatible with common imaging detectors,and abandons the use of an absorption analyzer grating to reduce the radiation dose.When using x-ray tubes,an absorbing source grating must be introduced into the dual phase grating interferometer.In order to attain a high fringe visibility,in this work we conduct a quantitative coherence analysis of dual phase grating interferometry to find how the source grating affects the fringe visibility.Theoretical analysis shows that with the generalized Lau condition satisfied,the fringe visibility is influenced by the duty cycle of the source grating and the transmission through the grating bar.And the influence of the source grating profile on the fringe visibility is independent of the phase grating type.Numerical results illustrate that the maximum achievable fringe visibility decreases significantly with increasing transmission in the grating bar.Under a given transmission,one can always find an optimal duty cycle to maximize the fringe visibility.These results can be used as general guidelines for designing and optimizing dual phase grating x-ray interferometers for potential applications.     

Optical response of tunable terahertz plasmon in a grating-gated graphene transistor

Tunable terahertz plasmon in a graphene-based device with a grating serving as a top gate is studied. Transmission spectra exhibit a distinct peak in the terahertz region when the terahertz electric field is perpendicular to the grating fingers.Our results show that the extinction in the transmission of single-layer graphene shields beyond 80%. Electronic results further show that the graphene plasmon can be weakly adjusted by tuning the gate voltage. Theoretical calculation also implies that the plasmon frequency of graphene can fall into the terahertz region of 1–2 THz by improving the sustaining ability and capacitance of the top gate.     

Novel optical fiber current sensor based on magnetic fluid

<正>A novel fiber optic Fabry-Perot(F-P) current sensor is developed based on magnetic fluid as the medium in F-P interference cavity.A signal demodulation method based on slanted fiber Bragg grating(FBG) wavelength measurement system is proposed.Theory and principle of electromagnetic-controlled refractive index of the magnetic fluid is described,as well as the structure of the sensor system.Preliminary experiments are carried out,and the results indicate that there is a fairly good linearity of the measurement characteristic.The thickness of magnetic fluid film and initial concentration will affect the measurement results.     

Dual wavelength erbium-doped fiber laser with a lateral pressure-tuned Hi-Bi fiber Bragg grating

Tunable dual wavelength erbium-doped fiber laser (EDFL) with stable oscillation at room temperature is proposed and demonstrated. This laser utilizes a Bragg grating fabricated in a high birefringence fiber as the wavelength-selective component, and then achieves the stable dual wavelength oscillation by introducing the polarization hole burning effect. Furthermore, by applying lateral strain upon the fiber Bragg grating (FBG), the space of the laser dual wavelengths can be tuned continuously.     

Optical terminal analysis of a multigrating tiled compressor in a PW-class CPA-laser

In the highest-power chirped-pulse amplification lasers, the pulse must be stretched in time, amplified, compressed in a grating compressor and subsequently focused by off-axis parabola to obtain a high peak power. In the optical terminal, the temporal and spatial effects of mismatched multigrating tiled compressor on the far-field pulse are critical factors to be analysed. In this paper, a k-space raytracing model is proposed for the temporal and spatial analyses of possible errors in a four-grating single-pass tiled compressor. The results show that the last grating affects mainly the partial focal spot, while the middle two gratings affect the temporal waveform, and the partial focal spot needs much higher error control than that in the temporal domain in a picosecond pulse compression.     

Measuring vibration by using fiber Bragg grating and demodulating it by blazed grating

A method of measuring vibration by using fiber Bragg grating (FBG) and demodulating the spectrum by blazed grating is introduced. The sensor system is made of a simple supported beam with a FBG adhered to its upper surface. A blazed grating is used to demodulate the changing spectrum that is got from the sensor system, and a line charge-coupled device (CCD) is used to accept the diffraction spectrum. Through analyzing the number of the CCD's pixels, we can get the amplitude of vibration and the change of the temperature. The experimental results show that the vibration amplitude of the exciter matches the detected signal under the stable frequency. The temperature shift and vibration signal are also successfully separated.     

Dual-wavelength fiber grating laser in linear overlapping cavity

We report a switchable dual-wavelength fiber grating laser in linear overlapping cavity. The laser features two overlapping cavities sharing a single Yb-doped gain medium fiber and two sets of fiber Bragg gratings. A coiling fiber setup is inserted into the 1035 nm laser cavity. Given that the bending loss is inversely related to the bending radius, the cavity loss of 1035-nm can be modulated. Modulating the bending loss facilitates the switching of the fiber grating laser to a single-or dual-wavelength output at 1030 or 1035 nm and convenient tuning of the power ratio of the two wavelengths. An approximately 152.6-mW output power and up to 38-dB polarization extinction ratio are observed. The simultaneous lasing at I030 and 1035 nm is a qualified seeder source for amplification to high power scale and can be applied to difference frequency generation of a terahertz signal. This dual-wavelength fiber grating laser is a potential pump source for generating terahertz radiation using a novel approach.     

A strain-induced birefringent double-clad fiber Bragg grating

A strain-induced birefringence double-clad (DC) fiber Bragg grating (FBG) is proposed and demonstrated.The grating is fabricated in the core of rectangular inner cladding double clad fiber by using phase mask method. By applying lateral strain on the grating, the birefringence is induced. In order to detect the birefringent effect of the grating, we use it as the output mirror of a laser. When lateral strain is applied,the grating becomes birefringent. Therefore, one reflection peak of double-clad fiber Bragg grating becomes two peaks and the laser also lases in two wavelengths. The wavelength spacing of the laser can be tuned from 0 to 0.8 nm. The absolute wavelengths for the two polarizations can be tuned 1.2 and 2.0 nm,respectively.     

Sampled phase-shift fiber Bragg gratings

A phase-shift fiber Bragg grating (FBG) with sampling is proposed to generate a multi-channel bandpass filter in the background of multi-channel stopbands.The sampled moiré fiber gratings are analyzed by Fourier transform theory first,and then simulation and experiment are performed,the results show that transmission peaks are opened in every reflective channel,the spectrum shape of every channel is identical.It can be used to fabricate multi-wavelength distributed feedback (DFB) fiber laser.     

Multiplexed reflective-matched optical fiber grating interrogation technique

The multiplexed-reflective matched fiber grating interrogation technique was presented in this paper. The interrogation technique was based on the use of two (or more) wavelength-matched fiber Bragg gratings (FBGs) to receive the reflected signal from the sensing FBG. The two (or more) matched gratings were arranged parallelly and used as filters to convert wavelength into intensity encoded information for interrogation. Theoretical and experimental results demonstrated that the interrogation system is immune to the source fluctuation and cross-sensitivity effect between temperature and strain, and also can enlarge the dynamic range.     

High OSNR and simple configuration dual-wavelength fiber laser with wide tunability in S+C+L band

A simple configuration dual-wavelength fiber laser,by combining the first-order Brillouin laser and the residual pump laser,is proposed and experimentally demonstrated.A 1 km long single-mode fiber is used as the stimulated Brillouin scattering gain medium pumped by a narrow linewidth tunable laser source(TLS).Through simply adjusting the TLS output power,power-equalized dual-wavelength lasing can be achieved with a high optical signal to noise ratio(OSNR) of 80 d B.With the good tunability of the TLS,the dual-wavelength fiber laser has a tunable range of ～130 nm,and simultaneously the beat frequency of the two lasing wavelengths can be tuned from 10.1875 to 11.0815 GHz with the tunable range of 0.8940 GHz.The high stability of the dualwavelength operation is experimentally verified by the measured beat frequency fluctuation of ≤6 MHz in 1 h and power fluctuation of ≤0.03 d B in 2 h.The temporal characteristics of the fiber laser are also investigated experimentally.The fiber laser will find good applications in fiber sensing and microwave photonics areas.