Tunable dual-wavelength fiber laser based on a single fiber Bragg grating in a Sagnac loop interferometer

We propose a simple dual-wavelength Er-doped fiber laser configuration based on a dual-wavelength fiber Bragg grating written on the splice joint between two different fibers for wavelength-selective filter in the Sagnac loop interferometer. The wavelength separation between the adjacent lasing wavelengths is 1.12 nm and the side-mode suppression ratio (SMSR) is over 55 dB. The output power variation is less than 0.8 dB over a two-minute period. Moreover, the lasing wavelength can be effectively tuned using the thermal heating method.     

Temperature sensitivity of long period fiber grating in SMF-28 fiber

In this paper we have presented long period fiber grating (LPFG) as temperature sensor. Temperature based sensors have found a number of applications in commercial and industrial fields. In LPFG based temperature sensors, they respond to shift in various peak resonant wavelengths corresponding to various attenuation bands of the transmission spectrum. Temperature effect on the various attenuation bands of a LPFG have been investigated to create a highly sensitive measurement device. The temperature sensitivities of various attenuation bands of a LPFG over the wavelength region of 1.1–1.7 μm, for a grating period of 280 μm period, are obtained by monitoring the wavelength shift of each peak resonant wavelength with temperature increment of 20 °C, ranging from 0 °C to 100 °C.     

Study and investigation of long period grating as refractive index sensor

In this paper, we have investigated the response of long period grating (LPG) as refractive index sensor. The response has been studied for refractive index variation ranging from 1 to 1.45. In this work, we found that the sensing mechanism is based on two different aspects. First is the change of coupling power from the guided core mode to other co-propagating cladding modes and second is the wavelength shift of the peak resonant wavelengths from their original positions due the change of refractive index of the environmental (external) medium surrounding the cladding of the grating.     

An interferometric structure with a dual-resonance long period grating for strain sensing

Spectral characteristics and amplitude tunability of a long period grating with a dual- resonance inside fiber loop mirror are studied in terms of applied stress caused by elongation. Inserting the polarization controller between grating and part of polarization maintaining fiber in the loop structure enables tuning of resonance and interferometric peaks. The maximum sensitivity of demonstrated sensor is of 1.943?dB/mε for the range of 1.1–4.4 mε. Combination of these two optical components allows to measure strain in a wider range comparing with sensors based on standard long period grating.     

Comparison of single and double cladding long period fiber grating sensor using an intensity modulation interrogation system

The performance of long period fiber grating (LPFG) sensors written in single cladding and double cladding fibers have been compared by using a fast responding interrogation system based on intensity modulation. Temperature and dynamic strain monitoring using this system have been demonstrated. This system is capable of resolving strain to 0.2 and 0.4 με at a loading frequency of 20 Hz, and temperature resolution to 0.02 and 0.19 °C by using LPFG in the single cladding (SC-LPFG) and double cladding (DC-LPFG), respectively.     

Novel fiber design for broadband long period gratings

We report a novel fiber design for obtaining long period gratings (LPGs) having a broad transmission spectrum. These gratings are based on guided to guided mode coupling in a staircase profile fiber. Numerical simulations predict 20 dB bandwidths as high as 100 nm and an estimated insertion loss of less than 1 dB. Such broadband LPGs are expected to find applications in many all fiber components.     

Analysis of period and visibility of dual phase grating interferometer

Dual phase grating interferometer may simultaneously achieve large field of view and high x-ray dose efficiency. Here, we develop a simple theoretical method to better understand the imaging process of the dual phase grating interferometer. The derivation process of fringe period and the optimal visibility conditions of the dual phase grating interferometer are given in detail. Then, we theoretically prove that the fringe period and optimal visibility conditions of the dual phase grating interferometer include that of the Talbot interferometer. By comparing our experimental results with those of other researchers, we find that when the positions of phase gratings are far away from the positions where the fringe visibility is optimal, the fringe period of the dual π -phase grating interferometer is twice the theoretical results under the illumination of polychromatic x-ray. This conclusion may explain the contradictory research results of dual phase grating interferometer among different researchers.     

Single-sideband modulated radio-over-fiber system based on long period fiber grating

We present a prototype for optical single-sideband (SSB) modulated radio-over-fiber (RoF) system by employing a long period fiber grating (LPFG). A LPFG with 13.78 nm base width of transmission spectrum and 0?23.2 dB of transmission depth was designed by using commercial software. Then it is used in RoF SSB modulation scheme. In the scheme, a Mach?Zehnder modulator modulates the light wave with millimeter-wave driving signals to realize optical double-sideband (ODSB) modulation, the generated ODSB modulation signals pass through a LPFG. Due to the negative slope in transmission spectrum, the lower sideband experiences higher attenuation than the upper sideband. Thus the conversion from ODSB to optical single sideband with carrier (OSSB + C) can be easily achieved by using only one LPFG. Also, the carrier to sideband ratio (CSR) can be reduced by using a LPFG, results show the CSR can be decreased from 12.49 dB to 1.1 dB.     

A quantitative analysis of an athermal design for a long period grating based tunable filter

In this paper, a quantitative analysis of an athermal design for a unique all-fiber tunable filter based on the combination of a single resonant band long period grating (LPG) with an electro-optic polymer second cladding layer is presented. The single resonant band LPG is used to select the resonant wavelength, which can be tuned by changing the refractive index of an electro-optic polymer second cladding layer via external electric field. Although the basic operational principle and implementation of this unique tunable filter have been previously reported, this paper is focused on the quantitative analysis of the athermal design, which may significantly enhance the practicability of the proposed tunable filter.     

Simultaneous strain and temperature measurement based on a photonic crystal fiber modal-interference interacting with a long period fiber grating

An alternative all-fiber sensor for simultaneous strain and temperature measurement based on a photonic crystal fiber (PCF) spliced between single-mode fibers cascaded with a long period grating (LPG) is proposed. By collapsing the air holes at two splicing regions along the PCF, a simple but effective modal-interference (MI) is occurred between the core and cladding modes of the PCF. Due to the different responses on the changes of strain and temperature on the MI and the cascaded LPG, the strain and temperature can be measured simultaneously. Experimental results show that the sensing resolution of 9.1 με in strain measurement is experimentally achieved over a range of 2640 με, while the temperature sensing resolution is 0.27 °C within a range of 30-100 °C.     

Wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating

A novel wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating (FBG) and erbium–ytterbium co-doped fibers is proposed and demonstrated for the first time. The strain-induced chirped FBG, which acts as a partial-reflecting mirror, is achieved by bending a uniform FBG bonded at a slant onto the lateral surface of a simply supported beam. A single wavelength lasing with a maximum wavelength tuning range of 17 nm has been achieved experimentally.     

Radio-frequency interrogation of a fiber Bragg grating sensor in the configuration of a fiber laser with external cavities

A fiber laser sensor, which consists of two coupled cavities based on three fiber Bragg gratings (two of them acting as sensing elements) and is interrogated via the longitudinal mode beating frequency, is presented. The two resonant cavities have lengths of 4250 m and 4297 m, respectively. Their beating frequency is of the order of 24 kHz, and its shift as a function of the variation of the period of one (or both) of the sensing gratings, induced by strain or temperature changes, can be measured by a radio-frequency analyzer. The system is suitable for long-distance sensing with high spatial resolution and high sensitivity.     

Fiber Bragg grating and its application in external cavity semiconductor laser

The FBG-ECL fabricated using the interactions of laser diodes with fiber Bragg gratings has important role in optical communications. In this paper the comparison analysis of fiber Bragg grating and it's application in FBG-ECL are described in detail. The important parameters such as the bandwidth, SMSR and optimal output power of the FBG-ECL were analyzed deeply in the paper. The influence of the two important parameters (the residual reflectivity and external cavity length) on the FBG-ECL characteristic was given.     

长周期光纤光栅耦合常数的研究

长周期光纤光栅表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长时的模式耦合。由纤芯基模、一阶包层模式的场分布推得其功率分布 ,进一步得到两种模式的归一化常量。推导了纤芯基模与一阶各次包层模式 (HE1l/EH1l)的耦合常数。结果表明 ,纤芯基模与HE1l包层模式产生的耦合常数远大于与EH1l包层模式的耦合常数。次数低时 ,耦合常数随包层模式次数的升高而增大     

Study on a low loss, low voltage, polarization-independent EO modulator based LPG

In this paper, a new type of high-speed EO modulator is presented. It is a multi-cladding structure with LPG written in the core, and polymer with high EO coefficient is fabricated as the outer layer. A special material with ultra-negative temperature coefficient is used for the temperature compensation. This modulator is polarization independent, it can be coupled with the optical fiber very easily and can work steadily with low voltage, high speed, is temperature insensitive, and its cost is low. To our knowledge, this is the first time a modulator has been designed with such a simple and effective structure.     

Numerical analysis of plasmon polarition refractive index fiber sensors with hollow core and a long period grating

The main principle of this design is based on the efficient energy transfer between the waveguide mode (WM) and the co-directional SPP provided by a properly designed fiber long period grating (LPG). This LPG is imprinted into a waveguide fiber layer of a specially designed hollow core optical fiber. The simulations are based on the finite element method (FEM) algorithm in electromagnetics and coupled mode theory for gratings. Compared to the previous proposed structure using a fiber Bragg grating (FBG), this novel kind of sensor can greatly enhance the refractive index sensitivity, e.g., from 5.93 nm/RIU (with FBG) to 817 nm/RIU (with LPG) at the sensing refractive index of 1.40. The other advantage is that the working conditions can be performed for the well-developed telecom wavelength windows 1500-1600 nm.     

基于长周期光纤光栅的新型地震检波器

基于长周期光纤光栅能透射特定波长的性质,以及其无后向反射的优点,提出一种新型的长周期光纤光栅地震检波器系统。以长周期光纤光栅相位匹配条件为基础,逐步推导出长周期光纤光栅地震检波器的灵敏度公式。建立长周期光纤光栅地震检波器的力学模型,并推导出检波器系统的传递函数。通过实例仿真得到该长周期光纤光栅地震检波系统的幅频特性图,从而证明了长周期光纤光栅用于地震检波器的可行性。通过调整光纤长度、质量块质量、阻尼系数等参数而得到适合的频率特性和灵敏度,为将来提高该系统的性能指明了研究方向。     

Detection and analysis of paraffin oil adulteration in coconut oil using fiber optic long period grating sensor

A fiber optic sensing system for the detection of adulteration of coconut oil by paraffin oil is experimentally demonstrated. The sensing mechanism is based on the sensitive dependence of the resonance peaks of a long period grating (LPG) on the changes of the refractive index of the environmental medium surrounding the cladding surface of the grating. The wavelength shift of the attenuation bands of the LPG was measured with the sensor immersed in a mixture of paraffin oil and pure coconut oil in different proportions. Detection limit of adulteration was found to be 3% for coconut oil-paraffin oil binary mixture.     

A laser feedback interferometer with an oscillating feedback mirror

A method is proposed to solve the problem of direction discrimination for laser feedback interferometers.By vibrating the feedback mirror with a small-amplitude and high-frequency sine wave,laser intensity is modulated accordingly.The modulation amplitude can be extracted using a phase sensitive detector(PSD).When the feedback mirror moves,the PSD output shows a quasi-sine waveform similar to a laser intensity interference fringe but with a phase difference of approximately ±π/2.If the movement direction of the feedback mirror changes,the phase difference sign reverses.Therefore,the laser feedback interferometer offers a potential application in displacement measurement with a resolution of 1/8 wavelength and in-time direction discrimination.Without using optical components such as polarization beam splitters and wave plates,the interferometer is very simple,easy to align,and less costly.     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.