Dependence of fringe spacing on the grating separation in a long-period fiber grating pair

Ultrahigh-channel-count OAM mode conversion utilizing a hybrid few-mode fiber configuration

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

Optical Fiber Interferometers Based on Arc-Induced Long Period Gratings at INESC TEC

In this work, we review the most important achievements of an INESC TEC long-period-grating-based fiber optic Michelson and Mach–Zehnder configuration modal interferometer with coherence addressing and heterodyne interrogation as a sensing structure for measuring environmental refractive index and temperature. The theory for Long Period Grating (LPG) interferometers and coherence addressing and heterodyne interrogation is presented. To increase the sensitivity to external refractive index and temperature, several LPG interferometers parameters are studied, including order of cladding mode, a reduction of the fiber diameter, different type of fiber, cavity length and the antisymmetric nature of cladding modes.

Strain-, curvature- and twist-independent temperature sensor based on a small air core hollow core fiber structure

Cross-sensitivity (crosstalk) to multiple parameters is a serious but common issue for most sensors and can significantly decrease the usefulness and detection accuracy of sensors. In this work, a high sensitivity temperature sensor based on a small air core (10 µm) hollow core fiber (SACHCF) structure is proposed. Co-excitation of both anti-resonant reflecting optical waveguide (ARROW) and Mach-Zehnder interferometer (MZI) guiding mechanisms in transmission are demonstrated. It is found that the strain sensitivity of the proposed SACHCF structure is decreased over one order of magnitude when a double phase condition (destructive condition of MZI and resonant condition of ARROW) is satisfied. In addition, due to its compact size and a symmetrical configuration, the SACHCF structure shows ultra-low sensitivity to curvature and twist. Experimentally, a high temperature sensitivity of 31.6 pm/°C, an ultra-low strain sensitivity of -0.01pm/µε, a curvature sensitivity of 18.25 pm/m^-1, and a twist sensitivity of -22.55 pm/(rad/m) were demonstrated. The corresponding temperature cross sensitivities to strain, curvature and twist are calculated to be -0.00032 °C/µε, 0.58 °C/m^-1 and 0.71 °C/(rad/m), respectively. The above cross sensitivities are one to two orders of magnitude lower than that of previously reported optical fiber temperature sensors. The proposed sensor shows a great potential to be used as a temperature sensor in practical applications where influence of multiple environmental parameters cannot be eliminated.

Sensitivity Characterization of Cascaded Long-Period Gratings Operating near the Phase-Matching Turning Point

We characterized a cascaded long-period gratings (LPGs)-based sensor that was operating at the phase-matching turning point (PMTP). The cascaded LPGs constructed an in-fiber Mach–Zehnder interferometer (MZI), which exhibited a series of high-quality-factor (Q) narrow-bandwidth resonance peaks. As the LPG operated at the PMTP, the proposed sensor showed an ultrahigh refractive index (RI) and temperature sensitivity, and high measurement precision. In this study, we took an in-depth look at the effects of grating separation on Q-factor and sensitivity. The results showed that the sensitivity to the surrounding refractive index (SRI) reached 4741.5 nm/RIU at 1.4255 and 2138 nm/RIU, over the range of 1.335–1.373. In addition, the temperature sensitivity was around 4.84 nm/°C. With a 0.02 nm wavelength resolution, the RI and temperature sensing limits were 9.3 × 10⁻⁶ RIU and 5.5 × 10⁻³ °C.

Compact and broad wavelength range tunable orbital angular momentum mode generator based on cascaded helical photonic crystal fibers

A compact and broad wavelength range tunable orbital angular momentum (OAM) generator was experimentally demonstrated by cascading two helical photonic crystal fibers (HPCFs) with opposite helicity, i.e., clockwise-twisted + anticlockwise-twisted HPCF. Such an OAM generator exhibited a length of approximately 9 mm and generated a high-quality OAM mode with a wavelength range of 35 nm. Moreover, the wavelength range is expected to be tuned from 17.9-51.3 nm by applying mechanical torsion.

Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain

Negative curvature hollow core fiber (NCHCF) is a promising candidate for sensing applications; however, research on NCHCF based fiber sensors starts only in the recent two years. In this work, an all-fiber interferometer based on an NCHCF structure is proposed for the first time. The interferometer was fabricated by simple fusion splicing of a short section of an NCHCF between two singlemode fibers (SMFs). Both simulation and experimental results show that multiple modes and modal interferences are excited within the NCHCF structure. Periodic transmission dips with high spectral extinction ratio (up to 30 dB) and wide free spectral range (FSR) are produced, which is mainly introduced by the modes coupling between HE₁₁ and HE₁₂. A small portion of light guiding by means of Anti-resonant reflecting optical waveguide (ARROW) mechanism is also observed. The transmission dips, resulting from multimode interferences (MMI) and ARROW effect have a big difference in sensitivities to strain and temperature, thus making it possible to monitor these two parameters with a single sensor head by using a characteristic matrix approach. In addition, the proposed sensor structure is experimentally proven to have a good reproducibility.

EDF ring laser using cascaded-chirped long period fiber grating for temperature measurement

A novel tunable erbium-doped fiber ring laser (EDFRL) using a cascaded-chirped long period fiber grating (C-CLPG) as a wavelength selection element is proposed. The oscillation wavelength is determined by the one of the spectral peaks of the C-CLPG used, and the oscillation output provides a high signal to noise (S/N) ratio detection and a highly sensitive measurement of the temperature due to its high power and narrow spectral output. In the experiment, it is confirmed that the wavelength of the output shifts in accordance with the temperature-induced spectral shift of the C-CLPG transmittance spectrum and the temperature sensitivity is obtained to be ∼-0.2 nm/°C within the wavelength range of 1567.30 ∼ 1575.78 nm. The oscillation wavelength range is to be limited depending on the fringe spacing of the channeled spectrum of C-CLPG, which limits the temperature measurement range, but a data processing approach to solve this problem is additionally proposed and its availability is also presented.

Design of a narrow-bandwidth refractive index sensor based on a cascaded few-mode long-period fiber grating

This work presents a design of a cascaded few-mode long-period fiber grating (FM-CLPFG) refractive index sensor with a narrow bandwidth that is based on the feature of narrow-bandwidth loss peak of few-mode long-period fiber grating (FM-LPFG) and that further reduces the loss-peak bandwidth of a single FM-LPFG by cascading. On the basis of the coupled-mode theory of FM-LPFG, the mutual interference between the loss peaks of each mode is reduced, and the loss-peak coupling intensity is ensured by selecting the appropriate grating period and grating length. Furthermore, the influence of the cascaded fiber length and the number of cascaded grating segments on the loss-peak bandwidth are analyzed. Based on the above designed parameters, the FM-CLPFG narrow-bandwidth sensor with a bandwidth of about 1 nm is designed. The results show that the sensitivity of this sensor is available to 2410 nm/RIU, with the surrounding refractive index changing from 1.4445 to 1.4475.

Ultra-high sensitivity of dual dispersion turning point taper-based Mach-Zehnder interferometer

We present here a detailed investigation into the sensitivity of the taper-based Mach-Zehnder interferometer as a function of external refractive index, with particular attention to the dispersion turning point (DTP) and possibilities for ultra-sensitive sensors. Our numerical simulation revealed that two DTPs exist with a decrease in the microfiber waist diameter; given this relationship, it is possible to obtain an ultra-sensitive operation. We then conducted experiments with fabricated devices with different waist diameters to achieve both positive and negative sensitivities at two DTPs. In particular, we achieved an ultrahigh refractive index sensitivity of approximately 95,832 nm/RIU at the second DTP when working with a diameter of 1.87 µm around the RI of air. These results show its potential for use in acoustic sensing and biochemical detection.

High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure

We have proposed a novel tapered-single mode-no core-single mode (TSNS) fiber refractometer based on multimode interference. The TSNS structure exhibits a high contrast ratio (>15 dB) and a uniform interference fringe. The influence of different lengths and diameters of the TSNS on the refractive index unit (RIU) sensitivity was investigated. The experimental investigations indicated a maximum sensitivity of 1517.28 nm/RIU for a refractive index of 1.417 and low-temperature sensitivity (<10 pm/°C). The experimental and simulation results are also in good agreement.

In-line reflective Mach-Zehnder interferometer based on a tilted in-fiber beam splitter

A fiber in-line reflective Mach-Zehnder interferometer is proposed and experimentally demonstrated, which is based on a tilted in-fiber beam splitter inscribed by a femtosecond laser. The beam splitter splits the incident light beam into two parts; one is directed to the fiber cladding-air interface, where it experiences total internal reflection and is directed to the fiber core. The other part of the light beam keeps traveling in the fiber core. The two parts of the light beams are recombined in the fiber core to generate interference before being reflected at the fiber cut end face. The device proposed is compact in size, robust in mechanical strength, easy in fabrication, and can be used for environmental refractive index sensing in a convenient manner.

Hydrogen sulfide sensor based on tapered fiber sandwiched between two molybdenum disulfide/citric acid composite membrane coated long-period fiber gratings

In this work, a novel hydrogen sulfide detection scheme based on tapered fiber seeded in two long-period fiber gratings (LPGs) coated by a molybdenum disulfide/citric acid composite membrane is proposed and fabricated. The input light of a broadband source is coupled twice by passing through two LPGs with identical parameters, from which a Mach-Zehnder interferometer can be formed. The composite sensitive membrane was prepared with molybdenum disulfide and citric acid, which was coated on the surface of the two LPGs. The experimental results show that in the range of 0-70 ppm of hydrogen sulfide, with the increase of gas concentrations the interference spectra appear to blueshift. In addition, a high sensitivity of 16.65 pm/ppm, an excellent linear relationship (R²=0.97721), and high selectivity for hydrogen sulfide are achieved. The effect of temperature is also discussed. The sensor has the advantages of low cost and small volume, and can be used for detection applications at sites where hydrogen sulfide is produced, such as natural gas plants, areas of magmatic activity, coal mines, etc.

Ultralow chirp photonic crystal fiber Mach-Zehnder interferometer

A photonic crystal fiber Mach-Zehnder interferometer design was optimized to obtain high performance and ultralow chirp. Two long-period gratings were used to excite the cladding modes, and the rich structure of the cladding was tailored to obtain a slightly chirped free spectral range, as required by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) Norm G.694.1. Finally, a fabrication tolerance analysis was performed. The advantages of the proposed device are an ultralow chirp, high bandwidth, and fabrication robustness tolerance.

Modal interferometer based on a single mechanically induced long-period fiber grating and a nanoparticles-coated film section

A modal interferometer by a single mechanically induced long-period fiber grating (MI-LPFG) using a half-length coating fiber is presented. The coating material used for this Letter is a film of silica nanoparticles doped with an organic chromophore. The silica nanoparticles, with diameters within the range of 40-50 nm, were deposited over 3.5 cm length of fiber by the dip-coating method, forming a film with a thickness between 500 and 1250 nm. Then the modal interferometer was implemented by inscribing the MI-LPFG over the coated fiber section and a similar fiber length of the uncoated fiber. The experimental results show high-contrast transmission bands, where the position and depth of the absorption envelope band are finely selected by the grating period, the pressure applied, and the film thickness. The novel modal interferometer architecture based on a single MI-LPFG, combined with a functionalized nanoparticles coating film, offers an attractive platform for the development of fiber sensors and other fiber-based devices.

In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber

An in-line optofluidic refractive index (RI) sensing platform is constructed by splicing a side-channel photonic crystal fiber (SC-PCF) with side-polished single mode fibers. A long-period grating (LPG) combined with an intermodal interference between LP₀₁ and LP₁₁ core modes is used for sensing the RI of the liquid in the side channel. The resonant dip shows a nonlinear wavelength shift with increasing RI over the measured range from 1.3330 to 1.3961. The RI response of this sensing platform for a low RI range of 1.3330-1.3780 is approximately linear, and exhibits a sensitivity of 1145 nm/RIU. Besides, the detection limit of our sensing scheme is improved by around one order of magnitude by introducing the intermodal interference.

In-fiber Mach-Zehnder interferometer for gas refractive index measurements based on a hollow-core photonic crystal fiber

We describe an in-fiber interferometer based on a gas-filled hollow-core photonic crystal fiber. Expressions for the sensitivity, figure of merit and refractive index resolution are derived, and values are experimentally measured and theoretically validated using mode field calculations. The refractive indices of nine monoatomic and molecular gases are measured with a resolution of δn_s < 10^-6.

Investigation on single taper-based all-solid photonic bandgap fiber modal interferometers

We demonstrate a single taper-based all-solid photonic bandgap (AS-PBG) fiber modal interferometer that consists of a central tapered fiber region connected to the untapered via two abrupt transitions. Modal interference is given by superimposing the bandgap-guided fundamental core mode with a lower effective index and a specific index-guided cladding supermode with a higher effective index. A series of interferometers with taper diameter of 50μm ~60μm and device length of ~3mm are fabricated and studied in contrast to the conventional counterparts. The temperature coefficient of the interferometer is closely determined by the fraction of the cladding supermode energy localized within the index-raised regions of the fiber. The refractive index (RI) responsivities associated to fiber taper sizes are investigated. The measured maximal RI sensitivity is ~3512.36nm/RIU at the taper diameter of 50μm around RI = 1.423. This research gives a deep understanding to the modal-interferometric AS-PBG structure, which we believe to be valuable for the future application of the related device.

All-fiber Mach-Zehnder interferometer based on two liquid infiltrations in a photonic crystal fiber

We propose a novel all-fiber Mach-Zehnder interferometer (MZI) fabricated by infiltrating two separated liquid sections along a PCF. Due to the reduced effective index difference between the core region and the liquid-filled cladding region, the guided field in the liquid sections possesses a larger mode field area and can simultaneously induce the core mode and cladding modes of the empty PCF to form a MZI. The measured results demonstrate that very clear interference spectra can be obtained. By increasing the length of the MZIs, the decreased average fringe spacing can be observed. We have also measured the temperature sensitivity of our proposed PCF-based MZIs. Due to the existence of the two liquid sections, the temperature sensitivities can be enlarged to -0.176nm/°C and -0.53dB/°C.

A thermally annealed Mach-Zehnder interferometer for high temperature measurement

An in-fiber Mach-Zehnder interferometer (MZI) for high temperature measurement is proposed and experimentally demonstrated. The device is constructed of a piece of thin-core fiber (TCF) sandwiched between two short sections of multimode fiber (MMF), i.e., a MMF-TCF-MMF structure. A well-defined interference spectrum is obtained owing to the core-mismatch, and the interference dips are sensitive to the ambient temperature. The experimental results show that the proposed interferometer is capable of high temperature measurement up to 875 °C with a sensitivity of 92 pm/°C over repeated measurements. The explored wavelength drop point may limit the measurement range, which can be improved by repeated thermal annealing.

Bending effect on modal interference in a fiber taper and sensitivity enhancement for refractive index measurement

We demonstrate the bending effect of microfiber on interference fringes in a compact taper-based modal interferometer and sensitivity for refractive index (RI) measurement. For the bend curvature ranging from 0 to 0.283 mm(-1), the measured RI sensitivity distinctively increases from 342.5 nm/RIU (refractive-index unit) to 1192.7 nm/RIU around RI = 1.333 and from 3847.1 nm/RIU to 11006.0 nm/RIU around RI = 1.430, respectively. Theoretical analysis reveals that such enhancement is determined by the dispersion property of the intermodal index rather than other parameters, such as the variation of the straightforward evanescent field. The magnitude of sensitivity varies as a function of the microfiber bend curvature. Approaching a critical curvature (the intermodal-index dispersion factor approaches zero), the sensitivity is significantly enhanced, exhibiting great potential in RI sensing areas.

Microfiber Mach-Zehnder interferometer based on long period grating for sensing applications

A Mach-Zehnder interferometer (MZI) composed by a pair of long period gratings (LPGs) fabricated in silica microfiber for sensing applications is demonstrated. Each LPG is fabricated with a pulsed CO2 laser by creating six periodical deformations along fiber length with only one scanning cycle. The length of the MZI can reach as short as 8.84 mm when the diameter of the microfiber is 9.5 μm. Compared with the ones fabricated in single-mode fibers, the present MZI is much shorter owing to the large effective-index difference between the fundamental and higher order modes. The microfiber MZI exhibits a sensitivity to surrounding refractive index (RI) of 2225 nm per refractive index unit and the temperature sensitivity of only 11.7 pm/°C. Theoretical analysis suggests that the performances of the MZI sensor can be improved by using thinner microfibers with a diameter down to 3.5 μm: The sensitivity can be greatly enhanced due to the stronger evanescent-field interaction and reduced dispersion factor; the transmission dips become narrower which benefits high-resolution measurement; the thinner fiber also allows further reduction in device length. The present device has great potential in biochemical and medical sensing due to the advantages including easy fabrication, excellent compactness and high sensitivity.

Spectrum evolution of cascaded mismatching long-period fiber gratings

Two identical long-period fiber gratings (LPFGs) are difficult to produce in practice, notably for CO(2)-laser or arc-discharge induced LPFGs. So it is meaningful to study the spectrum evolution of cascaded LPFGs with some deviation, named "mismatching" LPFGs. Theory and experiment demonstrate that the upper envelope of the fringe pattern is intrinsically curved but less dramatically than the lower envelope. Bending the grating pair to introduce proper cladding-mode loss can improve the contrast of the fringe pattern at a desired wavelength, which indicates that cascaded mismatching LPFGs can be used as high-quality comb filters or fiber sensors.

Highly sensitive refractive index sensor based on two cascaded special long-period fiber gratings with rotary refractive index modulation

We present a refractive index (RI) sensor based on a fiber Mach-Zehnder interferometer (MZI) formed by two cascaded special long-period fiber gratings (LPFGs) with rotary refractive index modulation (RLPFGs), in which the coupling occurred between the guided mode and the high-order asymmetric cladding mode. The experimental results show that the RI sensitivity of a refractometer with an interaction length of 40 mm is up to 58.8 nm/RI in the range of 1.3344 to 1.3637, which is 3.5 times higher than that of an MZI formed by two normal LPFGs. The temperature sensitivity for the same parameters of an RLPFG-MZI is about 0.03 nm/°C. Such a kind of high-sensitivity, easy-to-fabricate and simple-structure interferometer may find applications in the chemical or biochemical sensing fields.

Long period grating assistant photonic crystal fiber modal interferometer

A novel in-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode all-solid photonic bandgap fiber (AS-PBGF) is presented. After inserting a small piece of the AS-PBGF into two sections of standard single-mode fiber (SMF) via being spliced slight core offset, LPG is inscribed in the AS-PBGF. The LPG is especially designed to realize the coupling between two core modes of LP01 and LP11 in the AS-PBGF. Two core modes LP01 and LP11 of the AS-PBGF are excited firstly at the input spliced point and actualized energy exchange when they pass through the LPG. Then the two beams will interfere at the output spliced point to form a high-contrast in-fiber modal interferometer. The proposed interferometer has some advantages such as configuration compact, high interference contrast and the wavelength spacing well controlled by changing the position of the LPG without changing the total length of AS-PBGF.

In-fiber modal Mach-Zehnder interferometer based on the locally post-processed core of a photonic crystal fiber

We demonstrate a novel, compact and low-loss photonic crystal fiber modal Mach-Zehnder interferometer with potential applications to sensing and WDM telecommunications. By selectively collapsing a ~1-mm-long section of a hole next to the solid core, a pair of modes of the post-processed structure are excited and interfere at its exit. A modulation depth of up to ~13 dB and an insertion loss as low as 2.8 dB were achieved. A temperature sensitivity of -53.4 pm/°C was measured, making the device suitable for temperature sensing.

Photonic bandgap fiber tapers and in-fiber interferometric sensors

Nonadiabatic tapers in hollow-core air-silica photonic bandgap fibers (PBFs) were fabricated by the use of a fiber fusion splicer. In addition to the well-known scaling down of fiber dimensions, the innermost rings of air holes were found collapsed or significantly deformed, which results in almost doubling the diameter of the hollow core in the tapered region. The tapering of the PBF causes coupling of the fundamental core mode to a surface mode with approximately 10% higher effective mode index. An in-fiber core-surface mode interferometer was constructed by cascading two such tapers along the PBF. The interferometer was experimentally demonstrated for strain and temperature measurement, and the sensitivities of the interferometric peak wavelength to strain and temperature are measured to be -0.64 pm/microepsilon and 495.6 pm/( degrees C m), respectively.

All-optical switching application of germano-silicate optical fiber incorporated with Ag nanocrystals

Ag-nanocrystal-incorporated germano-silicate optical fiber with high resonant nonlinearity was fabricated by using modified chemical vapor deposition and solution doping techniques. An all-optical signal switching application based on cross-phase modulation was demonstrated in the cascaded long-period fiber gratings. Pumped with 499 nm argon laser, all-optical signal gating of the pi-phase shift was achieved with low pump intensities of less than 7.64 GW/cm2.

Wavelength-spacing-tunable multichannel filter incorporating a sampled chirped fiber Bragg grating based on a symmetrical chirp-tuning technique without center wavelength shift

We propose and experimentally demonstrate a simple and flexible scheme for a wavelength-spacing-tunable multichannel filter exploiting a sampled chirped fiber Bragg grating based on a symmetrical modification of the chirp ratio. Symmetrical bending along a sampled chirped fiber Bragg grating attached to a flexible cantilever beam induces a variation of the chirp ratio and a reflection chirp bandwidth of the grating without a center wavelength shift. Accordingly, the wavelength spacing of a sampled chirped fiber Bragg grating is continuously controlled by the reflection chirp bandwidth variation of the grating corresponding to the bending direction, which allows for realization of an effective wavelength-spacing-tunable multichannel filter. Based on the proposed technique, we achieve the continuous tunability of the wavelength spacing in a range from 1.51 to 6.11 nm, depending on the bending direction of the cantilever beam.

Reconstruction of long-period fiber gratings from their core-to-core transmission function

In order to reconstruct the structure of a long-period grating, both the complex core-to-core transmission function and the complex core-to-cladding transmission function should be known. However, in practice, only the core-to-core transmission function of the grating can be measured. We demonstrate theoretically the reconstruction of long-period gratings from only the core-to-core transmission function. The reconstruction is performed by extracting the complex core-to-cladding transmission function of the grating from its core-to-core transmission function. Generally, the extraction is not unique; however, we show that by writing an additional grating in cascade to the interrogated grating, a unique reconstruction can be obtained. In weak long-period gratings, only the amplitude of the core-to-core transmission function is needed to reconstruct the grating. The results of our work can enable the experimental reconstruction of long-period gratings from their transmission function as well as the development of novel distributed sensors.

All-optical 2 x 2 switching with two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair

We have demonstrated a new type of all-optical 2 x 2 switch by using two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair and a 3-dB fiber coupler. A 400-Hz square-wave pulse train at approximately 1549.4 nm was fully switched between the two output ports up to 200 Hz by modulated pump signals at 976 nm with a maximum pump power of approximately 35 mW, where the extinction ratio at approximately 1549.4 nm between the on and the off states was approximately 17.5 dB.

Synthesis of fiber Bragg grating parameters from experimental reflectivity: a simplex approach and its application to the determination of temperature-dependent properties

A simple, accurate, and fast method to synthesize the physical parameters of a fiber Bragg grating numerically from its reflectivity is proposed and demonstrated. Our program uses the transfer matrix method and is based on a Nelder-Mead simplex optimization algorithm. It can be applied to both uniform and nonuniform (apodized and chirped) fiber Bragg gratings. The method is then used to synthesize a uniform Bragg grating from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.

Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings

We demonstrate implementation of an all-fiber Mach-Zehnder interferometer formed in a photonic crystal fiber (PCF). We formed the all-PCF Mach-Zehnder interferometer by mechanically inducing two identical long-period fiber gratings (LPGs) in the PCF. The spectral properties of a LPG and a LPG pair were investigated. The interference fringe formed within the stop band of the LPG pair varied with the period and the strength of the gratings, and the fringe spacing was decreased with increasing grating separation. From the fringe spacing measurement the differential effective group index of the PCF was calculated to be deltam approximately equal to 2.8 x 10(-3).

Measurement of refractive-index variation with temperature by use of long-period fiber gratings

The thermo-optic coefficient of the core material of a fiber is analyzed by use of a pair of long-period fiber gratings. First the effective index difference between the core and the cladding modes is measured from the peaks of the interference fringe generated by the grating pair. The order of the cladding mode is decided by the cutoff wavelength and the numerical aperture of the fiber. The material index of the fiber core is obtained in terms of wavelength. At each wavelength the index is chosen to minimize the difference between the measured and the calculated spectra of the grating pair. Finally the thermo-optic coefficient of the fiber core is calculated by repetition of the measurement at different temperatures. With a germanosilicate-core fiber and a boron codoped germanosilicate-core fiber, the thermo-optic coefficients were 1.1x10(-5)/( degrees )C and 0.75x10(-5)/( degrees )C, respectively.

Cladding-mode-assisted recouplings in concatenated long-period and fiber Bragg gratings

We investigate new types of mode recouplings in a concatenated grating structure comprising a long-period grating and a fiber Bragg grating. It is demonstrated that the light coupled out to the cladding mode by one of these gratings can be recoupled back to the guided mode by the other grating. Theoretical analysis based on the coupled-mode theory is presented, together with experimental results.

Effect of CO2 laser irradiation on the refractive-index change in optical fibers

The effect of CO2 laser irradiation on the refractive-index change in optical fibers is investigated by measuring the interference fringe shift formed by a long-period fiber grating pair. The refractive-index decrease on CO2 laser irradiation was due to relaxation of the residual stress, which was formed in optical fibers during the drawing process, and the refractive-index decrease was found to increase linearly with the drawing force. The effect of the CO2 laser output power on residual-stress relaxation, and fiber elongation was also studied.

Resonant optical nonlinearity measurement of Yb(3+) / Al(3+) codoped optical fibers by use of a long-period fiber grating pair

A new method of measuring optical nonlinearity for resonant nonlinear optical fibers is proposed. A long-period fiber grating (LPG) pair was used to measure the nonlinear refractive index n(2) of a Yb(3+)/Al (3+) codoped optical fiber, which was spliced between the two LPGs, as the fiber was pumped with a laser diode. The nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm depended on the pump power. As the pump power increased, the nonlinear refractive index decreased. At launched pump powers of 2-8 mW, the nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm was ~7.5x10(-15)m (2)/W .

Design of binary long-period fiber grating filters by the inverse-scattering method with genetic algorithm optimization

An approach is presented to the design of binary long-period fiber grating (LPFG) filters based on the Gel'fand-Levitan-Marchenko (GLM) inverse-scattering method and genetic algorithm optimization. The nonuniform coupling strength of the binary grating can be realized by varying the local duty ratio. A coupled-mode theory combined with the Poisson sum formula for treating the binary index perturbation is developed for the application of the GLM synthesis method. Since the coupled-mode theory, which smears out the discrete coupling nature, can be regarded only as an approximation to the modeling of a binary LPFG, we use instead the transfer-matrix model to analyze the coupling behavior of a nonuniform binary LPFG. Based on the synthesized grating patterns from the GLM method, a real-coded genetic algorithm with the transfer-matrix model is used to compensate for the discrepancies resulting from use of the coupled-mode theory and to optimize the design. We exemplify the above procedure by designing a flatband LPFG filter and a high-visibility all-fiber Mach-Zehnder filter.