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High polarization extinction ratio (PER) in-fiber polarizers are essential in various fiber-based technologies. In this letter, we propose and demonstrate an in-fiber dispersion compensated polarizer (DCP) based on integrated fiber gratings, which is fabricated by a UV-inscribed 45° tilted fiber grating (TFG) and a chirped fiber Bragg grating (CFBG) in series. Such compact device could enhance the PER of 45° TFG with nearly double value and it is potential to achieve ultra-high PER with relatively short length and simple fabrication. Meanwhile, DCP could also function as a dispersion management component. Finally, with the integrated grating device, we have demonstrated an all fiber mode-locked laser system, generating soliton pulses at the normal and anomalous dispersion regime, respectively.

A multi-switchable generation between dissipative solitons (DSs) and noise-like pulses (NLPs) in a fiber laser is demonstrated. A 45° tilted fiber grating is applied as an effective in-line all-fiber polarizer to realize the nonlinear polarization rotation (NPR) mechanism. With the fixed orientations of polarization controllers, the operation regimes of the fiber laser can be alternatively switched between DS and NLP by monotonically increasing pump power only. We observed five switching stages between DS/NLP regimes with the pump power elevating from 200 to 660 mW. Remarkably, the whole procedure could be precisely controlled forwardly and backwardly. This multi-shuttle behavior between the two distinct types of output pulse can be attributed to the transmission of NPR mechanism together with the gain filtering effect. To the best of our knowledge, this is the first report on the multi-shuttle behavior between DS and NLP in a fiber laser under monotonic change of pump power. Our findings may contribute to in-depth understanding of nonlinear dynamics and laser physics.

A simple all-fiber Solc filter (AFSF), which consists of two 45°-tilted fiber gratings (45°-TFGs) UV-inscribed in the polarization-maintaining fiber (PMF) and a series of PMF cavities, is proposed and demonstrated. The performance of the proposed filter has been theoretically simulated and experimentally verified. Both the simulated and experimental results show that the bandwidth of the filter could be tuned by the PMF sub-cavity length and the number of PMF cavities. And the free spectral range (FSR) only depends on the sub-cavity length. As a proof of that, the bandwidths of AFSF with different number of PMF sub-cavity (N=2, N=3, N=4) and the same PMF sub-cavity length of 30cm are 4 nm, 2.6nm, and 2nm, respectively. The FSRs of 3-stage AFSF with different PMF sub-cavity length (L=20 cm and L=40 cm) are 15.3 nm and 7.97 nm, respectively. Furthermore, we have also investigated the tunability of the AFSF by controlling the temperature of PMF cavity with a tuning sensitivity around 1.205 nm°/c. Compared with existing fiber-optic Solc filters, the AFSF with prominent advantages such as extremely simple and robust structure, thermal tunability in wavelength, and low cost will bring a bright future for applications in optical communication and sensing systems.

We achieve Fano-like resonances in an all-in-fiber structure embedded with an in-line Mach-Zehnder interferometer (MZI). A fiber Bragg grating is inserted into MZI's one arm to form a resonance, which functions as the discrete state of the Fano-like resonance to couple with the continuum propagating mode of MZI in the fiber core. A theoretical model predicts the controllable resonance lineshape by changing the phase difference between the MZI's two interference pathways. Fano-like resonances with an extinction ratio over 20 dB are experimentally observed, which are reliably tuned into Lorentzian and electromagnetically induced transparency-like resonances by versatile methods. The realization of Fano-like resonances with broad tunability in this all-in-fiber structure holds potentials in fiber-based applications of sensing, signal processing and nonlinear optics.

Phase interrogation surface plasmon resonance (SPR) imaging is, in principle, suitable in multiple samples and high-throughput detection, but the refractive index difference of various samples can be largely varied, while the dynamic range of phase interrogation SPR is narrow. So it is difficult to perform multi-sample detection in phase interrogation mode. In this paper, we successfully designed a multi-channel phase interrogation detection SPR imaging sensing scheme based on a common optical interference path between p- and s-polarized light without using any mechanical moving components. The fixed optical path difference between p- and s-polarized light is introduced by a birefringence crystal to produce sinusoidal spectral interference fringes. We adopted a time-division-multiplexing peak-finding algorithm to track the resonance wavelength so that the detection range can cover every channel. The phase values which carry the high sensitivity signal of the corresponding samples are calculated by the iterative parameter scanning cross-correlation algorithm.

We propose and demonstrate a sensitive vector twist sensor based on a small period long period fiber grating (SP-LPFG) fabricated with a femtosecond (fs) laser. The fabricated SP-LPFG is compact in size (2.8 mm) and shows strong polarization dependent peaks in its transmission spectrum due to the vectorial behavior of high-order cladding modes. Twist sensing is realized by monitoring the polarization dependent peaks, since the polarization of input light changes with fiber twist. The proposed sensor can be interrogated by the peak intensity and wavelength, with high twist sensitivity that reaches 0.257 dB/deg and 0.115 nm/deg, respectively.

Small period long period fiber grating is fabricated by femtosecond laser line by line technique. High order Bragg resonances, which are useful for dual-/multi- parameter sensing, are significantly enhanced (from ~0.6 dB to ~11 dB).

We present a passively mode-locked Erbium-doped fiber laser with tunable parameters including central wavelength, 3-dB bandwidth, and pulse duration. The mode-locking mechanism of the laser is realized by using single-walled carbon nanotubes (SWCNTs) polyvinyl alcohol composite film as a saturable absorber. The tunable operation is implemented via a fiber birefringence filter consisting of a polarization maintaining (PM) fiber and a Brewster fiber grating. The laser achieves a maximum spectral tuning range of 36 nm with 8-cm PM fiber. The maximum spectral width variation of 5.19 nm is acquired when the PM fiber is 12 cm. Simultaneously, the spectral widths of pulses at different central wavelengths are also adjustable. Furthermore, the total cavity length is 8.28 m, which is the shortest cavity length to obtain such wide tuning range in an Erbium-doped fiber laser based on SWCNTs.

Via using an L-band optimized in-fiber polarizing grating device, a GHz L-band femtosecond passively harmonic mode-locked Er-doped fiber laser based on nonlinear polarization rotation is first demonstrated. In total, 4.22 GHz pulses with the duration of 810 fs and super-mode suppression ratio (SMSR) of 32 dB are obtained under the pump power of 712 mW corresponding to 215th harmonic order. The central wavelength of 4.22 GHz pulses is 1581.7 nm with 10.1 nm 3 dB bandwidth. Furthermore, under this fixed pump power, higher harmonic orders can also be attained by rotating the polarization controllers properly. The highest repetition rate we obtained is 7.41 GHz with the SMSR of 20.7 dB.

Recently, the integration of two-dimensional materials with optical fibers has opened up a great opportunity to develop all-fiber signal-processing devices. Graphene is an ideal material for all-optical signal processing via thermal-optic effect because of its high electrical and thermal conductivity, as well as broadband light-matter interactions with fast responses. Herein, we report the achievement of all-optical switching with fast response by integrating few-layer graphene onto a tilted fiber Bragg grating (TFBG) inscribed in a reduced-diameter fiber. Relying on graphene's decent photothermal effect, the transmission spectrum of the TFBG could be all-optically modulated by tuning the incident pump power. The all-optical switch can consequently operate at a series of wavelengths owing to the TFBG's comb-like resonances. The reduced diameter of the graphene-integrated TFBG and the pump at its resonant wavelength promise the all-optical switch to have a fast-dynamic response of around 1 μs and an extinction ratio exceeding 13 dB. This compact device with graphene integration has the potentials to be integrated into all-fiber system to extend the functions of all-optical signal processing.

In this paper, we report a highly effective relative humidity (RH) sensor implemented on graphene oxide (GO) coated long period grating (LPG). The GO nanocolloides bonded onto a cylindrical fibre cladding enables the LPG with strong evanescent waves to absorb more water molecules increasing its RH sensitivity. In an LPG, the phase matching condition occurs when a forward propagating core mode is coupled with the co-propagating lower order cladding modes generating evanescent waves to interact with the surrounding medium. This unique effect of LPGs can be more enhanced with multilayer GO deposition. There is an expansion of GO film with the absorption of more water molecules as RH increases. The absorption of water molecules on GO coating increases the conducting carrier (holes) density on it, thus decreasing the refractive index of GO film. The combined effect of increasing evanescent waves and modulated refractive index makes the GO coated LPGs as effective RH sensors. Our recently achieved results have shown the RH sensitivity of the GO coated LPG is about 0.01 dB/%RH. We have also investigated the effect on GO layer thickness, showing thicker layer increases the RH response of the LPG cladding mode resonances in lower wavelength region.

A phase surface plasmon resonance (SPR) sensing technology based on white light polarized interference in common-path geometry is reported. A halogen lamp is used as the excitation source of the SPR sensor. The fixed optical path difference (OPD) between p- and s-polarized light is introduced by a birefringence crystal to produce sinusoidal spectral interference fringes. The SPR phase is accurately extracted from the interference fringes using a novel iterative parameter-scanning cross-correlation algorithm. The dynamic detection range is expanded by tracking the best SPR wavelength, which is identified using a window Fourier algorithm. The experimental results show that the sensitivity of this SPR system was 1.3 × 10−7 RIU, and the dynamic detection range was 0.029 RIU. This sensor, not only simple to implement and cost efficient, requires no modulators.

We have proposed a compact and distributed near infrared detecting system based on 45 ºtilted fiber gratings. The system has been applied to measure the moisture level of flour sample, which have shown linear response between moisture level and testing signal. (tel:18062120349, e-mail: yanzhijun@gmail.com).

We propose a compact linear polarization spectrometer based on the in-fiber polarization-dependent diffraction grating. The beam profile of radiated light of the grating is shaped to be a Gaussian profile to improve the performance of the spectrometer, where the size of the focused light spot is reduced from 44 um to 33 um with the shaped radiation mode of the grating. Based on the experimental results, the proposed spectrometer can achieve 0.05 nm resolution and 115 nm wavelength responding range from 1495 nm to 1610 nm. To verify the performance of the proposed fiber spectrometer, we measure the transmission spectra of an excessively tilted fiber grating, which has a pair of orthogonal polarization transmission spectra. Compared with the traditional measuring method, the proposed fiber spectrometer integrates the polarizing and spectral analyzing functions in the measuring system and achieves the polarization-sensitive spectral analysis, which shows good wavelength consistency and perfect polarization characteristics.

We have systematically studied the 45° tilted fiber grating (45TFG) as a functional device for erbium-doped fiber laser (EDFL) mode locking. A number of 45TFGs with different polarization-dependent loss (PDL) have been fabricated. Mode-locked (EDFL) using these devices have been characterized in terms of threshold, pulse duration, signal-to-noise ratio, and spectral width. Our results show that a 45TFG with higher PDL could achieve better laser results. By using a 45TFG with 24 dB PDL, the mode-locked laser has 8.1% conversion efficiency and a threshold of 200 mW.

A high-sensitivity curvature sensing configuration is implemented by using a fiber Mach-Zehnder interferometer (MZI) with D-shaped fiber Bragg grating (FBG). A segment of D-shaped fiber is fusion spliced into a single mode fiber at both sides, and then a short FBG is inscribed in the D-shaped fiber. The fiber device yields a significant spectrum sensitivity as high as 87.7 nm/m -1 to the ultralow curvature range from 0 to 0.3 m -1 , and can distinguish the orientation of curvature experienced by the fiber as the attenuation dip producing either a blue or red wavelength shift, by virtue of the asymmetry of D-shaped fiber cladding. In addition, by tracking both resonant wavelengths of the MZI and embedded FBG, the temperature and curvature can be measured simultaneously.

Great efforts have been made to create a bright K-shell source using the thin metal cylindrical cavities in the past few decades. Several metal materials such as titanium (Ti), iron, and so on have been tested for high x-ray conversion efficiency mainly at the OMEGA and NIF laser facilities. Recently, x-ray sources in Ti K-shell transition energy range were investigated at the Shenguang-III prototype laser facility with ∼5 kJ laser energy. The experiments were aimed to reproduce the previous ones at OMEGA, but with an extraordinarily small volume of cylindrical cavity and detailed characterizations of the x-ray source. The cavities were 800 μm inner diameter, 800 μm length, and 30 μm thick plastic tubes supporting 1 μm thick Ti. Seven laser beams were focused to 200 μm diameter. The combination of the small cavity volume and the focused laser spots is intended to improve the electron temperature with limited laser energy since the electron temperature is a key issue for high x-ray conversion efficiency. Thomson scattering was adopted to experimentally probe the electron temperatures at special time and space zones as well as the average temperature obtained from the Ti K-shell spectrum. The evolutions of the electron temperature and density are predicted by the radiation hydrodynamic simulation. A top view and two photon energy bands of x-ray source images provide a way to directly observe the plasma movement toward the cavity axis and distinguish the different emission mechanisms between the Ti K-shell and lower energy x rays. Six Higher-energy x-ray detectors located at different angles were used to record the Ti K-shell x-ray emission and demonstrate its isotropic feature. The characteristics of the x-ray radiate intensity including the time evolution, the angular distribution, and the total yields for both the photon energy regions above and below 4 keV are compared between the cavity and planar targets. Obviously, different behaviors were found between the two photon energy regions and the two types of targets. The x-ray conversion efficiency of the Ti cylinder was determined to be ∼ 4 % and ∼ 21 % in 4 π sr in the Ti K-shell (4–7 keV) and

We demonstrate a multi-wavelength mode-locked Yb-doped fiber laser by incorporating a pair of polarization maintaining 45° tilted fiber gratings (PM-45°TFG) based Lyot filter. Thanks to the functions of the polarizer and the comb filtering introduced by the Lyot filter, dissipative soliton (DS) pulses centered at 1035.26 nm, 1044.93 nm, 1055.62 nm, 1066.11 nm and 1076.63 nm can be generated respectively by finely tuning the intracavity polarization controllers (PCs). Moreover, the laser also can operate in a multi-wavelength regime via appropriately adjusting the pump power and polarization orientation. The high nonlinearity induced by the long cavity length leads to the generation of h-shaped mode-locked pulse with a repetition rate of 566.27 kHz. In the absence of any disturbance, the laser can operate steadily, that can potentially be used in various fields including wavelength division multiplexing systems etc.

Normal serum amylase is uncommon even in acute hypertriglyceridemic pancreatitis (HTGAP). However, normal serum lipase and amylase activity in HTGAP with chylous ascites is exceptionally rare. We report a pregnant woman with HTGAP and chylous ascites that were misdiagnosed. She showed acute abdominal pain and significant systemic inflammatory response, but her serum amylase and lipase levels failed to increase, although ultrasonic imaging finding of the pancreas was normal. Early clinical recognition of chylomicronemia helps clinicians diagnose HTGAP rapidly during pregnancy.

We proposed a polarization-related fiber optic spectrometer based on in-fiber diffraction grating, with a resolution of 0.05nm and 117nm wavelength range. Using such spectrometer, we have tested the polarization dependent spectra of an Ex-TFG.

We present a distributed Bragg reflector (DBR) Yb-doped fiber laser based on a pair of type IA fiber Bragg gratings (FBGs). The high temperature resistant gratings are fabricated in high absorption hydrogen loaded Yb-doped silica fiber by use of a 244 nm argon laser and phase mask method. The DBR laser, with only 10 mm cavity length, exhibits high signal-noise ratio (SNR) of over 50 dB and can survive at 450°C in a long term with stable output power and central wavelength. Besides, the laser has a relatively low temperature sensitivity of 8.9 pm/°C, which contributes to cross-sensitization of stress and temperature.

A compact optic-fiber vector magnetometer is proposed and experimentally demonstrated, which is based on an excessively tilted fiber grating (Ex-TFG) assistant with the magnetic fluid (MF). Without any complicated processing, the cladding mode resonances of the bare Ex-TFG packaged by the MF show high sensitivity to slight perturbations by the magnetic field. Due to the excellent magneto-optical properties of the MF and the azimuth-dependent refractive index sensitivity of the Ex-TFG, such a magnetometer can achieve the magnetic field intensity sensitivity of 2.45 nm/mT and the orientation sensitivity of 0.41 nm/ deg. In addition, based on the spectral interrogation, the detection limit of the magnetic field intensity could reach around 8.1 μT at the minimum wavelength measurement accuracy of 0.02 nm.

We propose and experimentally demonstrate a hybrid grating, in which an excessively tilted fiber grating (Ex-TFG) and a fiber Bragg grating (FBG) were co-inscribed in a reduced-diameter fiber (RDF). The hybrid grating showed strong resonances due to coupling among core mode and a set of polarization-dependent cladding modes. This coupling showed enhanced evanescent fields by the reduced cladding size, thus allowing stronger interaction with the surrounding medium. Moreover, the FBG’s Bragg resonance confined by the thick cladding was exempt from the change of the surrounding medium’s refractive index (RI), and then the FBG can work as a temperature compensator. As a result, the Ex-TFG in RDF promised a highly sensitive RI measurement, with a sensitivity up to ~1224 nm/RIU near the RI of 1.38. Through simultaneous measurement of temperature and RI, the temperature dependence of water’s RI is then determined. Therefore, the proposed hybrid grating with a spectrum of multi-peaks embedded with a sharp Bragg resonance is a promising alternative for the simultaneous measurement of multi-parameters for many RI-based sensing applications.

In this paper, we have theoretically analyzed and experimentally demonstrated the spatial distribution of the radiation mode of 45° tilted fiber grating (TFG). The simulation results have shown the intensity distribution of the radiation mode along the fiber axis exhibited an exponential reduction. In experiment, we have observed the radiation mode pattern of a 5-mm long 45°TFG. The captured profiles along the radial direction and axial direction were in good agreement with the simulated results of the model.

We demonstrate an all-fiber mode locked laser based on hybrid structure of multimode fiber saturable absorber (SA) that can realize both conventional soliton and stretched-pulse states. Stable 16.44 MHz conventional soliton pulses are achieved by injecting 80 mW threshold pump power. By increasing the incident pump power to 420 mW, the laser evolves from soliton operation into stretched-pulse mode locking state. 310 fs stretched-pulse are obtained with the same repetition rate as the soliton pulses. The center wavelength and its 3 dB spectrum bandwidth are 1603 nm and 14.2 nm, respectively. For the first time, we experimentally confirm transition between conventional soliton and stretched-pulse in 1.5 μm mode-locked fiber laser by introducing multimode optical fiber SA. Moreover, the maximum single pulse energy of nearly 1 nJ is achieved. Such all-fiber mode-locked lasers based on hybrid structure of multimode fiber are attractive for practical applications without damage and the limitation of life time.

We demonstrate an all-fiber dissipative soliton Er-doped laser in L-band region using a 45° titled fiber grating for the first time. 697fs dissipative soliton with 30.19 nm spectral bandwidth at 1596.12 nm is realized.

We demonstrate a pump-controlled wavelength switchable Yb-doped fiber laser (YDFL) by nonlinear polarization rotation (NPR) for the first time. The polarizer replaced by a 45° tilted fiber grating (45°-TFG) combines with a section of polarization maintaining fiber (PMF) to form a fiber-based birefringent filter. Stable dissipative soliton (DS) with center wavelength of 1068.39 nm is generated under the mode-locked threshold of 277 mW. The operating wavelength switching between 1046.51 nm and 1067.90 nm can be realized via increasing the pump power simply while keeping the polarization controllers (PCs) in a fixed state. The laser maintains stable mode-locking operation at each wavelength, which can be regarded as a type of multi-wavelength ultrafast light source with precise control and integration potential.

We experimentally demonstrate a harmonic-order controllable L-band Er-doped passively mode-locked fiber laser based on nonlinear polarization rotation (NPR). Distinct from all previous reports, we find that the intracavity birefringence is able to control the harmonic order of a passively mode-locked fiber laser. Experimentally, under a constant pump power of 704 mW, the harmonic order can be tuned from 113th to 39th monotonically by adjusting the polarization controller orientation only. The corresponding repetition rate changes from 2.21 to 0.77 GHz simultaneously. Remarkably, the super-mode suppression ratio of each harmonic order we observed is higher than 29 dB with a maximum of 36.5 dB. Simulated transmission spectra of NPR prove that the changed transmission plays an important role in controlling the harmonic order.

We report on the detailed analyses of mode coupling from fiber core to cladding in excessively tilted fiber gratings (ETFGs). Cladding modes responsible for the typical dual peak pairs in the transmission spectrum of ETFGs are identified with phase matching condition, which suggests two set of dual peak pairs generated from coupling to cladding modes with even and odd azimuthal order. The polarization dependence of those dual peak pairs are also investigated by calculating the coupling coefficients of cladding modes for two orthogonal polarizations. With the calculated coupling coefficients, the measured polarization dependent spectra can be reproduced numerically.

It is a big challenge to weld two materials with large differences in coefficients of thermal expansion and melting points. Here we report that the welding between fused silica (softening point, 1720°C) and SiC wafer (melting point, 3100°C) is achieved with a near infrared femtosecond laser at 800 nm. Elements are observed to have a spatial distribution gradient within the cross section of welding line, revealing that mixing and inter-diffusion of substances have occurred during laser irradiation. This is attributed to the femtosecond laser induced local phase transition and volume expansion. Through optimizing the welding parameters, pulse energy and interval of the welding lines, a shear joining strength as high as 15.1 MPa is achieved. In addition, the influence mechanism of the laser ablation on welding quality of the sample without pre-optical contact is carefully studied by measuring the laser induced interface modification.

We demonstrate a directional curvature sensor based on tilted few-mode fiber Bragg gratings (FM-FBGs) inscribed by a UV laser. The eigenmodes of LP01 and LP11 mode groups are simulated along with the fiber bending. The directional curvature sensor is based on the LP11 mode resonance in the tilted FM-FBG. For curvature from 4.883 to 7.625 m−1, the curvature sensitivities at direction of 0° and 90° are measured to be −2.67 and 0.128 dB/m−1, respectively. The temperature variation barely affects the resonance depth of LP11 mode. The proposed curvature sensor clearly demonstrates the potential to simultaneous directional curvature and temperature measurement with the resolutions of 9.15×10−4 m−1 and 0.952°C, respectively.

We demonstrate a L-band passively all-fiber mode locked erbium-doped laser based on a single 45° titled fiber grating and the center wavelength of the fiber laser is 1598.5 nm.

We demonstrate a dispersion-managed Er-doped all-fiber laser based on nonlinear polarization rotation using a 45°-titled fiber grating. Ultrawide spectrum with full-width half-maximum of 96.41nm at 108MHz repetition rates can be generated directly.

We proposed and experimentally demonstrated a label-free biosensor based on enzyme-immobilized tilted fiber Bragg grating (TFBG) for low concentration glucose detection by using the fine spectrum structure of the TFBG.

We have demonstrated an all-fiber Erbium doped ultrafast laser by inserting a 45° tilted fiber grating. After optimizing the balance between intra-cavity nonlinearity and dispersion, an ultrashort pulse with sub-70 fs pulse duration can be achieved.

45° tilted fiber grating (TFG) possesses a series of merits, such as strong polarization dependant loss (PDL), low insertion loss, superior compatibility, and compactness. One of the most prominent features of such polarizing device is the all-fiber structure. This polarizing element plays an important role in all-fiber integrated systems which have been widely investigated in the last two decades. In this report, we reviewed the recent development in single wavelength, multiple wavelength, Q-switched, and mode locked fiber lasers based on the 45°TFG as in-fiber polarizer.

We demonstrate a dispersion managed Erbium doped all-fiber laser with 101.75MHz repetition rate using 45°TFG to realize mode locking based on nonlinear polarization rotation technique. In our experiment, stable dual-pulse tightly bound states were observed under the pump power of 505mW.

We report a refractive index (RI) and liquid level sensing system based on a hybrid grating structure comprising of a 45° and an 81° tilted fiber gratings (TFGs) that have been inscribed into a single mode fiber in series. In this structure, the 45°-TFG is used as a polarizer to filter out the transverse electric (TE) component and enable the 81°-TFG operating at single polarization for RI and level sensing. The experiment results show a lower temperature cross-sensitivity, only about 7.33 pm/°C, and a higher RI sensitivity, being around 180 nm/RIU at RI=1.345 and 926 nm/RIU at RI=1.412 region, which are significantly improved in comparison with long period fiber gratings. The hybrid grating structure has also been applied as a liquid level sensor, showing 3.06 dB/mm linear peak ratio sensitivity.

We report a highly sensitive refractive index (RI) sensor in the aqueous solution, which is based on an 81°-tilted fiber grating structure inscribed into a thin cladding fiber with 40 μm cladding radius. The numerical analysis has indicated that the RI sensitivity of cladding resonance mode of the grating can be significantly enhanced with reducing cladding size. This has been proved by the experimental results as the RI sensitivities of TM and TE resonance peaks in the index region of 1.345 have been increased to 1180 nm/RIU and 1150 nm/RIU, respectively, from only 200 and 170 nm/RIU for the same grating structure inscribed in standard telecom fiber with 62.5-μm cladding radius. Although the temperature sensitivity has also increased, the change in temperature sensitivity is still insignificant in comparison with RI sensitivity enhancement.

The L-shape waveguide was written in fused silica using a femtosecond laser with beam shaping. The guiding structure supports good light turning; 0.88 dB/turn was achieved at the silica-air interface. By using the finite-different time-domain method, the turn loss due to the turning structure and refractive index of the L-shape waveguide has been simulated. The results show that the proposed method has unprecedented flexibility in fabricating a 90°-bend waveguide.

An all-fiber passively picosecond mode locked Erbium doped laser using a 45° tilted fiber grating (45° TFG) and a fiber Bragg grating (FBG) is reported in this work. Due to the strong polarization dependent loss (PDL) of 45° TFG and narrow 3-dB bandwidth of FBG, the Erbium doped fiber laser (EDFL) can generate picosecond mode locked pulse based on the nonlinear polarization rotation (NPR) effect. The laser features a repetition rate of 9.67 MHz, a pulse duration of 33 ps, a signal-to-noise ratio (SNR) of 70 dB, an average output power of 1.2 mW, and a single pulse energy of 124 pJ under the pump power of 102 mW. Besides, the central wavelength of the laser can be continuously adjusted from 1550.65 nm to 1551.44 nm. The technique of using a 45° TFG to generate picosecond pulses can be readily extended to other wavelength such as mid-infrared (mid-IR) where fiber polarizing components are either expensive or not available. To the best of our knowledge, the spectral width is the narrowest among all-fiber passively mode locked Erbium-doped laser based on NPR.

We demonstrate a compact eccentric long period grating with enhanced sensitivity in low refractive index region. With a period designed at 15 µm for coupling light to high order cladding modes, the grating is more sensitive to surrounding refractive index in low refractive index region. The intrinsically low coupling coefficients for those high order cladding modes are significantly improved with the eccentric localized inscription induced by the femtosecond laser. The fabricated grating is compact with a length of 4.05 mm, and exhibits an average sensitivity of ~505 nm/RIU in low refractive index region (1.3328-1.3544). The proposed principle can also work in other refractive index region with a proper choice of the resonant cladding modes.

We demonstrate a continuous tunable mode locked Erbium fiber laser using a tilted fiber grating based birefringent filter and carbon nanotube based saturable absorber. The central wavelength can be tuned from 1558 nm to 1564 nm while maintaining ultrashort pulse emission.

We demonstrate a fiber specklegram sensor based on the twist-induced effect in tilted two-mode fiber Bragg gratings (TM-FBGs). The measured LP 21 mode specklegram rotates 4.73 (radm−1) with twist in the tilted TM-FBGs.

We generate a passively nanosecond Q-switching by inserting a Brewster fiber grating which induces the nonlinear polarization rotation (NPR) effect. In our experiment, Q-switched pulses with pulse width of 502 ns, repetition rate of 207.5 kHz, output power of 12.7 mW and single pulse energy of 61.2 nJ are achieved under the pump power of 450 mW.

Light modulation by means of polarization management is crucial to avoid signal fading and errors in coherent optic communications [1]. In recent years, graphene has exhibited a great potential in light modulation due to its unique interaction with the electromagnetic field, and attracted increasing attention for applications in photonics and optoelectronics. [2-4]. To provide sufficient interaction length, an optical fibre with enhanced evanescent field generated by etched or tilted fibre grating structures [5, 6] is a promising approach. In this paper, we report an integration of multi-layer graphene onto an excessively tilted fibre grating (Ex-TFG, the tilted angle of ∼82°) inscribed in thin-cladding fibre, and reveal its special polarization tuning characteristics. In the hybrid waveguide of graphene-integrated thin-cladding Ex-TFG, with strong and distinctive core-to-cladding mode coupling property, the grating can provide not only an accessible evanescent field for the enhancement of the light-graphene interaction, but also two sets of polarization-dependent cladding modes. As depicted in the upper part of Fig. 1(a), we can clearly see the two waveguide attenuation peaks corresponding to the TE and TM polarization status from the bare Ex-TFG. After the graphene coating, from the lower part of Fig. 1(a), we can observe that the larger change of coupling intensity in the TE mode and more pronounced wavelength shift in the TM mode, which attests to the polarization-dependent coupling and interaction with graphene.

We demonstrated a compact stable room-temperature multiwavelength erbium doped fiber laser by employing a 45° tilted fiber gratings (TFGs) based all-fiber polarization interference filter. Benefiting from the filter, the channel number, the linewidth, the uniformity and stabilization of the multiwavelength laser were greatly improved. The filter also worked as a polarizing functional device in nonlinear polarization rotation leading to multiwavelength operation. More than 60 wavelengths (within 3dB bandwidth) lasing with a linewidth of 0.03nm and a signal-to-noise ratio of 31dB were obtained. The wavelength spacing was 0.164nm agreeing with the value of the filter and it can be flexibly controlled by adjusting the length of the filter.

Nanosecond passive Q-switching generation from an all-fiber erbium-doped laser with a UV inscribed 45° tilted fiber grating (TFG) is systematically demonstrated. The 45° TFG is employed as a polarizer together with two polarization controllers (PCs) to realize nonlinear polarization rotation (NPR). Because of the NPR effect, stable Q-switched pulses with an average output power of 17.5 mW, a single pulse energy of 72.7 nJ, a repetition rate of 241 kHz, a pulse width of 466 ns, and a signal to noise ratio (SNR) of 58.8 dB are obtained with 600 mW pump power. To the best of our knowledge, the SNR is the highest among all-fiber passively Q-switched erbium-doped laser. The stability of this erbium-doped fiber laser (EDFL) is also examined by monitoring the laser consecutively for 5 h under laboratory conditions.

We have investigated experimentally and numerically the temperature and refractive index (RI) sensitivity characteristics of excessively tilted fiber gratings (Ex-TFGs) in detail. Both results have shown that the temperature and RI sensitivities of Ex-TFGs are mode order dependent. For temperature sensitivity, the higher order cladding mode of Ex-TFG exhibited lower temperature sensitivity, quantitatively, the temperature sensitivities of TM cladding modes at the resonance wavelength around 1550nm are 9pm/°C, 6.8pm/°C, 5.6pm/°C and, 4pm/°C for cladding mode 28th, 31st, 35th, 40th, respectively, indicating the overall temperature sensitivity of Ex-TFGs were lower than that of normal FBGs. The SRI sensing results have shown that the RI sensitivity of Ex-TFG at the special index value could be improved by choosing the cladding mode with effective index close to the refractive index of the detecting medium. The SRI sensitivities at the effective mode index were 2250nm/RIU at 1.408, 864nm/RIU at 1.395, 1536nm/RIU at 1.380 and 1360nm/RIU at 1.355, for the cladding mode of 28th, 31st, 35th, 43rd, respectively. The experimental results have also shown the SRI sensitivity of Ex-TFG was increasing with increasing of the resonance wavelength.

Long period fiber grating around the dispersion turning point, which exhibits high sensitivity to measurands such as temperature and refractive index, is fabricated in single mode fiber with a femtosecond laser for the first time.