Gamma-shaped long-cavity normal-dispersion mode-locked Er-fiber laser for sub-nanosecond high-energy pulsed generation

We propose the design of a novel ?-shaped fiber laser resonator and apply it to build a long-cavity normaldispersion mode-locked Er-fiber laser which features enhanced functionalities for management and optimization of pulsed lasing regimes. We report the generation of sub-nanosecond pulses with the energy of ~0.5 µJ at a kilohertz-scale repetition rate in an all-fiber system based on the new laser design. A combination of special design solutions in the laser, such as polarization instability compensation in the ultra-long arm of the resonator, intra-cavity spectral selection of radiation with a broadband fiber Bragg grating, and polarization selection by means of a tilted refractive index grating, ensures low amplified spontaneous emission (ASE) noise and high stability of the laser system output parameters.

Fibre laser using a microchannel based loss tuning element for refractive index sensing

We have proposed and demonstrated a fibre laser system using a microchannel as a cavity loss tuning element for surrounding medium refractive index (SRI) sensing. A ~6µm width microchannel was created by femtosecond (fs) laser inscription assisted chemical etching in the cavity fibre, which offers a direct access to the external liquids. When the SRI changes, the microchannel behaves as a loss tuning element, hence modulating the laser cavity loss and output power. The results indicate that the presented laser sensing system has a linear response to the SRI with a sensitivity in the order of 10-5. Using higher pump power and more sensitive photodetector, the SRI sensitivity could be further enhanced.

Formation and characterization of ultra-sensitive surface plasmon resonance sensor based upon a nano-scale corrugated multi-layered coated D-shaped optical fiber

We present experimental results on the performance of a series of coated, D-shaped optical fiber sensors that display high spectral sensitivities to external refractive index. Sensitivity to the chosen index regime and coupling of the fiber core mode to the surface plasmon resonance (SPR) is enhanced by using specific materials as part of a multi-layered coating. We present strong evidence that this effect is enhanced by post ultraviolet radiation of the lamellar coating that results in the formation of a nano-scale surface relief corrugation structure, which generates an index perturbation within the fiber core that in turn enhances the coupling. We have found reasonable agreement when we modeling the fiber device. It was found that the SPR devices operate in air with high coupling efficiency in excess of 40 dB with spectral sensitivities that outperform a typical long period grating, with one device yielding a wavelength spectral sensitivity of 12000 nm/RIU in the important aqueous index regime. The devices generate SPRs over a very large wavelength range, (visible to 2 mu m) by alternating the polarization state of the illuminating light.

Vectorial bending characteristics of long-period grating written in D-shaped fibre

The curvature- or bend-sensing response of long-period gratings (LPG) UV-inscribed in D-shaped fiber has been investigated experimentally. Strong fiber orientation dependence of the spectral response when such LPGs are subjected to dynamic bending has been observed and is shown to form the basis for new vector sensors.

Design and fabrication of fibre Bragg gratings with V-shaped dispersion profile for multi-channel signal processing

We have designed and fabricated a new type of fibre Bragg grating (FBG) with a V-shaped dispersion profile for multi-channel dispersion compensation in communication links.

Design and fabrication of fibre Bragg gratings with V-shaped dispersion profile

We propose a new type of fiber Bragg grating (FBG) with a V-shaped dispersion profile. We demonstrate that such V-shaped FBGs bring advantages in manipulation of optical signals compared to conventional FBGs with a constant dispersion, e.g., they can produce larger chirp for the same input pulsewidth and/or can be used as pulse shapers. Application of the proposed V-shaped FBGs for signal prechirping in fiber transmission is examined. The proposed design of the V-shaped FBG can be easily extended to embrace multichannel devices. © 2007 IEEE.

Bend sensors with direction recognition based on long-period gratings written in D-shaped fiber

The curvature- or bend-sensing response of long-period gratings (LPGs) UV inscribed in D-shaped fiber has been investigated experimentally. Strong fiber-orientation dependence of the spectral response when such LPGs are subjected to bending at different directions has been observed and is shown to form the basis for a new class of single-device sensor with vector-sensing capability. Potential applications utilizing the linear response and unique bend-orientation characteristics of the devices are discussed.

The spectral sensitivity of long period gratings fabricated in elliptical core D-shaped optical fibre

Long period gratings (LPGs) were written into a D-shaped optical fibre that has an elliptical core with a W-shaped refractive index profile and the first detailed investigation of such LPGs is presented. The LPGs’ attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15 nm between the two orthogonal polarisation states. A finite element method was successfully used to model many of the behavioural features of the LPGs. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature and bending. These LPG devices produced blue and red wavelength shifts depending upon the orientation of the bend with measured maximum sensitivities of -3.56 and +6.51 nm m, suggesting that this type of fibre LPG may be useful as a shape/bend orientation sensor with reduced errors associated with polarisation dependence. The use of neighbouring bands to discriminate between temperature and bending was also demonstrated, leading to an overall curvature error of ±0.14 m-1 and an overall temperature error of ±0.3 °C with a maximum polarisation dependence error of ±8 × 10-2 m-1 for curvature and ±5 × 10-2 °C for temperature.

Gamma-shaped long-cavity normal-dispersion mode-locked Er-fiber laser for sub-nanosecond high-energy pulsed generation

We propose the design of a novel ?-shaped fiber laser resonator and apply it to build a long-cavity normaldispersion mode-locked Er-fiber laser which features enhanced functionalities for management and optimization of pulsed lasing regimes. We report the generation of sub-nanosecond pulses with the energy of ~0.5 µJ at a kilohertz-scale repetition rate in an all-fiber system based on the new laser design. A combination of special design solutions in the laser, such as polarization instability compensation in the ultra-long arm of the resonator, intra-cavity spectral selection of radiation with a broadband fiber Bragg grating, and polarization selection by means of a tilted refractive index grating, ensures low amplified spontaneous emission (ASE) noise and high stability of the laser system output parameters.

Improved supercontinuum generation by dispersion tuning and dual wavelength pumping

Supercontinuum generation in ultra-long Raman fibre laser cavities is compared for a range of fibre dispersions in the anomalous and normal regimes. For normal dispersion improved performance and efficiency is achieved using dual wavelength pumping.

202nm continuous tuning from high-power external-cavity InAs/GaAs quantum-dot laser

Record broadly tunable high-power external cavity InAs/GaAs quantum-dot diode laser is demonstrated. A maximum output power of 455mW and a side-mode suppression ratio >45dB in the central part of the tuning range are achieved. ©2010 IEEE.

Generation and performance of localised surface plasmons utilising nano-scale structured multi-layered thin films deposited upon D-shaped optical fiber

A new generation of surface plasmonic optical fibre sensors is fabricated using multiple coatings deposited on a lapped section of a single mode fibre. Post-deposition UV laser irradiation using a phase mask produces a nano-scaled surface relief grating structure, resembling nano-wires. The overall length of the individual corrugations is approximately 14 μm with an average full width half maximum of 100 nm. Evidence is presented to show that these surface structures result from material compaction created by the silicon dioxide and germanium layers in the multi-layered coating and the surface topology is capable of supporting localised surface plasmons. The coating compaction induces a strain gradient into the D-shaped optical fibre that generates an asymmetric periodic refractive index profile which enhances the coupling of the light from the core of the fibre to plasmons on the surface of the coating. Experimental data are presented that show changes in spectral characteristics after UV processing and that the performance of the sensors increases from that of their pre-UV irradiation state. The enhanced performance is illustrated with regards to change in external refractive index and demonstrates high spectral sensitivities in gaseous and aqueous index regimes ranging up to 4000 nm/RIU for wavelength and 800 dB/RIU for intensity. The devices generate surface plasmons over a very large wavelength range, (visible to 2 μm) depending on the polarization state of the illuminating light. © 2013 SPIE.

Physical characteristics of localized surface plasmons resulting from nano-scale structured multi-layer thin films deposited on D-shaped optical fiber

Novel surface plasmonic optical fiber sensors have been fabricated using multiple coatings deposited on a lapped section of a single mode fiber. UV laser irradiation processing with a phase mask produces a nano-scaled surface relief grating structure resembling nano-wires. The resulting individual corrugations produced by material compaction are approximately 20 μm long with an average width at half maximum of 100 nm and generate localized surface plasmons. Experimental data are presented that show changes in the spectral characteristics after UV processing, coupled with an overall increase in the sensitivity of the devices to surrounding refractive index. Evidence is presented that there is an optimum UV dosage (48 joules) over which no significant additional optical change is observed. The devices are characterized with regards to change in refractive index, where significantly high spectral sensitivities in the aqueous index regime are found, ranging up to 4000 nm/RIU for wavelength and 800 dB/RIU for intensity. © 2013 Optical Society of America.

Controllable group delay in a θ-shaped microfiber resonator with coupled-resonator-induced transparency

The control of Light velocity is theoretically and experimentally demonstrated in a θ-shaped microfiber resonator with coupled-resonator-induced transparency. By adjusting the structure parameters, group delays from -60ps to 200ps are achieved in the all-fiber resonator.

73 nm wavelength tuning from a frequency-doubled quantum-dot laser in PPKTP waveguides

A compact all-room-temperature CW 73-nm tunable laser source in the visible spectral region (574nm-647nm) has been demonstrated by frequency-doubling of a broadly-tunable InAs/GaAs quantum dot external-cavity diode laser in periodically-poled potassium titanyl phosphate waveguides with a maximum output power in excess of 12mW and a maximum conversion efficiency exceeding 10%. Three waveguides with different cross-sectional areas (4×4μm2, 3×5μm2 and 2x6μm2) were investigated. Introduction - Development of compact broadly tunable laser sources in the visible spectral region is currently very attractive area of research with applications ranging from photomedicine and biophotonics to confocal fluorescence microscopy and laser projection displays. In this respect, semiconductor lasers with their small size, high efficiency, reliability and low cost are very promising for realization of such sources by frequency­doubling of the infrared light in nonlinear crystal waveguides. Furthermore, the wide tunability offered by quantum-dot (QD) external-cavity diode lasers (ECDL), due to the temperature insensibility and broad gain bandwidth [1,2], is very promising for the development of tunable visible laser sources [3,4]. In this work we show a compact green-to-red tunable all­room-temperature CW laser source using a frequency-doubled InAs/GaAs QD-ECDL in periodically-poled potassium titanyl phosphate (PPKTP) crystal waveguides. This laser source generates frequency-doubled light over the 574nm-647nm wavelength range utilizing the significant difference in the effective refractive indices of high-order and low-order modes in multimode waveguides [3]. Experimental results - Experimental setup used in this work was similar to that described in [3] and consisted of a QD gain chip in the quasi­Littrow configuration and a PPKTP waveguide. Coarse wavelength tuning of the QD-ECDL between 1140 nm and 1300 nm at 20°C was possible for pump current of 1.5 A. The laser output was coupled into the PPKTP waveguide using an AR-coated 40x aspheric lens (NA ~ 0.55). The PPKTP frequency-doubling crystal (not AR coated) used in our work was 18 mm in length and was periodically poled for SHG (with the poling period of ~ 11.574 11m). The crystal contained 3 different waveguides with cross-sectional areas of ~ 4x4 11m2, 3x5 11m2 and 2x6 11m2. Both the pump laser and the PPKTP crystal were operating at room temperature. The waveguides with cross-sectional areas of 4x411m2, 3x511m2 and 2x611m2 demonstrated the tunability in the wavelength ranges of 577nm - 647nm, 576nm -643nm and 574nm - 641nm, respectively, with a maximum output power of 12.04mW at 606 nm Conclusion - We demonstrated a compact all-room-temperature broadly­tunable laser source operating in the visible spectral region between 574nm and 647nm. This laser source is based on second harmonic generation in PPKTP waveguides with different cross-sectional areas using an InAs/GaAs QD-ECDL References [I] E.U. Rafailov, M.A. Cataluna, and W. Sibbett, Nat. Phot. 1,395 (2007). [2] K.A. Fedorova, M.A. Cataluna, I. Krestnikov, D. Livshits, and E.U. Rafailov, Opt. Express 18(18), 19438-19443 (2010). [3] K.A. Fedorova, G.S. Sokolovskii, P.R. Battle, D.A. Livshits, and E.U. Rafailov, Laser Phys. Lett. 9, 790-795 (2012). [4] K.A. Fedorova,G.S. Sokolovskii, D.T. Nikitichev, P.R. Battle, I.L. Krestnikov, D.A. Livshits, and E.U. Rafailov, Opt. Lett. 38(15), 2835-2837 (2013) © 2014 IEEE.