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We introduce PBGFs with the cladding made of our newly designed quasi-hexagonal air holes and demonstrate how it actually operates. This cladding structure is introduced for the first time to the best of our knowledge, and is realized by making use of the hydrofluoric acid's corrosive properties. The fibre corrosion can be accurately controlled, thus opening us the gate for the design and fabrication of new PBGFs with more complex and more efficient cladding structures. Numerical results and actual simulations indicate that PBGFs built with this cladding structure have improved bandgap properties and guiding bands as wide as 500 nm have been theoretically reached. Using the same method, we have also been able to design two other types of PBGFs with improved cladding structure.

Hollow core's inner surface coating in a photonic bandgap fibre (PBGF) is investigated by means of finite element method. The coat material and thickness-dependence dispersion curve and group velocity dispersion are numerically studied. The coating with materials of low index or small thickness will rise up the dispersion curve but will not induce surface modes. However, coating with materials of high index or big coat thickness will induce surface modes and avoided-crossings. By varying coat material's refractive index and thickness, the appearances of surface modes and avoided-crossings can be changed. It is found that the avoided-crossing can enormously enlarge the negative dispersion which can find applications in dispersion compensation. We numerically achieve a negative dispersion as large as -21416.15 ps/nm/km. The results give a physical insight into the propagation properties of PBGFs with the hollow core coated by a layer of material and are of crucial significance in the applications of PBGF coating.

Bragg gratings are straightforwardly written into a 10-ring photonic crystal fibre (PCF) with a highly Ge3+ -doped core by means of a 193 nm ArF excimer laser. The grating inscription efficiency is improved, due to enhanced photosensitivity and the large overlap between the fundamental mode and the photosensitive core. The exposure duration only lasts less than 180s. In addition, the high-index core changes the modal properties of the PCF and two groups of higher order mode resonance dips are therefore obtained. A strong cladding mode resonance is observed, because the LP02-like mode has a large overlap integral with the fundamental mode.

Birefringence of a microstructure fibre with irregular structures is investigated using the finite element method. The birefringence of each sample is statistically analysed, and the relation between the birefringence and the irregular structure of the microstructure fibre is derived. The results indicate that the irregular structure of the symmetrical microstructure fibre result in the birefringence which is more strongly affected by disorder of air holes than the nonuniformity. Moreover, lower birefringence can be obtained with smaller air holes and larger pitch in the fibre structure.

Fibre Bragg gratings (FBGs) are inscribed in homemade microstructured fibres by the standard phase mask method. Enhanced couplings between the forward fundamental mode and backward cladding modes are obtained. The mode coupling and spectral characteristics are investigated experimentally. The cladding mode resonances can be affected by filling active materials into the air holes, which will be useful for the implementation of tunable photonic devices in optical fibre communication and sensing systems.

Enhancing coherent combining efficiency via choosing appropriate lasing wavelength in a Michelson compound cavity based on two 3?dB fibre loop mirrors and one fibre Bragg grating (FBG) has been experimentally demonstrated. The FBG with 4.5% reflectivity is replaced at the cleaved facet with 4% Fresnel reflection. A high coherent combining efficiency of 93.5% is obtained when the FBG with central wavelength at 1559.845?nm is introduced into the cavity, while it is only 90.1% combining efficiency with the FBG at central wavelength 1557.830?nm. In comparison with other reports, the proposed compound-cavity laser has the advantage of needless tuning FBG to obtain the coherent condition, and it is facile to ascertain the seemly wavelength lasing for a Michelson compound cavity.

We numerically investigate the mode exciting properties of photonic crystal fibres by using the beam propagation method when the optical field is input from a traditional single mode fibre. The results show that both the excited mode spectrum and the coupling-efficiency of each excited mode depend on the normalized pitch ?/? and the normalized hole-size ?/?. Furthermore, we obtain the boundary profile of the optimizing coupling-efficiency for the excited fundamental mode: the boundary (?/?)* is linear to the boundary (d/?)*. All of these will pave the way for smoothing applications of photonic-crystal fibres, such as splicing and designing photonic-crystal-fibre functional devices.

A dual-wavelength laser system with wideband tunable wavelength spacing and more than 2 W output power is proposed and demonstrated. The operation principle of the laser system is based on the saturation gain characteristic of a cladding-pumped Er3+/Yb3+ co-doped fibre amplifier combining with a wideband tunable fibre laser with flat power spectrum. The tuning range of the wavelength spacing is continuous within a 35 nm spectral range, and the power difference between both lasing wavelengths can be easily adjusted and controlled by a variable optical attenuator. The total output power can approximately keep unchangeable when the wavelength spacing and the power difference between both lasing wavelengths are continuously tuned and changed. The maximum total output power of the laser system is about 2.22 W.

We propose and demonstrate a novel, all-fiber and wavelength tunable Er3+/Yb3+ co-doped cladding-pumped fiber-ring laser, which utilized amplified spontaneous emission (ASE) as a secondary source so that it can operate in L-band. The lasing threshold can be reduced to 30% by reflecting backward C-band ASE into the same Er3+/Yb3+ co-doped fiber (EYDF) using a fiber Bragg grating (FBG). By using a two-section high-birefringence fiber-loop mirror (HiBi-FLM) as the tunable wavelength filter, the tuning-wavelength range can be up to 60 nm, from 1559.2 to 1619.4 nm, and the output power excursion of the laser at different wavelengths can be less than 0.2 dB. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 892–895, 2006; Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/mop.21509

Novel highly birefringent photonic bandgap fibres (PBGFs) are obtained by filling of a high index material in the air holes of total internal reflection birefringent photonic crystal fibres. The effect of the filling high index material on the transmission characteristics has been theoretically investigated. The photonic bandgap has been achieved by using plane-wave method. Moreover, the phase and group modal birefringence have been studied by a full-vector finite-element method. Numerical results show that very high group and phase modal birefringence with magnitude of order of 10−2 and 10−3 has been respectively acquired, which is much higher than those of the non-filled fibres. Furthermore, strong coupling between surface modes and the fundamental modes has been found in the bandgap of the birefringent PBGFs, whose effect on the birefringence and confinement loss has also been discussed.

In this paper, a novel output-power controllable erbium-doped fibre laser, based on a strain-applied high-birefringence fibre loop mirror (HBFLM), was proposed and demonstrated. The reflectivity of HBFLM could be changed from about 0.8% to 100% because the fibre length and birefringence of the high-birefringence fibre (HBF) were altered via bonding a part of HBF to the centric surface of an uniform-strength cantilever beam and applying different stress at end of the beam. As a result, the laser output power could be adjusted and controlled in a dynamic range of more than 33 dB. The laser is not only a controllable and adjustable output-power laser with simple configuration and low cost, but also can be used for optimizing the output characteristics of a linear cavity laser via easily finding out the optimized reflectivity of the reflectors.

We have demonstrated a novel tunable linear cavity Er3+/Yb3+ co-doped fiber laser, which utilizes amplified spontaneous emission as a secondary pump source so that it can operate in L-band. The tuning wavelength range can be up to 34 nm, from 1588.6 to 1622.6 nm, and the output power excursion of the laser at different wavelengths can be less than 0.4 dB by using a two sections of high-birefringence fiber loop mirror as the wavelength filter. The high output power of 200 mW is realized by using the cladding-pump.

We propose an improved method to compress high-order femtosecond solitons, which combines effects of negative third-order dispersion and Raman self-scattering in the fibers. Based on the results of the numerical simulations, we can conclude that given a dispersion decreasing parameter, the higher the soliton order is, the shorter the optimum fiber length is needed, and the greater the relative peak intensity is. For a given initial soliton width and a given dispersion decreasing parameter, there exist both an optimum dispersion decreasing fiber (DDF) length and an optimum value of the negative third-order dispersion coefficient of the DDF at which the enhancement in higher-order soliton compression is maximum. We have also studied the dependence of the compression enhancement on both the initial soliton width and the dispersion decreasing parameter and found for a certain input pulse, the relative peak intensity will increase with the dispersion decreasing parameter. It is also shown that both Gaussian pulses and solitons are favorable for the highest quality compression in fibers with slowly decreasing dispersion.

A novel structure of high extinction ratio filter based on a Mach–Zehnder (M–Z) interferometer and a standard single-mode fiber loop mirror (SMFLM) is proposed for the first to our knowledge. The numerical simulation and experimental results show that the extinction ratio can be improved greatly in comparison with the conventional M–Z interferometer in the same parameters. The improvement of the extinction ratio depends on mainly the difference between the two arms and the measuring resolution. A single-channel optical switch is also demonstrated by using a fiber Bragg grating (FBG) instead of the SMFLM. The dynamic range of greater than 37.4 dB at wavelength-selective port is obtained.

We describe a novel erbium-doped fiber ring laser utilizing the backward amplified spontaneous emission (ASE) power as a secondary pump source so that it can operate in L-band stably. The output wavelength can be tuned in a wide range of 45 nm, from 1560 to 1605 nm. We also compared this scheme with the condition of not using the ASE as secondary pump source, and found this scheme could improve the performance of the laser when using the same components.

A simple and novel strain method is used to change a uniform grating into a linearly chirped grating. Independent tuning of the chirp and the center wavelength of the fiber grating is demonstrated. A 10?Gb?s-1 transmission over 100 km of standard single-mode fiber is realized using this strain method. It is deduced that the product of the maximum dispersion compensation bandwidth and the corresponding dispersion is nearly equal to a constant by numerical simulation.

To design and calculate the zero-dispersion wavelength is one of the important aspects for highly nonlinear photonic crystal fibres. By using the air filling fraction f defined as f = (6d)/(2πΛ) here for the cladding effective index, and the step effective index model, the relationship between the properties of chromatic dispersion and the two different structures has been analysed. It is pointed that the variation of the zero dispersion wavelength is insensitive to the core diameter change in one range of core diameter D, while keeping the air filling fraction f constant. In the other range of core diameter D, the photonic crystal fibres have the best nearly-zero ultra-flattened dispersion. These properties are significant to the design of chromatic dispersion and zero dispersion wavelength in photonic crystal fibres.

A passive Q-switched large-mode-area Yb-doped fibre laser is demonstrated using a GaAs wafer as the saturable absorber. A high Yb doping concentration double-clad fibre with a core diameter of 30 μm and a numerical aperture of 0.07 is used to increase the laser gain volume, permitting greater energy storage and higher output power than conventional fibres. The maximum average output power is 7.2 W at 1080 nm wavelength, with the shortest pulse duration of 580 ns and the highest peak power of 161 W when the laser is pumped with a 25 W diode laser operating at 976 nm. The repetition rate increases with the pump power linearly and the highest repetition rate of 77 kHz is obtained in the experiment.

A novel all-fiber self-organization coherent Erbium-doped fiber laser is proposed and demonstrated. The laser system is composed of two independent lasers. When each of the two branch lasers operates independently, the output power is 10. 41mW and 8.69 mW respectively. By adjusting a polarization controller (PC), the two lasers achieve coherent coupling, and the output power is 24.4 mW, which is more than two times that the single laser yields. Furthermore, we bring forward and discuss the factor estimating the effect of coherent combination—coherent coupling factor. The value of growth factor to evaluate the effect of coherent combining is 1.27. The coherent fiber laser has the advantages of simple structure, high efficiency and single frequency, which conduce to coherent coupling easily.

In this article, the influences of initial pulse and gain fiber parameters on the pulse's self-similar evolution are studied by numerical simulation. We propose and demonstrate that dispersion length is the determining factor of pulse's self-similar evolution. For a given input pulse, if dispersion length is comparable or smaller than fiber length, it can evolve into similariton, otherwise it can not. Under the self-similar evolution condition, increasing pulse energy and gain coefficient can accelerate self-similar evolution, otherwise the effect is not evident. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1785–1787, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22636

We propose and demonstrate a new concept of stable narrow-line-width and close wavelength spacing dual-wavelength lasing in an Er-doped fibre ring laser (EDFRL) by cleaving the spectrum with a wavelength-selective component in the EDFRL. A fibre loop mirror (FLM) combining with a polarization controller (PC) acts as the cleaver. The cleaver can produce a fine pectinate spectrum. By adjusting the PC, the fine pectinate spectrum can be so changeable that cleaving the spectrum of a fibre Bragg grating (FBG) into two parts. As a result, we obtain the dual-wavelength fibre lasering with a bandwidth of only 0.03 nm and a wavelength spacing of only 0.07 nm. Furthermore, the laser can also perform stable switchable single wavelength or stable different-bandwidth dual-wavelength by carefully adjusting the PC at room temperature.

A polarization-independent all-fibre comb filter with dynamically halved channel spacing is proposed. The filter is based on high-birefringent fibre Sagnac interferometers and its channel spacing can be dynamically halved by turning on an optical switch. A good agreement between the numerical simulation and the experimental results is achieved.

A high-power Er3+/Yb3+-codoped double-cladding all-fibre amplifier was successfully demonstrated and experimentally investigated. The amplifier could be operated with a maximum output power of 2.18 W and 2.11 W at 1541 nm and 1550 nm wavelengths, respectively, when the maximum pump power was 6.07 W. The power conversion efficiency was up to 35.6% and 34.4% at the two wavelengths, respectively. The output power and the gain were greater than 2.00 W and 20.0 dB, respectively, in the wavelength range from 1539 nm to 1565 nm for 20.0 mW input signal power. The gain fluctuation and the noise figure around 1550 nm wavelength were less than 0.3 dB and 6.0 dB, respectively.

Multiwavelength operation of a linear cavity erbium-doped fibre laser (EDFL) is proposed and demonstrated. A 3-dB fibre loop mirror and a high birefringence (HiBi) fibre loop mirror are utilized as the cavity reflectors. By utilizing the wavelength-dependent polarization rotation induced by the HiBi fibre loop mirror and by using a fibre polarizer to control the intracavity polarization state, the polarization states of different wavelengths are diversified. Therefore, the polarization hole burning (PHB) effect has been greatly enhanced and the homogeneous broadening of erbium-doped fibre is suppressed to a large extent. By simply tuning a polarization controller, we experimentally obtained simultaneous lasing of four and five wavelengths with wavelength spacing of ~1.8 nm and less than 2 nm at room temperature, respectively. The repeated scans show that all of these lasers have good stability.

All-fibre high power erbium–ytterbium co-doped double clad fibre lasers are proposed and demonstrated. By using different back-cavity mirrors, the different double clad fibre lasers are constructed. It is experimentally found that the output behaviour of laser can be controlled by a back-cavity mirror. The lower the reflectivity of the back-cavity mirror, the higher the output power and the high the slop efficiency. The maximum output power is about 1.6 W and the slop efficiency is 27.6%.

A tunable Yb-doped double-clad fibre (YDCF) laser by tuning the fibre Bragg grating (FBG) is successfully demonstrated. The FBG was fabricated in the YDCF directly, resulting in low splicing loss and compact configuration. A continuous tuning range of 15.6 nm from 1046.6 nm to 1062.2 nm is achieved. The laser linewidth is smaller than 0.1 nm in the whole tuning range. A maximum output power 117 mW is achieved when the pump power is 332 mW. The laser oscillation is quite stable in the whole operation.

A polarization controlled switchable multiwavelength erbium-doped fibre laser with overlapping cavities is proposed. The wavelengths are specified by two Bragg gratings in polarization-maintaining PANDA fibre. The proposed laser can be designed to be operated in stable four-wavelength or wavelength switching modes only by simple adjustment of two polarization controllers. For wavelength switching, four single-wavelength, six dual-wavelength, and four three-wavelength operations have been obtained. The minimum wavelength spacing is only about 0.4 nm.

A simple actively mode-locked fibre ring laser is proposed and successfully demonstrated to generate switchable dual-wavelength picosecond pulses using a Bragg grating in a polarization-maintaining fibre. The wavelength spacing specified by the grating is only 0.52 nm. The proposed laser can be made to operate in stable dual-wavelength or switch between wavelengths at room temperature, by simple adjustment of a polarization controller.

We present a simple method to induce mechanically a long-period fibre grating in a general sing mode fibre. The transmission spectrum of the new type of grating can be tailored by changing the external applied pressure or by introducing a phase-shift in the grating. The tunability this kind of gratings exhibit will be particularly useful for the filter design.

A novel Er3+/Yb3+ co-doped double-clad fiber-ring laser, which utilizes amplified spontaneous emission as a secondary pump source in order to operate in L-band, is demonstrated. Six laser diodes are used to cladding-pump Er3+/Yb3+ co-doped double-clad fiber. When the pumping power of entering fiber is 2731.8 mW, the maximum output power is 518.4 mW, and the slope efficiency is 19%. The relationship between the output power and the splitting ratio of the output coupler is also studied in this paper. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 43: 213–215, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20422

A novel Yb3+-doped double-clad fibre laser based on a double-clad fibre Bragg grating is presented. The fibre Bragg grating, as the output mirror, has been formed in Yb3+-doped double-clad fibre with the phase-mask method. When the input pump power is 2.8 W, the maximum laser output power is 570 mW, with

We report a novel method for measuring photo-induced birefringence of photosensitive fibres based on a fibre loop. The method is simple and is only slightly affected by the surroundings. The experimental results show that the photo-inducing normalized birefringence of the photosensitive fibre is multi-decaying-exponentially proportional to the exposure and it can attain 10−5.

A novel Yb3+-doped four-wavelength double clad fibre laser based on a Fabry-Perot filter, and pumped by a 976 nm laser diode is presented. Its output laser wavelengths are 1085, 1090, 1095 and 1010 nm, respectively. The laser exhibits 0.32 nm line-width, 1.5 W laser output power, 40 dB signal-to-noise ratio and 66% slope efficiency.

A simple method, by letting the pulses pass through an electrooptic intensity modulator twice, for the amplitude equalization of high repetition rate pulses generated from a p-order rational harmonic mode-locked fiber ring laser when p

A two-order cascaded Raman fibre laser based on GeO2-SiO2 fibre and fibre Bragg gratings is fabricated. Output power 309 mW of the second Stokes light around 1176.2 nm is obtained with a slope efficiency of 45.3%. The experimental results agree well with the simulation results obtained by using the double-pass model.

A wavelength-tunable actively mode-locked erbium fiber ring laser is demonstrated using an F–P semiconductor as a modulator and a tunable comb filler. Continuous wavelength tuning is enlarged to 6 nm by successively choosing different cavity modes of the F–P LD and tuning the temperature of the LD. The ∼2-GHz frequency pulse is generated with a side-mode suppression ratio over 30 dB in the entire tuning range. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 318–320, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10753

A novel actively mode-locked fiber laser for generating dual-wavelength picosecond pulses is proposed by using a Fabry-Perot laser diode as the modulator in the ring cavity. A linear chirped fiber grating was used as a wavelength selector and produces temporal-spectral multiplexing of the pulses in the gain medium to reduce gain cross-saturation effects. About 2-GHz stable dual-wavelength pulses with a wavelength spacing of 0.92 nm at room temperature were experimentally demonstrated and the wavelengths could be tuned in a range of about 3 nm.