Microcosm study reveals the microbial and environmental effects on tributyltin degradation in an estuarine sediment

Tributyltin (TBT) is one of the most harmful contaminants ever released into the aquatic environment. Despite being banned, it is still present at many locations throughout the world. Its degradation in sediment mainly occurs through microbial biodegradation, a process that remains unclear. This study therefore aimed at better understanding TBT biodegradation in estuarine sediment and the microbial community associated with it. Microcosm experiments were set up, embracing a range of environmental control parameters. Major community shifts were recorded, mainly attributed to the change in oxygen status. The highest percentage of degradation (36,8%) occurred at 4 °C in anaerobic conditions. These results are encouraging for the in-situ bioremediation of TBT contaminated muddy sediment in temperate ports worldwide. However, with TBT able to persist in the coastal environment for decades when undisturbed in anoxic sediment, further research is needed to fully understand the mechanisms that triggered this biodegradation observed in the microcosms.

Design and fabrication of elliptical-core few-mode fiber for MIMO-less data transmission

We propose a design strategy of elliptical core few-mode fiber (e-FMF) that supports three spatial modes with enhanced mode spacing between LP_11a and LP_11b, to suppress intra-mode coupling during mode-division multiplexing (MDM) transmission. Our theoretical investigations show that there exist two optimization regimes for the e-FMF, as a comparison with traditional circular core FMF(c-FMF). At the regime of three-mode operation, there occurs a trade-off between mode spacing and bending-induced loss. Meanwhile, in terms of five-mode regime, a trade-off between mode spacing and high-order mode crosstalk happens. Finally, we fabricate 7.94 km e-FMF with the optimal parameters, based on the commercial fiber manufacture facility. The primary characterizations at 1550 nm show that three spatial modes of e-FMF can be transmitted with a loss less than 0.3 dB/km. Meanwhile, -22.44  dB crosstalk between LP_11a and LP_11b is observed, even when the 2 km e-FMF is under stress-induced strong perturbation.

Comparison of bandwidth and sensitivity of long-period gratings in single-mode and few-mode fibers

We present a comparative study of the bandwidth and the sensitivity of the resonance wavelength of long-period gratings (LPGs) to external perturbation fabricated in single-mode fibers (SMFs) and few-mode fibers (FMFs), and their dependencies on the group indices and the dispersion properties of the phase-matched modes. Unlike SMFs, a relatively large core size of FMFs invariably leads to nonuniform index modulation across the fiber cross section under UV exposure, enabling the coupling between modes having dissimilar azimuthal symmetry. Simple analytical formulas for the group/effective index difference, dispersion difference, bandwidth, and wavelength sensitivities are derived for the case of SMFs where light is coupled from the fundamental core mode to the symmetrical cladding modes. Our results show that a two-mode fiber operating at a V-number close to 3 is capable of producing LPGs with broader bandwidth and higher sensitivity as compared with their SMF counterparts, except for a few special cases. Our analyses provide insights into the characteristics of LPGs and facilitate their designs for specific applications.

Role of wavelength dependent sensitivity in affecting the crosstalk mitigation of homogeneous multicore fiber: an analytical estimation approach

The mitigation of both crosstalk and its wavelength dependent sensitivity for homogeneous multicore fiber (MCF) is theoretically investigated using an analytical evaluation approach. It is found there exists a performance trade-off between the crosstalk mitigation and its wavelength dependent sensitivity suppression. After characterizing the fabricated homogeneous MCFs, we verify that although the increasing core pitch can mitigate the crosstalk, the wavelength dependent sensitivity is drastically degraded from 0.07dB/nm to 0.11dB/nm, which is harmful to the dense wavelength division multiplexing (DWDM) transmission over C + L band using MCF.

Phase sensitive SPR sensor for wide dynamic range detection

Phase detection has been utilized to enhance the sensitivity of surface plasmon resonance (SPR) sensors for a long time. However, an inherent drawback for phase sensitive SPR sensors are their limited dynamic range, which has greatly hindered wide applications of such sensors. In this Letter, a design combining phase detection and angular interrogation has been proposed to provide an SPR sensor with both high sensitivity and wide dynamic range. As a result, a resolution of 2.2×10^-7 RIU with a dynamic range of over 0.06 RIU has been achieved simultaneously. An added advantage of this design is the flexibility for sensitivity and dynamic range adjustment.

Optimum input states of polarization for Mueller matrix measurement in a system having finite polarization-dependent loss or gain

We present the theoretical and simulation results of the relationship between three input states of polarization (SOP) and the Mueller matrix measurement error in an optical system having birefringence and finite polarization-dependent loss or gain (PDL/G). By using the condition number as the criterion, it can be theoretically demonstrated that the three input SOPs should be equally-spaced on the Poincaré sphere and centered on the reversed PDL/G vector to achieve better measurement accuracy in a single test. Further, an upper bound of the mean of the Mueller matrix measurement error is derived when the measurement errors of output Stokes parameters independently and identically follow the ideal Gaussian distribution. This upper bound also shows that the statistically best Mueller matrix measurement accuracy can be obtained when the three input SOPs have the same relationship mentioned above. Simulation results confirm the validity of the theoretical findings.

Effect of input states of polarization on the measurement error of Mueller matrix in a system having small polarization-dependent loss or gain

For the measurement of Mueller matrix in an optical system with birefringence and small polarization-dependent loss or gain (PDL/G), we theoretically derive the statistical relationship between the Mueller matrix measurement error and three input states of polarization (SOP). Based on this theoretical relation and simulation results, it can be concluded that the three input SOPs, that are coplanar with an angle of 120 degrees between any two of them in Stokes space, can be considered as a substitute for the best input SOPs which can statistically lead to the minimum measurement error. This conclusion is valid when the PDL/G of the optical system under test is less than 0.35 dB.

Photonic-assisted microwave frequency measurement with higher resolution and tunable range

A photonic-assisted approach to microwave frequency measurement is proposed based on frequency-to-power mapping with the help of the so-called amplitude comparison function. The key component is a dual-output Mach-Zehnder modulator (MZM) working at chirped modulation. The proposed scheme is characterized as having simplicity, higher resolution, and tunable measurement range. Owing to experimental constraint, an equivalent experiment has been carried out using a common single-output MZM at different bias points to prove the concept.

Observation of spectral enhancement in a soliton fiber laser with fiber Bragg grating

We demonstrate that soliton generation in a fiber laser containing a fiber Bragg grating exhibits spectral enhancement near the Bragg resonance wavelength. The Bragg grating leads to a spectral hole on the soliton spectrum while the observed enhancement is always located at the long wavelength side of the Bragg resonance.

Coupled-mode analysis of Bragg-reflection filters based on asymmetric nonlinear dual-core fibers

We demonstrate a Bragg-reflection grating coupler using a nonlinear dual-core fiber with a long-period grating (LPG) and a fiber Bragg grating (FBG) inscribed in different cores. The LPG couples light from the primary core to the cladding, while the FBG operating in reflection acts to drop the channel from the secondary core. The coupler is nonreflective along the launching core. Theoretical analysis of this structure demonstrates a design for obtaining a flat-top bandpass filter with high reflectivity, steep band transitions, and negligible sidelobes. By launching an intense pump beam into the individual cores, a switchable and a wavelength tunable passband can be achieved. The results demonstrate the usefulness of the device as a grating-based all-optical element.

Experimental analysis of spectral characteristics of antiresonant guiding photonic crystal fibers

Both numerical and experimental analyses are carried out to investigate the spectral characteristics of antiresonant guiding photonic crystal fibers. The transmission minima were observed at the wavelengths where LP(ml) (m<or=4, l=1,2,3,(...)) modes of the high-index inclusions in the cladding are cut off. As the order of modes increases, the bandwidths of the corresponding stop bands gradually reduce.

Stress distribution and induced birefringence analysis for pressure vector sensing based on single mode fibers

We propose a theoretical approach to analyze the pressure stress distribution in single mode fibers (SMFs) and achieve the analytical expression of stress function, from which we obtain the stress components with their patterns in the core and compute their induced birefringence. Then we perform a pressure vector sensing based on approximately 2 km SMF. Using Mueller matrix method we measure the birefringence vectors which are employed to compute the pressure magnitudes and their orientation. When rotating the pressure around the fiber, the corresponding birefringence vector rotates around a circle with double speed. Statistics show the average deviation of calculated pressure-magnitude to practical value is approximately 0.17 N and it is approximately 0.85 degrees for orientation.

Photonic ultrawideband monocycle pulse generation using a single electro-optic modulator

A compact approach to photonic-assisted ultrawideband (UWB) monocycle pulse generation is proposed and experimentally demonstrated based on the wavelength dependence of the half-wave voltage of a Mach-Zehnder modulator (MZM). By employing a single MZM with dual-wavelength injection at around 1310 and 1550 nm, a pair of polarity-reversed monocycle pulses with the full width at half-maximum of about 80 ps and the fractional bandwidth of greater than 160% can be generated. The experiment results agree well with the theoretical prediction. The simple setup and the convenience to control the monocycle pulse polarity are favorable for future applications.

Ultra-wideband pulse generation with flexible pulse shape and polarity control using a Sagnac-interferometer-based intensity modulator

We propose a novel scheme to generate ultra-wideband (UWB) pulse by employing a Sagnac interferometer comprising a phase modulator. This structure performs a dual-input and dual-output intensity modulator (IM), ultimately resulting in the flexibility to select the shape and the polarity of the generated UWB pulse. The experiment results show a good agreement with the theoretical investigation in terms of both pulse profile and spectrum, which conforms to the definition of UWB signals by the U.S. Federal Communications Commission. Furthermore, the proposed scheme is independent of the voltage bias point.

Spectrally resolved reflectometric measurement of polarization mode dispersion in an optical fiber link with polarization-dependent loss

In an optical fiber link with polarization-dependent loss (PDL), we demonstrate that although the complex polarization mode dispersion vector cannot be fully obtained by the reflectometric measurement, the spectrally resolved differential group delay (DGD) and differential attenuation slope (DAS) can be explicitly determined by such measurements performed simultaneously in the optical frequency domain and the fiber length domain. In principle, this technique can be used to realize the spectrally resolved and spatially resolved measurement of DGD and DAS in an optical fiber link having PDL based on distributed Rayleigh backscattering. We report the experimental result based on the far-end Fresnel reflection to confirm the validity of the proposed method.

Cascaded dynamic eigenstates of polarization analysis for piezoelectric polarization control

We propose the cascaded dynamic eigenstates (DESs) of polarization to analyze multicomponent polarization control (PC) devices, and achieve the analytical expression of output state of polarization (SOP) as a function of voltage for piezoelectric polarization control (PPC). By measuring the DES at the output port of the device, the prestage DESs will rotate around subsequent ones. Experimental results in PPC confirm the validity of our analysis. The average error of our theoretical output SOP is 1.23 degrees, and the SOP response time is ~10 micros, which is promising to realize a quasi-open-loop high-speed PC.

Spectral-resolved backreflection measurement of polarization mode dispersion in optical fibers

An improved backreflection technique is proposed to perform the spectral-resolved measurement of polarization mode dispersion (PMD) in optical fibers. This technique is based on the PMD dynamical equation and realized by measuring the polarization state evolutions of the reflected signal in both frequency and time domains. Two experimental setups, employing the far-end Fresnel reflection, are constructed to verify this technique. The agreement between the results of the proposed backreflection technique and the conventional forward technique is observed.

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.

40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter

We propose a novel all-optical format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) based on single semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). By adopting the ultrafast SOA model associated with intraband mechanism, the 40Gb/s NRZ-to-RZ format conversion is successfully demonstrated with numerical simulation. We investigate the influence of the filter detuning, extinction ratio, conversion efficiency, output pulsewidth, and filter bandwidth. These simulation results confirm the feasibility of our scheme. The proposed scheme is robust and potential for applications in future optical networks.

Experimental demonstration of both inverted and non-inverted wavelength conversion based on transient cross phase modulation of SOA

We demonstrate experimentally ultrafast inverted and noninverted wavelength conversion (WC) based on a semiconductor optical amplifier (SOA) and an optical bandpass filter (OBF). In the case of small detuning, the WC is inverted regardless if the OBF is blue- or red-shifted with respect to the central wavelength of the converted signal. However the WC is non-inverted when the filter detuning is relatively large. An analytical formula for the transient cross phase modulation is applied to reveal the polarity variation of WC with respect to the OBF detuning. The theoretical detuning values are in good agreement with our experimental results.

Symmetry properties of two-dimensional anisotropic photonic crystals

A theoretical study of two-dimensional photonic crystals made of anisotropic material is presented. Detailed computation principles including a treatment of the TE and TM polarizations are given for a photonic crystal made of either uniaxially or biaxially anisotropic materials. These two polarizations can be decoupled as long as any one of the principal axes of the anisotropic material is perpendicular to the periodic plane of the photonic crystal. The symmetry loss due to the anisotropy of the material and the variation of the Brillouin zones relative to the tensor orientations are also analyzed. Furthermore, the symmetry properties of the two-dimensional photonic band structure are studied, and the resulting effect on the photonic bandgap and the dispersion properties of photonic crystal are analyzed as a function of the orientation of the anisotropic material.

Generalized Mueller matrix method for polarization mode dispersion measurement in a system with polarization-dependent loss or gain

A generalized Mueller matrix method (GMMM) is proposed to measure the polarization mode dispersion (PMD) in an optical fiber system with polarization-dependent loss or gain (PDL/G). This algorithm is based on the polar decomposition of a 4X4 matrix which corresponds to a Lorentz transformation. Compared to the generalized Poincaré sphere method, the GMMM can measure PMD accurately with a relatively larger frequency step, and the obtained PMD data has very low noise level.

Gain Properties of multi-wavelength time division multiplexed Raman amplifier

We report gain properties of the multi-wavelength pulsed Raman pump. Based on fiber dispersion property, we obtain the group velocity difference between pump pulses which imposes the upper limit of pulse repetition rate beyond which pump-to-pump interaction occurs. The pump-to-pump interaction can be severe when pump instantaneous power is high due to small duty cycle. However, bigger duty cycle imposes much lower limit to the pulse repetition rate. Therefore, an optimized level of duty cycle is required. In addition, shorter pump wavelength separation helps to increase the upper limit of pulse repetition rate. Ultimately this will improve the noise performance of TDM Raman amplifier.

Tunable fly's-eye lens made of patterned polymer-dispersed liquid crystal

A fly's-eye lens was fabricated using polymer-dispersed liquid crystals and its optical properties were evaluated. The morphologies were examined under an optical microscope. The forming process has been simulated based on a patterned photo-polymerization technique in which the spatially modulated reaction rate has been coupled with the time-dependent Ginzburg-Landau (TDGL) equations with the free energies relating to isotropic mixing, nematic ordering, and network elasticity incorporated. The simulated results are in good agreement with the experimental results. The beam profile was tested using a CCD. The results showed that this fly's-eye lens could modulate a Gaussian beam into a mesa-like beam. Such device is potentially useful in beam shaping and many illumination systems that require uniform beam profile.

On-line simultaneous monitoring of polarization mode dispersion and chromatic dispersion

We have developed new expressions for power fading and average power fading owing to polarization mode dispersion (PMD) and chromatic dispersion in terms of the angle of precession of the output state of polarization about the PMD vector. Based on these expressions, a simple and novel pilot-tone-based technique for simultaneous monitoring of chromatic dispersion and PMD is proposed. Experimental results demonstrate the effectiveness of the proposed technique; these results agree well with the theoretical analysis.

Multiwavelength Raman fiber laser with a continuously-tunable spacing

A spacing-tunable multiwavelength Raman fiber laser with an independently-adjustable channel number is proposed and demonstrated. It uses a novel free-spectral-range (FSR)-tunable comb filter based on a superimposed chirped-fiber Bragg grating (CFBG) and a linear cavity formed by a bandwidth-tunable CFBG reflector, a pumped highly-nonlinear fiber for Raman gain, and an optical circulator based loop mirror. Multiwavelength laser operations with spacing tuning from 0.3 to 0.6 nm and channel number adjustment from 2 to 10 have been achieved.

Tunable superprism and polarization splitting in a liquid crystal infiltrated two-dimensional photonic crystal made of silicon oxynitride

Tunable superprism and polarization splitting effects, based on liquid-crystal infiltrated two-dimensional photonic crystal with SiON as a background material, are demonstrated. Owing to the strong birefringence of the liquid crystal, the dispersion curves of the two polarizations are distinctly different, resulting in large splitting between the two polarizations. Extremely large splitting, as large as 80 degrees, can be obtained. Moreover, the splitting can be tuned to a substantial degree when the optic axis of the liquid crystal is reoriented. Fully controllable, incident-angle-dependent and wavelength-dependent superprism effects are also demonstrated. This large swing in the refraction can be completely switched off by reorientation of the optic axis of the liquid crystal, thus to resemble bulk media refraction.

Analysis of perturbed Bragg fibers with an extended transfer matrix method

We introduce an extended transfer matrix method (TMM) for solving guided modes in leaky optical fibers with layered cladding. The method can deal with fibers with circular but nonconcentric material interfaces. Validity of the method is verified by two full-vector numerical methods. The TMM is then used to investigate the guidance property of perturbed Bragg fibers. Our analysis reveals that the core modes will interact with each other when a perturbed Bragg fiber has only C(1) symmetry. Special attention is paid to the first transverse-electric (TE(01)) mode, which is found to experience severe degradation around spectral regions where its dispersion curve supposedly crosses a transverse-magnetic (TM) or hybrid mode.

Quasi-monochromatic fiber depolarizer and its application to polarization-dependent loss measurement

We theoretically derive the relationship between the degrees of polarization (DOPs) of input and output for an optical component with polarization-dependent loss (PDL) and birefringence. Based on the theoretical result, we propose a novel depolarizer for quasi-monochromatic light that can depolarize a fully polarized light with a 50 MHz linewidth to less than 0.2% in the DOP The depolarized light is then used to measure PDL in a single-mode optical fiber link. To the best of our knowledge, our new PDL measurement method is significantly faster than all known methods. Experimental results show excellent agreement with other methods.

Four-wave mixing assisted self-stable 4x10 GHz actively mode-locked Erbium fiber ring laser

We proposed and developed a self-stable multiwavelength (4 cbannels) 10-GHz actively mode-locked Erbium-doped fiber ring laser with 0.8 nm wavelength spacing. A 1-km highly nonlinear fiber is incorporated in the laser cavity to eliminate the strong gain competitions in the homogeneously broadened EDF by multiple parametric four-wave mixing processes. The fiber nonlinearity is also helpful to provide phase locking and stabilize the output pulse. Stable laser pulses at 1556.55, 1557.36, 1558.17, and 1558.98 nm are successfully obtained simultaneously with supermode noise suppression ratio greater than 50 dB. The corresponding time-bandwidth products of four channels are 0.39 ~ 0.41.

New approach to determine the effects of polarization mode dispersion and chromatic dispersion on pulse and RF signals

We present a novel analytical expression relating the output state of polarization (SOP) and the polarization mode dispersion (PMD) vector, including polarization-dependent chromatic dispersion (PCD), in terms of the angle of precession of the output SOP around the PMD vector. We derive a number of new expressions incorporating for the first time this angle of precession. First, a general relation to study the effect of differential group delay, PCD, and chromatic dispersion on pulses of arbitrary shapes is given. From this general relation, we derive expressions for pulse broadening and power penalty for Gaussian pulses. Moreover, a new expression for PMD-induced power fading for single-sideband modulated radio frequency signals is also derived. Measured experimental results are presented to support the derived expressions.

Dynamic switching of optical vortices with dynamic gamma-correction liquid crystal spiral phase plate

Generation and dynamic switching of optical vortices from charge 1 up to charge 6 at wavelength of 0.6328-microm by one dynamic gamma-correction liquid crystal spiral phase plate are reported. The liquid crystal spiral phase plate comprises 46 slices, which are driven by a 16- channel voltage output card. The spiral phase plate was designed based on the relationship between the topological charge purity of an optical vortex generated by the spiral phase plate and the total number of slices. The calculation results show that a minimum slice number of 44 is required for generating optical vortices up to charge 6 with charge purity above 94%.

Thermally switchable and discretely tunable comb filter with a linearly chirped fiber Bragg grating

We propose and experimentally demonstrate a novel switchable and discretely tunable comb filter based on a thermally induced linearly chirped fiber Bragg grating. Experimentally we achieved a thermally induced optical bandpass filter that has eight switchable passband peaks with peak separations of 1.6 and 3.2 nm, a very narrow bandwidth (as small as 10 pm) of each peak, a tunable range of 16.5 nm, and a greater than 25 dB rejection ratio. Two spectral peaks separated by only 0.8 nm were also obtained with this comb filter. This filter provides the unique advantages of an all-fiber structure, switchable spectral peaks, independent tuning of the center wavelength and wavelength spacing of the spectral peaks, low polarization sensitivity, simple structure, ease of fabrication, and low cost.

Improved air-silica photonic crystal with a triangular airhole arrangement for hollow-core photonic bandgap fiber design

We propose an improved photonic crystal (PC) cladding design for existing air-guiding photonic bandgap (PBG) fibers whose cladding airholes are arranged in a triangular lattice pattern. By increasing the sizes of concentrated silica regions in the cladding PC, we can have a larger degree of freedom in controlling the cladding bandgap regions. We predict that a fiber made from this type of cladding would perform better in terms of the PBG-guiding wavelength range, radiation loss owing to finite cladding size, and the ability to avoid surface mode problems.

Tunable microwave filter that uses a high-birefringent fiber and a differential-group-delay element

We propose and experimentally demonstrate a novel and simple photonic microwave notch filter that uses a high-birefringent fiber that gives a fixed differential group delay (DGD), together with a DGD element that gives a tunable DGD. This configuration overcomes the problems of optical coherence interference and chromatic dispersion, which may occur in schemes that use fiber delay lines or fiber gratings. Also presented is a theoretical analysis for the performance of the microwave filter that uses the present configuration. The present scheme provides a continuous tuning capability for changing the notch frequency. Measured notch rejection is greater than 40 dB. This scheme can operate over a wide wavelength range of the optical carrier. There is good agreement between experiment results and theoretical analysis.

Design of air-guiding honeycomb photonic bandgap fiber

We introduce a design procedure for hollow-core photonic bandgap (PBG) fiber with a cladding made of air-silica honeycomb photonic crystal (PC). It is found that air-guiding can be realized in both fundamental and secondary PBG regions of the cladding PC. Dispersion and radiation loss of the fundamental mode for two types of fiber structure are theoretically calculated. The fibers show promising waveguiding ability.

Bound twin-pulse solitons in a fiber ring laser

Bound states of twin-pulse solitons were experimentally observed in a passively mode-locked fiber ring laser. Similar to those of single-pulse solitons, the bound states of twin-pulse solitons are marginally stable and occur at some fixed, quantized soliton separations. Our experimental investigations revealed that the formation of such bound states might be resulted from the dispersive wave mediated long-range soliton interaction in the laser.

Bidirectional optical bistability in a dual-pumped erbium doped fiber ring laser

We investigate bidirectional optical wave propagations in a dual-pumped erbium doped fiber ring laser without isolator, and observe optical bistability behaviors. Consequently, we propose and construct a NOLM-NALM fiber ring laser to demonstrate and exploit this bidirectional optical bistability phenomenon in optical switching by introducing two tunable variable ratio couplers in the system. Numerical analyses based on the proposed laser structure have also been demonstrated corroborated with the experimental results.

Generating doughnut-shaped beams with large charge numbers by use of liquid-crystal spiral phase plates

Liquid-crystal spiral phase plates with cell gaps of 7 and 20 microm have been used to generate doughnut-shaped beams (doughnut beams) with charges of 1 and 4, respectively. Stacking these liquid-crystal spiral phase plates yielded doughnut beams with charge numbers up to 8. High efficiency and flexibility are the advantages of generating doughnut beams by stacking liquid-crystal spiral phase plates. Interference of doughnut beams generated by liquid-crystal spiral phase plates and plane waves has been studied. Fingerlike interference patterns were obtained with a doughnut beam tilted from a Gaussian beam; spiral fanlike patterns were obtained with a doughnut beam and a Gaussian beam collimated coaxially. The experimental results are supported qualitatively by simulation. By rotating a glass slide in the path of the Gaussian beam, one can rotate the fanlike interference pattern in a controlled fashion. With the liquid-crystal display technology that we have developed and report here, these liquid-crystal spiral phase plates should find applications in optical tweezers.

Antiguiding in microstructured optical fibers

Antiguiding, as opposed to positive index-contrast guiding (or index-guiding), in microstructured air-silica optical fibers is shown to have a significant influence on the fiber's transmission property, especially when perturbations exist near the defect core. Antiguided modes are numerically analyzed in such fibers by treating the finite periodic air-silica composite (including the central defect) as the core and outer bulk silica region as the cladding. Higher-order modes, which can couple energy from the fundamental mode in the presence of waveguide irregularities, are predicted to be responsible for high leakage loss of realistic holey fibers. The modal property of an equivalent simple step-index antiguide model is also analyzed. Results show that approximation from a composite core waveguide to a simple step-index fiber always neglects some important modal characteristics.

Thermally tunable narrow-bandpass filter based on a linearly chirped fiber Bragg grating

We propose a new method for the development of a tunable optical bandpass filter (TOBF) based on a linearly chirped fiber Bragg grating (LCFBG). A NiCr wire heater is used to heat the LCFBG at a small point to introduce a narrow passband within the stop band of the LCFBG. The central wavelength of the passband is tuned by scanning the wire heater along the LCFBG. As an example demonstrating the effectiveness of the proposed method, we demonstrate a TOBF with a very small 3-dB bandwidth of approximately 7 pm, a tuning range of 16.4 nm, and a rejection ratio of more than 25 dB. Compared with previously reported tunable-fiber-based bandpass filters, this method provides the advantages of a large tuning range, continuous tunability, a switchable passband, a simple tuning mechanism, low cost, and narrow bandwidth.

Effects of active fiber length on the tunability of erbium-doped fiber ring lasers

We numerically investigate the effects of the active fiber length on the tunability of erbium-doped fiber ring lasers for the cases of with and without pair-induced quenching (PIQ). The numerical results are confirmed by experiments. We have found that the tuning range shifts from C-band to L-band with an increase in the active fiber length. A maximum tuning range of over 100 nm, covering both the C- and L-band, can be achieved with an optimized active fiber length. It is also found that the PIQ is favorable for Lband lasing though it reduces the output power and degrades the power flatness. Using these findings, a novel method employing active fiber length switching is proposed to extend the tuning range of the laser, which is only limited by the free spectral range (FSR) of the tunable filter. A large tuning range of 102 nm is obtained using a tunable fiber Fabry-Perot filter with an FSR of 75 nm.

Wide bandwidth flat gain Raman amplifier by using polarization-independent interferometric filter

We propose a method to flatten Raman gain by using a polarization independent filter. It is formed by a fiber loop mirror contains a number of high birefringence fiber sections and polarization controller. The gain profile is observed for one and two sections of polarization maintaining fiber (PMF) with different lengths. By using single pump wavelength at 1455 nm and 5cm length of Hi-Bi FLM, we managed to flatten Raman gain to within +/-0.5 dB over a bandwidth of 43 nm at the center wavelength of 1555 nm. By using two sections of Hi-Bi Fiber and longer PMF, which are 5 cm and 8.5 cm PMF in Hi-Bi FLM, we are able to flatten Raman gain to within +/-0.5 dB over a bandwidth of 38 nm at the center wavelength of 1555 nm with lower gain suppression from its original gain profile.

A largely tunable CFBG-based dispersion compensator with fixed center wavelength

A largely tunable chirped fiber Bragg grating (CFBG)-based dispersion compensator with fixed center wavelength is demonstrated. Tunable dispersion ranging from 178 to 2126 ps/nm, corresponding to a large range of 3-db bandwidth from 0.42 to 5.04 nm, is realized by using a 10 cm-long CFBG with an original bandwidth of 1.61 nm. The variation in center wavelength is less than 0.2 nm.

660GHz Solitons Source Based on Modulation Instability in Short Cavity

Ultrashort soliton source, with very high repetition rate, is a necessity for next generation optical fiber communication. A very simple passively mode-locked fiber ring laser centered at 1566 nm, which is based on the modulation instability (MI) theory, is proposed. Using high Erbiumdoped (6470ppm) Bismuth fiber, a soliton pulse train, with 660 GHz repetition rate and pulse width of 420 fs, has been observed in a 11.5m short cavity for the first time.

Design of double-pass discrete Raman amplifier and the impairments induced by Rayleigh backscattering

We report on the investigation of discrete Raman fiber amplifier in double-pass configuration based on the dispersion-compensated fiber and high reflection FBG. We proved in simulation and experiments that the double-pass configuration requires nearly 50% less pump power and the same fiber length to provide the same Raman gain and double-dispersion-compensation performance compared to the typical counter-pumped Raman amplifier. We also analyzed the equivalent noise figure (NF) and the Rayleigh backscattering impairments. The theoretical results shown that the impact of multipath interference (MPI) noise is the dominating limitation factor of this system operated at very high Raman gain region.

Design and analysis of VCSEL based twodimension wavelength converter

A novel two-dimensional vertical cavity surface emission laser (VCSEL) based wavelength converter is proposed. We developed a twodimensional transmission line laser model (TLLM) to analyze the proposed wavelength converter. This model takes into account Bragg reflectors by using the modified connecting matrix. Therefore, accurate and efficient modeling of the VCSEL structure is achieved. Extinction ratio of the output signal is investigated by considering input signal power, wavelength, facet reflectivity and cavity diameter.