Modeling of a two-color, two-stage, ultrafast Yb-doped fiber amplifier

We have modeled a two-stage, Yb-doped fiber amplifier system to amplify two-color ultrafast laser pulses in near-infrared. The first and second stages namely preamplifier and power amplifier are single-clad, single-mode and double-clad, single-mode Yb-doped fibers respectively. From numerical simulations of the modeled fiber amplifiers, the optimal lengths of the two fibers to have equal power at two colors centered at 1035 nm and 1105 nm are in agreement with our experimental results. Numerical simulations have also been performed to demonstrate the impact of using a flat-top, two-color seed spectrum on the system performance.     

Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

We report a simple fabricating approach to control the mode couplings in long-period fiber gratings (LPFGs) through side exposing fiber to long-pulse-10.6-μm laser from a cheap, internally modulated CO₂ tube. By tuning focused-spot size on fibers, not only circularly symmetric mode coupling but also asymmetric mode couplings can be effectively achieved. Simulation of mode profiles in grating cross-section with Finite Element Method (FEM), and LPFG-cladding etching experiment with hydrofluoric acid (HF), support our explanation that asymmetric mode coupling in LPFGs depends on local refractive-index (RI) change within an azimuthally thin cladding layer, resulted from large-spot method induced deep melt flow on fiber surface during CO₂ laser irradiation.     

Simultaneous measurement of strain and temperature with a long-period fiber grating inscribed Sagnac interferometer

A Sagnac interferometer with a long-period fiber grating (LPG) inscribed in the polarization-maintaining fiber (PMF) is proposed and experimentally demonstrated for simultaneous measurement of strain and temperature. Due to the different responses of the LPG and the Sagnac interferometer to strain and temperature, simultaneous measurement can be achieved by monitoring the wavelength shifts and the intensity changes of a resonance dip of the sensor setup. The experimental results show that the achieved sensitivities to strain and temperature are 6.4 × 10^− 3 dB/με and 0.65 nm/°C, respectively.     

Design of Topas microstructured fiber with ultra-flattened chromatic dispersion and high birefringence

A microstructured polymer optical fiber (mPOF) with both ultra-flattened near-zero chromatic dispersion and high birefringence based on Topas cyclic olefin copolymer is designed. Three rings of uniform elliptical air holes are arranged in triangular lattice in the cladding and an extra small defected hole is introduced in the fiber core. Guided modes, dispersion, birefringence and mode confinement properties of the designed mPOF are investigated by using the full-vector finite element method. Dispersion values between ± 0.5 ps/km/nm over the wavelength 1.1-1.7 μm and high birefringence of the order of 10⁻³ are obtained for the optimized fiber structure. Low confinement losses and small effective mode area are obtained at the same time. The relatively simple architecture of the proposed Topas mPOF can be fabricated by our extrusion-stretching techniques.     

Simultaneous measurement of curvature and temperature based on PCF-based interferometer and fiber Bragg grating

A sensor head consisting of a photonic crystal fiber (PCF)-based Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. The MZI fabricated by splicing a short length of PCF between two single-mode fibers with the air-hole structure that completely collapsed near the splicing points, is sensitive to fiber bending and surrounding temperature, while the FBG is only sensitive to the later. Simultaneous measurement of curvature and temperature is therefore obtained. Sensitivities of 4.06 nm/m^− 1 and 6.30 pm/°C are achieved experimentally for curvature and temperature, respectively. And the corresponding resolutions are 5.2 × 10^− 4 m^− 1 and 1.25 °C for curvature and temperature, respectively, based on the wavelength measurement resolution of 10 pm.     

Impact of channel baud-rate on logarithmic digital backward propagation in DP-QPSK system with uncompensated transmission links

We numerically investigate the impact of channel baud-rate on the performance of logarithmic step-size based spilt-step Fourier method (SSFM). This algorithm is used to implement digital backward propagation (DBP) to efficiently compensate fiber chromatic dispersion (CD) and non-linearities (NL). The DBP method is implemented in N-channel dual-polarization quadrature phase shift keying (DP-QPSK) transmission over 2000 km standard single mode fiber (SMF) with no in-line optical dispersion compensation. We investigate the same-capacity and same-bandwidth transmission systems with 56 Gbit/s/ch (14 GBaud), 112 Gbit/s/ch (28 GBaud) and 224 Gbit/s/ch (56 GBaud). Each system has the bandwidth occupancy of 500 GHz with a total transmission capacity of 1.12 Tbit/s. Moreover, we have also compared the multiple channel transmission performance with single channel transmission to quantify the impact of inter-channel (cross-phase modulation ‘XPM’ and four-wave mixing ‘FWM’) and intra-channel (self-phase modulation ‘SPM’) non-linearities. The logarithmic step-size based DBP algorithm (L-DBP) depicts efficient mitigation of CD and NL impairments. The benefit of the logarithmic step-size is the reduced complexity and computational time for higher baud-rate transmission systems.     

A simple technique for evaluation of transmission coefficient at the splice with angular offset and transverse offset for first higher mode propagation in graded index fibers

We present simple and accurate analytical expressions for transmission coefficient at the splice with angular offset and transverse offset in case of first higher mode propagation in graded index fibers. We employ the simple series expression for first higher order modal field for graded index fiber. The evaluation of the concerned parameters based on our formalism requires very little computations. Here, we show that our estimations match excellently well with the exact results in case of step and parabolic index fibers. Further, splice being highly tolerant for longitudinal separation, our analysis is judiciously restricted to the case of transverse and angular mismatches only. The analysis should find application in dual mode graded index fiber in all optical technology.     

Concentration and refractive index sensor for methanol using short period grating fiber

This paper reports on the spectral (response) characteristics of fiber Bragg grating sensor to various methanol concentrations. A complete experimental analysis on the use of Ge-B co-doped fiber Bragg grating sensor for a concentration and refractive index measurement is presented. The spectral changes in the grating response due to variation of concentration have been investigated in relation to new sensing configurations. The results also highlight the possibilities of using the sensor for medical and biological applications.     

Erbium doped fiber ring laser for optical wavelength conversion

We experimentally investigate the gain saturation effect of a piece of 8 m long Erbium doped fiber (EDF), we introduce a theoretical model for the EDF ring loop, and our simulation results show very good cross-gain modulation (XGM) and wavelength conversion. We also experimentally investigate the XGM in an EDF ring loop system. Based on the study of the XGM in the EDF ring loop system, a wavelength conversion is designed with the EDF ring loop system. The EDF ring loop systems as a wavelength conversion is experimentally demonstrated by converting a sinusoidal modulated optical signal at wavelength of 1551 nm to an optical signal at wavelength of 1553.3 nm.     

Simulative investigation the impact of optical amplification techniques on single- and dual-electrode MZM and direct modulator's in single-tone RoF system

Since the introduction of erbium-doped fiber amplifier (EDFA) in optical fiber communication systems at 1550 nm, the fiber loss is no longer considered as the limiting factor but the fiber dispersion and chirp parameters play an important role in degrading the performance of optical communication systems. Further, the invention of external modulators of Mach-Zehnder (MS) type minimizes the transmission power penalty due to fiber chromatic dispersion as these modulators are consider almost chirp-free modulators. Also, better suppression of nonlinear distortions consists of harmonic distortions (HDs) and intermodulation distortions (IMDs) were achieved by using Mach-Zehnder external modulator [8]. In this work, a simulation comparison of external- and direct-modulation schemes using different optical amplifiers; SOA and EDFA in single-tone RoF system is reported. A single-tone channel of 20 GHz is transmitted over single-mode fiber (SMF) either externally modulated by using single-electrode (SEMZM) - and dual-electrode (DEMZM) - Mach-Zehnder modulator or by direct-modulation scheme. Better improvements in the measurement of received RF power of a 20 km linked single-tone RoF system are achieved by using SEMZM with EDFA, which means maximum optical link is achieved between Central Office (CO) and Base Station (BS). Further, this scheme provides better BER as compare to other schemes discussed in this paper.