Inherently gain flattened S-band erbium doped fiber amplifier based on dual core resonant leaky fiber

A co-axial dual core resonant leaky optical fiber (DCRLF) is designed for inherent gain equalization of S-band erbium doped fiber amplifier (EDFA). Resonance tail of leakage loss of the fiber into the S-band region is utilized to flatten the gain. We have numerically studied the effect of various design parameters and their fabrication tolerances on gain flattening. We show 23.5 dB flat gain with ± 0.9 dB ripple over 30 nm bandwidth (1490–1520 nm) using 120 mW pump. The study should be useful in designing optical fiber amplifiers for optical communication system employing wavelength division multiplexing.     

1.3-μm InGaAsP planar buried heterostructure laser diodes with AlInAs electron stopper layer

This study reports on the realization of 1.3-μm InGaAsP buried-heterostructure (BH) laser diodes (LDs) via an Fe-doped semi-insulating InP layer and an AlInAs electron stopper layer (ESL). Experimentally, the as-cleaved BH LD with an AlInAs ESL exhibited improved characteristics in terms of threshold current, slope efficiency, and maximum light output power at 90 °C as compared to those of the normal BH LD without an AlInAs ESL. In addition, high internal quantum efficiency or reduced threshold current density was observed, indicating increased modal gain in BH LDs fabricated with an AlInAs epilayer on top of the active region. It was also found that the temperature sensitivity of the BH LDs with an AlInAs ESL is more stable than that of the normal BH LDs. These results could be attributed to the suppression of thermal carrier leakage out of strain-compensated multiple-quantum-well by a large conduction-band offset of the AlInAs/InGaAsP heterojunction. Otherwise, without consideration of damping factor or coupling loss, the 3-dB bandwidth of the proposed BH LDs reaches a high value of 15.3 GHz. Finally, this TO-can packaged BH LD shows an eye-opening feature with the extinction ratio of 7.49 dB while operating at 10 Gbit/s at 50 mA.     

Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers

We propose refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers. The surface plasmon resonance modes and the sensing properties are theoretically analyzed using finite element method (FEM). In the calculation, Drude–Lorentz model is used to describe the Metal Dielectric constant. The calculation results show that the sensitivity of Ag-metalized SPR sensor can reach 1500 nm/RIU corresponding to a resolution of 6.67 × 10^?5 RIU. Comparing with conventional detecting material-Au under the same structure, the sensitivity and 3 dB bandwidth of our device are better.     

Dual-stage L-band erbium-doped fiber amplifier with distributed pumping from single pump laser

A dual-stage L-band erbium-doped fiber amplifier with a flat gain bandwidth over 36 nm is demonstrated using pump distribution technique. The pump power was distributed to two stages depending on the splitting ratio and the length of erbium-doped fiber that was used for this configuration. Both parameters are the key components for achieving a substantially flat gain response throughout the L-band region ranging from 1570 nm to 1605 nm. Although the input signal power was varied from ? 30 dBm to 0 dBm, gain of 17 dB with slight variations of less than 1.5 dB and a noise figure of less than 6.7 dB were achieved. All the results obtained show better performances when comparison was made with the conventional single-stage L-band optical amplifier.     

Broadband and low amplitude noise supercontinuum generated by using compressed pulse

In this paper, enhancement of bandwidth of supercontinuum generated in a normal dispersion-flattened microstructured fiber by using compressed pulse is demonstrated experimentally and numerically. Using high-order soliton compression effect, the standard single mode fiber is used as a pulse compressor. The experimental measured ?10 dB spectral width is broadened from 75 nm to more than 140 nm by adding a 20 m long standard single mode fiber. Numerical analysis shows that using pulse compressed by a certain length fiber can increase the spectral bandwidth without making extra amplitude noise.     

Filter-free ultra-wideband doublet pulses generation based on wavelength conversion and fiber dispersion effect

Filter-free ultra-wideband (UWB) doublet pulse generation is experimentally demonstrated in the optical domain based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) and UWB-over-fiber technology is implemented by exploiting dispersion-induced pulse broadening effect in single-mode fiber (SMF). In our proposed system, the SOA generates a polarity-inversed Gaussian pulse train with respect to the injected one through the XGM. After a piece of SMF, the bandwidth of polarity-reversed Gaussian pulse broadens due to the induced dispersion. After the combination of the two light waves with a suitable time delay between them, UWB pulse is obtained. The key parameters for UWB pulse, including central frequency, 10 dB bandwidth, as well as fractional bandwidth are experimentally obtained with 8 GHz, 9.9 GHz and 123% respectively. The generated UWB doublet pulse conforms with the UWB definition of Federal Communications Commission (FCC).     

A miniaturized HTS microwave receiver front-end subsystem for radar and communication applications

This paper presents a miniaturized high performance high temperature superconducting (HTS) microwave receiver front-end subsystem, which uses a mini stirling cryocooler to cool a high selective HTS filter and a low noise amplifier (LNA). The HTS filter was miniaturized by using specially designed compact resonators and fabricating with double-sided YBCO films on LAO substrate which has a relatively high permittivity. The LNA was specially designed to work at cryogenic temperature with noise figure of 0.27 dB at 71 K. The mini cryocooler, which is widely used in infrared detectors, has a smaller size (60 mm × 80 mm × 100 mm) and a lighter weight (340 g) than the stirling cryocoolers commonly used in other HTS filter subsystem. The whole front-end subsystem, including a HTS filter, a LNA, a cryocooler and the vacuum chamber, has a size of only φ120 mm × 175 mm and a weight of only 3.3 kg. The microwave devices inside the subsystem are working at 71.8 K with a consumed cooling power of 0.325 W. The center frequency of this subsystem is 925.2 MHz and the bandwidth is 2.7 MHz (which is a fractional bandwidth of 0.2%), with the gain of 19.75 dB at center frequency and the return loss better than ?18.11 dB in the pass band. The stop band rejection is more than 60 dB and the skirt slope is exceeding 120 dB MHz^?1. The noise figure of this subsystem is less than 0.8 dB. This front-end subsystem can be used in radars and communication systems conveniently due to it’s compact size and light weight.     

Crystallographic orientation-dependent optical properties of GaInSb mid-infrared quantum well laser

The optical properties of compressively strained GaInSb/GaInAlSb quantum well (QW) laser are numerically studied in different crystal orientations solving envelope function equation using finite difference method. The simulation results demonstrate that there is a strong correlation between the optical gain and its emission wavelength with crystal orientation of the QW. The maximum and minimum optical gains are evaluated in the (1 1 3) and (1 1 1) crystal orientations, respectively, due to band mixing effects. The peak emission wavelength can be tuned from 2.4 μm to 2.25 μm by changing the crystal orientation from (1 1 0) to (1 1 1). Typical optical gains are evaluated 3115, 3080, 2790, 3415, and 2940 cm^?1 in (0 0 1), (1 1 0), (1 1 1), (1 1 3), and (1 3 1) crystal orientations, respectively, when the injection carrier density is 3.5 × 10¹⁸ cm^?3. The highest optical power and lower threshold current are obtained in (1 1 3) orientation for the number of quantum wells three.     

Numerical analysis of a direct UV-written 1 × 8 double smooth octagon microrings resonant wavelength demulti/multiplexer

A microring resonant wavelength demulti/multiplexer (MRRWDM) based on UV-written technology is designed. By using a double smooth octagon microrings structure, a 1 × 8 device around the central wavelength of 1550.918 nm with the wavelength spacing of 1.4 nm is presented. Analytical results based on coupled mode theory show that the 3 dB bandwidth is about 0.22 nm, the insertion loss is less than 0.7 dB, and the crosstalk is below ?47 dB for every output channel of the designed device without tolerances.     

Supercontinuum broadening in all-normal dispersion photonic crystal fiber by means of soliton compression in standard single-mode fiber

Using standard single-mode fiber as high-order soliton compressor for broadening supercontinuum in an 80 m long all-normal dispersion photonic crystal fiber is investigated experimentally and numerically. An analytical formula for calculating proper fiber input power to generate the broadest supercontinuum is derived. The numerical results show that the formula is more accurate in high power level corresponding to the soliton order which is larger than two. The measured supercontinuum ? 20 dB bandwidth is broadened from 84.2 nm to 277.1 nm by using a 20 m long standard single-mode fiber without enhancing fiber input power. Numerical calculations of the amplitude noise in the output spectra show that using soliton compression effect can efficiently broaden the spectral bandwidth and not generate obvious noises.     

Multifrequency super-thin cloaks

Bandwidth and thickness have become the most troublesome problems for EM cloaks. In this paper, we propose to solve the two problems using connected patches based on the microwave network model. By covering an obstacle with combined connected patches, cloaking effect can be achieved at multiple frequencies so as to expand the operating band. As an explicit example, a dual-band super-thin cloak using two different connected patch unit cells is demonstrated. Cloaking effect can be achieved at 3.50 GHz and 4.14 GHz simultaneously with an 8 dB transmission enhancement. The cloak design method provides a new route to broadening the bandwidth of thin EM cloaks.     

V-band directional tandem coupler of monolithic uniplanar structure

We present a uniplanar coplanar-waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or bonded structures. Due to an optimized tandem structure and non-bonded crossovers minimizing the parasitic components, a maximum coupling of 2.5 dB is measured at 62 GHz with a 2 dB bandwidth of 83%, while a high directivity factor of 33 dB is simultaneously obtained at 58–62 GHz. Over the entire design frequency range of 30–90 GHz, we achieve good phase unbalance of 90 ± 6.0°, as well as return loss and isolation lower than −23 and −16 dB, respectively.     

Novel optical add-drop multiplexer based on dual racetrack resonators

In this paper, we designed and fabricated a four-channel optical add-drop multiplexer (OADM) based on dual racetrack resonators. The size of the fabricated device is only 2400 μm × 500 μm. The fabricated device can effectively and perfectly realize the signals upload and download. The free spectral range (FSR) of OADM is about 15.2 nm. We take the spectral responses near 1555 nm as an example. When the device acts as an optical drop multiplexer, the minimum insertion loss is 4.481 dB and the maximum extinction ratio is 31.931 dB. The maximum adjacent channels crosstalk is -9.845 dB. When the device acts as an optical add multiplexer, the minimum insertion loss is 0.944 dB and all of the extinction ratios are bigger than 25 dB. The maximum crosstalk is -16.531 dB which indicates the crosstalk can be neglected.     

Tunable multi-wavelength fiber laser based on a polarization-maintaining erbium-doped fiber and a polarization controller

A stable and tunable multi-wavelength fiber laser with a polarization-maintaining erbium-doped fiber (PM-EDF) and a polarization controller (PC) is proposed and demonstrated. A homemade PM-EDF incorporated in the ring cavity is used as the gain medium. Simultaneous multi-wavelength oscillation is achieved at room temperature. The theory of the PM-EDF and PC to suppress the wavelength competition is described in detail. The 3 dB bandwidth is less than 0.01 nm. The power fluctuation and wavelength shift are measured to be less than 0.5 dB and 0.05 nm over 32 min. The wavelength tuning between single-, double-, triple-, and four-wavelength is realized.     

A filterless optical millimeter-wave generation based on frequency octupling

A novel method of a filterless optical millimeter-wave (MMW) signal generation with frequency octupling using four nested Mach–Zehnder modulators (MZMs) is proposed for Radio-over-fiber systems. By symmetrically biasing the MZMs and using two RF driving signals with 90 deg phase delay, a cost-effective, high-quality and filterless optical millimeter-wave at 80 GHz with an optical harmonic distortion suppression ratio exceeding 40 dB is obtained. The proposed system is insensitive to the MZM bias drift, which demonstrates a relatively higher stability. So it is a viable solution for the future ultra-high frequency MMW applications.     

Optical millimeter-wave signal generation by frequency quadrupling using one dual-drive Mach–Zehnder modulator to overcome chromatic dispersion

We propose a novel approach to generate quadrupling-frequency optical millimeter-wave using a dual-drive Mach–Zehnder modulator (MZM) in radio-over-fiber system. By properly adjusting the phase difference in the two modulation arms of MZM, the direct current (DC) bias, the modulation index and the gain of base-band signal, the quadrupling-frequency optical millimeter-wave with signal only carried by one second-order sideband is generated. As the signal is transmitted along the fiber, there is no time shift of the codes caused by chromatic dispersion. Theoretical analysis and simulation results show that the eye diagram keeps open and clear even when the quadrupling-frequency optical millimeter-wave are transmitted over 110 km and the power penalty is about 0.45 dB after fiber transmission distance of 60 km. Furthermore, due to another second-order sideband carrying no signals, a full duplex radio-over-fiber link based on wavelength reuse is also built to simplify the base station. The bidirectional 2.5 Gbit/s data is successfully transmitted over 40 km standard single mode fiber with less than 0.6 dB power penalty in the simulation.     

Improving the characteristics of the modulation response for fiber Bragg grating Fabry–Perot lasers by optimizing model parameters

A unified and comprehensive study on the small-signal intensity and frequency modulation characteristics of a fiber Bragg grating Fabry–Perot (FBG–FP) laser are numerically investigated. The effect of injection current, temperature, external optical feedback (OFB), nonlinear gain compression factor, fiber grating (FG) parameters and spontaneous emission factor on modulation response characteristics are presented. The rate equations of the laser model are presented in the form that the effect of temperature (T) and external optical feedback (OFB) are included. The temperature dependence (TD) of laser response is calculated according to the TD of laser cavity parameters instead of directly using the well-known Parkove equation. It is shown that the optimum external fiber length (L_ext) is 3.1 cm and the optimum range of working temperature for FGFP laser is within ±2 °C from the FBG reference temperature (T_o). Also, the antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the modulation spectra. It is also show that modulation response is extremely sensitive to the OFB level, high injection current and gain compression factor. The study indicates clearly that good dynamic characteristic can be obtained by system parameters optimization.     

Influence and correction of temperature on optical measurement for fat and protein contents in a complex food model system

The influence of temperature on optical measurements has been studied for determining fat and protein contents in complex food systems. A model system consisting of mixtures of fat, protein, water and emulsion was developed to create an imitation of complex food systems. The changes in optical properties of the system from 25 °C to 40 °C were measured in the wavelength from 1100 nm to 1670 nm. Irregular changes as a whole were founded and therefore a statistical method was needed to correct the temperature effect. A method called global robust temperature calibration model is proposed and the correction effect was validated. The results indicated that it can significantly reduce the temperature effect on optical measurement.     

Performance evaluation of 64 × 10 Gbps and 96 × 10 Gbps DWDM system with hybrid optical amplifier for different modulation formats

In this paper, we investigated the performance of multi terabits DWDM system consisting of hybrid optical amplifier RAMAN-EDFA for different data format such as non-return to zero (NRZ), return to zero (RZ) and differential phase shift keying (DPSK). We find that in 64 × 10 and 96 × 10 Gbps, RZ is more adversely affected by nonlinearities, where as NRZ and DPSK is more affected by dispersion. We further show that RZ provide good quality factor (13.88 dB and 15.93 dB for 64 and 96 channels), less eye closure (2.609 dB and 3.191 dB for 64 and 96 channels) and acceptable bit error rate (3.89 × 10⁸ and 1.24 × 10⁹ for 64 and 96 channels) at the respective distance as compare to other existing modulation format. We further investigated the maximum single span distance covered by using existing data formats.     

A spectroscopic ellipsometry study of TiO2 thin films prepared by ion-assisted electron-beam evaporation

Film characterization based on variable-angle spectroscopic ellipsometry (VASE) is desirable in order to understand physical and optical characteristics of thin films. A number of TiO₂ film samples were prepared by ion-assisted electron-beam evaporation with 200-nm nominal thickness, 2.0 Å/s deposition rate and 8 sccm oxygen flow rate. The samples were maintained at 250 °C during the deposition, and annealed in air atmosphere afterwards. As-deposited and annealed films were analyzed by VASE, spectrophotoscopy and X-ray diffractometry. From ellipsometry modeling process, the triple-layer physical model and the Cody–Lorentz dispersion model offer the best results. The as-deposited films are inhomogeneous, with luminous transmittance and band gap of 62.37% and 2.95 eV. The 300 °C and 500 °C are transition temperatures toward anatase and rutile phases, respectively. Increasing temperature results in an increase of refractive index, transmittance percentage and band gap energy. At 500 °C, the highest refractive index and band gap energy are obtained at 2.62 and 3.26 eV, respectively. The developed VASE-modeling process should be able to characterize other TiO₂ films, using similar physical and optical modeling considerations.