A theoretical model used to analyze the output characteristics of fiber grating external cavity semiconductor lasers

Using the ray trace method, the expression of the output spectrum of the fiber grating external cavity semiconductor laser (FGESL) has been deduced and the fine structure of the output spectrum has been investigated. Due to the fiber grating (FG) external cavity, within the reflection bandwidth of the FG, the energy distribution varies seriously, there exist many peaks and the number of peaks is related to the length of the external cavity. With increase in the bias current, the output power and side-mode suppression ratio (SMSR) of FGSEL tend to increase accompanied by fluctuation, and the fluctuation will weaken with the decrease in the reflectivity of the front facet of the semiconductor laser.     

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.     

Flat and compact switchable dual wavelength output at 1060 nm from ytterbium doped fiber laser with an AWG as a wavelength selector

A dual-wavelength ytterbium doped fiber laser with a narrowest spacing of 0.53 nm and widest spacing of 12.2 nm at 1064 nm is presented in this paper. An arrayed waveguide grating (AWG) together with an optical channel selector (OCS) have also been incorporated in the proposed setup that works as a switchable mechanism giving 23 different wavelength tunings. Producing an average output power of ?8 dB m and side mode suppression ratio (SMSR) of 59.65 dB, this dual-wavelength fiber laser is quite stable with an output power variance as low as 0.47 dB giving it an advantage due to its switching ability and stable dual-wavelength output powers.     

THz optical pulses from a coupled-cavity quantum-dot laser

Optical pulses are generated from a coupled-cavity quantum-dot (QD) laser consisting of a short QD-waveguide Fabry–Perot (F–P) cavity and three long external fiber Bragg grating (FBG) cavities. When the laser is biased at low operation current, the feedback from the external cavities dominates and laser pulses have a 1.01 THz repetition rate, determined by the equal frequency difference of the three FBGs. We are thus able to decouple the repetition rate of a mode-locked laser from the cavity length. With much higher bias current, the QD F–P cavity dominates and the repetition rate is switched to 43.8 GHz, defined by the length of the F–P cavity.     

An external cavity diode laser using a volume holographic grating

This study presents an external cavity diode laser (ECDL) system, utilizing a volume holographic grating (VHG) and a microfabricated silicon flexure as the VHG holder. The laser design is aimed for easy assembly, controllability, and better stability of the laser cavity. The laser frequency was stabilized to a D₂ transition of rubidium at 780.247 nm, with a mode-hop-free tuning range of 16 GHz and 9.6 GHz with and without feed-forward on the diode injection current. The measured linewidth was 850 kHz in 500 s, qualified for laser cooling experiments.     

Multi-wavelength fiber ring laser based on a sampled chirped fiber Bragg grating with a hybrid gain medium

The novel multi-wavelength fiber ring laser based on a sampled chirped fiber Bragg grating with a single SOA or a hybrid gain medium is demonstrated, respectively. In case of a hybrid gain medium, the SMSR improvement of ~ 5 dB was obtained when compared with the system with a single SOA. The proposed fiber laser offers advantages such as simple structure, low loss, multi-wavelength lasing lines with moderate output power.     

High-power tunable Er,Yb ribbon fiber laser

A high-power Er,Yb double-clad ribbon fiber laser pumped by a 9-diode-bar pump module is reported. The laser yielded 102 W of continuous-wave output at 1566 nm for a launched pump power of 244 W, corresponding to a slope efficiency of ~ 44% with respect to launched pump power. Tunable operation was achieved using a simple external feedback cavity with a diffraction grating and the operating wavelength could be tuned from 1533 nm to 1567 nm. Temperature distribution in the ribbon fiber geometry and prospects of power scaling will be discussed.     

An energy-efficient tie-type architecture for stable and wavelength-tunable SOA-based fiber laser

A new and energy-efficient tie-type architecture for stable and wavelength-tunable semiconductor optical amplifier (SOA)-based fiber ring laser is proposed and experimentally investigated. Here, the tie-type laser configuration is constructed by two Sagnac fiber loops. The proposed laser also can extend the lasing wavelength to longer wavelength (L-band) even only the C-band SOA is used. The proposed tie-type architecture has >5 dB higher output optical power at bias current of 80 mA when compared with the single ring SOA-based fiber laser. In this measurement, the output power, wavelength tuning range, side-mode suppression ratio (SMSR) and output stability of proposed fiber laser have also been analyzed and discussed.     

Tunable dual-wavelength fiber laser based on an opto-VLSI processor and four-wave mixing in a photonic crystal fiber

A stable wavelength and wavelength spacing tunable dual-wavelength fiber laser based on an Opto-very-large-scale-integration (Opto-VLSI) processor and four-wave mixing (FWM) in a high-nonlinear photonic crystal fiber is experimentally demonstrated. The results show that the line width of the tunable dual-wavelength fiber laser is 0.02 nm, and the wavelength spacing can be tuned from 0.8 nm to 4 nm with a 0.15 nm step. Under the influence of the FWM, the uniformity is below 0.6 dB and the measured side mode suppression ratio (SMSR) is above 45 dB.     

Resolution and sensitivity enhancements in strong grating based fiber Fabry–Perot interferometric sensor system utilizing multiple reflection beams

We investigate an asymmetric intensive fiber Bragg grating (FBG) defined Fabry–Perot (F–P) sensor system decoded by a multiple-path-matched Michelson interferometer. The interrogation of higher order reflection beams cannot only solve the problem of the degraded resolution induced by the spectral mismatch of the FBGs, but also amplify the effect of the fiber strain on the phase of the light. We demonstrate multiple reflection beams in the F–P cavity based on the concept of the FBG effective length for constructing respective interrogation interferometers, and present a cost function with optimized system parameters to improve noise properties. The performances of interrogating the second, third and fourth order reflection beams are compared in a strain sensing experiment arrangement. Under the condition of the same optical path length mismatch, the interrogation of the fourth order reflection beam can achieve 9.8 dB sensitivity enhancement and 3 dB resolution promotion compared with the result using the second order reflection beam.     

Electric field controlled higher-order diffraction images of paraelectric potassium lithium tantalate niobate

We report some electric field controlled photorefractive higher-order diffraction phenomena of a paraelectric phase potassium lithium tantalate niobate crystal doped with iron. In experiments, a p-polarized semiconductor laser (532 nm) was used to record grating at a small incident angle. Higher-order diffraction images were observed when the signal beam was focused behind and in front of the crystal. Then the higher-order diffraction images were reconstructed by a p-polarized He–Ne laser (632.8 nm) in the direction perpendicular to the surface. The higher-order diffraction images could be controlled by the external electric field. A theory about the higher-order diffraction images of the K and 2K grating is developed. The results show that the even order diffraction images of the K grating and the odd order diffraction of the 2K grating overlap each other. The odd order diffraction images of the K grating are diffracted in unattached direction. The electric field controlled higher-order diffraction image provides a useful method for optical information processing.     

Design of a compact polarization splitter based on the dual-elliptical-core photonic crystal fiber

We propose a compact polarization splitter based on dual-elliptical-core photonic crystal fiber. Two elliptical cores are introduced to increase the difference of effective index between x-polarized and y-polarized mode and three elliptical modulation air holes are used to control the power transfer between the two cores. By optimizing the structure parameters, the length of the polarization splitter is distinctly shortened. Numerical results demonstrate that the compact splitter has the length of 775 μm and up to 50 dB extinction ratio at the central wavelength of 1.55 μm. The corresponding bandwidth of 32 nm could be achieved from the wavelength of 1.534–1.566 μm with the extinction ratio over 20 dB     

Incident angle and temperature dependence of WSi wire-grid polarizer

The dependences of the incident angle and thermal durability of a tungsten silicide (WSi) wire-grid polarizer were examined. A WSi grating with a 0.5 fill factor, 260 nm depth, and 400 nm period was formed on a Si surface using two-beam interference and dry etching. The TM transmission spectrum of the fabricated element was greater than 60% at the incident angle of θ = 40° (the angle between the incident direction and the perpendicular axis to the grating direction) in the 4–10 μm wavelength range. An extinction ratio of 22.2 dB was achieved at 2.5 μm wavelength. Additionally, results show that this polarizer has higher thermal resistance than that of commercial infrared polarizers. Therefore, this polarizer is effective for taking a polarized thermal image of high temperatures.     

Modulation of output power in the spatio-temporal analysis of a semi conductor laser

This paper examines the spatio temporal patterns of a set of a (2 + 1) dimensional partial differential equations (PDE) describing the dynamics of semiconductor external cavity THz lasers. The study is conducted in two phases: (a) first we show that the behaviour of the single mode system may be periodic or chaotic; (b) the output power may be optimized by proper choices of parameters, such as cavity length and injected current, which lead to high dimensional spatio-temporal chaotic regimes. This conclusion applies to systems with one and with two external cavities, and is robust with respect to variations of parameter values and initial-boundary conditions.     

Design and VNA-measurement of coplanar waveguide (CPW) on benzocyclobutene (BCB) at THz frequencies

The low permittivity and the low loss tangent of the benzocyclobutene polymer (BCB) offers to coplanar waveguides (CPW) a low dispersive propagation properties at THz frequency. These transmission lines have been designed, modeled with a three dimensional (3D) solver of Maxwell equations based on finite element method (FEM) from 20 to 1000 GHz at various characteristic impedances (Z_c). Their dispersion and losses (radiation, conduction and dielectric) have been investigated separately versus the waveguide size, the nature of the substrate (dielectric or semiconductor) to optimize the THz signal propagation. Monomode CPW on BCB numerically designed for various Z_c were realized and measured with vector network analyzer (VNA). S-parameters of CPW are de-embedded by optimization of the accesses’ model. A good agreement is found between experimental and numerical results with low attenuation constants of 2.7 dB/mm and 3.5 dB/mm at 400 GHz and 500 GHz, respectively.     

Coupled frequency-doubling optoelectronic oscillator based on polarization modulation and polarization multiplexing

A coupled frequency-doubling optoelectronic oscillator (OEO) is proposed and experimentally demonstrated, which is constructed based on the perfect combination of polarization modulation and polarization multiplexing. A fundamental microwave signal at 9.95 GHz or a frequency-doubled microwave signal at 19.9 GHz is generated with a wavelength-independent sidemode-suppression ratio (SMSR) as high as 78 dB obtained. The phase noise of the generated 19.9-GHz signal is ? 103.45 dBc/Hz at 10-kHz frequency offset, indicating a good short-term stability. The proposed scheme is simple and flexible, which can find applications in radars and wireless communications.     

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ = 2 nm around λ = 1565 nm with a group index of n_g = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 μm that exhibits stitching errors of typically 10–50 nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n_g = 60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.     

Multiwavelength fiber ring laser using a gain clamped semiconductor optical amplifier

We demonstrate a novel multi-wavelength fiber ring laser based on a gain clamped semiconductor optical amplifier. The number of lasing lines can be tuned by adjusting the loss inside the cavity. The wavelength interval between the wavelengths is 100 GHz. The proposed laser shows a stable operation with total intensity fluctuation for a single laser line within ±0.02 dB at room temperature for a period of 30-minutes.     

A novel polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber

A kind of polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber has been proposed. The polarization splitter is based on the phenomenon of resonant tunneling. We use the finite element method and the full-vector beam propagating method to analyze the characteristics of three-core photonic crystal fiber. Compare with the silica glass three-core PCF, the ZnTe tellurite glass three-core PCF have higher extinction ratios and lower coupling loss, the extinction ratios ERA = ? 164.2681 dB and ERC = ? 37.1742 dB at the wavelength λ = 1.55 μm, and the coupling loss is lower than 0.02 dB. The 8.7983-mm-long splitter is proposed to achieve extinction ratio better than ? 20 dB and a bandwidthof 20 nm.     

Simulation and optimization of deep violet InGaN double quantum well laser

The performance characteristics of a deep violet InGaN double quantum well laser diode (LD) such as threshold current (I_th), external differential quantum efficiency (DQE) and output power have been investigated using the Integrated System Engineering Technical Computer Aided Design (ISE-TCAD) software. As well as its operating parameters such as internal quantum efficiency (η_i), internal loss (α_i) and transparency threshold current density (J₀) have been studied. Since, we are interested to investigate the mentioned characteristics and parameters independent of well and barrier thickness, therefore to reach a desired output wavelength, the indium mole fraction of wells and barriers has been varied consequently. The indium mole fractions of well and barrier layers have been considered 0.08 and 0.0, respectively. Some important parameters such as Al mole fraction of the electronic blocking layer (EBL) and cavity length which affect performance characteristics were also investigated. The optimum values of the Al mole fraction and cavity length in this study are 0.15 and 400 μm, respectively. The lowest threshold current, the highest DQE and output power which obtained at the emission wavelength of 391.5 nm are 43.199 mA, 44.99% and 10.334 mW, respectively.