High Group Velocity Dispersion Research Articles

High reflectivity, ultraflat-spectrum chirped fiber Bragg grating written using low energy UV femtosecond pulses

Chirped fiber Bragg gratings (CFBGs) have been extensively used in applications such as ultrafast lasers, fiber sensors, and fiber communications. This work presents a comprehensive investigation of CFBG written using a 257 nm femtosecond laser and a linear chirped phase mask. The CFBG, written on a hydrogen-loaded polarization-maintaining fiber, exhibited a center wavelength of 1031.7 nm and a bandwidth of 11.97 nm. The CFBG achieved an average reflectivity of 98.2 % within its bandwidth, demonstrating an ultrahigh flatness of the reflection spectrum with a low spectral reflectivity variance of 0.00003 (σ2). The high group velocity dispersion (GVD) of 60.7 ps2 signifies its potential in fiber chirped pulse amplification systems and dispersion compensation system. Meanwhile, the fs-laser + PM writing method shows advantages of high efficiency and low energy consumption (as low as 2 nJ), owing to the high photosensitivity of the fiber to the UV band, which holds significant promise in fiber grating inscription.

Tutorial for analyzing coherent Raman spectra excited with chirped femtosecond sources

Coherent Raman spectroscopy (CRS) and imaging have been used successfully for the label-free visualization and analysis of functional and endogenous biomolecules in complex biological samples. The interest in implementing these methodologies worldwide in research laboratories has increased in recent years; however, many practical challenges are faced in the implementation process. One of them is identifying from scratch different coherent Raman signals when broadband femtosecond sources are utilized. We present a straightforward methodology to identify and analyze the multiple excited resonant SRS/CARS signals using a simple spectrophotometer in this work to expand the understanding of this particular branch of CRS. The coherent Raman experiments are conducted in the so-called temporal slit configuration using a broadband femtosecond laser source to obtain the excitation pump and Stokes beams and high group velocity dispersion glass to temporally stretch the pulses; but in a non-conventional and non-optimum configuration for CRS experiments neither for spectral focusing (single-frequency measurements) nor for mixing of a broadband pulse with a narrow band pulse (multiplexed detection). Despite that, we demonstrate our methodology's feasibility to extract and analyze complex to discern coherent Raman spectra. We use dimethyl sulfoxide (DMSO) and β-phase barium meta-borate crystal (BBO) and discuss molecular vibrations identified in the spectral range from 300 to 800 cm−1 from both kinds of CRS spectra, stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS).

Highly negative dispersion compensating fiber with low third order dispersion

Abstract In this work, a dispersion compensating photonic crystal fiber (DC-PCF) is proposed in which dispersion, dispersion slope, second order dispersion, third order dispersion, nonlinearity, effective mode area, V parameter are investigated. The suggested structure is very effective for compensating of chromatic dispersion about −951 to −3075.10 ps/(nm.km) over 1340–1640 nm wavelength bandwidth. With perfectly matched layer boundary condition, guiding properties are inspected applying finite element method (FEM). The investigated results conform the opportunity of large negative dispersion and high group velocity dispersion (GVD) of −2367.10 ps/(nm.km) and 3018.55 ps2/km respectively, at 1550 nm operating wavelength. The offered fiber also shows low third order dispersion about −637.88 ps3/km, high nonlinearity of 91.11 W−1 km−1. From overall simulation results, it can be expected that the suggested PCF will be an effective candidate in high bit rate long haul optical communication system as well as sensing applications.

Group velocity dispersion analysis of terahertz quantum cascade laser frequency comb

The frequency comb which is characterized by equally-spaced frequency lines with high mode coherence has received much attention since its first demonstration in near-infrared and optical frequency range. In the terahertz frequency range, the electrically-pumped terahertz quantum cascade laser (THz QCL) based on semiconductors is an ideal candidate for achieving frequency comb operation in a frequency range between 1 THz and 5 THz. The group velocity dispersion (GVD) is a key factor for the frequency comb. A higher GVD can pull the frequencies from their equidistant values and limit the comb bandwidth. Therefore the laser dispersion needs to be compensated for in order to make the total GVD sufficiently low and flat, such as using a Gires-Tournois interferometer (GTI) or the double chirped mirror (DCM). However, a successful design still depends on the knowledge of the total GVD in the laser. In this paper, we show how to calculate the GVD in metal-metal waveguide THz QCLs by taking into account the dispersions from the GaAs material, the waveguide, and the laser gain, which conduces to the understanding of the frequency comb behavior. The waveguide loss is modelled by the finite element method. The loss due to intersubband absorption is calculated by Fermi's gold rule. All the losses, i.e., waveguide loss, mirror loss, and intersubband absorption loss, are summed up to calculate the clamped gain. The material loss can be calculated by using the reststrahlen band model. Because of these losses and gain, the refractive index needs to be replaced by a complex refractive index. The real part of the complex refractive index is the refractive index, which can be calculated from the Kramers-Kronig relationship that connects the loss or gain with the refractive index. Then the GVD introduced by the material loss, waveguide loss, and clamped gain can be finally calculated. The results show that the total GVD of THz QCL is approximately –8 × 10<sup>5</sup>~8 × 10<sup>5</sup> fs<sup>2</sup>/mm which is strongly determined by the clamped gain. Finally, the developed numerical model is employed to study the dispersion compensation effect of a GTI mirror which is coupled into a QCL gain cavity. The design of the THz QCL based on GTI structure is more flexible and feasible than that of the DCM. The result shows that by carefully designing the geometry of GTI, the dispersion of a THz QCL can be compensated for, thus achieving the broadband terahertz frequency combs.

Bend Coupling Through Near-Zero GVD Slow Light Photonic Crystal Waveguides

Slow light propagation through photonic crystal (PhC) slab devices has great potential to reduce the size and power consumption of silicon photonic optical circuits. Most commonly, slow light routing through photonic crystals is achieved by using W1 waveguide bends operating near their cutoff frequencies. Unfortunately, this leads to optical pulse distortion due the high group velocity dispersion (GVD) associated with these designs. In this letter, however, we study the coupling between slow light waveguides optimized for near-zero GVD and 60 $^{\circ }$ PhC bends. Using numerical methods and the temporal coupled mode theory, we assess the performance of single bends coupled to input/output waveguides, and S-bends composed of two cascaded bends. In this latter, we observe that the bend-waveguide quality factor has great impact over transmission and dispersion. We propose a novel 60 $^{\circ }$ PhC bend design for routing optical modes while maintained reduced dispersion. This is achieved over a -3 dB bandwidth of around 50 nm in devices with slowdown factor up to 40. We show that this 60 $^{\circ }$ PhC bend has good stability under changes in S-bend length and fabrication induced disorder. These results can lead to great improvements in the design of monolithically integrated modulators, switches, (de)multiplexers, and filters based on photonic crystals, as well as on the routing of long optical buffers and delay lines.

On the Chiroferrite Medium-Based Waveguide Dispersion Compensator

The letter investigates a kind of dispersion compensator composed of dual-core planar waveguide separated by isotropic dielectric medium. Both the resonantly coupled cores are comprised of anisotropic chiroferrite materials. A dielectric clad layer of silicon dioxide is combined adjacent to one of the core sections. Supermodes are generated through mode-coupling at the resonance frequency that lead to the investigation of the group velocity dispersion (GVD) feature of the structure. The chiroferrite material parameters, particularly the chiral admittance and gyrotropy of the cores, play vital roles in achieving high GVDs. A fine tuning of resonance is achieved through proper choice of the core material parameters, and the results would be useful to devise dispersion compensator for usage in communication systems.

Tailoring of group velocity dispersion in dual-core chiral nihility waveguide

The article focuses on an approach to overcome the dispersion present in optical communication systems. The group velocity dispersion (GVD) remains an important issue in this context. The approach presented in this paper is based on the use of chiral nihility mediums with different chirality parameters in dual-core planar optical waveguide. It is found that the two cores can be resonantly coupled in the guide, and also, the waveguide would yield high GVD by choosing suitable design parameters. For the analytical investigation, transfer matrix method is used and the dispersion relation is derived by implementing appropriate boundary conditions. Excitation of supermodes in the guide may be achieved through the coupling of two particular core modes, which would ultimately result in giant GVD. This essentially demonstrates that the structure may find applications in dispersion compensating systems.

High-efficiency terahertz pulse generation via optical rectification by suppressing stimulated Raman scattering process

We experimentally demonstrate high-efficiency terahertz pulse generation via optical rectification in LiNbO3. The spectral broadening of an excitation pulse via the stimulated Raman scattering process coincides with high-efficiency terahertz pulse generation, which enhances undesired stretching of the excitation pulse owing to the very high group velocity dispersion in LiNbO3. We avoid this by the bandwidth control of the excitation pulse and achieve the highest reported efficiency of 0.21% for energy conversion into a THz pulse.

Design and analysis of large-core high-GVD planar optical waveguide for dispersion compensation

We present a large-core planar optical waveguide design showing high group velocity dispersion (GVD). The design is based on a coupled asymmetric dual-core structure with an additional layer of over cladding. The resonance between the two dissimilar cores, namely, core-1 and core-2, has been utilized to achieve high GVD. The core separation controls the magnitude of the GVD. The resonance wavelength can be fine tuned by varying the parameters of the core-2. The over-cladding helps in maintaining high GVD even with large core-1. We show a GVD of 7.8×10−25 s2/mm with the core-1 size as large as 6 μm. Such a structure should find applications in the design of high-power integrated-optic dispersion compensators for optical telecommunication and ultrafast waveguide lasers.

Group index and group velocity dispersion in silicon-on-insulator photonic wires

We determine group index and group velocity dispersion (GVD) of SOI single-mode strip waveguides (photonic wires) with 525x226nm cross-section over the entire telecommunication bandwidth by employing an integrated Mach-Zehnder interferometer. The measured GVD yields 4400 ps/(nm*km) at 1550 nm and exceeds that of standard single-mode fibers by almost three orders of magnitude. In the photonic wires the GVD is mainly determined by strong light confinement rather than by material dispersion. Our results indicate that despite this high GVD, dispersion-induced signal impairment is negligible in photonic circuits for data rates up to 100-Gb/s and total waveguide lengths as long as about 1 meter. The measured group index and GVD are used as benchmarks to compare model calculations originating from four different theoretical methods.

Sloped-wall thin-film photonic crystal waveguides

The effect of the slope of the groove walls in the behavior of thin-film one-dimensional photonic crystal waveguides is extensively studied. In this respect, we point out its influence on the modal dispersion relations and then on the bandgap structure in general. Likewise, we also prove the lack of accuracy in the evaluation of the bandgap edges when material dispersion is ignored. The extreme importance of both facts, the wall slope and the material dispersion, in the analysis and design of realistic photonic crystal devices is emphasized. In particular, we exploit the wall slope as a new design parameter. By suitably choosing the value of the above parameter, sloped-wall photonic crystal waveguides with high quality factor or high group velocity dispersion can be achieved.

Tracing interactions in HCGs through the H I

AbstractWe present a global study of Hɪ spectral line mapping for 16 Hickson Compact Groups (HCGs) combining new and unpublished VLA data, plus the analysis of the Hɪ content of individual galaxies. Sixty percent of the groups show morphological and kinematical signs of perturbations (from multiple tidal features to concentration of the Hɪ in a single enveloping cloud) and sixty five of the resolved galaxies are found to be Hɪ deficient with respect to a sample of isolated galaxies. In total, 77% of the groups suffer interactions among all its members which provides strong evidence of their reality. We find that dynamical evolution does not always produce Hɪ deficiency, but when this deficiency is observed, it appears to correlate with a high group velocity dispersion and in some cases with the presence of a first-ranked elliptical. The X-ray data available for our sample are not sensitive enough for a comparison with the Hɪ mass; however this study does suggest a correlation between Hɪ deficiency and hot gas since velocity dispersions are known from the literature to correlate with X-ray luminosity.