Magnetically tunable terahertz single high-order mode generator

The amplification properties of a tunable single transverse mode laser diode can be exploited to be used as an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n \times 1</tex> wavelength division switch. This kind of switch is expected to play an important role in telecommunications systems, especially when signals are not synchronous or when signal bit rates are various. The light amplification ratio between a desired main-mode wavelength and undesired submode wavelengths are improved by decreasing injection light power, and a ratio of 8.5 dB is achieved. This experimental result is in good agreement with theory. It is predicted theoretically using rate equations that the ratio is also improved by increasing the laser cavity mirror loss difference between the main mode and the submodes. A light amplification ratio of more than 11 dB is obtained when the cavity mirror loss difference is more than 50 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .

The paper reviews recent results obtained with diode lasers used in external hybrid cavities with frequency selective feedback. Such cavities attract continuing interest for several reasons. They generate a tunable single laser mode with very low linewidths (usually a few tens of kilohertz). Very wide discrete tunable ranges over 100 nm for Fabry-Perot type and over 200 nm for quantum well lasers are achieved. They can be made to oscillate in a tunable mode having the desired polarization state,TE orTM and, in some cases, simultaneously atTE andTM. This is done by designing a cavity that increases strongly theTM/TE intensity ratio and by using coatings on one laser facet that greatly lower bothTE andTM reflectivities. High-speed polarization switching in the gigahertz range is possible by inserting passive or active polarization selecting elements in the cavity. For all these reasons hybrid external cavities are attractive for applications in optical metrology, spectroscopy and optical communications. Moreover, the external cavity configuration allows the study of physical mechanisms in the laser diode by inducing on purpose phenomena that would have been otherwise impossible to achieve with free-running lasers.

In this paper, a nematic liquid crystal (NLC)-based tunable terahertz (THz) plasmonic metamaterials (MMs) with large modulation depth (MD) and low insertion loss (IL) is designed and experimentally verified at THz frequencies. The proposed structure includes two-layered MM that is immersed in LC. The metal MM is used directly as electrode. The tunable device with a 46×46 array of sub-wavelength circular air loops was fabricated on a quartz glass substrate, with 2×2 cm2 area and 220 µm thickness. The obtained results show that the amplitude MD and IL for normally incident electromagnetic (EM) waves are about 96% and 1.19 dB at 421.2 GHz, respectively, when the bias voltage applied to the NLC layer varies from 0 to 16 V. Meanwhile, the transmission peak frequency gradually decreases from 421.2 to 381.8 GHz, and the frequency tunability (FT) of the proposed structure is greater than 9.35%. This study provides a potential solution for THz modulators, filters, and switches.

A new scheme to generate narrow-band tunable terahertz (THz) radiation using a variant of the echo-enabled harmonic generation is analyzed. We show that by using an energy chirped beam, THz density modulation in the beam phase space can be produced with two lasers having the same wavelength. This removes the need for an optical parametric amplifier system to provide a wavelength-tunable laser to vary the central frequency of the THz radiation. The practical feasibility and applications of this scheme are demonstrated numerically with a start-to-end simulation using the beam parameters at the Shanghai Deep Ultraviolet Free-Electron Laser facility (SDUV). The central frequency of the density modulation can be continuously tuned by either varying the chirp of the beam or the momentum compactions of the chicanes. The influence of nonlinear rf chirp and longitudinal space charge effect have also been studied in our article. The methods to generate the THz radiation in SDUV with the new scheme and the estimation of the radiation power are also discussed briefly.

Tunable terahertz (THz)-wave generation from 0.5 to 4.5 THz has been achieved in a GaP crystal by collinear difference frequency mixing of dual wavelengths in the range 940–1000 nm. THz output energy of 5 nJ/pulse at 2.6 THz was obtained when the pump energy was 1.2 mJ/pulse.

In this work we present the experimental realization of electromagnetically induced transparency (EIT) in A-type and multi-V-type configurations in a sample of rubidium atoms inside a vapor cell at room temperature. Typical EIT windows are clearly visible in the Doppler- broadened absorption signal of the weak probe beam. The coherent optical pump and probe fields are produced by two tunable low-cost, low-power, continuous-wave (cw), free-running and single mode operated diode laser systems, temperature stabilized and current controlled, tuned to the D2 line of rubidium atoms at 780.2 nm wavelength. The continuum wave and single mode operation of our laser systems are confirmed by direct and saturated absorption spectroscopy techniques. Among other applications, these simple experiments can be used as a low-cost undergraduate laboratory in atomic physics, laser physics, coherent light-atom interaction, and high resolution atomic spectroscopy.

The wavelength, λ, range of 1.8 μm≤λ≤3.5 μm contains strong spectral absorption lines of many gases used in health, industry, safety, and medicine and whose sensitive and quantitative detection is desirable. However, the performance of InP diode lasers markedly deteriorates beyond λ∼2 μm. In this paper we present new results on developing tunable high power single mode laser diodes based on the GaSb material system with emission in the wavelength range of 1.8 μm≤λ≤2.2 μm.

A narrow linewidth tunable single longitudinal mode Ga-EDF fiber laser

A monolithic tunable single mode terahertz quantum cascade laser source based on intra-cavity difference frequency generation with a tuning range of ~580 GHz is demonstrated at room temperature.

A scheme is presented for generation of powerful coherent terahertz radiation by multiphoton resonant excitation of three-level hydrogenlike atoms and homonuclear diatomic molecules or molecular ions with strong laser pulses. The results of analytical and numerical investigations of the problem show that with the proposed mechanism one can achieve the implementation of widely tunable powerful terahertz sources, which may open new perspectives for terahertz science and its applications.

Generation of tunable millimeter-wave (MMW) and Terahertz (THz) signals from 117 GHz to 954 GHz is experimentally demonstrated with a 1310 nm Quantum Dot Distributed Feedback laser subject to single beam optical injection.

We report the operation and the theoretical modeling of an efficient, tunable, and low-loss single mode fiber coupler. The coupler design follows a scheme previously reported, in which two optical fibers mounted in curved grooves in separate quartz substrates are polished until sufficient cladding material has been removed to permit optical coupling between the mated polished faces of the fibers. The results of a computer analysis of the distributed coupling taking place between the fibers are discussed, emphasizing the intuitive dependence of the coupling coefficient and effective interaction length of the device on its geometrical parameters. A detailed experimental analysis of fiber couplers follows in which we characterize two types of couplers made with different brands of single-mode fibers. Operation up to 100 percent coupling ratio and 50 dB extinction ratio between coupled and direct branch as well as operation in overcoupling regimes are demonstrated, both at visible and infrared signal wavelengths. Tuning curves are shown that emphasize the excellent tunability properties of such couplers in which the coupling ratio can be smoothly and continuously tuned between 0 and 100 percent. Experimental evidence of the relatively low loss level and very low polarization dependence of the fiber couplers are also presented. All experimental results, including an analysis of the influence of the refractive index of the intermediate layer of index-matching liquid between the polished faces of the fibers, are found to be very well predicted by our theoretical model.

Microring resonators are excellent candidates for very large scale photonic integration due to their compactness, and fabrication simplicity. Moreover a wide range of all-optical signal processing functions can be realized due to the resonance effect. Possible applications include filtering, add/drop of optical beams and power switching, as well as more complex procedures including multiplexing, wavelength conversion, and logic operations. All-active ring components based in InGaAsP/InP are possible candidates for laser sources, lossless filters, wavelength converters, etc. Our work is based on measurement, characterization and proposal of possible exploitation of such devices in a variety of applications. We investigate the spectral characteristics of multi-quantum well InGaAsP(=1.55m)/InP microring structures of various ring diameters and different configurations including racetracks with one or two bus waveguides and MMI couplers. The latter configuration has recentl y exhibited the possibility to obtain tunable active filters as well as tunable laser sources based on all-active ring-bus-coupler structures. In the case of tunable lasers single mode operation has been achieved by obtaining sufficiently high side mode suppression ratio. The tuning capability is attributed to a coupled cavities effect, resembling the case of multi-section DBR lasers. However, in contrast to the latter, the fabrication of microring resonators is considered an easier task, due to a single step growth procedure, although further investigation must be carried out in order to achieve wide range tunability. Detailed mappings of achievable wavelengths are produced for a wide range of injection current values.

Electromagnetic metamaterials/metasurfaces have been widely concerned by researchers because of their rich resonance properties. Metamaterial absorbers with single resonance mode and narrow bandwidth are often used in sensing fields. However, the development of such metamaterial absorbers by optimizing the dielectric layer structure has not received much attention. In this paper, a metamaterial absorber containing dielectric layers of the same thickness (PMMA and STO dielectric layers) are proposed and confirmed. At room temperature, an individual absorption peak is excited, and the single resonance mode and narrow bandwidth characteristic is obtained (resonant frequency is 266.14THz, peak value is 0.902, the FWHM is 1.1THz). The amplitude and resonance frequency of the metamaterial sample can be controlled by changing the thickness of PMMA or STO dielectric layers. An equivalent LC resonant circuit is constructed and used to interpret the relationship between the resonant frequency and the thickness of the dielectric layer. In further experimental measurements, the metamaterial sample was placed in an environment with a gradual change in temperature. Two metamaterial units with the same dielectric layer structure strategy are also proposed and simulated, and similar resonance behaviors can also be excited by these two new metamaterial units. The results show that the absorptive properties of the metamaterial sample show remarkable repeatability in the process of temperature increase and temperature decrease. Therefore, based on the properties of single resonance mode, narrow bandwidth, and reversibility, the metamaterial sample has the potential to be used in smart packaging sensing.

Terahertz dielectric anisotropy enhancement in dual-frequency liquid crystal induced by carbon nanotubes