Mode Beating Research Articles

Modes in silver-iodide-lined hollow metallic waveguides mapped by terahertz near-field time-domain microscopy

Thin dielectric layers inside hollow metallic waveguides are used to improve the waveguide transmission characteristics as the dominant waveguide mode changes into the hybrid HE11 mode. We investigate the effect of 1 μm thick silver iodide (AgI) coatings on the fundamental modes in cylindrical waveguides at terahertz (THz) frequencies, in the regime of the dielectric layer being thinner than the optimal thickness hopt� 2 THz� ≈ 20 μm. In the region of 1–3.2 THz, the lowest-order modes are similar in profile to the TE11 and TM11 modes, as determined by the timeresolved near-field measurements and verified numerically. Higher-order modes are detected experimentally as mode mixtures due to the multimode propagation. Numerical electromagnetic modeling is applied to resolve the mode structure ambiguity, allowing us to correlate experimentally detected patterns with a superposition of the TM11 and the higher-order mode, TE12. Mode profiles determined here indicate that in the regime of ultrathin dielectric (h ≪ 0.1λeff), the dielectric layer does not transform the dominant mode into the low-loss HE11 mode. Experimental mode patterns similar to the HE11 and the TE01 modes nevertheless can be formed due to mode beating. The results indicate that the Ag/AgI waveguides can be used for guiding THz waves in the TE01 mode or the TE12 mode with high discrimination against other modes. © 2012 Optical Society of America OCIS codes: 110.6795, 180.4243, 320.7100.

Tripartite entanglement via coherently enhanced six-wave mixing

In this paper, we show that the tripartite continuous-variable entanglement can be obtained by using coherently enhanced six-wave mixing process in a triple Λ atomic system. Multiphoton resonances condition must be satisfied for the scheme. Due to atomic coherence, two of the cavity modes are pulled into a quantum beat and in the mixing interaction with the third cavity mode. This yields entanglement between the quantum-beat components and the third cavity mode. The pair of the beat modes is not entangled with each other, but they are both entangled with the third cavity mode. The generated tripartite entanglement is significantly dependent on the normalized detunings of the external coherent fields and the cooperativity parameters.

Modeling and design of anX-band rf photoinjector

A design for an $X$-band rf photoinjector that was developed jointly by SLAC National Accelerator Laboratory (SLAC) and Lawrence Livermore National Laboratory (LLNL) is presented. The photoinjector is based around a 5.59 cell rf gun that has state-of-the-art features including: elliptical contoured irises; improved mode separation; an optimized initial half cell length; a racetrack input coupler; and coupling that balances pulsed heating with cavity fill time. Radio-frequency and beam dynamics modeling have been done using a combination of codes including PARMELA, HFSS, IMPACT-T, ASTRA, and the ACE3P suite of codes developed at SLAC. The impact of lower gradient operation, magnet misalignment, solenoid multipole errors, beam offset, mode beating, wakefields, and beam line symmetry have been analyzed and are described. Fabrication and testing plans at both LLNL and SLAC are discussed.

Modeling of transient dynamics in two-dimensional circular microresonators using the pulsed complex source point beam concept

An analytical method for transient dynamics description in microresonators is used to characterize and visualize the transient effects. In the frame of this method, the pulsed complex source point concept is used to simulate an incident transient beam. The excited fields in the microcavity are described by means of a rigorous mathematical approach that is based on the analytical solution in the Laplace transform domain and accurate evaluation of residues at singular points corresponding to the excited modes. The effects of transient mode beating and ultrashort pulse splitting inside the microresonator for short pulse excitation are discussed.

Multimode fiber laser for simultaneous measurement of strain and temperature based on beat frequency demodulation

A multimode fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and demonstrated. Because of the long cavity and birefringence, longitudinal mode beat frequency and polarization mode beat frequency are achieved in the beat frequency signals of the multimode fiber laser. The strain and temperature can be obtained by monitoring both of them for their different strain and temperature responses. The experimental measurement errors are within ± 16.2 με and ± 1.9 °C. The usage of only one fiber laser and one demodulation system makes the system simple, cost-effective and portable.

High-purity transmission of a slow light odd mode in a photonic crystal waveguide

We demonstrate a novel scheme to control the excitation symmetry for an odd mode in a photonic crystal waveguide and investigate the spectral signature of this slow light mode. An odd-mode Mach-Zehnder coupler is introduced to transform mode symmetry and excite a high-purity odd mode with 20 dB signal contrast over the background. Assisted by a mixed-mode Mach-Zehnder coupler, slow light mode beating can be observed and is utilized to determine the group index of this odd mode. With slow light enhancement, this odd mode can help enable novel miniaturized devices such as one-way waveguides.

Enhanced longitudinal mode spacing in blue-violet InGaN semiconductor lasers

A novel explanation of observed enhanced longitudinal mode spacing in InGaN semiconductor lasers has been proposed. It has been demonstrated that e-h plasma oscillations, which can exist in the laser active layer at certain driving conditions, are responsible for mode clustering effect. The resonant excitation of the plasma oscillations occurs due to longitudinal mode beating. The separation of mode clusters is typically by an order of magnitude larger that the individual mode spacing.

Controlling surface plasmon interference in branched silver nanowire structures

Using quantum dot fluorescence imaging, we investigated the interference of surface plasmon beams in branched silver nanowire structures. Depending on the phases and polarizations of the incident light, interferences of plasmon beams modulate the plasmon propagation in the branched structures and the output light intensity in the distal ends. The interference visibility is strongly dependent on the incident polarization at the main wire terminal, and the mechanism is revealed by quantum dot fluorescence imaging of the near field distribution of propagating plasmons. The near field distribution pattern resulting from the beating of different plasmon modes plays a critical role in the plasmon interference. The overlap of the antinode in the near field pattern with the connection junction in the nanowire structure is required for a large interference visibility, since the overlap makes the electric field intensity difference of the two plasmon beams smaller. It is found that the plasmon interference is strongly dependent on the polarization of the excitation light at the main wire terminal, but weakly dependent on the polarization at the branch wire terminal.

Investigation of self-pulsing and self-mode-locking in ytterbium-doped fiber laser

The output characteristics of a continuous-wave diode pumped free-running double-clad ytterbium-doped fiber laser are investigated. In the experiment, not only self-pulsing is observed, but also self-mode-locking is observed for the first time in the free-running fiber laser. The physical mechanism responsible for these phenomena is analyzed. The analysis reveals that the signal is absorbed in the weakly pumped or non-pumped part of the active fiber, which leads the self-pulsing to be present, and self-mode-locking is induced by axial mode beating and self-phase modulation, which are enhanced by nonlinear effects, stimulated Raman scattering, and stimulated Brillouin scattering. The pulse width of self-pulsing decreases as pump power increases. The pulse width of self-mode-locking pulses becomes narrow with the increase of the pump power. When the pump power is higher than 8 W, self-pulsing and self-mode-locking disappear. The interval between adjacent self-mode-locking pulses is 224 ns, corresponding to the cavity round-trip time. The maximum and the minimum durations (full width at half maximum) of self-mode locked pulses are about 35.0 ns and 6.3 ns respectively, and the maximum and minimum spectral widths (full width at half maximum) of signal light are about 7.05 nm and 2.01 nm respectively.

Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems

Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.

Simultaneous strain and temperature fiber grating laser sensor based on radio-frequency measurement

We propose and experimentally demonstrate a novel simultaneous strain and temperature fiber optic sensor. The sensing head is formed by two concatenated ultra-short distributed Bragg reflector lasers that operate in single longitude mode with two polarization modes. The total length of the sensing head is only 18 mm. The two lasers generate two polarization mode beat notes in the radio-frequency range which show different frequency response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by radio-frequency measurement. This approach has distinctive advantages of ease of interrogation and avoidance of expensive wavelength detection.

Switchable multi-wavelength fiber ring laser using a side-leakage photonic crystal fiber based filter

A switchable multi-wavelength fiber ring laser is proposed and experimentally demonstrated with a novel side-leakage photonic crystal fiber (SLPCF) based filter incorporated into the ring cavity at room temperature. Stable multi-wavelength laser operations can be achieved due to the spatial mode beating, polarization hole burning and spectral hole burning effects. By adjusting the polarization controller appropriately, the laser can be switched among the single-, dual- and triple-wavelength lasing oscillations whose signal-to-noise ratio is up to 50 dB. In addition, the lasing wavelength can be also tuned and switched by applying the strain to the filter.

Frequency‐tunable terahertz electromagnetic‐pulses generation based on an optical Fabry–Perot microresonator with variable birefringence material

AbstractA frequency‐tunable terahertz (THz)‐frequency electromagnetic‐pulses generation method is proposed based on an optical Fabry–Perot (FP) microresonator with variable birefringence materials. At the output FP microcavity, polarization mode beat with high‐frequency can be achieved due to the phase mismatch effect of the two polarization modes in the birefringence materials in the FP cavity. One can generate frequency‐tunable electromagnetic pulse radiation through changing the birefringence of the materials. When the birefringence changed from 3.7355 × 10−4 to 7.4710 × 10−2, electromagnetic‐pulses with frequencies from 0.05 to 10.0 THz can be obtained. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2879–2882, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26422

Peculiarities of spike multimode generation of a superradiant distributed feedback laser

Using one-dimensional semiclassical Maxwell — Bloch equations with account for the coherent polarisation dynamics, we have studied spike generation regimes of a superradiant distributed feedback laser in the case of inhomogeneous broadening of the spectral line of an active medium. By analysing the dynamic spectra of inversion of the active medium and laser radiation, we have revealed the relationship of individual spikes of radiation and their modulation with specific parts in the spectral line of the active medium and mode beatings. It has been shown that the broadening and shift of the lasing spectrum with respect to the initial electromagnetic Bragg-cavity modes is accompanied by a strong spectral gradient of inversion that is typical of the superradiant regimes.

A Waveguide Polarization Toolset Design Based on Mode Beating

A toolset of waveguide elements is examined, which can be combined to produce polarization functional devices in a single contiguous waveguide. In particular, waveguide implementations of an optical isolator and a polarization modulator are discussed. The waveguide elements, i.e., quasi-phase-matched nonreciprocal polarization mode converter, reciprocal polarization mode converter (R-PMC), and a differential phase shifter, are all based on mode beating. A universal 3-dB R-PMC specification is identified, which suffices for all the polarization functional devices considered here. A full-vectorial modesolver is used to determine the modes in a number of example III-V waveguide structures, and the polarization state evolution is considered by using an averaged Stokes vector illustrated on the Poincare sphere construct.

Polarization maintaining highly birefringent small mode area photonic crystal fiber at telecommunication window

In this paper a highly polarization maintaining birefringent photonic crystal fiber at telecommunication window is reported via Full-Vectorial Finite Element Method (FV-FEM). By taking triangular lattice of three ring hexagonal structure PCF with suitable designing parameters such as symmetrical elliptical hole along radial axis of ellipticity a/b=5µm, air hole diameter d=1µm and pitch Λ=2.3µm, high birefringence (2.437×10-3) with small effective mode area(9.47µm2) and beat length (0.64cm) are found at wavelength 1.55µm.

Resonant self-pulsations in coupled nonlinear microcavities

A different point of view on the phenomenon of self-pulsations is presented, which shows that they are a balanced state formed by two counteracting processes: beating of modes and bistable switching. A structure based on two coupled nonlinear microcavities provides a generic example of a system with enhanced ability to support this phenomenon. The specific design of such a structure in the form of multilayered media is proposed, and the coupled-mode theory is applied to describe its dynamical properties. It is emphasized that the frequency of self-pulsations is related to the frequency splitting between resonant modes and can be adjusted over a broad range.

Yb-laser based on the LMA active fiber and multimode Bragg grating

An ytterbium-fiber laser based on an active fiber with an increased core diameter and a multimode grating is created and studied. It is demonstrated that the effective reflection of the Bragg grating amounts to 96%. The laser line width is about 3 nm, which indicates the propagation of a superposition of modes in the multimode fiber. The repetitively pulsed regime of lasing, which can be related to the mode beating, is demonstrated

Entanglement of three-mode light via six-wave mixing in a four-level Y-type atomic system

In this paper, we show that it is possible to achieve continuous-variable entanglement of three optical fields in a four-level Y-type atomic system. The three fields are amplified through the Rabi resonances of the different driven transitions. Two fields from the V configuration are in one quantum beat and in the mixing parametric interactions with the third field from the lower transition. This yields entanglement between the pair of beat modes and the third field. The best achievable state in the output approaches the Einstein–Podolsky–Rosen entangled state.

Theoretical and experimental study of laser beam propagation in capillary tubes for non-symmetrical coupling conditions

A non-symmetrical theoretical model is used to describe the propagation of laser beams in dielectric capillary waveguides under non-ideal coupling conditions. The displacement of the laser beam focusing point from the capillary axis, the deviation of the transverse energy distribution from a symmetric one, and an angle of incidence different from zero modify, through the excitation and beating of several modes, the energy repartition during the propagation in the waveguides. The results of modeling are in very good agreement with experimental results, obtained with a low-intensity laser beam on a test bench, where good control of the laser energy distribution in the focal plane, the focusing point position, and the angle of incidence was achieved.