Transverse Mode Discrimination Research Articles

A large mode area fiber with dual-helical-leakage-channels

This paper proposes a novel type of fiber with dual-helical-leakage-channels for large mode area and single mode operation. A coordinate transformation simulation technique is adopted to analyse the fiber performance, such as mode field area and loss coefficients, which can be modified by many fiber parameters. The simulation results show that loss coefficient L will decrease with the increase of helix pitch Λ and fiber core radius r, and increase with the increase of central angle of cladding 2 θ and wavelength λ. When θ = 12°~14°, Λ= 15 mm~16 mm, λ = 1.9 µm~2.1 µm, n1 = n3 = 1.438, n0 = n2 = n1 + 0.001, r = 30 µm, the fiber loss coefficients are L01 = 0.06 dB/m, L11 = 17.9 dB/m and L21 = 55.9 dB/m respectively, the transverse mode discrimination reaches the highest international standard of large mode area fiber, so the single mode fiber diameter is 60 µm. Especially, the helix pitch is 15 mm~16 mm, this indicates that the proposed fiber is easier to be drawn. In addition, the fiber has an all-solid-state structure, so it is convenient for cutting and splicing. The fiber may have a good application prospect in the field of high-power fiber lasers.

A segmented heterostructure cladding fiber designed for extreme large mode area

A segmented heterogeneous cladding (SHC) fiber was proposed in this work to obtain extreme large mode area. The mode propagation characteristics of the SHC fiber was analysed in detail, including loss coefficient, effective mode area and transverse mode discrimination et al. The research results show that the fiber parameters such as the central angle θ of cladding 2, the wavelengthλ, the fiber core radius r and the refractive index n2 of cladding 2 all have effects on loss coefficient and effective mode area, but transverse mode discrimination is almost unaffected. When r is 150 μm, the mode area is up to 34,343 μm2. The principle of single mode operation for the SHC fiber laser was also suggested. Thus, the SHC fiber can be designed for extreme large mode area and applied in high power fiber lasers.

Diffraction mode selection in planar Bragg resonators of optical and microwave wavelength ranges

Using the coupled waves approach complemented by the time-domain quasi-optical approximation, we solve a 2D diffraction problem which allows to evaluate the eigenfrequencies, quality factors and spatial structures of eigenmodes in planar Bragg resonators with a finite length and width of the corrugated area. We find the values of the Fresnel parameter determined by the geometrical dimensions of the system which allows for efficient transverse mode discrimination due to the larger diffraction losses of the modes with higher transverse numbers.

Polarization- and Modal-Control in a Vertical-Cavity Surface-Emitting Laser With an External-Cavity Formed by a Liquid Crystal Overlay

By utilizing a fully-vectorial three-dimensional cold-cavity optical simulation, we analyze the spectral, polarization, and modal-losses behavior of a vertical-cavity surface-emitting laser with a nematic liquid-crystal (LC) overlay. We identify phenomena responsible for mode and polarization selectivities and show how they can be controlled by varying design geometry parameters, such as LC thickness and oxide aperture diameter, or by electrooptically tuning the LC director. We present possibility of achieving voltage-controlled polarization switching of a fundamental mode or transverse mode switching within the same polarization. Furthermore, we show that it is theoretically possible to obtain improved single-mode operation by means of increased higher order transverse mode discrimination. Finally, we point out a way to destabilize the diode by equalizing the modal losses of all the analyzed modes.

Single mode lasing in transversely multi‐moded PT‐symmetric microring resonators

Conventional techniques for transverse mode discrimination rely on introducing differential external losses to the different competing mode sets, enforcing single‐mode operation at the expense of additional losses to the desirable mode. We show how a parity‐time (PT) symmetric design approach can be employed to achieve single mode lasing in transversely multi‐moded microring resonators. In this type of system, mode selectivity is attained by judiciously utilizing the exceptional point dynamics arising from a complex interplay of gain and loss. The proposed scheme is versatile, robust to deviations from PT symmetry such as caused by fabrication inaccuracies or pump inhomogeneities, and enables a stable operation considerably above threshold while maintaining spatial and spectral purity. The experimental results presented here were obtained in InP‐based semiconductor microring arrangements and pave the way towards an entirely new class of chip‐scale semiconductor lasers that harness gain/loss contrast as a primary mechanism of mode selectivity. image

Transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning

Transverse mode discrimination is demonstrated in long-wavelength wafer-fused vertical-cavity surface-emitting lasers using ring-shaped air gap patterns at the fused interface between the cavity and the top distributed Bragg reflector. A significant number of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing in particular the identification of a design that reproducibly increases the maximal single-mode emitted power by about 30 %. Numerical simulations support these observations and allow specifying optimized ring dimensions for which higher-order transverse modes are localized out of the optical aperture. These simulations predict further enhancement of the single-mode properties of the devices with negligible penalty on threshold current and emitted power.

Strong modes discrimination and low threshold in cw regime of 1300 nm AlInGaAs/InP VCSEL induced by photonic crystal

A self-consistent electrical-thermal-optical-gain modeling of threshold characteristics of an InP-based 1300 nm AlInGaAs photonic-crystal vertical-cavity surface-emitting diode laser is presented. It is shown that low threshold characteristics and strong transverse-mode discrimination can be simultaneously achieved for optimized photonic crystal structure and current aperture. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimal photonic-crystal parameters assuring single-mode operation of 1300 nm AlInGaAs vertical-cavity surface-emitting laser

We present a self-consistent analysis of InP-based 1300 nm AlInGaAs photonic-crystal vertical-cavity surface-emitting lasers (PhC VCSELs) and tunnel-junction PhC VCSELs, and analyze the influence of the electrical confinement, the PhC hole diameter and etching depth, and the size of the single defect optical aperture on the threshold current and the transverse mode discrimination. We also investigate the thermal performance of the two VCSEL configurations. As a result we determine the optimal PhC parameters assuring stable, single-mode operation in a broad range of driving currents.

Enhancement of the Mode Area and Modal Discrimination of Microchip Lasers Using Angularly Selective Mirrors

Angularly selective mirrors (ASMs) are proposed as a means to expand the mode area and modal discrimination of microchip lasers. ASMs used as output couplers selectively reflect incoming <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> vectors over a narrow angular range, while they transmit more inclined components. The eigenvalue problem of a microchip resonator equipped with a Gaussian ASM is solved analytically in the paraxial optics approximation using the ABCD matrix formalism. The narrow angular distribution of the reflected beam produces, through the laws of diffraction, a significant increase of the mode size and improved transverse mode discrimination, at the expense of higher oscillation threshold due to larger output coupling losses. Simulations performed using the parameters of Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped YAG material show that one order of magnitude increase of the mode area can reasonably be achieved without causing overheating and thermal fracture. ASMs can be directly deposited on the active material in the form of a resonant grating mirror. This technology involves only planar batch processes that retain the mass production advantage of microchip lasers. The significant increase of brightness of microchips expected from this innovation will give rise to more effective and more compact devices and new applications.

Improving both transverse mode discrimination and diffraction losses in a plano-concave cavity

The paper considers the interest in using two apertures rather than one in a plano-concave cavity for improving the discrimination between TEM00 and TEM10 modes while keeping the diffraction losses unchanged or even decreasing them. Such an investigation involves stochastic methods like simulated annealing that can be used if and only if the resonant field computation time is low. Fortunately, a quad–double arithmetic recurrence formula allows to compute round-trip operators faster than any adaptative numerical integrator with the same accuracy.

Transverse-mode control in vertical-cavity surface-emitting lasers via a patterned phase aperture

Transverse-mode discrimination in vertical-cavity surface-emitting lasers that contain a patterned phase aperture is analyzed numerically. The two lowest-order modes are calculated for different aperture shapes and sizes. They are then expanded in Laguerre-Gaussian modes to study the power distribution as well as their beam propagation factor. The mode selection depends on the aperture's size and degree of symmetry. The maximum value for the mode discrimination in the case of a specific phase aperture is determined, and an enhancement by a factor of 3, compared to the case without a phase aperture, is found.

Threshold Analysis of Vertical-Cavity Surface-Emitting Lasers With Intracavity Contacts

A numerical analysis of vertical-cavity surface-emitting lasers (VCSELs) incorporating intracavity contacts and distributed Bragg reflectors (DBRs) is presented. The model considers polarization dependent reflection at the DBRs, current spreading, and nonuniform carrier density distribution self consistently. Analytic expressions for the current spreading and the corresponding series resistance for VCSELs incorporating intracavity contacts are derived. It is shown that current spreading strongly affects the lateral gain profile, the threshold current density, the transverse mode shape and the transverse mode discrimination through the creation of intracavity optical phase and gain apertures. The series resistance and the depth of the dip in the current density distribution are used as figures-of-merit to provide guidelines for device optimization, as illustrated by means of two examples of long wavelength VCSEL designs

Analysis of eigenfields in the axicon-based Bessel-Gauss resonator by the transfer-matrix method

The axicon-based-Bessel-Gauss resonator (ABGR) has been proposed for the production of Bessel-Gauss beams. To analyze eigenfields of the ABGR with a plane or spherical output coupler, we present and demonstrate the transfer-matrix method. Since the method is slow to converge to eigenmodes of the ABGR by use of the Fox and Li iterative algorithm, in this paper the Huygens-Fresnel diffraction integral equations associated with ray matrices are converted into finite-sum matrix equations, and mode-fields and corresponding losses are described as eigenvectors and eigenvalues of a transfer matrix according to the self-reproducing principle of the laser field. By solving the transfer matrix for eigenvectors and eigenvalues, we obtain field distributions and losses of the dominant eigenmodes. Moreover, eigenfields across arbitrary interfaces between the axicon and the output coupler, and the propagation of output beams, are simulated by using the fast-Fourier transform (FFT). The calculation results reveal that because of the ABGR's poor transverse mode discrimination the ABGR should be improved to produce good-quality Bessel-Gauss beams.

Mode discrimination in vertical-cavity surface-emitting lasers including Bragg reflectors and limiting apertures

The effect of intracavity apertures and distributed Bragg reflectors (DBRs) on transverse-mode discrimination in vertical-cavity surface-emitting lasers is investigated theoretically. The effects of aperture shape and size as well as the stop-band width of the DBRs are examined with the goal of optimizing the mode discrimination for better lasing stability and higher single-mode power. It is found that decreasing the width of the DBRs' stop band, by means of controlling the material composition and number of layer pairs, increases significantly the optimum aperture size and hence the single-mode output power.

Microfluidic single-mode laser using high-order Bragg grating and antiguiding segments

We present a single-mode, single-polarization, distributed feedback liquid dye laser, based on a short high-order Bragg grating defined in a single polymer layer between two glass substrates. In this device we obtain single-mode operation in a multimode structure by means of transverse-mode discrimination with antiguiding segments. The laser is fabricated using microfabrication technology, is pumped by a pulsed frequency-doubled Nd:YAG laser, and emits narrow-line-width light in the chip plane at 577 nm. The output from the laser is coupled into integrated planar waveguides defined in the same polymer _lm. The laser device is thus well suited for integration, for example, into polymer based lab-on-a-chip microsystems.

Laterally periodic resonator for large-area gain lasers

Laterally periodic resonators, which can be constructed by use of transversely periodic phase- or amplitude-modulating elements in a cavity, are proposed for stabilization and generation of transversely coherent output from large-area gain. Lasers with periodic resonators have the combined features of conventional cavities and laser arrays. Significant low-order transverse modes and mode discrimination of a sample resonator with intracavity periodic phase elements are investigated numerically by the iteration method. Wave-propagation calculations are carried out by use of a fast Fourier transform, and a modified Prony method is used to evaluate wave functions and losses of transverse modes. Results of numerical calculations are consistent with expectations.

High transverse mode discrimination in apertured resonators using diffractive binary optics

We demonstrate theoretically that the discrimination between TEM 00 and TEM 01 modes achieved by a hard apertured resonator is greatly enhanced by the insertion of a simple binary diffractive optic inside the resonator. The latter, known as π/2 plate, introduces a constant phase shift in the central region of the beam.

Graded-phase mirror resonator with a super-Gaussian output in a CW-CO/sub 2/ laser

Two graded-phase mirror (GPM) resonators have been designed to produce super-Gaussian output beam profiles of order 4 and 6. An inverse propagation method was used to calculate the appropriate shape of the GPM's. Previous experimental work with a pulsed TEA-CO/sub 2/ laser has already confirmed the high transverse mode discrimination offered by such custom resonators, as anticipated by a theoretical analysis of their diffractional properties. Here, we extend these results by presenting new experimental measurements with a CW-CO/sub 2/ laser. The main objective of this paper is to demonstrate the super-Gaussian beam shape produced by these custom resonators. This is assessed with numerical simulations involving the incoherent superposition of transverses modes. The experimental results obtained with the GPM resonators, including measurements of the propagation of the output beam profile, are compared to those of a conventional semiconfocal cavity, and show higher monomode power extraction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of the longitudinal position of an aperture inside a cavity on the transverse mode discrimination

Two aspects of the fundamental mode selection in planoconcave resonators containing a circular aperture are analyzed. The first is the transverse mode discrimination (TMD), and the second is the fundamental mode volume. It is shown that the TMD decreases rapidly when the aperture is moved away from the plane or concave mirror position. Moreover, the half-confocal configuration has been found to be that which optimizes the TMD. For a fixed fundamental mode loss and a fixed TMD, the diffraction-induced increase of the fundamental mode volume is larger when the aperture is against the concave mirror than when it is against the plane mirror.

Transverse Mode Selection in Resonator with a Super-Gaussian Aperture

Abstract It is shown that the use of a super-Gaussian aperture of order five to ten for fundamental mode selection in plano-concave cavity gives better results (1) than for a Gaussian aperture for its lower TEM00 mode losses and, (2) than a hard aperture for its smoothing edge effects yielding more regular TEM00 mode profiles. Explicit results are given in hard and soft aperture cases for the transverse mode discrimination and fundamental mode losses.