High-order Mode Oscillation Research Articles

Suppression of high order mode oscillation in a C-band long pulse high efficiency relativistic backward wave oscillator

In order to realize mode control and long pulse operation of the proposed C-band high efficiency relativistic backward wave oscillator, the asymmetric high order mode oscillation and its influences on the working mode operation are investigated in the 3D particle-in-cell simulation. The simulation shows that the quasi-TM21 mode is the dominant competing mode, which is mainly located in the two-period extraction cavity of the device. Accordingly, a simplified model of a two-period extraction cavity is established to investigate the mechanism of the high order mode excitation. It is indicated that the distance between two gaps of the two-period extraction cavity has great impact on the startup time of the competing mode. By optimizing the gap distance to delay the startup of the competing mode, as well as shortening the rising time of the input pulsed power pulse, the competing quasi-TM21 mode can be suppressed sufficiently.

Self-starting high-order mode oscillation fiber laser.

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72nm with 3dB of 0.05nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

The Generation of 1.2 μJ Pulses From a Mamyshev Oscillator Based on a High Concentration, Large-Mode-Area Yb-Doped Fiber

We demonstrate a high-energy and high-average power Mamyshev oscillator based on a high performance large-mode-area step-index germanosilicate-cladding Yb-doped fiber. The oscillation of higher-order modes is effectively suppressed by employing appropriate intracavity spatial mode filtering, allowing significant scaling of single pulse energy. The laser directly generates a nearly diffraction-limited beam with an average power of 17 W operating at the fundamental repetition rate of 14.01 MHz, corresponding to a pulse energy of 1.2 μJ. The pulses could be externally dechirped to ∼58 fs, leading to a peak power of ∼13 MW. To the best of our knowledge, this is the highest pulse energy with sub-100 fs duration ever obtained directly from a mode-locked fiber oscillator.

Visible-Wavelength All-Fiber Vortex Laser

We report the first demonstration of a compact all-fiber visible-wavelength laser directly emitting cylindrical vector or vortex beams. The visible all-fiber cavity is constructed by a few-mode Pr/Yb-codoped ZBLAN fiber pumped by a 850 nm laser diode, a 634.6 nm few-mode fiber Bragg grating, and a visible-wavelength fiber mirror. Direct oscillation of high-order modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes) at 634.6 nm wavelength is achieved using core-shifted fiber coupling method. The visible cylindrical vector beams with the radially or azimuthally polarized state can be observed. Furthermore, by adjusting the rotation of the intracavity polarization controller appropriately, two orthogonal modes (i.e., LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">even</sup> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">odd</sup> ) can introduce a π/2 phase difference, and the 634.6 nm vortex beams with ±1-order orbital angular momentum modes are also obtained.

Experimental Investigation of Lasing Modes in Double-Lattice Photonic-Crystal Resonators and Introduction of In-Plane Heterostructures

Photonic-crystal surface-emitting lasers (PCSELs) are attractive for a wide range of applications such as material processing and remote sensing owing to their advantage of broad-area coherent lasing supported by a 2-D photonic crystal. Recently, we proposed double-lattice photonic-crystal resonators and achieved 10-W-class high-power and high-beam-quality (namely, high-brightness) operation. To further increase the brightness, it is important to understand the lasing mode behavior of PCSELs in detail. In this article, we experimentally investigate the change in the lasing mode depending on the current injection levels and compare the results with a theoretical analysis that accounts for the change in refractive index and gain distributions due to the current injection. We observed a transition from single-mode lasing to two-mode lasing at high injection currents. We develop a technique for resolving the near-field profiles of individual lasing modes and show that these profiles coincide with theoretical predictions. A comparison of experimental and theoretical results reveals that high-order modes appear due to a bandgap confinement effect induced by a change in the refractive index in the carrier injection area. To effectively remove this confinement effect and suppress the oscillation of high-order modes, we incorporate an in-plane heterostructure into the PCSEL.

Multimode oscillation Q-switched erbium-doped fiber laser with a few-mode fiber cavity

We propose and demonstrate experimentally a multimode oscillation Q-switched erbium-doped fiber laser with a few-mode fiber cavity. The different coupling processes of reflection peaks of fiber Bragg gratings (FBGs) which act as cavity mirrors allow the different oscillating states. We obtain microsecond light pulse output states whose oscillation and output beams have different mode compositions, including high order mode. Such a structure enables the high order mode oscillation in the whole cavity with an all-fiber structure, which has potential applications in mode-division multiplexed systems and research of multimode nonlinear effect.

High-order mode direct oscillation of few-mode fiber laser for high-quality cylindrical vector beams

Generation of high-order modes with high quality is important for the application of cylindrical vector beams in fibers. We experimentally demonstrated high-order LP11 mode generation and amplification with a broad bandwidth in an all few-mode fiber laser. A wavelength-division-multiplexing (WDM) mode selective coupler (MSC) is proposed to achieve efficient mode conversion from LP01 mode to LP11 mode, but also combine high-order LP11 modes at the wavelengths of 980/1550 nm. To the best of our knowledge, this is the first report on the high-order mode oscillation in an all few-mode fiber laser. LP11 mode and cylindrical vector beams including radially and azimuthally polarized beams are obtained with high modal purity. The purity of the generated high-order modes are all in excess of 95%.

A study on the high-order mode oscillation in a four-cavity intense relativistic klystron amplifier

The high-order mode oscillation is studied in designing a four-cavity intense relativistic klystron amplifier. The reason for the oscillation caused by high-order modes and a method to suppress these kinds of spurious modes are found through theoretical analyses and the study on the influence of major parameters of a high frequency structure (such as the oscillation frequency of cavities, the cavity Q value, the length of drift tube section, and the characteristic impedance). Based on much simulation, a four-cavity intense relativistic klystron amplifier with a superior performance has been designed, built, and tested. An output power of 2.22 GW corresponding to 27.4% efficiency and 61 dB gain has been obtained. Moreover, the high-order mode oscillation is suppressed effectively, and an output power of 1.95 GW corresponding to 26% efficiency and 62 dB gain has been obtained in our laboratory.

Analysis and Suppression of High-Order Mode Oscillation in an S-Band Klystron

High-order mode oscillation occurred in an S-band experimental klystron with 300 MHz instantaneous bandwidth. The cause of the oscillation was analyzed and simulated. A method was proposed to suppress the oscillation through adjusting the structure and size of a resonant cavity in the tube, in which the high-order mode frequency can obviously be changed and the variation of the fundamental mode frequency is very small. Improved designs for the resonant cavity with Ansoft HFSS code and corresponding experiment have been performed, which shows that the simulation results are in good agreement with the experimental measurements. The testing result of an experimental klystron with this improved resonant cavity indicates that the high-order mode oscillation can effectively be suppressed.

Research and Development of S-Band High Power Multibeam Klystron

The design, manufacture, and test of an S-band high power multibeam klystron (MBK) are presented in this paper. The test results indicate that the performance of the MBK satisfies the design requirement. Several technical problems, such as the oscillation of high-order modes, a decrease of the beam current under no-driver (oscillating) conditions, and the high-voltage breakdown are also discussed. Some improvements have been taken to overcome these problems. The improved tube has a beam transmission rate over 98%, and operates very stably under DC and RF conditions.

Experimental researches of a four-cavity intense relativistic klystron amplifier

This paper described a high-order mode oscillation phenomenon, in the simulation of four-cavity intense relativistic klystron amplifier(IRKA). And we have analyzed the cause of this kind of oscillation, and explored the suppression methods for the whole tube. Two-dimensional and three-dimensional electromagnetism simulation software are used to study the influence of parameter’s changes (such as the oscillation frequency of cavities, the cavity Q value, the length of drift tube, and the characteristic impedance) on the high-order mode oscillation. The effectiveness of the high-order mode oscillation suppression methods is verified by simulation. Then, by optimizing the structure of IRKA, we have achieved an output microwave with a power of 2.13 GW, a gain of 60 dB and an efficiency of 26% in simulation, and the high-mode oscillation is avoided effectively. Finally, an output microwave with a power of 1.9 GW, a gain of 61 dB, and an efficiency of 24% is gained in the experiment.

A single-spatial-mode semiconductor laser based on InAs/InGaAs quantum dots with a diffraction filter of optical modes

The concept of a diffraction optical filter is used for prevention of high-order mode oscillation in a design of stripe laser diodes with an active region based on InAs/InGaAs quantum dots emitting in the 1.3-μm wavelength range grown on GaAs substrates. Incorporation of such a filter made it possible to increase the width of the stripe and obtain an output power as high as 700 mW with retention of a single-spatial-mode character of lasing.

Suppression of High Order Mode Oscillation in a Broadband Klystron

该文描述了在一种S波段宽带速调管研制过程中出现的高次模式振荡现象，分析了抑制这种振荡的可能方式，提出了一种抑制这种振荡的方法。该方法通过改变谐振腔结构，在基本不影响基模频率的前提下，使高次模式频率产生较大的改变，从而使该高次模式的振荡得到抑制。利用Ansoft HFSS模拟设计软件，对谐振腔进行了改进设计并完成了相应的实验验证。利用改进设计的谐振腔，进行了速调管整管试验，有效地抑制了速调管中高次模式的振荡。

Development of Long Pulse and High Power 170GHz Gyrotron

Recent activities on the research and development of 170 GHz gyrotron were presented. In the 170 GHz gyrotron experiment in JAERI, pulse duration had been limited by the beam current decrease due to the emission cooling of the electron emitter. A preprogramming control of the heater power was introduced to keep the beam current constant and to avoid the oscillation mode shift to a lower mode during the operation. And, a built-in mode converter system was improved to reduce the stray radiation. In the preliminary experiment results of 0.2 MW/480 sec and 0.13 MW/600 sec were obtained. The pulse extension experiment at higher power will be followed. In parallel, a study of the high order mode oscillation was carried out using a short pulse gyrotron (∼1 msec). The oscillation mode is TE31,12, which allows 1.5 MW level CW operation. A stable oscillation of TE31,12 was demonstrated at the power of 1.56 MW. The maximum efficiency was 30% at 1 MW output. This result indicates that the high order mode up to the level of TE31,12 will be acceptable to increase the power or to reduce the heat load on the cavity wall at 1 MW output.

Effect of absorbed pump power on the quality of output beam from monolithic microchip lasers

The dependence of the beam propagation factor (M2 parameter) with the absorbed pump power in the case of monolithic microchip laser under face-cooled configuration is extensively studied. Our investigations show that the M2 parameter is related to the absorbed pump power through two parameters (α and β) whose values depend on the laser material properties and laser configuration. We have shown that one parameter arises due to the oscillation of higher order modes in the microchip cavity and the other parameter accounts for the spherical aberration associated with the thermal lens induced by the pump beam. Such dependency of M2 parameter with the absorbed pump power is experimentally verified for a face-cooled monolithic microchip laser based on Nd3+ -doped GdVO4 crystal and the values of α and β parameters were estimated from the experimentally measured data points.

Distributed-feedback grating used as a lateral-mode selector in phase-locked antiguided arrays

We show here, for the first time, that a distributed-feedback (DFB) grating can act as a lateral-mode discriminator if located below the active region of a phase-locked antiguided array. Spatial-mode selection from such a lower-DFB (LDFB) grating in a resonant antiguided structure (ROW-LDFB) relies on the fact that the optical field distribution below the active region is strongly array-mode dependent. In particular, it is shown that at and near resonance a ROW-LDFB structure strongly favors resonant-mode oscillation, while suppressing oscillation of high-order modes. ROW-LDFB devices thus accomplish both spatial- and frequency-mode selection in a single structure. Furthermore, for effective intermodal discrimination, there is no need for interelement loss or Talbot-type filters, thus eliminating all potential sources of self-pulsations.

我が国の外洋に面した漁港での長周期波に関する実態調査について

At fishery harbors facing to open-sea, we carried out a questionnaire survey which concerns the actual condition on long-period waves. Then a numerical calculation was performed on the basis of the received answers. The obtained results are summarized as follows : (1) As wave group causes the harbor oscillation of high order modes, long-period waves are not amplified so much, and in the harbor the height of long-period waves is about 1/10 times as high as offshore waves. (2) Even at the innermost point of harbor, the energy of long-period waves is not attenuated so much. (3) At fishery harbors facing to Open-sea, a water current and a vortex caused by long-period waves are easy to occur near the harbor entrance. (4) On the area of north side of the Pacific Ocean, ship entering into harbors is hard under a strong seasonal wind and a typhoon. (5) Even if the fishery harbor is facing to Open-sea, it is easy to enter the harbor and moored ships are safe, if the harbor is surrounded by natural geographical features. (6) In the Tohoku region of the Pacific Ocean and west side of Shikoku, most harbors are natural good harbors, and are not affected by long-period waves.

Lateral mode discrimination in AlGaInP selectively buried ridge waveguide lasers

The guiding mechanism of (AlxGa1−x)0.5In0.5P visible lasers is analysed using a simple effective index approach and is found to be analogous to that of AlGaAs channelled substrate planar lasers. Outside the ridge, the bound mode is leaky, resulting in a real lateral index guide. Furthermore, the loss outside the ridge provides good discrimination against oscillation of higher order modes. The best ridge widths, based on the tradeoff of low loss in the fundamental mode along with high lateral mode discrimination, are in the range 5–7 μm.

Terraced-heterostructure large-optical-cavity AlGaAs diode laser: A new type of high-power cw single-mode device

A new terraced lateral wave confining structure is obtained by liquid phase epitaxy over channeled substrates misoriented perpendicular to the channels’ direction. Single spatial and longitudinal mode cw operation is achieved to 50 mW from one facet, in large spot sizes (2×7.5 μm, 1/e2 points in intensity) and narrow beams (6°×23°, full width half-power). At 70 °C ambient temperature cw lasing is obtained to 15 mW from one facet. Weak mode confinement in an asymmetric lateral waveguide provides discrimination against high-order mode oscillation.