High-power Oscillator Research Articles

Raman suppression in a high-power single-mode fiber oscillator using a chirped and tilted fiber Bragg grating

We propose the design of a high-power fiber oscillator with chirped and tilted fiber Bragg gratings (CTFBGs) to suppress the stimulated Raman scattering within the fiber cavity and in the delivery fibers. A high-power single-mode fiber oscillator system was constructed for demonstration. With a CTFBG inserted into the fiber cavity, obvious Raman suppression was observed and the output signal laser power increased by ∼7%. With a CTFBG inserted between the oscillator and the delivery fibers, a maximum Raman suppression ratio of >20 dB was achieved, which implies a longer delivery distance for the same laser power level. By further improving the performance of CTFBGs, including lower inversion loss at signal wavelength, wider rejection bandwidth and deeper rejection depth, meanwhile optimizing the positions of CTFBGs, better Raman suppression could be obtained, which is significant for the further power scaling and delivery distance increasing of high-power fiber oscillators.

Watt-level and sub-100-fs self-starting mode-locked 2.4-µm Cr:ZnS oscillator enabled by GaSb-SESAMs.

Femtosecond lasers with high peak power at wavelengths above 2 µm are of high interest for generating mid-infrared (mid-IR) broadband coherent light for spectroscopic applications. Cr2+-doped ZnS/ZnSe solid-state lasers are uniquely suited since they provide an ultra-broad bandwidth in combination with watt-level average power. To date, the semiconductor saturable absorber mirror (SESAM) mode-locked Cr:ZnS(e) lasers have been severely limited in power due to the lack of suitable 2.4-µm SESAMs.For the first time, we develop novel high-performance 2.4-µm type-I and type-II SESAMs, and thereby obtain state-of-the-art mode-locking performance. The type-I InGaSb/GaSb SESAM demonstrates a low non-saturable loss (0.8%) and an ultrafast recovery time (1.9 ps).By incorporating this SESAM in a 250-MHz Cr:ZnS laser cavity, we demonstrate fundamental mode-locking at 2.37 µm with 0.8 W average power and 79-fs pulse duration. This corresponds to a peak power of 39 kW, which is the highest so far for any saturable absorber mode-locked Cr:ZnS(e) oscillator.In the same laser cavity, we could also generate 120-fs pulses at a record high average power of 1 W. A comparable laser performance is achieved using type-II InAs/GaSb SESAM as well. These results pave the way towards a new class of high-power femtosecond SESAM mode-locked oscillators operating directly above 2-µm wavelength.

Closed-loop controlled compensation of thermal lensing in high-power thin-disk lasers using spherically deformable mirrors

We present the closed-loop controlled compensation of the thermally induced lens in a high-power thin-disk oscillator to obtain fundamental-mode operation over a wide range of pump powers. The radius of the oscillating beam was measured in real time at the position of the thin-disk crystal. This signal was used by the control system to adapt the curvature of a spherically deformable mirror to maintain a constant radius of the oscillating laser beam.

高功率掺镱光纤振荡器: 研究现状与发展趋势

高功率掺镱光纤振荡器: 研究现状与发展趋势

Design and Experiment of a Quasi-Optical Mode Converter

Quasi-optical mode converter is an important component to realize the high-efficiency output for a high-power gyrotron oscillator. In this paper, design and experiments of a quasi-optical mode converter, consisting of a Denisov-type launcher and three quasi-optical mirrors, are carried out for the development of 140 GHz/TE28,8 mode gyrotron oscillator. Based on scalar diffraction method, the field distribution at the radiation aperture of the Denisov-type launcher is optimized to make the vector correlation of the aperture field with the ideal Gaussian field reach 96.2%. Based on the geometric optics method and the Gaussian beam matching method, the focusing mirror and the beam shaping mirrors were designed. A 3D full-wave analysis software Surf3D was used to obtain the field distribution on each mirror surface and the output window, which shows that the output beam with Gaussian mode content of 96.67% is obtained at the output window. The total power conversion efficiency of the quasi-optical mode converter is 93.98%. The design and experiments of the quasi-optical mode converter are performed to use the output signal of the self-developed TE28,8 mode generator as its input one under the conditions of the simulated demonstration for its conversion characteristics and the strict control for machining precision as well as the process of the converter assemble and testing. The tested results indicate that the design is in good agreement with the experiment, which is helpful for engineering design and experimental demonstration of the quasi-optical mode converter development.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Abstract Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot. At present, research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser. However, the suppression of spectral broadening, caused by self-phase modulation or four-wave mixing, is still a challenging attribute to the close distance between the broadened laser and signal laser. If using a traditional fiber grating with only one stopband to suppress the spectral broadening, the signal power will be stripped simultaneously. Confronting this challenge, we propose a novel method based on phase-shifted long-period fiber grating (PS-LPFG) to suppress spectral broadening in a high-power fiber master oscillator power amplifier (MOPA) laser system in this paper. A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique. The resonant wavelength of the fabricated PS-LPFG is 1080 nm, the full width at half maximum of the passband is 5.48 nm, and stopband extinction exceeds 90%. To evaluate the performance of the PS-LPFG, the grating is inserted into the seed of a kilowatt-level continuous-wave MOPA system. Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%, providing an effective and flexible way for optimizing the output linewidth of high-power fiber MOPA laser systems.

Novel constant-cladding tapered-core ytterbium-doped fiber for high-power fiber laser oscillator

Abstract Power scaling based on traditional ytterbium-doped fibers (YDFs) is limited by optical nonlinear effects and transverse mode instability (TMI) in high-power fiber lasers. Here, we propose a novel long tapered fiber with a constant cladding and tapered core (CCTC) along its axis direction. The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering (SRS) threshold and suppress higher-order mode resonance in the laser cavity. The CCTC YDF was fabricated successfully with a modified chemical vapor deposition (MCVD) method combined with solution doping technology, which has a cladding diameter of 400 μm and a varying core with a diameter of ~24 μm at both ends and ~31 μm in the middle. To test the performance of the CCTC fiber during high-power operation, an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally. As a result, a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%, although the TMI effect was observed at an output power of ~3.12 kW. The measured beam quality (M2 factor) was ~1.7, and no sign of the Raman component was observed in the spectrum. We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects, and further power scaling is promising by optimizing the structure of the YDF.

A Bipolar Transistor-Based High-Power Chaotic Oscillator with Selected Inertia

The feasibility of creating a high-power wideband source of chaotic microwave signals that is constructed around a single high-power bipolar transistor has been demonstrated for the first time. This has become possible after implementing a selected-inertia oscillator. Theoretical calculations supporting the idea of fabricating such an oscillator are presented. A hybrid integrated prototype of a selected-inertia chaotic oscillator that is based on a 2T982A-2 high-power transistor has been designed. The feasibility of generating chaotic microwave signals with a center frequency of 4.55 GHz and an overall power of 1.1 W has been proved. The effective width of the chaotic signal power spectrum equals 11% at a spectral characteristic ripple of 3 dB, the spectral density of noise oscillations is 2.2 × 10–3 W/MHz, and the e1fficiency is 15%.

A novel fiber laser oscillator employing saddle-shaped core ytterbium-doped fiber

A novel high-power fiber laser oscillator employing a saddle-shaped core ytterbium-doped fiber (SSCYDF) is proposed and demonstrated experimentally. The SSCYDF is designed and fabricated with a long-tapering core (diameter of ~ 30 µm at both ends and ~ 23 µm in the middle) and a constant inner cladding (diameter of ~ 400 µm) in longitudinal dimension. On the one hand, the small core section of the fiber can only support less than two modes, which is helpful for the mitigation of the transverse mode instability. On the other hand, the large core section provides a large mode area for suppression of stimulated Raman scattering. Therefore, this type of laser oscillator holds the potential advantages for both mitigation of transverse mode instability and suppression of stimulated Raman scattering. Based on the homemade SSCYDF, an all-fiber laser oscillator is constructed and investigated by pumping with laser diodes at wavelength of 976 nm and 915 nm, respectively. The maximum output power of 1312 W is achieved in the case of co-pumping at 915 nm. This is the first work, to the best of our knowledge, to validate the feasibility of using saddle-shaped core ytterbium-doped fiber with constant cladding for high-power fiber lasers.

Design and Simulation of an S-Band Relativistic Inverted Magnetron

A high-power relativistic inverted magnetron oscillator (IMO) operating in the S-band is designed and numerically simulated using the massively parallel electromagnetic particle-in-cell code [improved concurrent electromagnetic particle-in-cell code (ICEPIC)]. This inverted magnetron is designed for optimum operation at ~2.5 GHz for magnetic fields ranging from 0.09 to 0.15 T and voltages extending from ~190 to 360 kV. Simulations in the above mentioned voltage/magnetic field range predict a device capable of operating in a single dominant $\pi $ mode with little to no mode competition, fast startup times, and high output power exceeding 500 MW in many cases sampled. This design eliminates a common source of energy loss in standard relativistic magnetrons and downstream end-loss current. The IMO also eliminates the need for extended waveguide structures and combiners common for magnetrons that require radial RF extraction. This is done by radiating RF energy axially through the use of a dual-ring radiator. The dual-ring radiator excites the TM01 mode in a single downstream waveguide that maybe then directed toward an antenna. The S-band IMO-simulated RF output power efficiencies are typically near 15% with RF output power ranging from 250 MW to ~1 GW at the highest voltages sampled. ICEPIC models show key segments of the design that may undergo excessive electromagnetic field stress during saturation. Such stress may ultimately lead to plasma formation and performance degradation; however, this may be mitigated via alternate materials, voltage derating, and further design refinement. In the simulation, the S-band IMO exhibits stable, robust, and reliable performance in the desired mode over a range of voltages and magnetic fields.

Towards power improvement of all-fiber laser oscillators with 30 μm-core Yb-doped fibers by suppressing transverse mode instability

In this work, the transverse mode instability (TMI) behaviors in high-power all-fiber laser oscillators based on the ytterbium (Yb)-doped fiber (YDF) with a core diameter of 30 μm have been experimentally investigated for the first time. We have constructed all-fiber laser oscillators with the YDF with core/inner-cladding diameter of 30/400 μm and 30/600 μm, and investigated the corresponding TMI thresholds respectively. Experimental results manifest that the YDF with the larger cladding diameter corresponds to a higher TMI threshold for the two types of YDF with 30 μm core diameter. Compared with the 30/400 μm fiber, the 30/600 μm fiber can improve the TMI threshold by 2.0 times in the co-pumping configuration. By adopting bidirectional pumping configuration in the 30/600 μm fiber oscillator, a maximum output power is scaled to ∼ 3.6 kW without any sign of TMI, and no Raman Stokes light is observed in the output spectra. To the best of our knowledge, this is the first experimental demonstration of high-power all-fiber laser oscillators based on commercial 30/600 μm YDF. The experimental results show an excellent feasibility of all-fiber laser oscillators with large core and cladding diameter fiber for high-power operation.

A 300-GHz High-Power High-Efficiency Voltage-Controlled Oscillator With Low Power Variation

This letter presents a 300-GHz voltage-controlled oscillator (VCO) with high output power and efficiency. The VCO core employs a differential Colpitts topology with inductive degeneration to facilitate a fundamental oscillation at 300 GHz. A common-base output buffer regulates the output power to minimize the power variation during the frequency tuning. The 300-GHz VCO is fabricated in a 130-nm InP double heterojunction bipolar transistor (DHBT) technology. The VCO exhibits a measured frequency tuning range of 9.9 GHz (294.9–304.8 GHz). The peak output power is measured to be 4.7 dBm at 297 GHz. The dc power consumption is 75.6 mW, leading to a high dc-to-RF conversion efficiency of 3.9%. The VCO output power is maintained nearly constant with only a 0.3-dB variation over the entire frequency tuning range. The phase noise is −86.6 dBc/Hz at 10-MHz offset frequency.

Carrier-Envelope Offset Frequency Stabilization of a Thin-Disk Laser Oscillator via Depletion Modulation

We present a novel concept for the stabilization of the carrier-envelope offset (CEO) frequency of femtosecond pulse trains from thin-disk laser oscillators by exploiting gain depletion modulation in the active gain region. We shine a small fraction of the laser output power back onto the thin disk allowing the population inversion in the gain medium to be controlled. We employ this technique in our home-built Kerr-lens mode-locked Yb:YAG thin-disk laser and benchmark the performance against the proven technique of pump current modulation for CEO stabilization, showing that the two techniques have equivalent performance. The new method which only requires an additional AOM demonstrates a scalable and cost-effective method for CEO stabilization of high-power laser oscillators.

Cascaded nonlinearities in high-power femtosecond optical parametric oscillator

A femtosecond optical parametric oscillator (OPO) pumped by the second harmonic of a Yb:KGW solid-state oscillator was investigated. A comparison between lithium triborate (LBO) and barium borate (BBO) crystals was carried out, with particular attention to the impact of phase mismatch on pulse characteristics. The phase mismatch induces cascaded nonlinearity, which acts as an effective nonlinear refractive index. Negative group delay dispersion of the resonator and the effective nonlinear refractive index allows soliton-like pulse formation with varying pulse durations and a bandwidth-limited time-bandwidth product. Deviation from the soliton equation was observed when intracavity power was varied. The measured pulse durations were lower than the classical soliton duration, indicating that the pulses were not classical solitons. A Z-scan experiment was used to determine nonlinear refractive indices under conditions identical to those in the oscillator cavity. Unexpectedly, the signs of nonlinear refraction as measured by the Z scan and indirectly observed in the OPO proved to be the opposite. This sign reversal could be interpreted in terms of the competing cascaded nonlinearities corresponding to self-phase modulation and cross-phase modulation between the signal and the pump pulses in the nonlinear OPO crystal.

A 51.3 W, sub-kHz-linewidth linearly polarized all-fiber laser at 1560 nm

We demonstrate a continuous-wave high output power and narrow linewidth all-fiber master oscillator power amplifier laser at 1560 nm. The laser output power is 51.3 W with 36.38% optical-to-optical conversion efficiency and a linewidth of 0.865 kHz. The measured signal-to-noise ratio is 49 dB, and the polarization extinction ratio is greater than 20 dB. The laser can be used for high-efficiency nonlinear frequency conversion.

High Power and High Frequency CMOS Oscillator With Source-to-Drain Coupling and Capacitive Load Reduction Circuit

A design for a high-output-power, high-frequency CMOS oscillator is presented in this paper. The proposed oscillator can increase the output power by coupling the signal at the source of the core transistor to the drain of the buffer transistor. In addition, the source-to-drain coupling generates an optimum negative transconductance at the desired operating frequency. A capacitive load reduction circuit is also applied to increase the fundamental oscillation frequency. Using these techniques, the fundamental and push-push oscillators were implemented using 65 nm CMOS technology. The measurement results of the fundamental and push-push oscillators showed 2.7 and -25 dBm of peak differential output power at 194 and 394 GHz, respectively. In the 200 GHz frequency band, the proposed fundamental oscillator shows the highest output power among recently reported state-of-the-art CMOS oscillators.

Development Policy of a High-Power Millimeter-Wave Oscillator Gyrotron Intended for Microwave Rocket Research

Development Policy of a High-Power Millimeter-Wave Oscillator Gyrotron Intended for Microwave Rocket Research

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

Development of the Concept of High-Power Microwave Oscillators with Phase Locking by an External Signal

By simulations and experiments we develop a method for generation phase locking of a Ka-band subgigawatt nonstationary relativistic backward-wave oscillator (BWO) with an external radio pulse, the duration of which is considerably greater than the pulse duration of a driven BWO. The level of input power (about 20 kW), which provided phase locking with a total spread of about 1.2 rad, is determined both for the case of competition with an electromagnetic disturbance from a steep front of the beam current and the case of a slow current rise without phase locking in the self-excitation mode. The phase locking effect is achieved with a significant difference in the frequencies of the external radio pulse and driven BWO (about 1 GHz) and hence differs from the amplification mode.

Study of a 0.35 THz Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

A compact high-power extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS-sourced) sheet electron beam (SEB) is presented at 0.35 THz. It combines the advantages of a planar interaction circuit and a SEB generated from the PS discharge, including a large beam cross-section, high gain per unit length, and high current density with the additional benefit of not requiring an external focusing magnetic field. Staying within what is achievable with microfabrication techniques, the influence of tolerance on the Q value, resonance frequency, and characteristic impedance was investigated. The effect of surface roughness caused by the manufacturing method on Ohmic loss of the material surface was studied. The advanced microfabrication techniques of Ultra Violet Lithographie, Galvanik, and Abformung (UV-LIGA) and Nano-computer numerical control (Nano-CNC), which are capable of realizing high precision and a metal surface of sufficient smoothness, were proposed to manufacture the planar structures. The effect of plasma density in PS-sourced SEB on the resonance frequency of the EIO circuit was investigated. The simulation results showed that the output signal had a slight frequency upshift and a decrease of the output power as the plasma density increased at 0.35 THz, which is consistent with the theoretical analysis. Beam–wave interaction simulations for this planar EIO predicted a peak output power of 1.8 kW at ~0.35 THz using an effective value of conductivity of ${1.1} \times {10}^{{7}}$ S/m to take into account the skin depth and surface roughness.