Final Amplifier Stage Research Articles

All-Fiber Chirped-Pulse Amplifier Emitting 670 fs Pulses With 92 MW Peak Power

We demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The stretcher fiber was designed to match a diffraction grating compressor in the third-order dispersion and allowed the stretching of 6 ps 1026 nm chirped pulses up to ~500 ps. These pulses were amplified in the developed Yb-doped fiber and then compressed using a transmission grating compressor down to 670 fs duration with 3.5 W average power and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$61.8~\mu \text{J}$ </tex-math></inline-formula> pulse energy, corresponding to a record-high peak power of 92 MW.

A 102 W High-Power Linearly-Polarized All-Fiber Single-Frequency Laser at 1560 nm

A 1560 nm high-power linearly-polarized all-fiber single-frequency narrow-linewidth laser with near diffraction-limited beam quality is demonstrated. The Yb–Er energy transfer efficiency and the ability of the signal laser to capture pump light have been improved by specifically choosing the pumping wavelength and the input signal power in the final power amplifier stage of this laser system. Under the off-peak absorption pumping wavelength of 940 nm, along with the maximum input signal power of 6 W, a maximum output power of 102 W with a slope efficiency of 40.5% is acquired. At the highest output power status, a polarization extinction ratio (PER) of 15.5 dB, a linewidth of 3.05 kHz, and a beam quality of Mx2 = 1.14, My2 = 1.06 are obtained, respectively. This advanced single-frequency fiber laser has great potential for the long-range coherent Doppler lidar and the next generation of gravitational wave detection.

Versatile few-cycle high-energy MID-IR OPCPA for nonlinear optics, spectroscopy and imaging

High-power, high-energy, ultrashort, mid-infrared (MID-IR) laser systems operating at high repetition rates are of considerable interest for many science applications, such as coherent vibrational spectroscopy, label-free imaging, time-resolved pump-probe and high-harmonic generation studies. We developed an optical parametric chirped-pulse amplifier (OPCPA) system employing a difference-frequency generation in a lithium gallium sulfide nonlinear crystal in the final amplifier stage, which provides in principle the possibility for passive carrier-envelop-phase (CEP) stability. The OPCPA efficiently down-converts a 1 μm 200 μJ Yb-YAG pump pulse into the MID-IR spectral range generating μJ-level pulses at a repetition rate of 200 kHz. Two modes of operations providing complimentary MID-IR pulse properties are presented. Depending on the envisaged application, one can switch between (a) a wavelength-tunable (4.2–11 μm) source and (b) a broadband source centered at ≈8.5 μm by controlling the group-delay dispersion of the signal pulse. The broadband, high-energy MID-IR pulses have a short pulse duration of 74±2 fs, which corresponds to only ≈3 optical cycles at the central wavelength of 8.5 μm.

2차 고조파 최소화를 위한 광대역 중간주파수신기 개발

In this study, an eight-channel wide-band intermediate frequency (IF) receiver with an S-band input frequency, an output frequency of 200 MHz, and a signal bandwidth of 160 MHz was developed. The performance measurement results of the IF receiver showed that the fundamental signal and the second harmonic signal were involved in the bandwidth of the IF receiver output. This was caused by the nonlinearity characteristics of the final stage amplifier. Therefore, the final stage amplifier was changed to a differential amplifier, and the circuit of the IF receiver was modified to reduce the second harmonic signal. After improvement, an IF receiver with a gain of 33 dB, Output 1 dB Gain Compression Point(OP1dB) of &gt; 15.8 dBm, 3rd Output Intercept Point(OIP3) of &gt; 25.7 dBm was developed. The second harmonic suppressions were in the range of −83.30 dBc to −75.19 dBc when the fundamental signal output power was −10 dBm. This result satisfied the requirement of the system for the second harmonic suppression that was lower than −70 dBc.

Nonlinear spectral-phase-engineering strategies via quasiparametric chirped-pulse amplification

Ultrafast petawatt laser facilities require pulse compression strategies to deal with high-order dispersions. The third-order dispersion and especially the fourth-order dispersion need special compensation techniques. Quasiparametric amplification (QPA) has shown many benefits, such as high efficiency, and can bear the phase mismatch. In this paper we discuss the high-order dispersions induced in quasiparametric amplification and show the potential of the quasiparametric preamplifier with negligible pump depletion as a high-efficiency nonlinear phase compensator. We also discussed the high-order dispersions in QPA as a final stage amplifier where the pump is totally depleted and studied how those dispersions finally affect the amplified signal pulses.

Holmium-doped fiber amplifier for pumping a ZnGeP2 optical parametric oscillator.

We present a holmium-doped all-fiber master oscillator power amplifier (MOPA) system operating at 2108 nm targeting optical frequency conversion applications. The MOPA delivers pulses of 0.52 mJ energy at 10 kHz repetition rate after three amplification stages, with a close to square-shaped temporal profile of 50 ns duration, diffraction-limited beam quality and linear polarization. Challenges with achieving high gain and efficiency in the final amplification stage are discussed and attributed to quenching effects inferred from measurements of non-saturable absorption in the holmium fibers. Using this MOPA, we demonstrate a mid-IR conversion efficiency of 59% by direct pumping of a ZnGeP2 optical parametric oscillator.

Scaling of average power in sub-MW peak power Yb-doped tapered fiber picosecond pulse amplifiers.

Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S2 technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (105 hours) of the developed tapered fiber laser up to kilowatt output average power levels.

Design of Electron Tube and Semiconductor-Based and Voltage Stabilised Amplifier Circuits

Abstract In this project, an electron tube circuit consisting of a voltage stabiliser, preamplifier, and final stage amplifier circuit was designed. One of the main aims was to reuse as many components and materials as possible, thereby decreasing the project’s ecological footprint. The result is a hybrid electron tube amplifier. The function of the circuits was subsequently measured and analysed using various tests.

Comparative study on coherent noise in optical parametric and quasi-parametric chirped-pulse amplification

The coherent noise inherent to optical parametric chirped-pulse amplification (OPCPA) poses a fundamental limitation on the temporal quality of current ultrashort ultraintense lasers as well as their applications in strong-field physics research. Here we present a peer comparison of coherent noise between OPCPA and one of its alternatives — quasi-parametric chirped-pulse amplification (QPCPA). In the initial amplification stage termed as small-signal regime, the spectrum of chirped signal pulse in either OPCPA or QPCPA is exposed to the imperfections in pump pulse and optics, resulting in coherent noise pedestals in the time domain. While in the final amplification stage featured by amplifier saturation, the nonlinear growths of coherent noise become sharply different. We demonstrate that the non-parametric saturation behavior of QPCPA can greatly smooth the spectral distortions of signal, and thus bring down the coherent noise. Consequently, QPCPA scheme allows an improvement of pulse contrast by 2–3 orders of magnitude. The results presented in this paper provide a new perspective on high-contrast amplification of ultrashort pulses.

Narrow-linewidth linearly polarized nanosecond pulsed all-fiber amplifier with 75 kW peak power

We present a linearly polarized narrow-linewidth nanosecond fiber amplifier with a repetition rate of 400 kHz, pulse width of 2.8 ns, and a peak-power of up to 75 kW. The pulsed source is realized by modulating a continuous-wave 1064 nm single frequency laser fiber from an arbitrary waveform generator using an elctro-optic intensity modulator. The amplifier is achieved by Master Oscillator Power-Amplifier(MOPA) which is constituted by two Pre amplifier stage and a final amplifier stage. Good beam quality is found with measured beam propagation factor of M2 = 1.29.

Design, fabrication and testing of 7 kV, 6 A series connected IGBTs switch for triode based 20 kW stage ICRF amplifier

Many high power RF amplifiers, working at various frequencies are being used in ICRH group. High power RF output is obtained by amplification in stages i.e. 50 W, 2 kW, 20 kW, 200 kW. These stages are connected as chain and act as driver for the final stage amplifier of ∼1.5 MW. The high power tubes used in these amplifiers (e.g. Triodes, Tetrodes), need DC power sources up to few mega-watt working at ∼30 kV.The Triodes and Tetrodes that are in use are vulnerable to internal arc faults. To protect them from permanent damage, DC Voltages need to be removed within few tens of microseconds. The conventional power supplies that are being used are short circuited with a very fast switch known as crowbar switch. Ignitron based crowbar switches are widely in use but are being replaced because of the presence of large quantities of toxic Mercury. Further, above crowbar methods cause enormous stress (during the short circuit) of the power supply and can cause considerable disturbance in the power lines.With the development of solid state series switch such short circuits of conventional supplies can be avoided. It is envisaged that a series IGBT switch rated at 7 kV, 6 A would be suitable for HVDC power supply of 20 kW stage.This development involves complete switch design, selection of IGBTs, static and dynamic voltage balancing circuit design. Further, isolation level requirements of various circuits and components are analyzed and adequate isolation is provided. The 7 kV, 6 A series switch is fabricated and successfully tested.

Generation of 2- $$\mu$$ μ J 410-fs pulses from a single-mode chirped-pulse fiber laser operating at 1550 nm

We report on a simple, robust, femtosecond chirped-pulse-amplification system, based on Er- and Er-Yb-doped fibers, operating at a central wavelength of 1555 nm. The entire system is constructed from commercially available fiber components, except the grating compressor, for easy duplication by other researchers. The laser system produces chirped pulses with up to $$4\,\mu \hbox {J}$$ of pulse energy at 250 kHz. After dechirping, the pulse duration is 410 fs and the pulse energy is reduced to $$2\,\mu \hbox {J}$$ . The repetition rate of the laser is electronically tunable between 125 kHz and 60 MHz, limited by strong amplified spontaneous emission (ASE) generation at the low end. The amplifier system is almost completely fiber integrated, except for the pump delivery into the final amplifier stage, which is free-space backward-pumped to reduce undesired nonlinear effects, and the compressor which was designed using the gratings. The laser is practically free of misalignment and has exhibited excellent long-term stability during its use in various experiments for more than 600 working hours.

295-kW peak power picosecond pulses from a thulium-doped-fiber MOPA and the generation of watt-level >2.5-octave supercontinuum extending up to 5 μm.

We report a gain-switched diode-seeded thulium doped fiber master oscillator power amplifier (MOPA) producing up to 295-kW picosecond pulses (35 ps) at a repetition rate of 1 MHz with a good beam quality (M2 ~1.3). A narrow-band, grating-based filter was incorporated within the amplifier chain to restrict the accumulation of nonlinear spectral broadening and counter-pumping of a short length of large-mode-area (LMA) fiber was used in the final stage amplifier to further reduce nonlinear effects. Finally, we generated watt-level >2.5-octave supercontinuum spanning from 750 nm to 5000 nm by using the MOPA output to pump an indium fluoride fiber.

Phase error reduction of a digitally controlled phase shifter utilizing a variable phase and gain amplifier

This paper presents a variable phase and gain amplifier (VPGA), featured in a 4-bit digitally controlled phase shifter, that enables significant phase error reduction. The functionality of the VPGA is demonstrated by utilizing it between the third and fourth bits of a digitally controlled phase shifter. The first three bits are implemented using distributed active switches based on HBTs and the fourth bit is realized in a final amplification stage based on switching between common-base (CB) and common-emitter (CE) topologies. By the use of the VPGA, RMS phase error is reduced from 22° to 11° with the cost of reduced gain (0.1–2.5dB) and increased RMS gain error (1.0–2.2dB). A total of 360° phase shift is achieved in 4bit resolution with an RMS phase error of 0.1° at 10.5GHz, and a maximum 11° phase error in 4.5GHz bandwidth. The chip area is 2.150mm×1.040mm including pads, and the VPGA consumes only 0.32mm×0.410mm area. The chip is implemented in a 0.25-µm SiGe BiCMOS process. These performance parameters are attributed to the adjustment method by the VPGA applied in this work, which enables superior performance than the-state-of-the-art utilizing similar technologies.

Conceptual design of sub-exa-watt system by using optical parametric chirped pulse amplification

A 50 PW ultrahigh-peak-power laser has been conceptually designed, which is based on optical parametric chirped pulse amplification (OPCPA). A 250 J DPSSL and a flash- lamp-pumped kJ laser are adopted as new repeatable pump source. The existed LFEX-laser with more than ten kilo joules are used in the final amplifier stage and the OPCPA with the 2x2 tiled pump beams in random phase has been proposed with several ten centimeter aperture. A pulse duration of amplified pulses is set at less than 10 fs. A broadband OPCPA with ∼500 nm of the gain spectral width near 1 μm is required. A partially deuterated KDP (p-DKDP) crystal is one of the most promising nonlinear crystals and our numerical calculation ensured such ultra-broad gain width. p-DKDP crystals with several deuteration ratio have been successfully grown.

Practical technique for improving all-fiber coherent combination of multistage high-power ytterbium fiber amplifiers

We demonstrate coherent combination of two high-gain and high-power single-mode 1μm fiber amplifiers via direct pump current modulation. Each all-fiber amplifier channel, which is built as a master oscillator-multiple amplifier architecture, can operate either continuous waves or 4ns, 1MHz pulses with average power of 50W and more than 55dB gain. These two amplifiers are coherently combined by modulating the pump currents of the preamplifiers in a multidither arrangement. The key feature of the scaling scheme is the insertion of a designated second stage preamplifier between the first stage preamplifier and the final power amplifier stage, so as to improve the coherency and to minimize the gain variation of the preamplifier to the power amplifier. Otherwise, overall phase control of the fiber amplifiers with this method is not possible at such a high power level.

μJ-level, kHz-repetition rate femtosecond fiber-CPA system at 1555 nm

In this work, we demonstrate a high-power, fiber-based chirped pulse amplification (CPA) setup utilizing Er- and Er/Yb-doped fibers, operating at 1555nm central wavelength. The integrated all-fiber pulse-picker allows to reduce the repetition frequency down to the kHz-range, which enables generation of sub-picosecond pulses with energies above 2μJ and pulse peak power exceeding 1MW. The system utilizes an Er/Yb co-doped large mode area fiber in the final amplification stage. Thanks to the used mode-field adapters and fiber-based components, the setup is almost fully fiberized, except the bulk grating pulse compressor. In order to provide compactness and simplicity, the compressor was designed using dense 1100 lines per millimeter gratings, that allow to keep the small grating separation.

Fiber laser pumped burst-mode operated picosecond mid-infrared laser**Project supported by the National Natural Science Foundation of China (Grant No. 61078015) and the National Basic Research Program of China (Grant No. 2011CB311803).

We demonstrate a compact periodically poled MgO-doped lithium niobate (MgO:PPLN)-based optical parametric oscillator (OPO) quasi-synchronously pumped by a fiber laser system with burst-mode operation. The pump source is a peak-power-selectable pulse-multiplied picosecond Yb fiber laser. The chirped pulses from a figure of eight-cavity mode-locked fiber laser seed are narrowed to a duration of less than 50 ps using an FBG reflector and a circulator. The narrowed pulses are directed to pass through a pulse multiplier and to form pulse bunches, each of which is composed of 13 sub-pulses. The obtained pulse bunches are amplified by two-stage fiber pre-amplifiers: one-stage is core-pumped and the other is cladding-pumped. A fiberized acousto–optic modulator is inserted to control the pulse repetition rate (PRR) of the pulse bunches before they are power-amplified in the final amplifier stage with a large mode area (LMA) PM Yb-doped fiber. The maximum average powers from the final amplifier are 85 W, 60 W, and 45 W, respectively, corresponding to the PRR of 2.72 MHz, 1.36 MHz, and 0.68 MHz. The amplified pulses are directed to pump an MgO:PPLN-based optical parametric oscillator (OPO). A maximum peak power at 3.45 μm is obtained approximately to be 8.4 kW. Detailed performance characteristics are presented.

High power fiber laser system for a high repetition rate laserwire

We present the development of a high power fiber laser system to investigate its suitability for use in a transverse electron beam profile monitor, i.e., a laserwire. A system capable of producing individual pulses up to $165.8\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ at 1036 nm with a full width at half maximum of $1.92\ifmmode\pm\else\textpm\fi{}0.12\text{ }\mathrm{ps}$ at 6.49 MHz is demonstrated using a master oscillator power amplifier design with a final amplification stage in a rod-type photonic crystal fiber. The pulses are produced in trains of 1 ms in a novel burst mode amplification scheme to match the bunch pattern of the charged particles in an accelerator. This method allows pulse energies up to an order of magnitude greater than the steady-state value of $17.0\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ to be achieved at the beginning of the burst with a demonstrated peak power of $25.8\ifmmode\pm\else\textpm\fi{}1.7\text{ }\mathrm{MW}$ after compression. The system is also shown to demonstrate excellent spatial quality with an ${M}^{2}=1.26\ifmmode\pm\else\textpm\fi{}0.01$ in both dimensions, which would allow nearly diffraction limited focusing to be achieved.

Submicrojoule femtosecond erbium-doped fibre laser for the generation of dispersive waves at submicron wavelengths

We have demonstrated a femtosecond erbium-doped fibre laser system built in the master oscillator/power amplifier (MOPA) approach. The final amplifier stage utilises a specially designed large mode area active fibre cladding-pumped by multimode laser diodes. The system is capable of generating submicrojoule pulses at a wavelength near . We have obtained pulses with an energy of . The output of the system can be converted to wavelengths shorter than through the generation of dispersive waves in passive nonlinear fibre. We have obtained ultra-short pulses with a spectral width of and a centre wavelength of , which can be used as a seed signal in parametric amplifiers in designing petawatt laser systems.