Doped Fiber Laser System Research Articles

High repetition rate ultrafast laser technology for drivinghigh-order harmonic generation

The interaction of ultra-intense and ultrafast laser pulses with matter has paved the way for generation of ultrashort pulses in the attosecond timescale and coherent radiation in the extreme ultraviolet (XUV) and soft X-ray region at a table-top size. On one hand, it provides attosecond temporal resolution and atomic spatial resolution for ultrafast electron dynamics in atoms, molecules and condensed matters; on the other hand, it inspires the great potential of optical frequency comb (OFC) in precise measurements of transition frequencies and hyperfine splitting, a verification of basic physical constants such as the Rydberg constant, and the research on precise optical atomic clocks. With the expansion of the above-mentioned scientific researches, the repetition rate of the driving source has become the key factor to break through the measurement accuracy obstacles. Therefore, the research on high-repetition rate ultrafast laser technology for driving high-order harmonic generation (HHG) commences. In this paper, the high repetition rate HHG as the starting point, the method applied, the main parameter indices and the technical difficulties of the driving source are summarized. This paper can be a reference for the future development of high-repetition rate ultrafast laser technology for driving HHG. First of all, the research involving femtosecond enhancement cavity (fsEC) using passive amplification is introduced. The repetition rate of the high-order harmonics as obtained from this method has exceeded 100 MHz, which serves as the main technical means to obtain OFC in the XUV region. However, key issues, such as dispersion control of the high-precision resonator and stabilization of femtosecond laser frequency, are still the focus for future research and the current application difficulties of fsEC technology. Secondly, the research on Ti:sapphire laser system, ytterbium (Yb) doped fiber laser system and Yb all-solid-state laser system are reviewed. The few-cycle femtosecond pulse output by the Ti:sapphire laser system has an extremely high peak power, and it is the main driving source for generation of high-order harmonics and isolated attosecond pulses. However, constrained by the average power of the Ti:sapphire laser system, it is difficult to increase the repetition rate of XUV laser pulses to hundreds kilohertz or even megahertz. The thin-disk laser oscillator and the Yb fiber amplifier can produce femtosecond pulses with high average power. By means of nonlinear spectral broadening and pulse compression, an extreme ultraviolet radiation with a repetition rate of 10 MHz has been obtained in the HHG process of gases, yet the system structure remains complex and the technical difficulty is still high. Meanwhile, the Yb all-solid-state femtosecond amplifier, which can directly produce ultrashort pulses with high peak power, serves as an ideal driving source to realize HHG with the repetition rate on the MHz level. In view of the development trends of high-repetition rate driving sources, and based on the research experience on high-power Yb femtosecond lasers of our group, we propose high repetition rate ultrashort all-solid-state Yb amplifier for driving MHz repetition rate HHG with sub -40 fs all-solid-state Yb oscillator as seed and key techniques of gain narrowing suppression, precise dispersion control and efficient thermal management. Finally, the parameters of various technical means in passive amplification and gain amplification, as well as the key problems that need to be solved, are summarized, and the development trends of high repetition rate ultrafast laser technology for driving HHG are prospected.

910-MHz, watt-level, signal-power-enhanced, compact femtosecond optical parametric oscillator based on bidirectional pumping technique

We propose a high-efficiency and compact 910-MHz femtosecond optical parametric oscillator, which is harmonically pumped by a ∼101 MHz Yb doped fiber laser system. The OPO is capable of delivering watt-level, power-enhanced signals across the telecommunication waveband. The signal power enhancement is realized by exploiting the bidirectional pumping technique. A maximum signal power of 1.04W at 1502nm is obtained for an input pump power of 3.8W. Tunable near-infrared signal pulses with a wavelength range between 1350 and 1610nm are measured, and the pulse durations vary from 193 to 464fs. This compact and economic design provides a solution for efficient high repetition rate pulse generation over a large wavelength span, which will be beneficial for a variety of practical applications.

High Power 4.2-Cycle Mid-Infrared Pulses From a Self-Compression Optical Parametric Oscillator

We demonstrate the generation of few cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical, and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.

Generation of stable cnoidal waves in an erbium doped fiber laser system

In this work an all-fiber laser system able to produce stable cnoidal waves in both limits, i.e. in sinusoidal and soliton limits, is presented. The system is constructed using an erbium highly-doped fiber as an active media and an electrooptic modulator as the core of the control tool. This control tool is able to modulate the cavity losses. The direct modulation allows to control the pulses properties, shape, width and intensity. The proposed system is well described by a three level laser model based on the Statz-de Mars rate equations. Numerical and Experimental results are presented.

Evolution of the nth Stokes-wave due to stimulated Raman scattering in single-mode fibers

The evolution of the nth analytical solutions of commonly used Raman equations, analyzed by numerical simulation and experimentally, is reported. In the experiment, a 1 km undoped single-mode fiber was pumped with an ytterbium doped fiber laser system (FL) in continuous wave regime at 1064 nm in a free running configuration. We showed that it is possible to obtain up to the nth power thresholds and the maximum power for each Stokes wave by using compact analytical solutions as a first approximation in a simple, quick process.

Effect of Attenuation on Four-Wave Mixing Generation

Four-wave mixing (FWM) phenomenon can be produced by using a different refractive index of the optical fiber in an erbium doped fiber laser system. In our study, we used a 20 meter length of Photonics crystal fiber (PCF) to generate multiwavelength signals. Variable optical attenuator is employed in the setup to study its effect on the output signals. Results show that uniform signals were obtained by using the VOA to suppress other modes of wavelengths which occurred before placing the VOA. New lasing wavelength at 1530.445 nm, 1531.450 nm, 1531.960 and 1532.415 nm were produced uniformly.

Operating regimes of an optical coupler combined fiber laser system

Rate equations have been used to study an optical coupler combined Er 3+-doped fiber laser system, in which a common mirror attached at one of the coupler output ports is shared by two constituent lasers. Three regimes may be identified for the system depending on the relative magnitude of the wavelength difference δλ between the two constituent laser fields to the reflection band of the fiber gratings. Taking into consideration the transmission features of the coupler-mirror combination, appropriate boundary conditions have been established according to the operation regimes of the compound system. After neglecting the backgrounds losses of the fiber, rate equations have been simplified to a set of algebra equations. Based on the algebra equations, we discussed in detail the case where the laser field generated by the first laser with a wavelength slightly different from that of the field oscillating inside the second laser, is injected into the other laser by the common mirror, leading to a decreased gain for the oscillating field.

Front-end system used in PM-Yb fiber laser for high-peak power laser system

We have demonstrated the generation of optional pulse shape by Yb doped fiber laser system for EUV lithography laser and IFE front-end system. Yb fiber laser system operated the polarization-maintained pulses for single-transverse and -longitudinal mode using fiber Bragg grating (FBG). The several output waveforms obtained including rectangular pulses from 1ns to 12.5 ns with a 500-ps rise time, and Gaussian pulses from 1 ns to 25 ns. The increase in output power has been achieved to 30W (300μJ, 100kHz) with a 30-μm core Yb double-clad LMA fiber.

Octave wide tunable UV-pumped NOPA: pulses down to 20 fs at 0.5 MHz repetition rate

Femtosecond laser pulses, which are tunable from 440 to 990 nm, are generated at MHz repetition rates by noncollinear parametric amplification (NOPA). The pulses have durations of 20 to 30 fs over the major part of the tuning range and a high energy stability of 1.3% (rms). The NOPA is pumped with ultraviolet pulses from the third harmonic of an ytterbium doped fiber laser system and seeded by a smooth continuum generated in bulk sapphire. The residual second harmonic is used to pump an additional NOPA, which is independently tunable from 620 to 990 nm. Interference experiments show that the two NOPA systems have a precisely locked relative phase, despite of being pumped by different harmonics with a random phase jitter. This demonstrates that the phase of pulses generated by optical parametric amplification does not depend on the pump phase.

Generation of Optional Pulse Shape by Yb Doped Fiber Laser System

We have demonstrated the generation of optional pulse shape by Yb doped fiber laser system. Yb fiber laser system operated the polarization-maintained pulsed at 1064 nm for single-transverse and -longitudinal mode using fiber Bragg grating (FBG), and this system generates optional pulse shapes that sliced up the Yb fiber oscillator by a LN (LiNbO3) intense modulator with 12.5-GHz bandwidth. The developed electronic-pulseshape generator supplies an arbitrary-voltage pulse to an integrated optic LN amplifier modulator which modulates the laser pulse. The output laser waveforms can obtain corresponding to the electronic waveforms such as rectangular, Gaussian and multi pulses with a 300-ps resolution.