High-power CEP-stable few-cycle fiber lasers

Effects related to the use of high power and repetition rate lasers in ablation of metals and silicon were investigated.Numeric simulation and experiments were performed to find out conditions of the efficient use of laser energy.Accumulation of defects lowers the ablation threshold when a high pulse-repetition rate is applied.An optimum beam waist exists for certain pulse energy to maximize the ablation but the waist does not allow achieving a high processing accuracy.Energy efficiency of the laser processing falls down when pulse energy exceeds the material dependant limit.Plasma shielding was assumed to be the main limiting factor in processing efficiency of metals with the high power picosecond lasers.Intelligent control of process parameters is required to keep optimal conditions for the material removal by laser ablation.

We have developed a high-power single-frequency pulsed fiber laser system utilizing a Yb-doped rod-type large-pitch photonic crystal fiber. The maximal peak power is 33 kW with a 0.4-mJ pulse energy and a 70-MHz linewidth, and the average power is 60 W at a 150-kHz repetition rate. At a higher repetition rate, the average power can be increased to 190 W, which is limited by the onset of transverse-mode instability in the large-pitch photonic crystal fiber. The output beam is linearly polarized and shows a good beam quality with a M2 value of 1.2.

The quantum defect (QD) is an important issue that demands prompt attention in high-power fiber lasers. A large QD may aggravate the thermal load in the laser, which would impact the frequency, amplitude noise and mode stability, and threaten the security of the high-power laser system. Here, we propose and demonstrate a cladding-pumped Raman fiber laser (RFL) with QD of less than 1%. Using the Raman gain of the boson peak in a phosphorus-doped fiber to enable the cladding pump, the QD is reduced to as low as 0.78% with a 23.7 W output power. To our knowledge, this is the lowest QD ever reported in a cladding-pumped RFL. Furthermore, the output power can be scaled to 47.7 W with a QD of 1.29%. This work not only offers a preliminary platform for the realization of high-power low-QD fiber lasers, but also proves the great potential of low-QD fiber lasers in power scaling.

The recent demonstration of rare‐earth‐doped fiber lasers with a continuous‐wave output power approaching the 10‐kW level with diffraction‐limited beam quality proves that fiber lasers constitute a scalable solid‐state laser concept in terms of average power. In order to generate high peak power pulses from a fiber several fundamental limitations have to be overcome. This can be achieved by novel experimental strategies and fiber designs that offer an enormous potential towards ultrafast laser systems combining high average powers (> kW) and high peak power (> GW). In this paper the challenges, achievements and perspectives of ultrashort pulse generation and amplification in fibers are reviewed. This kind of laser system will have a tremendous impact on strong‐field physics experiments, such as the generation of coherent light by high‐harmonic generation. So far, applications in the interesting EUV spectral range suffer from the very low photon count leading to nonrelevant integration times with highly sophisticated detection schemes. High repetition rate high average power fiber lasers can potentially solve this issue. First demonstrations of high repetition‐rate strong‐field physics experiments using novel fiber laser systems will be discussed.

A theoretical model is developed which characterizes the intracavity pulse evolutions in high-power fiber lasers. It is shown that experimentally observed dynamics of the key pulse parameters can be described by a reduced model of ordinary differential equations. Critical in driving the intracavity dynamics is the amplitude and phase modulations generated by the discrete elements in the laser. The theory gives a simple geometrical description of the intracavity dynamics and possible operation modes of the laser cavity. Furthermore, it provides a simple and efficient method for optimizing the performance of complex multiparametric laser systems.

We present a high peak and average power laser system with ultrashort pulses at high repetition rates. Pulse shortening and peak power enhancement of a state-of-the-art fiber laser system is achieved by utilizing nonlinearity, namely selfphase modulation and subsequent compression in a chirped mirror compressor. The nonlinear interaction is achieved by propagation in a noble gas filled hollow core fiber with an inner diameter of 200 μm and a length of 0.5 m. A total second dispersion of -7000 fs² is applied by a chirped mirror compressor resulting in ultrashort pulses of 71 fs duration. This is achieved by coupling 400 MW, 800 fs pulses from the CPA system to the Xenon filled hollow core fiber. The average power at the output of the compressor is measured to be 10 W at 50 kHz repetition rate resulting in 200 μJ pulse energy. Hence, the compressed pulses have a peak power of more than 2 GW. Consequently, the pulses of the CPA system are shortened by a factor of ten and the peak power is enhanced by a factor of 5. In addition this approach offers further peak and average power scalability.

Nanosecond (ns) pulsed laser with high average power and high pulse repetition rate above 50 kHz is a potential solution for laser cutting, laser welding, laser cleaning and many other industry processing scenarios. Although Nd:YAG is a widely used solid-state gain medium, it is difficult to obtain ns pulsed laser with repetition rate above 50 kHz due to its limited stimulated emission cross section and thermal distortion under high pump intensity. In this paper, a kilowatt-level 100 kHz high repetition rate ns Nd:YAG master oscillator power amplifier (MOPA) laser system is reported, and a general optimization method was used to obtain a 205 W seed laser with a high repetition rate of 100 kHz. After beam shaping elements, the seed laser was amplified to 1008 W by a two-rod Nd:YAG preamplifier and a two-rod Nd:YAG main amplifier. The pulse-to-pulse stability factor of the pulsed laser was 0.961 and the pulse width was measured as 142.8 ns. The beam parameter product in the horizontal axis and vertical axis were measured as BPPx = 2.81 mm∙mrad and BPPy = 2.78 mm∙mrad respectively. This is the first time to obtain a kilowatt-level average power ns pulsed laser with repetition rate above 50 kHz using Nd:YAG, and the compact MOPA system is also suitable for power scaling and other practical use.

EUV lithography (EUVL) is the candidate for next generation lithography to be introduced by the semiconductor industry to HVM (high volume manufacturing) in 2013. The power of the EUVL light source has to be at least 115W at a wavelength of 13.5nm. A laser produced plasma (LPP) is the main candidate for this light source but a cost effective laser driver is the key requirement for the realization of this concept. We are currently developing a high power and high repetition rate CO 2 laser system to achieve 50 W intermediate focus EUV power with a Tin droplet target. A high conversion efficiency (CE) from laser energy to EUV in-band energy is the most important issue for the concept to be realized. A CE of more than 2 % has been obtained with a Tin target and a CO 2 laser which is also predicted by numerical simulation analysis. The high CE requires a short laser pulse of less than 15 ns. This paper describes the development of a CO 2 laser system that is based on RF-excited, axial flow CO 2 laser amplifiers. The system produces a short pulse length about 15 ns and a nominal average power of several kW at a repetition rate of 100 kHz, An output power of 2.6 kW has been achieved with a pulse length between 15~30 ns at 130 kHz repetition rate at small signal amplification. The phase distortion of the laser beam after amplification is negligible and the beam can be focused to about 150mm diameter in 1/e 2 . The conceptual design of the CO 2 laser system for LPP EUVL light source, and the laser performance are reported. The further increase of the amplification efficiency is discussed to obtain 5 kW average output power.

Summary form only given. 1.7 cycles midIR pulse has been measured via a tunable SEA-F-SPIDER arrangement providing both temporal and spatial characterisation. The intrinsic CEP stability of the pulse is monitored and stabilised from a simple interferometric measurement.A significant part of future development in attosecond science [1] and especially molecular high harmonic generation spectroscopy [2] is likely to require the use of intense and ultrashort infrared laser pulses in the mid-infrared (mid-IR) spectral range. Secondary sources based on Ti:Sapphire laser pumped Optical Parametric Amplifier (OPA) in nonlinear crystals are readily available as commercial systems and their extension to few-cycles mid-IR sources have been demonstrated [3,4]. MidIR pulses at 1.7um are generated by self phase modulation (SPM) of the idler pulse of a HE-TOPAS (Light Conversion) in an Argon filled hollow core fibre and subsequent compression in Fused Silica. The carrier envelope phase (CEP) of such short pulses is of prime interest for strong field physics experiment. Its pulse to pulse passive stability is here assured by the difference frequency generation mechanism in the last OPA stage [4]. Providing a stable pump source (~1% pulse to pulse energy fluctuation) the main source of CEP fluctuation is due to the temporal interferometric jitter between pump and seed in the DFG process.Our temporal characterisation relies on SEA-SPIDER [6] with direct spectral filtering for ancilla preparation (SEA-F-SPIDER [7,8]). A spatio-temporal reconstruction of our single cycle pulse is shown in fig.1 (inset) from which a lineout has been extracted (red curve) and compared to the Fourier Limit (in black). Apulse duration of 9.9fs has been obtained representing 1.7 cycles at 1.7um. Interference fringes at ~700nm are produced from SHG of the seed for one part and SPM of the pump in the last OPA crystal for the other part. The interference of these two almost balanced paths (about 3 meters each) creates fringes that are stabilised and controlled by a piezo actuator added on the pump arm. The evolution of the interferogram as a function of time with a slow loop stabilisation turned on is visible on figure 1 b) for different relative set phases. The stability in each interval is excellent and on the order of 100 mrad standard deviation. Since the stability of this interferometer is directly coupled to the CEP stability of the generated pulse, this measurement is a monitor of an important source of the CEP fluctuations.

Fibers for high-power lasers and amplifiers

We developed a compact and high-power mode-locked fiber laser for three-dimensional optical memory. Fiber lasers have the potential to be compact and stable light sources that can replace bulk solid-state lasers. To generate high-power pulses, we used stretched-pulse mode locking. The average power and pulse width of the output pulse from the fiber laser that we developed were 109 mW and 2.1 ps, respectively. The dispersion of the output pulse was compensated with an external single-mode fiber of 2.5 m length. The pulse was compressed from 2.1 ps to 93 fs by dispersion compensation. The pulse emitted from this fiber laser has an energy sufficient to generate two photon recording effectively, so the fiber laser we have developed is possible to use as a light source of three-dimensional optical memory.

In this contribution we demonstrate the nonlinear pulse compression of an ultrafast thulium-doped fiber laser down to 14 fs FWHM duration (sub-3 optical cycles) at a record average power of 43 W and 34.5 μJ pulse energy. To the best of our knowledge, we present the highest average power few-cycle laser source at 2 μm wavelength. This performance level in combination with GW-class peak power makes our laser source extremely interesting for driving high-harmonic generation or for generating mid-infrared frequency combs via intra-pulse frequency down-conversion at an unprecedented average power. The experiments were enabled by an ultrafast thulium-doped fiber laser delivering 110 fs pulses at high repetition rates, and an argon gas-filled antiresonant hollow-core fiber (ARHCF) with excellent transmission and weak anomalous dispersion, leading to the self-compression of the pulses. We have shown that ARHCFs are well-suited for nonlinear pulse compression around 2 μm wavelength and that this concept features excellent power handling capabilities. Based on this result, we discuss the next steps for energy and average power scaling including upscaling the fiber dimensions in order to fully exploit the capabilities of our laser system, which can deliver several GW of peak power. This way, a 100 W-class laser source with mJ-level few-cycle pulses at 2 μm wavelength is feasible in the near future.

In contrast to conventional laser amplifiers, optical parametric amplifiers (OPAs) offer an enormous gain bandwidth that can reach or even exceed one optical octave supporting extremely short pulses. At the same time, OPAs have the reputation to be scalable to high average powers due to the lack of energy storage in the nonlinear medium. In particular, pumping OPAs with pulsed high average power fiber lasers enabled the current record level of more than 20 W of output power. However, thermal effects similar to those observed in conventional lasers will ultimately limit average power scaling of OPAs. In this contribution, experimental and numerical results on thermal effects in high average power OPAs are presented.

We report an all-fiberized SBS-based high repetition rate sub-nanosecond Yb fiber laser for supercontinuum generation. The high repetition rate ns laser pulses were produced from a fiber Bragg grating (FBG)-constructed fiber laser cavity consisting of a piece of double cladding Yb fiber as the gain medium and a short piece of Bi/Cr-doped fiber as a saturable absorber (SA). By optimizing the fiber length of the Bi/Cr-doped fiber and the reflectivity of the FBG, the Q-switching state of the fiber laser can be adjusted so that the energy storing condition within the fiber cavity can assure the start of stimulated Brillouin scattering (SBS) and as a result, compress the laser pulse duration. The fiber laser had an average laser power output of 1.2 W at 1064 nm with pulse repetition rate of about 80 kHz, almost four times the reported results. The pulse duration was about 1 ns with peak power of about 15 kW. After one stage of amplification, the laser power was raised to about 3 W and was used to pump a 20 m long photonic crystal fiber (PCF). Supercontiuum (SC) laser output was obtained with average power up to 1.24 W and spectrum spanning from 550 to 2200 nm.

A high-power wavelength-tunable femtosecond fiber laser source is developed based on photonic-crystal fiber technology. Laser oscillator and amplifier stages in this system employ diode-pumped ytterbium-doped single-polarization large-mode-area photonic-crystal fibers in a stretcher-free configuration, delivering laser pulses with an average power of 10.4 W, a pulse width of 52 fs, and a peak power of 4 MW at a repetition rate of 50 MHz after pulse compression. Nonlinear transformation of such laser pulses in a highly nonlinear photonic-crystal fiber yields light pulses smoothly tunable within the range of wavelengths from 1.0 to 1.4 μm and allows the generation of supercontinuum stretching from 450 to at least 1750 nm. We report experiments on silicon microprocessing and chromium nanofilm patterning at a high repetition rate, demonstrating the potential of the developed fiber-laser source for fast micromachining, microfabrication, and microprocessing.