Saturable Bragg Reflector Research Articles

Probing the ultrafast gain and refractive index dynamics of a VECSEL

Typically, strong gain saturation and gain dynamics play a crucial role in semiconductor laser mode-locking. While there have been several investigations of the ultrafast gain dynamics in vertical-external-cavity surface-emitting lasers (VECSELs), little is known about the associated refractive index changes. Yet, such refractive index changes do not only have a profound impact on the pulse formation process leading to self-phase modulation, which needs to be compensated by dispersion, but they are also of particular relevance for assessing the feasibility of Kerr-lens mode-locking of VECSELs. Here, we measure both refractive index as well as gain dynamics of a VECSEL chip using the ultrafast beam deflection method. We find that, in contrast to the gain dynamics, the refractive index dynamics is dominated by an instantaneous (∼100 fs) and a very slow component (∼100 ps). The time-resolved measurement of nonlinear refraction allows us to predict a pulse-length dependent, effective nonlinear refractive index n2,eff, which is shown to be negative and on the order of 10−16m2/W for short pulse lengths (∼100 fs). It becomes positive for large excitation fluences and large pulse lengths (few ps). These results agree with some previous reports of self-mode-locked VECSELs for which the cavity design and pulse properties determine sign and strength of the nonlinear refractive index when assuming Kerr-lens mode-locking.

Towards a space-qualified Kerr-lens mode-locked laser.

We report a 1.5-GHz Kerr-lens mode-locked (KLM) Yb:Y2O3 ring laser constructed by directly bonding the cavity components onto an aluminum baseplate. Stable unidirectional operation with an output power ≥10mW was obtained for pump-diode currents of 300-500 mA, corresponding to a total electrical power consumption of 1.5 W. After repetition rate stabilization, a comparison with a conventionally constructed identical laser showed a 50% reduction in phase noise. In free-running operation the bonded laser showed superior passive repetition rate stability. The bonding process follows an already proven integration approach in space-borne instrumentation, mapping a development pathway for KLM lasers in aerospace applications.

Kerr-lens mode-locked Yb:LuAG ceramic thin-disk laser.

A Kerr-lens mode-locked (KLM) thin-disk laser with Yb:LuAG ceramic was demonstrated. Yb:LuAG ceramic is an attractive material for high-power lasers due to its high thermal conductivity and large emission cross section. The highest output power of 17 W with a pulse duration of 130 fs was achieved. Moreover, the pulse duration of 88 fs was also obtained with a high-Q factor cavity. To the best of our knowledge, this is the first demonstration of a KLM thin-disk laser based on Yb:LuAG, including both ceramic and single crystal. The results show the usefulness of ceramic thin disks for high-power ultrashort pulse laser sources.

Passively Q-switched Cr:LiCAF laser with a saturable Bragg reflector

We present a low-cost multimode diode pumped passively Q-switched Cr:LiCAF laser operating near 800 nm. AlGaAs-based saturable Bragg reflectors (SBRs) were used for passive Q-switching. The system is experimentally characterized in detail using four different output couplers and two SBRs with different modulation depths. Pulse widths in the 1.5–4 µs range at repetition rates between 18 and 40 kHz were achieved with average powers up to 125 mW. The experimentally obtained results are compared with basic Q-switching theory and further performance improvement in terms of pulse length shortening and peak power scaling is elaborated.

Diffractive saturable loss mechanism in Kerr-lens mode-locked lasers: direct observation and simulation.

Passive mode-locking relies critically on a saturable loss mechanism to form ultrashort pulses. However, in Kerr-lens mode-locking (KLM), no actual absorption takes place, but rather losses appear due to diffraction, and actual light must escape the cavity. The Kerr-lens effect works to generate through diffraction an effective instantaneous saturable absorber that depends delicately on the interplay between the spatial and temporal profiles of the pulse. Despite the importance of KLM as a technique for generating ultrafast pulses and the fundamental role of diffraction losses in its operation, these losses have never been observed directly. Here, we measure the light that leaks out due to diffraction losses in a hard-aperture Kerr-lens mode-locked Ti:sapphire laser, and compare the measured results with a numerical theory that explicitly calculates the spatiotemporal behavior of the pulse.

Intra-oscillator high harmonic generation in a thin-disk laser operating in the 100-fs regime.

We demonstrate that Kerr lens modelocking is well-suited for operating an ultrafast thin-disk laser with intra-oscillator high harmonic generation (HHG) in the 100-fs pulse duration regime. Exploiting nearly the full emission bandwidth of the gain material Yb:YAG, we generate 105-fs pulses with an intracavity peak power of 365 MW and an intracavity average power of 470W. We drive HHG in argon with a peak intensity of ∼7⋅1013 W/cm2 at a repetition rate of 11MHz. Extreme-ultraviolet (XUV) light is generated up to the 31st harmonic order (H31) at 37 eV, with an average power of ∼0.4 µW in H25 at 30 eV. This work presents a considerable increase in performance of XUV sources based on intra-oscillator HHG and confirms that this approach is a promising technology for simple and portable XUV sources at MHz repetition rates.

Tunable Q-switched mode-locked Cr:LiSAF laser

We have achieved stable passively Q-switched mode-locked (QML) operation in a Cr: LiSAF laser. In the experiments, a 10 mm long, %1.5 Cr-doped LiSAF crystal was pumped by two 150 mW single-mode diodes at 660 nm. An AlGaAs based saturable Bragg reflector (SBR) with a central wavelength of 850 nm and a modulation depth of 0.8% was used for initiating and sustaining QML operation. The QML laser was self-starting and robust against environmental fluctuations. In QML operation, the system produced sub-5-ps long mode-locked pulses at 150–200 MHz, inside 2–2.5 μs long Q-switched envelopes with 30–70 kHz repetition rate. The modulation depth of mode-locking stayed around 50%. We have also observed variation of mode-locked pulse parameters within the Q-switched envelope and also from Q-switched pulse to pulse. At an absorbed pump power level of around 280 mW, the QML pulses had an average power of 50 mW, whereas the estimated peak power of the mode-locked pulses were around 0.5 kW. Simple rotation of an intracavity birefringent tuning plate (BRF) enabled tuning of the central wavelength of the pulses between 835 and 865 nm. To our knowledge, this study is the first work focusing on QML operation in Cr: LiSAF lasers. The findings in this initial study is an initial attempt to understand the dynamics of QML operation in Cr:LiSAF. Further work will focus on reducing the mode-locked pulsewidths to sub-1-ps level and also on increasing the modulation depth of mode-locking for further peak power scaling.

Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO4 laser.

We demonstrate sub-100-fs Kerr-lens mode-locking of a Tm:MgWO4 laser emitting at ∼2µm assisted by a single-walled carbon-nanotube saturable absorber. A maximum average output power of 100 mW is achieved with pulse duration of 89 fs at a pulse repetition rate of ∼86MHz. The shortest pulse duration derived from frequency-resolved optical gating amounts to 76 fs at 2037 nm, corresponding to nearly bandwidth-limited pulses. To the best of our knowledge, these are the shortest pulses generated from any Tm-doped tungstate crystal and the first report on saturable absorber assisted Kerr-lens mode-locking of a Tm laser at ∼2µm.

High power diode pumped solid-state femtosecond laser systems

We demonstrate high power direct diode-pumped femtosecond laser based on a commercially available configuration and pumped by multi-diode laser modules delivering high brightness and average power. The multi-diode modules employ beam-combining scheme preserving linear polarization and focusing properties of individual laser diodes. Stable passive Kerr lens modelocked (KLM) laser operation was demonstrated, producing 30 fs pulses at ∼300 mW average output power. A simple approach to numerical modelling of KLM laser operation in cases when longitudinal pump beam exhibits strong astigmatism is also discussed.

Influence of pump beam shaping and noise on performance of a direct diode-pumped ultrafast Ti:sapphire laser.

One of the major factors that limits the widespread use of ultrafast titanium sapphire (Ti:S) lasers in life science is its expensive and complex pump source. Broad area diode laser (BA-DL) based pump sources have high potential to solve this problem, since they are compact, inexpensive and very efficient. However, their non-diffraction limited beam profile makes it challenging to achieve high powers in Kerr-lens mode-locked (KLM) operation of Ti:S lasers. In this work, we show that the ideal way to beam shape two spectrally combined BA-DLs with different beam qualities is to aim for a compromise between matching to the cavity mode diameter and the cavity mode Rayleigh range. We furthermore conclude that the relative intensity noise (RIN) of the BA-DL pumped Ti:S laser, another important parameter for imaging applications, is sufficiently low for a wide range of life science applications. However, for applications that are highly sensitive to noise, new laser diode designs are likely necessary to reduce inherent noise originating within the laser diode.

Distributed Kerr-lens mode locking based on spatiotemporal dissipative solitons in multimode fiber lasers

We introduce a mechanism of stable spatiotemporal soliton formation in a multimode fiber laser. This is based on spatially graded dissipation, leading to distributed Kerr-lens mode-locking. Our analysis involves solutions of a generalized dissipative Gross-Pitaevskii equation. This equation has a broad range of applications in nonlinear physics, including nonlinear optics, spatiotemporal patterns formation, plasma dynamics, and Bose-Einstein condensates. We demonstrate that careful control of dissipative and non-dissipative physical mechanisms results in the self-emergence of stable (2+1)-dimensional dissipative solitons. Achieving such a regime does not require the presence of any additional dissipative nonlinearities, such a mode-locker in a laser, or inelastic scattering in a Bose-Einstein condensate. Our method allows for stable energy (or "mass") harvesting by coherent localized structures, such as ultrashort laser pulses or Bose-Einstein condensates.

Geometrical analysis of Kerr-lens mode-locking for high-peak-power ultrafast oscillators

Ultrashort pulses from Kerr-lens mode-locked oscillators have inspired a variety of applications. The design and alignment of these laser resonators have thus far been theoretically supported by the conventional analysis of beam propagation. However, the well-established theoretical framework is sometimes beyond the scope of high-peak-power oscillators. In this paper, we analyze the geometry of ring resonators by extending the ABCD-matrix method to a high-peak-power regime. The guidelines to achieving stable Kerr-lens mode-locking is provided for high-peak-power pulses.

High-power passively mode-locked cryogenic Yb:YLF laser.

We report, to the best of our knowledge, the first mode-locked operation of Yb:YLF gain media at cryogenic temperatures. A saturable Bragg reflector was used for initiating and sustaining mode locking. Once aligned, the system was self-starting and quite robust. Using output couplers in the 10-40% range, 3-5 ps long pulses with an average power as high as 28 W were achieved. The repetition rate was 46.45 MHz, and the corresponding pulse energy and peak power were as high as 602 nJ and 126.5 kW, respectively. The central wavelength of the mode-locked pulses could be tuned in the 1013.5-1019 nm range using an intracavity birefringent filter. The achieved output power performance is two to three orders of magnitude higher than previous room-temperature Yb:YLF systems.

Kerr lens effect induced by a vortex LG0m laser beam

For a long time, there was this idea that it is impossible to mode lock a laser on a pure high-order transverse mode. Recently, it has been demonstrated experimentally the generation of femtosecond pulses by a Kerr-Lens-Mode-Locking (KLM) laser operating on a vortex LG0m mode. At the best of our knowledge, there is no studies on lensing effect induced by a doughnut laser beam inside a Kerr medium which is fundamental for modelling accurately the behaviour of a KLM laser and optimising its parameters. We have shown that the doughnut Kerr lens can be view as a ring-lens, or eccentric lens, showing two combined lensing effects. The first one is equivalent to a pure converging lens, and the second one to a diverging axicon. Both lensing effects have been analytically modelled, and provide values for the best focus position which are in a good agreement with the focal position calculated numerically by using a diffraction integral.

LD泵浦的高重复频率全固态飞秒激光器（特邀）

Compared with traditional ~100 MHz femtosecond lasers, the mode spacing is larger of GHz femtosecond lasers so that each comb can simply be resolved. Furthermore, the less dense of longitudinal modes results in higher average power. Therefore, it has more important application value in many research fields, such as comb-resolvabled spectroscopy, direct optical frequency comb spectroscopy, optical arbitrary waveform generation and astronomical spectrograph calibration. In this review, the generation schemes of GHz femtosecond pulses and the corresponding technical challenges of GHz-repetition-rate all-solid-state femtosecond lasers pumped by laser diode were introduced in detail firsly. Secondly, the international research progresses of all-solid-state GHz femtosecond lasers based on SESAM passively mode-locking and Kerr-lens mode-locking were summarized. Finally, the application value and research object of our group in all-solid-state GHz-repetition-rate femtosecond lasers were forcasted based on our preliminary research results.

Simulation of femtosecond pulse in a Kerr-lens mode locked Ti: sapphire laser

The Kerr-lens mode-locking (KLM) is known as a suitable method for generation of femtosecond pulses and mode-locked Ti:sapphire laser is now widely used sources of stable, energetic femtosecond pulses. We will present the simulation of KLM in Ti:sapphire laser cavities with a folded-cavity four-mirror by applying the ABCD ray-tracing technique for a Gaussian beam. Simulations will be performed for an asymmetric resonator design. Based on the numerical analysis, we will find the optimum design parameters (slit position, gain cavity spacing, gain medium position) for KLM.

Sub-100-fs Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating at 21 W average power.

We investigate power-scaling of a Kerr lens mode-locked (KLM) Yb:Lu2O3 thin-disk laser (TDL) oscillator operating in the sub-100-fs pulse duration regime. Employing a scheme with higher round-trip gain by increasing the number of passes through the thin-disk gain element, we increase the average power by a factor of two and the optical-to-optical efficiency by a factor of almost three compared to our previous sub-100-fs mode-locking results. The oscillator generates pulses with a duration of 95 fs at 21.1 W average power and 47.9 MHz repetition rate. We discuss the cavity design for continuous-wave and mode-locked operation and the estimation of the focal length of the Kerr lens. Unlike to usual KLM TDL oscillators, an operation at the edge of the stability zone in continuous-wave operation is not required. This work shows that KLM TDL oscillators based on the gain material Yb:Lu2O3 are an excellent choice for power-scaling of laser oscillators in the sub-100-fs regime, and we expect that such lasers will soon operate at power levels in excess of hundred watts.

Sub-100 fs pulse generation from a Kerr-lens mode-locked Yb:Lu2O3 ceramic thin-disk laser.

In this Letter, we demonstrate, to the best of our knowledge, the first Kerr-lens mode-locked (KLM) ceramic thin-disk laser. Ceramic laser gain media have many advantages for a high-power laser such as high fracture toughness, size scalability, low cost, and short time fabrication. By using the Yb:Lu2O3 ceramic thin disk, the pulse duration of 98fs with the output power of 3.7W was demonstrated. The shortest pulse duration was 86fs with the decreased output power of 0.36W. From our results, it was proven that a KLM thin-disk laser with ceramic gain media can become a new option for high-power, sub-100fs pulse laser sources.

Discrete Similariton and Dissipative Soliton Modelocking for Energy Scaling Ultrafast Thin-Disk Laser Oscillators

Since their first demonstration, modelocked thin-disk lasers have consistently surpassed other modelocked oscillator technologies in terms of achievable pulse energy and average power by several orders of magnitude. Surprisingly, state-of-the-art results using this technology have so far only been achieved in modelocking regimes where soliton pulse shaping is dominant (i.e., soliton modelocking with semiconductor saturable absorber mirrors or Kerr lens modelocking), in which only small nonlinear phase shifts are tolerable, ultimately limiting pulse energy scaling. Inspired by the staggering success of novel modelocking regimes applied to overcome these limitations in modelocked fiber lasers, namely the similariton (self-similarly evolving pulses) and dissipative soliton regimes, here, we explore these nonlinearity-resistant regimes for the next generation of high-energy modelocked thin-disk lasers, whereby millijoule pulse energies appear to be realistic targets. In this goal, we propose two possible implementations. The first is based on a passive multipass cell and designed to support dissipative solitons in an all-normal dispersion cavity. The second incorporates an active multipass cell and is designed to support similaritons. Our numerical investigations indicate that this is a very promising path to increase the pulse energy achievable directly from modelocked oscillators toward the millijoule level while additionally simplifying their implementation by eliminating the need for operation in cumbersome vacuum chambers.

70 femtosecond Kerr-lens mode-locked multipass-cavity Alexandrite laser.

We report, to the best of our knowledge, the shortest femtosecond pulses generated from a Kerr-lens mode-locked (KLM) Alexandrite laser operating near 750nm. The Alexandrite gain medium was pumped with a continuous-wave (cw), 532nm laser, and the performance of both the short and extended resonators was investigated. The use of an extended cavity eliminated the multi-wavelength spectral instabilities observed during the cw operation of the short cavity. Furthermore, since the repetition rate of the Alexandrite laser was reduced from 107 to 5.6MHz, the resulting increase in the intracavity pulse energy provided enhanced Kerr nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The KLM MPC Alexandrite laser produced nearly transform-limited, 70fs pulses at a pulse repetition rate of 5.6MHz with only 1W of pump power. The time-bandwidth product was further measured to be 0.331.