Mode-locked Oscillator Research Articles

Sub-30-fs Yb:YAG thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime.

We experimentally investigate the limits of pulse duration in a Kerr-lens mode-locked Yb:YAG thin-disk laser (TDL) oscillator. Thanks to its excellent mechanical and optical properties, Yb:YAG is one of the most used gain materials for continuous-wave and pulsed TDLs. In mode-locked operation, its 8-nm wide gain bandwidth only directly supports pulses with a minimum duration of approximately 140 fs. For achieving shorter pulses, a Kerr-lens mode-locked TDL oscillator can be operated in the strongly self-phase modulation (SPM) broadened regime. Here, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequencies via SPM inside the Kerr medium. In this work, we study and compare different laser configurations in the strongly SPM-broadened regime. Starting with a configuration providing 84-fs pulses at 69 W average power at 17 MHz repetition rate, we reduce the pulse duration by optimizing various mode-locking parameters. One crucial parameter is the dispersion control which was provided by in-house-developed dispersive mirrors produced by ion-beam sputtering (IBS). We discuss trade-offs in average power, pulse duration, efficiency, and intra-cavity peak power. For the configuration operating at the highest SPM-broadening, we achieve a minimum pulse duration of 27 fs, which represents the shortest pulse duration directly generated by any ultrafast TDL oscillator. The corresponding full width at half maximum (FWHM) spectral bandwidth exceeds more than five times the FWHM gain bandwidth. The average output power of 3.3 W is moderate for ultrafast TDL oscillators, but higher than other Yb-based laser oscillators operating at this pulse duration. Additionally, the corresponding intra-cavity peak power of 0.8 GW is highly attractive for implementing intra-cavity extreme nonlinear optical interactions such as high harmonic generation.

High-power modelocked thin-disk oscillators as potential technology for high-rate material processing.

High average power femtosecond lasers have made spectacular progress in the last decades – moving from laboratory-based systems with maximum average powers of tens of watts to kilowatt-class mature industrial systems in a short time. The availability of such systems opens new possibilities in many fields; one of the most prominent ones that have driven many of these technological advances is precise high-speed material processing, where ultrashort pulses have long been recognized to provide highest precision processing of virtually any material, and high average power extends these capabilities to highest processing rates. Here, we focus our attention on one high-average power technology with large unexplored potential for this specific application: directly modelocked multi-MHz repetition frequency high-power thin-disk oscillators. We review their latest state-of-the-art and discuss future directions and challenges, specifically with this application field in mind.

Energy scaling of an erbium-doped mode-locked fiber laser oscillator

We investigate a promising approach to the energy scaling of mode-locked erbium fiber lasers. It consists in a special cavity design, comprised of a really long polarization maintaining (PM) part and a short non-PM part. Two separate fiber parts ensure the principle of space division for highly chirped dissipative soliton formation and NPE-based self-amplitude modulation effects. As a result, the pulse energy was increased up to 4 nJ with an estimated transform-limited pulse duration of 150 fs. It was also found that further energy scaling is limited by transition to a noise-like pulse generation regime. Numerical simulations revealed that this transition is associated with a contrast of the employed spectral filter and the amplifier noise level, which agrees well with the experiment.

Photonic broadband signal frequency conversion and bandwidth multiplication based on a Fourier domain mode-locked optoelectronic oscillator

A photonic broadband signal frequency conversion and bandwidth multiplication system is demonstrated based on a Fourier domain mode-locked optoelectronic oscillator (FDML-OEO). In this method, frequency conversion is achieved by using the selected optical sideband as the optical source of the FDML-OEO. An experiment is conducted, and a linear frequency modulation (LFM) signal with different central frequencies are converted. The central frequency of the converted signal can be tuned simply by adjusting the wavelength of the optical carrier. Due to the feature of low phase noise of the OEO, the degradation of the converted signal is almost neglected. Broadband signals with different time widths are also converted in terms of fibre length regulation. Furthermore, bandwidth multiplication of the LFM signal is achieved by selecting a second-order optical sideband as the optical source of the FDML-OEO.

A thin-disk ring laser oscillator at a repetition rate beyond 120 MHz

We report a Kerr lens mode-locked thin-disk ring laser oscillator with an output power of 22 W and a pulse duration of 711 fs. A repetition rate of 120.7 MHz is the highest ever obtained from a thin-disk ring laser oscillator, to the best of our knowledge. This work is expected to open up another route in dual-frequency-comb spectroscopy using a single ring laser source.

All-optical repetition rate stabilization methods for ultrafast Yb-based SESAM mode-locked fiber oscillator

We demonstrate optical to radio frequency (RF) locking technique using all-fiber phase shifter. Phase shift at 1030 nm was achieved by pumping Er-doped fiber incorporated in the resonator. Pump induced refractive index change was fast enough to stabilize optical pulse repetition rate of ultrafast Yb-doped fiber oscillator and achieve sub-200 fs timing jitter. We also investigate the source of phase noise in soliton fiber oscillators and provide guidelines how to further reduce timing jitter.

Modeling an actively mode-locked optoelectronic oscillator based on electric amplitude modulation.

A theoretical model and its calculation method are proposed to simulate an actively mode-locked optoelectronic oscillator (OEO) based on electric amplitude modulation. The model includes electric amplitude modulation to achieve mode locking and convolution of electric signal and filter impulse response function to achieve mode selection. Numerical simulation is carried out through enhancing the calculating time window to an integral multiple of the roundtrip time and employing pulse tracking method with a precise delay. Through using this model, the waveform, the spectrum and the phase noise characteristic of the generated microwave pulse train from an actively mode-locked OEO are numerically simulated, where the simulation results fit in with the experimental results. This model can be used to design an actively mode-locked OEO based on electric amplitude modulation. More importantly, it is favorable for studying the dynamic process in an actively mode-locked OEO, which is difficult to grasp by carrying out an experiment.

Nonlinear fiber system for shot-noise limited intensity noise suppression and amplification.

We propose a nonlinear fiber system for shot-noise limited, all-optical intensity noise reduction and signal amplification. The mechanism is based on the accumulation of different nonlinear phase shifts between orthogonal polarization modes in a polarization-maintaining fiber amplifier in combination with an implemented sinusoidal transmission function. The resulting correlation between the input intensity fluctuations and the system transmission enables tunable intensity noise reduction of the input pulse train. In the experiment, the noise spectral density of a mode-locked oscillator is suppressed by up to 20 dB to the theoretical shot-noise limit of the measurement at -151.1dBc/Hz with simultaneous pulse amplification of 13.5 dB.

Connection between Optical Frequency Combs and Microwave Frequency Combs Produced by Active-Mode-Locked Lasers Subject to Timing Jitter

Optical frequency combs (OFCs) and microwave frequency combs (MFCs) are of intense interest in fields ranging from metrology to multifrequency light sources. MFCs in particular have attracted interest for applications in microwave photonics. Timing jitter is a key limiting factor of the quality of OFCs and MFCs, and despite the central importance of timing jitter, a clear generic account of its effect on OFCs and MFCs that provides a connection between the two is lacking. We consider the power spectral density (PSD) of the OFCs and MFCs produced by active and hybrid active-passive mode-locked lasers in which the active mode locking is effected by a high-spectral-purity oscillator, but with timing noise originating in both the active locking and the mode-locked oscillators. Based on an analytically tractable theoretical treatment, we account quantitatively for the characteristic comb line shape consisting of a narrow central peak lying atop a broad pedestal. The theory provides a quantitative comparison between characteristic OFC and MFC line shapes and identifies the origin of the difference.

The Investigation on Ultrafast Pulse Formation in a Tm-Ho-Codoped Mode-Locking Fiber Oscillator.

We experimentally investigate the formation of various pulses from a thulium–holmium (Tm–Ho)-codoped nonlinear polarization rotation (NPR) mode-locking fiber oscillator. The ultrafast fiber oscillator can simultaneously operate in the noise-like and soliton mode-locking regimes with two different emission wavelengths located around 1947 and 2010 nm, which are believed to be induced from the laser transition of Tm3+ and Ho3+ ions respectively. When the noise-like pulse (NLP) and soliton pulse (SP) co-exist inside the laser oscillator, a maximum output power of 295 mW is achieved with a pulse repetition rate of 19.85-MHz, corresponding to a total single pulse energy of 14.86 nJ. By adjusting the wave plates, the fiber oscillator could also deliver the dual-NLPs or dual-SPs at dual wavelengths, or single NLP and single SP at one wavelength. The highest 61-order harmonic soliton pulse and 33.4-nJ-NLP are also realized respectively with proper design of the fiber cavity.

Phase-locked and mode-locked multicore photonic crystal fiber laser with a saturable absorber.

Multicore photonic crystal fiber (MC-PCF) can scale the output power with the number of cores by spatial beam combining if the in-phase mode is selected. We demonstrated simultaneous realization of phase-locked and mode-locked laser using Yb-doped 7-core MC-PCF by a semiconductor saturable absorber placed in the near-field inside a resonator. High energy 333 nJ pulses were obtained directly from a mode-locked fiber laser oscillator at a 42.4 MHz repetition rate with an average power of 14.1 W at 24 W excitation. We observed the direct output pulse width of 52 ps assuming a sech2 profile. However, it might be noise-like pulses because of no variation when we performed pulse compression. Single-pulse operation was achieved by increasing the bandwidth of the intracavity filter. In this case, 137 nJ, 42.4 MHz pulses were generated with a 5.8 W average power and the compressed output pulse width was 8.6 ps.

Generation of few-cycle pulses from a mode-locked Tm-doped fiber laser

We report a compact, self-starting dispersion-managed mode-locked thulium-doped fiber oscillator that delivers 2.6 nJ pulses at 2 µm with a repetition rate of 250 MHz. The average output power and spectral bandwidth of the pulses reach impressive values of 648 mW and 103 nm, respectively. The generated pulses are near linearly chirped, capable of linearly compressing to 74 fs in a normal dispersion fiber after power attenuation. Using a nonlinear fiber compression scheme can even compress the pulses to 29 fs (4.3-cycle). The remaining pulse energy is 1.15 nJ, and the corresponding peak power is estimated as 39.4 kW. To the best of our knowledge, this is the first demonstration of nonlinearly compressing the pulse of a 2 µm fiber oscillator to the sub-5 cycle regime. Such a few-cycle fiber laser could be an ideal candidate source for short-wavelength mid-infrared frequency metrology and molecular spectroscopy applications.

Photonic Generation of Phase-Coded Microwave Signals Based on Fourier Domain Mode Locking

We report a novel photonic scheme to generate binary phase-coded microwave signals by using two cascaded phase modulators (PMs) based on a Fourier domain mode-locked (FDML) optoelectronic oscillator (OEO). The first PM is driven by a series of electrical coding signals to modulate the phase of the optical carrier. The second PM is a construction of the FDML OEO, which realizes phase-modulation to intensity-modulation conversion together with a phase-shifted fiber Bragg grating. Phase-coded microwave signals can be generated when the period of the electrical coding signal matches with the round-trip time of the OEO. A π phase shift of the phase-coded microwave signals can be achieved by properly adjusting the level of the electrical coding signal. The key point of our work is that phase-coded microwave signals can be generated directly from the FDML OEO cavity. Besides, the proposed system has great reconfigurability in terms of center frequency and bit rate. Our scheme is theoretically analyzed and experimentally verified. Phase-coded microwave signals with a bit rate of 420 Mb/s and center frequency of 9.3 GHz, as well as with a bit rate of 2 Gb/s and center frequency of 12.7 GHz are successfully generated in the experiment.

Generation of a 59 fs pulse with a 550 nm spectral range in a mode-locked Er-doped single-mode fiber laser system

We demonstrated a compact broad spectrum ultrashort fiber laser system that employed a similariton laser as a seed and fiber-optic nonlinearities in an erbium-doped fiber amplifier to broaden the optical spectrum. The system generated a stable 59 fs ultrashort pulse with a spectral bandwidth beyond 550 nm (i.e. 1490–2030 nm). The mode-locked oscillator produced an 85.9 fs similariton pulse at the wavelength of 1553 nm. The maximum average output power of the laser system was 251 mW. This new type of standard single-mode fiber-format energetic ultrashort system was suitable as a source for biological tissue imaging, broad-band spectroscopy, and optical frequency combs.

Microwave pulse generation via employing an electric signal modulator to achieve time-domain mode locking in an optoelectronic oscillator.

An actively mode-locked optoelectronic oscillator (OEO) is proposed and demonstrated to generate chirp-free microwave pulse trains with variable repetition rates. Time-domain mode locking is realized by using an electric signal modulator to achieve amplitude modulation of the generated microwave signal in the OEO cavity. Through setting the externally applied electric signal frequency to be equal to the integral multiple of the free spectral range, a microwave pulse train with a low close-to-carrier phase noise is generated. In the experiment, microwave pulse trains with repetition rates of 179.94 and 360.00 kHz are generated through fundamental mode locking and 2nd-order harmonic mode locking, respectively. Under fundamental mode locking, the phase noise of the actively mode-locked OEO at 100 Hz frequency offset is measured to be 30 dB lower than that in a free-running OEO.

2-GHz watt-level Kerr-lens mode-locked Yb:KGW laser.

We report on a 2-GHz high-power Kerr-lens mode-locked Yb:KGW laser pumped by a single-mode fiber laser. The output performance for two different output coupling rates was investigated. Stable bidirectional mode-locking operation at the repetition rate of 2.157 GHz was obtained with a 0.6% output coupler. The average output powers of bidirectional operation are 741 mW and 746 mW, with 123-fs and 126-fs pulse durations, respectively. By using a 1.6% output coupler, unidirectional mode-locking is achieved with 145-fs pulse duration and 1.7-W average output power, which, to the best of our knowledge, is the highest average power from Kerr-lens mode-locked GHz femtosecond oscillators.

Systematic exploration and characterization on the influence of dispersion to pulse characteristics in Tm-doped NPE mode-locked fiber oscillator

We present a systematic and experimental exploration on the influence of the second-order and third-order dispersion on pulse characteristics in an ultra-high numerical aperture (UHNA) fiber dispersion-managed, Tm-doped and nonlinear polarization evolution (NPE) mode-locked fiber laser cavity; and obtained the detailed regularity of the variation of the key parameters for mode-locked pulse, including pulse duration, pulse spectral profile, and pulse signal-to-noise ratio (SNR) with dispersion. In this process, we find that there are two interesting second-order dispersion points of −0.207 ps2 and 0.09 ps2, in which the pulse SNRs can be optimized to as high as 79 dB and 78 dB, respectively, considerably higher than that in other positions. Besides, the experimental results show that the value of third-order dispersion can not only impact the pulse spectrum but also monotonously affect the quality of pulse SNR. To the best of our knowledge, this is the first report on the detailed analysis of the effects of both the second and the third-order dispersion values on the pulse characteristics in 2-μm mode-locked fiber laser cavity and it will be an important reference to the realization of a Tm-doped mode-locked laser with high pulse contrast.

Observation of harmonic beams inside a Kerr lens mode-locked thin-disk ring laser oscillator beyond a repetition rate of 10 MHz

We report the highest intra-cavity pulse energy of 80 μJ inside a Kerr lens mode-locked thin-disk ring laser oscillator at a repetition rate beyond 10 MHz enabling observation of the 2nd and 3rd harmonic beams directly generated by the Kerr medium. By adding a tight focusing setup to the ring cavity, we could observe krypton gas flame at an intra-cavity peak intensity of ∼28 TW/cm2 yielding a conversion efficiency of 10−5 for the 3rd harmonic beam. A further upgrade of the ring oscillator could lead to realizing a table-top source of vacuum ultraviolet pulses for application in photoelectron spectroscopy.

Stable 5-GHz fundamental repetition rate passively SESAM mode-locked Er-doped silica fiber lasers.

A stable passively mode-locked Er-doped silica fiber laser with a fundamental repetition rate of up to 5 GHz is demonstrated, which, to the best of our knowledge, is the highest repetition rate for 1.5 μm semiconductor saturable absorber mirror (SESAM) mode-locked Er-doped silica fiber (EDF) lasers. A segment of commercially available EDF with a net gain coefficient of 1 dB/cm is employed as gain medium. The compact Fabry-Pérot (FP) cavity features a fiber mirror, namely multiple-layer dielectric films (DFs) directly coated on end facet of a passive fiber ferrule, enabling a short cavity length of 2 cm configured. The mode-locked oscillator operates at 1561.0 nm with a signal-to-noise ratio (SNR) of 62.1 dB, whose average power is boosted to 27 mW by a single-mode Er-doped fiber amplifier (EDFA) and spectral bandwidth is broadened form 0.69 nm to 1.16 nm with a pulse width of 3.86 ps. The fiber laser shows excellent spectral stability without conspicuous wavelength drifting for 3 hours. Moreover, the basic guidelines of selecting SESAM for high repetition rate passively mode-locked fiber lasers is given.

10-W-scale Kerr-lens mode-locked Yb:CALYO laser with sub-100-fs pulses.

We reported a high-power pure Kerr-lens mode-locked Yb:CALYO laser based on the dual-confocal cavity delivering sub-100-fs pulses. The output pulses at 81 MHz have an average power of 10.4 W and the pulse duration of 98 fs, corresponding to the peak power of 1.14 MW. This is, to the best of our knowledge, the highest average power ever reported for a Kerr-lens mode-locked Yb-bulk oscillator. Analysis of the dual-confocal cavity was also conducted, which indicates a way to achieve higher average power. We believe the result described in this Letter may pave a way to develop Kerr-lens mode-locked bulk lasers with much higher average power.