Mode-locked Oscillator Research Articles

Numerical study of passively mode-locked fiber optical parametric oscillators by self time-lensing

The recent development of fiber mode-locked lasers with fiber parametric gain has opened up an intriguing field of ultrafast pulse generation. Thanks to the tunability of the parametric gain in optical fibers, the gain media of mode-locked lasers are no longer limited to the choice of rare-earth-doped fibers. In our previous study, optical time-stretch was introduced into the cavity of a fiber optical parametric oscillator as a temporal diffracting distance to overcome localization of the parametric gain. The implementation of the diffracting distance for pulse stretching and compression relied on two dispersive elements with opposite sign dispersion. In this context, we propose a new design of passively mode-locked fiber optical parametric oscillators with only a single mono-dispersive element. We utilize the parametric process as a time lens at which the spectral components of the two oscillating parametric sidebands are phase-conjugated in the temporal domain. This allows both mode-locked sidebands to oscillate and switch periodically inside the fiber cavity with only a single dispersive element. We numerically simulate the pulse generation of this switching mode-locked fiber parametric oscillator.

Broadly Tunable and Passively Mode-Locked Operations of Yb3+,Gd3+:SrF2 Laser

Both tunable and passively mode-locked laser oscillators based on an efficient Yb,Gd:SrF2 crystal are reported for the first time. Charge compensation ions of Gd3+ enhanced the emission and absorption cross section of Yb,Gd:SrF2 crystal markedly. With the aid of birefringent filter, a 61.1-nm continuous-wavelength-tunable laser from 1020.2 to 1081.3 nm was obtained, making it a potential candidate for ultrafast lasers. Based on a semiconductor saturable absorber mirror, a diode-pumped passively mode-locked laser with the pulse width of 951 fs was realized. The oscillator operated at a repetition rate of 92 MHz.

Multiple-frequency measurement based on a Fourier domain mode-locked optoelectronic oscillator operating around oscillation threshold.

We propose and experimentally demonstrate a photonic-assisted approach to microwave frequency measurement based on frequency-to-time mapping using a Fourier domain mode-locked optoelectronic oscillator (FDML OEO) operating around oscillation threshold. A relationship between the frequency of the unknown input microwave signals and the time difference of the output pulses is established with the help of the frequency scanning capability of the FDML OEO and, thus, can be used for microwave frequency measurement. The proposed scheme is characterized as having broad bandwidth, high resolution, multiple-frequency detection capability, and tunable measurement range. Microwave frequency measurement with a measurement range up to 16GHz and a low measurement error of 0.07GHz is realized.

Improvement of dynamic range and repeatability in a refractive-index-sensing optical comb by combining saturable-absorber-mirror mode-locking with an intracavity multimode interference fiber sensor

A mode-locked fiber comb equipped with a multimode interference fiber sensor functions as a high-precision refractive-index (RI) sensor benefitting from precise RF measurement. However, its dynamic range and repeatability are hampered by the inherent characteristics of nonlinear-polarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with a wide dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness against cavity disturbance, the self-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10 × 10−8 for each mode-locking activation. The improved dynamic range and repeatability will be useful for enhancing the performance of RI sensing.

Deterministic Nonlinear Transformations of Phase Noise in Quantum-Limited Frequency Combs.

Optical phase noise of femtosecond lasers is analyzed over various steps of broadband nonlinear frequency conversion. The intrinsic phase jitter of our system originates from quantum statistics in the mode-locked oscillator. Supercontinuum generation by four-wave-mixing processes preserves a noise minimum at the optical carrier frequency. From there, a quadratic increase of the comb linewidth results with mutually anticorrelated phase fluctuations of both spectral wings. Passive phase locking by difference frequency generation strongly enhances the optical phase noise to a level equaling the carrier-envelope phase jitter of the fundamental comb. The same value results from quadratic extrapolation of the optical phase noise to radio frequencies. Our findings are consistent with a fully deterministic transformation of phase noise according to the elastic tape model.

Generating 84 fs, 4 nJ directly from an Yb-doped fiber oscillator by optimization of the net dispersion

We reported a simple and compact Yb-doped fiber femtosecond oscillator, which is based on a dispersion management and nonlinear polarization evolution (NPE) technique. Here, 84 fs, 4 nJ at 1030 nm pulses were generated directly from the mode-locked fiber oscillator by optimization of the net dispersion of the cavity. Only a pair of gratings was used to compensate the positive dispersion caused by the fibers and other optics in the cavity. This is, to our knowledge, the shortest pulse duration directly from a fiber oscillator without any extra compressor outside the cavity.

Kerr-lens mode locking above a 20 GHz repetition rate

We present a compact, Kerr-lens mode-locked oscillator consisting of a three-element optical setup and demonstrate femtosecond pulse generation at a repetition rate of 23.8 GHz.

High efficiency femtosecond laser ablation with gigahertz level bursts

The authors report on a simple and easy-to-use GHz amplified femtosecond laser source. The laser source is based on a passively mode-locked oscillator with a near GHz repetition rate. GHz pulses are then selected, and the obtained bursts of pulses are further amplified in a high-power amplifier chain. The presented GHz femtosecond laser source is used with a galvanometric scanner to perform ablation experiments on copper, aluminum, and stainless steel. Specific ablation rates of 0.7, 2.3, and 1.4 (mm3/min)/W are reached. The role of the important experimental parameters, such as the number of subpulses in the burst, is highlighted. Thanks to a specific ablation scheme in the GHz mode, the ablation efficiency is then comparable to the case of single nanosecond pulses, but with the usual quality of femtosecond processing.

Gain-guided soliton: Scaling repetition rate of passively modelocked Yb-doped fiber lasers to 12.5 GHz.

Fundamental repetition rates of 3.1 GHz, 7.0 GHz, and 12.5 GHz in passively modelocked Yb-doped fiber lasers are demonstrated. To the best of our knowledge, the fundamental repetition rate of 12.5 GHz is the highest value for 1.0 μm mode-locked fiber lasers. The mode-locked oscillator has a peak wavelength of 1047.5 nm and a pulse duration of 1.9 ps. The repetition rate signal has a signal-to-noise ratio of 57 dB. The peak wavelength of mode-locked spectra gradually makes a blue-shift and the modelocked threshold power increases with an increase in pulse repetition rate. Furthermore, in contrast to most of the all-normal-dispersion mode-locked fiber lasers, the present linear resonator (e.g., length < 1 cm) allows the buildup of gain-guided soliton without any filter effect. To unveil the underlying pulse shaping mechanism, a combined model comprising dynamic rate equations and the generalized nonlinear Schrödinger equation is established. Surprisingly, an essential gain filtering effect, which is contributed by a 26-nm gain bandwidth, is revealed and can verify the gain-guided pulse dynamics. Moreover, the pulse build-up in temporal and frequency domain, like spectral evolution and gain bandwidths, is numerically carried out in detail.

High power, microjoule-level diffraction-limited picosecond oscillator based on Nd:GdVO4 bulk crystal

1.2 μJ pulses with average power of 9 W were directly generated from a passively mode-locked picosecond oscillator based on a Nd:GdVO4 bulk crystal. Short cavity operation in continuous wave and mode-locking regimes was conducted first to confirm the resonator performance and proper alignment. With a carefully calibrated q-preserving multi-pass cell inserted into the laser cavity, the cavity length of the original short cavity was extended while the mode-matching condition was maintained fairly well. Compared with the short cavity, nearly fivefold energy enhancement was achieved while the diffraction-limited beam quality was undisturbed. To the best of our knowledge, this is the highest output power ever produced from a mode-locked oscillator based on a single bulk crystal at a repetition rate below 10 MHz without cavity dumping.

Harmonically Fourier Domain Mode-Locked Optoelectronic Oscillator

We experimentally demonstrate a harmonically Fourier domain mode-locked optoelectronic oscillator (FDML OEO). Harmonic Fourier domain mode locking operation is achieved when the cavity round-trip time is equal to integer multiples of the scanning period of the filter in the OEO loop. Compared with a fundamentally FDML OEO, frequency scanning microwave signals with increased tuning speed and large bandwidth can be easily generated by the proposed harmonically FDML OEO, which can find applications in spread-spectrum communication and modern radar systems. Up to fourth order, harmonic Fourier domain mode locking operation is achieved in the experiment. The time-bandwidth product of the harmonically FDML OEO is as large as tens of thousands. The maximum chirp rate of the generated $X$ -band linearly chirped microwave waveform is 0.725 GHz/ $\mu \text{s}$ , which is four times higher than that of the fundamentally FDML OEO.

High-Power and Large-Energy Dissipative Soliton Resonance in a Compact Tm-Doped All-Fiber Laser

We report the direct generation of high-power and large-energy dissipative soliton resonance square pulses at 1940.8 nm from a compact mode-locked Tm-doped all-fiber laser. A fiber loop mirror of 10/90 splitting ratio and a fiber Bragg grating with high reflectivity of >99% constitute the cavity mirrors together. The fiber loop mirror not only acts as a broadband reflective mirror and output port but also initiates the mode-locked fiber laser. The strictly all-fiber structure and proper selection of fiber types allow the laser to work in a high pump power state. By increasing the pump power, pulse duration extends from 5.9 to 14.5 ns, while the pulse peak power and 3-dB spectrum bandwidth almost keep constant. At the maximum pump power of 13 W, we acquire maximum output average power of 1.98 W and single pulse energy of 684 nJ. To the best of our knowledge, this is the highest average output power and single pulse energy in an all-fiber Tm-doped mode-locked oscillator. At the same time, the output power is also a record value in all-fiber net anomalous dispersion oscillators.

Active f-to-2f interferometer for record-low jitter carrier-envelope phase locking.

The f-to-2f interferometer plays a key role for carrier-envelope phase (CEP) measurement and subsequent stabilization. The CEP measurement typically relies on the application of two optical nonlinearities, namely supercontinuum generation and second-harmonic generation. Then the cascadation of these nonlinearities often leads to signal levels on the order of a few photons per pulse. We experimentally demonstrate that the introduction of optical gain into the infrared arm of an f-to-2f interferometer can mitigate this detection bottleneck and improve signal-to-noise ratios by 20dB compared to purely passive schemes. We further show that this measure allows for residual phase jitters between the carrier and envelope of about 10mrad, corresponding to record-breaking timing jitters in the single attosecond regime. Moreover, the method appears generally applicable to a wide range of oscillators in the near-infrared and may enable stable CEP locking of mode-locked oscillators that so far have resisted stabilization. Finally, we propose a parametric variant of the active f-to-2f interferometer that can be used for laser amplifiers with kilohertz repetition rates and below.

Femtosecond Mamyshev oscillator with 10-MW-level peak power

Ultrafast fiber lasers exhibit high broadband gain per pass, superior thermo-optical properties, and excellent beam quality, making them very suitable for practical use. For simplicity and efficiency, advanced mode-locked oscillator designs which can compete with the amplifier systems are always favorable. Here, we demonstrate a high-peak-power Mamyshev oscillator based on single-polarization, large-mode-area photonic crystal fibers. Using properly arranged filters, the fiber oscillator directly emits pulses with 9 W average power at 8 MHz repetition rate, corresponding to a single-pulse energy exceeding 1 μJ. The pulses are dechirped to 41 fs outside the cavity, leading to a record oscillator peak power as high as 13 MW. With such unprecedented performance, the proposed single-stage oscillator should be very attractive for various applications.

Enhanced self-phase modulation effect: an effective method of generating high average and peak power femtosecond laser pulses

This work experimentally explores the important application of the enhanced self-phase modulation (SPM) effect for the generation of high average power and high peak power femtosecond laser pulses in an all-fiber mode-locked laser oscillator pumped by a single-mode laser diode. Depending on the utilization of a section of nonlinear spectral broadening fiber with a 4.2 µm mode field diameter (MFD) and the optimization of the polarization-maintaining (PM) fiber length in a polarization-based all-fiber Lyot filter, the oscillator delivers 350 mW of average power at a 46.2 MHz repetition rate, resulting in 7.5 nJ pulse energy. The laser produces SPM-broadened spectrum and the spectrum features symmetrical multi-peak distribution. The resulting compressed pulse duration and average power are 107 fs and 271 mW, respectively, which corresponds to 54.8 kW of peak power. Additionally, comparative experiments have also been performed by replacing the nonlinear spectral broadening fiber with equal-length standard fiber with 6.2 µm MFD and shortening the length of the 45° aligned PM fiber of the all-fiber polarization-based Lyot filter, respectively. The presented experimental scheme demonstrates a notable improvement in terms of average and peak power compared to conventional experimental methods. Simultaneously, the experimental scheme also shows that the enhanced SPM effect in a mode-locked fiber laser oscillator has important application value for minimizing pulse duration and lowering the mode-locking threshold.

Nanotube mode-locked, wavelength and pulsewidth tunable thulium fiber laser.

Mode-locked oscillators with highly tunable output characteristics are desirable for a range of applications. Here, with a custom-made tunable filter, we demonstrate a carbon nanotube (CNT) mode-locked thulium fiber laser with widely tunable wavelength, spectral bandwidth, and pulse duration. The demonstrated laser's wavelength tuning range reached 300 nm (from 1733 nm to 2033 nm), which is the widest-ever that was reported for rare-earth ion doped fiber oscillators in the near-infrared. At each wavelength, the pulse duration can be regulated by changing the filter's bandwidth. For example, at ~1902 nm, the pulse duration can be adjusted from 0.9 ps to 6.4 ps (the corresponding output spectral bandwidth from 4.3 nm to 0.6 nm). Furthermore, we experimentally and numerically study the spectral evolution of the mode-locked laser in presence of a tunable filter, a topic that has not been thoroughly investigated for thulium-doped fiber lasers. The detailed dynamical change of the mode-locked spectra is presented and we observed gradual suppression of the Kelly sidebands as the filter's bandwidth is reduced. Further, using the polarization-maintaiing (PM) cavity ensures that the laser is stable and the output laser's polarization extinction ratio is measured to exceed 20 dB.

All-optical switching and real-time spectroscopy of soliton molecules in a few-cycle laser oscillator

Bound states of femtosecond solitons are generated and controlled in a commercial sub-10 fs Kerr-lens mode-locked ultrashort oscillator. Using real-time time-stretch interferometry, we resolve the resonance of vibrating soliton molecules and demonstrate all-optical switching between stable bound-states of different binding distance.

High power GHz femtosecond laser for ablation efficiency increase

Nowadays the relevance and the robustness of ultrafast lasers are well established for many industrial applications. The main limitation to the penetration of femtosecond processing in the industrial manufacturing market is the insufficient productivity compared to the needs of the application markets and alternative technologies. Increasing the power, pulse energy or repetition rate, of femtosecond lasers has been a general trend in in order to comply to the needs of the market for higher throughput. One of the strategies of increasing the power has resulted in the development of a GHz femtosecond laser. When increasing the repetition rate during the process beyond several hundred kHz, the delay between the laser pulses is less than the thermal relaxation time and there is thermal accumulation in the target material. The resulting rise in temperature leads to an ablation threshold lowering. The authors report on a simple and easy-to-use GHz amplified femtosecond laser source. The laser source is based on a passively mode-locked oscillator with a near GHz repetition rate. GHz pulses are then selected, and the obtained bursts of pulses are further amplified in a high-power amplifier chain. The presented GHz femtosecond laser source is used with a galvanometric scanner to perform ablation experiments on copper, aluminum, and stainless steel. Specific ablation rates of 0.7, 2.3, and 1.4 (mm3/min)/W are reached. The role of the important experimental parameters, such as the number of sub-pulses in the burst, is highlighted. Thanks to a specific ablation scheme in the GHz mode, the ablation efficiency is then comparable to the case of single nanosecond pulses, but with the usual quality of femtosecond processing.

Efficient near-infrared supercontinuum beam generation in ytterbium-doped double-clad passive fiber

We report the spectral behavior and averaged output power of a supercontinuum beam generated with an ytterbium (Yb)-doped double-clad passive fiber as a gain media for an amplifier with a mode-locked oscillator at a repetition rate of 10 MHz. The flat spectra was observed with a wavelength and average power of 6.2 W in the near-infrared spectral range of 1–2.3 μm, which represents a bandwidth of 1.1 μm at the 20-m-long Yb-doped double-clad passive fiber.

Optical rectification of a 100 W average power mode-locked thin-disk oscillator.

We demonstrate terahertz (THz) generation at megahertz repetition rate by optical rectification in GaP crystals, using excitation average power levels exceeding 100 W. The laser source is a state-of-the-art diode-pumped Yb:YAG SESAM-mode-locked thin-disk laser, capable of generating 580 fs pulses at an average power up to 120 W and a repetition rate of 13.4 MHz directly from a one-box oscillator, without the need for any extra amplification stages. In this first demonstration, we measure a maximum THz average power of 78 μW at a central frequency of 0.8 THz. Our results show that optical rectification of state-of-the-art high average power ultrafast sources in nonlinear crystals is within reach and paves the way toward high average power, ultrafast laser pumped THz sources.