Gigahertz Repetition Rates Research Articles

Dissipative coupling induced UWB magnonic frequency comb generation

Magnonic frequency combs have recently attracted particular attention due to their potential impact on spin-wave science. Here, we demonstrate theoretically the generation of ultra-wideband magnonic frequency combs induced by dissipative coupling in an open cavity magnomechanical system. A broadband comb with gigahertz repetition rates is obtained in the magnonic spectrum, and a typical non-perturbation frequency-comb structure is also observed. The total width of the magnonic comb in the robust plateau region can be up to ∼400 comb lines, which is much broader and flatter than that reported in the previous works. Furthermore, when the dissipative coupling strength is further increased, the chaotic motion is predicted in the magnonic spectrum. Our results provide an in-depth understanding of nonlinear magnomechanic dynamics in open quantum systems and fundamentally broaden the research range of magnon in wider spectral regimes.

Diode-pumped high-power gigahertz Kerr-lens mode-locked solid-state oscillator enabled by a dual-confocal ring cavity

Abstract Femtosecond oscillators with gigahertz (GHz) repetition rate are appealing sources for spectroscopic applications benefiting from the individually accessible and high-power comb line. The mode mismatch between the potent pump laser diode (LD) and the incredibly small laser cavity, however, limits the average output power of existing GHz Kerr-lens mode-locked (KLM) oscillators to tens of milliwatts. Here, we present a novel method that solves the difficulty and permits high average power LD-pumped KLM oscillators at GHz repetition rate. We propose a numerical simulation method to guide the realization of Kerr-lens mode-locking and comprehend the dynamics of the Kerr-lens mode-locking process. As a proof-of-principle demonstration, an LD-pumped Yb:KGW oscillator with up to 6.17-W average power and 184-fs pulse duration at 1.6-GHz repetition rate is conducted. The simulation had a good agreement with the experimental results. The cost-effective, compact and powerful laser source opens up new possibilities for research and industrial applications.

Ultrafast Yb:YAG laser oscillator with gigahertz repetition rate.

We present a SESAM modelocked Yb:YAG solid-state laser providing low-noise narrowband pulses with a pulse duration of 606 fs at a 1.09-GHz repetition rate, delivering up to 2.5 W of average output power. This laser provides access to a new parameter space that could previously not be reached by solid-state lasers and, to the best of our knowledge, is the first modelocked solid-state Yb:YAG laser in the gigahertz regime. This is achieved by introducing a single additional intracavity element, specifically a nonlinear birefringent YVO4 crystal, for soliton formation, polarization selection, and cavity intensity clamping. The isotropic pump absorption in Yb:YAG allows for stable and low-noise operation with multimode fiber pumping. This laser is ideally suited as a seed source for many commercial high-power Yb-doped amplification systems operating at a center wavelength around 1.03 µm. The laser exhibits a high power per comb line of 5.0 mW which also makes it interesting for applications in frequency comb spectroscopy, especially if it is used to pump an optical parametric oscillator. We measure a relative intensity noise (RIN) of 0.03%, integrated from 1 Hz to 10 MHz. Furthermore, we show that the laser timing jitter for noise frequencies >2 kHz is fully explained by a power-dependent shift in the center wavelength of 0.38 nm/W due to the quasi-three-level laser gain material. The narrow gain bandwidth of Yb:YAG reduces this contribution to noise in comparison to other SESAM modelocked Yb-doped lasers.

GHz-repetition-rate fundamentally mode-locked, isolator-free ring cavity Yb-doped fiber lasers with SESAM mode-locking.

A novel fundamentally mode-locked, GHz-repetition-rate ring cavity Yb-doped femtosecond fiber laser is demonstrated, which utilizes polarization-maintaining gain fiber and is enable by SESAM mode-locking. Thanks to the isolator-free structure, the ring cavity laser is operated bidirectionally and the two polarization-multiplexed output pulse trains are demonstrated synchronous. As a result, tunable waveforms one of which is with reduced pedestal and shorter pulse width in comparison with each individual, are generated by combination of the two orthogonal-polarized output pulses. Furthermore, a similar ring cavity structure that generates GHz picosecond pulses is demonstrated. We believe such high-repetition-rate polarization-multiplexed mode-locked fiber lasers could find further uses in various applications in need of gigahertz repetition rate and tunable waveforms.

Security of Decoy-State Quantum Key Distribution with Correlated Intensity Fluctuations

One of the most prominent techniques to enhance the performance of practical quantum key distribution (QKD) systems with laser sources is the decoy-state method. Current decoy-state QKD setups operate at gigahertz repetition rates, a regime where memory effects in the modulators and electronics that control them create correlations between the intensities of the emitted pulses. This translates into information leakage about the selected intensities, which cripples a crucial premise of the decoy-state method, thus invalidating the use of standard security analyzes. To overcome this problem, a security proof that exploits the Cauchy-Schwarz constraint has been introduced recently. Its main drawback is, however, that the achievable key rate is significantly lower than that of the ideal scenario without intensity correlations. Here, we improve this security proof technique by combining it with a fine-grained decoy-state analysis, which can deliver a tight estimation of the relevant parameters that determine the secret key rate. This results in a notable performance enhancement, being now the attainable distance double that of previous analyzes for certain parameter regimes. Also, we show that when the probability density function of the intensity fluctuations, conditioned on the current and previous intensity choices, is known, our approach provides a key rate very similar to the ideal scenario, which highlights the importance of an accurate experimental characterization of the correlations.

Multi-gigahertz femtosecond pulses from linear and nonlinear propagation of a phase-modulated laser.

We propose and demonstrate a non-mode-locking approach to generating multi-gigahertz repetition rate, femtosecond pulses in burst mode by shaping a continuous-wave (CW) seed laser in an all-fiber configuration. The seed laser at 1030 nm is first phase modulated and de-chirped to low-contrast, ∼2 ps pulses at a 17.5 GHz repetition rate, then carved to bursts at a 60 kHz repetition rate, and finally shaped to <2 ps clean pulses by a Mamyshev regenerator. This prepared high-quality picosecond source is further used to seed an Yb-doped fiber amplifier operating in the highly nonlinear regime, delivering output pulses at 23 nJ/pulse and $20\,\mathrm{\mu}$J/burst, compressible to ∼100 fs level. The system eliminates the need for mode-locked cavities and simplifies conventional ultrafast electro-optic combs to using only one phase modulator, while providing femtosecond pulses at multiple gigahertz repetition rate, enhanced pulse energy in burst mode and the potential of further power/energy scaling.

Efficient high-power sub-50-fs gigahertz repetition rate diode-pumped solid-state laser.

In this article we present a directly diode-pumped high-power Kerr-lens mode-locked Yb:CALGO bulk laser oscillator operating at 1-GHz repetition rate. We report on two laser configurations optimized for either highest average power or shortest pulse duration. In the first configuration optimized for high average power, the oscillator delivers up to 6.9 W of average power, which is the highest average power of any ultrafast laser oscillator operating at gigahertz repetition rate. The 93-fs pulses have a peak power of 64 kW, and the optical-to-optical efficiency amounts to 37%. In the second configuration optimized for short pulse duration, we demonstrate 48-fs pulses at 4.1 W of average power corresponding to a higher peak power of 74kW with 21% optical-to-optical efficiency. This is the shortest pulse duration and the highest peak power demonstrated by any GHz-class Yb-based laser oscillator. The compact laser setup is directly pumped by a low-cost multimode fiber-coupled laser diode and has a high potential as an economical yet powerful source for various applications.

Gigahertz-repetition rate, high power, ultrafast Tm-doped fiber laser source

The gigahertz (GHz)-pulse repetition rate, 2-μm harmonic mode-locking (HML) Tm-doped fiber oscillator is realized with the nonlinear polarization rotation method. The fundamental pulse repetition rate is 95.8 MHz, which is further increased to 1.25 GHz at 1962 nm, 1.63 GHz at 1982 nm, and 1.05 GHz at 2005 nm with a high super-mode suppression above 37 dB respectively. It is believed to be the first report on the GHz Tm-doped fiber oscillators, which can optionally operate in < 2000-nm or > 2000-nm region. The HML fiber oscillator delivers maximum output powers of 480 mW, 538 mW and 340 mW and at each wavelength, which are the largest output powers never reported from previous GHz 2-μm fiber oscillators. The pulse durations in each wavelength are all < 500 fs. The laser pulse with the highest pulse repetition rate is further scaled up to ∼ 4 W. The observed spectral narrowing and soliton self-frequency shift in the fiber amplifier are further analyzed with a theoretical model, which matches well with the experimental results.

High repetition rate actively mode-locked Er:fiber laser with tunable pulse duration

An actively mode-locked fiber laser with controllable pulse repetition rate and tunable pulse duration is presented, in which an optical delay line (ODL) is used to adjust the cavity length precisely for regulating the repetition rate, and a semiconductor optical amplifier (SOA) is introduced for enabling the pulse duration control. Experimentally, continuous tuning of the repetition rate from 2 GHz to 6 GHz is realized, which is limited by the availability of an even higher repetition rate radio-frequency (RF) source. Specifically, when the repetition rate is fixed at 2.5 GHz, the pulse duration can be tuned from 4 ps to 30 ps, which is, to the best of our knowledge, the widest tuning range of pulse duration ever achieved in a gigahertz (GHz) repetition rate actively mode-locked 1.5 µm fiber laser oscillator.

Electro-optic comb pumped optical parametric oscillator with flexible repetition rate at GHz level.

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5-1.7 µm from a MgO-doped periodically poled LiNbO3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

Microfiber-assisted gigahertz harmonic mode-locking in ultrafast fiber laser.

Optoacoustic interaction can be strongly enhanced in tiny core fibers, and it holds significant potential for stable harmonic mode-locking at gigahertz (GHz) and higher repetition rate. In this Letter, we propose and demonstrate a microfiber-assisted GHz harmonic mode-locking fiber laser, which is achieved by the enhanced optomechanical coupling between cavity modes in microfiber with the waist length of ∼16cm and the waist diameter of ∼1.56µm. The repetition rates can be stably locked at 2.3828 GHz and predominately locked at 1.7852 GHz, corresponding to the frequencies of radial R01 and torsional-radial TR21 acoustic modes, respectively. Our results provide novel insight into the design of a high-repetition-rate laser source and the application of microfibers in the optomechanical field.

Fully phase-stabilized 1 GHz turnkey frequency comb at 1.56 µm

Low noise and high repetition rate optical frequency combs are desirable for many applications, from timekeeping to precision spectroscopy. For example, gigahertz repetition rate sources greatly increase the acquisition speed of spectra in a dual-comb modality when compared to lower repetition rate sources, while still maintaining sufficient instantaneous resolution to resolve ro-vibrational signatures from molecules in a variety of conditions. In this paper, we present the stabilization and characterization of a turnkey commercial 1 GHz mode-locked laser that operates at telecom wavelengths (1.56 µm). Fiber amplification and spectral broadening result in high signal-to-noise ratio detection and stabilization of f ceo with 438 mrad of residual phase noise (integrated from 102 to 107 Hz). Simultaneously, we stabilize the beatnote between the nearest comb mode and a cavity stabilized continuous-wave laser at 1.55 µm with 41 mrad of residual phase noise (integrated from 102 to 107 Hz). This robust, self-referenced comb system is built with off-the-shelf polarization-maintaining fiber components and will be useful for a wide range of low noise frequency comb applications that benefit from the increased repetition rate.

All-optical non-linear activation function for neuromorphic photonic computing using semiconductor Fano lasers.

We predict that semiconductor Fano lasers can be used to realize an all-optical non-linear activation function for neuromorphic photonic computing. By exploiting optical control of a Fano mirror, the laser can generate optical pulses with low threshold energy, gigahertz repetition rates, and orders of magnitude suppression between the on- and off-states. Analytical estimates of the switching threshold energy, extinction ratio, and refractory period agree well with numerical results.

Gigahertz repetition rate thermionic electron gun concept

We present a novel concept for the generation of gigahertz repetition rate high brightness electron bunches. A custom design 100 kV thermionic gun provides a continuous electron beam, with the current determined by the filament size and temperature. A 1 GHz rectangular RF cavity deflects the beam across a knife-edge, creating a pulsed beam. Adding a higher harmonic mode to this cavity results in a flattened magnetic field profile which increases the duty cycle to 30%. Finally, a compression cavity induces a negative longitudinal velocity-time chirp in a bunch, initiating ballistic compression. Adding a higher harmonic mode to this cavity increases the linearity of this chirp and thus decreases the final bunch length. Charged particle simulations show that with a 0.15 mm radius LaB6 filament held at 1760 K, this method can create 279 fs, 3.0 pC electron bunches with a radial rms core emittance of 0.089 mm mrad at a repetition rate of 1 GHz.

L-band mode-locked femtosecond fiber laser with gigahertz repetition rate.

We demonstrate an L-band passively mode-locked femtosecond fiber laser with a fundamental repetition rate of 1.013GHz based on a semiconductor saturable absorber mirror. Compared with other reported L-band fiber lasers that use overlong Er-doped fiber, the laser here consists of 10cm heavily doped fiber to increase the fundamental pulse repetition rate to gigahertz level. An output ratio as low as 0.2% is used to make the central wavelength up to 1.6μm. The laser operates at the soliton regime with a 3dB spectral bandwidth of 13.6nm and a pulse duration of 229fs.

Temporal solitons in microresonators driven by optical pulses

Continuous-wave laser-driven, high-Q Kerr–nonlinear optical microresonators have enabled the generation of optical frequency combs, ultralow-noise microwaves and ultrashort optical pulses at tens of gigahertz repetition rate. Here, we break with the paradigm of the continuous-wave driving and instead use periodic, picosecond optical pulses. In a fibre-based Fabry–Perot microresonator we observe the deterministic generation of stable femtosecond dissipative cavity solitons ‘on top’ of the resonantly enhanced driving pulses. The solitons lock to the driving pulse, which enables direct all-optical control of the soliton's repetition rate and tuning of its carrier-envelope offset frequency. When compared with continuous-wave-driven microresonators or non-resonant pulsed supercontinuum generation, this new approach is more efficient and can yield broadband frequency combs at an average driving power significantly below the continuous-wave parametric threshold. Bridging the fields of continuous-wave-driven resonant and pulse-driven non-resonant nonlinear optics, these results enable efficient microresonator frequency combs, resonant supercontinuum generation and microphotonic pulse compression. By driving a high-Q fibre-based Fabry–Perot microresonator with periodic, picosecond optical pulses, deterministic generation of stable femtosecond dissipative cavity solitons has been experimentally realized.

Sub-100-fs 187 GHz mode-locked fiber laser using stretched-soliton effects

Current pulsed fiber lasers that are capable of delivering stable sub-100-fs pulses at megahertz repetition rates require intracavity pulse energies in the nanojoule range. Scaling these lasers to gigahertz repetition rates necessitates, therefore, very high average power levels and complex cladding-pumped configurations. Here we report a type of stretched-soliton all-fiber laser that generates broadband, soliton-like pulses at 1.55 μm with intracavity pulse energies of only tens of picojoules. In the laser cavity, strong dispersion management leads to a temporal breathing ratio of ∼70, while the weak residual anomalous dispersion is perfectly balanced by the low Kerr nonlinearity, resulting in the formation of temporally stretched, hyperbolic-secant pulses. A lumped wavelength-dependent attenuator compensates for the effects of the gain filtering on the pulse spectrum, ensuring intracavity pulse self-consistency. This unique stretched-soliton mechanism, combined with a harmonic mode-locking technique based on intense optoacoustic interactions in solid-core photonic crystal fiber, yields for the first time stable gigahertz-rate, sub-100-fs, dispersive-wave-free pulse trains at moderate pump powers.

Near perfect mode overlap between independently seeded, gain-switched lasers.

We drastically improve the mode overlap between independently seeded, gain-switched laser diodes operating at gigahertz repetition rates by implementing a pulsed light seeding technique. Injecting pulsed light reduces the emission time jitter and enables frequency chirp synchronization while maintaining random optical phases of the emitted laser pulses. We measure interference of these pulsed sources both in the macroscopic regime, where we demonstrate near perfect mode overlap, and in the single photon regime, where we achieve a Hong-Ou-Mandel dip visibility of 0.499 ± 0.004, thus saturating the theoretical limit of 0.5. The measurement results are reproduced by Monte-Carlo simulations with no free parameters. Our light source is an ideal solution for generation of high rate, indistinguishable coherent pulses for quantum information applications.

Multi-gigahertz repetition rate ultrafast waveguide lasers mode-locked with graphene saturable absorbers

We report the development of an approach to build compact waveguide lasers that operate in the stable fundamental mode-locking regime with multigigahertz repetition rates. The approach is based on the use of depressed cladding multi- or single-mode waveguides fabricated directly in the active laser crystal using the femtosecond laser inscription method and a graphene saturable absorber. Using this approach we achieve the stable self-starting mode-locking operation of a diode-pumped waveguide Nd:YAG laser that delivers picosecond pulses at a repetition rate of up to 11.5 GHz with an average power of 12 mW at a central wavelength of 1064 nm. The saturable absorbers are formed through the chemical vapor deposition of single-layer graphene on the output coupler mirror or directly on the end facet of the laser crystal. The stable self-starting mode-locking operation is achieved by controlling the group delay dispersion in the laser cavity with an intracavity interferometer. The method developed for the creation of compact ultrashort pulse laser generators with gigahertz repetition rates can be extended further and applied for the development of compact high-repetition rate lasers that operate at a wide range of IR wavelengths.

SESAMs structure design for GHz femtosecond lasers

Abstract SESAM mode locked femtosecond laser at higher frequency up to gigahertz repetition rate suffers from the tendency for Q-switching instabilities. Increasing the pump power and decreasing the spot size at both the laser crystal and SESAM could help to stabilize the cw mode-locking of a gigahertz ultrafast laser. However, this is limited by the available laser diode pump power and may result in SESAM damage. Two photon absorption (TPA) effect in SESAM will generate ‘roll over’ effect of nonlinear reflectivity curve, which will decrease the critical pulse energy and critical pump power for cw mode-locking. By designing SESAM structures, we could optimize the ‘roll over’ effect and suppress the Q-switching instabilities. From simulation, we find out that enhanced SESAM with a cap layer thickness of 116 nm and first GaAs layer thickness of 517 nm will lead to a minimum critical pump power of 12.1 W for our designed 1 GHz laser cavity.