Tunable Repetition Rate Research Articles

Dynamic gain and frequency comb formation in exceptional-point lasers

Exceptional points (EPs)—singularities in the parameter space of non-Hermitian systems where two nearby eigenmodes coalesce—feature unique properties with applications such as sensitivity enhancement and chiral emission. Existing realizations of EP lasers operate with static populations in the gain medium. By analyzing the full-wave Maxwell–Bloch equations, here we show that in a laser operating sufficiently close to an EP, the nonlinear gain will spontaneously induce a multi-spectral multi-modal instability above a pump threshold, which initiates an oscillating population inversion and generates a frequency comb. The efficiency of comb generation is enhanced by both the spectral degeneracy and the spatial coalescence of modes near an EP. Such an “EP comb” has a widely tunable repetition rate, self-starts without external modulators or a continuous-wave pump, and can be realized with an ultra-compact footprint. We develop an exact solution of the Maxwell–Bloch equations with an oscillating inversion, describing all spatiotemporal properties of the EP comb as a limit cycle. We numerically illustrate this phenomenon in a 5-μm-long gain-loss coupled AlGaAs cavity and adjust the EP comb repetition rate from 20 to 27 GHz. This work provides a rigorous spatiotemporal description of the rich laser behaviors that arise from the interplay between the non-Hermiticity, nonlinearity, and dynamics of a gain medium.

High-efficiency Yb:YAG thin-disk chirped pulse amplifier delivering 884-femtosecond laser with tunable repetition rates and high stability

We experimentally demonstrated a compact, highly efficient, stable, chirped pulse amplification (CPA) thin-disk regenerative system with a tunable repetition rate. The laser head consists of a 9 at. % Yb:YAG thin-disk with a diameter of 8.8 mm and a thickness of 150 μm, is designed for a 48-pump pass configuration. In CPA-based regenerative amplifier configuration, a maximal output power of 85 W at 500 kHz is achieved with a conversion efficiency of 53.1 %. This corresponds to a pulse energy of 170 μJ. To the best of our knowledge, this is the highest conversion efficiency in CPA thin-disk regenerative amplifiers. The pulse width is compressed to 884 fs with a pair of grating, resulting in a peak power of 192 MW. At 85 W, a near diffraction limit beam quality factor M2 of 1.40 is measured, which is essential in precision applications. In addition, remarkable long-term power stability is confirmed with a root-mean-square (RMS) fluctuation of 0.12 % over a 24-hour duration. The reported femtosecond amplifier is believed to be a promising tool for various applications such as extreme manufacturing, terahertz spectroscopy, etc.

Electro-optic frequency comb generation via cascaded modulators driven at lower frequency harmonics

Electro-optical modulation of a continuous wave laser is a highly stable way to generate frequency combs, gaining popularity in telecommunication and spectroscopic applications. These combs are generated by modulating non-linear electro-optic crystals with radio frequencies, creating equally spaced side-bands centered around the single-frequency seed laser. Electro-optic frequency comb architectures often choose between optical bandwidth (cascaded GHz combs) or higher mode density (chirped RF generation). This work demonstrates an electro-optic frequency comb with > 120 GHz of bandwidth and an 80 MHz repetition rate. The comb has three cascaded electro-optic modulators driven at sequentially lower harmonics, the last megahertz modulation dictating the repetition rate. This architecture can modulate at any individual harmonic and repetition rate without changes to the components. This comb can be used in any applications where a stable and tunable repetition rate is needed.

High-energy, high-peak-power diode-pumped Q-switched Tm:YAG laser at 2.02 μm with tunable repetition rate from 100 Hz to 1000 Hz

A high-energy, high-peak-power Q-switched Tm:YAG laser at 2.02 μm with a tunable repetition rate from 100 Hz to 1000 Hz and pulse duration in the hundreds of nanoseconds is demonstrated. The high energy output was achieved by employing a thermally stable linear cavity, including two compact laser diode continuous-wave side-pumped Tm:YAG laser modules. For Q-switched operation, stable Q-switched performance from 100 Hz to 1000 Hz was realized at near-room temperature. At a repetition rate of 100 Hz, the laser delivered a maximum pulse energy of 215 mJ with a pulse duration of 124.4 ns, resulting in a peak power of up to 1.73 MW. When the repetition rate was tuned to 200 Hz, 500 Hz, and 1000 Hz, pulse energies of 212 mJ, 125.6 mJ, and 68.2 mJ were obtained, with pulse durations of 133.6 ns, 176.3 ns, and 262 ns, respectively. To the best of our knowledge, these results represent the highest single pulse energies and peak powers ever accomplished for a diode-pumped Q-switched Tm-doped 2 μm laser operating at high repetition rates exceeding 100 Hz.

Tunable On‐Chip Electro‐Optic Frequency‐Comb Generation at 8 μm$\umu {\rm m}$ wavelength

AbstractThe development of compact systems operating in the long‐wave infrared wavelength range is of high interest for spectroscopic and sensing applications. There is currently a wide interest toward the development of optical frequency‐combs to enhance the performances of these systems. Amongst the different techniques to obtain optical frequency‐combs, electro‐optic frequency‐comb generation presents major advantages thanks to the tunable repetition rate only limited by the bandwidth of the used electro‐optical modulator. However, the development of integrated and efficient electro‐optical modulators operating in a wide long‐wave infrared spectral band is still at its infancy, and electro‐optical frequency‐comb has not been demonstrated so far beyond 3 wavelength. In this work, a Schottky‐based modulator embedded in a Ge‐rich graded SiGe waveguide is used for electro‐optic frequency‐comb generation. Considering the limited efficiency of the modulator, harmonically‐rich RF signals are used to enhance the generation of comb lines around the optical carrier. Interestingly, this allows us to demonstrate the generation of electro‐optical combs spanning over 2.4 GHz around 8 wavelength. This paves the way toward fully integrated and tunable mid‐infrared electro‐optic frequency‐comb generation systems.

30-100 kHz, 2 ns passively Q-switched laser for fast and efficient photoacoustic microscopy.

Actively Q-switched (AQS) fiber laser and solid-state laser (SSL) are widely used for photoacoustic microscopy (PAM). In contrast, passively Q-switched (PQS) SSL not only maintains most of the merits of AQS lasers, but also exhibits unique advantages, including the pulse width (PW), pulse repetition rate (PRR) tunability, wavelength, compactness, and cost. These advantages all benefit the PAM. However, there are few reports demonstrating the performance of PQS-SSL on PA imaging. Here, we demonstrate a compact PQS-SSL for fast and efficient PA imaging. The laser uniquely maintains a constant PW (~2 ns) and pulse energy (~3 μJ) during the PRR variation (30-100 kHz), which is valuable for preserving a stabilized imaging performance at different scanning rates. The PA imaging performance is compared by a resolution target and showcased by whole-body scanning of an embryonic zebrafish in vivo. The performance indicates that PQS-SSL is a promising candidate for PAM.

Harmonic active mode-locking optoelectronic oscillator with suppressed supermode noise based on pulse intensity feedback.

A harmonic active mode-locking optoelectronic oscillator (HAML-OEO) with pulse intensity feedback is proposed and experimentally demonstrated. It is capable of generating microwave pulses characterized by suppressed supermode noise, uniform intensity, and tunable repetition rates. Unlike traditional HAML-OEOs, active mode-locking and pulse intensity feedback are simultaneously achieved through the use of a dual-drive Mach-Zehnder modulator (DDMZM). By synchronously feeding back the generated microwave pulses to the DDMZM, each pulse undergoes a loss proportional to its intensity, facilitating pulse intensity equalization and supermode noise suppression. In the experiment, intensity-equalized microwave pulse trains with repetition rates of 499 kHz and 998 kHz are generated by the 5th- and 10th-order HAML-OEOs, respectively, with the measured supermode noise suppression ratios exceeding 40 dB.

Repetition rate tuning and locking of solitons in a microrod resonator.

Recently, there has been significant interest in the generation of coherent temporal solitons in optical microresonators. In this Letter, we present a demonstration of dissipative Kerr soliton generation in a microrod resonator using an auxiliary-laser-assisted thermal response control method. In addition, we are able to control the repetition rate of the soliton over a range of 200 kHz while maintaining the pump laser frequency, by applying external stress tuning. Through the precise control of the PZT voltage, we achieve a stability level of 3.9 × 10-10 for residual fluctuation of the repetition rate when averaged 1 s. Our platform offers precise tuning and locking capabilities for the repetition frequency of coherent mode-locked combs in microresonators. This advancement holds great potential for applications in spectroscopy and precision measurements.

Non-planar graphene directly synthesized on intracavity optical microresonators for GHz repetition rate mode-locked lasers

Generation of high-speed laser pulses is essential for sustaining today’s global, hyper-connected society. One approach for achieving high spectral and temporal purity is to combine optical nonlinear materials with spectral filtering devices. In this work, a graphene-coated microresonator integrates a nonlinear material and a spectral filtering platform into a single device, creating a tunable GHz repetition rate mode-locked fiber laser. The graphene is directly synthesized on the non-planar surface of microresonator, resulting in a uniform, conformal coating with minimal optical loss in the device. The whispering gallery modes of the resonator filter the propagating modes, and the remaining modes from the interaction with graphene lock their relative phases to form short pulses at an elevated repetition rate relying on inter-modal spectral distance. Additionally, by leveraging the photo-thermal effect, all-optical tuning of the repetition rate is demonstrated. With optimized device parameters, repetition rates of 150 GHz and tuning of 6.1 GHz are achieved.

Ultra-Short Pulse Generation at 914 nm from a CW laser source

We present an all-PM fiber laser system seeded by a single-frequency CW laser source generating 230fs pulses at 914nm with a tunable repetition rate between 5 and 20 MHz. Ultra-short pulse train was achieved using a Mach-Zehnder modulator for pulse carving, Nd-doped fiber amplifiers and a Mamyshev regenerator.

Towards 2-μm comb light source based on multiple four-wave mixing in a dual-frequency Brillouin fiber laser

In this study, we report the generation of multi-wavelength light sources through enhanced four-wave-mixing processes using a straightforward and adaptable dual-frequency Brillouin fiber laser. This passive optical and nonreciprocal cavity is first tested and analyzed with long fiber lengths up to 1 km in the 1.55 μm telecommunication C band and then in the 2-μm waveband. In the latter case, we demonstrate that our fiber cavity enables efficient multiple four-wave mixings, in the continuous-wave regime, which are commonly inaccessible in long silica-fibers due to increased losses. We also report on the tunable repetition rate from tens of GHz to hundreds of GHz, by simply changing the frequency spacing between the two continuous-wave pumps. The coherence limitations of our all-fiber system are discussed, along with the impact of the dispersion regime of the nonlinear fiber that forms the cavity.

GHz repetition rate tunable optical pulses generation using a SBS-based coupled optoelectronic oscillator

An optical pulse generator with a tunable repetition rate of GHz is proposed employing a coupled optoelectronic oscillator (COEO). The tunability is achieved with an equivalent tunable RF bandpass filter which is realized through phase modulation and stimulated Brillouin scattering (SBS) in a high nonlinear fiber. An injected seed laser is applied to stabilize the central wavelength of the mode-locked loop. The repetition rate of the generated optical pulses is the frequency difference between the Stokes wave and the seed laser. It is adjusted by changing the wavelength of the pump laser for SBS effect. In the experiments, optical pulse trains with a repetition rate of 7.3 GHz–9.3 GHz are generated, and RF oscillating signals at the corresponding frequency are simultaneously produced. The side-mode suppression ratios (SMSRs) of the generated RF signals are all higher than 50 dB, and the phase noise is lower than −80dBc/Hz @10-kHz offset frequency. The tunable range of the optical pulse rate is limited by the bandwidth of the used RF phase shifter in the oscillating loop.

Dual-comb generation with counter-propagating self-injection-locked solitons.

Microresonator-based optical frequency combs have been greatly developed in the last decade and have shown great potential for many applications. A dual-comb scheme is usually required for lidar ranging, spectroscopy, spectrometer and microwave photonic channelizer. However, dual-comb generation with microresonators would require doubled hardware resources and more complex feedback control. Here we propose a novel scheme for dual-comb generation with a single laser diode self-injection locked to a single microresonator. The output of the laser diode is split and pumps the microresonator in clockwise and counter-clockwise directions. The scheme is investigated intensely through numerical simulations based on a set of coupled Lugiato-Lefever equations. Turnkey counter-propagating single soliton generation and repetition rate tuning are demonstrated.

Wavelength tunable actively mode-locked Tm3+ doped fiber laser with GHz repetition rate

Using a phase modulator to synchronize the longitudinal modes in an all-fiber cavity, we demonstrate a wavelength tunable (1955–2045 nm) diode pumped actively mode-locked Thulium doped fiber laser with tunable repetition rate up to 10 GHz with the shortest measured pulse width of 42 ps and average output power of 5 mW. We have performed a detailed analysis of the stable mode-locking regimes, including the self mode-locking condition that challenges the harmonic mode-locked operation. We demonstrate a supermode suppression ratio of > 50 dB across the entire wavelength band of operation while maintaining a pulse width and timing jitter of < 50 ps and < 1 ps, respectively.

Versatile octave-spanning soliton crystals with high conversion efficiency in a Si3N4 microresonator.

Microresonator-based soliton crystals are a key recent advancement in the study of the rich nonlinear dynamics of soliton states. The soliton crystals are self-organized temporal pulses filling the microresonator cavity and have strong comb lines with wide spacing making them of great interest in many potential applications such as communication and meteorology. However, achieving a broad spectrum, tunable repetition rates, and high conversion efficiency are still a challenge. Here, we report the deterministic generation of versatile octave-spanning soliton crystals with various repetition rates via avoided mode crossings. In addition, we investigate the conversion efficiency of the obtained soliton crystals and achieved above ∼50% in one of the devices with a suitable coupling. Our results pave the way for accessing coherent broad and tunable on-chip soliton crystals, thus requiring a rigorous and viable microcavity design to engineer the desired mode coupling position.

High-power and low-threshold 1200 nm InGaAs/GaAs quantum-wells VECSEL grown by MOCVD

Here, we demonstrate an InGaAs/GaAs quantum-wells (QWs) based vertical-external cavity surface emitting laser (VECSEL) with multi-watt emitting beyond 1200 nm. The development is motivated by the need for high-performance 1200 nm laser sources, which are located in the C–H bond photoacoustic tomography window and atmospheric transmission band. In the exploration, strain compensation, epitaxy growth, and package strategy are all optimized for effective performance improvement. Highly strained InGaAs/GaAs QWs epitaxy structures are grown on GaAs substrate with high crystalline quality by metal-organic chemical vapor deposition (MOCVD) technology. With effective thermal management, the thermal impedance of the chip is as low as about 0.92 K/W. As a result, the 1200 nm VECSEL can emit a power of 9.4 W under continuous wave (CW) operation, and the threshold pump power density is as low as about 1.61 kW/cm2. The VECSEL can also operate at a quasi-continuous wave (QCW) emission with tunable pulse width and repetition rates. Under a duty cycle of ∼9.5%, the VCSEL is with a peak power of 16.6 W and a slope efficiency of about 15.5%, and the corresponding pulse energy is as high as about 1.57 mJ. No thermal rollover is observed. This exploration significantly broadens the VECSEL performance and can be regarded as an extension and enrichment of the earlier works on this topic.

Synchronized triple-wavelength fiber lasers at 1, 1.55, and 1.9 µm.

Synchronized lasers working at different wavelengths are of great significance for numerous applications, such as high-energy femtosecond pulse emission, Raman microscopy, and precise timing distribution. Here, we report synchronized triple-wavelength fiber lasers working at 1, 1.55, and 1.9 µm, respectively, by combining the coupling and injection configurations. The laser system consists of three fiber resonators gained by ytterbium-doped fiber, erbium-doped fiber, and thulium-doped fiber, respectively. Ultrafast optical pulses formed in these resonators are obtained by passive mode-locking with the use of a carbon-nanotube saturable absorber. A maximum cavity mismatch of ∼1.4 mm is reached by the synchronized triple-wavelength fiber lasers in the synchronization regime by finely tuning the variable optical delay lines incorporated in the fiber cavities. In addition, we investigate the synchronization characteristics of a non-polarization-maintaining fiber laser in an injection configuration. Our results provide a new, to the best of our knowledge, perspective on multi-color synchronized ultrafast lasers with broad spectral coverage, high compactness, and a tunable repetition rate.

Boost of cosmetic active ingredient penetration triggered and controlled by the delivery of kHz plasma jet on human skin explants

This work reports on the demonstration of the penetration of cosmetic active ingredients (caffeine and hyaluronic acid) in human skin explants following safe and controlled plasma jet exposure. First, temperature increase and immunohistochemistry in the stratum corneum and epidermis were characterized to check the safe delivery of plasma jets and to select two operation regimes at 1 and 20 kHz. Plasma exposure for tens of seconds is shown to induce transient modulations of skin pH, transepidermal water loss, and skin wettability, revealing a reversible skin barrier function modulation. Then, it is demonstrated that plasma exposure significantly accelerates the penetration of active ingredients. The tuning of the plasma jet pulse repetition rate allows controlling the penetration kinetics. Such ex vivo results agree with previous in vitro experiments also exhibiting a transient permeabilization time window. A preliminary demonstration of human skin wettability modulation with a low-power, user-friendly dielectric barrier discharge setup is documented, opening perspectives for plasma-based home cosmetic care device development. To the best of our knowledge, this work is one of the first demonstrations of safe and controlled plasma-assisted active ingredients’ skin penetration in the context of cosmetic applications.

Electro‐Optic Frequency Combs: Theory, Characteristics, and Applications

AbstractOptical frequency combs (OFCs) are a unique kind of light source, which are represented as a series of equally spaced coherent spectral lines in the frequency domain. OFCs can mainly be divided into mode‐locked lasers, Kerr frequency combs, and electro‐optic frequency combs (E‐O combs), which have broad applications in optical communications, frequency metrology, atomic clocks, distance ranging, spectroscopy, and arbitrary waveform generation. Among them, E‐O combs feature some unique advantages, such as fast tunable repetition rate, high sidebands power, and reconfigurability of the comb spectrum. Especially in recent years, with the development of micro–nano processing technology, on‐chip E‐O combs have become a dynamic research topic with many fundamental scientific problems as well as engineering applications for further exploration. To summarize the past development and envision the prosperous future of E‐O combs, the area of E‐O combs is reviewed from the following aspects: development of E‐O combs; theory including the generation process of E‐O combs and the electro‐optic modulation models; important techniques including flattening, broadening, noise, and stability controlling; applications including communications, ranging, spectroscopy, wavelength calibration of astronomical spectrographs, and microwave generation; and pros and cons when compared with other OFCs.

Versatile OSCAT time-domain THz spectrometer.

We report on a compact and versatile time-domain spectrometer operating in the THz spectral region from 0.2 to 2.5 THz based on ultrafast Yb:CALGO laser and photo-conductive antennas. The spectrometer operates with the optical sampling by cavity tuning (OSCAT) method based on laser repetition rate tuning, which allows at the same time the implementation of a delay-time modulation scheme. The whole characterization of the instrument is presented and compared to the classical THz time-domain spectroscopy implementation. THz spectroscopic measurements on a 520-μm thick GaAs wafer substrate together with water vapor absorption measurements are also reported to further validate the instrument capabilities.