Mode-locked Oscillator Research Articles

Watt-level mid-infrared continuous-wave Tm:YAG laser operating on the 3H4 → 3H5 transition

We report on the first continuous-wave mid-infrared Tm:Y3Al5O12 laser operating on the 3H4 → 3H5 transition. The 3.2 at.% Tm:Y3Al5O12 laser generated a maximum output power of 1.07 W at ~2.19 & 2.32 μm with a high slope efficiency of 46.3% (vs. the absorbed pump power) exceeding the Stokes limit. Depending on the output coupling and the pump power, a set of discrete laser lines in the range of 2.164–2.346 μm was observed. The spectroscopic properties of Tm:Y3Al5O12 relevant for the mid-infrared laser operation are revised. The maximum stimulated-emission cross-section for the 3H4 → 3H5 transition is 0.35 × 10−20 cm2 at 2324 nm and the emission bandwidth is ~37 nm. The overlap of the 3F4 → 3H6 and the 3H4 → 3H5 transitions is quantified revealing the formulas for Stokes and anti-Stokes phonon sidebands of emission bands. Tm:Y3Al5O12 is promising for broadly tunable (2.1–2.5 μm) and mode-locked mid-infrared oscillators.

Investigations of mode-locked Er-doped oscillators with record high-pulse energies

In our experiment, based on nonlinear polarization rotation (NPR), first, we demonstrate a large-energy square-wave mode-locked fiber laser under low pump power. The largest pulse energy of the square-wave pulses was 106.91 nJ. However, pulse breaking occurred when the pump power was higher than 1152 mW. Thus, under the maximum pump power of 2318 mW, several kinds of large-energy mode-locked operations were also investigated successfully. The maximum output pulse energy was recorded to be 881.34 nJ, which exhibits obvious enhancement in comparison with previous works. Additionally, in our work, we have successfully revealed that broadening of the pulse width, splitting of the pulse shape and increasing of the pulse repetition rate play significant roles in reducing the influence of large pulse energy and high peak power experimentally. Our experiment result will provide a useful guide for demonstrating large-energy pulse operations.

Soliton Mode-Locked Large-Mode-Area Tm-Doped Fiber Oscillator

We report a passively mode-locked fiber oscillator based on thulium-doped large-mode-area (LMA) fiber. Reliably mode-locking soliton operation can be readily achieved after the suppression of the higher-order modes by using mode field adapters in combination with coiling fiber method. The laser delivers 600-fs level pulses at $2~\mu \text{m}$ with an average power of up to 243 mW and a repetition rate of ~53.7 MHz. The output corresponds to a maximum pulse energy of 4.5 nJ, >5 times larger than that of a similar homemade mode-locked Tm laser with the LMA fibers replaced by single mode fibers, coinciding with theoretical prediction. The successful demonstration of our soliton mode-locked LMA fiber laser at $2~\mu \text{m}$ offers another strategy for high performance ultrafast fiber sources at mid-infrared region.

A passively mode-locked Holmium fiber oscillator based on a Nonlinear Amplifying Loop Mirror operating at 2050 nm

A passively mode-locked Holmium fiber oscillator based on a Nonlinear Amplifying Loop Mirror operating at 2050 nm

Passively Femtosecond Mode-Locked Erbium-Doped Fiber Oscillator with External Pulse Compressor for Frequency Comb Generation

Abstract Optical frequency comb (OFC) sources, for their highly desirable properties, they have revolutionized a wide range of applications specifically orthogonal frequency-division multiplexing (OFDM) system. OFC sources based on femtosecond erbium fiber laser has emerged as the most commonly used in many applications owing to their advantages of low cost, robustness, compactness, high performance, and flexibility. Here, 300 fs soliton mode-locked pulses were achieved using erbium fiber oscillator based on an efficient Graphene saturable absorber and an external all-fiber pulse compressor consisted of erbium-doped fiber amplifier (EDFA) and 200 m long single-mode fiber (SMF). The proposed system works with a central wavelength of 1557 nm at the desirable communication band. The output power of the system and its corresponding pulse energy were 9.29 mW and 1.858 nJ, respectively, at a maximum pump power of 92.6 mW. The designed system is entirely fiber based, with no free-space optics and uses only standard telecommunication equipment. The results of this experiment demonstrate that the simple design of mode-locked fiber oscillator with an external compressor can provide a compact, portable, robust, reliable and high-performance femtoseconds frequency comb.

Large Normal Dispersion Mode-Locked Erbium-Doped Fiber Laser

We report on a passively mode-locked oscillator based on an erbium-doped dual concentric core fiber combining high normal dispersion and large mode area. This large normal dispersion laser generates long pulses with 30 ps duration and 0.17 nm spectral width at 1530 nm wavelength. The source delivers an average power of 64 mW at a repetition rate of 16 MHz, corresponding to 4 nJ energy. This concept opens up new degrees of freedom in the design of mode-locked fiber lasers.

Mode-locked pulse oscillation of a self-resonating enhancement optical cavity

A power enhancement optical cavity is a compelling means of realizing a pulsed laser with a high peak power and high repetition frequency, which is not feasible using a simple amplifier scheme. However, a precise feedback system is necessary for maintaining the narrow resonance condition of the optical cavity; this has become a major technical issue in developing such cavities. We have developed a new approach that does not require any active feedback system, by placing the cavity in the outer loop of a laser amplifier. We report on the first demonstration of a mode-locked pulse oscillation using the new system.

Multi-wavelength, nano-second actively mode-locked Yb-fiber oscillator with 100 nm wide Raman broadened spectrum

An all-normal dispersion, actively mode-locked Yb-fiber oscillator has been demonstrated which delivers multi-pulsing output with Raman broadened spectrum spanning more than 100 nm. Pulses of nano-second duration at 953.81 kHz repetition rate were obtained due to an elongated cavity consisting of 200 m long single mode fiber introducing high nonlinearity. The individual pulses of the multi-pulsing output corresponded to different wavelengths in the output spectrum and concurrently generated multiple first order Raman Stokes sidebands at close wavelength intervals. This resulted in a smoother broadband spectrum compared to usually obtained Raman broadened spectrum in normal dispersion regime which contains multiple higher order Stokes lines at large wavelength intervals when pumped by a single nano-second pulse. Analysis of the spectral power content revealed that nearly 50% of the output power contained in the Raman band around the 1120 nm region. By amplification of the generated pulses and further non-linear broadening through an ultra-high numerical aperture (UHNA) fiber, a broad spectrum having a wavelength span of nearly 260 nm with 1.5 Watt average power has been demonstrated showing application capability of the designed source.

Milliwatt-class broadband THz source driven by a 112 W, sub-100 fs thin-disk laser.

We demonstrate a high repetition-rate, single-cycle THz source with a maximum average power of 1.35 mW, operating at a center frequency of 2 THz. This result was obtained by optical rectification (OR) in GaP using an amplifier-free, nonlinearly compressed modelocked thin-disk oscillator based on Yb:YAG, delivering 8.4 µJ pulses with 88 fs duration at a repetition rate of 13.4 MHz, resulting in driving pulses for OR with 112 W average power and 80 MW peak power. To the best of our knowledge, our result represents the highest average power so far achieved with OR in GaP. The demonstrated performance is very attractive for improving current linear THz time-domain spectroscopy experiments, which are currently restricted by low signal-to-noise ratio and long measurement times.

170 mW-level mode-locked Er-doped fiber laser oscillator based on nonlinear polarization rotation

In our work, by optimizing the length of the cavity and using high-power home-made pump source, based on the commonly-used nonlinear polarization rotation (NPR) technique, we designed a stable all-fiber high-power Er-doped mode-locked laser successfully. The maximum average output power was as high as 174 mW under the pump power of 2659 mW. To our knowledge, 174 mW was a new record average output power of NPR based all-fiber mode-locked Er-doped fiber laser, which was much higher than the results reported previously. Our work will provide a significant reference for future demonstrations of high-power mode-locked Er-doped fiber lasers and promote the applications of mode-locked Er-doped oscillators.

Passively mode-locked Yb-doped all-fiber oscillator operating at 979 nm and 1032 nm with the single wall carbon nanotubes as SA

We report on an all-fiber passively mode-locked ytterbium-doped (Yb-doped) fiber laser with single polymer composite saturable absorber (SA) based on the single wall carbon nanotubes (SWCNTs) with diameters of 0.8 nm. The SWCNTs are mixed with sodium carboxymethylcellulose (NaCMC) to form SWCNTs SA film. The non-saturable absorption coefficient, modulation depth and saturable intensity of the SWCNTs SA are measured to be 29%, 12% and 0.12 MW/cm2, respectively. By using the SWCNTs as SA, a passively mode-locked Yb-doped fiber laser has been achieved at 979 nm with pulse duration of 228 ps and repetition rate of 15.47 MHz. To our knowledge, this is the first time ever reported in an all-fiber Yb-doped mode-locked fiber laser using SWCNTs as SA at 980 nm, which overcome the difficulties of low gain and serious re-absorption effects in the quasi-three-level system. In order to verify the saturation absorption performance of the SWCNTs, mode-locking experiment in quasi-four-level system is also carried out around 1030 nm and the stable and uniform mode-locked pulses are obtained. A mode-locked fiber laser at 1032 nm is realized with a pulse width of 485 ps and repetition rate of 8.78 MHz. It is sufficiently manifested that SWCNTs with diameters of 0.8 nm have excellent saturable absorption performance as optical mode-locking elements in ultrafast photonic applications.

Kerr-lens mode-locked thin-disk oscillator with 50% output coupling rate.

Several approaches to power scaling of mode-locked thin-disk oscillators exist. One of these approaches is based on the increased gain provided by multiple passes through the thin-disk laser medium. For the first time, to the best of our knowledge, we applied this approach to a Kerr-lens mode-locked thin-disk oscillator. The so obtained additional gain allowed mode-locked operation with up to 50% output coupling rate. This first demonstration is of particular importance for gain media with inherently low-emission cross sections and paves the way to even more powerful Kerr-lens mode-locked thin-disk oscillators. Moreover, the experimental results indicate an increased self-amplitude modulation related to an overall increase in the soft-aperture Kerr-lens effect.

Efficient nonlinear compression of a mode-locked thin-disk oscillator to 27 fs at 98 W average power.

We demonstrate efficient pulse compression of a 13.4MHz, 534fs, 123W, Yb:YAG thin-disk oscillator down to 27fs at 98W average power, resulting in a record-high 166MW peak power from an amplifier-free oscillator-driven setup. Our compressor is based on two stages: one multipass cell allowing us to reduce the pulse duration to sub-90fs and, subsequently, a multiple-plate compressor, allowing us to reach 27fs. The overall average power compression efficiency is 80%, and the beam has excellent beam quality and homogeneity. In addition, we demonstrate further spectral broadening that supports a transform limit of 5fs in a second multiple-plate stage, demonstrating the potential for reaching a 100W class, amplifier-free, few-cycle source in the near future. The performance of this unique source is very promising for applications previously restricted to amplified sources, such as efficient generation of extreme ultraviolet light at high repetition rate, and the generation of high-power broadband THz radiation.

Investigation of stimulated Raman scattering in a mode-locked ytterbium-doped fiber amplifier setup

Studies on stimulated Raman scattering (SRS) in an ytterbium (Yb)-doped all-fiber amplifier setup seeded by a mode-locked all-normal dispersion Yb-doped fiber oscillator have been carried out. Oscillator pulses at 40 MHz repetition rate, centered at about 1060 nm having a spectral width of ~10 nm, were amplified in an all-fiber multi-stage amplifier setup in chirped pulse amplification configuration. We observed that the length of the fiber used as a stretcher plays a crucial role in SRS generation. Experimentally, 8.4 W power in the first SRS band at ~1120 nm was obtained from total amplifier output power of 10.8 W with a 30 m length of stretcher fiber, leading to as high as ~77% conversion efficiency. This is the highest average output power and conversion efficiency achieved without any external seed source at Stokes wavelength or with a synchronous pumping mechanism. Numerical simulations have also been carried out to show that there is an optimum stretcher fiber length for which maximum conversion of the amplifier power into the first SRS band takes place.

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.