Saturable Absorber Research Articles

Passively mode-locked fiber laser based on ethylene glycol

A passively mode-locked Er-doped fiber laser employing ethylene glycol as a saturable absorber (SA) has been proposed and experimentally validated. The ethylene glycol-SA is fabricated by filling the space between the end facets of two optical patch cords inserted into a cannula with ethylene glycol. Utilizing such an SA with a modulation depth of 9.6 %, stable mode-locking operation are manifested in the laser cavity, including the production of bright pulse and bright-dark pulse pair. The generated bright pulse has a center wavelength at 1562.28 nm, a 3 dB bandwidth of 0.21 nm, a repetition frequency of 20.24 MHz, and a signal-to-noise ratio (SNR) of 57 dB. Through precisely manipulating the polarization state and the pump power in the laser cavity, the bright-dark pulse pairs can be successfully attained at the same repetition frequency. This is the first demonstration of a passively mode-locked fiber laser utilizing ethylene glycol as the saturable absorber. Due to their superior optical absorption characteristics, low cost, simplicity in fabrication and non-volatile nature, ethylene glycol-SAs have enormous potential in the field of nonlinear optics and ultrafast photonics.

Mode-locked figure-9 fiber laser assisted by a SESAM for improving signal-to-noise ratio and stability

The mode-locked figure-9 fiber laser has been proved to exhibit excellent performance, but improvement of its output signal-to-noise ratio(SNR) and stability are restricted by the laser structure design. In this paper, we propose and verify a modified laser cavity to increase SNR and output stability by introducing a saturable absorber mirror(SESAM) to the single arm end of the figure-9 laser as a mode-locked auxiliary device instead of the typical mirror. Numerical simulations based on the model of the figure-9 fiber and SESAM lasers clearly indicate that modified structure has better output performance than typical one. Additionally, the related experimental results also demonstrate that at the same pump power of 150mW and compare the typical structure, the modified one outputs a higher SNR of 70 dB, together with 12 h output lower RMS of 0.23 %. We believe that the presented laser design could be a stable and reliable ultrafast optical seed for improving the performance of high-power laser facilities, which also has potential applications in optical frequency comb and all-optical modulation.

Self Q-switched single-frequency all-fiber laser

Here, we propose a self Q-switched single-frequency (SF) all-fiber laser. The intra-cavity Er:Yb co-doped fiber serves not only as an active fiber for lasing but also as a fiber-type saturable absorber (SA). The self-Q-switching behavior is induced by the ground-state reabsorption of the unpumped portion of the active fiber. To achieve SF operation, double fiber loops constructed by two 3 dB optical couplers (OCs) form a cascade sub-cavity, which serves as a mode selector with a narrow bandwidth. Taking the advantage of a high damage threshold of the fiber-type SA, microjoule-level self-Q-switched SF pulses are obtained with a narrow linewidth of ∼252kHz. Combed with the measured pulse width of ∼1.766µs, the corresponding time-bandwidth product is ∼0.445, which is very close to the Gaussian transform limit. The maximum output power of the pulsed SF laser can reach ∼100mW. By stretching the fiber Bragg grating (FBG), the wavelength of the pulsed SF laser can be tuned.

Multistability manipulation by reinforcement learning algorithm inside mode-locked fiber laser

Abstract Fiber mode-locked lasers are nonlinear optical systems that provide ultrashort pulses at high repetition rates. However, adjusting the cavity parameters is often a challenging task due to the intrinsic multistability of a laser system. Depending on the adjustment of the cavity parameters, the optical output may vary significantly, including Q-switching, single and multipulse, and harmonic mode-locked regimes. In this study, we demonstrate an experimental implementation of the Soft Actor–Critic algorithm for generating a harmonic mode-locked regime inside a state-of-the-art fiber laser with an ion-gated nanotube saturable absorber. The algorithm employs nontrivial strategies to achieve a guaranteed harmonic mode-locked regime with the highest order by effectively managing the pumping power of a laser system and the nonlinear transmission of a nanotube absorber. Our results demonstrate a robust and feasible machine-learning–based approach toward an automatic system for adjusting nonlinear optical systems with the presence of multistability phenomena.

Q-Switching in an Erbium-Doped Fibre Laser Using a Saturable Absorber Based on Vanadium Pentoxide Polyethylene Glycol in the Long-Wavelength Range

Q-Switching in an Erbium-Doped Fibre Laser Using a Saturable Absorber Based on Vanadium Pentoxide Polyethylene Glycol in the Long-Wavelength Range

Yb-doped mode-locked fiber laser with a hybrid structure of NPR and a Lyot filter as the saturable absorber.

Passively mode-locked fiber lasers based on a nonlinear polarization rotation (NPR) have attracted much attention due to their ability to generate short pulses with wide spectra and high peak power. However, environmental perturbations can easily cause the lasers to lose the mode-locked state and make it a challenge for practical application. The aim of this research is to improve the laser stability by inserting a Lyot filter into the mode-locked laser cavity. The experimental results indicate that the mode-locked state can be maintained when the radius of the fiber loop is changed from 7.5 to 1.5 cm, while the signal-to-noise ratio of the fundamental frequency remains almost the same. The tunability of the output power can be achieved by adding a half-wave plate (HWP) in the laser cavity without changing the pump power, while the mode-locked state remains stable. By adjusting the angle of the HWP2, the output power can be adjusted from 3.36 to 66.5 mW at repetition rate of 29.7 MHz.

Diode-pumped passively mode-locked femtosecond Yb:YLF laser at 1.1 GHz

We report femtosecond pulse generation at GHz repetition rates with the Yb:YLF gain medium for the first time. A simple, low-cost, and compact architecture is implemented for the potential usage of the system as a low-noise timing jitter source. The system is pumped by 250 mW, 960 nm single-mode diodes from both sides. The semiconductor saturable absorber mirror (SESAM) mode-locked laser is self-starting and generates transform-limited 210 fs long pulses near 1050 nm. The laser's average output power is 40 mW, corresponding to a pulse energy of 36 pJ at 1.1 GHz repetition rate. The measured laser relative intensity noise (RIN) from 1 Hz to 1 MHz is 0.42%. The performance obtained in this initial work is limited by the specifications of the available optics and could be improved significantly by employing custom-designed optical elements.

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Biological material spider silk by direct incorporation onto fiber ferrule for wavelength tunable Q-switched application

This study presents a novel structure saturable absorber (SSA) based on spider silk for wavelength tunable Q-switched erbium-doped fiber laser (EDFL) operation from S to L bands. The nonlinear optical absorption of spider silk was measured, showing a high modulation depth of 64.92%, a low saturation intensity of 0.03 MW cm−2, and a non-saturable loss of 24%. By adjusting the polarization controller, a wavelength tunable Q-switched EDFL was achieved, with a tuning range of 64 nm from 1522 nm to 1586 nm. The Q-switched pulses had a repetition rate varying from 20.62 kHz to 6.57 kHz and a pulse width ranging from 14.02 μs to 26.30 μs, corresponding to an output power from −11.31 dBm to −9.02 dBm at the maximum pump power of 151.40 mW. The proposed SSA using spider silk offers a low-cost, eco-friendly, and high-performance solution for wide wavelength tunable Q-switched EDFL applications in optical testing, fiber communication, optical fiber sensing, and ultrafast lasers.

Numerical study of transient absorption saturation in single-layer graphene for optical nanoscopy applications

Transient absorption, or pump–probe microscopy is an absorption-based technique that can explore samples ultrafast dynamic properties and provide fluorescence-free contrast mechanisms. When applied to graphene and its derivatives, this technique exploits the graphene transient response caused by the ultrafast interband transition as the imaging contrast mechanism. The saturation of this transition is fundamental to allow for super-resolution optical far-field imaging, following the reversible saturable optical fluorescence transitions (RESOLFT) concept, although not involving fluorescence. With this aim, we propose a model to numerically compute the temporal evolution under saturation conditions of the single-layer graphene molecular states, which are involved in the transient absorption. Exploiting an algorithm based on the fourth order Runge–Kutta (RK4) method, and the density matrix approach, we numerically demonstrate that the transient absorption signal of single-layer graphene varies linearly as a function of excitation intensity until it reaches saturation. We experimentally verify this model using a custom pump–probe super-resolution microscope. The results define the intensities necessary to achieve super-resolution in a pump–probe nanoscope while studying graphene-based materials and open the possibility of predicting such a saturation process in other light-matter interactions that undergo the same transition.

Passively Q-switched Er-doped fiber laser based on carbon-doped silver nanoparticles saturable absorber prepared using laser ablation method

In this work, we investigate the effect of carbon-doped silver nanoparticles on the Q-switched performance of erbium-doped fiber laser (EDFL). The carbon-doped silver nanoparticles were synthesized using a laser ablation method. The prepared nanoparticles were inserted between fiber ferrules using the adhesion process of the index-matched gel. Incorporating a saturable absorber inside the laser cavity initiates a stable Q-switched mechanism at 11.2 mW of threshold power. The measured results demonstrate that as the pump power of EDFL increases from 11.2 to 267 mW, the pulse repetition, and pulse width tuned from 21.33 to 95.2 kHz and 13.3–3.18 µs, respectively. At 267 mW of pump, the maximum average output power, pulse energy, and peak power were further measured to be 2.36 mW, 24.68 nJ, and 7.76 mW, respectively. Besides, the stability and threshold characteristics of EDFL based on carbon-doped silver nanoparticles were further explored. This study shows that the synthesis of metal nanoparticles using the laser ablation technique and their implementation as saturable absorber represents a promising avenue for advancing ultrafast laser technologies with improved stablility, efficiency, and tunability.

Sustainable ecological pervious concrete (SEPC) based on modified corn-cob coarse aggregate: Crushing morphology, strength, pore properties, and durability

In terms of agricultural products, corn is the most abundant crop in the world, yet a significant portion of its cobs are thrown away each year, which is a terrible resource waste. This study uses modified corn-cob coarse aggregate (MCCA) and recycled aggregate (RA) to prepare sustainable ecological pervious concrete (SEPC). Five different substitution rates (0%, 25%, 50%, 75%, and 100%) are designed to investigate the effect of substitution rate on the crushing morphology, strength, pore properties, and durability of SEPC. The results show that the saturated water absorption of MCCA is 45.4% and its numerical tube pressure reaches 0.9 MPa. The test results show that the mix of MCCA reduces the compressive strength of SEPC but is beneficial to improve the stress state of SEPC by increasing its continuous porosity, planting opening size, frost resistance and resistance to sulfate erosion. At 50% replacement rate, comprehensive performance of SEPC is at its best. Specifically, test group with 50% MCCA substitution rate exhibits the lowest rate of compressive strength loss during freeze-thaw cycles (7.58%) and the best corrosion resistance coefficient (97.97%) when subjected to sulfate erosion. Therefore, MCCA can be used as a sustainable and ecological lightweight aggregate to improve some properties of pervious concrete.

Sub-100 fs pulses lasers from Yb:GdScO3 crystal based on semiconductor saturable absorber mirrors

Sub-100 fs laser pulses are generated in a Yb:GdScO3 passively mode-locked laser based on a semiconductor saturable absorber mirror. The laser operates in the continuous wave mode-locked state with a repetition frequency of 81 MHz and a pulse width of 98 fs. The output pulses exhibit a high signal-to-noise ratio of 60 dB in the radio-frequency spectra, with a central wavelength of 1064.4 nm and a full width at half maximum of 12.4 nm. This achievement marks the first successful demonstration of an ultrafast laser in a Yb:GdScO3 crystal, validating Yb:GdScO3 as an ideal medium for generating ultrafast lasers.

2.8-µm polarization-maintaining Er fiber laser mode-locked by a GaSb-based SESAM

A GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) enables continuous-wave picosecond mode-locked operation with excellent stability of a polarization-maintaining mid-infrared Er:ZBLAN fiber laser. The GaSb-based SESAM mode-locked fiber laser delivers an average output power of 190 mW at 2.76 µm at a repetition rate of 32.07 MHz (corresponding to a pulse energy of ∼6 nJ) and exhibits a high signal-to-noise ratio of ∼80 dB. The polarization extinction ratio is more than 23 dB. By employing an intracavity diffraction grating, the laser wavelength is continuously tunable across 2.706–2.816 µm. Passively Q-switched operation of this laser is also demonstrated.

Upconversion-pumped femtosecond thulium laser at 2309 nm mode-locked by a GaSb-based SESAM

We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.

Seedless synthesis of Au nanoplates with tunable plasmonic peaks

Au nanoplates with tunable in-plane dipolar localized surface plasmon resonance peaks in a broad range from the visible to near-infrared region were obtained in high yield using a seedless wet chemical growth method after purification. Cetyltrimethylammonium chloride was used as a surfactant, while hydrogen peroxide and sodium borohydride were used as the weak and strong reducing agents, respectively. The edge length and in-plane dipolar localized surface plasmon resonance peak of the Au nanoplates could be adjusted by varying the amounts of hydrogen peroxide and sodium borohydride. The Au nanoplates were further used as the saturable absorber to generate pulsed laser output in a passively Q-switched solid-state laser at approximately 2 µm. Our study offers a new method for obtaining Au nanoplates with tunable plasmonic peaks over a broad range.

Phthalocyanine Covalent Organic Frameworks: Dimensionality Effect on Third‐Order Nonlinear Optical Properties

AbstractDimensionality of covalent organic frameworks (COFs), in terms of 2D and 3D, must greatly influence their properties and functionalities due to the different electronic and porous structures, but still less investigated in the 20 years history. Herein, on the basis of the imidization reaction of phthalocyanine‐based tetraanhydride monomers with either 1,4‐phenylenediamine or 1,3,5,7‐tetra(4‐aminophenyl)adamantine, 2D and 3D imide‐bonded COFs with sql and pts topology have been prepared to explore their third‐order nonlinear optical (NLO) properties. In contrast to metal‐free analogues, both 2D and 3D copper‐phthalocyanine COFs have the enhanced NLO behaviors according to the open aperture Z‐scan and optical limiting (OL) results recorded under a nanosecond laser at 532 nm. In particular, 2D COFs with strong π–π staking phthalocyanine units show the NLO property with bigger third‐order nonlinear susceptibility (Imχ(3)) and unique reverse saturation absorption response at 1064 nm than that of 3D species with dispersed conjugated modules. Furthermore, femtosecond transient absorption spectra disclose the longer excited lifetimes of copper COFs than metal free samples, helpful in their NLO property. These findings clearly disclose the materials advantages of 2D phthalocyanine COFs toward the NLO characteristics and provide new candidates for OL application.

Revealing photoluminescence and nonlinear optical absorption characteristics of PbMo0.75W0.25O4 single crystal for optical limiting applications

Abstract Nonlinear absorption properties of PbMo0.75W0.25O4 single crystal fabricated by the Czochralski method were studied. The band gap energy of the crystal was determined as 3.12 eV. Urbach energy which represents the defect states inside the band gap was found to be 0.106 eV. PbMo0.75W0.25O4 single crystal has a broad photoluminescence emission band between 376 and 700 nm, with the highest emission intensity occurring at 486 nm and the lowest intensity peak at 547 nm, depending on the defect states. Femtosecond transient absorption measurements reveal that the lifetime of localized defect states is found to be higher than the 4 ns pulse duration. Open aperture (OA) Z-scan results demonstrate that the PbMo0.75W0.25O4 crystal exhibits nonlinear absorption (NA) that includes simultaneous two-photon absorption (TPA) as the dominant mechanism at the 532 nm excitations corresponding to 2.32 eV energy. NA coefficient (βeff) increased from 7.24 x 10-10 m/W to 8.81 x 10-10 m/W with increasing pump intensity. At higher intensities βeff tends to decrease with intensity increase. This decrease is an indication that saturable absorption (SA) occurred along with the TPA, called saturation of TPA. The lifetime of the defect states was measured by femtosecond transient absorption spectroscopy. Saturable absorption behavior was observed due to the long lifetime of the localized defect states. Closed aperture (CA) Z-scan trace shows the sign of a nonlinear refractive index. The optical limiting threshold of PbMo0.75W0.25O4 single crystal at the lowest intensity was determined as 3.45 mJ/cm2. Results show that the PbMo0.75W0.25O4 single crystal can be a suitable semiconductor material for optical limiting applications in the visible region.

Cr2AlB2/PbS heterostructure as saturable absorber for ultrashort pulse generation

Rapid progress has been realized in the nonlinear optical device applications of 2D materials. 2D materials are attractive due to their characteristics optical nonlinearities and distinctive light-matter interactions, which assure them promising nonlinear optical devices for ultrashort pulses generating. Cr2AlB2, a member of the new series of 2D materials known as MBenes, has been predicted to have potential saturable absorption properties. In this study, the saturable absorption properties of Cr2AlB2 and Cr2AlB2/PbS vdW heterostructure SAs are investigated. In comparison with Cr2AlB2, the Cr2AlB2/PbS vdW heterostructure exhibits better saturable absorption. The enhancements of nonlinear optical properties can be attributed to the carrier migration between heterostructure. The simulation results based on the Ginzburg-Landu equation are employed to predict the laser performances of Cr2AlB2 and Cr2AlB2/PbS heterostructure SA. By incorporating SAs in the cavity, highly stable conventional solitons were observed. Both simulation and experimental results demonstrate that Cr2AlB2/PbS vdW heterostructure has superior pulse performance compared to the Cr2AlB2 SA. These results demonstrate that Cr2AlB2/PbS vdW heterostructure processes better saturable absorption properties and serves as a promising SA in ultrashort pulse generation.

GeS2 Nanosheets as Saturable Absorbers for High Signal-to-Noise Ratio in an L-Band Ultrafast Pulsed Laser

GeS₂ Nanosheets as Saturable Absorbers for High Signal-to-Noise Ratio in an L-Band Ultrafast Pulsed Laser