Showing papers in "Laser Physics in 2015"

Thirring optical solitons in birefringent fibers with spatio-temporal dispersion and Kerr law nonlinearity

Abstract: This work addresses Thirring optical solitons in birefringent ?bers. The dynamics of the vector coupled nonlinear Schr?dinger equation, which describes the propagation of Thirring solitons through birefringent optical ?bers with spatio-temporal dispersion and Kerr law nonlinearity, is investigated analytically. The tools of integration applied in this dynamical model are the Jacobian elliptic equation?expansion approach, Riccati equation?expansion scheme and soliton ansatz method. These algorithms lead to exact Thirring bright, dark and singular solitons along with the corresponding parameter constraints.

Bright, dark and singular optical solitons in a cascaded system

Abstract: This work studies nonlinear dynamics of optical solitons in a cascaded system with Kerr law nonlinearity and spatio-temporal dispersion. The mathematical model that describes the propagation of optical solitons through a cascaded system is given by the vector-coupled nonlinear Schrodinger equation. It is investigated analytically using three integration algorithms. The Jacobian elliptic equation expansion method, Bernoulli equation expansion approach and Riccati equation expansion scheme are the integration tools of this model that are recruited to extract singular, bright and dark solitons. The restrictions that need to hold for the existence of these solitons are derived.

Study of molecular iodine, iodate ions, iodide ions, and triiodide ions solutions absorption in the UV and visible light spectral bands

Abstract: The paper reports on experimental studies concerning the absorption spectra of molecular iodine and its , I−, anions in the spectral band of 180–600 nm. Values of the absorption cross-sections of the above mentioned substances have been measured, and relations of absorption coefficients to concentrations have been studied. The results obtained demonstrate that the spectral band under consideration is likely to be successfully used for simultaneous real-time detection of substances containing iodine with an absorption method using laser emission sources in the UV and visible light spectral bands.

A phase-sensitive optical time-domain reflectometer with dual-pulse diverse frequency probe signal

Abstract: In the present communication we propose a novel approach to the realization of a phase sensitive optical time-domain reflectometer (OTDR) which is capable of a precise reconstruction of the phase signal which impacts the arbitrary point of a fiber-optic line. The method uses a dual-pulse probe signal with diverse carrier optical frequency within each half of the double pulse. The quasi-periodic intensity pattern which emerges as a result of double frequency backscattered signal interference contains the information of the external action over the fiber. The phase signal is extracted with the aid of an I/Q quadrature demodulation scheme, realized at the receiving side of the OTDR. The feasibility and limitations of the proposed scheme are theoretically proved and experimentally demonstrated.

Photo-induced optical bleaching in Ge12Sb25S63 amorphous chalcogenide thin films: effect of 532 nm laser illumination

Abstract: The photo-induced effects of Ge12Sb25S63 films illuminated with 532 nm laser light are investigated from transmission spectra measured by FTIR spectroscopy. The material exhibits photo-bleaching (PB) when exposed to band gap light for a prolonged time in a vacuum. The PB is ascribed to structural changes inside the film as well as surface photooxidation. The amorphous nature of thin films was detected by x-ray diffraction. The chemical composition of the deposited thin films was examined by energy dispersive x-ray analysis (EDAX). The refractive indices of the films were obtained from the transmission spectra based on an inverse synthesis method and the optical band gaps were derived from optical absorption spectra using the Tauc plot. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. It was found that the mechanism of the optical absorption follows the rule of the allowed non-direct transition. Raman and x-ray photoelectron spectra (XPS) were measured and decomposed into several peaks that correspond to the different structural units which support the optical changes.

Generation and characterization of erbium-Raman noise-like pulses from a figure-eight fibre laser

Abstract: We report an experimental study of the noise-like pulses generated by a ~300 m long passively mode-locked erbium-doped figure-eight fibre laser. Non-self-starting mode locking yields the formation of ns scale bunches of sub-ps pulses. Depending on birefringence adjustments, noise-like pulses with a variety of temporal profiles and optical spectra are obtained. In particular, for some adjustments the Raman-enhanced spectrum reaches a 10 dB bandwidth of ~130 nm. For the first time to our knowledge, we extract information on the inner structure of the noise-like pulses, using a birefringent Sagnac interferometer as a spectral filter and a nonlinear optical loop mirror as an intensity filter. In particular we show that the different spectral components of the bunch are homogeneously distributed within the temporal envelope of the bunch, whereas the amplitude and/or the density of the sub-pulses present substantial variations along the envelope. In some cases, the analysis reveals the existence of an intermediate level of organization in the structure of the noise-like pulse, between the ns bunch and the sub-ps inner pulses, suggesting that these objects may be even more complex than previously recognized.

Plasmonic enhancement of Rhodamine dye random lasers

Abstract: We demonstrate improved characteristics in Rhodamine dye random lasers with the addition of gold nanoparticles. As a result of the strong plasmonic enhancement induced by gold nanoparticles, Rhodamine 640/gold random lasers have less than half the lasing threshold compared with Rhodamine 640/alumina random lasers in the weakly scattering regime for 10−3 M dye concentration. The optimum concentration of gold nanoparticles occurs at ~8 × 1010 cm−3, close to the transition between the weakly scattering and diffusive regimes. Rhodamine 640 has a better performance compared with Rhodamine 6G which is attributed to the greater spectral overlap of the Rhodamine 6G fluorescence spectrum with the plasmon resonance of gold, leading to an increased energy transfer and fluorescence quenching for Rhodamine 6G by gold. We also observe the contrasting trends of lasing threshold between random dye lasers incorporating dielectric and metal nanoparticles in the diffusive scattering regime. The effects of gold nanoparticles in random dye lasers are discussed in the context of the tradeoff between local field enhancement and fluorescence quenching.

Impact of geometrical parameters on the optical properties of negative curvature hollow-core fibers

Abstract: We analyze the impact of geometrical parameters on such important optical characteristics of negative curvature hollow-core fibers (NCHCFs) as waveguide dispersion, waveguide losses and the structure of transmission bands. We consider both theoretically and experimentally the resonance effects and formation of band edges under bending in NCHCFs.

Formation of plasma channels in air under filamentation of focused ultrashort laser pulses

Abstract: The formation of plasma channels in air under filamentation of focused ultrashort laser pulses was experimentally and theoretically studied together with theoreticians of the Moscow State University and the Institute of Atmospheric Optics. The influence of various characteristics of ultrashort laser pulses on these plasma channels is discussed. Plasma channels formed under filamentation of focused laser beams with a wavefront distorted by spherical aberration (introduced by adaptive optics) and by astigmatism, with cross-section spatially formed by various diaphragms and with different UV and IR wavelengths, were experimentally and numerically studied. The influence of plasma channels created by a filament of a focused UV or IR femtosecond laser pulse (λ = 248 nm or 740 nm) on characteristics of other plasma channels formed by a femtosecond pulse at the same wavelength following the first one with varied nanosecond time delay was also experimentally studied. An application of plasma channels formed due to the filamentation of focused UV ultrashort laser pulses including a train of such pulses and a combination of ultrashort and long (~100 ns) laser pulses for triggering and guiding long (~1 m) electric discharges is discussed.

Can an LED-laser hybrid light help to decrease hydrogen peroxide concentration while maintaining effectiveness in teeth bleaching?

Abstract: The aim of this study was to compare the bleaching efficacy of 35% hydrogen peroxide and 15% hydrogen peroxide with nitrogen-doped titanium dioxide catalysed by an LED-laser hybrid light. We studied 70 patients randomized to two groups. Tooth shade and pulpal sensitivity were registered. Group 1: 15% hydrogen peroxide with nitrogen-doped titanium dioxide. Group 2: 35% hydrogen peroxide. Both groups were activated by an LED-laser light. No significant differences were seen in shade change immediately, one week or one month after treatment (p > 0.05). Differences were seen in pulpal sensitivity (p < 0.05). The use of an LED-laser hybrid light to activate 15% hydrogen peroxide gel with N_TiO2 permits decreasing the peroxide concentration with similar aesthetic results and less pulpal sensitivity than using 35% hydrogen peroxide for bleaching teeth.

Research progress of chelate precursor doping method to fabricate Yb-doped large-mode-area silica fibers for kW-level laser

Abstract: With continuous efforts and practical managing experiences, the chelate precursor doping method has been justified as an effective way to dope rare-earth ions into silica host materials, a key technique in making large-mode-area silica fibers for high power laser applications. It is characterized by good controllability, stability and repeatability to accomplish different refractive index profiles. Different preforms with a large core, designed refractive index profile, good symmetrical shape and homogeneous elemental distribution were successfully fabricated. The home-made standard 20/400-type double-cladding Yb-doped large-mode-area silica fiber was drawn and presented a 1.6 kW laser output at 1064 nm, the highest power record publically reported with this method. With further optimization, chelate precusor doping method has potential to manufacture high power laser fibers for the next generation.

Effect of ytterbium co-doping on erbium clustering in silica-doped glass

Abstract: The effect of ytterbium co-doping on erbium ion clustering in silica-based glass was studied. It was experimentally shown that the pump-to-signal conversion efficiency could be significantly increased if the number of Yb3+ ions in the fiber core was 20 times higher than the number of Er3+ ions. Estimations of the number of Er3+ ions in clusters confirmed a decrease in their clustering at high Yb concentrations.

Wideband tunable graphene-based passively Q-switched Tm:YAP laser

Abstract: We investigate the characteristics of a graphene Q-switched Tm:YAP laser operating in a wavelength tunable regime. By using a volume Bragg grating (VBG) as the intra-cavity wavelength selection element, and graphene as the saturable absorber (SA), a tunable Q-switched Tm:YAP laser was realized with a continuous tunable wavelength range from 1965.2 to 2000.7 nm. Through careful alignment of the VBG and the graphene SA in coordination, stable pulsed laser operations were achieved at this wavelength range. A maximum output power of 920 mW was obtained at a wavelength of 1989.3 nm, corresponding to a pulse duration of 2.1 μs, a repetition rate of 91.6 kHz and a single pulse energy of 10.1 μJ.

Optical properties measurement of highly diffusive tissue phantoms for biomedical applications

Abstract: Extensive research in biomedical optics essentially requires the determination of optical properties of various biological tissues. Quantitative characterization of biological tissues in terms of optical properties is achieved with an integrating sphere. However, samples having significantly higher scattering and absorption coefficients such as malignant tissues potentially reduce the signal-to-noise ratio (SNR) and the accuracy of an integrating sphere. We describe the design, implementation and characterization of a modified sample holder (path length of up to 1 mm) for an integration sphere. Experiments conducted with various phantoms reveal significant improvement of the SNR for a wide range of optical properties. The alternative approach opens up potential applications in the measurement of optical properties of highly diffusive biological samples. For 20% intralipid µa = 0.112 ± 0.046 cm−1 and µs = 392.299 ± 10.090 cm−1 at 632.8 nm. For 1.0% Indian ink µa = 9.808 ± 0.490 cm−1 and µs = 1.258 ± 0.063 cm−1 at the same wavelength. The system shows good repeatability and reproducibility within a 4.9% error.

A passively Q-switched ytterbium-doped fiber laser based on a few-layer Bi2Se3 saturable absorber

Abstract: In this work, we demonstrate a simple Q-switched pulsed ring ytterbium-doped fiber laser based on a few-layer TI:Bi2Se3 saturable absorber (SA). Few-layer bismuth selenide within a suspension was induced onto a fiber ferrule at room temperature via an optical deposition method, resulting in a simple SA for the laser. Stable Q-switched pulsed lasing was achieved at a low pump threshold of 122.2 mW at 974 nm. The pulse repetition rate ranged from 18.97 to 45.41 kHz, and the narrowest pulse width and the maximum pulse energy were 13.1 μs and 5.88 nJ respectively. Results indicated that TI:Bi2Se3 was also compatible with the 1 μm waveband, and hence could be considered a potential broadband SA for passively mode-locked and Q-switched optical fiber lasers.

Optical bistability in nonlinear periodical structures with PT-symmetric potential

Abstract: The interaction between forward and backward waves in a periodical structure with PT -symmetric potential is investigated. The results demonstrate that the PT -term can change the bandgap of the periodical structure and affect the effective feedback mechanism. The linear solution, reflectivity, dispersion relation, and a generalized analytical solution of this periodical structure are obtained. The influences of the PT -term and detunings on the characteristic of bistability (or multistability) are also discussed.Keywords: optical bistability, periodical structures, PT-symmetric(Some figures may appear in colour only in the online journal) J Liu et al Optical bistability in nonlinear periodical structures with PT-symmetric potentialPrinted in the UK015102 LP © 2015 Astro Ltd201525Laser Phys.LP1054-660X10.1088/1054-660X/25/1/015102 Paper 1Laser Physics Astro Ltd CB 5 See, for example, a review by [1] and references therein. 1054-660X/15/015102+5$33.00Laser Phys. 25 (2015) 015102 (5pp) doi:10.1088/1054-660X/25/1/015102

Quantitative method to assess caries via fluorescence imaging from the perspective of autofluorescence spectral analysis

Abstract: A quantitative method to discriminate caries lesions for a fluorescence imaging system is proposed in this paper. The autofluorescence spectral investigation of 39 teeth samples classified by the International Caries Detection and Assessment System levels was performed at 405 nm excitation. The major differences in the different caries lesions focused on the relative spectral intensity range of 565–750 nm. The spectral parameter, defined as the ratio of wavebands at 565–750 nm to the whole spectral range, was calculated. The image component ratio R/(G + B) of color components was statistically computed by considering the spectral parameters (e.g. autofluorescence, optical filter, and spectral sensitivity) in our fluorescence color imaging system. Results showed that the spectral parameter and image component ratio presented a linear relation. Therefore, the image component ratio was graded as 1.62 to quantitatively classify sound, early decay, established decay, and severe decay tissues, respectively. Finally, the fluorescence images of caries were experimentally obtained, and the corresponding image component ratio distribution was compared with the classification result. A method to determine the numerical grades of caries using a fluorescence imaging system was proposed. This method can be applied to similar imaging systems.

Propagation dynamics of modified hollow Gaussian beams in strongly nonlocal nonlinear media

Abstract: We investigate here a new class of optical beams: modified hollow Gaussian beams (MHGBs) in strongly nonlocal nonlinear media (SNNM). A set of analytical expressions for the propagation properties is deduced and some numerical simulations are also carried out to illustrate the propagation properties. It is found that the evolution of the MHGBs in SNNM is periodical, which is the result of the competition between nonlinearity and diffraction. The second-order moment beam width of the MHGBs can keep invariant during propagation like a soliton when the input power equals the critical power, otherwise it varies periodically like a breather. However, the patterns of transverse intensity are always changing with the propagation distance increasing, which is different from solitons or breathers. It is also found that the evolution curve of on-axis intensity may manifest itself in a concave, a platform, or a Gaussian-like shape depending on the input power.

Yb-doped large-mode-area laser fiber fabricated by halide-gas-phase-doping technique

Abstract: In this manuscript, we designed a rare-earth-halide gas-phase-doping setup to fabricate a large-mode-area fiber for high power laser applications. YbCl3 and AlCl3 halides are evaporated, carried respectively and finally mixed with usual host gas material SiCl4 at the hot zone of MCVD system. Owing to the all-gas-phasing reaction process and environment, the home-made Yb-doped fiber preform has a homogeneous large core and modulated refractive index profile to keep high beam quality. The drawn fiber core has a small numerical aperture of 0.07 and high Yb concentration of 9500 ppm. By using a master oscillator power amplifier system, nearly kW-level (951 W) laser output power was obtained with a slope efficiency of 83.3% at 1063.8 nm, indicating the competition and potential of the halide-gas-phase-doping technique for high power laser fiber fabrication.

Yb:YAG thin disk laser passively Q-switched by a hydro-thermal grown molybdenum disulfide saturable absorber

Abstract: We demonstrate a passively Q-switched Yb:YAG thin disk solid-state laser based on nanoflake MoS2 as a saturable absorber. MoS2 is synthesized by a hydro-thermal process. The prepared MoS2 is transferred onto the BK7 glass for ease-of-use in the solid-state laser as a saturable absorber. The average output power could reach up to 250 mW, center wavelength 1030 nm corresponding to a pulse width, a pulse repetition rate, and a per pulse energy of 12 μs, 17 kHz, and 15 μJ, respectively. Our results show that nanoflake MoS2 could be a promising saturable absorber for Q-switching solid-state lasers. The over saturation of the MoS2 saturable absorber at a high pump strength limit in a solid-state laser could be also effective for high power operation.

From single pulse to double pulse ns laser ablation of silicon in water: photoluminescence enhancement of silicon nanocrystals

Abstract: In this paper we introduce a comparative approach for studying the emission properties of silicon nanocrystal (Si-nc) colloids prepared by single pulse and double pulse laser ablation processes of a silicon wafer in distilled water. Experiments were conducted to investigate the luminescence properties of the colloids considering the size distributions and surface characteristics of the synthesized Si-ncs. The results indicated that single pulse and double pulse laser ablation processes under similar experimental conditions can lead to the preparation of Si-nc colloids with almost the same size distributions and different surface chemistry. The results show that double pulse laser processing with an inter-pulse delay time of ~5 ns can produce Si-nc colloids with a much greater emission intensity (about fivefold to tenfold) in the orange–red region (550–700 nm) of the visible spectrum. Based on the detailed analysis of the Si-nc size distribution and surface characteristics, the observed prominent orange–red emission is possibly due to a different type of Si–OH surface termination that protects the nanocrystal core upon inward oxidation.

Effect of low-level diode laser on proliferation and osteogenic differentiation of dental pulp stem cells

Abstract: This study assessed the effect of low-level laser irradiation (LLLI) on the proliferation and osteogenic differentiation of dental pulp stem cells (DPSCs). DPSCs were exposed to 810 nm laser light (0.1, 0.2, or 0.3 J cm−2) for 7 d (60 s daily). The negative control group (cells in regular medium) and positive control group (cells in osteogenic medium (OM)) were not lased. One group of cells in OM was irradiated with laser operated at 0.2 J cm−2. Cell viability was evaluated at 24 h and one week after the last day of laser irradiation using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Osteogenic differentiation was assessed using real-time reverse transcriptase polymerase chain reaction (RT-PCR) and alizarin Red S staining. Cell proliferation was not affected by laser irradiation at 24 h except in one group (cells in OM exposed to laser at 0.2 J cm−2). However, one week after the last day of laser irradiation, it was significantly increased in groups exposed to laser at 0.1 or 0.2 J cm−2 and decreased in groups containing OM (P < 0.05). Osteoblast marker expression was observed in groups containing OM. LLLI at 0.2 J cm−2 dramatically enhanced cell differentiation. Laser at 0.3 J cm−2 increased bone sialoprotein (BSP) and decreased alkaline phosphatase (ALP). Mineralized nodules were only observed in groups containing OM. Considering these findings, LLLI may be used as a novel approach for preconditioning of DPSCs in vitro prior to bone tissue engineering.

Near-IR telecommunication diode laser based double-pass QEPAS sensor for atmospheric CO2 detection

Abstract: A novel double pass quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for atmospheric CO2 detection is developed by use of a 1.5 μm telecommunication diode laser. A low-cost high-reflection concave mirror is positioned behind a traditional QEPAS based acoustic detection module to enhance the absorption optical path and improve the detection sensitivity. The obtained minimum detection limit is 29 ppmV for 1 s averaging time which corresponds to a normalized noise equivalent absorption coefficient (NNEA, 1σ) of . With a 247 s averaging time, it demonstrates an ultimate detectable sensitivity of 1.74 ppmV.

Laser-ablated silicon nanoparticles: optical properties and perspectives in optical coherence tomography

Abstract: Due to their biocompatibility silicon nanoparticles have high potential in biomedical applications, especially in optical diagnostics. In this paper we analyze properties of the silicon nanoparticles formed via laser ablation in water and study the possibility of their application as contrasting agents in optical coherence tomography (OCT). The nanoparticles suspension was produced by picosecond laser irradiation of monocrystalline silicon wafers in water. According to transmission electron microcopy analysis the silicon nanoparticles in the obtained suspension vary in size from 2 to 200 nm while concentration of the particles is estimated as 1013cm−3. The optical properties of the suspension in the range from 400 to 1000 nm were studied by spectrophotometry measurements revealing a scattering coefficient of about 0.1 mm−1 and a scattering anisotropy factor in the range of 0.2–0.4. In OCT study a system with a central wavelength of 910 nm was employed. Potential of the silicon nanoparticles as a contrasting agent for OCT is studied in experiments with agarose gel phantoms. Topical application of the nanoparticles suspension allowed the obtaining of the contrast of structural features of phantom up to 14 dB in the OCT image.

80 fs passively mode-locked Er-doped fiber laser

Abstract: In this paper, 80 fs pulse generation from an all-fiber Er-doped laser is reported. The laser is mode-locked by a nonlinear polarization evolution mechanism in a stretched-pulse regime. Ultrashort pulses are generated directly from a ring-shaped, dispersion-managed cavity with a repetition rate of 31.9 MHz and 0.47 nJ of pulse energy. The laser has an all-fiber design, including the dispersion compensation and external compression stages, forming a robust and compact device.

Spatial mode switchable fiber laser based on FM-FBG and random distributed feedback

Abstract: We demonstrate an all-fiber laser capable of realising switchable spatial mode output, by utilising a few-mode fiber Bragg grating and random distributed feedback (RDFB). An offset splicing spot is used to assist the efficient exciting of higher order modes, and the laser output can be freely switched between the LP01 and LP11 modes by adjusting the polarisation controllers. The intensity distribution of the laser output beam profile, along with its spectral evolution and temporal characteristics has been investigated. To the best of our knowledge, this is the first spatial mode switchable RDFB fiber laser which may broaden the scope of the random fiber laser.

Coupled effect of stimulated Raman scattering and random lasing of dyes in multiple scattering medium

Abstract: Random lasing (RL) and stimulated Raman scattering (SRS) of dye in multiple scattering media (MSM) appears simultaneously and each couple with other. This coupling has considerable influence on the SRS regularities of dye in MSM. The main feature of this impact is that RL radiation promotes the Raman lines revealing in the RL spectrum range as part of total radiation. SRS initiation occurs owing to the CARS-like mechanism provided by the two component pump: incident monochromatic radiation (laser pump) and RL radiation arising inside the MSM. It leads to important consequences: the RL spectrum must overlap with the spectral region of the possible Stokes lines of the dye; only those Stokes lines appear which are in a range of the RL spectrum; all conditions which promote RL assist SRS also. It is shown MSM promotes the best conditions for SRS and RL coupling due to optimal matching of RL localization regions and pump radiation.

Behavior of optical bistability in multifold quantum dot molecules

Abstract: We analyze the optical bistability (OB) behavior in a multifold quantum dot (QD) molecule composed of five quantum dots controlled by the tunneling coupling. It is shown that the optical bistability can strongly be affected by the tunneling inter-dot coupling coefficients as well as detuning parameters. In addition, we find that the rate of an incoherent pump field has a leading role in modification of the OB threshold. We then generalize our analysis to the case of multifold quantum dot molecules where the number of the quantum dots is N (with a center dot and satellite dots). We compare the OB features that could occur in a multifold QD system consist of three (), four (), and five (N = 5) quantum dots. We realize that the OB threshold increases as the number of satellite QDs increases. Such controllable optical bistability in multiple QD molecules may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.

Nonlinear optical response of gold/silicon nanocomposite prepared by consecutive laser ablation

Abstract: A gold/silicon nanocomposite has been prepared by a consecutive laser ablation method and characterized by UV–visible absorption spectrometry, Fourier transform infrared spectrum, x-ray diffraction pattern and transmission electron microscopy methods. The Au/Si nanocomposite was formed by nanosecond pulsed laser irradiation on a gold plate in a silicon nanoparticle colloidal solution which has been prepared by laser ablation of Si plate ethanol. The UV–visible absorption spectrum of nanocomposite colloidal solution shows a 40 nm red shift of the surface plasmon peak compared with an Au nanoparticle in ethanol. The presence of Si nanoparticles in the solution was confirmed by the Fourier transform infrared spectrum. X-ray diffraction pattern of Au/Si nanocompsite powder only contains gold structures, and thus Si nanoparticles are amorphous. The diameter of Si nanoparticles and Au/Si nanocomposites are determined by transmission electron microscopy images about 13 and 30 nm, respectively. The nonlinear absorption coefficient of colloidal Au/Si was studied by the open aperture Z-scan method. The value of the nonlinear absorption is measured 5.8 × 10−3 cm W−1 with a positive sign which illustrates the two photon absorption phenomena.

Soliton molecules in a fiber laser mode-locked by a graphene-based saturable absorber

Abstract: We have experimentally investigated a novel kind of soliton molecule in a graphene-based mode-locking fiber laser with anomalous dispersion. The soliton molecule exhibits a stable rectangular profile on the oscilloscope, whereas it shows randomly distributed peaks in the autocorrelation trace, which indicates that the temporal separation of pulses in the molecule is varying all the time. The optical spectrum is modulated with a depth of about 7 dB over the whole profile, induced by the interaction of the intra-molecule solitons. The experimental results demonstrate that solitons in the pulse molecule oscillate randomly in temporal domain and the neighboring molecules are temporally separated by a fundamental cavity repetition rate, which is very different from that of the multi-solitons or bound-state solitons.