Increasing Pump Power Research Articles

MoS2 Q-switched 2.8 µm Er:ZBLAN fiber laser

Since the invention of graphene, two-dimensional (2D) materials have attracted great interest in both fundamental science and potential applications. Here, we report the saturable absorption features of 2D (multilayered structure) molybdenum disulfide (MoS2) in the mid-infrared (mid-IR) wavelength region, and use it to passively Q-switch an Er3+-doped ZBLAN fiber laser at 2.8 µm. This MoS2 saturable absorber has a moderate modulation depth of 5% at 2.8 µm, making it an appropriate candidate for light modulating in the mid-IR spectral range. In stable Q-switching, the Er3+-doped ZBLAN fiber laser is widely tuned in both repetition rate and pulse width. Through increasing pump power, the repetition rate and pulse duration are monotonically tuned from 36 to 70 kHz and from 1.84 µs to 806 ns, respectively. The maximum average power is 140 mW (at 70 kHz), giving a pulse energy of 2 µJ and peak power of ~2.48 W. Successful Q-switching operation of this 2.8 µm fiber laser confirms that 2D MoS2 has great potentials as efficient light modulating elements in mid-IR regions.

Tm3+-Doped Harmonic Dissipative Soliton Mode-Locked Fiber Laser at 1.93 $\mu$ m Based on Tungsten Disulfide in Anomalous Dispersion Regime

We report a harmonic dissipative soliton $1.93~\mu \text{m}$ thulium fiber laser based on a tungsten disulfide (WS2) saturable absorber (SA) in the anomalous dispersion regime. The multilayer WS2 nanosheets were prepared by liquid phase exfoliation method and the SA was fabricated by dropping WS2 solution onto a gold mirror. The transferred WS2 SA had a modulation depth of 2.5% and a saturation intensity of 0.82 MW/cm2. By incorporating the SA into a linear Tm3+ fiber laser cavity, harmonic mode-locked dissipative soliton laser was achieved at 1930 nm with the spectral width of 8 nm, the pulse energy of 3 nJ, the pulse width of 3.6 ns, and the repetition rate of 56.3 MHz. Based on the experimental results, it is shown that with the presence of harmonic mode-locking in $2~\mu \text{m}$ wavelength region, the multilayer WS2 serving as a SA was verified to be a good candidate for broadband high-energy mode-locking. The order of the harmonic dissipative soliton mode-locked pulses remains the same along with the increasing pump power.

Nickel oxide nanoparticles for Q-switching pulses generation

We report an observation of Q-switching pulses train in passively Q-switched Erbium-doped fiber laser (EDFL) employing Nickel oxide (NiO) film as a passive saturable absorber (SA). The NiO was prepared by the facile sonochemical method. It is embedded into Poly(ethylene oxide) (PEO) film, which is then sandwiched between two fiber ferrule connectors to form a fiber-compatible NiO SA. The Q-switched laser generates a stable pulses train operating at 1558 nm as the pump power was increased above the threshold pump power of 144 mW. The laser has a pulse repetition rate tunable from 50.3 to 66.0 kHz with the increasing pump power from 144 mW to 240 mW. The maximum pulse energy and minimum pulse width of 5.6 µs were observed at pump power of 240 mW. The SNR of the fundamental frequency was obtained at 67 dB, indicating the stability of the laser. These performances indicates a good prospect of NiO as an excellent SA for operation in 1550 nm region.

结构紧凑的双波长连续波掺铒光纤激光器

Multiwavelength erbium-doped fiber lasers can be applied to many fields, such as wavelength division multiplexing optical communication and others attracting considerable attention. To meet the requirements of different applications, we propose a compact dual-wavelength continuous-wave erbium-doped fiber laser based on nonlinear amplifying loop mirror. An all-polarization-maintaining fiber cavity is adopted in which there are only three intracavity devices besides fiber itself:a wavelength division multiplexer, a 2×2 fiber coupler and a fiber reflector. The intensity-dependent loss effect induced by the nonlinear amplifying loop mirror is used to equalize the intensity in the cavity. The input laser with higher power suffers a higher loss than the one with lower power. This feature can be used to suppress mode competition and achieve stable multiwavelength oscillation. With a pump power of 260 mW, dual-wavelength erbium-doped fiber laser can be achieved, with wavelengths of 1 560.5 nm and 1 563.2 nm, respectively. The side-mode suppression ratio is 46.8 dB. As pump power increases, the laser can operate in single-, dual-and triple-wavelength regions in proper order. As the multi-wavelength lasing oscillation is a balance between intensity-dependent loss and mode competition, the intensity change breaks the original balance state, resulting in the change in number and wavelength of lasing lines. This laser is simply structured and easy to operate. It can have applications in many fields.

Experimental study of supercontinuum generation in Yb<sup>3+</sup>-doped microstructure fiber pumped by femtosecond pulses

The nonlinear effects and supercontinuum generation by the concept of wavelength conversion and amplification are experimentally studied in two Yb<sup>3+</sup>-doped microstructure fibers (Yb<sup>3+</sup>-MSFs), with the Ti: sapphire femtosecond pulses used as pump. Firstly, two Yb<sup>3+</sup>-MSFs are pumped by continuous wave separately to obtain the emission spectrum. The relationship between the luminous efficiency and the deviation of pump light from the Yb<sup>3+</sup> absorption peak is studied for each of the two fibers. The experimental results indicate that the luminous efficiency decreases as the deviation increases. However, both fibers still have high luminous efficiency even when the deviation reaches to 85 nm. Secondly, the supercontinuum spectrum is generated by the femtosecond laser pumping the cores of the two fibers. The influence of the pump power, relative position between emission light and zero-dispersion wavelength <i>λ</i><sub>0</sub>, pump wavelength and fiber length on the supercontinuum generation are studied. The results demonstrate that the amplified emission light at 1035 nm is first captured by the pump light to evolve into ultrashort pulse, and nonlinear effects are subsequently generated. As the pump power increases, for Yb<sup>3+</sup>-MSF1 whose <i>λ</i><sub>0</sub> is located near the emission light of Yb<sup>3+</sup> irons, the fundamental soliton is generated and further shifts toward red region under Raman effect. Compared with Yb<sup>3+</sup>-MSF1, the Yb<sup>3+</sup>-MSF2 has a small core, which means that its <i>λ</i><sub>0</sub> is short and the emission light is located in its anomalous dispersion region far from the <i>λ</i><sub>0</sub>. Experimental results reveal that higher-order soliton and soliton fission are more likely to happen and supercontinuum spectrum can be formed. However, the further broadening of the supercontinuum spectrum is limited by OH- absorption at 1380 nm. Either increasing the deviation of pump light from the Yb<sup>3+</sup> absorption peak or shortening the fiber length reduces the accumulated power of the emission light, so the experimental results show that red-shift of Raman soliton is reduced and the supercontinuum spectrum is narrowed for both fibers. The supercontinuum generation efficiency in the output spectrum can reach 98% when the effect of pump light coupling efficiency and microstructure fiber loss are neglected. It means that almost all the residual pump light and emission light of Yb<sup>3+</sup> contribute to the generation of supercontinuum. Finally, the Yb<sup>3+</sup>-MSF2s are tapered to different taper lengths to study their influence on supercontinuum generation. The results indicate that the leakage after tapering weakens the energy of the Raman soliton, which further results in the decrease of red-shift. Eventually, the red edge of supercontinuum spectrum shrinks seriously with theincrease of the taper length. However, the decreasing of <i>λ</i><sub>0</sub> at the taper waist leads to blue-shift of dispersive wave that satisfies the phase matching condition with Raman soliton. This contributes to the blue-shift of the short wavelength boundary and widens the range of supercontinuum spectrum at short wavelength. Therefore, tapering is a promising method of expanding supercontinuum spectrum towards short wavelength. In conclusion, the supercontinuum spectrum is generated in Yb<sup>3+</sup>-doped microstructure fiber pumped by the Ti: sapphire femtosecond laser. The output spectrum can be adjusted flexibly by combining the merit of high peak power and wavelength tunability of Ti: sapphire femtosecond laser and the characteristics of wavelength conversion and amplification of Yb<sup>3+</sup> irons. Thus, the method presented in the paper provides a promising way to obtain tunable supercontinuum spectrum.

Alternated Q-switched and gain-switched dual-pulse Yb fiber laser with Tm-Ho co-doped fiber as saturable absorber

We demonstrate a dual-wavelength and dual-pulse Yb-doped fiber laser with a Tm-Ho co-doped fiber as saturable absorber (SA). The wavelength of the first pulse in the dual-pulsing output is at 1064 nm generated via SA-induced self Q-switching and the second pulse is at 2127.7 nm and generated by gain-switching with the laser at 1064 nm as the pump for the Tm ions in the Tm-Ho co-doped fiber and thus induced stimulated emission. With the increases of the pump power, the pulse duration of the two pulses is decreased depending on the level of the pump. The pulse duration of the laser at 1064 nm decreases from 1.4 μs to 296 ns when the pump power increases from 275 mW to 475 mW, while the pulse duration at 2127.7 nm is reduced from 440 ns to 246 ns when the pump increases from 300 mW to 475 mW. Accordingly, the repetition rate is decreased from 17.4 kHz to 6.8 kHz. The experimental result shows that Tm-Ho co-doped fiber is an excellent candidate for SA-induced Q-switching at 1064 nm.

Stimulated Brillouin scattering induced all-optical modulation in graphene microfiber

Graphene microfibers are burgeoning modulators with great potential in all-optical communication. One of the critical issues that remains to be understood is the dynamic mechanism of light–graphene interaction. Here, we propose a power dependent modulation by using 980 nm pump light and 1064 nm signal light via graphene microfiber, and the results show a strong transmission reduction and frequency blue shift with the increase of pump power. The experimental observation is attributed to a stimulated Brillouin scattering process induced by the pump light. Power and frequency variations are a result of energy transition of the scattered phonon in the fiber. This work reveals the nonlinear effect process in the light–graphene interaction and provides a new method for power and frequency control with graphene all-optical modulation.

Dual-band Tm3+-doped tellurite fiber amplifier and laser at 1.9\u2009\u03bcm and 2.3\u2009\u03bcm

Ultrabroadband amplification and two-color CW lasing simultaneously near 1.9 μm and 2.3 μm in a Tm3+-doped tellurite fiber were demonstrated experimentally, for the first time to the best of our knowledge. A low-loss Tm3+-doped core fiber from TeO2–ZnO–La2O3–Na2O glasses stable against crystallization was produced by a special technique, providing a low concentration of hydroxyl groups. Supercontinuum from a highly GeO2 doped silica fiber pumped by an Er fiber laser system was used as a seed for an amplifier. A maximum gain of 30 dB and 7 dB was measured at 1.9 μm and 2.3 μm, respectively. We report detailed experimental and theoretical studies, which are in a very good agreement, of laser amplification and generation in the manufactured fiber with carefully measured and calculated parameters. A quantitatively verified numerical model was used to predict power scalability at 2.3 μm in schemes with optimized parameters at increased pump power. The presented results show that a high-quality tellurite fiber is a promising candidate for developing lasers in the 2.3 μm atmospheric window which are particularly relevant for applications in gas sensing, eye-safe laser radars, breath analysis, remote sensing and stand-off trace gas detection.

Wavelength-adjustable mode-locked Tm-Ho co-doped fiber laser from 1839 nm to 1876 nm

Abstract We demonstrate a passively mode-locked Tm-Ho co-doped fiber laser using GaAs saturable absorber (SA). The laser has a very broad output spectrum covering from 1700 nm to 2100 nm with output wavelengths adjustable from 1839 nm to 1876 nm. The significant broadening produced by mode-locking is due to the laser-induced self-phase modulation (SPM). Results show that the peak wavelength is red-shifted with the increase of pump power. On the other hand, with the increase of the cavity length, the peak wavelength is blue-shifted. The red-shift generated by the increasing pump power may be due to the reabsorption of the short wavelength part of the laser by Ho3+ ions, while the increase in the blue shift due to the increasing cavity length is due to the absorption loss in the long wavelength direction inside the optical fiber at the broad-band laser since the long wave part of the output has already entered the absorption band of the fiber material. As the length of the fiber increases, the longer wavelength side of the laser output spectrum is absorbed more, causing a blue-shift of the peak wavelength. Therefore, the laser peak wavelength can be adjusted by changing the pump power or cavity length.

Effects of a seed pulse on rogue-wave formation for midinfrared supercontinuum generation in chalcogenide photonic crystal fibers

We numerically study the characteristics of optical rogue waves (RWs) in midinfrared supercontinuum (MIR-SC) generation with a femtosecond pump in the anomalous dispersion regime of a chalcogenide photonic crystal fiber. An approach based on seed-induced modulation instability is used to regulate the formation of optical RWs. By carrying out multiple simulations in the presence of noise, we show that optical RWs can be generated in a more controlled manner by providing a seed with an optimum modulation frequency. The redshift range of optical RWs is expanded to a considerable extent in the seed-induced case rather than the noise-induced case. Furthermore, the increase of pump power can facilitate the redshift of optical RWs. These results demonstrate how a seed pulse affects the formation of optical RW for MIR-SC generation with a femtosecond pump in the anomalous dispersion regime of fiber, indicating that optical RW can be used to generate ultra-broadband MIR-SC.

Engineering the color output in a single upconversion microtube via intervention of the electronic population

The engineering of upconversion (UC) light color output is of great importance due to its promising application in biological imaging, data storage and solar cells. In this work, we showed that a tunable red-to-green emission ratio can be achieved by rationally designing the depopulation mechanism of intermediate state via increasing pump power density, pulse duration, pulse interval and Yb3+ concentration accompanied with the waveguide effect in a single Yb3+/Er3+ codoped NaYF4 microtube. We revealed that the red UC luminescence can be enhanced by three-photon process. As expected, three-photon and two-photon processes could be effectively switched by controlling pulse duration and power density. Compared with varying the power density, elevating the Yb concentration accompanied with the waveguide effect is more effective in modifying the color output. Our results provide insight into underlying physical mechanisms of intervening electronic population and present a general approach to constructing a new class of luminescent materials with tunable UC emissions.

Surface gap solitons in exciton polariton condensates

A gap soliton is a solitonic state existing inside the band gap of an infinite-periodic exciton-polariton condensate (EPC). The combination of surface states and gap solitons forms the so named surface gap solitons (SGSs). We analyze the existence of SGSs near the interface between uniform and semi-infinite periodic EPCs. We find that SGSs exist only when the system is excited by a pump with low power and small width. As the pump power or width increases, SGSs become unstable.

Intensity and Wavelength Division Multiplexing FBG Sensor System Using a Raman Amplifier and Extreme Learning Machine

A fiber Bragg grating (FBG) sensor is a favorable sensor in measuring strain, pressure, vibration, and temperature in different applications, such as in smart structures, wind turbines, aerospace, industry, military, medical centers, and civil engineering. FBG sensors have the following advantages: immune to electromagnetic interference, light weight, small size, flexible, stretchable, highly accurate, longer stability, and capable in measuring ultra-high-speed events. In this paper, we propose and demonstrate an intensity and wavelength division multiplexing (IWDM) FBG sensor system using a Raman amplifier and extreme learning machine (ELM). We use an IWDM technique to increase the number of FBG sensors. As the number of FBG sensors increases and the spectra of two or more FBGs are overlapped, a conventional peak detection (CPD) method is unappropriate to detect the central Bragg wavelength of each FBG sensor. To solve this problem, we use ELM techniques. An ELM is used to accurately detect the central Bragg wavelength of each FBG sensor even when the spectra of FBGs are partially or fully overlapped. Moreover, a Raman amplifier is added to a fiber span to generate a gain medium within the transmission fiber, which amplifies the signal and compensates for the signal losses. The transmission distance and the sensing signal quality increase when the Raman pump power increases. The experimental results revealed that a Raman amplifier compensates for the signal losses and provides a stable sensing output even beyond a 45 km transmission distance. We achieve a remote sensing of strain measurement using a 45 km single-mode fiber (SMF). Furthermore, the well-trained ELM wavelength detection methods accurately detect the central Bragg wavelengths of FBG sensors when the two FBG spectra are fully overlapped.

Eye-Safe Picosecond Solid-State Nd:YAG Ceramic Laser at 1.44 <italic>μ</italic>m

We report on a passively mode-locked Nd:YAG transparent ceramic laser with semiconductor saturable absorber mirror at an eye-safe wavelength of 1.44 μ m. With the increase of incident pump power, the laser went through three oscillation stages: Q-switched mode locking (QML), continuous wave mode locking (CWML), QML and remained at 1443 nm single wavelength output. The maximum average output power of the CWML laser was about 842 mW. The pulse width was measured to be 20.8 ps assuming a Gauss shape.

Monolayer MoS2-based high energy rectangular pulse fiber laser

We have reported the generation of wave-breaking-free pulses operated in dissipative soliton resonance (DSR) of an Yb-doped fiber laser (YDFL) with monolayer molybdenum disulfide. The experiment demonstrates that the energy of single pulse operated in DSR regime (10.28 nJ) is much higher than that of dissipative soliton pulse (0.17 nJ) generated in the same YDFL. When the cavity length is 201 m, as the pump power increases, the duration of the wave-breaking-free pulse broadens from 1.9 to 10.56 ns and pulse energy rises from 51.81 to 208.89 nJ. The highest pulse energy of ∼496 nJ is obtained, when the cavity length and pump power are increased to 495 m and 450 mW, respectively. The experimental results suggest that MoS2 based YDFL can achieve wave-breaking-free, ultrahigh-energy pulse.

1.7-$\mu$m Tm/Ho-Codoped All-Fiber Pulsed Laser Based on Intermode-Beating Modulation Technique

A 1.7- $\mu$ m pulsed laser is of tremendous interest for biomedical imaging applications due to its low scattering loss in bio-tissues, but is relatively difficult to generate in a fiber laser. In this paper, we propose and demonstrate a compact 1.7- $\mu$ m Tm/Ho-codoped fiber pulsed laser based on a novel intermode-beating modulation (IM) technique. A 1211-nm continuous-wave (CW) Raman fiber laser is especially homemade to pump a $\sim$ 20-cm short-length Tm/Ho fiber for obtaining $\sim$ 1.7- $\mu$ m optical gain. A pair of fiber end-facet mirrors with 1.7–1.8- $\mu$ m reflective band is used to form the all-fiber linear cavity. 1.76- or 1.78- $\mu$ m lasing is successfully achieved, and surprisingly exhibits stable self-gain-switching operation with $\sim$ 1- $\mu$ s pulse duration. Different from the conventional gain-switching requiring pulsed pumping, herein the gain modulation in the 1.7- $\mu$ m laser is induced by strong intermode-beating of the CW pump source. The pulse repetition rate with an increasing pump power is fixed at 145 kHz, which exactly coincides with the longitudinal-mode beat frequency of the 1211-nm pump laser. Our results may provide a new type of optical modulation (i.e., intermode-beating modulation technique) for a low-cost, compact pulsed laser.

Experimental evaluation of temperature increase and associated detection sensitivity in shot noise-limited photothermal microscopy

Abstract Photothermal microscopy is useful for visualizing non-fluorescent chromophores in a light-scattering medium. However, it remains a major challenge to enhance sensitivity while reducing thermal damage to samples, particularly for biological tissues. In this study, a highly sensitive laser scanning photothermal microscope was constructed by implementing a low-noise balanced photodetector into a spatially segmented balanced detection which was proposed and demonstrated to be useful for high sensitivity detection in our previous studies. We confirmed that the noise level was only 1.05-fold greater than the shot noise. Temperature increase and the detection sensitivity for this parameter were evaluated by observing the motion of individual 20-nm-sized gold nanoparticles embedded in agarose gel in response to incident light at varied intensities. We found that the nanoparticles remained trapped in the gel structure at a low pump power and it exhibits random motion as increase in the pump power because local temperature of agarose gel around the nanoparticle reached the melting point of 338 K. The peak SNR is 290 at about the melting point with an integration time of 10 μ s per pixel. On an assumption that the SNR is proportional to the temperature increase, the estimated limit of detection of a temperature increase at the nanoparticle surface, which is defined as the ratio of temperature increase to the SNR, ranges from 0.1 to 0.2 K. Additionally, findings from single carbon nanotube imaging suggest that a larger or longer light absorber would improve the trade-off relationship between the signal-to-noise ratio and temperature increase.

Numerical simulation of Al2O3 – water nanofluid effects on the performance of a cross flow micro heat exchanger

The numerical model investigated in this paper was implemented with Finite Element Method (FEM) based Comsol Multiphysics software, starting from a real cross-flow micro heat exchanger manufactured from 50 stainless steel plates for cold and hot fluid transfer. Each plate (foil) had 34 microchannels with rectangular cross-section, at a depth of 100 μm and a width of 200 μm. The working fluids were water and Al2O3-water nanofluids with volumetric concentrations of 0.5%, 1% and 1.5% in the temperature domain of 293 – 333 K. The oxide-based nanofluids are widely investigated in thermal energy transfer applications due to an increased heat transfer performance by comparing with pure water as a cooling fluid. Analytical models for the physical properties of water and nanofluid, referring to density, viscosity, thermal conductivity and specific heat capacity were used here. Microchannel bulk temperature profiles for cold and hot plates of the model with 1% Al2O3-water nanofluid were presented here in order to examine the uniformity of temperature distribution in the channel flow field area. Simulation results indicated that the total heat flow rate in the heat exchanger models increased with the increase in volume concentration of the nanoparticles at mass flow rates of 41, 61, 81, 101 and 150 Kg/h, but with the expense of a bigger pressure drop (or Fanning friction factor) at an increased pumping power required to drive flow through the channels.

Pump-gain integrated functional laser fiber towards 10 kW-level high-power applications

To incorporate sufficient pump energy from multi-hundred watt-level 976 nm commercial laser diodes, a new kind of (8 + 1)-type pump-gain integrated functional laser ((8 + 1)-PIFL) fiber was fabricated by our newly optimized special fiber assembly and coating techniques for the first time. This (8 + 1)-PIFL fiber allowed for 10.66 kW aggregated pump power at 976 nm from six counter-pump ports in a master oscillator power amplifier laser setup and stably presented 8.72 kW laser output at 1079.4 nm with a high optical-to-optical efficiency of 81.0%. This is the highest power of the fiber amplifiers pumped by commercial laser diodes that has been reported so far, and only limited by the available pump power. Up to this power level, pure output spectrum with narrow 3 dB bandwidth Δλ3 dB of 2.76 nm and without any sign of stimulated Raman scattering and amplified spontaneous emission was obtained. Neither roll over of output power nor obvious thermal issue was observed with increased pump power, indirectly justifying strong photodarkening and transverse mode instability suppression. With another ten pump ports to collect more pump power, a further power scaling towards 10 kW or above can be expected.

Convective heat transfer enhancement using Ag-water nanofluid in a micro-fin tube combined with non-uniform twisted tape

This article presents an experimental study of convective heat transfer enhancement using combined techniques. One of these techniques was the use of twisted tape generating swirls along the whole tube length inserted in a micro-fin tube that effectively combined the features of extended surfaces, turbulators and artificial roughness. Nanofluids were used for improving the thermo-physical properties of the fluid. In this study, fluid flow of an Ag-water nanofluid in a micro-fin tube into which non-uniform twisted tapes were inserted was examined under turbulent flow. The effects of the twist ratios of non-uniform twisted-tapes (y/W in three series, 3.0 > 2.8 > 2.6, 3.0 > 2.6 > 2.2 and 3.0 > 2.4 > 1.8), tube and tape arrangements (counter and co-current arrangements), nanofluid concentrations (ϕ of 0.007, 0.016 and 0.03% by volume) were determined. The Nusselt numbers, friction factors and the thermal performance parameters of these devices were obtained and compared with the use of micro-fin tubes alone with a base fluid (water). Utilization of micro-fin tubes into which twisted tapes were inserted and through which nanofluids were allowed to flow tended to increase heat transfer rates over that of the base fluid. The experimental results revealed that heat transfer, friction loss and thermal performance factors increased with decreasing twist ratios for non-uniform twisted tapes and increasing nanofluid concentrations. Micro-fin tubes into which non-uniform twisted tapes were inserted in a counter-current arrangement gave higher heat transfer enhancement than the ones in a co-current arrangement. The optimum condition was found using the micro-fin tube with a non-uniform twisted-tape in a counter-current-arrangement with twist ratios in a series of 3.0 > 2.4 > 1.8 with a Ag-water nanofluid at a concentration of 0.3% by volume up to 112.5% for the heat transfer rate and 1.62 for thermal performance. Furthermore, the dependence of twist ratios, tape arrangement, nanofluid concentration and Reynolds number for enhancement of thermal performance and increased pump power due to higher pressure losses were analysed over the range of parameters considered.