Fixed Pump Power Research Articles

Bandwidth modulation and pulse characterization of passively Q-switched erbium-doped fiber laser

We demonstrate the modulation of laser bandwidth by utilizing an ultranarrow tunable bandpass filter (UNTBF) in a passively Q-switched erbium-doped fiber laser. The passive Q-switch mechanism is enabled by using carbon nanotubes as saturable absorber at a Q-switched threshold of 35.5 mW. Based on spectral filtering effect introduced by the UNTBF, the 3 dB laser bandwidth can be tuned from 0.016 nm to 0.478 nm at a fixed pump power of 75.9 mW. The corresponding pulse behavior for each different bandwidth is characterized, and the results reveals that the pulse width can be as well tuned from 7.8 to 2.6 μs against the laser bandwidth, which agrees with the rule of time-bandwidth product. Correspondingly, the pulse repetition rate and the pulse energy vary from 16.23 kHz to 26.16 kHz and from 0.67 to 1.03 μJ respectively across the laser bandwidth. Further investigation of the pulse performance is performed against the pump power increment up to 107.2 mW. To the best of our knowledge, this is the first demonstration of spectrum bandwidth modulation in a passively Q-switched fiber laser, which can be useful for fully exploiting the possibilities of Q-switched pulse applications.

Watt-level 815 nm lasing from Tm3+-doped fluorotellurite glass fibers

Tm3+-doped fluorotellurite fibers based on TeO2–BaF2–Y2O3(TBY) glasses were fabricated by using a rod-in-tube method. By using an 81 cm-long Tm3+-doped fluorotellurite fiber as the gain medium and a 1400 / 1570 nm dual-wavelength pump technique, lasing at 815 nm was obtained for a threshold pump power of 629 mW at 1400 nm and a fixed pump power of 960 mW at 1570 nm. As the 1400 nm pump power is increased to 1803 mW, the obtained maximum output power was about 1616 mW. The corresponding optical-to-optical conversion efficiency was about 58.5%. Our results show that Tm3+-doped fluorotellurite fibers are promising gain media for constructing 815 nm fiber lasers.

Wavelength-tunable soliton molecule in spatiotemporal mode-locked Yb-doped fiber laser

Spatiotemporal mode-locking (STML) fiber lasers could realize the synchronized locked of multiple transverse and longitudinal modes, which could be regarded as an ideal platform to investigate high-dimensional nonlinear dynamics. The observation of STML in the soliton molecule state could offer a unique opportunity to reveal the mechanism of molecular complexity. Due to the distinctive spatiotemporal characteristics, realizing the wavelength tunable of soliton molecule in STML fiber laser is still challenging. In this letter, we report wavelength-tunable soliton molecules in an STML Yb-doped fiber laser. By rotating the wave plates at a fixed pump power, both single dissipative soliton (SDS) and various soliton molecules, including loose bounded dissipative soliton pairs (BDSPs), tight BDSPs, soliton molecules of triplet pulses or soliton molecules of quadruplets pulses, have all realized wavelength tunable with a tuning range of ∼25 nm. The realization of the wavelength tunable and the formation of the soliton molecule can all be attributed to the tunable comb filter and saturable absorber induced by the nonlinear polarization rotation technique. The formation of SDS and BDSPs was further confirmed by the numerical simulations. And the numerical results contribute to further comprehending the nonlinear dynamics of the STML fiber laser.

High-efficiency photon-number-resolving detector for improving heralded single-photon sources

Heralded single-photon sources (HSPS) intrinsically suffer from multiphoton emission, leading to a trade-off between the source’s single-photon quality and the heralding rate. A solution to this problem is to use photon-number-resolving (PNR) detectors to filter out the heralding events where more than one photon pair is created. Here, we demonstrate an improvement of a HSPS by heralding photons using a high-efficiency parallel superconducting nanowire single-photon detector (P-SNSPD) with PNR power. Specifically, we show a reduction in the of the heralded single photon by for a fixed pump power, or alternatively, an increase in the heralding rate by a factor of for a fixed . We also demonstrate that such a PNR device can reveal thermal photon-number statistics of unheralded photons, which is enabled by our ability to construct its full input–output response function. These results are possible thanks to our P-SNSPD architecture that ensures non-latching operation with no electrical crosstalk, which are essential conditions necessary to obtain the correct photon-number statistics and also faster recovery times, therefore enabling fast heralding rates. These results show that our efficient PNR P-SNSPD architecture can significantly improve the performance of HSPSs and can precisely characterize them, making these detectors a useful tool for a wide range of optical quantum information protocols.

Scaling Law of THz Yield from Two-Color Femtosecond Filament for Fixed Pump Power

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions.

A study of the sign reversal from a transmission peak to an absorption dip in a V-type system using the D1 and D2 lines of 87Rb in the presence of a buffer gas

In this work, we have investigated the transmission peak and absorption dip in the Doppler broadened V-type system using the D1 and D2 lines of 87Rb atoms in the presence of nitrogen buffer gas. The effects of the probe power variations at a fixed pump power on these resonances are examined both experimentally and theoretically. We have explored the fact that the transmission peak essentially turns into an absorption dip when the probe power is stronger than the pump power. Moreover, the theoretical model of a five-level V-type system is found to be sufficient to support the experimental outcomes when the effect of collision due to the presence of buffer gas is included in the theoretical model. We have also shown the variation of the resonance amplitude and population distribution in the upper level (|5〉) with increasing probe power.

Investigation of noise-like pulse evolution in normal dispersion fiber lasers mode-locked by nonlinear polarization rotation.

Transition from a gain-guided soliton (GGS) to a fully developed noise-like pulse (NLP) is numerically demonstrated in fiber lasers operated in the normal dispersion regime, which explains well the experimental observation of spectrum evolution that the bottom of the averaged spectrum gradually broadens with pump power increasing. Numerical results suggest that the transition could also happen under the condition of cavity linear phase delay bias change with fixed pump power. It is demonstrated that the peak power clamping effect and the normal dispersion are the key factors leading to the spectrum evolution. In addition, intermittent meta-stable states between GGS and NLP can be obtained when the cavity dispersion is chosen at small normal dispersion.

Vector Staircase Noise-Like Pulses in an Er/Yb-Codoped Fiber Laser

We report on the experimental observation of vector staircase noise-like pulses (NLPs) produced by an Er/Yb-codoped fiber laser with a nonlinear optical loop mirror (NOLM) as the saturable absorber (SA). The staircase NLPs consist of two rectangular pulses with different energies and durations, such that the two pulse widths both increase linearly with increasing pump power with an almost constant slope while the pulse amplitude remains constant, similar to the case with Dissipative Soliton Resonance pulses. The polarization rotation state of the staircase NLPs has also been investigated along with multiple polarization dynamic patterns, including group velocity locked and polarization locked states. We have also observed harmonic mode-locked (HML) staircase NLPs by carefully adjusting the orientation of the intra-cavity polarization at fixed pump power. It was found that the HML staircase NLPs were identical to the fundamental staircase NLPs as regards the vector character and the changes in pulse duration. The experimental results are expected to enrich understanding of the vector nature of staircase NLPs and may lead to a new laser source for material processing.

Generation of controllable bound solitons from a thulium–holmium co-doped mode-locked fiber laser

The characteristics of wavelength tuning and pulse separation changing of soliton pairs at 2 μm are still largely unexploited although they are helpful for the application of bound states in the field of high-capacity soliton fiber communication systems. We report on the experimental observations of stable bound solitons in a thulium–holmium co-doped passively mode-locked fiber laser by using the nonlinear polarization rotation technology. At a fixed pump power, the stable single soliton operation is converted to the soliton pair operation by cautiously adjusting the polarization controllers. Tightly bound soliton pairs with continuously tunable wavelengths from 1960.48 nm to 1989.45 nm, with a soliton separation of ∼4 ps, are observed. Meanwhile, several different pulse separations from 5.2 ps to 11.2 ps of soliton pairs are also realized. With the increasing pump power, a triple-soliton bound state is also observed. The results help in understanding the nonlinear dynamics of fiber lasers.

Dual-wavelength harmonic mode-locked dissipative soliton resonance of Yb fiber laser

Dissipative soliton resonance (DSR) mode-locked fiber lasers have stimulated great interest recent years due to their unique pulse-breaking-free advantage for scaling soliton pulse energy. However, the mechanism accounting for pulse breaking to generate multipulses in DSR fiber lasers has not reached a consensus. Here, we show that harmonic mode-locked DSR 1-μm Yb-doped fiber laser in the dual-wavelength regime can be realized with a semiconductor saturable absorber mirror (SESAM). In the fundamental frequency (3.5 MHz) mode-locked state, the pulse width continuously broadens from 2 to 8.9 ns as the output power increases from 1.26 to 5.1 mW (pulse energy being from 0.36 to 1.46 nJ), while the pulse peak power is clamped at 0.17 W, highlighting the typical characteristics of DSR mode-locking operation. Through simply adjusting the SESAM’s feedback conditions, the DSR fiber laser can transits from the fundamental-frequency mode locking to harmonic mode-locked states. Harmonic states from the 2nd to 9th order have been observed, with pulse repetition rate enhanced from 7 to 31.5 MHz. Under fixed pump power, increasing harmonic orders leads to reduction of the pulse width and pulse energy but does not change the unique quantization feature of the peak power. The status of the SESAM also guarantees the generation of dual wavelengths centered at 1072.5 and 1079.5 nm with spectral widths of 2.7 and 1.9 nm, respectively. This dual-wavelength harmonic mode-locked DSR Yb fiber laser can find applications in various areas.

High-order Laguerre-Gaussian mode laser generated based on spherical aberration cavity

The high-order Laguerre-Gaussian (LG) mode output from an end-pumped Nd:YVO<sub>4</sub> laser cavity with strong spherical aberration (SA) induced by short-focal-length lens is studied in this work. A long-focal-length lens L1 is used in the cavity to expand and collimate the beam, so that the beam incident on another short-focal-length lens L2 in the cavity undergoes a strong SA. Since the ring-shaped LG modes with different values of angular index <i>m</i> have different beam radii, the actual focal points of each order of beam are then spatially displaced. A flat output coupler (OC) is located near the focal point of L2, which is composed of a cat-eye retroreflector together with the lens. Such a retroreflector can provide only ideal retroreflection to the incident beam with a focal point exactly on the OC. Given the focal point displacements of the LG beams with different orders, such a mechanism can be used for implementing the transverse mode selection. The mode which has an actual focal point on the OC has a smaller loss than the other defocused modes. With an <i>a</i>-cut Nd:YVO<sub>4</sub> as laser crystal, scalar (linear-polarized) single-mode LG output with radical index <i>p</i> = 0 and angular index <i>m</i>>0 is obtained. The laser mode-order is selectable from LG<sub>0, ±10</sub> to LG<sub>0, ±33</sub> under 878.6-nm incident diode pump power of 1.03 W, by simply adjusting the distance between the OC and L2 in a range of 0.5 mm, when using lens L1 with <i>f</i> = 150 mm and lens L2 of <i>f</i> = 33.9 mm. It is found that sufficient SA which makes the optical paths of the neighboring modes well distinguishable is essential for single-mode operation of a wanted order of LG mode. However, too strong an SA can stop the high-order mode beam from oscillating, since the width and radius of the ring-shaped LG mode are an increasing function of indices <i>p</i> and <i>m</i>, which bring a stronger loss to the corresponding mode. Based on this analysis, we turn to a focal-length combination of <i>f</i><sub>1</sub> = 100 mm and <i>f</i><sub>2</sub> = 51.8 mm, to reduce the SA to a level suitable for further higher mode operation. A highest-order LG<sub>0, ±75</sub> is obtained by such an SA mode-selecting technique under fixed pump power of 1.03 W.

Spectral filtering effect-induced temporal rogue waves in a Tm-doped fiber laser.

We have experimentally and theoretically investigated optical rogue waves (ORWs) in a net negative dispersion Tm-doped fiber laser with a long cavity, adopting nonlinear polarization evolution as a mode-locker as well as a spectral filter. We obtained a state with numerous pulses bunched in a burst accompanied by perturbation within the burst, in which the spectrum was partially perturbed. After statistical analysis, we found that ORWs have existed in this bunching state. By adjusting the intra-cavity polarization controllers, the perturbed pulse bunching turned into a chaotic pulse bunching state, which gave rise to giant pulses with ultra-high amplitudes, and the giant pulses were a precursor of a broad-spectrum noise-like pulse. The probability of occurrence of ORWs was increased in the chaotic state, which is caused by multi-pulse instability induced by the spectral filtering effect. Simulation results confirm the experimental results and demonstrate that the spectral filter bandwidth (SFB) is directly related to the probability of the emergence of ORWs. When increasing the SFB across the range of multi-pulse instability at a fixed pump power, the frequency with which ORWs appear increases.

A Novel Repetition Rate Adjustable SBS-Based Q-Switched Fiber Laser

We experimentally demonstrated a novel repetition rate adjustable SBS-based Q-switched fiber laser. An inserted Fabry-Perot interferometer formed by two non-contact end faces of two FC/PC fiber connectors was inserted to help the system stabilize the random output of SBS-based Q-switching. By mechanically fine-tuning the length of the inserted Fabry-Perot interferometer, a linear and continuous repetition rate adjustment under a fixed pump power was achieved. This method paves a new way for the indirect control of repetition rate in SBS-based Q-switched fiber lasers. It is also found out that higher pump power allows wider adjustment range. At 600 mW pumping which is highest pump power of the source, the repetition rate almost tripled from 11.22 kHz to 32.06 kHz only by varying the length of FP cavity in less than 3 μm. In the meantime, the variation of pulse width, output power and single pulse energy were recorded and analyzed under different fixed pump powers. Although the adjusting range of repetition rate is not very wide but the tuning of repetition rate due to such a small length changing is unique and valuable. This original repetition rate control technique has great potential in areas like environment sensing and material detections.

Flow Resistance in Dredged Soil under a Vacuum Preloading System with Vertical Drainage Boards

Zhu, W.; Zhang, M.; Yu, G.; Petukhov, V.; Mymrin, V., and Gidarakos, E., 2020. Flow resistance in dredged soil under a vacuum preloading system with vertical drainage boards. Journal of Coastal Research, 36(2), 327–338. Coconut Creek (Florida), ISSN 0749-0208.Vacuum preloading combined with prefabricated vertical drains is one of the most widely applied methods to improve soft-ground loading capacity in coastal engineering projects. The resistance of solid particles to the pore-water flow in vacuum-preloaded soil plays an important role in the soil consolidation and pore-water dissipation. However, the flow resistance is still not clearly known during the vacuum drainage process. It is not clear whether the formulae for calculating the flow resistance factor for flow in uniform non-deformational porous media are applicable for flow in vacuum-preloaded soils or not. Because of a lack of knowledge on flow resistance in the vacuum-preloaded soil, coastal engineers only rely on their experience to select vacuum pumps. In this paper, the hydraulic resistance to flow in vacuum-preloaded soils was investigated. Energy conservation in the vacuum preloading system during the drainage process was analyzed. Sixteen laboratory tests were conducted, in which the vacuum pressure, flow rate, seepage velocity, and particle Reynolds number were observed or calculated. Results showed that particle size, uniformity coefficient, and porosity of the soil have important effects on resistance in dredged soils under vacuum preloading condition. The flow resistance in the vacuum-preloaded soil behaved quite differently from that in uniform porous medium. The formulae for calculating the flow resistance factor for flow in uniform packed beds were not applicable for flow in vacuum-preloaded soils. A new formula was proposed to calculate the flow resistance factor in vacuum-preloaded soils with acceptable accuracy. Because the vacuum preloading system with fixed pump power consumes much energy, new vacuum preloading systems with low energy consumption are necessarily developed in future work.

L-Band GHz Femtosecond Passively Harmonic Mode-Locked Er-Doped Fiber Laser Based on Nonlinear Polarization Rotation

Via using an L -band optimized in-fiber polarizing grating device, a GHz L -band femtosecond passively harmonic mode-locked Er-doped fiber laser based on nonlinear polarization rotation is first demonstrated. In total, 4.22 GHz pulses with the duration of 810 fs and super-mode suppression ratio (SMSR) of 32 dB are obtained under the pump power of 712 mW corresponding to 215th harmonic order. The central wavelength of 4.22 GHz pulses is 1581.7 nm with 10.1 nm 3 dB bandwidth. Furthermore, under this fixed pump power, higher harmonic orders can also be attained by rotating the polarization controllers properly. The highest repetition rate we obtained is 7.41 GHz with the SMSR of 20.7 dB.

Influence of Average Cavity Dispersion and Spectral Bandwidth on Passively Harmonic Mode Locked L-Band Er-Doped Fiber Laser

We systematically study the impact of average cavity dispersion and pulse spectral bandwidth on harmonic mode locking in the long-wavelength band from an Er-doped fiber laser using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film mode locker. By carrying out pulse energy management through optimizing the average cavity dispersion and optical spectral bandwidth, 2.08 GHz repetition rate corresponding to 201st harmonic with 36.5 dB side mode suppression ratio (SMSR) can be realized under 205 mW pump power at 1597.53 nm. Furthermore, we systematically investigate the characteristics of the harmonically mode locked ultrashort optical pulses with different spectral bandwidths versus pump power under fixed cavity dispersion first. Key laser parameters have been studied including pumping efficiency, duration, energy, SMSR, and time bandwidth product. Enlightened by the experimental results, a simple law on preliminary and fast judgment of pulses frequency under fixed pump power is proposed, namely, by monitoring the pulse spectral bandwidth. The laser source featuring high repetition rate and L-band operation is greatly sought-after in some specific applications such as modern optical communication system etc.

Tunable and switchable harmonic h-shaped pulse generation in a 303 km ultralong mode-locked thulium-doped fiber laser

We experimentally demonstrated a type of tunable and switchable harmonic h-shaped pulse generation in a thulium-doped fiber (TDF) laser passively mode locked by using an ultralong nonlinear optical loop mirror. The total cavity length was ∼3.03 km, the longest ever built for a TDF laser to our best knowledge, which resulted in an ultralarge anomalous dispersion over −200 ps2 around the emission wavelength. The produced h-shaped pulse can operate either in a fundamental or in a high-order harmonic mode-locking (HML) state depending on pump power and intra-cavity polarization state (PS). The pulse duration, no matter of the operation state, was tunable with pump power. However, pulse breaking and self-organizing occurred, resulting in high-order HML, when the pump power increased above a threshold. At a fixed pump power, the order of HML was switchable from one to another by manipulating the PS. Switching from the 8th up to the 48th order of HML was achieved with a fixed pump power of ∼4.15 W. Our results revealed the detailed evolution and switching characteristics of the HML and individual pulse envelope with respect to both the pump power and PS. We have also discussed in detail the mechanisms of both the h-shaped pulse generation and the switching of its HML. This contribution would be helpful for further in-depth study on the underlying dynamics of long-duration particular-envelope pulses with ultralarge anomalous dispersion and ultralong roundtrip time.

Mega-Hertz repetition rate broadband nano-second pulses from an actively mode-locked Yb-fiber laser

This work demonstrates an actively mode-locked all-normal dispersion Yb-fiber ring laser, delivering pulses of few nano-seconds (ns) to sub-ns duration at a 4 MHz repetition rate. Employing a filterless cavity architecture and different, long-length intra-cavity nonlinear fibers; a broadband output spectrum in the 1060–1140 nm wavelength region was obtained. With an increasing pump power, the pulse width reduced along with the appearence of new pulses, resulting into a muti-pulsing output. For a fixed pump power, the number of pulses and the amount of spectral broadening varied for different non-linear fibers. In the multi-pulsing state, the pulses were found to be located seperately in both time and wavelength, where Raman stokes corresponding to individual pulses could also be observed. The maximum output pulse energy achieved in a single pulse state was around 8.7 nJ, which increased to 14.5 nJ in a multi-pulse state. To the best of the authors’ knowledge, this is the first demonstration of a filterless active mode-locked fiber ring laser producing high-repetition rate broadband nano-second pulses in single and multi-pulse states at 1 µm.

2.07-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m, 10-GHz Repetition Rate, Multi-Wavelength Actively Mode-Locked Fiber Laser

We propose and demonstrate a multi-wavelength actively mode-locked fiber laser around $2.07~\mu \text{m}$ . A holmium-doped fiber is used as the gain medium which is pumped by a 1.93- $\mu \text{m}$ fiber laser. The multi-wavelength laser is achieved by employing a fiber Lyot filter in cavity, and actively mode-locking is achieved by driving a sinusoidal radio frequency signal on the electro-optic modulator. Stable actively mode-locked pulses can be obtained with up to 10.078-GHz repetition rate. The shortest measured pulse width is 16 ps. The wavelength channels can be accurately tuned under a fixed pump power by adjusting the polarization controllers and the BIAS voltage. Up to 27 channels can be obtained under a stable mode-locked operation. However, the increase of output channels leads to a significant increase in the threshold, and the maximum output channels have a strong dependence on the repetition rate.

Facile active control of a pulsed erbium-doped fiber laser using modulation depth tunable carbon nanotubes

Short pulsed fiber lasers have been widely made using single-walled carbon nanotubes as a saturable absorber (SA). However, most of the currently used devices can only operate in one determined operation state with an unchangeable modulation SA depth in the cavity, which significantly limits their application in photonic devices. In this paper, well-aligned carbon nanotube arrays are synthesized using zeolite AlPO4-5 as a template, which features anisotropic optical absorption. The linear optical absorption of the as-synthesized carbon nanotube arrays can easily be tuned by adjusting a polarization controller, thus providing a tunable modulation depth for the carbon nanotube SA. By exploiting this SA in an erbium-doped fiber laser cavity, both Q-switched and mode-locked pulsed lasers are achieved by simply adjusting a polarization controller under a fixed pump power of 330 mW. In addition, the repetition rate of the Q-switching and pulse duration of the mode-locking can be tuned according to the variation of modulation depth. Moreover, soliton molecules can be obtained when the modulation depth of the SA is 4.5%.