Threshold Pump Research Articles

Spectroscopic investigations of Tm3+ doped CdF2 single crystals and infrared laser potentialities

By use of standard Bridgman technique, CdF2 single crystals doped with Tm3+ have been obtained with good optical quality. XRD analysis has been performed confirming the cubic structure. An ordered sequence of spectroscopic measurements (including absorption, excitation, emission and fluorescence dynamics) was carried out at room temperature. By exploiting both UV–Visible and near-infrared absorption spectra, the JO phenomenological parameters are determined by taking into account the complete refractive index dispersion, which allowed us to compute the probabilities of all transitions. Under 355 nm wavelength excitation, the Stokes emission spectrum exhibits mainly an intense blue emission line around 450 nm which is associated to 1D2→3F4 excited state transition. Whereas, the anti-Stokes blue emission around 480 nm associated to 1G4→3H6 transition is obtained under infrared excitation according to a two photon absorption mechanism. For both blue emissions, the CIE chromaticity coordinates of CdF2:Tm3+ are located in the blue region.As trivalent Tm3+ ions are known for their emissions in the near- and mid-infrared spectral ranges suitable for efficient laser operation, we have systematically studied the fluorescence behavior of the eye safe emission (pointed at ∼1.8 μm) and associated to the 3F4 → 3H6 transition. Such transition exhibit a relatively high emission cross-section (σem = 0.22 × 10−20 cm2) and a positive gain cross-section at low pumping threshold with a metastable fluorescence lifetime decay.

Investigating the Impact of SMF on Brillouin Fiber Laser Performance

Using erbium-doped fiber, a steady Q-switched erbium-doped laser (EDFL) was successfully demonstrated. With a 150 m of Single Mode Fiber to generate the Brillouin Fiber Laser and increase the non-linearity inside the cavity. In the absence of a Brillouin pump, a compact Brillouin fiber laser (BFL) is realized by combining an Erbium Doped Fiber (EDF) and a Single Mode Fiber (SMF), which serve as both the Brillouin and linear gain media. It is accomplished by self-Brillouin production in a laser cavity from 4.5 m of EDF and 150 m of SMF with strong nonlinearity. The SMF inside the ring cavity was used to generate self-generating BFL. The BFL in the linear cavity produced more Brillouin stokes at a lower pump threshold. In the experiments, the average of spacing is 0.088 nm was generated at the EDF pump power of 93.468 to 259.493 mW for the 150 m of SMF. This compact BFL has total cavity length (160.34 m). Eventually, this is the most recent gain medium length recorded for Brillouin erbium fiber laser production without the use of a Brillouin pump.

Quantum Dynamical Approach to Predicting the Optical Pumping Threshold for Lasing in Organic Materials.

The quantum dynamic (QD) study of organic lasing (OL) is a challenging issue in organic optoelectronics. Previously, the phenomenological method has achieved success in describing experimental observation. However, it cannot directly bridge the laser threshold (LT) with microscopic parameters, which is the advantage of the QD method. In this paper, we propose a microscopic OL model and apply time-dependent wave packet diffusion to reveal the microscopic QD process of optically pumped lasing. LT is obtained from the onset of output as a function of optical input pumping. We predict that the LT has an optimal value as a function of the cavity volume and depends linearly on the intracavity photon leakage rate. The calculated structure-property relationships between molecular parameters and the LT are in qualitative agreement with the experimental results, confirming the reliability of our approach. This work is beneficial for understanding the OL mechanism and optimizing the design of organic laser materials.

Silver indium selenide composite as a saturable absorber for passive Q-switched and mode-locked pulsed generation in erbium-doped fiber laser

We have successfully accomplished a Q-switched and mode-locked pulse in an Erbium-doped fiber (EDF) laser cavity by integrating a new composite material silver doped Indium Selenide (Ag/In2Se3)-based saturable absorber (SA). Ag/In2Se3 film was synthesized by using pulsed laser ablation and a common dry cast method to form a free-standing thin film SA. This SA device exhibits a nonlinear optical property of modulation depth, saturable intensity, and non-saturable loss of 2.27%, 847.20 MW/cm2, and 16.82% respectively. By implementing Ag/In2Se3 film into the EDF laser cavity, a Q-switched pulsed began to generate at a threshold pump power of 27.63 mW. Then as 100 m single-mode fiber (SMF) was implemented into the EDF laser cavity, a traditional soliton pulse began to appear with 1.873 MHz, at the threshold pump power of 76.63 mW with a narrow pulse width of 3.11 ps, and remarkable temporal stability, as demonstrated by a signal noise ratio (SNR) of 79.62 dB. The peak power of 2.61 kW was recorded. This contribution further verifies the improved optical qualities of the Ag/In2Se3 composite and offers a new possibility for developing broadband SA devices for various photonics applications.

Effect of Ca2+ ions on structure and spectroscopic properties of Er3+: KBaY(MoO4)3 crystals

This study successfully grew KBaY(MoO4)3 crystals doped with single Er3+ ions and co-doped with Ca2+/Er3+ ions using the top-seeded solution growth method. Thermal analysis of the KBaY(MoO4)3 crystals was performed by differential scanning calorimetry, and their phase and structure were characterized using X-ray diffraction and Fourier transform infrared spectroscopy. The luminescent properties of the crystals in the visible and near-infrared wavelength ranges under different concentrations of Er3+ ions were analyzed through emission spectroscopy, and the concentration quenching mechanism was investigated in depth. The results showed that the strongest emission intensity was obtained at an Er3+ ion concentration of 5 mol%. Additionally, by substituting Ba2+ ions with Ca2+ ions, the emission intensity was significantly enhanced, especially when the Ca2+ ion doping concentration reached 6 mol%, resulting in the maximum emission intensity. Furthermore, we investigated the effects of matrix structure and lattice distortion on the spectral properties by employing Judd-Ofelt (J-O) theory and studying energy transfer mechanisms. The mechanisms of Ca2+-induced fluorescence quenching and factors influencing the fluorescence decay time were analyzed. Through the analysis of the absorption cross-section, emission cross-section, and gain cross-section of crystals with different Ca2+ ion concentrations, it was found that Ca2+, Er3+: KBYM crystals exhibited a low laser pump threshold. The results demonstrate that the 6 mol% Ca2+, 5 mol% Er3+: KBYM crystals possess excellent performance and are a promising laser gain medium.

Efficiency optimization of E-band bismuth-doped ring-cavity fiber laser with low pump power

We demonstrate an efficient ring-cavity E-band bismuth-doped fiber laser with a low threshold power. The laser output power and efficiency are optimized for two operating wavelengths near the luminescence peak, 1430 nm and 1437.6 nm, by experimentally changing the laser output coupling ratio. Using a single co-propagating 1320 nm laser diode pump and a 100 m long bismuth-doped fiber, we achieve an output power of 114 mW with a slope efficiency of 24.5 % and an OSNR of 64 dB for an optimal coupling ratio of 70 % for a pump threshold below 64 mW. With added spectral tunability, this fiber laser will find application forthe characterization of E-band devices and components, such as optical amplifiers, and experimentation in transmission systems.

High-peak-power optically pumped passively mode-locked semiconductor laser with minimal components.

A high-peak-power sub-500-fs mode-locked optically pumped semiconductor laser is innovatively developed with only three components of a semiconductor gain chip, a semiconductor saturable absorber, and a focusing lens. The developed laser near the threshold pump power of 3.9 W can be operated with stable fundamental mode locking. The laser output can be naturally turned into the stable harmonic mode locking (HML) with the order gradually changing from 2nd to 8th by increasing the pump power from 4.0 W to 5.0 W. Due to the onset of the high-order transverse modes, the order of HML is fixed at 8th for a pump power greater than 5.0 W. For the HML with order less than 8th, the overall peak power and pulse width in the HML are approximately 0.36 kW and 550 fs, respectively. In the operation of 8th-order HML, the minimum pulse width and maximum peak power can reach 480 fs and 0.95 kW, respectively.

Growth, thermal properties and optical performance of Nd, Ho: NaY0.1Gd0.89(WO4)2 crystals

In this paper, Φ50 × 50 mm Nd, Ho: NaY0.1Gd0.89(WO4)2 crystals have been successfully grown by Czochralski. The crystal structure, doping concentration, thermal properties, fluorescence spectrum, emission cross section, fluorescence lifetime and energy transfer mechanism of Nd, Ho: NaY0.1Gd0.89(WO4)2 crystals have been systematically studied. The results show that the covalent bond effect of W–O bond makes NaY0.1Gd0.89(WO4)2 crystal have high doping ability. Low thermal expansion coefficient and excellent thermal conductivity reduce the possibility of dislocation defects. Under the excitation of 808 nm light source, a wide emission band at 2 μm is obtained. The absorption cross section, emission cross section and gain coefficient are obtained by absorption spectrum calculation. It shows that the incorporation of Y3+ ions can improve the luminous efficiency, widen the spectral line width and adjust the emission spectrum width. The low output pump threshold, high level lifetime and large absorption cross section of laser show great potential for high power laser systems.

Tunable backward terahertz-wave parametric oscillator centered at a high frequency of 0.87 THz with injection seeding.

We report the first demonstration of a frequency tunable backward THz-wave parametric oscillator (BW-TPO) centered at a high frequency of 0.87 THz using a slant-stripe-type magnesium oxide-doped periodically poled lithium niobate (PPLN) crystal as the nonlinear medium. Down-converted THz and idler beams generate upon excitation of the PPLN with a sub-nanosecond pulsed source of λ = 1064.44 nm. The resulting first idler has a wavelength of 1067.75 nm, equivalent to an oscillation frequency of 0.872 THz as per the spectral line separation from the pump. We also present angle tuning of the BW-TPO frequency ranging from 0.836-0.905 THz through PPLN rotation. The threshold pump intensity for BW-TPO is determined to be 5.6 GW/cm2 while obtaining a conversion efficiency as high as 12.3% at a pump energy (intensity) of 15.25 mJ (8.90 GW/cm2). A reduction of the BW-TPO threshold energy and improved pump-to-idler energy conversion efficiency resulted from injection seeding with a CW laser at the same wavelength as the first idler. The THz output is also directly proportional to seed power.

High-power and low-threshold 1200 nm InGaAs/GaAs quantum-wells VECSEL grown by MOCVD

Here, we demonstrate an InGaAs/GaAs quantum-wells (QWs) based vertical-external cavity surface emitting laser (VECSEL) with multi-watt emitting beyond 1200 nm. The development is motivated by the need for high-performance 1200 nm laser sources, which are located in the C–H bond photoacoustic tomography window and atmospheric transmission band. In the exploration, strain compensation, epitaxy growth, and package strategy are all optimized for effective performance improvement. Highly strained InGaAs/GaAs QWs epitaxy structures are grown on GaAs substrate with high crystalline quality by metal-organic chemical vapor deposition (MOCVD) technology. With effective thermal management, the thermal impedance of the chip is as low as about 0.92 K/W. As a result, the 1200 nm VECSEL can emit a power of 9.4 W under continuous wave (CW) operation, and the threshold pump power density is as low as about 1.61 kW/cm2. The VECSEL can also operate at a quasi-continuous wave (QCW) emission with tunable pulse width and repetition rates. Under a duty cycle of ∼9.5%, the VCSEL is with a peak power of 16.6 W and a slope efficiency of about 15.5%, and the corresponding pulse energy is as high as about 1.57 mJ. No thermal rollover is observed. This exploration significantly broadens the VECSEL performance and can be regarded as an extension and enrichment of the earlier works on this topic.

2D Semi-Metallic Hafnium Ditelluride: A Novel Nonlinear Optical Material for Ultrafast and Ultranarrow Photonics Applications.

2D semi-metallic hafnium ditelluride material is used in several applications such as solar steam generation, gas sensing, and catalysis owing to its strong near-infrared absorbance, high sensitivity, and distinctive electronic structure. The zero-bandgap characteristics, along with the thermal and dynamic stability of 2D-HfTe2, make it a desirable choice for developing long-wavelength-range photonics devices. Herein, the HfTe2 -nanosheets are prepared using the liquid-phase exfoliation method, and their superior nonlinear optical properties are demonstrated by the obtained modulation depth of 11.9% (800nm) and 6.35% (1560nm), respectively. In addition, the observed transition from saturable to reverse saturable absorption indicates adaptability of the prepared material in nonlinear optics. By utilizing a side polished fiber-based HfTe2 -saturable absorber (SA) inside an Er-doped fiber laser cavity, a mode-locked laser with 724 fs pulse width and 56.63dB signal-to-noise ratio (SNR) is realized for the first time. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser development with improved efficiency and reduced thermal impact. Finally, employing this HfTe2 -SA, a highly stable single-frequency fiber laser (SNR ≈ 74.56dB; linewidth ≈ 1.268kHz) is generated for the first time, indicating its promising ultranarrow photonic application.

Evidence of Yb3+ band broadening and continuous wave laser operation in bismuth modified KY1-x-yBixYby(WO4)2 mixed single crystals

To increase the Yb3+ optical bandwidths in highly anisotropic β:KY(WO4)2 (KYW) type laser single crystals, the simultaneous contributions of the Bi3+ 6 s2 lone electron pair and Y3+/Bi3+ ionic radii mismatch is proposed. The top seeded solution growth method with K2W2O7 as solvent was used to grow Yb doped single crystals of the KY(WO4)2/KBi(WO4)2 (KYBiW) solid solution. Crystals obtained across the used KY1xBixW solid solution range show always the low temperature β phase, with the monoclinic C2/c crystalline structure, but important differences between the Y/Bi composition of the starting solute and that of the crystal grown are found. The incorporation of Y and Yb into the crystal is preferential, resulting in a low Bi segregation coefficient, SBi= 13%, on the KYW side but near SBi= 90% in the KBiW side (Yb is still strongly incorporated filling the remaining 10%). The saturation temperature of the melt decreases as the Bi concentration in the precursor KY1-x-yBixYby(WO4)2/K2W2O7 mixture increases. Evidence of optical absorption and photoluminescence broadening of the Yb3+ bands with Bi3+ incorporation is derived from 6 K and 300 K measurements. Laser operation of KY1-x-yBixYby(WO4)2 crystals is shown to be comparable to that of KY1−yYby(WO4)2 ones. Up to ≈ 0.5 W of output power is obtained pumping in continuous wave regime with a Ti sapphire laser with slope efficiency η ≈ 65% and pump threshold< 200 mW, which is within the usual performance of KYW:Yb crystals, but the spectral distribution of the laser output in KYBiW:Yb crystals is larger than that for KYW:Yb. This spectral broadening may help in the reduction of ultrashort (fs) laser pulse duration generated by modelocking.

Sol-Gel growth of zinc oxide particles on graphene sheets for mode-locking in Thulium/Holmium-doped fiber laser

This work used Graphene/Zinc Oxide (G/ZnO) nanocomposite to generate mode-locked pulses in Thulium/Holmium-doped fiber (THDF). The sol-gel method was used to synthesize the ZnO particles decorated graphene sheets. The G/ZnO nanocomposite solution was deposited onto the arc-shaped fiber that had been fabricated to act as the saturable absorber (SA) device. The nonlinear optical response of G/ZnO-based SA was investigated via the twin detection approach, in which it exhibits a modulation depth of 14.11% and a saturation intensity of 4.23 MW cm−2. After incorporating the fabricated SA into the THDF laser cavity, mode-locked pulses were produced at a threshold pump power of 274.3 mW with a center wavelength of 1895.11 nm, fundamental frequency of 11.82 MHz, and a pulse duration of 1.7 ps. The maximum average output power and single pulse energy were determined to be 1.39 mW and 117 pJ, respectively. The mode-locked pulses generated were exceptionally stable, giving a signal-to-noise ratio (SNR) of 58.2 dB. According to the findings of the current research, it is anticipated that the G/ZnO-arc-shaped fiber-based SA has the potential to produce stable mode-locked lasers in the 2.0 μm region.

A temperature sensor based on Er3+/Yb3+ co-doped tellurite fiber for real-time thermal monitoring of transformers

In this paper, a temperature sensor based on fluorescence intensity ratio (FIR) technique is prepared by the dual curing method. The pumping threshold is only 0.1 mW for the application of real-time thermal monitoring on small transformers. This not only greatly reduces the thermal effects accumulated by laser irradiation but also contributes to the integration and portability of the light source. Based on the FIR, the temperature information could be demodulated through tracking the fluorescence spectral changes. The basic temperature sensing characteristics of this sensor were evaluated, which showed that the maximum stability deviation and reproducibility error were within ±1.1 K and ±0.8 K, respectively. Notably, it has a maximum measurement error of only 0.61 K when applied to the temperature monitoring of transformers. And a fast time response of 2.1 s was exhibited in the cyclic exhalation experiment. This proposed sensor is suitable for mass production due to its resistance to electromagnetic interference, low pumping power, fast time response, excellent stability and reproducibility.

Coherent phonon manipulation using single-mode circular electrostatic resonator

Coherent phonon manipulation is the mechanical analogy to photonic frequency combs while there is lack of a comprehensive evaluation system on the performance. In this paper, coherent phonon manipulation is demonstrated using a single-mode circular electrostatic resonator. Its manipulation mechanism for controlling phononic frequency comb (PFC) parameters is verified experimentally and also theoretically based on waveform derivation of the differential equations. The spacing of PFC system is regulated solely by the pump frequency and a spacing resolution of 0.1 Hz is achieved at room temperature. The amplitude modulation law is investigated and a complete PFC evolution process with a pump threshold of 2 mVpp is achieved. The sweeping experiment of electrostatic resonators is explained from the perspective of phonons. Additionally, feasibility of manipulating phonons using the electrostatic resonators is verified through nonlinearity modulation and damping loss experiments. This new methodology of phonon manipulation can be extended to other resonant energy carriers, with great potentials for frequency detection and phonon-quantum computing.

Tunable Random Fiber Laser Based on Dual-Grating Structure

In order to reduce the pumping threshold and achieve a short-cavity single-mode transmission with a narrow-linewidth random fiber laser, we propose a tunable random fiber laser based on the combination of random grating and highly reflective fiber Bragg grating (FBG). Theoretical modeling of a random refractive index-modulated fiber grating was carried out. Random grating is regarded as a linear combination of uniform fiber gratings with different periods. Simulation calculations were performed using the transfer matrix method to determine the preparation parameters. Under the premise of satisfying light localization, a point-by-point method was used to write a random grating in a single-mode fiber using a femtosecond laser according to the simulated parameters. We constructed a random fiber laser with a linewidth of 1.68 kHz and a threshold of 29.2 mW using a random grating and a highly reflective FBG combined with an erbium-doped fiber. Due to the broad scattered wavelength range of the random grating, by changing the central wavelength of the high-reflection FBG, the tunable wavelength of the output laser was realized, and the tunable range was 0.847 nm (1549.110–1549.957 nm). Moreover, the laser’s central wavelength and output power are stable for a long time. Compared with other lasers, the proposed laser has the advantages of a lower threshold, shorter cavity length, narrower linewidth, and a relatively simple structure.

High-Q lasing in Nd3+-doped phosphate glass microsphere resonators.

Nd3+-doped glasses are the most widely used laser gain media. However, Nd3+-doped non-silica microsphere lasers generally have lower quality (Q) factors due to the presence of non-radiative energy-loss impurities in traditional glass systems. In this work, we report the first, to the best of our knowledge, Nd3+-doped phosphate glass microsphere laser with the highest Q-factor of 1.54 × 106 among all Nd3+-doped non-silica glass microsphere lasers. Whispering gallery modes in the 1020-1120-nm band can be obtained for a typical microsphere with a diameter of 82.57 µm. When the pump power exceeds the threshold of 0.17 mW, single- and multi-mode microsphere lasing can be generated under 808-nm laser diode (LD) pumping. Typical Q-factors of the phosphate glass microspheres can reach 106, which is at least an order of magnitude higher than those of other Nd3+-doped non-silica glass microsphere lasers. The Nd3+-doped phosphate glass microsphere laser reported in this work can be considered as an active optical/photonic device with low pump thresholds.

Recent Progress in Low Threshold Plasmonic Nanolasers

AbstractPlasmonic nanolasers as a new class of coherent laser beyond the diffraction limit have attracted a lot of attention. However, the ultrahigh optical confinement caused by the plasmon effect is inevitably accompanied by metal absorption loss, thus increasing the pump threshold of the plasmonic nanolaser. In the past decade, many good results about low threshold plasmonic nanolasers have been realized and successfully extended to various applications. Here, this advance is discussed and some opinions are offered. First, based on the theoretical model and key parameters of the plasmonic nanolaser, the factors affecting the threshold are analyzed. Subsequently, the experimental efforts on the realization of low threshold plasmonic nanolasers from optical pumping to electrical pumping are reviewed. Their applications in on‐chip optical interconnects, biochemical analysis, and far‐field structure are then considered. Finally, feasible approaches to threshold reduction are discussed, as well as more possible applications in the future.

Multimode Graded Index Fiber with Random Array of Bragg Gratings and Its Raman Lasing Properties

Light propagation in multimode fibers is known to experience various nonlinear effects, which are being actively studied. One of the interesting effects is the brightness enhancement at the Raman conversion of the multimode beam in graded index (GRIN) fiber due to beam cleanup at Raman amplification and mode selective feedback in the Raman laser cavity based on fiber Bragg gratings (FBGs) with special transverse structure. It is also possible to explore random distributed feedback based on Rayleigh backscattering on natural refractive index fluctuations in GRIN fibers, but it is rather weak, requiring very high power multimode pumping for random lasing. Here, we report on the first realization of femtosecond pulse-inscribed arrays of weak randomly spaced FBGs in GRIN fibers and study Raman lasing at its direct pumping by highly multimode (M2~34) 940-nm laser diodes. The fabricated 1D–3D FBG arrays are used as a complex output mirror, together with the highly reflective input FBG in 1-km fiber. Above threshold pump power (~100 W), random lasing of the Stokes beam at 976 nm is obtained with output power exceeding 28 W at 174 W pumping. The beam quality parameter varies for different arrays, reaching M2~2 at the linewidth narrowing to 0.1–0.2 nm due to the interference effects, with the best characteristics for the 2D array.

Passively Q-switched ytterbium doped fiber laser based on a weak-photothermal saturable absorber

Recently, plasmonic nanocrystals (NCs) have attracted much attention due to their wide applications in bioimaging, photothermal therapy and photonics devices. Compared to other metal NCs, copper NCs show a weak photo-thermal effect, which shows that the copper NCs are a good candidate for nonlinear optical materials. In this paper, we demonstrated a Q-switched fiber laser at 1 μm wavelength by using Cu1.8S NCs as a saturable absorber (SA). By incorporating the Cu1.8S SA into an ytterbium doped fiber laser cavity pumped by a 980 nm laser diode, stable passive Q-switching at 1038 nm was obtained for a threshold pump power of 63 mW. On gradually increasing the pump power from 64 mW to 160 mW, the repetition rate of a Q-switched laser increases from 25.7 kHz to 54.5 kHz and the pulse duration decreases from 6.7 μs to 2.33 μs. Our results showed that Cu1.8S NCs is an effective SA for constructing pulsed lasers with a broadband operating wavelength.