Low Pump Threshold Research Articles

Random Raman Lasing in Diode-Pumped Multi-Mode Graded-Index Fiber with Femtosecond Laser-Inscribed Random Refractive Index Structures of Various Shapes

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

Coherent random fiber laser emission from CdSe/ZnS quantum dots.

We experimentally demonstrate the coherent random laser emission by combining CdSe/ZnS colloidal quantum dots (CQDs) with hollow optical fiber. Through the localized surface plasmon resonance (LSPR) induced by Ag nanoparticles (NPs), well-distinguished discrete spikes are observed from the Ag modified hollow fiber loaded with CdSe/ZnS QDs solution. In addition, coherent random laser action with low threshold is also realized in the hollow optical fiber filled with high packing-density CdSe/ZnS QDs even if the Ag NPs is not introduced. The self-assembled clusters of CdSe/ZnS QDs serve as the optical gain media as well as the strong scattering centers. The angle measurement experiments show that the directional emission of random laser can be adjusted by using different pumping manners. This facile, inexpensive, low pump threshold random laser could be widely used in photonic devices and display imaging.&#xD.

Analysis of Random Lasing in Human Blood.

Random lasing (RL) is an optical phenomenon that arises from the combination of light amplification with optical feedback through multiple scattering events. In this paper, we present our investigations of RL generation from human blood samples. We tested mixtures of rhodamine B dye solutions with different blood components, including platelets, lymphocytes, erythrocytes, and whole blood. Intense coherent RL was obtained in all cases at relatively low pump thresholds, except for erythrocytes. We also studied the potential of RL signal analysis for biosensing applications using blood samples from healthy individuals and patients suffering from Chronic Lymphocytic Leukemia (CLL). CLL is a blood disease characterized by a high count of lymphocytes with significant morphological changes. A statistical analysis of the RL spectra based on principal component and linear discriminant analyses was conducted for classification purposes. RL-based sample discrimination was conducted for whole blood, platelet, and lymphocyte samples, being especially successful (86.7%) for the latter. Our results highlight the potential of RL analysis as a sensing tool in blood.

Growth and optical properties of high-concentration Ho3+ ion-doped Ba(Y1−xLux)2F8 crystal for mid-infrared laser application

30mol% Ho:BaY2F8 (Ho:BYF) and 30mol% Ho, 20mol% Lu:BaY2F8 (Ho:BYLF) crystals with monoclinic phase were prepared by the Bridgman technique. After introducing Lu3+ ions, the thermal property of Ho:BYLF crystal was improved without significantly shortening the 5I5 level lifetime of Ho3+ ion compared with Ho:BYF crystal. Under laser diode pumping at 889 nm, the emission peaks of both Ho:BYF and Ho:BYLF crystals in the range of 3–5 μm could be detected. The spectral parameters of the crystal are calculated in detail, and the cross sections of absorption, emission and gain are given. During laser pumping, the two crystals showed low pumping threshold, and Ho:BYF crystal obtained a maximum energy output of 5.22 mJ, while Ho:BYLF crystal tended to be saturated at 1 mJ due to the high concentration of Lu3+ ion. These two crystals show great application potential for laser operating in the mid-infrared range.

A 1.55 μm laser gain medium based on Yb3+ and Er3+ co-doped KBa0.94Ca0.06Y(MoO4)3 crystal with in-situ operating temperature self-monitoring capability

The 1.55 μm laser technology is widely applied in military, information communication, biomedicine and other fields. With the deepening development of these application areas, the demand for novel 1.55 μm laser gain media is becoming increasingly urgent. This study reports a novel Yb3+, Er3+ co-doped KBa0.94Ca0.06Y(MoO4)3 (KBCYM) crystal. In this crystal, Yb3+ serves as a sensitizer, significantly enhancing the emission intensity of Er3+ in both visible and near-infrared bands. Notably, when the concentration of Yb3+ reaches 6 mol%, the emission intensity peaks at 1.55 μm. Optical cross-section calculations reveal that the crystal exhibits a low laser pumping threshold at this concentration, demonstrating its potential as a laser gain medium. However, the crystal inevitably generates thermal effects during operation, which may adversely affect its performance. Therefore, real-time monitoring of the operating temperature is crucial. The thermal stability of the crystal was evaluated by measuring the temperature dependence of its luminescence intensity in the near-infrared band. Remarkably, even when the temperature rises to 553 K, the emission intensity at 1.55 μm only decreases by 10.9%. Additionally, the temperature sensing performance was evaluated using fluorescence intensity ratio techniques, yielding absolute and relative sensitivities of 0.00981 K–1 at 453 K and 1.32%/K at 303 K, respectively, highlighting its potential for optical temperature sensing. Finally, through leveraging the unique properties of Yb3+, Er3+: KBCYM crystals, we successfully developed 1.55 μm luminescent optical devices with practical applications. These devices not only exhibit efficient luminescent performance, but also possess a self-temperature measurement function, opening up new avenues for the further development of laser technology.

Kilohertz-linewidth and low-threshold single-frequency all-fiber laser utilizing self-developed Nd3+-doped fluoro-sulfo-phosphate fiber.

Compared to Nd: YAG lasers, Nd3+-doped fiber lasers offer superior beam quality, compactness, and heat dissipation, especially in generating single-frequency lasers, which holds great promise for applications in optical atomic clocks, quantum computing, and high-precision bio-photonic imaging. In this study, theoretical simulations of the local environment and experimental analyses on the luminescent characteristics of what we believe to be a novel Nd3+-doped fluoro-sulfo-phosphate (FSP) laser glass were performed to mitigate the concentration and hydroxyl quenching effects. Based on that, a highly Nd3+-doped (4 mol%) FSP fiber with a large emission cross-section (3.24 × 10-20 cm2), wide bandwidth (33.7 nm), long lifetime (354 µs), and high gain coefficient (4.24 dB/cm) was designed. Utilizing this fiber, a 1065 nm SFFL with a low pump threshold of 18 mW, a narrow linewidth of 6.5 kHz, and a 0.9 µm compact all-fiber laser were demonstrated, highlighting the potential of Nd3+-doped FSP fiber in high-performance fiber lasers.

783 MHz fundamental repetition rate all-fiber ring laser mode-locked by carbon nanotubes

We demonstrate a 783 MHz fundamental repetition rate mode-locked Er-doped all-fiber ring laser with a pulse width of 623 fs. By using carbon nanotubes saturable absorber, a relatively low self-starting pump threshold of 108 mW is achieved. The laser has a very compact footprint less than 10 cm × 10 cm, benefiting from the all-active-fiber cavity design. The robust mode-locking is confirmed by the low relative intensity noise and a long-term stability test. We propose a new scheme for generating high repetition rate femtosecond optical pulses from a compact and stable all-active-fiber ring oscillator.

Low self-starting threshold polarization-maintaining Er-doped fiber optical frequency comb

We report the development of an all-fiber polarization maintaining (PM) optical frequency comb using a mode-locked figure-9 laser with a low self-starting pump threshold. We have achieved self-starting mode-lock for repetition rates (fr) from 70 MHz to 109 MHz. At a repetition rate of 109 MHz, mode-locking can be achieved for a pump power ranging from 187 mW to 880 mW. To the best of our knowledge, this is the lowest pump power reported for PM figure-9 erbium fiber lasers with repetition rate over 100 MHz. By optimizing the pump power to 238 mW, we have achieved an output power of 5 mW, center wavelength of 1566.2 nm, and 3-dB spectral bandwidth of 20.5 nm. The repetition rate has a high signal-to-noise ratio of 95 dB at a resolution bandwidth of 300 Hz. We have studied the spectral characteristics of the laser under different cavity lengths and pump powers. Additionally, we have stabilized the repetition rate using a GPS-Rb disciplined RF reference. The fractional instability of the repetition rate is measured to be 4.67×10-12 at 1 s and 9.22×10-13 at 10 s over a measurement of 11 h. Our findings demonstrate that the developed figure-9 comb is robust, compact and has the advantage of high stability and low power consumption. It offers a cost effective solution for future outdoor comb applications.

An All‐Colloidal and Eco‐Friendly Quantum‐Dot Laser

AbstractColloidal zinc‐chalcogenide quantum dots (QDs) are emerging as the promising heavy‐metal‐free light–emitters, however, the development of light amplification and lasing therein remains challenging. Here, corroborated by comprehensive in situ and transient spectroscopy, it is unraveled that the ultrafast photooxidization‐induced trapping process (≈2 ps) and the sub‐bandgap photoinduced absorption under intense excitation are the main hindrances to light amplification in ZnSeTe QDs. Upon tackling the hurdle by heterostructure engineering, this study demonstrates that the ZnSeTe QDs exhibit intrinsically superior gain performance, including the high gain cross‐section (5.6 × 10−16 cm2), long gain lifetime (615 ps), low pump threshold (≈25.9 µJ cm−2) and high fatigue resistance. On this basis, a microfluidic channel is designed to mass‐produce the micellar cavities whose surfaces are treated to combine with the ZnSeTe QDs. The active QD‐micelle hybrids show state‐of‐the‐art lasing performance and allow the fiber‐based micro‐manipulation toward the single‐mode lasers. These findings not only provide essential knowledge on the gain physics in ZnSeTe QDs, but also open a new avenue to develop high‐performance eco‐friendly lasers.

One-pot synthesis of heterostructured CsPbBr3/PdSe nanowires with excellent humidity stability.

Lead halide perovskite nanowires (NWs) have garnered increasing attention owing to their unique properties, including axial carrier transportation, low pumping thresholds, and polarized emission. However, their intrinsic instability significantly limits their applications. In this study, heterostructured CsPbBr3/PbSe NWs with a diameter of 10 nm and lengths ranging from several to tens of microns were successfully synthesized via a one-pot solution-phase process. Photoluminescence measurements revealed that the CsPbBr3/PbSe NWs emitted green fluorescence with a narrow width of 16 nm and a high photoluminescence quantum yield (PLQY) of 37.6%. The obtained CsPbBr3/PbSe NWs underwent anion exchange with I- ion precursors to transform into CsPbBr3-xIx/PbSe, but not with Cl- ion precursors. Remarkably, these heterostructured CsPbBr3/PbSe NWs remained stable when immersed in water or exposed to air for several months, demonstrating excellent humidity stability. This study offers an effective approach for preparing stable CsPbBr3 NWs and fosters their future applications in optoelectronic devices.

Analysis and optimization of optical frequency comb spectra of magnesium fluoride microbottle resonator

<sec>Optical frequency comb has shown great potential applications in many areas including molecular spectroscopy, RF photonics, millimeter wave generation, frequency metrology, atomic clock, and dense/ultra-dense wavelength division multiplexed high speed optical communications. Optical frequency comb in the microresonator supporting whispering-gallery mode has attracted widespread interest because of its advantages such as flexible repetition rate, wide bandwidth, and compact size. The exceptionally long photon lifetime and small modal volume enhance light-matter interaction, which enables us to realize intracavity nonlinear frequency conversions with low pump threshold. With the advantages of small size, low power consumption, wide spectral coverage and adjustable dispersion, the magnesium fluoride microresonator optical frequency comb has potential applications in optical communication and mid-infrared spectroscopy.</sec><sec>In this work, the spectral characteristics of the optical frequency comb generated by a magnesium fluoride whispering-gallery mode microbottle resonator platform are investigated. In order to optimize the spectral distribution of the optical frequency comb of the magnesium fluoride microbottle resonator, the second-order dispersion and higher-order dispersion of the bottle resonator structure under different curvatures and axial modes are solved iteratively by the finite element method, and the spectral evolutions of the optical frequency comb under different axial mode excitations are simulated by solving the nonlinear Schrödinger equation through the split-step Fourier method. The results show that near-zero anomalous dispersion tuning can be achieved in a wide bandwidth range by exciting low-order axial mode at an optimal radius of curvature, while the high-order axial mode will lead the microbottle resonator to present the weak normal dispersion. The weaker anomalous dispersion in the lower-order axial mode broadens the bandwidth of the optical comb, demonstrating that the third-order dispersion and the negative fourth-order dispersion can broaden the Kerr soliton optical comb; the weak normal dispersion in the higher-order axial mode suppresses the generation of the Kerr optical comb, and the Raman optical comb dominates. The selective excitation of Kerr soliton combs and Raman combs can be achieved by modulating the axial mode of the microbottle resonator under suitable pumping conditions. The present work provides guidance for designing the dispersion in magnesium fluoride microresonator and the experimental tuning of broadband Kerr soliton optical combs and Raman optical combs.</sec>

All-fibre mode-locked laser using SMF-GIMF-SMF structure as artificial saturable absorber

In this work, we proposed and experimentally demonstrated a combination of single mode fibre (SMF) with graded indexed multimode fibre (GIMF) to form an artificial saturable absorber. A SMF-GIMF-SMF structure is incorporated into Erbium-doped fibre laser (EDFL) system to induce mode-locking operation with very low threshold pump power of 41.6 mW. The highest pulse energy obtained is 406.65 pJ at pump power of 84.7 mW. With a relatively short GIMF used as artificial saturable absorber, high fundamental repetition rate with low threshold power is achieved. The proposed artificial saturable absorber is potentially to contribute to the development of sustainable industry, innovation, and infrastructure.

Highly Accurate Pneumatically Tunable Optofluidic Distributed Feedback Dye Lasers.

Optofluidic dye lasers integrated into microfluidic chips are promising miniature coherent light sources for biosensing. However, achieving the accurate and efficient tuning of lasers remains challenging. This study introduces a novel pneumatically tunable optofluidic distributed feedback (DFB) dye laser in a multilayer microfluidic chip. The dye laser device integrates microfluidic channels, grating structures, and vacuum chambers. A second-order DFB grating configuration is utilized to ensure single-mode lasing. The application of vacuum pressure to the chambers stretches the soft grating layer, enabling the sensitive tuning of the lasing wavelength at a high resolution of 0.25 nm within a 7.84 nm range. The precise control of pressure and laser tuning is achieved through an electronic regulator. Additionally, the integrated microfluidic channels and optimized waveguide structure facilitate efficient dye excitation, resulting in a low pump threshold of 164 nJ/pulse. This pneumatically tunable optofluidic DFB laser, with its high-resolution wavelength tuning range, offers new possibilities for the development of integrated portable devices for biosensing and spectroscopy.

1.76 μm on-chip gain characteristics of Tm3+ in a rib waveguide based on Tm3+-doped LiNbO3 thin-film on insulator

A theoretical study has been performed on 1.76 μm on-chip amplification characteristics of Tm3+ in a rib waveguide of LiNbO3 on insulator (Tm:LNOI) under 795 nm wavelength pumping. Steady-state rate equations and propagation equations of signal and pump waves are established on the basis of a three-level model of Tm3+ system with the cross relaxation process 3H4→3F4: 3H6→3F4 included. The validity of the three-level model is demonstrated by referencing previously reported experiment result of Er3+-doped LNOI waveguide amplifier and by examining the effect of Tm3+ blue upconversion emissions on the 1.76 μm amplification. Besides the relations of signal gain to propagation length, launched pump power and input signal power, the effects of waveguide cross-section geometry and Tm3+ concentration on the gain characteristics are also studied. The results show that the waveguide cross-section geometry affects definitely the gain performance via its effect on mode field. The Tm3+ concentration affects the signal gain also via its effect on the 3H4→3F4: 3H6→3F4 cross relaxation coefficient, besides the conventional effect that higher active ion concentration yields larger signal gain. A comparison with conventional Ti4+(Zn2+)-diffused Tm3+-doped LN [Ti(Zn):Tm:LN] waveguide shows that the Tm:LNOI waveguide has much better gain performance than the Ti(Zn):Tm:LN waveguide, including much stronger pump power and propagation distance dependences of signal gain, much higher signal gain and much lower threshold pump power. These features are associated with significantly increased Tm3+ population inversion extent due to ultra-compact mode field and larger overlapping factor of mode field and Tm3+ population profiles in the Tm:LNOI waveguide.

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.

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.

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.

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.

2.7 μm mid-infrared Er-doped thin-film LiNbO3-on-insulator photonic wire laser

The 2.7 μm continuous-wave laser was demonstrated theoretically on the basis of a 1480-nm-pumped Er-doped thin-film LiNbO3-on-insulator photonic wire (Er:LNOI PW) and a three-level rate equation model of Er3+. Under 1480 nm pumping, the excited state absorption (4I13/2 → 4I9/2) and cooperative upconversion process (4I13/2 → 4I15/2: 4I13/2 → 4I9/2) populate the 4I11/2 level meanwhile bypass significantly the 4I13/2 level, enabling a population inversion between these two levels. The cavity length and output coupler's reflectivity at laser wavelength were optimized for maximizing laser output. Under the optimum configuration, dependences of laser power on both pump power and Er3+ concentration were investigated. Simulation results show that lower threshold pump power, higher laser output and slope efficiency are obtained for a higher Er3+ concentration. The laser output reveals a linear relationship to the Er3+ concentration and the relationship is strong at higher pump level. Simulation results show that the threshold pump power, laser output and slope efficiency are on orders of several tens of mW, sub-mW and 3.1%, respectively, for an Er3+ concentration of 1.5 mol%. In addition, there is no need to account for photorefractive effect on the laser performance as it is far smaller than the refractive index contrast of the LNOI PW, ∼0.7. Present work opens up the possibility of implementing an on-chip 2.7 μm mid-infrared laser based on an Er:LNOI.