Net Gain Coefficient Research Articles

Nd3+-doped double-clad fluoro-sulfo-phosphate fiber for a compact kilohertz-linewidth single-frequency laser

The utilization of single-frequency fiber lasers spans critical domains including gravitational wave detection, coherent optical communication, and atomic physics. Glass fibers doped with rare earth ions serve as pivotal gain media for them, and the quest for advanced fiber matrices is paramount. Herein, a newly developed Nd3+-doped AlF3-Na2SO4-KPO3-Zn(PO3)2 glass is reported. The glass showcases exceptional attributes including high anti-crystallization stability (>150∘C), elastic modulus (75 GPa), chemical durability (8.1×10−7g⋅cm−2⋅min−1), Nd3+ concentration (>1020ions/cm3), broad effective linewidth (45 nm), and extended lifetime (465 µs), surpassing those of conventional silica and phosphate glasses. Moreover, a custom-designed double-clad Nd3+-doped fiber with a net gain coefficient of 3.2 dB/cm at 1064 nm is fabricated, and single-frequency laser output with a narrow linewidth of 11 kHz has been obtained utilizing a 1.4-cm-long fiber, indicating the potential of this fiber as a promising gain medium for ultra-narrow-linewidth single-frequency fiber lasers.

Toward a10 dB net-gain waveguide amplifier in an Er3+-Yb3+ co-doped phosphate glass.

High-gain materials and high-quality structures are the two main conditions that determine the amplification performance of optical waveguides. However, it has been hard to balance each other, to date. In this work, we demonstrate breakthroughs in both glass optical gain and optical waveguide structures. We propose a secondary melting dehydration technique that prepares high-quality Er3+-Yb3+ co-doped phosphate glass with low absorption loss. Additionally, we propose a femtosecond laser direct-writing technique that allows controlling the cross section, size, and mode field of waveguides written in glass with high accuracy, leveraging submicron-resolution multi-scan direct-writing optical waveguide technology, which is beneficial for reducing insertion loss. As a proof of concept demonstration, we designed and fabricated two kinds of waveguides, namely, LP01- and LP11-mode waveguides in the Er3+-Yb3+ co-doped phosphate glass, enabling insertion loss as low as 0.9 dB for a waveguide length of 2 mm. Remarkably, we successfully achieved an optical amplification for both the waveguides with a net gain of >7 dB and a net-gain coefficient of >3.5 dB/mm, which is approximately one order of magnitude larger than that in the Er3+-Yb3+ co-doped phosphate glass fabricated by the traditional melt-quenching method. This will open new avenues toward the development of integrated photonic chips.

Amplified spontaneous emission from inclusions containing cesium lead bromide in glasses.

Cesium lead halide (CsPbX3, X = Cl, Br and I) perovskite nanocrystals embedded glasses exhibit good optical properties and have potential as gain media. However, origins of the amplified spontaneous emission (ASE) from CsPbX3 nanocrystals are controversial. Here, it is found that ASE is from CsPbX3 nanocrystals in inclusions instead of CsPbX3 nanocrystals dispersed in the glass matrix. Inclusions with various sizes are capable of generating ASE, and ASE of the inclusions can sustain at energy densities as high as several tens of mJ/cm2. Thresholds of the fs laser energy densities increase with the increase in fs laser wavelength, and high net optical gain coefficient is obtained.

Colloidal Quantum Dot Infrared Lasers Featuring Sub-Single-Exciton Threshold and Very High Gain.

The use of colloidal quantum dots (CQDs) as a gain medium in infrared laser devices has been underpinned by the need for high pumping intensities, very short gain lifetimes, and low gain coefficients. Here, PbS/PbSSe core/alloyed-shell CQDs are employed as an infrared gain medium that results in highly suppressed Auger recombination with a lifetime of 485ps, lowering the amplified spontaneous emission (ASE) threshold down to 300µJcm-2 , and showing a record high net modal gain coefficient of 2180cm-1 . By doping these engineered core/shell CQDs up to nearly filling the first excited state, a significant reduction of optical gain threshold is demonstrated, measured by transient absorption, to an average-exciton population-per-dot 〈Nth 〉g of 0.45due to bleaching of the ground state absorption. This in turn have led to a fivefold reduction in ASE threshold at 〈Nth 〉ASE =0.70excitons-per-dot, associated with a gain lifetime of 280ps. Finally, these heterostructured QDs are used to achieve near-infrared lasing at 1670nm at a pump fluences corresponding to sub-single-exciton-per-dot threshold (〈Nth 〉Las =0.87). This work brings infrared CQD lasing thresholds on par to their visible counterparts, and paves the way toward solution-processed infrared laser diodes.

Optimal design of annular thermoelectric generator with twisted tape for performance enhancement

The adaptation of annular thermoelectric generator (ATEG) to cylindrical heat sources exhibits a better performance compared to widely studied flat plate type thermoelectric generator (FTEG). To enhance heat transfer and improve the overall performance, a novel structure of the heat exchanger of the twisted-tape annular thermoelectric generator (TT-ATEG) is proposed in this paper. A three-dimensional finite element model of TT-ATEG is established for the first time. The net power gain coefficient and the efficiency gain coefficient are defined to quantify the performance of TT-ATEG, and the gain effects of the length, twist ratio and radius of the twisted tape are analyzed respectively. To maximize the comprehensive performance of TT-ATEG, the optimal weighting factor of net power and efficiency are determined, and the full-parameter optimization of the twisted tape is completed. The results reveal that the optimal performance of TT-ATEG is obtained if the weight ratio of net power and conversion efficiency is 1:4. In our case study, dimensionless factors radius ratio Rr, length ratio Lr and twist ratio of the twisted tape are 0.714, 0.479 and 136.84 respectively. Compared with the ATEG without twisted tape, the net power and efficiency are improved by 10.41% and 22.51%, respectively. The results demonstrate that the novel design of TT-ATEG could accelerate the heat transfer effectively and enhance the overall performance.

Effect of structural modifications on the spectral and lasing characteristics of truxene-cored starbursts with/without diphenylamine end-cappers

This study examined the effect of chemical modifications on the spectral and lasing characteristics of truxene-cored starbursts as gain media for organic lasers. A series of conjugated starburst materials, consisting of a truxene core and oligofluorene bridges with and without diphenylamine (DPA), namely, TrXD and TrX, were assessed as model molecules to investigate the influence of DPA on the photophysical characteristics and organic lasing behaviors. TrXD with DPA could effectively restrain the aggregation and greatly suppress the aggregation-induced emission quenching, yielding high photoluminescence efficiency. A higher radiative decay rate kr was observed for TrXD than for TrX, suggesting that DPA is beneficial for enhancing the radiative decay process. TrXD showed low amplified spontaneous emission thresholds of 2.1–4.3 μJ cm−2 with high net gain coefficients of 80–101 cm−1, rather low loss coefficients of 2.6–4.4 cm−1, compared with TrX. The best performance with a lasing threshold of 0.31 kW cm−2 was obtained for Tr3D, which was superior to that of Tr3 (0.86 kW cm−2). The molecular systems with DPA have a large potential as attractive molecules for organic lasers because of the superior lasing properties induced by kr.

Enhanced Amplified Spontaneous Emission in Quasi-2D Perovskite by Facilitating Energy Transfer.

Despite the superior optoelectronic properties of quasi-two-dimensional (quasi-2D) Ruddlesden-Popper halide perovskites, the inhomogeneous distribution of mixed phases result in inefficient energy transfer and multiple emission peaks. Herein, the insufficient energy funneling process at the high-energy phase is almost completely suppressed and the excitonic understanding of gain nature is studied in the energy funneling managed quasi-2D perovskite via introducing poly(vinyl pyrrolidone) (PVP) additive. The energy transfer process is facilitated from 0.37 to 0.26 ps after introducing the PVP additive, accelerating the exciton accumulation in the emissive state, and increasing the ratio of the high-dimensional phase for enhancing radiative emission. The gain lifetime is promoted to be as fast as 28 ps to outcompete nonradiative recombination during the build-up of population inversion. Simultaneously, the net gain coefficient is increased by more than twofold that of the pristine perovskite film. Owing to the remarkable gain properties, room-temperature amplified spontaneous emission is realized with a low threshold of 11.3 μJ/cm2, 4 times lower than 43 μJ/cm2 of the pristine film. Our findings suggest that the PVP-treated quasi-2D perovskite shows great promise for high-performance laser devices.

Deep blue photoluminescence and optical gain from sodium-doped carbon dots

The photoluminescence (PL) from carbon dots can be dramatically modified through the engineering of surface oxygen-containing functional groups. Despite the recent surge of interest in the non-metal doping of carbon dots, the photo-physical properties of metal-doped carbon dots remain unexplored. Here, we report the highly efficient deep blue PL and amplified spontaneous emission from sodium (Na)-ion-doped carbon dots. Based on the structure- and temperature-dependent PL and the temporal evolution of PL, we reveal that the introduction of Na ions into carbon dots not only passivate the electronegative oxygen related defects but also create new Na–O related defect centers in the band gap, leading to deep blue PL with dramatically reduced bandwidth and significantly enhanced quantum yield. We establish a luminescence model with four energy levels to interpret the PL dynamics of Na+-doped carbon dots. We demonstrate for the first time the ASE from such carbon dots, which exhibit a net optical gain coefficient of 47.8 cm−1 at 440 nm. Our results open new horizons for improving the PL of carbon dots and pave the way for realizing optoelectronic devices based on carbon dots.

Net Optical Gain Coefficients of Cu+ and Tm3+ Single-Doped and Co-Doped Germanate Glasses.

Broadband tunable solid-state lasers continue to present challenges to scientists today. The gain medium is significant for realizing broadband tunable solid-state lasers. In this investigation, the optical gain performance for Tm3+ and Cu+ single-doped and co-doped germanate glasses with broadband emissions was studied via an amplified spontaneous emission (ASE) technique. It was found that the net optical gain coefficients (NOGCs) of Tm3+ single-doped glass were larger than those for Cu+ single-doped glass. When Tm3+ was introduced, the emission broadband width of Cu+-doped glass was effectively extended. Moreover, it was found that for the co-doped glass the NOGCs at the wavelengths for Tm3+ and Cu+ emissions were larger than those of Tm3+ and Cu+ single-doped glasses at the same wavelengths. In addition, the NOGC values of Tm3+ and Cu+ co-doped germanate glasses were of the same order of magnitude, and were maintained in a stable range at different wavelengths. These results indicate that the Tm3+ and Cu+ co-doped glasses studied may be a good candidate medium for broadband tunable solid-state lasers.

All-fiber mode-locked gigahertz femtosecond laser at 1610 nm using a self-developed long-wavelength gain fiber.

We report a compact all-fiber passively mode-locked ultrafast laser with a fundamental repetition rate of 1.6 GHz that uses a self-developed long-wavelength active fiber, i.e., a fluoro-sulfo-phosphate-based Er3+/Yb3+ co-doped fiber (only 6.2 cm in length). This active fiber can provide a net gain coefficient of 0.6 dB/cm at 1610 nm. The high-repetition-rate all-fiber mode-locked laser operates at a low pump power of only approximately 90 mW. The mode-locked pulse train has a period of 625 ps and a 3 dB bandwidth of 7.0 nm, which can support a transform-limited pulse width of 390 fs.

Silicate-based erbium-doped fiber extended to L-band and its amplification performance

With the continuous development of mobile internet, cloud computing, Internet of things, and 5G, the annual growth rate of the existing communication transmission capacity is far from meeting the growing demand. The gain bandwidth of L-band erbium-doped fiber amplification (EDFA) has become the main factor for limiting the bandwidth extension. Owing to its negative effect on the gain and noise figure after 1580 nm, the excited state absorption (ESA) is regarded as a key factor that directly limits the L-band expansion of EDFA. If the ESA coefficient has the same value as the stimulated emission coefficient, the net gain coefficient becomes zero. At the same time, signal amplification cannot be obtained no matter how the pump power and fiber length are adjusted. Improvements from the perspective of erbium-doped fibers, such as changing the host glass and doping components, are extremely beneficial to L-band extension based on simple amplifier configurations. In the present study, an erbium-doped fiber extended to the L-band is fabricated by the modified chemical vapor deposition technology. The saturation curve of the fiber is measured, and then the rate of clustering is obtained. In addition, the absorption coefficient and emission coefficient of the fiber are tested by the truncation method, then the effect of ESA is studied. We demonstrate that modulating the ratio of P to Al in the fiber core has a significant inhibitory effect on the ESA of Er<sup>3+</sup>. To better characterize the amplification performance of the fiber, a single-stage and a multi-stage amplifier system are built respectively. At an input signal injection with a power value of –9 dBm, the fiber exhibits a gain of 10.5 dB with a maximum noise figure of 5.9 dB at 1625.3 nm, based on a single-stage amplifier system under a 980 nm forward-pumping excitation. It indicates that the amplification performance of the fiber in the long-wavelength has improved, which is consistent with the conclusion of the suppression effect on the ESA. By optimizing the fiber length for the first, second and third stage to 19 m, 26 m and 10 m, respectively, a gain of 23.4 dB is obtained at the longest wavelength. The maximum noise figure is 6.3 dB, and the saturation output power is 24.8 dBm. In the meantime, the pump conversion efficiency arrives at 13%. This is the first report on the L-band extended silicate-based erbium-doped fiber that can achieve long-wavelength gain extension to 1625.3 nm.

Fabrication and Performance of a Heterogeneous-Helical-Cladding Fiber

Large mode area (LMA) fibers are widely used in high power fiber lasers to solve the nonlinear problems. In this work, a Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped phosphate glass LMA fiber with heterogeneous helical claddings (HHC) is fabricated successfully by rod in tube method for the first time. The performance of the preform glass was matched well and suitable for drawing the fiber, and the related fabrication method is helpful for other special structure fiber fabrication. The fiber performance such as loss and gain coefficients are both tested and compared respectively. The experiment results show that the loss coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α</i> will increase with the decrease of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Λ</i> (helix pitch) or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> (fiber core radius), furtherly, the net gain coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</i> will become larger with the increase of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Λ</i> . The experiment results are beneficial to the HHC fiber design furtherly. As the HHC fiber has an all-solid-state structure, it is easy to fabricate relatively and convenient for cutting, splicing, etc. Thus, the HHC fiber will have potential applications in the field of fiber lasers.

Stable 5-GHz fundamental repetition rate passively SESAM mode-locked Er-doped silica fiber lasers.

A stable passively mode-locked Er-doped silica fiber laser with a fundamental repetition rate of up to 5 GHz is demonstrated, which, to the best of our knowledge, is the highest repetition rate for 1.5 μm semiconductor saturable absorber mirror (SESAM) mode-locked Er-doped silica fiber (EDF) lasers. A segment of commercially available EDF with a net gain coefficient of 1 dB/cm is employed as gain medium. The compact Fabry-Pérot (FP) cavity features a fiber mirror, namely multiple-layer dielectric films (DFs) directly coated on end facet of a passive fiber ferrule, enabling a short cavity length of 2 cm configured. The mode-locked oscillator operates at 1561.0 nm with a signal-to-noise ratio (SNR) of 62.1 dB, whose average power is boosted to 27 mW by a single-mode Er-doped fiber amplifier (EDFA) and spectral bandwidth is broadened form 0.69 nm to 1.16 nm with a pulse width of 3.86 ps. The fiber laser shows excellent spectral stability without conspicuous wavelength drifting for 3 hours. Moreover, the basic guidelines of selecting SESAM for high repetition rate passively mode-locked fiber lasers is given.

Ultralow Threshold Optical Gain Enabled by Quantum Rings of Inverted Type‐I CdS/CdSe Core/Crown Nanoplatelets in the Blue

AbstractAchieving low‐threshold optical gain for solution‐processed materials is crucial for their real‐life applications and deployment as gain media. However, the realization of low gain threshold in the blue region has shown to be technically an extremely challenging task using colloidal nanocrystals as a result of fast nonradiative Auger rates in smaller nanocrystals. Here, ultralow‐threshold blue amplified spontaneous emission (ASE) (≈2.7 µJ cm−2) accompanied with a large net modal gain coefficient of 360 cm−1 in the blue enabled by blue‐emitting (≈455–465 nm) colloidal quantum rings (QRs) of inverted type‐I CdS/CdSe core/crown nanoplatelets (NPLs) is proposed and demonstrated. The synthesized QRs with controlled crown size outperform the best reported ASE thresholds and net modal gain coefficients from the solution‐processed materials by ≈2.5‐ and ≈4‐fold, respectively, in the similar blue spectral window. Utilizing this QR architecture, it is also shown that the ASE peak can be spectrally tuned by controlling the lateral size of the crown and hence quantum confinement in the lateral direction. These outstanding results support the prospects of these solution‐processed QRs made of 2D hetero‐NPLs in the challenging blue region as colloidal gain.

Fabrication and performance investigation of the Nd [formula omitted]-doped Heterogeneous helical cladding phosphate glass fiber

The heterogeneous helical cladding (HHC) fiber is a new type of extreme large mode area fiber proposed by our group firstly. In this work, Nd3+-doped phosphate glass HHC fibers with different core radius r (60μm –175μm) and different helix pitch Λ (5.25 cm and 10.5 cm) were fabricated successfully. Firstly, the fiber preform glass was designed and prepared, and the performance of which was measured. Secondly, the HHC fiber preform was made by rod-in-tube method. Thirdly, the HHC fiber was obtained by the spiral drawing method, then the loss and gain performance of which were measured. Based on the experiment results, the following conclusions can be drawn: the larger the Λ (or r), the lower the loss coefficient L, and when r (or Λ) is the same, the larger the Λ (or r), the higher the net gain coefficient. This work will facilitate the further exploration and application of the HHC fiber.

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

AbstractWhispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm−2 and ≈1.39 mJ cm−2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm−1 and a high net modal gain coefficient of ≈485 cm−1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.

Output property in loss-noise model of single mode laser

In order to optimize communication and output stability of laser system, with linear approximation method and loss-noise model of single mode laser driven by colored pump noise and quantum noise whose real and imaginary part are cross-correlated, the steady-state mean intensity correlation function and signal-to-noise ratio (SNR) are calculated with an input additive periodic signal. The new physical quantity K is defined to describing output property of laser system. The maximums are existed between the relationship of K and net-gain coefficient, self-saturation coefficient, cross-correlated coefficient of real and imaginary part of quantum noise. That is the best match of the laser communication system. Also the decrease of noise intensity and increase of input signal frequency all can optimize the output property of laser system.

Low–threshold sky-blue gain medium from a Triazine-capped ladder-type oligomer neat film

Abstract A new solution-processable non-doped sky-blue organic laser gain medium has been developed based on triazine end-capped ladder-type oligo-(p-phenylene). The planar and symmetric conjugation of electron-withdrawing triazine moiety enables fine tuning of the emission from deep-blue to sky-blue with lowered LUMO energy level at no significant expense of optical gain properties. The emitter exhibits low amplified spontaneous emission (ASE) threshold of 41 nJ pulse−1 (621 W cm−2), high maximum net gain coefficient of 73 cm−1 and low loss coefficient of 6.45 cm−1. Notably, a decent ASE phenomenon (at wavelength of 481 nm) can still be detected at film thickness of 42 nm with an ASE threshold of 99 nJ pulse−1 (1.5 kW cm−2). These results suggest a prospective design concept for developing high-performance laser gain materials to address the challenge of electrically pumped organic lasers.

Monodisperse Six-Armed Starbursts based on Truxene-Cored Multibranched Oligofluorenes: Design, Synthesis, and Stabilized Lasing Characteristics.

A series of monodisperse six-armed conjugated starbursts (Tr1F, Tr2F, and Tr3F) containing a truxene core and multibranched oligofluorene bridges capped with diphenylamine (DPA) units has been designed, synthesized, and investigated as robust gain media for organic semiconductor lasers (OSLs). The influence of electron-rich DPA end groups on their optoelectronic characteristics has been discussed at length. DPA cappers effectively raise HOMO levels of the starbursts, thus enhancing the hole injection and transport ability. Solution-processed electroluminescence devices based on the resulting six-armed starbursts exhibited efficient deep-blue electroluminescence with clear reduced turn-on voltages (3.2-3.5 V). Moreover, the resulting six-armed molecules showed stabilized electroluminescence and amplified spontaneous emission with low thresholds (27.4-63.9 nJ pulse-1 ), high net gain coefficients (80.1-101.3 cm-1 ), and small optical loss (2.6-4.4 cm-1 ). Distributed feedback OSLs made from Tr3F exhibited a low lasing threshold of 0.31 kW cm-2 (at 465 nm). The results suggest that the construction of truxene-centered six-armed conjugated starbursts with the incorporation of DPA units can effectively enhance EL properties by precisely regulating the HOMO energy levels, and further optimizing their optical gain properties.

Giant Modal Gain Coefficients in Colloidal II-VI Nanoplatelets.

Modal gain coefficient is a key figure of merit for a laser material. Previously, net modal gain coefficients larger than a few thousand cm-1 were achieved in II-VI and III-V semiconductor gain media, but this required operation at cryogenic temperatures. In this work, using pump-fluence-dependent variable-stripe-length measurements, we show that colloidal CdSe nanoplatelets enable giant modal gain coefficients at room temperature up to 6600 cm-1 under pulsed optical excitation. Furthermore, we show that exceptional gain performance is common to the family of CdSe nanoplatelets, as shown by examining samples having different vertical thicknesses and lateral areas. Overall, colloidal II-VI nanoplatelets with superior optical gain properties are promising for a broad range of applications, including high-speed light amplification and loss compensation in plasmonic photonic circuits.