Gain Medium Research Articles

Passively generated Q-switched pulses fiber laser in 2 μm wavelength using MXene Ti3C2Tx and Ti2CTx saturable absorber

Abstract In this paper, Q-switched Thulium-doped fiber lasers (TDFLs) operating near the 2 µm region were demonstrated, utilizing a newly developed Thulium doped fiber as the gain medium and titanium carbide MXene as the saturable absorber (SA). The Thulium-doped fiber with a composition of SiO2-GeO2-Al2O3-HfO2-Y2O3-Bi2O3, was produced from a preform prepared using the modified chemical vapor deposition (MCVD) process. Two variations of titanium carbide MXene-based SA, Ti3C2Tx and Ti2CTx, were tested and compared, both successfully generating Q-switched pulses. When Ti3C2Tx was used, the Q-switched pulse had a center wavelength of 1934.223 nm, a repetition rate of 55.39 kHz and a pulse width and 1.942 µs. For Ti2CTx, the generated Q-switched pulse has a center wavelength of 1932.229 nm, with a repetition rate of 77.28 kHz and a pulse width of 1.608 µs. The signal-to-noise ratio (SNR) values were 45.6 dB and 50.2 dB for Ti3C2Tx and Ti2CTx respectively. Based on the experimental results, both Ti3C2Tx and Ti2CTx show promise as SAs for generating Q-switched pulses in the 2 µm region within the TDFL cavity.

Effect of Different Doping Concentrations on the Properties of Fe2+:ZnSe and Fe2+, Cr2+:ZnSe Crystals Grown by the Chemical Vapor Transport Method.

The Fe2+:ZnSe crystal is an important material for 3-5 μm mid-infrared lasers. In this work, we have grown different doping concentrations of Fe2+:ZnSe and Fe2+, Cr2+:ZnSe by the chemical vapor transport method. The doped crystals have the same structure. The lattice constant increases slightly with a higher Fe2+ concentration, while the doped Cr2+ has a great influence on the lattice constant. Besides, with a higher Fe2+ concentration, the TO mode has no change and the LO mode presents a small blue shift. The doped ZnSe crystals all have a wide absorption peak in the range of 2.3-4.5 μm, and these absorption peaks widen with the increase of the Fe2+ doping concentration. Besides, in the range of 5-20 μm, the crystals have good transparency. The concentrations of 3, 2, and 1% Fe2+ samples are calculated to be 4.15 × 1019, 3.27 × 1019, and 3.02 × 1019 cm-3, respectively, and the Fe2+ concentration of the Fe2+, Cr2+:ZnSe sample is 3.71 × 1019 cm-3. The band gap decreases from 2.52 to 2.27 eV with a higher Fe2+ doping concentration. Therefore, we can modulate the absorption bandwidth and band gap by doping the ion concentration, which is more suitable for developing mid-infrared gain medium applications.

Mid-infrared arrayed waveguide gratings using a quantum cascade laser gain medium as core material

Mid-infrared photonics is a widely researched field with several applications, such as chemical sensing and spectroscopy. The development of photonic integrated circuits for the mid-infrared can enable the reduction in device size, weight, and power (SWaP) consumption. This paper demonstrates arrayed waveguide gratings working in the mid-infrared regime (5–5.4 µm). Our devices are fabricated on an InP-based quantum cascade laser platform with the gain medium as the waveguide core. To minimize the propagation losses caused by free carrier absorption and intersubband absorption in the unbiased QCL structure, we exposed the photonic chips to proton implantation. The performance of three sets of AWGs with different etch depths was characterized. The lowest waveguide losses were measured to be 2 dB/cm. The best performing 7×1 AWG and 13×1 AWG designs featured insertion losses of −2dB and −2.5dB, respectively. This study showcases the feasibility of applying such a platform for easy integration with active components like lasers and photodetectors, paving the path for on-chip mid-infrared applications.

Manipulating winding numbers and multiple topological bound states via long-range coupling in chiral photonic lattices

The topological bound states are limited to one pair in traditional one-dimensional (1D) topological chiral models as the winding number can only take the value of 0 or 1. It was increased to 2 or 3 via long-range couplings (LRCs) that were restricted to second- or third-order in recent experimental or theoretical researches. How to generate larger winding number to manipulate more topological bound states remains a challenge. Here, we propose an effective method by introducing arbitrary Nth order LRC stronger than the sum of other order couplings. The resulting winding number has been computed by getting the winding loop's envelope in parameter space. The coupling-inverted 1D photonic topological insulators supporting up to three pairs of highly localized multiple topological bound states are realized by fabricating photonic lattices with multi-layer waveguide arrays. Furthermore, these topological bound states are closely spaced, offering a promising implementation approach of multimode topological lasers in gain media. Our work highlights the advantages of LRCs in manipulating the topological phases of matter. Published by the American Physical Society 2024

Ultrafast visible fiber laser mode-locked by frequency-shifted feedback of an α-BaTeMo2O9 crystal acousto-optic modulator.

We report the first demonstration, to the best of our knowledge, of visible mode-locked fiber laser using frequency-shifted feedback (FSF) with a visible α-BaTeMo2O9 (α-BTM) crystal acousto-optic modulator (AOM). First, an α-BTM crystal is used as the visible high-quality AOM with a high diffraction efficiency of 85%, a fast rise/fall time of 79/98 ns, and a low insertion loss of 0.2 dB at 635 nm. Then, the 635 nm FSF mode-locked fiber laser is achieved using a Pr3+:doped ZBLAN double-clad fiber as a visible gain medium and the α-BTM AOM as a frequency-shifting element. Self-starting mode-locking at 635 nm directly generates red laser pulses with a pulse duration of 45 ps and a repetition frequency of 31.8 MHz. Furthermore, we investigate the evolution of mode-locking dynamics as the α-BTW-AOM feedback-frequency, which helps further understand the FSF dynamics by directly generating visible ultrashort pulses.

Enhancing yellow-green emission in Eu/Tb co-doped tellurite glasses controlled by Lu2O3

In this research, Lu2O3 was incorporated into Eu3+ and Tb3+ co-doped tellurite glasses to enhance structural stability and modify the yellow-green emission. A series of microstructural analyses, including Differential Scanning Calorimetry (DSC), Raman spectroscopy, and density measurements, confirmed that the addition of Lu2O3 causes the long-chain or cyclic Te-O-Te network structure to break, resulting in the formation of more [TeO3], which in turn leads to an increase in non-bridging oxygens (NBO) and lowers the phonon energy of the matrix material. Fluorescence spectral characterization revealed that both green and yellow luminescence intensities peaked when Lu2O3 concentration reached 15 %. Additionally, the Judd-Ofelt theory supports its superior laser performance. With the addition of Lu2O3,The radiative transition probability (Arad), lifetime (τrad), and branching ratio (β) of the excited states of Tb3+ and Eu3+ have all been enhanced.Under the regulation of 15% Lu2O3, The maximum absorption cross-section of at 544 nm is 1.88×10-20 cm2, and the maximum emission cross-section is 2.14×10-20 cm2. At 611 nm, the maximum absorption cross-section is 1.05×10-20 cm2, and the maximum emission cross-section is 1.44×10-20 cm2. The decay curve indicates a fluorescence lifetime enhancement from 0.6 ms to 1 ms with Lu2O3. These results underscore the potential of Lu2O3 modulated Eu3+/Tb3+ co-doped tellurium glass as an effective yellow-green laser gain medium.

Tm3+ doped CaF2 based oxyfluroborosilicate glasses and glass ceramics for visible and NIR lasers

The structural and luminescence properties of Tm3+ doped CaF2 based oxyfluoroborosilicate glasses and glass ceramics were discussed. The critical glass ceramic (BSTm1.0GC2) nature was obtained through different heat treatment processes as 450 oC for 1 h and the concentration of Tm3+ ions was optimized as 1.0 mol% for efficient emission. The spectroscopic and laser characteristic parameters were evaluated applying the standard Judd-Ofelt theory. The emission of blue light through 1D2 → 3F4 transition was studied by exciting at 360 nm UV wavelength. Upon 808 nm laser excitation, the glasses and glass ceramics produce 1.46 μm emission through 3H4 → 3F4 transition. The effective bandwidth (114.99 nm), stimulated emission cross-section (14.30×10–21 cm2), gain bandwidth (1644.74×10–28 cm3), figure of merit (10.21×10–24 cm2s) and the quantum yield (76%) of 3H4 → 3F4 transition made the BSTm1.0GC2 suitable as gain medium for 1.46 μm NIR fiber laser applications.

Multiple-beam output, high-peak power passively Q-switched Nd:YAG/Cr4+:YAG laser for ignition application - Modeling of laser pulse characteristics and thermal behavior

In this work we report on the characteristics of a passively Q-switched Nd:YAG/Cr4+:YAG laser with four beams, developed for studies on the laser ignition of different combustible mixtures. The compact laser contains a Nd:YAG/Cr4+:YAG ceramic composite medium, with a monolithic optical resonator. A configuration in which each beam contains laser pulses with an energy of 3.1 mJ and a duration of 0.9 ns (i.e. with a peak power of 3.4 MW) is discussed; operation can be performed in pulse-burst mode, with up to five pulses per pumping pulse. The characteristics of the laser pulses are described by a model based on the rate equations for the photon flux and population inversions in the Nd:YAG gain medium and in the Cr4+:YAG medium with saturable absorption. In addition, the thermal effects are simulated for the operation up to 100 Hz repetition rate, considering the Nd:YAG/Cr4+:YAG laser medium with two geometries, the first of the rod type and the second of the annular type (i.e. cylinder with a central hole) for a good management of thermal effects. This work is an important advancement towards developing a compact laser device with multiple beams, featuring laser pulses and structural properties suitable for igniting combustible mixtures in real engines.

Growth, spectroscopy properties, and laser operation of single crystal fiber: Nd3+-doped CNGG

Nd3+-doped calcium niobium gallium garnet single crystal fibers (Nd:CNGG SCFs) with varying Nd3+ concentrations were successfully grown using the laser-heated pedestal growth (LHPG) method. The study thoroughly examines the fundamental physical structure, optical properties, and laser characteristics of the as-grown Nd:CNGG SCFs. Results indicate that the Nd:CNGG SCF exhibits broader absorption and emission full width at half maximum (FWHM) compared to bulk single crystals. Moreover, pumped by a commercial 808 nm laser diode, continuous-wave laser operation at 1061 nm with a half-height width of 1.86 nm was achieved. The laser output power reached 1.04 W at an absorbed power of 7.46 W, with a slope efficiency of 25.4% relative to the absorbed pump power. These findings suggest that Nd:CNGG SCF holds promise as a practical and potent candidate for laser gain medium, offering significant research potential in the field of lasers.

Exploring the criterion for the self-pulsing suppression of mid-IR intracavity OPO with self-Raman scattering.

The stimulated Raman scattering (SRS) from the Nd:YVO4 gain medium is originally employed to eliminate the self-pulsing effect from the intracavity optical parametric oscillator (OPO) for achieving a continuous-wave (CW) mid-infrared output. The SRS with a second-order pump-wave power depletion is applied as the damping for the coupling between OPO pump-wave relaxation oscillation and signal-wave depletion. The SRS threshold conditions for different cavity and diode-pumped mode size designs are theoretically and experimentally explored. By using different resonators, the CW mid-infrared output from 120 mW to 1.56 W at the diode pump power from 3 to 19.3 W can be successfully demonstrated.

Performance evaluation of ∼2.1 μm microchip laser operation in Ho3+ doped germanate glass

An in-band pumped continuous wave (CW) ∼2.1 μm microchip laser is presented for the first time based on a short cavity Ho3+ doped germanate glass (GeO2-PbO-Ga2O3–Na2O: GPGN). A 1.94 μm, 5 W Tm3+ fiber laser was employed for the excitation of the Ho3+ ions. A 19% laser slope efficiency was achieved in a simple, unoptimized plane parallel Fabry-Perot cavity configuration. A positive thermal lens was estimated in the laser cavity with a sensitivity factor of S ∼31 m−1W−1 and an optical path distortion value exceeding 6 μm. The laser results along with the analysis of the thermal lens indicate that with improved thermal management and an optimized cavity configuration GPGN is a promising gain medium for microchip laser operation around 2.1 μm.

Synthesis and characterization of Er2O3-doped Y2O3 nanoparticle incorporated optical fiber for use as optical amplifier

Erbium oxide (Er2O3)-doped optical fibers (EDF) are well-known for their applications in optical amplification at 1550 nm. In this study, we present the synthesis of Er2O3-doped yttrium oxide (Y2O3) nanoparticles (NPs) using a homogeneous coprecipitation technique and their integration into optical fibers for enhanced optical amplification. A series of NP samples with varying Y/Er molar ratios were synthesized to identify the optimal composition for incorporation into optical fibers. X-ray diffraction (XRD) analysis revealed that the nanoparticles crystallize in a cubic geometry (space group Ia3) with crystallite sizes ranging from 20 to 41 nm. These sizes increased approximately to 90 nm as the calcination temperature was raised from 1000°C to 1400°C. Field emission scanning electron microscopy (FESEM) corroborated the XRD results, while high-resolution transmission electron microscopy (HRTEM) confirmed the crystalline structure and an average particle size of approximately 100 nm. Photoluminescence studies showed that emission and excitation intensities were functions of the Y/Er molar concentration ratio and calcination temperature, with the lifetime extending up to 6.86 ms for a sample with a 0.25:0.01 Y2O3:Er2O3 ratio calcined at 1400°C. To assess the performance of the synthesized NPs, an optical preform was prepared using the Vapor Phase Delivery (VPD) method combined with the Solution Doping (SD) technique. The preform was then drawn into fiber, and its amplification performance was evaluated. The resulting fiber demonstrated efficient amplification with a gain of 14.9 dB with respect to 0 dBm input signal at 1550 nm under 150 mW pump power from 9 m active fiber, indicating its potential as a gain medium for constructing erbium-doped fiber amplifiers (EDFAs).

Stable-to-unstable transition in quantum friction

We investigate the frictional force arising from quantum fluctuations when two dissipative metallic plates are set in a shear motion. While early studies showed that the electromagnetic fields in the quantum friction setup reach nonequilibrium steady states, yielding a time-independent force, other works have demonstrated the failure to attain steady states, leading to instability and time-varying friction under sufficiently low-loss conditions. Here, we develop a fully quantum-mechanical theory without perturbative approximations, and we unveil the transition from stable to unstable regimes of the quantum friction setup. Due to the relative motion of the plates, their electromagnetic response may be active in some conditions, resulting in optical gain. We prove that the standard fluctuation-dissipation leads to inconsistent results when applied to our system, and, in particular, it predicts a vanishing frictional force. Using a modified fluctuation-dissipation relation tailored for gain media, we calculate the frictional force in terms of the system Green's function, thereby recovering early works on quantum friction. Remarkably, we also find that the frictional force diverges to infinity as the relative velocity of the plates approaches a threshold. This threshold is determined by the damping strength and the distance between the metal surfaces. Beyond this critical velocity, the system exhibits instability, akin to the behavior of a laser cavity, where no steady state exists. In such a scenario, the frictional force escalates exponentially. Our findings pave the way for experimental exploration of the frictional force in proximity to this critical regime. Published by the American Physical Society 2024

Watt-level long-term stable ultra-narrow linewidth 1064 nm single-longitudinal-mode ring cavity fiber oscillator

This study presents a high-power, single-longitudinal-mode (SLM) fiber oscillator with a ring cavity design, operating at 1064 nm. Utilizing a double-cladding ytterbium-doped fiber as the gain medium, the system incorporates a fiber Bragg grating Fabry-Perot cavity and a dual coupler ring for step-by-step filtering to achieve SLM operation. With a pump power of 4.19 W, the oscillator delivered an output power of 1.01 W and narrowed the linewidth to 147 Hz, with the potential for further power increase. The oscillator demonstrated excellent longitudinal-mode stability, maintaining mode-hop-free operation for 8.7 hours after temperature stabilization. To the best of our knowledge, this work represents the longest duration of mode-hop-free operation and the narrowest laser linewidth achieved by a free-running ring-cavity SLM fiber oscillator at watt-level output powers.

Absorber-Free Mode-Locking of a Hybrid Integrated Diode Laser at Sub-GHz Repetition Rate

We demonstrate absorber-free passive and hybrid mode-locking at sub-GHz repetition rates, using a hybrid integrated extended cavity diode laser operating near 1550 nm. The laser is based on InP as a gain medium and a Si3N4 waveguide feedback circuit. Absorber-free Fourier domain mode-locking with ≈15 comb lines at around 0.2 mW total power is achieved with repetition rates around 500 MHz, using three highly frequency-selective micro-ring resonators that extend the on-chip cavity length to 0.6 m. To stabilize the repetition rate, hybrid mode-locking is demonstrated by weak RF modulation of the diode current. The RF injection reduces the Lorentzian linewidth component from 8.9 kHz to a detection-limited value of around 300 mHz. To measure the locking range of the repetition rate, the injected RF frequency is tuned with regard to the passive mode-locking frequency and the injected RF power is varied. The locking range increases approximately as a square-root function of the injected RF power. At 1 mW injection, a wide locking range of about 80 MHz is obtained. We also observe the laser maintaining stable mode-locking when the DC diode pump current is increased from 40 mA to 190 mA, provided that the cavity length is maintained constant with thermo-refractive tuning.

Efficient single-frequency Tm:YAG crystal-derived silica fiber laser at 1.7 µm.

A single-frequency distributed Bragg reflector (DBR) fiber laser operating at 1720 nm has been demonstrated for the first time using a Tm:YAG crystal-derived silica fiber (TCDSF), to the best of our knowledge. A single-frequency laser with an over 220 mW output power was achieved from a 1.5-cm-long TCDSF when being in-band pumped by a homemade 1610 nm fiber laser. The slope efficiency reached 25.30% for the absorbed pump power. The optical signal-to-noise ratio (OSNR) of the laser was ∼69.78 dB, and the laser linewidth was ∼84.5 kHz. Additionally, the measured relative intensity noise (RIN) remained at -130 dB/Hz at frequencies above 9 MHz. The experimental results indicate that the TCDSF is a promising gain medium for single-frequency lasers at 1.7 µm.

Gain performance and thermal effects of Nd:Glass and Nd,Y:SrF2 crystal

The gain performance and thermal effects of Nd:glass (N31) and Nd,Y:SrF2 crystal are investigated and compared. The results show that, under the same pump conditions, Nd,Y:SrF2 can provide a small-signal gain similar to that of N31. The main advantages of Nd,Y:SrF2 are its weaker thermal effects and greater thermal-fracture limitations compared with those of N31. In this paper, the two gain media are compared in the form of rods whose diameter is 5 mm and length is 60 mm. The small-signal gain coefficients of N31 and Nd,Y:SrF2 are 1.39 and 1.46, respectively, at a pump energy of 1.5 J/1 Hz. The pump energy is set at 1.5 J/8 Hz to experimentally investigate their thermal effects. The thermal wavefront of N31 is 0.987λ at a repetition rate of 10 Hz, whereas that of Nd,Y:SrF2 is 0.679λ. In thermal destructive experiments, N31 fractured at an average pump power of 15 W (1.5 J/10 Hz), whereas Nd,Y:SrF2 fractured at a higher power of 27 W (1.5 J/18 Hz) owing to its excellent thermal conductivity, which is 7.28 times that of N31. These results indicate the potential of Nd,Y:SrF2 crystal as a gain medium for high-repetition-rate laser amplifiers.

Ultra‐Broadband Emission in Bi/Ge Co‐Doped Silica Glass and Fiber via Bismuth Coordination Engineering

AbstractBismuth (Bi) and Germanium (Ge) co‐doped silica glass and fiber, as advanced gain media with broadband near‐infrared (NIR) emission and amplification, have promise for extending communication bandwidth. However, efficiently modulating the NIR emissions of bismuth to cover the C+L communication bands remain a significant challenge. In the study, a high‐temperature and high‐pressure reduction treatment on Bi/Ge co‐doped silica glass is employed to tailor the coordination environment around bismuth active center. This method facilitated the creation of new bismuth NIR luminescence centers, resulting in the luminescence spectrum with a peak position at 1550 nm and a FWHM exceeding 350 nm. The changes in the bismuth coordination environment are elucidated using HRTEM, photoluminescence decay, temperature‐dependent emission, EXAFS and CW‐EPR. Furthermore, the feasibility of this method in Bi/Ge co‐doped silica fiber is validated, and obtained >5 dB amplification in the range of 1400–1700 nm. This coordination engineering method holds significant potential for widespread application in Bi/Ge co‐doped silica glass and optical fiber is believed. It presents a promising prospect for expanding communication bandwidth by effectively modulating the NIR emissions of bismuth to cover the S to U communication band.

All-solid continuous-wave Nd: YVO4-diamond intracavity Raman laser

We demonstrated a continuous intracavity diamond Raman laser operating by linear-cavity and V-cavity design. The diamond crystal as the Raman medium and Nd: YVO4 as the laser gain medium was utilized to achieve high beam quality Stokes output with narrow line width. By employing linear and V-shaped cavity, we achieved Stokes light outputs of 1.68 and 2.26 W, respectively. Compared to the linear cavity, the V-shaped cavity has a higher conversion efficiency of 11.07% and a narrower linewidth of 0.07 nm. Mechanism for linewidth of different cavity design and methods for further increasing power range and high quality are discussed.

Polarization self-modulation in a coaxially end-pumped orthogonally polarized laser

A scheme of orthogonally polarized laser (OPL) with polarization self-modulation (PSM) in the coaxial end-pumping configuration is proposed in this work. The gain medium consisted of two Nd:YVO4 crystals with orthogonal optical axes, and a quarter-wave plate served as a polarization-switching device. By rotating the electric vector during each cavity round trip, frequency degeneration and correlated operation of two polarization components were achieved. The polarization eigenstates in two orthogonal directions were analyzed using the Jones matrix and eigenequation. The spectral distribution was also briefly discussed based on thermal effects. The results showed that two eigenmodes located at 45° and 135° to the crystal axis, respectively, with a frequency difference equaled to half the free spectral range of the resonator. In the experiments, it was found for the first time that the modulation of eigenstates can be realized by adjusting pump parameters (including pump power, beam size, focusing position, and pump wavelength) and cavity anisotropy, which enables the manipulation of degree of polarization (DOP) and monitoring the resonator properties. Overall, tuning the laser polarization eigenstate offers a flexible method to switch the operational states of the OPL, including the longitudinal mode frequency difference, DOP, polarization angle, etc. It is believed this scheme provides a valuable reference to realize compact, high-beam-quality, and robust OPLs for applications such as dual-frequency light source development, precision measurement and polarization control.