Gain Medium Research Articles

Monolithically Integrated Ultra‐Low Threshold GeSn‐on‐Insulator Laser Using Rapid Melting Growth

AbstractA low‐threshold, monolithically integrated laser on Si is considered a crucial missing ingredient in realizing efficient fully functional photonic‐integrated circuits (PICs). Owing to its compatibility with complementary metal‐semiconductor‐oxide (CMOS) processes, direct bandgap GeSn alloy has recently been studied intensively in hopes of making GeSn lasers the mainstream technology for PICs. However, the inevitable formation of harmful defects in GeSn directly grown on Si has thus far required the use of non‐monolithic approaches such as wafer bonding to obtain high‐quality GeSn layers, preventing the realization of practical, low‐threshold GeSn lasers. Here, ultra‐low threshold lasing in a monolithically‐grown, nearly‐defect‐free GeSn single‐crystal layer is demonstrated. The rapid melting growth method used in this study allows the fabrication of a compact, integrated laser that simultaneously achieves an ideal GeSn gain medium with built‐in tensile strain and an excellent GeSn optical cavity on an insulating layer. The measured threshold is ≈0.52 kW cm−2 under the optical pumping scheme at 10 K, which is the lowest among all the reported GeSn lasers. This work provides a new solution for building a truly CMOS‐compatible, monolithic laser that can complete the device library of the mature Si photonics foundries.

Secondary Ion Mass Spectrometry Approach for Measuring the Diffusion Range in Optical Fibers with Nanostructured Cores.

A new class of optical fibers with cores composed of various glass nanorods requires a method to determine the internal structure. In particular, the identification of various ion distributions at the fiber cross-sectionwith nanometer accuracy is required. For this purpose, the classical depth profiling mode of the secondary ion mass spectrometry technique is applied. Multiple sampling with various rotation angles allows to retrieve the 1D profile of distribution for particular ions and determine their diffusion ranges. As a proof-of-concept, a nanostructured fiber composed of germanium-doped silica and ytterbium-doped silica rods is measured and the accuracy of the measurement is 3 nm. We show that the average diffusion length of germanium and ytterbium is about 41.5 and 116.6 nm, respectively. It is only 23% and as much as 153% of the initial rod diameter for Ge and Yb ions, respectively. The real diffusion range of particular ions has a significant impact on the refractive and gain media distribution in the fiber and influences the optical properties of the fiber. Therefore, information on diffusion is necessary to design nanostructured optical fibers correctly.

LD-Pumped 228 nm Nd:GdVO4/Cr4+:YAG Passively Q-Switched Solid-State Laser

The 228 nm deep ultraviolet laser, leveraging its advantages of short wavelength, high photon energy, and low thermal effect, can significantly enhance the Raman signal in resonance Raman spectroscopy and demonstrates broad application potential in areas such as precision processing of photonic devices. This paper investigates a solid-state linear-cavity passively Q-switched 228 nm deep ultraviolet laser. Firstly, the laser employs an Nd:GdVO4 crystal as the gain medium, combined with Cr4+:YAG crystal passive Q-switching technology to generate 912 nm pulsed fundamental frequency light. Subsequently, a lithium metaborate (LBO) crystal is used to generate 456 nm second-harmonic light, and finally, a barium metaborate (BBO) crystal is utilized to achieve 228 nm fourth-harmonic laser output. In this paper, we investigate the variation in 456 nm and 228 nm laser output power under the cavity length of 63 mm. Ultimately, at a pump power of 41.75 W, the highest average power of 670 mW was achieved for a 456 nm blue laser output with a repetition rate of 12 kHz and a pulse width of 32 ns. Additionally, a maximum average power of 18 mW was obtained for a 228 nm deep ultraviolet laser output, featuring a repetition rate of 12 kHz and a pulse width of 33 ns.

Narrow-Pulse-Width, Straight-Type-Cavity, All-Solid-State Laser at 228.5 nm

Deep-ultraviolet (DUV) lasers operating at a wavelength of 228 nm offer distinct advantages in Raman spectroscopy and analysis, demonstrating significant potential in the field of surgical medicine. This paper details the development of a high-repetition-rate, narrow-pulse-width, short-cavity laser system functioning at 228.5 nm, which is based on Barium Borate (BBO) electro-optic Q-switching. The system utilizes a double-concave resonator structure and a pressure-applied electro-optic Q-switching technique, incorporating Lithium Borate (LBO) and BBO as frequency-doubling crystals. A low-concentration Nd:YVO4 crystal, measuring 4 mm × 4 mm × 5 mm, serves as the gain medium, with a high-reflectivity coating applied to its left end face to function as the total reflection mirror within the resonant cavity. Upon achieving a pump power of 37 W at a repetition rate of 12 kHz, the system produced a maximum average power of 32 mW, with a pulse width varying from 2.48 ns to 2.70 ns and a central wavelength of 228.5 nm, which is effectively applicable for deep-ultraviolet spectral detection.

Fast measurement of group index variation with optimum precision using Hong–Ou–Mandel interferometry

Hong–Ou–Mandel (HOM) interferometry has emerged as a valuable means for quantum sensing applications, particularly in measuring physical parameters that influence the relative optical delay between photon pairs. Unlike classical techniques, HOM-based quantum sensors offer higher resolution due to the intrinsic dispersion cancellation property of correlated photon pairs. Due to the use of single photons, HOM-based quantum sensors typically involve a large integration time to acquire the signal and subsequent post-processing for high-resolution measurements, restricting their use for real-time operations. Based on our understanding of the relationship between measurement resolution and the gain medium length that produces photon pairs, we report here on the development of an HOM-based quantum sensor for high-precision group index measurements. Using a 1 mm long periodically poled KTP (PPKTP) crystal for photon-pair generation, we have measured the group index with a precision of ∼6.75×10−6 per centimeter of sample length at an integration time of 100 ms, surpassing the previous reports by 400%. Typically, the measurement range reduces with the increase in the resolution. However, using a novel scheme compensating photon delay due to group index changes stepwise with an optical delay stage, we have measured the group index variation of PPKTP crystal over a range of 3.5 × 10−3 for a temperature change from 25 to 200 °C, corresponding to an optical delay adjustment of ∼200 μm while maintaining the same precision (∼6.75×10−6 per centimeter of sample length). The current results establish the usefulness of HOM-interferometer-based quantum sensors for fast, precise, and long-range measurements in various applications, including quantum optical coherence tomography.

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.

Fabrication of a Random Laser by Embedding Copper Oxide Nanoparticles in Nonmetallic Host Media.

In this work, a simple, low-cost method for fabricating random gain media from Rhodamine B dye solution containing highly-pure CuO nanoparticles in transparent resin hosts is proposed. The spectroscopic characteristics of the dye solution samples were investigated with varying dye concentrations, along with nanoparticles of different particle sizes and concentrations. The fabricated samples were excited using two laser sources (405 and 530 nm) to record the fluorescence spectra. Our findings indicate that nanoparticles ranging in size from 15 to 78 nm may not have a significant impact on the fluorescence intensity or the peak wavelength at which maximum fluorescence occurs. Additionally, the fluorescence peak width, centered around 595 nm showed minimal sensitivity to the variation in particle size. The optimal concentration of nanoparticles in the dye solution was determined based on the optimum values of scattering spectral width and anisotropy parameter. Subsequently, the gain coefficient was calculated and correlated with these two parameters. It was found that the samples with the highest fluorescence intensity can be achieved using dye dissolved in transparent resin at a molar concentration of 5 × 10-5 M, containing 0.03-0.04 mg of CuO nanoparticles with a particle size of 78 nm.

A Study on the Carbon Dioxide Laser

Abstract: One of the first gas lasers to be created was the carbon-dioxide laser (CO2 laser). One of the most practical kinds of lasers, it was created in 1964 by Bell Labs' Kumar Patel [22]. The most powerful continuous-wave lasers on the market right now are carbon-dioxide lasers. Additionally, they are quite efficient; the output power to pump power ratio can reach 20%. The primary wavelength bands of the infrared light beam produced by the CO2 laser are 9.6 and 10.6 micrometers (μm). [22] In order to improve its performance, a carbon dioxide laser uses carbon dioxide as its main gain medium. Nitrogen (N2), helium (He), and occasionally hydrogen (H2), water vapor, oxygen, or xenon (Xe) are added. This laser works by energizing the gas mixture to produce laser light by promoting the emission of radiation by an electrical gas discharge. In carbon dioxide lasers, the electrical gas discharge can be powered by radio frequency (RF), direct current (DC), or alternating current (AC). The wavelength of the light emitted by these lasers is 10.6 micrometers. In dermatology, they are frequently used for operations like wrinkle reduction, scar removal, and sun damage treatment. Furthermore, for accurate cutting and tissue removal, carbon dioxide lasers are employed as surgical instruments in specialties such as neurosurgery and gynecology.[24]

A Review on The Nitrogen Laser

Abstract: A nitrogen laser is a type of gas laser that uses molecular nitrogen as its gain medium to operate in the ultraviolet (UV) spectrum. First created in 1963, nitrogen lasers were first put to use in commerce in 1972. A nitrogen laser is a type of gas laser that uses molecular nitrogen as its gain medium to operate in the ultraviolet (UV) spectrum. First created in 1963, nitrogen lasers were first put to use in commerce in 1972.Applications for nitrogen lasers can be found in physics, chemistry, and medical research. The principle of operation for nitrogen lasers is a quick electrical discharge through nitrogen gas. Liquid nitrogen or a gas cylinder might be used to supply the nitrogen gas. The UV-wavelength laser light has a short pulse width and a high intensity. The principle of operation for nitrogen lasers is a rapid electrical discharge via nitrogen gas. Liquid nitrogen or a gas cylinder might be used to supply the nitrogen gas. The laser light has a short pulse width, high intensity, and is released into the ultraviolet spectrum. Electricity is used by the nitrogen laser to excite the nitrogen. The electrons in the laser strike the nitrogen atoms in the air when an electric spark passes through a spark gap, stimulating them into a metastable condition. A photon with a wavelength of 337 nm causes stimulated emission and the creation of a laser state in the excited nitrogen atoms. Electricity is used by the nitrogen laser to excite the nitrogen. The nitrogen atoms in the air are excited into a metastable condition when an electric spark passes through a spark gap in the laser. A photon with a wavelength of 337 nm causes stimulated emission and the creation of a laser state in the excited nitrogen atoms. [22]

Sub-100 Hz intrinsic linewidth 852 nm silicon nitride external cavity laser.

We demonstrate an external cavity laser with intrinsic linewidth below 100 Hz around an operating wavelength of 852 nm, selected for its relevance to laser cooling and manipulation of cesium atoms. This system achieves a maximum CW output power of 24 mW, a wavelength tunability over 10 nm, and a side-mode suppression ratio exceeding 50 dB. This performance level is facilitated by careful design of a low-loss integrated silicon nitride photonic circuit serving as the external cavity combined with commercially available semiconductor gain chips. This approach demonstrates the feasibility of compact integrated lasers with sub-kHz linewidth centering on the needs of emerging sensor concepts based on ultracold atoms and can be further extended to shorter wavelengths via selection of suitable semiconductor gain media.

Improved temporal characteristics for post-compressed pulses via application-tailored nonlinear polarization ellipse rotation.

Intense ultrashort laser pulses with high temporal quality are essential for fundamental science. Nonlinear polarization ellipse rotation (NER) is one way to ensure this high temporal quality, by suppressing weaker signals beyond the duration of the main pulse up to a few orders of magnitude. Post-compression schemes have revolutionized ultrafast lasers, enabling the generation of pulses with durations beyond the limit supported by laser gain media. However, high compression ratios lead to the formation of new pre- and post-pulses. Both NER and post-compression depend on the optical Kerr effect. This makes the combination of the two in a single setup both advantageous and straightforward. While NER cannot suppress the new pre- and post-pulses generated in post-compression, we show via simulations and experimental data that by combining the two, it is possible to shape the output spectrum and counteract temporal contrast degradation.

Emission dynamics and spectrum of a nanoshell-based plasmonic nanolaser spaser

Abstract We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle with an active core and a plasmonic metal shell. Using time-dependent equations for the gain medium and metal, we calculate the lasing threshold through an instability analysis. Below threshold, the nanoshell acts as an optical amplifier when excited by an external probe field, while above threshold, it enters a regime of autonomous lasing. At the gain threshold, the lasing starts at one frequency, typically a plasmon resonance of the nanoparticle. With increasing gain, the emission then broadens to additional frequencies. This result contrasts with previous findings reporting only a single emission wavelength above threshold. We also compute the full spectrum and linewidth of the nanolaser, revealing strong frequency shifts and an asymmetrical lineshape. Finally, we demonstrate that the emission line can be tuned across the visible spectrum by modifying the aspect ratio of the nanoshell.

Continuous-wave two-photon terahertz quantum cascade laser

We report on the implementation of a terahertz two-photon quantum cascade laser operating in a continuous wave mode. Lasers that can emit two photons as a result of the relaxation of a single electron between two states of the same parity have been discussed since the early days of the laser era, but implementation has been hampered by the lack of a suitable gain medium. The semiconductor structure of a quantum cascade laser seems to be an ideal medium for realizing such two-photon emission. Our work demonstrates dual-band laser radiation in the range of 3.1–3.9 THz (104–130 cm−1) at temperatures up to 90 K.

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.

Bismuth telluride as a passive Q-switcher for neodymium-doped fiber laser

The scene of fiber laser has progressed very rapidly with the demonstration of various materials as saturable absorbers in a near- and mid-infrared laser cavity. However, the researchers focused on erbium-, ytterbium-, and thulium-doped fiber as a gain medium. Here, we proposed a neodymium-doped fiber laser setup to generate a Q-switched laser in a 1-μm regime. The laser utilized bismuth telluride, a topological insulator as a Q-switcher for pulse generation. With a linear absorption of approximately 5 dB at 1089 nm, this SA can generate a stable Q-switched laser in a neodymium-doped fiber laser cavity. It generates a stable Q-switched laser at 1089 nm when the pump intensity is adjusted between 167 and 190 mW. The pulsed laser owns the shortest pulse width of 6.48 μs with a maximum repetition rate of 40 kHz. The stability of the laser is promising since it has a signal-to-noise ratio of 40 dB. In the authors’ knowledge, this is the first demonstration of bismuth telluride as a saturable absorber in an all-fiberized neodymium-doped fiber laser cavity.

Direct generation of Laguerre-Gaussian modes in an all-solid-state laser

By employing a hybrid scheme that combines annular pumping and off-axis pumping, this study achieves the direct generation of single-handed vortex beams from the laser resonator. The gain medium of the solid-state laser is subjected to annular end-pumping, while off-axis pumping is realized by tilting the output mirror. The adjustment of the gain medium’s longitudinal position ensures the matching of pump light to the Laguerre-Gaussian (LG0n) modes within the cavity. In the experiments, vortex lasers with LG01, LG02, and LG03 modes are successfully obtained at a pump power of 5.5 W. The corresponding maximum output powers are 0.75 W, 0.67 W, and 0.59 W, with optical conversion efficiencies of 13.6 %, 12.2 %, and 10.7 %, respectively. The handedness of the vortex beams is simply reversed by flipping the off-axis displacements. Numerical simulations are conducted to reconstruct the experimental results, employing the theory of coherent superposition of LG modes.

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.