Μm Wavelength Region Research Articles

Generation of mode-locked pulses assisted by reduced graphene oxide/cerium oxide (rGO/CeO2) embedded in arc-shaped fiber

Abstract This study demonstrates the synthesis of a reduced graphene oxide/cerium oxide (rGO/CeO2) composite and its successful use as a saturable absorber (SA) in ultrafast photonics. The rGO/CeO2 composite was synthesized using a facile hydrothermal approach, and an arc-shaped fiber was fabricated using a polishing technique. The arc-shaped fiber employed indirect evanescent field coupling, allowing interaction between the composite material and the laser, which facilitated the generation of mode-locked pulses. An arc-shaped fiber drop-casted with rGO/CeO2 was connected to a 2.0 µm laser source, achieving a modulation depth of 50.16%. The emitted laser pulses have a duration of 1.776 ps, an operational wavelength of 1904 nm, and a spectral bandwidth (FWHM) of 2.76 nm. The pulse energy was 89.36 pJ with a high signal-to-noise ratio (SNR) of 63 dB. This study introduces rGO/CeO2 embedded in an arc-shaped fiber as an effective SA, exhibiting favorable nonlinear optical absorption in the near 2.0 µm wavelength region, making it suitable for applications in ultrafast photonics.

Brillouin laser pumped tunable low-threshold mid-IR Kerr comb at 2 μm

Optical frequency combs in the 2 μm wavelength region are important for applications ranging from sensing of gases such as CO2 and CO to optical communications, LIDAR, and gravitational wave detection. The development of low-loss waveguides and high-Q microresonators with anomalous dispersion and the availability of tunable narrow linewidth lasers around 1.55 μm have enabled the realization of small footprint soliton combs and low-threshold Kerr combs in this wavelength region; demonstrations of microresonator frequency combs in the 2 μm wavelength region have been limited. Here, we harness an intracavity pumping scheme to demonstrate a low-threshold (<100 mW) microresonator Kerr comb at 2 μm. We exploit Brillouin lasing in a silica microsphere (∼310 μm diameter) to create an intracavity pump, which then generates a ∼140 nm wide Kerr comb in the backscattered Stokes direction. We demonstrate the tolerance of the comb generation scheme to microsphere dimensions and the input pump wavelength by achieving Kerr comb generation in microspheres of diameters ranging from 295 to 318 μm and also at different input pump wavelengths for a particular microsphere diameter. Intracavity pumping opens up opportunities for the development of soliton combs and Kerr combs in the mid-IR wavelength region for applications such as dual-comb spectroscopy, LIDAR, and optical communications.

A Brief Look Into Nonlinear Optical Properties Of MXenes (M2X) Ti2C, Nb2C, And V2C Through Four Wave Mixing Generation In 2 µm Fiber Laser

Generation of four wave mixing (FWM) effect in MXenes with chemical ratio of M2X, particularly Ti2C, Nb2C, and V2C were reported in this work. The setup was operated at 2 µm wavelength region with tapered fiber as the material host. The experiment managed to show conversion efficiencies of −43.93 dB, −42.60 dB, and −41.20 dB and idlers’ SNR of 7.73 dB, 9.10 dB, and 9.80 dB for Ti2C, Nb2C, and V2C, respectively. The relation of nonlinear optical properties such as nonlinear absorption coefficient and nonlinear refractive index to FWM generation were explored. The result of the investigation successfully showed the relationship of third order nonlinear optical properties and FWM effect in the specified MXenes with comparisons.

Hybrid integrated GaSb/Si3N4 narrow linewidth (<50 kHz) distributed Bragg reflector laser

A narrow linewidth hybrid integrated distributed Bragg reflector (DBR) laser platform operating at 2 μm wavelength region is demonstrated. The laser architecture comprises AlGaInAsSb/GaSb type-I quantum well reflective semiconductor optical amplifiers butt-coupled to a Si3N4 photonic integrated circuit (PIC), incorporating a narrow-band DBR. The DBR is realized with a long spiral-shaped waveguide structure with periodic circular posts placed adjacent to the waveguide. At room temperature operating conditions, the laser exhibits a maximum continuous wave output power of more than 17 mW for emission near 2 μm. Linewidth properties are analyzed with a heterodyne measurement technique, involving the mixing of the laser signal with a frequency comb phase-locked to an ultra-stable laser. The hybrid laser exhibits a narrow linewidth of ∼8 kHz in 1 ms timescale and ∼50 kHz in 10 ms timescale.

Long-wavelength infrared upconversion time-stretch spectroscopy

High-speed spectroscopy in the molecular fingerprint spectral region (≈6–12 μm) is essential for the detection of ultrafast molecular dynamic processes, rapid combustion analysis, and biological diagnostics. However, ultrafast spectroscopy in the long-wavelength infrared (LWIR) region remains a challenge due to the limitations of laser sources and the lack of ultrafast and sensitive detectors in this wavelength region. Here, we demonstrate broadband LWIR time-stretch spectroscopy, which can realize a single-shot high-speed spectral measurement in a 8–10 μm region, by combining the LWIR femtosecond (fs) light generation and upconversion time-stretching detection with specific dispersive fiber. Broadband tunable fs light generated in the 8–10 μm region is upconverted to the 1.1–1.2 μm near-infrared wavelength via difference-frequency generation with the 1 μm chirped pump pulse. Time-stretch detection of the upconverted light can then be realized by adopting dispersion shifted fiber, which has a superior dispersion-to-loss ratio in the 1.1–1.2 μm wavelength region, as the dispersive medium. As a result, we experimentally demonstrate LWIR time-stretch spectroscopy in the 8–10 μm region with a spectral resolution of 1.07 cm−1, at a rate of 200 kSpectra s−1, which is only limited by the repetition rate of the 1 μm pump source. The demonstration of high-speed time-stretch spectroscopy in the LWIR region would open the possibility in exploring the transient dynamics of molecular fingerprint spectroscopy.

High-energy noise-like pulsing in a thulium/holmium co-doped fiber laser with watt-level average output power

A Thulium/Holmium co-doped fiber laser with high-energy noise-like rectangular pulses at the central wavelength of 1985 nm is experimentally demonstrated. The experimental setup is based on a 298-m long nonlinear optical loop mirror. By varying the pump power from 3 to 10 W, the noise-like rectangular pulse width can be tuned from 2.8 to 15.2 ns, respectively, and under a maximum pump power of 10 W, 1.45 W of average output power is obtained. The pulse repetition rate is 671 kHz. Consequently, highly energetic optical pulses with 2.16 µJ pulse energy and an estimated peak power of 142 W are achieved. To the best of our knowledge, these pulses, which are generated directly from the laser cavity, possess the highest average output power and pulse energy for noise-like pulse emission in the near 2 µm wavelength region. The proposed laser source has the most straightforward all-fiber cavity design that has been proposed for generating high-energy noise-like rectangular pulses, and as such has potential applications in scientific research and as a pump source for mid-infrared supercontinuum generation.

Spectral characterization of a Cryo-Cooled CO laser operating with CO2 laser gas mixture

This study explores the emission spectra and performance characteristics of a cryo-cooled CO laser system operating with CO2 laser gas mixture under various operating conditions, viz., N2 free gas mixture, different coolant temperatures and in grating tuned configuration. The narrow operational window in case of N2 free gas mixture, increased output power and richer emission spectrum obtained with the addition of N2 gas indicates enhanced discharge stability and efficient excitation in presence of Nitrogen. The emission spectrum revealed shifting of the trapping levels towards lower vibrational quantum number with reducing coolant temperature and the same was also corroborated by theoretical calculations. No lasing was observed for vibrational levels V>15 which is attributed to the higher partial pressure of oxygen inherently present in the gain medium formed due to dissociation of CO2. The system lased on more than 20 transitions in grating tuned configuration with power exceeding 100mW on several lines. The presence of higher concentration of O2 in the discharge prevents carbon deposition − a common issue in CO lasers. This study underscores the significance of this unconventional CO laser system offering reliable operation in the 5 µm wavelength region without the usage of extraneous CO gas.

Advancing Q-switched pulse generation in the 1.9 µm wavelength region using novel thulium-doped fiber gain medium and MXene Nb2CTx saturable absorber

This paper presents the first demonstration of a Q-switched Thulium-doped fiber laser (TDFL) utilizing our newly developed Thulium-doped fiber (TDF) and a MXene niobium carbide - polyethylene oxide (Nb2CTx-PEO) thin film as a gain medium and saturable absorber (SA), respectively. The TDF core glass contains 6.15 wt% Al2O3, 2.2 wt% Y2O3, 0.47 wt% Tm2O3, 2.12 wt% GeO2, 1.57 wt% HfO2 and 0.05 wt% Bi2O3 with the absorption of 30 dB/m at 1550 nm. The outcome was the realization of a stable pulsed laser operating at a central wavelength of 1888.3 nm. The Q-switched pulses exhibited a maximum repetition rate of 46.7 kHz and a minimum pulse width of 1.995 µs at a pump power of 731.41 mW. The Q-switched operation maintained a signal-to-noise ratio (SNR) of 45 dB, confirming its stability. This breakthrough highlights the laser system's potential for diverse applications, including free-space optical communication, spectroscopy, and medical fields.

Experimental Study on Energy-Free Superhydrophobic Radiative Cooling Versatile Film with Enhanced Environmental Tolerance.

Clean, energy-free methods of cooling are an effective way to respond to the global energy crisis. To date, cooling materials using passive daytime radiative cooling (RC) technology have been applied in the fields of energy-efficient buildings, solar photovoltaic cooling, and insulating textiles. However, RC materials frequently suffer from comprehensive damage to their microstructure, resulting in the loss of their initial cooling effect in complex outdoor environments. Here, a superhydrophobic daytime passive RC porous film with environmental tolerance (SRCP film) was fabricated, which integrated strong solar reflectivity (approximately 90%), mid-infrared emissivity (approximately 0.97), and superhydrophobicity (water contact angle (WCA) of 160° and sliding angle of 3°). This study revealed that SRCP film had an average reflectivity of 14.3% higher than SiO2 particles in the 0.3-2.5 μm wavelength region, achieving a cooling effect of 13.2 °C in ambient conditions with a solar irradiance of 946 W·m-2 and a relative humidity of 74% due to the synergistic effect of effective solar reflection and thermal infrared emission. In addition, empirical results showed that the attained films possessed outstanding environmental tolerance, maintaining high WCA (156°), stable cooling effect (8.3 °C), and low SiO2 loss (less than 5.1%) after 30 consecutive days of UV irradiation and 14 days of corrosion with acidic and alkaline solutions. More importantly, this work could be flexibly prepared by various methods without the use of any fluorine-containing reagents, which greatly widens the practical application scope.

PDRs4All

Context. Extraordinary observations of the Orion Bar by JWST have shown, for the first time, the incredible richness of polycyclic aromatic hydrocarbon (PAH) emission bands and their variation on very small scales. These variations are the result of photochemical evolution of the PAH carrier. Aims. We aim to probe the photochemical evolution of PAHs across the key zones of the ideal photodissociation region (PDR) that is the Orion Bar using unsupervised machine learning. Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science programme PDRs4All (ID: 1288). We levered bisecting k-means clustering to generate highly detailed spatial maps of the spectral variability in the 3.2–3.6, 5.95–6.6, 7.25–8.95, and 10.9–11.63 μm wavelength regions. We analysed and subsequently described the variations in the cluster profiles and connected them to the conditions of the physical locations from which they arise. We interpreted the origin of the observed variations with respect to the following key zones: the H II region, the atomic PDR zone, and the layers of the molecular PDR zone stratified by the first, second, and third dissociation fronts (DF 1, DF 2, and DF 3, respectively). Results. Observed PAH emission exhibits spectral variation that is highly dependent on the spatial position in the PDR. We find the 8.6 μm band to behave differently than all other bands, which vary systematically with one another. Notably, we find a uniform variation in the 3.4–3.6 μm bands and 3.4/3.3 intensity ratio. We attribute the carrier of the 3.4–3.6 μm bands to a single side group attached to very similarly sized PAHs. Further, cluster profiles reveal a transition between characteristic profile classes of the 11.2 μm feature from the atomic to the molecular PDR zones. We find the carriers of each of the profile classes to be independent, and reason the latter to be PAH clusters existing solely deep in the molecular PDR. Clustering also reveals a connection between the 11 .2 and 6.2 μm bands and that clusters generated from variation in the 10.9–11.63 μm region can be used to recover those in the 5.95–6.6 μm region. Conclusions. Clustering is a powerful and comprehensive tool for characterising PAH spectral variability on both spatial and spectral scales. For individual bands as well as global spectral behaviours, we find ultraviolet processing to be the most important driver of the evolution of PAHs and their spectral signatures in the Orion Bar PDR.

Optimization of dispersions in benzene-core square photonic crystal fibers for mid-infrared supercontinuum generation with very low peak power

An ultra-broadband, very low peak power supercontinuum source is proposed based on a novel benzene-filled structured photonic crystal fiber. The modification of the structural parameters has been highly effective in improving dispersion properties, an ultra-flat all-normal dispersion is achieved with fluctuations of ±1.174 ps/(nm.km) in the 0.458 µm wavelength region. Three fibers with suitable optical properties best meet the requirements of supercontinuum generations with low peak power based on numerical simulation results. With anomalous dispersion regime, the first fiber offers the supercontinuum spectra in the mid-infrared region spanning from 0.784 µm to 3.973 µm (2.539 µm bandwidth at 30 dB) using laser pulses excited at 1.55 µm has a peak power of 150 W. A pulse with a peak power of 280 W propagates in a 5 cm length of the second fiber results in a supercontinuum spectrum covering from 0.744 µm to 1.859 (bandwidth 1.063 µm at 30 dB) at a pump wavelength of 1.3 µm with all-normal dispersion regime. A smooth, highly coherent supercontinuum spectrum spanning from 0.843 µm to 2.517 µm (bandwidth at 30 dB of 1.608 µm) is achieved when the third fiber has all-normal ultra-flat dispersion profile pumped at 1.75 µm wavelength with laser pulse peak power of 350 W. With very low peak power, these proposed three-fiber based broadband supercontinuum sources could be a low-cost alternative to traditional glass-core fiber, which has great potential for mid-infrared spectroscopy.

Harmonic mode-locking based on Tm, Ho:Ca(Gd,Lu)AlO4 laser near 2.1 μm wavelength region

In this paper, a laser diode (LD) pumped passive mode-locking (ML) Tm, Ho:Ca(Gd, Lu)AlO4 laser based on a semiconductor saturable absorber mirror (SESAM) is reported. With the introduced group delay dispersion (GDD) of −4000 fs2, a stable pulse train with the fundamental repetition rate of 105 MHz is achieved. By increasing the pump power, a stable harmonic mode-locking (HML) pulse with the repetition rate of 211.0 MHz and a 4th-order HML pulse train with the repetition rate of 420 MHz are obtained. By adjusting the introduced GDD to −2600 fs2, a stable 2nd-order HML pulse with repetition rate of 190.8 MHz at 2078.5 nm is also obtained. To the best of our knowledge, this is the first report on the HML Tm, Ho:Ca(Gd, Lu)AlO4 lasers and also the first report on LD pumped mode-locking Tm, Ho:Ca(Gd, Lu)AlO4 laser.

Low loss and ultra-broadband design of an integrated 3 dB power splitter centered at 2 µm.

Because chemical gas is sensitive to absorption in the 2µm band, and 2µm matches the absorption band of the remote sensing material, many remote sensors and optical sensors are designed to operate in the 2µm wavelength region. In this paper, we designed an integrated 3dB power splitter centered at 2µm. The study of this device is built on a silicon-on-insulator (SOI) platform. We introduced a subwavelength grating (SWG) to improve the performance of the device. We used the three-dimensional finite-difference time-domain (3D FDTD) method to analyze the effect of the structure on the power splitter. The insertion loss (IL) of the fundamental TE mode is only 0.04dB at 2µm and its bandwidth of IL <0.45d B is 940nm (1570-2510nm). It is suitable for multidomain and all-band photonic integrated circuits at 2µm.

All-fiberized amorphous carbon nitride (a-CNx) based passive Q-switcher

This research presents the first-ever demonstration of a passively Q-switched fiber laser in the 1.5 μm wavelength region using amorphous carbon nitride (a-CNx) as a saturable absorber (SA) in an all-fiber configuration. The a-CNx thin film was prepared using plasma-enhanced chemical vapor deposition. The laser operation showed remarkable stability, achieving a repetition rate of 57.99 kHz and a pulse duration of 1.36 μs at maximum input pump power. The peak power and pulse energy reached 100.56 mW and 136.77 nJ, respectively. The utilization of a-CNx as an SA offers potential benefits such as a simpler and more flexible cavity design, making it an attractive candidate for various applications, including material processing and medical laser equipment. This work contributes to the advancement of fiber lasers and expands the possibilities for utilizing carbon nitride-based SAs in practical laser systems.

Downconversion luminescence studies in Er3+ activated P2O5 + Gd2O3 + NaF + AlF3 glasses for 1.54 μm NIR laser applications

A series of PGNAEr:(60-x)P2O5 + 7Gd2O3 + 25NaF + 8AlF3 + xEr2O3 (here, x = 0.1, 0.5, 1.0, 2.0 and 4.0 mol%) glasses have been successfully fabricated using the melt quenching technique. X-ray diffractometer has been used to study the amorphous-like structure of the PGNAEr glasses. Spectral measurements of optical absorption and excitation of PGNAEr glasses are studied in both visible and near-infrared wavelength range. The density (2.87–3.16 (g/cm3)) and refractive index (1.535–1.544) are found to increase with Er2O3 concentration, and the absorption spectrum of PGNAEr20 glass plays a vital role in evaluating Ωλ (λ = 2, 4, 6) and laser parameters. The down conversion emission process of PGNAEr glasses is examined in the near-infrared (1400–1700 nm) range upon the radiations at 522 nm (visible) and 980 nm (near-infrared) wavelengths. The emission at 1.54 μm (1545 nm) is observed to be enhanced by a factor of 7 when excited at 522 nm compared to the 980 nm laser source. The observed near-infrared emission from the PGNAEr glass system covers the C + L (1530–1600 nm) bands and third telecommunication window (1440–1530 nm). Using the McCumber’s theory, the emission cross-section is calculated to be around 4.20 × 10−20 cm2 from the absorption cross-section of 2.70 × 10−20 cm2 for the PGNAEr glass system. The calculated gain bandwidth (636 × 10−28 cm3) and gain cross-section (6.93 × 10−21 cm2) indicate that PGNAEr glasses are a promising candidate for optical gain media in the 1.54 μm wavelength region.

120 nm wide band tunable pulsed laser generation employing vanadium carbide MXene/polyvinyl alcohol (V2C-PVA) film

MXene, a group of 2D materials, has garnered significant attention from researchers due to its impressive characteristics, such as large surface area, high metallic conductivity, and strong nonlinear saturable absorption. These properties make MXene an excellent material for exploring new possibilities in ultrafast photonics technology. The present study has demonstrated that vanadium carbide (V2C) MXene can function as a saturable absorber (SA) and effectively generate Q-switching pulses in the 1.9 μm wavelength region. The molten salt synthesis method was used to synthesize V2C MXene, which involved the selective etching of aluminum (Al) layers from the V2AlC MAX Phase precursor. The V2C MXene was transformed into a thin film by mixing it with polyvinyl alcohol (PVA) using a solution casting technique. The resulting V2C-PVA film was found to have saturable absorption properties, with a modulation depth of 8% and saturation intensity of 1.6 MW cm −2. Upon integrating the V2C-PVA film into the Thulium/Holmium-doped fiber laser (THDFL) cavity, stable Q-switching pulses were realized at a central wavelength of 1896.9 nm with 13.9 nm of 3-dB spectral bandwidth. At the maximum pump power of 448.7 mW, the 2.2 μs of pulse duration, 58.26 kHz of repetition rate, and 31 nJ of pulse energy were achieved. By adjusting the tunable bandpass filter (TBPF) integrated within the THDFL cavity, the system has a tunable spectral range of approximately 120.75 nm, from 1889.75 nm to 2010.5 nm. The exceptional performance of V2C-based SA for Q-switching operation showcases the immense potential of other MXene materials in the future of photonics applications.

Remote Measurements of Industrial CO2 Emissions Using a Ground-Based Differential Absorption Lidar in the 2 µm Wavelength Region

Carbon dioxide (CO2) is a known greenhouse gas and one of the largest contributors to global warming in the Earth’s atmosphere. The remote detection and measurement of CO2 from industrial emissions are not routinely carried out and are typically calculated from the fuel combusted or measured directly within ducted vents. However, these methods are not applicable for the quantification of fugitive emissions of CO2. This work presents the results of remote measurement of CO2 emissions using the differential absorption lidar (DIAL) technique at a wavelength of ~2 µm. The results from the DIAL measurements compare well with simultaneous in-stack measurements, these datasets were plotted against each other and can be described by a linear regression of y (t/h) = 1.04 x − 0.02, suggesting any bias in the DIAL data is likely small. Moreover, using the definition outlined in EN 15267-3 a lower detection limit of 0.12 t/h was estimated for the 2 µm wavelength DIAL data, this is three orders of magnitude lower than the corresponding CO2 detection limit measured by NPL in the 1.5 µm wavelength region. Thus, this paper demonstrates the feasibility of high-resolution, ground-based DIAL measurements for quantifying industrial CO2 emissions.

Low-loss operation of silicon-on-insulator integrated components at 2.6-2.7 µm.

Development of mid-infrared photonics is gaining attention, driven by a multitude of sensing applications requiring increasingly compact and cost-effective photonics systems. To this end, low-loss operation of µm-scale silicon-on-insulator photonic integration elements is demonstrated for the 2.6-2.7 µm wavelength region. The platform utilizes the 3 µm thick silicon core layer technology enabling demonstration of low-loss and low birefringence waveguides. Measurements of record low single mode waveguide propagation losses of 0.56 ± 0.09 dB/cm and bend losses <0.08 dB for various miniaturized bend geometries are presented and validated by simulation. Furthermore, a wavelength filter based on echelle grating that allows to select several operating channels within the 2.64-2.7 µm band, with a linewidth of ∼1.56 nm for each channel is presented.

Improved device performance of interband cascade lasers with hybrid cladding layers operating in the 3–4 µm wavelength region

In this work, we briefly review the development and status of interband cascade lasers (ICLs) as related to long-standing issues due to the InAs/AlSb superlattice cladding. By focusing on a hybrid cladding approach to alleviate these issues, we demonstrate substantially improved device performance of ICLs compared to earlier reported ICLs of a similar design in the 3–4 µm wavelength region. These improvements include a threshold current density for broad-area devices as low as 134 A/cm2 at 300 K and reduced threshold voltage with a peak voltage efficiency of 80%, which is more than 10% higher than that obtained from previously reported ICLs. Moreover, we have demonstrated continuous wave (cw) operation of a broad-area device up to 278 K, the highest cw operating temperature among epi-side up mounted broad-area type-II ICLs, implying improved thermal dissipation with the hybrid cladding approach. Additionally, by conducting a comparative study of ICLs with different GaSb layer thicknesses in the hole injector, we reveal and discuss an interesting correlation between the carrier transport, threshold voltage, and hole-induced absorption loss, which may help to guide device optimization for operation in a targeted temperature range.

Discretely Tunable (2594, 2629, 2670 nm) GaSb/Si3N4 Hybrid Laser for Multiwavelength Spectroscopy

AbstractA discrete, tunable photonic integrated laser is showcased for multiwavelength spectroscopy of CO2, H2S, and H2O. The laser combines an AlGaInAsSb/GaSb type‐I quantum well‐reflective semiconductor optical amplifier with a Si3N4 photonic integrated circuit (PIC). Operating at room temperature, the laser emits at 2670.42, 2629.12, and 2594.27 nm, with mW‐level average powers. The PIC employs a novel approach for achieving switching between three distinct emission wavelengths by using two cascaded tunable Mach–Zehnder interferometers (MZIs) that are connected to three spiral‐shaped narrow‐band distributed Bragg reflectors (DBRs). Phase tuning the arms of the MZIs allows the laser emission wavelength to be switched between the DBRs, while the DBRs themselves offer fine‐tuning of the emission wavelength. This design enables a simpler tuning mechanism with fewer control variables compared to hybrid PIC‐based lasers using Vernier architectures or a combination of multiple semiconductor amplifiers. Low‐loss and broadband MZIs are achieved by employing two interconnected directional couplers, which utilize asymmetric waveguides in the coupling region to effectively control the phase. Besides achieving state‐of‐the‐art performance for Si3N4‐based integrated lasers at 2.6 µm wavelength region, the demonstrated performance of passive components, including MZIs and spiral DBRs, opens new possibilities for mid‐infrared PIC technology.