Μm Wavelength Region Research Articles

Q-switched fiber laser with tunable wavelength operation utilizing a nonlinear saturable absorption of vanadium pentoxide

Q-switched fiber laser with a tunable wavelength output is experimentally demonstrated utilizing a nonlinear saturable absorption of vanadium pentoxide (V2O5) in conjunction with a tunable bandpass filter (TBF). The V2O5 saturable absorber (SA) was prepared by mixing V2O5 with polyethylene glycol. The SA film exhibits a modulation depth of 7% with a saturation intensity of 90 MW/cm2. By integrating the SA into an erbium-doped fiber laser cavity while controlling the passband wavelength of the TBF, a Q-switched pulsed laser is realized with a tunable wavelength range of 30 nm that extends from 1535 to 1565 nm. At 1565 nm operation, the laser exhibits the highest slope efficiency of 8.31%. The repetition rate varies from 13.32 to 15.25 kHz, while the pulse width shrinks from 21.3 10 12.75 μs when the pump power rises from 14.9 to 37.7 mW. The pulse energy is 102.6 nJ at the maximum pump power of 37.7 mW. This is the first demonstration of a tunable Q-switched fiber laser operating in a 1.5 μm wavelength region utilizing V2O5 SA.

Nanoscale Engineering of Optical nonlinearity Based on a Metal Nitride/Oxide Heterostructure.

The abilities to modulate linear and nonlinear optical response of materials in the nanoscale are of central importance in the design and fabrication of photonic devices for applications like optical modulators. Here, based on a simple transition metal oxide/nitride (TiO2/TiN) system, we show that it is possible to tune the optical properties by controlling the nanoscale architecture. Through controlled oxidation of the plasmonic TiN nanoparticle surfaces, we observe a continuous change of linear and nonlinear optical (NLO) properties with the increase of the thickness of the oxide layer in the TiN/TiO2 heterogeneous architecture. The NLO response is manifested by the strong saturable absorption with a structurally tunable negative NLO absorption coefficient. The variation in the NLO absorption coefficient by up to 7-fold can be connected to the relative change in the volume fraction of the metallic core and the dielectric shell. We demonstrate further that the optimized TiN-TiO2 heterostructures can be used to drive an optical switch for pulse laser generation in the 1.5 μm wavelength region. Our results delineate a topochemical process for optimization of the NLO properties of common plasmonic materials for photonic applications based on simple materials chemistry.

Fabrication of Ge-ZnS multilayered optical filters for mid-infrared applications

Germanium/Zinc Sulphide (Ge/ZnS) single and multilayers are deposited by ion assisted electron beam deposition on Silicon substrates for applications in the 2-15 µm wavelength region. Refractive indices of Ge and ZnS thin films are extracted from measured reflectance and transmittance spectra. Several types of mid-infrared filters (high-, anti-reflection coatings and dichroic filters) with a number of layers ranging from 3 to 29 are then designed and deposited. The optical responses of fabricated mid-infrared filters show a good agreement between experimental spectra and simulation in the case of high- and anti-reflection coatings in the long-wave infrared but also for dichroic filters with a low transmittance in the mid-wave infrared and broadband bandpass in the long-wave infrared.

Erbium energy levels in GaN grown by hydride vapor phase epitaxy

Erbium doped GaN (Er:GaN) is a promising candidate as a novel gain medium for solid-state high energy lasers (HELs) due to its superior physical properties over a synthetic garnet such as Nd:YAG. Er:GaN emits in the 1.5 µm region, which is retina-safe and has a high transmission in the air. We report photoluminescence (PL) studies performed on Er:GaN epilayers synthesized by the hydride vapor phase epitaxy (HVPE) technique. The room temperature PL spectra of HVPE grown Er:GaN epilayers resolved as many as 11 and seven emission lines in the 1.5 µm and 1.0 µm wavelength regions, respectively, corresponding to the intra-4f shell transitions between Stark levels from the first (4I13/2) and the second (4I11/2) excited states to the ground state (4I15/2) of Er3+ in GaN. The observed peak positions of these transitions enabled the construction of the detailed energy levels in Er:GaN. The results agree well with those of the calculation based on a crystal field analysis. Precise determination of the detailed energy levels of the Stark levels in the 4I11/2, 4I13/2, and 4I15/5 states is critically important for the realization of HELs based on Er:GaN.

Infrared signature of NEPE, HTPB rocket plume under varying flight conditions and motor size

In this study, radiation signatures of rocket plumes based on HTPB and NEPE propellants are simulated under varying altitudes, flight Mach numbers, and motor size. The flow field of plume is calculated by computational fluid dynamics, and the infrared signature is predicted using a Ludwig model. To verify the numerical method, experiments were conducted using small rocket motors and radiometric FTIR. The acceptable agreement between numerical calculation and the experiment was obtained in 2–5 μm wavelength region. The total radiance decreases with increasing altitude whether afterburning exists or not. A decreasing rate in the radiance of the plume with strong afterburning is larger than that of the plume with weak afterburning. The change in total intensity according to altitude depends on the afterburning and is directly affected by the radiating area. As the flight Mach number increases, total radiance, total intensity, and radiating area decrease. The motor size makes radiation intensity increase exponentially; exponent value is from 2.6 to 2.7. As the motor size increases, the effect of altitude increases, while the effect of flight Mach number decreases.

Bayesian analysis of Juno/JIRAM's NIR observations of Europa

Decades of observations of Europa's surface in the near-infrared (NIR), spanning spacecrafts like Galileo, Cassini and New Horizons, along with ground based observations, have revealed a rich mixture of species on Europa's surface. Adding to the NIR data of Europa, Juno spacecraft's spectrometer JIRAM has observed it in the 2–5 μm wavelength region. Here we present analysis of select spectra from this dataset, focusing on the two forms of water-ice - amorphous and crystalline. We were limited in our ability to include other dominant Europan species, like acid hydrate, due to unavailability of their optical constants over the entire JIRAM wavelength range. We also take this as an opportunity to present a novel Bayesian spectral inversion framework. Traditional spectral fitting methods, for example a grid-based search of parameter space, lack a systematic way to quantify detection significances of the species included in the model, statistically constrain surface properties and explore degenerecies of solution. Our Bayesian inference framework overcomes these shortcomings by confidently detecting amorphous and crystalline ice in the JIRAM data and permits probabilistic constraints on their compositions and average grain sizes to be obtained. We first validate our analysis framework using simulated spectra of amorphous and crystalline ice mixtures and a laboratory spectrum of crystalline ice. We next analyze the JIRAM data and, through Bayesian model comparisons, find that a two-component, intimately mixed model of amorphous and crystalline ice, henceforth referred to as TC-IM, is strongly preferred (at 26σ confidence) over a two-component model of the same materials but where their spectra are areally/linearly mixed. We also find that the TC-IM model is strongly preferred (at > 30σ confidence) over single-component models with only amorphous or crystalline ice, indicating the presence of both these phases of water ice in the data. Given the high SNR of the JIRAM data, abundances and grain sizes of amorphous and crystalline ice are very tightly constrained for the analysis with the TC-IM model. The solution corresponds to a mixture with a very large number density fraction (99.952−0.001+0.001 %) of small (23.12−1.01+1.01 microns) amorphous ice grains, and a very small fraction (0.048−0.001+0.001 %) of large (565.34−1.01+1.01 microns) crystalline ice grains. The overabundance of small amorphous ice grains we find is consistent with previous studies. The maximum-likelihood spectrum of the TC-IM model, however, is in tension with the data in the regions around 2.5 and 3.6 μm, and indicates the presence of non-ice components not currently included in our model. Our new technique therefore holds the promise of being able to identify these minor species hiding in Europan reflectance data in future work and constrain their abundances and physical properties.

Molecular water detected on the sunlit Moon by SOFIA

Widespread hydration was detected on the lunar surface through observations of a characteristic absorption feature at 3 µm by three independent spacecraft1–3. Whether the hydration is molecular water (H2O) or other hydroxyl (OH) compounds is unknown and there are no established methods to distinguish the two using the 3 µm band4. However, a fundamental vibration of molecular water produces a spectral signature at 6 µm that is not shared by other hydroxyl compounds5. Here, we present observations of the Moon at 6 µm using the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). Observations reveal a 6 µm emission feature at high lunar latitudes due to the presence of molecular water on the lunar surface. On the basis of the strength of the 6 µm band, we estimate abundances of about 100 to 400 µg g−1 H2O. We find that the distribution of water over the small latitude range is a result of local geology and is probably not a global phenomenon. Lastly, we suggest that a majority of the water we detect must be stored within glasses or in voids between grains sheltered from the harsh lunar environment, allowing the water to remain on the lunar surface. The Stratospheric Observatory for Infrared Astronomy (SOFIA) looked at the Moon in the 6 µm wavelength region and found a signature of molecular water, distinguishing it from other forms of hydration. The authors estimate water abundances between 100 and 400 µg g−1 at high latitudes, trapped within impact glasses or possibly in between grains.

Application of PRISMA satellite hyperspectral imagery to mineral alteration mapping at Cuprite, Nevada, USA

The PRISMA hyperspectral imaging satellite of the Italian Space Agency was launched into orbit on March 22, 2019. The PRISMA is a pushbroom sensor that records 250 hyperspectral bands in the 0.4-2.5 μm wavelength region at a spatial resolution of 30 m. The swath of the hyperspectral imagery is 30 km. This study evaluates the application of the PRISMA hyperspectral imagery to mineral exploration. The study area is the Cuprite in Nevada, USA. Cuprite has served as test-site for a number of airborne and spaceborne remote sensing imaging systems. The Cuprite PRISMA hyperspectral data were analyzed with the Advanced Coherence Estimator algorithm. The analysis of the hyperspectral imagery accurately mapped the spatial distribution of alunite, kaolinite, hydrated silica, muscovite and buddingtonite. The study shows that the PRISMA hyperspectral imagery is a useful tool for mineral exploration projects in arid and semi-arid environments.

Performance Analysis on Single- and Double-Pass Ytterbium-Doped Fiber Amplifier in the 1 µm Region

ABSTRACT In this work, single- and double-pass configurations of ytterbium-doped optical fiber amplifiers (YDFAs) have been investigated. This low quantum defect rare-earth dopant and their performance near 1 µm region has been comprehensively investigated on the basis of gain and noise figure in relation with input signal power and input pump power. The obtained results showed that at an input pump power of 100 mW, the gain was proportionally increased with ytterbium-doped fiber (YDF) length for both proposed configurations, but double-pass regime scored higher gain. This is due to the double-propagation of the forwarded and amplified spontaneous emission (ASE) signal into the active medium and hence will maximize the attainable gain near the 1 µm wavelength region. At YDF length of 5 m, the double-pass technique showed better amplification performance at a lower input signal power of −20 dB compared to the higher input signal power of 0 dB. The maximum gain and optical signal-to-noise ratio (OSNR) attained by double-pass configuration were 24.6 dB and 53.4 dB, respectively. Double-pass amplification at input signal power of −20 dB has increased the gain by 11.2 dB in comparison with the single-pass technique.

Semiconductor quantum plasmons for high frequency thermal emission

AbstractPlasmons in heavily doped semiconductor layers are optically active excitations with sharp resonances in the 5–15 μm wavelength region set by the doping level and the effective mass. Here, we demonstrate that volume plasmons can form in doped layers of widths of hundreds of nanometers, without the need of potential barrier for electronic confinement. Their strong interaction with light makes them perfect absorbers and therefore suitable for incandescent emission. Moreover, by injecting microwave current in the doped layer, we can modulate the temperature of the electron gas. We have fabricated devices for high frequency thermal emission and measured incandescent emission up to 50 MHz, limited by the cutoff of our detector. The frequency-dependent thermal emission is very well reproduced by our theoretical model that let us envision a frequency cutoff in the tens of GHz.

Chemical vapor deposition synthesized monolayer MoS2 for Q-switching pulse generation in the 2 µm spectral region

We demonstrate a monolayer MoS2-based saturable absorber (SA) synthesized by chemical vapor deposition (CVD). The as-grown SA has high spatial homogeneity and shows excellent saturable absorption properties in the 2 µm wavelength region. A higher modulation depth of 21.3% and lower saturation intensity of 0.47 MW/cm2 are achieved compared with those of other documented MoS2 SAs to date. When applying the MoS2 SA to a Tm:YAG ceramic laser, we observe a Q-switched pulse train with a shortest pulse duration of 388.0 ns under a pulse repetition rate of 82 kHz, which corresponds to a single pulse energy of 5.4 uJ and a peak power of 14 W. We believe that CVD constitutes an effective method towards high-quality MoS2 saturable absorbers that possess remarkable application potential in the 2 µm wavelength region.

Multiwavelength Brillouin Generation in Bismuth-Doped Fiber Laser With Single- and Double-Frequency Spacing

Multiwavelength fiber lasers operating in the 1.3 μm wavelength region are of great interest as they complement existing systems in the conventional optical communication band of 1.55 μm. In this work, a multiwavelength Brillouin fiber laser with single- and double-Brillouin frequency spacings operating in 1.3 μm is proposed and demonstrated using a dispersion compensating fiber (DCF) as the Brillouin gain medium. A bismuth-doped fiber serves as the laser gain medium. At the maximum pump power of 1021 mW and with the Brillouin pump set at 11 dBm, up to 7 Brillouin Stokes lines with a wavelength spacing of 0.074 nm are obtained in the single-spaced configuration. In the case of the double-spaced configuration, 14 Stokes lines are generated with a wavelength spacing of 0.15 nm between the even Stokes lines. By tuning the central wavelength of the Brillouin pump, both laser configurations are wavelength-tunable with a range of 93 nm and 19 nm for the single-spaced and double-spaced Brillouin lasers respectively. This is, to the best of the author's knowledge, the first demonstration of a multiwavelength/Brillouin laser using a bismuth-doped fiber as the laser gain medium.

Design of a large-mode-area pixeled leakage channel fiber with an ultrahigh loss ratio for 2 µm operation.

A novel bend-resistant large mode area fiber with pixeled structure and leakage channel is proposed for application with a tight bend radius at the 2 µm wavelength region. Two layers of discontinuous fluorine-doped silica rods, which are called pixels, are introduced in the cladding. By removing four pixels, the loss of high-order modes is enhanced significantly due to the leakage channel. Bend properties of the fiber are investigated thoroughly by numerical simulations. As a result, a robust single-mode operation is obtained with an ultrahigh loss ratio (the minimum loss of high-order modes to the loss of fundamental mode) exceeding 3×105 when the fiber is bent with a tight radius as 10 cm. The effective mode area can reach 1764µm2. In addition, the fiber can keep high performance within a broad waveband from 1.75 to 2.15 µm. Large mode area and robust single-mode operation with a tight bend radius make this fiber beneficial to the development of 2 µm miniaturized fiber lasers.

Study on water absorption characteristics of cubic Y2O3 films

Abstract Based on the electron beam evaporation with oxygen plasma assistance, Y2O3 films were prepared at different deposition temperatures and oxygen flow rates. X-ray diffraction showed that all deposited Y2O3 films had cubic phase with (2 2 2) plane preferred orientation. Spectrometer test results showed that water absorption decreases in transmittance and reflectance around the wavelength of 2.9 μm. According to Lorentz dispersion theory, a multi-oscillator model was established to calculate optical constants and the inhomogeneity of Y2O3 films in the wavelength range from 2.5 to 4 μm. The results indicate an abnormal dispersion phenomenon is appearing in the 2.5–4 μm wavelength region. It was found that under the same oxygen flow rate, higher substrate temperature leads to a greater refractive index and smaller extinction coefficient. When the oxygen flow rate is increased at the same substrate temperature, the extinction coefficient remains almost unchanged, while the refractive index decreases. In addition, the refractive index and the inhomogeneity of the films increase almost linearly as the grain size increases.

A new method for atmospheric correction and de-noising of CRISM hyperspectral data

We propose a new method to perform atmospheric correction and de-noising on hyperspectral image cubes acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board NASA’s Mars Reconnaissance Orbiter (MRO). The CRISM imager has had an important role in advancing our understanding of many aspects of Martian mineralogy. Many mineral detections from CRISM data have been facilitated by significant efforts in the development of the CRISM data processing pipeline to retrieve surface reflectance. However, some residuals remain in CRISM spectra after atmospheric correction, causing difficulty in the interpretation of processed reflectance spectra. In addition, CRISM images are occasionally corrupted with high noise levels exhibiting heterogeneous statistical properties. This paper identifies the cause of such spectral distortions and describe a technique that simultaneously performs both atmospheric correction and de-noising for each image cube individually. In particular, our method focuses on the 1.0–2.6 μm wavelength region of CRISM images and is applicable to images of non-icy surfaces. Experimental results show that our technique is able to significantly mitigate noise and distortions from various sources like gaseous absorptions, detector temperature, and water ice aerosols, compared with the atmospheric correction method in the CRISM official processing pipeline called volcano scan correction, for a variety of scenes. Careful validations that include the qualitative examination of noise and artifacts both on ratioed and non-ratioed spectra and comparison using multiple overlapping images strengthen confidence in our approach.

Passively Q-switched Er:Lu2O3 laser with MXene material Ti4N3Tx (T = F, O, or OH) as a saturable absorber.

In this Letter, we report a titanium nitride (Ti4N3Tx) passive Q-switched Er:Lu2O3 laser. The homemade two-dimensional Ti4N3Tx saturable absorber shows excellent passive Q-switching performance around a 2.85 µm wavelength region. Under the absorbed pump power of 7.4 W, the passive Q-switching laser yields a maximum output power of 0.778 W at a pulse repetition rate of 113.7 kHz, corresponding to a single pulse energy of 6.84 µJ and peak power of 24.57 W.

Optical frequency combs operating at 2 μm wavelength are used for dual-comb spectroscopy

A quantum well diode laser optical frequency comb is used to perform dual-comb spectroscopy measurements in the 2 μm wavelength region with 50 mW of optical power.

Analytical Model of a Holmium-doped Fiber Laser Including Up-conversion Transition

This paper presents theoretical investigation of a holmium-doped fiber laser. Analytical model is developed to consider the effect of up-conversion transition on the laser performance. In this model, the core pumped system is simulated at in-band pumped scheme to optimize the fiber length, pump wavelength, pump power threshold and laser wavelength. A good agreement is achieved between the simulation findings and the experiment results. The results indicate the negative effect of up-conversion transitions on the performance of holmium-doped fiber lasers via reducing the laser efficiency. Understanding the impact of this interaction is helpful to describe the reason for obtaining a lower laser efficiency than expected in experimental reports of holmium-doped fiber lasers pumped at 1.95µm wavelength region. The developed model is suggested as valid tools to simulate and optimize any rare-earth doped fiber lasers and amplifiers.

GeSe Evanescent Field Saturable Absorber for Mode-Locking in a Thulium/Holmium Fiber Laser

A mode-locked Thulium-Holmium doped fiber laser (THDFL) with a Germanium Selenide (GeSe) saturable absorber (SA) is demonstrated for operation in the 2.0 μm wavelength region. The SA device is fabricated by drop-casting the GeSe onto an arc-shaped fiber which is then incorporated into the THDFL to induce mode-locked pulses. Stable mode-locking is attained at 1908.78 nm, with pulse duration of 1.67 ps and output power of 2.74 mW at a maximum pump power of 476 mW. The results of this work show that the GeSe can be a viable alternative for ultrafast photonics applications in the 2.0 μm. region.

Mode-locked pulses from a Thulium-doped fiber Mamyshev oscillator.

We present experimental results of the generation of ultrashort pulses in the 2 µm wavelength region by a fiber Mamyshev oscillator, along with the simulation of the pulse propagation in the cavity. The Mamyshev oscillator emitted pulses with energies of 3.55 nJ at a repetition rate of 15 MHz and optical spectra with bandwidths of 48 nm. The pulses propagated in anomalous dispersive Thulium-doped fiber sections with dispersion compensation sections of normal dispersive fibers.