Alakai Defense Systems has recently developed what we believe is the first one-handed UV Raman sensor for standoff trace detection of chemicals, which we refer to as Argos. Argos, equipped with increased range and detection capability, is the higher performance version of the lower cost SAFR sensor. And because they are lightweight, both Argos and SAFR can be deployed onto unmanned ground vehicles (UGVs) or unmanned aerial vehicles (UAVs). Data will be presented showing Argos detection performance on residue and trace samples at ranges from 2 m to 15 m. Further, data will be presented from UGV and UAV experiments performed with the SAFR system in warehouse and outdoor applications.
We present recent results on high energy, high repetition rate 2090 nm Ho:YAG laser system resonantly pumped by the radiation of efficient Tm-doped CW fiber lasers. The laser consists of a master oscillator power amplifier (MOPA) with two additional amplification stages. We demonstrate maximum energy of 120 mJ per pulse with a 15 ns pulse width at 500 Hz, corresponding to a peak power of 8 MW. The results presented in a wide 100-1000 Hz range of repetition rates.
We report an approach to generation of optical frequency combs in the spectral range 2 – 20 µm. The 2-cycle, multi-Watt laser at the repetition rate 80 MHz is based on a polycrystalline Cr:ZnS. The bandwidth of the super-octave ultrafast Cr:ZnS laser source at the central wavelength 2.4 µm is extended to the long-wave IR range (5 – 20 µm) via optical rectification in non-oxide nonlinear materials: GaSe, ZGP. The key advantages of Cr:ZnS frequency comb technology is high efficiency of optical-to-optical conversion from low-cost cw EDFL light to fs MIR pulses, and ultra-low timing jitter. These advantages, in turn, has allowed us to implement shoe-box-sized, light-weight, frequency combs that open new avenues in imaging, sensing, and spectroscopy. Our preliminary evaluations confirm the applicability of the developed sources for dual-comb spectroscopy.
KEYWORDS: Frequency combs, Long wavelength infrared, Spectroscopy, Molecular spectroscopy, Mode locking, Femtosecond phenomena, Mid-IR, Optical amplifiers, High power lasers, Synchrotrons
Femtosecond mid-IR and long wave IR lasers provide tremendous opportunities for imaging and sensing because they combine high spatial coherence of the laser beams (crucial for remote sensing), high temporal coherence of mode-locked oscillators (enabling dual comb spectroscopy in important molecular fingerprint region), and few-optical-cycle pulses (enabling IR sensing with high dynamic range via electric field sampling with low cost room temperature near-IR photodetectors). The development of such a sources (especially compact and field-deployable sources) is a challenging task that relies on the most advanced bulk laser technologies, new gain and nonlinear materials for a wide range of laser wavelengths. We present a viable route to the generation of power and energy scalable few-cycle pulse trains in the IR and describe the application of the developed sources for dual comb spectroscopy. The laser architecture is based on a combination of laser and nonlinear interactions in polycrystalline Cr:ZnS media that enables simultaneous amplification of ultrashort pulses, nonlinear pulse compression to 2-optical-cycle, and nonlinear broadening of pulses’ spectrum to an optical octave. Importantly, all of the necessary optical signals for stabilization of the frequency comb with the large lever arm are generated directly inside polycrystalline Cr:ZnS. This, in turn, has allowed us to implement robust and reliable shoe-box sized middle-IR frequency combs with ultra-low timing jitter of the pulse trains, broad instantaneous spectra, and Watt-level average power.
We report a technique for generation of ultra-broadband coherent femtosecond continua in the infrared. The laser architecture is based on the Cr:ZnS–GaSe and Cr:ZnS–ZGP tandem arrangements that enable simultaneous amplification of ultrashort middle IR pulses and augmentation of pulses’ spectrum via a chain of intrapulse three-wave mixings. The first part of the tandems is based on a single-pass polycrystalline Cr:ZnS amplifier, which is optically pumped by off-the-shelf continuous wave Er-doped fiber laser and outputs 2-cycle pulses with multi-Watt average power at 80 MHz repetition rate, at the central wavelength 2.5 μm. The second stage of the tandems comprises a GaSe or ZGP crystals configured for intrapulse difference frequency generation. The Cr:ZnS–GaSe tandem has allowed us to achieve multi-octave 2–20 μm continuum with 2 W power in the range 2–3 μm and power in excess of 20 mW in the important range 3–20 μm. On the other hand, Cr:ZnS–ZGP tandem features long-wave infrared (6–12 μm) output pulses with record braking sub-Watt power level. Last but not least, Cr:ZnS–GaSe and Cr:ZnS–ZGP IR sources have small footprints and are easily convertible to the optical frequency combs with low carrier-to-envelope timing jitter.
There is strong demand for effective gain materials for the 3.0-3.9 μm spectral range not nicely covered by Cr:ZnSe and Fe:ZnSe amplification bands. We characterized, Fe:ZnAl2O4 ceramic sample, Fe:MgAl2O4 and Fe:InP single crystals as promising laser materials for this mid-IR spectral range. In all crystals, the absorption bands corresponding to 5E↔5T2 transition of Fe2+ ions in the tetrahedral sites were measured. In addition, absorption band of Fe2+ ions in the octahedral sites were observed in Fe:ZnAl2O4 sample with maximum absorption cross-section at ~1.0 μm. From the absorption measurements, the radiative lifetime of Fe:MgAl2O4 was calculated to be 60 μs. Saturation absorption of Fe2+ ions in Fe:ZnAl2O4 was studied using Ho:YAG@2.09 μm and Er:YAG@2.94 μm lasers. Saturation measurements were taken up to energy density of 2 J/cm2 and showed no saturation of absorption. This can be explained by a fast non-radiative (<100 ps) relaxation time from the 5T2 level of Fe2+ ions in the Fe:ZnAl2O4 sample at RT. A strong mid-IR photoluminescence (PL) signal in Fe:InP crystal was detected under the direct excitation of the 5E↔5T2 transition of Fe2+ ions using Er:YAG@2.94 μm laser as well as excitation using photo-ionization process under radiation from Nd:YAG@1.064 μm laser. This indicates that Fe:InP crystals could become promising mid-IR laser media with optimization of fabrication technology.
II-VI chalcogenides (e.g. ZnSe/S) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for fabrication of effective mid-IR gain media. TM:II-VI materials feature a favorable blend of laser spectroscopic parameters: a four-level energy structure, absence of excited state absorption, close to 100% quantum efficiency of fluorescence (for Cr doped II-VI media), broad mid-IR vibronic absorption and emission bands. This talk summarizes progress in fabrication of high quality Cr:ZnS/Se and Fe:ZnS/Se by cation vacancy and cation interstitial enhanced post growth thermal diffusion. We also describe recent breakthrough on recrystallization and effective doping of ZnS ceramics under hot isostatic pressing resulting in a large cm-scale monocrystalline domains formation and an increase of the Fe diffusion coefficient by three orders of magnitude.
We report recent advances in high-power Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W near 2500 nm, 32 W at 2940 nm, and 35 W at 4300 nm with corresponding optical efficiencies of 62%, 29%, and 35%.
Current improvements of output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime are reported: up to 2 W output power at 75-1200 MHz repetition rate, up to 2 cycle pulse duration (16 fs) with efficiency of 20-25% with regards to Er-fiber laser pump power. The effects of efficient up-conversion of mid-IR fs pulses in the laser medium as well as supercontinuum generation are demonstrated.
Further extension of mid-IR spectral coverage to 3-8 m is demonstrated by Cr:ZnS KLM laser pumped degenerate (subharmonic) parametric oscillators (OPOs) based on based on quasi-phase matching in orientation-patterned gallium arsenide, and random phase matching in polycrystalline ZnSe.
II-VI binary and ternary chalcogenides (e.g. ZnS, ZnSe; CdZnTe, ) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for effective mid-IR lasers potentially covering 1.8-9 µm spectral range. This talk summarizes progress in Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W, as well as Fe doped ternary chalcogenides with tunability potentially extended up to 9 um.
TM:II-VI media feature a unique combination of superb ultra-fast laser capabilities with high nonlinearity enabling exceptional output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime over 2-2.6 um and effective up and down conversion of fs pulses via random phase matching (RFM). Extension of mid-IR spectral coverage to 3-8 um is demonstrated by Cr:ZnS KLM laser pumped subharmonic parametric oscillators (OPOs) based on quasi-phase matching in OP-GaAs, and RFM in polycrystalline ZnSe.
Fe:II-VI semiconductors are complimentary to Cr doped compounds and 3-8 um KLM ultrafast oscillators based on Fe doped chalcogenides are feasible. Another important feature of Fe:II-VI media is excellent energy storage capability at 77-200K (~60 µs luminescence life time) enabling efficient Q-switched regime and high energy amplification of ns and ultrafast pulses.
One of the major problems in the development of CW, gain switched, Q-switched and KLM ultrafast Fe:II-VI lasers was the absence of convenient pump sources overlapping with absorption band (2.7-4.5 um) of Fe: gain media. Potential utilization of Quantum Cascade Lasers (QCL) as pump sources of Fe:II-VI lasers will be discussed in the form QCL-solid state laser hybrid platforms as well as Fe doped active layers integrated in QCL structures.
This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime. We developed a flexible design of femtosecond polycrystalline Cr:ZnS and Cr:ZnSe lasers and amplifiers in the spectral range 2–3 μm. We obtained few-optical-cycle pulses with multi-Watt average power in very broad range of repetition rates 0.08–1.2 GHz. We also report on efficient nonlinear frequency conversion directly in the polycrystalline gain elements of ultra-fast lasers and amplifiers. In this work we also report on recent progress in spinning ring gain element technology and report to the best of our knowledge the highest output power of 9.2 W Fe:ZnSe laser operating in CW regime at 4150nm.
Cr2+ doped ZnS and ZnSe possess a unique blend of physical, spectroscopic, and technological parameters. These laser materials feature ultra-broadband gain in 1.9 – 3.3 μm mid-IR range, low saturation intensities, and large pump absorption coefficients. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high second and third order nonlinearity. Cr:ZnS and Cr:ZnSe are available in polycrystalline form: the material consists of a multitude of microscopic single-crystal grains with a broad distribution of grain sizes and orientations, which results in random quasi-phase-matching (RQPM). The distinctive features of RQPM are a linear dependence of the conversion yield with length of the medium and ultra-wide bandwidth of three-wave mixing. We review resent experimental results on optically pumped mid-IR ultrafast lasers based on polycrystalline Cr:ZnS and Cr:ZnSe. We demonstrate that Kerrlens mode-locking of polycrystalline Cr:ZnS and Cr:ZnSe lasers allow for generation of few-cycle mid-IR pulses with MW-level peak power. This opens several avenues for efficient nonlinear frequency conversion of short optical pulses directly in the laser gain medium via RQPM process. We implemented Kerr-lens mode-locked Cr:ZnS oscillators, which feature high power (up to 0.25 W), spectrally broad (up to 22 THz) second harmonic generation (SHG) in the laser medium. We also demonstrate simple and robust ultrafast source based on single-pass continuously pumped polycrystalline Cr:ZnS laser amplifier: mid-IR pulses with 6.8 W average power and the spectrum spanning 2.0–2.6 μm as well as SHG pulses with 0.52 W average power and 1.05 – 1.25 μm spectral span were obtained.
We study coherence properties of a more-than-octave-wide (2.6-7.5 μm) mid-IR frequency comb based on a 2-μm Tmfiber- laser-pumped degenerate (subharmonic) optical parametric oscillator (OPO) that uses orientation-patterned gallium arsenide (OP-GaAs) as gain element. By varying intracavity dispersion, we observed a 'phase' transition from a singlecomb state (at exactly OPO degeneracy) to a two-comb state (near-degenerate operation), characterized by two spectrally overlapping combs (signal and idler) with distinct carrier-envelope offset frequencies. We achieve this by generating a supercontinuum (SC) from the mode-locked Tm laser that spans most of the near-IR range, and observing RF beats between the SC and parasitic sum-frequency light (pump + OPO) that also falls into the near-IR. We found RF linewidth to be <15 Hz (a resolution of our spectrum analyzer), which proves that coherence of the pump laser comb is preserved to a high degree in a subharmonic OPO. Transition to a two-comb state was characterized by a symmetric splitting of the RF peak. Low pump threshold (down to 7 mW), high (73 mW) average power and high (up to 90%) pump depletion make this comb source very attractive for numerous applications including trace molecular detection and chemical sensing with massively parallel spectral data acquisition.
Progress in fabrication and mid-IR lasing of Cr and Fe thermal-diffusion and radiation enhanced thermal diffusion doped II-VI binary and ternary polycrystals is reported. We demonstrate novel design of mid-IR Fe:ZnSe and Cr:ZnSe/S solid state lasers with significant improvement of output average power up to 35W@4.1 μm and 57W@2.5 μm and 20W@2.94 μm. We report significantly improved output characteristics of polycrystalline Cr:ZnS/Se lasers in gain-switched regime: 16 mJ at 200 Hz, pulse duration 5 ns with tunability over 2400-3000 nm as well as Kerr-Lens-Mode-Locked regime in terms of average power (up to 2 W), peak power and pulse energy (0.5 MW and 24 nJ, respectively), and pulse duration (less than 29 fs).
Absorption spectroscopy with frequency combs in the molecular fingerprint portion of the spectrum (2-10 μm) has great potential for trace molecular detection and in particular for such applications as monitoring of the atmosphere and medical breath analysis. Especially attractive is dual-comb Fourier transform spectroscopy where full advantage is taken of temporal and spatial coherence of frequency combs as well as of their broadband nature. The promise is high speed, broad spectral coverage, superior sensitivity, high spectral resolution, and the possibility of absolute frequency calibration of molecular resonances. Here we report a system suitable for performing dual-comb spectroscopy in the range of 2.5-6 μm and beyond. Broadband mid-IR frequency combs are obtained via a doubly-resonant near-degenerate synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned GaAs (OP-GaAs) pumped by a femtosecond Tm-fiber laser at 2-μm wavelength. Low pump threshold (7 mW), high coherence, and broad instantaneous spectral coverage make this system extremely promising for spectroscopic studies.
We present a method of designing a grating outcoupler to obtain the desired two dimensional-intensity profile and improved field distribution of the optical beam emitted by a grating coupled surface-emitting laser. The method is based on variation of the periodicity, the duty cycle, and the groove tilt angle of the grating. Grating design involves numerical analysis of the optical field propagated through the grating by applying the rigorous coupled wave approach method. Experimental evaluation of the designed grating components was done by fabrication and testing the broad area semiconductor lasers with the monolithically integrated grating outcouplers. We present a grating design that provides the spreading of a single optical output into multibeams at different outcoupling angles in the emitting plane. We also present an approach to provide uniform optical intensity profile from the grating outcoupler based on varying duty cycle.
KEYWORDS: Optical design, Etching, Near field optics, Waveguides, Wave propagation, Semiconductor lasers, Beam shaping, Broad area laser diodes, Optical fabrication equipment, Near field
We present a method of designing a grating outcoupler to obtain the desired 1D- and 2D-intensity profile of the optical
beam emitted by a grating coupled surface emitting laser. The method is based on variation of the periodicity, duty
cycle, and the groove tilt angle of the grating. Grating design involves numerical analysis of the optical field propagated
through the grating, by applying the Rigorous Coupled Wave Approach method. Experimental evaluation of the
designed grating components was done by fabrication and testing the broad area semiconductor lasers with the
monolithically integrated grating outcouplers. We also present a grating design which provides the spreading of a single
optical output into multi-beams at different outcoupling angles in the emitting plane.
Second harmonic generation (SHG) from near infrared (IR) diode lasers is an attractive solution for blue-light sources with high peak power and narrow linewidth. IR sources based on broad stripe devices with narrow linewidth makes it possible to achieve a wide range of wavelengths throughout the blue region. This paper summarizes recent results utilizing a configuration of external dual grating reflector coupled surface emitting laser array for blue light generation.
Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. We adjusted the spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. Our results provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.
Recent achievements in second harmonic generation (SHG) from mid-IR diode lasers have made the realization of
compact blue-light sources with high power a reality. Moreover, narrow linewidth control of IR sources based on broad
stripe high power devices makes it possible to achieve a wide range of wavelengths throughout the blue region. This
paper summarizes recent results utilizing a novel Master Oscillator Power Amplifier configuration for blue light
generation.
Many applications require the ability to image a scene in several different narrow spectral bands simultaneously.
Absorption filters commonly used to generate RGB color filters do not have the flexibility and narrow band filtering
ability. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength.
This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. This
paper extends the previous work in adjusting spectral transmission of a multi-layer dielectric filter by drilling a periodic
array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a
specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a
particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances
were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. It
was found that individual layer fluctuations have a minor impact on filter performance, whereas hole taper produces a
large peak shift. The results in this paper provide a reproducible methodology for similar multi-band monolithic filters in
either the optical or infrared regimes.
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