High-efficiency Transmission Gratings Research Articles

Standoff Deep Ultraviolet Raman Spectrometer for Trace Detection.

We developed a state-of-the-art, high-sensitivity, low-stray-light standoff deep-ultraviolet (DUV) Raman spectrometer for the trace detection of resonance Raman-enhanced chemical species. As an excitation source for Raman measurements, we utilized our recently developed, second-generation, miniaturized, diode-pumped, solid-state neodymium-doped gadolinium orthovanadate (Nd:GdVO4) laser that generates quasi-continuous wave 228 nm light. This 228 nm excitation enhances the Raman intensities of vibrations of NOx groups in explosive molecules, aromatic groups in biological molecules, and various aromatic hydrocarbons. Our DUV Raman spectrograph utilizes a custom DUV f/8 Cassegrain telescope with an ∼200 mm diameter primary mirror, high-efficiency DUV transmission gratings, custom DUV mirrors, and a custom 228 nm Rayleigh rejection filter. We utilized our new standoff DUV Raman spectrometer to measure high signal-to-noise ratio spectra of ∼50 μg/cm2 drop-cast explosives: ammonium nitrate (AN), trinitrotoluene, pentaerythritol tetranitrate as well as aromatic biological molecules: lysozyme, tryptophan, tyrosine, deoxycytidine monophosphate, deoxyadenosine monophosphate at an ∼3 m distance within 10-30 s accumulation times. We roughly estimate the average ultraviolet resonance Raman (UVRR) detection limits for the relatively homogeneous drop-cast films of explosives and biological molecules to be ∼1 μg/cm2 when utilizing a continuous raster scanning that averages Raman signal over ∼1 cm2 sample area to avoid quick analyte depletion due to ultraviolet (UV) photolysis. We determined 3 m standoff UVRR detection limits for drop-cast AN films and identified factors impacting UVRR detection limits such as analyte photochemistry and analyte morphology. We found a detection limit of ∼0.5 μg/cm2 for drop-cast AN films on glass substrates when the Raman signal is averaged over ∼0.5 cm2 of sample surface using a continuous raster scan. For a step raster scan, when the probed sample area is limited to the laser spot size, the detection limit is approximately tenfold higher (∼5 μg/cm2) due to the impact of UV photochemistry.

1 MHz, 273 W average power Ytterbium-doped rod-type fiber chirped pulse amplification system

Ytterbium-doped ultrafast fiber lasers are widely used in scientific research, industrial processing, medical diagnosis, and other fields due to their excellent beam quality and high power output. The larger mode area allows the fiber to transmit higher peak-pulse power. The commercial rod-type Ytterbium-doped fiber with a core diameter of 85 μm, produced by NKT in Denmark, can produce ultra-short pulses on the order of 100 watts and 100 microjoules. Based on this rod-type fiber, we construct a chirped-pulse amplification (CPA) system in which the high-efficiency transmission gratings and temperature-tunable chirped fiber Bragg grating (CFBG) are used to compensate for dispersion. We investigate the effect of power input on the amplified power and pulse compression quality, and find that higher power input slows down the gain saturation and improves amplification efficiency. At power inputs of 20 W and 30 W, we obtain power outputs of 305 W and 323 W respectively, with an amplification efficiency of about 80%. To reduce the accumulation of nonlinear phase shift, we use circular polarization amplification. At low power outputs (less than 160 W), the effect of nonlinear phase accumulation on the compressed pulse is negligible, and the increase in power input increases the amplification efficiency. When the power output exceeds 200 W, the cumulative increase of nonlinear phase shift reduces the pulse compression quality, which implies that the input power is appropriately reduced to the power range between 5 W and 20 W. With a power input of 20 W and pump power of 429 W, the power output can reach 305 W. After pulse is compressed by using a diffraction-grating pair, this rod-type fiber CPA system can deliver 1 MHz, 264 fs pulses with 273 W in average power. These results provide an important experimental basis for optimizing the performance of high-power and high-energy ultrafast fiber lasers.

Optimal Design and Analysis of 4.7 μm Hybrid Deep Dielectric High Efficiency Transmission Gratings

There is currently no transmission grating with good diffraction efficiency in the 4.7 μm band. Metal gratings at this wavelength are all reflective gratings which has a diffraction efficiency of lower than 90% and lower laser damage threshold. In this paper, we bring up a design of a multi-layer transmission grating with both high diffraction efficiency and wide working wavelength band. We have proved that the transmission grating made of composite materials has an average diffraction effectiveness of more than 96% throughout the whole spectral range of 200 nm. Meanwhile, the theoretically computed transmission grating has a highest first-order diffraction efficiency of more than 99.77% at 4746 nm. This multilayer dielectric film transmission grating’s optimized design may further boost spectral beam combining power, providing a practical technique for increasing SBC power and brightness.

Polymer-Based High Diffraction Efficiency and High Resolution Volume Holographic Transmission Gratings

We report on the optical characterization of very high-efficiency and high-resolution holographic volume phase transmission gratings. The gratings are recorded in a new photo-polymerizable mixture made by epoxy-resin and multi-acrylate. The epoxy-resin used is known to make tenacious acrylate-based films. The holographic mixture contains two photo-initiators, the synergic effect of which enables a reliable photo-polymerization process in the visible region of the electromagnetic spectrum. The recorded holograms are mechanically stable, show long-term temporal stability and very high values of diffraction efficiency, coupled with good angular selectivity due to a relatively narrow band of wavelengths. We measured the intensity of the transmitted beam and calculated the intensity of the diffracted beam at different wavelengths, deriving the refractive index modulation and the grating pitch by fitting the experimental data with a slightly modified theoretical approach. These kind of mixtures can be used in several fields of application, such as chemical or bio-sensors, high resolution optical sensors, high-density optical data storage, encryption and security.

Wafer-scale replicated gratings for compressing ultrafast laser pulses at telecom wavelengths

Wafer-scale, nanoimprint lithography-based approaches for manufacturing of high-efficiency transmission gratings at telecom wavelengths are reported. Two microns-deep, binary gratings are thereby fabricated and combined with a subwavelength, antireflective structure to achieve a cost-efficient and reliable manufacturing process. Diffraction efficiencies of 92% are experimentally achieved in the Littrow configuration. These gratings are used to compress 8 picosecond pulses with 1W of average output power at central wavelength of 1555nm to pulse duration of 378 femtoseconds

Reactive ion beam etching of highly dispersive, high-efficiency transmission gratings for the VIS range

Reflection losses due to refractive index mismatch limit the obtainable diffraction efficiencies for transmission gratings in the highly dispersive regime, i.e., with period to wavelength ratios smaller than 0.7. The design and fabrication of such gratings with high-diffraction efficiencies (≥94 % , Littrow configuration) will be discussed with an emphasis on process strategies to control the profiles in the reactive ion beam etching step. Experimental results from the manufacturing of monolithic fused silica pulse compression gratings with 3000 L / mm optimized for a center wavelength of 519 nm will be presented. The influence of different etching parameters such as etch gas mixture, ion incidence angle, and acceleration voltage of the ion source on profile depth, side-wall angle, duty cycle, and ultimately diffraction efficiencies will be discussed.

Manufacturing of highly-dispersive, high-efficiency transmission gratings by laser interference lithography and dry etching

Diffraction efficiency for transmission gratings with period-to-wavelength ratios smaller than 0.6 e.g. for laser pulse compression is mainly limited due to reflection losses from imperfect effective refractive index matching. In our contribution, different approaches for achieving high efficiencies by reducing these losses are discussed based on 1d and 2d rigorous coupled wave analysis. These include multilayers underneath the corrugation, sub-wavelength gratings and optimized non-binary profiles. Experimental results from the manufacturing of 1700 l/mm gratings for 1030 nm center wavelength using two-beam interference lithography and reactive ion beam etching are presented. Diffraction efficiencies of above 96% were achieved, significantly exceeding the theoretical limit for widely used monolithic binary profiles in this application.

A novel, cost-effective, multi-point Thomson scattering system on the Pegasus Toroidal Experiment (invited).

A novel, cost-effective, multi-point Thomson scattering system has been designed, implemented, and operated on the Pegasus Toroidal Experiment. Leveraging advances in Nd:YAG lasers, high-efficiency volume phase holographic transmission gratings, and increased quantum-efficiency Generation 3 image-intensified charge coupled device (ICCD) cameras, the system provides Thomson spectra at eight spatial locations for a single grating/camera pair. The on-board digitization of the ICCD camera enables easy modular expansion, evidenced by recent extension from 4 to 12 plasma/background spatial location pairs. Stray light is rejected using time-of-flight methods suited to gated ICCDs, and background light is blocked during detector readout by a fast shutter. This ∼103 reduction in background light enables further expansion to up to 24 spatial locations. The implementation now provides single-shot Te(R) for ne > 5 × 1018 m-3.

Ultrafast ion temperature and toroidal velocity fluctuation spectroscopy diagnostic design

High sensitivity measurements of localized, long-wavelength ion temperature, and toroidal velocity fluctuations (T(i)/T(i),v(parallel)/v(parallel)) are required to address critical issues pertaining to turbulent transport. This diagnostic design exploits emission from charge exchange recombination between neutral beam atoms and the intrinsic carbon impurity. The n=8-7 transition of C VI at lambda(0)=529.05 nm will be measured. The key difference between this diagnostic design and conventional charge exchange spectrometers is the use of high-efficiency prism-coupled transmission gratings, avalanche photodiode detectors, and high-throughput collection optics. The spectrometer achieves a spectral resolution of 0.25 nm, and observes 528.0-530.0 nm with eight discrete spectral channels, with an entrance throughput of 1.6 mm(2) sr, two orders of magnitude larger than conventional charge exchange system. The diagnostic will achieve a turbulence-relevant time resolution of 1 micros. System modeling demonstrates a sensitivity of T(i)/T(i) < or = 1%.

High-efficiency transmission gratings buried in a fused-SiO_2 glass plate

Binary gratings were fabricated with high first-order diffraction efficiency by conventional electron-beam drawing and subsequent inductive coupled-plasma dry etching upon the surfaces of SiO2 glass plates. The gratings were covered with a thin SiO2 film by plasma-enhanced chemical-vapor deposition without the grooves' being filled in. The buried gratings exhibited first-order diffraction efficiencies of 84% for transverse-electric and 87% for transverse-magnetic polarized light at a wavelength of 1.55 microm when the period and the depth were 1.5 and 2.8 microm, respectively.

High-efficiency fused-silica transmission gratings

We describe the design, fabrication, and performance of high-efficiency transmission gratings fabricated in bulk fused silica for use in high-power ultraviolet laser systems. The gratings exhibit a diffraction efficiency of 94% in order m=-1 and a damage threshold greater than 13>J/cm( 2) for 3-ns pulses at 351 nm. Model calculations and experimental measurements are in good agreement.

Chirped-pulse amplification of 85-fs pulses at 250 kHz with third-order dispersion compensation by use of holographic transmission gratings

We demonstrate pulse stretching and compression in a high-repetition-rate chirped-pulse Ti:sapphire regenerative amplifier, using high-efficiency holographic transmission gratings. A quantitative dispersion measurement technique is developed to characterize dispersion of the system to the third order. After recompression with third-order dispersion compensation, 3.1-microJ 85-fs, nearly transform-limited pulses are obtained.

Generalised coupled-wave theory of scalar volume diffraction

An extended coupled-wave theory of volume phase holography is developed using Fourier techniques. Fresnel, as well as Bragg, diffraction effects are incorporated for general high-efficiency transmission gratings illuminated by general beams. The method is illustrated by a numerical example.