Blazed Reflection Gratings Research Articles

Erratum: “Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration” (2020 ApJ, 891, 114)

Erratum: “Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration” (2020 ApJ, 891, 114)

Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration

Abstract Future observatories utilizing reflection grating spectrometers for extreme ultraviolet (EUV) and soft X-ray (SXR) spectroscopy require high-fidelity gratings with both blazed groove facets and custom groove layouts that are often fanned or feature a slight curvature. While fabrication procedures centering on wet anisotropic etching in monocrystalline silicon produce highly efficient blazed gratings, the precision of a nonparallel groove layout is limited by the cubic structure of the silicon crystal. This motivates the pursuit of alternative techniques to grating manufacture, namely thermally activated selective topography equilibration (TASTE), which uses gray-scale electron-beam lithography to pattern multilevel structures in resist followed by an optimized polymer thermal reflow to smooth the 3D patterns into continuous surface relief profiles. Using TASTE, a mold for a reflection grating with a periodicity of 400 nm and grooves resembling an asymmetric sawtooth was patterned in 130 nm thick poly(methyl methacrylate) resist on a silicon substrate over a 50 mm by 7.5 mm area. This structure was coated with 15 nm of gold by electron-beam physical vapor deposition using titanium as an adhesion layer and then tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the Advanced Light Source synchrotron facility. Results demonstrate a quasi-blaze response characteristic of a 27◦ blaze angle with groove facets smooth to 1.5 nm rms. Absolute peak-order efficiency ranges from 75% to 25%, while total relative efficiency measures ⪆90% across the measured bandpass of 15.5 nm > λ > 1.55 nm.

Blazed high-efficiency x-ray diffraction via transmission through arrays of nanometer-scale mirrors

Diffraction gratings are ubiquitous wavelength dispersive elements for photons as well as for subatomic particles, atoms, and large molecules. They serve as enabling devices for spectroscopy, microscopy, and interferometry in numerous applications across the physical sciences. Transmission gratings are required in applications that demand high alignment and figure error tolerances, low weight and size, or a straight-through zero-order beam. However, photons or particles are often strongly absorbed upon transmission, e.g., in the increasingly important extreme ultraviolet (EUV) and soft x-ray band, leading to low diffraction efficiency. We demonstrate the performance of a critical-angle transmission (CAT) grating in the EUV and soft x-ray band that for the first time combines the advantages of transmission gratings with the superior broadband efficiency of blazed reflection gratings via reflection from nanofabricated periodic arrays of atomically smooth nanometer-thin silicon mirrors at angles below the critical angle for total external reflection. The efficiency of the CAT grating design is not limited to photons, but also opens the door to new, sensitive, and compact experiments and applications in atom and neutron optics, as well as for the efficient diffraction of electrons, ions, or molecules.

Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers

The authors report a silicon-on-insulator (SOI) process for the fabrication of ultrahigh aspect ratio freestanding gratings for high efficiency x-ray and extreme ultraviolet spectroscopy. This new grating design will lead to blazed transmission gratings via total external reflection on the grating sidewalls for x rays incident at graze angles below their critical angle (about 1°–2°). This critical-angle transmission (CAT) grating combines the alignment and figure insensitivity of transmission gratings with high broadband diffraction efficiency, which traditionally has been the domain of blazed reflection gratings. The required straight and ultrahigh aspect ratio freestanding structures are achieved by anisotropic etching of ⟨110⟩ SOI wafers in potassium hydroxide (KOH) solution. To overcome structural weakness, chromium is patterned as a reactive ion etch mask to form a support mesh. The grating with period of 574nm is written by scanning-beam interference lithography (SBIL) which is based on the interference of phase-locked laser beams. Freestanding structures are accomplished by etching the handle and device layers in tetramethylammonium hydroxide and KOH solution, respectively, followed by hydrofluoric acid etching of the buried oxide. To prevent collapse of the high aspect ratio structures caused by water surface tension during drying, the authors use a supercritical point dryer after dehydration of the sample in pure ethanol. The authors have successfully fabricated 574nm period freestanding gratings with support mesh periods of 70, 90, and 120μm in a 10μm thick membrane on ⟨110⟩ SOI wafers. The size of a single die is 10×12mm2 divided into four 3×3.25mm2 windows. The aspect ratio of a single grating bar achieved is about 150, as required for the CAT grating configuration.

Extreme-ultraviolet efficiency measurements of freestanding transmission gratings.

We report new, near-normal-incidence, transmission grating efficiency results at selected extreme-ultraviolet wavelengths between 4.5 and 30.5 nm for two transmission gratings, one with a period of 200 nm and the other with a period of 400 nm. These gratings consist of opaque gold bars separated by open spaces that have been produced by photolithography techniques commonly used to produce electronic components. The gold bars and the open spaces are nominally of the same width. Both gratings have a thickness of 470 nm. The transmission efficiency at the central, first, and, when possible, second order of diffraction was measured. In addition, guided-wave phenomena at nonnormal angles of incidence, as well as transmission differences depending on which side of the grating was illuminated, were investigated. The observed guided-wave effects allow one to selectively enhance the transmission of the grating at desired wavelengths, as is realized with a blazed reflection grating.

Direct three-dimensional shape measurement by digital light-in-flight holography

A method for direct shape measurement with short laser light pulses and digital holography with a CCD array is proposed. An in-line holographic setup is used in which the reference beam is reflected from a blazed reflection grating, i.e., a Littrow setup. By this method a relatively large optical delay is created between the reference and the object beams even with a small object-reference angle, which is necessary because of the limited resolution of the CCD. The delay varies continuously across one axis of the CCD array. In this way different object sections are reconstructed from different parts of the CCD, which in turn correspond to a certain path length from the object. By putting the sections together, one can evaluate the three-dimensional shape. Theoretical as well as experimental results are presented.

Fabrication of 7×6 multimode optical fiber grating demultiplexer-star coupler using a single GRIN-rod lens

A multimode optical fiber grating demultiplexer-star coupler having seven demultiplexing channels and six fan-out channels is demonstrated. This device consists of an input-output fiber array, in which 42 output fibers are aligned radially around an input fiber, a single gradient-index (GRIN)-rod lens and a multifacet blazed reflection grating. In this device, the incident light beam is split into six beams, each of which is diffracted at the multifacet grating and couples to the output fibers. The device has a working band from 0.62 to 0.88 /spl mu/m, channel separation from 36 to 45 nm and 3 dB bandwidth from 13 to 24 nm. We also evaluate the unevenness in the center wavelength and the minimum excess loss among the fan-out channels and discuss the relationship between the unevenness and the fabrication accuracy of the optical components in detail.

Multimode optical fiber demultiplexer-star coupler using a single gradient-index-rod lens and a multi-facet grating

An optical fiber multi-function device consisting of a single gradient-index-rod lens and a multi-facet blazed reflection grating is proposed to simultaneously realize functions of wavelength demultiplexing and optical signal distribution in a multimode optical fiber transmission system. We analyzed the demultiplexing characteristics and the tolerance of optical components using the ray trace method. This device can realize not only low loss optical signal distribution but also offers improved demultiplexing characteristics in comparison with the previously proposed demultiplexer-multiposition switch. The following characteristics are expected from the design using commercially available optical components: a working band of 0.64–0.88 μm, channel separation of 34–36 nm, 3 dB bandwidth of 27–28 nm, channel cross-talk of less than - 40 dB and minimum excess insertion loss of 0.9–2.1 dB.

A 7×6 optical fiber grating demultiplexer-multiposition switch for 0.64-0.88 μm band

An optical fiber grating demultiplexer-multiposition switch having seven demultiplexing channels and six position-switching channels is demonstrated. This device consists of a unique fiber array in which 42 output fibers are aligned radially around an input fiber, a single gradient-index (GRIN)-rod lens and a rotatable blazed reflection grating. In this device, seven demultiplexing channels can be switched to six different directions by rotating blazed reflection grating. Therefore, this device can realize total transmission channels of 42. The device had a working band of 0.64-0.88 /spl mu/m, a channel separation of 35-42 nm and a 3 dB band width of 18-22 nm. The deviations of center wavelengths among the switching channels were suppressed less than 3 nm.

Optical Fibre Grating Demultiplexer-multiposition Switch Using a Single Gradient-index-rod Lens

Abstract An optical fibre device using a single gradient-index (GRIN)-rod lens and a rotatable blazed reflection grating is proposed for simultaneously realizing the functions of both demultiplexing and multiposition switching. An example of design using the commercially available GRIN-rod lens is shown for the 0·8 μm band multimode fibre use. An extensive tolerance analysis is also done as a basis for fabricating this device. In addition, the potential of a GRIN-rod lens is clarified in terms of the channel separation and the transmission loss.

Ray Trace Evaluation Method of a Multimode Optical Fibre Grating Demultiplexer Using a GRIN-rod Lens

Abstract An evaluation method based on a ray trace analysis is reported for a multimode optical fibre demultiplexer using a GRIN-rod lens and a blazed reflection grating. The validity of this evaluation method is assessed by reference to an existing demultiplexer and a fairly good agreement is obtained between the calculated performance and the experimental results for the previously reported device. An extensive tolerance analysis is also reported, to establish a basis for the fabrication of this device. The results of this analysis are given as a map of the 3 dB contour of the transmission loss.

Planar microoptical multiplexer and demultiplexer for multimode optical transmission lines

A matching pair of 10-channel planar grating multiplexer/demultiplexer has been numerically analysed and fabricated. Basic construction elements are a planar slab waveguide with a cylindrical mirror facet, a standard plane blazed reflection grating and an array of input/output fibres. The arrangement has proved to work sufficiently well without special measures to reduce aberration such as inclining the input/output facet. The ray tracing method satisfactorily describes the characteristics of the devices. Measured insertion losses range from 2.2 to 7.8 dB, full 1-dB passband widths are 14 nm for the demultiplexer and 25 nm for the multiplexer.

Optical wavelength-division multiplexer for the 1–1.4 μm spectral region

An optical wavelength-division multiplexer, for multimode fibre transmission systems, has been demonstrated at λ ~ 1.1 μm. The device used a blazed reflection grating and a graded-refractive-index (g.r.i.n.) lens. The spectral response was measured using a tunable fibre Raman oscillator. The insertion loss was ≤2.6 dB, and the crosstalk level was ≤−29 dB. The device is compared to other multiplexers.