Blaze Angle Research Articles

First results from the online variable line spacing grating spectrometer at FLASH

A high resolution online grating spectrometer has been set up at the free electron laser in Hamburg FLASH at DESY in Hamburg. This new photon diagnostic device allows to parasitically measure the spectral distribution of the FEL laser pulses on a single shot basis and therefore gives complementary spectral and temporal pulse information for user experiments and FEL operation. The spectrometer combines a plane mirror and two plane variable line spacing (VLS) gratings and is installed at the non-monochromized beamline branch at FLASH. In contrast to a standard grating the blaze angle is optimized for the zero-order radiation transferred to the experimental stations, leaving approximately 1–10% intensity for recording the wavelength in the spectrometer. The detector, an intensified CCD camera imaging the dispersed beam on a Ce:YAG crystal, can follow the focal plane of the gratings covering a wavelength ranging from 6 to 60 nm. The resolving power of the spectrometer is expected to range from 500 to 8000 depending on the FEL wavelength.

Micromechanical reflective diffraction grating with a variable blaze angle

This paper describes a micromechanical reflective diffraction grating controlled by a variable magnetic field. Periodic variation of the tilt angle of the mirror elements produces modulation of the radiation intensity in the principal maxima of the diffraction pattern. A graphoanalytical method makes it possible to determine the tilt angles in a wide range of frequencies of the torsional vibrations.

Direct evidence of tilted Bragg grating azimuthal radiation mode coupling mechanisms

A number of useful fiber optic devices depend on being able to predict and manipulate the radiation field emitted by tilted fiber Bragg gratings. Previously we demonstrated analytically the manner in which this radiation field is directionally dependent on the phase matching characteristics of a grating's three-dimensional structure as well as the polarization dependent dipole response of the medium itself. In this paper, for the first time, experimental measurements of the out-tapped field are presented which clearly illustrate and confirm the existence of the predicted trends associated with each of these physical mechanisms. Using an infrared camera and commercially available beam profiling software, these findings were gathered from a number of tilted fiber Bragg gratings written with an ultraviolet excimer laser at a variety of blaze angles.

Vacuum-ultraviolet blazed silicon grating anisotropically etched by native-oxide mask

We describe a simple and convenient method of controlling the profile of a blazed grating that consists of using a patterned native-oxide layer of off-cut silicon (111) wafers as the mask of anisotropic etching to maximize the smooth blaze facets of the desired blaze angle and to minimize the deficiencies of the groove apexes. With the blazed-grating profile well controlled by this technique, a 1200 g/mm blazed grating was fabricated that had a blaze angle of 5.0 degrees and smooth blaze facets of about 0.2 nm rms. It was measured to have blaze efficiency in the vacuum-ultraviolet wavelength region.

Micromachined silicon grisms for infrared optics

We demonstrate the successful fabrication of large format (approximately 50 mm × 50 mm) gratings in monolithic silicon for use as high-efficiency grisms at infrared wavelengths. The substrates for the grisms were thick (8-16 mm) disks of precisely oriented single-crystal silicon (refractive index, n ~ 3.42). We used microlithography and chemical wet etching techniques to produce the diffraction gratings on one side of these substrates. These techniques permitted the manufacture of coarse grooves (as few as 7 grooves/mm) with precise control of the blaze angle and groove profile and resulted in excellent groove surface quality. Profilometric measurements of the groove structure of the gratings confirm that the physical dimensions of the final devices closely match their design values. Optical performance of these devices exceeds the specifications required for diffraction-limited performance (RMS wave surface error <λ/20) in the near- and mid-infrared (1-40 μm). Peak diffraction efficiencies measured in the reflection range from 70-95% of the theoretical maximum. Tests of our grisms in the near infrared indicate transmission efficiencies of 30-48% uncorrected for Fresnel losses and confirm excellent performance. In infrared wavelength regions where silicon transmits well, the blaze control and high index permit high-resolution, high-order dispersion in a compact space. The first application of these grisms is to provide FORCAST, a mid-infrared camera on NASA's airborne observatory, with a moderate resolution (R=100-1000) spectroscopic capability.

Fabrication of extreme-ultraviolet blazed gratings by use of direct argon-oxygen ion-beam etching through a rectangular photoresist mask

We present a simple method to fabricate blazed gratings used in the extreme-ultraviolet wavelength region. The method uses an argon and oxygen ion beam to etch directly the fused silica substrate through a rectangular profile photoresist grating mask. The blaze angle can be significantly reduced by using oxygen, and the profile evolution under ion-beam bombardment can be simplified when a rectangular mask is used. A simple geometric model is built to analyze the etching process. The etched grating groove profile is approximately triangular with a sharp apex angle provided the aspect ratio of the mask ridge is properly chosen; the blaze angle is directly proportional to the ion-beam grazing incident angle for a fixed oxygen partial pressure. Gratings of a 2400 line/mm groove density and 0.5-3 degrees blaze angles have been fabricated, which confirms the convenience of this method and the effectiveness of the etching model.

Using the compensating effect for Rayleigh anomalies to design a type of broadband mid-IR grating

We suggest and numerically demonstrate a specific design method of broadband mid-IR echelette grating. The method, which applies the compensating of the efficiency blaze in TM-plane polarization to Rayleigh anomalies, is called compensating effect method. This idea allows the grating designer to realize fully the considerable advantages of this type of echelette grating with an apex angle 90 degrees and a blaze angle 19.47 degrees , especially, it can be fabricated easily with conventional (holographic and ruled) techniques. Its diffraction efficiency in unpolarized light has a minimum value of 40%. The cases of Littrow mount and off-Littrow mount are both discussed.

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.

Segmented chirped grating for coarse wavelength division demultiplexing with multimode optical fibers

A chirped grating segmented into partitions each having a constant blaze angle to use in a demultiplexer for coarse wavelength division multiplexing with multimode optical fibers is developed. Its designed configuration utilizes a resonance region to achieve high diffraction efficiency and large dispersion. The width, blaze angle, and diffraction order of each partition were optimized by vector diffraction analysis. The diffraction loss of the manufactured grating was less than 1.5 dB, and polarization-dependent loss was less than 0.6 dB within a wavelength width of at least 70 nm. It is confirmed that a demultiplexer with the developed chirped grating had a wide passband and low cross talk.

Micrograting fabricated by deep x-ray lithography for optical communications

A concave micrograting designed for optical communication applications was fabricated by deep x-ray lithography DXL. The micro- grating has a height of 125 m, a grating pitch of 3 m, a total of 2584 lines, and a sidewall root-mean-square surface roughness of 5 nm. The designed operational wavelength range is from 1475 to 1625 nm. This micrograting is embedded inside a mirror-planar waveguide to form a spectrometer chip. The Rowland-circle grating mounting scheme is used for the spectral detection. The calculated diffraction efficiency of the third-order diffraction reaches 65% when Au is coated on the grating surface and the blaze angle is suitably chosen. The measured spectral width is 1.1 nm, which is in very good agreement with the calculated result of 0.9 nm. This chip-based grating device can be used as an ul- tracompact spectrometer or an ultracompact wavelength-division multi- plexer in optical communications. Based on this work, our DXL technique can be further developed into an x-ray LIGA method for the mass pro- duction of such chip-based spectrometers. © 2007 Society of Photo-Optical In-

Double reflection in the concave reflective blazed grating

The miniature spectrometer has many applications in integrated optics and photonics. The blazed grating with the Rowland circle structure has the advantage of self-focusing and is chosen as the major component in the spectrometer chip. In the simulations for the blaze angle design in the visible spectrum, we discover the phenomenon of the double reflection diffraction. Its cause and parameter space are discussed. The spectrometer utilizing the phenomenon has similar performance to the standard blazed grating and is easier to manufacture in microelectromechanical systems (MEMS) technology. The discovery will greatly ease the design of the spectrometer chip.

Revealing the blaze angle: A simple experiment for visualizing diffraction effects using microscopic and macroscopic gratings

We describe a diffraction grating experiment for visible and microwave optics. The groove spacing for the visible-optics grating is determined through atomic force microscopy, so that an additional feature, the blaze angle, is revealed. From the blaze angle, students calculate the theoretical shift in the diffraction pattern and are then able to visualize this shift through measurements of a microwave diffraction pattern produced using a macroscopic grating. The experiment demonstrates the utility of an atomic force microscope in an engineering application and allows separation of diffraction pattern effects associated with slit width and slit spacing.

Efficiency of a grazing-incidence off-plane grating in the soft-x-ray region

Efficiency measurements of a grazing-incidence diffraction grating in the off-plane mount were performed using polarized synchrotron radiation. The grating had 5000 grooves/mm, an effective blaze angle of 14 degrees, and was gold coated. The efficiencies in the two polarization orientations (TM and TE) were measured in the 1.5-5.0 nm wavelength range and were compared with the efficiencies calculated using the PCGrate-SX code. The TM and TE efficiencies differ, offering the possibility of performing unique science studies of astrophysical, solar, and laboratory sources by exploiting the polarization sensitivity of the off-plane grating.

Replication of a holographic ion-etched spherical blazed grating for use at extreme-ultraviolet wavelengths: topography

We have measured the topography of a holographic ion-etched spherical blazed grating and three of its replicas using an atomic force microscope. The master grating had a roughness of less than 5 angstroms rms, a blaze angle of 2.5 degrees, and an antiblaze angle of 3.3 degrees. Thus the groove profile was more triangular than sawtooth. We find that the replication process did not significantly change the master grating. Moreover, we find no significant difference in roughness, blaze angle, or antiblaze angle between the master and its replicas before or after multilayer coating. However, bumps were observed on the gratings after coating, the cause of which is not understood. Although widespread, they occupy a relatively small fraction of the total area.

Three-dimensional patterning using fine step motion in synchrotron radiation lithography

A simple method of fabricating blazed diffraction gratings using synchrotron radiation lithography was developed that employs an ordinary binary mask with a line-and-space pattern, a step-and-expose procedure that provides a linear cumulative-dose distribution, and a thick resist with a linear sensitivity curve. This method enables the fabrication of blazed gratings with a grating pitch of 1μm and a blaze angle of 15°–40°. The angle and pitch can be changed by changing the dose from step to step and/or by using mask spaces of different widths.

N-fold symmetric gratings as gain-flattening filters

We present N-fold symmetric gratings as gain-flattening filters, since they are polarization insensitive and capable of exhibiting low back reflection. These gratings consist of N superimposed identical tilted gratings which are evenly distributed azimuthally. The coupling interactions between the forward propagating core mode and the backward propagating cladding modes are analyzed, including the limit when N/spl rarr//spl infin/ which is a rotationally symmetric grating. The fiber grating parameters are optimized to equalize the gain profile of a typical erbium doped fiber amplifier to within 0.07 dB. Finally, we examine how the performance of the gain-flattening filter depends on the blaze angle and the effect of an uneven azimuthal distribution of the grating.

Record high extreme-ultraviolet efficiency at near-normal incidence from a multilayer-coated polymer-overcoated blazed ion-etched holographic grating

We have measured the extreme-ultraviolet (EUV) efficiency of a polymer-overcoated blazed ion-etched holographic test grating. The grating had a magnetron-sputtered Mo2C/Si multilayer coating matched to the grating blaze angle of 2.78 degrees. At an angle of incidence of 5.6 degrees and a wavelength of 15.79 nm, the measured efficiency peaks in the second outside order at 29.9%. The derived groove efficiency is 53.0%. To the best of our knowledge these are the highest values obtained yet at EUV wavelengths from a holographic ion-etched blazed grating.

High fidelity blazed grating replication using nanoimprint lithography

We report progress in using nanoimprint lithography to fabricate high fidelity blazed diffraction gratings. Anisotropically etched silicon gratings with 200nm period and 7.5° blaze angle were successfully replicated onto 100mm diameter wafers with subnanometer roughness and excellent profile conformity. Out-of-plane distortion induced by residual stress from polymer films was also analyzed and found to be extremely low. The replicated blazed gratings were tested and demonstrated high x-ray diffraction efficiencies. This process was developed for fabricating blazed diffraction gratings for the NASA Constellation-X x-ray telescope.

Recording of concave diffraction gratings in a two-step process using spatially incoherent light

An original optical design for two-step recording of holographic diffraction gratings is presented. It combines the merits of the two previously developed methods. On the one hand, due to the two-step recording one can use wide possibilities for the aberrations compensation. On the other hand, one can record both the grating objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously, one must use spatially coherent light for at least one of two recording steps. Our method does not require the use of additional optics. Both surfaces of the grating objective blank have a spherical form. The blank of the final grating can be plane concave, which is usual for diffraction gratings. Since the blaze angle of the grating recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations taking into consideration the polarization are also provided. The transmission of the system is optimal for the s-polarized light, additional increasing of the transmission can be achieved by use of an immersion grating objective.

Fabrication of sawtooth diffraction gratings using nanoimprint lithography

We report a process which integrates interference lithography, nanoimprint lithography, and anisotropic etching to fabricate replicated diffraction gratings with sawtooth profiles. This new process greatly reduces grating fabrication time and cost, while preserving the groove shape and smoothness. Relief gratings with 400 nm period inverted triangular profiles and 200 nm period gratings with 7° blaze angle were replicated from silicon masters with surface roughness of less than 1 nm. This process was developed for fabricating the off-plane blazed diffraction gratings for the NASA Constellation-X x-ray telescope.