Volume Phase Holographic Gratings Research Articles

A Laser Source with High Spectral Purity for Simple Tunable Laser Raman Instrument

ABSTRACT The authors present an external cavity diode laser with high spectral purity for tunable laser Raman spectroscopy. To achieve high spectral purity required for Raman spectroscopy, a laser cavity utilizing a volume phase holographic grating, which has very low stray light, is developed and the first-order diffracted beam from the grating is taken as the output. Raman signals can be obtained from common samples with only a commercial laser rejection filter, greatly simplifying a tunable Raman instrument.

Study on measuring refractive index modulation of volume phase holographic gratings

In this paper, we present an approach to measure the refractive index modulation of volume phase holographic gratings (VPHG) by use of its angular selectivity, with the advantage of avoiding the diffraction efficiency errors induced by noise. Based on Kogelnik's coupled wave theory for volume holographic gratings, the wave vectors matching relationship for reflective VPHG at different readout wavelengths is deduced and the diffraction efficiency as a function of readout angle is also obtained. Then an all-fiber experimental system is designed to measure corresponding diffraction efficiency with different readout angles for VPHG recorded in dichromate gelatin. By seeking the Bragg angle where diffraction efficiency is maximum and the blank angle where diffraction efficiency drops to the minimum value, the refractive index modulation can be calculated. The experiment results proves the feasibility of the method.

A New Photopolymer-based VPHG for Astronomy: The Case of SN 2013fj

The spectroscopic studies of near-infrared emission arising from supernovae allow the derivation of crucial quantities that could better characterise physical conditions of the expanding gas, such as the CaII IR HVF spectral feature. For this reason, it is mandatory to have diffractive optical elements (DOEs) with a spectral coverage in the range 8000–10000 Å (for low-z sources) combined with a reasonable signal-to-noise ratio (S/N) and medium-low resolution. In order to cope with all of those requirements, we developed a volume phase holographic grating (VPHG) based on an innovative photosensitive material, developed by Bayer Material Science. We demonstrated the capabilities of this new DOE through observation of SN 2013fj as a case study at the Asiago Copernico Telescope, where an AFOSC spectrograph is available.

非均匀混合调制光栅衍射效率计算方法及特性分析

A method for calculating diffraction efficiencies of non-ideal volume phase holographic gratings is presented by introducing a function between modulation profiles and depths of gratings. By applying Kamiya rigorous delamination-calculation method, rigorous coupled-wave equation for the non-ideal conditions, such as mixed modulation and attenuated modulation profiles, is expressed. The diffraction characteristics of the phase gratings, mixed gratings, out-of-phase gratings and attenuated gratings are investigated. The response degree of diffraction efficiency to the numbers of sub-regions that have been divided is discussed. Comparing with Kogelnik theory and Sabol model, the accuracy of the algorithm is further demonstrated. The results show that, the peaks of diffraction efficiency change over average absorption constants, attenuated constants and the spatial dephasing between the absorption and phase gratings; absorption modulation causes a modest increase of the peak, but average absorption constants drag the efficiencies of the whole transmission orders. The effect of the numbers of sub-regions that have been divided on the angle selectivity and angular spectrum widths of the grating is small, but it decides the accuracy of the algorithm. The work have a certain reference value for calculating the efficiencies and analyzing the diffraction characteristics of the non-ideal modulation of volume holographic gratings.

Performances of volume phase holographic grating for space applications: study of the radiation effect

The special properties of volume phase holographic gratings make them promising candidates for spectrometry applications where high spectral resolution, low levels of straylight, and low polarization sensitivity are required. Therefore it is of interest to assess the maturity and suitability of volume phase holographic gratings as enabling technologies for future space missions, with demanding requirements for spectrometry. One of the main areas of research is related to grating ageing under space radiation. In the present paper, two volume grating technologies are analyzed and compared under gamma irradiation. The performances of both technologies, the photo-thermo-refractive glass and the Dichromated Gelatin, are tested on samples and assessed in the Hα and near-infrared bands. The diffraction efficiency degradation under gamma irradiation is assessed.

THE MULTI-OBJECT, FIBER-FED SPECTROGRAPHS FOR THE SLOAN DIGITAL SKY SURVEY AND THE BARYON OSCILLATION SPECTROSCOPIC SURVEY

We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999 on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha absorption of 160,000 high redshift quasars over 10,000 square degrees of sky, making percent level measurements of the absolute cosmic distance scale of the Universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near ultraviolet to the near infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.

The Brazilian Tunable Filter Imager for the SOAR Telescope

ABSTRACTThis article presents a description of a new Tunable Filter Instrument for the SOAR telescope. The Brazilian Tunable Filter Imager (BTFI) is a highly versatile new technology to be used both in seeing-limited mode and at higher spatial fidelity using the SAM Ground-Layer Adaptive Optics facility (SOAR Adaptive Module) which is being deployed at the SOAR telescope. Such an instrument presents important new science capabilities for the SOAR astronomical community, from studies of the centers of nearby galaxies and the insterstellar medium to statistical cosmological investigations. The BTFI concept takes advantage of three new technologies. The imaging Bragg Tunable Filter (iBTF) concept utilizes Volume Phase Holographic Gratings in a double-pass configuration as a tunable filter, while a new Fabry-Perot (FP) concept involves the use of commercially available technologies which allow a single FP etalon to act over a very large range of interference orders and hence spectral resolutions. Both of these filter technologies will be used in the same instrument. The combination allows for highly versatile capabilities. Spectral resolutions spanning the range between 25 and 30,000 can be achieved in the same instrument through the use of iBTF at low resolution and scanning FPs beyond R ∼ 2,000 with some overlap in the mid-range. The third component of the new technologies deployed in BTFI is the use of EMCCDs, which allow for rapid and cyclical wavelength scanning thus mitigating the damaging effect of atmospheric variability through the acquisition of the data cube. An additional important feature of the instrument is that it has two optical channels which allow for the simultaneous recording of the narrow-band, filtered image with the remaining (complementary) broadband light. This avoids the otherwise inevitable uncertainties inherent in tunable filter imaging using a single detector, which is subject to temporal variability of the atmospheric conditions. The system was designed to supply tunable filter imaging with a field-of-view of 3′ on a side, sampled at 0.12″ for direct Nasmyth seeing-limited area spectroscopy and for SAM’s visitor instrument port for GLAO-fed area spectroscopy. The instrument has seen first light, mounted on the SOAR telescope, as a visitor instrument. It is now in commissioning phase.

Features of the diffraction of supershort pulsed radiation in a system of superimposed transmitting volume phase holographic gratings

This paper discusses how the shape and spectrum of supershort radiation pulses are transformed as a result of diffraction at superimposed transmitting volume phase holographic gratings. It shows that de-interleaving of the light signal with simultaneous demultiplexing in the diffracted beam can be carried out in such a system.

A Thomson scattering diagnostic on the Pegasus Toroidal experiment

By exploiting advances in high-energy pulsed lasers, volume phase holographic diffraction gratings, and image intensified CCD cameras, a new Thomson scattering system has been designed to operate from 532 - 592 nm on the Pegasus Toroidal Experiment. The system uses a frequency-doubled, Q-switched Nd:YAG laser operating with an energy of 2 J at 532 nm and a pulse duration of 7 ns FWHM. The beam path is < 7m, the beam diameter remains ≤ 3 mm throughout the plasma, and the beam dump and optical baffling is located in vacuum but can be removed for maintenance by closing a gate valve. A custom lens system collects scattered photons from 15 cm < R(maj) < 85 cm at ~F∕6 with 14 mm radial resolution. Initial measurements will be made at 12 spatial locations with 12 simultaneous background measurements at corresponding locations. The estimated signal at the machine-side collection optics is ~3.5 × 10(4) photons for plasma densities of 10(19) m(-3). Typical plasmas measured will range from densities of mid-10(18) to mid-10(19) m(-3) with electron temperatures from 10 to 1000 eV.

A compact multichannel spectrometer for Thomson scattering

The availability of high-efficiency volume phase holographic (VPH) gratings and intensified CCD (ICCD) cameras have motivated a simplified, compact spectrometer for Thomson scattering detection. Measurements of T(e) < 100 eV are achieved by a 2971 l∕mm VPH grating and measurements T(e) > 100 eV by a 2072 l∕mm VPH grating. The spectrometer uses a fast-gated (~2 ns) ICCD camera for detection. A Gen III image intensifier provides ~45% quantum efficiency in the visible region. The total read noise of the image is reduced by on-chip binning of the CCD to match the 8 spatial channels and the 10 spectral bins on the camera. Three spectrometers provide a minimum of 12 spatial channels and 12 channels for background subtraction.

Transmitted wavefront error of a volume phase holographic grating at cryogenic temperature

This paper describes the results of transmitted wavefront error (WFE) measurements on a volume phase holographic (VPH) grating operating at a temperature of 120 K. The VPH grating was mounted in a cryogenically compatible optical mount and tested in situ in a cryostat. The nominal root mean square (RMS) wavefront error at room temperature was 19 nm measured over a 50 mm diameter test aperture. The WFE remained at 18 nm RMS when the grating was cooled. This important result demonstrates that excellent WFE performance can be obtained with cooled VPH gratings, as required for use in future cryogenic infrared astronomical spectrometers planned for the European Extremely Large Telescope.

Cryogenic Volume-Phase Holographic Grisms for MOIRCS

We have developed high-dispersion VPH (volume phase holographic) grisms with zinc selenide (ZnSe) prisms for the cryogenic optical system of MOIRCS (Multi-Object near-InfraRed Camera and Spectrograph) for Y-, J-, H-, and K-band observations. We fabricated VPH gratings using a hologram resin. After several heat cycles at between room temperature and 120 K, the VPH gratings were assembled to grisms by gluing with two ZnSe prisms. Several heat cycles were also carried out for the grisms before being installed into MOIRCS. We measured the efficiencies of the VPH grisms in a laboratory, and found them to be 70%–82%. The performances obtained by observations of MOIRCS with the 8.2 m Subaru Telescope have been found to be very consistent with the results in the laboratory test. This is the first astronomical application of cryogenic VPH grisms.

Grisms Developed for FOCAS

Faint Object Camera and Spectrograph (FOCAS) is a versatile common-use optical instrument for the 8.2 m Subaru Telescope, offering imaging and spectroscopic observations. FOCAS employs grisms with resolving powers ranging from 280 to 8200 as dispersive optical elements. A grism is a direct-vision grating composed of a transmission grating and prism(s). FOCAS has five grisms with replica surface-relief gratings including an echelle-type grism, and eight grisms with volume-phase holographic (VPH) gratings. The size of these grisms is 110 mm $\times$ 106 mm in aperture with a maximum thickness of 110 mm. We employ not only the dichromated gelatin, but also the hologram resin as a recording material for VPH gratings. We discuss the performance of these FOCAS grisms measured in the laboratory, and verify it by test observations, and show examples of astronomical spectroscopic observations.

Femtosecond spectral pulse shaping with holographic gratings recorded in photopolymerizable glasses

The majority of the applications of ultrashort laser pulses require a control of its spectral bandwidth. In this paper we show the capability of volume phase holographic gratings recorded in photopolymerizable glasses for spectral pulse reshaping of ultrashort laser pulses originated in an Amplified Ti: Sapphire laser system and its second harmonic. Gratings with high laser induce damage threshold (LIDT) allowing wide spectral bandwidth operability satisfy these demands. We have performed LIDT testing in the photopolymerizable glass showing that the sample remains unaltered after more than 10 million pulses with 0,75 TW/cm2 at 1 KHz repetition rate. Furthermore, it has been developed a theoretical model, as an extension of the Kogelnik's theory, providing key gratings design for bandwidth operability. The main features of the diffracted beams are in agreement with the model, showing that non-linear effects are negligible in this material up to the fluence threshold for laser induced damage. The high versatility of the grating design along with the excellent LIDT indicates that this material is a promising candidate for ultrashort laser pulses manipulations.

Generation of femtosecond paraxial beams with arbitrary spatial distribution

We present an approach to generate paraxial laser beams with arbitrary spatial distribution in the femtosecond time regime. The proposed technique is based upon a pair of volume phase holographic gratings working in parallel arrangement. It exploits the spatial coherence properties of the incoming laser beam in a compact and robust setup that mitigates angular and spatial chirp. The gratings were recorded in a photopolymerizable glass with a high optical damage threshold and a large optical throughput. Setup performance is studied and experimentally demonstrated by generating Laguerre-Gaussian femtosecond pulses.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) in Sloan Digital Sky Survey III (SDSS-III)

AbstractThe Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a large-scale, near-infrared (H-band), high-resolution (R ~ 30,000), high S/N (≳100) spectroscopic survey of Milky Way stellar populations. APOGEE will operate from 1.51–1.68μm, a region that includes useful absorption lines from at least fifteen chemical species including α, odd-Z, and iron peak elements. The APOGEE instrument has a novel design featuring 300 science fibers feeding light to a mosaiced VPH grating and a six-element camera encased in a liquid nitrogen-cooled cryostat. A three year bright-time observing campaign will enable APOGEE to observe approximately 100,000 red giants across the Galactic bulge, disk and halo.

Generation of polarization-shaped ultraviolet femtosecond pulses

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.

Propagation of linearly polarized Gaussian beam through the compound modulated diffraction grating

A new type of diffraction grating called the compound modulated diffraction grating is proposed, which incorporates both surface relief and periodic refractive index modulation. Based on the angular spectrum representation and strict modal theory, the propagation of linearly polarized Gaussian beam through the compound-modulated diffraction grating whose characteristic size is comparable with the wavelength is studied. It is shown that the propagation of beam in each layer is different and the intensity fluctuation frequency in the compound-modulated grating is smaller than that in the conventional relief grating at the same transmission depth. Using the compound modulated grating model, the influence of surface fluctuation of the sub-wavelength_volume phase holographic grating and waist width of the incident beam on the diffraction efficiency is analyzed.

Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings

ABSTRACTWe report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration. The ghost is caused by light reflected off the detector surface, recollimated by the camera, recombined by, and reflected from, the grating, and reimaged by the camera onto the detector. This recombination can occur in two different ways. We observe this ghost in two spectrographs being developed by the University of Wisconsin–Madison: the Robert Stobie Spectrograph for the Southern African Large Telescope, and the Bench Spectrograph for the WIYN 3.5 m telescope. The typical ratio of the brightness of the ghost relative to the integrated flux of the spectrum is of order 10−4, implying a recombination efficiency of the VPH gratings of order 10−3 or higher, consistent with the output of rigorous coupled wave analysis. Any spectrograph employing VPH gratings, including grisms, in Littrow configuration will suffer from this ghost, although the general effect is not intrinsic to VPH gratings themselves and has been observed in systems with conventional gratings in non‐Littrow configurations. We explain the geometric configurations that can result in the ghost, as well as a more general prescription for predicting its position and brightness on the detector. We make recommendations for mitigating the ghost effects for spectrographs and gratings currently built. We further suggest design modifications for future VPH gratings to eliminate the problem entirely, including tilted fringes and/or prismatic substrates. We discuss the resulting implications for the spectrograph performance metrics.

Measuring the wavelength of light using Bragg diffraction gratings

This paper proposes, substantiates, and applies transmission-type volume phase holographic gratings to measure the wavelength of quasi-monochromatic radiation. The measurement procedure consists of using a goniometric device to determine the diffraction angle of the unknown wave at a grating calibrated in spatial period. The requirements on diffraction gratings and calibration-and-measurement procedures are analyzed to determine how well they achieve the maximum accuracy. Regardless of the accuracy with which the zero of the goniometer is set, the calibration of holographic gratings is achieved by using two lasers with reliably set wavelengths. The extent to which the temperature of the grating affects the stability of its spatial period is determined. The technique thus developed is tested by measuring the temperature deviation of the radiation frequency of an injection laser.