Volume Phase Holographic Gratings: Polarization Properties and Diffraction Efficiency

Abstract

ABSTRACT. We discuss the polarization properties and first-order diffraction efficiencies of volume phase holographic (VPH) transmission gratings, which can be exploited to improve the throughput of modern spectrographs. The wavelength of peak efficiency can be tuned by adjustment of the incidence angle. We show that the variation of the Kogelnik efficiency versus Bragg angle depends only on one parameter, given by , where is semiamplitude of the refractive index modulation, n is the average index, d is P p (Dnd)/(nL) Dn tune the thickness of the active layer, and L is the grating period. The efficiency has a well-defined dependence on polarization. In particular, it is possible to obtain theoretical 100% diffraction efficiency with one linear polarization at any angle, or to obtain 100% efficiency with unpolarized light at specific angles. In the latter case, high efficiency is the result of aligning the peaks of the s-and p-polarization efficiency-versus-thickness curves. The first of these "s-p-phased gratings" for astronomy is in use with the 6dF spectrograph. Consideration of polarization is particularly important for high spectral resolution, which requires large incidence angles. We also discuss the possibility of separating polarization states for improved throughput along the entire optical train of a spectrograph.

Manufacturing of diffractive elements in fused silica using high repetition rate femtosecond Yb:KGW laser pulses

“…It is obvious that by choosing optimal design parameters it is theoretically possible to achieve diffraction efficiency equal to 100%. When the Bragg angle is out of tune, the overall efficiency decreases and has strong polarization dependence [16]. The most important parameters that play key roles in grating efficiency are grating thickness t and modulation of refractive index ∆n.…”

Novel gratings for next-generation instruments of astronomical observations

“…2 left), and when the lattice period is 0.7 to 4 times of the wavelength, the diffraction efficiency can be achieved up to 100% for S or P polarization of the 1st diffraction order [8][9][10]. The various VPH gratings and VPH grisms (Fig.…”

Polarization and phase-shift properties of high spatial frequency holographic gratings in a photopolymerizable glass

“…For the particular case ͑ B ͒ = 1, the grating splits the incident light into two beams with orthogonal polarization states, thus acting as an HPBS. However, this condition is rather restrictive, since the holographic grating has to satisfy a specific ratio between the sample thickness T and the refractive index modulation ⌬n [1].Several holographic materials, such as dichromated gelatins [2,5,6], photopolymers [7], and silver halides [3], have been used to record polarizing gratings in a 90°recording geometry. Typical challenges arising in some of these materials are shrinkage of the fringe pattern during the development process, low diffraction efficiency, grating defects, low values of the refractive index modulation, and optical scattering.…”

“…Several holographic materials, such as dichromated gelatins [2,5,6], photopolymers [7], and silver halides [3], have been used to record polarizing gratings in a 90°recording geometry. Typical challenges arising in some of these materials are shrinkage of the fringe pattern during the development process, low diffraction efficiency, grating defects, low values of the refractive index modulation, and optical scattering.…”

“…Interesting applications of the polarizing Bragg gratings, such as holographic polarizing beam splitters (HPBS) [2,3], switches and couplers [4], and diffractive optical elements incorporated in astronomical spectrographs [5], have been proposed. A good performance of these optical elements can be achieved providing holographic gratings separating the incident beam into two orthogonal polarization states, typically s and p, with large extinction ratios.…”