This article provides a basic overview of the capabilities of transmission gratings optimized for molecular spectroscopy. These gratings offer near 100%, polarization-insensitive efficiency over a broad wavelength range and are ideal for applications such as Raman spectroscopy where the available light intensity from the sample is very low. The design, manufacturing, and performance of the two most commonly used types of transmission gratings — the surface relief transmission grating (SRTG) and the volume phase hologram (VPH) — are described and examples of the two grating types are discussed.
Most diode array–based diffractive spectrometers for the ultraviolet (UV), visible (vis), and near-infrared (NIR) spectral ranges use reflection-type diffraction gratings and are generally optimized for high resolution rather than high optical throughput. During the past 5–10 years, transmission-type gratings have been developed that provide near 100% efficiency for any polarization. These gratings have found widespread use for Raman spectroscopy and astronomy, where very little light is available, but elsewhere the awareness of the distinct benefits provided by transmission gratings seems quite limited. The purpose of this article is to provide a basic overview of the capabilities of transmission gratings optimized for molecular spectroscopy. Specifically, we will describe the two most commonly used types of gratings: the surface relief transmission grating (SRTG) and the volume phase hologram (VPH).
Transmission Grating Basics
Angular Dispersion: The Grating Equation
Equation 1 is the general grating equation that is valid for any grating structure. The term m is the order of diffraction, and in the remainder of this article we will consider first-order diffraction (m = 1) only.
For a certain spectrometer design, the center wavelength as well as the period Λ (or groove density G = 1/Λ) for the grating are often given parameters that cannot be freely chosen. Furthermore, a spectrometer is used in a range of wavelengths around the center wavelength and equation 1 determines the angular dispersion of this wavelength range.
Although the grating equation determines the angular direction (β) of the diffracted light as a function of wavelength (λ), it does not say anything about the diffraction efficiency. The calculation of the diffraction efficiency for a transmission grating geometry generally requires complicated numerical methods such as rigorous coupled wave theory (1) for solving Maxwell's equations. However, Baldry and colleagues (2) approximated analytical equations based on Kogelnik's original work from 1969 (3) to provide a basic relationship between the physical parameters of the gratings and the diffraction efficiency and bandwidth of the grating. This analysis is based on a pure sinusoidal index modulation and is surprisingly accurate for a certain range of gratings. Based on this analysis, it can be shown that a grating's diffraction efficiency, for a fixed grating period and center wavelength, depends on three factors:
As will be demonstrated in the next section, such nearly 100% efficient, polarization-independent gratings can be produced as both SRTGs and VPHs. The high polarization-independent efficiency is what makes transmission gratings so powerful.
It has been assumed that the grating is used in the Littrow configuration where α = β. From equation 2 it can be seen that the bandwidth of the grating is inversely proportional to the grating thickness t.