# Echelle Grating Spectroscopic Technology for High-Resolution and Broadband Spectral Measurement
###### Abstract  
Echelle grating provides high spectral resolving power and diffraction efficiency in a broadband wavelength range by the Littrow mode. The spectrometer with the cross-dispersed echelle scheme has seen remarkable growth in recent decades. Rather than the conventional approach with common blazed grating, the cross-dispersed echelle scheme achieves the two-dimensional spatial distribution of the spectrum by one exposure without scanning in the broadband spectral range. It is the fastest and most sensitive spectroscopic technology as of now, and it has been extensively applied in commercial and astronomical spectrometers. In this review, we first highlight the characteristics of the echelle and then present the optical layout, detection approach, and method of calibration. Finally, we discuss the state-of-the-art implementations and applications of commercial and astronomical instruments.  
echelle grating; spectrometer; high-resolution; broadband spectral range +
Footnote †: journal: Applied Sciences  
of it were first described by [PERSON] in 1949 [7; 8; 9; 10; 11; 12]. The echelles were first applied in astronomy, both satellite- and ground-based, with specially designed instruments. Although there was some early analytical interest, it was not until the 1970s that instrumentation based on the echelle became generally available to analytical chemists [13]. This availability was due to the commercial acceptance and the astronomical applications. The dispersive system using an echelle can select the wavelength quickly and easily. Different from traditional gratings, echelles are used at large angles of incidence and diffraction with high diffraction orders, blaze in a full spectrum, and form a two-dimensional spectrum by a cross-dispersion scheme. It greatly increases the resolving power and intrinsic dispersion with a broad spectral coverage at fairly uniform efficiencies and increases the speed and compactness of spectrometers by the availability of shorter focal lengths, cyclic spectra, and the narrower geometry provided by the nearly identical exit angles of all wavelengths [10]. In higher orders, at least, the echelles are nearly free of polarization effects.  
The echelle and cross-dispersion spectroscopic scheme is used by many high-end analytical instruments, including ICP-OES [14; 15; 16; 17; 18], AAS [19; 20], and LIBS [21; 22; 23]. The spectral resolution is up to the picometer scale, and the wavelength covers the range from deep ultraviolet to near-infrared light. Due to the transient imaging and the high resolution at compact sizes, astronomical spectrometers also favor the echelle approach. Spectral analysis of the weak light collected by telescopes requires long-time exposure, and the echelle spectrometer can capture broadband spectra at one shot instead of scanning by ordinary blazed grating spectrometers [24]. It is very important for the analysis of the moving celestial body to seize the instantaneous state of the measured object [6].  
In this review, we present the specific theoretical underpinnings of the echelle's fundamental characteristics and illustrate typical optical layouts, detecting schemes and spectral calibration methods. We also describe the development and the state-of-the-art of commercial instruments and astronomical spectrometers. Finally, we discuss the future direction of this evolving field.