## 1 Introduction  
As the demands for high-speed and large bandwidth information processing are increasing, electro-optic (EO) devices such as modulators, sensors, deflectors, multipliers, switches, and spatial light modulators, which are developed using EO effect, are gaining important applications in data processing and signal transmission [1; 2; 3; 4; 5; 6]. EO coefficients are quantitative parameters of the EO effect, their accurate measurements are the key for designing these EO devices, and a study on the dispersion of the EO coefficients is of great significance for designing the EO devices that are suitable for wide spectrum applications [7; 8; 9; 10]. Furthermore, the accurate measurements of the EO coefficients and their dispersion are also very significant to develop new EO materials [11; 12].  
There are many measurement methods of the EO coefficients to be explored, such as Mach-Zehnder, Michelson, and Fabry-Perot interference methods for phase measurement, as well as half-wave voltage and Senarmont polarimetric methods for phase retardation measurement. Both the Mach-Zehnderand Michelson interference methods require a construction of a detection and reference beam [13; 14]. The EO sample is inserted into the detection beam path, and the phase induced by applying a voltage is measured by comparing the interference fringes between the detection beam and the reference beam. The accuracy of the measurement is limited by environmental stability. The Fabry-Perot interference method helps to obtain a high measurement sensitivity via multiple reflection interferences of incident light in the EO sample [15]. Half-wave voltage and Senarmont polarimetric methods determine the EO coefficients by measuring the phase retardation between two orthogonally polarized components that pass through the sample, and the intensity of light at the extreme value must be determined [16; 17]. However, the variations of light intensity around the extreme value are small, but the fluctuations are evident in the light source intensity; the accuracy and sensitivity of the two methods are limited. Thus, none of the above methods are suitable for studying the dispersion of the EO coefficients for a wide wavelength range coverage. Further, all the aforementioned methods are time-consuming, generally in the order of minutes. Although [PERSON], [PERSON], and [PERSON] et al. have done significant work on improving the measurement method of the EO coefficients, it is still necessary to study a new method for fast, highly accurate, and sensitive measurements of the dispersion of the EO coefficients [11; 12; 18].  
Photoelastic modulation has aroused great interests in applications in optical rotation measurement, circular dichroism measurement, and polarimetry [19; 20; 21; 22]. A photoelastic modulator (PEM) is a dynamic retarder with variable retardation based on the photoelastic effect, whereby the piezoelectric actuator excites and maintains the mechanical vibration in the photoelastic crystal when a sinusoidal driving voltage is applied, which typically forms a stress standing wave and results in a periodic variation of birefringence, and thus the phase and polarization states of the incident light are modulated by the temporal modulation birefringence [23]. The photoelastic modulation has a large modulation frequency, high modulation efficiency, and satisfactory modulation stability. Considering these excellent polarization modulation properties, we have realized a high-speed, accurate, and sensitive measurement for the EO coefficients by using a PEM in our previous work [24]. This paper focuses on the study of the dispersion of the EO coefficients in a wide wavelength range based on the photoelastic modulation. A PEM and an automatic scanning monochromator are applied to build a simple spectroscopic polarimetry for determining the EO coefficients, and the PEM control and the data processing were based on a field programmable gate array (FPGA). Thus, a novel measurement method for the dispersion of the EO coefficients is developed.