## 1 Introduction  
Frequency difference tunable dual-frequency lasers have drawn a lot of attention in the last few years for their applications in the fields of absolute distance interferometry, Light Detection and Ranging (LIDAR) detection and terahertz wave generation [1; 2; 3]. In recent years, significant achievements have been made in the research of dual-frequency lasers, such as the intensity balance ratio, beat note stability and beat effect-based Q-switch regime [4; 5; 6]. These achievements have led to dual-frequency laser technology with a complete system and rich connotation. LIDAR with a tunable dual-frequency laser as the light source has a better performance, for that the range and accuracy of speed measurement can be greatly improved. If the beat frequency can be tuned according to a target's speed, one can get the proper Doppler frequency shift which is more convenient for signal processing. If the beat frequency reaches several GHz or higher, the moving speed of m/s or even mm/s can be measured easier [7; 8]. As well as in terms of distance measurement, with an ability to change the wavelength of the beat signal and realize high-resolution detection of targets at different distances [9; 10]. Therefore, the tuning capability of frequency difference is an important property of the dual-frequency laser.  
The principle of birefringence is a common method implemented to split a single-frequency into a dual-frequency. For example, using two quarter-wave plates, one can continuously tune the frequency difference from a few gigahertz to 150 GHz [11; 12]. However, the inaccurate rotating of the quarter wave plate hampers its application under circumstances where high precision is required. Mechanical stress-induced birefringence can generate a several GHz beat frequency signal with good linear relation. However, the beat frequency is not stable enough and is susceptible to external forces. It is necessary to pay attention to the pressure to avoid crushing the crystal, which also limits the range of frequency difference adjustment with this method [5; 13]. Changing the temperature of the birefringent crystal can change its refractive index, which can tune the frequency difference over a wide range from 0 to 60 GHz [14]. Using the electro-optical effect of the crystal, an external electric field can be applied to tune the frequency difference with a short response time [15; 16]. However, to achieve the few gigahertz frequency difference tuning, usually an applied voltage of several thousand volts is required.  
This paper presents a dual-frequency microchip laser with a frequency difference that can be tuned by varying the cooling temperature and voltage applied to the laser. With a wide gain bandwidth, the Nd:YAG microchip is used as the gain medium to realize broad-range tuning of the beat frequency. LTO, as the birefringent crystal, has good thermal stability and a large electro-optical coefficient, making it an ideal candidate for dual-frequency generation. The single-frequency generated by the Nd:YAG is split into two perpendicularly polarized components by the LTO crystal. Furthermore, their frequency difference is first large-scale adjusted by controlling the cooling temperature, then fine-scale adjusted by the variable voltage applied to the LTO crystal. Combined with temperature- and voltage-tuning methods, it can ensure broad-range and high-accuracy tuning of the frequency difference simultaneously.