## 1 Introduction  
Due to the growing need for high data transmission budgets, progress in the development and deployment of optical communication systems has stimulated an increase in carrier and modulation frequencies of transmitted signals up to hundreds of gigahertz [1; 2; 3]. The next-generation target range of 120-350 GHz [4; 5], which belongs to the so-called millimeter or sub-terahertz (sub-THz) range, is located in the atmosphere's local transparency window and allows unidirectional wireless data transmission for kilometers-long distances [6; 7]. Further progress in communication systems will involve the development of the following bands in the vicinity of 667 and 856 GHz [8; 9; 10]. The functional abilities of silicon microelectronics drop sharply at such high frequencies [11; 12; 13], and it is urgent to search for new materials and methods that will form the basis of next-generation telecommunication devices. We use the term \"telecommunication wavelengths\" in this paper to refer to the \"conventional band\" or C-band of fiber optic telecom systems, which lies in the range of 1530-1565 nm (195.9-191.6 THz). The direct interaction of telecom wavelengths with sub-terahertz (sub-THz) frequencies in a nonlinear optical (NLO) medium is one of the approaches being discussed in the scientific community [14]. It may allow modulation and demodulation of the carrier optical radiation or even passively outcouple radiofrequencies into free space. The following requirements are presented for the desired NLO media: to be transparent at telecom wavelengths and in the sub-THz range; to have a large nonlinear coefficient; and to be suitable for implementation in planar optical design. The first two requirements are derived from nonlinear optics principles, which state that the efficiency of the interaction of two waves is higher the longer the time of their interaction, that is, the propagation length over which a beneficial energy exchange occurs between them, and the higher the effective nonlinear coefficient of the medium \(d\) (which is related to the electro-optical coefficient \(r\) as \(d\sim n^{4}r/4\pi\), where \(n\) is a refractive index at optical wavelength) [15; 16]. The most promising in this vein could be binary semiconductor compounds that can be formed into planar structures by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), such as gallium selenide (GaSe) crystals. The relevance of its application in telecommunication optics is discussed in recent papers [1]. GaSe is often used for NLO devices in the infrared (IR) range [17; 18]. It has an absorption coefficient \(\alpha\) below 0.03 cm\({}^{-1}\) at \(\lambda\approx 1.5\) um and less than 1 cm\({}^{-1}\) in the sub-THz range [19]. GaSe has a large quadratic susceptibility coefficient \(d_{22}\approx 54\) pm/V and demonstrates a sufficient laser-induced damage threshold at the level of tens of MW/cm\({}^{2}\) for 10 ns laser pulses [20]. The growth of gallium selenide by MBE methods has been demonstrated in several works [1; 21; 22]. The disadvantages of the crystal include an extremely low hardness coefficient of \(\approx 0\) on the Mohs scale and a pronounced splitting orthogonally to the optical axis, which is a cause of defects in the crystal structure and makes it difficult to process mechanically [23]. Doping with isovalent chemical additives, for example, such as S [24; 25; 26; 27], Te [25; 26; 28], Al [29; 30; 31; 32; 32], and In [27; 28; 33], is an effective way to improve the mechanical and optical properties of GaSe. Despite many works aimed at studying the properties of doped and undoped GaSe crystals, a small number of publications are devoted to the results on their linear and nonlinear properties at telecommunication wavelengths. Only several works are devoted to the study of sulfur-doped GaSe crystals in the optical range [34; 35; 36; 37] and in the terahertz range [19; 26; 38]. None of the works present the refractive index study of sulfur-doped crystals at a wavelength in the vicinity of 1.5 um.