## 1 Introduction  
Monolayer or few-layer black phosphorus is generally called phosphorene [1], which is a typical two-dimensional (2D) material. Compared with graphene [2], transition-metal sulphides, and other 2D materials [3] phosphorene has unique properties in acoustics [4], optics [5], thermotics [6], electrics [7, 8], and mechanics [9, 10], which are usually anisotropic, viz. intensively depending on its crystal orientation. These properties, especially owing to their anisotropy, prospectively make phosphorene a kernel material of microelectronic and optoelectronic devices of the new generation [11]. Hence the nondestructive, accurate, and rapid identification of its crystal orientation is a key point to the investigation and application of phosphorene [12]. For the identification of the crystal orientation of phosphorene, the existing methods mainly include microscopic observation based on TEM and AFM, angle-resolution electric conductivity, and spectroscopy methods such as infrared and Raman. Among them, the angle-resolution electric conductivity method is a contact measurement method with low spatial resolution and angle resolution sensitivity [11]. It is difficult to realize an on-line and quantitative measurement using the microscopic observation method. Besides, the spectral resolution and spatial resolution of most infrared spectrometers hardly match the precise requirement of the crystal orientation measurements for phosphorene samples at the micro-scale [7,11]. Because of these characteristics, such as high resolution, high sensitivity, nondestructive, and non-contact measurement [13,14], Raman spectroscopy has been widely applied in experimental investigations on the optical [5] and mechanical [9,15] behaviors of nanomaterials. It has been proved that polarized Raman spectroscopy can be used to identify the crystal orientation of phosphorene rapidly and accurately [11]. The existing studies about crystal-orientation identification using Raman usually adopted the parallel-polarized Raman configuration, in which the polarization directions of the detected scattering light are kept parallel to that of the incident exciting laser. To realize this configuration, the Raman system should be equipped a polarizer in the incident path and an analyzer in the scattering path. Furthermore, both the polarizer and the analyzer, or the sample stage, should be 360\({}^{\circ}\) in-situ rotatable with a certain angle step. By fitting the Raman intensities of A\({}_{\text{g}}\)\({}^{2}\) mode in the Raman information of phosphorene under different polarization directions, the crystal orientation can be estimated [11]. However, for most of the micro-Raman systems, neither the 360\({}^{\circ}\) rotatable analyzer nor a 360\({}^{\circ}\) in-situ rotatable sample stage is a standard accessory. Some published work had to rotate the sample manually, which shows it is difficult to keep the sampling spot in-situ, especially for the cases of complex structures or loading modes. At the same time, because of the inherent and complicated interactions of electron-photon and electron-phonon, laser wavelength and specimen thickness influence the angular dependence of Raman intensity [5,16,17] which may induce an inaccurate or even a mistaken identification of crystal orientation by using the existing methods for the phosphorene sample of multi-layer or under deformation state [5,9,10]. Therefore, it is necessary to develop a more general, convenient, and accurate method for determining the crystal orientation of phosphorene. In this paper, an identification method of phosphorene orientation was proposed by using a Raman system without analyzer. A 360\({}^{\circ}\) continuously rotatable polarizer was used to change the polarization direction of the incident laser. The Raman information was collected at different incident polarization directions without any analyzer.