# Development of a Holographic Waveguide with Thermal Compensation for Augmented Reality Devices
## 3 Scheme of Obtaining DOE  
The DOE in the proposed version are, according to their classification, relief-phase gratings [28] and are obtained in a layer of photosensitive material by analog holography [29, 30]. At the same time, the combination of various parameters of diffraction gratings, their number and location on the glass substrate of the holographic waveguide provides the required size of the angular field and the exit pupil, and the number of glass substrates in the holographic waveguide determines the color of the reproduced image. The technological process of manufacturing holographic waveguides with the above parameters is based on analog recording and consists of the following main stages: control of the glass substrate, application of photosensitive material, exposure (obtaining diffraction gratings), development, finishing operations (cutting and gluing). The layer of photosensitive material when applied should not exceed 1 um (from 0.5 nm to 1 um). The Microposit S1805 positive photoresist manufactured by Shipley is used as a sensitive material (for subsequent recording of gratings). Dilution of the Microposit S1805 photoresist is not carried out, because its viscosity is small enough and allows you to obtain sufficiently thin layers. The exposure process is a recording of diffraction gratings on a photosensitive material under the influence of laser radiation. To record the gratings, the method of analog holography is used, that is, to obtain an interference pattern of two waves (the angle of incidence of which on a substrate with a photosensitive resistor must be strictly defined) from a single coherent radiation source (in this case, a gas helium-cadmium laser with a power of 120 mW). This method can be implemented by assembling a specialized laser recording stand, the scheme of which is shown below in Figure 8[31]. To form a color image, it is necessary to connect three substrates of holographic waveguides, each of which operates mainly at its own wavelength (red, blue or green) [32, 33]. Thus, the radiation is divided according to the spectral composition and the angle of incidence at the input by three waveguides, passes through them, obeying the air defense condition, and then, when output from the last DOE, it is summed up and forms an RGB image. The period of each diffraction grating is determined by the recording angle, which is 42, 50 and 57 degrees for the grating periods of 610, 520 and 460 nm, respectively. The  
Figure 8: The scheme of analog recording of DOE. 1—laser; 2—shutter; 3, 5, 13—mirrors; 4—beam–splitting cube; 6, 9—micro lenses; 7, 10—aperture; 8, 11—lenses, 12—prism (mirror); 14—photographic substrate. orientation of the grating depends on the angle of rotation of the substrate 14 relative to the two interfering beams during recording, as shown in Figure 8. When recording grids 1a, 2b and 1b, 2a (Figure 6), the substrate should be rotated by an angle of 60 and \(-\)60 degrees, respectively, and when recording the output grid, the substrate is oriented along the larger side. The direction of the strokes in each case is perpendicular to the plane of incidence of the subject and reference beam (Figure 9). For the development of the photoresistive layer, the developer Microposit 303 developer is used (Figure 10), diluted to a concentration of 1:7. The low concentration of the developer provides slower etching of the photographic material with the formation of a high-frequency surface relief. Otherwise, the diffraction grating may not appear. The transparency of such an element, consisting of three RGB substrates, is about 65-70%. This transmission is due to the fact that the gratings are obtained in a photoresist layer, which itself has a certain yellowish color, i.e., absorption is present due to the features of the photographic material. The diffraction efficiency of the obtained individual DOE reaches 24% for R and G substrates and 18% for substrate B (the reduced efficiency is associated with very small grating periods for this wavelength).