Design and fabrication of wide angle phase control mirror

  Objective  In quantum communication, such as quantum key distribution, quantum entanglement, and quantum teleportation, photons are controlled to a specific polarized direction for transmission and decoding. There is a need to establish an effective and stable link of quantum channels between the transmitter and receiver to maintain max channel efficiency and reduce quantum bit error rate. Therefore, it is necessary to do phase control on the thin film optical components in the optical system. In published literatures, most phase control mirrors are designed with a single incidence angle (such as 10°, 22.5°, 35° and 45°), which cannot meet the requirement of optical-mechanics system used in new generation of quantum communication working at medium to high orbit any longer. This research developed a dielectric reflector with high reflectivity and wide angle range for efficient energy transfer and precise phase control, which has wide application prospects for this type of thin film component in next generation of quantum communication systems working at medium to high orbit.

  Methods  The design method is as below. Two materials with different refractive index are selected, the values of which are referred as H and L respectively. (HL) ^ n is used as the basic film system mechanism, and the design value of wavelength is determined based on the signal channel at the center of the cutoff band. Multiple layers equivalent to d₁L d₂H d₃L or d₁H d₂L d₃H for phase control are applied on the surface layer of the film base structure. The initial film structure is: G | (HL) ^ 14 d₁H d₂L d₃H d₄L d₅H d₆L d₇H d₈L | Air, where G is the substrate , d₁-d₈ are the thickness coefficients of each film layer respectively. Optimal goals are set based on task indicators, and optimization algorithms are used such as Global Modified LM or Global Simplex for optimal iterations to change film thickness to obtain the best design result. The preparation of this product was completed on Lab900-plus vacuum deposition machine produced by Leybold Company in Germany. The equipment is equipped with two e-type electron guns, with SiO₂ using a circular crucible and Nb₂O₅ using a seven hole crucible. Equipped with a 4-probe quartz crystal oscillator physical thickness control system, OMS5100 optical automatic control system, and a Veeco RF ion source with a grid aperture of 12 cm. The sample is Φ 50 ×5 mm quartz substrate.

  Results and Discussions  During the deposition, precisely controlling the layer of high sensitivity in the film system is needed. So the film thickness fitting analysis is carried out in terms of the evaporation consumption of the crucible material and crystal oscillator parameter correction, which ensured the success of development in the end. The results show that the reflectivity of the reflector reaches over 99.3% at 780 nm at incidence angles of 37.5°, 45°and 52.5°, and the phase difference is less than 3° (Fig.6), meeting the task requirements. It also passed the environmental reliability and firmness tests (Tab.6).

  Conclusions  The wide angle phase control mirror uses Nb₂O₅ and SiO₂ as high and low refractive index materials, quartz as the substrate, and a combination film system of high reflectivity film layer and multi layer phase control film as the initial film system. Optimization algorithms such as Global Modified LM or Global Simplex are used to design high reflectivity and phase control mirror in wide angle range. On Lab900-plus, the equipment of Leybold in Germany, by combining electron beam evaporation with Veeco RF ion source assisted deposition, deposition process was optimized. By combining optical monitoring and crystal oscillator monitoring, the development was successful. The results show that at 780 nm, with an incidence angle range of 45° ± 7.5°, the reflectivity is greater than 99.3%, and the phase difference is controlled within 3°. The product passed environmental reliability tests. The development of this product can enable the design of quantum communication optical systems with a wide angle field of view. How to ensure the product performance over the lifetime is the focus of next stage of work.