Figure 7. Reflection spectrum of a 25 period ZnS/PbF2 Bragg reflector, showing both the fundamental and first order lines. Figure 8. Transmission spectra of 25 period ZnS/PbF2 DBR design centred near 1250nm, measured at room temperature and at 175°C. The transmission minima shown are the second and third order resonances. Figure 9. Laser damage probabilities of a ZnS/BaF2 partial reflector at 1.06 m Figure 10. Laser damage probabilities of thin films of the component materials of the design used for the tests in figure (9). Manuscript Received 2-8-89 Laser-Induced Damage of Dielectric Systems with Gradual Interfaces at 1.064 μm D. Ristau, H. Schink, F. Mittendorf, J. Akhtar, J. Ebert and H. Welling Institut für Quantenoptik, Universität Hannover ABSTRACT of Previous work has shown that laser induced damage thresholds Α microprocessor controlled coevaporation technique is used for the production of high reflective and antireflective coatings. Damage thresholds and absorption data of these systems are compared to the performance of conventional systems. An increase of damage thresholds of up to 20% is observed for some materials. This improvement is discussed by comparing the influence of intense laser radiation on gradual and abrupt interfaces. key words: damage thresholds, gradual interfaces, codeposition, absorption, antireflective coatings, highreflecting mirrors, oxide materials. 1 Introduction From the standpoint of laser induced damage the e-beam process is still of current interest for the production of dielectric layer systems. At present only sol gel processes [1] are proven to produce coatings for special applications, which can withstand higher laser power levels than e-beam deposited coatings. In future, ion processes [2] like IAD (ion assisted deposition), IBS (ion beam sputter deposition) or IPD (ion-plating deposition) are expected to surpass the potentiality of the e-beam process because these techniques yield coatings with an improved microstructure. But, although extensive research has been done on ion processes, the aspect of laser induced damage thresholds (LIDT) has not totally been clarified [3]. Compared to the conventional process ion processes are more complicated, less economic, and up to now, they found only limited application in production. Therefore, e-beam deposition is still a major process in the field of high power coatings and its potentiality should be totally explored. °) Optics Laboratory, Pinstech, Islamabad, Pakistan as In the past several years the technique of coevaporation is discussed a means to improve the properties of e-beam deposited coatings. In preceeding studies [4] the technique of coevaporated interfaces has been demonstrated to decrease the total losses in e-beam deposited stacks with alternating materials. In some cases [5] also damage thresholds were higher in gradual 2-QWOTstacks than in conventional systems with abrupt interfaces. In this paper an extension of the technique to practical systems like high reflecting mirrors and antireflective coatings is described. For high reflecting mirrors we tested TiO2, Taz Os, HfO2 and ZrO2 in conjunction with SiO2. Damage thresholds of gradual quarterwave stacks and gradual designs with adapted standing wave field distribution were measured and compared to data of the corresponding abrupt systems. Antireflective coatings are double layer systems of Taz Os, HfO2 and ZrO2 in combination with SiO2 and a system of CeO2 /MgF2. A comparison of damage thresholds and absorption data for gradual and abrupt systems with the same designs are presented. 2 Experimental In the past, several techniques have been tested for the production of inhomogeneous layers. One of the eldest methods is the evaporation of materials with different evaporation temperatures from a single source [6,7]. During the production of the inhomogeneous region the mixing ratio is varied by adjusting the evaporation temperature. This technique does not need any mechanical alterations in the coating plant, but it suffers from the disadvantages that the properties of the layers are very sensitive to production parameters and the technique is restricted to soft coatings. A more sophisticated technique is based on an e-beam which is alternately switched between the crucibles containing the different materials [8]. The mixing ratio can be adjusted by the exposure times of the materials to the e-beam. For the production of coatings with gradual interfaces it is sufficient to regulate the deposition rates of the two materials forming the adjacent layers. If coating designs are restricted to types involving two materials, only two evaporation sources with seperate rate regulation circuits are necessary. In such an arrangement each source is working with constant rate during most of the deposition time. Solely for depositing the interface regions both sources have to be operated simultaneously with variing deposition rates. 2.1 Experimental setup The experimental setup for simultaneous deposition of two materials with arbitrary rates is shown in figure 1. A quartz crystal monitor head is attached to each e-beam source. Each monitor head is shielded against the evaporation flux of the adjacent source with the aid of an aperture. Thermal radiation from the sources is also kept off the monitor heads by these apertures. The rate regulation is carried out by a microprocessor system which registrates the depostion rates and controls the emission currents of both sources. In order to achieve a sufficient accuracy for the rate measurements the fundamental frequency of the quartz crystals is multiplied by a factor of 16 with the aid of a PLL-circuit. During rate regulation the frequency is |