Same thin film design deposited on different type of

In this case, for the WMS02, WDM100 and WMS15 substrates, the central wavelength decreases with increasing substrate temperature. Upon heating, the stresses tend to be more tensile, indicating that the films have a smaller coefficient of thermal expansion (CTE) than the WMS02, WDM100 and WMS15 substrates. For Fabry-Perot-type filters, the optical thickness of the spacer layers is the most critical factor in the determination of the filter central wavelength as well as other details of the filter passband. The decrease in the compressive stress is attributed to the effect of stress relaxation, causing reductions in filter spacer thickness and central wavelength.

However, when the substrate temperature is above 95^oC, the central wavelength of the WMS15 substrate increases with increasing substrate temperature, which is inversely proportional to that obtained when the substrate temperature is below 95^oC.

For the F7 substrate, the central wavelength increases with increasing substrate temperature, which is different from other substrate materials in this experiment. This is attributed to the fact that the films have a larger CTE than the glass substrates for temperatures above 95^oC. Upon heating, the stresses tend to be more compressive, causing increases in filter spacer thickness and central wavelength. Fig. 4-6 depicts

types of substrates, namely, WMS02, WMS15, WDM100, and F7.

1536.4 1536.5 1536.6 1536.7 1536.8 1536.9 1537 1537.1 1537.2

20 40 60 80 100 120 140 160 180 200

Substrate temperature (ºC)

CWL (nm)

F7

WMS 15

WMS 02

WDM100

Fig. 4-6. Results of four types of substrate deposited with same five-cavity 156-layer DWDM film design.

For the WMS02 and WMS15 substrates, by comparing the data shown in Fig. 4-6 and the glass data of Ohara Inc. The curves of central wavelength vs temperature and those of CTE vs temperature are found to be inversely proportional with each other, as shown in Fig. 4-7, i.e., the higher the CTE value, the lower the central wavelength shift. From this experiment, we can predict unknown CTE values of other substrate materials using in situ measured CWL shifts.

6.00E-06

Fig. 4-7. Comparison of curves of central wavelength vs temperature and curves of CTE vs temperature for WMS02 and WMS15 substrates.

The experimental results obtained in §4.3.2. and 4.3.3. are found to be in fairly good agreement with the theoretical statement. This series of experiments has showed that an important factor pertaining to this temperature-dependent center wavelength stability is the volumetric distortion of the film caused by stress applied to the substrate. The consideration of this phenomenon leads one to conclude that, if the coefficient of thermal expansion of the film is smaller than the coefficient of thermal expansion of the substrate, DWDM filters deposited on thermally expanded substrates are subject to considerable compressive stress from the substrate as the substrate temperature decreases after the evaporation. The stress applied to the film deposited at a step temperature is much greater than that applied to the film deposited at a lower temperature. The higher the temperature of the substrate during the evaporation process, the larger the refractive index of the film when cooled to a lower temperature, as revealed in Fig. 4-5, in which the central wavelength decreases with increasing temperature.

4.5 Summary

DWDM filters are strictly used in the optical fiber communication field. A small wavelength versus temperature drift of DWDM filters has a significant influence on the system performance of high-density wavelength division multiplexing applications. In this study, we investigated the central wavelength shift effect of stress on DWDM filters. The center wavelength shift dependence on temperature in DWDM filters made of multilayer films is described in relation to the thermal properties and stress of different optical thin film design and various substrates materials. The results of this study suggest that the temperature stability of the central wavelength of a DWDM filter, which has Ta2O5 and SiO2 as high- and low- refractive-index layers and produced by the IAD process is mainly dependent on its substrate and coating materials properties, i.e., the CTE’s of the substrate and coating materials.

From the results of this series of experiments, we can reveal the stress effects of DWDM filters and improve the methods of managing thermal effects. By process control, we can adjust DWDM filters stresses to meet ITU requirements, and use the substrate with moderate CTE’s to accurately control the position of the central wavelength outside the substrate’s effective area to our advantage.

Chapter 5

Optical and Mechanical Property Changes in Thin-Film Filters with Post Deposition Thermal Treatments

5.1 Introduction

Optical thin-film dichroic mirrors are usually used as color-splitting and color-combining components for projector systems [1]. Current trends in the development of projectors are focused on achieving a high brightness, a high color saturation, a large image area, a compact size, a light weight and a low price.

Improvements in brightness and color performance are especially important for projectors with a large image area. Metal halide lamps are used as light sources due to their high power efficiency. However, light emitted from metal halide lamps contains a high amount of IR radiations, which adversely affects brightness and causes a system temperature increase, which is detrimental to the performance of dichroic mirrors. The efficient separation of light into highly saturated colors is achieved using dichroic mirrors, which can easily change the color area to meet different color standards by optical thin film design. Because optical thin film components are operated at high temperatures, dichroic mirrors with an excellent temperature stability are required; otherwise the color area would change. With the above considerations, dichroic mirrors were designed using TiO2 and SiO2 as high- and low-refractive-index materials, respectively, and fabricated by reactive electron-beam deposition (REBD)

real-time-measured spectral curves, for dichroic mirrors fabricated at different temperatures on various types of substrate, we can determine the appropriate substrate for projectors with a wavelength shift minimum, i.e., with a color change minimum.

However, to our knowledge, there has been no report on the dichroic mirror wavelength shift mechanism of the temperature effect with different types of substrate.

Another type of filter studied here is a thin-film narrow-band-pass filter (NBPF).

An important application of narrow-band-pass filters is in high-density wavelength division multiplexing (DWDM) systems in fiber optic communications. DWDM filters are required to separate many wavelengths traveling through a single fiber[2].

NBPF is strictly used in optical fiber communications. A small wavelength versus temperature drift of the NBPF has a significant influence on the system performance of DWDM applications. The most important requirement of such a filter is to have a precise central wavelength. The channel spacing and center wavelength tolerance are 0.39 and 0.08 nm for 50 GHz at a wavelength of 1550 nm, respectively. A DWDM filter was fabricated by depositing more than 120 layers of oxygen based dielectric materials, namely Ta2O5 and SiO2. Since the characteristics of the sources Ta2O5 and SiO₂ are different, the thickness variation across the substrate in the radial direction differs between Ta2O5 and SiO2. Consequently, the center wavelengths markedly differ from each other and are not within a single channel. The coating uniformity for a DWDM filter is extremely critical; the variation in optical thickness must be less than 10^-4 to meet the required specifications.

Coating uniformity can be improved by many methods, such as the use of coater

source material, monitor system [5], and coating process [6-7]. However, the aforementioned techniques are necessary for film uniformity, but not sufficient. We report on a study of stresses in a DWDM filter that influence center wavelength shift in post deposition thermal treatments. By post deposition thermal annealing we can adjust out-of-specification DWDM filters stress to shift central wavelength to meet, the International Telecommunications Union (ITU) channel requirement, in other words, the effective filter uniformity area can be extended.

5.2 Experimental