Optical coating calibration

There are two typical coating thickness monitoring methods, one is the optical monitoring such as transmittance or reflectance measurement, and the other is the quartz crystal measurement.

The optical monitor is a multi-wavelength spectrophotometer capable of measuring reflection, transmission and color of a wide variety of substrates. The measurement location may be in a vacuum system or positioned external to vacuum

Incident angle (deg)

Reflectance (%)

environment. Employing a photo-diode array spectrometer which capable of measuring light from a collection probe and fiber optics cable. While measuring multiple wavelengths, the optical monitor can make on-line optical measurements formerly only available from off line laboratory instruments. Reflection or transmission can be measured on optically transparent substrates over the entire range of wavelengths from 300 to 900 nm (dependent on supplied light source and the photo diode array), which enables the system operator to compare each layer’s response as it is deposited on the coated substrate.

The measurement of film thickness and deposition rate is based on the measurement of the oscillating frequency of a simultaneously coated quartz crystal.

As the mass of coating material deposited on the quartz crystal increases its oscillation frequency decreases. The film thickness is derived from the change in resonant frequency of the quartz crystal. The calculation uses an algorithm incorporating the physical properties of the layer material. The deposition rate is calculated from the measurements of film thickness. In our case the frequency range of the sensor crystal is 6 MHz, and the thickness accuracy is around 2 %; this will be introduced part of the process inaccuracy such as thickness error.

Refractive index and absorption coefficient of a single layer coating can be determined by spectrophotometric transmission measurement in the 250-2000 nm wavelength range [88]. Spectrophotometry allows the values of the index of refraction n and extinction coefficient k to be obtained over a wide spectral range. The optical parameters n, k and thickness d of the samples have been calculated from transmittance verses spectra. The method is based on analysis of the transmittance spectrum of a weakly absorbing film, nonabsorbing substrate system [89-90]. Inverse of transmittance can be decomposed into two components:

( ) ( ) ( ) ( )

(

)

T and + T are the experimentally traced envelope curves of the transmission ⁻ spectra, and λ is the wavelength of the light. Optical parameters of a weakly absorbing film are given by

( ) [

( ) (

) ] [

^{( ) (}

⁾ ] ⁽

⁾

for the first-order approximation. ns is the index of refraction of the substrate, and λ_i, λ_i+1 are the wavelengths corresponding to two adjacent extreme.

Because different deposition processes provide different energy, as shown in Table 3.1, the material parameter, such as the refractive index (n) and the absorption coefficient (k), may not be the same because of the packing density difference.

Table 3.1: The comparison of several kinds of deposition methods [91]

area large large small small

Typically in commercial deposition process the test samples are the same material with the real substrates such as glasses, plastics. But in some special cases such as coating on crystal fiber or on a crystal with a small diameter, the glass is usually utilized to be the test substrate. Because bulk crystals are very expensive and the small diameter crystal which cladding with pyrex is hard to be measured.

Therefore, a convention method is needed to get the correct reflectance or transmittance on these substrates. In the next part, we will describe our calibration procedure to determine the transmittance of a coated crystal.

As an example, an AR coating on a Nd:YAG crystal for intracavity doubling blue laser generation was illustrated. As shown in Fig. 3.22, the blue rhombus markers are AR design on Nd:YAG for pumping wavelength (808 nm) and fundamental lasing wavelength (946 nm), the brown solid line is the same simulation on the glass with an incident angle of 5 degree. The red square markers are the

measured result using a spectrophotometer (HITACHI U-4100) with an incident angle of 5 degree. The abnormal behavior of the measured data around 850 nm is an artifact of the spectrometer due to grating switch.

Figure 3.22. The spectrum of a antireflection coating on the Nd:YAG crystal and the comparison result on the test glass substrate.

Figure 3.23 shows the standing-wave electric-field of the AR coating design. It is reasonable that the electric field distribution inside the thin film layers are reduced for the beam all pass the coating.

0 0.5 1 1.5 2 2.5 3

700 800 900 1000 1100

simulation, Nd:YAG simulation, glass measured, glass

Wavelength (nm)

Reflectance (%)

Figure 3.23. The standing-wave electric field distribution of the AR coating for 946 nm. The arrows indicate the interface of each layer.

The calibrated procedure is illustrated in table 3.2. Step 1 specifies our design on the Nd:YAG and the simulation in this case include four layers. From Fig. 3.22, we choose four critical points of the red curve such as 946 nm, 808 nm, 1064 nm, and 750 nm to make the calibrated curve match the experiment result. Step 2 is to change the incident angle to 5 degree. Step 3 is to change the substrate to glass. Step 4 shows the experimental result.

From Fig. 3.22, the measured curve has a little difference compared to the simulation curve, so we need to find out the exact value of this coating on the Nd:YAG. How we derive the correct value that we are interested?

20 40 60 80 100

-600 -400 -200 0 200 400 600 800

Electric Field (V/m)

Physical distance from medium (nm) air

Nd:YAG f

First we take the experiment result of the four points into the optimized condition (in the software this factor included in the item “target”) then optimized the thickness of each layer by the thin film design software, Macleod, as shown from step 5, the meaning will be described later. After transformation, the last step 7 is the reflectance for the four wavelengths on the Nd:YAG facet.

The thinking we calibrated is that the reflectance on the two substrates, glass and Nd:YAG can be obtained from the thin film software easily, by just changing the substrate in the design. When we put the measurement data in to the target part, the software will be adjusting the physical thickness to introduce the values to match the condition we give (the target we set); the reflectance we measured. As shown in Table 3.3, the micro-physical thickness difference is from three major factors. First one is the deposition process error of the quartz crystal, another is the calibration error from sampling numbers, the interface structure of the different material layers are also important; it can be see that the percentage error is very small so that the effective reflectance on the Nd:YAG is reasonable.

Step 7 change the incident angle to 0 deg A/N/N 0.0142 0.1937 0.534 0.1131

Step 6 replace the substrate to 5 deg A/N/N 0.0118 0.1977 0.5512 0.1014

Step 5 adjust the thickness using Macleod 5 deg A/G/G 0.735 1.559 2.797 0.422

Step 4 measured result 5 deg A/G/G 0.735 1.559 2.797 0.422

Step 3 adjusted simulation 5 deg A/G/G 0.7632 1.0171 3.2264 0.0392

Step 2 adjusted simulation 5 deg A/N/N 0.0272 0.0076 0.8581 0.6566

Step 1 first simulation 0 deg A/N/N 0.0243 0.0079 0.8375 0.6885

Step 0 design value <0.2 <0.2 ~ ~

Reflectance (%) Wavelength (nm) 946 808 1064 750

Table 3.2: The reflectance/transmittance calibration between the crystal substrate and the glass substrate Note: A stands for Air, N stands for Nd:YAG, G stands for Glass.

Thickness error (%) 0.2 1.8 0.1 0.2

Calibrated thickness (nm) 53.97 31.88 256.03 157.08

Design thickness (nm) 53.39 26.93 273.93 156.44

Material TiO2 SiO2 TiO2 SiO2

Layer 4 Layer 3 Layer 2 Layer 1

Table 3.3: The AR coating design for 808-nm pumping source and 946-nm lasing wavelength Note: Layer 1 is next to the Nd:YAG substrate, while Layer 4 is next to the Air.