Large-aperture ground glass surface profile measurement using coherence scanning interferometry.

Area of Science:

• Optical Engineering
• Metrology
• Surface Science

Background:

• Measuring rough glass surfaces with low reflectance is challenging.
• Sub-surface light scattering significantly degrades measurement accuracy.
• Existing interferometry techniques struggle with such demanding samples.

Purpose of the Study:

• To develop a coherence scanning interferometer (CSI) capable of accurately measuring rough, low-reflectance glass surfaces.
• To enhance correlogram contrast and lateral resolution for improved surface metrology.
• To validate the system's performance on a challenging ground glass sample.

Main Methods:

• A compound light source combining a superluminescent light-emitting diode (SLED) and an ytterbium-doped fiber amplifier (YDFA).
• Tuning the light source for a short temporal coherence length (15 μm) and high spatial coherence.
• Shifting the infrared spectral range to 1,038 nm (near-visible) for compatibility with high-resolution digital cameras.
• Utilizing a multi-mega pixel industrial machine vision camera.

Main Results:

• The developed interferometer successfully measured a ground Zerodur mirror (200 mm aperture, 0.9 μm rms roughness).
• The system achieved adequate correlogram contrast despite the low reflectance and rough surface.
• Improved lateral image resolution was obtained by using the digital camera.

Conclusions:

• The proposed coherence scanning interferometer effectively addresses the challenges of measuring rough, low-reflectance surfaces.
• The specialized light source and camera integration enhance measurement accuracy and resolution.
• This system offers a viable solution for advanced surface metrology applications.