What problems may occur in the processing of quartz optical glass lenses?

The processing of quartz optical glass lens (hereinafter referred to as "Shi Ying lens") involves multi-link high-precision operation. Due to the high hardness (Mohs hardness 7), brittleness, sensitivity to processing stress and strict optical performance requirements, Shi Ying glass is prone to various defects. The following are typical problems, causes and effects that may occur during processing:

First, the problem of rough machining stage (cutting, milling and fine grinding)

Glass cracking/edging

Performance: Cracks and notches appear on the edge or surface of the lens, and the whole glass is broken in severe cases.

Cause:

When cutting, the diamond saw blade is blunt or the feed speed is too fast, which leads to local stress concentration;

During milling/fine grinding, the grinding wheel has coarse grain size, excessive pressure (over 0.3MPa), or insufficient cooling (local high temperature caused by water cooling interruption);

There is internal stress in Shi Ying glass blank (without annealing treatment), and the stress release causes cracking during processing.

Influence: It directly leads to the scrapping of the workpiece. If the crack is not found, it may be enlarged in subsequent polishing or use.

Dimensional tolerance out of tolerance

Performance: The deviation of lens thickness and excircle diameter from the design value is too large (more than ±0.01mm), or the flatness/curvature radius does not meet the requirements.

Cause:

The cutting positioning accuracy is low, and the coarse material allowance is uneven;

The spindle of the milling machine jumps (more than 0.005mm), or the dimension is not calibrated in time after the grinding wheel is worn;

The thickness is not monitored in real time during fine grinding, resulting in excessive or insufficient grinding.

Influence: It affects the allowance control of the subsequent polishing process, or leads to the inability to match the lens holder during assembly.

Surface scratches/pits (rough grinding stage)

Manifestation: There are obvious linear or irregular scratches (depth > 1 μ m) or dense pits (pits) on the surface.

Cause:

Abrasive of grinding wheel falls off, or impurities (such as metal chips) are mixed in cooling water;

Improper selection of abrasive particle size (too coarse) or low concentration of grinding liquid (insufficient lubrication) during fine grinding.

Influence: It increases the difficulty of subsequent polishing (it takes longer to remove the damaged layer), and in severe cases, deep scratches that cannot be removed may remain.

Second, the problem of precision machining stage (polishing, centering and edge grinding)

Insufficient finish of polished surface (fog/scratch)

Performance: the surface is foggy (decreased light transmittance), or there are tiny scratches (Ra > 0.01μ m), which can be seen under strong light.

Cause:

Non-uniform particle size of polishing solution (mixed with coarse particles), or mismatch of hardness of polishing die (such as asphalt) (too hard will easily lead to scratches, too soft will easily lead to foggy surface);

The polishing pressure is unstable (big and small), or the rotating speed is too high (> 200 r/min), which leads to local overheating;

Before polishing, the surface roughness of fine grinding exceeded the standard (not below Ra 0.1μm), and polishing could not completely eliminate traces.

Influence: seriously reduce the light transmittance of the lens and destroy the imaging quality of the optical system (produce stray light).

Surface accuracy deviation (poor flatness/curvature)

Manifestations: the flatness of the plane lens exceeds λ/10(λ=632.8nm), the curvature radius deviation of the spherical lens is more than ＞0.01mm, or there are local bumps (such as "edge collapse").

Cause:

Uneven contact between the polishing mold and the lens (such as mold deformation), or levelness error of the polishing table;

Improper control of polishing time (too long leads to over-polishing of the center, and too short leads to the edge not being in place);

The lens is clamped too tightly, which leads to stress deformation, and the stress release after polishing leads to surface springback.

Influence: The aberration of the optical system is increased (such as spherical aberration and astigmatism), which can not meet the design requirements of focusing or imaging.

Centering deviation (misalignment between optical center and geometric center)

Performance: The deviation between the center of the lens excircle and the optical center (curvature center) exceeds 0.02mm, and eccentricity occurs after assembly.

Cause:

Fault of positioning sensor of centering edger (such as unstable vacuum adsorption);

The lack of surface smoothness of lens leads to the deviation of optical positioning signal during centering;

The feed speed of grinding wheel is too fast when edging, which leads to uneven grinding of excircle.

Influence: the optical system produces eccentricity difference, which affects the imaging clarity or the coaxiality of the laser light path.

Three, the surface treatment stage (cleaning, coating).

Incomplete cleaning (residual stains/particles)

Performance: polishing solution crystals, oil stains or tiny particles (> 0.5 μ m) remain on the lens surface, and defects are formed after coating.

Cause:

Improper selection of ultrasonic cleaning frequency (for example, low frequency 28kHz can not remove fine particles, and high frequency 130kHz may damage the film);

Insufficient concentration of cleaning agent, or incomplete washing with pure water (residual cleaning agent);

Dust pollution in the air during drying (cleanliness is lower than class 1000).

Influence: Pinholes or bulges are formed at the particles after coating, which destroys the continuity of the film and reduces the optical performance (such as the antireflection film becomes local reflection).

Coating defects (film peeling, pinhole, color difference)

Performance:

Local peeling and peeling of the film (insufficient adhesion);

Dense pinholes (pinholes) or uneven colors (chromatic aberration) appear on the surface;

Deviation of film thickness (e.g. transmittance of antireflection film at target wavelength < 95%).

Cause:

Before coating, the lens surface is not completely cleaned (residual oil stains affect the adhesion), or the vacuum degree of the vacuum chamber is insufficient (> ＞10⁻⁴Pa), resulting in impurities in the film;

The temperature of evaporation source is unstable during evaporation coating, which leads to uneven film accumulation;

During magnetron sputtering, the target is polluted, or the temperature of the substrate (lens) is too high (exceeding the tolerance temperature of Shi Ying glass), which leads to excessive film stress.

Influence: The optical performance is not up to standard (e.g., the reflectivity exceeds the standard), and the film layer is easily corroded, which shortens the service life of the lens.

Fourth, detect problems related to the environment

Excessive internal stress

Performance: obvious stress stripes (exceeding the specified level) were observed by polarized light detector.

Cause:

Local overheating during machining (such as insufficient cooling during cutting and grinding), resulting in residual thermal stress;

After polishing, no low-temperature annealing (usually 200-300℃ heat preservation) was carried out, and no processing stress was released.

Influence: When used in high and low temperature environment, stress release may lead to lens cracking or surface change.

Surface defect level is not up to standard.

Performance: The number or size of scratches and pockmarks exceeds the industry standard (for example, the "80-50" grade in MIL-PRF-13830B requires that the maximum scratch width is 80μm and the maximum pockmarks diameter is 50μm).

Cause:

The cleanliness of each processing link is not well controlled (for example, dust in the air falls on the surface of the lens and is pressed in when it is polished to form defects);

Poor lighting conditions during inspection led to the omission of tiny defects.

Influence: when used in laser system, scratches may lead to concentrated reflection of laser energy, causing film burning; When used in imaging system, the pockmarks will produce stray light, which will reduce the imaging clarity.

Environmental adaptability failure

Performance: Cracks appear after high and low temperature cycling test, the film falls off, or the lens breaks after vibration test.

Cause:

The edge of the lens is not chamfered (stress concentration), or the thermal expansion coefficient of the film and the glass does not match when coating (for example, the thermal expansion coefficient difference between the metal film and Shi Ying glass is too large);

There are tiny bubbles (raw material defects) in the lens, and the stress concentration at the bubbles leads to rupture during vibration.

Impact: Failure to meet the requirements of outdoor, high temperature or vibration environment, resulting in equipment failure.