Control strategies and technical practices for surface precision errors in optical prism processing

As the core optical component for optical signal deflection, dispersion, and imaging, the surface accuracy of an optical prism directly determines the performance of the optical system. If the surface flatness error exceeds 0.5 λ (λ=632.8nm), it will cause the light reflection/refraction path to shift, affecting the imaging clarity; If the surface roughness is greater than Ra0.01 μ m, it will increase light scattering loss and reduce the transmittance efficiency of the optical system. In the processing of optical prisms, surface accuracy errors mainly come from various factors such as raw material characteristics, processing technology, equipment accuracy, and environmental interference. It is necessary to finely control the entire process to control the errors at the micrometer or even nanometer level. Starting from the mechanism of error generation and combining with the entire processing flow, this article systematically elaborates on the control strategies and practical methods for surface accuracy errors.

1、 The core types and generation mechanisms of surface accuracy errors in optical prisms

Before formulating control strategies, it is necessary to clarify the specific types and causes of surface accuracy errors, in order to provide a basis for targeted prevention and control. The surface accuracy errors in optical prism processing are mainly divided into two categories: "shape errors" and "micro roughness errors", and there are significant differences in their generation mechanisms and influencing factors.

（1） Shape error: deviation in macroscopic geometric accuracy

Shape error refers to the deviation between the actual geometric shape of the prism surface and the ideal design shape, mainly including flatness error, parallelism error, and angle error. It is commonly seen in the milling and precision grinding stages

Flatness error: manifested as concave, convex or wavy surface of the prism, with an error range of usually 0.1-5 μ m, mainly caused by radial runout of the milling and grinding machine spindle (>0.005mm), poor flatness of the grinding tool (flatness>1 μ m), and uneven processing pressure (pressure fluctuation ± 0.5N). For example, when using a grinding wheel with a flatness error of 2 μ m to mill a K9 glass prism, if the grinding wheel plane is not corrected in time, it will cause the prism surface to replicate the flatness error of the grinding wheel, resulting in a final flatness deviation of 1.5-2 μ m;

Parallelism error: refers to the parallelism deviation between adjacent parallel surfaces of a prism (such as the two right angled surfaces of a right angled prism), usually requiring ≤ 10 "(arcseconds). If the prism clamping and positioning deviation (>0.01mm) and the worktable feed verticality error (>0.001mm/m) occur during processing, it will lead to parallelism deviation. For example, when precision grinding a right angle prism, if the perpendicularity error between the positioning reference plane of the fixture and the feed direction of the worktable is 0.002mm/m, the parallelism error of the prism after processing will reach 15-20 ", exceeding the requirements of the optical system;

Angle error: refers to the deviation between the angle of each edge of the prism and the design angle (such as the 60 ° angle deviation of an equilateral prism), with an allowable error usually ≤ 5 ", mainly caused by inaccurate correction of the grinding tool angle (angle deviation>2") and prism position deviation during processing (>0.005mm). For example, when processing a 30 ° -60 ° -90 ° right angle prism, if the angle correction deviation of the diamond grinding wheel is 3 ", it will directly cause the prism's 60 ° angle deviation to reach 3-4", affecting the accuracy of light refraction angle.

（2） Micro roughness error: defects in surface micro morphology

Micro roughness error refers to the degree of micro roughness on the surface of a prism, expressed as the arithmetic mean roughness Ra. Optical prisms typically require Ra ≤ 0.01 μ m (some high-precision prisms require Ra ≤ 0.005 μ m), mainly occurring during the precision grinding and polishing stages:

The roughness hazard during the precision grinding stage: If the abrasive particle size used for precision grinding is not selected properly (such as using W14 abrasive instead of W7 abrasive), it will leave deep grinding marks on the surface (depth 0.5-1 μ m), which are difficult to completely remove during subsequent polishing, and the final roughness Ra will increase to 0.02-0.05 μ m; In addition, insufficient supply of coolant during precision grinding (flow rate<50mL/min) can lead to the accumulation of debris, scratching the surface, and the formation of local roughness deviations;

The roughness during the polishing stage exceeds the tolerance: uneven hardness of the polishing pad (hardness deviation ± 5 Shore A), fluctuation of polishing solution concentration (concentration fluctuation ± 5%), and excessive polishing pressure (>3N/cm ²) are the main causes. For example, when using a polyurethane polishing pad with a hardness deviation of 8 Shore A, areas with excessively high hardness will be excessively polished, resulting in the appearance of "bright spots" on the surface, and the roughness Ra will decrease to 0.008 μ m. Areas with excessively low hardness will not be polished enough, and Ra will increase to 0.015 μ m, resulting in uneven surface roughness.

2、 Raw material pretreatment stage: laying the foundation for precision control

The physical properties (such as hardness, toughness, and internal stress) of optical prism raw materials (such as K9 glass, quartz glass, and calcium fluoride crystals) directly affect the error sensitivity during processing - for example, quartz glass has high hardness (Mohs hardness 7) but high brittleness, and is prone to edge breakage during processing, resulting in excessive edge roughness; Calcium fluoride crystals are prone to deliquescence, and if not pre treated properly, a hydration layer can form on the surface, affecting the subsequent polishing accuracy. The raw material pretreatment stage requires "stress relief", "surface cleaning", and "characteristic adaptation" to provide a stable raw material foundation for subsequent processing.

（1） Eliminating internal stress in raw materials

During the melting and forming process of glass raw materials, internal stresses are generated (such as stress values greater than 5nm/cm for K9 glass), and stress release during processing can cause surface deformation and flatness errors. Need to eliminate internal stress through annealing treatment:

Annealing of glass raw materials: Place K9 glass or quartz glass raw materials into an annealing furnace, heat them at a heating rate of 5-10 ℃/h to the annealing temperature (550-580 ℃ for K9 glass and 1100-1200 ℃ for quartz glass), and hold for 4-6 hours to evenly release internal stress; Then cool down to room temperature at a cooling rate of 2-5 ℃/h to ensure that no new stress is generated during the cooling process. After annealing, it is necessary to use a stress gauge to detect the stress value of the raw material, ensuring it is ≤ 2nm/cm. Otherwise, re annealing is required;

Annealing of crystal raw materials: For crystal raw materials such as calcium fluoride and sapphire, the "segmented annealing" process is used: first, the material is kept at a low temperature (200-300 ℃) for 2 hours to eliminate surface stress; Heat up to the high temperature range (1/2-2/3 of the crystal melting point, such as 600-700 ℃ for calcium fluoride) and hold for 3 hours to eliminate internal lattice stress; Slowly cool down to avoid crystal cracking. After annealing, the crystal lattice integrity is checked by X-ray diffraction to ensure no lattice distortion.

（2） Cleaning and pretreatment of raw material surfaces

Oil stains, dust, and oxide layers on the surface of raw materials can affect the subsequent clamping and positioning accuracy, leading to processing deviations that need to be removed through multi-step cleaning

Preliminary cleaning: Soak the raw materials in isopropanol (purity 99.9%) for 10-15 minutes to remove surface oil stains; Use ultrapure water (resistivity>18.2M Ω· cm) for ultrasonic cleaning (frequency 40kHz, power 300W) for 20 minutes to remove surface dust; Blow dry the surface moisture with a nitrogen gun (pressure 0.3MPa) to avoid residual water stains;

Oxidation layer removal: For easily oxidizable materials such as sapphire and silicon-based prisms, a 5-10nm thick oxide layer will form on the surface. It is necessary to soak in diluted hydrofluoric acid solution (concentration 5%) for 30 seconds to remove the oxide layer; Clean with ultrapure water and ultrasonically for 10 minutes to neutralize residual acid and prevent corrosion of the raw material surface;

Surface defect detection: After cleaning, use a differential interference microscope (magnification 100x) to inspect the surface of the raw material. If scratches (depth>0.5 μ m) or bubbles (diameter>10 μ m) are found, they need to be screened and removed to avoid the defects from expanding during subsequent processing and affecting surface accuracy.

（3） Adaptation of raw material characteristics and processing technology

The physical properties of different raw materials need to be matched with corresponding processing parameters to avoid errors caused by parameter mismatch:

Hardness adaptation: For high hardness raw materials (such as quartz glass, Vickers hardness 600HV), high hardness grinding tools (such as diamond grinding wheels, hardness HV10000) should be selected, and the processing feed rate should be reduced (5-10mm/min) to avoid accuracy deviation caused by rapid wear of the grinding tools; Low hardness raw materials (such as calcium fluoride with a Vickers hardness of 150HV) require the use of soft grinding tools (such as silicon carbide grinding wheels with a hardness of HV2000) and an increased feed rate (15-20mm/min) to prevent excessive wear of the raw materials;

Resilience adaptation: Brittle raw materials (such as sapphire) are prone to edge breakage during processing, and "low pressure, high speed" parameters (pressure 1-2N, speed 3000-4000r/min) need to be used during the milling and grinding stage to reduce the risk of edge breakage; Resilient raw materials (such as optical resins) are prone to sticking to the blade, and it is necessary to increase the coolant flow rate (80-100mL/min) and choose hydrophobic grinding tools to avoid surface roughness caused by debris adhesion.

3、 Milling and precision grinding stages: the key to controlling macroscopic shape errors

Milling and precision grinding are the "shape forming stages" of optical prism processing, which require equipment precision calibration, grinding tool optimization, and process parameter control to control flatness, parallelism, and angle errors within the design range. The error control in this stage directly determines the basic accuracy of subsequent polishing. If the flatness error in the milling and grinding stage exceeds 5 μ m, even if the subsequent polishing process is optimized, it is difficult to reduce the final flatness error to below 1 μ m.

（1） Calibration and maintenance of equipment accuracy

The accuracy of processing equipment is the foundation of shape error control, and it is necessary to regularly calibrate the accuracy of key components of milling and precision mills to ensure that the processing requirements are met:

Spindle accuracy calibration: The radial runout and axial displacement of the milling mill spindle should be ≤ 0.002mm, and a dial gauge (with an accuracy of 0.001mm) should be used for every 500 prisms processed. Calibration: Contact the dial gauge head with the spindle end face, rotate the spindle, and record the axial displacement value; Place the dial gauge head vertically in contact with the outer circle of the spindle, rotate the spindle, and record the radial runout value. If it exceeds the tolerance, replace the spindle bearing or adjust the spindle clearance;

Workbench accuracy calibration: The flatness of the workbench should be ≤ 0.005mm/m, and the feed verticality error should be ≤ 0.001mm/m. Calibration using a laser interferometer (measurement accuracy ± 0.5 μ m/m) every quarter: The laser interferometer emits laser and detects the verticality deviation in the feed direction through the reflection of the workbench movement; Use a flatness measuring instrument to check the flatness of the workbench surface. If it exceeds the tolerance, it needs to be corrected by adjusting the bolts at the bottom of the workbench;

Accuracy calibration of positioning fixture: The flatness of the reference surface of the positioning fixture for clamping prisms should be ≤ 0.001mm, and the coaxiality of the positioning pin should be ≤ 0.003mm. For every 200 prisms processed, a micrometer (with an accuracy of 0.001mm) should be used to calibrate the diameter of the positioning pin. The flatness of the reference surface should be detected using a differential interference microscope. If the reference surface is worn (flatness>0.002mm), it needs to be re ground and repaired with a diamond tool.

（2） Optimization and dynamic correction of grinding tools

The accuracy and wear state of the grinding tool directly affect the shape error of the prism surface. It is necessary to ensure the stability of the grinding tool accuracy through the process of "grinding tool selection pretreatment dynamic correction":

Grinding tool selection: During the milling and grinding stage, the grinding tool particle size is selected based on the hardness of the raw materials (80 # -120 # diamond grinding wheels are selected for K9 glass, and 120 # -180 # diamond grinding wheels are selected for quartz glass). The grinding wheel bonding agent is selected as a resin bonding agent (with good elasticity and reduced edge breakage); During the precision grinding stage, choose a finer grained grinding tool (W20-W7 silicon carbide grinding wheel) and a ceramic bonding agent (high hardness, ensuring flatness) as the bonding agent;

Grinding tool pretreatment: Before using the new grinding tool, it needs to undergo "shaping" treatment. Use a diamond polishing pen (pen tip diameter 0.5mm) to trim the surface of the grinding tool at a feed rate of 5mm/min, remove the protruding particles on the surface of the grinding tool, and make the flatness of the grinding tool ≤ 0.5 μ m; The precision grinding wheel requires a diamond grinding disc with a flatness error of ≤ 0.1 μ m for "finishing", ensuring that the surface roughness Ra of the grinding wheel is ≤ 0.1 μ m;

Dynamic correction of grinding tools: During the machining process, the grinding tools will gradually wear out and need to be dynamically corrected every 100 prisms processed. The milling grinding wheel should be trimmed 3-5 times with a trimming pen, removing 0.01-0.02mm of the grinding tool surface layer each time; Fine grinding wheels are polished 1-2 times with grinding discs to restore the flatness of the grinding tool and avoid the expansion of prism surface shape errors caused by tool wear.

（3） Fine tuning of process parameters

By optimizing the processing pressure, speed, feed rate and other parameters of milling and precision grinding, reducing processing stress and thermal deformation, and controlling shape errors:

Milling parameters: For the K9 glass right angle prism (size 20 × 20 × 20mm), the milling parameters are set as follows: grinding wheel speed 2800r/min, feed rate 8mm/min, processing pressure 2.5N, and coolant (water+5% grinding fluid) flow rate 60mL/min. Under this parameter, the flatness of the milled prism surface can be controlled at 2-3 μ m without obvious edge breakage;

Fine grinding parameters: During the fine grinding stage, the abrasive particle size needs to be gradually refined, divided into 3-4 processes: one process uses a W20 grinding wheel (speed 2000r/min, pressure 1.5N, feed rate 12mm/min) to reduce the flatness error to 1-1.5 μ m; The second step uses W14 grinding wheel (speed 1800r/min, pressure 1N, feed rate 15mm/min), reducing the flatness error to 0.5-1 μ m; The third step is to use a W7 grinding wheel (speed 1500r/min, pressure 0.8N, feed rate 18mm/min), with a flatness error controlled within 0.3-0.5 μ m, laying the foundation for subsequent polishing;

Collaborative control of pressure and speed: The processing pressure should be matched with the speed to avoid deformation of the raw material caused by excessive pressure (such as plastic deformation and surface indentation of K9 glass when the pressure is greater than 3N), or overheating of the grinding tool caused by excessive speed (such as thermal stress cracks on the glass surface when the grinding wheel temperature exceeds 50 ℃ when the speed is greater than 3500r/min). In actual processing, real-time monitoring is required through pressure sensors (accuracy ± 0.01N) and infrared thermometers (accuracy ± 1 ℃) to ensure parameter stability.

4、 Polishing stage: the core of controlling micro roughness errors

The goal of the polishing stage is to remove the grinding marks left by the precision grinding stage (depth 0.1-0.5 μ m), reduce the surface roughness to below Ra0.01 μ m, while maintaining macroscopic shape accuracy (flatness ≤ 0.5 μ m). The error control in this stage needs to focus on the collaborative optimization of polishing pads, polishing solution, and polishing parameters to avoid roughness exceeding the tolerance due to excessive or insufficient polishing.