The processing and manufacturing flow of the SHORT PASS FILTER

Processing and manufacturing flow (technical description) of SHORT PASS FILTER (short pass filter) 

The core function of UltraOpto's SHORT PASS FILTER (short-pass filter) is to efficiently transmit short-wave light and precisely cut off long-wave light. With the advantages of high transmittance, deep cut-off depth, and low ripple, it is widely used in spectral imaging, laser systems, biological detection, infrared detection, security monitoring, and other fields. The following is the full process of standardized processing and manufacturing, from substrate selection to finished product delivery. Each process strictly controls the precision and performance to ensure batch consistency and long-term stability of the products. 

I. Core Manufacturing Process (8 Key Procedures) 

1. Base material selection and pretreatment (performance foundation) 

The optical performance of short-wave pass filters depends on high-quality substrates. Precise selection based on the target working band is required, and the quality of raw materials must be strictly controlled throughout the process. 

Material selection: 

BK7 Grade A optical glass (visible light band) and fused silica (UV/near-infrared band) are chosen for general applications, while Ge (germanium) and ZnS (zinc sulfide) are selected for infrared short-pass scenarios. This ensures that the base materials have no absorption and low dispersion in the target short-wave band, meeting the requirements of different application scenarios. 

Raw material inspection: 

The internal uniformity and stress of the substrate are detected by a Twyman-Green interferometer, and the transmission rate in the target short-wave band is verified by a spectrophotometer. Bubbles, streaks, stones and other defects are removed to ensure that the transmission rate of the substrate in the target band is ≥ 92% and there is no obvious interference from impurities. 

Rough blank cutting and cleaning: 

The base material is cut into blanks close to the finished product size using a diamond wire saw, with a dimensional allowance controlled within 0.5-1.0mm. After cutting, the surface oil and dust are removed by ultrasonic cleaning (deionized water + neutral cleaner), and then the blanks are dried and sent to a 100-level clean room for standby to prevent impurities from affecting subsequent processing. 

2. Precise upper plate positioning (clamping core) 

The blank is fixed by using the glue-free contact method (Van der Waals force bonding) or the low-temperature soldering method to avoid the stress caused by the glue layer, which may lead to the deformation of the substrate and affect the surface shape accuracy. During clamping, the reference surface of the substrate is used as the positioning basis, and the flatness and parallelism of the reference surface are strictly controlled. The clamping deviation is ≤ 0.001mm/m, providing a high-precision reference for subsequent grinding and polishing processing and ensuring that the substrate does not shift during the processing. 

3. Rough grinding and fine grinding of the profile (key to forming) 

Coarse grinding: Use a diamond grinding wheel to roughly grind both sides of the substrate to the designed dimensions, removing most of the excess material to form a regular sheet-like shape. Focus on controlling the uniformity of the substrate thickness, and keep the surface roughness below Ra 1.6 μm to avoid deep scratches. 

Fine grinding: Using the reference surface as a benchmark, the incident surface and the exit surface are finely ground with resin-bonded grinding wheels. The double-sided separator is used for disk processing to strictly control the parallelism of the two surfaces to be ≤ 0.1 μm, with surface shape accuracy reaching λ/4 @ 632.8 nm and angle tolerance ≤ ± 3″, laying a flat foundation for subsequent ultra-precision polishing and coating. 

4. Ultra-precision polishing (surface shape core) 

Ultra-precision polishing is carried out in a 100-level clean environment with constant temperature (20 ± 0.5℃) and humidity. A polyurethane polishing pad is used in conjunction with cerium oxide (visible light band) or silica (UV/IR band) polishing liquid, and the polishing pressure and speed are precisely controlled. After polishing, the surface roughness of the substrate Ra is ≤ 0.2nm, the surface shape accuracy reaches λ/[email protected] (for high-end custom models, it can reach λ/20), and the surface quality meets the 20-10 (scratch/pit) standard, completely removing the residual scratches from fine grinding, ensuring that short-wave light has no scattering and no additional loss. 

5. Short-wave pass filter system design and vacuum coating (functional core) 

This is the core process in the manufacturing of short-wave pass filters, directly determining the performance of short-wave transmission and long-wave cutoff. It adopts a multi-layer dielectric film system design and high-precision vacuum coating technology: 

Film stack design: 

Based on the target cut-off wavelengths (such as 400nm, 550nm, 800nm), a multi-layer dielectric film stack is designed by alternately stacking high refractive index materials (TiO₂, Ta₂O₅, Ge, etc.) and low refractive index materials (SiO₂, ZnS, etc.). A mathematical model for film thickness deposition is established using Matlab software to simulate and adjust the parameters of the multi-layer films, reducing thickness errors^{(1)}, and ensuring that the short-wave transmission rate and long-wave cut-off depth meet the standards. 

Vacuum coating: 

The polished substrate is sent into a high vacuum coating chamber. The film system is deposited layer by layer on the substrate surface by using ion beam assisted deposition (IAD) or electron beam, resistance thermal evaporation technology. During the coating process, the thickness of the film layer is precisely controlled by a crystal controller, with a thickness accuracy of ±0.3nm, ensuring the uniformity of the film layer. 

Film layer inspection: 

After the coating is completed, a spectrophotometer is used to measure the short-wave transmittance and long-wave cut-off depth, and an ellipsometer is used to measure the refractive index and thickness of the film layer. It is ensured that the short-wave band transmittance of the target is ≥ 90% (≥ 95% for high-end customization), the long-wave cut-off depth OD is ≥ 4 (≥ 6 for high-end customization), and the transition band is steep without tailing. 

6. Annealing treatment (stability strengthening) 

Place the coated filter into a constant-temperature annealing furnace, and adopt a stepwise heating process (50℃/h) to 150-200℃. After maintaining a constant temperature for 2-4 hours, slowly cool down (30℃/h) to room temperature. This effectively releases the stress generated in the film during the coating process, preventing film peeling and cracking, and simultaneously enhances the adhesion of the film and the stability of its optical performance. This method is particularly effective for infrared substrates such as Ge and ZnS, significantly optimizing the compatibility of the film system. 

7. Secondary Processing and Edge Reinforcement 

Precision cutting: 

According to the designed dimensions, the filter is precisely cut into the final shape (round, square or irregular) by a precision cutting machine. The dimensional tolerance is controlled within ±0.1mm to ensure proper installation and fit. 

Chamfering treatment: 

Use diamond chamfering wheels to chamfer all sharp edges at 0.2-0.5mm × 45° to prevent chipping and corner loss, and enhance the product's impact resistance. 

Cleaning and drying: 

Wipe the surface of the filter with anhydrous ethanol and a lint-free cotton swab to remove impurities left during coating and cutting. Then, dry it with nitrogen and in a constant temperature oven to ensure that the optical surface is free of stains and fingerprints. 

8. Final Inspection and Packaging for Shipment (Quality Closed Loop) 

In the 100-level clean room, the finished products are subject to all-round testing to ensure that each product meets the standards. 

Optical performance testing: 

Use a spectrophotometer to test the short-wave transmittance, long-wave cut-off depth, and transition band width to ensure they meet the design requirements. 

Geometric accuracy inspection: Use a laser interferometer to inspect surface shape accuracy and parallelism, and use calipers to measure dimensional tolerances to ensure that all accuracy requirements are met. 

Reliability testing: 

Sampling for temperature and humidity tests (85°C/85% RH, 500 hours) and film adhesion tests (in accordance with ISO 15184 standard) to ensure no film peeling and no transmission rate drift. 

Packaging and Outbound: 

After passing the inspection, the products are independently packaged with anti-static and scratch-resistant materials, with desiccants inside and detailed inspection reports attached. This is to prevent damage during transportation and storage and ensure that the products are in good condition upon delivery. 

II. Process Characteristics and Application Compatibility 

1. High performance accuracy: 

Utilizing a multi-layer dielectric film system design and vacuum coating technology, combined with the mathematical model correction of film thickness deposition, it ensures high transmission in the short-wave band, deep cut-off in the long-wave band, a steep transition zone, and no stray light leakage, meeting the requirements of different wavelength bands. 

2. Strong stability: 

Through stress-free clamping, annealing treatment and strict quality control, problems such as film layer peeling and stress deformation are effectively avoided. After wet heat cycle testing, the transmittance drift is ≤1%, and it can work stably in the temperature range of -40℃ to 85℃. 

3. Strong customization: 

The cut-off wavelength (200nm - 2000nm), substrate (optical glass/fused silica/Ge/ZnS, etc.), size (round/square/irregular shape), and optical performance can be customized as needed, suitable for different scenarios such as ultraviolet, visible light, and near-infrared. 

4. Batch consistency: 

The entire process is managed in a closed loop with metrology-grade equipment. Every step from raw materials to finished products is subject to inspection. The performance deviation of products within the same batch is ≤ ±3%, meeting the requirements for bulk purchasing. 

UltraOpto SHORT PASS FILTER (short-pass filter) provides high-reliability core optical components for spectral imaging, laser systems, biological detection and other fields with strict process control, precise film design and customization capabilities, helping customers' systems achieve precise short-wave transmission and long-wave cut-off control.