How to manufacture high-precision long pass filters

How to Manufacture High-Precision LONG PASS FILTER (Long-Wave Pass Filter) (Technical Specifications)

UltraOpto's high-precision LONG PASS FILTER (Long-Wave Pass Filter) has core requirements of precisely transmitting long-wave light, efficiently blocking short-wave light, and featuring high transmittance, deep cut-off depth, narrow transition band, high surface shape accuracy, and excellent batch consistency. It is suitable for high-end scenarios such as laser systems, spectral imaging, and scientific research detection. The high-precision manufacturing process must be strictly controlled throughout the entire process, from substrate selection to finished product inspection, with meticulous attention to precision and details in each step. The following are the core manufacturing points and technical control standards.

I. Core Premise for High-Precision Manufacturing: Substrate Selection and Pre-treatment (Laying the Foundation for Precision) 

1.The optical uniformity and stability of the substrate directly determine the basic accuracy of the filter. It needs to go through strict selection and pretreatment to eliminate the influence of raw material defects on subsequent processing. 

Precise selection: 

Based on the target long-wave band (such as 500nm, 800nm, 1100nm and above), select the appropriate substrate - for the visible light/near-infrared band, use fused silica (with high uniformity and low dispersion); for the mid-infrared band, use infrared-transparent substrates such as Ge (germanium) and ZnSe (zinc selenide), ensuring that the substrate has no absorption and no stray light in the target long-wave band and no additional transmission in the short-wave band. 

2. Stringent Testing: 

The optical uniformity of the substrate is inspected using a Twyman-Green interferometer to ensure there are no bubbles, streaks, or lattice distortions. A spectrophotometer is used to verify the full-spectrum transmittance, with the target long-wave band substrate transmittance ≥ 92% and no obvious transmission peaks in the short-wave band. The internal stress of the substrate is detected by an X-ray stress analyzer to eliminate products with excessive stress and prevent deformation during subsequent processing. 

3. Stress-free pretreatment: 

Use diamond wire saw for low-temperature cutting, with a dimensional allowance controlled within 0.5-1.0mm. After cutting, clean the surface impurities with ultrasonic cleaning (deionized water + neutral detergent), dry it, and then send it to a 100-level clean room. Use a glue-free clamping pretreatment to avoid stress caused by the adhesive layer and ensure that the substrate is flat and free of deformation. 

II. Core Processes of High-Precision Manufacturing: Process Control and Precision Assurance 

Ultra-precision grinding and polishing: 

Controlling surface shape and parallelism accuracy 

The surface shape accuracy and parallelism are the key geometric indicators of high-precision long-pass filters, directly affecting the accuracy of light path transmission. They need to be precisely controlled through a two-stage grinding and polishing process. 

1.Fine grinding stage: 

Resin-bonded diamond grinding wheels are used in conjunction with double-sided separators for disc processing. The base material's reference surface is used for positioning, and the parallelism of both sides is strictly controlled to be ≤ 0.1 μm, with the surface roughness Ra ≤ 1.6 μm, to prevent deep scratches and stress concentration. 

Polishing stage: 

In a 100-level clean environment with constant temperature (20 ± 0.5℃) and humidity, a polyurethane polishing mold is used in conjunction with dedicated polishing liquid (cerium oxide for visible light and silicon dioxide for infrared), precisely controlling the polishing pressure and speed to achieve ultra-precision polishing. The final surface shape accuracy reaches λ/[email protected] (customized high-end can reach λ/20), the surface roughness Ra ≤ 0.2nm, and the surface quality meets the 20-10 (scratch/pit) standard, eliminating surface defects that cause light scattering. 

2. High-precision coating design and vacuum coating: Controlling the accuracy of optical performance 

The optical accuracy of long-wave pass filters (central cut-off wavelength, transmittance, and cut-off depth) mainly depends on the design of the coating system and the coating process, which are the core links in high-precision manufacturing. 

Precise coating design: 

Based on the target cut-off wavelength, a multi-layer dielectric coating system is designed with high refractive index materials (such as TiO₂, Ta₂O₅, Ge, etc.) and low refractive index materials (such as SiO₂, Al₂O₃, etc.) alternately stacked. Through professional optical simulation software, a model is established to optimize the thickness and number of layers of the coating, ensuring that the transition band width is ≤ 5nm (≤ 3nm for high-end customization), the long-wave transmission rate is ≥ 95%, the short-wave cut-off depth OD is ≥ 4 (≥ 6 for high-end customization), and there is no stray light leakage. 

High-precision vacuum coating: 

The polished substrate is sent into an ultra-high vacuum coating chamber (with a vacuum degree of ≥10⁻⁶Pa) and ion beam assisted deposition (IAD) technology is adopted. The thickness of each layer of the film is precisely controlled in real time by a crystal controller, with a thickness accuracy of ±0.3nm, ensuring the uniformity of the film layer is ≤±1%. During the coating process, the temperature and deposition rate are strictly controlled to avoid stress in the film layer, prevent film layer peeling and cracking, and enhance the adhesion of the film layer. 

Post-coating annealing: 

The coated filter is placed in a constant-temperature annealing furnace, and a stepwise temperature increase and decrease process is adopted (50°C/h for heating, 2-4 hours at constant temperature, and 30°C/h for cooling). This effectively releases the stress in the film layer, stabilizes the performance of the film system, and ensures that the drift of the central cut-off wavelength is ≤ ±1nm, with no performance degradation during long-term use. 

3. Precise cutting and edge strengthening: Controlling dimensions and installation accuracy 

High-precision filters need to be adapted to the installation requirements of optical systems, and the dimensional tolerances and edge quality must be strictly controlled. 

Precision cutting: 

Utilizing CNC precision cutting machines, we cut according to the designed dimensions (circular, square, or irregular shapes), with dimensional tolerances controlled within ±0.05mm (±0.03mm for high-end custom orders). During the cutting process, we prevent chipping and corner breakage to ensure a neat and regular shape. 

Edge chamfering and strengthening: 

Use a diamond chamfering wheel to precisely chamfer at 0.2-0.5mm × 45°, removing sharp edges and enhancing impact resistance; for high-power laser scenarios, additional edge strengthening treatment is performed to reduce edge scattering and increase the laser damage threshold (≥100mJ/cm² @ 1064nm, 10ns pulse). 

III. High-Precision Assurance: Full-Process Quality Control and Inspection 

1.The manufacturing of high-precision long-wave pass filters requires the establishment of a dual quality control system of "process inspection + full inspection of finished products" to ensure that each product meets the precision standards. 

Process inspection: 

Inspect uniformity and stress after substrate pretreatment; inspect surface shape accuracy, parallelism and surface quality after grinding and polishing; monitor film thickness and refractive index in real time during coating with an ellipsometer; inspect the stability of optical performance after annealing. 

2. Final inspection of finished products: 

In a 100-level clean room, a spectrophotometer is used to test the central cut-off wavelength, long-wave transmittance, short-wave cut-off depth and transition band width; a laser interferometer is used to test the surface shape accuracy and parallelism; a two-dimensional image measuring instrument is used to test the dimensional tolerance; high and low temperature and humidity tests (-40℃ to 85℃, 85% RH, 500 hours) and film adhesion tests are conducted on a sample basis to ensure that the optical performance, geometric accuracy and reliability all meet the standards. 

3. Batch consistency control: 

Standardized production processes are adopted, and 10% of each batch of products are randomly selected for full parameter testing to ensure that the optical performance deviation of products within the same batch is ≤ ±3%, and the dimensional deviation is ≤ ±0.05mm, meeting the requirements of high-end batch applications. 

IV. UltraOpto's Advantages in High-Precision Manufacturing 

With years of experience in manufacturing optical components, UltraOpto, through its core technology of "precise substrate selection + ultra-precision processing + high-precision coating + full-process quality control", can produce high-precision long-pass filters with a center cut-off wavelength accuracy of ±1nm, a surface shape accuracy of λ/[email protected], and a transition band width of ≤3nm. It supports customization in the 200nm-2000nm wavelength band and is suitable for high-end applications such as laser systems, spectral imaging, and scientific research and detection, providing precise and stable long-wave transmission and short-wave cut-off solutions for optical systems.