How to ensure precise alignment of the optical path during the installation of optical prisms?

As the core component for optical path turning, splitting, and dispersion, the installation alignment accuracy of optical prisms directly determines the imaging quality, energy transmission efficiency, and functional stability of the optical system. Even if the angle accuracy of the prism itself meets the standard (such as a right angle prism with a 90 ° angle deviation of ≤ 0.001 °), if there is a slight deviation during installation (such as a 0.1 ° angle deviation or a 0.01mm position deviation), it may still cause optical path deflection, energy loss, or imaging distortion, especially in high-precision optical systems (such as laser rangefinders, microscopes, lithography equipment), alignment errors may even cause the system to completely fail. Therefore, the installation of prisms needs to follow the process of "preparation, positioning, and calibration", combined with mechanical positioning and optical calibration methods, to ensure that the optical path accurately matches the design requirements. Below, we will provide a detailed overview of the implementation path for precise alignment of optical paths from four dimensions: pre installation preparation, core alignment methods, different prism adaptation techniques, and post installation verification, in order to provide guidance for practical operation.

1、 Preparation before installation: laying the foundation for precise alignment

The preparation work before installation is the "prerequisite guarantee" for aligning the optical path, which requires eliminating external interference from three aspects: environmental control, tool calibration, and prism preprocessing to ensure the accuracy of subsequent operations.

（1） Environmental control: eliminate external interference factors

Optical systems are highly sensitive to temperature, humidity, cleanliness, and vibration in the installation environment, and environmental fluctuations can directly affect alignment accuracy

Temperature and humidity control: The installation environment temperature should be stable at 20 ± 2 ℃, with a relative humidity of 30% -50%. Temperature changes can cause thermal expansion and contraction of prism substrates (such as glass and crystals) (such as K9 glass with a thermal expansion coefficient of 7.1 × 10 ^ -6/℃, and a temperature fluctuation of 1 ℃ can cause a length change of 0.71 μ m in a 100mm long prism), leading to optical path deviation; High humidity may cause condensation of water vapor on the surface of prisms, affecting the accuracy of optical calibration.

Control method: Operate in a clean laboratory (Class 1000) equipped with a constant temperature and humidity system (temperature accuracy ± 0.5 ℃, humidity accuracy ± 5%); 12 hours before installation, place the prism and installation components (such as mirror holder and bracket) in the laboratory environment for temperature balance to avoid temporary deformation caused by temperature differences.

Cleanliness control: If there are dust particles (such as dust with a diameter ≥ 0.1 μ m) on the surface of the prism, it will cause light scattering and affect the clarity of the calibration light path; Particles may also cause slight tilting of the prism during installation.

Control method: The installation operation should be carried out on a clean workbench (Class 100), and the operator should wear dust-free gloves, masks, and hair caps to avoid contaminating the prism with hand oil and dandruff; Clean the prism surface and installation components with compressed air (filtration accuracy of 0.3 μ m), and if necessary, use a dust-free cloth dipped in anhydrous ethanol (purity ≥ 99.7%) to gently wipe the transparent surface of the prism to ensure that there are no visible pollutants on the surface.

Vibration control: Vibration during installation (such as ground vibration, tool collision) can cause prism positioning deviation, especially in high-precision alignment (such as angular deviation ≤ 0.0001 °), which has a significant impact.

Control method: Select an anti vibration workbench (vibration amplitude ≤ 0.1 μ m, frequency ≤ 5Hz), and install an air spring shock absorber below the workbench; Prohibit operations that are prone to vibration, such as knocking or carrying heavy objects, near the installation area; The tools used (such as tweezers and wrenches) should be placed on anti slip pads to avoid slipping and collision.

（2） Tools and equipment calibration: ensuring accurate measurement benchmarks

If there are errors in the positioning tools and measuring equipment used during installation, they will be directly transmitted to the prism alignment results and need to be calibrated in advance:

Mechanical tool calibration:

Positioning fixtures (such as prism fixtures and mirror holders): Use a coordinate measuring instrument (with an accuracy of ± 0.0001mm) to detect the positioning reference plane of the fixture (such as the V-shaped groove of the fixture and the installation plane of the mirror holder), ensuring that the flatness of the reference plane is ≤ 0.0005mm/m and the perpendicularity is ≤ 0.001 °;

Tightening tools (such as torque wrenches and hex screws): The torque wrench needs to be calibrated with a standard torque meter (torque range 0-10N · m, accuracy ± 1%) to ensure that the tightening torque of the screw meets the requirements during installation (for example, M3 screws usually have a torque of 0.5-0.8N · m, which can cause deformation of the mirror seat if too large, and loosening if too small);

Angle measuring tools (such as angle rulers and level gauges): Use standard angle blocks (accuracy ± 0.0001 °) to calibrate the angle ruler, and use laser level gauges (accuracy ± 0.001 °/m) to calibrate the level gauge, ensuring that the angle measurement error is ≤ 0.0005 °.

Preparation of optical calibration equipment:

Optical equipment is required for real-time monitoring of the optical path during installation, and commonly used equipment includes:

Laser collimator (output wavelength 632.8nm, beam divergence angle ≤ 0.1mrad): used to provide high-precision reference optical path;

Four quadrant detector (position resolution ≤ 0.1 μ m): used to detect the offset of the beam center;

Autocollimator (measurement accuracy ± 0.0001 °): used to detect angular deviation of prisms;

Spectrometer (wavelength accuracy ± 0.1nm): used for spectral alignment verification during the installation of a prism.

These devices need to be preheated in advance (such as preheating the laser collimator for 30 minutes) and calibrated with standard optical components (such as standard flat mirrors) to ensure accurate measurement data.

（3） Prism and installation component pre-processing: eliminating inherent errors

The preprocessing of prisms and installation components can reduce the impact of inherent errors on alignment. Specific operations:

Prism performance retest: Before installation, use an autocollimator to test the angular accuracy of the prism (such as 90 ° and 45 ° angles for a right angle prism), and use an interferometer to test the flatness of the prism (flatness of the transparent surface ≤ 0.05 λ, λ=632.8nm) to ensure that the prism's performance meets installation requirements; If there is a slight error in the prism (such as an angle deviation of 0.002 °), the error value should be recorded and compensated during installation.

Cleaning and adaptation of installation components:

The installation holes and positioning pins of components such as mirror holders and brackets need to be tested with a special caliper (precision H7) to ensure that the aperture tolerance matches the screws and positioning pins (such as M4 screws adapted to a diameter of 4H7 holes);

The burrs and scratches on the surface of the components need to be polished flat with fine sandpaper (over 8000 grit) to avoid tilting the prism during installation;

If the prism and mirror holder are bonded and fixed, it is necessary to test the shrinkage rate of the adhesive (such as UV curing glue) in advance (shrinkage rate ≤ 0.5%) to avoid prism displacement caused by shrinkage after curing.

2、 Core alignment method: combination of mechanical positioning and optical calibration

The core of prism installation alignment is to first achieve rough positioning through mechanical structure, and then perform precise calibration through optical means. The combination of the two can control the alignment error within the design allowable range (usually angle deviation ≤ 0.001 °, position deviation ≤ 0.01mm).

（1） Mechanical coarse positioning: establishing preliminary optical path reference

Mechanical coarse positioning fixes the prism in a rough position through the mechanical structure of the installation components (such as positioning pins, V-grooves, reference surfaces), laying the foundation for subsequent fine calibration. There are three commonly used positioning methods:

Reference plane positioning: suitable for planar prisms (such as right angle prisms, isosceles prisms), using a precise reference plane (flatness ≤ 0.0005mm) on the mirror holder to align with one side of the prism for positioning:

Operation steps: Gently attach the positioning side of the prism (marked in advance, usually a non optical surface) to the reference surface of the mirror holder, ensuring that the bonding gap is ≤ 0.001mm (can be checked with a feeler gauge); Limit the translation direction of the prism through the limit block on the mirror holder (accuracy ± 0.001mm), and preliminarily fix the prism (such as gently pressing it with an elastic pressure plate to avoid excessive force causing prism deformation);

Accuracy control: At this stage, the position deviation of the prism can be controlled within 0.05mm, and the angle deviation can be controlled within 0.01 °.

V-shaped groove positioning: suitable for cylindrical prisms (such as cylindrical prisms) or prism components with cylindrical surfaces, using the centering effect of V-shaped grooves (angle accuracy ± 0.001 °, surface roughness Ra ≤ 0.02 μ m) to achieve positioning:

Operation steps: Place the cylindrical surface of the prism into the V-shaped groove, ensuring that the cylindrical surface is in uniform contact with both sides of the V-shaped groove (a micrometer can be used to detect the radial runout of the prism, with a runout of ≤ 0.001mm); Limit the axial displacement of the prism through the stoppers at both ends of the V-shaped groove and preliminarily fix it;

Advantages: The V-shaped groove positioning can automatically center, reducing human operation errors, especially suitable for batch installation.

Positioning pin positioning: Suitable for prisms that require high-precision repeated installation (such as beam splitters in lithography equipment), it is positioned by matching the positioning pin (accuracy h6) on the mirror base with the positioning hole (accuracy H7) on the prism base:

Operation steps: Align the positioning hole of the prism base with the positioning pin on the mirror base, gently insert it, and ensure that the fit gap between the positioning pin and the hole is ≤ 0.002mm; preliminarily fix the prism base on the mirror base with screws, and do not tighten the screws temporarily (leave 0.5-1 turn of clearance for subsequent fine adjustment);

Accuracy control: The repeatability accuracy of the positioning pin can reach 0.001mm, suitable for scenarios that require multiple disassembly and installation.

（2） Optical precision calibration: achieving precise matching of optical paths

After rough mechanical positioning, it is necessary to detect the deviation of the optical path through optical means and fine tune the position and angle of the prism until the optical path meets the design requirements. Common precision calibration methods are divided into three categories according to the prism function:

1. Turning prism (such as right angle prism, ridge prism): Ensure accurate deflection angle of the optical path

The core function of a turning prism is to change the direction of the optical path (such as a right angle prism that deflects the optical path by 90 °). Accurate calibration requires ensuring that the deviation angle error is ≤ 0.001 °. Common methods include "self collimation method" and "laser collimation method":

Self collimation method (suitable for small angle deviation calibration):

Principle: The parallel light emitted by the autocollimator is reflected by a prism, and the reflected light returns to the autocollimator. The prism angle deviation is calculated by detecting the offset of the reflected light.

Operation steps:

Fix the autocollimator on the optical platform, adjust the autocollimator so that its optical axis coincides with the incident optical axis of the designed optical path (observe through the cross reticle of the autocollimator to ensure that the incident optical axis is aligned with the center of the reticle);

Install the prism according to the mechanical coarse positioning, and the parallel light from the autocollimator is incident on the reflective surface of the prism (such as the 45 ° reflective surface of a right angle prism), and the reflected light returns to the autocollimator;

Observe the offset of the reflected light spot on the collimator reticle: if the light spot is offset in the horizontal direction, it indicates that there is a horizontal angle deviation in the prism; If there is a vertical deviation, it indicates the presence of a vertical angle deviation;

Fine tuning prism: Adjust the angle of the prism through the fine tuning screw on the mirror holder (such as a micro splitter, with an accuracy of 0.001mm). After each adjustment, observe the offset of the light spot of the autocollimator until the light spot returns to the center of the dividing plate. At this point, the prism deflection angle meets the design requirements (such as 90 ° deviation ≤ 0.001 °).

Laser collimation method (suitable for long-distance optical path calibration):

Principle: Using high parallelism laser (divergence angle ≤ 0.1mrad) emitted by a laser collimator as a reference optical path, adjust the position of the prism by detecting the deviation between the laser emitted from the prism and the designed optical path.

Operation steps:

Place a laser collimator and a four quadrant detector at the entrance and exit ends of the designed optical path, adjust their optical axes, and ensure that the laser is accurately incident on the center of the four quadrant detector (the detector displays an offset of 0);

After installing the prism, the laser is deflected by the prism and incident on the four quadrant detector. The X and Y direction offsets displayed by the detector are recorded (such as X direction offset of 0.1mm and Y direction offset of 0.05mm);

Calculate the prism adjustment corresponding to the deviation: According to the optical path deflection formula (offset=optical path length x tan θ, where θ is the angle deviation), if the optical path length is 1m and the X direction offset is 0.1mm, corresponding to θ ≈ 0.0057 °, the prism horizontal angle needs to be adjusted to reduce by 0.0057 °;

Adjust the angle and position of the prism with a fine adjustment frame (accuracy ± 0.0001 °) until the four quadrant detector displays an offset of ≤ 0.001mm, at which point the optical path deflection accuracy meets the standard.

2. Spectral prism (such as semi transparent and semi reflective prism, polarizing spectral prism): Ensure accurate spectral ratio and direction

The splitting prism needs to ensure that the angle deviation of the two outgoing beams is ≤ 0.001 ° and the splitting ratio error is ≤ 2% simultaneously. The commonly used calibration methods are the "dual detector method" and the "spectrometer calibration method":

Dual detector method (applicable for angle and energy calibration):

Operation steps:

Use a laser collimator to emit monochromatic laser (such as 632.8nm), which is incident on the splitting surface of a prism. The prism divides the laser into two paths: reflected light and transmitted light;

Place four quadrant detectors (to detect angle deviation) and power meters (to detect energy ratio) at the design positions of the two outgoing light paths;

Angle calibration: If the reflected light detector displays an offset of 0.002mm (with a light path length of 1m), the corresponding angle deviation is approximately 0.000115 °. By adjusting the tilt angle of the prism (such as adjusting the normal direction of the light splitting surface), the offset of both detectors should be ≤ 0.001mm;

Energy ratio calibration: If the designed spectral ratio is 1:1, and the power meter displays that the reflected light power accounts for 55% and the transmitted light accounts for 45%, the incident angle of the prism needs to be fine tuned (such as slightly rotating the prism to change the angle between the incident light and the spectral surface) until the error of the two power ratios is ≤ 2% (such as 49% -51%).

Spectrometer calibration method (applicable to broadband spectral prisms):

For spectral prisms that cover visible and infrared wavelengths (such as those used in spectrometers), it is necessary to calibrate the spectral accuracy at different wavelengths:

Use a broadband light source (such as a deuterium lamp+tungsten halogen lamp, covering 200-2500nm) to enter the spectral prism, and connect the two outgoing lights to the spectrometer separately;

Record the intensity ratio of two light channels at different wavelengths (such as 400nm, 600nm, 800nm) and compare it with the design ratio;

If the proportion deviation at a certain wavelength is too large (such as a 5% deviation in the proportion of transmitted light at 800nm), the position of the prism needs to be adjusted (such as shifting along the optical axis to change the position of the incident light on the beam splitter) until the proportion error of the entire wavelength band is ≤ 3%.

3. Dispersion prism (such as equilateral prism): Ensure that the dispersion wavelength matches the optical path

The core function of a dispersive prism is to decompose composite light into monochromatic light according to wavelength. Calibration requires ensuring that the angle of the emitted light at different wavelengths meets the design value (error ≤ 0.001 °). The commonly used method is the "monochromatic light tracking method":

Operation steps:

Use a monochromator to output monochromatic light of specific wavelengths (such as 450nm, 550nm, 650nm), which is incident on the incident surface of a dispersive prism;

Place an angle meter (with an accuracy of ± 0.0001 °) at the exit end of the prism to measure the exit angles of monochromatic light of different wavelengths;

Compare the measured angle with the design angle (e.g. the designed 550nm light emission angle is 30 °, while the measured angle is 30.002 °), and calculate the angle deviation;

Fine tune the incident angle of the prism (such as changing the angle between the incident light and the incident surface of the prism by rotating the prism holder) until the deviation of the exit angle of each wavelength is ≤ 0.001 °;

After calibration is completed, use composite light (such as white light) to enter and observe the dispersion spectrum through a spectrometer to see if it is uniform, without significant shift or distortion.