Reverse engineering of laser rangefinder optics from 3D

Reverse Engineering Laser Rangefinder Optics from 3D Model

This time-of-flight (TOF) laser rangefinder is dissected through reverse engineering of its 3D model. From the housing geometry and lens cell dimensions, we reconstruct the input laser beam diameter at up to 0.8 mm, optical element types, and glass materials. Zemax simulations validate a beam expander design featuring a three-lens setup.

Structure of the Optical System

The pulsed laser rangefinder's optical layout includes these key components:

• Emitter — laser diode with collimating optics.
• Beam expander — telescopic system to enlarge the beam diameter.
• Receiver objective.
• Optical filter.
• Detector — avalanche photodiode.

The analysis zeroes in on the beam expander. The 3D model reveals lens cells, enabling reconstruction of the light paths. Lenses are numbered along the light path: 1 (negative diverging), 2 and 3 (positive components).

Geometry sets the max beam diameter on lens 1 at ~1.35 mm. Factoring in diffraction from the collimated diode, the input beam stays within lens 2's cell.

Analysis of Emitter and Output Beam

The emitter mirrors real Chinese units: blue sealant secures the optics, while the cantilever mount of the first expander lens on a glass block compromises centering accuracy. A pinkish tint on the exit lens suggests anti-reflective coating tuned to the laser diode wavelength (~905 nm).

The output beam is modeled as a Gaussian with ~0.8 mm diameter and apodization (13% edge energy). This is the limit derived from reference light paths between lenses.

Zemax Modeling

Geometry is imported into Zemax:

• Input beam: 0.8 mm, Gaussian profile.
• Lens 1: negative, concave surfaces.
• Materials: tested with CDGM glasses (H-FK71 low n_d=1.497; H-K9L like BK7).

The beam diameter ratio on lens 1 (0.256/0.15 ≈1.7) points to n=1.7 dense crown glass. With all H-K9L, it produces a diverging beam (165 mrad RMS, 21.4 mm exit vs. expected 14 mm).

Key deviations from beam expander expectations:

• Virtual image left of lens 1.
• Vignetting beyond D>0.8 mm.
• Non-afocal system, no parallel output.

Glass selection (not all H-K9L) and radius tweaks (R=117.38 mm nearly flat) are needed for afocality.

Comparison with Design Scheme

Author's beam expander design:

• Matching radii on negative lens (tooling efficiency).
• Flat surface instead of R=117.38 mm.
• Uniform glass (BK7/H-K9L).

The Chinese version is more complex: varied glasses, suboptimal radii. Geometry matches, confirming the telescopic scheme's versatility for laser beams.

Calculated beam diameters:

| Lens | Input D (mm) | Output D (mm) | Limit |

|------|--------------|---------------|-------------|

| 1 | 0.8 | 1.35 | Mount |

| 2 | ~2.5 | ~4 | Housing |

| 3 | ~12 | 14 | Exit |

Key Takeaways

• Input laser beam diameter ≤0.8 mm from cell geometry.
• Negative lens 1 needs n_d≈1.7 to cut vignetting.
• Afocal only with tailored CDGM glasses (not all H-K9L).
• Cantilever mount hurts precision, impacts range specs.
• Zemax aligns with independent calculations.

— Editorial Team