Schematic diagram of TOF LiDAR principle

Introduction to Laser Ranging Technology

Laser ranging technology, a cornerstone of precision measurement, has evolved significantly with advancements in laser systems. Widely used in military, civilian, and autonomous driving applications, this technology relies on high-performance components like narrow bandpass filters to ensure accurate signal detection. In particular, 905nm laser ranging systems are popular in LiDAR for autonomous vehicles due to their cost-effectiveness and compatibility with silicon detectors.

Why 905nm Lasers and Filters?

Laser ranging systems commonly use 905nm or 1550nm laser sources. The 905nm wavelength is preferred in applications like autonomous driving and guidance systems because it pairs well with affordable silicon-based receivers, unlike 1550nm, which requires more expensive Ge/InGaAs detectors. Narrow bandpass filters are critical for suppressing ambient light and secondary emissions, ensuring reliable signal capture. These filters must be carefully selected to match the laser’s characteristics and operational conditions.

Key Factors in Selecting 905nm Narrow Bandpass Filters

Choosing the right filter parameters for a 905nm laser ranging system involves several considerations to optimize performance. Below are the primary factors to evaluate:

1. Laser Wavelength Tolerance
   Manufacturers typically specify a wavelength tolerance for 905nm lasers, such as ±5nm. This range accounts for variations in production and ensures the filter’s center wavelength aligns with the laser’s output.

2. Temperature-Induced Wavelength Shift
   The center wavelength of 905nm semiconductor lasers shifts with temperature, typically by 1nm per 10°C increase. For example, in high-temperature environments, the laser wavelength may shift from 905nm to 910nm or higher, requiring a filter with a slightly longer center wavelength to accommodate this drift.

3. Measurement Angle Range
   The angle of incidence affects the filter’s effective center wavelength. Larger measurement angles, such as ±20° in autonomous driving, cause the filter’s passband to shift to shorter wavelengths. To compensate, a wider bandwidth and a slightly longer center wavelength may be necessary. For instance, a 20° angle can shift the filter’s center wavelength by approximately 13-23nm, depending on the filter’s effective refractive index.

4. Manufacturing Tolerances
   Filter production introduces tolerances in center wavelength and bandwidth. High-quality filters minimize these variations, but designers must account for potential deviations to ensure consistent performance across different units.

Recommended Filter Parameters for Autonomous Driving

For a typical 905nm laser ranging system used in autonomous driving with a measurement angle of around 20°, the following filter parameters are suggested based on practical experience:

These parameters ensure the filter accommodates wavelength shifts due to temperature and angle variations while maintaining high transmission and effective blocking of unwanted light. The 66nm bandwidth, though wider than typical 10nm filters, is chosen to cover the laser’s spectral width, temperature-induced shifts, and angle-dependent effects, ensuring robust performance in dynamic environments.

Applications and Considerations

The choice of filter parameters varies by application. In autonomous driving, where systems operate in diverse conditions, a wider bandwidth may be preferred for reliability. In contrast, applications requiring ultra-precise measurements, such as scientific instruments, may use narrower bandwidths (e.g., 10nm) for enhanced signal-to-noise ratios. Always consult with filter manufacturers to verify specifications and consider custom solutions for specific needs.

Conclusion

Selecting the right 905nm narrow bandpass filter is crucial for optimizing laser ranging systems, particularly in demanding applications like autonomous driving. By carefully considering laser wavelength tolerance, temperature effects, measurement angles, and manufacturing tolerances, designers can ensure reliable performance. The recommended parameters provide a practical starting point, but customization may be necessary to meet unique operational requirements.