785nm Bandpass Filter

785nm light, a near-infrared wavelength with moderate penetration depth and reduced interference from ambient visible light, enables precise wavelength isolation.

  • Application 1: In Raman spectroscopy, the 785nm Bandpass Filter blocks stray light and enhances signal-to-noise ratio by allowing only the specific 785nm excitation wavelength to reach the sample, improving spectral accuracy.
  • Application 2: For fluorescence microscopy, it isolates the 785nm emission from fluorescent markers, eliminating background noise and enabling clear visualization of low-abundance biological targets in tissue samples.
  • Application 3: In optical communication systems, the filter ensures only the 785nm bandpass signal is transmitted, minimizing crosstalk with adjacent wavelengths and optimizing data transmission efficiency in short-range infrared links.

785nm Wavelength Filter Selection Guide for Raman Spectroscopy and LiDAR Systems

This guide provides technical specifications for selecting 785nm bandpass filters tailored to two key applications, explaining configuration rationales and problem-solving mechanisms without vendor recommendations.

1. Raman Spectroscopy Analysis Systems

Application Requirements

In Raman spectroscopy, 785nm lasers are used to excite samples, generating Raman scattering signals. Since Rayleigh scattering intensity is 10^6-10^8 times stronger than Raman signals, filters must efficiently separate excitation and scattered light while ensuring high transmission for Raman signals (wavelengths >785nm) to prevent feature peak masking by background noise.

Filter Configuration Scheme

Excitation Optical Path: 785nm Narrow-Bandpass Filter
  • Key Parameters:

- Central wavelength: 785±1nm

- Full width at half maximum (FWHM): ≤5nm

- Peak transmittance: ≥90%

- Blocking depth: OD ≥5 (200–1200nm range)

  • Selection Rationale:The narrow bandwidth precisely matches typical laser linewidths (≈0.1nm), while high blocking depth suppresses stray laser light, preventing excitation light leakage from interfering with detection.
Detection Optical Path: Long-Pass Filter + Notch Filter Combination
  • Long-Pass Filter:

- Cutoff wavelength: 780nm

- Transition bandwidth: ≤0.2% (e.g., LP01-780RU/S-25 series)

- Blocking depth: OD ≥6 (below 780nm)

  • Notch Filter:

- Central wavelength: 785nm

- Bandwidth: 9nm

- Blocking depth: OD ≥9

- Out-of-band transmittance: ≥90%

  • Synergistic Effect:The long-pass filter blocks >90% of Rayleigh scattering, while the notch filter further suppresses residual excitation light. Combined, they reduce background noise to <10⁻⁹ of original intensity, significantly improving signal-to-noise ratio (SNR).

Solved Key Issues

  • Signal Overwhelming:The two-stage filtering design reduces excitation light interference from \(10^6\) times to negligible levels, enabling clear visualization of Raman characteristic peaks (e.g., 50–4000cm⁻¹ fingerprint region).
  • Wavelength Selectivity:The narrow-bandpass filter ensures only 785nm laser irradiates the sample, avoiding fluorescence interference from other wavelengths. The long-pass filter precisely transmits Raman signals while blocking UV/visible background light.

2. Light Detection and Ranging (LiDAR) Systems

Application Requirements

In 785nm LiDAR systems, filters must enable efficient laser signal transmission and ambient light suppression. This wavelength is ideal for low eye-safety risk scenarios like indoor navigation and close-range detection in service robots and industrial automation equipment.

Filter Configuration Scheme

Transmitter Side: High-Transmittance Bandpass Filter
  • Key Parameters:

- Central wavelength: 785±2nm

- Bandwidth: 8±2nm

- Peak transmittance: ≥95%

- Angular response: ≤±0.5nm (0–45° incidence)

  • Selection Rationale:The narrow bandwidth minimizes spectral broadening of laser pulses, while high transmittance ensures <5% power loss. Angular stability guarantees wavelength shift within ±0.5nm across all incident angles, maintaining ranging accuracy.
Receiver Side: Ultra-Narrow Bandpass Filter + Bandstop Filter
  • Ultra-Narrow Bandpass Filter:

- Bandwidth: ≤4nm

- Blocking depth: OD ≥6 (outside 785±10nm)

- Thermal drift coefficient: ≤0.07nm/℃

  • Bandstop Filter:

- Suppression band: 900–1100nm

- Blocking depth: OD ≥5 (covering major near-infrared components of solar spectrum)

  • Synergistic Effect:The ultra-narrow bandpass filter allows only 785nm echo signals, while the bandstop filter blocks strong infrared interference (e.g., 940nm ambient light). Together, they maintain >99% signal purity under 1000lux illumination.

Solved Key Issues

  • Ambient Light Interference:The two-stage filtering design reduces ambient noise to <0.1% of signal intensity, tripling detection range in high-light environments.
  • Wavelength Stability:Thermal drift compensation (matching VCSEL lasers) ensures filter passband tracks laser wavelength shift within –40℃ to 85℃, preventing temperature-induced signal degradation.

Core Selection Principles

  • Spectral Matching Priority:Central wavelength deviation should be ≤±1nm from the laser source, with filter bandwidth <2× laser linewidth to avoid signal loss or background leakage.
  • Quantified Blocking Depth:Use OD ≥6 filters for Raman systems to suppress Rayleigh scattering, and OD ≥5 for LiDAR to block ambient light.
  • Dynamic Performance Consideration:Prioritize low angular sensitivity (wavelength shift ≤±1nm at 45° incidence) and low thermal coefficients (≤0.1nm/℃) for vibration/temperature-sensitive environments.
  • Material Compatibility:Choose fused silica or UV-grade glass substrates with low expansion coefficients (≤0.5ppm/℃), capable of withstanding >100mW laser power long-term without damage.

This configuration ensures optimal balance between signal quality and system stability for 785nm applications, meeting stringent requirements in research, industrial inspection, and automation fields.

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