Context: A compact, handheld Raman spectrometer designed for field identification of chemical substances. The system uses a 785nm laser diode as the excitation source.
Usage of Filter: The bandpass filter is placed in the excitation optical path, immediately after the laser diode and collimating lens, but before the light hits the dichroic mirror.
Function: It acts as a "Laser Line Clean-up" filter. Diode lasers often emit minor "sidebands" or amplified spontaneous emission (ASE) at wavelengths other than the peak 785nm.
Result: The filter transmits only the pure 785nm narrow line and blocks the spectral noise. This prevents "false" Raman signals or background noise from overwhelming the detector, ensuring that the detected spectrum comes solely from the sample's interaction with the 785nm light.
US 11,835,525 B2 - Multi-spectral microparticle-fluorescence photon cytometry
Context: A high-throughput cytometry system that analyzes microparticles (like cells) using multiple wavelengths of light simultaneously. 785nm is used as one of the specific Near-Infrared (NIR) interrogation channels.
Usage of Filter: The filter is positioned in front of one of the dedicated avalanche photodiode (APD) or photomultiplier tube (PMT) detectors.
Function: It serves as a detection channel selector. In a multi-spectral system, light scatter and fluorescence from 405nm, 488nm, 532nm, and 638nm sources are all bouncing around the flow cell. This filter isolates only the 785nm scatter signal (or specific NIR fluorescence) while rejecting all other excitation wavelengths.
Result: It allows the system to accurately count particles or measure side-scatter (granularity) without cross-talk from the visible light channels, providing a distinct data parameter for cell classification.
US 10,416,081 B2 - Optical system and method for spectroscopy
Context: A Shifted Excitation Raman Difference Spectroscopy (SERDS) system. This technique uses two excitation wavelengths that are very close together (e.g., 784.5nm and 785.5nm) to subtract background fluorescence.
Usage of Filter: The filter is used within the laser stabilization module or as part of the external cavity locking mechanism.
Function: It strictly defines the excitation envelope. Unlike standard Raman where a broad 785nm center is acceptable, SERDS requires precise locking. The filter ensures that when the laser is tuned to 785nm, it does not drift or broaden.
Result: It enables the effective subtraction of fluorescence. Because the filter guarantees a clean 785nm shape, the "background" remains consistent between the two shifted scans, allowing the mathematical algorithm to remove the noise and reveal hidden Raman peaks.
