How to Assemble a Confocal Raman Instrument

Confocal Raman spectroscopy is a highprecision analytical method built on conventional Raman by integrating a confocal microscope optical path. By placing a confocal pinhole in the beam path, only the Raman signal from the sample's focal plane is allowed to pass through, effectively rejecting outoffocus stray light and fluorescence. This enables micronscale spatial resolution and depthsectioning capability.

This technique is especially suitable for microareas, thin films, multilayer interfaces, heterogeneous samples, and highly fluorescent systems. It also supports Raman imaging and 3D depth profiling. Confocal Raman has become a core characterization tool for "localization, microzone analysis, and phase identification" in materials science, minerals, biomedicine, polymers, and microcontaminant analysis.

Below is a componentbycomponent guide to assembling a confocal Raman instrument.

1. Laser Excitation System (Energy Source)

Provides monochromatic, highly stable, narrowlinewidth excitation light, determining signal strength and the degree of fluorescence interference.

Laser options:

• 532 nm (green) – common for inorganic materials and carbon materials
• 633 nm (red)
• 785 nm (nearIR) – for biological samples and highfluorescence materials
• 1064 nm – for strong fluorescence suppression
• UV options available for resonance Raman
• Multiwavelength switching module for different sample types

Auxiliary components:

• Neutral density (ND) filters – adjust laser power to prevent sample damage
• Beam shaper / beam expander – optimize spot size and fill the objective back aperture for better focusing
• Shutter – controls exposure, protects the sample and detector

2. Confocal Microscope Optical Path (Core for Spatial Resolution)

Responsible for laser focusing, signal collection, and confocal spatial filtering — the heart of "confocal" capability.

Dichroic mirror: Reflects the excitation laser and transmits the Raman scattered light, separating incident and signal paths while maintaining coaxial alignment.

Microscope objective (critical component):

• High numerical aperture (NA 0.7–0.95)
• Focuses laser to a micronsized spot and efficiently collects backscattered Raman light
• Determines lateral and axial resolution and collection efficiency
• Available in long / short working distance, dry / oil immersion, suitable for transparent or thick samples

Confocal pinhole (critical component):

• Micronsized adjustable aperture located at a plane conjugate to the sample focal plane
• Allows only light from the focal plane to pass, rejecting outoffocus stray light and fluorescence — enables 3D optical sectioning (Zaxis depth resolution)
• Pinhole diameter is matched to objective NA and wavelength, typically set to 1–2 Airy units

Imaging and observation components: White light illumination, CCD camera (brightfield / fluorescence imaging), eyepieces — used for sample positioning, morphology observation, and focus confirmation.

3. Filter System (Signal Purification)

Filters out the extremely intense Rayleigh scatter (same wavelength as the laser, 10⁶–10⁸ times stronger than Raman) while preserving the weak Raman signal.

Notch filter / Edge filter (core filtering element):

• Deeply suppresses Rayleigh light (OD > 6–8) and transmits Stokes / antiStokes Raman light
• Longpass or notch filters for low wavenumber (<100 cm⁻¹) detection, suitable for lowfrequency vibration analysis

Bandpass / shortpass filters: Further remove stray light and ambient background interference.

4. Dispersion System (Wavelength Separation)

Spatially separates Raman light of different wavenumbers to achieve spectral resolution.

• Monochromator / Spectrometer: Consists of an entrance slit, collimating mirror, diffraction grating (holographic / ruled — the core dispersive element), and focusing mirror.
• Grating line density (600 / 1200 / 1800 / 2400 grooves/mm) determines resolution
• Slit: Controls light throughput and spectral resolution; works together with the pinhole to optimize the signaltonoise ratio.

5. Detector (OpticaltoElectrical Conversion)

Converts the dispersed Raman light signal into an electrical signal, determining sensitivity and detection range.

• CCD (ChargeCoupled Device) – mainstream choice: multichannel, high sensitivity, low noise; suitable for visible light (532/633 nm)
• EMCCD (ElectronMultiplying CCD) – for extremely weak signals and singlephoton detection
• InGaAs detector – suitable for nearinfrared (785/1064 nm); covers longwavelength Raman and suppresses fluorescence

6. Precision Sample Stage and Control System

• XYZ nanoprecision translation stage: Step accuracy <100 nm; enables pointbypoint scanning (mapping), line scans, area scans, and 3D depth scans to generate chemical images.
• Autofocus and Zstep module: Maintains confocal condition and acquires depthseries data.
• Computer and software: Controls laser, wavelength switching, pinhole / slit, grating, detector, and sample stage; acquires spectra; processes data (baseline correction, fitting, imaging); exports results.

For more information, please contact:

Email: optoskyphotonics@gmail.com

Web: www.optosky.net