IS-Instruments developed a new transmission Raman spectrometer based on spatial heterodyne spectroscopy (SHS), designed to maximise light throughput and improve signal collection for challenging Raman measurement applications.

The project explored how the inherently high étendue of SHS systems could support Raman transmission observations over larger sampling areas while maintaining high spectral resolution. The resulting instrument demonstrated compatibility with fibre inputs ranging from 50 µm up to 3 mm core diameter without degradation in spectral performance.

The Challenge

Conventional Raman spectroscopy systems typically operate in a backscatter configuration, where laser light is focused onto the sample surface and the scattered light is collected from the same side. While effective for many applications, this approach can be sensitive to surface variations and sample heterogeneity. Transmission Raman spectroscopy offers an alternative approach by measuring Raman light transmitted through the bulk of a sample. This enables bulk-average chemical analysis and can reduce the influence of localised surface inconsistencies.

The technique has attracted significant interest for:

• Pharmaceutical quality assurance
• Detection of counterfeit medicines
• Bulk chemical analysis
• Industrial process monitoring

However, transmission Raman measurements present substantial optical challenges. Raman scattering signals are inherently weak and distributed across a large collection area, resulting in high optical étendue. Conventional dispersive spectrometers struggle to efficiently collect this light while maintaining the spectral resolution required for accurate analysis.

The Solution

To address these limitations, IS-Instruments developed a transmission Raman spectrometer based on spatial heterodyne spectroscopy (SHS).

Unlike traditional dispersive systems, the SHS architecture provides a significantly higher étendue for a given spectral resolution, enabling substantially greater light-collection efficiency. This allows the instrument to capture Raman signals over larger sampling areas without requiring narrow entrance slits or complex fibre-bundle arrangements. The static Fourier Transform design also eliminates moving parts, improving robustness while simplifying the optical configuration. The system was designed to operate with fibre inputs of up to 3 mm diameter and numerical apertures up to 0.22 without requiring internal instrument adjustments.

Experimental Demonstration

The instrument was tested using approximately 5-mm-thick paracetamol tablet samples in a transmission Raman configuration.

Measurements were conducted using a range of fibre input geometries, including:

• 50 µm core fibres
• 2 mm fibre bundles
• 3 mm fibre bundles

The system demonstrated successful transmission of Raman observations across all tested fibre configurations while maintaining spectral resolution performance. Comparisons between fibre configurations showed that larger fibre inputs enabled significantly improved signal-collection efficiency while preserving spectral fidelity.

Key Features

• Spatial heterodyne Raman spectrometer
• High-etendue optical architecture
• Transmission Raman capability
• Fibre inputs up to 3 mm core diameter
• No entrance slit required
• Static Fourier Transform design with no moving parts
• High-throughput Raman signal collection
• Compatible with pharmaceutical and quality assurance applications

Project Outcome

The project demonstrated the successful application of spatial heterodyne spectroscopy for high-throughput transmission Raman measurements. By exploiting the high-étendue characteristics of SHS technology, the system enabled efficient collection of weak Raman signals from large sampling areas while maintaining the spectral resolution required for chemical identification.

The work highlighted the potential of SHS-based Raman systems to support future applications in pharmaceutical quality assurance, counterfeit drug detection, and industrial process monitoring.

Further Information

Read the full paper as published by Optica.