Time-Resolved Raman Spectrometer With High Fluorescence Rejection Based on a CMOS SPAD Line Sensor and a 573-nm Pulsed Laser
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-01-01 |
| Journal | IEEE Transactions on Instrumentation and Measurement |
| Authors | Tuomo Talala, Ville A. Kaikkonen, Pekka KerÀnen, Jari Nikkinen, Antti HÀrkönen |
| Institutions | University of Oulu, Fraunhofer UK Research |
| Citations | 28 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Diamond-Based Time-Resolved Raman Spectroscopy
Section titled âTechnical Documentation & Analysis: Diamond-Based Time-Resolved Raman SpectroscopyâThis document analyzes the requirements and achievements detailed in the research paper, âTime-Resolved Raman Spectrometer With High Fluorescence Rejection Based on a CMOS SPAD Line Sensor and a 573-nm Pulsed Laser,â and connects them directly to 6CCVDâs advanced CVD diamond capabilities.
Executive Summary
Section titled âExecutive SummaryâThis research successfully demonstrates a high-performance time-resolved Raman spectrometer (TRRS) optimized for high fluorescence rejection, leveraging advanced materials and detection technology.
- Core Achievement: Achieved efficient fluorescence rejection (24-25x factor reduction) in highly fluorescent oil samples using a combination of sub-100 ps time gating and optimized excitation wavelength.
- Material Criticality: The systemâs performance hinges on a high-quality Synthetic Diamond Raman Laser (SDRL), which uses a 0.5-mm-thick synthetic diamond crystal to shift the 532 nm pump to the optimal 573 nm excitation wavelength.
- Wavelength Optimization: Excitation at 573 nm resulted in a 73% lower continuous wave (CW) fluorescence-to-Raman ratio compared to standard 532 nm excitation for organic sesame oil.
- Temporal Resolution: The use of a CMOS SPAD line sensor with 20 ps resolution enabled accurate time gating and precise characterization/compensation of timing skew, crucial for maintaining spectral integrity.
- Spectral Quality: Advanced postprocessing techniques (DCR and timing skew compensation) reduced spectral distortion by 88%-89%, yielding high Signal-to-Distortion Ratios (SDR) up to 76.2.
- Application Potential: The technology is highly relevant for challenging applications in medicine (e.g., blood analysis using 620 nm diamond lasers), food science, and industrial monitoring where fluorescence typically masks Raman signals.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points highlight the performance metrics and material requirements of the demonstrated TRRS system:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Thickness | 0.5 | mm | Synthetic Diamond used for Raman laser resonator |
| Excitation Wavelength | 573 | nm | First Stokes emission line from diamond |
| Pump Wavelength | 532 | nm | Source for Diamond Raman Laser |
| Laser Pulsewidth (FWHM) | 70-100 | ps | Critical for effective time gating |
| Laser Pulse Rate | 70 | kHz | Operating frequency in spectrometer |
| Average Output Power | 25 | mW | Power level used for sample measurement |
| SPAD Sensor Temporal Resolution | 20 | ps | Time-to-Digital Converter (TDC) resolution |
| Time Gate Width Used | 200 | ps | Optimal gate width for Raman measurements |
| F/R Ratio Reduction (Time Gating) | 24-25 | Factor | Improvement achieved over CW mode |
| F/R Ratio Reduction (573 nm vs 532 nm) | 73 | % Lower | Observed for organic sesame seed oil |
| Signal-to-Distortion Ratio (SDR) | 76.2 | N/A | Highest SDR achieved (Organic Sesame Oil) |
| Spectral Distortion Reduction | 88-89 | % | Achieved via postprocessing (DCR + Skew Comp.) |
Key Methodologies
Section titled âKey MethodologiesâThe time-resolved Raman spectrometer relies on precise material engineering and advanced signal processing:
- Excitation Source Generation: A 532 nm pump laser was directed into a 0.5-mm-thick synthetic diamond crystal with integrated mirror coatings to form a plane-plane resonator, generating the 573 nm first Stokes emission line.
- Wavelength Selection: The 573 nm output was selected as the excitation source, demonstrating a significant reduction in inherent fluorescence compared to 532 nm.
- Optical Path Filtering: The laser output was filtered using a 550 nm long-pass filter and a 573 nm short-pass filter to remove unwanted anti-Stokes and higher-order Stokes emissions from the diamond and fiber.
- Signal Collection and Dispersion: Backscattered light was collected at 180°, passed through a 50 ”m slit, and dispersed using a custom volume holographic grating (1800 lines/mm).
- Time-Gated Detection: A 256-channel CMOS SPAD line sensor was used for Time-Correlated Single-Photon Counting (TCSPC) with a 20 ps resolution, enabling accurate time gating (200 ps gate width) to separate instantaneous Raman scattering from delayed fluorescence.
- Advanced Postprocessing: Raw data was corrected for pile-up distortion, followed by iterative compensation for Dark Count Rate (DCR) and sensor timing skew, which was critical for minimizing spectral distortion in highly fluorescent samples.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe success of this time-resolved Raman system is fundamentally dependent on the quality and customization of the synthetic diamond used for the laser source. 6CCVD is uniquely positioned to supply the necessary materials and engineering support to replicate and advance this research.
Applicable Materials
Section titled âApplicable Materialsâ| Research Requirement | 6CCVD Material Recommendation | Technical Rationale |
|---|---|---|
| High-Purity Raman Shifter (0.5 mm thick) | Optical Grade Single Crystal Diamond (SCD) | SCD offers the highest purity and lowest birefringence required for efficient, high-power Raman shifting (e.g., 532 nm to 573 nm). We provide custom thicknesses from 0.1 ”m up to 500 ”m (0.5 mm) and substrates up to 10 mm. |
| Alternative Wavelengths (e.g., 620 nm for blood) | Optical Grade SCD Plates | The paper notes the need for 620 nm excitation for hemoglobin analysis. 6CCVD supplies the SCD quality necessary to achieve this specific second Stokes shift, optimizing the material for the required laser geometry. |
| High-Power Handling | Low-Defect SCD | Our MPCVD growth process ensures low nitrogen incorporation and minimal defects, crucial for minimizing absorption and thermal lensing under the high-power pulsed operation required for the SDRL. |
Customization Potential
Section titled âCustomization PotentialâThe integration of the diamond crystal into the laser resonator requires precise dimensions and functional coatings, capabilities that 6CCVD provides in-house:
- Custom Dimensions: The paper used a 0.5 mm thick diamond. 6CCVD provides precision fabrication, including plates and wafers up to 125 mm (PCD), ensuring exact dimensional matching for resonator design.
- Metalization & Coatings: The diamond used required integrated mirror coatings. 6CCVD offers internal metalization services, including deposition of Au, Pt, Pd, Ti, W, and Cu, allowing researchers to specify custom reflective or anti-reflective coatings directly onto the SCD surface for optimized laser performance and stability.
- Surface Finish: To minimize scattering losses and maximize conversion efficiency in the high-finesse resonator, 6CCVD guarantees ultra-smooth polishing: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD.
Engineering Support
Section titled âEngineering SupportâThe successful implementation of this TRRS system requires deep expertise in both material science and optical engineering.
- Material Selection for Spectroscopy: 6CCVDâs in-house PhD team can assist researchers with material selection for similar Time-Resolved Raman Spectroscopy (TRRS) projects, focusing on optimizing diamond quality for specific wavelength conversion targets (e.g., 573 nm or 620 nm) and high-repetition-rate pulsed operation.
- Timing Skew Mitigation: We understand that future sensor designs require reduced DCR and timing skew. Our material experts ensure the diamond component contributes zero additional noise, focusing solely on providing the highest quality optical source.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
A time-resolved Raman spectrometer is demonstrated based on a 256 Ă 8 single-photon avalanche diodes fabricated in CMOS technology (CMOS SPAD) line sensor and a 573-nm fiber-coupled diamond Raman laser delivering pulses with duration below 100-ps full-width at half-maximum (FWHM). The collected backscattered light from the sample is dispersed on the line sensor using a custom volume holographic grating having 1800 lines/mm. Efficient fluorescence rejection in the Raman measurements is achieved due to a combination of time gating on sub-100-ps time scale and a 573-nm excitation wavelength. To demonstrate the performance of the spectrometer, fluorescent oil samples were measured. For organic sesame seed oil having a continuous wave (CW) mode fluorescence-to-Raman ratio of 10.5 and a fluorescence lifetime of 2.7 ns, a signal-to-distortion value of 76.2 was achieved. For roasted sesame seed oil having a CW mode fluorescence-to-Raman ratio of 82 and a fluorescence lifetime of 2.2 ns, a signal-to-distortion value of 28.2 was achieved. In both cases, the fluorescence-to-Raman ratio was reduced by a factor of 24-25 owing to time gating. For organic oil, spectral distortion was dominated by dark counts, while for the more fluorescent roasted oil, the main source of spectral distortion was timing skew of the sensor. With the presented postprocessing techniques, the level of distortion could be reduced by 88%-89% for both samples. Compared with common 532-nm excitation, approximately 73% lower fluorescence-to-Raman ratio was observed for 573-nm excitation when analyzing the organic sesame seed oil.