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 | Tampere University, University of Oulu |
| Citations | 28 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis analysis details the design and performance of a time-resolved Raman spectrometer engineered for high fluorescence rejection, utilizing advanced CMOS single-photon avalanche diode (SPAD) technology and a specialized pulsed laser source.
- Core Technology: The system integrates a 256-channel CMOS SPAD line sensor (20 ps Time-to-Digital Converter (TDC) resolution) with a 573-nm pulsed diamond Raman laser (70-100 ps Full-Width at Half-Maximum, FWHM).
- Fluorescence Rejection: Efficient rejection is achieved through the combination of sub-100 ps time gating and the use of the 573-nm excitation wavelength, which is optimized for low fluorescence in oil samples.
- Performance Gain: Time gating reduced the Fluorescence-to-Raman (F/R) ratio by a factor of 24-25x for both organic and roasted sesame seed oils, despite their short fluorescence lifetimes (2.2-2.7 ns).
- Wavelength Optimization: The 573-nm excitation resulted in a 73% lower F/R ratio for organic sesame oil compared to common 532-nm excitation.
- Timing Skew Mitigation: An improved iterative postprocessing technique for timing skew compensation was developed, reducing spectral distortion by 88%-89% for both samples, and pushing effective timing skew down to picosecond levels.
- Spectral Quality: High Signal-to-Distortion Ratios (SDRs) were achieved for the main Raman peak (1440 cm-1): 76.2 for organic oil and 28.2 for the highly fluorescent roasted oil.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Excitation Wavelength | 573 | nm | Pulsed diamond Raman laser output. |
| Laser Pulsewidth (FWHM) | 70-100 | ps | Used for time-gated measurement. |
| Laser Average Output Power | 25 | mW | Power level used during measurements. |
| Laser Pulse Rate | 70 | kHz | Limits the overall measurement speed. |
| SPAD Sensor Type | 256 x 8 | Array | CMOS SPAD line sensor (0.35-”m technology). |
| Time-to-Digital Converter (TDC) Resolution | 20 | ps | Temporal resolution of the detector. |
| TDC Range Used | 81.92 | ns | Corresponds to 4096 TDC bins. |
| Total Timing Skew (Max Difference) | 86 | ps | Maximum difference between channels 1 and 256. |
| Wavenumber Range | 247-1826 | cm-1 | Spectral range covered by the line sensor. |
| Theoretical Spectral Resolution | 6.2 | cm-1 | Based on 256 channels covering the full range. |
| IRF FWHM (Average) | 152 | ps | Instrument Response Function width. |
| F/R Ratio Reduction (Time Gating) | 24-25 | Factor | Achieved for sesame seed oils. |
| F/R Ratio Reduction (573 nm vs 532 nm) | 73 | % lower | Observed for organic sesame seed oil. |
| Optimal Time Gate Width Used | ~200 | ps | Chosen to maximize timing skew compensation efficiency. |
| Organic Oil Fluorescence Lifetime | 2.7 | ns | Sample characteristic (CW F/R ratio 10.5). |
| Roasted Oil Fluorescence Lifetime | 2.2 | ns | Sample characteristic (CW F/R ratio 82). |
Key Methodologies
Section titled âKey MethodologiesâThe spectrometer relies on precise synchronization and advanced postprocessing to isolate the instantaneous Raman signal from the delayed fluorescence decay.
- Pulsed Laser Excitation: A diamond Raman laser was used to generate 573-nm pulses (70-100 ps FWHM) at 70 kHz. The 573-nm wavelength was selected to minimize intrinsic fluorescence absorption in edible oils (absorption lowest at 570-590 nm).
- Optical Delay and Synchronization: A 5-m graded-index multimode fiber was used to deliver the excitation light, creating sufficient optical delay for the SPAD sensor to synchronize its Time-to-Digital Converters (TDCs) with the laser pulse.
- Time-Correlated Single-Photon Counting (TCSPC): The collected light was dispersed onto the 256-channel CMOS SPAD line sensor, which recorded photon arrival times with 20 ps resolution.
- Pile-Up Distortion Correction: A standard correction algorithm was applied to the raw hit counts to account for the probability of multiple photons arriving within a single measurement cycle.
- Improved Iterative Timing Skew Characterization: The sensorâs timing skew was characterized using a short-lifetime reference sample (Erythrosin B). The resulting timestamps were iteratively adjusted to minimize sharp spectral distortion peaks in the reference spectrum, ensuring highly accurate picosecond-level compensation data.
- Time Gating: A narrow time gate (approximately 200 ps width) was applied in postprocessing. This gate was positioned to capture the maximum number of instantaneous Raman photons while excluding the delayed fluorescence photons, maximizing the efficiency of the timing skew compensation.
- Dark Count Rate (DCR) Compensation: DCR was estimated using hit counts from late TDC bins (2200-2800, >30 ns after excitation) and subtracted from the measured spectra.
- Baseline Correction and Filtering: Final spectra were refined using baseline correction (fitting a curve through non-Raman peak regions) and computational filtering (using the
sgolaymethod) to remove residual noise.
Commercial Applications
Section titled âCommercial ApplicationsâThe combination of optimized excitation wavelength and picosecond-level time gating makes this technology highly suitable for analyzing materials traditionally challenging due to high fluorescence backgrounds.
- Food and Oil Quality Control: Rapid, non-destructive detection of adulteration and quality assessment in edible oils (e.g., olive oil, sesame oil, biodiesel) by operating in the low-fluorescence 570-590 nm window.
- Biomedical and Clinical Diagnostics: High-sensitivity analysis of biological fluids and tissues (e.g., blood components, human teeth, cancer detection) where intrinsic fluorescence from hemoglobin or other biomolecules is a major impediment. The system is adaptable to 620-nm excitation (using a different diamond Raman laser) which is optimal for blood analysis.
- Pharmaceutical Manufacturing: Quality assurance and analysis of pharmaceutical compounds, enabling clear Raman signal acquisition despite fluorescent excipients or active ingredients.
- Geological and Planetary Science: Development of miniature, high-speed Raman spectrometers for in situ mineral and organic identification in environments where low-pulse-energy operation is required (e.g., planetary rovers).
- Integrated Sensor Development: The use of CMOS SPAD line sensors allows for the development of compact, robust, and potentially portable time-resolved Raman devices suitable for field deployment or industrial process monitoring.
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.