Time-wavelength optical sampling for real-time spectroscopy

Spectrometers find applications in a wide range of fields. Effective analysis of the dynamics of the biological and chemical reaction requires real-time spectroscopy, which has attracted significant research attention in recent years. Conventional spectroscopic techniques like Fourier-transform infrared and grating-based spectrometers suffer from several setbacks, such as slow updating rates and long acquisition times, making them unsuitable for real-time spectroscopy. New techniques based on ultrafast lasers, specifically time-stretch spectroscopy (TSS), have exhibited great potential for realizing high spectral resolution with ultrafast frame rates.

Although TSS is one of the simplest spectroscopic tools available, its simplicity comes with some drawbacks that have posed a great threat to its practical applications. For instance, in conventional TSS based on direct-power detection (DPD), high spectral resolution requires fast photodetection and long pulse stretching. However, high-speed photodetectors are not only expensive but also restricted within very few wavelength ranges with limited response time and bandwidth. Similarly, tradeoffs between spectral resolution and signal-to-noise ratio (SNR) have also been observed with pulse stretching.

To address the limitations of DPD-based TSS, a new technique known as time-wavelength optical sampling (TWOS) has been proposed. The key innovation that differentiates TWOS from the conventional DPD-based TSS is the use of optical sampling in probing the stretched pulses via femtosecond laser pulses. This is beneficial as it allows greater pulse stretching and better spectral resolutions and enables the TWOS to work even in low-light conditions. Among the available optical sampling methods, optical sampling by laser cavity tuning (OSCAT) is the most suitable for TWOS. It combines intracavity tuning of the pulse repetition rate with an extracavity Mach-Zehnder interferometer to obtain rapid scan of relative delay. While the underlying principles of TWOS have been demonstrated, some important questions remain unanswered. These include, for example, the limit for spectral resolution, the applicability of TWOS under low light, and the range of frame rate for TWOS spectrometers, among others.

Herein, Dr. Srikamal Soundararajan and Professor Lingze Duan from the University of Alabama in Huntsville proposed and investigated the real-time TWOS spectroscopy technique with the main aim of finding answers to some of the raised questions. By combining the concepts of TSS and ultrafast optical sampling, TWOS addresses some of the inherent challenges facing conventional TSS, such as overreliance on the detector speed and the tradeoff between SNR and spectral resolution. In their most recent effort, the pair of researchers focused on optimizing the performance of the TWOS spectrometer in terms of power sensitivity, spectral resolution and frame rate. The original research work is currently published in the journal Optics Express.

Among some of the notable achievements, the authors obtained a spectral resolution as high as 710 MHz within the 1.5 µm wavelength range using their OSCAT-based TWOS spectrometer. They also demonstrated a 15-dB improvement in low-power tolerance compared with DPD-based TSS. Furthermore, they performed real-time spectroscopic measurements on an HCN gas sample with frame rates as high as 2 kHz and achieved excellent accuracy, demonstrating the effectiveness of the new technique.

In summary, the new study by Dr. Srikamal Soundararajan and Professor Lingze Duan conducted a comprehensive study of a new time-stretch and real-time spectroscopy technique called TWOS to address some of the inherent challenges of conventional spectroscopic schemes. Their findings demonstrated the feasibility of TWOS as a viable option for high-resolution, real-time spectroscopy. Moreover, it was pointed out in their recent paper that TWOS is a generic concept and can find applications in a broader range of fields that include optical imaging, sensing and metrology. In a statement to Advances in Engineering, Professor Lingze Duan stated that “Our study outlines an innovative approach to achieve real-time spectroscopy, with the greatest benefits being its simplicity and cost-effectiveness.”