The real-time measurement of fast non-repetitive events is arguably the most challenging problem in the field of instrumentation and measurement. These instruments are needed for investigating rapid transient phenomena such as chemical reactions, phase transitions, protein dynamics in living cells, and impairments in data networks. Optical spectrometers are the basic instrument for performing sensing and detection in chemical and biological applications. Unfortunately, the scan rate of a spectrometer is often too long compared with the time scale of the physical processes of interest. In terms of conventional optical spectroscopy, this temporal mismatch means that the instrument is too slow to perform real-time single-shot spectroscopic measurements because it either employs a moving component, such as a translating slit, or relies on a detector array (e.g., a CMOS image sensor) with limited refresh rate (typically up to ~10 kHz). Although pump–probe methods offer the ability to perform time-resolved spectroscopic analysis with extremely fine temporal resolutions of less than 1 ps, they are based on the stroboscopic measurement technique and hence do not operate in real time, thus making them unable to capture non-repetitive and rare events such as those found in complex physical, chemical and biological systems.
Optical time-stretch (also known as dispersive Fourier transformation) is a powerful method that overcomes the speed limitation of traditional spectrometers and hence enables fast real-time spectroscopic measurements. Optical time-stretch is an example of the analogy between paraxial diffraction (that is, Fraunhofer diffraction) and temporal dispersion. This analogy, known as space–time duality, emerges from Maxwell's equations and has been employed to produce a wide variety of elegant methods (including optical time-stretch) for high-speed all-optical signal processing such as Fourier optics and temporal imaging (the temporal equivalent of a classical lens). In the application of space–time duality to optical time-stretch, the concept that diffraction in the far-field regime causes the spatial frequency spectrum of light to appear as an intensity image is extended to the time domain. Specifically, using chromatic dispersion, optical time-stretch maps the temporal frequency spectrum of an optical pulse to a temporal waveform whose intensity envelope mimics the spectrum. This occurs when the pulse propagates inside a dispersive medium with group-velocity dispersion (GVD). For this to occur, the pulse must propagate sufficiently to satisfy the temporal equivalent of the far-field condition in diffraction.
The ability of optical time-stretch to perform fast continuous single-shot measurements is made possible by replacing the diffraction grating and detector array in traditional spectrometers with a temporal dispersive element and single-pixel photodiode. The temporal waveform is stretched in time (bandwidth compressed) so that it is slow enough to be captured by a photodetector and real-time digitizer. Simultaneously, the waveform can be optically amplified to overcome the thermal noise floor of the high-speed photodiode. Because the time-stretched waveform is sufficiently slow, optical time-stretch allows the optical spectrum to be measured directly in the time domain. By sampling the temporal waveform, a real-time analog-to-digital converter (ADC) samples the optical spectrum at a scan rate significantly beyond that of a conventional grating-based spectrometer. With its unique ability to perform ultrafast real-time measurements, optical time-stretch has been employed for a diverse range of spectroscopy and imaging applications.
Currently, we aim to develop new types of optical time-stretch and establish new biomedical applications based on them. In the past decade, we have pioneered optical time-stretch imaging (also known as serial time-encoded amplified imaging) and an integration of optical time-stretch imaging with microfluidics to demonstrate cancer detection in blood, microalgal lipid production analysis, anticancer drug efficacy evaluation, and noninvasive surface inspection for manufacturing.
- C. Lei, H. Kobayashi, D. Wu, M. Li, A. Isozaki, A. Yasumoto, H. Mikami, T. Ito, N. Nitta, T. Sugimura, M. Yamada, Y. Yatomi, D. Di Carlo, Y. Ozeki, and K. Goda, "High-throughput imaging flow cytometry by optofluidic time-stretch microscopy", Nature Protocols 13, 1603 (2018)
- C. Lei, B. Guo, Z. Cheng, and K. Goda, "Optical time-stretch imaging: principles and applications", Applied Physics Reviews 3, 011102 (2016)
- K. Goda and B. Jalali, "Dispersive Fourier transformation for fast continuous single-shot measurements", Nature Photonics 7, 102 (2013)
- K. Goda, A. Ayazi, D. R. Gossett, J. Sadasivam, C. K. Lonappan, E. Sollier, A. Fard, S. C. Hur, J. Adam, C. Murray, C. Wang, N. Brackbill, D. Di Carlo, and B. Jalali, "High-throughput single-microparticle imaging flow analyzer", PNAS 109, 11630 (2012)
- K. Goda, K. K. Tsia, and B. Jalali, "Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena", Nature 458, 1145 (2009)
- Field leader: Yuqi Zhou
- Funding: JSPS Core-to-Core Program, White Rock Foundation, Konica Minolta Foundation
- Collaboration: Serendipity Lab