We present a novel real-time frequency-encoded spatiotemporal (FEST) focusing technology using a programmable two-dimensional optical frequency comb. This technique enables, for the first time, simultaneous spatial and temporal focusing at microseconds through thick scattering media

Hemphill, A. S.

Jing, J. C.

Shen, Y.

Wang, L. V.

Wei, X.

Xu, S.

Yang, C.

Yang, Z.

English

Caltech Authors - Main

STh1M.2.pdf CLEO 2020 © OSA 2020
Real-time frequency-encoded spatiotemporal focusing 
X. Wei1†, Y. Shen1,2†, J. C. Jing1†, A. S. Hemphill1, C. Yang3, S. Xu3, Z. Yang3*, and L. V. Wang1*
1Caltech Optical Imaging Laboratory, Andrew and Peggy Cherng Department of Medical Engineering, Department of Electrical Engineering, 
California Institute of Technology, 1200 East California Boulevard, Mail Code 138-78, Pasadena, California 91125, USA. 
2Present address: School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510006, China.
3State Key Laboratory of Luminescent Materials and Devices, Guangdong Engineering Technology Research and Development Center of Special 
Optical Fiber Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China 
University of Technology, Guangzhou 510640, China.
†
These authors contributed equally to this work.
*yangzm@scut.edu.cn or LVW@caltech.edu 
Abstract: We present a novel real-time frequency-encoded spatiotemporal (FEST) focusing 
technology using a programmable two-dimensional optical frequency comb. This technique enables, 
for the first time, simultaneous spatial and temporal focusing at microseconds through thick scattering media.  
 
OCIS codes: (140.3518) Lasers, frequency modulated; (030.6140) Speckle; (170.7160) Ultrafast technology 
1. Introduction
In the fields of biophotonic imaging, optogenetics, and laser surgery, focusing deep into living tissues is essential for 
precise diagnosis, manipulation, and treatment [1]. The microscopic inhomogeneity inherent in biological objects, 
however, randomizes incident light and limits high-resolution biomedical applications to small depths, usually <1 
mm in soft tissues [2], preventing whole-body animal and human studies. To overcome this obstacle, the 
concept of wavefront shaping [3–5] has been developed and applied to focus photons deep into or through scattering 
media. The existing wavefront shaping techniques can be mainly classified into three categories: feedback-
based wavefront shaping [3], transmission matrix measurement [4], and optical phase conjugation (OPC) [5]. 
The former two approaches obtain the optimal wavefront through an iterative process, and their runtimes 
are usually long, conventionally up to a few hours. MEMS-based spatial light modulators (SLMs) associated with 
field programmable gate array (FPGA) electronics and one-dimensional GLV phase modulators have recently been 
employed to reduce system runtimes [6,7], but with limited number of control modes. In contrast, OPC approaches 
determine the optimal wavefront in a few shots using two-dimensional (2D) cameras [8]. However, a considerable 
amount of time is required to read out the data from the camera sensor arrays. Up until, the state-of-the-art 
OPC system still takes a few milliseconds to measure the optimal wavefront and transfer the data to the computer 
or processor [9]. Here we present a novel technology, termed frequency-encoded spatiotemporal (FEST) 
focusing, that can generate a sharp spatiotemporal focus through thick dynamic scattering media at microsecond 
scales.  
2. Experimental setup and results
Figure 1a illustrates the principle of the real-time FEST focusing technology. The backbone of the FEST focusing 
system is a programmable 2D optical frequency comb (2D-OFC) that is generated from a homebuilt CW laser with 
an ultrafine linewidth of 1.8 kHz. The 2D-OFC is employed for both high-speed wavefront measurements 
and spatiotemporal focusing, through parallel optical beating and localized mode locking, respectively. As shown in 
Fig. 1a, a single input laser beam is acousto-optically diffracted into a 2D-OFC beam array by driving two 
orthogonal acousto-optic deflectors (AODx,y) with specially designed multitone radiofrequency (RF) signals 
(e.g., Fig. 1b showing a 23 × 24 beam array). Each sub-beam of the 2D-OFC beam array has a unique frequency and 
angle. The 2D-OFC beam array then illuminates a high-speed SLM (HS-SLM), where the beam array is pixel-
matched with the HS-SLM to perform independent 2D phase compensation for spatiotemporal focusing. A scattering 
medium (SM) is placed after the HS-SLM, where the 2D-OFC beam array is randomized into a time-variant multi-
frequency speckle pattern. The time-variant speckle light is then combined with a reference beam at a different 
optical frequency through a beam splitter (BS), and beats at a high-speed photodetector (PD). The beating signal, 
e.g., Fig. 1c, is then digitized by a high-speed data acquisition (DAQ) card and sent to a graphics processing unit 
(GPU) for fast phase retrieval. To this end, the beating signal is first converted to the frequency domain through fast 
Fourier transformation (FFT), resulting in M × N distinct peaks in the frequency spectrum, e.g., Fig. 1d. The phases 
of all frequencies are calculated by the four-quadrant inverse tangent operation, followed by a subtraction of the 
calibration term (if necessary). The phase sequence is then shaped into a 2D conjugated phase map according to the 
pixel matrix (Fig. 1b), and addressed to the HS-SLM, after which a spatiotemporal focus is generated, as shown in 
Fig. 1e, which is generated by using a 35 × 36 beam array. The complete FEST focusing system is controlled by a 
customized C++ program with low latency that 
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STh1M.2.pdf CLEO 2020 © OSA 2020
operates in a continuous streaming mode. Thanks to the unprecedented speed of FEST technology, we have 
successfully applied it to demonstrate, for the first time, spatiotemporal bright-field and fluorescence imaging through 
highly scattering biological tissues, as well as focusing through an in vivo mouse ear, all at microsecond scales (the 
results are not shown here due to the length limit). 
 
 
Fig. 1. (a) The principle of the real-time FEST focusing technology. (b) A typical 2D-OFC beam array that has a spot number of 23 × 24. (c) A 
typical beating signal. The inset shows small features of the beating signal. (d) The frequency spectrum of the beating signal after performing fast 
Fourier transformation. (e) The spatiotemporal focus spot after FEST focusing. 
 
3. Conclusion 
In conclusion, we have reported a novel real-time wavefront shaping technology — frequency-encoded 
spatiotemporal focusing using a programmable 2D ultrafine optical frequency comb — that enables, for the first time, 
simultaneous spatial and temporal (3D) focusing at microsecond scales through thick dynamic scattering media. We 
anticipate that this work opens a new avenue for high-speed wavefront shaping with the potential to revolutionize the 
field of photonics. 
 
4. Acknowledgment 
This work was partially supported by Local Innovative and Research Teams Project of Guangdong Pearl River Talents 
Program No. 2017BT01X137, China; US National Institutes of Health (NIH) Grants No. DP1 EB016986 (NIH 
Director’s Pioneer Award), and R01 CA186567 (NIH Director’s Transformative Research Award). 
 
5. References 
[1] S. H. Yun and S. J. J. Kwok, Nat. Biomed. Eng. 1, 0008 (2017). 
[2] A. P. Mosk, A. Lagendijk, G. Lerosey, M. Fink, Nat. Photon. 6, 283–292 (2012). 
[3] I. M. Vellekoop, A. P. Mosk, Opt. Lett. 32, 2309–2311 (2007). 
[4] S. M. Popoff, G. Lerosey, R. Carminati, M. Fink, A. C. Boccara, and S. Gigan, Phys. Rev. Lett. 104, 100601 (2010).  
[5] Z. Yaqoob, D. Psaltis, M. S. Feld, C. Yang, Nat. Photon. 2, 110–115 (2008). 
[6] B. Blochet, L. Bourdieu, S. Gigan, Opt. Lett. 42, 4994–4997 (2017). 
[7] O. Tzang, E. Niv, S. Singh, S. Labouesse, G. Myatt, and R. Piestun, Nat. Photon. 33, 788–793 (2019). 
[8] M. Cui, C. Yang, Opt. Express 18, 3444–3455 (2010). 
[9] D. Wang, E. H. Zhou, J. Brake, H. Ruan, M. Jang, and C. Yang, Optica 2, 728–735 (2015). 
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Real-time frequency-encoded spatiotemporal focusing

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Real-time frequency-encoded spatiotemporal focusing

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