Bit-efficient, sub-millisecond wavefront measurement using a lock-in camera for time-reversal based optical focusing inside scattering media

Abstract

Time-reversed ultrasonically encoded optical focusing measures the wavefront of ultrasonically tagged light, and then phase conjugates the tagged light back to the ultrasonic focus, thus focusing light deep inside the scattering media. In previous works, the speed of wavefront measurement was limited by the low frame rates of conventional cameras. In addition, these cameras used most of their bits to represent an informationless background when the signal-to-background ratio was low, resulting in extremely low efficiencies in the use of bits. Here, using a lock-in camera, we increase the bit efficiency and reduce the data transfer load by digitizing only the signal after rejecting the background. With this camera, we obtained the wavefront of ultrasonically tagged light after a single frame of measurement taken within 0.3 ms, and focused light in between two diffusers. The phase sensitivity has reached 0.51 rad even when the SBR is 6 × 10−4.

Focusing light inside dynamic scattering media with millisecond digital optical phase conjugation

“…Specifically, a digital camera is used to measure the wavefront of the scattered light with digital holography. Then, an SLM, with pixels that are one-to-one matched with the pixels of the camera by a camera lens, is used to reconstruct the conjugate wavefront of the scattered light to achieve optical phase conjugation/time reversal [11,12,23,24,32–34]. In most wavefront shaping experiments, nematic liquid crystal based SLMs (NLC-SLMs) are used for phase modulation [7–9,11,12].…”

“…To take full advantage of the SLM and further improve the speed of ultrasound-guided DOPC, we develop a double-exposure binary wavefront measurement method. The speed of our system is one to two orders of magnit ude higher than those of previous ultrasound-guided DOPC systems [16,24–31], and our method achieves the fastest light focusing inside a scattering medium among all the digital wavefront shaping methods developed to date [3–6]. …”

“…The low speeds prevent DOPC from being applied to thick living biological tissue, because the motion of the scatterers inside tissue causes the speckles on the phase conjugate mirror (camera + SLM) to decorrelate (on a time scale of 0.1–10 ms [15,21–23]) and breaks the time reversal symmetry. Although a bit-efficient, sub-millisecond wavefront measurement method was developed based on a lock-in camera [24], the net speed of the system was limited by the low speed of data transfer and wavefront modulation. Recently, a fast DOPC system controlling 1.3 × 10 5 optical degrees of freedom was developed, and it focused light through scattering media with an effective latency of 5.3 ms and a total system runtime of 7.1 ms [23].…”

Optical focusing through biological tissue and tissue-mimicking phantoms up to 9.6 centimeters thick with digital optical phase conjugation

“…To achieve wavefront shaping, it is required to determine the optimum wavefront and shape the incident light before the speckle pattern associated with the scattering medium decorrelates [13][14][15] . However, even for ex vivo biological tissue, it has been shown that the speckle correlation time decreases as the scattering medium becomes thick 16,17 , due to the Brownian motion of the scatterers.…”

Lock-in camera based heterodyne holography for ultrasound-modulated optical tomography inside dynamic scattering media

“…In this work, we overcome the above drawbacks in heterodyne holography based UOT by using a lock-in camera [31][32][33] (heliCam C3, Heliotis; 300 Â 300 pixels, 40 lm pixel size), in which each pixel performs analog lock-in detection and outputs only the information of the AC signal, with a frame rate of up to 3800 frames per second to an on-chip memory. Specifically, the lock-in circuitry generates the in-phase (S I ðr i Þ) and the quadrature (S Q ðr i Þ) components of the AC signal for each pixel with a position of r i , i ¼ 1, 2,…, 300 Â 300, which are digitized by a 10-bit ADC.…”