Abstract
Optical phase conjugation (OPC) has enabled many optical applications such as aberration correction and image transmission through fiber. In recent years, implementation of digital optical phase conjugation (DOPC) has opened up the possibility of its use in biomedical optics (e.g. deep-tissue optical focusing) due to its ability to provide greater-than-unity OPC reflectivity (the power ratio of the phase conjugated beam and input beam to the OPC system) and its flexibility to accommodate additional wavefront manipulations. However, the requirement for precise (pixel-to-pixel matching) alignment of the wavefront sensor and the spatial light modulator (SLM) limits the practical usability of DOPC systems. Here, we report a method for auto-alignment of a DOPC system by which the misalignment between the sensor and the SLM is auto-corrected through digital light propagation. With this method, we were able to accomplish OPC playback with a DOPC system with gross sensor-SLM misalignment by an axial displacement of up to~1.5 cm, rotation and tip/tilt of ~5° and in-plane displacement of ~5 mm (dependent on the physical size of the sensor and the SLM). Our auto-alignment method robustly achieved a DOPC playback peak-to-background ratio (PBR) corresponding to more than ~30 % of the theoretical maximum. As an additional advantage, the auto-alignment procedure can be easily performed at will and, as such, allows us to correct for small mechanical drifts within the DOPC systems, thus overcoming a previously major DOPC system vulnerability. We believe that this reported method for implementing robust DOPC systems will broaden the practical utility of DOPC systems.
Highlights
Optical phase conjugation is an optical process by which a phase-conjugated light wave can be made to retrace the original light wave in the backward direction
Optical phase conjugation (OPC) has been of interest in many optical applications including aberration correction [1,2,3,4], optical resonators [5,6], pulse compression [7,8], image transmission through fibers [1,9], and high-resolution imaging [10,11] for more than 40 years
OPC-assisted deep tissue optical focusing has been demonstrated with a second harmonic generation (SHG) active nanomaterial and with fluorescence beads [16,17]
Summary
Optical phase conjugation is an optical process by which a phase-conjugated light wave can be made to retrace the original light wave in the backward direction (time-reversal property). [14], and [15] reported the successful application of OPC in combination with ultrasound tagging to render optical focusing in a prior unattained tissue depth. This class of methods is termed time-reversed ultrasonically encoded (TRUE) optical focusing. OPC-assisted deep tissue optical focusing has been demonstrated with a second harmonic generation (SHG) active nanomaterial and with fluorescence beads (with a narrow band-width filter) [16,17]. The symmetric optical propagation property of OPC further enables the generation of a quasi-isotropic optical focal spot within a scattering medium [18]
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