High-Resolution Multimode Fiber Imaging with Wavelength-Scanned Integrated Optical Phased Array

Introduction

Multimode fibers (MMFs) are promising for minimally-invasive endoscopy applications due to their capability of realizing high-resolution imaging with a small diameter (~100 μm). This enables in-vivo imaging of organs that are difficult to access. In previous laboratory demonstrations, spatial light modulators (SLMs) were used to control the field distribution at the facet of an MMF. However, integrated optical phased arrays (OPAs) provide advantages of lower cost, higher reliability, and higher modulation speed, emerging as an alternative to bulky free-space SLMs.

Principle

The proposed imaging system, as shown in Fig. 1(a), employs an integrated OPA to modulate light from a tunable laser and couple it into an MMF. The light is equally divided into 8 waveguides by cascaded multimode interference (MMI) couplers, and each waveguide is independently modulated by a phase shifter and coupled into the MMF by a grating-based nanoantenna, as illustrated in Fig. 1(b).

Fig. 1. (a) Schematic illustration of the wavelength-scanning multimode fiber imaging system with an 8-channel optical phased array (OPA). (b) 3D view of the integrated OPA. SMF: single-mode fiber, MMF: multimode fiber.

The target to be imaged is placed at the distal facet of the MMF and illuminated by the speckle patterns out from the MMF. The total transmitted power is measured by a single-pixel photodiode (PD). Different speckle patterns are generated by simultaneously scanning the laser wavelength and modulating the OPA.

The relationship between the speckle patterns (I) and measured PD signals (S) can be formulated as a set of linear equations S=I∙T, where T denotes the transmittance distribution of the target. By pre-calibrating the illumination matrix I and measuring the total-transmitted power S, the image can be retrieved from I^+∙S, where I^+ is the pseudo-inverse of the illumination matrix I.

To ensure accurate reconstruction, the illumination matrix I must be close to full rank, meaning a sufficient number of linearly independent speckle patterns must be generated. For an 8-channel OPA, the available number of orthogonal speckle patterns is derived to be 57, which is much less than the number required for MMF imaging (typically on the order of a few thousands). The wavelength of light offers an additional degree of freedom that can be utilized to produce more independent speckles. Due to intermodal dispersion, a slight wavelength shift will result in accumulated phase differences between different modes, thereby decorrelating speckle patterns. By scanning over N_λ different wavelengths, the number of orthogonal speckle patterns can be potentially increased from 57 to 57×N_λ.

Experimental Demonstration

The OPA chip was fabricated on a silicon-on-insulator (SOI) multi-project wafer with a 220-nm-thick silicon layer. Fig. 2(a) shows the microscope image of the fabricated 8-channel OPA chip, with a compact footprint of 500×800 μm^2.

Fig. 2. (a) Microscope image of the fabricated OPA chip. (b) Experimental setup. VSA: voltage source array. PC: polarization controller. DUT: device under test. TEC: thermo-electric cooler. MMF: multimode fiber. OL: objective lens. NPBS: non-polarizing beam splitter. PD: photodiode.

The experimental setup is shown in Fig. 2(b). The light from a tunable laser was aligned to transverse-electric (TE) polarization and coupled into the chip. The modulated light was then launched into a one-meter-long MMF (Thorlabs FG105LCA) with a core diameter of 105 μm and a numerical aperture (NA) of 0.22. The near-field pattern from the distal facet of the MMF was magnified 40 times by an objective lens and divided into two paths by a beam splitter. One path was directed to an InGaAs camera for calibration, while the other path had the target placed with a photodiode to measure the total-transmitted power. The wavelength was scanned from 1300 nm to 1310 nm at a step of 0.2 nm, which was measured to be large enough to decorrelate the speckle patterns.

The reconstruction results of the USAF-1951 resolution test chart are shown in Fig. 3. Truncated singular value decomposition (SVD) was used for calculating the pseudo-inverse of the illumination matrix. In the case with a single wavelength, the image reconstruction failed even though 3000 different speckle patterns were used, due to the low rank of the illumination matrix. By scanning more wavelengths, more effective information about the target was acquired, and the reconstruction quality became better. This improvement saturated when N_λ reached 50, because the illumination matrix almost reached the maximum possible rank of 3000, limited by the number of modes in the MMF.

Fig. 3. Reconstruction results of the USAF-1951 resolution test chart with different number of scanned wavelengths Nλ, and K represents the number of speckle patterns generated for each wavelength by modulating the OPA. The blue circles represent the core region of MMF.

The finest line pairs that could be resolved exist at Group 2 Element 6, as shown in the red box in Fig. 3, which has a linewidth of 70 μm. These feature sizes, when demagnified 40 times to the distal facet of the multimode fiber, correspond to a linewidth of 1.75 μm in a field of view (FOV) of 105 μm.

Conclusion

In conclusion, a high-resolution MMF imaging system using an integrated OPA with only 8 physical channels was demonstrated. By scanning the wavelength and employing wavelength-dependent speckles out from the MMF, an equivalent resolution of 1.75 μm in a FOV of 105 μm was experimentally achieved. The number of resolvable points was derived to be around 3000, which was solely limited by the number of modes in the MMF and was two orders of magnitude greater than the number of OPA channels.

Reference

[2] G. Hu, Y. Qin, and H. K. Tsang, "Multimode-Fiber Imaging Using a Wavelength-Scanned Integrated Optical Phased Array," Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, China, 2024.