The relentless pursuit of miniaturization in confocal endoscopic imaging systems encounters a fundamental physical paradox at submillimeter scales: the mutually exclusive demands between scanning precision and device compactness, governed by the mechanical energy density ceiling of microelectromechanical system (MEMS) scanners and the unyielding conservation of optical étendue. This study introduces a waveguide spatial optical scaling transformation technique that circumvents these fundamental limitations through strategic reconstruction of optical field phase-energy distributions. Central to this breakthrough is the resolution of dual constraints plaguing fiber-coupled systems: the mode-selective transmission characteristics of micro-scale imaging fibers that enforce stringent light field propagation rules, and the nonlinear dynamics of MEMS micromirrors that induce counteractive speed-precision tradeoffs within the critical 1°?6° mechanical tilt range. These limitations manifest as significant spatial frequency response degradation, particularly evident in edge-field performance deterioration under high-resolution imaging conditions.

These limitations originate from the planar geometry of imaging fiber endface (non-aspheric surfaces), where two-dimensional laser scanning introduces angle-dependent coupling efficiency variations across different fields of view. This geometric limitation propagates through the photomultiplier chain, resulting in pronounced moiré patterns at the imaging plane. Analysis reveals a critical design paradox: increasing the fiber endface diameter nonlinearly amplifies moiré modulation depth through combined mode interference and wavefront vector mismatch, while reducing endface dimensions compresses angular tolerance to arc-second levels-an impractical requirement for practical endoscopic maneuvering. Additionally, MEMS scanner design confronts an inherent trilemma among drive voltage, angular precision, and scanning speed. As shown in Fig. 3(a), y-axis mechanical tilt exhibits progressive precision degradation with increasing scan frequency, reaching 6.5% positioning error at mechanical angular displacement below 3.5° tilts (calibration error is ±0.1°). These results from insufficient driving torque in millimeter-scale devices, where energy storage capacity scales positively with actuation force. Figure 3(b) illustrates the speed-resolution tradeoff: while 2°?6° tilt expansion improves line-scan resolution by 25%, exceeding 12 Hz scan rates induces inertial-delay-induced resolution loss due to coupled damping dissipation. These limitations fundamentally stem from insufficient energy transfer efficiency in microscale electromechanical systems, making conventional designs unable to achieve both high frame rates and superior resolution.

To quantitatively evaluate the optimization efficacy of waveguide optical cones on spatial frequency response, we established a full-field modulation transfer function (MTF) measurement system based on the knife-edge method (Fig. 6). Utilizing a Thorlabs Nano Max precision translation stage (10 nm stepping accuracy) to scan a standard knife-edge along the optical axis, we captured diffraction patterns at the edge transition region (interference fringe spacing around 2.8 μm, Fig. 6 inset). Three independent experimental trials with Fourier-transform-derived MTF curves demonstrated repeatability errors below ±3.5% across 0?350 lp/mm spatial frequencies. In conventional direct coupling configurations, the system exhibited marked spatial non-uniformity (uniformity coefficient is 0.58), with central field MTF15 (15% contrast threshold) reaching 317.71 lp/mm versus edge-field MTF10 (10% contrast) declining to 214.55 lp/mm. This performance degradation stems from phase mismatch in higher-order mode groups during scanning angle escalation, where mechanical angles below 3° induce 23.7% intensity fluctuation standard deviation through multimodal interference, significantly deteriorating imaging signal-to-noise ratio. The introduced waveguide optical cone architecture addresses these limitations through dual physical mechanisms: synergistic effects between graded refractive index distribution and conical geometry concentrate beam energy while suppressing higher-order mode noise. Experimental data confirm transformative improvements- at the original edge-field resolution benchmark of 214.55 lp/mm, the MTF value increases from 0.24 to 0.28 (18% contrast enhancement), with central field performance at 317.71 lp/mm, and MTF is improved to 0.33. System-wide uniformity coefficient uniformity coefficient increases to 0.91, achieving near-perfect field consistency. The MTF curve slope moderates from -0.42 dB·lp^-1·mm^-1 to -0.28 dB·lp^-1·mm^-1, reflecting a 67% efficiency gain in transmitting 50?100 lp/mm high-frequency components that correspond to 5?10 μm feature resolution. These metrics validate the waveguide cone’s capacity to optimize beam propagation paths through conical structural modulation, counteracting the inherent limitations of direct-coupled systems where uncontrolled beam divergence exacerbates mode mismatch and scattering, particularly in peripheral fields.

The fundamental challenge in confocal endoscope miniaturization arises from the intrinsic physical incompatibility between the conservation of optical étendue at microscales and the energy density limits of scanning components. Through the development of a waveguide spatial optical transformation model, this work achieves synergistic optimization of light field manipulation and scanning precision at submillimeter dimensions, providing both theoretical foundations and engineering solutions to overcome conventional technical barriers. The primary innovation lies in uncovering the phase reconstruction mechanism enabled by graded-index waveguide coupling effects: modal gradient compression in tapered geometries effectively suppresses higher-order mode excitation. Experimental validation demonstrates an 18% enhancement in edge-field MTF contrast ratio alongside optimized full-field uniformity coefficient (uniformity coefficient is 0.91 vs baseline uniformity coefficient is 0.58), enabling high-resolution imaging within a 6.2 mm-diameter probe. These advancements provide quantifiable modeling support for device-oriented design of confocal endoscopic systems, bridging the critical gap between theoretical predictions and clinical-grade miniaturization requirements.