Anisotropic laser-feedback polarimetry enables highly sensitive, self-calibrating birefringence measurement in low-transmittance materials

Birefringent crystals, with intrinsic optical anisotropy, are foundational to nonlinear and integrated optics, biomedicine, and materials science, and the field has advanced rapidly in recent years: new compounds exhibiting giant or deep-UV birefringence and a growing family of tunable artificial birefringent components (e.g., liquid-crystal waveplates, vortex plates, metasurface waveplates) have greatly broadened applications, while stress-induced residual birefringence arising in precision optical fabrication can critically degrade high-accuracy optical systems. These emerging materials and effects create a pressing need for birefringence metrology that is accurate, fast, and versatile enough to quantify phase retardance and retardance-axis azimuth (stress direction), track dynamic changes, map spatial distributions, resolve weak signals, and handle low-transparency samples.

In a new PhotoniX article, a highly sensitive, self-calibrating birefringence measurement method based on anisotropic laser-feedback polarization effects is reported. The platform features a unified polarization modulation–analysis architecture. Leveraging the distinctive optical-path topology of laser-feedback interferometry, it removes the need for an additional reference arm and merges the polarization-modulation path of the source with the polarization-analysis path at detection into a single compact unit. Using the anisotropic microchip laser itself as the sensing element, the system is highly sensitive to subtle variations of intracavity birefringence parameters, enabling high-precision measurements. Moreover, the feedback signal is shifted to a 2-MHz carrier, effectively suppressing low-frequency noise. The measured standard deviations of phase retardance and retardance-axis azimuth are 0.0453° and 0.0939°, respectively. Together with the ultrahigh effective optical-gain amplification (up to 10⁶) of microchip lasers, the configuration provides sufficient SNR for high-sensitivity weak-light detection. Consequently, the microchip-laser feedback gain enables measurements even when sample transmittance drops to 10⁻⁵, i.e., nearly opaque or highly scattering specimens.

Beyond static characterization, the system supports large-range continuous retardance tracking and spatial mapping. The authors demonstrate dynamic birefringence monitoring in a voltage-tuned liquid-crystal variable retarder, as well as two-dimensional birefringence distributions in stressed glass, highlighting broad applicability across common birefringence scenarios.

Potential applications span optical component quality assessment (lens and crystal inspection), biomechanical stress mapping in biomedical systems, functional characterization of engineered anisotropic structures, and non-destructive testing of photonic integrated circuits and other extreme-anisotropy devices. The team plans to extend the approach toward higher-speed operation, broader anisotropy types beyond linear birefringence, and scalable in-situ large-area monitoring.