In modern applications of polarized optical technology, linear and circular polarizations are the predominant types of polarized light. The transformation between these two forms of polarized light requires the use of optical phase retarders. Zero-order waveplates measure only a few tens of micrometers thick, and high precision is required during their production to achieve such thickness, which poses considerable fabrication challenges. This study introduces an innovative design for a binary composite-structure waveplate using two quartz crystals of equal thickness. By customizing the optical axis angle of one crystal, a zero-order phase delay can be achieved for a designated light wavelength. This novel zero-order waveplate structure can effectively mitigate the effects of thickness variations on the phase delay of the output light during the manufacturing process.

The proposed zero-order waveplate features a binary structure with equal-thickness components comprising two parallel crystal plates fabricated using identical birefringent materials, each with thickness d, bonded together (Fig. 1). We define a coordinate system, as shown in Fig. 1(a), where the bonding interface of the two crystal plates is aligned with the yoz plane. Figure 1(b) shows a cross-sectional view of the device. The optical axis of the crystal on the left side of the xoy plane, denoted as crystal 1, lies within the xoy plane and is perpendicular to the xoz plane. Meanwhile, the optical axis of the crystal on the right side, denoted as crystal 2, is within the xoy plane and forms an angle γ with the x-axis while maintaining a thickness equal to that of crystal 1. The phase delay of light incident on this binary structure can be fine-tuned by adjusting the optical axis angle of crystal 2, thus resulting in a smaller phase delay for a specific light wavelength. The differential phase delay is quantified as shown in Eq. (8). A zero-order phase delay of 1/4 wavelength corresponds to a specific relationship among the optical axis angle of crystal 2, the crystal thickness, and the light wavelength, as expressed in Eq. (9).

By applying Eqs. (8) and (9), we can ascertain the design parameters for a zero-order waveplate with a equal-thickness binary structure suitable for any uniaxial birefringent crystal. Our investigation into the spectral characteristics, thickness variations, deviations in the optical axis angle, as well as the effect of temperature fluctuations on the phase delay using optical quartz crystals shows that the equal-thickness binary structure waveplate possesses achromatic qualities similar to those of conventional zero-order waveplates. Additionally, it indicates that a phase-delay deviation owing to thickness inconsistencies is less than 1.3% of that observed in conventional zero-order waveplates. Moreover, the phase-delay variations remain within ±0.1° for temperature shifts of ±10 °C, which is smaller than the corresponding variation in conventional zero-order waveplates. Notably, the accuracy of phase delay for our waveplate design is primarily affected by the precision of the optical axis angle of crystal 2. We constructed two samples with different wavelengths using an optical quartz crystal as the base material and performed spectral phase-delay testing using a polarimeter via a transmission method. The experimental results (Figs. 6 and 7) confirm that the observed phase-delay variation across the wavelengths is consistent with theoretical predictions, with the polarimeter measurements of the device’s phase-delay deviations being less than 0.5°.

This paper presents a novel equal-thickness binary structure design for a zero-order waveplate. Based on the light-propagation properties of uniaxial crystals, we developed a universal design formula that delineates the relationship among the optical axis angle of crystal 2, the intended wavelength, the desired delay, and the thickness of an individual crystal. Our analysis shows that the equal-thickness binary structure zero-order waveplate is comparable to classical designs in terms of optical performance and is affected less by temperature and thickness variations. In conclusion, the innovative equal-thickness binary structure zero-order waveplate successfully minimizes the effects of manufacturing inaccuracies on the phase delay accuracy, thereby offering considerable practical advantages.