Fig. 1. Concept of pancake optics systems. (a) Device configuration and (b) operation mechanism of conventional pancake optics system. (c) Configuration and (d) operation mechanism of double path pancake optics system. LCP, RCP, and LP represent left-handed circular polarization, right-handed circular polarization, and linear polarization.

Fig. 2. Schematic of reciprocal and nonreciprocal polarization rotators. Polarization rotation in (a) a reciprocal polarization rotator during forward propagation and (b) backward propagation. Polarization rotation in (c) a nonreciprocal polarization rotator through forward propagation and (d) backward propagation.

Fig. 3. Working principle of the proposed novel pancake optics system. (a) Polarization conversion process in the proposed novel pancake optic system with a FR. (b) A possible configuration of the proposed novel pancake optics. (c) Polarization conversion process in the proposed novel pancake optic system without a FR.

Fig. 4. Experiments using a laser source. The folded beams in pancake optics system (a) without FR, (b) with FR.

Fig. 5. Characterization of the FR in the novel pancake optics system. (a) Transmission spectrum of the FR. (b) Measurement setup for characterizing polarization rotation. LP stands for linear polarizer. (c) Measured and calculated normalized transmission spectra (zero means perfect polarization rotation) of the FR.

Fig. 6. Experiments using a micro-OLED panel. (a) Original image. (b) 0^th order folded image and (c) 1^st order image in the pancake system without a FR. (d) 1^st order image in the pancake system with a FR operating in 510-550 nm. (e) Original image. (f) 0^th order folded image and (g) 1^st order image in the pancake system without a FR. (h) 1^st order image in the pancake system with a FR operating in 603–663 nm.

Fig. 7. Mechanism of ghost images in the novel pancake optics system. (a) Light path of ghost images generated by the transmission of the RPs in block state. (b) Ghost images and (c) suppressed ghost images by an extra linear polarizer captured at its own focal plane. (d) Light path of ghost images caused by surface reflection of FR. (e) Light path of ghost images produced from imperfect polarization rotation in FR. (f) Light path of ghost images induced by panel reflection and reflection of the RPs in transmission state.

Fig. 8. Achieving broadband FR. (a) 1^st order white image in the pancake system with a FR operating in 510-550 nm. (b) Broadband FR design of sequences of FRs and QWPs. (c) Spectrum of polarization rotation ability using a single piece FR, two sequences of FRs and QWPs and three sequences of FRs and QWPs.