Ultra-broadband achromatic imaging with diffractive photon sieves

Abstract

Diffractive optical elements suffer from large chromatic aberration due to the strong wavelength-dependent nature in diffraction phenomena, and therefore, diffractive elements can work only at a single designed wavelength, which significantly limits the applications of diffractive elements in imaging. Here, we report on a demonstration of a wavefront coded broadband achromatic imaging with diffractive photon sieves. The broadband diffraction imaging is implemented with a wavefront coded pinhole pattern that generates equal focusing power for a wide range of operating wavelength in a single thin-film element without complicated auxiliary optical system. Experimental validation was performed using an UV-lithography fabricated wavefront coded photon sieves. Results show that the working bandwidth of the wavefront coded photon sieves reaches 28 nm compared with 0.32 nm of the conventional one. Further demonstration of the achromatic imaging with a bandwidth of 300 nm is also performed with a wavefront coded photon sieves integrated with a refractive element.

Figure 1. Schematic configurations of a CPS and a WFCPS with a coding parameter α=30π.

(a) Schematic of a CPS and the ray path. (b) Schematic of a WFCPS with wavefront coding and the ray path. (c) Illustration of pinhole distribution of a CPS with an aperture of 50 mm, focal length of 500 mm at 632.8 nm. (d) Illustration of pinhole distribution of a WFCPS with the same geometrical dimensions as (c).

Figure 2. The simulated PSFs, MTFs and imaging behaviors of a CPS and a WFCPS at different wavelengths from λ=618.8 nm to 646.8 nm.

(a) PSFs of a CPS. (b) PSF of a WFCPS with a coding parameter α = 30π. (c) MTFs of the corresponding PSFs in (a,b). (d) CPS imaging. (e) WFCPS intermediate imaging. (f) WFCPS restored imaging.

Figure 3. PSF and image measurements of a CPS and a WFCPS.

(a) PSF of a CPS at single wavelength 632.8 nm. (b) Image of a CPS at single wavelength 632.8 nm. (c) PSF of a CPS with a broadband source of 28 nm bandwidth. (d) Image of a CPS with the 28 nm-bandwidth broadband source. (e) PSF of a WFCPS with broadband source of 28 nm bandwidth. (f) Intermediate blurred image produced by the WFCPS with the 28 nm-bandwidth broadband source. (g) Restored image of the WFCPS with 28 nm-bandwidth broadband source.

Figure 4. Hybrid achromatic element.

(a) Schematic of a hybrid achromatic element and the ray path. (b) The diffractive field distributions of a designed conventional hybrid element along the optical axis with different illumination wavelengths. (c) PSFs of the conventional hybrid element at different wavelengths from 400 nm to 700 nm. (d) PSFs of an ultra-broadband wavefront coded hybrid element with a coding parameter α = 10π. (e) MTFs of the corresponding PSFs in (c,d).

Figure 5. The simulated imaging behaviors of a conventional hybrid element and an ultra-broadband wavefront coded hybrid element at different wavelengths from 400 nm to 700 nm.

(a) Conventional hybrid element imaging. (b) Wavefront coded hybrid element intermediate imaging. (c) Wavefront coded hybrid element restored imaging.

Figure 6. PSF and image measurements of a conventional hybrid element and a wavefront coded hybrid element.

(a) PSF of a conventional hybrid element under illumination centered at 632.8 nm with a FWHM bandwidth of 28 nm. (b) Image of a conventional hybrid element under illumination of a 28 nm bandwidth. (c) PSF of a conventional hybrid element under illumination with a bandwidth of 300 nm. (d) Image of a conventional hybrid element under illumination of a 300 nm bandwidth. (e) PSF of a wavefront coded element under illumination of a 300 nm bandwidth. (f) Intermediate blurred image generated by the wavefront coded element under illumination of a 300 nm bandwidth. (g) Restored image of the wavefront coded element under illumination of a 300 nm bandwidth.