Angle-resolved photoemission spectroscopy (ARPES) based on synchrotron radiation requires tunable photon energies to perform three-dimensional electronic structure analysis, covering both surface states (20~200 eV) and bulk phases (>200 eV). However, single-period undulator cannot cover the entire required photon energy range due to limitations in magnetic field period and intensity.

This study aims to characterize the energy spectral performance of a double elliptically polarized undulator (DEPU) designed to expand the photon energy coverage for comprehensive electronic structure studies.

Firstly, a DEPU system consisting of a low-energy insertion device (LEID) with 148 mm period length and a high-energy insertion device (HEID) with 58 mm period length was installed at beamline BL09U of Shanghai Synchrotron Radiation Facility (SSRF). Then, the spectral characteristics were measured using ionization chambers with gas absorption spectra of argon, nitrogen, and neon for energy calibration across different energy ranges. Finally, the gap-energy correspondence relationships were systematically determined by measuring fundamental peak positions at various magnetic gap settings using photodiodes, and photon flux measurements were conducted with appropriately sized white slits to maintain 4σ apertures.

Measurement results show that the LEID effectively covers the energy range of 22~250 eV through fundamental harmonics with gap variations, achieving photon flux above 10¹² ph·s^-1 at energies below 100 eV. The HEID fundamental harmonics cover 250~1 700 eV efficiently, while third harmonics extend the coverage to 1 700~2 000 eV, maintaining flux levels above 10¹² ph·s^-1 up to 800 eV. Energy calibration measurements show deviations of -0.14 eV to -5.3 eV compared to literature values, confirming reliable energy positioning across the entire range.

The DEPU successfully achieves comprehensive photon energy coverage from 20 eV to 2 000 eV in the EUV to soft X-ray domain, with measured flux levels consistent with theoretical simulations. This dual-period design enables depth-resolved electronic structure research by providing both surface-sensitive low-energy photons and bulk-sensitive high-energy photons, demonstrating its effectiveness for advanced materials characterization including topological quantum materials and unconventional electronic systems.