Highly Sensitive Slope Sensor for Surface Metrology Using an External Cavity Laser

Characterization and metrology of super-polished X-ray mirrors are critical for the next-generation synchrotron light sources. Currently, optical metrologists commonly use phase-measuring interferometry as an accurate, non-contact method to measure these aspheric or freeform surface shapes to guide the polishing process. However, this technique usually requires specifically designed, one-of-a-kind null optics. An alternative approach is slope-measuring profilometry, which has been used to inspect X-ray mirrors since the 1980s. Instruments utilizing this method are based on the most advanced industrial electronic autocollimators and have achieved very high accuracy (below 50 nrad) on flat X-ray mirrors. The main drawbacks are the necessity of using a pinhole in front of the autocollimator's aperture to achieve reasonable spatial resolution (approximately 1.5 mm) on the surface of the mirror under test. Raytum Photonics proposes to develop a state-of-the-art, highly sensitive angular detection and calibration system utilizing an external cavity laser that employs the Littman-Metcalf configuration. This configuration relies on an external feedback mirror to maintain laser oscillation at a wavelength entirely determined by the mirror’s angular position. The slight angular change of the mirror directly translates to a wavelength (frequency) change in the laser emission. By measuring the change in laser emission wavelength, the change in the mirror’s angular position can be deduced with extremely high accuracy. Raytum Photonics proposes a two-phase approach to meet the requirements of the solicitation. In the first phase, Raytum Photonics proposes to develop the external cavity laser with the Littman-Metcalf configuration and demonstrate that the proposed resolution and dynamic range can be achieved. In the second phase, we will make further efforts to develop the entire system into a fieldable device capable of performing precision angular calibrations on demand, with the inclusion of an angular reference in the fieldable device. All developmental work will be conducted at the Raytum Photonics facility. We also propose to collaborate with scientific users at Brookhaven National Laboratory to conduct tests using the proposed angular detection and calibration system once developed.