Reflective focused laser differential interferometer
US20260110529A1
Abstract
A reflective focused laser differential interferometry (R-FLDI) system includes: a light source for generating a beam of light; a beam expanding optic; a first linear polarizer; a quarter-wave plate; a first birefringent prism; a beam discriminating optic; a focusing lens; a reflector; a second birefringent prism; a second linear polarizer; and a detector. In a first pass, the beam of light passes through the plano-concave lens, the first linear polarizer, the quarter-wave plate, the first birefringent prism, the beam discriminating optic, and the focusing lens such that it propagates through a measurement location, after it passes through the measurement location, it is reflected by the reflector such that it passes through the focusing lens and is redirected by the beam discriminating optic through the second birefringent prism and a second linear polarizer and is detected by the detector.
Description (excerpt)
CROSS-REFERENCE TO RELATED APPLICATIONS Pursuant to 37 C.F.R. § 1.78 (a)(4), this application claims the benefit of and priority to prior filed co-pending Provisional Application Ser. No. 63/710,770, filed Oct. 23, 2024, which is expressly incorporated herein by reference in its entirety. RIGHTS OF THE GOVERNMENT The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. FIELD OF THE INVENTION The present invention relates generally to focused laser differential interferometry and, more particularly, to a reflective-focused laser differential interferometer. BACKGROUND OF THE INVENTION Focused laser differential interferometry (FLDI) is a non-intrusive optical technique that measures the difference in optical path length between an interfering beam pair. It features excellent spatial and temporal resolution (for example, 100 μm and 10 MHz, respectively) and high sensitivity. FLDI can be utilized to obtain high bandwidth, non-intrusive measurements in facilities such as wind tunnels to make reliable measurements of convective velocity between two closely spaced FLDI probe volumes, disturbance-level characterization, and has been widely adopted in hypersonic ground testing and other high-speed applications (e.g., supersonic aerodynamics, jet dynamics, propulsion systems, plumes/wakes, shock waves, etc.). Traditional FLDI has been adapted into variants that each provide unique measurement capabilities. One of the primary limitations of traditional FLDI instruments is that they are typically only able to provide measurements at a single point during a test. To overcome this limitation, some variants of FLDI create multiple measurement locations. This is achieved through the use of additional beam-splitting optics (typically birefringent prisms or diffractive optical elements), which can be introduced into the optical path of the setup to create multiple, focused, beam pairs in the test region. These multi-point FLDI setups are typically referred to as double-FLDI (D-FLDI), two-point-FLDI, or linear-array-FLDI (LA-FLDI). The focusing optical path of traditional FLDI also can limit the instruments utility over wide, flat geometries where the walls of the test article can intrude on the beam's propagation. To overcome this limitation, a cylindrical lens can be utilized in the FLDI optical path to flatten the beam pair along one of the dimensions as it is focused down. This variant is typically referred to as cylindrical-FLDI (C-FLDI). All of the traditional variants of FLDI require “pitch” optics (the aspects of the optical system associated with generating a beam) and separate “catch” optics (the aspects of the optical system associated with receiving the beam) set up on either side of the measurement location. Therefore, to incorporate a traditional FLDI setup in ground testing facilities (e.g. wind tunnels), two points of optical access are required, typically located on either side of the test region with the measurement location positioned within the optical line-of-sight. This often limits the number of available measurement locations a traditional FLDI setup can access. Additionally, the nature of having two separate optical setups on either side of the test region requires precise alignment between the pitch and catch sides for a traditional FLDI to properly function. This requirement for precise alignment makes traditional FLDI arrangements susceptible to vibrations and oscillations that often occur during or between tests, making it challenging to get consistent and repeatable results using a traditional FLDI setup, especially at facilities with large impulses or vibrations (e.g., shock tunnels, propulsion facilities, etc.) The reflective-FLDI (R-FLDI) systems and features described herein would solve one or more problems encountered in the current state of the art. SUMMARY OF THE INVENTION The present invention overcomes the foregoing problems and other shortcomings, drawbacks, and challenges of FLDI systems. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention. According to one embodiment of the present invention, a reflective focused laser differential interferometry (R-FLDI) system includes: a light source for generating a beam of light; a beam expanding optic; a first linear polarizer; a quarter-wave plate; a first birefringent prism; a beam discriminating optic; a focusing lens; a reflector; a second birefringent prism; a second linear polarizer; and a detector. In a first pass, the beam of light passes through the plano-concave lens, the first linear pola
Filing details
- Inventors
- Jonathan Hill
- Assignee
- Government Of The United States As Represented By The Secretary Of The Air …
- Filed
- Sep 29, 2025
- Granted
- Application pending
Bibliographic data and excerpted text sourced from Google Patents (public record) as part of IP TechMatch's current-filings monitor. This filing is not part of the 2019 historical archive. For the authoritative full text, drawings, and legal status, see the source links above or consult USPTO records directly.