OpenRRI is an open-source software project designed to assist scientific researchers in acquiring and operating a cost-effective interferometer based on the RRI technique [1]. An RRI-based interferometer offers flexible, wide measurement capabilities [2, 3, 4, 5, 6] while maintaining a low cost relative to commercially available interferometer products.
The mentioned interferometer operates using a laser diode, whose optical frequency is sinusoidally modulated through a laser injection current; the demodulation process is based on the RRI technique. This interferometer enables the measurement of relative displacement with sub-nanometer precision [5] and allows to resolve multiple interference sources within a single optical signal, thereby opening up possibilities for new interferometer configurations. Additionally, the RRI technique offers absolute distance measurement with precision up to 10µm over a wide range [6], which can be utilized for measuring interferometer dead paths and characterizing optical configurations.
Below is a list of the basic hardware required to operate the interferometer:
- A laser diode that allows for mod-hop-free frequency modulation within a range of at least 6 GHz (~50 pm for 1550nm central wavelength).
- A corresponding laser driver.
- A photodetector corresponding to the wavelength in use.
- A data acquisition board (with at least 1 DAC and 1 ADC) that meets the requirement of having the ADC and DAC share the same trigger or clock. In this project, Red Pitaya STEMlab 125-14 is used, and the prepared hardware image is made available by us.
You can find a list of hardware units that we have tested and utilized here.
The OpenRRI project consists of the following parts:
- A hardware image containing the FPGA bitfile and Linux drivers for the Red Pitaya STEMlab 125-14 board.
- A Python library, which contains all RRI signal processing functions and an easy-to-use data acquisition API for communicating with the Red Pitaya STEMlab 125-14 Board.
Please note: The OpenRRI code provided is not a real-time implementation of the RRI algorithms and can only aquire data over a limited recording period, depending on the chosen sample rate.
1. Kissinger, Thomas, Thomas OH Charrett, and Ralph P. Tatam. „Range-resolved interferometric signal processing using sinusoidal optical frequency modulation.“ Optics express 23.7 (2015): 9415-9431.
2. Kissinger, Thomas, et al. „Fiber segment interferometry for dynamic strain measurements.“ Journal of Lightwave Technology 34.19 (2016): 4620-4626.
3. James, Stephen W., et al. „Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 1. Measurement of strain.“ Smart Materials and Structures 31.7 (2022): 075014.
4. Kissinger, Thomas, et al. „Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 2. Measurement of shape.“ Smart Materials and Structures 31.7 (2022): 075015.
5. Shmagun, Vitalii, et al. „Comparison of fiber interferometric sensor with a commercial interferometer for a Kibble balance velocity calibration.“ Measurement Science and Technology (2023).
6. Shmagun, Vitalii, et al. „Absolute distance measurements for in-situ interferometer characterisation using range-resolved interferometry.“ Measurement Science and Technology 33.12 (2022): 125024.