Twyman–Green interferometer (a modern derivative), Fiber-based Fizeau systems
Purpose
The Fizeau interferometer is used to measure small differences in optical path length, often to characterize surface flatness, refractive index variations, wavefront distortions, or relative velocities (via Doppler shifts). It is particularly valued for high-precision optical metrology, material characterization, and fundamental physics experiments involving light propagation.
Operational Principle
The Fizeau interferometer is based on the principle of optical interference: when coherent light (typically from a laser or monochromatic source) is split into two or more paths and then recombined, the phase difference between the paths results in interference fringes. These fringes are sensitive to changes in optical path length, which can arise from variations in distance, refractive index, or surface topography.
In its classical form, a beam of light is directed toward a partially reflective surface (e.g., a glass plate), which produces two reflections—one from the top surface and one from the bottom surface or an adjacent reference mirror. The resulting interference pattern depends on the difference in optical path length between these two beams.
Design and Components
- Coherent Light Source: Typically a laser or filtered lamp (e.g., sodium vapor lamp).
- Beam Splitter/Partially Reflective Surface: Divides and recombines the incoming beam.
- Reference Surface: A flat or known-quality optical surface used for comparison.
- Test Surface: The object or surface being measured (e.g., a lens or mirror).
- Detector or Screen: Used to capture the interference fringes (photographic plate, CCD, or eye).
- Fringe Analysis System: Software or visual methods to interpret fringe spacing and shape.
The classic Fizeau setup consists of two closely spaced glass surfaces, one being a reference and the other under test, arranged so that their partial reflections interfere.
Measurement Capabilities
- Measures: Surface flatness, optical path length difference, refractive index, velocity (via Doppler shift)
- Resolution: Down to fractions of a wavelength of light (sub-nanometer precision)
- Fringe Spacing Sensitivity: Related to λ/2 changes in path difference (λ = light wavelength)
- Velocity Sensitivity (Fizeau Doppler mode): Used in determining light speed in moving media
Applications
- Optical Metrology: Assessing surface figure, wavefront deformation, and lens/mirror quality.
- Refractive Index Studies: Measuring index variations in transparent media or gas flows.
- Relativity Tests: Used historically to detect effects of motion on light propagation.
- Thin Film Characterization: Measuring film thickness and uniformity via interference.
- Velocity Measurements: In modified configurations, used to measure relative motion of light in moving media (e.g., Fizeau’s 1851 water flow experiment).
Historical and Scientific Significance
The Fizeau interferometer was introduced by Hippolyte Fizeau in the mid-19th century. One of its most notable uses was in Fizeau’s 1851 experiment to measure the speed of light in moving water, which provided early empirical support for the partial aether-drag hypothesis and later influenced Einstein’s development of special relativity.
The Fizeau design laid the groundwork for numerous optical instruments and inspired derivatives such as the Twyman–Green interferometer, which adapted the Fizeau principle for improved coherence and stability using laser sources.
Today, Fizeau-type interferometers remain central in precision optics manufacturing, aerospace quality control, and fundamental optical physics due to their robustness, simplicity, and unmatched accuracy in fringe-based measurements.