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Interferometry

A laser interferometer is an instrument that performs high-precision measurements based on the principle of optical interference. Its core working principle involves the superposition of two or more beams of coherent light to form interference fringes; by analyzing changes in these fringes, physical quantities such as length, displacement, surface topography, and refractive index can be measured.

1. The Basic Principles of Laser Interference 
Interference conditions:  
Two light beams must satisfy coherence (temporal coherence and spatial coherence).The optical path difference must be less than the coherence length of the laser (ΔL < Lc), where ΔL denotes the optical path difference and Lc denotes the coherence length.
Interference formula:  
Light intensity distribution I = I₁ + I₂ + 2√(I₁I₂)cos (∆ф) where ∆ф = 2π/λ ΔL represents the phase difference. Each movement of the interference fringes corresponds to a change in optical path difference by one wavelength. λ) .

2. Classification of Laser Interferometers 

2.1 Classification by Optical Path Structure
(1) Michelson Interferometer  
Principle: A beam splitter divides the laser into a reference beam and a measurement beam. After being reflected, the two beams recombine to produce interference. Featuring a simple structure, this setup is suitable for displacement measurement (e.g., LIGO gravitational wave detection).

(2) Mach-Zehnder Interferometer  
Principle: Two beams of light travel along independent paths without using mirrors, making it ideal for measuring changes in refractive index (such as fluid flow velocity or plasma diagnostics).

(3) Fabry-Perot Interferometer  
Principle: Multi-beam interference, high finesse, used for spectral analysis or laser cavity length control.

(4) Fizeau Interferometer 
Principle: This method utilizes direct light interference between the surface under test and a reference plane, making it suitable for optical surface flatness inspection.

2.2 Categorized by Measurement Object 
(1) Displacement/Length Interferometer  
Measure target displacement with nanometer-level precision, such as for machine tool guide rail calibration. 
(2) Wavefront/Shape Interferometer 
Detect surface errors of optical components (such as lenses and mirrors with λ/10 accuracy). 
(3) Refractive Index/Gas Interferometer  
By inverting the medium's refractive index based on changes in optical path difference, such as air refractive index compensation. 
2.3 Classification by Signal Processing Method 
(1) Zero-Offset Interferometer 
Directly detecting the interference light intensity is structurally simple but easily affected by noise.
(2) Heterodyne Interferometer 
By introducing a frequency difference (such as in acousto-optic modulation), the anti-interference capability can be enhanced through beat-frequency signals—for example, in precision positioning applications within semiconductor manufacturing. 

3. Typical Application Areas 
3.1 Industrial Manufacturing and Metrology 
Position feedback for CNC machines and lithography equipment (nanometer-level repeat positioning accuracy). 
Optical component surface shape inspection (e.g., telescope lenses, smartphone camera modules).

3.2 Scientific Research  
Gravitational wave detection (Michelson interferometer). 
Plasma Density Measurement (Mach-Zehnder Interferometer).

3.3 Biomedical 
Optical Coherence Tomography (OCT) uses low-coherence light interference to achieve micrometer-level imaging of biological tissues.

3.4 Environmental Monitoring 
Atmospheric turbulence analysis, gas concentration detection (via refractive index changes).

3.5 Semiconductor Industry 
Wafer thickness measurement, lithography alignment (dual-frequency laser interferometer).

4. CNI's Typical Interference Laser System 

CNI offers highly reliable, narrow-linewidth, single-longitudinal-mode (single-frequency) lasers that feature stable mode operation, long coherence length, low noise, and excellent beam quality—making them the ideal choice for interference applications. 
For detailed information, please visit www.cnilaser.com. 

Laser interferometers, with their ingenious optical path design and advanced signal processing techniques, achieve ultra-precise measurements spanning from macroscopic to microscopic scales. Their applications extend across high-end fields such as industry, scientific research, and healthcare, and they are poised for further advancements toward higher precision, enhanced anti-interference capabilities, and multifunctional versatility in the future.