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Coherent Length
The essential nature of laser coherence refers to the ability of light waves to maintain a stable phase relationship during vibration. This "unified rhythm" characteristic is the core hallmark that distinguishes lasers from ordinary light.
Definition of Coherence
The coherence of light refers to the phenomenon where two light waves maintain the same phase difference, have the same frequency, or exhibit completely consistent waveforms during propagation. Such two light beams can produce stable interference in the course of propagation, namely constructive interference and destructive interference.
Laser coherence requires meeting the following three conditions:
1. Same frequency
2. Vibrations are in the same direction
3. Constant phase difference
The superposition of light waves produces an interference pattern with fixed bright and dark fringes (as shown in the figure).
Coherence fringe contrast (639nm single-frequency laser 85%)
Fringe Contrast: A quantitative measure of coherence. It is the ratio of the difference between the maximum and minimum intensities of interference fringes to their sum.
Fringe contrast relates to the relative intensity of bright and dark fringes. When Imin = 0, K = 1 (excellent contrast, ideal state: complete coherence); when Imax = Imin, K = 0 (fringes vanish entirely: incoherence).
Two key dimensions of coherence
1. Temporal coherence: The "memory of history" capability of light waves.
Definition: Whether the light waves emitted by a light beam at successive moments maintain phase correlation along its propagation path.
Key indicator: Coherence length (Lₘ) — maximum propagation distance for light waves to maintain coherence. The mentioned linewidth describes the laser’s monochromaticity (spectral purity): narrower linewidth means longer coherence length.
Here is the formula for coherence length: (
: Director of the Center, : Line width)
2. Spatial Coherence: The "Teamwork" ability of light waves
Definition: Whether light waves from different points in a beam’s cross-section are phase-synchronized simultaneously.
Key indicator: Coherence area — the range in a beam’s cross-section where phase consistency is maintained (lasers: nearly entire cross-section; ordinary light: only microregions). This stems from lasers’ excellent directionality and wavefront flatness. Laser resonators select the fundamental transverse mode (TEM₀₀), outputting plane waves (uniform phase across all points) with high spatial coherence.
Typical long-coherence laser
CNI offers high-reliability single-frequency (single longitudinal mode) or narrow-linewidth lasers, featuring long coherence, high stability, excellent beam quality, long lifespan and easy operation. This series is an ideal choice for applications like interferometry, holography, Raman scattering, Brillouin scattering, radar and high-precision metrology.
Typical products: CNI single-frequency or narrow-linewidth lasers | |||||
Ultraviolet | Blue light | Green light | Yellow light | Red Light | Infrared |
266, 320nm 355, 360 nm | 405, 457 nm 473 nm, 488 nm | 515, 532 nm 550 nm, 561 nm | 570 nm, 577 nm 589, 594 nm | 607, 633 nm 639, 660 nm | 785, 1064 nm 1342, 1550nm |
High Stability DPSS Laser | High Stability Fiber Laser | High Stability Diode Laser |
By the way, in some applications (e.g., microscopic illumination, laser display), laser coherence may cause field fringes or speckles. For this, laser decoherence devices are needed, or incoherent light sources like wide-linewidth LEDs, SLDs, ASE, tungsten-halogen lamps, tritium-halogen lamps, mercury-argon lamps and tritium lamps can be used.For more details, refer to www....
Changchun New Industries Optoelectronics Technology Co., Ltd.
Address: No. 888 Jinhu Road, High-Tech Zone, Changchun, 130103, P.R.China
International Tel:+86-431-85603799
Email: sales@cnilaser.com
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