Table of Contents
What Is Laser Linewidth?
Laser linewidth describes the spectral width of the laser output, indicating how monochromatic the emitted light is. A narrower linewidth means a purer single frequency, which is critical for applications such as spectroscopy, interferometry, coherent communications, and gravitational wave detection. The fundamental lower limit on linewidth is set by quantum mechanics through the Schawlow-Townes formula.
In practice, technical noise sources such as vibrations, thermal drift, and pump noise broaden the linewidth well beyond the Schawlow-Townes limit. Modern stabilized lasers can approach sub-hertz linewidths using active feedback and ultra-stable cavities.
The Schawlow-Townes Formula
Where Δν is the laser linewidth (Hz), h is Planck's constant (6.626 x 10-34 J·s), ν is the laser frequency, Δνc is the cold cavity linewidth, nsp is the spontaneous emission factor, and Pout is the output power. The cavity linewidth depends on the round-trip loss and cavity length.
Factors Affecting Linewidth
| Parameter | Effect on Linewidth | Typical Range |
|---|---|---|
| Output Power | Inversely proportional | 0.1 mW - 10 W |
| Cavity Length | Longer cavity = narrower | 1 cm - 1 m |
| Mirror Reflectivity | Higher R = narrower | 0.95 - 0.9999 |
| Spontaneous Emission | Higher nsp = broader | 1 - 5 |
| Wavelength | Shorter = broader | 200 nm - 10 μm |
Linewidths of Common Lasers
| Laser Type | Wavelength | Typical Linewidth |
|---|---|---|
| HeNe | 632.8 nm | 1 - 1500 MHz |
| Nd:YAG | 1064 nm | 1 - 10 kHz (stabilized) |
| DFB Diode | 1550 nm | 1 - 10 MHz |
| External Cavity Diode | Various | 10 - 500 kHz |
| Fiber Laser | 1550 nm | 1 - 100 kHz |
Frequently Asked Questions
What is the Schawlow-Townes limit?
The Schawlow-Townes limit is the fundamental quantum noise floor for laser linewidth. It arises from spontaneous emission events that add random phase perturbations to the coherent laser field. In practice, most lasers operate well above this limit due to technical noise, but it represents the best achievable linewidth for a given set of cavity and power parameters.
How does cavity length affect linewidth?
A longer cavity has a narrower free spectral range and a higher quality factor for each longitudinal mode. This reduces the cold cavity linewidth, which enters the Schawlow-Townes formula squared, making cavity length a very effective parameter for narrowing linewidth.
Why does higher power give narrower linewidth?
Higher output power means more photons in the cavity mode. Since spontaneous emission adds one random photon at a time, the relative phase perturbation per spontaneous event decreases as the photon number increases, leading to a narrower linewidth proportional to 1/P.