1. What is a narrow linewidth laser?
A narrow linewidth laser refers to a laser with a very narrow spectral linewidth, usually at the kHz (kilohertz) level or even lower. Compared with ordinary lasers (linewidth is usually at the MHz or GHz level), narrow linewidth lasers have extremely high frequency stability and coherence, and are widely used in:
Optical communication: improve signal transmission rate and distance;
Fiber optic sensing: improve measurement accuracy and sensitivity;
Spectral analysis: achieve high-resolution spectral measurement;
Scientific research: such as atomic cooling, gravitational wave detection, etc.
2. The core structure of narrow linewidth laser: DFB short straight cavity
According to the content of the picture, narrow linewidth lasers usually use active fiber DFB (Distributed Feedback) short straight cavity structure. This structure is the key to achieving ultra-narrow linewidth.
- What is DFB structure?
DFB structure is a technology that introduces periodic refractive index modulation in the laser cavity. By writing Bragg gratings in optical fibers or semiconductor materials, strong feedback for specific wavelengths is formed, thereby achieving:
Single longitudinal mode output: only one frequency mode is amplified and other modes are suppressed;
Strong frequency stability: Bragg gratings are insensitive to temperature and mechanical disturbances;
Extremely narrow linewidth: The feedback mechanism is highly selective and frequency drift is suppressed.
- Advantages of short straight cavity
The short straight cavity structure means that the physical length of the laser cavity is shorter, which helps to:
Increase the longitudinal mode spacing: so that only one longitudinal mode falls within the gain bandwidth;
Reduce phase noise: reduce intracavity mode competition;
Improve frequency stability: less sensitive to external disturbances.
3. How to achieve ultra-narrow linewidth at kHz level?
To achieve ultra-narrow linewidth at kHz level or even sub-kHz level, the laser must be extremely optimized in the following aspects:
- Precision writing of Bragg grating
The period, depth and uniformity of the Bragg grating directly affect the feedback intensity and linewidth of the laser. The phase mask method or femtosecond laser writing technology can achieve a grating structure with nanometer precision to ensure the singleness and stability of the feedback.
- Low-noise pump source
The intensity noise and frequency drift of the pump source will directly affect the output stability of the laser. Using a low-noise current driver and temperature control system can effectively reduce the pump noise and improve the frequency stability of the laser.
- Temperature and mechanical stability design
Temperature fluctuations and mechanical vibrations are the main sources of laser frequency drift. Narrow linewidth lasers are usually equipped with:
High-precision temperature control system (±0.01°C);
Vibration isolation packaging structure;
Low expansion coefficient materials (such as Invar alloy) as cavity support.
- Active frequency stabilization technology (optional)
For extreme applications (such as gravitational wave detection), Pound-Drever-Hall (PDH) locking technology can also be introduced to lock the laser frequency to an ultrastable cavity or molecular absorption line to achieve Hz-level linewidth.
4. Scientific research and industrial applications: the "highlight moment" of narrow linewidth lasers
- Optical communication
In high-speed coherent optical communication systems, narrow linewidth lasers are used as local oscillator light sources to greatly improve signal demodulation accuracy and transmission distance.
- Fiber optic sensing
In distributed fiber optic sensing systems (such as DAS, DTS), narrow linewidth lasers are used for interferometric measurement to improve the spatial resolution and sensitivity of the system.
- Spectral analysis
Narrow linewidth lasers can be used in high-resolution spectrometers to achieve fine spectral analysis of gas, liquid or solid materials.
- Scientific research
Atomic cooling and trapping: Narrow linewidth lasers are used to stimulate specific atomic transitions and achieve laser cooling;
Gravitational wave detection: For example, ultra-narrow linewidth lasers are used for interferometric measurement in the LIGO project;
Quantum optics: used to build high-coherence quantum light sources.
Conclusion: A single ray of light can measure everything
Narrow linewidth lasers, with their extreme frequency stability and coherence, have become the core light source for modern precision measurement and information transmission. From optical communications to quantum physics, from spectral analysis to gravitational wave detection, they are silently playing an irreplaceable role.