Laser linewidth measurement

Laser linewidth measurement
There are many methods for measuring laser linewidth:
1. When the laser linewidth is large ( > 10 GHz, when multiple modes oscillate in multiple laser resonators), traditional spectrometers using diffraction gratings can be used for measurement. However, this method is difficult to achieve high frequency resolution.
2. Another method is to use a frequency discriminator to convert frequency fluctuations into intensity fluctuations. The discriminator can be an unbalanced interferometer or a high-precision reference cavity. The resolution of this measurement method is also limited.
3. Single-frequency lasers usually use the heterodyne method, which records the beat frequency of the laser output frequency and its own frequency after offset and delay.
4. For linewidths of a few hundred hertz, the traditional heterodyne method is difficult to apply due to the need for a large delay length. A loop fiber loop and built-in fiber amplifier can be used to extend the delay length.
5. By recording the beat frequency of two independent lasers, extremely high resolution can be achieved. In this case, the noise of the reference laser is much lower than that of the measured laser, or their performance indicators are similar. Locking loops or methods based on mathematical recording to calculate the instantaneous frequency difference can be used. This method is very simple and stable, but requires another laser (with a frequency close to the measured laser). If the measured linewidth requires a wide spectral range, a frequency comb is very convenient.
Optical frequency measurement usually requires providing a specific frequency (or time) reference at a certain point. For narrow linewidth lasers, just one reference beam is needed to provide sufficient accurate reference. The heterodyne technique obtains the frequency reference by applying a sufficiently long time delay on the test device itself. Ideally, the time coherence between the initial beam and its delayed beam should be avoided. Therefore, long optical fibers are usually used. However, due to stable fluctuations and acoustic effects, long optical fibers introduce additional phase noise.