Line width measurement of narrow-linewidth laser

Line width measurement of narrow-linewidth laser

The linewidth of narrow-linewidth laser, especially that of single-frequency lasers, refers to the width of the laser spectrum (usually half-width to full-width FWHM). More precisely, the width of the power spectral density of the radiated electric field is expressed in terms of frequency, wavenumber or wavelength. The line width of the laser has a very close correlation with time and is characterized by coherence time and coherence length. If the phase undergoes an unbounded shift, then the phase noise generates a linewidth, which is the case with a free oscillator. Phase fluctuations confined within a very small phase range result in 0 linewidths and some noise sideband. The offset of the resonant cavity length also contributes to the line width and makes it dependent on the measurement time. This indicates that merely the line width or even the shape of the spectrum (line type) cannot provide all the information about the laser spectrum.

Many techniques can be adopted to measure the linewidth of a laser:

When the linewidth ratio is large (>10GHz, when there are multiple mode oscillations in the resonant cavities of multiple lasers), a traditional spectrometer using a diffraction grating can be used for measurement. It is very difficult to obtain high frequency resolution by using this method.

Another approach 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 very limited.

3. Single-frequency lasers typically employ the self-heterodyne method, which records the beat between the laser output and itself after frequency offset and delay.

When the line width is several hundred Hertz, the traditional heterodyne technique is not practical because a large delay length is required at this time. A cyclic fiber loop and an internal fiber amplifier can be used to extend it.

5. A very high resolution can be achieved by recording the beats of two independent lasers. At this time, the noise of the reference laser is much lower than that of the test laser, or the performance indicators of the two are similar. The instantaneous frequency difference can be obtained by using a phase-locked loop or through calculation based on mathematical records. This method is very simple and stable, but it requires another laser (operating near the frequency of the test laser). If the measured line width requires a very wide spectral range, it is very convenient to use a frequency comb.

Optical frequency measurement usually requires a certain frequency (or time) reference at some point. For narrow-linewidth laser, only one reference light is needed to provide a sufficiently accurate reference. The heterodyne technique obtains the frequency reference by applying a sufficiently long time delay from the test device itself. Ideally, it avoids the time coherence between the initial beam and its own delayed light. Therefore, long optical fibers are usually adopted. However, due to stable fluctuations and acoustic effects, long optical fibers can cause additional phase noise.