Compared with semiconductor lasers, single-frequency fiber lasers have a much longer resonant cavity. Take the distributed feedback (DFB) single-frequency fiber laser pointer with a short straight cavity structure as an example. The resonant cavity length is about 2-10cm, while the single-frequency semiconductor The resonant cavity of the laser is on the order of hundreds of um to mm, while the single-frequency fiber laser of Shanghai HannStar’s CoSF-R “optimized traveling wave cavity” has a cavity length of more than 10 meters and has extremely low phase noise.
The index that determines the relative intensity noise level of a single-frequency laser in the middle and low frequency bands is the relaxation oscillation peak. Single-frequency semiconductor lasers benefit from the composition of active gain medium and resonant cavity. The electrode and gain medium have a high degree of “overlap”, exhibiting excellent relative intensity noise (RIN) performance, and the relaxation oscillation peak is usually as high as GHz. . The fiber laser is essentially end-pumped. The pump excitation of the active medium is uneven in length. At the same time, because the resonant cavity is longer, the relaxation oscillation peak frequency is lower. Distributed feedback (DFB) single The peak frequency of relaxation oscillation of a high-frequency fiber laser is usually around several hundred kHz to 1 MHz.
Because of the low peak frequency of relaxation oscillation, single-frequency fiber green laser pointer needs to suppress the relative intensity noise (RIN) of single-frequency fiber laser. Classical methods include: current negative feedback, saturation amplification, external cavity injection locking, etc. The CoSF-D Distributed Feedback (DFB) single-frequency fiber laser developed by Shanghai HannStar has been suppressed by Relative Intensity Noise (RIN) and obtained good results, as shown in the figure below. Single-frequency fiber laser: It has the advantages of long coherence and narrow linewidth, playing an important role in many special fields
The relative intensity noise of the suppressed CoSF-D distributed feedback (DFB) single-frequency fiber laser in the low frequency band is lower than -145dBc/Hz, the relaxation oscillation peak value is lower than -140dBc/Hz, and the relative intensity noise of the high frequency band is lower than －150dBc/Hz, close to shot noise level. It can be said that the suppressed CoSF-D distributed feedback (DFB) single-frequency fiber laser has a better relative intensity noise level than a single-frequency semiconductor laser, and it has both low phase noise and low relative intensity noise characteristics, which can be stabilized. Single longitudinal mode operation, single frequency, single polarization, single longitudinal mode output.
The line width can be understood as the spectral line width of the spectrum, and the phase noise can directly reflect the line width of a single-frequency laser. Compared with single-frequency semiconductor lasers, single-frequency fiber lasers have lower phase noise. Therefore, the intrinsic linewidth of single-frequency fiber lasers is much narrower than that of single-frequency semiconductor lasers. Determined by the structure and the length of the cavity.