A mode control method that uses a saturable absorbing filter and spatial hole burning mechanism in an Erbium-doped fiber laser is described and demonstrated. In a Fabry-Perot-type laser cavity, the intensity of a resonant mode varies along the cavity length with a spatial frequency corresponding to twice that of the wavelength of the mode. When only one mode is oscillating, a gain grating(i.e. a periodic variation of population inversion) is formed in the gain medium, and an absorption grating is formed in the saturable absorber, because of the saturation effect by the optical standing wave pattern. The induced gain and absorption gratings act as mode selective filters, and the filtering characteristics depend on their lengths and positions in the laser cavity. It is shown that, with the proper position control of gain medium and saturable absorber, a laser having a few stable longitudinal modes can be achieved.
In experiments, two fiber lasers with different locations for the Erbium-doped fiber were constructed. The unpumped Erbium-doped fibers, which act as the saturable absorbers, were located near the one end of the laser cavity for both setups. The gain fibers were located at the center of the cavity for one setup(Setup A), and at the 1/4 position for the other setup(Setup B). For the Setup A laser, two modes oscillated at low pump power, and the frequency difference between the modes corresponded to the free spectral range(F.S.R.) of the cavity. At high pump power, three neighboring modes were observed. The intensity ratio of the lasing modes was about 1:1 for the 2 mode operation, and about 1:2:1 for the 3 mode operation. Mode hopping occurred every few minutes, and the change in the cavity mode number of the oscillating modes was one in most cases. For the Setup B laser, the mode properties were almost the same except that the mode spacing between the adjacent lasing modes was twice that of the Setup A.
To understand the mode selection mechanism in detail, a theoretical model was developed that include an induced grating effect. A simulation of the mode structure for the Setup A was performed using the model, and the results were in good agreement with the experimental results.
A new form of a rotation sensor using the mode controlled fiber laser is proposed and demonstrated. The rotation sensor consisted of a mode-controlled Fabry-Perot type fiber laser and an interferometer which is used to combine two output beams from both ends of the laser. The rotation rate is obtained from the change in the phase difference between the two output beams, which is linearly proportional to the rotation rate. The phase difference is independent of the cavity length and the random birefringence in the cavity because the laser should satisfy the resonance condition. The experimental results agreed well with the theoretical prediction.
