- Jan Szczepanek
- Tomasz M. Kardas
- Czesław Radzewicz
- Yuriy Stepanenko
The technology of the ultrafast mode-locked laser using nonlinear polarization evolution in polarization maintaining fibers works as follows: the linearly polarized light is divided at the input into a beam of low intensity and a beam of high intensity. The medium of the absorber can be adjusted in such a way, that both parts will detect a slightly different refractive index, and effectively travel with somewhat dissimilar velocities. Due to the velocity difference, the plane of polarization starts rotating. The output of the absorber contains a polarizing filter eliminating any waves, but for those oscillating perpendicularly to the plane of polarization of the incoming light. When the laser is operating in continuous mode, the light in the beam is of a relatively low intensity, the difference in the optical paths does not thus occur, the polarization does not change and the output filter blocks the light. At a high enough intensity, typical for femtosecond pulses, the rotation of polarization causes the pulse to pass through the absorber. In order for the saturable absorber with polarization rotation to work, the medium has to detect different refractive indices in distinct directions, thus it has to be birefringent. The problem posed by ordinary optical fibers, however, is that birefringence occurs accidentally in them, for example as an effect of stress caused by the press of a finger. Lasers built in this manner are exceptionally sensitive to mechanical influences.
The creators of the device were the first ones to demonstrate how a saturable absorber can be constructed with the use of polarization maintaining fibers: we divide the optical fiber into segments of an appropriate length and then reconnect them, rotating each consecutive segment by 90 degrees in relation to the one preceding it. The more rotated segments there are, the better the quality of the pulses generated in the fiber. In the presented laboratory version of the laser, the saturable absorber consisted of an optical fiber of approx. 3 m in length, divided into three segments, and a filtering polarizing element. The number of rotated segments could potentially be increased even up to around a dozen.