Effect of laser linewidth and fiber length on self-pulsing dynamics and output stabilization of single-mode Yb-doped double-clad fiber laser

Abstract

We report an experimental study of the effect of fiber length and laser linewidth on self-pulsing dynamics and output stabilization of a single-mode Yb-doped double-clad CW fiber laser. It is found that initiation of self-pulsing under low-level pumping conditions is due to relaxation oscillations and saturable absorption in the weakly pumped region of the doped fiber, irrespective of the fiber length and the laser linewidth. However, with an increase in pump power, depending on fiber length and laser linewidth, the pulses initiated due to relaxation oscillation get amplified, and result in short-duration giant pulses due to either stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). In the case of fiber lasers that employ a broadband mirror and wherein the fiber length is sufficient to reach the SRS threshold, the giant self-pulses are generated by SRS, whereas in the case of fiber lasers using a fiber Bragg grating, characterized by narrowband reflection and with sufficient fiber length to reach the SBS threshold, the giant self-pulses are generated by SBS. Output stabilization and, hence, elimination of self-pulsations can be achieved either by suppressing the relaxation oscillations with the addition of an appropriate length of a passive fiber to sufficiently increase the cavity photon lifetime, or by increasing the pump power to achieve gain uniformity along the doped fiber such that relaxation oscillations and reabsorption effects are suppressed.

Self-induced laser line sweeping in an ytterbium fiber laser with non-resonant Fabry-Perot cavity

“…It is known that spectral dynamics of an ytterbium fiber laser (YFL) exhibits, even in the simplest case of continuous wave (CW) pumping, more complexity than it could be expected [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Among the spectral effects captured yet on early stages of the YFL exploration notice mode hopping [1,2], bi-stable operation near threshold [2,3], spectral hole burning (SHB) with its constructive or destructive role in establishing single-frequency operation [2][3][4].…”

Self-Q-Switch and CW Operation of a Tunable Dual-Wavelength Er/Yb Double-Clad Fiber Laser

“…In the SQS operation region of the laser, when low pump power level is applied, the weakly pumped section of the EYDCF is acting as saturable absorber due to the formation of erbium ion pairs (on highly doped fibers, which is the case), regardless of the EYDCF length. With the increase of the pump power, the population inversion is replenished to a faster rate contributing to generate high energy pulses at increased frequency due to the amplification of the relaxation oscillations at lower pump level, depending on the EYDCF length [21]. Therefore, laser pulse trains with typical characteristics of passive Q-switched pulses were detected in a range from 322 to 890 mW in which SQS laser operation is obtained.…”

“…The obtained experimental results were used to define the laser operation regions shown in Figure 5. With a pump power of 1.1 W, CW laser operation is obtained when the gain along the EYDCF is more uniform reaching a level in which the effects of saturable absorption and relaxation oscillations are eliminated [21].…”

“…With the increase of the pump power, the repetition rate of SQS pulses increases to a limit in which the gain along the EYDCF is more uniform leading to a reduction of the saturable absorption and suppressing SQS operation at high pump. As a result, when a passive fiber with enough length to increase the photon lifetime in the resonator is not added, the full repetition rate of the SQS operation of the laser is mainly determined by the physical length of the EYDCF [21]. SQS laser pulses are observed in a range from 21.7 to 71.63 kHz for a pump range from 322 to 980 mW.…”

“…As the pump power is increased, the saturable absorption is reduced due to the increase of gain uniformity along the EYDCF, reaching a limit in which CW laser operation is obtained. As a consequence, in case of the proposed configuration when pump power is increased, the full repetition of the SQS laser operation depends on the EYDCF length [21]. In order to avoid possible damages in fiber systems caused by high energy SQS pulses generated at low pump power when a CW laser source is required or otherwise to establish possible applications of the SQS laser source with a wide repetition rate, the existence of SQS pulses in double-clad-fiber-based lasers has to be studied establishing operation limits, the conditions which contribute to their generation and the mechanisms to suppress or increase their operation according with the laser application.…”