Research News

Period multiplication in mode-locked figure-of-9 fiber lasers

Period multiplication is a bifurcation phenomenon, which depicts a typical roadmap from periodic steady-state to chaos. Nonlinear system, whose output is periodic, may go through period bifurcation before its output becomes chaos. Examples could be as simple as droplets’ patterns of a water tap or as complex as discrete-time neural networks. There has been particular interest in studying period multiplication properties in mode-locked lasers, but the mechanism behind is still not well-studied.

Recently, we experimentally demonstrated a figure-of-9 fiber laser which can generate period-multiplied pulses with long-term stability. The presented laser design could be a stable and reliable ultrafast optical platform to study bifurcation and chaos.

Jiaqi Zhou, Weiwei Pan, and Yan Feng, “Period multiplication in mode-locked figure-of-9 fiber lasers,” Opt. Express 28, 17424-17433 (2020).

Figure-of-9 fiber laser is a type of nonlinear amplified loop mirror (NALM) mode-locked fiber laser. The reported Yb-doped figure-of-9 laser cavity was constructed by entirely polarization maintaining (PM) fibers and PM fiber-pigtailed components. A nonreciprocal phase shifter providing a linear π/2 phase bias was inserted into the NALM loop to promote self-starting of the mode-locked operation.

By adjusting pump power, the working state of the laser could be switched between fundamental mode-locking and period-quadrupling mode-locking. In contrast with the fundamental mode-locked state, the pulse intensity of the period-quadrupling was no longer uniform, but alternated between four different values, which indicated that steady-state period switched from 1 to 4 round trip times. Other periods can be observed if one slightly modifies the configuration, a small change in passive fiber length, for example.

Based on the experimental observation, we proposed a simplified iteration model to prove that pulse multiplication is a result of overdriving the artificial saturable absorber regardless of pulse formation mechanism. The reliability of the iteration model was also verified experimentally.

Further theoretical insight was obtained by numerical simulations based on the cubic Ginzburg–Landau equation and the split-step Fourier method, which clearly showed that the ratio between the distances of each bifurcation point was approaching the Feigenbaum constant.

The results agree with the classical chaos theory and strongly suggest that the laser design presented here could be a stable and reliable ultrafast optical platform to study bifurcation and chaos.

Research News

Update in 2013

In 2013, we continued our studies on Raman fiber laser/amplifier and mode locked fiber laser, and development of lasers for guide star and cold atom physics.

We found a method for power scaling of single mode linearly polarized Raman fiber laser. In a proof of principle experiment, an output power of 300 W has been achieved, limited by available power. In the single frequency Raman fiber amplifier direction, we have achieved more than 80 W at 1178 nm in the CW case and more than 120 W in the QCW long pulse case. Consequently, after frequency doubling, more than 50 W CW and 80 W QCW (peak power) laser at 589 nm have been demonstrated. With these results, we are confident that power scaling of Raman fiber amplifier based guide star laser to over 100 W is feasible.

We are always interested in applying our expertise in wavelength flexible high power narrow linewidth fiber amplifier to atomic physics. In this year, we have scale the room temperature 1014.8 nm single frequency fiber amplifier to ~ 20 W, and carried out the frequency doubling and quadrupling experiment to 253.7 nm, and absorption and Doppler-free absorption spectral measurement of mercury atoms.

Together with Prof. Gu of Ryerson University, we also studied mode locked Yb fiber lasers with chirped FBGs, and demonstrated dual wavelength switchable dissipative soliton fiber laser and studied the effect of large normal and anomalous dispersion.

Research News

A short summary of what we have done in 2012

To continue the guide star laser work, we have first researched ways to generate linearly polarized Yb doped fiber laser, and are now able to build100 W class linearly polarized 1120 nm Yb-doped fiber laser with cross-axis-matched FBG pairs written in polarization maintaining fiber. We improved the SBS suppression technique further, and achieved a 44 W single frequency Raman fiber amplifier at 1178 nm with an optical efficiency of 52 %. Up to 25 W at 589 nm has been demonstrated with a home-built frequency doubling cavity, which is not yet optimized.

Mode locking of fiber laser is another fascinating field we are interested. We studied mode locking of Raman fiber laser with graphene absorber. The idea is simple: To achieve wavelength versatile mode locked fiber laser by combining the shared advantage of Raman gain and graphene saturable absorption, both of which is broadband. During the path, we have demonstrated passively Q-switched Yb-doped fiber laser by graphene, and the use of single-multi-single mode fiber structure as bandpass filter for building all fiber tunable dissipative soliton fiber laser.

The SBS suppression technique was applied to single frequency Yb doped fiber laser, and achieved 170 W linearly polarized laser with a 10 micron core PM fiber by 7 time increase of SBS threshold. We also find Yb doped fiber laser at the wing of gain spectrum interesting. One example is a demonstration of high power single frequency 1014.8 nm fiber amplifier working at room temperature for mercury cooling after frequency quadruplication. These works are not yet published.