Author: Yan

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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.

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Diamond guide star laser

The physical properties of CVD diamond make it excellent medium for high average power Raman laser generation. We have been thinking of using diamond to generate sodium guide star lasers. In the past few years, we have been cooperating with Prof. Mildren’ group at Macquarie Universty on fiber laser pumped diamond Raman lasers. A particular interest is to develop a new diamond Raman guide star laser technology, an important alternative to the already developed fiber Raman technology. Recently, we have made breakthough in this topic, thanks to the hard work of Mr. Xuezong Yang, who is a Ph.D. student jointly suported by UCAS and MU. The results was published on Optics Letters：

Xuezong Yang, Ondrej Kitzler, David J. Spence, Zhenxu Bai, Yan Feng, and Richard P. Mildren, “Diamond sodium guide star laser,” Opt. Lett. 45, 1898-1901 (2020)

Abstract: Laser guide stars based on the mesospheric sodium layer are becoming increasingly important for applications that require correction of atmospheric scintillation effects. Despite several laser approaches being investigated to date, there remains great interest in developing lasers with the necessary power and spectral characteristics needed for brighter single or multiple guide stars. Here we propose and demonstrate a novel, to the best of our knowledge, approach based on a diamond Raman laser with intracavity Type I second-harmonic generation pumped using a 1018.4 nm fiber laser. A first demonstration with output power of 22 W at 589 nm was obtained at 18.6% efficiency from the laser diode. The laser operates in a single longitudinal mode (SLM) with a measured linewidth of less than 8.5 MHz. The SLM operation is a result of the strong mode competition arising from the combination of a spatial-hole-burning-free gain mechanism in the diamond and the role of sum frequency mixing in the harmonic crystal. Continuous tuning through the Na D line resonance is achieved by cavity length control, and broader tuning is obtained via the tuning of the pump wavelength. We show that the concept is well suited to achieve much higher power and for temporal formats of interest for advanced concepts such as time-gating and Larmor frequency enhancement.

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High power 780 nm laser for quantum science and technology

Diffraction-limited single frequency 780 nm lasers are required to cool and manipulate rubidium atoms for various quantum applications. For advanced quantum technology applications like large scale atom interferometers, high power (several tens watts and more) 780 nm laser is required. Several effords had been reported to produce over ten watts 780 nm laser by frequency doubling of a 1560 nm Er fiber amplifier.

We have made our contribution recently based on our knowledge in high power Raman fiber laser and high efficiency freqeuncy doubling. A home-made a high power 1480 nm Raman fiber laser is used it to core-pump a Er fiber amplifier directly into the up level of the laser transition. It allows high power and high efficiency amplification of the 1560 nm single frequency laser before the onset of stimulated Brillouin scattering. With the 50 W 1560 nm output, a 21.2 W continuous-wave single frequency 780 nm laser was achieved by utilizing single-pass frequency doubling in a MgO:PPLN crystal.

The result is reported recently on Optics Letters.
J. Dong, X. Zeng, S. Cui, J. Zhou, and Y. Feng, “More than 20 W fiber-based continuous-wave single frequency laser at 780 nm,” Opt. Express 27(24), 35362 (2019).

The work is an excellent demonstration of our combined experience on high power Raman fiber lasers, high power single frequency fiber amplifiers, and high efficiency second harmonic generation.

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In the past five years…

It is already five years that I blogged nothing here. Just give a short outlines here to show the main research achievements.

We have improved and maturized the Raman fiber amplifier based guide star lasers. The laser can now operate at CW, 100 microsecond quasi-CW, and pulsed at Larmor frequency. Laser prototypes were tested at telescopes for sodium guide star, and also for mesospheric magnetometry.

With cascaded random Raman fiber laser, we have demonstrated up to 11 th Raman Stokes light generation. With a Yb pump laser at 1 micron, continuous wavelength tuning up to 2 micron were demonstrated. With this technology, fiber lasers can now output more than 100 W at any wavelength from 1030 nm to 2000 nm.

We continued our study on mode locked Raman fiber lasers, have made some interesting demonstrations including NPR mode locked dissipative soliton, figure of 8 dissipative soliton, rectangular pulse generation etc. We believe we have improved the understanding of mode locked Raman fiber lasers now.

Amplified spontanesou emission sources were found to be useful for pumping cascaded random Raman fiber lasers and mode locked Raman fiber lasers.

Specialty lasers at various wavelengths were demonstrated with wavelength tuning of fiber lasers and second harmonic generations.

Besides, we have made contributions in scattered topics. Many of them are actually more interesting and look for futher developments. Check out our publication list.

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Raman fiber laser goes to kilowatt level

Using the integrated Ytterbium-Raman fiber amplifier architecture, we are now able to generate over kilowatt Raman fiber laser.

A kilowatt-level Raman fiber laser is demonstrated with an integrated Ytterbium-Raman fiber amplifier architecture. A high power Ytterbium-doped fiber master oscillator power amplifier at 1080 nm is seeded with a 1120 nm fiber laser at the same time. By this way, a kilowatt-level Raman pump laser at 1080 nm and signal laser at 1120 nm is combined in the fiber core. The subsequent power conversion from 1080 nm to 1120 nm is accomplished in a 70 m long passive fiber. A 1.28 kW all-fiber Raman amplifier at 1120 nm with an optical efficiency of 70% is demonstrated, limited only by the available pump power. To the best of our knowledge, this is the first report of Raman fiber laser with over one kilowatt output.

The work is published recently on Optics Express: L. Zhang, C. Liu, H. Jiang, Y. Qi, B. He, J. Zhou, X. Gu, and Y. Feng, “Kilowatt Ytterbium-Raman fiber laser,” Opt. Express 22, 18483 (2014).