Cavity-birefringence-dependent h-shaped pulse generation in a thulium-holmium-doped fiber laser

Generation of pulses is an essential aspect of fiber lasers. Pulses are considered to have different profiles and parameters such as fiber gain, nonlinearity, and dispersion. Proper management of such resonant parameters can effectively realize the various pulse profiles and shaping.  Recently, researchers have taken advantage of the advanced technology to explore and uncover different types of pulses. As a result, several particular pulses have been reported up to date. Additionally, some of the laser pulse generation has been performed using 2D-based material devices.

A group of researchers at Jiangsu Normal University, School of Physics and Electronic Engineering in China: Dr Junqing Zhao, Dr Lei Li, Professor Luming Zhao, Professor Dingyuan Tang and Professor Deyuan Shen investigated and unfolded a new laser pulse profile generation technique. The h-shaped pulse was generated in a laser containing thulium-holmium-doped fiber (THF). One most striking point in their contribution is that all desired pulse duty circles (PDCs) could be achieved by managing the cavity birefringence. They also focused on studying the factors that affect its generation such as pump power requirements and its cavity birefringence. This fascinating research work has been published in the journal, Optics Letters.

In their experiments, the research team used an asymmetric nonlinear optical loop mirror (NOLM) for generating the pulses in the THF laser and a wavelength division multiplexer (WDM) to feed the THF with the required pump light. A long piece of high nonlinear fiber (HNLF) was incorporated in the NOLM to enhance the cavity nonlinearity and enhance the peak-power-clamping effect. The light was then reflected and transmitted as a signal. A fiber polarization controller and a fiber optical coupler were employed for the purpose of determining the state of polarization of the light signals transmitted and extracting the output light respectively. Furthermore, the challenge of changing the numbers of cavity birefringence was solved by using polarization maintain fiber (PMF) at varying lengths.

Pump-power tuning indicated that PPC effect could only occur on the flat trailing portion of each achieved h-shaped pulse, whereas the peak power of the leading edge was determined by the launched pump power. Polarization state (PS) switching also resulted in the change of clamped peak power, proving that PPC effect is PS-dependent.

During the study, the authors observed and reported for the first time the highest ever pulse duty cycle (PDC) value for a fiber laser. For instance, the value was approximately 98.2% for a PMF length of 2.3m and pulse duration of only 318.14 ns. Through observation of the formed pulse profiles, the authors confirmed the significance of linear cavity birefringence as a tool for pulse tuning.

As an important finding of this study, the research presented a method of generating h-pulses in a THF laser. For the pulses to be generated more effectively, various parameters were to be taken into consideration. For example, the nonlinearity of the NOLM was improved by using it as an artificial saturable absorber at a much higher aperture. Although the generated h-pulses portrayed some similarities when compared to other pulses uncovered before, it had its unique properties which acted as a distinguishing factor. The difference could be spotted in their various spectral profiles, trailing edge and leading portion. For example, unlike h-pulses, a chair-like pulse possesses a massive trailing edge and more extended leading portion. For the case of the power requirement, it was observed that the longer the PMF length, the more power required for the system.

According to the authors, proper management of the cavity birefringence is the secret of achieving better pulse tuning. It is possible to change birefringence cavity settings by adjusting fiber polarization controller. Varying PMF lengths result in different linear birefringence amount thus further determining the leading portion and trailing edge intensities.