A simple method reveals temporal and spectral dynamics of ultrafast pulses

Fiber lasers utilize active grain medium in which an optical fiber doped with rare-earth elements such as ytterbium, dysprosium and erbium are used. Over the years, they have evolved into powerful light sources. Fiber lasers offer a high degree of compactness, are cheaper, easy to operate on and possess the capability of generating mode-locked pulses. To date, several passive mode-locking methods have been developed so as to enhance the production of bright, ultra-short pulses with great simplicity and high stability. Investigations on polarization evolution locking (PEL) are still ongoing in a bid to comprehend the mode-locking mechanisms of fiber lasers. Recent technological advances have enabled researchers establish that time-stretch spectroscopy could be harnessed to reveal the spectrum dynamics of the polarization-rotating pulses at a frame rate as high as the laser repetition frequency, when used as a real-time method. Unfortunately, experimental studies on the pulse temporal evolution are quite challenging. Worse off, no report on PEL effect from perspective of individual frequency lines of a fiber laser has been published.

Recently, a team of researchers at East China Normal University: Ming Yan, Xuling Shen, and Heping Zeng in collaboration with Qiang Hao at University of Shanghai for Science and Technology thoroughly characterized the time-evolved temporal and spectral features of pulses from a stretched-pulse fiber laser using a novel facile technique. The technique they intended to employ was based on a fast electrooptic modulator, that had the capability to reveal the temporal and spectrum evolution of the polarization evolution locking (PEL) pulses they were investigating. Their work is currently published in the research journal, Optics Express.

In brief, the research method employed entailed the utilization of a typical NPR mode-locked erbium-doped fiber laser oscillator of specific attributes. Next, they set the pump power to 250 mW. They then used an optical spectrum to measure the laser output from the PBS that was coupled into a fiber collimator. The output beam was then split into two, where 10% of the filtered light is detected by a fast photodetector and monitored by a digital oscilloscope and the rest was used for beat note measurements with narrow-linewidth cw lasers.

The authors found the PEL-induced rf sidebands to be highly sensitive to the laser emission wavelength. In addition, the strongest PEL-induced modulation was observed at the spectral edges of the fiber laser. They also observed that by using the optical comb nature of the fiber laser as an ultrahigh resolution spectrometer, they were able to reveal the wavelength dependence of PEL at the comb-line level through beat note detection.

In summary, the Ming Yan and colleagues study presented the development of a facile technique for studying the PEL effect in a broadband stretched-pulse fiber laser. They observed that their technique had the capability to indirectly reflect the temporal and spectral dynamics of the pulses without using high-speed devices such as an oscilloscope of tens of GHz bandwidth. Altogether, the results obtained here could bring novel insights for the PEL phenomenon and the methods presented here have potential to benefit further experimental and theoretical studies on fiber lasers.