R. Grajales-Coutiño

Adjustable noiselike pulses from a figure-eight fiber laser

We propose and study experimentally and numerically a passively mode-locked figure-eight fiber laser scheme generating noiselike optical pulses, or subns wave packets with a fine inner structure of subps pulses presenting random amplitudes and durations. The particular design of the nonlinear optical loop mirror (NOLM) used in this laser, relying on nonlinear polarization rotation, allows adjusting the switching power through input polarization control. Experimental results show stable pulsed operation over a limited range of the NOLM input polarization angle. Interestingly, the spectral and temporal characteristics of these pulses are observed to be widely variable over that range. In particular, the spectral width varies from 16 to 52 nm and this spectral variation is associated with an inverse evolution in the durations of the bunch and of the inner ultrashort pulses. Simulation results are in good agreement with the experiment. They confirm the strong dependence of the pulse properties on the value of the NOLM switching power, although NOLM switching is not alone responsible for the appearance of the noiselike pulsing mode.

Experimental investigation of the nonlinear optical loop mirror with twisted fiber and birefringence bias

We examine the transmission characteristics of a NOLM device using a symmetrical coupler, highly twisted fiber, and a quarter-wave (QW) retarder plate introducing a polarization asymmetry in the loop. We demonstrate high dynamic range with controllable transmissivity, and good stability over long times. We experimentally study the transmission behavior for different input polarization states and distinguish between different polarization components of the output beam. Experiments are in good agreement with our theoretical approach previously published. Appropriate choice of the input and output polarizations allows a very high dynamic range. The adjustment of the QW retarder and input polarization enables tuning the critical power over a wide range.

Generation of long broadband pulses with a figure-eight fiber laser

In this paper we performed the experimental and numerical study of a passively mode-locked fiber laser that generates packets of sub-picosecond pulses instead of individual pulses. The proposed configuration is a figure-eight fiber laser scheme, which includes a Nonlinear Optical Loop Mirror with polarization asymmetry inserted into a ring cavity. No experimental evidence of self-starting mode locking operation of the laser was observed; however, for proper adjustments of the wave retarders included in the setup, a mechanical stimulation triggers the onset of mode locking. The autocorrelation of the generated pulses shows a narrow sub-picosecond peak riding a large sub-nanosecond pedestal whose intensity is half that of the peak, and the optical spectrum is smooth and wide. We show that contrary to conventional ultrashort pulses, these pulses do not vanish rapidly after propagation through a long dispersive fiber, which makes then attractive for super-continuum generation as well as for applications in metrology. Finally, we study the pulse formation in the laser and present arguments based on experimental data and numerical simulations that the observed pulses are actually sets of a large number of solitons.

Experimental investigation of a passively mode-locked fiber laser based on a symmetrical NOLM with a highly twisted low-birefringence fiber

We experimentally investigate the passive mode-locking operation of a figure eight-fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by the polarization asymmetry between the counterpropagating beams in the loop. The most efficient switching is obtained when we have linear polarization for one of the beams and the circular polarization for the other. We used a quarter-wave retarder (QWR) in the NOLM loop to break the polarization symmetry. Through the QWR position, it is possible to adjust the transmission behavior from a maximum to a minimum at a low input power. With our configuration, it is possible to get self-starting mode-locking operation at a specific position of the QWR. This QWR position corresponds to a value close to the minimal transmission. The pulse repetition frequency was 0.8 MHz. The mode-locked laser ran in a stable operation for hours. We achieved a stable generation of picosecond pulses with milliwatts of an average output power.

Novel figure-eight fiber laser scheme including a power-symmetric nonlinear optical loop mirror with adjustable switching power

We propose and study experimentally a novel passively mode-locked figure-eight fiber laser scheme based on a polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). In contrast to conventional power-imbalanced structures, the NOLM used in the proposed laser relies on a difference of nonlinear polarization rotation between the counter-propagating beams to provide switching. In this experiment, the polarization state at the NOLM input is set to linear. By controlling the polarization orientation at the NOLM input through a half-wave retarder plate, it is possible to adjust the NOLM switching power. This property of the NOLM is attractive in the frame of a figure-eight laser. Firstly, the switching power can be readily set to a value ensuring stable mode-locking operation, without having to cut into the loop and modify the NOLM length. On the other hand, we observed that stable pulsed operation is maintained over a certain range of the NOLM input polarization angle, whereas the pulse properties vary over that range. In particular, the spectral width varies from 16 to 52 nm over that range. This spectral variation is associated with a variation of the pulse temporal properties. This result can be explained by the fact that the input polarization angle allows controlling the critical power of the NOLM, which in turn affects the pulses characteristics. The proposed device thus offers a convenient way to adjust the pulses properties (in particular their spectral bandwidth and duration), simply by controlling the angle of a wave retarder, a property which is attractive for some applications.

Pulses with adjustable characteristics from a figure-eight fiber laser

In this work we study experimentally a novel passively mode-locked erbium-doped figure-eight fiber laser based on a polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). The NOLM operation strongly depends on the polarization state at its input. In this experiment, the input polarization state is set to linear, and its orientation is controlled through a half-wave retarder plate. The variation of the input polarization angle allows adjusting the NOLM switching power over a wide range. In this work we show experimentally that this adjustment makes it possible to tune the spectral bandwidth and the temporal properties of the generated pulses over a wide range.

Experimental investigation of a figure-eight fiber laser with a symmetrical NOLM and highly twisted fiber-in-the-loop

We report experimental results that demonstrate the mode-locking operation of a figure eight fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a twisted low-birefringence fiber in the loop. The mode-locking operation is achieved by nonlinear polarization rotation in the NOLM, where the counterpropagating beams accumulate a differential nonlinear phase shift when they have different polarizations. By adjusting a single quarter wave (QW) retarder in the NOLM it was possible to change the NOLM transmission behavior from a maximum to a minimum at low input power. Self-starting mode-locking operation was found for only specific settings of the QW retarder. The pulse repetition frequency was around 0.8 MHz. The mode-locked laser operated stably for hours and the adjustment procedure was straightforward. We achieved stable generation of picosecond pulses with milliwatts of average output power.

Nonlinear Optical Loop Mirror with a Twisted Fiber and Birefringence Bias

We examine the transmission characteristics in a symmetrical NOLM using a highly twisted fiber, and a quarter-wave (QW) plate in the loop. The QW plate adjustment enables tuning the critical power over a wide range.

Polarization properties of nonlinear optical loop mirror with twisted fiber and birefringence bias in the loop

The nonlinear optical loop mirror (NOLM) is used in application like optical switching and demultiplexing, all-optical active mode locking, passive mode locking, pedestal suppression, pulse shaping, etc. This device offers a versatile way to obtain a nonlinear transmission behavior through the nonlinear differential phase shift between the two interfering beams due to the self-phase modulation. Recently we discussed a NOLM device using a symmetrical coupler, highly twisted fiber, and a quarter-wave (QW) retarder introducing the polarization asymmetry in the loop. We have shown high contrast operation, flexibility of characteristics, and stability in time. In this report we analyze theoretically and study experimentally the transmission behavior for different input polarization considering as well different output polarizations. We propose a simple description of the NOLM transmission for right- and left-hand circular output polarization at different output polarization states. The nonlinear characteristics depend on the QW retarder plate angle, but also on the polarization state at the NOLM input. Experiments were carry out with the NOLM consisted of a 500-m length SMF-28 fiber with twist rate of 7 turns/meter. Experiments show a good agreement with our theoretical approach. Appropriate choice of the input and output polarizations allows very high contrast, at least higher than 5000. Our measurements were restricted by sensitivity of our detection system to measure low power pulses at conditions when theoretically transmission must be equal zero. Experiments show flexibility and stability of characteristics. Adjustment of the QW retarder and input polarization allows tuning the critical power over a wide range.

Nonlinear optical loop mirror with a symmetrical coupler and a quarter-wave retarder in the loop

We provide a detailed theoretical analysis and we report a novel experimental method to vary the transmission through a fiber optical loop mirror with a symmetrical coupler and a quarter-wave retarder in the loop. When the quarter-wave retarder is rotated the transmission is adjusted over a wide range.