Georgina Beltrán-Pérez

Actively Q-switched dual-wavelength laser with double-cladding Er/Yb-doped fiber using a Hi-Bi Sagnac interferometer

An actively Q-switched double-wavelength Er/Yb fiber laser is experimentally demonstrated. The linear cavity is formed by a pair of fiber Bragg gratings on one side and a Sagnac interferometer (SI) with high birefringence fiber in the loop on the opposite side. A 3 m of double-cladding Er/Yb-doped fiber used as a gain medium is pumped by a 978 nm laser diode. The SI is used to adjust the internal cavity losses for simultaneous dual-wavelength laser generation. The adjustment is performed by temperature variations of the high birefringence fiber in the SI loop. The maximum average output power for the Q-switched laser operation in dual-wavelength mode was around 68 mW with a repetition rate of 40 kHz for 2 W of pump power. The minimum pulse duration was around 280 ns. The maximum pulse energy was 1.75 µJ.

Dissipative soliton resonance in a full polarization-maintaining fiber ring laser at different values of dispersion.

We investigated the dissipative solitons resonance in an ytterbium-doped fiber ring laser in which all the elements are polarization maintaining (PM). A semiconductor saturable absorber mirror was used as a mode-locker. The cavity included a normal dispersion single-mode fiber (SMF) and an anomalous dispersion photonic crystal fiber. The change of the length of the PM SMF allows the variation of the net-normal dispersion of the cavity in the range from 0.022 ps2 to 0.262 ps2. As the absolute value of the net-normal dispersion increases from 0.022 ps2 to 0.21 ps2, a square-shaped single pulse transformed to a single right-angle trapezoid-shaped pulse, and, at the dispersion of 0.262 ps2, to multiple right-angle trapezoid-shaped pulses, per round-trip.

Fiber Laser Thermally Tunable by a Filter Composed of Two Sagnac Interferometers

Many applications, such as fiber Bragg grating characterization, wavelength division multiplexing, etc. require tunable laser sources with one emission line in a certain wavelength range. Generally, such sources are laser systems combined with tunable filters. In this work an all-fiber laser tunable by a filter composed of two Sagnac interferometers in series is described. The filter loops were constructed of high birefringence fibers (HiBiFi) and the filter was thermally tunable with a single transmission maximum in the Erbium gain spectrum. The results showed that for interferometers with 14 cm HiBiFi length a bandwidth reduction of the transmission peak, a single-line laser emission, a tuning sensitivity of approx. -0.8 nm/degC and a spectral full-width at half-maximum bandwidth of 0.03 nm in the range of 1538-1580 nm were obtained.

Instantaneous frequency measurement of dissipative soliton resonant light pulses

We measured the instantaneous frequency profile of two different dissipative soliton resonant (DSR) light pulses, the usual flat-top and less-common trapezoid-shaped light pulses. The DSR light pulses were provided by an ytterbium-doped polarization-maintaining fiber ring passively mode-locked laser using the adequately selected amount of net-normal dispersion. We confirmed that the DSR light pulses have a (moderately) low linear chirp across the pulse, except at the edges, where the chirp changes exponentially. This unique instantaneous frequency behavior can be succinctly resumed by the following parameters: linear chirp slope and leading and trailing chirp lifetimes. As the pump power increases, the linear chirp slope decreases, whereas the leading and trailing chirp lifetimes do not show an appreciable change. The results are compared with previous theoretical works.

Polarization evolution of vector wave amplitudes in twisted fibers pumped by single and paired pulses

Nonlinear polarization dynamics of single and paired pulses in twisted fibers is experimentally and numerically studied. Accompanying a dramatic difference in the output spectrum when a single- or double-amplified soliton pulse is launched in the fiber, the output polarization for the two cases also reveals very different characteristics.

Initial conditions for dissipative solitons in a strict polarization-controlled passively mode-locked Er-Fiber laser

We report the dynamics of dissipative solitons in a ring cavity passively mode-locked fiber laser with a strict control of the polarization state. We study the relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. We have found that at pulse ellipticities between 5° and 15°, the laser generates one bunch of pulses in the cavity, while at higher ellipticities the laser generates multiple bunches. At constant ellipticity we rotated the polarization azimuth and observed a regime transition from the generation of noise-like pulses (NLP) to that of soliton crystal. The NLP regime was found when the azimuth was rotated towards smaller low-power transmission through the polarizer. The number of solitons in the soliton crystal also depended on the azimuth in a straightforward way: the higher the initial transmission, the bigger the number of solitons.

Development and implementation of a system to measure the response of quartz crystal resonator based gas sensors using a field-programmable gate array

The development of gas sensors based on quartz crystal resonators requires the use of a frequency counter to measure the sensor response. However, commercial frequency counters have at most two channels, and in odour sensory systems (electronic noses), arrays of at least eight sensors are used. Furthermore, it is also desirable that the signal processing and data analysis stage can be included in the same system. A field-programmable gate array (FPGA) can provide a solution to these kinds of problems. This work involves the development of a system to measure the response of quartz crystal resonators, which is implemented using a high resolution frequency counter using an FPGA capable of processing and storing the data in 32-bit registers. Data transfer and acquisition were performed using a microcontroller. The virtual instrumentation software LabVIEW was used for the sensor response display and data storage in a computer. A frequency range from 0 Hz to 100 MHz with a resolution and stability of 1 Hz was achieved using a timebase of 1 s. Tests were performed using gas sensors to obtain real data. The system was escalated to four channels, verifying that it behaves in a correct and stable way.

Thermally Tunable All-Fiber Filter Based on Two Series Connected Sagnac Interferometers With Different Loop Lengths

In this paper, the characterization of a thermally tunable fiber optic filter is presented. It consists of two Sagnac interferometers (SIs) connected in series. Each SI contains a high-birefringence fiber (HiBiFi) segment with different length in the loop, which generates a reduction in the bandwidth of the filter transmission peak. In a previous work, a filter composed by two SIs with equal HiBiFi segments connected in series was reported. An experimental bandwidth reduction of approximately 31%, compared with a filter composed by a single SI was obtained. Using the configuration proposed in the present work, a reduction of approximately 52% in the range of 1533 to 1586 nm and a sensitivity of -1.27 nm/degC was obtained.

Suppression of noise of soliton pulses using a polarization-imbalanced nonlinear loop mirror

The generation of clean solitons is important for a number of applications such as optical analog-to-digital conversion (ADC) based on soliton self-frequency shift. In real sources the quality of the pulses is deteriorated by dispersive waves, continuous wave (CW), amplified spontaneous emission (ASE). The dispersive waves appear in the spectral profile as side-lobe components that would limit the resolution of ADC. Spectral compression techniques cause the appearance of a pedestal on the spectrum. All of these imperfections of pulses have to be eliminated to improve the performance of alloptical systems. The nonlinear optical loop mirror (NOLM) is a good candidate for these tasks. In the present work we report the implementation of a polarization-imbalanced NOLM for soliton cleaning. The NOLM consists of a nearly symmetrical coupler with a 51/49 coupling ratio, 100 m of twisted OFS Truewave fiber whose dispersion value is 9 ps/nm/km at 1550 nm, and a tunable in-line fiber polarization controller (PC) asymmetrically inserted inside the loop. The use of the nearly symmetrical coupler allows very low transmission for low power components of radiation. At the same time adjustment of the PC allows the adjustment of the nonlinear characteristic to have a maximum transmission for solitons with different durations. We used two sources of pulses, SESAM based and a ring fiber laser. At the appropriate adjustment of PC, we obtained a rejection of ASE by 220 times, rejection of the dispersion waves and the pedestal by more than 200 times. The maximum transmission reached 70%.

Spectral Dynamics of Actively Q-Switched Erbium-Doped Fiber Lasers

We report the spectral dynamics of actively Q-switched erbium-doped fiber lasers implemented in two common laser configurations: linear Fabry-Pérot cavity with a pair of fiber Bragg gratings as reflectors and one-directional ring cavity without spectrally selective elements. In both geometries, the spectral width of sub-pulses, forming a Q-switch pulse, depends on the sub-pulse number, the spectrum of the most powerful sub-pulse is much broader than those of the rest, less powerful, ones. In the case of the ring laser, the optical spectra of sub-pulses shift to the Stokes side with increasing a sub-pulse’s number. Possible reasons for such laser spectral dynamics are discussed. We also reveal the details of supercontinuum generation in a 100-m-long telecom fiber when such Q-switch pulses are employed as pump.