Baldemar Ibarra-Escamilla

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.

Fiber laser generating switchable radially and azimuthally polarized beams with 140 mW output power at 1.6 μm wavelength

We report a fiber laser design that is capable of producing switchable radially and azimuthally polarized beams at 1.6 μm. A c-cut calcite crystal within a three-lens telescope is inserted in the cavity of an erbium doped fiber laser design. Due to the axially symmetric birefringence of the calcite crystal, radially, and azimuthally polarized modes have spatially separated foci in the cavity. Switching between the radially and azimuthally polarized outputs can be achieved by simply translating one of the lenses of the telescope. Output power more than 135 mW for both polarizations has been obtained.

Fine adjustment of cavity loss by Sagnac loop for a dual wavelength generation

We experimentally demonstrate a fine adjustment of cavity loss by Sagnac loop for a dual wave-length generation. The single or dual wavelengths are obtained by controlling the losses on both cavities through a fiber optical loop mirror (FOLM). Wavelength separation on the dual laser is 0.98 nm. The dual or single wavelength is obtained by changes in temperature in the order of 10−1°C around the maximum in the FOLM. Also, we investigate energy fluctuations on signal level saturation effect in the cavity through different pumping power that act on the EDF, where we note that from the 60-mW pumping begins to generate dual-wavelength and 80-mW stabilizes.

Theoretical and experimental analysis of tunable Sagnac high-birefringence loop filter for dual-wavelength laser application

We present detailed investigations of the spectral dependencies of the transmission of a fiber optical loop mirror (FOLM) consisting of a coupler with output ports spliced at arbitrary angles to a high-birefringence (Hi-Bi) fiber. The application for dual-wavelength lasers is discussed. For this aim, the spectral dependence of the reflection is tuned by the temperature of the Hi-Bi fiber that allows a fine adjustment of the cavity loss for generated wavelengths. The ratio between maximum and minimum reflection can be adjusted by the twist angle of the fiber at the splices, which also provides useful possibilities for the adjustment of cavity losses. We used the twist and temperature variation of the Hi-Bi fiber to change the operation from single wavelength to stable dual-wavelength generation with either equal or unequal powers of wavelengths.

Measurement of beat length in short low-birefringence fibers using the fiber optical loop mirror

Abstract In this paper we provide a detailed analysis and investigation of a novel experimental method to measure the birefringence in short low-birefringence fibers. This method is based on the fiber’s twist in a fiber optical loop mirror. It allows simple birefringence measurement in fibers with beat length within the range of 0.4–100 m. The shortest fiber we were able to measure was 0.034 of beat length. Statistical error of measurements was less than 3%. To demonstrate the measurement technique we present experimental results for three optical fibers whose beat lengths were in the range from 1 to 19 m.

Dynamics of noise-like pulsing at sub-ns scale in a passively mode-locked fiber laser.

We report an original noise-like pulse dynamics observed in a figure-eight fiber laser, in which fragments are continually released from a main waveform that circulates in the cavity. Particularly, we report two representative cases of the dynamics: in the first case the released fragments drift away from the main bunch and decay over a fraction of the round-trip time, and then vanish suddenly; in the second case, the sub-packets drift without decaying over the complete cavity round-trip time, until they eventually merge again with the main waveform. The most intriguing result is that these fragments, as well as the main waveform, are formed of units with sub-ns duration and roughly the same energy.

Polarization independent nonlinear fiber sagnac inteferometer

In general, the propagation of light in a twisted, birefringent fiber evolves with a period and phase change non-linearly dependent on the initial polarization state. We show both analytically and numerically that a critical value for the ratio of twist rate to birefringence can be chosen so that the period and nonlinear phase change are no longer sensitive to the input polarization state. This makes possible the design of a input-state independent Sagnac interferometer for mode-locked laser applications.

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.

Tunable dual-wavelength fiber laser based on a polarization-maintaining fiber Bragg grating and a Hi-Bi fiber optical loop mirror

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization maintaining fiber Bragg grating (PM-FBG) as an end mirror that defines two closely spaced laser emission lines. The PM-FBG is also used to tune the laser wavelengths. The total tuning range is ∼8 nm. The laser operates in a stable dual-wavelength mode for an appropriate adjustment of the cavity losses for the generated wavelengths. The high birefringence (Hi-Bi) fiber optical loop mirror (FOLM) is used as a tunable spectral filter to adjust the losses. The FOLM adjustment was performed by the temperature control of the Hi-Bi fiber.

Intra-pulse Raman frequency shift versus conventional Stokes generation of diode laser pulses in optical fibers

We report experimental observations of stimulated Raman scattering in a standard fiber using a directly modulated DFB semiconductor laser amplified by two erbium-doped fibers. The laser pulse width was variably controlled on a nanosecond-scale; the laser emission was separated into two distinct regimes: an initial transient peak regime, followed by a quasi steady-state plateau regime. The transient leading part of the pump pulse containing fast amplitude modulation generated a broadband Raman-induced spectral shift through the modulation instability and subsequent intra-pulse Raman frequency shift. The plateau regime amplified the conventional Stokes shifted emission expected from the peaks of the gain distribution. The output signal spectrum at the end of a 9.13 km length of fiber for the transient part extends from 1550 nm to 1700 nm for a pump pulse peak power of 65 W. We found that the Raman-induced spectral shift is measurable about 8 W for every fiber length examined, 0.6 km, 4.46 km, and 9.13 km. All spectral components of the broadband scattering appear to be generated in the initial kilometer of the fiber span. The Stokes shifted light generation threshold was higher than the threshold for the intra-pulse Raman-induced broadened spectra. This fact enables the nonlinear spectral filtering of pulses from directly modulated semiconductor lasers.