A. Carrascosa

Excitation and interrogation of whispering-gallery modes in optical microresonators using a single fused-tapered fiber tip

We show that whispering-gallery modes (WGMs) in optical microresonators can be excited and detected using a fused-tapered fiber tip (FTFT). The fabrication of FTFTs is simple and inexpensive; they are robust and allow the excitation and interrogation of the resonances with a single fiber. Excitation of high-Q WGMs in silica microcylinders and microspheres is demonstrated.

Phase recovery by using optical fiber dispersion

We propose a simple and fast procedure to retrieve the phase profile of arbitrary light pulses. It combines a first experimental stage, followed by a one-step numerical stage. To this end, it is necessary to perform a Fresnel transform, which is obtained just by propagating the light pulses through an optical fiber. We experimentally test this proposal recovering the phase profile in the light pulses provided by a passively mode-locked laser. The proposal is then compared with a temporal variation of the Gerchberg-Saxton recursive algorithm, which is specially modified for this purpose.

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.

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.

Photonic fractional Fourier transformer with a single dispersive device

In this work we used the temporal analog of spatial Fresnel diffraction to design a temporal fractional Fourier transformer with a single dispersive device, in this way avoiding the use of quadratic phase modulators. We demonstrate that a single dispersive passive device inherently provides the fractional Fourier transform of an incident optical pulse. The relationships linking the fractional Fourier transform order and scaling factor with the dispersion parameters are derived. We first provide some numerical results in order to prove the validity of our proposal, using a fiber Bragg grating as the dispersive device. Next, we experimentally demonstrate the feasibility of this proposal by using a spool of a standard optical fiber as the dispersive device.

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%.

An approach to the measurement of the nonlinear refractive index of very short lengths of optical fibers

A method for the measurement of the nonlinear-refractive index coefficient in single-mode optical fibers is presented. It takes advantage of the high sensitivity of the acousto-optic interaction effect in optical fibers to the fiber properties. Direct measurement of the nonlinear-refractive index change resulting from cross-phase modulation between a probe and a pump signal is obtained from the fibers acousto-optic interaction performance. It is a non-interferometric method in which a very short length of fiber (<0.25 m) is required.A method for the measurement of the nonlinear-refractive index coefficient in single-mode optical fibers is presented. It takes advantage of the high sensitivity of the acousto-optic interaction effect in optical fibers to the fiber properties. Direct measurement of the nonlinear-refractive index change resulting from cross-phase modulation between a probe and a pump signal is obtained from the fibers acousto-optic interaction performance. It is a non-interferometric method in which a very short length of fiber (<0.25 m) is required.

A new technique for the measurement of the nonlinear refractive index in optical fibers

In the last years, acousto-optic (AO) interaction has been proved to be a powerful and highly sensitive method for fine characterization of optical fibers. The key of the technique relies on the high sensitivity of the AO phase-matching condition with the structural and material properties of the fiber, enabling the detection of sub-nanometer fiber radius variations, or core refractive index variations in the order of 10−8 in single-mode fibers. In this contribution, we present our results on the use of in-fiber AO interaction for the measurement of the nonlinear refractive index in single-mode optical fibers.