Single- and few-moded lithium aluminosilicate optical fiber for athermal Brillouin strain sensing.

Results are presented toward realizing a true single-mode fiber whose Brillouin frequency shift is independent of temperature, while its dependence on strain is comparable to conventional high-silica-content single-mode fibers. Demonstrated here is a fiber with a negative thermal sensitivity dν/dT of -0.26 MHz/K and a strain sensitivity of +406 MHz/%. The suppression of the Brillouin thermal response is enabled by the large thermal expansion coefficient of the group I oxide-containing silica glass network.

Spectrogram approach to S2 fiber mode analysis to distinguish between dispersion and distributed scattering.

We report on the significant effect that intermodal dispersion can have on spatially and spectrally resolved interferometric (S(2)) fiber mode analysis. This dispersion can significantly broaden the measured intermodal group delay and could be misinterpreted as distributed scattering. In our new approach, the spectral interference data is analyzed over multiple wavelength windows staggered across the measurement bandwidth and assembled together to form a spectrogram that reveals the wavelength dependence of the intermodal group delay. Measurements on standard telecom single-mode and large-mode-area fibers are presented. This spectrogram analysis is a more accurate map of mode conversion along the fiber and is essential for evaluating fibers and fiber devices.

Coherence of supercontinua generated by ultrashort pulses compressed in optical fibers.

Femtosecond fiber lasers together with nonlinear fibers are compact, reliable, all-fiber supercontinuum sources. Maintaining an all-fiber configuration, however, necessitates pulse compression in an optical fiber, which can lead to nonlinearities for subhundred femtosecond, nanojoule pulses. In this work we show that using large-mode-area fibers for pulse compression mitigates the nonlinearity, resulting in compressed pulses with significantly reduced satellite pulses. Consequently, supercontinua generated with these pulses are shown to have as much as a 10 dB increase in coherence fringe contrast. By using a hybrid highly nonlinear fiber-photonic crystal fiber, the continuum can be extended to visible wavelengths while still maintaining high coherence.

Spatially and spectrally resolved imaging of modal content in large-mode-area fibers.

A new measurement technique, capable of quantifying the number and type of modes propagating in large-mode-area fibers is both proposed and demonstrated. The measurement is based on both spatially and spectrally resolving the image of the output of the fiber under test. The measurement provides high quality images of the modes that can be used to identify the mode order, while at the same time returning the power levels of the higher-order modes relative to the fundamental mode. Alternatively the data can be used to provide statistics on the level of beam pointing instability and mode shape changes due to random uncontrolled fluctuations of the phases between the coherent modes propagating in the fiber. An added advantage of the measurement is that is requires no prior detailed knowledge of the fiber properties in order to identify the modes and quantify their relative power levels. Because of the coherent nature of the measurement, it is far more sensitive to changes in beam properties due to the mode content in the beam than is the more traditional M(2) measurement for characterizing beam quality. We refer to the measurement as Spatially and Spectrally resolved imaging of mode content in fibers, or more simply as S(2) imaging.

Diffraction limited amplification of picosecond pulses in 1170 microm2 effective area erbium fiber.

Robust fundamental mode propagation and amplification of picosecond pulses at 1.56 microm wavelength is demonstrated in a core-pumped Er fiber with 1170 microm2 effective area. Record peak power exceeding 120 kW, and 67 nJ pulse energy are achieved before the onset of pulse breakup. A small increase in input pulse energy results in a temporal collapse of the pulse center to 58 fs duration, with peak powers approaching 200 kW.

Demonstration of bend-induced nonlinearities in large-mode-area fibers.

We present what we believe to be the first direct measurements of enhanced nonlinearities in large-mode-area fibers due to bend induced reductions in effective area. Both Raman scattering and self-phase modulation are observed to increase in tightly coiled fibers. The measured increase in nonlinearity compares well with predictions from simulations of the modal effective area.

Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber.

Amplification in a single-clad, large-mode-area erbium fiber as an alternative to double-clad Er-Yb amplifiers is presented. Both signal and pump are coupled through a mode-matched splice into the fundamental mode, which ensures preferential gain in the fundamental mode while minimizing the amplified spontaneous emission (ASE). The 875 microm(2) effective area of the Er fiber enables amplification of 6 ps pulses at 1.55 microm wavelength by approximately 33 dB in a single stage to >25 kW peak power with low nonlinear pulse distortion and a diffraction-limited output beam with M(2)<1.1.

Simultaneous direct amplification and compression of picosecond pulses to 65-kW peak power without pulse break-up in erbium fiber.

Picosecond pulses at 1.56 micro mm wavelength are directly amplified with a diffraction limited beam quality in a core-pumped Er fiber with an 875 micro m(2) effective area. The interplay between nonlinear spectral broadening and anomalous fiber dispersion compresses the input pulse duration during amplification so that 42 nJ energy pulses with approximately 65 kW peak power are achieved without pulse break-up.

Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier.

Suppression of stimulated Raman scattering (SRS) is demonstrated in a cladding-pumped fiber amplifier. The Yb-doped amplifier fiber design incorporates a high-index ring that resonantly couples SRS wavelengths out of the gain material, thus filtering the gain. Modeling shows that fiber asymmetry plays an important role in the filtering spectrum.

Supercontinuum generation in ultraviolet-irradiated fibers.

We demonstrate that UV exposure of highly nonlinear, germanosilicate fibers causes a strong change in their chromatic dispersion and can significantly alter the infrared supercontinuum generation in these fibers. By varying the level of UV exposure to the fiber, we show that the dispersion zero and the short-wavelength edge of the supercontinuum can be changed by more than 100 nm. A nonlinear Schrödinger equation model of the continuum generation in the nonlinear fiber shows that the short-wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in the refractive index of the fiber core.

Track of a fiber fuse: a Rayleigh instability in optical waveguides.

The phenomenon colloquially known as a fiber fuse occurs when an optical fiber carrying high power is damaged or in some way abused. Beginning at the damage site a brilliant, highly visible plasmalike disturbance propagates back toward the optical source at speeds ranging from 0.3 to approximately 3 m/s, leaving in its wake a trail of bubbles and voids. We suggest that the bubble tracks in fused fibers are the result of a classic Rayleigh instability that is due to capillary effects in the molten silica that surrounds the vaporized fiber core. We report measurements of the bubble distribution and the collapse time that are consistent with this contention.

Measurement of tissue absorption coefficients by use of interferometric photothermal spectroscopy.

We describe a spectroscopic technique called interferometric photothermal spectroscopy (IPTS) that can measure the absorption coefficient of pulsed laser radiation in nonscattering tissue samples. The technique is suitable for measuring effective absorption coefficients from 10(3) to 10(5) cm(-1). IPTS is particularly attractive because it requires minimal disturbance of the sample. These features indicate potential use for in vivo measurements of tissue absorption coefficients. To validate the technique, the absorption coefficient of pulsed Q-switched Er:YSGG (2.79-microm) radiation in pure water was measured to be 5200 (+/-500) cm(-1) when IPTS was used, in agreement with other published values. IPTS was also used to measure the absorption coefficient of pulsed ArF excimer laser radiation (193 nm) in bovine corneal stroma (in vitro), giving a value of 1.9 (+/-0.4) x 10(4) cm(-1).