Andrew D. Yablon

High power, single mode, all-fiber source of femtosecond pulses at 1550 nm and its use in supercontinuum generation

We present a source of high power femtosecond pulses at 1550 nm with compressed pulses at the end of a single mode fiber (SMF) pigtail. The system generates 34 femtosecond pulses at a repetition rate of 46 MHz, with average powers greater than 400 mW. The pulses are generated in a passively modelocked, erbium-doped fiber laser, and amplified in a short, erbium-doped fiber amplifier. The output of the fiber amplifier consists of highly chirped picosecond pulses. These picosecond pulses are then compressed in standard single mode fiber. While the compressed pulses in the SMF pigtail do show a low pedestal that could be avoided with the use of bulk-optic compression, the desire to compress the pulses in SMF is motivated by the ability to splice the single mode fiber to a nonlinear fiber, for continuum generation applications. We demonstrate that with highly nonlinear dispersion shifted fiber (HNLF) fusion spliced directly to the amplifier output, we generate a supercontinuum spectrum that spans more than an octave, with an average power 400 mW. Such a high power, all-fiber supercontinuum source has many important applications including frequency metrology and bio-medical imaging.

Measuring the Modal Content of Large-Mode-Area Fibers

Spatially and spectrally resolved imaging of mode content in fibers, or more simply, S2 imaging, is a new measurement technique for analyzing the mode content of large-mode-area (LMA) fibers. It works by spatially resolving the spectral interference that occurs when light in a few-mode fibers scatters into different modes that then propagate with different group delays. A scanning spatial filter in the form of a single-mode fiber probe coupled to an optical spectrum analyzer is utilized to provide both spatial and spectral resolution, and the data are analyzed via the Fourier transform of the optical spectrum. The wealth of data allows for imaging multiple modes simultaneously propagating in the fiber under test as well as quantifying their relative power levels. In addition, the ability to analyze mode images as a function of modal group delay allows distinguishing between discrete scattering at fiber surfaces and distributed scattering that occurs along the length of the LMA fiber. The all-fiber nature of the setup makes the measurement sufficiently stable to measure phase images of the higher order modes (HOMs). Because the method is interferometrically based, even very weak HOMs can be detected.

Low-loss high-strength microstructured fiber fusion splices using GRIN fiber lenses

Gradient-index fiber lenses are used to fabricate high-strength (> 100 kpsi) fusion splices between microstructured optical fibers. High coupling efficiencies are attainable (< 0.6 dB loss), providing the mode field diameter is at least about 3.5 /spl mu/m.

Multi-Wavelength Optical Fiber Refractive Index Profiling by Spatially Resolved Fourier Transform Spectroscopy

A non-destructive technique to measure an optical fibers refractive index profile with sub-?m spatial resolution over a wavelength range spanning more than one octave (from 480 to 1040 nm) in a single measurement is described. Data showing the variation of refractive index with wavelength for several fiber types is presented.

Diffraction-Limited Fundamental Mode Operation of Core-Pumped Very-Large-Mode-Area Er Fiber Amplifiers

Diffraction-limited fundamental mode amplification is demonstrated in Er-doped fibers with mode areas ranging from 900 to 1800 mum2. The amplifiers are core-pumped with Raman fiber lasers with both signal and pump selectively launched into the fundamental mode. This scheme results in differential gain for the fundamental mode and stabilizes it against mode mixing caused by perturbations in the core. Gains that are greater than 30 dB are demonstrated from a single stage without significant amplified spontaneous emission. The low nonlinearity of the large mode areas enables amplification to high peak powers without resorting to unconventional microstructured or higher order mode fibers.

11.2 dB SBS gain suppression in a large mode area Yb-doped optical fiber

11.2 dB suppression of stimulated Brillouin scattering (SBS) in an Yb-doped, Al/Ge co-doped large mode area (LMA) gain fiber is demonstrated with a ramp-like acoustic index profile exhibiting an acoustic index contrast of 0.09 and acoustic index slope of 0.01/μm.

Dependence of Brillouin Frequency Shift in Optical Fibers on Draw-Induced Residual Elastic and Inelastic Strains

The dependence of Brillouin frequency shift (BFS) in optical fibers on the residual elastic and inelastic strains induced by different draw tensions during fiber fabrication is investigated experimentally and theoretically. The BFS is linearly proportional to the draw tension with a measured coefficient of -42.0 MHz/100 g, which agrees with the theoretical value of -41.96 MHz/100 g. Theoretical analysis further shows that the elastic strain in fiber core influences predominantly the BFS due to the second-order nonlinearity of Youngs modulus while the effect of the inelastic strain in fiber cladding is less than 1.0%.

Improved supercontinuum generation through UV processing of highly nonlinear fibers

We demonstrate that UV exposure of highly nonlinear, germano-silicate fibers can significantly broaden the infrared supercontinuum generated by femtosecond pulses in these fibers. Both simulations and measurements of the fiber chromatic dispersion show that UV-induced refractive index changes increase the waveguide dispersion by up to 5 ps/(nm-km) at 1570 nm and shift the dispersion zero by over 100 nm. We examine fibers with a range of UV exposure levels and show that the short wavelength edge of the supercontinuum can be continuously changed by more than 100 nm. We also show that the long wavelength edge is extended beyond that of the unexposed fiber. The resulting continuum spans from 0.85 to 2.6 /spl mu/m. Cutback measurements show that the supercontinuum in the exposed fiber is generated in as little as 1 cm of fiber. A nonlinear Schro/spl uml/dinger equation (NLSE) model of the supercontinuum 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 refractive index of the fiber core.

Diffraction limited amplification of picosecond pulses in 1170µm 2 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.

Increased pulsed amplifier efficiency by manipulating the fiber dopant distribution

In the pulse application, a significantly increased efficiency of high power largemode area fiber amplifiers is demonstrated by improving the overlap of the doped region with the fundamental mode of the fiber.