Eric M. Monberg

Light propagation with ultralarge modal areas in optical fibers

We demonstrate robust single-transverse-mode light propagation in higher-order modes of a fiber, with effective area A(eff) ranging from 2,100 to 3,200 microm(2). These modes are accessed using long-period fiber gratings that enable higher-order-mode excitation over a bandwidth of 94 mm with greater than 99% of the light in the desired mode. The fiber is designed such that the effective index separation between modes is always large, hence minimizing in-fiber mode mixing and enabling light propagation over lengths as large as 12 m, with bends down to 4.5 cm radii. The modal stability increases with mode order, suggesting that A(eff) of this platform is substantially scalable.

All-fiber, octave-spanning supercontinuum

We present an all-fiber supercontinuum source based on a passively mode-locked erbium fiber laser and a small-effective-area, germanium-doped silica fiber. The parallels between this system and the continuum generated in microstructured fibers with 800-nm pulses are discussed, and the role of dispersion is investigated experimentally. We construct a hybrid fiber by fusion splicing lengths of different-dispersion fiber together, generating more than an octave of bandwidth.

Anomalous dispersion in a solid, silica-based fiber

We demonstrate an all-solid (nonholey), silica-based fiber with anomalous dispersion at wavelengths where silica material dispersion is negative. This is achieved by exploiting the enhanced dispersion engineering capabilities of higher-order modes in a fiber, yielding + 60 ps/nm km dispersion at 1080 nm. By coupling to the desired higher-order mode with low-loss in-fiber gratings, we realize a 5 m long fiber module with a 300 fs/nm dispersion that yields a 1 dB bandwidth of 51 nm with an insertion loss of approximately 0.1 dB at the center wavelength of 1080 nm. We demonstrate its functionality as a critical enabler for an all-fiber, Yb-based, mode-locked femtosecond ring laser.

Raman fiber laser with 81 W output power at 1480 nm.

We demonstrate a Raman fiber laser with an operating wavelength of 1480 nm and record output power of 81 W. High-power operation is enabled by a long-period grating used to frustrate backward lasing at the Stokes wavelength in the Yb-doped fiber amplifier. A cascaded Raman fiber with a long-wavelength fundamental mode cutoff enables efficient multiple Stokes scattering from 1117 to 1480 nm while preventing further unwanted scattering to 1590 nm.

Surface nanoscale axial photonics:robust fabrication of high-quality-factor microresonators

Recently introduced surface nanoscale axial photonics (SNAP) makes it possible to fabricate high-Q-factor microresonators and other photonic microdevices by dramatically small deformation of the optical fiber surface. To become a practical and robust technology, the SNAP platform requires methods enabling reproducible modification of the optical fiber radius at nanoscale. In this Letter, we demonstrate superaccurate fabrication of high-Q-factor microresonators by nanoscale modification of the optical fiber radius and refractive index using CO2 laser and UV excimer laser beam exposures. The achieved fabrication accuracy is better than 2 Å in variation of the effective fiber radius.

Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber

We demonstrate a seven-core erbium-doped fiber amplifier in which all the cores were pumped simultaneously by a side-coupled tapered multimode fiber. The amplifier has multicore (MC) MC inputs and MC outputs, which can be readily spliced to MC transmission fiber for amplifying space division multiplexed signals. Gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band.

High-energy (nanojoule) femtosecond pulse delivery with record dispersion higher-order mode fiber

Delivery of high peak-power femtosecond pulses with fibers is constrained by nonlinear distortions accumulated during pulse propagation. We address this problem with a novel, to our knowledge, fiber schematic, where the pulse propagates in a small Aeff (18 microm2) but highly dispersive (record value of approximately -900 ps/nm km) medium, enabled by transmission in the LP02 mode of a few-mode fiber. The novel fiber yields a low dispersion-to-nonlinear-length ratio (due to its large dispersion) despite its small Aeff, hence enabling mitigation of nonlinearities. This enables fiber delivery of distortion-free <150 fs, approximately 1 nJ, and 840 nm pulses--an order-of-magnitude improvement over single-mode fibers of similar Aeff.

Lifting polarization degeneracy of modes by fiber design : a platform for polarization-insensitive microbend fiber gratings

Polarization dependence in microbend gratings is an inherent problem, even in perfectly circular fibers, since antisymmetric modes are almost degenerate linear combinations of four distinct, polarization-sensitive modes. We demonstrate a novel fiber design that lifts polarization degeneracies of the antisymmetric modes to solve this problem. By intentionally exacerbating the polarization splittings, we achieve coupling to only the polarization-insensitive doublet, over wavelength ranges exceeding 100 nm, thus demonstrating a device with practical usable bandwidths. This allows all previous applications envisaged with UV-induced long-period gratings to be realized with the significantly lower-cost microbend technology platform.

Optical fibers by a hybrid process using sol–gel silica overcladding tubes

Abstract We have successfully developed a sol–gel process to produce large silica glass bodies to be used as optical fiber preform overcladding tubes which meet the demands of optical fiber. We have made and tested tubes weighing approximately 4.5 kg, which comprise about 90% of the eventual fibers mass. This sol–gel process uses colloidal silica dispersed in high pH water. The sol is cast, gelled by reducing the pH and dried to a porous tube. The dried body is heat treated to remove organics, to dehydrate and to purify by removing both refractory oxide particles and transition metal ions to the parts/billion range and then sintered to transparency in He. These tubes are competitive with vapor deposited synthetic silica tubes and produce fiber meeting current commercial standards. Net shape formation of large precision glass bodies by gel casting is demonstrated.

Low-loss hollow-core fibers with improved single-modedness

Hollow-core fibers (HCFs) are a revolution in light guidance with enormous potential. They promise lower loss than any other waveguide, but have not yet achieved this potential because of a tradeoff between loss and single-moded operation. This paper demonstrates progress on a strategy to beat this tradeoff: we measure the first hollow-core fiber employing Perturbed Resonance for Improved Single Modedness (PRISM), where unwanted modes are robustly stripped away. The fiber has fundamental-mode loss of 7.5 dB/km, while other modes of the 19-lattice-cell core see loss >3000 dB/km. This level of single-modedness is far better than previous 19-cell or 7-cell HCFs, and even comparable to some commercial solid-core fibers. Modeling indicates this measured loss can be improved. By breaking the connection between core size and single-modedness, this first PRISM demonstration opens a new path towards achieving the low-loss potential of HCFs.