P. W. Wisk

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.

Amplification and noise properties of an erbium-doped multicore fiber amplifier

A multicore erbium-doped fiber (MC-EDF) amplifier for simultaneous amplification in the 7-cores has been developed, and the gain and noise properties of individual cores have been studied. The pump and signal radiation were coupled to individual cores of MC-EDF using two tapered fiber bundled (TFB) couplers with low insertion loss. For a pump power of 146 mW, the average gain achieved in the MC-EDF fiber was 30 dB, and noise figure was less than 4 dB. The net useful gain from the multicore-amplifier, after taking into consideration of all the passive losses, was about 23-27 dB. Pump induced ASE noise transfer between the neighboring channel was negligible.

Refractive index perturbations in optical fibers resulting from frozen-in viscoelasticity

We show that frozen-in viscoelasticity has a significant impact on the refractive index profile of optical fibers. Viscoelastic strains can be frozen into an optical fiber during draw, or by cooling down a drawn fiber from its fictive temperature while under tension. The resulting refractive index perturbation is concentrated in the highest viscosity region of the fiber, exhibits minimal birefringence, and can significantly alter critical performance characteristics. Our quantitative theory for the refractive index perturbation is in excellent agreement with measured data. Frozen-in viscoelasticity can also be harnessed to serve as the basis for optical fiber devices.

Stability of carbon and beryllium‐doped base GaAs/AlGaAs heterojunction bipolar transistors

GaAs/AlGaAs heterojunction bipolar transitors (HBTs) utilizing highly Be‐doped base layers display a rapid degradation of dc current gain and junction ideality factors during bias application at elevated temperature. For example, the gain of a 2×10 μm2 device with a 4×1019 cm−3 Be‐doped base layer operated at 200 °C with a collector current of 2.5×104 A cm−2 falls from 16 to 1.5 within 2 h. Both the base emitter and base collector junction ideality factors also rise rapidly during device operation, and this current‐induced degradation is consistent with recombination‐enhanced diffusion of Be interstitials producing graded junctions. By sharp contrast, devices with highly C‐doped (p=7×1019 cm−3) base layers operated under the same conditions show no measurable degradation over much longer periods (12 h). This high degree of stability is most likely a result of the fact that C occupies the As sublattice, rather than the Ga sublattice as in the case of Be, and also has a higher solubility than Be. The effect...

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.

All-fiber grating-based higher order mode dispersion compensator for broad-band compensation and 1000-km transmission at 40 Gb/s

We use a novel fiber-grating device to demonstrate the first polarization-insensitive all-fiber higher order mode dispersion compensator for broad-band dispersion compensation. Its low loss and high effective area have enabled transmission through 1000 km (10/spl times/100 km) of nonzero dispersion-shifted fiber (NZDSF) at 40 Gb/s.

Space-, wavelength-, polarization-division multiplexed transmission of 56-Tb/s over a 76.8-km seven-core fiber

We report the first experimental demonstration of space-division-multiplexed DWDM transmission of PDM-QPSK channels over a multicore fiber. A total capacity of 56-Tb/s (7×80×107-Gb/s) is transmitted over a 76.8-km seven-core-fiber with a record spectral-efficiency of 14-b/s/Hz.

Improved performance of carbon-doped GaAs base heterojunction bipolar transistors through the use of InGaP

Carbon‐doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) typically exhibit severe leakage at the base‐emitter interface which limits their utility for low‐current applications. Furthermore, the device breakdown voltage, and hence power handling capability, is limited due to the band gap of the GaAs collector material. In this letter we will demonstrate for the first time that both of these limitations can be overcome through the use of InGaP. Since InGaP is not readily doped with carbon, it does not suffer from compensation due to carryover of carbon from the GaAs base. Hence, the ideality factor of the base‐emitter junction improves from 1.3 to 1.09 when the conventional n‐AlGaAs emitter layer is replaced with n‐InGaP. Moreover, InGaP eliminates the crossover of the base and collector currents typically observed in heavily carbon doped GaAs HBTs. This results in the maintenance of gain even at very low collector currents. As a collector material, we have found that InGaP produces significantly ...

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.