A.M. Vengsarkar

Long-period fiber-grating-based gain equalizers

Long-period fiber gratings are used to f latten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3.

Birefringence reduction in side-written photoinduced fiber devices by a dual-exposure method.

An in situ birefringence measurement in conjunction with an atomic force microscope study shows that the geometric asymmetry of the side-writing process is a major cause of the induced birefringence in grating-based fiber devices. Measured refractive-index profiles of UV-exposed fibers clearly show the asymmetry in the induced index change. We demonstrate the use of a dual-exposure technique for producing low-birefringence devices.

Long-period fiber gratings as band-rejection filters

Optical fiber communication systems that use optical amplifiers are increasingly seeking high-performance devices that function as spectrally-selective band-rejection filters. For example, ASE filters that improve erbium amplifier performance and band-rejection filters in Raman lasers/amplifiers1 must have low insertion losses and low back-reflections. In addition, they must be relatively inexpensive to mass-produce and should be compact after packaging. While bulk-optic filters and short-period Bragg gratings2,3 can be used for some of the applications, all the aforementioned requirements are rarely met. In this paper, we present a novel class of photoinduced, long-period fiber gratings that function as highly-efficient band-rejection filters.

Dispersion-compensating single-mode fibers: efficient designs for first- and second-order compensation.

Single-mode dispersion compensating fiber designs with absolute dispersion values of greater than 100 ps/(nm km) are described. A multiclad fiber with a triangular refractive-index profile in the core gives a dispersion of −250 ps/(nm km), resulting in a 15:1 compensation scheme. All fiber designs use only the fundamental LP01 mode for dispersion compensation and can account for first- and second-order dispersion effects.

A gain-flattened ultra wide band EDFA for high capacity WDM optical communications systems

An 84 nm gain-flattened ultra wide band erbium-doped silica fiber amplifier for high capacity WDM optical communication systems is demonstrated with a noise figure of 6 dB and an output power of 25 dBm. We demonstrated an ultra wide band EDFA with a two-section, split band structure. The total 3dB bandwidth is 84.3 nm, which should be able to support 100 WDM channels with 100 GHz channel spacing or 200 WDM channels with 50 GHz channel spacing. The split bands allow independent optimization of each band for dispersion compensation and span loss variations.

Transmission of 32-WDM 10-Gb/s channels over 640 km using broad-band, gain-flattened erbium-doped silica fiber amplifiers

We report 640-km transmission of 32 wavelength-division-multiplexed (WDM) 10-Gb/s channels with 100-GHz spacing (covering 25-nm bandwidth) using wide dynamic range, gain-flattened erbium-doped silica fiber amplifiers (EDSFAs) with an optical bandwidth of 35 nm. The EDSFA separation is 80 km, the system gain is 140 dB and the total end of system gain nonuniformity among the 32 channels is 4.9 dB.

Transmission of eight 20-Gb/s channels over 232 km of conventional single-mode fiber

As a demonstration of the upgradeability of conventional single-mode fiber systems, we demonstrate the transmission of eight 20-Gb/s channels over 232 km of nondispersion-shifted fiber using dispersion-compensating fiber with negative dispersion-slope. All channels were received without discernible transmission penalties. Due to fiber nonlinearity, differences were observed in system performance depending on the arrangement of the fibers.<<ETX>>

Atomic force microscopy study of uv‐induced anisotropy in hydrogen‐loaded germanosilicate fibers

Thermally treated and uv‐exposed hydrogen‐loaded germanosilicate fibers are profiled with an atomic force microscope after cleaving the fiber in the exposed region and etching the cleaved endface in hydrofluoric acid. Thermally treated fibers etch symmetrically throughout the core region, but the uv‐exposed fibers etch differently. In the uv‐exposed fibers, both the etch depth and the refractive index are asymmetric. They are highest at the core‐cladding interface on the side exposed to the uv radiation. We propose that a photolytic process increases the refractive index across the entire core, but the asymmetry is the result of transient heating due to uv absorption. Furthermore, we demonstrate that uniform circumferential exposure symmetrizes the etch depth and refractive index change and reduces the induced birefringence.

Thermally enhanced ultraviolet photosensitivity in GeO2 and P2O5 doped optical fibers

The 248 nm UV photosensitivity of H2 loaded optical fibers is shown to be significantly enhanced by increasing the temperature during the UV exposure. Heating to 250–400 °C resulted in dramatic UV reaction rate increases in GeO2 doped fibers. Increasing the core temperature during 248 nm irradiation caused P2O5 doped fibers to become photosensitive.

Stabilized extrinsic fiber-optic Fizeau sensor for surface acoustic wave detection

A surface acoustic wave sensor based on an in-line extrinsic Fizeau interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used solely as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot or Fizeau cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap. Strains in the thin-walled silica tube housing the two fibers change the length of the air-gap, thereby altering the phase difference between the reference and sensing reflections. A theoretical analysis of the interaction between the strain induced by elastic stress wave fields and the fiber sensor housing is presented. >