Paul J. Lemaire

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.

Decay of ultraviolet‐induced fiber Bragg gratings

We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium‐doped silica fiber. The decay is well characterized by a ‘‘power‐law’’ function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power‐law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated‐aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet‐induced refractive‐index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and ...

Mode-locked hybrid soliton pulse source with extremely wide operating frequency range

The authors report a mode-locked pulse source with extremely wide operating frequency range and very stable operation, through the use of a long, linearly chirped Bragg reflector as the output coupler integrated in a fiber external cavity. A 1.55 mu m strained MQW laser diode is used, with one facet high reflectivity (HR) coated for improved cavity Q, and the other antireflection (AR) coated to allow coupling to the external cavity and suppress Fabry-Perot modes. Near-transform-limited pulses are obtained over a frequency range of 700 MHz around a system operating frequency of 2.488 GHz, with pulsewidths of 50 ps, as required for a practical soliton transmission system.<<ETX>>

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.

Ultraviolet laser fabrication of ultrastrong optical fiber gratings and of germania-doped channel waveguides

We report the fabrication of low loss (<0.2 dB) fiber Bragg gratings of 10 nm reflection bandwidth, and of 19 nm bandwidth lossy gratings. With this technology we also demonstrate the direct writing of channel waveguides in commercial germania‐doped wafers without the use of photoresist and etching.

Stable single mode hybrid laser with high power and narrow linewidth

We describe hybrid lasers combining a semiconductor gain section and fiber cavity with integrated chirped Bragg reflector. These devices have produced output powers of 27.5 mW in a narrow linewidth (400 KHz) stable single longitudinal mode. The use of a chirped reflector to stabilize the single mode output, and correct grating orientation are described. The laser output has a side‐mode suppression ratio of over 55 dB at 27.5 mW output, and relative intensity noise (RIN) below 160 dB/Hz.

Volume holographic storage in hydrogen treated germano‐silicate glass

Strong permanent gratings (index change Δn∼3×10−3) have been formed in bulk hydrogen treated germano‐silicate glass with 242 nm exposure. Diffraction efficiencies of 6% and initial sensitivity (Δn/incident energy density) of 2×10−5 cm2/J have been obtained. The observed sensitivity is comparable to the values achieved in other holographic storage materials such as LiNbO3. It is estimated that by more uniform hydrogen loading of the sample, diffraction efficiencies of 100% can be obtained in 3 mol % GeO2 samples of only 175 μm thickness. These materials are potentially useful as fast access time (sub‐μs), high capacity (∼0.8 Tbits), robust, inexpensive write‐one‐read‐many storage media, narrow (subangstrom) linewidth optical filters, and other optical components.

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.

ULTRAVIOLET LASER FABRICATION OF STRONG, NEARLY POLARIZATION-INDEPENDENT BRAGG REFLECTORS IN GERMANIUM-DOPED SILICA WAVEGUIDES ON SILICA SUBSTRATES

Strong Bragg reflectors (≳4 nm width) have been fabricated in Ge‐doped silica waveguides on silica substrates by ultraviolet (UV)‐induced refractive index change. Index changes in excess of 3×10−3 were obtained by deuterium sensitization. The passband quality, polarization dependence, and loss of the reflectors are reported.

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.