Peter A. Krug

Passive temperature-compensating package for optical fiber gratings

We demonstrate a compact, passive temperature-compensating package for fiber gratings. The grating is mounted under tension in a package comprising two materials with different thermal-expansion coefficients. As the temperature rises the strain is progressively released, compensating the temperature dependence of the Bragg wavelength. A fiber grating mounted in a package 50 mm long and 5 mm in diameter exhibited a total variation in Bragg wavelength of 0.07 nm over a 100 °C temperature range, compared with 0.92 nm for an uncompensated grating.

Measurement of index modulation along an optical fiber Bragg grating

We analyze a fiber Bragg grating by focusing the beam of a probe laser (λ = 633 nm) through the side of the fiber onto its core. Designed for Bragg retroreflection at some longer wavelength (λB = 1290 nm), the grating causes Bragg reflection of the probe beam if the input angle is chosen suitably. The reflected power is proportional to the square of the amplitude of the refractive-index modulation at the probed position. By scanning the probe beam along the fiber we measure the axial profile of the modulation amplitude with a spatial resolution of 10 μm. The measured profile is consistent with a single-photon UV writing process.

Experimental demonstration of compression of dispersed optical pulses by reflection from self-chirped optical fiber Bragg gratings

Dispersion compensation is demonstrated experimentally by pulse compression with the use of chirped optical fiber Bragg gratings. The gratings chirp is self-induced by the Gaussian intensity profile of the 240-nm wavelength beam used for holographic sidewriting of the grating. Chirped pulses generated by a 1.55-microm gain-switched distributed-feedback laser with an initial pulse duration of 21 ps and a spectral width of 0.7 nm are compressed to 13 ps, in good agreement with theory.

Intermediate excited-state absorption in erbium-doped fiber strongly pumped at 980 nm

We report the observation of fluorescence at 530 to 550 nm from erbium-doped germanosilicate fiber under strong pumping in the 980-nm band. The observed nonlinear dependence on pump power is shown to arise from a sequential two-photon absorption from the ground state to the (4)F(7/2) state through the short-lived (4)I(11/2) intermediate state. Rate-equation modeling shows that the pump power dependence of this intermediate excited-state absorption is influenced by population cycling induced by amplified spontaneous emission in the fiber from the metastable (4)I(13/2) state.

Annular-slot arrays as far-infrared bandpass filters

Arrays of both annular and square annular slots in a conducting sheet on a dielectric substrate have been fabricated photolithographically. The structures are shown to behave as bandpass filters in the far infrared, with a resonant wavelength slightly larger than the average circumference or perimeter of the slot. The measured far-infrared transmittance of the annular array is approximately 76% of that predicted by theory, while its resonant frequency agrees with theory to within 5%.

Nanosecond-to-picosecond pulse compression with fiber gratings in a compact fiber-based chirped-pulse-amplification system

A chirped, 12-cm-long fiber Bragg grating with 10-nm spectral bandwidth was employed in place of a conventional diffraction grating pair compressor in a compact fiber-based high-energy chirped-pulseamplification system. Initial nanosecond chirped pulses from a fast-tuned laser diode were compressed to produce 1.9-ps-long, 300-nJ pulses.

Photoviscous annealing: dynamics and stability of photorefractivity in optical fibers

The mechanisms of photorefractivity in germanosilicate fibers are considered. Evidence is presented that high intensity irradiation significantly changes the state of the glass network in the irradiated core. The large changes of refractive index (>>10-4) are attributed to these structural changes. The possible roles of stress relief, nanocavitation and spinodal decomposition are discussed.

Effects of hydrogen loading and grating strength on the thermal stability of fiber Bragg gratings

Summary form only given. We have demonstrated that gratings of different strengths in unhydrogenated fiber display different stabilities at elevated temperatures, however, gratings written in hydrogenated fiber did not show this trend. These observations should provide further insight into the mechanism of photo-induced index change during both writing and annealing.

Stark-tuned, far-infrared laser for high-frequency plasma diagnostics

A Stark-tuned optically pumped far-infrared CH(3)OH laser operating at 119 microm has been built. The laser is designed to operate at high power while exhibiting a well-separated Stark doublet. At a pump power of 65 W and an electric field of 1 kV/cm the laser has delivered over 100 mW cw while exhibiting a frequency splitting of 34 MHz. These parameters indicate that this laser would be suitable for use in the present generation of modulated interferometers on large thermonuclear plasma devices. The achieved modulation frequency is more than an order of magnitude higher than could be achieved by using standard techniques.

Rapid scan phase modulator for interferometric applications

A new rapid scan phase modulator has been studied in a range of interferometric applications from the visible to the millimeter wave region. The modulator consists of two parallel mirrors mounted on a rotating platform. It causes little attenuation and is capable of large changes in optical path length at high speeds. The system is relatively insensitive to vibration and can yield a nearly flat transmission envelope.