Michel J. F. Digonnet

Analysis of a tunable single mode optical fiber coupler

We report the operation and the theoretical modeling of an efficient, tunable, and low-loss single mode fiber coupler. The coupler design follows a scheme previously reported, in which two optical fibers mounted in curved grooves in separate quartz substrates are polished until sufficient cladding material has been removed to permit optical coupling between the mated polished faces of the fibers. The results of a computer analysis of the distributed coupling taking place between the fibers are discussed, emphasizing the intuitive dependences of the coupling coefficient and effective interaction length of the device on its geometrical parameters. A detailed experimental analysis of fiber couplers follows in which we characterize two types of couplers made with different brands of single-mode fibers. Operation up to 100 percent coupling ratio and 50 dB extinction ratio between coupled and direct branch as well as operation in overcoupling regimes are demonstrated, both at visible and infrared signal wavelengths. Tuning curves are shown that emphasize the excellent tunability properties of such couplers in which the coupling ratio can be smoothly and continuously tuned between 0 and 100 percent. Experimental evidence of the relatively low loss level and very low polarization dependence of the fiber couplers are also presented. All experimental results, including an analysis of the influence of the refractive index of the intermediate layer of index-matching liquid between the polished faces of the fibers, are found to be very well predicted by our theoretical model.

Theoretical analysis of optical fiber laser amplifiers and oscillators

Using the formalism of mode overlap, a theoretical analysis of optically pumped fiber laser amplifiers and oscillators is developed. The concept of normalized overlap coefficients is introduced to account for the effects of the transverse structure of the interacting signal and pump modes on the device characteristics. Simple and accurate closed-form expressions are derived for the gain of fiber amplifiers and the threshold and energy conversion efficiency of fiber laser oscillators in terms of the fiber and laser material parameters and the pump and signal modes. When applied to step-index Nd:YAG fiber lasers, this study predicts optimum fundamental mode oscillation in fibers with a V number of 5-25 with submilliwatt thresholds and nearly quantum-limited conversion efficiencies.

Characteristics of erbium-doped superfluorescent fiber sources for interferometric sensor applications

The characteristics of 1.55 /spl mu/m Er-doped superfluorescent fiber sources (SFSs), intended for fiber-optic gyroscope (FOG) applications, are explored theoretically and experimentally. With proper selection of the source configuration, fiber length, pump wavelength, pump power, and fiber composition, we show that it is possible to meet the stringent requirements of the FOG, including a high output power, broad emission bandwidth, and excellent spectral thermal stability. Variations of the mean wavelength, spectral width, and output power of the SFS with fiber length, pump power, pump wavelength, and temperature are modeled for representative sources pumped near 980 nm or 1.48 /spl mu/m, and are shown to be in good agreement with experimental results. The effects of a multimoded pump, erbium ion pair, and optical feedback are also assessed. This study indicates that the Er-doped SFS is an excellent candidate for the FOG and for other applications requiring spatial coherence and low temporal coherence. >

Reduced Thermal Sensitivity of a Fiber-Optic Gyroscope Using an Air-Core Photonic-Bandgap Fiber

A 6.5-fold reduction in thermal sensitivity is demonstrated experimentally in a fiber-optic gyroscope made of a 235-m length of quadrupolar-wound air-core fiber, compared to the same gyro operated with a similar coil of conventional (SMF28) fiber. This result is in good agreement with the theoretical value of 6.6 and the value of 7.5 expected from independent thermal measurements carried out on short pieces of the same fibers

Wavelength multiplexing in single-mode fiber couplers

Theoretical and experimental studies of wavelength-division multiplexing in a single-mode fiber optic coupler fabricated by mechanical polishing are reported. The variable spacing geometry of the device allows fine tuning of the center wavelength of operation. Wavelength selectivities ranging from 200 to 35 nm have been experimentally demonstrated, with cross talk ranging from 50 to 10 dB. Selectivity control is simply achieved by proper choice of the interaction length of the coupler. The dependence of the multiplexer behavior on all relevant parameters is investigated and found to satisfy predicted results.

Phase sensitivity to temperature of the fundamental mode in air-guiding photonic-bandgap fibers.

Because in an air-core photonic-bandgap fiber the fundamental mode travels mostly in air, as opposed to silica in a conventional fiber, the phase of this mode is expected to have a much lower dependence on temperature than in a conventional fiber. We confirm with interferometric measurements in air-core fibers from two manufacturers that their thermal phase sensitivity is indeed ~3 to ~6 times smaller than in an SMF28 fiber, in agreement with an advanced theoretical model. With straightforward fiber design changes (thinner jacket and thicker outer cladding), this sensitivity could be further reduced down to ~11 times that of a standard fiber. This feature is anticipated to have important benefits in fiber optic systems and sensors, especially in the fiber optic gyroscope where it translates into a lower Shupe effect and thus a greater long-term stability.

High-Power

We report on the development of novel high-power light sources utilizing a Yb3+-doped phosphate fiber as the gain element. This host presents several key benefits over silica, particularly much higher Yb2 O3 concentrations (up to 26 wt%), a 50% weaker stimulated Brillouin scattering (SBS) gain cross section, and the absence of observable photodarkening even at high population inversion. These properties result in a greatly increased SBS threshold compared to silica fibers, and therefore, potentially much higher output powers out of either a multimode large mode area or a single-mode fiber, which means in the latter case a higher beam quality. To quantify these predictions, we show through numerical simulations that double-clad phosphate fibers should produce as much as ~ 700 W of single-frequency output power in a step index, single-mode core. As a step in this direction, we report a short phosphate fiber amplifier doped with 12 wt% Yb2 O3 that emits 16 W of single-frequency single-mode output. We also describe a single-mode phosphate fiber laser with a maximum output power of 57 W. The laser slope efficiency is currently limited by the fairly high fiber loss ( ~ 3 dB/m). Measurements indicate that 77% of this loss originates from impurity absorption, and the rest from scattering.

\hbox{Yb}^{{\bm 3}{\bm +}}

We report the demonstration of the first air-core photonic-bandgap fiber gyroscope. Because the optical mode in the sensing coil travels largely through air, which has much smaller Kerr, Faraday, and thermal constants than silica, far lower dependencies on power, magnetic field, and temperature fluctuations are predicted. With a 235-m fiber coil, we observe a minimum detectable rotation rate of ~2.7deg/h and a long-term stability of ~2deg/h, which are consistent with the Rayleigh backscattering coefficient of the fiber and comparable to that measured with a conventional fiber

-Doped Phosphate Fiber Amplifier

Exact and approximate theoretical models of the small-signal single-pass gain in single-mode three-level laser fibers are presented. These models are relevant to the behavior of rare-earth-doped (in particular Er/sup 3+/) silica-based fibers. They account for the effects of both the pump and signal-mode structures and pump excited-state absorption (ESA), the latter being present for some pump wavelengths in Er-doped silica fibers. By relying on the modal overlap integral formalism for four-level lasers, relatively simple closed-form expressions for the pump absorption and the gain are derived. These expressions are also applicable to four level laser fibers by a simple adjustment of some of the model parameters. >

Air-core photonic-bandgap fiber-optic gyroscope

A linearly polarized, narrow-linewidth, diode-pumped, Yb-doped silica-fiber oscillator operating at 1150 nm was frequency doubled to produce 40 mW of 575 nm radiation. The oscillator generated 89 mW of cw linearly polarized output power and was tunable over 0.80 nm. The laser output was coupled to a periodically poled LiNbO3 waveguide that converted 67% of the coupled power to the yellow. The system was fully integrated, with no free-space optics, and had an overall optical-to-optical efficiency of 7.0% with respect to the incident diode-laser pump power.