Monica K. Davis

Thermal effects in doped fibers

A theoretical analysis of the pump-induced temperature change and associated thermal phase shift occurring in a pumped doped fiber is presented. Although the primary devices targeted are all-optical switches based on doped fibers, where such effects can be detrimental, this analysis is also applicable to lasers, amplifiers, and other doped fiber devices. The effects of a single pump pulse, multiple pulses and continuous wave (CW) pumping are investigated, both in the dynamic and steady-state regimes. Simple expressions are derived for the thermal relaxation time constant of a fiber, and for its steady-state temperature rise and thermal phase shift under CW pumping. This study predicts that in all-optical fiber switches utilizing a reasonably good dopant the thermal effect due to a single short pulse is negligible in all interferometers, while the steady-state effect can be sizable in a standard fiber Mach-Zehnder but is negligible in a twin-core fiber, a two-mode fiber, and a specially designed Mach-Zehnder interferometer.

Measurements of thermal effects in fibers doped with cobalt and vanadium

Thermal index changes due to nonradiative relaxation in optically pumped Co/sup 2+/-doped and V/sup n+/ doped fibers are studied experimentally by an interferometric method. In both dopants these effects are shown to be very strong and to mask any residual resonantly enhanced nonlinearity. The measured magnitude and time constants of thermal effects are well explained by a new theoretical model, which confirms its validity and usefulness. These measurements also demonstrate a new and simple method to differentiate between nonlinear and thermal phase shifts in doped fibers based on the dependence of the phase change on the pump pulsewidth. This study provides new information on the spectroscopy of these two dopants, including the percentage of absorbed power they transform into heat (/spl sim/38% for Co/sup 2+/ and 56% for V/sup n+/), and the likely presence of clusters in Co/sup 2+/-doped silica even at very low concentrations (8 wt ppm CoO).

Grating stabilization design for high-power 980-nm semiconductor pump lasers

Wavelength stabilization of high-power pump lasers for fiber amplifier applications involves complex interactions between the laser chip and the fiber Bragg grating (FBG). We present a comprehensive theoretical model for the window of operation in the coherence collapse (CC) regime while maintaining wavelength locking in 980-nm FBG-stabilized pumps. Experimental data from the development of a 500-mW grating stabilized pump verifies the theory. We also provide the first evidence that the critical feedback distance for CC operation of longer laser diode chips is sensitive to detuning.

Polarization-induced feedback effects in fiber Bragg grating-stabilized diode lasers

Enhanced performance in fiber Bragg grating-stabilized laser diode designs using polarization-maintaining fiber pigtails requires tight control of polarization extinction ratio. Polarization extinction ratio (PER) affects device power stability, photocurrent behavior, and spectral characteristics. In this paper, the impact of PER on the final device performance is quantified and significant performance improvement when PER is maintained at or above 20 dB is demonstrated.

Switching power reduction using a pumped nonlinear directional coupler

We show that in a nonlinear directional coupler (NLDC) in which the signal is controlled by a pump of appropriate and different wavelength, the power needed for full switching is 3 times lower than for the same device used as a self switch. This power requirement is only /spl radic/3 times higher than for a Mach-Zehnder switch, which makes the NLDC even more appealing for practical multiplexing and switching applications.

Polarization extinction ratio impact on spectral stability of Bragg grating stabilized laser diodes

We demonstrate that polarization extinction ratio (PER) affects the spectral instability of 980-nm band fiber Bragg grating stabilized pump lasers using a polarization-maintaining fiber pigtail. Enhanced spectral stability better than 0.01 nm is demonstrated in devices with superior PER.

Fast stable thermal fiber switch in a Sagnac interferometer

We demonstrate a fast, stable, all-optical switch using a thermal effect in cobalt-doped fiber. 46 ns switching is accomplished in a 10-m Sagnac loop containing a 2.55-cm length of doped fiber. Compared to other demonstrated all- optical switches using doped fibers, this switch offers a significant increase in speed and stability, as well as a significant reduction in fiber length.

Theoretical and experimental characterization of thermal effects in doped fibers

Nonradiative decay mechanisms in doped fibers introduce heat into the fiber core and often decrease the upper state lifetime of a dopant. This can be detrimental to doped-fiber lasers, amplifiers, and in some cases all-optical switches. In this paper we report theoretical and experimental studies of thermal effects in doped fibers, with particular emphasis on their impact on all-optical nonlinear switches using resonant nonlinearities. We observe significant thermal effects in transition metal-doped silica fibers. We determine that sub-microsecond(s) switching using a resonant nonlinear effect in a conventional switch architecture requires dopant oscillator strengths of 3.6 10-3 or greater when nonradiative processes are the predominant decay mechanism.

Novel techniques to characterize clustering in doped fibers

We present rate equations describing the effects of clusters on Nd-doped fibers. Two models are presented, based on formalisms found in the literature. The first one, valid for any cluster size, describes the interaction between an excited ion and a ground state ion in proximity to each other as a non-radiative process referred to as cross relaxation. The second model, valid only for large clusters, describes this interaction as cooperative upconversion. The effects of clustering on two experimental applications are explored: nonlinear phase shift in all-optical fiber switches and cw transmission saturation behavior of clustered fibers. Both measurements suggest that in some fibers as many as 60% of the available dopant ions are clustered. The extent of clustering is experimentally linked to the concentration of Nd in the fibers tested, and perhaps to the use of co-dopants as well.