ABSTRACT
We report measurements of the metastable state lifetime in a high-concentration erbium-doped fiber. We use a single-wavelength pump-probe technique that allows us to measure the average lifetime over the entire fiber length. A reduction in the lifetime of approximately 20% caused by a cooperative upconversion effect is clearly observed when the input power is increased up to 10 mW. The input power dependence of the lifetime is analyzed using a simple model. The cooperative upconversion coefficient is determined based on this analysis.
ABSTRACT
We report on the recovery of population inversion in a CW-pumped erbium-doped fiber amplifier. Spontaneous emissions after the input of signal pulses were temporally resolved. The recovery time for population inversion was estimated to be a few hundred microseconds. The pump power and signal width dependences of the recovery were measured. We discuss the dependences by applying an analytical formula derived for the temporal evolution of population inversion.
ABSTRACT
Accuracy degradation at a dielectric interface in simulations using the finite-difference time-domain method can be prevented by assigning suitable effective permittivities at the nodes in the vicinity of the interface. The effective permittivities with exact second-order accuracy at the interface inclined to the Yee-lattice axis are analytically derived for what we believe to be the first time. We discuss two interfaces with different inclined angles between their normal and the Yee-lattice axis in the case of two-dimensional TE polarization. The tangent of the angle is 1 for one interface and 1/2 for the other. With the derived effective permittivities, reflection and transmission at the interface are simulated with second-order accuracy with respect to cell size. The accuracy is demonstrated by numerical examples.
ABSTRACT
The fourth-order finite-difference time-domain (FDTD) method using a symplectic integrator propagator can calculate the propagation of the electromagnetic waves with very low dispersion error in the region of a constant or smoothly varying index profile. An additional technique is required for the problem with the discontinuous dielectric interfaces. We derived the third-order effective permittivities at dielectric interfaces for the fourth-order FDTD method in the case of 2D TE polarization. As the required accuracy level is increased, the memory resources used by the fourth-order FDTD method with the effective permittivities are reduced severalfold or more compared with the standard FDTD method. The accurate performance of the proposed method is demonstrated through numerical examples.
ABSTRACT
We have designed and demonstrated a chirped ladder-type tunable filter and discussed its potential application for a tunable diode laser. A ladder interferometric filter normally has a periodic passband, which makes it impossible to stabilize laser oscillation frequency. To overcome this drawback, we have designed, fabricated, and characterized a novel chirped tunable ladder filter. We have successfully demonstrated a single-passband response in the fabricated device. Furthermore, a tuning operation of more than 30 nm was achieved by introducing a current injection structure and optimizing electrode lengths at each single-stage ladder interferometer.