We describe a two-mode fiber switch made of a short length of erbium-doped fiber in which switching is accomplished by a nonlinearity that is resonantly enhanced by the dopant. In a 0.95-m fiber pumped with a 1.48-μm laser diode, a 906-nm signal is switched with only 8 mW of absorbed power at a signal absorption loss of 0.25 dB.
We report low loss, low pump power, optical-optical switches in rare-earth-doped fibers based on the third-order optical nonlinearity resonantly enhanced by the dopant. In a 0.95-m Er- doped two-mode fiber switch pumped with a 1.48-micrometers diode laser, the absorbed pump power required for switching a 906-nm signal was 8 mW, for a signal loss of only 0.25 dB. This is an enhancement by a factor of 6200 in power-length product over undoped silica. The phase shift was found to be due in part to a non-resonant contribution, thought to arise from a strong UV-VUV transition, and in part to a resonant term from the 980-nm transition. In a 0.98-m Nd-doped, elliptical-core, two-mode fiber switch, switching of a 632.8-nm signal was achieved with only 6.6 mW of absorbed power at 900 nm. The dynamic response of the switch was found to have two components, a slow component equal to the metastable level lifetime (approximately equals 380 microsecond(s) ) and a fast component (approximately equals 2 microsecond(s) ). The latter is believed to arise from rapid cross-relaxation between paired ions, a mechanism which shows promises for low- power, microsecond switching in fibers.
A resonance-enhanced nonlinearity is used to demonstrate a digital switching response in a two-mode fiber interferometer constructed from ytterbium(III)-doped optical fiber. The fiber used was single-mode at the pump wavelength (980 nm) and two-moded at the signal wavelength (514 nm). Interferometer-based optical switches are generally sensitive to fluctuations in pump power, because of the sinusoidal dependence of the output state on pump power. However by allowing the doped fiber to lase, thus clamping the phase shift, this sensitivity can be eliminated to yield a bi-stable, digital switching response. In our interferometer configuration, a laser cavity was defined in the ytterbium-doped fiber by a matched pair of Bragg gratings, and the magnitude of the clamped phase shift was controlled by stretching one of the gratings to adjust the laser threshold. Digital switching of a 514 nm signal with 11 dB isolation and a power-length product of 1.3 mW.m was demonstrated. The relaxation time of the switch was determined by the ytterbium(III) excited state lifetime, while the rise time could be reduced by increasing the pump power without compromising the isolation. The clamping technique demonstrated in this paper should be applicable to any optical switch incorporating a gain medium.
A theoretical model of switches utilizing the resonantly enhanced nonlinearity present in doped fibers is presented. It establishes the dependence of the switching pump power, fiber length, residual resonant signal loss, and response time of the switch on the dopant spectroscopic parameters. Simple factors of merits are derived for the power and the length requirements and are evaluated for selected rare-earth transitions. The most promising candidates require only a fraction of a mW and a few cm of fiber for full switching. Similar power characteristics, but with considerably shorter response times (ns) and lengths (sub-mm), are predicted for dopants with a high oscillator strength transition, or a power-length product ten orders of magnitude smaller than for switches based on the Kerr effect in undoped silica fibers.>
We report on optically induced optical switching of a 632.8-nm signal in a Nd3+-doped, two-mode fiber based on a nonlinearity resonantly enhanced by the dopant. A fast, 0.38-μs component was observed in the switched signal, which we believe is due to cross relaxation between clustered ions. We demonstrated a 600-kHz switch in a 1.14-m elliptical-core fiber with 0.31 μJ of launched 807-nm pump energy. Information about cluster size and speed of the cross-relaxation process is inferred from the switch dynamics.