Optical waveguides – Directional optical modulation within an optical waveguide – Light intensity dependent
Reexamination Certificate
1998-12-18
2002-05-21
Bovernick, Rodney (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
Light intensity dependent
C385S016000, C385S032000, C385S122000
Reexamination Certificate
active
06393167
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a new architecture for an all-optical fiber or waveguide switch based on a fiber Sagnac interferometer.
2. Description of the Related Art
In an all-optical waveguide switch, a light signal is switched from one output port to another by the application of either another optical signal of different wavelength (pump-induced switching) or by the light signal itself (self-switching). This is typically accomplished in an optical interferometer by placing an element possessing an optical third-order nonlinearity in one of the two arms of the interferometer. For example, in the case of pump-induced switching, in the absence of pump light, the interferometer is adjusted (or fabricated) such that all the signal power comes out of one of the two output ports of the interferometer. When the pump light is applied, it modifies the index of refraction of the nonlinear element, and thus the phase of the signal traveling in this arm. When the phase shift has the right value (which depends on the interferometer, but which is, as an example, &pgr; in a Mach-Zehnder interferometer), the signal is switched from one port to the other.
Because third-order nonlinear effects are generally weak, they tend to require relatively high intensities and/or long nonlinear media to produce this kind of large phase shift. The switching is then characterized by a high intensity-length product. Thus, an optical fiber which preserves a high optical intensity over very long lengths (kilometers) can produce a large phase shift at low optical powers. In fibers, however, only a few types of third-order nonlinearities are available. The most commonly used type is the Kerr effect. The Kerr effect is, however, notoriously weak in silica fibers. To make a Kerr-based switch in a silica fiber requires either a long fiber and a relatively low switching power, or a high power and a short fiber (or waveguide). In the former situation, the fiber arm needs to be so long that most interferometers are unstable and impractical. This is particularly true of the commonly-used Mach-Zehnder interferometer, which needs to be in the sub-centimeter length range for its bias point to be stable over reasonable fiber temperature changes. In the latter situation, the fiber can be short and thus the interferometer can be more stable, but the power required to switch is too high. A high switching power is detrimental because it leads to breakdown of the fiber, because it is expensive, or both.
Other materials and other types of nonlinearity are much stronger than the Kerr effect in silica, and thus require smaller intensity-length products. One particular example is so-called resonantly enhanced nonlinearities, which occur in materials and/or dopants that possess suitable electronic transitions. Examples include semiconductors, such as CdSe
x
S
l−x
, or GaAs, and chalcogenide glasses. (See, M. Asobe, Low power all-optical switching in a nonlinear optical loop mirror using chalcogenide glass fibre,
ELECTRONICS LETTERS
, Jul. 18, 1996, Vol. 32, No. 15, pp. 1396-1397.) A resonantly enhanced nonlinearity can also be observed in dopants that can be introduced into a silica fiber, for example, a trivalent rare earth like erbium (Er
3+
) or neodymium (Nd
3+
). (See, M. J. F. Digonnet, et al., Resonantly Enhanced Nonlinearity in Doped Fibers for Low-Power All-Optical Switching: A Review,
OPTICAL FIBER TECHNOLOGY
Vol. 3, 1997, pp. 44-64.) The advantage of the latter type of nonlinearity is that one can still utilize a silica-based fiber, i.e., retain all the basic low-loss, low-dispersion properties of the silica fiber, which may be eventually beneficial to produce a low-loss, ultrafast switch. However, with existing resonantly enhanced nonlinear materials, if one wishes to keep the switching power low, the length required for the nonlinear element is still too long for most interferometers to be stable.
In summary, the search for a suitable all-optical switch is strongly connected to (1) the development of materials with strong third-order nonlinearities, and to (2) the identification of a switch architecture that can be stable even with long lengths of fiber in its arms.
The Sagnac fiber loop was recognized years ago as a potential solution to this last problem. The primary reason is that unlike most interferometers, the Sagnac loop is a true commonpath interferometer, which means that it is reciprocal. Therefore, even with very long loop lengths, the Sagnac loop is extremely stable to slow external perturbations (slow being defined on the scale of the time it takes light to propagate around the Sagnac loop). Thus, it is possible to utilize a very long Sagnac loop of silica fiber (up to kilometers) and obtain, via the Kerr effect of the fiber, a sizeable phase shift with a low switching power.
The Sagnac interferometer has been used in several ways to demonstrate all-optical switching. The most common approach utilizes the Kerr effect of the silica fiber and an effect known as cross-phase modulation. (See, N. J. Doran, et al., Experimental Investigation of All-Optical Switching in Fibre Loop Mirror Device,
ELECTRONICS LETTERS
, Vol. 25, No. 4, Feb. 18, 1989, pp. 267-269; and M. C. Farries, et al., Optical fiber switch employing a Sagnac interferometer,
APPLIED PHYSICS LETTERS
, Vol. 55, No. 1, Jul. 3, 1989, pp. 25-26.) In this scheme, the pump pulse that causes the switching propagates only in one direction of the loop, and the pump pulse is much shorter than the loop length. The signal traveling in the loop in the same direction as the pump (copropagating) sees the pump during its entire passage through the loop, while the signal traveling in the other direction as the pump (counterpropagating) sees the pump only during the brief time they happen to be at the same location in the loop. Since the Kerr effect is extremely fast (femtoseconds), for pump pulses
100
femtoseconds or longer (which covers most experimental situations), the counterpropagating signal experiences a nonlinear index change over a very short fraction of the loop length. On the other hand, the copropagating signal experiences a nonlinear index change over the entire loop length (assuming negligible walk-off). Thus, the two signals experience a differential phase shift. When the pump power is such that this differential phase shift is equal to &pgr;, the signal has been fully switched from one port to the other.
A self-switching application of the Kerr effect in a Sagnac loop utilizes the fact that if the two signals counterpropagating in the loop have different powers, which can be induced by adjusting the coupling ratio of the Sagnac loop coupler away from 50%, then one signal will experience a larger Kerr phase shift than the other. (See, N. J. Doran, et al., cited above.) By adjusting the signal power, this power imbalance can be such that the differential phase shift between the counterpropagating signals is &pgr;, and again the signal is fully switched.
Another embodiment utilizes the Kerr effect again but counterpropagating signals with orthogonal polarizations in the Sagnac loop. (See, M. Jinno, et al., Demonstration of laser-diode-pumped ultrafast all-optical switching in a nonlinear Sagnac interferometer,
ELECTRONICS LETTERS
, Vol. 27, No. 1, Jan. 3, 1991, pp. 75-76.) The loop is made of polarization-maintaining fiber to ensure. that the polarizations of the two optical signals and the pump remain the same relative to each other along the entire loop. The signal with a polarization parallel to the pump polarization then experiences a larger phase shift than the signal with a polarization orthogonal to the pump polarization. Again, by adjusting the pump power to. a suitable level, this differential phase shift can be made equal to &pgr;, and the signal is fully switched. This effect was also demonstrated using a dye-doped polymer fiber as the nonlinear element. (See, D. W. Garvey, et al., Characterization of the Switching Properties of a Singlemode Polymer Opti
Davis Monica K.
Digonnet Michel J. F.
Bovernick Rodney
Kang Juliana K.
Knobbe Martens Olson & Bear LLP
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