1. Field of the Invention
This invention relates generally to an optical inverter and, more particularly, to an optical inverter that employs an intensity dependent optical inverting switch that includes a variable refractive index material at an optical interface.
2. Discussion of the Related Art
The art of digital logic systems consistently requires the need for greater processing speeds, increased channel bandwidths, and improved transmission reliability for information processing and transmission fields. Because of this continued need for improved system performance, the art is moving towards an increased focus on the optical domain. Digital logic systems typically incorporate a plurality of inverters. An inverter is a device which operates upon digital signals. These signals are composed of pulses having either a xe2x80x9chighxe2x80x9d state or a xe2x80x9clowxe2x80x9d state. The inverter produces a low output for a high input, and a high output for a low input. The output signal thus contains no more or less information than the input, and is merely a reversal of the pulse state. In an optical system, a logical xe2x80x9chighxe2x80x9d might be represented by the presence of a light pulse at a specific time in an optical waveguide, and a logical xe2x80x9clowxe2x80x9d might be represented by the absence of a light pulse at a specific time in the waveguide. The opposite representation is also possible, where the presence of light indicates a xe2x80x9clowxe2x80x9d and the absence of light indicates a xe2x80x9chigh.xe2x80x9d The mapping is arbitrary as long as it is consistent with the system in which the inverter is to be used.
Inverters are important in digital systems for many reasons, such as the application of Demorgan""s theorem. Demorgan""s theorem allows the combination of only AND gates and inverters, or the combination of only OR gates and inverters, to form a complete gate set. Also, optical inverters are a vital component of optical analog-to-digital (A/D) converters.
FIG. 1 shows a representation of an optical inverter 10 suitable for a typical optical digital logic application. The optical inverter 10 receives an optical input beam of a predetermined intensity on an input fiberoptic cable 12, or some other suitable optical waveguide medium, and outputs an optical output beam on an output fiberoptic cable 14, or some other suitable optical waveguide medium. The optical inverter 10 acts as an inverter in that when the optical input beam on the cable 12 is applied to the optical inverter 10, the optical output beam on the cable 14 is dark, or at a predetermined low optical intensity level, and represents a logical zero. Likewise, when the optical input beam on the cable 12 is off, or at a low optical intensity level, a predetermined intensity optical output beam is provided on the output cable 14, and represents a logical one. Thus, the optical output beam of the optical inverter 10 will be a high or low intensity optical output beam, depending on the optical input beam.
Various types of optical inverters are known in the art. One known optical inverter is referred to as a semiconductor optical amplifier (SOA) inverter. The SOA inverter is a saturable optical amplifier that includes a first optical input having a first wavelength, a second optical input having a second wavelength and an optical output that is a combination of the first and second inputs. The first input goes high and low as a digital high and a digital low, and the output conversely goes low and high as an inversion of the first input. The second input is maintained high. When the first input is low or zero, the output is simply the second input, thus representing a high state or logical one. When the first input goes high, the intensity of this first input is designed high enough to saturate the SOA, significantly reducing its gain for both wavelengths. As a result, the amount of light intensity in the output from the second input is reduced. The output beam is then passed through a filter which removes the wavelength of the first input leaving only the second input. This filtered output will then appear to go down when the first input goes high and come back up again when the first input goes low.
Because the main use of the SOA is a wavelength conversion device for use in wavelength division multiplexing technology, it is limited in its ability to be used as an inverter. Further, the SOA is an active device that is fairly complex and is generally not efficient as an optical inverter. What is needed is an optical inverter that is simpler and more effective than the optical inverters known in the art.
U.S. patent application Ser. No. 09/133,032, filed Aug. 11, 1998, titled xe2x80x9cSaturable Absorber Based Optical Inverter,xe2x80x9d now U.S. Pat. No. 6,035,079 and assigned to the assignee of the instant application, discloses an optical inverter that employs a saturable absorber to distinguish between a logical one and a logical zero. A laser generates an optical beam that is split into a first beam that propagates along a first beam path and a second beam that propagates along a second beam path. The first beam and the second beam are then recombined as an optical output beam by an optical combiner. The first beam path and the second beam path have a length relative to each other such that the first and second beams are 180xc2x0 out of phase when they reach the optical combiner. The saturable absorber acts as an optical switch, and is positioned in the first beam path. The saturable absorber receives an optical input signal that causes the absorber to saturate and switch from an opaque mode to a transparent mode to allow the first beam to pass through the absorber. Therefore, if the saturable absorber is switched to the transparent mode, the first and second beams combine destructively and the optical output beam is dark, or a logical zero. When the optical input signal is not applied to the saturable absorber, the absorber is in the opaque mode, and the first beam is blocked so that the optical output beam is the second beam, providing a logical one.
Although the saturable absorber optical inverter has been used for inverting an optical beam, an optical inverter employing different optical inversion schemes may be better suited for different applications. It is therefore an objective of the present invention to provide an optical inverter.
In accordance with the teachings of the present invention, an optical inverting system is disclosed that employs an intensity dependent optical inverting switch. The inverting switch includes a first optical structure having an index of refraction that varies with the intensity of an incident beam and a second optical structure having a substantially constant index of refraction forming an interface therebetween. An optical digital pulse stream to be inverted is combined with a continuous laser beam, and the combined beam is applied to the first optical structure. The combined beam enters the first optical structure, effecting its index of refraction, and impinges the interface at a predetermined angle of incidence. If the angle of incidence is less than the critical angle defined by Snell""s Law at the interface, most of the combined beam is refracted into the second optical structure through the interface, and exits the second optical structure as an inversion of the optical digital pulse stream. Part of the combined beam is also reflected off of the interface. If the angle of incidence is greater than the critical angle, the combined beam is completely reflected off of the interface, and exits the first optical structure as a scaled version of the optical digital pulse stream.
Additional objectives, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.