Light can be useful to transmit information across optical fiber that is the optical conduit in a photonic link. A photonic link can include an optical transmitter, a transmission medium, and an optical receiver. The transmission medium can include free space, optical fiber, and water, among other media. As an example of transmission, light can be useful to transmit voice and/or data information across optical fiber in long-distance telecommunication systems. As other examples, it can be useful to transmit RF analog sensor information across optical fiber in electronic warfare, radar, and communication systems, such as those found in aircraft, ships, and land transmission systems.
In order to transmit information across an optical fiber (or through another suitable medium, such as free space) the information can be encoded into an optical signal. A modulator can be used to encode information onto an optical signal.
A modulator can act as an iris to change the intensity of the light beam, i.e., amount of light, passing through the modulator to various intensity levels. This type of modulation is often used in transmitting analog information.
A modulator can also act as a shutter to control the intensity of the beam by changing the intensity between two intensity levels, such as by turning the beam of light on and off. These types of modulators are often used in transmitting digital information.
An electrooptic modulator can be used to convert information in an electronic signal to an optical signal. Electrooptic modulators can be of an analog or digital type. Examples of electrooptic modulators include electroabsorption modulators and interferometric Mach-Zehnder modulators. Mach-Zehnder modulators can include lumped and traveling wave designs, among others.
There are several factors that determine the performance of a modulator, such as, bandwidth, modulator drive, and transfer function nonlinearity, to name a few. Bandwidth refers to the number of times that the modulator can effectively perform a change through its entire light beam intensity range during a period of time. Modulator drive refers to the electrical voltage or current used to actuate the modulator to change the intensity of the light beam. Transfer function nonlinearity determines the extent to which a modulator produces distortions of an analog signal voltage applied to it. Distortions can include harmonics of an input signal and mixing products of multiple simultaneous input signals, among others.
Mach-Zehnder interferometric modulators rely on two physical effects to vary the light intensity. These effects are: a susceptibility of the velocity of light to an electric field, as the light travels through a material, and the concept of optical interference. In a Mach-Zehnder interferometer, an optical splitter divides the incoming light beam into two optical paths and a combiner recombines the beams at the outputs of the optical paths.
An electrical adjustable delay element controls the optical path length in one of the optical paths resulting in a phase difference between the two beams when they are recombined. The adjustable delay element is provided through use of an electric field that is applied to the optical paths in which the split light beams are traveling.
For example, a voltage creates an electric field across the optical path which causes the light beam traveling through the optical path to either be in phase or out of phase with the light beam traveling in the other optical path. When the light beams are recombined, the phases of the light beams can: cancel each other out, subtract from each other, or add together. This results in the light being passed through the modulator at various intensities. In this way, the light beam can be encoded with information as a series of changes in intensity for transmitting analog information or as on/off pulses of light for transmitting digital information.
Various devices have been used for RF transmission in free space or on transmission lines. These devices can exhibit nonlinear transfer functions. Many approaches have been introduced to compensate for or circumvent nonlinearities in electrical RF transmission. In one approach, switched attenuators are used to limit signal intensity when a signal is sufficiently intense to cause a distortion when passed through a transmission link. Further, this approach utilizes additional devices and extra circuitry and can limit performance in other aspects of a transmission system.