Opto-electronic oscillator
Encyclopedia
An opto-electronic oscillator (OEO) is an optoelectronic circuit that produces repetitive electronic sine wave
and/or modulated optical continuous wave signals.
An opto-electronic oscillator is based on converting the continuous light energy from a pump laser to radio frequency
(RF) and microwave
signals. The OEO is characterized by having very high quality factor (Q) and stability, as well as other functional characteristics that are not readily achieved with electronic oscillator
s. Its unique behavior results from the use of electro-optical (E/O) and photonic components, which are generally characterized with high efficiency, high speed, and low dispersion
in the microwave frequency regime.
to convert light energy into stable, spectrally pure
RF/microwave reference signals. Light from a laser
is introduced into an E/O modulator, the output of which is passed through a long optical fiber and detected with a photodetector
. The output of the photodetector is amplified
and filtered and fed back to the electric port of the modulator. This configuration supports self-sustained oscillations, at a frequency determined by the fiber delay length, the bias setting of the modulator, and the band pass characteristics of the filter. It also provides for both electric and optical outputs. The conditions for self-sustained oscillations include coherent addition of partial waves each way around the loop and a loop gain
exceeding losses for the circulating waves in the loop. The first condition implies that all signals that differ in phase
by some multiple of 2π from the fundamental signal may be sustained. Thus the oscillation frequency is limited only by the characteristic frequency response of the modulator and the setting of the filter, which eliminates all other sustainable oscillations. The second condition implies that, with adequate light input power, self-sustained oscillations may be obtained without the need for the RF/microwave amplifier in the loop.
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
and/or modulated optical continuous wave signals.
An opto-electronic oscillator is based on converting the continuous light energy from a pump laser to radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) and microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
signals. The OEO is characterized by having very high quality factor (Q) and stability, as well as other functional characteristics that are not readily achieved with electronic oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
s. Its unique behavior results from the use of electro-optical (E/O) and photonic components, which are generally characterized with high efficiency, high speed, and low dispersion
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
in the microwave frequency regime.
Operation
Most OEOs utilize the transmission characteristics of a modulator together with a fiber-optic delay lineDelay line
Delay line may refer to:* Propagation delay, the length of time taken for something to reach its destination* Analog delay line, used to delay a signal...
to convert light energy into stable, spectrally pure
Spectral purity
Spectral purity is a quantification of the monochromaticity of a given light sample. This is a particularly important parameter in areas like laser operation and time measurement...
RF/microwave reference signals. Light from a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is introduced into an E/O modulator, the output of which is passed through a long optical fiber and detected with a photodetector
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
. The output of the photodetector is amplified
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
and filtered and fed back to the electric port of the modulator. This configuration supports self-sustained oscillations, at a frequency determined by the fiber delay length, the bias setting of the modulator, and the band pass characteristics of the filter. It also provides for both electric and optical outputs. The conditions for self-sustained oscillations include coherent addition of partial waves each way around the loop and a loop gain
Loop gain
Loop gain is an engineering term used to quantify the gain of a system controlled by feedback loops. As such, the concept of loop gain is useful in a variety of disciplines. Traditionally, most of those have been in the field of electronics, telecommunications, or control systems...
exceeding losses for the circulating waves in the loop. The first condition implies that all signals that differ in phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
by some multiple of 2π from the fundamental signal may be sustained. Thus the oscillation frequency is limited only by the characteristic frequency response of the modulator and the setting of the filter, which eliminates all other sustainable oscillations. The second condition implies that, with adequate light input power, self-sustained oscillations may be obtained without the need for the RF/microwave amplifier in the loop.