Injection locked dual opto-electronic oscillator

An injection locked dual opto-electronic oscillator having a master oscillator which generates a high Q RF output signal with a plurality of harmonic signals within a predetermined pass band. A slave oscillator has a modulation input coupled to the output signal from the master oscillator as well as an output signal. The slave oscillator has a cavity length selected to produce a single mode operation within the pass band. An electronic phase shifter in the slave oscillator is adjustable to produce constructive interference at a single harmonic of the output signal from the master oscillator and destructive interference of all other harmonics within the pass band and to bring the slave oscillator into injection locked condition with the master oscillator. Therefore the slave OEO is used as a filter for the spurious radiation generated by the master OEO and at the same time preserves the high Q of the RF carrier signal from the master OEO.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensed by or for the United States Government.

1. Field of the Invention

The present invention relates to opto-electronic oscillators.

2. Description of the Prior Art

Microwave radio frequency (RF) resonators or oscillators are used in numerous different applications including high frequency communication, navigation, timing, global position, radar detection and the like. The previously known microwave resonators have operated completely in the electronic domain. These previously known microwave electronic oscillators, however, all suffer from a number of common disadvantages.

One disadvantage of the RF electronic oscillators is that such RF oscillators operate with a relatively narrow bandwidth. As such, the amount of information that can be contained on modulated microwave frequencies is likewise necessarily limited.

A still further disadvantage of these previously known RF oscillators or modulators is that the electronic components used to construct the RF oscillators are relatively bulky, subject to EMI interference, and are also prone to high signal loss. Additionally, the quality factor or Q of the previously known electronic microwave resonators is typically in the range of 1,000–100,000. This relatively low Q factor of the currently known microwave RF oscillators results in a relatively low signal-to-phase noise ratio for the oscillator.

An opto-electronic oscillator (OEO) was previously invented to benefit the high Q by long optical fiber cavity. However this type of fiber OEO suffers from spurious radiation due to the supermodes introduced by the long cavity.

SUMMARY OF THE INVENTION

The present invention provides an injection locked dual opto-electronic microwave oscillator assembly which overcomes all of the previously known disadvantages of the previously known RF microwave oscillators.

In brief, the opto-electronic oscillator assembly of the present invention comprises a master opto-electronic oscillator having a laser which generates an optical output signal. The output signal from the laser is coupled as an input to an optical modulator having an RF input as well as an output.

The RF modulated optical output from the optical modulator is then coupled by an optical fiber to an optical input of a photodetector. The photodetector, upon receipt of the signal from the optical fiber, converts the optical signal into an RF signal. The RF signal is then filtered by bandpass filter so that the output signal from the bandpass filter is within a predetermined pass band. That signal is in turn amplified by an RF amplifier and a portion of the output signal from the amplifier is coupled as a feedback signal to the modulation input of the optical modulator.

The length of the optical fiber for the master oscillator is selected to produce a long cavity, high Q factor for the master oscillator, i.e. a Q factor in excess of 109. Consequently, the output signal from the master oscillator includes not only the primary frequency, but also harmonics at evenly spaced intervals within the pass band of the filter. The frequency spacing of the harmonics is approximately equal to

CnL
where C equals the speed of light, n equals the index of refraction of the optical fiber, and L equals the length of the optical fiber.

A portion of the output signal from the master oscillator is then coupled as an inject lock input signal to a slave opto-electronic oscillator. The slave opto-electronic oscillator is substantially identical to the master electro oscillator with two exceptions. First, the length of the optical fiber within the slave oscillator is selected with a cavity length which permits only a single mode oscillation within the pass band. As such, the optical fiber used in the slave oscillator is much shorter than the length of the optical fiber in the master oscillator. Although this results in a relatively low Q for the slave oscillator, a high Q output signal from the slave oscillator is still maintained due to the injection locked high Q input signal received from the master oscillator.

Secondly, the slave oscillator includes an RF phase shifter in series with the RF signal within the slave oscillator. Adjustment of the phase of the RF signal by the phase shifter enables the RF signal from the master oscillator to injection lock the slave oscillator to achieve positive interference at a single harmonic within the pass band and, simultaneously, achieve negative interference at every other harmonic of the input signal received from the master oscillator. This, in turn, effectively cancels out all of the harmonics other than the single harmonic while simultaneously maintaining the high Q, high signal-to-phase noise value of the master oscillator.

In practice, the combination master and slave oscillators form an injection locked oscillator assembly in which the high Q signal from the master oscillator is injected into the slave oscillator and locks the oscillation frequency and phase of the slave oscillator. Since the cavity length of the optical fiber in the slave oscillator is selected such that only one oscillation mode is allowed within the RF pass band, the high Q of the master oscillator is maintained on the output from the slave oscillator and all unwanted harmonics are simultaneously eliminated.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

With reference first toFIG. 1, a preferred embodiment of the dual opto-electronic oscillator assembly10of the present invention is shown and includes both a master opto-electronic oscillator12as well as a slave opto-electronic oscillator14. Both the master oscillator12and slave oscillator14will be separately described.

Still referring toFIG. 1, the master oscillator12includes a continuous wave laser16having its output connected as an input signal to an optical modulator18. Any conventional laser16may be used and it will be understood that the output signal from the laser16may be either in the visible or invisible spectrum.

The optical modulator18includes both an RF microwave input20as well as an optical output22. The optical modulator may be of any conventional construction, such as a Mach-Zehnder modulator or an electro-absorption modulator. Since the frequency of the laser16is much higher than the RF microwave modulation signal at the modulator input20, the optical signal on the output22from the optical modulator18consists of an optical signal modulated at the frequency of the RF modulating signal. It is also possible to use a directly modulated laser instead of the laser16and modulator18to provide the optical output22.

The output22from the optical modulator18is coupled by an optical fiber24to an optical input28of a photodetector26. In the well-known fashion, the photodetector26converts the optical signal on its input28to an RF signal on its output30.

The output30from the photodetector26is amplified by a microwave RF amplifier32. The output from the RF amplifier32passes through an RF bandpass filter34to an RF coupler36. The RF coupler36then electrically connects a portion of the RF signal from the filter34to the RF input20of the modulator18. The RF signal applied to the modulator input20thus forms a feedback signal to cause the master oscillator12to oscillate at the desired microwave frequency, e.g. 10 gigahertz.

The optical fiber24serves as a long cavity optical delay to create a temporally shifted feedback signal to the optical modulator in order to achieve the oscillation. Furthermore, the length of the optical fiber24is selected to produce the necessary high Q for the master oscillator12in order to achieve the required signal-to-phase noise level. In practice, a length of several kilometers of the optical fiber24will produce Q values in excess of 109for the master oscillator12.

In an optical oscillator, the mode spacing, i.e. the spacing between adjacent harmonic signals, is inversely proportional to the oscillator delay created by the optical fiber24. The space in between the harmonic frequencies can be determined by the following formula:

Δ⁢⁢f≈CnL
where Δf equals the spacing between adjacent harmonic frequencies, C equals the speed of light, n equals the index of refraction of the optical fiber, and L equals the length of the optical fiber.

Consequently, assuming that the optical fiber has an index of refraction of approximately 1.46, as would be the case with Corning SMF28 optical fiber, and a length of 6 kilometers, the spacing between adjacent harmonics is about 34 kilohertz.

In practice, only a single mode or signal frequency of oscillation is desired from the master oscillator12. However, the microwave RF filter34exhibits a pass band of 8 megahertz or greater and, as such, passes not only the desired center frequency but many harmonics at the frequency spacing Δf.

With reference now toFIG. 2, an exemplary output signal on an output38from the RF coupler36is shown. The output from the RF coupler36includes a plurality of evenly spaced radio frequency peaks40each of which has a high Q shape and high signal-to-noise ratio. The peaks40are evenly spaced from each other by the frequency Δf.

With reference again toFIG. 1, the RF coupler36also provides an RF output signal on its output38to the slave oscillator14. The slave oscillator14is also an opto-electronic oscillator and, as such, includes a continuous wave laser50which generates an optical input signal to an optical modulator52. An output from the optical modulator52is optically coupled by an optical fiber54to an input of a photodetector56.

The photodetector56converts the optical signal from the optical modulator to an RF signal which is then amplified by RF amplifier58and coupled as an input signal to an RF filter60. The RF filter60has substantially the same pass band as the RF filter34, e.g. 8 megahertz, and thus filters out all harmonics outside the pass band.

The output from the RF filter60is then coupled through an RF phase shifter62to an RF combiner64which combines the signal from the phase shifter62with the input signal from the master oscillator12. This combined signal65is then fed to an RF coupler66which divides the combined RF signal from the RF combiner64into an RF output68from the opto-electronic oscillator as well as an RF input70of the optical modulator52.

Unlike the master oscillator12, the optical fiber54of the slave oscillator14is relatively short, i.e. about 50 meters for a 10 gigahertz oscillator, such that the Q of the slave oscillator14is very low. The length of the optical fiber54, furthermore, is selected such that the slave oscillator14can only operate in a single oscillation mode within the pass band of the RF filter60.

With reference now particularly toFIG. 3, a frequency spectrum output signal is illustrated of the slave oscillator14operating in a closed loop mode, i.e. in the absence of a signal from the master oscillator12. As can be seen inFIG. 3, the output signal from the slave oscillator14provides a single peak72within the pass band of the RF filter60. However, unlike the output from the master oscillator12, the output signal from the slave oscillator14exhibits excessive noise due to the low Q of the slave oscillator14.

The multi-mode signals on the output38of the RF coupler36of the master oscillator12, however, are injected into the RF combiner64of the slave oscillator. The phase shifter62is then adjusted, by any conventional means, e.g. electronically or manually, to bring the slave oscillator into the locking range of one of the strongest modes or peaks40of the master oscillator12. When thus locked, the phase shifter62produces constructive interference at a single harmonic or oscillation mode of the output signal from the master oscillator12together with destructive interference of all other harmonics or modes within the pass band. This, in turn, produces a single mode high Q output signal on the output68from the slave oscillator14as illustrated inFIG. 4since the high Q signal from the master oscillator12is maintained by the slave oscillator14.

As can be seen from the foregoing, the present invention provides an opto-electronic oscillator assembly which, by using a master oscillator with a high Q value as an injection locking signal to an opto-electronic oscillator with a low Q, produces a single mode output signal with extremely high signal-to-phase noise ratio. Furthermore, the utilization of opto-electronics in the master oscillator enables the master oscillator to achieve an oscillator with Q values that are several magnitudes of order greater than Q values that can be obtained by previously known electronic oscillators alone which, in turn, produces the high signal-to-noise ratio in the output signal from the oscillator assembly.

Having described my invention, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the appended claims.