System for Optical Injection Phase Locking and Switching of Microwave Oscillators

This invention relates generally to electronic oscillator circuits and more particularly to such oscillator circuits which may be "phase locked" and/or switched "on" and "off" by carriers optically generated in semiconductor elements of microwave solid state oscillators.

Many applications, such as radar systems having antenna arrays, for example, require phase coherency between a number of remotely located microwave subsystems. Such phase coherency has been generally implemented in present systems either by using a single microwave oscillator and dividing the output for distribution, or by phase locking separate oscillators by injection of a microwave reference signal. In accordance with either of these signal distribution techniques, microwave energy is routed to the remotely located subsystems by metal waveguides or coaxial cables which add undesirable weight to the system.

An article entitled "Illumination Improves Trapatt Performance" on page 14 of the January, 1977, issue of the magazine "Microwaves" relates to a scheme for using optical illumination to improve the performance of Trapatt oscillators. The article also suggests that by varying the optical illumination level, the Trapatt output frequency can be varied to obtain ultra-fast intrapulse frequency switching. By way of contrast, the subject invention uses modulated optical signals as the illuminating source and a solid state oscillator detects and interacts with the "dc" component and the modulation component of the optical signals to achieve switching and phase locking, respectively, of the oscillator.

Briefly, in accordance with one embodiment of the subject invention, optical signals are generated by a light source which includes a first IMPATT diode oscillator which modulates a GaAlAs injection laser diode and the optical signals are applied to a second oscillator by means of an optical fiber waveguide. The second oscillator includes an IMPATT diode in a tuned microwave cavity having a feedthrough port which allows the optical fiber waveguide to be butt coupled to the junction region of the IMPATT diode. Since the diode junction is reverse biased under normal IMPATT operating conditions, it functions also as a photodiode and incident photons from the optical signal generate hole-electron pairs which contribute to the avalanche current of the IMPATT diode. As a result of the photon action, the oscillations of the second oscillator are locked in phase with the modulation component in the optical signal. When the IMPATT diode of the second oscillator is biased just below the threshold of oscillation, said oscillator can be triggered into oscillation by optical illumination of sufficient intensity, and if the illuminating signal has the proper "dc" component, as well as modulation component, the oscillator can be switched "on and off" while maintaining phase coherence.

It is therefore a primary object of the subject invention to provide means for phase locking remotely located oscillators in response to modulated light signals.

Another object of the subject invention is to provide means for switching remotely located oscillators in response to applied light signals.

A further object of the subject invention is to provide means for phase locking and/or switching remotely located oscillators by means of data transmitted over light weight optical fiber waveguides.

Yet another object of the subject invention is to provide means for optically synchronizing solid state microwave oscillators in which the avalanche breakdown of the diode element is triggered optically by injecting a light pulse into the reverse-biased junction region thereof, whereby the light pulse stimulates the onset of avalanche-breakdown current pulse generation and resulting microwave emission.

A still further object of the subject invention is to provide means for synchronizing a microwave oscillator, i.e. provide phase coherent microwave emissions by means which utilize light weight optical fibers for interconnecting the oscillator to a synchronizing source.

These and other objects, features and advantages of the invention will become apparent to those skilled in the art in view of the following detailed description taken in conjunction with the accompanying drawings wherein like reference numerals indicate like or corresponding parts and wherein:

FIG. 1 is a schematic and block diagram of one preferred embodiment of the subject invention which comprises a microwave oscillator that is adapted for phase locking and switching in response to optical energy applied through an optical fiber waveguide;

FIG. 2 is an expanded view of the active solid state component of the microwave oscillator of FIG. 1;

FIG. 3 is a graph of output optical power versus input signal for the optical source 10 shown in FIG. 1; and

FIG. 4 is a schematic diagram of a second embodiment of an oscillator adapted for phase locking and switching in response to optical energy and which may be used in lieu of oscillator 25 of FIG. 1.

Referring first to FIG. 1, optical signals are produced by a source 10 which includes a cw GaAlAs injection laser diode 12, a microwave energy source 14, and a dc bias source 16. The microwave oscillator 14 is coupled through a capacitor 17 to the input terminal 18 of a switch 20 and the dc bias source 16 is coupled through a coil 22 to the terminal 18. The laser diode 12 is connected between the output terminal 24 of switch 20 and ground. The switch 20 is shown as a mechanical device for clarity of illustration; however, it will be readily recognized by those skilled in the art that an electronic device such as an FET gate may preferably be used in lieu of the mechanical switch shown. The optical signals are generated by direct current modulation of the laser diode 12. As shown in FIG. 3, the laser diode 12 is prebiased (by source 16) to slightly above threshold so that the operating point is on the linear section of the laser diode input-output characteristic curve and the rf signal from microwave oscillator 14 is superimposed on the dc prebias level. The modulation depth of the output signal from the laser 12 is shown in FIG. 3 as about 50 percent, and illustrates the presence of a "dc" component 45 as well as a modulation component 43 in the output light energy from laser diode 12.

Again referring primarily to FIG. 1, a microwave oscillator 25 comprises a GaAs IMPATT diode 26 mounted within a tuned microwave cavity structure 28 which may be conventional except for the incorporation of a feedthrough port 30 that allows an optical fiber waveguide 32 to be butt coupled (shown in greater detail in FIG. 2) to the junction (active) region 34 of the diode 26. The other end of the waveguide 32 is butt coupled to the laser diode 12. Diode 26 is biased by means of a dc source 36 and a bias pin 38 in a conventional manner and the cavity is tuned by means of a waveguide tuning short 40. Microwave energy is coupled from cavity structure 28 to a utilization device 42 by output coupling probe assembly 44. Utilization device 42 might comprise the mixer for one channel of an array of radar receivers, for example.

The junction of IMPATT diode 26 is reverse biased under normal operating conditions and consequently it also functions as a photodiode. The incident photons of the optical signal generate hole-electron pairs which contribute to the avalanche current of the IMPATT diode. Therefore, the oscillations of the oscillator 25 will lock in phase with the modulation component in the optical signal generated by the source 10. Also, in accordance with the invention optical injection phase locking may be achieved not only at the fundamental frequency of the modulating signal, but also at various harmonics thereof.

Diode 26 of oscillator 25 may be biased by the dc source 36 to just below the threshold of oscillation, and the diode can be triggered into oscillation by optical illumination having a sufficient "dc" component, i.e. the oscillator 25 is set so that it will oscillate only when diode 26 is illuminated. Since there is no light output from diode 12 when switch 20 is in the open position, the oscillator 25 may be switched on and off by closing and opening, respectively, switch 20. In this regard, the output optical signal from the source 10 (shown in FIG. 3) may be considered as comprising two components, one being the modulation component signal 43 which provides rf synchronization between the output signals from oscillators 14 and 25, and the other optical signal component is the "dc" component 45 which primarily controls the "turn on/turn off" of oscillator 25. Hence by controlling switch 20 of optical source 10, on/off switching of oscillator 25 may be obtained while maintaining phase coherency between microwave oscillator 14 of optical source 10 and oscillator 25.

It is noted that while oscillator 25 in the embodiment shown in FIG. 1 incorporates an IMPATT diode, that any "active oscillator device" which has sufficient response to light illumination may be utilized in accordance with the invention, e.g. Gunn devices, field effect transistors, etc. Also the principles of the subject invention are applicable to low frequency as well as microwave and millimeter oscillators. FIG. 4 shows an example of a transistor oscillator circuit 25' which may be used in lieu of the oscillator 25 of FIG. 1. Oscillator 25' is conventional except a portion of metal cap 27 is removed to allow optical fiber waveguide 32 to be butt coupled to silicon transistor 26'. The operating quiescent base current point for a transistor oscillator circuit is normally determined by the biasing voltages. However, optical illumination can generate equivalent base current which effectively varies the operating point. Another effect of illumination is the modification of the transistor base resistance and collector barrier capacitance due to the additional carriers generated, and transistor oscillators of the type shown in FIG. 4 can experience phase locking and/or switching in response to optical illumination in a manner analogous to the operation of oscillator 25 described previously. In particular if the transistor bias voltage is adjusted such that circuit 25' is slightly below the threshold of oscillation, the circuit will oscillate only when transistor 26' is illuminated. As was the case with the IMPATT oscillator 25, the transistor oscillator 25' is switched on and off primarily in response to the "dc" component 45 (FIG. 3) in the output energy from laser diode 12 (FIG. 1) and the output signal of oscillator 25' is phase locked to the output signal from oscillator 14 (FIG. 1) in response to the modulation component in the output energy from laser diode 12.

Although in the illustrated embodiment only one microwave oscillator (25) is driven by the optical signal source 10 it will be apparent to those skilled in the art that the optical signal from source 10 may be "branched" to a plurality of optical fiber waveguides, and the energy from each fiber waveguide would be used to switch and/or phase synchronize an associated remotely located oscillator such as oscillator 25.