Optically Controlled Resonant Tunnel Diode Oscillator

This application is related to U.S. Pat. No. 5,144,261 issued Sep. 1, 1992 application No. 07/821,729 entitled, "OPTICALLY INJECTION LOCKED RESONANT TUNNEL DIODE OSCILLATOR," filed by the inventors herein on Jan. 15, 1992.

The invention described herein relates to optically controlled oscillators, and more particularly to optically controlled frequency modulated or injection locked oscillators which utilize resonant tunnel diodes.

A growing interest has developed for the direct optical control of semiconductor devices due to the potential applications in phased array radar and communications. The following list of documents comprises a sample of articles (1-6) which concern the optical control of such devices, and a sample of articles (7-14) which concern the design and fabrication of resonant tunnel diodes.

1. A. S. Daryoush, "Optical Synchronization of Millimeter-Wave Oscillators for Distributed Architectures," IEEE Trans. Microwave Theory Tech., Vol. 38, pp. 467-476, May 1990; and a related article by, K. Kurokawa, "Injection Locking of Microwave Solid-State Oscillators," Proc, IEEE 61, 1386 (1973).

2. A. J. Seeds and A. A. A. DeSalle, "Optical Control of Microwave Semiconductor Devices," IEEE Trans. Microwave Theory Tech., Vol. 38, pp. 577-584, May 1990.

3. T. C. L. G. Sollner, E. R. Brown and H. Q. Le, "Microwave and Millimeter-Wave Resonant-Tunneling Devices," Lincoln Lab. Jour., Vol. 1, pp. 89-105, 1988.

4. S. C. Kan, S. Sanders, G. Griffel, G. H. Lang, S. Wu, and A. Yariv, "Optical Switching of a New Middle Trace in an Optically Controlled Parallel Resonant Tunneling Device-Observation and Modeling," Appl. Phys. Lett., Vol. 58, pp. 1548-1550, 1991.

5. P. England, J. Yee, L. T. Florez, J. P. Harbison and J. E. Golub, "Optical Switching in Resonant Tunneling Structures," Conference on Quantum Electronics Laser Science, May 12-17, 1991, Baltimore, Md., 1991 Technical Digest Series, Vol. 11, p. 34, 1991.

6. D. J. Struzbecher, J. F. Harvey, T. P. Higgins, A. C. Paolella, and R. A. Lux, "Direct Optical Frequency Modulation of a Resonant Tunnel Diode Oscillator," submitted to IEEE Electron Device Lett., 29 Jul. 1991.

7. L. L. Chang, L. Esaki and R. Tsu, "Resonant Tunneling in Semiconductor Double Barriers," Appl. Phys. Lett., vol. 24, pp. 593-595, 1974.

8. I. Mehdi, R. K. Mains, and G. I. Haddad, "Effect of Spacer Layer Thickness on the Static Characteristics of Resonant Tunneling Diodes," Appl. Phys. Lett. 57, 899 (1990); and a related article by J. E. Oh, I. Mehdi, J. Pamulapati, P. K. Bhattacharya, and G. I. Haddad, "The Effect of Molecular Beam Epitaxial Growth Conditions on the Electrical Characteristics of In.sub.0.52 Al.sub.0.48 As/In.sub.0.53 Ga.sub.0.47 As Resonant Tunneling Diodes," J. Appl. Phys. 65, 842 (1989); I. Mehdi, R. K. Mains, G. I. Haddad, and U. K. Reddy, "Properties and Device Applications of Deep Quantum Well Resonant Tunneling Structures," Surf. Sci. 228, 426 (1990); and also by the same authors, R. K. Mains, I. Mehdi, and G. I. Haddad, "Effect of Spatially Variable Effective Mass on Static and Dynamic Properties of Resonant Tunneling Devices," Appl. Phys. Lett. 55, 2631 (1989).

9. C. J. Arsenault and M. Meunier, "Proposed New Resonant Tunneling Structures with Impurity Planes of Deep Levels in Barriers," J. Appl. Phys. 66, 4305 (1989).

10. R. L. Wang, Y. K. Su, Y. H. Wang, and K. F. Yarn, "Negative Differential Resistance of a Delta-Doping-Induced Double Barrier Quantum-Well Diode at Room Temperature," IEEE Electron Dev. Lett. 11, 428 (1990).

11. K. Reddy, A. J. Tsao, S. Javalagi, G. K. Kumar, D. R. Miller, and D. P. Neikirk, "Quantum well injection transit time (QWITT) diode oscillators," in Conference Digest Fifteenth International Conference on Infrared and Millimeter Waves, 10-14 Dec 1990, Orlando, Fla., ed. R. J. Temkin, (SPIE vol. 1514), p. 88.

12. J. M. Gering, T. J. Rudnick, and P. D. Coleman, "Microwave Detection Using the Resonant Tunneling Diode," IEEE Trans. Microwave Thry. and Tech. 36, 1145 (1988).

13. T. C. L. G. Sollner, P. E. Tannenwald, D. D. Peck, and W. D. Goodhue, "Quantum Well Oscillators," Appl. Phys. Lett. 45, 1319 (1984); and a related article by, E. R. Brown, T. C. L. G. Sollner, W. D. Goodhue, and C. D. Parker, "Millimeter-Band Oscillations Based on Resonant Tunneling in a Double-Barrier Diode at Room Temperature," Appl. Phys. Lett. 50, 83 (1987).

14. G. Keiser, Optical Fiber Communications, (McGraw-Hill, N.Y., 1983) and see C. K. Kao, Optical Fiber Systems: Technology, Design, and Applications, (McGraw, 1982).

As is evident from the above cited references, the technology of building resonant tunnel diodes (RTDs) is well established. As well, the technology of microwave light generation and delivery using lasers, light emitting diodes, plasma tubes and the like in combination with intensity modulators and fiber optics is also well known. Therefore, as is suggested by the above identified references, one skilled in the art would readily be able to design any number of optical or integrated optic systems to deliver light intensity modulated at microwave or millimeter wave frequencies in the nominally 2 mW power range with photon energies above the bandgap of a semiconductive device material to the semiconductive device and incorporate such a semiconductive device in oscillator circuit applications.

An example of important technical fields where such semiconductor devices would be able to be directly incorporated is in phased array radar and communication systems and in remote antenna systems. As suggested by references 1 and 2, intensity modulated light would modulate active oscillator modules using these semiconductor devices and distributed over an antenna array remote from the rest of the radar or communication system. One means of achieving modulation, as suggested by reference 2, is by modulating oscillators with a direct optical signal delivered over optical fibers. Reference 2 describes the various means which are available to optically control semiconductor devices, including the direct optical modulation of oscillators utilizing IMPATTS, FETs, and HEMTs. However, these semiconductor devices lack some of the enhanced performance characteristics of RTDs and, therefore, modulation or locking of oscillator modules would be further optimized by optically controlling RTDs incorporated in such oscillators.

The present invention addresses this present need for direct optical control of an RTD to modulate or lock an RTD oscillator.

One object of the invention is to modulate or lock the natural frequency of a semiconductor oscillator with minimum noise using an externally modulated optical signal.

Another object of the invention is to provide this control using an external signal which can be easily transmitted over a media which is not influenced by other electrical signals and which is light weight and inexpensive.

In accordance with these objectives, the present invention provides an oscillator based on a resonant tunneling structure whose natural oscillating frequency is frequency modulated or locked by an external intensity modulated optical signal.

In the most generic embodiment of the present invention, the optically controlled, frequency modulated or injection locked oscillator comprises a resonant tunnel diode (RTD) which, when voltage biased, oscillates at a free running frequency; an optical signal delivery system, such as a light intensity modulator connected to optical fibers; and other oscillator circuitry which one skilled in the art could readily adapt to the concepts of the present invention. In operation, the free running oscillation of the RTD can be frequency modulated or injection locked to the frequency of the intensity modulated optical signal delivered via the optical signal delivery system. This injection locking occurs as the modulation frequency approaches the free running oscillation frequency. In experiments conducted by the inventors herein, injection locking of the oscillator occurred over a bandwidth of 150 Kilohertz with a noted reduction in FM noise; and frequency modulation of the oscillator occurred at modulation frequencies as high as 100 MH.sub.z.

These and other objects, features and details of the present invention will become apparent in light of the ensuing detailed disclosure, and particularly in light of the drawings wherein:

FIG. 1 is a schematic circuit diagram of an optically modulated resonant tunnel diode oscillator assembly according to the present invention;

FIG. 2 is an equivalent circuit diagram of a portion of FIG. 1;

FIG. 3 is an edge elevation view of an embodiment corresponding to the circuit diagram of FIG. 1;

FIG. 4 is a partial planar view as taken along the line 4--4 of FIG. 3;

FIG. 5 is a schematic block diagram of a test circuit used as an initial demonstration of the direct optical frequency modulation and injection locking of an RTD oscillator as embodied by the present invention;

FIG. 6 is a curve of output power versus frequency showing the power spectrum of the unmodulated and unlocked RTD oscillator;

FIG. 7 is a curve of output power versus frequency of the oscillator subassembly portion of FIG. 5 showing the power spectrum of the unlocked oscillator together with the optically injected signal which is shown as the left most peak;

FIG. 8 is a curve of the output power versus frequency for the oscillator subassembly portion of FIG. 5 for showing the power spectrum of the injection locked oscillator;

FIG. 9 is a superimposition of the curves from FIGS. 6 and 8;

FIG. 10 is a plot of the power spectrum versus frequency of the RTD oscillator with the laser light on and off;

FIG. 11 is a plot of the output power spectrum versus frequency of the RTD oscillator when being frequency modulated using laser light which is intensity modulated at 1 KH.sub.z ; and

FIG. 12 is a plot of the output power spectrum versus frequency of the RTD oscillator when being frequency modulated using laser light which is intensity modulated at 75 MH.sub.z.

As shown in FIGS. 1, 2, 3 and 4, a circuit or assembly 10 is provided. Assembly 10 includes an input light emitter subassembly 11 and an oscillator subassembly 12. Assembly 10 may also have an optional add-on output power sensor or test subassembly (not shown) which would be part of the load impedance Z.sub.L 25.

As shown in FIGS. 1 and 2, oscillator subassembly 12 receives modulated light from emitter subassembly 11. Incorporated in oscillator subassembly 12 is the resonant tunnel diode 22. FIG. 2 shows an approximate equivalent circuit of the RTD 22 with a shunt capacitance C.sub.d 43 caused by the charge separation in the RTD layers; a negative shunt resistance -R.sub.d 42 caused by the negative differential resistance portion of the RTD I-V characteristic; and a series resistance r.sub.d 44 caused by the resistance of the contacts, of the semiconductor layers on each side of the tunneling structure, and of the substrate. Oscillator subassembly 12 also has a transmission line propagation circuit 24, which terminates in a load impedance 25; a bias voltage 27; and an AC isolation circuit 28 for the DC bias voltage. In operation, the transmission line 24 matches the load impedance Z.sub.L 25 to the impedance of the RTD 22. The load impedance Z.sub.L 25 produces a Vs output 26 and a variable impedance to tune the oscillator and thus, load impedance Z.sub.L would obviously have to include any other necessary load circuits, such as subsequent amplifier stages or an antenna circuit.

As shown in FIGS. 3 and 4, emitter subassembly 11 has a modulated laser 29 and an optical fiber 30 which projects light 21. RTD diode 22 has an interface 31 with an opening for an optical signal from the optical fiber 30. As shown in structural detail in FIG. 3, oscillator subassembly 12 may comprise an upper metal strip of transmission line 32, a metal ground strip or plane of transmission line 33, a transmission line dielectric or dielectric layer 34 sandwiched between metal strips 33 and 34 and a ground plane 38 attached to the bottom portion of metal ground strip 33. RTD diode 22 is connected to transmission line 32 and ground plane 33 via ohmic contacts 35 and 36. The circuit is completed by grounding ground plane 33 via transmission line ground connection 37.

FIG. 5 is explained hereafter.

In operation, RTD 22 is biased in the negative differential resistance (NDR) region. When so biased, the RTD oscillates at a free-running frequency which is determined by the internal and external capacitances and the external inductance of the device. The free-running oscillation of RTD 22 is then frequency modulated or injection locked by the intensity modulated optic signal 21 which is delivered via subassembly 11. Intensity modulation of the optical signal results in frequency modulation of the RTD oscillator.

When tested, it was found that, as the modulation frequency in the experiment approached the free running oscillator frequency, injection locking over a bandwidth of 150 kilohertz occurred with a marked reduction in FM noise.

FIG. 6 shows the free running signal (at 2.7574 GH.sub.z) of the RTD oscillator with no modulating signal and with no injected signal.

FIG. 7 shows the RTD oscillator signal with an optically injected signal, as previously described.

FIG. 8 shows the oscillator signal after it has locked to the injected signal.

FIG. 9 shows a superimposition of the signals from FIG. 6 and FIG. 8 which compares the free running oscillator signal to the injection locked oscillator signal. As shown in FIG. 9, the injection locked signal has lower FM noise and a narrower frequency spread centered on the injected signal frequency. This behavior is typical of injection locked oscillators.

FIG. 10 shows the shift in frequency of the RTD oscillator as the injected laser light is turned on and off.

When the intensity modulated laser light is injected into the RTD, the resulting frequency shifts are responsible for the frequency modulation of the RTD oscillator.

FIG. 11 shows the frequency modulated oscillator signal when the RTD is being modulated by light which is intensity modulated at 1 KH.sub.z. Note that for this figure the free running oscillator frequency was 1.2762 GH.sub.z. For this relatively small modulation frequency, the modulation index is very large.

FIG. 12 shows the frequency modulated oscillator signal when the RTD is intensity modulated at 75 MH.sub.z. For this modulation frequency, the modulation index is small.

The oscillator circuit, the RTD structure, and the optical coupling were all non-optimized. Therefore, the curves in FIGS. 6-12 demonstrate the physical effect and they do not depict the optimum conditions or limitations of this technique.

The physical mechanisms responsible for this behavior involve the generation by the absorbed light of electron-hole pairs. Light absorbed in the GaAs material (primarily in the substrate) creates electron-hole pairs, changing the carrier density, and hence the series resistance (r.sub.d in FIG. 2) of the RTD at the same frequency as the variations in the light intensity. The varying r.sub.d causes a varying current in the RTD, changing the operating point on the IV characteristic at the same frequency as the modulated light signal. There will also be a contribution to the injected signal from holes generated by the absorbed light in the substrate, in the barriers, in the GaAs layers on each side of the barriers, and in the quantum well. These holes become trapped at the RTD interfaces, causing a change in the voltage drop across each layer of material in the RTD and therefore a change in the operating point. The total overall injected signal causes modulation of the frequency of the RTD oscillator.

As shown in FIG. 5, which is the schematic diagram of the initial test circuit, laser 29 has a laser diode 51, a laser bias unit 52, and an RF generator 53. Optical fiber 30 connects at a first end thereof to laser diode 51 and directs light beam 21 at the other end thereof onto RTD diode 22. RTD diode 22 has a top portion or diode mesa 54 which receives light 21, and has a bond wire 55 which connects to strip 32, and has a coplanar fixture 56. The test circuit also includes a slide screw tuner 57, a bias tee 58, an RTD bias unit 59, and a spectrum analyzer 60. Tuner 57 has an inboard coaxial line 61, which connects to RTD diode 22, and has an outboard coaxial line 62 which connects to bias tee 58. Analyzer 60 has an input conductor 63 which connects to bias tee 58. RTD bias 59 has an output conductor 64 which connects to bias tee 58.

The advantages of assembly 10 are indicated hereafter.

Assembly 10 has the potential for external optical modulation of the frequency of a semiconductor oscillator 12 operating at higher frequency and with less noise than oscillators constructed from other semiconductor devices.

Since resonant tunnel diodes (RTDs) have demonstrated operation at frequencies as high as 700 Gigahertz with very low noise, it is expected that frequency modulated oscillators, in addition to injection locked oscillators can be built which operate at higher frequencies and with lower noise than oscillators using other semiconductor devices.

Assembly 10 can be used in various applications, including phase array radar, phased array communication systems, remote control of microwave and millimeter wave antennas, EMI/EMP hardening by optical devices for low power signal processing, and optical isolation of microwave subsystems to reduce vulnerability to reflected microwave power and to permit separate shielding from EMI and EMP.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than words of limitation and that changes may be made within the purview of the appended claims without departing from the true scope and spirit of the invention in its broader aspects.

For example, other ways can be used to couple the light into the RTD 22. Other circuits can be used to propagate the output signal, such as transmission line, waveguide, or resonators of various geometries. Other ways can be used to connect the propagation circuit to the RTD 22. Other ways can be used to terminate the propagation circuit in a load. And other ways can be used to isolate the DC bias circuit from the AC circuit. As well, various materials can be used for substrate 38.