Optical synchronization of millimeter-wave oscillators fordistributed architecture

A review of various methods of phase and frequency synchronization of active MMIC based transmit/receive modules is presented, and particular emphasis is placed on the synchronization of oscillators through the use of an indirect subharmonic optical injection locking technique. In this approach, the nonlinear behavior of large-signal modulated laser diodes and solid-state oscillators is exploited to extend the bandwidth of the synchronizing link to the millimeter-wave frequency range. Experimental results of the phase and frequency coherency of two 21.5 GHz FET oscillators are reported. Optimum performance is achieved at a subharmonic factor of 1/4, with a locking range of 84 MHz and a phase noise degradation of only 14 dB. The phase coherency measurement of two injection-locked oscillators points to a phase shift, which is introduced as a result of the frequency detuning between the slave and master oscillator signals. A scheme to correct for this phase error is presented     

Indirect Subharmonic Optical Injection Locking of a Millimeter-Wave IMPATT Oscillator

Large aperture phased-array antennas operating at millimeter-wave frequencies are designed for space-based communications and imaging. Array elements are composed of active transmit-receive (T/R) modules that are phase and frequency synchronized to a reference signal at the central processing unit by a fiber-optic (FO) distribution network. The implementation of FO links, synchronizing the millimeter-wave Iocal oscillators (LO's), imposes a great challenge. This paper presents results of indirect optical injection locking of a free-running 38-GHz (Ka-band) IMPATT oscillator over the Iocking range of 2-132 MHz, depending on the injected power level (amplifier gain). In the experiment, the nonlinearity of both the laser diode and the IMPATT oscillator is exploited to achieve 12th subharmonic injection locking. The overall system FM noise degradation of the reference signal is 16 dB at 500-Hz offset. The FM noise degradation is dominated by the theoretical limit of 20 log N, where N is the frequency multiplication factor used in subharmonic injection locking. Methods by which optical injection locking may be extended into 60 and 90 GHz are demonstrated.     

Optical Control of GaAs MESFET's

Theoretical and experimental work for the performance of GaAs MESFET's under illumination from light of photon energy greater than the bandgap of the semiconductor is described. A simple model to estimate the effects of light on the dc and RF properties of MESFET'S is presented. Photoconductive and photovoltaic effects in the active channel and substrate are considered to predict the change in the dc equivalent circuit parameters of the FET, and from these the new Y- and S-parameters under illumination are calculated. Comparisons with the measured S-parameter's without and under illumination show very close agreement. Optical techniques can he used to control the gain of an FET amplifier and the frequency of an FET oscillator. Experimental results are presented showing that the gain of amplifiers can be varied up to around 20 dB and that the frequency of oscillators can be varied (tuning) around 10 percent when the optical absorbed power in the active region of the FET is varied by a few microwatts. When the laser beam is amplitude-modulated to a frequency close to the free-running FET oscillation frequency, optical injection locking can occur. An analytical expression to estimate the locking range is presented. This shows a fair agreement with the experiments. Some suggestions to improve the optical locking range are presented.     

Analysis of oscillators with external feedback loop for improvedlocking range and noise reduction

A simple scheme for enhancing the locking/capture range and phase-noise performance of FET-based voltage-controlled oscillators (VCO's) is presented using a low-pass feedback loop from the oscillator output to the varactor tuning port. The nonlinearity of the FET provides for mixer or phase detector behavior (a self-oscillating mixer). The resulting feedback oscillator advantageously combines the principles of a conventional injection-locked oscillator (ILO) and phase-locked loop (PLL), which we refer to as an injection-locked phase-locked loop (ILPLL). The analysis suggests that the ILPLL can be designed for superior near-carrier phase-noise performance compared with conventional ILO or PLL circuits. A 10-GHz prototype was fabricated, which demonstrated a locking range more than double that of the isolated VCO injection-locking range over the same range of injected signal power     

Dither-free absolute frequency locking of a 1.3 μm DFB laser on87Rb

A technique for frequency locking a semiconductor laser in the 1.3 μm region by observing the absorption in a ⁸⁷Rb vapor optically pumped by another frequency-modulated semiconductor laser is presented. This technique offers well-defined strong atomic resonances and does not require frequency dithering of the 1.3 μm laser to observe the discrimination pattern required for frequency locking. Dither-free absolute frequency locking of a DFB (distributed feedback) laser at 1324 nm was achieved using a pump laser diode operating at 795 nm (D₁-line). Applications include master oscillator of multiwavelength and coherent communication systems and reference source for an optical frequency synthesizer. Resonances in the 1.55 μm region are also available through this approach     

Indirect optical injection-locking of multiple X-band oscillators

Experimental results of indirect optical injection-locking of two X-band oscillators are presented. An S-band master source is used to lock both oscillators over 18 MHz using the laser diode nonlinearities. System FM noise degradation is 14 dB and the means to improve system performance are suggested.     

A theory of noise in GaAs FET microwave oscillators and its experimental verification

A theory has been developed which provides an entirely analytical approach to the calculation of AM and FM noise in free-running GaAs FET microwave oscillators. The theory is based on the model that low-frequency device noise is mixed with the carrier signal via the nonlinearity of the FET and upconverted to microwave frequencies. Because of the analytical nature of the theory, all the important device and circuit parameters on which the noise generation depends are explicitly given. Two GaAs FET oscillators have been fabricated and used to investigate the FM noise. The theory predicts well both the spectral dependence and the absolute magnitude of the FM noise in both oscillators. The noise performance of the oscillators differs by 19 dB. The theory indicates that no single factor is responsible for this, and moreover that attention should be given to the optimization of many device and circuit features in the design of a low noise FET oscillator.     

Isolated locked hybrid junction power combiner

The isolated locked hybrid junction power combiner, consisting of a single hybrid junction, appropriate phase-shifting element and circulator, combines three 1-port oscillators (one master oscillator strongly locking the other oscillators). Combiner frequency, noise and locking properties are controlled by the master oscillator, and combining efficiency close to 100%, can be achieved. The combiner can be extended in a modular fashion.     

Optical phase control of an optically injection-locked FETmicrowave oscillator

A simple technique is proposed and demonstrated for controlling the phase of an optically-injection-locked 7.2-GHz FET oscillator. The relative phase φ between the oscillator and the locking signal is adjusted by optically tuning the oscillator frequency. Locking characteristics described include locking bandwidth (2.6 MHz), phase tuning range (187°), phase modulation (β=0.69 at 500 kHz), and optical tuning (125 MHz)     

Investigations on optically controlled millimeter wave Gunn oscillators

There is a growing interest in optically controlled millimeter wave oscillators. In this paper, we have investigated the external-circuit impedances of an optically controlled millimeter wave subharmonic Gunn diode oscillator, which is illuminated by GaAs/GaAlAs laser beam. The variation of the external-circuit impedances looking outward from the Gunn diode with respect to the optical injection plasma density are calculated based on a field analysis method. The results give some useful conclusions for optically controlled millimeter wave Gunn diode oscillator design. Experimentally an optical tuning frequency shift of 7MHz is achieved at W-band.     

Waveguide cavity FET oscillator

A new single-tuned oscillator, applicable to power combining circuits, is described in which a probe antenna is used to provide coupling between an active device and the cavity. It is shown that output power, oscillation frequency and injection locking range of the oscillator can be controlled independently in the circuit design. The experiments with low-power FET oscillators demonstrate output power of 44 mW at 9.2 GHz and DC-RF conversion efficiency of 33.2% from a single-device oscillator and about 100% of power combining efficiency in the case of two- and three-device circuits.     

Experimental relative frequency stabilization of a set of lasersusing optical phase-locked loops

A relative frequency stabilization technique using optical phase locking of miniature diode pumped Nd:YAG ring lasers is described. The master laser is RF phase modulated with a modulation index up to 7.4, and slave lasers are locked up to 21 master laser sidebands with a frequency stability better than 3 kHz     

A study of injection locking and pulling in oscillators

Injection locking characteristics of oscillators are derived and a graphical analysis is presented that describes injection pulling in time and frequency domains. An identity obtained from phase and envelope equations is used to express the requisite oscillator nonlinearity and interpret phase noise reduction. The behavior of phase-locked oscillators under injection pulling is also formulated.     

PHASE AND FREQUENCY LOCKING OF MICROWAVE AND MILLIMETER WAVE POWER COMBINING

The phase and frequency locking of microwave, millimeter wave power combining were analysed and summarized in an all-round way. The master/slave phase locking of cavity oscillators, the peer phase locking of mutually coupled oscillators, and the peer phase locking of ring-connected multiple oscillators were investigated. The results of numerical calculations, and the relations of phase to phase locking model and oscillator parameters were given. And the cavity and space power combining aspects for microwave and millimeter wave were presented.     

Noise-reduction techniques of high-frequency oscillator-high- order multipliers and avalanche-diode-type microwave sources

Results of noise-reduction techniques by injection phase locking to a master oscillator and through the use of high-Q cavities are presented for a class of high- frequency oscillator-high-order multiplier and avalanche-diode oscillator microwave sources. Frequency and amplitude noise spectra are described for two states of oscillation: free-running and injection phase-locked for the video range 300 hertz to 10 MHz off the carrier frequency. At X band, a rms noise deviation less than 0.15 hertz in a 100-hertz bandwidth has been achieved at frequencies greater than 10 kHz off the carrier for the free-running high- frequency oscillator-high-order multiplier source. Corresponding results obtained for a free-running avalanche- diode oscillator was a flat 6-hertz spectrum across the video range 1 kHz to 100 kHz.     

A 16 element quasi-optical FET oscillator power combining arraywith external injection locking

The authors present analysis, design and experimental results of a 16 element planar oscillator array for quasi-optical power combining. Each element in the array consists of a single FET oscillator with an input port for injection of the locking signal and an output port which is connected to a patch radiator. The array is synchronized using a 16-way power dividing network which distributes the locking signal to the oscillating elements. The array is constructed using a two-sided microstrip configuration, with the oscillators and feed network on one side of a ground plane, and the patch radiators on the opposite side. An effective radiated power (ERP) of 28.2 W CW with an isotropic conversion gain of 9.9 dB was measured at 6 GHz. For an injected power of 10.3 dBm, a locking range of 453 MHz at a center frequency of 6.015 GHz was obtained; a bandwidth of 7.5%. Because of the simple nature of the individual oscillator elements, this approach is well suited to MMIC implementation     

Initial observations of optical injection locking of oscillators using heterojunction bipolar transistors

For the first time, optical phase control of a hybrid heterojunction bipolar transistor (HBT) oscillator has been achieved. The results strongly indicate that the HBT is a suitable device for optically controlled oscillators. At a frequency of oscillation of 500 MHz a locking range of more than 15 MHz was measured ( Delta f/f equivalent to 3%). The optical tuning range of the oscillator exceed 25 MHz.<>     

Subharmonic injection locking phenomena in synchronous oscillators

Subharmonic locking behaviour of a synchronous oscillator has been demonstrated. It is shown that the oscillator can be locked using all subharmonics down to the tenth. Locking behaviour is similar to that observed for IMPATT diode and Gunn microwave injection locked oscillators.<>     

A Waveguide-Cavity Multiple-Device FET Oscillator

A waveguide-cavity oscillator, applicable to power-combining circuits, has been developed using probe for coupling between active device and, cavity. No lossy stabilizing element is required. The control of output power, oscillation frequency, and injection locking bandwidth are performed easily. Output power of 44 mW and dc-RF conversion efficiency of 33.2 percent were obtained at 9.2GHz for a single-device low-power FET oscillator. A simple technique of cascading the pretuned oscillator modules was used to construct multiple-device oscillators incorporating up to four FET's with combining efficiency of about 100 percent.     

Transmission-Type Injection Locking of GaAs Schottky-Barrier FET Oscillators

Transmission-type injection-locked oscillators equipped with both signal-input and power-output ports are studied. A comparison with traditional reflection-type injection-locked oscillators, in which a signal is injected into the output port of the oscillator, is presented theoretically. It is shown that the Iocking range of transmission types always differs from the reflection type by a factor of G/sub s// G/sub p/ where G/sub s/ represents the maximum stable gain of the two-port oscillator and G/sub p/ represents the square root of the output power ratio of the two ports. Experiments on common-source injection-locked oscillators using GaAs FET chips are described and show that, with transmission types, a 1.8 times wider Iocking range can be obtained than with reflection types. Furthermore, investigation of FM noise for both types of injection revealed lower off-carrier FM noise for transmission types than reflection types, even though the Iocking gain of the transmission types was kept the same as that of reflection types. Thus overall features of transmission-type injection locking were found to be advantageous for FM signal amplification even though there is a minimal power loss at the signal input port.