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Photonic Generator of Arbitrary RF Waveforms Disclosure Number: IPCOM000201129D
Publication Date: 2010-Nov-08

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This invention is a generator of wideband arbitrary RF waveforms according to a set of lower bandwidth digital control signals. The generator produces a time-sequence of picosecond duration optical pulses whose intensities are specified by the digital control signals. These pulses are supplied by optical fiber to an optical-to-electrical (O/E) transducer, such as a photoconductor, that has picosecond response. The optical pulses, when incident on the photoconductor, produce pulses of electrical charge that momentarily changes the resistance of that photoconductor. The photoconductor is part of the bias circuit of an electronic amplifier that has a low-pass filter response. These optical pulses act like the sampled impulses of an ideal digital-to-analog converter (DAC) based on the Shannon sampling theory. Thus, the generator is capable of forming any arbitrary waveform consistent with the sampling rate or the frequency of the optical pulses and with the resolution of the digitally specified intensities of the optical pulses. Use of short optical pulses allows the timing of the digital control signals to be less precise and also alleviates the constraints on the sharpness of the low-pass filter of the DAC. The bandwidth of the generated RF waveform can be many times larger than the frequency of each digital control waveform. For example, the RF waveform can have a bandwidth of 40 GHz and yet be defined on a cycle to cycle basis with full control of the amplitude, phase and frequency of each cycle. Thus, the sampling rate needed for accurate construction of this RF waveform, with realistic low-pass filters, would be much more than the 80 GHz Nyquist rate. The digital control waveforms can have much lower clock frequency, such as 5 GHz. If the amplitude of the arbitrary RF waveform can have 8 bits (> 48 dB) of resolution, the digital control signals must comprise at least 8 channels of control waveforms, with one channel specifying each bit. Also, the number of sets of digital control signals is equal to S/D, where S is the sampling rate for the RF waveform generation and D is the clock frequency of the control waveforms. These sets of control signals can be time staggered, or time interleaved, such that they specify the intensity of different ones of the succession of optical pulses. The waveform generator comprises a mode-locked laser producing a train of optical pulses, an optical pulse compressor (or pulse-spectrum expander), an optical splitter/combiner coupled to an optical time-delay element, an optical wavelength-carving demultiplexer (such as an arrayed waveguide grating) that spectrally carves a set of pulses of differing wavelength that have picosecond pulse width, an array of optical attenuators, sets of optical time-delay elements, sets of optical on/off modulators, a picosecond O/E transducer and an electronic amplifier. The waveform generator also may include at least one optical circulator. The on/off modulators may comprise chirped optical gratings or optical resonators. This waveform generator acts like a digital-to-analog converter controlled by time-interleaved sets of digital control signals. A novel feature of this generator is the combination of optical attenuators to specify differing amplitudes of optical pulses of a specific relationship (such as a binary, 2N, relationship), together with the sets of optical delay elements (wherein each set imposes a specific time delay corresponding to a multiple of the sampling interval), and further together with the sets of on/off modulators (wherein each set specifies a particular digital value for the intensity of the optical pulses corresponding to that set). Another novel feature is the optical splitter/combiner coupled to a time-delay element when used together with the other elements of the waveform generator. Yet another novel aspect, although not a required aspect, is the picosecond O/E transducer coupled to the electronic amplifier that acts as a low-pass filter. A preferred realization of the set of optical modulators is an array of optical grating modulators wherein each modulator of the set can be controlled to reflect or to transmit optical pulses of a specific wavelength, with that wavelength corresponding to the wavelength of those optical pulses coupled to that modulator. Different optical modulators of a set respond to different wavelengths of light. In a first embodiment, the different modulators of a set, operating to transmit the optical pulses, are coupled to time-delay elements that produce the same time delay. In a second embodiment, the different modulators of a set, operating to reflect the optical pulses, are coupled to time-delay elements that produce different time delays. The time delays are selected so that the various optical pulses transmitted through a set (first embodiment) or reflected from a set (second embodiment) will coincide in time when they are combined together. In some embodiments, the optical grating is chirped to control the time delay or the intensity modulation imposed on different optical-wavelength components of a pulse. In some embodiments, the chirp also is designed to further compress the optical pulse that is reflected from the grating.

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Figure 2 illustrates a photonic generator capable of producing wideband arbitrary RF waveforms. This photonic generator makes use of the Shannon impulse-sampling concept of waveform reconstruction. Any RF waveform can be represented as a time sequence of impulses whose intensities match the amplitude of RF waveform at those instances of time corresponding to the occurrence of the impulses. The frequency of these impulses or samples must be at least as great as the Nyquist rate. Thus, for an RF waveform with a bandwidth extending from DC to 40 GHz, the Nyquist rate is twice the maximum frequency of the RF waveform, i.e., 80 GHz. In practical implementations, the actual frequency of the samples typically is somewhat greater than the Nyquist rate, e.g., 100-120 GHz sampling rate for a 40 GHz signal bandwidth, so that the low-pass filter used in reconstructing the RF waveform can have a more gradual drop-off at the edge of its passband while still avoiding aliasing.

DC power

RF antenna element

Figure 2. Photonic generator of arbitrary RF waveforms.

The photonic generator comprises a laser source that generates a train of optical pulses. These optical pulses are delivered to an optical pulse-pattern generator. This pulse pattern generator produces a sequence of optical output pulses whose intensities are controlled by sets of digital control signals that have a clock frequency much lower than the frequency of the output pulses. The width of these output pulses preferably is 3 psec or less, but the pulses can be wider if the bandwidth of the RF waveform to be generated is smaller. The width of the optical pulse preferably is much shorter than the desired sampling interval. For example, a sampling rate of 100 GHz would correspond to a sampling interval of 10 psec. Preferably, the optical pulses are short enough that they almost be considered as impulses. The crosstalk between adjacent output pulses or samples limits the achievable dynamic range of the waveform generator. Thus, it is preferable to have each output pulse decayed by many tens of dB before the occurrence of the next output pulse.

The optical pulse-pattern generator can supply the output pulses to an optical fiber that carries those pulses to an optical-to-electrical (O/E) transducer. A preferred O/E transducer illustrated in Figure 2 is a picosecond photoconductor that produces electrical charge carriers according to the intensity of the incident light. Thus, this photoconductor produces a pulse of electrical carriers, with a corresponding pulsed reduction in the electrical resistance of this photoconductor, according the number of photons that are in

Short-pulse laser

Multi-channel, low-frequency digital control signals

 Optical pulse- pattern generator

 Pico-second photoconductor & RF mixer

RF power amplifier

Optical fiber

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the pulse of incident light. The photoconductor changes the bias of the input stage of the RF power amplifier to which it is connected...