M.R.H. Khan

Multiwavelength-Integrated Optical Beamformer Based on Wavelength Division Multiplexing for 2-D Phased Array Antennas

A novel, hardware-compressive architecture for broadband and continuously tunable integrated optical true-time-delay beamformers for phased array antennas is proposed and experimentally demonstrated. The novel idea consists in employing the frequency-periodic response of optical ring resonator (ORR) filters in conjunction with on-chip wavelength division multiplexing (WDM), in order to create multiple signal paths on an individual beamformer channel. This novel idea dramatically reduces the network complexity and, in turn, its footprint on the wafer. This allows the integration of an unprecedented number of delay channels on a single chip, ultimately overcoming the main limitation of integrated optical beamformers, that is, the difficulty to feed antenna arrays with many elements using a single integrated chip. A novel beamformer has been realized based on this technique, using the ultra-low-loss TriPleX waveguide platform with CMOS-compatible fabrication equipment, and its functionality is demonstrated over an instantaneous bandwidth from 2 to 10 GHz. This result, at the best of our knowledge, represents at the same time the record instantaneous bandwidth (8 GHz) for an optical beamformer based on ORR, and the first demonstration of an integrated beamformer where the periodic response of ORRs is exploited to process signals from different antenna elements, simultaneously, using a single delay line.

Novel microwave photonic fractional hilbert transformer using a ring resonator-based optical all-pass filter

We propose and demonstrate a novel wideband microwave photonic fractional Hilbert transformer implemented using a ring resonator-based optical all-pass filter. The full programmability of the ring resonator allows variable and arbitrary fractional order of the Hilbert transformer. The performance analysis in both frequency and time domain validates that the proposed implementation provides a good approximation to an ideal fractional Hilbert transformer. This is also experimentally verified by an electrical S21 response characterization performed on a waveguide realization of a ring resonator. The waveguide-based structure allows the proposed Hilbert transformer to be integrated together with other building blocks on a photonic integrated circuit to create various system-level functionalities for on-chip microwave photonic signal processors. As an example, a circuit consisting of a splitter and a ring resonator has been realized which can perform on-chip phase control of microwave signals generated by means of optical heterodyning, and simultaneous generation of in-phase and quadrature microwave signals for a wide frequency range. For these functionalities, this simple and on-chip solution is considered to be practical, particularly when operating together with a dual-frequency laser. To our best knowledge, this is the first-time on-chip demonstration where ring resonators are employed to perform phase control functionalities for optical generation of microwave signals by means of optical heterodyning.

System integration and radiation pattern measurements of a phased array antenna employing an integrated photonic beamformer for radio astronomy applications

In this paper we describe the system integration and the experimental demonstration of a photonically beamformed four-element receiving array antenna for radio astronomy applications. To our knowledge, the work described here is the first demonstration of the squint-free, continuously tunable beamsteering capability offered by an integrated photonic beamformer based on optical ring resonator true-time-delay units, with measured radiation patterns. The integrated beamformer is realized in a low loss, complementary metal-oxide-semiconductor (CMOS) compatible optical waveguide technology. The measurements show a wideband, continuous beamsteering operation over a steering angle of 23.5 degrees and an instantaneous bandwidth of 500 MHz limited only by the measurement setup.

Dual-Frequency Distributed Feedback Laser With Optical Frequency Locked Loop for Stable Microwave Signal Generation

We demonstrate the photonic generation of microwave signals by using a dual-frequency distributed feedback waveguide laser in ytterbium-doped aluminum oxide (Al2O3:Yb3+). An optical frequency locked loop (OFLL) was implemented to stabilize the center frequency of the microwave signal. This approach resulted in a microwave frequency at ~14 GHz with a phase noise of -75 dBc/Hz at 1 MHz offset from the center frequency. The frequency stability of the photonic microwave signal has an Allan deviation of more than 1 × 10-10 for an averaging time of 1000 s. The combination of the dual-frequency laser and the OFLL scheme holds great potential for the photonic generation and distribution of highly stable microwave or millimeter-wave signals.

A novel measurement technique to estimate the RF beat-linewidth of free-running heterodyning system using a photonic discriminator

We propose a novel technique to our knowledge to estimate the beat spectrum linewidth of a free-running heterodyning scheme using an optical discriminator. Utilizing a dense wavelength division multiplexing (DWDM) filter as an optical discriminator, the phase modulation (PM) to intensity modulation (IM) conversion is achieved. Thus converted phase noise of the laser into RIN (relative intensity noise). We establish a relationship between the converted RIN and the beat spectrum linewidth. The proposed concept on beat spectrum linewidth estimation is demonstrated and compared with direct beat spectrum measurement. We show that our proposed technique estimates the beat linewidth more accurately for a free-running heterodyning scheme.

Optical phase synchronization in coherent optical beamformers for phased array receive antennas

An optical phase synchronization system using a power feedback loop technique is experimentally demonstrated. The system allows coherent combining of signals modulated on the same optical carrier in a hybrid optical beam forming system setup.

On-chip, CMOS-compatible, hardware-compressive integrated photonic beamformer based on WDM

We propose and experimentally demonstrate a novel, hardware-compressive architecture for broadband and continuously tunable integrated optical true-time-delay beamformers. The architecture is based on on-chip wavelength division multiplexing (WDM) that, in conjunction with the frequency-periodic response of optical ring resonator (ORR) filters, dramatically reduces the network complexity and, in turn, its area occupation on the wafer. This allows the integration of an unprecedented number of delay channels on a single chip, overcoming the main limitation of current integrated beamformers, that is, the limited capability to feed very large arrays when using a single chip. Based on this technique, a novel device is realized with TriPleXTM waveguide technology, using CMOS-compatible fabrication equipment, and its functionality is demonstrated over the instantaneous 2-10 GHz bandwidth. At the best of our knowledge, this results represent at the same time the record instantaneous bandwidth (8 GHz) for an optical beamformer based on optical ring resonators (ORR), and the first demonstration of an integrated beamformer where signals from different antenna elements are processed simultaneously by individual delay lines, exploiting the periodic response of ORRs.

CMOS-compatible integrated optical delay line for broadband K u -band satellite communications

We present the first demonstration of a broadband and continuously tunable integrated optical true-time-delay line, operating over the complete DVB-S satellite communication band, realized with a CMOS-compatible TriPleXTM waveguide technology. The integrated optical chip includes a delay line and an optical sideband filter (OSBF) based on reconfigurable optical ring resonators (ORR). The filter and the delay responses are software programmable and have been configured for Ku-band operation. For the first time, the continuous tunability between 0 and 277 ps is demonstrated over the complete for DVB-S satellite communication band (10.7-12.75 GHz).

Investigation on the performance of an optically generated RF local oscillator signal in K u -band DVB-S systems

We investigate a way to externally generate the local oscillator (LO) signal used for downconversion of the Ku-band (10.7–12.75 GHz) RF signal received from a phased array antenna (PAA). The signal is then translated to an intermediate frequency (950–2150 MHz) at the output of the mixer of standard LNBs (Low Noise Block) of a DVB-S (Digital Video Broadcasting-Satellite) reception system. The LO is generated optically by heterodyning of two lasers. We analyze the impact of the laser parameters (optical power and linewidth) on the quality of the generated LO (i.e. phase stability and carrier-to-noise ratio). In addition, a complete system analysis is presented for a 25×64 elements PAA system. We also report the progress on the development of the distributed feedback (DFB) lasers to be used in the heterodyne process. We analyze and experimentally demonstrate that, in order to have an optically-generated LO with equivalent performance to the one of an electronic LO, the laser linewidth should be in the order of tens of Hz, with a maximum relative intensity noise (RIN) of −123 dB/Hz and a minimum optical power of 0.125 mW.

Highly stable microwave carrier generation using a dual-frequency distributed feedback laser

Photonic generation of microwave carriers by using a dual-frequency distributed feedback waveguide laser in ytterbium-doped aluminum oxide is demonstrated. A high-performance optical frequency locked loop is implemented to stabilize the microwave carrier. This approach results in a microwave frequency at ~14 GHz with a phase noise of -75 dBc/Hz at 1 MHz offset from the center frequency. The frequency stability of the photonic microwave carrier has an Allan deviation of better than 1×10-10 for an averaging time of 1000 s with a loop settling time of 15 μs.