Photonic Delay Line Research Articles

Electronically controlled agile lens-based broadband variable photonic delay line for photonic and radio frequency signal processing

To the best of our knowledge, proposed for the first time is the design of an optically broadband variable photonic delay line (VPDL) using an electronically controlled variable focus lens (ECVFL), mirror motion, and beam-conditioned free-space laser beam propagation. This loss-minimized fiber-coupled VPDL design using micro-optic components has the ability to simultaneously provide optical attenuation controls and analog-mode high-resolution (subpicoseconds) continuous delays over a moderate (e.g., <5 ns) range of time delays. An example VPDL design using a liquid-based ECVFL demonstrates up to a 1 ns time-delay range with >10 dB optical attenuation controls. The proposed VPDL is deployed to demonstrate a two-tap RF notch filter with tuned notches at 854.04 and 855.19 MHz with 22.6 dB notch depth control via VPDL attenuation control operations. The proposed VPDL is useful in signal conditioning applications requiring fiber-coupled broadband light time delay and attenuation controls.

Silicon-photonics-based wideband radar beamforming: basic design

Proposed is silicon-photonics-based phased array antenna beamforming for high-resolution long-range radars with wide instantaneous radio frequency (rf) bandwidth. Specifically, the proposed silicon-photonics beamformer platform offers the potential for cost-effective monolithic chip-scale integration of photonic delay lines, 2×2 optical switches, variable optical attenuators, and optical amplifiers that form the base unit of a rf transmit/receive array signal processor. In effect, the proposed silicon-photonics devices empower the design of a powerful proposed photonic beamformer with one time-delay unit per antenna element. Device-level designs studies are shown that promise meeting the high-resolution radar mission-critical requirements via time delays of up to 2.5 ns, switching times of less than 100 ns, optical isolations as good as 50 dB, and optical gains of up to 6 dB. Longer delays are achieved off chip using optical fibers.

Microwave Photonic Delay Line With Separate Tuning of the Optical Carrier

Significant performance improvement for photonic delay lines in microwave-photonics based on a new concept of separately tuning the optical carrier is proposed and analyzed. Optical microresonator delay lines using a balanced SCISSOR design with additional resonators to separately tune the carrier phase delay to provide a true-time-delay for the broadband microwave signal are presented. Significant improvements in broadband tunable delay and devices losses are predicted.

Compact photonic delay line constructed by holographic optical elements

To fully support phased-array antenna and other applications, a 3-range (long time, moderate time, and short time) delay line structures with holographic optical elements is proposed. Flexibility, light-weight, on-axis coupling, easy alignment, easy fabrication, and compactness are their advantages. When use holographic optical elements to build the photonic delay line system, all of the delay and non-delay paths in these three photonic delay lines are setting in a compact structure. They do not need any extra components, such as mirrors and optical paths in free space, and can bear stronger vibration. Therefore, the holographic optical elements are more suitable to design the photonic delay lines. In these three structures, their losses and crosstalk s are balanced. All of delay and non-delay cases in these three delay line structures, their crosstalk, signal-to-noise ratio, and loss are 1/81,000, 59.1 dB, and 3.2 dB, respectively. Finally, to support complete applications, a polarization independent photonic delay line system with holographic optical elements is proposed, too.

Design of photonic crystal delay lines based on enhanced coupled-cavity waveguides

We present an accurate systematic analysis of coupled-cavity waveguide (CCW)-based delay lines, in photonic crystal (PhC) structures with both square and hexagonal patterns. The group velocity profile of different CCW devices, obtained by varying the geometries and the coupling factors, is evaluated by using both the plane-wave expansion (PWE) method and tight binding (TB) model. We determine the optimal geometrical parameters to reduce the group velocity, and demonstrate that the delay line performance can be further enhanced by using high-Q cavities.

Comment on "Wavelength controlled photonic true time delay for wide-band applications"

For original paper see Raz et al., ibid., vol.17, no.5, p.1076 (2005). Recently, motivated by wide-band radio frequency antenna control applications, Raz proposed and demonstrated a wavelength controlled photonic true time delay (TTD) line based on a single thin-film wavelength-division multiplexer. Nabeel comments on this paper.

Optical source coherence controller for fibre optic delay line RF/microwave signal processors

A technique for optical source coherence control that can suppress the important coherent interference effects in photonic delay line RF/microwave signal processors is presented. The concept is based on phase modulation of the optical carrier to transform the laser coherence to meet the requirements of the photonic signal processor that suppresses coherent interference. It enables conventional narrow-linewidth, high quality DFB lasers to be used for high frequency photonic signal processors, and tailors their coherence length to meet the processor design requirements. Experimental results show that the optical source coherence controller can realise photonic RF/microwave filters with robust response using narrow-linewidth sources, demonstrating the effectiveness of this technique for coherent interference suppression in photonic signal processors.

Photonic-delay technique for phase-noise measurement of microwave oscillators

A photonic-delay line is used as a frequency discriminator for measurement of the phase noise - hence the short-term frequency stability - of microwave oscillators. The scheme is suitable for electronic and photonic oscillators, including the optoelectronic oscillator, mode lock lasers, and other types of rf and microwave pulsed optical sources. The approach is inherently suitable for a wide range of frequency without reconfiguration, which is important for the measurement of tunable oscillators. It is also insensitive to a moderate frequency drift without the need for phase locking.

Tunable microwave photonic passive delay line based on multichannel fibre grating matrix

A discretely tunable passive microwave photonic delay line based on a multichannel grating matrix is demonstrated. This topology demonstrates high-speed high-resolution time delaying of microwave signals, along with requiring a reduced number of discrete gratings for any given delay range. High channel isolation and low insertion loss are also features of this topology, making this suitable for many wideband applications.

Broadening compensation for ultrashort pulses in photonic crystals

Extremely large group velocity dispersion of both signs can be achieved at the band edges of guided modes in photonic crystal waveguides. The selection of a proper value and sign of this parameter allows the design of short-length waveguides to compensate for pulse broadening. This pulse broadening can be caused, for instance, when a photonic crystal delay line is introduced. We present a theoretical study of the possibilities of using photonic crystal waveguides as intra-circuital dispersion compensation elements for ultrashort pulses, so the width of a transmitted pulse can be reduced. However, we also demonstrate that recovering the original pulse shape is not possible for these large-bandwidth pulses due to higher-order dispersion terms.

Phase Performance of an Eight-Channel Wavelength-Division-Multiplexed Analog-Delay Line

The design of an eight-channel 25 /spl mu/s wavelength-division-multiplexed (WDM) microwave photonic delay line is described. The performance of the delay line was studied by measuring the temperature dependent relative phase drift between channels and the interchannel crosstalk. A simple method for measuring the maximum phase errors produced by the interchannel crosstalk is described and used to measure the maximum error on a channel in this system. The use of polarization interleaving and dispersion compensation to reduce the effects of crosstalk in the system is examined experimentally.

Optical characterization of photonic crystal delay lines based on one-dimensional coupled defects

We performed optical characterizations of optical delay lines based on photonic crystal waveguides. The delay lines were composed of cylindrical air holes in silicon-on-silicon-dioxide ridge waveguides, defects were periodically created by means of increasing the separation of two neighboring air holes, and the structure was designed to have flat-transmittance impurity band. We clearly observed an impurity band with a bandwidth of ~30 nm and a maximum transmittance of ~0.5 at an operating wavelength of ~1.55 microm . A 600-fs delay was confirmed with a 20-microm -long delay line. A good agreement was confirmed between the experimental results and the calculations.

Multipath and ground clutter analysis for a UWB noise radar

An ultrawideband (UWB) random-noise radar operating in the 1-2 GHz frequency band has been developed and field-tested up to a 200 m range at the Environmental Remote Sensing Laboratory (ERSL) of the University of Nebraska. A unique heterodyne correlation technique based on a delayed transmitted waveform using a photonic delay line has been used to inject coherence within this system. The performance of this radar in the presence of ground reflections is investigated analytically and experimentally, and the mitigating effects of UWB waveform on multipath-induced interference are analyzed. In addition, the ground clutter statistics, in a look-down mode, are theoretically established and experimentally verified. The performance of this radar in detecting clutter embedded targets with small radar cross section (RCS) is also experimentally examined.

Design of impurity band-based photonic crystal waveguides and delay lines for ultrashort optical pulses

We investigate the transmission of ultrashort pulses through impurity band-based photonic crystal waveguides. It is found that in the general case the transmission behavior depends strongly on the pulse width with respect to the resonance linewidth in impurity bands. By controlling the configuration of the waveguides, quasiflat impurity bands can be obtained in which the dependence of transmission on pulse width is very weak. As long as the pulse width is much narrower than the bandwidth, pulses can transmit through the quasiflat impurity bands with negligible distortion and attenuation. The conditions necessary for achieving quasiflat impurity bands are derived by examining waveguides of different configurations and properties. The mechanism responsible for the formation of quasiflat impurity bands is revealed from the discussion of the symmetry of single defect and their coupling.

Receiver operating characteristics for the coherent UWB random noise radar

An ultrawideband (UWB) random-noise radar operating in the 1-2 GHz frequency band has been developed and held-tested at a 200 m range at the University of Nebraska. A unique heterodyne correlation technique based on a delayed transmitted waveform using a photonic delay line has been used to inject coherence within this system. The performance of this radar, assuming a point target, has been investigated from a statistical point of view by developing the theoretical basis for the system's receiver operating characteristics (ROC). Explicit analytical expressions for the joint probability density function (pdf) of the in-phase (I) and quadrature (Q) components of the receiver output have been derived under the assumption that the input signals are partially correlated Gaussian processes. The pdf and the complementary cumulative distribution function (cdf) for the envelope of the receiver output are also derived. These expressions are used to relate the probability of detection (P/sub d/) to the probability of false alarm (P/sub f/) for different numbers of integrated samples, and the results are analyzed.

Demonstration of an all-digital 7-bit 33-channel photonic delay line for phased-array radars

A 7-bit multichannel photonic delay line for phased-array antenna control is demonstrated. Multichannel (33-pixel) ferroelectric liquid-crystal (FLC) devices are used as polarization rotation elements, and polarization beam-splitter cubes are used as polarization elements that route the optical signals to different paths. The controller is remotely fed by a unique fiber-optic-array design that uses gradient-index lens collimators for the input single-mode polarization-maintaining fibers. The optical signal is collected by a similar fiber array that uses multimode fibers for improved coupling efficiency. Photonic delay-line (PDL) design issues such as multiport assemblies, multipixel FLC designs, and delay-line architectures are discussed. Furthermore, various PDL parameters are examined. High electrical isolation numbers are obtained for both the within-channel leakage noise (e.g., less than -70 dB) and the interchannel cross talk (e.g., less than -90 dB). Optical and electrical insertion loss is examined for the PDL as well as for the overall system. A high-compression dynamic range of 149 dB x Hz and a spurious free dynamic range of 105 dB x Hz(2/3) are presented.

Doppler estimation using a coherent ultrawide-band random noise radar

The University of Nebraska has developed an ultrawide-band (UWB) coherent random noise radar operating over the 1-2 GHz frequency range. The system achieves phase coherence by using heterodyne correlation of the received signal with a time-delayed frequency-shifted replica of the transmit waveform. Knowledge of the phase of the received signal and its time dependence due to target motion permits the extraction of the mean Doppler frequency from which the target speed can be inferred. Theoretical analysis, simulation studies, and laboratory measurements using a microwave delay line showed that it was possible to estimate the Doppler frequency from targets with linear as well as rotational motion. Field measurements using a photonic delay line demonstrated the success of this technique at a range of about 200 m at target speeds of up to 9 m/s. Analysis shows that the accuracy with which the Doppler frequency can be estimated depends not only on the phase performance of various components within the system, but also upon the random nature and bandwidth (BW) of the transmit waveform, and the characteristics of unsteady target motion.

Synchronous amplitude and time control for an optimum dynamic range variable photonic delay line

A synchronous-amplitude-controlled and time-delay-controlled photonic controller for phased-array antenna applications is proposed and demonstrated. Amplitude control is based on a variable optical attenuator system that operates in synchronism with the photonic delay line (PDL). This amplitude control system can provide both the signal calibration for the different PDL channels and settings required for driving the antenna elements of a phased-array radar and the optimum optical power levels that impinge on the photodetector for optimum fiber-optic-link performance. Various variable amplitude control modules based on ferroelectric liquid crystals, polymer-dispersed liquid crystals, and photoconductive devices are proposed. We show that the dynamic range loss due to a switched-PDL inherent structure loss can be compensated when we control the optical power from the laser, using the synchronous optical attenuation system. For the first time to our knowledge, full dynamic range loss compensation is demonstrated for an external-modulation-fed 3-bit switched PDL with a structure optical insertion loss of 5.5 dB. A compression dynamic range of 158 dBxHz was measured at 6 GHz, and a spurious free dynamic range of 111 dBxHz(2/3) was estimated. Feasibility of the dynamic range compensation technique for multichannel, higher-insertion-loss PDL systems is discussed.

Fault-tolerant polarization-insensitive photonic delay line architectures using two-dimensional digital micromirror devices

A binary multichannel photonic delay line (PDL) module is introduced that gives balanced loss switched states and a polarization-insensitive operation via the use of binary operation Digital Micromirror Devices (DMDs). Experimental demonstration of a DMD-based PDL architecture is performed for a 6.84-ns time delay design. Experimental results include a 25-beam feed interchannel crosstalk test indicating a <−60 dB optical interchannel crosstalk level for a 0.381 mm interchannel distance in the multichannel PDL. An average optical signal-to-leakage noise ratio of 35.33 dB is measured for this PDL. A butterfly design PDL optical architecture is proposed for minimizing loss and improving assembly accuracy. These DMD-based variable PDLs can be used in applications ranging from radio frequency (RF) fiber-optic signal processing systems to adaptive optics for astronomical and laser radar arrays.

Polarization selective hologram-based photonic delay lines

The use of polarization selective holograms as optical signal routing elements for the implementation of photonic delay lines (PDLs) is proposed. A single bit PDL using ferroelectric liquid crystal (FLC) devices as active polarization switches and polymer dispersed liquid crystal (PDLC) devices as polarization selective holograms for optical path routing is experimentally demonstrated and characterized. Different within-channel leakage noise filters for improved PDL performance are discussed and experimentally demonstrated. Record high optical signal-to-leakage noise ratios (>45 dB) are obtained for both PDL settings using a combination of the proposed noise filters. An alternative reflective PDL architecture is also proposed. This reflective architecture requires half the physical length for each path compared with the transmissive design to obtain the same time delays. Other polarization dependent optical router designs based on birefringent-mode nematic liquid crystal (NLC) devices are also proposed.