The design for a photonic microwave filter tunable in both bandwidth and operating frequency is proposed and experimentally demonstrated. The circuit is based on a single sideband modulator used in conjunction with two or more transmission fiber Bragg gratings (FBGs) cascaded in series. It is demonstrated that the optical filtering characteristics of the FBGs are instrumental in defining the shape of the microwave filter, and the numerical modeling was used to optimize these characteristics. A multiphase-shift transmission FBG design is used to increase the dynamic range of the filter, control the filter ripple, and maximize the slope of the filter skirts. Initial measurements confirmed the design theory and demonstrated a working microwave filter with a bandwidth tunable from approximately 2 to 3.5 GHz and an 18 GHz operating frequency tuning range. Further work is required to refine the FBG manufacturing process and reduce the impact of fabrication errors.
Optical waveguide crossings based on silica-on-silicon technology are investigated. The effect of crossing angle
(θ) on light transmitted at through and cross-port on a sequence of waveguide crossings with angle varying from
7 to 28° is modeled and experimentally validated. Results demonstrate that structures with small footprint
(θ≈9°) can achieve low crosstalk of -32 dB with high throughput, insensitivity to wavelength of operation, low
polarization dependent loss of 0.6 dB, and low sensitivity to fabrication tolerances. As a result, waveguide
crossings with small crossing angle present an attractive approach to reducing the overall component footprint
without compromising the performance.
There are many applications where a very wideband phase shifter is required. Analog pre-distorters to linearize Ka-band
amplifiers require a frequency-independent phase shift over at least 1 GHz. The same requirement applies to phased-array
antennas or antenna feeds, as well as direct radiating array antennas. Most electrical phase shifters have a fixed
operating frequency, discrete phase shift steps (e.g., 5-bit control) and some frequency and temperature dependent
responses which result in sub-optimum system performance. The requirements in the 50/40 GHz band will be even more
demanding where the bandwidth to be covered could extend up to 5 GHz.
The use of photonic technology mitigates the limitations of electrical phase shifters. Operation over a wide range of
frequencies (e.g., 4 to 50 GHz) using a single design is possible, and a flat phase response over many GHz's can be
achieved. This paper discusses the use of novel microwave photonic technologies to enhance the performance of a
broadband phase shifter with respect to power, mass, volume, electromagnetic interference and compatibility of future
on-board satellite subsystems. The targeted phase shifter is equally applicable to analog linearizers, phased-array
antennas/feeds or other smart antenna schemes where relative phase shifts are required.
The results of a prototype phase shifter are presented showing a broadband response over several GHz. Limitations of
this device and justification for an integrated version will be discussed. Finally, preliminary results for an integrated
device are presented.
The increasing demand for broadband mobile communications has generated interest in exploring new frequency bands
and modifying network structures. In such systems, photonic technologies can bring both cost reduction as well as an
increase in performance, mainly due to the low-loss properties of optical fibers. An optical source capable of producing
tunable, high-quality microwave/mm-wave signals would be of great interest not only in such communications systems,
but in fiber sensors and numerous other applications as well.
One potentially cost-effective method to fabricate such a system is via optical heterodyning. In this approach, the
difficulties in generating a high-quality signal are two-fold. The first issue is in maintaining a specific frequency
difference (i.e. microwave signal) between the lasers for an extended period of time. The second is in narrowing the
inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to
values practical for a communications system. Both of the above requirements are facilitated by the newly developed
doped-fiber, external cavity laser (DFECL), which offers relatively stable single-longitudinal-mode operation in addition
to narrow linewidth operation.
This paper will demonstrate frequency locking of a DFECL using a delay-line discriminator. The RF linewidth, initially
10-15MHz, is reduced to levels conducive to optical PLL locking. Optical power levels are approximately -3 dBm and
unamplified microwave power output levels are typically -35 dBm, depending on photodetector responsivity. Carrier-to-noise
ratios are generally 40-45 dB. The physical mechanisms underlying the observed laser dynamics are discussed,
including laser-to-fiber alignment and thermal fluctuations.
Photonic true-time delay (TTD) beamforming has been considered a promising technique for wideband phased-array
antenna (PAA) systems. An efficient way to achieve optical TTD beamforming is through the use of fiber Bragg grating
(FBG) delay lines. In an FBG-based TTD beamforming system, a microwave signal carried by an optical carrier is
required, which is usually obtained by modulating the microwave signal on the optical carrier using an external
modulator. In this paper, we propose a novel approach to economically generate high-frequency microwave signals
using two wavelengths from two phase-locked laser diodes through optical heterodyning. Since no optical modulator is
required, the cost is significantly reduced. In addition, since the system uses only two wavelengths, the power-penalty
problem caused by chromatic dispersion is minimized. In the proposed approach, the two phase-locked wavelengths are
generated using an optical phase-locked loop (OPLL). A TTD beamforming system using an OPLL in combination with
FBG-based delay lines to achieve tunable time delays is investigated. Experimental results are provided.
The beam propagation method (BPM), in both two-dimensional and three-dimensional versions, is a widely used tool for modeling optical building blocks of photonic integrated circuits (PIC) and integrated optics devices. Such optical building blocks include bent waveguides, couplers, splitters, angled waveguides, etc. Most of the time, trial BPM runs need to be executed to properly set the grid density and propagation step in order to obtain stable and repeatable results. Often, these practice runs can consume a large quantity of valuable design time and computational resources, especially when modeling devices that require short propagation steps and a very dense computation grid. We propose a method that helps the BPM user to quickly assess a range of values for the grid density and propagation step, which enables adequate modeling without resorting to numerous BPM runs. This straightforward and highly intuitive method is based on what we have called the Overlap Quantization Error (OQE). It is also independent of the BPM algorithm used for the simulations. To illustrate the technique, several simulation results are presented for both high- and low-contrast curved waveguides.
The applications of optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs) are inevitable in most optical transmission links. These optical amplifiers employed in a transmission link will provide amplification to the optical signals to be transmitted, at the same time the amplifiers will also add amplified spontaneous emission (ASE) noise to the amplified optical signals. In radio-over-fiber systems, optical links are used to distribute high quality radio frequency (RF) signal, microwave signal or millimeter-wave (mm-wave) signal over optical fiber for low loss long-distant transmission. In this paper, the effects of amplified spontaneous emission (ASE) noise of optical amplifiers on the quality of the optically generated electrical signals are theoretically studied.
Software Defined Radio (SDR), a radio that provides software control of a variety of modulation techniques over a broad frequency range, is an emerging technology that offers numerous advantages over conventional radio designs. With SDR, one would implement a common hardware platform and accommodate the different communications standards and technologies via software modules and firmware. This platform must be compatible with the high degree of versatility of SDR-based communication systems. SDR technology is being promoted by the US Department of Defence to replace tens of thousands of single protocol, single use radios with a common platform that could be reprogrammed to ensure interoperability. Military and public safety organisations from around the world are also considering this technology to solve their interoperability problems.
Although SDR can be easily implemented below 6 GHz using conventional electronics, it is increasingly difficult to do so at the higher operating frequencies proposed by many new wireless and SATCOM standards. To take full advantage of the SDR concept, a hardware platform is required that is capable of continuous operation from frequencies where electrical sources have difficulty providing continuously tunable operation, up to 60 GHz. In addition, various signal modulation schemes will need to be supported.
We present here a prototype for such a transmitter based on optical technology. It can generate a RF carrier tunable from about 18 to more than 40 GHz, which can be modulated using both intensity and phase modulation techniques. Simulations and experimental results are presented.
The nonlinear effects of an electro-optic intensity modulator in an optical up-conversion system for millimeter-wave (mm-wave) over fiber applications are investigated in this paper. In the analysis, general nonlinearities caused by the Mach-Zehnder modulator used for optical up-conversion are analyzed and discussed. Electrical fields of the up-converted optical signal are expressed in power series form, which are widely used in electronics in expressing memoryless nonlinearity. Harmonic distortion and inter-modulation distortion generated in an optical up-conversion, a scheme recently proposed for mm-wave over fiber applications, are analyzed in detail. One-tone and two-tone measurements are performed to validate the analyses based on our newly proposed optical up-conversion configuration.
Wireless technology is a cost-effective means to bring broadband communications to both mobile users and home consumers; however, deploying next generation, multi-GHz wireless systems is currently too expensive. For these systems, photonic technologies can bring cost reduction as well as an increase in performance, mainly due to the ultra low-loss property of optical fibers. One approach to signal distribution is to capitalise on the vast fibre-optic distribution networks deployed within and between cities. A microwave carrier can be optically deployed from central offices to remote antenna sites using these optical links. This paper will discuss the generation of such a microwave carrier using a dual-wavelength, external-cavity laser (ECL).
Two different dual-wavelength ECL's, constructed with fiber-Bragg-gratings (FBG's), have been investigated. One uses a semiconductor gain chip with a dual-FBG acting as an external reflector. The other uses two similar dual-FBG reflectors on each side of a semiconductor optical amplifier (SOA). In both cases the wavelength separation between the gratings is 0.25 nm.
We will demonstrate that a dual-wavelength emission can be temporarily stabilized in the gain-chip ECL if a specific phase relation, between the external feedback from the FBG's and the residual feedback from the gain chip, is satisfied at both lasing wavelengths. The power of the RF beat signal generated by the dual-wavelength optical signal was typically 25 dB above the noise floor. The 3-dB linewidth of the RF signal was approximately 2 MHz and it can be tuned over a frequency range of 200 MHz. The physical mechanisms underlying the observed laser instability will be briefly discussed.
Wireless access systems operating at higher microwave/mm-wave frequencies (10-60 GHz) are attractive because of ample spectrum availability and the potential for large bandwidth allocations. The difficulty with such a system is in the high cost associated with the microwave sources and signal distribution. Optical fibers are an obvious means of alleviating distribution problems, but optical modulation techniques are not currently capable of economically generating mm-wave frequencies. One potentially cost-effective method to fabricate such a system is via optical heterodyning. In such a system, it is relatively simple to generate microwave and mm-wave carriers. However, the difficulties in generating a high-quality signal are two-fold: The first is in maintaining a specific frequency difference (i.e. microwave signal) between the lasers for a prolonged period of time. The second is in narrowing the inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to values more practical for a communication system. The applications in which this microwave signal can be used are determined by the degree to which the linewidth can be reduced. This presentation will discuss the design and implementation of an 11 GHz discriminator-aided, optical phase-lock loop constructed from commercially available external-cavity lasers incorporating fiber Bragg gratings.
The effective bandwidth of a wireless communication system is proportional to the carrier frequency, shifting the operating frequency of the wireless system from the crowded microwave L and S bands to the unregulated mm-wave band is a trend for future broadband wireless services. Intensity modulation and direct detection scheme (IM/DD) has been considered a simple method to impose a millimeter-wave signal onto optical fiber. However, for systems using IM/DD, the chromatic dispersion introduce significant power penalty, which limits the transmission distance. Remote heterodyne schemes have been proposed to solve the dispersion problem. Several approaches have been proposed to generate two phase-correlated optical wavelengths that are separated at a required millimeter-wave frequency. These approaches include single-side band with carrier modulation, optical carrier-suppressed modulation, optical offset injection locking and optical offset phase locking of two laser sources. All these methods provides phase-correlated optical wavelengths, but with complicated system configuration and high system cost. In this paper, we propose a simple method to generate phase-correlated wavelengths using a phase modulator and two fiber Bragg gratings. Two wavelengths are generated by modulating the phase modulator with an RF frequency. The required millimeter-wave signal is obtained by selecting two sidebands using the two narrowband fiber Bragg gratings. Theoretical analysis and experimental results will be reported in the paper.
Distribution of millimeter-wave signals over optical fiber has been considered a promising technology for future broadband wireless access networks, thanks to the low loss and broad bandwidth of optical fibers operating at the 1550 nm window. Different schemes have been proposed to distribute millimeter-wave signals using optical fiber, which include intensity modulation and direct detection (IM/DD) scheme and remote heterodyne (RHD) scheme. In a millimeter-wave-over-fiber system using IM/DD scheme, two sidebands located at the two sides of the optical carrier are generated. For frequencies higher than 20 GHz, the chromatic dispersion becomes a serious problem which leads to high power penalty. The dispersion problem can be solved if RHD scheme is used. In an RHD scheme, two wavelengths that are phase correlated are generated using single-side band with carrier modulation, optical carrier-suppressed modulation, optical offset injection locking or optical offset phase locking of two laser sources. Ideally, the laser sources are considered to have very narrow linewidth, which will not introduce phase noise at the remote side when beating the two wavelengths. However, in real applications laser diodes usually have a finite linewidth, which leads to the phase de-correlation in the fiber links; phase noise is then generated at the remote end. In this paper, we will analyze the effects of the finite linewidth of optical sources on the performance of millimeter-wave over fiber systems. Simulation and experimental results will be provided.
A new hybrid remote-sensing method using active high-power microwave (HPM) illumination and passive infrared (IR) detection is presented for the detection of shallow buried landmines. A 2.45 GHz, 5 kW microwave source was used for illumination. The thermal signature of the mine at the soil surface was detected in the 8 - 12 micrometer region both in near real-time as well as after a brief time-delay following illumination. The thermal signature at the soil surface is primarily made up of two components. A thermal signature occurs at the soil surface in near real-time due to the interference of the incident beam and the beam reflected by the mine. A second thermal signature is generated when the variations in heating due to differential microwave absorption by the mine and the surrounding soil is conducted upwards from the mine location to the surface. Both signatures are dependent on the complex dielectric constants of the mine and the soil. Results will be presented from laboratory experiments and field trials with different types of metallic and non-metallic mine surrogates, dummy mines without explosives and live mines with explosives but without fuses.
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