We review the electroabsorption (EA) device as a linear mixer for frequency conversion. We explain the operating principle and compare the operation of the EA device as a modulator/mixer with the operation as a detector/mixer. We present experimental results of EA devices with the quantum- confined Stark effect and the Franz-Keldysh effect. Using a InAsP/GaInP multiple-quantum-well EA device, a conversion loss of 18.4 dB is obtained at 10-mW optical local oscillator power when the device is operated as a detector/mixer. The two-tone measurement on the same device shows a sub-octave, spur-free dynamic range of 120.0 dB-Hz. Phase noise measurement demonstrates that very low phase noise is added in the conversion process. We propose a novel full-duplex operation of the EA device as a detector/mixer for the application in the antenna remoting or fiber-radio systems.
The best RF link gain and the multi-octave SFDR are obtained when the semiconductor EA modulator is biased at the point where the photocurrent has maximum slope efficiency. Since the maximum slope efficiency corresponds directly to the maximum slope of the photocurrent, the photocurrent output is monitored as the applied bias voltage is swept1. The voltage at which the modulator photocurrent changes fastest (highest slope) corresponds to the optimum bias point for maximum link gain. During the sweep, the photocurrent output from the modulator is amplified and sent to a microcontroller. The microcontroller records the photocurrent during the sweep and determines the optimum bias. This is preferable to the common way of determining maximum RF gain by tapping off a portion of the optical power because it does not induce optical loss or require additional optical components. This paper will present the design and testing of a module that will stand alone and apply the optimum dc bias for a modulator automatically.
Semiconductor electroabsorption modulator (EAM) is a promising alternative to lithium niobate modulator for digital and analog fiber optic links due to its inherent small size, high modulation efficiency, and potential of monolithic integration with other electronic and optoelectronic components. For high-speed application, the bandwidth of the lumped element EAM is known to be RC-time limited. To achieve an ultra large bandwidth in lumped element EAM, the modulation efficiency has to be greatly sacrificed. This is especially critical in analog operation where RF link loss and noise figure must be minimized. To overcome the RC bandwidth limit and to avoid significantly compromising the modulation efficiency, the traveling wave electroabsorption modulator has been proposed and experimentally investigated by several authors.
Waveguide npin heterojunction phototransistors (HPTs) are investigated for analog fiber-optic link applications. The device is fabricated on a semi-insulating InP substrate with an integrated CPW transmission line. At low optical power, an incremental DC responsivity of approximately 16 A/W is observed at 1.3 micrometers wavelength. It is demonstrated that the HPT responsivity at RF frequencies can be increased through a second optical beam. Signal mixing experiment using two RF-modulated optical beams shows that the waveguide npin HPT can be used as an effective integrated photodetector/mixer for RF signal up- and down-conversion.
The effect of RF phase noise and phase delay fluctuations on the performance of a Quadrature Amplitude Modulated fiber optic link is investigated. Plots showing the bit error rate against the phase noise for 16, 64 and 256 QAM systems are presented. Two techniques are used for evaluating the average error probability performance namely, numerical integration and Taylor series approximation. To alleviate the difficulties in the simulations, phase noise is assumed to have a zero mean Gaussian probability distribution function.
High-frequency self-pulsing effects in combined index and gain coupled two-section DFB lasers in a single cavity laser are investigated by numerical simulations. The gain coupling increase the modulation index and frequency of the intensity oscillations. Simulation results show that the signals of oscillation frequencies up to 200 GHz may be possible to generate using such devices. Preliminary experimental results agree well with simulation.
A novel npin waveguide structure for the dual-function electroabsorption modulator/detector is proposed and fabricated. With the addition of an n-layer to the conventional pin-structure, the device exhibits phototransistor behavior in the detector model. The device has an InGaAsP intrinsic layer with Franz-Keldysh electroabsorption at 1.3 micrometers wavelength. Preliminary results show optical gain in the detector mode and good modulator characteristics.
Wideband microwave/millimeter wave photonic link technology suitable for shipboard RF antenna remoting is presented. High performance 1.3 micrometers laser diode based direct modulation links with bandwidths exceeding 3 GHz, indirect modulation links to 20 GHz using Mach-Zehnder and semiconductor electroabsorption modulators, and frequency conversion links with operation beyond 40 GHz using a series modulator configuration are described. The performance currently attained by the broadband analog links makes them strong candidates to replace copper-based shipboard antenna remoting systems operating in the HF to EHF frequency range. Modulation efficiency improvements required for analog photonic links to approach RF transparency are discussed. The photonic links are suitable for insertion into both transmit and receive single- and multi-element navigation, radar, communication, and electronic warfare antenna systems.
Two L-band phase-matched fiber optic delay line channels and a broadband fiber optic RF signal processing filter have been designed, fabricated, tested, and evaluated. These two related RF photonic system development efforts are potentially useful in ELINT signal processing of ultrawideband signal. Specifically, two high performance optical delay lines operating at 1 GHz with a 500 MHz bandwidth have been prototype and show prototyped and show improved dynamic range and environmental phase tracking performance over conventional SAW delay lines. In addition, an eight-tap fiber optic transversal filter using wavelet amplitude weighting has been designed, fabricated, and tested in the 50 MHz to 20 GHz frequency range. A high pass wavelet filter useful for ultrawideband signal detection has been optically implemented, and test result presented for sensitivity and dynamic range are promising.
Analog fiber optic links can be used to transmit microwave and millimeter wave signals in applications such as cable TV, antenna remoting and active phased array. In this paper, we examine various issues pertaining to an integrated laser- modulator transmitter module for analog fiber optic links: (1) the performance requirements of the lasers, (2) the performance requirements of the electroabsorption modulator, (3) the bias control of the electroabsorption modulator.
Planar photoelastic effect on compound semiconductor structures has been investigated for integrated optical transmitter in rf photonics system. While our prior works emphasized the investigation of low-loss photoelastic waveguide, photoelastic waveguide modulator, and photoelastic optical splitter, the present work focuses on the attainment of high performance laser which employs the photoelastic effect for waveguiding. Planar separate- confinement, double-heterostructure, single-quantum-well photoelastic GaAs/AlGaAs lasers have been fabricated using WNi stressors for waveguiding and ion implantation for isolation. Even without bonded on heat-sinks, these planar photoelastic lasers operate continuous wave at room temperature. The lowest threshold is 29 mA for a cavity length of 178 micrometers and a stressor width of 5 micrometers . The main waveguiding mechanism of the photoelastic lasers is determined to be weak index-guiding with the beam waist in the junction plane measured at 10 micrometers behind the end- facet.
A Franz-Keldysh effect InGaAsP electroabsorption waveguide device is utilized as the high-frequency, high-linear dynamic range modulator and photodetector. The dual-function modulator/photodetector can be useful in compact transmit/receive front end antenna architectures. Adjusting the electrical bias to the reverse-biased p-i-n diode, either efficient optical modulation or detection is demonstrated. As an electroabsorption modulator, a fiber optic link with a minus 17.4 dB rf loss and a 124 dB-Hz4/5 sub-octave spurious-free dynamic range is obtained with electrical biases in the 2 to 3 V range. As a waveguide photodetector, a 0.47 A/W fiber coupled responsivity, photocurrents up to 20 mA, and an output second-order intercept of plus 34.5 dBm are achieved at 7 V electrical bias. Supporting measurements on additional test devices show a trend toward larger intercept point with longer device lengths.
A tunable millimeter wave source is demonstrated using the frequency multiplication properties of a Mach-Zehnder modulator based analog fiber optic link. Extremely low phase noise millimeter wave output signals (minus 86 dBc/Hz at 1 kHz from a 40.96 GHz carrier) have been obtained using harmonic carrier generation by electrically overdriving the Mach-Zehnder modulator. This technique can be useful for high quality signal synthesis to 100 GHz, well beyond the modulation bandwidth of present optical modulators.
Planar electroabsorption InP/InGaAsP waveguide modulators suitable for RF applications have been fabricated using the photoelastic effect. The planar device structure is achieved by using WNi thin film surface stressors for lateral waveguiding and helium implantation for electrical isolation between devices. These are the first reported frequency measurements on a photoelastic InP/InGaAsP waveguide modulator.
Large spurious-free dynamic range (SFDR) has been measured in a fiber optic link using an InGaAsP electroabsorption (EA) waveguide modulator. Link phase noise is investigated and conversion of AM noise to phase noise appears to be an issue in the EA device. Preliminary measurements of the EA link show inferior close-to-carrier phase noise compared to a link using a Mach-Zehnder modulator with similar SFDR. Optical feedback from the output coupling fiber is shown to contribute partially to close-in phase noise.
The Franz-Keldysh effect is utilized for high-linearity, electroabsorptive InGaAsP waveguide modulators. Two-tome RF measurements are performed in which the DC bias is adjusted to maximize the spurious-free linear dynamic range. With the proper choice of bias, the sub- octave link dynamic range increases to 123 dB in a 1 Hz bandwidth for 0.25 mA photocurrent.
The phase and amplitude stability of directly and externally modulated analog fiber optic links are investigated and factors which limit link stability are discussed. Link gain, phase noise, and phase delay fluctuation measurements have been performed on fiber-optic links which are currently being considered for Navy shipboard antenna remoting and phased array antenna applications. Measurements indicate that good amplitude and adequate phase stability can be obtained using either directly or externally modulated analog photonic links provided certain design considerations are met.
A fully packaged and connectorized 1.32 micrometers InGaAsP electroabsorption modulator for analog fiber optic link applications is described. This Franz-Keldysh effect modulator has a 3-dBe bandwidth exceeding 20 GHz, a fiber-to-fiber optical insertion loss of less than 9 dB, and a high modulation efficiency (equivalent switching voltage less than 10 V). A fiber link spurious free dynamic range of greater than 100 dB in a 1 Hz bandwidth is achievable using this modulator, which makes it attractive for Navy shipboard applications.
The use of III-V semiconductor heterojunction and quantum well optical modulators is considered for monitoring of shipboard radar and communication emissions. Both antenna- coupled and all-dielectric electro-optic electromagnetic environment monitoring systems are investigated with respect to sensitivity, linear dynamic range, and system bandwidth at optical wavelengths of 1.3 and 1.5 micrometers . Results are presented which indicate that these devices can, if carefully designed and utilized, be a useful alternative to interferometric modulators for this ultra-wideband analog fiberoptic application.