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.
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.
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.
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.
Strained balanced InAsP/InGaP superlattices for optoelectronic applications were studied with materials grown by low pressure MOVPE. For 20 and 30 period InAsP/InGaP superlattices with a 23 percent As mole fraction in the InAsP layers, sharp x-ray satellite peaks are obtained. The superlattices show an average lattice constant close to that of the InP substrate. Strong photoluminescence with narrow emission linewidth are observed at room temperature around 1.11 micrometers . PIN diodes with an intrinsic region consisting of the InAsP/InGaP superlattice show efficient electroabsorption at wavelengths around 1.15 micrometers with a small residual absorption of 59 cm-1.