J. T. Zhu

Enhanced linear dynamic range property of Franz-Keldysh effect waveguide modulator

The spurious-free dynamic range (SFDR) of an InGaAsP-InP Franz-Keldysh effect (FKE) waveguide modulator is studied. Low-biasing the FKE modulator is shown to enhance the SFDR by 22 dB with a 3-dB optical power penalty. Polarization-independent RF operation is also obtained with low bias. At 2 mA photocurrent, the simulated narrowband and broadband SFDR are, respectively, 127 dB and 104 dB in a 1-Hz bandwidth. The measured result projects a narrowband SFDR of 129 dB-Hz/sup 4/5/ at this photocurrent.<<ETX>>

Measurement of IP3 in p-i-n photodetectors and proposed performance requirements for RF fiber-optic links

The nonlinearities generated by wide-band, high-current photodetectors are discussed in the context of the performance of an externally modulated analog radio-frequency fiber-optic link. The quantitative impart of the detectors third-order intermodulation product intercept (IP3) on the links spur-free dynamic range is analyzed. Waveguide p-i-n photodiodes were developed to specifically address these stringent IP3 goals. The IP3 was measured using a four laser heterodyne system resulting in an IP3 of 27 dBm measured at a center frequency of 20 GHz with 20 mA of de photocurrent. The authors believe that this is the highest photodetector IP3 reported to date at these operating conditions.

High-saturation-power waveguide photodetectors for analog fiber optic links

A four-layer asymmetric waveguide structure using a nonabsorbing 1.08 eV bandgap InGaAsP waveguiding layer has been studied for high saturation power, high speed waveguide photodiode operating at 1.32 micrometers wavelength. A peak photocurrent of 32 mA corresponding to an optical power of 76 mW has been obtained for 40 GHz waveguide photodiode.

Properties of Schottky contacts of aluminum on strained Si1−x−yGexCy layers

Schottky contacts of Al/Si1−x−yGexCy were fabricated using conventional Si technology. Effects of thermal processing of the alloys on the electrical properties of the Al/Si1−x−yGexCy Schottky diodes were investigated. Current–voltage (I–V), capacitance–voltage (C–V), and x‐ray diffraction measurements were performed. These thick alloy films (100–150 nm) experienced strain relaxation upon annealing at 700 °C. Nearly ideal I–V and C–V behaviors were obtained for strain‐compensated samples. I–V and C–Vcharacteristics show evidence of dislocation‐related traps for strain‐relaxed samples. Carbon incorporation improves the I–V and C–V characteristics by lessening the extent of lattice relaxation due to thermal processing.

Harmonic signals from electroabsorption modulators for bias control

Harmonic signals reflected from electroabsorption modulators are measured and analyzed as a function of modulator bias. The second-harmonic signal exhibits a dip close to the bias where the maximum of RF link gain occurs, over a significant optical power range. Using an equivalent circuit analysis we show this is caused by the inherent electroabsorption effect. The second-harmonic signal can be exploited for dynamic self-bias control of electroabsorption modulators in analog fiber-optic links.

High-speed and high-saturation power semiconductor waveguide photodetector structures

In the present work, two new approaches are investigated to simultaneously improve the saturation power and linearity: a) using a three layer waveguide structure with an InGaAsP (/spl lambda//sub g/ = 1.24 /spl mu/m) active layer, that operates through the Franz-Keldysh effect (FKE), and b) using a four-layer large optical cavity (LOG) asymmetric waveguide with InGaAs absorber and a passive InGaAsP layer.

Analysis of IP3 of photodiode using equivalent circuit

The IMD3 of a p-i-n waveguide photodiode is characterized up to 18 GHz. The observed frequency dependence of the IP3 closely agrees with that obtained from an equivalent circuit analysis of the photodiode.

Novel structure for dual-function electroabsorption waveguide modulator/phototransistor

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.

Dual function electroabsorption waveguide modulator/detector for photonic rf signal processing

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.

Electroabsorption waveguide modulators for high performance analog fiber links

Analog fiber optic links have been shown to be useful in antenna remoting and active phased array applications. Semiconductor electroabsorption (EA) waveguide modulators can be used in these links in view of their potential for low voltage operation, large bandwidth, and monolithic integration with other components. Important considerations of an optical modulator for analog applications are RF efficiency and the spurious free dynamic range (SFDR). In our prior work we demonstrated dual-function properties of a Franz-Keldysh effect waveguide which can function as a modulator and as a detector. As an electroabsorption modulator, a fiber optic link with a -17.4 dB RF loss and a 124 dB/Hz/sup 4/5/ sub-octave SFDR is obtained with electrical biases in the 2-3 V range. As a waveguide photodetector, a 0.45 A/W fiber coupled responsivity, photocurrents up to 20 mA, and an output third-order intercept of >34.5 dBm, are achieved at -7 V. As extensions of this work, we present (a) the results on the novel bias control of the electroabsorption waveguide modulator for maximum RF efficiency based upon the behavior of the modulator photocurrent, (b) the progress in the study of high SDFR electroabsorption waveguide modulator based upon a combination of the Franz-Keldysh effect and quantum confined Stark effect.