G. D. Thurmond

17‐GHz bandwidth electro‐optic modulator

A high‐speed integrated‐optic Ti:LiNbO3 Mach–Zehnder interferometric modulator for 0.83‐μm wavelength operation has been fabricated and characterized. The modulator exhibits smooth, resonance‐free frequency response with a 17‐GHz 3‐dB bandwidth. The modulator has a built‐in phase bias of π/2 for maximum linearity. Complete intensity modulation can be achieved with 120‐mW drive power. Optical modulation was measured up to 18 GHz directly by using a very high‐speed photodiode and indirectly using the swept frequency technique.

Minimizing dc drift in LiNbO3 waveguide devices

Improved stability against electrical dc bias drift has been demonstrated in LiNbO3 electro‐optic modulators by replacing the commonly used SiO2 buffer layer with indium tin oxide (ITO), a transparent conductor. The long term drift of the modulators having an ITO buffer layer with a sheet resistivity of ∼20 Ω/⧠ is less than 0.3% in 8 h. The mechanism of the dc drift phenomenon is discussed using an electrical equivalent circuit model of the modulator.

Fabrication and characterization of GaAs Schottky barrier photodetectors for microwave fiber optic links

High‐Speed GaAs Schottky barrier photodiodes have been fabricated and characterized. These detectors have 3‐dB bandwidths of 20 GHz and quantum efficiencies as high as 70%. The response of the detectors to light modulated at 1–18 GHz has been directly measured. Microwave modulated optical signals were obtained by using a LiNbO3 traveling wave modulator and by heterodyning two laser diodes.

Traveling-wave electrooptic modulator

An integrated-optics Mach-Zehnder interferometric modulator in LiNbO3 has been designed and fabricated. The electrodes are 3-μm thick asymmetric coplanar striplines formed by ion-beam etching techniques. The push–pull design and the r33 electrooptic coefficient of LiNbO3 are utilized for efficient modulation. Complete modulation is achieved with 6.5 V for the 6-mm long device at 0.83-μm wavelength and with 18 V at 1.3-μm wavelength. The 3-dB bandwidth of the modulator is 3.5 GHz, being limited by the excessive resistive loss of the stripline electrodes. Since this particular modulator retains a dc electrical bias, it performs either as an intensity modulator by applying a π/2 dc phase bias to achieve maximum linearity or as a frequency shifter by changing the dc bias point to π. In addition, we analyzed the principle of operation of the Y junction by observing both the in-phase and the out-of-phase modes of a multimode waveguide modulator.

Radar applications of X-band fiber optic links

High-speed fiber-optic delay lines for use in radar phase noise and repeater test sets are described. Both FM and AM signal-to-noise performance measurements of the X-band modulated (10-GHz) fiber-optic links for these applications are presented. Using long delay lines, improved measurement capability and flexibility was achieved with small components.<<ETX>>

10 GHz RF Fiber Optic Links

A 10-GHz direct laser modulation and an external modulation link were demonstrated. Signal-to-noise ratios of 130 dB/Hz and 115 dB/Hz have has been measured for the external modulation link at 1.3 µm wavelength and for the direct modulation link at 0.83 µm wavelength, respectively.

Radar measurement applications of fiber optic links

Novel applications of fiber-optic delay lines to improve the capability and performance of radar repeater and phase noise test sets are described. Using a 2.25-km-long fiber-optic link, the generation of an ideal target for a radar repeater test set and the measurement of phase noise within 100 Hz of a 9.6-GHz radar carrier signal is demonstrated using a delay line discriminator phase noise test set. FM and AM signal-to-noise measurements were made to determine the performance capability of the 10-GHz modulated fiber-optic links for radar applications.<<ETX>>

Fiber Optic Delay Lines For Radar Applications

Novel applications of fiber optic delay lines to improve the capability and performance of radar repeater and phase noise test sets are described. Using a 2.25 km long fiber optic link, we demonstrated generation of an ideal target for a radar repeater test set and measurement of phase noise closer than 100 Hz to a 9.6 GHz radar carrier signal using a delay line discriminator phase noise test set. FM and AM signal-to-noise measurements were made to determine the performance capability of the 10 GHz modulated fiber optic links for the radar applications.

X-Bana Rf Fiber Optic Links

Two 10-GHz direct laser modulation links and an external modulation link were demonstrated. A signal-to-noise ratio of 130 dB/Hz was measured for the 1.3 μm wavelength external modulation link and 115 dB/Hz for the 0.8 pm and 1.3 μm wavelength direct modulation links.

Long microwave delay fiber optic link for radar testing

A unique application of a long fiber-optic delay line as a radar repeater to improve radar testing capabilities is described. It is shown that, with the use of a 1.3- mu m wavelength distributed feedback laser and an external modulator connected to a long (31.6-km) fiber cable to provide long (152- mu s) delays with minimum dispersion at X-band (10 GHz) frequencies, performance measurements can be made on modern Doppler radars utilizing long transmitter pulses. Tests demonstrate that these fiber-optic links can meet the stringent signal/noise ratio and phase linearity and stability requirements for Doppler radar systems that use pulse compression waveforms to obtain long-range operation with high resolution.<<ETX>>