Jeffrey D. Bull

40 GHz electro-optic polarization modulator for fiber optic communications systems

A novel ultrahigh-speed electro-optic polarization modulator is introduced. The modulator uses a mode converter and a static polarization controller to change the output polarization state in a circular path, following a great circle, around the Poincaré sphere. Any two states on the Poincaré sphere can be connected. The mode converter is constructed using an AlGaAs ridge waveguide combined with slow-wave travelling wave electrodes. The travelling wave electrodes are designed to match the velocity of the electrical modulating signal, the data signal, to the optical carrier signal over a broad frequency range. This modulator demonstrates a 3 dB bandwidth in excess of 40 GHz. The polarization modulator exhibits extremely low differential group delay, on the order of a few 10s of femto-seconds, and low drive voltage, on the order of 5 V.

A new hybrid current sensor for high-voltage applications

This article presents a new hybrid electro-optic/inductive current sensor for metering and protective relaying applications. The sensor combines a Rogowski coil with a passive integrator located in the high-voltage environment. An integrated-optic Pockels cell (IOPC) having integrated electrodes provides optical isolation. Test results from 30 A to 30 kA show the sensor to be highly linear, exceeding 0.2% linearity standards. Temperature cycling tests show the ratio error to have a maximum value of 0.3% over the -30 to +70/spl deg/C range.

Noise Reduction in Class-AB Microwave-Photonic Links

A Class-AB microwave-photonic link is presented as a means to reduce noise associated with DC-bias. By operating with two complementary, approximately half-wave-rectified, optical sources and generating the difference in a balanced detector, linear modulation with high modulation efficiency is achieved with near zero DC. Using two mode-converter intensity modulators, we demonstrate a 5 dB reduction in shot noise and elimination of intensity noise, relative to a quadrature-biased Mach-Zehnder, for the same received RF signal power.

Broadband class-AB microwave-photonic link using polarization modulation

A novel broadband class-AB (CAB) microwave-photonic link is demonstrated using an AlGaAs-GaAs electrooptic mode-converter-based polarization modulator (PolM). The polarization-modulated signal is split into two paths, each independently optically biased to provide two electrooptic transfer functions operating close to, but on opposing sides of, their nulls. Experimental results at 2 and 4 GHz confirm that detection of the transfer functions in a differential detector provides excellent cancellation of second-order distortion and recreates the same third-order distortion as a quadrature-biased Mach-Zehnder modulator. Biasing the operating points near the null reduces optical carrier power, and hence, carrier-related noise, thereby increasing achievable spur-free dynamic range. The use of the PolM allows the CAB link to be implemented with a single modulator, reducing the matching requirements of dual modulator schemes and allowing third-order distortion-limited broadband operation.

Intensity-noise suppression in microwave-photonic links using polarization modulation

A novel microwave-photonic link is demonstrated using a unique AlGaAs-GaAs electrooptic mode-converter-based polarization modulator in a balanced output configuration. This enables suppression of common-mode intensity noise and optical-amplifier-induced beat noise using a polarization-selective balanced optical receiver. Unlike the conventional approach using a dual-output Mach-Zehnder modulator, the complementary output signals are combined naturally as orthogonal polarizations into one transmission fiber.

Microwave Photonic Signal Detection Using Phase-Matched Optical Rectification in an AlGaAs Waveguide

We demonstrate a novel approach for detecting microwave-modulated optical signals through velocity-matched optical rectification in second-order optical nonlinear materials. Optical intensity modulation is down-converted to microwave frequency (5-20 GHz) using a traveling-wave GaAs electrooptic modulator as an optical square-law detector. Phase matching between microwave and optical waves is achieved through a velocity-matched slow-wave asymmetric coplanar-strip electrode structure.

Negative Tap Photonic Microwave Filter Based on a Mach–Zehnder Modulator and a Tunable Optical Polarizer

A negative tap photonic microwave filter based on a Mach-Zehnder modulator (MZM) and a tunable optical polarizer is proposed. In the proposed filter, the output light from the MZM, after experiencing a time delay difference between the two orthogonal modes in a polarization-maintaining fiber, is sent to the tunable optical polarizer. By adjusting the dc bias of the MZM and the polarization angle of the tunable polarizer with respect to the two orthogonal modes, two positive or one positive and one negative coefficient are generated. A theoretical analysis is presented which is verified by experiments. A two-tap microwave filter with two positive or one positive and one negative coefficient is demonstrated.

Optical Heterodyne Method for Amplitude and Phase Response Measurements for Ultrawideband Electrooptic Modulators

We present an optical heterodyne method for measuring amplitude and phase responses of electrooptic modulators over an ultrawide frequency range. The microwave input signal for the modulator under test (MUT) is produced by heterodyning the outputs of two lasers. The output of one of these two lasers is modulated by a subcarrier to produce dual tones separated by twice the subcarrier frequency. The output of the MUT is optically filtered to extract the components carrying the amplitude and phase information. Experimental results of measurements made up to 50GHz are presented

Ultrahigh-speed polarization modulator

Recent measurements on an ultrahigh-speed polarization modulator, capable of circumscribing a great circle on the Poincare sphere, are presented. Such a device, when fully packaged, has been demonstrated with a bandwidth of /spl sim/50 GHz.

Asymmetrically Strained Ridge Waveguide for Passive Polarization Conversion

A novel passive polarization converter is presented using the elasto-optic effect in a ridge waveguide. A compressively stressed Si 3N 4 layer is deposited onto one side of the waveguide while a tensile layer is deposited on the opposing side. The average stress of the tensile and compressive layers is used to cancel the modal transverse-electric-transverse-magnetic (TE-TM) birefringence of the ridge waveguide. The difference in the tensile and compressive stresses induces birefringence with principal axes rotated 45deg from the TE-TM orientations. The method is demonstrated on an AlGaAs-GaAs waveguide at 1550 nm. We measure the orientation of the axes to be 46deg plusmn 1deg, demonstrating that complete mode conversion can be obtained without using periodic phase matching. Complete conversion is obtained in 1.16 mm with a waveguide loss of 0.1 dB/cm. Stability over temperature is also demonstrated.