Wavelength switching technique for phase interrogation of Mach Zehnder interferometer-based optical sensors.

A method for determining the phase shift of a Mach Zehnder interferometer (MZI) is presented. It is based on switching the wavelength of continuous wave (CW) laser light illuminating the MZI and measuring the interferometer output amplitudes at DC and switching frequency. The method can measure the MZI phase shift unambiguously over the entire phase shift range of 2π. A practical proof of concept demonstration shows that the method can perform real-time measurement with high repeatability and accuracy limited by the optical frequency drift and power fluctuation of the lasers. The method does not require modifications of the sensor or accessing to the laser electronics and also uses simple detection. It is, therefore, suitable for bio and medical sensing applications.

Mach-Zehnder modulator condition for a low chirp factor versus DC bias.

An analysis of the Mach-Zehnder modulator's chirp factor α versus DC bias is presented. Unlike the well-known belief that α is infinite at 0 and π biases, it is shown that α can be zero at these biases under a careful balance of the modulations in the two modulator arms. Derivation of the condition to achieve this effect is given, and practical verifications are presented, where an excellent agreement with the theory is obtained. It is shown that modulators with a low residual chirp over a wide range of DC bias are achieved by designing the RF driving such that α would be minimized at zero bias instead of at quadrature bias as used currently in the modulator design.

Characterization of optical Mach-Zehnder modulator using DC measurements.

This Letter presents a simple but effective method for characterizing the frequency response of broadband Mach-Zehnder optical modulators. The method measures the modulator's direct current output versus the modulating frequency to determine the frequency response and requires no calibrated broadband photodetector or measurements of the electrical or optical spectrum or radio frequency power. Therefore, it significantly simplifies characterization. The method is suitable for in-situ measurements and can be automated.

Remoted all optical instantaneous frequency measurement system using nonlinear mixing in highly nonlinear optical fiber.

A novel remoted instantaneous frequency measurement system using all optical mixing is demonstrated. This system copies an input intensity modulated optical carrier using four wave mixing, delays this copy and then mixes it with the original signal, to produce an output idler tone. The intensity of this output can be used to determine the RF frequency of the input signal. This system is inherently broadband and can be easily scaled beyond 40 GHz while maintaining a DC output which greatly simplifies receiving electronics. The remoted configuration isolates the sensitive and expensive receiver hardware from the signal sources and importantly allows the system to be added to existing microwave photonic implementations without modification of the transmission module.

Amplitude independent instantaneous frequency measurement using all optical technique.

A novel all-optical system which independently measures both the amplitude and frequency of an RF signal is proposed and demonstrated. A photonic Hilbert transformer provides two orthogonal measurements of an RF signal. These are compared using four wave mixing in a highly nonlinear fiber, producing two independent outputs enabling determination of both signal frequency and amplitude. This all optical approach requires only simple, low cost DC electronics at the receiver. The system is demonstrated up to 20 GHz but can be scaled to 40 GHz and beyond.

Wideband RF photonic in-phase and quadrature-phase generation.

A photonic implementation of a practical broadband RF Hilbert transformer is demonstrated by using a four-tap transversal system. An almost ideal 90 degrees phase shift with less than 3 dB of amplitude ripple has been achieved from 2.4 to 17.6 GHz. An efficient method to realize both transformed (quadrature-phase) and reference (in-phase) signal has been achieved by using a coarse wavelength division multiplexing coupler. Extension of the transformer bandwidth and further improvements of its implementation are discussed.

Multichannel vector sum phase shifter.

A novel multichannel vector sum phase shifter that is suitable for phased array antenna applications is demonstrated. Each channel is implemented using a distinct optical wavelength. Selective control of each channel is performed using an acousto-optic polarization coupler. The concept is successfully demonstrated for two individually controlled channels. For each channel, a continuously variable frequency linear phase shift is demonstrated between DC and 7 GHz, with the phasing range exceeding 100 degrees.