Christopher E. Sunderman

Photodiode linearity requirements for radio-frequency photonics and demonstration of increased performance using photodiode arrays

The impact that photodiode nonlinearities have on intensity-modulation direct-detection analog photonic links is described. The second- and third-order linearity requirements for photodiodes, quantified in terms of output intercept points, are calculated. Measured data at 1 GHz for a high-current (60-mA) partially-depleted-absorber photodiode are presented. A four-photodiode 26-mA array at 5 MHz is demonstrated to achieve modulation-limited third-order performance and a second-harmonic output intercept point of 84 dBm.

Even-order harmonic cancellation for off-quadrature biased Mach-Zehnder modulator with improved RF metrics using dual wavelength inputs and dual outputs

We present a technique using a dual-output Mach-Zehnder modulator (MZM) with two wavelength inputs, one operating at low-bias and the other operating at high-bias, in order to cancel unwanted even-order harmonics in analog optical links. By using a dual-output MZM, this technique allows for two suppressed optical carriers to be transmitted to the receiver. Combined with optical amplification and balanced differential detection, the RF power of the fundamental is increased by 2 dB while the even-order harmonic is reduced by 47 dB, simultaneously. The RF noise figure and third-order spurious-free dynamic range (SFDR(3)) are improved by 5.4 dB and 3.6 dB, respectively. Using a wavelength sensitive, low V(pi) MZM allows the two wavelengths to be within 5.5 nm of each other for a frequency band from 10 MHz to 100 MHz and 10 nm for 1 GHz.

An Optical Technique for Radio Frequency Interference Mitigation

A new photonics-based approach for interference mitigation utilizing the nonlinear response of optical modulators is described. The technique is analyzed both theoretically and experimentally with excellent agreement. Proof-of-concept experiments demonstrate upward of 78-dB suppression of a large interfering signal, while degrading the response to small signals of interest by only 8 dB.

Long-Reach Analog Photonics for Military Applications

In the increasingly digital world of electronic warfare, defense and signals intelligence, analog fiber optics and photonics still play a crucial enabling role.

Microwave photonic delay line signal processing.

This paper provides a path for the design of state-of-the-art fiber-optic delay lines for signal processing. The theoretical forms for various radio-frequency system performance metrics are derived for four modulation types: X- and Z-cut Mach-Zehnder modulators, a phase modulator with asymmetric Mach-Zehnder interferometer, and a polarization modulator with control waveplate and polarizing beam splitter. Each modulation type is considered to cover the current and future needs for ideal system designs. System gain, compression point, and third-order output intercept point are derived from the transfer matrices for each modulation type. A discussion of optical amplifier placement and fiber-effect mitigation is offered. The paper concludes by detailing two high-performance delay lines, built for unique applications, that exhibit performance levels an order of magnitude better than commercial delay lines. This paper should serve as a guide to maximizing the performance of future systems and offer a look into current and future research being done to further improve photonics technologies.

Cancellation of chromatic dispersion-induced second harmonic using dual wavelengths and balanced photodetection

A method for canceling dispersion-induced second harmonic by simultaneously modulating two optical wavelengths combined with balanced photodetection is demonstrated. The second harmonic is reduced by ∼30dB while the fundamental is increased by 6dB.

Microwave Phase Shifting Using Coherent Photonic Integrated Circuits

An optically coherent silicon-on-insulator circuit for microwave phase shifting is presented. Such photonic integrated circuits provide advantages over more-complicated incoherent methods and coherent techniques implemented in bulk fiber-optic components. The circuit is described theoretically with supporting experimental data. Continuous microwave phase control at 49 GHz is demonstrated, with an electrical power to achieve 2π phase shift of 80 mW and a 3-dB phase modulation bandwidth of 170 kHz.

Nonlinear Optical Angle Modulation for Suppression of RF Interference

A new optical technique for suppression of electronic interference is detailed theoretically and experimentally. The method is based on operating an angle-modulated photonic link in its nonlinear regime, where Bessel functions govern the response. This novel signal-processing technique is analyzed for optical intensity and phase modulation. Experimental results demonstrate suppression of continuous-wave, pulsed, and chirped signals at levels ranging from at least 30 dB to upwards of 70 dB relative to small-signal conditions.

Control of residual amplitude modulation in Lithium Niobate phase modulators

This work provides a simple model for Residual Amplitude Modulation observed in Lithium Niobate phase modulators. It operates under two key assumptions: the optical field incident on the modulator is not perfectly aligned to the preferred axis, and the two linear polarizations become spatially separated while travelling down the waveguide. These assumptions allow for a straight forward transfer matrix based approach. The effects of chromatic dispersion present in the optical fiber following the modulator are included, as they become important for modulation frequencies over 20 GHz. The result is a closed form expression for the intensity modulated signal seen by the photodetector in a phase modulated system. The model describes a near-instantaneous control mechanism, which is useful in minimizing the residual amplitude modulation in fielded systems by offering over 40 dB of suppression. The model is compared to direct measurements, validating the polarization effects and control mechanism proposed. Furthermore, etalon effects are ruled out by doing a course temperature dependence measurement.

Using a microcontroller to optimize the bias voltage of balanced photodiodes to minimize even-order distortion in analog fiber-optic links

Use of a microcontroller-based bias board for balanced photodiodes improves even-order distortion cancellation due to voltage dependent responsivity nonlinearities. The bias board improves the second order intermodulation distortion in balanced photodiodes by 8.3 dB.