Joseph M. Singley

High sustained average power cw and ultrafast Yb:YAG near-diffraction-limited cryogenic solid-state laser

We report what we believe to be record performance for a high average power Yb:YAG cryogenic laser system with sustained output power. In a CW oscillator-single-pass amplifier configuration, 963 W of output power was measured. In a second configuration, a two amplifier Yb:YAG cryogenic system was driven with a fiber laser picosecond ultrafast oscillator at a 50 MHz repetition rate, double-passed through the first amplifier and single-passed through the second, resulting in 758 W of average power output. Pulses exiting the system have a FWHM pulsewidth of 12.4 ps, an energy/pulse of 15.2 μJ, and a peak power of 1.23 MW. Both systems are force convection-cooled with liquid nitrogen and have been demonstrated to run reliably over long time periods.

Mitigation of Photodiode Induced Even-Order Distortion in Photonic Links With Predistortion Modulation

A new class of predistortion techniques for suppressing photodiode generated even-order distortion is presented. Modulation induced distortions can be generated to cancel the photodiode even-order contributions. This method is described conceptually, theoretically, and experimentally in a generalized fashion to include intensity-, phase-, and polarization-modulation implementations. Measured suppression of photodiode second-order distortion upwards of 34 dB is demonstrated with frequencies ranging from 1 to 35 GHz.

Suppression of even-order photodiode distortions via predistortion linearization with a bias-shifted Mach-Zehnder modulator

A new technique to cancel photodiode-induced even-order distortion in microwave photonic links is demonstrated. A single Mach-Zehnder modulator, biased slightly away from the quadrature point, is shown to suppress photodiode second-order intermodulation distortion in excess of 40 dB without affecting the fundamental power. The technique is theoretically described with supporting experimental results.

Heat-fraction-limited CW Yb:YAG cryogenic solid-state laser with 100% photon slope efficiency

We report the demonstration of a heat-fraction-limited CW Yb:YAG laser operating near 77 K with output at 1029 nm, pumped with a diffraction-limited room-temperature CW Nd:YAG laser operating at 946 nm. With a 50% reflectivity outcoupler, the average threshold absorbed pump power was 18.8 mW and the average slope efficiency 91.9%, close to the heat-fraction limited value of 91.5%. Average optical to optical and photon slope efficiencies are 84% and 100% respectively. To the best of our knowledge this solid-state laser is the first to operate at the heat-fraction-limit and demonstrates record slope, photon slope and optical-optical efficiencies for optically-pumped solid-state lasers.

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.

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.

Simultaneous optical phase and intesnity modulation for analog signal processing

Photonic processing of radio-frequency (RF) signals provides an attractive alternative to all-electrical processing due to increased bandwidth and scalability of the former. One particularly elusive capability for electronic warfare applications is wideband (near DC to > 100 GHz) RF energy detection achievable in a short timeframe [O(s)] with high resolution (< 500 MHz). A photonic approach to this end is to encode RF energy onto an optical carrier and then conduct wideband down conversion and/or channelization via optical or microwave-photonic filters [1]. Some unresolved issues with this straightforward approach are realizing narrow pass bands (narrow 3-dB bandwidth, BW3dB) over large processing bandwidths (large free-spectral range, FSR) with high sensitivity (high off-resonance rejection and low on-resonance insertion loss). However, there has been recent interest in photonic-assisted instantaneous frequency measurement (IFM) where post-filter processing allows for frequency resolution well below the filter BW3dB [2]–[8]. In this work, we discuss various IFM techniques and suggest new signal processing architectures based on simultaneous optical phase and intensity modulation.

Laser noise considerations for phase modulated links

Photonic links employing intensity modulation with direct detection are typically unaffected by laser phase noise without a mechanism to convert that phase noise into intensity noise. Such mechanisms in long links include chromatic dispersion and double-Rayleigh scattering, the latter of which introduces multi-path interference (MPI). However, these processes are generally not a concern in short links for avionic platforms except when reflections from fiber optic connectors produce strong MPI. Phase modulated links provide distinct advantages over intensity modulation for digital [1] and analog [2] applications; however, they are severely impacted by laser phase noise. Here we describe considerations for a phase modulated link employing an asymmetric Mach-Zehnder interferometer (MZI).