Optimizing the linearity in high-speed photodiodes.

Analog photonic links require high-fidelity, high-speed optical-to-electrical conversion for applications such as radio-over-fiber, synchronization at kilometer-scale facilities, and low-noise electronic signal generation. Photodetector nonlinearity is a particularly vexing problem, causing signal distortion and excess noise, especially in systems utilizing ultrashort optical pulses. Here we show that photodetectors designed for high power handling and high linearity can perform optical-to-electrical conversion of ultrashort optical pulses with unprecedented linearity over a large photocurrent range. We also corroborate and expand upon the physical understanding of how the broadband, complex impedance of the circuit following the photodiode modifies the linearity - in some cases quite significantly. By externally manipulating the circuit impedance, we extend the detector's linear range to higher photocurrents, with over 50 dB rejection of amplitude-to-phase conversion for photocurrents up to 40 mA. This represents a 1000-fold improvement over state-of-the-art photodiodes and significantly extends the attainable microwave power by a factor of four. As such, we eliminate the long-standing requirement in ultrashort pulse detection of precise tuning of the photodiode's operating parameters to coincide with a nonlinearity minimum. These results should also apply more generally to reduce nonlinear distortion in a range of other microwave photonics applications.

Compact, thermal-noise-limited reference cavity for ultra-low-noise microwave generation.

We demonstrate an easy-to-manufacture 25-mm-long ultra-stable optical reference cavity for transportable photonic microwave generation systems. Employing a rigid holding geometry that is first-order insensitive to the squeezing force and a cavity geometry that improves the thermal noise limit at room temperature, we observe a laser phase noise that is nearly thermal noise limited for three frequency decades (1 Hz to 1 kHz offset) and supports 10 GHz generation with phase noise near -100 dBc/Hz at 1 Hz offset and <-173 dBc/Hz for all offsets >600 Hz. The fractional frequency stability reaches 2×10-15 at 0.1 s of averaging.

Measurement of carrier envelope offset frequency for a 10 GHz etalon-stabilized semiconductor optical frequency comb.

We report Carrier Envelope Offset (CEO) frequency measurements of a 10 GHz harmonically mode-locked, Fabry-Perot etalon-stabilized, semiconductor optical frequency comb source. A modified multi-heterodyne mixing technique with a reference frequency comb was utilized for the measurement. Also, preliminary results from an attempt at f-2f self-referencing measurement are presented. The CEO frequency was found to be ~1.47 GHz for the particular etalon that was used.

Dynamic line-by-line pulse shaping with GHz update rate.

We introduce a novel scheme for dynamic line-by-line pulse shaping with GHz update rates. Four lines of an optical frequency comb source are used to injection-lock four individual VCSEL, which are subsequently electrically modulated at 0.4 to 1 GHz through current modulation. This concept could be considered a completely new way of pulse shaping as the light is not simply modified, but rather regenerated with the desired properties. We also discuss an important drawback of line-by-line pulse shapers that ultimately limits the modulation speed capability.