Ravi P. Gollapalli

Radio-frequency clock delivery via free-space frequency comb transmission

We characterize the instability of an rf clock signal caused by free-space transmission of a frequency comb (FC) under typical laboratory conditions. The phase-noise spectra show the involvement of multiple random processes. For a 10 m transmission, the rms timing jitter integrated over 1-10(5) Hz is 95 fs, and the root Allan variance over 1 s is 4x10(-13). The measured Allan variance has a tau(-1) behavior and an excellent agreement with the phase noise measurement. These results indicate the feasibility of FC-based free-space rf clock distribution over short distances.

Atmospheric Timing Transfer Using a Femtosecond Frequency Comb

We have experimentally demonstrated atmospheric transfer of microwave timing references using a femtosecond frequency comb. The excess timing jitter induced by the atmospheric propagation has been characterized, and evidence is provided to show that such characterization is not compromised by the parasitic effect of power-to-phase coupling in the photodetector. The fractional frequency stability for a 60-m total transmission distance is on the order of 10-12 with a 1-s averaging time. The Allan deviation shows a τ-1 dependence up to 500 s. Scale estimate confirms that the measured excess timing noise is caused by clear-air turbulence. Comparisons with previous works show that our results offer a more precise characterization of atmospheric timing transfer. The work may potentially help the development of high-fidelity synchronization for future free-space optical communications.

Multiheterodyne Characterization of Excess Phase Noise in Atmospheric Transfer of a Femtosecond-Laser Frequency Comb

We report an experimental investigation on remote transfer of a femtosecond-laser frequency comb through an open atmospheric link. Optical multiheterodyne is used to measure the excess phase noise and the frequency stability of the transferred comb. The dispersion of air is found to have a minimal impact on the multiheterodyne signal, and the effectiveness of the technique to characterize the behaviors of comb lines under the influence of turbulence is theoretically analyzed. Large phase modulation due to the index fluctuation of the air over a 60-m transmission link is found to cause a significant linewidth broadening. Under low-wind conditions, a fractional frequency stability in the order of 10-14 has been achieved over several minutes with a 1-s averaging time. A comparison of this work with previous tests based on continuous wave (CW) lasers indicates that pulsed lasers can work as well as CW lasers for remote transfer of optical frequency references through the atmosphere.

A System On Chip design for fast time domain impedance spectroscopy

Impedance spectroscopy is a powerful technique that can be employed to determine the physical properties of different materials. In principle the technique can be implemented in the time domain using pulsed signal excitation rather than sweeping across frequencies. The advantage is particularly apparent when the measuring instrument is either traveling through the material medium rapidly, as in the case of a satellite moving through space plasma, or if the medium is for example a fluid that flows past an instrument quickly. Here we describe a Time Domain Impedance Probe (TDIP) circuit design that is used to measure the absolute density of ionospheric plasmas. A preliminary version of this instrument was flown on a sounding rocket, but here we outline the system and circuit design that is being implemented for a Low Earth Orbit (LEO) micro-satellite. The design employs a bridge architecture together with a software adaptive filter and LMS algorithm for fast calibration and data compression. We propose that the design can be generalized, and we present a System On Chip (SOC) concept based on the time domain architecture. The proposed concept appears to be well suited to ultra-fast time domain spectroscopic measurements, but does have some inherent limitations such as increased noise. We suggest that this unavoidable shortcoming can be somewhat mitigated through repetitive pulsing and averaging during the measurement process.

First results from a time domain impedance probe for measuring plasma properties in the ionosphere

A new Time Domain Impedance Probe (TDIP) is presented in this paper. The new instrument is able to make measurements of absolute electron density and electron neutral collision frequency in the ionosphere at temporal and spatial resolutions not previously attained. A single measurement is made in 100 microseconds, which yields an instantaneous spatial resolution of 0.1 meters for sounding rocket experiments. A prototype of this instrument was integrated into the payload of a NASA USIP sounding rocket launched out of Wallops Island on March 1 2016. The sounding rocket launched at 8:50 am and reached an reached an altitude of 170 km, passing through the D and E and F layers of the ionosphere. The TDIP was active for 206 seconds during the flight. Here we describe the instrument, and present some time domain data obtained from the sounding rocket experiment. A 6 Volt amplitude Gaussian derivative excitation was applied to a dipole probe structure, and the current through the probe terminals measured with a balanced active bridge circuit. The time domain current response was sampled at 5 MS/s, at 12 bit resolution. In the course of the flight, the instrument measured what appeared to be a highly nonlinear response of the plasma because of the large input voltage signal applied. These are the first measurements of this type of response, to our knowledge. Post-flight laboratory calibration indicated that the instrument worked correctly through the flight. Further modeling, simulation and theoretical work needs to be performed to understand and interpret the measurements.

Atmospheric Clock Transfer Based on Femtosecond Frequency Combs

Precise timing and frequency synchronization has become ubiquitously needed as the world enters the age of global connectivity. Today’s communication and computer networks are running under the regulation of synchronized time bases to ensure efficient data routing and information transfer. As the carrier frequencies of these networks continue to rise to accommodate the ever-increasing data traffic, the need for high-fidelity clock distribution becomes imperative (Prucnal et al., 1986). Within the scope of basic sciences, ultra-stable timing dissemination also becomes increasingly important as more and more highly sophisticated instrument and research facilities, such as space-borne atomic clocks (Chan, 2006), long-baseline radio telescope arrays (Cliche & Shillue, 2006), and particle acceleratorbased X-ray pulse sources (Altarelli, 2007), require unprecedented level of synchronization in order to explore the uncharted physical parameter regimes.

A numerical study on instantaneous-phase evolution of femtosecond pulses in an erbium-doped fiber amplifier

Femtosecond pulse amplification using erbium-doped fiber amplifiers (EDFAs) is a critical technique for fiberoptic communications, supercontinuum generation as well as optical frequency comb technology. When designing an ultrafast fiber amplifier, it is necessary to understand the evolution of pulse properties, such as temporal intensity and phase and frequency chirp, during pulse propagation in the gain fiber. While a lot of previous numerical research has carefully studied various pulse property variations of femtosecond pulses inside EDFAs, little work has been seen to map out the instantaneous phase evolution throughout the amplification process with all the relevant dispersion and nonlinear effects considered. Here we present our numerical results on this topic.