Nonlinear wave mixing in optical microresonators offers a route to chip-level optical frequency combs with many promising applications. The properties of the combs generated depend crucially on the interaction between nonlinearity and dispersion. This paper will discuss our research on Kerr comb generation in silicon nitride chip-scale microresonators, with an emphasis on distinct features observed in the normal and anomalous dispersion regimes. The topics covered include comb initiation, comb coherence and mode-locking, power conversion efficiency, and second-harmonic involved comb generation.
High resolution spectroscopy is the foundation for many of the most challenging and productive
of all astronomical observations. A highly precise, repeatable and stable wavelength calibration
is especially essential for long term RV observations. The two wavelength references
in wide use for visible wavelengths, iodine absorption cells and thorium/argon lamps, each
have fundamental limitations which restrict their ultimate utility.
We are exploring the possibility of adapting emerging laser frequency comb technology in development
at the National Institute of Standards and Technology in Boulder, Colorado, to the
needs of high resolution, high stability astronomical spectroscopy. This technology has the
potential to extend the two current wavelength standards both in terms of spectral coverage and
in terms of long term precision, ultimately enabling better than 10 cm/s astronomical radial
Femtosecond pulse shaping for generating nearly arbitrarily shaped ultrafast optical pulses is now a well-established technology and has been widely adopted for applications ranging from high-speed communications to coherent laser control of chemical reactions. Arbitrary waveform generation (AWG) capabilities for millimeter-wave, microwave and THz electromagnetic signals, however, are quite limited. Commercial radio frequency AWG instrumentation is currently limited to ~2 GHz bandwidth. In this talk we review work at Purdue in which shaped optical pulses are used to drive an optical-to-electrical (O/E) converter. This leverages our femtosecond optical AWG technology to achieve cycle-by-cycle synthesis of arbitrary voltage waveforms in the range between a few GHz and ~1 THz. Such capabilities could open new possibilities for applications in areas such as wireless communications, electronic countermeasures, sensing, and pulsed radar.
Recently our work has focused on the range from GHz to tens of GHz. A particular focus has been on the generation of signals appropriate for ultrawideband (UWB) wireless communications using "monocycle" pulses with very large fractional bandwidths. UWB technology provides high immunity to multipath interference, low probability of intercept, and high spatial resolution (for position location). Potential defense applications include tactical sensor networks and RFIF tags for inventory control. Our experiments demonstrate the ability to generate programmable monocycles with spectra that can be tailored to match emission limits and with durations and bandwidths that improve on the mainstream electronic technology. Additional potential applications include predistortion of transmit waveforms in order to precompensate distortions associated with broadband antennas and waveform optimization for enhanced target discrimination in pulsed radar.
Speckle intensity correlations are used to study polarized coherent
light propagating through scattering media. In particular, using measured speckle patterns as a function of frequency, second and third order intensity correlations with frequency are formed and then employed to determine the co-polarized and cross-polarized temporal impulse responses. The polarized impulse response provides information on the scattering medium that could aid in characterization. Determination of the temporal response from intensity only data is especially convenient in the optical domain.
A new micromachining technique using user-defined trains of amplified femtosecond laser pulses is described. In this method, a 2-fold Michelson interferometer is used to split each output pulse of an amplified femtosecond laser system operating at 1 kHz into four different pulses at desired seperations ranging from 1 ps to 1 ns. These quadruple pulses are then focused on metal, semiconductor and dielectric samples and the material removal characteristics are noted. The experimental results show that there is a distinct effect of the pulse separation on the machining characteristics. It is observed that, in some cases, use of the quadruple pulses separated by 1 ns provides better material removal than the original pulses separated by 1 ms. The femtosecond laser-material interaction is also modeled for the case of metal samples using the two-temperature model. Numerical simulations that were carried out show that irradiation with quadruple pulses lead to a reduction in the predicted melting threshold fluence, which agrees with the experimental observation.
We demonstrate a convenient technique for determining the temporal response and scattering parameters of a diffusive medium using laser speckle pattern frequency correlations. Experimental results using an external-cavity tunable laser diode are presented. This approach can be extended to provide data for image reconstruction based on a diffusion model.
Large-aperture biased photoconductive emitters which can generate high-power narrow-band terahertz (THz) radiation are developed. These emitters avoid saturation at high fluence excitation and achieve enhanced peak power spectral density by employing a thick layer of short lifetime low- temperature-grown GaAs (LT-GaAs) photoconductor and multiple-pulse excitation. THz waveforms are calculated from the saturation theory of large-aperture photoconductors, and a comparison is made between the theory and the measurement. A direct comparison of the multiple-phase saturation properties of terahertz emission from semi-insulating GaAs and LT-GaAs emitters with different carrier lifetimes reveals a strong dependence of the multiple pulse saturation properties of terahertz emission on the carrier lifetime. In particular, the data demonstrate that saturation is avoided only when the interpulse spacing is longer than the carrier lifetime.
We discuss Fourier optics methods for ultrafast optical pulse shaping, waveform synthesis, and signal processing, which are achieved by spatial manipulation of the dispersed optical frequency spectra of femtosecond pulses.
We present the bit-error-rate (BER) analysis of an ultrashort light pulse code-division multiple access (CDMA) communication system, which employs a practical nonlinear detector. The system considered here assumes a CDMA network, in which M user stations are connected via a star coupler and a common fiber channel. The transmitter at each station consists of a mode locked femtosecond fiber laser at 1550 nm whose ultrashort pulses are encoded by a spectral phase encoder and uses an on- off-keying scheme. The receiver consists of a decoder, a nonlinear detector, and an integrate and dump data circuit. In this work, we develop an efficient Monte Carlo simulation using importance sampling scheme to estimate the error probabilities of the system in addition to the analytical technique. The simulation results are found to be in good agreement with the analytical results.
Considerable interest has arisen recently in the prospect of using specially crafted ultrashort laser pulses for coherent control of atomic and molecular systems. The aim in these studies is to utilize control over femtosecond optical waveforms as a tool to manipulate constructive and destructive interferences associated with quantum mechanical wave packets motions, which could ultimately lead to optical manipulation of chemical reactions and in the shorter term should make possible preparation of well-defined quantum mechancial states for precise spectroscopic determination of molecular Hamiltonians. At Purdue we have initiated a project aimed at applying these coherent control concepts in a new setting--namely, in specially designed layered semiconductor materials. A key motivation for using layered semiconductors as a coherent control laboratory is the ability to engineer the Hamiltonian through the epitaxial growth process, so that one may have better knowledge of and control over the Hamiltonian than one has in studies of complex molecules. Here we review the femtosecond pulse shaping technology crucial for our coherent control studies and discuss our plans and progress in applying pulse shaping to manipulate coherent charge oscillations in double coupled quantum wells and superlattices in the GaAs/GaAlAs material system.
We present a comprehensive report of time-resolved reflectivity and transmission measurements of the superconducting system Pr<SUB>x</SUB>Y<SUB>1-x</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ). For a fixed photon energy (1.98 eV), varying the Pr content permits optical probing of energy states both above and below the Fermi level where a superconducting gap is expected to occur. We find qualitatively different behavior in the sign, magnitude, and temporal responses as a function of Pr fraction. Our results indicate that a simple two-fluid model interpretation cannot account for the observed response and that the intrinsic band structure of Pr<SUB>x</SUB>Y<SUB>1-x</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) plays a significant role in the dynamics of these systems.
Timed sequences of femtosecond pulses produced by pulse-shaping techniques
have been used to achieve improved optical control over molecular motion in crystalline
solids. Selected lattice vibrational modes in an organic molecular crystal have been driven
repetitively by appropriately timed pulse sequences in a manner analogous to that in which
a child on a swing is pushed repetitively with timed mechanical forces. Repetitive driving
with a pulse sequence results in larger lattice vibrational amplitudes and improved modeselectivity
compared to driving with a single pulse. Numerous applications of pulseshaping
techniques in femtosecond spectroscopy are anticipated.