Research Accomplishments (with various collaborators)

Pulse Shaping

• Invention of Fourier synthesis techniques for generating arbitrarily shaped femtosecond pulses (1988). Subsequent introduction of liquid crystal modulator arrays leading to the first demonstration of computer programmable femtosecond pulse shaping (1990-92). Programmable pulse shaping has been adopted in laboratories world-wide for applications including fiber optics, coherent control of photochemical reactions, and optical pulse generation at the few cycle limit.
• First femtosecond operation of a direct space-to-time pulse (DST) shaper suitable for ultrafast parallel-to-serial conversion applications; first elucidation of the chirp behavior of the DST pulse shaper (1999-2001).
• First demonstration of a novel time domain functionality of integrated optic arrayed waveguide gratings useful for generation of femtosecond pulse sequences at rates in the hundreds of GHz to THz range (2001-2004).
• Demonstration of techniques for frequency conversion of shaped femtosecond pulses to shorter (blue) and longer (mid-infrared) wavelengths (2000-04).
• First demonstration of programmable line-by-line pulse shaping in which single spectral lines from a mode-locked laser are manipulated individually; first recognition of time-dependent pulse shape noise arising from frequency fluctuations of spectral lines (2005). This work links the field of pulse shaping to the field of frequency-stabilized mode-locked lasers.
• First demonstration of pulse shaping applied to combs generated via nonlinear wave mixing in microresonators, resulting in pulse compression to the bandwidth limit and identification of coherent as well as partially coherent modes of operation (2011-2012, with NIST).

Applications of Pulse Shaping

• First application of shaped femtosecond pulse sequences to enhance laser control over molecular motion (1990). Subsequent application of femtosecond pulse sequences to manipulate and enhance terahertz radiation emitted from coherent charge oscillations in semiconductor multiple quantum wells (1993-94) and from ultrafast photoconductive antennas (1996-99).
• Demonstration of encoding and decoding of femtosecond optical pulses. Proposal and analysis of this technique as the basis for an ultra-high-speed optical code-division multiple-access (O-CDMA) communications network (1988-90). First systems demonstrations of coherent O-CDMA (1998). Experimental demonstration of error-free operation with up to 4 users at 10 Gb/s using low power nonlinear processing via second harmonic generation in periodically poled lithium niobate (PPLN) waveguides (2004-05).
• First experimental observation of the fundamental dark soliton in optical fibers (1988).
• Application of programmable pulse shaping for spectral phase equalization of chromatic dispersion for femtosecond pulses in fibers, resulting in distortionless transmission of sub-500-fs pulses over 50 km of fiber (1998-2005).
• First generalization of pulse shaping for realization of programmable compensation for all-order polarization-mode dispersion (PMD), a complex transmission impairment of significant concern for high-speed fiber communications, and invention of wavelength-parallel polarization sensor for characterization of same (2004-05).
• Application of optical pulse shaping technology to cycle-by-cycle synthesis of arbitrarily shaped, ultrabroadband burst electrical signals in the GHz to tens of GHz range, surpassing the capabilities of purely electrical arbitrary waveform generator technology (2002-2005). First application of radio-frequency arbitrary waveform generation for precompensation of phase distortion in ultrawide band antenna systems (2005-2008).
• First demonstration of silicon photonic pulse shaping to realize dynamic RF generation (2008-2010).
• Application of hyperfine-resolution pulse shaping for radio-frequency photonic filtering, including demonstration of nearly arbitrary programmable phase filters over 20 GHz RF bandwidth and their application to ultrabroadband RF matched filtering and pulse compression (2006-09).
• Application of pulse shaping to quantum optics: shaping of the time correlations between time-frequency entangled Photon pairs (biphotons). Generalization of the nonlocal disperson compensation concept to higher order polynomial phase and demonstration on the femtosecond time scale. Demonstration of orthogonal spectral phase coding of biphotons. Proposal and experimental realization of a new concept for tunable biphoton delay via frequency shifting the spontaneouls parametric down conferation pump laser (2013-2015).

Other Contributions

• Experimental demonstration of holographic storage, recall, and processing of shaped femtosecond waveforms (1992).
• Collaborated on experiments demonstrating time-to-space conversion of femtosecond signals using dynamic spectral holography (1994). Further experiments demonstrating the first time-to-space conversion based on second harmonic generation spectral nonlinear optics at telecommunications wavelengths and demonstrating orders of magnitude sensitivity improvements (1998).
• First observation of nondiffracting optical spatial soliton beams in a nonlinear solid-state waveguide (1990-91).
• Demonstration of 100-femtosecond switching, at that time the fastest switching time yet, by an all-optical fiber device (1988-89).
• First elucidation of the effect of phase matching bandwidth on ultrashort pulse measurements based on second harmonic generation (1983). First application of bandwidth-engineered aperiodically poled lithium niobate waveguides for femtosecond pulse measurements, leading to autocorrelation and FROG pulse measurements at unprecedented (~1 nW) low power levels (2004-05).
• Experiments and theory elucidating the relation between time-domain statistics of photon propagation in thick scattering media and frequency-domain correlation properties of resulting laser speckle (1997-2004).
• Simultaneous independent first demonstration of photoemissive sampling technique for contactless testing of ultra-high-speed electronics (1986-87).
• Fiber and grating pulse compression for generation of optical pulses as short as 16 femtosecond {at that time, the shortest ever reported} (1984).
• Invention of novel three-pulse transient grating technique for measurement of femtosecond dephasing in condensed matter (1984-85).
• Theory elucidating behavior of virtually-imaged phased-array (VIPA) spectral dispersers and demonstration of their application to two-dimensional spectral dispersion, hyperfine resolution pulse shaping, dispersion and PMD compensation, and RF photonic filtering (2004-2010).
• Development of characterization techniques for optical arbitrary waveforms capable of line-by-line spectral resolution and single-shot waveform capture. Application to interferometric characterization of dispersion of fibers up to 50 km [~3-4 orders of magnitude longer than previously demonstrated using interferometric techniques] (2009-2010).
• Demonstration of electro-optic modulation scheme capable of generating high-repetition-rate (10 GHz) optical frequency combs with unprecedented flatness (2010).
• Cavity filtering of mode-locked frequency combs, including application to enhanced generation of low phase noise, 10 GHz RF tones (2007-2009, with NIST).
• Invention of novel RF photonics filters based on optical frequency combs, with demonstrations of very high stopband suppression, rapid tunability, and pulse compression functionality (2010-2012).
• Demonstration of high contrast optical diode action on a silicon photonic chip (2011-2012).
• Contributions to the development and understanding of microresonator frequency combs, including introduction of the drop port geometry (suppresses noise background, provides direct access to the intracavity waveform), microheater tuning, and observation of novel mode-locked dark pulse states in normal dispersion microresonators. Convincing demonstrations that comb generation can take place in normal dispersion microresonators (previously anomalous dispersion was widely believed necessary) and identification of mode interations as the mechanism responsible for comb generation under normal dispersion conditions (2012-2016).
• Studies of ultrabroadband radio-frequency propagation over highly scattering (multipath) indoor wireless channels, including high accuracy measurements of the channel response over ca. 10 GHz bandwidths, precompensation of the channel response via waveform design, and measurement of the resultant temporal and spatial focusing. Introduction of a phase compensation technique for channel compensation with substantially reduced temporal sidelobes compared to the more common time reversal technique (2011-2016).
• Investigation of frequency bin entanglement in quantum optics. Demonstration of two- and three-dimensional frequency bin of photon pairs generated from on-chip microring resonators via spontaneous four wave mixing. Collaboration on experimental realization of two- and three-dimensional single photon gates for frequency encoded photons.