Fangxin Li

Complete temporal pulse characterization based on phase reconstruction using optical ultrafast differentiation (PROUD).

A simple and general technique for recovering the phase profile of a given optical waveform from temporal intensity measurements is introduced and experimentally demonstrated. The proposed method involves the measurement of the temporal intensity profiles at the input and output of a linear optical time differentiator. The signal phase profile can be unambiguously recovered from these intensity measurements using a direct and noniterative algorithm. Given that ultrafast optical differentiators can be readily implemented in all-fiber or free-space platforms, the proposed technique could be applied over time waveforms with durations ranging from the subpicosecond to the nanosecond regime.

Characterization and optimization of optical pulse differentiation using spectral interferometry

A simple fiber-based spectral interferometry setup is implemented for characterizing and monitoring the amplitude and phase of ultrafast temporal waveforms generated by optical differentiation with a long-period fiber grating (LPFG). In particular, the system is applied to characterize subpicosecond odd-symmetry Hermite-Gaussian (HG) pulses, consisting of two pi phase-shifted temporal lobes, obtained by temporal differentiation of Gaussian-like pulses. This technique is ideally suited for optimizing the experiment conditions (e.g., wavelength shifting between the input pulse and LPFG transmission characteristic) so as to achieve a nearly ideal odd-symmetry HG temporal waveform (with a sharp discrete pi phase shift at its center), of potential interest as a higher order soliton in dispersion-managed optical communication systems

Linear Characterization of Optical Pulses With Durations Ranging From the Picosecond to the Nanosecond Regime Using Ultrafast Photonic Differentiation

In this paper, we extend the recently introduced linear technique for temporal phase reconstruction using optical ultrafast differentiation (PROUD) to achieve full characterization of ultrashort optical pulses with durations down to the picosecond regime using a well-characterized temporal stretcher (e.g., dispersive optical fiber). The proposed method is experimentally demonstrated by precisely characterizing the amplitude and phase temporal profiles of microwatt-power picosecond pulses ranging from 4 to 20 ps with both continuous and discrete temporal phase variations. Using this simple mechanism, the same PROUD setup can be used to characterize optical pulses with durations ranging from the picosecond to the nanosecond regime. We provide a comprehensive mathematical analysis of this general PROUD technique: we evaluate in detail the influence of the key specifications (e.g., different sources of noise) of the used components and instruments, namely, optical differentiator, linear temporal stretcher, and time-domain intensity test equipment, on the performance of the PROUD measurement system, particularly in terms of phase sensitivity in the optical pulse characterization.

Synchronized Generation of Reconfigurable Microwave Sinusoidal Wave Packets From a Free-Running Pulsed Laser

Generation of customized microwave sinusoidal wave packets precisely synchronized with a free-running pulsed laser is required for many electrooptic modulation-based optical pulse processing and measurement applications. We propose and experimentally demonstrate a simple technique for this purpose. This technique is based on optical frequency-to-time conversion of a spectral interference in a fiber-optic two-arm interferometer with a wavelength-dithering feedback control. The same platform can be used to operate over a wide range of input repetition rates and can be optically reconfigured for tuning the frequency, amplitude, and phase of the generated microwave sinusoids.

Precise and simple group delay measurement of dispersive devices based on ultrafast optical differentiation

We propose and demonstrate a very simple technique for accurately measuring the group delay of dispersive devices. It only requires an optical differentiator (uniform long-period fiber grating), a pulsed laser source and a high-speed oscilloscope.

Pulse Phase Reconstruction using Optical Ultrafast Differentiation

We introduce a simple, linear technique based on all-optical temporal differentiation for recovering the phase profile of optical waveforms from intensity measurements. We demonstrate characterization of low-power complex pulses in the sub-picosecond to nanosecond range.

Full characterization of picosecond pulses by phase reconstruction using optical ultrafast differentiation (PROUD)

The recently introduced PROUD technique for ultrashort pulse characterization is extended down to the picosecond regime using a well-characterized temporal stretcher (e.g. dispersive optical fiber). The proposed method is demonstrated by precisely characterizing optical pulses ranging from 4~20-ps with microwatts average powers..

Generation and Full Characterization of High-Repetition Rate Odd-Symmetry Hermite-Gaussian Pulses

We experimentally demonstrate the generation of a continous sequence of picosecond odd-symmetry Hermite-Gaussian (HG) temporal pulses at 10 GHz repetition rate. The HG waveforms are fully characterised using an improved Fourier transform spectral interferometry technique

Simplified time-lens based system configuration for transform-limited real-time Fourier transformation of optical pulses

We demonstrate that a time lens (quadratic phase temporal modulator) followed by a dispersive device can be used to implement real-time Fourier transformation (RTFT) of temporal optical pulses without introducing any additional temporal phase distortion. In this so-called transform-limited RTFT operation, the time and frequency domains are fully interchanged from the input to the output of the device; in other words, the temporal waveform of the pulse at the output of the device is a replica of the input energy spectrum and at the same time, the output energy spectrum is proportional to the temporal intensity shape of the input signal. As compared with the conventional methods, the proposed configuration does not require the use of an input dispersive device preceding the time lens, thus resulting in a much simpler and more practical alternative for implementing transform-limited RTFT of optical signals. Transform-limited RTFT has enormous application in optical signal processing especially for reconfigurable, ultra-fast pulse filtering in the all-optical domain. Ultrafast optical pulse filtering enables other important optical pulse processing operations, such as all-optical temporal correlations or convolutions. We propose and analyze a novel ultra-fast optical pulse filtering design based on the above-simplified configuration for transform-limited RTFT. In this proposed filtering configuration, the time lens process is implemented using a phase electro-optic modulator driven by a RF tone. Our proposal results in a much more compact and practical design than the conventional 4-f ultra-fast optical pulse filtering system. We further carried out the analytical study of the proposed filtering system and demonstrated its simplicity and feasibility.