Pulse Characterization Method Research Articles

Simple and highly sensitive optical pulse-characterization method based on electro-optic spectral signal differentiation

A very simple self-referenced, linear pulse-characterization technique based on spectral phase reconstruction by frequency-domain signal differentiation is introduced. This technique can be implemented using electro-optic intensity modulation of the pulse under test with a synchronized RF sinusoid. The pulse spectral phase profile can be accurately and unambiguously reconstructed from only two measured energy spectra, i.e., at the input and at the output of the modulator, using a direct analytic equation. The method is experimentally demonstrated by precisely characterizing microwatt-power picosecond pulses after linear dispersion through short sections (50-700 m) of conventional single-mode fiber.

Direct measurement of spectral phase for ultrashort laser pulses

We present an intuitive pulse characterization method that provides an accurate and direct measurement of the spectral phase of ultrashort laser pulses. The method requires the successive imposition of a set of quadratic spectral phase functions on the pulses while recording the corresponding nonlinear spectra. The second-derivative of the unknown spectral phase can be directly visualized and extracted from the experimental 2D contour plot, without any inversion algorithm or mathematical manipulation.

Structures of interferometric frequency-resolved optical gating

We report on a novel method for the characterization of complex ultrashort pulses, in both amplitude and phase. This method is the straightforward extension of interferometric autocorrelation (IAC) toward frequency-resolved optical gating (FROG) and therefore displays all advantages of a collinear method for measuring few-cycle pulses. The interferometric FROG (IFROG) trace contains two independent time-frequency distributions, namely the standard second-harmonic (SH)-FROG trace and a novel non-semidefinite FROG trace, which requires new retrieval algorithms. We discuss one suitable retrieval concept that is based on an adaption of generalized projections and demonstrate it with different types of short laser pulses. The IFROG trace contains additional information that can be used for intrinsic calibration of the FROG traces. The strong redundancy of data increases the robustness of the method. IFROG is a pulse characterization method that is particularly suited for the complex sub-10-fs pulse shapes as they can arise during supercontinuum compression and gives detailed insight into the satellite structures of partly compressed continua.

Measurement of the polarization dynamics of ultrashort pulses by using nonlinear phase modulation and channelled spectroscopic polarimetry

Recently we presented a vectorial pulse characterization method based on the spectral polarimetric analysis of femtosecond laser pulses before and after propagation through a Kerr medium. In this work we present an improvement of the method. We propose to carry out the spectral polarimetric analysis by using the technique named spectroscopic polarimetry with a channelled spectrum which has been developed by Oka and Kato (1999 Opt. Lett. 24 1475). This combination supposes an important improvement of the data acquisition process both in velocity and accuracy.

Time-frequency signal processing of terahertz pulses.

We demonstrate that some signal-processing techniques, such as the fractional Fourier transform and the spectrogram, which are known to be effective for optical signals, can be implemented at terahertz frequencies and with available terahertz devices. These techniques, contrary to the frequency-resolved optical-gating method for pulse characterization, do not require nonlinear media, which do not exist at terahertz frequencies. Thus the fractional Fourier transform or the spectrogram offers the only possibility of characterizing terahertz pulses simultaneously in time and frequency.

Frequency-resolved pump–probe characterization of femtosecond infrared pulses

A novel method for ultrashort IR pulse characterization is presented. The technique utilizes a frequency-resolved pump-probe geometry that is common in applications of ultrafast spectroscopy, without any modifications of the setup. The experimental demonstration of the method was carried out to characterize 70-fs IR pulses centered at 3 microm .

Generation and characterization of polarization-shaped femtosecond laser pulses

We demonstrate the generation and complete characterization of femtosecond laser pulses which change their intensity, frequency, and light polarization almost arbitrarily within a single pulse employing the new technique of femtosecond polarization pulse shaping. Specifically, the degree of polarization ellipticity as well as the orientation of the elliptical principal axes can be varied as a function of time in a completely controllable manner, using a 128-pixel, two-layer, liquid-crystal display (LCD) inside a zero-dispersion compressor. A mathematical formalism is presented with which polarization-shaped pulse parameters can be calculated and used to generate intuitive quasi-three-dimensional electric field representations. However, laboratory realization requires accurate and complete experimental pulse characterization methods. For this purpose, the technique of dual-channel spectral interferometry is employed. Furthermore, Jones calculus for polarization-shaped pulses with experimentally determined Jones matrices is developed. It can be used to predict and account for all pulse-shape modifications occurring in various optical elements of the pulse shaper and the experimental setup.

Precision and consistency criteria in spectral phase interferometry for direct electric-field reconstruction

We define and study precision and consistency criteria for spectral phase interferometry for direct electric-field reconstruction (SPIDER), an interferometric technique for the characterization of ultrashort optical pulses. Precision quantifies the similarity of different estimates of the electric field reconstructed from a single experimental data set. Consistency is a measure of the match between the reconstructed field and the data. These powerful experimental criteria allow one to monitor the fidelity of the reconstruction of the electric field from the experimental data by tracking systematic errors and random noise. Because of such imperfections, such criteria are necessary even for pulse characterization methods based on a direct algebraic inversion. In the case of SPIDER, the redundancy of data in each interferogram and the principle of shearing interferometry allow simple calculation of the precision and consistency measures.

New directions in sub-10-fs optical pulse generation

Sub-10-fs pulse generation has reached a maturity and robustness that gave rise to a rapid increase of new ultrafast applications. Some of these novel applications rely on the octave-spanning spectral width, the coherence properties, or the extremely high peak intensity of femtosecond lasers, which sets them apart from the traditional use of femtosecond lasers in ultrafast spectroscopy. In this review, we concentrate on the sub-10-fs regime and first discuss the state of the art in ultrashort pulse generation. We then report on ultrabroadband wavelength conversion schemes and dispersion control by chirped mirrors. We also review advances in pulse characterization methods, with which we can measure pulse properties with much higher accuracy than simple autocorrelation methods and which support even a bandwidth in the single-cycle regime. With the recent demonstrations of pulses with a duration of merely two optical cycles, a regime is entered where the relative phase between the carrier and the envelope of the pulse starts to play a role. We discuss methods to measure and control the carrier–envelope phase and point out applications in frequency metrology. A wealth of further novel directions and developments in femtosecond research is reviewed in this article.

Fast-frequency-hopping modulation and detection demonstration

We demonstrate fast-frequency-hopping modulation that exploits the unique features of acousto-optic-modulator based laser pulse shaping and the spectrally and temporally resolved upconversion technique (STRUT) pulse-characterization method. These pulses have been specifically designed so that they can be characterized by the STRUT, without any processing of the STRUT data set. We present examples of complex fast-frequency-hopping laser pulses that have been generated and characterized by our pulse-shaping system and STRUT. We discuss the theoretical limitations on the data rate that can be obtained with such a technique.

Feedback-controlled femtosecond pulse shaping

We describe the experimental implementation of feedback-optimized femtosecond laser pulse shaping. A frequency-domain phase shaper is combined with different pulse characterization methods and appropriate optimization algorithms to compensate for any phase deviation. In particular, bandwidth-limited, amplified laser pulses are achieved by maximizing the second-harmonic generation (SHG) of the shaped laser pulses with the aid of an evolutionary algorithm. Real-time measurement of the absolute phases is achieved with spectral interferometry where the reference pulse is characterized by FROG, the so-called TADPOLE method. Using the complete electric field as feedback, arbitrary laser pulse shapes can be optimally generated in two different ways. First, a local convergence algorithm can be used to apply reliable and accurate spectral chirps. Second, an evolutionary algorithm can be employed to reach specific temporal profiles.

Acousto-optic method for ultrasonic pulse characterization

The light distribution in a diffraction pattern produced by a sequence of ultrasonic pulses depends on a number of parameters characterizing the pulse shape, modulation and sequencing as well as changes in the spectral composition due to non-linearities in the propagation medium. Based on calculations of such light diffraction patterns it is possible to consider the inverse problem, i.e. the determination of parameters mentioned above by evaluating certain features of the diffraction pattern produced by the pulses. A number of examples are given to illustrate these possibilities.