Janne Hyyti

Ultrahigh precision nonlinear reflectivity measurement system for saturable absorber mirrors with self-referenced fluence characterization

Measurement of nonlinear optical reflectivity of saturable absorber devices is discussed. A setup is described that enables absolute accuracy of reflectivity measurements better than 0.3%. A repeatability within 0.02% is shown for saturable absorbers with few-percent modulation depth. The setup incorporates an in situ knife-edge characterization of beam diameters, making absolute reflectivity estimations and determination of saturation fluences significantly more reliable. Additionally, several measures are discussed to substantially improve the reliability of the reflectivity measurements. At its core, the scheme exploits the limits of state-of-the-art digital lock-in technology but also greatly benefits from a fiber-based master-oscillator power-amplifier source, the use of an integrating sphere, and simultaneous comparison with a linear reflectivity standard.

Pulse retrieval algorithm for interferometric frequency-resolved optical gating based on differential evolution

A novel algorithm for the ultrashort laser pulse characterization method of interferometric frequency-resolved optical gating (iFROG) is presented. Based on a genetic method, namely, differential evolution, the algorithm can exploit all available information of an iFROG measurement to retrieve the complex electric field of a pulse. The retrieval is subjected to a series of numerical tests to prove the robustness of the algorithm against experimental artifacts and noise. These tests show that the integrated error-correction mechanisms of the iFROG method can be successfully used to remove the effect from timing errors and spectrally varying efficiency in the detection. Moreover, the accuracy and noise resilience of the new algorithm are shown to outperform retrieval based on the generalized projections algorithm, which is widely used as the standard method in FROG retrieval. The differential evolution algorithm is further validated with experimental data, measured with unamplified three-cycle pulses from a mode-locked Ti:sapphire laser. Additionally introducing group delay dispersion in the beam path, the retrieval results show excellent agreement with independent measurements with a commercial pulse measurement device based on spectral phase interferometry for direct electric-field retrieval. Further experimental tests with strongly attenuated pulses indicate resilience of differential-evolution-based retrieval against massive measurement noise.

Improved phase retrieval for dispersion scan

Ultrashort pulse generation has come of age, along with several well established methods that allow characterization of femtosecond pulses in amplitude and phase. While all these methods have been demonstrated to work well for pulses with >10 fs pulse duration, the reliable measurement of few-cycle pulses still poses a challenge that requires sophisticated adaptation of these existing techniques. In the sub-2-cycle regime, the recently devised dispersion-scan (d-scan) technique [1] has recently attracted much interest since it relies on single-beam geometry, is easy to implement, robust against adjustment errors, and can be completely immune against phase-matching limitations [2]. Formally, it is similar to frequency-resolved optical gating (FROG), yet with the noted difference of scanning the added group delay dispersion in the beam path rather than the delay between two replicas of the pulse under test. Spectrally resolving the pulses after nonlinear conversion then gives rise to two-dimensional d-scan traces, from which the pulse shape can be reconstructed by a retrieval algorithm. To this end, the Nelder-Mead (NM) algorithm has been nearly exclusively employed. While NM is a very flexible algorithm and does not require gradients, it turns out to be very slow for complex pulse shapes that require a large number of points, i.e., a large dimension for the retrieval algorithm, for which one often observes local stagnation, slow convergence speed, and the build-up of erroneous spectral phase oscillations. To overcome this limiting problem of the otherwise very promising d-scan technique, we investigated a number of possible solutions, including regularization, generalized projections (GP), and differential evolution (DE), and combinations thereof. Here we demonstrate that the last method can overcome all above-mentioned problems of d-scan retrieval and is remarkably resilient against detection noise. Furthermore, we demonstrate the possibility of retrieving both phase and amplitude of the test pulse.

Third-harmonic interferometric frequency-resolved optical gating

The ability to generate third-harmonic light with a broad phase-matching bandwidth at any dielectric interface offers a promising tool for the characterization of ultrafast laser pulses. Third-harmonic, interferometric frequency-resolved optical gating (TH-iFROG) exploits this nonlinearity. The TH-iFROG traces contain an abundance of information, but their complicated analytical form poses a problem for standard FROG algorithms. Here we solve the pulse retrieval problem with an evolutionary algorithm known as differential evolution. The algorithm processes a TH-iFROG trace, extracting three novel types of FROG traces that are subsequently used for pulse retrieval. The analytical forms of these FROG traces are also presented. Built-in error correction mechanisms and the large amount of redundant data make the pulse retrieval robust against measurement noise and systematic errors. The technique is demonstrated via the characterization of unamplified few-cycle pulses using a simple fused silica window as the nonlinear medium. Comparison to a reference measurement made with a commercial pulse characterization device shows excellent agreement between the measured complex electric fields.

Non-instantaneity of χ (3) nonlinear optical effects

We present direct experimental evidence for a non-instantaneous nonlinear response in TiO2. An asymmetry in interferometric FROG measurements indicates a relaxation time constant of about 5 fs.