# A Single Device for Complete Temporal Characterization of Ultrafast Laser Pulses Across Six Octaves

Antoine Espaignol, Franck Billard, Matthieu Didier, Olivier Faucher, Edouard Hertz, Pierre Bejot

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Antoine Espaignol, Franck Billard, Matthieu Didier, Olivier Faucher, Edouard Hertz, et al.. A Single Device for Complete Temporal Characterization of Ultrafast Laser Pulses Across Six Octaves. 2026. [hal-05477857](https://hal.science/hal-05477857v1)

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# A Single Device for Complete Temporal Characterization of Ultrafast Laser Pulses Across Six Octaves

ANTOINE ESPAIGNOL, FRANCK BILLARD, MATTHIEU DIDIER, OLIVIER FAUCHER, EDOUARD HERTZ, AND PIERRE BEJOT*

Université Bourgogne Europe, CNRS, Laboratoire Interdisciplinaire Carnot de Bourgogne ICB UMR 6303, 21000 Dijon, France

*pierre.bejot@cnrs.fr

**Abstract:** We report the first reconfiguration-free temporal characterization of a femtosecond light source spanning more than six octaves, from the ultraviolet (210–220 nm) to the mid-infrared (15–16 µm), using the recently developed plasma-induced frequency-resolved optical switching (PI-FROSt) technique. A commercial optical parametric amplifier equipped with harmonic-generation and difference-frequency modules is characterized, including the signal, idler, second- and third-harmonic beams, and mid-infrared extensions. All measurements are performed with the same optical setup and switching medium, without any realignment or change of nonlinear elements. These results establish PI-FROSt as a unique, broadband, phase-matching-free, and damage-free diagnostic tool for ultrafast sources.

**Keywords:** Pulse characterization, nonlinear optics, ionization.

## Introduction

Ultrafast lasers are now essential tools in industry, research, and medicine, where their exceptional temporal precision and low thermal load enable accurate and reproducible material processing and measurement. Reliable characterization of these sources is a fundamental requirement to ensure measurement accuracy, operational safety, long-term stability, and to support the development of advanced laser-based systems. In this context, the rapid evolution of femtosecond laser technologies drives the need for increasingly robust and precise metrology instrumentation, capable of characterizing ultrashort pulses over extended temporal and spectral ranges. Among these sources, optical parametric amplifiers (OPAs) are widely used sources of ultrashort laser pulses, offering broad tunability and extremely large
amplification bandwidths. By exploiting phase-matching in a nonlinear crystal, OPAs enable the generation and amplification of femtosecond pulses across the visible and near-infrared spectral ranges. Their high gain, excellent beam quality, and intrinsic compatibility with femtosecond pump lasers make them particularly suited for applications requiring high temporal resolution, such as ultrafast spectroscopy [1–5], strong-field physics [6, 7], and attosecond science [8]. Moreover, by exploiting cascaded nonlinear processes - including second- and third-harmonic generation as well as sum- and difference-frequency generation - commercial OPAs can now deliver pulses with tunable central wavelengths spanning more than six octaves, from the ultraviolet (≈ 200 nm) to the mid-infrared (≈ 16 µm), in a fully automated manner. Despite these remarkable capabilities, manufacturers typically provide only limited information on the wavelength-dependent temporal and spectral properties of the emitted pulses, usually restricted to nominal pulse duration and bandwidth specifications [9]. This paucity of data partly reflects the intrinsic difficulty of characterizing ultrashort pulses over such an exceptionally broad spectral range. To the best of our knowledge, no single instrument has yet demonstrated the ability of temporally characterizing pulses spanning from the ultraviolet to the mid-infrared. Consequently, measuring the temporal profile of pulses spanning the UV–MIR range requires the use of multiple characterization devices, each operating over a limited spectral bandwidth. This approach inherently hampers direct comparison between the resulting measurements, as the devices rely on different experimental geometries. Recently, a phase-matching–free technique was reported for the temporal characterization of pulses spanning from the near infrared (NIR; ≈ 0.77 µm) to the mid-infrared (MIR; 10 µm), establishing a record operational bandwidth of approximately 3.7 octaves [10]. This technique, termed frequency-resolved optical switching (FROSt), exploits the ultrafast modulation of the transient optical transmission of a solid sample induced by a pump pulse. The switching mechanism originates from photoexcited carrier dynamics and operates as a temporal knife-edge, slicing the probe pulse waveform, with the transmitted spectrum subsequently recorded as a function of the pump–probe delay. However, despite its exceptionally broad operational bandwidth, this technique is limited by the transparency window of the bulk material (zinc selenide [10]), preventing the characterization of pulses below 450 nm. More recently, an alternative approach, termed plasma-induced frequency-resolved optical switching (PI-FROSt), was developed to overcome the intrinsic limitations of the original FROSt design [11, 12]. In brief, PI-FROSt exploits the defocusing of the probe pulse through a plasma lens. In this technique, a plasma is generated in a gas via non-resonant multiphoton ionization induced by a moderately intense pump pulse. Because of the bell-shaped profile of the pump beam, the spatial distribution of the plasma acts as a divergent
lens. Consequently, a probe beam traversing this low-density plasma undergoes defocusing, resulting in an increased beam size in the far field. The plasma lens is created almost instantaneously, serving as a temporal switch that diffracts only the portion of the probe pulse arriving after the pump. A coronagraph in the far field isolates the diffracted signal, which is then recorded by a spectrometer as a function of the pump-probe delay, producing a spectrogram that enables comprehensive retrieval of the probe field's temporal and spectral characteristics.

This approach, successfully applied to characterize conventional ultrashort IR and UV femtosecond laser pulses with high fidelity, offers several significant advantages. It is straightforward to implement, supports both self- and cross-referenced configurations, allows in-situ measurements without damage at ultrahigh repetition rate, and provides robust reconstruction through both custom and standard algorithms. Furthermore, it is free from phase-matching constraints and can operate over an exceptionally broad spectral range. For instance, this technique was used to simultaneously characterize a set of five low-order harmonics generated in a ZnO crystal spanning over approximately 2.6 octaves [13]. The present work reports a complete temporal characterization of a commercial OPA (ORPHEUS-HE, Light Conversion), including the signal and idler pulses, the second- and third-harmonics of the signal (SHS-THS), the second harmonic of the idler (SHI), and the difference-frequency (DFG) wavelength extensions. The measurements, performed using the same PI-FROSt setup without any reconfiguration, span approximately 6.2 octaves, from 210 nm to 15 µm, thereby establishing a record in the field. Beyond the technical performance, which highlights the exceptional flexibility of the PI-FROSt method, these measurements will benefit all users seeking an accurate determination of the temporal pulse profile at the output of the ORPHEUS system. More generally, PI-FROSt emerges as a unique powerful tool for the characterization of ultrafast sources in spectral regions that are not readily accessible with commercial diagnostic devices.

## EXPERIMENTAL SETUP

The experimental setup is shown in Fig. 1. The characterized OPA is a commercial ORPHEUS-HE system (Light Conversion). The pump source is a 10 kHz ytterbium-based amplified laser (PHAROS, Light Conversion) delivering 175 fs vertically polarized pulses with an energy of 2 mJ at a central wavelength of 1035 nm. The OPA is pumped with 1.5 mJ, while the remaining 500 µJ are available to other experiments. In the present work, this residual energy is redirected to a motorized delay line and then used as the pump beam for
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Experimental PI-FROSt setup for characterizing a commercial OPA delivering pulses the ultraviolet to the mid-infrared. OAP: off-axis parabolic mirror, HWP: half-waveplate, BD: beam dump, and FS: fused silica.</figcaption></figure>

producing the plasma lens in the PI-FROSt measurements. The OPA configuration used in this work includes several wavelength-extension modules - namely, the third harmonic of the signal (THS), the second harmonic of the signal and of the idler (SHS and SHI), as well as two distinct difference-frequency generation (DFG) modules - enabling continuous and fully automated tuning of the output wavelength from 210 nm to 16 µm. Using a combination of dichroic mirrors and waveplates mounted on motorized, computer-controlled wheels, the THS, SHS, SHI, signal, and idler beams are directed to a common output port, although they cannot be operated simultaneously. In contrast, the DFG modules deliver their output through a separate port. At the OPA output, all beams are vertically polarized. To avoid nonlinear propagation of the OPA beam, only a weak reflection from a fused-silica wedge is used as the probe. The pump beam polarization is set horizontal with a zero-order half waveplate in order to minimize both Kerr and Raman-induced molecular alignment effects. The pump (≈ 250 µJ) and probe beams are combined collinearly and focused at a small angle (≈ 2°) by a UV-enhanced aluminum parabolic mirror with a focal length of f = 15.2 cm, intersecting at the focal point located approximately 50 cm from the OPA output. The use of a UV-enhanced aluminum coating enables operation over the entire spectral range of the OPA. After the interaction in air, the probe beam is collimated by a second UV-enhanced aluminum parabolic mirror (f = 15.2 cm), while the pump beam is blocked by a beam dump. Instead of employing a coronagraph, as in our previous implementations [11–13], an iris diaphragm is placed at the Fourier plane of the collimating parabolic mirror to selectively transmit only the portion of the probe beam diffracted by the plasma generated by the pump.
Depending on the spectral region of interest, flip mirrors are used to direct the signal beam toward one of the three spectrometers employed in this study. The signal is then focused by a third UV-enhanced aluminum parabolic mirror (f = 15.2 cm) onto the entrance slit of the selected spectrometer. For the UV-visible spectral range (200 nm – 1 µm), spectra are recorded using a SR-750-D1 spectrometer (Andor) with a 0.5 m focal length and an 1800 lines/mm grating blazed at 500 nm, coupled to a cooled CCD camera (2048 × 512 pixels, 13.5 µm pixel size). The typical spectral resolution in this range is approximately 70 pm. For the near-infrared region (1–5 µm), a Mozza MIR spectrometer (Fastlite) with a spectral resolution of 5 cm⁻¹ is employed. Finally, mid-infrared spectra are acquired using a f =320 mm iHR320 monochromator (Horiba) equipped with three different gratings (600, 150, and 60 lines/mm, respectively) coupled to a liquid-nitrogen-cooled HgCdTe detector (J15D22-M204-S02M-60, Teledyne Judson Technologies) operating up to 22 µm. As in our previous works [11–13], the spectrum of the diffracted signal is measured as a function of the pump-probe delay τ to produce a spectrogram $S_{\text{exp}}(\omega, \tau)$, which enables comprehensive retrieval of the temporal and spectral characteristics of the probe field.

## RESULTS

Typical experimental (top row) and corresponding retrieved (middle row) PI-FROSt spectrograms are shown in Fig. 2 for probe beam central wavelengths ranging from UV to MIR. While the spectral amplitude $A(\omega)$ is directly retrieved at negative delays (i.e., when the plasma induced by the pump is stationary and diffracts the entire probe pulse), the spectral phase $\phi(\omega)$ is obtained using a standard Levenberg-Marquardt algorithm. This algorithm minimizes the least-squares cost function $|S_{\text{exp}} - \mu S_{\text{theo}}|^2$, where $S_{\text{exp}}$ is the experimental spectrogram, $S_{\text{theo}}$ is the theoretical spectrogram calculated as

$$S_{\text{theo}}(\omega, \tau) = \left| \int \rho(t - \tau) \varepsilon_{\text{OPA}} e^{i\omega t} dt \right|^2, (1)$$

and $\mu = \sum S_{\text{exp}}S_{\text{theo}}/ \sum S_{\text{theo}}^2$ is a normalization factor that minimizes the least-squares error for each evaluation of $S_{\text{theo}}$. In Eq. 1, $\varepsilon_{\text{OPA}}$ is the complex electric-field envelope of the OPA pulse, given by

$$\varepsilon_{\text{OPA}} = \int A(\omega) e^{i[\phi(\omega) - \omega t]} d\omega. (2)$$

The plasma density $\rho(t)$ is modeled as

$$\rho(t) \propto \int_{-\infty}^{t} I^{K}(t') dt', (3)$$
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Examples of experimental (top row) and retrieved (middle row) PI-FROSt spectrogram as a function of the OPA central wavelength, from UV (left) to the MIR (right). The last row depicts the retrieved spectral intensity (solid lines) and phase (dotted lines).</figcaption></figure>

where *I* is the pump-pulse intensity, assumed to be Gaussian with a full-width at half-maximum of 175 fs, and *K* represents the effective nonlinearity of the ionization process. After a few iterations, the algorithm converges, providing access to the OPA spectral phase, as exemplified in the last row of Fig. 2. The temporal envelope is then directly retrieved using Eq. 2. Experimental PI-FROSt spectrograms were analyzed over the full operational range of the OPA, except for spectral regions affected by strong atmospheric absorption, namely the 2.6–3 µm and 5.5–8 µm ranges. In these regions, the PI-FROSt signal clearly shows that the main pulse is followed by multiple relatively intense satellite pulses extending over tens to hundreds of picoseconds, a characteristic signature of optical free-induction decay induced by strong infrared rovibrational absorption lines [14]. Figures 3(a-c) show the OPA beam spectrum as a function of the central wavelength. In the UV-visible-NIR spectral range, most spectra exhibit a pronounced low-frequency shoulder, which may originate from imperfect phase or group-velocity matching during the frequency-conversion process, or from Raman scattering in the β-barium borate (BBO) crystal during the amplification stage [15]. Near the wavelengths corresponding to the second (≈ 515 nm) and third (≈ 340 nm) harmonics of the pump beam, the OPA spectrum exhibits pronounced modulation. In the MIR [beyond 13 µm, see Fig. 2(c)], a clear signature of the vibrational mode centered at 15 µm of CO₂ is observed. The temporal intensity distribution as a function of the output central wavelength of the OPA beam is shown in Fig. 2(d-f). In the UV-visible-NIR spectral range, the main pulse is almost always followed by a post-pulse, which can be attributed to the low-frequency shoulder observed in the spectral domain.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Experimental spectra (top row), retrieved intensity profiles (middle row), and full-width at half-maximum pulse duration (bottom row) as a function of the wavelength of the OPA beam. The gray line in (g) shows the Fourier transform limited full-width at half-maximum pulse duration. The grey area in (g) represents the atmospheric absorption windows.</figcaption></figure>

From measured spectra and retrieved temporal intensity profiles, both the Fourier-transform-limited (FTL) and the actual FWHM pulse durations were extracted [see Fig. 2(g)]. As shown, the measured pulse duration varies from approximately 110 fs to 220 fs across the full spectral range of the OPA output, whereas the Fourier-transform-limited (FTL) duration ranges from 55 fs to 190 fs. Analysis of the retrieved spectral phase reveals that, except in the 1.5–2.2 µm and 10–15 µm spectral ranges - where negative group-velocity dispersion is observed - the OPA output beam exhibits a positive quadratic chirp. As a result, the actual pulse duration ranges from one to three times the Fourier-transform-limited (FTL) value across the full spectral range, underscoring that knowledge of the OPA spectrum alone is insufficient to accurately assess the pulse duration.
Conclusion

In this work, we have demonstrated the complete, reconfiguration-free temporal
characterization of a widely tunable femtosecond light source spanning more than six octaves,
from the deep ultraviolet to the mid-infrared, using the PI-FROSt technique. By relying on an
air-plasma-induced ultrafast optical switch, rather than on nonlinear phase-matching or
material transparency, PI-FROSt overcomes the fundamental limitations that have so far
prevented a single device from accessing such an extreme spectral range. The same
experimental geometry, optical elements, and retrieval procedure were used for all
wavelengths, enabling a consistent and quantitative comparison of pulse properties across the
entire output of the OPA. Beyond its metrological significance, the present study reveals the
complex and strongly wavelength-dependent temporal structure of a modern commercial
OPA. The ability of PI-FROSt to retrieve both the spectral phase and the full temporal profile
over such a broad bandwidth therefore represents a major advance for ultrafast laser
diagnostics. It provides users of broadly tunable sources - such as OPAs, OPCPAs, or
frequency-converted systems - a practical mean to directly assess and optimize pulse quality
at any wavelength, without changing instrumentation. Looking forward, this approach opens
the door to real-time or in-situ characterization of complex ultrafast sources, including high-
harmonic generation, mid-infrared filamentation, and multidimensional spectroscopy setups,
where large spectral spans and high peak powers make conventional diagnostics impractical.
PI-FROSt could thus establish a new paradigm for universal, broadband, and robust
femtosecond pulses metrology.

**Acknowledgments.** The authors would like to thank B. Kibler for fruitful discussions. This work was supported by the CNRS, the EIPHI Graduate School (contract "ANR-17-EURE-0002"), the HICONE contract "ANR-25-CE57-6106"), the Région Bourgogne Franche-Comté, and benefited from the facilities of the SMARTLIGHT platform funded by the Agence Nationale de la Recherche (EQUIPEX+ contract "ANR-21-ESRE-0040").

The authors declare no competing financial interest.

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