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Abstract
An intense white light (WL) continuum from 1600 to 2400 nm is generated in a 20-mm-long YAG irradiated by 1-ps, 1030-nm pulses. Long filamentation formed in the YAG is proven to be responsible for the enhancement of the longer-wavelength spectral part of the WL. The WL is compressed down to 24.6 fs ( 3.9 cycles at 1900 nm) after optical parametric chirped-pulse amplification in a lithium niobate crystal near degeneracy, confirming that its spectral phase is well behaved. The pulse compression experiment reveals that the group delay introduced in the WL generation process is dominated by the dispersion of YAG.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Generation of white light (WL) in bulk dielectric, which is alternatively termed as supercontinuum generation, is the fundamental and indispensable step to obtain an extremely broad coherent continuum in the ultraviolet, visible (VIS), and/or infrared (IR) regions [1]. In the shorter-wavelength part of the WL, VIS [2] and Near-IR [3] pulses have been demonstrated to be compressible down to a few-cycle regime.
Generation of a WL continuum using femtosecond laser pulses has been extensively studied [4] and triggered various applications in ultrafast laser technology such as seed pulse generation in optical parametric amplification (OPA) [5] or optical parametric chirped-pulse amplification (OPCPA) [6,7] and ultrafast spectroscopy [8]. In an early stage of WL generation experiments, most of them rely on femtosecond ultrashort laser pulses from Ti:sapphire-based chirped-pulse amplification systems, where a sapphire plate has been frequently chosen in VIS WL generation. About investigation of the longer-wavelength part of the WL, Bradler et al., [9] demonstrated stronger WL generation in the longer-wavelength part of the WL in YAG than in sapphire using femtosecond 800-nm pulses.
Recently emerging diode-pumped Yb-lasers have shown a huge potential in their high power operation, order of magnitude higher than Ti:sapphire lasers. High-repetition-rate pulsed operation is attractive in a large variety of scientific and industrial applications such as high harmonic generation [10,11] and attosecond pulse generation [12] in the soft x-ray, photoelectron photoion coincidence spectroscopy such as cold target recoil-ion momentum spectroscopy [13], multiphoton microscopy, and laser material processing. It is a drawback of Yb-doped laser media that they show much narrower emission spectra than Ti:sapphire, resulting in the generation of a hundreds of femtosecond to picosecond pulses from amplifiers. Generation of a WL continuum using sub-picosecond to picosecond pulses is not an easy task compared to femtosecond pulse irradiation, because the laser intensity required for WL generation becomes close to the damage threshold of bulk material. In addition, WL in bulk dielectric excited by VIS to near-IR pulses tends to show an asymmetrical spectral broadening, ultrawide broadening in the blue-shifted part and modest broadening in the red-shifted part, due to different self steepening between the blue- and red-shifted spectral regions [4]. These factors make the extension of WL toward a long-wavelength side more difficult than toward a short-wavelength side.
The IR extension of WL in YAG using sub-picosecond to picosecond pulses at a wavelength of around 1000 nm has shown a monotonous decay from 1400 to 1700 nm [14], from 1200 to 2000 nm [15], (Note that Ref. [15] claims the extension of WL to 2500 nm, which is very weak in their plot) and from 1060 to 2000 nm [16], all of which employed a 10-mm-long YAG. Reference [17] employed a 15-mm-long YAG and discussed its extension to 2200 nm in the case of a loose focusing condition, however, a detailed description is not given. Pulse compressions of IR-extended WL continua have been demonstrated down to 49 fs from 1500 to 1700 nm [18] and to 197 fs from 1700 to 1800 nm [15], which shows a substantial amount of chirp compared to the transform-limited pulse duration of 53 fs. A cross-correlation method was employed to measure the temporal profile of WL with a 165-fs gating pulse [16], much longer than a pulse duration that the WL can support. Past works [17–20] have utilized an IR WL seed using a 10-mm-YAG to generate mid-IR pulses around 3000 - 4000 nm based on difference frequency generation between a 1000-nm pump and the WL.
In this study, we quantitatively investigate generation of a WL continuum in a long YAG irradiated by 1-ps, 1030-nm pulses and its pulse compression. We observed a significant enhancement of the WL in a wavelength range from 1600 nm to 2400 nm in a 20-mm-long YAG, which is more than 10 times stronger than in a customarily chosen 10-mm-long YAG. The temporal profile of the WL is characterized by second-harmonic generation-based frequency resolved optical gating (SHG-FROG) after OPA in a lithium niobate crystal pumped by picosecond 1030-nm pulses near degeneracy. The WL is compressed down to 24.6 fs, which is close to the transform-limited pulse duration of 21.5 fs and corresponds to 3.9 cycles at 1900 nm.
2. WL generation and characterization methods
A schematic of our white-light generation experiment is depicted in Fig. 1.
Fig. 1. A schematic of WL generation and characterization. Pulse diagnostics includes pulse energy calibration by a photodiode and spectrum measurement by an optical multichannel analyzer. HWP, half-wave plate at 1030 nm; TFP, thin film polarizer; YAG, yttrium aluminum garnet crystal.
To generate a WL continuum in YAG, we use a partial output (pulse energy: 80 µJ, pulse duration: 1.0 ps, center wavelength: 1030 nm, repetition rate: 3 kHz) of an Yb:YAG thin disk regenerative amplifier, which is a modified version of that used in a previous work [21]. The output beam profile is a near Gaussian (M$^2$<1.3) with a diameter of 7.1 mm (full width at half maximum in intensity). The beam size and energy of the input pulses are controlled by an iris and a variable attenuator (a half-wave plate and a thin film polarizer), respectively. It has been known that the generation of WL is quite sensitive to a focusing condition [22]. After the attenuator, the input pulses are focused into 10- and 20-mm-long YAG crystals with a plano-convex spherical lens of $f$ = 300 mm with anti-reflection coating at 1030 nm. The focused position is set at 3-mm downstream from the input surface of the YAG crystals. The actual beam focus position in the YAG crystal is unknown because the focus position is measured in the air and the YAG crystals are slided into the beam path afterwards by a translation stage. A generated WL continuum is collimated by a CaF$_2$ lens without anti-reflection coating and sent to pulse diagnostics. The Fresnel reflection loss in the CaF$_2$ lens is accounted for when the spectral energy density of the WL continuum is calibrated. In the diagnostics, we switch back and forth a pulse energy measurement and a spectral measurement using a flipper. We measure the spectral energy density of the WL around 1550 nm by using an interference filter at 1550 nm (FB1550-12, Thorlabs) followed by a large-area InGaAs photodiode (DET20C2, Thorlabs). The wavelength-dependent responsivity of the optical multichannel analyzer (NIR-QUEST 512-2.5, Ocean Optics) is calibrated by measuring black body radiation (2960 K) from a tungsten halogen light source (HL-2000, Ocean Optics). The calibrated spectrum is normalized to the spectral energy density of the WL at 1550 nm. To focus on the longer-wavelength part of the WL, a long-pass filter (FELH1300, Thorlabs) is inserted to reject the strong 1030-nm component.
3. Experimental results
The spectral energy density of a WL continuum is measured with different focusing conditions (Fig. 2(a)) and input energies (Fig. 2(b)).
Fig. 2. Spectra of WL continua with different focusing conditions (a) and input energies (b) obtained with the 20-mm-long YAG. As a reference, the black dashed curve in (a) plots the best WL spectrum obtained with a 10-mm-long YAG crystal. (c) Side views of filamentation formed in the 20-mm-long YAG crystal with three input pulse energies and a common $f$-number of 60. After determining the focus position in the air as indicated by the white dotted line, we slide the YAG into the laser beam path. Note that the contrast of the three photographs is changed with the same amount to clarify the onset of filaments.
In the case of the 10-mm long YAG, the extension of WL toward a long-wavelength side has been achieved in a limited range of experimental conditions such as a YAG position, a focusing condition, and an input energy. Because of this limitation, we plot only one spectrum best in its intensity around 2000 nm (black dashed curve in Fig. 2(a)) with an $f$-number of 75 (an iris diameter of 4 mm) and a pulse energy of 14 µJ, which corresponds to a laser power of 14 MW equal to 10 times the critical power of YAG (1.4 MW at 1030 nm) [4,15].
Using the 20-mm-long YAG, in contrast to the 10-mm-long YAG, the longer-wavelength part of the WL continuum is generated in a wide range of experimental conditions with various input pulse energies from 6 to 9 µJ ($f$/60) and $f$-numbers from 43 to 60. Figure 2(a) shows dependence of the WL spectra on an $f$-number of the input beam with an identical pulse energy of 7 µJ.
An $f$-number of less than 37.5 is found to be too tight to enhance the IR WL. This fact is consistent with past works [17,22,23]. In a wide range of an $f$-number from 42.9 to 60.0, the WL continuum from 1600 nm to 2400 nm is drastically enhanced compared to the case with the 10-mm-long YAG. With an $f$-number of more than 75.0, we have not observed the generation of the IR WL around 2000 nm with a pulse energy of 7 µJ. We note that, with an $f$-number of more than 75.0, it is possible to generate the WL by increasing the input pulse energy. However, the WL with an increased input pulse energy tends to produce spatial multiple filaments. Therefore, we do not perform a systematic measurement.
Secondly, we measure dependence of the WL spectra on the input pulse energy with a fixed $f$-number of 60. The results are summarized in Fig. 2(b). It is clearly shown that an increase in the pulse energy results in an enhancement of the WL spectrum around 1800 nm. The interference pattern observed in the black curve around 1400 nm appears coincidentally with the onset of a spatial multiple filament. The pulse energy of the spectral components at wavelengths longer than 1600 nm amounts to 5.2 nJ with the 7-µJ input pulse energy, corresponding to a conversion efficiency of 0.074 %. This level of pulse energy, comparable to a typical pulse energy from an oscillator, is sufficient for seeding successive amplifiers without suffering from amplified spontaneous emission in a laser amplifier or superfluorescence in OPA.
Figure 2(c) presents side views of filamentation formed in the 20-mm-long YAG crystal taken by a digital camera. In this measurements, we varies an input pulse energy with a fixed $f$-number of 60. The white dotted line in Fig. 2(c) indicates the focus position measured in the air. In the photographs, it is clear that the filamentation lasts close to the exit surface of the 20-mm-long YAG, indicating the necessity of a long YAG crystal. These photographs clearly present the onset of two filaments formed in the propagation direction. Taking the spectrum into consideration, the second filament is closely related to the enhancement of the IR part of the WL. A robust and damage free operation is ensured in a long YAG crystal because the laser intensities are substantially reduced on both input and output surfaces. It should be noted that, in the 20-mm-long YAG, we have not observed any visible optical damage with the input pulse energy up to 21 µJ.
4. Pulse compression and characterization of the WL
It is not obvious that a WL generated in long bulk media using picosecond input pulses is compressible with a shot-to-shot pulse energy fluctuation of 1.8%. Pulse-energy instability may cause shot-to-shot spectral phase fluctuation that cannot be corrected in practice. As an example, it has been demonstrated that a WL produced in a long microstructure fiber using femtosecond Ti:sapphire oscillator pulses is practically not compressible due to the fluctuation of spectral coherence extremely sensitive to the input pulse energy [24,25]. We note that, in the VIS region, the blue shifted part of the WL was compressed down to 12.7 fs using 3-ps pulse irradiating a 130-mm-long YAG rod [7].
We characterize the spectral phase of the WL generated in the 20-mm-long YAG by a second-harmonic generation-based frequency resolved optical gating (SHG-FROG) apparatus [26]. The WL is parametrically amplified in a lithium niobate (LN) crystal pumped by 1030-nm picosecond pulses prior to a FROG measurement. This is because the nano-joule-level WL pulses are too weak to measure their dispersion directly with the 3-kHz repetition rate. Similar OPA [27] and OPCPA [28] operating around 2000 nm have been demonstrated using seed pulses generated as a difference frequency based on the WL generation.
Figure 3 shows a schematics of the pulse stretch, OPA, and pulse compression. The WL generated in the 20-mm YAG (input energy: 7 µJ, focusing condition: f/60) is stretched to $\sim$ 500 fs (1750 - 2250 nm) in a 5-mm-thick ZnSe window with an anti-reflection coating (WG71050-D, Thorlabs). The WL is amplified to $\sim$20 µJ in a 5% MgO-doped, 3-mm-thick LN crystal (type I phase-matching, $\theta$ = 42.8 deg., anti-reflection coating at 1030 nm and from 1500 nm to 2500 nm) The amplified pulses are compressed by four bounces on a specially designed chirp mirror (Tokai Optical Co. Ltd.), which compensates the dispersion introduced in 1.5-mm-thick ZnSe from1750 nm to 2250 nm.
Fig. 3. A schematic of our OPCPA system. HWP, half-wave plate at 1030 nm; TFP, thin film polarizer; YAG, yttrium aluminum garnet crystal; LN, 5% Mg-doped, 3-mm-thick lithium niobate crystal; CM, chirp mirror to compensate the dispersion introduced in ZnSe; FROG, second-harmonic generation-based frequency-resolved optical gating apparatus .
The compressed pulses are characterized by a home-made SHG-FROG apparatus employing a 100-µm-thick BBO as a SHG crystal (type I phase-matching, $\theta$ = 21 deg., protection-coating from 1000 to 2000 nm, Crylight). The group delay mismatch in the BBO crystal is calculated to be 5 fs in type I interaction: 2000 nm ($o$) + 2000 nm ($o$) $\rightarrow$ 1000 nm ($e$). The FROG results are summarized in Fig. 4. The duration of the compressed pulse (Fig. 4(d)) is retrieved to be 24.6 fs (full width at half maximum in intensity), which is close to the transform-limited pulse duration of 21.5 fs within 14%. The retrieved result of the OPA output spectrum from 1700 to 2250 nm well corresponds to the OPA gain bandwidth obtained in simulations [29,30] as well as in an experiment [30]. As is seen clearly in Fig. 4(c), the residual group delay is well behaved and dominated by the third-order dispersion, which should be compensatable by combining an appropriately designed chirp mirror or chirp mirror pair with a variable dispersion controller such as a wedge pair. In Fig. 4(c), the large deviations of the group delays around 1650 nm and beyond 2250 nm are attributed to the limited bandwidth of the chirp mirror.
Fig. 4. Summary of SHG-FROG results: (a) measured FROG trace, (b) reconstructed FROG trace (the FROG error is 0.58% over 256 $\times$ 256 grids), (c) retrieved spectral intensity (black curve) and group delay (blue dashed curve), and (d) retrieved temporal profile with a pulse duration of 24.6 fs (full width at half maximum in intensity). Note that the temporal width of the transform-limited pulse assuming a flat phase is 21.5 fs (full width at half maximum in intensity).
From the residual group delay of the compressed WL, it is possible to derive the dispersion of the WL after the YAG as shown by the blue curve in Fig. 5. To calculate this group delay, we subtract the group delays of the CaF$_2$ lens, the ZnSe pulse stretcher, the LN OPA crystal, and the four bounces on the chirp mirror. The spectral phase dispersion introduced in an unsaturated OPA process, which has been derived in Ref. [31], is calculated to be below 0.2 fs in a wavelength range from 1700 to 2250 nm in our OPCPA system. Therefore, we neglect this term from the group delay subtraction. The spectral phase dispersion introduced in an unsaturated OPCPA system has been measured to be a fraction of the period of a signal electric field [32].
Fig. 5. Group delay introduced in the WL process with the 20-mm-long YAG crystal derived from the measured group delay of the compressed WL pulses by SHG-FROG and the simulated group delays of the CaF$_{2}$ lens, the ZnSe pulse stretcher, the LN OPA crystal, and the four bounces on the chirp mirror.
The residual group delay is approximated by the dispersion of a 4-mm-long YAG. This approximation is meant to emphasize that the group delay of the WL after exiting the long YAG is smooth enough so that post pulse compression can be performed by compensating a similar dispersion to that of a 4-mm-long YAG. It is obvious that the dispersion from the WL generation in a long material does not have a huge impact, which is consistent with a past work [16] that demonstrated a self steepening effect of the longer-wavelength part of the WL during the nonlinear interaction.
5. Conclusion
We quantitatively investigate the generation of a WL continuum in a long YAG irradiated by 1-ps, 1030-nm pulses and its pulse compression. We observed significant enhancement of the WL in a wavelength range from 1600 to 2400 nm in a 20-mm-long YAG, which is more than 10 times stronger than in a customarily chosen 10-mm-long YAG. The input pulse energy used in the WL generation is five times the critical power of YAG, low enough to ensure a robust operation. The temporal profile of the WL is characterized by SHG-FROG after OPA in a lithium niobate crystal pumped by picosecond 1030-nm pulses near degeneracy. The WL is compressed down to 24.6 fs, which is close to the transform-limited pulse duration of 21.5 fs and corresponds to 3.9 cycles at 1900 nm. The pulse compression of the IR WL down to a few-cycle level has been demonstrated for the first time to the best of our knowledge. The group delay retrieved in the FROG measurement reveals that the output pulses from the YAG are compressed easily thanks to a well-behaved spectral phase dominated by the dispersion of YAG.
This demonstration holds a significant impact in ultrafast laser technology by expanding a spectral range of femtosecond laser pulses for both technical and spectroscopic applications. The WL extension from 1600 to 2400 nm would also lead to a new scheme toward middle-to-far IR as well as THz generation that can be achieved by additional parametric frequency conversion such as difference frequency generation. Femtosecond pulses with a more than nanojoule pulse energy are also beneficial for seeding and synchronizing optical amplifiers. Laser amplifiers in this wavelength region include Tm- and Ho-doped laser media, Cr:ZnSe, and ZnS. This ultrabroad WL is also indispensable for high-power, few-cycle IR pulse generation based on OPA and/or OPCPA.
Funding
Japan Society for the Promotion of Science (19H02623); Precursory Research for Embryonic Science and Technology (JPMJPR2002); Ministry of Education, Culture, Sports, Science and Technology (JPMXS0118068681).
Acknowledgments
This research is supported by QST President’s Strategic Grant (Creative Research).
Disclosures
The authors declare no conflicts of interest.
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