The temporal intensity contrast of the short-pulse laser oscillator pulse that seeds a high-intensity laser system sets a baseline noise level for amplification to terawatt to petawatt peak powers that can be focused onto targets at optical intensities approaching ${10}^{22}\mathrm{W}/{\mathrm{cm}}^2$. The femtosecond to nanosecond dynamics of laser–matter experiments using these sources are crucially altered by any optical intensity noise above $\sim {10}^9-{10}^{10}\mathrm{W}/{\mathrm{cm}}^2$ that can ionize matter and form a plasma that rapidly expands away from the target before the arrival of the primary laser pulse [1,2].

In addition to a high-contrast output, sources that are suitable for seeding high-energy, high-peak-power laser systems are typically $<500\mathrm{fs}$ in duration and their emission bandwidth is typically selected to be compatible with large-aperture amplifiers, for example, Nd:glass for 1054 nm amplification, to achieve terawatt to petawatt peak powers.

Previous investigations into the temporal contrast of short-pulse oscillators by Braun et al. compared Kerr lens (KLM) and saturable absorber mode-locked sources using a high-dynamic-range second-order autocorrelator [3]. They observed that cavities employing saturable absorbers exhibit strong suppression of amplified spontaneous emission (ASE) and other weak temporal noise features to generate $>{10}^8$ contrast pulses and that KLM sources produced considerably greater optical noise, with pedestals observed at the ${10}^{-5}$ intensity level.

With recent advances in laser technology, particularly with compact turn-key fiber-based ytterbium lasers [4,5] and femtosecond-pumped optical parametric oscillators (OPOs) [6,7], a broader and more detailed comparison of these laser sources is now needed. Here, we present an investigation of the temporal intensity contrast across a wide range of mode-locking schemes used in laser oscillator sources suitable, and currently used, for seeding high-power laser systems in the 1 μm region, underpinning a wide range of high-intensity laser–matter interaction experiments. These new measurements have used a third-order autocorrelator that provides observations of asymmetric temporal processes with up to ${10}^{10}$ dynamic range for the $\sim 1–15\mathrm{nJ}$, $<350\mathrm{fs}$, 70–80 MHz pulse trains emitted from these sources. To the best of our knowledge, this is the first time such a comprehensive comparative survey of short-pulse mode-locked oscillator contrast has been undertaken using third-order autocorrelation measurements and is also the first time the temporal contrast of fiber-based and OPO sources suitable for seeding high-energy laser systems have been measured and compared directly to saturable absorber and KLM oscillators. We have used the same diagnostic system in all of these studies to facilitate a direct comparison of their contrast performance, but a detailed theoretical treatment of these systems is beyond the scope of this work.

To measure the temporal intensity envelope of the low-power nanojoule laser pulses emitted from a range of laser oscillator sources, a scanning third-order autocorrelator diagnostic (TOAD) was developed (Fig. 1). This was based upon the design described by Luan et al. [8]. Focused nonlinear interactions maximize the second-harmonic generation (SHG) of the probe from a 1.5 mm thick $\beta$-barium borate (BBO, type I) crystal and sum-frequency third-harmonic generation (THG) signal from a 1 mm long BBO (type II) sum-frequency crystal. After SHG, the fundamental and second-harmonic pulses were recollimated with an achromatic doublet and then spatially separated with a dichroic mirror. A variable delay on the subsequent fundamental wavelength beam path produces temporal scanning of the pulses. The two parallel beams enter either side of an achromatic doublet lens and focus into the sum-frequency nonlinear crystal to generate the 351 nm third-harmonic autocorrelation signal. A noncollinear phase-matching geometry permits spatial separation of the emerging beams and background-free measurements. The third harmonic was detected with a high-gain, low-noise photomultiplier (ET Enterprises 9124B), and a set of twelve calibrated density filters were used to maintain the third-harmonic signal within the linearity range of the photomultiplier. A custom low-noise instrumentation preamplifier with $\sim 100\mathrm{\mu s}$ integration time was used to increase the dynamic range by integrating over many hundreds of autocorrelation events of the 70–80 MHz pulse trains measured.

 

Fig. 1. Optical layout of the TOAD for high-dynamic-range characterization of laser pulses. L1 and L2, focusing and recollimation optics used to maximize the intensity onto an SHG crystal, a 1.5 mm thick BBO (type I); BS1, a wedged dichroic beam splitter separates the fundamental and second-harmonic pulses; TS1, translation stage for temporal scanning; L3, an achromatic doublet lens used to focus the fundamental and second-harmonic beamlets and maximize the intensity onto the THG crystal, a 1 mm thick BBO (type II); A1 and A2, apertures used to spatially filter THG light from the residual fundamental and second harmonic; UVD, calibrated UV density filters; UVP, UV colored glass bandpass filter (UG11).

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The TOAD was found to have a temporal resolution of $\sim 200\mathrm{fs}$, limited by the nonlinear crystal lengths, where there is a trade-off between the phase-matched bandwidth versus conversion efficiency that is required in order to achieve up to ${10}^{10}$ dynamic range with $\sim 10\mathrm{nJ}$ energy in the fundamental input pulse. Accurate measurement of the pulse duration below this limit is more routinely achieved with low-dynamic-range second-order autocorrelation techniques with thin crystals to permit higher resolution optical characterization.

The TOAD was aligned to each laser source with the minimum number of beam delivery optics to avoid introduction of secondary pulses. The dynamic range was primarily limited by the available pulse energy from each source. Each contrast trace was fitted against a squared hyperbolic secant or Gaussian pulse to highlight deviations of the temporal intensity from these ideal cases. In comparison with discrete prepulses, which are device specific and can be $>10\mathrm{ps}$ ahead of the primary pulse, we present only the close-in noise pedestal features surrounding the primary pulse, which are more indicative of the general mode-locking characteristics to be expected across all laser sources using these schemes. High-contrast amplification techniques such as picosecond pumping in optical parametric chirped pulse amplification (OPCPA) will further improve the contrast against long-range noise sources [9–11].

Semiconductor saturable absorber mirrors (SESAMs) are widely used for mode-locking Yb-doped Ti:sapphire and Nd:glass lasers. The contrast of SESAM mode-locked Nd:glass lasers was previously found to exhibit $>{10}^8$ contrast [3]. Our contrast measurements of a SESAM mode-locked Nd:glass laser emitting 3 nJ, 310 fs pulses (Time-Bandwidth GLX200HP [12]) are presented in Fig. 2. We observed $>2\times {10}^9$ contrast, limited by the noise floor of the TOAD diagnostic.

 

Fig. 2. Measured third-order autocorrelation of $\sim 3\mathrm{nJ}$ pulses emitted from a SESAM mode-locked Nd:glass oscillator (Time-Bandwidth GLX200). Red dashes: an ideal 400 fs squared hyperbolic secant profile, which equates to a 310 fs FWHM pulse duration after deconvolution. DL, detection limit of the TOAD, which is $\sim 2\times {10}^{-9}$ in this measurement.

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The fast, few hundred femtosecond, impulse response combined with modest $<1\%$ modulation depth in the absorber effectively suppresses ASE [12,13]. The relatively slower recovery of available absorbers, typically tens of picoseconds [14], has no measureable effect on contrast. This is potentially a result of the primary pulse extracting much of the available gain in the stable mode of operation and suppressing ASE immediately behind the pulse before the population inversion can recover. Laser pulses as short as 60 fs have been obtained with this technique [15], and 40 fs pulses have been produced from Nd:glass SESAM oscillators with use of mixed gain media [16]. Such a source, in combination with bandwidth broadening amplification from optimization of OPCPA [17], offers an attractive high-contrast seed that can potentially produce $<20\mathrm{fs}$ equivalent amplified pulse bandwidths. The useful self-starting properties of saturable absorbers have also been used in conjunction with Kerr-lens mode-locked cavities [18].

The optical Kerr effect is most notably harnessed for mode-locking of Ti:sapphire oscillators, frequently achieving $<20\mathrm{fs}$ pulse durations at 800 nm and $<120\mathrm{fs}$ for 1 μm operation. These laser sources offer an attractive route to quickly achieve ultrafast amplified pulses. However, previous investigations by Braun et al. [3] and Jung et al. [19], using second-order autocorrelators, found short-range noise pedestals in these systems, which they attributed to a combination of self-phase modulation, the temporal dependence of the Kerr effect, and uncompensated third-order dispersion (TOD).

We have similarly characterized a range of KLM oscillators that lase at 1054 nm central wavelength and are suitable for seeding high-energy Nd:glass amplifier systems. These sources output $\sim 120\mathrm{fs}$, 5 nJ pulses. Figures 3(a) and 3(b) show the contrast performance of nominally identical commercial models of laser oscillators (Spectra Physics Tsunami), however, with very different noise pedestal features that both start at an $\sim {10}^{-6}$ intensity level. A large asymmetric prepulse foot from one of these systems is seen in Fig. 3(a), which is indicative of negative net TOD or higher odd-order dispersion possibly in combination with self-phase modulation. In Fig. 3(c), a similar KLM Ti:sapphire laser design (Coherent MIRA) was characterized and showed similar pedestal noise features starting between ${10}^{-6}–{10}^{-7}$ relative intensity. Similar features have also been seen in a 100 fs KLM Yb:KGW oscillator (Light Conversion Flint), indicating these features are most likely manifested from the KLM operation rather than being a characteristic of the Ti:sapphire gain media, but variabilities in the pedestal length and time direction between devices are possible.

 

Fig. 3. Measured third-order autocorrelations of KLM laser sources. (a),(b) An AOM initiated KLM Ti:sapphire (Spectra Physics Tsunami). (c) A KLM Ti:sapphire (Coherent MIRA). All KLM sources were found to exhibit noise pedestals at $\sim {10}^{-6}$ relative intensity. Red dashes: all sources are fitted against a 260 fs Gaussian intensity profile, corresponding to $\sim 210\mathrm{fs}$ FWHM duration after deconvolution, the TOAD resolution limit. DL, detection limit of the TOAD.

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Recent technical advances have led to compact, robust mode-locked fiber laser sources with $>10\mathrm{nJ}$ outputs using a normal dispersion scheme and external compression [20]. However, long glass propagation lengths can lead to a significant accumulation of higher-order phase terms, particularly positive third-order phase. Additionally, as NPE depends upon the optical Kerr effect to produce elliptical rotation, strong self-phase modulation effects are likely to occur, producing spectral modulations that may then translate to temporal modulations after dispersion.

We have characterized the contrast of a short-pulse Yb-doped fiber laser producing 50 fs pulses that are externally compressed with a prism pair (Coherent Fidelity 2). The results in Fig. 4 show the clear presence of a postpulse foot starting at $\sim {10}^{-5}$ relative intensity with a decaying oscillatory modulation indicative of positive higher odd-order dispersion, also likely in combination with self-phase modulation. We observed a $>3\times {10}^8$ prepulse contrast, limited by the noise floor of the TOAD diagnostic. However, nonlinear mechanisms that act to convert the large postpulse noise intensity into prepulse noise will need to be controlled to maintain this low level of prepulse [21].

 

Fig. 4. Measured third-order autocorrelation of the NPE mode-locked Yb-doped fiber laser (Coherent Fidelity 2). Red dashes: the $\sim 210\mathrm{fs}$ Gaussian FWHM TOAD resolution limit. DL, detection limit of the TOAD, which is $\sim 3\times {10}^{-8}$ in this measurement.

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Significant advances have also been made in the area of OPOs. These are external cavities with a broadly phase-matched difference frequency nonlinear crystal pumped by a mode-locked laser. The signal and idler pulse durations are closely linked to the pump duration driving the nonlinear amplification. The use of short crystal lengths and all-reflective designs with minimal dispersion control can achieve sub-50 fs pulse durations with broad wavelength tunability when pumped with ultrafast Ti:sapphire lasers [6].

In Fig. 5, we present the contrast characterization of a multiwatt Yb-fiber laser-pumped OPO that produces 12 nJ, 120 fs pulses at 1054 nm wavelength (Spectra Physics Insight DS + [7]). The TOAD was able to measure the pulse temporal intensities with $2\times {10}^{10}$ dynamic range.

 

Fig. 5. Measured third-order autocorrelation of a Yb-fiber laser-pumped optical parametric oscillator (Spectra Physics Insight DS +). Red dashes: the $\sim 210\mathrm{fs}$ Gaussian FWHM TOAD resolution limit. DL, detection limit of the TOAD, which is $\sim 2\times {10}^{-10}$ in this measurement.

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Short-lived prepulse features at $-1.5\mathrm{ps}$, ${10}^{-8}$ relative intensity and $-3\mathrm{ps}$, ${10}^{-9}$ relative intensity were observed, which are likely either residual negative odd-order phase features or discrete prepulses generated by thin optics in the cavity. The contrast performance of the OPO significantly improves upon that of the KLM and NPE lasers. The wide tunability of an OPO, high output powers, and comparable $<150\mathrm{fs}$ pulse durations, in addition to the now-confirmed high-contrast output, offers an attractive option for seeding advanced high-power lasers.

In summary, we have experimentally characterized the temporal intensity contrast of SESAM, KLM, NPE, and OPO mode-locked laser oscillators that are suitable for seeding amplifier chains that operate in the 1 μm region with a high-dynamic-range third-order autocorrelator. We found SESAM mode-locked laser sources produced the most ideal pulses, with no noise pedestal detected within the $2\times {10}^9$ dynamic range of the measurements, limited by the optical intensities from these sources without additional amplification. Suspected unwanted nonlinearities and/or higher-order phase produce symmetric and asymmetric noise features at $\sim {10}^{-5}$ and ${10}^{-6}$ relative intensity in NPE and KLM lasers, respectively, which both use the optical Kerr effect as their primary mode-locking mechanism. This level of optical noise may become contrast-limiting sources for short-pulse, high-power, high-contrast laser systems. A potentially manageable prepulse at the ${10}^{-8}$ level was observed from an OPO source, which also offers broader wavelength tunability in comparison with other mode-locked sources.