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Abstract
In this article we present a directly diode-pumped high-power Kerr-lens mode-locked Yb:CALGO bulk laser oscillator operating at 1-GHz repetition rate. We report on two laser configurations optimized for either highest average power or shortest pulse duration. In the first configuration optimized for high average power, the oscillator delivers up to 6.9 W of average power, which is the highest average power of any ultrafast laser oscillator operating at gigahertz repetition rate. The 93-fs pulses have a peak power of 64 kW, and the optical-to-optical efficiency amounts to 37%. In the second configuration optimized for short pulse duration, we demonstrate 48-fs pulses at 4.1 W of average power corresponding to a higher peak power of 74 kW with 21% optical-to-optical efficiency. This is the shortest pulse duration and the highest peak power demonstrated by any GHz-class Yb-based laser oscillator. The compact laser setup is directly pumped by a low-cost multimode fiber-coupled laser diode and has a high potential as an economical yet powerful source for various applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast laser sources have become a cornerstone technology for numerous industrial applications and fundamental research systems. Low-cost ultrafast laser oscillators operating at repetition rates in the gigahertz range are highly attractive for many areas such as spectroscopy [1,2], medical imaging [3–5], and quantum optics [6–8]. For instance, in frequency-comb-based spectroscopy, higher repetition rates result in a larger comb-mode spacing and a higher power per comb mode, which facilitates the use of individual lines. Repetition rates in the gigahertz range are also beneficial in quantum applications relying on entangled photons. As the generation rate must remain smaller than one pair of entangled photons per driving pulse, gigahertz repetition rates enable higher entangled-photon flux, offering the potential for an improved signal-to-noise ratio [9].
These different applications typically rely on nonlinear processes such as supercontinuum or second harmonic generation which strongly benefit from higher peak power and higher pulse energy. Although tens of kilowatts of peak power are routinely available from MHz-class bulk laser oscillators (see e.g., [10–16]), such performance is more challenging to achieve at gigahertz-level repetition rates as it requires more than a ten-fold increase in the average power or decrease in the pulse duration. As a result, only a few GHz-class laser oscillators, capable of delivering a high peak power combined with high average power, have been demonstrated sofar [ Fig. 1].
Fig. 1. Overview about state-of-the-art GHz-class laser oscillators operating at ≥ 1 GHz of repetition rate and demonstrating (a) > 1 W of average power and (b) directly generating pulses with > 1 kW of peak power. The different laser technologies are distinguished by Kerr-lens mode-locking (KLM) and mode-locking using a saturable absorber mirror (SESAM). Presented results of this work are highlighted as green stars.
A first technology capable of delivering GHz-repetition-rate pulses are fiber-laser systems [17–20]. They provide compactness, high efficiency, and excellent reliability, but typically need an amplification process to achieve several watts of average power, increasing the cost and complexity of the system [21–23]. Additionally, the amplified pulses usually exhibit strong spectral features and a low temporal contrast which ultimately limits the achievable peak power.
Laser oscillators based on a semiconductor gain medium have already provided watts of average power at gigahertz repetition rates directly at the output of the laser cavity. In 2013, a vertical-external-cavity surface-emitting laser (VECSEL) reached up to 3.3 W of average power at 1.7 GHz repetition rate [24]. However, the comparably long pulse duration of 400 fs led to a peak power of only 4.3 kW. Although this technology has the advantage to be spectrally flexible by band-gap engineering, further scaling of the peak power is challenging due to a strong trade-off between shorter pulse duration and higher average power and efficiency [25].
Solid-state laser oscillators have also already demonstrated high average power and ultrashort pulse durations at gigahertz repetition rate. Here, the broad gain bandwidth of Ti:sapphire is beneficial for the generation of extremely short pulses and allows for high peak power. For instance, an octave-spanning optical spectrum supporting sub-6-fs pulses at 1 GHz repetition rate was reported from a Kerr lens mode-locked (KLM) Ti:sapphire laser oscillator in [26]. Up to date, the shortest pulse duration at GHz repetition rate of 12 fs at an average output power of 1.2 W was reported in [27], which results in 88 kW of peak power assuming transform-limited soliton pulses [Fig. 1(b)]. However, although cost-effective and efficient diode-pumping has started to be developed [28–30], standard Ti:sapphire lasers still typically rely on more complex and expensive green pump lasers. Additionally, Ti:sapphire features a high quantum defect which can lead to significant thermal effects and limits the average power. So far, a maximum average power of 1.2 W was achieved at GHz repetition rates [Fig. 1(a)].
The development of ultrafast Yb-based diode-pumped solid-state lasers (Yb-DPSSLs) overcame these limitations. In the last decade, semiconductor saturable absorber mirror (SESAM) mode-locked Yb-DPSSL oscillators achieved both high average and peak power at gigahertz repetition rate [31–36]. One remarkable result has been the development of a SESAM mode-locked Yb:CALGO laser oscillator delivering 4.6 W of average output power in 96-fs pulses at 1.1 GHz repetition rate, leading to a peak power of 38 kW [37].
In comparison, KLM Yb-DPSSL oscillators offer the potential for even higher peak powers at GHz repetition rates. Although KLM laser oscillators usually require a more astute cavity design than SESAM mode-locked ones, they typically achieve shorter pulse durations and do not require special solutions to overcome Q-switched mode-locking instabilities [36]. Very recently, 145-fs pulses with 1.7 W of average power and 4.8 kW of peak power at 2.16 GHz repetition rate and 17% optical-to-optical efficiency were demonstrated by a KLM Yb:KGW laser oscillator [38]. This is the first KLM Yb-based laser oscillator capable of exceeding the watt-level of average power and the kilowatt-level of peak power at a gigahertz repetition rate. However, this source applied soft-aperture Kerr-lens mode-locking and relied on a complex and expensive single-mode fiber-laser pump system.
In this article, we present two configurations of a directly diode-pumped KLM bulk laser oscillator based on the Yb:CALGO gain material delivering high average output power combined with short pulse duration at ∼1 GHz repetition rate. Careful optimization of the cavity components and the mode-locking parameters enabled us to generate 93-fs pulses at 6.9 W of average power and 64 kW of peak power in a first configuration (config. 1) and sub-50-fs pulses at 4.1 W of average power with 74 kW of peak power in a second configuration (config. 2).
2. Experimental setup
The experimental setup is shown in Fig. 2. The laser oscillator is based on a 3-mm-long a-cut anti-reflection (AR) coated Yb(3 at.%):CALGO crystal. Yb:CALGO is a gain material that combines a flat and broadband emission cross-section [39] with low quantum defect allowing for the generation of sub-100-fs pulses at high efficiency and high average power [11,12,14,40]. Additionally, its nonlinear refractive index of ∼9×10−20 m2/W is high enough for direct use as Kerr medium. The crystal is wrapped in indium foil and mounted in a water-cooled copper holder. It is optically pumped at a wavelength of 980 nm by a commercially available 20-W multimode fiber-coupled laser diode (0.15 NA and 105 µm core diameter resulting in a beam quality factor M2 ≈ 25) from BWT Beijing Ltd. The crystal is placed such that its c-axis, which features the highest absorption cross-section [39], is aligned with the p-polarization of the pump. Accordingly, one maximizes the portion of pump power in the p-polarization by bending the pump delivery fiber, which results in a ratio of 80%/20% of p-polarization/s-polarization. The total output power is used to pump the crystal. The central pump wavelength is adjusted to the 980-nm absorption peak of the Yb:CALGO by maintaining the diode at a temperature of 38 °C using a Peltier element. The pump beam is collimated by a 60-mm focal length aspherical lens and subsequently focused into the crystal with another identical lens. In this configuration, the pump beam radius of the focus at the position of the gain crystal was measured to be 65 µm (1/e2) which was experimentally optimized for achieving stable KLM operation at highest power levels and highest optical-to-optical efficiency. We estimate the laser beam radius in the crystal to be around 70 µm in CW and up to 75 µm in KLM operation (1/e2) based on a formalism of ray transfer matrices for Gaussian beams. The laser oscillator is based on a ∼150-mm-long standing-wave cavity composed of 4 mirrors resulting in ∼1 GHz repetition rate. As shown in Fig. 2, the gain medium is pumped through a flat dichroic mirror M1, which is highly reflective (HR) for the laser wavelengths above 1000 nm and highly transmissive for the pump wavelength around 980 nm. The second (M2) and third (M3) mirror are concave with a radius of curvature of -38 mm and -75 mm, respectively. The last mirror is a plane output coupler (OC) with a transmission TOC = 10% for config. 1 and TOC = 5% for config. 2. For achieving a stable KLM operation and a high beam quality, a hard aperture constituted of an uncooled copper plate with a 1.0-mm diameter hole is inserted between M3 and OC where the laser beam is collimated. The phase shift arising from self-phase modulation and the positive group delay dispersion (GDD) in the gain crystal is balanced by the dispersive mirror (DM) M2 which provides -500 fs2 per bounce between 1020 nm and 1080 nm. In config. 2 optimized for shortest pulse duration, an AR-coated 3-mm-thick wedged fused-silica plate introducing a round-trip GDD of ∼100 fs2 is inserted inside the cavity. The dispersive coating on M2 and the AR coating on both faces of the fused-silica plate were designed and grown in our own ion beam sputtering coating facility. The total round-trip GDD introduced by the gain medium, the DM and the fused-silica plate (for config. 2) at the wavelength of 1050 nm is estimated to be -400 fs2 for config. 1 and -300 fs2 for config. 2. The introduced GDD of M1, M2, and M3 per cavity round-trip is shown in Fig. 3(a). Both cavity configurations differ only in the out-coupling rate, the round-trip GDD, and the adjusted pump power.
Fig. 2. Experimental setup of the laser oscillator for both configurations (config. 1 and config. 2). The inset shows the beam profile of the laser output in mode-locked operation for config. 2. L1, L2: lenses; M1-M4: cavity mirrors, HA: 1-mm diameter hard aperture; FS: AR-coated 3-mm thick fused-silica plate used in config. 2; DM: dispersive mirror; OC: output coupler with the transmission (TOC) for each configuration; AC: autocorrelator; SO: sampling oscilloscope; OSA: optical spectrum analyser; RF: radio-frequency analyser. Cavity lengths for config. 1 are indicated by the black arrows and labelled respectively. In config. 2, the length Yb:CALGO-M2 was adjusted accordingly to compensate for the added fused silica plate. The repetition rate is adjusted to ∼1 GHz by freely tuning the distance M3-M4.
Fig. 3. (a) Measured optical spectrum (blue) with sech2 fit (red) for soliton pulses and round-trip group delay dispersion (GDD) introduced by the mirrors M1, M2 and M3 (black) for both configurations. (b), (e) Intensity autocorrelation trace (blue) of the pulse and its sech2 fit (red). (c), (f) Radio-frequency (RF) spectrum of the laser fundamental repetition rate frep ≈ 1 GHz measured with 300-Hz resolution bandwidth (RBW). Inset: RF spectrum of the higher repetition rate harmonics measured with 10-kHz RBW. (d), (g) Sampling oscilloscope trace (the ringing in the signal trace at 0.5 ns is attributed to the electronics of the detection setup). λc: central laser wavelength; Δλ: full width at half maximum spectral bandwidth.
3. Experimental results
The system is operated in two configurations optimized for either highest average power or shortest pulse duration. Operation in the KLM regime is initiated by gently pushing the output coupler which is mounted on a translation stage. The experimental results for each configuration are discussed below and summarized in Table 1.
Table 1. Laser parameters of the KLM Yb:CALGO laser oscillator generating pulses at 1 GHz repetition rate for config. 1 (highest average power) and config. 2 (shortest pulse duration).a
In config. 1 optimized for the highest average power, the oscillator is operated with a 10% output coupling rate and the total round-trip GDD is approximately -400 fs2. Using 18.5 W of pump power, the oscillator generates 93-fs pulses [Fig. 3(b)] at 6.9 W of average output power and 1.0 GHz repetition rate. This results in an optical-to-optical efficiency of 37% with respect to the pump power, a peak power of 64 kW, and 6.8 nJ of pulse energy. The optical spectrum is centered at 1045 nm and has a full width at half maximum (FWHM) bandwidth of 12.5 nm [Fig. 3(a)]. The least-square-based fit for sech2-pulses drawn on top of the optical spectrum (sech2 in frequency domain) and the autocorrelation trace show a very good agreement with the expected soliton regime. Additionally, the resulting time-bandwidth product of 0.320 indicates the generation of transform-limited soliton pulses. The short-term stability of the mode-locked regime is proven by the radio-frequency spectrum [Fig. 3(c)]. The single pulse regime is confirmed by observing the pulse train using a 40 GHz sampling oscilloscope with an 18.5-ps rise time photodetector [Fig. 3(d)].
In config. 2 optimized for shortest pulse duration and highest peak power, the output coupling rate was reduced to 5% and the cavity GDD adjusted to approximatively -300 fs2 per cavity round-trip by inserting an AR-coated 3-mm-thick fused-silica plate (dashed component in Fig. 2). Using 19.5 W of pump power, the oscillator generates pulses as short as 48 fs at 4.1 W of average output power. The laser beam profile is a clean TEM00 Gaussian mode [inset of Fig. 2] with a beam quality factor M2 < 1.1. Although the pulse energy decreased to 4.1 nJ, the peak power increased to 74 kW due to the shorter pulse duration, which is the highest peak power so far demonstrated by an Yb-based GHz-class laser oscillator. Similar to config. 1, we measured the optical spectrum, intensity autocorrelation trace, radio-frequency spectrum and confirmed single pulse operation [Fig. 3(a,e-g)]. The FWHM spectral bandwidth increased to 24.3 nm while the central wavelength at 1052 nm shifted by 7 nm towards the longer wavelength side which is expected for an Yb:CALGO laser operating at a lower resonator loss [40]. The sech2-fit of the optical spectrum and the autocorrelation trace indicate a clean soliton mode-locking regime. However, it is worth noting the presence of a slight pedestal and dispersive wave on the blue and red side of the optical spectrum, respectively. In this configuration, the spectral bandwidth expands much larger and the observed features can be attributed to a strong variation of the introduced GDD below 1010 nm and above 1100 nm [Fig. 3(a)]. It is expected that these features will limit a further spectral expansion for the efficient generation of even shorter pulses and, respectively, reaching higher peak powers. We expect that shorter pulses and corresponding higher peak powers are within reach by further improving the intracavity GDD for a broader flat spectral coverage and applying for instance the broadband cross-polarized pumping scheme developed in our group [40]. In both configurations, once mode-locking was initiated it remained stable over several hours.
4. Conclusion
We demonstrated a directly diode-pumped KLM Yb:CALGO bulk laser oscillator allowing for high average power (config. 1) and short pulses (config. 2) at GHz repetition rates. Using 18.5 W of diode pump power, we demonstrated 6.9 W of average power in a first configuration. By decreasing the net GDD of the cavity and the output coupler transmission, the oscillator generated pulses as short as 48 fs at 4.1 W in a second configuration. This system demonstrates the shortest pulse duration, highest average power and peak power with respect to previous GHz-class Yb-based laser oscillators.
We believe shorter pulse durations are within reach of the technology as sub-30-fs pulse durations at high average power have already been demonstrated for Yb:CALGO at MHz repetition rates [40]. Improving the dispersion management in the cavity will resolve the current limitation regarding the pulse duration and should allow for a further increase in peak power. The system shows a good long-term stability by operating for several hours without neither interruption of the mode-locking nor considerable degradation of the output performance.
This simple, compact and low-cost Yb-DPSSL oscillator competes with current cutting-edge GHz-class laser sources and is attractive for industrial applications benefitting clean soliton pulses with high peak powers at GHz repetition rates.
Funding
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200020_179146, 200021_188456, 200021_200774); Spark (CRSK-2_190593); R'Equip (206021_144970, 206021_198176).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Ref. [41]
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