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Abstract
A novel type of Yb:YAG transparent ceramic thin disk laser amplifier is demonstrated, based on fabricating a composite gain element with a paraboloidal interface between gain and no-gain regions and employing side-pumping of the thin disk. The pump-induced gain and the impact of amplified spontaneous emission on the achievable inversion are reported.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Since the original work by Adolf Giesen on thin disk lasers [1], his invention has spurred continuous development toward higher average powers and increasing energy/pulse in short-pulse lasers [2–5]. Such designs have proven capable of continuous wave (CW) or quasi-CW high-power operation due to highly effective thermal management via conduction through a short thickness of the gain medium and across a high surface area of contact with the heat sink. There is growing interest in operating thin disk lasers in a pulsed amplifier mode [4,5], where the intent is to store the maximal energy in a thin disk amplifier followed by extraction. However, under these conditions the pumping process will encounter competition from amplified spontaneous emission (ASE) at higher energies [6]. The basic processes in solid state amplifiers are depicted in Fig. 1, where the gain element is pumped to inversion while spontaneous emission occurs, and then a pulse is injected that generates the amplified output via stimulated emission in the thin direction of the disk. However, amplified spontaneous emission (ASE) parasitically “de-pumps” the amplifier by way of “unwanted” stimulated emission during pumping, depleting the gain in the transverse (i.e., longer) dimension. Previous work has shown that some degree of ASE mitigation is enabled via the introduction of an undoped “cap” into the gain element, allowing some ASE-generated photons to escape the gain region; this results in a composite thin disk structure with enhanced energy-scaling capabilities [6–8]. Although prior work has demonstrated the capabilities of composite thin disk laser elements [3,6–8], control of the gain profile has only been achievable via the properties of the pump spot due to the limited control of the geometry of the element afforded by the diffusion bonding fabrication method employed in previous work [7,8]. The present work describes an alternative pathway to tailoring the gain profile by way of additive manufacturing, while also enabling side-pumping in thin disk laser gain media. Tailored gain profiles have previously been explored for other gain medium geometries such as rods and waveguides [9,10].
Fig. 1. Basic processes of pumping, gain, and ASE in amplifying laser media. All three of these processes can occur simultaneously although they are separated for clarity.
This work is the first report, to the authors’ knowledge, of a ceramic thin disk with an inherent radial gain gradient, present by virtue of the structure of the thin disk itself rather than by the properties of the pump spot, fabricated via additive manufacturing methods. The concept is portrayed in Fig. 2 below, where side-pumping is employed rather than the usual multi-pass pumping via the face of the disk. The gain region shown in green is a paraboloidal shape that induces the aforementioned gain gradient via increasing absorption as the diode pump light travels inward. This geometry inherently provides mode control by way of the gain profile created with the embedded doped structure. The use of side-pumping should simplify the usual multi-pass pump architecture of thin disk lasers while sustaining a centrally-located gain profile, although the exact shape of the Yb-doped region could be optimized to support the desired extraction beam profile. At this initial stage, this work provides a straightforward proof-of-principle by deploying a simple parabolic profile. As discussed in the next section, the parabolically-shaped profile can be generated in the ceramic green body via the shape of the concave “punch” in a press or by programming the parabolic shape into the printer software for a material jetting method of additive manufacturing. In the present work, the equation of the parabola was chosen such that the maximum thickness of the resulting Yb-doped dome in the finished thin disk was 500-900 µm in the center, decreasing to zero at the edge of the disk.
The following sections describe the means by which this laser optic was fabricated (Section 2), followed by a description of a series of laser experiments that indicate promising performance is likely possible with additional work (Section 3). Finally, the obtained results are interpreted in terms of the interplay between the pump-induced gain and ASE (Section 4).
2. Fabrication of the ceramic thin disk gain element
10 at% Yb:YAG and 16 at% Lu:YAG nanopowders were obtained from Nanocerox Inc. (Ann Arbor, MI). The Lu dopant was included in the region without Yb doping to match the refractive index change induced by Yb [11], thus avoiding a discontinuity in refractive index at the interface between the gain and clad (undoped cap) regions. The dome-like geometry of the Yb:YAG|Lu:YAG interface was created via either use of a (inverse) dome-shaped graphite punch or by material jet printing on a pressed powder bed substrate. These two fabrication routes are illustrated in Fig. 3(a) and Fig. 3(b), respectively.
Fig. 3. Schematics of the (a) shaped punch and (b) inkjet printing routes used to fabricate the thin disk laser gain element.
The “punch method” is conceptually straightforward. First, a concave punch was fabricated with a parabolic shape as detailed in Section 1. The die was then filled with Yb:YAG and compressed to yield a paraboloidal shape. Next, Lu:YAG powder was added, and the structure was compressed again, this time with a flat punch. At this point, the part was processed using the usual ceramics fabrication procedures of calcining, hot-pressing, vacuum-sintering and use of hot isostatic pressing, as discussed below.
Material jetting (Fig. 3(b)) was conducted with a Nordson EFD E4V material jetting printer (Westlake, OH). The ink consisted of the 10% Yb:YAG powder, deionized water, 0.3 wt% TEOS, and 300 MW PEG, which were mixed and sonicated before use. The profile of the Yb-doped/Lu-doped interface was designed to be roughly parabolic for inkjet printing. A lower limit of approximately 20 µm on the per-layer thickness of the prints, in combination with the requirement of a ∼500 µm height from the base of the dome to its peak, placed a resolution limit on the ability of the print profile to match the intended parabolic geometry (Fig. 3(b)). After printing, a flat punch was used to press the printed dome into the substrate, thus inverting its geometry. This was done, rather than printing the dome on a pressed Yb:YAG substrate without inversion, in order to preserve the integrity of the boundary between the two regions, as the printed region tended to exhibit cracking upon drying, so pouring powder onto the print before any pressure application could allow Lu:YAG powder into the structure of the Yb:YAG print. After inversion, spray dried Yb:YAG powder was added on top, and the entire compact was pressed just like in the punch method.
After formation of the green body, each sample was then subjected to cold isostatic pressing at 30 ksi, followed by a burnout step at 1000°C in air, and then a vacuum sintering procedure at 1610°C for one hour. This densified the samples to the point of closed porosity and a subsequent hot isostatic pressing at 1850°C for 4 hours in 29 ksi argon yielded fully dense and transparent ceramics. Samples came out of the HIP with clearly distinguishable doped and undoped regions due to a small percentage of Yb3+ ions having been reduced to Yb2+ under the reducing conditions of the vacuum sinter and HIP, giving the doped region a characteristic green color (Fig. 4). These disk samples were then cut into square plates (Fig. 4) and ground down on the doped side such that the thickness of the doped region reached zero right at their outer edges (Fig. 4) followed by polishing. After the correctness of the final part geometry was verified, an oxygen anneal at 1100°C for 24 hours was employed to re-oxidize all remaining Yb2+ ions to the trivalent state. Finally, highly-reflective (HR) and anti-reflective (AR) coatings were applied to the bottom and top lasing faces for some of the thin disks, respectively, (see Fig. 2) and the finished thin disks were tested for laser gain performance.
Fig. 4. Thin disk ceramic after HIP and cutting, and before grinding to the final geometry (between the dotted lines), annealing in O2, and applying coatings. The Yb-doped region appears green due to a small concentration of Yb2+, which is converted to transparent Yb3+ after the O2 anneal.
The following section presents results from testing conducted on two “punch method” samples and two “inkjet method” samples. In all cases, test results from each of the four samples were highly similar, so each figure presents data from only one sample but is considered representative of the results from all samples.
3. Optical and laser assessments
After the ceramic laser optic was fully processed, ground to shape and polished, an absorption spectrum was obtained, as plotted in Fig. 5. This data was used to determine the thickness of the doped section (determined to be 750 µm for this part for 10% Yb-doping). In addition, it is evident that the absorption (and therefore the anticipated gain level) dropped 50% between the center and ¾ of the way to the edge.
Fig. 5. Absorption spectra of Yb:YAG|Lu:YAG composite structure, showing radial dependence of optical pathlength and Yb2+ “green absorption” bands – which are subsequently annealed away in oxygen after the thin disk is completely fabricated into the desired shape and size.
Two types of laser experiments were performed by: (1) including the thin-disk as the gain element (and HR mirror) for an oscillator, and (2) utilizing a probe beam at 1030 nm to measure the small-signal gain, as pictured in Fig. 6(a) and Fig. 6(b), respectively. All the data was acquired with a pulsed 15-bar diode array from Lumibird (Lannion, France) using 2 msec pulses and capable of 2 kW of peak power.
Fig. 6. Pictures of the laser test setups for gain measurements (a) and oscillator experiments (b). PD = photodiode, BS = beam splitter, M = mirror, TD = thin disk, HR = high reflector, OC = output coupler, LDA = laser diode array (940 nm), LD = laser diode (1.03 µm), PM = power meter, F = long pass filter.
While this laser design is for two-sided pumping, the initial experiments reported here were pumped on one side using the single available 940 nm diode array to demonstrate the basic feasibility. Figure 7(a) shows a schematic for one of the thin disks, which is 1 cm2 in area and 1.4 mm thick with a parabolic doped region 0.94 mm high. The two unpumped small sides received a fine-grind and the pump side was used without an AR-coating. Pumping with the diode array from one side gave the result shown in the inset photograph in Fig. 7(b) of the well-known, weak green emission due to an up-conversion effect centered at 2.5 mm from the pumped edge, which is consistent with the calculated gain profile in Fig. 7(b). (The model here entailed iteratively solving the standard time- and position-dependent equations for the excited state density and pump irradiance, accounting for ground state depletion and gain at the pump wavelength.) The calculated small increase in the “double-pass gain” between 0.8-1.0 cm is due to the parabolically-shaped Yb-doped region at the edge where the ground state absorption path length is decreasing. The scatter level of the gain element was < 3%/cm (at 543 nm), which is low given that the thickness is only 0.14 cm.
Fig. 7. (a) Measured geometry of one of the thin disks, where the grey curve is the demarcation of the Yb-doped region. (b) Observed (inset) and calculated gain profile for pumping on one-side of the thin disk.
The double-pass gain data for diode array amperages ranging from 20 A to 150 A (0.06 J to 3.44 J in a 2 msec pulse) is plotted in Fig. 8(a). As this data employs a probe beam, the reduction in the absorption at 1030 nm is indistinguishable from the actual gain. Here the thin disk was emplaced on an HR mirror with index-matching fluid and no AR-coating, which was found to yield the same results as applying the HR directly on the gain element. It is obvious that the gain becomes saturated at higher amperages, which is interpreted as arising from ASE (as mentioned in the Introduction). The data were numerically fitted with the following formula:
where τEM is the spontaneous emission lifetime, τASE is the effective lifetime that includes the impact of ASE, g(t) is the double-pass small signal gain (so G = eg), and gSS is the steady-state gain that would be achieved in the absence of ASE (i.e., if τASE = τEM). This equation is essentially a description of the Yb3+ excited state population, which is proportional to the small-signal gain, g. As the first term on the right-hand side is the constant pumping term, gss can be freely defined as the steady-state gain that would have been attained were there no ASE – a quantity of interest to calculate. Next, the observed waveforms were fit to the effective lifetime, τASE, which accounts for the shortening of the lifetime due to ASE. The fitted results that empirically account for the ASE are: $ \tau_{A S E}(m \mathrm{sec})=1.2-1.13 g(t)^{2} $. However, to achieve these fits, gSS was also varied as a free parameter. In Fig. 8(b), the fitted value of gSS is plotted against the diode current.Fig. 8. (a) Gain curves (G = eg) and model fits (dashed lines) as a function of time for 2 msec pulses. The fits are based on Eq. (1) where a small thermal deflection at long time was accounted for. (b) Fitted double-pass steady-state small signal gain (gSS) plotted against the diode current.
The gain data of Fig. 8 is well-fit by adjusting the gSS parameter for each waveform while using the same $\mathrm{\tau }ASE(m\sec ) = 1.2 - 1.13g{(t)^2}$ function for all the waveforms. The question arises as to what the curvature of the fitted gSS values versus diode current is due to, as it may have been expected to be a simple linear function. One possibility is that it is due to bleaching of the ground state population. As a test of this possibility, the dashed curve in Fig. 8 follows the usual saturation formula where the intensities are replaced by the diode currents (JDIODE), accounting for the diode threshold (JTHRESH = 18A): [1 + (JDIODE -JTHRESH)/JSAT]−1, resulting in a fitted value of JSAT = 160A. This explanation is plausible since the maximum pump intensity at 150A in the gain element is ∼35 kW/cm2 while the Yb:YAG pump saturation intensity is 29 kW/cm2. Although the essential physics have seemingly been captured via the presented equation fitting, the origin of the small dip in the plateau for the highest amperages cannot be accounted for using Eq. (1). Nevertheless, it is suspected that this dip is caused by ground state depletion, although the side-pumped geometry proved difficult to accurately include in the model.
The results of the oscillator output measurements are presented in Fig. 9. These gave a threshold value of ∼1.0 J for output coupling (O.C.) of 5% and 10%, while increasing to 1.5 J for 25% O.C. A calculation indicates that for 1 J of diode absorption, the double-pass gain is 1.08, which is reasonably consistent with the oscillator threshold observations. Despite this, the slope efficiencies are all rather low, between 5.9% and 8.2%. It is suspected that this is due to the (deduced) 1.4 mm cavity mode being much smaller than the pumped area, which is ∼4 × 4 mm2, due the well-known difficulty in sustaining a cavity mode of several millimeters. Higher efficiencies were sought by trying several mirror curvatures and cavity lengths, as well as experimenting on other thin disks, but without any improvement – which is consistent with the suggested smaller mode size. It was surprising that higher-order modes did not arise to fully extract the pumped region, but visual observation of the beam output was consistent with mainly TEM00 output.
Fig. 9. Oscillator efficiency data for output couplings (OC) between 5% and 25% as annotated on each plot.
To test out the theory of poor overlap between the oscillator mode and the pump spot, an experiment was conducted on a similar thin disk, where the fine-grind side emission for the oscillator was monitored for the oscillator either being aligned to lase or (purposely) misaligned to defeat the lasing, as presented in Fig. 10 using 10% output coupling. Based on the observed reduction of side emission when lasing, it is concluded that only a small fraction of the excited-state volume was extracted, consistent with the cavity mode being smaller than the pumped area. Based on the areas of the lasing and non-lasing decays at 130A, the extraction efficiency is estimated to be 15.7%. While accounting for the quantum defect (91%) and considering the limiting slope from Fig. 9 (i.e., at the point of greatest upward curvature), a value of 11.8% is derived; although admittedly a qualitative procedure, rough agreement is obtained.
Fig. 10. Plot of the rise and decay direct signal voltages at different pump currents, which shows the decrease in the side emission caused by aligning the laser oscillator and thereby extracting a fraction of the energy from the thin disk.
A final experiment that complements the ASE interpretation of pump losses is presented in Fig. 11, which shows the emission originating from the side of the thin disk for 30 to 150 A and for the emission emanating from the large face at 150 A. The emission exhibits some saturation, but not as much as observed for the gain measurements because emission response is averaged over the entire pump region rather than just the peak value where the probe beam is located. Moreover, this measurement is inherently different because it is not simply an assessment of the excited state population since the signal includes the ASE itself. The emission decay nevertheless shortens substantially as pump power becomes stronger, i.e., both rising more quickly and falling more precipitously. Interestingly, the emission from the large face at 150 A showed a somewhat longer decay and slower rise than that from the thin disk side for the same pump power. This suggests that the emission from the large face has a smaller contribution from ASE owing to the short dimension, while the side emission also includes a contribution from the increased amount of ASE light propagating along the full transverse length of the part.
Fig. 11. Normalized side emission for diode pump currents from 30 to 150A, and emission from the large face of the thin disk for 150A.
4. Discussion and conclusion
A method of fabricating a transparent ceramic thin disk with a gain profile by building a structure into the ceramic green body with a parabolic interface between the gain (Yb:YAG) and no-gain (Lu:YAG) regions has been developed, with the ultimate goal of designing an amplifier module. By virtue of this structure, the gain profile can be adjusted toward the center of the thin disk with side-pumping, a significant simplification compared to the usual multi-pass pumping configuration. The presented initial work based on one-sided pumping proves that this approach is feasible, but ultimately the thin disk would need to be pumped on both sides and the gain structure would need to be optimized carefully for the intended gain profile and amplifier performance. The small signal gain has been measured as a function of the diode pump power and the operation of an oscillator has been demonstrated. The gain was observed to increase, as expected, at lower pump power but then saturates due to losses from amplified spontaneous emission (ASE). It may be possible to better suppress the ASE by introducing absorptive edge cladding on the two sides parallel to the pumping direction. In addition, it may be advantageous to adjust the angle of the side faces to near 45° to better expel the ASE rays. Performance data was fit to the combined effects of the ASE and pump saturation. The oscillator achieved the expected threshold and, by way of monitoring the side emission, it was verified that the large pumped region was not fully extracted by the smaller cavity mode.
In future work we hope to better suppress ASE by averting any re-entrant rays arising from side reflections by including Cr4+-doped edge cladding on unpumped sides and/or strongly tilting the edges, although the high single-pass transverse gain inherent to the thin disk structure will likely remain a limiting feature at high pump levels. However, this work has proven that the desired structure can be fabricated and has offered experimental verification of the basic physics that is operative.
Funding
Small Business Innovation Research (DE-SC0018722); Office of Science (DE-AC52-07NA27344).
Acknowledgements
We wish to thank Walter Aguirre and Eric Strang for precisely grinding and polishing the samples. LLNL Information Management release number is: LLNL-JRNL-836234. This work was funded by the Office of Science under Award Number: DE-SC0018722 to Radiation Monitoring Devices (RMD); LLNL was funded via a subcontract from RMD. The LLNL work was performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344.
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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