The fast evolution of high power laser systems in the peak power range of 100 terawatt (TW) to petawatt (PW)[
High Power Laser Science and Engineering, Volume. 7, Issue 1, 010000e1(2019)
Performance demonstration of the PE
We report on the energetic and beam quality performance of the second to the last main amplifier section HEPA I of the PE
1 Introduction
The fast evolution of high power laser systems in the peak power range of 100 terawatt (TW) to petawatt (PW)[
High-energy lasers using neodymium doped glass extend their capability toward shorter pulse duration and improved temporal pulse contrast necessary for such applications using optical parametric chirped-pulse amplification (OPCPA)[
With the increasing availability of laser diodes as optical pump sources, rare-earth-doped gain media became attractive for high average power laser systems up to the kW level[
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Notably two fully, direct laser diode-pumped laser projects are located in Germany: the Polaris laser system in Jena relies on ytterbium doped glass and calcium fluoride
The PE
2 Setup description
Figure
The initial set of amplifiers boosts the energy from the nanojoule to the joule level and therefore does the highest amount of work in terms of energetic gain, while the initial spectral bandwidth should be preserved. They are consequently named high-gain broadband amplifiers (HGBAs).
With
Unfortunately,
The second amplifier section consists of two high-energy power amplifiers (HEPA I and II). Despite the majority of the gain performed already, the lion’s share of the diode pump power, thus the related thermal management, concentrates in those instruments. Consequently it is crucial to thoroughly design and test the average power capacity of those amplifiers after the initial confirmation of energetic amplification capacity.
With that many passes through a large size, high-energy amplifier a fully imaged setup is preferable, as free propagation would lead to a significant loss in spatial beam quality due to static and pump induced wavefront aberration. As long as the main source of wave front error lies in (or sufficiently close to) the respective image planes, a careful placement of a correction system (either static or adaptive) can eliminate most of the aberration impact without sacrificing spatial beam quality too much. The correction should be as close as possible to the origin of the major wavefront aberration. Otherwise, one would need at least two deformable mirrors to correct near- and far-field independently[
As highest extraction efficiency is desired, a flat top pump and extraction profile yields the best results, while minimizing laser induced damage issues. In order to optimize beam quality and to mitigate laser induced damage, a fully imaged amplifier setup was chosen for both amplifiers HEPA I and II with an emphasis on a minimal free propagation distance between the optical elements for each of its passes.
The general setup of HEPA I is shown in Figure
Estimating the amplified spontaneous emission (ASE) impact on the energy storage capacity was performed by HASEonGPU[
To avoid chromatic aberration and ghost reflection as far as possible and to increase the compactness of the system, a fully reflective setup was chosen using telecentric telescopes. In order to avoid breakdown close to the foci, the telescopes are under vacuum. The spherical mirrors have focal lengths of 1 m and 1.35 m (magnification 1.35) to reduce the average energy density, especially after the last amplification pass and the following transport optics at the same time. The overall challenge in constructing such a setup lies in reducing the losses due to the finite reflectivity, notably for the laser gain medium slabs.
The full setup of HEPA I was modeled with Zemax. Considering perfect surfaces and transmission optics (gain media slabs, windows), one can achieve a Strehl ratio (SR) of more than 95% in 12 passes, as shown in Figure
Primary ghost reflections, especially from the slabs, were suppressed by applying a wedge and blocking those unwanted reflections by reasonably sized pinholes close to the focal planes of the individual telescopes.
In the case of the 12 passes in HEPA I, we found a total transmission of 95% in the experiment, when the doped gain medium is replaced with similar coated fused silica slabs, and 15% transmission inserting the
Amplifier stage HEPA II is designed in a similar fashion as HEPA I, but scaled to the 200 J level. The pump peak power is 1.2 MW at 976 nm with an increased brightness of the laser diode pump source to more than
In order to experimentally prove the SSG capacity of the combined setup of HEPA I and II on the one hand and to show the energetic and spectral capacity on the other, a double pass through HEPA I using polarization coupling was set up. Now, instead of the originally intended 12 passes, 24 passes are implemented, effectively simulating both amplifiers in terms of total gain and spectral gain narrowing impact using HEPA I alone.
However, during testing the high pressure windows, an excessive amount of birefringence was found. In order to perform the polarization coupling, the high pressure windows had to be removed from the assembly, leaving the laser slabs to the same condition as the remaining amplifier section (i.e., under vacuum as the telescopes or under ambient air) and restricting the operation to single shot only.
3 Experimental results
In order to assess the performance of this double-pass configuration, we opted for three performance tests. The first was the probing of the SSG using a continuous wave (cw) laser, where the whole setup was left under ambient condition. The second step was the energetic activation of the amplifier with an additional emphasis on the reproduction of the SSG experiments on the one hand and to gain insight into the spectral behavior of the whole setup on the other. We investigated the spectral performance of the entire amplifier chain to support sub-150 fs pulses. This step at high pulse energies had to be performed under vacuum, where due to the absence of the high pressure windows also the amplifier operated under vacuum condition. The last step was the observation of the near- and far-field quality, which is an important feature for the reliable pulse compression and beam transport into the envisioned target area.
3.1 Small signal gain probing
With an SSG of about 1.3 per pass only, one can expect a total SSG of 543 in 24 passes, while 1.31 will ultimately result in an SSG of 652. Both results are easily distinguishable and therefore we can measure the SSG per pass with a precision otherwise not accessible in a single pass regime.
We injected a narrow-band cw source at 1030.0 nm with a beam diameter of 2 mm, effectively probing the gain medium at the center only. The pump light distribution is homogeneous (less than 2% rms, see the inset in Figure
As we used the setup in 24 passes, only 2.2% transmission was found, which matches very well the expected absorption for the ytterbium doped slabs at ambient temperature. Figure
For a short pump duration (i.e.,
Thermal roll-over (TRO) can be ruled out, as for this particular setup the whole laser was under normal pressure. It was not necessary to put in the vicinity of the foci under vacuum, as only a cw laser was used. Consequently all four slabs were air cooled during the individual single shots with about one shot per 5 min. Reference shots were taken at the beginning and the end of the individual shot series that showed no sign of thermal impact on the SSG.
As already mentioned, about 1 ms of pump duration is needed to surpass an SSG of 1, even at the highest available pump intensity. This indicates that in our case with a maximum pump duration of 4 ms, a significant amount of pump energy is lost due to the ground state absorption, which is a common drawback of any ytterbium doped gain medium at room temperature.
To mitigate this issue we plan to use the amplifiers at a lower temperature in the future, notably in the 200 K to 250 K range to reduce the reabsorption and harness the improved thermo-mechanical properties at the same time. Keeping the operation in this range, spectral modulations are still negligible, which appear at even lower temperatures[
3.2 Energy extraction using nanosecond pulses
With the SSG for a narrow-band cw laser far exceeding the needed factor of 200, one can go forward to estimate the gain performance for a broader spectrum, much closer to the spectral distribution out of HGBA III.
In order to assess the scaling of the spectral bandwidth behavior of the large amplifier setup and in order to verify the results obtained using the narrow-band laser, we opted for a broadband, nanosecond cavity-dumped oscillator with 6 ns (FWHM) long pulses as a seed source for HGBA II instead of the chirped-pulse amplification (CPA) oscillator together with its front end. Using this approach we avoid the danger to generate a short pulse by narrowing the spectrum, as it would happen in a stretched CPA pulse due to the spectral–temporal coupling.
By adapting the regenerative amplifier HGBA I to its cavity-dumped mode, we have an easy way to manipulate the bandwidth without changing the temporal width of the pulse. We used the aforementioned output of HGBA II and varied the energy input into the double-12-pass HEPA I setup by the means of a half-wave plate and a polarizer.
In order to verify the SSG results obtained by a cw laser at 1030 nm, a narrow-band input of 0.7 nm FWHM (case A) was prepared. The 5 nm FWHM bandwidth case (case B) corresponds to the natural spectral behavior of an ytterbium doped CaF
At this point we have to point out, that the whole setup was under vacuum condition, including the amplifier head with the four gain medium slabs. As it was mentioned before, the high pressure windows showed too much birefringence that did not allow using any polarization coupling and were therefore omitted.
Figure
The gain drops at an output energy of 10 J from 820 to 610 by increasing the input spectral width from 0.7 nm (case A) to 5 nm (case B) and again to 444 with 16 nm (case C) at the input, when interpolating the spline fit.
As mentioned earlier, for this setup the gain medium slabs were under vacuum in contrast to the SSG experiments discussed before. This leads to the inevitable buildup of heat, resulting in a slight reduction of the gain per pass with an increasing number of successive shots. Considering 24 passes, this leads to a rather significant impact on the gain curve in the form of a TRO, even when working in a single-shot regime.
Between individual following shots (more than 5 min between each shot) no thermal effect was directly visible. However, over the course of a day a deviation from the expected behavior was found (TRO was visible). A separate shot on the next day revealed the full energetic performance for the cases B and C eliminating the TRO. An energy of 12.6 J was found in both cases.
Figure
The most narrow-band case (A) stays virtually the same, as one can expect from the rather broadband nature of the gain medium. The intermediate case (B) behaves as expected with only a minor bandwidth reduction and a slight shift toward longer wavelengths.
The most broadband case (C) however shows the expected gain narrowing between 1030 nm and 1035 nm. A Fourier transform of the amplified spectrum reveals 146 fs for this first try, proving the bandwidth capacity of the whole amplifier setup and spectral shaping strategy of PE
3.3 Near- and far-field observation
Despite that the setup of HEPA I was initially not designed to support a double-12-pass amplification scheme, the near-field image quality could be sustained. Taking a look into the beam profile in Figure
The far-field quality of such laser pulses is of utmost importance to reach highest in-focus intensities, with the Fourier limit as its target. We concentrated on three different aspects. At first we wanted to assess the laser amplifier design in terms of far-field quality, second the energy extraction setup including gain medium and transport optics without any far-field correction applied. As a last point we wanted to explore the future potential of a wavefront correction to achieve a focal spot as close as possible to the Fourier limit in the initially designed 12-pass setup, as HEPA I and II have individual deformable mirror systems.
As mentioned before, the laser amplifier simulation using Zemax indicated for HEPA I a limiting SR close to 95%, omitting the individual mirror and gain medium quality. Nevertheless, any random nonrotational symmetric wavefront error from the optics outside the image planes of the single-12-pass setup statistically balances each other. Optics situated in the individual image planes is hit under each pass respectively, which in return will generate the majority of the accumulated wavefront error. After 12 passes we measured an SR of 75% using the wavefront sensor with the laser gain medium replaced by fused silica slabs. We have to consider about
The next step was to exchange the fused silica slabs with the doped gain medium slabs. Measuring those slabs using a Fizeau interferometer (Accufiz, 4DTechnology), we can estimate a wavefront error per pass of about 0.2
For this particular setup we decided to measure the near-field by imaging the near beam profile of a 1064 nm cw laser onto a camera (SP620U, Spiricon). Taking advantage of the focal spot of the first lens in the image relaying telescope, the far-field was measured directly. Out of the near-field we calculated the corresponding far-field with a perfectly flat wavefront. Dividing the measured intensity by the calculated ideal one yields by definition the SR[
The measured far-field is shown in Figure
Figure
4 Conclusion
We installed and successfully used the fully imaged and reflective second to the last amplifier section HEPA I to amplify broadband nanosecond pulses. In order to prove the energetic and spectral performance of both last amplifier sections relying on
With an SSG of 900 and the measured energetic gain of more than 500 for broadband nanosecond pulses at room temperature under single-shot condition we far exceeded the necessary gain of 200 with enough margin considering the envisioned case of a sub-room temperature operation of the amplifier head.
In our experimental campaign we achieved a pulse energy of 12.6 J using broadband nanosecond pulses out of the front end amplifier section. CPA pulses with a similar spectrum would show a Fourier limit below 150 fs without any further spectral shaping campaign necessary.
Small scale near-field spatial modulations result from imperfections of the gain medium, while at the same time the very first iteration of a far-field optimization shows a measured SR of 76% under 12 passes. Together with the relatively smooth near-field output beam profile this is to our knowledge the best beam quality for multi-pass 10 J scale amplifiers relying on
With this benchmark of HEPA I we are therefore confident to ultimately deliver ultrashort high quality laser pulses on the 150 J level with good spectral properties and the desired high quality in near- and far-field to the target area with the upcoming activation of the last amplifier section HEPA II for future laser particle acceleration campaigns.
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D. Albach, M. Loeser, M. Siebold, U. Schramm. Performance demonstration of the PE
Special Issue: HIGH ENERGY DENSITY PHYSICS AND HIGH POWER LASERS 2018
Received: Jul. 30, 2018
Accepted: Nov. 9, 2018
Published Online: Jan. 16, 2019
The Author Email: D. Albach (penelope@hzdr.de)