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We propose an original optical architecture for the construction of an Integrated Dumper – Regenerative Amplifier, by combining pulse generation and pulse slicing together with downstream regenerative amplification within a common amplifying unit and resonator. This design provides relatively short pulses at high energy, using a fairly simple and robust two-path resonator. The demonstration is performed with the help of a diode-pumped Yb3+: YAG slab operated at room temperature at 1Hz PRF, in the energy range of 5 to 50mJ per pulse with 500ps to 5ns FWHM.
©2007 Optical Society of America
1. Introduction
High energy waveform Shaping (WS) of laser pulses may represent a central issue when the interaction of laser beams with materials is considered. Depending on the nature of the interaction, more or less short pulses may be required. In the field of large scale laser facilities, Inertial Confinement Fusion is one example [1]. Other examples within the framework of more usual industrial applications are metal working or cutting, surface cleaning and conditioning, or other plasma-related applications. The use of shaped pulses can considerably improve the efficiency of the process. This paper is not specially dedicated to WS, but to its implementation within the framework of a basic laser source, without any decrease in the overall laser efficiency. We propose an original resonator concept to generate and shape high energy, nanosecond and sub-nanosecond wide pulses. The architecture of the resonator uses a three mirror design, in the form of an Integrated Dumper-Regenerative Amplifier (IDRA).
The usual configurations involving WS use either clipping electro–optics, downstream of the output of a Q-Switched oscillator, or external signal injection, at the expense of more complex optical configurations. A standard configuration is based on the injection of a low energy pulse at the input [1] of a Regenerative Amplifier (RA). External injection to initiate the RA involves the implementation of a dedicated sub–system together with a number of coupling optics and proper optical isolation. Fibre based designs [1,4] may be preferred to free space beam configurations [2,3] in some cases, to benefit from greater flexibility, despite the need for in-line pre-amplification and integrated optics. The implementation of external WS downstream of a Q-Switched oscillator may appear simpler but the overall optical efficiency can be reduced dramatically, especially in the field of pulses where the Full-Width at Half-Maximum (FWHM) ranges from some hundreds of picoseconds to a few nanoseconds. The problem of a poor temporal overlap between the required profile to be delivered after WS and the natural Q-Switched pulse shape may justify the choice of an injected RA configuration. The same limitations are found with gain-switching from a microchip laser, if used as the external injector, even though such lasers can produce pulses with less than 500ps FWHM. Cavity Dumping (CD) is a solution to provide pulses which are shorter than those obtained with Q-Switching. Similar limitations remain, however, with the decrease in the overall efficiency due to WS. Moreover, an additional drawback in using pure CD comes from the near-field beam shape, which is difficult to control and stabilize when the whole pulse energy is extracted from the resonator during a single round trip. In the field of high energies, the minimum values of FWHM with CD also remain limited by the shortest cavity length, due to the size of the opto-mechanical components, and by the transition times of the Pockels Cells (PC) in the resonator. For that reason, we proposed the IDRA, another concept which combines the advantages of externally injected RAs together with those of a simple architecture. Assuming that High Voltage (HV) electronics can be used, the IDRA will help in the definition of a highly robust and efficient source for the delivery of shaped and relatively short pulses, together with the benefit of stabilized output energy and near field.
At very low FWHMs, the most critical issue to be addressed for WS is the performance of the available HV drivers. Commercially available electronics based on avalanche transistors enable the delivery of faster than 80ps transition steps from 0 to 4kV into 50 ohm loads. With photoconductive Silicon bars and planar micro-strip lines to drive a PC from 0 to 4kV, previous works [5] have shown that arbitrary WS can be conveniently operated for 5 - 10ns wide pulses and 100 – 200ps temporal resolution. Therefore, WS also makes sense to shorten pulses in the range of 80 – 100ps FWHMs. In another step, the arbitrary profile of nanosecond and sub – nanosecond wide pulses can be shaped with similar 80 – 100ps temporal resolutions.
2. The optical design
The basic IDRA architecture resembles a single stage Master Oscillator – Power Amplifier design, which combines a low energy - shaped CD process together with downstream RA, within one and the same resonator, using the same amplifying element. Because of re - circulation, RA up to the gain saturation delivers highly stable pulsed beams within the mode distribution of the resonator. The complete optical performance will be demonstrated with the help of a single diode–pumped Yb3+: YAG slab.
Our current design (Fig. 1) uses a single pumping head, the amplifying medium being a side pumped YAG slab [6] with 5 % Yb3+ doping level. The pumping head is operated at room temperature T=20°C together with a stack of QCW collimated diode bars at 940nm. The slab is 2mm thick and 20mm long, and its bottom face is plugged into a metal radiator to provide good heat exchange. The bottom face has been coated with a high reflection layer to ensure double pass pump deposition. We use large aperture cylindrical lenses to focus the pump power into the central part in the form of a narrow band, approximately 1.5mm wide and 20mm long. The available small signal gain can be varied from about 2 to 5 in a single pass by varying the peak diode power from 0.8 to 2kW at 1Hz Pulse Repetition Frequency (PRF).
The set-up incorporates three PCs. Two of them (PC 1, PC 3) are made of a large - aperture single KD * P crystal, while the third (PC 2) consists of a 50 ohms - impedance structure dedicated to WS. PC 1 and PC 1 include three electrode assemblies [7] deposited onto the same crystal, to benefit from a two-step mode of operation. The HV drivers generate voltage steps from 0 to 5kV with 3ns transitions times. The IDRA resonator exhibits two coupled optical paths. The top main path is used for the generation of the initial CD pulse and forthcoming RA, following the round - trip WS inside the bottom path. Apart from PC 2, the bottom path includes a quarter - wave plate and the third Rmax mirror.
Fig. 1. The optical architecture of the IDRA: the main path consists of two Pockels Cells, PC 1 and PC 3, while the shaping path in the bottom is operated with the help of the third cell PC 2. Cavity Dumping and Regenerative Amplification need to be properly synchronized, which means that the minimum length of the shaping path is equal to that of the main path.
Fig. 2. The operating chronograms to switch the IDRA, by using three electrodes – KD*P Pockels Cells in the main path: TRT is the round trip time in the upper main path and switching is operated at the quarter wave voltage, i.e. 4.5kV steps with 3ns transition times. N represents the number of round trips for Regenerative Amplification.
In a first step (Fig. 2), the system is triggered in the CD mode of operation by switching PC 3. Shortly afterwards, i.e. the delay due to the build-up time for CD, a nearly square pulse is dumped with the help of PC 1 throughout the polarizer P 1, down to the bottom path. This pulse is shaped by PC 2 as required to provide the expected output FWHM and / or waveform. The latter pulse is re-injected into the top main path of the IDRA to start the sequence of RA and extract the remaining – but most important - part of the stored energy (Esto) in the slab. From the theoretical point of view, for the purpose of efficiency, the CD and WS have to be operated at very low energy. This keeps decrease of Esto before the forthcoming RA as small as possible. The effectiveness of pulse re-injection and the search of gain saturation towards the end of RA simply requires selecting the right values of switching times for the two rear steps in PC 1 and PC 3. The three cavity mirrors (Fig. 1) are high reflection coated at the laser wavelength (1030nm), to confine the laser energy.
The energy distribution between the CD pulse and the output pulse can be varied in a wide range of values, versus the shaping–induced losses inside the bottom path and versus the required optical contrast at the output. The IDRA is highly tolerant with respect to the WS optical losses, which include the optical transmission throughout PC 2, the temporal overlap between the profiles of the CD pulse and the shaper, and the spatial mode overlap at the re – injection for RA. Even high losses do not imply any major limitation in performance, until the remaining value of Esto is kept above 80% of its initial value at the starting time of RA. In the situation of our short pulse experiments, the total losses were estimated in excess of 20dB.
Since P 1 can work alone in principle, a question could arise regarding the need of two polarizers instead of just one in the central part of Fig. 1 (P 1, P 2). Firstly, cascading enables a higher optical extinction when switching the resonator. Secondly, the leakage of some unwanted pre-pulses during CD prevents the extraction of the output pulse from P 1 if one wants to optimise the temporal output contrast. The main limiting factor in the contrast therefore is mainly defined by the amount of the Amplified Spontaneous Emission (ASE) at the time of dumping.
Figure 1 also includes a magnifying lens. This lens helps to account for the lack of reliable information on the maximum intensity permitted in our PCs for sub – nanosecond FWHMs. It ensures additional margins regarding the expected optical damage limitations. The focal length must be selected to compensate partly for the gap between the optical damage level in the KD * P, typically 250 - 500MW/cm2 for nanosecond FWHMs, while that in YAG exceeds 5 – 10GW/cm2. In the set-up, we consider F = 100 - 200cm and the peak diode power is varied from 1 to 1.8kW. Since the actual pump power and slab geometry cannot be optimally matched for the complete range of experimented FWHMs, this is the obvious solution to preserve RA up to gain saturation.
3. Experimental results
The optical performance of the IDRA can be demonstrated at varying FWHMs and output energies, within the limits specified above. Given that the first step to validate the concept consists of proper management of the CD and WS modes of operation, we start by determining the variation of the build-up time (TCD) for CD versus the pump current. The top side of Fig. 3 shows the following range of values: 30ns < TCD <2800ns. Such a plot provides a useful control of the optical isolation along the main path of the IDRA, and for the effectiveness of energy locking between the three Rmax mirrors up to the higher gain values. The bottom side of Fig. 3 right shows the shorter electrical pulse shape that we used with PC 2 for WS, within a slightly limited measurement bandwidth (∼2GHz at -3dB), i.e. about 500ps FWHM at 3.5kV peak voltage. Experiments were made with a number of output energy values at 1Hz PRF within the range E = 5 - 50mJ, without any transverse mode selection. The beam’s near field cross-section is about 4mm2, this value being governed by the actual width of the slab pumping band. When the output pulses from the IDRA are extracted near the saturation of the gain in the resonator, the long-term stability of output pulse-shape and energy is shown to be better than 5%. To give an example using F= 200cm, 1ns FWHM pulses are produced up to more than E= 20mJ. By limiting somewhat the peak pump power for the generation of shorter pulses, 5 to 10mJ pulses have been demonstrated at sub - nanosecond FWHMs.
Fig. 3. Build-up time in the Cavity Dumping mode of operation versus the pump current (top), the high – voltage electrical shaping waveform (bottom). The selected values of build-up times in the demonstration of the IDRA range from 100 to 200ns, depending on the actual pump power.
For the purposes of illustration, the top side of Figure 4 shows the temporal evolution of the power inside the resonator at FWHM= 500ps. By monitoring the leakage signal out of the main cavity with a photodiode near the right Rmax mirror in the main path, the rear electrical transition on PC 3 can be correctly synchronized. The circulating pulse may thus be extracted just before (Fig. 4, top, left blue curve) or slightly after (Fig. 4, top, right red curve) the gain saturation. The bottom side of Fig. 4 shows the related output pulse shape from the IDRA at E= 5mJ with 500ps FWHM. The limitation in the temporal contrast for this example is due to the lack of specifically designed wave-plates and polarizers for the actual wavelength (1030nm) at the time of our experiments. Higher contrast values, typically in excess of 20dB, could be obtained with optimized optical components.
Fig. 4. The leakage signal, through the flat Rmax mirror in the main path, to monitor the sequence of Regenerative Amplification (top) and the output pulse downstream P 2 (bottom). The pulse extraction can be operated just before or just after the gain saturation. The envelope of the successive pulses in the main path simply describes the shape of a standard Q-Switching process.
4. Conclusions
To summarize from a prospective viewpoint, we have shown that the IDRA is a fairly robust and simple concept for the production of high energy shaped pulses. Using a unique side – pumped gain medium, made of Yb3+: YAG in the related design, the architecture is very easy to operate. Relatively short pulses are produced, within the limits of optical damage inside currently available polarizers and PCs. The waveform and the FWHM of such pulses can be adjusted relatively flexibly. With our current optical configuration, the implied peak power limitations result in energy values in the range 5 - 50mJ at 1Hz PRF, for 0.5 to 5ns FWHM pulses. The applicability of the concept to other gain media appears to be straightforward: any gain material can be used, either Ytterbium or Neodymium, with classical diode pumping, or any other one, under the assumption of a minimum small signal gain in the main path. To overcome the optical losses throughout the PCs and polarizers, this minimum value typically ranges from 1.5 to 1.8 in a single pass. Even though side pumping appears to be simpler and more efficient because of the linear architecture, axis pumping could also be used. In a further stage in the near future, the optical design of the main cavity will have to be optimized with proper intra-cavity optics. An attractive option is the use of phase plates to provide a single – mode flat top beam. That should enable the generation of pulses as short as 100ps FWHM, up to about 100mJ energy levels at 1 - 10Hz PRF, as well as longer pulses together with a high optical efficiency. Depending on the ultimate performance of PCs and associated drivers, such a design may be of interest in the field of highly efficient lasers for the generation of sub - picosecond and high – energy pulses at a low PRF.
Acknowledgments
This work was sponsored by the Laser R&D funding program at CEA/DAM, within the framework of studies dedicated to Ytterbium doped lasers. The opinions and interpretations are those of the authors and may not be necessarily endorsed by the external commissions.
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