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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 2 — Jan. 22, 2007
  • pp: 466–472
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Generation of variable width pulses from an Yb3+: YAG Integrated Dumper – Regenerative Amplifier

Alain Jolly, Nelly Deguil Robin, Jacques Luce, and Gérard Deschaseaux  »View Author Affiliations


Optics Express, Vol. 15, Issue 2, pp. 466-472 (2007)
http://dx.doi.org/10.1364/OE.15.000466


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Abstract

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

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

1. A. Jolly, J.F. Gleyze, D. Penninckx, N. Beck, L. Videau, and H. Coic, ⟪ Fiber Lasers integration for LMJ,⟫ Elsevier, Académie des Sciences - CR de Physique 7,198–212 (2006) [CrossRef]

] 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

1. A. Jolly, J.F. Gleyze, D. Penninckx, N. Beck, L. Videau, and H. Coic, ⟪ Fiber Lasers integration for LMJ,⟫ Elsevier, Académie des Sciences - CR de Physique 7,198–212 (2006) [CrossRef]

,4

4. LLE Review, “ Highly stable, Diode-Pumped, Cavity-Dumped Nd:YLF Regenerative Amplifier for the OMEGA Laser Fusion Facility,” LLE - Q. Report91,103–107 (2002)

] may be preferred to free space beam configurations [2

2. S. Biswal, J. Nees, A. Nishimura, H. Takuma, and G. Mourou, “Ytterbium-Doped Glass Regenerative Chirped-Pulse Amplifier,” Opt. Commun. 160,92–97 (1999) [CrossRef]

,3

3. M. Saeed, D. Kim, and L.F. DiMauro, “Optimization and characterization of a high repetition rate, high intensity Nd: YLF regenerative amplifier,” Appl. Opt. 29,1752–1757 (1990) [CrossRef] [PubMed]

] 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.

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

6. A. Jolly and E. Artigaut, “Theoretical design for the optimisation of a material’s geometry in diode-pumped high-energy Yb3+:YAG lasers and experimental validation at 0.5-1J,” J. Appl. Opt. 43,6016–6022 (2004) [CrossRef]

] 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

7. J. Luce, in CEA patent CESTA ZD132, n°2772149 (1997).

] 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.

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.

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

Acknowledgments

References

1.

A. Jolly, J.F. Gleyze, D. Penninckx, N. Beck, L. Videau, and H. Coic, ⟪ Fiber Lasers integration for LMJ,⟫ Elsevier, Académie des Sciences - CR de Physique 7,198–212 (2006) [CrossRef]

2.

S. Biswal, J. Nees, A. Nishimura, H. Takuma, and G. Mourou, “Ytterbium-Doped Glass Regenerative Chirped-Pulse Amplifier,” Opt. Commun. 160,92–97 (1999) [CrossRef]

3.

M. Saeed, D. Kim, and L.F. DiMauro, “Optimization and characterization of a high repetition rate, high intensity Nd: YLF regenerative amplifier,” Appl. Opt. 29,1752–1757 (1990) [CrossRef] [PubMed]

4.

LLE Review, “ Highly stable, Diode-Pumped, Cavity-Dumped Nd:YLF Regenerative Amplifier for the OMEGA Laser Fusion Facility,” LLE - Q. Report91,103–107 (2002)

5.

A. Jolly and Ph. Estraillier, “Generation of arbitrary waveforms with electro-optic pulse-shapers for high energy - multimode lasers,” J. Opt. Laser Technol. 36,75–80 (2004) [CrossRef]

6.

A. Jolly and E. Artigaut, “Theoretical design for the optimisation of a material’s geometry in diode-pumped high-energy Yb3+:YAG lasers and experimental validation at 0.5-1J,” J. Appl. Opt. 43,6016–6022 (2004) [CrossRef]

7.

J. Luce, in CEA patent CESTA ZD132, n°2772149 (1997).

OCIS Codes
(140.3280) Lasers and laser optics : Laser amplifiers
(140.3480) Lasers and laser optics : Lasers, diode-pumped

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: August 16, 2006
Revised Manuscript: December 11, 2006
Manuscript Accepted: December 11, 2006
Published: January 22, 2007

Citation
Alain Jolly, Nelly Deguil Robin, Jacques Luce, and Gérard Deschaseaux, "Generation of variable width pulses from an Yb3+: YAG Integrated Dumper – Regenerative Amplifier," Opt. Express 15, 466-472 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-2-466


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References

  1. A. Jolly, J.F. Gleyze, D. Penninckx, N. Beck, L. Videau and H. Coic, « Fiber Lasers integration for LMJ,» Elsevier, Académie des Sciences -CR de Physique 7, 198-212 (2006) [CrossRef]
  2. S. Biswal, J. Nees, A. Nishimura, H. Takuma and G. Mourou, "Ytterbium-Doped Glass Regenerative Chirped-Pulse Amplifier," Opt. Commun. 160, 92-97 (1999) [CrossRef]
  3. M. Saeed, D. Kim and L.F. DiMauro, "Optimization and characterization of a high repetition rate, high intensity Nd: YLF regenerative amplifier," Appl. Opt. 29, 1752-1757 (1990) [CrossRef] [PubMed]
  4. LLE Review, "Highly stable, Diode-Pumped, Cavity-Dumped Nd:YLF Regenerative Amplifier for the OMEGA Laser Fusion Facility," LLE - Q. Report 91, 103-107 (2002)
  5. A. Jolly and Ph. Estraillier, "Generation of arbitrary waveforms with electro-optic pulse-shapers for high energy - multimode lasers," J. Opt. Laser Technol. 36, 75-80 (2004) [CrossRef]
  6. A. Jolly and E. Artigaut, "Theoretical design for the optimisation of a material’s geometry in diode-pumped high-energy Yb3+:YAG lasers and experimental validation at 0.5-1J," J. Appl. Opt. 43, 6016-6022 (2004) [CrossRef]
  7. J. Luce, in CEA patent CESTA ZD132, n°2772149 (1997).

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