New design tool developed for MOPAs

A collaboration between Advanced Optical Technology (AOT; Basildon, England) and the Department of Physics at Hull University (Hull, England) has produced a new tool to help in the design of Q-switched master-oscillator and power-amplifier (MOPA) diode-pumped solid-state (DPSS) lasers working in the nanosecond regime.

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A collaboration between Advanced Optical Technology (AOT; Basildon, England) and the Department of Physics at Hull University (Hull, England) has produced a new tool to help in the design of Q-switched master-oscillator and power-amplifier (MOPA) diode-pumped solid-state (DPSS) lasers working in the nanosecond regime. The team has developed an analytical expression that predicts the output of a multipass amplifier from a given input; the developers claim that this can replace the numerical methods currently used to model amplifier performance.

The collaboration includes names that are well known in European laser-design circles. AOT was founded in 1999 by Clive Ireland and John Ley to undertake feasibility studies, contracted projects, and product developments in new areas of photonics. The Hull team is headed by Peter Dyer. The DPSS lasers involved in the study typically operate at kilohertz rates, with oscillators producing pulses up to around 50 kW but at subwatt average power. For many applications, amplification is required to raise the average power into the multiwatt range. Multipass amplification can be important in situations in which the oscillator input does not heavily saturate the amplifier in a single or double pass-for example, where the input fluence (energy per unit area) to the amplifier is low and/or the amplifier saturation fluence is high. This is the case when Nd:YAG or Nd:glass is used for the amplifier.

The Franz-Nodvik equation, developed in 1963, is a powerful tool widely used for helping to predict amplifier performance. It links output fluence Eout to saturation fluence Es, input fluence Ein, stimulated-emission cross section σ, and the initial length-integrated inversion No:

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Maximum amplifier performance, in terms of extraction efficiency, for a MOPA is calculated for different oscillator input-pulse energies in the range 1 µJ to 1 mJ for a multipass Nd:YAG amplifier with beam overlap.
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The team claims, however, that where there are co- or counterpropagating beams that overlap in the amplifier, no comparable analytical expression has been available. In such cases, numerical methods have had to be used to model amplifier performance. The new research shows that the Franz-Nodvik equation with pulse overlap can be generalized, and for an M number of passes of the amplifier has the form:

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This allows straightforward modeling of different amplifier configurations. As an example, the team modeled the case of a short Nd:YAG amplifier end-pumped in a 1-mm diameter spot by a 75-W-peak-power diode bar with a fiber pigtail (see figure).

Data presented in this way can be used to help make early design decision. For this particular example, it is evident that with an input of around 1 mJ/pulse a double-pass amplifier arrangement would allow good stored energy extraction (around 80%). However, if the input from the oscillator was in the region of 100 µJ/pulse, a four-pass arrangement would be required for good efficiency, and for around 10 µJ/pulse, eight passes. In the limit of a low input-pulse energy-for example, less than 1 µJ/pulse-the calculation points to the possibility of using a regenerative-amplifier instead of a multipass arrangement to efficiently extract amplifier stored energy.

“Many of the papers written in the 1960s have stood the test of time, and today, almost half a century on, they remain as useful to the laser engineer as in those early days,” says Ireland. “Despite the huge growth in laser research since then, uncovering some useful fundamental laser physics that might have come to light in the interim is not that common. However, this we feel is the case with this work. We find that the availability of this generalized analytical expression for multipass amplifier performance provides a very useful aid in laser-system design, and hope and expect that others will too.”

Bridget Marx

REFERENCE

1. Optics Communications, doi:10.1016/j.optcom.2005.06.013, in press.

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