Abstract
Frequency combs in the mid-IR wavelength are usually implemented by difference-frequency generation (DFG) that mixes pump pulses and signal pulses. Different from most optical parametric amplifiers that operate at a typical low repetition rate of <0.1 MHz, mid-IR frequency combs require that pump/signal pulse repetition rate must be at least as high as tens of MHz (normally >30 MHz). The DFG mixing high repetition rate (HRR) pulses limits the allowed pulse energy to prevent crystal damage. In this paper, we numerically investigate HRR DFG with a focus on the energy scalability of idler pulses. We show that HRR DFG-unlike optical parametric amplifiers-may operate in the linear regime, in which the idler pulse energy scales linearly with respect to the pump/signal pulse energy. Our simulation results suggest an efficient approach to energy scaling the idler mid-IR pulses in a HRR DFG: increase the signal pulse energy to the same level as the pump pulse energy. We also show that DFG seeded by pump/signal pulses at ∼2-µm range benefits from reduced group-velocity mismatch and exhibits better idler energy scalability. For example, 44.2-nJ pulses at 9.87 µm can be achieved by mixing 500-nJ, 2.0-µm pump pulses and 100-nJ, 2.508-µm signal pulses in a 2-mm-thick GaSe crystal. At the end of this paper, we show that such high-energy signal pulses can be derived from the pump pulses using a recently invented fiber-optic method. Therefore, implementation of high-power (>2 W) longwave mid-IR frequency combs is practically feasible.
Highlights
Ultrafast laser sources that operate in the longwave mid-infrared wavelength range of 6 to 20 μm with 10s of MHz repetition-rate have attracted much attention in recent years [1]
We show that high repetition rate (HRR) differencefrequency generation (DFG)–unlike optical parametric amplifiers–may operate in the linear regime, in which the idler pulse energy scales linearly with respect to the pump/signal pulse energy
Different from optical parametrical amplifier (OPA) based on low repetition-rate DFG, HRR DFG may operate in the linear regime, in which the idler pulse energy scales almost linearly with respect to the pump/signal pulse energy
Summary
Ultrafast laser sources that operate in the longwave mid-infrared (mid-IR) wavelength range of 6 to 20 μm (molecular “fingerprint” region) with 10s of MHz repetition-rate have attracted much attention in recent years [1]. For DFG operating in the OPA regime, increasing the pump energy (rather than increasing the signal energy) constitutes a more efficient method of energy scaling the idler. Our experimental results showed that at 30 MHz repetition rate, around 200 nJ, 200 fs pulses at 1.03 μm can damage a GaSe crystal even for a loosely focused beam radius of 200 μm [14] This sets an upper limit for the total input power at 6-W, leading to a low pulse energy (and low average power) for the mid-IR output pulses. Analysis naturally leads to the following questions for constructing high-power mid-IR frequency combs: (1) For HRR DFG, how does the idler power scale with respect to input pump/signal power? Implementation of high-power mid-IR frequency combs is practically feasible
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