Abstract
By frequency quadrupling a picosecond pulse train from a Ti:sapphire laser at 820 nm we generate a frequency comb at 205 nm with nearly bandwidth-limited pulses. The nonlinear frequency conversion is accomplished by two successive frequency doubling stages that take place in resonant cavities that are matched to the pulse repetition rate of 82 MHz. This allows for an overall efficiency of 4.5 % and produces an output power of up to 70 mW for a few minutes and 25 mW with continuous operation for hours. Such a deep UV frequency comb may be employed for direct frequency comb spectroscopy in cases where it is less efficient to convert to these short wavelengths with continuous wave lasers.
Highlights
Frequency conversion in nonlinear crystals has been used for a long time to convert laser radiation to shorter wavelengths that is difficult, or impossible to generate directly with existing laser systems
In a real crystal, phase matching can not be achieved for all wavelengths simultaneously so that the second harmonic spectrum I2(Ω) is modified
The spectrum of the fundamental wave is given by I1(Ω) with the detuning from the carrier frequency Ω for which we can always adjust for proper phase matching, i.e. Δk = 0
Summary
Frequency conversion in nonlinear crystals has been used for a long time to convert laser radiation to shorter wavelengths that is difficult, or impossible to generate directly with existing laser systems. The shortest wavelengths of continuous wave (cw) radiation are generated in this way The limit for this method is set by the phase matching and transparency range of available nonlinear crystals. While previous authors have used this strategy with ps [1, 2, 3] and fs [4, 5] pulses, the required temporal and spatial overlap of two different color pulses adds further complications that may compromise performance, when one desires the enhancement provided by a resonant optical cavity This seems to be unnecessary if one is interested in wavelengths above the shortest phase matchable for SHG, which is 205 nm in BBO. For 2-photon spectroscopy, a frequency comb can be entirely equivalent to cw excitation [13, 14, 15]
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