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Description
In an FEL oscillator, it is not uncommon to see spectral content at the 3rd and even 5th harmonic of the fundamental lasing wavelength. This arises from on-axis emission from the undulator at the fundamental and odd harmonics. Under proper conditions, FEL oscillations also at the 2nd harmonic are possible, but even harmonics are strongly suppressed due to off-axis emission from the undulator. In the results here from the FELBE IR FEL, we demonstrate spectral emission from an IR FEL oscillator at both odd and even harmonics and remarkably, up to extremely high orders (> 50th order).
We have found that the harmonics, while falling rapidly in power at low orders, exhibit a plateau-like behavior at higher orders. The power and range of the harmonics increases with increasing K-parameter. While lasing in the mid-IR, the harmonics reach into the near-IR and visible. This allows imaging of the harmonics with a standard CMOS array, where we see a transverse mode that is zero on center and appears more like a Laguerre-Gaussian mode. Measurements of the high harmonics will be described along with an outline of potential applications for this surprising spectral content.
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