Modulation instability, three mode regimes and harmonic generation at the Novosibirsk terahertz free electron laser

Kubarev, V. V.; Кулипанов, Г.Н.; Kolobanov, E. I.; Матвеенко, А. В.; Medvedev, L. E.; Ovchar, V. K.; Salikova, T. V.; Scheglov, M.A.; Serednyakov, S. S.; Винокуров, Н.А.

doi:10.1016/j.nima.2008.12.122

Cited by 19 publications

(14 citation statements)

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“…This peculiarity allows us to have a narrow laser line. However, this is possible only when the modulation instability is suppressed [2]. The instability period T = L s /c = Nλ/c = 30−60 ps, where L s is the slippage length, N = 74 is the number of poles in the undulator, and λ is the wavelength.…”

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confidence: 99%

Detailed observation of modulation instability on THz NovoFEL

Kubarev

2012

2012 37th International Conference on Infrared, Millimeter, and Terahertz Waves

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Detailed experimental observations of the modulation instability in the frequency and time domains were made on the THz NovoFEL by a Fourier spectrometer and an ultrafast Schottky diode detector. The regime stabilized by negative detuning between the electron and light frequencies is shown to transform into a multimode one with coherent modulation of the light pulse. Further decrease in the stabilization factor leads, for a high gain, to an unstable regime at the resonance frequency. In this regime, a light pulse splits into incoherent sub-pulses with a duration equal to the slippage length divided by the light velocity. The generation of one short sub-pulse is possible for small positive detuning. I. RESULTST he Novosibirsk terahertz free electron laser (THz NovoFEL) generates a continuous train of sufficiently long (100−150 ps) pulses [1]. This peculiarity allows us to have a narrow laser line. However, this is possible only when the modulation instability is suppressed [2]. The instability period T = L s /c = Nλ/c = 30−60 ps, where L s is the slippage length, N = 74 is the number of poles in the undulator, and λ is the wavelength. Negative detuning between the electron and light repetition frequencies is an efficient method of instability suppression (Figs.1 and 2). Earlier we observed the modulation instability and its suppression in the frequency domain by a Fourier spectrometer showing spectra and coherence. The instability was practically not observed in the time domain, because it was impossible to measure it by our sampling oscilloscope because of a jitter of perturbations from pulse to pulse [2]. Now we use our ultrafast Schottky diode detector [3] connected to a direct 30 GHz LeCroy oscilloscope, and have a full and clear picture of the phenomenon in the time domain (Fig.2). One can see that full coherent stabilized regime #1 transforms to multimode regime with correlated sub-pulses #2, where the sub-pulses with a slippage period appear as a result of the stabilization factor decrease. Then the sub-pulses became practically uncorrelated at resonance point #3. In the resonance, we have the shortest coherence length, which is approximately equal to the length of one sub-pulse and the widest spectrum. For positive detuning we observe a decrease in the number of generated low correlated sub-pulses with a practically constant maximal pulse power due to the saturation effect (#4−#5). The last regime (#5) can be used for some applications when a shortpulse radiation is needed. The measurement of the light pulse structure is not only necessary for studying the instability, but has a routine application in determination of a real laser pulse power. Thus, the maximal pulse power of NovoFEL was about 400 kW in the regime with a repetition frequency of 11.2 MHz and a cw power of 400 W, and about 250 kW in the regime with a repetition frequency of 22.4 MHz and a cw power of 500 W.Modulation instability greatly affects the possible level of high harmonics in the NovoFEL facility. In the entire THz NovoFEL spectral r...

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confidence: 99%

Detailed observation of modulation instability on THz NovoFEL

Kubarev

2012

2012 37th International Conference on Infrared, Millimeter, and Terahertz Waves

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“…The third harmonic radiation is much more sensitive to the quality of the electron beam alignment and intracavity light coherence [6]. However, for a carefully aligned electron beam, the third harmonic lasing was so stable that it was impossible to achieve a pure first harmonic radiation by manipulations with the optical resonator only.…”

Section: Resultsmentioning

confidence: 99%

Third Harmonic Lasing on Terahertz NovoFEL

Kubarev

Кулипанов

Shevchenko

et al. 2011

J Infrared Milli Terahz Waves

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“…Other large-scale spectral peculiarities (~ GHz) of the NovoFEL radiation caused by different fast instabilities were investigated early in works [2][3][4][5][6][7]. We found two types of side-band modes caused by classical FEL instability on trapped electron [3,5,7] and by some instability of an unknown nature when the NovoFEL pulses have two-humped form [2,6,7].…”

Section: Introductionmentioning

confidence: 96%

“…We found two types of side-band modes caused by classical FEL instability on trapped electron [3,5,7] and by some instability of an unknown nature when the NovoFEL pulses have two-humped form [2,6,7]. These mode structures were measured both in a relatively stable regime [2,3,5,7] and a strongly unstable regime [6,7], where we used our original method of one-pulse spectral measurements. Full stabilization of all side-band instabilities was also demonstrated by using stabilization frequency detuning between electron and light pulses [3,[5][6][7].…”

Section: Introductionmentioning

confidence: 99%

“…These mode structures were measured both in a relatively stable regime [2,3,5,7] and a strongly unstable regime [6,7], where we used our original method of one-pulse spectral measurements. Full stabilization of all side-band instabilities was also demonstrated by using stabilization frequency detuning between electron and light pulses [3,[5][6][7].…”

Section: Introductionmentioning

confidence: 99%

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