P.J.M. van der Slot

High-efficiency mid-infrared ZnGeP2 optical parametric oscillator directly pumped by a lamp-pumped, Q-switched CrTmHo:YAG laser

We report a singly resonant optical parametric oscillator (SRO) based on a ZnGeP(2) crystal directly pumped by a lamp-pumped Q-switched CrTmHo:YAG laser. The IR was tunable from 4.7 to 7.8 microm via crystal angle tuning. A maximum optical to optical efficiency of 56% was obtained from the pump (2.09 microm) to total IR at a pump energy of 6.5 mJ. The corresponding idler energy was 1.45 mJ. The SRO was measured to have a slope efficiency of 64% and a threshold of 1 mJ. The spatial beam quality of the idler, characterized by the M(2) parameter, was 1.38 when the SRO was pumped at 2.5 times threshold. These results show that ZnGeP(2) optical parametric oscillators directly pumped by a CrTmHo:YAG laser can be operated efficiently, while maintaining good IR beam quality.

Energy and frequency measurements on the Twente Raman free-electron laser

Results of energy and frequency measurements on the Twente Raman FEL are presented and discussed. A highest energy of 230 mJ was measured using a joule meter. The spectrum was determined using waveguide cutoff filters. These measurements generally showed a broad spectrum containing two or more peaks. Far from magneto-resonance this indicates that the radiation field grows at the upper as well as at the lower interaction of the slow space-charge wave with the the electromagnetic waveguide modes. Near magneto-resonance, the analysis of the spectra shows the presence of the cyclotron instability together with the FEL instability. The two instabilities are resonant with different waveguide modes. The latter was confirmed by a measurement of the radial profile of the emitted radiation for a single setting of the guide and undulator field.

Comparison between a FEL amplifier and oscillator

Previous experiments with the Raman FEL, situated at the Twente University, showed that the output was influenced by the rather strong increase of the current density with time. The field emission diode has been modified to produce a more constant current pulse to simplify the analysis of the measurements. This resulted in a lower current density of the electron beam. With this new diode two set-ups are studied. In the first set-up the laser is still configured as an amplifier whereas in the second set-up the laser configuration is changed into an oscillator using a Bragg reflector with a space-variable corrugation height. For both set-ups we measured the frequency spectrum for specific values of undulator and guide magnetic fields. The relative performance of the amplifier and the oscillator configuration will be presented.

A large core polymer optical fibre sensor for x-ray dosimetry based on luminescence occurring in the cladding

An optical fibre sensor for short pulse duration x-ray dosimetry is presented. The sensor is based on luminescence generated in the cladding of a 1 mm core diameter polymer optical fibre which has been doped with a radioluminescent phosphor. On interaction with x-rays, this phosphor emits visible light, part of which is coupled to the fibre core through a combination of surface roughness at the core?cladding interface and through evanescent wave coupling of these guided waves. From here it is transmitted to an optoelectronic photodetector for monitoring. A 15 cm fibre sensor was used for the experiment which was conducted using a pulsed x-ray source normally employed for the preionization of excimer lasers. The results are ncalibrated against the emission intensity from a scintillating plastic block and a pen dosimeter. The peak output signal of the fibre sensor increases linearly with the dose produced by the x-ray source. There is also a discussion on the long-term stability of such a sensor, the expansion of this sensor into a multi-point device and methods to improve the efficiency of the luminescence coupling from cladding to core for large core polymer fibres included.

Phase velocity fluctuations and gain in Cherenkov free-electron lasers

We report on the influence that waveguide-induced fluctuations in the phase velocity of the radiation wave have on the gain of Cherenkov-type free-electron lasers. We theoretically analyze low-gain Cherenkov free-electron lasers with a compact design that require resonator feedback for operation. We find that even small spatial variations of the waveguide parameters along the propagation direction can lead to significant degradation of the gain, and we present typical values for the required degree in waveguide precision that would eliminate such effects

A Cherenkov free electron laser with high peak power

A Cherenkov FEL can be a suitable source for radiation from the millimeter wavelength region down to the far infrared. With only a few different dielectric materials the laser can range from 6 mm down to ? 600 ?m. Nonlinear theory shows, for an amplifier configuration, power levels of about 100 kW in the far infrared up to several MW at millimeter wavelengths. Good coupling with the evanescent wave supported by the dielectrically lined waveguide requires a thin walled annular beam of good quality. The characteristics of such an electron gun, a special design for a Cherenkov FEL with parameters given in this paper, will be presented.

Photonic Free-Electron Lasers

A photonic free-electron laser (pFEL) produces coherent Cerenkov radiation from a set of parallel electron beams streaming through a photonic crystal. The function of the crystal is to slow down the phase velocity of a copropagating electromagnetic wave, such that also mildly relativistic electrons (of about 10-keV energy) can emit coherent Cerenkov radiation. Starting from spontaneous emission, the feedback of the radiation on the electrons results in bunching of the electrons on the scale of the radiation wavelength, and consequently, coherent radiation can build up. The frequency of the coherent mode is set by the electron velocity and wave dispersion of the photonic crystal and can, a priori, be continuously varied by varying the electron energy. The scale invariance of Maxwells equation allows operation from Gigahertz to Terahertz and possible infrared (IR) frequencies without the need to increase the electron beam energy. Therefore, the pFEL is a very attractive, compact, and coherent radiation source that has the potential to significantly enhance the power available in the THz domain.

First lasing of a Cherenkov free-electron laser with annular electron beam

A Cherenkov FEL with an annular electron beam has been succesfully operated. During the first lasing experiments output was found for accelerating voltages ranging from 85 to 150 kV and currents from 1 to 10 A. At a particular accelerating voltage lasing was observed for several distinct settings of the magnetic field (0.7–1.6 T). This field is used to compress and guide the electron beam along the dielectric liner. Depending on the magnetic field setting, the output was reproducible or showed large fluctuations within a single laser pulse as well as from pulse to pulse. Due to a misalignment of the cathode the total output power is only of the order of 100 W. From theoretical considerations the spectrum is expected to center around 19 GHz.

Design of a 30 GHz bragg reflector for a Raman FEL

A design of a Bragg reflector for a Raman FEL is described. It is shown that mode conversion occurs whenever the axial wavenumbers of the two modes fulfil the Bragg condition. With a constant ripple of the corrugation it is shown that the reflected radiation also contains higher order modes, assuming that the incident radiation consists only of a TE11 mode. The mode purity can be increased by increasing the length of the reflector at the expense of a smaller reflection bandwidth. A more flexible method is by applying a Hamming window to the corrugation of the reflector. Contributions of other modes to the reflected radiation can in that case be neglected. The reflector will be installed in a Raman laser to be able to compare the amplifier with the oscillator configuration. Therefore some preliminary results are also presented about the start-up of the Raman laser.

Temporal model for quasi-phase matching in high-order harmonic generation

We present a model for quasi-phase matching (QPM) in high-order harmonic generation (HHG). Using a one-dimensional description, we analyze the time-dependent, ultrafast wave-vector balance to calculate the on-axis harmonic output versus time, from which we obtain the output pulse energy. Considering, as an example, periodically patterned argon gas, as may be provided with a grid in a cluster jet, we calculate the harmonic output during different time intervals within the drive laser pulse duration. We find that identifying a suitable single spatial period is not straightforward due to the complex and ultrafast plasma dynamics that underlies HHG at increased intensities. The maximum on-axis harmonic pulse energy is obtained when choosing the QPM period to phase match HHG at the leading edge of the drive laser pulse.