ORPHEUS User’s Manual
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5.2 Pump Beam Delivery and Splitting
Pump beam from the laser is directed into ORPHEUS using external mirror BRM1 and mirror BRM2,
which is attached to OPA housing. The beam then passes an iris aperture A1 and a polarization rotator RP1
(λ/2 plate). A1 should always be fully open, unless aligning the device at very low pump intensities only.
Beam height from the base plate of ORPHEUS at the input is 25 mm. A polarizer P1 reflects most of the
pump beam energy for second harmonic generation and transmits 0.2–1.5 µJ for white light continuum
generation (depends on configuration).
5.3 White Light Continuum Generator
After passing the polarizer P1, pump beam polarization is rotated to vertical by RP2. The beam then
travels to the retro reflector which consists of two mirrors M1-M2, placed on a computer-controlled
translation stage (“Delay 1” in the software). Computer controlled adjustment of the optical path length of
pump beam ensures the temporal overlap of white light and pump pulses in the first amplification stage.
The lens L1 is set to focus the beam into the WLG – white light continuum generation substrate. The
white light should be stable and single filament. The best way to check the shape of the WLC is to place a
business card right after the iris A2. If the WLC is not visible, its shape is asymmetric or has an interference
pattern - proceed to the troubleshooting section of this manual.
Iris aperture A2 blocks the outer rings of the WLC. Under normal operation it is closed to about 1-2
mm diameter. The beam passes through the L2 lens. Dichroic mirror DM1 reflects the residual pump beam
downwards and transmits the white light continuum. The reflected residual pump is blocked inside DM1
mount.
Lens L3 is used to ensure proper diameter of the WLC at the amplification crystal Crystal 1. The pulse-
width of the continuum pulse at lens L2 is roughly the same as the initial pump pulse (~300 fs). The TD
element disperses the white light in time due to group velocity dispersion – different wavelengths travel
through the substrate at different velocities. After the TD the longest wavelength (1040 nm) arrives earlier
than the shortest one (620 nm). The resulting pulse-width of the chirped white light after the TD element is
roughly 12-20 ps, depending on the thickness and substrate of the material of TD element used. This white
light continuum will be overlapped at the nonlinear crystal with a ~200 fs long pump pulse. By tuning the
“Delay 1” translation stage a different spectral region of white light is overlapped temporally with the pump
pulse, which allows selective seeding of the amplification process.
5.4 Generation of the Second Harmonic of Pump
Major part of pump beam, which is reflected by the polarizer P1 and mirror M9, is used for
generation of the second harmonic of laser radiation. Lenses L8 and L9 constitute a down-collimating
telescope. At very low pump pulse energies (<20 μJ), the beam after the telescope is not collimated, rather
it is set to focus near the nonlinear crystal. The reduction factor and focusing condition is chosen depending
on the pump pulse parameters during the installation.
The second harmonic generation SHG Crystal is placed on a computer-controlled rotation stage
(“SHG Crystal” in the software). By changing the angle of the crystal, it is possible to some extent to
attenuate the power of the second harmonic radiation, and in turn – signal and idler radiation, however it
can also change the wavelength, pulse duration, beam size of the parametric radiation pulses. The crystal
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