The goal of this work is to prove the feasibility of building a laser system that can generate mid-infrared radiation with
the parameters required for the measurement of the hyperfine splitting in the ground state of the mounic hydrogen
The first experimental results of a very straightforward scheme that, to our knowledge, has not been considered in the
literature, are presented. We study a laser test bench system emitting nanosecond pulses of infrared tunable radiation in
the spectral range 6.78 μm with high energy and narrow line-width, based on direct difference frequency generation
(DFG), in non-oxide nonlinear crystals, using as pump lasers a single-mode Nd:YAG laser and tunable narrowbandwidth
The investigated system is based on lithium thioindate (LiInS2) and silver thiogallate (AgGaS2) crystals cut for type II difference frequency generation. The pulses of the Nd:YAG laser (1,064 μm) are combined with the pulses at ~ 1.262 μm of the Cr:forsterite laser through a dichroic mirror and sent to the nonlinear crystals in different optical geometries.
The generated radiation reaches an output energy up to 80 μJ in a single pass optical geometry, has 10 ns long pulses at
50 Hz frequency repetition rate and is tunable in the range 6595 – 6895 nm. These first results prove the suitability of
such an approach for building the laser system for the muonic-hydrogen experiment.
M2-factor of the master-oscillator power-amplifier (MOPA) CuBr laser emission compliant with ISO 11146 is studied
for first time. M2 is an invariant that gives how many times diffraction-limited is a laser beam compared to a perfect
Gaussian TEM00 beam. Statistical parameters of the near and far fields of MOPA laser radiation are measured by a beam
analyzing technique. Two patterns of the MOPA laser emission are examined: annular that is typical for lasers without
addition of hydrogen, and of filled-center (top-hat and Gaussian-like) with addition of hydrogen. 2D intensity profile
changes of the near and far fields are recorded as functions of delay time of laser excitation current pulses. The MOPA
gain curve is found and the influence of gain on the input signal (from MO into PA) due to the absorption/amplification
in PA on the field profiles is shown. The change of position and waveform of laser pulses is given too. For annular
radiation M2 range is from 13-14 (small delays) to 5-6 (large delays) and for filled-center radiation M2 is 6-7 (small
delays) and at the end of gain curve is as much as 4.
Great improvement of CuBr laser beam spatial coherence was made by a special design of the laser resonator, the generalized diffraction filtered resonator. Utilizing it diffraction-limited beam divergence can be easily obtained throughout the laser pulse. Since the spatial coherence is in inverse relation with the beam divergence, decreasing the latter we increase the former. The temporal evolution of beam divergence for the more intense green (λ=510nm) laser line was measured within laser pulse of MO (master oscillator) CuBr laser system fitted with a stable plane-plane resonator (PPR), a confocal unstable resonator of positive branch (PBUR) and a generalized diffraction filtered resonator (GDFR). With the MOPA (master oscillator power amplifier) system only GDFR was used. The estimations were verified by direct coherence measurements by means of a reversal shear interferometer that was a modified Michelson interferometer. The estimations as well as the direct measurement of spatial coherence show that coherence degree increases from PPR through PBUR to GDFR. Moreover, with GDFR it is time-independent. With MOPA system the coherence degree goes up further. So the degree of coherence measured interferometrically with MO is: for PPR - 0.16, for PBUR - 0.28 and for GDFR - 0.36. For MOPA the measured degree of coherence reaches 0.65. The estimated and the measured coherence trends show similarity. Based on the Michelson interferometer and having just four optical components (a spherical lens, an optical wedge and two plane mirrors), a new rigid instrument for spatial coherence analysis of optical beams was introduced as well.