SBS phase conjugation of a cw pumped acusto-optical Q-switched Nd:YAG laser

SBS phase conjugation of a cw pumped acusto-optical Q-switched Nd:YAG laser

Phase conjugation based on stimulated Brillouin scattering can be used to compensate thermal effects in the active medium of solid-state-lasers at high average output power and enhance the beam quality to the diffraction limit (TEM₀₀). With this concept e.g. 200 W in TEM₀₀ operation were achieved with a flashlamp-pumped Nd:YALO-MOPA-system. In our group we developed a flashlamp-pumped Nd:YALO oscillator with a phase conjugating mirror based on SBS, which provides output powers up to 50 W in TEM₀₀ operation with a total efficiency of 1.7 %.

In industrial applications most common used laser are pumped cw with lamps or diode lasers. In these systems the average output power with a good beam quality is in the range of 30 W. The lowest power thresholds achieved in bulk materials and focusing geometry are in the range of 10 kW in a material with high Brillouin gain, like e.g.CS₂. Typical peak powers, that can be achieved with periodical Q-switch and beam quality in cw pumped lasers, are below 10 kW (see Fig. 1).

For these system you need a phase conjugating mirror with a much lower power threshold. Commercially available glass fibers with a low Brillouin gain, a long interaction length and a small diameter have power thresholds in the range of Watts. In order to use the interaction length the laser must have a long coherence length in the same range. Moreover due to the small diameter the quality of the phase conjugation, the fidelity and the peak power is limited. It is possible to combine the advantages of the high Brillouin gain of CS2 and the longer interaction length of fiber optics if you fill CS2 in a small capillary which works as a wave guide.

Fig. 1: Typical pulse peak power of a cw pumped, periodically Q-switched laser as a function of the repetition frequency.

Another way to reduce the SBS threshold is a special designed glass fiber with an SBS oscillator and amplifier in one fiber. In our group a fiber phase conjugating mirror was developed, which has a power threshold of about 100 W and high reflectivity of more than 90 % at a interaction length of 1 m.

In Fig. 2 the used laser is depicted. It is a conventional cw lamp-pumped TEM₀₀ laser, which was acusto-optically Q-switched with repetition rates of up to 10 kHz. The telescope build with the two lenses L₁ and L₂ the mode volume in the Nd:YAG crystal (diameter 4 mm, length 77 mm) was enlarged. The etalon E increases the coherence length of the laser.

Two different etalons were used in this laser:

R = 40 %, length = 3 cm single longitudinal mode pulse energy app. 0.4 mJ pulse duration app. 200 ns maximum output power app. 1.4 W	Fig. 3: Temporal pulse form with strong mode selection
R = 10 %, length = 0.5 cm several longitudinal modes pulse energy app. 1.0 mJ pulse duration app. 150 ns maximum output power app. 6 W	Fig. 4: Temporal pulse form with weak mode selection

With the first etalon the laser operates in single longitudinal mode but the losses in the laser resonator are so high, that the maximum peak power of 2 kW is below or in the same range than the SBS threshold of the new phase conjugating mirror. With the second etalon the mode selection is weaker and the laser operates with several longitudinal modes. The coherence length is in the range of few centimeters. The pulse peak power is higher than 6.5 kW. The phase conjugating mirror is a CS₂ filled capillary with a diameter of 80 µm and a length of 30 cm, at the endfaces the capillary is sealed with a optical window which is antireflection coating on the outside. The capillary is drawn by hand, so the diameter is not homogenous over the length but parabolic. 80 µm is the diameter in the minimum in the middle of the capillary, therefore the resulting interaction length is less than 30 cm.

The emitted laser pulses are diminished by the polarizer P. The polarizer and the Faraday rotator work as an optical isolator to ensure that no light reflected in the capillary is coupled into the laser resonator. The light was coupled into the capillary with the lens ( focal length 80 mm). The reflected light was reflected at a glass plate onto energy detector and a CCD camera.

Fig. 6: Spatial profile of the reflected beam

The profile of the reflected beam is Gaussian similar to the incident beam. The beam path is also similar even when another lens with a long focal length is inserted between the glass plate and the lens L.

Fig. 7: Temporal pulse form of the transmitted and reflected light

The temporal pulse form of the transmitted and the reflected light is depicted in Fig. 7. The reflectivity is a function of the incident power, when a certain power is exceeded the reflected portion increases clearly and the transmitted power saturates. This is a distinct evidence, that the stimulated Brillouin scattering is in this experiment in the non-transient region, the reflectivity of the SBS mirror shows no threshold relative to the incident pulse energy but relative to the incident power.

Fig. 8: Energy reflectivity as a function of the incident pulse energy

The threshold of this SBS-mirror with a wave guide structure is almost ten times smaller than the SBS threshold of bulk CS2 in focusing geometry. The energy reflectivity as a function of the incident pulse energy is depicted in fig. 8. The strong fluctuations in the reflectivity are likely due to the fluctuations in the longitudinal mode structure. The maximum reflectivity is more than 45 %, it is limited to the Pulse peak power of the pump laser and show no saturation effects. The reflected average power is in the range of some mW, at higher incident power gas bubbles appear in CS2 and the reflectivity breaks down. It last some minutes until the bubbles left the interaction zone and the reflectivity raises up again.