20 mJ, 1 ps Yb:YAG Thin-disk Regenerative Amplifier

The overall goal of this protocol is to demonstrate the design and operation of a high energy, high power, Yb:Yag thin disk regenerative amplifier as a front end for fuse cycle optical parametric chirped pulse amplifiers. Ytterbium YAG thin disk lasers in combination with optical parametric chirped pulse amplifiers, or the so-called third generation femtosecond technology gives us the chance to generate sub-cycle to few-cycle pulses at high energy and average power simultaneously. The main advantage of the laser system discussed in this movie is this capability of average power and big power in the same time for laser pulses as short as picoseconds.

Though this laser can serve as a pump for few cycle lasers, it can also be used as the front end of other systems, such as ultraviolet to mid-infrared widely tunable lasers. These benches hold the different stages of the experimental set-up. The stages are best understood using the schematic.

Note the labeling of the oscillator, stretcher, regenerative amplifier and the compressor stages. First focus on the oscillator stage. It's 13 meter linear cavity has a copper hard aperture, as sapphire cur medium, and a 13%transmission output coupler.

The mirror system includes three high dispersion mirrors, and two mirrors with a negative radius of curvature. Pump the oscillator via fiber coupled diodes coupled to ytterbium doped yttrium aluminum garnet thin-disk amplifier. After the output coupler, a barium boron oxide crystal serves as a pulse picker for the pulse stretcher stage.

The pulse stretcher temporally stretches seed pulses to approximately two nanoseconds. The pulse next enters the regenerative amplifier stage. There, the seed pulse is held in amplifier cavity.

The cavity has a high voltage Pockels cell with a barium boron oxide crystal. The regenerative amplifier uses another ytterbium doped yttrium aluminum garnet thin-disk that is pumped by fiber coupled diodes. The final stage is a pulse compressor stage, where the amplified pulses are compressed to one picosecond.

Note that throughout the system, the pump diodes, thin-disk heads, and the bread board require chillers to keep them cool. Begin by starting the water cooling for the system. Turn on the water flow and the cooling chillers for the oscillator components.

From the chiller control, move to the pump diode power supply, and turn it on. Next turn on the pump diode output. Adjust the current to pump the thin-disk at 940 nanometers in continuous wave mode.

At this point, arrange to observe the output spectrum before the pulse picker. Connect the fiber to a spectrometer, and place it in the beam path. The center of the spectrum is at 1030 nanometers.

After observing the spectrum, replace the fiber with a power meter at the same location. Observe the output power of the continuous wave mode. Next, move on to pulsed mode operation and mode locking.

Work at the oscillator mirror that is on a translation stage. Perturb the mirror by mechanically pushing the stage from the back. Once again, observe the output spectrum before the pulse picker after the pulsed mode is initiated.

The center of the spectrum is at 1030 nanometers with a bandwidth of about four nanometers. Next, once again, observe the output power. Now make arrangements to observe the output pulsed terrain with an oscilloscope.

Connect the fast photodiode to the oscilloscope with appropriate attenuation. As before, perform the measurement before the pulse picker. Place the photodiode in the beam path.

On the oscilloscope, tune the trigger level knob to stabilize the repeating wave-forms, and allow observation of the output pulsed train. Use the peak to peak amplitude from this observation to find the pulse to pulse stability. After this, substitute the photodiode in the beam path with a beam profiler.

Use the beam profiler software to display the beam profile and to perform beam pointing stability measurements. Proceed to work with the pulse picker by first configuring its software. From there, move to the electronics.

Turn on the high-voltage to the pulse picker crystal to produce the repetition rate. After the pulse picker, use a thin film polarizer to select pulses. These seed pulses go through the grating stretcher, and then undergo regenerative amplification.

Prepare the regenerative amplifier by starting it's cooling system. Turn on the cooling water and set the chillers for the laser head and Pockels cell. At the Pockels cell computer, start the Pockels cell software.

Go to the Pockels cell driver's power supply, and turn it on. Then turn on the power supply of the pump-diode unit, and start diode output. Adjust the current from the power supply to amplify the seed pulses.

Next, choose a position before the compressor stage to observe the output. Use a spectrum analyzer and fiber to observe the spectrum of the regenerative amplifier. When done, read the output power from the power meter.

Note the power reading at this point before the compressor stage. Replace the spectrum analyzer with a fast photodiode connected to an oscilloscope. Observe the pulse train on the oscilloscope screen and then determine the pulse to pulse stability.

After replacing the fast photodiode by a beam profiler, observe the output beam profile, and measure the beam pointing fluctuations. After amplification, send the output pulses through the compressor stage. Here this is done with a half-wave plate with a motorized rotation mount.

Adjust the mount's controls to allow a few Watts of laser power through. Next in the path, have a beam stabilizer in place and aligned. With the stabilizer on, use its software to lock the laser beam and avoid beam drift.

Now, return to the half-wave plate, and allow the full amplifier output though. The pulses travel onto the compressor stage, which compresses them to one picosecond. The final optical elements are represented in this schematic.

After the compressor stage, prepare the beam for parametric chirped pulse amplification. On the bench, use a telescope to columnate and adjust the beam size. Obtain feed back on the beam from a beam profiler.

Guide the fundamental beam through a non-linear crystal to generate the second harmonic. Have a harmonic separator at 45 degrees to separate the second harmonic beam from the fundamental beam. Use the beam profiler to observe the Gaussian beam profiles of the second harmonic beam, and the profile of the unconverted fundamental beam.

This is the output beam profile of the regenerative amplifier. Seed pulses are amplified to 125 Watts while being pumped at 280 Watts continuous wave. A plot of the power output of the regenerative amplifier as a function of time demonstrates the system's stability while operating at an optical efficiency of 45%Here, the amplifier spectrum is plotted in green and is referred to horizontal axis along the top of the plot.

The retrieved temporal intensity after compression to one picosecond full-width at half-maximum is plotted in blue using the lower horizontal axis. These two curves give the experimental second harmonic generation efficiency versus input pump peak intensity in a 1.5 millimeter thick barium boron oxide crystal. The black curve is for 0.5 millijoules of amplifier output.

The green curve is for 20 millijoules of amplifier output. The points labeled A, B, and C, label different second harmonic generation efficiencies at different peak intensities. Here are the retrieved spectral intensities plots for each labeled value.

A is plotted in red, B in orange, and C in blue. This laser, when it is combined with a broad band optical parametric amplifiers or a high harmonic generation system, allows us to generate attosecond pulses or femtosecond pulses at higher energy and average power. These parameters open a new era for researchers to explore, for example, light driven electronics in fast time scales.

After watching this video, you should have a good understanding of how high-power, high-energy ytterbium:YAG thin disk laser is designed and operated. Don't forget that working with high-energy, high-power lasers can be extremely hazardous and sensitive to dust. Therefore, precautions such as safety goggles and cleanroom coats should always be used while operating this laser.