3.4 Experimental Set-up
3.4.1 The Pump Source
A commercial Tm-fibre laser manufactured by IPG Photonics (Model TLR-80-1940) was used as pump source (Shown in Figure 3.5). The fibre laser required water cooling at a temperature setpoint of 20◦C which was supplied by a 1 kW cooling capacity circulatory water chiller. The fibre laser delivered a maximum of 86.5 W in a near- diffraction-limited beam with a specified M2 of 1.05. The fibre-laser’s wavelength was specified to match the Ho:YLF absorption peak at 1940 nm. It was subsequently
Tm-fiber laser
Master
Oscillator Ho:YLF
AOM
r-~-~----------------~~--'
1---+-;1
Ho:YLF
AMP
ToOPO
HR
CHAPTER 3. COMPACT HIGH POWER HO:YLF OSCILLATOR AND AMPLIFIER
Figure 3.5: The IPG fibre laser used as pump source for the Ho:YLF oscillator-amplifier system.
measured with a Jarrell Ash monochromator and found to be 1938.7±2.5 nm at full power. The output power of the fibre laser as a function of setpoint was measured using a LM-100 Coherent power meter. The results are shown in Figure 3.6.
Since there are strong water absorption lines in the 1.9µm wavelength region, it was necessary to investigate if water vapour in the atmosphere would pose a problem with the Tm:fibre laser as pump source for the Ho:YLF oscillator-amplifier system. In order to do so, the transmission spectrum of air in the region of 1940 nm was calculated using HITRAN (Rothman et al., 2003), and is shown in Figure 3.7. This calculation was for a distance of 1 m in air with a relative humidity of 50 % at 300 K. The fibre laser’s emission spectrum was superimposed on the graph to determine if there are any water absorption lines overlapping with the laser’s output spectrum.
It was concluded that there are water absorption lines and that it should be inves- tigated further. An experiment was devised where the fibre laser’s beam profile was measured with a Spiricon Pyrocam after propagating a certain distance. The resultant beam profile is shown in Figure 3.8.
The absorption of the light led to heating of the air, resulting in turbulence which in
[g]1 . I
CHAPTER 3. COMPACT HIGH POWER HO:YLF OSCILLATOR AND AMPLIFIER
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80 90 100
Fibre Laser Setpoint [%]
Optical Power [W]
Figure 3.6: Power slope of the fibre laser with regard to the setpoint.
turn led to beam distortion and beam wandering. The beam profiles shown in Figure 3.8 illustrate how this affected the beam intensity profile. After propagating only 1.85 m, the collimated beam was distorted at lower powers and severe beam wandering was observed. This was despite the fact that only 4 % of the light was absorbed in the air with the fibre laser running at 50 %. At higher fiber laser powers, these detrimental effects decreased and the total amount of laser power absorbed was negligible. This is attributed to a shift in wavelength of the fibre-laser with an increase of its output power, possibly caused by the heating-up of the internal fibre Bragg grating of the fibre laser.
From these measurements it became apparent that water absorption of the fibre laser’s light had to be taken into careful consideration when designing a system pumped by this laser, as pump beam distortion and beam wandering would affect an end- pumped system adversely.
There are several ways in which this problem is typically resolved:
• A fibre bragg grating can be used to shift the fibre-laser’s wavelength to a re- gion where there is no water absorption. There are limits to how far the laser’s
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CHAPTER 3. COMPACT HIGH POWER HO:YLF OSCILLATOR AND AMPLIFIER
Transmission
Wavelength [µm]
IPG fibre laser output Transmission through air, 50 % rel. humidity at 300 K
Figure 3.7: The transmission spectrum of air in the region of 1940 nm over a length of 1 m with a relative humidity of 50 %, and temperature 300 K. The fibre laser’s wavelength at full power is also shown.
Figure 3.8: Beam profile measurements of the fibre-laser beam 1.85 m away from the collimator at 50 % (left), and 100 % output power (right).
CHAPTER 3. COMPACT HIGH POWER HO:YLF OSCILLATOR AND AMPLIFIER
Tm: Fibre laser
Amplifier Output
82 W Pump
HR 2065 HT 1940
HR 2065
HT 1940 HR 2065
HT 1940
AOM
Ho:YLF c-axis vertical Ho:YLF
c-axis horizontal f=350 mm
HR 2065 r=500 mm
HR 2065
OC
HR 2065 Laser output
18 W transmitted
pump
Laser output (2065 nm) Pump (1940 nm)
Laser resonator (2065 nm)
Amplifier output f=100 mm
f=-50 mm
Figure 3.9: A schematic diagram of the fibre-laser-pumped Ho:YLF oscillator-amplifier system.
wavelength can be shifted though. This has already been implemented in the fibre-laser. The fibre laser could also be run at full power since the beam is least affected at full power. The beam must then be attenuated to obtain the desired pump power.
• The system could be encased in a box which is flushed with dry air. The subse- quent decrease in humidity would lessen the water absorption of the laser light.
This is the traditional solution to the problem.
• The distance the pump beam propagates in air before entering the laser crystal could be minimised. This imposes spatial restrictions on the setup.
From these options, it was decided to implement a scheme where the beam’s prop- agation distance is minimised as it would be the most practical solution. This would entail having the oscillator crystal’s pump-face as close to the fibre laser’s end as pos- sible, and the oscillator and the amplifier crystal close to each other. The spatial restrictions on the layout of the system imposed by this resulted in a compact laser.