Diode pumped solid state laser + SHG

This is quite a beginner level optical project.

As you may see, this project is divided into three parts, the diode laser, the pumping part and the SHG part.

The Diode Laser

A multi-mode laser diode is chosen as the core of the diode laser part. This diode laser is rated at 3W with wavelength around 808nm. In order to drive this powerful laser diode, we need to design a laser diode driver capable of providing constant current up to 3A. A picture of our 3W laser diode is shown below.

Considerable amount of heat also needs to be dissipated when the diode is running at full load. However, due to characteristics of laser diode, its frequency will drift greatly when temperature changes, which will make the pumping unstable. Hence thermoelectric cooler (TEC) is introduced here together with a self-made digital controller to set the laser diode at nearly any temperature (restricted by TEC).

With some tiny but carefully designed mechanical part, I may mount the laser diode rigidly on a water cooled dock. A rendered picture of the mount part and a picture of assembled laser diode module is shown below.


The Pumping Part

The pumping part will receive 808nm laser from the diode laser and pump a 2.5*2.5*10 size Nd:YVO4 crystal (0.3% doped) to produce 1064nm laser.

First, we need to focus the laser beam into a tiny spot to create sufficient power density in the crystal. I use two 30mm focus length lens to finish the task, which works great with this fast axis compressed (FAC) laser diode. The focused laser spot is really powerful, which even burns the ceramic IR viewing card (black lines and spots).


The crystal need a giant heat sink too. The crystal mount is made of copper. The crystal will be installed in the slot with a small copper block pressing on it. I also place some indium foil between the crystal-copper contact surfaces to ensure good thermal contact.



In this setup, one surface of the crystal is coated with 808HT+1064AR, and another is 1064HR. which means 808nm pumping light will go right into the crystal and be absorbed but the back surface will reflect any 1064nm laser to form a part of the cavity mirror. If I place a mirror that partially reflective 1064nm laser in front of the crystal  to form a whole optical cavity, we may get the 1064nm beam. The mirror in this experiment is a plain mirror, 90% transparent 10% reflective for 1064nm. I place it 30mm in front of the crystal. After some adjustment, we get the 1064nm laser spot! (The dark red spot is expand 808nm laser while the bright green spot is 1064nm laser)


The SHG Part

I choose KTP crystal in this experiment. KTP is a very common SHG crystal, often found in high power CW DPSSL lasers. The KTP I use is 3*3*5 in size, mounted on a copper adapter to fit into ordinary lens holders.


If the KTP crystal is directly placed into the Nd:YVO4 laser cavity, a strong green laser beam can be observed. However, if we want to study the characteristic of SHG, it may be a better choice to put it outside the cavity. Adding a small prism after the KTP, I can split the 532nm beam and 1064nm beam. (Take a picture using a camera with IR filter, you can see a green 532nm spot on the left and a shining 1064nm spot on the right, indicating low SHG efficiency without cavity)


Some Measurement

Some results from Ocean Optics QE65 Pro spectrometer:

The left one shows the result when the output coupler mirror is not present. This spectrum clearly shows the fluorescence of the Nd:YVO4 crystal. Several emitting peaks and strongly absorbed 808nm peak can be found in this graph.

The right one shows the result after KTP is installed outside the cavity. Three peaks of 532/808/1064 wavelength are shown on the graph.



A picture of the whole setup:


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