Lab Report 4: Diode Pumped Nd-YAG Laser

LIN PEI-YING, BAIG JOVERIA

Introduction

Neodymium Yttrium Aluminum Garnet lasers (Nd:YAG) have numerous applications in the world today. These include the medical and scientific fields for processes such as laser spectroscopy and surgery. A Nd:YAG laser can be optically pumped using lasers of several different wavelengths. This report will discuss some of the basic characteristics of the Nd:YAG laser such as the efficiency, atomic levels, optical pump source and modes of operation.

Background theory

Pulse Laser

Since the invention of the laser, the delivered peak power has increased steadily (see Figure 1). To date, powers up to a few Peta Watt (1 PW = 1015 W) are generated in a few facilities around the world with state-of-the-art technology. Clearly, such high powers cannot be sustained for a long period of time, therefore high peak powers are obtained by means of pulsed laser operation. In fact, extremely intense laser sources can emit light only for a fraction of a picosecond (1ps =10-12 s).

Fig.1. Progress of laser peak power in the last 5 decades. (from [Mourou]).

Characteristics of optical pumping:

Optical pumping is a process in which light is radiated into a specimen under investigation and the effect of the light on the specimen is examined. It was in this way that the strange physical phenomenon was observed of atoms only being able to accept or release energy in well-defined quantities. This observation led to the conclusion that atoms only have discrete energy states or energy levels. When light is absorbed or emitted, a transfer takes place between the energy levels.

Nd:YAG laser system

Fig.2. Energy level structure and common pump and laser transitions of the trivalent neodymium ion in Nd:YAG. (from [RP Photonics Encyclopedia])

Nd:YAG is an abbreviation of Neodymium-doped YAG (Nd:YAG). It is a kind of solid-state lasers. It can be used for everything from low-power continuous-wave lasers to high-power Q-switched (pulsed) lasers with power levels measured in the kilowatts. Most Nd:YAG lasers produce infrared light at a wavelength of 1064 nm. Light at this wavelength is rather dangerous to vision, since it can be focused by the eye's lens onto the retina, but the light is invisible and does not trigger the blink reflex. Nd:YAG lasers can also be used with frequency doubling or frequency tripling crystals, to produce green light with a wavelength of 532 nm or ultraviolet light at 355 nm. The dopant concentration in commonly-used Nd:YAG crystals usually varies between 0.5 and 1.4 molar percent. Higher dopant concentration is used for pulsed lasers; lower concentration is suitable for continuous-wave lasers. Nd:YAG is pinkish-purple, with lighter-doped rods being less intensely colored than

heavier-doped ones. Since its absorption spectrum is narrow, the hue depends on the light under which it is observed.

Q-switching mode operation:

Fig.3. A scheme of principal of Q-switching The quality of a resonator is the quotient of the resonance frequency and the half width of the resonance curve. The definition is the same as for oscillator circuits known from electronics. A low Q figure for a resonator signifies high losses and vice versa. In a laser resonator with low Q, a high inversion can be produced without laser oscillation, because the threshold is high. If the resonator Q is then suddenly switched to a higher value, a high photon density is formed and a large part of the inversion stored in the laser-active material transfers into the photon field.

Intra-cavity frequency doubling of the laser wavelength:

In this step we observe that the 1064nm Infrared laser convert to green light with wavelengths of 532nm. That is because of the nonlinear property of KDP crystal. When KDP crystal satisfies the phase matching condition, the amplification for the second harmonic can be obtained. Frequency doubling is also a kind of nonlinear optical effect. Certainly we can use these effects to achieve the conversion of different light.

Experiment :

Fig.4. Experimental setup (A = pump laser diode; B = anamorphic collimator; C = focusing lens; D = Nd:YAG crystal; G = power meter)

Preliminary Alignment:

The experimental setup used is shown above. The beam is first aligned very carefully by adjusting the collimation and anamorphic system such that it is parallel to the bench. The mirrors are also aligned to make the back reflection collinear with the forward pump beam.

Optimization:

The temperature of the laser diode also has an impact on the absorption peak of the Nd:YAG, hence it is important to set the temperature of the laser diode to 22°C at 808 nm to optimize it according to figure 5. Figure 5: Wavelength vs Temperature

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Figure 7: Pump Power vs Absorbed Power

The power characteristic of the pump diode are plotted as a function of pump current right after the focalization lens. The result is shown in Figure 6. As expected, the characteristic shows that above the threshold value of about 300 mA, the power increases approximately linearly with current as was expected. To analyze how much of this power is absorbed by the pump diode, the power transmitted from the pump diode is also recorded. The graph of absorbed power versus the pump power is shown in Figure 7. It shows a fairly constant transmission over all pump powers, hence showing a linear relationship between absorbed power and pump power. The next part of the experiment involves the use of a function generator to provide a supply square signal. The RG 1000 filter is then inserted in to the setup. This filter is used to exclude the wave that maybe passing through the system from the incidence. Hence, this filter acts as a low pass filter and its behavior is observed on the oscilloscope by adjusting the load resistance. This result is shown in Figure 8. Since this circuit behaves like an RC resonant circuit, the sensitivity is seen to increase with the load resistance as more voltage is tapped and the bandwidth is inversely proportional to the resistance as the resonance frequency is determined by the inverse of the product of resistance and capacitance. Hence, this

load resistance is chosen to be 1 MΩ so that the lower frequencies are allowed to pass through and blocking the contribution from the 808nm incidence light.

Figure 8: Oscilloscope showing the behavior of RG 1000

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Quantum efficiency

Using the method of slope at the origin, the photon lifetime was measured to be 0.243 which is found to be very close to the tabulated value of 0.230 found in the datasheet of the Nd:YAG. The next setup involved analysis of the lasing effect by creating a cavity. For this purpose, the setup was modified to that shown in Figure 9. The cavity is aligned to obtain the TM0 mode. This is a tricky alignment and in order

to create lasing in the cavity, two effects must be considered. The diffraction of the beam will result in a beam divergence and it must be insured that the length of the cavity is kept shorter than the radius of curvature of the lens. Additionally in order for must be ensured that mirror is placed close enough that the light

intensity reaches it since the light diffracts and has a beam divergence associated with it. The laser power output power is measured and plotted against the pump power taking in to account the transmission of RG 1000. The plot is as shown in Figure 10. The graph shows small deviations from a linear fit. At this stage, it is interesting to quantify the quantum efficiency of the system since this determines how much of the incident photons are finally emitted in lasing. This result

Figure 9: Setup for lasing

Figure 11

is displayed in Figure 11.

Frequency Doubling:

The experiment is then modified slightly to observe frequency doubling as shown in Figure 12. A highly reflective output mirror is used for this purpose since the frequency doubling phenomenon is based on a non linear effect.

The cavity is realigned with this mirror and a KTP crystal is introduced in to the cavity. This is a fairly tricky alignment. Hence, initially, the beam is made to pass through the KTP crystal and power is monitored while placing it out of the cavity in order to align the beam parallel to the bench. Then this crystal is introduced in to the cavity. With the correct alignment, the set up begins to perform lasing at a wavelength of 532 nm. A BG18 filter is used to exclude the power from the wavelength of 1064 nm to interfere with the useful required component. The results are shown in Figure 13. A power of 132 µW was observed.

Conclusion:

The lab report focused on the demonstration of lasing in a stable cavity created with the help of Nd:YAG and highly reflective mirrors. The laser output power was determined and quantum efficiency was calculated for this system. Finally, intra cavity frequency doubling was achieved by the use of KTP crystal.