S.
Blair
October 19, 2010
ECE 5410 - Optical Electronics Laboratory Assignment #2
Introduction In this lab you will design and construct a working laser based on an open-cavity HeNe laser tube, as shown in Figure 1. From a set of high reectance mirrors, you are to choose one mirror
Figure 1: Laser resonator schematic. pair which results in a stable resonator conguration. You will then construct the cavity, obtain lasing, and make some simple measurements. Cavity stability is determined by the product of the g parameters. The g parameters are calculated given the total mirror-to-mirror cavity length L and the mirror radius of curvature R: L g2 = 1 (1) R2 for the front mirror (the output coupler) for example. The laser cavity is stable when 0 < g1 g2 < 1 (2)
is satised, where g1 is the g parameter of the back mirror (the high reector). There will be a contest associated with this lab exercise. The group that obtains the highest output power from the laser will get extra credit for the lab. In addition, since this is a group exercise, you will need to evaluate (in your own lab book) the other group members in terms of their participation and contribution to the completion of this lab. This is not a team exercise. Your TA will also rate each team member. Grade the other members in the following categories on a 1 to 3 scale (1 = poor, 2 = average, 3 = good): 1) Participation in pre-lab analysis 2) Participation in laser alignment 3) Participation with measurements 4) Participation in post-lab analysis 5) Contribution to the success of the lab Initial Design Before coming to lab, each group should design one stable resonator. The laser tube is about 70 cm in length, but the tube housing and external mirror mounts set the minimum cavity length to about 82 cm. Thus your total cavity length L should be 82 cm or greater (but no greater than about 130 cm due to the size of the optical rail). The available mirrors are listed in Table 1. Note
S. Blair
October 19, 2010 Mirror 1 2 3 4 5 6 7 8 9 10 T% 0.0 0.0 0.0 0.0 0.0 1.0 2.0 2.0 2.0 5.0 R (cm) 30 50 300 600 60 300 500 30
Table 1: List of HeNe mirrors available. Each mirror has diameter 1.5 cm. The combined absorption and scatter of each mirror is about 0.2%.
that each mirror has about 0.2% scattering loss. Choose one mirror pair and cavity length L that will result in a stable resonator. For the mirror pair, the back mirror should have high reectance, or low transmittance of 0.04%. The other mirror (output coupler) should have transmittance T of greater than 0.5%. The other design constraint that you need to deal with is to make sure that the fundamental Gaussian mode of this cavity ts within the bore of the 64 cm long laser tube without appreciable power loss due to aperture clipping, but large enough to eciently ll the volume of excited gas for maximum power. The bore diameter of the glass tube is approximately 1.4 mm. You must therefore calculate the beam size at each mirror (remember that the spot size is 2w) and the position of the beam waist inside the resonator in order to determine whether the beam will t snugly inside the tube. With your chosen mirrors, calculate the total loss within the cavity based only on the mirror reectances. The minimum waist of the Gaussian beam within the resonator cavity is calculated from:
4 w0
L(R1 L)(R2 L)(R1 + R2 L) , (R1 + R2 2L)2
The length of the cavity can be written L = z2 z1 , where z2 and z1 are the distances between the mirrors and the position of the beam waist, as shown in Figure 2. These positions can be calculated from z1 = z2 L(R2 L) R1 + R2 2L L(R1 L) = . R1 + R2 2L
In general, the mathematical form for a Gaussian beam is written w0 (x2 +y2 )/w2 (z) ik(x2 +y2 )/2R(z) i(z) E(x, y, z) = E0 e e , e w(z) where w(z) = w0 1 + (z/z0 )2 is the beam size at some position z from the waist w0 , R(z) = 2 z + z0 /z is the radius of curvature of the phase front, (z) = kz tan1 (z/z0 ) is the propagation
S. Blair
October 19, 2010
Figure 2: Laser resonator schematic.
2 phase, and z0 = w0 n/ is the beam Rayleigh range. The intensity distribution is given by
E 2 e2(x +y )/w (z) , I(x, y, z) = 0 2 2 1 + z 2 /z0 which is what you will measure in lab. Note that the beam width is given by represents the full-width between 1/e intensity points. Lab Procedure
2w(z), which
HeNe is a low gain laser (about 9.3% gain per pass for the one in the lab), so precise alignment is critical to obtain lasing. Because this is a low gain laser, the end mirrors must be kept free from dust and ngerprints that would induce scattering losses exceeding the available gain. Therefore, all mirrors should be handled carefully and stored face up inside the covered box. You will be responsible for cleaning any smudged mirrors! THE OPEN CAVITY HeNe TUBE HAS EXPOSED HIGH-VOLTAGE ELECTRODES. DO NOT TOUCH THESE ELECTRODES WHILE THE POWER SUPPLY IS TURNED ON. YOU SHOULD DO ALL MAJOR ALIGNMENT WITH THE POWER SUPPLY OFF. ONLY TURN ON THE POWER WHEN YOU ARE READY TO MAKE THE FINAL ADJUSTMENTS TO OBTAIN LASING. Because alignment is critical, a separate closed HeNe laser has been set up and aligned to the optical rail for use in aligning your open cavity. Do not disturb this external laser. If you do, you will have to realign the whole system. The open HeNe tube has already been aligned to this laser, however, if realignment is necessary, use the following procedure. Remove the open cavity mirrors. Adjust the height and translation at each end of the open tube so that the alignment beam passes unobstructed down the bore. The easiest way to get alignment is to observe scattering of the HeNe beam within the open tube. Good alignment is achieved when very little scattering is evident. Finally, with a piece of paper, observe the pattern of the alignment beam after it passes through the open tube. The pattern should be small and bright, almost free from extraneous rings or halos. Rings in the pattern indicate that the beam is reecting o the sides of the bore at some position. Make minor adjustments to eliminate those rings. Now you are ready to align the external mirrors. Start rst with the high reector, which is on the side of the tube farthest from the alignment laser. When the high reector is aligned, the
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October 19, 2010
alignment beam will retroreect back down the open tube. You can use the laser spots on the near Brewster window as a guide. The reected spot from the end mirror should overlap the spot from the transmitted alignment beam. When you have obtained perfect alignment, these overlapped spots will begin to icker. This is an indication that the alignment beam has exactly retroreected into the alignment HeNe laser. The retroreected beam intereferes with the longitudinal modes of that cavity and causes unstable operation. This is your best indication that the high reector is aligned. The output coupler is aligned by observing the reected beam o its back surface. This reected beam should be aligned to the output aperture of the alignment laser. When perfect alignment is achieved, the beam will once again icker due to intereference in the small HeNe. Now you are ready to turn on the power supply to the open cavity laser. Block the output of the alignment laser and look for red light scattering o the front surfaces of the open cavity mirrors. You may (most likely) have to make ne adjustments to the mirrors to obtain lasing. Slowly and carefully search for lasing by simultaneously varying both angular adjustments of one of the mirrors in tiny (1/10 turn) sweeping motions to cover an area of alignment. If this does not produce lasing, return this mirror to its original position and perform the same procedure on the other mirror. If lasing cannot be found, you may have to realign the tube to the small HeNe beam. Consult your lab TA. 1) Before placing your mirrors in the cavity, turn the open HeNe on and measure the single-pass (small-signal) gain using the alignment laser. Based on this value and the cavity loss, will you be able to get the laser to lase? 2) Once lasing is obtained, maximize the output power by making ne adjustments to the mirrors. This is an iterative process since the adjustment of one component will aect the adjustment of another. When youve maximized power, record the meter reading. Compare this output power to that of the alignment laser. 3) With the measured output power of your laser, calculate the saturation intensity of the gain medium. 4) Use a mirror to direct the laser output onto a far wall. Measure the beam size at this position and compare to the value calculated for your resonator design. A precise measure of the beam width can be obtained by passing the beam through an iris closing down the iris until and only 63% of the power is transmitted. The diameter of the iris is 2w(z). 5) Play with the alignment of the output coupler. You may be able to observe higher transverse modes other than TEM00 . Describe what you see and identify the mode(s) (see section 2.8). 6) Calculate the optimal value of output coupler transmission (i.e. the value that maximizes laser output intensity). Use the closest mirror in the set and realign the laser. Does the output match what youve predicted? 7) Verify the 63% transmission used in problem 4).
