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An alignment technique for a ring laser cavity
Technical
note
An alignment
technique
for a ring laser cavity
Z.-H. FANG, H. THIENPONT
We present a practical method for fast alignment of a ring laser cavity. With the aid of an auxiliarv HeNe laser a closed oath is formed in the cavity so that laser action can be qu/ckly attained. KEMNORDS: lasers, alignment
Introduction It can be difftcult obtaining laser action in a ring laser cavity, owing to the critical alignment required. A laser with a low gain active medium makes the demands even more severe; so to overcome such difficulties a procedure is proposed to simplify adjustment. The method is very successful for aligning a HeNe ring laser.
As shown in Fig. 2, aperture AZ has to be put into the ring in such a way that the incident beam passes through both apertures A, and A2 That is, any beam travelling through diaphragm AZ will pass exactly
Plosmo tube
Figure 1 shows a conventional set-up for an active ring resonator with an auxiliary HeNe laser used for alignment and a plasma tube between mirrors M, and Mr. All devices are fixed on an optical table. The figure shows that a symmetrical set-up will shorten the time needed for alignment.
Principle
of operation
The principle of this technique is based on the fact that a geometrical plane is determined by a straight line and a point that is not on that line. Within that plane a laser beam has to form a closed loop, with any light stimulated from the noise being continuously reflected until lasing begins. Therefore, after a complete ‘round-trip’ the beam of the auxiliary laser has to retrace the same route. During the second round-trip the beam is called the second beam.
Fig. 1 Apparatus for ring cavity. Mt. Ma-plane concave mirrors with radius greater than 3 m
mirrors; M,. M,-
Aperture.
It is clear that the plasma tube’s axis is the straight line mentioned above and the plane-defining point is determined by an aperture A, which is located close to mirror M, (Fig. 2).
Alignment
procedure
By adjusting M, until the beam hits the centre of the plasma tube, and then by aligning this tube. the beam can be made to travel along its axis. This is the most important condition for obtaining perfect alignment. If this condition is fulfilled, the laser spot coming out of the tube will be circular and have homogeneous intensity. Moreover, no scattered light will be visible along the tube. The authors are currently at the Free University Brussels, Department ALNA T.W.. Pleinlaan 2, 1050 Brussel, Belgium. Received 15 December 1993.
0030-3992/84/050269-02/$03.00 Of’TlCS
AND LASER TECHNOLOGY.
OCTOBER
Auxiliary HeNe loser
Fig. 2 The same set-up as in Fig. 1. Dottedlines represent the beam path in the ring cavity with a miaaliinment
@ 1984 1984
Butterworth & Co (Publishers) Ltd 269
along the tube’s axis and therefore adds no supplementary conditions. Errors are avoided by considering the tube’s diameter to be infinitely small. As soon as this condition is fulfilled we can say that the incident and the second beam are in the same plane and it should be possible to obtain laser action. Initially the second beam will not pass through A,, so tuning of M3 and M, will be necessary. This process should be repeated several times depending on the original positions of the mirrors. When the two beams coincide interference rings with varying intensities show up. From the theoretical point of view, that is enough for obtaining laser action. However, due to the short distance between A, and A2, it is difficult to judge whether both beams are collinear or not. For precise verification we suggest moving A2 to point C. so that alignment is more accurate. Sometimes a third spot
270
which is very weak in intensity may appear on screen A,. If this occurs, very careful trimming is necessary for perfect alignment so that both beams travel through the same diaphragm. Of course. during the process the first beam should always pass through the centre of the aperture. otherwise all the above procedures are useless.
Conclusion When interference is good, a spot with varying intensity can be observed on a sheet of paper placed behind M,. At that point A, and A1 can be removed and the power supply of the plasma tube can be switched on. With the aid of this flashing spot, it is easy to obtain laser action. Sometimes, however, one needs to rotate the plasma tube about its axis to match the polarization.
OPTICS AND LASER TECHNOLOGY.
OCTOBER 1984
note
An alignment
technique
for a ring laser cavity
Z.-H. FANG, H. THIENPONT
We present a practical method for fast alignment of a ring laser cavity. With the aid of an auxiliarv HeNe laser a closed oath is formed in the cavity so that laser action can be qu/ckly attained. KEMNORDS: lasers, alignment
Introduction It can be difftcult obtaining laser action in a ring laser cavity, owing to the critical alignment required. A laser with a low gain active medium makes the demands even more severe; so to overcome such difficulties a procedure is proposed to simplify adjustment. The method is very successful for aligning a HeNe ring laser.
As shown in Fig. 2, aperture AZ has to be put into the ring in such a way that the incident beam passes through both apertures A, and A2 That is, any beam travelling through diaphragm AZ will pass exactly
Plosmo tube
Figure 1 shows a conventional set-up for an active ring resonator with an auxiliary HeNe laser used for alignment and a plasma tube between mirrors M, and Mr. All devices are fixed on an optical table. The figure shows that a symmetrical set-up will shorten the time needed for alignment.
Principle
of operation
The principle of this technique is based on the fact that a geometrical plane is determined by a straight line and a point that is not on that line. Within that plane a laser beam has to form a closed loop, with any light stimulated from the noise being continuously reflected until lasing begins. Therefore, after a complete ‘round-trip’ the beam of the auxiliary laser has to retrace the same route. During the second round-trip the beam is called the second beam.
Fig. 1 Apparatus for ring cavity. Mt. Ma-plane concave mirrors with radius greater than 3 m
mirrors; M,. M,-
Aperture.
It is clear that the plasma tube’s axis is the straight line mentioned above and the plane-defining point is determined by an aperture A, which is located close to mirror M, (Fig. 2).
Alignment
procedure
By adjusting M, until the beam hits the centre of the plasma tube, and then by aligning this tube. the beam can be made to travel along its axis. This is the most important condition for obtaining perfect alignment. If this condition is fulfilled, the laser spot coming out of the tube will be circular and have homogeneous intensity. Moreover, no scattered light will be visible along the tube. The authors are currently at the Free University Brussels, Department ALNA T.W.. Pleinlaan 2, 1050 Brussel, Belgium. Received 15 December 1993.
0030-3992/84/050269-02/$03.00 Of’TlCS
AND LASER TECHNOLOGY.
OCTOBER
Auxiliary HeNe loser
Fig. 2 The same set-up as in Fig. 1. Dottedlines represent the beam path in the ring cavity with a miaaliinment
@ 1984 1984
Butterworth & Co (Publishers) Ltd 269
along the tube’s axis and therefore adds no supplementary conditions. Errors are avoided by considering the tube’s diameter to be infinitely small. As soon as this condition is fulfilled we can say that the incident and the second beam are in the same plane and it should be possible to obtain laser action. Initially the second beam will not pass through A,, so tuning of M3 and M, will be necessary. This process should be repeated several times depending on the original positions of the mirrors. When the two beams coincide interference rings with varying intensities show up. From the theoretical point of view, that is enough for obtaining laser action. However, due to the short distance between A, and A2, it is difficult to judge whether both beams are collinear or not. For precise verification we suggest moving A2 to point C. so that alignment is more accurate. Sometimes a third spot
270
which is very weak in intensity may appear on screen A,. If this occurs, very careful trimming is necessary for perfect alignment so that both beams travel through the same diaphragm. Of course. during the process the first beam should always pass through the centre of the aperture. otherwise all the above procedures are useless.
Conclusion When interference is good, a spot with varying intensity can be observed on a sheet of paper placed behind M,. At that point A, and A1 can be removed and the power supply of the plasma tube can be switched on. With the aid of this flashing spot, it is easy to obtain laser action. Sometimes, however, one needs to rotate the plasma tube about its axis to match the polarization.
OPTICS AND LASER TECHNOLOGY.
OCTOBER 1984