Lasers are widely used to align optics. In most cases a unique laser beam is enough for alignment of “open” optics as shown in Figure 1 and Figure2. However, there are cases where the optical set up does not allow the use of a single laser beam in one direction as for example in collinear mirrors (Figure 3). In this case a device with two laser beams travelling in opposite directions on the same axis is extremely useful for alignment of the optics in opposite directions.
This article describes the design, integration and alignment of a Dual-beam laser.
Principle of a single dual-beam laser:
A commercial 70% – 30% beam splitter divides a laser in two beams in such a way that the transmitted beam has 70% intensity of the original beam and the reflected (upper beam) with 30% as shown in Figure 4.
The transmitted beam is then reflected by a mirror perpendicular to the laser axis. The beam reaches again the beam-splitter and a beam is reflected down and opposite to the 30% beam. The non-useful transmitted beam will come back to the laser. The intensity of the useful down beam will be therefore 0.7 x 0.3 = 21% (assuming an ideal reflective mirror). With a 50% – 50% beam splitter the intensities of the two beams will be 50% and 25% respectively.
Improved design with a lambda/4 retarding plate:
In order to optimize the efficiency of the opposite beams, a retarding lambda/4 plate in front of the mirror may be used.
The input laser beam is rotated in such a way that its polarization direction is at 45 deg with respect to a 50 – 50% polarizing beam-splitter (Figure 5).
The reflected and transmitted beams have now 50% each and with perpendicular polarization. The transmitted beam reaches the lambda/4 plate which is pre-rotated to twist the incident polarized beam by 45 deg. The beam is later reflected back by the mirror and when it passes the lambda/4 plate, the beam suffers another 45 deg rotation. The final polarization is then 90 deg with respect to the incident beam. Under this condition the reflected beam is again reflected by the beam-splitter with full efficiency. Finally, we have 2 beams travelling in opposite directions with a theoretical efficiency of 50% each.
Here, we show two ways to align the emerging beams of the dual-beam laser:
- By tilting the flat mirror when the laser is fixed with respect to the beam-splitter
- By tilting the laser device when the mirror is “glued” to the cube-beam-splitter
In the first case the alignment is easier to carry out but it requires a tip-tilt alignment support for the mirror. This support should be small and usually is rather expensive
In the second case the mirror is literally glued to the cube beam-splitter. The laser needs to be mounted in a tip-tilt structure. The easy way is to hold it inside a tube and adjusted with screws. A 4 x 90 deg screw array is inexpensive and enough for an accurate alignment, however, the alignment procedure is more laborious.
Flat mirror tip-tilt
- The dual-beam splitter is mounted in such a way that the fix beam (the one reflected directly by the beam-splitter which we call it: reference beam) passes a 1mm pinhole-screen and is reflected by a mirror previously placed at least 1m away of the pinhole-screen (Figure 7)
- The mirror is aligned in auto-collimaton. Note that the reflected reference beam pass through the beam-spliter and goes out together with the secondary beam (the one reflected by the internal mirror). Note also that the reference beam is much less intense that the secondary beam
- Place a white screen at least 1 m away from the laser and put a mark in the screen where the transmitted beam hits the screen (optical reference, Figure 8)
- Adjust the tip-tilt of the internal mirror to send the secondary beam on the mark
- A reference laser is first used to define an optical axis as shown in Figure 9. The laser passes a 1 mm pinhole-screen and hits a remote screen. A mark is placed where the laser fall on the screen.
- The dual-beam splitter is placed in such a way that the red reference beam is reflected by auto-collimation on the beam-splitter (Figure 10)
- The two green beams from the dual-laser will be tilted symmetrically with respect to the pinholes. Note also that the beams will move simultaneously when the laser is tilted into its support (Figure 10)
- Tip-tilt the laser until both beams reach the pinholes and marks on the screens as shown in Figure 11. Note that every time you tilt the beams, you must fine-adjust the position of the laser to pass the 2 beams through the pinholes and reach the marks.
List of material for a Dual-beam with tip-tilt laser
- CPS532 – Collimated Laser Diode Module, 532 nm, 4.5 mW, Round Beam, Ø11 mm Housing
- BS007 – 50:50 Non-Polarizing Beamsplitter Cube, 400 – 700 nm, 5 mm
- Silver Circular Plane Mirror. 10 mm Ø λ/ 4, Standard Quality
- LDS5-EC – 5 VDC Regulated Power Supply, 2.5 mm Phono Plug, 230 VAC
Mechanical drawings of a dual-beam with tip-tilt laser:
Carlos Guirao and Gerardo Avila
European Southern Observatory
Dual laser beam by CAOS group is licensed under a Creative Commons Attribution-Non-Commercial-No Derivative Works 3.0 Germany License.
Based on a work at spectroscopy.wordpress.com.