Introduction

First, we assume that you know the safety rules for manipulating lasers.

In this article, we want to turn a classic laser spot to a laser sheet. Laser sheets can be made easily from commercial construction tools used for leveling. Often, it is a red laser diode with different modes like lines and grids.

Here, we needed a relatively powerful laser with a preference for the green color. Indeed, our application consists in visualizing a liquid-air interface. The liquid is a solution of Rhodamine-B, a fluorescent dye, which absorbs green light and emits in red.

On this picture, the laser sheet has a 2 m size on a screen positioned at a distance of about 80 cm. It is not the best we could make, we were able to vanish almost completely the central spot. But, still, it gives a good idea.

On this picture, the laser sheet has a 2 m size on a screen positioned at a distance of about 80 cm. It is not the best we could make, we were able to vanish almost completely the central spot. But, still, it gives a good idea.

In this page, we use a 40 mW green laser diode.

Protocol

The main objective is to make a cylindrical lens. The idea is to use a glass capillary tube. Form a list of refractive indices, we can see that filling the tube with glycerol will almost match the refractive index of glass. This is how we are going to made a cheap cylindrical lens. We do not have strong requirements for the laser sheet quality, in terms of thickness and uniformity.

Setup

It will be important to center the laser beam with the cylindrical lens. Thus, we use microcrontrollers to adjust the relative position. A fine tuning can make a laser sheet even better than the picture presented above, with an almost complete vanish of the laser spot.

Our setup.

Our setup.

We use a SM1 tube to retain the cylindrical lens. The tube diameter is 1 inch, i.e. 2.5 cm.

The capillary will be maintained in a SM1 tube with a retaining ring.

The capillary will be maintained in a SM1 tube with a retaining ring.

Preparation of the cylindrical lens

First, we cut a capillary tube to get a final length of 2.5 cm. To do so, we use a glass cutter to mark the glass all around the perimeter at the desired position. We prefer to cut a slightly longer tube, by 1 mm or 2. The cracked end is smoothed with a fine sand paper, until the capillary tube fits in the SM1 tube.

Capillary tubes

Capillary tubes

The second steps consists in filling one end with epoxy glue. Pay attention to preserve the cleaness of the glass surface... we want a transparent surface at the end. We deep the end in a small amount of epoxy and the outer part of the tube is cleaned with a tissue. The clog holds by capillarity before hardening.

Capillary tube clogged at one end with epoxy glue

Capillary tube clogged at one end with epoxy glue

With a syringe and a needle, the tube is filled with glycerol.

Capillary tube filled with glycerol

Capillary tube filled with glycerol

Again, with epoxy (and a syringe), we clog the upper part of the tube. The lens is ready to use! If you need to clean the outer part of the tube, you can use acetone, which dissolves epoxy.

Second sealing at the top with epoxy glue

Second sealing at the top with epoxy glue

Then, you just need to assemble everything as shown above. Use the microcrontrollers to center the capillary tube. To do so, point the laser on a black and non-reflective surface and tune until you get a laser sheet.

Application

As we said in the introduction, our objective is to visualize a liquid-air interface. Here, we have an example of a glass slide deepen in water containing Rhodamine-B. The laser is positioned on the side, with an angle of 45°. To avoid on the picture some reflections of the laser on the glass slide, we put in front of the objective a filter that cuts the laser wavelength. The camera is a Nikon D5100 with a 200mm macro lens.

A vertical glass slide (1mm thick)

A vertical glass slide (1mm thick)

Authors

  • This page has been realized by Sepideh Khodaparast and François Boulogne.