BREVE REFERENCIA DEL SECTOR METALMECANICO

W hen interference patterns are formed from two sources which are on a line parallel to the interference plane then they are of a Young's slits type. W hen interference patterns are formed from two sources

which are on a line perpendicular to the interference plane then they are of a Mach-Zehnder, or Michelson, type.

Light from a HeNe laser was divided by a beam splitter and coupled into two singlemode optical fibres each Im long. The light then travelled through the fibres and when emitted from the fibre ends it was collimated by a 50mm focal length f/1 lens and recombined using a second beam splitter, figure 4.7. W hen the fibres were aligned correctly circular interference fringes were seen on a screen. The size of the central fringe and also the total num ber of fringes in the field of view could be altered by changing the relative positions of the ends of the two fibres or by adjusting either of the lenses to change the divergence of the beams. W hen viewed on a monitor using a ccd cam era the fringes were sym m etrical and of high contrast. One of the fibres was phase modulated using a piezoelectric cylinder. This caused the fringes to be modulated about the centre of the fringe p attern so th a t a photodetector placed a t the centre of the p attern

m easured the maximum and minim um of the fringe intensity. The fringe visibility was found to be unity.

The fibres were replaced by two Im long 50/125 micron fibres. Interference fringes were produced as before but this tim e they were superimposed on a background of speckle. When viewed on a monitor they were seen to be symmetrical and have high contrast. W hen one of the fibres was phase modulated, the fringe visibility at the centre of

the pattern was determined to be unity. If a single interference fringe filled the whole field of view then the speckle pattern appeared to be

modulated but different regions had different phases.

The fibres were then replaced by two Im long plastic optical fibres w ith a core diam eter of 400 microns. The fringes were seen to have good contrast but were distorted. The circular fringes seen in previous experim ents now showed discontinuities and were no longer circularly symmetric. The effect was even more m arked when fibres w ith 970 micron cores were used. Fringes showing good co n trast were p resen t b u t the circular sym m etry w as almost completely distroyed. The centre fringes were very difficult to distinguish from the speckle. As it was not possible to phase

modulate the two large core fibres using the piezoelectric cylinder, phase modulation was achieved by warming one fibre w ith a finger; then, the visibility could be m easured as before. The visibility of the fringes formed by the 400 micron core fibre was m easured to be 0.7 and, for the 970 micron core fibre, the visibility was m easured to be

0.6. . The interference fringes present at the screen were recorded by

a cam era on black and white film. The lens of the cam era was

removed and the interference fringes allowed to fall directly onto the

film. Photographs of the fringe patterns formed with 50/125 pm, 400

pm, and 970 pm fibre are given in figures 4.8, 4.9, and 4.10. mapsi

The phase front of a beam of light emitted by an optical fibre affects the uniformity of the interference fringes which the beam can

produce. A m ap of the phase front of the beam can be obtained by allowing the light to interfere w ith a uniform reference beam. The

interference fringes obtained are a m ap of the phase difference between the phase front of the light emitted from the fibre and the phase front of the reference beam.

As the distortion of the wavefront of a beam emitted from a fibre m ust depend upon the num ber of guided modes, we looked a t the phase front of a 970pm core fibre, figure 4.11. The experim ental arrangem ent was as follows. Light from a HeNe laser was divided into two beams by a pellicle beam splitter. One beam was coupled into the 970 micron core fibre and the other beam was spatially filtered, expanded and collimated to provide the reference. The beam s were recombined by a second beam splitter. Interference fringes were produced which fell onto a screen at the output of the interferometer.

The radius of curvature and divergence of the two beam s were

m atched by focusing the reference beam to a point the same distance from the interference plane as the fibre end. This was done using a lens w ith a focal length which produced a beam with the same divergence as the light from the fibre. A slight difference in the radii of curvature at the screen caused a num ber of circular fringes to be present. The distortion of the fibre wavefront was seen as distortion of the circular fringes, figure 4.12.

4.4 Measiarement of visibility curves bv fibre interferometer.

In order th a t experim ents could be carried out w ith non-zero p ath differences it was necessary to determine the visibility curves for our sources. The two sources were a 5mW HeNe laser (633nm) and a

15mW HeCd laser (442nm).

The visibility curves were m easured by introducing the light to an air p ath Michelson interferom eter consisting of a pellicle beam splitter,

which divided the light into two beam s. The beam s were each reflected off a mirror and recombined by the same beam splitter. C ircular interference fringes w ere obtained. One arm of the

interferom eter was phase modulated using a rotating glass slide and th e m axim a and minim a of in te n sity w ere m easured by a

photodetector at the centre of the pattern. The path difference between the two arm s of the interferom eter was increased in steps from zero

to around 160cms. A graph of visibility against path difference was plotted for each source (figures 4.13 and 4.14). The graph of visibility against p ath difference for the HeNe laser shows a periodicity of around 70cm. W hen radiation from the laser was m easured by a spectrum analyser a frequency of 409MHz was detected which would beat over a distance of 73.3cm in space. The HeCd shows a visibility curve which has a coherence length of about 12cm.

Using the graph of visibility against path difference for each source as a reference the same experim ent was carried out using a fibre M ach-Zehnder interferom eter. Each source, in tu rn , was coupled into two fibres which were singlemode a t 1300nm. The fibres were initially of equal length. One arm of the interferom eter was phase

modulated and the two beam s emitted from th e fibres were

superimposed by a beam splitter in order to obtain circular fringes. The fringe visibility was m easured by a photodiode a t the centre of the circular fringe pattern. A known length of fibre was cut from one arm and the visibility m easured again. In th is way a graph of visibility against p ath difference was plotted as before (figures 4.15 and 4.16). It was seen th a t the only difference between the results tak en w ith air path s and those w ith fibre p ath s was th a t the geometrical path difference for a given change in visibility to occur

was about 1.5 tim es sm aller in the later case due to the refractive index of the fibre.