Delay line interferometer

Transcription

Michelson Interferometer: Aim: To determine the wavelength of the laser using Michelson interferometer and refractive index of the thin film. Apparatus: Laser source, Michelson interferometer kit, optical bench, glass slide meter scale. Theory: When a single source of light is used, the distribution of energy in a medium is uniform. Instead if two or more coherent sources are present, the distribution of energy will not be uniform due to the superposition of the waves. This phenomenon of modification in the distribution of light energy due to the superposition of two or more waves is termed as interference. If the crest of one wave meets with the trough of the other, the resultant intensity will be zero and the waves are said to interfere destructively. Similarly if the crest of one wave meets with the crest of the other, the resultant intensity will be a maximum. Now the waves are said to interfere constructively. This superposition of waves results in interference fringes. For constructive interference to occur, path difference between two waves must be equal to an integral multiple of the wavelength i.e. path difference , where n is the order. n = 0, ±1, ±2, ±3... If the path difference between two waves is , then the interference between them will be destructive. The phenomenon where incident light waves reflected from both the upper and lower boundaries of a thin film gets interfered to form a new wave, is commonly called thin film interference. In optics, a medium with thickness of the order of one wavelength of light in the visible region can be considered as a thin film. Interference causes thin films of oil on water, soap bubbles, and thin glass plates to appear coloured when seen under sunlight. The brilliant colors seen in butterflies, peacock feathers etc are also due to interference. Michelson interferometer is the best example of amplitude splitting interferometers, invented by Albert Michelson in 1893. In a Michelson interferometer, light from a monochromatic source S is divided by a beam splitter (BS), which is oriented at an angle 450 to the beam, producing two beams of equal intensity. The transmitted beam (T) travels to mirror M1 and it is reflected back to BS. 50% of the returning beam is then deflected by 900 at the beam splitter and is made to strike the screen, E. The reflected beam (R) travels to mirror M2, where it is reflected. Again 50% of the beam passes straight through the beam splitter and reaches the screen. It can be seen clearly from the figure that, the light ray starting from the source, S and undergoing reflection at the mirror M2 passes through the beam splitter three times. But the ray reflected at M1 travels through BS only once. This causes a difference in the optical path difference. To avoid this, a compensating glass plate (CP) of same thickness as that of BS is introduced between M1 and BS. These two beams interfere at the screen, and produce interference fringes. Procedure: To find wavelength of the laser source: • Attach the diode laser with mount, adjustable mirror and screen as in the figure. • Align the laser source so that the beam is parallel with the top of the base. The laser beam must strike at the centre of the movable mirror and should be reflected directly back into the laser aperture • Adjust the position of the beam splitter so that the beam is reflected to the fixed mirror. Adjust the angle of beam splitter to be 45 degree. • We will get two sets of bright spots on the screen, one set from fixed mirror and other from movable mirror. Adjust the angle of the beam splitter and make the two sets of beams as close together as possible. Tighten the screws securing the beam splitter and the mirror mount. • With the screws on the back of the adjustable mirror, adjust the mirror's tilt until the two sets of dots on the screen coincide. • Expand the laser beam slowly by rotating the collimating lens in front of the laser diode. • Align the laser with the interferometer and make certain that the fringes are moving when the micrometer screw is turned. Then fix a position on the screen and note the micrometer reading. • Then, as the screw is moved, the fringe begins to move. Count the number of fringes (say, N) that move past the fixed point (either inward or outward). Note the micrometer reading. Then compute the distance the mirror is moved which is 'd'. • Repeat the procedure several times. Average the readings. • We can calculate the wavelength of the laser source used, ? from the equation, To find the refractive index of the glass slide: • Align the laser source and the interferometer. • Place the rotation stage perpendicular to the optical path, between the beam splitter and the movable mirror. • Mount the glass slide on the rotation stage and align the glass slide perpendicular to the optical path. • When we introduce the glass plate, the fringe will be shifted and become blurred. Sharpen the fringe again by moving the mirror mount to and fro. • Rotate the rotation stage slowly and count the number of fringe translations that occur as we rotate the table to an angle ?. • Then the refractive index of the glass slide, 'n' can be calculated using the equation where t is the thickness of the thin film introduced and ? is the wavelength of the laser used. Applications: 1. Interferometers are used to measure the wavelength of optical beams precisely. The Michelson interferometer is a historically important device which provides simple interferometric configuration for introducing basic principles. 2. Michelson interferometers are used for the detection of gravitational waves. 3. It is widely used in DWDM (Dense Wavelength Division Multiplexing) networks. In this, Michelson interferometers are used to produce a delay line interferometer which converts phase modulation into amplitude modulation. 4. Widely used in astronomical interferometry.