43

CHAPTER – 4

INSTRUMENTATION AND CALIBRATION

4.1 Instrumentation

1

2

3

4

5

6

7

D1 D2 8

9REF. INPUT

LOCK IN AMPLIFIER

Fig 4.1 Designed Rotating polarizing technique

44

The block diagram of simple Polarimeter and its experimental set up are shown

in Fig-4.1.

1. Two laser sources 2.Polarizer 3.DC Motor 4.Rotating Polaroid cum analyzer

5.Heating chamber 6.Sample cell (dielectric cell) 7.Thermometer 8.Photodetectors

9.Lock in Amplifier.

The Instrument consists of a

1) A light source- Two Laser diode of same wavelength.

2) A primary fixed linear polarizing strip. –Polarizers.

3) Specially designed Heating chamber that can accommodate the liquid Crystal

samples

4) Digital thermometer with an accuracy of 0.1◦C, fixed in the Heating Chamber.

5) Rotating polarizer (analyzer) mounted on a constant speed DC brush less motor,

6) Photo detector- Two phototransistors (L14G2) (characteristics in annexure).

7) Digital Thermometer.

4.2 Laser source

Laser diodes are complex semiconductors that convert an electrical current into

light. The conversion process is fairly efficient in that it generates little heat compared

to incandescent lights.

Most common Laser diodes have inside two semiconductors: a LD (laser diode)

and a PD (photo diode). The laser diode will be forward biased and its cathode (LDC)

will connect to a driver transistor & network to regulate the LD current based on the

photodiode current (feedback network). The photodiode will be reverse biased, its

anode (PDA) will feed a driver regulator and thus the control will give a feedback

45

signal for the LD driver. The laser diode is driven by a Laser diode driver iC-WJ, (iC

Haus, Germany). The laser diode driver circuits details are given in Fig 4.2.

For the present study a switchable diode laser with the following specifications have

been used.

Model number: LD 660-50 B

Type : switch able diode laser.

Wave length : 660 nm

CW power : 50 m W

Working temperatures: 60o

Pin Connection : Model No.: LD-660-50A LD-660-50B

46

Absolute Maximum Ratings

PARAMETER SYMBOL RATINGS

Optical Output Power P0 55 m W

LD Reverse Voltage VRL 2 V

PD Reverse Voltage VRD 30 V

Operation Temperature To -10 to +60 oC

Storage Temperature TSTG -40 to +75oC

Electrical – Optical Characteristics

ARAMETER SYMBOL MIN. TYP MAX. UNITS TEST CONDITION

Lasing

Wavelength

λ p 650 660 665 nm P0=50 m W, R T

Threshold

Current

I th - 45 70 M A P0=50 m W

Operation

Current

I op - 100 120 M A P0=50 m W

Operation

Voltage

V op - 2.5 2.8 V P0=50 m W

Slope

Efficiency

Η 0.7 1 1.3 M W/

m A

P0=35-45 m W

Beam

Divergence

Θ - 9 - deg P0=50 m W (parallel)

Beam Θ┴ - 20 - deg P0=50 m W

47

Divergence (perpendicular)

Monitor

Current

Im - 360 - µ A P0=50 m W, V R = 5V

Astigmatism As 11 mm P0=50 m W, NA=0.4

Laser diodes are sensitive to ESD rapid turn-on currents, and over voltage

conditions. To avoid damage to Laser Diode, some form special driver circuit is

required. Automatic power control is very much required for proper protection and

operation of Laser Diode. Lasers are highly monochromatic. One main drawback with

laser sources is their limited wavelength tunability. The advantages of Laser diodes are

they are highly coherent, monochromatic and have high intensity.

The laser diode emits a monochromatic light at a wavelength of 660nm and is

coherent and can pass through an aperture 2 µm or less to avoid stray light. The whole

set-up is so arranged that the light passes through the polarizer, the liquid crystal sample

placed in the special heating chamber, through the rotating polarizer and falls on to the

first photo-transistor .The output of the photo transistor is given to the input of the lock

in amplifier.

The reference signal from the second laser passes through the fixed polarizer,

rotating analyzer, and on to the phototransistor. The signal from the phototransistor is

connected to the reference input of the lock-in amplifier.

A small polarizing sheet is placed in the path of the diode laser to linearly

polarize its light. The polarizing sheets are supplied by Edmund Company limited;

48

U.S.A. The laser diode was driven by a special driver I.C for Automatic Power Control

(APC). The frequency of the rotating polarizer and reference signal are always same

because both beams are passing through the same rotating polarizer. Both the

frequencies change simultaneously if there is a change in the speed of the motor thereby

producing no error in the lock in amplifier output. The signal outputs from both the

photo detectors is sinusoidal in nature whose phase varies with any material introduced

in the path of the light beam. The phase difference can be measured by using a lock-in-

amplifier whose output voltage is proportional to change in the phase difference

between reference and the input signals. This output signal of the lock-in-amplifier is in

turn proportional to optical rotation introduced by the Liquid Crystal Sample.

For the calibration of the setup, the variation in the lock-in amplifier output is

measured by taking two optically flat glass microscope slides without any liquid in

between them and is placed in a specially designed heating chamber. The output of the

lock in amplifier was monitored by varying the temperature. The temperature was

controlled by varying the current through the heating element using a Triac controller.

The temperature was measured with an accuracy of 0.1oC using a digital thermometer.

The output of the lock in amplifier was found to be constant and stable without any

variation. The detailed calibration procedure is given in chapter 5.

For the measurement of phase transition in liquid crystals, the sample was

placed between the same two glass plates and placed in a specially designed

temperature-controlled chamber. The photo detector outputs from reference signal and

the sample are given to the lock-in-amplifier. The output of the photo detectors are the

sinusoidal waves, because of the rotating polarizer shown in fig4.2. Due to the

49

symmetry of the linear polarizer, this behavior is repeated twice for one revolution.

Therefore, the detected signal shows double the frequency of the rotation of the linear

polarizer. The advantage of lock in amplifier is that its output is independent of the

changes in the intensity of laser diode.

The requirement in the Driver circuit under analog modulation for CW

Operation is the Modulation cut off frequency is determined by the capacitor C1 as well

as by the operating point set with the resistor RSET.With C1=100nF and

RSET=R2=10k the cut off frequency is approx.40KHz withC1=22nF and the same

resistor value of about 230KHz. The Diode Driver circuit is shown in Fig 4.3.

50

Fig 4.2 Detector output for reference and sample signals

51

Fig 4.3 Laser Diode Driver Circuit.

52

4.3 Polarizer Identification:

A significant percentage of linear polarizer listed on eBay is described as

circular, apparently because they are round, with rotating mounts. However, shape &

mount are the same for both types. Linear PL’s are generally marked as PL or Polarizer,

while CPL’s are marked Circular Polarizer (sometimes abbreviated Cir.), PL Cir., or CPL.

If in doubt about a filter, find some glare to polarize & view it from the male threaded

side of the filter while rotating. Now try flipping the filter over & look from the female

threaded side (or unthreaded side w/slim designs). Linear PL’s work the same both

sides, but CPL’s must be properly oriented. A circular model has no polarizing effect

when the filter is reversed from its normal camera orientation. (Another check is to

hold the filter in contact with a mirror. A linear PL will appear gray, & so will a CPL with

front threads contacting the mirror, but when the CPL is reversed it will appear black).

4.4 Power Supply Unit

The circuit diagram of the power supply unit for rotating polarizer mounted on a

constant speed DC motor is shown in Figure 4.4

The LM317T is an adjustable 3 terminal positive voltage regulator capable of supplying

in excess of 1.0 amp over an output range of 1.25 to 37 volts. The device also has built

in current limiting and thermal shutdown which makes it essentially blow-out proof.

53

Fig 4.4 Variable Power Supply for Speed Control of Motor

54

4.5 Heating Chamber:

The substance to be observed is sandwiched between glass plate (rubbed

uniformly in one direction for the alignment of molecules) and cover slip. There is a

provision for placing the substance along with the thermometer (or) sensor of the

temperature indicator. The temperature of the stage is controlled by a Triac power

controller while temperature variations of the block are recorded using a Digital

thermometer of + 0.10C accuracy. Its construction is given below.

Two hollow groves of width 12.5mm and depth 35mm are made on the lower

part of plate A and upper part of plate B on the opposite sides. These are for inserting

two 35 watts ceramic heating elements. The two plates are kept intact by fixing screws.

Two holes one on the lower plate B and other on the upper plate having a depth of

25mm and 3mm diameter are drilled on the side of the slide slot. These are for inserting

the digital thermometer and temperature sensor.The heating chaber photo is shown in

fig 4.5.

Heating elements are inserted into the hollow groves and wires are taken out

from the rectangular heating block and connected to a Triac Controller. The temperature

of the block is varied using a Triac Controller. The Triac Power controller for

temperature control is shown in fig4.6. The power is controlled by phase control by

varying the potentiometer R1.

55

Fig 4.5 Heating chamber photo.

56

Fig.4.6 Triac Power Controller for Temperature Control

57

4.6 Specially Prepared Surface Stabilized Liquid Crystal Cell.

(Sample cell/Dielectric cell)

A liquid-crystal cell consists of a thin LC layer placed between two parallel

glass plates that are coated with thin layers of transparent conductive material and

rubbed so that the molecules are parallel to each other along the reflecting direction.

This is called the homogeneous alignment of the molecules. When electric field is

applied to the cell, Liquid Crystal molecules align in the direction of electrified which is

perpendicular to the plane of glass plates. This alignment of molecules along the

electrified is called to homeotropic alignment.

For the purpose of studying the electro-optic properties of liquid crystals, a specially

fabricated glass cell was imported from USA with 6 micrometers separation and a pre

tilt angle. One of the critical components for studying the electro-optic properties of

liquid crystals is the sample cell. A small separation of 5.0 +_0.2μm will provide very

high electric field with lower voltages. The specification of the cell is given below.

The conductive active ITO area = 5mmx5mm.

ITO resistance = 100 ohms.

Parallel alignment layer with 10 to 30 pre-tilt angles.

Cell Gap, d=5.0 0.2μm.

Cell thickness = 1.5mm

Transparent conductive tracks are on the small projection of one of the glass plates.

Teflon wires are soldered to the tracks by a special soldering machine imported and

available at Central University of Hyderabad. A small pin hole is provided to the cell

58

for filling up the sample. Specialized techniques for filling the cell with solid and liquid

samples were adopted. The sample cell is shown in fig 4.7.

4.7 Quarter Wave Plate

Quarter-wave plate in the setup is used to turn plane-polarized light into

circularly polarized light. To do this, the wave plate is oriented such that equal amounts

of fast and slow waves are excited. We may do this by Orienting an incident plane-

polarized62 wave at 45° to the fast (or slow) axis, as shown below.

On the other side of the plate, we again examine the wave at a point where the fast-

polarized component is maximum. At this point, the slow-polarized component will be

passing through zero, since it has been retarded by a quarter-wave or 90° in phase.

The analyzer is rotated manually without quarter wave plate for beam

extinction. Then the quarter-wave plate is inserted between the polarizer and

sample holder and rotated to retain the extinction.

59

Fig 4.7: Specially Prepared Surface Stabilized Liquid Crystal Cell

60

Electro optic variable wave plate to delay the time of flight of a beam of light passing

through the sample cell is fabricated. It can be designed to be a quarter wave or half

wave plate

4.8 Detectors

The two phototransistors (L14G2) were placed along the same horizontal

axis of the rotating polarizer. Here the role of the phototransistor is to detect the light

falling on them. When light falls on the phototransistor its current changes, producing

direct voltage changes at the collector. The dual detection system is unaffected by any

gain mismatch between the two detectors. The circuit diagram is shown in fig 4.8.

Specifications of the photo transistor are given in the annexure.

61

Fig 4.8 Circuit photo transistor

62

4.9 Lock-in-amplifier

Lock-in amplifier is used to extract the signal buried in noise. It is easy,

reliable and simple to detect the phase angle between reference and actual signal which

is proportional to the change of optical rotation introduced by the liquid crystal sample

with change in temperature and electric field.

A lock in amplifier is an instrument with dual capability.

1) It measure signals whose amplitude are much smaller than those of noise

components.

2) It can provide high resolution measurements of relatively clean signals over several

orders of magnitude and frequency.

Stanford dual channel digital lock-in amplifier with a phase resolution of 0.0010 is used

for the study of electro optic effects.

Designed rotating polarization spectrometer is shown in the figure 4.9 and the complete

setup for rotating polarizing technique is shown in fig 4.10

63

64