INVERTED ACTIVE MEDIUM

IN ABSENCE OF FEEDBACK MIRRORS THE EXCITED MOLE- CULES DECAY BY SPONTANEOUS EMISSION OR BY COLLI- SIONAL NON-RADIATIVE TRANSITIONS.

IF FEEDBACK IS WEAK THE EXCITED MOLECULES CAN DECAY BY STIMULATING EMISSION OR BY COLLISIONAL PROCESSES.

IMPORTANT

TOTAL REFLECTOR PARTIAL REFLECTOR

INVERTED ACTIVE MEDIUM

IN PRESENCE OF FEEDBACK MIRRORS THE EXCITED MOLE- CULES DECAY DOMINANTLY BY STIMULATING EMISSION IF INTRACAVITY INTENSITY IS EQUAL TO SATURATION INTENSITY

go Is = Maximum power that can be extracted.

EXTRACTING LASER FROM INVERTED MEDIUM

Optical Resonator Classification

Where gi = 1-L/Ri (i= 1,2)

L is resonator length

R is ROC of the mirror

On the basis of geometry they can be divided in to two categories If ray retrace its path after each round Stable resonator

0 ≤ g1g2 ≤1If ray change its path after each round Un-Stable resonator

-1≤g1g2≥1

Stability diagram

Stability Condition :-0g1g2 1Where gi = 1-L/Ri (i= 1,2)L is resonator lengthR is ROC of the mirror

(1,1)

PLANE PARALLEL RESONATOR

(-1,-1)

HEMISPHERICAL(0,1)

HEMISPHERICAL(1,0)

1-d/R

1-d

/R POSITIVE BRANCH UNSTABLE RESONATOR

NEGATIVE BRANCH UNSTABLE RESONATOR

1

2

All the stable resonator which fall on blue line are must be confocal

All the resonator along green line are hemispherical

Types of Stable Resonator

Plane-Parallel

L R1= R2=

Confocal

Concave-Convex

Hemispherical

R2=R1=L

R2=- (R1-L)R1L

R1=L/2 R2=L/2

Unstable Resonator

P

2

P1

M2 M1

Unstable resonator in general are non periodic focusing system

Are characterized by a geometrical magnification parameter,

Having characteristic magnifying geometrical eigen waves according to a purely geometric or paraxial analysis

XYYXX

YXYX

L

x

IIII

IIII 2

2121

2112

Electrode X2

Focused laser spot falling on 1st quadrant 0f duo-lateral PSD

YYYXX

YXYX

L

y

IIII

IIII 2

2121

1122

LX

Y1

Y2

X1 X2

X

YLY

Ly X2

X1

Y2

Y1

Lx

Focused laser spot falling on1st quadrant 0f pincushion PSD

Output Ix2Output Ix1

XB

XA Incident light

Electrode X1

Common electrode

Resistance length Lx

Sectional view of PSD

P layer

I layer

N layer

Technical specificationsMaterial Silicon Geometry SquareActive Area 12X12 mm2

Sensitivity 0.6 A/W @ 920nmRise time 3 sDark current 1 nAResistance length 14 mmInter electrode resistance 10 K Position resolution 1 mSpectral response 320 nm -1060 nmTerminal capacitance 300 pF

Quadrant Detector (S1880)

``

A

B

C

D

A'

B'

C'

D'

A

B

C

D

A'

B'

C'

D'

`

A

B

C

D

A'

B'

C'

D'

`

A

B

C

D

A'

B'

C'

D'

15°

`

A

B

C

D

A'

B'

C'

D'

30°

45°60°

Current is applied to the A and A’ windings so the A winding is north

Current is applied to the B and B’ windings so the B winding is north

Current is applied to the C and C’ windings so the C winding is north

Current is applied to the D and D’ windings so the D winding is north

Current is applied to the A and A’ windings so the A’ winding is north

Rotor

Stator

A A’

B B’

C C’

D D’

V

N NN SS

`

Permanent magnet S M Variable-reluctance S M Hybrid Stepper Motor

360

rs

rs

NN

NN

= step angle in degree

Ns=Number of teeth on stator core

Nr=Number of teeth on rotor core

6

)/( ssN

N= motor speed in RPM

= step angle in degree

s/s= number of steps per second

Stepper motor parametersSpeed 800 rpmDimension 60X60 mmEncoder shaft diameter 8 mmStep angle full 0.720 Maximum torque 2 N Shaft diameter 8 mm

Parameter for digital encoder Configuration Hollow (no

attachment) Shaft diameter 8 mm Power supply 5 V DC Resolution 40 to 1024

impulse /turn

Digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements.

Types: Linear and rotary configurations but the most common type is rotary

Rotary encoders come in two configurations Absolute encoder where a unique digital word

corresponds to each rotational position of the shaft.

Incremental encoder, which produces digital pulses as the shaft rotates, allowing measurement of relative position of shaftRotary encoders are composed of a glass or plastic code disk with photographically organized tracks. Digital pulses are produced when radial lines in each track interrupt the beam between a photoemitter-detector pair.

Code disk

ShaftTracks

IR emitters Phototransist

ors

Specifications Selectable step angle

F (full step) position : 0.72 degree per step H (half step) position : 0.36 degree per step

Current down

With auto current cut back feature when the motor is at stand still (idle) to reduce motor heat buildup when not running.

Input pulse type selector switch is given

Output current adjustment Potted at max current value for the motor Power supply : 24V DC

PCI-bus mastering for data transfer 16-channel single-ended or 8 differential A/D inputs 12-bit A/D conversion with up to 100kHz sampling rate Programmable gain for each input channelOn board samples first input first output (FIFO) buffer 2-channel analog to digital output (PCI-1710)

Motor controller card ৡ Number of axis : 1-4ৡMaximum rate of steps : 3 MHzৡ Memory for application : 1000 lines x 80 characterৡ Multitasking of application program: 8 simultaneouslyৡ Power requirements : 12 VDC (20 mA)

Y -tive

DCBA

DCBAY

DCBA

CBDAX

)()(

)()(

Start

Stop

Drive the motor along + Y axis

Yes

No Y + tive)()( DCBA

Drive the motor along – Y axis

Stop No

X + tive)()( CBDA

No

Yes

Drive the motor along –X axisX -tive

)()( DACB

Yes

Drive the motor along + X axis

No

YesYes

Y -tive)()( BADC

Stop

Stop

B A

C D +X-X

+Y

-Y

Alignment accuracy 20 arc secMirror misalignment angle 1/3˚Time of alignment 10 sec

Diode laser beam diameter 2 mm Diode laser beam divergence 0.7 mrad

Data acquisition card

Motor controller

Stepper motordrivers

Digital encoder

Biasing circuits& amplifier

Stepper motor

Schematic of alignment of single mirror

Active Area of position sensing detector (PSD) 12X12 mm2

Resetting of reference

Manual misalignment

Powering

along x axis with focusing

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 1774 3547 5320 7093 8866 10639 12412

Distance from centre (a.u)

ph

oto

sig

nal

(a.

u)

Movement of spot in along X and Diagonally X

Diode laser spot on PSD scanned from left to right and diagonally

Diagonal

-1-0.5

0

0.51

1.52

2.53

3.5

0 1 2 3 4 5

Distance (a.u)

Ph

oto

curr

ent

(a.u

)