Embed Size (px)
Citation preview
Lecture 16
Photonic Signals and Systems- An Introduction
- By- Nabeel A. Riza *
25/11/2019 N. A. Riza Lectures 1
• Text Book Reference: N. A. Riza, Photonic Signals and Systems – An Introduction, McGraw Hill, New York, 2013.
Acousto-Optic Interferometers
Topics:
Optical Interferometry – The Basics Mathematics Needed for System Design
AO heterodyne interferometer System design and working principles using opposite Doppler shifts
and 2-beam creation and combining 2 beams to form a high phase stability RF output from the
photo-detector. This system can be used for measuring Optical Path Length of test materials using
relative RF phase shift measurements of the optical system RF output
1-D Scanning AO heterodyne interferometer system design where changing the RF frequency
driving the AODs provides a moving test beam to sample in the 1-D direction a possible test
material
AO Homodyne Interferometer Design that uses the same doppler shifts for both beams to
produce a DC electrical signal from Photo-Detector
25/11/2019 N. A. Riza Lectures 2
2 Beam Optical Interferometry - Basics
Incident optical wave’s Electric Field is:Light Wave
PD
i(t)
PD: Photo-DetectorGd: PD Gain Conversion (A/W)Pd: Optical Power (W)Ad: Area of PD (m2)I: Irradiance (Intensity) W/m2
Opticalfrequency
(Hz)
Opticalphase in
the beam
Real Signal
Photo current:
]2cos[0 tEEinc
2
02)( inc
dddddd E
c
AGIAGPGti
20
0
||2
)( Ec
AGti dd
2
0 ||)( Eti 25/11/2019 N. A. Riza Lectures 3
2 Beam Optical Interferometry - Basics
E1 beam PD
i(t)
E2 beam
2 Beams that are parallel and overlap fall on PD.
Beam 1:
Beam 2:
Both beams are mutually coherent
2 Beam interference Term:
Heterodyne:
Homodyne:
Argument beam 2 – Argument beam 1
RF carrier in
12=
]2cos[ 11011 tEE
]2cos[ 22022 tEE
221
2 )()( EEEti
)cos(2)( 1202012
022
01 EEEEti
]2[]2[ 112212 tt
])(2cos[2)( 121202012
022
01 tEEEEti
12
)cos(2)( 1202012
022
01 EEEEti12
25/11/2019 N. A. Riza Lectures4
AO Bragg Cell Interferometry - Basics
INPUT Optical Field is
Positive Doppler Beam 1
Beam 2
Glass
nd
+1
DC
INPUT
-1
DC
Negative Doppler
Beam 2
Beam 1
)2cos( t
)2cos(1 tfA c
)2cos(01 tA
])(2cos[1 tfA c ])(2cos[1 tfA c
nd
2
)2cos(01 tA
)2cos(02 tA
])(2cos[2 tfA c)2cos( t
)2cos(2 tfA c 25/11/2019 N. A. Riza Lectures 5
Two AO Bragg Cell In-line Heterodyne Riza Interferometer - Basics
+1
DC
Block the light
PD
A1cos2fctA2cos2fct
nd
i(t)
Light IN @ λ
+1
-1
+1
-1
+1
-1
2 Beams
on PD
Output of AO
Heterodyne
Interferometer
RF Carrier
of 2fc
Phase of
text sample
)2cos( t
)2cos( t
])(2cos[1 tfA c
])(2cos[2 tfA c
i(t) < [ “+1” Beam Optical field + “-1” Beam Optical field ]2 >
]22cos[2)(
])(2)(2cos[2)(
]})(2cos[])(2cos[{)(
21
2
2
2
1
21
2
2
2
1
2
21
tfAAAAti
tftfAAAAti
tfAtfAti
c
cc
cc
nd
225/11/2019 N. A. Riza Lectures
6
High-Stability AO Heterodyne Riza Interferometerfor Test and Measurement
The 2 in-line Interfering Beams on the PD generating the current i(t) are:
+1 (Positive Doppler) Generated from AOD1 using the input incident light
- 1 (Negative Doppler) from AOD2 using the DC light from AOD125/11/2019 N. A. Riza Lectures
7
1D Spatial-Scanning AO Heterodyne InterferometerIn many cases, different zones of a test sample require optical sampling to read the test object phase and amplitude transmittance information.
n=1,2,3, ..for discrete scan spots on Test Array
The 2 in-line Interfering Beams on the PD generating the current i(t) are:
+1 (Positive Doppler) Generated from AOD1 using the input incident light undergoes a second +1 Positive Doppler Shift in AOD2 creating a +1 x +1 or double Doppler Diffracted Beam
The DC light beam from AOD1 is the reference beam 25/11/2019 N. A. Riza Lectures 8
D: Baseband
Point DetectorCollimated
Laser Beam
Irradiance I0
+1
DC
Collinear
+1 and +1
order beams
Spatial
Block
B1DC
AOD1 AOD2
D
S1S2
S3 S4
Baseband Signal
At DC.
Test Object
+
A cos2fct
A1cos2fct
A2cos2fct
x
x
x
High-Stability AO Homodyne Riza Interferometer- Optical E-field Analysis
The 2 in-line Interfering Beams on the PD generating the current i(t) are:
+1 (Positive Doppler) Generated from AOD1 using the input incident light
+ 1 (Positive Doppler) from AOD2 using the DC light from AOD1
25/11/2019N. A. Riza Lectures
9
