This topic will provide a detailed study about the optical heterodyne detection and its working. A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called heterodyning.
Heterodyning is used to shift one frequency range into another, new frequency range, and is also involved in the processes of modulation and demodulation.
The two input frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer.
1. PRESENTED BY
VAISHALI.K
M.Tech (ECE) - Ist yr
21304023
DEPARTMENT OF ELECTRONICS & COMMUNICATION
ENGINEERING
PONDICHERRY UNIVERSITY
SUBMITTED TO
Dr.R.NAKKERAN,
Head of the department,
Dept.Of Electronics Engineering
3. INTRODUCTION
A heterodyne is a signal frequency that is
created by combining or mixing two other
frequencies using a signal processing
technique called heterodyning.
Heterodyning is used to shift one frequency
range into another, new frequency range, and
is also involved in the processes
of modulation and demodulation.
The two input frequencies are combined in
a nonlinear signal-processing device such as
a vacuum tube, transistor, or diode, usually
called a mixer.
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4. INTRODUCTION
In coherent detection the local carrier generated at the receiver is
phase locked with the carrier at the transmitter.
Hence it is also called synchronous detection.
In non coherent detection the local carrier generated at the
receiver not be phase locked with the carrier at the transmitter.
Hence it is also called asynchronous detection.
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5. COHERENT DETECTION
In optical fiber communication the term coherent refers to any
technique which employs non linear mixing between two optical
waves.
In this technique gain is provided to the incoming optical signal
by combining or mixing it with locally generated continuous
wave (CW) optical field.
The device used for creating the CW signal is a narrow line width
laser called a local oscillator (LO).
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7. MATHEMATICAL ANALYSIS OF
COHERENT DETECTION
Let us consider the electric field of the optical signal to be plane
wave having the form
To send information one can modulate the amplitude, frequency
or phase of the signal carrier.
Thus one of the following three modulation techniques can be
implemented
ASK
FSK
PSK
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8. MATHEMATICAL ANALYSIS OF
COHERENT DETECTION
In direct detection system the electrical signal coming into the
transmitter amplitude modulates the optical power level of the
light source.
Thus the optical power is proportional to the signal current level.
At the receiver the incoming optical signal is converted directly
into demodulated electrical output.
This directly detected current is proportional to the intensity Idd
of the optical signal
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9. MATHEMATICAL ANALYSIS OF
COHERENT DETECTION
Because the response capability of the receiver
If the local oscillator field is
Now the coherent detected output at the receiver is
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10. MATHEMATICAL ANALYSIS OF
COHERENT DETECTION
Since the optical power P(t) is proportional to the intensity at
the photodetector we have
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11. HOMODYNE DETECTION
When the signal carrier and local oscillator frequencies are
equal, it is called a homodyne detection.
Homodyne detection brings the signal directly to the baseband
frequency, so that no further electrical demodulation is required.
Homodyne receivers yield the most sensitive coherent system.
It is most difficult to build, since the local oscillator lasers to
have the same frequencies.
In this since
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12. HETERODYNE DETECTION
In heterodyne detection, the intermediate frequency is nonzero
and an optical phase locked loop is not needed.
In heterodyne detection, the local-oscillator frequency (LO)
ωLO is chosen to differ from the signal-carrier frequency ω0 such
that the intermediate frequency ωIF is in the microwave region
(ωIF = 1 GHz).
Heterodyne receivers are much easier to implement than
homodyne receivers.
In heterodyne detection, 3dB degradation in sensitivity
compared to homodyne detection.
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13. HETERODYNE DETECTION
Let us consider the output current at the receiver
The receiver output current then contain a dc term given by
The dc current is normally filtered out in the receiver, and IF
current gets amplified. One then recovers the information from
the amplified current using conventional RF demodulation
techniques.
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15. HETERODYNE DETECTION
Heterodyne detection
Information can be transmitted through amplitude, phase, or
frequency modulation of the optical carrier wave in heterodyne
detections.
Heterodyne detection is used for the construction of Doppler
velocimeters and laser range finders and as well as in
spectroscopy (particular LIDAR systems).
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17. ADVANTAGES OF HETERODYNE
Direct encoding of the spectrum of the incoming signal over a
given wavelength range.
Signals are down-converted to frequencies where low noise
electronics can be used.
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18. APPLICATIONS OF HETERODYNE
The heterodyne detection used in radio as they boost the angular
resolution through inter-ferometry.
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