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Incoherent and Coherent Optical Receiver System Schematics

Radobs 17

 

Many of you will have read about my repeated references to (coherent)
optical heterodyne detection, but may not have understood how it really
differs from (incoherent) direct detection.  Here now is a short primer:
Incoherent Detection:
The basic incoherent optical receiver front-end is shown below, and for
simplicity I have omitted the lenses.  The received optical signal Pr and
background radiation noise Npl, is filtered by a narrow-band filter to
reject as much Planckian starlight and background radiation as possible,
which would only add to the detected noise.  In a photon-counting receiver,
an optical (or image) intensifier may be used between the optical filter and
the photodetector.  In this case, a simple PIN photodetector or
photodetector array without any current gain it used to detect the optical
signal.  If an intensifier is not used, the photodetector must have gain for
very sensitive detection, i.e., quantum noise limited sensitivity, and thus
the device used may be an Avalanche Photodetector (APD) biased in the
geiger-mode, or a photomultiplier.
                                                           I
                              Intensifier        ----------<--------
                     --         ******          |
       Signal Pr    |  |  Er   *      *         |
       -----------> |  | ----> *      * ----> -----  PIN (or APD)
       -----------> |  | ----> *      * ----> /   \  Photodetector
       -----------> |  | ----> *      * ----> -----
       Background   |  |       *      *         |        -----
          Npl        --         ******          |       |     |
                 Narrow-Band                     --->---|     |---->
               Optical Filter                           |     |
                     Bo                                  -----
                                               Low-Pass Electrical Filter
                                                           Be
                Incoherent (Direct) Detection Optical Receiver.
The photon detection rate and signal current I is proportional to the square
of the electric field Er^2 or optical power Pr, because the photodetector is
a square-law device.  Assuming that the post-detection bandwidth Be is equal
to the modulation bandwidth, and the receiver is background limited, then
the sensitivity of the receiver is inversely proportional to the optical
detection bandwidth Bo.  Generally Bo >> Be, and the best conventional
spectrographic gratings provide a Bo of about 1 part in 100,000, i.e., about
5 GHz.  Significantly smaller optical bandwidths may be obtained with state-
of-the-art Fabry-Perot interferometers, and this can be as low as 1 MHz.
Coherent Detection:
The basic coherent optical receiver front-end is shown below, and again for
simplicity I have omitted the lenses.  The received optical signal may be
pre-filtered by a narrow-band filter to reject as much Planckian starlight
Npl and background radiation as possible, but for diffraction limited
Optical SETI this should not be required.  In a heterodyning receiver, a
local-oscillator is used to beat or mix with the signal on the
photodetector.  In this case, a simple PIN photodetector or photodetector
array without any current gain it used to detect the optical signal.  The
powerful local-oscillator effectively provides the gain and ensures quantum
noise limited detection.  In the quantum limit for identical optical
bandwidths, the signal-to-noise ratio of the incoherent photon-counting
receiver is within 3 dB of the coherent heterodyne receiver.
                                                            I
                                                  ----------<-------
                 --          Beamcombiner        |
   Signal Pr    |  |    Er     -------           |
   -----------> |  | -------> |     . | ---->> -----      PIN
   -----------> |  | -------> |   .   | ---->> /   \  Photodetector
   -----------> |  | -------> | .     | ---->> -----
   Background   |  |           -------           |        -----
      Npl        --            ^ ^ ^ ^           |       |     |
              Optional         | | | |            --->---|     |--->
             Narrow-Band       | | | | Eo                |     |
           Optical Filter      | | | |                    -----
                            -------------         Intermediate Frequency
                           |             |          Electrical Filter
                           |    Local    |                 Bif
                 Eo >> Er  | Oscillator  |
                           |    Laser    |
                           |             |
                            -------------
                                  |
                                  |
                                   ----------------<----------------
                                           Frequency Control
                     Coherent Optical Heterodyne Receiver.
The signal current I is proportional to the product of the signal electric
field and the local-oscillator electric field, and a difference or
Intermediate Frequency (I.F.) is produced because the photodetector is a
square-law device.  Let us see how this comes about.
Consider two optical beams mixing on a photodiode (square-law detector). 
Let the beams be given by:
Received signal beam = Er.cos(wrt + phi),
Local-oscillator beam = Eo.coswot.
The photodetector current is given by:
I = k(Er + Eo)^2
where k is a constant of proportionality.
I = k[Er.cos(wrt + phi) + Eo.coswot]^2
I = kEr^2.cos2(wrt + phi) + 2kEr.Eo.cos(wrt + phi).coswot + kEo^2.cos2wot
I =   0.5kEr^2[1+ cos2(wrt + phi)]
    + kEr.Eo[cos{(wr - wo)t + phi}] + kEr.Eo[cos{(wr + wo)t + phi)}]
    + 0.5kEo^2[1 + cos2wot]
Rejecting all but the difference frequency term,
I = kEr.Eo[cos{(wr - wo)t + phi}]
where (wr - wo) = Bif, is the difference (intermediate) frequency.
In this coherent detection case, where the local oscillator power Po >> than
the received power Pr, the effective optical detection bandwidth which
determines the noise floor of a signal, is the I.F. filter post-detection
bandwidth Bif.  Depending on the linewidth of the local-oscillator laser,
this bandwidth could be as small as 1 kHz, so it can be made significantly
smaller than the best incoherent optical filters.  By employing dye lasers
which have wide tunability, it is possible to effectively tune the receiver
over the entire visible spectrum and part of the near-infrared.
January 14, 1991
RADOBS.17
BBOARD No. 320
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1991        *
* AMIEE, SMIEEE                                                           *
* Consultant                            "Where No Photon Has Gone Before" *
*                                                   __________            *
* FIBERDYNE OPTOELECTRONICS                        /          \           *
* 545 Northview Drive                          ---   hf >> kT   ---       *
* Columbus, Ohio 43209                             \__________/           *
* United States                            ..    ..    ..    ..    ..     *
* Tel. (614) 258-7402                     .  .  .  .  .  .  .  .  .  .  . *
* skingsle@magnus.ircc.ohio-state.edu         ..    ..    ..    ..    ..  *
* CompuServe: 72376,3545                                                  *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 
 

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