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EJASA - Part 5



        Table 2, Line 26 -

        This is the bottom line, showing the SNR (CNR) normalized to a
    1 Hz bandwidth.  The 34 dB CNR for the 656 nm system corresponds to a
    photon detection rate of 2,640 per second (Equ. 36).  For practical
    Professional Optical SETI searches, we should be looking for signals
    with minimum bandwidths of about 100 kHz.  As long as the Signal-To-
    Planck and Signal-To-Daylight ratios are larger than the quantum SNR,
    the former do not reduce the system performance.  It should be noted
    that at a frequency of 1.5 GHz (wavelength = 20 cm), the full
    6.4-kilometer diameter microwave Cyclops Project [5], which in 1971
    would have cost about ten billion dollars, only achieves an SNR of
    60 dB (see Table 1, Page 19).  This is about 26 dB greater than for a
    10-meter diameter symmetrical visible system.

        Other than the fact that interstellar absorption at microwave
    frequencies for distances in excess of a few thousand light years is
    significantly less than in the visible spectrum, the Microwave Cyclops
    system has little to commend it for communications within the solar

   
                                                                     Page 27

    neighborhood, particularly as the cost of the receiver is about one
    hundred times that of a single-aperture ground-based optical counter-
    part.  This is good grounds for thinking "small is beautiful".  For
    some strange reason, while free-space laser communications appears to
    be fine for future terrene GEO (Geosynchronous Earth Orbit) to LEO
    (Low Earth Orbit) and deep-space communications (much of this work is
    being coordinated by NASA [63-66]), the SETI community appears to be
    convinced that ETIs would not use such technology for interstellar
    communications!  This is illogical.  A presently favored operating
    wavelength for terrene free-space communications systems is 530 nm
    (green), obtained by frequency-doubling the 1,060 nm wavelength
    produced by a laser-diode pumped Nd:YAG laser.

        As previously mentioned, terrene SETI programs appear to have been
    distorted by poor assumptions in the Cyclops study (see Table 1,
    Page 19). [5]  As we showed earlier, the efficacy of the optical
    approach was severely hampered by constraining the near-infrared
    transmitting telescope size to 22.5 cm.  It boggles the mind to think
    that ETIs would be trying to contact us with their equivalent of a
    Celestron or Meade telescope.  This would put the onus on us to build
    very large and expensive multi-aperture receiving telescopes to pick up
    their weak signals; surely the very opposite would be the case!  The
    Cyclops study was unable even to predict the rise in ascendancy of the
    ubiquitous semiconductor chip over the following five years, and the
    effect it would have on SETI signal processing, even though integrated
    circuits were being developed in the editors' backyard!

        Present-day experimental ground-based free-space communications
    links are already using receiving telescope apertures as large as
    1.5 meters. [66]  Since the overall performance of symmetrical systems
    is proportional to the telescope diameter raised to the sixth to eighth
    power (allowing for power density limitations due to heating effects
    at the transmitter mirror), poor estimations about transmitting and
    receiving telescope apertures can drastically skew a comparative
    systems analysis.  In practice, transmitting and receiving telescopes
    are likely to be extremely asymmetric.  If we do discover an optical
    ETI signal in the next few decades, it will probably be found to have
    been transmitted by a huge optical array, while our receiving antenna
    will be a relatively puny telescope.

        Figure 4 shows a graph of received signal spectral density,
    superimposed on the Planckian spectral density curve for a (solar-type)
    black body radiator at a temperature of 5,778 K.  The microwave system
    performance shown in this graph is based on the 300-meter diameter
    Arecibo telescope; producing a CNR some 19 dB greater than for the
    100-meter radio telescope system modelled in Table 2 (Page 22).

        The reader is encouraged to compare this graph to that given in
    FIRST CONTACT [26] (Chapter 4, Page 151, by Dr. Michael Klein).  The
    first impressions from that graph (Figure 1 of Chapter 4) is again
    that optical communications are useless.  This is far from the truth.
    Indeed, the graph is very misleading.  One might be forgiven for
    thinking that in this model the ETIs are using Compact Disc-type
    laser-diodes and/or hobby model-type telescopes!  The assumed

   
                                                                     Page 28

    Spectral Density, W/m^2.Hz
      |
    10^-15 |
           |                              EIRP = 2.3 X 10^18 W  23rd Mag.
           |                                                  *
           |                    EIRP = 8.7 X 10^15 W          * CNR = 34 dB
           |                                        *         *         .
    10^-20 |                            CNR = 22 dB *         *.    Quantum
           |                                        *.        *      Noise
           |                              .         *         *
           |                                        *         #656 nm Beacon
           | EIRP = 2.2 X 10^10 W                   *    #   |  #(10 m Dia.)
    10^-25 |       *               10,600 nm Beacon #        | |
           |       * CNR = 20 dB   (10 m Dia.)  #            |V| #
           |    . .*. . . . . . .           #                |I| 2nd Mag.
           |       * Temp. = 10 K      #    ^                |S|  #Starlight
           |       *              #         |                |I|
    10^-30 |       *         #          Planckian            |B|   #
           |       *    #              Black Body            |L|
           |       # 1.5 GHz Beacon       Curve              |E|    #
           |   #     (300 m Dia.)                            | |
           |#                                                |L|     #
    10^-35 |                                                 |I|
           |                                                 |G|      #
           |                                                 |H|
           |       Microwave      Millimeter      Infrared   |T| Ultra #
           |                                                 | | Violet
    10^-40  ----------------------------------------------------------------
         10^8   10^9   10^10   10^11   10^12   10^13   10^14   10^15   10^16

                                    Frequency, Hz

    Figure 4 -

    Spectral density and interstellar CNR for 1 kW (SETI) signals at
    ten light years.  Quantum Efficiency at Visible and Infrared = 0.5.
    Microwave system is based on 300-meter diameter Arecibo-type
    telescopes.  Optical systems are based on perfect 10-meter diameter
    telescopes as modelled in Table 2.  The Carrier-To-Noise Ratios (CNRs)
    are normalized to a 1 Hz bandwidth.  The EIRP of a solar-type star =
    3.9 X 10^26 W, and has an apparent magnitude equal to 2.2.


    optical EIRPs are much too low.  Also, the graph is plotted in terms
    of EIRP, and therefore exaggerates the efficacy of the microwave
    approach for an electronic receiver (instead of an observer), because
    it does not show the typical 10 K noise floor of a high-quality
    microwave receiver, only the radio brightness of a quiet G-type star.
    The latter is about 54 dB beneath the 10 K systems noise floor, as
    shown in Figure 4, and could only be detected after considerable signal
    integration.  At 1.5 GHz, it is generally the Cosmic Background, i.e.,
    the 2.73 K aftermath of the theoretical Big Bang, and the electronic
    noise in the microwave front-end that limits signal detectability, not
    Planckian radio noise from the star.


                                                                     Page 29

                                    LASERS

        Table 3 gives a list of many of the more important laser types
    presently known. [79]  As previously mentioned, the CO2 wavelength of
    10,600 nm has been identified as an "optical magic wavelength".
    [46-47,51-53,57]  However, there are many laser wavelengths in the
    visible and infrared spectrums that might be suitable for ETI trans-
    mitters and local-oscillators.  We should not discount the possibility
    that ETIs may use efficient frequency-doubled lasers, so we might
    consider exploring the visible spectrum for near-infrared lasers at
    half the wavelengths quoted below.  For example, the 532 nm wavelength
    corresponding to the frequency-doubled Nd:YAG 1,064 nm transition may
    be a suitable wavelength; one that is presently favored for terrene
    optical communications.


    =====================================================================
   |  Table 3  Important laser types and wavelengths                     |
   |=====================================================================|
   |               Type               |          Wavelength (nm)         |
   |----------------------------------|----------------------------------|
   | Free-Electron                    | Ultra-violet to far-infrared*    |
   | Krypton-Fluoride Excimer         | 249                              |
   | Xenon-Chloride Excimer           | 308                              |
   | Nitrogen Gas (N2)                | 337                              |
   | Organic Dye (in solution)        | 300-1,000 (tunable)**            |
   | Krypton Ion                      | 335-800                          |
   | Helium-Cadmium                   | 422.0                            |
   | Argon Ion                        | 450-530 (main lines 488 & 514.5) |
   | Helium Neon                      | 543, 632.8, 1,150                |
   | Semiconductor (GaInP)            | 670-680                          |
   | Ruby                             | 694                              |
   | Semiconductor (GaAlAs)           | 750-900                          |
   | Neodymium YAG                    | 1,064                            |
   | Semiconductor (InGaAsP)          | 1,300-1,600                      |
   | Hydrogen-Fluoride Chemical       | 2,600-3,000                      |
   | Semiconductor (Pb-salt)          | 3,300-27,000 (tunable)**         |
   | Deuterium Fluoride               | 3,600-4,000                      |
   | Carbon Monoxide                  | 5,000-6,500                      |
   | Carbon Dioxide (CO2)             | 9,000-11,400 (main line 10,600)  |
    =====================================================================
    *  Extremely high peak powers available within the decade (> 100 GW).
    ** Suitable for wide-tunability receiver local-oscillators.


        Carbon Dioxide and Semiconductor lasers are very efficient.  In
    addition to the types listed above, there are a variety of chemical
    lasers, including: Iodine, Hydrogen Bromide, Xenon Hexafluoride,
    Uranium Hexafluoride, and Sulphur Hexafluoride.  These chemical lasers
    are efficient and very powerful.

        Lasers like the Helium-Cadmium and Helium-Neon can be discounted
    because of their very poor efficiency and low power, even though their
    temporal coherence is excellent.  Similarly, the original Ruby laser is


                                                                     Page 30

     =====================================================================
    | Table 4  The most intense Fraunhofer lines from the Sun{1}          |
    |=====================================================================|
    | Wavelength, nm      Bandwidth, nm   Bandwidth, GHz    Element       |
    |---------------------------------------------------------------------|
    | 410.1748               0.3133          558.7          H_delta       |
    | 413.2067               0.0400           71.0          Fe I{2}       |
    | 414.3878               0.0466           81.4          Fe I          |
    | 416.7277               0.0200           34.5          Mg I          |
    | 420.2040               0.0326           55.4          Fe I          |
    | 422.6740               0.1476          247.9          Ca I          |
    | 423.5949               0.0385           64.4          Fe I{2}       |
    | 425.0130               0.0342           56.8          Fe I{2}       |
    | 425.0797               0.0400           66.4          Fe I{2}       |
    | 425.4346               0.0393           65.1          Cr I{2}       |
    | 426.0486               0.0595           98.3          Fe I          |
    | 427.1774               0.0756          124.3          Fe I          |
    | 432.5775               0.0793          127.1          Fe I{2}       |
    | 434.0475               0.2855          454.6          H_gamma       |
    | 438.3557               0.1008          157.4          Fe I          |
    | 440.4761               0.0898          138.9          Fe I          |
    | 441.5135               0.0417           64.2          Fe I{2}       |
    | 452.8627               0.0275           40.2          Fe I{2}       |
    | 455.4036               0.0159           23.0          Ba II         |
    | 470.3003               0.0326           44.2          Mg I          |
    | 486.1342               0.3680          467.2          H_beta        |
    | 489.1502               0.0312           39.1          Fe I          |
    | 492.0514               0.0471           58.4          Fe I{2}       |
    | 495.7613               0.0696           85.0          Fe I{2}       |
    | 516.7327               0.0935          105.1          Mg I{2}       |
    | 517.2698               0.1259          141.2          Mg I          |
    | 518.3619               0.1584          176.9          Mg I          |
    | 525.0216               0.0062            6.7          Fe I{3}       |
    | 526.9550               0.0478           51.6          Fe I{2}       |
    | 532.8051               0.0375           39.6          Fe I          |
    | 552.8418               0.0293           28.8          Mg I          |
    | 588.9973               0.0752           65.0          Na I(D2){2}   |
    | 589.5940               0.0564           48.7          Na I(D1)      |
    | 610.2727               0.0135           10.9          Ca I          |
    | 612.2226               0.0222           17.8          Ca I          |
    | 616.2180               0.0222           17.5          Ca I          |
    | 630.2499               0.0083            6.3          Fe I{3}       |
    | 656.2808 _____________ 0.4020 ________ 280.0 ________ H_alpha       |
    | 849.8062               0.1470           61.1          Ca II         |
    | 854.2144               0.3670          150.9          Ca II         |
    | 866.2170               0.2600          104.0          Ca II         |
     =====================================================================

    Table reproduced from "Astrophysical Formulae", edited by K.R. Lang,
    Springer-Verlag, 1978, p. 175. [90]

    {1} After MOORE, MINNAERT, and HOUTGAST.
    {2} Blended line.
    {3} Magnetic sensitive line.


                                                                     Page 31

    inefficient and low power.  Probably, one of the more important
    considerations for an ETI transmitting laser is that it should be
    capable of being deployed in space, be able to produce extremely high
    C.W. or pulse powers, and be nuclear or stellar (solar) pumped.

        Organic dye lasers are suitable for local-oscillators, with their
    wide tunability and narrow linewidth (< 5 kHz).  Lead-salt semi-
    conductor lasers are suitable for infrared local-oscillators.


                               FRAUNHOFER LINES

        Table 4 is a list of the most intense Fraunhofer lines from the Sun
    and their effective bandwidths.  The H_alpha Hydrogen line upon which
    the visible Optical SETI model is based, has a wavelength of
    656.2808 nm (frequency = 4.57 X 10^14 Hz), and an effective linewidth
    or bandwidth of 0.402 nm (280 GHz). [88-90]  The actual FWHM linewidth
    is somewhat less that 280 GHz.


                              THE OPTICAL SEARCH

        An "All Sky Survey" of the type planned for the Microwave Observing
    Project (MOP), which pixelizes the entire celestial sphere, does not
    make sense in the optical regime. [40-45]  The 10^16 beams (Equ. 20)
    for a diffraction limited 10-meter diameter visible-wavelength
    telescope are mainly wasted looking out into empty (local) space.  For
    a celestial sphere one thousand light years in radius, containing one
    million solar-type stars, the average angular separation between stars
    is 0.23 degrees (see Figure 10).  A 34-meter diameter radio telescope at
    1.5 GHz has a typical field-of-view (FOV) of 0.41 X 0.41 degrees, and
    thus, on average, its FOV encompasses several stars.  It is efficient
    when conducting a radio "All Sky Survey" to continuously scan the
    celestial sphere in consecutive or adjacent strips or sectors.

        The 10-meter diameter Professional 656 nm Optical SETI Telescope
    would have a typical FOV = 0.33 X 0.33 degrees and a 128 X 128
    photodetector array FOV = 2.1" X 2.1".  Since the average separation
    between stars is 0.23 degrees, the average number of stars in the
    optical array FOV is 6.4 X 10^-6.  Thus, the narrow diffraction-limited
    field-of-view means that for most of the time the optical detector(s)
    would be viewing empty space.  A similar situation prevails for the
    smaller, single detector amateur optical telescopes to be discussed
    later.  The argument has been advanced by Dr. Bernard Oliver, in
    correspondence with the author and at the author's SETI Institute talk,
    that because an "All Sky Survey" would be out of the question at
    optical frequencies, this implies that ETIs would not use these
    frequencies.

        The author's response to this is that there is nothing "holy" about
    the "All Sky Survey" approach.  What we may wish to do is to have a
    Targeted Search of tens of thousands of stars, instead of a mere eight
    hundred as presently planned for MOP (see Page 11).  However, each time
    we wish to scan another star in the frequency domain, we will move the


                                                                     Page 32

    telescope to an adjacent sector of the sky that contains the desired
    object.

        While there is the possibility that ETI transmitters exist in the
    interstellar voids, far from their home stars, the author thinks that
    this scenario is unlikely (except perhaps within our own solar system,
    i.e., von Neumann-type probes), if for no other reason than it would
    place the energy-intensive transmitters far from a "cheap" and
    plentiful energy source.

        One of the many objections made to the optical approach to SETI is
    that there are just too many frequencies to search.  As Figure 5
    illustrates, under the author's rationale, this is more a perception
    than a reality because of the wider signal bandwidths assumed.


      21-cm Water-Hole                               Channel or Bin
         |                                                  |
       ------------------------------------------------------------------
      |  *                                                  #            |
      |  *                      MICROWAVE HAYSTACK          #            |
      |  *                                                  #            |
       ------------------------------------------------------------------
      |                                  |               --> <--         |
    1 GHz                              10 GHz              1 Hz      100 GHz

        Number of 1 Hz frequency channels or bins between
        1 GHz and 10 GHz = 9 Billion.


                10,600 nm                                 656 nm
                   |                                         |
       ------------------------------------------------------------------
      |            *                                   #     *           |
      |            *             OPTICAL HAYSTACK      #     *           |
      |            *                                   #     *           |
       ------------------------------------------------------------------
      |                                  |          --> <--              |
    10 THz                            100 THz       100 kHz        1,000 THz

        Number of 100 kHz frequency channels or bins between
        20 THz and 920 THz = 9 Billion.

    Figure 5 -

    The Microwave and Optical Cosmic Haystack frequency domains.  This
    demonstrates that the number of frequencies to search in the microwave
    and optical haystacks are of similar magnitude.


        Wide bandwidth means that laser linewidths, Doppler shifts, and
    chirps (drifts) are less significant, and the number of frequencies to
    search in the optical spectrum is more manageable.  Just because
    visible frequencies are over five orders of magnitude higher than


                                                                     Page 33

    microwave frequencies does not mean that there are over 10^5 more
    frequencies to search in the optical frequency domain.  The modulation
    bandwidth of proposed optical ETI signals as a percentage of the
    carrier frequency may be as large or larger than the percentage
    modulation bandwidth of proposed microwave ETI signals.  In fact,
    assuming minimum bin bandwidths of 100 kHz, the number of frequencies
    to search in the entire optical spectrum may not be much greater than
    the number of 1 Hz frequencies between 1 and 10 GHz, i.e., nine
    billion!  This is illustrated diagrammatically in Figure 5.  This
    clearly has important ramifications in terms of the search time.

        The reader should note that for a drifting carrier signal, i.e.,
    one subjected to Doppler Chirp, the optimum detection bandwidth is
    equal to the square root of the frequency drift rate. [5,8]  This
    assumes that the local-oscillator laser is not de-chirped.  Thus, the
    optimum bandwidth for a monochromatic 1.5 GHz signal drifting at a
    local Doppler Chirp rate of 0.17 Hz/s (see Table 2, Line 30, Page 22)
    is about 0.4 Hz, while for a monochromatic 656 nm signal drifting at
    51 kHz/s, the optimum bandwidth is 226 Hz.  If the bin bandwidth is
    excessive, too much system noise is detected, and the CNR is degraded.
    On the other hand, if the bin bandwidth is too small, the response time
    of the filter (approximately 1/Bif) is insufficient to respond to all
    the energy in the signal as it sweeps by, again leading to a reduction
    in CNR and detectability.

        It is an interesting exercise to estimate the time that would be
    required at visible wavelengths for both an All Sky Survey and a
    Targeted Search.  We will assume the use of a 10-meter diameter
    receiving telescope, a 128 X 128 photodetector array (16,384 pixels),
    and initially, a single 10 GHz bandwidth Multi-Channel Spectrum
    Analyzer (MCSA) that sequentially samples all 16,384 photodetectors.
    These MCSAs could have final bin bandwidths of about 100 kHz.  At this
    time, 10 GHz MCSAs do not exist, and the state-of-the-art for single-
    chip devices employed in Microwave SETI is about 10 MHz.  However, it
    is only a question of time before these more powerful 10 GHz devices
    are developed.

        For the purposes of this brief analysis we shall not concern
    ourselves with the huge amount of data storage that must be provided,
    or the data reduction time overhead required.  Equ. 20 (Page 81) shows
    that the number of received beams for such a telescope is about 10^16.
    Since the minimum sampling time per pixel for a 10 GHz bandwidth is
    100 ps, the time to sample the entire array of 16,384 instantaneous
    beams is 1.64 microseconds.  The number of array sets of beams in the
    celestial sphere consisting of 10^16 beams is 6.1 X 10^11.  Thus, the
    time just to "look" at one 10 GHz wide band of the visible spectrum,
    assuming that a continuous scan of the sky could be made with no dead
    time or overlap, is 10^6 s, i.e., 11.6 days!  This is a substantial
    amount of time for a single band just 10 GHz wide.

        Since there are 42,857 bands of 10 GHz bandwidth between 350 nm and
    700 nm, the time required to search the entire sky and all visible
    frequencies, is at a minimum, 1,360 years!  Even if we had 128
    parallel MCSAs (don't even consider having 16,384 - 10 GHz MCSAs!), the


                                                                     Page 34

    time to search even a 10 GHz band is long, notwithstanding the "slight"
    data storage problem.  Clearly, we can forget about this form of
    optical All Sky Survey, since it is a grossly inefficient way of
    scanning or pixelizing the sky.  Almost all the data bins will be empty
    bins, having been derived from beams pointing to empty (near) space.
    The situation for an Optical All Sky Survey is actually much worse than
    just implied, due to the additional time that each pixel must be
    sampled to ensure a high probability of detecting the fewer, but more
    energetic optical photons - more about this in a moment.

        On the other hand, if we only consider a Targeted Search, the time
    required is much shorter and allows for the search to be done across
    the entire optical spectrum, not just at selected laser frequencies or
    Fraunhofer lines.  As we have just seen, if the photon arrival rate is
    sufficiently high, the time with a single 10 GHz MCSA for a single scan
    of the entire array is 1.64 microseconds.  To scan for one star over
    the entire 350 nm to 700 nm band would take 0.070 seconds (assuming
    suitable L.O. lasers).  This is a trivial amount of time, and the amount
    of data that has to be collected and stored is relatively insignificant.
    Indeed, it is the time to do the FFTs and move the telescope to a new
    position that will be the most significant overheads here.

        The above times are highly optimistic because the basic flux
    sensitivity of any kind of receiver, be it microwave or optical, depends
    on the sampling or integration time.  Hence, before we can estimate the
    realistic length of time for a given search, we must decide what are the
    minimum detectable flux levels that we wish to detect.  This, in turn,
    will determine the minimum sampling time for each pixel.  Usually, SETI
    minimum detectable flux estimates are based on integrating a very weak
    signal for a period of time, and not for providing sufficient SNR to
    allow actual demodulation.  We must also decide if we want to model a
    system based on short pulses or on continuous wave (C.W.) signals.

        Of course, it is extremely unlikely that the signal flux would be
    sufficiently high to allow for a high probability of detecting the
    photons in a sampling bandwidth of 10 GHz.  In reality, our minimum
    MCSA bin bandwidths would be about 100 kHz, and the sampling
    (integration) time is at least a factor of 10^5 longer.  For the
    purposes of this further analysis, we shall assume a C.W. signal and a
    100 kHz minimum bin bandwidth, so that the pixel sampling time is now
    10 us.  For our 10-meter diameter 656 nm symmetrical heterodyning
    telescope system, we can estimate the minimum detectable signal flux
    density by calculating the flux required to reduce the CNR to 0 dB.

        We have already shown (Table 2, Line 12, Page 22), that a flux
    intensity of 2.04 X 10-^17 W/m^2 will produce a CNR = 34 dB re 1 Hz.
    Therefore, in a 100 kHz bandwidth, the CNR will be -16 dB.  To increase
    the CNR to 0 dB means that the intensity must be increased by 16 dB to
    8.12 X 10^-16 W/m^2.  Thus, the minimum detectable signal flux for
    this bandwidth and sampling rate is 8.1 X 10^-16 W/m^2.  This is
    equivalent to saying that during the 10 microsecond sampling time, if
    an ETI signal is present on one pixel, we would have a reasonable
    probability of detecting one photon (Equ. 36).  This signal flux would
    be produced by a ten meter diameter transmitter at a range of ten light


                                                                     Page 35

    years, with a power of 16 dB re 1 kW, i.e., 40 kW.  This is a trivial
    amount of power for an ETI.



Continued

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