Laser transition in atomic iodine for passive and active SETI

Acta Astronautica 67 (2010) 1384–1390

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Laser transition in atomic iodine for passive and active SETI Yu.F. Kutaev a, S.K. Mankevich a, O.Yu. Nosach b, E.P. Orlov b, a b

Astrofizika Research and Production Association, Volokolamskoe shosse 95, 124424 Moscow, Russia P.N. Lebedev Physics Institute, Russian Academy of Sciences, Leninsky prosp. 53, 119991 Moscow, Russia

a r t i c l e i n f o

abstract

Article history: Received 21 February 2009 Accepted 1 February 2010 Available online 12 March 2010

It is proposed to use the wavelength 1.315 mm of the laser transition 2P1/2-2P3/2 in atomic iodine both for passive and active SETI. The search for an extraterrestrial signal at this wavelength is promising because an active quantum filter (AQF) with a quantum sensitivity limit and with strong fixing in the spectrum luminescence line of the width less than 0.01 cm  1, has been developed for the noted wavelength. Such AQF is capable of receiving the laser signals, consisting of only a few photons, against the background of emission from a star under study. In addition, the high-energy iodine lasers emitting diffraction-limited nanosecond pulses of energy of 2 kJ in a single beam at the wavelength 1.315 mm have been created. The spectral tuning of radiation of these lasers allows one to compensate the frequency Doppler shift. A weak absorption of the 1.315 mm radiation in the Earth atmosphere allows the search for extraterrestrial signals by using the ground optical telescopes equipped with adaptive optical systems. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Extraterrestrial signals Iodine laser Active quantum filter Quantum sensitivity limit Diffraction-limited divergence Spectral tuning Absorption in the Earth atmosphere Optical telescope Adaptive optical system

1. Introduction The problem of a search for extraterrestrial intelligence (ETI) signals involves a number of basic technological tasks, which should be solved to make this problem real. These tasks are: 1. The choice of radiation wavelengths suitable for the ETI signal search. 2. The signal separation against the emission of a star around which a planet with the assumed ETI rotates. 3. Detection of the ETI signal with the probability no less than 0.9. 4. The provision of the required energy of the communicated signal for its detection at interstellar distances. In 1959 G. Cocconi and P. Morrison substantiated the expediency of searching for ETI signal at the wavelength of 21 cm (1420.4 MHz) of the radio line of hydrogen [1]. In  Corresponding author.

E-mail address: [email protected] (E.P. Orlov). 0094-5765/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2010.02.002

1961, R.N. Schwartz and C.N. Townes pointed out that in the optical range ‘‘yperhaps it would be appropriate to examine high-resolution stellar spectra for lines which are unusually narrow, at peculiar frequencies, or varying in intensity’’, and that ‘‘ythe choice probably being dictated by the availability of suitable maser material to produce the desired frequency’’ [2]. For passive and active SETI in optical range the following devices are required. The narrowband filters, which virtually do not absorb the useful signal, and photon detectors with the 100% quantum yield, i.e. with quantum limit of the sensitivity—in the receivers, and the high-energy diffraction-limited lasers—in the transmitters. The attempts to create receivers and transmitters with such ideal parameters have been unsuccessful for a long time. The experimental studies of the possibility of detecting the weak laser signals by using optical quantum amplifiers were initiated in papers [3,4]. The quantum limit of the sensitivity was achieved [5] in the iodine active quantum filter (AQF) based on a photodissociation iodine quantum amplifier operating at wavelength of

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1.315 mm (frequency 228.1 THz) [6,7]. At present a receiver with a narrowband and virtually ideal parameters has been developed, and the diffraction-limited high energy and high power iodine lasers created. 2. The iodine AQF External view of the iodine AQF is shown in Fig. 1. The laser transition in atomic iodine is shown schematically in Fig. 2. The exited iodine atoms I* in 2P1/2 state (upper laser level) are originated as a result of photo-dissociation of molecules, for example, C3F7I under UV radiation at l ffi 270 nm: C3 F7 I þhnp -C3 F7 þI ; where hnp is the quantum of UV radiation (pumping radiation). The laser transition 2P1/2-2P3/2 is characterized by strictly fixed frequency of the luminescence line with FWHM ffi0.01 cm  1 and a large lifetime of the exited iodine atoms, which is equal to 0.13 s [8]. The following characteristic features of the active medium of photodissociation lasers give a possibility of using the iodine optical quantum amplifier as an AQF for receiving extremely weak signals against a strong illumination background. 1. The active medium has high optical homogeneity [6,7]. 2. The recombination of iodine atoms is very fast, and the iodine atoms do not occupy the lower level of the laser transition [6,7]. 3. The gain of the active medium a 40.1 cm  1 [8] greatly exceeds its absorption coefficient b o10  4 cm  1 [10]. 4. The gain-line FWHM is Dn  0.01 cm  1 [8].

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3. The sensitivity of the iodine AQF The first characteristic feature allows one to provide single mode amplification [9]. Fig. 3 illustrates this. The second characteristic feature allows one to provide the minimum of quantum noise. As is well known [3,4], the spectral density of spontaneous radiation brightness (quantum noise) of the optical amplifier is described by the formula Bqn ¼ Bvac n2 =ðn2 ðg2 =g1 Þn1 ÞðKðoÞ1Þ; where Bvac ¼ ‘ o3 =8p3 c2 is the spectral density of brightness of vacuum; n1 and n2 , the populations of the lower and upper laser levels; g1, g2 , their statistical weights; K(o) , the gain factor on the frequency o. Due to rapid depletion of the lower energy level [6,7] and the absence of iodine atoms because of their rapid recombination, the AQF noise is minimal. The third characteristic feature, namely, the inequality acb provides the quantum yield g = 1. The gain exceeding 0.1 cm  1 provides the signal gain factor K 4106, and this is much more than the gain (K 103), at which the noise of the receiver is determined only by the quantum noise of

Fig. 2. The laser transition in atomic iodine.

Fig. 1. External view of the iodine AQF developed at the N.G. Basov Department of Quantum Radiophysics, Lebedev Physics Institute (LPI), Russian Academy of Sciences (RAS).

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the AQF (the shot and thermal noise of photodiodes and elements of electrical circuits at K 4103 are negligibly small) [5]. The single mode amplification, the minimum of quantum noise, a 100% quantum yield (g = 1) and the signal gain Kc103 allow one to achieve an ultimately high sensitivity of iodine AQF: one photon per mode for the time 1/cDn  10 ns.

source: dS=S0 ¼ Bpls =Bvac ffi ðexpð‘ o=kTÞ1Þ1 , where dS is the decrease of sensitivity; Bpls , the spectral density of brightness of high-power natural light source. Thus, if a signal is detected against the background of the Sun disc the sensitivity will decrease only by 15%. If it is detected against the background of the emission of a star with T= 7500 K the sensitivity will decrease only by 30%.

3.1. Experimental verification The principle scheme of the experiment [5] is shown in Fig. 4. 3.1.1. Experimental conditions

Fig. 4. Principle scheme of experiment. (1)—Iodine AQF; (2)—focusing lens; (3)—diaphragm; (4)—photodetector; (5)—video amplifier.

 The optical pulse duration, tp = 40 ns.  The effective average time of an electron amplifier, te =30 ns.

 The signal reception angle= three diffraction AQF angles. 3.1.2. Experimental results The oscilloscope trace of the output voltage of an electron amplifier is shown in Fig. 5. For the ratio signal/noise= 1, the sensitivity S0  3 photons was achieved. 4. The sensitivity of the iodine AQF against the background of high-power light source Due to small width of the gain line (the fourth characteristic feature) in combination with high gain of the AQF, the sensitivity of the AQF receiver, detecting the signals spectrally matched with the gain line, remains virtually invariable even upon observation of a signal against the background of any high-power natural light

Fig. 5. The oscilloscope trace of the output voltage of an electron amplifier upon detection of an optical pulse from the AQF by a photodiode. Input signal is E20 photons.

Fig. 3. Image of a diffraction spot without (a, c) and with (b, d) pumping AQF. The signal was attenuated to 1/5000th of its magnitude by filter and amplified in AQF by a factor of 3000 (b). The photographs in experiments (a) and (b) were specially overexposed in order to reveal the first diffraction ring in the Airy diffraction pattern. A comparison of the images (a) and (b) shows that almost the diffraction-limited resolution was achieved in both cases. The diameters of the central peaks in the first (a) and the second (b) images are almost identical, and only the intensity distribution in the first diffraction ring changes after intensification. Fig. 3c and d show the images of two diffraction spots obtained in a similar way, but with normal exposure. For this reason, the first diffraction ring in the Airy pattern is not observed.

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Fig. 6. Optical schematic of the experimental setup: (1), (5)—cavity mirrors of the master oscillator (MO); (2)—optical modulator; (3)—MO cell; (4)—diaphragm of 4 mm in diameter; (6)—totally reflecting spherical mirror with a focal length f =50 cm; (7)—diaphragm of 0.2 mm in diameter; (8)—dielectric mirror, R= 30%; (9)—photodiode; (10)—optical filters (attenuators); (11)—diaphragm of 12 mm in diameter; (12)—semitransparent spherical mirror with a reflectivity of E55% and f= 75 cm; (13)—cell of iodine AQF; (14)—interstage filters; (15)—diaphragm of 12 mm in diameter; (16)—totally reflecting spherical dielectric mirror with f = 50 cm; (17)—totally reflecting plane mirror; (18)—totally reflecting spherical dielectric mirror with f =75 cm; (19)—diaphragm of 12 mm in diameter; (20)—KS-14 red light filter; (21)—polariser (Glan prism); (22)—LFD-2 photodiode; (23)—lens with f= 60.3 cm; (24)—discharge chamber of a standard light source of the ISI-1 type (Podmoshenskii sours) with a protective glass (25).

4.1. Experimental verification This statement was verified in the model experiments [11] by detecting the signal against the background of a plasma radiation source (ISI-1 Podmoshenskii source) with the brightness temperature of 40 000 K.The optical schematic of the experimental setup [11] is shown in Fig. 6.

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Fig. 7. The oscillogram of the output voltage of an electron amplifier upon detection of an optical pulse and the ISI-1 radiation from the AQF by a photodiode. The ISI-1 radiation pulse starts within 8 ms after the time counting origin, immediately after the high-frequency electric interference from the ignition pulse; the useful signal pulse appear at the 13th microsecond.

signals are received against the background of radiation from such stars as mentioned above. The methods of detecting and processing the weak pulsed laser signals in iodine AQF systems were verified and protected by patents [14–19].

5. High energy diffraction-limited iodine lasers 4.1.1. Experimental conditions

 The optical pulse duration, tp =40 ns.  The effective average time of an electron amplifier, te = 90 ns.

 The signal reception angle= diffraction AQF angles. 4.1.2. Experimental results The oscilloscope trace of the output voltage of an electron amplifier is shown in Fig. 7. For the ratio signal/noise= 1, outside the ISI-1 pulse the sensitivity S0  3 photons were achieved. Within the ISI-1 pulse, the sensitivity S 6 photons were achieved. If tp = te = 10 ns, then the probability of detection of three photons against the background emission of the stars similar to the Sun will be more than 0.9. We can assume that the probability of life origination is the highest for the single stars of the initial main sequence, which belong to the spectral classes located between classes F5 and K5 [12]. The surface temperature of these stars does not differ strongly from the Sun surface temperature [13], and the brightness of their radiation is close to that of the Sun. Therefore, the sensitivity of the iodine AQF receiver is not almost changed, when the

The specific features of the active medium considered in Section 2, and the high-power pump sources elaborated at present led to the development of high-energy 1.315 mm iodine photodissociation lasers pumped by xenon flash lamps [6,7], high-current open electric discharges [6,20,21], and by the radiation of strong shock waves initiated by the explosives [22–24]. High-power repetitively pulsed and cw oxygen–iodine lasers were also developed [25]. For example, the lasers pumped by pulsed xenon flashlamps and high-current open electric discharges emit the nanosecond and subnanosecond pulses with the energy up to a few kilojoules in one beam [26,27], while the lasers pumped by a strong shock wave initiated by the explosives emit the megajoule microsecond pulses [21]. The output of cw oxygen–iodine lasers approaches the megawatt level [25]. The high optical homogeneity of the active gas medium of the iodine lasers in combination with phase-conjugation methods allows the generation of diffraction-limited pulses. At present such lasers are being further developed in many laboratories worldwide. As the examples the performances of iodine photodissociation lasers developed for the most famous laser setups ‘‘Iskra’’ [27] and ‘‘Asterix IV’’ [26] are presented in Table 1.

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Table 1 Performances of iodine photodissociation lasers developed for laser setups ‘‘Iskra’’ and ‘‘Asterix IV’’. No. Described object or parameter

Iodine laser developed for setup ‘‘Iskra’’, Russian Federal Nuclear Center—The All-Russian Research Institute of Experimental Physics (RFNC—VNIIEF), Sarov, Russia

Iodine laser developed for setup ‘‘Asterix IV’’, ¨ Max-Planck-Institut fur Quanten Optik, Garching, Germany

1.

High-current open electric discharge 50

Xenon flash lamps

2.0–2.5

2.1

0.25–1.0

5.0

10  5

–

2.

3.

4.

5.

Pump source Beam diameter (cm) Energy in one beam (kJ) Pulse duration (ns) Divergence of laser beam (rad)

29

6. An ideal transmitter–receiver pair and a natural frequency reference Thus, the laser receivers with an ultimate sensitivity and high-energy diffraction-limited lasers operating at a wavelength of 1.315 mm have been developed. In other words, an efficient transmitter–receiver pair has been realized. This pair has a vast dynamic range of output power and pulse energy, as well as the diffraction-limited divergence of radiation and ultimate sensitivity. In addition, the receiver of this pair can separate efficiently the 1.315 mm signal against the background of the star emission just with a weak loss of the sensitivity. The strictly fixed frequency of luminescence line of the iodine atom and, respectively, the gain-line frequency require the precision frequency tuning of the sent signal. It is no problem when the relative velocity of moving of the receiver is known, because there is a possibility to tune the frequency of radiation of the iodine lasers within several reverse centimeters by magnetic field [28]. A large lifetime of the exited iodine atoms allows continuous observation of the star chosen. All these properties of the active medium of the atomic-iodine lasers, which can be used for communication with ETI, make the 1.315 mm (428.1 THz) line of the 2 P1/2-2P3/2 transition of the atomic iodine a candidate for the natural frequency reference [29,30]. This solves the first problem of optical communication with ETI formulated in Section 1, since it is reasonable to assume that the above consideration and approach to the solution of space-communication problems are also accessible to the representatives of the assumed ETI, which can be even at a higher development level than the Earth civilization. In this case, they also should conclude that the laser transition of the atomic iodine is promising for generating the signals directed to other civilizations.

7. Principal scheme of an ETI signal receiver Fig. 8 presents the principle scheme of the receiver for SETI. An optical pulsed signal sent by the ETI is incident on the primary mirror (1) of a receiving telescope directed to the star under study and is focused to AQF (2). The signal behind the AQF is focused by the optical system (3) on the photo detector (4) whose output electric signal is fed to an electron video amplifier (5). The output signal of the video amplifier is fed to the processing unit (6). It is very important that the 1.315 mm radiation is virtually not absorbed in the Earth atmosphere. As shown in [31], the absorption coefficient in the near-ground layer is  2  10  7 cm  1. Absorption is mainly caused by the water vapor, whose concentration decreases with height. The signal is attenuated after its propagation through the entire Earth atmosphere by less than z = 20%, especially when a detector is located high over the sea level. A weak absorption of the 1.315 mm radiation in the Earth atmosphere allows the search and transmission of ETI signals almost without a decrease in communication range by using ground-based optical telescopes equipped with adaptive optical systems, which are capable of realtime compensation of the atmospheric spread of images [32–34]. Several of such telescopes are listed in Table 2. We see that the diameters of primary mirrors have already reached 8–10 m, and there are the real projects on the creation of optical telescopes with the diameter of 30 and 42 m [35,36]. A group delay of the lower frequency of the pulse spectrum and the higher frequency at l = 1.315 mm is just at intergalactic distances less than 5  10  13 s. A group delay, stipulated by the dispersion of an atmospheric refractive index, is much smaller than 2  10  16 s. This expels the shape pulse distortion on the way from the transmitter to the receiver.

8. The range of communication with ETI The main elements of a laser transmitter are a laser and an astronomical telescope playing the role of a transmitting antenna. Let us assume that the ETI transmitter contains the same elements and the losses z of

Fig. 8. Principle scheme of the ETI signals receiver: (1)—primary mirror of the receiving telescope; (2)—iodine AQF; (3)—focusing lens; (4)—photodiode; (5)—video amplifier; (6)—signal processing unit.

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Table 2 Modern optical telescopes. Telescope name

Diameter of primary mirror

Construction of primary mirror

Dislocation

KECK I

10

Parabolic, multiple segments active

Mauna Kea, Hawaii, USA

KECK II VLT GEMINI north GEMINI south SUBARU

10 4  8.2 8

Thin active Thin active

8

Thin active

8.2

Thin active

GTC

10

Analog of KECK II

Paranal, Chili Mauna Kea, Hawaii, USA Cerro Pachon, Chili Mauna Kea, Hawaii, USA La Palma, Canaries, Spain

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diameter of 10 m, a receiver with an AQF mounted on a ten-meter ground based optical telescope, equipped with adaptive optical system, can detect this signal at a distance of up to 100 pc, irrespective of the civilization position on the celestial sphere. The realization of the projects for manufacturing the ground-based optical telescopes of the diameter of 30 m will increase the research range up to 1000 pc, if the hypothetical galactic technological civilization uses similar telescopes. The required pulse energy of sending the signals being received by this civilization will be about 90 J, if a distance to the star is 25 pc and the diameters of telescopes are 10 m. By using a 30 m telescope the required pulse energy at the same distance should be approximately 1 J; if the distance to the star is 100 pc 20 J. 9. Conclusions

1.315 mm radiation in the atmosphere of the ETI planet are equal to the losses in the Earth atmosphere. If the total energy Et of a pulse emitted by the transmitter is distributed uniformly over the aperture, then it is known [37] that the angular energy density in the direction to the center of the diffraction pattern is I0 =(1  z)EtAt/l2, where At is the transmitter-aperture area equal to pD2t =4 for a circular aperture of the diameter Dt. If the receiver is located at the center of the diffraction pattern, the energy received by the detector is 2

Er ¼ ð1zÞI0 Ar =R2 ¼ ð1zÞ2 At Ar Et =ðl R2 Þ;

ð1Þ

where Ar is the receiver-aperture area (if it is circular, then Ar ¼ pD2r =4); Dr is the diameter of the primary mirror of the receiving telescope; and R is the distance between the transmitter and receiver. Consider first the process of signal received from a radiation source, which is used by the assumed ETI to communicate with our solar system. Let us discuss the reception of single pulses in which each of the pulses should be found and received; in this case, the pulse repetition rate is not important for detecting. If the development level of the ETI is such that the ETI knows the parameters of the laser transition in atomic iodine, the optimal duration of the pulse received by the AQF is also known. Therefore, we will assume that the ETI transmitter sends the diffraction-limited 10 ns laser pulses. As mentioned above, the pulses of such duration are well spectrally matched with the gain line of the iodine AQF for the gain K  103–106. For the signal-detection probability to exceed 0.9, the energy of the detected signal should exceed the quantum noise of the AQF by a factor of three, i.e. the signal-tonoise ratio should be equal to three, and, then, from expression (1), we get the condition to which the distance R should satisfy for the signal sent by the transmitter to be detected by the receiver: R rðp=4Þð1zÞðEt =3 ‘ oÞ1=2 Dt Dr =l : 2

Using this formula we obtain, that if a hypothetical galactic technological civilization sends in our direction a diffraction-limited 10 ns, 2 kJ laser pulse with the beam

The pointed properties of radiation with l =1.315 mm, and those of the receivers and transmitters which were created on the laser transition 2P1/2-2P3/2 in the atomic iodine allows one to make conclusion about possibility of serving of the wavelength l = 1.315 mm as a natural frequency reference point and the use the laser transition 2 P1/2-2P3/2 in the atomic iodine both for passive and active SETI.

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