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Soviet Journal of Quantum Electronics

Passive switch for 1.315 μ radiation

V. A. Gaǐdash, Valentin A. Eroshenko, S G Lapin, V I Shemyakin, V K Shurygin

Published Aug 31, 1976• 2 citations
Publisher

Abstract

The physical basis of the operation of an atomic iodine passive switch (for iodine photodissociation lasers) is considered. Atomic iodine is obtained by thermal dissociation of I 2 . An experimental dependence of the switch transmission on the radiation intensity is reported; it is in satisfactory agreement with the theory. A weak-signal attenuation by a factor of ~ 100 is achieved for 70–80 % transmission of a saturating signal ( I ~5 × 10 8 W/cm 2 ). It is shown that a switch of this kind can be used as a decoupler in multistage amplifying systems.

DOI
10.1070/qe1976v006n08abeh011752

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 1, 1987•0 citations
The physical basis of the operation of an atomic iodine passive switch (for iodine photodissociation lasers) is considered. Atomic iodine is obtained by thermal dissociation of I 2 . An experimental dependence of the switch transmission on the radiation intensity is reported; it is in satisfactory agreement with the theory. A weak-signal attenuation by a factor of ~ 100 is achieved for 70–80 % transmission of a saturating signal ( I ~5 × 10 8 W/cm 2 ). It is shown that a switch of this kind can be used as a decoupler in multistage amplifying systems.
An absorption cell for the 1.315 mu m iodine laser wavelength has been constructed using thermal dissociation of molecular iodine vapour to produce ground-state iodine atoms. Measurements of the small-signal absorption coefficient for iodine pressures up to 5 Torr and temperatures up to 750 degrees C have been compared with the predictions of a simple thermodynamic model of the iodine dissociation equilibrium, and satisfactory agreement obtained. The model readily allows calculation of operating conditions required for the use of the iodine absorber as a saturable absorber, for free-induction decay pulse generation and for coherent pulse propagation experiments.
A superradiant and amplified spontaneous transition pulse is obtained from an iodine photodissociation laser amplifier which uses i-C3F7I as an active molecule and is pumped with a flashlamp (full width at half maximum=3 μs). We have observed a sharp leading pulse of superradiance whose rise time is less than 50 ns, and the amplified spontaneous emission follows the superradiance during the pumping flash. By measuring the population inversion density in the active medium of the amplifier, we have obtained the threshold condition for the superradiance to take place. Also, the buildup time and peak intensity of the pulse are measured as a function of the flashlamp input energy. By placing a reflecting mirror at a varying distance (from 173 to 673 cm) behind the amplifier, we have found a periodic intensity fluctuation in the superradiance; the period (Δt) depends linearly on the distance (d), i.e., Δt=2d/c, where c is the light velocity.
The effect of background light on the sensitivity of a laser receiver with an iodine active quantum filter (λ=1.315 μm) was theoretically and experimentally investigated. Upon the reception of a 40-ns pulsed signal against the background of 2.5-fold attenuated radiation of a pulsed light source with a brightness temperature of 4 × 10 4 K, the sensitivity of this receiver for a signal-to-noise ratio of three and a diffraction-limited acceptance angle was experimentally shown to be equal to 20 photons, which exceeds the quantum limit by about a factor of two. This is consistent with the results of theoretical treatment and suggests that upon the detection of optical signals against the background of the solar disk, the sensitivity of this receiver should decrease by only 12%. This receiver was compared with a receiver employing a photomultiplier of the visible range. Upon the reception of optical signals with the same parameters against the background of the solar disk and an interference filter with a transmission band width of 5 nm, the sensitivity of a receiver equipped with an FEU-115 photomultiplier was shown to be equal to about 1400 photons for a signal-to-noise ratio of three.
An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3)×10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2×10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4)×10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
Recently, Jaroszynski and King found superradiance(SR) occurring in photodissociatively created systems using NaI and n-C3F7I moleculesl) , and Hahn and Lee have performe2)d the amplified spontaneous emission(ASE) experiment in an iodine photodissociation laser(IPL) amplifier . In general, to observe SR experimentally, a very short intense pumping pulse of which FWHM is about a few tens of nanoseconds, is used for making a totally inverted system. But in this work, the amplifier is pumped with conventional flashlamps of FWHM=3us, yet we have detected a output pulse which is a superposition of ASE and SR. The temporal behaviour of the pulse is investigated in detail, and the experimental results are compared with theoretical results.