, Ryabtsev, Boguslavski oe , Beterov. † The nonlinear interference effect [1] underlies numerous studies on coherent population trapping [2,3], inversionless amplification [4], electromagnetically induced transparency [5], atomic laser cooling and capture [6], phase control of atomic states [7], etc. Initiation of electromagnetically induced transparency (EIT) in a medium with coherent pumping is one of the most prominent interference phenomena and manifests itself as narrow dips in the absorption spectra of atoms interacting with multifrequency resonance radiation.As a rule, the experiments on EIT of alkali-metal atoms are carried out using the Λ -scheme of transitions in the laser radiation field modulated at a frequency equal to the ground-state hyperfine-or Zeeman-splitting frequency [8] or in the field of two phase-locked lasers [9]. The scanning of modulation frequency (or frequency difference in the case of two lasers) gives rise to a narrow interference resonance at the point corresponding to the exact two-photon Raman resonance between two ground-state sublevels coupled to the same excited state.The use of the four-level N-type schemes of resonance transitions, for which Zeeman coherence can be spontaneously transferred from the excited to the ground level [10], opens up new opportunities for studying nonlinear phenomena. Such a scheme was investigated both theoretically [11] and experimentally [8,12] for the quasidegenerate upper and lower states † Deceased.interacting with a two-frequency laser emission in a weak magnetic field. It was shown that the coherence transfer can give rise not only to the EIT but also to the electromagnetically induced absorption. This paper reports the results of experimental implementation of the nondegenerate four-level N-type scheme for the observation of the EIT in a three-frequency laser radiation field. The experiments were carried out using the radiations of two independent semiconductor lasers, one of which was modulated at a frequency equal to the hyperfine-splitting frequency of the excited 5 P 3/2 state of the 87 Rb atom. Therefore, the three-photon Raman resonance N-type scheme has been implemented for initiating EIT.Experiments were carried out on the optical transitions between the hyperfine-structure components of the 87 Rb D 2 absorption line (the 5 S 1/2 5 P 3/2 transition at a wavelength of 780 nm). The experimental setup is schematically shown in Fig. 1a. Two HL7851MG diode lasers with external cavities were used as radiation sources. The generation linewidth did not exceed 1 MHz. For tuning to different transitions, a portion of the radiation was diverted into two independent saturated-absorption sections 1 and 2 (according to [13]). The remaining portion of the radiation was combined in a collinear geometry. The wave fronts were carefully brought into coincidence by means of a set of mirrors and diaphragm 3 . The polarization plane of one of the radiations was turned through 90 ° by a Abstract -A nondegenerate four-level N-type scheme was experimentally...

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