ABSTRACT
The results of an optoelectronic system-frequency-shifted feedback (FSF) laser experimental examination are presented. The considered FSF laser is seeded only with optical amplifier spontaneous emission (ASE) and operates in the mode-locked regime, whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit. In the frequency domain, the spectrum of such a pulse sequence is an optical frequency comb (OFC). These OFCs we call initial. We consider the possibility of tunable acousto-optic (AO) dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime. The examined system applies a single frequency shifting loop with single AO tunable filter as the frequency shifter that is fed with several radio frequency signals simultaneously. The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width, envelope shape and position in the optical spectrum may be tuned. The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable filter (AOTF). The dual-combs frequency spacing is determined by the frequency difference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply. The quad-combs are obtained with three initial OFCs, forming a pair of dual-combs, appearing when three ultrasound frequencies feed the AOTF transducer. The quad-combs frequency spacing is defined by the difference between the frequency spacing of dual-combs. Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed. The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.
ABSTRACT
The quasicollinear geometry of acousto-optic (AO) diffraction is notable as makes it possible to achieve an extremely high AO interaction length and, consequently, an anomalously high spectral resolution for AO devices. This geometry is especially convenient for the implementation of multifrequency AO diffraction, which has found wide application for solving the problems related to the laser pulse shaping. Since acoustic beams propagate over long distances in quasicollinear AO devices, and optical radiation spectral components diffract in the acoustic field in different parts of the AO crystal, accurate calculation of the characteristics of such devices requires knowing the distribution of the acoustic field amplitude inside the AO cell. The acoustic beam structure is affected by several factors in quasicollinear AO cells: the dimensions of the piezoelectric transducer, the geometry of acoustic wave propagation in the AO cell, acoustic anisotropy and the acoustic energy absorption along the chosen direction in the crystalline material used. In this paper, we propose a generic method to measure the acoustic beam power distribution along the direction of its propagation in the quasicollinear AO cell in the presence of ultrasound power absorption and media acoustic absorption. The measurements were carried out for the ultrasound frequency range from 72 to 176 MHz, for the case when the wave vector of the acoustic beam is directed at an angle of 1.58∘ to the [110] axis in the (11¯0) plane of the paratellurite crystal. The ultrasound attenuation coefficients were obtained for frequency interval between 87 and 176 MHz and their linear dependence on ultrasound frequency was confirmed.
