## 1 Introduction  
Nowadays, the diffusing light techniques are one of the most powerful tools for characterization of the structure and dynamics of the complex random media at the microscopic level (for spatial scales comparable with the wavelength of the probe light). Among the variety of these techniques, a particular emphasis should be placed on a group of methods primarily based on the analysis of spatial, temporal, and frequency correlations of the coherent or partially coherent light propagating in the probed medium. In particular, the diffusing wave spectroscopy (DWS) pioneered by several research groups in the late eighties and early nineties of the past century [1; 2] as a probe for scatter dynamics in relatively simple and well-defined multiple-scattering Brownian systems, was widely used to characterize complex non-stationary and non-ergodic media (colloidal and quasi-crystal systems with strong interactions between the particles, mixtures with separating phases, drying paints, biological tissues, etc. [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]). An important condition for the fair DWS applicability in the diagnostics of the dynamic random media is the coherence length of the probe light that must exceed the maximal propagation path of the light in the medium. In the opposite case, fluctuations of the detected multiple-scattered light, which are caused by the scatter motions in the medium, will be partially or totally suppressed, depending on the ratio of the average propagation path of light to its coherence length. At the same time, the suppression effect associated with the blur of the dynamic speckle patterns in the scattered, partially coherent light fields, can be used to recover the path-length distributions of the probe light in the static multiple-scattering media. In turn, this opens the way for evaluation of the structural properties of the examined scattering system using an appropriate scattering model, which establishes the relationships between the structural properties of the system (e.g., the average size of the scattering units and their volume fraction), and its optical transport parameters (e.g., the transport mean free path of light propagation in the system and the absorption length, [20]). A successful attempt of the scattering media characterization using the effect of the laser speckle suppression due to an increasing ratio of the average propagation path of the light in the medium to the coherence length was reported by [PERSON], [PERSON], and [PERSON] [21]. Beginning from this work, several investigations were conducted on the improvement and expansion of this approach [22; 23; 24; 25]. Application of the tunable semiconductor lasers governed, for example, by the temperature, allows for a controllable decorrelation [26] of the acquired speckle patterns, which must be manifested as the speckle blur at large acquisition time intervals. Another way to obtain a controllable speckle blur in the course of the scattering media probing with a tunable laser light is application of the binary modulation of laser light frequency together with a long-exposure recording of time-varying speckles. In particular, this approach can be realized using a semiconductor laser with a diffraction grating as an external tunable element of the laser cavity; the binary modulation of the light frequency is achieved due to repeatable sequences of the mode hops under a condition of periodic changes in the grating tilt with respect to the beam axis [27]. Controllable changes in the coherence length of the probe light can also be provided using a slow modulation of the pumping current in the laser diode in the vicinity of the laser generation threshold [28]. At certain conditions, this approach makes it possible to tune the coherence length of emitting radiation within the range from several tens of micrometers to several millimeters.