**3. Laser active elementary holographic structure on the basis of dyedoped polymer film**

Spatial modulation of laser emission controlled by the structure of the excitation light field was obtained also in the dye-doped polymer film [21–28]. The dye-doped polymer film as an active medium was sandwiched between two laser mirrors forming a laser cavity. The pumping was performed by an interference pattern, formed with two mutually coherent beams of the second harmonic of a Q-switched Nd:YAG laser (532 nm), located in the plane of the laser cell. The laser emission was observed normally to the plane of the laser cell.

The cross section of the obtained laser emission was modulated in intensity with an interval between maximums that depends on the period of the interference pattern of the pumping. Thus, the emitted light field qualitatively looks like a diffraction from an elementary dynamic hologram, i.e., a holographic diffraction grating.

An elementary hologram (holographic grating) usually represents a passive diffractive device. In obtaining information about the recorded holographic structure, an external light source is required.

However, as it is known, diffraction is the result of interference of secondary waves from all lines of the optical heterogeneity of the periodical structure of the grating [29, 30]. Therefore, if we have a periodical structure each strip of which is emitting mutually coherent light waves, the total light field will be analogous to a passive diffraction picture. Such a result was already observed during the study of the coherence of emission of the DD CLC laser. In this case, the excitation also was performed in the form of an interference pattern of pumping beams.

The interference of the laser beams is used in various spheres of science and technology and, among them, for achieving laser emission. In particular, double-beam coherent pumping has long been used for obtaining of the distributed feedback (DFB) in dye lasers [31–36]. In these cases the mutually coherent pumping beams in the active medium form an interference pattern whose bright and dark strips are distributed along generated laser emission (**Figure 9**). But the correlation between the emitting centers in the emitting strips of the active medium allows not only formation DFB in the dye lasers. For instance, as it was shown for the DD CLC laser, the excitation by the interference pattern gives rise to the spatial modulation of the laser emission.

**Figure 9.** Dye laser with longitudinally distributed excitation.

(**Figure 8(a)** and **(b)**). The new type of a laser, which combines the properties of a laser and a hologram, was firstly realized on the basis of a DD CLC layer. The field of emission of this laser has a spatial modulation with the periodical distribution of the intensity, controlled by the transversely distributed excitation. Therefore, the spatial distribution of the emission intensity in this case carries out information about the interference pattern of the pumping that makes it similar to the elementary hologram, i.e., holographic diffractive grating. Thus, according to

**3. Laser active elementary holographic structure on the basis of dye-**

cell. The laser emission was observed normally to the plane of the laser cell.

Spatial modulation of laser emission controlled by the structure of the excitation light field was obtained also in the dye-doped polymer film [21–28]. The dye-doped polymer film as an active medium was sandwiched between two laser mirrors forming a laser cavity. The pumping was performed by an interference pattern, formed with two mutually coherent beams of the second harmonic of a Q-switched Nd:YAG laser (532 nm), located in the plane of the laser

The cross section of the obtained laser emission was modulated in intensity with an interval between maximums that depends on the period of the interference pattern of the pumping. Thus, the emitted light field qualitatively looks like a diffraction from an elementary dynamic

An elementary hologram (holographic grating) usually represents a passive diffractive device. In obtaining information about the recorded holographic structure, an external light

However, as it is known, diffraction is the result of interference of secondary waves from all lines of the optical heterogeneity of the periodical structure of the grating [29, 30]. Therefore, if we have a periodical structure each strip of which is emitting mutually coherent light waves, the total light field will be analogous to a passive diffraction picture. Such a result was already observed during the study of the coherence of emission of the DD CLC laser. In this case, the excitation also was performed in the form of an interference pattern

The interference of the laser beams is used in various spheres of science and technology and, among them, for achieving laser emission. In particular, double-beam coherent pumping has long been used for obtaining of the distributed feedback (DFB) in dye lasers [31–36]. In these cases the mutually coherent pumping beams in the active medium form an interference pattern whose bright and dark strips are distributed along generated laser emission (**Figure 9**). But the correlation between the emitting centers in the emitting strips of the active medium allows not only formation DFB in the dye lasers. For instance, as it was shown for the DD CLC laser, the excitation by the interference pattern gives rise to the spatial modulation of the

results presented in **Figures 6** and **7** regarding to the spectral.

hologram, i.e., a holographic diffraction grating.

**doped polymer film**

470 Holographic Materials and Optical Systems

source is required.

of pumping beams.

laser emission.

In this part of chapter, the lasing from the dye-doped polymer film is investigated for the transversely distributed pumping (**Figure 10**). In this case, two mutually coherent pumping beams form in the active medium an interference pattern whose bright and dark strips are distributed perpendicular to the generated laser emission. The luminescent areas of the active medium inside of laser cavity of laser cell can generate laser emission separately. Due to correlation between the emitting centers of different lasing areas, the conditions for interference of the beams from these areas arise.

Therefore, the emission of such a laser should have spatial modulation and will form a pattern similar to the diffraction from the holographic grating. The aim of this study was to obtain and investigate the spatially modulated laser emission from a dye-doped polymer film and to get an improved pattern of lasing by improving the laser emission coherency as compared with DD CLC laser [21].

The experimental setup was the same as that used for holographic recording and for pumping of the DFB lasers which is shown above (**Figure 2**). The second harmonic (532 nm) of a Q-switched Nd:YAG laser with pulse duration of 15 ns was used for the coherent pumping. The repetition frequency of pulses was 12.5 Hz. The laser was ensured a coherence length of approximately 100 mm. With the beam splitter, the beam was divided into two beams of equal intensity. The beam splitter was composed of two interference mirrors 1 and 2 reflecting 50 and 100% accordingly. The distance between the mirrors (15–20 mm) ensured a stable interference pattern. The laser cell consisted of a polyvinyl alcohol (PVA) film doped with Rhodamine-6G and sandwiched between cavity mirrors enough transparent (≈75%) for the pumping emission. The total energy of the pumping radiation was 20–30 mJ, so the real effective energy of the pulse (i.e., the energy incident on the laser cell) was 14–20 mJ. The mirrors were placed with their reflective surfaces inward to the laser cell and by these surfaces have optical contact with the polymer layer. The radius of curvature of the concave mirror was 2 m. The pumping was carried out at the angles of the convergence of the pumping beams **0.6**°, **0.9**°, and **1.8**°. Concentration of the dye was 0.148% and the thickness of the polymer film was 130 μm. The pattern of the laser emission of this laser cell is shown in **Figure 11**. The photos **a, b,** and **c** correspond to the convergence angles of the pumping beams of **0.6**°, **0.9**°, and **1.8**°, respectively. As can be seen, along the cross section of the light bundle here, the smooth distribution of intensity typical for conventional lasers does not take place.

**Figure 10.** Dye laser with transversally distributed excitation.

**Figure 11.** The emission pattern of the dye-doped polymer laser cell with the transverse distribution of the pumping at the convergence angles 0.6°, 0.9°, and 1.8° for the pumping beams—(a), (b), and (c).

But the intensity has a spatially distributed form and qualitatively looks like a diffraction pattern from a diffraction grating. The angles between the intensity maximum directions correspond to the formula (3):

cave mirror was 2 m. The pumping was carried out at the angles of the convergence of the pumping beams **0.6**°, **0.9**°, and **1.8**°. Concentration of the dye was 0.148% and the thickness of the polymer film was 130 μm. The pattern of the laser emission of this laser cell is shown in **Figure 11**. The photos **a, b,** and **c** correspond to the convergence angles of the pumping beams of **0.6**°, **0.9**°, and **1.8**°, respectively. As can be seen, along the cross section of the light bundle here, the smooth distribution of intensity typical for conventional lasers does not

**Figure 11.** The emission pattern of the dye-doped polymer laser cell with the transverse distribution of the pumping at

the convergence angles 0.6°, 0.9°, and 1.8° for the pumping beams—(a), (b), and (c).

take place.

472 Holographic Materials and Optical Systems

**Figure 10.** Dye laser with transversally distributed excitation.

$$\varphi = 2 \arcsin\left(\frac{\lambda\_p}{2d}\right) \tag{3}$$

where, in our case, *λ***p** is the wavelength of lasing and *d* is the period of the interference pattern of the pumping [4, 29, 30]. The diameter of the excited region was 1.5–2.0 mm. In this area, a sufficient number of the lines of interference pattern of the pumping, i.e., microlasers, were located. When shutting one of the pumping beams, the pattern of the spatial modulation, of the laser emission, disappears (**Figure 12**). The elongated shape of the emitted light field in all photos is a result of the plano-concave structure of the laser cavity. To avoid Fabry-Perot interference of the generated emission, the pumping was performed not at the central but at the peripheral part of the resonator.

**Figure 12.** The emission pattern of the laser cell with single-beam pumping.

Because of pumping, the nonlinear effects can be induced in the polymer film. So the dynamic grating could be formed with enough modulation depth for observation of diffraction. To check this possibility, the area of lasing was tested with a beam of He-Ne laser (632 nm). But no signs of diffraction and, thus, no signs of any grating were detected.

The structure of the emitting spot was investigated under a microscope. In **Figure 13**, the photos of the spot demonstrating the modulated by intensity laser emission are shown. The convergence angles of the pumping beams were **0.6**°, **0.9**°, and **1.8**°, and spatial frequencies of emitting areas were **19, 28,** and **57** lines per millimeter accordingly.

As seen, the laser emission is observed from all the area of the pumping where the peaks of emission are allocated as microlaser stripes. Naturally, the peaks of lasing of these strips correspond to the intensity maximums of the interference pattern formed by the pumping beams.

**Figure 13.** Microphotographs of the structure of the emitting area of the laser cell. The convergence angles of the pumping beams accordingly are (left to right) 0.6°, 0.9°, and 1.8°—(a), (b), and (c).

In **Figure 14**, the laser emission spectrum is shown. The spectrum along the cross section of the lasing of the radiation is strongly constant. The obtained spectrum of lasing is caused by the dye concentration, polymer matrix properties, and spectral reflection characteristics of the cavity mirrors.

**Figure 14.** Spectrum of the laser emission.

The aim of this study was realization of the laser with the transversely distributed pumping performed by double-beam coherent excitation of the dye-doped polymer film. According to the author's opinion, the emission field of such a laser should be spatially modulated and must carry information about the spatial distribution of the excitation field analogically described above DD CLC laser. The results shown in **Figure 10** confirmed these assumptions. By the opinion of the author, the emitted spot represented a one-dimensional array of mutually coherent microlasers which gives the interference field.

As it can be seen from **Figure 11**, the pictures of lasing do not contain the central maximum of intensity. There are observed only intensity maximums located symmetrically with respect to the pattern center. So the cross section of the laser emission is not quite similar to the diffraction. As it was noted above, the absence of such a diffraction grating was confirmed by the absence of any signs of diffraction when probing the lasing area with a beam of He-Ne laser (632 nm). By the interference of the coherent microsheaves, symmetrically located intensity maximums were formed. Therefore, we can say that the obtained pattern of emission is not a result of diffraction from a nonlinear grating formed in the active medium. The observed spatial modulation of lasing could be only the result of the interference of the mutually coherent microlaser emission. Thus, during the collective lasing of all strips, according to the Huygens-Fresnel principle [29, 30], the interference pattern shown in **Figure 11** was formed. The obtained laser emission carries information about the periodical distribution of the intensity of the pumping. Qualitatively it is almost similar to an elementary hologram whose diffraction orders also carry information about its periodical structure. So, we can say that the obtained laser operates like an active elementary dynamic hologram.

Thus, a dye-doped polymer film laser with transversely distributed excitation is investigated. Similar to the described DD CLC laser, the emission pattern of this laser is spatially modulated. However, the intensity maximums in this case are more visible due to the enhanced lasing conditions. The intensity distribution of laser emission contains information about the pumping interference field as it takes place in the case of elementary dynamic hologram. But unlike the passive diffraction of incident light, the pattern is formed due to the own radiation of the emitting areas.

According to future plans, the possibility of the reconstruction of the image of a two-dimensional transparent object on the basis of such approach will be investigated.
