**6.1 Diffraction efficiency**

Principal difficulties in measurement of parameters of high-selectivity volume holograms stem from the rigid requirements on the illuminating beam in its divergence (spatial frequency spectrum of radiation) and monochromaticity (radiation wavelength spectrum). To correctly find the maximum values of diffraction efficiency of a hologram under Bragg conditions, meeting the following conditions is necessary.


#### **6.2 Angular selectivity contour**

A very common measurement procedure for angular selectivity contour of volume holograms provides for angular scanning of parameters, i. e. rotation of a hologram relative to the reading beam when using collimated monochromatic laser radiation (Fig. 12b). Advantages of the technique are obvious:


Drawbacks of the technique are less obvious, but render the measurements of highselectivity holograms rather involved:


For a single measurement act to yield information, incorporating complete data of angular selectivity contour, use can be made of а divergent beam of monochromatic radiation (Fig. 12c) rather than collimated one. Hologram-reading radiation beam is to have spatial frequency spectrum (Δψ) in an interval, greater than the width (∆θ) of spatial spectrum of diffracted radiation. A radiation detector, placed into diffracted or zero beam, as Fig. 12c shows, records spatial distribution of beam intensity in given spatial domain. Used in practice to this end are CMOS-matrices (including matrices of digital cameras). Fig. 13 (bottom) shows distribution of intensity in diffracted (a) and in zero (b) radiation beams, recorded on camera matrix.

Advantages of the technique are:

66 Holograms – Recording Materials and Applications

5. Feasibility of producing samples of thickness from tens of microns to several

Principal difficulties in measurement of parameters of high-selectivity volume holograms stem from the rigid requirements on the illuminating beam in its divergence (spatial frequency spectrum of radiation) and monochromaticity (radiation wavelength spectrum). To correctly find the maximum values of diffraction efficiency of a hologram under Bragg

• Divergence of reading radiation beam (Δψ) should be much narrower than angular selectivity of a hologram (Δθ): Δψ < Δθ. The contour measured in the case is a convolution of the true angular selectivity contour of a hologram and the contour, defined by spatial frequency spectrum of the reading radiation beam (see Fig. 12a). • Spectral distribution of reading radiation beam (ΔΛ) should be much narrower than

• At given experiment geometry and transmission hologram thickness (Т) the region of overlap of zero and diffracted beams on the exit surface of the sample (*А*out) with respect to that on the entry surface (*А*in) should satisfy relationship: *А*out/*А*in > 0.8.

A very common measurement procedure for angular selectivity contour of volume holograms provides for angular scanning of parameters, i. e. rotation of a hologram relative to the reading beam when using collimated monochromatic laser radiation (Fig. 12b).

• comparative simplicity of optical scheme for measurements; ease of data processing

Drawbacks of the technique are less obvious, but render the measurements of high-

• rigid requirements on divergence and geometry of the reading radiation beam (see 6.1); • long duration of measurements of a contour, which increases with growing accuracy of

• complexity of forming a collimated radiation beam with divergence on order of

For a single measurement act to yield information, incorporating complete data of angular selectivity contour, use can be made of а divergent beam of monochromatic radiation (Fig. 12c) rather than collimated one. Hologram-reading radiation beam is to have spatial frequency spectrum (Δψ) in an interval, greater than the width (∆θ) of spatial spectrum of diffracted radiation. A radiation detector, placed into diffracted or zero beam, as Fig. 12c shows, records spatial distribution of beam intensity in given spatial domain. Used in practice to this end are CMOS-matrices (including matrices of digital cameras). Fig. 13 (bottom) shows distribution of intensity in diffracted (a) and in zero (b) radiation beams,

• possibility to carry out measurements in a strictly localized area of a hologram.

**6. Specifics of parameter measurements of high-selectivity volume** 

conditions, meeting the following conditions is necessary.

spectral selectivity of a hologram (Δλ): ΔΛ < Δλ.

millimeters.

**6.1 Diffraction efficiency** 

**6.2 Angular selectivity contour** 

operation;

construction;

Advantages of the technique are obvious:

selectivity holograms rather involved:

fractions of a milliradian.

recorded on camera matrix.

**holograms** 


Drawbacks of the technique are also obvious:


Fig. 12. a - effect of halfwidth of spatial spectrum of reading radiation (Δψ) on ratio Δθexp/Δθtr at different values of Δθtr: 1 – Δθtr = 0.5 mrad; 2 – Δθtr = 1 mrad; Δθtr = 3 mrad. Δθexp is the halfwidth of experimentally measured contour;Δθtr is the halfwidth of true contour of the hologram under study. b,c,d - schematic diagram of measurement of angular selectivity contour by collimated (b) and divergent (c) beams of monochromatic radiation; measurement of spectral selectivity contour by collimated radiation beam with a wide wavelength spectrum (d): 1 – radiation source, 2 – optical system for formation of divergent radiation beam; 3 – spectral instrument for expansion of wavelength spectrum in spatial frequencies; H – hologram; RD – radiation detector.

Light-Sensitive Media-Composites for

Recording Volume Holograms Based on Porous Glass and Polymer 69

Fig. 14. Spectral distribution of intensity of laser radiation, passed the hologram outside Bragg conditions (curves 1) and under Bragg conditions of hologram reading (curves 2); spectral selectivity contour of a hologram (curves 3): a – femtosecond laser (λmax = 808 nm);

One of the cardinal problems of 3D holography is provision of research in the field with recording materials (Denisyuk, 1980). Volume recording media for holography are at present manufactured in laboratory conditions in the form of isolated specimens or small batches. Obtaining samples with stable and reproducible performance is, as the authors'

The current studies reveal those properties of devised materials, which open up new application opportunities far beyond narrow professional use of recording media for holography. A number of special features of recording media, considered in the paper, can be quite in demand to accomplish unconventional tasks in various fields of science and

AgHal-PG-media exhibit a set of parameters, pertaining to commonly used traditional AgHalmedia: possibility to achieve high sensitivity, the width of spectral sensitization, the variety of techniques of post-exposure treatment etc. The list of the most important parameters of silverhalide media is supplemented by AgHal-PG-media with new opportunities: obtaining samples with thickness of several millimeters; shrinkproof; limitation of the maximum particle

Polymeric medium with diffusion enhancement has a modulation transfer function, which is untypical for traditional light-sensitive materials and allows excluding the region of low spatial frequencies during the information recording. A no less important and rather unique property is the possibility to obtain the structure of high-efficiency hologram as a latent image at the recording stage and thus achieve a distortionless recorded interference structure in a wide dynamic range after post-treatment. It should be also noted that enhancement and fixation of holograms recorded on such a medium require no treatment in water solutions. Advancement of volume holography and provision of this line of research with experimental base for comprehensive studies makes it necessary to investigate the processes taking place in the bulk of recording media during hologram construction, which, in its turn, calls for improvement of research techniques and methods to control the parameters of target processes.

b – semiconductor radiation source (λmax = 654 nm).

experience shows, still possible even in such conditions.

size in the light-sensitive agent and post-treatment products.

**7. Conclusion** 

engineering.

Fig. 13. Distribution of radiation intensity in diffracted (a) and zero (b) radiation beams, recorded on a camera matrix (bottom); processing result for experimental data (top).

#### **6.3 Spectral selectivity contour**

When measuring the spectral selectivity contour of a volume hologram according to schematic diagram, shown on Fig. 12b, the hologram is to be scanned with a collimated radiation beam of a wavelength, varying within the spectral range of the contour. At different times, this was carried out in different ways. In work (Denisyuk et al., 1970), the measurements involved a monochromator, with a collimated beam of radiation with a wide spectral range being formed in front of its entrance slit. In work (Sukhanov et al., 1984), the reading radiation wavelength was changed with the help of frequency-tuned dye laser with excimer pump: in this case, the laser radiation divergence was less than 0.5 mrad and the spectral width of scanning radiations was 0.01 nm. The halfwidth of spectral selectivity contour, measured in the present work for a reflection hologram, was Δλ = 0.16 nm (at DE = 80%). Such measurements, naturally, require special equipment and cannot be accomplished using standard techniques for measuring spectral characteristics of optical elements.

Applying the collimated laser radiation with wide spectral interval of wavelengths to measurement of the spectral selectivity contour of volume holograms was proposed by the authors and was implemented with the use radiation of femtosecond laser and semiconductor laser. The schematic diagram of measurements is given on Fig. 12d. A collimated beam of laser radiation with a wide spectrum illuminates hologram, which can be placed in positions "outside Bragg conditions" (Id = 0) and "under Bragg conditions" (Id is at the maximum for wavelength λBr within the reading radiation wavelength range).

A radiation beam, having passed a hologram with the direction unchanged (I0), arrives at the entrance slit of spectral instrument (3 on Fig. 12d), the spatial pattern of wavelength spectrum expansion being recorded behind it on a CMOS-matrix as a spectrogram. With the hologram placed "outside Bragg conditions", reading radiation spectrum is recorded (curves 1 on Fig. 14); with the hologram "under Bragg conditions", spectrum of zero diffraction beam (curves 2 on Fig. 14) is recorded, in similarity to the distribution, given on Fig. 13c. Spectral selectivity contour of a hologram represents in this case a differential contour (curves 3 on Fig. 14), resulting from comparison of two spectrograms.

Fig. 14. Spectral distribution of intensity of laser radiation, passed the hologram outside Bragg conditions (curves 1) and under Bragg conditions of hologram reading (curves 2); spectral selectivity contour of a hologram (curves 3): a – femtosecond laser (λmax = 808 nm); b – semiconductor radiation source (λmax = 654 nm).
