Latest Achievements in Chemical Composition Optimization of
Photo-Thermo-Refractive Glass and Its Applications
S. A. Ivanov, N. V. Nikonorov and A. I. Ignatiev
ITMO University, 49 Kronverksky ave., St.Petersburg, Russia
Keywords: Hologram, Refractive Index, Volume Bragg Gratings, VBG, Photo-Thermo-Refractive Glass, PTR Glass,
Photo-Thermo-Inducted Crystallization, PTI Crystallization.
Abstract: Review on latest modification applied to chemical composition of PTR glass was made. Advancements of
updated chemical composition of PTR glass was shown in comparison with commercially produced glass.
Such properties as refractive index change, optimal exposition and optical losses in visible range for the
glass with recorded hologram was studied. In work samples of two chemical compositions were studied.
Conditions of matching included equal regimes of thermal treatment and expose dosages as well as
optimized parameters for each composition. Also the study of holograms received at optimal parameters for
each glass was made on three different wavelengths. Moreover several new applications for holograms on a
modified PTR glass were tested: such as holographic marks in telescopic systems and complex linked
holograms. Due to high transparency in visible range, PTR glass now can be applied for creating
holographic marks in telescopic systems. Studies show transparency of 92% with Fresnel losses. Also it is
found that spectral selectivity is maintained for such holograms, thus it is opening a new way of optical
solutions in telescopic systems. As it was measured, spectral selectivity of recorded hologram corresponds
to 400mkm efficient thickness according to calculations. Though, it needs further studies to increase the
effective thickness of such holograms as well as investigations of different Bragg angles at recording step.
Complex (linked) holography is another way of multiplexing inside a bulk glass. It leads to combination of
reflecting and transmitting Bragg gratings as a unite element with proper functions. This, for instance, can
provide positive feedback for complexes of laser diode crystals on a small size site. Simultaneously, such
element can combine emission from all emitting surfaces in one beam. This study may lead to creation of
high power coherent diode laser sources at small size site with ultra-narrow emitting bandwidth and high
quality spatial beam characteristics.
1 INTRODUCTION
Not as long ago as a holographic medium was
mostly used thin mediums, efficient thickness of
hologram for those was below 1mm. Such
restrictions had place due to low homogeneity as
well as incapability of achieving thick
photosensitive layers. For complex elements with
high spatial and wavelength selectivity, it is
necessary to record holograms at high deep. It’s
necessary, first of all, because selectivity of Bragg
grating depends from efficient thickness of
hologram and this dependence is linear. For
instance, selectivity of grating with 500mkm
thickness is twice lower than such for 1000mkm
one. Due to this fact, such material as a photo-
thermo-refractive (PTR) glass, which allows
recording of holograms with high efficient thickness,
keep getting more and more popular as a material for
amplitude-phase hologram and diffraction elements
creation (
Adibi, Buse, and Psaltis, 2001). PTR glass is
manufactured by several companies: Corning(USA),
Optigrate(USA), PD-LD(USA) and University
ITMO(Russia). On the basis of commercially
produced (classical) PTR glass a variety of
holographic diffractive optical elements can be
produced: spectral and spatial selectors, narrow-
filters intracavity Bragg mirrors, Bragg chirped
gratings for compression of light pulses, combiners
for powerful laser beams, etc. (Efimov et al, 1999).
78
Ivanov S., Nikonorov N. and Ignatiev A..
Latest Achievements in Chemical Composition Optimization of Photo-Thermo-Refractive Glass and Its Applications.
DOI: 10.5220/0005334400780084
In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 78-84
ISBN: 978-989-758-092-5
Copyright
c
 2015 SCITEPRESS (Science and Technology Publications, Lda.)

2 RECORDING PROCESS
PTR glass is a multicomponent material which
includes such components as glass formers and
modifiers Na
2
O, A1
2
O
3
and ZnO as well as different
dopants. Ce
3+
is a donor of photoelectrons which
defines the photosensitivity of material. Ions of Sb
5+
and Sn
4+
,
at first, act as photoelectron acceptors and
trap photoelectrons from cerium, at second, act as
donors of electron for silver ions, during the process
of thermal treatment. Silver ions are responsible for
formation of colloidal particles, which are becoming
a core for the crystalline phase growth. Halides and
bromides are crystalized on the silver cores, during
the process of thermal treatment, which leads to
refractive index change.
The hologram recording process is divided on two
steps. First is UV expose at 325nm wavelength,
which is close to absorption band of Ce
3+
(λ max ≈
310 nm). During the expose cerium gives away
electron with oxidation (figure 1a) according to
following reaction:
Ce
3+
+ hν → e
-
+ [Ce
3+
]
+
Approximately 20% of photoelectrons gained at
photo ionization process are trapped by ions of
silver, which create molecular clusters (Ag
2
+
,
Ag
3
2+
et al.) rest of them are accepted by ions of tin
and antimony (figure 1b):
e
-
+ Sb
5+
→ [Sb
5+
]
–
e
-
+ Sn
4+
→ [Sn
4+
]
–
Subsequent thermal treatment at temperatures
around 300
о
С leads to discharge of tin and antimony
and formation of molecular clusters and colloidal
particles of silver Ag
o
n (figure 1c):
[Sb
5+
]
-
→ [Sb
5+
]
-
+ e
-
[Sn
4+
]
-
→ [Sn
4+
]
-
+ e
-
nAg
+
+ ne
–
→ nAg
o
Further temperature increase leads to the shell
growth on the colloidal particle of silver (figure 1.d).
And finally crystalline phase NaF grows on the shell
(Pierson and Stookey, 1999) (figure 1.e).
The UV dose defines concentration of colloidal
particles and thus concentration of microcrystals.
Then temperature and duration of thermal treatment
defines the size of microcrystals and their volume
fraction. As a result, difference in refractive index,
between exposed and unexposed parts of glass,
appears. In exposed part refractive index is defined
by refractive index of crystalline phase and its
volume fraction as well as refractive index of
residual glass phase from which fluorite and sodium
are crystallized. In unexposed part refractive index
doesn’t change compare to virgin glass (Pierson and
Stookey, 1999). Summary, the refractive index
change in PTR glass depends from UV exposure
dose and temperature and time of thermal treatment
and known to reach values of n = 510
-4
. With
Figure 1 photo-thermo-inducted crystallization of PT
R
glass:
A) Cerium photoionization and accept of electrons by
Sb and Sn ions;
B) Discharge electrons by Sn and Sb and accept with
Ag ions with formation of neutral silver;
C) Colloidal particles formation at heating up to 400 ºС;
D) Shell growth (Ag,Na)Br on colloidal particle o
f
silver T>500ºС;
E) Growth of microcrystals NaF T>500 ºС.
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high efficient thickness of holograms, such value is
enough for achieving 99% diffraction efficiency. In
addition, lifetime of such elements is nearly
unlimited due to high stability of NaF crystals.
Holograms recorded in PTR glass are high resistant
to mechanical and chemical treatment almost like
BK7 glass. Optical breakdown for PTR glass is
1kJ/cm
2
at  = 1.06μm. Optical and spectral
characteristics of holograms are stable with heating
up to 450 ºС.
An important advantage of PTR glass as a
material for hologram recording is its high
homogeneity (refractive index fluctuations in the
order of 10
-5
) and reproducible characteristics of the
glass. PTR glass, like BK7 optical glass, allows
traditional methods of machining - grinding and
polishing, as well as a variety of forming techniques
(e.g., sagging and aspheric surfaces creation). It is
worth noting that the glass itself is flexible material
and allows various ways for composition
modification, for example, it can be doped with rare
earth ions, or it can be ion exchanged for waveguide
structures creation or material strength increase.
Production of PTR glass can be carried out both in
the laboratory (600 g) and industrial (300 kg) scales
using simple and non-toxic technology. Wherein the
chemical reagents required for the synthesis of glass
are commercially available and inexpensive.
To date, the properties of PTR glass still are
actively studied and improved. One of the important
aspects is the study of influence of different
modifications of the chemical composition on the
holographic properties of the glass.
3 CHEMICAL COMPOSITION
IMPROVEMENTS
A study of tin influence on holographic
characteristics was held. It was found that the
addition of Tin in the glass composition adversely
affects characteristics of the recorded diffraction
elements. In the course of this work, was compared
several key parameters of holographic gratings for
different glass compositions. These parameters were
efficient thickness of the hologram, and refractive
index modulation amplitude (RIMA). RIMA is a
quantity equal value to half of the refractive index
change resulting from the process of photo-thermo-
inducted (PTI) crystallization. During the hologram
recording process in the glass sinusoidal distribution
of the refractive index is formed. While the two
amplitudes of the distribution fall within the
dynamic range of refractive index change
(corresponds to refractive index change caused by
PTI crystallization process). Thus, by measuring the
RIMA at the optimum recording conditions, it is
possible to obtain data on the maximum dynamic
range of the refractive index change. A study, based
on a comparison of the RIMA for the glass
composition with different tin concentration, showed
that the presence of tin does not affect the dynamic
range of the refractive index change (figure 2a).
a
b
Figure 2: amplitude of the refractive indexmodulation with
respect to exposure dose in optimal exposure range (a) and
over-exposure range (b).
RIMA curves for the investigated glass were almost
identical. The difference in the maximum value of
the refractive index change is actually missing.
Further studies revealed that in range of over-
exposure (figure 2.b), beyond the optimal dose of
UV irradiation, the fall of the RIMA is different. We
suppose that it’s connected with scattering
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difference at the recording step. This assumption
was confirmed by comparison of efficient thickness
of obtained holographic gratings. Gratings, recorded
in the glass containing tin, had a smaller efficient
thickness compare to the gratings in the glass
without tin, at the same doses of UV irradiation and
thermal treatment regime.
a
b
Figure 3: efficient thickness with respect to exposure dose
sample with tin (a) and without (b).
Moreover, the efficient thickness dependence of
exposure for the composition containing tin differs
from linear (figure 3 a), whereas for glass without
tin dependence is strictly linear (figure 3 b).
Exclusion of tin from composition of the modified
PTR glass, allows reducing of stray formation of
silver clusters in the unexposed areas of interference
pattern, i.e. reducing scattering of recorded
holograms, as well as adjusting growth kinetics of
silver particles in the irradiated areas.
Further study of chemical composition allowed
complex optimization of components, with main
goal to decrease optical losses in visible spectral
range caused by absorption band of colloidal silver
(
Ivanov et al, 2014) . Concentration optimization
undergone following elements: halides (fluorides
and bromides), responsible for the growth of
microcrystalline shell and crystalline phase;
Antimony ions, which play a key role in the
acceptance and donation of the photoelectrons
received upon irradiation of cerium and subsequent
thermal treatment of PTR glass; also was lowered
the concentration of impurities capable of
photoelectrons capture. In work, mainly was
compared RIMA and induced losses spectra in the
visible range. As well as comparison between the
value of the optimal exposure for the classic and
improved glass was made. During the work, we also
had to upgrade the regime of thermal treatment,
because the new glass composition reveals its
potential in other regime than classic PTR glass
composition. As a result was improved a number of
parameters exceeding commercially produced
material. First of all, problem with absorption in
visible spectral range was solved, resulting in great
reduction of induced optical losses caused by
colloidal silver. The new composition of PTR glass
after the FTI crystallization process shows no
absorption band of the colloidal particles in the
optical losses spectra of PTR glass with recorded
hologram (figure 4).
The absence of the absorption band in visible
spectral range allows production of pure phase
volume holographic gratings, which positively
affects the characteristics of the following elements,
and as a consequence the quality of the diffracted
beam.
Figure 4: Absorption coefficient spectra of modified PTR
glass with recorded hologram.
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Thus, the absence of the induced losses in the visible
spectrum allows usage of the elements on this
material in schemes with high requirements for
transmission in optical channel. Because holograms
are purely phase, i.e. they lack the contribution of
the amplitude component; contours of angular and
spectral selectivity have good quality and symmetry
(figure 5) that positively affects the optical quality of
the beam in diffracted order. In addition, chemical
composition optimization increased RIMA up to n
1
= 10.210
−4
which means that refractive index
dynamic range of the new PTR glass has value of
2×10
-3
.
Figure 5: Contour of angular selectivity of hologram
recorded on modified PTR glass.
Optimization of antimony concentration led to shift
of optimal exposition towards shorter times (figure
6), i.e. for modified PTR glass maximum RIMA is
achieved with 4 times lesser recording times than
that for a commercially produced material and lays
in range of 0.5J/cm
2
.
Figure 6: amplitude of the refractive indexmodulation with
respect to exposure dose.
This is beneficial for the quality of obtained
diffractive elements, because the process of
hologram recording is very sensitive to air
turbulence and vibrations of the optical scheme, high
recording times are extremely undesirable and lead
to decrease of contrast in the interference pattern.
Synthesis of modified PTR glass from high purity
reagents allowed to lower impurities concentration
(mainly iron oxide), which are responsible for
capturing and irretrievable loss of photoelectrons,
required during PTI crystallization process. Also,
it’s improved the transmission of the virgin glass in
the UV optical range.
4 NEW APPLICATION
All the above optimizations led to the new field of
application for PTR glass as a holographic medium
for holographic marks for telescoping systems
(
Ivanov et al, 2014) . Since transmission of glass,
containing hologram, is above 90% without AR
coating, it can took its place in problems with strict
requirements to transmission in observation channel
such as collimator sight. Application of PTR glass
can solve problem of mark image stabilization,
which is necessary due to the instability of laser
diode source used in such scopes. To date this
problem is solved by addition in optical scheme
achromatizing diffraction elements such as
additional thin gratings, complex two cavity mirrors
or compound objectives. Wavelength shift, caused
by laser diode temperature changes, can be nullified
by spectral selectivity of thick hologram recorded on
PTR glass. While the central wavelength of laser
diode shifts, recorded hologram continues to
reconstruct image of mark on proper angle – thus
maintaining the position of mark in target plane.
And redistribution of energy in diode output spectra
leads to insignificant lowering of intensity of the
mark which can be easily leveled by diode power
output adjustment. Since the diffraction efficiency of
holograms on PTR glass can achieve values of 99%,
intensity required for mark observation is pretty low.
Important to note that current materials used for
mark recording are vulnerable to external impact
such as moisture and mechanical damage, that leads
to need in additional cover for holograms. With
application of PTR glass, since it is high resistant to
external impacts, there is no need in additional
protection of observation channel.
In pictures below are photo of reconstructed
image of mark (figure 7 a) and spatial shift of central
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dot in the mark image with respect to wavelength
change (figure 7 b).
a
b
Figure 7: photo of reconstructed image of mark in target
plane at 180 cm (a) and spatial shift of central dot in the
mark image with respect to wavelength (b).
Due to the complexity of measuring spectral
selectivity of image hologram, in the experiment
was measured spatial shift of central dot in the mark
image. From that spatial shift spectral selectivity of
recorded hologram was calculated. Spectral
selectivity matches theoretical predictions and
corresponds to 400μm efficient thickness of
recorded hologram. Such a low thickness is a result
of huge losses of intensity during recording process,
what means that expose times should be readjusted
to achieve desirable spectral selectivity and spatial
stability of mark image.
5 COMPLEX MULTIPLEXATION
Another promising application of holograms on PTR
glass is complex (linked) multiplexation, which
means recording multiply gratings in a single
volume, where each grating corresponds to Bragg
conditions inside the medium for another grating.
Reciprocal gratings combination within the medium
can be various, and they can perform their functions
simultaneously. For instance, it is possible to create
a combination of reflective and transmitting
hologram for spatial and spectral filtering at the
same time; that can be used to create arrays from
emitting diode elements at the small size site.
Complex element in such application will provide
positive feedback for each emitting area, stabilizing
the emission wavelength and adjusting the spatial
characteristics of the beam. In one optical path
inside the element radiation is directed onto a
reflective grating, which has high spectral
selectivity. Reflective hologram is responsible for
the spectral stabilization. Reflected radiation is
directed back into the crystal along the same path.
Another path within the complex element is for
output. Transmitting grating provides radiation
output at the appropriate angle for all emitting areas.
Presence of multiple transmitting gratings allows
correction of the spatial characteristics of the beam
emerging from the system. It is noteworthy that in
this implementation is possible to create a single
cavity for a large number of emitting semiconductor
crystals, this leads to the creation of a coherent high
power source of radiation with extremely narrow
bandwidth. Due to the large dynamic range of the
refractive index change in PTR glass, it is possible
to record a plurality of holographic elements in one
volume. In pictures below are shown spatial
contours of single hologram and complex hologram
(figure 8). Gratings period and their spatial
orientation within the medium are selected following
next conditions: first diffraction order from first
grating meets Bragg condition in the medium for the
second grating.
6 CONCLUSIONS
These studies, aimed at material characteristics
improvement led to: refractive index dynamic range
increase; optimal exposure lowering; scattering
during recording process and after thermal treatment
reduction; and allowed to get rid of the absorption
band of the colloidal particles in the visible spectral
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a
b
Figure 8: angular selectivity contours of usual (a) and
complex linked hologram (b).
range. All of the above makes it possible to extend
the application area of this material, including such
as: recording of holographic marks for telescopic
systems; complex elements for semiconductor laser
diodes creation and powerful coherent radiation
sources based on them; usage the elements on PTR
glass as intracavity selectors and filters for pulse
Raman lasers.
In summary, PTR glass and optical elements
based on it has the following advantages: high
refractive index dynamic range (Δn ≈ 2×10
-3
); High
diffraction efficiency (up to 99%); large efficient
thickness of the hologram (above few mm), which
allows you to create narrow spectral (Δλ ≈ 0,05nm)
and spatial (0.2degree) filters; unlimited lifetime of
hologram (up to ten years), high thermal (up to 450 °
C), mechanical and chemical resistance. Optical
breakdown threshold of the hologram on the PTR
glass close to the breakdown threshold of
commercial optical glass BK7 (1 kJ/cm2 under
pulsed irradiation at λ= 1.06μm).
This work was financially supported by Russian
Scientific Foundation (Agreement # 14-23-00136).
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