# Physical and Spectroscopic Properties ofYb3+-Doped Fluorophosphate Laser Glasses

Authors: Shujiang Liu; Anxian Lu
Journal: Laser Chemistry
 (2008)

---

## Abstract

The physical properties including refractive index, Abbe number, nonlinear refractive index, microhardness and thermal expansion coefficient, and spectroscopic properties ofYb3+-doped fluorophosphate laser glasses were investigated. The results show that due to the addition of fluoride, mechanical and thermal properties are promoted, emission cross-section σemi is also greatly enhanced. The largest gain coefficient σemi·τm (0.824 pm2·ms) can be obtained with the minimum pump intensity Imin (1.112 kw/cm2). This kind of Yb3+-doped fluorophosphate glass is an excellent candidate material for Yb3+-doped host for high-power generation.


---

## Body

## 1. Introduction

With the rapid development of laser diode
(LD) recently, Yb3+ doped laser materials as the gain medium in high-energy
solid-state laser systems have obtained much attention [1–3]. There are only
two manifolds in the Yb3+ energy level scheme, namely, the F7/22 ground state and
the F5/22 excited state, and the
absorption band is located at about 970 nm, with a large cross-section, which
enables efficient pumping by high-power III–V diode lasers
that are commercially available. The Yb3+ ions are also of interest
not only as high-power lasers for nuclear fusion but also as energy transfer
sensitizers for infrared lasers and up conversion lasers [4, 5]. The main obstacle
is mechanical and thermal problems in the development of high-average powder
solid-laser materials because more than half of pumping energy precipitate in
gain medium, though effective semiconductor diode is used as pumping source,
which results in various problems such as thermal load, mechanical stress. Therefore,
in addition to high optical and spectroscopic properties, excellent thermal and
mechanical properties are necessary, to improve the repetition rate of the
laser glass.For a long term, Yb3+ doped phosphate glasses have been regarded as ideal host matrix for high-power
laser due to larger absorption and emission cross-section, less nonlinear
refractive index [6–8]. However, the
line-like network of phosphate glass results in higher thermal expansion
coefficient (TEC), inferior physical properties such as mechanical and chemical
durability [9, 10]. Furthermore, the hygroscopic tendency of phosphor lowers the
fluorescence lifetime of Yb3+ ions. Some early researches [11, 12] have
shown that formation of P–O–B(4) bonds in borophosphate glass could greatly improve
physical properties along with broad emission bandwidth. However, the photo
energy of B–O bond (~1400 cm−1) is larger, so that the fluorescence
lifetime of borate glass is less as compared to other glass. In order to
improve the water resistance of phosphate glass, fluorophosphate glass is available
because fluorophosphate glass has m advantages such as long fluorescene life
time, low nonlinear refractive index [13, 14].To obtain
excellent physical and spectroscopic properties of Yb3+-doped laser
glass, we investigated the relationship between composition and mechanical,
thermal and spectroscopic properties of fluorophosphate glasses.


## 2. Experimental

Two series of
fluorophosphate glasses were chosen, and the glass compositions (mol%) listed in
Table1 were prepared by melting 100 g batches using analytical grade (NH4)2HPO4,
Al2O3, carbonate, Fluoride, Nb2O5, and Yb2O3 with a purity of above 99.99%. When each batch was slowly heated from room
temperature up to 1000°C in a Al2O3 crucible, the crucible was covered to minimize the volatilization of
phosphor, then the batch was melted at 1300~1320°C depending on the
glass composition. Melts were quenched in stainless steel moulds and properly annealed.
The final compositions of the glasses were checked by chemical analysis and found
to be within ±1%.Table 1
Chemical composition of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4P2O5636363636344444444Al2O3777777777Nb2O5111114444Li2O107.552.5—————BaO1813.594.5—————LiF—2.557.510101010—NaF—————1010—10MgF2—————24—2020CaF2—4.5913.518—241414Yb2O3111111111The refractive
index (nD, nF, and nC) was measured on an Abbe
refractometer (WZS-S) at room temperature at the wavelength of 589.3, 486.1,
and 656.3 nm, respectively.The microhardness
of the investigated samples was measured using Vickers’s microhardness indentor
(MET-4). The eyepiece on the microscope of the apparatus allows measurements
with an estimated accuracy of ±0.5μm for the indentation diagonal. Grinding and well polishing were
necessary to obtain polished and flat parallel surfaces glass samples before
indentation testing. At least five indentation readings were made and measured
for each sample. Testing was conducted with a load of 30 g and loading time 15 seconds.
The measurements were carried out under normal atmospheric condition.Thermal
expansion coefficient of the investigated glass was carried out on 2.0 cm long rods using AS-100 automatic
recording multiplier dilatometer with heating rate of 5°C/min. The
uncertainty of linear thermal expansion from room temperature to 300°C  is ±5×10-7/°C.The samples
for measurement of spectroscopic properties were cut to a size of 20 mm×20 mm×2 mm with two larger sides polished. Absorption spectra were recorded with
Perkin-Elmer (Lambda 900) UV/VIS/NIR spectrophotometer, at room temperature, in
the range of 870–1150 nm. Emission
spectra were measured with Triax 550 spectrophotometer through exciting the
samples with a diode laser operating around 940 nm. The emission from the sample was
focused to a monochrometer and detected by the Ge detector. The signal was
intensified with a lock in amplifier and processed by a computer. Fluorescence
lifetimes were measured by exciting the samples with a Xenon lamp and detected
by an S-1 photomultiplier tube. The fluorescence decay curves were recorded and
averaged with a computer-controlled transient digitizer.


## 3. Results

### 3.1. Physical Properties

Table2
summarizes the data of measured refractive index, Abbe number, nonlinear
refractive index, microhardness, and thermal expansion coefficient for
fluorophosphate glass samples.Table 2
Physical properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4Refractive indexnD1.5561.5511.5481.5431.5401.5091.5101.5101.510Abbe numberν62.2366.8667.9067.3264.3263.0762.7264.2063.94Nonlinear refractive
indexn2(×10−13esu)1.361.201.161.151.231.151.161.121.13Microhardness (GPa)3.683.753.893.954.104.404.164.204.11Thermal expansion coefficientα(×10−7/K)105.9291.4182.6178.8675.2471.9889.1863.7188.14The knowledge
of low nonlinear refractive indexn2 is required for laser
applications to prevent spatial intensity fluctuations in the wavefront and
self-focusing which lead to damage of optical components. n2 can be
calculated using the formula [15]:  (1)n2=[68(nD-1)(nD2+2)2]×10-13ν[1.517+(nD2+2)(nD+1)ν/6nD]1/2esu, where ν is
Abbe number,  ν=(nD−1)/(nF−nC).
nD is refractive index. It is clear that nD constantly decreases
in series A with increasing content of LiF and CaF2 at the expense
of Li2O and BaO. It could mainly correlate with the difference of
the polarizability of F− and O2-, furthermore,
introduction of Ca2+ with higher polarizing power than Ba2+ promotes this refractive index behavior. n2 is minimized at A3. In
the samples of series B, n2 is only related to ν, since the values
of nD are nearly same, as shown in Table 2. Furthermore, n2 appear inverse to ν according to (1), such B3 glass has the minimum n2 due to its largest ν.As shown in
Table1, with increasing fluoride content in the samples of series
A, the microhardness increases gradually along with a decrease of thermal
expansion coefficient. In series B, B1, and B3 glasses exhibit high
microhardness and low-thermal expansion coefficient when compared to B2 and B4
on the same condition.


### 3.2. Spectroscopic Properties

The spectroscopic properties of Yb3+ ions in glasses are
determined by transition between the four subenergy levels of F7/22 and three subenergy levels of F5/22, and the emission
cross-section is associated with integral absorption cross-section ∑abs which
can be obtained by (2) [16]: (2)∑abs=∫σabs(λ)dλ,(3)σabs(λ)=(2.303log(I0/I)Nl), where σabs is absorption cross-section, N is
the Yb3+ ion concentration (ions/cm3), and l is the
thickness of the sample, log (I0/I) is absorbance. The
reciprocity method was used to calculate the emission cross-section of Yb3+ ions
[17]: (4)σemi(λ)=σabs(λ)zlzμexp(Ezl−hcλ−1kT), where Zl/Zu is the partition function; T is the
absolute temperature;  Ezl is the zero line energy, which is defined as the energy separation between the
lowest components of the upper (F5/22)
and lower states (F5/22);
k, h, and c are Boltzman's constant, the Plank constant, and the
velocity of light, respectively.Spontaneous emission probabilityArad  is calculated as follows [18]:(5)Arad=1τrad=8πcn2λp42J′+12J+1∑abs,where λp is the peak wavelength of absorption band, n is the refractive index at the peak wavelength, which is
obtained from Cauchy’s equation n(λ)=A+B/λ2 according to the measured
nD, nC, or nF. J  and J′ are the total momentums for the upper and lower levels.Figure1 shows the absorption and emission spectra of the
samples of series A. The line shape of absorption spectra is similar except
intensity in all samples, the main
absorption peak is around 975 nm (as shown in Figure 1(a)), which corresponds to the energy transition
of the lowest subenergy level of F5/22 and F7/22. The absorption spectra is characterized by broader line widths due to out-of-order
glass structure in which Yb3+ ions are localized in different
coordination site and some portion of stark splitting energy overlap. As seen
in Figure 1(b), the main emission peak of all samples is around 975 nm, and
subemission peak is around 1006 nm which is mostly concerned. Other
spectroscopic properties have been shown in Table 3.Table 3
Spectroscopic properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4Concentration (1020 ions/cm3)2.7532.7452.7402.7462.7443.1273.1203.0753.027Σabs(104pm3)3.933.863.543.653.734.764.845.495.51σemi(pm2)0.5980.6010.6080.6490.6530.9150.9471.0651.032Arad(s−1)98210189219519681188120813711376τm(ms)0.840.870.981.031.080.890.870.730.75Absorption and emission spectra of Yb3+ in A0, A2, and A4 glasses.
(a)(b)In the Table3,
emission cross-section σemi and fluorescence lifetime τm gradually increase as the fluoride content
increases from 0 to 28 mol% in series A glasses. Series B glasses shows larger
integral absorption cross-section ∑abs and emission cross-section σemi,
but less fluorescence lifetime τm compared to series  A glasses.


## 3.1. Physical Properties

Table2
summarizes the data of measured refractive index, Abbe number, nonlinear
refractive index, microhardness, and thermal expansion coefficient for
fluorophosphate glass samples.Table 2
Physical properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4Refractive indexnD1.5561.5511.5481.5431.5401.5091.5101.5101.510Abbe numberν62.2366.8667.9067.3264.3263.0762.7264.2063.94Nonlinear refractive
indexn2(×10−13esu)1.361.201.161.151.231.151.161.121.13Microhardness (GPa)3.683.753.893.954.104.404.164.204.11Thermal expansion coefficientα(×10−7/K)105.9291.4182.6178.8675.2471.9889.1863.7188.14The knowledge
of low nonlinear refractive indexn2 is required for laser
applications to prevent spatial intensity fluctuations in the wavefront and
self-focusing which lead to damage of optical components. n2 can be
calculated using the formula [15]:  (1)n2=[68(nD-1)(nD2+2)2]×10-13ν[1.517+(nD2+2)(nD+1)ν/6nD]1/2esu, where ν is
Abbe number,  ν=(nD−1)/(nF−nC).
nD is refractive index. It is clear that nD constantly decreases
in series A with increasing content of LiF and CaF2 at the expense
of Li2O and BaO. It could mainly correlate with the difference of
the polarizability of F− and O2-, furthermore,
introduction of Ca2+ with higher polarizing power than Ba2+ promotes this refractive index behavior. n2 is minimized at A3. In
the samples of series B, n2 is only related to ν, since the values
of nD are nearly same, as shown in Table 2. Furthermore, n2 appear inverse to ν according to (1), such B3 glass has the minimum n2 due to its largest ν.As shown in
Table1, with increasing fluoride content in the samples of series
A, the microhardness increases gradually along with a decrease of thermal
expansion coefficient. In series B, B1, and B3 glasses exhibit high
microhardness and low-thermal expansion coefficient when compared to B2 and B4
on the same condition.


## 3.2. Spectroscopic Properties

The spectroscopic properties of Yb3+ ions in glasses are
determined by transition between the four subenergy levels of F7/22 and three subenergy levels of F5/22, and the emission
cross-section is associated with integral absorption cross-section ∑abs which
can be obtained by (2) [16]: (2)∑abs=∫σabs(λ)dλ,(3)σabs(λ)=(2.303log(I0/I)Nl), where σabs is absorption cross-section, N is
the Yb3+ ion concentration (ions/cm3), and l is the
thickness of the sample, log (I0/I) is absorbance. The
reciprocity method was used to calculate the emission cross-section of Yb3+ ions
[17]: (4)σemi(λ)=σabs(λ)zlzμexp(Ezl−hcλ−1kT), where Zl/Zu is the partition function; T is the
absolute temperature;  Ezl is the zero line energy, which is defined as the energy separation between the
lowest components of the upper (F5/22)
and lower states (F5/22);
k, h, and c are Boltzman's constant, the Plank constant, and the
velocity of light, respectively.Spontaneous emission probabilityArad  is calculated as follows [18]:(5)Arad=1τrad=8πcn2λp42J′+12J+1∑abs,where λp is the peak wavelength of absorption band, n is the refractive index at the peak wavelength, which is
obtained from Cauchy’s equation n(λ)=A+B/λ2 according to the measured
nD, nC, or nF. J  and J′ are the total momentums for the upper and lower levels.Figure1 shows the absorption and emission spectra of the
samples of series A. The line shape of absorption spectra is similar except
intensity in all samples, the main
absorption peak is around 975 nm (as shown in Figure 1(a)), which corresponds to the energy transition
of the lowest subenergy level of F5/22 and F7/22. The absorption spectra is characterized by broader line widths due to out-of-order
glass structure in which Yb3+ ions are localized in different
coordination site and some portion of stark splitting energy overlap. As seen
in Figure 1(b), the main emission peak of all samples is around 975 nm, and
subemission peak is around 1006 nm which is mostly concerned. Other
spectroscopic properties have been shown in Table 3.Table 3
Spectroscopic properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4Concentration (1020 ions/cm3)2.7532.7452.7402.7462.7443.1273.1203.0753.027Σabs(104pm3)3.933.863.543.653.734.764.845.495.51σemi(pm2)0.5980.6010.6080.6490.6530.9150.9471.0651.032Arad(s−1)98210189219519681188120813711376τm(ms)0.840.870.981.031.080.890.870.730.75Absorption and emission spectra of Yb3+ in A0, A2, and A4 glasses.
(a)(b)In the Table3,
emission cross-section σemi and fluorescence lifetime τm gradually increase as the fluoride content
increases from 0 to 28 mol% in series A glasses. Series B glasses shows larger
integral absorption cross-section ∑abs and emission cross-section σemi,
but less fluorescence lifetime τm compared to series  A glasses.


## 4. Discussion

### 4.1. Effect of Fluoride on Mechanical and Thermal Properties

Excellent
mechanical and thermal properties are indispensable to the laser driver for
inertial confinement fusion (ICF), especially low-thermal expansion coefficient
can reduce the thermal load of laser glasses, enhancing thermal shock
toughness. As seen in Table2, the microhardness increases and thermal
expansion coefficient decreases when fluoride content gradually increases, moreover,
the mechanical and thermal properties of series B glasses with higher fluoride
content are superior to those of series A glasses. This anomalous behavior
should be due to the structural change caused by the fluorides. It is clear
from the glass composition as shown in Table 1 that the A0 glass without fluoride
content mainly consists of metaphosphate (ΣMO: P2O5=1)
group. Addition of fluorides leads to rupturing of long metaphosphate chains,
(PO3−)n and the formation of short structural
fragments of P2(O,F)7 and P(O,F)4. However,
these smaller fragments are linked up to a greater extent by the Al(O,F)6 polyhedra [19], which leads to strengthening of the glass network. In
particular, Nb5+ with higher field strength also promotes the
linkage of smaller fragments in series B glasses. In addition, the fact that B1
and B3 glasses display better mechanical and thermal properties is also
explained by the high cation field strength for Mg2+ and Li+ ions.


### 4.2. Effect of Fluoride on Spectroscopic Properties

The emission
cross-sectionσemi has an important effect on laser properties of Yb3+ ions because a larger σemi indicates higher laser gain [20]. From
(4), it can be seen that the value of emission cross-section is only determined
by the absorption cross-section, which removes the errors of reabsorption in the
experiment of fluorescence spectrum. As seen in Table 3, the σemi increases with
increase in fluoride content, and the increase of σemi depends on the change of structure of
fluorophosphate glasses. Addition of fluoride leads to the reduction of P–O–P linkages due to
a gradual transformation of (PO3−)n to P2(O,F)7 and P(O,F)4, which decreases the connectivity of the
glass network. This behavior is strengthened by the concentration of F− ions. Moreover, the surrounding coordination Yb3+ ions are also
changed due to present mixed anions, fluorine, and oxygen. In particular, the
series B glasses with higher Nb2O5 and fluoride content
contain different types of structural units such as P2(O,F)7,P(O,F)4,Al(O,F)6, and Nb(O,F)6 in the framework of fluorophosphate glass,
which increases asymmetry of the Yb3+ site environments and results
in larger absorption and emission cross-section as shown in Table 3.Generally, the decay rate for an excited state population,Γm=1/τm, is comprised
of three processes: the radiative decay rate (Γr), the nonradiative
decay rate (Γnr), and the additional nonradiative loss decay rate (Γq).
The total decay rate is thus [21]: (6)Γm=Γr+Γnr+Γq=1τr+1τnr+1τq, where τr,τnr, and τq are radiative
decay, nonradiative decay, and additional nonradiative decay lifetime
correspondingly. The radiative decay rate (Γr) is influenced by
variations of the local crystal field symmetry at the rare-earth site. These
variations are determined by the host matrix into which the ions are placed. Thus, τr depends on the ingredients around
the Yb3+ ions and structure of host matrix. Since Yb3+(F5/22→F7/22) possesses the
simple electronic energy level structure and nonradiative decay does not exist,
the second process (Γnr) is negligible (i.e., Γnr=0).
The third process, Γq, represents an additional nonradiative loss
mechanism which involves impurity or OH−
group [22]. For our glass samples starting with high purity materials, the
effect of impurities can be little, so τq is dependent mostly on OH−
groups. Based on that, the
total lifetime τm is influenced
mostly by radiative decay and additional nonradiative loss by OH−  groups. OH−
groups decrease, Γq also decreases, then τq and τm increase
correspondingly. As shown in Table 3, the τm increase with increase in fluoride content in series
A glasses. This can be explained as follows: the hydroxyl and fluorine ions are
isoelectronic with a similar ionic size, so that hydroxyl ions can be easily
replaced by fluorine during melting, which decreases the traps of OH−
and
prolongs the measured fluorescence lifetime. Whereas series B glasses exhibit
shorter τm compared to
series A glasses, this could be related to the difference of local environment around
the Yb3+ ions in two host matrixes.


### 4.3. Effect of Fluoride on Laser Parameters

The minimum pump intensityImin is a measure for the ease of
pumping the laser material to get laser action. It is a very important
parameter to evaluate the potential laser property. The lower the minimum pump
intensity, the higher the pump efficiency, then the better laser property. Imin describes the minimum absorbed pump intensity that is required for transparency
to be achieved at the extraction wavelength. Imin is calculated by the
following equation [23]: (7)Imin=βmin⋅Isat, where (8)βmin=σabs(λlaser)σabs(λlaser)+σemi(λlaser)={1+ZlZuexp(Ezl−hcλlaser−1kT)},Isat=hcλpumpσabs(λpump)τm,  where βmin is defined as the minimum fraction of Yb3+ ions that should be
excited to balance the gain exactly with the ground state absorption at the
laser wavelength. Isat is the pumping saturation intensity that
characterizes the pumping dynamics. Minimum values of βmin,Isat, and Imin are apparently preferred in terms of laser properties. Imin is mainly determined by σemi and τm according to (7).
The figure of merit of the Yb3+-doped laser materials is given by
the Imin and σemi, and it turns
out to be given by the emission cross-section σemi and fluorescence lifetime τm. Therefore, the combination of higher σemi, longer lifetime τm, and lower minimum pump intensity Imin give a better Yb3+-doped laser material. The parameters such as
laser gain coefficient σemi⋅τm and minimum pump intensity Imin are given in
Table 4.Table 4
Lasering properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4σemi⋅τm(pm2⋅millisecond)0.5020.5230.5960.6680.7050.8140.8240.7770.774Imin(kw/cm2)1.7611.681.5161.6151.271.0731.1121.1761.172Theσemi⋅τm increases with increasing fluoride content in series A glasses, since addition
of fluorine promotes emission cross-section σemi and fluorescence
lifetime τm. In spite of
shorter τm, series
B glasses exhibit higher values of σemi⋅τm and Imin due to higher σemi and the Imin of series B glasses is superior to the known QX/Yb glass [24]. Therefore, we
believe that the fluorophophate glasses are promising laser glasses for
high-peak power and high-average power.


## 4.1. Effect of Fluoride on Mechanical and Thermal Properties

Excellent
mechanical and thermal properties are indispensable to the laser driver for
inertial confinement fusion (ICF), especially low-thermal expansion coefficient
can reduce the thermal load of laser glasses, enhancing thermal shock
toughness. As seen in Table2, the microhardness increases and thermal
expansion coefficient decreases when fluoride content gradually increases, moreover,
the mechanical and thermal properties of series B glasses with higher fluoride
content are superior to those of series A glasses. This anomalous behavior
should be due to the structural change caused by the fluorides. It is clear
from the glass composition as shown in Table 1 that the A0 glass without fluoride
content mainly consists of metaphosphate (ΣMO: P2O5=1)
group. Addition of fluorides leads to rupturing of long metaphosphate chains,
(PO3−)n and the formation of short structural
fragments of P2(O,F)7 and P(O,F)4. However,
these smaller fragments are linked up to a greater extent by the Al(O,F)6 polyhedra [19], which leads to strengthening of the glass network. In
particular, Nb5+ with higher field strength also promotes the
linkage of smaller fragments in series B glasses. In addition, the fact that B1
and B3 glasses display better mechanical and thermal properties is also
explained by the high cation field strength for Mg2+ and Li+ ions.


## 4.2. Effect of Fluoride on Spectroscopic Properties

The emission
cross-sectionσemi has an important effect on laser properties of Yb3+ ions because a larger σemi indicates higher laser gain [20]. From
(4), it can be seen that the value of emission cross-section is only determined
by the absorption cross-section, which removes the errors of reabsorption in the
experiment of fluorescence spectrum. As seen in Table 3, the σemi increases with
increase in fluoride content, and the increase of σemi depends on the change of structure of
fluorophosphate glasses. Addition of fluoride leads to the reduction of P–O–P linkages due to
a gradual transformation of (PO3−)n to P2(O,F)7 and P(O,F)4, which decreases the connectivity of the
glass network. This behavior is strengthened by the concentration of F− ions. Moreover, the surrounding coordination Yb3+ ions are also
changed due to present mixed anions, fluorine, and oxygen. In particular, the
series B glasses with higher Nb2O5 and fluoride content
contain different types of structural units such as P2(O,F)7,P(O,F)4,Al(O,F)6, and Nb(O,F)6 in the framework of fluorophosphate glass,
which increases asymmetry of the Yb3+ site environments and results
in larger absorption and emission cross-section as shown in Table 3.Generally, the decay rate for an excited state population,Γm=1/τm, is comprised
of three processes: the radiative decay rate (Γr), the nonradiative
decay rate (Γnr), and the additional nonradiative loss decay rate (Γq).
The total decay rate is thus [21]: (6)Γm=Γr+Γnr+Γq=1τr+1τnr+1τq, where τr,τnr, and τq are radiative
decay, nonradiative decay, and additional nonradiative decay lifetime
correspondingly. The radiative decay rate (Γr) is influenced by
variations of the local crystal field symmetry at the rare-earth site. These
variations are determined by the host matrix into which the ions are placed. Thus, τr depends on the ingredients around
the Yb3+ ions and structure of host matrix. Since Yb3+(F5/22→F7/22) possesses the
simple electronic energy level structure and nonradiative decay does not exist,
the second process (Γnr) is negligible (i.e., Γnr=0).
The third process, Γq, represents an additional nonradiative loss
mechanism which involves impurity or OH−
group [22]. For our glass samples starting with high purity materials, the
effect of impurities can be little, so τq is dependent mostly on OH−
groups. Based on that, the
total lifetime τm is influenced
mostly by radiative decay and additional nonradiative loss by OH−  groups. OH−
groups decrease, Γq also decreases, then τq and τm increase
correspondingly. As shown in Table 3, the τm increase with increase in fluoride content in series
A glasses. This can be explained as follows: the hydroxyl and fluorine ions are
isoelectronic with a similar ionic size, so that hydroxyl ions can be easily
replaced by fluorine during melting, which decreases the traps of OH−
and
prolongs the measured fluorescence lifetime. Whereas series B glasses exhibit
shorter τm compared to
series A glasses, this could be related to the difference of local environment around
the Yb3+ ions in two host matrixes.


## 4.3. Effect of Fluoride on Laser Parameters

The minimum pump intensityImin is a measure for the ease of
pumping the laser material to get laser action. It is a very important
parameter to evaluate the potential laser property. The lower the minimum pump
intensity, the higher the pump efficiency, then the better laser property. Imin describes the minimum absorbed pump intensity that is required for transparency
to be achieved at the extraction wavelength. Imin is calculated by the
following equation [23]: (7)Imin=βmin⋅Isat, where (8)βmin=σabs(λlaser)σabs(λlaser)+σemi(λlaser)={1+ZlZuexp(Ezl−hcλlaser−1kT)},Isat=hcλpumpσabs(λpump)τm,  where βmin is defined as the minimum fraction of Yb3+ ions that should be
excited to balance the gain exactly with the ground state absorption at the
laser wavelength. Isat is the pumping saturation intensity that
characterizes the pumping dynamics. Minimum values of βmin,Isat, and Imin are apparently preferred in terms of laser properties. Imin is mainly determined by σemi and τm according to (7).
The figure of merit of the Yb3+-doped laser materials is given by
the Imin and σemi, and it turns
out to be given by the emission cross-section σemi and fluorescence lifetime τm. Therefore, the combination of higher σemi, longer lifetime τm, and lower minimum pump intensity Imin give a better Yb3+-doped laser material. The parameters such as
laser gain coefficient σemi⋅τm and minimum pump intensity Imin are given in
Table 4.Table 4
Lasering properties
of Yb3+-doped fluorophosphate glasses.
A0A1A2A3A4B1B2B3B4σemi⋅τm(pm2⋅millisecond)0.5020.5230.5960.6680.7050.8140.8240.7770.774Imin(kw/cm2)1.7611.681.5161.6151.271.0731.1121.1761.172Theσemi⋅τm increases with increasing fluoride content in series A glasses, since addition
of fluorine promotes emission cross-section σemi and fluorescence
lifetime τm. In spite of
shorter τm, series
B glasses exhibit higher values of σemi⋅τm and Imin due to higher σemi and the Imin of series B glasses is superior to the known QX/Yb glass [24]. Therefore, we
believe that the fluorophophate glasses are promising laser glasses for
high-peak power and high-average power.


## 5. Conclusion

Yb3+-doped
fluorophosphate laser glasses have successfully been developed. A systematic
investigation of physical properties including refractive index, Abbe number, nonlinear
refractive index, microhardness, and thermal expansion coefficient has been
performed as a function of fluoride content. With the increase of fluoride content, the microhardness increases gradually along with the decrease of
thermal expansion coefficient. The structure around Yb3+ is simultaneously
changed which greatly influences the spectroscopic properties and laser
parameters. The best laser performance is found in 44P2O5-7Al2O3-4Nb2O5-10LiF-20MgF2-14CaF2-1Yb2O3 glass system with the gain
coefficient σemi⋅τm(0.824pm2⋅ms) and minimum pump intensity Imin(1.112kw/cm2). The favorable combination of outstanding physical, spectroscopic
properties and laser parameters indicates that current Yb3+-doped fluorophosphate
glass is an excellent candidate material for Yb3+-doped host for
high-power generation.



---

*Source: 656490-2008-09-25.xml*