Rare Earth Doped Photonic Glass Materials for the Miniaturization

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					   Rare Earth Doped Photonic Glass Materials for the Miniaturization
               and Integration of Optoelectronic Devices

Ju H. Choi1, Alfred Margaryan2, Ashot Margaryan2, Wytze van der Veer3 and Frank G. Shi1
   1
       University of California Irvine, Dept. of Chemical and Materials Science Irvine, CA 92697
                       2
                         AFO Research Inc., P.O. Box 1934, Glendale, CA 91209
                 3
                   University of California Irvine, Dept of Chemistry, Irvine, CA 92697
                                1
                                  Phone: 949.824.7385, Fax: 949.824.2541
                                           1
                                             Email: juc@uci.edu


                                              Abstract
         3
       Er doped alkaline-free glass systems based on ,MgF2-BaF2-Ba(PO3)2-Al(PO3)3 (MBBA
system), was investigated with the aim of using as high gain media The absorption spectra were
recorded to obtain the intensity parameters (Ωt) which are found to be Ω2= 4.47×10-20 cm2,
Ω4=1.31×10-20 cm2, Ω6=0.81× 10-20 cm2 for the MBBA system. The emission cross section for the
4
  I13/2 →4I15/2 transition is determined by the Fuchtbauer-Ladenburg method and found to be 2.35
×10-20 cm2 for the MBBA systems. Comparison of the measured spectroscopic values to those of
Er3+ transitions in other glass hosts suggests that new MBBA systems are good candidates for
broadband compact optical fiber and waveguide amplifier applications.

Keywords: Rare earth ion, Fluorophosphate glass

1.0 Introduction                                       size. In order to evaluate the potential for
     The use of compact lasers has attracted           compact laser media, we conducted a
increasing interest operating in the infrared          systematic investigation of the spectroscopic
region for optical communications, medical             properties of Er3+ doped MBBA system. The
and eye-safe light detecting and ranging               previous results on Nd3+, Yb3+ doped
applications in the visible region for data            fluorophosphate glass systems already
storage, undersea communications [1-3].                showed the strong potentials for each
With the development of 980nm laser diodes,            characteristics transitions of rare earth ions
the diode pumped solid state lasers can                [5-9] to develop new host materials with a
provide a compact and efficient device with            high emission cross section for compact gain
the advantage of easy coupling with fiber              media. We obtained intensity parameters,
integrated optical systems. Specifically, the          the radiative lifetime and gain coefficient for
optically excited luminescence originating             the 4I13/2 →4I15/2 transition of Er3+. Finally,
from the dipole-forbidden 4I13/2         4
                                           I15/2       the potential of these systems for short fiber
                3+
transition of Er has a wavelength of 1.54              amplifiers or planar waveguides is evaluated
µm that matches one of the minimum loss                by comparison to other reported glass hosts.
windows of commercial silica-based optic
fibers. Typical Er3+ doped fiber amplifiers            2.0 Experimental procedures
utilize approximately several meters of silica         2.1 Glass formation:
fiber doped with a few hundred ppm weight                  The batch materials of 40MgF2-40BaF2-
Er3+ ions [4]                                          10Ba(PO3)2-10Al(PO3)3 system (the MBBA
     In the construction of integrated light           system) was purchased from reagent grade
amplifiers it is desirable to obtain the               materials (City Chemicals, except for Er2O3,
maximum gain with minimum component                    Spectrum Materials), all have better than
99.99% purity. The ingredients of the             doped MBBA system was used in this work.
glasses were weighed with 0.1% accuracy           The refractive index nD, and Abbe number
and mixed thoroughly for 3 hours. Next, the       are 1.5885 and 68.1, respectively.
raw mixed materials were melted in a              3.2.     Absorption spectrum properties
vitreous carbon crucible in Ar-atmosphere at      of Er3+ in MBBA system.
1200 - 1250 °C. The melt was quenched by
pouring it in a room temperature stainless          3.00E-024
                                                                                                  MBBA system
steel mold. Next, the samples were annealed
below the glass transition temperature,             2.00E-024

around 400 - 430 °C, to remove internal
stress, which was verified by examination           1.00E-024


with a polariscope (Rudolph Instruments).
                                                            400     600   800      1000    1200   1400   1600
The samples for optical and spectroscopic                                       Wavelength (nm)
measurements were cut and polished to a
size of 15×10×2mm3.                               Figure 1: The absorption cross section of
2.2 Spectroscopic measurements:                   Er3+ in the range of 300nm to 1700nm.
     The refractive index nd of the samples
was measured at 588 nm, using an Abbe                  The absorption spectrum of Er3+ ion
refractometer (ATAGO). The absorption             consists of 11 absorption bands centered at
spectra were obtained at room temperature         1532, 804, 650, 544, 520, 488, 451, 406, 377,
in the range of 400-1700 nm with a Perkin-        365, and 356 nm, corresponding to the
Elmer photo spectrometer (Lambda 900).            absorptions from the ground state 4I15/2 to
The lifetime and fluorescence spectrum of         the excited states in the 4f11 electronic
both samples was recorded using a chopped         configuration respectively. The radiative
Ti-Sapphire laser (Coherent 890) tuned to         nature of trivalent rare earth ions in a variety
800 nm, pumped by the 514 nm line of an           of laser host materials is usually investigated
Ar laser (Innova 300), see Figure 3. The          using the Judd-Ofelt model [10, 11]. The
fluorescence signal was recorded with a 0.25      observed oscillator strengths fmed at each
m monochromator (Oriel 77200), using a            absorption peak is calculated by integration
InGaAs PIN detector (Thorlabs DET 410), a         the optical absorption spectra over each peak,
trans-impedance amplifier and a Lock-In           as given by following expression Eq. (2):
amplifier (Oriel Merlin 70100). The
lifetimes of both samples, was recorded with
                                                                 mc 2 α (λ )
the same system, recording the temporal           f med =             ∫      dλ
behavior of the fluorescence signal with a                      πe 2 N λ2       .                           (2)
100 MHz digital Oscilloscope.
                                                  Here c is the velocity of light, N is the Er3+
3.0 Results and discussion                        ion      concentration    (ion/cm3).     α(λ)
3.1 Optical properties                            (=2.303Do(λ)/d) is the optical absorption
    The value of (nF-nC) and Abbe number          coefficient at a particular absorption
(νd) normally describes dispersion in glasses     wavelength λ, which is calculated from the
as below                                          sample thickness d and the measured
                                                  absorption density D0(λ). The oscillator
νd =
        (n D − 1 )              (1)               strengths as predicted by Judd-Ofelt model
       (n F − n C )                               fcal were also calculated. The oscillator
                                                  strengths of the observed electronic
where nD, nF and nC are the refractive indices    transition are due to three interactions,
at the D, C and F spectral lines. The variation   electrical dipole (ed), magnetic dipole (md)
of refractive index as a function of rare earth   and electric quadrupole (eq). In most
dopant concentration was systematically           instances in the Er3+ system, the oscillator
investigated in previous work. 2wt% of Er3+
strength of the eq component is of the order                              deviation δrms of the fits was 8.52 x 10-6,
of 10−10, and the md component is of the                                  which indicates that these fits are reliable. In
order of 10−8. These contributions are thus                               Table I [12-15] these values are compared to
unimportant compared with the ed                                          those for other reported laser glasses. The
contribution to the oscillator strength, which                            value of Ω2 indicates the strength of the
is in the order of 10−6 [1]. However, a                                   covalent binding between the tri-valent rare
significant contribution of the md                                        earth ion and the host material [16, 17]. The
component is involved for the 4I15/2 → 4I13/2                             value of Ω2 of the MBBA systems is smaller
absorption transition for the Er3+. Therefore,                            than those of BK20 oxide glass and
theoretical oscillator strengths f(aJ, bJ’) of                            phosphate glass, which show strong co-
the J → J’ transition at the mean frequency                               valent bonds. The measured value is higher
ν is given for both the electric and the                                  than that of ZBLAN, which has a very high
magnetic dipole transition by below Eq. (3)                               fluoride content causing strong ionic bonds
                                                                          and thus weaker co-valent bonds. The values
fcal(aJ,bJ' =
          )        8π2mν
                              [ χedSed (aJ,bJ') + χmdSmd(aJ,bJ')]   (3)   of Ω2 of the MBBA, system are comparable
                3(2J +1)he2n2
                                                                          to those of other fluorophosphate glass with
                                                                          similar fluorine content.
where me is the mass of the electron. e and h                                    The effect of co-valent bonding between
are the charge of the electron and plank’s                                the Er3+ ions and the host material can be
constant, respectively. χed =n(n2+2)2/9 and                               understood in terms of the Judd-Ofelt
χmd =n3 are local field corrections and are                               parameters. In case of a Er3+ doped system
functions of the refractive index n of the                                the t=2 transition matrix elements [U(2)]2 of
medium. Sed and Smd are the electrical dipole                             the transitions between the 4I11/2, 4I13/2 and
and the magnetic dipole line strength                                     4
                                                                            I15/2 states are very small. The quality of
respectively                                                              these transitions for laser operation is thus
3.2. Intensity parameters & quality factor                                characterized by Ω4 and Ω6 via the
     The Judd-Ofelt intensity parameters                                  spectroscopic quality factor Q (=Ω4/Ω6), as
were determined by a least squares fit of the                             introduced by Kaminskii [18]. The Q values
theoretical (free ion) oscillator strengths to                            are found to be 1.62 for the MBBA system.
the measured (glass matrix) values obtained                               These values are larger than those found in
from optical absorption spectra. By fitting                               most laser glasses as well as in FP20, see
the measured oscillator strengths fmed to the                             Table I. The MBBA glass system is thus
calculated values fcal we obtained the                                    better suitable for laser applications than
following values for three Judd-Ofelt                                     other published glass systems
parameters Ω2= 4.47×10-20 cm2, Ω4=                                        .
1.31×10-20 cm2 and Ω6= 0.81× 10-20 cm2 are
obtained for the MBBA system. The
     .
       Glasses    Ω2 (10-20 cm2) Ω4 (10-20 cm2) Ω6 (10-20 cm2) Ω4 /Ω6        Ref
        BK20         5.66              1.84        1.18        1.56           12
       ZBLAN         2.20              1.40        0.91        1.54           13
    Phosphate        6.65              1.52        1.11        1.34           14
        FP20         4.71              1.61        1.62        0.99           15
       MBBA          4.47              1.31        0.81        1.62     Current work
Table I: Comparison of Judd-Ofelt parameters of Er3+ doped MBBA system and other reported
laser glasses

3.3 The emission cross section and gain                                       The efficiency of a laser transition is
coefficient of the 4I13/2 → 4I15/2 transition.                            evaluated by considering stimulated
                                                                          emission cross-section (σem(λ)). In this work,
σem(λ) was determined from the emission                                                                            3 as a function of the population inversion γ,
spectrum   using    Fuchtbauer-Ladenburg                                                                           using the relation below
method (FL) [19]
                                                                                                                   G(λ) = γσ em ( λ ) − (1 − γ )σ abs ( λ )                                        (5)
                                   β J − > J ' λ 4p A ra
σ em                          =
                                8 π cn (λ p ) ∆ λ eff
                                                 2
                                                                            (4)                                    Using this equation, we calculated the gain
                                                                                                                   spectra as shown in Fig. 3.
where λp is the peak wavelength of the
emission, λeff is the width of the emission
line, βJ->J’ is the branching ration, which is in                                                                                                     4
                                                                                                                                                                                           γ=1
case of the 4I13/2 → 4I15/2 transition equal to 1,                                                                                                    3                                    γ=0.8

c is the speed of light in vacuums, and n(λp)
                                                                                                                                                                                           γ=0.6
                                                                                                                                                      2




                                                                                                                       Gain coefficient (x10 , cm )
                                                                                                                                                                                           γ=0.4




                                                                                                                      2
is the refractive index at emission peak                                                                                                              1
                                                                                                                                                                                           γ=0.2
                                                                                                                                                                                           γ=0




                                                                                                                      -20
wavelength. In our case an effective line                                                                                                             0
width is used instead of the full width at half
                                                                                                                                                      -1
maximum to compensate for the a-
symmetric profile of the emission line. Fig.                                                                                                          -2

2 shows the absorption cross section, σabs(λ),                                                                                                        -3

and the emission cross section, σem(λ),                                                                                                               -4
                                                                                                                                                       1400   1450   1500   1550   1600   1650     1700
determined by FL method.                                                                                                                                              Wavelength (nm)


                                                                                                                   Figure 3: Gain coefficient in the eye-safe
                              6
                                                                                        0.7
                                                                                                                   range of Er3+ in the MBBA system
                                                                       MBBA system

                                                                                        0.6
                              5
                                                                                                                    Note that the gain will be positive at 1536
  Emission Intensity (a.u.)




                                                                                                                   nm, when the population inversion is larger
                                                                                              Absorbdance (a.u.)




                              4
                                                                                        0.5
                              3                                                                                    than 0.5. The maximum value for the gain is
                              2
                                                                                        0.4                        achieved in the case of complete population
                                                                                                                   inversion (γ = 1), in this case cross section
                              1                                                         0.3                        for stimulated emission is 2.35 × 10−20 cm2
                              0                                                                                    for the MBBA system.
                                                                                        0.2
                              1350   1400   1450     1500   1550     1600   1650     1700
                                                   Wavelength (nm)                                                 4.0 Conclusions
                                                                                                                   The novel MBBA system was successfully
Figure 2: Absorption cross section and
                                                                                                                   developed and the absorption and emission
measured emission cross section of Er3+ in
                                                                                                                   spectra of Er3+ were measured and analyzed.
the MBBA system.
                                                                                                                   Three intensity parameters are found to be
    For the MBBA system, the peak                                                                                  Ω2= 4.47×10-20 cm2 Ω4=1.31×10-20 cm2
absorption cross sections of σabs(λ) turned                                                                        Ω6=0.81× 10-20 cm2 for the MBBA system.
out to be 1.58 × 10-20 cm2 and the peak                                                                            The strong emission bands were observed at
                                                                                                                   1536 nm and the effective bandwidths were
emission cross sections of σem(λ) are 1.86 ×
                                                                                                                   found to be 91 nm. Emission cross section
10-20 cm2. Comparing the Er3+ ion to a
                                                                                                                   determined by FL method for the 4I13/2
simplified two level system, we assume the
                                                                                                                   →4I15/2 transition are found to be 2.35 ×
population is either in the 4I15/2 ground state
                                                                                                                   10−20 cm2 and population inversion of above
or the 4I13/2 excited state. In this case the
                                                                                                                   50 % were obtained. These spectroscopic
optical gain properties are directly
                                                                                                                   results show that these novel materials are
associated with the absorption and emission
                                                                                                                   strong candidates for developing broadband
cross sections. Gain spectra is shown in Fig.
                                                                                                                   optical amplifiers and compact fiber lasers.
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