D Krol

E C Honea

F D Patel

J S Hayden

S A Payne

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UCRL-JC-12SS17
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Nd3+and Yb3+Doped Phosphate Glass Waveguides
Fabricated Using Electric Field Assisted Ag+Diffusion
F. D. Patel, E. C. Honea, D. Krol,
S. A. Payne, J. S. Hayden
This paper was prepared for submittal to the
Advanced Solid State Lasers and Radiative Processes and
Dephasing in Semiconductors Topical Meetings
Coeur d’Alene, ID
February 2-4,1998
December 17,1997
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Nd3+AND Yb’+ DOPED PHOSPHATE GLASS WAVEGUIDES FABRICATED USING
ELECTRIC FIELD ASSISTED Ag+ DIFFUSION
Falgun D. Patel, Eric C. Honea, Denise Krol, Stephen A. Payne
Lawrence Livermore National Laboratory
P O 60X 808, L-441
Liverrnore CA 94551
Tel: (51 O) 424-5986
Fax: (510) 423-6195
email: falgun@LLNL.GOV
Joseph S. Hayden
Schott Glass Technologies, Inc.
400 York Ave.
Duryea, PA 18642
Tel: (717) 457-7485
L INTRODUCTION
Solid-state waveguide lasers offer several attractive features that may make high
efficiency and effective thermal management possible. Due to the ability to confine
pump light to high intensity over distances much longer than the Rayleigh range, as
well as maintaining good overlap between the pump and Iasing modes over the entire
guiding region, e~cient operation with high slope efficiency should be possible, even for
quasi-three level laser systems. Since the waveguide region is typically only a few
microns of thickness, heat can be extracted efficiently from the structure. The effects of
heating are of less significance than in bulk solid-state lasers because mode
confinement is maintained by an index of refraction difference, usually much larger than
tnat induced by dn/dT or stress-optic effects. Rare earth doped waveguide ~aser action
has been reported in numerous ‘papers [14]. The processes for fabricating waveguides
include film deposition methods such as epitaxial growth, RF sputtering, and most
recently, thermal bonding of precision finished crystals [5]. In addition, ion implantation,
ion exchange in a molten salt and electric field assisted solid film diffusion [6] have
been utilized. The ion exchange method remains the simplest, particularly for many
common laser glasses that already have mobile ions, and has received considerable
attention in recent years. An excellent review is found in reference [7].
Our work has focused on developing process conditions for the fabrication of
waveguides in phosphate laser glasses using solid silver film diffusion. These
processes are important in determining the overall structure and properties of the
guiding region, such as propagation loss, modal profile, and modal overlap between the
pump and laser wavelengths. Phosphate laser glass was chosen as the solid state
laser medium due to the useful spectroscopic properties of rare earth ions in these
materials, as well as the range of material properties and compositions possible.
Il. WAVEGUIDE FABRICATION
Fabrication begins with thermally evaporating silver films on both sides of the
phosphate glass substrate. A quartz crystal monitor is used to measure the thickness
of the silver film during evaporation. The sample is then placed into a furnace where an
electric field is applied across the substrate. The diffusion temperature and time are
varied from 270 ‘C to 350 ‘C and 5 minutes to 45 minutes, respectively. The excess
silver is then etched in a solution of nitric acid. A post fabrication anneal may be
performed to reduce the overall index of refraction change and increase the diffusion
depth.
The typical range of waveguide properties achieved using different process
conditions is summarized in Table 1. Malone[4] has previously characterized the
properties of the phosphate glass used in our experiments (NIST-I ). In this case
instead of being neodymium doped, the NIST-I glass was prepared with 7 wt.’Mo YbQ
corresponding to an input ytterbium ion density of 6 x 10m Yb3+ ions/cm3. The
designation for the glass melt used in this work was GWO-LI B. Waveguides were
prepared by first coating a glass sample substrate (2 inch square by 1 mm thick) with a
200 nm thick silver film. The sample was then cleaved into four pieces and planar
waveguides were fabricated under different process conditions. Measurement of the
effective indexes using the prism coupling technique is described in Reference [6].
Laser light was coupled into and out of the waveguide by means of a high index prism
(n=l .844). The index profile was determined by inverting the wave equation using a
WKB analysis. The method is described in detail by Chiang [8]. The smooth index
profile was well described by a Gaussian distribution,
n(x) = nW~@ + An Exp[-(x/d)A2] (1)
where d is the I/e diffusion depth, An is the total change in index, and nW_ is 1.5178
for Yb: phosphate glass and 1.5222 for Nd:phosphate glass (measured at L = 632.8
rim).
[
T
7
0.014>indicatesslightlygreater than 0.014; 2.71< indicatesslightlyless than 2.71
‘able 1. Summary of parameters for Ybphosphate glass waveguides
Yb:phosphateglass sample # 1 2a 2b 3a 3b 3C 4
Diffusiontemperature, T (OC) 300 300 — 330 — — 330
Diffusiontime, t (rein) 15 30 — 5—— 15
Appliedvoltage (V) 15 15 — 15 — — 15
Anneal temperature ~C) —— 300 — 330 330 –
Anneal time (rein) -. — 10 — 10 20 —
An 0.014> 0.038 0.027 0.020 0.015 0.013 0.011
Diffusiondepth (l/e), d (urn) 2.71< 2.71 3.80 6.23 7.86 9.16 9.78
Number of bound modes 2 3 4 5 5 6 6
.. . . —. .
Table I summarizes the results, an overall decrease in the change in index of
refraction, an increase in the I/e diffusion depth, and additional bound modes for longer
postbake times. Notice that the index change in sample 4 is lower than for sample 3a.
This indicates that the silver was depleted before the prescribed 15 minutes were
completed. Once the silver source was consumed, thermal migration of the diffused
Ag+ ions occurred in the substrate, reducing the overall surface index of refraction and
increasing the diffusion depth. One might expect to achieve a similar index change by
annealing sample 3a at 330 “C. Fig. 1 shows the results of successive postbake steps,
where the index distribution of Sample 3a is approaching that of Sample 4.
1.540
1.535
1.530
1.525
1.520
Fig.1
1.515 ~
o 5 15 20
DaP&8n)
1.545,
1.540
1.535
1.530
1.525
1.520
Fig.2
1.515 ~
o 5 10 15 20
Diffusion Depth (urn)
Fig. 1. Theprofiles are as follows: Square Sample 3a; diamond: Sample 3b; triangle: Sample 3G
circle Sample 4. The bulk index of refractionof the substrate is 1.5178. The general trend of the graded
indexafter annealing is a decrease in Anand an increase in the l/e diffusiondepth.
Fig.2 Varyappliedvoltagefordiffusiontemperatureandtimeof300 “Cand30 minutes,respectively.
Triangle:5 V Circle: 7 V Square: 15V. Forallthreecasesthe thickness of the Ag film, the sourcefor
iondiffusion,was approximately250 nm.
Another set of experiments was performed to determine the effect of varying the
applied voltage while keeping constant the diffusion time and temperature. As shown in
Fig. 2, the overall index change was approximately the same for the three cases while
the l/e diffusion depth and the number of bound modes increased. Similar results, both
experimental and numerical, are reported in Reference [6].
Ill. LOSS CHARACTERIZATION
The formation of Ag colloids during the fabrication is a problem because of
scatter and absorption losses. In our experiments a yellowish tint was obsemd in
glass samples fabricated with diffusion temperatures at or above 300”C. It is well
known that this discoloration may indicate the presence of ‘Ag colloids[9]. The glass
melt GWO-LI B was prepared without the use of refining agents or antisolorants that
possibly participate in the generation of reduced silver through a redox interaction.
Indeed, in the earlier work with this glass by Malone[4], silver colloid formation was not
noted, and the low loss exhibited by the resultant waveguides indicates colloidal silver
was probably not present in any significant amount. However, it should be noted that,
in this case, visual detection of any yellow discoloration may have been difficult since
neodymium-doped variant of this glass was employed.
Loss is an important issue in determining performance characteristics for
waveguide lasers. We have estimated the loss in our waveguides to vary from 0.71
dB/cm to 0.99 dB/cm using a two-prism measurement technique, a variation on the ,
three-prism method [1O]. Two prisms are held in contact with the waveguide sutiace by
index matching oil. A HeNe laser beam is focused into the first prism and the power of
the output light from the second prism, a distance z from the first, is recorded by a
photodiode. The exponential loss coefficient a (dB/cm) is then calculated from the
following equation: P-s = PM exp(-a z/4.34). The reference power P@, is taken to be
the value at the smallest prism separation. The second (output) prism is moved a small
distance and another measurement is made. The coupling efficiencies of both prisms
are assumed to be the same because large deviations from
behavior were not detected (Fig. 3).
,..
aa
33.
31-
29.
27.
●
the exponential loss
Fig. 3 Loss measurement of Nd:phosphate glass waveguide.
Using the Lorentz-Lorenz equation [11], one can obtain a relationship between
the change in refractive index and the Ag concentration diffused into the substrate. The
electronic polarizability is given by
()3 n2–1
(2)
a=—
47cN n2 + 1
where n is the index of refraction and N is the number density. Assuming a one to one
exchange of silver for sodium (NM = N~~),we can define a change in polarizability
3
[1
nf –1 xf –1
Aa=fxAg.qa =— —–—
471NAgrf +1 rf +1
(3)
where n, and ni are the final and initial indexes of refraction, respectively. Defining An =
nfni, and approximating An- ni, n,, the following equation results:
.
All values in Eq. (4) are
polarizabilities for Na and
‘a=(2mNA~i+n,~)An
(4)
known: ni is 1.5178 (at L = 632.8 nm) and the electronic
Ag are obtained from Ref. [12]. Rewriting the equation into a
compact form with units of weight percent, a linear relationship between the index of
refraction and the diffused Ag concentration results,
An= 0.0026NAg[wt.% ] (5)
Experimental data is in close agreement with the theoretical result obtain in Eq. 5
[13]. Electron microprobe analysis was performed to determine the Ag concentration in
the waveguides and to correlate this result to the graded index distribution using (5).
The theoretical result in Fig. 4 is obtained by using the measured index profile which is
best described by (1) and using (5) to find the silver concentration as a function of
depth. Due to rounding of the sample edge during preparation, the silver concentration
at the sutiace could not be clearly determined. The data in Figure 4 is shifted by
several microns to reflect that measurements were made away from the true edge of
the waveguide. The microprobe analysis was done to confirm silver diffision into the
phosphate glasses and that the depth of the diffusion was in agreement with calculated
results.
0
‘h
. Meas. Run 1
$7 . Meas. Run 2
6 _ TheoreticalAg cone
al \* II
‘k--
● *a
● *
o 5 10 15 20 25
Depth (urn)
Figure 4. Electronmicroprobeanalysis on a Nd:phosphate glass
waveguide with a measured l/e depth of 11 urnand a Anof 0.018.
IV. CONCLUSION
In conclusion, we have studied electric field assisted Ag ion exchange in Nd and
Yb-doped phosphate glasses to understand the role of diffusion temperature, time, and
electric field during waveguide fabrication. Understanding the fabrication process
allows one to produce waveguides of prescribed parameters determined for optimal
Iasing characteristics. The high index changes result in large numerical apertures (NA),
which are preferable for diode pumping schemes. Lasing can be theoretically achieved
with the current losses in our waveguides, but lower losses, typically on the order of 0.1
dB/cm, are preferable. We are currently building a diode pumped Yb:phosphate glass
waveguide laser with both planar and channel guiding regions.
This work was perfo&ed under the-auspices of the U.S. Department of Energy
by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
REFERENCES
1. T. Feuchter, et. al., “Erbium-Doped Ion-Exchanged Waveguide Lasers in
BK-7 Glass,” /EEE I??otonics Technology Leftefs, 4, No. 6 (1992) pp. 542-544.
2. D. C. Hanna, et. al., “Quasi-three level 1.03 urn laser operation of a planar
ion-implanted Yb:YAG waveguide,” Optics Communications, 99 (1993), pp. 211-215.
3. D. P. Shepherd, et. al., ”1.9 urn operation of a Tm:lead germanate glass
waveguide laser,” Optics Letfecs, 19, No. 13 (1994) p. 954-956.
4. K. Malone, N. Sanford, J. Hayden, “Integrated optic laser emitting at 906,
1057, and 1358 rim,” Hectronics Letters, 8, p. 691 (1993).
5. C. T. A. Brown, et. al., “Thermally bonded planar waveguide lasers, “App/ied
Physics Letters, 71, No. 9, Sept. 1997, pp. 1139-1141.
6. S. 1.Najafi, cd., Introduction to Glass Integrated ODtics, Artech House, Inc.,
1992.
7. R. V. Ramaswamy, R. Srivastava, “Ion-Exchanged Glass Waveguides: A
Review,” Journal oftightwave Technology, 6, No. 6, June 1988, pp. 984-1002.
8. K. S. Chiang, “Construction of Refractive-Index Profiles of Planar Dielectric
Waveguides from the Distribution of Effective Indexes,” Jouma/ of Lighhvave
Technology, LT-3, No.2, April 1985, pp. 385-391.
9. C. F. Bohren, D. R. Huffman, “Surface Modes in Small Padicles,” Absomtion
and Scattering of Liaht by Small Particles, John Wiley& Sons, Inc., 1983, pp. 372-374.
10. Ming-Jun Li, Chap. 5, “Optical Waveguide-Characterization Techniques,”
Introduction to Glass Integrated ODtics, S.1. Najafi, cd., Artech House, Inc., 1992, pp.
105-136.
11. J. D. Jackson, “Multiples, Electrostatics of Macroscopic Media, Dielectrics,”
Classical Electrodynamics, John Wiley & Sons, Inc., 1975, 2ndcd., p. 155.
12. J. R. Tessman, A. H. Kahn, W. Shockley, “Electronic Polarizabilities of Ions
in Crystals,” Physical Review, 92, No. 4, Nov. 15, 1953, pp. 890-895.
13. K. J. Malone, “Nd Phosphate Glass Waveguide Laser,” Ph.D. Thesis,
University of Colorado, Dept. of Electrical and Computer Engineering, July 1994, Fig.
5.14, p. 151.
T
echnical Inform
ation D
epartm
ent  • Law
rence Liverm
ore N
ational Laboratory
U
niversity of C
alifornia • Liverm
ore, C
alifornia  94551


UCRL-JC-12SS17 PREPRINT Nd3+and Yb3+Doped Phosphate Glass Waveguides Fabricated Using Electric Field Assisted Ag+Diffusion

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UCRL-JC-12SS17 PREPRINT Nd3+and Yb3+Doped Phosphate Glass Waveguides Fabricated Using Electric Field Assisted Ag+Diffusion

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