Chromium
Jo&
diffusion
in lithium
niobate for active
optical
waveguides
M. Almeida,a) Gerard Boyle,b) and Ant6nio P. Leite
Centro de Fisica do Porto, Faculdade de Ci&tcias, Universidade do Porte, P Comes Teixeira, 4000 Porto,
Portugal
Richard M. De La Rue and Charles N. lronside
Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland
Federico Caccavale
and P. Chakrabortf)
Universita di Padova, Dipartitnento di Fisica, via F Marzolo 8, 35131 Padova, Italy
I. Mansour
Universita di Padova, Dipartimento di Elettronica ed Informatica, via Gradenigo 6/A, 35131 Padova, Italy
(Received 14 February 1995; accepted for publication 26 April 1995)
A method to fabricate chromium-doped lithium niobate substrates in which the active ions are
introduced by thermal diffusion from a film is reported. Chromium concentration depth profiles have
been obtained by secondary-ion-mass spectrometry and the relevant diffusion parameters have been
derived. Fluorescence spectrum and upper laser level lifetime of chromium diffused
proton-exchanged and chromium/titanium-diffused
lithium niobate waveguides have been
measured. A simple model has been used to estimate the performance of such structures as
waveguide optical amplifiers and lasers. 0 199.5 American Institute of Physics.
I. INTRODUCTION
Laser action in chromium bulk-doped crystals with
broad tunability has. been demonstrated using various hosts,
(LiCAF),
LiSrAlF,
(MSAF),
such as LiCaAlF6
GdsSc,Ga,Otz (GSGG),’ and Y,Al,O,, (YAG)?
Waveguide lasers and optical amplifiers have been fabricated in bulk and film-doped lithium niobate substrates
doped with the rare earths Nd and Er.3-7 Optical amplification in proton-exchanged chromium bulk-doped lithium niobate has also been measured.’ These results indicate a significant potential for an integrated, broad-band tunable laser
in the 750- 1150 mn spectral range, with the additional prospect of diode laser pumping. The attraction of lithium niobate as a host material for laser active ions is that it has a
well developed waveguide technology and well characterized electro-optic properties. Combining electro-optic properties with laser active ions offers the possibility of extra
functionality, such as integration of a high speed modulator
with a waveguide laser to produce an integrated modelocked lasetg The particular attraction of Cr as a laser active
ion is that it has a broad emission spectrum and, combined
with the electro-optic properties of lithium niobate, this offers the possibility of high speed electro-optic tuning of the
laser to produce a wavelength agile laser. Furthermore, with
Cr ions it is possible to pump the material at around 670 nm
with a laser diode.
There are also significant drawbacks associated with
lithium niobate as a laser host material. It suffers photorefractive damage at shorter pump wavelengths. It also has a
short upper laser state lifetime, less than 1 ,us, compared to
typical values close to 100 w for other Cr host materials.*
3Electxonic [email protected]
b)On leave from Department of Electronics and Electrical Engineering,
University of Glasgow, Glasgow G12 SQQ, Scotland.
‘IPermanentaddress:Saha Institute of Nuclear Physics, l/AF Bidhan Nagar,
Calcutta-700064.India.
J. Appl. Phys. 78 (4), 15 August 1995
From published data on the spectroscopy of Cr-doped host
crystals, very different behavior of Cr in LiNbOs is expected
as compared to other hosts referred to above.‘~2V10-‘4However, the potential for tunable laser action and monolithic
integration with performant devices in the same LiNbOs substrate is rather attractive, due to the well established technology involving this material.‘5.16
Diffusion of the active dopant from a film into the crystal offers the possibility of tailoring the doping profiles,
avoiding important limitations imposed by a bulk-doped substrate: It should become possible to exploit undoped LiNbOs
substrates with excellent optical quality and large dimensions, with the definition of active areas at any position in the
substrate and efficient pump/signal interaction through tailoring of the doping depth profile. Optical amplification and
Iaser action in doped LiNbOs will preferably be obtained in
locally diffusion-doped substrates, rather than bulk ones.
A process to fabricate chromium-doped lithium niobate
substrates, in which the chromium ions are introduced by
thermal diffusiotrfrom a film, has been developed. The corresponding diffusion profiles .of Cr in LiNbO,, as obtained
from secondary-ion-mass spectrometry (SIMS) measurements, are presented. Application of standard diffusion
theory enables the extraction of fundamental data for optimization of the device design, such as diffusion depth and surface concentration. Data obtained from samples fabricated at
different temperatures and diffusion times provide the effective diffusion coefficient and the activation energy for chromium diffusion into z-cut LiNbO,.
Using a simple model of waveguide optical amplification, an estimate of the behavior of the gain attainable has
been carried out.
II. EXPERIMENT
A. Sample preparation
Samples were fabricated by using optical grade z-cut
LiNbO, wafers. The wafers were cut into 10X5X1 mm3
0021-8979/95/78(4)/2193/5/$6.00
Q 1995 American Institute of Physics
2193
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TABLE I. Times and temperaturesused in the fabrication of a set of
samplesfor SIMS analysis.
Temperature(“C)
Time
04
1050
950
1000
8
4
1
10
12
14
6
8
10
2
4
6
pieces with a diamond saw. After cleaning, the samples were
coated with a lo-nm-thick layer of chromium by thermal
evaporation using an electron beam.
Diffusion was carried out in a tubular furnace with a
flow of dry oxygen at a rate of approximately 0.5 dm3/min.
Diffusion temperatures from 950 to 1100 “C and diffusion
times from 1 to 24 h were used. The samples were placed in
a ceramic crucible and slowly moved into the middle section
of the furnace. Warm-up from room temperature to the diffusion temperature and cooling-down times were maintained
constant at five minutes.
Planar diffused samples fabricated in this way were used
to measure the Cr concentration profiles. Table I presents the
parameters (time and temperature) used in the fabrication of
a set of samples for SIMS analysis.
In order to measure the waveguide fluorescence spectrum and the lifetime of the upper laser level, proton exchanged and Ti-indiffusion channel waveguides were fabricated on top of these structures. For proton-exchanged
waveguides,i7 pure benzoic acid was employed with annealing in a dry oxygen atmosphere. For Ti-indiffusion waveguides a layer of Ti was electron-beam evaporated and subsequently diffused in a dry oxygen atmosphere. The mask
widths for channel waveguide fabrication were from 1 to
10 pm.
B. Characterization
The diffused samples were measured by SIMS using a
Cameca IMS 4f ion microscope equipped with a normal
incidence electron flood gun used to compensate the charge
buildup while profiling insulating samples like LiNbO, crystals. Concentration profiles were obtained using 14.5 keV
Cs+ bombardment and negative secondary ion detection. The
caesium beam current was 200 nA over a rastered area of
125X 125 pm2 and the secondary ions, emitted from a central circular area (10 ,um diameter), were collected by the
spectrometer. The erosion speed was evaluated by measuring
the depth of the erosion crater at the end of each analysis by
means of a Tencor Alpha-step profilometer. The depth resolution in our case was mainly determined by the roughness
of the crater under analysis, being S30 nm.
The calibration of the chromium concentration was obtained by means of the measurement of the total Cr dose in
as-deposited films using Rutherford backscattering spectrometry (RBS) analysis with a 2.2 MeV 4He+ beam (Q= 160”),
and invoking mass conservation in the diffusion process.
2194
J. Appl. Phys., Vol. 78, No. 4, 15 August 1995
III. RESULTS AND DISCUSSION
A. Modelling of the concentration
profile
In order to interpret the experimental results and extract
useful data, a diffusion model of the fabrication technique
was applied.
The diffusion equation for a one-dimensional process
is’*
ac d2C
-z--D-p
(0
where C is the concentration and D is the diffusion coefticient. For constant .D and a planar source, a solution to Eq.
(1) is
x2
c=s exp
--z
i 19
(2)
where A is an arbitrary constant.
For an initial state (thin film layer of thickness 7) defined
by
- r<x<o
C(x)= o”09
i , x>o ,
the diffusion profile can be calculated considering the extended source distribution to be composed of an infinite
number of line sources and by application of the principle of
superposition.” This leads us to the concentration profile:
C(x)=Co
x-l- 7
erf --e’.f-.-...2fi
i
x
(4)
2JDt 1 *
If the film thickness is small, the profile can be approximated by
C(x) = sexp(
-&)=Ctiexp(
-$),
(5)
where C,,,,.ris the surface concentration and d is the diffusion
depth.
The diffusion coefficient D is related to the diffusion
temperature by the well-known Arrhenius equation
h
1.0
%
0.8
r;
0
l2
0.6
-.i5
0.4
s
g
8
0.2
5
0.0
1.0
2.0
depth
3.0
4.0
(elm)
FIG. 1. Concentrationprofiles obtainedby SIMS for samplesdoped with 10
mn of Cr and diffused at 1000‘C for different times.
Almeida et
a/.
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3.0
2.5
0.8
$
2.0
r
z
1.5
2
1.0
0.5
0.0
2.0
1.0
depth
3.0
4.0
where Do is the diffusion constant, Q is the activation energy, k is the Boltzmann constant, and T is the diffusion
temperature in Kelvin. Knowing D at different temperatures,
one can calculate Do and Q.
B. SIMS results
The main parameters that affect and control the chromium concentration depth profiles are the initial thickness of
the chromium film, the diffusion temperature and the diffusion time.
Figure 1 shows SIMS concentration profiles of chromium atoms in lithium niobate after the diffusion process,
applied on IO-rim-thick Cr film samples, at the same temperature (1000 “C), but for various diffusion times.
Figure 2 presents SIMS concentration profiles of diffused chromium atoms in lithium niobate for the same diffu-~
\
0”
0.8 -
b
0.0 I-0.0
0.4 -
\
&
. ..w_.-...?.
+ .-,
-----------.- -..______-.__
_____.
-.-.-._.
----
\
,
4.0
8.0
12.0
16.0
20.0
t (h)
I
I
2.0
3.0
I
4.0
(dh)
FIG. 4. Plot of the diffusion depth found by fitting gaussianfunctions to the
experimental data (as obtained by SIMS) against the square root of the
diffusion time.
sion time (10 h) but for two different diffusion temperatures
(950 and 1000 “C). The Cr distribution is seen to be strongly
affected by the temperature.
According to the concentration profile model, the Cr ion
distribution in LiNbOs should have the form given by Eq.
(5). Using the set of analyzed samples (Table I) we found the
parameters Csurfand d that give the best fit of the experimental values to this function.
Figure 3 shows the plot of chromium surface concentration versus diffusion time. The lines in this figure represent
the theoretical behavior from mass conservation.
The diffusion depth, similarly found by fitting gaussian
functions to the experimental data as obtained by SIMS, is
plotted against the square root of the diffusion time in Fig. 4.
From the slopes of the straight lines in Fig. 4 we calculated
the diffusion coefficients for three temperatures, Table II.
Using these values, an Arrhenius plot was made, Fig. 5,
from which we calculated the diffusion constant Do and the
activation energy Q. We obtained values for these constants
of 4.7X 10” ,um2 h-t and 2.95 eV, respectively. The activation energy is approximately 8.5% larger than we have estimated previously2’ and the diffusion constant is one order of
magnitude larger. The present values result from additional
data as presented in this paper. Nevertheless, it is well known
that the estimated diffusion constant value in the Arrhenius
law depends very strongly on the precision of the diffusion
coefficient versus the inverse of temperature curve.
TABLE II. Calculated diffusion coefficients for three diffusion temperatures.
WC)
950
PIG. 3. Plot of chromium surface concentrationvs diffusion time, plotted
using the values obtainedfrom gaussianfits to the experimentalpoints. The
lines representthe theoretical behavior from mass conservation.
J. Appl. Phys., Vol. 78, No. 4, 15 August 1995
1.0
dt
W-N
PIG. 2. Concentrationprofiles obtainedby SIMS for samplesdoped with 10
mn of Cr and diffused for 10 h at different temperatures.
‘s
.l=
ss
8
F
0.0
1000
1050
DgLrn’ h-‘)
0.033
0.098
0.273
Almeida et a/.
2195
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d
900
1150
Wave1 engt h (nm)
i
7.6
a.0
7.8
1 +
(XI o-~)
a.2
a.4
K’)
FIG. 5. Arrhenius plot made using the values presentedin Table IL
FIG. 7. Waveguidefluorescencespectrumfor a sampledopedwith 10 um of
Cr and diffused at 1050“C for 4 h.
time resolution of 100 ns. In all cases and at room temperature, the value obtained was 900flOO ns. These results are
in close agreement with published values.”
C. Guided wave fluorescence
The waveguide fluorescence has been measured using
the setup shown in Fig. 6.
A sample was coated with 10 nm of Cr and diffused at
1050 “C for 4 h, then a channel waveguide was fabricated by
proton-exchange (in pure benzoic acid at 220 “C for 4 h followed by annealing at 400 “C! for 2 h in a dry oxygen atmosphere), using a 8 pm window mask width. The corresponding fluorescence spectrum is shown in Fig. 7. For
comparison, the fluorescence spectra of Cr bulk-doped z-cut
LiNbOs (Cr concentration: 3.2~10~’ cme3, supplied by
Union Carbide) and of waveguides fabricated by Tiindiffnsion and proton exchange in the same material were
also measured, and all presented similar features. The pump,
in all cases, was the 488 nm line from an argon-ion laser; the
polarization of the pump beam was parallel to the c-axis of
the crystal for all the samples measured.
The lifetime of the upper laser level has. been measured
on the same samples, using a fast mechanical chopper and a
laser beam focused in the plane of the,.chopper blade, together with a fast amplified silicon detector, Fig. 6, giving a
D. Optical amplification
A simple model of laser amplification was applied to
estimate the behavior of Cr:LiNbO? channel waveguides in
both bulk- and diffusion-doped structures, using the parameters measured as above and reasonable estimates for the
scattering losses and waveguide modal profiles for pump and
signal.
The rate equations for a four-level laser system in the
steady state were used, as in Ref. 21, for the cases of bulkand locally doped amplifiers.
For the waveguide bulk-doped amplifier a Cr concentration of 1.OX1O2o cmm3 was used, and for the waveguide
locally doped amplifier a crystal doped with 10 run of Cr and
diffused at 1050 “C for 4 h was assumed. For both
waveguides we consider scattering losses on the order of 0.4
dB/cm, an effective pump area of 15 pm* and a lifetime of 1
,YS (room-temperature operation). We also considered a signal wavelength of 910 nm (peak of the fluorescence intensity) and a pump wavelength of 670 nm (close to the peak of
the absorption spectrum, which is shown in Fig. 8, and compatible with laser diode pumping). For these wavelengths the
signal emission and pump absorption cross sections are, respectively, 1.15X10-t* and 1.43X1O-2o cm*.”
Argon Laser488 ““I
i
-2
$i/ 10 -
Ed
Spectrum
Analyzer
FIG. 6. Experimental setup for measurementof fluorescencespectrumand
lifetime.
2196
J. Appl. Phys., Vol. 78, No. 4, 15 August 1995
I
0
400
I
I
650
Wavelength
I
I
L-L-_-
900
(nm)
FIG. 8. Chromium bulk-dopedLiNbO:, absorptionspectrum (Cr concentration 3.2X 1020cm-.3).
Almeida et al.
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It was found that, fdr a launched pump power of 10 mW,
theoretical maximum gains of 0.18 and 0.33 were obtained
for waveguide lengths around 8 and 6 mm, in bulk and locally doped amplifiers, respkctively (the gain factor being
defined as the natural logarithm of the amplitude of the signal power as a function of the length).
IV. CONCLUSIONS
A study of Cr diffusion in LiNbO, crystal has been done
as a function Of parameters like temperature and diffusion
time. SIMS measurements were performed in order to obtain
detailed information on concentration depth profiles.
The chromium diffusion process has been analyzed in
terms of the classical mathematical diffusion model. Consequently, diffusion constan@ and activation energies have
been derived. The strongest factor which controls the diffusion process is found to be the temperature.
The fluorescence behavior of the diffusion-doped
Cr:LiNbO? has been compared with that of bulk-doped
Cr:LiNb03, and found to be similar in practically all respects.
The possibilities of designing and fabricating channel
waveguide amplifiers based on chromium-doped LiNbO, has
now been established-with major advantages being apparent for diffusion-doped Cr:LiNb03. Further studies of Cr
dimsion in LiNb03 for the realization of channel waveguide
amplifiers are in progress.
ACKNOWLEDGMENTS
A. P. Leite and J. M. Almeida acknowledge the financial
support of JNICT under contract STRDA/C/TIT/l19/92 and
access to laboratory facilities at the Optoelectronics Centre,
INESC-Porto.
J. Appl. Phys., Vol. 78, No. 4, 15 August 199.5
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