Waveguide mode filters fabricated using
laser-induced forward transfer
K.S.Kaur1,*, A.Z. Subramanian1, Y.J.Ying1, D.P.Banks1, M.Feinaeugle1, P. Horak1,
V. Apostolopoulos2, C.L.Sones1, S. Mailis1, R.W.Eason1
1Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK.
2School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
Abstract: Titanium (Ti)-in-diffused lithium niobate waveguide mode filters
fabricated using laser-induced forward transfer followed by thermal
diffusion are presented. The mode control was achieved by adjusting the
separation between adjacent Ti segments thus varying the average value of
the refractive index along the length of the in-diffused channel waveguides.
The fabrication details, loss measurements and near-field optical
characterization of the mode filters are presented. Modeling results
regarding the device performance are also discussed.
©2010 Optical Society of America
OCIS codes: (140.3390) Laser materials processing; (140.7090) Ultrafast lasers;
(230.7370) Waveguides; Laser-Induced Forward Transfer.
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Lithium Niobate (LN) is a very important optical material which is widely used by the
photonics industry mainly due to its excellent electro/acousto-optical properties . Channel
waveguides, which are at the heart of any photonics circuit, are fabricated in LN by doping it
with titanium (Ti) metal using the in-diffusion technique where Ti is locally deposited on a
LN substrate using photolithography and lift off methods followed by thermal treatments [2,
3]. This method is suitable for mass production of devices and is compatible with the wafer-
scale parallel techniques used in the microelectronics industry. However, for cases that require
rapid prototyping of devices more flexible and faster techniques are needed.
In our recent work the laser-induced forward transfer (LIFT) technique was used to
deposit segmented Ti metal lines onto LN substrates which produced low loss optical
waveguides after thermal diffusion . LIFT is a direct-write technique first demonstrated by
Bohandy et al [5, 6] for depositing metals to repair damaged photomasks. Due to its simplicity
and flexibility its use was quickly extended to print a variety of metals, semiconductors,
dielectrics, ceramics and biomaterials [7-11]. This new approach offers both the flexibility,
less stringent experimental conditions and rapid prototyping associated with LIFT, and the
large refractive index change, low optical loss and stability of the waveguides associated with
the thermal diffusion method. Additionally, printing of multiple diffusion sources in a single
shot, from specifically tailored donors and deposition on non-planar substrates are the other
advantages offered by this technique. In this paper, we present results of fabrication and
optical characterisation of index-tapered waveguides produced by this method by adjusting
the density of adjacent titanium segments during the LIFT process that defines the average
refractive index which light experiences in different sections of the waveguide. We
demonstrate here the use of such a refractive index tapered waveguide as a mode filter thereby
taking this technique into a new realm of fabricating custom-made complex refractive index
profiles and photonic devices.
The fabrication of tapered waveguides using LIFT was a two-step process. Firstly, a linear
arrangement of Ti metal segments was printed on top of LN substrates using the LIFT
technique as shown in fig. 1. The LIFT samples were prepared by depositing thin films (~ 150
nm) of Ti (the donor) on top of transparent glass substrates (the carrier) by e-beam
evaporation. Femtosecond pulses with a Gaussian spatial profile from a mode-locked Ti:
sapphire laser (800 nm, 150 fs, FWHM ~ 4 mm) were centred on a 450 µm diameter circular
aperture resulting in a reasonably spatially uniform incident pulse profile. A highly de-
magnified image of the aperture was then relayed onto the carrier-donor interface using a
commercial micromachining workstation (New Wave UP266, USA) thereby printing circular
Ti discs of diameter ~10 µm (comparable to the incident laser spot size) onto a congruent
undoped z-cut LN substrate on the –z face along the crystallographic y-direction. The
separation between the carrier and the donor was maintained at ~ 1 µm separation using a
Mylar spacer. The donor-receiver assembly was mounted on a 3-axis precision (10 nm
resolution), fast (max. speed ~ 250 mm/s) computer-controlled translation stage to allow the
carrier-donor complex to be scanned in front of the incoming laser pulses. All experiments
were performed under a background pressure of 10-1 mbar. Single laser pulses were used to
print each Ti dot and the laser was operated at 250 Hz. The laser threshold fluence for transfer
of Ti discs was ~ 0.4 J/cm2. The separation between two adjacent Ti dots was controlled by
varying the scan speed of the translation stages. After printing, the deposited metal lines were
diffused into the LN crystal by heating it to 1050oC in an oxygen atmosphere for 10 hours.
Index tapered waveguides were produced by varying the writing speed along the
segmented Ti lines at a constant acceleration. The reason for writing lines at constant
acceleration was to avoid sudden changes in the final refractive index profiles to minimize
losses and obtain a smooth mode filter effect from the tapered waveguides. The idea behind
this technique is to have the ability to alter the behaviour of the waveguides just by
manipulating the scan speed and hence the segment separation along the length of the
waveguides. The amount of material deposited and then diffused per unit length decreases
with increasing scan speed, which in turn decreases the average effective index of the mode.
The mode confinement should therefore decrease with increasing speed and the waveguide
modal behavior should change from multi-mode to single-mode. By increasing the segment
separation and hence decreasing the index contrast along the waveguide a mode filter can be
produced that allows only the fundamental mode to propagate. In the present set of
experiments three different values of constant acceleration of 0.3, 0.4 and 0.5 mm/s2 were
used for fabricating the tapered waveguides. The initial velocity was kept constant at 2.5 mm/s
for all the tapers. Uniform waveguides were also fabricated for comparison by depositing Ti
segments with a constant separation (10 µm between the centres of adjacent segments)
obtained by scanning the sample with a constant velocity of 2.5 mm/s (fig. 1). The samples
were then end-polished for optical characterisation and loss measurements.
Fig. 1. Schematic of the LIFT technique for printing segmented Ti lines onto LN substrates. The exaggerated
version of how the Ti dots separate out by increasing speed from one end (port 1) of the substrate to the other (port 2)
for fabrication of a tapered waveguide along with constant velocity lines for comparison are also shown.
The waveguide losses at 1550 nm were measured using the fibre mismatch technique
. Light from a tuneable laser (1500-1600 nm) was launched into the waveguides using a
single mode fibre (SMF) and the output was first collected using a similar SMF and then with
a multimode fibre (MMF). The difference in the collection efficiency gave the coupling loss
of 6 dB for the SMF. In this technique it is assumed that the MMF collects all the output light
from the waveguide. The propagation loss was then calculated by taking the difference
between the insertion loss (total loss due to the waveguide) and the coupling loss. The
insertion loss of the waveguides was measured to be 11 dB resulting in a propagation loss of ~
3.1 dB/cm, (the waveguide length was 16 mm). The variation in the velocity along the
waveguide length leads to higher values of optical loss in tapered waveguides than the
previously reported constant velocity waveguides . In comparison, overall optical losses of
1.6 dB and 1.05 dB have been reported for mode filters with lengths of 0.6 mm and 1.6 mm
respectively prepared using soft proton exchange (SPE) method .
The optical characterization of the waveguides was performed using the set-up outlined in
fig. 2. Light from a tuneable fiberised laser (1500-1600 nm, TM polarization) was coupled
into the waveguides from port 1 using an objective lens (40 x). The output was collected using
another objective lens (40 x) and the mode profiles were observed using an IR camera. All
these measurements were performed at 1550 nm. The waveguide samples and objective lenses
were mounted on a 3-axis (x-y-z) translational stage.
Fig. 2. Experimental set-up used for optically characterizing the waveguides
Figure 3(b) shows optical mode profiles of segmented waveguides corresponding to Ti
deposition at constant velocity (2.5 mm/s). The waveguides supported two modes (TM00 and
TM01). Figure 3 (d), (e) and (f) show the optical mode profiles of index tapered waveguides
corresponding to Ti deposition with constant acceleration of 0.3, 0.4 and 0.5 mm/s2
respectively with an initial velocity of 2.5 mm/s when light was launched from port 1 (fig. 2).
The images clearly show that the tapers supported only the fundamental mode (TM00) thereby
exhibiting the mode filtering operation. Similar results were obtained even when the coupling
conditions were altered by moving the waveguides with respect to the input beam in the
transverse direction to excite higher order modes indicating that the port 2 of the tapered
waveguides could support only the fundamental mode. When the acceleration value was
increased beyond 0.5 mm/s2 the waveguides ceased to guide altogether due to the waveguide
reaching its cut-off value. Another important feature to be noticed in these images is that as
the writing speed/acceleration was increased the mode size increased as well, as a direct
consequence of the decrease in the index contrast with increasing segment separation that lead
to a broader and less tightly confined mode. The Gaussian function fit mode field diameter
(MFD) values for mode profiles captured from tapers fabricated with 0.3, 0.4 and 0.5 mm/s2
acceleration respectively are presented in table 1. It clearly depicts the increase in the MFD
with increasing acceleration value.
Fig. 3. (b) Near field intensity profiles captured from a waveguide written with a constant velocity of 2.5 mm/s. (d-f):
near field intensity profiles of tapered waveguides written at accelerations of 0.3, 0.4 and 0.5 mm/s2 respectively
when the light was launched from port 1. (a) and (c): near field intensity profiles corresponding to the waveguide
written with an acceleration of 0.3 mm/s2 when the light was launched from port 2.
When the laser light was launched from port 2 (fig. 2) a much better confined
fundamental mode was obtained on the higher index port 1 of the tapers (fig. 3 (a)) as
expected. The Gaussian fit MFD value for this mode is also presented in table 1. However
upon altering the launching angle a higher order mode was monitored at the same port as
shown in fig. 3 (c) for a waveguide written with an acceleration of 0.3 mm/s2. This behavior is
not expected for an adiabatic taper however the corrugation in the refractive index distribution
along the waveguides caused by the shape of the printed Ti dots is believed to be responsible
for this non-adiabatic behavior of the device. Modeling results, which will be discussed in the
next section, confirmed this observation.
Table 1. Gaussian Fit MFD Values for Mode Profiles Captured from Waveguides Written with Acceleration of
0.3, 0.4 and 0.5 mm/s2 respectively When the Light was Launched from Port 1 along with the MFD Value for
the Fundamental Mode on the Higher Index Port 1 for Tapered Waveguide Written with 0.3 mm/s2
Acceleration When Light was Launched from Port 2.
Waveguides written with constant acceleration of Gaussian fit MFD (µm)
0.3mm/s2 (light launched form the port 1)
0.4 mm/s2 (light launched form the port 1)
0.5 mm/s2 (light launched form the port 1)
0.3 mm/s2 (light launched form the port 2)
3. Theoretical modeling
To understand the non-adiabatic nature of the segmented tapers fabricated using LIFT the
light propagation both along segmented Ti:LN and continuous index tapers was modeled.
First a 3D model of Ti diffusion of LIFT-deposited dots at high temperature was developed
with dot separation varying from zero to 3 µm in steps of 0.5 µm. This Ti distribution was
then converted to the corresponding 3D refractive index profile using the method discussed in
. The zero and 3 µm separation values corresponded to the initial and final Ti dot
separation for the tapered waveguides written using 0.3 mm/s2 acceleration. The maximum
refractive index contrast values over this range of segment separation vary from 0.0406 to
0.0322. In the actual experiments the samples were ~ 16 mm long but theoretical modeling
was not possible for these lengths due to excessively large computer memory requirements.
The simulation of light propagation in the waveguides was therefore restricted to shorter
lengths (~ 700 µm) and qualitative results were obtained using Comsol multiphysics software.
The structure was built by drawing 7 sections each section containing 10 Ti discs with 0, 0.5,
1, 1.5, 2, 2.5 and 3 µm separations respectively. The corresponding refractive index profile
was calculated and is illustrated in fig. 4 (a) as variation of the intensity of the segments along
the waveguide. The structure which is visible in some of the segments in fig. 4 (a) is due to
limitations in the image generating capabilities of the software. Cross sections of the profiles
showed a smooth refractive index distribution. The light propagation pattern as shown in fig. 4
(b) was obtained using a Gaussian (TM00) distribution as an input to the port corresponding to
the 3 µm separation side of the waveguides. The results revealed that during propagation, part
of the TM00 mode gradually converted to TM01 mode at the zero separation port of the taper.
The intensity profiles obtained from the 3 µm and zero end of the tapered waveguide at the
positions marked by red lines are shown in fig. 5 (i) and 5 (ii) respectively.
Fig. 4. (a) Shows the refractive index profile for the segmented Ti:LN waveguide with the brighter regions
corresponding to higher index. (b) Shows the light propagation pattern when TM00 mode was launched from the 3 µm
end of the waveguide.
Fig. 5. Mode profiles obtained from the (i) 3 µm end and (ii) 0 µm end of the segmented Ti:LN waveguide. The
positions where the modes were captured are marked as red in fig. 4 (b).
The case of light propagation through a continuous Ti:LN tapered waveguide was
simulated using the beam propagation method (BPM) in the commercially available RSoft
Beamprop software, with the refractive index contrast values varying linearly from 0.0406 to
0.0322 throughout the length ( ~ 1 cm in this case) of the taper. The results depicted that the
mode size increases from the higher index port of the taper to the lower index port, as
expected for an adiabatic taper. The simulated mode profiles both for the higher and lower
index ports are presented in figs. 6 (a) and 6 (b) respectively. This confirms the dot-induced
non-adiabatic refractive index structure observed for the segmented Ti:LN waveguides.
Fig. 6. Simulated near field intensity profiles obtained from (a) the high index and (b) the low index port of a
continuous Ti:LN waveguide. The mode size increases as the refractive index contrast decreases along the length of
The control over the waveguide refractive index contrast obtained by adjusting the separation
between adjacent segments of Ti which were LIFT-deposited and diffused into LN substrates
has been used to fabricate a refractive index tapered waveguide device to be used as a mode
filter. The propagation losses of the tapers were measured to be ~ 3.1 dB/cm at 1550 nm using
the fibre mismatch technique. The mode profile pictures captured confirmed the mode
filtering action performed by the index tapered waveguides. The corrugations introduced in
the refractive index profile due to the segmented geometry of the deposits induce a non-
adiabatic behavior in the tapers and this was confirmed by the theoretical modeling results.
Financial support from the Engineering and Physical Sciences Research Council (EPSRC),
UK, (under Grant no. EP/C515668/1) and the European community (under Grant no.
508581101, e-LIFT) are gratefully acknowledged.