Continuously tunable delay line based on SOI

tapered Bragg gratings

Ivano Giuntoni,1,* David Stolarek,2 Dimitar I. Kroushkov,1 Jürgen Bruns,1
Lars Zimmermann,1,2 Bernd Tillack,1,2 and Klaus Petermann1
1
Technische Universität Berlin, Fachgebiet Hochfrequenztechnik, Einsteinufer 25, 10587 Berlin, Germany
2
IHP, Im Technologiepark 25, D-15236 Frankfurt (Oder), Germany
*ivano.giuntoni@tu-berlin.de

Abstract: The realization of an integrated delay line using tapered Bragg

gratings in a drop-filter configuration is presented. The device is fabricated
on silicon-on-insulator (SOI) rib waveguides using a Deep-UV 248 nm
lithography. The continuous delay tunability is achieved using the thermo-
optical effect, showing experimentally that a tuning range of 450 ps can be
obtained with a tuning coefficient of −51 ps/°C. Furthermore the system
performance is considered, showing that an operation at a bit rate of 25
Gbit/s can be achieved, and could be extended to 80 Gbit/s with the addition
of a proper dispersion compensation.
©2012 Optical Society of America
OCIS codes: (130.3120) Integrated optics devices; (130.7408) Wavelength filtering devices;
(200.4490) Optical buffers.

References and links

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#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11241
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1. Introduction
Optical delay lines constitute an important component for several applications in optical
signal processing. In the field of optical communications they are required for bit
synchronization for multiplexing and interleaving [1], equalization and dispersion
compensation [2] and data buffering in optical switches [3]. Other possible applications
concern the optical beam forming in phased-array antennas [4] or optical coherence
tomography [5]. In all cases a continuous tunability is required, together with a wide
bandwidth and a large tunability range. Furthermore an amplitude and phase preservation of
the signal is highly desirable to avoid distortion.
Different approaches for the implementation of optical tunable delay lines were proposed,
mostly based on fiber Bragg gratings [6,7]. However the realization of integrated devices on
silicon is an important step for the realization of more complex integrated systems for fast
optical signal processing. Integrated delay lines have been realized making use of all-pass
filters (APF) [8] or coupled resonator optical waveguides (CROW) [9,10], both based on ring
resonators. Slow light effects in photonic crystal waveguides were used as well for the aim
[10–12]. Delays larger than 100 ps could be achieved, however with relatively high losses and
limited bandwidth, limiting the potential application field. Recently integrated delay lines
based on Bragg gratings on rib waveguides have been proposed [13], with combination of a p-
i-n junction to perform the tuning via free-carrier plasma effect. This solution would provide a
high tuning range with much lower losses, however not yet experimentally proved.
In this paper we demonstrate the realization of optical delay lines based on tapered Bragg
gratings in a drop filter configuration, which allows in-line operation without the necessity of
an external circulator to out-couple the delayed signal. The tuning is obtained exploiting the
thermo-optical effect in silicon.
2. Principle
In a chirped Bragg grating different wavelengths are reflected at different positions along the
grating length, with a consequent delay difference between them. The wavelength reflected at
the end of the grating will exhibit a delay difference of ∆τ = 2nLg/c with respect to the
wavelength reflected at the beginning of the grating, where Lg is the overall grating length, n
the waveguides effective index and c the speed of light in vacuum.
Considering the wavelength of the input signal fixed and within the reflection bandwidth
of the grating, the added delay is then τ = 2nz1/c, where z1 is the position where the reflection
takes place. If the grating is properly uniformly perturbated in a way to slightly shift the
reflection bandwidth, the unchanged wavelength of the input signal will be reflected at a
different position z2, corresponding to a different added delay. With the perturbation a delay
difference of ∆τ = 2n(z1 – z2)/c is achieved.
It was shown that the thermo-optical effect produces a shift of 80 pm/°C of the reflection
bandwidth of a Bragg grating [14,15]. Our idea is therefore to heat the Bragg grating
uniformly and hence vary the position where the signal is reflected within the grating and
consequently the added delay.
To implement the chirp in silicon rib waveguides a waveguide tapering was proposed
[16,17], linearly varying the rib width along the grating (see Fig. 1(a)). A linear variation of
the effective index and hence of the reflected wavelength is produced. This solution is more
robust than varying the grating period [15,17]. To out-couple the reflected signal without an
optical circulator, whose integration is not trivial, we inserted the tapered grating in the drop-
filter structure described in [17] (see Fig. 2(b)). It allows to separate the delayed signal and

#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11242
forward it to a separate output waveguide. Furthermore the use of two grating pairs G1 and G2
permits to double the achievable delay.

Fig. 1. (a) Uniform Bragg grating on a tapered rib waveguide to achieve a chirp. (b) Scheme of
the integrated Drop-filter.

3. Fabrication and characterization

The proposed device was manufactured using a double lithographic process based on Deep-
UV 248 nm lithography, a planar technology which proved to be able to reliably pattern
stitching-free gratings with a length of several millimeters and very high performances [18].
First the gratings were realized with a double patterning lithography on the plain substrate,
subsequently the rib waveguides were added with a second lithographic step. Waveguides
were fabricated using a SOI substrate with a silicon guiding layer thickness of 1.4 µm on a 1
µm thick buried oxide. They were designed with a rib width of 1.5 µm and a rib height of 0.5
µm to assure single mode operation and linear loss around 0.2 dB/cm. All the gratings exhibit
a period of 224 nm and an etching depth of 50 nm. A taper of ∆w = 150 nm over a grating
length of 1 cm was chosen to implement the chirp. The high overlay precision of the DUV
exposure and the full control over the etching depth allow to precisely determine insertion
loss, bandwidth and grating strength [15].
The characterization of the fabricated samples was performed with a modulation phase
shift method. A polarization controller allowed setting the polarization of the propagating
light, and an efficient in- and out-coupling was achieved using lensed fibers. Facets were
covered with an anti-reflection coating to avoid undesired resonances. The device
transmissivity was calculated as the ratio between the power transmitted by the drop-filter
(port Out3, see Fig. 1(b)), and the one measured from a common waveguide, isolating the
grating performance from the waveguide characteristics [15]. Using a Peltier element and a
temperature controller, the temperature of the measured sample could be varied with a
precision of 0.1°C, allowing to tune the optical delay.
On Fig. 2 the transmission spectra and the respective group delay measured and the output
of the drop filter (reflection of the cascaded grating pairs) for different temperatures between
32°C and 41°C are shown. At the starting temperature of 32°C a bandwidth of 1.2 nm
centered at λ = 1535.4 nm was measured, with an insertion loss varying from 1.7 dB at the
short-wavelength side and 6 dB at the long-wavelength side of the transmission bandwidth,
corresponding to a reflection at the end of the grating. Over this bandwidth a linear increase of
the group delay of 500 ps was measured, corresponding to 250 ps for each grating pair. A
noticeable group delay ripple can be observed and is due to the absence of a grating
apodization.

#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11243
 Fig. 2. Transmission spectra and group delay of the drop-filter with tapered gratings at
different temperatures between 32°C and 41°C. TE polarization is considered. The dashed line
indicates the position of the signal wavelength λs = 1535.8 nm.

As expected, the increase of the device temperature produces a shift of 80 pm/°C of the
Bragg wavelength to larger values, keeping the spectrum shape unchanged. If the wavelength
of the input signal is placed in the middle region where the transmission is kept high for all
temperatures, the delay applied to it can be tuned. Choosing a wavelength within this range,
e.g. λs = 1535.8 nm, the signal delay can be varied between almost 0 and 440 ps (see Fig. 3).
The tuning is linear within the temperature range between 32°C and 38°C, with a tuning
coefficient of −51 ps/°C. For larger temperatures the effect of the group delay ripple
predominates producing a sharper decrease of the group delay. Since the grating reflection
spectrum is not perfectly flat, the signal experienced different losses at different temperatures,
varying between 1.7 and 6 dB.

Fig. 3. Variation of the added delay (red curve) and the inserted loss (blue curve) at different
temperatures. The signal wavelength is λs = 1535.8 nm.

4. System performances
To evaluate the performance of the measured tunable delay line in real systems, different
emulations with the VPItransmissionMaker simulation software were performed using the
experimental data previously shown. Two limitations for data stream transmission are
intrinsic in the device. First of all, the chirped gratings introduce a chromatic dispersion on the
signal of 500 ps/nm. Furthermore, as previously discussed, the larger the tuning rage of the

#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11244
delay, the larger the required temperature variation, with a consequent larger shift of the
grating spectrum and a reduction of the usable bandwidth. Both phenomena affect the signal
bandwidth and hence the data rate which can be transmitted without errors.

Fig. 4. Scheme of the used setup for the system emulation.

On Fig. 4 the virtual setup used to estimate the system performance of the device is
shown. The input signal consists on a QPSK modulated signal with a stream of 1018 random
symbols. After propagating along a standard fiber span (80 km @ 17 ps/nm·km), a dispersion
compensating fiber and an EDFA, the signal is coupled into the tunable delay line (TDL). To
estimate the effect of the dispersion, the delayed signal is then coupled to a fictitious drop
filter (CDC), analogous to the one described, but with reversed chirp in order to compensate
the added dispersion. This filter is obtained by mirroring and properly shifting the measured
spectra achieving a transmission bandwidth centered around λs and a dispersion of −500
ps/nm. The filter is not tuned, in order to add always a fixed extra delay of 270 ps.
Figure 5(a) shows the bit error rate (BER) for different data rates with and without
dispersion compensation. In the former case it can be observed that the dispersion added by
the tunable delay line strong affects the quality of the signal, leading to a BER larger than 10−3
for data rates beyond 25 Gbit/s. The performance can be strongly improved if the dispersion
compensation is included. In this case a safe data transmission until 80 Gbit/s can be
achieved.

Fig. 5. (a) Bit error rate (BER) at different data rates with and without dispersion
compensation. (b) Signal shape of a 10 Gbit/s QPSK-NRZ bit sequence at the output of the
transmitter (TX) and after propagating through the delay line at different temperatures with
dispersion compensation.

Finally the transmission of a bit sequence at a data rate of 10 Gbit/s was emulated, to
verify the delay tuning. On Fig. 5(b) the output signal for three different tuning temperatures
is shown. At 40°C, corresponding to a delay close to zero (see Fig. 3), only the delay of 270
ps added by the dispersion compensator filter can be seen. Decreasing the temperature to

#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11245
36°C and 32°C the signal is delayed of 197 ps and 417 ps respectively. The dispersion
experienced by the signal is not equal at all temperatures, leading to a slight distortion of the
signal shape, which anyway does not compromise a good detection.
Table 1. Comparison with the state of the art silicon based tunable delay lines
(from [13])a

Device type Optical loss (dB/ns) Delay (ps) Data rate (Gbit/s) Delay·Bandwidth
APF [8] 43 510 5 2.55
CROW [8,10] 45 [8] 220 [8] 4 [8] 0.88 [8]
60 [10] 80 [10] 100 [10] 8 [10]
PhC waveguide [10] 35 82 100 8.2
Gratings with p-i-n (*) 3.3 660 20 13.2
[13]
This work 9.6 6.5–450 25(80) 11.25 (36)
a
The data in parenthesis about this work refer to the system with dispersion compensation. (*) Only theoretical
analysis, without experimental verification so far.

5. Conclusions
We presented the realization of integrated delay lines based on tapered Bragg gratings in SOI
rib waveguides. The delay can be furthermore continuously tuned varying the operating
temperature, exploiting the thermo-optical effect. A comparison of the achieved device
performances with the state of the art of integrated delay lines is shown on Table 1. The
device demonstrated in this work exhibits a large tuning range (until 450 ps) comparable to
the one achievable with APFs [8] or in chirped gratings tuned with free-carrier plasma effect
[13], but with a lower insertion loss and the possibility to operate at larger data rates, until 25
Gbit/s. We demonstrated also that the maximum affordable data rate can be increased up to 80
Gbit/s adding an analogous drop-filter with reversed grating taper for the dispersion
compensation.
Acknowledgments
The financial support of the German Research Foundation (DFG) in the frame of the research
group FOR653 is gratefully acknowledged.

#163806 - $15.00 USD Received 28 Feb 2012; revised 20 Apr 2012; accepted 23 Apr 2012; published 1 May 2012
(C) 2012 OSA 7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 11246