EpiPho
otonics

December, 2011

Na
ano-Seccond Speeed PLZ
ZT Waveeguide S
Switchess and Filtters
Keeiichi Nashim
moto
EpiPhotonicss Corp, 770 C
E
Charcot Ave,, San Jose, CCA 95131, USSA
EpiPhootonics Corp, 4-1-9-508 Chuo,C
Yamatoo-shi, Kanagawa 242-002
21, Japan
ed optical sw
witches and fifilters fabricatted in epitaxiially grown (PPb,La)(Zr,La))O3 (PLZT)
Nano--second speewaveguiides are intro
oduced. The optical switcches includingg 1x2, 1x4, 11x8, 1x16, 2xx2, and 4x4 ports haveg time less tthan 10 ns anda the AWG
G based 8x1 wavelength selective filtter has 15 nss switchingswitchingtime. A PLZT buriedd waveguide
e is successffully develop
ped to minim
mize insertionn loss and ppolarizationence in the op
ptical switchees.
dependeI. INTRODUCTTION
There
e have beenn increasing
g interests in high-spee
ed switchingg and filterinng devices for packet
based ooptical netwo
orks in teleccommunicatio
ons and datta centers. H
High-speed switching annd filtering
device ttechnologiess include seemiconductoors [1] and LiNbO
Lsy to meet
3 [2]. However, itt is not eas
various requirementts such as reesponse spe
eed, power consumptionn, polarizatio
on independ
dence, andn loss.
insertion
We have developped (Pb,La)(Zr,Ti)O3 (PLZT) [3] wa
aveguide devvices [4,5] b
by unique solid-phasen efficient an
nd polarization insensitivve electroepitaxy (SPE) [6] motivated by the fact thatt PLZT is an
optic ma
aterial althou
ugh it has b
been very difficult to gro
ow high-quality PLZT op
ptical waveg
guides andfabricate
e channel w
waveguides. The epitaxia
al PLZT wavveguide growwn on a sem
miconductorr substrateis also attractive fo
or further minimizing drriving voltages. We rep
port nano-se
econd speedd MZ type
path switche
es and a A
AWG based wavelength
h selective ffilter using eepitaxial PLZT optical
optical p
waveguiides grown on Nb-doped SrTiO3 (NST)(semiconductor wwafer. Thosee high-spee
ed devices
have exxcellent pe
erformances in terms of low-drivving voltage
e, low-poweer consump
ption, lowpolarizattion indepen
ndence, andd low insertio
on loss suitaable not onlyy for optical packet/bursst switched
networkss but also foor existing op
ptical networrks.
II. PLZT WAVEGUIDE FABRICATION
NThe SPES
processs for PLZT thin-film is ccomposed oof a precursor solution ddeposition stepsand arapid theermal annea
aling step. A PLZT botttom clad layyer and a P
PLZT core laayer are gro
own by theSPE onn a NST wafer.
wA 3 μm-wide chhannelwaveguiide is formedd by an ICP etching process. Aridge shhape channel waveguidde is fabricated inthis wayy. A buried channel wavveguide req
quires aPLZT toop clad layeer. The coree layer has higher
index thhan the cladd layers to cconfine the light inthe coree layer. The clad layeers prevent opticalabsorption by the NST and ttop electroddes. Anp electrode layer with an Au
indium-ttin oxide top
contact layer is fab
bricated on the surface
e of thehannel wave
eguides usinng sputteringg and a
PLZT chlift-off prrocess. A ccross-section
nal structure
e of thePLZT bu
uried waveg
guide is shoown in Fig. 1. The
top elecctrodes are cconnected too bond padss by Auinterconnects. Edgee facets are
e optically p
polished
after diccing.
Fig. 1. PLZT buried w
waveguide structure.
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EpiPhotonics

December, 2011

III. PLZT SWITCHES
A building block for PLZT optical switches is a modified balance bridge type 1x2 switch. Thebalance bridge type switch is composed of a MZ modulator with top electrodes and input and output
3dB couplers as shown in Fig. 2. An input optical signal is delivered to the cross port and bar port
under no voltage application (3 dB splitting state). By applying a voltage to one of top electrodes (Ais on or B is on), the signal is promptly switched to the cross port or the bar port due to the resulting
electro-optic index change.
AIN

Bar

MZ Modulator
3 dB Coupler

Cross

3 dB Coupler

BFig. 2. 1x2 MZ optical switch element.
Fig. 3 shows a typical switching curve at 1550 nm wavelength in the buried waveguide type 1x2switch as a function of voltage application to the electrode B. The insertion loss, the PDL, and thecrosstalk/extinction ratio are 3 dB, 0.3 dB, and 27 dB at 7.5V, respectively. A typical crosstalk in amatrix 2x2 switch consisted of four 1x2 switch elements is better than 35 dB due to a two stage
switch architecture. Switching rise and fall time of the 1x2 switch element is about 3 ns as shown inFig. 4.
The 1xN switch has a simple tree structure as shown in Fig. 5 so that the introduced optical signal
is delivered to one of N output ports. A 1x8 switch chip has a length of about 17 mm and typicalinsertion loss is 5 dB. A 4x4 switch has a strictly non-blocking tree structure as shown in Fig. 6. An
optical signal from any input port is delivered to any output port without disturbing other connections.
The tree structure 4x4 has two stage switch nature as well as the 2x2 switch. The first stage
switches and the second stage switches work as delivery switches and the third stage switches andthe fourth stage switches work as gate switches so that cross talk can be minimized. A 4x4 switch
chip has a length of about 22 mm and typical insertion loss is 6 dB. Fig. 7 shows the 4x4 PLZTswitch module with a high-speed driving circuit. The driving circuit gives 10V at around 5 ns speed tothe twenty four 1x2 switch elements in the 4x4 switch. The switching time is less than 10 ns.
8.E-05

2.4 ns‐15

7.E-05‐20

6.E-05
Amplitude

Output (dBm)‐10‐25‐30
P1/Bar TE
P2/Cross TEP1/Bar TM
P2/Cross TM

‐35
‐40

5.E-05
4.E-05
3.E-05
2.E-05
1.E-050.E+00‐450510015

12345678910Time (ns)

Voltage (V)Fig. 3. Switching curve in the 1x2 PLZT switch.

Fig. 4. Switching rise and fall response.

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December, 2011

Fig. 5. Tre
ee structure 1x8 switch.

Fig. 6. Non--blocking 4xx4 switch stru
ucture.

A nettwork basedd on the na
ano-second PLZT
switch can switch frequentlyy to allocatte thebandwid
dth efficiently. That is because thee highswitching speeds allow gua
ard time tto beminimize
ed. On the other hand, a network basedon a coonventional milli-secondd speed sw
witch isead isnot efficcient because its switcching overhe
very largge as compa
ared with daata size. The
erefore,this drasstic reductio
on in guard time makes datatransfer very efficie
ent and rea
alistic in ne
etworks
includingg optical paccket switchinng [7], optica
al burst
switching, and time division slott switching.

Fig. 7. 4x4 PLZT optica
al switch subbsystem.

IV. PLZT WAV
VELENGTH SE
ELECTIVE FIL
LTERS
A PLZ
ZT waveleng
gth selectivee filter is bassed on an 8-ch,8500 GHHz or 200 GGHz-spacingg, AWG as
shown in Fig. 8. Wh
hen the control voltage is applied too electrodess located onn arrayed wa
aveguides,
ath differencce is createdd between th
he adjacent arrayed-wavveguides, annd the shift
an addittional light pa
of a foccal point willl occur. The phase shhift of 2π in each waveeguide is suufficient to selectsanywavelenngth from thee certain outtput waveguide.

Fig. 8. Wa
avelength seelective filterr.

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December, 2011

The 3-dB bandwidth is 2.3 nm in the ridge waveguide filter. The adjacent crosstalk isapproximately -18.5 dB. Fig. 9 shows the transmission spectra for the output port 4 when thevoltages were applied to the electrodes. When the voltage approaches 22V, the phase shift due tothe voltage become π and the complete wavelength tuning from 1559 nm to 1543 nm is achieved.
The maximum tuning range is 4,000 GHz (approximately 32 nm around 1.55 um) since the channel
spacing is 500 GHz and the number of channels is 8. The tuning speed of about 15 ns wasconfirmed at the peak-to-peak driving voltage of 14.0 V.
50

Loss [dBm]4030
No voltage
Va = 15V
Va = 19V
Va = 22V

20

10
1525

1535

1545

1555

1565

1575

1585

Wavelength [nm]Fig. 9. Transmission spectra in the wavelength selective filter.

V. SUMMARY
High-speed switches and filters using PLZT waveguides are introduced. MZ type switches
including 1x1, 1x2, 1x4, 1x8, 1x16, 2x2, and 4x4 ports show the switching speed less than 10 ns. An
AWG type wavelength selective filter showed the switching/filtering speed of 15 ns. The high-speed
PLZT switches and filters will be of key enabler for optical packet/burst based networks.

ACKNOWLEDGMENT
The author is very thankful for the collaborative work on AWG devices with Prof. H. Tsuda at Keio
University.

REFERENCES[1] I. M. Soganci, T. Tanemura, K. A. Williams, N. Calabretta, T. d. Vries, E. Smalbrugge, M. K. Smit, H.
J. S. Dorren, and T. Nakano, IEEE Photon. Technol. Lett, Vol. 22, pp. 143-145, 2010.
[2] R. Krähenbühl, M. M. Howerton, J. Dubinger, and A. S. Greenblatt, J. Lightwave Tech., vol. 20, pp.
92-99, 2002.
[3] G. H. Haertling and C. E. Land, J. Am. Ceram. Soc., vol. 54, pp. 1-11, 1971.
[4] K. Nashimoto, N. Tanaka, M. LaBuda, D. Ritums, J. Dawley, M. Raj, D. Kudzuma, and T. Vo,BroadNets 2005, The Fifth International Workshop on Optical Burst/Packet Switching, pp. 195-200,2005.
[5] J. Ito, M. Yasumoto, K. Nashimoto, H. Tsuda, IEICE Trans. Electron., Vol.E92-C, No.5, pp. 713-718,
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[6] K. Nashimoto, S. Nakamura, H. Moriyama, M. Watanabe, and E. Osakabe, Appl. Phys. Lett., vol. 73,pp. 303-305, 1998.
[7] N. Wada, N. Kataoka, T. Makino, N. Takezawa, K. Nashimoto, T. Miyazaki, in Proc. ECOC 2007, vol.
6, PD.3.1, 2007.
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