All-Optical Analog-to-Digital Conversion Using Inherent Multiwavelength Phase Shift in LiNbO Phase Modulator

Abstract

All-optical analog-to-digital conversion utilizing inherent multiwavelength phase shift in lithium niobate phase modulator is proposed. In the experimental demonstration, a wavelength-tunable continuous-wave laser diode and a lithium niobate phase modulator are used to quantize the sinusoidal tone electrical analog signal. Using 16 different wavelengths, an effective number of bits of 4.3-bit has been obtained after software sampling measurement. Benefits of the presented approach in this letter are its simple realization of the phase shift and high stability.

Full text

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1036IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 12, JUNE 15, 2008
All-Optical Analog-to-Digital Conversion Using
Inherent Multiwavelength Phase Shift in LiNbO?
Phase Modulator
Qingwei Wu, Hongming Zhang, Minyu Yao, and Wei Zhou
Abstract—All-optical
inherent multiwavelength phase shift in lithium niobate phase
modulator is proposed. In the experimental demonstration, a
wavelength-tunable continuous-wave laser diode and a lithium
niobate phase modulator are used to quantize the sinusoidal
tone electrical analog signal. Using 16 different wavelengths, an
effective number of bits of 4.3-bit has been obtained after software
sampling measurement. Benefits of the presented approach in this
letter are its simple realization of the phase shift and high stability.
analog-to-digital conversionutilizing
Index Terms—Analog-to-digital conversion, phase modulation,
photonic analog-to-digital converter (ADC), photonic sampling.
I. INTRODUCTION
T
has attracted researchers’ interests in recent years [1], and
many schemes of optical ADC have been developed, including
photonic-assisted ADC [2]–[4] and photonic-sampled or/and
quantized ADC [5]–[11]. In [10], an interferometric optical
ADC scheme was proposed, using only one standard phase
modulator to realize optical quantization, which is much sim-
pler, compared with others schemes. And in [11], a free-space
interference architecture based on this scheme was experi-
mentally demonstrated. An improvement of this scheme using
polarization-differential interference has also been developed
to avoid free-space interference [12]. The key issue in both
interferometric ADC approaches proposed in [11] and [12]
is to realize the desired phase shift between the transmission
characteristics of two adjacent channels in order to achieve op-
tical quantization. It is realized through free-space adjustment
in [11] and a fiber stretcher in [12]. However, both of them
require additional mechanical adjustment, which will result in
instabilityforlong-termoperation.Inthisletter,a newapproach
HE potential of using optical technology to improve the
performance of the analog-to-digital converter (ADC)
ManuscriptreceivedAugust10,2007;revisedDecember10,2007.Thiswork
wassupportedbytheNationalNaturalScienceFoundationofChina(60607008)
and by the National High Technology Research and Development Program of
China (2007AA01Z271).
The authors are with the State Key Laboratory on Integrated Optoelectronics,
Tsinghua National Laboratory for Information Science and Technology,
Department of Electronic Engineering, Tsinghua University, Beijing 100084,
China (e-mail: wqw04@mails.tsinghua.edu.cn; zhhm@mail.tsinghua.edu.cn;
yaomy@tsinghua.edu.cn).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2008.924189
Fig. 1. Schematic illustration of the proposed all-optical ADC.
will be presented to achieve the desired phase shift, which uti-
lizes the dispersion effect introduced by multiwavelength light
propagating in a lithium niobate LiNbO
and an experiment of optical ADC using this approach will also
be carried out.
phase modulator,
II. PRINCIPLE OF OPERATION
In general, a synchronized ultrashort optical pulse train with
differentwavelengthsisusedasthesamplingpulsesourcefor
the proposed optical ADC, the schematic illustration of which
is shown in Fig. 1.
After a wavelength-division-multiplexing (WDM) coupler, a
synchronized multiwavelength optical pulse train with the same
polarization state enters into a commercial LiNbO phase mod-
ulatortosample theelectricalanalog signal.Apolarizationcon-
troller (PC) is used to make sure that two orthogonal polariza-
tion states with the same amplitude will exist in the phase mod-
ulator. And a polarizing beam splitter follows the phase modu-
lator, performing as an in-line analyzer. The transmission axis
of the analyzer is parallel to the polarization state of the input
pulse train, and then the maximal polarization interference will
happen through this analyzer. The phase difference induced by
the electrooptic effect of the phase modulator linearly changes
with the voltage of the applied electrical analog signal. And due
to the dispersion effect in LiNbO , an additional phase shift of
each wavelength will be additionally introduced.
In order to realize optical quantization, the phase shift be-
tween the transmission characteristics of each two adjacent
channels should be
(or
number of channels.
separate channels can be obtained
through a wavelength demultiplxer. When the
electrical analog signal is
the modulator),
quantized levels can be achieved, corre-
sponding to an ideal resolution of
be noted that in the processing unit of Fig. 1, there are some
electrical threshold circuits, but all the sampling and encoding
), where is the
of the
(is half-wave voltage of
[10]. It should
1041-1135/$25.00 © 2008 IEEE

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WU et al.: ALL-OPTICAL ANALOG-TO-DIGITAL CONVERSION USING INHERENT MULTIWAVELENGTH PHASE SHIFT 1037
Fig. 2. One channel proof-of-principle experiment setup.
are accomplished in the optical domain, and that is the reason
why we call this proposed scheme all optical ADC.
As we know, the relation between the refractive index of
LiNbO crystal and the light wavelength follows the Sellmeier
equation:
(1)
where
extraordinary light, respectively. Assumed that the length of
LiNbO crystal in the phase modulator is
through the crystal, the phase difference between ordinary
light and extraordinary light at
, respectively. And due to the dispersion effect, there
is a difference between
follows:
andare the refractive index of the ordinary and
, after traveling
andare and
and, shown as
(2)
where the phase difference
phase shift.
This equation shows that for a givenphase modulator, we can
find out
separate wavelengths to achieve the phase shift of
(or) between each two adjacent channels, thus
it ispossible tomake useof theinherentphaseshiftto buildupa
wavelength multi/demultiplexing all-optical ADC architecture,
asshowninFig.1.Thephaseshiftisrealizedautomaticallyafter
the light travelling through the modulator, without any mechan-
ical adjustment, and due to the available pulse source with ul-
trastable wavelength, the phase shift can be much more stable
than the free-space adjustment and the fiber stretcher.
is referred to as the inherent
III. EXPERIMENTAL SETUP AND RESULTS
A one channel proof-of-principle experimental setup has
been implemented, as shown in Fig. 2.
A wavelength tunable continuous-wave (CW) laser source
was usedintheexperimentinorder todirectlyobservethemod-
ulated curves on the oscilloscope and simulate the multiwave-
length channels. The wavelength of the laser source could be
tuned from 1526.95 to 1560.55 nm with a 0.2-nm spectrum
separation. A 26-dBm sinusoidal tone 2.5-GHz radio-freqency
(RF) signal was applied on the phase modulator, the half-wave
voltageofwhichwasabout5.66V. AdjustingPCs,themaximal
TABLE I
DESIRED PHASE SHIFT AND CORRESPONGDING WAVELENGTH
OF THE LASER SOURCE
Fig. 3. Relation between phase shift relative to the starting wavelength
(1.55 ?m in this figure) and the wavelength (the length of the LiNbO crystal
is 5 cm).
polarization interference could be obtained and the modulated
curves were observed and recorded on the digital sampling os-
cilloscope.
Sixteenwavelengthswerechoseninourexperimenttorealize
a
(or ) phase shift between each two adjacent
channels, shown in Table I.
The presented wavelengths in Table I seem to be irregular,
which is because that the laser source cannot be tuned con-
tinuously, where the mode jumping may happen, and also the
number
in may be different for different adja-
cent channels. Fig. 3 shows the relation between the phase shift
and wavelength with a given length of LiNbO crystal (5 cm).
From this figure, we can see that the phase shift has an excellent
linearrelationwiththewavelength.If thelengthofthecrystalin
the phase modulator is well fabricated, through an appropriate
bias voltage, the chosen wavelengths can well correspond to the
standardizedWDM channels,whichmeans thatthecommercial
available WDM laser source can be used in the proposed ADC.
Themodulatedcurvesofthese16wavelengthswererecorded
ontheoscilloscopeandshowntogetherinFig.4.Then,software
sampling was executed to get the digitized values by setting a
fixed time interval just as the sampling pulse train would do.
Fig. 5 showsthese digital values and a corresponding sinusoidal
fit.Fromtheseresults,asignal-to-noiseratioof27.6dBhasbeen
obtained, corresponding to an effective number of bits (ENOB)
of 4.3-bit, with a deviation from the ideal value of 0.7 bits.
The phase shift between two adjacent channels depends on
thewavelengthoftheeachchannelandthelengthoftheLiNbO

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1038IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 12, JUNE 15, 2008
Fig. 4. Modulated curves of 16 channels. Corresponding phase shift between
two adjacent channels shown in the figure may have an additional value of ?n?.
Fig. 5. Digitized value (dotted) and corresponding sinusoidal fit (solid line).
crystal in the phase modulator. From Fig. 3, we can see that
when length of the crystal is 5 cm, 1 nm of wavelength differ-
encecorrespondstoaphaseshiftofabout4rad.Thewavelength
stability of the commercially available laser source overlife and
temperature ( 5 C to 70 C) can be within 10 pm. That is to
say, the possible drift of the phase shift can be as fractional as
0.04 rad, which is negligible for our proposed ADC when the
resolution is less than 7 bit. Compared with the free-space ad-
justment[11]andthefiberstretcher[12],ourproposedapproach
to realize the desired phase shift is much more stable, especially
under long-term operation.
IV. CONCLUSION
A new approach of phase shift applied in interferometric
all-optical ADC has been proposed and demonstrated, utilizing
the dispersion effect of the LiNbO crystal in a commercial
phase modulator. In our presented all-optical ADC, a synchro-
nized multiwavelength optical pulse train is used to sample the
electrical analog signal through only one single phase mod-
ulator, and at the same time, the desired phase shift between
the transmission characteristics of each two adjacent channels
is achieved. In the proof-of-principle experiment, using a
wavelength-tunable CW laser source and software sampling,
we obtain an ENOB of 4.3-bit for sinusoidal 2.5-GHz input
RF signal. The desired phase shift between adjacent channels
is achieved automatically through the dispersion effect of the
crystal, without any requirement of mechanical adjustment. It
is compact, stable, and simple to be carried out. Using avail-
able commercial components, the system may be operated at
sampling rate greater than 100 GSamples/s, and the resolution
can be up to 5 bit at bandwidth of tens of gigahertz.
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