IEEE Photonics Technology Letters:
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December 2001 VOLUME XX NUMBER X IPTLEL (ISSN 1041-1135)
PAPER
Copyright © 2001 IEEE.
Reprinted from
IEEE Photonics Technology Letters, vol. 13, no. 12, pp. 1298-1300, Dec. 2001
Return-to-Zero Modulator Using a Single NRZ Drive Signal and
an Optical Delay Interferometer
P.J. Winzer, J. Leuthold
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 12, DECEMBER 2001
Return-to-Zero Modulator Using a Single NRZ Drive
Signal and an Optical Delay Interferometer
Peter J. Winzer and Juerg Leuthold
Abstract—In this letter, we experimentally demonstrate and discuss a new technique for variable duty cycle return-to-zero (RZ)
modulation, employing a phase modulator driven by a single electrical nonreturn-to-zero signal, and an optical delay interferometer. Unlike other methods for generating RZ, no synchronization
between two electrical driving signals is needed.
Index Terms—Delay lines, modulation, optical communication,
optical pulse generation, optical transmitters.
I. INTRODUCTION
Fig. 1. Block diagram of RZ source.
O
PTICAL RETURN-TO-ZERO (RZ) signals are becoming increasingly important in optical communication
systems. They have proven to be superior to the nonreturn-to-zero (NRZ) format both in terms of receiver sensitivity
[1]–[4], and in terms of fiber transmission performance [5]–[7].
The two most commonly used techniques to generate optical
RZ data streams either employ a sinusoidally driven intensity
modulator or an actively mode-locked laser [8]–[10] in addition
to a NRZ data modulator. Apart from the need for two or
more high-power RF components, these techniques necessitate
accurate synchronization between the data modulator and
the pulse source. Two less frequently used techniques make
use of electrical RZ generation by means of gating the NRZ
clock with the NRZ data signal [11], which necessitates high
modulation bandwidths, however, or drive a Mach–Zehnder
intensity modulator with the NRZ data between its transmission
minima [12], with the drawback of having to use exactly twice
the modulator’s maximum-to-minimum transmission drive
voltage V for good extinction performance. In this letter,
we discuss and experimentally demonstrate a new scheme for
RZ generation using a single, NRZ-driven phase modulator
followed by a (passive) optical delay interferometer. This
technique eliminates the need for any synchronization between
two signals, and considerably alleviates the requirements on the
driver amplifiers. While under review, the method described
in this letter has been independently proposed by another
group [13], who interpret the resulting modulation format as
duobinary, carrier-suppressed RZ (DCS-RZ). Our in-depth discussion in Section II will yield more insight into the properties
of the modulation format and show that a DCS-RZ format is
only obtained under the assumption of very high bandwidth
modulator driving, or by using a dual-drive Mach–Zehnder
modulator (MZM) for phase modulation [15].
Manuscript received July 9, 2001; revised August 21, 2001.
The authors are with Bell Laboratories, Lucent Technologies, Holmdel, NJ
07733 USA (e-mail: peter.winzer@ieee.org).
Publisher Item Identifier S 1041-1135(01)09971-2.
II. PRINCIPLE OF OPERATION AND SIMULATION
A block diagram of the transmitter is shown in Fig. 1. The
light from a continuous-wave (CW) laser source is first passed
through a phase modulator that is driven by the differentially
encoded NRZ data signal, which exhibits a change in its logic
level for each logical “1” bit to be sent; differential encoding
can be done entirely in the digital domain using a one bit delay
feedback and an exclusive or (XOR) gate [14]. The subsequent
(passive) optical delay interferometer, which is adjusted for destructive interference in the absence of optical phase changes,
then converts the NRZ phase modulation into RZ pulses with a
width corresponding to the optical delay.
Fig. 2 visualizes the (simulated) waveforms at various points
within the setup. The binary signal to be transmitted in RZ
format is shown in (a). Its differentially encoded version, whose
low-pass filtered edges take into account finite-bandwidth
driving electronics, is given in (b). The phases of the two
interfering signals at the output of the interferometer are shown
as solid and dashed lines in (c). As indicated in (d), any phase
difference between the two arms gives rise to an RZ pulse with
of
a complex field envelope
(1)
denotes the differentially encoded, elecwhere
trical NRZ driving voltage normalized to V (the voltage necessary for a phase shift of ), stands for the interferometer’s
takes into account any deviation from destrucdelay, and
tive interference in the absence of phase modulation. The (field)
splitting ratios of the interferometer’s two splitters are denoted
s and s ; their squares represent the fractions of the splitters’
1041–1135/01$10.00 © 2001 IEEE
WINZER AND LEUTHOLD: RETURN-TO-ZERO MODULATOR
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Fig. 3. The spectrum E (f ) of the RZ optical field e(t) is made up of two
components that are frequency shifted relative to the optical carrier.
Fig. 2. Principle of operation. (a) The data to be cast into RZ form
is differentially encoded, and (b) amplified by band-limited RF driving
electronics. (c) At the output coupler of the delay interferometer, two fields
with delayed phases are superimposed, giving rise to RZ pulses (d) with phases
shown in (e).
input powers that are coupled from their inputs to one of their
outputs.
Note that the achievable extinction ratio is primarily determined by the interferometer’s ability to produce destructive interference, which depends on setting the splitting ratios equal,
and on tuning for destructive interference, but not on dynamic
features, such as achieving an exact amount of drive voltage
swing (which is a critical issue in a similar technique described
most importantly influences the
in [12]). The maximum of
insertion loss of the device, since it determines the degree of
achievable constructive interference. (For example, if the available drive voltage swing amounts to only V /2, we find an additional 3-dB insertion loss.)
The phase of the modulated optical field, as calculated from
,
(1) with ideal interferometer parameters and
i.e., with a phase modulator drive voltage swing of V , is shown
in Fig. 2(e). Note that there are two differently shaped phase
transitions between adjacent RZ pulses, which is a direct consequence of the first term in (1), characterizing basically the output
optical field’s phase, while the phase rises by
during each leading edge of the differentially encoded NRZ seduring each trailing edge. The
quence, it falls by
phase jumps at the beginning of an RZ pulse represent changes
caused by the term in brackets in (1). For
in the sign of
high bandwidth (nearly rectangular) driving of the phase modulator with V , the phase transitions occur instantaneously at the
beginning of a pulse, leading to a duobinary type of RZ format
[13]. A duobinary-like format is also acheived using a dual-drive
MZM for phase modualtion [13], [15]; this inherently leads to
instantaneous phase shifts, independent of the drive bandwidth,
but at the cost of an additional drive signal, and consequently,
the need for temporal alignment of two RF signals.
Fig. 4. Optical eye diagram recorded from 2
delay was 100 ps, leading to 25% RZ duty cycle.
0 1 PRBS at 2.5 Gb/s. The
Due to the different slopes of two adjacent RZ pulses’ phase
waveforms, the center frequency of every other RZ pulse is
shifted toward a different direction relative to the unmodulated
carrier, which influences the spectrum of the RZ sequence
Fig. 3 shows the simulated absolute magnitude of the spectrum
of
resulting from a 2
PRBS (solid). As expected
from the phases of the RZ pulses, the spectral envelope is given
by a superposition of the spectrum of a single downshifted
pulse (dashed) and that of a single upshifted pulse (dotted). The
amount of this frequency shift (and thus, the overall spectral
width of the RZ signal) is determined mostly by the drive
voltage swing. In general, low drive voltages produce less
frequency shift, and thus, narrower RZ spectra. In Fig. 3, we
.
assumed
III. EXPERIMENTAL DEMONSTRATION
For an experimental demonstration at a data rate of 2.5 Gb/s,
we used a CW semiconductor laser operating in the 1550-nm
wavelength range, a LiNbO phase modulator, and a delay interferometer with 100-ps optical delay. In the absence of phase
modulation, the interferometer was tuned for maximum destructive interference at the laser wavelength by means of a current-tunable phase section in one of its arms, which led to a
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 12, DECEMBER 2001
sitivity of 40 dBm could be obtained for RZ, showing 2-dB
sensitivity gain over NRZ.
IV. CONCLUSION
We have discussed and experimentally demonstrated a new
method of generating optical RZ signals using a single, NRZ
driven phase modulator in combination with a passive optical
delay interferometer. The technique does not require synchronization of two electrical drive signals.
ACKNOWLEDGMENT
0
Fig. 5. Measured heterodyned spectrum of a 2
1 PRBS at 2.5 Gb/s with
100-ps delay (black) and simulation according to (1) (gray).
The authors would like to thank J. F. Bailey, K. H. Bogart,
M. A. Cappuzzo, L. T. Gomez, R. W. Long, S. Patel, E. J.
Laskowski, A. E. White, and L. W. Stultz for manufacturing the
delay interferometer. Also, A. H. Gnauck, M. Zirngibl, and G.
Raybon are acknowledged for valuable discussions.
REFERENCES
Fig. 6. BER for the proposed RZ modulation scheme (circles) and NRZ
baseline (rectangles) measured at with an optically preamplified receiver. The
amounts to 40 dBm, showing a gain of 2 dB
sensitivity for BER = 10
over NRZ.
0
(static) extinction ratio of 20 dB. The measured optical eye diPRBS is shown in Fig. 4,
agram corresponding to a 2
yielding a RZ duty cycle of 25%, as expected for the chosen
delay. The heterodyned spectrum of the optical field is shown
in Fig. 5 (black), together with the results of a simulation (gray)
,
according to (1). The simulation parameters are
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accounting for a slight phase mismatch at
splitters
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circles represent the performance of the proposed RZ modulation scheme, while the rectangles apply to NRZ, generated with
, a receiver sena dual-drive LiNbO MZM. At BER
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