![Measured spectra of (a) demodulation process, (b) recovered clock, (c) the RF spectrum of the NRZ-DPSK (upper) and recovered clock (lower). [The resolution bandwidth is 0.05 nm in (a) and (b)].](https://www.researchgate.net/profile/Xinliang-Zhang-3/publication/3294606/figure/fig4/AS:1076695822020609@1633715696121/Measured-spectra-of-a-demodulation-process-b-recovered-clock-c-the-RF-spectrum-of_Q320.jpg)
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2356 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 22, NOVEMBER 15, 2006
All-Optical Clock Recovery From NRZ-DPSK Signal
Yu Yu, Xinliang Zhang, and Dexiu Huang
Abstract—All-optical clock recovery (CR) from 20-Gb/s
nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK)
signal is demonstrated, with an all-fiber delay interferometer (DI)
and a mode-locked semiconductor optical amplifier (SOA) fiber
laser. The tunable DI serves as an all-optical DPSK demodulator
and the phase-modulated NRZ-DPSK signal is converted into the
intensity-modulated pseudoreturn-to-zero (PRZ) signal, with the
enhancement of the clock component. Followed SOA fiber-laser
is used to achieve CR from the PRZ signal. Fixed bit pattern and
pseudorandom binary sequence NRZ-DPSK signals are
used to test the performance of the proposed system. It is shown
that the recovered clock signal with the extinction ratio over 10 dB
and the root-mean-square timing jitter of 800 fs can be achieved.
Index Terms—Clock recovery (CR), delay interferometer
(DI), demodulation, mode-locked semiconductor optical amplifier
(SOA) fiber laser, nonreturn-to-zero differential phase-shift keying
(NRZ-DPSK).
I. INTRODUCTION
ALL-OPTICAL clock recovery (CR) is one of the key tech-
nologies for realizing optical time-division-multiplexed
networks that require synchronous operations such as demul-
tiplexing and 3R regeneration. Several schemes for all-optical
CR are proposed and demonstrated [1]–[4], but all the presented
schemes are for the return-to-zero (RZ) and nonreturn-to-zero
(NRZ) format data. Recently, differential phase-shift-keying
(DPSK) is a potential modulation format for high-speed optical
fiber communication systems, because of its robustness to fiber
nonlinearities and polarization-mode dispersion [5], [6]. It has
also received much interest in optical packet/burst switching,
orthogonal modulation, and labeling techniques [7], [8]. How-
ever, the ideal NRZ-DPSK signal does not contain amplitude
information and clock component due to its constant amplitude.
It cannot be directly detected, and thus, the clock signal cannot
be extracted from it. Previous publications have demonstrated
CR from RZ-DPSK based on a phase-locked loop [9] or CR
from NRZ-DPSK with optical–electrical conversion based on a
balanced photodetector [10]. But to the best of our knowledge,
there is no report on all-optical CR from NRZ-DPSK signals.
In this letter, we report a scheme of all-optical CR from a
20-Gb/s NRZ-DPSK signal, with an all-fiber delay interferom-
eter (DI) combining an injection mode-locked semiconductor
Manuscript received July 14, 2006; revised September 3, 2006. This work
was supported by the Ministry of Education of China under the New Century
Excellent Talent Project (Grant NCET-04-0715).
The authors are with the Wuhan National Laboratory for Optoelectronics and
the Institute of Optoelectronics Science and Engineering, Huazhong University
of Science and Technology, Wuhan 430074, China (e-mail: xlzhang@mail.hust.
edu.cn).
Color versions of Figs. 1, 2, and 5 are available at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2006.885294
Fig. 1. Experimental setup for all-optical CR from NRZ-DPSK signal based
on DI (BPG: bit pattern generator; OSA: optical spectrum analyzer; CSA: com-
munication signal analyzer; C1, C2, C3: 50 : 50 couplers; C4: 10 : 90 coupler).
optical amplifier (SOA) fiber laser. The passive DI, without ad-
ditional noise and pattern effect, serves as an NRZ-DPSK to
pseudoreturn-to-zero (PRZ) demodulator. The PRZ signal is in-
jected into the free-running SOA fiber laser for CR. The prin-
ciple of the demodulation and the CR are analyzed with the help
of spectra analysis. A stable clock signal with 10-dB extinction
ratio (ER) can be achieved.
II. EXPERIMENTAL SETUP AND OPERATION PRINCIPLE
The schematic diagram for the proposed all-optical CR from
NRZ-DPSK data is shown in Fig. 1. In order to illustrate the op-
eration principle clearly, the diagram is divided into three main
parts: the NRZ-DPSK signal generator, the DPSK demodulator
(DI), and the all-optical CR unit. In this scheme, the DPSK
signal is first converted into an intensity-modulated PRZ signal
by a tunable DI, which is a comb filter. The PRZ signal is then
used for an injection of a free-running SOA fiber laser to achieve
mode-locking and perform a complete CR.
The SHF 40-Gb/s optical communication system is used to
generate an input NRZ-DPSK signal at a wavelength of 1555 nm
and an average power of 7 dBm. Its bit rate is fixed at 20 Gb/s
in our experiment. The signal power can be controlled by fol-
lowed erbium-doped fiber amplifier and the attenuator. The po-
larization controller (PC) is set before the DI to optimize the
output result. Our proposed DI has a time delay of about 25 ps
in one arm which is corresponding to about 5.2-mm fiber length
difference, and the temperature of the lower arm can be con-
trolled. The fiber laser consists of a reflective-SOA with a cir-
culator, a 50 : 50 coupler (C3), an isolator, a tunable bandpass
filter, a tunable delay line (TDL), and a 10 : 90 coupler (C4). Its
fundamental oscillating frequency is 15.8 MHz, which is corre-
sponding to about a 19-m fiber length. But it can be changed by
adjusting the TDL. The demodulated PRZ signal is introduced
into the fiber ring laser by C3. The cavity gain is provided by
1041-1135/$20.00 © 2006 IEEE
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YU et al.: ALL-OPTICAL CR FROM NRZ-DPSK SIGNAL 2357
Fig. 2. Principle for all-optical CR from NRZ-DPSK signal.
the 1000- m-strained multiquantum-well InGaAsP–InP mate-
rial reflective-SOA. Its small signal gain is 18 dB and peak gain
wavelength is 1550 nm when biased at 200 mA. The isolator
is used at the output of the SOA to ensure unidirectional trans-
mission in the ring. A filter with a full-width at half-maximum
bandwidth of 1.6 nm is used for wavelength selection; its center
wavelength is 1549.8 nm. The TDL is used to adjust the cavity
length in order to satisfy the mode-locking condition. The recov-
ered clock signal is extracted from the mode-locked fiber laser
with C4.
Fig. 2 shows the principle for our proposed CR. Along the
two arms, the NRZ-DPSK signal from the generator is split into
two paths through a 50 : 50 coupler (C1). A time delay is
introduced into the upper arm, while a phase shift is in-
troduced into the lower arm. In our converter, the phase shift
can be tuned by controlling the operation temperature of one
arm. The characteristic of the DI is the function of and ,
which are interacted by each other. The two lights superim-
pose at another 50 :50 coupler (C2) after traveling the DI. If
and are adjusted properly, the demodulation process can be
achieved. The phase modulation can be converted to the inten-
sity modulation, so a PRZ signal can be achieved, as shown in
Fig. 2(a), (b), and (c). Simultaneously, the enhancement of the
clock component can be achieved. The PRZ signal with optimal
power is then introduced into the free-running SOA fiber laser
by a 50 : 50 coupler (C3). The CR principle is similar to [11] and
[12], but we employ a reflective-SOA, which has better carrier
modulation performance [13]. By adjusting the TDL and the PC
carefully, the line frequency of the data pattern becomes a har-
monic of the fundamental frequency of the fiber ring, and the
fiber ring laser generates mode-locked pulse trains.
III. RESULTS AND DISCUSSION
Fig. 3 is the experimental result of “11001110”NRZ-DPSK
signal demodulation. As shown in Fig. 3, the phase modulation
information in the bit stream can be detected and converted to
the intensity-modulated PRZ signal. The dips at the boundary
of “1”and “0”in the DPSK signal are due to the characteristic
of our modulator.
The eye diagrams of input NRZ-DPSK, the demodulated
PRZ, and the recovered clock are shown in Fig. 4. The length
of input pseudorandom binary sequence is , which
means the maximal number of consecutive “0”sor“1”sin
DPSK bit stream is 30 or 31 (equals 29 or 30 consecutive “0”s
in demodulated PRZ). It reflects the tolerance of the CR circuit.
Fig. 3. Experimental result of 11001110 NRZ-DPSK (upper) to PRZ (lower)
demodulation.
Fig. 4. Eye diagrams of NRZ-DPSK (upper), PRZ (middle), and clock (lower)
at 20 Gb/s.
Due to the high performance of our passive demodulator, the
demodulated PRZ shows clear and open eyes, resulting in the
good quality of the recovered clock signal. The average power
of the NRZ-DPSK before the DI is about 4 dBm. Depending
on our SOA and the loss of the fiber ring cavity, the optimal
power of the PRZ signal injected into the fiber laser is about
1 dBm. This power should be controlled within a certain range.
When it is below 8 dBm, the modulation of the fiber laser
is too weak to establish a mode-locking process, and the CR
would fail. The output ER of the recovered clock is over 10 dB,
and no pattern effect can be found. The root-mean-square (rms)
timing jitter of the extracted clock is about 800 fs.
The measured spectra are shown in Fig. 5. It can be seen from
Fig. 5(a), before demodulation, the spectrum of the NRZ-DPSK
is smooth. There is no discretely separated spectral line in the
optical spectrum. The middle spectrum in Fig. 5(a) is the trans-
mission spectrum of the DI. It is a comb filter with a wavelength
spacing of 0.32 nm, and one notch aims at the center wavelength
of the input NRZ-DPSK by controlling the operation temper-
ature. After passing through the DI, the spectrum of the de-
modulated PRZ shows two main modes with a mode spacing
of 0.16 nm, which is corresponding to a frequency spacing of
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2358 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 22, NOVEMBER 15, 2006
Fig. 5. Measured spectra of (a) demodulation process, (b) recovered clock,
(c) the RF spectrum of the NRZ-DPSK (upper) and recovered clock (lower).
[The resolution bandwidth is 0.05 nm in (a) and (b)].
Fig. 6. Measured timing jitter of PRZ and extracted clock signals.
20 GHz. After passing through the CR circuit, most data mod-
ulation is removed, and a clock signal is achieved. Fig. 5(b)
shows that the spectrum of the recovered clock. The sidebands
exist at 20 GHz from the carrier wavelength of 1549.8 nm.
Compared with the incoming data, the optical spectrum of the
recovered clock reveals suppressed pedestal caused by modula-
tion response of the laser (i.e., mode-locking).
Fig. 5(c) shows the measured RF spectra of the NRZ-DPSK
signal and the recovered clock. Because of the dips in the NRZ-
DPSK signal, its RF spectrum (upper) contains a weak clock
signal stream. It can be shown that the mode-locked SOA fiber
laser provides a uniform and stronger clock signal stream, which
is synchronized to the injected NRZ-DPSK signal.
In Fig. 6, the rms timing jitter measurements for the demod-
ulated PRZ and the extracted clock are measured. By changing
the DI’s temperature slightly, the jitter of PRZ will increase, and
the optical signal-to-noise ratio (OSNR) will drop, simultane-
ously. The proposed CR system provides low jitter signals, a
remarkable jitter reduction, and an improvement of OSNR can
be achieved. Due to the passive interferometer, the OSNR for
the PRZ signal would be improved, the CR process can still be
achieved even if the input OSNR is poor.
IV. CONCLUSION
We have demonstrated a scheme of all-optical CR from
a 20-Gb/s NRZ-DPSK signal. A tunable DI is utilized as
the DPSK demodulator. It converts the NRZ-DPSK signal
to intensity-modulated PRZ signal. The PRZ signal, which
contains stronger clock components, is directly introduced
into a free-running SOA fiber laser for mode-locking. A stable
clock with ER over 10 dB and a timing jitter of 800 fs are
achieved. The clear and open eye diagrams and the timing jitter
measurements show the good performance of our proposed CR
system for NRZ-DPSK signal.
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