Self-pulsating DFB lasers are developed
for all-optical clock recovery. The system performance of these devices
is demonstrated and their ultra high speed potential is pointed out.
Optical signal processing offers the potential for ultra-high speed
operation (faster than electronics), for improved functionality (needed
e.g. in packet switched networks), and for cost reduction (by avoiding
the expensive opto-electronic conversions). An optical clock is a key
device needed for triggering the signal processing functions in digital
optical communication systems /1/. In this paper an overview on self-pulsating
DFB lasers (section 2), their application for all-optical clock recovery
(section 3) and their speed potential (section 4) is given.
2. Self-Pulsating DFB Lasers for Clock Recovery
Self-pulsating DFB lasers are compact devices about 800 µm
long comprising two DFB sections with detuned gratings and an
integrated phase tuning section (Fig. 1). The devices are driven by
dc currents. However, they emit optical pulses at a high repetition
rate /2,3/. The self-pulsation frequency is electrically tunable. Fig.
2 shows for example rf spectra of one device at various currents. A
continuous tuning range from 6 to 46 GHz can be noticed /4/. This electrical
tuning function is quite different to the characteristics of mode-locked
lasers where the repetition frequency is defined by the length of the
|Fig. 1. 40 Gb/s input data signal and 40 GHz
output clock signal of a self-pulsating DFB laser.
|Fig. 2. Electrical frequency tuning of
a self-pulsating DFB laser
The application for clock recovery requires the synchronization of
the self-pulsation relative to the data signal. This synchronization
can be achieved easily by injecting the optical data signal into the
self-pulsating laser whose frequency is tuned electrically close to
the data rate (locking range about 100 MHz). Fig. 1 shows a 40 Gb/s
PRBS data signal (upper trace) injected via a circulator into the device
/4/. The lower trace shows the 40 GHz self-pulsation signal emitted
from the device. The oscilloscope is triggered in parallel to the data
stream and thus only synchronized signals are displayed. The display
of the self-pulsation trace thus verifies the locking of the optical
clock. Analyzing the optical clock signal more in detail one finds sinus
shaped pulses, stable in amplitude and with low timing jitter. The jitter
derived from phase noise measurements is in the 300 fs range and can
be to some amount - lower than the jitter of a degraded input
signal (regenerative function).
3. System Applications of Self-Pulsating Lasers
for Optical Clock Recocvery
The most critical test of a device is its application in a regenerator.
There the device has to operate with degraded input data signals that
have to be transformed to improved output signals. The 40 GHz all-optical
clock has been applied in an all-optical 3R regenerator (3R: re-amplification,
re-timing, re-shaping) based on the synchronous modulation
scheme /5/. This 3R regenerator has been evaluated in loop experiments.
Error free transmission of 40 Gb/s PRBS signals over more than 10.000
km was achieved /5/. Fig.3 shows the according eye diagrams which are
clearly open. This result demonstrates the good system performance and
the regenerative function of the self-pulsating laser as 40 GHz all-optical
|Fig. 3. Eye Diagrams of 40 Gb/s signal
and demultipexed 10 Gb/s channel after 10.000 km transmission
|Fig. 4. Synchronization time for a 10 GHz
clock (lower trace) to injected data packet with 1 bits
An important function needed in future packet switched networks is
the ultra-fast synchronization of clock recoveries to asynchronous data
packets. Optical solution here can offer strong advantages compared
to the electronics. In Fig. 4 the synchronization time of a 10 GHz self-pulsating
laser (lower trace) to a packet of 1 bits (upper trace)
is investigated. The non-synchronized noisy signal changes rapidly within
1 ns after injecting the data signal to a clear clock pulse trace. Only
ten 1 bits are needed for the synchronization /6/. Analysis
of the behavior at the end of the packet resulted in a stable clock
signal for more than hundred 0 bits. The ultra fast locking
and the long hold time make the self-pulsating laser ideally suited
for future packet switching applications.
4. Speed Potential of Self-Pulsating DFB Lasers
The frequency for different configurations of self-pulsating DFB lasers
has been analyzed by modeling. Fig. 5 shows a calculated (VPI component
maker) rf spectrum indicating 160 GHz self-pulsation. Even higher frequencies
seem to be achievable by designing the DFB gratings properly and by
using suited operating conditions. First devices towards higher speed
have already been fabricated. Fig. 6 shows a measured rf spectrum indicating
80 GHz self-pulsation /7/. The locking function to 80 Gb/s PRBS signals
has been verified, Fig. 7 shows the according stable 80 GHz pulse trace.
The jitter performance of the clock has been evaluated by applying the
device in a transmission experiment. The phase noise jitter of the clock
recovered after 160 km transmission was less then 300 fs and close to
the jitter of the 80 Gb/s transmitter /8/. This indicates an excellent
system performance of the high speed all-optical clock recovery based
on the self-pulsating laser.
|Fig.5. Modeling of 160 GHz SP.
||Fig.6. Measured 80 GHz self-pulsation.
||Fig.7. Pulse trace of 80 GHz clock.
Optical clock recovery is a key function for signal processing in future
high speed and flexible all-optical networks. Self-pulsating DFB lasers
are developed for these applications. They are compact semiconductor
devices, easy to operate, and tuneable in frequency via the driving
dc currents. Their good performance as optical clock has been demonstrated
in several system experiments. Important advantages are the ultra high
speed potential exceeding that of electronics and the fast locking function,
needed for operation in asynchronous packet switched networks.
1. B. Sartorius, OFC 2001, invited paper MG7, Anaheim, Cal, USA
2. M. Radziunas et al, IEEE J. QE 36, pp.1026, 2000
3. M. Möhrle et al, IEEE J. Sel. Topics in QE, 7, pp. 217,
4. C. Bornholdt, et al, Electron. Lett. 36, pp.327, 2000.
5. B. Sartorius et al, OFC 2000, paper PD 11, Baltimore, USA
6. S. Bauer, et al., OFC 2000, paper TuF5, Baltimore, USA
7. C. Bornholdt et al, ECOC 2001, paper Th.F.1.2, Amsterdam,
8. C. Bornholdt et al, OFC 2002, paper TuN, Anaheim, Cal, USA
©2002 Optical Society of America
OCIS codes: (070.4340) Non-linear optical signal processing, (230.1150)
All-optical devices, (320.7090) Ultrafast lasers