Abstract
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.

1. Introduction
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 laser cavity.

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 clock recovery.

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 (upper trace)

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.

5. Summary
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.

6. References
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, 2001

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, NL

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