Optical Clock Recovery

Optical Clock Recovery Z. Chen, H. Sun, S. Ma, N.K. Dutta Department of Physics, University of Connecticut, Storrs, CT, 06269

I. ABSTRACT A scheme for high speed clock and data recovery using an electroabsorption modulated laser and a semiconductor optical amplifiers arranged in an optical-electrical-optical (OEO) loop has been demonstrated. By injecting the 80Gb/s optical data into the OEO ring, the 10GHz clock tone is traced and amplified in the loop. A 10GHz electrical clock and, a 10 GHz optical clock are recovered simultaneously.

II. INTRODUCTION Clock and data recovery (CDR) is one of the most important and sometimes overlooked functions that are required by almost all optical transceiver or regenerator. Most current clock recovery systems use a fast photo-detector followed by an electrical phase lock circuit to synchronize the clock with the incoming data streams. The limited speed of photo-detectors and electrical circuits prevents these systems from being used in future optical time division multiplexed (OTDM) systems where the single-channel data rate could exceed 40 Gb/s. Optical clock recovery systems can overcome this limitation by using ultrafast nonlinearities to replace high-speed electronic components. Several nonlinear processes have been exploited for this purpose including four-wave mixing in fiber [1] or semiconductor waveguides [2], cross-absorption modulation in an electro-absorption modulator (EAM) [3], and phase-modulation in semiconductor amplifiers [4]. Other approaches utilize two-photon absorption (TPA) in photo-detectors [5] or self-seeding in laser cavity [6]. In this work, we investigated a clock recovery system using self starting oscillation in optical-electrical-optical (OEO) ring. In this scheme, a SOA acts as a gain and phase modulator driven by the incoming OTDM data signals. A pin-diode and a band-pass filter are used as OE converter. The filtered electrical signal is forwarded to a slave EML which generate a signal at a different wavelength than the incoming data signal. The EML signals are modulated by the incoming data in SOA each round trip after another hence results in a self starting oscillation. The 80Gb/s ultrahigh speed optical

Broadband Access Communication Technologies III, edited by Benjamin B. Dingel, Raj Jain, Katsutoshi Tsukamoto, Proc. of SPIE Vol. 7234, 72340H · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.802457

Proc. of SPIE Vol. 7234 72340H-1

data signal is produced by a harmonic mode locked fiber ring laser and a pulse compressor. The recovered optical clock is monitored both electronically and optically.

III. SOA-OEO CDR SETUP In fiber Implementation of all-optical clock recovery has been performed using various techniques as in section II. We present a new scheme that extract electrical and optical clock signals simultaneously. The experimental setup is similar to scheme suggested in [7].

80G or 160G OTDM data Apogee 155OEML

ODL

SOA 250 m SM F

mm BPF

RF Amplifier (9-10Ghz)

Photodetector Recovered Optical Clock

(

RF Amplifier (940Ghz) Electrical BPF 10Mhz @ 10Ghz

Recovered Electrical Clock Figure 1: OEO self starting clock recovery setup

Scheme shown in Figure 1 consists of a self starting SOA-OEO optoelectronic oscillator. 10GHz rate optical and electrical clock are recovered from the incoming 80Gb/s or 160Gb/s OTDM optical signal interleaved from a 20GHz harmonic mode lock fiber ring laser driven by a 10GHz synthesized electrical sinusoidal wave. This input data signal has a wavelength of 1557nm. When the optical signals are directly injected, the 10GHz component of the injected data stream creates periodic switching windows in SOA (Q-Photonics) for SOA-OEO self starting signals to lock to. Light from an EML (Apogee 10G butterfly TOSA) is coupled to SOA. The wavelength of this EML signal is 1552nm. The EML output passes through a 250m single mode fiber where the 10GHz clock tone is amplified by dispersion induced pulse broadening. After SMF spool, a 1 nm optical bandpass filter is used to select and feed the 1552nm signal to a 45GHz photodetector. The output of the photodetector is amplified, filtered by a high Q 10-GHz microwave bandpass filter of 10-MHz bandwidth, and fed back through a driver to the electrical input of the EML. Within the locking range, the SOA-OEO output


tracks the clock of the injected data stream, which is useful for electrical clocking applications. Outside the locking range, however, the recovered clock’s jitter follows the very quiet spectrum of the SOA-OEO, which is useful for optical clocking applications. Therefore, our scheme offers a simple and scalable solution for timing extraction with excellent jitter-transfer capabilities for use in future OTDM networks.

IV. 80GB/S AND 160GB/S OTDM DATA GENERATION In high data rate OTDM lightwave systems, several signals are individually modulated at the bit rate B with the same carrier wavelength, and are multiplexed optically to form a composite optical signal at the bit rate of NB, where N is the number of multiplexed temporal channels, while B represents the baseline clock bit rate. Generally, the periodic optical pulse train is generated by the laser, and it’s split equally into N branches. In each branch, after the electric-optical modulation, the pulse train is converted to an independent bit data stream at the bit rate of B. It is required that each channel of optical signal has a high clock rate and short pulse duration.

4000mDSF 300mSMF

PC

Oscilloscope

BOG PMF Loop Mirror

Figure2: Experimental Setup for 80Gb/s data generation

One approach in generating high repetition rate short pulses is the combination of a gain-switched DFB laser diode and the soliton-effect compression technique [8-10], which has been shown to be a promising way to generate compact and stable femtosecond pulse sources. The other approach is based on the MLL technique [11-12]. Among various types of pulse sources, actively mode-locked-laser (MLL) is a very promising candidate because it is compact, environmentally stable and can generate dropout-free pulses with a large extinction ratio, and a very low phase noise. Therefore, the pulses generated by MLL can directly be compressed to ultrashort pulses through soliton compression using a dispersion-decreasing fiber (DDF) [11], or supercontinuum compression [12]. In this work, a MLL and a DCF-SMF pulse compressor are employed as in Figure 2. The principle can be simulated by the nonlinear Shrögdinger wave propagation equation (NLSE) in fiber [13]. The


measured compressed 20GHz pulse train has a FWHM pulsewidth of 1ps, and the autocorrelation trace is shown in Figure 3. The 1ps 20GHz pulse is interleaved into 80Gb/s or 160Gb/s OTDM data signal by a 80GHz polarization maintaining fiber loop mirror (PML). In this work, the PMF loop mirror [14] is employed to serve as the delayed interferometer. Olsson etc. have previously used the PMF loop mirror as the notch filter to filter out the original CW signal [15]. The character of the 80GHz notch filter is depicted in figure 5, the 3-dB bandwidth of the notch is tested to be 0.317nm with the peak-to-notch contrast ratio of 35dB. By tuning the PC, the time delay between clockwise and counterclockwise optical pass can be adjusted. In the frequency domain, this time delay corresponds to a notch position shift in spectrum. After the PML, 20GHz pulse is encoded to 11001100 80Gb/s serial data stream, or 160G 10100000 as shown in Figure 4. 100 80 60 40

'0 0

-20

0

Figure3:

20 40 60 80 100 120 140 160 180 200 Trnie(ps)

Autocorrelation trace of 20GHz pulse train after compression, measured pulsewidth is 1ps

20 ps/Div

Figure4: Oscilloscope trace of OTDM 80G 11001100 or 160G 10100000 data


0 -10 -20 0.317nm

-30 -40 -50 -60 -70

-80 1550.5 1551.5 1552.5 1553.5 1554.5 1555.5 Wavelength (nm) Figure5:

measured transmission of 80GHz PMF loop mirror OT

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(a) Oscilloscope trace of recovered 10GHz optical clock (b) Optical spectrum of the recovered optical

clock

V. 80GB/S CLOCK RECOVERY RESULT To test the clock recovery scheme in section II, we used either 20Gb/s or interleaved 80Gb/s signal data streams described in section III. The 10-GHz-rate switching windows in the SOA, created by the 10-GHz spectral component of the input data, allow the self-starting SOA-OEO signal to be injection-locked to the input stream. The SOA is saturated by the input data stream. Optical clock is readily coupled out through a 1nm optical band pass filter from the EML. In locked status, the EML is


driven by a locked electrical 10GHz signal. A 53GHz optical detector is used to monitor the optical clock. The Agilent 81600C oscilloscope mainframe is triggered by the source 10GHz signal that drives the MLL in Figure 2. In Figure 6(a), recovered 10GHz optical signal is shown. The recovered optical clock spectrum is illustrated in Figure 6(b).

Time (2OpDiv) Figure 7: Oscilloscope trace triggered by original source clock (left) and from recovered clock (right) 0

50GHz BW

1MHz BW

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Frequency (Ghz)

Figure8: ESA trace (upper) source 10GHz clock (lower) recovered 10GHz clock


10.000503

The recovered electrical clock, which is monitored by an electrical spectrum analyzer (ESA) at the output of the electrical filter, also serves as the trigger for monitoring the data stream. The data oscilloscope result triggered by recovered clock is compared to the result that triggered by source clock. The two oscilloscope traces are shown in Figure 7 and the ESA traces are shown in Figure 8.

VI. CONCLUSION In conclusion, a scheme of OEO self starting oscillating ring used as high speed clock and data recovery has been proposed and investigated. The 80Gb/s patterned data was generated by interleaving 20GHz harmonic mode locked pulse signal. By injecting the 80Gb/s data into the OEO ring, the 10GHz clock tone is traced and amplified in the loop. The 10GHz optical clock is recovered simultaneously.

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