JTh2A.13.pdf Advanced Solid-State Lasers Congress Technical Digest ©OSA 2013

Synchronous modulator incorporated re-circulating comb laser

sources for Tbps superchannel transmission

Le Nguyen Binh, Mao Bangning, Nebojša Stojanović, Changsong Xie, Ning Yang Huawei Technologies, European Research Center, Riesstr. 25, C-3.0G80992 Munich, Germany.

Email: le.nguyen.binh@huawei.com

Abstract: Stable generation of comb lasers by re-circulating fiber loop incorporating integrated

synchronous optical modulators is presented allowing transmission of Tb/s supechannels over long

haul dispersion non-compensating optically amplified lines. OCIS codes: (130.4110) Modulators;(140.3560) Lasers, ring

1. Introduction

Superchannels reaching total capacity of a few to tens of Tbps have attracted significant attention for the emerging

optical transport networks. Comb generation of optical narrow line optical sources of high stability and minimum

jittering, from a single primary source offers significant advantage in the superchannel optical transmitters [1,2].

This paper presents the generation and modulation of super-channels defined as multi-channels generated by the

lightwaves coming from the same reference laser source or the primary laser line, by generation of multi-sub-carrier

laser sources from a primary lightwave carrier re-circulating sideband (SSB) frequency shifting techniques. The

uniqueness of the SSB generation presented here is the synchronous modulation of the lightwave field re-circulating

in the loop employing an integrated optical synchronous modulator (IOSM) which consists of a master

interferometer whose two arms of two embedded intensity or phase modulators. Their electrodes are controlled in

such a way that the synchronization of the modulated lightwaves are in constructive interference. The formation of

Tbps superchannels (SC) can be 10x100G (1Tbps) or 20x100G (2Tbps) with the amplitudes of the sub-channels can

be equalized using wavelength selective switches. Pulse shaping of the data sequence is also implemented using

56GSa/s digital to analogue converter (DAC) so as to packing modulated sub-channels more effectively with

minimizing the channel cross-talk. Employing the comb line laser sources, modulation of individual sub-carriers are

implemented with and without pulse shaping to generate SC in which each individual channels can be modulated

using two polarized modes at 28GSa/s so that 1 Tbps and 2 Tbps SC can be generated for transmission over long

haul optical transmission lines consisting of several 100km spans of non-dispersion compensating standard single

mode optical fibers.

2. Generation of multi-subcarriers for Tbps superchannels

Multi-carrier modulation using comb-generator is constructed and tested in the laboratory for advanced optical

communication systems. The schematic of the constructed re-circulating frequency shifting (RCFS) comb generator

is shown in Fig. 1 in which a re-circulating loop is the basic configuration incorporating an optical modulator, an

optical amplifier to compensate the insertion loss of various devices and an optical attenuator whose coefficient can

be adjusted so that the overall loop gain is slightly less than unity. This loop gain less than unity is required to ensure

no lasing would occur. An optical coupler with a coupling ratio of 50:50 or 3dB split is employed to coupling into

and out of the loop the lightwaves and combed sub-carrier lines respectively. A synthesizer is used to generate a

sinusoidal radio wave whose frequency determines the frequency shifting of the optical lines of the sub-carriers of

the generated comb, injected into the electrodes of the modulator.

The IOSM incorporated in the loop is a synchronous modulator whose structure can be similar to that of an

IQ modulator, biased such that synchronization of the slave interferometers which are embedded in the two optical

paths of the master interferometer, can be constructive rather than destructive at the combined output as shown in

the insert of Fig. 1. Two arms of IOSM are driven with a pi/2 phase shift with respect to each other so that single side

band (SSB) shifting of the lightwave can be achieved at the output of the modulator. When the loop is open, SSB

spectrum is observed at the output of the modulator as shown in Fig. 2(a) and (b). Note that this frequency shifting

can be either shifting left or right depending on the relative phase shift between the two arms of the synchronous

modulator. The control of the stability of the generated comb lines is implemented by a control feedback mechanism

with feedback signals coming from the synchronization of the jittering of the referenced RF synthesizer and

injection of the optical power into the ring, the pumping level of pump laser into the optical amplifying devices

incorporated in the ring. To set spectral spacing between the comb lines, a synthesizer launches the RF waves to the

two arms of the IOSM with RF amplifiers to boost the electrical signals to appropriate level so that maximum

JTh2A.13.pdf Advanced Solid-State Lasers Congress Technical Digest ©OSA 2013

modulation depth can be achieved but minimum nonlinear effects. We note that although the modulator 3dB

bandwidth is specified at 23GHz, it is still possible to operate the modulator at 33 GHz although some RF signal loss

would be expected. This is because the driving signal is purely sinusoidal and very narrow band unlike data signals

employed to drive the data modulator. We have successfully generated comb lines from 28GHz to 33 GHz for

channel spacing 28G, 30G and 33GHz.

Fig. 1: Schematic structure of the RCFS comb generator. ECL: external cavity laser; EDFA: Er;diped fibre amplifiers. FC = fiber coupler; PS =

phase shifter; EDFA= Er:doped fiber amplifier; ECL=external cavity laser.

(a) (b)

9A (a) (b)

Fig. 2: Spectrum of the SSB lightwave at output of the synchronous optical modulator (a) shifting right; and (b) shifting right in frequency scale. Resolution of horizontal scale (0.2 nm/div.)

(a) (b)

Fig. 3: Spectrum of multi-subcarriers, comb llines generated by the RCFS loop. Vertical scale: power amplitude of arbitrary units and

horizontal scale: frequency of 10GHz/div.

An external cavity laser is employed as the primary lightwave which is fed into the SSB frequency-shifted fiber

ring under non-resonant condition to generate comb sub-channels. By non-resonance we mean that the resonance

condition for lasing is not permitted by ensuring that the total gain and loss of the lightwaves circulating in the ring

is less than unity. This laser output power is set at about 13 dBm which is the highest power level. Higher power

LiNbO3 Synchronous

optical modulator

FC

Fiber loop

Synthesizer pi/2 PS

Bias

ECL

Primary source

Generated comb line

laser sources

EDFA

RF amplifiers

slave

slave

master Electrodes

(sig & bias)

IN

OUT

JTh2A.13.pdf Advanced Solid-State Lasers Congress Technical Digest ©OSA 2013

level can be used if EDFA booster amplifier is used. Thus we note the power level of the SBS line at -10dBm and -

4dBm for the comb lines which has also been passing through an EDFA at saturation power of 16dBm which is

distributed to all sub-carrier lines. The spectra of the comb lines generated from the RCFS comb generate shown in

Fig. 3. Both sides of the primary laser lines can also be generated by employing two RCFS rings with the SSB shift to

the right or the left of the primary line. This ultra-wide spectral comb line source has also been generated in our

laboratory.

The optical amplifier incorporated in the re-circulating ring is operating in the saturation mode only if when the

power of the lines is sufficient to boost it into the saturated region otherwise it would operate in the linear region.

For this reason, we observe that the frequency shifting lines would not be in the saturated level for the first few sub-

carrier lines), the far most right side of the spectrum. The tuning of the DC bias supply voltage to the electrodes is

quite sensitive and would be in the 10’s of mV range. One most important point we must point out is that the output

fiber port must be angled connector type so that no reflection of sub-carrier combed lines can be feedback into the

recirculating to avoid the noise oscillation of the generated comb lines. Note that the suppression of the other

harmonic lines is more than 25dB.

(a) (b)

Fig.4: Spectrum of QPSK super-channel modulation with narrow spacing , 18 channels under Nyquist pulse shaping after transmission over

2000km optically amplified SSMF spans (a) non-shaping QPSK modulation 10 sub-channels of 28GS/s polarization muxed QPSK for 1 Tbps

superchannels (b) Nyquist shaping superchannels, 20 at 28Gb/s for 2Tbps.

The comb generator has been employed to generate the referenced laser lines which are then modulated with

Nyquist pulse shaping implemented in a 56GSa/s digital to analogue converter (DAC). The modulated spectra of

sNyquist shaping and non-shaping are shown in Fig.4 (b) and (a) respectively with 20 and 10 channels spaced as

closed as possible and transmitted over 2000 km of standard single mode fibres incorporating optical amplifiers but

not dispersion compensating fibres. The equalization in the power amplitude of the superchannels is implemented

using WSS (wavelength selective switch). Individual combed modulated channels are coherently detected via the

use of a pi/2 polarized hybrid coupler and then digitally process off lines with an achieved bit error rate in the range

of 1e-3 which is acceptable for forward error coded transmission systems. The power sensitivity is also obtained

with launched power in the normal acceptable range so that not significant nonlinear impairments.

4. Concluding remarks

We have presented the generation of comb source whose spectra cab be controlled by modulating the amplitude of

the lightwave circulating in a re-circulating fiber loop. Synchronization of the optical fields of the comb lines is

achieved via the use of an integrated synchronous optical modulators such that constructive interference of the

lightwaves is achieved. 1 Tbps and 2Tbps superchannels are obtained by modulating these comb laser lines and

transmitted over long dispersion non-compensating optical transmission lines with obtained bit error rate acceptable

for high quality transmission systems. It is noted that 5x200G can also be used in which the 200G can be generated

by using the high level modulation format e.g. 16QAM or Mary-QAM instead of QPSK (quadrature phase shift

keying).

5. References [1] Fred Buchali, “Technologies towards Terabit Transmission Systems “, ECOC 2010, , paper We.6.C.1, September, 2010, Torino, Italy.

[2] Nebojša Stojanović, Changsong Xie, Yu Zhao, Bangning Mao, Neil Guerrero Gonzalez, Juan Qi, Le Nguyen Binh , “Modified Gardner Phase Detector for Nyquist Coherent Optical Transmission Systems”, OFC/NFOEC Technical Digest, OFC 2013, Paper, JTh2A.50, L.A., 2013.