920 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 8, APRIL 15,
2016
Fig. 2. On the left: pulse shapes after the Mach-Zehnder
modulator, afterthe SOA and after filter in part 1. On the right:
the corresponding spectra.
part 2 of solitonic compression and part 3 for
pedestalsuppression. After each part, 10% of the power is used
formonitoring. An optical switch is employed to facilitate
theswitching between signals after each stage. The fibers beforethe
optical switch are of length that can be ignored.
In part 1, an NRZ waveform generator generates
periodicrectangular waveform as driven signal for the modulator.
Theintensity modulated laser beam is then injected in a quantumwell
SOA (model JDS Uniphase CQF871/0) that inducesfront edge distortion
due to its gain nonlinearities (Fig. 2 onthe left). As shown by the
non-symmetrical spectrum afterSOA in Fig. 2, the distortion
contribution has a frequencyshift as the linewidth enhancement
factor is non-zero. Thenby taking benefit of an optical filter, we
are able to achieve40 ps pulse train [17].
Part 2 consists of an Erbium doped fiber amplifier (EDFA)and a
given length of standard single mode fiber (SSMF).While the SSMF is
anomalous dispersive, in this case, opticalpulse of Gaussian-like
shape evolves like high order solitons,which always experience an
initial pulse compression in anom-alous dispersive fibers [21].
Simulation results show that fordifferent fiber lengths of 12, 4.2
and 2 km the best compressionratios are respectively 4.92, 15.31
and 35.36 with proper initiallaunched powers. The launched power is
always higher forshorter fiber length.
In experiment, a 4.2 km SSMF is chosen as a tradeoffbetween
initial power level and compression ratio. Thus therequired initial
peak power is 541.8 mW that is equivalent to5.25 pJ per pulse, and
it corresponds to 7.2 dBm at repetitionrate of 250 MHz. As a
result, the full width at half maxi-mum (FWHM) is compressed to
4.49 ps under assumptionof hyperbolic secant pulse shape. The
autocorrelation tracein Fig. 3 reveals an important issue, the
pedestal, which canlargely reduce the pulse quality.
Part 3 consists of an EDFA, a SSMF, a polarizationcontroller and
an optical filter. The high output power ofEDFA induces
nonlinearities inside the SSMF, which includethe higher order terms
in nonlinear Schr’́odinger equationsuch as 3rd order dispersion,
the intrapulse stimulated Ramanscattering and the self-steepening
when pulse width becomecomparable to 1 ps [19].
Fig. 3. Autocorrelation trace of pulses after the pulse
compression in part 1.The launched power is 7.2 dBm at the
repetition rate 250 MHz while the fiberlength is 4.2 km. The inset
figure is the corresponding spectrum.
Fig. 4. On the top: normalized autocorrelation trace and
spectrum in part 3before the optical filter. On the bottom:
normalized autocorrelation trace andspectrum in part 3 after the
filter.
This is further proved by the asymmetric spectrum in topright of
Fig. 4, as only the higher order terms can bringasymmetric spectrum
broadening when the input pulse issymmetric.
Then by carefully tuning the optical filter, we obtain
pulseautocorrelation trace as shown in Fig. 4 bottom left. The
pulsethat we finally obtain benefits largely from the higher
ordernonlinear effects during propagation in SSMF in part 3.
Under assumption of hyperbolic secant pulse shape, theFWHM of
filtered pulse is 1.1303 ps. The peak level topedestal level ratio
of autocorrelation trace is 39. The timebandwidth product is
calculated to be 0.4969, which is veryclose to the
bandwidth-limited value of 0.315 for hyperbolicsecant shape
pulses.
In experiment, the launched power in part 3 is around835.7 pJ
per pulse for 1.2 km SSMF. The optical filteringinduces a loss of
energy about 12 dB, which is defined as theproportion of the output
power of the filter to the input one.The remaining energy after the
optical filter is about 21 pJper pulse. For pulses of smaller
launched power, the spectrum
YOU et al.: WIDELY CONTINUOUS TUNABLE ps PULSE TRAIN GENERATION
921
Fig. 5. Pulse profile simulation for 1200 m SSMF. The input
pulse is ofFWHM 4.49 ps that correspond to the output of part 2.
The input peak power is60 W or 491.5 pJ per pulse. For parameters
of SSMF, the 3rd order dispersionslope at 1540 nm is 0.09
ps·nm−2·km−1, the value of intrapulse Ramanscattering coefficient
is 3 fs, the dispersion parameter is 20.3 ps·nm−1·km−1,and the
nonlinear coefficient is 0.002 W−1·m−1.
broadening induced by nonlinear effects is not enough
forseparating the pulse and the pedestal using optical filter.
As the observation using autocorrelator loses the informa-tion
of pulse shape in time domain, simulation with split-stepFourier
method is performed to get a view of pulse shapeevolvement during
the propagation. All the parameters aretaken as typical values of
SSMF shown in the description ofFig. 5. By varying the initial peak
power, we find that witha power of 60 W we can obtain relatively
good simulationresults. As the parameters of SSMF are all taken as
typicalvalues, it is difficult to obtain perfect fit between the
simulationand the experiment.
In Fig. 5 we observe the split of pulse during propagation.Due
to the anomalous dispersion, the fact that the new largepeak
propagates slower than the initial peak means a transferof energy
to the low frequency side in the frequency domain.This result is
consistent with Fig. 4, and this new large peakis experimentally
what we observed after the filter.
III. RESULTS AND DISCUSSION
For application in optical sampling, the pulse width,
thesignal-to-noise ratio (SNR) and the timing jitter are the
keyparameters to characterize the quality of pulse train. The
pulsesFWHM are derived from the autocorrelation trace under
theassumption of a hyperbolic secant shape. The extinction ratioof
the autocorrelation trace is taken to characterize the pulsequality
instead of SNR.
A. Timing Jitter Measurements
The RF spectrum analysis for timing jitter measurementshas been
extensively studied [22]–[25]. In experiment we areusing the method
described in [22]. The main difficulty lies inthe determination of
lower frequency boundary f1 and upperfrequency boundary fu for the
integration.
Fig. 6. Results of RMS timing jitter value as a function of the
lowerfrequency f1. The upper frequency fu is set to be 118 kHz in
this measurement.
Fig. 7. Results of RMS timing jitter value as a function of the
upperfrequency fu. The lower frequency f1 is set to be 2 kHz.
Both timing jitters from 15th and 19th order harmonicsare
calculated (Fig. 6 and 7). Ideally the two curves
shouldcoincide.
In Fig. 6, the 2 curves increase rapidly for lower
frequencyvalue smaller than 2 kHz, which suggests that the spectra
atthe range larger than 2 kHz is the major contribution to
thetiming jitter [22].
In Fig. 7, the 2 curves become almost flat above 118 kHz.It
means that the integration after 118 kHz is only accumula-tion of
background noise.
We repeat the measurements for 6 times and we deter-mine the
integration frequency range in a similar way. Theaverage value
respectively for 15th and 19th harmonics are1.647 and 2.52 ps.
Comparing with the RMS timing jitter estimation of 40 pspulses
at the output of part 1 (Fig. 1) of about 2.5 ps,we conclude that
the major contribution to the timing jittercomes from the
electrical part of the system. This timing jitterestimation is a
functionality providing by the Agilent DCA-Xoscilloscope with
optical input for pulses larger than 30 ps.
B. The Tunability on Repetition Rate
In part 2 of experimental diagram (Fig. 1), an EDFA beforethe
SSMF is used to amplify optical pulses to the required
922 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 8, APRIL 15,
2016
peak power, which also means that the average output powerwill
increase as the repetition rate increases. The output powerrange of
EDFA appears to be the limiting factor for the upperrange of
repetition rate tunability.
For example, a 40 ps 250 MHz repetition rate pulsetrain at the
output of part 1 is amplified to 7.5 dBm asrequired for solitonic
compression in 4.2 km SSMF. TheEDFA in our experiment has an output
power range between2 and 12.5 dBm. This means that the repetition
rate can onlybe tuned between 70 MHz and 790 MHz.
Then in the part 3, the average power required is
experi-mentally determined to be 23.2 dBm for 1.2 km SSMF withpulse
repetition rate at 500 MHz. The EDFA before 2nd stagehas an output
power range from 12 dBm to 26.75 dBm,which means that the limit of
repetition rate is between40 MHz and 1.1 GHz.
Experiments are performed at different repetition rates of100,
250 and 500 MHz. The FWHM pulse widths are respec-tively 0.986,
0.895 and 1.136 ps under hyperbolic secant shapeassumption.
As for the solitonic compression, higher compression ratioalways
means higher peak power. At the same time higherrepetition rate
also requires higher average power. Thereforewe need to make a
tradeoff between the tunability and thepulse width due to the
limited output range at the EDFA.
IV. CONCLUSION
We have performed the ps optical pulse train
generationexperiment based on SOA nonlinear dynamics and
solitoniccompression. The result shows that this technique is
verypromising for versatile source for ultrashort pulse
generation.The tunability range and the pulse quality are very
suitablefor optical sampling. The wavelength can cover the full
rangeof telecommunication band. The major contribution to
theobserved timing jitter comes from electrical part meaningthat a
better jitter performance by improving jitter noise ofelectrical
devices in the future can be expected.
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