1998Coherence Degradation in the Process of

Ž.
OPTICAL FIBER TECHNOLOGY4,215᎐2231998
ARTICLE NO.OF980253
Coherence Degradation in the Process of
Supercontinuum Generation in an
Optical Fiber
Masataka Nakazawa,Kohichi Tamura,Hirokazu Kubota,and Eiji Yoshida
Optical Lightwa¨e Communication Laboratory,NTT Optical Network Systems Laboratories,
Tokai,Ibaraki-ken319-11,Japan
E-mail:nakazawa@nttlsl.iecl.ntt.co.jp
Received January7,1998;revised January15,1998
Changes in coherence during the process of supercontinuum generation in
Ž.Ž.
a dispersion-shifted fiber DSF,a dispersion-flattened fiber DFF and a
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dispersion-decreasing fiber DDF have been investigated in detail for the
first time.It is shown that both time and frequency coherence are greatly
degraded in DSF and DFF.This is due to the four wave mixing effect caused
by self-phase modulation and group velocity dispersion in the presence of
amplified spontaneous emission.Such a light pulse may be unsuitable for
long-distance,high-speed transmission because it contains timing jitter and
amplitude fluctuations.On the other hand,DDF can maintain its coherence
as the spectral broadening occurs coherently through adiabatic N s1soli-
ton compression.This fiber is useful for ultra-high-speed communication.
ᮊ1998Academic Press
Key Words:Supercontinuum;soliton;four wave mixing;coherence.
I.INTRODUCTION
Ž. With the advent of high-power pico-femtosecond lasers,a supercontinuum SC, which occurs due to a combination of self-phase modulation,cross-phase modula-tion,four wave mixing,and the stimulated Raman effect,has been generated in w x
many materials1.The SC,in other words‘‘white light,’’is very useful for the
w x time-resolved spectroscopy of materials and multi-wavelength optical sources1.It is also well known that an SC has been used for the generation of femtosecond
w x
pulses with a pair of fiber gratings2.We used an SC in a multi-wavelength optical time domain reflectometer to measure not only fault locations but also the
w x
wavelength dependence of the fiber loss3,4.
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Copyrightᮊ1998by Academic Press
All rights of reproduction in any form reserved.
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NAKAZAWA ET AL.
Recently there has been increasing interest in the application of the SC as a
Ž
multi-wavelength optical source for high-speed WDM wavelength division multi-.w x
plexed optical communication5᎐8.However,the transmission distance is rather Ž.
short of the order of40᎐100km and it is not clear to what extent the SC source is suitable for long-distance,high-speed communication.It has also been reported
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that dispersion-decreasing fiber DDF can broaden the spectral width of the SC
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significantly more than dispersion-shifted fiber DSF or dispersion-flattened fiber Ž.w x
DFF9.However,there has as yet been no investigation of the extent to which timing jitter and amplitude fluctuation exist,and hence how time and frequency coherence are maintained,during the process of SC generation.Such an investiga-tion is of great importance if the SC technique is to be applied to high-speed, long-distance communication.
In this paper,we report for the first time that SC generation causes a large coherence degradation due to the random excitation of high-order solitons initi-
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ated by amplified spontaneous emission ASE.This process occurs mainly in DSF and DFF as we couple a high-intensity pulse to the low-dispersion fiber.However, we show that there is no such coherence degradation when a DDF is excited with fundamental or low-order solitons.
II.MEASUREMENT OF THE SC IN FIBERS
In order to measure the coherence degradation in an SC in fiber,we prepared Ž.
three kinds of fiber A,B,and C.Fiber A was a dispersion-decreasing fiber which made it possible to shorten the input pulse width adiabatically via optical soliton compression.The length of fiber A was0.91km and the group velocity dispersion Ž.
GVD was9ps r km r nm at the input end and nearly0ps r km r nm at the output end.Fiber B was a1-km-long dispersion-shifted fiber with a zero GVD around 1539nm.Fiber C was a2-km-long dispersion-flattened fiber with a GVD of0.2 ps r km r nm at1.55␮m and a dispersion slope as small as q0.004ps r km r nm2in the1.5-␮m region.
The input pulse into these fibers needed to produce the SC was generated by a
w x
regeneratively and harmonically mode-locked fiber laser10.This laser can emit a 3-ps transform-limited Gaussian pulse train at10GHz.Regenerative mode-locking was accomplished by feeding back the longitudinal self-beat signal which was detected with a high-speed photodetector and a high-Q filter.The phase between the pulse and the modulation signal was adjusted so that the pulses would always experience the maximum transmission when they passed through the modulator. Thus,complete mode-locking was achieved automatically because we used an ideal feedback signal as the modulation frequency.This reflected the instantaneous frequency change between the longitudinal beats even when perturbations were applied.This enabled us to realize a stable mechanism and long-term operation with neatly repetitive longitudinal modes.
Here,it is important to use a pulse train with a high repetition rate in order to measure the longitudinal mode spacing clearly with a high-resolution Fabry᎐Perot resonator.It is generally difficult to measure coherence degradation using a
COHERENCE DEGRADATION IN SUPERCONTINUUM GENERATION217
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femtosecond laser with a low repetition rate10᎐100MHz,as the longitudinal mode spacing cannot be resolved sufficiently without employing a monochromator with an ultrahigh resolution.However,if there is a longitudinal mode spacing of10 GHz,it is easy to measure the change in the longitudinal mode.Therefore,this stable laser source made it possible for us to undertake a detailed study of the way in which the coherence is degraded for the first time.The finesse of the Fabry᎐Perot resonator we used was higher than100and the FSR was170GHz.The wavelength of the input pulse was1542nm.In order to generate spectral broadening due to nonlinear effects,the pulse was amplified with a high-power erbium-doped fiber Ž.
amplifier EDFA and then the output pulse with a peak power of as high as a few watts was coupled into the test fibers.
III.THE SC CHARACTERISTICS OF A DISPERSION-DECREASING FIBER Figure1shows the spectral broadening generated by adiabatic soliton compres-Ž.
sion in fiber A DDF.The input power was approximately set at an N s1soliton to achieve adiabatic soliton compression.Figure1a shows the broadened spectrum at around1542nm.The spectrum was broadened from1500to1570nm.The
Ž.Ž.
FIG.1.Supercontinuum characteristics of fiber A dispersion-decreasing fiber,DDF:a spectral Ž.Ž.Ž. broadening,b change in the longitudinal modes coherence degradation,and c output pulse
waveform measured with an autocorrelator.
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NAKAZAWA ET AL.
power of the SC in the longer wavelength region was higher than that in the shorter wavelength region,while the spectral broadening toward the shorter wavelength region was broader than that toward the longer wavelength region. Figure1b shows the detailed structure of the spectra sliced every10nm between 1510and1570nm.The longitudinal modes of the input pulse are clearly seen every 10GHz in each spectrum,and the corresponding pulse shape in the time domain maintains a clean pulse waveform.The measured pulse width was approximately16 ps and was the same in each spectrally sliced component.
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It is interesting to note that there are no continuous wave CW spectral components between the longitudinal modes in the spectral broadening in the longer wavelength region,while there is a gradual increase in the CW components between the modes toward the shorter wavelength region.This can be understood as follows.In the process of spectral broadening toward the longer wavelength region,the spectral components always experience anomalous GVD even though the GVD becomes gradually closer to zero.Here the nonlinear phase change can be balanced with the anomalous GVD due to the soliton effect and phase matching which occurs in the nonlinear process.This is the origin of soliton or modulational instability,in which the phase between the modes is conserved.In fiber A,an N s1soliton is excited and pulse spikes do not appear on the top of the soliton. On the other hand,when the spectrum is broadened into the shorter wavelength region,some of the spectral components experience small GVD or even normal GVD as the GVD at the output end is almost zero at1542nm.These shorter wavelength components,which experience small GVD,cause the excitation of high-order solitons.When there is ASE,the evolution of high-order solitons is disturbed and each pulse has a different waveform.This effect causes coherence degradation and eventually a CW spectral component appears between the longitu-dinal modes.The output pulse measured with an autocorrelator is shown in Fig.1c. The pulse is compressed from3ps to260fs with this adiabatic process.This coherent SC has the same origin as pulse compression using DFF configurations w x
11,12.
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It should be noted that there is Gordon᎐Haus G-H jitter even when coherence is maintained throughout the SC process.If the ASE noise is large and the input power into the DDF is low,the G-H jitter becomes significant and this may limit the application rage of the SC to ultra-high-speed communication using femtosec-ond pulses.In addition,if the DFF is excited with a higher-order soliton pulse at the input,the coherence may deteriorate as described later.However,the order of the soliton number is relatively low compared with a DSF or a DFF with low GVD, and the contribution is smaller than those of the DSF or DFF.The other important factor is the nonuniformity in the adiabatic process.If the change in the GVD as a function of distance is very drastic and deviates from the adiabatic change,it also causes an incoherent SC.
IV.THE SC CHARACTERISTICS OF A
DISPERSION-SHIFTED FIBER
Figure2shows the spectral broadening generated by SC generation in fiber B Ž.
DSF.Figure2a shows the broadened spectrum at around1542nm,in which the
COHERENCE DEGRADATION IN SUPERCONTINUUM GENERATION219
Ž.Ž.
FIG. 2.Supercontinuum characteristics of fiber B dispersion-shifted fiber,DSF:a spectral Ž.Ž.Ž. broadening,b change in the longitudinal modes coherence degradation,and c output pulse waveform measured with an autocorrelator.
spectrum is broadened between1510and1580nm.Similar to Fig.1a,the continuum in the longer wavelength region is higher than that in the shorter wavelength region.The spectral broadening toward the longer wavelength region is
Ž.
rather broader than that for fiber A DDF.This is due to the fact that the GVD of Ž.Ž.
the DSF fiber B is much smaller than that of the DDF fiber A and therefore nonlinear interaction can be easily enhanced when the same power is coupled to the fiber.Figure2b shows the detailed structure of the spectra sliced every10nm between1510and1570nm.There is a noticeable difference between Fig.1b and Fig.2b.In Fig.2b,the longitudinal modes every10GHz have almost disappeared in each spectrum and CW components cover the whole spectral region.Such a spectral profile indicates that the output waveform has become incoherent.There is a slight indication of the longitudinal modes at1550nm,but it is already partially coherent.The degradation of the coherence is more rapid in the DSF than in the DDF.The reason for this degradation is that the evolution of very-high-order solitons is initiated by the ASE noise when optical amplifiers are used.It is
Ž
important to note that the origin of this evolution is MI or FWM modulational
.w x instability or four wave mixing which occurs at the top of the soliton pulse13. When there is no ASE noise,the evolution of higher-order solitons is deterministic
w x
and the waveform varies repeatedly along the fiber13.Hence the coherence is
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NAKAZAWA ET AL.
not degraded.However,it is degraded when there is ASE noise.The output pulse width shown in Fig.2c is4.5ps.It is important to note that the measured pulse width in each spectrally sliced component is almost the same but there is no coherence between the pulses.Moreover,when such a pulse train is measured with an autocorrelator,it is difficult to evaluate how much jitter and random amplitude fluctuation it contains.
The nonlinear interaction can be reduced when the signal wavelength is set in the normal GVD region because phase matching to initiate MI process is not achieved.This may lead one to believe that,in terms of maintaining the coherence, it is better to use the normal GVD region.However,it is important to note that a broad SC is not obtained when the wavelength of the input pulse is set in the normal dispersion region.This is because the pulse is soon broadened thus halting the parametric process.This situation is different from the femtosecond pulse
w x
compression experiment described in Ref.2since there the fiber was as short as a few centimeters to a few tens of meters in length and the input power was extremely high.In such a case,a highly broadband SC can be obtained with
Ž.
relatively little pulse broadening a few picoseconds pulse width.
V.THE SC CHARACTERISTICS OF A DISPERSION-FLATTENED FIBER Figure3shows the spectral broadening generated by SC generation in fiber C Ž.
DFF.Figure3a shows the SC broadened from1470to1630nm,which is the broadest among the three fibers.The continuum is broadened fairly symmetrically in both the longer and shorter wavelength regions compared with Figs.1a and2a.
Ž.
However,in the same way as with fiber B DSF,the longitudinal modes soon disappear when the wavelength is detuned from the input wavelength.This result indicates that,although it is possible to generate a broader SC,the use of DFF is not effective for maintaining the coherence.This can be understood as follows.As with fiber B,high power excites a high-order soliton in the low GVD region,which is initiated with ASE noise.Hence the waveform and spectrum are different for each pulse,resulting in a degradation in the coherence.However,the spectrum is broadened greatly since the GVD is kept constant over a wide wavelength region. The output pulse is shown in Fig.3c.This pulse is greatly distorted due to the group delay characteristic of fiber C.
VI.JITTER AND RANDOM EVOLUTION OF THE SC PULSES AND
THEIR SUPPRESSION
In the next step,we measured the output pulse characteristics of the SC with an electrical sampling scope to see how much difference there is in the waveforms. We simultaneously investigated the RF spectrum to see how much noise appears around10GHz.The wavelength of the sliced spectrum was set at1553nm.The results are shown in Fig.4,where fibers A,B,and C correspond to a,b,and c, respectively.In Fig.4,the RF spectra are shown on the left and the output of the sampling scope is shown on the right.We used0.25-nm optical filters to extract the
COHERENCE DEGRADATION IN SUPERCONTINUUM GENERATION221
Ž.Ž.
FIG.3.Supercontinuum characteristics of fiber C dispersion-flattened fiber,DFF:a spectral Ž.Ž.Ž. broadening,b change in the longitudinal modes coherence degradation,and c output pulse
waveform measured with an autocorrelator.
spectral component.With fiber A,a neatly repetitive10GHz component was
clearly obtained as shown in Fig.4a-1,which is also proven by the RF spectrum
shown in Fig.4a.A clean10-GHz component appeared and there was no noise
floor.These results indicate that there is no coherence degradation in fiber A.This
means that a spectral slice using DDF can be used as an optical source for WDM w x8.However,with fibers B and C,the waveforms shown in Fig.4b-2and4c-2are very different from that in Fig.4a-1and have large amplitude fluctuations.This is due to randomly excited high-order solitons initiated by ASE.Although there is a pulse-like waveform,there is a very large amplitude fluctuation of almost100%. Such pulses are not applicable for communication purposes since they even contain timing jitter.This is also confirmed with the RF spectra shown in Fig.4b-1and 4c-1,in which there is a uniform RF noise component over a broad frequency region.The line below y80dBm is the noise level of the measurement system. Because of the amplitude fluctuation and timing jitter at the top of the pulse,the noise floor appears at y70dBm.The integrated noise power becomes comparable to that of the10GHz components,which means that the amplitude fluctuation is almost100%.
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NAKAZAWA ET AL.
FIG.4.Output waveform characteristics of the SC measured with a high-speed electrical sampling scope and an RF spectrum analyzer.RF spectrum of the SC measured with a pin photo detector for Ž.Ž.Ž.Ž. a-1DDF,b-1DSF,and c-1DFF.Output waveforms measured with a sampling scope for a-2Ž.Ž.
DDF,b-2DSF,and c-2DFF.
VII.SUMMARY
We have described for the first time changes in coherence during the SC generation in DSF,DFF,and DDF.It is shown that both time and frequency coherence are greatly degraded in DSF and DFF,when a higher-order soliton is excited in the presence of ASE noise.This can also be understood as a nonlinear pulse evolution in the presence of ASE noise through the MI effect.It should be noted that a high-order soliton does not evolve randomly even when there are perturbations such as the self-Raman effect or third-order dispersion.Although the waveform itself is greatly distorted due to the perturbations,each pulse of a pulse train is distorted to exactly the same degree.Thus there is no coherence degradation.When there is ASE noise at the beginning,however,it acts as a seed for the random evolution of high-order solitons,which degrades the coherence between the pulses.These pulses may not be suitable for long-distance,high-speed transmission.
COHERENCE DEGRADATION IN SUPERCONTINUUM GENERATION223 On the other hand,adiabatic N s1soliton compression in a DDF can maintain its longitudinal mode coherence.Therefore,such a fiber is very useful for produc-ing a coherent SC and will be applicable to ultra-high-speed communication.It should be noted that the excitation of high-order solitons in a DDF also degrades the coherence in the presence of ASE.
ACKNOWLEDGMENTS
The authors express their thanks to Drs.I.Yamashita and I.Kobayashi of NTT Optical Network Systems Laboratories for their continuing encouragement and fruitful comments.
REFERENCES
w x1R.R.Alfano,Ed.,‘‘The Super Continuum Laser Sources,’’Springer-Verlag,Berlin r New York, 1989.
w x2C.V Shank,R.L.Fork,R.Yen,and R.H.Stolen,‘‘Compression of femtosecond optical pulses,’’
Ž.
Appl.Phys.Lett.,vol.40,no.9,7611982.
w x3M.Nakazawa and M.Tokuda,‘‘Continuum spectrum generation in a multimode fiber using two
Ž.
pump beams at1.3␮m wavelength region,’’Jap.J.Appl.Phys.,vol.22,no.4,L2391983.
w x4M.Nakazawa and M.Tokuda,‘‘Measurement of the fiber loss spectrum using fiber Raman
Ž.
optical-time-domain reflectometry,’’Appl.Opt.,vol.22,no.12,19101983.
w x5K.Mori,T.Morioka,H.Takara,and M.Saruwatari,‘‘Continuum tunable optical pulse generation utilizing supercontinuum in an optical fiber pumped by an amplified gain-switched LD pulse,’’in Proc.of Optical Amplifiers and Their Applications,OA&A’93,p.190,MD11,1993.
w xŽ.
6T.Morioka et al.,‘‘1Tbit r s100Gbit r s=10channel OTDM r WDM transmission using a single
Ž.
supercontinuum WDM source,’’Electron.Lett.,vol.32,no.10,906,1996.
w x7S.Kawanishi et al.,‘‘Single-channel400Gbit r s time-division-multiplexed transmission of0.98ps pulses over40km employing dispersion slope compensation,’’Electron.Lett.,vol.32,no.10,916Ž.
1996.
w x8K.Tamura,E.Yoshida,and M.Nakazawa,‘‘Generation of10GHz pulse trains at16wavelengths
Ž.
by spectrally slicing a high power femtosecond source,’’Electron.Lett.,vol.32,no.18,1691,1996. w x9T.Okuno,M.Onishi,and M.Nishimura,‘‘Dispersion-flattened and decreasing fiber for ultra-broadband supercontinuum generation,’’Proc.of ECOC’96,Edinburgh,UK,p.80,PDP,1997.
w x10M.Nakazawa,E.Yoshida,and Y.Kimura,‘‘Ultrastable harmonically and regeneratively mode-
Ž.
locked polarisation-maintaining erbium fibre ring laser,’’Electron.Lett.,vol.30,no.19,16031994. w x11S.V.Chernikov,D.J.Richardson,E.M.Dianov,and D.N.Payne,‘‘Picosecond soliton pulse
Ž.
compressor based on dispersion decreasing fibre,’’Electron.Lett.,vol.28,18421992.
w x12M.Nakazawa, E.Yoshida,H.Kubota,and Y.Kimura,‘‘Generation of a170fs,10GHz transform-limited pulse train at1.55␮m using a dispersion-decreasing,erbium-doped active soliton
Ž.
compressor,’’Electron.Lett.,vol.30,no.24,20381994.
w x13M.Nakazawa,K.Suzuki,H.Kubota,and H.A.Haus,‘‘High-order solitons and the modulational
Ž.
instability,’’Phys.Re¨.A,vol.39,no.11,57681989.。