3
ROBERTSON, P, and WOE, T.: 'Bandwidthefficient turbo trellis-coded
modulation using punchlred component codes', IEEE J Se/, Areas
Commun.. 1998, 16, pp. 20G2218
4 DIVSALAR, D.,and SIMON, M.K.: 'The design of trcllis coded MPSK for
fading channel: performance criteria', IEEE Trans. Commun., 1988, 36,
--..,
pp. 100+1012
5 AL-SEMARI. s., and FUJA, T.: 'I-Q TCM: reliable communication over the
Rayleigh fading channel close to the cuttoff rate', IEEE E". Inf
The03 1997,43, pp. 2SW262
6 JELICIC,B.D.,and ROY, s : 'Design oftrellis coded QAM for flat fading and
AWGN channels', IEEE Trans. Yeh. k h n o l . , 1994,44, pp. 192-201
7 ALAMOUTI, s.M.: 'A simple transminer diveeity scheme for wireless
communications', IEEEJ Se/. Areos Commuri., 1998,16, pp. 1451-1458
constant effective refractive index n,,, a period chirp C,,, and an
apodisation profile A(zj (z is along the fibre axis). As shown in [7],
the apodisation profile A(=) can be 'mapped' into the magnitude of its
complex transmission coefficient 1HLi)l (so-called space-to-wavelength
mapping):
where % =
1. - 20 (with
being the Bragg wavelength at the input cnd
of the grating, z = 0), and L is the grating length. However, an efficicnt
space-to-wavelength mapping is ensured only ifthe following condition
is verified [7]
IC,~ Q K
Generation of ultra-high repetition rate
optical pulse bursts by means of fibre
Bragg gratings operating in transmission
J. Azafia, R. Slavik, P Kockaert, L.R. Chen
and S . LaRochelle
An experimental demonstnition of the usc af specially apodised
linearly chirped fibre Brvgg gratings operating in transmission for
generating a custamised ultra-high repetition rate optical pulse burst
(325 GHz, in the example shown) from B single ulea-shon pulse.
Introduction: Generation of optical pulse bursts at repetition rates
beyond those achievable by conventional modelocking techniques is
becoming increasingly important for many areas, including ultrahigh-speed optical communications, photonic signal processing, and
optical computing. Ultra-high repetition rate pulse bursts can be
generated by amplitudefphase spectral filtering of a single ultrashort input pulse using pulse shaping techniques based on bulk
optics [I], integrated arrayed waveguide gratings [2], or fibre Bragg
gratings (FBGs) [3-51. The advantages of FBGs over othcr technologies are inherent to an all-fibre approach compactness, low insertion
loss, and the potential for low cost.
In this Letter we experimentally demonstrate the usc of specially
apadised linearly chirped FBGs (LCFBGs) operating in transmission as
amplitude filtering stages for generating a customised ultra-high repetition rate optical pulse burst from a single ultra-short input pulse, or
alternatively from a lower rate pulse sequence. In particular, we use a
single apodised LCFBG specifically designed to generate a 325 GHr
optical pulse bunt by transmission of two identical narrow spectral
bands. This technique offers all the advantages of an FBG-based
solution and in addition avoids the use of additional devices (e.g.
optical circulators or Mach-Zehnder interferometers) to remeve the
reflected and processed signals.
Principle oJoperarion: High repetition rate optical pulse bursts can
be produced by amplitudefphase spectral filtering of a single ultrashort input pulse [I]. In general, the required filter must be spectrally
periodic in both amplitude and phase. Amplitude-only and phase-only
filtcring represent two extremes. In the amplitude-only filtering
approach, a periodic set of frequency components of the original
pulse is blacked the group delay of the pcnodic filter is constant
within cach period and all passhands experience the same group delay.
This results in a periodic pulse train the repetition rate of which is
equal to the frequency spacing of the filter bands. Furthermore, the
amplitude response within one spectral period of the filter determines
the shape of the temporal envelope of the generated pulse burst.
All-fibre periodic amplitude filters can he implemented using
sampled or superimposed FBG smcNres operating in reflection
[3, 51, or FBG-based Fabry-Perot (FP) cavities operating in transmission [6]. The FP approach presents the advantages of a large number of
transmission peaks 161. However, the spectral passbands in an FP filter
always exhibit a Lorcntzian shape, which would m u l t in a rapidly
decaying temporal envelope for the burst, causing undesirable pulse-topulse intensity fluctuations.
Alternatively, we show in this Letter that all-fibre periodic amplitude
filters can be implemented using a single specially apodised LCFBG
operating in transmission. Consider an apodised LCFBG with a
ELECTRONlCS LETTERS 21st November 2002
A
~
(2)
where K is a constant dependent on the shape of the apodisation profile
and AI is the single passband bandwidth. As a consequence of (2). the
choice of the grating chirp is limited by the bandwidth which has to be
resolved (the narrower the bandwidth, the lower the grating chirp).
Since we operate the grating in transmission where the group delay is
constant within cach transmitted spectral band (neglecting fibre dispersion) and all the hands experience the same group delay, we are
performing an amplitude-only filtering process.
Note that we also have high flexibility in tailoring the spectral shape
of the passbands, which is cssential to control the temporal envelope of
the generated pulse burst: as discussed above, we only need to record
the desired spectral shape in the grating apodisation profile (see (I)).
Results and discussion: For our experiments, we used a single
apodised LCFBG specifically designed to generate a 325 GHz optical
pulse burst. For simplicity, we consider here only two transmitted
bands but of course the grating can be similarly designed to transmit a
larger number of spectral bands. Thc apodired LCFBG was written
using a phase-mask scanning method with dithering to realise the
apodisation profile [E]. The phase mask (Tcraxion Inc.) had a chirp of
I .2S nmfcm (C,,
=O.625 nmfcm). To satisfy the condition expressed
in (2) with the available phase mask, the repetition rate needed to he
more than 300 GHz (to obtain a lower repetition rate, a phase mask
with lower chirp would be nccessary). The grating was -49 mm long,
which ensures that the grating reflection bandwidth ( 2 9 n m ) is
broader than that of the input pulse ( 2 5 nm). The apodisation profile
of the LCFBG (Fig. I , inset) consists of two separate valleys which,
by virtue of the space-to-wavelength mapping process, translates into
two transmitted spectral bands. The amplitude shape of these bands
was optimised using numerical simulations [5] to achieve an appraximately square-like temporal envelope for the generated pulse burst.
To achieve the desired repetition rate, the spectral separation between
transmitted bands must be -2.57 nm ( 2 3 2 5 GHr), i.e. using (l), a
spatial separation between the valleys in the apodisation profile of
-13.7 mm is reauired.
l
i
,ne
,se
?&a
I "
1552
15Ed
i.
"m
Fig. 1 Measured transmission characteristics of FBG
-lmansmissivity - - - mnsmissinn group delay
Inset: Apodisation profile of FBG
Vol. 38 No. 24
1555
0 IEE 2002
16 September 2002
Electronics Lelleis Online No: 20021048
Dol: 10.1049/e1:20021048
J. Azaiia and L.R. Chen (Photonic SyJrems Croup. Deportment o/
Electrical and Computer Engineering, McCill LJniverviq, Montreal$_i/
Quhbec, Canada H3A 2.40
R. Slavik, P. Kockaert and S. LaRochelle (Centre d'oprique, Phoronique et Laser Dhparirmenr d e Ghnie Elecrrique et de G k i e
lnformorique, Universith Laval, Sainte-Fo.~. QuGbec. Cunndu G I K
7P4)
P. Kockaert (Service d'Optique el d'Acoustique. Uxiversitk Libre de
Bruelles, CP194/1. 8-1050 Brussels, Belgium)
R. Slavik: On leave from IREE AS CR, Prague, Czech Republic
References
I
"ms 0s
2
Fig. 2 Measured autocorrelution trace o/trannitted signalfrom FBG
Inset: Autocomelation trace of typical pulses incident upon FBG
Fig. 1 shows thc measured transmissivity (solid curvc) and group
delay in transmission (dashed curve) of the apodised LCFBG. As
expected two spcctral bands separated by -2.57 nm (-325 GHz) are
transmitted. The two transmitted bands exhibit similar amplitude and
group delay characteristics, the group dclay being nearly constant
within each transmitted band. The transmission peak of each passband
is close to 50%. To ensure that practically all the transmitted energy
falls within the two passbands, the reflectivity out of these bands
(within the gnting bandwidth) must he maximisea in this case it was
>99% Fig. 2 shows the measured autocorrelation traces of the
transmitted signal and the input pulse (inset) in thc apodised LCFBG.
The input pulse was generated from a wavelength-tunable 20 MHr
modelocked erbium-doped fibre laser (Pntel, FFL) centred at the
grating central wavelength. The laser generates non-transform limited
Gaussian pulses with a full-width-half-maximum time width of -1 ps.
The beating between the two transmitted spectral bands produces the
desired optical pulsc burst: the repetition rate of the pulse burst,
325 GHz, is fined by the spectral scpamtion between the transmitted
bands. The triangular envclope of the autocorrelation trace indicates
that the generatcd pulse burst exhibits a square-like envelope of finite
duration (-lops). Note that thc temporal duration of the pulsc bunt
depcnds on the bandwidth of the transmitted spectral bands (the
narrower the bandwidth, the longer the pulse bunt). As mentioned
above, to efficiently solve a narrower bandwidth, a lower grating chirp
would be requircd (see (2)). The duty cyclc of the generated sequence
(defined as the ratio between the pulse FWHM-time-width to the burst
period) is =50% The degraded extinction ratio between pulses in the
measured train is due to the fact that the autocorrelation trace corresponding to each individual pulse is broader than the pulse itself.
Obviously, the duty cycle can be improved by increasing the number of
transmitted spectral bands.
Conc1usion.r: We have experimentally demonstrated the use of an
apodiscd LCFBG operating in transmission as amplitude filtering
stages for generating a ultra-high repetition rate pulse burst from a
single ultra-short pulse. This technique has all the advantages o f t h e
previously proposed FBG-bascd solutions, but avoids the need for
additional devices to retrieve the reflected and processed signals.
Moreover, it can be applied for multiplying the repetition rate of a
given periodic pulse train by spectral mode selection.
Acknowledgments: The authors acknowledge financial support from
the Natural Scicnces and Engineering Research Council (Canada), the
Canadian Institute for Photonic Innovations and its industrial affiliates, the Ministerin de Educacion y Cultura (Govemment of Spain)
and the Fonds National de la Recherche Scientifique (FNRS,
Belgium).
1556
3
4
5
6
7
8
R'EINER, A.M., and LEAIRD, U.E.: 'Generation of terahem-rate trains of
femtosecond pulses by phase-only filtering', Opt. Lett.. 1990, 15, (I),
pp. 51-53
LBAIRU. D.E.. et 01.: 'Generation of high-repetition-rate WDM pulse trains
From an arrayed-waveguide grating', IEEE Photonics Techno/. Lett.,
2001, 13, (3). pp. 221-223
PETROPOULOS, P, et 01.: 'Generation of 40-GHr pulsc stream by pulse
multiplication with a sampled fibre Bragg grating', Opt. Len, 2000, 25,
(8J, pp. 521-523
LONGHI. s., er 01.: '40-GHz pulse-train generation at 1.5 pm with a
chimed fiber eratine as a frewencv
.~ multidier', Om Lett., 2000, 25,
(19j, pp. 1 4 s l - 1 4 s j
AZANA.~,etul.: 'Generation ofultrahigh repetition rate pulse bursts using
supenmpased fibre Bragg gratings'. 2 f h European Conf. on Optical
Communication (ECOC'20021, Copenhagen, Denmark, September
2002, paper 08.3.3.
UOUCET. s., SLAVIK,
K., and LAROCHELLE, s.: 'High-finesse large band
Fably-Perof fibre filter with superimposed chirped Bragg gratings',
E I ~ C I Len.,
~ ~ .2002,38, (9), pp. 4 0 2 4 0 3
AZANA,I.. and CHEN, L.R.: 'Synthesis oftemporal optical waveforms using
fibre Bragg gratings: A new approach bared on spafe-lo-frequency-totime mapping', J Opt. Soc. A m B, 2002 (to appear)
MARTIN, I , and OUELLBTTE, F.: 'Novel writing technique of long and
highly reflective in-fibre gratings', Elrcrmn. Len, 1994,30, pp. 91 1-812
Four-channel parallel 3.125 Gbit/s/ch/s fibre
optic receiver/transmitter chip signal
detection circuit
Jae J. Chang, M. Abrams, Young Kim, E Bien
and Myunghee Lee
A signal detect/loss of signal (SD/LOSj circuit for a four-channel
parallel TXjRX chip was designed and fabncafed in a 0.35 pm SiGe
HBT process. The measurement was performed in power supply range
(3.63.6 VJ and case temperatures (&IOO'Cj under the influence of
cross-talk from the other RX channels and tinsmiher channels. The
result shows a typical assen level of -20.5 dBm and a de-assen level
of -21.5 dBm with a typical hysteresis of I dB.
Introduction: In parallel optical fibre communication receiver
systems, it is highly recommendcd to have a feature to discem
meaningful data from the meaningless, unwanted noise to inform
the customers whether the signal is missing or not [I]. The circuit
component performing this feature is called signal detect (SD) or loss
of Signal (LOS).
This feature is primarily used for the following purposes: (i) to
monitor the status of the link, (ii) to squelch thc receiver channel when
the channel is not being used or whcn the input signal is missing, to
save power and keep the dice temperature down.
The hnctional specification of SD/LOS is summansed by assert/deassert level of input signal power and hysteresis of the IcvcIs, which is
provided to prevent chattering when the input signal level is close to the
threshold of assertlde-assert level. The dcsign of SD/LOS is challenging because this assert/de-asscn level is usually set at a very low
optical input power range even under the sensitivity level of required
receiver operation specification, and also because these levcls have to
ELECTRONICS LETTERS 21st November 2002
Vol. 38 No. 24
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