Download How optics has changed data communications

Invited Paper
How optics has changed data communications
Ivan Gleska,b, * and Paul R. Prucnalb
a
Department of Experimental Physics, Comenius University, Bratislava, Slovakia
b
Department of Electrical Engineering, Princeton University, USA
ABSTRACT
Today’s voice/data communication packet or circuits switched networks are characterized by hybrid architectures. The
relatively low transmission bandwidth of electronics and optoelectronic interfaces makes it difficult to fully utilize the large
bandwidth of optical fiber. This problem can be overcome if the data is left in optical form during signal processing steps
such as address recognition, demultiplexing, switching, routing, regeneration, dispersion compensation, and clock recovery.
This requires high bandwidth, ultrafast, all-optical devices to perform these signal processing functions. These will allow
move away from electronic bandwidth of a few tens of gigahertz to a terahertz bandwidth offered by all-optical devices thus
fully utilize the enormous capacity of an optical fiber. This requires new data formats as well as a whole new class of
ultrafast all-optical devices. We will describe a new generation of such devices based on different optical phenomena such
as wavelength filtering, phase shifting, optical interference, mixing, and controlled nonlinear index changes which enable us
to perform various very complex functionalities not only in today’s networks but also for the future use in all-optical data
networks.
Keywords: Nonlinear optics, fiber-optic devices, all-optical data processing, nonlinear interferometers
1. INTRODUCTION
In the backbone of today’s high performance networks, optical fibers provide enormous point-to-point communications
capacity. To accommodate the rapidly growing volume of communications traffic, each fiber’s capacity can be augmented
by using “Dense Wavelength Division Multiplexing (DWDM)”, Commercial DWDM equipment is being deployed which
multiplexes tens of 10 Gbit/s, with future target of 40 Gbit/s channels into each optical fiber, enabling an aggregate
throughput on the order of a few Terabit/sec per fiber. Optical Time Division Multiplexing (OTDM) is another technique
used to increase the transmission speed by carrying data on narrow, single wavelength optical pulses which are time
interleaved. However, as the capacity of a single fiber extends to a few Terabits per second, a new communications
bottleneck is emerging at the endpoints of the optical fibers, where routing and switching are performed to direct the traffic
to its destination. Currently, the majority of backbone routers rely upon electronic crossbar switches to route packets.
Electronic crossbars, however, do not provide sufficient capacity to interconnect multiple fibers due to their low speed and
scalability. They rely on integrated devices made of silicon, gallium arsenide, and indium phosphide. The switching speed
of these devices is determined by the frequency response of the material and the architecture of the transistors in the switch.
Complementary metal oxide semiconductor technologies, which are used in the majority of high performance digital
devices like microprocessors, are not likely to scale beyond tens of giga Hertz. The highest performance electronics based
upon III-V materials like indium phosphide have only reached a few hundred giga Hertz1, which is far from Terahertz speed
needed for terabit routers required for optical networks in the near future.
Growing demand for higher switching speeds has led to the development of all-optical mechanisms for all-optical ultra-fast
data processing. Thanks to advances in optics all-optical devices could be employed in optical networks for different
applications such as demultiplexing, packet routing in networks and clock extraction. All-optical devices are also finding
applications in high speed all-optical signal processing such as wavelength conversion, all-optical 3R regeneration (reamplification, re-shaping, re-timing), and high speed all-optical analog sampling.
*
[email protected]
15th Czech-Polish-Slovak Conference on Wave and Quantum Aspects of Contemporary Optics,
edited by Miroslav Miler, Dagmar Senderáková, Miroslav Hrabovský, Proc. of SPIE Vol. 6609, 660904, (2007)
0277-786X/07/$18 · doi: 10.1117/12.739364
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2. THE USE OF OPTICAL NONLINEARITIES FOR ALL-OPTICAL SWITCHING
All-optical switching generally takes advantage of a nonlinear interaction between optical waves. The nonlinearity in a
variety of materials including fiber, semiconductors and crystals like lithium niobate has been used to demonstrate alloptical switching.
2.1 All-optical switching based on four wave mixing
Four wave mixing (FWM) is a parametric phenomenon arising from third order optical nonlinearity and has been used to
develop all-optical switches. To understand the origin of FWM, let us consider the basic equation of third order
nonlinearity:
P (t ) F ( 3 ) u E (t ) 3
(2.1)
(3)
Here, P(t ) is the instantaneous polarization, F
is the third order nonlinearity coefficient and E(t) is the instantaneous
applied electric field. If the applied field consists of three frequency components
(2.2)
E ( t ) E 1 ( t ) u e i Z 1t E 2 ( t ) u e i Z 2 t E 3 ( t ) u e i Z 3 t
Substituting (2.2) in (2.1) shows that P(t) has frequency terms at Z1, Z2, Z3, 3Z1, 3Z2, 3Z3, (+/-Z1+/- Z2+/- Z3), (2Z1 +/Z2), (2Z1 +/- Z3), (2Z2 +/- Z1), (2Z2 +/- Z3), (2Z3 +/- Z1), (2Z3 +/- Z2). These time-varying polarization components act as
sources of new components of electromagnetic field at these frequencies. By general convention, all the terms in
parentheses are called the FWM mixing terms while the 3Z1,2,3 terms are called the third harmonic generation terms.
Thus, the interaction of three electromagnetic waves in a medium with third order nonlinearity results in the generation of
electromagnetic waves at new frequencies. However, the power created at the new frequencies by the four wave mixing
process depends on the efficiency of the parametric coupling. Higher efficiency of FWM mixing requires a matching of the
frequencies as well as wave vectors, a condition known as “phase matching”. In general, this phase matching condition is
more difficult to satisfy for the terms where a single photon is produced by the annihilation of three photons, as in the case
of the terms, (Z1+ Z2+ Z3), (2Z1 + Z2), (2Z1 + Z3), (2Z2 + Z1), (2Z2 + Z3), (2Z3 + Z1), (2Z3 + Z2). The phase matching
condition is easier to satisfy for the other mixing terms - (Z1+ Z2- Z3), (2Z1 - Z2), (2Z1 - Z3), (2Z2 - Z1), (2Z2 - Z3), (2Z3 Z1), (2Z3 - Z2) – thus, resulting in more power at these frequencies.
FWM occurs even when the number of input waves is less than three. Thus, in the presence of two beams of frequencies Z1
and Z2 in a medium having third order nonlinearity, FWM mixing terms are produced at the frequencies, (2Z1 - Z2) and
(2Z2 - Z1). Although the above equations are strictly valid only for continuous wave beams, FWM mixing terms are
generated even when the beams are pulse trains, if the pulses from the two beams at Z1 and Z2 temporally overlap. This
technique can be used to all-optically demultiplex a single channel from an OTDM stream. Thus, the information present in
one channel can be duplicated onto the mixing wavelengths, which can be filtered out from the rest of the data channels.
One disadvantage of all-optical switching using FWM is the low efficiency associated with the mixing process. An average
power ratio between the FWM and the control pulses of about 6% was only obtained. Another disadvantage of FWM
switching is the degradation in performance due to the tendency of the control and data pulses to walk off from each other
because of their different group velocities. In the experiments done by Andrekson2, this was reduced by the use of
dispersion shifted (DS) fiber, which has very low chromatic dispersion at the operating wavelength thus minimizing the
walk off to 0.6ps. Thus, the DS fiber serves two purposes – it reduces the effect of walk off between the control and data
signals caused by group velocity dispersion; it also increases the nonlinear interaction by providing the condition of phase
matching for the four wave mixing process.
In optical networks, data signals quickly reach an arbitrary state of polarization after transmission in the fiber. Since alloptical switches usually handle signals after transmission, it is important that they should have uniform switching
performance for different polarizations. For efficient four-wave mixing, it is necessary that the data and control pulses have
their polarizations aligned. This essentially makes the switch mentioned above polarization dependent. Polarization
independent operation of FWM based switch at 100 Gbit/s was obtained using a polarization maintaining (PM), polarization
rotating fiber loop mirror (PRLM)3. The technique essentially involved the addition of two independent four-wave mixing
processes for the two orthogonal polarizations with equal efficiency, ensured by providing equal pump powers in both
polarization directions.
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All-optical switching based on FWM in semiconductor optical amplifier (SOA) was attempted because of the compact size
of the SOA. Error free demultiplexing of a 6.3Gbit/s channel from a 100 Gbit/s aggregate stream was demonstrated in 1994
by Kawanishi4. The power penalty of demultiplexing was about 5dB and the efficiency of the FWM mixing process was
about 4%.
A polarization independent FWM process was demonstrated for 200Gbit/s demultiplexing by using two PM fibers before
and after the SOA5. In this method, the first PM fiber separates the two orthogonal polarizations temporally, thus
depolarizing the signal to suppress polarization coupling in the SOA. The two orthogonal polarizations then undergo
independent FWM mixing in the SOA before being combined temporally using the second PM fiber, which has its principal
axis rotated 90 with respect to that of the first PM fiber. This switch demonstrated stable, error free operation with less than
0.5dB polarization dependency and a FWM mixing efficiency of 15%.
2.2 The use of cross phase modulation for all-optical switching
Cross phase modulation (XPM) induced carrier frequency shifts in optical fiber is another technique that has been used for
high speed all-optical demultiplexing. In this method, optically amplified control (clock) pulses are used to shift the center
wavelength of every coincident signal (data) pulse by adjusting the temporal synchronization such that the signal pulses
coincide with the clock pulses edges. The center frequency shift depends on the power of the clock pulses. At the output end
of the fiber, an ordinary frequency filter can be used to separate out the demultiplexed data channel.
Multiple data channels can be simultaneously demultiplexed by an extension of the same method. If the control pulses used
have a width that spans multiple data channels, different data channels “see” different control pulse powers and hence have
different center frequency shifts. At the output end of the fiber, the different channels can be demultiplexed by using a
grating. A 4-output demultiplexing operation was demonstrated at 60Gbit/s by Uchiyama6.
3. NONLINEAR OPTICAL INTERFEROMETERS FOR ULTRA-FAST
ALL-OPTICAL PROCESSING
3.1 Sagnac interferometer
Fig. 1 shows the schematic of a Sagnac Interferometer also called Fiber Loop Mirror. It consists of a 2x2 directional coupler
whose two ports (3 and 4) are connected together using a loop of optical fiber. Light is
2
CW
CCW
Fiber Loop
(Length L)
3
4
K:(1-K)
Input
1
2
Output
Fig. 1. Structure of a fiber loop mirror
input into the interferometer through port 1. In general, the coupler has a coupling ratio K/(1-K) where (K<1). When
K=0.5, fifty percent of the input light will travel clockwise (CW) around the loop and fifty percent will travel
counterclockwise (CCW). In addition, light coupled across the coupler (between port 1 and port 4) suffers a S/2 phase lag
with respect to light traveling straight through the coupler (between port 1 and port 3). Since the light components traveling
in CW and CCW directions traverse the same piece of fiber, they experience the same amount of phase change. The
transmitted intensity at port 2 (called also the output port) is therefore the sum of a CW field of arbitrary phase I and a
CCW field with relative phase (I - S). Assuming K=0.5, the two components will have equal amplitudes,
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resulting in a zero transmitted intensity at the output port 4. In this case all the input light is reflected back along the input
port 1. Hence, this architecture is called an optical loop mirror.
For K z 0.5, the transmission coefficient T, defined as the ratio of power output to power input is given by the equation:
(3.1)
T 1 4 K (1 K )
Here we assumed that the light components traveling in the CW and CCW directions have the same amount of phase
change after passing through the fiber loop. More detailed analyses model of a fiber loop mirror, which includes coupler
excess loss, fiber loss, and birefringence in the loop, etc, can be found in 7.
The Sagnac interferometer is inherently stable due to the fact that both CW and CCW components travel through the same
piece of optical fiber and therefore they do not “pick-up” any additional relative phase change due to slow and random
environmental fluctuations.
3.1.1 Nonlinear Sagnac interferometer
By taking advantage of nonlinear effects in optical fiber, the CW and CCW components can be made to experience different
phase changes even though they pass through the same piece of fiber. This nonlinear configuration of the Sagnac
interferometer is known as Nonlinear Optical Loop Mirror (NOLM). An additional phase difference 'I between CW and
CCW signals can be obtained due to the non-resonant nonlinear interactions within the fiber loop. In this case the
transmission coefficient T, of the NOLM can be expressed as follows
§ 'I ·
(3.2)
T 1 4 K (1 K ) cos 2 ¨
¸
© 2 ¹
If the optical power is high enough, the signal acquires a phase change that is intensity dependent through self phase
modulation (SPM). This phase shift ( I ) is given by
2ʌn2 E
I
2
L
(3.3)
O
where n2 is the nonlinear Kerr coefficient, O is the signal wavelength, E is amplitude of the electric field inside of the optical
fiber and L is the distance traversed in the fiber. If the coupling coefficient (K) is not balanced, the CW and the CCW
components of the signal can acquire different phase shifts.
Because K is not 0.5, the relative phase difference between CW and CCW is
2 ʌn 2 E
2
L
K 2 ʌn 2 E
2
L
2ʌn2 E
2
O
L
(1 K )
2 ʌn 2 E
2
L
(3.4)
( 2 K 1)
O
O
O
This equation shows that for any value of K z 0.5, all of the input power will emerge at the output port (port 2) whenever
'I
( 2 K 1)
m ʌ , for m odd
(3.5)
The minimum power transmission to port 2 occurs for m even and can be obtained from Eq. (3.1).
This type of NOLM was first proposed by Doran and Wood8 to study interactions between counterpropagating light,
including optical solitons, in the loop.
3.1.2 SOA-based nonlinear Sagnac interferometers
As already mentioned, NOLM is a very good candidate for ultra-fast all-optical switching applications since it depends on
the ultra-fast nonlinearity in fiber. Therefore, the switching window of NOLM can be made very short (less then 1ps).
However, the nonlinear interaction used in NOLM is relatively weak. Hence, the operation of the NOLM usually requires
high control pulse energies and long fiber loop lengths to generate a significant phase shift. Although there are many
techniques to reduce both the control pulse energy requirements and loop lengths9, practical NOLMs for commercial optical
communication systems have yet to be realized.
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Experiments with SOAs inserted into the loop of the Sagnac interferometer demonstrated that low energy optical pulses
could change the gain of the amplifiers sufficiently to produce significant phase shifts in subsequent pulses passing through
the amplifier. Since these integrated semiconductor amplifiers were very short (<1mm), they are a practical alternative to
generating an optically induced nonlinearity. Additionally, the temporal onset of the phase shift is as fast as the rising edge
of the control pulse10. Unlike the fiber nonlinearity, however, this resonant interband nonlinearity in the semiconductor
material has a long relaxation time. Efforts were soon underway to form a new class of switching devices using the efficient
nonlinearity in SOAs to induce a differential phase change between the two signal pulses counter-propagating in the fiber
loop of. The first device developed ,using a differential phase change induced by the nonlinearity in SOA, was known as a
semiconductor laser amplifier in a loop mirror (SLALOM) and was used to investigate “contrast enhancement and optical
correlation” by Eiselt11. Although the rising edge of the temporal switching window was a few picoseconds, the window’s
falling edge depended upon the gain recovery time of the SOA which was several hundred picoseconds.
The last innovation to produce picosecond switching windows with SOAs was an architectural realization. It was
discovered that the temporal width of the switching window could be linearly controlled by adjusting the displacement 'xsoa
of the SOA from the midpoint of the loop (see fig. 2) 2. Due to the dynamics of
o
Contr
l
'xsoa
CW
center
SO
A
CC
W
TOAD
Control
50:50
Input
Filter
Output
Fig. 2. Schematic diagram of the TOAD
this configuration, the switching window closes earlier than the recovery time of the SOA as the SOA is moved closer to the
midpoint. Fig. 2 shows a schematic diagram of such a device, which is known as a Terahertz Optical Asymmetric
Demultiplexer (TOAD)12.
The TOAD. Since its first demonstration, the TOAD has been used in various applications for all-optical data processing 1317
. Its Sagnac version, shown also in Fig. 3(a), uses an intraloop 2x2 coupler to inject control pulses to the nonlinear element
(a polarization insensitive SOA). When the data signal enters the TOAD, it splits into equal clockwise and counterclockwise
components which counter-propagate around the loop and arrive at the SOA at slightly different times. The control pulse
arrives at the SOA just before the counterclockwise component of the signal, which is to be gated, but just after its
clockwise counter propagating component and induces nonlinearities in the SOA. This cause the two components to
experience different phase shifts, as shown in Fig. 3(b), and consequently recombine and exit the loop at the output port. All
other data signals, for which the counter propagating components do not straddle the control pulse arrival at the SOA, exit
the loop at the input port. The TOAD switching window (e.g. transfer function) is shown in Fig. 3(c), and its response can
be described by the following equation:
Pout (t )
Pin (t )
Gcw (t ) Gccw (t ) 2 Gcw (t )Gccw (t ) cos(Icw (t ) Iccw (t ))
4
^
`
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(3.6)
where Gcw, Gccw, Icw, and Iccw are the gain and phase experience by the clockwise and counterclockwise propagating signals
respectively. The offset 'x defines the width of the switching window from the equation 'W 2'xsoa /cfiber . If 'W is set
small (say a few picoseconds), one can obtain optical signal processing at ultra-fast speed5.
Working characteristics of the TOAD. By precisely controlling the offset position of the SOA, very short switching
windows can be achieved. Demultiplexing of a single channel from a 250-Gbit/s data stream was demonstrated by Glesk13.
Since the size of the device only depends upon the SOA length and offset from the center position in the loop, compact
TOADs based upon discrete components have been constructed with loop lengths of less than 1 meter.
\
0'
1x
' r—
3.5
(a)
— Phase change for CW pulses
C
10
(b)
—-—Phase change for CCW pulses
E
ti—A TOAD)
Control
A—
Input
__
6
C
C
H
02
2.C
34
(c)
8
3.C
0-
5D:50
12
ibo
output
250
350
Time (ps)
400
100
200
300
4u0
Time (ps)
Fig. 3. (a) Schematic of a TOAD, (b) calculated phase evolution for CCW and CW pulses,
(c) the corresponding TOAD transmission
To achieve full switching, the differential phase shift between the counter-propagating data pulses should approach S. For a
500 Pm long InGaAsP SOA ( nSOA 3.3 ) at 1.55 Pm, the index change is on the order of 10-3. It has been estimated that the
carrier density change associated with this phase shift is on the order of 1017 cm -3, which is approximately a 10% change in
the carrier density of an appropriately biased SOA in population inversion. Depending upon the SOA parameters, current
bias, and wavelength, control pulse energies of < 500 fJ are typically needed. This value can fluctuate by nearly 20%
without significantly degrading the signal-to-noise ratio (SNR) at the switched output13.
The TOAD is robust to temperature variations and can be reliably operated without stabilization as data signals propagating
in both directions around the loop experience the same effective medium. The architecture can also be adapted to
accommodate other short, highly nonlinear materials in place of the SOA to operate the device with different wavelengths
or to achieve performance enhancements using holographic based polarizing filter elements22. The TOAD device and its
variations may prove to be the most practical approach to all-optical switching as they can be integrated using a variety of
techniques that are discussed later in this section.
3.2 Other types of interferometers with SOA as optical nonlinearity
Although the TOAD is based upon Sagnac interferometer, other interferometric configurations such as Mach-Zehnder and
Michelson are possible using a similar operating principle. These architectures improve the integratability of the device,
although they may require active stabilization if constructed from discrete components. Of these geometries one of the
better known is called UNI.
3.2.1 UNI: Ultra-fast Nonlinear Interferometer
Like the other all optical interferometric switches already mentioned, the UNI is also based on the gain saturation of SOA
by high-energy control pulses. The UNI18 uses one SOA in a single-arm interferometer (SAI) structure as shown in Fig. 4.
In the UNI, a polarization-sensitive delay element is used
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PM
Fiber
PSI
PC
PM
Fiber
Signal
Output
PSI
50:50
SOA
Filter
Control
Fig. 4. Block diagram of UNI
to provide a temporal offset between the two orthogonal data polarizations. The principle of operation of the UNI is as
follows. An input signal (data pulse) enters the switch through polarization sensitive optical isolator (PSI). Pulses are split
by a polarization-sensitive delay (length of polarization maintaining (PM) birefringent fiber) into two orthogonal
components, which will also separate them temporally. Due to the large difference in refractive indices for the orthogonally
polarized modes in the birefringent fiber, these two data components will be delayed from each other by an offset
determined by the length of the birefringent fiber and by the degree of birefringence. If nx and ny are the mode indices for
the orthogonal fiber modes, then the temporal offset ('t) is given by
L ,
(3.7)
't n n u
x
y
c
where L is the length of the PM fiber and c is the velocity of light in vacuum. Typically birefringent fiber has n x n y ~10-4.
Thus, for an offset of a few picoseconds, a few meters of birefringent fiber are required.
Now, the two data signal components travel through the SOA into which high-energy controls pulses are injected using the
50:50 coupler. If the control pulses are injected into the SOA such that they arrive at the SOA in between the arrival of the
two data pulse components, the two components acquire different phase shifts due to the saturation of the SOA by the highenergy control pulse. The two signal components are then re-timed to overlap in a second polarization sensitive delay
(another length of birefringent fiber) and are subsequently interfered by use of a polarizer set at 450 to the orthogonal signal
polarizations. The control pulse is filtered out at the output of the device. A counter-propagating structure can be used to
avoid the wavelength or polarization discriminating element at the output. Since the UNI is a single arm interferometer
device, all the signals travel along the same path and are exposed to identical fluctuations in optical path length. Therefore,
the device is stable and no active interferometric bias stabilization is necessary. The switching window duration is
determined by the temporal offset produced by the polarization sensitive delay element. Thus, changing the length of the
birefringent fiber can vary the switching window size of the UNI.
4. MULTI WAVELENGTHS SUORCES BASED ON SPECTRAL SLICING
OF SUPERCONTUNNUUM
Self phase modulation (SPM) is a well known nonlinear phenomenon in optics which is widely used in many applications in
today’s modern optics. The one application we will describe here is based also on SPM with the goal to construct multi
wavelength optical picosecond source via spectral slicing of optical supercontinuum. The technique of supercontinuum
generation is based on sending high intensity optical pulses from a mode-locked Erbium doped fiber laser into a piece of
dispersion decreasing fiber (DDF), where self phase modulation (SPM) induced spectral broadening will take place.
Leff w
2
GZ (T ) (
)
U (0, T )
(4.1)
LNL wT
As the relation shows, the broadening of the phase GZ is proportional to the pulse amplitude U. The DDF will help the
broadening process even more due to the soliton compression relation
N2 = (JP0 (T0)2 / |E| ) = 1
(4.2)
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To maintain its low soliton order N, the width of the optical pulse T0 narrows because of the gradually decreasing E 2 in the
DDF and approaching zero at the very end. As the amplitude of the optical pulse in the process of compression increases
further, high-order dispersion and nonlinearity comes into play and assist further spectral broadening. The set of WDM
filters such as TFFs are used to slice out the designated WDM channels from the wide spectrum. A typical supercontinuum
source can provide as much as a 100 nm of spectral bandwidth, thus capable of supplying over 100 DWDM channels.
5. ALL OPTICAL APPROACH TO 3R DATA REGENERATION
In fiber-optic communication systems, the fidelity of an optical signal is impaired by attenuation, dispersion and
nonlinearities in optical fiber. To achieve long haul optical fiber transmission, “3R” regeneration (re-amplifying, re-shaping
and re-timing) has become a critical technology. The cost and complexity of current WDM regenerators increases in
proportion to the number of wavelengths because separate OEO (optical-electrical-optical) conversion is required to
regenerate each channel individually. All optical techniques toward 3R regeneration, such as SOA-based interferometers19,
EAM-based cross absorption modulators20, and injection locking laser diode devices21 have been exploited recently. Those
approaches, though emphasizing high data rate and significant SNR improvement, still come short in scalability due to
single channel processing. A regeneration technique to process multiple WDM channels using a single device is needed. An
all-optical 3R regenerator we have developed can regenerate multiple WDM channels simultaneously. This novel all-optical
3R regeneration system which is capable of processing multiple WDM channels using TOAD switching and
supercontinuum spectral slicing as we will see has several advantages.
5.1 Regeneration capabilities and bit-parallel-gating of the TOAD
Our novel approach relies on a patented all-optical sampling technology, based on the Terahertz Optical Asymmetric
Demultiplexer (TOAD). TOAD-based signal processing has shown already very promising results. Kailight Photonics
reported all-optical 2R on single channel using NRZ-data format14. In paper15, optical data regeneration was demonstrated
by removing timing jitter from one single channel. Experimental results showed that jitter amplitude up to 17ps for 10Gb/s
signal (or 0.17 UIpp, unit interval peak-to-peak) can be eliminated with the TOAD-based regenerator. Using the ultrafast
gating properties of the TOAD, three parallel OC-48 WDM channels after traveling over 27.5 km in optical fiber in NRZ
format were re-synchronized. Bit error rates less than 10-9 were measured for each channel.16.
The schematic of our approach to the simultaneous WDM 3R regenerator is shown in Fig. 5. The signal
Bit parallel
gating control
demux
TOAD 1
Out
B
A
C
In
TOAD 2
Out
D
4u1
TFFs
Super Continuum
Source
Control
4u1
4u10Gb/s
NRZ WDM
In
TFFs
Control
DCF
mux
Fig. 5. Simultaneous WDM multi channel 3R regenerator
waveforms at various stages in the regenerator (A, B, C, and D) are illustrated in this figure. The input to the regenerator are
4u10 Gbps WDM non-return-to-zero (NRZ) signals that are impaired by noise and timing jitter (see Fig. 6(a)). First,
dispersion compensation fiber (DCF) is used to compensate transmission
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(A) Input WDM signals
(B) After TOAD1
(C) After interleaving
(D) Regenerated outputs
O1
O2
O
O
Fig. 6 Signal format at specified locations in the regenerator
dispersion. Then TOAD 1 performs bit-parallel gating during which all four WDM channels are gated simultaneously, thus
converting NRZ WDM signal into a perfectly re-timed return-to-zero (RZ) amplified signal (see Fig. 6(b)). Only one TOAD
is required for re-timing, independent of the number of WDM channels. The next stage performs bit-interleaving of the retimed RZ signals. This stage utilizes a 1x4 thin film filter (TTF) demux-delay lines-4x1 power combiner structure, to
interleave, or “stagger” the four WDM channels (see Fig. 6(c)). Using TOAD’s picosecond switching window, multiple
WDM channels can be interleaved. The stage output feeds control port of TOAD 2 after passing an EDFA. Input port of
TOAD2 is fed with multi-wavelength pulses generated by spectral slicing of an optical supercontinuum with minimal noise
and extremely low timing jitter (about 25 fs). TOAD 2 serves as an optical modulator, using the interleaved WDM data to
gate the clean multi-wavelength pulses from the supercontinuum source (see Fig. 6(d)). With a fast gain recovery SOA to
provide the rapid gating, all four WDM channels can be simultaneously re-shaped. Because, as discussed earlier, the
supercontinuum source can expand as much as a 100 nm in spectral width, it can provide for more than hundred of DWDM
channels.
The operation of the regenerator was carried out both by simulation and experiment. We demonstrated the system
functionalities and measured the performance of the test bed. Strong jitter-reduction ability of the regenerator was observed.
Not only can the regenerator tolerate up to 0.8 UIpp in jitter amplitude with a negative power penalty, the output signal BER
also breaks the floor obtained at the input. Separate TOADs are used to handle the reshaping of signals due to the speed of
the current SOAs. The main task in the future is to find assistance to speed up the SOA recovery time. Once the solution is
found, the scalability of our regenerator will increase multiple folds with possibility of processing tens or even hundreds of
WDM channels with reasonable number of TOADs.
6. CONCLUSION
Today perhaps more than ever before one can see a remarkable influence of the field of optics on our daily lives regardless
we realize it or not. A development of optical fiber, DFB lasers, narrowband optical filters, and others have resulted in the
deployment of commercial multi-wavelength optical networks with aggregate throughputs exceeding terabits per second. As
a result of these advances, there is a growing discrepancy between the rate of electronic data rate generated by individual
customers and the aggregate rate of data transported in optical networks. This discrepancy known as “electronic
bottleneck,” is most apparent at points in the network where it is necessary to add/drop, switch or somehow process the
optical data. Using all-optical technology to process the data will provide performance benefits in terms of speed and
parallelism. This is because in some cases the data can be processed efficiently in optical form by only handling a few
needed bits of information, while the bulk of the data could remain untouched.
All-optical data processing is an essential technology for building future ultra-fast optical networks. Any advancement in
this area is expected to have a major impact on to the whole network performance. With continued efforts by academia and
industry, significant progress is being made toward building ultra-fast all-optical devices for different applications in current
optical networks.
We have reviewed and discussed several all-optical approaches to building ultra-fast all-optical deices for different
applications, in which the data remain in optical form while passing through a switch or device, controlled via an optical
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non-linearity, using an optical control signal. Fully functional all-optical switches in networks will require switching in both
space, time and wavelength domains. As discussed, a class of SOA based all-optical devices provides superior performance.
If they operate based on the differential onset of a fast optical non-linearity (for example, the TOAD), they can operate at
ultra high speeds because their switching window is much shorter than the recovery time of the SOA. The gain of the SOA,
together with the phase sensitivity of the optical interferometer, allows these switches to be controlled with extremely low
switching energy. They are integratable, cascadable, while providing signal amplification.
The system demonstration we discussed illustrates the use of ultra-fast all-optical switches in a laboratory testbed setting for
all-optical 3R data regeneration at 40 Gbit/s.
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