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Experimental Demonstration of a 2-Stage Continuously
Tunable Optical Tapped-Delay-Line in which N+M Pump
Lasers Produce N×M Taps
A. Mohajerin-Ariaei1, M. R. Chitgarha1, M. Ziyadi1, S. Khaleghi1, A. Almaiman1, J. Touch2,
M. Tur3, L. Paraschis4, C. Langrock5, M. M. Fejer5, and A. E. Willner1
1) Department of Electrical Engineering, University of Southern California, 3740 McClintock Ave, Los Angeles, CA 90089, USA
2) Information Sciences Institute, University of Southern California, 4676 Admiralty Way, Marina del Rey, CA, 90292, USA
3) School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel
4) Cisco Systems, 170 W. Tasman Drive, San Jose, CA 95134, USA
5) Edward L. Ginzton Laboratory, 348 Via Pueblo Mall, Stanford University, Stanford, CA 94305, USA
[email protected]
Abstract: We experimentally demonstrate a 2-stage continuously tunable optical tapped-delayline in which N+M pump lasers produce N×M number of taps. A 3×2-taps optical correlator is
implemented to search multiple patterns among 20-Gbuad QPSK signals using nonlinearities and
coherent comb source.
OCIS codes: (070.4340) Nonlinear optical signal processing, (060.4370) Nonlinear optics, fibers.
1. Introduction
Tapped-delay-lines are a fundamental building block for many data signal processing functions, including
correlation, equalization, filtering, and almost any element that provides a transfer function [1]. Moreover, it may be
beneficial to enable an optical tapped-delay-line (OTDL) so that: (i) certain functions may be performed natively on
optical data streams, and (ii) there exists the potential for optical functions to be performed at a very high rate [2].
There have been several reports on different ways to achieve an optical tapped-delay-line [2-5]. One technique to
achieve a continuously tunable OTDL is the use of chromatic dispersion as well as wavelength conversion and
mixing [3,5] In this approach, an input data stream experiences a transfer function, and a set of wavelength pumps
is used such that each wavelength acts as one of the taps in the OTDL.
A key challenge of this OTDL approach is that the number of taps is limited to the number of pump wavelengths,
and yet there are applications for which there might be a need for a fairly large number of taps. In such a scenario,
the large number of needed pumps may make this approach impractical. Therefore, a laudable goal would be to
enable a large number of taps with a fewer number of pump lasers.
In this paper, we experimentally demonstrate a 2-stage continuously tunable OTDL in which N+M pump lasers
produce N×M number of taps. A 3×2-taps OTDL correlator is implemented to search multiple patterns among 20Gbuad QPSK symbols using nonlinearities and coherent comb source.
2. Concept
Fig. 1(a) depicts the generic form of a proposed tunable
-taps OTDL. The structure composed of two
cascades OTDL with and number of taps respectively. For this structure, the relation between the input signal
∑
∑
and the output
is determined by
where
, representing an
OTDL with
number of taps. By choosing appropriate values for the delays, for instance,
and
,
∑
the output function can be simplified as
where
denotes one symbol time interval.
x(t)
|a0|e j∠a0
T
|a1|e j∠a1
+
b)
+
N-Optical
Comb Fingers
…
T
•••
|aN-1|e j∠aN-1
T
|a2|e j∠a2
Modulator
|b0|e j∠b0
+
N-Comb Fingers
T
λ
•••
+
+
Frequency
Dependent
Optical Delay
a0
Multiplexing
|b1|e j∠b1
y(t)
M-Pump Lasers
a1… aN-1
…
PPLN-1
+
z(t)
|bM-1|e j∠bM-1
Ƭ
Ƭ
Ƭ
Pump
Frequency
Dependent
Optical Delay
Multicasting
•••
+
•••
a)
M-Lasers
b0
…
b1 bM-1
Multiplexing
PPLN-2
SFG
SFG
x(t) x(t-T) x(t-NT)
…
DFG
DFG
a0 aN-2 aN-1
x(t)x(t-T) x(t-NT) y(t) y(t) y(t)
λ
Modulated signals
…
…
…
…
λ
z(t)
y(t)
y(t-M Ƭ)
b0 b1 bN-1
…
Fig. 1. (a) Generic block diagram of an OTDL with N×M number of taps. (b) The principle of operation of the proposed OTDL.
…
λ
The principal of operation of our proposed OTDL is shown in Fig.1(b). fingers from an optical comb source are
selected and modulated using an I/Q modulator. These signals travel through a chromatic dispersive element at
various speeds incurring a different time delay ( ). The delayed signals with another set of fingers ( ) which are
passed through the phase/amplitude filter multiplexed together in a PPLN-1 waveguide and by injecting
pump
lasers, replicas are generated. The replicas are delayed relatively
and with the
lasers ( ) which are
passed through another phase/amplitude filter are multiplexed together in PPLN-2 to generate the output signal
.
The proposed method can be used to implement a N×M-taps optical correlator. To find the specific pattern on the
stream, the values of stream{A, B, C, D} are mapped onto the four QPSK symbols. Then, the optical signal is
multiplied with the conjugate of the target pattern to achieve the correlation output. For instance, to identify the
appearance of a six-length pattern [BDCABD] in a long stream of optical QPSK symbols, a 3×2-taps OTDL might
be used in which the correlator weights are set to the phase conjugate of the target pattern [B*D*C*A*B*D*].
3. Experimental Setup
The experimental setup for 3×2-taps OTDL is illustrated in Fig. 2. A flat and broad spectrum 20-GHz frequency
comb can be produced using the method of [6]. A SLM phase/amplitude filter is used to select and write complex
weights on comb fingers ( ) and separate them into two paths. For each path, three comb fingers with a spacing of
1.92 nm are selected. A nested Mach-Zehnder modulator generates the 40-Gbit/s optical QPSK data on comb
fingers. The signals sent through a ~650 m DCF to introduce one symbol-time relative delay between the two
adjacent signals. The delayed signals with comb fingers are multiplexed together and with two injected pumps
produce two replicas in PPLN-1. The two replicas and pumps are sent to another SLM to adjust appropriate phases
on the pumps ( ). They are all with another pump are injected to PPLN-2 to create the correlator output signal.
MLL
HNLF
DLI
SLM
Filter1
90
22.5
dBm9 nm
20 Gbaud PRBS (215-1)
650 m
PPLN-1
Waveguide
DCF
(dBm)
Power(dBm)
Power
Coherent
Receiver
-20
1 nm
PPLN-2
Waveguide
9 nm
9 nm 9 nm
21.2 dBm
-30
SLM
Filter2
-40
1546
1548
1550
1552
1 nm
1554
1565
Pumps (bi)
OutPump
Correlator PPLN-1
output
QPM
λp
Wavelength (nm)(nm)
Wavelenghth
QPM
1551
Wavelenghth (nm)
1535
PPLN-2
26 dBm
-10
Pumps
Comb fingers(ai)
OutPPLN-1
Power (dBm)
20
dBm 9nm
450 m
29
dBm 2.5nm
30
dBm 5.7nm
QPSK
signals
PPLN-1
b)
Flat 20GHz Comb Generation
Power (dBm)
a)
1551
Wavelenghth (nm)
1535
1565
Fig. 2. (a) Experimental setup. DCF: Dispersion compensating fiber, SLM: Spatial light modulator, (b) Optical spectra of PPLN-1, PPLN-2.
[C A D B C A]
[B D C A B D]
Pattern Peak
1
Phase (rad)
3pi/4
Target Pattern:
[B D C A B D]
0.5
[D B A C D B]
[C D A B B C]
[A C B D A C]
0
-500
500
1500
2500
3500
QPSK Data Target Phase:
[3π/4 -π/4 -3π/4 π/4 3π/4 –π/4]
Phase
pi/4
-pi/4
-3pi/4
1505 Symbol
Symbol
[B C D A A B]
Pattern Peak
1
3pi/4
Phase (rad)
(b)
Coherent detection
(a)
Coherent detection
4. Results and Discussion
Figure 3(a) shows the 3×2taps correlator result for the specific pattern of [BDCABD]. All output symbols in all four
corner points of the IQ-plane can be detected as the target pattern with 0°, 90°, 180°, and 90° rotations, separately
[6]. Analyzing the upper right corner of the constellation yields the location of the matched patterns in a 4096 QPSK
symbol stream shown as the correlator peaks in Fig. 3(a). Figure 3(b) shows the similar results for [BCDAAB].
Target Pattern:
[B C D A A B]
0.5
1525
QPSK Data Target Phase:
[3π/4 -3π/4 -π/4 π/4 π/4 3π/4]
Phase
pi/4
-pi/4
0
-500
[D A B C C D]
[A B C D D A]
500
1500
2500
Symbol
3500
-3pi/4
3580
3590
Symbol 3600
3610
Fig. 3. Experimental results for 3×2-taps OTDL correlator. (a) Target pattern [B D C A B D]. (b) Target pattern [B C D A A B].
Acknowledgments
The authors would like to thank the support of NSF.
4. References
[1] J. G. Proakis, Digital Communications. McGraw-Hill (2000).
[2] C. R. Doerr, JLT, vol. 22, no. 1, pp. 249–256, (2004).
[3] J. Sharping, Opt. Exp.,vol.13, no.20, (2005).
[4] M. S. Rasras, IEEE Photon. Lett., vol. 20, no. 9, (2008).
[5] S. Khaleghi, et al., IEEE Photonics Journal 4, 1220 (2012).
[6] M. Ziyadi, Opt. Exp., vol. 22, no. 1, pp. 84-89, (2014).