A hybrid WDM/OCDMA ring with a dynamic add/drop function based on Fourier code for local area networks

A hybrid WDM/OCDMA ring with a dynamic

add/drop function based on Fourier code for

local area networks

Yong-Kyu Choi,1 Kenta Hosoya,

2 Chung Ghiu Lee,

3 Masanori Hanawa,

2

and Chang-Soo Park4*

1School of Information and Communications, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro,

Buk-Gu, Gwangju 500-712, South Korea 2Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu,

Yamanashi 400-8511, Japan 3Department of Electronic Engineering, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-759,

South Korea 4School of Information and Communications, Graduate Program of Photonics and Applied Physics, Gwangju

Institute of Science and Technology, 261 Cheomdan-gwagiro, Buk-Gu, Gwangju 500-712, South Korea

*[email protected]

Abstract: We propose and experimentally demonstrate a hybrid

WDM/OCDMA ring with a dynamic add/drop function based on Fourier

code for local area networks. Dynamic function is implemented by

mechanically tuning the Fourier encoder/decoder for optical code division

multiple access (OCDMA) encoding/decoding. Wavelength division

multiplexing (WDM) is utilized for node assignment and 4-chip Fourier

code recovers the matched signal from the codes. For an optical source well

adapted to WDM channels and its short optical pulse generation, reflective

semiconductor optical amplifiers (RSOAs) are used with a fiber Bragg

grating (FBG) and gain-switched. To demonstrate we experimentally

investigated a two-node hybrid WDM/OCDMA ring with a 4-chip Fourier

encoder/decoder fabricated by cascading four FBGs with the bit error rate

(BER) of <109

for the node span of 10.64 km at 1.25 Gb/s.

©2011 Optical Society of America

OCIS codes: (060.2340) Fiber optics components; (060.2330) Fiber optics communications;

(060.4510) Optical communications; (060.4262) Networks, ring.

References and links

1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical

processing,” J. Lightwave Technol. 4(5), 547–554 (1986). 2. P. J. Urban, B. Huiszoon, R. Roy, M. M. de Laat, F. M. Huijskens, E. J. Klein, G. D. Khoe, A. M. J. Koonen, and

H. de Waardt, “High-Bit-Rate Dynamically Reconfigurable WDM–TDM Access Network,” IEEE J. Opt.

Commun. Netw. 1(2), A143–A159 (2009). 3. J.-F. Huang, Y.-T. Changa, and C.-C. Hsua, “Hybrid WDM and optical CDMA implemented over waveguide-

grating-based fiber-to-the-home networks,” Opt. Fiber Technol. 13(3), 215–225 (2007).

4. X. Wang, N. Wada, T. Miyazaki, G. Cincotti, and K. Kitayama, “Hybrid WDM/OCDMA for next generation access network,” Proc. SPIE 6783, 678328, 678328-14 (2007).

5. X. Chen, G. Xia, D. Huang, and X. Yuan, “Experimental demonstration of 40 Gbit/s hybrid optical code-division

multiplexing/wavelength-division multiplexing system,” Opt. Eng. 46(11), 115006 (2007).

6. C. Zhang, and K. Qiu, “Design and analysis of coherent OCDM en/decoder based on photonic crystal,” Opt.

Lasers Eng. 46(8), 582–589 (2008).

7. P. C. Teh, M. Ibsen, J. H. Lee, P. Petropoulos, and D. J. Richardson, “Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s quarternary phase coding gratings,” IEEE Photon. Technol.

Lett. 14(2), 227–229 (2002). 8. S. Boztas, R. Hammons, and P. V. Kumar, “4-phase sequences with near-optimum correlation properties,” IEEE

Trans. Inf. Theory 38(3), 1101–1113 (1992).

9. M. Hanawa, “Fourier code: A novel orthogonal code for OCDM systems,” in Opto-Electronics and Communications Conference and Australian Conference on Optical Fibre Technology (OECC/ACOFT’ 2008),

Sydney, 1–2 (2008).

#140518 - $15.00 USD Received 5 Jan 2011; revised 14 Feb 2011; accepted 8 Mar 2011; published 18 Mar 2011(C) 2011 OSA 28 March 2011 / Vol. 19, No. 7 / OPTICS EXPRESS 6243

10. M. Hanawa, “Multiple access interference reduction by limiting receiver bandwidth on Fourier code based-

OCDM system,” in Opto-Electronics and Communications Conference (OECC’ 2009), Hong Kong, 1–2 (2009). 11. J. Wu, and C.-L. Lin, “Fiber-Optic Code Division Add-Drop Multiplexers,” J. Lightwave Technol. 18(6), 819–

824 (2000).

12. C. -S. Bre's, I. Glesk, R. J. Runser, and P. R. Prucnal, “All-Optical OCDMA Code-Drop Unit for Transparent Ring Networks,” IEEE Photon. Technol. Lett. 17, 1088–1090 (2005).

13. Advanced Optics Solutions Gmb, (http://www.aos-fiber.com/eng/FBG/Athermalen.html).

14. K. Hosoya, M. Hanawa, and K. Nakamura, “Programmable FBG-based variable optical correlator for optical code division multiplexing,” in Asia-Pacific Conference Communications (APCC’2009), Shanghai, 560–563

(2009).

15. N. Wada, “Optical Code Processing System, Device, and its Application,” JNW 5(2), 242–250 (2010).

1. Introduction

In local area networks (LANs) that include a large number of users, bandwidth as well as

optical power distribution to each user is an important issue. Recently, an increase in the

bandwidth occupied by services necessitates that the transmission capacity of LANs be

upgraded to support such services. Time division multiplexing (TDM), wavelength division

multiplexing (WDM), and optical code division multiplexing (OCDM) can be used alone or

in their combined form [1–5] as a part of increasing bandwidth in LANs. To increase

transmission capacity, the WDM could be combined with TDM or OCDM because it has to

avoid collisions with its own wavelength. By introducing hybrid WDM/OCDM, the number

of required wavelength could be drastically reduced and node configuration could be very

simple. Also, the WDM/OCDMA configuration provides asynchronous access to the ring by

OCDM coding the synchronous optical network (SONET) or Ethernet signal to be added to

the ring without data collision. On the other hand, in WDM-SONET ring, TDM access

protocol for the upstream signal should be used. Therefore, the hybrid form of WDM and

OCDM could be an especially good candidate for LANs requiring high bandwidth with

security and easy accessibility.

In a ring configuration consisting of WDM and OCDM, system performance depends on

wavelength channel cross-talk and code interference. The former is due to imperfect channel

filtering in WDM, and the latter is caused by cross-correlation between the neighboring

optical coded channels. In optical code division multiple access (OCDMA), the code

interference depends on the cross-correlation between the orthogonal codes of users. To

decrease the interference, a few orthogonal codes such as binary phase shift keying (BPSK)

using a photonic crystal [6] and quarternary phase shift keying (QPSK) using a super-

structured fiber Bragg grating (SSFBG) [7] have been used. The QPSK orthogonal code has

been known to provide a greatly improved cross-correlation characteristic compared with the

BPSK orthogonal code [8]. Furthermore, the Fourier code, as one of the QPSK orthogonal

codes, inherits the lowered code interference from the orthogonality [9,10]. Furthermore, the

encoder/decoder based on the Fourier code has an advantage of easy code adaptation because

the Fourier code has a same phase difference among the orthogonal codes. This characteristic

of the encoder/decoder is very useful in a ring configuration with the requirement of

dynamically adding or dropping coded channels on the operating wavelengths [11,12].

In this paper, we propose and experimentally demonstrate a two-node hybrid

WDM/OCDMA ring with a dynamic add/drop function based on the Fourier code for LAN

applications. The Fourier encoder/decoder is fabricated by cascading four fiber Bragg gratings

(FBGs). Different from the conventional methods using a wavelength-managed mode lock

laser diode (MLLD) to generate a short pulse train [4,5], a reflective semiconductor optical

amplifier (RSOA) is used with a wavelength-selective reflector to provide a WDM capability

and is gain-switched to generate a short pulse train. This short pulse train is coupled to

another RSOA and is on-off modulated by the modulation signal of 1.25 Gb/s.


2. Proposed WDM/OCDMA ring configuration

Figure 1 shows the proposed WDM/OCDMA on a single fiber ring which is composed of

multiple nodes. And the lower part of Fig. 1 shows the signal flows between the nodes and

their block diagram. At each node, one wavelength is assumed to be added and dropped, and

N different Fourier codes from the N-chip Fourier encoder/decoder can be generated. Among

the wavelengths guided along the ring, the wavelength assigned to the node is dropped, and

the same wavelength or different wavelength with new information can be added to the ring

through the node. To generate a Fourier coded optical signal, an optical source with a specific

wavelength is necessary. Here, we use an RSOA which has broadband spectrum but its lasing

wavelength can be selected with the external wavelength-selective reflector. To produce an

optical short pulse, it can be operated at a gain-switching mode. This optical short pulse is

blocked or passed by another RSOA #2 depending on bit 0 or bit 1 of the data signal based on

external modulation technique. Then, the modulated optical short pulses pass through the N-

chip Fourier encoder and are finally encoded and added into the ring. On the other hand, the

OCDMA signals carrying on the dropped wavelengths are decoded by the N-chip Fourier

decoder.

λj, add λi, drop

Node i Node j

λj, drop λi, add

Photo-

detector

A hybrid WDM/OCDMA ring

Amplifier

- Limiting

bandwidth

N-chip

Fourier

encoder

Short pulse

generator

using RSOA#1

Modulation

using RSOA#2

Tx

N-chip

Fourier

decoder

Rx

Rx

Tx

Node i

Node j

Node

Node

Fig. 1. Proposed hybrid WDM/OCDMA ring for LANs

The generation process of the optical short train from the two RSOAs is described in detail

here. An RSOA has inherently broadband spectrum and thereby a same RSOA can be used as

different WDM optical sources. To produce a specific wavelength from the RSOA, it needs to

be wavelength-locked by an external wavelength selective reflector with the same wavelength

as the node. The wavelength-selective reflector is placed in front of the anti-reflective (AR)

coated facet of the RSOA, here FBG. Then the RSOA #1 is self-injection locked to the

reflected light with the selected wavelength. To generate a short pulse train, the RSOA #1 is

gain-switched by a clock signal with the same repetition rate of the modulation signal. The

pulse width is determined by the gain switching dynamics in the gain medium, i.e. the RSOA

#1 in this case. It is possible to tune the pulse width by adding some dispersive medium like a

chirp FBG or a dispersion compensation fiber after the short pulse generator. The final short

pulse train whose wavelength is matched to the specific wavelength of the node is shown in

the upper part of Fig. 2. For the assignment of different wavelength, the Bragg wavelength of


the FBG needs to be changed. Also, the wavelength shift caused by thermal deviation can be

reduced by using athermal FBG with < 0.7 pm/Κ [13].

light reflected by FBG

lB (Bragg wavelength)

wavelength

lB

RSOAhigh

reflectivecoating

clock

DC bias

optical short pulse train

time

amplifier

clock period

lB

Fig. 2. Schematic of the short pulse generator based on the RSOA and the external FBG

The Fourier code is a kind of complex-valued Hadamard code. This code can be obtained

from the orthogonal property between the rows of the Fourier matrix FN which is well-known

as the twiddle factor in the Cooley-Tukey fast Fourier transform (FFT) algorithm. The

elements of the Fourier matrix FN are exp(2πj(m-1)(n-1)/N), where j is the imaginary unit, m

and n are the positive integer indices for rows and columns from 1 to N. Equation (1) shows

the Fourier matrix of N=4 as an example. Each row represents a 4-chip Fourier code as a

QPSK code (Ci). Each element of the matrix shown in Eq. (2) reflects the phase shifts

between neighboring elements of the corresponding row of Eq. (1). Also, as we see in Eq. (2),

the orthogonal code corresponding to the second row can be obtained by giving a phase shift

of π/2 to the first orthogonal code and the other codes can be obtained in a same way without

considering individual elements.

1

2

4

3

4

1 1 1 1

1 1

1 1 1 1

1 1

C

Cj jF

C

Cj j

(1)

2/32/32/3

2/2/2/

000

4

342312

F

(2)

The Fourier encoder/decoder is made of four FBGs with the partial reflectivity separated

from each other by the same distance and Bragg wavelength in a cascaded form as shown in

Fig. 3. The phase difference(ΔØ) between neighboring gratings is adjusted by the refractive

index change or the Bragg wavelength as shown in Eq. (3), where L is the FBG spacing (5

mm in our experiment), n is the refractive index of the fiber core between two neighboring

gratings, and λB is the Bragg wavelength of the grating. For each 4-chip Fourier code, one

optical short pulse on single wavelength becomes encoded four optical short pulses reflected

from four gratings in time domain and the encoded four optical short pulses have same phase

difference between neighboring optical short pulses. In addition, to avoid the interference

between the encoded neighboring optical short pulses, the pulse width should be less than the


round trip optical delay (2ΔT=48.5-ps) between the chips as shown in Fig. 3. This type of

encoder/decoder based on binary Hadamard code was reported to be stable on thermal

variation by using a feedback controller as a thermal stabilizer [14].

2

{(2 ) mod }B

B

Ln

ll

(3)

input pulse

matched case12 23 34

5mm

encoded pulse12 23 34

5mm

(a) (b)

encoded pulse

unmatched case

FBG

T

T

Fig. 3. Fabricated coder for 4-chip Fourier code, (a) Encoder, (b) Decoder

The amount of phase shift is controlled by the spacing between FBGs or changing the

operating wavelength for the given spacing. This wavelength-dependent phase shift induces a

code mismatch and naturally acts as a WDM filter to the wavelength of the unwanted signal.

In conclusion, the QPSK code using Fourier code provides lower cross-correlation property

and easier code adaptation than those using binary Hadamard code like BPSK code [14].

Besides, this easy code adaption can be used for dynamic channel add/drop function because

each code element is simultaneously controlled in the optical ring network.

3. Experiment

To evaluate the performance of the proposed ring, an experimental setup with two nodes was

organized as shown in Fig. 4. In general, multiple nodes should be considered. However, due

to the limitation in the available encoders and decoders, we demonstrated with two

wavelengths and four Fourier codes. In fact, there could be two types of ring. If we consider

only drop case, each node can drop multiple wavelengths and multiple codes, multiple

wavelengths and single code, single wavelength and multiple codes, or single wavelength and

single code. For single wavelength and single code, we can construct eight nodes with two

wavelengths and four codes. For higher capacity, we can increase the number of available

wavelengths along the ring. Or, initially, we can design enough codes. Then we assign the

codes to the SONET or Ethernet signals as many as we need leaving other codes for

expansion. For tunable add and drop case, at each node, the specific wavelength is dropped

with multiple codes and at the receiver side, we can select a particular code by tuning the

decoder. For the case of signal add, if each node has only one code, then we can reroute a

signal path from node inode j to node inode k by changing the encoder at the transmitter

of node i. To show the proof of concept without the loss of generality, the wavelength spacing

is set to 0.8 nm according to the ITU-T Recommendation for WDM channel allocation. Also,

Fourier encoder and decoder acts as an optical filter for other wavelengths including ASE

noise except for its own wavelength. Furthermore, the multiple interference noise from the

other nodes due to cross-correlation can be further reduced by using Fourier code [9]. To

show the feasibility of the proposed scheme, wavelength channel cross-talk, code interference,

and add/drop function are investigated. Node 1 included Encoder#1 with the code word C1 for

λ1, i.e., (C1, λ1). In addition, instead of tuning Encoder#1, Encoder#2 with the code word C2

for λ1, i.e., (C2, λ1) was used for investigating add/drop function and measuring code

interference. On the other hand, Node 2 had Encoder#3 with the code word C1 for λ2, i.e., (C1,


λ2) and Decoder#1 with the variable code Ci for λ1, i.e., (Ci, λ1: index i means a variable code,

when i = 1, it shows auto-correlation with (C1, λ1), cross-correlation with (C2, λ1), WDM

channel cross-talk with (C1, λ2)). By changing Ci from C1 to C4, add/drop function can be

simulated. From Node 1, the generated pulses were injected into RSOA#2 through an optical

circulator and RSOA#2 was wavelength-locked to that of the injected light as a λ1. Then, the

modulation signal of 1.25 Gb/s was applied to RSOA#2 through a Bias-T. Depending on bit 1

or bit 0 of the modulation signal, the injected pulses remained or disappeared. Finally, the

output of RSOA#2 showed a gapped short pulse train corresponding to the data pattern of the

signal. This gapped signal passed through the circulator, split into two to be encoded by C1

and C2, and was optically amplified with an erbium doped fiber amplifier (EDFA) before

being launched to the ring. The signal propagating through the single mode fiber (SMF) of

10.64 km along the ring is transferred to another EDFA at Node 2. This amplified signal is

dropped and decoded by Decoder#1, simultaneously. The output was detected by using a

photo-detector and amplified by a 4-GHz limited electrical amplifier [10]. For WDM channel

cross-talk, optical short pulses with the wavelength of λ2 were encoded by Encoder#3, not

modulated for simple demonstration and decoded by Decoder#1. The encoder/decoder

fabricated with Fourier code has a code adaptation from C1 to C4 by the wavelength difference

of about 0.15 nm on the operating wavelength. If we consider the typical WDM channel

spacing of 0.8 nm, this change to the code characteristic can be neglected.

Short pulse generator

using RSOA#1, λ1

RSOA#2

Bias-TDC

PPG1.25GHz Clock 1.25Gb/s data

EDFA

PD

BERT

DCA

4GHz

AMP

Ring path

Add λ2

Drop λ2

Drop λ1

Coupler

Add λ1

Ring path

Transmitter

Receiver

SMF

10.64 km

Short pulse generator

using RSOA#3, λ2

PPG

EDFA

1.25GHz Clock

Transmitter

Decoder#1 (C1-4, λ1)

ISO

ISO

Coupler

Encoder#2 (c2, λ1)

Encoder#3 (c1, λ2)

Coupler

Encoder#1 (c1, λ1)

EDFA

Switch#1 (On-off )

Switch#2 (On-off )

Coupler

Tunable

Node 1

Node 2

Fig. 4. Experimental setup. RSOA: reflective semiconductor optical amplifier, AMP: electrical

amplifier, EDFA: erbium-doped fiber amplifier, SMF: single mode fiber, ISO: isolator, PPG:

pulse pattern generator, PD: photo detector, BERT: bit error rate tester, DCA: digital communication analyzer

4. Results and discussion

Figure 5 shows the optical short pulses with the repetition rate of 1.25 GHz from RSOA#1

and RSOA#3 (biased at 18 mA and modulated by peak-to-peak voltage of 2 V), an optical

spectra with optical power of 20 dBm and a side mode suppression ratio (SMSR) of about

30 dB. The Bragg wavelengths of the FBGs used in the short pulse generator of Node 1 and

Node 2 were 1552.07 nm and 1552.82 nm, respectively. The reflectivity and pass band of the

FBGs used at both nodes for self-injection locking were about 45% and 0.37 nm, respectively.


Each optical short pulse has 32-ps pulse-width which satisfies the 48.5-ps chip interval of

each encoder/decoder. This pulse width is good enough to generate a 4-chip code word at 1.25

Gb/s.

500uW/div 300ps/div

500uW/div 50ps/div

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

(a)

500uW/div 300ps/div

500uW/div 50ps/div

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

(b)

1551.0 1551.5 1552.0 1552.5 1553.0-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

1552.0 1552.5 1553.0 1553.5-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

Fig. 5. Optical short pulse trains and their optical spectra: the signals at (a) pulse width: 32 ps, λ1: 1552.07 nm, (b) pulse width: 32 ps, λ2: 1552.82 nm

The modulated and encoded signals are shown in Fig. 6. The bias current to RSOA#2 was

43 mA and the data pattern was „1011‟ at 1.25 Gb/s. As we expected, the modulated signal

shows gapped pulses with a missing pulse at the time of bit 0 as shown in Fig. 6 (a). Figure 6

(b) and (c) shows the modulated signals encoded by two different code words, C1 and C2,

respectively. The ASE noise from RSOAs was effectively suppressed after passing through

the encoders due to the filtering effect of the encoder/decoder. Figure 7 shows the

wavelength-dependency of the encoder fabricated. It was displayed over the wavelength range

of 1551.3 – 1552.8 nm. If the operating center wavelength is shifted by about 0.05 nm, then

the code is changed from C1 to C2 (i.e., C1 code to 1552.07 nm and C2 to 1552.12 nm). This

means that the wavelength channel spacing should be larger than 0.2 (0.05 × 4) nm to

discriminate between neighboring orthogonal codes in WDM applications.

500uW/div 300ps/div

500uW/div 50ps/div

500uW/div 300ps/div

500uW/div 50ps/div

500uW/div 300ps/div

500uW/div 50ps/div

(a) (b) (c)

1011 (C1, λ1) (C2, λ1)

Fig. 6. (a) the pulse train modulated by the data pattern „1011‟, (b) the modulation signal

encoded by C1, and (c) the modulation signal encoded by C2


(a)

1551.3 1551.6 1551.9 1552.2 1552.5 1552.8-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

C1

(b)

1551.3 1551.6 1551.9 1552.2 1552.5 1552.8-60

-50

-40

-30

-20

-10

Po

wer (

dB

m)

Wavelength (nm)

C2

Fig. 7. The optical spectra of the modulated signals on λ1: (a) by C1, (b) by C2

To investigate the contrast ratio of the OCDMA encoder/decoders, their auto-correlation

peak (ACP) and cross-correlation peak (CCP) were measured and theoretically calculated by

adjusting two optical switches at Node 1 and fixing C1 at Node 2 as shown in Fig. 8. In Fig. 8,

left side represents experimentally measured results and right side represents theoretical

results, respectively. The measured results were consistent with the theoretical results. The

ratio of ACP over CCP appeared to be more than 5, respectively. These results prove that the

Fourier encoder/decoder has a good contrast ratio [15]. Also, we did not use an additional

WDM filter to separate the wavelength λ2 from Node 2 because other wavelengths except for

λ1 were removed by the wavelength-dependent characteristic of the encoder/decoder.

a.u./div 300ps/div500uW/div 300ps/div

500uW/div 50ps/div

(a)

(C1, λ1)×(C1, λ1)(C1, λ1)×(C1, λ1)

(C2, λ1)×(C1, λ1)

500uW/div 300ps/div

500uW/div 50ps/div

(b)

(C2, λ1)×(C1, λ1)(C2, λ1)×(C1, λ1)

a.u./div 300ps/div

a.u./div 50ps/div

a.u./div 50ps/div

Fig. 8. Measured (left) and theoretical (right) correlation waveforms after being decoded, (a) auto-correlation waveform for (C1, λ1) × (C1, λ1), (b) cross-correlation waveform for (C2, λ1) ×

(C1, λ1)

To simulate add/drop function, the outputs of Encoder#1 and #2 were combined at Node 1

and detected with the Decoder#1 by mechanically tuning C1 to C4 and their results are shown

in Fig. 9 with theoretical results. In theoretical results of Fig. 9, their auto-correlation

waveforms were not symmetrical for the center pulses. The difference between the

experimental and theoretical results came from the phase error and the difference of timing.

The theoretical results were obtained under the assumption of complete synchronization


between two channels and simple Gaussian shape as the source pulse shape without red chirp

caused by gain-switching mode of RSOAs. Under the existence of two encoded signals, the

peaks were observed only for the cases of C1 to C1 and C2 to C2 and the ratios of ACP over

CCP were also more than 5, respectively. These results show that the Fourier encoder/decoder

can be used as a dynamic add/drop filter.

500uW/div 300ps/div

500uW/div 50ps/div

(a)

(C1+C2, λ1)×(C1, λ1)(C1+C2, λ1)×(C1, λ1)

a.u./div 300ps/div

a.u. /div 50ps/div

500uW/div 300ps/div

500uW/div 50ps/div

(b)

(C1+C2, λ1)×(C2, λ1)(C1+C2, λ1)×(C2, λ1)

a.u./div 300ps/div

a.u./div 50ps/div

500uW/div 300ps/div

500uW/div 50ps/div

(c)

(C1+C2, λ1)×(C3, λ1)

a.u./div 300ps/div

(C1+C2, λ1)×(C3, λ1)

a.u./div 50ps/div

500uW/div 300ps/div

500uW/div 50ps/div

(d)

(C1+C2, λ1)×(C4, λ1)

a.u./div 300ps/div

(C1+C2, λ1)×(C4, λ1)

a.u.div 50ps/div

Fig. 9. Measured (left) and theoretical (right) correlation waveforms after code switching to

Decoder#1, (a) auto-correlation waveform for (C1+C2, λ1)×(C1, λ1), (b) auto-correlation waveform for (C1+C2, λ1)×(C2, λ1), (c) cross-correlation waveform for (C1+C2, λ1)×(C3, λ1), (d)

cross-correlation waveform for (C1+C2, λ1)×(C4, λ1)


Finally, we measured the code interference-induced power penalty and WDM channel

cross-talk induced power penalty through their bit error rate (BER) curves and plotted them in

Fig. 10. From the BER curves, without code interference, each auto-correlation considering

wavelength channel cross-talk shows almost the same BER curves. This means that the

channel spacing of 0.8 nm does not make any channel cross-talk. By contrast, about 0.4-dB

power penalty resulted with code interference. From the scalability point of view for higher

number of users, the code-length of the Fourier code is the same with the maximum number

of OCDM channels. The more the number of codes, the less the phase margin in the Fourier

code. The code interferences caused by multiple Fourier codes can be reduced just by limiting

the receiver bandwidth [10], instead of nonlinear signal processing technique. To measure the

stability due to the drift in the operating wavelength, we gave a wavelength shift of 0.018 nm

to λ1 at Node 1 and measured its BER. The corresponding power penalty appeared to be about

0.8 dB, and the wavelength drift within 0.018 nm was negligible. If an athermal FBG whose

thermal dependency is less than 0.7 pm/Κ [14] is used as a reflector, this power penalty could

be more improved.

-12 -11 -10 -9 -8 -7 -6 -5

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Lo

g (

BE

R)

Received Power (dBm)

Fig. 10. Measured BER curves,

: BER for (C1, λ1) × (C1, λ1),

: BER for (C1, λ1) × (C1, λ1)

on wavelength shift of 0.018nm,

: BER for (C1, λ1 + λ2) × (C1, λ1),

: BER for (C1 + C2, λ1)

× (C1, λ1),

: BER for (C1 + C2, λ1) × (C2, λ1)

5. Conclusion

A hybrid WDM/OCDMA ring with a dynamic add/drop function was proposed and

demonstrated. For the dynamic add/drop function, Fourier code with the same phase

difference between the orthogonal codes was employed. For an optical source well adapted to

WDM channels, RSOAs were used with FBGs and were gain-switched to generate the optical

short pulse train for driving the encoder/decoder. The results of the experiment showed that

the proposed ring has a power penalty of 0.4 dB between different orthogonal codes, and

WDM channel cross-talk is negligible. Also, through the proposed ring, error free

transmission over a fiber length of 10.64 km at a BER of <109

was achieved.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) Grant (NRF-

2008-F01-2008-000-10012-0) and by the (Photonics 2020) research project through a grant

provided by the Gwangju Institute of Science &Technology in 2011.


Documents

A hybrid WDM/OCDMA ring with a dynamic add/drop function based on Fourier code for local area networks