a s e r s , o p ti kifayat aser pt photonics 218 5:3 l p ... · aser pt photonics an open access...

Volume 5 • Issue 3 • 1000192J Laser Opt Photonics, an open access journalISSN: 2469-410X

Open AccessResearch Article

Journal of Lasers, Optics & PhotonicsJo

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asers, Optics & Photonics

ISSN: 2469-410X

Kifayat, J Laser Opt Photonics 2018, 5:3DOI: 10.4172/2469-410X.1000192

*Corresponding author: Numan Kifayat, Comsats Institute of Information Technology, Islamabad, Pakistan, Tel: +92 51 9247000; E-mail: [email protected]

Received July 10, 2018; Accepted October 24, 2018; Published October 31, 2018

Citation: Kifayat N (2018) Improved Optical Buffer Architectures Based on Fiber Delay Lines. J Laser Opt Photonics 5: 192. doi: 10.4172/2469-410X.1000192

Copyright: © 2018 Kifayat N. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Keywords: Random access memory (RAM); Optical buffer; Semiconductor optical amplifier (SOA); Dispersion compensation fiber Bragg gratings (DCFBG); Fiber delay line (FDL); Erbium doped fiber amplifier (EDFA)

IntroductionOptical fiber communications has enabled a technological

revolution by providing immediate and inexpensive access to information and communication [1]. The international community has felt the revolution changes in both culture and commerce. Optical fiber communication has been a key enabler of globalization where emerging markets are becoming competitive and spatial boundaries are fading [2,3]. However, the telecom market did not adequately track the growing information technology usage. In 2001, speculation caused the field to grow too quickly and that created a depression in the field [4]. For years, the capacity of the United States’ existing infrastructure was easily meeting demand. Now, the need is catching up with supply as new applications are being developed that will require larger bandwidths [5].

Optical communications must provide customers with the capacity they need while lowering the cost per bit. Optical routers are efficient solutions that hold promise in reducing the cost of the hardware at the core of the network while increasing its scalability and flexibility. Electronic routers can handle greater capacities, but at the cost of larger footprints, higher power consumption and challenges in heat extraction. In an electronic router, incoming optical data is converted to the electrical domain to be forwarded and buffered and then converted back to optical data. The major benefits of electrical processing are simple data regeneration and the availability of inexpensive, compact memory. However, optical routers may be necessary in order to reduce power consumption as they have potential to offer advantages such as transparency for data packets at any bit-rate and protocol flexibility [6]. The key challenges in building a competitive optical router are finding optical equivalents for functions that are currently purely electrical.

Photonic packets nowadays are stored momentarily by optical buffers. Optical buffering is achieved using fiber delay lines (FDL)

combined with other components such as SOA gates, optical couplers and similar devices that can act as switches [7-10]. Figure 1 shows the basic idea of optical buffering (Figure 1).

The random lengths packets and asynchronous traffic increases the difficulty level of the contention resolution process. In the optical domain, the control logic of contention resolver is tough to understand that’s why the control logic is electronically implemented. In all optical switches, to resolve the problem of contention. Wavelength conversion, Space deflection and optical buffering are the three strategies.

Fiber delay lines (FDL) are essentially better solutions to implement optical buffers, to maintain the pulse length, polarization and repetition

Improved Optical Buffer Architectures Based on Fiber Delay LinesNuman Kifayat*Comsats Institute of Information Technology, Islamabad, Pakistan

AbstractData center networks involve optical fiber links that are connected by electrical nodes. Dense Wavelength

Division Multiplexing technologies can achieve bit rates over 1Tb/s but the problem is that light has to be converted into electronic domain to switch the data to their respective destinations. Due to increased channel capacities, switching capacity is becoming a bottleneck for the system. Presently, in optical communication research activities are focused on optical switching technologies that include all optical switching, i.e., without converting from optical to electronic domain. However, due to the lack of proper optical buffers blocking in these systems is a huge problem. In this paper, we propose improved optical buffer designs based on Erbium Doped Fiber Amplifier and Dispersion compensation Fiber Bragg Grating to resolve the problem of contention in all-optically switched networks, and enhanced the power performance of the current designs. We compare our improved proposed optical buffer designs with existing optical buffers in terms of their signal degradation (power), Eye height, and Jitter. In our proposed designs, optical power of 129.4 mW and 101.13 mW was recorded for up to 100 rounds for the first and second design respectively with buffering time of 300 µs. These results are much better than Re-circulating Regenerative Fiber Loop for optical Buffering and SOA based All Optical Variable Buffer where optical power of 84.4 mW and 77.9 mW was noted respectively for up to 50 rounds using buffering time of 150 µs. Photonic packets, nowadays are stored temporally by optical buffers.

Figure 1: Demonstration of optical buffer.


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Simulation of Re-circulating regenerative fiber loop for optical buffering

Now we discuss the simulation layout of existing architecture and its main parameters. The transmitting wavelength of the laser was 1551 nm and power the was set to 0dBm. In the simulation layout for cross gain modulation, we used the WDM Mux and the Demux along with SOA. The Channel wavelength for the Mux and the Demux were 1551 nm and 1558 nm, respectively. The biased current of the SOA was 0.3A. The gaussian optical band pass filter was used at a centered wavelength of 1558 nm and its bandwidth was 0.6 nm [16]. The schematic of the existing architecture is shown in Figure 2a.

In Figure 2b SOA1 is used for amplification after a pump co-propagating coupler. The centered wavelength of Gaussian optical filter was set to 1551 nm and its bandwidth is 0.1 nm. A 600 m optical fiber is inserted in the loop control for optical storage. Figure 2c shows the pre-receiver design of the optical buffer [16].

Simulation of SOA based all optical variable buffer

The simulation layout and its important parameters are discussed below.

Figure 3a contains a CW laser whose transmitting wavelength was 1551 nm and power was fixed to 0dBm. Isolators are used before and after the SOAs to minimize the reflections. Two SOA with biased current of 0.15A are used. One SOA is used before the storage part and the other SOA is used before the buffer output for amplification. An optical band pass filter is centered on wavelength of 1551 nm and its bandwidth is 0.6 nm. In the data storage part, we have used the optical fiber of length 1000 m having a loop delay of 5 µs along with a variable optical attenuator [17]. The variable optical attenuator inside the loop is used for setting the proper attenuation to solve the problem of cavity lasting (Figure 3b).

At the pre-receiver side, the semiconductor optical amplifier and Gaussian band pass filter is used. The SOA biased current is 0.35A and the Gaussian band pass filter is centered on wavelength of 1551 nm with 0.1 nm bandwidth.

rate of the pulses. FDL is simply fit into the optical communication systems with slight in and out coupling problems and minimum losses. Several delay designs can be made from fiber elements such as degenerated or dilated buffer, re-circulating or looped buffer, feed-forward buffer and folded path buffer [11-15].

This paper introduces the advancement and challenges in the future optical fiber communication systems, and discusses the major problem of contention in optical fiber communication on the physical layer along with its possible solutions, and focuses on the need for optical buffering. Different optical buffering techniques are reviewed and fiber delay lines and SOA based architectures are selected to improve performance in terms of power loss and higher buffering time, without being part of any optical switched network. The architectures selected and re-simulated in terms of simulation layouts and working principles. The details of the improved designs. The comparison of the enhanced proposed designs with the existing designs in terms of power loss, eye height and jitter.

DescriptionOptical amplification of the signal to be buffered is a key factor

to circulate an optical packet for a certain finite number of rounds. The higher the optical power of the circulating packet, the higher the number of rounds it will be able to circulate in the loop and this results in higher buffering time and vice versa. On the other hand, an increased number of rounds (higher buffering time) also introduces dispersion and decreases SNR. So, in order to mitigate dispersion and maintain the reduction in SNR for higher buffering time we used Dispersion Compensation Fiber Bragg Grating (DCFBG). Optical components such as the Erbium doped fiber amplifier (EDFA) and SOA’s are used to maintain the strength/power of an optical signal. Further details of our enhanced proposed designs. As previously mentioned different optical buffering techniques are reviewed and fiber delay lines and a SOA based architectures are selected, to improve the performance in terms of power loss and higher buffering time, without being part of any optical switched network. The existing architectures simulated are also “Fiber delay lines and SOA” based architectures. We simulated the existing designs in optisystem.

Figure 2a: Schematic layout 1 of re-circulating optical buffer.


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Figure 2b: Schematic layout 2 of re-circulating optical buffer.

Figure 2c: Schematic layout 3 of re-circulating optical buffer.

Figure 3a: Schematic layout 1 of SOA based variable optical buffer.


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is shown below in Figures 5a-c. When the packet completes the fiber delay line the dispersion compensation is achieved by using DCFBG. The parameters of DCFBG are: Frequency is 1551 nm, Bandwidth is 125 GHz and dispersion is -10 ps/nm.

Results and DiscussionIn this section we discuss the simulation results of the existing

architectures and compare these with our proposed architectures in terms of power, eye height and jitter. A significant level of power we can circulate the optical signal for a higher number of rounds and thus achieve a higher buffering time. Eye height shows the signal SNR and dispersion. Jitter shows the time error that is received at the receiver for the transmitted signal.

Comparison of proposed architecture with existing architectures

Power comparison: We compared our proposed architecture in terms of power vs. number of rounds with existing architectures. In our first proposed architecture, we inserted the EDFA and DCFBG in the loop of Re-circulating Regenerative Fiber Loop for optical Buffering. This improves the performance in terms of power vs. number of rounds. For 50 rounds our proposed architecture is showing better performance in terms of power decay compared to other architecture.

The Enhanced Proposed DesignsSimulation of optical buffer design based on EDFA and DCFBG

Figure 4a is our proposed architecture that is similar to Re-circulating fiber loop for optical buffering [18], but its performance is enhanced by introducing the Erbium Doped Fiber Amplifier (EDFA) and Dispersion Compensation Fiber Bragg grating (DCFBG) inside the loop control. The simulation schematic is shown below.

In this design two transmitters with wavelength of 1551 nm are used. A digital optical switch 2x2 is used to solve the problem of contention (Figure 4b).

For the loop control Fiber delay line (FDL) length is 600 m. After the fiber delay line, the EDFA amplifies the packet. The EDFA gain is 0.1dB and the noise figure is 1dB. DCFBG after the EDFA is used for dispersion compensation. DCFBG is tuned to the input packet wavelength of 1551 nm.

Simulation of optical buffer design based on DCFBG

This architecture is enhanced compared to Re-circulating fiber loop for optical buffering [17], by introducing Dispersion Compensation Fiber Bragg Grating (DCFBG) inside the loop. The layout of the design

Figure 3b: Schematic layout 2 of SOA based variable optical buffer.

Figure 4a: Schematic layout 1 of optical buffer design based on EDFA and DCFBG.


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Figure 4b: Schematic layout 2 of Optical buffer design based on EDFA and DCFBG.

Figure 5a: Schematic layout 1 of optical buffer design based on DCFBG.

For the 50th rounds the value of power for Re-circulating Regenerative Fiber Loop for optical Buffering is 84.4 mW and for our proposed Optical buffer design based on EDFA and DCFBG it is 150.8 mW. On the other hand if we record the value from the graph for the same number of 50th rounds of SOA based All Optical Variable Buffer, the value is 77.9 mW and for our proposed design power is 150.8 mW as shown in Figure 6a. The reasons for improvement are that we have used the EDFA for amplification of signal power as it gets reduced after passing through the fiber delay lines of different lengths. In addition, we have also used DCFBG for dispersion compensation and filtering after the FDL and EDFA.

The second proposed Optical buffer design based on DCFBG is compared with the other architecture in terms of power vs. number of rounds as shown in Figure 6b. From Figure 6b, it is evident that

our proposed design is better in terms of power compared to existing designs. If we compare the values for the 50th round, the value of the power is 119.4 mW. However the values for the other architectures are 84.4 mW and 77.9 mW. The reasons for enhancement are that we have used DCFBG after the Fiber Delay line. DCFBG compensates for the dispersion of the optical signal and reflects the desired wavelength.

Eye height comparison: In this section we compare the performance of our proposed architectures with the existing architectures in terms of eye height vs. number of rounds. Eye height generally decreases with increasing number of rounds for all the designs. Our proposed design Optical buffer based on EDFA and DCFBG is better compared to the existing designs. But the problem with this design is that its eye height abruptly decreases with an increasing number of rounds and its value decreases quite significantly for higher numbers of rounds


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as illustrated in Figure 7a. The reduction in eye height is because of dispersion and a decrease in SNR. So for improvement of our proposed designs we introduce EDFA and DCFBG in the first design. EDFA is used to improve SNR and DCFBG is used to minimize dispersion. The performance of the other proposed design can also be observed from Figure 7b.

Jitter comparison: In terms of Jitter vs. number of rounds, our proposed design named Optical buffer based on EDFA and DCFBG performs better as compared to the existing designs for higher number

of rounds as illustrated in Figures 8a. The value of jitter for the 50th round for our design, Optical buffer design based on DCFBG is recorded to be 0.2 compared to existing architectures whose corresponding values are 0.35 and 0.6 as shown in Figure 8b.

This work will be practically implemented in the future and will help to improve the optical power performance of the existing optical buffer designs if used in the all-optical switched network systems, for future optical fiber based communication systems.

Figure 5b: Schematic layout 2 of optical buffer design based on DCFBG.

Figure 5c: Schematic layout 3 of optical buffer design based on DCFBG.


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ConclusionWe selected two architectures, Re-circulating Regenerative Fiber

Loop for optical Buffering and SOA based All Optical Variable Buffer. We simulated the existing designs in OPTI-SYSTEM software and found that these architectures were not suitable for higher number of rounds in terms of power loss.

We compared Re-circulating Regenerative Fiber Loop for optical Buffering and SOA based All Optical Variable Buffer with our proposed architectures in terms of power, eye height, Jitter. We enhanced Re-

circulating Regenerative Fiber Loop for optical Buffering for our proposed architectures. Simulation results and tabulated data shows that our proposed architectures are superior in performance in terms of power, eye height, and jitter. For all architectures with increasing number of rounds, optical power gets reduced but we achieve a higher buffering time so there is a tradeoff between Optical power and buffering time. We also achieve buffering time of 300µs along with contention resolution without any conversion from optical to electrical or electrical to optical domain.

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Figure 7a: Eye height vs. number of rounds.

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Figure 8a: Jitter vs. number of rounds.

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Figure 8b: Jitter vs. number of rounds.


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The architecture which is based on EDFA along with DCFBG respectively is better in performance as compared to DCFBG based architecture. Since, DCFBG based architecture just minimizes the dispersion without amplification. Dispersion compensation also improves the overall power performance but it does not provide amplification like EDFA.This work will help to improve the optical power performance of the original optical buffer designs.

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https://ieeexplore.ieee.org/document/908817



E:\Journals\JLOP\JLOPVolume.5\JLOPVolume 5.3\JLOP 5.3_W\JLOP-18-227 (192)\constructing optical LIFO buffers of size B with 3 log2 B fiber delay lines










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https://www.researchgate.net/publication/318647143_Optical_Buffer_Design_based_on_SOA_and_DCFBG

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