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Wavelength Division Multiplexing

Jigisha patel, Nirma University, Richa Gandhi,Nirma University.

Abstract-This letter presents the concept of

WDM,Wavelength division multiplexing optical module using a planar lightwave circuit for full duplex operation, Wavelength-Division Multiplexing and Demultiplexing, Waveguide Input Grating Coupler for Wavelength-Division Multiplexing and encoding.

I.Introduction

n fiber-opticcommunications, wavelength-division multiplexing is a technology which multiplexes a Inumber of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. A waveguide grating coupler is a grating structure etched into the surface of an optical waveguide. Conventionally, the grating coupler is used to either couple a guided wave out of the waveguide or to couple an incident free-space wave into the waveguide.[2] Full duplex technique in WDM enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. Full duplex optical modules are key devices for 1.3/1.55- m bidirectional wavelength-division multiplexing optical subscriber systems and reducing their cost is particularly important. We have already developed an optical module for use in the optical network units of synchronous transfer mode passive optical networks by using planar lightwave circuit platform technology

. The module cost is lower than that of conventional optical modules because we have integrated its optical functions and simplified its assembly.[1]

II. MULTIPLEXING AND DEMULTIPLEXING

Wavelength-division multiplexing (WDM) technology, by which multiple optical channels can be simultaneously transmitted at different wavelengths through a single optical transmission medium, like optical fiber, is a useful means of making full use of the low-loss characteristics of optical fibers over a wide-wavelength region. Most wavelength division multiplexers employ one of three technologies: arrayed waveguide grating (AWG), filter and dispersive element, primarily diffraction grating . Although AWG technology is widely used for WDM devices, its strong temperature dependence often requires thermal regulation. Multiplexers and demultiplexers based on filters exhibit high insertion loss for devices with many channels. The devices based above two technologies have difficulties in applications of multimode and bi-directional transmission. They are not suitable in application of high throughput optical link in parallel processing and

computing. Since a grating-based WDM device can offer these advantages and other advantages of low cost for many channels, low loss, and little crosstalk, it has received much attention. We employed this technology to explore the WDM mulitplexer/demultiplexer for the application of high throughput optical link in parallel processing and computing. In regard to the structure of grating-based WDM multiplexers/demultiplexers there are two main types: the Czerny-Turner structure, which has different lenses for input and output, and the Littrow structure, which has one common lens. Since Littrow WDM multiplexers/demultiplexers use fewer components they are more cost-effective. Most bulk grating-based WDM multiplexers/demultiplexers that have been developed recently employ a Littrow-type structure. Here we give the working principle of Littrow-type structured WDM multiplexer/demultiplexer. To examine the operating principle of the Littrow structured grating-based WDM multiplexers/demultiplexers, we refer to the structure shown in Figure 1. An input fiber and multiple output fibers are arranged on the focal plane of the lens. Wavelength-multiplexed light signals from the input fiber are collimated by the lens and reach the diffraction grating. The light is angularly dispersed, according to different wavelengths, and simultaneously reflected. Then the different wavelengths pass through the lens and are focused to their corresponding output fibers. Each wavelength is fed to one individual output fiber. This functions as a demultiplexer. When working in the reverse direction, the device serves as a multiplexer.

Fig 1. The diagram for the structure of the Littrow type WDM.

If we put two layers of fiber arrays, the devices can transmit WDM signal in bi-direction, that is, it can function as both mux and demux at the same time. Figure 2 shows the structure of devices [3].

Fig 2 Scheme of double-duck of grating WDM multiplexer/demultiplxer

III.FULL DUPLEX

Fig. 3 shows the single fiber ATM-PON system which is the application target of our optical module. Module is used as an optical transceiver called an optical network unit(ONU) oroptical line terminal (OLT). The ATM-PON employs an optical WDM signal in that 1.3 and 1.55 m wavelength lights are used for the upstream and downstream signals, respectively.To utilize the WDM system, the transceiver must be capable of full duplex operation in addition to having a WDM circuit. The system has a passive double star (PDS) configuration to accommodate many subscribers. The optical module for this configuration is required to have a high optical output power and a high sensitivity since it has to receive a divided signal from an optical splitter. Therefore, we must suppress the crosstalk from the transmitter LD to the receiver PD caused by the high output power operation, to achieve high sensitivity in full duplex operation. The crosstalk between the LDand the PDin amodulecan be divided into optical crosstalk and electrical crosstalk. For an integrated optical transceiver module, the LD and PD are assembled in a small area, and the optical crosstalk is caused by uncoupled stray light from the LD at the LD-waveguide butt coupling. Therefore, we designed an optical circuit using a PLC platform to isolate the PD from the stray light with a 1.3/1.55 m WDM filter. Fig. 4 shows the optical module configuration. In anONU for ATM-PON, 1.3 and 1.55 mwavelength lights are used as output and input lights, respectively. The circuit consists of a 1.3/1.5 m WDM circuit, a 1.3 mLD, a 1.3 mPDas a power monitor photodiode (M- PD) for the LD, and a 1.55 m receiver PD (R-PD). The WDM circuit is composed of a silica waveguide and aWDMfilm inserted in a groove formed on the waveguide. The film, which is fixed in the groove with an adhesive, reflects 1.3 m light and allows 1.55 m light to pass. The LD and PDs are flip-chip-bonded on the silicon terrace of the PLC platform, and encapsulated in a transparent silicone resin. This configuration has two advantages as regards reducing both the electrical and optical crosstalk between the LD and

Fig. 3. ATM passive double star system configuration.

Fig. 4. Full duplex 1.3/1.55 micrometer WDM optical module configuration with PLC platform

R-PD. First, the WDM filter optically isolates the R- PD from the LD by reflecting uncoupled stray light from the LD as well as the output signal light. This reduces the optical crosstalk directly. Second, the LD and PD are separated on the small substrate by placing the LD on the opposite side of the WDM filter to the PD. This effectively reduces the electrical and optical crosstalk because the crosstalk decreases greatly with distance [1].

IV. CROSS TALK IN FULL

DUPLEX SYSTEM

To fabricate an optical module with the above configuration, we first determined the LD-PD distance by investigating the relationship between distance and optical and electrical crosstalk. We fabricated three types of optical module with LD-PD distances of 6, 9, and 14 mm for the experiment. In all the modules, the LDs and R-PDs were encapsulated in a transparent silicone resin in the same way as an actual module. We used a WDM filter, which had an isolation of more than 50 dB between 1.3 and 1.55 m. Fig. 5 shows the relationship between optical crosstalk and LD-PD distance before and after WDM filter installation. The optical crosstalk [dB] is defined as

Fig 5 Relationship between LD-PD distance and optical crosstalk.

The optical crosstalk was effectively suppressed by the WDM filter and was less than -43 dB when the LD-PD distance was more than 9 mm. The improvement range of 20–30 dB was not identical to the WDM filter isolation of 50 dB and the suppression of the optical crosstalk was saturated at distances greater than 9 mm. This is because the stray light, which propagated unhindered through the waveguide cladding, was randomly reflected at the WDM filter and passed through it with a large incident angle. In this case, the optical crosstalk was almost independent of the LD-PD distance. When the LD-PD distance was 6 mm, the stray light included other propagating lights in addition to the cladding mode and this also propagated to the

filter while it was eliminated at greater distances[1].

V.GRATING STRUCTURE

The waveguide grating couplers described in this letter consist of a grating structure where the grating lines are arranged in a complicated manner determined by the design method. The freedom in choosing the local position of the grating lines makes it possible to impose a two-dimensional phase

modulation onto the incoupled light. This can be used to obtain a grating structure that in addition to coupling light into the waveguide performs functions such as focusing, beam splitting, and wavelength demultiplexing of the incoupled light. The design algorithm aims at optimizing the local position of each grating line so that as much light as possible of each wavelength is focused into the desired position. Two mechanisms are responsible for the wavelength discriminating capability of the grating coupler: First, the incident wave, which is a spherical wave emitted from the tip of the fiber, has a phase that varies over the grating area where the variation depends on the wavelength. Second, the optical path length in the waveguide is different simply because the vacuum wavelength is different and, to a lesser extent, because of the small change in the effective refractive index of the waveguide mode for the different wavelengths (waveguide and material dispersion). There is also a third mechanism contributing to the wavelength sensitivity: the local interaction between the optical field and the grating structure that determines the coupling strength. However, this effect was not taken into account because of the relatively small wavelength separation used. An accurate description of this interaction requires rigorous electromagnetic modeling using, e.g., the finite difference time domain method (FDTD). The coupler design was carried out using an efficient optimization algorithm. The computer code was written in MATLAB and the calculation time on a 300-MHz computerwas approximately 1 h for designing one of the grating couplers[2].

VI. APPLICATIONS

A fiber optic system designed to transmit over a single optical fiber asynchronous signals previously carried by up to 48 separate metallic cables was developed as a test bed for aircraft applications of fiber optics. Advanced wavelength-division multiplexing and time-division multiplexing techniques were used to achieve the above-mentioned cable count reduction. WDM is widely used in telecommunication network[3].

VII. CONCLUSION

In this letter we discussed the implementation of WDM using the systems described above. These systems efficiently multiplexes and demultiplexes different wavelengths used in telecommunication networks, and we also discussed full duplex system in which bidirectional link can be established.

VIII. REFERENCES

[1] A 1.3/1.55-_m Wavelength-Division Multiplexing Optical Module Using a Planar Lightwave Circuit for Full Duplex Operation Toshikazu Hashimoto, Takeshi

Kurosaki, Masahiro Yanagisawa, Yasuhiro Suzuki, Member, IEEE, Yuji Akahori, Yasuyuki Inoue, Member, IEEE, Yuichi Tohmori, Member, IEEE, Kazutoshi Kato, Member, IEEE, Yasufumi Yamada, Noboru Ishihara, and Kuniharu Kato, Member, IEEE.

[2]Wavelength-Division Multiplexing and Wavelength Encoding Johan Backlund, Jörgen Bengtsson, Carl-Fredrik Carlström, and Anders Waveguide Input Grating Coupler for Larsson, Member, IEEE.

[3]Wavelength division multiplexers/demultiplexers for highthroughput optical links The University of Texas at Austin, PRC, MER 1606H, 10100 Burnet Road

[4]Applications of wavelength- division multiplexing and time-

division multiplexing to aircraft data links by garry duck,Canada.