- Email: [email protected]
Novel integrated CWDM demultiplexer and channel monitor in silica
Optics Communications 305 (2013) 131–136
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Novel integrated CWDM demultiplexer and channel monitor in silica Ahmad Atieh n Electrical Engineering Department, Taibah University, Medina Munawarah, Kingdom of Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2012 Received in revised form 31 March 2013 Accepted 10 April 2013 Available online 20 May 2013
A novel integrated demultiplexer and channel monitoring device is proposed and analyzed. The device uses multilayer graded index profile structure with α-parameter less than one. The effects of different relevant parameters such as refractive index profile parameter, thickness, input signal incident angle, number of layers and channel wavelength spacing on the structure are investigated. The component is recommended for course wavelength division multiplexing (CWDM) optical communication system applications. & 2013 Published by Elsevier B.V.
Keywords: CWDM demultiplexer Optical channel monitor Integrated optical devices Graded index profile with α-parameter less than one Optical communication devices
1. Introduction The demand for increasing data rates in existing optical communication systems exploded recently due to increased number of subscribers and offered triple play related applications. Recently, 40 G and 100 G optical communication systems became commercially available to support that. Newly deployed or upgraded systems suffer from higher costs due to premiums set by equipment manufacturers and initial costs of modules and devices. On the other hand, end users are not willing to pay extra money for newly offered applications and faster services. As a result, pressure on communication system providers and carriers has increased in order to provide more services at lower cost. This pressure is passed to equipment manufacturers, who reduced the cost by using off-shore manufactured devices and integrating more functionality in the same module. An example of functionality integration is combining signal demultiplexing and channel monitoring. Different technologies have been developed in the market to build signal demultiplexers such as multilayer thin film filter (TFF), arrayed waveguide (AWG), fiber Bragg grating (FBG) filters and planner structures with graded index profiles [1–6]. Most of these technologies do not offer integrated demultiplexer and channel monitoring feature within the same device. In addition, demultiplexer module's insertion loss is relatively high and increases with increased number of channels, especially for FBG and TFF technologies. For example, typical insertion loss of an 8-channel thin
n
Tel.: +966 58 140 3020; fax: +966 4 847 5837. E-mail addresses: [email protected], [email protected]
0030-4018/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.optcom.2013.04.010
film CWDM demultiplexer module is about 4 dB. Also, packaging complexity increases when adding more channels and requires more real-state. In this work, a simple and relatively low loss integrated demultiplexer and channel monitor device is introduced. The device is based on a graded index planer structure with α-parameter less than one. The device is an (N+1) layers structure made of doped silica (SiO2), where spatial dispersion property of light is exploited. Wavelength dispersion occurs due to material refractive index wavelength dependence, which is known as material dispersion. When dispersed wavelengths propagate from one layer to another layer through the proposed device, they refract at different angles and get separated. This phenomenon is similar to what happens when light propagates through a prism. However, in multilayer structure the wavelength separation (spatial shift) occurs at every interface across the device. The spatial shift causes wavelengths to be separated along the length of the device (x-axis). The place where the separated wavelengths (demultiplexed signal) are collected is known as the x-exit position of the structure. The different wavelengths of the incident signal on the device follow different paths in the structure due to refractive index wavelength dependence of each layer which is controlled by the refractive index profile of the structure. The last layer of the device is designed to be a splitting layer which has theoretically 100% reflecting mirror at the top of it. The structure has two sets of output light signals which are used for monitoring and demultiplexing purposes. The number of layers, thickness of each layer including splitting layer, input signal incident angle, channel spacing and effect of refractive index profile parameter are investigated and analyzed in this work.
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2. Integrated device structure design The proposed integrated device structure is made up from (N +1) layers as shown in Fig. 1. The last layer which is between the Nth-layer and 100% reflecting mirror is called splitting layer. All Nlayers have the same thickness and the same base material which is silica. The external layer (from the bottom) refractive index is denoted by no. The other layers have refractive indices in the range between 1.44445 and 1.7 and denoted by ni. Typically, the refractive index of each layer can be changed by adding substantial concentration of impurities to the silica. In this work, the refractive indices of the N-layers structure are assumed to fit the graded index profile with the form h y α i nðyÞ ¼ n1 1−Δ ; a
0oyoa
ð1Þ
where Δ¼ (n1−n2)/n1, a is the structure thickness, α-parameter describes the refractive index profile shape, n1 and n2 are the refractive indices at y ¼0 and y¼ a, respectively. Fig. 2 illustrates the refractive index profile for different values of α. Values of α more than 1 are typically used in graded index optical fibers to compensate for modal dispersion. However, in this work a value of α-parameter smaller than 1 is used to maximize spatial shift between different rays propagating in the structure. The α-profile in a multilayer structure of thickness a is created by approximating the actual graded index profile using single point refractive index for each layer (ni). The area under the approximated point for any
layer which is (a/N) ni; where (a/N) is the thickness of each layer, is set to equal the area under the actual curve as shown in Fig. 3. Equating the approximated and actual areas of the ith layer is given by Z ða=NÞðiþ1Þ Z ða=NÞðiþ1Þ h a y α i ni ¼ dy ð2Þ nðyÞdy ¼ n1 1−Δ N a ða=NÞi ða=NÞi After mathematical manipulation, the refractive index of the ith layer is given by h a iαþ1 N a Δ ði þ 1Þ− ði þ 1Þ ni ¼ n1 a N ðα þ 1Þaα N h a iαþ1 a Δ ðiÞ− ðiÞ − ð3Þ N ðα þ 1Þaα N Light propagation in the structure follows the ray theory. An incident ray at the interface of two layers with different refractive indices refracts away from the normal to the interface if the ray propagates from higher refractive index layer to a lower refractive index layer as described by Snell's law ni sinðθi Þ ¼ niþ1 sinðθiþ1 Þ
ð4Þ
where niþ1 is the refractive index of (i+1)th layer, θi is the incident angle on the interface between the incident ray and the normal to the surface, and θiþ1 is the transmission angle measured from the normal to the refracted ray. The rays will bend away from the normal as they penetrate the structure's N-layers. The distance traveled by the different rays along the x-axis can be found using ray geometry. The point where the light rays strike the splitting layer of the structure is defined as (xo, yo), where yo represents the N-layers structure thickness, while xo is given by N a ni −1 tan sin xo ¼ ∑ sinðθi Þ ð5Þ niþ1 i¼1 N The incident angle at each interface must be less than the critical angle to guarantee that light rays penetrate all layers of the structure. This can be ensured if the input signal incident angle at the input of the structure is set to the following angle nN θo o sin−1 ð6Þ no
Fig. 1. Integrated multilayer device structure made of (N+1) layers. A 100% reflecting mirror is placed at the top of the structure.
Fig. 2. Refractive index profile for different values of α-parameter.
where no is the refractive index of the material from which light enters the device as shown in Fig. 1. Rays that are incident at the interface of the splitting layer at (xo, yo) will either be reflected back or transmitted through it. The reflected rays will follow an
Fig. 3. Graded index profile approximation process for N-layers structure.
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identical path to their original path from the bottom of the structure to the top of it. The transmitted beams will hit the 100% reflecting mirror and are totally reflected back. Then these rays will follow a path similar to their original forward path. Thus, two different groups of rays travel downward and arrive at two different positions at the bottom of the structure. One group represents the main output (demultiplexed signal) and the other group represents the auxiliary output (channel monitoring). The splitting layer thickness (ts) could be the same as the other N-layers' width or different. The overall structure dimensions are controlled by the thickness of the splitting layer and the exit position of the auxiliary output along the x-axis. The main output “demultiplexed signal” exits at 2 xo along the x-axis. The splitting layer refractive index must be chosen carefully because it affects the ratio of transmitted and reflected power levels. As well as it affects rays spatial shift.
3. Structure design parameters and results When a multi-wavelength signal is incident on the device at angle θο, it experiences wavelength separation due to refractive index wavelength dependence of the material of the structure [7]. The signal continues to encounter wavelength separation during propagation through the structure due to using values of α-parameter of graded index profile less than one. Fig. 4 illustrates the acquired spatial shift by a multi-channel signal spaced by 1 nm when incident at five layers structure at angle θο of 1.0 rad for different structure thicknesses. Different wavelength separation cases were investigated in this work. A 1-nm wavelength separation was chosen to match available lasers in our lab in order to verify the device once it is manufactured. It is clear that as the value of α decreases the spatial shift between different channels increases. This is due to rapid change of index of refraction in first layers which result in larger distances traveled in the x-direction. Hence, more spatial shift is achieved. The effect of structure thickness (a) on spatial shift is shown in Fig. 5 for two wavelength separation cases. When the structure thickness increases, the distance traveled by light rays in both x and y directions increases. Hence, adjacent channels get separated more and the achieved spatial shift is larger. The x-exit position also increases with increasing structure thickness as shown in Fig. 6 for two wavelength separation cases. The input signal incident angle effect on spatial shift is investigated while bearing in mind the concept of total internal reflection. The incident angle at each interface must be smaller than the critical angle to
Fig. 4. Achieved spatial shift as function of α-parameter when θo ¼ 1 rad, N ¼5 layers, and 1 nm channel spacing.
Fig. 5. Achieved spatial shift as function of structure thickness a for various values of α-parameter when θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing, and (b) 20 nm channel spacing.
guarantee penetration of all layers and to achieve maximum spatial shift. The condition set on the incident angle at each interface matches that of the incident angle at the first layer of the structure given by Eq. (5). If the incident angle is greater than −1 θmax ¼ sin ðnN =n0 Þ rays penetrate limited number of layers and experience total internal reflection before reaching splitting layer. This behavior is not sought because channel monitoring feature of the device will be eliminated. Thus, one must take care when choosing the incident angle to guarantee penetration of rays to all layers. Fig. 7 shows that θmax must be around 1.0 rad. Fig. 8 illustrates the effect of number of layers on achieved spatial shift in the structure. As the number of layers increases the refractive indices difference between successive layers decreases which causes smaller change in refraction angles between adjacent channels. It is clear that the change in spatial shift is rapid when the number of layers is small because the refractive indices difference between successive layers is large which causes considerable change in refraction angles, hence increases spatial shift. When the number of layers is large enough, then adding more layers will have a minor effect on spatial shift. The effect of channel spacing on spatial shift is illustrated in Fig. 9. It is clear that the larger the wavelength spacing between adjacent channels, the farther channels are separated spatially along the x-axis. This is due to the fact that refractive indices difference between adjacent wavelengths is larger. It is important
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Fig. 8. Achieved spatial shift as function of number of layers for various values of α-parameter when θo ¼ 1 rad, a¼ 100 mm, and 1 nm channel spacing.
Fig. 9. Effect of channel spacing on channels' spatial shift along the x-axis for different values of α-parameter. Fig. 6. The x-exit position as function of structure thickness a for various values of α-parameter when θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
The effect of adding a splitting layer to the N-layers structure on spatial shift of the transmitted signal “monitored” is shown in Fig. 10 for two wavelength separation cases. The splitting layer is assumed to be made of silica with refractive index of 1.7 at 1510 nm and assumed to have the same thickness as the other N-layers. The x-exit position as function of structure thickness for the transmitted signal is depicted in Fig. 11 for two wavelength separation cases. When comparing Fig. 11 to Fig. 6, it is clear that the monitored signal is picked at a different position along the x-axis.
4. Power analysis in the structure
Fig. 7. Spatial shift as function of input signal incident angle for various values of α-parameter when structure thickness a¼ 100 mm, N ¼ 5 layers, and 1 nm channel spacing.
to have larger spatial separation between different wavelengths in order to enable and simplify packaging, where each wavelength will be collected through a separate lensed tapered fiber.
When a multi-channel signal is incident on the structure, it encounters power losses due to light attenuation of the device in addition to reflection of rays at each interface between successive layers. In this work, the attenuation of the device is assumed negligible for CWDM applications due to relatively short structure width (a o50 mm). The absorption of light in silica glass can be intrinsic caused by interaction of light with major components of glass or extrinsic caused by interaction of light with impurities in the glass. Given that commercial optical fiber attenuation in the operating wavelength range of interest (1510–1610 nm) is approximately 0.3 dB/km, implies that the attenuation of a structure with less than 50 mm long can be assumed negligible. However, the layers of the demultiplexer are doped with impurities in order to change their refractive indices. As a result, an increase in extrinsic
A. Atieh / Optics Communications 305 (2013) 131–136
Fig. 10. Transmitted signal (monitored) spatial shift as function of structure thickness a for various values of α-parameter when splitting layer is introduced and θo ¼1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
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Fig. 11. The x-exit position as function of structure thickness a for different values of α-parameter when splitting layer is introduced and θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
absorption is acquired which is expected to be relatively negligible as it is assumed in this work due to structure dimensions. The actual value of the device attenuation can only be found once the device is actually built. The power evolution of transmitted (refracted) and reflected rays at each interface for parallel polarization at which the electric field E is in the plane of incident is given by [8] Pr ¼ Pi
n2 cosðθ1 Þ−n1 cosðθ2 Þ 2 n2 cosðθ1 Þ þ n1 cosðθ2 Þ
Pt ¼ Pi
n2 cosðθ1 Þ−n1 cosðθ2 Þ 1− n2 cosðθ1 Þ þ n1 cosðθ2 Þ
ð7Þ 2 ! ð8Þ
where Pi is the incident power at the interface, Pt is the transmitted power, Pr is the reflected power, θ1 is the angle of incidence at the interface and θ2 is the angle of refraction. The total power losses represent the accumulated reflected power at each interface in the forward path and backward path from the mirror to the exit point along the x-axis. The incident signal at the device input is collected at two different output ports; the main exit port and auxiliary exit port. The same analysis can be done for perpendicular polarization. Typically, optical devices' manufacturers align polarization to one polarization axis. Fig. 12 illustrates the
Fig. 12. Normalized main power level as function of number of layers when θo ¼1 rad, and a ¼50 mm.
normalized main output power level as function of number of layers, while Fig. 13 shows the normalized auxiliary output power level. It is clear that most of the incident power is present at the main output port while small partial percentage of power is present at the monitored output port. The percentage of power presented at the monitor output port can be controlled by
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Fig. 13. Normalized auxiliary power level as function of number of layers when θo ¼1 rad, and a ¼50 mm.
Fig. 15. Normalized auxiliary power level as function of splitting layer refractive index when N ¼5 layers, θo ¼1 rad, and a ¼50 mm.
Figs. 14 and 15 illustrate the effect of changing splitting layer refractive index on the main and auxiliary output power levels. It is clear that when the splitting layer has refractive index equals to that of the Nth layer, the main output power level is maximum and the auxiliary power level is minimum because there will be no reflection at the interface between the Nth layer and the splitting layer. Thus, the choice of splitting layer refractive index controls the achieved monitoring power level.
5. Conclusions
Fig. 14. Normalized main power level as function of splitting layer refractive index when N ¼ 5 layers, θo ¼ 1 rad, and a¼ 50 mm.
choosing proper splitting layer refractive index. Note that when the number of layers increases, the power level at the main output port decreases and the power level in the auxiliary output port increases. This is due to the fact that as the number of layers increases, the difference between the refractive indices between the splitting layer (nt ¼1.7) and the Nth layer gets larger. As a result, more reflection occurs at the interface which causes lower main output power levels and higher auxiliary output power levels. Moreover, as the value of α-parameter decreases the graded index profile gets steeper and the change in refractive indices between splitting layer and the Nth layer is larger. As a result, the overall power collected at both main and auxiliary output ports get smaller. However, increasing the value of α-parameter decreases spatial shift as shown in Fig. 4.
A novel integrated demultiplexer and channel monitoring device is presented and analyzed. The structure of the component is designed exploiting dispersive feature of the graded index profile material with α-parameter less than one. The device is recommended for CWDM applications with channel separation of 20 nm. Proper choice of splitting layer refractive index controls the percentage of power levels in the main and auxiliary output ports. This device has relatively low insertion loss compared to other available technologies which their insertion loss increases with increased number of channels.
References [1] [2] [3] [4] [5] [6]
M. Gerken, D. Miller, Journal of Lightwave Technology 22 (2004) 612. Martina Gerken, David Miller. Applied Optics 42(7) (2003) 1330–1345. Yoshinori Hibino. IEEE Circuits and Devices 16(6) (2000) 21–27. Z. Sheng, S. He, L. Lou, Optics Communications 225 (2003) 95. A. Atieh, I. Mansour, Z. Dalala, Proceedings of SPIE 8307 (2012) 830704. Haythem A. Bany Salameh, Mansour I. Irshid. Journal of Lightwave Technology 24(6) (2006) 2401–2408. [7] G.P. Agrawal, Nonlinear Fiber Optics, second ed., Academic Press Inc., 1995. [8] F.Graham Smith, Terry A. King, Dan Wilkins, Optics and Photonics: An Introduction, second ed., John Wiley & Sons Inc., 2007.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Novel integrated CWDM demultiplexer and channel monitor in silica Ahmad Atieh n Electrical Engineering Department, Taibah University, Medina Munawarah, Kingdom of Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2012 Received in revised form 31 March 2013 Accepted 10 April 2013 Available online 20 May 2013
A novel integrated demultiplexer and channel monitoring device is proposed and analyzed. The device uses multilayer graded index profile structure with α-parameter less than one. The effects of different relevant parameters such as refractive index profile parameter, thickness, input signal incident angle, number of layers and channel wavelength spacing on the structure are investigated. The component is recommended for course wavelength division multiplexing (CWDM) optical communication system applications. & 2013 Published by Elsevier B.V.
Keywords: CWDM demultiplexer Optical channel monitor Integrated optical devices Graded index profile with α-parameter less than one Optical communication devices
1. Introduction The demand for increasing data rates in existing optical communication systems exploded recently due to increased number of subscribers and offered triple play related applications. Recently, 40 G and 100 G optical communication systems became commercially available to support that. Newly deployed or upgraded systems suffer from higher costs due to premiums set by equipment manufacturers and initial costs of modules and devices. On the other hand, end users are not willing to pay extra money for newly offered applications and faster services. As a result, pressure on communication system providers and carriers has increased in order to provide more services at lower cost. This pressure is passed to equipment manufacturers, who reduced the cost by using off-shore manufactured devices and integrating more functionality in the same module. An example of functionality integration is combining signal demultiplexing and channel monitoring. Different technologies have been developed in the market to build signal demultiplexers such as multilayer thin film filter (TFF), arrayed waveguide (AWG), fiber Bragg grating (FBG) filters and planner structures with graded index profiles [1–6]. Most of these technologies do not offer integrated demultiplexer and channel monitoring feature within the same device. In addition, demultiplexer module's insertion loss is relatively high and increases with increased number of channels, especially for FBG and TFF technologies. For example, typical insertion loss of an 8-channel thin
n
Tel.: +966 58 140 3020; fax: +966 4 847 5837. E-mail addresses: [email protected], [email protected]
0030-4018/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.optcom.2013.04.010
film CWDM demultiplexer module is about 4 dB. Also, packaging complexity increases when adding more channels and requires more real-state. In this work, a simple and relatively low loss integrated demultiplexer and channel monitor device is introduced. The device is based on a graded index planer structure with α-parameter less than one. The device is an (N+1) layers structure made of doped silica (SiO2), where spatial dispersion property of light is exploited. Wavelength dispersion occurs due to material refractive index wavelength dependence, which is known as material dispersion. When dispersed wavelengths propagate from one layer to another layer through the proposed device, they refract at different angles and get separated. This phenomenon is similar to what happens when light propagates through a prism. However, in multilayer structure the wavelength separation (spatial shift) occurs at every interface across the device. The spatial shift causes wavelengths to be separated along the length of the device (x-axis). The place where the separated wavelengths (demultiplexed signal) are collected is known as the x-exit position of the structure. The different wavelengths of the incident signal on the device follow different paths in the structure due to refractive index wavelength dependence of each layer which is controlled by the refractive index profile of the structure. The last layer of the device is designed to be a splitting layer which has theoretically 100% reflecting mirror at the top of it. The structure has two sets of output light signals which are used for monitoring and demultiplexing purposes. The number of layers, thickness of each layer including splitting layer, input signal incident angle, channel spacing and effect of refractive index profile parameter are investigated and analyzed in this work.
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2. Integrated device structure design The proposed integrated device structure is made up from (N +1) layers as shown in Fig. 1. The last layer which is between the Nth-layer and 100% reflecting mirror is called splitting layer. All Nlayers have the same thickness and the same base material which is silica. The external layer (from the bottom) refractive index is denoted by no. The other layers have refractive indices in the range between 1.44445 and 1.7 and denoted by ni. Typically, the refractive index of each layer can be changed by adding substantial concentration of impurities to the silica. In this work, the refractive indices of the N-layers structure are assumed to fit the graded index profile with the form h y α i nðyÞ ¼ n1 1−Δ ; a
0oyoa
ð1Þ
where Δ¼ (n1−n2)/n1, a is the structure thickness, α-parameter describes the refractive index profile shape, n1 and n2 are the refractive indices at y ¼0 and y¼ a, respectively. Fig. 2 illustrates the refractive index profile for different values of α. Values of α more than 1 are typically used in graded index optical fibers to compensate for modal dispersion. However, in this work a value of α-parameter smaller than 1 is used to maximize spatial shift between different rays propagating in the structure. The α-profile in a multilayer structure of thickness a is created by approximating the actual graded index profile using single point refractive index for each layer (ni). The area under the approximated point for any
layer which is (a/N) ni; where (a/N) is the thickness of each layer, is set to equal the area under the actual curve as shown in Fig. 3. Equating the approximated and actual areas of the ith layer is given by Z ða=NÞðiþ1Þ Z ða=NÞðiþ1Þ h a y α i ni ¼ dy ð2Þ nðyÞdy ¼ n1 1−Δ N a ða=NÞi ða=NÞi After mathematical manipulation, the refractive index of the ith layer is given by h a iαþ1 N a Δ ði þ 1Þ− ði þ 1Þ ni ¼ n1 a N ðα þ 1Þaα N h a iαþ1 a Δ ðiÞ− ðiÞ − ð3Þ N ðα þ 1Þaα N Light propagation in the structure follows the ray theory. An incident ray at the interface of two layers with different refractive indices refracts away from the normal to the interface if the ray propagates from higher refractive index layer to a lower refractive index layer as described by Snell's law ni sinðθi Þ ¼ niþ1 sinðθiþ1 Þ
ð4Þ
where niþ1 is the refractive index of (i+1)th layer, θi is the incident angle on the interface between the incident ray and the normal to the surface, and θiþ1 is the transmission angle measured from the normal to the refracted ray. The rays will bend away from the normal as they penetrate the structure's N-layers. The distance traveled by the different rays along the x-axis can be found using ray geometry. The point where the light rays strike the splitting layer of the structure is defined as (xo, yo), where yo represents the N-layers structure thickness, while xo is given by N a ni −1 tan sin xo ¼ ∑ sinðθi Þ ð5Þ niþ1 i¼1 N The incident angle at each interface must be less than the critical angle to guarantee that light rays penetrate all layers of the structure. This can be ensured if the input signal incident angle at the input of the structure is set to the following angle nN θo o sin−1 ð6Þ no
Fig. 1. Integrated multilayer device structure made of (N+1) layers. A 100% reflecting mirror is placed at the top of the structure.
Fig. 2. Refractive index profile for different values of α-parameter.
where no is the refractive index of the material from which light enters the device as shown in Fig. 1. Rays that are incident at the interface of the splitting layer at (xo, yo) will either be reflected back or transmitted through it. The reflected rays will follow an
Fig. 3. Graded index profile approximation process for N-layers structure.
A. Atieh / Optics Communications 305 (2013) 131–136
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identical path to their original path from the bottom of the structure to the top of it. The transmitted beams will hit the 100% reflecting mirror and are totally reflected back. Then these rays will follow a path similar to their original forward path. Thus, two different groups of rays travel downward and arrive at two different positions at the bottom of the structure. One group represents the main output (demultiplexed signal) and the other group represents the auxiliary output (channel monitoring). The splitting layer thickness (ts) could be the same as the other N-layers' width or different. The overall structure dimensions are controlled by the thickness of the splitting layer and the exit position of the auxiliary output along the x-axis. The main output “demultiplexed signal” exits at 2 xo along the x-axis. The splitting layer refractive index must be chosen carefully because it affects the ratio of transmitted and reflected power levels. As well as it affects rays spatial shift.
3. Structure design parameters and results When a multi-wavelength signal is incident on the device at angle θο, it experiences wavelength separation due to refractive index wavelength dependence of the material of the structure [7]. The signal continues to encounter wavelength separation during propagation through the structure due to using values of α-parameter of graded index profile less than one. Fig. 4 illustrates the acquired spatial shift by a multi-channel signal spaced by 1 nm when incident at five layers structure at angle θο of 1.0 rad for different structure thicknesses. Different wavelength separation cases were investigated in this work. A 1-nm wavelength separation was chosen to match available lasers in our lab in order to verify the device once it is manufactured. It is clear that as the value of α decreases the spatial shift between different channels increases. This is due to rapid change of index of refraction in first layers which result in larger distances traveled in the x-direction. Hence, more spatial shift is achieved. The effect of structure thickness (a) on spatial shift is shown in Fig. 5 for two wavelength separation cases. When the structure thickness increases, the distance traveled by light rays in both x and y directions increases. Hence, adjacent channels get separated more and the achieved spatial shift is larger. The x-exit position also increases with increasing structure thickness as shown in Fig. 6 for two wavelength separation cases. The input signal incident angle effect on spatial shift is investigated while bearing in mind the concept of total internal reflection. The incident angle at each interface must be smaller than the critical angle to
Fig. 4. Achieved spatial shift as function of α-parameter when θo ¼ 1 rad, N ¼5 layers, and 1 nm channel spacing.
Fig. 5. Achieved spatial shift as function of structure thickness a for various values of α-parameter when θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing, and (b) 20 nm channel spacing.
guarantee penetration of all layers and to achieve maximum spatial shift. The condition set on the incident angle at each interface matches that of the incident angle at the first layer of the structure given by Eq. (5). If the incident angle is greater than −1 θmax ¼ sin ðnN =n0 Þ rays penetrate limited number of layers and experience total internal reflection before reaching splitting layer. This behavior is not sought because channel monitoring feature of the device will be eliminated. Thus, one must take care when choosing the incident angle to guarantee penetration of rays to all layers. Fig. 7 shows that θmax must be around 1.0 rad. Fig. 8 illustrates the effect of number of layers on achieved spatial shift in the structure. As the number of layers increases the refractive indices difference between successive layers decreases which causes smaller change in refraction angles between adjacent channels. It is clear that the change in spatial shift is rapid when the number of layers is small because the refractive indices difference between successive layers is large which causes considerable change in refraction angles, hence increases spatial shift. When the number of layers is large enough, then adding more layers will have a minor effect on spatial shift. The effect of channel spacing on spatial shift is illustrated in Fig. 9. It is clear that the larger the wavelength spacing between adjacent channels, the farther channels are separated spatially along the x-axis. This is due to the fact that refractive indices difference between adjacent wavelengths is larger. It is important
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Fig. 8. Achieved spatial shift as function of number of layers for various values of α-parameter when θo ¼ 1 rad, a¼ 100 mm, and 1 nm channel spacing.
Fig. 9. Effect of channel spacing on channels' spatial shift along the x-axis for different values of α-parameter. Fig. 6. The x-exit position as function of structure thickness a for various values of α-parameter when θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
The effect of adding a splitting layer to the N-layers structure on spatial shift of the transmitted signal “monitored” is shown in Fig. 10 for two wavelength separation cases. The splitting layer is assumed to be made of silica with refractive index of 1.7 at 1510 nm and assumed to have the same thickness as the other N-layers. The x-exit position as function of structure thickness for the transmitted signal is depicted in Fig. 11 for two wavelength separation cases. When comparing Fig. 11 to Fig. 6, it is clear that the monitored signal is picked at a different position along the x-axis.
4. Power analysis in the structure
Fig. 7. Spatial shift as function of input signal incident angle for various values of α-parameter when structure thickness a¼ 100 mm, N ¼ 5 layers, and 1 nm channel spacing.
to have larger spatial separation between different wavelengths in order to enable and simplify packaging, where each wavelength will be collected through a separate lensed tapered fiber.
When a multi-channel signal is incident on the structure, it encounters power losses due to light attenuation of the device in addition to reflection of rays at each interface between successive layers. In this work, the attenuation of the device is assumed negligible for CWDM applications due to relatively short structure width (a o50 mm). The absorption of light in silica glass can be intrinsic caused by interaction of light with major components of glass or extrinsic caused by interaction of light with impurities in the glass. Given that commercial optical fiber attenuation in the operating wavelength range of interest (1510–1610 nm) is approximately 0.3 dB/km, implies that the attenuation of a structure with less than 50 mm long can be assumed negligible. However, the layers of the demultiplexer are doped with impurities in order to change their refractive indices. As a result, an increase in extrinsic
A. Atieh / Optics Communications 305 (2013) 131–136
Fig. 10. Transmitted signal (monitored) spatial shift as function of structure thickness a for various values of α-parameter when splitting layer is introduced and θo ¼1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
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Fig. 11. The x-exit position as function of structure thickness a for different values of α-parameter when splitting layer is introduced and θo ¼ 1 rad, and N ¼ 5 layers. (a) 1 nm Channel spacing and (b) 20 nm channel spacing.
absorption is acquired which is expected to be relatively negligible as it is assumed in this work due to structure dimensions. The actual value of the device attenuation can only be found once the device is actually built. The power evolution of transmitted (refracted) and reflected rays at each interface for parallel polarization at which the electric field E is in the plane of incident is given by [8] Pr ¼ Pi
n2 cosðθ1 Þ−n1 cosðθ2 Þ 2 n2 cosðθ1 Þ þ n1 cosðθ2 Þ
Pt ¼ Pi
n2 cosðθ1 Þ−n1 cosðθ2 Þ 1− n2 cosðθ1 Þ þ n1 cosðθ2 Þ
ð7Þ 2 ! ð8Þ
where Pi is the incident power at the interface, Pt is the transmitted power, Pr is the reflected power, θ1 is the angle of incidence at the interface and θ2 is the angle of refraction. The total power losses represent the accumulated reflected power at each interface in the forward path and backward path from the mirror to the exit point along the x-axis. The incident signal at the device input is collected at two different output ports; the main exit port and auxiliary exit port. The same analysis can be done for perpendicular polarization. Typically, optical devices' manufacturers align polarization to one polarization axis. Fig. 12 illustrates the
Fig. 12. Normalized main power level as function of number of layers when θo ¼1 rad, and a ¼50 mm.
normalized main output power level as function of number of layers, while Fig. 13 shows the normalized auxiliary output power level. It is clear that most of the incident power is present at the main output port while small partial percentage of power is present at the monitored output port. The percentage of power presented at the monitor output port can be controlled by
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A. Atieh / Optics Communications 305 (2013) 131–136
Fig. 13. Normalized auxiliary power level as function of number of layers when θo ¼1 rad, and a ¼50 mm.
Fig. 15. Normalized auxiliary power level as function of splitting layer refractive index when N ¼5 layers, θo ¼1 rad, and a ¼50 mm.
Figs. 14 and 15 illustrate the effect of changing splitting layer refractive index on the main and auxiliary output power levels. It is clear that when the splitting layer has refractive index equals to that of the Nth layer, the main output power level is maximum and the auxiliary power level is minimum because there will be no reflection at the interface between the Nth layer and the splitting layer. Thus, the choice of splitting layer refractive index controls the achieved monitoring power level.
5. Conclusions
Fig. 14. Normalized main power level as function of splitting layer refractive index when N ¼ 5 layers, θo ¼ 1 rad, and a¼ 50 mm.
choosing proper splitting layer refractive index. Note that when the number of layers increases, the power level at the main output port decreases and the power level in the auxiliary output port increases. This is due to the fact that as the number of layers increases, the difference between the refractive indices between the splitting layer (nt ¼1.7) and the Nth layer gets larger. As a result, more reflection occurs at the interface which causes lower main output power levels and higher auxiliary output power levels. Moreover, as the value of α-parameter decreases the graded index profile gets steeper and the change in refractive indices between splitting layer and the Nth layer is larger. As a result, the overall power collected at both main and auxiliary output ports get smaller. However, increasing the value of α-parameter decreases spatial shift as shown in Fig. 4.
A novel integrated demultiplexer and channel monitoring device is presented and analyzed. The structure of the component is designed exploiting dispersive feature of the graded index profile material with α-parameter less than one. The device is recommended for CWDM applications with channel separation of 20 nm. Proper choice of splitting layer refractive index controls the percentage of power levels in the main and auxiliary output ports. This device has relatively low insertion loss compared to other available technologies which their insertion loss increases with increased number of channels.
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