**8. Design of equalized holographic ROADMs for application in CWDM metro networks**

These type of ROADMs are designed for application in CWDM (Coarse Wavelength Division Multiplexing) networks, where the distance between the different wavelengths allow the use of DML (Direct Modulation Lasers) without cooling, reducing the cost and the tolerances of the network components. Application in METRO networks and its interconnection with some PON (Passive Optical Network), as a part of the access to the subscriber, is reviewed.

Different technologies have been proposed for the implementation of ROADMs (Ma & Kuo, 2003), (Homa & Bala, 2008). Each of them has its own advantages and drawbacks. The main characteristic of holographic ROADMs is the easy way of changing the tuning and power level of the signal at the output fibers by the dynamic implementation of different holograms on the SLM according to the requirements of the network management.

### **8.1 Holographic ROADM structure**

The working principle of the dynamic holographic device is based on the wavelength dispersion produced in a diffraction component (grating, spatial light modulator) as explained in Section 3.

We use for this application a phase reflective spatial light modulator (SLM) and a fixed transmissive diffraction grating to select the corresponding output wavelength from an set of channels in the input, as shown in Fig 11. The active element of the SLM is a Ferroelectric Liquid Crystal (FLC) with a low switching time (less than 50 μs) that allows a real time

Application of Holograms in WDM Components for Optical Fiber Systems 273

Fig. 12. Four different tuned wavelengths at the output of the holographic router

1998).

account.

**8.2.2 Holographic device losses** 

**8.2.3 Channel power equalization** 

the following causes:

(3x.0.8) ≈ 3 dB.

As we have commented in Section 4.1, for wavelengths close to the central value, the filter response is very similar to the Gauss filter; for wavelengths far from the central value, the filter response is similar to a 3nd order Bessel filter with less out band attenuation. Both of them have a linear phase characteristic, which means a constant group delay. These simulations are in agreement with experimental measurements shown in (Parker et al..,

The losses produced in this holographic router, as we have commented before, are due to



In total, with an optimized holographic device, a loss about 6 dB has to be taken into

Power equalization at the all output channels is necessary to compensate the different response of the network components and distances for the used channel wavelengths. To reach it and to compensate for the holographic device losses, a gain component, such as a Semiconductor Optical Amplifier (SOA), has to be employed. The total equalization takes into account the gain-wavelength variation of this amplifier, ΔGA, whose typical response is


drawn in Fig. 13, (the maximum gain, GA is about 20~25 dB).

operation. The role of the fixed diffraction grating is to provide more wavelength tuning range and greater total diffraction angle.

Fig. 11. Reflective holographic router

One of the reasons because we have chosen this type of "2f-folded"implementation, , is the reduced size of the device in comparison with the other possible structure, "linear-4f", where the length of the optical axis is four times the focal distance of the lens used. Its working operation has been described in Section 3.

#### **8.2 Holographic ROADM design**

#### **8.2.1 Dynamic wavelength tuning**

At the input of the router there are different wavelengths λ1, λ2,…. λn according to some ITU Rec. For the design of this holographic router, these wavelengths are in agreement with the G.694 Rec. for use in CWDM systems. The range of wavelengths is from 1271 nm to 1611 nm with 20 nm as separation between channels; 4, 4+4, 8, 12 and 16 groups of channels are specified distributed along the complete range.

In a holographic router the tuning of this wavelength range is achieved by changing the spatial period of the hologram *ND/n*, where *n* is the number of pairs of bars (2-phases) or number of four bars (4-phases), *N* is the number of pixels and *D* the size of the SLM pixels. The expression which allows the selection of the output wavelength *λ,* according to the physical parameters and structure of the device, is (Martin Minguez & Horche, 2007):

$$
\lambda \approx \frac{\chi}{f} \cdot \frac{1}{\left(\frac{n}{\text{ND}} + \frac{2}{\text{(M/2)} \, d}\right)}\tag{13}
$$

where *x* is the distance from the optical axis to the output fiber, *f* is the focal length of the lens, *d* is the spatial period of the fixed diffraction grating and M is the number of phases. Fig. 12 shows some tuned wavelengths according to different values of *n*, for a typical holographic device.

operation. The role of the fixed diffraction grating is to provide more wavelength tuning

One of the reasons because we have chosen this type of "2f-folded"implementation, , is the reduced size of the device in comparison with the other possible structure, "linear-4f", where the length of the optical axis is four times the focal distance of the lens used. Its

At the input of the router there are different wavelengths λ1, λ2,…. λn according to some ITU Rec. For the design of this holographic router, these wavelengths are in agreement with the G.694 Rec. for use in CWDM systems. The range of wavelengths is from 1271 nm to 1611 nm with 20 nm as separation between channels; 4, 4+4, 8, 12 and 16 groups of channels are

In a holographic router the tuning of this wavelength range is achieved by changing the spatial period of the hologram *ND/n*, where *n* is the number of pairs of bars (2-phases) or number of four bars (4-phases), *N* is the number of pixels and *D* the size of the SLM pixels. The expression which allows the selection of the output wavelength *λ,* according to the

> 1 2 ( / 2).

(13)

 

where *x* is the distance from the optical axis to the output fiber, *f* is the focal length of the lens, *d* is the spatial period of the fixed diffraction grating and M is the number of phases. Fig. 12 shows some tuned wavelengths according to different values of *n*, for a typical

*ND M d*

physical parameters and structure of the device, is (Martin Minguez & Horche, 2007):

*x f n*

range and greater total diffraction angle.

Fig. 11. Reflective holographic router

**8.2 Holographic ROADM design 8.2.1 Dynamic wavelength tuning** 

holographic device.

working operation has been described in Section 3.

specified distributed along the complete range.

Fig. 12. Four different tuned wavelengths at the output of the holographic router

As we have commented in Section 4.1, for wavelengths close to the central value, the filter response is very similar to the Gauss filter; for wavelengths far from the central value, the filter response is similar to a 3nd order Bessel filter with less out band attenuation. Both of them have a linear phase characteristic, which means a constant group delay. These simulations are in agreement with experimental measurements shown in (Parker et al.., 1998).

#### **8.2.2 Holographic device losses**

The losses produced in this holographic router, as we have commented before, are due to the following causes:


In total, with an optimized holographic device, a loss about 6 dB has to be taken into account.

#### **8.2.3 Channel power equalization**

Power equalization at the all output channels is necessary to compensate the different response of the network components and distances for the used channel wavelengths. To reach it and to compensate for the holographic device losses, a gain component, such as a Semiconductor Optical Amplifier (SOA), has to be employed. The total equalization takes into account the gain-wavelength variation of this amplifier, ΔGA, whose typical response is drawn in Fig. 13, (the maximum gain, GA is about 20~25 dB).

Application of Holograms in WDM Components for Optical Fiber Systems 275

f f

Fig. 15. Losses in the incident light due to different ND x ND hologram apertures

The EH-ROADM is able to select at the output fibers any combination of wavelengths at the input fiber, from all input wavelengths in just one output fiber to each input wavelength at the corresponding output fiber, including all intermediate cases. This operation mode is done by the selection in the SLM of a mixed hologram composed of all individual

Fig. 16 shows an example for three input wavelengths and its holograms, formed, in this case, by black and white bars (2-phases). For every input wavelength (channel) a hologram is assigned, where *ni* (spatial period) produces the pass-band filter for the channel and *Ni* 

**Mixed holograms espectra**

1501 1511 1521 1531 1541 1551 1561

**λchannel(nm)**

**n2N2 n3,N3**

**n** (hologram spatial period): λchannel

=

Mixed hologram

**N** (hologram surface): Atchannel

1/e2

*o*

*core NDf*

<sup>4</sup>

holograms corresponding to each input wavelength.


SLM equalization

**n1,N1**

Fig. 16. Mixed holograms operation

+ +

λ1,At1 λ 2,At2 λ 3,At3

**Atchannel (dB)**

collimating lens

φcore

**8.2.4 Mixed hologram operation** 

λ0

gaussian light

ND

SLM

ND

Fig. 13. Incremental gain (ΔGA) of a typical CWDM SOA

The target is to have at the output fibers a net loss of 0 dB (GT), according to the equation:

$$\mathbf{G}\_{\rm r} = \mathbf{G}\_{\rm A} - \Delta \mathbf{G}\_{\rm A} - 10 \log(\text{number of channels}) - L\_{\rm rse} - \Delta \mathbf{A} \, t = 0 \tag{14}$$

where Δ*At* is the total attenuation range for channels to be equalized at the input of the device; L*HR* is the intrinsic holographic router losses (≈ 6 dB) and the term Δ*LHR* =10 x log (number of channels) has taken into account the additional loss due to the mixed holograms utilized for equalizing all the input channels. This point will be explained in detail in the following paragraphs. Fig. 14 shows the structure of an Equalized Holographic ROADM (EH-ROADM) for 4 input channels with full routing of them to the 4 output fibers. A way to obtain at the output fibers tuned wavelengths with different relative attenuation between them is to control the losses due to the SLM aperture, as pointed out in Fig. 15.

The minimum losses due to the SLM aperture are obtained when the incident light, with a Gaussian distribution, fills the complete surface *ND x ND* of the SLM. Therefore, the losses are proportional to the quantity of SLM aperture illuminated by the collimated light coming from the lens, as in Fig. 15. A practical way to reach the former condition is by changing the size of the hologram according to the number of active pixels.

Fig. 15. Losses in the incident light due to different ND x ND hologram apertures

#### **8.2.4 Mixed hologram operation**

274 Holograms – Recording Materials and Applications

The target is to have at the output fibers a net loss of 0 dB (GT), according to the equation:

10log( .. ) 0 *G G G number of channels L At TA A HR* (14)

where Δ*At* is the total attenuation range for channels to be equalized at the input of the device; L*HR* is the intrinsic holographic router losses (≈ 6 dB) and the term Δ*LHR* =10 x log (number of channels) has taken into account the additional loss due to the mixed holograms utilized for equalizing all the input channels. This point will be explained in detail in the following paragraphs. Fig. 14 shows the structure of an Equalized Holographic ROADM (EH-ROADM) for 4 input channels with full routing of them to the 4 output fibers. A way to obtain at the output fibers tuned wavelengths with different relative attenuation between

**SOA HReq**

**in out**

**Holograms**

λ1

λ2

λ<sup>4</sup> λ<sup>3</sup>

**GA , ΔGA LHR , <sup>Δ</sup>LHR**

**LT**

The minimum losses due to the SLM aperture are obtained when the incident light, with a Gaussian distribution, fills the complete surface *ND x ND* of the SLM. Therefore, the losses are proportional to the quantity of SLM aperture illuminated by the collimated light coming from the lens, as in Fig. 15. A practical way to reach the former condition is by changing the

them is to control the losses due to the SLM aperture, as pointed out in Fig. 15.

Fig. 13. Incremental gain (ΔGA) of a typical CWDM SOA

λ1 λ2 λ3 λ<sup>4</sup>

Fig. 14. Equalized Holographic ROADM 1x4

size of the hologram according to the number of active pixels.

The EH-ROADM is able to select at the output fibers any combination of wavelengths at the input fiber, from all input wavelengths in just one output fiber to each input wavelength at the corresponding output fiber, including all intermediate cases. This operation mode is done by the selection in the SLM of a mixed hologram composed of all individual holograms corresponding to each input wavelength.

Fig. 16 shows an example for three input wavelengths and its holograms, formed, in this case, by black and white bars (2-phases). For every input wavelength (channel) a hologram is assigned, where *ni* (spatial period) produces the pass-band filter for the channel and *Ni* 

Fig. 16. Mixed holograms operation

Application of Holograms in WDM Components for Optical Fiber Systems 277

**Fibre 3** 113 95 78 61 **9538 Fibre 4** 139 121 103 85 **9718 Fibre 5** 165 146 128 110 **9898 Fibre 6** 191 172 153 135 **10078** 

In an equalized holographic router, the directing of the input wavelengths to the output fibers is done by the choice of three parameters: *nij* for wavelength tuning, *Ni* for power equalization and *Δxj* for placing the output optical fibers. Subscript *i* is related to the number of input wavelengths and subscript *j* related with the number of output fibers. Haqving fixed the separation between fibers, in our case *Δx* = 180 μm, we obtain the corresponding value of *nij* from (13), according to the input wavelength(s) and output fiber(s) considered. As we are managing different sets of *nij* values, all of them have to be different in order to

Table 8 shows the holograms (*nij*) and number of active pixels (*Ni*) for a 4-channels grid according to the ITU G.695 Rec. For instance, in Fig. 11, a mixed hologram 113+95+78+61 addresses the 4 input wavelengths (λ3+ λ4+ λ5+ λ6) to the output fibre 3; a mixed hologram 113+121+128+135 addresses λ3 to fibre 3, λ4 to fibre 4, λ5 to fibre 5 and λ6 to fibre 6. In each case, every λi has the corresponding *Ni* range to assure the power equalization at the output. Table 9 is a summary of the losses in the device (SOA+EH-ROADM) according to the different input channels, whose variation in wavelength is in agreement with Fig. 15. In this case, there is a net gain of 10 dB to compensate for the power variation due to different paths of the input channels along the network. The *Ni* range, 256÷1024, in Table 8 is to compensate a total of 12 dB of attenuation; with a step of ΔNi = 16 the ripple at the output is

Additional mixed hologram loss (dB) ΔLHR

1511 **1.5 13.5**  1531 **2 14**  1551 **1 13**  1571 **0 12** 

**Total min. net gain, GT (dB) 10** 

Table 9. SOA gain, EH\_ROADM losses and total net gain

Max. gain of SOA, GA (dB) **@ 1531 nm**  Min. gain of SOA, GA (dB) **@ 1571 nm** 

λ (nm) ΔGA (dB) Total loss (dB)

**6** 

LHR

**[10 x log (4)]** 

**24** 

**22**

**Ni (pixels) 256÷1024 256÷1024 256÷1024 256÷1024** 

Table 8. Holograms and active pixels for an EH\_ROADM 1x4

avoid cross-talk between wavelengths on different output fibers.

Intrinsic router loss (dB) **6** 

**λ3 = 1511 nm λ4 = 1531 nm λ5 = 1551 nm λ6 = 1571 nm xj (μm)** 

nij (holograms)

< 0.5 dB.

takes into account the number of active pixels to reach the correct attenuation, *Atchannel*, to equalize the output signals. This mixed hologram produces the additional loss in the holographic router, *10 x log (number of channels).* A more fitted equalization can be obtained by monitoring the outputs with a feed-back loop to adjust the size of the holograms according to the wanted output signal level.

#### **8.2.5 Design calculations**

Having chosen the SLM, the focal length of the lens, *f*, to illuminate with collimated light the complete active surface *ND x ND* of the SLM (see Fig.15), is related to the number of pixels *N* and their size *D* according to expression (8):

$$f = \pi \phi\_{\text{surr}} \frac{ND}{4\lambda\_o} \tag{15}$$

where *Φcore* is the input fiber core diameter and λ0 the central wavelength in the operation region. The 3 dB pass-band filter bandwidth of the device, *BW*, is (Parker et al., 1998)

$$BW \ge \Phi\_{\text{core}} \frac{\text{(M / 2)} \, d}{f} \cdot \left( 1 - \frac{\lambda\_{\text{o}}^2}{\text{(M / 2.d)}^2} \right)^{3/2} \tag{16}$$

if the condition *8D >> d* is reached, where *d* is the fixed grating spatial period. For our calculations, we have a reflective 4-phases SLM with *N* = 1024 and *D* = 8 μm (*ND* = 8.192 mm). Then, the focal distance for the lens is 37.655 mm and the *BW* ≥ 1.52 nm (190 GHz), *d* being = 6.5 μm, the spatial period of a 4-phases transmissive diffraction grating and *Φcore* = 9 μm the core diameter of a singlemode fiber. By changing *d* we can adjust the *BW* of the holographic filter.

In the expression (13) the selected wavelength of operation is calculated. The value of *n* is varied from *n* = 0 (for maximum wavelength) and *n* = *N/4* (for minimum wavelength); the central wavelength λ0 is obtained when *n* = *N/8*. For the design of a 1x4 router working in the upper band of the CWDM grid, 1471-1611 nm, we take λ0 = 1541 nm. In this case λmin = 1407 nm and λmax = 1693 nm; these values cover the entire CWDM upper band. The distance from the optical axis to the output fiber, (see Fig. 11), where the 1st order of the total diffraction is produced, is *x* = 9.808 mm and the total diffraction angle *φ* ≈ 14.6º.

From ITU G.694 Rec., all CWDM channels are spread Δλ = 20 nm to allow Direct Modulated Lasers (DML) wavelength variation with temperature and filter tolerance; therefore, an ΔΛ = (8-1).Δλ = 1611-1471 = 140 nm range assumes *ΔX* = 1260 μm, according to the relation:

$$
\Delta \mathbf{x} \approx \Delta \lambda. f\left(\frac{n}{\text{ND}} + \frac{2}{\text{(M/2)}.d}\right) \tag{17}
$$

This is the maximum interval, at the output plane, where all the output fibers have to be placed. That mean, a separation between fibers of *Δx* = *ΔX* /(8-1) = 1260 / 7 = 180 μm. Single mode fibers have a cladding diameter of 125 +/- 1 μm, so, we can reduce this separation to aprox. 130 μm, if it is needed.

takes into account the number of active pixels to reach the correct attenuation, *Atchannel*, to equalize the output signals. This mixed hologram produces the additional loss in the holographic router, *10 x log (number of channels).* A more fitted equalization can be obtained by monitoring the outputs with a feed-back loop to adjust the size of the holograms

Having chosen the SLM, the focal length of the lens, *f*, to illuminate with collimated light the complete active surface *ND x ND* of the SLM (see Fig.15), is related to the number of pixels

<sup>0</sup> 4 *core*

where *Φcore* is the input fiber core diameter and λ0 the central wavelength in the operation

( / 2. ) *core*

if the condition *8D >> d* is reached, where *d* is the fixed grating spatial period. For our calculations, we have a reflective 4-phases SLM with *N* = 1024 and *D* = 8 μm (*ND* = 8.192 mm). Then, the focal distance for the lens is 37.655 mm and the *BW* ≥ 1.52 nm (190 GHz), *d* being = 6.5 μm, the spatial period of a 4-phases transmissive diffraction grating and *Φcore* = 9 μm the core diameter of a singlemode fiber. By changing *d* we can adjust the *BW* of the

In the expression (13) the selected wavelength of operation is calculated. The value of *n* is varied from *n* = 0 (for maximum wavelength) and *n* = *N/4* (for minimum wavelength); the central wavelength λ0 is obtained when *n* = *N/8*. For the design of a 1x4 router working in the upper band of the CWDM grid, 1471-1611 nm, we take λ0 = 1541 nm. In this case λmin = 1407 nm and λmax = 1693 nm; these values cover the entire CWDM upper band. The distance from the optical axis to the output fiber, (see Fig. 11), where the 1st order of the total

From ITU G.694 Rec., all CWDM channels are spread Δλ = 20 nm to allow Direct Modulated Lasers (DML) wavelength variation with temperature and filter tolerance; therefore, an ΔΛ = (8-1).Δλ = 1611-1471 = 140 nm range assumes *ΔX* = 1260 μm, according

> <sup>2</sup> . ( / 2). *<sup>n</sup> x f ND M d*

This is the maximum interval, at the output plane, where all the output fibers have to be placed. That mean, a separation between fibers of *Δx* = *ΔX* /(8-1) = 1260 / 7 = 180 μm. Single mode fibers have a cladding diameter of 125 +/- 1 μm, so, we can reduce this separation to

diffraction is produced, is *x* = 9.808 mm and the total diffraction angle *φ* ≈ 14.6º.

*f Md* 

region. The 3 dB pass-band filter bandwidth of the device, *BW*, is (Parker et al., 1998)

( /2). <sup>1</sup>

*M d BW*

*ND <sup>f</sup>* (15)

(16)

(17)

3/2 <sup>2</sup> 0 2

according to the wanted output signal level.

*N* and their size *D* according to expression (8):

**8.2.5 Design calculations** 

holographic filter.

to the relation:

aprox. 130 μm, if it is needed.


Table 8. Holograms and active pixels for an EH\_ROADM 1x4

In an equalized holographic router, the directing of the input wavelengths to the output fibers is done by the choice of three parameters: *nij* for wavelength tuning, *Ni* for power equalization and *Δxj* for placing the output optical fibers. Subscript *i* is related to the number of input wavelengths and subscript *j* related with the number of output fibers. Haqving fixed the separation between fibers, in our case *Δx* = 180 μm, we obtain the corresponding value of *nij* from (13), according to the input wavelength(s) and output fiber(s) considered. As we are managing different sets of *nij* values, all of them have to be different in order to avoid cross-talk between wavelengths on different output fibers.

Table 8 shows the holograms (*nij*) and number of active pixels (*Ni*) for a 4-channels grid according to the ITU G.695 Rec. For instance, in Fig. 11, a mixed hologram 113+95+78+61 addresses the 4 input wavelengths (λ3+ λ4+ λ5+ λ6) to the output fibre 3; a mixed hologram 113+121+128+135 addresses λ3 to fibre 3, λ4 to fibre 4, λ5 to fibre 5 and λ6 to fibre 6. In each case, every λi has the corresponding *Ni* range to assure the power equalization at the output. Table 9 is a summary of the losses in the device (SOA+EH-ROADM) according to the different input channels, whose variation in wavelength is in agreement with Fig. 15. In this case, there is a net gain of 10 dB to compensate for the power variation due to different paths of the input channels along the network. The *Ni* range, 256÷1024, in Table 8 is to compensate a total of 12 dB of attenuation; with a step of ΔNi = 16 the ripple at the output is < 0.5 dB.


**Total min. net gain, GT (dB) 10** 

Table 9. SOA gain, EH\_ROADM losses and total net gain

Application of Holograms in WDM Components for Optical Fiber Systems 279

Fig. 19 shows the simulation of this device, composed of three different blocks: a CW tunable laser, a wavelength conversion semiconductor optical amplifier and a wavelength holographic router. In Fig. 20, the response of the Wavelength Conversion and Routing Holographic Device (WCR-HD) is represented for a 2.5 Gb/s input signal, λi = 1540 nm, which is converted to an output signal, λo = 1520 nm, where the losses of the holographic

> All optical switching (λ conversion + λ routing)

> > HOLOGRAPHIC λ ROUTER

Control

λCW1 λCW2 λCW3 λCW4

router are compensated by the gain of the SOA.

λCWj

Tunable laser

Control

SOA

λCW1 λCW2 λCW3 λCW4

gain: 10 dB losses: 10 dB

Insertion losses: 0 dB

Fig. 19. Wavelength Conversion and Routing Holographic Device (WCR-HD) simulation

Fig. 18. Device composed of an optical λ converter and a holographic λ router

λi (modulated)

## **8.2.6 CWDM METRO networks application**

The use of tunable holographic devices in Access and Metro networks, like demultiplexers or routers has been studied in different papers (Koonen, 2006), (Martin Minguez & Horche, 2010). In Fig. 17 an application for the equalized holographic ROADM is represented.

**ROADM: Reconfigurable Optical Add-Dropp Multiplexer**

Fig. 17. Application of an EH\_ROADM in a CWDM METRO network

A double ring CWDM METRO topology is used to connect this primary access network, through an Optical Line Termination (OLT), with some Fiber to the Office (FTTO) or Fiber to the Home (FTTH) networks with Passive Optical Network (PON) structure; on the other side, a connection to a DWDM METRO network, by an OXC (Optical Cross Connect) with λ conversion, is provided. The target is to address the wavelengths of the double ring network, λ1, λ2, λ3 and λ4 to four different PONs with the possibility of wavelength reallocation.
