1. Cascaded Solid Spaced Filters for DWDM applications
J. Floriot, F. Lemarchand, M. Lequime

2. Narrow Bandpass filters: Requirements
Transmittance
Example: 50GHz
Dl -0.5db > 0.22nm
Dl -3db < 0.3nm
Dl -20db < 0.6nm l
sharpness
-0.5 dB
-3 dB
Steepness /
shape factor
-20 dB
Isolation band nm

3. Narrowing the  bandwidth
Fabry Perot Cavities
Spacer
layer
Incident Light
T()
R()
substrate
Mirror
Mirror
Narrowing the  bandwidth
Higher Spacer thickness
Or Better mirror reflectance

4. Single FP cavities 35 layers – 17 layer mirrors – 2.12µm spacer
nL = 1.46 / nH = 2.09
35 layers – 17 layer mirrors – 2.12µm spacer
27 layers – 13 layer mirrors – 11.68µm spacer
15 layers – 7 layers mirrors – µm spacer

5. Dielectric mirrors 5 to 7 layers
Solid Spaced Filters
Dielectric mirrors 5 to 7 layers
T()
FSR
Wafer thickness
Transparent wafer
µm thick
Low Absorption, low scattering Level, few sensitive to deposition errors

6. Cascaded Solid Spaced Filters
air gap
T()

7. Experimental Demonstration: 2 fused silica wafers
< 3 arc-second
< 3 arc-second
108.7µm
146.0µm
1.443
Refractive 0
1.443
0 = nm
402L
540L
Both Sides Coated : 5 layer Ta2O5 / SiO2 IAD mirrors

8. lightpathTM collimators
Measurement set-up
tunable laser source
InGaAs Photodiode + amplifier+ DAC
lightpathTM collimators
 – 1570 nm
 = 10 pm
w0 = 250µm

9. -3db = 0.79nm
Tmax = 99.9%
-3db =0.55
Tmax = 98.9%
Residual Transmission Level = -16dB

10. -3db =0.47
Tmax = 98.1%

11. Triple Cavity Filters
how extending the rejection band ?

12. Use of a blocking filter
BW1
BW2 >>BW1

13. Cavities with different FSR

14. Different Phase Dependence Mirrors
RM1()  RM2 ()
But M1 () ≠  M2 ()
FSR1 ≠ FSR2

15. Conclusion ? Vs Number of layers low high errors sensitivity low high
Absorption, scattering
Rejection Band
low high
low high
reduced broad
Compactness  Optical contacting