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

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 1528-1568nm

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

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 – 107.22µm spacer

Dielectric mirrors 5 to 7 layers Solid Spaced Filters Dielectric mirrors 5 to 7 layers T() FSR Wafer thickness Transparent wafer 50 -150µm thick Low Absorption, low scattering Level, few sensitive to deposition errors

Cascaded Solid Spaced Filters air gap T()

Experimental Demonstration: 2 fused silica wafers < 3 arc-second < 3 arc-second 108.7µm 146.0µm 1.443 Refractive index @ 0 1.443 0 = 1560.86 nm 402L 540L Both Sides Coated : 5 layer Ta2O5 / SiO2 IAD mirrors

lightpathTM collimators Measurement set-up tunable laser source InGaAs Photodiode + amplifier+ DAC lightpathTM collimators  1520 – 1570 nm  = 10 pm w0 = 250µm

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

-3db =0.47 Tmax = 98.1%

Triple Cavity Filters how extending the rejection band ?

Use of a blocking filter BW1 BW2 >>BW1

Cavities with different FSR

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

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