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Advanced Cascaded Solid Spaced Filters for DWDM Applications

This paper discusses the design and performance of advanced cascaded solid spaced filters tailored for Dense Wavelength Division Multiplexing (DWDM) applications. Emphasis is placed on narrow bandpass filters with specific requirements for transmittance and isolation across the 1528-1568 nm band. The role of Fabry-Perot cavities, mirror reflectance, and spacer thickness in shaping filter performance, particularly in terms of steepness and rejection band, is explored. Experimental setups using tunable laser sources are detailed, showcasing measurement results and considerations for optimization.

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Advanced Cascaded Solid Spaced Filters for DWDM Applications

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  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 1528-1568nm

  3. 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 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

  5. Solid Spaced Filters Dielectric mirrors5 to 7 layers T() FSR Wafer thickness Transparent wafer 50 -150µm 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 1.443 Refractive index @ 0 0 = 1560.86 nm 402L 540L Both Sides Coated : 5 layer Ta2O5 / SiO2 IAD mirrors

  8. Measurement set-up tunable laser source InGaAs Photodiode + amplifier+ DAC lightpathTMcollimators  1520 – 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 • howextending the rejection band ?

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

  13. Cavities with different FSR

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

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

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