Derickson, Dennis J.

Fortenberry, R. M.

Kastanis, W. P.

Scobey, M. A.

Stokes, L. F.

English

DigitalCommons@CalPoly

398 I OFC 2002 I THURSDAY MORNING ThC5 9:45am Thin film Interference filters for 25 GHz channel spacIng M.A. Scobey, R.M. Fortenberry, L.P. Stokes, W.P. Kastanis, D.l. Derickson. Gerra Photonics, Inc. 3640 Westwind Blvd, Santa Rosa Ca. 95403, Email: Mike_Scobey@CierraPhtonics.com 1. IntroductIon Optical demultiplexing of 25 GHz spaced chan~ nels presents a number of challenges to the opti­cal DWDM component designer. In particular, technical challenges around insertion loss, chro­matic dispersion, and filter squareness have pre­vented the adoption of thin film interference fil~ ter technology for 25 GHz or 50 GHz channel spaced systems. This paper discusses the first ex­perimental results of five and six cavity thin film optical filters for 25 GHz spaced DWDM systems, Power penalty due to dispersion is simulated for a cascade of eight 25 GHz six cavity filters. Thin film filters are shown to be a practical solution for optical multiplexing and demultiplexing for 25 GHz channel spaced DWDM systems. 2. Thin film filter background Thin film optical filters are grown in vacuum deposition chambers using physical vapor depo­sition (PVD) techniques. They are in general composed ofalternately varying high and low re­fractive index layers that form coherently stacked Fabry-Perot resonant cavities. Design techniques to create low rifgle multi-cavity thin film filters are well known. For the 25 GHz designs used in 1551.6 1551.7 this paper, each cavity has roughly 40 layers. Typ­ical thin film coating materials are metal oxides such as tantalum pentoxide and silicon dioxide. The spacer (cavity) layers for the designs in this paper are silica. Substrates are selected for a high Coefficient of Thermal Expansion (CTE) to pro­vide passive temperature stabilization.3 The number of cavities in the design gives a good in­dication of the squareness, or "Figure-of-Merit" (FOM) of the filter. For this paper FOM is defined as the 0.5 dB bandwidth divided by the 25 dB bandwidth. One of the more challenging issues with the fabrication of high cavity count 25 GHz filters is the growth of thin films with sufficiently low op­tical loss required by the high normalized electric field within the filter. Normalized fields can reach values as high as 800 within the central cavities. Besides creating simple insertion loss, absorption and scatter in thin film resonant cavity filters also create wavelength dependent loss that has the ef­fect of rounding the amplitude response of 25 GHz filters. 3. Experimental Results Five and six cavity 25 GHz filters were grown using a proprietary PVD deposition process. A measured spectral scan of the five cavity filter is shown in Fig. I. Tabulated values of theoretical and measured results for both a five cavity and a six cavity filter are shown in Table 1. Theoretical insertion loss (IL) values in the table assume an absorption coefficient (k) of 2 ppm. Spectral measurements were made at normal incidence using Agilent's 81640a swept laser. The high resonance of thin film filters pro­duces a wavelength dependent phase shift on transmission that manifests itself as chromatic dispersion (CD). the power penalty due to CD on real world transmission systems is difficult to predict as the CD values vary across the passband. In general, for symmetrical amplitude bandpass filters, the CD passes through zero near or at the center of the bandpass filter, which is typically where the carrier signal strength is largest. Mea­0 -5 -10 iii -/5~ is .... -20 c 'f -25 .s ~ -30 -35 -40 1551.2 1551.3 1551.4 Slued CD on a packaged 25 GHz five cavity filter is shown in Fig. 2. The CD was measured using Agilent's 86037c CD measurement system. Power penalty due to filter amplitude and CD was simulated using a OC192 NRZ pseudo r~n­dam binary sequence with repeat length of 27 -- I generated using commercially available opti':al modeling software as shown in Fig. 3. The model assumed the modulated signal was tnmsmitted through an eight filter cascade. The performance of the filter was modeled as a dispersion power penalty for a range of filter detuning from perfect centering of the signal source. An amplitude-only power penalty as a function ofwavelength decen­tering with no phase effects was generated. Also shown is the effect of dispersion-only power penalty as a function of wavelength decentering. Excess loss of the filter is not included in the analysis as it is contained in the total link power budget. The conclusion of this simulation is that for up to QC192 data rates, the power penalty de­rived from CD effects dominates over the ampli­tude driven power penalty and can be kept at manageable levels. 4. Optical bandwidth budget As signal bandwidth increases as a fraction of channel width, the tolerancing budgeting be­comes more complex. For 100 and 200 GHz channel spacing and 0C48 data rates, the optical bandwidth of each modulated laser is a small fraction of the filter bandwidth and could be neg­le<:ted when designing a filter's 0.5 and 25 dB bandwidths. For 25 GHz channel spacing and OC192 data rates, however, each laser's optical bandwidth is a sizeable fraction of the filter's 0.5 dB bandwidth, Further, allowances for filter cen­1551.5 Wavelength (om) l'hC5 Fig.1. Five cavity 25 GHz measured spectral results. ThC5 Table 1, Experimental data on 5 cavity and 6 cavity filters. Filter 5 cavity measured 5 cavity theoretical 6 cavity measured 6 cavity theoretical IL(dB) 0.65 0,63 0.50 0.73 0.5 dB BW (pm) 129 145 167 155 IdBBW 25dBBW FOM (pm) (pm) (%) 151 275 47 155 276 53 186 304 55 178 262 59 THURSDAY MORNING / OFC 2002/ 399 500 frllr---====================-----l 0 ~ ~ ·10300 E 200 -iL-1I-'+l·-------------J.--.-I1+\1 ·ItfH1c--.---'j- -15 iil 1100 -20 ~ - .2.§ 0 -25:• E& -100 -30 :l .!!! I! c ·200 -35 .... ·300 -I--------"'I------------------1I-1-1+-U -40 -400 +-----------------------~+_l-_U_45 -500 -50 1551.17 1551.22 1551.27 1551.32 155-1.37 Wavelength (om) ThC5 Fig. 2. Amplitude function and CD of packaged 5 cavity 2S GHz thin film filter. 14 12 iii 10:;!. ~ ~ 8c•.. ~ 8• ~ 0.. 4 2 \ f-+-Dispersion Penalty 7 -a- Arrp~tude Penalty I\ (norrmlized) I ~ '\ 7 "/ ~ ."'--­ ~ -It o -80 -60 -40 -20 0 20 40 60 80 Wavelength Offset (pm) ThC5 Fig. 3. Calculated power penalty of OC192 signals due to dispersion and filter amplitude func­tion as a function of filter or signal source decentering. The simulation transmits through eight cascaded six cavity 25 Ghz filters. ter wavelength drift over operating temperature, have an advantage over other technologies in that aging, and measurement uncertainties, as well as non-adjacent channels are strongly rejected and laser drift, must be made, These sources of wave­ can be ignored. length centering error must be added to the re­quired clear channel bandwidth to obtain the 5. Summary necessary filter bandwidth budget, For example, a Thin film interference filters using five and six. clear channelbandwidth requirement may be 100 cavity designs are shown to be a suitable and pm while adding the various centering errors may practical method to demultiplex 25 Ghz channel give a total 0,5 dB bandwidth budget of ISO pm, spaced data signals for up to 0C192 data rates, Adjacent channel crosstalk budgets are com­ Experimental results show good agreement with plicated by signal modulation broadening as well, theoretical values and low overall loss and square Adjacent channel rejection of 25 dB is interpreted filter functions. Chromatic dispersion and group as allowing no more than -25 dB of either adja­ delay of the filters are shown to be manageable at cent channel total (modulated) power to transmit OC192. through the filter, The weighted combination of the adjacent channels spectral function and the References filter slope through the adjacent channel then de­ 1. H.A. Macleod, Thin Film Optical Filters, Sec­termines the allowable 25 dB bandwidth. Addi­ ond Edition, Macmillan publishing, New tionally, various sources of wavelength centering York, 1986. error must be subtracted from that value to ob­ 2. P.W Baumeister, Optical Interference Coat­tain the necessary filter bandwidth budget. For ing Technology, Lecture Notes, UCLA. example, the 2S dB bandwidth requirement may 3. H. Takahashi, "Temperature stability of thin­be 2 channels minus the weighted clear channel film narrow bandpass filters produced by bandwidth, or 325 pm, while subtracting the var­ ion-assisted deposition': Proc. SPIE 2253, ious centering errors could give a total 25 dB 1343 (1994). bandwidth budget of 275 pm, Thin film filters 

Thin Film Interference Filters for 25GHz Channel Spacing

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Thin Film Interference Filters for 25GHz Channel Spacing

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