Passive plant: cables, panels, splices, splitters, filters

Between two transceivers lies the passive plant — everything that carries or joins the light without being powered. It decides most of the link's loss, all of its reflections and nearly all of its faults, and it outlives several generations of the optics plugged into it. This page catalogues the components — cable types and constructions, patch panels and pigtails, fusion and mechanical splices, connectors and adapters, attenuators, PON splitters, WDM filters, enclosures — with their loss allowances and the design and handling rules that keep the budget honest.

Cables

Type	Construction	Fibre count	Where	Notes
Simplex / duplex patch cord	1–2 fibres, 900 µm tight buffer, 2–3 mm jacket, connectors both ends	1–2	rack, module to panel	LC-LC, LC-SC, MPO-MPO; length 0.5–30 m
Distribution (indoor)	900 µm tight-buffered fibres, aramid, LSZH/OFNR/OFNP jacket	4–144	building risers, between rooms	terminated on panels or pre-terminated cassettes
Breakout / fan-out	each fibre in its own 2 mm sub-cable	2–24	direct connectorisation without panels	rugged, bulky
Loose tube (outdoor)	250 µm fibres in gel- or dry-filled tubes around a strength member, water-blocking, PE jacket	12–288	ducts, direct burial, aerial	temperature range −40…+70; must be spliced to pigtails indoors
Ribbon / rollable ribbon	12 fibres bonded side by side, mass-spliced	144–6 912	high-count backbone, data-centre interconnect	12-fibre mass fusion splicing
Armoured	corrugated steel or FRP layer	any	rodent and crush protection, direct burial	grounding of metallic armour
ADSS (all-dielectric self-supporting)	aramid-strength aerial cable	12–144	pole lines, power-line rights of way	spans 100–1 000 m
OPGW	fibres inside the ground wire of HV lines	12–96	utilities	—
Micro-cable / air-blown	200 µm fibres, thin cable blown into microducts	12–432	FTTH, metro	—
Drop cable (FTTH)	1–4 G.657 fibres, flat, self-supporting or indoor	1–4	last 50–500 m to the subscriber	bend-insensitive fibre
Trunk MPO (data centre)	12/24/144 fibres pre-terminated with MPO connectors	12–144	leaf–spine, structured DC cabling	polarity method A/B/C (Breakout & MPO)

Fibre and colour coding inside cables: 12 colours per TIA-598 (blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua), tubes colour-coded the same way (Labels & colour codes).

Termination: panels, pigtails, cassettes

Component	What it does	Loss
Pigtail	1–2 m of fibre with a factory connector on one end, bare on the other; fusion-spliced to the cable	connector 0.1–0.3 dB + splice 0.05
Patch panel / ODF (optical distribution frame)	holds adapters (LC/SC duplex, MPO) where pigtails meet patch cords; splice trays behind	one mated pair per port
Splice cassette / tray	organises and protects splices with bend-radius control	—
Pre-terminated cassette	MPO trunk in, 12/24 LC ports out (fan-out inside)	0.3–0.5 dB per path (MPO + LC pairs)
Field-installable connector	mechanical or fusion-spliced connector installed on site	0.3–0.5 dB (mechanical), 0.1–0.2 (splice-on)
Wall outlet / terminal box	subscriber or work-area termination	one pair

Design rule: every mated pair in the path counts — module to patch cord (1), patch cord to panel (2), panel to far patch cord (3), patch cord to module (4); cassettes add MPO pairs.

Splices

Type	Loss (SMF)	Loss (MMF)	Return loss	Use
Fusion, single-fibre	0.02–0.1 dB (typ. 0.03)	0.05–0.2	> 60 dB (no reflection)	all permanent joints
Fusion, ribbon (mass)	0.05–0.15 dB	—	> 60 dB	high-count cables
Mechanical	0.2–0.5 dB	0.3–0.5	35–45 dB (index-matching gel)	temporary, restoration

Splice loss is measured with an OTDR (bidirectional average); a splice reading > 0.3 dB is redone (Testing). Splicing G.652 to G.657 or to DSF adds 0.05–0.3 dB from mode-field mismatch (Light in fibre).

Connectors and adapters

Detail in Connectors & fibre types; the plant-side essentials:

Item	Values
Insertion loss grades (IEC 61755)	grade A ≤ 0.07 dB (rare), B ≤ 0.12 (0.25 max), C ≤ 0.25 (0.5 max), D ≤ 0.5 (1.0 max)
Return loss	UPC ≥ 50 dB (grade 1), ≥ 45 (grade 2); APC ≥ 60 dB
Mating cycles	500–1 000 before loss degrades
Adapter sleeves	zirconia ceramic (precision) vs phosphor bronze (cheap, wears); keep dust caps on empty adapters
MPO	male (pins) / female (no pins), key up/down, polarity A/B/C; low-loss ≤ 0.35 dB, standard ≤ 0.75

Attenuators

Type	Range	Use
Fixed plug-in (bulkhead)	1, 2, 3, 5, 7, 10, 15, 20 dB; ±0.5 dB; UPC or APC	protect receiver from overload on short links (Link budget)
Variable (VOA), mechanical	0–30 dB, screw-adjusted	commissioning, lab
Variable, electronic (eVOA)	0–20 dB, controlled	DWDM channel equalisation (DWDM components)
Air-gap vs doped-fibre	air-gap reflects (avoid on long links); doped-fibre absorbs	prefer absorptive for laser sources

Attenuator wavelength dependence is small (±0.5 dB across 1310–1550) but not zero; specify the working wavelength.

Splitters (PON)

Ratio	Insertion loss (typ. / max)	Note
1:2	3.5 / 3.9 dB	—
1:4	6.9 / 7.4	—
1:8	10.2 / 10.8	—
1:16	13.5 / 14.1	—
1:32	16.8 / 17.5	GPON class B+ typical
1:64	20.2 / 21.0	class C+ / XGS-PON
1:128	23.5 / 24.5	rare; needs E-class budgets

PLC (planar lightwave circuit) splitters are wavelength-flat 1260–1650 nm and uniform; FBT (fused biconic taper) splitters are cheaper, wavelength-dependent and used for 1:2/1:4 or unbalanced (e.g. 90/10 tap) ratios. Cascading 1:4 then 1:8 costs ~0.5 dB more than a single 1:32 (How PON works).

WDM filters and muxes

Device	Loss	Isolation	Use
CWDM mux/demux 4/8/16-ch (TFF)	1.0–2.5 dB per pass	30 dB adjacent	CWDM
DWDM AWG 40/48/96-ch	3–6 dB	25–30 dB	DWDM
DWDM TFF 8-ch	1.5–3 dB	30 dB	small counts
OADM 1–4 ch	1–2 dB drop, 0.8–1.5 dB pass	30 dB	ring add/drop
BiDi / WDM coupler 1310/1550	0.5–1 dB	20–30 dB	single-fibre working
PON WDM1r (1490/1310/1550 video overlay)	0.5–1 dB	30 dB	RF video overlay
Monitoring tap 1625/1650 nm	0.3–0.5 dB	—	live OTDR

Enclosures and environments

Enclosure	Where	Concerns
Splice closure (dome / inline)	manholes, poles, direct burial	sealing (IP68), gel or mechanical seals, re-enterable
Street cabinet / FDH	FTTH distribution	splitters, temperature, humidity, security
Wall box / FAT	building entry	bend radius, slack storage
Rack ODF	equipment rooms	labelling, slack management, front/rear access
Rack cable management	data centres	bend radius ≥ 30 mm (SMF) / 10 × diameter, no tight ties, separate fibre from copper weight

Design and handling rules

Count every mated pair and splice; use design allowances, not typical values.
Prefer fusion splices and factory-terminated pigtails; avoid mechanical splices in the permanent link.
Bend radius: ≥ 30 mm for standard SMF cords in service (≥ 15 mm G.657.A1, ≥ 7.5 mm A2), ≥ 20 × cable diameter during installation.
Dust caps on every open adapter and connector; clean before every mating (Connectors & fibre).
Label both ends of every cord with the far-end position; record fibre numbers and colours in the plant database.
Keep 3–5 m of slack in closures and 1–2 m in panels for future re-splicing.
Separate APC (green) and UPC (blue) worlds; never mate them.
Test after installation and keep the results (Testing).

In CodingBox

The plant fixes the loss and the wavelength; the module has to match both. CodingBox shows the module's connector code, wavelength and rated fibre lengths so a module is paired with the right plant — APC plant needs APC-compatible modules on the patch cord side, PON optics need the class the splitter budget demands (Check transceiver).

Making the joints in this plant — fusion and mechanical splicing, pigtails, field connectors, acceptance criteria: Splicing and termination; getting the cable there — ducts, direct burial, aerial, indoor pathways, bend and tension limits: Cable routes and installation.

The cable itself in depth — layers, families from loose tube to OPGW and rollable ribbon, fire ratings, colour codes, reading a data sheet: Cable construction; the other half of the link, everything with a power supply: Active optical infrastructure.