Units & conversions

Optical work mixes logarithmic power, wavelengths, frequencies, bit rates and baud rates. This page collects the conversions you need at the bench, with the mental shortcuts engineers actually use.

dB and dBm

dB is a ratio: 10 · log10(P₁/P₂). It describes gain or loss.
dBm is an absolute power referenced to 1 mW: P(dBm) = 10 · log10(P/1 mW).
Adding a loss in dB to a level in dBm gives a level in dBm: −3 dBm − 7 dB = −10 dBm. Never add two dBm values.

dB	Power ratio	Rule of thumb
0	×1	—
1	×1.26	about a quarter more
3	×2	double / half
6	×4	—
10	×10	one decade
20	×100	—
30	×1000	—

dBm	mW	µW	Typical meaning
+10	10	10 000	booster output, DWDM launch
+3	2	2 000	strong LR/ER transmitter
0	1	1 000	typical single-mode Tx
−3	0.5	500	typical short-reach Tx
−10	0.1	100	healthy Rx on a short link
−18	0.016	16	PIN receiver sensitivity (10G)
−24	0.004	4	APD receiver sensitivity
−30	0.001	1	very weak — check the link
−40	0.0001	0.1	DDM resolution floor: "no light"

Raw DDM power is stored in 0.1 µW steps: raw = µW × 10 (0 dBm → 10 000). Details in DDM parameters & units.

Link budget arithmetic

Budget      = Tx power (min)  −  Rx sensitivity            [dB]
Losses      = fibre (km × dB/km) + connectors + splices + mux/demux + margin
Margin left = Budget − Losses                                (should stay ≥ 0, ideally ≥ 3 dB)

Worked example and component losses: CWDM mux/demux & link budget.

Item	Typical loss
Single-mode fibre, 1310 nm	0.33–0.35 dB/km
Single-mode fibre, 1550 nm	0.18–0.22 dB/km
Multi-mode fibre, 850 nm	2.5–3.5 dB/km
Connector pair (clean)	0.1–0.3 dB
Fusion splice	0.05–0.1 dB
Mechanical splice	0.2–0.5 dB
1:2 splitter	~3.5 dB; each further doubling +3–3.5 dB
CWDM mux (8 ch)	1.0–2.5 dB
DWDM AWG mux (40 ch)	4–7 dB

Wavelength ↔ frequency

λ · f = c, with c ≈ 299 792 458 m/s. Convenient forms:

λ (nm)   = 299 792.458 / f (THz)
f (THz)  = 299 792.458 / λ (nm)

Wavelength	Frequency	Note
850 nm	352.7 THz	multi-mode VCSEL
1310 nm	228.8 THz	O-band
1490 nm	201.2 THz	GPON downstream
1528.77 nm	196.1 THz	C-band top (C61)
1550 nm	193.4 THz	C-band centre
1552.52 nm	193.1 THz	ITU anchor (C31)
1563.86 nm	191.7 THz	C-band bottom (C17)

In the C-band 100 GHz ≈ 0.8 nm and 50 GHz ≈ 0.4 nm. Channel numbering and the full grid: DWDM and the ITU grid, Wavelength bands & channel plans.

Bit rate, baud and line coding

The number on the label (10G, 25G, 100G) is the payload rate. On the fibre the signal runs faster because of line coding and FEC, and PAM4 carries two bits per symbol.

Nominal	Line coding / FEC	Signalling rate	Modulation	Symbol rate
1 GbE	8b/10b	1.25 Gb/s	NRZ	1.25 GBd
10 GbE	64b/66b	10.3125 Gb/s	NRZ	10.3125 GBd
8GFC	8b/10b	8.5 Gb/s	NRZ	8.5 GBd
16GFC	64b/66b	14.025 Gb/s	NRZ	14.025 GBd
25 GbE (per lane)	64b/66b + RS-FEC	25.78125 Gb/s	NRZ	25.78 GBd
32GFC	64b/66b + FEC	28.05 Gb/s	NRZ	28.05 GBd
50G lane	RS(544,514)	53.125 Gb/s	PAM4	26.5625 GBd
100G lane	RS(544,514)	106.25 Gb/s	PAM4	53.125 GBd
200G lane	RS(544,514)	212.5 Gb/s	PAM4	106.25 GBd

Rules: 64b/66b adds 3.125 % (×66/64); RS(544,514) adds another ~5.8 % (×544/514); PAM4 halves the symbol rate for a given bit rate. This is why a "100G" lane runs at 53.125 GBd and why DDM nominal signalling rate bytes read 103 (×100 Mb/s) for 10GbE, not 100. Background: FEC & link training.

Bit-error ratio

BER	Meaning	Time between errors at 10 Gb/s
10⁻⁹	poor	0.1 s
10⁻¹²	classic error-free target (NRZ optics)	~100 s
10⁻¹⁵	post-FEC target for 25G+	~28 h
2.4×10⁻⁴	typical pre-FEC threshold for RS(544,514)	— corrected to <10⁻¹⁵

DDM raw units

Monitor	Raw unit	Convert
Temperature	1/256 °C, signed	°C = raw / 256
Vcc	100 µV	V = raw × 0.0001
Tx bias	2 µA	mA = raw × 0.002
Tx / Rx power	0.1 µW	mW = raw × 0.0001; dBm = 10·log10(mW)

Distances and fibre units

Module length fields: SMF in km (byte 14) and 100 m (byte 15); multi-mode in 10 m units (bytes 16–19) or metres for copper — see Memory map quick reference.
1 dB of extra loss ≈ 3 km of 1310 nm single-mode, ≈ 5 km of 1550 nm, ≈ 300 m of 850 nm multi-mode.

In CodingBox

CodingBox performs all DDM conversions and shows values in engineering units on the DDM screen, with the raw bytes available in the EEPROM editor when you need to check a conversion by hand.