Launch power

Launch power is the optical power that enters the fiber at the start of a link or a span: the output of a transceiver measured at its connector, or the output of an amplifier that feeds the next span. It is an average power and is quoted in dBm, decibels relative to 1 mW,

$$P_\mathrm{dBm} = 10\log_{10}\!\left(\frac{P}{1\ \mathrm{mW}}\right)$$

so 0 dBm is 1 mW, −5 dBm is 0.32 mW and +3 dBm is 2.0 mW (see decibel). Single-channel transceivers typically launch in that range, and the amplified line systems of long-haul networks launch around 0 to a few dBm per channel, which with many channels amounts to about 100 mW or more in one fiber. The launch power is the starting entry of every link budget, and in long links it is bounded from both sides: too little and noise dominates, too much and the fiber's nonlinearity distorts the signal.

Launch power in the link budget

The received power is the launch power minus every loss along the path. A transmitter launching 0 dBm into 80 km of fiber at 0.2 dB/km, with 1 dB of connector and splice loss, delivers −17 dBm to the receiver. That figure is compared with the receiver sensitivity, the lowest power at which the receiver meets its error-rate target, and the difference is the margin. The receiver also has a maximum input, its overload level, so a high-power transmitter on a short link can need an attenuator to bring the received power down into range. Datasheets give the launch power as a minimum and maximum over temperature and life.

Average power and modulation

A power meter reads the average power, and launch power is specified the same way. For on-off keying the average lies midway between the one and zero levels. At 0 dBm average with an extinction ratio of 6 dB, the one level is 1.60 mW and the zero level 0.40 mW. A higher extinction ratio at the same average power puts more of the power into the useful difference between the levels, which is why transmitter specifications give both quantities, and in newer standards also the modulation amplitude.

Per-channel and total power in WDM

In a wavelength-division multiplexed system, launch power is quoted per channel, and the total is

$$P_\mathrm{tot} = P_\mathrm{ch} + 10\log_{10} N$$

in dBm for $N$ equal channels. Eighty channels at 0 dBm each give 19.0 dBm, or 80 mW; at +3 dBm per channel the total is 22.0 dBm, 160 mW. Read the other way, an amplifier with 20 dBm of total output shared among 96 channels gives each about +0.2 dBm.

Nonlinear limits

Silica has a weak Kerr nonlinearity, and the phase it adds grows with power: $\phi_\mathrm{NL} = \gamma P L_\mathrm{eff}$. With $\gamma \approx 1.3$ /(W·km) for standard fiber and an effective length of 21.7 km for a long span at 0.2 dB/km, 10 mW produces about 0.28 rad. This phase drives self-phase modulation within a channel and cross-phase modulation and four-wave mixing between channels.

In an amplified link, each amplifier adds noise that is fixed by its noise figure, so raising the launch power improves the OSNR dB for dB. The nonlinear interference, however, grows roughly with the cube of the launch power, and beyond some point it degrades the signal faster than the higher power helps. The result is an optimum launch power per channel, typically a few dBm in dense WDM over standard fiber, that depends on the fiber's effective area and dispersion, the channel spacing and symbol rate, and the number of spans. The OSNR and Noise Figure Calculator computes the noise side of that balance.

A narrow-linewidth continuous-wave signal meets a lower ceiling: stimulated Brillouin scattering reflects power back toward the source above a threshold of a few milliwatts over a long span. Modulated data spreads the spectrum and raises this threshold, and analog transmitters dither the laser frequency for the same purpose.

Measuring launch power

Launch power is measured with an optical power meter set to the signal wavelength, connected through a short, clean patch cord to the transmitter output. A dirty or damaged connector can remove a dB or more, so connectors are inspected before the measurement. In WDM systems an optical spectrum analyzer, or the per-channel monitor of the line system, separates the channels. Transceivers report an estimate of their own output through digital diagnostics, typically within a few dB; it serves for monitoring, and acceptance tests use a calibrated meter. The How to Check an Optical Power Meter article covers verifying the meter itself.

Accessible launch power is also limited by laser safety classification, which depends on wavelength and beam geometry and is set by the IEC 60825 series.

Common questions

What is a typical fiber launch power?

For single-channel transceivers, roughly −5 to +3 dBm, depending on reach. Amplified WDM systems launch around 0 to +3 dBm per channel, which totals about 19 to 23 dBm with 80 to 100 channels.

Why not raise the launch power to reach further?

Above an optimum, fiber nonlinearity adds distortion faster than the extra power improves the signal-to-noise ratio, and narrow continuous-wave signals also reach the Brillouin threshold. Receivers also have a maximum input power.

References: G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010). G. P. Agrawal, Nonlinear Fiber Optics, 6th ed. (Academic Press, 2019). P. Poggiolini, "The GN model of non-linear propagation in uncompensated coherent optical systems," Journal of Lightwave Technology 30, 3857 (2012). IEC 60825-1, Safety of laser products: Part 1: Equipment classification and requirements.