Optical Amplifier

Generic optical amplifier

Optical Amplifiers Important Parameters: Gain Saturation Output Power Erbium Doped Fiber Amplifier Semiconductor Optical Amplifier 980 nm Pump Signal Amplified Signal ~10 m Erbium Doped Fiber Important Parameters: Gain Saturation Output Power Noise Figure

Optical Amplifier Applications * B. Verbeek, JDSU

Applications of optical amplifiers

Gain bandwidth of optical amplifiers

Amplifier Comparison

Gain Saturation Output saturation power is defined as the output power when gain drops by 3db Power amplifiers usually operate at saturation. Saturation gain is lower than the unsaturated one.

Amplifier gain versus power

Gain versus Amplifier length

Gain versus pump level

Noise Sources

Noise Figure Noise Figure definition is similar as for electrical amplifiers. Essentially a degradation of signal. However, we do not use the optical SNR, but rather the SNR that would be measured with an “ideal” square-law detector at the input and output of the amplifier. Where E=Electric Field, I=Detector Current, A=Signal amplitude, x,y=Amplifier Spontaneous Emission

Noise Figure NF definition assumes shot-noise limited source. Laser noise is ignored. Detector thermal noise is ignored/negligible. Noise Figure 3 2.5 2 1.5 1 0.5 5 10 15 20 25 30 Gain (dB) 3 dB NF limit, for complete inversion, high gain

EDFA EDFA has revolutionized optical communications All optical and fiber compatible Wide bandwidth, 20~70 nm High gain, 20~40 dB High output power, >200mW Bit rate, modulation format, power and wavelength insensitive Low distortion and low noise (NF<5dB)

Pump Source 980 nm 1480 nm low ASE, low noise amplifier higher power pump laser high output power not as efficient degree of population inversion is lower

Gain Spectrum Amorphous nature of silica and the codopants inside the fiber affects the spectrum considerably.

Gain Spectrum Population at different levels are different resulting gain dependence on wavelength Different pumping level has different spectrum

EDFA configurations

EDFA Gain Transient Channel turn-on, re-routing, network reconfiguration, link failure….

Gain Transient Power may become too high (nonlinearity) or too low (degrade SNR) when add/drop channels transient happens in us to ms transient penalty depends on data rate, number of EDFAs and number of channels. power increase degrades performance due to SPM

EDFA Transient Dynamics

Semiconductor Optical Amplifier

Operating Principle: device physics same as EEL. difference is that R< 10-5: AR, angled stripe, window region SOA can be operated in saturation, or unsaturated. gain clamping single pass chip gain: G=exp (g_modal * L) packaging: TEC, high coupling efficiency, isolators

Gain vs. Wavelength Single SOA 40-80 nm, InGaAs/InGaAsP. Spanning from 1250-1650 nm

Gain vs. Output Power An SOA has a Saturation Output Power

Output Power SOAs are linear for small input powers.

Gain Dynamics

Saturation Output Power Saturation Output Power decreases for higher energy photons.

Noise Figure

Noise Figure SOAs are noisier than EDFAs because the coupling efficiency is lower. Otherwise, they have the same theoretical limitations. Thus, integrated SOAs should be less noisy.

Cross Gain Modulation Saturating the SOA with a signal affects the overall gain spectrum. Thus, all wavelengths will be slightly modulated.

Cross Gain Modulation: solutions low input power (linear regime). SOA not in saturation. 8x20 Gbs 160 km. Spiekman et al, 1999 reservoir channel, SOAs in saturation. 32x2.5 gbs. 125 km. Sun et al 1999 Gain-Clamped SOA Solution: Fixed Gain SOA want fixed gain to eliminate XGM Note: a laser has fixed gain above threshold (gain clamping)

Gain Clamped SOA gain medium is shared between SOA and a laser. lasing at a different wavelength.

In-Line Optical Amplifier Distance Noise Figure can limit performance of links * B. Verbeek, JDSU