Optoamplifier Basics: Types, Specifications, and Applications

An optical amplifier is a device that boosts the strength of an optical signal. Typical fiber cables experience a loss of about 0.2dB per kilometer for 1.5 micrometer light signals. This means that over a distance of 100km, a signal can lose around 20dB. To compensate for these losses at regular intervals and maintain a good Signal-to-Noise Ratio (SNR) or Bit Error Rate (BER), the signal needs to be amplified.

Traditionally, optical amplification involved converting the signal from optical to electrical form, and then back to optical. However, the Erbium Doped Fiber Amplifier (EDFA) allows signal amplification in the optical domain itself, eliminating the need for costly high-speed electronic devices that would otherwise be required for conversions at frequencies exceeding 10GHz.

An optical amplifier’s performance is typically characterized by parameters like gain, gain efficiency, gain bandwidth, and gain saturation, which are described below:

• Gain: The ratio of output power to input power, measured in Decibels (dB).
• Gain Efficiency: The gain as a function of the input power.
• Bandwidth: The range of wavelengths over which the amplifier operates effectively and provides maximum gain.
• Gain Saturation: The maximum output power the optical amplifier can deliver. Amplification is not possible beyond this limit.

Types of Optical Amplifiers

The main types of optical amplifiers include:

• EDFA (Erbium Doped Fiber Amplifier)
• Semiconductor Optical Amplifiers (SOA)
• Raman Amplifier

Rare Earth Doped Fiber Amplifiers are further categorized into:

• EDFA (Erbium Doped): Operates in the 1500-1600nm band.
• PDFA (Praseodymium Doped): Operates in the 1300nm band.

SOA’s work in a broader range, from 400-2000nm.

EDFA (Erbium Doped Fiber Amplifier)

EDFAs have been commercially available since around 1990 and perform best in the 1530 nm to 1565 nm band, providing gains of up to 30dB.

As shown in Figure 1, an Erbium Doped Fiber is pumped using a semiconductor laser at either 980 nm or 1480 nm. Efficient amplification is achieved when stimulated emission dominates over spontaneous emission.

Light is typically absorbed as it propagates through a medium. However, if the population at the higher energy state is greater than that at the lower energy state, the light will be amplified as it travels through the fiber amplifier.

The 980 nm pump is generally preferred due to its low noise amplification characteristics compared to the 1480 nm pump. However, silica fiber exhibits lower loss at 1480 nm, allowing the pump to propagate alongside the input signal being amplified. The pump can also be placed remotely.

EDFAs are available in L-band and C-band configurations. L-band EDFAs typically require higher pump powers than C-band EDFAs. A gain equalizing filter can be employed at the output of the EDFA to equalize the gain across the bandwidth of interest, if needed.

Here are typical specifications for an EDFA:

EDFA Specifications

• Gain: 40dB or greater
• Wavelength Range: 1530 nm to 1560 nm
• Maximum Saturation: 22 dBm
• Noise Figure: ~5dB
• Pump Power: ~25 dBm
• Bandwidth (3dB): 30 to 60 nm
• Polarization Sensitivity: Not available with EDFA

EDFAs offer high pump utilization of power and are often used with optical filters to equalize the gain response. While they offer many advantages, their size prevents seamless integration with other semiconductor devices in a chain.

SOA (Semiconductor Optical Amplifier)

Photons traveling through the active region of an SOA can cause electrons to lose energy by releasing photons. If the wavelength of these released photons matches that of the initial photons, amplification is achieved. This is accomplished by passing an electric current through the device. As a result, the optical signal passing through the active region is amplified with a certain gain.

SOA Specifications

• Gain: 30dB or greater
• Wavelength Range: 1280 nm to 1650 nm
• Maximum Saturation: 18 dBm
• Noise Figure: ~8 dB
• Pump Power: Less than 400 mA
• Bandwidth (3dB): ~60 nm
• Polarization Sensitivity: Available with SOA

Optical Amplifier Application in CATV

Figure 3 illustrates the application of EDFA in CATV transmission and reception. As shown, the CATV signal is amplified by an EDFA with a gain of 10dB. An optical splitter is then used to divide the signal, providing individual connections to users.