Optical amplifiers serve as an integrated part of long haul data transmission. The most prominent feature of optical amplifiers is that the device can amplify optical signal directly, which removes the need to first convert the signal into an electrical one before amplifying. As a critical enabler of optical communication network over long distance, optical amplifiers, together with WDM technology providesa path to boost network capacity required by current and future communication systems. This article illustrates several commonly used optical amplifiers, from the perspective of their configurations, advantages and possible drawbacks.
Before we go any further, let’s firstly grasp some basic information about optical amplifiers. It is generally accepted that when transmitting signals over long distance (>100 km), it is necessary to compensate for attenuation losses within the fiber to ensure the quality of signal. This was initially accomplished by an optoelectronic module consisting of an optical receiver, a regeneration and equalization system, and an optical transmitter. However, the function of it is limited by the optical to electrical (O-E) and electrical to optical (E-O) conversion. Because of this, optical amplifiers have since been demonstrated to replace this conventional devices. Optical amplifiers rivals their former counterparts by eliminating the need for O-E and E-O conversions. The general form of an optical amplifier is shown below.
The widely used optical amplifiers today can be broken into three groups: erbium-doped fiber amplifier (EDFA), raman amplifier and semiconductor optical amplifier (SOA). Each of them will be discussed in the following section.
The amplifying medium of erbium-doped fiber amplifier is a glass optical fiber doped with erbium ions. The erbium-doped glass optical gain medium amplifies light at wavelengths that are in the neighborhood of 1550 nm—the optical wavelengths that suffer minimum attenuation in optical fibers. EDFA works best in the range 1530 to 1565 nm, and it possesses low noise and can amplify many wavelengths simultaneously, making it the amplifier of choice for most applications in optical communications.
- EDFAs have high pump power utilization (>50%)
- Directly and simultaneously amplify a wide wavelength band (> 80 nm) in the 1550 nm region, with a relatively flat gain.
- Flatness can be improved by gain-flattening optical filters.
- Gain in excess of 50 dB.
- Low noise figure suitable for long haul applications.
- EDFAs are not small
- It cannot be integrated with other semiconductor devices.
The gain medium of raman amplifier is an undoped optical fiber. Power is transferred to the optical signal by a nonlinear optical process known as the Raman effect. An optical pump supplies the power to the optical gain. The wavelengths that experience optical gain are determined by the wavelength of the optical pump. So it is possible for raman amplifier to amplify a given optical wavelength by proper selection of the pump wavelength. The optical gain in a raman amplifier is distributed over a long span of optical fiber.
- Variable wavelength amplification possible
- Compatible with installed SM fiber
- Can be used to “extend” EDFAs
- Can result in a lower average power over a span, good for lower crosstalk.
- Very broadband operation may be possible
- High pump power requirements, high pump power lasers have only recently arrived
- Sophisticated gain control needed
- Noise is also an issue
SOAs generally utilize a semiconductor as the gain medium. The input and output faces of the amplifier are anti-reflection coated in order to prevent optical feedback to the gain medium and lasing. SOA is pumped electronically (directly via an applied current), and does not need a separate pump laser. SOAs are noisier than EDFAs and generally handle less power. However, they are less expensive and are proving well suited for use in local area network (LAN) where cost matters.
- The semiconductor optical amplifiers are of small size and electrically pumped.
- It is potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc.
- All four types of nonlinear operations (cross gain modulation, cross phase modulation, wavelength conversion and four wave mixing) can be conducted.
- SOAs are with compact design—only a small semiconductor chip with electrical and fiber connections.
- The output powers are significantly smaller.
- The gain bandwidth is smaller, but devices operating in different wavelength regions can be made.
- The upper-state lifetime and thus the stored energy are much smaller, so that the gain reacts to changes in pump power or signal power within nanoseconds (instead of milliseconds).
- The performance of SOA still cannot be compared with that of EDFA.
- SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time.
Optical amplifiers are extensively used in optical network to increase the strength and enhance the performance of optical data links. This article mainly introduces three types of optical amplifiers: erbium-doped fiber amplifiers (EDFAs), raman amplifiers and semiconductor optical amplifiers (SOAs), including the merits and possible drawbacks of them. Hope it could help when choosing your optical amplifiers that best suited your system.
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