Table of Contents
- List of Figures.
- List of Tables.
- Preface.
- Acknowledgments.
- Foreword.
- 1. Introduction.
- 2. Optical Network Devices.
- Part I: LAN/MAN Architectures.
- 3. Optimizing Amplifier Placements: The Equally-Powered Case.
- 4. Optimizing Amplifier Placements: The Unequally-Powered Case.
- Part II: WAN Architectures.
- 5. Wavelength Conversion.
- 6. Impact of Transmission Impairments.
- 7. Conclusions.
- Appendices: Switch Model.
- EDFA Model. References.
- Index.
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Chapter 2: Optical Network Devices
Although an optical signal can propagate a long distance before it needs amplification, both long-haul and local lightwave networks can benefit from optical amplifiers. All-optical amplification may differ from opto-electronic amplification in that it may act only to boost the power of a signal, not to restore the shape or timing of the signal. This type of amplification is known as 1R (regeneration), and it provides total data transparency (the amplification process is independent of the signal's modulation format). 1R-amplification is emerging as the choice for transparent all-optical networks of tomorrow. However, in today's digital networks (e.g., Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH)), which use the optical fiber only as a transmission medium, the optical signals are amplified by first converting the information stream into an electronic data signal, and then retransmitting the signal optically. Such amplification is referred to as 3R (regeneration, reshaping, and reclocking). The reshaping of the signal reproduces the original pulse shape of each bit, eliminating much of the noise. Reshaping applies primarily to digitally-modulated signals, but in some cases may also be applied to analog signals. The reclocking of the signal synchronizes the signal to its original bit timing pattern and bit rate. Reclocking applies only to digitally-modulated signals. Another approach to amplification is 2R (regeneration and reshaping), in which the optical signal is converted to an electronic signal which is then used to directly modulate a laser. 3R and 2R techniques provide less transparency than the 1Rtechnique; and in future optical networks, the aggregate bit rate of even just a few channels might make 3R and 2R techniques less practical.
Also, in a WDM system, each wavelength would need to be separated before being amplified electronically, and then recombined before being retransmitted. Thus, in order to eliminate the need for optical multiplexers and demultiplexers in amplifiers, optical amplifiers must boost the strength of optical signals without first converting them to electrical signals. A drawback is that noise, as well as the signal, will be amplified.
Optical amplification uses the principle of stimulated emission, similar to the approach used in a laser. The two basic type of optical amplifiers are semiconductor laser amplifiers and rare-earth-doped-fiber amplifiers, which will be discussed in the following sections. A general overview of optical amplifiers can be found in (O'Mahony, 1993).
3.1. Optical Amplifier Characteristics
Some basic parameters of interest in an optical amplifier are gain, gain bandwidth, gain saturation, polarization sensitivity, and amplifier noise.
Gain measures the ratio of the output power of a signal to its input power. Amplifiers are sometimes also characterized by gain efciency which measures the gain as a function of input power in dB/mW.
The gain bandwidth of an amplifier refers to the range of frequencies or wavelengths over which the amplifier is effective. In a network, the gain bandwidth limits the number of wavelengths available for a given channel spacing.
The gain saturation point of an amplifier is the value of output power at which the output power no longer increases with an increase in the input power. When the input power is increased beyond a certain value, the carriers (electrons) in the amplifier are unable to output any additional light energy. The saturation power is typically defined as the ouput power at which there is a 3-dB reduction in the ratio of output power to input power (the small-signal gain).
Polarization sensitivity refers to the dependence of the gain on the polarization of the signal. The sensitivity is measured in dB and refers to the gain difference between the TE and TM polarizations.
In optical amplifiers, the dominant source of noise is amplified spontaneous emission (ASE), which arises from the spontaneous emission of photons in the active region of the amplifier (see Fig. 2.6). The amount of noise generated by the amplifier depends on factors such as the amplifier gain spectrum, the noise bandwidth, and the population inversion parameter which specifies the degree of population inversion that has been achieved between two energy levels. Amplifier noise is especially a problem when multiple amplifiers are cascaded. Each subsequent amplifier in the cascade amplifies the noise generated by previous amplifiers.
3.2. Semiconductor-Laser Amplifier
A semiconductor laser amplifier (see Fig. 2.6) consists of a modified semiconductor laser. A weak signal is sent through the active region of the semiconductor, which, via stimulated emission, results in a stronger signal being emitted from the semiconductor.
The two basic types of semiconductor laser amplifiers are the FabryPerot amplifier, which is basically a semiconductor laser, and the traveling-wave amplifier (TWA). The primary difference between the two is in the reflectivity of the end mirrors. Fabry-Perot amplifiers have a reflectivity of around 30%, while TWAs have a reflectivity of around 0.01% (O'Mahony, 1993). In order to prevent lasing in the Fabry-Perot amplifier, the bias current is operated below the lasing threshold current. The higher reflections in the Fabry-Perot amplifier cause Fabry-Perot resonances in the amplifier, resulting in narrow passbands of around 5 GHz. This phenomenon is not very desirable for WDM systems; therefore, by reducing the reflectivity, the amplification is performed in a single pass and no resonances occur. Thus, TWAs are more appropriate than Fabry-Perot amplifiers for WDM networks.
Today's semiconductor amplifiers can achieve gains of 25 dB with a gain saturation of 10 dBm, polarization sensitivity of 1 dB, and bandwidth range of 40 nm (O'Mahony, 1993).
Semiconductor amplifiers based on multiple quantum wells (MQW) are currently being studied. These amplifiers have higher bandwidth and higher gain saturation than bulk devices. They also provide faster on-off switching times. The disadvantage is a higher polarization sensitivity.
An advantage of semiconductor amplifiers is the ability to integrate them with other components. For example, they can be used as gate elements in switches. By turning a drive current on and off, the amplifier basically acts like a gate, either blocking or amplifying the signal...
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