Bandwidth-expandable EDFA offers options for growth

Kevin Zhang

Lesley Rogers

A typical single-mode fiber has approximately 300 nm of available bandwidth, with the S-, C-, and L-band transmission windows falling in the region of lowest attenuation. The speed of the signal (that is, the bit rate) depends on the dispersion and the nonlinear limits of an optical fiber. The traditional non-dispersion-shifted fiber (NDSF) has zero dispersion at 1310 nm, but attenuation is higher than that of the 1550-nm region. Given the availability of newly developed non-zero dispersion-shifted fiber (NZ-DSF) and of erbium-doped fiber amplifiers (EDFAs), the 1550-nm region is an ideal bandwidth window for DWDM technology.

The NZ-DSF minimizes four-wavelength mixing in a DWDM transport system. However, the challenge of managing the dispersion due to the wider bandwidth and the slope of the fiber dispersion still exists. A single dispersion-compensation module (DCM) cannot compensate for the entire bandwidth because the amount of dispersion for each wavelength is different and its slope is varied.

Each telecommunications equipment company takes its own approach to using the optical fiber bandwidth to reach the highest capacity. Combining time-division multiplexing (TDM) and WDM technologies allows a systems provider to optimize its use of the available bandwidth. Time-to-market and system cost also are important factors in determining system configurations.

The most challenging task of designing an optical network system is the infrastructure planning. Systems engineers must determine the maximum capacity of the transmission system and choose the low first-cost technology with scalability for future capacity requirements. It is very difficult to design a system in which the capacity is adequate now, the cost is low, and the scalability unlimited. The most significant factor to consider in the design and implementation of DWDM technology is scalability.

Another challenge is how to manage the multiwavelength "traffic" in an optical network mode. System integrators must design-in channel add/drop capability, multiplexing and demultiplexing, amplification, and fast dynamic response time, while minimizing the dispersion distortion within the large bandwidth. The optical amplifier should play a big role in addressing these design criteria.

To cover the wider bandwidth and speed signal transmission, systems require more amplifier gain or a decrease in the fiber span length to compensate for the signal level loss. Four times less energy will occur in OC-768 than in OC-192 with the same amount of amplification, for example, and the number of channels is proportional to the gain of the optical amplifier.

In addition, system engineers need to think more globally about how to address the dispersion, four-wavelength mixing, and channel power regulation in conjunction with optical add/drop multiplexing functions added. A modular optical amplifier is a possible solution.

Bandwidth-expandable amplifier

A bandwidth-expandable optical amplifier is a potential solution to address these optical networking challenges (see Fig. 1). Essentially, the amplifier consists of two components: narrowband filters (NBFs) and narrowband amplifiers (NBAs). The NBF selects the bandwidth currently required by the capacity or allocated to different systems such as SONET, ATM, or IP. The future bandwidth is reserved by the NBF with a negligible insertion loss. It could be an express line if the amplifier is used in an OADM node. The NBA boosts the line level only within the selected bandwidth.

These expandable optical amplifiers can be used as boosters, as in-line amplifiers, and as preamplifiers. In an in-line application the transmission distance limit is no longer determined by the edge channel dispersion. At the optical node, active control of the gain and power regulation for the small optimized segment of wavelengths are inherent features of its design.

The device is expandable and can be cascaded in various configurations, such as parallel or series. The parallel cascade (amplifier array) renders the scalability that leads to cost savings. In this case capacity can be added when there is a real need and the large up-front investment for unused capacity can be avoided. This pay-as-you-grow tactic is an economical way of addressing the up-front planning for the infrastructure blueprint.

It is worthwhile to note that the bandwidth-expandable architecture offers more flexibility to manage the entire fiberoptic bandwidth and for transmission in both directions. The traditional C-band can be split into sub-bands, such as "red" and "blue," offering a means for bidirectional communications.

Additionally, populating the C-band first and migrating to the L-band in the long-wavelength side and to the S-band for the short-wavelength side allows the system to expand as required. Various components such as L-band amplifiers, Raman fiber amplifiers in the S-band, and semiconductor amplifiers are compatible with this approach. Each segment of the bandwidth can be managed and manipulated by one optical amplifier.

The expandable optical amplifier array focuses dispersion management over a narrow band of wavelengths-a solution that enables more optimized dispersion compensation with the correct slope, which results in increased transmission length and better immunity to nonlinear effects.

The modularity of the optical amplifiers inherently offers power-level control over the bandwidth. By controlling the pump power of each module, a greater degree of control is realized for a large dynamic range of signal inputs. This feature is for systems that contain a group of signals coming from various locations and going to different destinations. If the NBA provides center-stage access, the drop and add signals can experience gain to compensate for the losses accumulated in OADM components.

We have developed this bandwidth-expandable optical amplifier by leveraging thin-film filter and fiberoptic component technology. The amplifier consists of an optical gain module (OGM) and a gain driver (OGD). Each is extremely compact- 65 × 65 × 6.5 mm3 and 50 × 50 × 13 mm3 respectively-a footprint that allows the OGM to be easily integrated into customer-specific terminal cards.

The OGM is essentially a gain block in which all passive components including the NBF and erbium-doped fiber (EDF) are enclosed (see Fig. 2). It provides gain over small wavelength spans, typically 3-6 nm, and can be cascaded to provide gain over a greater number of channels as the need arises. In addition, the OGM offers center-stage taps that give systems designers the ability to add functions required by the network such as dispersion compensation or power management.

Optical amplifiers use a variety of filters to ensure superior performance in gain, noise figure, and gain flatness. The bandwidth add/drop (BA/D) filter is the enabling element of the bandwidth-expandable optical amplifiers. The filter characteristics determine the crosstalk in an array and the concatenation capability.

With the integration of these BA/D filters, the OGM will amplify only four consecutive channels rather than wasting valuable pump power providing gain over wavelengths that don`t need amplification. Additional channels across the entire erbium gain spectrum can be added as needed, without disrupting service to the operational channels, by cascading additional OGMs. Because more channels can be added easily, the hardware installed to provide service for today`s fixed DWDM channels will not become obsolete as the network grows. In addition, the added functionality of the add/drop filters allows the OGM to perform as an add/drop node, depending on where it is incorporated into the network.

A common question asked regarding passive filter-cascaded DWDM components is about the accumulated insertion loss and its variation within the filter bandwidth caused by the edge-rounding effect. Because our device is a narrowband modular amplifier, the designer can manipulate the gain and gain filters to level the outputs of the amplifier array over the wide bandwidth.

Optical gain driver

The OGD contains a pump laser, two photodetectors and electronic circuits (see Fig. 3). With a +5-V dc power supply, the OGD provides many operational modes such automatic gain control, automatic level control, constant current, and constant pump power. The built-in laser safety protection prevents any possibility of damage to the pump laser due to current surge, temperature control failure, and so on. The advanced temperature control of the OGD keeps the laser operating precisely at the set operating temperature (within ±0.05°C).

Comparison analysis

In a typical optical amplifier for a 40-channel DWDM system, the 40-channel signals are amplified first by a broadband optical amplifier and then demultiplexed into individual channels to compensate for the dispersion in each channel and to regulate the power level (see Fig. 4). This is a desirable architecture because one can correct for exactly the amount of dispersion that occurs for each channel and control the power level of each channel. Obviously, it increases the system complexity, and operation and system costs.

One way to trade off the complexity and costs with narrowband accessibility that meets the needs of dispersion compensation and power regulation in an optimized narrow-bandwidth system is to use bandwidth-expandable modular optical amplifiers. Ten narrowband optical amplifiers provide the optimized bandwidth (four channels) to compensate for the dispersion and to regulate the power level. This could be the better solution if the performance, cost, and complexity must be balanced.

In addition, the added value of the narrowband amplifier architecture should be recognized. Scalability is designed in so it is not necessary to invest now for future bandwidth requirements.

Simplicity can reduce the operations cost-the bandwidth-expandable amplifier incorporates four features in one package (demux, amplifier, variable optical attenuator, and mux) compared to the traditional solution that incorporates discrete components. The result is a much more compact package with increased functionality and increased reliability.

FIGURE 4. Broadband optical amplifier enables fine control of signal dispersion in a 40-channel DWDM long-haul system.