Micro-optic integrated components improve optical amplifiers
Next-generation EDFAs will require advanced components to accommodate smaller channel spacings.
Ronnie Chua and Bo Cai
Demand for transmission capacity continues to increase as a growing number of people send data, voice, and video signals through the Internet and other emerging multimedia applications. Fiber-optic links and networks have become the primary vehicle of choice for meeting such a demand.
In both long-haul telecommunications and metropolitan area networks, the enabling technology for optical-fiber transmission systems is the erbium-doped fiber amplifier (EDFA). The EDFA has an amplification range that coincides with fiber`s third transmission window, around 1550 nm. However, next-generation EDFAs will need to accommodate very narrow channel spacings and other demands placed upon them by developments in optical networking. New components will provide EDFAs with the ability to meet these challenges.
Key in fiber-optic telecommunications
The EDFA amplifies transmission signals in the optical realm by relying on rare-earth ions such as erbium. When doped into optical fibers, the erbium ions can cause the loss spectrum of the fiber to be drastically altered. As photons from a laser at either 980 or 1480 nm are launched into the same fiber, they will excite the rare-earth ions to higher energy levels. When a signal photon in the 1550-nm window passes through the fiber, it will collide with the excited rare-earth ions, causing the erbium ion to return to a relaxed state while releasing additional photons to amplify the transmission signal.
The advantages of EDFAs over amplifiers that require an optical-electrical-optical conversion are obvious. In regenerative-repeater technology, the optical signal must first be converted into electrical form before it is amplified. The amplified signal then undergoes another electro-optical signal conversion before it is retransmitted. This regeneration process is repeated indefinitely. Thus, such an amplification structure requires complex and high-speed electronic circuitry as well as highly reliable electro-optical devices. Repeaters are transmission protocol-dependent and have only a limited repeating range. Even so, the typical data-transmission rate is limited to 560 Mbits/sec.
EDFAs eliminate the need for such high-speed electronics because they bypass electro-optical conversions and can easily amplify transmission signals in the optical realm regardless of protocol. Furthermore, the transmission capacity of an EDFA can be increased to gigabits per second. Recent developments in dense wavelength-division multiplexing (DWDM) have even demonstrated the possibility of terabit-per-second transmission rates by relying on the EDFA`s extended amplification bandwidth beyond the conventional 1535 to 1565 nm to include the long wavelengths around 1570 to 1610 nm.
With the dominance of DWDM technology in telecommunications today, the EDFA has become an even more critical element in optical networking. DWDM systems are designed to carry multiple optical signals over a single fiber by transporting each signal on a separate wavelength. The EDFA complements--and in fact enables--DWDM systems by amplifying optical signals in different wavelengths around 1550 nm independently.
A conventional EDFA system with a 30-nm amplification bandwidth can amplify up to 40 DWDM wavelength signals separated by 100-GHz channel spacings on the ITU wavelength grid. But as more wavelength channels are added and channel spacings narrow considerably to achieve terabit transmission rates, next-generation EDFAs must be designed with several key considerations, including a wider and flattened gain spectrum; lower noise figure; higher output power; and larger dynamic range for add/drop or other optical-networking functions.
To meet these requirements, much research has focused on the development of new EDFA components, including integration of multiple discrete component functions by relying on technologies such as interference filters.
Building blocks in EDFAs
There are six basic elements in an EDFA system: isolators, tap couplers, wavelength-division multiplexers (WDMs), pump lasers, photodetectors, and erbium-doped fiber (see Fig. 1). The optical isolators used in an EDFA system are inline fiber devices with polarization-insensitive isolation properties. They are used to prevent the backward propagation of amplified spontaneous emission (ASE) noise that degrades amplifier performance.
In a typical polarization-insensitive inline isolator, there are magnets, a garnet crystal as Faraday rotator, and two birefringent wedges with their optical axes aligned at 45 relative to each other. The isolator uses the nonreciprocal Faraday polarization rotation and the polarization walk-off effect of the birefringent wedges to prevent back reflection of light. In the forward propagation direction, the ordinary (o) and extraordinary (e) polarized beams remain unchanged for both wedges. As the beams pass through the second birefringent wedge, the output o and e beams are parallel to each other and can thus be simultaneously coupled into an output fiber. But in the backward propagation direction, the reversed polarization rotation of the Faraday rotator will cause both o and e beams to travel in two different directions and not parallel to each other. Thus, the beams cannot be coupled back into the input fiber.
Tap couplers are used in EDFAs to siphon off a small amount of the signal for monitoring and feedback purposes. Most tap couplers are manufactured using a fused biconic taper (FBT) process. FBT is a low-cost coupler fabrication technique that requires a section of two fibers to be fused to create a taper. Controlling the length and duration of this fused taper allows power to transfer between the two fiber cores with specific coupling ratios. Typically, only 1% to 5% of the optical signal is tapped for power monitoring in EDFA systems. The tapped signal is then converted into an electric current by a photodetector, another important component in the optical amplifier.
The photodetector allows the EDFA signal to be monitored for integrity and strength, usually both before and after amplification. The electric current generated by the detector is proportional to the power in the incident optical signal. For proper monitoring, the detector itself must have high sensitivity at the signal wavelength and minimum noise. There are different types of optical detectors available, but the most commonly used is the positive-intrinsic-negative (PIN) photodiode. Photoconductive, the PIN diode is formed by doping impurities on three layers of crystal with the positive type on top, the intrinsic layer in the middle, and the negative type on the bottom.
Pump lasers are also critical components in the EDFA. By introducing a pump laser into the EDFA, power from the pump wavelength can be transferred to the signal wavelength because of the inherent physics of the erbium ions that are doped into the optical fiber. EDFA pump lasers usually operate at either 980 or 1480 nm because these wavelengths are optimized for effective energy transfer in the erbium fiber.
The WDM is another critical element in the EDFA; it is essential in enabling the pump laser to introduce the pump wavelength into the amplifier. Amplification is achieved by combining both pump and signal wavelengths into a single optical fiber where the rare-earth element is present. WDMs designed for this purpose are usually manufactured with the same FBT process as the tap couplers used in EDFA power monitoring.
Integrated components in next-generation amplifiers
In a typical EDFA, the noise figure is determined mainly by loss in the amplifier`s input where the signal is weakest. Saturation power and gain are mainly determined by gain in the EDFA`s output where the signal reaches its maximum strength.
Depending on the application, different EDFA configurations may be adopted. To minimize the noise figure in preamplifiers, maximum gain and minimum loss can be achieved with a forward-pumping design in the amplifier`s input section, where the pump wavelength propagates in the same direction as the signal. To maximize the saturation power and gain in post-amplifiers, backward pumping from the output end can be configured to increase the gain at the output section and maximize pump efficiency. For certain high-performance inline amplifiers, the forward and backward pumping configurations are combined into a double-stage or dual-pump design. Not only can such an architecture achieve both low noise figure and high output power, but it also can accommodate loss components such as add/drop devices, gain flattening or blocking filters, and dispersion-compensating elements between the amplifier`s input and output.
To optimize EDFA performance, components with low insertion loss, a flat transmission window, and high power handling capability are desired. Other considerations include compactness, reliability, and low component and assembling costs. Thus, even as discrete single-function components have reached product maturity for use in conventional EDFAs, much research has been devoted to the development of integrated fiber-optic components for next-generation EDFA systems. These components essentially combine two or more discrete component functions into a single compact device or module. They offer distinct advantages over discrete EDFA components--most notably in performance, cost, size, inventory levels, and simplicity of assembly for EDFA manufacturers.
Micro-optic integration technology
Integration can be achieved through micro-optic filter technology. By using a combination of thin-film interference filters and coatings to control the wavelength or amount of light signal, component functions such as wavelength-division multiplexing and tap coupling can be emulated. With proper free-space packaging techniques, these functions can be combined with an isolator or photodetector to achieve different levels of component integration (see Fig. 2).
The most basic integration involves a combination of two component functions. Common among these are the following hybrids:
isolator and WDM
isolator and tap coupler
tap coupler and WDM
tap coupler and photodetector.
Packaging a WDM or tap coupler with an isolator can be achieved by inserting a properly coated WDM or tap filter into an isolator that measures just 5.5 mm in diameter ¥ 32 mm long. Replacing the isolator collimator with a dual-fiber collimator creates an optical signal multiplexing or tap filtering function.
Integrating the tap coupler with the photodetector requires one dual-fiber collimator with a tap filter and a PIN photodetector. By controlling the reflective coating on the tap filter, a small amount of light that enters the input fiber will pass through the filter and onto a wide-area PIN photodiode, while the rest of the light is reflected into the output fiber of the same collimator.
In amplifiers with a middle-stage access architecture, integration enables a diversity of useful components to be inserted for controlling the gain shape or amplifier noise. By relying on the same integration technique used in the above isolator-WDM or isolator-tap coupler combinations, an integration of an isolator with a gain-flattening filter or ASE noise rejection filter can similarly be achieved in the same compact package. These kinds of integration simplify the conventional EDFA design with fewer components while expanding its capability to accommodate new telecommunications network requirements.
For instance, the gain spectrum of an EDFA is wavelength-dependent. In a multichannel DWDM system, the conventional EDFA will create uneven channel amplification. In long-haul telecommunications, such channel discrepancies will accumulate over a cascade of amplifiers, resulting in unacceptably high bit-error rates among the channels. To correct this problem, the amplifier gain has to be flattened. A useful method is to design an interference filter with the inverse characteristics of a specific EDFA gain spectrum. The filter can then be integrated with a collimator or isolator device and inserted into the EDFA to increase its usable bandwidth for multichannel amplification.
A second level of integration can be achieved with a three-function combination of tap coupler, isolator, and WDM in the same compact package. Meanwhile, a third level of integration that has isolator, tap coupler, WDM, and photodetector components in one compact 14-pin standard butterfly package has been developed. This Integrated Fiber Amplifier Module (IFAM) package is hermetically sealed for greater reliability and stability. As packaging technology improves, a combination of the IFAM with the erbium-doped fiber and pump laser will offer an attractive option for future amplifier system designers.
Advantages of integration
Integrated components offer significant advantages over discrete devices in EDFA systems. They enable EDFAs to meet the demanding requirements of today`s long-haul telecommunications and metropolitan networks. Integration relies on a free-space packaging technique that eliminates the extra fiber coupling and splices inherent in conventional EDFA designs, thus improving amplifier performance.
For instance, a one-stage optical isolator spliced to a tap coupler in a conventional EDFA will have an insertion loss of 0.8 dB or higher. But the typical insertion loss of an integrated isolator-tap coupler component can be as low as 0.5 dB. By using thin-film filter technology to replace fused-fiber technology, hybrid components assembled on micro-optics have very low polarization-dependent loss and much flatter passbands. Furthermore, the component`s optical path can be epoxy-free for high-power EDFA operation. The table shows the typical specifications of an integrated fiber amplifier module.
Integration is also a more cost-effective option for EDFA manufacturers because of lower component prices, the reduction in the number of devices to specify or inventory, the simplification of the amplifier assembly process, smaller size, and the decrease in the number of components to qualify.
These advantages are particularly important for cost-sensitive optical system designs, particularly in metropolitan area and cable-TV networks. The flexibility of micro-optic integration also enables different component function integrations for the different kinds of amplifier architecture discussed here.
With these advantages, integrated components will eventually define the future design of EDFAs to meet the high-capacity, high-performance, and low-cost requirements of today`s telecommunications systems. u
Ronnie Chua is marketing manager and Bo Cai is new product technical service manager in the Technical Service Group of E-TEK Dynamics (San Jose, CA).