Waveguide amplifier arrays enable metro design options
Erbium-doped waveguide amplifier (EDWA) arrays promise to become an attractive alternative to erbium-doped fiber amplifiers (EDFAs). EDWAs are smaller and more cost-effective amplification solutions, particularly for the metro and access applications. The physical concept of an EDWA is the same as that of an EDFA: in both cases, erbium ions in rare-earth-doped glass media provide optical gain in the 1550-nm fiberoptic transparency window.1 However, unlike erbium-doped fibers, EDWAs are planar waveguides and can be manufactured as an integral part of a more complex planar lightwave circuit (PLC).2 In general, PLC chips can contain a large variety of planar waveguides with different functionalities, which is similar to the way electronic integrated circuits (IC) are built (see Fig. 1).3
The integrated functions can include amplification, variable attenuation, combining, splitting, filtering, and pump-sharing functions. The latter function enables several amplifiers to be driven by a single 980-nm pump laser. Since much of the cost of an amplifier resides in an individually packaged pump laser, the pump sharing provides an alternative way of decreasing the cost per amplifier. This feature particularly benefits amplifiers for sub-band and per-channel amplification because each amplifier requires only modest power.
An EDWA array module can consist of several distinct parts: a PLC chip, a few fiber-based components, an electrical control board, and a metal housing. One or two 980-nm pump lasers can be coupled into a pump distribution network, which consists of a pump mixer and four independent variable optical attenuators (VOAs ).
The mixer combines the pump energy from the two pump lasers and evenly distributes it among four different pump paths to the VOAs, which in turn control the amount of pump energy that is transmitted and eventually coupled into each erbium-doped waveguide. After the VOAs, the pump light is combined with signal light by four coarse WDM couplers. These are followed by four erbium-doped waveguides (EDWs).
The core material of our EDWs is characterized by ultrahigh delta of approximately 9% (delta is defined as a percent difference between refractive indices of the core and the cladding). This feature allows very small bending radii—on the order of 150 µm—to be used in the chip layout, and leads to a very compact chip design. The EDWs are followed by pump filters, which filter out the remaining pump light. The PLC device is 9 mm wide and 25 mm long, yet performs optical functions of 27 individual fiber-based components interconnected by 28 fusion splices.
The EDWA is electrically connected to the control board of the module. The board monitors the signal from all PIN taps and also controls the VOA settings. Pump lasers are mounted separately and coupled to the chip using fiber pigtails. It is possible to use either cooled or un-cooled pump lasers. In the later case electrical power requirements are much lower. In both cases, the lasers are wavelength-stabilized with external fiber Bragg gratings. The electronic control board also controls and monitors the pump lasers and associated thermo-electric coolers. Additional fiber-based isolators are provided at all four input and output ports.
All PLC elements can be divided into two categories: passive waveguides and active waveguides. A passive waveguide element such as an integrated optical tap makes possible on-chip monitoring of both input and output signal powers, thus enabling both automatic gain control (AGC) and automatic power control (APC) modes. The taps must be efficient, compact, broadband and polarization independent (see Fig. 2).
Another important feature is the output pump filter, which filters out the remaining pump light from the output waveguides carrying the signals. The filter should have a small insertion loss across the full C-band and a high extinction ratio at the pump wavelength around 980 nm. It should be characterized by less than 0.4-dB insertion loss across the C-band, and an extinction ratio of more than 40 dB at 980 nm.
An integrated pump-sharing network with pump variable optical attenuators (VOAs) allows a user to change the gain independently on each amplifier, by varying the amount of coupled pump power. An integrated VOA has to be compact and low-loss; it also should have a high dynamic range and a low switching power. The VOA design can be a thermo-optically controlled Mach-Zehnder interferometer.
The active elements of a PLC are erbium-doped waveguides (EDWs). There are four separate EDWs on the chip, which when pumped provide amplification for four independent signals. Each amplified signal could be either a single channel at one wavelength or a small number of DWDM channels at different wavelengths. This amplification is bit-rate independent, so that the EDWA will work with any type of signals within the C-band (see Fig. 3).
General applications for EDWA arrays include receivers, transmitters, and in-line amplifiers. These applications can address several market segments, including the 10-Gbit/s metro and metro core, and future 40-Gbit/s metro markets. Targeted applications for EDWA arrays include channel balancing in optical add/drops, pre- and post-amplification in wavelength multiplexers and demultiplexers, and loss compensation in switching matrices.
Products such as optical crossconnects and reconfigurable optical add/drop multiplexers will benefit greatly from the incorporation of low-cost, low-noise, wavelength-independent amplification by EDWA arrays. Amplifier arrays also offer new alternatives in in-line amplification: lower-cost, simpler-to-maintain EDWA arrays may replace more complex in-line EDFAs in metro core networks. EDWA arrays enable loss-less components, such as loss-less splitters and combiners, in which inherent losses are compensated by gain.
- E. Desurvire, Erbium doped fiber amplifiers, (John Wiley & Sons, New York, 1994).
- S. Frolov, T. shen, and A, Bruce, SPIE Proc. 4990, Photonics West 2003.
- Y. P. Li and C. H. Henry, "Silicon Optical Bench Waveguide Technology" in Optical Fiber Telecommunications IIIB ed. by I.P. Kaminow and T. L. Koch (Academic Press, San Diego, 1997).
Sergey V. Frolov is director of device design and Joe Shmulovich is CEO of Inplane Photonics, 600 Corporate Court, South Plainfield, NJ 07080. Sergey Frolov can be contacted at email@example.com.