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Defense & Security

New prospects for high-power, high-efficiency analog electroabsorption modulators

A novel device for light-intensity modulation with enhanced spurious-free dynamic range is shown to be suitable for application in most radar systems.
14 July 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1185

Modern communications rely heavily on data transmission using fiber-optic links. The encoding of information is usually done by modulating the amplitude of the light-signal intensity. The use of electroabsorption modulators1 (EAMs) is attractive, thanks to their small size, wide optical-transmission bands, and potential for monolithic integration with other hardware components. EAMs have been used successfully in digital telecommunications2 at 1.55μm. Common challenges in analog applications include the development of devices capable of handling high optical power with low optical-insertion loss. These conditions also affect the packaging design, which must ensure low signal loss, optimal temperature operation, and high radio-frequency (RF) transmission.

Figure 1. Electroabsorption-modulator (EAM) mechanical design, showing the EAM's position relative to the coplanar waveguide (CPW), lensed fibers, thermoelectric cooler (TEC), and radio-frequency (RF) input circuitry.

We recently validated novel EAM prototypes. We produced a dilute-core-waveguide (DCW) EAM specifically designed to handle RF-signal transmission at high optical power with enhanced spurious-free dynamic range (SFDR) compared with conventional electro-optic modulators. The module was built to handle an optical power of 100mW (+20dBm) over a bandwidth (BW) of 20GHz, with a performance optimized1 for applications such as radar.3 Initial bench testing of the DCW EAM chip showed promise, but it needed packaging for performance characterization in non-laboratory conditions. In the new configuration, the RF-modulation layer and optical waveguide are decoupled, posing new challenges to the package-assembly process.

In an optical-fiber link, the EAM modulator is coupled to fibers at both facets of the chip and has an electrical RF input. To optimize performance for any single-mode optoelectronic module, active fiber alignment is essential. In general, a deviation of 0.25μm from the ideal placement results in a 10% light-transmission penalty. In a conventional EAM, the RF-modulation layer is also the optical waveguide, so a lensed fiber generating maximum photo current will also achieve the lowest insertion loss. Since the new DCW design decouples the photo current from the photo power, it is imperative to first determine the decoupling distance. The difference between the optical modes of the EAM waveguide and the glass fiber requires an efficient optical-mode converter. The RF input circuitry includes a coplanar waveguide design, a direct-current block, and a bias circuit, which were designed to ensure proper signal transfer and to minimize capacitive losses. A schematic of the EAM module's mechanical design4 is shown in Figure 1.

Figure 2. Photo-current and photo-power intensity of the dilute-core-waveguide EAM prototype as a function of lensed-fiber vertical (Z) position.

Figure 3. Spurious-free dynamic range (SFDR) input–output RF behavior showing suppression of the third-order intermodulation (IM3) for all bandwidths (BWs) >1MHz, with device bias at Vb = −1.4V.

An optical bench test was set up with laser-light output from the lensed-fiber coupling into one facet of the EAM chip. The light exiting from the other facet passed through a lens and was focused on a detector. At the same time, two probes were attached to the EAM's electrical pads. The photo current and the photo power could thus be measured simultaneously as the lensed fiber was scanned along the vertical (Z) direction. Figure 2 shows typical curves of photo current and photo power as a function of lensed-fiber Z position. There is a separation of about 0.4μm between the photo-current maximum and the photo-power maximum, consistent with the design.

Thus far, a number of DCW EAM modules have been fully packaged, with an average insertion loss of about 7.6dB and some operating at up to 100mW optical power. The instantaneous BW can be tuned rapidly over a 20GHz range with a high SFDR (see Figure 3), meeting the requirements of many operational and developmental radar systems. Emphasis on optimizing the metallization and ferrulization of the polarization-maintaining fiber should decrease the insertion loss. We continue to examine other performance parameters for system insertion, such as temperature sensitivity.

Infotonics acknowledges funding from ARL under contract W911NP- 07-2-004. We also acknowledge CyOptics for wafer processing.

Songsheng Tan, Nancy Stoffel
Infotonics Technology Center
Canandaigua, NY