Tech Briefs

The modulators improve the transmission of RF signals on optical fibers.

Externally coupled electroabsorption modulators (EAMs) are commonly used in order to transmit RF signals on optical fibers. Recently, an alternative device design with diluted waveguide structures was developed. Bench tests show benefits of lower propagation loss, higher power handling (100 mW), and higher normalized slope efficiency. Bench tests were performed in order to characterize the optical coupling of the EAM. The photo current maximum was offset from the optical power output maximum. The transmissions vs. bias voltage curves were measured, and an XY scanner was used to record the mode field of the light exiting from the EAM waveguide in each position. The Beam Propagation Method was used to simulate the mode field and the coupling efficiency. Based on the bench tests and simulation results, a design including mechanical, optical, and RF elements was developed.

The device studied is a dilute core waveguide (DCW) electroabsorption modulator (EAM). It has a similar structure as that of a high-saturation-power waveguide photodetector. The DCW EAM was designed to enhance the optical power handling and provide a greater spur-free dynamic range relative to a more conventional EAM design. The approach used to achieve this goal was to reduce the optical confinement factor at the electroabsorption (EA) layer in order to enhance the dynamic range and the maximum power of the EAM. This approach has some effects on the assembly methods that must be utilized to form a low-loss packaged device.

A dilute waveguide EAM device was designed in an effort to increase the maximum power and the dynamic range of the device. This design of the DCW EAM was intended to spatially separate the region of maximum photocurrent and photointensity. Using an optical test setup with a lensed fiber test method, it was shown that the axes of maximum photocurrent and photointensity are separated by a distance of ~0.4 um. Using these observations, an assembly process was developed for the DCW EAM.

The EAM Module Components. A butterfly package was used with seven electrical connection pins, one RF connector, and two fiber inlets. Two metal ferrules surround each fiber. One of the ferrules is used to fix the fiber to the submount. This is done via laser welding wherein a metal clip is placed over the ferrule. The base of the ferrule is welded to the submount,and the shoulder of the clip is welded to the ferrule. The second ferrule was located so it resides within the fiber port of the butterfly package.The module size, excluding the RF connector and electrical pins, is about 26 x 18 x 10 mm3
Measurements were reported for six DCW EAM modules. The average insertion loss of the module is about 7.6 db. Improvements on this insertion loss are possible with the use of a lensed fiber with an optimized mode field. Based on data, it was projected that an improvement of 0.7 db is expected if it were possible to substitute a lensed fiber with a mode field of 4 um instead of the lensed fiber used with a mode field of 3.3 um. It is also expected that with the maturation of the assembly process, a final insertion of 6 db would be achievable for the PCW EAM in its packaged state.