Fiber optical communications uses light confined in an optical fiber for transferring information over long distances. For high speed transmission over long distances, an important light source for fiber optical communication is the Distributed Feed-Back laser (DFB-laser).
Two major types of DFB-lasers exist: Buried Heterostructure (BH) and Ridge. These two types are briefly described in connection with FIGS. 1a and 1b. Both have their advantages, e.g. the BH in general gives better performance and the Ridge is simpler to manufacture.
Even better performance may be obtained by adding a modulator to the DFB laser, e.g. an integrated Electro Absorption modulator (DFB-EA), since it introduces less chirp than direct modulation of the laser.
The DFB-EA component is made of a laser (DFB) and a modulator (EA) The device may be manufactured in many different ways and a popular way is to first epitaxially grow the laser material, then etch away all material not needed for the laser part and regrow new material around the laser (Butt Joint) to use for the modulator. Then a Ridge DFB-EA could be made if a contact layer is grown on top of the laser and the modulator material followed by etching the ridge.
This way to manufacture the DFB-EA device would introduce problems with current leakage/recombination in the butt jointed rim around the material forming the laser part due to defects at the interface and only slightly higher bandgap in the surrounding regrown material forming the modulator part. This causes high threshold currents. A typical value for the bandgap energy for the laser material is 0.80 eV and for the modulator material 0.83 eV. It is generally difficult to control the composition and quality of the modulator material close to the laser material due to diffusion of material from the masked region, so called SAG—Selected Area Growth.
These issues may be taken care of by manufacturing a BH DFB-EA laser instead. In this case a narrow mesa (waveguide) is etched down and semi-insulating (SI) InP is regrown around the laser and the modulator waveguide (see FIG. 1a), thus forcing all current to run through the component. The etching may also be done in front of the modulator, hence the regrown SI-InP will create a window some 15-30 μm adjacent to where the device finally will be cleaved. The window is used to suppress the light reflected by the cleaved facet of the chip back into the laser. This is achieved by spreading the light leaving the modulator waveguide material before it is reflected from the cleaved facet (see FIG. 2). The window thus relaxes the requirements on the anti-reflective coating of the facet. However, this method of manufacture of a DFB-EA requires one or two extra epitaxial process steps and hence increases the complexity (and price) of the device. Also, it is well known that the regrowth of the current-blocking layer around the mesa in the BH-laser mate cause reliability problems.
Insulating materials, such as BCB (Benzocyclobutene), has been used in micro chip fabrication for a long time, e.g. see article by R. A. Kirchhoff, C. J. Carriere, K. J. Bruza, N. G. Rondan, and R. L. Sammler with the title “Benzocyclobutenes: A new class of high performance polymers” Science-Chemistry, Vol A28, Nos. 11 & 12, 1991, pp. 1079-1113. The material has been used in a variety of electronics applications ranging from conductive, metal-filled adhesives to high planarizing and insulating layers on silicon wafers.