Semiconductor lasers are commonly used as light sources in applications involving optical communications. Such lasers can generate light in response to an application of an electrical signal. A common laser structure is called an “edge-emitting laser” that usually includes a flat junction formed between two pieces of semiconductor material, each having been treated with a different type of impurity. When an electrical current is passed through the laser, light emerges from a plane of the junction region, at an edge of the laser. Examples of such a laser include the Distributed Feedback (DFB) laser, the Fabry-Perot laser, and the Distributed Bragg Reflector Laser (DBR).
A completed semiconductor laser is generally made in four phases, similar to other semiconductor devices. A first phase involves an extraction and purification of raw semiconductor materials, such as sand for silicon. Next, a method called “crystal growth and wafer preparation” is used to form wafers, which are thin disks of semiconductor material. In a third stage, “chip fabrication” , a plurality of lasers are created in and on the wafer surface. After chip fabrication, the lasers on the wafer must be tested. The electrical test (also called a “wafer sort”) may be part of the last step of chip fabrication or an initial step in the packaging process, which involves separating the wafer into individual lasers (or “chips”) and placing them into protective packages
A large problem with semiconductor lasers is a high cost of manufacturing associated with them. In order to confirm that a laser is properly functioning, an individual laser must first be cut from the wafer, separated, and at least partially packaged before it can be tested. If a laser is found to be defective, it is discarded. The yield of a wafer may be a key indicator of costs, and often times the yield may be low. Costs become much more problematic especially since the overall costs may include packaging costs for what tests ultimately determine to be a defective chip.
One response to this problem, has been the creation of the Vertical Cavity Surface-Emitting Laser (VCSEL), which in contrast to the edge-emitting laser, can emit light from the wafer perpendicular to the plane of the junction region. A light output perpendicular to the surface of the wafer allows VCSELs to be tested at the wafer-level stage by the monitoring of emitted light. In contrast, it is difficult to test an edge-emitting laser at the wafer-level stage, because the laser light is not accessible until the lasers have been cut from the wafer, since the individual placement of the individual lasers on the wafer does not provide sufficient room to insert light monitoring devices between each laser.
In summary, the horizontal emission of light by edge-emitting lasers make it difficult to perform testing at the wafer-level stage. As a result, edge-emitting lasers must be partially packaged before test, resulting in additional costs for the manufacturer that are eventually passed on to consumers. Competing lasers, such as the VCSEL, have a testing advantage over the edge-emitting laser, but cannot replace the edge-emitting laser in many applications.