Looking at LD Module Internal Structure
Many electronic and optical semiconductor devices are packaged in metal and resin assemblies for protection against the external environment. These packages have multiple pins and leads that are connected via wiring to the internal semiconductor chip and other parts. In optical semiconductors, such as semiconductor lasers (LDs) and semiconductor laser amplifiers (SOAs), etc., a window is required for input and output of light to and from the package. The optical module has a packaged optical semiconductor chip for outputting light using electric current.
The LED light is radiated from a transparent window mounted on the package. However, most optical modules for communications applications output the light from the semiconductor chip to outside the package via an optical fiber mounted on the package. This type of module with attached optical fiber is called a pigtail type.
Fig. 1 External View of Butterfly Module
This section explains the structure of a typical pigtail butterfly module, which gets its name from the two rows of seven leads at right angles on each side of the metal package plus an optical fiber pigtail at one end (Fig. 1). Let’s look at the internal structure (Fig. 2) of a common butterfly module with built-in Fabry-Perot etalon.
Fig. 2 Internal Structure of Butterfly Module
First, the key parts are explained below.
(1) LD Chip
he back face of the LD chip has a high-reflectance film while the front face has a low-reflectance film to facilitate high optical output at the front face. The LD chip itself uses a chip on carrier (CoC) form with a carrier block composed of a material such as aluminum nitride (AlN). All LD chips are subjected to checks of characteristics and current tests before assembly in the module.
Since the output LD laser light is diffused, a high-performance condensing lens is required to collect the light efficiently at the optical-fiber core.
When light from the LD is output via the optical fiber, there is a risk that stray light from outside and reflected light from the optical fiber may be injected back to the LD, possibly causing unstable LD operation. The optical isolator is commonly used with optical modules; it allows passage of forward light only and blocks passage of backward light.
(4) Optical Receiver (PD)
Variations in the LD optical output can be checked by monitoring the current at the PD at the back face of the LD chip. There is also an Automatic Power Control (APC) function which monitors the PD current to control the LD driver current to keep the fiber output constant.
(5) TEC (Thermo Electric Cooler)
When a current is passed across a junction between two different metals in series, heat is radiated or absorbed according the current direction (Peltier Effect). Using this effect, heat from the LD chip can be either radiated outside the package, or if the ambient temperature is extremely low, the chip can be warmed.
A thermistor is an electronic part with variable resistance depending on temperature. It is used to detect the LD temperature.
These parts are soldered in place and connected with gold wires to the package leads before the package is filled with dry nitrogen gas and hermetically sealed. Light from the LD passes via the lens, optical isolator, and sapphire glass window mounted on the package and via the optical fiber. To maximize the output, the position of the mounted optical fiber is precisely adjusted before bonding to the package.
Module Design Points
There are two main points to consider when designing a pigtail optical module. First, the coupling efficiency of the fiber to assure transmission of as much light as possible from the LD chip to the outside via the pigtail fiber. Each maker has its own unique method for the lens system, which is the key to high coupling efficiency. As shown in Fig. 2, Anritsu Devices uses a simple one-piece lens configuration. It is also possible to use a dual-lens configuration for converting the LD light to parallel light and then condensing the light at the fiber. Additionally, rather than using a lens in the package, the shape of the tip of the coupled fiber can be polished to function as a lens. Both methods have advantages and disadvantages, and different makers use each method as necessary.
The second point is heat radiation design. Not only does the LD output power drop as the chip temperature rises, but also the center wavelength tends to drift to longer wavelengths. To stabilize operation without any impact from external conditions, the LD temperature must be kept constant. Stable output can be achieved by controlling the TEC current according to the resistance of the thermistor mounted near the LD chip. High-optical-output parts also have high power consumption. In addition to the LD element characteristics, the power consumption is greatly affected by thermal design, including the choice of TEC technology and parts layout. Anritsu Devices has successfully manufactured the world’s best-in-class, high-output lasers with a single-mode-fiber output of 650 mW.
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