Chapter 2: Package of Laser Diode

In this chapter, packages of laser diodes are explained. There are several packages such as TO-Can, C-mount, and fiber-coupled package and so on. TO-Can and fiber-coupled package are explained in detail.

Table of Contents

2. Package of Laser Diode

2.1. About package of Laser Diode

The laser diode itself is a semiconductor chip, electric wires and electrodes. However commercially available laser diode are in a metal casing. This is due to the semiconductor device being weak to heat. Oscillation efficiency of the laser diode is higher compared to other lasers, but there is still energy that is not turned in to light, which becomes heat. In order to protect the semiconductor device from its own heat by dispatching it, laser diode are sold in metal packaging. Below shows, Thorlabs, inc. company’s typical laser diode  packages. More information will be explained in the package description page.

Chip on Submount (COS)
Fig. 2.1.1. Chip on Submount (COS)

Fig. 2.1.2. TO-Can

Fig. 2.1.3. C-Mount

14-Pin Butterfly Mount
Fig. 2.1.4. 14-Pin Butterfly Mount

LD and LED are a similar light-emitting device using semiconductors. However, since LD’s output is higher than the LED’s, it is impossible to use a lens made of epoxy resin for the LD (the LED does), so the appearance of the package is very different.

2.2. TO-CAN package of Laser Diode

The most popular package of the laser diode is the TO-Can package in the Fig. 2.1.2. The TO-Can package has various diameters.

The TO-Can has three pins. Two are the cathode and the anode of the laser diode chip. The remaining one is a cathode for the output monitoring. The top of the silver cylinder is the output window, the laser light is emitted from here. At the golden base part, it is possible to attach the radiating plate. The following is a video that shows how the laser is emitted from the laser diode of a TO-Can package.

Pin placement of a TO-Can package

There are various types pin placement points of a TO-Can package, as shown billow (Quoted from Thorlabs, Inc. web site). You’ll have to conform to the pin arrangement when you connect them.

Fig. 2.2.1. TO-Can pin alignment(click to enlarge)

2.3. Fiber-Coupled package of Laser Diode

Normal LDs of today used as an excitation light source are modulated and connected to an optical fiber, as shown in the Fig. Most commonly used LD for fiber lasers and optical amplifiers is 14-pin butterfly package.

A typical 14-pin butterfly package is a single emitter type, which consists of a LD chip, optical isolator, Thermoelectric Cooler:TEC [1], and Photo Diode:PD for Laser beam monitoring (Wavelength or output monitor), and is outputted by a optical fiber attached to the ferrule.

Fig. 2.3.1. The (a)appearance (with FBG) and (b) package content of a 14-pin butterfly package’s fiber pigtail.

The LD is sub-mounted to a Peltier device (TEC) for temperature control purpose. The laser outputted from the LD is focused into the optical fiber (single-mode, multi-mode, polarization-maintaining, large-diameter fiber etc.) by a micro lens. If there is a monitor PD (here a PIN Diode:p-intrinsic-n Photo Diode[2]), output can be monitored. After precise alignment they are fixed by YAG laser welding. Table shows the fiber pigtail LD used as excitation LD for fiber lasers [6-9]. A single-mode fiber: SMF pigtail LD can be fused with a SMF optic fiber, and mainly used for exciting rare-earth-doped SMF. A multi-mode fiber: SMF pigtail LD can be fused with the excitation port of an excitation combiner, and used mainly for exciting double-clad fiber: DCF.

Table 2.3.1. Fiber pigtail LD used as excitation LD for fiber lasers.

LD type Fiber type Core diameter Maximum output @CW Reference
Single emitter type SM ~10 μm 0.8 W [3]
MMF ~100 μm ~25 W [4]
Bar type MMF ~100 μm ~30 W [5]
Stack type MMF ~600 μm ~400 W [6]

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