Notes on Monitor Photodiodes in Photodigm butterfly-packaged laser diodes
The monitor photodiode included in the Photodigm butterfly-packaged laser diode is included to allow monitoring of the average laser output power about a fixed operating point for a given set of operating conditions. The monitor Photodiode is positioned on the back side of the butterfly package. It tracks primarily the back facet emission of the DBR laser. The fixed operating conditions primarily include the operating current and device operating temperature. The actual photodiode current characteristics for a given butterfly-packaged device will vary slightly as functions of drive current, operating temperature, and photodiode bias conditions.
A typical curve showing the measured fiber output power and photodiode current versus injection current is given in Figure 1. The curves of Fig. 1 show that the back facet emission of DBR laser diodes is highly non-linear with respect to the front facet emission. As the DBR laser approaches a mode hop with increasing current, the slope of the back facet emission significantly increases from the slope response just after a mode hop. As shown in Fig 1, the photodiode response is approximately 1.2mA/mA at 240mA, and increases to 3.3mA/mA just below the kink at 280mA. This non-linear characteristic can be extremely useful when locking the DBR laser to a specific wavelength of power level. Note that due to hysteresis effects, the position of the mode hops with decreasing current occur at lower current values than with the increasing current curves shown in Fig. 1.
Figure 1. Typical Photodigm butterfly-packaged laser diode fiber output power and monitor photodiode response as functions of laser injection current.