**Acknowledgements**

shown, the launcher optical assembly is mounted on a motion control stage. The demulti‐ plexed red laseris made incident on a quadrant photodiode which serves as a position sensitive detector. The error signal generated by the quadrant photodiode is used as a feedback signal to actively align the motion control stage to keep the red beam centered on the quadrant photodiode. As a consequence, the ammonia spectroscopy beam also remains centered on the 1.5 μm photodiode. This leads to overcoming of measurement errors induced by thermal

Experimental results demonstrating the performance and value of the alignment control system are shown in **Figure 8**. **Figure 8a** shows the scenario with no vibrations/misalign‐ ments. As expected, the 2f/1f signal stability is the same with the alignment control system on or off. This is a key check to ensure that the alignment control system does not introduce any artifacts or errors by itself. **Figure 8b** shows the scenario under induced vibrations/misalign‐ ments. With the alignment control off, the total normalized transmitted power drops close to zero in a matter of seconds, hence indicating a severe misalignment of the system. However, with the alignment control on, the system is able to actively maintain alignment over time. About 20–25% powerfluctuations are still observed which are experimentally found to be well within the correction capability of the 2f/1f WMS technique. For more details on the tests demonstrating performance ofthe hybrid sensor, the readeris advised to referto reference [14].

**Figure 8.** Experimental results showing the performance of the alignment control system. (a) 2f/1f WMS signals (with and without alignment control) as a function of time under no vibrations. (b) Transmitted spectroscopy laser power (with and without alignment control) as a function of time under induced vibrations. Figure taken with permission

In summary, this chapter presented the value and potential benefits of diode laser‐based industrial harsh environment sensors. The chapter started with an overview of the industrial Internet and discussed the importance of sensors toward achieving enhanced output from industrial assets such as gas turbines, aircraft engines, and turbomachinery equipment . The discussion showed that real‐time decision making through online sensors (that monitor the desired machine parameters) can enable optimized operation, performance enhancement, and extension of asset life. Subsequently, optical harsh environment sensors were introduced and the capabilities of diode laser‐based techniques, that is TDLAS and QCLAS, were discussed.

from SPIE (From our published paper).

408 410High Energy and Short Pulse Lasers

**5. Summary**

misalignment and enables long‐term reliable operation of the sensor.

The authors would like to acknowledge R&D support from the General Electric Company. The authors would also like to thank the Optics & Instrumentation team at GE Global Research, Bangalore for their contributions to the work presented in sections 4.1 and 4.2.
