Development of infrared technologies: all-optical modulation and rangefinding

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Collins, Jack Donal Peter (2022). Development of infrared technologies: all-optical modulation and rangefinding. University of Birmingham. Ph.D.

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Abstract

The longer wave infrared bands, including mid-wave infrared, spanning 3 \(\mu\)m to 8 \(\mu\)m, and the long-wave infrared, spanning 8 \(\mu\)m to 14 \(\mu\)m, remain a relatively unexplored field beyond thermal imaging. While their visible counterparts have found their way into a plethora of devices, mid-wave infrared and long-wave infrared see little to no use outside of defence and security, and laboratory settings. Due to these limited uses, thermal imaging and its accompanying technologies have not seen the same level of development as their visible counterparts. The effect of this limited development can be seen in the lack of choice of optical modulators for these wavelengths, and the lack of uses outside thermal imaging. However, the use of these longer wavelength infrared bands could be beneficial in areas where a high transmission in the atmosphere is required, due to their relative low attenuation compared to the visible range. Some of these use cases involve long distance rangefinding and optical wireless communication.

To enable these new applications, high speed modulators are required. One of the topics of this thesis involves the development and benchmarking of an all-optical shutter for the short-wave, mid-wave and long-wave infrared, for use in compact and rugged systems. This all-optical technology consists of a semiconductor optical window that on absorption of a high-energy optical pump in the near infrared, attenuates short-wave to long-wave infrared. The primary mechanism for the attenuation of the short-wave to long-wave infrared is intraband absorption caused by excited free carriers in the semiconductor optical window. This shutter technology was used to improve the temporal accuracy of a slow mercury cadmium telluride detector in a femtosecond system, enabling observations on the order of picoseconds to be made. The shutter technology was then implemented in a smaller, breadboard system, which was designed to test the shutter for real world applications such as time of flight rangefinding and active gated imaging in the long-wave infrared. With the system it was found that a microbolometer thermal camera could be externally gated using the all-optical shutter technology on the scale of microseconds.

An electronic solution for improving the temporal accuracy of mid-wave infrared and long-wave infrared was developed alongside the solid state shutter technology. As nothing similar exists on the market for mid-wave infrared rangefinding, custom electronics needed to be designed and made including a high-speed preamplifier, constant fraction discriminator and time-to-digital converter. Rather than externally gating a detector, a constant fraction discriminator was designed and custom built to provide high temporal accuracy. This constant fraction discriminator was able to negate the issues of amplitude walk and variation that would otherwise make the system unsuitable for measuring distances. To verify the performance of the system a visible wavelength benchmark was also built. The performance of the final implementation of the constant fraction discriminator system for the mid-wave infrared was compared to the threshold level triggering in the visible system. It was found that the performance of the mid-wave infrared rangefinder came close to the 633 nm benchmarking system, showing centimetre resolution, making the mid-wave infrared a viable option for rangefinding.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):