Introduction

Light acts as a stimulus and induces a multitude of responses from any surface it is incident on. In this chapter, we make a brief overview of the theories of light, lasers and detectors used in optical laboratories around the world. Further, the chapter states the benefits of using imaging sensors for profiling laser beams and provides an overview of the book.

Applications using light for inquiring about physical or biological phenomena are incomplete without an optical radiation detector. In the visible regions, photon detectors have gained a high degree of popularity, given the extent of their utility. These detectors are semiconductor-based electronic components that detect the amount of light 'falling' on their surface and convert it into electrical signals. Optical radiation carries with it energy that is capable of inducing a proportional conversion to electrical signals when incident on the detector material. In this book, we are keen on presenting a method of using silicon-based sensors for beam profiling. The book covers lasers and their working, silicon-based charge-coupled device (CCD) and complementary metal–oxide–semiconductor (CMOS) sensors before combining them for use in the laboratory. The book is in no way an exhaustive thesis on either of these concepts, but attempts to introduce them at an accessible level and provide a guide to using them in the laboratory. The method detailed in the book for using silicon-based imaging detectors as a laser diagnostics tool serves two purposes of power measurement and beam profiling. While laser power is usually measured using a power meter, this method enables the end user to use a single device for quantifying the output of the laser and understanding the geometry of the beam simultaneously. Understanding the beam characteristics and profile is key in many applications where knowledge of the initial and the final measured beam can provide information about the object/medium. This resource includes a video guide on how to calibrate the components needed and to profile the beam itself.

Early scientists such as Pierre Gassendi and Sir Isaac Newton long believed that light was made up of particles called corpuscles. These corpuscles, travelling in straight lines, result in light rays. Until the discovery of light bending around sharp edges (through a phenomenon called diffraction), the particle theory explained most of light's behaviour such as creating shadows, and brightness and darkness due to presence or absence of light. Subsequently, with the discovery of diffraction, light began to be described as a wave. After initially being disregarded, the work of Christiaan Huygens and Augustin Jean Fresnel were proven to show wave behaviour of light. Much later, in the twentieth century, quantum mechanics fused both theories and accepted the dual wave and particle nature of light [1].

Subsequent to Albert Einstein's contributions ¹ on the photoelectric effect, he further laid the foundation for understanding stimulated and spontaneous emissions at atomic levels. His paper 'On the quantum theory of radiation' in 1916 was pivotal in the eventual realisation of the first working laser by Theodore Maiman in 1960. Through a deceivingly short article [2] on stimulated emission using ruby, Maiman presented a light amplification device that is one of the most commonly used technologies today. The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Although lasers were initially analogous to an optical maser, due to its functionality enforcing that of a maser (amplification of microwave radiation) working in the optical wavelength region, this definition was not regarded as the most accurate for the laser [3].

Lasers are varied and widespread, both in terms of type and application. In modern communication systems and audio systems such as compact disc players, the laser is the most important component. Industrial applications utilise the focused power and intensity of the laser for manufacturing, cutting, cleaning and heat treating. Medicine, a domain constantly searching for new and efficient tools, has found lasers to be a handy tool in surgery, whether they be oncology or dermatology. The military utilise it for range finding and targeting, while the supermarkets use lasers at checkout counters [4]. A full list of laser applications would be endless and forever incomplete as with every passing day, research and innovation extends its applications into more and more domains.

Any device that receives and responds to a stimulus can be considered a sensor [5]. When this definition is applied to optical sensors, the analogy unravels to 'an electrical signal as a result of material–light interaction.' Light interacts with objects or media and after phenomena such as reflection, refraction, absorption, interference and polarisation before interacting with an optical sensor or detector. Detectors capable of measuring electromagnetic radiation in the ultraviolet to far-infrared are known as light or optical detectors [6]. From a sensor's standpoint, the detected electromagnetic radiation could result in a quantum or thermal response, which effectively defines the broad definition of optical detectors. Quantum detectors function in ultraviolet to mid-infrared regions while thermal detectors function from mid- and far-infrared spectral ranges.

Materials used in detector sensors are sensitive to incident light. The changes in the characteristics of the material cause a change in the current in the detector circuit. The change in current, in turn, results in a change of voltage, creating an output signal. Optical detectors must be chosen with due care and consideration of the application, as the operating wavelengths decide the type of detector and optics ² . Currently, many detectors are available, capable of capturing ultraviolet, visible and infrared wavelength of the spectrum [7]. The significance of imaging systems operating across all wavelengths of the electromagnetic spectrum is unquestionable, as the number of applications would fill volumes of textbooks. Extensive and exhaustive guides to laser physics or optical detectors are quite the task to formulate. We would like to mention, at this stage, that this is not the motivation for this literature. However, we will share relevant resources through textbooks and pivotal research articles for your reference through footnotes or in dedicated sections for additional resources.

In this text, we provide a basic overview of lasers and silicon-based imaging systems; particularly charge-coupled devices (CCD) and complementary metal–oxide–semiconductor (CMOS) sensors. This overview is to serve as a 'fuel to inspire' introduction to a novice, a reference guide to a laboratory technician setting up laser equipment, and a refresher to experienced researchers and engineers. It will discuss conventional laser power meters before moving on to discuss the benefits of using an imaging system for laser diagnostics. Chapter 2 will provide an introduction to lasers. Thereafter, we will discuss imaging systems and a broad overview of the sensors in chapter 3. A detailed discussion and guide to using imaging systems for laser profiling and measurement (chapter 4) is followed by a description of the potential sources of errors when using this method for laser diagnostics is detailed in chapter 5. Finally, the conclusions presented in chapter 6 complete this work.

Most laser systems' power is measured using power meters. Conventional power meters, like the Lasermet ADM 1000 (figure 1.1) ³ , use integrating spheres combined with photodiode detectors for maximising the degree of light entering and being measured by the detector. However, they do not provide information regarding the beam profile. They are dependable sources of information when measuring the amount of light in a specific direction, either from a laser or from other sources. The digital meter provides a wide range of metrics related to lasers, including peak power, energy per pulse, graphical illustrations and computer interfacing to utilise the data in a multitude of ways. This information is useful in most applications where the beam power and profile are important to monitor.

Zoom In Zoom Out Reset image size

CCD and/or CMOS sensors can acquire two-dimensional information of the input beam. While the power cannot be measured directly with these sensors, they can measure the footprint and intensity distribution (within the dynamic range of the sensor). Therefore, knowledge of the optical power and/or measuring the beam profile can be combined. This guide provides a step-by-step method of combining beam profiling using imaging sensors while simultaneous monitoring laser power. The advantages of this method are:

• •  
  The beam profile information is intuitively available. The method provides an option of both snapshot and continuous monitoring of the beam profile. An analysis of the beam's shape, intensity distribution and profile can be made.
• •  
  The laser power can be monitored continuously and simultaneously with the beam profile.
• •  
  The consistency of the laser's output can be monitored on a regular basis.
• •  
  The distribution of laser hotspots can be mapped using this method. It is important to identify this, as it defines the locations in the beam of the laser where it presents maximum intensity on incidence. This intensity defines the application and utility of the laser.