When you superimpose the laser diodes gain band on top of the Fabry-Perot modes, as shown in the figure below, you are now left with the total number of allowed spectral modes. These modes are referred to as the longitudinal modes of the laser and are the only allowable wavelengths at which the laser can amplify and emit light.  If the gain at a particular longitudinal mode is greater than the total loss in the cavity (the gain threshold), that mode will be amplified and lasing will occur.   This interplay between the gain threshold, gain bandwidth and longitudinal mode spacing is what ultimately determines whether or not a diode operates as a single or multi-mode device.   If the gain threshold is significantly high and the gain bandwidth substantially narrow to allow for only one mode to lase it is considered to be single longitudinal mode (SLM).  Inversely, If the gain band is substantially wide and threshold significantly low to allow for many modes to lase at once, it is considered to be multi-longitudinal mode (MLM).

All lasers must contain three key components, a gain material, a resonator cavity, and an excitation source.  For us to understand what a longitudinal mode is and where it comes, we will first ignore the gain medium and excitation source, focusing solely on the resonator.  To further simplify our initial analysis we are also going to assume that both mirrors in the resonator are 100% reflective, this is known as an ideal resonator.  In this case, all of the light waves are then trapped inside of this cavity, creating a standing wave inside as the light travels back and forth between the two mirrors.  Just like in a vibrating string an infinite number of standing waves can exist inside of this cavity, but they all must have a node at the two mirrors.  Therefore, as is shown in the figure below the wavelength or frequency of these standing waves are governed by the length of the resonator, with the lowest order mode having a wavelength of one half the cavity length and each higher order mode have a wavelength equal to ½n , where n in the mode number.  By taking into account the relationship between frequency, wavelength, and index of refraction it can be shown that each mode in the cavity will be spaced out according to the relationship , called the free spectral range, where c is the speed of light, n is the index of refraction, and L is the cavity length.

Objective lenses are roughly classified basically according to the intended purpose, microscopy method, magnification, and performance (aberration correction).

We live in a beautiful world – and that beauty and complexity extends far beyond what humans can see unaided. From plant and animal anatomy to cells and proteins and even down to the level of atoms, there are worlds within worlds of detail to be explored on the microscopic scale.

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Jan 20, 2023 — Lens Chief Ray Angle Mismatch and Impact on Image Quality ... The chief ray angle (CRA) of a lens and the chief ray of a sensor affect image ...

As a result of this quantization of allowable frequencies/wavelengths in the laser cavity, we can see that there are an infinite number of modes that can fit into an optical cavity all with equal frequency spacing.  These are known as Fabry-Perot modes, and heavily influence the spectral characteristics of the laser.  In reality, no mirror is perfectly reflective, and in fact, if they were, we wouldn’t be able to get any light out of a laser rendering it useless.  Since every resonator will have some loss at the mirrors, the Fabry-Perot modes are not pure delta functions, but instead, they have a certain linewidth dependent on the reflectivity of the mirrors.  A detailed analysis of the causes of this line broadening is beyond the scope of this blog post, but the figure below shows several examples of how decreasing the mirror reflectivity effects the mode structure.

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Free Series Divergence Test Calculator - Check divergennce of series usinng the divergence test step-by-step.

Here at RPMC Lasers, we have over 20 years of experience with diode lasers and are readily available to assist you in not only deciding which of our standard off-the-shelf laser diodes are ideal for your application, but we are also able to offer countless custom laser diode packaging configurations with a wide range of integrated optics include all of the designs discussed above.  For more information about our wide variety of laser diodes you can click here, and for more information about laser diode fundamentals be sure to visit our Lasers 101 page.

20241013 — The discovery of a World War II-era bomb forced the partial evacuation of the Sternschanze nightlife district in the German city of Hamburg late Saturday.

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The Exploring with microscopes – question bank provides a list of questions about microscopy and places where their answers can be found. The questions support an inquiry approach.

While doing so, it contributes a base magnification of anywhere from 4x (for a scanning objective lens, typically used to provide an overview of a sample) to ...

Our microscope resources emphasise the link between microscope technology and the science that microscopes have helped uncover. The activity Which microscope is best? is a good starting point for learning how specialised microscopes can help answer different scientific questions.

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Associate Professor Tony Poole shares his story about the primary cilium, a structure of the surface of cells that seems to monitor what’s going on in the cell surroundings. This elusive structure was first tracked down using microscopes, and many aspects of how it works remain mysterious. In the article A closer look at the cell’s antenna, see how Tony is using microscopes to build a 3D computer model of the primary cilium.

Sugar GRIN (gradient index) lens · Light doesn't always go straight! · Here, I poured a bag of sugar into the bottom of a fish tank, added a little water to ...

At the University of Otago, Dr Bronwyn Lowe and Māori weavers have been working closely together to explore several properties of harakeke (New Zealand flax). In the article Harakeke under the microscope, learn about the differences between harakeke varieties on the microscopic scale and explore how mātauranga Māori (traditional Māori knowledge) can shed light on scientific research.

There are two critical parameters that all lasers diodes must meet to begin the lasing process.  The first one of these parameters is that there must be more gain than loss inside the laser cavity, the point at which this condition is satisfied is known as the gain threshold and was covered in the last installment in our Laser Diode Fundamentals series.   The second condition that must be met, is that there must be a longitudinal mode present inside the optical cavity which coincides with the laser’s gain curve.  In this blog post, we are going to explore what precisely longitudinal modes are and how they affect the performance of a laser diode.

The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image captured with a camera.

Dr Bronwyn Lowe describes her use of scanning electron microscopy (SEM) to explore harakeke leaves. Bronwyn found that different harakeke varieties have differently patterned waxes on the leaf surface. She also explored the distribution of fibre (muka) in the leaves of different varieties.

John Wollaston Born England, active 1742–75. John Wollaston was one of several painters who introduced English rococo portraiture—with its emphasis on ...

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Our microscope resources invite students to share in the sense of wonder that scientists have felt for centuries looking through the microscope. We look at the diversity of objects on the microscopic scale and introduce several New Zealand scientists who use microscopes to explore the things that interest them. At the same time, we show how microscopes themselves have evolved to look more and more closely at the world around us.

Microscopes are the tools that allow us to look more closely at objects, seeing beyond what is visible with the naked eye. Without them, we would have no idea about the existence of cells or how plants breathe or how rocks change over time. Our understanding of the world around us would be severely limited – and this is why many scientists see microscopes as the most important scientific instrument there is.

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This SEM image of tin spheres of various sizes (used to calibrate the microscope) was taken by Liz Girvan. It won an image competition in Otago and has attracted worldwide interest.

The student activities provide plenty of hands-on experiences. Modelling animal cells in 3D imitates what can be seen under high-resolution microscopes. Using lolly slices to build 3D images and Using shadows to build 3D images model how scientists interpret microscopic data. Ferns under the microscope demonstrates how increasing the power of magnification leads to much greater detail. For younger students use the Making a simple microscope activity – it uses accessible technology to increase students’ ability to observe closely.

There are advantages and disadvantages to both multi-longitudinal mode and single longitudinal mode diode lasers.  For example, multi-longitudinal mode diode lasers are generally more straightforward and more cost effective to manufacture.  Additionally, the larger number of modes typically allows for much higher power to be generated by the device.  Single longitudinal mode diode lasers though have a much narrower bandwidth which makes them more desirable for applications where precise knowledge of the wavelength is required.  But as a result, these diode lasers are typically much lower power and more challenging to manufacture.

New Zealanders are only too aware of how devastating a major earthquake can be. Professor Dave Prior and his group are looking for clues to how and why earthquakes happen. In the article Squishy rocks and earthquakes and the interactive From mountains to microscopes, follow Dave and the team as they collect rock samples from deep in the Alpine Fault (Westland) and see how microscopy of rocks can shed light on the history of movement in the fault.

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Dr Rebecca Campbell (University of Otago) discusses the importance of fluorescent molecules in confocal laser scanning fluorescence microscopy (‘confocal microscopy’) of cells. She explains how green fluorescent protein (GFP) from jellyfish can be used to make specific neurons glow green.

Dr Rebecca Campbell is studying a small group of brain cells (GnRH neurons) that control fertility. Learn about her remarkable discoveries about how these cells interconnect – all done using microscopes of course!

Using the earliest microscopes, scientists glimpsed a world of unimaginable complexity – and they wanted to know more. To satisfy this urge, microscope technology became more sophisticated over time, letting us look more and more closely at objects. We’ve been able to ask more specific questions about the object we’re viewing: What does its surface or internal structure look like? What is it made up of? How does it change over time? For each of these questions, specialised microscopes have now been developed that can provide the answers.

It becomes phase contrast microscopy by inserting a phase contrast objective lens with the same PH code into the optical path. Microscopy using a phase contrast ...

Now that you have developed an understanding of the basic principles underlying longitude mode structure, in the next blog post for our laser fundamentals series we will explore how you can design a laser to ensure that it operates with only a single longitudinal mode.  In that post, we will discuss how this can be done either by adding external optical elements to a tradition laser diode  or  by modifying laser diodes structure itself to allow only one mode to reach gain threshold.