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How laser light is different from ordinary light

Fri, 10 Sep 2021 15:29:36 Z

The sun, stars and fire are natural sources of ordinary light. We recognize the light received by the human eye as white, whether it is the light of a lamp or the sun. However, we do not notice the spectrum of colors that make white, except through a prism by which light can be decomposed into different components.

Let’s find out about the differences between ordinary light and laser light.

Ordinary light

Ordinary light sources mainly emit polychromatic light and their emission is spatial. The source of ordinary light emits many rays in all directions, randomly. Ordinary light consists of a mixture of rays with different wavelengths, where each wavelength corresponds to one color that your eyes register.

Then why is the laser light different from ordinary light?

Because laser light is:

Monochromatic - It contains only one specific wavelength and hence one color.
Coherent - The motion of all photons is coordinated.
Directional - The laser beam is very narrow, concentrated and therefore, it is a high intensity source.

Laser (Light Amplification by Stimulated Emission of Radiation) is an artificial source of light radiation that emits a coherent beam of photons, as the source is stable in frequency, wavelength, and power. Unlike the light emitted by common sources, such as light bulbs, laser light is mostly monochromatic, i.e. only one specific wavelength (color).

When we talk about highly monochromatic lasers, we consider that the beam linewidth is very narrow. You can use these lasers especially for applications such as laser spectroscopy or coupling with optical fibers in fiber communications.

Take a look at other types of light sources: incoherent light sources (light bulbs, LED, stars). These sources produce radiation by spontaneous emission in all directions, with a spread of wavelengths and no interrelationships among individual photons. Spontaneous emission is a random process. On the other hand, lasers use stimulated emission, producing photons with identical properties (all photons move at the same wavelength and direction).

You can be surprised by the fact that measured linewidths of real solid-state lasers, such as diode-pumped YAG lasers, are <1 kHz. This is not the bottom line. In fact, linewidths less than 1Hz are also achievable by suppression of external noise influence. Narrow-linewidth lasers are used in holography, frequency metrology and light detection and ranging (LIDAR).

The laser beam is highly coherent, which means that the electromagnetic waves are in the same phase with each other and propagate in the same direction. You can get a laser beam of high intensity and directivity by superposition of electromagnetic waves that are in phase. Such a highly directional beam can be focused on a very sharp point, which is not possible with ordinary light.

However, real laser light is not purely directional because some propagation effects can distort the laser beam, especially if it is interacting with nonlinear media. When you propagate laser beam it gets broadened with distance. In order to achieve an adequate beam size and directivity you need to additionally precisely control, measure and direct laser beam with lenses and mirrors.

Laser light properties are used all around us

Specific properties of artificial light obtained by lasers are advantageous for transmitting data for hundreds of miles, up to terabits per second, by a convenient, single wavelength coherent source. Nowadays, you can transmit data with laser beams through free-space optics technology over moderate distances with many Gbit/s data rates.

The laser has also become an important research instrument and has found its application in many fields, such as the correction of vision, the sharpening of the astronomical image from space, testing the DNA molecule and in obtaining pure energy by laser fusion of atoms.

Since the discovery of the laser, more than 30 Nobel Prizes have been awarded in the field of natural sciences for scientific discoveries directly related to lasers. In 2018, Donna Strickland became the second woman to receive a Nobel Prize in physics, which she was awarded for her work on ultra-short and high-intensity laser pulses. Measuring the laser's properties with precision and accuracy is key to such research, which is why Gentec-EO develops and manufactures high-accuracy laser power meters and laser energy meters.

How to calculate laser pulse energy

Tue, 10 Jul 2018 13:32:03 Z

Facing a hard time calculating laser pulse energy? Or maybe you graduated from university decades ago and simply can’t remember? Don’t worry, we’ll cover the essentials in this blog post.

The laser pulse energy calculation explained, with numerical examples

If you are trying to calculate the amount of energy that is contained in your laser pulses, you are either working with a pulsed laser and want to know the energy in each individual pulse, or with a CW or pulsed laser that is fired for a known and finite amount of time.

Don’t worry, in both cases, the equations are really simple:

For a pulsed laser, you will need to divide the average power of your source by its repetition rate. Laser specification sheets usually give these two parameters, but for a better accuracy, they can also be measured by suitable instruments.

For example, a Gentec-EO laser power meter can be used to measure average power.

Pulsed laser: Pulse Energy (Joules) = Average Power (Watts) / Repetition Rate (Hertz)

Let’s put some real values in there and assume that you are working with a laser that has a fixed 200 W output and a repetition rate that can be tuned from 20 Hz to 1 kHz. If you set the control at 20 Hz, we have 200 W / 20 Hz = 10 J per pulse.

In order to provide a constant 200 W output with 20 pulses being fired each second, then each of your pulses has to contain 10 J of energy.

If you set the control at 1 kHz (1000 pulses every second), then we have 200 W / 1000 Hz = 0.2 J = 200 mJ in each pulse. For a fixed average power, the higher the repetition rate, the lower the energy per pulse.

For a CW or pulsed laser that is fired for a known and finite amount of time, you will need to multiply the average power of your source by that time, which is often called “Pulse Duration”.

As mentioned previously, the average power of your source is usually something that is given in its specification sheet.

As for the amount of time that you are going to fire your laser, well, for obvious reasons, the specification sheet does not know that, unless the laser has pre-programmed pulse durations that you can use.

CW or pulsed laser: Pulse Energy (Joules) = Average Power (Watts) * Pulse Duration (Seconds)

Again, let’s use some real values and assume you are working with a CW laser that outputs 500 W. If you fire this laser at an object for exactly 5 seconds, then you have 500 W * 5 s = 2500 J.

This is the total amount of energy that you directed towards this object during this long pulse.

Another example: if your 500 W laser has a 200 ms pre-programmed pulse duration, that will give you 500 W * 0.2 s = 100 J of total energy. The total energy can be controlled by modulating the pulse width/duration.

Why is pulse energy calculation / measurement important?

Well, if you are reading this article about how to calculate laser pulse energy, you probably need to do it and thus know better than me why this is important for you!

For those who are reading out of curiosity, here is a real-life example: In several medical and surgery applications, pulsed lasers are used treat and cut many types of body tissues.

In order to avoid permanent damage to one’s body (pulse energy higher than what is required) and on the other side avoid incomplete or incorrect treatment (pulse energy lower than what is required), it is mandatory to measure pulse energy to make sure it is stable around the desired sweet spot.

This will ensure both safety and efficiency during treatments.

What is much better than calculating your laser pulse energy?

The answer is simple, measure it using a Gentec-EO laser joulemeter!

If you calculate your pulse energy using average power and repetition rate specifications, you might come close to the real value (what your laser really outputs), but you are more likely to find something that is quite far from it. Why?

The main reason is simple. Your laser does not necessarily output the exact average power that is written in its specification sheet. Same thing for the repetition rate.

In addition, the average power can vary significantly through time with the aging of the laser. What you get on day one can be much higher than what you will get five years after.

You can avoid all this uncertainty by directly measuring your pulses’ energy with a joulemeter. That will give you the best possible accuracy. Gentec-EO offers a very wide range of pyroelectric, photodiodes and thermopiles detectors in order to cover all applications and all laser specifications.

If you would like to find the best Gentec-EO joulemeter for your needs, you can contact us directly or visit our laser energy meter page.

You see, laser pulse energy calculation isn’t that complicated after all!

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