Laser Physics and its groundbreaking inventions #2: The Chirped Pulse Amplification (CPA)

Introduction

Has it ever occurred to you whenever you hold your smartphone that you are in a possession of a great device made possible by a 2018 Nobel Prize-winning technology developed at a private university situated in New York? The Chirped Pulse Amplification (CPA) is the latest efficient way of increasing very short laser pulses to a very large unimaginably level that has ever existed and one of its various uses is in the production of glass screen for mobile phones.

The state-of-the earth technique was birthed by Donna Strickland with the support of her supervisor, Gerard Mourou in which both were awarded a Nobel Prize for Physics. Strickland's fell in love with laser physics at the mere sight of the red and green beams of laser light illuminating her laboratory in a beautifying way; and also to fulfill her supervisor's imagination of being able to amplify short laser pulses to higher energy pulses.

Before I go further in the discussion, here's a quick reminder on how Laser light is being created.

In my last post on

, I shed more light on what laser is and how it is being created. In that post, I explained that the word "LASER" is an acronym for Light Amplification for the Stimulated Emission of Radiation. I also went further in saying that laser light works in a similar way to a lightbulb in a way that lights get emitted when some atoms move from a higher state to a lower state i.e when the atoms are excited. Also, in a laser, the emissions are focused, organized, monochromatic, and unidirectional. A laser can be made through some series of steps. Basically, two things are needed to make a laser.

^{CW Single-frequency Ring Dye Laser
Author:Sergey100, Laser systems laboratory of Novosibirsk State University; public domainlicense}

The first is to get a large number of atoms with electrons that can be energized and the second is to get a flash tube and a laser crystal to energize the atoms. The flash tube is just like a fluorescent light having a coil around the crystal that flashes without interruption. Laser beams are produced in the laser beam when a very high voltage of electricity makes the tube to flash on and off, as a result of which energy is now fed into the crystal as photons. Afterward, the atoms inside the crystal absorb the energy, soak it up and get excited thereby giving off photons in return when they are back to the ground state. This is a process called spontaneous emission.

The emitted photons become excited as they move randomly at a speed of light in the crystal thereby getting other atoms excited and then giving off more photons in the process. As the process continues to occur, more and more photons are released which results in the emission of more light. A mirror is then placed at the end of the flash tube to make sure the rebounding photons keep rebounding while another mirror is placed partially at the other end to allow some of the rebounded photons to escape from the crystal. It is those escaped photons that eventually form a laser light with a very strong beam.

So, what brings about the CPA?

Just like we have known or probably just getting to know from my brief introduction above that laser light can be made from a kind of chain reaction by which photons produce more photons of light. These generated photons are emitted in pulsations. Since the discovery of lasers about 60 years ago, it has always been a great challenge for scientists and researchers to generate more powerful laser pulses. It was virtually impossible to amplify the strength of the light without causing damage to the amplifying medium.

^{Schematic layout of a grating-based stretcher of laser.
Author:Han-Kwang Nienhuys, own work; CC SA-BY 3.0license}

The Chirped Pulse Amplification, CPA invented by Strickland and Mourou was designed in such a simple way as to accept short laser pulse, expand the pulse, amplify it, and then compress it back again.

How does the CPA work?

To understand how the CPA works, the first thing is to get a pulse you would like to amplify then expand it by putting a "chirp" into the pulse. Putting a chirp into the signal is like adding a kind of dispersive material like a prism which serves to split the pulse into different colors of wavelength. It allows the longer wavelengths to be sent first before the shorter ones so as to reduce the pulse intensity. It is that upward or downward rapid movement of frequencies over the pulse that gives bird chirps their usual sound. After the amplifying and stretching of the pulse, the next is to pass the already reduced pulse intensity down the laser amplifier. Afterward, make the pulse go through an altered set of equally spaced parallel wires which will nullify the gap between the two set of different wavelengths thereby making them whole as a single pulse having the same length and shape like the earlier pulse.

^{Diagram showing process of chirped pulse amplification of ultrahigh-intensity lasers.
Author:LLNL S&TR, United States Department of Energy; Public domainlicense}

The most distinctive attribute that makes this method of CPA fulfills all the necessary requirements when done properly, is that the lengthening of the pulse will totally maintain the consistency between the conflicting frequency, which literally translates to easy reversibility of the pulse back to its original shape. Also, the method banks on the reality that stimulated emission will totally double the photons that it is intensifying. This means that the amplified photons will have the same phase and frequency as the original pulse.

Furthermore, Strickland and Mourou observed that whenever a pulse is extended promptly, its maximum power becomes very low and that makes it easy for the pulse to be amplified without destroying the amplifier. What this implies is that it becomes easier to pack more light in an extremely small space thereby increasing the intensity of the pulse steadily.

The problems encountered.

A lot of years and hard work was infused by both Strickland and Mourou to see the success of the CPA. Many challenges were encountered on the journey, some of which was in the process of stretching the pulse. The 2.5 km fiber optic cable used kept breaking in the middle and that took a lot of difficulties before they eventually used a cable of 1.4km-long which was later successful. Another serious problem faced was in making the different phases in the equipment to occur at the same time. So, also is the problem of making the beam stretcher to harmonize with the compressor. All these challenges were later solved and the two scientists were able to show the entire world that their vision wasn’t just a theory but a reality.

^{Chirped pulse amplification
Source:Warrior, public domainlicense}

The various awe-inspiring applications of the CPA.

The CPA is a great invention from Strickland and Mourou that came to revolutionize laser physics. It became an agreed level of quality for all future high-intensity lasers. It also paved the way to study new fields of laser and its application to physics, medicine, and chemistry. So, here are some of the mind-blowing applications of the CPA…..

In the Industry: CPA lasers have made possible of industries that deal in perfect machining of a great collection of materials. This procedure makes use of heat energy to extract material from the surface of metals and non-metals.

Prof. Dustin Froula, a senior scientist at the Laboratory for Laser Energetics (LLE) in Rochester said, “Industry has transformed CPA for a variety of laser materials processing techniques, which includes the machining of fragile materials such as the glass cover on smartphones.”

As the world’s fastest film camera: CPA is used for high-speed illumination of what transpires within atoms and molecules in the ever-evolving microscopic world. Things occur within atoms very rapidly, so fast that for quite a long time it was only likely to report the hitherto and aftermath. But, with the advent of CPA with pulses as brief as a million of a billionth of a second, it becomes achievable to see matters that are formerly seen to be rapid.

A tool for data storage: As a result of the very high intensity of a laser, its light can be used as an instrument for altering the properties of materials. What I am saying, in essence, is that laser light can be used to transform a non-conductor to a conductor and its razor-sharp beams can be used to drill holes in materials, even human bone, as easy as possible. Taking, for instance, lasers can be used to produce a very reliable and long-lasting storage device, as the storing medium isn’t built on the exterior part of the device but inside very tiny holes drilled into the device. The technology behind the CPA can also be used to produce surgical stents and very tiny cylinders of expansible metal for strengthening blood vessels and other major passageways in the body.

For eye surgery and cancer treatment: CPA is very effective in Lasik eye surgery for cutting open the lens of the eye without affecting the adjoining tissue and millions of American citizen have benefitted from this great technique. A Rochester Professor of Optics, Wayne Knox, has gone a step ahead with CPA to rectifying eye defects. He is working on the extremely fastness of lasers to improve contact lenses rather than patients going through laser eye surgery.

The Professor said, “CPA is a tool for future vision correction.” CPA can be used to treat chronic brain tumors through proton therapies. It is the only way through which a tumor can be located in the brain and accurately killing the cancerous cells without affecting the other part of the brain.

^{A podiatrist preparing for foot Laser microsurgery
Author:Dr. Michael Wynn; CC BY-SA 3.0}

In-laser driven diffraction: CPA has really helped to advance the study of light-matter relationship beyond their normal limit. It has made it easy to extract and control electrons from molecules, thereby giving rise to the manufacturing of tools like the laser-driven electron diffraction. This is a great tool mainly used to capture the behavior of molecules when they are bent or in vibration.

In Laser Micromachining and Microsurgery: CPA is very effective in controlling and sending a large amount of energy into materials which is very applicable in laser micromachining and microsurgery. The short burst of power is used to engrave waveguides in dielectrics for quantum informational and computational chips.

Other areas of the CPA that is yet to be fully explored…

There are many areas of and applications of the CPA that are yet to be fully researched. Some of which is in the area of attosecond physics; a world of a billionth of a billionth of a second. This study will make it possible for laser pulses shorter than a few hundred attoseconds to reveal what happens inside electrons. Then, the electrons can be both scrutinized and regulated.

Another area of research is in the possibility of increasing laser power beyond 100 petawatts to a zettawatt, which is a million petawatts and also by shortening the pulses down to zeptoseconds, a tiny fragment of time approximately 10^-21 seconds.