Tag: inertial confinement

How do you make a mode-locked laser?

Given

Mode-locked lasers are lasers that are capable of producing intense ultra-short pulses of light at a very high rate.

Concepts

Set 1

Take a bunch of atoms, excite them and place them in a box covered with mirrors in all directions. Send in one photon, a particle of light, to intercept one of these atoms. Unable to get more excited, the atom will get de-excited by emitting the interceptor photon and another photon identical to it. Because the box is covered with mirrors, these two photons bounce off a wall and intercept two more atoms. The same thing happens, over and over. A hole in the box allows the ‘extra’ photons to escape to the outside. This light is what you would see as laser light. Of course it’s a lot more complicated than that but if you had to pare it down to the barest essentials (and simplify it to a ridiculous degree), that’s what you’d get. The excited atoms that are getting de-excited together make up the laser’s gain medium. The mirror-lined box that contains the atoms, and has a specific design and dimensions, is called the optical cavity.

Set 2

Remember wave-particle duality? And remember Young’s double-slit experiment? The photons bouncing back and forth inside the optical cavity are also waves bouncing back and forth. When two waves meet, they interfere – either constructively or destructively. When they interfere destructively, they cancel each other out. When they interfere constructively, they produce a larger wave.

A view of a simulation of a double-slit experiment with electrons (particles). The destructively interfered waves are ‘visible’ as no-waves whereas the constructively interfered waves are visible as taller waves. Credit: Alexandre Gondran/Wikimedia Commons, CC BY-SA 4.0

As thousands of waves interfere with each other, only the constructively interfered waves survive inside the optical cavity. These waves are called modes. The frequencies of the modes are together called the laser’s gain bandwidth. Physicists can design lasers with predictable modes and gain bandwidth using simple formulae. They just need to tweak the optical cavity’s design and the composition of the gain medium. For example, a laser with a helium-neon gain medium has a gain bandwidth of 1.5 GHz. A laser with a titanium-doped sapphire gain medium has a gain bandwidth of 128,000 GHz.

Set 3

Say there are two modes in a laser’s gain medium. Say they’re out of phase. Remember the sine wave? It looks like this: ∿. A wave’s phase denotes the amount of the wave-shape it has completed. The modes are the waves that survive in the laser’s optical cavity. If there are only two modes and they’re out of phase, the laser’s light output is going to be sputtering – very on-and-off. If there are thousands of modes, the output is going to be a lot better: even if they are all out of phase, their sheer number is going to keep the output intensity largely uniform.

Two sinusoidal waves offset from each other by a phase shift θ. When θ = 0º, the waves will be in phase. Credit: Peppergrower/Wikimedia Commons, CC BY-SA 3.0

But there’s another scenario in which there are many modes and the modes are all in phase. In this optical cavity, the modes would all constructively interfere with each other and produce a highly amplified wave at periodic intervals. This big wave would appear as a short-duration but intense pulse of light – and the laser producing it would be called a mode-locked laser.

Like in the previous instance, there are simple formulae to calculate how often a pulse is produced, depending on the optical cavity design and the gain medium’s properties. These formulae also show that the wider the modes’ range of frequencies – i.e. the gain bandwidth – the shorter the duration of the light pulse will be. For example, the helium-neon laser has a lower gain bandwidth, so its lowest pulse duration is around 300 picoseconds. The titanium-doped sapphire laser has a higher gain bandwidth, so its lowest pulse duration is 3.4 femtoseconds. In the former duration, light would have travelled around 9 cm; in the latter, it would have travelled only 1 µm.

Brief interlude

  • An optical cavity of the sort described above is called a Fabry-Pérot cavity. The LIGO detector used to record and study gravitational waves uses a pair of Fabry-Pérot cavities to increase the distance each beam of laser light travels inside the structure, increasing the facility’s sensitivity to a level required to be affected by gravitational waves.
  • Aside from the concepts described above, ensuring a mode-locked laser works as intended requires physicists to adjust many other parts of the device. For example, they need to control the cavity’s dispersion (if waves of different frequencies propagate differently), the laser’s linewidth (the range of frequencies in the output), the shape of the pulse, and the physical attributes of the optical cavity and the gain medium (their temperature, e.g.).

Method

How do you ‘lock’ the modes together? The two most common ways are active and passive locking. Active locking is achieved by placing a material or a device that exhibits the electro-optic effect inside the optical cavity. In such a material, its optical properties change if an electric field is applied. A popular example is the crystal lithium niobate: in the presence of an electric field, its refractive index increases, meaning light takes longer to pass through it. Remember that the farther a light wave propagates, the more its phase evolves. So a wave’s phase can be ‘adjusted’ by passing it through the crystal and then tuning the applied electric field (very simplistically speaking), to get its phase right. What actually happens is more complicated, but by repeatedly modulating the light waves inside the cavity in this manner, the phases of all the waves can be synchronised.

A lithium niobate wafer. Credit: Smithy71, CC0

Passive locking dispenses with an external modulator (like the applied electric field); instead, it encourages the light waves to get their phases in sync by repeatedly interacting with a passive object inside the cavity. A common example is a semiconductor saturable absorber, which absorbs light of low intensity and transmits light of high intensity. A related technique is Kerr-lens mode-locking, in which low- and high-intensity waves are focused at different locations inside the cavity and the high intensity waves are allowed to exit. Kerr-lens mode-locking is capable of producing extremely intense pulses of laser light.

Conclusion

Thus, we have a mode-locked laser. They have several applications. Two that are relatively easier to explain are nuclear fusion and eye surgery. While ‘nuclear fusion’ describes a singular outcome, there are many ways to get there. One is to heat electrons and ions to a high temperature and confine them using magnetic fields, encouraging them to recombine. This is called magnetic confinement. Another way is to hold a small amount of hydrogen in a very small container (technically, a hohlraum) and then compress it further using ultra-short high-intensity laser pulses. This is the inertial containment method, and it can make use of mode-locked lasers. In refractive eye surgery, doctors use a series of laser pulses, each only a few femtoseconds long, to cut a portion of the cornea during LASIK surgery.

Addendum

If your priority is the laser’s intensity over the pulse duration or the repetition rate, you could use an alternative technique called giant pulse formation (a.k.a. Q-switching). The fundamental principle is simple – sort of like holding your farts in and letting out a big one later. When the laser is first being set up, the gain medium is pumped into the optical cavity. Once it is sufficiently full, the laser will start operating. In terms of energy – remember that the atoms making up the gain medium are excited. In the giant pulse formation technique, an attenuator is placed inisde the cavity: this device prevents photons from being reflected around. As a result, the laser can’t operate even when the gain medium is more than dense enough for the laser to operate.

After a point, the pumping is stopped. Some atoms in the medium might spontaneously emit some energy and become de-excited, but by and large, the optical cavity will contain a (relatively) large amount of energy that also remains stable over time – certainly more energy than if the laser had been allowed to start earlier. Once this steady state is reached, the attenuator is quickly switched to allow photons to move around inside the cavity. Because the laser then begins with a gain medium of higher density, its first light output has very high intensity. The ‘Q’ of ‘Q-switching’ refers to the cavity’s quality factor. On the flip side, in giant pulse formation, the gain medium’s density also drops rapidly, and subsequent pulses are not so intense. This compromises the laser’s repetition rate.

What is a fusion reaction?

The Copernican
February 21, 2014

Last week, the National Ignition Facility, USA, announced that it had breached the first step in triggering a fusion reaction. But what is a fusion reaction? Here are some answers from Prof. Bora – which require prior knowledge of high-school physics and chemistry. We’ll start from their basics (with my comments in square brackets).

What is meant by a nuclear reaction?

A process in which two nuclei or a nucleus and a subatomic particle collide to produce one or more different nucleii is known as a nuclear reaction. It implies an induced change in at least in one nucleus and does not apply to any radioactive decay.

What is the difference between fission and fusion reactions?

The main difference between fusion and fission reactions is that fission is the splitting of an atom into two or more smaller ones while fusion is the fusing of two or more smaller atoms into a larger one. They are two different types of energy-releasing reactions in which energy is released from powerful atomic bonds between the particles within the nucleus.

Which elements are permitted to undergo nuclear fusion?

Technically any two light nuclei below iron [in the Periodic Table] can be used for fusion, although some nuclei are better than most others when it comes to energy production. Like in fission, the energy in fusion comes from the “mass defect” (loss in mass) due to the increase in binding energy [that holds subatomic particles inside an atom together]. The greater the change in binding energy (from lower binding energy to higher binding energy), more the mass lost, results in more output energy.

What are the steps of a nuclear fusion reaction?

To create fusion energy, extremely high temperatures (100 million degrees Celsius) are required to overcome the electrostatic force of repulsion that exists between the light nuclei, popularly known as the Coulomb’s barrier [due to the protons’ positive charges]. Fusion, therefore, can occur for any two nuclei provided the temperature, density of the plasma [the superheated soup of charged particles] and confinement durations are met.

Under what conditions will a fusion chain-reaction occur?

When, say, a deuterium (D) and tritium (T) plasma is compressed to very high density, the particles resulting from nuclear reactions give their energy mostly to D and T ions, by nuclear collisions, rather than to electrons as usual. Fusion can thus proceed as a chain reaction, without the need of thermonuclear temperatures.

What are the natural forces at play during nuclear fusion?

The gravitational forces in the stars compress matter, mostly hydrogen, up to very large densities and temperatures at the star-centers, igniting the fusion reaction. The same gravitational field balances the enormous thermal expansion forces, maintaining the thermonuclear reactions in a star, like the sun, at a controlled and steady rate.

In the laboratory, the gravitational force is replaced by magnetic forces in magnetic confinement systems whereas radiation force compresses the fuel, generating even higher pressures and temperature, and resulting in a fusion reaction in the inertial confinement systems.

What approaches have human attempts to achieve nuclear fusion taken?

Two main approaches, namely magnetic containment and inertial containment, have been attempted to achieve fusion.

In the magnetic confinement scheme, various magnetic ‘cages’ have been used, the most successful being the tokamak configuration. Here, magnetic fields are generated by electric coils. Together with the current due to charged particles in the plasma, they confine the plasma into a particular shape. It is then heated to an extremely high temperature for fusion to occur.

In the inertial confinement scheme, extremely high-power lasers are concentrated on a tiny sphere consisting of the D-T mixture, creating tremendous pressure and compression. This generates even higher pressures and temperatures, creating a conducive environment for a fusion reaction to occur.

To create fusion energy in both the schemes, the reaction must be self-sustaining.

What are the hurdles that must be overcome to operate a working nuclear fusion power plant to generate electricity?

Fusion power is in the form of fast neutrons that are released, of an energy of 14 Mev [although MeV is a unit of energy, it denotes a certain mass of the particle according to the mass-energy equivalence; to compare, a non-excited proton has an energy of 938.2 MeV]. This energy will be converted to thermal energy which then would be converted to electrical energy. Hurdles are in the form of special materials that need to be developed that are capable of withstanding extremely high heat flux in a neutron environment. Reliability of operation of fusion reactors is also a big challenge.

What kind of waste products/emissions would be produced by a fusion power plant?

All the plasma facing components are bombarded by neutrons, which will make the first layers of the metallic confinement radioactive for a short period. The confinement will be made of different materials. Efforts are being made by materials scientists to develop special-grade steel to have weaker effects struck by neutrons. All said, such irradiated components will have to be stored for at least 50 years. The extent of contamination should be reduced with the newer structural materials.

Fusion reactions are intrinsically safe as the reaction terminates itself in the event of the failure of any sub-system.

India is one of the seven countries committed to the ITER program in France. Could you tell us what its status is?

ITER project has gradually moved into construction phase. Therefore, Fusion is no more a dream but a reality. Construction at site is progressing rapidly. Various critical components are being fabricated in the seven parties through their domestic agencies.

The first plasma is expected in the end of 2020 as per the 2010 baseline. Indian industries are also involved in producing various subsystems. R&D and prototyping of many of the high tech components are progressing as per plan. India is committed to deliver its share in time.