In the previous article, we explored the historical developments of the laser and the principles of laser physics. Briefly, we mentioned their wider applications that propelled industries and science into the modern world.
This modern world has seen revolutionary tools go through decades of self-transformation. A common odyssey is that of the stone-age arrow heads changing into the common chisels and machining tools we use today. The laser has seen its own adaptions and changes in the form of changing a continuous lasing radiation into a pulsed laser.
A pulsed laser, as oppose to a continuous laser beam, is a series of fragmented laser pulses that repeat cyclically. Shorter pulsed lasers with the same emitted energy will have a larger intensity (i.e. number of photons). Consequently, as matter which is being lased absorbs this energy, it will undergo a higher increase in temperature than if it was lasered with a longer duration pulse. With this increase in temperature, after the pulse is complete, the temperature decreases with the squared of the duration of the pulse across the penetration of the matter. In other words, due to the higher intensity of a pulsed laser we can apply a higher temperature to a material with a shorter penetration depth. This gives us far more precision in lasering surfaces while causing minimal thermal disturbances to regions below the surface –an advantage which is very desirable in increasing the precision of laser illumination, laser cutting, and spatial measurements.
The transmission of a laser pulse may be demonstrated by a cyclical Gaussian function:
This function, when applied to an oscillating electromagnetic field is chopped up into a pulse profile:
The width of a single wave cycle relates to the frequency (or colour) of the wave:
When a laser pulse travels through the modulator, it is chopped by a time-dependent phase function:
This consequently perturbs the frequency of the pulse, shifting the frequency by a marginally small amount, on the order of GHz from a 100THz initial pulse.
Although simply irradiating laser light has many power enhancement applications, can we do anything with the pulsed laser for communication purposes? Can we adjust this constant beam to something that can transmit information?
Some of the most useful laser-dependent technologies is not based on the laser’s ability to generate heat. Rather, it is based on the laser’s ability to transmit coherent and precisely directed signals.
Light, visible light which we can see, is part of the Electromagnetic Spectrum which also compromises radio waves. Radio waves transmit signals of light at a specific frequency at varying amplitudes, which are received by a receiver that then in turn is converted into electrical signals which we can process to a speaker.
Lasers can operate in the same way, where the amplitude of the signal can fluctuate in the form of pulses. Pulsed lasers can send bits and blanks of information which are encoded in a laser’s fluctuation in intensity at the transmitter.
For example, pulsed lasers can send bits and blanks of information which are encoded in a laser’s fluctuation in intensity at the receiver. We’ll discuss later how pulsed lasers work and the benefits of sending visible EM radiation as oppose to radio waves in communication.
LiDAR (Light Detection and Ranging) implements the transmission and receiving of light into directional scanning technology. By computing the time delay between sending a laser signal and receiving the same signal from a reflected object, the distance to the object can be determined. In the LiDAR’s surrounding, since the speed of light is constant, the distance between the detector and the object may be computed, thus building a comprehensive 3D map.
To generate a pulsed laser, we have to adjust our resonator by introducing a attenuator mechanism that switches the transmission of the laser beam through the output mirror as a function of time. This is known as gain switching and Q-switching.
Gain Switching
How are the pulses generated in a laser? When a laser is pumped with a constant power supply, the population inverse of the media increases to its maximum threshold. This induces an initial pulse of laser light to be emitted. This initial pulse is so strong that this stimulated emission of radiation drains the population inversion. As the power is constantly supplying energy to the gain medium, the population increases yet again to the threshold limit, before releasing another pulse of photons, and draining the population inversion, and so on in an oscillating behaviour. As the power is continued to be supplied to the media, the population inversion and the photon density stabilises to a constant value. This constant value continues until the resonator switches the power off, thus completing the pulse.
Q-switching
To physically switch the laser between pulses, an operation called Q-switching is used. Before the pulse escapes from the laser, an attenuator is placed between the output mirror and the gain medium. To physically determine the laser’s operation between Q-switches, we use a value known as the quality factor: a ratio between the energy stored and the dissipated energy per cycle as the laser radiation travels back and forth between mirrors. The attenuator allows the build up of stored energy in the resonator. Once the attenuator is removed, the loss of energy per cycle is minimised, increasing the quality factor and releasing a high photon density during the pulse. Once the optical losses are immediately increased, the quality of the resonator will decrease, leading to no emission of photons and the pump power will rather contribute to the build up of population inversion.
Types of Q-switches
Saturable absorber
Saturable absorbers are the most desired forms of attenuators due to the low point of failure. The attenuator mechanism is intrinsic to the absorber’s material and also has minimal points of failure as it does not require any mechanical adjustments.
Punched disk
The punched disk is a circular series of holes on a disk. When the disk rotates and transmits light through the resonator, emission is created. The design of the disk can be modified to alter the profile of the pulse. The rotational speed of the disk may also be adjusted to change the rate of the pulse.
Acousto-Optical Modulator
Some materials have their lattice structure modified sending a resonant wave through the material. This wave is induced by a piezo-electric mechanism which turns the material, often a quartz solid, into a grating. Incoming laser light from the resonator is deflected, preventing it from further spontaneous emission out with the gain medium. This system is very desirable since it can be modulated by an alternating current, allowing the pulses to be controlled based on generated electrical frequencies.
Electro-optical Modulator
The function of an EOM is primarily dependent on the central Pockel cell. A Pockel cell uses an electric field to apply electro-optical effects to fix the axis of polarisation. When polarised laser light is sent through an actively charged Pockel cell, an electric field is created within the cell and the polarisation of the light is rotated by 90° by a circular polarisation effect. An output polariser rotated perpendicular to the output will subsequently block the polarised laser beam. To allow transmission of the light, the electric field can simply be turned off. Just as in the case of the AOM, the EOM may be controlled using an alternating high voltage.
Pulsed lasers have revolutionized both fundamental science and practical applications, offering unprecedented control over light-matter interactions. Innovation and technological development has reshaped our tools in society, and pulsed lasers have been part of that reshaping. I have no doubt that pulsed lasers will continue to reshape our lives as we move forward into the future.
