Editor’s Note: This is a transcript of a lecture delivered by Dr. Garbald Hansen in 3025 to the faculty of the Tamar War College as part of the college’s reopening celebration. The Tamar War College was destroyed by Draconis Combine forces in the early 2900’s, reconstruction began in 3017 and finished in 3025. After delivering this lecture, Dr. Hansen was escorted offstage by LIC operatives. He never spoke publicly again.
“The ubiquitous medium laser. A model of efficiency. Weighing a single ton, with a heat index of three, damage rating of five, ton for ton, no other weapon system even comes close in terms of efficacy. But before we get to the true substance of today’s lecture, you must be familiar with a number of technical terms.
First of all, the word ‘laser’ is actually an acronym. It stands for ‘Light Amplification by Stimulated Emission of Radiation’. You have all undoubtedly seen the Gaussian beam of light emitted by the military grade lasers found on almost every BattleMech in the Inner Sphere, but today we are going to dig a little deeper into that beam and how it’s created.
‘Spatial coherence’ refers to the ability of a laser to be focused on a given area while ‘Collimation’ refers to the ability of a beam to retain its integrity over distance. ‘Temporal coherence’ refers to the ability of a laser to emit light only within a specified spectrum range. ‘Irradiance’ is measured in watt per square meter and is, effectively, the amount of energy put on a given target surface.
Every laser consists of three major components: An energy source, commonly called the pump, a gain medium, and a series of mirrors that form an optical resonator.
Let’s take a look at the Diverse Optics Type 2 medium laser, arguably the oldest laser still in production today. Only 25 years after Skobel MechWorks ushered in the age of the BattleMech with its Mackie, the Type 2’s predecessor was found on the WSP-1 Wasp when it was introduced by General Mechanics in 2464.
Only seven years later, in 2471, production of the WSP-1A Wasp began. Armed in part with the Type 2, it remains a common sight on the battlefield over 500 years later. The Diverse Optics Type 2 is currently produced by every Great House with the sole exception of House Kurita. Defiance Industries, Achernar BattleMechs, Kali Yama Weapons Industries, Irian BattleMechs, and Hellespont Industrials all manufacture the classic WSP-1A armed with this model.
It is a Diode-pumped solid-state laser, meaning the gain medium is a solid, in this case a neodymium-doped yttrium aluminum garnet crystal rod, abbreviated as Nd:YAG. ‘Diode-pumped’ refers to the energy source used to power the laser. It is a gross simplification, but you can think of these diodes as extreme versions of the common Light Emitting Diode. Electrical current, usually produced by a fusion engine, powers this pump but other power sources can be used as well. Nd:YAG lasers typically emit light with a wavelength of 1064 nm, which is in the infrared portion of the spectrum. However, other transitions ranging between 940 and 1440 nm are also possible.
Incidentally, this is also where the laser’s green color comes from. The infrared light is frequency-doubled using lithium triborate to obtain a wavelength of 532 nm, which you would recognize as ‘green’.
Which brings us to the optical cavity, also called a resonating cavity or optical resonator. As we can see here, the optical cavity contains the Nd:YAG rod, aka the gain medium. On either side of the rod are a series of mirrors. Curved mirrors are placed in tandem to form multiple confocal sections with the rest of the cavity being quasi-collimated using plane mirrors. It is important to note that one of the mirror arrays is highly reflective while the other is only partially reflective. The laser beam you see originates from this partially reflective mirror array.
There are a number of different resonator types. The exact type is determined by the focal length of the mirror and the distance between them. As stated earlier, the Diverse Optics Type 2 uses a confocal arrangement, fairly common among medium class lasers. A Concave-Convex arrangement is often used in heavier class lasers, while a hemispherical or even plane-parallel configuration can be found among the lighter lasers.
Now lets take a look at the Diverse Optics Type 18 medium laser found on both the Thunderbolt and Archer. Notice that the Nd:YAG gain medium has been replaced with a Titanium-doped sapphire rod giving it a wavelength in the 660 to 986 nm range, in this case it is tuned for 800 nm, which is in the red portion of the visible light spectrum. Again we see the confocal arrangement in the optical cavity, but notice the pump.
The Type 2 pump source uses relatively simple Double Heterostructure laser diodes consisting of layers of gallium arsenide and aluminum gallium arsenide. The Type 18 improves upon that by adding a layer of indium phosphide between the other two materials creating what is referred to as a Separate Confinement Heterostructure laser diode, a much more efficient arrangement. These SCH diodes are what provide energy to the laser system.
Take a look at the Martell, Defiance B3M, and Magna II medium lasers. Generally speaking there are 38 different BattleMech models still in production today. Of these, 15, or 40%, all use one of these model medium lasers. They can be found on the Locust, Commando, Firestarter, Cicada, Trebuchet, Shadow Hawk, Wolverine, Rifleman, Jagermech, Warhammer, Marauder, Zeus, Battlemaster, Stalker and Atlas.
And they are all almost identical to the Diverse Optics Type 18 model discussed earlier. Incidentally, the Martell medium laser was first found on the Ostwar, a primitive BattleMech developed on Terra by Ostmann Industries in 2470, although it did not enter production until almost 30 years later. This is where the controversy comes in. Given the WSP-1A was developed in 2471, one year after the Ostwar, some academics believe the Martell model is in fact the oldest continuously produced laser in the Inner Sphere, not the Diverse Optics Type 2. Unfortunately, the exact specifications of the medium laser used by the primitive WSP-1 from 2464 are lost to us, so there is no way to definitively say whether it carried a Type 2, as the subsequent model did, its prototype, or some other model all together.
Which segues perfectly into the next medium laser model, the Fuersturm-b also produced by Ostmann Industries on Terra. Found on their Ostroc, which debuted in 2511, there are a number of striking differences between it and the models currently in use. The first is the use of an advanced composite ceramic gain medium as opposed to the Nd:YAG or Titanium-doped sapphire commonly used.
But the most interesting difference is in the design of the optical cavity. While it appears to be a confocal configuration, notice the off-axis fixed insertion mirror here and the mobile pickup mirror on the other side. The causes the beam to travel in a circular zigzag path basically folding the light beam and resulting in a path-length significantly longer then that found in other models. This creates a wavelength of an astounding 1523 nm.
Also pay particular attention to the pump source. While it also uses laser diodes, the thickness of the active region in the Separate Confinement Heterostructure is so thin it acts as a quantum well. Technically speaking, the Fuersturm-b uses Quantum cascade laser diodes as its pump source. Unfortunately, between the Amaris Civil War and the First Succession War, Ostmann Industries’ sole production facility was reduced to ashes by 2777 and the secrets of the diode manufacture, as well as the composition of the ceramic gain medium, went up in smoke along with it. It is interesting to note, however, that before its destruction Ostmann Industries licensed both its Ostscout and Ostsol to Kong Interstellar, but for whatever reasons both of those models used the Tronel line of lasers as opposed to the Fuersturms.
In fact just three years earlier, in 2774 on Caph, another famous BattleMech manufacturer was reduced to rubble by the excessive violence of the First Succession War. Stormvanger Assemblies, maker of the Javelin and Cyclops BattleMechs. While the Cyclops is best known for the holographic Tacticon B-2000 battle computer and Olmstead 840 tight beam communications systems, it is the Diverse Optics Type 20 medium laser that fascinates me. It is the only Type 20 to have ever been produced.
For those of you who are not familiar with Caph, it was one of the first planets settled by the Terran Alliance, and subsequently ended up in the Alliance Core region of the Terran Hegemony. The Caph Institute of Technology was located in the original planetary capital of Brunnel, and almost every major weapons manufacturer at the time had a facility on the planet. Notably, Nirasaki Computers Collective known best for its engineering work on the control systems of the SLDF Space Defense System. New Earth Trading Company, the first true interstellar corporation, maintained a regional headquarters there, and it is rumored this is where the artificial intelligence of the Casper drone ships was designed. General Systems produced the vaunted Exterminator there, used exclusively by the Royal and Special Forces Commands of the SLDF. Lang Industries, Martinson Armaments, Skobel MechWorks, and of the course the aforementioned Stormvanger Assemblies all had extensive research and manufacturing centers here. Suffice it to say, Caph was likely the most advanced center for military research and development outside of Terra itself.
But I digress. The Type 20 is revolutionary in two ways. First, the use of an Ytterbium doped glass rod as the gain medium. But second, and most importantly, is the pump source itself. Whereas all of the previous pump sources for the lasers described up to this point have been variations of the laser diode, the Type 20 actually uses a micro laser as its pump source. Note, the idea itself is not revolutionary, but the miniaturization required to assemble this complex apparatus while keeping within the traditional weight and bulk range of medium lasers is nothing short of amazing. The result is the Type 20 has an optical power output technically placing it within the range of a heavy laser, although it lacks the effective range of its much larger cousins.
Which brings me to the classification of lasers, specifically those lasers found on BattleMechs. We are all familiar with the generic groupings of light, medium, and heavy. These classes are defined first by the power output of the laser. Generally speaking, light lasers fall between .8 – 1.5 megawatts, medium between 1.5 and 3.0 MW, and heavy lasers fall between 3.0 – 5.0 MW. The power output is then plotted over distance to make the final classification. The overall weight of the laser is also factored in.
The power output is determined by the laser’s peak power multiplied by the laser pulse duration. Let’s return to the Diverse Optics Type 18 for a moment. The Type 18 has a peak power of 2000 megajoules and a pulse duration of 1 millisecond, giving it a power output of 2.0 MW.
Now here is the Type 20 again. Remember the Type 20 uses a micro laser as its pump source, giving it a peak power of 3000 megajoules while maintaining the same pulse duration resulting in a power output of 3.0 MW.
And here is our old friend, the Fuersturm-b. Its peak power is only 1500 megajoules, however due the superior heat dissipation properties of the ceramic gain material, it can maintain a pulse duration of twice that of the either the Type 18 or Type 20, giving it also a power output of 3.0 MW.
This next one is an Omicron 3000, carried in the right arm of the omnipresent Stinger. Introduced only eight years after the WSP-1A Wasp, it was originally produced by Earthwerks Incorporated, but has since spread throughout the Successor States, including models produced by both Coventry Metal Works and Bergan Industries. This one is remarkable in that it has a peak power of 3000 megajoules, 50% greater than that of the Type 18, and fully twice that of the Fuersturm-b, but it can only maintain a pulse duration of half of a millisecond, giving it the lowest power output of the group and just barely qualifying for the medium bracket at 1.5 MW.
So, when classifying lasers, it is critical to realize that the pulse length can be very short resulting in very high peak powers with relatively low energy, i.e. the Omicron 3000, or can be very long resulting in low peak power and high energy, such as with the Fuersturm-b.
What you are looking at now is a graph. The bottom is distance measured in meters, the vertical axis is power output in megawatts, and each colored line corresponds to a specific model medium laser. You can clearly see that both the Diverse Optics Type 2 and the Omicron 3000’s power output are the first to begin to drop off. Incidentally, this is just the issue addressed in the later Omicron 4000, found on the Quickdraw. Increasing the peak power to 4000 megajoules by switching the type of laser diodes used by the pump, the Omicron 4000 delivers a power output of 2 MW. Diverse Optics addressed the same shortcoming in their Type 18 model.
Notice also the Fuersturm-b and Diverse Optics Type 20 are the best in class at maintaining power output over distance, with most of the others clustered nicely in between, with one exception.
Are there any Spider pilots out there? Well, congratulations. Among all of the medium laser models still capable of being produced by the Inner Sphere, the Abderdovey Mk III is technically at the best. Originally manufactured by Newhart Industries, their production lines were all destroyed, the last one in 2776. However, during the collapse of the Terran Hegemony, the Free Worlds League managed to obtain the blueprints for the Spider, along with the Vulcan, and charged Nimakachi Fusion Products to renew production of the two models at their plants on Lapida II and Tematagi.
The Aberdovey Mk III is remarkable for two reasons, the first having little to do with the technical specifications of the laser itself, but rather its mount. The alignment of the mirrors in the optical cavity is absolutely critical to the efficacy of the laser. As these mirror arrays drift out of alignment the power output of the laser is negatively impacted. This is often caused by excessive kinetic force, usually the result of taking damage from another weapon system, or even jumping, falling, or running on rough terrain. The battlefield is full of kinetic energy. However, the Aberdovey features integrated shock absorbers and motion dampeners to assist in maintaining the optimal mirror alignment, and hence maintain maximum performance of the entire laser assembly. In addition, special attention has been paid to the optical resonator by hermetically sealing it. This serves as an effective defense against particulates penetrating the cavity and degrading the performance of the laser by causing excessive dispersion of the Gaussian light beam and impacting the laser’s collimation. A clever design requiring significantly less maintenance than other models.
As for the laser itself, its spatial and temporal coherence are exceptional. While this model uses the common Nd:YAG gain medium and a traditional confocal optical cavity, it differs from all the others in its use of high pressure Krypton flashtubes as the pump source. While not as efficient as Xenon which is capable of converting 50% of electrical energy into light, the Krypton's spectral-line output in the near-IR range very closely matches the narrow absorption-profile of Nd:YAG. Interestingly enough, the use of Krypton flashtubes also gives the Aberdovey the shortest pulse duration of any medium laser on the market today at .33 milliseconds. However, due to its spectral purity it can operate at 6000 megajoules peak power, giving it an output of 1.98 MW. It also has an unusually high irradiance due to its relatively narrow band. A fine piece of technology.
At last count, there are 19 different models of medium lasers across the 38 BattleMech models still in production. Of these, I have only spoken about eight, the other two being out of production.
Now, before I move onto the heavy class lasers, I will open the floor for questions.”
[Question]
“All of the models I have described thus far operate in a continuous wave mode. Lasers can also be built to operate in a pulsed mode, such as the Star-League era Blakenburg medium pulse or the Defiance P5M medium pulse laser. The beginnings of this can, in fact, be seen in the Aberdovey’s use of a Krypton flashtube as a pump source. One of the easiest ways to establish pulse mode operation is to use large capacitors to strobe the Krypton flashtube. That is to say, pulse the pump source itself, and the output of the laser will be pulsed as well. There are, of course, others techniques, such as Q-switching, but they are significantly more complex.
Next question.”
[Question]
“The increased range of the Star-League era heavy laser was due in large part to the advanced gain medium used inside the optical cavity. Take the Fuerstrum-b discussed earlier. It used a composite ceramic gain medium, the production of which has been lost. However, research is already underway on a Neodymium-doped glass medium, Nd:Glass, that had the potential to remain stable under high energy conditions. In addition, the idea of using another laser as the pump source, such as we saw in the Diverse Optics Type 20, also holds great potential for significantly increasing the energy output. Consider the titanium-sapphire, Ti:Sapphire, laser. Using a light, frequency-doubled Nd:YAG laser as a pump source and a Ti:Sapphire gain medium, the theoretical energy output could potentially result in a significantly less energy loss over distance given high enough collimation.
Even more exciting is the possibility of introducing a mode-locked oscillator to the Ti:Sapphire laser.”
[Indistinct noises. The recording abruptly ends.]
