Blue Laser Power Beaming for Planetary Exploration

A blue laser-based, remote power beaming system may enable practical power distribution for explorers on the surfaces of the Moon and Mars.

Any plan for the long-term exploration or habitation of the Moon and Mars will almost certainly entail the use of multiple habitats, vehicles and remotely located equipment, all of which will require power sources.

While the specifics of how multiple remote locations or mobile instruments might be powered depend on particulars of the distances, surface topography, and power requirements involved, the most efficient and practical way to power anything beyond the smallest instruments at remote locations will most probably mean setting up a central power source and delivering it using some mode of distribution network. This process will eliminate the need to transport bulky solar cells or other energy sources to remote locations or mount them on vehicles.

Figure 1. Simplified schematic of a laser-based power beaming system. Input energy, in this case generated by solar cells, is converted into laser light. The laser beam is expanded to minimize its spread of the propagation distance. Another solar cell on the receiving end converts the laser light back to electricity.

On Earth, the simple solution to getting power to all but the most remote or inaccessible locations is a wired distribution network. However, for the Moon and Mars, NASA deems this approach impractical due to exorbitant launch costs. To illustrate, transporting a 1 km spool of 12-gauge aluminum wire to the Moon costs $19,800,000. For copper wire (a superior conductor), the cost skyrockets to over $50,000,000. Power beaming offers a realistic alternative.

Power Beaming Basics

Power beaming is a form of wireless power transfer that is conceptually simple, as it can be accomplished using many different wavelengths along the electromagnetic spectrum.

With visible and near infrared wavelengths, lasers are the source of choice for power beaming because of their unique characteristics. In particular, the small apparent source size and high brightness of a laser enable a beam containing significant optical power to be highly collimated and projected over long distances with little beam spread.

Figure 1 shows the basic elements of a system of this type. A laser draws electrical power from a source and converts it into light. This light is expanded and collimated to a diameter that spreads minimally over the chosen propagation distance. At the receiving end, an optical detector converts the laser light back to electricity.

Figure 2. Lower diffraction at shorter wavelengths makes it easier to deliver a smaller beam at a longer distance with a blue laser.

The advantage of this type of power beaming is that the laser beam can provide substantially higher power density than the solar radiation reaching the surface of the Moon or Mars (especially the latter). This increased power density reduces the required size of the receiver panels by as much as 100X. For example, generating 500 W of power with sunlight might require about 2 m² of solar panel area. A laser system can deliver enough light to yield the same power with a solar panel area of only 0.02 m². This provides interplanetary explorers with a much more portable system.

Power beaming at visible and near infrared wavelengths isn’t very practical on the Earth’s surface due to our atmosphere. Constant air movement creates local refractive index changes which distort the wavefront of the beam. This makes it difficult to get high overall efficiency (the ratio of received optical power to energy in) for a power beaming system without the addition of expensive adaptive optics systems.

The complete lack of atmosphere on the Moon, and the relatively thin Martian atmosphere, present much more ideal environments for power beaming at optical wavelengths. In particular, there is almost no loss and no beam distortion when propagating over even multi-kilometer distances on the Moon.

NASA is aware of these factors and is actively pursuing power beaming technology. In 2023, they awarded NUBURU a Small Business Innovation Research (SBIR) grant to investigate the feasibility of laser power beaming for space-based and off-planet uses.

Our efforts for this grant are focused on implementing power beaming using the high-brightness blue laser technology we have developed primarily for industrial materials processing applications. Numerous high-power, near-infrared lasers are already commercially available that could be readily adapted to a power beaming system, but a blue laser source could reduce the size, weight, and cost of the transmitter and receiver components needed to for a 5 km range link as compared to near infrared lasers. There are several factors that contribute to this, including optimized propagation, high electrical efficiency and visible beacons among others.

Optimized Propagation

A primary reason is that blue light is the shortest of the visible wavelengths. This minimizes the spreading of the beam due to diffraction as it propagates. This permits a smaller beam (with a higher power density) to be delivered to the receiver with the minimal diameter beam expander.

The drawing in figure 3 illustrates the comparison of diffractive beam spreading for a 450 nm laser as compared to a 1,060 nm (near infrared) laser. Using a 50 mm diameter transmitter, the minimum beam spread at 5 km for the blue laser is achieved by forming a beam waist at 2.2 km. This produces a final beam diameter of 57 mm at 5 km; at 6 km, the blue beam has only spread to 69 mm in diameter.

Figure 3. Optimum transmitter size as a function of range for a fixed detector size for blue and near infrared lasers.

Using the same 50 mm diameter transmitter, the minimum beam spread at 5 km for the near infrared laser is achieved by forming a beam waist at 0.6 km. This produces a final beam diameter of 136 mm at 5 km; at 6 km, the near infrared beam has enlarged to 162 mm in diameter. That means a larger, heavier solar panel is required on the receiving end to collect all the light. Or, the beam can be allowed to overfill the detector, which results in throwing power away.

We can also look at the system the other way – namely constrain the size of the detector and calculate the diameter of the transmitter required not to overfill it. This comparison is shown in the chart. Clearly, as range increases, a power beaming system based on a near infrared laser becomes impractically large.

Tabel 1. A table comparing the theoretical internal quantum efficiency ratings of bandage materials.

High Electrical Efficiency

A power beaming system based on blue lasers also has the potential to deliver extraordinarily elevated electrical efficiency. Specifically, this means the conversion efficiency of the received light back into electricity.

The table shows the conversion efficiency of several types of solar cells at 450 nm. For some of the most popular solar cells in current use, conversion efficiency isn’t that high in the blue. This is especially true of the c-Si cell, which is the most employed material for commercial photovoltaic systems.

But there is a straightforward path, based completely on existing technology, that can yield solar cells with much higher efficiency. In fact, the top layers of many multi-junction cells (typically InGaP, AlGaInP or AlGaAs) are already moderately well-matched to the blue. But a multi-junction cell isn’t inherently optimum for use with a single wavelength source.

However, there’s no technical reason why a single junction cell can’t be fabricated from one of these higher bandgap materials, like AlP. This delivers a theoretical internal quantum efficiency of over 90 percent, as compared to other materials outlined in table 1. In practice, losses in the coatings and beam delivery optics might yield a real-world efficiency for the entire optical part of the power beaming system of about 85 percent.

Visible Beacon

Another NASA requirement also favors blue lasers (or any visible wavelength) over an infrared source. Specifically, NASA prefers a power beaming system to double as a navigational beacon, as this could aid explorers in finding their way from point-to-point should other navigational aids fail (NASA likes redundancy!).

Rayleigh scattering from CO₂ in the Martian atmosphere should be sufficient to make a multi-kW blue laser beam easily visible to the eye, but even on the airless Moon, solar radiation ionizes regolith particles causing the smallest of them to become suspended above the surface at heights of up to 10 m. Mie scattering from these particles would make a blue laser beam readily visible against the jet-black lunar “sky.”

Future Work

Work to date at NUBURU on the NASA SBIR actively involves the characterizing of the current-voltage (I-V) curves of solar cells and panels under laser illumination and building a power beaming demonstration system. Testing this system, which will be capable of transmitting power over relatively short ranges (15 – 30 m), is the next step in our work.

Additionally, NUBURU continues to work on increasing source brightness of our blue lasers – that is, making lasers with higher output power and a lower beam parameter product (BPP). These types of sources will expand the range of industrial processing applications we can service and are essential to building efficient, longer range power beaming systems, as well.

This article was written by Adam Paricio-Moreau, Research & Development Manager, NUBURU. For more information, visit here .