Continuous Power, Average Power, Peak Power and the Limiting Parameters for Ruby Laser Applications


The first laser in history was a ruby laser. Ruby lasers were powerful and they could vaporize holes in the hardest substances known to man. In the years that followed the development of those early ruby lasers however, more efficient laser types were discovered.

Laser technology has come a long way since the first laser was built in 1960. This page briefly provides some insight into what ultimately limits the ruby laser, as well as what differentiates pulsed lasers from continuous output (CW) lasers.

Advantages of Ruby:

Disadvantages of Ruby:


Ruby makes a great hobby laser: it has a huge gain, a high energy output, a visible beam and it can be pumped using xenon lamps and electrolytic capacitors. It can be a very simple laser as long as the user is content with pulses at a very low repetition rate. But that's where the advantages end - in order to operate the laser at a high repetition rate (such as those ruby laser types that are used to make welds or remove tattoos), it becomes necessary to remove a huge amount of waste heat. Aside from the power requirements and catastrophic damage that would result in the absence of presumably involved, complex or expensive cooling measures, the laser output from ruby diminishes when the rod becomes warm.

It is possible to obtain a continuous output from a ruby laser. It was done in 1962 using a mercury arc lamp and in more recent times, using diode laser pumping. Technically speaking, the 1962 version was not a true CW laser as it produced "a continuous train of pulses", undoubtedly owing to the nature of ruby as a relaxation oscillator (600kHz?). Regarding the much more recent work with continuous diode laser pumping of ruby, I cannot say for sure whether the results were truly continuous or if they were pulsed in the same manner that occurred in the early 60s.

As a side note regarding continuous lasing of ruby, if these pulses and spikes are indeed one in the same, perhaps lasing does not entirely cease between pulses (judging from my interpretation of the word "spike"), thereby maintaining coherence across all spikes or pulses. I was under the impression that q-switched ruby lasers were more coherent than free running ruby lasers, because the phase changed between the spiked pulses. I can only speculate however, as these details extend beyond the scope of what I have explored as of this writing.

In any case, practical ruby lasers are pulsed and are pumped with high energy capacitors and a flashlamp. As such, they are valued for the powerful high energy laser pulses that they provide. CW ruby lasers have a limited output power at a wavelength (694.3nm) that is less than 1/160th as bright as the wavelength of peak sensitivity of the human eye (555nm) or 1/40th as bright as the 632.8nm wavelength of the helium neon gas laser. Unless future technology enables a substantial improvement in CW ruby laser performance, it is likely that the CW ruby laser will be of only specialized or limited use.

One of the most common solid state laser materials is Nd:YAG. Nd:YAG is a 4 level laser that can be run CW or quasi-CW (pulsed at a high repetition rate). This makes it suitable for cutting, as opposed to merely drilling holes, because the repetition rate can be high enough to make continuous cuts, but differences in peak power and average power enable it to perform in ways that are not possible with a CW laser of the same continuous output power.



Assuming I didn't make any careless errors (feel free to check my calculations), one can quickly see why ruby is totally inadequate for cutting. In order to cut, one must have a high repetition rate (my guess would be that even 1000 pulses per second, which was just an example, would result in slow cuts). Drilling is another matter, as a ruby can store high energy resulting in very powerful pulses (especially when q-switched). Ruby has long since been replaced by much more efficient laser types, such as Nd:YAG. Ruby is great for a hobby laser but if one is considering a laser to make actual cuts, one will need a powerful continuous laser output or a pulsed laser output with high peak power at a reasonably high repetition rate. Because of the kind of work that is performed by a laser when it is used to drill or cut, peak power rather than the average power is the basis for the work performed by a pulsed laser. In my view, this would seem to suggest that pulsed lasers are of greater efficiency than continuous lasers in cutting applications (assuming an equal wall plug efficiency).