Late-1960s Philips SPI 1000W

The highly loaded mercury arc lamps (SP and CS) introduced by Philips in 1935 proved very useful in technical and scientific applications. For four decades these remained lamps of choice whenever an intense light source with a very high brightness was needed for the projection of light and images, for photochemistry, and for other special applications. It is therefore only natural that these lamps were tested for the optical pumping of the ruby laser shortly after its invention in 1960. Experiments at Bell Labs (USA) following this particular approach permitted the first continuous output operation of the laser (W. Boyle, 1962).

The key in properly exciting the trivalent chromium ions present in the synthetic ruby material is to deliver a large amount of optical power in the visible domain (the spectral range of strongest optical absorption from the ions) so as to cause a population inversion, i.e., a situation where the density of excited ions become larger that that of ions at lower energy states. This condition is needed in order to obtain the emission and amplification of coherent light (i.e., photons with all the same characteristics in terms of frequency, phase and polarization), also known as LASER operation. The process of creating this population inversion is called optical pumping in this particular case, and there are many different ways to create a population inversion though. With a strong continuous optical output in the visible domain, the mercury arc lamp was thus an interesting light source for the optical pumping of ruby.

Because a significant part of the mercury lamp’s input power is lost in the production of ultraviolet radiation (useless to the ruby laser), these were not the most efficient light sources for this particular application. High-power xenon flash tubes proved more effective because of their much higher optical power output. In fact, T. Maiman used an helical xenon flash lamp in his first laser in 1960, but such method of excitation was limited by a pulsed mode of operation with a long duty cycle. Interests towards continuous laser operation thus made the use of the less effective mercury arc lamps the focus of research and development efforts.

At that time, during the early 1960s, an interesting innovation was being applied to mercury discharge lamps for general and stage & studio lighting applications: metal halide additives. It did not take long for lamp engineers to figure out that salt additives could also benefit the production of light for laser excitation. In late 1962, A. Timmermans and T. Tol from Philips (Eindhoven, the Netherlands) began experimenting with mercury arc lamps provided with metal halide additives for the optical pumping of ruby lasers. More specifically, they used the company’s SPP and SP mercury capillary arc lamps as development platforms because of their long and thin arc which simplified the optical coupling between the lamp(s) and the laser crystal. Their experiments showed that the optical power output at 694.3 nm of the lasers increased by 50 % when thallium is added to the mercury arc.

As a result, they developed modified SPP and SP lamps with a higher power density and with a dose of thallium iodide, which became the SPPI and SPI 1000W, the latter being shown here. Around the mid-1960s, Philips went on to set-up a very small production of these highly specialized sources in the company’s lamp factory in Emmasingel, Eindhoven, for applications requiring a more efficient continuous lasing operation of the ruby laser. This particular technology remained used until the early 1970s, when the more efficient Nd:YAG laser rose to prominence in technical and industrial applications. Since this particular type of laser is optically pumped in the near infrared, a more effective excitations source was then developed in the form of the krypton capillary arc lamp, which is suitable for both continuous and pulsed operation.