Access Laser’s temperature-stabilized lasers make use of special materials such as INVAR, which have a low coefficient of thermal expansion to minimize variations of the resonator length. By combining this material improvement with a temperature feedback control system, we ensure excellent performance.
Typical CO2 lasers will exhibit “line-hopping” when the optical output shifts from one molecular transition to another due to thermal expansion of the resonator. The performance impact from this is not only instability in the produced wavelength, but in the measured power. However, we can control this typically uncontrolled variation and leverage the small changes to intentionally select the wavelength produced in our compact lasers, such as the L3, and L4.
An important caveat for utilizing this technique is that the laser must reach thermal equilibrium while RF power is being applied. This warm-up period typically requires 30 minutes of operation. The wavelength-vs-temperature table of information, provided by Access Laser’s final test technicians, are recorded after such a process. The values in these tables represent the temperature ranges for particular wavelengths. The center value typically provides the most stable operation at any such line. This approach to wavelength selection and power stability is based upon maintaining control of the resonator length by expanding the resonator structure. Therefore, the number of wavelengths that can be selected is higher for a shorter laser, such as the L3, when the relative change in length is greater than those of longer lasers, such as the AL50. Following these steps, and properly implementing the temperature stability technology, enables single longitudinal mode and single transverse mode operation at the desired wavelength.
Due to the limited responsiveness of the fan and water cooling approaches implemented, it’s not practical to stay on one spectral line, which has a very narrow range of temperature (on the order of 0.5oC). Typical applications for temperature stabilized (-S) lasers require one stable line near the center of the wavelength band (i.e. P20 or R20). Temperature stabilized (-S) models offer the best spectral purity and frequency stability when water cooled with a good chiller (water temperature control of +0.1oC). Furthermore, lasers with this technology have an instantaneous linewidth less than 100kHz and a frequency drift less than 5MHz per minute. To further improve the average power stability, and enhance frequency stability, the Line Tracker (-T) technology is available for all –S models.
Wavelength table for a typical model L3S, 400 mW laser, 14 lines
Wavelength table of a typical model L4S, 1 W laser, 11 lines
The Wavelength-vs-Temperature Table
The Wavelength-vs-Temperature table, provided with most –S lasers, is only for reference. In practice, you may not achieve the desired line at the given temperature. It depends upon the ambient conditions in which you operate the laser. The table is recorded during a temperature ramp-up immediately following turn-on. Once the laser stabilizes, its temperature range may shift. To achieve a particular wavelength, you will need to find out exactly where that line is in your own ambient working environment, by experiment. If the laser is equipped with a piezo actuator (-Z), then the Wavelength-vs-Piezo Voltage table is recorded when the piezo is set to a single ramp-up after the laser has stabilized at a particular room’s temperature.
The most important and accurate information from a Wavelength-vs-Temperature table is the relative range of spectral lines. The wider this range, the more stable the laser will be when operating at a specific line. The range can be temperature or piezo voltage, depending on the specific laser model and options. Another piece of highly useful information is the sequence of spectral lines. Most spectral lines will appear in the sequence as recorded during the temperature or piezo voltage ramp-up.
Example of Using the Wavelength-vs-Temperature Table
Looking at the above line table of a Merit-S, we can see that the 10P20 transition has the widest temperature range— from 82.3˚ to 87.5˚ F. Therefore, this line would provide the highest level of stability, if selected. Suppose we’d like to produce the 10P20 wavelength by setting the laser body temperature at 84.9 degrees, which should be the middle of the range for 10P20, but instead we measure the 10P22 wavelength as produced. The Wavelength-vs-Temperature table shows that 10P22 at a lower temperature than 10P20, so we should increase the temperature setting to produce 10P20.