Spectrum Analyzer for Other IR Lasers
The wavelength scale is viewed from the rear through an adjustable opening located on the top of the cover. This method of viewing insures a uniform level of UV illumination on the fluorescent display surface without interference from room lights. The instrument can also be operated with the case entirely removed for wide angle viewing.
The Model 16A was designed primarily for CO2 lasers (12C 16O2 isotope). However this model can also be used with several other lasers. For example, the NO2 laser emits at wavelengths within the design range of this instrument. A portion of the carbon monoxide laser spectrum can also be displayed since the second order of the diffraction grating covers a spectral range between 4.55 microns and 5.65 microns. Even the 1.06 micron region can be displayed using the 10th order of the grating.
Spectrum Analyzers are also available for monitoring the spectral range of any other IR laser which emits more than 10 milliwatts of power. Spectrum Analyzers for CO, HF and DF lasers are standard (see appropriate data sheet). Analyzers for N2O, CS2, isotopic CO2 and broad band wavelength coverage are available on request.
The CO2 Spectrum Analyzer uses a unique folded optical design which allows a 7 1/2 foot path length to be contained in an instrument only 18" long. the above picture demonstrates the unobstructed portion of the optical path using a He-Ne laser in a partially assembled Analyzer.
The Analyzer can be used as a monochrometer with external detectors mounted to monitor individual lines. The display screen is removed to allow the light to pass onto the detector. The removal of the thermal sensitive screen does not effect the calibration of the instrument.
The large number of wavelengths obtainable from CO2 lasers arise from rotational line splitting of three vibrational transitions 0001 - -1000, 0001 - 0200, (0111 - 1110). The collision coupling of these levels is sufficiently rapid that a continuous single mode CO2 laser will lase at only one length at a time. The laser, however, will usually jump from one rotational line to another at the slightest variation of cavity length unless it is equipped with some wavelength selection device.
It is quite common for CO2 lasers to sequentially lase at 5 or 10 rotational lines as the cavity length is changed approximately 1/2 wavelength. Laser action normally occurs in the high gain lines near 10.6, 10.2, and 9.6 microns, but variations in reflectivity due to either dielectric coatings or Fabry Perot resonances at a window may broaden this range. Multi-mode lasers can oscillate at more than one rotational line at a time, but only by having different rotational lines oscillate in different volumes of the laser discharge.
The ability of CO2 to lase at a number of wavelengths offers the flexibility of being able to tune the laser over a 20% wavelength interval. However, for many experiments multiple wavelengths can present problems. For example, when attempting to heterodyne two lasers, they both must be oscillating in the same rotational line to detect the beat. Also, lasers which jump between rotational lines can emit nearly constant power, but the wavelength variations can become amplitude variations if the beam is passed through windows or beam splitters with Fabry-Perot resonances.
In the past, the only way of monitoring rotational lines was to use IR scanning spectrometers which were designed for other applications. These instruments are not only too large to be easily introduced into laser systems, but they simply do not give an accurate representation of the laser's behavior.
Typically, high resolution wavelength scans can take many seconds. During this period, the laser may have changed its wavelength several times. Since the wavelength changes are discontinuous, it's possible for scanning spectrometers to completely miss detecting any of the wavelengths. On other scans, it may catch an unstable laser wavelength several times and give the impression these lines are running simultaneously.
Macken Instruments CO2 Spectrum Analyzer displays all the lines simultaneously, giving an accurate representation of the complete spectrum. The use of a thermal sensitive display screen permits parallel processing of wavelength information. If a person attempted to replace the thermal sensitive screen with a non-scanning detector array, it would take 1,000 detectors to obtain the same wavelength resolution!
The thermal sensitive phosphor offers the additional advantage of displaying spatial wavelength variations along the height of the slit. This useful feature often reveals the upper portion of a multi-mode beam lasing at one wavelength while the lower portion is lasing at a different wavelength.
The wavelength of a CO2 laser is a parameter every bit as important as output power or mode structure. Lack of wavelength information can affect the accuracy and repeatability of laser tests. Macken Instruments' Model 16A fills a need for an instrument which gives easy access to this fundamental information.