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  | Frequently asked questions about... |
 | Optics | |
| Lasers | |
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| I have an older Melles Griot
Laser, part number 05 LHP 151 that I need to replace, but I can't find the same
part number in the new catalog. Is it still a current product? Yes it is. In our previous catalogs, we sold the laser heads and the power supplies separately, whereas in our new catalog we have grouped them together to be sold as systems. We found that most people ordered both the laser head and its appropriate power supply on the same orders, so we decided to sell them as a system to make it easier on the consumer. The 05LHP151 is the part number for just the laser head while the new system part number 25 LHP 151-249 designates the same laser head with its corresponding US power supply. We understand that our customers may only need to buy just the laser head or just the power supply, so we still sell them individually. Commonly, we can still supply a laser head or power supply, but just haven't listed it in the catalog or on the website. This particularly applies to our older components. Most of the time these are still available and can be purchased by calling us directly. Simply contact the Melles Griot's Applications Engineering department to find the appropriate parts: techsupport@catalog.mellesgriot.com. or 1-800-835-2626. |
| What is the difference between the two stabilized lasers that you offer?
This is a common question. The first obvious reason is the size. As you will notice one is much smaller than the other. The smaller unit can be operated from an AC or DC input lab- or modular-style power supply as well, which makes ideal for OEM systems. One very important difference is noted on page 40.14 in Catalog X. The larger unit (25 STP 901, 903 and 905) can be set to use either a "frequency-stabilized mode" (Comparison Method) or an "intensity-stabilized mode" (Slope Method) whereas the compact unit (25 STP 910 and 912) uses only the "intensity-stabilized mode" (Slope Method). The difference between the two methods is the way each laser monitors the beam. In the Comparison Method the laser monitors the intensity of two individual modes under the gain curve whereas in the Slope Method the intensity of the entire Gain Curve is monitored. Typically, the Comparison Method works best when the application requires no change in frequency for the long term (months). Many customers have noted that the Comparison Method is able to lock into the same exact frequency under the gain curve every time and remain there, whereas the Slope Method may not lock into the same exact frequencies under the gain curve every time, but will maintain the intensity of the beam by maintaining the lock in that single frequency. |
| What is the coherence length of a Helium Neon laser?
Coherence length is defined as the length over which energy in two separate waves remains constant. With respect to the laser, it is the greatest distance between two arms of an interferometric system for which sufficient interferometric effects can be obtained: Lc = c / Dn L Coherence length will vary from laser to laser as a function of the Doppler broadened gain width; however, for a HeNe 20 - 30 cm is typical. |
| What is the Doppler broadened gain width of a Helium Neon laser?
Helium Neon lasers can range from 800 MHz to 1575 MHz full-width-at-half-maximum (FWHM) depending on the design. The typical 632.8nm HeNe is 1400 MHz. The width of a single mode located under the gain curve is typically 1 MHz. |
| Can a Helium Neon laser be re-gassed?
While re-gassing can provide some extension of the output performance in some gas lasers like the CO2, Argon and the higher powered side arm HeNes (which have external optics), it is not recommended or provided for smaller internal mirror coaxial tubes. Typical end-of-life failure for a HeNe is cathode sputtering. This occurs when the protective oxide layer on the cathode is expended through continuous bombardment by the laser discharge. There is no cost effective way of regenerating this layer. When the oxide layer is expended, the discharge itself vaporizes the "raw" aluminum and deposits this material, in its vapor state, on other surfaces such as the optics and the glass bore. |
| What agency approvals are available for the Melles Griot Helium Neon lasers?
Most of our HeNe laser heads are CDRH / CE certified. Certified product for CDRH falls under one of the following classifications: Class II (<1.0 mW), Class IIIa (<5.0 mW) or Class IIIb (<575mW). For CE our lasers are certified as a class 2 or 3B. Plasma tubes including some laser heads are not certified. In this case, it is the responsibility of the end-user to certify their system by meeting agency approvals. If CDRH / CE certification is a requirement for your system, be sure to verify the laser you select meets your needs. |
| Which power supply can I use to operate my Helium Neon laser?
All Melles Griot HeNe lasers have a recommended Laboratory or Module power supply (AC and DC input versions available), typically listed on the laser's specifications sheet. If you are unsure which one is appropriate, please call us at 1-800-835-2626 and ask for a Laser Applications Engineer. Rule of thumb in choosing a power supply: If you know the operating voltage and current of your laser, the power supply must have a voltage range large enough to fit the operating voltage for the laser and must be set at the appropriate operating current. |
