Lasers Frequently Asked Questions

Beam expanders, whether Galilean or Keplerian style, can be used in reverse. There are however several factors to consider before doing so. The first factor is alignment. The exiting aperture will now be much smaller than the entrance aperture, making any tilt in the alignment pronounced. The second factor is clipping. All laser beam expanders will have a limit to the acceptance angle of the smaller optic. For example, in the case of our Fixed Power HeNe Beam Expanders, the largest beam that can exit the system is 27mm. Thus when used in reverse, the largest beam diameter that can be accepted is 27mm. This condition applies to any expander system and should be supplied by the expander manufacturer. The third factor to consider is angle divergence. Although the beam is reduced by a magnification such as 10X, the divergence of the initial beam will increase by the same magnitude. Thus a 5mm diameter beam with 1mrad divergence will become a 0.5mm diameter beam with a 10mrad divergence. In cases where a laser or other light source must be kept as collimated as possible, the increase in divergence may be unacceptable. Fourth factor to consider is the increase in power density. Unless the optics in the laser beam expander are damage threshold tested, there is no guarantee that damage will not occur when reducing a mid or high power beam. If the laser beam expander is designed for high power density lasers, the damage threshold specification will be listed. For more information, please visit Beam Expanders.

The Modes or Mode Spacing of a laser beam refer to the quality of the laser beam. The Transverse Electromagnetic Mode (or TEM) describes the pattern that the laser spot makes. A TEM₀₀ means a single laser spot (or beam cross-section) and it is typically used to specify how much of the laser output is in this mode. Due to its symmetrical beam intensity, this mode is typically selected. The total spectral bandwidth (sometimes referred to as the linewidth) is determined by multiplying the mode spacing value by (the number of modes minus one). For example, a He-Ne laser with a longitudinal mode spacing value of 730 MHz with 3 modes will have a total spectral bandwidth of 1.460 GHz. This defines the frequency range that most of the energy is distributed over.

Our Helium-Neon lasers are typically available as either linearly polarized (to a stated ratio) or randomly polarized. Randomly polarized lasers are not "unpolarized". When the laser is first started, the polarization shifting follows a sinusoidal pattern but becomes randomized after warm-up as the effective length of the cavity lengthens and is stabilized. The axis is random, but the polarization state is always linear. For linearly polarized lasers, the ratio represents the amount of light in one polarization axis compared to an orthogonal axis. The plane polarization "E" vector orientation for horizontally positioned rectangular-housed lasers is vertical and for cylindrical-housed lasers it is also vertical, if the laser head is rotated such that the power cable is positioned at 270° (6 o'clock).

For low-end laser devices where the beam quality and divergence angle are not as important, you can often get away with a single aspherical lens. But whenever circularization, divergence, and confocality are essential, cross cylinders are generally the most cost-effective option.

Strictly speaking, there are optical designs which enable fiber coupling without first collimating the output of the diode laser, but all of these designs suffer from massive astigmatism which can significantly reduce the efficacy of the fiber coupling. Having a collimated beam inside of the laser package allows for the addition of optical elements such as micro-optical isolators and bandpass filters.

Laser Diode Modules are the simplest commercial packaging for laser diodes. They consist of the actual diode, a photodiode for monitoring output, and the can that holds them. Unfortunately, there is no single “best” lens to focus or collimate their output. Lasers diodes pose special challenges because their outputs are irregularly shaped (elliptical rather than circular) and vary widely amongst the on the market. It is difficult to compensate for these factors with a single lens and therefore the best lens will depend on the specifications of one’s laser diode (e.g. the beam divergence in each axis) as well as the desired beam characteristics (e.g. desired spot size and distance). In many cases, the desired results can only be obtained with multiple elements. Luckily, if the main concern is beam quality without the need for a custom multiple-element solution, Edmund Optics® offers many Laser Diode Modules and other packaged diode lasers that come with the diode and integrated optics to maximize the quality of the beam.

Operating the diode at the low end of the temperature range can extend its lifetime. High operating temperatures are often the reason for laser failure. The lifetime of a typical module will be cut in half for every 8°C increase in temperature. Note the specific operating and storage temperature ranges for each specific laser diode module. Power supplies should also be selected to run at the lowest voltage value in order to extend lifetime. Heat sinks are recommended in order to transfer heat away from the laser diode module and must be used if operating near maximum voltages. The heat sink should be mounted toward the front of the module (this is the location of the diode) and a 1-inch outer diameter sleeve is typically sufficient. Our laser diode pivot mount offers both heat sinking and alignment features. Handling can also effect lifetime and performance. Although our laser diode modules offer varying degrees of ESD protection (Electro-Static Discharge), high-energy discharges or energy spikes can cause permanent damage. Diode modules typically have a lifetime of 10-20,000 hours compared to the typical 100,000 hours of the raw diode component. Please note that due to the construction of green diode-pumped solid-state laser modules, they are extremely sensitive to temperature.

Unless specifically labeled as "modulated", our laser diode modules are continuous wave (CW) and cannot be modulated due to an internal feedback loop that reads the laser as "on" for a frequency higher than 40 Hz. Since this is rather slow, it doesn't make modulation practical. Modulated units are specifically designed for faster "on-off" applications. For specific available modulation frequency ranges, please review our various series of Modulatable Laser Diode Modules.

Almost all our laser diode modules are preset internally to accept only a certain amount of voltage in order to operate the laser. "Power variable" lasers however typically operate on a 3-wire lead principle, where the additional lead can be used to adjust the internal potentiometer by varying the voltage. The other two electrical leads are the common positive (red) and negative (black) used for operating power. The case housing is typically set to a positive potential, which should be taken into consideration in order to properly ground the housing and not damage the laser. Power supplies should be selected to run at the lowest voltage value in order to extend lifetime. Heat sinks are recommended and must be used if operating near maximum voltages.

Yes, laser diodes are linearly polarized. Diodes are polarized parallel to the short axis for an elliptical beam. However, the polarization ratio is difficult to specify since it varies with the type of diode. For example, a typical 5mW diode can have a ratio of 400:1 or 500:1 at an NA of 0.4 and a temperature of 25°C. In general, as the optical output power increases, the ratio increases.

The major difference stems from the fact that laser pointers are meant for a consumer market and laser diodes are meant for an industrial/technical market. Laser pointers are designed to be low cost and are manufactured in high volume. Factors such as lifetime, beam quality, and ruggedness of design are not primary considerations in the design of a laser pointer. Diode modules are meant for more demanding applications and are designed and toleranced for better beam quality, longer lifetimes, and more rugged packaging. Laser pointers are meant for short periods of use, while a laser diode module can be operated for longer periods of time. Thus, laser diode modules are preferred if the size of the spot, the quality of the spot, divergence of the beam, pointing stability, or lifetime of the unit is important.

Many people still hold very strong ideas on the relative superiority of CCD versus CMOS. There are certain non-typical applications where one would be preferable to the other, but generally any good CCD is equivalent to any good CMOS. Beam profiling requires a high-quality sensor with a good dynamic range. In this particular case, since your laser is pulsed, each image measurement will also be more sensitive to noise, so a low-noise sensor is critical for your application. Traditional criticisms of CMOS technology include that it is both noisier than a CCD and has a lower dynamic range. Before some recent advances in CMOS manufacturing techniques, this was true. Our USB Laser Beam Profiler uses a high quality CMOS sensor and control electronics that have been specifically designed to optimize its linear dynamic range and keep the noise below the level of most lasers.

No. These warning labels or stickers, which specify a laser product's class, output power, and wavelength, must be placed on all finished products for retail that conform to the standards set by CDRH (the Center for Devices and Radiological Health). For more info, see the CDRH website at http://www.fda.gov/cdrh/index.html. You can also contact the LIA (the Laser Institute of America) at http://www.laserinstitute.org/ or OSHA (the Occupational Safety and Health Administration) at http://www.osha.gov/ for specific safety recommendations and requirements.

Laser safety eyewear will not allow the user to see the laser beam since the beam is only visible if it reflects off a surface and creates a laser `spot'. The majority of safety eyewear available is designed to completely block the designated wavelength(s) and will not make the beam spot or reflections visible. In fact, depending on the optical density at a specific wavelength (see above question), the protective filtering will completely block out the related visual color. For example, eyewear designed to block a He-Ne laser at 632.8nm will completely block the color red and make it look more like brown. This type of eyewear is typically designed for a narrow spectrum (specific laser) or broadband (covers a range of laser applications) and will provide maximum protection without sacrificing luminous transmission, allowing the user to see clearly while performing their tasks. Our Laser Line series of safety eyewear makes it possible to see part of the beam at contact when using alignment lasers, while providing a degree of protection for low power lasers. Please note that just like selecting the incorrect eyewear, using the alignment type of eyewear with high power lasers can cause permanent damage. Eyewear is typically available in a variety of styles; including goggles and several types of spectacle styles.

Spatial filters are used to "clean up" laser beams by filtering out unwanted multiple-order energy. The resulting beam intensity will still have a Gaussian profile. Spatial filters are particularly useful in interferometric and holographic applications. For a more in-depth discussion of what components make up a spatial filter system and how to use a spatial filter, view Understanding Spatial Filters.