Frequently Asked Questions

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).

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 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.

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.

It seems like you are experiencing some temporally-localized interference. If you look at a short enough slice of time, the photodiode might pick up the constantly shifting course of constructive and destructive interference that is basically what its called speckle.

Laser Diode Modules have a fair amount of noise if you look on a very short time scale. The total noise is a combination from multiple sources including but not limited to power supply noise, mode hopping, thermal effects, and optical feedback. These noise sources can be severely reduced by using a low-noise power supply and a heat sink or preferably a Thermo-electrically cooled (TEC) laser. In order for a laser to have a perfectly consistent output even on that small of a time scale, the necessary controls need to be designed in from the ground up, which is generally not the case with 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.

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.

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.

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.

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.

Any visible pits and scratches will create light scatter which would be insignificant if there is plenty of light available but would decrease the signal in a low-light level application. An optic with a 20-10 scratch-dig, for example, will have better surface quality than an optic with 60-40. Provided the thicknesses are the same, glass transmission for uncoated fused silica is similar to the transmission for uncoated float glass in the visible range of the spectrum. UV Fused silica, however, does have much better transmission in the UV compared to float glass, if you would like a window with a higher surface quality rating.

A laser beam expander is designed to provide larger collimated beams in order to decrease spot sizes at large distances. Not only will the beam size be increased by a certain factor, but the beam divergence will also be decreased by the same factor. The result is a smaller spot size at long distances when compared to what the laser could produce by itself. For a more detailed explanation of the advantages of laser beam expanders, view Beam Expanders.

A refractive line generator uses a cylinder or rod lens to focus a laser in only one axis only (drop-moved placement) in order to create a line of light. A diffractive line generator uses a flat optic with an etched microstructure that breaks apart a laser beam and forms an interference pattern in the shape of a laser line. Refractive optics do not correct for the inherent Gaussian profile of a laser beam and form a line with a "hot spot" in the center and fading edges. Diffractive optics will create a line that is uniform in thickness over its length, but is segmented. Diffractive line generators also cause a small portion of the light to be redirected into different diffractive orders, which causes additional faint lines to appear. Some laser line generators use a unique patented Powell glass lens design in order to achieve a continuous (not segmented) line with an even distribution along the length of the line.

Though we don’t have a complete package to automatically modulate beam waist, we do offer laser beam expander systems that allow one to focus down the spot size of a laser source. You could attach, for example, one of our laser beam expanders to a motor for them to automatically adjust the spot size of a laser.

Reflective laser beam expanders are achromatic by design. This enables them to easily be used for broadband applications including those using multiple laser wavelengths or for tunable lasers. This achromatic nature makes them ideally suited for ultrafast applications or for wavelengths that are difficult to find transmissive designs for, such as certain QCL wavelengths. 

Reflective laser beam expanders are suited for most types of lasers. TECHSPEC Monolithic Reflective Beam Expanders (Mark I) are not designed for use with high power lasers, but are suitable for any laser with a beam size smaller than the aperture, including Nd:YAG lasers, Quantum cascade lasers, Ti:Sapphire ultrafast lasers, and fiber lasers at various wavelengths into the IR. For more information, please contact our technical support.

These laser beam expanders offer two different mounting options. There are ¼-20 through holes, which can hold the laser beam expander on its side so the optical axis of the part is parallel to your table. Additionally, there are 6-32 threaded holes on the bottom of the laser beam expander that can also be used.

Ideally, they would be mounted to a goniometer to reach the optimal alignment with respect to your input beam in order to reach the specified wavefront performance. However, in cases where a user is looking to mount these laser beam expanders without any alignment compensation, mounting the laser beam expander against either of the ideal banking surfaces noted on the downloadable web drawing would be best.

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.

The flat cuts are optical and mechanical reference surfaces for alignment and mounting. These flats are cut in the same diamond turning operation as the parabolic surfaces of the laser beam expander, and as such are perpendicular to the optical axis of these mirrors within the machine capability. They are designed to serve as retroreflection alignment fiducials; once your laser beam is being reflected back upon itself, the beam is properly aligned with the optical axis of the input mirror.

The wavelength spectrum of laser pulses increases when pulse duration decreases, so the short pulse durations of ultrafast lasers result in wide bandwidths. This wide bandwidth is then significantly affected by chromatic dispersion as ultrafast pulses travel through optical media, such as microscope objectives, acousto-optic modulators, windows, lenses, and filters. More information can be found here.

There are three alignment flats created in the same diamond turning process. Any of the three can be used as a convenient retroreflection fiducial or used for pre-aligned fixturing to align to the optical axis of the laser beam expander.

The expansion power and physical dimensions are readily customized per customer request. Additional coating options, including protected gold and protected silver, may also be available. Please contact our technical support for more information.

To clean the mirror surfaces, use compressed air to blow dirt and dust off the surface of the mirrors. To remove fingerprints or other contaminants, the “Drag Method” should be used. In the Drag Method, a lens tissue is saturated with reagent-grade isopropyl alcohol or reagent-grade acetone and is slowly dragged across the surface. If done correctly, the solvent will evaporate uniformly without leaving streaks or spots. If your laser beam expander features a bare metal coating, the Drag Method should be avoided as it can damage the delicate bare metal coating surface. Dirt and dust can easily be blown off with compressed air. However, fingerprints will permanently damage a bare metal coating and preventative measures should be taken to prolong the lifetime of the coating. Review our application note on cleaning optics for more information.

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.

The use of larger beams will lead to increased wavefront error. This may or may not be acceptable depending on your application requirements, such as wavelength. For more information, please contact our technical support engineers.

Fundamentally, these laser beam expanders can work in reverse. However, the specified wavefront values are not guaranteed for reverse operation, and care should be taken to ensure that the input beam diameter is no greater than the specified maximum exit aperture. Also, note that divergence will increase proportionally with the laser beam expander’s magnification.

Yes, short ultrafast pulses interact with optical coatings and substrates in a way that is different from other laser pulses, leading to different damage mechanisms. For more information, please read our LIDT for Ultrafast Lasers application note.

CRDS can only be used to measure mirrors with a reflectivity above 99.5% because lower reflectivities result in ring down times that are too fast for the system to detect. The best technique to determine reflectivity depends on the reflectivity level and application requirements.

Some of the most common materials for use in the 2µm spectral region are fused silica, zinc selenide, calcium fluoride (CaF₂), germanium, and sapphire. More information about compatible optical materials and their properties can be found at our Characteristics of 2µm Lasers application note.

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.

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.

CRDS can only be used to measure mirrors with a reflectivity above 99.5% because lower reflectivities result in ring down times that are too fast for the system to detect. The best technique to determine reflectivity depends on the reflectivity level and application requirements.

To specify the correct goggle for your application, we need to know the Optical Density (OD) required and the wavelength of the laser you are using. The OD is the attenuation density provided by the eyewear and indicates how much energy will be blocked. This information can often be obtained from the laser manufacturer. Each goggle is color coded and stamped with the type of laser beam against which it protects and the OD values provided. We can supply the guaranteed OD values as well as typical values at other wavelengths.

If you do not know the OD needed, our Engineering Department can help you to find the appropriate information. The specifications required to process the request are: the laser type, wavelength(s), and if being used for intrabeam or point source diffuse viewing and at what distance. If continuous wave (CW), then please supply the laser output power (watts) and laser exposure time (sec.). If single pulsed (< 1 Hz), please supply the laser pulse length (sec.) and laser pulse energy (Joules). If repetively pulsed or scanning (> 1 Hz), please supply the laser pulse length (sec.), pulse repetition rate (> 1 Hz), laser pulse energy (Joules) and total emission time (sec.). Please let us know if there is also any hazardous secondary radiation produced (such as laser beam interaction with metals or ceramics during cutting or welding). Also, please indicate if your application requires you to see the reflection from a diffuse beam.

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.

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.