Free-Space Single-Mode Laser Diodes

Learn More About Single-Mode Lasers

The term “single mode” is sometimes misused and misunderstood within the laser community. It refers to the number of modes in the laser beam along two axes: the transverse electromagnetic (TEM) plane and the longitudinal axis. The TEM modes determine the spatial distribution of the laser’s intensity, while the longitudinal modes determine the spectral properties of the laser, including the frequency content and linewidth. A laser is considered single-mode if it has only one mode along either the TEM or longitudinal axis or both. Single longitudinal mode lasers are ideal for applications where a narrow linewidth and precise wavelength are required and are commonly referred to as single-frequency lasers. However, they still have a defined linewidth.

Laser diodes, for example, are categorized as single-mode or multimode based on the far-field distribution in the lateral direction (slow axis). A single-mode laser has a bell-shaped distribution with one peak, while a multimode laser has a distribution with multiple peaks. Single-mode lasers typically have lower power but higher brightness. The output beam quality of a laser, as determined by M2, can be used to determine whether it is single-mode or multimode. A pure single-mode laser has M2 less than 1.3, a quasi-single-mode laser has M2 between 1.3 and 2.0, and a multimode laser has M2 greater than 2.0.

Single-mode lasers have a better beam quality, including a smaller focus diameter, lower divergence, and higher power density, compared to multimode lasers. However, single-mode fiber lasers have limitations, including a thin fiber core with a low damage threshold and low output power, making them unsuitable for applications requiring high-energy light. Multimode lasers, on the other hand, have higher output power and a larger end area, but are less suitable for long-distance transmission due to their greater dispersion and energy loss.

The spatial mode structure of a diode laser is determined by the ridge width of the waveguide, similarly as the core of a fiber-optic cable.   For single-mode laser diodes, the ridge width is determined by the wavelength of the laser but is typically between 3 to 10 microns wide.  Such a narrow waveguide allows for single transverse electric (TE) and transverse magnetic (TM) modes to be supported resulting in a perfect TEM^­₀₀ beam profile.  The limited volume associated with having such a narrow active area results in far less power generation and much larger power densities at the facet, but it also results in a much lower capacitance allowing for quicker switching, therefore, making single-mode laser diodes far superior for high-speed applications.  Single-mode laser diodes are available in both free space and fiber coupled packaging configurations.

Only looking for Single Longitudinal Mode or Narrow Linewidth Lasers? See our Narrow Linewidth Lasers page!

Our Single-Mode Laser Products

Here at RPMC Lasers, we offer a wide range of wavelengths from UV as low as 349 nm to LWIR up to 16 µm with single-mode outputs, including DFB, DBR, and VBG stabilized, single-frequency diodes, as well as VCSELs, QCLs, single-mode CW DPSS and CW Fiber lasers, gas lasers, and more. These single-mode lasers are the ideal for a diverse range of applications including fiber laser seeding, confocal fluorescence microscopy, gas sensing, and telecommunications, just to name a few.

Deeper Dive into Single-Mode Lasers

Multimode vs Single-Mode Lasers for Raman Spectroscopy

Raman spectroscopy is one of the fastest growing and most diverse applications in all of laser spectroscopy.  As a result, it can be rather challenging at times to sift through the wide-ranging laser options all being marketed for Raman spectroscopy.  In this application note we will tackle one of the most common questions that arises when picking a laser for Raman spectroscopy; “Should I chose a single-spatial mode or multi-spatial mode laser for my application?”  On the surface, this seems like a simple question since Raman is a nonlinear optical effect and therefore the tighter the beam can be focused the higher the conversion efficiency.  Seemingly a single-mode laser would be preferable, but in practice there are other factors that can complicate the situation.

Read the full article here.

What is Single Longitudinal Mode?

Most people seem to have a relatively intuitive understanding of what it means for a laser to be single transverse mode, commonly referred to as TEM₀₀, but understanding what it means to be single longitudinal mode is often harder to understand.  This is most likely because unlike the TEM mode structure, which you can physically see when you look at the beam profile, the longitudinal mode structure instead affects the spectral properties of the laser, which aren’t as directly observable.  To really understand the longitudinal mode structure of a laser beam first you need to take a step back and think about how standing waves function inside of a resonator.

Whenever a wave has two fixed points, whether it is the mirrors in a laser cavity or the ends of a guitar string, the frequencies that the wave can vibrate at are solely dependent the length of the cavity.  This process is illustrated in the figure below showing the fundamental frequency and the first two overtones for a cavity of length L.  If you extrapolate this out, it is easy to understand that while there are an infinite number of frequencies that can exist within the cavity, each one will have a discreet spacing between them which is dependent on the geometry of the cavity.

Read the full article here.

Laser Diode Fundamentals: Beam Properties

An in-depth analysis of the physics of PN-junctions isn’t necessary to understand the beam properties, but it is essential to realize that this junction, which is only about 1 micron in thickness is the only part of the diode where electron-hole recombination is possible.  As a result, no matter how powerful the laser diode is, this thin layer is the single location where light generation and amplification is possible.  Therefore, it is critical to ensure that all of the laser light is tightly contained within the PN-junction.  This is accomplished by etching a ridge into the top layer of the diode which creates a waveguide due to the extreme difference in index of refraction of the semiconductor (~3.5) and the air (~1.0) interface.  If this ridge is significantly narrow as to only allow a single spatial mode to propagate through the waveguide, for example, 3 microns at 785nm, then it will result in a single mode laser.  However, this limits the total volume of the diode’s laser cavity and increases the power density at the facet, limiting the maximum laser power.  For higher power applications the diode ridge width is typically expanded to approximately 100 microns to increase the cavity volume and decrease the front facet power density. For extremely high-power applications multiple waveguides can be placed on the same diode producing an array of single emitters on one diode bar.

Read the full article here.

Why Should Single-Mode Fibers Have an Angle Polished?

When you look through a window at night and see your reflection, that is because on average 4% of the incident light is reflected at the interface between air and glass.   While 4% may seem like a small amount when dealing with lasers, a 4% back reflection can have more than enough power to destabilize or even permanently destroy the laser.    If the laser beam is perfectly aligned through the system, so too will the back reflections be perfectly aligned to go back into the laser cavity.

While back reflections are definitely problematic for lasers in general, they are most problematic for single-mode lasers.  Especially single-mode diode lasers.  There are two primary reasons for this extreme sensitivity.  As we discussed in our previous blog post titled “Laser Diode Fundamentals: Single Longitudinal Mode Diodes,” injection seeding can change the gain threshold of a laser.  In that blog, we explained that by controlling the feedback into the laser you could select specific longitudinal modes.  Similarly, when unwanted back reflections exist in the system, it will change the gain threshold in unintended ways and cause the longitudinal mode structure to destabilize.

Read the full article here.

How Can We Help?

If you have any questions, or if you would like some assistance please Contact Us here. Furthermore, you can email us at info@rpmclasers.com to talk to a knowledgeable Product Manager.

Alternatively, use the filters on this page to assist in narrowing down the selection of Single-Mode lasers for sale. Finally, head to our Knowledge Center with our Lasers 101 page and Blogs, Whitepapers, and FAQ pages for further, in-depth reading.

Finally, check out our Limited Supply – In Stock – Buy Now page: This page contains an ever-changing assortment of various types of new lasers at marked-down/discount prices.