Multi-line lasers (or multiline lasers or multi-wavelength lasers) are laser sources emitting radiation with multiple spectral lines – usually all in a single laser beam, or with a single fiber output. In some cases, the emission occurs at all those lines simultaneously with a stable distribution of the optical power. For example, for display applications or for special illumination purposes there are RGB sources, emitting red, green and blue laser light, which can be regarded as triple-line lasers. In other cases, one can only switch between the different lines, i.e., obtain only one laser line at a time. Another possibility (e.g. common for ion lasers) is that one obtains multiple lines simultaneously, but not with a stable power distribution. Here, multi-line operation is not an attractive feature as such, but may allow for the highest total output power.
There are various applications where multi-line outputs of the one or other kind are required, for example in the context of optical spectroscopy (e.g. CARS), fluorescence microscopy, microwave and terahertz generation (through beat notes or difference frequency generation) and communications, and medical lasers.
In principle, a frequency comb source could also be called a multi-line laser system, although that is not common.
A special and somewhat atypical case is that a pilot beam is superimposed on a main laser beam, for example in laser material processing. Such a device would usually not be called a dual-line laser.
There are also cases where a multi-line laser is meant to be a laser with a single laser line not concerning the spectrum, but a spatial pattern exhibiting multiple lines – usually obtained with some kind of beam shaper. Such devices are used as alignment lasers.
In cases where substantially different wavelengths are involved, the question arises how the beam parameters should be chosen for optimum spatial overlap. Usually, one will prefer a constant ratio of beam radii for the different wavelengths along the beam. For achieving this, the different wavelength component need to have their beam focus at the same position, and they must all have the same Rayleigh length. That implies that the shorter-wavelength beams have smaller beam radii. These conditions may be naturally fulfilled e.g. when all the radiation is generated in a common bulk laser resonator.
Single Laser or Multiple Lasers
In principle, a single laser can be made to emit on multiple laser lines simultaneously. This works particularly well when cooperative lasing in a cascade of laser transitions can be realized. However, it is rare that a combination of laser transitions with wavelengths suitable for a specific application is available.
Multiple Laser Transitions from a Single Starting Level
It is more common that a laser gain medium has multiple laser transitions beginning from a common upper laser level. A common example is Nd:YAG, exhibiting the well-known 1064-nm transition, but also additional transitions at 946, 1123, 1319, 1338, 1415 and 1444 nm. An example for a visible laser with multiple emission lines is the praseodymium-doped fiber laser for emission in the red (635 nm), orange (605 nm), green (520 nm) and blue (491 nm) spectral region.
It is often relatively simple to realize switching between multiple laser lines, at least when there is a substantial spectral separation of all lines. Typically, one has a dispersive optical element inside the laser resonator (e.g. a prism) and an end mirror the angular alignment of which can be changed. For one specific setting of that mirror, only one spectral component will be sufficiently well aligned to enable lasing.
On the other hand, with a common upper laser level it is often not easy to obtain simultaneous lasing on multiple lines with a stable power distribution, because the competition between such laser lines is usually rather delicate. The laser will usually prefer the transition with the highest net gain, even if the gain difference is very small. One could in principle use some kind of stabilizing mechanism, for example a nonlinear limiting mechanism for each laser line. An often more practical approach is to use a dispersive optical element in the laser resonator such that the beam position in the gain medium is somewhat wavelength-dependent. That way, the competition between the different lines is avoided; each part of the gain medium works only with a single wavelength. Nevertheless, most of the optical resonator can be made for all optical components together, such that one requires relatively few optical elements and automatically obtains an accurate superposition of the different laser beams at the output. Relative beam pointing fluctuations are then usually quite weak.
Multiple Gain Media
It is possible to build a solid-state bulk laser, for example, incorporating two or more different laser crystals, each one with an active doping for different wavelengths – all in a common laser resonator with suitable reflectivities of the laser mirrors. Alternatively, one can sometimes use a single laser crystal containing multiple dopants.
Ideally, all wavelength channels can be served with a single pump source.
Nonlinear Frequency Conversion
Some multi-wavelength laser sources utilize some kind of nonlinear frequency conversion, for example frequency doubling, and output the frequency-doubled light together with some residual laser light. For obtaining more closely spaced wavelengths, one can use one or more Raman lasers. With optical fiber technology, one can realize multi-stage Raman converters generating multiple wavelength outputs.
Spectral Beam Combining
Another possibility is to realize a multi-line laser by spectral beam combining with the outputs of separate lasers. With those and the beam combining optics in a single housing, the device is essentially usable like a single laser. However, it must then be ensured that the focusing and alignment of the different beams are accurately and stably arranged. It depends on the application to which extent such requirements are critical. They may not be critical e.g. if the different wavelength components are anyway spatially separated for the application.
Dual-wavelength Laser Diodes
Dual-wavelength laser diodes can also be made and are commercially available. Some, however, have two different laser diodes with different wavelengths realized in close proximity on a single semiconductor chip. Their radiation will therefore not exhibit a full spatial overlap. Such devices are used for reading digital data from different types of optical media such as CD-ROMs, DVD and blue-ray disks.
In other cases, a single laser cavity is used and has different (separately pumped) sections for different wavelengths .
Laser Diodes + Fiber Amplifier
There are also there are also multi-line laser sources based on multiple seed laser diodes with different wavelengths and a fiber amplifier for boosting the output power. This approach allows for high power with perfect spatial overlap. Also, it gives one high flexibility concerning the distribution of powers over the different wavelength channels, wavelength tuning, common or separate power modulation of the different channels, etc. However, this approach is limited to cases where all involved wavelength are within the amplification band of a certain active fiber.
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