What are Laser Specifications?

                   What are Laser Specifications?

For commercially sold lasers and laser systems, it is common to have a set of laser specifications,  descriptions of their properties with some precision. Such specifications (in short: specs) are made by suppliers of lasers (usually by the manufacturers), but a buyer also will usually write down the required specifications for his application before selecting lasers. 

Laser specifications are important for different purposes:

-The buyer of a laser needs to know about the essential properties of a laser system before buying and when applying a system. Some key performance figures such as output power and beam quality need to be guaranteed, and various details must be known for proper application, for example: combining the laser with other devices. 

-Specifications are often included in quotations and purchase orders in order to define duties of the supplier and conditions for use. In cases of conflict between user and supplier, specifications may be used to determine whether or not a laser system is qualified

Types of Specifications

Specifications can address very different aspects, such as performance figures, device lifetime, required ambient condition, and other properties such as dimensions, weight and mounting details.

Some typically important Performance specifications concern the following aspects:

-The output power must stay within certain limits. Note that the output power may depend on additional conditions. For example, for pulsed lasers the average output power may depend on the chosen pulse repetition rate, and for tunable lasers it normally varies with the chosen wavelength.

-Various properties of the output laser beam need to be defined, in particular its transverse size: the beam radius (or diameter) at the exit from the laser housing, whether the beam is collimated, otherwise (for divergent or convergent beams) where the beam waist is located. Note that different definitions for the beam radius exist.

-The beam quality may be specified via the beam parameter product (BPP) or the M2 factor.

-The polarization may be linear with a given direction (vertical or horizontal). In other cases, the polarization is undefined, or rarely an unpolarized output is even guaranteed.

-For pulsed lasers, relevant quantities can be the pulse repetition rate, pulse energy and duration, pulse shape and possibly a chirp. If the repetition rate can be varied, specifications of the other pulse parameters may be required for different values of the repetition rate. (For Q-switched lasers, for example, higher repetition rates normally imply lower pulse energies and longer pulses.)

-The laser wavelength (usually understood as the central wavelength, defined via the “center of mass”) must stay within certain limits. For tunable lasers, a range of wavelength needs to be defined, and how other properties (e.g., the output power) vary in that range, or at least what minimum specifications are valid for that range.

-There is often an upper (and sometimes a lower) limit for the optical bandwidth, which is related to the temporal coherence of the laser light. One may also specify the limits for the coherence length. For example, a sufficiently short coherence length may be required to avoid problems with interference effects.

-Various kinds of fluctuations (laser noise) may be limited. For example, one may specify the maximum r.m.s. (root mean squared) fluctuations within a certain measurement bandwidth, possible in the form of relative intensity noise (RIN). Phase noise is related to the optical bandwidth, but more comprehensive (frequency-resolved) phase noise specifications may be required in the form of a maximum power spectral density versus noise frequency. The beam pointing stability may be specified as an r.m.s. value of the angular deviations within a certain measurement time and bandwidth.

Usually, specifications are valid only after the certain warm-up time after switching on a laser system.