Optical Power

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

The term optical power occurs in the literature with two totally different meanings:

• It can be the energy of light per unit time, as is delivered by a laser beam, for example.
• It can be a focusing power, also called dioptric power, for example of a lens or microscope objective.

In the following, both meanings are discussed.

Power as Energy per Time

Definition

A power with that meaning is usually specified in watts = joules per second. Particularly in the area of optical fiber communications, optical powers are also often specified in dBm, which means decibels relative to the reference power 1 mW. For example, a power of 10 μW = 0.01 mW corresponds to −20 dBm (= 20 dB less than 1 mW).

In radiometry, one uses the term radiant flux (or radiant power), which may be considered as more general, since it can be applied not only to light.

Strictly speaking, the optical power of linearly polarized monochromatic light, when defined as delivered energy per unit time, exhibits a fast oscillation with a frequency of twice the optical frequency. However, that fast oscillation can hardly be measured and is usually of no practical relevance. Therefore, one usually considers the optical power and intensity as time-averaged quantities, not exhibiting that oscillation.

Measurement of Optical Power

Optical powers of laser beams are often measured with so-called optical power meters. Some of these devices can handle multiple kilowatts of laser power from a high-power laser; they are usually water-cooled. Other power meters, often based on photodiodes, can be used for measuring very low optical powers of microwatts, nanowatts or even less. Optical power monitors are widely used as permanent parts of optical systems, e.g. in optical fiber communications.

The optical power of light which is radiated in a wide range of directions cannot easily be measured with a power meter, as it is difficult to collect it. In such cases, an integrating sphere can be used.

Note that the optical power of some source may be spread over a substantial frequency or wavelength range. As the responsivity of a photodetector (e.g., a photodiode) maybe frequency-dependent, one may then not easily obtain a correct measurement of the total optical power.

Power of Light Pulses

The power of an optical pulse varies with time (for ultrashort pulses, within femtoseconds); its maximum is called the peak power. The peak power can be estimated from the pulse energy and pulse duration. Particularly for a regular pulse train with a high pulse repetition rate, one often specifies the average power. When such the pulse train is sent into a power meter with a low temporal resolution, that device will often just display the average power.

Noise

Even for a continuous-wave laser, the output power is not perfectly constant, but exhibits some level of intensity noise. (The better term would actually be power noise, but this not common.) See also the article on relative intensity noise.

Power and Intensity

From the power of a laser beam, one can estimate the maximum optical intensity (power per area) within its transverse profile, if the shape and width of that profile are known. On the other hand, the total optical power of a beam is the integral of its intensity over the full beam area.

Focusing Power

For this meaning, it is recommended to use a term like focusing power, dioptric power or refractive power rather than optical power to avoid the possible confusion with the energy per unit time (see above). Qualitatively, what is meant is the ability of an optical element (for example, a lens or a microscope objective, which is a combination of several lenses) to focus light. Quantitatively, the focusing power is the reciprocal of the focal length of the device, and its units are inverse meters (m⁻¹), also called diopters (dpt). The article on focal length contains many more details.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 78 suppliers for optical attenuators. Among them:

LASEROPTIK

LASEROPTIK can provide coatings for variable optical attenuators. One can change the transmission by tilting an edge filter, with highest transmission at normal incidence and typical operation for angles of incidence between 0° and 45°.

Optogama

Optogama designs and manufactures motorized laser power attenuators (LPA) for laser power control. LPA are available for use in the UV, visible, and NIR spectral ranges from 250 nm to 2000 nm. These devices feature a large clear aperture dedicated for considerable size beam applications. All optical elements of these laser power attenuators are made for high LIDT and provide a stable and reliable performance even when using them with high power lasers in industrial applications.

EKSMA OPTICS

EKSMA Optics offers variable attenuators for linearly polarized laser beams with high performance optics. They are available for Nd host laser applications in the 1040–1070 nm range and for femtosecond applications with broadband Ti:sapphire and Yb:KGW/KYW laser pulses. Manual and motorized versions available, highly customizable and designed for use in laboratory settings. Compact motorized laser power attenuators provide stable and reliable performance even in high power lasers for industrial applications.

Gentec Electro-Optics

Gentec Electro-Optics offers optical attenuators for measurements on high-power laser beams.

Vortex Optical Coatings

We design and manufacture custom attenuator coating across a range of 300 nm – 6000 nm for a variety of applications. The main uses of attenuators are protecting sensitive detectors from overload, attaining accurate laser power measurements and being used in filter wheels to balance the system for particularly strong signals.

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