A laser accelerometer is an

accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is accele ...

that uses a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

to measure changes in velocity/direction.

Mechanism

It employs a frame with three

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

input axes and multiple

proof mass A proof mass or test mass is a known quantity of mass used in a measuring instrument as a reference for the measurement of an unknown quantity. A mass used to calibrate a weighing scale is sometimes called a ''calibration mass'' or ''calibration w ...

es. Each proof mass has a predetermined blanking surface. A flexible beam supports each proof mass. The flexible beam permits movement of the proof mass on its axis. A

light source provides a light ray. The laser source has a transverse field characteristic with a central null intensity region. A

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

transmits a beam of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

to a detector. The detector is positioned to be centered on the light ray and responds to the light's intensity to provide an intensity signal. The signal's magnitude is related to the intensity of the light ray. The proof mass blanking surface is centrally positioned within and

normal Normal(s) or The Normal(s) may refer to: Film and television * ''Normal'' (2003 film), starring Jessica Lange and Tom Wilkinson * ''Normal'' (2007 film), starring Carrie-Anne Moss, Kevin Zegers, Callum Keith Rennie, and Andrew Airlie * ''Norma ...

to the light ray null intensity region to provide increased blanking of the light ray in response to transverse movement of the mass on the input axis. In response to acceleration in the direction of the input axis, the proof mass deflects the beam and moves the blanking surface in a direction transverse to the light ray to partially blank the light beam. A control responds to the intensity signal to apply a restoring force to restore the proof mass to a central position and provides an output signal proportional to the restoring force.

Applications

Accelerometers are added to many devices, including (smart) watches, phones and vehicles of all kinds. Accelerometers oriented vertically function as

gravimeters Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Units of measurement G ...

, useful for mining. Other applications include

medical diagnostics Medical diagnosis (abbreviated Dx, Dx, or Ds) is the process of determining which disease or condition explains a person's symptoms and signs. It is most often referred to as diagnosis with the medical context being implicit. The information re ...

and satellite measurements for

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

studies.

Lasers

Basic lasers operate with a frequency range (line width) of some 500

mHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

. The range is widened by small temperature changes and vibrations, and by imperfections in the laser cavity. The line width of a specialised scientific laser approaches 1mHz.

History

2021

An accelerometer was announced that used

infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

to measure the change in distance between two micromirrors in a Fabry–Perot cavity. The proof mass is a single silicon crystal with a mass of 10–20 mg, suspended from the first mirror using flexible 1.5 μm-thick

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...

() beams. The suspension allows the proof mass to move freely, with nearly ideal translational motion. The second (concave) mirror acts as the fixed reference point. Light of a certain frequency resonates – bounces back and forth – between the two mirrors in the cavity, increasing its intensity, while other frequencies are discarded. Under acceleration, the proof mass displacement relative to the concave mirror changes the intensity of reflected light. The change in intensity is measured by a single-frequency laser that matches the cavity's

resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

.The device can sense displacements under 1

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(10⁻¹⁵ m) and detect accelerations as low as 3.2 × 10-8 ''g (''the acceleration due to Earth's gravity) with uncertainty under 1%. An accelerometer was announced with a line width of 20 Hz. The SolsTiS accelerometer has a titanium-doped sapphire cavity that is shaped in a way to encourage a narrow line width and to rapidly dissipate waste heat. The device exploits the wave qualities of atoms. The laser is divided into multiple beams. One beam strikes a diffuse rubidium gas refrigerated to around 10⁻⁷ K. This temperature is achieved by using

Doppler cooling Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques. History Doppler cooling was si ...

with six beams to slow/cool the atoms. The atoms split into two quantum waves. A second pulse reverses the split, while a third allows them to interfere with each other, creating an interference pattern that reflects acceleration the waves underwent while separated. Another laser pulse detects the interference patterns in the various atoms, which reflects the amount of acceleration. Military-grade laser accelerometers, drift (accumulate errors at the rate of) kilometres a day. The new devices reduce drift to 2 km a month.

References

External links

* {{Cite conference, last=Melkoumian, first=Baghrat V., date=2001-08-21, title=Laser accelerometer for guidance and navigation, url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4365/0000/Laser-accelerometer-for-guidance-and-navigation/10.1117/12.438048.short, conference=Acquisition, Tracking, and Pointing XV, publisher=International Society for Optics and Photonics, volume=4365, pages=206–213, doi=10.1117/12.438048 Laser applications Gravity Accelerometers Sensors