A laser accelerometer is an

accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...

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'' originated as an acronym for light amplification by stimulated emission of radi ...

to measure changes in velocity/direction.

Mechanism

It employs a frame with three

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

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 ...

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, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

transmits a beam of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

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 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, useful for mining. Other applications include medical diagnostics and satellite measurements for

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

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), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

. 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 that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those o ...

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. (''Trisilicon tetranitride'') is the most thermodynamically stable and commercially important of the silicon nitrides, and the term ″''Silicon nitride''″ commonly re ...

() 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 is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

.The device can sense displacements under 1

femtometre The femtometre (American spelling femtometer), symbol fm, (derived from the Danish and Norwegian word 'fifteen', ) is a unit of length in the International System of Units (SI) equal to 10−15 metres, which means a quadrillionth of one metre. ...

(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 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, url-access=subscription Laser applications Gravity Accelerometers Sensors