|
Laser interferometry technology The technology behind the Renishaw laser system The Basics Laser measurement is accomplished through interferometry, a technique that uses the wavelength of light as the unit of measurement. A laser is used because the laser light is coherent, meaning that all the rays have exactly the same wavelength and are exactly in phase (see Properties of light). This means that the peaks and valleys of the component light waves are in perfect synchronisation. The wavelength of laser light from a helium-neon (HeNe) laser is 0.633 micron (0.000025 inch) and by further sub-dividing the wavelength, Renishaw has achieved a resolution of 1.25 nanometres (0.00000005 inch). The long term wavelength stability of the Renishaw ML10 Laser is (in vacuum) better than 0.l ppm. An interferometer system measures the change in distance by counting the number of wavelengths of light seen by detector optics. As the wavelength is known to great accuracy, then the overall distance can be calculated to great accuracy. An interferometer system or Michelson interferometer, comprises three optics : a polarising beam-splitter, and two retro-reflectors, or corner-cubes. The following is a brief description of the optical interference principles which the Renishaw ML10 system uses to measure relative changes in optical path length, between measurement and reference arms of the interferometer. The figure shows a schematic diagram of the ML10 laser head, linear measurement optics, and the interference detectors, together with the laser beam paths. The laser beam (1) emerging from the laser head is a circular polarised laser beam of single frequency light. When this beam reaches the polarising beam-splitter it is split into two components. The reflected beam (2) is vertically polarised light, and transmitted beam (3) is horizontally polarised light. As these beams travel to their retro-reflectors they have exactly the same frequency. These two retro-reflectors reflect these beams, (2) and(3), back towards the beam-splitter.
fig 1 In linear distance measurement, one retro-reflector is usually rigidly attached to the beam-splitter, to form the fixed length reference arm of the interferometer, and other retro-reflector moves relative to the beam-splitter and forms the variable length measurement arm. The laser system tracks any change in the separation between the measurement arm retro-reflector and the beam-splitter, as the retro-reflector moves. Whilst it is moving, the frequency of the reflected laser beam from the moving retro-reflector is Doppler shifted, so that during movement, the frequencies of the two returned beams are not the same. This difference in frequency is directly proportional to the speed of movement. When the reflected laser beams reach the beam-splitter they are reflected or transmitted (as before), so that both beams are recombined and returned to the laser head. This recombined beam (4) consists of two superimposed beams of different polarisations. One component is vertically polarised, and has travelled around the reference arm. The other component is horizontally polarised, and has travelled around the measurement arm. At this stage the two beams do not interfere with one another because they have different polarisations. When the beams enter the laser head, they pass through a special optic (5) which causes the two beams to interfere with one another to produce a single beam (6) of plane polarised light. The angular orientation of the plane of this polarised light depends on the phase difference between the light in the two returned beams. So, if the measurement arm retro-reflector is moving away from the beam-splitter, then the plane of polarisation spins in one direction, and if the measurement arm retro-reflector is moving towards the beam-splitter, the plane of polarisation spins in the opposite direction. The beam (6) is then spilt into three and focused onto three polarisation sensitive detectors. As the plane of polarised light spins, each detector produces a sinusoidal output waveform. The polarisation sensitivity of the detectors are adjusted so that their outputs are in phase quadrature. (Three detectors are used to allow the system to eliminate background light, to distinguish the direction of movement, and to measure the distance moved.) These signals are very similar to the feedback signals obtained from many linear and rotary encoder systems, and are processed in a similar way, using very high speed direction discrimination, counting and interpolation electronics. This allows the laser system to measure changes in distance as the measurement arm retro-reflector moves. Because the laser wavelength is very short (633nm), and can be determined very accurately (to one part per million in air), the Renishaw laser can provide measurements with very high accuracy and resolution. Properties of light The interferometer system uses a laser because it needs a source of coherent light. A coherent light source emits electromagnetic radiation that is all the same frequency (monochromatic) and all the same phase (the peaks and troughs of all light packets emitted line up). Due to laser light being both coherent and monochromatic. It also has another property that we exploit, that of having predictable interference patterns. If two in-phase beams of coherent light meet, they effectively add (fig 2), the result being a bright band of light, where their amplitudes have added, and is known as 'constructive interference'. Figure 2. Constructive interference Conversely, if the two beams are 180o out of phase they
will cancel each other out (the peaks of one beam align with the troughs
of the second beam, and the troughs of the first align with the peaks
of the second). This results in a dark band, and is known as 'destructive
interference'. Figure 3. Destructive interference
Michelson interferometer A Michelson interferometer consists of a monochromatic light source, a beam-splitter and two retro-reflectors. For ease of alignment, we use retro-reflectors instead of plane mirrors. The beam-splitter is commonly called an interferometer, but it is the combination of optics that is required for a full interferometer system. The Michelson interferometer works by having a coherent light source that is split at the surface of a half mirror (fig 4). The beam splitter contains a 45o splitting layer which is specially made to transmit horizontally polarised light and to reflect vertically polarised light. When the circular polarised light from the laser hits this layer, it is split into horizontal and vertically polarised components, which are then reflected or transmitted accordingly. Then one half of the light is reflected towards a fixed distance mirror while the remaining half is allowed to pass through to a variable distance mirror. The two beams are then reflected back towards a detector via similar paths.
Figure 4. The Michelson Interferometer If each of the mirrors is exactly the same distance from the half mirror, then the light will arrive at the detector in phase and constructive interference will occur. Positioning the movable mirror further away so that its position is shifted by one quarter wavelength, then the beam will return to the detector 180o out of phase and destructive interference will occur. Therefore provided the beam is not broken, the distance moved by the movable mirror can be measured by counting the light fringes produced. Retro-reflector A retro-reflector comprises three reflecting surfaces mounted at right angles (90o) to each other. A retro-reflector always returns a light beam parallel but displaced to the incoming light beam. The optics may be hollow or solid. In the case of a solid, it can be described as a corner of a cube, hence the common name of a 'corner-cube'. Polarising beam-splitter A beam-splitter is an optical device for dividing a beam into two or more separate beams. A simple beam-splitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A polarising beam-splitter has the additional property of transmitting only one polarisation of light and reflecting the other polarisation.
Figure 5. A simple beam splitter Polarisation With respect to light radiation, polarisation is the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarisation direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forward). The polarisation vector is always in the plane at right angles to the beam propagation direction. Near some given stationary point in space, the polarisation direction in the beam can vary at random (unpolarised beam), can remain constant (plane-polarised beam), or can have two coherent plane-polarised elements whose polarisation directions make a right angle. In the latter case, depending on the amplitude of the two waves and their relative phase, the combined electric vector traces an ellipse and the wave is said to be elliptically polarised. Elliptical and plane polarisations can be converted to each other by means of birefringent optical systems. Circularly polarised light has its plane of polarisation rotating at the optical frequency. Thus, as circularly polarised light propagates, the electric vector rotates around the direction of propagation producing a spiral in space. Some materials interact differently with light, depending upon whether the light is left or right circularly polarised. For example, crystalline quartz, which demonstrates a refractive index difference between left and right circularly polarised light propagating along an axis of 71 x 10-6. Such crystals are sometimes referred to as being optically active. Another famous material is dextrose (sugar) solution. Birefringent material A quarter wave plate made of birefringent material has the property of retarding light in one polarization with respect to the other. This results in the two polarization states being 90o out of phase and this gives us circularly polarised light. Laser cavity The term 'laser' is an acronym for "Light Amplification by Stimulated Emission of Radiation". The Renishaw laser is an enclosed cavity filled with Helium-Neon gas that, when a high voltage is applied, will produce a very narrow frequency band of light. The ends of the cavity are mirrored so that the light is reflected back and forth and is allowed to interfere. The wavelengths that are not equal to the length of the cavity or multiples of it undergo destructive interference, while those that are resonant (have a wavelength that fits completely within the cavity) will undergo constructive interference and will be amplified. Therefore, we get coherent light bands within the spectral range of HeNe gas that are resonant to the laser cavity. The light is emitted from one end of the cavity as a near parallel (collimated) beam of light through a partially transmitting mirror (reflectivity ~98%, transmission ~2%). The HeNe spectrum curve (fig 6) shows the spectrum of the light emitted by HeNe gas. The vertical lines, V1 andV2 represent frequencies of light that will resonate within the cavity. HeNe gas has a narrow bandwidth, therefore two peaks coincide and two wavelengths are emitted from the laser. For some laser cavities, these two modes are plane polarised, and are also orthogonally polarised.
Figure 6. Doppler effect The Doppler effect is the effect produced on a wave frequency due to the relative motion of a source or an observer. The radiation emitted from a source that moves away from an observer appears to be of a lower frequency than the radiation emitted from a stationary source. The radiation emitted from a source that moves toward an observer appears to be of a higher frequency than the radiation emitted from a stationary source. The classic example, in sound, of the Doppler effect is the change in pitch of the whistle on a train as the train approaches, passes and then moves away from an observer. Deadpath error Deadpath is an error associated with changes in the air refractive index which may occur during a linear displacement measurement where EC10 automatic compensation is being used. The deadpath error of the laser measurement is related to the distance between the two optical elements, when the system is datumed. If there is no motion between the interferometer and the reflector and the environmental conditions surrounding the laser beam change, then the wavelength (in air) will change over the entire path (L1+L2). If the "Environmental Factor" changes to correct for the new environmental conditions, the laser measurement system will be compensated over the distance L2 only.
It should be noted that deadpath error will be negligible if the stationary and moving optic are abutted when the datum is set. Where the geometry of a machine is such that the two optics are furthest apart when the moving optic is at the zero datum of the axis, the preset facility can be used as follows to avoid the potential deadpath error associated with datuming the laser interferometric system at this point:
Some definitions on this page are based upon definitions which appear in the Photonics Dictionary.
|
