Laser interferometer with optical feedback isolation

Abstract: Apparatus for isolating optical feedback in a laser interferometer having a laser light source (12) from which a linear polarised outgoing laser beam (28) of certain polarisation orientation is derived. The interferometer also redirects the outgoing laser beam to form a return laser beam (29) directed to a receiver (13). The outgoing and the return laser beams are physically close and are substantially parallel. The linearly polarised outgoing laser beam is converted to circular polarisation. A portion of the circularly polarised return laser beam directed toward the laser light source is converted back to linear polarisation with polarisation orientation orthogonal to the polarisation orientation of the outgoing laser beam. A portion of the circularly polarised return laser beam directed toward the receiver converted back to linear polarisation with polarisation orientation being substantially the same as the polarisation orientation of the outgoing laser beam. The portion of the return laser beam with polarisation orientation orthogonal to the polarisation orientation of the outgoing laser beam is directed away from the laser light source, and the portion of the return laser beam with polarisation orientation of the outgoing laser beam is directed toward the receiver.

Description: This invention relates to a laser interferometer with optical feedback isolation. The problem of optical feedback affecting lasers and laser interferometers has been known for at least two decades and is noted and described in a paper by N. Brown in the November 1981 issue of Applied Optics , Vol. 20, No. 21 at pages 3711 to 3714. When the configuration of laser interferometer optics and the distance between those optics becomes such that laser light, which originates from the cavity inside the laser, becomes re-directed and re-enters the laser cavity, then optical feedback can occur. Such feedback then detrimentally affects the properties of the original laser beam, possibly including but not limited to, its intensity, its frequency, and its phase. When the distance between the optics is relatively long -- for example, 15 metres or more -- then, due to the natural divergence of the laser light beam and/or slight beam redirection caused by changes in atmospheric refractive index, the problem often becomes severe. Some kinds of polarization isolation have been proposed to mitigate optical feedback in some interferometer systems by suppressing reflections which directly retrace the path of the outgoing beam; however, they are not suitable for use with laser interferometers such as the modified Michelson interferometer, which uses offset outgoing and returning beams. The paper by Brown suggests some such approaches, and approaches with other kinds of laser interferometers have been described in a paper by C. N. Man, et al. In The Journal of Physical Electronic Science Instrumentation (J. Phys. E. Sci. Instrum.). Vol. 11, 1978, pages 19 - 21. All these approaches fall when applied to laser interferometers such as the modified Michelson interferometer, because they block the entire return beam. US-A-3601490 discloses a Michelson laser interferometer in which a polarising beamsplitter is placed in front of a laser source and a quarter-wave retarder is placed immediately in front of this beamsplitter to convert linearly polarised light from the beamsplitter into a circularly polarised measurement outgoing laser beam. A movable plane mirror provides a reversed handed circularly polarised measurement return laser beam in the opposite direction to and on a coincident path with the outgoing beam. The measurement return laser beam is converted by the quarter-wave retarder into a linearly polarised beam which is reflected by the beamsplitter in a direction orthogonally away from the laser source, thus providing optical feedback isolation, and a receiver is located so that it is reached by this orthogonally directed measurement return beam. The polarising beamsplitter also provides via a second quarter-wave retarder, a circularly polarised reference outgoing laser beam in a direction orthogonal to the measurement outgoing laser beam; and this is returned by a fixed plane mirror on a coincident path to be converted by the second quarter-wave retarder into a linearly polarised beam which is transmitted by the beamsplitter on a coincident path with the return measurement beam towards the receiver. The return measurement and reference beams existing the beamsplitter are converted by a third quarter-wave retarder to oppositely circularly polarised components of a composite interference beam which reaches the receiver. The arrangement of US-A-3601490 just described does not solve the problem of optical feedback for the measurement beam of a modified Michelson laser interferometer, which uses offset outgoing and return beams and where the return beams are aimed towards a receiver located on the same side of the polarising beamsplitter as the laser source. Two principal mechanisms which can bring about optical feedback in a modified Michelson interferometer, which uses offset outgoing and return beams are: (1) diffraction and (2) atmospheric beam deviation. They both arise from the fact that in typical configurations of interferometer optics, there frequently exists, originating from the laser cavity, an outgoing laser beam which is positioned close to and parallel to a returning laser beam travelling in the opposite direction, often but not always aiming toward a receiver. In such an optical configuration, diffraction can bring about optical feedback, as follows: If the distance that these parallel beams travel is sufficient, the natural divergence of the returning laser light due to diffraction may cause the diameter of the returning laser beam to increase to the point that it partly or entirely encompasses the outgoing beam. When this occurs, the portion of the returning beam which is sufficiently collinear with the outgoing beam may travel along the path of the outgoing beam until it re-enters the laser cavity. When the intensity and the alignment of this re-entered light are sufficient, this light interferes with the electromagnetic fields inside the laser cavity; this condition is known as optical feedback. The consequences are that the properties of the outgoing laser beam are then degraded, as described above. In the same optical configuration, atmospheric beam deviation can bring about optical feedback as follows: When refractive index fluctuations occur in the region traversed by the laser beams, outgoing or returning, the beams may be temporarily angularly deviated. For example, such index fluctuations may arise routinely in air as a result of turbulence, temperature gradients, pressure gradients, humidity gradients, concentrations of chemical vapors, or any combination of these effects. When the returning laser beam sufficiently deviates from its intended path, some of its intensity may re-enter into the laser cavity and may cause optical feedback. The longer the beam path, the less angular deviation that is required to cause re-entry into the cavity; also, the larger the returning beam's diameter, which increases with distance due to the aforementioned divergence, the less angular deviation that is required to cause the same effect. Consequently, when the distance between optics is relatively large, the problem may become severe. The result of optical feedback is modulation of the output beam from the laser cavity, causing frequency and intensity fluctuations in the output laser beam. If these fluctuations cause errors in the interferometer measurement system, the measurement may cease entirely or an error message may be generated. An object of the invention is to prevent the optical feedback in a modified Michelson interferometer by preventing a suitably directed returning beam from reaching the laser cavity and influencing the emitted laser light beam, while allowing the returning beam to be used in making the actual measurement for which the interferometer is used. Another object of the invention is to prevent optical feedback due to the returning beam while introducing no substantial degradation of any optical properties of the portions of the returning beam needed to make a measurement, for example, the portions desired to reach any receiver. Such optical properties may include but are not limited to, intensity, frequency, and phase. According to the present invention there is provided a laser interferometer comprising: a laser light source producing a first laser beam; polarising beamsplitting means placed in front of said laser source in order to form a second laser beam which is linearly polarised as an outgoing laser beam; retroreflecting means arranged for redirecting the outgoing laser beam as a return laser beam substantially close to, but not coincident with said outgoing laser beam; and a receiver located to the same side of the polarising beamsplitting means through which the first laser beam is applied whereby the return laser beam reaches the receiver via the polarising beamsplitting means; said interferometer being characterised in that: a first quarter-wave retarder is located in the path of said outgoing second laser beam with its fast axis at 45 degrees to the polarisation orientation of said outgoing second laser beam, in order to form an outgoing third laser beam which is circularly polarised, wherein said retroreflecting means are effective to reverse the handedness of circularly polarised light; and a second quarter-wave retarder is located in the path of the return laser beam to the same side of the polarising beamsplitting means as the first quarter-wave retarder, said second quarter-wave retarder not intercepting the path of the outgoing second or third laser beams and being in a plane which is parallel to the plane of the first quarter-wave retarder, the fast axes of the two quarter-wave retarders being orthogonal to each other; such that divergent and/or deviated light in the return laser beam directed towards said laser light source and which is not intercepted by the second quarter-wave retarder is intercepted by the first quarter-wave retarder and is then directed by said polarising beamsplitting means away from said laser light source, whereby optical feedback isolation is provided, and whereby return laser beam light directed towards the receiver and intercepted by the second quarter-wave retarder is allowed to be transmitted by said polarising beamsplitting means and reach the receiver. The use of the combination of the polarising beamsplitter and a quarter-wave retarder for optical feedback isolation is known per se in an unmodified Michelson interferometer from US-A-3601490 as described and discussed earlier in this specification. The first retarder in the arrangement of the invention is inserted into the path of the outgoing laser beam, between the polarising beamsplitting means and the retroreflecting means, with its fast axis at either +45 or -45° with respect to the polarization plane of the incident beam. This converts the outgoing beam's linear polarization into circular polarization of a certain handedness. The returning beam will now be circularly-polarized with opposite handedness compared to the outgoing beam, due to the handedness reversal caused by the retroreflecting means. If any of the returning light is positioned such that it passes back through the first retarder, its polarization becomes linear with an orientation that is orthogonal to that of the outgoing beam. When it reaches the polarizing beamsplitting means, this returning light will not be transmitted toward the source of the outgoing beam, but will be reflected in some other direction. Thus, this portion of the returning beam will not reach the laser cavity, and optical feedback is thereby prevented. The second retarder in the arrangement of the invention is inserted into the path of the returning laser beam, also between the polarizing beamsplitting means and the retro-reflecting means, with its fast axis orthogonal to that of the first retarder. The consequence of this is that the polarization of the returning beam will be converted to linear in the original orientation of the outgoing beam. Note that this portion of the returning beam cannot cause optical feedback because it is not coaxial with any portion of the outgoing beam. when this returning light reaches the polarizing beamsplitting means, it will follow the original path, i.e. to the receiver, just as it would in the absence of any retarders at all. Thus, the isolator of the invention leaves this portion of the returning beam substantially unaffected in its optical properties. In this invention, the first retarder and the second retarder are mutually shaped and positioned such that: 1) the entire outgoing light beam passes through the first retarder only; and 2) the portion of the returning light beam intended for the receiver passes back through the second retarder only, and 3) undesired portion of the returning light beam, which is to be optically isolated from reaching the laser cavity, passes back only through the first retarder. Generally, if the first and second retarders are contiguous and coplanar, then most configurations which meet the requirement 1) will automatically satisfy requirement 3). In many configurations, the optimum location of the pair of retarders is as close as possible to the laser cavity while remaining between the beamsplitter means and the retroreflector means. Fig. 1 is a diagrammatic view of a prior art laser interferometer system which is experiencing the optical feedback problem, due to returning laser light re-entering the laser cavity. Fig. 2 is a diagrammatic view of a similar laser interferometer system incorporating the principles of the present invention to suppress optical feedback while permitting the desired returning light to travel without impediment. Fig. 3 is a diagrammatic view of the front of a laser head, illustrating how a spread retroreflected beam can reenter the laser cavity through the exit aperture. Fig. 4 is a similar view showing the effect of the present invention in restricting the retroreflected beam and preventing its reentry into the cavity, while permitting its entry into the receiver aperture. Fig. 5 is a top plan view of a circular quarter-wave retarder plate made of mica, showing the fast axis at 45 degrees. Fig. 6 is a similar view with the retarder plate of Fig. 5 cut in half. Fig. 7 is a similar view with the lower half of the retarder plate flipped over 180 degrees. Fig. 8 is a top plan view of the two semi-circular retarder plates of Fig. 7 sandwiched between a pair of protective windows and then assembled into a protective metal holder. Fig. 9 is a view in elevation and in section of the complete assembly of Fig. 8. Fig. 1 is a diagrammatic view of a prior art modified Michelson laser system which is experiencing the optical feedback problem described above. An interferometer system may comprise a laser head 11 which contains a light-generating laser cavity 12 and a receiver 13. A laser beam 14 may be emitted through an exit aperture 15 and enter a polarizing beamsplitter 16 which separates the laser beam 14 into an outgoing measurement beam 17 having one linear polarization and a reference beam 18 having orthogonal linear polarization. The outgoing beam 17 reflects from the retroreflector 19 and emerges as a returning beam 20, having the same linear polarization When the distance from the laser head 11 to the retroreflector 19 is long, for example, 50 metres, then the outgoing beam 17 and the returning beam 20 spread or diverge with distance, due to diffraction, this is represented for simplicity, as multiple diverging rays forming the returning beam 20. Most of the returning beam 20 passes back through the beamsplitter 16 and reaches the front of the laser head 11. A portion of the returning beam 20 enters a receiver aperture 21 and reaches the receiver 13, as desired. However, another portion of the returning beam 20 re-enters the exit aperture 15 and re-enters the laser cavity 11. Optical feedback may occur, therefore. Thus, Fig. 1 represents the prior art and the problem. Fig. 2 is a similar diagrammatic view of a modified Michelson laser interferometer system which utilizes the principles of the present invention to suppress optical feedback without detriment to any desired returning beam. The optical configuration in Fig. 2 differs from that of Fig. 1 only by the addition of an optical feedback isolator 25, which comprises two quarter-wave retarders 26 and 27. Quarter-wave retarders convert incident linearly-polarized light into transmitted circularly-polarized light of a selected handedness when the orientation of the incident polarization is appropriate. Such retarders are reciprocal devices, in the sense that they will also convert incident circularly-polarized light into transmitted linearly-polarized light whose polarization orientation is determined by the retarder orientation. According to customary usage, the orientation of the retarders may be be specified with regard to either a "fast axis" or an orthogonal "slow axis" of the retarder. When the outgoing beam 17 emerges from the beamsplitter 16, it has a certain linear polarization, for example, vertical. This beam 17 passes through the first quarter-wave retarder 26, whose fast axis is at +45 or -45° to the incident polarization, and emerges as a beam 28 with a certain circular polarization, for example, left-handed. The beam 28 then enters the retroreflector 19 which reverses the handedness of circular polarization as it reflects. A returning beam 29, which is circularly polarized, now has the opposite handedness, for example, right-handed. Part of the returning beam 29 passes back through the first retarder 26, thereby becoming linearly polarized with an opposite orientation to the outgoing beam 17; this part of the beam 29 is then reflected by the beamsplitter 16, instead of being transmitted, and emerges as a harmless beam 30. Laser light can no longer re-enter the laser cavity 12. Optical feedback is thereby prevented. Another part of beam 29 passes through the second quarter-wave retarder 27, whose fast axis is orthogonal to that of the the first retarder 26, thereby becoming linearly polarized with original polarization of the outgoing beam 17; this part of the beam 29 then can pass directly though the beamsplitter 16, then through the receiver aperture 21, and finally reach the receiver 13 substantially without detriment. The ability of the optical feedback isolator 25 to prevent any laser light from re-entering the laser cavity 12 is clearly seen by comparing Fig. 3 and Fig. 4. Both figures show a diagrammatic view of the front of the laser head 11, along with the exit aperture 15 and the receiver aperture 21. In Fig. 3, the shaded circular area 31 represents the returning laser beam which reaches the laser head 11 when no optical feedback isolator is utilized; this corresponds to the configuration in Fig. 1. In Fig. 4, the shaded nearly semi-circular area 32 represents the returning laser light which reaches the laser head 11 when an optical feedback isolator is utilized; this corresponds to the configuration in Fig. 2. Clearly, optical feedback is prevented because the optical feedback isolator effectively screens out the light which might re-enter the laser cavity. The optical feedback isolator 25 may be constructed as illustrated in Figs. 5-9. Fig. 5 shows a circular quarter-wave retarder plate 33, which may be made of a thin sheet of mica of proper thickness. The fast axis of the retarder 33 lies at 45 degrees to the diametrical line 34. The retarder 33 is then cut in half along the diametrical line 34, to form two semi-circular retarders 35 and 36, as shown in Fig. 6. As shown in Fig. 7 one of the semi-circular retarders 36 is then flipped over 180 degrees, which causes its fast axis to be orthogonal to the fast axis of the other retarder 35. These two semi-circular retarders 35 and 36 may then be permanently sandwiched between a pair of antireflection coated windows 37 and 38 having substantially no wedge, and being secured with optical cement 39. Lastly, the resulting sandwich assembly of elements 35, 36, 37, 38, 39 may be mounted in a protective metal holder 41. The completed optical feedback isolator 25 is depicted in plan view in Fig. 8 and cross-sectional view in Fig. 9. It will be realized by workers skilled in the art that the construction depicted in Figs. 5-9 is only a preferred way of providing two suitably-shaped, practical retarders at a reasonable manufacturing cost. Numerous departures from this construction are possible which still function essentially identically. For example, the use of mica is not exclusive; other retarder materials such as quartz will work. Also, the use of windows is optional, since they serve only to protect the mica which is typically fragile. The antireflection coatings are an optional enhancement as well; and they may be applied directly to the mica if the glass windows are omitted.

Claims: A laser interferometer comprising: a laser light source (12) producing a first laser beam (14); polarising beamsplitting means (16) placed in front of said laser source (12) in order to form a second laser beam (17) which is linearly polarised as an outgoing laser beam (17); retroreflecting means (19) arranged for redirecting the outgoing laser beam (17,28) as a return laser beam (29) substantially close to, but not coincident with said outgoing laser beam (28); and a receiver (13) located to the same side of the polarising beamsplitting means (16) through which the first laser beam (14) is applied whereby the return laser beam (29) reaches the receiver (13) via the polarising beamsplitting means (16); said interferometer being characterised in that: a first quarter-wave retarder (26) is located in the path of said outgoing second laser beam (17) with its fast axis at 45 degrees to the polarisation orientation of said outgoing second laser beam (17), in order to form an outgoing third laser beam (28) which is circularly polarised, wherein said retroreflecting means are effective to reverse the handedness of circularly polarised light; and a second quarter-wave retarder (27) is located in the path of the return laser beam (29) to the same side of the polarising beamsplitting means (16) as the first quarter-wave retarder (26), said second quarter-wave retarder (27) not intercepting the path of the outgoing second or third laser beams (17, 28) and being in a plane which is parallel to the plane of the first quarter-wave retarder, the fast axes of the two quarter-wave retarders being orthogonal to each other; such that divergent and/or deviated light in the return laser beam (29)directed towards said laser light source and which is not intercepted by the second quarter-wave retarder (27) is intercepted by the first quarter-wave retarder (26) and is then directed by said polarising beamsplitting means (16) away from said laser light source, whereby optical feedback isolation is provided, and whereby return laser beam light (29) directed towards the receiver (13) and intercepted by the second quarter-wave retarder (27) is allowed to be transmitted by said polarising beamsplitting means (16) and reach the receiver (13). A laser interferometer according to claim 1, wherein said retroreflecting means (19) is a movable retroreflector in order to provide said return laser beam (29) as a measurement return laser beam (29), and said beam splitting means (16) separates said first laser beam (14) into said second outgoing laser beam (17) and into a reference outgoing beam (18) having a linear polarisation which is orthogonal to that of said second laser beam (17), said beam splitting means (16) directing said reference outgoing beam (18) towards a fixed retroreflector located outside of the paths of any of said first, second, third and measurement return laser beams (14,17,28,29) and arranged to retroflect said reference outgoing beam (18) incident thereon as a reference return beam on a path such that said reference return beam is directed by said beamsplitting means (16) to reach the receiver (13). A laser interferometer according to claim 1 or claim 2, wherein said first and second quarter-wave retarders (26,27) are contiguous and coplanar. A laser interferometer according to claim 3, wherein said first and second quarter-wave retarders (26,27) are sandwiched between a pair of optically transmitting windows (37,38).

Inventors: GLIEVEF JOHN H, PARKER DAVID R, GLIEVEF, JOHN H., PARKER, DAVID R.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1989
Main classification: G02B5
IPC codes: G02B27, G01J9, G01B9