Abstract:
An absolute distance meter (ADM) that determines a distance to a target includes a light source that emits an emitted light beam. The ADM also includes a fiber switching network having at least one optical switch that switches between at least two positions in response to a switch control signal, a first one of the positions enabling a measure mode in which the emitted light beam is emitted from the fiber switching network towards the target and is reflected back as a measure light beam into the fiber switching network, a second one of the positions enabling a reference mode in which the light beam comprises a reference light beam within the fiber switching network. The ADM further includes a single channel detector that detects the measure and reference light beams in a temporally spaced multiplexed manner and provides an electrical signal which corresponds to the detected measure and reflected light beams. Also, the ADM includes a single channel signal processor that processes the electrical signal and provides a conditioned electrical signal in response thereto, and a data processor that processes the conditioned electrical signal to determine the distance to the target.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/232,222 entitled “ABSOLUTE DISTANCE METER WITH OPTICAL SWITCH”, filed Aug. 7, 2009, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to absolute distance meters, and more particularly to an absolute distance meter having an optical fiber switching network that reduces undesirable drift within the absolute distance meter, thereby providing for more accurate distance measurements. 
     BACKGROUND 
     Generally, an absolute distance meter (ADM) is a device that determines the distance to a remote target. It does this by sending laser light to the target and then collecting light that the target reflects or scatters. An ADM may be used to measure distances in one dimension, as might be seen, for example, in a consumer product available at a hardware store. It may be attached into a more complex device having the ability to measure quantities corresponding to additional dimensions (degrees of freedom). 
     An example of a device of the latter type is the laser tracker, which measures three-dimensional spatial coordinates. Exemplary systems are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al. The laser tracker sends a laser beam to a retroreflector target held against a surface of interest or placed into a fixed nest. The most common type of retroreflector target is the spherically mounted retroreflector (SMR), which may comprise a cube-corner retroreflector mounted within a sphere with the vertex of the cube-corner at the sphere center. 
     A device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. Another device closely related to the laser tracker is the total station, typically used by surveyors. The broad definition of laser tracker, which includes laser scanners and total stations, is used throughout this document. 
     A radar device is similar to a laser tracker in that it emits and receives electromagnetic waves and analyzes the received waves to learn the distance to a target. Radars usually emit waves in the RF, microwave, or millimeter region of the electromagnetic spectrum, whereas laser trackers usually emit waves in the visible or near-infrared region. Radars may be either bistatic or monostatic. Monostatic radars emit and receive electromagnetic energy along a common path, whereas bistatic radars emit and receive on different paths. Total stations may also be either bistatic or mono static. Laser trackers used for high accuracy industrial measurement, however, are monostatic. 
     To understand why laser trackers are monostatic, consider a beam emitted by the laser tracker that travels to a retroreflector target and is retroreflected back on itself. If a bistatic mode were used in the tracker, the incident laser beam would strike off the retroreflector center and the reflected laser beam would shift relative to the incident beam. Small-size retroreflector targets of the sort often used with laser trackers would not be compatible with such a bistatic device. For example, a common type of retroreflector target is the 0.5-inch diameter SMR. The cube-corner retroreflector in such an SMR typically has a clear aperture diameter of about 0.3 inch, which equals about 7.5 mm. The 1/e 2  irradiance diameter of a laser beam from a tracker might be about this large or larger. Consequently, any shift in the laser beam would cause the beam to be clipped by the SMR. This would result in an unacceptably large drop in optical power returned to the tracker. 
     Bistatic geometry would also be problematic for a fiber-optic based ADM system. In a monostatic laser tracker that launches laser light from an optical fiber, a laser collimator can be made by placing the end face of the optical fiber at the focal point of a collimating lens. On the return path from the distant retroreflector, collimated laser light again strikes the collimating lens, although in general the returning laser beam may be off center with respect to the outgoing laser light. The fiber end face is located at the focus of the collimating lens, which has the effect of causing the light from the retroreflector target to be efficiently coupled back into the fiber, regardless of where the beam strikes the lens. In a bistatic device, alignment of the fiber-optic receiving optics is much more challenging and coupling efficiency is much lower. 
     One type of laser tracker contains only an interferometer (IFM) without an absolute distance meter. If an object blocks the path of the laser beam from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner, as described in more detail below. Some laser trackers contain only an ADM without an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,455,670 to Payne, et al. Other laser trackers typically contain both an ADM and an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,764,360 to Meier, et al. 
     A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest. 
     Angular encoders attached to the mechanical azimuth and zenith axes of the tracker may measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR. 
     One of the main applications for laser trackers is to scan the surface features of objects to determine their geometrical characteristics. For example, an operator can determine the angle between two surfaces by scanning each of the surfaces and then fitting a geometrical plane to each. As another example, an operator can determine the center and radius of a sphere by scanning the sphere surface. 
     Prior to U.S. Pat. No. 7,352,446 to Bridges et al., an interferometer, rather than an ADM, was required for the laser tracker to scan moving targets. Until that time, absolute distance meters were too slow to accurately find the position of a moving target. To get full functionality with both scanning and point-and-shoot capability, early laser trackers needed both an interferometer and an ADM. 
     A general comparison of interferometric distance measurement and absolute distance measurement follows. In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between a plurality of targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. 
     Although there are several sources of error in an interferometer measurement, in most cases the dominant error is in the value of the average wavelength of the laser light over its path through the air. The wavelength at a point in space is equal to the vacuum wavelength of the laser light divided by the index of refraction of the air at that point. The vacuum wavelength of the laser is usually known to high accuracy (better than one part in 10,000,000), but the average refractive index of air is known less accurately. The refractive index of air is found by first using sensors to measure the temperature, pressure, and humidity of the air and then inserting these measured values into an appropriate equation, such as the Ciddor equation or the Edlin equation. 
     However, the temperature, pressure, and humidity are not uniform over space, and neither are the sensors perfectly accurate. For example, an error in the average temperature of one degree Celsius causes an error in the refractive index of about one part per million (ppm). As mentioned above, the wavelength of light in air is inversely proportional to the air refractive index. 
     Similarly, in an ADM, the so-called ADM wavelength of the amplitude modulation envelope (also known as the ambiguity range) is inversely proportional to the air group refractive index. Because of this similarity, errors in measuring temperature, pressure, and humidity cause errors in calculated distance that are approximately equal for ADM and interferometer systems. 
     However, ADMs are prone to errors not found in interferometers. To measure distance, an interferometer uses an electrical counter to keep track of the number of times that two beams of light have gone in and out of phase. The counter is a digital device that does not have to respond to small analog differences. By comparison, ADMs are usually required to measure analog values, such as phase shift or time delay, to high precision. 
     In most high-performance ADMs, laser light is modulated, either by applying an electrical signal to the laser source or by sending the laser light through an external modulator such as an acousto-optic modulator or electro-optic modulator. This modulated laser light is sent out of the ADM to a remote target, which might be a retroreflector or a diffuse surface. Light reflects or scatters off the remote target and passes, at least in part, back into the ADM. 
     To understand the difficulties faced by ADMs, we consider two common ADM architectures: temporally incoherent architecture and temporally coherent architecture. In some temporally coherent systems, the returning laser light is mixed with laser light from another location before being sent to an optical detector that converts the light into an electrical signal. This signal is decoded to find the distance from the ADM to the remote target. In such systems, modulation may be applied to the amplitude, phase, or wavelength of the laser light. In other temporally coherent systems, several pure laser lines having different wavelengths are combined before being sent to the retroreflector. These different wavelengths of light are combined at the detector, thereby providing “synthetic” modulation. 
     In temporally incoherent optical systems, light is not usually mixed with light of another wavelength in an optical detector. The simplest type of temporally incoherent system uses a single measure channel and no reference channel. Usually laser light in such systems is modulated in optical power. Light returning from the retroreflector strikes an optical detector that converts the light into an electrical signal having the same modulation frequency. This signal is processed electrically to find the distance from the tracker to the target. The main shortcoming of this type of system is that variations in the response of electrical and optical components over time can cause jitter and drift in the computed distance. 
     To reduce these errors in a temporally incoherent system, one approach is to create a reference channel in addition to the measure channel. This is done by creating two sets of electronics. One set of electronics is in the measure channel. Modulated laser light returned from the distant retroreflector is converted by an optical detector to an electrical signal and passes through this set of electronics. The other set of electronics is in the reference channel. The electrical modulation signal is applied directly to this second set of electronics. By subtracting the distance measured in the reference channel from the distance found in the measure channel, jitter and drift are reduced in ADM readings. This type of approach removes much of the variability caused by electrical components, especially as a function of temperature. However, it cannot remove variability arising from differences in electro-optical components such as the laser and detector. 
     To reduce these errors further, part of the modulated laser light can be split off and sent to an optical detector in the reference channel. Most of the variations in the modulated laser light of the measure and reference channels are common mode and cancel when the reference distance is subtracted from the measure distance. 
     Despite these improvements, drift in such ADM systems can still be relatively large, particularly over long time spans or over large temperature changes. All of the architectures discussed above are subject to drift and repeatability errors caused by variations in optical and electrical elements that are not identical in the measure and reference channels. Optical fibers used in ADM systems change optical path length with temperature. Electrical assemblies used in ADM systems, such as amplifiers and filters, change electrical phase with temperature. 
     A method and apparatus for greatly reducing the effects of drift in an ADM within a laser tracker is taught in U.S. Pat. No. 6,847,436 to Bridges, the contents of which are herein incorporated by reference. This method involves use of a chopper assembly to alternately redirect returning laser light to a measure or reference path. Although this method works well, there is a limitation in the maximum rate of rotation of the chopper wheel and hence in the data collection rate of the ADM. 
     A method of measuring the distance to a moving retroreflector is taught in U.S. Pat. No. 7,352,446 to Bridges et al., the contents of which are herein incorporated by reference. To obtain the highest possible performance using the method of U.S. Pat. No. 7,352,446, the distances are recomputed at a high rate, preferably at a rate of at least 10 kHz. It is difficult to make a mechanical chopper as in U.S. Pat. No. 6,847,436 with a data rate this high. Hence another method needs to be found to solve the ADM drift problem. 
     It is possible to correct for drift in a distance meter by mechanically switching an optics beam between two free-space optical paths. One optical path, which is called the reference path, is internal to the instrument. The second optical path, which is called the measure path, travels out from the instrument to the object being measured and then back to the instrument. Light from the measure and reference paths strikes a single optical detector. Because of the action of the mechanical switch, the light from the two reference paths does not strike the single optical detector at the same time. The mechanical switch may be a mechanically actuated optical component such as a mirror, prism, beam splitter, or chopper wheel. The actuator may be a solenoid, motor, voice coil, manual adjuster, or similar device. Because the optical detector and electrical circuitry is the same for the measure and reference paths, almost all drift error is common mode and cancels out. Examples of inventions based on this method include U.S. Pat. No. 3,619,058 to Hewlett et al.; U.S. Pat. No. 3,728,025 to Madigan et al.; U.S. Pat. No. 3,740,141 to DeWitt; U.S. Pat. No. 3,779,645 to Nakazawa et al.; U.S. Pat. No. 3,813,165 to Hines et al.; U.S. Pat. No. 3,832,056 to Shipp et al.; U.S. Pat. No. 3,900,260 to Wendt; U.S. Pat. No. 3,914,052 to Wiklund; U.S. Pat. No. 4,113,381 to Epstein; U.S. Pat. No. 4,297,030 to Chaborski; U.S. Pat. No. 4,453,825 to Buck et al.; U.S. Pat. No. 5,002,388 to Ohishi et al.; U.S. Pat. No. 5,455,670 to Payne et al.; U.S. Pat. No. 5,737,068 to Kaneko et al.; U.S. Pat. No. 5,880,822 to Kubo; U.S. Pat. No. 5,886,777 to Hirunuma; U.S. Pat. No. 5,991,011 to Damm; U.S. Pat. No. 6,765,653 to Shirai et al.; U.S. Pat. No. 6,847,436 to Bridges; U.S. Pat. No. 7,095,490 to Ohtomo et al.; U.S. Pat. No. 7,196,776 to Ohtomo et al.; U.S. Pat. No. 7,224,444 to Stierle et al.; U.S. Pat. No. 7,262,863 to Schmidt et al.; U.S. Pat. No. 7,336,346 to Aoki et al.; U.S. Pat. No. 7,339,655 to Nakamura et al.; U.S. Pat. No. 7,471,377 to Liu et al.; U.S. Pat. No. 7,474,388 to Ohtomo et al.; U.S. Pat. No. 7,492,444 to Osada; U.S. Pat. No. 7,518,709 to Oishi et al.; U.S. Pat. No. 7,738,083 to Luo et al.; and U.S. Published Patent Application No. US2009/0009747 to Wolf et al. Because all of these patents use mechanical switches, which are slow, none can switch quickly enough to be used in an ADM that accurately measures a moving retroreflector. 
     Another possibility is to correct drift only in the electrical, and not the optical, portion of a distance meter. In this case, light from the reference optical path is sent to the reference optical detector and light from the measure optical path is sent to the measure optical detector. The electrical signals from the reference and optical detectors travel to an electrical switch, which alternately routes the electrical signals from the two detectors to a single electrical unit. The electrical unit processes the signals to find the distance to the target. Examples of inventions based on this method include: U.S. Pat. No. 3,365,717 to Hölscher; U.S. Pat. No. 5,742,379 to Reifer; U.S. Pat. No. 6,369,880 to Steinlechner; U.S. Pat. No. 6,463,393 to Giger; U.S. Pat. No. 6,727,985 to Giger; U.S. Pat. No. 6,859,744 to Giger; and U.S. Pat. No. 6,864,966 to Giger. Although the use of an electrical switch can reduce drift in the electrical portion of an ADM system, it cannot remove drift from the optical portion, which is usually as large or larger than the drift in the electrical portion. In addition, it is difficult to implement an electrical switching system that can switch quickly enough to avoid a phase shift in electrical signals modulated at several GHz. Because of their limited utility and difficulty of implementation, electrical switches are not a good solution for correcting drift in an ADM. 
     For a bistatic distance meter, there are two references that discuss the use of fiber optic switches. U.S. Published Patent Application No. US2009/0046271 to Constantikes teaches a method in which one fiber switch is placed in the outgoing beam path and a second fiber switch is placed in the returning beam path. These two fiber optic switches are switched at the same time to either permit light from the measure or reference path to reach the optical detector. U.S. Pat. No. 4,689,489 to Cole teaches use of a fiber switch in which light from the return port of the bistatic distance meter is into one port of a switch and light from the outgoing beam is fed into the second port of the switch. The fiber-switch architectures described in these references apply only to bistatic devices and cannot be used with laser trackers for reasons discussed earlier. 
     There is a need for an ADM that accurately measures moving targets with little drift. It must be monostatic and minimize drift in both optical and electrical components. 
     SUMMARY 
     According to an aspect of the present invention, an absolute distance meter (ADM) that determines a distance to a target includes a light source that emits an emitted light beam. The ADM also includes a fiber switching network having at least one optical switch that switches between at least two positions in response to a switch control signal, a first one of the positions enabling a measure mode in which the emitted light beam is emitted from the fiber switching network towards the target and is reflected back as a measure light beam into the fiber switching network, a second one of the positions enabling a reference mode in which the light beam comprises a reference light beam within the fiber switching network. The ADM further includes a single channel detector that detects the measure and reference light beams in a temporally spaced multiplexed manner and provides an electrical signal which corresponds to the detected measure and reference light beams. Also, the ADM includes a single channel signal processor that processes the electrical signal and provides a conditioned electrical signal in response thereto, and a data processor that processes the conditioned electrical signal to determine the distance to the target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  is a perspective view of an exemplary laser tracker sending a laser beam to an external retroreflector; and 
         FIG. 2A  is a block diagram of a tracker electro-optics assembly including an ADM with an optical fiber switching network, visible laser, and tracker optics; and 
         FIG. 2B  is a block diagram of a tracker electro-optics assembly including an ADM with an optical fiber switching network, incremental distance meter assembly, and tracker optics; and 
         FIG. 3  is a block diagram of a tracker electro-optics assembly including an ADM with an optical fiber switching network and tracker optics; and 
         FIG. 4  is a block diagram of a tracker electro-optics assembly including an ADM with an optical fiber switching network and simplified optics; and 
         FIG. 5  shows an optical fiber switching network that includes a fiber optic switch, optical coupler, and a partial fiber retroreflector according to an embodiment of the present invention; and 
         FIG. 6  shows an optical fiber switching network that includes a fiber optic switch, optical circulator, and partial fiber retroreflector according to another embodiment of the present invention; and 
         FIG. 7  shows an optical fiber switching network that includes two fiber optic couplers and a fiber-optic switch according to yet another embodiment of the present invention; and 
         FIG. 8  shows an optical fiber switching network in which multiple fiber optic switches are combined to increase optical isolation according to still another embodiment of the present invention; and 
         FIG. 9  shows an optical fiber switching network in which the switching action is performed by optical modulators or optical attenuators according to another embodiment of the present invention; and 
         FIG. 10  is a block diagram of exemplary ADM electronics used in embodiments of the present invention; and 
         FIG. 11  is a block diagram of the data processor used in embodiments of the present invention; and 
         FIG. 12  is a graph of an exemplary signal from an ADM system; and 
         FIG. 13  is a graph of an exemplary switching signal; 
         FIG. 14  is a graph of an exemplary gating signal; 
         FIG. 15  is a block diagram of a processing system used in embodiments of the present invention; and 
         FIG. 16  is a block diagram of ADM electronics used in embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary laser tracker  10  is illustrated in  FIG. 1 . An exemplary gimbaled beam-steering mechanism  12  of laser tracker  10  comprises zenith carriage  14  mounted on azimuth base  16  and rotated about azimuth axis  20 . Payload  15  is mounted on zenith carriage  14  and rotated about zenith axis  18 . Zenith mechanical rotation axis  18  and azimuth mechanical rotation axis  20  intersect orthogonally, internally to tracker  10 , at gimbal point  22 , which is typically the origin for distance measurements. Laser beam  46  virtually passes through gimbal point  22  and is pointed orthogonal to zenith axis  18 . In other words, laser beam  46  is in the plane normal to zenith axis  18 . Laser beam  46  is pointed in the desired direction by rotation of payload  15  about zenith axis  18  and by rotation of zenith carriage  14  about azimuth axis  20 . Zenith and azimuth angular encoders, internal to the tracker (not shown), are attached to zenith mechanical axis  18  and azimuth mechanical axis  20  and indicate, to high accuracy, the angles of rotation. Laser beam  46  travels to external retroreflector  26  such as the spherically mounted retroreflector (SMR) described above. By measuring the radial distance between gimbal point  22  and retroreflector  26  and the rotation angles about the zenith and azimuth axes  18 ,  20 , the position of retroreflector  26  is found within the spherical coordinate system of the tracker. 
     Laser beam  46  may comprise one or more laser wavelengths, as will be described in the discussion that follows. For the sake of clarity and simplicity, a steering mechanism of the sort shown in  FIG. 1  is assumed in the following discussion. However, other types of steering mechanisms are possible. For example, it would be possible to reflect a laser beam off a minor rotated about the azimuth and zenith axes. The techniques described here are applicable, regardless of the type of steering mechanism. 
     Elements of the Laser Tracker 
     Tracker electro-optics assembly  250 A is shown in  FIG. 2A . It comprises ADM assembly  2000 , visible laser  110 , and optical assembly  190 . ADM assembly  2000  comprises ADM electronics  300 , ADM laser  102 , optical fiber switching network  200 , and data processor  400 . Optical assembly  190  comprises ADM beam collimator  140 , visible-beam launch  150 , tracking assembly  170 , and beam expander  160 . 
     There are many ways to modulate light. One type of modulation is of optical power, with the modulation signal usually either sinusoidal or pulsed. Another type of modulation is of optical wavelength. This type of modulation is sometimes used in coherent laser distance meters. Modulation may be applied directly to the light source or to an external modulator, such as an electro-optic modulator, to vary the power, polarization, or phase of the laser light. The method described in this disclosure is applicable to any of these types of modulation. Light can come from a laser, superluminescent diode, or any other type of optical emitter. In the text below, the light source is often referred to as a laser, but this should not be taken to limit the type of light source that could be used. 
     Light from ADM laser  102  is injected into optical fiber  104  and travels to fiber switching network  200 . Some light from fiber switching network  200  travels through fiber optic cable  501  to ADM beam collimator  140 . ADM beam collimator  140  comprises stable ferrule  142  and positive lens  144 . Optical fibers are preferably of the single-mode type. 
     In the event that ADM laser  102  operates at an infrared wavelength, it is convenient to provide a visible laser beam to help make the beam easier to find. Visible laser  110  sends visible light through fiber optic cable  215  to visible-beam launch  150 , which comprises stable ferrule  152 , positive lens  154 , and dichroic beam splitter  114 . Dichroic beam splitter  114  transmits ADM beam  108  but reflects visible beam  112 . To the right of beam splitter  114 , composite laser beam  116  comprises visible laser beam  112  and ADM laser beam  108 , which are substantially collinear. Laser beam  116  passes through beam splitter  118  and beam expander  160 , emerging as a larger collimated laser beam  46 . Beam expander  160  comprises negative lens  162  and positive lens  164 . 
     In some applications, it is desirable to include an interferometer (IFM) in addition to an ADM. Tracker electro-optics assembly  250 B is shown in  FIG. 2B . It comprises the same elements as electro-optics assembly  250 A except that visible laser  110  is replaced with incremental distance meter assembly  180 . Incremental distance meter assembly  180  comprises stable laser  182  and interferometer (IFM) assembly  184 . Stable laser  182  is preferably a frequency stabilized helium-neon laser that produces a red beam. IFM assembly  184  comprises optics and electronics (not shown) that measure the incremental change in distance to retroreflector  26 . 
     Laser beam  46  travels to external retroreflector  26 , as shown in  FIG. 1 . Beam  46  reflects off retroreflector  26  and returns to laser tracker  10  as beam  48 . If laser beam  46  strikes the center of retroreflector  26 , reflected laser beam  48  retraces the path of the incident laser beam  46 . If laser beam  46  strikes the retroreflector  26  off the center, reflected laser beam  48  returns parallel to incident beam  46  but offset from it. Reflected laser beam  48  re-enters tracker  10  through beam expander  160  and retraces the path back through the optical system. 
     Tracking assembly  170  comprises beam splitter  118 , optional optical filter  128 , and position detector  130 . Some of reflected laser beam  48  bounces off beam splitter  118  and passes through optional optical filter  128  to strike position detector  130 . Optical filter  128  blocks undesirable wavelengths of light, such as ambient light in the vicinity of retroreflector  26 . 
     Position detector  130  produces an electrical signal that indicates the position of the spot of light on position detector  130 . Position detector  130  may be any type of detector that indicates the position of the returning light beam. For example, it may be a position sensitive detector such as a lateral effect detector or quadrant detector or it may be a photosensitive array such as CCD or CMOS array. The retrace point of the position detector is defined as the point that laser beam  126  strikes if laser beam  46  strikes the center of retroreflector  26 . When laser beam  46  moves off the center of retroreflector  26 , laser beam  126  moves off the retrace point and causes the position detector  130  to generate an electrical error signal. A servo system (not shown) processes this error signal to activate motors (not shown) that turn laser beam  46  from laser tracker  10  toward the center of the external retroreflector  26 . By this means, the laser beam from tracker  10  is made to track the movement of retroreflector  26 . 
     Dichroic beam splitter  114  transmits the returning ADM laser light through ADM beam collimator  140 , where it is coupled into optical fiber  501 . The laser light travels back into fiber switching network  200 , and a part of it travels through optical fiber  230  to ADM electronics  300 . ADM electronics  300  converts the optical signal into an electrical signal and conditions the electrical signal in a way appropriate for the particular type of modulation applied to the laser light. The signal from ADM electronics  300  is sent to data processor  400 , which processes the signal to find result  420 , the distance from tracker gimbal point  22  to retroreflector target  26 . 
     The components of tracker electro-optics assembly  250 A,  250 B may be located entirely within tracker payload  15 , located partly within tracker payload  15  and partly within azimuth base  16 , or located entirely within azimuth base  16 . If ADM or interferometer components are located in azimuth base  16 , these may be connected to optical components by routing fiber optic cables through the mechanical azimuth and zenith axes into payload  15 . This method is shown in WO 2003/062744, which is incorporated herein by reference. Alternatively, if ADM or interferometer components are located in azimuth base  16 , the light emitted by ADM laser  102  or stable laser  182  may be sent through free space to a beam steering mirror located in the payload. This method is shown in U.S. Pat. No. 4,714,339 to Lau et al. 
     Optical fiber switching network  200  provides a means of routing and switching optical signals to and from optical assembly  190 . Fiber switching network  200  is described in more detail below. 
     It is possible to eliminate visible-light laser  110  in  FIG. 2A  or incremental distance meter assembly  180  in  FIG. 2B . In this case, visible-beam launch  150  is not necessary. The resulting electro-optics assembly  350  is shown in  FIG. 3 . This architecture might be appropriate if an IFM were not needed and if ADM laser  102  emitted visible laser light. It might also be appropriate if the IFM were not needed and if a visible pointer beam was not needed. 
     For handheld distance meters or other instruments that do not track, the architecture can be further simplified by eliminating tracking assembly  170  and possibly beam expander  160 . The resulting ADM distance meter  450  is shown in  FIG. 4 . 
       FIGS. 2A ,  2 B,  3 , and  4  all contain ADM assembly  2000 , which contains optical fiber switching network  200 . The benefit of fiber switching network  200  is that it enables a reduction in the drift of the ADM distance readings. The reason for this reduction can be understood by considering ADM electronics  300  in more detail. A specific embodiment for the ADM electronics is considered in the discussion that accompanies  FIGS. 10 and 11 ; that is, in conjunction with a laser tracker. However, the advantages of the fiber switching network for reducing drift in an ADM system applies more generally to ADM systems and may include for example pulsed time-of-flight ADMs, chirped ADMs, and coherent as well as incoherent ADMs. To explain how fiber switching network  200  enables the reduction in drift, reference is now made to  FIG. 16 , which describes the elements of ADM electronics  300  in more general terms. 
     In  FIG. 16 , ADM electronics  300  comprises laser transmitter  310 , single channel laser receiver  320 , single channel signal line  332 , and interconnection lines  330 ,  334 , and  336 . Laser transmitter  310  may generate a variety of signals. A signal from interconnection line  330  is used to modulate ADM laser  102 . In addition, most types of ADM systems generate one or more additional signals used in processing of the signal in single channel receiver  320 . The combination of such signals is referred to here as the single channel signal  332 , for reasons that will become clear in the discussion that follows. 
     Single channel receiver  320  comprises single detector  322  and single channel electronics  324 . Light arrives at single detector  322  over interconnection line  336 , which is a fiber optic cable attached to fiber switching network  200 . Single detector  322  converts the optical signal from  336  to an electrical signal. This electrical signal is processed by single channel electronics, and the resulting processed signal is sent over interconnection line  334  to data processor  400 . 
     The drift seen in ADM systems is generally the result of changes in the electrical and optical systems over time and especially with respect to changes in temperature. In the Background section of this document, it was explained that ADM systems often try to remove the effects of such changes by subtracting the readings of a reference channel from those of a measure channel. As explained, the signal in the reference channel can be optical or electrical, with an optical reference signal generally producing the highest performance. The use of two channels in this way can only correct drift to a limited degree because two separate electrical channels are required in the receiver unit—one for the measure channel and one for the reference channel. If the reference signal is optical, the receiver unit must also provide two separate optical detectors—one for the measure channel and one for the reference channel. However, the electrical and optical components within the two channels are not identical and neither are the temperatures of the components in each of the channels. Consequently, drift seen within the measure and reference channels is not completely common mode and does not completely cancel out. 
     By using a fiber switching network to multiplex optical signals, it is possible to use a single detector to serve both measure and reference channels. It is also possible to use a single electrical channel, rather than two electrical channels, in the receiver. Because there is only a single electrical receiver channel, any electrical signals supplied by transmitter  310  need to be provided in only a single channel. The result of the single optical detector, the single electrical receiver channel, and the single channel signals from the transmitter is a nearly complete cancellation of drift effects. The resulting ADM system is nearly drift free. 
     Fiber Switching Network 
     Several possible embodiments of an optical fiber switching network  200  in accordance with the present invention are discussed here. These are labeled as  200 A- 200 E in  FIGS. 5-9 , respectively.  FIG. 5  shows ADM system  550 , which comprises ADM laser  102 , fiber switching network  200 A, ADM electronics  300 , and stable ferrule  142 . Fiber switching network  200 A comprises fiber optic coupler  206 , fiber-optic switch  500 , partial fiber retroreflector  505 , interconnecting optical fibers  104 ,  230 ,  501 ,  502 ,  503 ,  510 , and electrical connection  470 . Light travels from ADM laser  102  through optical fiber  104  into optical coupler  206 . Part of the light from fiber coupler  206  travels to low-reflection termination (LRT)  208 , which absorbs almost all of the light. Preferably, the reflectance of LRT  208  is less than 1/50000. The rest of the light from fiber coupler  206  travels through optical fiber  503  to optical switch  500 . In this case fiber-optic switch  500  is a single-pole double throw (SPDT) switch, but other types of switches could be used. 
     Electrical connection  470  sends to fiber-optic switch  500  an electrical signal that controls whether the optical signal is routed to optical fiber  501  or optical fiber  502 . If switch  500  routes light to optical fiber  501 , light passes from stable ferrule  142  through the tracker and out to retroreflector  26 . The returning laser light travels to fiber-optic switch  500 , through coupler  206 , through fiber  230 , and into ADM electronics  300 . Light that travels along this path to and from the retroreflector is said to be in the measure path and, during this time, the tracker is said to be in the measure mode. 
     If switch  500  routes light to optical fiber  502 , light passes to partial fiber retroreflector  505 , which reflects a fraction of laser light back through coupler  206 , through fiber  230 , and into ADM electronics  300 . Light that travels internal to the tracker by reflecting off partial fiber retroreflector  505  is said to be in the reference path and, during this time, the tracker is said to be in the reference mode. 
     Fiber coupler  206  is preferably a 50/50 coupler, also known as a 3 dB coupler. For light injected into a 50/50 coupler  206  by ADM laser  102 , 50% of the laser light goes to optical fiber  510  and 50% goes to optical fiber  503 . For light injected into coupler  206  from the reverse direction, 50% of the returning light goes to ADM laser  102  and 50% of the returning light goes to ADM electronics  300 . Faraday isolation is provided within ADM laser  102  to prevent light that passes through fiber coupler  206  to ADM laser  102  from destabilizing the laser. 
     The amount of light returned to optical fiber  501  after the light has traveled to retroreflector  26  depends on a number of factors including the distance to the retroreflector, the diameter and tilt of the retroreflector, and the coupling efficiency of the ADM beam collimator  140 . The reflectance of partial fiber retroreflector  505  is preferably selected to reflect laser power approximately equal to the average of powers returned by retroreflector  26  under different measurement conditions. 
     Fiber-optic switch  500  should preferably have optical isolation between the two switching positions of at least 20 dB. This means that, when the switch is in the up position, the amount of optical power that leaks into the down position is less than that applied to the up position by a factor of at least 100. After reflecting and retracing the path, isolation is reduced by another factor of 100, so that the overall effective isolation is a factor of 10 4 , or 40 dB. Switches with lower levels of isolation can be used by combining them to increase their overall isolation, as explained below. 
     In addition to optical isolation, fiber-optic switch  500  should preferably have optical return loss of at least 40 dB. This means that the light reflected back by the switch should be reduced by a factor of at least 10,000 compared to the incident light. This ensures that excessive unwanted light is not reflected onto the light traveling on the desired path and thereby reducing the accuracy of the measurement. 
     A second fiber switching network  200 B is shown in ADM system  650  of  FIG. 6 . ADM system  650  comprises ADM laser  102 , fiber switching network  200 B, ADM electronics  300 , and stable ferrule  142 . Fiber switching network  200 B comprises optical circulator  610 , fiber-optic switch  500 , partial fiber retroreflector  505 , interconnecting optical fibers  104 ,  230 ,  501 ,  502 ,  503 , and electrical connection  470 . Light travels from ADM laser  102  through optical fiber  104  into port  601  and out port  602  to fiber  503 . From fiber  503 , the light travels as described above for ADM system  550 . Return light passes back through port  602  and out port  603  to optical fiber  230 . 
     The advantage of a three-port circulator, such as  610  in  FIG. 6 , compared to a four-port fiber optic coupler, such as  206  in  FIG. 5 , is that no power is lost to the fourth port, which in  206  of  FIG. 5  is dissipated in low-reflection termination  208 . The disadvantage of a circulator is that it will generally have some level of polarization mode dispersion (PMD). As a result, any change in polarization state of light returned on optical fiber  501  or  502  can result in a delay in the phase of the modulated light, thereby producing an error in the reported ADM distance. 
     A third fiber switching network  200 C is shown in ADM system  750  of  FIG. 7 . ADM system  750  comprises ADM laser  102 , fiber switching network  200 C, ADM electronics  300 , and stable ferrule  142 . Fiber switching network  200 C comprises fiber-optic coupler  204 , fiber-optic coupler  206 , low-reflection terminations  208 ,  715 , fiber switch  700 , interconnecting optical fibers  104 ,  230 ,  501 ,  510 ,  701 , and  716 , and electrical connection  470 . Light travels from ADM laser  102  through optical fiber  104  to first optical coupler  204 . Part of the light from first optical coupler  204  travels through a reference optical fiber  702  to switch  700 , and the other part travels through optical fiber  716  to second optical coupler  206 . Part of the light from second optical coupler  206  travels through optical fiber  510  to low reflection termination  208 , and the other part travels through optical fiber  501  to stable ferrule  142 . 
     Light returned to stable ferrule  142  travels back through optical fiber  501  to second optical coupler  206 . Part of the return light from second optical coupler  206  travels to optical switch  700 . Another part of the return light from second optical coupler  206  travels back through optical fiber  716  to first optical coupler  204 . Part of this return light goes through optical fiber  104  to ADM laser  102 , where it is blocked by a Faraday isolator built into the laser. Another part of the return light travels through optical fiber  715  to low reflection termination  210 . 
     In the measure mode, electrical connection  470  causes switch  700  to connect optical fiber  701  to ADM electronics  300 . In the reference mode, electrical connection  470  causes switch  700  to connect optical fiber  702  to ADM electronics  300 . Compared to ADM system  550 , ADM system  750  has the advantage of not requiring partial fiber retroreflector  208 . It has the disadvantage of requiring an extra fiber-optic coupler, an extra low-reflection termination, and an additional optical fiber  702 . 
     A fourth fiber switching network  200 D is shown in ADM system  850  of  FIG. 8 . ADM system  850  comprises ADM laser  102 , fiber switching network  200 D, ADM electronics  300 , and stable ferrule  142 . Fiber switching network  200 D comprises fiber-optic coupler  206 , fiber switches  500 ,  810 ,  820 , low-reflection terminations  208 ,  816 ,  826 , partial fiber retroreflector  505 , interconnecting optical fibers  104 ,  230 ,  501 ,  502 ,  503 ,  510 ,  812 ,  814 ,  822 ,  824 , and electrical connection  470 . Fourth fiber switching network configuration  850  is a modification of ADM system  550  shown in  FIG. 5  to increase the isolation between the measure and reference channels by adding cascaded switches  810  and  820 . 
     In the measure mode, switch  500  connects optical fiber  503  to optical fiber  812 , and switch  810  connects optical fiber  812  to optical fiber  501 . Also, in the measure mode, switch  820  connects optical fiber  502  to optical fiber  822  that leads to low-reflection termination  826 . Suppose that the isolation of each switch  500 ,  810 ,  820  is 20 dB. This means, for example, that less than 0.01 of the optical power will pass through to the undesired path in a particular switch. In this case, less than 0.01 of the optical power present on optical fiber  503  will pass to optical fiber  502 , and less than 0.0001 will pass to fiber  824 . This light reflected by partial fiber retroreflector  505  will be further reduced by a factor of 0.0001 in passing back to optical fiber  503 . In other words, the reflected optical power is decreased by a factor of at least 10 −8 =−80 dB compared to the outgoing optical power on optical fiber  503 . 
     In the reference mode, switch  500  connects optical fiber  503  to optical fiber  502 , and switch  820  connects optical fiber  502  to optical fiber  824  that leads to partial fiber retroreflector  505 . Also, in the reference mode, switch  810  connects optical fiber  812  to optical fiber  814  that leads to low-reflection termination  816 . As in the previous case, for switches each having 20 dB of isolation, the resulting power returned to optical fiber  503  is reduced to less than 10 −8 =80 dB times the original amount. 
     A fifth fiber switching network  200 E is shown in ADM system  950  of  FIG. 9 . ADM system  950  comprises ADM laser  102 , fiber switching network  200 E, ADM electronics  300 , and stable ferrule  142 . Fiber switching network  200 E comprises fiber-optic coupler  206 , optical modulators or attenuators  910 ,  920 , partial fiber retroreflector  505 , interconnecting optical fibers  104 ,  501 ,  503 ,  510 ,  922 ,  230 , and electrical connection  470 . ADM system  950  is like ADM system  550  of  FIG. 5  except that  910 ,  920  are optical modulators or attenuators driven between minimum and maximum levels to act as single pole single throw (SPST) switches. If  910 ,  920  are optical modulators, these are preferably polarization independent and bidirectional in their operation. The operation of ADM system  950  is like that of ADM  850  described above. 
     A specific embodiment of ADM electronics  300  is now considered. This particular embodiment will be referred to as ADM electronics  3000  as is shown in  FIG. 10 . ADM electronics  3000  converts the light output of fiber switching network  200  in either the measure mode or reference mode into a digital electrical signal for processing by the data processor  400  and also generates modulation signal for ADM laser  102 . The input to ADM electronics  3000  is fiber optic  230  and the outputs are electrical modulation signal  360  and conditioned electrical signal  460 . U.S. Pat. No. 7,352,446 to Bridges et al., which is incorporated by reference, discloses details for similar ADM electronics  3000 . 
     ADM electronics  3000  of  FIG. 10  comprises frequency reference  3002 , synthesizer  3004 , detector  3006 , mixers  3010 , amplifiers  3014 ,  3018 , frequency divider  3024 , and analog-to-digital converter (ADC)  3022 . Frequency reference  3002  provides the time base for the ADM and should have low phase noise and low frequency drift. The frequency reference may be an oven-controlled crystal oscillator (OCXO), rubidium oscillator, or any other highly stable frequency reference. Preferably the oscillator frequency should be accurate and stable to within a small fraction of a part per million. The signal from the frequency reference is put into the synthesizer, which generates three signals. The first signal is at frequency f RF  and modulates the optical power of ADM laser  102 . This type of modulation is called intensity modulation (IM). Alternatively, it is possible for the first signal at frequency f RF  to modulate the electric field amplitude, rather than the optical power, of the laser light from ADM laser  102 . This type of modulation is called amplitude modulation (AM). The second and third signals, both at the frequency f LO , go to the local-oscillator ports of mixer  3010 . 
     Fiber-optic cable  230  carries laser light. The light in this fiber-optic cable  230  is converted into electrical signals by detector  3006 . This optical detector  3006  sends the modulation frequency f RF  to amplifier  3014  and then to mixers  3010 . Mixer  3010  produces two frequencies, one at |f LO −f RF | and one at |f LO +f RF |. These signals travel to low-frequency amplifier  3018 . Amplifier  3018  blocks the high-frequency signals so that only the signals at the intermediate frequency (IF), f IF =|f LO −f RF | pass through to the analog-to-digital converter (ADC)  3022 . The frequency reference  3002  sends a signal into frequency divider  3024 , which divides the frequency of the reference  3002  by an integer N to produce a sampling clock. In general, the ADC may decimate the sampled signals by an integer factor M, so that the effective sampling rate is f REF /NM. This effective sampling rate should be an integer multiple of the intermediate frequency f IF . 
     The timing electronics  472  may comprise a frequency divider chip and a microprocessor or field-programmable gate array. The frequency divider chip divides the frequency of the signal from frequency reference  3002  to a lower frequency. This frequency is applied to the microprocessor or field-programmable gate array that uses its internal processing capability to provide the required timing signals shown in  FIGS. 13 and 14 . 
     Here are frequencies for an exemplary ADM: The frequency reference is f REF =20 MHz. The synthesizer RF frequency that drives the laser is f RF =2800 MHz. The synthesizer LO frequency that is applied to the mixers is f LO =2800.01 MHz. The difference between the LO and RF frequencies is the intermediate frequency of f IF =10 kHz. The frequency reference is divided by N=10, to produce a 2-MHz frequency that is applied to the ADC as a sampling clock. The ADC has a decimation factor of M=8, which produces an effective sampling rate of 250 kHz. Since the IF is 10 kHz, the ADC takes 25 samples per cycle. 
     The ADC sends the sampled data to data processor  400  for analysis. Data processors include digital signal processor (DSP) chips and general-purpose microprocessor chips. The processing performed by these processors is described below. 
     As shown in  FIGS. 2-4 , ADM electronics  3000  generates a signal that travels over electrical connection  470  to switch fiber switching network  200  between measure and reference modes. In addition, data processor  400  converts the digital output of ADM electronics  3000  to result  420 , which is a numerical distance value. One exemplary embodiment of data processor  400  is data processor  400 A shown in  FIG. 11 . The input to data processor  400 A is electrical interface  460  to ADM electronics  3000  and the output is result  420 . U.S. Pat. No. 7,352,446, incorporated by reference above, also discloses details for a similar data processor  400 . 
     Data processor  400  of  FIG. 11  takes the digitized data from ADC  3022  and derives from it the distance from the tracker to external retroreflector  26 .  FIG. 11  refers to this distance as the RESULT  420 . Data processor  400  comprises digital signal processor  410 , microprocessor  450 , and crystal oscillators  402 ,  404 . 
     Analog-to-digital converter  3022  sends sampled data to DSP  410 . This data is routed to a program that runs within the DSP. This program contains three main functions: phase-extractor function  420 , compensator function  422 , and Kalman-filter function  424 . The purpose of the phase-extractor function is to determine the phases of the signals, that is, the phases of the signals that pass through the detector  3006 . To determine these phases, the modulation range must first be calculated. Modulation range is defined as the round-trip distance traveled by the ADM laser light in air for the phase of the laser modulation to change by 2 pi radians. 
     To synchronize the ADM measurement to the measurements of the angular encoders and position detector, counter  414  determines the difference in time between the sync pulse and the last state distance. It does this in the following way. Crystal oscillator  404  sends a low-frequency sine wave to frequency divider  452 , located within microprocessor  450 . This clock frequency is divided down to f SYNC , the frequency of the sync pulse. The sync pulse is sent over a device bus to DSP, angular encoder electronics, and position-detector electronics. In an exemplary system, the oscillator sends a 32.768 kHz signal through frequency divider  452 , which divides by 32 to produce a sync-pulse frequency f SYNC =1.024 kHz. The sync pulse is sent to counter  414 , which resides within DSP  410 . The counter is clocked by crystal  402 , which drives a phase-locked loop (PLL) device  412  within the DSP. In the exemplary system, oscillator  402  has a frequency of 30 MHz and PLL  412  doubles this to produce a clock signal of 60 MHz to counter  414 . The counter  414  determines the arrival of the sync pulse to a resolution of 1/60 MHz=16.7 nanoseconds. The phase-extractor function  420  sends a signal to the counter when the ADC  322  has sent all the samples for one cycle. This resets counter  414  and begins a new count. The sync pulse stops the counting of counter  412 . The total number of counts is divided by the frequency to determine the elapsed time. Since the time interval in the above equations was set to one, the normalized time interval t NORM  is the elapsed time divided by the time interval. The state distance x EXT  extrapolated to the sync pulse event is
 
 x   EXT   =x   k   +v   k    t   NORM .
 
The Kalman-filter function  424  provides the result, which is the distance from the tracker to external retroreflector  26 .
 
     It is important to recognize that the method of using fiber-optic switches described herein is not limited to a phase-based distance measurement method, of which the exemplary embodiment of  FIG. 10  is one example. For example, fiber optic switches can equally well be used with a pulsed time-of-flight distance meter. 
       FIG. 12  shows an example of the multiplexed  1300  signal that emerges from signal conditioner  3018  of  FIG. 10  and enters analog-to-digital converter (ADC)  3022  of the same figure. This type of multiplexed signal might be produced by a phase-based ADM. In  FIG. 12 , the larger amplitude represents the signal from the measure channel, and the smaller amplitude represents the signal from the reference channel. The reference and measure signals are multiplexed together by fiber switching network  200 . In the example shown in  FIG. 12 , the frequency of the sinusoidal is 100 kHz, and the corresponding period is 0.01 milliseconds=10 microseconds. Numerical result  420  has, in this example, an output frequency of 10 kHz and a corresponding period of 0.1 milliseconds=100 microseconds. 
     In general, the act of switching between measure and reference signals causes some transients to appear in the output signals of electrical and opto-electric components of ADM electronics  3000 . If these transient signals, which are read by ADC  3022 , were included in the calculations of data processor  400 , an erroneous result  420  would occur. To avoid this problem, it is important that transients have died out in the raw data processed by data processor  400  to get result  420 . 
     In the example considered here, only 80 microseconds of each 100 microsecond period are processed, and the other 20 microseconds are discarded. Of the 80 microseconds that are retained, 20 microseconds (2 sinusoidal periods) are retained from the reference channel and 60 microseconds (6 sinusoidal periods) are retained from the measure channel. 
       FIG. 13  shows timing signal  1200  from electrical connection  470 . Measure mode begins when timing signal  1200  goes to high value  1210 , and reference mode begins when timing signal  1200  goes to low value  1230 .  FIG. 14  shows the gating signal  1250  that indicates when data  460  is considered valid. A high gating signal  1260  indicates that the reference signal is valid. A high gating signal  1265  indicates that the measure signal is valid. A low gating signal  1255  indicates that no signal is valid. 
     The methods of algorithms discussed above are implemented by means of processing system  1500  shown in  FIG. 15 . Processing system  1500  comprises tracker processing unit  1510  and optionally computer  80 . Processing unit  1510  includes at least one processor, which may be a microprocessor, digital signal processor (DSP), field programmable gate array (FPGA), or similar device. Processing capability is provided to process information and issue commands to internal tracker processors. Such processors may include position detector processor  1512 , azimuth encoder processor  1514 , zenith encoder processor  1516 , indicator lights processor  1518 , ADM processor  400 , interferometer (IFM) processor  1522 , and camera processor  1524 . Auxiliary unit processor  1570  optionally provides timing and microprocessor support for other processors within tracker processor unit  1510 . Preferably, it communicates with other processors by means of device bus  1530 , which preferably transfers information throughout the tracker by means of data packets, as is well known in the art. Preferably, computing capability is distributed throughout tracker processing unit  1510 , with DSPs and FPGAs performing intermediate calculations on data collected by tracker sensors. The results of these intermediate calculations are returned to auxiliary unit processor  1570 . Auxiliary unit  1570  may be attached to the main body of laser tracker  10  through a long cable, or it may be pulled within the main body of the laser tracker so that the tracker attaches directly (and optionally) to computer  80 . Preferably, auxiliary unit  1570  is connected to computer  80  by connection  1540 , which is preferably an Ethernet cable or wireless connection. Auxiliary unit  1570  and computer  80  may be connected to the network through connections  1542 ,  1544 , which are preferably Ethernet cables or wireless connections. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.