Abstract:
Techniques for producing higher fidelity interferometer measurements by reducing sensitivity to spurious sources include reducing the coherence length of an electromagnetic beam. In addition, multiple surfaces within an optical system may be measured by electronically tuning the position of a coherence plane along the optical paths of an interferometer. A phase modulator is used in conjunction with a long coherence length electromagnetic source to generate beams for each leg of an interferometer. Providing a controlled broadband RF signal to the phase modulator increases the bandwidth of the beam and thereby reduces the coherence length of the beam. This reduces the spurious contributions to the output interference fringes from undesired surfaces along the beam path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/951,399 filed on Dec. 6, 2007, now U.S. Pat. No. 8,045,251, entitled “Coherence Length Controller,” which is expressly incorporated herein by this reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This application relates generally to reducing the coherence length of electromagnetic beams, and more specifically to removing spurious interference fringes at the output of an interferometer system by reducing the coherence length of an electromagnetic source used within the interferometer. 
     BACKGROUND 
     Coherence is a property of waves, such as those in the electromagnetic spectrum, in which light is a subset. The property of coherence enables two waves to exhibit interference. When waves interfere, parts of the waves may add constructively or subtract destructively. Coherence, or the degree of coherence, is a parameter that quantifies the ability of the two waves to interfere with each another. The property of coherence is used in various applications such as interferometry, positioning, optical testing, holography, optical strain sensors, radio antenna arrays, optical tomography, telescope interferometers, radio astronomy and many other optical and radio frequency (RF) applications. 
     In addition to coherence relating to the ability of two waves to interfere, coherence may also relate to one wave&#39;s ability to interfere with itself. More specifically, the degree of coherence of a wave can imply how much a wave is like a copy of itself shifted slightly in time or space. For example, any wave is perfectly identical to a copy of itself that is not shifted. However, shifting the copy of the wave a small amount might cause the copy to appear nothing like the original wave. Such a wave has a lower degree of coherence than a wave that can be shifted by a larger amount and still be similar to itself. So the higher the degree of coherence, the more self-similar the wave is. In other words, the wave has a higher autocorrelation, since it correlates more strongly with itself. 
     Electromagnetic waves, such as light or RF waves, have a constant speed of propagation represented by the symbol “c” which is approximately 300,000,000 meters per second in vacuum. Since the speed is constant, an electromagnetic wave can be counted on to move a specific distance when a specific amount of time passes. Also, a specific amount of time passes when an electromagnetic wave moves a specific distance. As such, shifting, or moving, an electromagnetic wave can be described as a change in time or a change in distance. These descriptions (temporal and spatial) can generally be used interchangeably as the speed of propagation “c” can be used to convert between the time and the distance. 
     Any wave will correlate perfectly to a copy of itself that is not shifted. A slightly coherent wave will also correlate somewhat with a slightly shifted copy of itself, but will likely not correlate at all with a copy of itself that is shifted a large amount. In contrast, a perfectly coherent wave will correlate perfectly with a copy of itself that is shifted by any amount. A wave that can be transversely shifted a large amount and remain correlated to itself is said to have high spatial coherence, whereas a wave that can be longitudinally shifted a large amount and remain correlated to itself is said to have high temporal coherence. Temporal coherence can be quantified by the coherence length or the coherence time, which is the coherence length divided by “c”. 
     The spectral content of a signal, or a wave, can be considered from a “frequency domain” point of view. Such a frequency domain view can be obtained by performing the Fourier transform of the time-domain representation of the signal. The bandwidth of the signal is the span of frequencies over which its spectral content is non-zero. For example, lasers generally have a very narrow bandwidth. A theoretically perfect laser might have only one frequency component, and thus an infinitesimally small bandwidth. The frequency domain view of the signal from such a perfect laser would appear as a single line at the frequency of the laser. This line can be represented mathematically by the delta function. Accordingly, such a source of electromagnetic energy can be referred to as a line source. In contrast, white light is composed of a broad range of frequencies that are visible to the human eye. That is, white light has many frequency components, and thus a wide bandwidth. A source of white light, far from being a line source, is instead a broadband, or wideband, source. 
     Since a laser can emit a (very nearly) single frequency of radiation, the light is monochromatic, or generally always the same, or highly self-similar. Thus, laser light can be very coherent. In other words, laser light can have a very long coherence length. For example, a frequency stabilized helium-neon (HeNe) laser can produce light with coherence lengths of several kilometers. In contrast, white light is made up of many different frequencies. With such great diversity of spectral content, a wave from a white light source may not be very similar if examined at different times or places along the wave. That is, the wave is not highly self-similar, not very coherent, and may have an extremely short coherence length. With such a short coherence length, the white light wave will only interfere with a copy of itself that is very minimally shifted in time or space. 
     Interferometry is a technique where interference between two or more waves creates an interference pattern that can be analyzed to determine differences between the waves. Interferometry is often used for measuring small path length differences such as would occur from small distance or refractive-index differences. One or more interference patterns are typically processed to extract phase maps or other useful data. Interferometers often use two waves having the same frequency. This can be accomplished by splitting a single source into two, in which case each of the splits might be called a leg or branch of the interferometer. Where the two waves are in phase, they will interfere constructively (add to each other), while where they are out of phase, they will interfere destructively (cancel each other out). Thus the constructive or destructive interferences shown within the interference pattern can indicate differences in the path lengths between the arms of the interferometer. There are many types of interferometers all of which employ the same basic principles. Some examples include the Michelson interferometer, the Twyman-Green interferometer, the Mach-Zehnder interferometer, the Sagnac interferometer, and the Fabry-Perot interferometer. 
     It is often desirable for the interference pattern of interest to have the highest contrast possible. This occurs when regions of destructive interference produce nearly complete cancellation of the waves, and regions of constructive interference have the greatest wave amplitude. In the past, this has typically meant that the path length difference between the two paths of interest would have to be much smaller than the coherence length of the source. So, for example, if a white-light source is used, its extremely short coherence length necessitates that the interferometer&#39;s two path lengths would have to be almost exactly the same. 
     In an interferometer setup there are generally many surfaces within the system off of which the waves may reflect or scatter creating wave paths in addition to those of the main interfering legs. Although these are not the main interfering wave paths, they can still introduce extraneous interference patterns if their path length differences are shorter than the source&#39;s coherence length. These extraneous patterns can mix with the desired interference pattern, making the resulting pattern more difficult to process, thereby reducing the effectiveness of the interferometer. Various techniques may be used to attempt to mitigate the occurrence of these extraneous fringe patterns, including the use of: wedged optics, anti-reflection coatings, field stops, software processing, phase shifting, polarization adjustment, and other methods. These techniques may be difficult to employ, expensive, and/or have limited efficacy. 
     It is with respect to these considerations and others that the disclosure made herein is presented. 
     SUMMARY 
     Technologies are described herein for controlling the coherence length of an electromagnetic beam to remove spurious interference fringes at the output of an interferometer system. Through the utilization of the technologies and concepts presented herein, the coherence length of an electromagnetic beam can be controlled by applying a modulating signal to a phase modulator and applying the phase modulator to the electromagnetic beam. By adjusting the bandwidth of the modulating signal provided to the phase modulator, the coherence length of the electromagnetic beam can adjustably reduced. Moreover, technology presented herein supports introducing a phase delay into an electromagnetic beam for positioning the region of measurement within an interferometer system. 
     According to various embodiments presented herein, a system for controlling the coherence length of a coherent radiation beam includes a signal generator, a variable control, and a phase modulator. The signal generator is used to produce a broadband signal that is provided to the phase modulator. The bandwidth of the broadband signal is controlled using the variable control. The phase modulator modifies the frequency spectrum of the coherent radiation beam in response to the bandwidth adjustment of the broadband signal. By controlling the bandwidth of the signal, the coherence length of the coherent radiation beam can be controlled. 
     According to other embodiments, a method for controlling the coherence length of a coherent radiation beam is provided. A coherent radiation source is provided to generate the coherent radiation beam. The coherent radiation beam is then provided to a phase modulator. The phase modulator is used to modulate the coherent radiation beam using the broadband signal to reduce the coherence length of the coherent radiation beam. 
     According to further embodiments, a method is provided for controlling the coherence length of a coherent radiation beam within an interferometer system. A coherent radiation source is used to generate the coherent radiation beam. A reference beam and a measurement beam are derived from the coherent radiation beam. The measurement beam is applied to a sample. An output pattern is generated in response to an interference between the reference beam and the measurement beam after the measurement beam is applied to the sample. A phase modulator is applied to both the reference beam and the measurement beam, and a broadband signal is provided to each of the phase modulators. The broadband signal is then controlled in order to improve the contrast of the output pattern. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an optical system diagram illustrating a coherence controller system according to embodiments described herein; 
         FIG. 2  is an optical system diagram illustrating an interferometer system using two modulated coherence controllers according to embodiments described herein; 
         FIGS. 3A-3C  are frequency domain graphs illustrating the spectral content of signals used within a coherence control system according to embodiments described herein; and 
         FIG. 4  is a logical flow diagram illustrating a process for reducing the coherence length of a coherent radiation beam according to embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to technologies for controlling the coherence length of an electromagnetic beam. As stated above, embodiments presented herein allow for the control of the coherence length of an electromagnetic beam through the application of a modulating signal to a phase modulator, and applying the phase modulator to the electromagnetic beam. Furthermore, a phase delay may be introduced into the electromagnetic beam to provide adjustable positioning of the coherence plane within an interferometer system. 
     According to various embodiments described below, phase modulators are used to modify electromagnetic beams used in each leg of an interferometer. Even though the path lengths between the various desired and spurious paths of an interferometer may be different for each permutation of path pairs, all pairs whose path-length differences are shorter than the coherence length of the source may yield spurious fringe patterns. However, by reducing the coherence length, such as applying a phase modulation to a beam from the source according to the embodiments discussed below, the contrast, and hence the visibility of the spurious interference patterns may be substantially reduced by only allowing interference to occur in the region where the path lengths are equal, or very nearly equal. Adjusting the coherence length by modifying the electrical modulating signal can reduce the interfering region (also known as the coherence region) to include only the desired measurement planes. Note that the center of this coherence region can be referred to as “the plane of coherence” 
     Additionally, the disclosure provided herein allows for the reduced region of measurement to be intentionally shifted, or positioned along the beam paths of the interferometer system. The modulation of the beams can be electronically advanced or retarded to allow positioning the region of measurement within the interferometer so as to increase the fringe contrast from a desired source while substantially reducing the fringe contrast from all other sources. This may be accomplished by introducing a time delay into one or both of the phase modulating signals within the interferometer as needed. Thus, phase modulation can reduce the depth of the coherence plane within the interferometer where high contrast interference may occur, while the introduction of phase delay can position the coherence plane as desired. 
     While the subject matter described herein may be presented in the general context of optical systems, laser beams, lenses, and mirrors, one skilled in the art will recognize that other implementations may be performed in combination with other types of electromagnetic waves and beams. Furthermore, one skilled in the art will recognize that any techniques for modulating the electromagnetic beams and thus spreading the bandwidth of the beams utilizing the disclosure provided below may be used. 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and which are shown by way of illustration specific embodiments or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, aspects of a coherence length controller will be described. Turning first to  FIG. 1 , details will be provided regarding an illustrative optical system for controlling beam coherence. In particular,  FIG. 1  is an optical system diagram illustrating a coherence controller assembly  100  according to various embodiments. A long coherence length source  102  emits an electromagnetic, long coherence length beam  103 . For example, a frequency stabilized polarized laser may be used to emit a laser light beam. The long coherence length beam  103  can be processed by a coherence controller assembly  100  to provide a reduced coherence length beam  120 . 
     Polarizing beam splitters are often used to split a beam according to two separate polarizations. According to the embodiment illustrated by  FIG. 1 , the polarizing beam splitter  104  allows long coherence length beam  103  to pass through while reflecting the return beam as described below. The long coherence length beam  103  proceeds through the beam splitter  104  to a quarter-wavelength waveplate  106 . The quarter-wavelength waveplate  106  converts the beam to a circular polarization. The polarization of an electromagnetic wave can describe the direction of the transverse electric field of the wave. The circularly polarized beam proceeds from the quarter-wavelength waveplate  106  to a lens  108 , which focuses the beam into a phase modulator  112 . 
     The phase modulator  112  can be an acousto-optic modulator (AOM, also known as a Bragg cell), an electro-optic modulator, a magneto-optic modulator, or any other type of phase modulator. The example of the phase modulator embodied in  FIG. 1  is an AOM. Within the phase modulator  112 , a broadband RF signal  113  interacts with the electromagnetic beam. This interaction can both diffract (deviate) and frequency modulate (broaden) the electromagnetic beam. The portion of the beam that proceeds straight through the phase modulator  112  is not deflected. This non-deflected portion of the beam can be blocked by a beam stop  114 . The diffracted beams can be retro-reflected from a spherical mirror  116  back into the phase modulator  112 . After being retro-reflected back into the phase modulator  112 , the beams can again interact with the RF modulation. This second interaction within the phase modulator  112  can once again frequency modulate the beams and diffract the beams back along their original path. Considering both passes through the phase modulator  112 , the bandwidth of the electromagnetic beam can be broadened by twice the bandwidth of the broadband RF signal  113 . Thus, control of the broadband RF signal  113 , such as by a digital controller, can also control the final bandwidth of the electromagnetic beam being modulated. Spreading the bandwidth of the electromagnetic beam can reduce the coherence length of the electromagnetic beam. For example, broadening a narrow bandwidth laser source out to about 1 GHz of bandwidth can provide a coherence length of about one third of a meter (about one foot). 
     After being retro-reflected from the spherical mirror  116 , the beams are diffracted back along their original path by the phase modulator  112 . Any beams that are not diffracted back along the original path can be blocked by an iris  110 . The beam that is diffracted back along the original path is collimated by the lens  108 . After being collimated by the lens  108 , the beam is converted from circular polarization to linear polarization by the quarter-wavelength waveplate  106 . The linear polarization provided to this reverse path through the quarter-wavelength waveplate  106  may be orthogonal to the original polarization of the long coherence length beam  103 . Due to the orthogonal polarity of the return path beam, the polarizing beam splitter  104  reflects the return beam. This reflected beam proceeds to a mirror  118  to direct the output beam of the coherence controller assembly  100 . This reduced coherence length beam  120  can have a reduced coherence length that is determined by the broadband RF signal  113 . 
     Referring now to  FIG. 2 , additional details will be provided regarding the embodiments presented herein for interferometry using a coherence controller  100 . In particular,  FIG. 2  is an optical system diagram illustrating an interferometer system  200  using two modulated coherence controller assemblies  100 A- 100 B according to one exemplary embodiment. 
     The long coherence length beam  103  output from a long coherence length source  102  (such as a laser) can be split into two beams to form each leg of an interferometer  200 . A beam splitter  205  can be used to split the long coherence length beam  103  emitted from the long coherence length source  102 . A first beam  210 A from the beam splitter  205  passes through a first coherence controller assembly  100 A. A second beam  210 B from the beam splitter  205  can reflect off of a mirror  215  and pass through a second coherence controller assembly  100 B. 
     After passing through the first coherence controller assembly  100 A, the first beam  210 A emerges as a first reduced coherence length beam  120 A. This first reduced coherence length beam  120 A can be reshaped (for example expanded) by a lens assembly  235 A. After being reshaped, the first reduced coherence length beam  120 A passes through a beam splitter  240  to interact with a sample  245 . After interacting with the sample  245 , the beam  120 A can reflect off of a mirror  250  and reflect off of the beam splitter  240  to an output plane. The output plane is where the output interference fringes  220  can be displayed, scanned, or captured. The sample  245  is generally the object to be measured with the interferometer. Because the first reduced coherence length beam  120 A can interact with the sample  245 , it may be called the measurement beam or the measurement leg of the interferometer  200 . 
     After passing through the second coherence controller assembly  100 B, the second beam  210 B emerges as a second reduced coherence length beam  120 B. This second reduced coherence length beam  120 B can be reshaped (for example expanded) by a lens assembly  235 B. After being reshaped, the second reduced coherence length beam  120 B reflects off a mirror  242  and passes through a beam splitter  240  to an output plane. The output plane is where the output interference fringes  220  can be displayed, scanned, or captured. Because the second reduced coherence length beam  120 B generally does not interact with the sample  245 , it may be called the reference beam or the reference leg of the interferometer  200 . The reference beam and the measurement beam can finally interfere with one another at the output plane to generate interference fringes  220 . 
     The first coherence controller assembly  100 A modulates the first beam  210 A to generate the first reduced coherence length beam  120 A. The modulation is controlled by a first broadband RF modulating signal  113 A, which is generated by an RF mixer  280 . The RF mixer  280  mixes an RF carrier signal  265  with an RF noise signal  275 . The RF carrier signal  265  is generated by an RF carrier source  260 , may be an oscillator, crystal oscillator, function generator, radio frequency oscillator, optical oscillator, or any other source for generating an RF carrier. The RF noise signal  275  is generated by a random noise source  270 , which may be a pseudorandom source. 
     The second coherence controller assembly  100 B modulates the second beam  210 B to generate the second reduced coherence length beam  120 B. The modulation is controlled by a second broadband RF modulating signal  113 B generated by an RF mixer  285 . Similar to the RF mixer  280  described above, the RF mixer  285  mixes the RF carrier signal  265  with an RF noise signal  275 . If desired, prior to entering the RF mixer  285 , the RF noise signal  275  may be delayed in time by passing through a variable delay device  277 , which will be described in greater detail below. 
     As discussed above, the RF modulating signals  113 A,  113 B can be derived from the RF carrier signal  265  that is broadened in bandwidth by mixing with the RF noise signal  275 . The frequency spectrum of the RF noise signal  275 , which may be a random or pseudorandom noise signal, can extend from zero frequency (DC) to a controllable upper frequency. The optical frequency spectrum, or bandwidth, of a long coherence length beam  103  from a long coherence length source  102  (such as a laser) can be broadened as this controllable upper frequency of the random noise source  270  is increased. Broadening the optical frequency, or bandwidth, of the beam from the long coherence length source  102  can decrease the coherence length of the beam. Representative spectral plots of the RF signals  265 ,  275 ,  113 A,  113 B are addressed in more detail with respect to  FIG. 3 . 
     The interferometer  200  can be set up so that the path length of the measurement beam as it interacts with the sample  245  is approximately equal to the path length of the reference beam. Such a set up may allow the interference fringes that relate to the interactions from the sample  245  to appear with the highest contrast at the output plane  220 . Without the use of coherence length control assemblies  100 A and  100 B to reduce the coherence lengths, the reference beam and the measurement beam may have a long enough coherence length to allow, not only the desired interference related to the sample  245 , but also various other interferences caused by beams reflecting and interacting with many other surfaces within the interferometer  200 . These other interference can form unwanted, extraneous interference fringes at the output plane  220  in addition to the desired interference fringes from the interaction with the sample  245 . 
     However, by reducing the coherence length of the beams, these extra paths may lie outside of the coherence length of the beam and thus will generate interference fringes with sufficiently low contrast as to be negligibly visible. For example, the interference length may be reduced to limit the coherence plane to only the area immediately around the sample  245  where the reference beam and the measurement beam are of nearly equal path lengths. In this case, an extra reflection off of another surface may generate a measurement beam path and a reference beam path having different path lengths such that the difference in the path lengths exceeds the (now reduced) coherence length of the beams and thus may not form the unwanted interference fringes that mix with the desired interference fringes  220  at the output plane. 
     The interferometer  200  may be configured so that the length of the measurement beam as it interacts with the sample  245  is not exactly equal to the length of the reference beam. This may be remedied using the variable delay device  277 . The variable delay device  277  is used to delay or advance the phase of the second broadband RF modulating signal  113 B. This effectively increases or decreases the path length of the reference beam of the interferometer  200 . This adjustment can be made using an electronic control associated with the variable delay device  277 . Using the variable delay device  277  to effectively increase or decrease the path length of the reference beam of the interferometer  200  allows positioning of the coherence plane at various locations within the interferometer  200 , which in turn allows the examination of reflections or interactions from various positions along the beam path within the interferometer  200 . 
     Turning now to  FIGS. 3A-3C , additional details will be provided regarding the embodiments presented herein for coherence length control. In particular,  FIG. 3A  is a frequency domain graph illustrating the spectral content of the RF carrier signal  265  generated by the RF carrier source  260 .  FIG. 3B  is a frequency domain graph illustrating the spectral content of the RF noise signal  275  generated by the random noise source  270 .  FIG. 3C  is a frequency domain graph illustrating the spectral content of the broadband RF modulating signal  113 . 
     As discussed above and further illustrated now in  FIGS. 3A-3C , the broadband RF modulating signal  113  is formed by mixing the RF noise signal  275  with the RF carrier signal  265  at the RF mixers  280  and  285 . The spectrum of the random noise signal  275  can be flat (white noise, or band limited white noise), Gaussian, or of any other random distribution and extending from zero frequency (DC) to a controllable upper frequency. Increasing the controllable upper frequency broadens the random noise signal  275 . The broadband RF modulating signal  113  includes the random noise signal  275  positioned as an upper sideband at the carrier frequency and a reflected copy of the random noise signal  275  positioned as a lower sideband. As such, broadening the random noise signal  275  by a fixed amount can increase the bandwidth  310  of the broadband RF modulating signal  113  by twice that fixed amount. As discussed above, increasing the bandwidth  310  of broadband RF modulating signal  113  can operate through the coherence controller assembly  100  to reduce the coherence length of the long coherence length beam  103  to form the reduced coherence length beam  120 . Since the bandwidth  310  is controllable, the coherence length of the reduced coherence length beam  120  is also controllable. 
     Turning now to  FIG. 4 , additional details will be provided regarding the embodiments presented herein for controlling coherence length. In particular,  FIG. 4  is a flow diagram showing a routine  400  performed by a coherence controller assembly  100  for reducing the coherence length of a coherent radiation beam. It should be appreciated that the operations described herein may be performed in parallel, or in a different order than in the illustrative examples described herein. It should also be appreciated that more or fewer operations may be performed than shown in the figures and described herein. Although several examples used in describing the method relate to interferometry, applications to other systems requiring, or benefiting from, coherence control may apply. 
     The routine  400  can begin with operation  402  where a desired coherence length is determined. In an exemplary interferometer system  200 , such as illustrated in  FIG. 2 , the coherence length may be selected to be smaller than the distance between the sample  245  and the elements nearest to the sample within the interferometer  200 . These neighboring elements may be a beam splitter  240  and a mirror  250 . In other exemplary configurations, the neighboring elements may be other components. Reducing the coherence length to be within this distance may be useful for placing beam interactions from other elements, such as the nearest neighboring elements of an interferometer  200 , outside of the coherence length. Placing other beam interaction outside of the coherence length may remove unwanted fringe patterns from the output plane  220 . The desired coherence length may also be selected based on various system parameters or required aspects of the output reduced coherence length beam  120 . 
     Next, at operation  404 , a phase related to the desired coherence plane can be determined. The phase can be the required beam delay or advancement to be applied to the reference beam to modify the path length of the reference beam and effectively position the coherence plane within a system. For example, such a system may be an interferometer system  200 . This phase that may be introduced to the system using a variable delay device  277 . This phase may be incorporated into the path length of the measurement beam, the reference beam, or in part to both beams. 
     In operation  406 , a coherent radiation source is provided to generate the electromagnetic beam. The source may be a long-coherence length source  102  to generate the long coherence length beam  103 . Some examples of coherent radiation sources include but are not limited to RF, microwave, optical, x-ray, and infrared. If the source is an optical source, any type of laser or any other optical source may be used. Some examples are a frequency stabilized HeNe laser, any other gas laser, a semiconductor laser, a crystal laser, a vertical-cavity surface-emitting laser (VCSEL), and a Fabry-Perot laser. 
     In operation  408 , a broadband RF signal  113  may be provided in response to the desired coherence length determined in operation  402 . As previously discussed, the broadband RF signal  113  can be generated by mixing the RF carrier signal  265  with the RF noise signal  275 . The RF noise signal  275  is generated by the random noise source  270  with a frequency spectrum extending from zero frequency (DC) to a controllable upper frequency. In operation  410 , a delay may be provided to the broadband RF signal in response to the delay, or phase, determined in operation  404 . The phase, or delay, may be introduced to the system using the variable delay device  277 . This phase may be incorporated into the path length of the measurement beam, the reference beam, or in part to both beams. 
     In operation  412 , a beam  103  from the coherent radiation source  102  may be phase modulated using the broadband RF signal  113  to generate a beam of reduced coherence length  120 . The optical frequency of a beam  103  from a laser can be made broader as a controllable upper frequency of the random noise source  270  is increased. Broadening the optical frequency, or bandwidth, of the beam  103  from the laser  102  can decrease the coherence length of the beam. The phase modulator may be an electro-optical modulator or an AOM. More generally, the phase modulator may be any mechanism of electromagnetic modulation or mixing via mechanical, electrical, optical, magnetic, or any other exemplary techniques. Routine  400  ends after operation  412 . 
     Based on the foregoing, it should be appreciated that technologies for coherence length control are presented herein. Although the subject matter presented herein has been described in language specific to optical assemblies, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features, acts, or media described herein. Rather, the specific features, acts and mediums are disclosed as example forms of implementing the claims. 
     The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.