Abstract:
An optical alignment system for optimizing a gap distance between an optical fiber end and an optical device uses an atomic force present when the gap distance approaches optimal alignment according to changes in an oscillating fiber amplitude at a fiber resonance frequency. A driving force flexurally vibrates the fiber to produce an oscillation of the fiber at a resonance frequency that produces maximal oscillation. A measurement system detects the amplitude shift at the resonance frequency while adjusting the gap distance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to fiber-coupled optical assemblies and, more particularly, to a method of precisely aligning an optical fiber to an optical device according to a flexural vibration of the optical fiber.  
       BACKGROUND OF THE INVENTION  
       [0002]     The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment difficult both to achieve and to maintain. For example, one problem in the construction of laser transmitters is that of efficiently coupling the optical output from a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. When such an alignment is achieved, the fiber is then fixed in place, ideally by a method that ensures alignment is sustained throughout the device lifetime.  
         [0003]     Typically, precise alignment of the fiber involves aligning the end of the fiber in at least one direction relative to the optical device to provide a maximum energy transfer from the optical device to the fiber. A further optical device such as a photodiode may be used to measure optical power coupled into the optical fiber. The fiber may be adjusted in vertical and lateral alignment until a maximum power coupling is achieved. A predetermined gap distance may be used for horizontal alignment or the gap distance may be adjusted while visually monitoring the distance to avoid direct contact between the fiber and the optical device.  
         [0004]     It is typically difficult, however, to determine an optimal gap distance. Even if visual means are used, the resolution of cameras or other monitoring devices available may not be sufficient to allow accurate determination of an optimal coupling position. The resolution of the image may also make the position of the edge of the fiber and/or the optical device difficult to determine. This uncertainty may result in a premature contact between the fiber and the optical device or in the optical device being separated from the optical fiber by too large a gap.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is embodied in an optical fiber alignment system for aligning an end of an optical fiber with an optical device to provide a gap distance between the optical fiber and the optical device. The optical fiber alignment system includes a first actuator for applying a force having a component that is normal to a central axis along the length of the optical fiber. The force provides a flexural vibration of the optical fiber to cause an oscillation of the optical fiber and is provided at a resonance frequency of the optical fiber. The optical alignment system further includes a sensing apparatus for sensing the oscillation of the optical fiber and a detector for determining a shift in amplitude of the oscillation. The optical alignment system further includes a second actuator for adjusting the gap distance between the optical fiber and the optical device according to the shift in the amplitude of the oscillation determined by the detector. An atomic force between the end of the optical fiber and the optical device varies according to the gap distance provided by the second actuator, the variation in the atomic force causing the shift in the amplitude of the oscillation.  
         [0006]     In an exemplary embodiment, the optimal gap distance may be on the order of microns. Embodiments of the present invention may determine an absolute relative distance between the laser facet and the fiber edge to nanometer precision. Once the absolute relative distance has been determined with nanometer precision in one fiber position, a second actuator may be used to move the fiber position back to the optimal position.  
         [0007]     The present invention is further embodied in a method of aligning an end of an optical fiber to an optical device according to a gap distance between the end of the optical fiber and the optical device. The method aligns the end of the optical fiber to be substantially at a predetermined position relative to the optical device and applies a first driving force normal to a central axis along a length of the optical fiber to provide a flexural oscillation of the optical fiber at the resonance frequency of the optical fiber. The method further measures the flexural oscillation of the optical fiber, monitors a shift in amplitude of the flexural oscillation and adjusts the gap distance between the optical fiber and the optical device. The gap distance changes an atomic force between the optical fiber and the optical device which provides a shift in the amplitude of the flexural oscillation. The method further monitors the shift in amplitude and adjusts the gap distance until the amplitude of the flexural oscillation is reduced by at least a threshold amount. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:  
         [0009]      FIG. 1   a  is a side view illustrating a relationship between optical fiber oscillation and an atomic force provided by an optical device according to an exemplary method of the present invention;  
         [0010]      FIG. 1   b  is a cross-section of an optical fiber illustrating exemplary directions of flexural force according to an exemplary method of the present invention;  
         [0011]      FIG. 2  is a flowchart illustrating an exemplary method of aligning the optical fiber to the optical device to optimize the gap distance according to the present invention;  
         [0012]      FIG. 3  is a flowchart illustrating an alternate exemplary method of aligning the optical fiber to the optical device using a first and second perpendicular flexural forces according to the present invention;  
         [0013]      FIG. 4  is a flowchart illustrating a first exemplary method for measuring a flexural oscillation according to the present invention;  
         [0014]      FIG. 5  is a flowchart illustrating a method for measuring a flexural oscillation at a non-aliasing sampling rate according to the present invention;  
         [0015]      FIG. 6   a  is a side view illustrating a first exemplary optical fiber alignment system according to the present invention;  
         [0016]      FIG. 6   b  is a side view illustrating an alternate exemplary optical fiber alignment system according to the present invention;  
         [0017]      FIG. 7   a  is a side view illustrating a second exemplary optical fiber alignment system according to the present invention;  
         [0018]      FIG. 7   b  is an overhead view of a fiber mount platform illustrating an arrangement of a fiber mount area and an electrode shape according to the second exemplary optical fiber alignment system of the present invention;  
         [0019]      FIG. 7   c  is an overhead view of a fiber mount platform illustrating an arrangement of a fiber mount area and an alternate electrode shape according to the second exemplary optical fiber alignment system of the present invention;  
         [0020]      FIG. 8   a  is a system diagram illustrating an exemplary detector for detecting a shift in amplitude of the flexural oscillation according to the present invention;  
         [0021]      FIG. 8   b  is a system diagram illustrating an alternate exemplary detector for detecting a shift in amplitude of the flexural oscillation according to the present invention; and  
         [0022]      FIG. 9  is a graph of a gap distance versus oscillation amplitude which is useful for describing the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing, FIG. la illustrates a relationship between an optical fiber oscillation  110  caused by a vibration and an atomic force  112  provided by an interaction between an optical device  104  and the tip of optical fiber  102  according to an exemplary method of determining an optimal gap distance. As gap distance  106  decreases, an interaction between oscillation  110  and atomic force  112  may change the vibration conditions, thus providing an indication of proximity to the optical device  104 .  
         [0024]     According to an exemplary method, a flexural driving force  108  may be provided to optical fiber  108  in a first direction. Flexural driving force  108  causes a vibration providing a flexural oscillation  110  of optical fiber  102 . Flexural driving force  108  is desirably provided at a resonance frequency of optical fiber  102  to maximize a vibration response of the fiber. In addition to flexural oscillation  110 , an atomic force  112  provided by optical device  104  acts to change a loading condition on the end of the fiber according gap distance  106 . The vibration condition may thus vary with the loading condition and may cause a shift in the resonance frequency. An optimal gap distance  106  may thus be determined by adjusting the gap distance in the z direction toward optical device  104  while monitoring flexural oscillation  110 . An optimal gap distance may occur in a region beginning where the atomic force changes the loading condition to where there is contact between the end of optical fiber  102  and optical device  104 . The inventors have determined that the atomic force may interact with the oscillation when the gap distance is between approximately 10-100 nm.  
         [0025]     Flexural driving force  108  provided in the first direction according to the y-axis is used to determine an optimal gap distance  106  according to oscillation in the y-direction. In an alternative exemplary embodiment, a second flexural driving force  114  may be similarly provided in the x-direction, as illustrated in FIG. lb. A cross-section of optical fiber  102  illustrates a desired relationship between first flexural driving force  108  and second flexural driving force  114 , namely that the two forces,  108  and  114  are perpendicular to each other. It is understood that first flexural driving force  108  may be provided at any direction and is not limited to the y-direction as illustrated in  FIGS. 1   a  and  1   b  and that the two flexural forces, when used need not be orthogonal.  
         [0026]     Referring now to  FIG. 2 , an exemplary method of determining an optimal gap distance according to the present invention is described. In step  200 , an end of the optical fiber is aligned according to a predetermined position. The predetermined position may be a predetermined z-position, x-position, y-position or any combination thereof to initially align the optical fiber with the optical device. The fiber may be precisely aligned in the x and y directions by measuring a coupling efficiency between the optical device and the fiber, for example using an optical power measure. The coupling efficiency measures the amount of energy coupled to or by the optical fiber. If the optical fiber is precisely aligned, a high portion of energy will be transferred to or from the fiber, resulting in a high coupling efficiency.  
         [0027]     In step  202 , a flexural driving force is applied to the optical fiber desirably at a resonance frequency of the optical fiber. The flexural driving force causes a flexural vibration providing an oscillation in the optical fiber. Because the driving force is applied at the resonance frequency only force to compensate for frictional losses is applied to maintain the oscillation at a substantially constant amplitude. In step  204 , the flexural oscillation at the resonance frequency is measured by methods described below.  
         [0028]     In step  206 , the gap distance is decreased. Decreasing the gap distance may cause the atomic force to interfere with the oscillation by changing the loading condition, thus providing an amplitude shift. In step  208 , an amplitude shift in the flexural oscillation is monitored. It is understood that steps  206  and  208  may be switched or combined into a single step. Step  210  checks whether the amplitude shift of the flexural oscillation has occurred and if it is reduced by at least a threshold amount. The threshold amount may be determined by experimentation. The amplitude shift may be a function of fiber type, optical device type and optical package. A range of values may be determined to account for the differences in fiber, device and package conditions. If the oscillation is reduced by the threshold amount, step  210  leads to step  212  and the process is complete.  
         [0029]     If the oscillation is not reduced by the amount, step  210  leads to step  206 . Steps  206  through  210  are repeated until the oscillation is reduced by the threshold amount. When the oscillation is reduced by the threshold amount, step  210  leads to step  212  and the process is complete.  
         [0030]     It is contemplated that the exemplary method illustrated in  FIG. 2  may be repeated for a number of positions of the optical fiber, for example, to improve a horizontal or lateral alignment of the optical fiber relative to an optical device.  
         [0031]     For example, after the end of the fiber is aligned with the optical device, step  212 , the fiber may be further aligned in a horizontal direction relative to the optical device. The end of the fiber may then be adjusted to the optimal gap distance previously determined from steps  200 - 212 . The horizontal alignment, however, may cause the optimal gap distance to change. The gap distance may thus be readjusted using the exemplary method of steps  200 - 212 .  
         [0032]     Referring now to  FIG. 3 , an alternate exemplary method of determining an optimal gap distance according to the present invention is described. In step  300 , an end of the optical fiber is aligned according to a predetermined position as described above.  
         [0033]     In step  302 , a first flexural driving force is applied to the optical fiber desirably at a resonance frequency of the optical fiber. The first flexural driving force causes a flexural oscillation in the optical fiber in a first direction. In step  304 , the flexural oscillation at the resonance frequency according to the first direction is measured.  
         [0034]     In step  306 , the gap distance is decreased. In step  308 , an amplitude shift in the flexural oscillation is monitored. It is understood that steps  306  and  308  may be switched or combined into a single step. Step  310  checks whether the amplitude shift of the flexural oscillation has occurred and if it is reduced by the threshold amount as described above. If the oscillation is reduced by the threshold amount, step  310  leads to step  312 .  
         [0035]     If the oscillation is not reduced by the threshold amount, step  310  leads to step  306 . Steps  306  through  310  are repeated until the oscillation is reduced by the threshold amount. After step  310  control passes to step  312 .  
         [0036]     Step  312  checks whether or not a second flexural driving force has been applied. If a second driving force has been applied, step  312  leads to step  318  and the process is complete.  
         [0037]     If a second driving force has not been applied, step  312  leads to step  314 . In step  314 , the first driving force is inhibited. In step  316 , a second driving force is applied in a second direction at a second resonance frequency of the fiber. The second resonance frequency may be equivalent to or different from the resonance frequency in the first direction. The atomic forces described above, a skew direction of the fiber, a skew direction of the applied force or a different application point for the second force may cause the second resonance frequency to be different from the resonant frequency of the first direction measurement. The second direction is related to the first direction as described above.  
         [0038]     The second driving force causes a flexural oscillation in the second direction. The gap distance may be further optimized by using a force in the second direction. For example, the optical fiber may be skewed relative to the alignment with the optical device in at least one of the vertical and lateral directions such that a further optimization at the second direction may provide a more optimal alignment than measuring the gap distance in a single direction.  
         [0039]     Step  316  leads to step  304 . Steps  304  through  310  are repeated for the flexural oscillation in the second direction as described above until the oscillation in the second direction is reduced by a further threshold amount. The further threshold amount may be the same the same as the threshold amount described above or may be a smaller amount to account for the gap distance adjusted using the first driving force. After step  312  control passes to step  318 , and the process is complete.  
         [0040]     Referring now to  FIG. 4 , a first exemplary method of measuring a flexural oscillation, step  204  or step  304  according to the present invention is described. In step  400 , the optical device is activated, sending light into the fiber according to an x and y direction alignment of the fiber end with the optical device. In step  402 , an optical power of reflected light is measured, by using a photodiode, for example. Although not shown, it is contemplated that the optical power of light transmitted through the fiber may also be measured. The exemplary method causes the fiber to oscillate, and thus the optical power of the light transmitted through or reflected by the fiber will vary with the oscillation of the fiber. Step  402  leads to step  404  and the process is complete.  
         [0041]     A second exemplary method of measuring a flexural oscillation, step  204  or step  304 , measures a capacitance formed between a conductive coating on the optical fiber and an optical package which includes an electrode. The capacitance is a function of distance between the conductive coating and the electrode, and will thus vary with the oscillation of the fiber.  
         [0042]     The oscillation measurement is desirably timed so that no aliasing occurs due to beats between the oscillatory frequency and the sampling frequency. Referring now to  FIG. 5 , a method to sample the flexural oscillation measurement to avoid aliasing is described. In step  500 , a resonance frequency of the optical fiber is determined according to an exemplary embodiment of the present invention. In step  502 , the sampling frequency for measuring the flexural oscillation is desirably chosen to be at least twice the highest desired frequency component. The highest frequency component desirably includes at least the resonance frequency determined in step  500  and may include a higher frequency component such that a shift in frequency amplitude changes when the gap distance decreases does not induce aliasing. Alternatively, an anti-aliasing filter (not shown) may be applied prior to sampling to remove frequency components above the desired frequency range that includes at least the resonance frequency determined in step  500 . Step  502  leads to step  504  and the measurement sampling frequency selection is complete.  
         [0043]     Referring now to  FIG. 6   a , a first exemplary optical alignment system  600  for measuring an optimal gap distance according to the present invention is described. Actuator  602  provides driving force  108  to optical fiber  102 . Optical fiber  102  may be metallized or non-metallized. Optical fiber  102  may be at least one of wedge-lensed, ball, conical or flat-cleaved optical single mode or multi-mode fiber.  
         [0044]     Optical device  104  provides an optical signal  604  to the end of optical fiber  102  that is oscillating at its resonance frequency. Photodetector  606  measures an optical power and provides measurement signal  608  to detector  610 . Detector  610  monitors the amplitude shift in oscillation and provides output signal  612 . Output signal  612  may be used to determine whether to adjust  616  the gap distance, using actuator  614 .  
         [0045]     Optical device  104  may be any surface that may receive an optical signal or from which an optical signal may radiate. For example, optical device  104  may be a photodiode, a semiconductor laser, an optical mirror, a second optical fiber, a semiconductor optical amplifier, an optical concentrator, and a light-emitting diode.  
         [0046]     Actuator  602  desirably provides a flexural driving force  108  with a travel displacement of less than 500 μm. It is contemplated that actuator  602  may be at least one of a moving coil actuator, a piezoelectric actuator an ultrasonic actuator and a capacitance actuator.  
         [0047]     Actuator  602  desirably provides a driving force  108  at a resonance-frequency of fiber  102 . Driving the fiber at the resonance frequency may provide a maximum oscillation measurement signal. The resonance frequency of the fiber may be determined by applying a signal with an adjustable frequency, such as a chirp signal to actuator  602 . A frequency where a maximum oscillation is observed is desirably chosen as the fiber resonance frequency. Actuator  602  may subsequently be driven at this resonance frequency. The maximum oscillation may be observed from signal  608 . It is understood that a maximum oscillation may be determined in the time domain, the frequency domain, or any combination thereof. Once the resonance frequency is determined, the resonance frequency may be monitored for an amplitude shift.  
         [0048]     If a second driving force  114  is applied as described above, actuator  602  desirably applies second driving force  114  at about the same z-value and perpendicular to first driving force  108 . A fiber resonance frequency for second driving force  114  may be different than for the first driving force as described above. It is desirable that a new resonance frequency be determined for second driving force  114 .  
         [0049]     Actuator  614  may be used to adjust  616  the gap distance between optical fiber  102  and optical device  104 . Actuator  614  is desirably a linear actuator with a travel range of about 100 μm. A linear actuator may be a precision motion system such as, for example, a piezo actuator, a combination of an ultrasonic motor, a high precision feedback encoder, and a high precision linear bearing or a motion system using a friction drive.  
         [0050]      FIG. 6   b  shows an alternate exemplary optical alignment system  600 ′ for measuring an optimal gap distance according to the present invention. Actuator  602  provides driving force  108  to optical fiber  102 . Optical device  104  provides an optical signal  604  to the end of optical fiber  102  that is oscillating at its resonance frequency. Photodetector  606 ′ measures an optical power and provides measurement signal  608  to detector  610 . Photodetector  606 ′ may be the same as photodetector  606  except that photodetector  606 ′ measures an optical power of light  604 ′ transmitted through the fiber  102 .  
         [0051]     Detector  610  monitors the amplitude shift in oscillation and provides output signal  612 . Output signal  612  may be used to determine whether to adjust  616 ′ the gap distance, using actuator  614  coupled to clamp  618 . Clamp  618  may be attached to optical fiber  102  such that the action of actuator  614  may adjust  616 ′ the gap distance in the direction indicated by the arrow.  
         [0052]     Referring now to  FIGS. 7   a - c , a second exemplary optical alignment system  700  for measuring an optimal gap distance according to the present invention is described. Optical device  104  may be provided on substrate  702  that may include a fiber mount area  704  and an electrode  706 . Actuator  602  provides a flexural driving force  108  to fiber  102 ′. Other components described in first exemplary system  600  are similar for second exemplary system  700 .  
         [0053]     Optical fiber  102 ′ desirably includes a conductive material on at least a portion of the fiber  102 ′, proximate to electrode  706 . Although the conductive material is illustrated as extending to the tip of the fiber, only a segment of the fiber may include conductive material provided the segment is proximate to electrode  706 . Optical fiber  102 ′ may be at least a wedge-lensed, ball, conical or flat-cleaved optical single mode or multimode fiber. Conductive material may include at least a metallization, a conductive plastic, or a conductive composite.  
         [0054]     Electrode  706  may be coupled to capacitance measurement circuit  712  with electrical probe  708 . A contact  710  on fiber  102 ′ may be coupled to capacitance measurement circuit  712  using a second electrical probe  708 . A measurement signal  714  may be provided to detector  610 ′. Detector  610 ′ desirably provides output signal  716  to monitor an amplitude shift in the oscillation of fiber  102 ′. Output signal  716  may be used to determine whether or not to adjust  616  the gap distance, using actuator  614 .  
         [0055]     In this embodiment of the invention, it may be desirable to couple the capacitance measurement circuit to the conductively coated optical fiber at a location that does not interfere with the oscillation of the fiber, for example at a position to the left of actuator  602  as shown in  FIG. 7   a.    
         [0056]     It is contemplated that the conductively coated optical fiber may be coupled to a structure having electrode properties. For example, the fiber may be attached to an electrically conductive fiber gripper which makes electrical contact with the conductively coated fiber. The capacitance measurement circuit may thus be coupled to the structure having electrode properties rather than directly to the optical fiber.  
         [0057]     Electrode  706  and the conductive coating provides a capacitance measure corresponding to fiber oscillation and atomic force interaction when the optical fiber oscillates in the y-direction. A capacitance measure of x-direction oscillation may further be provided by the same electrode  706  and conductive coating if actuator  602  applies second driving force  114 .  
         [0058]     An optical package may alternatively include a feed through connection  718  within substrate  702  to connect electrode  706  to contact  720  on an optical package base  722 . The optical package may thus be directly connected through contact  720  to capacitance measurement circuit  712 .  
         [0059]     As shown in  FIGS. 7   b  and  7   c  two alternate electrode patterns are illustrated for measuring x and y-direction oscillation. In  FIG. 7   b , electrode  706  is triangular shaped. In  FIG. 7   c , electrode  706 ′ is rectangular shaped and does not extend across the x-direction of the top surface of substrate  702 . Electrode  706  is desirably provided on the top surface of substrate  702 . Electrodes  706  and  706 ′ may be provided with contacts  724  for allowing contact with capacitance measurement circuit  712 .  
         [0060]     Detector  610 ,  610 ′ may be amplitude detectors. The amplitude of measurement signals  608 ,  714  may be monitored for a change in the amplitude or the phase if an oscillatory driving force  108  is used. Alternatively, driving force  108  may be an impulse. A change in an impulse response may be monitored, such as a decay rate of the impulse response, to determine an optimal gap distance.  
         [0061]     Amplitude detection of the measurement signal may have less accuracy as compared with frequency detection due to, for example, laser power fluctuation or amplification drift in the capacitance measurement circuit. Frequency detection may not be subject to a significant amount of amplitude fluctuation. Exemplary detector  610  ( 610 ′) uses a phase-locked loop (PLL) circuit for frequency detection caused by a resonance frequency shift and may thus provide an improved accuracy over amplitude detection.  
         [0062]     Referring now to  FIG. 8   a , exemplary detectors  610  and  610 ′ are described. Detectors  610  and  610 ′ are desirably PLL circuits. Detector  610  is the same as detector  610 ′ except that detector  610  receives the photodiode signal  608  while detector  610 ′ receives capacitance measurement signal  714 . PLL circuit  610  ( 610 ′) includes phase detector  802  for receiving measurement signal  608  ( 714 ) and an oscillatory signal  806  from voltage controlled oscillator (VCO)  804 . VCO  804  provides an oscillatory signal  806  responsive to a control signal. Oscillatory signal  806  may also be provided to actuator  602 .  
         [0063]     Phase detector  802  measures a phase difference between measurement signal  608  ( 714 ) and oscillatory signal  806  and provides a phase difference signal  810  to loop filter  812 . Loop filter  812  filters phase difference signal  810  to provide output signal  612  ( 716 ) which is a frequency difference signal. Loop filter  812  also provides a control signal to VCO  804 .  
         [0064]     In an exemplary embodiment, loop filter  812  is a first-order infinite-impulse response (IIR) filter which integrates the phase difference signal  810  provided by the phase detector  802  to develop a frequency difference signal  612  ( 716 ) that is applied to the VCO  804  to adjust its frequency in a sense that tends to minimize the phase difference between the output signal  806  of the VCO  804  and the measurement signal  608  ( 714 ). It is understood that loop filter  812  may be any stable filter. The oscillatory signal applied to actuator  602  causes the optical fiber to resonate at its resonance frequency. A frequency shift may be thus detected from frequency difference signal  612  ( 716 ).  
         [0065]     The inventors have determined that exemplary optical alignment systems  600  and  700  each provide a resolution in the nanometer range to monitor the gap distance while observing changes in the amplitude shift. The present invention thus allows a precise determination of gap distance as well as optical device position, such as a laser facet position and may provide a reliable fiber alignment procedure.  
         [0066]     Referring now to  FIG. 8   b , an alternate embodiment of exemplary detectors  610  and  610 ′ is described. Phase detector  802 ′ receives photodiode signal  608  (detector  610 ) or capacitance measurement signal  714  (detector  610 ′). Phase detector  802 ′ further receives reference frequency signal  814 . Phase detector  802 ′ provides a phase difference signal  810 ′ to loop filter  812 ′. Loop filter  812 ′ filters the phase difference signal  810 ′ to provide output signal  612  ( 716 ) which is a phase shift signal.  
         [0067]     Phase detector  802 ′ is the same as phase detector  802  except that phase detector  802 ′ detects a phase shift of the measurement signal  608  ( 714 ) with respect to reference frequency signal  814 . Reference frequency signal  814  may be provided at a fixed frequency that is the resonance frequency of the optical fiber before the atomic force interferes with the fiber oscillation. A phase shift may be detected when the atomic force begins to interfere with the oscillation of the fiber.  
         [0068]     Phase difference signal  810 ′ is the same as phase difference signal  810  except that phase difference signal  810 ′ detects a phase difference between measurement signal  608  ( 714 ) and reference frequency signal  814 . It is contemplated that loop filter  812 ′ may be any low pass filter that removes oscillatory components at or near the reference frequency.  
         [0069]     The invention is illustrated by reference to an illustrative example. The example is included to more clearly demonstrate the overall nature of the invention. The example is illustrative, not restrictive of the invention.  
       EXAMPLE  
       [0070]     Referring now to  FIG. 9 , an illustrative example of an amplitude shift according to gap distance according to an exemplary embodiment of the present invention is now described. Amplitude shift  902  is illustrated as a gap distance increases between an optical fiber end and an optical device. In region  908 , the gap distance is large and the atomic force does not act on the oscillatory motion. In region  906 , an atomic force interacts with the oscillatory motion, thus changing the loading conditions and causing an amplitude shift. In region  904 , the fiber is approaching or in contact with the optical device, thus providing a minimal motion.  
         [0071]     It is desirable to adjust the gap distance such that the amplitude shift is provided as in region  906 . Region  906  allows the fiber to be placed within about 10 to 100 nm of the optical device. Operation in region  904  may cause damage to the optical fiber if the optical fiber is directly in contact with the optical device and still in oscillation. In region  908 , the gap distance is not optimized. Although  FIG. 9  illustrates that the atomic force begins to act on the oscillatory motion within about 100 nm, this is only an illustrative number. An atomic force interaction distance may depend on the fiber conditions, for example, the fiber edge geometry and fiber size.  
         [0072]     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.