Abstract:
A purpose of this invention is allow sensors to be placed at locations as close to a region of interest as possible, i.e. a few tenths of a millimeter from a probe, the target material and each other. The invention allows the user to reproduce and actively follow these locations from a remote location and at a distance, connected with to the locations with an arbitrary length of single mode optical fiber cable, and perform the sensor functions in a controlled environment using the sensor of choice without affecting the experiment.

Description:
This application claims priority from provisional application No. 61/966,357, filed Feb. 22, 2014, the entire contents of which are herewith incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Nanotechnolgy, encompassing imaging, measuring and manipulating matter at scales 100 nanometers and smaller requires precision control of position and of distances between probes, tools, devices and materials at a nano-scale, i.e. nano-positioning. It also requires nanopostioners and other precision mechanical systems designed to move objects over a small range (1-100 micron) with resolution down to a fraction of an atomic diameter (Santosh Devasia et. al (2007), and Alexander H. Slocum (1992)). The objective is to achieve extremely high resolution, accuracy, stability, and reproducibility with a fast response time. 
     The optimum solution in each application is achieved by judicious mechanical design to package the precision actuators (Neal B Hubbard et. al (2006)) and position sensors (M. Tabib-Azar and A. Garcia-Valenzuela (1995)) for active feedback control of motion. The continuous mechanical structure connecting the material to the probe/tool determining their relative position is only rendered flexible by an actuator allowing change in relative position. Along the mechanical path across the actuator a distance sensor is placed and the information provided by the sensor is fedback to the actuator to control nanomotion, i.e. relative position of the material and the nano tool over time. 
     In reality the system controls the distance between the end points of the sensor which can be some distance away from the probe and the material we are interested in, leaving centimeters—a short distance, but 10 7 -10 9  times larger than what we intend to control—of the mechanical path uncontrolled, subject of vibration and thermal expansion. In fact, the practical sizes of the sensors, the complexity of the two or three dimensional arrangement of the actuators and sensors and the required functionalities often require the sensors to be located a “safe distance” from the region of interest, leaving no other options to minimize the effect of the uncontrolled mechanical structure than the often unreliable recourse to special materials and environmental (temperature, pressure, etc.) control of the whole positioning system. 
     SUMMARY OF THE INVENTION 
     A purpose of this invention is allow the sensors to be placed at locations as close to the region of interest as possible, i.e. a few tenths of a millimeter from the probe, the target material and each other. The invention allows the user to reproduce and actively follow these locations from a remote location and at a distance, connected with an arbitrary length single mode optical fiber optics cable, and perform the sensor functions in a controlled environment using the sensor of choice without affecting the experiment. 
     The invention described here is for single axis position control; 2D and 3D position control could require two or three independent systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Simplified illustration of the principles of reproducing nano measurements at a distance employing two identical Fabry-Perot resonators. 
         FIG. 2 : Interferogramm of superluminescent light from two Fabry-Perot resonators as a function of their length difference. 
         FIG. 3 : Block diagram of an apparatus that reproduces nano measurements at a distance employing two Fabry-Perot resonators for small amplitude modulation. 
         FIG. 4 : Interferogramm of modulating the light from the originating Fabry-Perot resonator on the second Fabry-Perot resonator as a function of their length difference. 
         FIG. 5 : Interferogramm of modulating the light from the originating Fabry-Perot resonator on the second Fabry-Perot resonator as a function of their length difference in the immediate vicinity of the point where the length of one resonator equals that of the other. 
         FIG. 6 : Block diagram of a higher precision, faster response apparatus that reproduces nano measurements at a distance employing two Fabry-Perot resonators for small amplitude modulation and employs a Pound-Drever-Hall laser locking scheme to improve sensitivity. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a block diagram of a simplified illustration of the present invention, a device which allows nano measurements to be reproduced at a distance employing two identical Fabry-Perot resonators, FP 1  and FP 2 . The resonators, FP 1  and FP 2 , are formed from two highly reflecting mirrors,  1  and  2  for FP 1  and  3  and  4  for FP 2 . As shown one of these mirrors is a convex mirror the other a plane mirror, but both mirrors could be convex mirrors. L 1  indicates the optical length of the Fabry-Perot resonator FP 1 , and L 2  the optical length of the Fabry-Perot resonator FP 2 . The method of operating the invention is based on making L 1  identical to L 2 . 
     In  FIG. 1  the light from a typical superluminescent diode  5 , having a nominal wavelength of about 1300 nm, is coupled into Fabry-Perot resonator FP 1 . This resonator acts as a filter and only a comb of wavelengths λ n =2L 1 /n, spaced at Δλ n =λ n −λ n+1 =2L 1 /n(n+1)=λ n λ n+1 /2L 1 , will be transmitted. More than 90% of the light will be reflected away from FP 1 . 
     Superluminescent diode  5 , being a typical such diode, has line-width of about 30 nm. Therefore even if Fabry-Perot resonator L 1  is very short, say L 1 =0.3 mm, more than 10 modes will be excited by the light, as n would be of the order of 500 and the mode separation, the Free Spectral Range, n, about 2 nm. 
     The light transmitted from FP 1  to FP 2  is coupled into FP 2  through a circulator  6 , which directs the intensity of the light back-reflected from FP 2  to a photodiode  7 . The signal of the photodiode  7  sums the intensities of all components of the wavelength comb transmitted by Fabry-Perot resonator FP 1 .  FIG. 2  shows the signal of photodiode  7  as a function of the differences of the optical lengths L 2 −L 1  of each component of the wavelength comb, as well as the voltages of each optical length. As shown the optical length differences have a periodicity of about λ/2˜655 nm. The narrowest and deepest minimum difference is observed at L 2 −L 1 =0, as every constituent of the wavelength comb is able to couple into the same resonant mode (same n) in FP 2  as the one which selected them in FP 1 . Moving away from the narrowest and deepest minimum difference, minima are observed in both directions at integer multiples of λ/2, i.e. when L 2 −L 1 =(+/−)m*λ/2 (where m is the number of periods from the narrowest and deepest minimum difference, so that for example (+/−)m*λ/2 equals+1 times 1310 nm/2 at the first period greater than the narrowest and deepest minimum difference). However, this test can be satisfied only for one element of the wavelength comb at a time as Δλ n  also depends on the value of n, Δλ n =2L 1 /n(n+1), and therefore the mode separations are not identical in the two cavities, resulting in broader and less deep minima for increasing |m|. For |m|=6 the separate coupling of each individual line of the wavelength comb as a function of L 2 −L 1  is clearly resolved, as shown on the enlarged scale inset of  FIG. 2 . It might be noted that had a white light source instead of a superluminescent diode been used, only one interference would be observed. All other minima would have disappeared; white-light interference is imperfect due to the finite width of the light source. 
     However, from the interference pattern observed in  FIG. 2 , the interference corresponding to L 2 =L 1  is the narrowest, and it is easy to identify. Our invention uses this narrow (and deep) minimum to make the distance L 2  equal to the distance L 1 , and maintain these distances identical, with sub-picometer precision. 
       FIG. 3  shows a block diagram of the simplest embodiment of the present invention. As shown, the embodiment employs single-mode fiber coupled components, but a similar embodiment could be assembled for any wavelength of light using free space or fiber coupling, as is practical for the situation. In  FIG. 3 , unlike  FIG. 1 , an isolator  8  and a circulator  9  are inserted between the superluminescent diode  5  and FP 1 . These components serve to provide a monitor signal on a photodiode  10  to facilitate adjustment of FP 1 . Furthermore, in  FIG. 3 , the mirrors  1  and  2  of FP 1  are held mechanically by holder rings  11  and  12 , respectively, allowing precise control of their separation by means of high precision piezoelectric actuator  13  inserted between the holder rings  11  and  12 . Similarly the mirrors of FP 2 ,  3  and  4  are held by holder rings  14  and  15 , respectively, again allowing precise control of their separation by means of high precision piezoelectric actuator  16 . Photodiode  17  detects the light transmitted to FP 2  and helps to tune that component. Finally as with the  FIG. 1  illustration, circulator  6  directs the intensity of the light back-reflected from FP 2  to a photodiode  7 . 
     In order to lock L 2  to L 1  the wavelength comb generated by FP 1  a small amplitude sine wave, ˜5 kHz, is applied to piezo  13 , giving a sub-angstrom amplitude modulation to the light transmitted from FP 1  to FP 2 . The lock-in amplifier  18  connected to photodiode  7  detects the response to this modulation. The resulting ‘derivative’ signal as a function of L 2 −L 1  over the entire ˜7 micron range of piezoelectric actuator  16  is shown on  FIG. 4 ; and over the immediate vicinity of L 2 =L 1  is shown in  FIG. 5 . The 250 picometer/volt slope signal shown in  FIG. 5  is fedback to piezo  16  locking and holding the FP 2  resonator length to that of FP 1  with an rms noise of ˜3 picometers over a 1 kHz bandwidth. It will be noted that the locking range of the system is only ˜2 nm, as infered from  FIG. 5 . 
     The precision achieved with the apparatus of  FIG. 3  can be improved using higher Q resonators, i.e. with the use of higher reflectivity mirrors (the ones shown are R=0.98, which is consistent with the observed ˜250 pm amplifier lock-in range as shown in  FIG. 5  which arises from both cavities having resonant modes of about 10 MHz width). However, this technique would result in a further reduction of the lock-in range making it inconveniently small for smaller resonance widths (higher Q). Using higher frequency modulations than the resonance width, such as in a Pound-Drever-Hall laser lock-in technique, the lock-in range becomes the modulation frequncy, f 1 =30-50 MHz, independent of the resonator qualty. 
     A higher precision, faster response preferred embodiment of the invention uses a generalized form of such a Pound-Drever-Hall laser lock-in scheme as shown in  FIG. 7 . As shown there an electro-optical phase modulator  18  is inserted between the two Fabry-Perot resonators FP 1  and FP 2 , and driven by a f 1 =30-50 MHz signal so that FM side-bands are generated for every line of the wavelength comb coming out of FP 1 . Each of these lines will lock to the identical mode of FP 2 , all of which reinforces the distance locking strength. 
     The f 1  modulation signal is generated by one of the channels of a dual channel direct digital synthesizer (DDS)  19 , which could be an Analog Devices model AD9958 The AD9958 can generate equal frequency signals (with 32 bit resolution) on both outputs, with independently adjustable phase (14 bit/0.02° resolution) and amplitude (10 bit resolution). 
     The signal-of-interest from the electro-optical phase modulator  18  is carried in the photodiode  7  output at frequency f 1 . The photodiode  7  output is filtered and connected to the RF input of the mixer  20  whose local oscillator (LO) input is connected to the second channel of the DDS  19 . The programmable phase-offset between the two DDS  19  channels allows phase adjustment for optimal detection of the f 1  component. The low-pass filtered IF output of the mixer  20  is the PDH error signal, which is brought after appropriate gain  21  to the piezoelectric actuator  16  resulting in a broad lock-in range and sub-picometer precision reproduction of movement. 
     It is important to realize that the precision of the motion control as well as the range of the motion reproduced is not dependent on wavelength. The invention provides continuous operation over a range 10-1000 times the light wavelength used. The range is only limited by the actuator used and the design of the Fabry-Perot resonator. 
     Typically, short and compact Fabry-Perot resonator designs are preferred. The invention does not measure the distance. It reproduces distance with better precision than the measurement technology available, and putting the distance in a different location, meters away from where it started. 
     The invention will perform the best when L 1  and L 2  are the shortest, i.e. an optical length less than a millimeter. It is best used to position two macroscopic objects, a probe and a sample, or a tool and a work piece, relative to each other with atomic precision. Ideally the two objects would be attached to the reference (inner) surfaces of rings  14  and  15  of  FIG. 6 , when the distance between the objects is minimal. Placing a precision measurement device directly between the tool/probe and its target is typically more cumbersome and adds centimeters, sometimes tens of centimeters, extra support structure between the reference surfaces and the distance between the tool/probe and the target. Even if the distance between the reference surfaces kept constant, the distance of interest is the subject of thermal variations of the supporting structure. Reducing this to a few tenth of a millimeter, as required for a well-designed miniature Fabry-Perot resonator, reduces the thermal drift in the tool/probe or target distance by two to three orders of magnitude. 
     While implementing FP 2  in the near vicinity of the tool/probe and target imposes minimal congestion, the environment of FP 1  can be created independently from the experiment. Distance is measured and controlled between the mirrors  11  and  12  of FP 1  at a different location than FP 2 , and the two Fabry-Perot resonators are connected only with a single-mode fiber. The temperature drift of the of the distance measurement can be minimized by careful temperature and vibration control, such as enclosed, sealed, high vacuum or a UVH environments. There is no need to access this environment as measurements and sample changes happen elsewhere, at FP 2 . 
     Applications which can benefit from the use of the invention disclosed here include but are not limited to scanning probe microscopy, nano-fabrication processing, and in general any process, where the positions of two macroscopic objects should be controlled and stabilized relative to each other with atomic scale, sub-nanometer precision. 
     Although only a few embodiments have been disclosed in detail above, other embodiments are possible and the inventors intend these to be encompassed within this specification. The specification describes specific examples to accomplish a more general goal that may be accomplished in another way. This disclosure is intended to be exemplary, and the claims are intended to cover any modification or alternative which might be predictable to a person having ordinary skill in the art. For example, other devices, and forms of modularity, can be used. 
     Also, the inventors intend that only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. The computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation. The computer may be a Pentium class computer, running Windows XP or Linux, or may be a Macintosh computer. The computer may also be a handheld computer, such as a PDA, cellphone, or laptop. 
     The programs may be written in C, or Java, Brew or any other programming language. The programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium. The programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein.