WO1989009413A1

WO1989009413A1 - Electro-optic probe

Info

Publication number: WO1989009413A1
Application number: PCT/US1989/001230
Authority: WO
Inventors: Roger S. Jones
Original assignee: Princeton Applied Research Corporation
Priority date: 1988-03-25
Filing date: 1989-03-23
Publication date: 1989-10-05

Abstract

An electro-optic probe (10) for measuring an externally-applied field, e.g., an electric or magnetic field, is provided comprising a housing (12a); a beamsplitter (22b) mounted in the housing (12a) and optically coupled to a light source (16) and to an optical detector (20) via flexible fiber optic cables for conditioning an optical beam received from the light source (16) by extracting a portion of the optical beam having a first polarization, and for analyzing the portion of the optical beam by extracting a subportion of the optical beam having a second polarization and transmitting the subportion to the optical detector (20); and a crystal (28) mounted in the housing (12a) in fixed relation to the beamsplitter (22b) and optically coupled to the beamsplitter (22b), for receiving the portion of the optical beam, changing the first polarization to the second polarization for the subportion of the optical beam in response to an externally-applied field, and returning the subportion of the optical beam to the beamsplitter (22b).

Description

ELECTRO-OPTIC PROBE

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to electro-optic measurement in¬ struments and, more specifically, to an electro-optic probe for use in measuring electric or magnetic fields, for example, asso¬ ciated with a waveguide or a microelectronic component. Description of the Related Art

Devices have been proposed for usefully exploiting physical phenomena in which the optical characteristics of a material are altered by the presence of an electric or magnetic field. For example, in accordance with a phenomenon known as the Pockels effect, certain crystals, many of which are normally biaxially birefringent (propagate light having a given polariza¬ tion at a different velocity than that of light having a differ¬ ent polarization for certain directions in the crystal) change their birefringent properties in the presence of an externally-applied electric field. The presence of an externally-applied electric field in such a crystal causes the polarization of a light beam projected through the crystal to change in proportion to the magnitude of the electric field. By measuring the change in polarization of the light beam, the cor¬ responding magnitude of the electric field can be obtained.

A number of devices for measuring Electric fields have recently been proposed which utilize phenomena such as the Pockels effect. For example, U.S. Patent No. 4,446,425, issued to Valdmanis et al. on May 1, 1984, discloses such an apparatus for measurement of electric signals with picosecond resolution. The apparatus includes a crystal which exhibits the Pockels effect in the presence of an electric field, a light source for projecting a pair of light beams along respective first and sec¬ ond optical paths, one of the paths intersecting the crystal, and a pair of photodetectors for detecting the respective light beams.

Various approaches have been used to improve the pratical utility of a device such as that proposed by Valdmanis et al. For example, an electro-optic probe utilizing the Pockels effect has been proposed which includes a microprobe tip having a crystal that exhibits the Pockels effect in the. pres^¬ ence of an electric field. The microprobe tip is optically coupled to a polarizing beamsplitter through a lens and a beam retarder. The beamsplitter is optically coupled to a diode laser and an optical detector. The diode laser projects a light beam through a rigid optically-transparent rod to a beamsplitter, which transmits a portion of the light beam having a preselected polarization. The light beam then passes through the beam retarder to the crystal. The presence of an externally-applied electric field at the crystal causes the crystal to vary the polarization of the light beam in accordance w.ith the Pockels effect. The probe reflects the light beam back to the beamsplitter, which analyzes the beam by selectively re¬ flecting to the optical detector portions of the light beam having polarization other than the initial preselected polariza¬ tion. The optical detector detects the change in intensity of this portion of the light beam, which corresponds to the inten¬ sity of the electric field.

Devices such as these have signicantly improved the ability to measure electric fields with high resolution. They are not, however, always suitable for practical field applica¬ tions where durability can be an important consideration. Nor are they as amenable as they might be to flexible coupling to test instrumentation such as an electro-optic oscilloscope mainframe. Improvements in the isolation of the field-sensing crystal from unwanted field effects, and improvements in the flexible movement of the field-sensing crystal relative to larger components of such a device would greatly enhance the utility of these devices.

Accordingly, it is an object of the present invention to provide an electro-optic probe that may be compact in size and sufficiently durable for use in an operational field envi¬ ronment.

It is also an object of the present invention to pro¬ vide an electro-optic probe the operation of which is not sig¬ nificantly affected by unwanted interference from the electric field being measured.

It is yet another object of the invention to provide an electro-optic probe that may be coupled to an associated test instrument by flexible optical couplers, such as flexible fiber optic cables. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by prac¬ tice of the invention. The objects and advantages of the inven¬ tion may be realized and obtained by means of the instrumen¬ talities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described here, an electro-optic probe is provided which is adapted to be optically remotely coupled by a first flexible op¬ tical conduit to a light source which transmits an optical beam, and by a second flexible optical conduit to an optical detection device. By virtue of its design, the probe is movable relative to the light source and the optical detection device.

The probe comprises support means having coupling means for coupling the first and the second optical conduits to the support means; beam filtering means mounted to the support means for receiving the optical beam from the first optical con¬ duit and conditioning the optical beam by extracting a portion of the beam having a first polarization, and for analyzing the portion of the optical beam by extracting a subportion of the optical beam having a second polarization and transmitting the subportiion to the second optical conduit; and sensing means, such as a crystal which exhibits the Pockels effect, mounted to the support means in fixed relation to the beam filtering means and optically coupled to the beam filtering means, for receiving the portion of the optical beam, changing the first polarization to the second polarization for the subportion of the optical beam in response to an externally-applied field, and returning the subportion of the optical beam to the beam filtering means.

According to one embodiment of the invention, the beam filtering means includes a polarizing beamsplitter optically coupled to the first and second optical conduits and to the crystal for both conditioning and analyzing the optical beam. In a second embodiment of the invention, the beam filtering means includes a polarizer for conditioning the optical beam and a polarizing beamsplitter for analyzing the beam. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. Of the drawings:

Fig. 1 illustrates an electro-optic probe according to a first preferred embodiment of the invention coupled to a light source and to an optical detector;

Fig. 2 is a graph of the portion of an optical beam having a specific (second) polarization as a function of the magnitude of an external electric field;

Fig. 3 illustrates an electro-optic probe according to a second preferred embodiment of the invention coupled to a light source and to a pair of optical detectors; and

Fig. 4 illustrates a use of the preferred embodiment shown in Fig. 1 in testing an integrated circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferred embodiments of the invention as illustrated in the accompanying drawings.

One might assume that probes such as those proposed in the past and described above could be adapted for use with a flexible cable merely by designing the diode laser, the laser driving ciruitry, the beamsplitter and other optics, and the photodetector into the probe so that a flexible electrical cable from an associated test instrument could be connected to the probe to drive the diode laser driving ciruitry and to receive the output from the photodetector. This approach is deficient, however, since placement of the fast electrical components, i.e., the diode laser driving ciruitry and photodetector, in the probe and proximate to the electric field source being tested causes electrical interference which can interrupt the indepen¬ dent operation of these components and the field source to be measured.

One might then assume that a probe based on the fore^¬ going principles but which overcomes the limitations noted above could be constructed merely by moving the crystal into the probe while positioning the diode laser, its driving circuitry, the beamsplitter, and the photodetector in a test instrument coupled to the probe by a flexible fiber optic cable. The principal drawback of this design is that it fails to preserve that criti¬ cal polarization relationship of the light beam as it is projected to the crystal and back to the analyzing beamsplitter. As is well known in the art, a linearly polarized light beam traveling in a bending or twisting fiber optic cable quickly loses its original polarization. Since devices utilizing the Pockels effect, such as those described above, depend on accurate measurement of small changes in polarization (approxi¬ mately 10^~° radians, for example), separation of the laser, beamsplitter, and photodetector from the crystal by a bending or twisting fiber optic cable, even of the polarization preserving type, is unsatisfactory.

The electro-optic probe of the invention provides in¬ herent design features that allow a high degree of electrical isolation while preserving the critical polarization relation¬ ships needed to successfully utilize phenomena such as the Pockels effect.

An electro-optic probe 10 according to a first pre¬ ferred embodiment of the invention is shown in Fig. 1. Probe 10 includes support means such as a housing 12. Housing 12 in¬ cludes a probe body 12a and a microprobe tip 12b. Microprobe tip 12b has the shape of a frustum, the base or inner end 12c of which is coupled to probe body 12a and the truncated apex or outer end of which forms an end face 12d spaced from and sub¬ stantially perpendicular to base 12c. A longitudinal axis 12e extends down the center of microprobe tip 12b perpendicular to base 12c and end face 12d. The wall of microprobe tip 12b com¬ prises an outer shell 12f which is durable. Outer shell 12f preferably comprises a material having a low dielectric con¬ stant, such as a ceramic or alumina material. Microprobe tip 12b preferably has nominally small dimensions appropriate for specific electric field measurement applications, such as inser¬ tion in a test aperture of a waveguide or into microelectronic circuit components, for example, onto conducting paths of an integrated circuit. Housing 12 can be approximately the size and shape of a conventional oscilloscope probe or, preferably, smaller.

Probe 10 is coupled by a flexible optical conduit such as a fiber optic cable 14 to an external light source 16 which produces an optical beam of pulsed light. The optical beam as used here may include may include electromagnetic radiation both within and outside the visible spectrum to encompass, for exam¬ ple, infrared and ultraviolet radiations. A number of devices may be used as light source 16. Examples include colliding pulse mode-locked (CPM) lasers, sync-pump dye lasers, frequency-doubled fiber-compressed Nd:YAG lasers, and semicon¬ ductor lasers, each having a pulse duration on the order of ten picoseconds or less, although other light sources may be suit¬ able or desirable. The most suitable device will vary depending on the application. Light source 16 of this embodiment includes a laser diode 16a and associated driving circuitry 16b for pro¬ ducing a pulsed optical beam having a pulse duration of approxi¬ mately 10 picoseconds. A lens 14a is positioned between light source 16 and cable 14 to couple the optical beam from the light source to the cable.

Probe 10 is also coupled by a flexible optical conduit such as a fiber optic cable 18 to an external optical detector 20 capable of detecting the presence and intensity of the opti¬ cal beam projected by light source 16 and converting the detected light intensity into a corresponding electric signal. A lens 18a is positioned between cable 18 and optical detector 20 to couple the optical beam from cable 18 to detector 20.

Coupling of light source 16 and optical detector 20 to probe 10 via flexible fiber optic cables 14 and 18 or the like allows probe 10 to be movable independent of the position of light source 16 and optical detector 20, which provides distinct advantages over prior devices as described above. For example, optical detector 20 may be located with and coupled to test in¬ strumentation such as an electro-optic oscilloscope or a signal processor for analyzing the optical signal received at optical detector 20 via cable 18 while probe 10 is easily moved to various test points. Probe 10 also includes beam filtering means, prefer¬ ably a polarizing beamsplitter 22, mounted in housing 12 and op¬ tically coupled to light source 16 and to optical detector 20. As shown in Fig. 1, beamsplitter 22 is coupled to housing 12 at probe body 12a by a mounting bracket 22a which optically couples the beamsplitter to cables 14 and 18 via lenses 24 and 26, re¬ spectively. Accordingly, the optical beam projected from light source 16 is transmitted along cable 14, collimated at lens 24, and directed essentially as a pencil beam to beamsplitter 22. Beamsplitter 22 conditions the beam by extracting a portion of the beam having a first or reference polarization. Beamsplitter 22 extracts the portion of the optical beam by transmitting this portion and reflecting the remainder of the beam, for example, to a light absorber, the reflected portion of the beam not further being used in this embodiment.

Probe 10 further includes sensing means, such as a crystal 28 which changes the polarization of a light beam pass¬ ing through the crystal from the first polarization in response to an externally-applied field to be measured. In this embodi¬ ment, the field to be measured is an electric field. Therefore, crystal 28 is selected to be responsive to the presence of an externally-applied electric field by changing the polarization of the optical beam in response to the electric field intensity. Suitable crystals would include those which exhibit the Pockels effect in the presence of an externally-applied electric field. Examples of such a crystal include lithium tantalate (LiTaθ3) and lithium niobate (LiNbθ3), although many others are known. The dimensions of crystal 28 are approximately 50 microns x 50 microns x 200 microns in this embodiment. These dimensions may vary depending on the specific application of the probe.

Crystal 28 is mounted in housing 12 and is optically coupled to beamsplitter 22 via a beam retarder 30 and a lens 32. Crystal 28 is mounted at or near end face 12d of microprobe tip 12b so that it is positioned near the external field source when microprobe tip 12b is inserted into small _spaces where electric field measurements are desired. The cross sectional dimensions of end face 12d in the plane of end face 12d perpendicular to axis 12e are slightly larger than those of crystal 28. Crystal 28 is embedded along axis 12e in an optically-transparent dielectric support material 34 encased in outer shell 12f of microprobe tip 12b. Support material 34 is selected to have appropriate durability to protect crystal 28 without interfering with its operation, and without affecting the polarization of light passing through it, such as Siθ2- Lens 32 focuses the portion of the optical beam from beamsplitter 22 through support material 34 and onto cyrstal 28.

The change in polarization of the portion of the opti¬ cal beam from the first polarization induced by crystal 28 in response to the externally-applied electric field can be illus¬ trated as follows. A rectilinear coordinate system can be surperimposed on probe 10 with its origin at a point 22b in beamsplitter 22 at which the optical beam intersects the po¬ larizing material in the beamsplitter. The z-axis of the coor¬ dinate system is assumed to lie along axis 12e of microprobe tip 12b. The polarization axis of beamsplitter 22 is assumed to lie along the x-axis of the coordinate system.

Changes in polarization of the portion of the optical beam from the first polarization, other than changes correspond¬ ing to integer multiples of 180°, will cause the polarization of the portion of the beam to have a component along the y-axis. Retardation other than integer multiples of 90° will cause the polarization to have components along both the x- and y-axes. Each of these components can be extracted and separately mea¬ sured in the invention, as explained more fully below.

Probe 10 further includes reflecting means positioned opposite crystal 28 from beamsplitter 22 for receiving the por¬ tion of the optical beam from crystal 28 and reflecting this portion of the beam back to beamsplitter 22 through crystal 28. The reflecting means preferably includes a dielectric mirror coating 36 applied to a face 28a of crystal 28 adjacent to end face 12d or microprobe tip 12b. Coating 36 is capable of fully or essentially fully reflecting the various optical components of the optical beam. The portion of the beam traveling along axis 12e enters and passes through crystal 28. Upon encoun¬ tering coating 36 at crystal face 28a, the portion of the beam is reflected back through crystal 28 toward beamsplitter 22. It will be understood by those of ordinary skill in the art that the reflecting means are not limited to the illus¬ trative example of coating 38, nor to the location described above. For example, crystal face 28a may be facetted so that the portion of the optical beam is reflected back to beamsplitter 22 through crystal 28 by total internal reflection. Alternatively, the reflecting means may include a reflecting surface such as a mirror located opposite crystal 28 from beamsplitter 22 and spaced from crystal 28.

As the portion of the optical beam initially passes through crystal 28, which is in the vicinity of an externally-applied electric field generated by, for example, a waveguide or a microelectronic component, the polarization of the portion of the optical beam is changed from the first polar¬ ization in response to the electric field. The degree of change of polarization from the first polarization is a function of the electric field intensity at crystal 28 for a given crystal.

As the portion of the optical beam encounters and is reflected by dielectric mirror coating 36 at crystal face 28a, it undergoes phase reversal. Upon passing back through crystal 28 and returning to beamsplitter 22, the polarization of the portion of the optical beam is again changed by crystal 28 under the effect of the electric field. Thus, the change in polariza¬ tion of the portion of the beam from the first polarization is essentially twice the change that would be expected if this por¬ tion of the beam made only a single pass through crystal 28. This has the advantageous effect of essentially doubling the sensitivity of the device over designs utilizing only a single pass of the beam through the crystal. Therefore, measurement of the change in polarization of the portion of the optical beam as it passes through crystal 28 provides an accurate indication of the magnitude of the electric field in the vicinity of crystal 28.

After being reflected by reflective coating 36 and passing back through crystal 28, the portion of the optical beam is directed back through lens 32 and retarder 30 to beamsplitter 22. With regard to the function of beam retarder 30, it was stated above that crystal 28 is normally birefringent i.e., many crystals which exhibit the Pockels effect are birefringent even in the absence of an externally-applied electric field. A single light beam passing through a birefringent crystal is split into two light beams, an ordinary ray and an extraordinary ray, differing in phase from one another. Furthermore, the pulsed light comprising the opti¬ cal beam includes photons of various wavelengths as represented by the Fourier transform of the beam, and photons at each of these wavelengths have a characteristic response in passing through birefringent crystal 28. These various wavelength com¬ ponents of the optical beam can emerge from crystal 28 having various changes in polarization with respect to the first polar¬ ization. These phenomena are generally referred to as static birefringence. Static birefringence is generally undesirable in crystal 28 in that it causes unwanted disruptions in the optical beam which can interfere with the desired polarization measure¬ ments. Beam retarder 30 is used to offset these undesirable effects.

Accordingly, beam retarder 30 comprises a birefringent crystal rigidly and adjustly mounted to housing 12 by a support 30a and positioned between beamsplitter 22 and crystal 28. The optic axis of beam retarder 30 is rotated approximately 90° rel¬ ative to the optic axis of crystal 28 to correct static birefringent effects by reversing these effects. If of the same material as beam retarder 28, the crystal of beam retarder 30 is preferably somewhat thicker or thinner than the corresponding dimension of crystal 28 and it is tilted with respect to the path of the optical beam passing through it. This orientation allows the degree of offset of static birefringent effects to be finely adjusted by controlling the tilt angle of beam retarder 30 with respect to the beam path and the optic axis of crystal 28.

Upon returning to beamsplitter 22, the polarization of the portion of the optical beam will be changed relative to the first polarization in response to the externally-applied elec¬ tric field. Beamsplitter 22 extracts a subportion of the por¬ tion of the optical beam having polarization other than the first polarization by transmitting light having the first polarization and reflecting light having other than the first polarization, the latter comprising the subportion extracted. Since the same polarizing beamsplitter is used in this embodi¬ ment for beam conditioning and beam analysis, light returning from crystal 28 having the first polarization (along the x-axis) is transmitted to lens 24. The y-axis component of light re¬ turning from crystal 28 is reflected by beamsplitter 22. Thus, the y-axis corresponds to a second polarization axis perpendicu¬ lar to the first polarization. Beamsplitter 22 reflects the subportion of the beam to a mirror 38, which reflects the subportion to lens 26. Lens 26 focuses the subportion and cou¬ ples it to fiber optic cable 18, which transmits the beam subportion to detector 20. Optical detector 20 receives the beam subportion via collimating lens 18a and detects the inten¬ sity of the beam subportion, this intensity corresponding to the magnitude of the electric field sensed by crystal 28. Optical detector 20 converts this detected optical beam subportion into a corresponding electrical signal which can be provided to amplification circuitry and test instrumentation such as an electro-optic oscilloscope or signal processor for analysis or display of the measured electric field, for example, as a func¬ tion of time or frequency.

The operation of probe 10, together with light source 16 and optical detector 20, as an electro-optic field measuring system can be enhanced by biasing the optical detector to enhance the sensitivity of the system. This can be explained as follows.

After passing through crystal 28 and returning through beam retarder 28, the portion of the optical beam enters beamsplitter 22, which analyzes the beam by selectively re¬ flecting a subportion of the portion of the optical beam having the second polarization to optical detector 20. In the absence of an externally-applied electric field at crystal 28, the opti¬ cal beam portion will return to beamsplitter 22 having the first polarization. Beam retarder 30 will have offset any static birefring ent effects imposed by crystal 28. Polarizing beamsplitter 22 transmits the entire optical beam portion, and none of the beam will be reflected to optical detector 20 since none has the second polarization. Thus, the intensity of the optical beam subportion as it emerges from beamsplitter 22 to¬ ward optical detector 20 is zero for an electric field intensity of zero.

Upon introducing an electric field at crystal 28 and increasing the magnitude of the electric field from zero, the change in the polarization of the optical beam portion, and the corresponding change in intensity of light reflected to optical detector 20 from beamsplitter 22, increase according to a sine squared function 40 as shown in Fig. 2. The lower porition 42 and the upper porition 44 of the sine squared function, corre¬ sponding to small and large electric field magnitudes, respec¬ tively, are characterized by relatively small and nonlinear changes in optical beam subportion intensity for a given change in electric field intensity. The central or steep portion 46 of curve 40, however, is characterized by relatively large and lin¬ ear changes in optical beam subportion intensity for a given change in electric field intensity. Operating along this steep portion 46 of curve 40 allows optical detector to detect the small variations in optical beam subportion intensity corre¬ sponding to the small changes in the polarization of the optical beam portion induced by crystal 28. Accordingly, the system of this embodiment uses a biasing technique to cause the system to operate along the steep portion of curve 40.

One technique for biasing the system is to couple a DC electrical bias to crystal 28. The DC bias increases the inten¬ sity of the electric field across crystal 28 by a known amount, for example, by E^~- as shown in Fig. 2. Electrical biasing can be undesirable, however, in that dielectric breakdown of crystal 28 may occur, and the electric field generated by the DC bias may adversely affect the field source, such as the microelectronic circuit under test.

Another technique for biasing optical detector 20 in¬ volves optically biasing detector 20 by imposing predetermined change in polarization on the optical beam portion. Optical biasing of detector 20, which is the preferred biasing tech¬ niques in this embodiment, is achieved here by aligning beam re¬ tarder 30 with respect to the optic axis of crystal 28 to change the retardation to the same point on the curve as would have been caused by applying an electrical bias E_D, as shown in Fig. 2.

Beam retarder 30 is positioned sufficiently far from crystal 28 and the external electric field source that changes in polarization induced by beam retarder 30 in response to the externally-applied electric field are neglible. Thus, the crys¬ tal of beam retarder 30 offsets the undesirable effects of stat¬ ic birefringence and optically biases the probe while being essentially independent of the effects of the externally-applied electric field. It should be noted that certain crystals may be selected for crystal 28 which do not have static birefringent effects, in which case it would be unnecessary to use beam re¬ tarder 30 to offset such effects.

The operation of probe 10 in measuring an externally-applied electric field can be summarized in the fol¬ lowing manner. Probe 10 is positioned by the user so that end face 12d of microprobe tip 12b is in the immediate vicinity of the field source generating the external electric field to be measured. Light source 16 is activated to generate a pulsed op¬ tical beam as described above, and to transmit the beam along cable 14 to probe 10. Beamsplitter 22 tran mits the portion of the optical beam having the first polarization and reflects other portions of the beam. The portion of the beam transmitted by beamsplitter 22 then passes through beam retarder 30, which partially or fully offsets the effects of static birefringence introduced by crystal 28 and optically biases the probe as described above. The beam portion travels from beam retarder 30 to lens 32, which focuses the beam portion onto crystal 28. Crystal 28 changes the polarization of the beam portion in re¬ sponse to the magnitude of the external electric field in the vicinity of crystal 28. The beam portion passes through crystal 28 and is reflected at crystal face 28a, where it under¬ goes a phase reversal and returns back through crystal 28. Dur¬ ing this second pass of the beam through erystal 28, the polari¬ zation of the beam portion is further changed in response to the magnitude of the external electric field in the vicinity of crystal 28. Because the phase of the beam portion is reversed upon reflection, the change in polarization occurring during each pass of the beam portion through crystal 28 (which gener¬ ally will be essentially equal to each other in magnitude) add constructively to produce an enhanced change in polarization of the beam portion.

After passing back through crystal 28, the beam por¬ tion is collimated by lens 32 and directed to beam retarder 30, which further adjusts the beam portion to offset static birefringent effects of crystal 28 and optically bias the opti¬ cal beam portion. The beam portion then enters beamsplitter 22, which analyzes it by selectively reflecting the subportion of the beam having the second polarization and transmitting other subportions of the beam. The beam subportion reflected at beamsplitter 22 is directed to mirror 38, which directs the subportion to lens 26. Lens 26 couples the beam subportion to cable 18, which transmits it to optical detector 20 via collimating lens 18a. Optical detector 20 measures the intensi¬ ty of the beam subportion and translates this optical intensity into a corresponding electrical signal representative of the externally-applied electric field intensity in the vicinity of crystal 28 in microprobe tip 12b. Since crystal 28 is very small, and since changes in polarization induced by the electric field at crystal 28 occur at speeds on the order of picoseconds, both absolute measurements and measurements of the change in the electric field over time can be made with high spatial and tem¬ poral resolution.

Advantages of this first preferred embodiment over prior devices will be apparent from its design as described above. For example, the positioning of an optical element such as beamsplitter 22 in housing 12 while locating light source 16 and optical detector 20 external to probe 10 allows the use of flexible optical conduits such as flexible fiber optic cables 14 and 18 without upsetting the polarization relationships needed to measure the subportion of the optical beam having the second polarization. The design of housing 12, particularly the use of outer shell 12f and support material 34 to support crystal 28, results in a durable probe suitable for field use. Furthermore, this embodiment avoids the problem of electrical interference of the electrical components, e.g., between laser diode driving circuitry 16b and optical detector 20. Note also that it is possible with the configuration of this embodiment to replace diode laser 16a its driving circuitry 16b other light sources, for example, to improve the temporal resolution of the probe.

An electro-optical probe 50 according to a second pre¬ ferred embodiment of the invention is shown in Fig. 3. This embodiment includes support means such as a housing 52 having a probe body 52a, a microprobe tip 52b, a frustum base 52c, an end face 52d, a longitudinal axis 52e, and a dielectric outer shell 52f, each essentially identical in design and dimensions to cor¬ responding parts of the housing described above with regard to the first embodiment.

Probe 50 is coupled by a flexible fiber optic cable 54 to external light source 16 which produces the optical beam as described above. A lens 54a is positioned between light source 16 and cable 54 to couple the optical beam from light source 16 to cable 54.

Probe 50 is also coupled to an external optical detec¬ tion device comprising a pair of external optical detectors 20a and 20b, each of the optical detectors having essentially the same design as detector 20. Optical detector 20a is coupled to probe 50 by a flexible fiber optic cable 56. A lens 56a posi¬ tioned between detector 20a and cable 56 collimates light from cable 56 and directs it to detector 20a. Similarly, optical detector 20b is coupled to probe 50 by a flexible fiber optic cable 58. A lens 58a positioned between detector 20b and cable 58 collimates light from cable 58 and directs it to detector 20b. The electrical output signal of detector 20a is coupled to the inverting terminal of a differential amplifier 60 and the output of detector 20b is coupled to the non-inventing terminal of amplifier 60. Amplifier 60 may in turn be coupled to a lock-in amplifier (not shown) or other external circuitry.

Probe 50 includes beam filtering means mounted in housing 52 which, in this embodiment, includes beam conditioning means such as a polarizer or polarizing beamsplitter 62 and beam analyzing means such as beamsplitter 64, each fixedly mounted to a support wall 52g in probe body 52a. Beamsplitter 62 is optically coupled to light source 16 by cable 54. A lens 66 collimates the optical beam projected by light source 16 and transmitted by cable 54, and directs it onto beamsplitter 62. Beamsplitter 62 conditions the optical beam by extracting a por¬ tion of the beam having a first polarization from portions of the beam having other than the first polarization. Beamsplitter 62 accomplishes this function by transmitting the portion having the first polarization and reflecting other portions.

The transmitted portion of the optical beam is directed to a beam retarder 68 and to sensing means such as crystal 70, both fixedly mounted in housing 52. Crystal 70 is rigidly mounted at or near end face 52d microprobe tip 52b along axis 52e, and is encased in an optically-transparent, dielectric support material 72 occupying the interior portion of microprobe tip 52b surrounded by outer shell 52f. Crystal 70 exhibits the Pockels effect in the presence of an externally-applied electric field and, therefore, light passing through this crystal having a given polarization will experience a change in polarization, the magnitude of the change being proportional to the intensity of the electric field at the crystal. Beam retarder 68 is pref¬ erably a crystal similar or identical to crystal 70 which is rigidly and adjustably positioned on a support 68a and oriented relative to crystal 70 to offset adverse effects of static birefringence in crystal 70 and to optically bias the system, as explained above.

Reflecting means are included to reflect the portion of the optical beam from beamsplitter 62 to beamsplitter 64 through beam retarder 68 and crystal 70. The reflecting means of this embodiment includes a mirror 74 mounted in housing 52 and positioned to receive the portion of the optical beam trans¬ mitted from beamsplitter 62 and reflect this beam portion to crystal 70. The reflecting means also includes a mirror 76 mounted in housing 52 and positioned to receive the optical beam portion from crystal 70 and reflect the beam portion to beamsplitter 64. Mirrors 74 and 76 are mounted in microprobe tip 52b at or near end face 52d and adjacent to crystal 70. They may be formed by bevelling the end portion of support mate¬ rial 72 to create a surface with the appropriate angle, and coating the surface with a reflective coating material having good bonding properties with support material 72 and outer shell 52f. The reflective coating material is selected to reflect the beam portion while substantially preserving the polarization of the optical beam portion.

Analyzing beamsplitter 64 is fixedly mounted in hous¬ ing 52 and is optically coupled to crystal 70 via mirror 76. The polarization axis of polarizing beamsplitter 64 (the second polarization axis) preferably is perpendicular to the polariza¬ tion axis of beamsplitter 62 (the first polarization axis). Beam retarder 68 is oriented to polarize the optical beam por¬ tion so that, in the absence an externally-applied electric field at crystal 70, beamsplitter 64 reflects half of the beam portion and transmits half of the beam portion. The presence of an externally-applied electric field at crystal 70 will change the polarization of the beam portion, thus upsetting the balance of the reflected and transmitted parts of the beam in relation to the magnitude of the electric field. Positive increases in the electric field intensity at crystal 70 will cause a change in polarization of the optical beam away from the balance and toward the second polarization. This will increase the intensi¬ ty of the subportion of the beam portion transmitted by beamsplittter 64 relative to the parts of the beam portion re¬ flected by beamsplitter 64. Negative increases in the electric field intensity at crystal 70 relative to zero will cause a change in polarization of the optical beam away from the balance point, thus decreasing the intensity of the transmitted subportion of the beam portion and increasing the reflected parts of the beam portion.

Beamsplitter 64 is coupled to a mirror 78 which re¬ ceives parts of the beam portion reflected from beamsplitter 64 and reflects them to a focusing lens 80. Lens 80 couples these parts of the beam to cable 56, which directs them to optical detector 20a. Beamsplitter 64 is coupled to a focusing lens 82, which couples the subportion of the beam portion transmitted from beamsplitter 64 to cable 58 and directs it to detector 20b. Amplifier 60, which is coupled to the outputs of detectors 20a and 20b, compares the electrical signals outputted by the detectors to produce an amplified output or difference signal. The second preferred embodiment of the invention may be used to measure the intensity of an externally-applied elec¬ tric field in the following manner. Retarder 68 is first tuned or calibrated to produce an output from amplifier 60 of zero in the absence of an externally-applied electric field at crystal 70. As noted above, the optical bias caused by beam retarder 68 causes the part of the beam portion having the first polariza¬ tion to be essentially equal to the subportion of the beam por¬ tion having the second polarization. In this situation, the intensities of the respective parts of the beams arriving at op¬ tical detectors 20a and 20b are essentially equal. Accordingly, the output of amplifier 60 is essentially zero.

After retarder 68 is tuned, end face 52d of microprobe tip 52b is placed in the vicinity of the field source generating the electric field to be measured, and light source 16 is again activated to generate a pulsed optical beam. Light source 16 transmits the beam along cable 54 to lens 66, which collimates the beam and directs it onto beamsplitter 62. Beamsplitter 62 transmits the portion of the optical beam having a first polari¬ zation and reflects other parts of the beam. The transmitted portion of the beam passes through beam retarder 68, which off¬ sets undesired effects of static birefringence caused by crystal 70 and optically biases the probe. The beam portion is the re¬ flected by mirror 74 to crystal 70. Crystal 70 changes the po¬ larization of the beam portion in response to the magnitude of the externally-applied electric field as the beam portion passes through crystal 70.

After passing through crystal 70, the beam portion is reflected by mirror 76, which reflects it to analyzing beamsplitter 64. Beamsplitter 64 selectively transmits the subporiton of the received beam portion having the second polar¬ ization, and reflects the remaining parts of the beam portion. The transmitted beam subportion is directed to lens 82_r which focuses and couples the beam subportion to cable 58. Cable 58 transmits the beam subportion to optical detector 20b. The parts of the beam subportion corresponding to the first polari¬ zation reflected by beamsplitter 64 are reflected at mirror 78 to lens 80, which focuses and couples these parts to cable 56. Cable 56 transmits these parts to optical detector 20a. Optical detectors 20a and 20b convert the received op¬ tical signals into corresponding electrical signals representa¬ tive of the intensity of the externally-applied electric field. These electrical signals are provided to amplifier 60, which compares the signals to produce a difference signal. Since changes in polarization of the optical beam portion can be caused by increases or decreases in electric field intensity relative to the reference or zero setting established during calibration, these changes in polarization will cause the dif¬ ference signal outputted by amplifier 60 to be either positive or negative. The difference signal can be provided to a lock-in amplifier or other circuitry for amplification and processing of the signal.

The second embodiment of the invention provides a num¬ ber of advantages over the prior devices as well as over the first embodiment of the invention. In this embodiment, the op¬ tical beam portion makes only a single pass through crystal 70. Although the advantage of increased sensitivity associated with the double pass arrangement of the first embodiment is not obtained, this single pass design is advantageous in that it im¬ proves the temporal resolution of the probe by reducing the length of time the optical beam portion is In crystal 74. Fur¬ thermore, use of a separate beamsplitter as a beam analyzer facilitates the use of two separate optical detectors. This design thus provides first order or common mode rejection of amplitude noise from light source 16.

The operation of probe 10 of the first preferred embodiment in testing an integrated ciruit will now be described with reference to Fig. 4. It will be understood that probe 50 of the second preferred embodiment could be used to test the in¬ tegrated circuit in like manner. It will also be understood that this example is merely illustrative of the utility and applicability of the invention, and that the invention is not limited to this application.

With reference to Fig. 4, a conventional integrated circuit 100 to be tested is rigidly mounted to a table ⁽not shown) . A micro-positioning device having a movable arm with three-axis movement relative to the table (not shown) is positioned on the table. Integrated circuit 100 includes a plu¬ rality of conductor paths 102 which are selectively activated to conduct electrical current. Integrated circuit 100 also in¬ cludes a photoconductive switch 104 electrically coupled to a selected one 102a of conductive paths 102 to selectively enable integrated circuit 100 in response to illumination of switch 104 by a light beam. It is assumed here that path 102a is activated and begins to conduct electrical current in response to optical illumination and activation of switch 104. As the current through switch 104 increases, the current through path 102a also increases albeit with a slight delay. The increasing current in path 102a generates a corresponding electric field in the imme¬ diate vicinity of path 102a in accordance with known principles. The response of integrated circuit 100 may be measured by the time interval between illumination of switch 104 by the light beam and the achievement of a given level of electrical current through path 102a. This response in terms of magnitude of cur¬ rent through path 102a as a function of time from illumination of switch 104 is referred to here as the integrated circuit re¬ sponse function.

At lease one probe 10 in accordance with the first preferred embodiment is mounted on the movable positioning arm of the micro-positioning device and located above integrated circuit 100. Probe 10 is coupled by cable 14 to external light source 16, which in this illustrative example includes a CPM laser capable of generating a pulsed optical beam having a pulse duration of approximately 0.5 picoseconds. Probe 10 is also coupled to optical detector 20 by cable 18. Where a plurality of probes is used, each may be similarly configured and inde¬ pendently movable.

Light source 16 is coupled by a fiber optic cable 106 to a variable optical delay. Projection of an optical beam by light source 16 thus propagates simultaneously along fiber optic cables 14 and 106. Variable optical delay 108 is coupled by a fiber optic cable 110 to switch 104. Variable delay 108 intro¬ duces a predetermined delay into the optical beam propagating along cable 106 to compensate for the difference in optical path lengths between, on one hand, light source 16 and crystal 70 via cable 14, and on the other hand, light source 16 and switch 104. In addition, variable delay 108 controls the relative timing of the optical beams at switch 104 and crystal 70 by introducing a predetermined sequence of delays into the optical beam trig¬ gering switch 104. This causes successive sweep measurements to be made and corresponding segments or time intervals of the in¬ tegrated circuit response function to be measured. These seg¬ ments can be combined to reconstruct the complete integrated circuit response as measured at path 102a. It will be recog¬ nized by those of ordinary skill in the art that variable delay 108 could be located at other positions, e.g., along cable 14, and still perform these functions. It will also be recognized that an electrical delay may be substituted for the optical delay introduced by variable delay 108.

Probe 10 thus configured may be used to measure the integrated circuit response function of integrated circuit 100 in the following manner. The movable arm of the micro-positioning device is moved to position end face 12d of microprobe tip 12b in the vicinity of path 102a so that crystal 28 is within the effective range of the electric field generated by path 102a. Light source 16 is energized to cause an optical beam to propagate along cable 106 to variable delay 108 and si¬ multaneously along cable 14 to probe 10. Variable delay 108 in¬ troduces an appropriate delay and transmits the beam to switch 104 via cable 110. Switch 104 is activated in response to the illumination and begins conducting, which causes path 102a to begin conducting and generate the electric field to be measured. Simultaneously, a portion of the optical beam directed to probe 10 impinges upon crystal 28, which changes the polarization of a subportion of the beam portion from the first polarization in response to the electric field about path 102a at a time corre¬ sponding to the delay set by variable delay 108. Beamsplitter 22 extracts the beam subportion having the second polarization by reflecting it to optical detector 20, which converts the beam subportion into a corresponding electrical signal representing the electrical field intensity as path 102a for the segment of the integrated circuit response function corresponding to the delay. A new or successive delay value is then set by vari¬ able delay 108 and light source 16 is activated to repeat the process described above so that a new sample value of the elec¬ tric field intensity at path 102a corresponding to a successive segment of the integrated circuit response function can be obtained in the same fashion. This process is repeated until samples are obtained for a desired number of segments of the in¬ tegrated circuit response function. The various samples can then be used, for example, in an electro-optic oscilloscope, to reconstruct the integrated circuit response function.

Additional advantages and modifications will readily occur to those skilled in the art. For example, the sensing crystal of the preferred embodiments (28 and 70) is described here as exhibiting the Pockels effect in response to an externally-applied electric field. Materials which alter the polarization characteristics of a light beam in response to mag¬ netic phenomena may also be used in accordance with the inven¬ tion. For example, materials are known which change their opti¬ cal properties in response to a magnetic field in accordance with phenomena such as the Faraday effect and the Voigt effect.

Furthermore, the sensing crystal need not be located in a microprobe tip as described above. For example, the sensing crystal may be mounted in a housing spaced from the field generating source, in which case the field may be sampled by extending the effects of the field to the crystal. Thus, a conventional electrical connector coupled to the probe of the invention may be used to transmit the electrical signal to be measured to the sensing crystal of the electro-optic probe.

Therefore, the invention in its broader aspects is not limited to the specific details, respresentative devices, and illustrative examples shown and described. Accordingly, depar¬ tures may be made from such details without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An electro-optic probe adapted to be optically and remotely coupled by a first flexible optical conduit to a light source which transmits an optical beam and by a second flexible optical conduit to an optical detection device which are movable relative to said probe for measuring an externally-applied field, said probe comprising: support means having coupling means for coupling the first and the second optical conduits to said support means; beam filtering means mounted to said support means for receiving the optical beam from the first optical con¬ duit and conditioning the optical beam by extracting a portion of the optical beam having a first polarization, and for analyz¬ ing the portion of the optical beam by extracting a subportion of the optical beam having a second polarization and trans¬ mitting said subportion of the second optical conduit; and sensing means, mounted to said support means in fixed relation to said beam filtering means and optically cou^¬ pled to said beam filtering means, for receiving said portion of the optical beam, changing said first polarization to said sec¬ ond polarization for said subportion of the optical beam in re¬ sponse to the externally-applied field, and returning said subportion of the optical beam to said beam filtering means.

2. An electro-optic probe according to claim 1, wherein said support means includes a housing having a probe body for supporting said beam filtering means and a microprobe tip having an inner end coupled to said probe body and an outer end for supporting said sensing means spaced from said probe body.

3. An electro-optic probe according to claim 2, wherein said microprobe tip includes an outer shell containing a transparent support material for supporting said sensing means.

4. An electro-optic probe according to claim 2, wherein said sensing means includes a crystal operative to change the polarization of light passing through said crystal in response to the externally-applied field.

5. An electro-optic probe according to claim 4, fur¬ ther including reflecting means positioned opposite said crystal from said beam filtering means for receiving said portion of the optical beam from said crystal and reflecting said portion back through said crystal to said beam filtering means.

6. An electro-optic probe according to claim 1, fur¬ ther including a beam retarder positioned between said beam fil¬ tering means and said sensing means for offsetting static birefringence of said sensing means.

7. An electro-optic probe according to claim 6, wherein said beam retarder has a polarization axis positioned at an angle relative to said second polarization to optically bias the optical detector.

8. An electro-optic probe according to claim 1, wherein said beam filtering means includes a polarizing beamsplitter.

9. An electro-optic probe according to claim 1, wherein said beam filtering means includes: a polarizer optically coupled to said sensing means for receiving the optical beam from the first optical con¬ duit and conditioning the optical beam; and a polarizing beamsplitter optically coupled to said sensing means for receiving said portion of the optical beam from said sensing means and analyzing said portion.

10. An electro-optic probe according to claim 9, fur¬ ther including reflecting means for reflecting said portion of the optical beam from said beam conditioning means to said beam analyzing means through said sensing means.

11. An apparatus for measuring an externally-applied field, comprising: a light source for generating an optical beam; optical detection means for detecting at least a part of the optical beam; a first flexible optical conduit coupled to said light source; a second flexible optical conduit coupled to said optical detection means; and a probe adapted to be optically and remotely cou¬ pled to said light source by said optical conduit and to said optical detection means by said second optical conduit, said probe including, support means having coupling means for coupling said first and said second optical conduits to said support means; beam filtering means mounted to said support means for receiving the optical beam from said optical conduit and conditioning the optical beam by extracting a portion of the optical beam having a first polarization, and for analyzing the portion of the optical beam by extracting a subportion of the optical beam having a second polarization and transmitting said subportion to said second optical conduit; and sensing means, mounted to said support means in fixed relation to said beam filtering means and optically cou¬ pled to said beam filtering means, for receiving said portion of the optical beam, changing said first polarization to said sec¬ ond polarization for said subportion of the optical beam in re¬ sponse to the externally-applied field, and returning said subportion of the optical beam to said beam filtering means.

12. An apparatus according to claim 11, wherein said support means includes a housing having a probe body for sup¬ porting said beam filtering means and a microprobe tip having an inner end coupled to said probe body and an outer end for sup¬ porting said sensing means spaced from saidN robe body.

13. An apparatus according to claim 12, wherein said microprobe tip includes an outer shell containing a transparent support material for supporting said sensing means.

14. An apparatus according to claim 11, wherein said sensing means includes a crystal operative to change the polari¬ zation of light passing through said crystal in response to the externally-applied field.

15. An apparatus according to claim 14, further including reflecting means positioned opposite said crystal from said beam filtering means for receiving said portion of the op¬ tical beam from said crystal and reflecting said portion back through said crystal to said beaming filtering means.

16. An apparatus according to claim 11, further including a beam retarder positioned between said beam filtering means and said sensing means for offsetting static birefrigence of said sensing means.

17. An apparatus according to claim 16, wherein said beam retarder has a polarization axis positioned at an angle relative to said second polarization to optically bias the opti¬ cal detector.

18. An apparatus according to claim 11, wherein said beam filtering means includes a polarizing beamsplitter.

19. An apparatus according to claim 11, wherein said beam filtering means includes: a polarizer optically coupled to said sensing means for receiving the optical beam from the first optical con¬ duit and conditioning the optical beam; and a polarizing beamsplitter optically coupled to said sensing means for receiving said portion of the optical beam from said sensing means and analyzing said portion.

20. An apparatus according to claim 19, further including reflecting means for reflecting said portion of the optical beam from said beam conditioning means to said beam ana¬ lyzing means through said sensing means.