Universal cushion support for photoelastic modulator

An apparatus includes an elastically deformable optical element holder situated to receive an optical element having a plurality of holder contact surfaces, the optical element holder including a plurality of receiving portions adjacent to an aperture and corresponding to respective holder contact surfaces, each receiving portion displaceable through deformation of the optical element holder so that the optical element is insertable in the aperture so as to be cushionably supported in a predetermined position with the receiving portions in contact with the respective holder contact surfaces.

FIELD

This application relates to holders for optical elements.

BACKGROUND

A photoelastic modulator (“PEM”) is an optical device that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the optical element is different for different components of a beam of polarized light that propagates through the optical element.

A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element. The transducer vibrates at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz, and in some examples, higher frequencies. The mass of the element is compressed and extended along the axis of the optical element as a result of the vibration. The combination of the optical element and the attached transducer may be referred to as an optical assembly. The axis along which the optical element vibrates is referred to as the optical axis of the PEM.

The optical assembly is mounted within a housing or enclosure that normally includes apertures through which the light to be modulated is directed through the optical element in a direction generally perpendicular to the optical axis of the PEM. The housing supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within it to achieve the above-noted photoelastic effect while at the same time holding the PEM in place.

PEMs are commonly used in measuring polarization properties of either a light beam or a sample. Many instruments use two or more PEMs to provide measurements of certain polarization properties. When two PEMs are used in a single instrument, they are typically arranged so that their optical axes are oriented to be 45 degrees apart (as considered in a direction perpendicular to those two optical axes). Examples of typical, two-PEM instruments include complete Stokes polarimeters, Tokomak polarimeters, and a number of other polarimeters and ellipsometers.

In the past, the mechanism for holding the optical element in the housing of the PEM used rigid barrel supports with grommet extensions to contact surfaces of the optical element. The rigid supports required additional machining steps and the grommets added parts and complexity to installation. Moreover, it has been found that the existing mechanism can have performance limitations. The resonant frequency and oscillation efficiency are sensitive to any mechanical coupling to the PEM housing so that an increase of such mechanical coupling decreases oscillation efficiency and absolute retardation and destabilizes resonant frequency. By decreasing post support rigidity, driving voltage for the PEM and/or power requirements can be significantly reduced. For instance, in some embodiments, the driving voltage can be reduced a third and power requirements by half.

SUMMARY

In some examples of the disclosed technology, an apparatus includes an elastically deformable optical element holder situated to receive an optical element having a plurality of holder contact surfaces, the optical element holder including a plurality of receiving portions adjacent to an aperture and corresponding to respective holder contact surfaces, each receiving portion displaceable through deformation of the optical element holder so that the optical element is insertable in the aperture so as to be cushionably supported in a predetermined position with the receiving portions in contact with the respective holder contact surfaces.

In further examples of the disclosed technology, methods can include inserting an photoelastic modulator optical element, having a plurality of holder contact surfaces, in an aperture of an elastically deformable optical element holder having a plurality of receiving portions adjacent to the aperture and respectively corresponding to the holder contact surfaces, the inserting including elastically displacing at least one of the receiving portions so that the receiving portions contact the respective holder contact surfaces and the optical element is cushionably supported by the optical element holder in a predetermined position.

DETAILED DESCRIPTION

FIGS. 1A-1Bare perspective views of an example unitary elastic optical element holder100. InFIG. 1A, the optical element holder100is shown without the optical element106, and inFIG. 1B, the optical element holder100is shown with the optical element106. InFIGS. 1A and 1B, the example of the optical element holder100includes a unitary and elastic holder housing102, or sleeve, formed of a suitable extrusion printable resilient thermoplastic elastomer material. In other examples, the holder housing102can be injection molded or waterjetted. The example holder housing102generally extends in the form of a square or ring, though various other shapes are possible, so as to provide a central region (cavity)104configured to define an aperture105that is sized to receive and, at one or more portions of the aperture105, frictionally engage an optical element106, such as a photoelastic modulator optical element. The holder housing102is typically deformable and resilient, with the thermoplastic elastomer typically having a hardness of about 90 A or less on a Shore A hardness scale. In some examples, the optical element holder housing102can include a plurality of portions that are affixed or in contact with each other to form the optical element holder100.

The illustrated holder housing102includes a plurality of receiving portions108(also referred to specifically as108a-108d) that are shaped to receive a feature of the optical element106. As shown, four receiving portions108are situated in respective corners of a generally square or rectangular arrangement with each receiving portion108having a concave, spherical notch110located in an interior angled housing surface112of the holder housing102. The optical element106includes a plurality of holder contact surfaces114(also referred to specifically as114a-114d) that are spherical and convexly shaped to match (complement) or agree with the shape of the concave notches110. As shown, four holder contact surfaces114a-114dprotrude from respective angled corner surfaces116of the optical element106. For clarity, inFIG. 1Bthe optical element106has hidden surfaces shown with dashed lines while hidden surfaces associated with the optical element holder100are not shown inFIG. 1B(but are illustrated inFIG. 1A).

The optical element holder100can resiliently deform allowing the optical element holder100to be bent and folded in an arbitrary manner, further allowing the receiving portions108to become temporarily displaced from the optical element106during optical element replacement and/or assembly of the optical element106in the housing100. The receiving portions108can also be made from the same material as the remainder of the holder housing102so as to provide a similar deformative resilience. With the temporary displacement, the optical element106, which is typically a rigid transparent object, can be easily inserted into the central region104in communication with the aperture105so that the holder contact surfaces114are received by the concave notches110. The concave notches110can return to an original position before the displacement or to a new displaced position. The optical element106is frictionally supported and held in a suitable predetermined position by the optical element holder100and is free to controllably vibrate at a higher vibrational efficiency than conventional optical element holding arrangements. Further, in certain embodiments, the elastomeric construction of the optical element holder100provides a housing for the optical element with improved damping and higher performance efficiency in applications where the optical element106is driven as part of a PEM system. In such applications, the deleterious vibrational effects that can be transferred to the housing of the optical element and to the interior region of the housing can be reduced relative to traditional rigid housings (e.g., formed from metals, alloys, or other such rigid structures).

Representative applications for the optical element holder100include cushionably and frictionally supporting the optical element106in various photoelastic modulating devices, including ellipsometers and CD (circular dichroism) analyzers. The optical element106can be transparent, semi-transparent, or opaque at different wavelengths, including in the visible range. Some examples of the optical element106can include non-linear optical crystals, phase plates, solid state media, resonators, diffractive elements, optical slides, optical filters, lenses, reflective elements, acousto-optic or electro-optic modulators, etc.

In representative examples, the optical element holder100includes a support member118that extends around the holder housing102so as to provide a structural continuity of the holder housing102and which can also define a holder base120. The holder base120can include inward extending portions122that provide additional support for the optical element106but also forms a protective barrier for the optical element106in the central region104(e.g., during insertion, transport, and movement in relation to proximate rigid objects that can damage the optical element106). The inward extending portions122can also provide an extended area for a bottom surface124that is situated to contact or align with an interior surface of an optical assembly housing (not shown) to provide further support for the optical element106that is cushionably held at the receiving portions108.

The optical element holder100also includes a circumferential wall126that extends around at least a majority of the holder housing102but defines an interruption that forms an opening128in the holder housing102. The circumferential wall126includes a thin member129adjacent to the opening128that is flexible and can be bent considerably due to its small thickness. The opening128is situated so that a transducer (e.g., a piezoelectric transducer) or other device (not shown) can be inserted through the opening128or otherwise coupled to the optical element106. The opening128also creates opposing open member portions130a,130bin the circumferential wall126that can be gripped and bent elastically away from each other, indicated directionally with arrows131so that receiving portions108a,108bcan be displaced in a similar manner as the thin member129is bent back on itself (e.g., during PEM assembly or optical element replacement). The receiving portions108c,108dalso displace but typically to a lesser extent due to the continuity of the circumferential wall126along a closed side132opposite the opening128. The displacement associated with the opening128and the resilience of the holder housing102allows the optical element106to be inserted into the central region104of the holder housing102so that the holder contact surfaces114can be aligned with the concave notches110. A return displacement of the open member portions130a,130bproduces a similar return displacement of the concave notches to frictionally and cushionably grasp the holder contact surfaces114, with reduced vibrational translation to and/or from the optical element106.

InFIG. 2, an example of a photoelastic modulator assembly200includes a housing202that is typically made of a rigid plastic or metal material and that includes an exterior surface204and an interior surface206. The interior surface206can define an interior region (cavity)208in which a photoelastic modulator optical assembly210can be received. A lid (not shown) can attach to a top surface212of the housing202and secured with fasteners, e.g., at one or more fastener holes214so that the photoelastic modulator optical assembly210is secured in the interior region (cavity)208. The photoelastic modulator optical assembly210includes flexible optical element holder216(which can be the optical element holder100in some examples), a photoelastic optical element218(e.g., optical element106), and piezoelectric transducer220attached to a receiving surface222of the photoelastic optical element218. The optical element holder216, also referred to herein as a “sleeve”) is configured to cushionably and removably support and hold a photoelastic optical element218in the housing202so that the piezoelectric transducer220can vibrate the photoelastic optical element at a characteristic frequency (e.g., frequencies in the range of 20 kHz to 100 kHz and higher) at high power efficiency. A transducer cushion224can also be situated between an end of the piezoelectric transducer220and the interior surface206to provide a protective cushion for the piezoelectric transducer220. The high power efficiency is achieved through the cushionable support provided to the photoelastic optical element218. The high power efficiency allows, for example, application of a lower voltage to the piezoelectric transducer or allows increases in driving amplitude. Thus, greater performance can be achieved from the same optical assembly, or smaller or more robust power supplies may be used. In some examples, by using the holder sleeve216additional absolute retardation cycles can be observed before noise reduces signal quality, thereby providing an improvement to applications preferring large amounts of absolute retardation or by allowing applications for longer wavelengths. In one example, the holder sleeve216allows for a decrease in driving voltage of about one third (e.g., 36 V decreased to 24 V) without decreasing PEM modulation amplitude. In another example, the holder sleeve216allows for an increase in PEM modulation amplitude, from 1200 nm to 2400 nm, without increasing a driving voltage.

The housing202also includes an aperture226on its lower surface adjacent to the photoelastic optical element218situated in the interior region208. A lid aperture (not shown) is typically defined in the lid opposite the aperture226so as to define a measurement axis227exiting the plane ofFIG. 2and a corresponding optical path through the aperture226and photoelastic optical element218. The piezoelectric transducer220extends perpendicularly with respect to the measurement axis227and parallel to an optical axis229of the photoelastic optical element218so that the photoelastic optical element218can be controllably vibrated to vary a strain of the optical element along the optical axis229. In some examples, a transparent window230or filter extends over the aperture226and is situated within a rotatable ring232to protect the photoelastic modulator optical assembly210from debris and damage. The aperture226can be circular in shape so as to provide a rotatable path for flange mounting of the rotatable ring232. In some embodiments, the rotatable ring232is rotatable about the measurement axis227so that the photoelastic modulator optical assembly210also is rotatable about the measurement axis227so as to vary the azimuth of the optical axis229(typically between 0° and 90°) according to different polarimetric application requirements. The rotatable ring232can be formed from any suitable materials, including plastic, metal, or alloy.

The optical element holder (holder sleeve)216includes four receiving portions234that are notched with a spherical concave shape formed into an interior surface236of the holder sleeve216. The concave shape can be formed, e.g., in the process of extrusion printing the holder sleeve216, so as to match and frictionally engage corresponding spherical convex supports238protruding from an exterior surface240of the photoelastic optical element218. The convex supports238are typically made of a rigid material and attached, e.g., with adhesive, to corresponding angled side surfaces242of the photoelastic optical element218. The convex supports238can be larger (as shown) or smaller than the respective receiving portion234, and shapes other than convex are possible, such as conical, frustoconical, parallelpiped, cylindrical, etc., and the shapes need not correspond with an inverse shape of the receiving portions234. For example, concave receiving portions can receive conical supports and conical depression receiving portions can receive convexly surfaced protrusions. In typical examples, the receiving portions234form depressions and corresponding surfaces of the convex supports238protrude from the photoelastic optical element218. In some embodiments, depressions are formed in the photoelastic optical element218and the receiving portions234protrude from the holder sleeve216.

A bottom surface244of the optical element holder216contacts a bottom surface246of the interior region208of the housing202so as to support the photoelastic modulator optical assembly210. An exterior side surface248of the optical element holder216can form a snugly deformed or frictional fit with one or more portions of the interior surface206of the housing202so that the holder sleeve216remains tightly secured in the interior region208of the housing202and the photoelastic optical element218is cushionably supported for efficient photoelastic modulation. In some examples, the optical element holder (holder sleeve)216is suitably resilient so that the optical element holder216can be inserted into the interior region208for a snug fit with the bottom surface246and the interior surface206, and the photoelastic optical element218can be inserted into the interior region208with the receiving portions234of the optical element holder216resiliently deforming to allow the insertion.

In further examples, the optical element holder216, with or without the photoelastic optical element218inserted and cushionably held, can be affixed in the interior region208. Additionally, for ease in assembly of the photoelastic modulator assembly200, the photoelastic optical element218can be inserted and cushionably secured in the optical element holder216and the piezoelectric transducer220can then be affixed to the photoelastic optical element218in a selected position. The assembly of the optical element holder216, photoelastic optical element218, and piezoelectric transducer220can then be inserted into the housing202. The shape and size of the optical element holder216and the housing202can be determined in relation to the size of the optical element218held by the optical element holder216. Thus, different optical element holders (holder sleeves)216can hold optical elements218of different sizes in the same housing202by varying the dimensions associated with the receiving portions234while maintaining the dimension of the exterior side surface248.

FIG. 3Ashows a photoelastic modulator assembly300that includes another resilient optical element holder302situated in a housing304. For clarity, a portion of the housing304is omitted, represented by a jagged line. The optical element holder302includes a bottom member306that surrounds a central aperture region308and that is in contact with a bottom surface310of the housing304. The optical element holder302also includes a top member312that partially surrounds the central aperture region308so as to define an opening314situated to allow a piezoelectric transducer (not shown) or other frequency controlled device insertion and connection or communication with an optical element (not shown) supported in the central aperture region308. The top member312includes a top surface316that can be aligned with or near a top surface318of the housing304. In some examples, a lid (not shown) can be secured to the top surface318so as to enclose the housing304and secure the optical holder302within. The top surface316of the top member312can be raised above the top surface318of the housing304so that the optical holder302can deform for a snug fit in the housing304with the lid secured to the top surface318. The optical element holder302includes a plurality of concave displaceable receiving portions320situated to receive and frictionally engage (contact) corresponding convex portions on the optical element so that optical element can be cushionably held and supported by the optical element holder302in the central aperture region308. In some examples, the housing304can receive separate optical element holders that can support optical elements of different size than the optical element supported by the optical element holder302.

Any of the optical element holders disclosed herein (e.g., the optical element holder100, the optical element holder216, and/or the optical holder302) can be made of an extruded resilient material that is 3-D printed. Suitable material includes the “ninjaflex” brand thermoplastic elastomer filament material manufactured by NinjaTek Corporation. Typically, during 3-D printing, a continuous thread of thermoplastic material is delivered to an extrusion head that heats the material and delivers the heated material to a surface in a controlled manner. The heated material fuses with adjacent material as it is delivered to a stage and multiple passes are performed in a controlled manner so that a three-dimensional object is formed. For instance, and using optical element holder302as an example, a plurality of elastically deformable filaments are fused to adjacent filaments during the 3-D printing process to create striated structures (examples of which are shown as322) that form the body of the optical element holder302. The adjacently fused filaments provide a resilience associated with flexure for the body of the optical element holder in addition to the resilience associated with the thermoplastic material itself. The diameter of the filaments can vary from embodiment to embodiment, with smaller diameters typically increasing printing duration and providing higher resolution for complex shapes. Further, and as more fully explained below, the in-fill percentage (fill density) and/or cross-section along the body of the optical element holder can also vary. In this regard, and in certain embodiments of the disclosed technology, the surfaces of the optical element holder (e.g., at the various walls that form the shape of the holder) are substantially continuous and have a high fill density (e.g., ≥90%, such as 100%) whereas the interior volume of the optical element holder has a lower fill density (e.g., <90%, <50%, <20%, <10%), effectively creating a semi-hollow or substantially hollow structure. The fill density of the interior is typically referred to as the “in-fill percentage”.

An example pair of optical holder cross-sections324,326of the optical element holder302are shown with additional reference toFIGS. 3B-3C. Although this discussion is with reference to optical element holder302, it applies to any of the optical element holders disclosed herein. The cross-section324includes striated members325(not necessarily oriented with the cross-hatching as shown) that provide a selected portion of the total area of the cross-section324. The cross-section324includes a series of square-shaped voids328so that the ratio of the area of the striated members325to the total area of the cross-section324corresponds to an in-fill percentage of the cross-section324. The square-shaped voids328have an alternating pattern, but the in-fill pattern can be other than alternating. The cross-section326includes striated members327forming a pattern of hexagonally-shaped voids330. It will be appreciated that other in-fill patterns can be used, including triangular, rectangular, circular, etc., and in-fill patterns can vary from region to region in the optical holder302, e.g., vary from the top member312to the bottom member306. The in-fill pattern and percentage can be selected in relation to various performance parameters, such as oscillation efficiency and absolute retardance, associated with oscillation of the optical element situated in the optical holder302. In some examples, in-fill percentages of about 5%, 10%, 15%, 25%, and 35% are used. Thus, a density and shape of the optical holder302can be selected to adjust performance associated with the optical element held by the optical holder302. By holding the optical element firmly but not rigidly, the optical holder302holds the optical element so that the optical element is allowed to oscillate in improved fashion. In one example, fill density was varied so that a power reduction of 10% was achieved.

FIGS. 4A and 4Bshow a photoelastic modulator assembly holder400that includes a base portion402, and a first pair of vertical support members404a,404band second pair of vertical support members406a,406bextending from the base portion402. In representative embodiments, the photoelastic modulator assembly holder400is unitary and resilient so that various portions can flex and move so as to allow insertion and cushionable support of a photoelastic modulator assembly407. Each of the vertical support members404a,404bincludes a respective receiving portion408a,408b. The receiving portions408a,408bare oppositely situated and associated with respective protruding portions410a,410bof a photoelastic optical element412of the photoelastic modulator assembly407shown inFIG. 4B. The vertical support member406aincludes a plurality of receiving portions414a,416aand the vertical support member406bincludes an opposite plurality of receiving portions414b,416b. As shown inFIG. 4B, the opposing pair receiving portions414a,414bare situated to receive corresponding protruding portions418a,418bthat protrude from a transducer block420coupled to the photoelastic optical element412. The opposite plurality of receiving portions416a,416bis situated to receive corresponding protruding portions (not shown) for a photoelastic modulator assembly different from the photoelastic modulator assembly407and having a photoelastic optical element with a shorter length. Thus, the same photoelastic modulator assembly holder400can support various photoelastic modulator assemblies. The base portion402includes an opening422defining an aperture that is situated below the photoelastic optical element412with the photoelastic modulator assembly407cushionably supported by the receiving portions408a,408b,414a,414band without contacting other portions of the photoelastic modulator assembly holder400. In some examples, one or more side surfaces424a-424iof the photoelastic modulator assembly holder400contact corresponding surfaces of a rigid housing (not shown) so as to provide support for the photoelastic modulator assembly407during oscillation of the photoelastic optical element412driven by the transducer420. InFIG. 5, an example of a method500of manufacturing a photoelastic modulator includes extruding with a resilient extrusion printable elastomeric material a resilient optical element holder (sleeve) having displaceable receiving portions, at502. For example, the resilient optical element holder (sleeve) can be 3-D printed with a selected resilient material, and an in-fill percentage of the resilient sleeve can be varied to correspond to an improved performance efficiency for operation of the photoelastic modulator. Efficiency improvements typically allow application of a lower voltage for a particular low-noise signal quality and reduced power requirements. At504, a piezoelectric transducer is aligned with an optical axis of a photoelastic optical element having a characteristic photoelastic frequency and bonded to a corresponding surface of the photoelastic optical element. A photoelastic optical element assembly is formed so that photoelastic optical element can be controllably vibrated at the characteristic frequency. At506the receiving portions of the resilient optical element holder (sleeve) are displaced outwardly. At508, the photoelastic optical element assembly is inserted into the resilient optical element holder (sleeve) so that protruding surfaces are in cushioned contact with the receiving portions. The resilient optical element holder (sleeve) cushionably supporting the photoelastic optical element and attached piezoelectric transducer is then inserted into and snugly secured in a rigid photoelastic modulator housing, at510.

In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only representative examples and should not be taken as limiting the scope of the disclosure. Alternatives specifically addressed in these sections are merely exemplary and do not constitute all possible alternatives to the embodiments described herein. For instance, various components of systems described herein may be combined in function and use. We therefore claim all that comes within the scope and spirit of the appended claims.