DE102015013026B4 - Polarization-maintaining vacuum cell for the application or measurement of electromagnetic waves in a vacuum, processes for their production and their use - Google Patents

Abstract

Process for the production of a polarization-maintaining vacuum cell (1) for the application or measurement of electromagnetic waves, the birefringence Δn <10-6 and the production of the vacuum cell (1) by joining at least four side windows (2) and an end piece ( 8) is carried out on a carrier plate (3) with a connecting material, characterized in that the connecting material contains an adhesive made of epoxy resin.

Description

The invention relates to a method, a device and the use of a device for producing a polarization-maintaining vacuum cell for the application or measurement of electromagnetic waves, the birefringence Δn <10 ^-6 and the vacuum cell by joining at least four side windows and an end piece is made on a carrier plate with a connecting material.

Vacuum-compatible measuring cells are already an integral part of many experiments in quantum optics, for example in the area of precision measurements and quantum information with cold atoms or ions. These measurements must be carried out in an ultra-high vacuum environment in order to avoid collisions with air molecules. Furthermore, these experiments require good optical access for numerous laser beams from different directions to control the atoms / ions as well as to measure the emitted light and to image the atoms / ions. Many experiments in quantum optics require precise control of the polarization of laser beams. For example, in protocols for the physically secure transmission of information in quantum cryptography, the information is encoded in the polarization state of light. In the case of atomic clocks with trapped cold atoms, the purity and stability of the polarization of the laser beams used to trap the atoms determine the accuracy. In the case of quantum simulators with neutral atoms in optical lattices, the quality of the control of the polarization of the laser beams used for the optical lattice determines the performance of the quantum simulator. The examples mentioned are already in the transition to technical application.

In the state of the art, the optical access of laser beams into the areas of vacuum apparatus takes place via built-in vacuum windows (sight glasses) or part of the vacuum apparatus is designed as a vacuum glass cell (cuvette). Vacuum cells enable a much more compact construction of the vacuum apparatus than conventional metallic vacuum chambers, which are equipped with optical access in the form of vacuum windows. However, the birefringence of the window elements distorts the polarization of transmitted laser beams (or other electromagnetic waves such as infrared radiation), the strength of the birefringence Δn determining the extent of the polarization distortion.

The birefringence can be an intrinsic property of the window material, or it can be induced by the stress-optical coefficients of the window material from mechanical stresses which occur due to the installation of the window or the cell and due to the pressure difference between outside air and vacuum. Achieving a birefringence Δn <10 ^-6 requires special attention when installing a vacuum cell or a window. Even smaller birefringence and thus even less polarization distortion cannot be achieved with conventional vacuum windows, sight glasses, viewports or vacuum cells.

For example, from the US 2012 258 022 A1 known that vacuum apparatuses are used as containers for laser-cooled atoms for applications in quantum information, for atomic clocks, for inertial navigation and for magnetic field and gravitational measurements. A serious obstacle to the development of these applications has been found to be the complexity and size of the vacuum systems required. As a solution to this, a compact vacuum glass cell is disclosed, which is produced by melting together, diffusion bonding, anodic bonding or optical contacting. However, information about the strength of the birefringence is not described there.

Furthermore, from the US 2009 009 54 14 A1 the production of vacuum cells is known in which a tubular glass cell body is applied to a silicon substrate which has a coefficient of thermal expansion similar to that of glass. An anodic bonding process is used for the connection, in which a bond is created between the silicon substrate and the glass body and thus a closed vacuum cell is obtained. A voltage of around 1 KV is applied for around two minutes in order to connect the substrates via a direct current flow. Birefringence Δn smaller than 10 ^-6 are not disclosed in the application.

Also disclosed Steffen, A. et al: In situ measurement of vacuum window birefringence by atomic spectroscop. In: Rev. Sci. Instrum. 84, 126103 (2013) , a polarization-maintaining vacuum cell for use or measurement in quantum optics with birefringence Δn <10 ^-6 . SF57 optical glasses with an unusually low stress coefficient are used, which can also be used in vacuum cells.

In addition, in Solmeyer, N. et al: Mounting ultra-high vacuum windows with low-stress-induced birefringence. In: Rev. Sci. Instrum. 82, 066105 (2011) described that for the production of vacuum cells with low birefringence indium in particular can be used to connect individual components.

In the DE 10 2005 061 984 A1 describes a pressure chamber with which microscopic recordings can be made, the polarization properties of the windows of the pressure cell enabling polarization-sensitive observation even under high pressure. A window of the pressure chamber takes on the function of a lens of a microscope objective, whereby the window consists of heavy flint glass with special cooling and can therefore be used for polarization-sensitive microscopy. This invention is a conventional windowed metal chamber, not a glass cell. Information on the strength of the birefringence is not given in the application.

From the DE 10 2007 019 695 A1 a measuring cell is known which is used for the optical analysis of small volumes. The measuring cell is usually designed as a cuvette and has a structured layer (carrier substrate) for the optical analysis of small volumes, which is provided with at least one channel, with at least the underside of the structured layer being sealed with a thin, optically transparent layer . However, the measurements are not carried out in a vacuum and polarization measurements in a range of birefringence Δn <10 ^-6 are not disclosed in the application.

The US 7 126 112 B2 describes a vacuum cell in which a microchip forms a wall of the vacuum cell. This makes the chamber compact, light and offers a large optical access in combination with a small overall size. B. remains accessible for cooling.

The present invention is based on the object of providing a vacuum cell for measurements in a vacuum with polarized electromagnetic waves, which largely suppresses the effect of stress-induced birefringence and thus the change in the polarization state of electromagnetic radiation when passing through the cell, with a high-density connection of the individual windows of the vacuum cell can be guaranteed to a carrier plate and a connection for vacuum pumps is generated in parallel.

The object of the invention is achieved according to the invention by the features of independent claims 1 and 9.

According to the invention it is provided that a vacuum cell takes place by joining at least four side windows in a holder by means of a connecting material, the stress occurring during the joining process of the individual windows being kept as low as possible. For this purpose, the individual side windows are placed in a holder and connected to one another by applying the connecting material to partial areas of the side surfaces of the individual side windows. After the respective connection of the side windows, a curing process follows, i.e. the side windows in the holder are first brought to the desired curing temperature over approx. 1 hour. When using adhesive as the connecting material, in particular epoxy resin, the curing temperature is in a range of 140-180 ° C. After approx. 3 to 4 hours, the cooling process is initiated with a temperature change of approx. 0.5 ° C / min. This process is repeated until a structure created by the connection of the side windows is present, which has a hollow cylindrical shape. Finally, the hollow cylindrical structure is acted upon with axial delimitations, such as an end piece or a bottom glass, at one end and the other, in order to complete the production of the vacuum cell body, the joining of the axial delimitations being carried out analogously to the preceding process steps. Alternatively, it is also possible to connect the side windows with a metal or a metal alloy, in particular with the aid of indium, whereby the connection of the side windows and / or the axial boundaries can be made by a soldering process or by pressure.

A low birefringence of Δn <10 ^{-6 of} the side windows is only achieved if there is complete and uniform wetting of the side surfaces during the production of the structure, ie the partial areas of at least two side windows are completely wetted by the connecting material. A leakage of connecting material at the partial areas of the side windows, ie the contact surfaces, could otherwise lead to the formation of drops and thus to an inhomogeneous stress distribution in the glass. To prevent this, a motorized application can be used, which includes, for example, a linear displacement table in the case of connecting the side windows or a rotating displacement table in the case of connecting the end piece or the carrier plate, with an amount of about 45mm ^{3 of} the connecting material in the form of adhesive is applied by syringe to the contact surfaces of the side windows.

The adhesive can be pressed through a needle with a diameter of 0.9 mm using a pressure of 8 hPa and applied to a contact surface at a feed rate of 4 mm / s. This creates an approx. 1 mm wide and 0.3 mm high line, which spreads over the entire surface when at least two side windows are contacted and in this way a complete and tight connection of the Side window guaranteed. The amount of connecting material applied to the contact surfaces of the side windows is generally 0.2 to 0.4 g / cm, the amount being able to vary depending on the length and width of the side windows.

After the radial side windows have been joined, the axial side surfaces for producing the vacuum cell are attached to both ends of the resulting structure and connected to the side windows. At one end, this can be a cover slip, for example. At the other end of the structure produced, a carrier plate can serve as an axial side surface. The carrier plate can be designed as a bottom glass and / or as a metallic flange connection. If the carrier plate is designed as a bottom glass, the bottom glass can be connected to a metallic flange. The connection is made with connection material that either contains adhesive, in particular epoxy resin, or a metallic compound, in particular indium. However, it is also possible for the bottom glass to have a flange-like connection at least in partial areas in order to ensure a direct vacuum pump connection to the vacuum cell.

It is also possible to connect the carrier plate, which can be designed as a metallic flange, directly to the individual windows, with titanium and / or tantalum in particular being able to be included as part of the flange-like connection. Due to their chemical and physical properties, titanium and tantalum are particularly preferred materials as components of the metallic, flange-like carrier plate, since their corrosion and temperature resistance (expansion coefficients in the range of 6-8 × 10 ^-6 / K is comparable to the expansion coefficients of the individual windows ) a good connection of the individual windows to the carrier plate can be guaranteed and the connections for the metallic flange-like carrier plate on vacuum pumps are standardized. In particular, tantalum in the form of a thin tantalum lip, due to its easy deformability, offers the advantage of being able to compensate for deformations of the metal flange that occur during assembly and thus minimize tension in the glass.

The type and configuration of the carrier plate depend on the respective requirements for the vacuum cell and its connections, as well as the planned application when operating the vacuum cell according to the invention. It may be necessary to design the carrier plate as a direct connection piece, i.e. as a flange-like floor glass or flange-like metallic carrier plate or also as an indirect connection piece, i.e. floor glass without a flange to which a flange-like metallic carrier plate or to which a suction glass flange can be connected as a carrier plate. In the event that the carrier plate is designed as a direct or indirect connecting piece, the resulting structure of the connected individual windows is subjected to a subsequent hardening process analogous to the above procedures.

For the connection of the side windows, a connection material is provided which takes into account the requirements for tightness or leakage and the prevention of an inhomogeneous stress distribution of the individual windows in the vacuum cell. For this purpose, it is provided according to the invention that either a metal, in particular indium, is used as the soft metal with a low melting point or an adhesive which has a high melting point and only binds a small amount of gases or outgasses them again quickly. According to the invention, an epoxy adhesive, in particular the epoxy adhesive H77 from Epoxy Technologies Inc., which contains small particles in the range <50 μm, has been found as the connecting material, in particular as the adhesive. Evidence was provided that this filled adhesive only generates low stresses in the glass during the gluing process.After joining the respective epoxy adhesive components, they are first evacuated in a desiccator in order to remove gas inclusions that were created when the two components of the adhesive were mixed are. The evacuation process is carried out for approx. 2 minutes and is repeated again after transferring it into a syringe in order to remove any gas inclusions that may have arisen during transferring. The following measurements showed a very low leakage rate. The leakage rate for the vacuum cell according to the invention in relation to helium is less than 8 × 10 ^-11 mbar / s.

According to the invention, it is provided in a special embodiment that the side windows consist of a glass with a low stress-optical coefficient, particularly preferably of flint glass. The use of glass with a low stress-optical coefficient has the advantage that only a low stress-induced birefringence occurs and changes in the polarization of electromagnetic radiation when transmitted into the vacuum cell through the individual side windows are minimized. This ensures the accuracy and reproducibility of the measurements to be carried out.

To ensure the shape of the vacuum cell according to the invention, it is provided that a holder with a stop is used during the joining process. This is done so that the structure and subsequently the stability and tightness of the vacuum cell remains guaranteed after manufacture. The bracket usually consists of a polygonal body made of aluminum with axial bearing surfaces, which are present at one end and the other end of the corpus and are placed on the individual windows. Depending on how many side windows are to be joined to form a vacuum cell, the body has four or more contact surfaces. The bracket is limited at one end by a stop that fixes the individual windows with the bracket.

In the proximal area of the bearing surfaces, axially extending recesses are let into the body at least in partial areas. The recesses could be designed like a groove or also concave. The recesses run in the axial direction at least in partial areas along the proximal area of the holder and continue as openings in the area of the radial delimitation of the holder or the stop. This has the advantage that an exit of the connection material in the direction of the holder and a connection between the holder and the individual windows in the area of the contact surfaces of the individual windows is prevented. Any excess connecting material can be removed through the recesses or openings and is not available for connecting the individual windows to the bracket or the stop.

In a particular embodiment of the vacuum cell and its production, it is provided that the vacuum cell has a polygonal structure with at least four, in particular twelve side windows. The polygonal structure with at least four, in particular twelve, side windows has the advantage that an almost tension-free connection between the individual windows is guaranteed, so that the forces that occur can be distributed over the entire hollow cylindrical area. An increased number of side windows also enables electromagnetic radiation to pass through a large number of side windows in a largely vertical direction without changing the polarization in all radial and both axial directions of the vacuum cell. However, since the total cross-sectional area of all connections, which together determine the leakage rate depending on the cell geometry and dimensions as well as the pressure to be achieved including the available pumping rate, a cross-sectional area of an estimated 2cm ² should not be exceeded.

In order to ensure the functionality of the vacuum cell according to the invention, leakage must be avoided, but the largest possible non-refractive, transparent area must be obtained and the birefringence of induced or emitted electromagnetic radiation should be very low. Measurements have shown that the value of the birefringence of the vacuum cell produced by the method according to the invention assumes ^{a value of Δn <10 -7.} This is due to the protection provided by the manufacturing method of low stresses within the cell and the use of glass with low stress-optical coefficient, which moves according to the invention particularly in a range of 2 × 10 ⁸ mm ² / N.

According to the invention, a trapezoidal configuration of the side windows of the vacuum cell has proven to be particularly advantageous. Due to the trapezoidal design of the side windows, the vacuum cells are intrinsically stable under external pressure and can be used for measurements in which electromagnetic radiation, in particular laser light, is required in a vacuum. By choosing the design of the side windows, the equipment requirements can be adapted to the application. It has also proven to be advantageous to optically polish the glasses and / or to coat them in an anti-reflective manner.

Furthermore, according to the invention, the use of a polarization-maintaining vacuum cell for the application or measurement of electromagnetic waves is provided, the birefringence of the vacuum cell being Δn <10 ^-6 . The use of the vacuum cell according to the invention with a polygonal structure and with more than four side windows on a carrier plate and an end piece takes place in physical precision measurement technology, in particular for measurement in atomic clocks, spectroscopy, magnetometry, quantum technology and quantum cryptography. It has proven particularly advantageous to use side windows with a birefringence index of Δn <10 ^-7 in order to improve the precision of the measurements of incoming and outgoing electromagnetic radiation.

• zeigt eine erfindungsgemäße Vorrichtung in einer vertikalen Schnittzeichnung. Man erkennt, dass die Vakuum-Zelle 1 aus mehreren Seitenfenstern 2 besteht, die einenends von einem Abschlussstück 8 und anderenends von einer Trägerplatte 3 begrenzt sind. Die Trägerplatte 3 kann in einer besonderen Ausführungsform wie vorliegend dargestellt mit einem Tantalring 4 verbunden werden. Dieser kann wiederum mit einem Flansch 9 verbunden werden, der als Anschlussstück für eine externe Vakuumpumpe dient (nicht dargestellt), an die der Flansch 9 mit Schrauben 12 (nicht dargestellt) über Bohrungen 11 fixiert werden kann. Ferner sind lateral an den Seitenflächen 2 Seitenflächen 13 zu erkennen, auf die das Verbindungsmaterial zur Herstellung einer Verbindung der Seitenfenster 2 aufgetragen werden kann.
• In ist eine Vakuum-Zelle 1 in einer vertikalen Aufsicht dargestellt. Im vorderen Bereich der Vakuum-Zelle 1 ist ein Flansch 9 mit Bohrungen 11 für Schrauben 12 (nicht dargestellt) angeordnet, an die sich in distaler Richtung ein Tantalring 4 mit Trägerplatte 3 anschließt. An diese Trägerplatte 3 sind axial miteinander verbundene Seitenfenster 2 befestigt, die durch ein Abschlussstück 8 beaufschlagt sind. Man erkennt, dass die Vakuumzelle 1 im inneren eine annähernd hohlzylindrische Form aufweist, die durch die lateral angeordneten Seitenfenster 2 begrenzt ist.
• stellt die erfindungsgemäße Vakuumzelle 1 in einer vertikalen Explosionszeichnung dar. Die lateral miteinander verbindbaren Seitenfenster 2 weisen eine annähernd trapezförmige Form auf. Die Seitenfenster 2 werden axial durch das Abschlussstück 8 und die Trägerplatte 3 begrenztEs schließt sich axial an die Trägerplatte 3 ein Tantalring 4 an, der mit einem Flansch 9 mit Bohrungen 11 über Schrauben 12 (nicht dargestellt) mit einer externen Vakuumpumpe verbunden werden kann.
• In erkennt man den schematischen Aufbau einer erfindungsgemäßen Vakuum-Zelle 1 in einer horizontalen Schnittzeichnung. Die Vakuum-Zelle 1 ist in eine Halterung 5 eingebettet, die während der Herstellung der Vakuum-Zelle 1 den Seitenfenstern 2 beim Verbinden die notwendige Struktur und den notwendigen Halt gibt. Unterhalb der Kontaktflächen der Seitenfenster 2 befinden sich Aussparungen 7, die axial verlaufen, wobei nutförmige oder konkave Vertiefungen zumindest in Teilbereichen im Corpus der Halterung 5 zu erkennen ist. Die Aussparungen 7 dienen als Hohlraum, um zu verhindern, dass eventuell überschüssiges Verbindungsmaterial bei der Verbindung der Seitenfenster 2 miteinander an die Halterung 5 gelangt und eine Verbindung der Halterung 5 mit den Seitenfenstern 2 erfolgt.
• zeigt eine perspektivische Ansicht der Vakuum-Zelle 1 nebst Halterung 5 und einem Anschlag 6. Die einzelnen Seitenfenster 2 sind lateral an ihren Seitenflächen miteinander verbunden. In axialer Richtung sind in der Halterung 5 zumindest teilweise nutförmige oder konkave Aussparungen zu erkennen, die eine Verbindung der Seitenfenster 2 mit der Halterung 5 unterbinden sollen, indem etwaiges überschüssiges Verbindungsmaterial bei der Herstellung der Vakuum-Zelle 1 in den Aussparungen 7 entfernt werden kann. Die Aussparungen 7 verlaufen in axialer Richtung zumindest in Teilbereichen entlang des lateralen Bereichs der Halterung 5 und setzen sich im Bereich der radialen Seitenfläche des Anschlags 6 als Durchbrüche fort. Man erkennt, dass der Anschlag 6 mit Senkkopfschrauben 10 an die Halterung 5 verbunden ist.

The invention is explained again on the basis of the following figures:

• shows a device according to the invention in a vertical sectional drawing. You can see that the vacuum cell 1 from several side windows 2 consists of one end of a terminator 8th and at the other end from a carrier plate 3 are limited. The carrier plate 3 can in a special embodiment as shown here with a tantalum ring 4th get connected. This can in turn with a flange 9 be connected, which serves as a connection piece for an external vacuum pump (not shown) to which the flange 9 with screws 12th (not shown) via holes 11 can be fixed. Furthermore, are laterally on the side surfaces 2 Side faces 13th to recognize on which the connecting material to produce a connection of the side windows 2 can be applied.
• In is a vacuum cell 1 shown in a vertical plan view. In the front area of the vacuum cell 1 is a flange 9 with holes 11 for screws 12th (not shown) arranged on which a tantalum ring is attached in the distal direction 4th with support plate 3 connects. To this carrier plate 3 are axially interconnected side windows 2 attached by a terminator 8th are acted upon. You can see that the vacuum cell 1 has an approximately hollow cylindrical shape on the inside, which is defined by the laterally arranged side windows 2 is limited.
• represents the vacuum cell according to the invention 1 in a vertical exploded view. The laterally interconnectable side windows 2 have an approximately trapezoidal shape. The side windows 2 are axially through the end piece 8th and the carrier plate 3 It closes axially on the carrier plate 3 a tantalum ring 4th at that with a flange 9 with holes 11 about screws 12th (not shown) can be connected to an external vacuum pump.
• In one recognizes the schematic structure of a vacuum cell according to the invention 1 in a horizontal sectional drawing. The vacuum cell 1 is in a holder 5 embedded during the manufacture of the vacuum cell 1 the side windows 2 gives the necessary structure and support when connecting. Below the contact surfaces of the side windows 2 there are recesses 7th which run axially, with groove-shaped or concave depressions at least in partial areas in the body of the holder 5 can be seen. The recesses 7th serve as a cavity to prevent any excess connecting material when connecting the side windows 2 together on the bracket 5 arrives and a connection of the bracket 5 with the side windows 2 he follows.
• Figure 3 shows a perspective view of the vacuum cell 1 including bracket 5 and a stop 6th . The individual side windows 2 are laterally connected to one another on their side surfaces. In the axial direction are in the holder 5 to recognize at least partially groove-shaped or concave recesses that connect the side windows 2 with the bracket 5 intended to prevent by removing any excess connecting material during the manufacture of the vacuum cell 1 in the recesses 7th can be removed. The recesses 7th run in the axial direction at least in partial areas along the lateral area of the holder 5 and settle in the area of the radial side surface of the stop 6th continued as breakthroughs. You can see that the stop 6th with countersunk screws 10 to the bracket 5 connected is.

List of reference symbols

1 -

Vacuum cell

2 -

sidewindow

3 -

Carrier plate

4 -

Tantalum ring

5 -

bracket

6 -

attack

7 -

Recesses

8th -

End piece

9 -

flange

10

- Countersunk screws

11

- holes

12th

- screws

13th

- side face

Claims (8)

Process for the production of a polarization-maintaining vacuum cell (1) for the application or measurement of electromagnetic waves, the birefringence Δn <10 ^-6 and the production of the vacuum cell (1) by joining at least four side windows (2) and an end piece (8) is carried out on a carrier plate (3) with a connecting material, characterized in that the connecting material contains an adhesive made of epoxy resin. Procedure according to Claim 1 , characterized in that the individual side windows are placed in a holder (5) and connected to one another by applying the connecting material to partial areas of the side surfaces of the individual side windows (2), the holder (5) having a stop (6) to ensure the shape of the Vacuum cell and recesses (7) which run in the axial direction at least in partial areas along the proximal area of the holder (5) and continue as openings in the area of the radial delimitation of the holder (5) or the stop (6). Method according to one of the preceding claims, characterized in that the side windows (2) consist of glass, preferably of a glass with a low stress-optical coefficient, particularly preferably of flint glass. Method according to one of the preceding claims, characterized in that characterized in that the adhesive on one side of a side window (2) an amount of 0.2 - 0.4 g / cm2 comprises. Method according to one of the preceding claims, characterized in that the vacuum cell (1) is constructed polygonally with twelve side windows (2). Method according to one of the preceding claims, characterized in that the vacuum cell (1) has a birefringence Δn smaller than 10 ^-7 . Polarization-maintaining vacuum cell (1) for the application or measurement of electromagnetic waves, the birefringence Δn <10-6 and the vacuum cell (1) by joining at least four side windows (2) and an end piece (8) on a carrier plate (3) is made with a connecting material, characterized in that the connecting material contains an adhesive made of epoxy resin. Use a vacuum cell (1) after Claim 7 in physical precision metrology, especially for measurements in atomic clocks, spectroscopy, magnetometry, quantum technology and quantum cryptography.

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