WO2005111710A1

WO2005111710A1 - Apparatus for heating a flat plate crystal for a laser

Info

Publication number: WO2005111710A1
Application number: PCT/GB2005/001608
Authority: WO
Inventors: Richard Bird Williams; Peter George Robin Smith; Craig George Sawyers
Original assignee: Stratophase Ltd
Priority date: 2004-05-14
Filing date: 2005-04-27
Publication date: 2005-11-24
Also published as: GB0410869D0

Abstract

Apparatus (2) for heating a flat plate crystal (4) for a laser, which apparatus (2) comprises a heating device (6) for heating the flat plate crystal (4), a holder (8) for the flat plate crystal (4), and mounting means (10) for mounting the holder (8) to an outer surface (12) of the heating device (6) such that heat from the heating device (6) is ale to transfer to the holder (8) and thereby to heat the flat plate crystal (4).

Description

APPARATUS FOR HEATING A FLAT PLATE CRYSTAL FOR A LASER

This invention relates to apparatus for heating a flat plate crystal for a laser. Non-linear optics is a technology by which the wavelength or colour of a laser can be changed. For example, non-linear optics may be employed to double the frequency of a laser, say from 1064nm in the infrared wavelength region to 532nm in the green wavelength region. Also, for example, non-linear optics may be employed to provide a tunable source of light, where a pump laser with a fixed wavelength can be converted into another laser with a longer wavelength, which can be changed. Such a tunable source of light is known as an optical parameter oscillator. Conventionally, the non-linear optics utilises an optical crystal which is chosen from approximately 20 commonly used materials for this application. These materials often need to be operated at an elevated temperature in order to ensure efficient conversion from one wavelength to another. Also, some optical crystals absorb water and they need to be heated in order to prevent moisture damage. Small ovens are available into which the crystal is mounted, the ovens enabling the heating of the chosen optical crystal. A typical size for an optical crystal may be in the range of 2 x 2 x 5 mm to 10 x 10 x 40 mm, although the crystals may be much smaller or much larger if desired. The crystal is often mounted in the oven by introducing the crystal into the oven, and then gently tightening a screw in order to press the crystal on to the oven and ensure that it does not fall out. This process is not without risk of damage to the crystal. In addition, the known ovens are custom made to suit a particular size of crystal, or a fixed selection of ovens is made available to cope with a selected standard crystal size. The need for so many ovens is not always convenient and it tends to increase manufacturing costs. An alternative material which is used in non-linear optics is that known as periodically poled lithium niobate. This material is in the form of a flat plate which may be approximately 0.1mm - 1mm thick. Typical size ranges are from 1 x 0.5 x 0.5mm to 10 x 40 x 1mm. The use of the flat plate crystal gives rise to even more problems in the mounting of the flat plate crystal in an oven. This is especially so since the optimum required temperature may be as high as 200° C. Generally, custom made ovens are produced, with a different oven design being necessary for each size of the periodically poled lithium niobate flat plate crystal. This severely compromises the usability of the oven since it is frequently necessary in product development or research to change a crystal for one which is longer, or shorter, or has different design parameters. Existing oven designs make this change difficult. In addition to the above problems, a number of active regions can be formed in the periodically poled lithium niobate flat plate crystal. For example, in the case of a 10mm wide crystal, 5 - 20 different active regions may be identified, with each active region having different optical parameters. Ideally, it should be possible easily to select any one of these active regions, and this is not possible with existing oven designs. It is an aim of the present invention to obviate or reduce the above mentioned problems. Accordingly, in one non-limiting embodiment of the present invention, there is provided apparatus for heating a flat plate crystal for a laser, which apparatus comprises a heating device for heating the flat plate crystal, a holder for the flat plate crystal, and mounting means for mounting the holder to an outer surface of the heating device such that heat from the heating device is able to transfer to the holder and thereby to heat the flat plate crystal. With the apparatus of the present invention, the flat plate crystal is able to be heated on the outside of the heating device, and this avoids the above mentioned problems associated with introducing the heating device into the known ovens. The flat plate crystal may be heated to any desired temperature suitable for the intended and efficient operation of the flat plate crystal. Thus, for example, the flat plate crystal may be heated to a temperature at which the flat plate crystal operates at a different wavelength to that at which it previously operated. The mounting means may advantageously enable the holder to be located precisely where required. The apparatus may be one in which the outer surface of the heating device is a flat outer surface, and in which the mounting means has a flat bottom which rests on the flat outer surface of the heating device. Other designs may be employed if desired. The flat outer surface of the heating device is preferably a top surface of the heating device. The flat outer surface may however be other than the top surface if desired. The heating device may have four flat sides, the flat top surface, a flat bottom, a heater, and a temperature sensor. The heating device may be regarded as an oven, but with the heating being effected through conduction from an outer surface of the oven rather than from inside the oven. The holder may have a groove which receives the flat plate crystal. Other formations by which the holder holds the flat plate crystal may be employed. The groove preferably has straight sides which extend at 90° to a flat base of the groove. The sides may alternatively be sloping sides. Preferably, the mounting means is positioned on the heating device. The mounting means may be positioned elsewhere if desired. The mounting means is preferably a dowel mounting means. Other types of mounting means may be employed. The apparatus is preferably one in which the dowel mounting means comprises a pair of dowels which upstand from the heating device and which are for engaging in holes which are precisely positioned one on each of two opposite sides of the holder. The mounting means may include biasing means for biasing the holder into contact with the heating device and thereby to facilitate the transfer of heat from the heating device to the holder. The biasing means preferably comprises a plurality of spring loaded pins. Other biasing means may however be employed. When the biasing means comprises the plurality of the spring loaded pins, then there are preferably two, four, six or eight of the spring loaded pins. Other numbers of the spring loaded pins may be employed. Advantageously, the apparatus includes an insulating outer housing. The insulating outer housing is for reducing unwanted heat loss. Preferably, the apparatus is one in which the insulating outer housing has a bottom part and a top part, and in which the top part is separable from the bottom part in order to facilitate access to the holder on the heating device. Also preferably, the top part houses the biasing means such for example as the spring loaded pins. The apparatus of the present invention may include selector means by which a desired active region in the flat plate crystal is chosen. The selector means may be inside the heating device and, for example, may be accessed by a screw on an outside face of the heating device. Alternatively, the selector means may be externally positioned with, for example, the heating device attached to the selector means. The selector means may have detents at the same spacing as the active regions in the flat plate crystal. This helps to ensure accurate alignment between the apparatus and the crystal, and therefore the laser and the crystal. The flat plate crystal is preferably a poled lithium niobate flat plate crystal but other types of flat plate crystals may be employed. The flat plate crystal may be covered with a conductive plate to neutralise surface charges. This conductive plate may be a metal plate, or a glass plate coated with a conductive material. The coating may be of a metal. Any suitable and appropriate metal may be employed including gold and aluminium. The coating may alternatively be an indium tin oxide coating. The conductive plate or surface is in contact with the flat plate crystal. The conductive plate is conveniently of similar thickness to the flat plate crystal and the same size in length and width. Other dimensions may however be employed. The present invention also extends to a laser when provided with the apparatus of the invention. Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is an exploded view of part of first apparatus for heating a flat plate crystal for a laser; Figure 2 is a view like Figure 1 but not in exploded form; Figure 3 is a view like Figure 2 and shows the apparatus additionally being provided with a bottom part of an insulating outer housing; Figure 4 is a view like Figure 3 but shows the apparatus additionally being provided with a top part of the insulating outer housing; " Figure 5 is a view like Figure 2 but shows part of second apparatus for heating a flat plate crystal for a laser; Figure 6 is a cross section through the second apparatus; and Figure 7 is a longitudinal section through the second apparatus as shown in Figure 6. Referring to Figures 1 - 4, there is shown apparatus 2 for heating a flat plate crystal 4 for a laser (not shown). The apparatus 2 comprises a heating device 6 for heating the flat plate crystal 4, a holder 8 for the flat plate crystal 4, and mounting means 10. The mounting means 10 is for mounting the holder 8 to an outer surface 12 of the heating device 6. The mounting is such that heat from the heating device 6 is able to transfer to the holder 8 and thereby to heat the flat plate crystal 4 to a temperature at which the flat plate crystal 4 operates efficiently at a different wavelength to that at which it previously operated. The outer surface 12 of the heating device 6 is a flat outer surface 12. The mounting means 10 has a flat bottom 14 which rests on the flat outer surface 12 of the heating device 6. As shown in the drawings, the flat outer surface 12 is a top surface of the heating device 6. More specifically, the heating device 6 has four flat sides 16, the outer surface 12, a flat bottom 18, a heater 20 and a temperature sensor 22. The holder 8 has a groove 24 which receives the flat plate crystal 4. The groove has straight sides 26 which extend at 90° to a flat base 28 of the groove. The mounting means 10 is positioned on the heating device 6. The mounting means 10 is a dowel mounting means. The dowel mounting means 10 comprises a pair of dowels 30 which upstand from the heating device 6 and which are for engaging in holes which are precisely positioned one on each of two opposite sides 32, 34 of the holder 8. It will be noticed that the dowels 30 engage the sides 32, 34 at different positions along the length of the sides 32, 34, thereby to help to ensure that the flat bottom 14 of the mounting means 10 rests squarely on the outer surface 12 of the heating device 6. The mounting means 10 includes biasing means 36 as shown in Figure 4. The biasing means 36 is for biasing the holder 8 into contact with the heating device 6. This facilitates the transfer of heat from the heating device 6 to the holder 8. The biasing means 36 comprises four spring loaded pins 38. As shown in Figures 3 and 4, the apparatus 2 includes an insulating outer housing 40. The insulating outer housing 40 has a bottom part 42 and a top part 44. The top part 44 is separable from the bottom part 42 in order to facilitate access to the holder 8 on the heating device 6. Removal of the top part 44 from the bottom part 42 releases the spring pressure from the spring loaded pins 38 on the holder 8. The holder 8 can then be removed if it is desired to heat another flat plate crystal. Referring now to Figures 5, 6 and 7 there is shown second apparatus 46 for heating a flat plate crystal for a laser. Similar parts as in the apparatus 2 shown in Figures 1 - 4 have been given the same reference numerals for ease of comparison and understanding. " Referring to Figure 5, it will be seen that the holder 8 is of the same size in plan as the heating device 6. It will also be seen that the dowels 30 are opposite each other rather than being staggered as shown in Figure 1. As can be appreciated from Figures 6 and 7, the insulating outer housing 40 is tubular, rather than rectangular as shown in Figure 4. Thus the bottom part 42 and the top part 44 of the insulating outer housing 40 as shown in Figures 6 and 7 are part circular, whereas the corresponding parts 42, 44 shown in Figure 4 are straight sided. Figure 6 shows how the insulating outer housing 40 of the apparatus 46 is such that the bottom part 42 comprises a cover portion 48 and insulation 50. The top part 44 similarly comprises a cover portion 52 and insulation 54. In Figure 7, the cover portions 48, 52 have been removed in order to show how opposite ends of the apparatus 46 are closed by end cheek members 56. Figure 7 also shows how the apparatus 46 is provided with adaptors 58 at each end. The adaptors 58 are for mounting the apparatus 46 into standard optical mounts, for example mirror mounts, microscope objective mounts, etc. The adaptors 46 are removable and interchangeable. Figure 6 also shows how the apparatus 46 also has a screw 60 which is a peek screw 60, for holding the heating device 6 onto the insulation 50. The screw 60 is made of a thermally insulating polymer and it provides thermal insulation between parts that it secures together. Also used is an insulating spacer 62. If metal parts were used, heat would be conducted and this could double the power required by the heating device 6. The main thermal insulation is the air gap between the oven and the parts 50, 54. Thus items 60, 62 need to have low thermal conductivity. It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the flat plate crystal 4 is a periodically poled lithium niobate crystal but in all embodiments of the invention, another type of flat plate crystal may be employed. The flat plate crystal employed may have different active regions, for example ten different active regions. Each active region has different optical parameters and the apparatus of the present invention allows the easy selection of any one of these regions. The selection may be effected by indexing the crystal or oven. The indexing may typically be in increments of 0.5mm. The flat plate crystal is held in the groove 24 in the holder 8 by a spring clip (not shown). The spring clip presses the flat plate crystal into good thermal contact with the adjacent surface of the holder 8. In the present invention, other means may be employed for holding the flat plate crystal in position so that, for example, the flat plate crystal may be adhesively bonded in position. Formations other than the groove 24 may be employed, as may be other designs for the holder 8. The heating device 6 may employ any suitable heater 20 and temperature sensor 22.

Claims

1. Apparatus for heating a flat plate crystal for a laser, which apparatus comprises a heating device for heating the flat plate crystal, a holder for the flat plate crystal, and mounting means for mounting the holder to an outer surface of the heating device such that heat from the heating device is able to transfer to the holder and thereby to heat the flat plate crystal.

2. Apparatus for according to claim 1 in which the outer surface of the heating device is a flat outer surface, and in which the mounting means has a flat bottom which rests on the flat outer surface of the heating device.

3. Apparatus according to claim 2 in which the flat outer surface of the heating device is a top surface of the heating device.

4. Apparatus according to claim 3 in which the heating device has four flat sides, the flat top surface, a flat bottom surface, a heater, and a temperature sensor.

5. Apparatus according to any one of the preceding claims in which the holder has a groove which receives the flat plate crystal.

6. Apparatus according to claim 5 in which the groove has straight sides which extend at 90° to a flat base of the groove.

7. Apparatus according to any one of the preceding claims in which the mounting means is positioned on the heating device.

8. Apparatus according to any one of the preceding claims in which the mounting means is dowel mounting means.

9. Apparatus according to claims 7 and 8 in which the dowel mounting means comprises a pair of dowels which upstand from the heating device and which are for engaging in holes which are precisely positioned one on each of two opposite sides of the holder.

10. Apparatus according to any one of the preceding claims in which the mounting means includes biasing means for biasing the holder into contact with the heating device and thereby to facilitate the transfer of heat from the heating device to the holder.

11. Apparatus according to claims 10 in which the biasing means comprises a plurality of spring loaded pins.

12. Apparatus according to any one of the preceding claims and including an insulating outer housing.

13. Apparatus according to claim 12 in which the insulating outer housing has a bottom part and a top part, and in which the top part is separable from the bottom part in order to facilitate access to the holder on the heating device.

14. Apparatus according to any one of the preceding claims and including selector means by which a desired active region in the flat plate crystal is selected.

15. A laser when provided with apparatus according to any one of the preceding claims.