WO2020025438A1

WO2020025438A1 - Device and method for welding of glass fibers to a micro lens array to manufacture a fiber collimator array

Info

Publication number: WO2020025438A1
Application number: PCT/EP2019/070030
Authority: WO
Inventors: Peter Schmidt
Original assignee: Schleifring Gmbh
Priority date: 2018-07-30
Filing date: 2019-07-25
Publication date: 2020-02-06
Also published as: EP3695257A1

Abstract

A device and a method for fabrication of fiber optic collimators is described. The fibers are attached to a lens or a micro lens array without the need for prefabricated fiber arrays with v-grooves or holes. A device for welding of a glass fiber to a lens comprises a retroreflector reflecting light back into the collimator, a detector coupled to the currently processed fiber for receiving light from the collimator and generating an output indicating the received light power and a positioning means for positioning the glass fiber relative to the lens based on the detector output. The amount of light received by the detector is a measure for the attenuation of the collimator. The signal output of the detector is used to move the fiber into the position with the lowest attenuation and weld it at this position to the lens. All irregularities and deviation of the individual lenses from a perfect lens caused by the fabrication tolerances are compensated by this method.

Description

Device and Method for Welding of Glass Fibers to a Micro Lens Array to manufacture a Fiber Collimator Array

Field of the invention The invention relates to a device and a method for welding a glass fiber to a lens to manufacture fiber optic collimators or fiber optic collimator arrays. Such fiber collimators or fiber collimator arrays are able to transmit a high optical power, have a very low pointing error and a high channel density and may be used in in optical rotary joints. Description of the related art

Optical transmission systems and specifically optical rotary joints must meet a high level of requirements on precision, transferrable power, insertion loss and return loss.

US 9,726,824 B1 discloses an optical circuit switch collimator, where a plurality of fibers is held in a fiber array matching to a lens array. Here, the lens array and the fiber array must match precisely, which requires a complex and expensive manufacturing process. While the precision of such a fiber array is limited by mechanical tolerances, the maximum power handling is limited by the damage threshold of the glue between fiber array and lens array. The maximum transferable power of such glued collimators is limited to a few lOOmW.

Advanced fiber collimator arrays for high power applications with glass fibers directly welded to a micro lens array suffer from increased optical losses caused by relatively low precision of the lateral fiber position. A method for welding glass fibers in a 2D-arrangement and a high package density is described in DE10204012B4. The welding increases the reliability of the connection and addi- tionally allows the transmission of optical power up to some watts over a single mode fiber, which is an advantage over a glued connection. The above men tioned relatively low precision of the fiber position relative to the lens is in the range of some micrometers and results in a pointing error of every single colli- mator, which in turn leads to an increased insertion loss of the collimators. At greater working distances, as typical for fiber optic rotary joints, not only the insertion loss increases, above all the variation of the insertion loss during rota tion. This is a disadvantage of collimators or collimator arrays manufactured by means of this existing process.

Summary of the invention

The problem to be solved by the invention is to provide a device and a method for welding of glass fibers to a lens or a micro lens array, which allows to minimize the positioning error of the glass fiber relative to a lens down to the sub-micrometer range and to minimize the pointing error of every single collimator of a fiber collimator array resulting from this process.

Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention.

A collimator is a lens with an attached fiber. A fi ber collimator array, also called FCA may comprise one or a plurality of lenses, preferably comprising glass or any other optical transparent material with at least one optical fiber, preferably glass fiber attached to the at least one lens. Normally, one fiber is attached to one lens. The collimator and/or collimator array has a fiber side to which the fibers are connected and oposing thereto an output side. The embodiments disclosed herein are alternatively usable for fiber collimators and fiber collimator arrays. The fibers may be attached by welding of individual fibers each and preferably one fiber at one time. For welding an individual glass fiber may be selected. The glass fiber is positioned into close proximity of the lens to which it has to be welded by a fiber positioning device. Welding of the glass fiber may be done by heating up the fiber and/or lens or MLA (micro lens array) laterally with a high- power laser producing high-power laser pulses of a first wavelength. Such a laser may be a C02 Laser or any other appropriate laser source with a wavelength that is absorbed sufficiently to heat up the used materials. The first wavelength may be in a range of 9,4 to 10,6 pm. At the same time or between the high-power laser pulses, low power light of a second, different wavelength (e.g. 1550nm, 1310nm, 850nm or any value in between) may be coupled into the glass fiber. This light is forwarded by the lens associated with the glass fiber to a mirror mounted at the output side of the lens array and distant to the lens array at a first distance. The distance may be half of the operational distance of the lens array. The light reflected by the mirror may be coupled back through the lens and the fiber to a detector. The detector provides an output signal indicating the optical power received by the detector.

A perfect alignment of fiber results in a maximum detector output signal whereas the signal decreases with position (lateral or along the optcal axis of the fiber) or angular displacement of the fiber. The detector and the second wavelength light source may be coupled by a coupler which may be a 3 dB coupler to the glass fiber. Furthermore, a wavelength selective filter, configured for selectively blocking the first wavelength may be provided between the coupler and the glass fiber. The detector output signal may be used to control the fiber positioning device, which may further be a closed loop control. The fiber positioning device may be configured to adjust the fiber position for maximum detector output signal which results in a minimum insertion loss. Such an adjustment of the glass fiber may be done before and/or during the welding until the weld is mechanically stable.

In an embodiment, all fiber collimators of an FCA fiber collimator array manufactured with the described process resulting in parallel optical axis.

Instead of a mirror, a reference fiber collimator may be used. This reference fiber collimator has at least one reference fiber which is coupled to the light detector. A wavelength selective filter configured for selectively blocking the first wavelength may be provided between the at least one reference fiber and the light detector.

Alternatively, to adjusting the fiber in relation to the MLA also the MLA can be positioned relatively to the fiber kept static.

Another advantage of the present invention is the compensation of the devia tions of the lens shape between the single lenses of a micro lens array as well as the deviation from the ideal lens shape. Differences within the fiber or the quali ty of the fiber end faces are compensated as well. The pointing error (angular deviation) is therefore no longer limited by the absolute position accuracy of the used machine, but rather by the smallest increment of the positioning unit itself.

The material of the each component used to manufacture a welded FCA can be different, e.g. fused silica and Pyrex or Borofloat or any other optically and / or near infrared transparent material e.g. silicon. Preferably the softening point of the lens material is lower than that of the fiber.

To improve the adjustment accuracy, an insertion loss profile of each lens of an MLA can be recorded before welding. When recording such a profile the optical axis of a fiber, preferably a referential fiber, is adjusted parallel to the surface normal of the plane side of the lens. Afterward the fiber is moved in x-, y- and z- direction. The stored relationship between position of the fiber and the insertion loss can be used to optimize the adjustment during welding, e.g. by moving the fiber to a position where slight variations does not affect the insertion loss. During the record of the insertion loss profile the fiber endface can be optionally held in direct contact, at a defined distance or alternatively coupled with an index matching gel to the surface of the micro lens array.

Although the laser energy of the high-power laser is not coupled into the core of the fiber directly, a part of the energy might be coupled into the fiber and manipulate measurement results or even destroy the detector. Thus the detector may be protected from the high-power laser by a wavelength selective filter which may allow light of the second wavelength to pass and blocks light of the first wavelength.

Also the mirror, reflecting the light back into the currently processed fiber can be coated to absorb any stray light of the first wavelength but be transparent for the second wavelength. The embodiments enables the manufacturing of fiber collimators or fiber collimator arrays simultaneously having a high channel density, a low insertion loss and the capability to transmit high optical power. Such fiber collimators or fiber collimator arrays may be used in areas in which not only a reliable transmission of high optical power and low insertion loss is required but also a temporarily or permanent contact with fluids is present. The heat, resulting from unavoidable losses, can be dissipated much better with welded than with glued fibers as the thermal conductivity of a welded joint by far exceeds that of a glued joint. This is of particular importance to prevent fiber collimator arrays from thermal destruction when high channel count, high optical power and high temperature environment are combined. Another advantage of fiber collimators or fiber collimator arrays is the improved stability of the return loss during the temperature changes compared to a glued connection.

In an embodiment, high precision 2D-fiber arrays may be manufactured. To achieve this, a spacer may be inserted temporarily between the fibers and a micro lens array, whereas the latter functions as a reference micro lens array. The pitch of the manufactured 2D-fiber array corresponds to that of the reference micro lens array. The Precision of such 2D-fiber arrays outperforms conventionally produced ID- and 2D-fiber arrays that are manufactured by grinding, sawing or drilling. They can be used in applications or with micro lens arrays of the same pitch.

Double clad fi brers in contrast to standard optical fibers provide two optical paths, a higher power in the outer channel (cladding) and a measurement or communication signal in the core. In an embodiment, a double-clad fiber may be used where the inner channel serves as a measurement channel for providing the signal necessary for the adjustment of the fiber relative to a lens.

In a further embodiment, a plurality of fibers may be positioned simultaneously and welded simultaneously and/or without repositioning individual fibers. For this purpose a plurality of fibers is temporarily clamped in a holder comprising a plurality of V-grooves. After the fibers have been positioned and welded, the holder is removed. The holder may be produced in a way that all fiber cores are on a straight line and the pitch of the V-grooves corresponds to that of the micro lens array. As a result a higher channel density compared to conventional glued 1-D fiber arrays can be achieved as the V-groove holder is removed after welding and only the fibers remain on the micro lens array.

The usage of fiber collimators or fiber collimator arrays produced using the process of the present invention is not limited to optical rotary joints. They may also be used in applications, requiring single- or multi-mode fibers in the wavelength range of 800nm...l700nm and, at the same time, high optical power in the range of a few watt, such as (D)WDM based long haul connections, status scans of fiber Bragg-sensors or antennas and status scans at offshore oil exploration.

The embodiments may be applied to micro lens arrays with similar or varying lens shapes or materials. A combination of different fibers, e.g. single-mode fi bers, multi-mode fibers and / or other fiber types in a single fiber collimator ar ray is also possible. A method comprises the steps of:

- transmitting light of a second wavelength through a fiber into a FCA

reflecting the light output from the FCA by a mirror back to the FCA coupling the reflected light by said fiber to a receiver

positioning the fiber dependent on the receiver output

These steps may be repeated multiple times to increase precision.

Further optional steps are welding of the fiber to the MLA by using light of a second wavelength.

The above steps may also be performed simultaneously with welding. Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings. Figure 1 shows an embodiment with a mirror

Figure 2 shows a further embodiment with a reference collimator Figure 3 shows a fiber holder module

In figure 1, a first embodiment is shown. A collimator 100 comprises a lens array 110 and at least one fiber 121, 122, 123. The lens array 110 may be monothithic or may include a lens arry with a spacer (113) which may be attached by glueing or optical contact bonding or any other suitable method. In case of a monolithic lens array the bodies 110 and 113 are one part. The lens array 110 has a fiber side 111 for attaching fibers and opposing thereto an output side 112, where light leaves or enters the lens array. In this embodiment, a glass fiber 121 has been selected to be connected to the lens array. There may be further fibers which have already been connected or will be connected in the future.

The collimator or lens array may be held by a support 200. The support 200 may comprises at least one lens array base 210, a main spacer 220, which may be a tube or a hollow cylinder, and a mirror 230. The distance 221 from the mirror 230 to the lens array may be defined by the main spacer 220. The total first distance between the lens array and the mirror is determined by the main spacer 220 and the lens array base 210. The length of the main spacer 220, distance 221 may be about one half of the operating distance of the fiber collimator. The mirror 230 is parallel to the output side 112 of the lens array and/or parallel to the lens array 110.

An attenuation measurement device 300 may comprise asecond wavelength light source 310 connected to a coupler 320 which further is connected to glass fiber 121. The fiber may be connected by means of a fiber connector 131.

Furthermore, a wavelength selective light filter 330 for passing the second wavelength may be in the light path between the glass fiber 121 and the second wavelength light source 310 and/or the light detector 340. Light from the second wavelength light source 310 is coupled via coupler 320, which may be a 3dB coupler, and the wavelength selective light filter 330 into the glass fiber 121. Before or during the welding, this fiber 121 is with one end in close proximity to a lens of the lens array 110 and therefore may couple light into the fiber cited 111 of the lens array 110. This light is guided to the output side 112 of the lens array 110 and propagates along the light path 380 towards the mirror 230 from which it is reflected back to the output side 112 of the lens array 110. This light further propagates back through the lens and the glass fiber 121 into coupler 320 and light detector 340, which generates a light detector output signal 341 indicating the optical power received.

A fiber positioning device 400 comprises an actuator 410 for moving the glass fiber, preferably with high precision. Such movements may comprise

perpendicular and lateral movements of the fiber relative to the surface of the fiber side 110. The movement may also comprise an angular movement around an axis perpendicular to the the surface of the fiber side 110, which may be a tilting movement of the fiber. The fiber positioning device device 400 may position the fiber before or during welding dependent on predetermined and/or preprogrammed position values and/or detector output signal 341. The fiber positioning device 400 may further comprise a control unit 420 which may be configured for a closed-loop control of the fiber position based on the detector output signal 341 such that the detector output signal 341 reaches its optimum or maximum value. Alternatively the fiber may be kept stationary in a welding apparatus and the positioning device 400 with the micro lens array fixed to it may be moved in relation to the fibre.

A first wavelength, high-power light source 500, which may be a laser is focused on a lens of the lens array or a surface of the lens array close to a lens and/or the glass fiber for welding the glass fiber to the lens array. The first wavelength, high- power light source 500 may radiate light while or after the glass fiber was put in place by the fiber positioning device 400.

Figure 2 shows a further embodiment. Here, a reference collimator 610 is provided. Such a reference collimator is preferably the same as the collimator 100. The reference collimator is arranged opposing to the collimator with a second distance of a second length of spacer 222 and lens array base 210. To provide this second length of spacer, a different spacer may be provided. This second length of spacer preferably is the full operating distance of the collimator. At the fiber side one reference fiber 611 or a plurality of fibers is connected to a light detector 340. There may be a fiber connector 621 for connecting the fiber and/or a wavelength selective light filter 620 in the light path. In addition a filter 330 may be provided. A reference fiber 611 may be selected at a lens of reference collimator 610 which corresponds to the lens of the glass fiber 121 of lens array 110. In another embodiment, a plurality of fibers 611 of the reference collimator may be connected at the same time.

As in figure 1 the lens array 110 may be monothithic or may include a lens array with a spacer (113) attached.

Here, in contrast to the previous embodiment, the second wavelength light is guided through a first lens array and a second lens array to the light detector, instead of reflecting the light emitted from the first lens array by a mirror back through the first lens array.

In figure 3, an embodiment of a fiber holder module 700 is disclosed. Such a module may be used to hold a plurality of fibers at the same time for welding. This may significantly increase the welding speed in a 2D (2-dimensional) collimator array. The module comprises a fiber holder base 701, the base comprising a plurality of V-grooves 711, 712, and a fiber holder plate 702 which holds fibers 721, 722 in place in their V-grooves. List of reference numerals

100 collimator

110 lens array 111 fiber side of lens array

112 output side of lens arrayll3 spacer

121 glass fiber

122, 123 fibers

131 fiber connector

200 support

210 lens array base

220 main spacer

221 first length of spacer

222 second length of spacer

230 mirror

300 attenuation measurement device

310 second wavelength light source

320 coupler

330 wavelength selective light filter (blocking the first wavelength)

340 light detector

341 detector output signal

380 propagation of second wavelength light

400 fiber positioning device

410 actuator

420 control unit

500 first wavelength, high-power light source

610 reference collimator

611 reference fiber

620 wavelength selective light filter (blocking the first wavelength) 621 fiber connector

700 fiber holder module fiber holder base fiber holder plate, 712 V-grooves

, 722 fibers

Claims

1. A device for positioning of a glass fiber (121) on a micro lens array (110), comprising:

a micro lens array (110) having a fiber side (111) and an opposing output side (112),

a mirror parallel to the output side (112) of the micro lens array (110) at a first distance,

a second wavelength light source (310) coupled to the glass fiber (121) and configured to emit light of a second wavelength into the micro lens array (110),

a light detector (340) coupled to the glass fiber (121) for receiving light from the micro lens array (110), and generating a detector output signal (341) indicative of the optical power received, and

a fiber positioning device (400) configured to move the glass fiber (121).

2. The device according to claim 1,

characterized in, that

the second wavelength light source (310) and the light detector (340) being connected by a coupler (320).

3. A device for positioning of a glass fiber (121) on a micro lens array (110), comprising:

a reference collimator (610) parallel to the output side (112) of the micro lens array (110) at a second distance,

a second wavelength light source (310) coupled to the glass fiber (121) and configured to emit light of a first wavelength into the micro lens array (110),

a light detector (340) coupled to the reference collimator for receiving light from the micro lens array (110), and generating a detector output signal (341) indicative of the optical power received, and

a fiber positioning device (400) configured to move the glass fiber (121).

4. The device according to 3,

characterized in, that

a wavelength selective light filter blocking the first wavelength (620) is provided between the reference collimator (610) and the light detector

(340).

5. The device according to 3 or 4,

characterized in, that

the first wavelength differs from the second wavelength.

6. The device according to any of the previous claims,

characterized in, that

a wavelength selective light filter blocking the first wavelength (330) is provided between the glass fiber and the second wavelength light source (310) and/or the light detector (340).

7. The device according to any of the previous claims,

characterized in, that

the fiber positioning device (400) comprises a control unit (420) configured to receive the detector output signal (341) and to provide a closed loop control of the the glass fiber (121) position.

8. A device for welding of a glass fiber (121) to a micro lens array (110), to manufacture a fiber collimator array comprising: a device for positioning of a glass fiber (121) on a micro lens array (110) according to any of the previous claims, and

a first wavelength, high-power light source (500) focused on a lens of the lens array or a surface of the lens array close to a lens and/or the glass fiber for welding the glass fiber to the collimator.

9. A device for welding of a glass fiber (121) to a micro lens array (110), to manufacture a fiber collimator array comprising a device for positioning of a glass fiber (121) according to any of the previous claims,

characterized in, that

the first wavelength, high-power light source (500) is configured to radiate light while or after the glass fiber was put in place by the fiber positioning device (400).

10. A method of manufacturing a collimator (100) comprising the steps of: transmitting light of a second wavelength through a glass fiber (121) into a lens array (110),

reflecting the light output from the lens array by a mirror parallel to and spaced apart from the output side of the lens array (110), receiving the light by the lens array (121),

coupling the light from the lens array (121) into a receiver (340), generating a detector output signal (341) indicative of the received optical power,

positioning the glass fiber (121) by a fiber positioning device (400) dependent on the detector output signal (341),

providing light from a first wavelength, high-power light source (500) focused on a lens of the lens array or a surface of the lens array close to a lens and/or the glass fiber for welding the glass fiber to the collimator.