Patent Application
20060110094
The invention relates to an optoelectronic module for coupling light signals into and out of an optical waveguide. In particular, the invention relates to an optoelectronic micromodule arrangement that is employed in the field of micromodule technology.
Bidirectional optoelectronic transmitting and receiving modules (transceivers) which can both transmit and receive optical signals are known. Bidirectional modules based on micromodule technology are used in particular as TO components (TO=transistor outline). A bidirectional transmitting and receiving module transmits light having a first wavelength and detects light having a second wavelength. A preferred, but not exclusive, area of use is WDM (wavelength division multiplex/demultiplex) communications.
It is furthermore known to use, for a compact construction, bidirectional transmitting and receiving modules that use a common carrier for the light transmitter and the receiver. In order, in this case, to prevent the receiver from receiving scattered light transmitted by the light transmitter, essentially two types of construction of bidirectional transmitting and receiving units are known.
WO 2004/051894 A1 discloses a bidirectional transmitting and receiving module in which a light transmitter is fixed on the top side of a carrier and a receiver is arranged on the opposite underside of the carrier. The transmitting element emits light signals parallel to the top side, which light signals are deflected such that they run perpendicular to the surface of the carrier and are coupled into an optical waveguide. Light signals coupled out of the optical waveguide penetrate the carrier in the perpendicular direction and are detected by the receiver on the underside of the carrier.
A different form of configuration of a bidirectional module uses so-called PLC technology. In this case, on the carrier, optical waveguides are laid directly up to the light transmitter and directly up to the receiver in order thereby to reduce scattered light (so-called optical crosstalk). In this embodiment light transmitter and receiver can also be arranged on the same side of the carrier. In this case, however, it is not possible to first incorporate the module and then align it in such a way as to optimize the coupling between an optical waveguide and the module, since the optical waveguides are already integrated into the carrier. Furthermore, it is necessary to use a plurality of optical waveguides.
There is a need for a bidirectional optoelectronic module which is distinguished by signal conversion that is as precise as possible and, at the same time, is as cost-effective as possible in terms of production.
In accordance with the present invention, the optoelectronic module has a carrier having at least a first side, on which both a light transmitter for emitting light signals and a receiver for detecting light signals are arranged. A first light-shaping element (e.g. a microlens, a mirror, prism, etc.) serves for coupling light signals of the light transmitter out of the module and into an optical waveguide, and at the same time for coupling light signals out of the optical waveguide into the receiver. In this case, the coupling between the first light-shaping element and the receiver does not have to be effected directly, but rather may be effected for example by means of deflection mirrors and lenses. The light transmitter and also the receiver are both arranged on the same side of the carrier. A shielding means is provided in order to shield the carrier from optical and/or electrical interference signals.
An essential problem in the provision of bidirectional optoelectronic modules is so-called crosstalk, that is to say undesirable interference or scattered signals that are not intended to be detected by the receiver but are present. The main source for optical crosstalk is the light transmitter, which may be formed e.g. as a laser diode or LED. If light from the light transmitter is not radiated directly into an optical waveguide (as in the case of PLC technology) rather the signals are first coupled in via light-shaping elements as coupling means, optical interference signals occur as a result of scattering and reflection at optical diffraction, reflection and coupling means. As a result of the electrical modulation of the transmitter diode for transmission of light signals to be transmitted, an electromagnetic alternating field is furthermore generated which would induce electrical interference signals in the (unprotected) receiver (electrical crosstalk). The light transmitter is not the only source that causes optical and/or electrical interference signals, but is generally the strongest.
According to the invention, the shielding means protects the receiver by shielding it from optical and/or electrical interference signals, that is to say either absorbing and/or reflecting the interference signals.
Thus, in contrast to the prior art, the carrier no longer has to be formed in transparent fashion for the light signals, which reduces the costs for the carrier. Receiver and light transmitter are arranged on the same side of the carrier, it not being necessary to lay optical waveguides on the carrier up to the light transmitter and receiver. At the same time, the coupling means make it possible to couple the outgoing signals into a single-mode fiber and to optimize the coupling by active alignment of the light-shaping element. In the case of PLC technology, this can only be realized with very great difficulty technically since optical waveguides are laid through trenches formed on the carrier up to the transmitter and receiver, respectively, into which coupling is effected directly on the carrier. Alignment is very difficult to realize in this case.
In a preferred embodiment, the shielding means is formed as a separate component and arranged on the first side of the carrier. In this case, it encloses the receiver in such a way that the receiver is arranged within the shielding means, that is to say is surrounded by the shielding means like being surrounded by a housing. In this case, it is formed in box-shaped or parallelepipedal fashion, by way of example.
The shielding means is advantageously formed in micromechanically etched fashion. In this case, a multiplicity of shielding means joined together in the wafer are etched, for example, in the form of parallelograms.
Particularly preferably, the shielding means is formed in hollow fashion and defines a three-dimensional spatial region that is bounded by the shielding means and the carrier. It has a taplike form, by way of example, and is arranged on the submount in such a way that the receiver is positioned within the spatial region defined by the shielding means and the submount bounds one or more sides of the spatial region not bounded by the shielding means. As a result, the receiver is shielded on all sides either by the shielding means or by the carrier. The shielding effect is maximized if the receiver is enclosed by the shielding means as far as possible on all sides.
Particularly preferably, the shielding means is formed as a Faraday cage and the receiver is arranged within the Faraday cage. The Faraday cage protects the receiver in particular from electrical interference signals, which cannot pass into the interior of the Faraday cage. This prevents induction of electrical signals on the basis of electromagnetic waves that arise during the modulation of the laser diode, by way of example. For this purpose, the shielding means has been metallized in a coating installation, by way of example.
In one embodiment, the shielding means has a coupling-in opening through which light signals from the first light-shaping element are coupled into the receiver. In this case, the coupling-in opening is preferably the only opening of the shielding means and thus the only possible entry or exit for electromagnetic radiation. The coupling-in opening is for example sawn into the shielding means by means of a wafer saw.
The module advantageously has a second light-shaping element, in particular a further microlens, which is arranged in the beam path between the first light-shaping element and the receiver in such a way that it couples light signals passing from the first light-shaping element to the receiver onto an optically active area of the receiver. Furthermore, it is also possible to provide a third light-shaping element for the light transmitter, which images the signals coupled out of the light transmitter onto the first light-shaping element, it being possible for further optical elements to be arranged between the beam-shaping elements. All the beam-shaping elements are preferably formed as microlenses made of silicon.
The side of the second light-shaping element that is remote from the receiver is advantageously provided with an optical filter, which filters out scattered light of the light transmitter from the receiver. However, the optical filter transmits the signals to be received, which are coupled into the receiver from the optical waveguide via the first light-shaping element and via the second light-shaping element. The optical filter is thus transmissive either only at a specific angle or only for the wavelength of the light signals to be received.
In one embodiment, a layer (blocking layer) that is nontransparent to the light of the light transmitter is formed on at least one side of the carrier. Preferably, the carrier has such a blocking layer both on the first side with the receiver and the laser diode and on the opposite side. The blocking layer is nontransparent at least to the wavelength at which the light transmitter emits light signals. The blocking layer protects the receiver from detecting light signals that have possibly been scattered or sent through the carrier. Consequently, not only the directions that are shielded by the shielding means outside the carrier are protected from interfering signal influence, but also the path via the carrier. The blocking layers may also be formed in metallized fashion, in which case they block both optical and electrical interference signals.
Preferably, apart from the first light-shaping element, all the components of the module that are required for coupling in and out are arranged on the same, first side of the carrier. The components thus comprise, by way of example, the light transmitter, the receiver, a light transmitter coupling means, a receiver coupling means and a beam splitter.
Particularly preferably, the first light-shaping element is arranged on an end side of the same carrier on which the light transmitter and the receiver are also arranged. An additional carrier such as a deflection prism, by way of example, on which the first light-shaping element is arranged in embodiments known heretofore is thereby obviated, which significantly reduces the production costs for the module.
The module is advantageously formed in a manner free of waveguides. Thus, PLC technology is not used, but the alignability of the coupling means is obtained despite the arrangement of light transmitter and receiver on one side of the carrier.
This means, inter alia, that a free-radiating region is present in the beam path between the light transmitter and the first light-shaping element and/or between the receiver and the second light-shaping element.
In a particularly preferred embodiment, the module can be coupled to an optical waveguide in such a way that the beam path runs essentially rectilinearly between the light transmitter and the first light-shaping element (and, if appropriate, moreover, up to the optical waveguide). The use of a deflection means such as a mirror thereby becomes superfluous. The emission direction of the light transmitter thus essentially correlates with the coupling-in direction into the optical waveguide. In this case, it must be taken into consideration that the light signals from the laser are not coupled exactly rectilinearly into the optical waveguide, but rather at a squint angle of 1° to 3° in order to avoid interference with the laser by retroreflected light. The squint angle can be set through active alignment of the microlens.
As an alternative to this, the detection direction of the receiver may also correlate with the coupling-out direction from the optical waveguide. In this case, the module can be coupled to the optical waveguide in such a way that the beam path runs essentially rectilinearly between the receiver and the first light-shaping element (and, if appropriate, moreover with the optical waveguide).
In an advantageous manner, the light transmitter transmits light having a first wavelength, while the receiver detects light having a second wavelength that differs from the first. In this case, a wavelength-selective beam splitter is furthermore provided, which reflects either the emitted wavelength or the wavelength to be received. The light transmitter and the receiver are then arranged on the carrier in such a way that the beam paths assigned to them, that is to say the beam paths to be detected and to be transmitted, run at an angle of approximately 90° with respect to one another at the location of the beam splitter. However, other arrangements with angles of between 40° and 140° are also possible. In this case, the beam splitter may be arranged in a groove of the carrier.
As an alternative to a wave-selective beam splitter, it is also possible to use a partly transmissive filter, that is to say, for example, a 3 DB or 5 DB filter or the like. The transmitter then transmits light signals at the same wavelength at which the receiver receives light signals. As a result of the filter's property of transmitting one portion of the light signals and reflecting another portion, in this embodiment the signal transmission/detection is qualitatively not as good as with a WDM filter. In return, however, the costs of such a construction are lower.
Preferably, a carrier is used which is nontransparent to the light signals to be transmitted and to be received. This enables particularly good shielding of the signals and prevents interference signal transport through the carrier. In this case, the first light-shaping element can advantageously be actively aligned, so that it can be aligned after the module has been incorporated into a TO housing by way of example, in such a way that both the coupling-in efficiency and the coupling-out efficiency into and out of an optical waveguide can be optimized.
The bidirectional module is preferably integrated into a TO housing with a baseplate in such a way that the carrier is arranged perpendicular to the baseplate. This means, inter alia, that the module is placed onto the TO baseplate, connected there (e.g. with bonding wires or by means of soldering connections), and is then actively aligned and then encapsulated.
In an advantageous manner, the module is coupled to an optical waveguide via the light-shaping element such that a free-radiating region is formed between the module and the optical waveguide. An optical waveguide such as a fiber or else a fiber STUB may be used as the optical waveguide.
In one embodiment, the carrier has a second side and metallizations for electrical contact-connection of the receiver are formed on the second side of the carrier. The second side may be opposite the first side, by way of example. This reduces mutual influencing or interference of the electrical signals conducted to and from the module.
The invention is explained in more detail below on the basis of exemplary embodiments shown in the figures, in which:
a and 1b show a perspective view of a bidirectional optoelectronic module with a shielding cap above the receiver;
a shows a plan view of a TO housing in which a bidirectional module with a perpendicularly arranged carrier plate is arranged and whose individual components are electrically contact-connected partly by means of bonding wires and partly by means of solder balls; and
b shows a plan view of a TO housing analogous to
In the figures, mutually corresponding or similar features have the same reference symbols.
a, 1b and 2 show a bidirectional electro-optical transmitting and receiving module 20. The module 20 has a basic carrier plate 2 as carrier, which is formed as a silicon submount, by way of example. All the essential components of the module 20 are arranged on one side, the top side 2′ illustrated in the figures, of the carrier 2.
A laser diode 1 serves as a light transmitter, whose signal output is recorded by the monitor diode 8. In the further beam course, light signals of the laser diode 1 are detracted by a transmitter microlens 6 and radiate through a wavelength-selective beam splitter 7 before the light signals are coupled out of the module without beam deflection through the microlens 4 (
In this case, the optical waveguide runs approximately in the plane of the top side 2′ of the carrier 2.
The laser diode 1 is formed as an edge emitting laser diode and emits laser signals essentially parallel to the first side 2′ of the carrier 2. In this case, the laser diode 1 is oriented in such a way that light signals emitted by it pass rectilinearly to the microlens 4 and from there at a squint angle into the optical waveguide (not illustrated).
Before the light signals reach the microlens 4, they pass through the light-shaping element—assigned to the light transmitter—in the form of light transmitter microlens 6, by which they are focused precisely onto the microlens 4. At the side opposite to the emission direction of the laser diode 1, a monitor diode 8 is arranged on the carrier plate 2, and records radiation which is emitted by the laser diode 1 in the opposite beam direction and makes up a fraction of the forward emitted radiation.
The photodiode 3 is furthermore arranged on the first side 2′ of the carrier 2, said photodiode serving as a receiver and being positioned in such a way that it detects incoming light signals from a direction running approximately perpendicular to the beam path of the light signals emitted by the light transmitter 1. A pin diode rotated through 90° is involved here, which is adhesively bonded onto the carrier 2 or soldered thereon by means of so-called “solder ball bumps”. Light signals to be received from the optical waveguide (not specifically illustrated) firstly pass through the microlens 4, are focused by the latter and transmitted along a grooved cutout 14 in the direction toward the laser diode 1. The beam splitter 7 inclined by 45° relative to the direction of incidence is situated in the beam path, which beam splitter may be formed e.g. as a WDM filter and deflects the light signals to be received by 90° in the direction of photodiode 3. What is involved in this case is e.g. a wavelength division multiplexer/demultiplexer which enables light having different wavelengths to be simultaneously coupled into and out of a fiber. The beam splitter 7 here separates the incoming from the outgoing light signals by virtue of the fact that it reflects only the incoming light signals by 90° and allows the outgoing light signals emitted by the laser diode 1 to be transmitted unimpeded. After reflection at the beam splitter 7, the light signals to be received pass through the receiver coupling means 5, which is formed as a receiver microlens. The receiver microlens 5 focuses the light signals to be detected directly onto an optically active area of the receiver photodiode 3.
A shielding means in the form of a rhomboid shielding cap 10 is slipped over the photodiode 3. The shielding cap 10 is composed e.g. of patterned silicon 110 and has in its interior a vapor-deposited or sputtered metal layer that acts as a Faraday cage. It thus keeps not only optical but also electrical interference signals away from the photodiode 3. The shielding cap 10 is opaque. The photodiode 3 is surrounded from all sides either by the shielding cap 10 or (on one side) by the carrier 2. The only opening through which electromagnetic waves can pass to the photodiode 3 is formed by an essentially square coupling-in opening 13. The coupling-in opening 13 is formed in a wall of the shielding cap 10 at the location that lies on the direct connecting line of the optically active area of the photodiode 3 and the receiver microlens 5. Light signals to be received which have been reflected from the beam splitter 7 to the receiver pass centrally through the coupling-in opening 13. Due to the dictates of production, a second opening 13′ is arranged on that side of the parallelogram—which shapes the shielding cap 10—which is opposite to the coupling-in opening 13. Said second opening 13′ is not necessary and is not desirable either, but is formed as a byproduct during the simultaneous sawing-out of the coupling-in openings into a plurality of shielding means by a single sawing process in the shielding means.
An optical filter 11 is arranged on that side of the receiver microlens 5 which is opposite to the photodiode 3. The optical filter 11 is arranged in the direct connecting line between the laser diode 1 and the coupling-in opening 13. The optical filter 11 reflects light having a wavelength which is emitted by the laser diode 1. Consequently, the filter 11 shields the coupling-in opening 13 from penetrating optical interference signals. In an alternative embodiment, provision may be made for introducing the filter 11 directly into the coupling-in opening 13.
The first side 2′ of the carrier 2 is completely protected by a blocking layer 12, which is opaque to any type of electromagnetic radiation. Consequently, electromagnetic radiation is prevented from penetrating into the carrier 2 and thereby possibly passing through to the photodiode 3 through reflection at the underside of the carrier 2 under the shielding means 10.
The photodiode 3 is connected to the rear side of the carrier 2 by means of metallic contacts (vias) 15 in order to relay the detected signals on the rear side (in this respect, cf.
In the case of the bidirectional module 20 shown in the figures, the beam path of the light signals to be received, the beam path of the light signals to be transmitted, the beam path of the light signals to be coupled into the optical waveguide and the beam path of the light signals to be coupled out of the optical waveguide all lie in the same plane, here in the plane of the first side 2′ of the carrier 2. The microlens 4 is fixed directly on the carrier on which light transmitter 1 and receiver 3 are also arranged. All the microlenses used, that is to say microlens 4, light transmitter microlens 6 and receiver microlens 5, are produced from silicon. Instead of being produced from silicon, the microlenses used may also be produced from glass.
The way in which the shielding cap 10 formed in a rhomboid fashion is placed over the photodiode 3 can be seen particularly well in
In the same way as on the opposite side 2′, a blocking layer 12 may be provided between the metallic contacts 19 and the actual carrier 2, which blocking layer prevents electromagnetic scattered radiation from penetrating into the carrier 2. The blocking layer 12 is formed below the contacts 17 in order to minimize possible interactions or signal influencing.
As an alternative, it is also possible to metallize only that region of the surface 2′ of the carrier 2 on which the photodiode 3 is arranged and which is covered by the shielding cap 10.
a shows, in a schematic illustration, a plan view of a bidirectional module that has already been inserted into a TO housing 21. The TO housing has a baseplate 21, through which a plurality of contact pins 16 project in a manner known per se. The carrier 2 with the optical components arranged on a side 2′ of the carrier is inserted into the TO housing perpendicularly to the baseplate. Signals are coupled out upwardly via the lens 4, the microlens 4 being fixed on the sawing edge of the carrier 2.
Consequently, in this illustration, the module 20 is rotated through 90° both with respect to
The electrooptical components on the one side 2′ of the carrier, in particular the laser diode and the receiver, are connected to the contact pins by means of bonding wires or soldering contacts (solder ball bumps). By way of example, in
In the exemplary embodiment of
During assembly, firstly the electrooptical module 20 as illustrated in
In an alternative embodiment, the shielding cap 10 is made of metal, for example an injection-molded MIM part. The bidirectional optoelectronic module need not be incorporated into a standard TO platform, but rather can also be used differently, preferably in the form of a hermetic package with pins.
The structures in the silicon may be produced both isotropically and anisotropically.
List of Reference Symbols
