Patent Publication Number: US-2004051932-A1

Title: Integrated variable optical attenuator and isolator, components therefor and method of assembly

Description:
[0001] The invention relates to arrangements for providing the functions of variable attenuation and isolation.  
       [0002] In the prior art, the variable attenuation and isolation functions are currently provided by separate subassemblies. For instance, the isolator may be co-packaged with a laser in an industry standard “butterfly” package while the variable attenuation function is provided by a variable optical attenuator packaged in a separate hermetic pigtailed package. The pigtail of the “butterfly” package is connected to the pigtail of the attenuator package either by using optical connectors or by splicing.  
       [0003] This kind of prior art arrangement is shown in FIG. 1, where a laser  100  and an isolator  102  are co-packaged in an industry standard “butterfly” package  104  and a variable optical attenuator (briefly VOA)  106  is separately packaged in a hermetic package  108 . Light from the laser  100  is focussed by a ball lens  110  through the isolator  102  into a fibre pigtail  112 , which is secured to a wall  114  of the package  104  by a ferrule  116 . Electrical connections (not shown) are made from the laser  100  to the leads  118  of the package  104 .  
       [0004] Similarly, the fibre pigtail  120  of the VOA package  108  is secured to a wall  122  of the package  108  by a ferrule  124 . Electrical connections (not shown) are made from the VOA  106  to the leads  126  of the package  108 . The pigtail  112  is connected to the pigtail  120  by a splice  128 .  
       [0005] This arrangement is disadvantageous firstly in that it requires two hermetic pigtailed packages, which are costly and bulky, and secondly in that it requires the pigtails to be joined. Splicing has the advantage of a low loss and stable connection, but is a difficult process and requires that the transmitter and VOA are mounted on the same circuit pack, thus reducing the number of transmitters which can be mounted on the circuit pack, and hence in the rack.  
       [0006] Patchcords are simpler to use but connectors have generally higher and more variable losses than splices. Generally there would be a connection between the transmitter pigtail and the patchcord at the front panel of the transmitter circuit pack and a connection between the patchcord and the VOA pigtail at the front panel of the VOA circuit pack. The VOA circuit packs may be positioned on a shelf below the transmitter circuit packs, hence reducing the number of shelves available for transmitters and the number of transmitters which can be mounted in the rack.  
       [0007] There is a further disadvantage. With conventional pigtails and patchcords the state of polarisation at the input to the VOA is unknown and so the VOA has to be of the polarisation independent type, which is typically larger, more complex and more expensive than a polarisation dependent type. Alternatively polarisation maintaining (PM) pigtails and patchcords could be used. PM fibre is more expensive than standard fibre and assembling transmitters and patchcords with PM fibre is more difficult than with standard fibre.  
       [0008]FIG. 2 is a perspective view of an optical isolator  102  suitable for co-packaging in a package with a laser. The isolator shown is a development, disclosed in U.S. Pat. No. 6,055,102, of a type first described in “Compact Optical Isolator”, F. J. Sansalone, Applied Optics, Vol. 10 No. 10, pp. 2329-2331, October 1971. A garnet material is used as the Faraday rotator and a rare earth magnet provides the magnetic field. The isolator  102  comprises a magnet  200  having a square sided opening  202  in which is located an isolator element  204 . The magnet  200  is generally U-shaped in cross-section, and comprises two mounting areas  206 . which lie within the same plane, for mounting the optical isolator  102  on a planar surface. The isolator element  204  is a laminate comprising a Faraday rotator  208  sandwiched between an “input” polariser  210   a  and an “output” polariser  210   b.    
       [0009]FIG. 3 is a schematic of a liquid crystal variable optical attenuator  300 . A polarised collimated light beam  302  is incident from the left through a first glass plate  304 , a thin layer of liquid crystal material  306  and a second glass plate  308 . The inner faces of the glass plates  304 ,  308  are covered with transparent, conductive material, such as indium tin oxide, to form electrodes  312 ,  314 . The components  304  to  314  comprise a liquid crystal cell  316 . Finally the light is incident on a polariser  318 . As the light propagates through the liquid crystal material  306  its plane of polarisation rotates, and the angle of rotation depends on the RMS value of the potential between the electrodes, and so the fraction transmitted by the polariser  318  varies in response to the RMS value of the potential between the electrodes, which is controlled by a controller (not shown). It will be appreciated that in order to reach the maximum attenuation it is necessary that the plane of polarisation of the collimated light beam  302  is aligned accurately, generally orthogonally, to the plane of polarisation of the polariser  318 .  
       [0010]FIG. 4 is a schematic of an alternative liquid crystal variable optical attenuator  300  that, in addition to the elements shown in FIG. 3, comprises a second polariser  322  arranged before the first glass plate  304  to establish the polarisation state of the light incident on the liquid crystal cell  316  as the polarisers  318 ,  322  can be aligned more conveniently as part of the same assembly than the plane of polarisation of the collimated light beam  302  can be aligned to the plane of polarisation of the polariser  318 .  
       [0011]FIG. 5 is a schematic section of a Faraday rotator variable optical attenuator  400 . A polarised collimated light beam  402  is incident from the left through a Faraday rotator material  406  and a polariser  408 . The Faraday rotator material  406  and polariser  408  are located within a solenoid  410 . As the light propagates through the Faraday rotator material  406  its plane of polarisation rotates, and the angle of rotation depends on the value of the magnetic field generated by the current flowing in the solenoid  410 , and so the fraction transmitted by the polariser varies in response to the current flowing in the solenoid  410 , which is controlled by a controller (not shown).  
       [0012] It will be noted that the variable optical attenuators  300  and  400  essentially comprise a polarisation rotating means (e.g. the liquid crystal cell  316  or the Faraday rotator material  406 ) followed by a polariser  318  or  408 .  
       [0013] A somewhat similar arrangement is disclosed in U.S. Pat. No. 5,978,135. There, an arrangement substantially like an optical isolator, that is with a polariser arranged before a magneto-optic element to establish the polarisation state of the light incident on the magneto-optic element (see FIG. 4) is disclosed where appropriate choice of the composition of the magneto-optic element can result in a thermally variable rotation of the plane of polarisation and thus a variable optical attenuator.  
       [0014] The object of the present invention is to provide the functions of variable attenuation and isolation to an optoelectronic module in a manner that has reduced piece part count and is more compact and cheaper than the prior art.  
       [0015] According to the present invention, that object is achieved thanks to an arrangement having the features set forth in the claims that follow. The invention also relates to components for such an arrangement as well as to a method for assembling such an arrangement.  
       [0016] The invention provides an integrated variable optical attenuator (VOA) and isolator, which has the advantages of: compact size, because only one hermetic package is required and there are no connecting pigtails; ease of use, because there is no requirement to join pigtails; and reduced cost, because only one hermetic package is required and there is no requirement to join pigtails.  
       [0017] There are additional advantages to further reduce the size and cost as, because the VOA and isolator are integrated, there is no uncertainty in the polarisation state and hence the VOA can be of the cheaper and more compact polarisation dependent type. Furthermore isolators and some VOAs have polarisers at their inputs and outputs and by integrating the VOA and isolator one polariser can be eliminated, further reducing size and cost. 
     
    
    
     [0018] The invention will now be described, by way of example only, with reference to the annexed figures of drawing, wherein:  
     [0019] FIGS.  1  to  5 , related to the prior art, were described previously,  
     [0020]FIGS. 6 and 7 are cross sectional views of two embodiments of an arrangement according to the invention, and  
     [0021]FIG. 8 schematically depicts a method of assembling an arrangement according to the invention starting from components thereof. 
    
    
     [0022]FIG. 6 shows a variable optical attenuator  500  placed in the same polarised collimated light beam  502  as an isolator  102 , the VOA  500  and the isolator  102  comprising an integrated assembly designated  600 .  
     [0023] The VOA essentially comprises a polarisation rotating medium  504  and a polariser  506 . The polarisation rotation medium can be e.g. a liquid crystal cell  316  with associated electrodes  312 ,  314  or a Faraday rotator material  406  having associated a solenoid  410 , such details having been shown in FIGS.  3  to  5 .  
     [0024] The isolator  102  is essentially of the kind shown in FIG. 2, and includes a laminate comprising a Faraday rotator  208  sandwiched between an input polariser and an output polariser.  
     [0025] For that reason, throughout FIGS.  6  to  8 , elements or parts identical or equivalent to elements or parts already described in connection with FIGS.  2  to  5  are denoted by the same reference numerals already appearing in those previous figures.  
     [0026] The arrangement of the invention is based on the recognition that the VOA  500  and the isolator  102  may share a common polariser that is in fact represented by the polariser designated  506 . This is interposed between the polarisation rotation medium or material  504  of the variable optical attenuator and the rotator  208  of the polariser, so that in polariser  506  also plays the role of the input polariser of the optical isolator.  
     [0027]FIG. 7 shows an alternative embodiment of an integrated VOA and isolator assembly  600  of the present invention in which the VOA  500  is supplemented with an additional polariser  604  to improve the maximum attenuation. In such an alternative arrangement the polarisation rotation medium  504  of the variable optical attenuator  500  is thus sandwiched between the additional polariser  604  and the polariser  506  that is common to the variable optical attenuator and the optical isolator.  
     [0028] It will be appreciated that in the prior art arrangement shown in FIG. 1 there are many potential sources of reflection between the isolator and the VOA and so, if the positions of the isolator and VOA were exchanged, the laser would be more susceptible to perturbation.  
     [0029] In the arrangements of FIGS. 6 and 7, the only additional upstream component is the VOA, which can be designed to minimise back reflection, and so the order of the components may be determined using other criteria, e.g. geometrical.  
     [0030] While an integrated assembly such as shown in FIG. 6 or FIG. 7 is most advantageous, it may be that such a component cannot be conveniently sourced as the VOA and isolator technologies may not both be available to a single component company.  
     [0031] In this case, the assembly  600  may be separated into two portions  700 ,  702  as shown in FIG. 8, where the VOA function and the isolation function are primarily performed by a first portion or component  700  and by a second portion or component  702 , respectively.  
     [0032] The arrangements shown in FIGS. 6 and 7 can thus be assembled by:  
     [0033] providing the first component  700  including the polarisation rotation medium  504  of the VOA  500 , the common polariser  506  connected in optical alignment, and possibly the polariser  604 ,  
     [0034] providing the second component  702  including the optical rotator  208  of the isolator  102  connected to the output polariser  210   b , and  
     [0035] assembling the first  700  and second  702  component by causing the common polariser  506  to be interposed between the polarisation rotation medium  504  and the optical rotator  208 .  
     [0036] It is advantageous for the first component  700  to comprise the “common” polariser  506  (rather than the second component  702  to comprise the polariser  506 ) as the polariser  506  can be well aligned to polariser  604  to reach the maximum attenuation while the alignment between the first component  700  and second component  702  is less critical for the values of isolation typically required.  
     [0037] The two Faraday rotators included in the isolator  102  and the VOA  500  could be both garnet materials. Advantageously, they may be different materials. For example, the isolator material could be the ‘latching’ type referred to in U.S. Pat. No. 5,978,135, which does not respond to small external magnetic fields, whereas the VOA material must respond to the variable field of the solenoid.  
     [0038] Of course, without prejudice to the underlying principle of the invention, the details and embodiments may vary with respect to what has been described and shown by way of example only, without departing from the scope of the invention as defined by the claims that follow.