Abstract:
An optical control modulator comprises an input to receive light from a light source; a polarization divider to divide the light into two orthogonal plane-polarized light components; a Faraday material which, upon being energized, rotates the plane of polarization of the plane-polarized light as it passes through the material; and an output. The light travels through the Faraday material in opposite directions on a common path. In one embodiment, the light passes once through the Faraday material on a loop path. In another embodiment, the light passes twice through the Faraday material by being reflected by a mirror disposed at a back face of the Faraday material.

Description:
FIELD OF THE INVENTION 
   The invention relates to an optical control element. It is particularly, but not exclusively, related to an optical control element, such as an optical switch or an optical modulator. 
   BACKGROUND OF THE INVENTION 
   In order to provide broadband telecommunications, optical transmission systems have been developed. Typically such systems transmit optical signals along optical fibre in the wavelength range of 1300 to 1550 nm. In order to provide multiple access for a plurality of users, it has been proposed for optical transmission systems to use wavelength division multiple access (WDMA). 
   SUMMARY OF THE INVENTION 
   An optical transmission system  110  is shown in  FIG. 1 . The system  110  comprises a central office (CO) or hub  112  connected to groups of users or optical network units (ONUs)  114 . In this embodiment only one group  116  is shown although it will be understood that there would be a plurality of such groups. The hub  112  is connected by a common optical fibre  118  to a wavelength division multiplexer (WDM)  120 . The users  1   14  are connected separately to the WDM  120  by respective optical fibres  122 . 
   Communication in the system occurs across a wavelength range, typically 1530 to 1565 nm. Each user is assigned a wavelength band, typically 0.8 nm, with which to communicate. The hub  112  is equipped with individual laser sources producing light at different wavelengths suitable for different users. Therefore, in transmitting to individuals of the users  114 , data is modulated onto appropriate light wavelengths which are then transmitted down the common optical fibre  118  to the WDM  120 . The WDM  120  separates the light according to wavelength and then directs particular wavelengths to particular users  114 . 
   Since it is impractical to provide every user  114  with a laser source operating within its assigned wavelength band, two arrangements have been proposed. In the first arrangement, spectral slicing is used, in which each user  114  has a broadband light source, from which a specific optical wavelength band is sliced and then used in the network. For bit rates of up to 500 Mbit/s modulation is effected by switching the broadband light source on and off. For higher bit rates, an external modulator may be used. In the second arrangement, a laser at the hub  112  transmits continuous wave (CW) light at appropriate wavelengths for each of the users  114 . Modulators located at the users  114  modulate data onto this CW light and send it to the WDM  120  on the respective optical fibres  122 . It has been proposed to use loopback based modulators (IEEE Photonics Technology Letters, November 1994, volume 6, number 11, pages 1365 to 1367) or mirror based modulators (disclosed in IEEE Photonics Technology Letters, September 1996, volume 8, number 9, pages 1175 to 1177). Different wavelengths of light from different users  114  are multiplexed together by the WDM  120  and transmitted together on the common optical fibre  118  to the hub  112 . Such a system is disclosed in IEEE Photonics Technology Letters, November 1994, volume 6, number 11, pages 1365 to 1367. 
   Intensity modulation is the most common modulation method used in optical transmission systems. Typically data is modulated onto the light by turning a laser on and off. Alternatively, external modulation may be provided, for example by electro-absorption devices, Mach-Zehnder interferometer modulators, mechanical based modulators or polarisation modulators. Polarisation modulators to achieve intensity modulation by using a Pockels cell are disclosed in Fundamentals of Photonics, B. E. A. Saleh, pages 703 to 705 and in Lasers &amp; Applications, R. Goldstein, April 1986, volume 5, number 4, pages 67 to 73. 
   Optical modulators are known which use the Faraday effect. This causes the rotation of the plane of polarisation of plane-polarised radiation to be rotated as the radiation passes through an isotropic medium in the direction of a magnetic field in which the medium is placed. The angle of the rotation is proportional to the strength of the magnetic field. Examples of media or Faraday rotators exhibiting the Faraday effect (Faraday materials) are Yttrium-iron-garnet and Terbium-gallium-garnet. 
   U.S. Pat. No. 4,789,500 discloses an arrangement using the Faraday effect to provide an optical isolator. This arrangement has a polariser, a Faraday material surrounded by a coil and a mirror. A voltage is applied to the coil to cause current flow so that the coil generates a magnetic field which extends into the Faraday material. Light passing through the polariser becomes plane-polarised and its plane of polarisation is rotated as it passes through the Faraday material. The plane-polarised light leaves the Faraday material and is then reflected by the mirror on a return journey. On its return journey, as the plane-polarised light passes through the Faraday material, its plane of polarisation is rotated further. Consequently, the plane-polarised light is stopped by the polariser. 
   One problem in optical communications systems is back reflection. Connectors and other optical components present in an optical fibre transmission path reflect light from discontinuities such as their end faces. Back reflection can have a destabilising effect on oscillation of laser sources and on the operation of optical fibre amplifiers, thus resulting in a poor transmission performance. For this reason, optical isolators are used to reduce back reflection. 
   A polarisation-independent optical isolator which uses the Faraday effect is disclosed in IEEE Photonics Technology Letters, March 1989, volume 1, number 3, pages 68 to 70. The isolator comprises, in series, in a forward direction in which light is able to pass, a first birefringent crystal element, a 45° Faraday rotator, a second birefringent crystal element and a third birefringent crystal element. A lens is placed at each end of the isolator to provide coupling with upstream and downstream optical fibres. Forward-travelling light entering the optical isolator is separated into ordinary and extraordinary rays by the first birefringent crystal element. By taking advantage of the spatial walk-off experienced by the ordinary and extraordinary rays, and the reciprocal and non-reciprocal natures of the birefringent crystal elements and the Faraday rotator respectively, the forward-travelling light is coupled into the downstream optical fibre whilst ordinary and extraordinary rays of backward-travelling light are spatially separated from the axis of propagation of the forward travel light and so are not coupled into the upstream optical fibre. 
   Polarisation-independent isolators are frequently used in optical fibre amplifiers. 
   According to a first aspect of the invention there is provided an optical control element having an input to receive light from a light source, a polarisation divider to convert the light into plane-polarised light, a material which upon being energised rotates the plane of polarisation of the plane-polarised light as it passes through the material and an output wherein the light travels through the material in opposite directions on a common path. 
   In the invention the intensity of the light may be modulated by changing its polarisation state. In one embodiment, the light is divided into two components having different respective polarisation states. The polarisation states of each of the light components may then be changed. Thus by changing the polarisation state of the light components prior to them entering the polarisation divider, the output from the polarisation divider can be intensity modulated. 
   Preferably the optical control element comprises a polarisation combiner. Advantageously, the polarisation divider and the polarisation combiner are the same element. 
   In one embodiment the light passes once through the material. In this embodiment, the light may be split into a first part and a second part wherein the first part travels on the common path in a first direction and the second part travels on the common path in a second opposite direction. 
   In another embodiment, the light passes a first time and then a second time through the material. In this embodiment, the light first travels on the common path in a first direction and then travels on the common path in a second opposite direction. Preferably there are two common paths. In this case, the light may be split into a first part and a second part wherein the first part travels on a first common path firstly in a first direction and then secondly in a second opposite direction and the second part travels on a second common path firstly in a first direction and then secondly in a second opposite direction. 
   Preferably the optical control element comprises a reflecting element such as a mirror. The reflecting element may be used to reflect incident light back along the common path from which it came. Preferably, the light travels on the common path from the input to the reflecting element and then travels back on the common path from the reflecting element to the polarisation divider. Whether it is output by the output is determined by whether the material is energised. 
   Preferably the input and the output commonly comprise an input/output. 
   Preferably the material is energised when subjected to a magnetic field. The material may be a magneto-optical material such as a Faraday material. Alternatively, the material is energised when subjected to an electric field. The material may be an electro-optical material such as NH 4 H 2 PO 4 , KH 2 PO 4 , LiNbO 3 , LiTaO 3 , and CdTe. 
   Preferably the optical control element comprises a magnetic field generator for applying a magnetic field to the material. In one embodiment, the magnetic field generator is a coil and a power supply for causing current flow in the coil. Preferably the material is located inside the coil. 
   Preferably the optical control element comprises an electrical field generator for applying an electrical field to the material. In one embodiment, the electrical field generator is a pair of plates and a power supply for applying an electrical field across the plates. Preferably the material is located between the plates. 
   Preferably the optical control element comprises a modulation control unit for controlling the power supply. The modulation control unit may modulate the power supply so as to modulate the amount of current flowing in the coil and thus the magnetic field that it generates. Alternatively, the modulation control unit may modulate the power supply so as to modulate the amount of electrical field generated across the plates. 
   Preferably the polarisation divider has a single input to receive light which is not plane-polarised and two outputs to output first and second light components orthogonally polarised with respect to each other. Preferably, the polarisation divider has two inputs to receive two beams of light which are orthogonally polarised with respect to each other and a single output. Preferably, the polarisation divider comprises an outward path and a return path in which the light passes through the polarisation divider on the outward path, passes through the material and then travels back through the polarisation divider on the return path. 
   Preferably the optical control element is a modulator. It may be used to provide a digital signal. Alternatively, it may be used to provide an analogue signal. 
   According to a second aspect of the invention there is provided a method of modulating light comprising the steps of: 
   receiving light from a light source; 
   using a polarisation divider to convert the light into plane-polarised light; 
   sending the light in opposite directions on a common path; and 
   rotating the plane of polarisation of the plane-polarised light as it travels on the common path. 
   According to a third aspect of the invention there is provided an optical network unit for an optical transmission system comprising an optical control element, wherein the optical control element comprises an input to receive light from a light source, a polarisation divider to convert the light into plane-polarised light, a material which upon being energised rotates the plane of polarisation of the plane-polarised light as it passes through the material and an output wherein the light travels through the material in opposite directions on a common path. 
   According to a fourth aspect of the invention there is provided an optical transmission system comprising at least one optical control element, wherein the or each optical control element comprises an input to receive light from a light source, a polarisation divider to convert the light into plane-polarised light, a material which upon being energised rotates the plane of polarisation of the plane-polarised light as it passes through the material and an output wherein the light travels through the material in opposite directions on a common path. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  shows a WDMA system; 
       FIG. 2  shows a modulator; 
       FIGS. 3   a  and  3   b  show operational states of the modulator of  FIG. 2 ; 
       FIG. 4  shows an alternative embodiment of a modulator; and 
       FIGS. 5   a  and  5   b  show operational states of the modulator of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is applicable in an optical WDMA communications system such as that described in relation to  FIG. 1 . As mentioned above each user  114  has a broadband light source which is spectrally sliced to provide a specific optical wavelength band or each user  114  receives CW light transmitted from the hub  112 . Digital modulators which are used at the users  114  to modulate data onto this CW light will now be described. 
     FIG. 2  shows a modulator  210  according to the invention. This will be referred to in the following as the “loop embodiment”. The modulator comprises an input/output  212 , a polarisation divider (otherwise known as a polarisation splitter)  214  having a first light path  216  and a second light path  218  and a Faraday rotator  220  driven by a modulating power source  222 . The polarisation divider  214  is coupled to the Faraday rotator  220  by coupling paths  224  and  226 . The Faraday rotator  220  comprises a Faraday material  228  surrounded by a coil  230 . Ends of the coil are connected to the modulating power source  222 . 
   The polarisation divider  214  may be an integrated device such as that described in Journal of Lightwave Technology, November 1993, volume 11, number 11, pages 1806 to 1811. 
     FIGS. 3   a  and  3   b  show operational states of the modulator  210  of  FIG. 2 . In both of these states, CW light is supplied by a laser located at the hub  112  and enters the modulator  210 . The light enters the input/output  212  and is divided according to its polarisation by the polarisation divider  214 . Light of one polarisation travels along the light path  216  and light of an orthogonal polarisation travels along the light path  218 . The two light components enter the coupling paths  224  and  226  and then pass through the Faraday material  228  in opposite directions. The state of the light produced by the modulator  210  depends on whether the Faraday material is energised by the coil  230  as will now be described. 
   Referring now to  FIG. 3   a,  if the modulating power source  222  is not applying a driving current to the coil, no magnetic field is produced and no Faraday rotation is applied to the light passing through the Faraday material  228 . In this case, the light components return to the polarisation divider and attempt to travel along light paths  216  and  218  which each previously allowed the transmission of a light component having an orthogonal polarisation with respect to the returned light components. In this case, the light components cannot travel along the light paths and thus the light components are either stopped or are refracted away from the input/output so that they are not output by the modulator  210 . Therefore, no (or at least very little) light is produced by the modulator  210  and thus a “0” signal is produced. 
   Referring now to  FIG. 3   b,  the modulating power source  222  is applying a driving current to the coil  230  and a magnetic flux density B is generated which extends into the Faraday material  228 . Accordingly, the light components have their planes of polarisation rotated as they pass through the Faraday material  228 . It is arranged so that the length of the Faraday material, its Verdet constant, V, that is its rotation power per unit length per magnetic flux density, and the magnetic flux density B are such that the planes of polarisation of the light components each undergo a rotation of 90° as they pass through the Faraday material  228 . In this case the light components leaving the Faraday material  228  return to the polarisation divider having planes of polarisation which are appropriate to allow them to travel along light paths  216  and  218 . Therefore, the light components are able to travel along the light paths  216  and  218  and are thus recombined at the input/output  212  so that they are output by the modulator  210 . In this case, the modulator  210  produces a “1” signal. 
   Of course, the modulator can be arranged so that it produces a “0” signal when there is power from the modulating power source  222 , and a “1” signal when there is no power from the modulating power source  222 . 
     FIG. 4  shows a modulator  410  according to the invention. This will be referred to in the following as the “mirror embodiment”. The modulator comprises an input/output  412 , a polarisation divider  414  having a first light path  416  and a second light path  418  and a Faraday rotator  420  driven by a modulating power source  422 . The polarisation divider  414  is coupled to the Faraday rotator  420  by coupling paths  424  and  426 . The Faraday rotator  420  comprises a Faraday material  428  surrounded by a coil  430 . Ends of the coil are connected to the modulating power source  422 . 
     FIGS. 5   a  and  5   b  show operational states of the modulator  410  of  FIG. 4 . In both of these states, CW light is supplied by a laser located at the hub  112  and enters the modulator  410 . The light enters the input/output  412  and is divided according to its polarisation by the polarisation divider  414 . Light of one polarisation travels along the light path  416  and light of an orthogonal polarisation travels along the light path  418 . The two light components travel along the coupling paths  424  and  426  and then enter through a front face  432  of the Faraday material. A mirror  434  is located at a rear face  436  of the Faraday material  428  opposite to the front face  432 . The two light components pass through the Faraday material  428  and then are reflected by the mirror  434  and pass back through the Faraday material  428 . The state of the light produced by the modulator  410  depends on whether the Faraday material  428  is energised by the coil  430  as will now be explained. 
   Referring now to  FIG. 5   a,  if the modulating power source  422  is not applying a driving current to the coil  430 , no magnetic field is produced and no Faraday rotation is applied to the light passing through the Faraday material  428 . In this case, the light components leaving the Faraday material  428  return to the polarisation divider  414  having planes of polarisation which are appropriate to allow them to travel along light paths  416  and  418 . Therefore, the light components are able to travel along the light paths  416  and  418  and are thus recombined at the input/output  412  so that they are output by the modulator  410 . In this case, the modulator  410  produces a “1” signal. 
   Referring now to  FIG. 5   b,  the modulating power source is applying a driving current to the coil  430  and a magnetic flux density B is generated which extends into the Faraday material  428 . Accordingly, the light components have their planes of polarisation rotated as they pass through the Faraday material  428 . It is arranged so that the length of the Faraday material, its Verdet constant, V and the magnetic flux density B are such that the planes of polarisations of the light components each undergo a rotation of 45° as they pass through the Faraday material  428 . In this way, there are two passes, and thus the planes of polarisation are rotated by a total of 90°. In this case the light components return to the polarisation divider  414  and attempt to travel along light paths  416  and  418  which each previously allowed the transmission of a light component having an orthogonal polarisation with respect to the returned light components. Therefore, the light components cannot travel along the light paths  416  and  418  and thus the light components are either stopped or are refracted away from the input/output  412  so that they are not output by the modulator  410 . In this case, no (or at least very little) light is produced by the modulator  410  and thus a “0” signal is produced. 
   Of course, the modulator can be arranged so that it produces a “0” signal when there is no power from the modulating power source  422 , and a “1” signal when there is power from the modulating power source  422 . 
   It should be noted that in the loop embodiment of  FIG. 2 , a “0” signal is produced when the Faraday material  228  is not energised and a “1” signal is produced when the Faraday material  228  is energised whereas in the mirror embodiment of  FIG. 4 , a “1” signal is produced when the Faraday material  428  is not energised and a “0” signal is produced when the Faraday material  428  is energised. 
   It will be understood by those skilled in the art that, for each modulator, a lens is used to couple the light from its transmission medium, typically an optical fibre, to the input/output. 
   Although it is possible that the planes of polarisation of light travelling in the modulators may be unintentionally rotated, for example while light components are travelling in the polarisation dividers, along the coupling paths or reflected by the mirror in the mirror embodiment, this can be compensated for by defining power settings of the modulating power control which provide maximum and minimum attenuation of the light and using these power settings for modulating “1” and “0” signals respectively. Of course, the power settings for modulating “1” and “0” signals can be set so that they provide other than maximum and minimum attenuation. 
   In order to provide a compact size, these modulators are fabricated as integrated optical circuits. Of course, it is not essential for all of the modulator to be fabricated on an integrated optical circuit and certain parts, such as the lenses, may be provided as bulk optics. In this case, various of the elements such as the polarisation dividers and the Faraday material in the Faraday rotators may be provided with anti-reflection coatings to minimise back reflection. Other parts, such as the coil, may be fabricated separately. 
   These modulators can be fabricated to have an insertion loss of less than 1 dB and possibly as low as 0.5 dB. 
   A significant advantage of these modulators is that they can be driven at high rates, such as 100 Mbit/s or even in the region of 1 Gbit/s. In order to provide very high rates, rather than using a single multi-coil, in another embodiment, a number of separate coils having a single turn or a small number of turns is used. In this way, the modulating power source drives a number of coils having small inductances rather than a single multi-turn coil having a relatively large inductance. 
   Although digital modulators have been described, in another embodiment of the invention the modulators can be used to provide analogue modulation. Since the amount of light extinction provided by the modulators depends on the amount of rotation imparted to the light components, which is, in turn, dependent on the current flowing in the coils, then analogue signals can be provided by controlling the amount of driving current provided by the modulating power supplies. Intermediate polarisation rotation may be provided in order to provide intermediate signal states between “0” and “1”, for example for analogue signals, because the polarisation state depends linearly on the magnetic field and thus linearly on the driving current. 
   An advantage of the mirror embodiment over the loop embodiment is that the rotation efficiency is double because the light goes twice through the Faraday rotator  420 . This means that in the mirror embodiment the thickness of the Faraday material  428  or value of the magnetic flux density B needs to be only half of the equivalent amount necessary in the loop embodiment. 
   In each of the embodiments, the polarisation divider enables the modulator to modulate light having a randomly orientated plane of polarisation. 
   Particular implementations and embodiments of the invention have been described. For example, rather than using a broadband light source in the users or ONUs, wavelength stabilized lasers may be used. Rather than using a CW light source, a non-CW light source may also be used such as a very fast modulated transmitter or a pulse source. It is clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention. The scope of the invention is only restricted by the attached patent claims.