FIELD  
       [0001]     This invention provides a control for a tunable laser particularly for use in telecommunication systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Tunable lasers for use in optical communications systems, particularly in connection with wavelength division multiplex (WDM) telecommunication systems, are known. A known tunable system comprises stacks of single wavelength distributed Bragg reflectors (DBR) lasers, which can be individually selected; or tuned over a narrow range, or by a wide range tunable laser that can be electronically driven to provide the wavelength required. Limited tuning range tunable lasers that rely upon thermal effects for tuning are also known.  
         [0003]     Atypical optical fibre telecommunications band is the 1550 nm C-band, located in the infra-red spectrum with International Telecommunication Union (YrU) 200, 100 or 506 GHz channel spacing (the so-called ITU Grid) spread between 191 THz and 197 THz.  
         [0004]     U.S. Pat. No. 4,896,325 discloses a wavelength tunable laser having sampled gratings at the front and rear of its gain region. The laser described in that specification relies on the use of two different gratings which produce slightly different reflection combs in the front and rear gratings. These provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Co-incidence of a maximum from each of the front and rear gratings is referred to as a supermode. To switch the device between supermodes requires a small incremental electrical current into one of the gatings to cause a different pair of maxima to coincide in the manner of a vernier. By applying electrical currents to the two gratings so that the corresponding maxima track, continuous tuning within a supermode can be achieved.  
         [0005]     Alternative forms of electrically tunable comb reflection gratings are commonly known as superstructure gratings such as described in “Broad-Range Wavelength-Tunable Superstructure Grating (SSG) DBR Lasers”, Y Thomori et al, IEEE Photonics Technical Letters, Volume 5, No. 6, July 1993.  
         [0006]     The applicant&#39;s co-pending UK application 0106790.9 discloses a tunable laser having a gain section, a phase section and a segmented Bragg grating reflector comprising a series of grating units, at least two of which have a different pitch, wherein an electrical current is applicable to the grating having a longer pitch such that the wavelength of the longer pitch grating can be tuned to the wavelength of the shorter pitch grating.  
         [0007]     Another tunable laser is disclosed in the applicant&#39;s copending UK application 0118412.6 in which there is disclosed a tunable laser including a gain section bounded at one end by a first reflector in the form of a distributed Bragg grating reflector producing a first comb of reflective peaks and on the other end by a second reflector in the form of a segmented distributed Bragg grating reflector with each segment capable of producing a peak corresponding to one of the peaks in the first comb, some at least of the segments in the segmented distributed Bragg grating reflector being capable of modification so as to reflect light at the wavelength of another peak so as to form a reinforced peak, so that the laser is capable of lasing at the wavelength of the reinforced peaks.  
         [0008]     The segmented distributed Bragg grating reflector comprises a plurality of segments, typically eight or nine, each comprising gratings, the different segments having a grating which may be each of different pitch and their preferred organisation being the longest pitch is at the facet end of the reflector, and the shortest pitch is closest to the gain section, with progressive pitch size change therebetween.  
         [0009]     The segments are modified to change the reflecting wavelength by altering the refractive index of the material of which the segments are formed, and the refractive index may be varied by passing an electrical current through the segments. Each segment has an associated electrode to permit the passage of current through the segment arid each electrode is actuable independently of the other electrodes.  
         [0010]     The common method of controlling multi-section lasers is to provide one digitally controlled current source per electrode and it is common to implement each current source digitally using a digital-to-analogue-converter (DAC). However for a tunable laser having multiple sections and a plurality of independently actuable grating segments, to provide one digitally controlled DAC per section or segment has major disadvantages in circuit size, control complexity and consequential costs.  
         [0011]     It is an object of the present invention to provide a suitable control for a tunable laser having a segmented distributed Bragg grating reflector, which ameliorates these problems.  
       STATEMENT OF THE INVENTION  
       [0012]     According to the invention there is provided a control for a tunable laser having a gain section, a comb reflection section, a phase change section and a segmented Bragg grating reflector section comprising a series of grating units each of a different pitch, and an electrode associated with each grating so that an electrical current is applicable to each individual grating, wherein the control includes a plurality of controlled. sources of electrical current which are each connected to switch means operable to independently connect each of said sources to a respective grating being one of a subset of consecutive gratings selected from said series of grating units.  
         [0013]     Each subset comprises ‘N’ gratings and each source may be connected to a respective switch means having a plurality of output terminals each being respectively connectable to every Nth gratings in said series of grating units. The respective switch means preferably each comprise a multi-channel, preferably four channel, multiplexer. The multiplexers may be each controlled by an embedded controllers, preferably a micro-processor, digital signal processor, or look-up table, operable via an interface means; preferably a N-wire interface having a maximum of 2*n (two to the power of n) output channels, or a serial control interface with encoded address data, that defines the active path within the multiplexer.  
         [0014]     Also according to the invention, there is provided a tunable laser having a gain section, a phase change section and a segmented Bragg grating reflector section comprising a series of grating units each of a different pitch, and an electrode associated with each grating so that an electrical current is applicable to each individual grating, and a control according to the first aspect of the present invention, the control activating at least one of a subset of consecutive gratings so as to reflect light at a wavelength of another peak to form a reinforced peak so that the laser is capable of lasing at the wavelength of the reinforced peak  
         [0015]     In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 700 nanometres (nm) and 3000 nm.  
         [0016]     Preferably the gain section of the laser is bounded at one end by a segmented Bragg grating reflector and its other end by a phase change section, which on its other side is, bounded by comb refection section, conveniently in the form of a distributed Bragg grating reflector producing a comb of reflective peaks.  
         [0017]     Preferably, the distributed Bragg grating reflector is a phase grating distributed Bragg reflector of the type disclosed in GB 2337135.  
         [0018]     Alternatively, the distributed Bragg grating reflector is a superstructure grating reflector, of the type described in “Broad-Range Wavelength Tunable Superstructure Grating (SSG) DBR Lasers”, Y Thomori et al., IEEE Photonics Technical Letters, Volume 5, No. 6, July 1993.  
         [0019]     Alternatively, the distributed Bragg grating reflector is a sampled grating Bragg reflector of the type described in U.S. Pat. No. 4,896,325.  
         [0020]     In a preferred embodiment, the Bragg grating segments are each kept short so that the cumulative waveguide length associated with the Bragg gratings is minimised, thereby keeping the waveguide attenuation losses low. With short Bragg grating segments the associated reflectivity of each is broad and this causes a degree of overlap between neighbouring segment reflectivity. However, by appropriate design the segment reflectivity discrimination can be sufficient for stable lasing operation in combination with reduced light power losses.  
         [0021]     The grating comprising the distributed Bragg grating reflector is made long so that its comb reflection peaks have finesse and constitute well defined wavelengths. These comb wavelengths are typically centred on channels in the communications band of interest.  
         [0022]     Electrical current passing through the distributed Bragg reflector alters the wavelengths at which the comb of reflecting peaks reflects.  
         [0023]     Electrical current passing through the phase change section alters the refractive index of the material of the phase change section to effect the phase and thereby minimise mode hoping.  
         [0024]     Tunable lasers are&#39;suitable for use in telecommunications systems, for example, in the C-band, namely within the band 1530 nm to 1570 nm.  
         [0025]     According to another aspect of the invention there is provided a method of control for a tunable laser having a gain section, a phase change section and a segmented Bragg grating reflector section comprising a series of grating units each of a different pitch, with an electrode associated with each grating so that an electrical current is applicable to each individual grating to vary its refractive index, wherein said method includes providing a plurality of controlled sources of electrical current, connecting the sources to switch means, and operating the switch means to independently connect each of said sources to one of a subset of consecutive gratings selected from said series of grating units.  
         [0026]     Preferably the laser is provide with a comb reflection section and passing an electrical current through the comb reflection section alters the wavelengths at which the comb of its reflecting peaks reflects.  
         [0027]     Preferably, the laser emits light through the segmented distributed Bragg grating reflector and end facet. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0028]     The invention will be described by way of example and reference to the accompanying drawings in which:-  
         [0029]      FIG. 1  is a schematic cross section through a laser suitable for operation in association with a control according to the present invention; and  
         [0030]      FIG. 2  is a schematic drawing of a laser as shown in  FIG. 1  having a first control system according to the present invention; and  
         [0031]      FIG. 3  is a schematic drawing of a laser as shown in  FIG. 1  with a second control system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The invention will be described by reference to a tunable laser as is described in UK patent application 0118412.6, which will be described herein only as is sufficient for an understanding of the present invention.  
         [0033]     Common reference numbering has been used across all Figures for drawing elements with equivalent functionality.  
         [0034]     Referring to  FIG. 1 , this shows a schematic cross sectional view of a laser in accordance with the present invention. As is conventional in semiconductor lasers the laser is built up in a series of layers, with an active layer  1  formed between a lower layer  2  and an upper layer  3 . There will typically be a plurality of layers in the structure, but they are not material to the invention and for better understanding of the invention they are not shown. The active layer  1  also acts as a waveguide.  
         [0035]     The laser itself has four principal sections. A gain section  4 , a phase change section  5  and front and rear reflecting section  6  and  7  respectively. The rear reflecting section  7  has a phase grating distributed Bragg grating reflector  8  formed in the layer  3 . Such a reflector produces a comb of reflectance peaks at separated wavelengths, and each peak is of substantially the same amplitude. The front reflector  6  is made up of a series of segments  9 - 17 , each segment being a distributed Bragg grating reflector unit, but each segment reflecting at nominally a single wavelength only. The wavelengths of the individual peaks of the segments  9  to  15 , and  17 , corresponding to one of the peaks of the comb reflectance produced by the front reflector Bragg grating reflector.  
         [0036]     A common electrode  18  acts as the electrical return for the semiconductor laser device.  
         [0037]     Each of the sections  4 ,  5 ,  7 , and segments  9 - 17  is provided with an associated electrode  4   a,    5   a,    7   a,    9   a - 17   a  through which electric current can be applied to the respective section or segment. Grating unit  17  does not of necessity have to have an associated electrode  17   a,  as its function is to form a lowest wavelength (band edge) down to which the next highest wavelength unit may be tuned.  
         [0038]     The material of which layer  3  is formed in the sections corresponding to the reflectors  6  and  7 , is such as to experience a reduction in refractive index when an electrical current is passed through it—the so-called electrorefractive effect. When light passes through a medium of refractive index n eff  the actual wavelength of the light within that medium, which will be referred to herein as λ 1 , is the wavelength λ divided by the value for the refractive index n eff . In other words:- 
 
λ 1   =λ/n   eff    (1) 
 
 where n eff  is the effective refractive index of the medium as seen by the propagating light of wavelength λ in free space. 
 
         [0039]     Thus if, for example, a current is passed through the electrode  12   a,  the wavelength at which the Bragg reflector in segment  12  reflects light will be reduced. If the reduction is such that it now reflects light at the-same wavelength as the Bragg grating reflector of the adjacent lower wavelength segment, segment  1 I, then the intensity of the peak for segment  11  is reinforced such that it is much higher and more intense than the other reflection peaks.  
         [0040]     Now, however, if a current is passed through the electrode  4   a  to create light in the gain section  4 , at an intensity above the lasing threshold, the light at the wavelength corresponding to the reinforced intensity segment peak is preferentially operable and the laser will commence to lase at that wavelength. Thus the laser will be tuned to is that wavelength. If a current is now passed through electrode  7   a  this will effectively move the whole comb of peaks for that reflector to lower wavelengths. If at the same time a current is passed through electrode  11   a  and the current passing through electrode  12   a  are increased, then the reinforced peak will also move to a lower wavelength.  
         [0041]     With reference now to  FIG. 2 , in order to supply an electrical current to different sections and selected pairs of adjacent Bragg grating segments a control arrangement  20  is required. Control arrangement  20  provides one digitally controlled current source  21 ,  22 ,  23  per electrode  4   a,    5   a,    7   a  and each current source is controlled digitally using a respective DAC  25 . An exemplary DAC is supplied by Analog Devices Inc., type AD5324, which is a quad 12 bit DAC.  
         [0042]     Since in the segmented distributed Bragg reflector section  6 , of  FIG. 1 , only two adjacent grating segments e.g.  11  and  12 , are activated at any one time this section is operated through only two current sources  31  and  32 , which are again each implemented using a respective DAC  25 . The two digitally controlled sources of electrical current  31  and  32  are each connected to switch means  33 . The switch means  33  is connected to the electrodes  9   a - 16   a  and is operable to independently connect each of said sources  31  and  32  to one of two adjacent gratings e.g.  11  and  12 , selected from said series of gratings  9 - 16  A suitable switch  33  may be a linear commutator. The switch means  33  may be operated by a control  34 , which one of ordinary skill will appreciate may be from a programmable controller (not shown) which also controls the DACs  25 .  
         [0043]     With reference now to  FIG. 3 , each current source  31  and  32  is connected to one of a pair of 4-way multiplexers  35  and  36  respectively. As an example, a suitable multiplexer is a MAX4634 available from Maxim Integrated Products. Each multiplexer  35 ,  36  has four outputs that are singularly connected to alternate electrodes of the segmented Bragg grating section as shown in  FIG. 3 , i.e.  35  is connected to alternate electrodes  9   a,    11   a,    13   a,    15   a  and  36  is connected to alternate electrodes  10   a,    12   a,    14   a  and  16   a,  so that pairs of adjacent segments can be concurrently activated with current. Each multiplexer  35 ,  36  is controlled via a 2-wire interface (A 0  and A 1 ) that defines the active path within the multiplexer and hence which segment receives current. This interface may be provided with an embedded micro-controller  38 . To change the laser wavelength and/or channel the micro-controllers  38  configure the multiplexers according to Tables 1 and 2 below:-  
                                             TABLE 1                                       ACTIVE SWITCH            0/P   1   3   5   7               A 0     L   H   L   H       A 1     L   L   H   H                  
 
         [0044]    
       
         
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                   
               
               
                   
                 ACTIVE SWITCH 
               
             
          
           
               
                 0/P 
                 2 
                 4 
                 6 
                 8 
               
               
                   
               
               
                 A 0   
                 L 
                 H 
                 L 
                 H 
               
               
                 A 1   
                 L 
                 L 
                 H 
                 H 
               
               
                   
               
             
          
         
       
     
         [0045]     In Table 1 and 2 L=Logical Low, H=Logical High, and output (O/P) numbers correspond with those given on  FIG. 3 .  
         [0046]     The two multiplexers therefore have unique switch settings for adjacent pairs of activated Bragg grating segments as is shown in Table 3 below:-  
                                                         TABLE 3                                       MULTIPLEXER 35       MULTIPLEXER 36                PAIR   A 0     A 1     A 0     A 1                 1 &amp; 2   L   L   L   L       2 &amp; 3   H   L   L   L       3 &amp; 4   H   L   H   L       4 &amp; 5   L   H   H   L       5 &amp; 6   L   H   L   H       6 &amp; 7   H   H   L   H       7 &amp; 8   H   H   H   H                  
 
         [0047]     In Table  3 , L=Logical Low, H=Logical High  
         [0048]     The multiplexer micro-processors  3   8 , and the DACs  25 , are controlled by a programmable controller, not shown, to produce the required operating wavelength.  
         [0049]     In many telecommunication applications the tunable laser will be required to operate on ITU Grid channels for which a look-up table, not shown, may be used to set the control means to the required condition, and the laser thereby switched between channels. Alternative applications may require the laser to be continuously tunable for which applications the programmable controller may be dynamically driven using variable controls not shown, but of an obvious nature to those of ordinary skill in the art.
A control ( 20) and a method of controlling a tunable laser having a gain section ( 4), a phase change section ( 5) and a segmented Bragg grating reflector section ( 6) comprising a series of grating units ( 9 - 17) each of a different pitch, and an electrode ( 9 a - 17 a) associated with each grating so that an electrical current is applicable to each individual grating. The control ( 20) includes a plurality of digitally controlled sources ( 31, 32) of electrical current which are each connected to switch means ( 33 or 35, 36) operable to independently connect each of said sources ( 31, 32) to a respective electrode (e.g. 11 a or 12 a) associated with a grating (e.g. 11 or 12) which is one of a subset of consecutive gratings (e.g. 11 & 12) selected from said series of grating units ( 9 - 17).