Abstract:
The optical power in a waveguide of an optical circuit is monitored by mounting an optical sensor, such as a photodiode, laterally apart from the waveguide but sufficiently close to the waveguide to detect light emerging laterally from the waveguide, and by receiving signals from the sensor that are representative of the optical power. In a DWDM circuit, a bandpass filter is placed between the waveguide and the sensor for monitoring only one of the wavelengths carried by the waveguide. To minimize crosstalk, the monitored portions of the waveguides are isolated from each other, for example by trenches or by optically absorptive barriers. Suitably calibrated processing of signals from several sensors that monitor several waveguides eliminates crosstalk.

Description:
[0001]    This is a continuation in part of U.S. provisional patent application Ser. No. 60/380,255 filed May 15, 2002. 
     
    
     
       FIELD AND BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to integrated optical circuits and, more particularly, to a method of monitoring optical power in a planar lightwave circuit Is (PLC) chip and to the circuit so monitored.  
           [0003]    Optical power and signal monitoring is an essential part of any system employing routing of optical signals. Currently the largest foreseen application is in dense wavelength division multiplexing (DWDM) optical networks as used in data communication and in telecommunication. In order to operate the network and maintain its quality of service, the network management needs to continuously monitor both power levels and signal integrity in many places along the network. Since any such DWDM optical network is built to carry many wavelengths (typically more than 40 wavelengths), monitoring of all these channels in a reliable and cost-effective way becomes a very important and complicated task.  
           [0004]    Other applications for optical communication and PLC devices are anticipated in the near future. Optical routers and optical computing will inevitably use the same principles for extracting part of the optical signal for monitoring and for similar uses.  
           [0005]    Current available solutions for power monitoring involve using discrete splitters introduced in a main optical path (optical fiber), to divert part of the passing light to a monitor channel where the light is dispersed to its different wavelengths and monitored.  
           [0006]    Alternatively part of the signal on each channel can be tapped after the signal passes a dispersion element, such as an arrayed waveguide grating (AWG) already present in the network. These elements usually are fabricated within a PLC chip, so that the light extraction is done within the chip, and the extracted light needs to be coupled out of the chip and into a light detector. The most common way of doing this coupling is by routing a waveguide to the edge of the chip, where the waveguide is coupled to an optical fiber. The other end of the fiber is connected to a photodetector.  
           [0007]    These prior art monitoring methods all require the fabrication of additional elements within the monitored optical circuit itself. This adds significantly to the cost of the PLC chip. In addition, it is inherent to these methods that light that otherwise would be used for the optical circuit&#39;s primary purpose is diverted for the purpose of monitoring optical power. There is thus a widely recognized need for, and it would be highly advantageous to have, a method of monitoring the optical power in a PLC without fabricating additional elements within the PLC itself and without explicitly diverting light from the circuit.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the present invention there is provided an optical circuit including: (a) a waveguide; and (b) an optical sensor, laterally displaced from the waveguide and sufficiently close to the waveguide to detect light emerging laterally from the waveguide.  
           [0009]    According to the present invention there is provided a method of monitoring optical power in a waveguide embedded in an optical layer of an optical circuit, including the steps of: (a) mounting an optical sensor laterally apart from the waveguide and sufficiently close to the waveguide to detect light emerging laterally from the waveguide; and (b) receiving signals, from the optical sensor, that are representative of the optical power.  
           [0010]    According to the present invention there is provided a method of monitoring optical power in a plurality of waveguides embedded in an optical layer of an optical circuit, including the steps of: (a) for each waveguide, mounting a respective optical sensor laterally apart from the each waveguide and sufficiently close to the each waveguide to detect light emerging laterally from the each waveguide; and (b) for each waveguide, receiving respective signals, from the respective optical sensor, that are representative of the optical power in the each waveguide.  
           [0011]    The present invention exploits the fact that PLC waveguides are inherently lossy.  
           [0012]    In a PLC, an optical circuit is created on the surface of a planar wafer using thin film deposition techniques similar to those used in the field of microelectronics.  
           [0013]    The light propagates in waveguides that have cross sections of a few microns and that are located close to the upper surface of the wafer, typically within 20 microns of the upper surface of the wafer. Any such waveguide has an inherent loss, typically at least 0.01 dB/cm, implying that a small fraction of the light that propagates within the waveguide is scattered per unit length along the waveguide and emerges laterally from the waveguide.  
           [0014]    The basic idea of the present invention is to mount an optical sensor on the upper surface of the wafer to intercept some of the light that emerges laterally from the waveguide. By being mounted on the upper surface of the wafer, the optical sensor is laterally displaced from the waveguide, meaning that the optical sensor is off to the side of the waveguide, and receives only light that emerges laterally from the waveguide, and not light that emerges from the end of the waveguide after propagating via the waveguide. Nevertheless, the optical sensor is close enough to the waveguide to detect the light that emerges laterally from the waveguide. The optical sensor responds to the light that it intercepts by generating signals that are representative of the optical power carried by the waveguide.  
           [0015]    With the optical sensor mounted on the upper surface of the wafer, the optical sensor is vertically displaced from the waveguide, with the “vertical” direction being defined by the wafer geometry, and in particular by the optical layer in which the waveguide is embedded.  
           [0016]    Preferably, the optical sensor is a photodiode.  
           [0017]    Preferably, the optical sensor is laterally displaced from a structure of the waveguide, for example, from a bend in the waveguide, from a gap in the waveguide, from an intersection of the waveguide with another waveguide, or from a scatterer in the waveguide.  
           [0018]    Preferably, the optical sensor is at most 50 microns away from the waveguide. Preferably, the optical sensor subtends an angle of at least about 124 degrees relative to the waveguide. Most preferably, the optical sensor subtends an angle of at least about 165 degrees relative to the waveguide.  
           [0019]    Typically, the waveguide is fabricated by suitable doping of layer of a material that is optically transparent to the light that is carried by the waveguide. Examples of such materials include, for example, Silicon-based glasses such as SiO 2  and SiON, Silicon, Lithium Niobate and Indium Phosphide. Preferably, the optical sensor is mounted in a depression in the optical layer in which the waveguide is embedded.  
           [0020]    Preferably, a bandpass filter, for example a grating or an interference filter, is placed between the waveguide and the optical sensor, to filter the light that emerges laterally from the waveguide.  
           [0021]    Preferably, an interface is provided for connecting the optical sensor to an external electrical circuit. In one embodiment of the present invention, the interface includes an electrical conductor deposited on the optical layer in which the waveguide is embedded. In another embodiment of the present invention, the interface is adapted to connect the optical circuit to an external electrical circuit on a printed circuit board.  
           [0022]    Preferably, the sensor is isolated from crosstalk, for example by means of one or more trenches, parallel to the waveguide, in the optical layer in which the waveguide is embedded, possibly with a metal deposited therein, or by means of an optically absorptive, most preferably metallic, barrier that at least partly surrounds the optical sensor.  
           [0023]    Preferably, when several optical sensors are used to monitor optical power in several waveguides in the same optical circuit, the signals from the optical sensors are processed in a manner that compensates for crosstalk. Most preferably, this processing relies on crosstalk coefficients, representative of the crosstalk among the optical sensors, that are measured in a prior calibration step. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0025]    [0025]FIG. 1 is a schematic cross section of a PLC of the present invention;  
         [0026]    [0026]FIG. 2 is a plan view of a PLC showing four kinds of waveguide structures;  
         [0027]    [0027]FIG. 3 is a plan view of another PLC of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    The present invention is of a method of monitoring optical power propagating via the waveguides of an optical circuit.  
         [0029]    The principles and operation of optical power monitoring according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0030]    Referring now to the drawings, FIG. 1 is a schematic cross section of a PLC  10  configured according to the present invention with photodiodes  22 ,  24  and  26  for monitoring optical power propagating via respective waveguides  16 ,  18  and  20 . PLC  10  consists of a substrate  12  above which is deposited an optical layer  14 . Optical layer  14  is made of a material that is transparent to the light that propagates via waveguides  16 ,  18  and  20 . Waveguides  16 ,  18  and  20  are formed in optical layer  14  by doping optical layer  14  locally to raise the local index of refraction. Typical materials that are used for substrate  12  and optical layer  14  include Silicon-based glasses such as SiO 2  and SiON, Silicon, Lithium Niobate and Indium Phosphide. Note that the cross section illustrated in FIG. 1 is transverse to waveguides  16 ,  18  and  20 . The geometry of optical layer  14  defines a vertical direction  50 , so that photodiodes  22 ,  24  and  26  are vertically displaced from their respective waveguides  16 ,  18  and  20 .  
         [0031]    Optical layer  14  typically is  40  microns thick, or thinner, and photodiodes  22 ,  24  and  26  typically are  300  microns wide, as shown. With waveguide  16  running through the middle of optical layer  14 , so that the center of waveguide  16  is  20  microns below photodiode  22 , this means that photodiode  22  subtends an angle θ of  165  degrees relative to waveguide  16 , in order to intercept a substantial fraction of the light emerging locally from waveguide  16 . Alternatively, photodiode  22 ,  24  or  26  could be only 75 microns wide, and so subtend an angle of only 124 degrees relative to waveguide  16 ,  18  or  20 .  
         [0032]    Photodiodes  22 ,  24  and  26  are representative of optical sensors generally. The scope of the present invention includes the use of any suitable optical sensor. In general, the separation between the optical sensor and the waveguide that it monitors preferably is at most about 50 microns, to ensure that the optical sensor intercepts a sufficient portion of the light that emerges laterally from the waveguide. The optical sensor is mounted directly on the upper surface of optical layer  14  without any special preparation of the upper surface of optical layer  14 , as illustrated in FIG. 1 for photodiode  22 . Alternatively, the optical sensor is mounted in a shallow depression in optical layer  14 . Photodiode  24  is illustrated in FIG. 1 as mounted in a shallow depression  28  in optical layer  14 .  
         [0033]    Preferably, the optical sensor is mounted in close proximity to a structure in the monitored waveguide at which lateral leakage of the light propagating via the waveguide is enhanced. FIG. 2 shows plan views of four such structures in waveguides in optical layer  14 . FIG. 2A shows a bend  34  in a waveguide  32 . FIG. 2B shows a gap  38  in a waveguide  40 . FIG. 2C shows two waveguides  42  and  44  crossing at an intersection  43 . FIG. 2D shows a scatterer  46  in a waveguide  48 . Some of these structures, such as bend  34  and intersection  43 , typically are present in PLC  10  anyway. Other structures, such as gap  38  and scatterer  46 , are introduced deliberately to locally enhance the leakage of light from the monitored waveguide.  
         [0034]    Returning to FIG. 1, a bandpass filter  32  is shown between waveguide  20  and photodiode  26 . The purpose of bandpass filter  32 , in a DWDM context, is to select only one of the wavelengths, that propagate in waveguide  20 , for monitoring by photodiode  26 . Bandpass filter  32  is realized by directing two crossed coherent ultraviolet beams on the portion of optical layer  14  that is to be modified. The resulting periodic interference pattern produces a corresponding change in the local index of refraction of optical layer  14 . Bandpass filter  32  is illustrated as being adjacent to waveguide  20 ; but bandpass filter  32  could be formed anywhere between waveguide  20  and the upper surface of optical layer  14 . Alternatively, bandpass filter  32  is realized as an interference filter by forming a stack of thin films in optical layer  14  above the portion of waveguide  20  that is to be monitored. Alternatively, bandpass filter  32  is realized as an external device between optical layer  14  and photodiode  26 .  
         [0035]    Also shown in FIG. 1 are two electrically conductive leads  52  and  54  for connecting photodiodes  22  and  26 , respectively, to an external electrical circuit. Leads  52  and  54  are part of a metal layer that is deposited on optical layer  14 , for example to drive optical gates that are part of the optical circuit. Alternatively, photodiodes  22 ,  24  and  26  are interfaced to an external electrical circuit in an external printed circuit board via bumps or wire bonds that directly contact the outward-facing surfaces of photodiodes  22 ,  24  and  26 .  
         [0036]    Also shown in FIG. 1 is a pair of trenches  30  in optical layer  14  that flank waveguide  20 . The purpose of trenches  30  is to minimize crosstalk by scattering away light that emerges laterally from waveguides, such as waveguide  18 , that are not to be monitored by photodiode  26 , and that propagates towards waveguide  20  and photodiode  26  in optical layer  14 . To this end, trenches  30  are deep trenches that traverse the full thickness of optical layer  14 . Optionally, a metal (not shown) is deposited in trenches  30  to enhance the isolation of waveguide  20  from neighboring waveguides.  
         [0037]    [0037]FIG. 3 is a partial plan view of another PLC  10 ′, showing another mechanism for isolating a photodiode  58 , that is used to monitor a waveguide  56 , from crosstalk. The metal layer that is deposited above optical layer  14  includes a ring  60  that surrounds photodiode  58 . Metal ring  60  absorbs light that emerges laterally from adjacent waveguides and propagates towards waveguide  58 .  
         [0038]    Crosstalk is reduced further by appropriate processing of the signals from the photodiodes. To this end, it is necessary to calibrate PLC  10  by measuring the crosstalk between every waveguide and every photodiode. Assume a PLC  10  with M waveguides indexed by an index mε[1,M] and J photodiodes indexed by an index jε[1,f]. The “crosstalk coefficient” A mj  between waveguide m and photodiode j is defined as the signal obtained from photodiode j when waveguide m carries unit optical power and none of the other waveguides carry any optical power. It is assumed that the photodiodes are linear, so that the response of photodiode j to the optical power P m  carried by waveguide m is S j =A mj P mj . Calibrating PLC  10  consists of sending known optical power successively through each of the M waveguides and measuring the resulting signals from the J photodiodes. Then, when PLC  10  is used operationally, with each waveguide m carrying optical power P m , the signal S j  from photodiode j is:  
         S   j     =       ∑     m   =   1     M                       A   mj          P   m                               
 
         [0039]    If J&gt;M, this is a set of overdetermined equations, for the powers P m , that can be solved by standard methods. For example, if waveguide n is to be monitored by photodiode k, then to a first approximation (i.e., ignoring crosstalk), P n =S k /A nk . These approximate powers are the first estimates  
       P   m     (   1   )                           
 
         [0040]    in an iterative scheme that converges to the desired solution. In the q-th iteration,  
         P   n     (   q   )       =       1     A   nk            [       S   k     -       ∑     m   ≠   n                                 A   mk          P   m     (     q   -   1     )             ]                             
 
         [0041]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.