Abstract:
Compactness is preserved while enabling beam monitoring of optical properties of an output beam by employing a combination of reflection and diffraction. An input beam is reflected, divided using reflection/diffraction, and re-reflected. As a consequence, both a light source and one or more beam monitoring detectors may be disposed along a single side of an optical module. In one embodiment, an input beam is introduced from a first side of an optical module, is reflected by a 45 degree mirror, and is divided by a diffraction grating which redirects a minor portion of the beam energy back to the 45 degree mirror. Following the second reflection from the mirror, the returned portion of the beam is used to measure one or more optical properties.

Description:
BACKGROUND ART  
       [0001]     In many optical applications, modules are used to couple a light source to an optical fiber. The module may include one or more lenses that promote efficient coupling between the optical fiber and the light source. The light source may be formed of a succession of thin films on a semiconductor substrate, so as to define a Vertical Cavity Surface Emitting Laser (VCSEL). A VCSEL is a surface emitting laser. Another type of semiconductor laser used in telecommunications applications is referred to as an edge emitting laser, which may be further divided into subtypes that include Fabry Perot (FP) and Distributed Feedback (DFB) lasers.  
         [0002]     Particularly within the field of data communications via optical signals, consistency with respect to certain optical properties is important in assuring proper operations. The power output (i.e., the light intensity) must remain above a predetermined level. The wavelength of the signal may also be significant. Various factors will cause changes in the optical properties. For example, a change in the temperature of the environment in which a laser diode resides will affect the laser emission wavelength. As another example, the bias current of the laser controls its output power. The aging of a laser diode also may affect its power output.  
         [0003]     Techniques for monitoring and controlling properties of an output beam are known.  FIG. 1  shows a prior art approach to monitoring and controlling an output beam of an edge emitting laser diode  10 . The diode is shown as being mounted on a substrate  12 . The laser diode emits an output beam  14  from a front facet  16  and emits a monitoring beam  18  from a rear facet  20 . The output beam may be directed through optics  22 , such as a lens which provides beam collimation. The beam is reflected by a 45 degree mirror  24  to an optical fiber  26  that has an optical axis perpendicular to the substrate  12 . The 45 degree mirror may be used for applications in which the desired orientation of the beam from an edge emitting laser is to be the same as the conventional output beam orientation of a module that uses VCSELs.  
         [0004]     Within the path of the monitoring beam  18  from the rear facet  20  of the edge emitting laser  10  is a detector  28  that generates a signal indicative of power. Because there is a known ratio between the power of the output beam  14  and the power of the monitoring beam, the signal from the detector may be used to identify the output power to the fiber  26 . The electrical signal from the detector is directed to a controller  30  that is able to adjust the bias current of the laser  10 . Thus, the signal from the detector provides feedback for maintaining the laser in a constant output power state. While not shown, the controller may also receive a signal from a temperature sensor. Then, the controller may adjust operations of a thermo-electric cooling (TEC) device or a heating device.  
         [0005]     While the monitoring and controlling techniques described with reference to  FIG. 1  operate well for their intended purpose, there are concerns. For example, the known ratio of the power of the two beams  14  and  18  is less reliable with respect to maintaining the output power to the fiber  26  if the output beam  14  is manipulated in a manner different than the monitoring beam  18 . For example, in an Externally Modulated Laser (EML), the modulation which occurs for telecommunications or other applications does not affect the monitoring beam  18 . Thus, a feedback signal from the detector  28  will not show all fluctuations of output power to the fiber.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, a combination of reflection and diffraction is used to cause a monitoring beam portion to substantially retrace (subtend) angles followed by an input beam for which monitoring is of interest. An optical monitoring system includes a beam input that defines an input segment of a beam path. A reflection-inducing structure positioned along the input beam segment reflects light from the input beam segment to a reflected beam segment. A diffraction-inducing structure positioned along the reflected beam segment diffracts a minor portion of the light, so as to return to the reflection-inducing structure. The minor portion is again reflected and is directed to a detector which generates a signal indicative of an optical property of this diffracted and reflected beam portion. The major portion of the light energy is not reflected by the diffraction-inducing structure, but instead exits as the output beam.  
         [0007]     In one embodiment, the optical monitoring system is formed as an optical module. A front side of the optical module includes a beam input and at least one beam monitor output. An internal mirror has a substantially 45 degree angle relative to the front side. The internal mirror is positioned to be impinged by a beam propagated through the beam input. A lid of the optical module is substantially transparent with respect to the beam, so as to enable passage of the output beam to an optical fiber or the like. However, a diffractor is disposed within the output path of the beam in order to reflect the minor portion, which again impinges the internal mirror. The diffractor in effect optically couples the diffracted portion to each beam monitor output via the reflection at the internal mirror. A detector may be aligned with each beam monitor output.  
         [0008]     A method in accordance with the invention includes receiving the input beam, reflecting the input beam, transmitting a major portion of the reflected beam as an output signal and diffracting a minor portion such that a monitoring beam portion is directed rearwardly, reflecting the monitoring beam portion so as to subtend generally the same angle as the input beam, and detecting at least one optical property of the monitoring beam portion.  
         [0009]     In a power monitoring application, a single detector, such as an edge detector, may be aligned with a single beam monitor output at the front side of the module. The detector generates a signal indicative of the intensity of the diffracted portion of the original input beam, which may be generated by an edge emitting laser mounted on a same substrate as the edge detector. The signal may be used to determine the intensity of the output beam and to provide feedback control to maintain a constant output power. Alternatively, in a wavelength-locking application, two detectors may be used. The first detector may monitor total power of the output beam as in the power monitoring application. A second detector is aligned with the second beam monitoring output at the front side of the module and is configured to generate a signal that is strongly dependent on wavelength. For example, a wavelength-specific filter may be positioned in the path to the second detector. The output of the second detector may be used to control the wavelength of the light source. As one possibility, the wavelength control may be provided by dynamically adjusting the temperature of a laser that is used as the light source. The relationship between temperature and the emitted wavelength of a laser is known. Thus, the wavelength and power of a laser can be controlled by adjustments to the temperature and bias current of the laser. For an edge emitting laser, the “feedback” is determined from the front facet emission, rather than from light emission from the rear facet of the edge emitting laser. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross sectional representation of a prior art optical arrangement for monitoring and controlling power of a beam output.  
         [0011]      FIG. 2  is a perspective view of a module in accordance with one embodiment of the invention.  
         [0012]      FIG. 3  is a top view of the use of the present invention in a power monitoring application.  
         [0013]      FIG. 4  is a top view of the use of the present invention in a wavelength-locking application.  
         [0014]      FIG. 5  is a block diagram of components for enabling dynamic adjustments of optical properties of an output beam in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 2  shows one embodiment of an optical module  40  that enables feedback control of either or both of output power and emission wavelength (or other beam property), while retaining generally the same size requirements as a module without such a capability. Compactness is maintained by employing a combination of reflection and diffraction. In the embodiment shown in  FIG. 2 , a front side  42  of the module includes a beam input  44  and a pair of beam monitor outputs  46  and  48 . Typically, the front “side” is not defined by structure, since even a transparent component would have an effect on beam propagation (i.e., refraction). Rather, the optical elements for directing and redirecting light define the positions of the input and the outputs. Moreover, the components of  FIG. 2  may be only a subset of components of a more complete module, such as one that includes electrical components. The invention is considered to be well suited for use for a module that houses the components of  FIG. 2  and the light source  12 , as well as a light source and at least one detector.  
         [0016]     A light source, such as a laser, emits an input beam  50  that is collimated by a ball lens  52 . In other embodiments, the collimation is achieved using alternative means, such as other types of optical devices. The light source can be an edge emitting laser that is mounted on a substrate that supports the ball lens and other components of  FIG. 2 . The input beam  50  then represents the emission from the front facet of the laser.  
         [0017]     Following the collimation of the input beam  50  by the ball lens  52 , the light follows an input beam segment  54  of the path through the module  40 . A 45 degree mirror  56  is positioned such that the light is reflected upwardly to a reflected beam segment  58  of the path through the module.  
         [0018]     The reflected beam segment  58  of the beam path passes through a lid structure  60 . For embodiments in which the 45 degree mirror  56  and lid are housed in common with other optical and electrical components of a more complete module, the lid may be easily held at a fixed but spaced-apart position relative to the mirror  56 . The lid structure is transparent to the wavelength of the light source, so as to allow an output beam  62  to exit at an output  64  of the module  40 . As one possibility, the lid structure may be a silicon substrate for beam wavelengths of longer than 1 μm. While not shown, a lens may be placed at the output  64  of the module. The lens may be used to focus the beam  62  onto an optical fiber or other element.  
         [0019]     Within the beam path through the module  40  is a diffraction-inducing structure  66 , such as a diffraction grating. While the major portion of the input beam  50  propagates through the diffraction grating, a minor portion is directed rearwardly for a second reflection from the 45 degree mirror  56 . In the embodiment of  FIG. 2 , first and second diffracted beam portions  68  and  70  are reflected by the mirror for exit via the beam monitoring outputs  46  and  48 , respectively. In other embodiments, a single diffracted beam portion is utilized for optical monitoring. Also in the embodiment of  FIG. 2 , the 45 degree mirror is shown as a continuous structure. In other embodiments, the mirror may be segmented such that the input beam and the diffracted portions are directed to different segments.  
         [0020]     Each diffracted beam portion  68  and  70  is reflected at an angle on the power of the reflected light depends upon the incident beam power and the design of the grating. When using a grating, more than one beam of diffracted light will be reflected, as shown in  FIG. 2 . In the design of the grating, care should be taken to ensure that reflected power back to the laser is less than that which might affect operation of the laser. Lamellar gratings and blaze gratings are two of the available options.  
         [0021]      FIG. 3  illustrates an embodiment for monitoring power. A laser  72  directs an input beam through the ball lens  52 , which provides beam collimation. The input beam may be emitted from a front facet of an edge emitting laser. The light travels along the input beam segment  54  of the beam path and is reflected by the 45 degree mirror  56  upwardly along a reflected beam segment  58 . As described with reference to  FIG. 2 , the major portion of the beam provides the output, but a minor portion is reflected by the diffraction-inducing structure to provide the diffracted beam portion  68  shown in  FIG. 3 . This beam portion is again reflected by the 45 degree mirror to the beam monitor output  46 . The axis of the beam monitor output is generally along the same horizontal plane as the axis of the input beam. That is, the axis is parallel to the base of the 45 degree mirror. The beam portion from the diffraction-inducing structure subtends the same angles as the original beam, but with the Bragg&#39;s diffraction angle. Therefore, a monitoring device, such as an edge detector  74  mounted on the same substrate as the mirror and the laser, can be aligned to collect the energy of the diffracted beam portion. The intensity of the diffracted beam portion is dependent upon the intensity of the input beam from the laser  72 . Therefore, the signal generated by the monitoring device can be used to control the laser so as to maintain a constant intensity. For example, the bias current of the laser may be dynamically adjusted on the basis of the signal from the monitoring device.  
         [0022]     Because the input beam is collimated following passage through the ball lens  52 , the beam can undergo multiple reflections and can propagate along a long path without losing significant intensity. Only a small diffraction angle is required, so that a diffraction grating may have a long period, thereby easing manufacturing requirements. Additionally, because the beam monitor output  46  is generally parallel to the base of the mirror, an edge detector monitor  74  can be used.  
         [0023]     While the diffraction-inducing structure for dividing the input beam has been described as being a diffraction grating, other approaches to partially reflecting the input beam may be used.  
         [0024]      FIG. 4  shows another application of the invention. For each of the applications of  FIGS. 3 and 4 , the illustrated optical and electrical components may be housed in a single module and can be mounted along a surface of a single substrate. In the application of  FIG. 4 , wavelength locking is enabled. Both of the diffracted beam portions  68  and  70  of  FIG. 2  are utilized. Thus, the beam along the reflected beam segment  58  is divided into the output beam and a pair of smaller intensity monitor beams  46  and  48 . The first monitoring device  74  operates in the same manner as described with reference to  FIG. 3 . Thus, the total power emitted by the laser  72  may be monitored. The second beam monitor output  48  has an optical path that passes through a filter  76  before reaching a second monitoring device  78 . If the filter is wavelength-specific, the output of the second monitoring device will have an intensity that is strongly dependent on wavelength. The monitoring devices  74  and  78  may be edge detectors that generate signals that are responsive to changes in intensity. In this configuration, the output of the first edge detector is used to control total emitted power by dynamically adjusting the bias current of the laser  72 . The output of the second edge detector  78  is used to control the wavelength of the laser. As one possibility, wavelength control is achieved by dynamically adjusting the temperature of the laser. Since the emitted wavelength of the laser is varied by changes in the operating temperature, the emission wavelength can be locked by the combination of controlling laser output using the measurements by the first edge detector  74  and controlling laser temperature using the output of the second edge detector  78 .  
         [0025]      FIG. 5  is a block diagram of components for implementing the invention of  FIGS. 2 and 4 . The laser  72  provides an input beam to the optical module  40 . The combination of diffraction and reflection divides the input beam between an output to an output device (such as an optical fiber) and a pair of lower intensity beam monitor outputs. One beam monitor output is directed to a power detector, such as an edge detector, which generates signals indicative of laser output power. The signals from the detector  74  are used by a bias current controller  82  to maintain a constant intensity of the laser output. The second monitor beam is received by the wavelength detector  78 . As in  FIG. 4 , a filter  76  may be used to ensure that the output of the detector  78  is strongly dependent upon an optical property at a specific wavelength. The output of the wavelength detector is used by a temperature controller  84  to regulate the temperature of the environment in which the laser resides. Therefore, the temperature can be adjusted to ensure that the wavelength of the laser emission is locked.  
         [0026]     While the optical module  40  is shown as being separate from the laser  72 , and the detectors  74  and  78 , the components may be housed in common. Thus, the lid of the optical module may form a portion of a hermetical seal for the laser and the detectors. Signals generated by the detectors within the housing could be output to the controllers  82  and  84 . However, there are advantages to providing the controllers within the same housing, so that all of the components are integrated. Thus, with a heating element, such as a resistor within the housing, the environment in which the laser (e.g., an Externally Modulated Laser (EML)) resides may be easily controlled. Similarly, persons skilled in the art would readily recognize means for controlling the bias current of the laser. For an embodiment in which the laser is an EML, the light that is monitored is the light emission from the front facet and after the modulation, so that there is a greater accuracy than would be achieved by monitoring emission from the rear facet.  
         [0027]     Where all of the components shown in  FIG. 5  either define or are contained within a single housing, the housing can be compact as a result of the above-described combination of reflection and diffraction. In another embodiment, a single hermetically sealed housing is used for multiple channels. Thus, there is a separate laser for each channel, as well as a separate power detector  74  for each laser  72 . Moreover, separate temperature control is provided for each channel. Identically formed lasers will emit at substantially the same wavelength, but can be induced to emit at the different wavelengths of the various channels by individually setting the temperatures of the lasers. The different wavelengths/channels can then be combined and transmitted over a single fiber, so as to greatly increase the bandwidth of data transmission via the fiber.