Abstract:
Method and apparatus of attenuating an optical signal without adding extra components is presented. The drive current of the optical signal source is set to meet a predetermined bandwidth requirement and exceed a predetermined amplitude requirement. An optical isolator that is used to prevent back-reflections from reaching the optical signal source is used to achieve the desired amount of attenuation. More specifically, the invention includes controlling the attenuation by tuning an angle θ between the transmission axis of a polarizer that is part of the optical isolator and the original polarization state of the optical signal. By increasing the angle θ, the amount of attenuation is increased; by decreasing the angle θ, the amount of attenuation is decreased. The invention allows continuous tuning of the angle θ.

Description:
RELATED APPLICATION 
   This patent application claims the benefit, under 35 U.S.C. § 119(e), of U.S. Provisional Application No. 60/422,396 filed on Oct. 30, 2002, which is incorporated herein in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to optical transmitters, and more particularly to the attenuation of optical signals. 
   BACKGROUND OF THE INVENTION 
   Transmitter Optical Sub-Assemblies (TOSAs) are well known in the art of optical networks. A TOSA operates as an electro-optical converter for use in data communications and telecommunications applications. It transforms electrical signals into corresponding optical signals that are then focused into an optical fiber. Once the optical signal reaches its destination, it is typically focused into a ROSA (Receiver Optical Sub-Assembly) for conversion back into a corresponding electrical signal. 
   A TOSA typically includes a diode laser for producing an optical signal and a lens for focusing the optical signal into the input end of an optical fiber. Diode lasers (e.g. distributed feed-back diode lasers) are typically sensitive to back reflected light (e.g. light reflected off of the input face of the optical fiber back into the diode laser). Therefore, TOSAs also typically include an optical isolator located between the laser and the optical fiber that allows light to pass from the diode laser to the optical fiber while preventing any back-reflected light from reaching the diode laser. 
   A common optical isolator is a Faraday rotator device having an input linear polarizer, a garnet crystal and an output linear polarizer. Normally, the transmission axis of the input linear polarizer is aligned to the linear polarization of the diode laser output to maximize light transmission through the input linear polarizer. The garnet crystal is subjected to a saturating magnetic field, making it a Faraday rotator having a thickness chosen such that the polarization of transmitted light is rotated by approximately 45 degrees. The polarization is rotated in the same direction regardless of the propagation direction of the light. The transmission axis of the output linear polarizer is oriented at approximately 45 degrees relative to that of the input linear polarizer to maximize the transmission of the light from the diode laser that has passed through the input polarizer and Faraday rotator. 
   Any light that is reflected back toward the diode laser is first incident upon the output polarizer, which passes only light linearly polarized along its axis. The polarization of this admitted light is rotated by approximately 45 degrees by the garnet crystal, and ends up being orthogonal to the transmission axis of the input polarizer. At the input polarizer, this light that is polarized orthogonal to the transmission axis is either absorbed or reflected away from the fiber. Optical isolators are used extensively and have excellent performance with typical insertion loss of &lt;0.3 dB and isolation of&gt;25 dB (reduction factor for reflected light). 
   TOSAs must satisfy certain bandwidth and power requirements in order to function properly for use in practical networking applications. These characteristics both vary as a function of the electrical current through the diode laser. Therefore, many times it is not possible to satisfy both the power requirement as well as the bandwidth requirement simply by adjusting the diode laser current. One solution is to set the diode laser current to meet the bandwidth requirement even though it may possibly exceed the power requirement, and then attenuate the diode laser output as needed to then meet the power requirement. A prior method of achieving this attenuation is to insert an optical plate with an attenuating thin-film coating, or to use a neutral density filter to attenuate the optical power. Such an optical filter has been used with an optical isolator (see for example U.S. Pat. No. 6,297,901). Adding additional optical element(s), however, is disadvantageous because of the cost of the optical element(s), the creation of additional optical interfaces, and the increased possibility of a failure related to the added optical element(s) (e.g. coating failure, mechanical mounting failure, alignment failure, surface contamination, etc.). Additionally, the attenuation of the added passive optical element(s) is not adjustable, thus requiring a series of optical elements to be created and stocked. 
   A variety of active devices have also been used to achieve variable attenuation. For example, it is possible to vary the magnetic field applied to the Faraday rotator to adjust the rotation angle of the light relative to the output polarizer (see for example U.S. Pat. No. 6,384,957). However, this solution increases the complexity and cost of the TOSA, and decreases the effectiveness of the optical isolator in completely blocking back-reflected light. 
   There is a need for a simple, inexpensive way to achieve arbitrary and continuously adjustable attenuation values for laser light focused into an optical fiber, without adding to the cost or complexity of the TOSA. 
   SUMMARY OF THE INVENTION 
   The invention solves the aforementioned problems by providing a continuously adjustable attenuation by using the optical isolator itself, without compromising its ability to block back-reflected light. The variable optical attenuation device of the invention includes an optical signal source generating an optical signal that is polarized in an original polarization direction D, an optical isolator that receives the optical signal and transmits only light that is substantially polarized in a polarizer direction T 1 , and a means for rotating at least one of the optical isolator and the optical signal source to adjust an angle θ, which is the angle between the original polarization direction D and the polarizer direction T 1 . 
   The attenuation method of the invention includes tuning the angle θ. By increasing the angle θ, a greater attenuation is achieved because a smaller portion of the original optical signal is transmitted. By decreasing the angle θ, the amount of attenuation that is achieved can be decreased, increasing the signal intensity. 
   Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of an optical signal source and an optical isolator that are combined for the present invention; 
       FIG. 2  is a perspective view of the optical isolator in  FIG. 1 ; 
       FIG. 3  is a graph showing the attenuation of the optical isolator as a function of rotation angle θ; and 
       FIG. 4  is a cross sectional view of an optical module built in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention is a method and device for attenuating the output of an optical signal source (e.g., a diode laser) without extra components. 
     FIG. 1  depicts an embodiment of a variable optical attenuation device  8  that includes a diode laser  10  that produces a diverging optical beam  12 , one or more lens elements  14 , an optical isolator  16  and an optical fiber  18 . The optical fiber  18  may or may not be a part of the TOSA device. The lens element  14  focuses the optical beam  12  from the diode laser into the input end  20  of the optical fiber. Depending upon the type of diode laser and optical fiber used, lens element  14  could be a single optical element as shown in  FIG. 1 , or multiple optical elements placed before and/or after the optical isolator  16 . 
   The optical isolator  16  further includes an input linear polarizer  22 , an output linear polarizer  24 , an optical element (such as a garnet)  26  disposed between the polarizers  22 ,  24 , and a magnetic field source (such as a static magnet)  28  for immersing the garnet  26  in a magnetic field B. The garnet and magnetic field are set to rotate the polarization of light traversing therethrough by 45 degrees in one direction regardless of whether the light is traveling toward or away from the diode laser  10  (e.g. in the direction shown by the arrow A in  FIG. 2 ). The transmission axis T 2  of the output linear polarizer  24  is fixed at a predetermined orientation relative to the transmission axis T 1  of the input linear polarizer  22 . The optical element  26  is an asymmetric polarization rotator, such as a Faraday rotator. 
   The optical beam  12  from laser diode  10  is generally linearly polarized in a direction D. To align the optical isolator for minimum attenuation of the forward traveling light while blocking the back-reflected light, the transmission axis T 1  of the input linear polarizer  22  is aligned with the polarization direction D of the optical beam  12  from diode laser  10  to maximize the transmission of the optical beam  12  through polarizer  22 . The garnet crystal  26 , immersed in magnetic field B, rotates the polarization of optical beam  12  by a predetermined angle that matches the orientation of T 2  relative to T 1 . Thus, after passing through the garnet crystal  26 , the polarization direction of optical beam  12  is aligned with transmission axis T 2 . Substantially all of the optical beam  12  is transmitted through output polarizer  24 . Preferably, the transmission axis T 2  is oriented at approximately 45° angle relative to the transmission axis T 1  and the garnet crystal  26  rotates a light beam propagating through it by approximately 45°. The invention is described herein in the context of this preferred embodiment. 
   Once the optical beam  12  exits the optical isolator  16 , it is incident upon, and is coupled into, the input end  20  of optical fiber  18 . A small amount of the optical beam  12  entering optical fiber  18  may be reflected back toward the optical isolator  16  either by fiber input end  20  or other optical components at either end of optical fiber  18 . The output optical polarizer  24  transmits only that portion of this back-reflected light that is aligned with transmission axis T 2 . The transmitted back-reflected light then undergoes a 45 degree polarization rotation by the garnet  26  in the same direction that the optical beam  12  was rotated while propagating toward the optical fiber  18 . Having experienced two sets of 45-degree rotations in the same direction, the back-reflected light now has a polarization state that is orthogonal to the transmission axis T 1 . Therefore, the input polarizer  22  generally absorbs or reflects away all of the back reflected light incident thereon, preventing the back-reflected light from reaching the diode laser  10 . 
     FIG. 2  depicts the polarization rotations involved in the variable optical attenuation device of  FIG. 1 . The optical beam  12  that is polarized along the direction T 1  passes through the input polarizer  22  and feeds into the garnet crystal  26 . The garnet crystal  26  rotates the optical beam  12  by approximately 45° in the direction A so that the optical beam  12  is polarized in the direction T 2 . Since the output polarizer  24  transmits light polarized along the direction T 2 , substantially all of the optical beam  12  passes through the output polarizer  24  and is coupled into an optical fiber (not shown). If any part of the optical beam  12  is back-reflected, substantially all of this back-reflected light also passes through the output polarizer  24  toward the garnet crystal  26  since reflection does not significantly affect the polarization state. However, because the garnet crystal  26  again rotates the back-reflected light by 45° along the direction A, the back-reflected light is polarized in the direction T 3  when it exits the garnet crystal  26 . Since the input polarizer  22  only transmits light that is polarized in the T 1  direction and T 3  is orthogonal to T 1 , the back-reflected light passing through the input polarizer  22  is greatly reduced, limited by the extinction ratio of the polarizer  24 . 
   The variable optical attenuation system  8  can be used to continuously adjust the power of the optical beam  12  by rotating the optical isolator  16  relative to the laser diode  10  about the optical axis. The power is maximized (i.e., attenuation is minimized) when the polarization state D of the optical beam  12  is aligned with the direction T 1  of the input polarizer  22 . Attenuation is achieved as the optical isolator  16  is rotated by an angle θ, wherein the angle θ is the angle between the polarization state of the optical beam  12  and the direction of maximum transmission T 1  of the input polarizer  22 . It should be noted that the optical isolator  16  could be fixed, and the diode laser  10  can be rotated. As θ is gradually increased from 0 to 90 degrees, the input polarizer gradually absorbs more of the optical beam  12 , increasing the amount of attenuation that is achieved. The portion of optical beam  12  that is transmitted through input polarizer  22  is polarization rotated by garnet crystal  26  and transmitted by output polarizer  24  as described above. 
     FIG. 3  depicts the amount of attenuation that is achieved as a function of angle θ. The intensity I(θ) of the optical beam  12  that is transmitted by the optical isolator  16  as a function of the rotation angle θis:
   I (θ)= I   0 ·cos 2  θ 
wherein I 0  is the light intensity incident upon the optical isolator  16 . As shown in  FIG. 3 , maximum transmitted power occurs where θ is zero. As θ is increased from zero to 90 degrees, the light intensity I(θ) of optical beam  12  transmitted by the optical isolator  16  drops to nearly zero. The power can be reduced up to the maximum isolation of the device, typically 40 dB or 10000×. Thus, by rotating the isolator from zero to 90 degrees, effectively any signal intensity up to I 0  can be obtained. It should be noted that regardless of the rotation angle θ, generally all of the back reflected light is still absorbed by optical isolator  16 .
 
   The rotation angle θ can be set in several ways. For example, the laser diode current can be set to provide the desired bandwidth performance, and then the rotation angle θ can be adjusted while the power entering or exiting the optical fiber  18  is actively measured until the desired power intensity is produced. Alternately, the required rotation angle θ can be calculated using the above equation, depending upon the desired attenuation factor. As an added benefit, the isolation of back-reflected light is improved by the projection factor of cos 2  θ when the isolator is rotated. 
   An optical isolator is often used in TOSAs to avoid optical feedback. The method of obtaining variable attenuation requires no additional optical elements or mechanical parts, but advantageously utilizes this existing optical isolator without modification. Unlike filter plates, which would necessarily only be manufactured at discrete values, the isolator attenuation technique of the present invention can achieve arbitrary attenuation values that can be varied to suit each diode laser. Adjusting the rotation angle θ is a fast and inexpensive manufacturing step, yet makes it possible to precisely control either the output power at a fixed diode laser current, or the operating slope efficiency (ΔP/ΔI), both of which are critical for many practical applications. 
     FIG. 4  depicts a possible implementation of a TOSA  40  in accordance with the invention. The TOSA  40  includes a housing  42  holding the isolator  16  (see  FIG. 1 ), a lens  44 , and part of the diode laser  10  (see  FIG. 1 ). The isolator  16  is press-fit or glued into the housing  42  at an arbitrary angular orientation. Although not clearly shown, the isolator  16  has a groove  17  that runs across a surface, indicating its input polarization axis. The lens  44  is securely fixed near the isolator  16  by any conventional mechanical means. The laser diode  10  is inserted into the housing  42  such that the diode laser  10  can still be rotated. 
   The housing  42  and/or the isolator  16  are rotated relative to the diode laser  10  until the desired power level is achieved. The housing  42 , for example, may be rotated by being placed on a motorized or manual rotation stage. A large-area detector power-meter  46  may be positioned near the output of the TOSA  40  to measure or characterize the transmitted power. The isolator orientation can be determined experimentally by maximizing transmission, minimizing transmission and rotating by 90°, or choosing an angle between two transmission minima. This method is advantageous in that it avoids problems due to light “clipping” on the isolator, which is not at its final position during the adjustment. Alternatively, the isolator orientation can be determined by measuring the transmission power as a function of θ while adjusting θ. The θ adjustment is stopped when the power-meter reading indicates that the desired degree of attenuation is achieved. Once the isolator orientation is known, varying the rotation angle θ (defined above) will attenuate the output by cos 2  θ. 
   It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, while the output of the optical isolator  16  is shown focused into an optical fiber  18 , the optical beam  12  can be delivered to the intended application directly, or with any other conventional optical delivery system (e.g. mirrors, gratings, lenses, etc.).