Abstract:
The present invention provides low cost methods and apparatuses for filtering out polarized light reflections in a free-space optical isolator. In one embodiment, a laser directs a non-polarized optical signal through a series of polarizers and rotators in order to isolate an optical signal having a specific polarization. The present invention also includes a quarter-wave plate placed in series with the rotators and polarizers, to help filter away reflections occurring while the signal passes through free space. The inclusion of the quarter-wave plate helps filter away a greater amount of near-end reflections from going back to the laser, even with the use of low cost polarizers. Accordingly, the present invention can polarize an optical signal more efficiently than with prior methods, and at a much lower cost.

Description:
This application claims the benefit of priority to U.S. Provisional Application No. 60/424,228, filed on Nov. 5, 2002, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The invention generally relates to isolating a laser or light emitting diode in a fiber optic network from back reflections. More specifically, a more economical component arrangement is used to minimize the cost of an optical isolator. 
   2. Description of the Related Art 
   In the field of data transmission, one method of efficiently transporting data is through the use of fiber optics. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire. 
   One goal in modern fiber-optic communication configurations is to maintain the integrity of the signal generated by the laser or the light emitting diode. One common problem that degrades the integrity of the signal generated occurs when portions of the signal are reflected back into the laser. The reflections reaching the laser are generally an aggregation of the reflections caused by the individual connections within a fiber-optic network. While general care is taken to ensure that individual connections minimize reflection back to the laser, the aggregation of such reflections may result in unacceptably high reflections into the laser. Further, carelessness in the installation of a small number of connectors may also result in unacceptably high reflections being reflected back into the laser. Such reflections can cause increased transmission noise or bit error rates due to the reflections bouncing around the optical fibers, increased laser noise due to the reflections causing optical resonance in the laser and other similar problems. 
   One common cause of reflections occurs when a laser beam leaves a medium having a first index of refraction and enters a medium with a second index of refraction. An example of this situation is when a Distributed Feedback (DFB) laser is interfaced with a fiber-optic pigtail with free space between the transmitting end of a network component and the receiving end of the fiber-optic pigtail. Reflections of the laser beam that are reflected into the laser are commonly referred to as “back reflections.” Back reflections are commonly measured in terms of a ratio of the amount of the laser beam that is reflected as compared to the transmitted part of the laser beam. This value is commonly expressed as a logarithmic ratio. 
   In terms of this logarithmic ratio, DFB lasers commonly require back reflection levels as low as −40 dB to operate properly. One specific type of reflection that needs attention is near-end back reflection. A near-end back reflection is one caused by the first couple of connections from a laser transceiver to a fiber optic pigtail and to a communications panel. Because these first connections generally occur in fiber-optic cable that is not subjected to bending and heat stresses, the state of polarization of the laser beam can be predicted fairly accurately. 
   One prior art method of controlling near-end back reflections is shown in  FIG. 1 , which generally shows a Transmitter Optical Subassembly (TOSA) designated generally as  100 . The TOSA  100  comprises a DFB laser  102  coupled to an optical isolator  104 . The optical isolator  104  includes a 0° polarizer  106  coupled to a Faraday rotator  108  coupled to a 45° polarizer  110 . In operation, the DFB laser  102  emits a beam  114  which may be of any polarization as illustrated by the polarization indicator  112 . The beam  114  passes through the 0° polarizer  106  which allows only the portions of the beam polarized at 0° to pass through causing the beam  114  to be polarized at 0° as shown by the polarization indicator  116 . The beam  114  then passes through the Faraday rotator  108 , which is designed to rotate the beam  114  by 45° in the positive direction. 
   The Faraday rotator  108  may be latching magnetic material or non-latching magnetic material. For non-latching material, an external magnet  109  may be used to apply a magnetic filed while latching material does not need an external magnetic field. This rotation causes the beam  114  to be polarized at 45° as is shown by the polarization indicator  118 . The beam  114  then passes through the 45° polarizer  110  without disruption as the optical axis of the 45° polarizer  110  and the polarization of the beam  114  are aligned. The beam  114  remains polarized at 45° as is shown by the polarization indicator  120 . The beam  114  is then propagated through an air space  122  into a fiber-optic pigtail  124 . 
   Although shown here as a single discrete component, the fiber-optic pigtail  124  actually represents the various connections that are made throughout a fiber-optic network that include multiple fiber-optic pigtail, communication panel, transceiver, and other connections. Due to the difference in the index of refraction of the fiber-optic pigtail  124  (about 1.47) and the air space  122  (about 1.0) at various connections within the network, a reflected beam, denoted at  126 , is propagated back towards the DFB laser  102 . Because the reflected beam  126  is caused by various components within the network, the reflected beam  126  may be any state of polarization as shown by the polarization indicator  128 . 
   A major part, however, of the reflected beam  126  is the near-end reflection caused by the first few components into which the beam  114  is transmitted. If these components are not subjected to mechanical and thermal stress, these portions of the reflected beam will be polarized at 45°. The reflected beam  126  passes through the 45° polarizer  110  such that only the portions of the reflected beam  126  that are polarized at 45° are allowed to pass through. This causes the reflected beam  126  to be polarized at 45° as shown by the polarization indicator  130 . The reflected beam  126  then passes through the Faraday rotator  108  where it is rotated by positive 45° such that it is polarized to 90° as shown by the polarization indicator  132 . Note that the Faraday rotator  108  rotates all beams passing through the Faraday rotator  108  by positive 45° irrespective of the direction of travel. The reflected beam  126  polarized at 90° has no 0° components and is therefore totally rejected from passing through the 0° polarizer  106 . In this way back reflections into the DFB laser  102  are minimized. 
   While in theory this method appears to completely block any back reflections into the DFB laser  102 , in practice this may not be the result. An ideal polarizer only allows beams to pass through at the angle of polarization. However, actual polarizers allow small portions of the beam perpendicular to the angle of polarization to leak through. One characteristic that determines the quality and often the price of a polarizer is the polarizer&#39;s ability to minimize the leakage of perpendicular beams passing through the polarizer. This characteristic is known as the polarizer&#39;s extinction ratio. 
   Commonly, the polarizers used in a TOSA  100  of the type described above have a perpendicular beam extinction ratio of about −40 to −45 dB. While using such polarizers effectively meets the operating criteria for most DFB lasers, the use of such polarizers can be expensive. For example, the polarizers can represent as much as 70% of the isolator cost. It would therefore be beneficial to construct an optical isolator using polarizers that are less expensive. Understandably, such polarizers may not have as high of extinction ratios, and therefore an alternate configuration of the other components within the isolator would need to be implemented. 
   SUMMARY OF A REPRESENTATIVE EMBODIMENT OF THE INVENTION 
   Example embodiments of the present invention solve one or more of the foregoing problems in the prior art by introducing methods and apparatuses for filtering out polarized light reflections in a free-space optical isolator. In one example embodiment, a laser directs non-polarized light through a series of polarizers and rotators in order to isolate an optical signal having a specific polarization. To increase the efficiency of the isolation, the embodiment includes a quarter-wave plate placed in series with the rotators and polarizers to aid in preventing back reflections of light to the laser. 
   These back reflections occur when the polarized optical signal is transmitted over free space. The inclusion of the quarter-wave plate allows a much greater amount of reflected light to be filtered away and thereby prevented from reflecting back to the laser. In this way, free-space optical isolators can use less expensive polarizers than used previously, and can thus block a much greater amount of reflected light at a much lower cost. 
   These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In order that the manner in which advantages and features of the invention are obtained, a description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  illustrates a prior art example of an optical isolator used to minimize back reflections to a DFB laser source. 
       FIG. 2  illustrates an embodiment of the present invention that uses more economical components to accomplish the minimization of back reflections. 
       FIGS. 3A–3C  illustrate the electromagnetic components of an optical signal as the optical signal passes through a quarter-wave plate. 
       FIGS. 4A–4C  illustrate some physical characteristics of a quarter-wave plate as contemplated by the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One device intended to optimize cost and still maintain an appropriate aggregate back reflection extinction ratio is shown in  FIG. 2 . In some respects, the device of  FIG. 2  operates in a manner similar to the device of  FIG. 1 . For example, the DFB laser  202  emits a beam  214  that may be any state of polarization as shown by the polarization indicator  212 . The beam passes through, for example, a 0° polarizer  206 . As will be described, the 0° polarizer  206  need not be as efficient (and thus, as expensive) of a polarizer as described in  FIG. 1 , but may only require, for example, an extinction ratio of around −30 dB. The beam  214  travels through the 0° polarizer  206  and is polarized at 0° as shown by the polarization indicator  216 . The beam  214  then travels through a Faraday rotator  208  similar to the type of Faraday rotator described in connection with  FIG. 1 . 
   As illustrated, the Faraday rotator  208  rotates the beam  214  to be polarized at 45°, shown by the polarization indicator  218 . The beam  214  then passes through a 45° polarizer  210  unaffected, since the beam&#39;s  214  polarization and the 45° polarizer axis are aligned. Of note, the 45° polarizer  210  may be of the type with a higher extinction ratio such as −40 to −45 dB. The reasons for using such a polarizer will become more apparent as the return path for a reflected beam is shown. After leaving the 45° polarizer, the beam  214  then passes through a quarter-wave plate  236 , illustrated in series with the 45° polarizer. 
   By way of explanation,  FIGS. 3A–3C  illustrate the results of using a quarter-wave plate  236  when a standard light beam in a linear polarization passes through the quarter-wave plate  236 . Linearly polarized light comprises two orthogonal components, including an electric field component  300 , and a magnetic field component  302 , both appearing as a series of up  310  and down  320  arcs about an axis (e.g., a sinusoidal wave about an X axis). Two arcs (one up λ/2  310  and one down λ/2  320 ) represent a full wave (or cycle) λ  322  about an axis for any orthogonal component, and the mid-point λ/4  314  of each arc constitutes a quarter-wave. A quarter-wave plate comprises a material that causes one of the orthogonal components in a light beam to shift relative to the other component, partly as a function of differences in speed for each component through the material. In a quarter-wave plate, the shift is one half of one arc, or a quarter of one wave λ/4  314 . By contrast, in a half wave plate, the light beam travels through the material such that one component shifts back (or forward) a full arc, or one half wave λ/2 ( 310 ,  320 ). 
   When a linearly polarized beam passes through a quarter-wave plate at a 45° angle, the resulting light beam appears to approach the source as either circular to the left or circular to the right, depending on which component the quarter-wave plate shifted (i.e., the faster or slower component through the material). Consider, for example  FIG. 3B , a light beam approaching a viewer, where the electrical field component can be viewed on a Z axis, and the magnetic field component can be viewed on an Y axis. A left-handed circularization  330  occurs when the magnetic component  302  shifts backward (i.e., travels more slowly through) a quarter-wave relative to the electrical component  300 , so that, as the light approaches, the first thing a viewer sees is the electrical field component, then to the left by a quarter-wave, the magnetic field component, and so on circularly around the X axis. A right-handed circularization  340 , (e.g.,  FIG. 3C ) occurs under the exact opposite circumstance. 
   Quarter-wave plates can be particularly useful for optical signal isolation since they respond more to wavelength and degree of alignment than to temperature fluctuations.  FIGS. 4A–4C  illustrate this principal, where the Y axis represents an amount of light transmitted through the wave plate, and the X axis represents the wavelength of approaching light. The wave plate is shown centered about a wavelength of 1.55 nm, and the four different optical signals are plotted over three separate parameters. The three different parameters are: a wavelength λ parameter (in this case 1.55 nm), a temperature (20, 85, −40, and 20) parameter, and an angle alignment parameter as between the approaching optical signal and the wave plate (0, 0, 0, and 1). The parameters are plotted by the following formula, which expresses the temperature and wavelength dependence of phase retardation (shifting of an optical signal component) of a wave plate as: 
         δ   ⁡     (     λ   ,   T     )       :=       π   2     ·     [     1   +         λ   ⁢           ⁢   c     -   λ     λ     +     CTE   ·     (     T   -   T0     )       +         d   ⁢           ⁢   Δ   ⁢           ⁢   ndT       Δ   ⁢           ⁢   n       ·     (     T   -   T0     )       +         d   ⁢           ⁢   Δ   ⁢           ⁢   nd   ⁢           ⁢   λ       Δ   ⁢           ⁢   n       ·     (     λ   -   λc     )         ]           
 
   Accordingly, optical signal  410  is plotted by curve  412 , optical signal  415  is plotted by curve  417 , optical signal  420  is plotted by curve  422 , and optical signal  425  is plotted by curve  427 . As illustrated, a change in temperature for each different optical signal has little overall effect on the amount of transmitted light (shifting the signal left or right, essentially within the same transmission). By contrast, a misalignment of optical signal angle (signal  420 ) by one degree between the transmitted light and wave plate shows a significant drop in transmission of the optical signal through the wave plate. 
     FIG. 4B  illustrates three optical signals  430 ,  435 , and  440  transmitted through a wave plate, the wave plate also centered at a wavelength of 1.55 nm. Optical signal  430  is plotted by curve  432 , optical signal  435  is plotted by curve  437 , and optical signal  440  is plotted by curve  442 .  FIG. 4B  illustrates that changes in optical signal wavelength also cause a significant difference in transmission through the wave plate, when holding temperature and alignment angle constant. In particular, curve  432  is at a much higher transmission level through the wave plate since it is aligned at 1.55 nm, in contrast with curves  437  and  442 , which are of different wavelengths from the wave plate. 
     FIG. 4C  further emphasizes this nature of the wave plate, showing a plot of optical signal transmission  445  (having a wavelength of 1.55 nm) through a wave plate centered at 1.55 nm. As illustrated in  FIG. 4C , even 1 degree of misalignment between the optical signal and the wave plate causes a significant change in transmission of the optical signal through the wave plate. Accordingly, quarter-wave plates are particularly useful in a free-space optical isolator since they are essentially independent of temperature for purposes of transmission, and provide a useful filter of optical signals approaching the wave plate at a misaligned angle. 
   Returning to the embodiment illustrated in  FIG. 2 , the quarter-wave plate receives the portion of beam  214  aligned at 45°, and causes the beam  214  to have a left-handed circular polarization, as shown by the polarization indicator  238 . Of course in other embodiments, the quarter-wave plate may be composed of materials that cause the oz beam  214  to take on a right-handed circular polarization. In either case, after the now-circularized beam  214  exits the quarter-wave plate  236 , the beam  214  is then propagated into the fiber-optic pigtail  224  and onto the fiber-optic network. 
   Due to the phenomenon described above regarding light traveling in a medium having a first index of refraction into a medium having a second index of refraction, a reflection beam  226  is reflected back towards the DFB laser  202 . For purposes of this illustration, the reflected beam  226  only represents a near-end reflection. Because the reflected beam  226  is generally caused by fiber-optic components that are not subjected to physical and heat stresses and because a circularly reflected beam is generally also circular and opposite in polarization to the original beam, the reflected beam  226  is a right-hand, circularly polarized beam as shown by the polarization indicator  240 . 
   Returning to  FIG. 2 , when the reflected beam  226  passes through the quarter-wave plate, the quarter-wave plate  236  acts essentially as a half wave plate (relative to the initial beam  214 ) since the reflected beam  226  has already been shifted one quarter-wave. The quarter-wave plate  236 , therefore, causes the reflected beam  226  to become linearly polarized due to the quarter shift, but this time at −45°, as shown by the polarization indicator  228 . This polarization is perpendicular (or 90°) to the optical axis of the 45° polarizer  210 . Hence, nearly all the reflected beam  226  is blocked by the 45° polarizer  210  because of this perpendicular relationship. A high-quality polarizer exhibiting an extinction ratio of around −40 to −45 dB can be used for the 45° polarizer  210  to maximize the extinction of the near-end back reflection. 
   Of course, inexpensive materials may still allow passage of a small amount of reflected beam  226  through the quarter-wave plate  236  at a variety of angular planes  234 , including the 45° plane  230 . Similarly, a small portion of beam  214  could pass through the quarter wave plate  236  on the first pass without becoming circularized, and become circularized only upon passing through the second time upon reflection as a portion of reflected beam  226 . Typically, however, only that minute portion of the reflected beam  226  that is made linear in the 45° plane will pass through the second polarizer  210  back to the Faraday rotator  208 . Thus, the second polarizer  210  also blocks circularized light from passing through upon reflection. 
   Consequently, the primary reflected signal that the Faraday rotator  208  receives will be any remaining linear portions of reflected beam  226  that are angled at 45°, and that passed through the second polarizer  210 . The Faraday rotator  208  then rotates those remaining portions counter-clockwise so that the remaining portions of reflected beam  226  are then vertical  230 . Since the vertical position  230  in this case is perpendicular to the polarization axis of the first polarizer  206 , the first polarizer  206  filters away the remaining near-end reflected light  226  from reaching the DFB laser  202 . 
   Accordingly, these representative embodiments demonstrate an economically efficient alternative for constructing an optical isolator with a high extinction ratio for use in a TOSA (or similar optical environment). The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.