Abstract:
A device and method for optical isolation for use in optical systems is disclosed. The device provides for a waveguide optical isolator fabricated using two arms, made of optical waveguides comprising magneto-optical material, in a Mach-Zehnder interferometer configuration. The device of the present invention operates using the TM mode of a light wave and, thus, does not require phase-matching of TM and TE modes. Further, the present invention does not use polarizers to extinguish the optical feedback.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0001] The invention described herein was funded in part by a grant from AFOSR/DARPA Program, Contract No. F49620-99-1-0038, and in part by DARPA FAME Program, Contract No. N0017398-1-G014. The United States Government may have certain rights under the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to optical communication systems. In particular, the present invention relates to waveguide optical isolators.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical isolators are essential elements in many optical systems for protecting a light source, such as a laser, from being exposed to light which is reflected back at the light source. Such reflected light, known as “optical feedback,” may cause the light source to become unstable or may even damage the light source. The problem is especially difficult in optical systems employing lasers that emit a relatively high output beam power where even surfaces of transmissive optical elements, or relatively small discontinuities or mismatches in optical waveguides can produce sufficient reflections to give rise to deleterious optical feedback.  
           [0004]    It is known to incorporate an optical isolator in the path of the laser output beam, near the laser cavity exit aperture, to isolate the laser from reflected laser light and thereby avoid or reduce optical feedback. An optical isolator permits the forward transmission of a radiation beam, in this case the laser output beam, while simultaneously preventing the reverse transmission of the same radiation beam, with a high degree of extinction. Thus, the laser energy reflected back towards the laser from various sources of reflection is trapped, extinguished or reflected by the optical isolator.  
           [0005]    Optical isolators based on the Faraday polarization rotation effect are available for use in laser systems. Such a conventional optical isolator is illustrated in FIG. 1. The conventional optical isolator  50  includes, a first polarizer  58  for linearly polarizing a light wave in a first direction  62  and a second polarizer  60  for linearly polarizing a light wave in a second direction  64 , a longitudinal magnet  52  surrounding a magneto-optical medium  54 , which may be in the form of an optical waveguide, for example. The magnet  52  applies a longitudinal magnetic field  56  to the magneto-optical medium  54 .  
           [0006]    In operation, an incident light wave is polarized by first polarizer  58  in a first direction  62 . If the incident light wave is plane polarized, the first direction  62  of polarization should coincide with the polarization of the incident light wave as it leaves the light source. This polarized light wave then enters the magneto-optical medium  54 , where a permanent magnet  52 , or alternatively an electromagnet, applies a magnetic field  56  that causes a rotation of the plane of polarization of the light wave by 45 degrees, as shown by directional arrows  70 , to align the direction of polarization of the light wave with the second polarizer  60  having a direction  64  of polarization set at 45 degrees from that of the first linear polarizer  58 . In this way, a forward propagating light wave passes through the conventional optical isolator  50  with little attenuation.  
           [0007]    A light wave of unknown polarization  74  propagating in the backward direction is first linearly polarized by the second polarizer  60 . Since the polarization of light waves traveling in the backward direction is unknown, only light waves traveling in the backward direction with the polarization direction  64  of the second polarizer  60  will pass second polarizer  60  and enter the magneto-optical medium  54 . Once propagating in the magneto-optical material  54 , the polarization of the backward propagating light wave is rotated by 45 degrees, in the same sense as the rotation of the forward propagating light wave, causing the direction of polarization of the backward propagating light wave exiting the magneto-optical medium  54  to be polarized at 90 degrees with respect to the direction of the first polarizer  58 . Therefore, the backward propagated light wave will not pass the first polarizer  58 .  
           [0008]    With such a conventional optical isolator  50 , however, there has been the problem of the need for a bulky magnet for applying a longitudinal magnetic field, stringent modal phase-matching for the TM and TE modes of light waves propagating in the magneto-optical medium and addition of auxiliary components such as polarizers  58 ,  60 . Further, non-uniformities in the longitudinal magnetic field introduce non-uniform polarization rotation across a light wave passing through the magneto-optical medium  54 . Unless the light wave dimension is made equal to or smaller than the cross-section of uniform Faraday rotation, these non-uniformities limit the extinction ratio obtainable by the conventional optical isolator.  
           [0009]    There exists a need for an optical isolator which eliminates the need for a bulky magnet for applying a longitudinal magnetic field, modal phase-matching and the need for polarizers.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the present invention, there is provided a waveguide optical isolator having first and second optical waveguide arms formed at least in part with a material that exhibits a transverse magneto-optic non-reciprocal phase shift effect, the two optical waveguide arms being arranged in a Mach-Zehnder interferometer configuration. Transverse magnetic fields of equal magnitude are respectively applied to the two magnetically active optical waveguide arms of the Mach-Zehnder interferometer in opposite transverse directions to cause non-reciprocal phase shifts of equal magnitude but of opposite signs for light waves propagating in the two magnetically active optical waveguide arms. Further, by adjusting the path lengths of the two optical waveguide arms, a 90 degree reciprocal phase shift of a light wave propagating in the first optical waveguide arm with respect to a light wave propagating in the second optical waveguide arm of the device is achieved. During forward propagation of a light wave in the first magnetically active optical waveguide arm, the 90 degree reciprocal phase shift is combined with a −45 degree forward propagation non-reciprocal phase shift so as to provide a net phase shift of +45 degrees for the light wave after propagation through the first magnetically active optical waveguide arm. Forward propagation of a light wave through the second magnetically active optical waveguide arm results in a +45 degree forward propagation non-reciprocal phase shift. Accordingly, when light waves that were initially in phase have propagated in the forward direction through the first and second magnetically active optical waveguide arms are combined, the two light waves which are remain in phase interfere constructively. For backward propagation of a light wave through the first magnetically active optical waveguide arm, the light wave undergoes a 90 degree reciprocal phase shift combined with a +45 degree backward propagation non-reciprocal phase shift to cause a net phase shift of 135 degrees of the light wave after propagating through the first magnetically active optical waveguide arm. A light wave after propagating through the second magnetically active optical waveguide arm is phase shifted by −45 degree backward propagation non-reciprocal phase shift. Accordingly, when two light waves, which were initially in phase propagating through the first and second magnetically active optical waveguide arms, respectively, are combined, they are 180 degrees out of phase and interfere destructively. In this manner, a light wave propagating in the forward direction through the waveguide optical isolator of the present invention, which is approximately equally divided for propagation in the first and second optical waveguide arms and then recombined, will pass through the device with relatively low attenuation, while a light wave propagating in the reverse direction in the waveguide optical isolator, which is approximately equally divided for propagation in the first and second optical waveguide arms and then recombined, is extinguished by destructive interference in the Mach-Zehnder interferometer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features, components and method steps, and wherein:  
         [0012]    [0012]FIG. 1 is an illustration of a conventional optical isolator based on the Faraday polarization rotation effect;  
         [0013]    [0013]FIG. 2 is an illustration of a waveguide optical isolator fabricated in a Mach-Zehnder interferometer configuration in accordance an exemplary embodiment of the present invention;  
         [0014]    [0014]FIG. 3 is an illustration of the structure of a magnetically active optical waveguide in accordance with an exemplary embodiment of the present invention; and  
         [0015]    [0015]FIG. 4 is an illustration of a test apparatus for testing waveguide optical isolators fabricated in the Mach-Zehnder interferometer configuration of FIG. 2 in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Now referring to the drawing, FIG. 2 illustrates a waveguide optical isolator  100  fabricated in a Mach-Zehnder interferometer configuration in accordance with an exemplary embodiment of the present invention. The waveguide optical isolator  100  includes an input optical waveguide section  102 , an input optical waveguide Y-branch  132 , two optical waveguide arms  104 ,  106 , an output optical waveguide Y-branch  130  and an output optical waveguide section  108 . The input optical waveguide Y-branch  132  approximately equally divides a light wave propagating in the forward direction in the input optical waveguide section  102  into two divided light waves and provides a respective one of the two divided light waves to each of the optical waveguide arms  104 ,  106  without changing the mode of propagation. The input optical waveguide Y-branch  132  also combines respective light waves propagating in the backward direction in the two optical waveguide arms  104 ,  106  and provides the combined light wave to the input waveguide section  102  without changing the mode of propagation. The output optical waveguide Y-branch  130  combines respective light waves propagating in the forward direction and provides the combined light wave to the output optical waveguide section  108  without changing the mode of propagation. The output optical waveguide Y-branch  130  also approximately equally divides a light wave propagating in the backward direction in the output optical waveguide section  108  into two divided light waves and provides each of the two divided light waves to respective ones of the two optical waveguide arms  104 ,  106  without changing the mode of propagation.  
         [0017]    Referring to FIG. 3, in an exemplary embodiment, the waveguide is fabricated by growing a bismuth-, lutetium-, neodymium-iron garnet film  240  (Bi,Lu,Nd) 3 (Fe,Al)  4 O 12  by liquid phase epitaxyo on a [111] oriented gallium gadolinium garnet (GGG) substrate. The bismuth-, lutetium-, neodymium-iron garnet film  240  is a magnetically active material which can cause non-reciprocal, controllable phase shift of a light wave propagating through it based upon a transverse magnetic field applied thereto. Alternatively, this film  240  could be bismuth-, lutetium-iron garnet or yttrium iron garnet (YIG), rare-earth substituted yttrium-iron garnet or rare-earth substituted iron garnet. Using a bismuth-, lutetium-, neodymium-iron garnet film  240 , the film-substrate lattice mismatch is 0.001 nm, causing minimum stress-induced anisotropy. The film  240  has in-plane magnetization, and a refractive index of 2.2403 for the TM mode at λ=1.55 μm. The film  240  initially has a thickness of approximately 1.65 μm.  
         [0018]    Before the rib waveguides are patterned, the films  240  are thinned to optimize the non-reciprocal response to a thickness ranging from 0.3 μm to 1.0 μm; in an exemplary embodiment this optimum thickness is approximately 0.5 μm. Since thickness tuning improves the phase shift per length, proper tuning yields a shorter device and hence reduces the total absorption loss in the waveguide optical isolator  100 . Straight ridge waveguides are then patterned on the film  240  by conventional photolithographic and etching techniques. As illustrated in FIG. 3, the ridge waveguides have a width ranging from 0.5 μm to 6.0 μm, in an exemplary embodiment this optimum width is approximately 2.0 μm; a 0.5 μm waveguide height; and a 0.07 μm rib height and are fabricated by photoresist patterning and phosphoric-acid wet etching. In this exemplary embodiment the etch rate is 0.01 μm/min at 57 degrees Celsius.  
         [0019]    A waveguide optical isolator  100  is then patterned onto a single chip by a photolithographic direct laser writing system. The resist patterns are made by focusing an Argon (Ar) laser beam (λ=360 nm) directly onto a photoresist-coated sample with computer-controlled XYZ translation stages and shutter. In an exemplary embodiment the total length of the fabricated waveguide optical isolator  100  is 8.0 mm, which includes 3.3 mm long optical waveguide arms, 0.4 mm long output and input waveguide Y-branches  130 ,  132 , and 3.9 mm long input and output waveguide sections  102 ,  108 . The separation between the optical waveguide arms  104 ,  106  is 24.4 μm, where the output and input waveguide Y-branches  130 ,  132  are each formed at a non critical angular separation of the branches ranging from 0.1 to 3 degrees.  
         [0020]    The reciprocal phase shift is obtained by forming the optical waveguide arms  104 ,  106  with a difference in length, herein referred to as “path length.” The path length has a direct impact on the reciprocal phase shift of each light wave  112 ,  114  after propagating through respective optical waveguide arms  104 ,  106 . In this exemplary embodiment the top optical waveguide arm  104  has a shorter path length than the bottom optical waveguide arm  106 . The path lengths are selected such that a light wave  114 , originating from the input waveguide Y-branch  132  propagates in a forward direction through the bottom optical waveguide arm  106  to reach output waveguide Y-branch  130  with a phase difference of +90 degrees with respect to a light wave  112  also originating from input waveguide Y-branch  132  propagating in a forward direction through the top optical waveguide arm  104  and reaching output waveguide Y-branch  130 . Further, a light wave  114 , originating from the output waveguide Y-branch  130  propagates in a backward direction through the bottom optical waveguide arm  106  to reach the input waveguide Y-branch  132  having a phase difference of 90 degrees with respect to a light wave  112  also originating from the output waveguide Y-branch  130  and propagating in the backward direction through the top optical waveguide arm  104  to reach the input waveguide Y-branch  132 . This reciprocal phase shift is the result of the different path lengths of the two optical waveguide arms  104 ,  106 . In the exemplary embodiment of the present invention described above with reference to FIG. 3, the top optical waveguide arm  104  is a quarter wavelength ±30%, which is 0.2 μm, shorter than the bottom optical waveguide arm  106  in order to achieve the reciprocal phase shifts described above. This 0.2 μm total path length difference produces less than 0.006 degrees of additional non-reciprocal phase shift, and thus the unequal optical waveguide arm lengths do not otherwise affect the operation of the device.  
         [0021]    As described above the waveguides contain magneto-optical material in the waveguides of the optical waveguide arms  104 ,  106 , which provide non-reciprocal phase shifts when a transverse magnetic field is applied to each optical waveguide arm  104 ,  106 . The amount of non-reciprocal phase shift depends upon the path lengths of each optical waveguide arm  104 ,  106 , and the magnitude and direction of the transverse magnetic field applied thereto. As described above, in this exemplary embodiment the top optical waveguide arm  104  is 3.3 mm minus 0.2 μm and the bottom optical waveguide arm  106  is 3.3 mm. As shown in FIG. 2, each of the top and bottom magnetically active optical waveguide arms  104 ,  106  have a respective transverse magnetic fields applied thereto. The respective transverse magnetic fields are of the same magnitude but are opposite in direction. The magnitude and transverse direction of the magnetic fields are arranged so as to produce a non-reciprocal 45 degree phase shift of a light wave propagating through the top optical waveguide arm  104  in a forward direction, a non-reciprocal −45 degree phase shift of a light wave propagating through the bottom optical waveguide arm  106  in a forward direction, a non-reciprocal −45 degree phase shift of a light wave propagating through the top optical waveguide arm  104  in a backward direction, and a non-reciprocal 45 degree phase shift of a light wave propagating through the bottom optical waveguide arm  106  in a backward direction. Thus, as illustrated on FIG. 2, the end result is constructive interference at the output waveguide Y-branch  130  of light waves propagated in the forward direction in the top and bottom optical waveguide arms  104 ,  106 , and +180 out of phase destructive interference at the input waveguide Y-branch  132  of light waves propagated in the backward direction in the top and bottom optical waveguide arms  104 ,  106 . This allows a forward propagating light wave to pass through the optical isolator  100  while extinguishing a backward propagating light wave.  
         [0022]    [0022]FIG. 4 illustrates a waveguide optical isolator testing apparatus to test a sample waveguide optical isolator  100  of the present invention. Testing apparatus  300  includes a laser source  302 , e.g. laser diode, an optical fiber half-wave plate and an optical fiber quarter-wave plate  304 , optical isolator  100 , a camera  320 , monitor  318 , and a photodetector  322 .  
         [0023]    The waveguide optical isolator  100  is tested using testing apparatus  300  by end fire coupling a light wave from the laser source  302  to an optic fiber  324 , connected to an optical fiber half-wave plate and an optical fiber quarter-wave plate  304 , focusing the light from the optical fiber half-wave plate and quarter-wave plate so as to cause the propagation of TM mode light waves in the ridge waveguides of the optical waveguide isolator  100 , and monitoring the light wave from the output waveguide section  108  with a silicon photodiode  322 , a camera  320  and monitor  318 . An output spatial filtering comprises a lens  326  and an aperture  328  is used to couple light only from the waveguide optical isolator  100  and, therefore, to eliminate any extraneous light before photodetection.  
         [0024]    Isolation measurements are made after applying opposing magnetic fields to the optical waveguide arms  104 ,  106 , of the waveguide optical isolator  100 , thus yielding an opposite sense of non-reciprocal phase retardation shift between optical waveguide arms  104 ,  106  in accordance with the non-reciprocal phase shift requirements described above. This is done by placing electromagnets on opposite sides of the waveguide optical isolator  100  with a separation of 6 mm. The electromagnets are mounted on XYZ translation stages for fine spatial adjustment of the magnetic field as applied to the first and second magnetically active optical waveguide arms  104 ,  106 . Further, the backward propagation for a light wave is simulated by reversing the polarities for both electromagnets, and the ratio in output light intensities for the two polarities of magnetic fields is taken as the isolation ratio.  
         [0025]    Measured extinction ratios of 19 dB with 2 dB excess loss at a wavelength of λ1.54 μm were obtained, where excess loss is defined as any loss other than material absorption, input mode coupling and waveguide Y-branch loss. In addition, the above extinction ratio and excess losses have been observed for wavelengths ranging from λ=1.4 μm to λ=1.7 μm.  
         [0026]    The present invention provides for a waveguide optical isolator fabricated using two arms, made of optical waveguides comprising magneto-optical material, in a Mach-Zehnder interferometer configuration. Because the waveguides of the device operate in the TM mode of propagation, there is no need to phase match TM and TE modes. It is noted that operation of the waveguides in the TE mode, is also possible by introducing horizontal asymmetries in the ridge waveguides. Furthermore, other optical waveguide configurations may be used instead of the ridge waveguide.  
         [0027]    Although the present invention has been described in detail with reference to specific exemplary embodiments thereof, various modifications, alterations and adaptations may be made by those skilled in the art without departing from the spirit and scope of the invention. In particular, the above described invention can be implemented with materials that exhibit the transverse magneto-optic non-reciprocal phase shift effect other than BiLuNe—IG or YIG. It is intended that the invention be limited only by the appended claims.