Abstract:
An integrated optical device functioning as optical isolator, shutter, variable optical attenuator, and modulator is disclosed. The device employs a Pockels cell for dynamically rotating with nanosecond speed the polarization state of incident light for attenuation and modulation. The invention provides a compact, high performance and reliable device without moving parts for use in laser systems and particularly in fiber optic telecommunication system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a free-space, polarization-dependent optical isolator, shutter, variable optical attenuator, and modulator device, and more particularly to a device which uses an optical Pockels cell, and Faraday rotator to control the optical polarization state. 
         [0003]    2. Description of the Related Art 
         [0004]    An optical isolator, shutter, variable attenuator, and modulator are all widely used in laser systems and particularly in modern fiber optical telecommunication networks. 
         [0005]    An optical isolator eliminates unwanted or reflected optical signals from interfering with a desired optical function. In fiber optic communications systems, some light may be reflected back from the fiber network. This reflected light affects the operation of the laser diode by interfering with and altering the frequency of the laser output oscillations. For this reason, an optical isolator is typically provided between the laser diode and the optical fiber to minimize the reflection from the fiber network. 
         [0006]      FIG. 1A  shows a conventional optical isolator configuration as disclosed in the prior art. The isolator includes two polarizers ( 14 - 1  and  14 - 2 ) and a Faraday rotator  16  disposed between the polarizers  14 - 1  and  14 - 2 .  FIG. 1B  shows the polarization plane orientation of two polarizers with polarization plane  11  of first polarizer  14 - 1  aligned with the x-axis and polarization plane  15  of second polarizer  14 - 2  aligned at 45 degree angle from x-axis. After passing through the first polarizer  14 - 1 , the Faraday rotator  16  rotates the polarization of the input beam of light  10 - 1  by 45 degrees, so that the beam of light  10 - 2  can then pass without change through the second polarizer  14 - 2 . 
         [0007]    The rotation of the polarization plane provided by the Faraday rotator  16  for a light ray traveling in one direction allows light to pass through both polarizers  14 - 1  and  14 - 2 , whereas for a light ray traveling in the direction opposite to beam  10 - 1 , the plane of polarization is rotated so that the passage of the back reflected light of light  10 - 3  through the Faraday rotator  16  is blocked by the first polarizer  14 - 1 . 
         [0008]    An optical attenuator is a very important element of an optical circuit for controlling an optical signal transmission. In fiber optic communication systems, variable optical attenuators are broadly employed to regulate the optical power levels to prevent damage to the optical receivers caused by irregular optical power variations. As the optical power fluctuates, a variable optical attenuator is employed, in combination with an output power detector and a feedback control loop, to adjust the attenuation and to maintain at a relatively constant level the optical power transmitted to a photo-receiver. Optical signal attenuation can be accomplished in a variety of ways by diverting all or a portion of an optical signal from an original pathway. 
         [0009]    Variable optical attenuators (VOA) have been developed with a variety of technologies. Currently, there are several types of commercially available variable optical attenuators in the market, such as opto-mechanical VOA devices using stepping motor or magneto-optical crystal, devices using liquid crystal (LC) technology, and devices using micro-electro-mechanical systems (MEMS) technology. 
         [0010]    Optical shutters or switches implemented with Pockels cells, which are based on an electro-optical crystal&#39;s birefringence, are usually found in non-telecommunications applications, mainly because of the cell&#39;s very high voltage requirement. The Pockels electro-optic effect produces birefringence in an optical medium by a constant or varying electric field. The electric field can be applied to the crystal medium either longitudinally or transversely to the light beam. Longitudinal Pockels cells need transparent or ring electrodes. Transverse voltage requirements can be reduced by lengthening the crystal. A Pockels cell combined with two polarizers can be used for a variety of applications.  FIG. 2A  shows a simple configuration of a Pockels cell-based device having multiple functions such as an optical variable attenuator and modulator as disclosed in the prior art. 
         [0011]      FIG. 2B  shows the polarization plane orientation for a normally closed shutter (the shutter is closed without applied voltage) of two polarizers with the polarization plane  21  of the first polarizer  24 - 1 , in  FIG. 2A , aligned with x-axis, and the polarization plane  25  of second polarizer  24 - 2  aligned with y-axis. When a variable electric field generated by Pockels cell driver  29  has a value between zero and the half-wave voltage, i.e., the voltage under which the plane of polarization of incident light is rotated 90 degrees by Pockels cell, the input light beam  20 - 1  can be attenuated from completely off to completely transparent when light beam  20 - 2  exits polarizer  24 - 2 . 
         [0012]    Such a configuration is not often used in fiber optical telecommunication system mainly because of the extremely high voltage (half wave voltage is about a few thousand volts or even higher) requirements, even though the response time is ultrafast, on the order nanoseconds. However, with the development of the new materials, the voltage required to create birefringence has been substantially reduced, and therefore the present invention is a viable approach for fiber optical telecom networks, especially in transmitters with capability of directly modulating the signal emitted from at laser at very high frequencies and low voltage. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    One embodiment of the present invention is an integrated, multifunctional optical device. The device includes a first and second polarizer, a Pockels cell, a Faraday rotator, at least one electric driver for the attenuator and shutter and an electric driver for the modulator. Each of the polarizers has a particular polarization plane. The polarization plane of said second polarizer is oriented at a 45 degree angle to the plane of said first polarizer. The first polarizer receives the input light beam and the second polarizer provides output light beam. The Pockels cell is configured to rotate an input beam based on an external voltage. The Faraday rotator disposed between the first polarizer and the Pockels cell. The Pockels cell is disposed between said Faraday rotator and said second polarizer to receive the input beam from the Faraday rotator. The Pockels cell has two birefringent axes aligned at 45 degrees symmetrically to the polarization plane of the said second polarizer. One electric driver is configured to drive the attenuator and the shutter. Another electric driver is configured to drive the modulator. 
         [0014]    It is an object of the present invention to provide an integrated, compact and multi-functional optical device operating as optical isolator, shutter, variable attenuator, and optical modulator without moving parts for applications in variety of laser systems and, in particular, in fiber optic telecommunication networks. 
         [0015]    It is also an object of the present invention to provide ultrafast (nanosecond level) variable attenuation and shuttering of optical signals. 
         [0016]    With such a simple and integrated design leading to better manufacturability, it is further an object to make such a device mass producible at low cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0018]      FIG. 1A  shows an optical isolator design utilized in the prior art; 
           [0019]      FIG. 1B  illustrates the polarization plane orientation of two polarizers in an isolator design; 
           [0020]      FIG. 2A  shows an attenuator design utilized in the prior art; 
           [0021]      FIG. 2B  illustrates the polarization plane orientation of two polarizers in the attenuator design; 
           [0022]      FIG. 3A  shows an embodiment of present invention for an integrated device with multiple functions; 
           [0023]      FIG. 3B  illustrates the polarization plane orientation of two polarizers in an embodiment of the present invention; and 
           [0024]      FIG. 3C  illustrates the orientation of two axes of the birefringent Pockels cell with respect to the polarization plane orientation of two polarizers. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    In a preferred embodiment, the configuration of an integrated device performing the functions of isolator, attenuator, shutter, and modulator is schematically shown in  FIG. 3A . For easy understanding of the description, the axes of coordinates are set as follows. Let a z-direction (toward the right in the drawing) indicate the direction in which the optical components are aligned and let the x-direction (vertical direction) and y-direction (horizontal direction) indicate the two directions orthogonal to the z-direction. The polarization plane  31  of the first polarizer  34 - 1  is aligned with the x-axis, and the polarization plane  35  of second polarizer  34 - 2  is aligned at 45 degrees to the x-axis as shown in  FIG. 3B . In this system, the collimated input light beam  30 - 1  is substantially linearly polarized, substantially coherent, monochromatic and collimated. The input light beam  30 - 1  is also oriented with the polarization axis of the first polarizer  34 - 1  to pass through the first polarizer  34 - 1 , which performs the Jones matrix operation 
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         [0000]    The polarization orientation of light beam  30 - 2  is rotated 45 degrees (in x and y plane) after passing through the Faraday rotator  36 , i.e., 
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         [0000]    which is designed to work for the prescribed wavelength of the input light since the rotation angle by Faraday rotator is wavelength dependent. Typically, the Faraday rotator is configured for a single wavelength or within a certain wavelength range to meet the wavelength requirement for the application. 
         [0026]    The Pockels cell  37  includes a transparent isotropic or non-birefringent medium. The Pockels cell medium is selected to meet specific wavelength requirement according to the application. Without an applied electric field, the cell  37  allows the light  30 - 3  to pass through the second polarizer  34 - 2 , which is oriented at 45 degrees from the polarization plane  31  of first polarizer  34 - 1 . Under these conditions, for the input light propagating in the z-direction, the system is transparent to the input light beam  30 - 1  but blocking to the reflected light of light beam  30 - 4 . The configuration depicted in  FIG. 3A  operates in a fashion similar to the isolator depicted in  FIG. 1A  which provides substantial attenuation in a backward direction. 
         [0027]    When the electric field generated by driver  32  and/or  33  is applied, the Pockels 
         [0000]    cell  37  becomes a voltage-controlled wave plate, i.e., 
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         [0000]    with birefringent axes  39 - 1  and  39 - 2  aligned with x-axis and y-axis as shown in  FIG. 3C . In order for the system to work as a shutter, drive  32  applies a voltage V sufficient to make Pockels cell  37  become a half wave plate, i.e., 
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         [0000]    which rotates the polarization direction of the input light  30 - 2  exiting from the Faraday rotator  36  by 90 degrees in x-y plane. Such a voltage is usually called half-wave voltage. The light beam  30 - 3 , with its polarization plane 90 degrees to that of the polarizer  34 - 2 , is then completely blocked by the polarizer  34 - 2 . Due to the ultrafast response time of a typical Pockels cell, such a shutter can be made with nanosecond switching time as disclosed in the art. 
         [0028]    With the applied voltage less than the half-wave voltage, the system acts as an attenuator. By varying the applied voltage, the intensity of the incident light beam  30 - 1  reaching the output  30 - 4  via the second polarizer  34 - 2  varies from being fully transmitted to being completely blocked. In practice, some insertion losses are incurred even without the applied electric field due to the material absorption, scattering, reflection, and misalignment of the polarization axes, etc. It should be noted that even though the back reflection isolation of light beam  30 - 4  of the preferred system configuration is degraded when the system is used as an attenuator, the back reflection is also substantially reduced for the attenuated input light beam  30 - 1 . Therefore, the total back reflection isolation for the system is not significantly sacrificed. It is clear that the system can modulate the input light  30 - 1  when Pockels cell  37  is driven by modulator driver  33 . Due to the high half wave voltage needed, a high modulation frequency is difficult to achieve for on-off states, but small amplitude modulation of light  30 - 1  is practical. 
         [0029]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the device can be configured as a normally-closed shuttle (i.e., the device will completely block the input light beam without applying voltage to Pockels cell  37 ) by rotating the polarization orientation plane  35  of the second polarizer  34 - 2  90 degrees from its normally-open position depicted on  FIG. 3B . In another version, all facets of the optical components including the polarizers, the Faraday rotator, and the Pockels cell can be coated with multiple layers of anti-reflection dielectric thin films to eliminate the reflections and reduce the light insertion loss. Additionally, the first polarizer, the Faraday rotator, the Pockels cell and the second polarizer can be bonded with adhesives which are transparent to the selected optical wavelength, or bonded by adhesives without covering or interfering with areas through which the light beam passes. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.