Abstract:
An optical test instrument, for testing response of an optical medium to optical energy, comprises a light source for emitting optical energy in response to electrical energy, a detector for providing an electrical output signal in response to incident optical energy, and a propagation path for delivering optical energy from the light source to the medium under test and for delivering optical energy from the medium under test to the detector. A positioning mechanism displaces an optical attenuator element between a first position, in which it is out of the propagation path, and a second position, in which it is interposed in a segment of the path in which the optical energy is propagated as a beam.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an optical fiber test instrument with a mechanically positioned optical attenuator. 
     It is conventional to use an optical fiber to communicate an information signal from an optical driver at a proximal end of the fiber to an optical receiver at a distal end of the fiber. The optical driver includes a laser diode that generates pulses of optical power at a frequency that depends on the information signal and launches the pulses into the fiber at the proximal end thereof. Some of the optical energy that is launched into the fiber is returned to the proximal end of the fiber due to Rayleigh back-scattering. It is of interest to the manufacturer of an optical fiber to determine the variation with distance along the fiber of the intensity with which optical energy undergoes Rayleigh back-scattering. This may be achieved using an optical time domain reflectometer (OTDR). 
     The conventional OTDR shown in FIG. 1 comprises a laser diode 2 which emits light at, for example, 1310 nm and is optically coupled through a single mode fiber 4, a directional coupler 6, a second single mode fiber 8, and a front panel connector 10 to the proximal end of a fiber under test, or target fiber, 12, which also is a single mode fiber. Current pulses are applied to the laser diode by a pulse generator 14 through a laser driver amplifier 16 and cause the laser diode 2 to emit brief pulses of light that are launched into the target fiber 12. 
     Optical energy that is reflected and back-scattered within the fiber 12 is coupled through the front panel connector 10, the fiber 8, the directional coupler 6, and a multi-mode fiber 18 to a photodiode detector 20. The detector 20 generates a current signal depending on the power with which return optical energy is emitted from the fiber 18 at its proximal end. A first amplifier 24 converts the current signal provided by the detector 20 to a voltage signal, and a second amplifier 28 amplifies the voltage signal and applies it to an analog-to-digital (A/D) converter 32. The digital signal provided by the A/D converter is processed by a processor 36, which is used to provide a display on a display device 40, e.g. a cathode ray tube, of the level of return power as a function of distance. Operation of the reflectometer is controlled by a controller 44. 
     The dynamic range of the optical signal received at the detector of an OTDR is enormous. In general, the intensity of the optical signal depends upon the distance along the fiber at which the back-scattering took place: the intensity of the back-scattered energy decreases with distance from the proximal end of the fiber under test. Therefore, when using the OTDR to examine a range that is close to the proximal end, the intensity of the back-scattered energy is higher than when examining a range that is far from the proximal end. 
     In order for the digital values provided by the A/D converter 32 to represent accurately the level of return power received at the detector 20, it is necessary that the detector not be overloaded by the return power, that amplifiers 24 and 28 not be saturated, and that the voltage signal applied to the A/D converter be within the converter&#39;s range. 
     U.S. Pat. No. 4,960,989 (Liebenrood et al) discloses an OTDR in which the gain of the detector is adjusted as a function of time relative to pulsing of the laser diode in order to prevent overdriving of the first amplifier. U.S. Pat. No. 4,960,989 is not concerned with the variation in power level due to change in intensity of Rayleigh back-scattering but with masking the effect of reflections, for example at connections between lengths of fiber and at breaks in the fiber. 
     It has also been proposed that the intensity of back-scattered light received at the detector should be controlled by controlling the current that is used to drive the laser diode. However, as the current is varied, the spectral characteristics of the light emitted by the laser diode also vary, and therefore the measurement results are not accurate. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is provided an optical test instrument, for testing response of an optical medium to optical energy, comprising a light source means for emitting optical energy in response to electrical energy, a detector for providing an electrical output signal in response to incident optical energy, a propagation means defining a path for delivering optical energy from the light source means to the medium under test and for delivering optical energy from the medium under test to the detector, at least one segment of the path defined by the propagation means being a segment in which the optical energy is propagated as a beam, an optical attenuator element, and a positioning means for displacing the optical attenuator element between a first position, in which it is out of said path, and a second position, in which it is interposed in said one segment of the path. 
     In accordance with a second aspect of the present invention there is provided an optical device for selectively switching optical energy received at a first optical input port or a second optical input port to an optical output port, comprising an optical coupling element, a first positioning means for displacing the optical coupling element between a first position, in which the first optical input port is coupled to the optical output port, and a second position, in which the second optical input port is coupled to the optical output port, whereby the first or second optical input port is selected, an optical attenuator element, and a second positioning means for displacing the optical attenuator element between a first position, in which it is out of the path from the selected optical input port to the optical output port, and a second position, in which it is interposed in said path. 
     In accordance with a third aspect of the present invention there is provided an optical device for selectively switching optical energy received at a first, second, or third optical input port to an optical output port, comprising a first optical coupling element, a second optical coupling element, a first positioning means for displacing the first optical coupling element between a first position, in which it receives light from the first optical input port and directs it toward the optical output port, and a second position, and a second positioning means for displacing the second optical coupling element between a first position, in which it receives light from the second optical input port and directs it toward the optical output port, and a second position, the second positions of the first and second optical coupling elements being such that when they are both in their second positions, the third optical input port is coupled to the optical output port and when either optical coupling element is in its first position, the third optical input port is optically isolated from the optical output port, whereby the first, second, or third optical input port is selected, and the device further comprising an optical attenuator element, and a third positioning means for displacing the optical attenuator element between a first position, in which it is out of the path from the selected optical input port to the optical output port, and a second position, in which it is interposed in said path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: 
     FIG. 1 illustrates schematically a conventional OTDR, 
     FIG. 2 is a partial schematic illustration of a first OTDR embodying the present invention, 
     FIG. 3 is a partial schematic illustration of a second OTDR embodying the present invention, 
     FIG. 4 is a schematic plan view of a conventional 4 by 1 optical switch, and 
     FIG. 5 is a partial schematic illustration of a third OTDR embodying the present invention. 
    
    
     DETAILED DESCRIPTION 
     The reflectometer shown in FIG. 2 is similar to that shown in FIG. 1 except that the fiber 4 is divided into two segments 4a and 4b, which are connected to respective graded index (GRIN) lenses 48a and 48b. The lenses are in coaxially aligned confronting relationship across a free space 50. Accordingly, a light pulse from the laser diode 2, traveling to the directional coupler 6, is propagated through the fiber segment 4a in confined manner and is converted to a collimated light beam by the GRIN lens 48a. The light beam traverses the space 50 and is focused into the fiber segment 4b by the GRIN lens 48b, and the light then continues in confined manner. 
     A neutral density filter 54 is mounted adjacent the space 50 on an arm 58 that is movable under control of a solenoid 62 to displace the arm between a first position in which the filter 54 is outside the space 50 and a second position, shown in dashed lines, in which the filter 54 is inside the space 50. Thus, in the second position of the arm 58, the filter 54 is traversed by the light beam passing from the GRIN lens 48a to the GRIN lens 48b. In this manner, an optical attenuator is selectively interposed in the fiber 4 under control of the solenoid 62. 
     The solenoid 62 is energized by current supplied by an amplifier (not shown) operating under control of the controller 44. During a far probing interval, in which the controller controls operation of the A/D converter so that the digital values are representative of the level of power with which light is back-scattered from a range that is far from the front panel connector 10, the solenoid places the arm 58 in its first position so that the optical pulse launched into the fiber under test is not attenuated by the filter. On the other hand, during a near probing interval, in which the controller controls the A/D converter so that the OTDR examines a range that is close to the connector 10, the solenoid is controlled to position the arm in its second position, so that the light pulses that are launched into the fiber 12 are attenuated by the neutral density filter and the intensity of return light received at the detector is reduced correspondingly. The optical power incident on the detector is lower than if the arm were in its first position and so the detector is protected from overloading and the signal applied to the A/D converter is within the range of the A/D converter. 
     It will be appreciated that when testing a single target fiber 12 in the manner described above, two sets of digital values will be obtained, depending on the position of the neutral density filter, for the two different ranges from the connector 10, and if these two sets of values are used, without change, to provide a display of return power versus distance, the display will comprise two traces that are offset from each other along the power axis. Generally, such a result would be undesirable, and accordingly the values acquired when the arm is in the second position are scaled so that the two traces are aligned and merged to form a single trace. 
     It is not essential to the OTDR shown in FIG. 2 that the GRIN lenses and the space 50 be inserted in the fiber 4. The same result would be obtained if the attenuator were placed in the fiber 18 at the location 64. Alternatively, the attenuator may be placed at the location 66 in the fiber 8. In the case of the attenuator&#39;s being placed in the fiber 8, the desired attenuation of the attenuator would be half that in the case of the attenuator&#39;s being placed in the fiber 4 or the fiber 18, because the attenuator is traversed twice by light passing from the laser diode to the detector. 
     By using a mechanically positioned optical attenuator to reduce the intensity of light incident on the detector when examining the fiber 12 over a range that is close to the connector 10, it is unnecessary to alter the current supplied to the laser diode, and accordingly the current can be kept constant and the spectral characteristics of the light emitted by the laser diode are not affected by change in current. 
     In certain applications of an OTDR, it is desirable to be able to test the target fiber at multiple wavelengths. FIG. 3 illustrates an OTDR that is able to test the target fiber 12 at two wavelengths. The OTDR shown in FIG. 3 comprises two laser diodes 2 1  and 2 2 , emitting light at 1310 nm and 1410 nm respectively, for example. The two diodes are coupled through respective fibers 68 1  and 68 2  to a 1410/1310 wavelength division multiplexer 70 having an output port connected to the fiber 4. The wavelength division multiplexer 70 is a directional coupler that defines two optical paths. Light at 1410 nm that enters the coupler along one path is confined within that path and exits the coupler on the same path, whereas light at 1310 nm that enters the coupler along one path crosses over and exits the coupler on the other path. Thus, the wavelength division multiplexer couples both the diode 2 1  and the diode 2 2  to the fiber 4. The attenuator is connected between the fiber segments 4a and 4b, as shown in FIG. 2. The OTDR shown in FIG. 3 therefore enables the target fiber to be tested at the wavelength of light emitted by either or both laser diodes. 
     FIG. 4 illustrates a 4 by 1 optical switch that comprises a rectangular housing 80 having four input ports 82 1  -82 4  along one edge and one output port 84 at the opposite edge. Each input port comprises a GRIN lens 88 having an optical fiber 90 pigtailed thereto. Similarly, the output port comprises a GRIN lens 94 having an optical fiber 96 pigtailed thereto. The axes of the input ports 82 1  -82 3  are in spaced parallel relationship with the axis of the output port 84, whereas the axis of the input port 82 4  is aligned with the axis of the output port 84. Three prisms 98 1  -98 3  are mounted on respective arms 102 1  -102 3  and are displaceable under control of respective solenoids 104 1  -104 3  to positions in which they couple the ports 82 1 , 82 2 , and 82 3  respectively to the output port 84, and isolate the input port 82 4  from the output port. Thus, by energizing the solenoid 104 1 , for example, the input port 82 1  is coupled to the output port 84 and the input port 82 4  is isolated from the output port, whereas if no solenoid is energized, the input port 82 4  is coupled to the output port. 
     An optical switch of the kind described above is sold by JDS FITEL, Inc. of Ontario, Canada under the designation SW14AZ-00 NC-068. This switch also has a fourth mounting arm 108 and a fourth solenoid 110 that are redundant to the function of the switch as a 4 by 1 switch but allow it to control a fourth prism, shown in dashed lines in FIG. 4, and implement a 5 by 1 switch. In this application, only one solenoid would ever be energized at any one time. 
     The OTDR shown in FIG. 5 comprises four laser diodes 2 1-  2 4  operating at 1310 nm, 1360 nm, 1410 nm, and 1550 nm respectively. The four laser diodes are connected by respective fibers to the four input ports 82 1  - 82 4  respectively of a 4 by 1 optical switch 112 of the general type described above with reference to FIG. 4. The output port 84 of the switch 112 is connected to the fiber segment 4b. By controlling the solenoids 104 1  -104 3 , optical pulses from any one of the laser diodes 2 1  -2 4  can be coupled into the fiber segment 4b and used for testing the fiber 12. 
     The switch 112 shown in FIG. 5 differs from the switch shown in FIG. 4 by virtue of the fact that a neutral density filter 114 is mounted on the mounting arm 108 and the solenoid 110 can be energized without regard to the states of the solenoids 104 1  -104 3 . By energizing the solenoid 110, the filter 114 can be positioned immediately upstream of the output port 84 of the switch. Therefore, if the laser diode 2 1 , for example, is selected by energizing the solenoid 104 1 , by controlling the state of the solenoid 110 it is possible to select whether the optical pulses from the selected laser diode are attenuated by the filter 114 or are coupled into the fiber segment 4b without being attenuated. 
     It will be appreciated that the invention is not restricted to the particular embodiments that have been described, and that variations may be made therein within departing from the scope of the invention as defined in the appended claims and equivalents thereof. For example, the invention is not restricted to the particular wavelengths mentioned in connection with the description of the various embodiments of the invention. Also, although the embodiments shown in FIGS. 3 and 5 employ two laser diodes and four laser diodes respectively, the invention as implemented in FIG. 3 is not limited to use of only two laser diodes and similarly the invention as implemented in FIG. 5 is not limited to use of four laser diodes.