Abstract:
A light beam on a single optic fiber carries time division multiplexed information originating from multiple sensors driven from a single light source. An optical switch apparatus coupled in series between the source and the sensors switches the light beam of the source between ON and OFF states. The switch apparatus includes an electro-optic switch and an acousto-optic switch coupled in series with the electro-optic switch. A controller and drivers for the electro-optic and acousto-optic switches produce in an appropriate time sequence control signals resulting in a desired ON and OFF time of the light beam being switched.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application contains subject matter that is related to the subject matter of the following application, which is assigned to the same assignee as this application. The below-listed application is hereby incorporated herein by reference in its entirety:
         “ELECTRO-OPTIC SWITCHING APPARATUS NOT REQUIRING DC BIAS,” by Dr. David B. Hall and Mr. Carl Bathelt, Ser. No. 11/083,736, filed concurrently herewith.       

     TECHNICAL FIELD 
     The invention relates generally to optical switches and more particularly to optical switches capable of significant ON to OFF attenuation ratios. 
     BACKGROUND 
     Time division multiplex (TDM) architectures often demand data at high throughput sampling rates. Such architectures often use a low duty cycle optical ON/OFF switch with fast rise and fall times to generate a stream of pulses. Because of the high coherence of the laser source, leakage light caused by the incomplete turn-OFF of the optical switch can produce excessive phase noise at the receiver and drastically reduce the overall signal-to-noise ratio. This may be due to bleed-through light during the OFF time interval of a single optical source causing unwanted signal output from a plurality of sensors all driven by the light output from the same optical source. This may be a problem where there are a large number of optical light sources in the system producing stray and/or bleed-through light that contaminates the desired light signal traveling in an optical fiber in a given time slot in the TDM system. 
     The required ON/OFF attenuation ratio for an optical switch increases with the number of sensors driven by the light from a single optical source and with the number of laser sources that generate light beams that are switched through the fiber. For example, fiber optic acoustic sensor systems, such as using a plurality of Mach Zehnder interferometers, may operate from a light beam from a single laser source and employ a single fiber-optic “return” path where the output from each interferometer is time division multiplexed onto the return path. A plurality of multiplexed signals carried by a single fiber in such a system gives rise to the need for an optical switch with a significant OFF attenuation factor. For such a system using 64 sensors with 64 corresponding TDM light outputs, the required OFF attenuation for the switch is 60–75 dB to maintain a good signal to noise ratio. A known approach to achieve this requirement has been to use a pair of expensive lithium niobate electro-optic switches connected in series in order to achieve the needed OFF attenuation. There exists a need for a cost-effective way to provide optical switching that can satisfy the requirements of such a system. 
     SUMMARY 
     It is an object of the present invention to provide an optical switch that satisfies this need. 
     The invention in one implementation encompasses an optical switching apparatus. The apparatus includes an electro-optic switch connected in series with an acousto-optic switch. A signal input of one of said switches receives an input light beam from a corresponding optical transmitter and a signal output of the other of the switches is coupled to the single optic fiber that carries the switched light beam. The sum of the switching transition times required for the apparatus to turn from OFF to ON and from ON to OFF being substantially less, e.g. preferably less than 5%, of the OFF time interval of the apparatus. This permits the higher attenuation level of the acousto-optic switch in combination with faster switching speed of the electro-optic switch to be combined to form a cost effective optical switching apparatus with desired characteristics. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
         FIG. 1  is a representation of one implementation of an apparatus that comprises an optical switch in accordance with the present invention. 
         FIG. 2  is a block diagram of an exemplary system employing an ON/OFF optical switch. 
         FIGS. 3 and 4  are graphs illustrating the switching times and attenuation levels achieved by the optical switch as shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of the present invention in which it is desired to provide optical switching of a light beam carried by optical fiber  10 . An electro-optic (EO) switch  12  receives the input light beam from fiber  10  and provides ON/OFF switching in response to a control signal provided an input  14 . The electro-optic switch  12  may comprise an electro-optic switch available from switch supplier JDS Uniphase. The switched output of electro-optic switch  12  is carried by optical fiber  16  and serves as an input to acousto-optic (AO) switch  18  which may comprise an acousto-optic switch available from switch suppliers Brimrose or Isomet. A control signal carried at input  20  of the acousto-optic switch  18  controls the ON/OFF switching of the switch. The switched output is carried by optical fiber  22 . 
     A controller  24  receives an input (clock signal)  26  representative of the desired ON and OFF duty cycle for switching the light beam on fiber  10 . The generator creates ON/OFF control signals that control EO driver  28  and AO driver  30  that provide appropriate ON and OFF gating signals on inputs  14  and  20  for electro-optic switch  12  and acousto-optic switch  18 , respectively. The signal, e.g. a series of pulses having first and second voltage levels, received on input  26  will typically be generated by a system clock (not shown) associated with the TDM system. The control signal supplied to inputs  14  and  20  will vary depending upon the required type of input signal required by each switch. For example, the signal supplied as input  20  for the acousto-optic switch  18  may comprise a frequency modulated control signal to produce a corresponding acoustic wave in a crystal or other piezoelectric material utilized as the active switching element in the acousto-optic switch. The signal supplied as input  14  for the electro-optic switch  12  may comprise an electrical signal utilized to produce and control the magnitude of an electrical field that in turn controls the active optical switching element in the electro-optic switch. AO and EO drivers for controlling the switching of such optical switches are known. Specific timing requirements concerning the generation of the ON and OFF control signals for each switch are discussed below. 
     Using an AO switch in series with an EO switch results in a cost-effective optical switching arrangement that can be constructed with commercially available off the shelf switches for switching applications requiring substantial attenuation during OFF times and having total rise and fall times that are only a small percentage of either the ON or OFF times. The EO switch has relatively fast rise and fall times (2 nanoseconds “ns”) and a moderate OFF attenuation level (25 decibels “dB”). The AO switch is generally complementary to the EO switch in regard to its characteristics. The AO switch has slower rise and fall times (20 ns) but a substantially greater OFF attenuation level (50 dB). The indicated levels of attenuation are exemplary. In general, the EO and AO switches may provide attenuation of 15–25 dB and 35–50 dB, respectively. 
       FIG. 2  shows an exemplary system incorporating an optical switch such as described in  FIG. 1 . A laser  40  supplies a light beam input to ON/OFF optical switch  42  that provides its switched output to an optical amplifier  44 . An array of sensors  46  receives the amplified light output from amplifier  44  as an input and provides a TDM series of outputs that are transmitted to optical receiver  48 . The array of sensors  46  may comprise a plurality of individual sensors S 1 , S 2  . . . SN that each receives the switched light input delayed in time by delay elements  50 . The input light pulse is delayed in time relative to each sensor in order to produce a sequential series of output pulses that do not overlap in time. Each of the illustrated sensors may comprise an interferometer that functions as an acoustic sensor. 
       FIGS. 3 and 4  are graphs illustrating the individual switched outputs of an AO switch, an EO switch, and the total switching output of the switching arrangement as shown in  FIG. 1 . As indicated in the keys of these figures, the output response of the AO switch is shown as a dashed line, the output response of the EO switch is shown as a solid line, and the total output response for the switching arrangement of  FIG. 1  is shown as a dotted line. The individual switch characteristics are explained for each switch as if the subject switch was the only switch being tested. The output response of the AO switch is in response to a 280 ns control pulse centered about T(0). The output response of the EO switch is in response to a 200 ns control pulse centered about T(0). 
       FIG. 3  shows the output response  60  of the AO switch, output response  62  of the EO switch and the combined response  64  of the AO and EO switches. These outputs are shown in  FIG. 3  on a linear amplitude (attenuation) vertical scale. As seen in this scale, the response  62  of the faster EO switch is substantially the same as the combined output response  64 . To better appreciate the contribution made by the AO switch, the output responses should be viewed with a vertical scale having greater dynamic range; see  FIG. 4 . The 10% to 90% rise and fall times are approximately 2 ns for the EO switch and 20 ns for the AO switch. The time required for state transitions by the AO switch is greater than 5 times that for the EO switch. 
     As seen in  FIG. 4 , the AO switch provides an OFF attenuation level of 50 dB relative to an ON level of 0 dB in response  66 . The EO switch provides an OFF attenuation level of 25 dB relative to an ON level of 0 dB in response  68 . The total switching characteristic for the switch combination as shown in  FIG. 1  is depicted as dotted line  70 . This ON/OFF response characteristic reflects the summation of the individual characteristics for the AO switch and EO switch as explained above. Assuming the control pulses are applied to the respective switches as described above, the switch arrangement provides an OFF attenuation level of 75 dB relative to an ON level of 0 dB. It will be apparent that the ON level of 0 dB is only a reference level and that some amount of actual insertion loss will be incurred during the ON state. It takes approximately 60 ns for the combined switch to transition between ON and OFF states. It will be seen that from approximately −120 ns to +120 ns the cumulative switch output  70  is approximately the same as the EO switch output  68 . This is because this time frame falls within the ON time cycle of the AO switch. At approximately −130 ns to +130 ns the rising and falling edges of the AO switch began to contribute to the cumulative attenuation. The timing of the control pulses as generated by the controller  24  plays an important role in the overall operation of the switch combination is shown in  FIG. 1 . For example, because the acousto-optic switch has a slower response time than the electro-optic switch, the control signal to the acousto-optic switch must start earlier than the control signal to the electro-optic switch so that both switches are in an ON state at the desired ON start time. 
     The controller  24  may comprise available digital circuitry such as shift registers and control gates connected to supply the required timing of the control signals for the switches. 
     Alternatively, controller  24  may comprise a microprocessor and operate under control instructions to generate the required timing for the control signals for the switches. 
       FIG. 2  shows the switching apparatus of  FIG. 1  utilized in a time division multiplexed system. In accordance with TDM techniques, only one output pulse from one of the sensors is to be transmitted at any given time. While the switch is in the OFF state, it must provide sufficient attenuation to prevent significant amounts of the light bleed-through from reaching the sensors. Such bleed-through light represents noise that will degrade the signal-to-noise ratio. When sensor S 2  is receiving the switched light pulse and generating a corresponding output pulse containing encoded sensor information, the other sensors should ideally be receiving no light and hence producing no outputs. Any bleed-through light during the OFF state of the optical switch will cause the other sensors to produce an undesired output that functions as noise concurrent with the output of S 2 . 
     In an exemplary TDM system the illustrative optic switching arrangement of  FIG. 1  is desired to be ON for approximately 200 ns and OFF for 10 microseconds (10,000 ns). During the OFF to ON transition for time from −160 to −100 (60 ns) and for a similar interval for the ON to OFF transition, the switch provides an attenuation level of less than 75 dB. For this example, the ON time is defined from time −100 ns to +100 ns. The OFF cycle time with less than 75 dB of attenuation is 120 ns/10,000 ns=1.2%. This percentage of time should be less than 20% and preferably less than 5% to minimize adverse impact on the signal to noise level. Further, it should be remembered that even during almost all of this 120 ns transition time at least 25 dB of attenuation is being provided. The 1.2% of the total OFF time having less than 75 dB of attenuation has a negligible impact on system performance. For systems having an OFF time that is substantially longer, e.g. 10 times longer, than the cumulative times for state transitions of the switch arrangement as shown  FIG. 1 , the combination of an AO switch in series with an EO switch behaves substantially like a single EO switch with an attenuation level of 75 dB. This of course requires that the control input of the AO switch receive a control pulse that is longer than and overlaps in time duration the control pulse delivered to the EO switch so that the AO switch is fully ON at the time the EO switch transitions from OFF to ON and from ON to OFF. 
     The exemplary optical switch of the present invention utilizes a single EO switch in combination with an AO switch to provide a cost effective optical switch that satisfies a required attenuation level in a TDM system. The performance of the system, especially with regard to noise caused by bleed-through light, is maintained at a high level since a maximum required attenuation level is provided for substantially all of the OFF system time. This provides a cost-effective solution as compared with one or more custom chips containing integrated optical switches as utilized in the past. 
     Although an exemplary implementation of the invention has been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.