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:
The invention relates to optical switches and more particularly to the control of the ON/OFF switching of electro-optic switches.
Electro-optic switches are used for a variety of applications in which a laser light beam is switched between ON/OFF states. For example, electro-optic switches utilized in telecommunication systems are typically maintained in an OFF state by an applying a bias DC voltage, e.g. −2 volts, to the control input and are switched to a maximum light transmission state (ON) by causing +2 volts to be applied to the control input. This can be accomplished by adding a +4 volt pulse to the −2 volt DC bias. Such electro-optic switches have generally proved successful in such applications.
Problems are associated with a drift of the applied DC bias necessary to keep the switch in a normally OFF state. Charge migration effects within the optical switch cause this drift over time and/or temperature. Failing to maintain the proper DC bias voltage results in the electro-optic switch not being fully OFF, i.e. the maximum attenuation that can be provided by the switch will not be attained. This causes in a decrease in the ON to OFF signal ratio and increases the amount of bleed-through light passing through the switch during the OFF state. Thus, there exists a need for an improvement in controlling electro-optical switches that will minimize this problem.
It is an object of the present invention to substantially solve this problem.
The invention in one implementation encompasses an apparatus. The apparatus includes an electro-optic attenuator (switch) that provides controllable attenuation of a light beam. The electro-optic attenuator has a control input and has a maximum attenuation level that results when either a first or second control voltage is applied to the control input. A driver of the control input includes an AC generator that generates minimum and maximum peak voltages that correspond to the first and second control voltages, respectively, so that the maximum attenuation level is provided by the electro-optic attenuator while either of the minimum and maximum peak voltages is applied to the control input. This keeps the electro-optic switch in a maximum OFF state as a normal condition without requiring a DC voltage source for providing a DC bias voltage to the control input.
Another implementation of the invention encompasses a method for controlling an electro-optic switch having a control input and switching a light beam between an ON and OFF state. The ON or OFF state results when either first or second control voltages are applied to the control input and the other state results when a third control voltage different from the first or second voltages is applied to the control input.
Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:
One of the aspects of the present invention resides in the recognition that an AC waveform, with appropriate peak-to-peak voltages, can be employed to replace the normally used DC bias voltage to maintain the electro-optic switch in either a normally ON or OFF state. As will be explained below, an EO driver can include a square wave generator with an appropriate peak-to-peak voltage so as to provide a normally maximum OFF (null) state of the electro-optic switch without requiring the use of a bias voltage supplied by a DC voltage source.
A controller 38 receives clock information on input 40 where the clock information provides a periodic time frame that defines a frequency of operation. In the illustrative example, the switch 14 is switched ON in accordance with the periodic time frame to provide a corresponding series of light pulses were the duration of the light pulse is relatively small compared to the time duration between light pulses. The controller 38 provides an output 42 to the EO driver 44 and an output 46 to the AO driver 48. The AO switch 36 may comprise a commercially off the shelf available component such as from Brimrose Company and the EO switch 32 may likewise comprise a commercially off the shelf available component such as from JDS Uniphase Company. The AO driver 48, which may be integrated with the switch itself, may comprise a known driving arrangement for the AO switch and may require a nominal TTL output voltage states to define corresponding OFF and ON states. The EO switch 32 has an input terminal that is responsive to an applied control voltage to cause the switch provide a corresponding attenuation level as will be explained in the discussion of
The exemplary EO driver 44 in accordance with the present invention preferably includes a square wave signal generator in which the peak voltage amplitudes of the square waves correspond to different first and second voltages each associated with a maximum OFF state of the EO switch. Of course, it will be understood by those skilled in the art that the first and second voltages could be each associated with a maximum ON state of the EO switch if it was desired to maintain the EO switch in a normally ON state instead of a normally OFF state. The EO driver 44 further includes the ability to generate a voltage pulse that corresponds to the ON state of the switch where the magnitude of the voltage pulse corresponds to a maximum ON state of the switch. The EO switch may comprise a conventional square wave voltage generator selected to have a frequency/time frame compatible with the duration of the ON pulse time. For example, for an ON pulse duration of 0.2 microseconds (μs) with a 10 μs period between ON pulses, the period/frequency of the square wave can be selected to be 10 μs (100 kHz) or some sub-multiple of a 100 such as 50 or 25 kHz.
An ON/OFF electro-optic switch is characterized by a half wave voltage Vπ that moves the optical transmission state from a minimum to a maximum. Twice Vπ moves the switch from one minimum to an adjacent minimum. Generally, twice Vπ moves the switch from any transmission state to the same transmission state. Therefore, an electro-optic switch driven by a square wave with a peak-to-peak amplitude of twice Vπ has a constant optical output.
A duty cycle α can range between 0 and 1, and represents the fraction of time spent at the upper voltage V+Vπ; 1−α represents the fraction of time spent at the lower voltage V−Vπ. AC coupling means that the time averaged voltage is zero so the following equation describes such a duty cycle:
α(V+Vπ)+(1−α)(V−Vπ)=0 (1)
Solutions to equation (1) are:
upper voltage: V+Vπ=2(1−α)Vπ (2)
lower voltage: V−Vπ=−2αVπ (3)
As can be seen from equations (2) and (3), a 50-50 duty cycle, e.g. α=0.5, gives an upper voltage of +Vπ and a lower voltage of −Vπ. As the duty cycle a varies from 0 to 1, the upper voltage level varies from 2 Vπ to zero, and the lower voltage level varies from zero to −2 Vπ.
The raised cosine output response of an interferometer switch as a function of voltage U with a built-in optical bias VOB is given by:
I(U)=0.5 I0+0.5 I0 cos [(π/Vπ)(U+VOB)] (4)
Setting the voltage U to either the upper voltage level V+Vπ or lower voltage level V−Vπgives:
I=0.5 I0−0.5 I0 cos [(π/Vπ)(V+VOB)] (5)
I=0.5 I0−0.5 I0 cos [(π/Vπ)(Vπ+VOB−2αVπ)]
Adjustment of the duty cycle a to set the optical switch output to zero yields:
α=(0.5/Vπ)(Vπ+VOB) (6)
The upper voltage V+Vπon the square wave corresponds to one OFF state and the lower voltage V−Vπcorresponds to an adjacent OFF state. The mid-point voltage V corresponds to a maximum ON state. The ON state pulse is generated by a voltage excursion of Vπ from the lower voltage V−Vπ. Alternatively, the ON state pulse could be generated by a voltage excursion of −Vπ from the upper voltage V+Vπ. Equation (1) is modified in view of the ON state pulse and produces a minor shift in the duty cycle α.
Let the duty cycle α be the fraction of time spent at V+Vπ, β the fraction of time spent at V, and 1−α−β the fraction of time spent at V−Vπ to yield:
α(V+Vπ)+(1−α−β)(V−Vπ)+βV=0 (7)
Solutions to equation (7) are:
upper voltage V+Vπ=2(1−α)Vπ−βVπ (8)
mid-point voltage V=(1−2α)Vπ−βVπ (9)
lower voltage V−Vπ=−2αVπ−βVπ (10)
From equations (5) and (9), the optical bias required to set the OFF state to zero, and the ON state to the peak value is:
VOB =−V=(2α−1)Vπ+βVπ (11)
Equation (11) differs from equation (6) only in that the additional term βVπis present. Thus, VOB is changed by the presence of the ON pulse T3 by only a very small amount: (2α−1+β)/(2α−1). For a duty cycle of 25% (α=0.25) and T3=2% (β=0.02) and a half wave voltage Vπ of 3.6 volts, the VOB=−1.728 volts, i.e. the ON pulse changed the optical bias voltage VOB by only 0.072 volts.
The present invention is suited for a variety of applications. In general, any type of electro-optic attenuator/switch having two or more control levels that define the same condition can benefit by not having to utilize a DC bias voltage to maintain the switch normally in a given state. The exemplary embodiment explained herein operates in a normally OFF state and is switched to an ON state. Alternatively, an electro-optic switch could also be operated in a normally ON mode by selecting appropriate upper and lower voltage levels for the AC waveform that correspond to adjacent ON states.
It will be apparent to those skilled in the art that an optical switching device can be operated in an attenuation mode as opposed to an ON/OFF mode. Such an attenuation usage can be advantageous for required applications such as for modulators. In such an application, the optical switching device could be operated to cause attenuation of the light beam to be other than a maximum attenuation level (OFF state) or a minimum attenuation level (ON state). Such an attenuator acting as a modulator could be normally operated at an OFF, ON or intermediate state, and cause various changes in the attenuation level in proportion to a control signal, whereby amplitude (intensity) modulation of the light beam in accordance with the control signal is accomplished. Even if the optical switching device is operated as a modulator, stability benefits can be obtained by utilizing an AC waveform to establish and maintain a defined normal state without requiring a DC bias voltage source.
Although a separate AC generator 80 and an AC signal source 82 are shown, it will be understood that the outputs could be generated by a single voltage generator. Other hardware and software based implementations of an EO driver can be utilized to perform the required functions. One or a plurality of ON/OFF switches can be utilized depending upon the required application. In the illustrative example of
Although exemplary implementations of the invention have 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.
Number | Name | Date | Kind |
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5247387 | Matsubara et al. | Sep 1993 | A |
5448058 | Arab-Sadeghabadi | Sep 1995 | A |
5852507 | Hall | Dec 1998 | A |
Number | Date | Country |
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0 436 344 | Jul 1991 | EP |
03 073916 | Mar 1991 | JP |
WO 89 11675 | Nov 1989 | WO |
Number | Date | Country | |
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20060209389 A1 | Sep 2006 | US |