Acousto-optic tunable filter controller

AOTF controller that monitors output power of a plurality of wavelengths of an AOTF and scans the frequency of corresponding RF input signals to an AOTF acoustic transducer and searches for the RF frequency corresponding to each desired wavelength that provides maximum optical output for each wavelength. The controller includes a plurality of sensor inputs for monitoring the power of each wavelength output from the AOTF, and alternatively, also monitors other AOTF parameters such as temperature and/or reads AOTF identification performance data that can be stored in a EPROM on a AOTF housing. The controller includes facility for input of modulation data, and in response to the data modulates the corresponding wavelength parameter such as power. A USB bus is provided for input of programming to the controller, and for output of performance data from the controller.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to control circuitry for optic filters, and more specifically to a control circuit for an acousto-optic tunable filter that automatically optimizes filter control parameters.

2. Description of the Prior Art

An acousto-optic tunable filter (AOTF) is used to select particular light wavelengths from an incident beam. Wavelength selection is needed in many areas of technology, such as fluorescence spectroscopy, microscopy, and optical communication systems. In addition to wavelength selection, AOTFs provide a means for light modulation of either or both wavelength and amplitude. AOTF performance is sensitive to various parameters including environmental temperature, acoustic power applied and combinations of frequencies, which can alter the AOTF crystal material properties and cause drift of output intensity. Due to this sensitivity, an AOTF crystal may be placed in a temperature controlled environment, which only partially stabilizes the crystal performance.

SUMMARY

It is an object of the present invention to provide an improved AOTF controller.

It is a further object of the present invention to provide an AOTF controller that automatically optimizes a specific wavelength filter output.

It is a still further object of the present invention to provide an AOTF controller that can be programmed for a variety of control functions.

It is another object of the present invention to provide an AOTF controller that provides performance output data for display.

It is another object of the present invention to provide an AOTF controller that can respond to various parameters for optimizing AOTF output.

Briefly, a preferred embodiment of the present invention includes an AOTF controller that monitors output power of a plurality of wavelengths of an AOTF and scans the frequency of corresponding RF input signals to. an AOTF acoustic transducer and searches for the RF frequency corresponding to each desired wavelength that provides maximum optical output for each wavelength. The controller includes a plurality of sensor inputs for monitoring the power of each wavelength output from the AOTF, and alternatively, also monitors other AOTF parameters such as temperature and/or reads AOTF identification performance data that can be stored in a EPROM on a AOTF housing. The controller includes facility for input of modulation data, and in response to the data modulates the corresponding wavelength parameter such as power. A USB bus is provided for input of programming to the controller, and for output of performance data from the controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the method and apparatus of the present invention will now be described in reference toFIG. 1of the drawing. An acousto-optic-tunable filter controller (AOTF)10according to the present invention is shown in operation in a system12for controlling light output from an acousto-optic-tunable filter14. A typical acousto-optic-tunable filter14includes an AOTF crystal16(e.g., a tellurium dioxide, quartz, and so on). An incident beam of light18(also referred to as an optical input signal) from a source20impinges on the AOTF crystal16. A typical AOTF14further includes an acoustic transducer22bonded to one side of the AOTF crystal16and an acoustic absorber24bonded to the opposite side of the AOTF crystal16. Acoustic waves are generated by the acoustic transducer22in response to an input RF signal26. The frequency of the acoustic waves depend on the frequency of the applied RF signal, and the waves propagate through the AOTF crystal16and get absorbed in the acoustic absorber24. The acoustic waves that propagate through the AOTF crystal16can generate a diffraction grating within the AOTF crystal such that a substantial portion of the incident beam of light18is diffracted. As a result, an output28of a typical acousto-optic-tunable filter includes a diffracted beam28that, for example, can be used as input to another optical device and an undiffracted beam30that is absorbed by a beam stop32.

In addition, the diffracted beam28(also referred to as the optical output signal) is a “filtered” version of the incident beam18. That is, the optical output signal28includes only a subset of wavelengths (i.e., “selected” wavelengths) that are present in the optical input signal18.

Wavelength selection is controlled by the frequencies of the acoustic waves that are generated by the acoustic transducer22bonded to the AOTF crystal16. That is, a selected wavelength of the optical output signal28depends on the acoustic frequency of an acoustic wave generated by the acoustic transducer22. Moreover, the frequency of an acoustic wave generated by the acoustic transducer22is controlled by the control RF signal26supplied to the acoustic. transducer22. That is, an acoustic frequency of an acoustic wave generated by the acoustic transducer22(which substantially determines a selected wavelength of the optical output signal28) is substantially determined by the base RF frequency of the control RF signal26supplied to the acoustic transducer22. Consequently, a selected wavelength of the optical output signal28is “tuned” by the base RF frequency of the control RF signal26.

The output28contains a spectrum, which can be further separated by any known device for the purpose, such as a prism30. The output wavelengths32are detected by a detector/sensor34which provides input36to the controller10of the present invention. The controller functions to provide what will be referred to as a wavelength locker44(FIG. 1), wherein a series of RF input scans are applied at26to the AOTF transducer22, with each consecutive scan of reduced span. The controller responds to the resultant outputs36, selecting the best frequency of each scan to determine the center of the next narrower scan to determine an optimum RF frequency (base frequency)26to drive the AOTF for achieving the desired AOTF output signals32. This function will be more fully described in reference toFIG. 2.

According to a further embodiment of the present invention the controller10is configured to receive data from an AOTF sensor38for sensing a condition of the AOTF such as an AOTF temperature. The controller10is configured to respond to the sensor38output by adjusting the RF signal at26to provide an optimum output at32. This operation will be fully described in reference toFIG. 3.

According to a still further embodiment of the present invention, the controller provides an interface40providing convenient communication apparatus to a computer42, and alternatively or in addition to a personal digital assistant (PDA)44. These and other features of the interface40will be fully described in reference toFIG. 4.

The wavelength locking feature will now be described in detail in reference to the flow chart ofFIG. 2. The wavelength locker44determines a base RF frequency for the control RF signal26needed to substantially maintain an optical output signal at32at a desired wavelength. Referring toFIG. 2, the wavelength locking process46can be initiated (48) in a variety of ways. The process can be initiated by an end user (e.g., a technician) of the AOTF controller10through a user interface to the AOTF controller10(e.g., using the computer42connected to the AOTF controller10through a USB port, or through an RS232 port. Alternative, the AOTF controller10can be configured to initiate the process for determining the base RF frequency for the control RF signal at26at a predetermined time (e.g., on power up). Moreover, the AOTF controller10can be configured to initiate the process46at regular intervals.

Once the process46for determining the base RF frequency for the control RF signal26is initiated (48) in the wavelength locker44, the wavelength locker44inputs (scans) a sequence of RF signals into the acoustic transducer22(block50), and then receives an output detected signal32through bus36(block52). The controller evaluates the detected data to determine which data is closest to the desired result, and then uses the corresponding applied RF signal as a new base RF frequency (block52) for the next scan. The controller then determines a new sequence of RF frequencies centered around the new base frequency, wherein the new sequence is a reduced RF spectrum span from the preceding spectrum span (block54). The controller then checks to determine if the new span is less than a pre-determined minimum (block56). If it is less58, then the base frequency of the new span is used to drive the acoustic transducer22(block60). If the new span is greater than the minimum62, then the process repeats wherein the new span/sequence is applied (block50) and the resultant detected signals are analyzed (block52), etc. This process will now be described in still more detail.

The scanning process ofFIG. 2includes repetitions of steps for automatically adjusting the controller RF drive to the AOTF for optimizing optical power output from the AOTF. A range of RF frequencies is set to be applied to the AOTF acoustic transducer, which most generally can be from any lower frequency (Flow) to any higher frequency (Fhigh). The spectrum is divided (Flowto Fhigh) into a sequence, for example of 1000 equal increments, or more generally “n” increments. The n+1 frequencies (sequence) are then sequentially applied (scanned) and the sensor output read for each point. A smaller scanning range is then selected by dividing the previous frequency spectrum span by some number (for example by 4). The center for this more narrow scan is selected as the frequency yielding the highest output (detected output) resulting from the previous scan just completed. Then the new sequence of RF signals is applied to the acoustic transducer22, etc. This process is continued until the frequency spectrum span is less than 100 Hz or other selected value. The RF frequency yielding the largest AOTF output is then used as the center/base RF frequency for driving the acoustic transducer of the AOTF.

As a further embodiment, because the optimum RF frequency may change with time due to a variety of factors, such as temperature, at some pre-determined interval of time or event, a new scan can be initiated (block64). The center RF frequency for this new scan can be the previously determined base RF frequency. The span of the frequencies is selected to be large, as done initially, to be certain that the span includes the best RF frequency. Each succeeding scan is then narrowed until a new base frequency is determined.

The controller can additionally be configured to provide output to a computer42(FIG. 1) for display of a chart/graph showing the optimum RF frequency26as a function of a desired wavelength output at32. This data can be arrived at by systematically stepping the temperature, and for each temperature, performing the wavelength locker process as described in reference toFIG. 2.

In a further embodiment of the present invention, the controller senses an output of the AOTF sensor38and in response makes an adjustment of the RF frequency at26to optimize the output of32. One embodiment of this feature requires a pre-calibration of the controller10, for example by setting the AOTF sequentially at various temperatures, and determining the optimum RF signal frequency for a particular wavelength at each temperature. This data of RF frequency vs. temperature for each of a plurality of selected wavelengths, can be stored in an RF frequency correction data base64(FIG. 1). The process of using this data in operation of the system is illustrated in reference toFIG. 3. The controller begins by reading the AOTF sensor output (block66). The controller then finds the closest sensor output data in the frequency correction data base (block68). Alternatively, block70can be included, wherein the controller10stores a last/previous sensor data and compares the new data with the previous data. If the new data does not differ by more than a pre-set amount72, the process skips to block80and the controller waits for a pre-set time interval or other event before repeating the process as indicated by line32. If the new data does differ by at least the pre-set amount74, then the controller finds the closest sensor output data in the frequency correction. data base (block68) and selects the corresponding RF frequency data from the database to apply to the acoustic transducer (block76). Then the controller outputs the RF frequency to the acoustic transducer (block78). At this point, the controller can wait for an interval, such as a pre-set time period, or until the controller receives a command to repeat the process (block80).

FIG. 4illustrates various alternative communication features of the present invention, including the use of a custom interface apparatus40(FIG. 1). The interface40includes an input-output bus84for input of analog and/or digital modulation, for FSK (frequency shift keying) and blanking control inputs, and RS232 communications. A corresponding FSK and blanking communication bus86between the controller10and interface40is provided. The interface40may provide modulation88, FSK90and blanking92inputs to the controller. RS232 and12C buses94and96between the controller10and interface40are included, as well as a DAC bus98.FIG. 4also shows the AOTF sensor38and a bus100interconnected to the controller. Bus26provides the RF signal to the AOTF acoustic transducer22. Sensor input bus36brings the wavelength detected signals to the controller. A USB bus102and blue tooth bus104are also provided. Further details concerning these features are described in U.S. Provisional, Patent application No. 60/585,248 file Jul. 1, 2004, the entire contents of which is included in the present disclosure by reference.

An embodiment of an acousto-optic-tunable filter (AOTF) controller10of the present invention is shown in more detail inFIG. 5. The controller10has a plurality. of sensor inputs104for detecting signals representing any of various AOTF parameters, such as light output power, temperature, AOTF identification data, etc. The controller10, in response to these detected signals, performs any of various functions. For example, in response to a detected AOTF output wavelength power, the controller10seeks to optimize the power output of the AOTF by adjusting a frequency of an RF drive signal from controller port106to an AOTF acoustic transducer22(FIG. 1). The AOTF16(FIG. 1) is placed in a control loop with the controller10. The information fed back can be any of various parameters including for example optical intensity of a selected wavelength output from the AOTF, a temperature of the AOTF, and/or AOTF device parameter data stored in a EPROM in an AOTF housing, etc. . . . The controller10has modulation inputs108for application of data to each of one or more wavelengths passed by the AOTF16.

A USB bus110is provided for input of controller programming data from a computer42(FIG. 1), and for output of monitoring performance data from the controller10to the computer42. Alternate additional RS232 communication line94is shown. FSK and blanking inputs114are shown for switching a particular RF frequency for the purpose of selecting or de-selecting a particular wavelength, or selecting any one of a plurality of wavelengths through an AOTF. Other input types than FSK are also included in the present invention for this purpose, and also for the purpose of adjusting the amplitude of a selected wavelength.

Each DDS116operates to provide an RF signal to a combiner118for output to the AOTF. Filters120and modulators122are shown in line with each DDS116output to the combiner118for filtering out unwanted signals/noise and for modulating the signal.

FIG. 5also shows a custom interface40as described in reference toFIG. 4, showing the RS232 bus94and a bus112including all other appropriate buses, such as those described in reference toFIG. 4. The interface40is provided with connectors selected to mate with a particular user's hardware, and has programmability for adopting input signals to conform to requirements of the controller.

FIG. 1Dshows the daughter board70having input connectors100and102, for example, where the choice of connectors100and102is specific to the requirements of the user. For example, an interface connector can mate with a specific user's connector for input of signals to the amplitude modulator input line108and frequency selection buses126. Another connector could be for signals between a computer and a USB transceiver, or to the RS232 transceiver94. All of these signals would be altered as required by the interface40and sent to the controller. An example of signal modification by the interface40would be to perform an A/D conversion for converting a user's analog input signal to a digital signal required by the amplitude modulator122. The reverse D/A conversion could also be performed as required. In any situation, the interface10is custom configured to provide the proper adaptation from the user to the mother board/controller10.

FIG. 5shows alternate ways of controlling the DDS modules116. A computer42can input signals via line110to the lines108and114(bus not shown), or it can input signals to the DDS 116 and modulator122via the bus84to the RS232 module128, or it can send directions via bus84to bus112to the USB module130to a DDS module116.FIG. 5also shows a blue tooth module124providing a wireless connection for communication with the controller for providing inputs and receiving data.

The controller10can also be used for other control functions, such as for controlling other mechanical and/or electrical functions. For example, the controller could direct and/or maintain an AOTF crystal physical orientation through electromechanical apparatus. A positioning system controlled by the controller is symbolically illustrated inFIG. 6.FIG. 6illustrates an AOTF132upon which is incident a beam134.FIG. 6shows a refracted beam136impinging onto a sensor138, and an un-refracted beam140onto a beam block142. A positioning system144is symbolically illustrated for orienting/positioning the AOTF132. The controller10is shown in communication with the positioning system144, for positioning the AOTF, for example to adjust the beam136onto the sensor138. The controller10outputs RF through line146to the acoustic transducer148, and receives a detected/sensed signal through line150from the sensor138. As discussed above, the AOTF can also have other sensors or data storage attached either directly as in an AOTF housing for providing useful input to the controller10. The sketches of AOTFs inFIGS. 1 and 6are simply given as symbolic representations of an AOTF. The present invention includes use of the controller10for controlling any controllable function of any kind of AOTF.

Although preferred embodiments of the present invention have been described above, it will be appreciated that certain modifications or alterations thereon will be apparent to those skilled in the art. It is therefore requested that the appended claims be interpreted as covering all such alterations and modifications that fall within the true spirit and scope of the invention.