WO2004042466A1 - Acousto-optic gain equalization filter and gain equalization system - Google Patents

Acousto-optic gain equalization filter and gain equalization system Download PDF

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Publication number
WO2004042466A1
WO2004042466A1 PCT/US2003/035210 US0335210W WO2004042466A1 WO 2004042466 A1 WO2004042466 A1 WO 2004042466A1 US 0335210 W US0335210 W US 0335210W WO 2004042466 A1 WO2004042466 A1 WO 2004042466A1
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optical
gain
plurality
aotf
acousto
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PCT/US2003/035210
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French (fr)
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Kenneth J. Bures
Dogan Gunes
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Unaxis Usa, Inc.
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    • GPHYSICS
    • G02OPTICS
    • G02FDEVICES OR ARRANGEMENTS, THE OPTICAL OPERATION OF WHICH IS MODIFIED BY CHANGING THE OPTICAL PROPERTIES OF THE MEDIUM OF THE DEVICES OR ARRANGEMENTS FOR THE CONTROL OF THE INTENSITY, COLOUR, PHASE, POLARISATION OR DIRECTION OF LIGHT, e.g. SWITCHING, GATING, MODULATING OR DEMODULATING; TECHNIQUES OR PROCEDURES FOR THE OPERATION THEREOF; FREQUENCY-CHANGING; NON-LINEAR OPTICS; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics
    • G02F1/01Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
    • G02F1/11Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
    • G02F1/125Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves in an optical waveguide structure

Abstract

A gain equalization acousto-optic filter according to the present invention includes a first plurality of series connected AOTF devices (208, 210, 212, 214) having an optical input that is coupled to an input of the gain equalization acousto-optic filter and an optical output that is coupled to an output of the gain equalization filter. A second plurality of series connected AOTF devices (216, 218, 220, 222) having an optical input that is coupled to the input of the gain equalization acousto-optic filter and an optical output that is coupled to the output of the gain equalization filter. Each AOTF device in the first and the second plurality of series connected AOTF devices comprises a separate RF signal input (225) for receiving an RF signal having a unique frequency.

Description

ACOUSTO-OPTIC GAIN

EQUALIZATION FILTER AND GAIN

EQUALIZATION SYSTEM

Cross Reference To Related Applications

This patent application claims priority to U.S. provisional patent application Serial Number 60/319,670, filed on November 5, 2002, the entire disclosure of which is incorporated herein by reference.

Background of Invention

[0001 ] The present invention relates to acousto-optic tunable filters. In particular, the present invention relates to gain equalization and gain flattening filters that use acousto-optic tunable filters. Acousto-optic tunable filters (AOTFs) are electrically- tunable optical filters. Wavelength tuning in an AOTF is accomplished by varying a surface acoustic wave frequency applied to the AOTF. AOTFs are useful for optical filtering and add-drop multiplexing in wavelength division multiplexing (WDM) optical transport systems.

[0002] WDM is an optical transport technology that propagates many wavelengths in the same optical fiber, thereby effectively increasing the aggregate bandwidth per fiber to the sum of the bit rates of each wavelength. Dense Wavelength Division Multiplexing (DWDM) is a technology that implements WDM technology with a large number of wavelengths. DWDM is typically used to describe WDM technology that propagates more than 40 wavelengths in a single optical fiber.

Brief Description of Drawings

[0003]

The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0004] FIG. 1 illustrates a known integrated acousto-optic tunable filter (AOTF).

[0005] FIG. 2 illustrates a schematic diagram of a gain equalization system for an optical communication system according to the present invention.

[0006] FIG. 3 illustrates a known acousto-optic tunable gain flattening filter that can be used for equalization.

[0007] FIG. 4 illustrates one embodiment of an acousto-optic tunable gain flattening filter according to the present invention.

Detailed Description

[0008] FIG. 1 illustrates a known integrated acousto-optic tunable filter (AOTF) 10.

The AOTF 10 includes an input polarization beamsplitter 1 2, a polarization mode- converter 14, and a polarization beam combiner 1 6. The polarization beamsplitter 1 2 receives an input light beam at a first input 1 8 and separates the input light beam into two orthogonal polarization states, which are typically the TE and TM modes. The two modes propagate through the polarization mode-converter 14 and are combined by the polarization beam combiner 1 6. The modes are either coupled straight through or crossed over to a first 20 and second output port 22 of the polarization beam combiner 1 6.

[0009] The polarization mode-converter 14 changes one polarization mode to another polarization mode by propagating light through an acousto-optic interaction region 24. The polarization mode-converter 14 includes a pair of parallel optical waveguides 26, 26' that are formed in the surface of a substrate. Strain is induced in the acousto-optic interaction region by the piezoelectric effect. The substrate is a piezoelectric and a birefringent material that includes an off-diagonal term in the substrate material's strain-optic tensor. The elasto-optic tensor Prelates the mechanical strain in the material to the optical index of refraction of the material. For example, lithium niobate has an off-axis elasto-optic tensor term of p

[001 0]

The polarization mode-converter 14 also includes a surface acoustic wave

(SAW) transducer 27, which in one configuration, is a set of inter-digitated conducting fingers 28 that are formed over or proximate to the pair of optical waveguides 26, 26'. The optical waveguides 26, 26' carry the separated TE and TM modes that are formed by the polarization beamsplitter 1 2. A sinusoidal oscillator

(not shown) that generates an acoustic waveform having a frequency f is a electrically connected to the conducting fingers 28 of the SAW transducer 27. The sinusoidal oscillator drives the conducting fingers 28 and generates a surface acoustic wave (SAW) that propagates approximately collinearly along the pair of optical waveguides 26, 26'. In one configuration, the SAW itself is guided through the use of an acoustic waveguide structure.

[001 1 ] The SAW causes an anisotropic perturbation of the indices of refraction in the pair of optical waveguides 26, 26'. This perturbation causes a mode conversion. By mode conversion, we mean a conversion of one polarization mode to the other polarization mode (i.e., TE becomes TM, and TM becomes TE). The mode conversion occurs gradually as the optical signals propagate through the pair of optical waveguides 26, 26'. Mode conversion only occurs when a phase matching criterion is satisfied. This is when the optical wavelength λ and the acoustic drive frequency f are related by Equation 1 as follows: a

where Δ n = n - n is the birefringence of the optical waveguide material, and V is the speed of sound in the substrate material. Eventually, complete mode conversion of the phase-matching optical signals occurs. This is when substantially the entire TE mode is converted to the TM mode in one waveguide 26' and substantially the entire TM-mode is converted to the TE mode in the other waveguide 26 of the pair of optical waveguides 26, 26'.

[001 2] Mode conversion continues to occur as long as the acoustically generated perturbation is present. That is, after complete mode conversion, the just-formed TM mode begins to convert back to TE mode and the just-formed TE mode begins to convert back to the TM mode. The TE mode and the TM mode that are propagating through the pair of optical waveguides 26, 26', thus could convert cyclically back and forth from pure TE to pure TM and then back again.

[001 3]

The AOTF 10 halts the mode conversion by terminating the acoustic signal with acoustic absorbers 29 that are positioned on the pair of optical waveguides 26, 26' at specific locations. This ensures that the light beam having the phase-matching optical wavelength λ will undergo substantially complete mode conversion. If the optical wavelength λ of the light beam is not phase-matched to the acoustic frequency f , then substantially no mode conversion occurs, and the TE Mode and a the TM mode of the light beam simply propagate down the pair of waveguide 26, 26' with no change in polarization.

[0014] The polarization beam combiner 16 is physically identical to the polarization beamsplitter 12. However, the polarization beam combiner 16 is configured to combine rather than split the light beams. The polarization beam combiner 1 6 has a first 30 and a second input port 32 that receives the TM mode and the TE mode.

[001 5] Integrated AOTFs combine the polarization beamsplitter 12, polarization mode-converter 14, and the polarization beam combiner 1 6 on a planar substrate that is birefringent, photoelastic, and piezoelectric, such as lithium niobate. Discrete AOTFs use physically separate polarization beamsplitters, polarization beam combiners, and acousto-optic interaction regions. The principles of operation of integrated and discrete AOTFs are similar.

[001 6] In operation, a single-mode optical beam including, for example, three channels centered at optical wavelengths λ , λ , and λ enters the polarization beamsplitter 12 through the first input 1 8. The polarization beamsplitter 1 2 separates the optical beam into TE and TM modes. The TE and TM modes propagate down separate waveguides 26, 26' in the polarization mode-converter 14. Portions of the TE and TM modes are mode-converted by the polarization mode-converter 14. The TE and TM modes are then combined in the polarization beam combiner 1 6.

[001 7] The mode-converter oscillator frequency is chosen to phase-match to one of the three optical channels. For example, the oscillator frequency can be chosen to phase-match to λ . In this configuration, the portions of the TE and TM modes centered at λ are mode-converted to TM and TE modes, respectively, while the 2 portions of the TE and TM modes centered at other wavelengths propagate down the waveguides without any polarization mode conversion. The TE mode couples straight through the polarization beam combiner 16 and the TM mode cross- couples in the polarization beam combiner 1 6. Alternatively, polarization splitters and combiners can be designed to couple the TM mode straight through and to cross-couple the TE mode. In this alternative configuration, the overall operation of the filter is the same.

[001 8] The second output port 22 of the polarization beam combiner 1 6 produces the combined TE+TM components centered at the phase-matching wavelength λ

2, whereas the first output port 20 of the polarization beam combiner 1 6 produces the combined TE+TM components for all the other wavelengths. The AOTF 10 has essentially "dropped" the phase-matching wavelength selected by the oscillator frequency f and has passed through all other wavelengths. Therefore, the AOTF a

1 0 performs the function of a tunable bandpass filter. The center frequency of the bandpass filter can be modified by changing the oscillator frequency f , and a therefore, the phase-matching wavelength.

[001 9] Thus, the AOTF 10 can be configured as an add/drop multiplexer that drops one particular wavelength, and passes all other wavelengths. Add-drop multiplexers are used in WDM optical transport systems for adding and dropping one or more channels while preserving the integrity of the other channels.

[0020] The AOTF 1 0 can be configured to simultaneously add and drop an optical signal. In this configuration, the signal propagating through the AOTF 10 has an empty spectral "slot" that corresponds to the spectral slot of the dropped signal. A new locally generated signal is then applied to a second input port 34 of the polarization beamsplitter 1 2. This can be accomplished by using an oscillator signal that is a superposition of several sinusoids at different frequencies. The AOTF 1 0 inserts this signal into the empty slot at λ in the "through" output. This is done simultaneously with the "dropping" of the input channel centered at λ .

[0021] AOTFs are particularly advantageous for use in WDM optical transport systems because they can achieve narrow passbands and broad tuning ranges. In fact, an AOTF can have a tuning range that is substantially the entire wavelength range of an optical fiber communication system, which can typically be approximately from 1 .3 μ m to 1 .6 μ m. Also, AOTFs have the unique capability of simultaneous multichannel filtering. By simultaneous multi-channel filtering, we mean that an AOTF can select several wavelength channels simultaneously by applying multiple acoustic wave signals.

[0022]

FIG. 2 illustrates a schematic diagram of a gain equalization system 100 for a

WDM optical communication system according to the present invention. Known long-haul WDM optical communication systems sometimes use fixed gain equalizing filters. In these known communication systems, the power in the individual channels can vary significantly because the operating conditions of a WDM network are dynamic. The power in the individual channels can also vary significantly because the gain of the repeater amplifiers varies with dynamic input load. Consequently, such systems may require dynamic gain equalization. A dynamic gain equalizer of the present invention dynamically adjusts the equalization in the individual channels to meet the requirements of the network.

[0023] The gain equalization system 100 includes an optical amplifier. The present invention is described in connection with an erbium-doped fiber amplifier (EDFA) 1 02. However, any type of optical amplifier can be used. For example, other types of optical fiber amplifiers such as thorium doped optical fiber amplifiers can be used. Solid state optical amplifiers can also be used.

[0024] The EDFA 1 02 has an input 104 that is optically coupled to a WDM channel. The

EDFA 1 02 is a repeater that extends the range of transmission of optical signals. The EDFA 102 amplifies the gain in the WDM channels. The gain spectrum of an EDFA is generally a function of the channel wavelength. Typically, the gain spectrum of an EDFA has a peak that is centered around 1 530 nm. The gain can reduce somewhat at longer wavelengths. WDM communication systems propagate many channels simultaneously in time. The channels falling within the peak gain spectrum of the EDFA 102 are amplified more than the other channels. Gain equalization is necessary when the gain disparity among channels falling in different sections of the amplifier band increases to an unacceptable level.

[0025] The gain equalization system 100 also includes an AOTF equalizer 106 having an input 108 that is optically coupled to an output 1 1 0 of the EDFA 1 02. The AOTF equalizer 1 06 dynamically adjusts the channel gain. AOTF equalizers have been constructed using planar integrated acousto-optic filter technology. These AOTF equalizers adjust the gain of each channel individually. See, for example, S.F. Su, et. al. "Gain Equalization in Multiwavelength Lightwave Systems Using Acoustooptic Tunable Filters", IEEE Photonics Technology Letters, Vol. 4, No.3, pp. 269-271 , March 1 992.

[0026]

The gain equalization system 100 also includes a dynamic feedback control circuit 1 10 that controls the gain of the AOTF equalizer 106. An output 1 1 2 of the

AOTF equalizer 106 is coupled to an input 1 14 of the dynamic feedback control circuit 1 10. The dynamic feedback control circuit 1 10 processes the signal generated by the AOTF equalizer 1 1 2 and generates a plurality of control signal at a plurality of outputs 1 1 6. The plurality of outputs 1 1 6 of the dynamic feedback control circuit 1 10 are coupled to a plurality of inputs 1 1 8 of the AOTF equalizer 106. The control signal generated by the dynamic feedback control circuit 1 1 0 controls the gain of the AOTF equalizer 106.

[0027] The present invention is described in connection with a gain flattening equalization filter. However, the present invention applies to any type of equalization and is not limited to flat equalization. FIG. 3 illustrates a known acousto-optic tunable gain flattening filter 1 50 that can be used for equalization. The gain flattening filter 1 50 includes a first 1 52 and a second AOTF device 1 54 that are optically coupled in a parallel configuration. Each of the first 1 52 and the second AOTF device 1 54 includes a SAW transducer 1 56 having an RF input 1 57. Also, each of the first 1 52 and the second AOTF device 1 54 includes acoustic absorbers 1 58 at both the inputs and the outputs of the AOTF devices 1 52, 1 54. An optical input 160 of the gain flattening filter 1 50 is coupled to an input 1 62 of the first AOTF 1 52 device and an input 1 64 of the second AOTF device 1 54 with a first optical waveguide 1 66. An output 1 68 of the first AOTF device 1 52 and an output 1 70 of the second AOTF device 1 54 are optically coupled to an optical output 1 72 of the filter 1 50 with a second optical waveguide 1 74.

[0028] The gain of the gain flattening filter 1 50 is changed in order to achieve gain equalization. The gain of the filter 1 50 is changed by adjusting the insertion loss of the filter 1 50. The insertion loss of the filter 1 50 is controlled by changing the acousto-optic (AO) conversion efficiency, which is a function of the acoustic power that is applied to the acousto-optic interaction region 24 (FIG. 1 ) of the AOTF devices 1 52, 1 54. The acoustic power applied to the SAW transducers 1 56 of the AOTF devices 1 52, 1 54 is controlled by adjusting the RF power level of the signals that are applied to the SAW transducers 1 56 that generate the acoustic waves in the AOTF devices 1 52, 1 54. Adjusting the RF power level also changes the amplitude of the optical signal.

[0029] jne gajn adjustment can be performed for multiple wavelengths in order to shape the gain response of the gain flattening filter 1 50 over the desired frequency spectrum. Multiple (e.g. 6 to 8) RF frequencies at different power levels can be applied to the saw transducers 1 56 of the AOTF devices 1 52, 1 54 in order to shape the gain response of the filter 1 50. However, applying multiple frequencies at different power levels can result in undesirable inter-channel interference that is caused by multi-wavelength operation of the AOTF 1 52, 1 54 devices.

[0030] FIG. 4 illustrates one embodiment of an acousto-optic tunable gain flattening filter 200 according to the present invention. The gain flattening filter 200 includes a first and a second plurality of AOTF devices that are optically coupled in a parallel configuration 202. The parallel configuration 202 is a parallel combination of a plurality of series connected AOTF devices in a first optical path 204 and plurality of series connected AOTF devices in a second optical path 206.

[0031 ] The gain flattening filter 200 of FIG. 4 includes eight AOTF devices with separate RF inputs. However, the gain flattening filter of the present invention can use any number of AOTF devices. The number of AOTF devices may be limited by limitations on the maximum physical length of the filter. The optimal number of AOTF devices depends on the desired bandwidth of the signal being processed by the filter 200. For example, an acousto-optic gain flattening filter according to the present invention including six to eight AOTF devices with separate RF inputs is sufficient to provide relatively flat gain over the entire erbium doped fiber amplifier (EDFA) bandwidth.

[0032] The first optical path 204 includes a first 208, second 210, third 21 2, and a fourth AOTF device 214 that are optically coupled in series. The second optical path 206 includes a fifth 216, sixth 21 8, seventh 220, and eighth AOTF device 222 that are optically coupled in series. In other embodiments, the number of AOTF devices in the first optical path 204 does not equal the number of AOTF devices in the second optical path 206. In one embodiment, the acousto-optic interaction region 24 (FIG. 1 ) is adjusted to change the bandwidth of the gain flattening filter 200. The length of the acousto-optic interaction region 24 (FIG. 1 ) is inversely proportional to the optical bandwidth of the AOTF device.

[0033] Each of the AOTF devices in the first 204 and the second optical path 206 includes a SAW transducer 224 having an RF input 225 that is used to launch an acoustic signal in the inter-digitated conducting fingers 28 (FIG. 1 ). In one embodiment, the RF input 225 of each of the AOTF devices in the first 204 and the second optical path 206 is configured to be electrically isolated from each RF input 225 of the other AOTF devices. In this embodiment, a different frequency signal can be applied to the RF inputs 225 of each of the SAW transducers 224.

[0034] |n one embodiment, each of the AOTF devices in the first 204 and in the second optical path 206 are acoustically isolated from the other AOTF devices in order to reduce or to eliminate inter-channel interference in the resulting filtered optical signal. In some embodiments, only certain AOTF devices are acoustically isolated from other AOTF devices in order to reduce or to eliminate certain inter- channel interferences. In the embodiment shown in FIG. 4, at least one AOTF device in the parallel configuration 202 includes two acoustic absorbers 226 so that each AOTF device has an acoustic absorber 226 at both its input and its output. The acoustic absorbers 226 suppress the acoustic waves proximate to the input and to the output of each of the AOTF devices, thereby preventing the acoustic surface waves propagating in one AOTF device from interfering with the acoustic surface waves propagating in the other AOTF devices.

[0035] An optical input 228 of the gain flattening filter 200 is coupled to an input 230 of the first optical path 204 including the first 208, second 210, third 21 2, and fourth AOTF device 214 with an optical waveguide 232. The optical input 228 of the gain flattening filter 200 is also coupled to an input 234 of the second optical path 206 including the fifth 216, sixth 21 8, seventh 220, and eighth AOTF device 222 with an optical waveguide 236. An output 238 of the first optical path 204 is coupled to an optical output 242 of the gain flattening filter 200 with an optical waveguide 240. An output 244 of the second optical path 206 is coupled to the optical output 242 with an optical waveguide 246.

[0036] In operation, the acousto-optic gain flattening filter 200'can be used to equalize gain of an optical channel in an optical network. The method includes amplifying an optical signal propagating in the optical channel of the optical network. The amplified optical signal is then acousto-optically converted with the RF signals that are applied to each of the RF inputs 225 of the filter 200. In one embodiment, the signals applied to each of the separate independent electrical inputs has a unique frequency that is different from the frequency of the signals applied to the other AOTF devices. In one embodiment, at least one of the signals applied to each of the RF inputs 225 is acoustically isolated from the other signals.

[0037] jhe power applied to each of the SAW transducers 224 in the AOTF devices is relatively low. In known gain flattening filters, multiple frequencies are applied to the SAW transducers 224. The number of frequencies that can be applied to known gain flattening filters is typically limited by the maximum power that a single SAW transducer can handle. The gain flattening filter of the present invention, therefore, can greatly increase the number of frequencies that can be applied to the filter.

[0038] The frequency response of the resulting filtered amplified optical signal is then measured. At least one of the plurality of RF signals in then changed in response to the measured frequency response of the filtered amplified optical signal in order to equalize the gain of the optical signal propagating in the optical channel. In one embodiment, the gain is equalized over the bandwidth of the amplified optical signal. In one embodiment, the gain equalization is substantially flat gain equalization of the optical signal. However, the acousto-optic gain flattening filter of the present invention can equalize gain to any network specification.

[0039] In one embodiment, the frequency response of the gain flattening filter 200 is dynamically adjusted in response to the present requirements of the optical network and/or in response to changes occurring in the optical network. In this embodiment, the frequencies of the signals applied to the RF inputs 225 of the AOTF devices are dynamically adjusted to change the frequency response of the filter 200 in response to the present requirements and/or changes occurring in the optical network. The speed at which the frequency response is dynamically adjusted is dependent upon the specific requirements of the optical network. For example, the speed at which the frequency response is dynamically adjusted can be on the order of a few microseconds.

[0040] In one embodiment, the dynamic adjustment can be used to optimize the performance of an optical network, such as a DWDM network. The dynamic adjustment can be used to optimize certain network parameters. In this embodiment, a frequency of at least one signal that is applied to the RF inputs 225 is adjusted to optimize at least one parameter of the optical network.

Equivalents

[0041 ] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined herein.

[0042] What is claimed is:

Claims

Claims
[cl ] A gain equalization acousto-optic filter comprising: a) a first plurality of series connected AOTF devices having an optical input that is coupled to an input of the gain equalization acousto-optic filter and an optical output that is coupled to an output of the gain equalization filter, each AOTF device in the first plurality of series connected AOTF devices comprising a separate RF signal input for receiving an RF signal having a unique frequency; and b) a second plurality of series connected AOTF devices having an optical input that is coupled to the input of the gain equalization acousto-optic filter and an optical output that is coupled to the output of the equalization filter, each AOTF device in the second plurality of series connected AOTF devices comprising a separate RF signal input for receiving an RF signal having a unique frequency.
[c2] The filter of claim 1 wherein the gain equalization acousto-optic filter comprises a gain flattening acousto-optic filter.
[c3] The filter of claim 1 wherein at least one AOTF device in the first and the second plurality of series connected AOTF devices comprises at least one acoustic absorber.
[c4] The filter of claim 1 wherein at least one AOTF device in the first and the second plurality of series connected AOTF devices comprises an input and an output acoustic absorber.
[c5] The filter of claim 1 wherein a number of AOTF devices in the first plurality of series connected AOTF devices and a number of AOTF devices in the second plurality of series connected AOTF devices are equal.
[c6] The filter of claim 1 wherein the first plurality of series connected AOTF devices and the second plurality of series connected AOTF devices each comprise four series connected AOTF devices.
[c7] The filter of claim 1 wherein a number of AOTF devices in the first plurality of series connected AOTF devices and a number of AOTF devices in the second plurality of series connected AOTF devices is determined by a desired bandwidth of the gain equalization acousto-optic filter. [cδ] A gain equalization system comprising: a) an optical amplifier having an input that is optically coupled to an optical channel of an optical communication system, the optical amplifier generating an amplified optical signal at an optical output; b) a gain equalization acousto-optic filter comprising an optical input that is optically coupled to the optical output of the optical amplifier, a plurality of RF signal inputs, and an optical output, the gain equalization acousto-optic filter generating a gain equalized optical signal in response to a plurality of RF signals being applied to the plurality of RF signal inputs; and c) a feedback control circuit having an input that is optically coupled to the optical output of the gain equalization acousto-optic filter and a plurality of electrical outputs that are coupled to the plurality of RF signal inputs of the gain equalization acousto-optic filter, the feedback control circuit generating the plurality of RF signals.
[c9] The gain equalization system of claim 8 wherein the optical amplifier comprises an optical fiber amplifier.
[cl 0] The gain equalization system of claim 9 wherein the optical fiber amplifier comprises an erbium-doped fiber amplifier.
[cl 1 ] The gain equalization system of claim 9 wherein the optical fiber amplifier comprises a thorium-doped fiber amplifier.
[cl 2] The gain equalization system of claim 8 wherein the optical amplifier comprises a solid state optical amplifier.
[cl 3] The gain equalization system of claim 8 wherein the gain equalization acousto-optic filter comprises a gain flattening acousto-optic filter.
[cl 4] The gain equalization system of claim 8 wherein the gain equalization acousto-optic filter comprises at least one acoustic absorber that reduces inter-channel interference in the gain equalized optical signal.
[cl 5] The gain equalization system of claim 8 wherein the gain equalization acousto-optic filter comprises a first and a second plurality of series connected AOTF devices that are optically coupled together in a parallel configuration. [cl 6] The gain equalization system of claim 1 5, wherein each of the AOTF devices comprises a separate RF signal input.
[cl 7] The gain equalization system of claim 1 5 wherein each of the AOTF devices in the first and the second plurality of series connected AOTF devices comprises an input and an output acoustic absorber that reduces inter- channel interference in the gain equalized optical signal.
[cl 8] The gain equalization system of claim 8 wherein the gain equalized optical signal is equalized over substantially the entire bandwidth of the optical fiber amplifier.
[cl 9] The gain equalization system of claim 8 wherein a response time of the feedback control circuit is less than ten microseconds.
[c20] A method of equalizing gain of an optical channel in an optical network, the method comprising: a) amplifying an optical signal propagating in the optical channel of the optical network; b) acousto-optically converting the amplified optical signal with a plurality of RF signals, thereby generating a filtered amplified optical signal; c) measuring a frequency response of the filtered amplified optical signal; and d) changing at least one of the plurality of RF signals in response to the measured frequency response of the filtered amplified optical signal in order to equalize the gain of the optical signal propagating in the optical channel.
[c21 ] The method of claim 20 wherein the changing at least one of the plurality of the RF signals equalizes the gain of the optical signal propagating in the optical channel over a bandwidth of the amplified optical signal.
[c22] The method of claim 20 wherein the equalization of the gain of the optical signal propagating in the optical channel comprises a substantially flat equalization of the gain of the optical signal.
[c23] The method of claim 20 further comprising acoustically isolating at least one of the RF signals in order to reduce inter-channel interference in the equalized optical signal.
[c24] The method of claim 20 further comprising dynamically adjusting a frequency of at least one of the plurality of RF signals in response to a change of an operating parameter of the optical network.
[c25] The method of claim 24 wherein the dynamic adjustment of the frequency occurs in less than ten microseconds.
[c26] The method of claim 20 further comprising dynamically adjusting a frequency of at least one of the plurality of RF signals in order to optimize an operating parameter of the optical network.
[c27] A gain equalization system comprising: a) a means for amplifying an optical signal propagating in an optical channel of an optical network; b) a means for acousto-optically converting the amplified optical signal with a plurality of RF signals, thereby generating a filtered amplified optical signal; c) a means for measuring a frequency response of the filtered amplified optical signal; and d) a means for changing at least one of the plurality of RF signals in response to the measured frequency response of the filtered amplified optical signal in order to equalize the gain of the optical signal propagating in the optical channel.
PCT/US2003/035210 2002-11-05 2003-11-04 Acousto-optic gain equalization filter and gain equalization system WO2004042466A1 (en)

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20010033710A1 (en) * 1997-06-06 2001-10-25 Novera Optics, Inc. Gain flattening tunable filter
WO2002025321A2 (en) * 2000-09-20 2002-03-28 Brimrose Corporation Of America High speed optical gain flattener

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20010033710A1 (en) * 1997-06-06 2001-10-25 Novera Optics, Inc. Gain flattening tunable filter
WO2002025321A2 (en) * 2000-09-20 2002-03-28 Brimrose Corporation Of America High speed optical gain flattener

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