System and method for generating beams of light using an anisotropic acousto-optic modulator

A light source system includes a beam source generating a first input beam of light. An anisotropic acousto-optic modulator (AOM) is positioned to receive the first input beam. The AOM includes a plurality of transducers for receiving control signals and generating corresponding acoustic waves that operate on the first input beam to generate first and second output beams with different frequencies and orthogonal linear polarizations. The first and second output beams have a combined optical power that is substantially the same as an optical power of the first input beam for a first input beam with one polarization and for a first input beam with two polarizations.

Measurement optics in a polarization-based or multiplexed heterodyne interferometer such as used for precision measurements in semiconductor device manufacturing equipment generally use a light beam including orthogonal polarization components that have different frequencies. In heterodyne interferometry, a dual frequency/dual polarization source of light is used. The frequency difference between the two orthogonally polarized beam components is important because it can be the limit to how fast something can move and the distance still be measured accurately by this type of measurement system. Zeeman split HeNe lasers can provide orthogonally polarized light components, but the difference frequency is limited to a maximum of about 8 MHz. A two-mode frequency stabilized HeNe laser can also provide two orthogonally polarized beams with frequency separation, but this frequency difference is in the 500 -1500 MHz range and cannot be easily utilized by the processing electronics. The desired frequency range that will fulfill the lithography industries need for speed, but is compatible with current electronic technology is about 7 -30 MHz.

Several methods of producing a desired frequency split in a heterodyne interferometer have been used in the past. Most of these prior solutions involve conditioning the light to get the desired frequency after the stabilized laser source. One prior solution is to use two high frequency acousto-optic modulators (AOMs) to generate the desired difference frequency. The laser source beam is split into two beams of orthogonal polarization. Each linearly polarized beam is sent through an AOM. The first order diffracted beams from each AOM are redirected using mirrors and recombined using a second beam splitter to become collinear and co-bore again. While the absolute frequency of the AOMs in this prior solution is typically too high to be ideal (e.g., 80 MHz) the difference in frequency between the two different AOMs can be adjusted (e.g., one at 80 MHz and the other at 90 MHz) so that when the two orthogonal, linearly polarized beam components are recombined, they have the desired difference frequency. Unfortunately, this is a more costly solution, because two AOMs are used to achieve the desired results (along with a beam splitter, two turning mirrors and a second beam splitter which acts as a beam recombiner). Other solutions using two AOMs exist, but all have the disadvantage of multiple components (e.g., minimum of two AOM units and a beam splitter), which tends to increase the cost of the solutions.

Another prior approach is to use a single low frequency isotropic AOM with a single acoustic wave and a birefringent recombination prism. While this method reduces the number of components as compared to the previously described two-AOM solution, it has significant issues of its own. The major disadvantages include: a significant portion of the source light is discarded, (even with a single polarization output laser); the solution takes a lot of space to accomplish; and the solution does not fully accomplish a secondary benefit of AOM frequency shifters in providing isolation for the laser because it only isolates feedback on one polarization. In this prior method, only a single polarization and frequency are desired prior to the AOM device, so for a Zeeman split HeNe laser, a polarizer is typically used to filter out the other polarization/frequency component from the source laser. Thus, half the source light is eliminated before the beam enters the AOM.

In the isotropic acoustic wave interaction of this prior solution, there is no effect on the beam's polarization, so the diffracted (first order), frequency shifted beam is the same polarization as the zero order or un-diffracted beam. Exiting the AOM, the zero order and first order beams have a frequency difference of around 20 MHz in a current device on the market. The job of making the beams collinear again is accomplished by passing the beams through a birefringent recombination prism. The beam separation angle exiting this type of AOM is small, so no compensation is made for making the beams co-bore again after they are made parallel with the recombination prism. Typically, the optic axis of the recombination prism is at a forty-five degree angle to the polarization of the beams. The recombination prism splits each beam into two orthogonally polarized components. One component sees the index of refraction of neand the other component sees the index of refraction of no. The two beams refract differently at the entrance and exit prism/air interfaces due to this index difference. The apex angle of the prism is optimized to allow one polarization component of each beam to become parallel again. The other two unwanted polarized beams exiting the recombination prism are not parallel to the desired beams and are apertured. This recombination scheme effectively throws away half the optical power in the first and zero order beams. The net result is that three-fourths of the original source optical power for a Zeeman split laser (more if the AOM device operates in the Raman Nath regime) is lost using this prior single isotropic AOM method of increasing the frequency split.

It is desirable to have a small footprint or package for a heterodyne interferometry light source, as this light source is often installed in a customer's equipment. The single low-frequency isotropic AOM solution has issues that demand more space than desired. To get adequate efficiency for a low frequency isotropic AOM, a long interaction length is necessary, so the device itself is quite long. Also, the separation angle between the diffracted orders on this device is small, so a long distance is typically used to get adequate beam separation to aperture off the unwanted beams following the recombination prism. Thus, a long footprint, additional optics to focus the light to a pinhole spatial filter, or additional optics to fold the beam path in the package may be used to address this issue.

In addition, when using a single low frequency isotropic AOM with zero and first order beams, the zero order beam does not protect the laser from feedback because the frequency in that path is still the laser frequency (not shifted up or down). Reflections from this beam upstream that make it back to the laser will cause wavelength stability problems and a possible loss of lock for the laser.

In another prior approach, two shifted frequency beams are generated in the same isotropic AOM. The frequency shifts for both beams are accomplished in a couple of different ways. The first is to use one acoustic wave in the AOM. There is a polarizing beam splitter before (or attached to the AOM) to split a single frequency polarized beam into two orthogonally polarized beams. The polarizing beam splitter also does the task of orienting the two orthogonally polarized beams at the plus and minus Bragg angle of the AOM device so that one beam is up-shifted and one beam is down-shifted by the single acoustic wave frequency. The AOM itself is isotropic and does not affect the polarization of the beams. The frequency difference between the output beams is two times the AOM frequency. In another form of the single isotropic AOM solution, a longer crystal is used, and each polarized beam traverses through two acoustic waves in series, which are generated by two transducers of the AOM. The net result on the output beams is a frequency difference of two times the difference in frequency of the two AOM transducers. Again, this is an isotropic interaction (i.e., it does not affect polarization), and a beam splitter is used before the AOM device to generate two beams of orthogonal polarization and moving in diverging directions.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown specific and illustrative embodiments according to the present teachings. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the appended claims. The following Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

FIG. 1is diagram illustrating a system100A for producing frequency-shifted beams with orthogonal linear polarizations in a first embodiment according to the present teachings. System100A includes laser light source (laser)102and acousto-optic modulator (AOM)108. Laser102acts as a source of a heterodyne beam104having two distinct frequency components (f1and f2) with orthogonal linear polarizations (e.g., horizontal and vertical). A beam or beam component with a vertical polarization, such as beam component106A, is represented in the Figures by an upward and downward pointing arrow, and a beam or beam component with a horizontal polarization, such as beam component106B, is represented by a circle. An exemplary embodiment of laser102is a commercially available He—Ne laser such as a Model 5517B available from Agilent Technologies, Inc., which uses Zeeman splitting to generate the two frequency components in the same laser cavity. Zeeman splitting in this manner can generate a laser beam having frequency components with frequencies f1and f2and a frequency difference (f2−f1) of about 2 MHz. The two frequency components f1and f2have opposite circular polarizations, and a quarter-wave plate is used to change the polarizations of the frequency components so that the two frequency components have orthogonal linear polarizations. In another specific embodiment, laser102is a two-mode frequency stabilized laser. A Zeeman laser has better frequency stability than a laser using the two-mode frequency stabilization method.

In the illustrated embodiment, AOM108is an anisotropic, low frequency shear wave AOM with a TeO2uniaxial crystal. One example of an AOM device that is suitable for use in implementing AOM108is the FS1102AOM produced by Isomet Corporation, having headquarters located at 5263 Port Royal road, Springfield, VA 22151. AOM108includes electro-acoustic transducers110for receiving control signals. The electro-acoustic transducers110convert electrical signals into sound waves that are launched through the crystal of the AOM108. The transducers110excite the AOM108with two acoustic waves of the same or different frequencies, and with a small angle between their propagation directions (i.e., the two waves have different propagation vectors, which are identified inFIG. 1by K1and K2). Two acoustic waves or beams are used so that correct phase matching can exist for both input beam components106A and106B. A first one of the acoustic waves acts on the horizontal polarization106B of the orthogonally polarized laser source beam104, and a second one of the acoustic waves is phase matched to the vertical polarization106A of the source beam104.

For a laser beam104with a given propagation direction in the crystal of the AOM108, the laser field can be decomposed into two components according to the polarization. One of the components is call the ordinary wave while the other is called the extraordinary wave. The propagation speed of the ordinary wave is different from the propagation speed of the extraordinary wave. If there is no acoustic field in the crystal of the AOM108, the ordinary wave and the extraordinary wave preserve their propagation directions as well as their polarizations. The propagation directions of the laser beam104and the acoustic fields in the crystal of the AOM108are chosen so that the extraordinary wave in the input laser beam104is phase matched to be down-converted (diffracted) into the ordinary wave by one of the acoustic fields. Simultaneously, the ordinary wave in the input laser beam104is phase matched to be up-converted (diffracted) into the extraordinary wave by the other acoustic field.

AOM108is operated in a low-frequency shear mode. AOM108diffracts the two input beam components106A and106B in opposite directions, thereby producing plus first order beam114A, which corresponds to component106A, and minus first order beam114B, which corresponds to component106B. AOM108causes an increase in the frequency of component106A, a decrease in the frequency of component106B, and causes a ninety-degree rotation in the polarization of both components106A and106B. The net effect is that while both input beam components106A and106B change polarization (i.e., horizontal becomes vertical and vertical becomes horizontal), the beams stay orthogonally polarized, and now have a frequency difference given by the following Equation I:
fsplit=(f1+faomtransducer1)−(f2−faomtransducer2)  Equation I
where:fsplit=difference in the frequency of beam114A and the frequency of beam114B;f1=frequency of beam component106A;f2=frequency of beam component106B;faomtransducer1=frequency of first signal provided to transducers110; andfaomtransducer2=frequency of second signal provided to transducers110.

In a specific embodiment according to the present teachings, faomtransducer1and faomtransducer2are both in the range of about 10 to 450 MHz. In the embodiment shown inFIG. 1, faomtransducer1is the same as faomtransducer2, and this common frequency is identified inFIG. 1by the term “fAOM”.

As an example, if the transducers110are provided with a 10 MHz RF control signal, both output beams114A and114B will be frequency shifted by 10 MHz, but in opposite directions (i.e., plus 10 MHz and minus 10 MHz), for a frequency split or difference of 20 MHz caused by the AOM108. If the Zeeman laser light source102provides a 2 MHz split (i.e., |f2−f1|=2 MHz), system100A provides a total frequency split (fsplit) in beams114A and114B of 18 MHz or 22 MHz. In a specific embodiment according to the present teachings, the total frequency split (fsplit) in beams114A and114B is in the range of 8 to 30 MHz.

Using an anisotropic AOM108is optimal for a Zeeman split laser102, which already has two orthogonal polarizations and frequencies. In the embodiment shown inFIG. 1, a single, low frequency, anisotropic AOM108is used to increase the frequency difference, |f1−f2|, between input beam component106A and input beam component106B. The same embodiment can also be used, with the proper frequency applied to AOM108, to decrease the frequency difference, |f1−f2|, between input beam component106A and input beam component106B. In a specific embodiment according to the present teachings, beams114A and114B have a combined optical power that is substantially the same as the optical power of beam104.

FIG. 2is a diagram illustrating a system100B for producing frequency-shifted beams with orthogonal linear polarizations in a second embodiment according to the present teachings. In the illustrated embodiment, system100B includes the same laser light source (laser)102and acousto-optic modulator (AOM)108as system100A (FIG. 1), and the laser102and AOM108shown inFIG. 2operate in the same manner as described above with respect toFIG. 1. One difference between system100B and system100A is that system100B also includes a second AOM202, which is positioned between the laser102and AOM108. AOM202is a high-frequency isotropic AOM that up-shifts or down-shifts the frequency of both components106A and106B of the beam104by the same amount (e.g., 30-500 MHz).

As described above with respect toFIG. 1, AOM108up shifts the frequency (f1) of the first component106A of beam104, and down shifts the frequency (f2) of the second component106B of beam104. In some applications, the frequency shifts provided by AOM108may not be sufficient to isolate the laser102from optical feedback. To provide additional isolation, AOM202is added between laser102and AOM108. AOM202is an isotropic high-frequency AOM that is used to up shift the frequency of both components106A and106B of the beam104by the same relatively large amount (e.g., 80 MHz) to provide better optical isolation. Since AOM202is isotropic, the polarizations of the orthogonal beam components106A and106B are not affected by AOM202. In a specific embodiment according to the present teachings, AOM108and AOM202are pre-aligned in a single package.

AOM202generates an output beam204with up-shifted frequency components (f1+fAOM1and f2+fAOM1), where fAOM1represents the signal frequency applied to AOM202(e.g., 80 MHz). The first up-shifted frequency component206A (f1+fAOM1) has a vertical linear polarization, and the second up-shifted frequency component206B (f2+fAOM1) has a horizontal linear polarization. The output beam204from AOM202is provided as an input beam to AOM108.

In the embodiment shown inFIG. 2, faomtransducer1(Equation I) is the same as faomtransducer2(Equation I), and this common frequency is identified inFIG. 2by the term “fAOM2”. AOM108up-shifts the frequency of the first component206A (f1+fAOM1) of beam204by an amount fAOM2, and changes the polarization of the first component from vertical to horizontal, resulting in a horizontally polarized beam212A that has a frequency of f1+fAOM1+fAOM2. Similarly, AOM108down-shifts the frequency of the second component206B (f2+fAOM1) of beam204by an amount fAOM2, and changes the polarization of the second component from horizontal to vertical, resulting in a vertically polarized beam212B that has a frequency of f2+fAOM1−fAOM2.

In the embodiment shown inFIG. 2, beam212A is coupled into optical fiber216A by lens214A, and beam212B is coupled into optical fiber216B by lens214B. Optical fibers216A and216B carry beams212A and212B downstream to a beam-combining unit that combines the beams212A and212B into a combined beam for use in interferometer optics at a measurement site. In a specific embodiment according to the present teachings, fibers216A and216B are polarization-maintaining (PM) fibers.

The use of fibers216A and216B allows the laser102and AOMs108and202to be positioned remotely from the interferometer optics so that the laser102and the AOMs108and202do not affect the thermal environment of the interferometer optics. Sending the separate beams212A and212B on corresponding separate fibers216A and216B avoids cross-talk between the polarization components. The use of fibers216A and216B to deliver the light downstream provides several other advantages, including: (1) compensation for pointing stability issues caused by ambient temperature variations is not necessary when the light is delivered with an optical fiber; (2) there is no need for additional optics to make the beams216A and216B co-bore, and the co-linearity specification is much looser; and (3) fiber delivery combined with the increased split frequency provided by system100B reduces or eliminates the need for electronics at the downstream metrology stage area that might generate heat.

In the embodiment shown inFIG. 2, AOM202is an isotropic AOM. In another embodiment, AOM202is an anisotropic AOM that changes the polarization of both beam components106A and106B.

In the embodiment shown inFIG. 2, AOM202is positioned between the laser102and AOM108. In another embodiment, AOM202is positioned between AOM108and the lenses214A and214B.

FIG. 3is a diagram illustrating a system100C for producing frequency-shifted beams with orthogonal linear polarizations in a third embodiment according to the present teachings. In the illustrated embodiment, system100C includes the same laser light source (laser)102and acousto-optic modulators (AOMs)108and202as system100B (FIG. 2), and the laser102and AOMs108and202shown inFIG. 3operate in the same manner as described above with respect toFIG. 2. One difference between system100C and system100B is that system100C includes birefringent recombination prism or wedge302rather than the lenses214A and214B, and the optical fibers216A and216B shown inFIG. 2.

The anisotropic AOM108has a larger diffraction angle than the previous solutions that use a single isotropic AOM, so the co-bore (in addition to the co-linearity) of the output beams212A and212B should be addressed if fiber delivery is not used. In the embodiment shown inFIG. 3, the co-linearity angles are adjusted with birefringent recombination prism302. Prism302receives beams212A and212B from AOM108, and re-directs these beams212A and212B to produce corresponding parallel beams306A and306B. The optic axis of the prism302is identified at304inFIG. 3. The two beams212A and212B refract differently at the entrance and exit prism/air interfaces, and the prism302is appropriately positioned to cause the input beams212A and212B to become corresponding parallel beams306A and306B. The beams212A and212B exiting the AOM108are orthogonally polarized, so very little light is lost in the recombination prism302.

FIG. 4is a diagram illustrating the combining of parallel beams in a first embodiment according to the present teachings. As shown inFIG. 4, the parallel beams306A and306B produced by prism302(FIG. 3) are provided to lens402. Lens402combines beams306A and306B, thereby producing a combined beam that is directed into a polarization maintaining optical fiber404. Optical fiber404carries the combined beam downstream to interferometer optics at a measurement site.

FIG. 5is a diagram illustrating the combining of parallel beams in a second embodiment according to the present teachings. As shown inFIG. 5, the parallel beams306A and306B produced by prism302(FIG. 3) are provided to walk off prism502. Walk off prism502“walks” beams306A and306B back together so that they are co-bore, thereby producing combined beam504. Combined beam504has one component506A with a horizontal polarization, and another component506B with a vertical polarization. The combined beam504is provided to interferometer optics at a measurement site. Adjusting the tilt of the prism302can compensate for any errors in co-linearity caused by an imperfect walk off prism.

FIG. 6is a block diagram illustrating a two-frequency interferometer system600in one embodiment according to the present teachings. Interferometer600includes laser light source102, AOM108, lenses602A and602B, optical fibers650and655, beam-combining unit660, analysis system680, and interferometer optics690. Laser102and AOM108operate as described above with respect toFIG. 1to produce linearly-orthogonally polarized beams114A and114B. Laser102uses Zeeman splitting to generate a heterodyne beam104, and anisotropic AOM108flips the polarization of the two beam components106A and106B, and increases the frequency difference between the two beam components106A and106B, and thereby produces linearly-orthogonally polarized beams114A and114B. In another embodiment, system600is configured as a free-beam system, rather than using fiber delivery as shown inFIG. 6. In yet another embodiment, system600includes a second AOM202positioned between the laser102and anisotropic AOM108or after AOM108, as shown inFIGS. 2 and 3and described above.

The use of optical fibers650and,655allows laser102, and AOM108to be mounted away from interferometer optics690. Accordingly, heat generated in laser102and AOM108does not disturb the thermal environment of interferometer optics690. Additionally, laser102and AOM108do not need to have fixed positions relative to interferometer optics690, which may provide significant advantages in applications having limited available space near the object699being measured.

Beam-combining unit660precisely aligns input beam114A (INR) and input beam114B (INT) from optical fibers650and655for combination in beam combiner670to form a collinear output beam COut. Beam combiner670can be a coated polarizing beam splitter that is used in reverse. Combined beam COut is input to interferometer optics690. In interferometer optics690, a beam splitter675reflects a portion of beam COut to analysis system680, and analysis system680uses the two frequency components of the light reflected in beam splitter675as first and second reference beams. The remaining portion of combined beam COut can be expanded in size by a beam expander (not shown) before entering a polarizing beam splitter692.

Polarizing beam splitter692reflects one of the polarizations (i.e., one frequency beam) to form a third reference beam directed through optics696toward a reference reflector698and transmits the other linear polarization (i.e., the other frequency) as a measurement beam through optics694toward an object699being measured. In an alternative version of the interferometer optics690, a polarizing beam splitter transmits the component that forms the measurement beam and reflects the component that forms the reference beam.

Movement of the object699being measured causes a phase change in the measurement beam that analysis system680measures by combining the measurement beam with the third reference beam to form a beat signal. To accurately determine the phase change caused by the movement of the object699, the phase of this beat signal can be compared to the phase of a reference beat signal generated from a combination of the first and second reference beams. Analysis system680analyzes the phase change to determine the speed of and/or distance moved by the object699.

FIG. 7is a diagram illustrating a system100D for producing frequency-shifted beams with orthogonal linear polarizations in a fourth embodiment according to the present teachings. In the illustrated embodiment, system100D includes the same acousto-optic modulator (AOM)108as systems100A-100C, but system100D uses a different laser702than the laser102of system100A. In the embodiment shown inFIG. 7, laser702acts as a source of a beam704having a single frequency (f1) with a single linear polarization. The single linear polarization is a 45 degree polarization in the illustrated embodiment, which is represented inFIG. 7by arrow706.

AOM108acts as a polarizing beam splitter and splits the input beam704into a horizontally polarized beam component and a vertically polarized beam component. AOM108diffracts these two orthogonally polarized beam components in opposite directions, thereby producing plus first order beam714A, and minus first order beam714B. AOM108causes an increase in the frequency of one beam component, a decrease in the frequency of the other beam component, and causes a ninety-degree rotation in the polarization of both beam components. The net effect is that while both beam components change polarization (i.e., horizontal becomes vertical and vertical becomes horizontal), the beams stay orthogonally polarized, and now have a frequency difference given by the following Equation II:
fsplit=(f1+faomtransducer1)−(f1−faomtransducer2)  Equation II
where:fsplit=difference in the frequency of beam714A and the frequency of beam714B;f1=frequency of beam704;faomtransducer1=frequency of first signal provided to transducers110; andfaomtransducer2=frequency of second signal provided to transducers110.

In a specific embodiment according to the present teachings, faomtransducer1and faomtransducer2are both in the range of about 10 to 450 MHz. In the embodiment shown inFIG. 7, faomtransducer1is the same as faomtransducer2, and this common frequency is identified inFIG. 7by the term “fAOM”.

As an example, if the transducers110are provided with a 10 MHz RF control signal, both output beams714A and714B will be frequency shifted by 10 MHz, but in opposite directions (i.e., plus 10 MHz and minus 10 MHz), for a frequency split or difference of 20 MHz caused by the AOM108. In a specific embodiment according to the present teachings, the total frequency split (fsplit) in beams714A and714B is in the range of 8 to 30 MHz.

In another embodiment according to the present teachings, the input beam704has a polarization state other than 45 degrees. Regardless of what polarization state is chosen for beam704, when the beam704enters the crystal of AOM108, the polarization is decomposed into two orthogonal eigen polarizations. The optical power in each eigen polarization depends on the polarization state of the input beam704. Linear polarization oriented at 45 degrees from the optical axis of the crystal of AOM108is used for beam704in one form of the invention because it results in two output beams714A and714B with equal optical power. In a specific embodiment according to the present teachings, beams714A and714B have a combined optical power that is substantially the same as the optical power of beam704.

In another embodiment according to the present teachings, system100D includes a second AOM202positioned between the laser702and anisotropic AOM108or after AOM108, as shown inFIGS. 2 and 3and described above. In a specific embodiment according to the present teachings, interferometer system600(FIG. 6) uses a single frequency, single polarization laser, such as laser702, rather than the two frequency, two polarization laser102shown inFIG. 6.

Specific embodiments according to the present teachings provide several advantages over prior solutions. Specific embodiments according to the present teachings provide a significant optical power savings, can be implemented in less space, and have better optical isolation than prior solutions. In the anisotropic low frequency AOM device108, the interaction length is shorter, and the beams exit the AOM108with a larger separation angle as compared with the prior solution of using a single isotropic AOM device. Both of these properties lead to a smaller, more compact package for the final product. The shorter interaction length means the AOM device108can be much smaller. The larger separation angle means that the unwanted beams can be apertured in a shorter distance. Both beams produced by AOM108are shifted in frequency, so AOM108is a better optical isolator for the laser102than the prior solution of using a single isotropic AOM.

Specific embodiments according to the present teachings are less complex than the prior solutions that use two high frequency AOMs, in that a polarization beam splitter is not used prior to the AOM device108to divide a single polarization beam into two orthogonally polarized beams and alter the direction of the beams. In addition, AOM108is further distinguishable over many prior solutions in that AOM108uses an anisotropic interaction, rather than the isotropic interaction used in these previous solutions.

In one prior approach, an anisotropic AOM is used with two acoustic frequencies in series to generate orthogonally polarized frequency shifted beams from a single polarization optical source. In contrast, specific embodiments according to the present teachings that use the dual acoustic wave AOM108do not generate two orthogonal polarizations from a single input polarization. Rather, the AOM108preserves the two polarizations of the laser102throughout the device108, while up shifting one beam and downshifting the other beam.

In other specific embodiments according to the present teachings, AOM108is configured to generate orthogonally polarized frequency shifted beams from a single polarization optical source. Regardless of whether AOM108is used with a single polarization source, or a two polarization source, such as a Zeeman laser, the AOM108according to a specific embodiment preserves or maintains the optical power of the input beam that is provided to the AOM108. Thus, in both of these cases, the output beams that exit the AOM108have a combined optical power that is substantially the same as the optical power of the input beam that enters the AOM108.