Patent Publication Number: US-2023139746-A1

Title: Optical isolation module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 16/840,714, filed Apr. 6, 2020, and titled OPTICAL ISOLATION MODULE, now allowed, which is a divisional of U.S. patent application Ser. No. 15/790,576, filed Oct. 23, 2017, now U.S. Pat. No. 10,645,789, and titled OPTICAL ISOLATION MODULE, which is a divisional of U.S. patent application Ser. No. 14/970,402, filed Dec. 15, 2015, now U.S. Pat. No. 9,832,855, and titled OPTICAL ISOLATION MODULE, which claims the benefit of U.S. Provisional Application No. 62/236,056, filed Oct. 1, 2015, and titled OPTICAL ISOLATION MODULE. Each of these prior applications is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an optical isolation module. The optical isolation module can be used in an extreme ultraviolet (EUV) light source. 
     BACKGROUND 
     Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers. 
     Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment. 
     SUMMARY 
     In one general aspect, an optical source for a photolithography tool includes a source configured to emit a first beam of light and a second beam of light, the first beam of light having a first wavelength, and the second beam of light having a second wavelength, the first and second wavelengths being different; an amplifier configured to amplify the first beam of light and the second beam of light to produce, respectively, a first amplified light beam and a second amplified light beam; and an optical isolator between the source and the amplifier, the optical isolator including: a plurality of dichroic optical elements, and an optical modulator between two of the dichroic optical elements. 
     Implementations can include one or more of the following features. The optical modulator can include an acousto-optic modulator. Each of the dichroic optical elements can be configured to reflect light having the first wavelength and to transmit light having the second wavelength; and the acousto-optic modulator can be positioned on a beam path between two of the dichroic optical elements, the acousto-optic modulator can be positioned to receive reflected light from the two of the dichroic optical elements, the acousto-optic modulator can be configured to transmit the received light when the received light propagates in a first direction relative to the acousto-optic modulator and to deflect the received light away from the beam path when the received light propagates in a second direction relative to the acousto-optic modulator, the second direction being different from the first direction. The first and second beams of light can be pulsed beams of light. An energy of the first amplified light beam can be less than an energy of the second amplified light beam. The first amplified light beam can have an energy sufficient to deform target material in a target material droplet into a modified target, the modified target including target material in a geometric distribution that is different than a distribution of the target material in the target material droplet, the target material including material that emits extreme ultraviolet (EUV) light when in a plasma state, and the second amplified light beam has an energy sufficient to convert at least some of the target material in the modified target to the plasma that emits EUV light. 
     The acousto-optic modulator can be positioned on a beam path between two of the dichroic optical elements and can be positioned to receive light reflected from the two of the dichroic optical elements, the acousto-optic modulator can be configured to receive a trigger signal, and the acousto-optic modulator can be configured to deflect received light from the beam path in response to receiving the trigger signal, and to otherwise transmit received light onto the beam path. 
     The optical source also can include a second optical modulator between the source and the amplifier. The second optical modulator is between two of the dichroic optical elements, and the second optical modulator is on a different beam path than the optical modulator. 
     The source can include a laser source. The source can include a plurality of sources, the first light beam being produced by one of the sources, and the second light beam being produced by another one of the sources. The source can include one or more pre-amplifiers. 
     In another general aspect, an apparatus for an extreme ultraviolet (EUV) light source includes a plurality of dichroic optical elements, each of the dichroic optical elements being configured to reflect light having a wavelength in a first band of wavelengths and to transmit light having a wavelength in a second band of wavelengths; and an optical modulator positioned on a beam path between two of the dichroic optical elements, the optical modulator positioned to receive reflected light from the two dichroic optical elements, and the optical modulator configured to transmit the received light when the received light propagates in a first direction on the beam path and to deflect the received light away from the beam path when the received light propagates in a second direction on the beam path, the second direction being different from the first direction, where the first band of wavelengths includes a wavelength of a pre-pulse beam, and the second band of wavelengths includes a wavelength of a main beam. 
     Implementations can include one or more of the following features. The optical modulator can be an acousto-optic modulator. The apparatus also can include a control system configured to provide a trigger signal to the acousto-optic modulator, and the acousto-optic modulator can be configured to deflect light away from the beam path in response to receiving the trigger signal and otherwise transmits light onto the beam path. 
     The apparatus also can include a second optical modulator, where the second optical modulator is between two of the dichroic optical elements, and the second optical modulator is positioned to receive light transmitted by the two dichroic optical elements. The optical modulator and the second optical modulator can be between the same two dichroic optical elements, and the second optical modulator can be on a second beam path that is different from the beam path. 
     In another general aspect, a method includes reflecting a first beam of light at a first dichroic optical element, the reflected first beam of light passing through an optical modulator and an amplifier to produce an amplified first light beam; transmitting a second beam of light through the first dichroic optical element, a second dichroic optical element, and the amplifier to produce an amplified second beam; receiving a reflection of the amplified first light beam at the second dichroic optical element, wherein an interaction between the reflection of the amplified first light beam and the second dichroic optical element directing the reflected amplified first light beam to the optical modulator; and deflecting the reflection of the amplified first light beam at the optical modulator to thereby direct the reflection of the amplified first light beam away from a source of the first beam of light. 
     Implementations can include one or more of the following features. A trigger signal can be provided to the optical modulator after the first beam of light passes through the optical modulator and before the reflection of the amplified first light beam is at the optical modulator. The trigger signal can cause the optical modulator to be in a state in which the optical modulator deflects incident light. 
     The amplified first light beam can propagate toward an initial target region. The reflection of the first amplified light beam can be produced through an interaction between the first amplified light beam and a target material droplet in the initial target region. The second amplified light beam can propagate toward a target region, and an interaction between target material and the second amplified light beam can produce a reflection of the second amplified light beam, the method further including: transmitting the reflection of the second amplified light beam through the second dichroic optical element, and deflecting the reflection of the second amplified light beam at a second optical modulator to thereby direct the reflection of the second amplified light beam away from a source of the second beam of light. The source of the first beam of light and the source of the second beam of light can be the same source. The source of the first beam of light can be a first optical subsystem in the source, and the source of the second beam of light can be a second optical subsystem in the source. 
     Implementations of any of the techniques described above may include a method, a process, an optical isolator, a kit or pre-assembled system for retrofitting an existing EUV light source, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1  and  2    are block diagrams of exemplary optical systems. 
         FIGS.  3  and  6    are block diagrams of exemplary optical isolators. 
         FIGS.  4 A and  4 B  are block diagrams of exemplary optical arrangements that can be used in the optical isolators of  FIGS.  3  and  6   . 
         FIGS.  5 A and  5 B  are timing plots associated with an exemplary optical modulator. 
         FIG.  7    is a block diagram of an exemplary control system. 
         FIGS.  8 A and  8 B  are a block diagram of a drive laser system for an extreme ultraviolet (EUV) light source. 
         FIGS.  9 ,  10 A,  10 B,  11 A,  11 B,  12 A- 12 C, and  13 A- 13 C  are examples of experimental data collected with and without an optical isolator. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , a block diagram of an exemplary optical system  100  is shown. The optical system  100  is part of an extreme ultraviolet (EUV) light source. The optical system  100  includes an optical source  102  that produces a light beam  110 . The light beam  110  is emitted from the optical source  102  and propagates along a path  112  in a direction z toward a target region  115 . 
     The target region  115  receives a target  120 , which includes material that emits EUV light when converted to plasma. The target  120  is reflective at the wavelength or wavelengths of the light beam  110 . Because the target  120  is reflective, when the light beam  110  interacts with the target  120 , all or part of the beam  110  can be reflected along the path  112  in a direction that is different from the z direction. The reflected portion of the beam  110  is labeled as the reflection  113 . The reflection  113  can travel on the path  112  in a direction that is opposite to the z direction and back into the optical source  102 . Reflections of a forward-going beam (a beam that propagates from the optical source  102  toward the target region  115 ), such as the reflection  113 , are referred to as “back reflections.” 
     The optical source  102  includes a light-generation module  104 , an optical isolator  106 , and an optical amplifier  108 . The light-generation module  104  is a source of light (such as one or more lasers, lamps, or any combination of such elements). The optical amplifier  108  has a gain medium (not shown), which is on the beam path  112 . When the gain medium is excited, the gain medium provides photons to the light beam  110 , amplifying the light beam  110  to produce the amplified light beam  110 . The optical amplifier  108  can include more than one optical amplifier arranged with the respective gain mediums on the path  112 . The optical amplifier  108  can be all or part of a drive laser system, such as the drive laser system  880  of  FIG.  8 B . 
     The light-generation module  104  emits the light beam  110  onto the beam path  112  toward the optical isolator  106 . The optical isolator  106  passes the light beam  110  in the z direction to the optical amplifier  108  and toward the target region  115 . However, the optical isolator  106  blocks the back reflection  113 . Thus, and as discussed in greater detail below, the optical isolator  106  prevents the back reflection from entering the light-generation module  104 . By preventing the back reflection from entering the light-generation module  104 , additional optical power can be delivered to the target  120 , which can lead to an increase in the amount of generated EUV light. 
     Referring to  FIG.  2   , a block diagram of an EUV light source  200  that includes an exemplary optical source  202  is shown. The optical source  202  can be used in place of the optical source  102  in the optical system  100  ( FIG.  1   ). The optical source  202  includes a light-generation module  204 , which includes two optical subsystems  204   a,    204   b,  the optical amplifier  108 , and the optical isolator  106 . The optical isolator  106  is on the path  112  and between the optical amplifier  108  and the light-generation module  204 . 
     The optical subsystems  204   a,    204   b  produce first and second light beams  210   a,    210   b,  respectively. In the example of  FIG.  2   , the first light beam  210   a  is represented by a solid line and the second light beam  210   b  is represented by a dashed line. The optical subsystems  204   a,    204   b  can be, for example, two lasers. In the example of  FIG.  2   , the optical subsystems  204   a ,  204   b  are two carbon dioxide (CO 2 ) lasers. However, in other implementations, the optical subsystems  204   a,    204   b  are different types of lasers. For example, the optical subsystem  204   a  can be a solid state laser, and the optical subsystem  204   b  can be a CO 2  laser. 
     The first and second light beams  210   a,    210   b  have different wavelengths. For example, in implementations in which the optical subsystems  204   a,    204   b  include two CO 2  lasers, the wavelength of the first light beam  210   a  can be about 10.26 micrometers (μm) and the wavelength of the second light beam  210   b  can be between 10.18 μm and 10.26 μm. The wavelength of the second light beam  210   b  can be about 10.59 μm. In these implementations, the light beams  210   a,    210   b  are generated from different lines of the CO 2  laser, resulting in the light beams  210   a,    210   b  having different wavelengths even though both beams are generated from the same type of source. The light beams  210   a,    210   b  also can have different energies. 
     The light-generation module  204  also includes a beam combiner  209 , which directs the first and second beams  210   a,    210   b  onto the beam path  112 . The beam combiner  209  can be any optical element or a collection of optical elements capable of directing the first and second beams  210   a,    210   b  onto the beam path  112 . For example, the beam combiner  209  can be a collection of mirrors, some of which are positioned to direct the first beam  210   a  onto the beam path  112  and others of which are positioned to direct the second beam  210   b  onto the beam path  112 . The light-generation module  204  also can include a pre-amplifier  207 , which amplifies the first and second beams  210   a,    210   b  within the light-generation module  204 . 
     The first and second beams  210   a,    210   b  can propagate on the path  112  at different times, but the first and second beams  210   a,    210   b  follow the path  112  and both beams  210   a,    210   b  traverse substantially the same spatial region to the optical isolator  106 , and through the optical amplifier  108 . As discussed with respect to  FIGS.  3  and  6   , the first and second beams  210   a,    210   b  are separated within the optical isolator  106 , and then propagate on the path  112  to the optical amplifier  108 . 
     The first and second beams  210   a,    210   b  are angularly disbursed by a beam delivery system  225  such that the first beam  210   a  is directed toward an initial target region  215   a,  and the second beam  210   b  is directed toward a modified target region  215   b,  which is displaced in the −y direction relative to the initial target region  215   a.  In some implementations, the beam delivery system  225  also focuses the first and second beams  210   a,    210   b  to locations within or near the initial and modified target regions  215   a,    215   b,  respectively. 
     In the example shown in  FIG.  2   , the initial target region  215   a  receives an initial target  220   a  and the first beam  210   a.  The first beam  210   a  has an energy that is sufficient to modify the geometric distribution of target material in the initial target  220   a  (or to initiate the spatial reconfiguration of the target material) into a modified target that is received in the modified target region  215   b.  The second beam  210   b  is also received in the modified target region  215   b.  The second beam  210   b  has an energy that is sufficient to convert at least some of the target material in the modified target  220   b  into a plasma that emits EUV light. In this example, the first beam  210   a  can be referred to as a “pre-pulse”, and the second beam  210   b  can be referred to as the “main pulse.” 
     The first beam  210   a  can reflect off of the initial target  220   a,  giving rise to a back reflection  213   a  that can propagate along the path  112  in a direction other than the z direction and into the optical amplifier  108 . Because the first beam  210   a  is used to modify a spatial characteristic of the initial target  220   a  and is not intended to convert the initial target  220   a  into the plasma that emits EUV light, the first beam  210   a  has a lower energy than the second beam  210   b.  However, reflections of the first light beam  210   a  can have more energy than reflections of the second light beam  201   b.    
     The first beam  210   a  (and the reflection  213   a ) propagates through the optical amplifier  108  before the second beam  210   b.  Thus, the gain medium of the optical amplifier  108  can still be excited when the reflection  213   a  passes through the gain medium of the optical amplifier  108 . As a result, the reflection  213   a  can be amplified by the amplifier  108 . Further, the initial target  220   a  can be substantially spherical in shape, dense, and highly reflective, whereas the modified target  220   b  can be a disk-like shape (or other non-spherical shape), less dense and less reflective. Due to the non-spherical shape, the modified target  220   b  can be positioned to reduce the amount of light that reflects back onto the path  112  due to an interaction between the second beam  210   b  and the modified target  220   b.  For example, the modified target  220   b  can be tilted in the x-z and/or y-z plane relative to the direction of propagation of the light beam  210   b,  or the modified target  220   b  can be away from the focus of the second beam  210   b.    
     In some implementations, the modified target  220   b  is not tilted in the x-z and/or y-z plane, and the modified target  220   b  is instead oriented such that the side of the modified target  220   b  that has the greatest spatial extent is in a plane that is perpendicular to the direction of propagation of the second beam  210   b.  Orienting the modified target  220   b  in this manner (which can be referred to as a “flat” target orientation) can enhance the absorption of the second beam  210   b.  In some implementations, such an orientation can increase the absorption of the second beam  210   b  by about 10% as compared to instances in which the modified target  220   b  is tilted 20 degrees (°) relative to a plane that is perpendicular to the direction of propagation of the second beam  210   b.  Orienting the modified target  220   b  in a flat orientation can increase the amount of reflected light that propagates back into the optical source  202 . However, because the optical source  202  includes the optical isolator  106 , the modified target  220   b  can have a flat orientation because the optical isolator  106  acts to reduce the impact of reflections that can arise from the modified target  220   b  in a flat orientation. 
     Finally, because the second beam  210   b  has a relatively large energy, the forward propagation of the second beam  210   b  through the amplifier  108  saturates the gain medium, leaving little energy that the amplifier  108  can provide to a back reflection of the second beam  210   b.  As such, even though the first beam  210   a  has a lower energy than the second beam  210   b , the back reflection  213   a,  which arises from the first beam  210   a,  can be substantial and can be larger than a back reflection arising from the second beam  210   b.    
     As discussed below, the optical isolator  106  prevents back reflections arising from the first beam  210   a  from entering the light-generation module  204 . The optical isolator  106  also can prevent back reflections arising from the second beam  210   b  from entering the light-generation module  204 , and an example of such an implementation is shown in  FIG.  6   . Because the optical isolator  106  prevents potentially damaging back reflections from reaching the light-generation module  204 , higher energy light beams can be generated from the light-generation module  204 , resulting in more energy being delivered to the modified target  220   b  and more EUV light. In some implementations, the average amount of EUV light produced can be increased by about 20% by using the optical isolator  106 . 
     Referring to  FIG.  3   , a block diagram of an exemplary optical isolator  306  is shown. The optical isolator  306  can be used as the optical isolator  106  in the optical source  102  ( FIG.  1   ), the optical source  202  ( FIG.  2   ), or in any other optical source. The optical isolator  306  is discussed with respect to the optical source  202 . 
     The optical isolator  306  includes a dichroic optical element  331 , reflective elements  332 , an optical modulator  335 , and a dichroic element  336 . The optical isolator  306  also can include optical arrangements  333 ,  334 . The dichroic elements  331  and  336  are on the beam path  112 . The dichroic elements  331  and  336  can be any optical component that is capable of separating or filtering light according to its wavelength. For example, the dichroic elements  331  and  336  can be dichroic mirrors, dichroic filters, dichroic beam splitters, or a combination of such elements. The dichroic elements  331  and  336  can be identical to each other, or they can have different configurations. In the example of  FIG.  3   , the dichroic elements  331  and  336  reflect the wavelength (or wavelengths) of the first beam  210   a  and transmit the wavelength (or wavelengths) of the second beam  210   b.    
     The first beam  210   a  is reflected from the dichroic element  331  onto a beam path  314 , which is between the dichroic elements  331  and  336  and has a spatial extent and form defined by the reflective elements  332 . The beam path  314  is different from the beam path  112 . Thus, in the optical isolator  306 , the first beam  210   a  does not remain on the beam path  112 , and the first and second beams  210   a,    210   b  are spatially separated from each other. The first beam  210   a  propagates on the beam path  314  through the optical arrangements  333 ,  334 , and the optical modulator  335 , before reaching the dichroic element  336 , which reflects the beam  210   a  back onto the beam path  112 . The second beam  210   b  passes through the dichroic element  331  and through the dichroic element  336 , remaining on the beam path  112  while propagating through the optical isolator  306 . 
     The optical modulator  335  is on the beam path  314  between the dichroic elements  331  and  336 . The optical modulator  335  is an optical element that is capable of deflecting incident light away from the path  314 . The optical modulator  335  is adjustable between an open state and a closed state such that the optical modulator  335  can transmit the first beam  210   a  and block the reflection  213   a  (the reflection of the first beam  210   a  from the initial target  220   a ). 
     The optical modulator  335  can be, for example, an acousto-optic modulator (AOM). An acousto-optic modulator includes a medium (such as quartz or glass) connected to a transducer (such as a piezo-electric transducer). Motion of the transducer causes sound waves to form in the medium, creating a spatially varying index of refraction in the medium. When the medium includes the sound waves, light incident on the medium is deflected. When the sound waves are not present in the medium, the acousto-optic modulator transmits incident light without deflection. Other optical modulators can be used as the modulator  335 . For example, the optical modulator  335  can be a Faraday rotator or an electro-optic modulator (EOM). The modulator  335  can be a combination of such devices, and can include more than one of the same type of device. 
     In implementations in which the optical modulator  335  is an acousto-optic modulator, the transducer moves at a time when the reflection  213   a  is expected to enter the path  314 . At other times, the transducer is not moved or vibrated. Thus, the beam  210   a  (the forward-going “pre-pulse”) passes through the optical modulator  335 , remaining on the path  314  and ultimately rejoining the path  112 . However, the reflection  213   a  is deflected (shown as deflection  217   a  in  FIG.  3   ) away from the path  314 . As a result, the reflection  213   a  does not reach the light-generation module  204  ( FIG.  2   ). 
     Because the optical modulator  335  can be configured to transmit incident light only at certain times, the optical isolator  306  provides a time-gate based isolation technique as opposed to one that is based on polarization. Additionally, the optical isolator  306  can be used in combination with a polarization-based isolation technique. For example, the polarization of the back reflections can be different than the polarization of the forward-going beams  210   a,    210   b,  and a polarization isolator  303 , which includes a polarizing element (such as a thin film polarizer), can be placed between the optical isolator  306  and the optical amplifier  108  ( FIGS.  1  and  2   ) to provide additional blocking of back reflections. The polarizing element of the polarization isolator  303  can be configured to primarily reject reflections of the second light beam  210   b,  allowing the optical isolator  306  to be tailored to reject reflections of the first light beam  210   a.  By using different techniques to reject reflections of the first light beam  210   a  and the second light beam  210   b,  the overall amount of reflections reaching the light-generation module  204  from any source can be reduced. 
     In some implementations, the optical isolator  306  includes first and second optical arrangements  333 ,  334 . The first beam  210   a  passes through the first optical arrangement  333  before reaching the optical modulator  335 . The first optical arrangement  333  can be any optical element or a collection of optical elements that reduces the beam diameter of the first light beam  210   a.  After passing through the optical modulator  335 , the first beam  210   a  passes through the second optical arrangement  334 . The second optical arrangement  334  can be any optical element or a collection of optical elements that enlarge the beam diameter of the second light beam  210   b.  The speed at which the optical modulator  335  can be transitioned between being opened (in a state in which incident light is transmitted by the optical modulator  335 ) or closed (in a state in which incident light is deflected or blocked by the optical modulator  335 ) increases as the beam diameter decreases. Thus, by reducing the diameter of the first beam  210   a,  the first optical arrangement  333  allows the optical modulator  335  to switch between being opened and closed, and vice versa, more quickly than in implementations that lack the first optical arrangement  333 . In some implementations, the beam diameter of the beam  210   a  can be reduced to about 3 millimeters (mm). 
     The second optical arrangement  334  enlarges the diameter of the first light beam  210   a  prior to directing the first light beam  210   a  onto the path  112 . Additionally, the second optical arrangement  334  reduces the beam diameter of the reflection  213   a  before the reflection  213   a  reaches the optical modulator  335 . By reducing the beam diameter of the reflection  213   a,  the speed at which the optical modulator  335  must be transitioned between the open and closed states to block the reflection  213   a  is reduced. 
     Referring to  FIGS.  4 A and  4 B , block diagrams of exemplary optical arrangements  433  and  434 , respectively, are shown. The optical arrangements  433 ,  434  can be used as the optical arrangement  333 ,  334 , respectively, in the optical isolator  306  ( FIG.  3   ). The optical arrangements  433 ,  434  are Galilean telescopes, which have one convex lens and one concave lens. In the optical arrangement  433 , a concave lens  442  is between a convex lens  441  and the optical modulator  335 . In the optical arrangement  434 , a concave lens  443  is between the optical modulator  335  and a convex lens  444 . Both of the arrangements  433 ,  434  reduce the diameter of a beam that propagates toward the optical modulator  335 . When the optical arrangements  433 ,  434  are used together in the configuration shown in  FIG.  3   , the beam diameter of the beam  210   a  is reduced prior to being incident on the optical modulator  335 , and the beam diameter of the beam  210   a  is enlarged by the optical arrangement  434  after passing through the optical modulator  335 . The beam diameter of the reflection  213   a  is reduced by the optical arrangement  434  prior to reaching the optical modulator  335 . The reflection  213   a  does not pass through the optical arrangement  433  because the optical modulator  335  deflects the reflection  213   a  from the beam path  314 . 
     The optical arrangements  433  and  434  can be identical Galilean telescopes or the arrangements  433  and  434  can includes lenses that have different characteristics (such as different focal lengths). 
     Referring to  FIG.  5 A , an exemplary plot that shows the state of the optical modulator  335  as a function of time is shown.  FIG.  5 B  shows the relative placement of pulses of a beam  510   a  and a reflection  513   a  on the same time axis that is shown in  FIG.  5 A . The pulse  510   a  is a pulse of a beam that propagates through the system  200  ( FIG.  2   ) when the system  200  is configured to use the optical isolator  306  ( FIG.  3   ) as the optical isolator  106 , and the reflection  513   a  is a reflection of the pulse  513   a  from the initial target  220   a.  The pulse  510   a  is a pulse of a pulsed light beam that is used as a “pre-pulse” to shape the initial target  220   a.    
     The optical modulator  335  is closed (deflects light from the path  314  or otherwise prevents incident light from remaining on the path  314 ) from the time t 1  to the time t 2 . At the time t 2 , the optical modulator  335  begins to transition to the open state. The optical modulator  335  is open between the times t 2  and t 3 , and, during this time range, the optical modulator  335  transmits incident light. The optical modulator  335  transitions to be closed at the time t 3 , and becomes closed again at the time t 4 . As discussed above, the transition times (the time between the time t 2  and t 3  and the time between t 3  and t 4 ) can be reduced by reducing the beam diameter of the light that is gated by the optical modulator  335 . 
     Referring also to  FIG.  5 B , the times t 2  and t 3  are selected such that the pulse  510   a  is incident on the optical modulator  335  at a time when the modulator  335  is open. Thus, the pulse  510   a  passes through the optical modulator  335  to reach the initial target  220   a.  The times t 3  and t 4  are selected so that the optical modulator  335  begins to close after transmitting the pulse  510   a  and is closed when the reflection  513   a  is incident on the optical modulator  335 . In this way, the optical modulator  335  provides a time-gate based isolation of the pre-pulse reflection  513   a.    
     In some implementations, the beam diameter of the pre-pulse  510   a  and the reflection  513   a  can be 3 mm. In implementations in which the optical modulator  335  is an acousto-optic modulator, the time that the optical modulator takes to transition from open to close and vice versa is determined by the beam diameter of the incident light and the speed of sound in the material of the optical modulator. The material can be, for example, germanium (Ge), which has an acoustic wave speed of 5500 meters/second (m/s). In this example, the transition time (the time for the optical modulator to transition from closed to open) is 375 nanoseconds (ns). The delay between the pre-pulse  510   a  and the reflection  513   a  can be, for example, 400 ns. Thus, the pre-pulse  510   a  is transmitted by the optical modulator  335  and the reflection  513   a  is deflected off of the path  314 . 
     In some implementations, the optical modulator  335  is closed except for the period of time at which the pulse  510   a  is expected. By remaining closed at other times, the optical modulator  335  prevents the reflection  513   a  from entering the light-generation module  204 . Additionally, by remaining closed, the modulator  335  also prevents or reduces the impact of secondary reflections of the pulse  510   a.  Elements, such as filters, pinholes, lenses, and tubes, on the path  112  are sources of glint and reflect incident light. These elements can reflect the pulse  510   b  and cause secondary reflections that propagate on the path  112  and the path  314 , and these secondary reflections are in addition to the reflection  513   a.  By keeping the modulator  335  closed except when the pulse  510   a  is incident on the modulator  335 , the secondary reflections are also prevented from entering the light-generation module  204 . Furthermore, the secondary reflections are removed from the path  314  and are thus prevented from propagating back onto the path  112 . In this way, the secondary reflections cannot reach the initial target region  215   a,  the modified target region  215   b,  or the region between the regions  215   a  and  215   b.  If the secondary reflections are able to reach these regions, the reflections can harm the target by breaking it apart before the target reaches the modified target region  215   b.  The secondary reflections can be referred to as forward pulse excited by reverse pulses (FERs). The optical isolator  306  can help mitigate self-lasing, which can limit the maximum about of optical power delivered to the target region  215   b.    
     Referring to  FIG.  6   , a block diagram of another exemplary optical isolator  606  is shown. The optical isolator  606  can be used instead of the optical isolator  106  in the system  100  ( FIG.  1   ) or the system  200  ( FIG.  2   ). Additionally, the optical isolator  606  can be used in any other optical system where the prevention of back reflections is desirable. The optical isolator  606  is discussed with respect to a configuration in which the optical isolator  606  is used as the optical isolator  106  in the system  200  ( FIG.  2   ). The optical isolator  606  can be used with the polarization isolator  303  discussed above with respect to  FIG.  3   . In implementations that include the polarization isolator  303 , the polarization isolator  303  is between the optical isolator  606  and the optical amplifier  108  ( FIGS.  1  and  2   ) to provide additional blocking of back reflections. 
     The optical isolator  606  is similar to the optical isolator  306  ( FIG.  3   ), except the optical isolator  606  includes a second optical modulator  637 . The second optical modulator  637  is on the path  112 , and is positioned between the dichroic optical element  331  and the dichroic optical element  336 . Similar to the optical modulator  335 , the second optical modulator  637  transmits incident light when in an open state and deflects or blocks incident light when in a closed state. The second light beam  210   b  is emitted from the light-generation module  204  and propagates on the path  112  to the dichroic optical element  331 . 
     As discussed above, the dichroic optical element  331  transmits the wavelength of the second light beam  210   b.  Thus, the second light beam  210   b  passes through the dichroic optical element  331  and is incident on the second optical modulator  637 . The second optical modulator  637  is controlled to be in the open state when the second light beam  210   b  is incident on the modulator  637 , and the second light beam  210   b  passes through the modulator  637  and the dichroic optical element  336 , remaining on the path  112  and reaching the modified target region  215   b  ( FIG.  2   ). Part of the second light beam  210   b  is reflected from the modified target  220   b  (in addition to converting at least some of the target material to plasma that emits EUV light) and can propagate as a reflection  213   b  along the path  112  in a direction other than the z direction. 
     The reflection  213   b  is transmitted by the dichroic optical element  336  and remains on the path  112 . The optical modulator  637  is closed when the reflection  213   b  is incident on the modulator  637 , and the reflection  213   b  is deflected from the path  112  as deflected light  217   b.  Thus, the second modulator  637  prevents the reflection  213   b  from reaching the light-generation module  204  or reduces the amount of the reflection  213   b  that reaches the light-generation module  204 , reducing or eliminating self-lasing from the light-generation module  404  and allowing the second light beam  210   b  to be of greater energy. In some implementations, the optical modulator  637  deflects 30-40% of the reflection  213   b.  The time during which the optical modulator  637  is open can be reduced to further reduce the amount of self-lasing. For example, reducing the open time from 20 microseconds (μs) to 2 μs can reduce the self-lasing by 90%. 
     The second modulator  637  is closed except for the period of time at which the beam  210   b  is expected. By remaining closed at other times, the second modulator  637  prevents the reflection  213   b  from entering the light-generation module  204 . Additionally, by remaining closed, the second modulator  637  also prevents or reduces the impact of secondary reflections from of the second beam  210   b.  Elements, such as filters, pinholes, lenses, and tubes, on the path  112  are sources of glint and reflect incident light. These elements can reflect the second beam  210   b  and cause secondary reflections that are in addition to the reflection  213   b  (which is caused by an interaction between the second beam  210   b  and the modified target  220   b ). By keeping the modulator  637  closed except when the second light beam  210   b  is incident on the modulator  637 , the secondary reflections are also prevented from entering the light-generation module  204  and the secondary reflections are removed from the path  112 . 
     The second optical modulator  637  can be the same as the modulator  335 , or the second optical modulator  637  and the modulator  335  can be different types of modulators. 
     Referring to  FIG.  7   , a block diagram of a system  700  is shown. The system  700  includes a light-generation module  704 , a control system  740 , and an optical modulator  735 . The light-generation module  704  can be the light-generation module  104  ( FIG.  1   ), the light-generation module  204  ( FIG.  2   ), or any other system that generates light beams having different wavelengths. The optical modulator  735  can be the optical modulator  335  ( FIG.  3   ) and/or the optical modulator  637  ( FIG.  6   ). 
     The control system  740  provides a trigger signal  747  to the optical modulator  735 . The trigger signal  747  is sufficient to cause the optical modulator  735  to change state or to begin to change state. For example, in implementations in which the optical modulator  735  is an acousto-optic modulator, the trigger signal  747  can cause the modulator to transition to a closed state by causing a transducer to vibrate to form sound waves in the modulator. The control system  740  also can receive data from the light-generation module  704  through a signal  741 , and can provide data to the light-generation module  704  through a signal  742 . Further, the control system  740  also can receive data from the optical module  735  via a signal  742 . 
     The control system  740  includes an electronic storage  743 , an electronic processor  744 , and an input/output (I/O) interface  745 . The electronic processor  744  includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, a processor receives instructions and data from a read-only memory or a random access memory or both. The electronic processor  744  can be any type of electronic processor. 
     The electronic storage  743  can be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage  743  can include both non-volatile and volatile portions or components. The electronic storage  743  can data and information that is used in the operation of the optical modulator  735 . For example, the electronic storage  743  can store timing information that specifies when the first and second beams  210   a,    210   b  are expected to propagate through the system  200  ( FIG.  2   ). The electronic storage  743  also can store instructions, perhaps as a computer program, that, when executed, cause the processor  744  to communicate with other components in the control system  740 , the light-generation module  704 , and/or the optical modulator  735 . For example, the instructions can be instructions that cause the electronic processor  744  to provide a trigger signal  747  to the optical modulator  735  at certain times that are specified by the timing information stored on the electronic storage  743 . 
     The I/O interface  745  is any kind of electronic interface that allows the control system  740  to receive and/or provide data and signals with an operator, the light-generation module  704 , the optical modulator  735 , and/or an automated process running on another electronic device. For example, the I/O interface  745  can include one or more of a visual display, a keyboard, or a communications interface. 
     Referring to  FIG.  8 A , an LPP EUV light source  800  is shown. The optical systems  100  and  200  can be part of an EUV light source, such as the source  800 . The LPP EUV light source  800  is formed by irradiating a target mixture  814  at a target location  805  with an amplified light beam  810  that travels along a beam path toward the target mixture  814 . The target location  805 , which is also referred to as the irradiation site, is within an interior  807  of a vacuum chamber  830 . When the amplified light beam  810  strikes the target mixture  814 , a target material within the target mixture  814  is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture  814 . These characteristics can include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma. 
     The light source  800  also includes a target material delivery system  825  that delivers, controls, and directs the target mixture  814  in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture  814  includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin can be used as pure tin (Sn); as a tin compound, for example, SnBr 4 , SnBr 2 , SnH 4 ; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture  814  can also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture  814  is made up of only the target material. The target mixture  814  is delivered by the target material delivery system  825  into the interior  607  of the chamber  630  and to the target location  605 . 
     The light source  800  includes a drive laser system  815  that produces the amplified light beam  810  due to a population inversion within the gain medium or mediums of the laser system  815 . The light source  800  includes a beam delivery system between the laser system  815  and the target location  805 , the beam delivery system including a beam transport system  820  and a focus assembly  822 . The beam transport system  820  receives the amplified light beam  810  from the laser system  815 , and steers and modifies the amplified light beam  810  as needed and outputs the amplified light beam  810  to the focus assembly  822 . The focus assembly  822  receives the amplified light beam  810  and focuses the beam  810  to the target location  805 . 
     In some implementations, the laser system  815  can include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system  815  produces an amplified light beam  810  due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system  815  can produce an amplified light beam  810  that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system  815 . The term “amplified light beam” encompasses one or more of: light from the laser system  815  that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system  815  that is amplified and is also a coherent laser oscillation. 
     The optical amplifiers in the laser system  815  can include as a gain medium a filling gas that includes CO 2  and can amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800. Suitable amplifiers and lasers for use in the laser system  815  can include a pulsed laser device, for example, a pulsed, gas-discharge CO 2  laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and high pulse repetition rate, for example, 40 kHz or more. The optical amplifiers in the laser system  815  can also include a cooling system such as water that can be used when operating the laser system  815  at higher powers. 
       FIG.  8 B  shows a block diagram of an example drive laser system  880 . The drive laser system  880  can be used as part of the drive laser system  815  in the source  800 . The drive laser system  880  includes three power amplifiers  881 ,  882 , and  883 . Any or all of the power amplifiers  881 ,  882 , and  883  can include internal optical elements (not shown). 
     Light  884  exits from the power amplifier  881  through an output window  885  and is reflected off a curved mirror  886 . After reflection, the light  884  passes through a spatial filter  887 , is reflected off of a curved mirror  888 , and enters the power amplifier  882  through an input window  889 . The light  884  is amplified in the power amplifier  882  and redirected out of the power amplifier  882  through an output window  890  as light  891 . The light  891  is directed toward the amplifier  883  with a fold mirror  892  and enters the amplifier  883  through an input window  893 . The amplifier  883  amplifies the light  891  and directs the light  891  out of the amplifier  883  through an output window  894  as an output beam  895 . A fold mirror  896  directs the output beam  895  upward (out of the page) and toward the beam transport system  820  ( FIG.  8 A ). 
     Referring again to  FIG.  8 B , the spatial filter  887  defines an aperture  897 , which can be, for example, a circle having a diameter between about 2.2 mm and 3 mm. The curved mirrors  886  and  888  can be, for example, off-axis parabola mirrors with focal lengths of about 1.7 m and 2.3 m, respectively. The spatial filter  887  can be positioned such that the aperture  897  coincides with a focal point of the drive laser system  880 . 
     Referring again to  FIG.  8 A , the light source  800  includes a collector mirror  835  having an aperture  840  to allow the amplified light beam  810  to pass through and reach the target location  805 . The collector mirror  835  can be, for example, an ellipsoidal mirror that has a primary focus at the target location  805  and a secondary focus at an intermediate location  845  (also called an intermediate focus) where the EUV light can be output from the light source  800  and can be input to, for example, an integrated circuit lithography tool (not shown). The light source  800  can also include an open-ended, hollow conical shroud  850  (for example, a gas cone) that tapers toward the target location  805  from the collector mirror  835  to reduce the amount of plasma-generated debris that enters the focus assembly  822  and/or the beam transport system  820  while allowing the amplified light beam  810  to reach the target location  805 . For this purpose, a gas flow can be provided in the shroud that is directed toward the target location  805 . 
     The light source  800  can also include a master controller  855  that is connected to a droplet position detection feedback system  856 , a laser control system  857 , and a beam control system  858 . The light source  800  can include one or more target or droplet imagers  860  that provide an output indicative of the position of a droplet, for example, relative to the target location  805  and provide this output to the droplet position detection feedback system  856 , which can, for example, compute a droplet position and trajectory from which a droplet position error can be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system  856  thus provides the droplet position error as an input to the master controller  855 . The master controller  855  can therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system  857  that can be used, for example, to control the laser timing circuit and/or to the beam control system  858  to control an amplified light beam position and shaping of the beam transport system  820  to change the location and/or focal power of the beam focal spot within the chamber  830 . 
     The target material delivery system  825  includes a target material delivery control system  826  that is operable, in response to a signal from the master controller  855 , for example, to modify the release point of the droplets as released by a target material supply apparatus  827  to correct for errors in the droplets arriving at the desired target location  805 . 
     Additionally, the light source  800  can include light source detectors  865  and  870  that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector  865  generates a feedback signal for use by the master controller  855 . The feedback signal can be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production. 
     The light source  800  can also include a guide laser  875  that can be used to align various sections of the light source  800  or to assist in steering the amplified light beam  810  to the target location  805 . In connection with the guide laser  875 , the light source  800  includes a metrology system  824  that is placed within the focus assembly  822  to sample a portion of light from the guide laser  875  and the amplified light beam  810 . In other implementations, the metrology system  824  is placed within the beam transport system  820 . The metrology system  824  can include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that can withstand the powers of the guide laser beam and the amplified light beam  810 . A beam analysis system is formed from the metrology system  824  and the master controller  855  since the master controller  855  analyzes the sampled light from the guide laser  875  and uses this information to adjust components within the focus assembly  822  through the beam control system  858 . 
     Thus, in summary, the light source  800  produces an amplified light beam  810  that is directed along the beam path to irradiate the target mixture  814  at the target location  805  to convert the target material within the mixture  814  into plasma that emits light in the EUV range. The amplified light beam  810  operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system  815 . Additionally, the amplified light beam  810  can be a laser beam when the target material provides enough feedback back into the laser system  815  to produce coherent laser light or if the drive laser system  815  includes suitable optical feedback to form a laser cavity. 
     Referring to  FIG.  9   , a plot  900  of example test data for an optical isolator such as the optical isolator  306  ( FIG.  3   ) is shown. The plot  900  shows the measured power of a reverse-going pre-pulse beam as a function of time with the optical isolator in an ON state and in an OFF state. The reverse-going pre-pulse beam can be a beam such as the reflection  213   a  ( FIG.  2   ), which arises from the interaction between the first beam  210   a  ( FIG.  2   ) and the initial target  220   a  ( FIG.  2   ) as discussed above. In the ON state, the optical isolator blocks or reduces the effects of the reflection  213   a  by deflecting all or part of the reflection  213   a  from the beam path  314  so that the reflection  213   a  that reaches the light-generation module  204  is reduced or eliminated. In the ON state, the optical isolator can operate, for example, as discussed with respect to  FIGS.  5 A and  5 B . In the OFF state, the optical isolator is not active and the system operates as if the optical isolator is not present. 
     In the example of  FIG.  9   , the optical isolator is in the OFF state between the times  905  and  910 , and otherwise is in the ON state. When the optical isolator is in the ON state, the power of the reflection  213   a  that reaches the light-generation module  204  is very low, and is close to zero Watts (W). For example, the power of the reflection  213   a  that reaches the light-generation module  204  can be about or below 0.1 W. As discussed above, it is desirable to reduce the power of the reflection  213   a  that reaches the light-generation module  204 . In contrast, when the optical isolator is in the OFF state, the power of the reflection  213   a  that reaches the light-generation module  204  is greater than 0 and can be between about 4.2 W and 18.2 W. Furthermore, when the optical isolator is in the OFF state, the power of the reflection  213   a  that reaches the light-generation module  204  varies quite a bit, which can lead to instabilities in the system. Thus, in addition to reducing the amount of power in the reflection  213   a,  the optical isolator also reduces the variation of the power of the reflection, resulting in a more stable system. 
     Referring to  FIGS.  10 A and  10 B , additional example test data are shown.  FIG.  10 A  shows the energy of the produced EUV light as a function of pulse number when the optical isolator (such as the optical isolator  306 ) is not present in the system, and  FIG.  10 B  shows the energy of the produced EUV light as a function of pulse number when the optical isolator is present in the system. When the optical isolator is not present, the average energy of the EUV light is 3.4 milliJoules (mJ). When the optical isolator is present, the average EUV energy increases to 4.1 mJ. 
     Referring also to  FIGS.  11 A and  11 B , the produced EUV light is also more stable when the optical isolator is present in the system.  FIG.  11 A  shows a distribution of particular values of the energy of the produced EUV light when the optical isolator is not present, and  FIG.  11 B  shows the distribution of particular values of the energy of the produced EUV light when the optical isolator is present. The distribution of energy values of  FIG.  11 B  (when the optical isolator is used) shows that higher energy values occur more often and that all of the energy values are contained in a smaller range as compared to a system that does not employ the optical isolator. Thus, using an optical isolator (such as the optical isolator  306 ) results in EUV light of higher energy and also results in EUV light that is more stable (varies less). 
     Referring to  FIGS.  12 A- 12 C and  13 A- 13 C , additional example test data are shown.  FIGS.  12 A- 12 C  show a target  1200  at three times in a system that lacks an optical isolator such as the optical isolator  306 , and  FIGS.  13 A- 13 C  show a target  1300  at three times in a system that includes an optical isolator such as the optical isolator  306 . The targets  1200  and  1300  include the target material that emits EUV light when in a plasma state. The targets  1200  and  1300  are shown at times that coincide with the targets  1200  and  1300  being in a location that receives a pre-pulse (such as the initial target region  215   a  of  FIG.  2   ) and a location that receives a main pulse (such as the modified target region  215   b  of  FIG.  2   ). 
     As discussed above with respect to  FIGS.  5 A and  5 B , the optical isolator can reduce or eliminate secondary reflections from objects such as pin holes, lenses, tubes, and optical elements. When present, the secondary reflections can reach the target as it moves from the initial target region  215   a  to the modified target region  215   b.    FIGS.  12 A- 12 C  show an example of the secondary reflections interacting with the target  1200  over time. As shown in  FIGS.  12 B and  12 C  as compared to  12 A, the target  1200  spreads out spatially as time passes and breaks apart.  FIGS.  13 A- 13 C  show an example of a system that uses the optical isolator (such as the optical isolator  306 ) to reduce or eliminate the secondary reflections. As compared to the target  1200  ( FIGS.  12 A- 12 C ), the target  1300  ( FIGS.  13 A- 13 C ) has a cleaner spatial profile, which can lead to increased absorption of an incident light beam and more target material available for interaction with the second beam  210   b  (and thus more EUV light produced). Additionally, because the target  1300  is used with an optical source that includes the optical isolator, the target  1300  can have a flat orientation relative to the direction of propagation of the incident light beam while still reducing or eliminating the effects of back reflections and secondary reflections on the optical source. 
     Other implementations are within the scope of the claims. 
     In implementations in which the optical subsystems  204   a,    204   b  ( FIG.  2   ) are different types of optical subsystems, the optical subsystem  204   a  can be a rare-earth-doped solid state laser (such as a Nd:YAG or an erbium-doped fiber (Er:glass)), and the wavelength of the first light beam  210   a  can be 1.06 μm. The optical subsystem  204   b  can be a CO 2  laser, and the wavelength of the light beam  210   b  can be, for example, 10.26 μm. In these implementations, the first and second beams  210   a,    210   b  can be amplified in separate optical amplifiers and can follow separate paths through the system  200 . Also, two separate optical isolators can be used, one for the first light beam  210   a  and its corresponding reflections, and another for the light beam  210   b  and its corresponding reflections. 
     The pre-amplifier  207  ( FIG.  2   ) can have multiple stages. In other words, the pre-amplifier  207  can include more than one amplifier in series and placed on the path  112 . 
     The light beams  110 ,  210   a,  and  210   b  can be pulsed light beams. The power of a pulse of the first light beam  210   a  (or the pulse  510   a ) can be, for example, 20-40 Watts (W). The power of a pulse of the second light beam  210   b  can be, for example, 300-500 W. 
     The first beam of light  210   a  can be any type of radiation that can act on the initial target  220   a  to form the modified target  220   b.  For example, the first beam of light  210  can be a pulsed optical beam generated by a laser. The first beam of light  210  can have a wavelength of about 1-10.6 μm. The duration of a pulse of the first beam of light  210   a  can be, for example, 20-70 nanoseconds (ns), less than 1 ns, 300 picoseconds (ps), between 100-300 ps, between 10-50 ps, or between 10-100 ps. The energy of a pulse of the first beam of light  210   a  can be, for example, 15-60 milliJoules (mJ). When the pulse of the first beam of light  210   a  has a duration of 1 ns or less, the energy of the pulse can be 2 mJ. The time between a pulse of the first light beam  210   a  and a pulse of the second light beam  210   b  can be, for example, 1-3 microseconds (μs). 
     The initial target  220   a  and the target  115  can have any the characteristics of the target mixture  814 . For example, the initial target  220   a  and the target  115  can include tin. 
     The optical systems  100  and  200  can include the polarization isolator  303 . In these implementations of the optical system  100 , the polarization isolator  303  is between the optical isolator  106  and the optical amplifier  108 .