ATOMIC GYROSCOPE AND ATOMIC INTERFEROMETER
20230011067 · 2023-01-12
Assignee
Inventors
Cpc classification
G02F1/093
PHYSICS
G02F1/11
PHYSICS
G01P3/36
PHYSICS
International classification
G01C19/72
PHYSICS
G01P3/36
PHYSICS
Abstract
An atomic interferometer includes: an optical system including an optical modulating device that includes: an optical fiber for a first laser beam to propagate therein; and a frequency shifter connected to the optical fiber and configured to shift the frequency of the first laser beam, the optical system being configured to generate a moving standing light wave from counter-propagation of the first laser beam from the optical modulating device and a second laser beam; and an interference system for making an atomic beam interact with three or more moving standing light waves including the moving standing light wave.
Claims
1. An atomic gyroscope comprising: an optical system including an optical modulating device that includes: an optical fiber for a first laser beam to propagate therein; and a frequency shifter connected to the optical fiber and configured to shift a frequency of the first laser beam, the optical system being configured to generate a moving standing light wave from counter-propagation of the first laser beam from the optical modulating device and a second laser beam; an interference system for making an atomic beam interact with three or more moving standing light waves including the moving standing light wave; and a monitor for detecting angular velocity or acceleration by monitoring the atomic beam from the interference system.
2. The atomic gyroscope according to claim 1, wherein the optical modulating device further includes an optical circulator, the optical circulator has a first port thereof connected to one end of the frequency shifter, the optical circulator has a third port thereof connected to the other end of the frequency shifter, and the optical circulator has a second port thereof for the first laser beam to enter.
3. The atomic gyroscope according to claim 1, wherein the first laser beam enters the optical fiber without crossing the atomic beam.
4. The atomic gyroscope according to claim 1, wherein the frequency shifter is an acousto-optic modulator or an electro-optic modulator.
5. An atomic interferometer comprising: an optical system including an optical modulating device that includes: an optical fiber for a first laser beam to propagate therein; and a frequency shifter connected to the optical fiber and configured to shift a frequency of the first laser beam, the optical system being configured to generate a moving standing light wave from counter-propagation of the first laser beam from the optical modulating device and a second laser beam; and an interference system for making an atomic beam interact with three or more moving standing light waves including the moving standing light wave.
6. The atomic gyroscope according to claim 2, wherein the frequency shifter is an acousto-optic modulator or an electro-optic modulator.
7. The atomic gyroscope according to claim 3, wherein the frequency shifter is an acousto-optic modulator or an electro-optic modulator.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0032] A description of an embodiment of the present invention is given in connection with, for instance, a Mach-Zehnder atomic interference scheme. It is to be noted that the drawings are provided for the understanding of the embodiment and the size of each component illustrated therein is different from the actual size. For the sake of convenience, a description of the embodiment is based on a Mach-Zehnder atomic interferometer; it is to be noted that the gist of the present invention can be applied to any atomic interference scheme using a moving standing light wave.
[0033] A Mach-Zehnder atomic interferometer 500 of the embodiment includes “an optical modulating device that adopts an optical configuration including an AOM and guiding a laser beam by an optical fiber” in place of the above-mentioned “retroreflection optical configuration to which an AOM is added”.
[0034] An optical system 300 included in the Mach-Zehnder atomic interferometer 500 of the embodiment includes three optical modulating devices 510a, 510b, and 510c corresponding to three moving standing light waves 200a, 200b, and 200c. The optical modulating device 510x (x∈{a, b, c}) includes optical fibers 511x and 514x in which a laser beam propagates and a frequency shifter 513x that is connected to the optical fibers 511x and 514x and shifts the frequency of the laser beam. Any frequency shifter can be used as the frequency shifter 513x; for example, the frequency shifter 513x is an AOM or EOM.
[0035] Hereinafter, descriptions are given of a first example (see
First Example
[0036] A laser beam L (for example, a laser beam which is circularly polarized light) from a laser source 311 is frequency-shifted by a predetermined frequency by passing through an EOM 312. The frequency-shifted laser beam L is equally divided by an optical fiber coupler 313a. One of the two laser beams L coming out of the optical fiber coupler 313a is equally divided by an optical fiber coupler 313c, and the other of the two laser beams L coming out of the optical fiber coupler 313a is equally divided by an optical fiber coupler 313b. One of the two laser beams L coming out of the optical fiber coupler 313b is equally divided by an optical fiber coupler 313d, and the other of the two laser beams L coming out of the optical fiber coupler 313b is equally divided by an optical fiber coupler 313e.
[0037] One of the two laser beams L coming out of the optical fiber coupler 313c is attenuated by a variable optical attenuator (VOA) 314a and then subjected to shaping so as to be a desired beam L.sub.a, 1 (for example, a Gaussian beam σ.sub.a+ which is circularly polarized light) by a beam shaper 315a built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.a, 1 enters an interference system 200. The other of the two laser beams L coming out of the optical fiber coupler 313c is guided to an AOM 513a, without crossing an atomic beam, by an optical fiber 511a with one end thereof connected to the optical fiber coupler 313c by an unillustrated optical connector. An intermediate part of the optical fiber 511a is not shown in
[0038] One of the two laser beams L coming out of the optical fiber coupler 313d is attenuated by a VOA 314b and then subjected to shaping so as to be a desired beam L.sub.b, 1 (for example, a Gaussian beam σ.sub.b.sup.+ which is circularly polarized light) by a beam shaper 315b built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.b, 1 enters the interference system 200. The other of the two laser beams L coming out of the optical fiber coupler 313d is guided to an AOM 513b, without crossing an atomic beam, by an optical fiber 511b with one end thereof connected to the optical fiber coupler 313d by an unillustrated optical connector. An intermediate part of the optical fiber 511b is not shown in
[0039] One of the two laser beams L coming out of the optical fiber coupler 313e is attenuated by a VOA 314c and then subjected to shaping so as to be a desired beam L.sub.c, 1 (for example, a Gaussian beam σ.sub.c.sup.+ which is circularly polarized light) by a beam shaper 315c built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.c, 1 enters the interference system 200. The other of the two laser beams L coming out of the optical fiber coupler 313e is guided to an AOM 513c, without crossing an atomic beam, by an optical fiber 511c with one end thereof connected to the optical fiber coupler 313e by an unillustrated optical connector. An intermediate part of the optical fiber 511c is not shown in
[0040] The other end of the optical fiber 511x (x∈{a, b, c}) is connected to the frequency shifter 513x by an optical connector, thus the laser beam L enters the frequency shifter 513x. The frequency of the laser beam L is shifted by the frequency shifter 513x. The amount of shift is determined by the frequency f.sub.x of a signal input to the frequency shifter 513x. As a result, the laser beam L is phase-modulated. One end of the optical fiber 514x is connected to the frequency shifter 513x by an optical connector, thus the laser beam L coming out of the frequency shifter 513x enters the optical fiber 514x. The laser beam L comes out of an optical connector attached to the other end of the optical fiber 514x and is subjected to shaping so as to be a desired beam L.sub.x, 2 (for example, a Gaussian beam σ.sub.x.sup.+ which is circularly polarized light) by a beam shaper 316x built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.x, 2 enters the interference system 200.
[0041] As a result, the laser beam L.sub.x, 1 that has not passed through the optical modulating device 510x and the laser beam L.sub.x, 2 that has passed through the optical modulating device 510x counter-propagate in free space to generate a moving standing light wave 200x (x∈{a, b, c}).
Second Example
[0042] The second example is a modification of the first example. A laser beam L (for example, a laser beam which is circularly polarized light) from a laser source 311 is frequency-shifted by a predetermined frequency by passing through an EOM 312. The frequency-shifted laser beam L is equally divided by an optical fiber coupler 313a. One of the two laser beams L coming out of the optical fiber coupler 313a is attenuated by a VOA 314a and then subjected to shaping so as to be a desired beam L.sub.a, 1 (for example, a Gaussian beam σ.sub.a.sup.+ which is circularly polarized light) by a beam shaper 315a built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.a, 1 enters an interference system 200.
[0043] The other of the two laser beams L coming out of the optical fiber coupler 313a is equally divided by an optical fiber coupler 313b. One of the two laser beams L coming out of the optical fiber coupler 313b is attenuated by a VOA 314b and then subjected to shaping so as to be a desired beam L.sub.b, 1 (for example, a Gaussian beam σ.sub.b.sup.+ which is circularly polarized light) by a beam shaper 315b built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.b, 1 enters the interference system 200. The other of the two laser beams L coming out of the optical fiber coupler 313b is attenuated by a VOA 314c and then subjected to shaping so as to be a desired beam L.sub.c, 1 (for example, a Gaussian beam σ.sub.c.sup.+ which is circularly polarized light) by a beam shaper 315c built up of, for example, a lens, a collimator, and so forth. The obtained beam L.sub.c, 1 enters the interference system 200.
[0044] The optical modulating device 510x (x∈{a, b, c}) in the second example includes a three-port optical circulator 515x in addition to the components of the optical modulating device 510x in the first example. In the three-port optical circulator 515x, the light that has entered a port 1 comes out of a port 2 and the light that has entered the port 2 comes out of a port 3. One end of the frequency shifter 513x is connected to one end of the optical fiber 511x, the other end of the optical fiber 511x is connected to a first port of the optical circulator 515x, the other end of the frequency shifter 513x is connected to one end of the optical fiber 514x, and the other end of the optical fiber 514x is connected to a third port of the optical circulator 515x. The optical fibers 511x and 514x are each connected to the frequency shifter 513x by an optical connector.
[0045] The beam L.sub.x, 1 obtained by the beam shaper 315x is introduced into a second port of the optical circulator 515x by an optical connector attached to the second port of the optical circulator 515x. This optical connector is an optical connector having a lens collimator, for example. The beam L.sub.x, 1 is transmitted from the second port to the third port, travels along the optical fiber 514x connected to the third port and then enters the frequency shifter 513x. The frequency of the beam L.sub.x, 1 is shifted by the frequency shifter 513x. The amount of shift is determined by the frequency f.sub.x of a signal input to the frequency shifter 513x. As a result, the beam L.sub.x, 1 is phase-modulated. The beam L.sub.x, 1 coming out of the frequency shifter 513x is introduced into the first port of the optical circulator 515x via the optical fiber 511x connected to the frequency shifter 513x. The phase-modulated beam L.sub.x, 1 is transmitted from the first port to the second port and comes out of the optical connector as a beam L.sub.x, 2. As a result, the laser beam L.sub.x, 1 that has not passed through the optical modulating device 510x and the laser beam L.sub.x, 2 that has passed through the optical modulating device 510x counter-propagate in free space to generate a moving standing light wave 200x (x∈{a, b, c}).
[0046] A four-port optical circulator may be used in place of the three-port optical circulator 515x. In this case, a fourth port is not used. In the four-port optical circulator 515x, the light that has entered a port 1 comes out of a port 2, the light that has entered the port 2 comes out of a port 3, and the light that has entered the port 3 comes out of a port 4.
Third Example
[0047] A Mach-Zehnder atomic interferometer 500 of the third example is the same as the Mach-Zehnder atomic interferometer 900 except that the Mach-Zehnder atomic interferometer 500 includes “an optical modulating device that adopts an optical configuration including an AOM and guiding a laser beam by an optical fiber” in place of the above-mentioned “retroreflection optical configuration to which an AOM is added”. Therefore, the following description deals with a difference between the Mach-Zehnder atomic interferometer 500 and the Mach-Zehnder atomic interferometer 900, that is, the optical modulating device. Repetitive descriptions of the common matter of the Mach-Zehnder atomic interferometer 500 and the Mach-Zehnder atomic interferometer 900 are omitted by incorporating the description of the above-mentioned Mach-Zehnder atomic interferometer 900 into the description of the third example.
[0048] The laser beam L.sub.1, x and the laser beam L.sub.2, x (x∈{a, b, c}) obtained by the beam divider 306 in the manner described above enter a polarizing beam splitter PBS3x. The laser beam L.sub.1, x which is linearly polarized light passes through the polarizing beam splitter PBS3x, and the laser beam L.sub.2, x which is linearly polarized light is reflected from the polarizing beam splitter PBS3x at an angle of 90°. The laser beam L.sub.1, x passes through a ¼ wave plate QWP1x and becomes right-handed circularly polarized light σ.sub.1, x.sup.+, i. The right-handed circularly polarized light σ.sub.1, x.sup.+, i passes through a ¼ wave plate QWP2x and enters a polarizing beam splitter PBS4x. The linearly polarized light corresponding to the right-handed circularly polarized light σ.sub.1, x.sup.+, i is reflected from the polarizing beam splitter PBS4x at an angle of 90° and enters an unillustrated optical isolator.
[0049] The laser beam L.sub.2, x reflected from the polarizing beam splitter PBS3x passes through a ½ wave plate HWP3x and is introduced into the optical fiber 511x, without crossing an atomic beam, by an optical connector attached to one end of the optical fiber 511x. An intermediate part of the optical fiber 511x is not shown in
Fourth Example
[0050] The fourth example is a modification of the third example. The optical modulating device 510x in the fourth example includes a three-port optical circulator 515x in addition to the components of the optical modulating device 510x in the third example. One end of the frequency shifter 513x is connected to one end of the optical fiber 511x, the other end of the optical fiber 511x is connected to a first port of the optical circulator 515x, the other end of the frequency shifter 513x is connected to one end of the optical fiber 514x, and the other end of the optical fiber 514x is connected to a third port of the optical circulator 515x. The optical fibers 511x and 514x are each connected to the frequency shifter 513x by an optical connector.
[0051] The laser beam L.sub.1, x and the laser beam L.sub.2, x (x∈{a, b, c}) obtained by the beam divider 306 pass through a ¼ wave plate QWP1x and become right-handed circularly polarized light σ.sub.1, x.sup.+, i and left-handed circularly polarized light σ.sub.2, x.sup.−, i. The right-handed circularly polarized light σ.sub.1, x.sup.+, i and the left-handed circularly polarized light σ.sub.2, x.sup.−, i then pass through a ¼ wave plate QWP2x and enter a polarizing beam splitter PBS5x. The linearly polarized light corresponding to the right-handed circularly polarized light σ.sub.1, x.sup.+, i is reflected from the polarizing beam splitter PBS5x at an angle of 90° and enters an unillustrated optical isolator. The linearly polarized light corresponding to the left-handed circularly polarized light σ.sub.2, x.sup.−, i passes through the polarizing beam splitter PBS5x and is introduced into a second port of the optical circulator 515x by an optical connector attached to the second port of the optical circulator 515x. This optical connector is an optical connector having a lens collimator, for example. This linearly polarized light is transmitted from the second port to the third port, travels along the optical fiber 514x connected to the third port and then enters the frequency shifter 513x. The frequency of the linearly polarized light is shifted by the frequency shifter 513x. The amount of shift is determined by the frequency f.sub.x of a signal input to the frequency shifter 513x. As a result, the linearly polarized light is phase-modulated. The linearly polarized light coming out of the frequency shifter 513x is introduced into the first port of the optical circulator 515x via the optical fiber 511x connected to the frequency shifter 513x. The phase-modulated linearly polarized light is transmitted from the first port to the second port and comes out of the optical connector. The linearly polarized light derived from the laser beam L.sub.2, x passes through the polarizing beam splitter PBS5x, then passes through the ¼ wave plate QWP2x, and becomes right-handed circularly polarized light σ.sub.2, x.sup.+, r. As a result, the right-handed circularly polarized light σ.sub.1, x.sup.+, i, which is derived from the laser beam that is from the polarization-maintaining fiber PMF, and the right-handed circularly polarized light σ.sub.2, x.sup.+, r, which is derived from the laser beam that is from the optical modulating device 510x, counter-propagate in free space to generate a moving standing light wave 200x (x∈{a, b, c}).
[0052] An four-port optical circulator may be used in place of the three-port optical circulator 515x. In this case, a fourth port is not used.
[0053] As is clear from the embodiment described above, unlike the retroreflection optical configuration that removes diffracted light of any unnecessary order in the space between the AOM 307x and the retroreflector RRx, the optical configuration that guides a laser beam by an optical fiber can eliminate diffracted light of any unnecessary order by selection or separation of a propagation mode of an optical fiber. Furthermore, the optical configuration that guides a laser beam by an optical fiber is not subject to design constraints derived from the focal distance of the lens LS2x. In addition, since a core diameter of an optical fiber—for example, a mode field diameter of a single-mode optical fiber is typically about 0.005 mm—is sufficiently smaller than a beam diameter of a laser beam incident on the AOM 307x—the beam diameter is sufficiently greater than an acoustic wave in a crystal and is typically about 0.5 mm—, it is possible to make the focal distance between an optical fiber and a lens for introducing the laser beam propagated in free space into the optical fiber shorter than the distance between the lens LS1x and the AOM 307x. Thus, the optical configuration that guides a laser beam by an optical fiber contributes to implementation of a compact atomic gyroscope and a compact atomic interferometer.
[0054] There is a possibility that the retroreflection optical configuration is affected by the influence of an air current or vibration of a retroreflector or other influences, so that a mechanism for eliminating these influences is needed. According to the present embodiment, the optical modulating device 510x (x∈{a, b, c}) is not affected by the influence of vibration of a retroreflector because the optical modulating device 510x (x∈{a, b, c}) does not include a retroreflector. Since the optical modulating device 510x includes the optical fibers 511x and 514x, it would be an overstatement to say that there is no possibility that the optical fibers 511x and 514x are affected by the influence of an air current; however, some techniques are already known to cancel errors occurred by vibration of the optical fibers 511x and 514x due to an air current or the like (see Reference Literature 1). For that matter, the securing of the optical fibers 511x and 514x with, for instance, fastenings helps to reduce the possibility that the optical fibers 511x and 514x are affected by the influence of an air current. [0055] (Reference Literature 1) Longsheng Ma, et al., “Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Optics letters (1994), Vol. 19, No. 21, 1777-1779.
[0056] When the Mach-Zehnder atomic interferometer 500 is used as an atomic gyroscope, the monitor 400 may detect angular velocity or acceleration on the basis of the population of atoms in an excited state. The detecting of angular velocity or acceleration on the basis of the population of atoms in an excited state is a well-known technique and therefore descriptions thereof are omitted.
[0057] The embodiment described above adopts the Mach-Zehnder atomic interference scheme; a Ramsey-Borde atomic interference scheme, for example, may be adopted instead.
[0058] A moving standing light wave 200x (x∈{a, b, c}) may be obtained from left-handed circularly polarized light σ.sub.1, x.sup.−, i, which is derived from a laser beam that is from a polarization-maintaining fiber, and left-handed circularly polarized light σ.sub.2, x.sup.−, r, which is derived from a laser beam that is from the optical modulating device 510x (x∈{a, b, c}).
[0059] Furthermore, the embodiment described above refers to, as an example, Mach-Zehnder atomic interference in which one splitting, one reversal, and one mixing are performed using three moving standing light waves; the present invention is not limited to such an embodiment and can be carried out as, for example, an embodiment using multistage Mach-Zehnder atomic interference in which more than one splitting, more than one reversal, and more than one mixing are performed. See Reference Literature 2 about such multistage Mach-Zehnder atomic interference. [0060] (Reference Literature 2) Takatoshi Aoki et al., “High-finesse atomic multiple-beam interferometer comprised of copropagating stimulated Raman-pulse fields,” Phys. Rev. A 63, 063611 (2001)—Published 16 May 2001.
[0061] In the claims and the specification, unless otherwise noted, the term “connected” and every inflected form thereof do not necessarily deny that one or more intermediate elements are present between two elements “connected” to each other.
[0062] In the claims and the specification, unless otherwise noted, an ordinal numeral is not intended to limit an element modified by or coupled to the ordinal numeral by an ordinal position or the amount of the element. Unless otherwise noted, an ordinal numeral is merely used as a convenient expression method to distinguish two or more elements from one another. Thus, for example, the phrase “a first X” and the phrase “a second X” are expressions to distinguish between two Xs and do not necessarily mean that the total number of Xs is 2 or do not necessarily mean that the first X has to come before the second X. The term “first” does not necessarily mean “coming before all others in order”.
[0063] In the claims and the specification, the term “include” and inflected forms thereof are used as non-exclusive expressions. For example, the sentence “X includes A and B” does not deny that X includes a component other than A and B, for example, C. Moreover, when a certain sentence includes a phrase in which the term “include” or an inflected form thereof is coupled to a negative word, for example, “not include”, the sentence only makes mention of the object of the phrase. Thus, for example, the sentence “X does not include A and B” acknowledges a possibility that X includes a component other than A and B. In addition, the term “or” is not intended to mean an exclusive OR.
[0064] The foregoing description of the embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive and to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teaching. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
DESCRIPTION OF REFERENCE NUMERALS
[0065] 100 atomic beam source [0066] 100a atomic beam [0067] 100b atomic beam [0068] 200 interference system [0069] 200a first moving standing light wave [0070] 200b second moving standing light wave [0071] 200c third moving standing light wave [0072] 300 optical system [0073] 301 master laser source [0074] 302 EOM [0075] 303 first slave laser source [0076] 304 second slave laser source [0077] 305 beam shaper [0078] 306 beam divider [0079] 307a AOM [0080] 307b AOM [0081] 307c AOM [0082] 311 laser source [0083] 312 EOM [0084] 313a optical fiber coupler [0085] 313b optical fiber coupler [0086] 313c optical fiber coupler [0087] 313d optical fiber coupler [0088] 313e optical fiber coupler [0089] 314a VOA [0090] 314b VOA [0091] 314c VOA [0092] 315a beam shaper [0093] 315b beam shaper [0094] 315c beam shaper [0095] 316a beam shaper [0096] 316b beam shaper [0097] 316c beam shaper [0098] 400 monitor [0099] 408 probe beam [0100] 409 photodetector [0101] 500 Mach-Zehnder atomic interferometer [0102] 510a optical modulating device [0103] 510b optical modulating device [0104] 510c optical modulating device [0105] 511a optical fiber [0106] 511b optical fiber [0107] 511c optical fiber [0108] 513a frequency shifter [0109] 513b frequency shifter [0110] 513c frequency shifter [0111] 514a optical fiber [0112] 514b optical fiber [0113] 514c optical fiber [0114] 515a optical circulator [0115] 515b optical circulator [0116] 515c optical circulator [0117] 900 Mach-Zehnder atomic interferometer [0118] BS1 beam splitter [0119] HWP1 ½ wave plate [0120] HWP2 ½ wave plate [0121] HWP3a ½ wave plate [0122] HWP3b ½ wave plate [0123] HWP3c ½ wave plate [0124] LS1a lens [0125] LS1b lens [0126] LS1c lens [0127] LS2a lens [0128] LS2b lens [0129] LS2c lens [0130] M1 mirror [0131] M2 mirror [0132] ML master laser beam [0133] PBS1 polarizing beam splitter [0134] PBS2a polarizing beam splitter [0135] PBS2b polarizing beam splitter [0136] PBS2c polarizing beam splitter [0137] PBS3a polarizing beam splitter [0138] PBS3b polarizing beam splitter [0139] PBS3c polarizing beam splitter [0140] PBS4a polarizing beam splitter [0141] PBS4b polarizing beam splitter [0142] PBS4c polarizing beam splitter [0143] PBS5a polarizing beam splitter [0144] PBS5b polarizing beam splitter [0145] PBS5c polarizing beam splitter [0146] PMF polarization-maintaining fiber [0147] QWP1a ¼ wave plate [0148] QWP1b ¼ wave plate [0149] QWP1c ¼ wave plate [0150] QWP2a ¼ wave plate [0151] QWP2b ¼ wave plate [0152] QWP2c ¼ wave plate [0153] RRa retroreflector [0154] RRb retroreflector [0155] RRc retroreflector [0156] SL1 first slave laser beam [0157] SL2 second slave laser beam