JP2004163334A - Interferometer device and its light source device - Google Patents

Abstract

Kind Code: A1 The present invention does not cause fluctuation of interference fringes due to heat generated from a laser, does not require complicated adjustment at the time of replacement due to failure or life of a laser as a light source, and can simultaneously use lasers of various wavelengths. To obtain a reliable interferometer device and its light source device.
A collimator lens of an interferometer main body is an achromatic optical system, a light splitting unit and an optical superimposing unit perform a coating process corresponding to a used wavelength band, and a light source device includes a laser and a polarization maintaining fiber. It has a cord and can be connected to the main body of the interferometer by an optical connector.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light source device and an interferometer device using the same, and can be suitably applied to an interferometer device using a laser as a light source.
[0002]
[Prior art]
Conventionally, a laser capable of easily obtaining coherent light has been generally used as a light source for an interferometer or the like. In recent years, semiconductor lasers of various wavelengths have been readily available and are frequently used in various measuring devices.
[0003]
In addition, as an interferometer for splitting light from a laser light source with a light splitter and superimposing the light again, a Michelson interferometer, a Mach-Zehnder interferometer, and a Twyman-Green interferometer. A Green interferometer, a Fizeau interferometer, and the like are used depending on the application.
[0004]
As an example of these conventional interferometers, FIG. 8 shows a schematic configuration of a Twyman-Green interferometer device used for measuring transmitted wavefront aberration of an objective lens for an optical pickup, a collimator lens, a prism, and the like. In this type of interferometer, the light splitting means and the light superimposing means are constituted by the same member, ie, a beam splitter.
[0005]
In the interferometer apparatus shown in FIG. 8, reference numeral 12 denotes a semiconductor laser, reference numeral 21 denotes a laser driver for driving the semiconductor laser, reference numeral 13 denotes a collimator lens, reference numeral 14 denotes a beam splitter which is a light splitting means and superimposing means, and a surface 14a is a semi-transparent mirror. A membrane has been applied. Reference numeral 16 denotes a reference plane mirror which is a surface mirror, 15 denotes an object stage, and 23 denotes a stage driving device. Reference numeral 17 denotes an image pickup lens, and an image pickup element (for example, a CCD) 18 for photoelectrically converting an image formed by the image pickup lens is disposed behind the image pickup lens. Reference numeral 26 denotes an image processing means for performing desired image processing on the output from the image sensor 18, and 27 denotes a computer which captures and displays and analyzes the data after the image processing, and controls the laser driver 21 and the stage driving device 23. It is.
[0006]
The general operation of the conventional Twyman-Green interferometer configured as described above will be described. First, the test lens ML is set on the test subject stage 15. Next, the laser driver 21 is driven to emit light of a specific wavelength from the semiconductor laser 12. The collimator lens 13 converts the light oscillated from the semiconductor laser into a parallel light beam. This parallel light beam is reflected by the mirror 30, guided to the beam splitter 14, and split by the semi-transparent mirror surface 14a into the direction of the reference plane mirror 16 and the direction of the test lens ML, that is, the reference light and the test light. The reference light beam in the direction of the reference plane mirror 16 is reflected by the surface 16 a and returned toward the beam splitter 14. On the other hand, the test light beam traveling in the direction of the test lens is a lens as shown in the drawing, and when measuring the transmitted wavefront performance, the reference concave mirror 31 is placed behind the test lens ML, and the center of curvature and the test object are measured. The converging points of the inspection lens ML are arranged so as to coincide with each other, the test light passing through the test lens ML is reflected, and the light is converted into a substantially parallel light beam again through the test lens ML and returned to the semi-transparent mirror surface 14a of the beam splitter 14. Interfere with the reference light. In the case where the test lens ML is a lens such as an objective lens for an optical disk, which is designed to collect light through a predetermined parallel flat plate, the optical path length of the disc is set between the reference concave mirror 31 and the test lens ML. A corresponding cover glass 32 is arranged.
[0007]
On the image sensor 18, an image of interference fringes generated by superimposing the reference light and the test light by the imaging lens 17 is projected. The image of the interference fringes is photoelectrically converted into an electric signal by the imaging device 18, sent to the image processing unit 26, subjected to image processing, and displayed on the monitor of the computer 27. If necessary, the image data can be recorded in a memory built in the computer 27 or another recording medium. The above is an example of a conventional interferometer.
[0008]
Further, as a light source of a device such as a rotary encoder, there is a light source unit configured to emit a parallel plane wave with a semiconductor laser and a collimator lens, and this light source unit is integrally detachable (for example, see Patent Document 1).
[0009]
[Patent Document 1]
JP-A-8-82533
[Problems to be solved by the invention]
Semiconductor lasers have a lifetime due to bulk deterioration, reflection surface deterioration, destruction of the light emitting end face due to surge, and the like, and at present it is necessary to replace them. Since the lenses, prisms, and beam splitters, which are the components of the interferometer, are adjusted with reference to the optical axis of the laser, which is the light source, the optical axis may deviate from the original position when the light source is replaced. In such a case, it is necessary to readjust the components or adjust the position of the light source. For example, if the above-mentioned interferometer measurement is part of a manufacturing line for optical components, the process would require enormous time and labor for laser replacement and subsequent alignment adjustment. It stops and it is uneconomical to have one spare.
[0011]
In the laser, part of power consumption is converted to heat and emitted into the housing. In addition, in order to prevent the laser from deteriorating due to the temperature fluctuation of the laser and the heat due to heat, the temperature around the laser is controlled by a Peltier element, and the heat is dissipated by the structure of the interferometer and the shape of the members. However, these heats cause a thermal gradient inside the housing, resulting in a change in the optical path length, causing fluctuations in the interference fringes, and a problem of lowering the measurement accuracy. In other words, in the method described in Patent Document 1 described above, replacement may be easy, but the same problem is involved in this heat.
[0012]
Furthermore, when the test lens to be measured is changed, for example, 780 nm for a CD and 650 nm for a DVD, the use wavelength of the lens differs depending on the use, and therefore, depending on the use wavelength of the test lens, It is necessary to replace with a semiconductor laser of a wavelength. In this case as well, in addition to the above-described problems relating to the adjustment, the trouble of replacing the optical system corresponding to the wavelength is unavoidable, which greatly hinders the versatility of the interferometer apparatus.
[0013]
In view of the above problems, the present invention does not cause fluctuation of interference fringes due to heat generated from a semiconductor laser, and does not require the above adjustment at the time of replacement due to failure or life of a semiconductor laser as a light source. It is an object of the present invention to obtain a versatile interferometer device capable of using a semiconductor laser having a wavelength and a light source device thereof.
[0014]
[Means for Solving the Problems]
The purpose of the above is
1) a light source device, a collimator lens that converts a light beam from the light source device into a parallel light beam, a light splitting unit that splits the parallel light beam into reference light and test light, and An interferometer apparatus having reference light reflected by a reference surface and test light passing through the subject or test light reflected by the subject, and a light superimposing unit, wherein the light source device is at least a laser, A light source device of an interferometer device, which has a polarization-maintaining fiber code, and a light beam emitted from the laser passes through the polarization-maintaining fiber code and enters the collimator lens;
By doing so, the above object is achieved, and the light source section that generates heat from the interferometer main body section can be separated.
[0015]
2) The light source device of the interferometer device according to 1), wherein the laser is a semiconductor laser.
By doing so, a compact light source device having various wavelengths can be obtained.
[0016]
3) An optical connector is provided at the end of the polarization-maintaining fiber cord, and is fixed to the collimator lens incident surface side by the optical connector. The device can be exchangeable.
[0017]
4) When the focal length of the collimator lens is L, and the maximum optical axis shift amount when the optical connector is connected derived from the dimensional tolerance of the optical connector is T,
Tan ⁻¹ (T / L) <0.025 °
By using the light source device of any one of 1) to 3) using an optical connector that satisfies the above condition, accuracy can be ensured even when the light source device is replaced, and the above-mentioned adjustment becomes unnecessary.
[0018]
5) the light source device and the interferometer device are not arranged on the same optical axis;
By doing so, a free layout becomes possible.
[0019]
6) A light source device having a laser and a polarization-maintaining fiber code, and a light beam emitted from the laser is emitted through the polarization-maintaining fiber code, and a collimator lens for converting the light beam from the light source device into a parallel light beam Light dividing means for dividing the parallel light beam into reference light and test light; reference light reflected on the reference surface after being incident on the reference surface and the test object; and test light passing through the test object or the test light. A light superimposing means for superimposing the test light reflected by the sample, wherein the collimator lens is an achromatic optical system, and the light splitting means and the light superimposing means correspond to a used wavelength band. Interferometer device characterized by having undergone a non-phase, non-polarization coating process
Accordingly, if light source devices of each wavelength are prepared, it is possible to cope with the light source device only by replacing the light source device, and the versatility of the interferometer can be improved.
[0020]
That is, the present inventor has found a method that can satisfy the necessary conditions and the overall accuracy of the interferometer even if the light source unit is separated from the main body of the interferometer by the polarization maintaining fiber and the optical connector, and have reached the present invention. is there.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to embodiments, but the present invention is not limited thereto.
[0022]
FIG. 1 is a diagram showing the overall configuration of an interferometer device and its light source device of the present invention. The same parts as those in FIG. 8 are denoted by the same reference numerals, and redundant description will be omitted. In FIG. 1, reference numeral 200 denotes a light source device of the present invention, 150 denotes an analysis and control unit, and a portion surrounded by a dashed line shown by 100 is an interferometer main unit, which includes an optical system. Reference numeral 35 denotes a light source section containing a semiconductor laser or the like, and 36 denotes a laser driver for driving the semiconductor laser, which is controlled by the computer 27. Reference numeral 37 denotes a polarization-maintaining fiber cord, and reference numeral 38 denotes a plug of an optical connector, which is provided at an end of the polarization-maintaining fiber cord 37. Reference numeral 39 denotes a receptacle of an optical connector. The receptacle 39 is fixed after its position is adjusted with respect to the collimator lens 13 in the interferometer main body 100. By connecting the plug 38 to the receptacle 39, the light beam of the semiconductor laser in the light source 35 is guided into the interferometer main body 100 via the polarization-maintaining fiber cord 37. The light source unit 35 may include a laser driver 36. That is, since the light source device 200 is connected to the interferometer main body 100 by the optical connector, and also connected to the laser driver 36 or the computer 27, the light source device 200 can be easily separated from the interferometer device. ing.
[0023]
The collimator lens 13 and the imaging lens 17 included in the interferometer body 100 of the present invention are designed as an achromatic optical system for various wavelengths of a semiconductor laser to be used, and are divided into a light splitting unit and a light superimposing unit. In the beam splitter 14, a semi-transmissive mirror surface 14a is similarly provided with a beam splitter coat that does not change the splitting ratio and polarization state for various wavelengths, and the other transmission surface has a maximum transmission for 405 nm, 650 nm, and 780 nm, for example. An anti-reflection film having a ratio and a polarization state is not changed. These coats applied to the beam splitter 14 are generally called a non-phase coat and a non-polarization coat.
[0024]
FIG. 2 is a diagram schematically showing the inside of the light source unit 35 of the present invention. In FIG. 2, reference numeral 40 denotes a semiconductor laser, 41 denotes a collimator lens, and 42 denotes a condensing lens for condensing a light beam converted into a parallel light beam by the collimator lens 41 onto a fiber core. Reference numeral 43 denotes a holder for fixing the fiber cord. 44 is a beam splitter and 45 is a photodiode.
[0025]
An outline of the operation of the light source unit 35 configured as described above will be described. A drive start command is issued from the computer 27 to the laser driver 36, and the laser driver 36 is turned on. As a result, the semiconductor laser 40 starts oscillating, and this light is converted into a parallel light beam by the collimator lens 41 and travels toward the condenser lens 42. A part of the light emission amount of the semiconductor laser is reflected toward the photodiode 45 by a beam splitter 44 between the collimator lens 41 and the condenser lens 42. The photodiode 45 outputs an output corresponding to the amount of received light to the laser driver 36. The laser driver 36 adjusts the injection current to the semiconductor laser 40 so that the output becomes a desired output, whereby the semiconductor laser 40 is controlled to have a predetermined light emission amount. On the other hand, most of the light transmitted through the beam splitter is condensed by the condenser lens 42 and is incident on the end face of the polarization maintaining fiber cord 37 held by the holder 43.
[0026]
The polarization preserving fiber cord 37 has its end face polished obliquely (for example, 8 °), and its end face is provided with an antireflection coat corresponding to the wavelength of the semiconductor laser 40 to be used. It is processed so as not to return to 40 directions. This is to prevent the oscillation of the semiconductor laser 40 from becoming unstable due to the returned light. The light oscillated from the semiconductor laser 40 in this way propagates inside the polarization-maintaining fiber cord 37 and is guided to the plug 38 of the optical connector at the other end. The above is the outline of the inside of the light source unit 35 and the operation.
[0027]
Although omitted from the light source section 35, it is desirable to provide a temperature controller for wavelength stability of the semiconductor laser and laser destruction due to increase in laser temperature.
[0028]
That is, alignment adjustment is performed between the semiconductor laser 40, the collimator lens 41, the condenser lens 42, and the incident side end face of the polarization plane preserving fiber 37 in the light source section 35, and the collimator lens 13 is arranged on the interferometer main body 100 side. If the plug 38 is connected to the receptacle 39 whose position has been adjusted, the light source can be easily replaced, and the problem of the conventional light source replacement in the interferometer body can be solved.
[0029]
Next, the polarization maintaining fiber used in the light source device of the present invention will be described.
FIG. 3 shows an example of a polarization maintaining fiber used in the light source device of the present invention. FIG. 3A is a conceptual diagram of polarization maintaining of a polarization-maintaining fiber. FIG. 3B is a sectional view thereof. As shown in FIG. 3 (b), the core is placed at the center, stress is applied to the X axis of the core, and the X and Y axes have different refractive indexes. This structure includes a panda type, an elliptical cladding type, and the like, and FIG. 3 shows a panda type.
[0030]
When the polarization direction of the incident light beam is incident on the polarization-maintaining fiber shown in FIG. 3A in accordance with the X-axis (or Y-axis), the light propagates through the fiber while maintaining this polarization state, and is emitted. Even at the end, a light beam polarized in the X-axis (or Y-axis) direction can be obtained. Further, the transverse mode preservability of the polarization maintaining fiber is related to the wavelength.
[0031]
FIG. 4 is a conceptual diagram for explaining the lateral mode preservation. FIG. 4 shows Gaussian-beams of various wavelengths incident on a polarization-maintaining fiber corresponding to a wavelength of 405 nm and observing the light quantity distribution on the exit surface. (A) is for a wavelength of 405 nm, (b) is for 633 nm, and (c) is for 1550 nm. The accompanying graph shows the transverse mode (cross-sectional intensity distribution). Thus, it was observed that the Gaussian distribution was preserved at 405 nm, but split at 633 nm into two beams and at 1550 nm into four beams.
[0032]
That is, in the light source device of the present invention, the polarization direction of the light beam oscillated from the semiconductor laser 40 and the polarization maintaining direction of the incident-side end face of the polarization-maintaining fiber 37 are made to coincide with each other. The polarization maintaining direction of the end face on the emission side of the wavefront preserving fiber 37 and the attachment of the plug 38 are also assembled so as to enter the collimator lens 13 on the interferometer main body 100 side in a desired polarization direction. Further, an appropriate selection is made in consideration of the characteristics of the polarization maintaining fiber 37 corresponding to the oscillation wavelength of the semiconductor laser 40 to be used.
[0033]
Next, the optical connector used in the present invention will be described.
The optical connector used in the present invention is preferably a single-core optical fiber connector of a plug-receptacle connection type having an alignment ferrule structure, for example, a JIS-C5970-F01 type single-core optical fiber connector having a positionable structure. If the equivalent function and accuracy are guaranteed, this is not the case.
[0034]
FIG. 5 is a diagram showing an example of the relationship between the plug and the receptacle of the optical connector. FIG. 5 shows a cross section of an optical connector coupling portion when a plug class B is selected from JIS-C5970-F01 type single core optical fiber connectors. Reference numeral 51 denotes a receptacle-side ferrule receiving portion; 52, a plug-side ferrule; and 53, a fiber core.
[0035]
For example, suppose that the class B is selected. In this case, the coaxiality of the ferrule 52 and the core 53 is 0.0014 mm or less, and the size of the ferrule 52 is in the range of 2.4985 mm to 2.4949 mm from the ferrule dimensions and the dimensional tolerance. On the other hand, the ferrule receiving portion 51 on the receptacle side also has a width of 2.504 mm to 2.501 mm. That is, the maximum gap D between the plug-side ferrule 52 and the receptacle-side ferrule receiving portion 51 is 0.0055 mm. Further, 0.0079 mm, which is the sum of the coaxiality of the ferrule 52 and the core 53 of 0.0014 mm, is the maximum deviation amount of the core portion from the ferrule receiving portion 51 on the receptacle side.
[0036]
On the other hand, when the focal length of the collimator lens 13 provided in the interferometer main body 100 is 125 mm, for example, and a JIS-C5970-F01 type single-core optical fiber connector and a grade B of the plug are selected, this optical connector is attached and detached. From the maximum deviation amount, the inclination θ of the optical axis of the light beam emitted from the collimator lens 13 is given by the following equation:
θ = Tan ^-1 (0.0079 / 125) = 0.0036 °
Is required. That is, the light beam emitted from the collimator lens 13 is inclined at a maximum of 0.0036 ° from the optical axis that should be. Generally, the allowable amount of the tilt of the optical axis of the interferometer is said to be about 0.025 ° although it varies depending on the application, and it can be seen that sufficient accuracy can be secured.
[0037]
That is, by taking into account the error due to the tolerance of the optical connector used, the permissible error in adjusting the assembling position of the collimator lens 13 of the interferometer main body 100 and the receptacle of the optical connector, and the measurement accuracy required for the subject, The required accuracy of the connector is required, and an optical connector with an accuracy that can meet the required accuracy is selected.
[0038]
Next, a method for easily changing the polarization direction of the light source light beam incident on the collimator lens 13 will be described.
[0039]
FIG. 6 is a diagram showing a mounting direction of a plug of an optical connector to be mounted on a polarization-maintaining fiber. 43 is a holder for fixing the fiber cord shown in FIG. 2, 37 is a polarization maintaining fiber, and 38 is a plug of an optical connector.
[0040]
As shown in FIG. 6, it is assumed that a light beam polarized in the X-axis direction is incident on the incident-side end face of the polarization-maintaining fiber 37. It is assumed that the positioning portion 39c for the plug of the receptacle 39 mounted on the interferometer main body 100 side is mounted so as to be upward as shown in FIG. 6C. At this time, as shown in FIG. 6A, if the positioning portion 38a of the plug 38 is assembled upward with respect to the X-axis of the polarization-maintaining single-mode fiber 37, the polarization direction will be the same as that shown in FIG. The light beam polarized in the axial direction is guided to the collimator lens 13 of the interferometer main body 100.
[0041]
On the other hand, as shown in FIG. 6B, when the positioning part 38a of the plug 38 is assembled in the X-axis direction of the polarization-maintaining fiber 37, when the plug 38 is coupled to the receptacle 39 of the interferometer main body 100, the polarization direction The light flux polarized in the illustrated Y-axis direction is guided to the collimator lens 13 of the interferometer main body 100.
[0042]
That is, by changing the attachment direction of the plug of the optical connector attached to the polarization-maintaining fiber, the polarization direction of the light beam incident on the collimator lens 13 of the interferometer main body 100 can be freely changed. This makes it possible to eliminate the need to insert and remove the half-wave plate in the optical path of the interferometer, adjust the rotation angle of the main shaft, and the like.
[0043]
FIG. 7 is a diagram in which the light source device of the present invention is applied to a Fizeau interferometer. Parts having the same functions as those in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted.
[0044]
The light beam oscillated from the semiconductor laser 40 in the light source unit 35 passes through the polarization-maintaining fiber cord 37 and enters the collimator lens 13 from the receptacle 39 connected to the plug 38 of the optical connector. This light beam passes through the semi-transparent mirror 33 and is split by the beam splitter 14 having the reference surface 14a. The test light goes to the test lens ML, is reflected by the reference concave mirror 31 behind the test lens ML, and is again parallelized. The light returns to the reference surface, and the interference fringes that interfere with the reference light are projected onto the image sensor by the imaging lens 17 via the semi-transparent mirror 33.
[0045]
That is, the light source device of the present invention can be used as a light source unit of various interferometers.
[0046]
【The invention's effect】
As described above, since the interferometer main body and the light source are separated while maintaining the coupling accuracy thereof, even if the light source in use is defective, if there is a spare replacement light source, the interferometer main body is This makes it possible to immediately and easily replace the device without removing the component parts of the device or adjusting the position.
[0047]
In addition, since the optical components provided in the interferometer main body are achromatized, if light source units having lasers of various wavelengths are prepared, the light source units used in other wavelength bands can be replaced only by replacing the light source units. It is also possible to measure a sample, and the versatility of the interferometer can be dramatically increased.
[0048]
Furthermore, by separating the interferometer main body and the light source, the semiconductor laser serving as a heat source and the temperature adjusting mechanism of the semiconductor laser can be separated from the interferometer main body. The length change can be prevented, so that the fluctuation of the interference fringes is eliminated, and the cause of the measurement instability is eliminated, and at the same time, the thermal measures of the interferometer itself become unnecessary.
[0049]
In addition, the light source unit can be used for other types of interferometers as long as the light source unit can be connected with a similar optical connector. High performance. In addition, it is possible to easily change the polarization direction of the light source to be incident on the interferometer main body.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of an interferometer device and a light source device of the present invention.
FIG. 2 is a diagram schematically showing the inside of a light source unit of the present invention.
FIG. 3 shows an example of a polarization maintaining fiber used in the light source device of the present invention.
FIG. 4 is a conceptual diagram illustrating the transverse mode preservation of a polarization maintaining fiber.
FIG. 5 is a diagram showing an example of a relationship between a plug of an optical connector and a receptacle.
FIG. 6 is a diagram showing a mounting direction of a plug of an optical connector to be mounted on a polarization-maintaining fiber.
FIG. 7 is a diagram in which the light source device of the present invention is applied to a Fizeau interferometer.
FIG. 8 is a diagram showing a schematic configuration of a conventional Twyman-Green interferometer device.
[Explanation of symbols]
13 Collimator lens 14 Beam splitter 15 Subject stage 16 Reference plane mirror 17 Imaging lens 18 Image sensor 23 Stage driving device 26 Image processing means 27 Computer 31 Reference concave mirror 35 Light source 36 Laser driver 37 Polarization preserving fiber cord 38 Plug ( Optical connector)
39 Receptacle (optical connector)
100 Interferometer main body 200 Light source device

Claims (6)

A light source device, a collimator lens that converts a light beam from the light source device into a parallel light beam, a light splitting unit that splits the parallel light beam into reference light and test light, and a reference surface and the reference surface after being incident on a subject. In the interferometer apparatus having a reference light reflected by the light and a light superimposing means for superposing the test light passing through the subject or the test light reflected by the subject,
The light source device has at least a laser and a polarization-maintaining fiber code, and a light beam emitted from the laser is incident on the collimator lens via the polarization-maintaining fiber code. Light source device. 2. The light source device for an interferometer device according to claim 1, wherein said laser is a semiconductor laser. 3. The light source device according to claim 1, wherein an optical connector is provided at an end of the polarization-maintaining fiber cord, and the optical connector is fixed to the collimator lens incident surface side by the optical connector. When the focal length of the collimator lens is L and the maximum optical axis shift amount when the optical connector is connected, which is derived from the dimensional tolerance of the optical connector, is T,
Tan ⁻¹ (T / L) <0.025 °
The light source device of the interferometer device according to any one of claims 1 to 3, wherein an optical connector satisfying the following is used. The light source device of the interferometer device according to any one of claims 1 to 4, wherein the light source device and the interferometer device are not arranged on the same optical axis. A laser, having a polarization-maintaining fiber code, a light beam emitted from the laser, a light source device that emits via the polarization-maintaining fiber code, and a collimator lens that converts the light beam from the light source device into a parallel light beam, A light splitting unit that splits the parallel light beam into a reference light and a test light; and a reference light reflected by the reference surface and the test light or the test light passing through the test subject after being incident on the reference surface and the test subject. An optical superposition means for superimposing the reflected test light,
An interferometer device wherein the collimator lens is an achromatic optical system, and the light splitting means and the light superimposing means are subjected to a non-phase and non-polarization coating process corresponding to a wavelength band to be used.

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