CN107664848B

CN107664848B - Light source with controllable linear polarization

Info

Publication number: CN107664848B
Application number: CN201610608330.8A
Authority: CN
Inventors: R·P·泰拉; D·C·布里克; Y·韩; A·克莱克泽夫斯基
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2016-07-28
Filing date: 2016-07-28
Publication date: 2021-02-05
Anticipated expiration: 2036-07-28
Also published as: CN107664848A

Abstract

Disclosed is a light source having: first and second wire grid polarizers; and a laser emitting a linearly polarized light beam characterized by a transmission direction. The first wire-grid polarizing filter is characterized by a first linear polarization transfer direction and a first actuator for rotating the first linear polarization transfer direction with respect to the linearly polarized light beam. The second wire-grid polarization filter is characterized by a second linear polarization transfer direction and a second actuator for rotating the second linear polarization transfer direction with respect to the linearly polarized light beam. The controller sets the first and second linear polarization transfer directions to provide linearly polarized light having the specified polarization direction.

Description

Light source with controllable linear polarization

Background

Quantum cascade lasers provide a tunable mid-infrared (MIR) light source that can be used for spectrometer measurements and images. Many chemical compositions of interest have molecular vibrations excited in the MIR region of the spectrum spanning wavelengths between 5 and 25 microns. Thus, measuring the absorption of MIR light or the reflection of MIR light at various locations on the sample can provide useful information about the chemistry of the sample as a function of location on the sample.

Disclosure of Invention

The invention comprises a light source having: first and second wire grid polarizers; and a laser emitting a linearly polarized light beam characterized by a transmission direction. The first wire-grid polarizing filter is characterized by a first linear polarization transfer direction and a first actuator for rotating the first linear polarization transfer direction with respect to the linearly polarized light beam. The first wire grid polarizing filter is positioned such that the linearly polarized light beam passes through the first wire grid polarizing filter. The second wire-grid polarization filter is characterized by a second linear polarization transfer direction and a second actuator for rotating the second linear polarization transfer direction with respect to the linearly polarized light beam. The second wire grid polarization filter is positioned such that the linearly polarized light beam passes through the second wire grid polarization filter after passing through the first wire grid polarization filter. The controller sets the first and second linear polarization transfer directions to provide linearly polarized light having the specified polarization direction.

In one aspect of the invention, the first wire grid polarizing filter comprises a parallel grid of metallized lines on a first transparent flat substrate, the first transparent flat substrate being angled with respect to the transmission direction such that light reflected from the first wire grid polarizing filter is not transmitted in a direction parallel to the transmission direction.

In another aspect of the invention, the second wire grid polarizing filter comprises a parallel grid of metallized lines on a second transparent flat substrate, the second transparent flat substrate being angled with respect to the transmission direction such that light reflected from the second wire grid polarizing filter does not transmit in a direction parallel to the transmission direction and such that light reflected from the second wire grid polarizing filter does not transmit in a direction parallel to light reflected from the first wire grid filter.

In another aspect of the invention, the first laser is a quantum cascade laser that emits light in a wavelength band between 2 and 14 microns.

In another aspect of the invention, the first and second planar substrates are transparent to light of a wavelength between 2 and 14 microns. The first planar substrate may be a substrate comprising a material selected from the group consisting of BaF₂Glass of substances of the group consisting of ZnSe, ZnS, ZnTe, diamond and Si.

In another aspect of the invention, the light source includes a second laser and a beam director. The second laser emits a linearly polarized light beam in a different wavelength band from the first laser. The beam director causes light from the second laser to enter the first wire grid polarizing filter on a path that coincides with a path that light from the first laser traverses through the first wire grid polarizing filter.

In yet another aspect of the invention, a light source includes first and second optical assemblies. The first optical assembly directs light exiting the second wire grid polarizing filter onto the specimen. The second optical assembly collects light exiting the specimen and directs the collected light onto a polarization analyzer, which measures light intensity as a function of polarization direction. The controller causes the polarization analyzer to measure light intensity for a plurality of different first and second linear polarization transfer directions.

Drawings

FIG. 1 shows one embodiment of a spectrometer using a light source according to the present invention.

Fig. 2 shows an embodiment of a light source according to the invention.

Fig. 3 illustrates a light source utilizing multiple quantum cascade lasers and a single polarization rotator to provide a desired wavelength and polarization direction.

Detailed Description

An imaging system based on MIR light sources can provide information about the underlying chemistry of the sample being imaged. Furthermore, by measuring the reflection and absorption of incident MIR light as a function of wavelength, a more accurate identification of the chemistry of the sample at the point where data is being taken can be obtained.

In absorption spectroscopy, a sample is illuminated with light, and the amount of light reflected from the sample is measured. The process is repeated for a plurality of light wavelengths to generate a spectrum comprising reflected light intensity as a function of wavelength. The fraction of incoming light reflected from the sample is related to the intensity of light absorbed by the sample. The absorption spectrum may be used to identify chemical compounds in the sample. Thus, an image of a sample in which each pixel of the image comprises an absorption or reflection spectrum as a function of wavelength is useful in visualizing the distribution of different chemical compounds in the sample.

The light reflected from the specimen depends on the properties of the surface of the specimen. Typically, the reflected light is a mixture of specularly reflected light from a flat surface (e.g., a tangent plane of a crystal in the sample) and diffusely reflected light from a rough surface or particle. The spectrum generated by specular reflection light is different from the spectrum generated by diffuse reflection light. Since many specimens of interest generate complex spectra of two types of reflections with unknown ratios, interpreting the image with respect to the chemical composition of the sample as a function of position on the sample poses significant challenges. These challenges can be significantly reduced if the contribution of each type of reflection to the measured spectrum at each point in the specimen can be separated.

The invention is based on the observation that polarized light undergoing specular reflection preserves polarization. In contrast, diffusely reflected polarized light is depolarized. Thus, the diffuse reflected light can be selectively measured by means of a linear polarization filter. If the incident laser light is linearly polarized, the specularly reflected light will be linearly or elliptically polarized. Elliptically polarized light may be characterized by two linear polarizations that are orthogonal to each other, measured on a coordinate system fixed relative to the specimen. The blocking of the linearly polarized filter has an orthogonal polarization axis defined on the filterLinearly polarized light of the polarization direction of (1). If the beam is linearly polarized in a direction parallel to this axis, all light passes through the filter. If the beam is linearly polarized along a direction orthogonal to this axis, all light is blocked. In general, if light is linearly polarized along an axis at an angle θ relative to the polarization axis, the light can be viewed as having a component parallel to the polarizer axis and a component orthogonal to the polarizer axis. The parallel component passes through the filter and the quadrature component is blocked by the filter. Thus, for a linear polarization component I with respect to a coordinate system on the sample_sAnd I_dThe portion of light in each component will pass through the filter. The amount of light depends on the angle between the polarization axis on the filter and the polarization of each linearly polarized component. By taking many measurements with different relative angles between the polarization axis and the coordinate system on the sample, the diffuse and specular light intensities I can be measured_d、I_sAnd I_pDifferent combinations of (a). Furthermore, a measurement of the direction of polarization of incoming light on the sample is required to provide other combinations of diffuse and specular light intensities. These measurements can be combined to obtain diffuse and specular components.

Referring now to FIG. 1, FIG. 1 shows one embodiment of a spectrometer using a light source according to the present invention. The imaging system 10 includes a light source 11 that generates a collimated light beam 18 having a narrow wavelength band in the MIR. In one aspect of the invention, light source 11 comprises a quantum cascade laser having a tunable wavelength under the control of controller 29. The light source 11 generates a linearly polarized light beam, the polarization direction of which is also under the control of the controller 29. The way in which the polarization direction of the light leaving the light source 11 is set will be discussed in detail below.

The partial mirror 12 divides the collimated beam 18 into two beams. The beam 18a is directed to a first optical assembly that directs the light in the beam onto the specimen 16. In particular, the lens 15 focuses the beam onto a specimen 16 mounted on an xy stage 17 that can position the specimen 16 relative to the focal point of the lens 15. Light reflected back from the specimen 16 is collected by a second optical assembly and directed to a polarization analyzer. Specifically, light exiting from the specimen 16 is collimated into a second beam having a diameter determined by the aperture of the lens 15 and returned to the partial mirror 12 along the same path as the beam 18 a. Although the first and second beams are shown in fig. 1 as having the same cross-section, it is understood that the second beam may have a different cross-section than the first beam. A portion of the second beam is transmitted through the partial mirror 12 and impinges on the first photodetector 13, as shown at 18 b. The photodetector 13 generates a signal related to the light intensity in the beam 18 b. The controller 29 calculates an image as a function of position on the specimen 16 by moving the specimen 16 relative to the focal point of the lens 15 using the xy stage 17.

The controller 29 also monitors the beam intensity in the collimated beam 18 using the second light detector 14 which receives part of the light generated by the light source 11 through the partial mirror 12. The light source 11 is typically a pulsed source. The light intensity may vary significantly with the pulse, and therefore, by dividing the intensity measured by the light detector 13 by the intensity measured by the light detector 14, the variation of the pixels of the image with respect to intensity is corrected. Furthermore, since the light intensity from light source 11 is zero between pulses, controller 29 sums only the ratio of the intensities from

photodetectors

13 and 14 during those times when the output of photodetector 14 is greater than some predetermined threshold. This aspect of the invention improves the signal-to-noise ratio of the resulting image, since the measurements between pulses only contribute to the noise that is removed by not using the measurements between pulses.

The imaging system 10 utilizes a linear polarizing filter 23 under the control of a controller 29 to separate the specular reflected light from the diffuse reflected light. The linear polarization filter 23 rotates through a plurality of angles. For each (x, y) coordinate of xy stage 17 and the wavelength of light from light source 11, controller 29 measures the intensity of light reflected back from specimen 16 as a function of the angle of rotation of the polarization axis of linear polarizing filter 23. In one aspect of the invention, the intensity measured by photodetector 13 is normalized to the output of photodetector 14 to correct for variations in the intensity of light generated by light source 11.

Further, the polarization direction of the light from the light source 11 is changed, and the output of the photodetector 13 is measured again for a plurality of angles of the linear polarization filter 23. Thus, the entire measurement set at any given light wavelength from light source 11 includes the light intensities detected by

light detectors

13 and 14 for a plurality of different angles of linear polarization filter 23 and a plurality of different polarization directions of the light from light source 11. The measured values are then fitted to a function to determine the intensity of the specular and diffuse reflected light. Details of this fitting process are discussed in co-pending U.S. patent application 14/683,841 filed on 10.4.2015, which is incorporated herein by reference, and will not be discussed in detail herein because of the improved embodiments of the present application for light source 11.

The light source 11 must operate over a wide range of wavelengths and intensities and provide a plurality of different output linear polarization angles. Referring now to fig. 2, fig. 2 illustrates one embodiment of a light source according to the present invention. The light source 30 includes: a quantum cascade laser 31; and a polarization rotator 35 that sets the output beam polarization direction. To simplify the following discussion, a polarization filter that passes light having a particular linear polarization while reflecting or absorbing light having another linear polarization will be referred to as a polarizer.

The polarization rotator 35 includes first and second

wire grid polarizers

63 and 64, respectively. Each wire grid polarizer has a pattern of parallel metallized lines disposed on a substrate that is transparent to the wavelength of light of the linearly polarized light. Linearly polarized light that is correctly aligned with the direction of the metallization lines is transmitted through the polarizer. Light having a different linear polarization is reflected. An actuator that causes the wire grid polarizer to rotate relative to the rods sets the angle of the line pattern relative to the light exiting the quantum cascade laser 31. Exemplary actuators and rods are labeled at 65 and 66, respectively. To simplify the drawing, the control line connections for the quantum cascade laser 31 and the wire grid polarizer have been omitted from the drawing.

In one aspect of the invention, the angle of the plane of the substrate relative to the beam direction of the quantum cascade laser 31 is 90 degrees different to prevent light reflected from the wire grid polarizer from entering the optical train. In one exemplary embodiment, the plane of the substrate is disposed at an angle between 3 and 6 degrees. However, other angles may be utilized. If the angle is too large, the size of the wire grid polarizer must be increased. Since the substrate must be made of a material that is transparent to MIR light over a wide range of wavelengths, cost considerations favor smaller angles.

Quantum cascade lasers generate linearly polarized light. In order to provide light of any desired linear polarization direction, it is necessary to utilize at least two wire grid polarizers. For the purposes of this discussion, it will be assumed that the polarization direction of the light exiting the quantum cascade laser 31 is 0 degrees. Assume that a 90 degree polarization is desired. Since the input beam has no polarization component at 90 degrees, a single polarizer with the pass direction set to 90 degrees will not pass any light. Thus, the first wire grid polarizer was arranged to pass light with a polarization angle of 45 degrees. The incident light has a component parallel to this direction which has an intensity reduced by a factor of 2. The second polarizer is then set to pass the light at 90 degrees to the initial beam polarization direction at an angle of 45 degrees relative to the light exiting the first polarizer. Thus, the light exiting the polarization rotator 35 will be reduced in amplitude by a factor of 4.

Typically, a second polarizer is provided to pass light of a desired polarization direction. The first polarizer must be set at an angle that is not orthogonal to the linear polarization direction of the laser. The first polarizer may be used to set a desired light attenuation by choosing a pass angle that provides the attenuation, which when combined with the attenuation introduced by the second polarizer provides the desired total attenuation.

In principle, other forms of polarization rotators may be utilized. However, conventional polarization rotators using dichroic mirrors are very sensitive to the wavelength of the light being rotated. Therefore, providing a polarization rotator that operates over a wide range of wavelengths presents significant challenges. For example, the intensity of light rotated varies significantly with wavelength.

In the above embodiment, the light source uses a single quantum cascade laser. Although quantum cascade lasers can be tuned over a significant wavelength range, the range available from a single quantum cascade laser is insufficient for many MIR imaging and spectroscopy applications. In many applications, a tuning range of 5 to 14 microns is required. Such tuning ranges may be operated separately from each otherFour quantum cascade lasers over different parts of the range. It should be noted that BaF is used₂Or ZnSe, is transparent over the entire range, and thus a single set of wire grid polarizers can be used over the entire range. In addition, there are other materials (e.g., ZnS, diamond, Si or Ge windows) that are also transparent/partially transparent to this range.

Referring now to fig. 3, fig. 3 illustrates a light source utilizing multiple quantum cascade lasers and a single polarization rotator to provide a desired wavelength and polarization direction. Again, the control wires for the actuators in the individual qc lasers and polarization rotator 35 have been omitted to simplify the drawing. In the light source 40, light from the individual qc lasers 71-74 is directed to the same input optical path by the partial mirrors 75-77. In the simplest mode of operation, only one qc laser is active at any given time. The reflectivities of the partial mirrors 75 to 77 are set such that: the light intensity reaching the polarization rotator 35 is the same for each quantum cascade laser.

In one mode of operation, a qc laser is typically pulsed at some predetermined pulse rate. By using different pulse rates for different qc lasers, light from different qc lasers may be detected separately and, therefore, measurements may be taken at multiple wavelengths simultaneously. Here, it should be noted that the polarization rotator 35 has the same arrangement for the wire grid polarizer independent of the wavelength of the incident light. Thus, if two qc lasers are providing light at the same time, the light from each qc laser will have the same linear polarization direction. Referring again to fig. 1, the contribution of each wavelength to the light reflected from the specimen 16 may be confirmed by separately measuring the light intensity at the

photodetectors

13 and 14 at each frequency. Thus, in addition to extending the range of wavelengths of incident light, the use of multiple lasers may also reduce the time required to generate a spectrum of the sample at one point of the sample by measuring the reflectivity of the sample at multiple wavelengths.

The above-described embodiments have utilized an imaging system as an exemplary system for operating a light source according to the present invention. However, it will be appreciated from the foregoing discussion that light sources according to the present invention may provide advantages in other systems where MIR light with controllable linear polarization is desired.

The above-described embodiments of the present invention have been provided to illustrate various aspects of the present invention. However, it is to be understood that different aspects of the present invention, as illustrated in the various specific embodiments, may be combined to provide further embodiments of the invention. In addition, various modifications of the invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the invention is to be limited only by the scope of the following claims.

Claims

1. A light source, comprising:

a first laser emitting a linearly polarized light beam characterized by a transmission direction;

a first wire-grid polarizing filter characterized by a first linear polarization transfer direction and a first actuator for rotating the first linear polarization transfer direction relative to the linearly polarized light beam, the first wire-grid polarizing filter positioned such that the linearly polarized light beam passes through the first wire-grid polarizing filter;

a second wire-grid polarizing filter characterized by a second linear polarization transfer direction and a second actuator for rotating the second linear polarization transfer direction relative to the linearly polarized light beam, the second wire-grid polarizing filter positioned such that the linearly polarized light beam passes through the second wire-grid polarizing filter after passing through the first wire-grid polarizing filter; and

a controller for setting the first and second linear polarization transfer directions to provide linearly polarized light having a specified polarization direction, the first linear polarization transfer direction being different from the second linear polarization transfer direction for at least one of the specified polarization values.

2. The light source of claim 1, wherein the first laser is a quantum cascade laser that emits light in a wavelength band between 2 and 14 microns.

3. The light source of claim 1, further comprising:

a second laser that emits a linearly polarized light beam, the second laser emitting light in a different wavelength band than the first laser; and

a beam director to cause light from the second laser to enter the first wire grid polarizing filter on a path coincident with a path of light from the first laser traversing through the first wire grid polarizing filter.

4. The light source of claim 1, further comprising:

a first optical assembly that directs light exiting the second wire grid polarizing filter onto a specimen; and

a second optical assembly that collects light exiting the specimen and directs the collected light to a polarization analyzer that measures light intensity as a function of polarization direction, the controller causing the polarization analyzer to measure the light intensity for a plurality of different first and second linear polarization transfer directions, the first linear polarization transfer direction being different from the second linear polarization transfer direction for at least one of the specified polarization values.

5. The light source of claim 1, wherein the first wire grid polarizing filter comprises a parallel grid of metallized lines on a first planar substrate, the first planar substrate being angled between 3 and 6 degrees relative to a perpendicular to the transmission direction such that light reflected from the first wire grid polarizing filter is not transmitted in a direction parallel to the transmission direction.

6. The light source of claim 5, wherein the first planar substrate is transparent to light of a wavelength between 2 and 14 microns.

7. The light source of claim 5, wherein the first flat surfaceThe substrate is composed of BaF₂Glass of a substance selected from the group consisting of ZnSe, ZnS, ZnTe, diamond and Si.

8. The light source of claim 1, wherein the second wire grid polarizing filter comprises a parallel grid of metalized lines on a second planar substrate angled between 3 and 6 degrees relative to a perpendicular to the transmission direction such that light reflected from the second wire grid polarizing filter does not transmit in a direction parallel to the transmission direction and such that light reflected from the second wire grid polarizing filter does not transmit in a direction parallel to light reflected from the first wire grid polarizing filter.

9. The light source of claim 8, wherein the second planar substrate is transparent to light of a wavelength between 2 and 14 microns.

10. A method for illuminating a specimen, comprising:

illuminating a first wire-grid polarizing filter with a first laser emitting a linearly polarized light beam characterized by a transmission direction, the first wire-grid polarizing filter characterized by a first linear polarization transfer direction; the first wire grid polarizing filter is positioned such that a portion of the linearly polarized light beam passes through the first wire grid polarizing filter;

illuminating a second wire-grid polarizing filter characterized by a second linear polarization pass-through direction, the second wire-grid polarizing filter positioned such that the linearly polarized light beam passes through the second wire-grid polarizing filter after passing through the first wire-grid polarizing filter, the first and second linear polarization pass-through directions adjusted such that light exiting the second wire-grid polarizing filter is linearly polarized light having the specified polarization direction and magnitude, and a first portion of the light exiting the second wire-grid polarizing filter is directed onto the specimen and a second portion of the light exiting the second wire-grid polarizing filter is directed onto a detector that measures light intensity in the second portion.

11. The method of claim 10, wherein the first wire-grid polarizing filter comprises a parallel grid of metallized lines on a first planar substrate angled between 3 and 6 degrees relative to a perpendicular to the transmission direction such that light reflected from the first wire-grid polarizing filter is not transmitted in a direction parallel to the transmission direction.

12. The method of claim 11, wherein the second wire-grid polarizing filter comprises a parallel grid of metalized lines on a second planar substrate angled between 3 and 6 degrees relative to a perpendicular to the transmission direction such that light reflected from the second wire-grid polarizing filter does not transmit in a direction parallel to the transmission direction and such that light reflected from the second wire-grid polarizing filter does not transmit in a direction parallel to light reflected from the first wire-grid polarizing filter.

13. The method of claim 12, further comprising: light exiting the specimen is collected and the collected light is directed to a polarization analyzer that measures light intensity as a function of polarization direction for a plurality of different first and second linear polarization transfer directions.