Method and apparatus for performing magnetic field measurements using magneto-optic kerr effect sensors

• US 5,631,559 A
• Filed: 06/07/1995
• Issued: 05/20/1997
• Est. Priority Date: 03/05/1993
• Status: Expired due to Fees

• Current Assignee: Northeastern University
• Original Assignee: Northeastern University
• Inventors: Oliver, Steven A., DiMarzio, Charles A.
• Primary Examiner(s): O'Shea, Sandra L.
• Assistant Examiner(s): Phillips, Roger
• Application Number:
  US08/474,692
• Time in Patent Office: 713 Days
• Field of Search: 324/244.1, 324/175, 324/207.25, 324/207.13, 324/96, 324/97, 324/117 R, 324/117 H, 356/364, 356/33, 356/34, 356/365-368, 250/225, 359/280-283
• US Class Current: 324/244.1
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01D 5/344 using polarisation G01D5/35303 takes precedence
• CPC Class Codes: G01R 33/0325 using the Kerr effect

Abstract

A sensor system includes a magneto-optic Kerr effect magnetic field sensing element and an optical system including a light source, at least one polarizer or polarizing element and a detector disposed about the sensing element. The sensing element is responsive to an external magnetic field. In one embodiment, the optical system and sensing element can be arranged to measure the strength of a magnetic field. In another embodiment the optical system and sensing element can be arranged to measure the rotational speed of rotating members.

Claims

1. A magneto-optic Kerr effect magnetic field threshold sensor comprising:

a light source;

a polarizing beamsplitter disposed to intercept incident light provided from said light source, wherein said polarizing beamsplitter provides maximum transmission, along a first direction, of a first polarization vector of said incident light, and substantially reflects, along a second direction, a second polarization vector of said incident light, wherein a relatively small, predetermined component of said second polarization vector, compared to said transmitted first polarization vector, is also transmitted along said first direction, said transmitted second polarization vector being an information-independent second polarization vector component;

a thin-film, magneto-optic Kerr effect magnetic field sensing element, said sensing element comprising a plurality of thin-film layers disposed on a substrate, at least one of said layers being of a reflective metallic ferromagnetic material and having;

an in-plane magnetization orientation;

a well-defined in-plane uniaxial anisotropy;

a predetermined coercive field value; and

a known magneto-optic Kerr effect value, wherein said sensing element is disposed such that an in-plane easy axis of said at least one sensing element ferromagnetic material layer lies in the light plane of incidence, collinear to an external magnetic field to be measured, said at least one sensing element ferromagnetic material layer disposed to intercept light transmitted through said polarizing beamsplitter in said first direction and reflect light that is modified by converting a portion of said first polarization vector to said second polarization vector by an amount determined by the orientation of the magnetization of said at least one sensing element ferromagnetic material layer relative to said incident light, the size of said first polarization vector, and a magneto-optic Kerr effect value, said converted light being an information-containing second polarization vector component;

a highly-reflective mirror disposed to intercept light reflected from said sensing element and to reflect said intercepted light back toward said sensing element for reflection of light, modified by converting a further portion of said first polarization vector to said second polarization vector, back to said polarizing beamsplitter, said converted further portion also contributing to said information-containing second polarization vector component, wherein a remaining portion of said first polarization vector is transmitted through said polarizing beamsplitter in a direction opposite said first direction, and said information-containing second polarization vector component and said information-independent second polarization vector component are reflected in a third direction; and

a detector disposed to intercept said reflected information-containing and information-independent second polarization vector components from said polarizing beamsplitter in said third direction, to mix said information-containing and information-independent second polarization vector components, and to output a mixed output signal having an offset component providing an amplitude which is substantially independent of the external magnetic field, and a signal component dependent on the external magnetic field.

2. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein the light source is selected from the group consisting of:

a laser light source; and

a light emitting diode (LED).

3. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said information-containing second polarization vector component has an amplitude which is smaller than the amplitude of said information-independent second polarization vector component.

4. The magneto-optic Kerr effect magnetic field sensor of claim 1, wherein said in-plane magnetization orientation of said at least one sensing element ferromagnetic material layer is provided by said external magnetic field, measured along said in-plane easy axis of said at least one sensing element ferromagnetic material layer, relative to said coercive field value of said at least one sensing element ferromagnetic material layer.

5. The magneto-optic Kerr effect magnetic field sensor of claim 1, wherein the size of said information-containing second polarization vector component is provided by a vector component of said at least one sensing element ferromagnetic material layer magnetization that lies along said light plane of incidence, a size of said first polarization vector, and said magneto-optic Kerr effect value.

6. The magneto-optic Kerr effect magnetic field sensor of claim 1, wherein said mixed output signal is an output current comprising the sum of:

an information-containing current which is linearly proportional to the vector component of said at least one ferromagnetic material layer magnetization that lies along said light plane of incidence, which is provided by said external magnetic field value, measured along said at least one magnetic material layer in-plane easy axis, relative to said coercive field value of said at least one magnetic material layer; and

an information-independent current which is independent of said external magnetic field value and is dependent on the amplitude of said information-independent second polarization vector.

7. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said sensing element is aligned with respect to said external magnetic field such that said external magnetic field is collinear to said at least one sensing element ferromagnetic material layer easy axis.

8. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said first polarization vector is s-polarized light, and said second polarization vector is p-polarized light, the light polarizations defined with respect to said sensing element.

9. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said first polarization vector is p-polarized light and said second polarization vector is s-polarized light, the light polarizations defined with respect to said sensing element.

10. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said sensing element is disposed on a fiber optic coupler half-block.

11. The magneto-optic Kerr effect magnetic field sensor of claim 1 wherein said sensing element further comprises a flux concentrator to increase the external magnetic field at said sensing element.

12. The magneto-optic Kerr effect magnetic field sensor of claim 1, further comprising a highly transmissive, nonreciprocal optical element, wherein said first polarization vector and said information-containing and information-independent second polarization vector components reflect off said sensing element toward said nonreciprocal optical element and are transmitted through said nonreciprocal optical element toward said mirror, off said mirror back toward said nonreciprocal optical element, and thence back toward said sensing element.

13. The magneto-optic Kerr effect magnetic field sensor of claim 12, wherein said highly transmissive, nonreciprocal optical element further comprises a polarization rotation element enabling the minimization of polarization noise along a light propagation path.

14. The magneto-optic Kerr effect magnetic field sensor of claim 13, wherein said highly transmissive, nonreciprocal optical element is provided as a Faraday rotator.

15. The magneto-optic Kerr effect magnetic field sensor of claim 1, wherein said reflective metallic ferromagnetic material is amorphous iron boron.

16. The magneto-optic Kerr effect magnetic field sensor of claim 1, wherein said detector is chosen from the group consisting of:

a semiconductor photodiode; and

a photomultiplier tube.

17. The magneto-optic Kerr effect magnetic field sensor of claim 10 further comprising:

a polarization maintaining fiber optic cable coupled between each element of said magnetic field sensor.

18. The magneto-optic Kerr effect magnetic field sensor of claim 12 further comprising:

a fiber optic cable coupled between each element of said magnetic field sensor.

19. A rotation sensor comprising:

a rotatable member;

at least one permanent magnet coupled to said rotatable member to provide a circumferentially oriented switching magnetic field proximate the member as the member is rotated;

a light source;

an optical polarizing element disposed to intercept incident light provided by said light source, wherein said optical polarizing element provides maximum transmission, along a first direction, of a first polarization vector of said incident light, and substantially reflects, along a second direction, a second polarization vector of said incident light, wherein a relatively small, predetermined component of said second polarization vector, compared to said transmitted first polarization vector, is also transmitted along said first direction, said transmitted second polarization vector being an information-independent second polarization vector component;

a reflective magneto-optic Kerr effect sensing element, said sensing element comprising a reflective metallic ferromagnetic film having;

a well defined in-plane uniaxial anisotropy;

a predetermined coercive field value;

a known magneto-optic Kerr effect constant; and

a predominantly oriented in-plane magnetization vector, said sensing film disposed proximate said rotatable member such that said sensing element is exposed to said switching magnetic field generated by said at least one magnet and oriented such that an in-plane easy axis is oriented both collinear to the switching magnetic field and in the light plane of incidence, said ferromagnetic film disposed to intercept said light provided by transmission through said polarizing beamsplitter, and reflect light modified by converting a portion of said first polarization vector to said second polarization vector by an amount determined by the orientation of the magnetization of said ferromagnetic film relative to said incident light, the size of said first polarization vector, and the magneto-optic Kerr effect value, said converted light being an information-containing component of said second polarization vector;

and

a detector disposed to intercept light which travels from said sensing element and which is reflected off said optical polarizing element.

20. The rotation sensor of claim 19 wherein a direction of said switching magnetic field is perpendicular to the radius of said rotatable member and said switching magnetic field switches a magnetization vector of said reflective ferromagnetic film between saturation positions.

21. The rotation sensor of claim 19 further comprising:

a polarization maintaining fiber optic cable coupled between each element of said rotation sensor.

22. The rotation sensor of claim 19 comprising:

a signal generating circuit coupled to said detector for generating a series of signal pulses to indicate a position of said rotating member.

23. The rotation sensor of claim 22 wherein said signal generating circuit comprises:

a power source;

a first amplifier having a first input terminal coupled to said power source wherein said power source is adapted to provide a suppression current to said amplifier and having a second input terminal coupled to a first reference potential;

a first resistor having a first terminal coupled to the first input terminal of said first amplifier and having a second terminal coupled to an output terminal of said first amplifier;

a capacitor having a first terminal coupled to said output terminal of said first amplifier and having a second terminal;

a second amplifier having a first input terminal coupled to the second terminal of said capacitor and having a second input terminal coupled to a second reference potential and having an output terminal; and

a second resistor having a first terminal coupled to the first input terminal of said second amplifier and a second terminal coupled to the output terminal of said second amplifier.

24. The rotation sensor of claim 19 wherein said light intercepted at said detector comprises:

said information-containing component of said second polarization vector having a value proportional to an instantaneous strength of said switching magnetic field at said sensing element; and

an information-independent component of said second polarization vector which is constant with respect to said switching magnetic field.

25. The rotation sensor of claim 19 further comprising a highly reflective element disposed to intercept light reflected from said sensing element and reflect light back to said sensing element, wherein said reflected light is then reflected back toward said polarizing element, and thence toward said detector.

26. The rotation sensor of claim 25 wherein said highly reflective element is selected from the group consisting of:

a highly reflective mirror; and

a nonreciprocal optical element disposed to intercept light reflected from the sensing element and transmit light having modified polarization characteristics towards a highly reflective mirror, wherein said highly reflective mirror then reflects said light back towards said nonreciprocal optical element where the polarization characteristics are further modified before transmitting said light back towards said sensing element.

27. The rotation sensor of claim 26 wherein said nonreciprocal optical element and highly reflective mirror are disposed to provide modifications to the polarization characteristics of said light reflected back towards said sensing element, minimizing the effects of polarization noise along the light propagation path.

28. The rotation sensor of claim 26 wherein said nonreciprocal optical element is provided as a Faraday rotator.

29. The rotation sensor of claim 27 further comprising a single-mode fiber optic cable coupled between each optical element of said magnetic field sensor.

30. The rotation sensor of claim 19 wherein said first polarization vector is s-polarized light, and said second polarization vector is comprised of p-polarized light, the light polarizations defined with respect to said sensing element.

31. The rotation sensor of claim 19 wherein said first polarization vector is p-polarized light and said second polarization vector is comprised of s-polarized light, the light polarizations defined with respect to said sensing element.

32. The rotation sensor of claim 21 wherein said sensing element is disposed on a fiber optic coupler half block.

33. The rotation sensor of claim 21 wherein said optical polarizing element is provided as a polarization splitter.

34. The rotation sensor of claim 19 wherein said sensing element further comprises a flux concentrator to increase the external magnetic field at said sensing element.

35. The rotation sensor of claim 25 further comprising:

a polarization maintaining fiber optic cable coupled between each element of said rotation sensor.

36. The rotation sensor of claim 25 wherein:

said light source provides a coherent light signal; and

said detector combines said information-containing and information-independent components of said second polarization vector to provide an output current as the sum of an information-independent current having an amplitude which is substantially constant regardless of a value of said switching magnetic field and an information-containing current proportional to the value of said switching magnetic field.

37. A magneto-optic Kerr effect magnetic field sensing system comprising:

a light source;

an optical polarizing element disposed to intercept light provided from said light source and to provide maximum transmission of light having a first polarization vector to be transmitted in a first direction and substantially reflects, along a second direction, a second polarization vector of said incident light, wherein a relatively small, predetermined component of said second polarization vector, compared to said transmitted first polarization vector, is also transmitted along said first direction, said transmitted second polarization vector being an information-independent second polarization component;

a magneto-optic Kerr effect magnetic field sensing element comprising a reflective thin film comprising at least one metallic ferromagnetic layer having;

known values for effective magnetization and magneto-optic Kerr effect constant; and

a predominantly oriented in-plane magnetization vector region, wherein said sensing film is disposed such that the direction of an external magnetic field to be measured is normal to a plane in which said sensing element is disposed and such that said sensing element intercepts said light transmitted through said optical polarizing element in the first direction and wherein a first portion of the first polarization vector of light intercepted by said sensing element undergoes polarization conversion to said second polarization vector depending upon the value of said magnetic field, the value of said at least one ferromagnetic layer effective magnetization, and the amplitude of said first polarization vector, wherein said first portion comprises said information-containing component of said second polarization vector and said first portion reflects off of said sensing element and returns toward said optical polarizing element, while a second portion of the first polarization vector and said information-independent component of said second polarization vector do not undergo polarization conversion as said second portion and said information-independent component reflects towards said optical polarizing element; and

a detector disposed to intercept light comprised of said information-containing component of said second polarization vector and said information-independent component of said second polarization vector which travels from said sensing element and which is reflected off said optical polarizing element.

38. The magneto-optic Kerr effect magnetic field sensing system of claim 37 further comprising a polarization maintaining fiber optic cable disposed between each element in said sensing system.

39. The magneto-optic Kerr effect magnetic field sensing system of claim 38 wherein said sensing element is disposed on the end of said polarization maintaining fiber optic cable.

40. The magneto-optic Kerr effect magnetic field sensing system of claim 38 wherein the light beam directed from said optical polarizing element toward said sensing element along said fiber optic cable is normally incident to said sensing element.

41. The magneto-optic Kerr effect magnetic field sensing system of claim 37 wherein said optical polarizing element is provided as a polarizing beamsplitter.

42. The magneto-optic Kerr effect magnetic field sensing system of claim 37 wherein:

said optical polarizing element intercepts light reflected back from said sensing element and reflects light comprised of the information-independent and information-containing components of said second polarization vector to said detector; and

said detector detects said information-independent and information-containing components of said second polarization vector as a mixed electrical signal.

43. The magneto-optic Kerr effect magnetic field sensing system of claim 42, wherein said mixed electrical signal is comprised of:

an information-containing electrical signal linearly proportional to the ratio of said external magnetic field value to the effective magnetization value of said at least one layer of ferromagnetic material; and

an information-independent electrical signal which is larger in magnitude than the information-containing electrical signal but which remains constant with respect to the value of said external magnetic field.

44. The magneto-optic Kerr effect magnetic field sensing system of claim 37 wherein said first polarization vector is provided as s-polarized light, and said second polarization vector is provided as p-polarized light, the light polarizations defined with respect to said sensing element.

45. The magneto-optic Kerr effect magnetic field sensing system of claim 37 wherein said first polarization vector is provided as p-polarized light, and said second polarization vector is provided as s-polarized light, the light polarizations defined with respect to said sensing element.

46. The magneto-optic Kerr effect magnetic field sensing system of claim 37 wherein said sensing element further comprises a flux concentrator to increase the external magnetic field at said sensing film.

47. The magneto-optic Kerr effect magnetic field sensing system of claim 37, wherein said thin ferromagnetic film is provided as amorphous iron boron.

48. The magneto-optic Kerr effect magnetic field sensing system of claim 38, wherein said optical polarizing element is provided as a polarization splitter.

49. A rotation sensor comprising:

a rotatable member;

at least one permanent magnet coupled to said rotatable member to provide a circumferentially oriented switching magnetic field proximate the member as the member is rotated;

a light source;

an optical polarizing element disposed to intercept light provided by said light source, wherein said optical polarizing element provides maximum transmission, along a first direction, of a first polarization vector of said light, and substantially reflects, along a second direction, a second polarization vector of said incident light, wherein a relatively small, predetermined component of said second polarization vector, compared to said transmitted first polarization vector, is also transmitted along said first direction, said transmitted second polarization vector being an information-independent second polarization vector component;

a reflective magneto-optic Kerr effect sensing element, said sensing element comprising a reflective metallic ferromagnetic film having;

a well defined in-plane uniaxial anisotropy, a predetermined coercive field value;

a known magneto-optic Kerr effect constant; and

a predominantly oriented in-plane magnetization vector, said sensing element disposed proximate said rotatable member such that said sensing element is exposed to said switching magnetic field generated by said at least one magnet and oriented such that an in-plane easy axis is oriented both collinear to the switching magnetic field and in the light plane of incidence, said sensing element of magnetic material further disposed to intercept said light provided by transmission through said polarizing beamsplitter, and reflect light modified by converting a portion of said first polarization vector to said second polarization vector by an amount determined by the orientation of the magnetization of said layer of ferromagnetic material relative to the incident light, the size of said first polarization vector, and said magneto-optic Kerr effect value, said converted light being said information-containing component of said second polarization vector;

a detector disposed to intercept light which travels from said sensing element and which is reflected off said optical polarizing element; and

a signal generating circuit coupled to said detector for generating a series of signal pulses to indicate a position of said rotating member, wherein said signal generating circuit comprises;

a power source;

a first amplifier having a first input terminal coupled to said power source wherein said power source is adapted to provide a suppression current to said amplifier and having a second input terminal coupled to a first reference potential;

a first resistor having a first terminal coupled to the first input terminal of said first amplifier and having a second terminal coupled to an output terminal of said first amplifier;

a capacitor having a first terminal coupled to said output terminal of said first amplifier and having a second terminal;

a second amplifier having a first input terminal coupled to the second terminal of said capacitor and having a second input terminal coupled to a second reference potential and having an output terminal; and

a second resistor having a first terminal coupled to the first input terminal of said second amplifier and a second terminal coupled to the output terminal of said second amplifier.

50. A magneto-optic Kerr effect magnetic field threshold sensor comprising:

a light source;

an optical polarizing element disposed to intercept incident light provided from said light source, wherein said optical polarizing element provides maximum transmission, along a first direction, of a first polarization vector of said incident light, and substantially reflects, along a second direction, a second polarization vector of said incident light, wherein a relatively small, predetermined component of said second polarization vector, compared to said transmitted first polarization vector, is also transmitted along said first direction, said transmitted second polarization vector being an information-independent second polarization vector component;

a thin-film, magneto-optic Kerr effect magnetic field sensing element, said sensing element comprising a plurality of thin-film layers, at least one of said layers being of a reflective metallic ferromagnetic material and having;

an in-plane magnetization orientation;

a well-defined in-plane uniaxial anisotropy;

a predetermined coercive field value; and

a known magneto-optic Kerr effect constant, wherein said sensing element is disposed such that an in-plane easy axis of said at least one sensing element ferromagnetic material layer lies in the light plane of incidence, collinear to an external magnetic field to be measured, said at least one sensing element ferromagnetic material layer disposed to intercept light transmitted through said optical polarizing element in said first direction and reflect light that is modified by converting a portion of said first polarization vector to said second polarization vector by an amount determined by the orientation of the magnetization of said at least one sensing element ferromagnetic material layer relative to said incident light, the size of said first polarization vector, and a magneto-optic Kerr effect value, said converted light being an information-containing second polarization vector component,

wherein said reflected light from said at least one sensing element ferromagnetic material layer returns to said polarizing beamsplitter for transmission of said first polarization vector in a direction opposite said first direction, and for reflection of said information-containing and information-independent second polarization vector components in a third direction; and

a detector disposed to intercept said reflected information-containing and information-independent second polarization vector components from said polarizing beamsplitter in said third direction, to mix said information-containing and information-independent second polarization vector components, and to output a mixed output signal having an offset component providing an amplitude which is substantially independent of the external magnetic field, and a signal component dependent on the external magnetic field.

51. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said light source is selected from the group consisting of:

a laser light source; and

a light emitting diode (LED).

52. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said optical polarizing element is provided as a polarizing beamsplitter.

53. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, wherein said in-plane magnetization orientation of said at least one sensing element ferromagnetic material layer is provided by said external magnetic field, measured along said in-plane easy axis of said at least one sensing element ferromagnetic material layer, relative to said coercive field value of said at least one sensing element ferromagnetic material layer.

54. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, wherein the size of said information-containing second polarization vector component is provided by a vector component of said at least one sensing element ferromagnetic material layer magnetization that lies along said light plane of incidence, a size of said first polarization vector, and said magneto-optic Kerr effect value.

55. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, wherein said mixed output signal is an output current comprising the sum of:

an information-containing current which is linearly proportional to the vector component of said at least one ferromagnetic material layer magnetization that lies along said light plane of incidence, which is provided by said external magnetic field value, measured along said at least one ferromagnetic material layer in-plane easy axis, relative to said coercive field value of said at least one ferromagnetic material layer; and

an information-independent current which is independent of said external magnetic field value and is dependent on the amplitude of said information-independent second polarization vector.

56. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said information-containing second polarization vector component has an amplitude which is smaller than the amplitude of said information-independent second polarization vector component.

57. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said sensing element is aligned with respect to said external magnetic field such that said external magnetic field is collinear to said at least one sensing element ferromagnetic material layer easy axis.

58. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said first polarization vector is s-polarized light, and said second polarization vector is p-polarized light, the light polarizations defined with respect to said sensing element.

59. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said first polarization vector is p-polarized light and said second polarization vector is s-polarized light, the light polarizations defined with respect to said sensing element.

60. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said sensing element is disposed on a fiber optic coupler half-block.

61. The magneto-optic Kerr effect magnetic field threshold sensor of claim 60 further comprising:

a polarization maintaining fiber optic cable coupled between each element of said magnetic field sensor.

62. The magneto-optic Kerr effect magnetic field threshold sensor of claim 61 wherein said optical polarizing element is provided as a polarization splitter.

63. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50 wherein said sensing element further comprises a flux concentrator to increase the external magnetic field at said sensing element.

64. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, further comprising a highly transmissive, nonreciprocal optical element, wherein said first polarization vector and said information-containing and information-independent second polarization vector components reflect off said sensing element toward said nonreciprocal optical element and are transmitted through said nonreciprocal optical element toward said mirror, off said mirror back toward said nonreciprocal optical element, and thence back toward said sensing element.

65. The magneto-optic Kerr effect magnetic field threshold sensor of claim 64, wherein said highly transmissive, nonreciprocal optical element further comprises a polarization rotation element enabling the minimization of polarization noise along a light propagation path.

66. The magneto-optic Kerr effect magnetic field threshold sensor of claim 65 wherein said highly transmissive, nonreciprocal optical element is provided as a Faraday rotator.

67. The magneto-optic Kerr effect magnetic field threshold sensor of claim 65 further comprising a fiber optic cable coupled between each element of said magnetic field sensor.

68. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, wherein said thin metallic ferromagnetic film is comprised of amorphous iron boron.

69. The magneto-optic Kerr effect magnetic field threshold sensor of claim 50, wherein said detector is chosen from the group consisting of:

a semiconductor photodiode; and

a photomultiplier tube.