OPTICAL INSTRUMENT AND METHOD FOR DETERMINING A WAVELENGTH OF LIGHT GENERATED BY A LIGHT SOURCE, AND OPTICAL SYSTEM COMPRISING THE OPTICAL INSTRUMENT

Abstract

The invention refers to an optical instrument for determining a wavelength of light generated by a light source, comprising a signal generator for generating a modulation signal, a tunable optical filter device configured to receive the modulation signal, the tunable optical filter device configured to modulate the light generated by the light source based on the modulation signal, an optical detector device configured to detect a degree of modulation of light modulated by the tunable optical filter device, and an analyser configured to determine the wavelength of the light based the degree of modulation.

Claims

1. An optical instrument for determining a wavelength of light generated by a light source, comprising a signal generator for generating a modulation signal, a tunable optical filter device configured to receive the modulation signal, the tunable optical filter device configured to modulate the light generated by the light source based on the modulation signal, an optical detector device configured to detect a degree of modulation of the light modulated by the tunable optical filter device, and an analyser configured to determine the wavelength of the light based the degree of modulation.

2. The optical instrument of claim 1, wherein the tunable optical filter device includes an acousto-optic tunable filter (AOTF).

3. The optical instrument of claim 1 or 2, wherein the tunable optical filter device is configured to diffract the light generated by the light source based on the modulation signal.

4. The optical instrument of any preceding claim, wherein the signal generator generates the modulation signal which includes a sweep of a parameter of the modulation signal for determining the wavelength of the light, wherein the degree of modulation by the tunable optical filter device is highest if the wavelength of the light corresponds to a particular value of the parameter of the modulation signal.

5. The optical instrument of any preceding claim, wherein the signal generator generates the modulation signal which is a frequency-modulated wave whose frequency is swept from a minimum frequency to a maximum frequency.

6. The optical instrument of claim 5, wherein a nominal frequency of the wave is greater than 1 GHz and/or a frequency of the sweep is between 50 MHz to 200 MHz.

7. The optical instrument of any preceding claim, wherein the signal generator includes an arbitrary waveform generator.

8. The optical instrument of any preceding claim, wherein the signal generator is configured to generate a trigger signal simultaneous to the generation of the modulation signal for indicating a start of the modulation signal.

9. The optical instrument of any preceding claim, wherein the signal generator is configured to be coupled to the light source, the signal generator configured to supply the trigger signal to the light source for starting the generation of a light pulse.

10. The optical instrument of any preceding claim, wherein the signal generator is coupled to the analyser for supplying the trigger signal to the analyser.

11. The optical instrument of any preceding claim, wherein the analyser includes a calibration means configured to store a relationship between the wavelength of a light and the time since the generation of the trigger signal.

12. The optical instrument of claim 11, wherein the relationship is linear function.

13. The optical instrument of any preceding claim, wherein the optical detector device includes a first photodiode which is positioned to detect first-order diffracted light.

14. The optical instrument of claim 13, wherein the first photodiode is configured to measure an intensity of the first-order diffracted light and to supply the measured intensity to the analyser.

15. The optical instrument of any preceding claim, wherein the optical detector device includes a second photodiode which is positioned to detect zeroth-order diffracted light.

16. The optical instrument of claim 15, wherein the second photodiode is configured to measure an intensity of the zeroth-order diffracted light and to supply an inverse of the measured intensity to the analyser.

17. The optical instrument of any preceding claim, wherein the optical detector device includes an analog-to-digital converter.

18. The optical instrument of any preceding claim, further comprising a beam splitter and an optical detector configured to measure an intensity of received light and to supply the measured intensity to the analyser, wherein the beam splitter is configured to split incoming light in to a first path directed to the tunable optical filter device and a second path directed to the optical detector.

19. The optical instrument of claim 18, wherein the optical detector includes a photodiode.

20. The optical instrument of any preceding claim, wherein the signal generator generates a plurality of identical modulation signals one after another for measuring the wavelength of the light at various points of time.

21. The optical instrument of any preceding claim, wherein the signal generator generates the modulation signal after the generation of the trigger signal by a predetermined time lag for varying the point in time at which the wavelength of the light is determined.

22. The optical instrument of any preceding claim, wherein the signal generator generates a plurality of identical modulation signals one after another for measuring the wavelengths of the light source at various points of time, wherein the signal generator generates a first modulation signal of the plurality of modulation signals after the generation of the trigger signal by a predetermined time lag.

23. The optical instrument of any preceding claim, further comprising a diffraction device configured to diffract light depending on its wavelength, wherein the diffraction device is positioned to diffract the light modulated by the tunable optical filter device.

24. The optical instrument of claim 23, wherein the diffraction device is positioned to diffract the zeroth-order diffracted light and/or the first-order diffracted light.

25. The optical instrument of claim 23 or 24, wherein the diffraction device is a diffraction grating.

26. The optical instrument of any preceding claim, wherein the optical detector device is configured to detect the light modulated by the tunable optical filter device at spatially separated locations.

27. The optical instrument of any preceding claim, wherein the optical detector device includes a camera or a plurality of photodiodes.

28. An optical system, comprising the optical instrument of any preceding claims, and a light source generating light which is input into the tunable optical filter device.

29. The optical system of claim 28, wherein the light source includes a laser and/or a light emitting diode (LED), wherein optionally the light source is configured to be run in continuous or pulsed operation.

30. The optical system of claim 29, wherein the light source includes a power driver configured to output a drive current to the laser and/or a light emitting diode (LED), the drive current controlling an output of the light.

31. A method for determining a wavelength of light generated by a light source, comprising the steps of generating a modulation signal, modulating the light generated by the light source based on the modulation signal, detecting a degree of modulation of the modulated light, and determining the wavelength of the light based on the degree of modulation.

32. The method of claim 31, wherein the light is modulated by a tunable optical filter device, optionally by an acousto-optic tunable filter (AOTF).

33. The method of claim 31 or 32, wherein the light is modulated by diffracting the light based on the modulation signal.

34. The method of any one of the claims 31 to 33, wherein the modulation signal includes a sweep of a parameter of the modulation signal for determining the wavelength of the light, wherein the degree of modulation is highest if the wavelength of the light corresponds to a particular value of the parameter of the modulation signal.

35. The method of any one of the claims 31 to 34, wherein the modulation signal is a frequency-modulated wave whose frequency is swept from a minimum frequency to a maximum frequency.

36. The method of claim 35, wherein a nominal frequency of the wave is greater than 1 GHz and/or a frequency of the sweep is between 50 MHz to 200 MHz.

37. The method of any one of the claims 31 to 36, wherein the modulation signal is generated by an arbitrary waveform generator.

38. The method of any one of the claims 21 to 37, further comprising a step of generating a trigger signal simultaneous to the generation of the modulation signal for indicating a start of the modulation signal.

39. The method of claim 38, wherein the trigger signal is supplied to the light source for starting the generation of a light pulse.

40. The method of any one of the claims 31 to 39, further comprising a step of storing a relationship a relationship between the wavelength of a light and the time since the generation of the trigger signal.

41. The method of claim 40, wherein the relationship is linear function.

42. The method of claim 40 or 41, further comprising determining the wavelength of the light generated by the light source using the relationship.

43. The method of any one of the claims 31 to 42, wherein the step of detecting the degree of modulation of the modulated light includes detecting first-order diffracted light.

44. The method of claim 43, wherein the step of detecting the degree of modulation of the modulated light includes measuring an intensity of the first-order diffracted light, wherein the step of determining the wavelength is based on the measured intensity of the first-order diffracted light.

45. The method of any one of the claims 31 to 44, wherein the step of detecting the degree of modulation of the modulated light includes detecting zeroth-order diffracted light.

46. The method of claim 47, wherein the step of detecting degree of modulation of the modulated light includes measuring an intensity of the zeroth-order diffracted light, wherein the step of determining the wavelength is based on an inverse of the measured intensity of the zeroth-order diffracted light.

47. The method of any one of the claims 31 to 46, wherein the step of detecting the degree of modulation of the modulated light includes using an analog-to-digital converter.

48. The method of any one of the claims 31 to 47, further comprising a step of splitting light coming from the light source into a first path which is modulated based on the generated modulation signal and a second path, wherein an intensity of the light of the second path is measured.

49. The method of claim 48, wherein the intensity of light of the second path is measured by a photodiode.

50. The method of any one of the claims 31 to 49, wherein a plurality of identical modulation signals is generated one after another for measuring the wavelength of the light at various points of time.

51. The method of any one of the claims 31 to 50, wherein the modulation signal is generated after the generation of the trigger signal by a predetermined time lag for varying the point in time at which the wavelength of the light is determined.

52. The method of any one of the claims 31 to 51, wherein a plurality of identical modulation signals is generated one after another for measuring the wavelengths of the light source at various points of time, wherein a first modulation signal of the plurality of modulation signals is generated after the generation of the trigger signal by a predetermined time lag.

53. The method of any one of the claims 31 to 52, further comprising a step of additionally diffracting the modulated light depending on its wavelength.

54. The method of claim 53, wherein the zeroth-order diffracted light and/or the first-order diffracted light is additionally diffracted.

55. The method of claim 53 or 54, wherein a diffraction grating is used for additionally diffracting the modulated light.

56. The method of any one of the claims 33 to 55, wherein the step of determining the wavelength includes detecting the modulated light at spatially separated locations.

57. The method of any one of the claims 33 to 56, wherein the step of determining the wavelength includes using a camera or a plurality of photodiodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0093] FIG. 1 is a is a block diagram of a light source and an optical instrument for determining a wavelength of the light source;

[0094] FIG. 2 shows time evolutions of a trigger signal, a parameter of a modulation signal, and a measured intensity as observed when operating the optical instrument of FIG. 1;

[0095] FIG. 3 shows a calibration curve of the optical instrument of FIG. 1;

[0096] FIG. 4 shows time evolutions of a laser pulse, a parameter of a modulation signal, and a measured intensity as observed when operating the optical instrument of FIG. 1 according to a second embodiment;

[0097] FIG. 5 shows the measured wavelength over time of the measurement according to FIG. 4;

[0098] FIG. 6 shows the measured wavelengths over time of the measurement according to FIG. 4;

[0099] FIG. 7 shows time evolutions of a laser pulse, a parameter of the modulation signal, and a measured intensity as observed when operating the optical instrument of FIG. 1 according to a third embodiment;

[0100] FIG. 8 shows the measured wavelength over time of the measurement according to FIG. 7;

[0101] FIG. 9 is a is a block diagram of a light source and an optical instrument for determining a wavelength of the light source according to a further embodiment;

[0102] FIG. 10 is a is a block diagram of a light source and an optical instrument for determining a wavelength of the light source according to a further embodiment; and

[0103] FIG. 11 is block diagram showing steps of a method for measuring a wavelength of a light source.

DETAILED DESCRIPTION

[0104] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of an optical instrument provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized.

[0105] The present optical instrument 10 can be used to determine a wavelength of a light generated by light source 12. For example, the optical instrument 10 can be used to validate the correct functioning of the light source 12. The light source 12 includes a laser or LED 14, which can be a device-under-test (DUT), and a power driver 16. The power driver 16 generates a drive current for powering and controlling the laser or LED 14. The power driver 16 generates to drive current immediately after receiving a trigger signal from a signal generator 18.

[0106] The optical instrument 10 further includes a signal generator 18, a tunable optical filter device 20, an analyser 24, a beam splitter 26, and/or an optical detector 28. The tunable optical filter device 18 may include an acousto-optic tunable filter (AOTF) and is positioned to receive the light generated by the laser 14. The tunable optical filter device 18 is electrically or electronically connected to the signal generator 18. The tunable optical filter device 18 is configured to diffract impinging light depending on a modulation signal received from the signal generator 18. In other words, the signal generator 18 generates the modulation signal based on which the light is modulated by the tunable optical filter device 20. For example, if a parameter of the modulation signal is a particular value, the intensity of them modulated (diffracted) light is highest resulting, for example, in a maximum in the first order diffraction. This can be detected by optical detector device 22 which includes one or more photodiodes. For example, a first photodiode is configured to detect the intensity of the first-order diffracted light and a second photodiode is configured to detect the intensity of the zeroth-order. A maximum in the intensity detected by the first photodiode and a minimum in the intensity detected by the second photodiode indicate that the current parameter of the modulation signal corresponds to the wavelength generated by the light source 12.

[0107] The signal generator 18 continuously varies the parameter of the modulation signal which may be the frequency of a carrier wave as long as the light source 12 generates light (see FIG. 2). For example, the signal generator 18 generates a sweep of the frequency from a minimum frequency to a maximum frequency (see middle graph of FIG. 2). This sweep of the frequency results in a change of detected intensity which reaches a maximum at the first photodiode (see lower graph of FIG. 2) and a corresponding minimum at the second photodiode.

[0108] Alternatively, the signal generator 18 generates a trigger signal which is forwarded to the analyser 24. The time since the receipt of the trigger signal which also starts to sweep of the modulation signal can be used to determine the current value of the modulation signal such as the current frequency of the acoustic wave generated by tunable optical filter device 24 for modulating or diffracting the light generated by the light source 12. If the time that has passed since the receipt of the trigger signal is stored in relation to known wavelengths during a calibration (see FIG. 3), this relationship can be used to determine the wavelength of the light source 12. To this end, the analyser 24 determines when the intensity of the signal generated by the first photodiode is maximal and/or the intensity of the signal generated by the second photodiode is minimal. The relationship is then used to determine the wavelength based on this point of time.

[0109] In a different embodiment, the signal generator 18 may be configured to repeatedly generate a sweep of the parameter of the modulation signal resulting in a saw-tooth profile of the parameter of the modulation signal (see middle graph in FIG. 4). In this way, the wavelength can be determined at various points of the times during the generational slide of the light source 12. This allows an observation of a temporal behaviour of the wavelength (see FIG. 5).

[0110] FIG. 6 shows an actual measurement result, whereby the average delay of 100 optical signal peaks obtained for 100 laser pulses (upper graph) is translated to an average wavelength value and its 95% confidence interval for each frequency sweep window (middle graph). The deviation from the steady state optical frequency is shown in the lower graph.

[0111] The temporal distance between the point of time when wavelength can be determined (i.e. the periodicity of the measurement) depends on the duration of the sweep which can be determined by external factors such as the rise time of the tunable optical filter device 20. In order to provide wavelength measurements between those points of time, the start of the first sweep of the plurality of sweeps may be delayed by a predetermined delay time (see FIG. 7). By varying the delay time and repeating the measurement of FIG. 4, more measurements of the wavelength over the same time range can be achieved (see FIG. 8).

[0112] The beam splitter 26 splits the light generated by light source 12 into a first path which leads to the optical tunable optical filter device 20 and a second part which leads to the optical detector 28. The optical detector 28 includes a photodiode and is configured to measure the intensity of the light generated by the light source 12. The optical detector 28 is electrically connected to the analyser 24. This allows to measure the intensity of the light generated by the light source and address potential changes in the intensity.

[0113] The analyser 24 may include a microprocessor and a memory unit is further electrically coupled to the optical detector device 22. The analyser 24 may include a functional unit characterised as a calibration means which allows recording and storing the relationship as shown in FIG. 3.

[0114] The embodiment of FIG. 9 includes the same features and characteristics as the embodiment of the optical instrument 10 of FIG. 1. The embodiment of FIG. 9 differs from the embodiment of FIG. 1 in that the light source 12 includes a laser array 30 instead of the laser 14. The laser array 30 is configured to provide light having multiple wavelengths. As the laser array 30 provides light of several wavelengths, the optical instrument 10 needs to be able to determine which wavelength is measured or, in other words, which laser of the laser array 30 corresponds to which filter function or parameter of the modulation signal. The intensity peaks detected at the first photodiode of the light source 12 can be resolved using the method of FIG. 7, i.e. by varying the delay time. In other words, the time delay is a way of encoding which laser of the laser array 30 is being measured.

[0115] The embodiment of FIG. 10 includes the same features and characteristics as the embodiment of FIG. 9. The embodiment of FIG. 10 differs from the embodiment of FIG. 9 in that the optical instrument 10 includes a diffraction device 32 such as a grating. The diffraction device 32 is positioned between the tunable optical filter device 20 and the optical detector device 20. The diffraction device 32 is provided to spatially separate the modulated light depending on its wavelength. This allows to simultaneously measure the intensity of the modulated light at several frequencies. In this case, the optical detector device 22 includes array of detectors (such as but not limited to a camera) to detect the response of each laser of the laser array 30 simultaneously.

[0116] As method for measuring the wavelength of the light source 12 is described with reference to FIG. 11. In step S1, the relationship between the wavelength and the time delay since the start of the modulation signal is recorded. For this calibration step, various laser source having known and fixed wavelength are used with the optical instrument 10 described above. The time point of maximum intensity for the first-order diffraction is recorded. All the recorded time points are input into the relationship. In step S2, the light source 12 (as described above) or DUT is coupled to the optical instrument 10 and the modulation signal is generated by the signal generator 18 (as described above). In step S3, the tunable optical filter device 20 modulates the light by the light source 12 depending on the modulation signal, for example diffracts the light. In step S4, the optical detector device 22 measures the intensity of the modulated light and forwards the intensity to the analyser 24. In step S5, the analyser 24 determines the point of time since the receipt of the modulation signal when the measured intensity is maximal. The analyser 24 uses the stored relationship and the determined point of time to determine the wavelength of the light source 12.