GB2168146A

GB2168146A - Optical wavelength measurement

Info

Publication number: GB2168146A
Application number: GB08430830A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-12-06
Filing date: 1984-12-06
Publication date: 1986-06-11
Also published as: GB8430830D0; GB2168146B

Abstract

The wavelength of an optical signal 12 is determined by attenuating at 1 the optical signal to a predetermined level and transmitting the attenuated optical signal along an optical fibre 7 to a detector 9. Since there is a monotonic change in loss in the optical fibre over a wavelength range covering the visible and near infrared, the detector current is related directly to the loss in the optical fibre and thus the wavelength of the transmitted optical signal. The determined wavelength is displayed on a meter 11. The optical fibre 7 may be omitted if the detector current itself varies with wavelength. <IMAGE>

Description

SPECIFICATION Optical wavelength measurement This invention relates to a method and apparatus for measuring optical wavelength.

According to one aspect of the present invention there is provided a method of measuring the wavelength of an optical signal, comprising the steps of attenuating the optical signal to a predetermined level, transmitting the attentuated signal in or to an optical element whose loss characteristic varies monotonically with wavelength and detecting the transmitted optical signal with a detector, the electrical output of the detector being directly related to the optical signal wavelength.

According to another aspect of the present invention there is provided an optical wavelength meter including means for attenuating an optical signal of unknown wavelength to a predetermined level, an optical element having a loss characteristic which varies monotonically with wavelength and a detector arranged to detect the attenuated signal following transmission thereof in or to said optical element, the electrical output of the detector in use of the meter being directly related to the wavelength of the optical signal.

Embodiments of the invention will now be described by way of example with refrence to the accompanying drawing which shows somewhat schematically the layout of an embodiment of fibre optic wavelength meter.

The wavelength meter illustrated in the drawing comprises a motor driven variable attenuator 1, a beamsplitter 2, a first detector 3, a reference electric signal source 4, a comparator 5, input optics 6, an optical fibre 7, output optics 8, a second detector 9, detection electronics 10 and panel meter 11.

An incoming optical signal of unknown wavelength and unknown power is applied to the beamsplitter 2 via a path in which the variable attenuator 1 is effective. The portion 13 of the optical signal thus split off is detected by the first detector 3. The electrical output of detector 3 is compared with the reference signal in the comparator whose output is applied to the variable attenuator 1 which is then automatically driven until the detector signal is the same as the reference level. Thus the incoming optical signal 12 is attentuated to a preset level. The detection electronics 10 are now actuated. The portion 14 of the attenuated optical signal 12 is transmitted along the optical fibre 7, via the input and output optics 6 and 8, to the second detector 9 and the attenuated signal as transmitted along the fibre 7 detected.The output current of the second detector 9 is related directly to the loss of the fibre and hence the wavelength of the optical signal 12 may be obtained since there is a monotonic change in loss in a fibre over the range covering the visible and the near infra-red. Thus the electrical signal level at the second detector is directly related to wavelength and the panel meter 11 can be calibrated to read out directly in wavelength. The detection electronics 10 contain some memory locations containing the calibration of the signal level as a function of fibre loss and detector sensitivity.

The accuracy of the wavelength measurement will be within +5nm. The power level of the incoming signal may also be determined from the amount of attentuation required to equalize the signals at the comparator 5. The optical fibre 7, together with the input and output optics if necessary, may be replaced by optical elements having a similar loss characteristic such as a heat absorbing filter. Alternatively the fact that the material of the detector itself, for example germanium or silicon, varies in its responsivity with wavelength.

In this case the optical element is thus the detector itself. Thus there is provided a method of measuring the wavelength of an optical signal which does not not require the use of dispersing elements as in conventional methods and in which the direct photo-current output (detector 9) may be read off in wavelength.

1. A method of measuring the wavelength of an optical signal, comprising the steps of attenuating the optical signal to a predetermined level, transmitting the attenuated signal in or to an optical element whose loss characteristic varies monotonically with wavelength and detecting the transmitted optical signal with a detector, the electrial output of the detector being directly related to the optical signal wavelength.

2. A method as claimed in claim 1, wherein the optical element is a length of optical fibre.

3. A method as claimed in claim 1, wherein the optical element is the detector.

4. A method as claimed in claim 1, claim 2 or claim 3, wherein the attentuation step comprises directing the optical signal to a beamsplitter via a path in which a variable attenuator is effective, directing a split-off--part of the signal to a further detector whose electrical output is applied to a comparator with a reference electrical signal and driving the variable attenuator until the further detector output equals the reference electrical signal.

5. A method as claimed in claim 4 wherein a direct part of the attenuated optical signal is applied to the optical element.

6. A method as claimed in claim 2, and for measuring wavelengths in the range from the visible to the near infra-red.

7. A method as claimed in any one of the preceding claims further including the step of determining the power level of the optical sig

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

SPECIFICATION Optical wavelength measurement This invention relates to a method and apparatus for measuring optical wavelength.

According to one aspect of the present invention there is provided a method of measuring the wavelength of an optical signal, comprising the steps of attenuating the optical signal to a predetermined level, transmitting the attentuated signal in or to an optical element whose loss characteristic varies monotonically with wavelength and detecting the transmitted optical signal with a detector, the electrical output of the detector being directly related to the optical signal wavelength.

According to another aspect of the present invention there is provided an optical wavelength meter including means for attenuating an optical signal of unknown wavelength to a predetermined level, an optical element having a loss characteristic which varies monotonically with wavelength and a detector arranged to detect the attenuated signal following transmission thereof in or to said optical element, the electrical output of the detector in use of the meter being directly related to the wavelength of the optical signal.

Embodiments of the invention will now be described by way of example with refrence to the accompanying drawing which shows somewhat schematically the layout of an embodiment of fibre optic wavelength meter.

The wavelength meter illustrated in the drawing comprises a motor driven variable attenuator 1, a beamsplitter 2, a first detector 3, a reference electric signal source 4, a comparator 5, input optics 6, an optical fibre 7, output optics 8, a second detector 9, detection electronics 10 and panel meter 11.

An incoming optical signal of unknown wavelength and unknown power is applied to the beamsplitter 2 via a path in which the variable attenuator 1 is effective. The portion

13 of the optical signal thus split off is detected by the first detector 3. The electrical output of detector 3 is compared with the reference signal in the comparator whose output is applied to the variable attenuator 1 which is then automatically driven until the detector signal is the same as the reference level. Thus the incoming optical signal 12 is attentuated to a preset level. The detection electronics 10 are now actuated. The portion

14 of the attenuated optical signal 12 is transmitted along the optical fibre 7, via the input and output optics 6 and 8, to the second detector 9 and the attenuated signal as transmitted along the fibre 7 detected.The output current of the second detector 9 is related directly to the loss of the fibre and hence the wavelength of the optical signal 12 may be obtained since there is a monotonic change in loss in a fibre over the range covering the visible and the near infra-red. Thus the electrical signal level at the second detector is directly related to wavelength and the panel meter 11 can be calibrated to read out directly in wavelength. The detection electronics 10 contain some memory locations containing the calibration of the signal level as a function of fibre loss and detector sensitivity.

The accuracy of the wavelength measurement will be within +5nm. The power level of the incoming signal may also be determined from the amount of attentuation required to equalize the signals at the comparator 5. The optical fibre 7, together with the input and output optics if necessary, may be replaced by optical elements having a similar loss characteristic such as a heat absorbing filter. Alternatively the fact that the material of the detector itself, for example germanium or silicon, varies in its responsivity with wavelength.

In this case the optical element is thus the detector itself. Thus there is provided a method of measuring the wavelength of an optical signal which does not not require the use of dispersing elements as in conventional methods and in which the direct photo-current output (detector 9) may be read off in wavelength.

1. A method of measuring the wavelength of an optical signal, comprising the steps of attenuating the optical signal to a predetermined level, transmitting the attenuated signal in or to an optical element whose loss characteristic varies monotonically with wavelength and detecting the transmitted optical signal with a detector, the electrial output of the detector being directly related to the optical signal wavelength.

2. A method as claimed in claim 1, wherein the optical element is a length of optical fibre.

3. A method as claimed in claim 1, wherein the optical element is the detector.

4. A method as claimed in claim 1, claim 2 or claim 3, wherein the attentuation step comprises directing the optical signal to a beamsplitter via a path in which a variable attenuator is effective, directing a split-off--part of the signal to a further detector whose electrical output is applied to a comparator with a reference electrical signal and driving the variable attenuator until the further detector output equals the reference electrical signal.

5. A method as claimed in claim 4 wherein a direct part of the attenuated optical signal is applied to the optical element.

6. A method as claimed in claim 2, and for measuring wavelengths in the range from the visible to the near infra-red.

7. A method as claimed in any one of the preceding claims further including the step of determining the power level of the optical sig nal prior to attenuation thereof from the amount of attenuation required to equalise the optical signal to the predetermined level.

8. An optical wavelength meter including means for attenuating an optical signal of unknown wavelength to a predetermined level, an optical element having a loss characteristic which varies monotonically with wavelength and a detector arranged to detect the attenuated signal following transmission thereof in or to said optical element, the electrical output of the detector in use of the meter being directly related to the wavelength of the optical signal.

9. A meter as claimed in claim 8 wherein the optical element is a length of optical fibre.

10. A meter as claimed in claim 8 wherein the optical element is the detector.

11. A meter as claimed in claim 8, claim 9, or claim 10, wherein the attentuating means comprises a motor driven variable attenuator, a beamsplitter, a further detector, a reference electrical signal source and a comparator for driving the attenuator in response to the difference between the further detector electrical output and the reference electrical signal.

12. A meter as claimed in claim 9 and including detector electronics, which contains memory locations containing calibration of the signal level as a function of fibre loss and detector sensitivity, and a panel meter directly calibrated in wavelength.

13. A method of measuring the wavelength of an optical signal substantially as herein described with reference to and as illustrated in the accompanying drawing.

14. An optical wavelength meter substantially as herein described with reference to and as illustrated in the accompanying drawing.