GB2350184A - Apparatus and method to measure wavelength - Google Patents

Abstract

An incident beam of light is initially divided into at least two beams. A first beam enters a wavelength measuring device (WMD) and a first signal is produced indicative of the wavelength. A second beam passes through an electro-optical modulator (EOM) and then through a delay fibre which imparts a wavelength dependent delay to the beam. A receiver then outputs a second signal and the two signals are used to determine the wavelength of the light. In a second embodiment, the light from the beam divider or the modulator is incident upon an optical device under test (DUT) and a third signal is produced. An optical characteristic of the DUT is then determined, based on the first, second and third signals.

Description

2350184 APPARATUS AND METHOD TO MEASURE WAVELENGTH The present invention

relates to an apparatus and method for rapidly determining the wavelength of an input lightwave, e.g. an electromagnetic wave output from a laser device.

In particular, the present invention relates to a means with which to perform fast and accurate wavelength measurements of a discretely or continuously scanning lightwave, either in isolation, e.g. for the purpose of evaluating scan linearity, or in conjunction with a second fiber-optic measurement system that measures a separate property of interest.

It is a common requirement in the field of fiber- optics to measure a wavelength dependent response of a fiber-optic device. In general, the task of a fiberoptic measurement system is to interrogate a device under test (DUT) with a lightwave of accurately known wavelength, and simultaneously measure a property of 20 interest. For example, it is possible to measure the dispersive properties of a fiber-optic device by measuring the time delay induced by the device as a function of the interrogation wavelength. The wavelength resolution of such a measurement system would typically be expected to be of the order of a f ew picometres.

In order to interrogate a DUT with such fine 2 resolution it is necessary to use a tunable single frequency semiconductor diode laser as the light source, and to scan the wavelength, either discretely or continuously, across the range of interest. In the dispersion measurement example mentioned above, it would be necessary for the fiber-optic measurement apparatus to measure the time delay induced by the DUT, and to attach to each delay value an accurate absolute wavelength value. Due to the poor absolute wavelength accuracy of all semiconductor diode tunable lasers, however, it is not sufficient either to command the laser to tune to a particular wavelength and expect it to do so accurately, or to query the laser for the current wavelength. Therefore, in order to obtain accurate absolute wavelength values, it is necessary for the fiber-optic measurement apparatus to measure the wavelength values concurrently with the measurement of the delay values.

Manufacturers of tunable lasers are aware that their lasers are used for wavelength scanning in fiber- optic measurement apparatus, for which task wavelength scan linearity is important. Typically, scan linearity is compromised by the occurrence of laser mode-hopping, as illustrated in Figure 1. Therefore it is of interest to manufacturers to have a method with which to observe the linearity of the wavelength scan produced by their laser.

There currently exist wavelength measurement 3 devices (WMDs), based upon interf erometric techniques, which are capable of subpicometre accuracy, and measurement rates of a few (up to 10) hertz. Such devices can be used to sample the wavelength of a scanning laser for the purpose of evaluating scan linearity; alternatively, they can be implemented into a measurement system, such as the one described above, and be given the task of measuring the interrogation wavelength.

If an existing WMD is used to test the linearity of a tunable laser scan, the accuracy of the test is seriously compromised by the update speed of the WMD. Typically, the WMD will have a maximum update speed of 1OHz, which is equivalent to sampling the wavelength signal only ten times per second. Furthermore, the 1OHz update rate of the WMD implies that the integration time of the device is of the order of 0.1 seconds. During this time span a scanning wavelength can change by a significant amount, with the result that the measured wavelength value is not representative of the true wavelength value. Consequently, no existing WMD is capable of accurately testing the linearity of a tunable laser scan.

If an existing WMD is incorporated into a fiber- optic measurement apparatus to track the interrogation wavelength, it will experience the same problems caused by the slow update speed. In addition, it will be the 4 limiting factor of the operation speed of the measurement apparatus, in a manner dependent upon whether the tunable laser is scanned continuously or discretely.

If the interrogation wavelength is scanned continuously, the WMD will be required to sample the wavelength at known intervals, in synchronisation with the measurement of the property of interest of the DUT. As a result, the 10Hz maximum update rate of the WME) will limit the update rate of the measurement apparatus as a whole to 10Hz. Moreover, in order to achieve high wavelength resolution, the wavelength scanning speed must be slow.

If the wavelength is stepped discretely, the WMD will be required to perform a measurement at each step.

Therefore the wavelength resolution of the entire measurement apparatus is limited by the minimum step size of which the tunable laser is capable. A high quality tunable laser may be capable of a sufficiently small step size. However, the speed of operation will be slow due to the time required by the tunable laser to shift wavelength, and due to the integration time of the WMD (typically 0.1 seconds or more), for the duration of which the apparatus is delayed at every measurement point.

An object of the present invention is to provide a system having an update rate and an integration time which are superior to the existing solution. The speed of operation of the present invention is preferably sufficiently high to be used to accurately evaluate the linearity of a tunable laser scan, and to be incorporated into a fiber-optic measurement apparatus without limiting the speed of operation of the apparatus.

An apparatus according to the present invention may be used as an independent device to evaluate the linearity of the scan of a tunable laser. Alternatively, it may be used in conjunction with a fiber-optic measurement apparatus, to be referred to herein as a "secondary optical apparatus", which requires the measurement of the wavelength of a single frequency laser lightwave. The present invention may enable this measurement to be performed more quickly and accurately than a standard WMD. If an apparatus according to the present invention is used in conjunction with a secondary optical apparatus, it can be operated in parallel with the secondary optical apparatus, and only requires that a portion of the lightwave power from the secondary optical apparatus be tapped off for analysis.

In its most general terms, the present invention proposes applying an amplitude modulation to the laser lightwave, and passing the modulated lightwave through a delay fiber. If the refractive index of the delay fiber varies linearly with wavelength it will induce a linearly wavelength dependent time delay to the lightwave, which is manifested in a linear shift of the modulation phase 6 at the f ar end of the f iber. From the relative phase shift data it is possible to calculate relative wavelength shifts, since the relationship between the two is linear. In order to convert to absolute wavelength values one may use the initial and final absolute wavelengths of the measurement range, which correspond to the initial and final observed phase values. A standard WMD is employed in the present invention for the purpose of measuring the initial and final wavelengths. 10 Accordingly, in a first aspect the invention provides an apparatus f or measuring the wavelength of EM radiation, the apparatus comprising: radiation dividing means for receiving input EM radiation, and dividing it into at least two beams; 15 a wavelength measurement device for receiving a first one of said divided beams and producing a first signal indicating the wavelength thereof; modulator for receiving a second one of said divided beams and modulating it; 20 delay means for receiving said modulated second beam and transmitting it with a wavelength dependent delay; and a receiver for receiving said modulated second beam from the delay means and outputting a second signal indicative of said delay caused by said delay means; means for determining the wavelength of the EM radiation based on said first and second signals.

7 The wavelength measurement device (WMD) may be of a conventional form and is present for the purpose of measuring the initial and final wavelengths (e.g. at opposite ends of a frequency range of interest). In the case that an apparatus according to the present invention is used in combination with a secondary optical apparatus which is a measurement apparatus that measures a wavelength dependent property of a fiber DUT, it is important that the sampling of the wavelength is accurately synchronised (by a control computer) with the measurement by the secondary optical apparatus.

In a second aspect, the present invention provides a method for measuring the wavelength of EM radiation, the method comprising:- modulating the EM radiation; transmitting the modulated EM radiation to a receiver with a delay dependent upon the wavelength of the EM radiation; detecting said delay caused to the modulated radiation, and thereby determining the wavelength of the EM radiation on the basis of the delay caused to the modulated second beam.

Preferably, the method further includes the step of determining the wavelength of the radiation at a first time and at a second time using an additional wavelength measurement device, and determining the wavelength at a third time (when the additional wavelength measurement 8 device is not used to measure the wavelength of the radiation) based on the detected delay at the first, second and third times and the determined wavelength at the first and second times.

In further aspects, the present invention relates to using the principle explained above to determine the optical characteristics of an optical component (device under test).

Accordingly, in a third aspect the invention provides an apparatus for determining an optical characteristic of an optical component, the apparatus comprising: radiation dividing means for receiving input EM radiation, and dividing it into at least two beams; 15 a wavelength measurement device for receiving a first one of said divided beams and producing a first signal indicating the wavelength thereof; a modulator for receiving a second one of said divided beams and modulating it; 20 delay means for receiving said modulated second beam and transmitting it with wavelength dependent delay; means for transmitting to said optical component a beam derived from the radiation dividing means or from said modulator; 25 a first receiver for receiving said modulated second beam from the delay means and outputting a second signal indicative of said delay caused by said delay 9 means; a second receiver for receiving from the optical component said beam transmitted to the optical component, and outputting a third signal indicative of the optical characteristic of the optical component; means for determining the optical characteristic of the optical component based on said first, second and third signals.

In a fourth aspect, the invention provides a method for determining an optical characteristic of an optical component, the method comprising:

receiving scanning EM radiation, and dividing it into at least two beams; receiving a first one of said divided beams and producing a first signal indicating the wavelength of said first beam; modulating a second one of said divided beams, transmitting the modulated beam to a first receiver with a delay dependent upon the wavelength of the EM radiation, and deriving a second signal indicative of said delay; transmitting to said optical component a third one of said beams or a beam derived from said modulated beam, and deriving a third signal from said beam transmitted to the optical component; determining the optical characteristic of the optical component based on said first, second and third to signals.

In particular, said step of determining the optical characteristic of the optical component may use the values of said first and second signals at first and second times, and the values of said second and third signals (not the first signal) at at least one third time.

Preferable features of the first and second aspects of the invention apply also to the third and fourth aspects.

Embodiments of the invention will now be described, as non-limiting examples only, with reference to the accompanying drawings in which:

Fig. 1 illustrates the phenomenon of laser mode- hopping; Fig. 2 shows a first embodiment of the present invention; Fig. 3 illustrates a use of the first embodiment of the invention; Fig. 4 illustrates a second use of the f irst embodiment of the invention; and Fig 5 illustrates a second embodiment of the invention, in use as part of a system for DUT interrogation.

The first embodiment of the invention comprises six optical components and three radio frequency (RF) components. The six optical components are: a polarisation maintaining (PM) power divider, a standard WMD, an electrooptic modulator (EOM), a length of optical fiber, a fiber-optic isolator and a photoreceiver. The three RF components are: an RF amplifier, an RF phase measurement device and an RF modulation signal source. The operation of the components is managed by a control computer, which is in contact with the WMD, the modulation signal source, the phase measurement device and the secondary optical apparatus (if there is one). The role and required specifications of each component are described below, and the configuration of the components is illustrated in Figure 2.

The PM power divider splits a single input lightwave into two paths. The two paths are labelled 1 and 2 in Figure 2. Path 1 leads to the standard WMD, which makes absolute measurements of the initial and final wavelengths of the measurement range. Path 2 leads to the EOM.

The optical power ratio of the two paths is dependent upon the splitting ratio of the power divider, and should be adjusted to optimise the use of the input optical power. The input lightwave is linearly polarised and aligned to the slow axis of the PM fiber of the PM power divider pigtail. This is preferable due to the presence of the polarisation sensitive electro-optic modulator (EOM) in path 2.

12 The WMD is located in path 1. The task of the WMD is to measure the absolute values of the initial and final wavelengths of the scan.

It is not important that the device has a fast update speed, since these two measurements can respectively be made before the scan has started and after the scan has finished. The accuracy of the WMD affects the absolute accuracy of the whole apparatus, and should preferably be of the order of one picometre. The WMD should have a means of interfacing with the control computer in order to transfer the measured wavelength values for data processing.

The electro-optic modulator (EOM) is located in path 2. The task of the EOM is to apply a sinusoidal modulation envelope to the input lightwave.

The EOM is driven at a frequency within the RF band by a sinusoidally varying voltage signal, supplied by a modulation signal source. The EOM is a polarisation sensitive device, and requires a linearly polarised input lightwave that is aligned to the slow axis of the PM fiber input pigtail. The EOM should be biased at the quadrature point to ensure maximum linearity of response to the modulation drive signal. In the event that the EOM is not biased at the quadrature point, there should be a dc port on the EOM to which a small dc voltage can be applied to adjust the bias.

Immediately following the EOM is a length of delay 13 fiber. This fiber can be either standard single mode fiber (SMF) or dispersion compensating fiber (DCF) The length of the f iber should be suf f icient to induce dispersion of several hundred picoseconds per nanometre.

The task of the fiber is to create a wavelength dependent phase delay of the modulation envelope at the f ar end of the fiber.

The modulated lightwave is detected at a photoreceiver whereupon it is converted to an electrical RF signal. The phase of the RF signal will be the same as the phase of the optical signal that created it, and will therefore be dependent upon the optical wavelength. A fiber-optic isolator may be placed between the delay fiber and the photo-receiver in order to eliminate stray reflections from the photo-receiver, if required.

The modulation signal source supplies the sinusoidally varying voltage signal that drives the EOM. The frequency of the sinusoidal variation should lie in the RF band and be tunable. The modulation signal source should have a means of interfacing with the control computer in order that the modulation frequency can be changed if required.

The electrical RF signal output by the photoreceiver is boosted by an RF amplifier in preparation for phase measurement at channel A of the PMD. The PMD may have a means of interfacing with the control computer in order to transfer the measured phase values for data 14 processing. The PMD should also measure the phase of the modulation drive signal at channel B, at the same time as it measures the wavelength dependent signal at channel A. This is in order to eliminate the effects of any phase 5 discontinuities that may occur in the driving signal.

Examples of the Uses of the Invention will now be given. First ExamDle A first use is illustrated with reference to Fig.

3. The embodiment is used as an independent device to evaluate the linearity of a tunable laser scan.

A schematic diagram of the configuration is shown in Figure 3. It consists of the first embodiment (which has the internal configuration shown in Figure 2), a tunable laser diode and a control computer. All these devices have IEEE interfaces, through which they can communicate.

The tunable diode laser outputs a scanning single frequency lightwave signal. The lightwave is injected into the first embodiment, which performs wavelength measurements in the manner previously described. The control computer manages the operation of the current invention and the tunable laser diode throughout the sweep via the IEEE bus. A typical result of a wavelength scan measurement is shown in Figure 1 (described above) Second Examnle Fig. 4 illustrates a second example of a use of the embodiment of Fig. 2, that is with the f irst embodiment of the invention in combination with a secondary optical apparatus. The complete system shown in Fig. 4 is a spectral characteristics test (SCT) apparatus.

The secondary optical apparatus in this example measures the reflection and/or transmission spectrum of a DUT by measuring the optical power reflected and/or transmitted by the DUT as a function of wavelength. A typical DUT to be measured by the SCT apparatus would be a spectral filter, such as a fiber Bragg grating. The apparatus employs a scanning tunable laser as the interrogating light source in order to achieve higher resolution than is Possible with an LED light source and an optical spectrum analyser.

A schematic diagram of the secondary optical apparatus incorporating the current invention is shown in Figure 4. It consists of the first embodiment (which has the internal configuration shown in Figure 2), a dual channel optical power meter, a tunable semiconductor diode laser (both with IEEE interfaces), a control computer (fitted with an IEEE interface card), a number of optical components and the DUT. All the items shown, with the exception of the first embodiment of the invention and the DUT, belong to the secondary optical apparatus.

The tunable diode laser outputs a single frequency lightwave signal. The lightwave is divided and a portion 16 is injected into the first embodiment of the invention, which performs wavelength measurements in the manner previously described. The remaining portion of the lightwave passes on to the secondary optical apparatus; as such, the current invention is operating in parallel with the secondary optical apparatus. The secondary optical apparatus directs the lightwave to the DUT via a circulator. The optical power transmitted by the DUT is measured at the first channel of the optical power meter.

The optical power reflected by the DUT is re-directed by the circulator to be measured at the second channel of the optical power meter.

The control computer manages the operation of the first embodiment of the invention and the secondary optical apparatus, and synchronizes the measurements taken. In this example, the tunable laser is scanned continuously across the wavelength measurement range and the reflection and transmission optical powers are measured as a function of the current wavelength. In this way, the reflection and transmission spectra of the DUT are obtained. It is important that the measurement of the current wavelength (by the first embodiment of the invention) and the measurement of the reflection and transmission optical powers (by the secondary optical apparatus) occur at exactly the same time. If synchronization of these measurements is not achieved the measured spectra will be incorrect.

17 Third Exa=le A second embodiment of the invention will now be described with reference to Fig. 5 as part of a third example of the use of the invention. The second embodiment is illustrated as part of a system for interrogating a DUT, by a fiber dispersion test (FDT) apparatus. However, unlike the second example, in this third example some of the components which perform the wavelength measurements are used also to perform DUT interrogation itself.

The apparatus in this example uses a "phase-shift" technique for measuring the dispersion of a fiber DUT. This technique employs an amplitude modulated tunable lightwave signal to interrogate the DUT. The phase of the modulation envelope of the lightwave is measured af ter it has interacted with the DUT, over a range of wavelengths. The measured phase change induced by the DUT as a function of wavelength is converted to a time delay value as a function of wavelength (using a linear relationship), and is presented in the form of a group delay curve.

In this example, the basic principle of interrogating the DUT is identical to that of measuring the wavelength of the lightwave. As such, it is possible for the two functions to share a number of key components. These components are the EOM, the modulation signal source and the RF phase measurement device.

18 Thus, it is not possible in this example to logically separate the current invention from the secondary optical apparatus - they are effectively merged into a single hybrid apparatus, a schematic diagram of which is shown in Figure 5. The hybrid apparatus consists of three instruments (each with IME interfaces), a control computer (fitted with an IEEE interface card) and numerous optical and RF components. The three instruments are a tunable semiconductor diode laser, a standard WMD device and a network analyser. In this example, the network analyser serves as both the modulation signal source and as the phase measurement device.

The tunable semiconductor diode laser, which in Fig. 4 was part of the secondary optical apparatus, outputs a single frequency lightwave signal. The lightwave is injected into a polarisation maintaining tap coupler with a splitting ratio of 90:10, which in Fig. 2 was part of the first embodiment of the invention. The wavelength of the lightwave from the 10% port of the tap coupler is measured by the conventional WMD, equivalent to that employed as a component in Fig. 2. This path is equivalent to path 1 in Figure 2.

The output from the 90% port of the tap coupler is modulated with the use of an EOM, which is a common component which is used for both the wavelength measurement function and DUT interrogation. The EOM is 19 biased at the quadrature point by supplying a D.C. voltage to the D. C. input port of the EOM, and is driven by a modulation signal f rom the output port of the network analyser. The modulated output signal from the EOM is subsequently divided into two paths by a second power divider, a 60:40 directional fiber coupler. In contrast to the second example of the use of the first embodiment, there exist two modulated lightwaves in this example. One modulated lightwave interrogates the DUT, the other modulated lightwave is used for wavelength measurement using the principles of the present invention.

The signal in the 4096 path (the lower path in Fig. 5) is directed to measure the DUT. All the subsequent components in this path may be designated as part of the secondary optical apparatus. The modulated lightwave is directed to the DUT via a circulator. After interaction with the DUT, the modulated lightwave is returned to the circulator and re-directed to a photo-receiver, which converts the modulated lightwave to an RF modulated electrical signal. An RF amplifier boosts the signal before detection at channel 1 (CH1) of the network analyser. The network analyser measures both the phase and the amplitude of the signal. The DUT is always measured in reflection. In the case of a dispersion compensating fiber grating (which is by its nature a reflective device) the fiber is terminated using an isolator to prevent back reflections form the far fiber end (as indicated in Fig. 5). In the case of a transmissive fiber device, such as DCF or dispersion shif ted f iber (DSF), ref lection is f orced by splicing a fiber-coupled mirror to the far end of the test fiber (not shown in Fig. 5).

The signal in the 60% Path (the upper path in Fig. 5) is directed for wavelength measurement. This path is equivalent to path 2 in Figure 2, and all the subsequent components in this path are parts of the second embodiment of the invention. The first component is a 5.6km length of DCF delay fiber, followed by a photoreceiver, which converts the modulated lightwave, which has passed through the delay fiber, into a modulated RF electrical signal. The RF electrical signal is boosted by an RF amplifier before detection at channel 2 (CH2) of the network analyser, where the phase and amplitude of the signal are measured.

The control computer reads both sets of phase data from the network analyser, and reads the absolute initial and final wavelengths measured by the wavelength measurement device. The control computer uses the data to calculate the delay induced by the DUT and the wavelength at which the delay was induced. In order to ensure that the delay and the wavelength correspond exactly, the network analyser measures both signals simultaneously. In order to eliminate the effects of 21 phase noise in the modulation drive signal the network analyser calibrates the measurements against a portion of the drive signal which has been split off at the output of the network analyser, by an RF coupler, and measured at reference port R. This reference phase measurement is performed in synchronization with the other two phase measurements.

One measurement cycle consists of scanning the modulated interrogation lightwave over a user-defined wavelength range, and measuring the returned signal phases at CH1 and CH2 of the network analyser. From this data can be calculated the group delay curve of the DUT for the scanned wavelength range. The cycle is under the command of the control computer, via IEEE interface with the three instruments. In terms of tasks performed by the apparatus's instruments and components, a measurement cycle consists of the following steps: the tunable laser is set to the user-defined start 20 of the wavelength scan range. the wavelength measurement device accurately measures the initial wavelength, and sends this value to the control computer. the network analyser starts to perform a "sweep" of 25 measurements over a time just greater than the sweep time of the tunable laser wavelength scan, and measures and stores the phase and amplitude of 22 CH1 and CH2, all the time self -calibrating against the phase and amplitude measured at port R (this eliminates the effects of any phase discontinuities that may accidentally occur in the RF driving signal produced at the output port).

the tunable laser starts to scan wavelength at a user-defined speed.

the network analyser continues to measure and store the signals at CH1, CH2 and R. the tunable laser stops scanning when it reaches the user-defined end of

the wavelength range.

the network analyser stops measuring.

the wavelength measurement device accurately measures the final wavelength, and sends this value to the control computer.

the computer transfers the data from the network analyser, and performs signal processing; CH1 data is used to calculate the group delay curve of the DUT, CH2 data is used, in conjunction with the initial and final wavelength measurements made by the wavelength measurement device, to calculate wavelength.

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Claims (26)

1. An apparatus f or measuring the wavelength of EM radiation, the apparatus comprising:

radiation dividing means for receiving input EM radiation, and dividing it into at least two beams; a wavelength measurement device f or receiving a first one of said divided beams and producing a first signal indicating the wavelength thereof; a modulator for receiving a second one of said divided beams and modulating it; delay means for receiving said modulated second beam and transmitting it with a wavelength dependent delay; and a receiver for receiving said modulated second beam from the delay means and outputting a second signal indicative of said delay caused by said delay means; means f or determining the wavelength of the EM radiation based on said first and second signals.

2. An apparatus according to claim I in which said radiation dividing means is a fiber-coupled power divider and said delay means is a length of delay fiber, said modulator comprising a fiber-coupled electro optic modulator, and an RF modulation signal source for supplying an RF modulation signal for controlling said fiber-coupled electro-optic modulator; and said receiver comprising a fiber-coupled photo- receiver, an RF amplifier for amplifying the output of 24 said photo-receiver and an electrical phase measurement device which receives the output of the RF amplifier.

3. An apparatus system according to claim 2, wherein the power divider has one or more inputs and two outputs.

4. An apparatus according to claim 3, wherein the path formed by one of the output ports of the power divider leads directly to said wavelength measurement device.

5. An apparatus according to claim 4, wherein the wavelength measurement device interfaces with a control computer.

6. An apparatus according to claim 3, wherein the power divider is a polarisation maintaining directional coupler.

7. An apparatus according to claim 4, wherein the other of the output ports of the power divider leads directly to the electro-optic modulator.

8. An apparatus according to claim 7, wherein the electro-optic modulator is driven by a sinusoidally varying voltage signal produced by the RF modulation signal source, and modulates the lightwave to have an amplitude modulation envelope of the same frequency and phase as the driving signal.

9. An apparatus according to claim 8, wherein the output port of the EOM is connected to said length of delay fiber.

10. An apparatus according to claim 9, wherein the delay f iber is dispersion compensating f iber (DCF).

11. An apparatus according to claim 9, wherein the delay fiber is single mode fiber (SMF).

12. An apparatus according to claim 2, wherein the amplitude modulated lightwave that passes through the delay fiber is converted to an electrical signal by said photo-receiver.

13. An apparatus according to claim 2, wherein a fiberoptic isolator is placed in the fiber path prior to the receiver, in order to prevent potentially deleterious reflections from the receiver.

14. An apparatus according to claim 12, wherein the phase of the converted electrical signal is measured using said electrical phase measurement device.

15. An apparatus according to claim 14, wherein the electrical phase measurement device interfaces with a control computer.

16. An apparatus according to claim 15, wherein the electrical phase measurement device also measures the phase of the RF modulation signal that drives the electro-optic modulator.

17. An apparatus according to claim 2, wherein the electrical phase measurement device is a vector voltmeter.

18. An apparatus according to claim 2, wherein the electrical phase measurement device is a network analyser.

19. An apparatus according to claim 18, wherein the 26 network analyser acts as the RF modulation signal source.

20. An apparatus according to claim 16, wherein the measured phase data of the RF modulation drive signal and the measured phase data of the converted amplitude modulated lightwave are processed and used to calculate a current relative wavelength.

21. An apparatus according to claim 20, wherein the current relative wavelength is converted to absolute wavelength using the data from the wavelength measurement device.

22. A method for measuring the wavelength of scanning EM radiation, the method comprising: modulating the EM radiation; transmitting the modulated EM radiation to a receiver with a delay dependent upon the wavelength of the EM radiation; detecting said delay caused to the modulated radiation, and thereby determining the wavelength of the EM radiation on the basis of the delay caused to the modulated second beam.

23. A method according to claim 21 further including a step of determining the wavelength of the radiation at a first time and at a second time using an additional wavelength measurement device, and determining the wavelength at a third time based on the detected delay at the first, second and third times and the determined wavelength at the first and second times.

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24. An apparatus for determining an optical characteristic of an optical component, the apparatus comprising: radiation dividing means f or receiving input EM radiation, and dividing it into at least two beams; a wavelength measurement device for receiving a f irst one of said divided beams and producing a f irst signal indicating the wavelength thereof; a modulator for receiving a second one of said divided beams and modulating it; delay means for receiving said modulated second beam and transmitting it with wavelength dependent delay; means for transmitting to said optical component a beam derived from the radiation dividing means or from said modulator; a first receiver for receiving said modulated second beam from the delay means and outputting a second signal indicative of said delay caused by said delay means; a second receiver for receiving from the optical component said beam transmitted to the optical component, and outputting a third signal indicative of the optical characteristic of the optical component; means for determining the optical characteristic of the optical component based on said first, second and third signals.

25. A method for determining an optical characteristic 28 of an optical component, the method comprising: receiving scanning EM radiation, and dividing it into at least two beams; receiving a first one of said divided beams and producing a first signal indicating the wavelength of said first beam; modulating a second one of said divided beams, transmitting the modulated beam to a first receiver with a delay dependent upon the wavelength of the EM radiation, and deriving a second signal indicative of said delay; transmitting to said optical component a third one of said beams or a beam derived from said modulated beam, and deriving a third signal from said beam transmitted to the optical component; determining the optical characteristic of the optical component based on said first, second and third signals.

26. A method according to claim 25 in which said step of determining the optical characteristic of the optical component is based on the values of said first and second signals at first and second times, and the values of said second and third signals at at least one third time.