GB2427023A

GB2427023A - Light monitoring device

Info

Publication number: GB2427023A
Application number: GB0511790A
Authority: GB
Inventors: Stephen Pope; Colin Edge
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2005-06-10
Filing date: 2005-06-10
Publication date: 2006-12-13
Also published as: GB0511790D0

Abstract

A monitoring device for monitoring a light beam, comprises a beam splitter and a first photodetector arranged such that, in use, an input light beam to be monitored is split into first and second separate beam portions by the beam splitter. The first and second beam portions are routed via separate respective optical paths having differing optical path lengths within the beam splitter, and subsequently are at least partially recombined such that interference occurs between them and a resultant recombined beam is detected by the photodetector. The monitoring device may monitor the wavelength of the input light beam, and thus the device may be a wavelength locker or a transmitter incorporating a wavelength locker.

Description

Light Monitoring Device The present invention relates to the monitoring of

a light beam, and in particular the monitoring of the wavelength of a light beam, for example in order to detect and correct any drift in the wavelength from a predetermined wavelength (known as "wavelength locking") . The invention has particular utility in the field of optical communications (and will be described primarily in relation thereto) but at least the broadest aspects of the invention are not limited to optical communications applications.

In this specification, the terms "light" and "optical" will generally be used to refer not only to visible light but also to other wavelengths of electromagnetic radiation, for example in the wavelength range of about 200nm to about 1mm, i.e. from ultraviolet to the far infrared.

Wavelength lockers are well known and are used, for example, to ensure that an optical signal generated by a laser for transmission over an optical communications network has the correct wavelength. (Wavelength drift may otherwise occur due to ageing effects, temperature variations, or power fluctuations, for example.) This is particularly important, for example, in wavelength division multiplex (WDM) optical communications systems, and is even more important in dense wavelength division multiplex (DWDM) systems, in which a plurality of wavelength channels is used to transmit optical signals via a single optical fibre. If the wavelength of one or more of the optical signals does not fall within its correct pre-assigned wavelength channel, corruption of the signals and/or problems with detection of the signals may occur, for example.

There are currently two principal telecommunications bands, namely the C Band (191.6 - 196.2 THz) and the L Band (186.4 - 191.6 THz). Within these bands there are standard wavelength channels defined by the International Telecommunications Union (ITU) at spacings of 100 GHz (0.8nm), 50 GHz (0.4nm), or 25 GHz (0.2nm). (In the future, additional bands, and narrower spacings of wavelength channels within the bands may be used.) There is therefore a need to "lock" optical signal wavelengths at these standardised wavelengths for example, and wavelength lockers are used for this purpose.

Thus, a wavelength locker typically monitors the light output of a laser and provides electronic feedback to the laser to control its wavelength. The locker typically comprises an etalon and/or other filter, with one or more photodetectors. The etalon or other filter transmits light that is a function of wavelength, and the level of light that is detected by the photodetector can therefore be related to the wavelength. Figure 1 of the accompanying drawings illustrates, schematically and in plan view, a wavelength locker of the type disclosed in international patent application WO01/091756 (Bookham Technology), the entire contents of which are incorporated herein by reference.

In this wavelength locker, a portion of the light beam 1 emitted from a source (e.g. a laser) is sampled from the beam by a cube-type beam splitter 3, and sampled light that is transmitted or reflected from a separate etalon or other filter 4 is detected by a pair of separate photodetectors 5. The wavelength of the light can be monitored by monitoring the difference between the photodetector signals produced by the two photodetectors. The power of the light can be determined from the sum of the two photodetector signals. In other known wavelength locker designs, one or more light splitting devices may be used, with one light path providing a power level, and another light path using an etalon or other wavelength-selective component to provide a signal for wavelength discrimination, for example.

Because known wavelength lockers use etalons or other types of filter, they can suffer from practical problems. For example, etalons are generally expensive to manufacture. Other types of filter (e.g. interference filters) typically exhibit wide variability from one filter to another, and therefore when wavelength lockers are fabricated on an industrial scale it is often necessary to discard significant numbers of filters because their variability is too great. It is also frequently necessary to stock a large inventory of different grades of filter in order to cater for each particular application.

The present invention seeks, among other things, to provide a novel and inventive solution to this problem.

Accordingly, a first aspect of the invention provides a monitoring device for monitoring a light beam, comprising a beam splitter and a first photodetector arranged such that, in use, an input light beam to be monitored is split into first and second separate beam portions by the beam splitter, wherein the first and second beam portions are routed via separate respective optical paths having differing optical path lengths within the beam splitter, and subsequently are at least partially recombined such that interference occurs between them and a resultant recombined beam is detected by the first photodetector. The monitoring device of the invention preferably monitors the wavelength of the input light beam.

The invention has the advantage that because it functions by providing differing optical path lengths for first and second beam portions, and recombining the beam portions such that interference occurs between them, the invention can dispense with the need for an etalon or other filter. (This is explained below.) It is to be understood that, at least in its broadest aspects, although the invention dispenses with the necessity of using an etalon or other filter, an etalon or other filter can, if desired, be used in conjunction with the invention to perform one or more functions. It is further to be understood, however, that the beam splitter itself (according to the invention), i.e. the component that actually splits the input light beam, is not an etalon.

In operation, the relative phases of the first and second beam portions, and thus the intensity of the recombined light detected by the first photodetector, will depend upon the difference between the optical path lengths of the first and second beam portions, and will also depend upon the wavelength of the input light beam. Consequently, the monitoring device according to the invention can monitor the wavelength of the input light beam by detecting the intensity of the recombined light received by the first photodetector.

More preferably, the monitoring device according to the first aspect of the invention is a wavelength locker for locking the wavelength of the input light beam substantially to a predetermined wavelength. Advantageously, therefore, the device may include a control signal generator arranged to generate a signal for controlling a laser or other source of the input light beam, the control signal being dependent upon the recombined beam detected by the first photodetector.

A second aspect of the invention provides a transmitter comprising a laser arranged to emit a light beam, and a monitoring device according to the first aspect of the invention arranged to monitor the light beam emitted by the laser.

The monitoring device forming part of the second aspect of the invention may be a wavelength locker for locking the wavelength of the light beam emitted by the laser substantially to a predetermined wavelength.

The light beam monitored by the monitoring device may, for example, be emitted from a rear facet of the laser (and a front facet of the laser may emit an output light beam). Alternatively, the monitored light beam may be the main transmitted light beam emitted by the laser, or at least a sampled portion thereof.

The beam splitter used in the invention preferably comprises a cube beam splitter or a modified cube beam splitter. The modified cube beam splitter preferably is modified such that the optical path of the first beam portion within the beam splitter is longer than the optical path of the second beam portion within the beam splitter. Advantageously, the optical path of the first beam portion within the beam splitter may be longer than the optical path of the second beam portion within the beam splitter at least partly because the distance travelled by the first beam portion within the beam splitter is longer than the distance travelled by the second beam portion within the beam splitter.

Additionally or alternatively, the beam splitter may include at least two regions of differing refractive index, and the optical path of the first beam portion within the beam splitter may be longer than the optical path of the second beam portion within the beam splitter at least partly because a refractive index of the beam splitter experienced by the first beam portion is greater than a refractive index of the beam splitter experienced by the second beam portion. Preferably, a region of the beam splitter in which the input light beam is split has a lower refractive index than a region of the beam splitter through which the first beam portion propagates.

In preferred embodiments of the invention, the beam splitter includes a partially reflective interface between two regions of the beam splitter, at which interface the input light beam is split, in use. Preferably, the input beam is split because the first beam portion passes through the interface, and the second beam portion reflects from the interface. Advantageously, the first beam portion may subsequently reflect from the interface, for example from an opposite side of the interface, e.g. after reflecting from another surface (i.e. another surface of the beam splitter and/or one or more other reflectors of the device). One or each of the first and second beam portions preferably is at least partially reflected from a respective surface of the beam splitter.

Other preferred and optional features of the invention are described below, and in the dependent claims.

Some preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a plan view schematic representation of a known wavelength locker; Figure 2 is a plan view schematic representation of a known beam splitter, as shown in Figure 1; Figure 3 is a plan view schematic representation of an embodiment of a beam splitter according to the present invention; Figure 4 is a plan view schematic representation of another embodiment of a beam splitter according to the present invention; Figure 5 is a plan view schematic representation of an embodiment of a monitoring device (e. g. a wavelength locker) according to the present invention; and Figure 6 is a graphical representation of electrical signals produced by photodetectors of a monitoring device (e.g. a wavelength locker) according to the invention, of the type illustrated in Figure 5, which electrical signals are representative of light intensities detected by the photodetectors.

Figure 1 has been described above. Figure 2 illustrates schematically and in plan view a known cube-type beam splitter 3 of the type used in known wavelength lockers, for example. The known beam splitter 3 comprises two prisms 7 and 9 of isosceles right-angled triangular section, bonded together along the hypotenuse of each triangular section to form a prism of square cross- section, having a diagonal interface 11 where the two triangular-section prisms are bonded together. The triangular-section prisms 7 and 9 normally are bonded by means of a resin or other bonding agent selected to provide the interface 11 with a known partial reflectivity (commonly 50% reflectivity and 50% transmission) at the wavelength of light with which the beam splitter is intended to be used. The prisms 7 and 9 are formed from a material (e.g. glass) having a high degree of transparency to the wavelength of light with which the beam splitter is intended to be used. As shown in Figure 1, in use an incident light beam 1 is partially reflected from the diagonal interface 11 and is partially transmitted through the interface. In this way, the beam 1 is split into first and second beam portions.

Figure 3 shows, schematically and in plan view, one embodiment of a novel beam splitter 13 used in a monitoring device (e.g. a wavelength locker) according to the present invention. This beam splitter 13 also comprises two prisms bonded together to produce a partially reflective interface 11. However, in this case, while one of the two prisms (prism 7) has an isosceles right-angled triangular section, the other prism (prism 15) does not have a triangular section, but instead has a quadrilateral section. In particular, prism 15 has a partial rectangular section, in which a corner of the rectangle in the form of an isosceles right-angled triangle is absent. The prism 15 is bonded to the right-angled prism 7 such that the prism 7 replaces the absent corner of the rectangular section of prism 15. Consequently the beam splitter 13 has a rectangular section (rather than a square cross-section), and the longer (or wider) portion of the beam splitter provided by the prism 15 provides an increased optical path length for one of the beam portions split from the input beam. This is explained below.

Figure 4 shows (schematically and in plan view) a beam splitter combination 17 that is a variant of the beam splitter 13 of Figure 3. The beam splitter combination 17 also has a rectangular cross-section, but in this case the rectangular cross-section is formed by bonding a square or rectangular part 19 to one peripheral surface 21 of a conventional beam splitter 3 of the type shown in Figure 2. The part 19 may advantageously have a higher refractive index than the parts 7 and 9. This has the advantage that the increased optical path length provided by the part 19 is provided by means of a shorter actual dimensional length than is the case with the part 15 of the beam splitter 13 of Figure 3.

Consequently, the beam splitter combination 17 occupies less space than does the beam splitter 13, and this can have the advantage that monitoring devices (e.g. wavelength lockers), or transmitters including monitoring devices, incorporating such beam splitters can be more compact, thereby allowing a greater number to be accommodated in a given volume.

From this description and the accompanying illustrations, the skilled person will now understand that preferred beam splitters used in monitoring devices according to the invention can be formed in any of a variety of ways.

Figure 5 is a plan view schematic representation of an embodiment of a monitoring device 23 (e.g. a wavelength locker) according to the present invention. The monitoring device 23 comprises a beam splitter 13 (e.g. of the type shown in Figure 3), a first photodetector 25 (e.g. a photodiode), a second photodetector 27 (e.g. a photodiode), and a collimating lens 29. Also shown is a light source 31 (e.g. a laser, especially a semiconductor laser). For embodiments of the invention comprising a transmitter, the transmitter includes the monitoring device 23 and the laser 31.

The beam splitter 13 has optical surfaces AB, BC, AC, DE and AE, having the following preferred reflectivities: AB - minimal reflectivity (e.g. coated with an anti-reflection coating) BC - 50% reflective, 5O% transmissive AC - 5O% reflective, 50% transmissive DE - 50% reflective, 5O% transmissive AE - minimal reflectivity (e.g. coated with an antireflection coating) and the functioning of the monitoring device 23 is as follows.

An input light beam 33 is emitted from the laser 31, which preferably is a semiconductor laser. The beam 33 normally will be emitted from a rear facet of the laser, and a main light beam, e.g. for optical communications transmission, will normally be emitted from an opposite (front) facet of the laser. However, it is of course possible for the light beam 33 to be the main beam, or a sampled portion of the main (sampled for monitoring purposes). The light beam 33 preferably propagates through a collimating lens 29 and enters the beam splitter 13 through a front surface AB that has an anti-reflection coating to minimise reflections back towards the laser 31. At the partially reflective (preferably 5O% reflective) interface AC the input beam 33 is split into first and second separate beam portions having substantially equal intensity. The first beam portion 35 propagates through the interface AC into the extended (or elongate) region 15 of the beam splitter, whereas the second beam portion 37 is reflected from the interface AC in a direction almost orthogonal to the first beam portion 35. (The direction of reflected second beam portion 37 is almost, but not quite, orthogonal to the first beam portion because the interface AC is oriented at an angle of 45 degrees to the front surface AB of the beam splitter 13, but the beam splitter is oriented such that the front surface AB is not quite orthogonal to the input beam 33, to minimise the possibility of stray reflections being received by the laser 31.) The first beam portion 35 is partially reflected from the partially reflective surface (preferably 50% reflective) DE at the end of region 15 of the beam splitter 13, and thus returns to the interface AC where it is again partially reflected (preferably 50% reflected) and is thus oriented towards the first photodetector 25. Because of the orientation of the beam splitter 13, the part 39 of the first beam portion 35 that is not reflected by the interface AC is oriented away from the laser 31.

The second beam portion 37 is partially reflected from the partially reflective (preferably 50% reflective) surface BC. The surface BC is substantially orthogonal to the surface DE, and thus the second beam portion 37 returns to the interface AC where it propagates through the interface towards the first photodetector 25 and at least partially recombines with the first beam portion 35 that has been reflected from the interface. The (at least partial) recombination of the first and second beam portions 35 and 37 causes interference between the first and second beam portions, and the resultant recombined beam is detected by the first photodetector 25. The part of the second beam portion 37 that is not reflected by the partially reflective surface BC, but instead propagates through the surface BC, is detected by the second photodetector 27, which is situated on the opposite side of the beam splitter 13 to the first photodetector 25. The angle between the surface BC and the surface DE preferably is 90 degrees 0.25 degrees. Each of the surfaces AB, BCD, DE, AE, and interface AC, is substantially planar.

It will be appreciated that because the surfaces BC, DE and AC are substantially 5O% reflective and 50% transmissive, the intensities of the first and second beam portions 35 and 37 which recombine, are substantially equal. This is because each of the beam portions reflects from two of the surfaces and propagates through one of the surfaces. Consequently, the intensity of each beam portion immediately prior to its interference with the other beam portion, is approximately 12.5% of the intensity of the input beam 33 from the laser. While it is not essential for the first and second beam portions 35 and 37 to have equal intensity immediately prior to their recombination, it is generally preferred that they have approximately equal intensity, in order to maximise the variations in the intensity of the recombined beam which occur due to differences between their respective phases when the first and second beam portions interfere.

- 10 - As indicated schematically by figures 3, 4 and 5, and as described above, the beam splitter 13 introduces an optical path length difference between the first and second beam portions 35 and 37. More particularly, the first beam portion 35 experiences a longer optical path length than does the second beam portion 37. Consequently, the relative phases of the two beam portions when they recombine will depend upon the difference between their optical path lengths, and the wavelength of the input light beam 33 from the laser 31. Because the relative phases of the first and second beam portions determines the amplitude, or intensity, of the recombined beam detected by the first photodetector 25, the device 23 provides a means of monitoring the wavelength of the input light beam 33.

A modelled example of the variation in the light intensity detected by the first photodetector 25 as the wavelength of the input light beam 33 varies, is shown in Figure 6. Figure 6 is a graphical representation of electrical signals produced by the first and second photodetectors 25 and 27 of a monitoring device 23 of the type illustrated in Figure 5, as the frequency (or wavelength) of the input light beam 33 varies. The electrical signals are (as the skilled person knows) representative of the light intensities detected by the photodetectors.

The signal produced by the first photodetector 25 is referred to as the "wavelength signal", and the signal produced by the second photodetector 27 is referred to as the "reference signal". As shown, the intensity of the wavelength signal (and thus, the intensity of the recombined beam that results from the interference between the first and second beam portions) varies periodically (in fact, sinusoidally) as the wavelength (or frequency) of the input light beam 33 from the laser 31 varies. In the specific modelled example represented in Figure 6, the period of the variation in the intensity of the recombined beam is approximately 100 GHz. Such a periodicity results from a length CD of the beam splitter 13 of Figure 5 being approximately 1.0 mm. In contrast, the intensity of the reference signal from the second photodetector 27 (which detects a proportion of the second beam portion 37, without interference with the first beam portion 35) does not vary with the wavelength of the input light beam 33.

In practice, any variation in the intensity of the reference signal is a result of a - 11 - variation in the intensity of the input light beam 33 emitted by the laser.

Consequently, by subtracting the reference signal from the wavelength signal (or at least taking account of the reference signal), any variations in the intensity of the input light beam 33 are removed from the wavelength signal, and the wavelength signal merely indicates variations in the wavelength of the input light beam. It will be understood that the particular manner in which the wavelength signal varies with the wavelength of the input light beam is not important. All that matters is that variations in the wavelength of the input light beam are detected and compared with a standard value.

The invention thus preferably is arranged to monitor (and preferably lock) the wavelength of light emitted by a "fixed wavelength" laser, rather than being arranged to lock a tuneable laser to each of a variety of different wavelengths, because the latter arrangement normally requires an etalon (or other wavelength selective device) having a precisely defined free spectral range. However, it is to be understood that the present invention could be used in the latter arrangement (e.g. for locking to a variety of different wavelengths) if the variation of the wavelength signal with input beam wavelength were sufficiently well known, e.g. from empirical testing of the device prior to installation.

The monitoring device preferably functions by comparing the difference (the "difference signal") between the wavelength signal and the reference signal with a standard signal value held in a memory of the device (e.g. a computer chip or hard drive), which standard signal value is representative of the required wavelength of the input light beam. For those embodiments of the invention in which the monitoring device comprises a wavelength locker, any difference between the measured difference signal and the standard signal value stored in the memory causes a control signal generator of the wavelength locker to generate a signal to control the laser such that the deviation from the required wavelength of the light beam emitted by the laser is corrected. For those embodiments of the invention comprising a transmitter comprising the monitoring device and the laser, the wavelength of light emitted by the transmitter is thus continuously or periodically corrected.

- 12 - The invention consequently provides a simple, yet effective, monitoring device, wavelength locker, or transmitter that avoids the necessity for a wavelength selective filter or etalon in order to monitor variations in the wavelength of an input light beam.

Claims

- 13 - Claims 1. A monitoring device for monitoring a light beam,

comprising a beam splitter and a first photodetector arranged such that, in use, an input light beam to be monitored is split into first and second separate beam portions by the beam splitter, wherein the first and second beam portions are routed via separate respective optical paths having differing optical path lengths within the beam splitter, and subsequently are at least partially recombined such that interference occurs between them and a resultant recombined beam is detected by the first photodetector.
2. A monitoring device according to claim 1, wherein the beam splitter comprises a cube beam splitter or a modified cube beam splitter.
3. A monitoring device according to claim 2, wherein the modified cube beam splitter is modified such that the optical path of the first beam portion within the beam splitter is longer than the optical path of the second beam portion within the beam splitter.
4. A monitoring device according to claim 3, wherein the optical path of the first beam portion within the beam splitter is longer than the optical path of the second beam portion within the beam splitter at least partly because the distance travelled by the first beam portion within the beam splitter is longer than the distance travelled by the second beam portion within the beam splitter.
5. A monitoring device according to claim 3 or claim 4, wherein the beam splitter includes at least two regions of differing refractive index, and the optical path of the first beam portion within the beam splitter is longer than the optical path of the second beam portion within the beam splitter at least partly because a refractive index of the beam splitter experienced by the first beam portion is greater than a refractive index of the beam splitter experienced by the second beam portion.

- 14 -
6. A monitoring device according to claim 5, wherein a region of the beam splitter in which the input light beam is split has a lower refractive index than a region of the beam splitter through which the first beam portion propagates.
7. A monitoring device according to any preceding claim, wherein the beam splitter includes a partially reflective interface between two regions of the beam splitter, at which interface the input light beam is split, in use, such that the first beam portion passes through the interface, and the second beam portion reflects from the interface.
8. A monitoring device according to claim 7, wherein the first beam portion subsequently reflects from the interface.
9. A monitoring device according to any preceding claim, wherein one or each of the first and second beam portions is at least partially reflected from a respective surface of the beam splitter.
10. A monitoring device according to any preceding claim, further comprising a second photodetector arranged to detect at least a proportion of: the input light beam; or the first beam portion; or the second beam portion; thereby to produce a reference signal.
11. A monitoring device according to claim 10, wherein the second photodetector is arranged to detect a proportion of the second beam portion only, and thereby detects any variations in the intensity of the input light beam.
12. A monitoring device according to claim 10 or claim 11, wherein the second photodetector is situated on an opposite side of the beam splitter to the first photodetector.
13. A monitoring device according to any preceding claim, wherein the relative phases of the first and second beam portions, and thus the intensity of the - 15 - resultant recombined light detected by the first photodetector, depends upon the difference between the optical path lengths of the first and second beam portions, and also depends upon the wavelength of the input light beam.
14. A monitoring device according to any preceding claim, arranged to monitor the wavelength of the input light beam, by detecting the intensity of the recombined light received by the first photodetector.
15. A monitoring device according to claim 14, wherein the first photodetector produces a wavelength signal representative of the detected intensity of the recombined light.
16. A monitoring device according to claim 15 when dependent upon claim 10, wherein the device is arranged to monitor the wavelength of the input light beam by monitoring a difference signal produced by subtracting the reference signal produced by the second photodetector from the wavelength signal produced by the first photodetector.
17. A monitoring device according to claim 15 or claim 16, wherein, in use, the wavelength signal or the difference signal is compared with a standard signal value stored in a memory of the device, which standard signal value is representative of a particular wavelength of the input light beam.
18. A monitoring device according to any preceding claim, which is a wavelength locker for locking the wavelength of the input light beam substantially to a predetermined wavelength.
19. A monitoring device according to claim 18, further comprising a control signal generator arranged to generate a signal for controlling a laser or other source of the input light beam, the control signal being dependent upon the recombined beam detected by the first photodetector.

- 16 -
20. A transmitter comprising a laser arranged to emit a light beam, and a monitoring device according to any preceding claim arranged to monitor the light beam emitted by the laser.
21. A monitoring device, wavelength locker or transmitter substantially as hereinbefore described with reference to, and/or substantially as illustrated in, the accompanying figures 3 to 6.