This invention relates to an apparatus for propagating optical radiation. The invention can take various forms, for example a passive cavity, a laser and a ring laser. The invention has application for sensing and communication systems.
Fibre lasers can be configured as sensing elements. Examples are distributed feedback DFB fibre lasers which comprise a fibre Bragg grating written into a single mode optical fibre doped with a rare-earth dopant pumped by optical pump radiation. The laser emits light at a wavelength defined by the Bragg grating. The wavelength is modulated by a physical parameter such as acoustic pressure, and this modulation can be measured by analysing the light emitted by the laser using an interferometer coupled into a demodulator. Such a configuration is particularly attractive because it offers the possibility of concatenating many such gratings in a linear array, each grating emitting at a different wavelength. Such a grating can be individually interrogated using wavelength division multiplexing WDM technology to separate each wavelength channel individually, and thus the technology promises scalability into very large arrays of DFB gratings.
Unfortunately, work into the development of such arrays has been limited because the lasers interact with each other and this leads to temporal instability. To date, the largest known laser array has comprised only four to five sensors.
Although the above discussion has focussed on DFB fibre lasers, similar comments can be applied to distributed Bragg reflector (DBR) fibre lasers, or other fibre lasers. Concatenation of lasers leads to feedback between the lasers resulting in temporal instability.
There is therefore a requirement for an array of lasers that can be integrated together without resulting in temporal instability. This requirement exists in sensing, as well as other fields such as telecoms where a source that emits at many individual wavelengths is also desirable.
There is also a requirement for a laser that has increased immunity from external reflections and which does not require the use of an isolator. Such a laser has application in many fields including communications and sensing.
An aim of the present invention is to provide an apparatus for propagating optical radiation that reduces the above aforementioned problems.
According to a non-limiting embodiment of the present invention, there is provided apparatus for propagating optical radiation in a first optical mode having a first spatial mode shape, and a second optical mode having a second spatial mode shape, which apparatus comprises an optical path, mode transforming means, and propagating means, wherein the mode transforming means transforms at least a portion of the first optical mode to the second optical mode, the propagating means is configured such that in use at least some of the optical radiation propagates along the optical path more than once, and the apparatus is characterised in that the first spatial mode shape is different from the second spatial mode shape.
The mode transforming means may be an integral feedback means and mode transformer. The mode transforming means may be a first grating.
The first grating may be characterised by a first coupling coefficient between the second mode incident upon the first grating and the second mode output by the first grating. The first grating may be characterised by a second coupling coefficient between the second mode incident upon the first grating and the first mode that is output by the first grating. The magnitude of the second coupling coefficient may be greater than the magnitude of the first coupling coefficient. The magnitude of the first coupling coefficient may be substantially zero. The magnitude of the second coupling coefficient may be uniform along the first grating. The magnitude of the second coupling coefficient may vary along the first grating.
The propagating means may be a reflector selected from the group comprising a grating, a dielectric surface, a mirror, a dichroic mirror, and a fibre Bragg grating.
The propagating means may be an integral feedback means and mode transformer.
The propagating means may be a second grating. The second grating may be characterised by a third coupling coefficient between the first mode incident upon the second grating and the first mode output by the second grating. The second grating may be characterised by a fourth coupling coefficient between the first mode incident upon the second grating and the second mode that is output by the second grating. The magnitude of the fourth coupling coefficient may be greater than the magnitude of the third coupling coefficient. The third coupling coefficient may be substantially zero.
The magnitude of the fourth coupling coefficient may be uniform along the second grating. The magnitude of the fourth coupling coefficient may vary along the second grating.
The apparatus may comprise both the first grating and the second grating. The reflectivity of the first grating may be greater than the reflectivity of the second grating. The reflectivity of the first grating may be less than the reflectivity of the second grating. The reflectivity of the first grating may be the same as the reflectivity of the second grating. The first and second gratings may overlay.
The mode transforming means may be a long period grating.
The propagating means may be a reflector selected from the group comprising a grating, a dielectric surface, a mirror, a dichroic mirror, and a fibre Bragg grating.
The propagating means may be provided by a ring configuration.
The apparatus may include a waveguide comprising at least one cladding and at least one core. The apparatus may include stress-applying parts. The waveguide may be twisted along its length. The core may be circular. The core may be non-circular. Additionally or alternatively, the core may comprise a ring. The core may be offset from the centre of the waveguide.
The cladding may be circular. The cladding may be non-circular. The cladding may comprise at least one flat portion.
The first optical mode may be the fundamental mode of the waveguide. The waveguide may be a single mode waveguide.
The waveguide may comprise a gain medium. The gain medium may comprise at least one rare earth dopant selected from the group comprising Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium and Dysprosium. The gain medium may comprise a transition metal or semiconductor.
The waveguide may comprise a photosensitive region. The photosensitive region and the gain medium may be in different areas of the waveguide. At least a portion of the photosensitive region may overlap the gain medium.
The apparatus may include a source of pump radiation configured to pump the gain medium. The source of pump radiation may be a semiconductor laser.
The apparatus may be configured to emit optical radiation having an optical wavelength. This embodiment of the invention can be in the form of a laser. A plurality of these apparatus may be connected in series. Additionally or alternatively, a plurality of these apparatus may be connected in parallel. The optical wavelengths emitted by each of these apparatus may be unique. The apparatus may comprise a demultiplexer and a plurality of modulators, wherein the demultiplexer directs the optical radiation to the modulators, the optical radiation received by each modulator having a different wavelength.
The apparatus may include enhancing means for enhancing the interaction of the apparatus to a measurand. This embodiment of the invention can be in the form of a sensor. The enhancing means may comprise a coating. The enhancing means may comprise a mechanical lever or diaphragm. The measurand may be pressure, hydrostatic pressure, acoustic energy, seismic energy, acceleration, vibration, fluid flow, mechanical strain, temperature, magnetic field, electric current, or electric field. The apparatus may include readout instrumentation.
The apparatus may include an isolator.
The apparatus may be in the form of a passive cavity, a laser, an array of lasers, a single longitudinal mode laser, an array of single longitudinal mode lasers, a sensor, or a sensor array.
The apparatus may be in the form of a laser array, in which the laser array comprises a plurality of lasers and at least one signal coupler, in which the lasers are configured to emit laser radiation at unique wavelengths, and in which the signal coupler is configured such that coupling between lasers is below a threshold that induces temporal instability.
At least one laser may comprise a DFB fibre laser grating.
At least one laser may comprise a DBR laser comprising at least one Bragg grating.
The laser array may comprise a plurality of gratings written into a single mode rare-earth doped waveguide.
The apparatus may comprise a signal waveguide, in which the signal coupler is configured to couple the laser radiation into the signal waveguide.
The apparatus may comprise a pump waveguide and a pump coupler, in which the pump coupler is configured to couple pump radiation guided by the pump waveguide into the lasers.
The pump waveguide may be the signal waveguide.
The signal coupler may be a taper, a long-period grating, a blazed grating or a perturbation.
The pump coupler may be a taper, a long-period grating, or a blazed grating.
The grating that comprises the laser may also comprise the pump coupler and the signal coupler.
Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:
With reference to
There is shown in
The first and second gratings 21, 22 have a reflectivity that can be defined as the proportion of power that is coupled between the first and the second optical modes 1, 2.
The first grating 21 can have a higher reflectivity than the second grating 22. For example, the reflectivity of the first grating 21 may be between 50% and 100%, and the reflectivity of the second grating 22 may be between 1% and 25%. The laser 20 will then emit laser radiation 34 more in one direction than the other and the laser 20 can be described as a unidirectional laser. The laser can be configured to emit laser radiation 34 in the other direction (i.e. towards the source 33) by having the reflectivity of the first grating 21 less than the reflectivity of the second grating 22. Alternatively, for bidirectional operation, the reflectivities of the first and second gratings 21, 22 should be approximately equal and in the range 1% to 100%, the exact figure being dependent upon the gain and round-trip loss within the cavity 29 defined by the first and second gratings 21, 22.
The first optical mode 1 is shown as being the fundamental mode of the waveguide 23, and the second optical mode 2 is shown as being one of the second modes of the waveguide 23. Alternatively, the first optical mode 1 can be an odd mode of the waveguide 23, and the second optical mode 2 can be an even mode of the waveguide 23—or vice versa.
The laser 20 may support many longitudinal lasing modes (by having the length of the cavity 29 between around 5 cm and 100 cm) and many transverse lasing modes (by ensuring that the waveguide 23 is multimoded). The waveguide 23 can be a dual mode waveguide.
Preferably, the waveguide 23 is a single mode waveguide in which the round trip loss in the cavity 29 is less than the round trip gain. Thus although the second and higher-order modes will be leaky, the single mode waveguide can be designed such that their loss within the cavity 29 is sufficiently low so as to permit lasing. Higher order modes emitted by such a laser 20 will tend to leak away—for example in a single mode waveguide connected to the cavity 29 or in a subsequent single mode waveguide that may be connected to the laser 20. The laser 20 can then be considered to support only a single transverse mode. The laser 20 can be configured to oscillate in a single longitudinal mode by decreasing the separation of the first and second gratings 21, 22. The laser 20 will then be a single longitudinal mode laser.
The first and second gratings 21, 22 can overlay as shown in the apparatus of
It is preferred in
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The laser emission 34 is directed to read-out instrumentation 52 via a coupler 47. The read-out instrumentation 52 may comprise an instrument for measuring the wavelength λ 55 of the laser 40. Alternatively, the read-out instrumentation 52 may comprise a phase or frequency demodulator such as found in radio receivers for demodulating phase or frequency modulated signals.
The sensor 57 can be configured as a hydrophone, an accelerometer, a geophone, an acoustic sensor, a flow sensor, a strain sensor, a temperature sensor, a pressure sensor, a magnetic field sensor, an electric current sensor, or an electric field sensor.
The apparatus shown in
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Several of the embodiments of the invention (for example
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The coupling coefficient between two modes of a waveguide can be calculated by coupled mode theory. The coupling coefficient is proportional to the product of the fields of the two modes and the perturbation of the refractive index, integrated over the perturbation. In this case, the grating defines a perturbation of the refractive index. The coupling coefficient is thus a function of the angle of the grating. An analysis of the properties of angled gratings in planar waveguides can be found in Riziotis and Zervas, Journal of Lightwave Technology, Vol 19, No 1, January 2001, pages 92-104. This reference also contains an extensive bibliography.
Referring to
FIGS. 10 to 13 and their accompanying figure descriptions detail various design features, such as stress applying parts, multiple claddings, multiple cores, circular cores, and cores shaped in the form of a ring. Waveguides may be fabricated using one or more of these design features in any combination. For example, it may be desirable to cladding pump a fibre having a plurality of cores. Such a fibre would then comprise the first cladding 111, the second cladding 112, the first core 131 and the second core 132. Alternatively or additionally, the fibre may include the stress applying parts 101. The cores 24, 131, 132 may be circular or non-circular.
In order to fabricate the first and second gratings 21, 22 described with reference to the preceding figures, the waveguide 23 preferably comprises a photosensitive region 102 as shown with reference to FIGS. 10 to 13. Silica can be made photosensitive by doping with germanium, tin or antimony. Germanium-doped silica can be codoped with boron in order to modify its refractive index and/or to increase its photosensitivity.
The waveguide 23, 100, 110, 120 or 130 can be hydrogenated with hydrogen and/or deuterium prior to writing the grating. If the waveguide 23, 100, 110, 120 or 130 also includes a gain medium 26, then it may be preferable to separate the disposition of the gain medium 26 and the photosensitive region 102. This is advantageous if the gain medium 26 comprises erbium codoped with Ytterbium as described in U.S. Pat. No. 5,771,251 which is herby incorporated herein by reference. Ring doping of the gain medium 26 can also be used to improve the efficiency or to control the gain of a fibre laser, as is described in U.S. Pat. No. 6,288,835 B1, which is hereby incorporated herein by reference. Alternatively, the photosensitive region 102 may overlap the gain medium 26, either completely or partially.
Several different techniques can be used to fabricate the gratings described in the preceding embodiments. A preferred technique is to write the grating through a phase mask placed at an angle to the waveguide 23, 100, 110, 120 or 130. The gratings shown in
The laser array 140 may be spliced to another laser array and time division multiplexing techniques used to separate out signals having the same wavelengths.
The laser array 140 shown in
The laser array 140 is shown fabricated in a single optical fibre 142. Alternatively, the signal waveguide 143 and the rare-earth doped waveguide 144 can be fabricated in separate fibres, and the signal coupler 145 can be a fused taper coupler, or any other form of coupler. The two fibres can also be coated in the same coating and brought together to form couplers at various intervals along the fibre.
Each signal coupler 145 couples laser radiation emitted from the respective laser 146 into the signal waveguide 143.
In the embodiment shown in
The rare earth doped waveguide 144 comprises rare earth dopant. The rare earth dopant may be erbium, erbium co-doped with ytterbium, ytterbium, or another rare earth dopant. The rare earth doped waveguide 144 is preferably doped with germanium to increase its photosensitivity. Other dopants such as tin and antimony can also be used to induce photosensitivity. If the waveguide 144 is doped with erbium co-doped with ytterbium, then it is preferable to separate at least a portion of the waveguide containing the rare earth dopant from the germanium dopant.
If the waveguide 144 contains erbium doping, or erbium co-doped with ytterbium, then the pump wavelength can contain radiation having at least one wavelength in the wavelength ranges used to pump such lasers, i.e. for erbium/ytterbium from around 915 nm to 980 nm and around 1450 to 1480 nm.
If the waveguide 144 contains erbium, then the laser 146 can be configured to operate in the L-band, and the pump radiation can contain at least one wavelength in the wavelength ranges that are used to pump L-band amplifiers, i.e. wavelengths of around 1450 to 1480 and around 1530 to 1540 nm. This is advantageous because pump radiation at high powers (1 W to 100 W or higher) can be transmitted over long distances (1 km to 100 km) at these pump wavelengths.
Transmitting multiple pump wavelengths can be advantageous as different ones of the lasers 146 can be pumped by different pump wavelengths.
In
If the signal coupler 145 is a taper, then it may be preferable that the signal waveguide 143 and the rare-earth doped waveguide 144 are configured such that the propagation constants of optical radiation propagating along the waveguides 143, 144 are substantially similar within the signal coupler 145.
The signal coupler 145 can also be post processed for example by the application of ultra violet irradiation in order to tune the coupling between the waveguides 143, 144. The ability to tune the coupling ratio, for example by irradiation through the coating, is particularly advantageous for improving manufacturing yield or for reworking or repair.
The signal waveguide 143 is preferably concentric with the optical fibre 142, thus facilitating fusion splicing between a down lead connecting instrumentation with the optical fibre 142.
The optical fibre 142 may be fabricated by drilling a standard single mode preform and inserting a suitably designed rare-earth doped preform rod alongside the core of the standard single mode preform.
If the signal waveguide 143 also serves as a pump waveguide, then it is preferable that the coupling ratio at the pump wavelength is configured such that each of the lasers 146 receives adequate pump radiation in order for the laser to emit laser radiation. The coupling ratio at the pump wavelength may be between 1% and 100%. Preferably the coupling ratio increases along the laser array 140, and it is preferable that the coupling ratio is between 50% and 100% for the last laser 146 in the array 140, and between 1% and 25% for the first laser 146 in the array 140.
The signal coupler 145 should be configured to reduce cross-coupling between the lasers 146. If a taper is utilized, then laser radiation coupled from the signal waveguide 143 into the rare-earth doped waveguide 144 can be advantageously absorbed by the rare earth dopant between the lasers 146. The coupling ratio of the signal coupler 145 at the signal wavelength 147 is advantageously arranged to be small, say from 1% to 50%, or even 0.5% to 10% in order to reduce the loss of signal from the signal waveguide 143 into the rare-earth doped waveguide 144.
The signal coupler 145 can either comprise a single coupler, or multiple couplers arranged such that the desired coupling ratios at the pump wavelength and the signal wavelengths are achieved. The multiple couplers can be separate or combined, for example a superstructure long-period grating in which two long-period gratings are overwritten.
In
The signal coupler 151 is preferably a blazed grating that reflects the laser radiation at the signal wavelength and couples the laser radiation between the rare-earth doped waveguide 144 and the signal waveguide 143. The advantage of using a blazed grating configured to reflect and couple the optical radiation is that it can have a narrower wavelength bandwidth than a long-period grating or a taper, thus reducing reflection for adjacent wavelengths. Unwanted reflections at other signal wavelengths will result in energy coupled from the signal waveguide 143 into the rare earth doped waveguide 144, whereupon such energy will be absorbed by the rare earth doped waveguide 144. The coupling ratio of the signal coupler 151 at the signal wavelength can be 1% to 100%. Preferably the coupling ratio is between 50% and 90%.
Referring to
The schemes shown in FIGS. 14 to 17 have several features that are in common:
It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional components may be provided to enhance performance. In addition, the invention can be considered to be a passive cavity, a laser, an array of lasers, a single longitudinal mode laser, an array of single longitudinal mode lasers, a sensor, a sensor array or a source for a communication system.
The present invention extends to the above-mentioned features taken in isolation or in any combination.
Number | Date | Country | Kind |
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GB0500277.9 | Jan 2005 | GB | national |
The present application claims priority under 35 U.S.C. 119 to United Kingdom (Great Britain) Patent Application Ser. No. GB0500277.9, filed in The United Kingdom on 7 Jan. 2005.