WO2003005121A1

WO2003005121A1 - Optical devices

Info

Publication number: WO2003005121A1
Application number: PCT/AU2002/000873
Authority: WO
Inventors: Simon J Hewlett; Martin Elias; Peter Jacob; Paul Scherl
Original assignee: Aofr Pty Ltd
Priority date: 2001-07-02
Filing date: 2002-07-02
Publication date: 2003-01-16
Also published as: AUPR606501A0

Abstract

An optical device to pass a selected transmission wavelength band(μt) and launch a selected injection wavelength band (μp). The device (1) includes two optical fibres (3,4) optically coupled at two spaced locations (C1,C2). Substantially identical wavelength selective devices (10,11) are written in each fibre (3,4) between the spaced locations (C1,C2) to substantially reflect all light of the selected injection wavelength band (μp) and substantially transmit light of other wavelengths. The optical path lengths of the respective fibres (3,4) between the spaced locations (C1,C2) provide a phase difference of (2n+1)π (where n=0, 1, 2,.....) at a selected intermediate wavelength between the injection wavelength band (μp) and the transmission wavelength band (μt).

Description

OPTICAL DEVICES

Field of the Invention

This invention relates to the injection of multiple discrete wavelength bands into optical fibres. The invention has particular application to Raman amplification in optical transmission fibres. As used in this specification the term "wavelength" or "wavelengths" will be understood as the context requires to refer to a discrete band of wavelengths centred on a particular wavelength.

Background of the Invention

Raman amplification has been recognised as an amplification technique with the potential to enable the transmission of optical signals over very substantial distances without the use of regenerators. Other amplification techniques such as erbium doped fibre amplifiers are available however the Raman amplification technique has advantages in that standard fibre can be used and it also promises improved signal-to-noise ratios.

The Raman amplification technique relies on the launching of pump wavelengths from a suitable light source into the fibre. Generally the pump wavelength band is launched into the transmission fibre in the opposite direction to the transmission direction. This provides for amplification of the transmitted signal in the region it is most attenuated. The wavelength at which gain occurs is dependent upon the fibre material but for silica fibre the gain occurs at a wavelength roughly 100 nm greater than the pump wavelength. The gain bandwidth is about 20 nm so multiple pump wavelengths are required to provide a flat gain profile in a desired wavelength band suitable for WDM (wavelength division multiplexing) applications. This creates a requirement for the introduction of multiple pump wavelengths to the transmission fibre without substantially affecting the transmission wavelength band. Relatively high power pumping is required compared to erbium doped fibre amplifiers, for example, and this can present significant difficulties for the use of micro-optic components. The use of fused fibre components has been proposed for the introduction of pump wavelengths in "Raman components gain some new requirements" by Dr Bob Shine and Dr Jerry Bautista Lightwave, March 2001. Difficulties arise however in concatenating sequential fused optical devices. These difficulties stem from the competing requirements for coupling ratios at the respective pump wavelengths and transmission wavelength bands. Typically there is a significant difference in coupling ratio between the pump wavelengths and transmission wavelengths and this presents difficulties for efficient transmission of the pump wavelength.

Disclosure of the Invention

In a first aspect the present invention provides an optical device to pass a selected transmission wavelength band and launch a selected injection wavelength band, said device including two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1 , 2 ) at a selected intermediate wavelength between the injection wavelength band and the transmission wavelength band.

In a second aspect the present invention provides a method of producing an optical device including the steps of optically coupling two optical fibres at spaced locations; forming substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a selected injection wavelength band and substantially transmit light of other wavelengths, and adjusting the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1, 2 ) at a selected intermediate wavelength between the injection wavelength band and the transmission wavelength band. In the preferred form the invention is used to launch or inject pump wavelengths for Raman amplification.

In one form of the invention the injection wavelength band is launched so as to be counter-propagating to said transmission wavelength band.

In another form of the invention the injection is launched so as to be co- propagating to said transmission wavelength band.

Preferably, the intermediate wavelength between the pump wavelength band and the transmission wavelength band is about half way between these wavelength bands. In a typical transmission system involving transmission at a band centred around 1550 nm and pumping wavelength centred around 1450 nm this corresponds to a phase difference of π at a wavelength of about 1500 nm. By choosing the π phase shift to occur at this midpoint the power transfer in both the transmission wavelength band and injection or pump wavelength band are maximised.

Preferably, the coupling ratios of the spaced apart locations are each 50% at the injection wavelength.

The wavelength selective devices are preferably Bragg gratings written into the optical fibres. The optical device of this invention is effectively a Mach-Zehnder interferometer with a phase shift in one arm. In practice the phase shift is preferably introduced by adjusting the optical path length of the two arms of the interferometer. This can be achieved by passing light of the desired frequency through the device and tuning it by physically altering the optical path length difference between the arms until a null is achieved at a selected port.

In a third aspect the present invention provides a multiple wavelength combiner to pass a selected transmission wavelength band and launch selected injection wavelength bands, said combiner including a first optical device having two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a first selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1 , 2 ) at a first selected intermediate wavelength between the first injection wavelength band and the transmission wavelength band, and a serially connected second optical device having two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a second selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1 , 2, ) at a second selected intermediate wavelength between the second injection wavelength band and the transmission wavelength band, whereby said transmission wavelength band sequentially passes through said first and second optical devices, said first injection band is launched from said first optical device and said second injection band is launched from said second optical device.

It will be apparent that in this form the invention uses a series of devices that can be connected sequentially to provide for the launching of each of the required pump wavelengths to produce the desired gain profile. In the preferred form of the invention a relatively flat gain spectrum is produced across the transmission range of 1530 to 1570 nm.

In one form of the invention at least one of the first injection wavelength band and the second injection wavelength band are launched so as to be counter propagating to the transmission wavelength band. Preferably, both the first injection wavelength band and the second injection wavelength band are launched so as to be counter propagating to the transmission wavelength band.

In another form of the invention at least one of the first injection wavelength band and the second injection wavelength band are launched so as to be co- propagating to the transmission wavelength band. Preferably, both the first injection wavelength band and the second injection wavelength band are launched so as to be co-propagating to the transmission wavelength band.

Any suitable optical fibre can be used to form the optical device of the invention.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic diagram of an optical device according to the present invention;

Figure 2 is a schematic diagram of two of the optical devices shown in Figure 1 sequentially connected to form a multiple wavelength combiner;

Figure 3 is a graph of transmission power versus wavelength for ports of an optical device according to the present invention;

Figure 4 is a schematic diagram of two sequentially connected comparative devices;

Figure 5 is a graph of transmission power versus wavelength for ports of the device of Figure 4;

Figure 6 shows graphs of coupling ratio and excess loss versus wavelength for a coupler forming part of an optical device according to the present invention;

Figure 7 shows graphs of insertion loss versus wavelength for signal input to signal output (upper line) and signal input to signal null (lower line) for an example optical device according to the present invention;

Figure 8 shows graphs of insertion loss versus wavelength for pump input to pump output (upper line) and pump input to pump input (lower line) for an example optical device according to the present invention; and Figure 9 shows graphs similar to Figure 7 for a modified optical device according to the present invention.

Best Modes for Carrying Out the Invention

Figure 1 shows the optical device 1 according to this invention. The device is formed by two lengths of standard communications grade optical fibre 2, 3 coupled at spaced apart locations 4, 5 by means of fused tapered couplers C1 , C2 of known type. The optical device thus has four ports 6, 7, 8, 9 being the ends of fibres 2, 3. Identical Bragg gratings 10, 11 are formed in each of the optical fibres 2, 3 between the couplers C1 , C2.

The Bragg gratings 10, 11 are formed using known techniques such as by the selective irradiation of the fibre material using ultraviolet light to change the refractive index of the fibre. This can be effectively achieved using standard telecommunications grade fibre although in some instances a pretreatment with high pressure hydrogen is used to improve the effectiveness of the UV irradiation. The Bragg gratings can be written into the fibre to provide for reflection of light of a selected wavelength. Gratings as fine as 0.02 nm and as coarse as 150 nm have been formed in optical fibres. These gratings readily provide for reflection of light having a wavelength as low as 400 nm to the upper wavelength transmission limit for optical fibres. By using a grating of composite spacing a band of selected wavelengths can be reflected by the Bragg grating. In practice very sharply defined reflection characteristics can be achieved and a discrimination of 27 dB over 0.2 nm has been found to be possible. The Bragg gratings 10, 11 are formed such that the gratings reflect light over a wavelength band corresponding to a selected pump wavelength λ_p. Light of other wavelengths is transmitted by the gratings.

A phase change schematically indicated by the component 12 is introduced in one of the transmission paths. In practice this is achieved by adjusting the respective optical path lengths of the two fibres 2, 3 between couplers C1 , C2 so as to introduce a phase change of π at a selected wavelength. The phase change is set after production of the device by tuning as will be described below.

Port 7 of the device 1 is the input port for the transmission signal λ_t and the output port for the pump wavelength λ_p. It will be apparent that light entering port 7 will be split by coupler C1 into two components respectively in the two fibre arms 2, 3 according to the coupling ratio CR1 of the coupler. Light in the arm 2 will be transmitted through the Bragg grating 10 in that arm since the Bragg grating has a reflection characteristic determined for the pump wavelength λ_p. A π phase shift is introduced before the light reaches coupler C2. At coupler C2 an amount of light determined by the coupling ratio CR2 will be coupled to port 8. Similarly the light coupled to fibre 3 by coupler C1 will be transmitted through the Bragg grating 11 but will have undergone a phase shift of π/2 at the coupler. On passing through coupler C2 a component will be coupled back to fibre 2 according to the coupling ratio CR2 with a π/2 phase shift. The light emerging at port 8 will thus be the combination of the light not coupled out of fibre 2 at coupler C2 and the light coupled into fibre 2 at coupler C2. Both of these components will have undergone a π phase shift and therefore add because they are in phase. The light at port 9 will be the combination of the light not coupled out of fibre 3 at coupler C2 and the light coupled from fibre 2 at C2. These two components will be out of phase by π and will thus tend to cancel.

Pump wavelength light λ_p provided to port 6 will have a component coupled into fibre 2 at coupler C1 in accordance with the coupling ratio CR1. The component remaining in fibre 3 and the component coupled to fibre 2 will be reflected back to coupler C1 by the respective Bragg gratings 10,11. On reaching the coupler C1 components will be respectively coupled to the other fibre in accordance with the coupling ratio CR1. Since at each coupling to the other fibre a phase change of π/2 is introduced, the components returning to port 6 will tend to cancel because they are out of phase. The components emerging at port 7 are in phase and thus add. Optical isolators may be required at port 6 of the device 1 to prevent transmission of light to the laser or other device supplying the λ_p wavelength light. It will be apparent that this provides a way in which the pump wavelength λ_p can be introduced in the reverse direction into an optical fibre connected to port 7. It will also be apparent that in the alternative the pump wavelength λ_p can be provided to the port 9 rather than port 6. This will result in the pump wavelength λ_p being co-propagated with the transmission signal λ_tfrom port 8.

In order to launch a further pump wavelength it is necessary to connect a further device V to form a multiple wavelength combiner. This is schematically shown in Figure 2 where the corresponding integers are identified with the same reference numerals denoted with a prime symbol. In this arrangement port 7' of a second device 1' is connected to port 8 of a first device 1. The second device 1' works in the same manner as described above except that the Bragg gratings 10', 11' are written to reflect a narrow wavelength band λ_q and the second pump wavelength λ_q is introduced at port 6'.

The second pump wavelength λ_q is thus introduced to port 8 of the first device 1. A component is coupled to fibre 3 by coupler C2. The component remaining in fibre 2 undergoes a π phase shift before passing through Bragg grating 10 which is only reflective at the first pump wavelength λ_p. Similarly the component in fibre 3 passes through Bragg grating 11 to coupler C1. At coupler C1 the respective components again have a component coupled to the other fibre. Each of the coupled components undergo a π/2 phase shift. As a result the components at port 6 are out of phase and tend to cancel. The components at port 7 are in phase and thus add.

The coupling ratios CR1 and CR2 of each device are designed to be 50% at the respective pump wavelength λ_p. That is the various concatenated devices required for a number of pump wavelengths have their coupling ratios chosen to be 50% at the respective pump wavelengths. The configuration of the multiple wavelength combiner according to this invention provides for an efficient transmission of the second pump wavelength λ_q through the first device. This is represented by the difference between the amount of power transferred to port 7 in comparison to port 6 sometimes referred to as the "margin". Figure 3 shows a plot of transmission power in dB versus wavelength for the following coupling ratios:

CR1 @ 1450 nm = 50% CR1 @ 1550 nm = 59.2%

CR2 @ 1450 nm = 50% CR2 @ 1550 nm = 59.2%

An optical path length difference of 750 nm between the Mach-Zehnder interferometer arms results in a π phase change at 1500 nm. The desired transmission band wavelength λt is centred on about 1550 nm. Thus the pump wavelengths of interest are centred on about 1450 nm. The graph thus shows the transmission of ports 6 and 7 for the range 1400 nm to 1600 nm. It will be apparent that a second pump wavelength λ_q travelling through the first device 1 can have a margin in excess of 20 dB. The same characteristics can be obtained for the transmission of further pump wavelengths from further devices sequentially connected to introduce desired pump wavelengths in the counter propagating direction. In the alternative the pump wavelengths λ_p, λ_q can be provided to ports 9, 9' to provide co-propagating pump wavelengths from ports 8 and 8'.

For comparative purposes Figure 4 shows a sequential connection of two standard optical fibre Mach-Zehnder interferometers to introduce pump wavelengths λ_p, λ_q respectively. The components are the same as those described above in relation to Figures 1 and 2 and corresponding parts have been labelled with corresponding reference numerals. It will be noted however that the absence of the π phase change in one of the fibres results in the output of the transmission band and thus the input for the sequential pump band being via port 9. This introduces difficulties for the choice of coupling ratios since the balance required to provide for efficient transmission of the transmission wavelength band results in less efficient transmission of the pump band in the opposite direction. By way of illustration Figure 5 shows a plot of transmission power against wavelength of port 6 and port 7 of the device shown in Figure 4. The coupling ratios are as follows:

CR1 @ 1450 nm = 50% CR1 @ 1550 nm = 59.2% CR2 @ 1450 nm = 31.7% CR2 @ 1550 nm = 40.8%

and are designed to provide 100% transmission at 1550 nm.

It will be seen that the second pump wavelength band λ_q achieves a considerably lower margin than that achieved by the arrangement of the present invention as shown in Figure 3.

Example 1

A wavelength selective device according to the invention was fabricated in the manner described above with reference to Figure 1. Two lengths of Corning SMF- 28 standard communications fibre were used. Couplers C1 and C2 were formed as fused tapered couplers using known techniques. Both couplers were designed to have a 50% coupling ratio at wavelengths close to a grating central wavelength of 1468 nm chosen for the Bragg gratings written into the fibres as described below. The excess loss of a single coupler typically reaches about 0.1 dB between 1450 to 1600 nm. The wavelength dependence of the coupling ratio and excess loss for each of the couplers is shown in Figure 6.

Bragg gratings were written into each of the fibres by first increasing the photosensitivity of the SMF-28 fibre by soaking the fibre in a high pressure, high temperature hydrogen atmosphere for several hours prior to grating fabrication. Gratings then were written by exposing the fibre to 248 nm radiation from an excimer laser using a phase mask technique as described by K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson and J. Albert in "Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask", Applied Physics Letters, Vol. 62, 1035-1037, 1993. A chirped phase mask was used to generate a grating centred on 1468 nm with a -3 dB bandwidth of 4.6 nm.

The optical device was tuned to introduce the necessary phase difference for optimum performance by exposing one of the fibres in the arms to a uniform UV beam to change the optical path length difference in the two arms. The optimum optical path length difference is determined by passing light of a desired intermediate transmission wavelength (in this case 1510 nm) through the device via signal input port 7 and adjusting for the minimum transmitted signal on output port 9.

The insertion loss versus wavelength spectra for the device were measured and are shown in Figures 7 and 8. It will be appreciated from Figure 7 that the attenuation of the transmitted signal from input port 7 to output port 8 is modest across the desired transmission band centred around 1550 nm. The noticeable loss seen on the low wavelength side of the grating transmission dip in Figure 7 is the result of cladding mode coupling by the grating and is characteristic of gratings written in Corning SMF-28 fibre. In general such cladding mode loss can be reduced or avoided where necessary by using a cladding mode suppressed specialty fibre.

Figure 8 shows in the upper curve the insertion loss for injection or pump wavelength input to port 6 as transmitted to output port 7. This shows that the loss of the pump input is modest across a relatively narrow wavelength band centred on 1468 nm. The slope across the top of the peak in the upper line is due to the hydrogen loading technique used to increase the photosensitivity of the Corning SMF-28 fibre. This slope can be avoided by using deuterium in place of hydrogen or by using photosensitive fibre that does not require hydrogen loading.

The lower curve in Figure 8 shows the amount of injection or pump power reflected to the pump input port 6.

The optical device as described above was tested for power handling using a 3 watt Raman laser at 1480 nm. The insertion loss (signal input at port 7 to signal output at port 8) of this device was 0.4 dB. There was no degradation of output power observed after 40 hours of testing with 3 watts of input power.

Environment testing conducted on the wavelength selective device described above included the following:

(1) four temperature cycles from -20 to 80°C; (2) a two day damp heat soak (85°C, 85% relative humidity); and

(3) four additional temperature cycles from -20 to 80°C.

There was no observable insertion loss performance change after these tests.

The device used for the testing was not athermalised but various temperature compensation packages or techniques of known type are available when demanded by the application. Without temperature compensation the grating wavelength shifts with temperature by approximately 0.01 nm/°C.

Example 2

A wavelength selective device as described in Example 1 was fabricated except that the tuning procedure was modified so that the π phase shift occurred at a wavelength below the mid point of the injection or pump wavelength and transmission wavelength band. This increases the margin in the pump band and decreases the margin in the signal band so that a selected amount of signal power is transmitted to port 9 (see Figure 1) to be used a port monitor. The wavelength selective device of this Example was trimmed to provide a 2% tap at 1550 nm on port 9 of the device. The insertion loss versus wavelength graph for this device is shown in Figure 9.

The foregoing describes only one embodiment of the present invention and modifications can be made without departing from the scope of the invention.

Claims

CLAIMS:

1. An optical device to pass a selected transmission wavelength band and launch a selected injection wavelength band, said device including two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1, 2 ) at a selected intermediate wavelength between the injection wavelength band and the transmission wavelength band.

2. An optical device as claimed in claim 1 wherein said injection wavelength band is launched so as to be counter propagating to said transmission wavelength band.

3. An optical device as claimed in claim 1 wherein said injection wavelength band is launched so as to be co-propagating to said transmission wavelength band.

4. An optical device as claimed in claim 1 wherein said intermediate wavelength is about half way between said injection wavelength band and said transmission wavelength band.

5. An optical device as claimed in claim 4 wherein said injection wavelength band is centred on about 1450 nm, said transmission wavelength is centred on about 1550 nm, and said intermediate wavelength is about 1500 nm.

6. An optical device as claimed in claim 2 wherein said device includes a monitoring port and said intermediate wavelength is selected to provide a predetermined output of said transmission wavelength band at said monitoring port.

7. An optical device as claimed in claim 6 wherein said predetermined output is about 2% of the transmission wavelength band passed by the device.

8. An optical device as claimed in any one of claims 1 to 7 wherein the optical coupling at said spaced locations each has a coupling ratio of about 50% over the injection wavelength band.

9. An optical device as claimed in any one of claims 1 to 8 wherein said wavelength selective devices are Bragg gratings written into each of said optical fibres.

10. A multiple wavelength combiner to pass a selected transmission wavelength band and launch selected injection wavelength bands, said combiner including a first optical device having two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a first selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1, 2 ) at a first selected intermediate wavelength between the first injection wavelength band and the transmission wavelength band, and a serially connected second optical device having two optical fibres optically coupled at two spaced locations, substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a second selected injection wavelength band and substantially transmit light of other wavelengths, the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1, 2, ) at a second selected intermediate wavelength between the second injection wavelength band and the transmission wavelength band, whereby said transmission wavelength band sequentially passes through said first and second optical devices, said first injection band is launched from said first optical device and said second injection band is launched from said second optical device.

11. A multiple wavelength combiner as claimed in claim 10 wherein at least one of said first injection wavelength band and said second injection wavelength band are launched so as to be counter propagating to said transmission wavelength band.

12. A multiple wavelength combiner as claimed in claim 11 wherein both said first injection wavelength band and said second injection wavelength band are launched so as to be counter propagating to said transmission wavelength band.

13. A multiple wavelength combiner as claimed in claim 10 wherein at least one of said first injection wavelength band and said second injection wavelength band are launched so as to be co-propagating to said transmission wavelength band.

14. A multiple wavelength combiner as claimed in claim 13 wherein both said first injection wavelength band and said second injection wavelength band are launched so as to be co-propagating to said transmission wavelength band.

15. A multiple wavelength combiner as claimed in claim 10 wherein said first intermediate wavelength is about half way between said first injection wavelength band and said transmission wavelength band; and said second intermediate wavelength is about half way between said second injection wavelength band and said transmission wavelength band.

16. A multiple wavelength combiner as claimed in any one of claims 10 to 15 wherein the optical coupling at said spaced location in said first and second optical devices each has a coupling ratio of about 50% over the corresponding first and second injection wavelength bands.

17. A multiple wavelength combiner as claimed in any one of claims 10 to 16 wherein said wavelength selective devices are Bragg gratings written into said optical fibres.

18. A method of producing an optical device including the steps of optically coupling two optical fibres at spaced locations; forming substantially identical wavelength selective devices in each fibre between said spaced locations to substantially reflect all light of a selected injection wavelength band and substantially transmit light of other wavelengths, and adjusting the optical path lengths of the respective fibres between said spaced locations providing a phase difference of (2n+1)π (where n=0, 1, 2 ) at a selected intermediate wavelength between the injection wavelength band and the transmission wavelength band.

19. A method as claimed in claim 18 wherein said optical coupling includes the step of forming a fused tapered coupling.

20. A method as claimed in claim 18 or claim 19 wherein said intermediate wavelength is about half way between said injection wavelength band and said transmission wavelength band.

21. A method as claimed in claim 20 wherein said injection wavelength band is centred on about 1450 nm, said transmission wavelength is centred on about 1550 nm, and said intermediate wavelength is about 1500 nm.

22. A method as claimed in any one of claims 18 to 21 wherein the optical coupling at said spaced locations each has a coupling ratio of about 50% over the injection wavelength band.

23. A method as claimed in any one of claims 18 to 22 wherein said wavelength selective devices are formed by writing Bragg gratings into each of said optical fibres.