GB2380313A

GB2380313A - Raman Device

Info

Publication number: GB2380313A
Application number: GB0123306A
Authority: GB
Inventors: Keith John Dowling; Hayden Scott Fews
Original assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-09-28
Filing date: 2001-09-28
Publication date: 2003-04-02
Also published as: GB0123306D0

Abstract

A Raman device has a length of optical fibre, means 102 for introducing pump radiation 101 into the fibre, the fibre having a first optical cavity defined by first and second spatially separate reflectors 104, 115 thereby forming a fibre laser. The device further comprises a plurality of pairs of spatially separate grating pairs 120,121,130,131 which pairs of gratings define an optical cavity for a pre-determined wavelength. The pairs of gratings being arranged in the fibre such that, in use, stimulated Raman scattering occurs, wherein at least some of the Raman scattering occurs within the fibre laser cavity. As an alternative to grating pairs, inter-coupled wavelength division multiplex coupler feedback means may be used.

Description

Raman Device The invention relates to a Raman device, in particular, but not exclusively a pump source for a distributed Raman amplifier.

Stimulated Raman scattering (SRS) in optical fibre is a non-linear optical effect wherein at a certain light power threshold the existing photonic energy is annihilated to create an optical phonon and a lower frequency (longer wavelength) Stokes photon. For sufficiently high optical powers the process becomes stimulated and optical gain is generated at the outgoing photon frequency. This process is well documented in the literature such as in chapter 6 of"Fiber-Optic Communications Technology", Djafar K. Mynbaev and Lowell L. Scheiner, Prentice Hall, ISBN 0-13-962069-9.

Optical communication matters are usually referred to in terms of the light wavelength as this is a more appropriate dimension for signals at this part of the electro-magnetic spectrum. However, the Stokes effect is usually referred to in terms of frequency. The C and L band communication wavelengths correspond approximately to frequencies in the 190-210THz region of the spectrum.

The Stokes frequency shift is constant in a given material e. g. 13. 2THz in Silica glass, so the gain always appears at this frequency shift away from that of the incoming pump photon frequency. Therefore, changing the wavelength of the pump varies the gain peak of a Raman amplifier. Stimulated Raman scattering exhibits the characteristic of self-phase matching between the Stokes photons and any existing signal at this lower frequency. Through this process SRS may be used to provide optical gain to optical communication signals. This finds particular application with wavelength division multiplexed (WDM) systems.

The pump powers required for Raman amplifiers in communication systems are relatively high, typically of the order of 0. 5-1. 5 W. It is non trivial to get these high optical powers constrained in the very small cores of optical fibres such as used with communication systems. Typically communication fibres are optimised for single mode propagation.

One method of generating high power radiation in a single mode optical fibre is to pump rare-earth doped optical fibre with high power laser diodes. This produces the required power in the single mode fibre. However, to generate Raman gain in the C or L communication bands (1550nm) the pump wavelength needs to be of the order of 1400-1500nm. These wavelengths are for illustrative purposes only in order that an understanding of the process may be gained.

A Ytterbium-doped (Yb-doped) fibre laser can be made to lase efficiently at about HOOnm, but it is very difficult to get a rare-earth fibre laser for the required longer wavelengths. One way of shifting the high power single-mode output from the Ybdoped laser is to use a Raman cascade.

In a Raman cascade, the high power input is launched into a long length of optical fibre and generates Stokes shifted Raman radiation. This radiation is trapped in the fibre by wavelength selective reflectors placed at either end of the length of Raman shifting fibre. This forms a Raman fibre laser, as there is a cavity (the fibre bounded by the reflectors) with gain at the Stokes shifted frequency. The trapped radiation builds up to a high intensity inside the cavity, which can then generate its own Raman shifted gain. This process is repeated as each subsequent Stokes shifted frequency is trapped in its own cavity, until the desired wavelength is achieved. For example, Cband Raman amplification requires a 1450nm pump wavelength, which can be obtained via five Stokes frequency shifts from the HOOnm Yb-doped Bbre laser output.

Such a device is known from US5323404,"Optical fibre laser or amplifier including high reflectivity gratings", which discloses an amplifier having in series a multi-mode pump source, a laser, and a Raman cascade with the feedback provided by fibre Bragg gratings. It is also possible to achieve a similar effect using a plurality of wavelength couplers in place of the fibre Bragg gratings to generate the feedback. Such a device is disclosed in W096/37936.

The known devices suffer from the problem that they are quite inefficient. In a typical device a multi-mode diode pump source can output 10W at say 1100nu,

whereas the cascade output at 1450nm will be only about 0. 5W i. e. 5% of the input power.

The present invention seeks to provide a Raman amplifier pump source having higher wavelength conversion efficiency and a higher output power at the desired wavelength than the known devices. The device can also be used more generally as a wavelength shifter.

According to a first aspect of the invention there is provided a Raman device comprising a length of optical fibre, means for introducing pump radiation into the fibre, the fibre having a first optical cavity defined by first and second spatially separate reflectors thereby forming a fibre laser, the device further comprising a plurality of pairs of spatially separate reflector means, which pairs of reflector means define an optical cavity for a pre-determined wavelength, the pairs of reflector means being arranged in the fibre such that, in use, stimulated Raman scattering occurs, wherein at least some of the Raman scattering occurs within the fibre laser cavity.

Preferably the pairs of means comprise pairs of reflectors and one of which reflectors may be arranged in said fibre laser cavity. Preferably, the fibre laser cavity comprises at least in part a rare-earth-doped double-clad fibre. Preferably a plurality of one reflectors of said plurality of pairs of reflectors is located in said fibre laser cavity.

Preferably, said plurality of pairs of reflectors is located in the fibre laser cavity. Preferably, the second spatially separate reflector defining the fibre laser cavity is located intermediate to the reflectors of at least one said plurality of pairs of reflectors.

Preferably, said reflectors comprise fibre Bragg gratings.

According to a second aspect of the invention there is a Raman device comprising a length of optical fibre, means for introducing pump radiation into the fibre, the fibre having a first optical cavity defined by first and second spatially separate reflectors thereby forming a fibre laser, which optical cavity, in use, forms a laser cavity, the device further comprising a plurality of pairs of spatially separate inter-coupled wavelength division multiplex coupler feedback means, which pairs of means define an optical cavity for a pre-determined wavelength, the pairs of means being arranged

in the fibre such that, in use, stimulated Raman scattering occurs, wherein at least some of the Raman scattering occurs within the fibre laser cavity.

The device advantageously includes the double-clad fibre laser and Raman cascade in one device by bringing the Raman cascade inside of the laser cavity. This permits the reflecting means count to be reduced by one over the known devices. More importantly, the light intensity is always higher inside a laser cavity than outside, and the efficiency of the Raman cascade is intensity dependent, so placing the cascade inside the fibre laser cavity increases the conversion efficiency of the device as a whole.

An exemplary embodiment of the invention will now be described in greater detail with reference to the drawings in which Fig. 1 shows a schematic representation of a known device; Fig. 2 shows a schematic representation of the device of the invention; Fig. 3 shows a schematic representation of a further embodiment; Fig. 4 shows a comparative graph of power output.

Figure 1 shows a schematic representation of a known Raman pump source. In use, pump radiation 12 from a multi-mode pump source 11 is coupled into a double-clad rare-earth-doped optical fibre 13 such as described in US5566196. The pump radiation will have a first wavelength, ko, which in a typical case where the pump source is a GaAs laser diode, or array of laser diodes, will be 980nm. The multi-mode pump radiation is coupled both into the single mode core and the multi-mode cladding of the fibre.

The double-clad rare-earth-doped optical fibre 13 is bounded by in-line refractive gratings 14, 15, which gratings form a matched reflector pair having a centre wavelength kp, which will typically be around 1 loom. The refractive grating 14 has a high reflectivity, ideally greater than 98%, whereas the refractive grating 15 is lower at around 50% and therefore functions as an output coupler for the laser optical cavity formed by the gratings.

The fibre 13 uses a single-mode guide for the data signal surrounded by a concentric multi-mode pump guide. The single-mode core is doped with rare-earth-ions to provide the desired gain medium. The glass cladding surrounding the core is itself surrounded by a low index polymer second cladding which allows it to become a guiding structure. In use, pump light is launched into the fibre into the undoped cladding, where it propagates in multi-mode fashion interacting with the doped core as it travels along the fibre. Therefore, in use, the cavity formed by the gratings 14 and 15 forms a laser in a single-mode fibre with a suitable power output.

However, to produce a useful optical component it is necessary to wavelength shift the output of the fibre laser to a desired wavelength, which in the case of a Raman amplifier is 1400-1500mn for C and L-band devices. One way to achieve this is to use a Raman cascade.

The output of the rare-earth-doped optical fibre laser 13 is coupled into a single-mode Raman cascade fibre 16. The Raman cascade is provided with in-line fibre Bragg gratings 20,21, 30,31, 40,41 etc. , where the gratings 20 and 21,30 and 31 and 40 and 41 etc., form matched reflector pairs. The wavelength of the grating pairs will correspond to the wavelength of an order cascade and the gratings should generally be of the highest reflectivity possible. The first order cascade corresponding to gratings 20 and 21, in the exemplary embodiment, is at 1156nm. Although in the exemplary embodiment, the grating pairs are shown with the first order grating pair 20,21 adjacent to the opposite ends of the cascade fibre, it is possible to place the gratings in other orders, which will affect the output characteristics of the device.

Intermediate to the gratings 20,21 at the end of the cascade fibre remote from the rare-earth-doped fibre 13, is a further in line fibre Bragg grating 17 having a wavelength % p provided to reflect the output of the laser optical cavity back through the Raman cascade fibre.

The outermost pair of gratings 40,41 comprise the nth order, which is the desired output wavelength of the device, which will be around 1450nm for Raman

amplification. In this case, the grating 41 remote from the laser will be a second output coupler with a reflectivity significantly less than 100%, and typically 50%.

Alternatively the pump source radiation 12 may be coupled into single-clad rareearth-doped fibre bounded by gratings 14 and 15 to form a fibre laser operating at the exemplary 11 OOnm Figure 2 shows a schematic representation of the device of the invention. In use, pump radiation 102 from a multi-mode pump source 101 is coupled into a double-clad rare-earth-doped optical fibre 103. The pump radiation will have a first wavelength, Xo, which in a typical case where the pump source is a GaAs laser diode, or array of laser diodes, will be 980nm. The double-clad rare-earth-doped optical fibre 103 is provided with an in-line grating 104 having a centre wavelength of Xp at the end adjacent to the pump source 101. The grating 104 has a high reflectivity, ideally greater than 98%.

The end of the double-clad rare-earth-doped optical fibre 103 remote from the pump source 101 is coupled to a Raman cascade 105, which is provided with in-line fibre Bragg gratings 120,121, 130,131, 140,141 etc. , where the gratings 120 and 121,130 and 131 and 140 and 141 form matched reflector pairs. The wavelength of the grating pairs will correspond to the wavelength of an order cascade and the gratings should generally be of the highest reflectivity possible. The first order cascade corresponding to gratings 120 and 121, in the exemplary embodiment, is at 1156nm. Although in the exemplary embodiment, the grating pairs are shown with the first order grating pair 120,121 adjacent to the opposite ends of the cascade fibre, it is possible to place the gratings in other orders, which will affect the output characteristics of the device. The grating 141 further remote from the gain fibre 103 is an output coupler with a reflectivity significantly less than 100%, and typically 50%.

A further in-line grating 115 is provided at the output of the Raman cascade fibre remote from the gain fibre section 103, which grating has a wavelength of, su thereby forming a matched reflector pair with grating 104. The grating 115 should have a

high reflectivity, again preferably of the order of 98% or higher. The matched grating pair 104,115 therefore define, in use, a fibre laser cavity.

Alternatively the pump source 101 may be coupled into single-clad rare-earth-doped fibre bounded by gratings04 and 115 to form a fibre laser operating at the exemplary HOOnm.

The effect of the change besides reducing the reflector count is that the efficiency of the Raman cascade is increased. As the efficiency of wavelength conversion is intensity dependent in a Raman cascade, by incorporating the Raman cascade into the laser cavity, the power in the cascade is greater and hence it performs more efficiently.

In a further embodiment, which is schematically illustrated in Figure 3, the device comprises a multi-mode pump source 101 which is coupled into a double-clad rareearth-doped optical fibre 103, thereby coupling pump radiation 102 into the fibre 103.

The pump radiation will have a first wavelength, ko, which in a typical case where the pump source is a GaAs laser diode, or laser diode array, will be 980nm. The doubleclad rare-earth-doped optical fibre 103 is provided with an in-line grating 104 having a centre wavelength of Ap at the end adjacent to the pump source 101. The grating 104 has a high reflectivity, ideally greater than 98%. Again the laser cavity is defined by the second grating 115 at the end of the fibre remote from the gain section.

As with the embodiment of Figure 2, the end of the double-clad rare-earth-doped optical fibre 103 remote from the pump source 101 is coupled to a Raman cascade 105, which is provided with wavelength division multiplexed (WDM) fused fibre couplers 220,221, 230,231, 240,241 etc, where the couplers 220 and 221,230 and 231, and 240 and 241 etc. form matched coupler pairs and are joined by a fibre 222, 232, and 242 etc. respectively. The wavelength of the coupler pairs will correspond to the wavelength of an order cascade. The WDM fused fibre couplers comprise respective fibres brought closely together, and are fused in a position adjacent to one another, so that optical coupling occurs at the selected cascade wavelength, thereby forming a feedback path via respective fibre 222,232, 242 etc. The feedback path

ensures the cascade wavelength re-enters the Raman cascade 105. The first order cascade corresponding to wavelength division multiplexed (WDM) fused fibre couplers 220,221 and fibre 222 is at 1156nm. Although in the exemplary embodiment, these feedback means are shown with the first order cascade wavelength feedback 220,221, 222 adjacent to the opposite ends of the cascade fibre, it is possible to place the feedback means in other orders, which will affect the output characteristics of the device. The re-entrant photonic energy is then further wavelength translated until such time as there is no further feedback via the wavelength division multiplexed coupler assemblies and the translated energy out couples via grating 115.

Alternative feedback, or reflection means, within the Raman device described, may be used to achieve equivalent functionality within the spirit of the invention e. g. fibre loop mirrors.

Besides the reduction in the number of reflectors and hence slightly reduced manufacturing cost, the device according to the invention performs more efficiently than the prior art devices. Figure 4 shows a graph of the comparative output power against laser diode pump power between an intra-cavity Raman shifter according to the invention and a prior art device according to Figure 1.

As the graph shows both the device according to the invention and the prior art device have a lasing threshold at around 4W of diode pump power with a slightly higher laser threshold for the intra cavity cascade brought about by the increased loss of the laser cavity incorporating both the rare-earth-doped fibre and the Raman cascade fibre.

Additionally, the slope efficiency (the gradient of the curves after the laser threshold) of the intra cavity device is significantly higher than the prior art device.

The invention finds, amongst others, application with WDM and DWDM systems wherin the Raman pump laser source feeds a Raman amplifier stimulated with communications data at the target Raman amplification wavelength.

Claims

1. A Raman device comprising a length of optical fibre, means for introducing pump radiation into the fibre, the fibre having a first optical cavity defined by first and second spatially separate reflectors thereby forming a fibre laser, the device further comprising a plurality of pairs of spatially separate reflector means, which pairs of reflector means define an optical cavity for a pre-determined wavelength, the pairs of reflector means being arranged in the fibre such that, in use, stimulated Raman scattering occurs, wherein at least some of the Raman scattering occurs within the fibre laser cavity.

2. A Raman device according to Claim 1, wherein one of which pairs of reflector means is arranged in said fibre laser cavity.

3. A Raman device according to Claim 1 or Claim 2, wherein the fibre laser cavity comprises at least in part a rare-earth-doped double-clad fibre.

4. A Raman device according to Claim 1 or Claim 2, wherein the fibre laser cavity comprises at least in part a rare-earth-doped single-clad fibre.

5. A Raman device according to any one of Claims 2 to 4, wherein a plurality of one reflectors of said plurality of pairs of reflectors is located in said fibre laser cavity.

6. A Raman device according to any one of Claims 2 to 4, wherein said plurality of pairs of reflectors is located in the fibre laser cavity.

7. A Raman device according to any one of Claims 1 to 6, wherein the second spatially separate reflector defining the fibre laser cavity is located intermediate to the reflectors of at least one said plurality of pairs of reflectors.

8. A Raman device according to any one of Claims 1 to 7, wherein said reflectors comprise fibre Bragg gratings.

9. A Raman device comprising a length of optical fibre, means for introducing pump radiation into the fibre, the fibre having a first optical cavity defined by first and second spatially separate reflectors thereby forming a fibre laser, which optical cavity, in use, forms a laser cavity, the device further comprising a plurality of pairs of spatially separate inter-coupled wavelength division multiplex coupler feedback means, which pairs of coupler feedback means define an optical cavity for a predetermined wavelength, the pairs of coupler feedback means being arranged in the fibre such that, in use, stimulated Raman scattering occurs, wherein at least some of the Raman scattering occurs within the fibre laser cavity.

10. A Raman device according to Claim 9, wherein the fibre laser cavity comprises at least in part a rare-earth-doped double-clad fibre.

11. A Raman device according to Claim 9, wherein the fibre laser cavity comprises at least in part a rare-earth-doped single-clad fibre.

12. A Raman device according to any one of Claims 9 to 12, wherein a plurality of the coupler feedback means are located in said fibre laser cavity.