GB2366447A

GB2366447A - Apparatus for amplifying a signal

Info

Publication number: GB2366447A
Application number: GB0122822A
Authority: GB
Inventors: Johan Samsung Nilson; David Colin Hanna; John Douglas Minelly; Ruediger Eberhard Paschotta
Original assignee: Southampton Photonics Ltd
Current assignee: Trumpf Laser UK Ltd
Priority date: 1996-12-04
Filing date: 1997-12-04
Publication date: 2002-03-06
Anticipated expiration: 2017-12-04
Also published as: GB2335074B; GB2366447B; GB0122822D0; GB9912603D0; GB2335074A

Abstract

Apparatus for amplifying a signal beam having a normalised intensity distribution (Figure 2) comprises, an optical waveguide with a core 30, and a first cladding 10 surrounding core. There is at least one pump beam source configured to supply optical pumping. The gain medium 20 is configured to be pumped by the optical beam, and the gain medium 20 is situated in a region of the optical waveguide where the intensity of signal beam is smaller than its peak intensity. The apparatus is configured to have an effective area ratio of between one and 10. The gain medium 20 may be doped with Ytterbium to produce an amplified wavelength in the range from 950 nm to 1050 nm. The gain medium 20 may be doped with erbium to produce an amplified wavelength in the range from 1450 nm to 1600 nm. The gain medium 20 may be doped with Nd to produce an amplified wavelength in the range 850 nm to 950 nm. The first cladding 10 which may guide the optical pump beam can be surrounded by a second cladding 210 having a lower refractive index in the first cladding.

Description

2366447 OPTICAL AWLIFIER AND LIGHT SOURCE The invention relates to optical

amplifiers and light sources. By way of example, though not exclusively, the invention relates to single- or few- moded waveguiding lasers, 5 superfluorescent sources, optical amplifiers, high pulse-energy devices, energy-storage devices, cladding-pumped devices, and semiconductor signal amplifier& The tightly confined modal fields of single- or few-moded waveguiding lasers, superfluorescent sources, and amplifiers lead to a very strong interaction between any waveguided light and the active medium in the waveguiding core. Therefore, a comparatively io small amount of gain medium is sufficient for providing the gain in these devices. Specifically, the gain for a given stored energy, as well as for a gi ven absorbed pump power, is high..'This is often beneficial, since it means that the pump power requirements for a given desired laser output power or amplifier gain can be low.

However, for several devices, this efficient interaction between mode and gain medium 15 can be detrimental. The following example refers to certain types of amplifiers and lasers, but of course the skilled man will realise that the same or similar problems can occur in, for example, superfluorescent sources.

In a laser or amplifier, the achievable single-pass gain is limited to, say, 50 dB. The reason is that at this gain, a significant fraction of the pump power is converted to amplified 20 spontaneous emission (ASE). A 10 dB higher gain results in approximately 10 dB more ASE, so at these gains, the extra pump power required to increase the gain further will be prohibitively high. Since the ASE limits the gain of the device, it also limits the energy stored in the gain medium. This in turn obviously limits the amount of energy that a pulse can extract from the device. Consequently, the pulse energy that can be obtained from waveguiding lasers and amplifiers is limited. Instead, bulk (i.e., not waveguiding) lasers and amplifiers for which the extractable energy for a given gain can be several orders of magnitude lower are often employed 5 to provide much higher pulse energies. However, the robustness and stability of bulk lasers is often inferior to waveguiding ones.

Moreover, the gain limit can also be problematic for lasers and amplifiers irrespective of whether the stored energy is a major concern, if the high gain appears at another wavelength than the desired one. The reason is that ASE (or lasing) at the gain peak will suppress the gain lo achievable at the desired wavelength, possibly to a value below what is required for a good amplifier or laser. This applies to all types of amplifiers and lasers.

Furthermore, in optically pumped lasers and amplifiers, a suitable interaction between the gain medium and the amplified or generated signal beam is not enough; also the interaction between the pump beam and the gain medium must be appropriate. However, in some types of 15 lasers and amplifiers (typically cladding-pumped ones), the interaction with the pump beam is significantly smaller than the interaction with the signal beam. Then, for a device that efficiently absorbs the pump, the interaction with the signal beam will be much stronger than is required. Unfortunately, this excess interaction is often accompanied by excess losses for the signal beam, since:

1. The scattering -loss of an active medium is normally higher than it can be for a passive medium. For instance, rare-earth-doped fibres have scattering losses of, e.g., several orders of magnitude higher than standard, passive, single-mode fibres.

2. A fraction of the active medium often has inferior properties. For instance, in Er-doped fibres, pairs of Er+:-ions may form. These result in an unbleachable loss. The strong interaction then leads to a high loss.

3. The active medium in its amplifying state may also absorb light (socalled excited-state 5 absorption, ESA). Again, a stronger interaction leads to more power lost through ESA.

Clearly, although often beneficial, the tight confinement of the guided light is a problem for some devices.

Various aspects of the invention are defined in the appended claims, and in passages throughout the present application.

10 According to the present invention, there is provided an amplifying optical device for amplifying a signal beam having a normalised intensity profile comprising: a first waveguiding structure which comprises a first core and cladding and which is configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump 15 power; and an amplifying region; wherein the second core is at least partly formed by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly formed by at least part of the second core; wherein the pump source is optically coupled to the second core; and wherein in use the normalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value.

20 Substantially all of the first core may be formed of a non-amplifying medium.

The first and second waveguiding structures may comprise an optical fibre.

The first waveguiding structure may support a single transverse mode at the signal wavelength.

The second core may be adjacent to at leastone region having a lower refractive index than the second core.

5 The amplifying region may comprise at least one rare earth dopant.

The amplifying region may be doped with Ytterbium, and the amplifying region may be characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are configured such that the amplifying optical device amplifies in a wavelength range from 950nm to 1050 rim. The lo amplifying region may absorb at least 32% of the optical pump power launched into the second waveguiding structure. The wavelength range may be from 975 rim to 985 rim.

The amplifying region may be doped with Erbium, and the amplifying region may characterized by a dopant concentration, a disposition and a length; and whecin the dopant concentration, the disposition and the length of the amplifying region are configured such that the 15 amplifying optical device amplifies in a wavelength range from 1480 rim to 1570 rim.

The amplifying region may be doped with Irbiurn co-doped with Ytterbium, and the amplifying region may be characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are configured such that the amplifying optical device amplifies in the wavelength range from 20 1480 rim to 1570 rim.

The amplifying region may be doped with Neodymium, and wherein the amplifying region may be characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are arranged such that the amplifying optical device amplifies in a wavelength range from 850rim to 950 nm.

The first waveguiding structure may be defined by a cross-section defining a central area of the first core and the amplifying region may surround the central area of the first core.

5 The first core may be circular, and the amplifying region may be disposed in a circular ring concentric with the first core. The circular ring may have an inner diameter between one and one point eight times the diameter of the first core. The circular ring may have an outer diameter between one and three times the diameter of the first core.

The amplifying region may be disposed in a circular ring having an inner radius and a 10 radial thickness. The inner radius of the amplifying region may be between 4.2 microns and 6.6 microns. The radial thickness of the amplifying region.may be between 1. 1 micrort 'and 1.5 microns.

The first waveguiding structure may be configured such that in use the optical radiation guided by the first waveguiding structure has a Gaussian equivalent spot size greater than about 15 eight times the wavelength as measured in vacuum of the optical radiation guided by the first waveguiding structure.

The first and second waveguiding structures may be made glass. The glass may be an oxide glass. The oxide glass may be selected from the group comprising silica, doped silica, silicate, aluminosilicate and phosphate.

20 The amplifying optical device may be configured such that at least some of the optical pump power propagates along the amplifying region at least two times.

The amplifying optical device may be such that higher-order signal-modes are made lossy by deliberately bending -the waveguide.

The first waveguiding structure may be configured to guide at least one desired signal mode and unwanted higher-order modes, and wherein the unwanted higher-order modes are 5 suppressed by the introduction of a signal-absorbing medium in a region of the cladding where the normalized intensity of the desired signal mode is weak compared to the normalized intensity of the unwanted higher-order modes.

The amplifying optical device may be configured as a light source comprising an optical amplifying device including feedback for promoting light generation. The feedback may lo comprise one or more reflectors. At least one of the reflectors may be a fibre Bragg grating. The light source may be a laser.

The amplifying optical device may be configured with an input signal optically coupled to the first waveguiding structure, wherein the amplifying optical device amplifies the input signal to provide highpower laser radiation.

15 The amplifying optical device may have an intrinsic saturation energy, and may further comprise an input signal in the form of an optical pulse optically coupled to the first waveguiding structure, wherein the disposition of the amplifying region is configured such that the amplifying optical device has high energy storage at its intrinsic lasing threshold, and the amplifying optical device is configured to be operated such that the input signal is amplified so that it has an energy 2o exceeding the intrinsic saturation energy of the amplifying optical device.

The amplifying optical device may further comprising an optical switch, and the amplifying optical device may be configured to be operable such that energy is stored in the amplifying region with the optical switch in a blocking state, the energy being released in the form of at least one optital pulse when the optical switch is in a non-blocking state.

The invention also provides a method of a method of pumping at least one optical fibre amplifier with a fibre laser, the fibre laser comprising: a first waveguiding structure which 5 comprises a first core and cladding, and which is configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump power; andan amplifying region; wherein the second core is at least partly formed by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly formed by at least lo part of the second core; wherein the pump source is optically coupled to the second core; and wherein in use the normalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value; the method comprising; configuring the fibre laser to provide optical feedback; pumping the fibre laser with optical pump power to provide a lasing output; and optically pumping the optical fibre amplifier with the lasing output. The amplifying region 15 of the second waveguide may be characterized in that it absorbs at least 32%of the optical pump power guided in the second waveguiding structure.

The invention also provides a method of amplifying optical pulses to energies exceeding the intrinsic saturation energy of an amplifying optical device, the amplifying optical device comprising: a first waveguiding structure which comprises a first core and cladding, and which is 20 configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump power; and an amplifying region; wherein the second core is at least partly formed by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly formed by at least part of the second core; wherein the pump source is optically coupled to the second c-ore; and wherein in use the normalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value; the method comprising; guiding optical radiation using the first waveguiding structure; and guiding the optical pump power using 5 the second waveguiding structure such that the amplifying region interacts with the optical radiation guided in the first waveguiding structure and the optical pump powerguided in the second waveguiding structure.

Embodiments of the invention provide devices that are considerably improved by a predetermined reduction of the interaction between a signal light beam and an active medium lo (per unit volume) compared to prior-art designs, without necessarily changing the properties of the gain medium or reducing the confinement of the signal light (although a reduced confinement can also be beneficial for the disclosed devices). The active medium serves to amplify or generate the signal light beam, or, if unpumped, can act as a saturable absorber.

The reduction in interaction is achieved by placing the bulk of the active medium in 15 regions where the intensity of the signal beam is substantially smaller than its peak intensity, in a cross-section of the waveguiding device perpendicular to the direction of propagation of the signal beam. This can provide advantages for the following devices:

I. Lasers (e.g., Q-switched and gain-switched ones) and amplifiers in which it is d6sirable to store large energies. In these devices (as well as for so-called energy- storage devices in general), the reduced interaction leads to a larger stored energy before practical upper limits on the gain is reached.

2. Optical amplifiers (typically semiconductor ones) for which even the energy of a single signal bit may be comparable to the stored energy. In those, already the amplification of a bit extracts enough energy to reduce the gain. This leads to four-wave mixing, cross-talk, and inter-symbof interference. This can be reduced with the higher stored energy that, for a given gain, accompanies the reduced interaction.

3. Amplifiers and lasers in which an efficient pump absorption necessitates large amounts of 5 gain media, which in prior-art devices leads to excessive small-signal absorption, background absorption, or excited state absorption at the operating wavelength, or excessive gain at another wavelength. A reduced interaction then leads to reduced losses.

Moreover, a reduced interaction can reduce the gain at the undesired wavelength relative to that at the desired one, and thereby the problems associated with a too high gain at the 10 wrong wavelength. This applies to lasers in which there is a significant unpurnped loss (typically, reabsorption loss or out-coupling loss). These points are especially relevant for cladding-pumped devices. For example, to ensure sufficient pump absorption, the fibre may need to be so long that one or both of those problems arise.

Embodiments of the invention can overcome or alleviate some ofthe problems described 15 above and can at least partially achieve one or more of the following: I.

1. To reduce the susceptibility to so-called quenching and background losses, in particular for cladding-pumped devices.

2. To obtain efficient emission at wavelengths otherwise inaccessible for devices where there is a significant unpumped loss, in particular for cladding-pumped devices.

20 3. To improve the energy storage capabilities, for energy-storage devices.

4. To reduce signal cross-talk and inter-symbol interference for signal amplifiers.

The invention can also be used with one of the following features:

I An amplifying optical fibre comprising an active medium placed partly or wholly outside the waveguiding core, e.g., in a ring around the core. The gain medium may also reside inside the core in regions where the normalized modal intensity of the signal beam is small. The fibre may be made of a glass, partly doped with P+, TM3+, Sm'+, Ho'+, Nd 3+, Er 3+' or Yb 3+' 5 or a combination thereof, and it may be cladding-pumped.

2. A cladding-pumped amplifier or laser in which the difference between the overlaps of the pump and signal beams with a gain medium is substantially reduced compared to prior-art designs.

3. A ring-doped, cladding-pumped ytterbium-doped fibre for amplification or generation of 10 light in the range 950 nm to 1050 rim.

4. A ring-doped, cladding-pumped neodymium-doped fibre for amplification or generation of light in the range 850 nm to 950 rim.

5. A ring-doped, cladding-pumped erbium-doped fibre for amplification or generation of light in the range 1450 nm to 1600 nm.

15 6. An amplifying planar waveguide structure comprising an active medium placed partly or wholly outside the waveguiding core, thus interacting with the signal beam only where the normalized intensity of the modal field is small. The design may be specifically adapted to correspond to any of the fibre devices listed above.

7. A semiconductor amplifier for signal amplification, in which the gain region is placed partly 20 or wholly outside the waveguiding core, thus interacting with the signal beams only where their normalized modal intensities are small. Thereby, the saturation energy of the device will be increased, which subsequently reduces the inter-symbol interference and interwavelength cross-talk.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referenced to by like references, and in 5 which:

Figure I illustrates a ring-doped optical fibre; Figure 2 illustrates the dependencies of the refractive index, the gain medium, and the modal field across a transverse cross-section through the center of the fibre in Figure 1;

Figure 3 illustrates a planar waveguide structure amplifying the evanescent field of a 10 signal beam;

Figure 4 illustrates a double-clad ring-doped optical fibre; Figures 5a and 5b illustrate examples of the proposed devices; Figure 6 (not in accordance with the invention) illustrates the extractable energy and small-signal gain at 1550 nm for a ring-doped erbium-doped fibre (EDF) pumped by 0. 1 W, is 0.2 W, and 0.5 W at 1480 run in the core; Figure 7 (not in accordance with the invention) illustrates the extractable energy and small-signal gain at 1550 nm for a ring-doped erbium-doped fibre (EDF) pumped by 0. 1 W, 0.2 W, and 0.5 W at 980 nm in the core; Figure 8 illustrates the normalized modal intensity VI versus ring position for the ring20 doped EDFs of Figures 6, 7, and 10; Figure 9 (not in accordance with the invention) illustrates the extractable energy ("pulse energy above ew") versus. launched pump power for a core-pumped fibre amplifier with an Yb 3±doped ring; Figure 10 illustrates the extractable energy and small-signal gain at 1550 rim for a ring5 doped EDF cladding-pumped by I W and 5 W at 980 rim; Figure I I (not in accordance with the invention) illustrates a view of afibre having a saturable absorber in the central part of the core and a ring-shaped gain medium around the absorber; Figure 12 illustrates a semiconductor amplifier for signal amplification; and 10 Figure 13)a to I _3)c illustrates devices in which unwanted, higher- order modes are suppressed by the inclusion of an absorber.

Figure I depicts a ring-doped optical fibre. A transparent cladding (10) (typical radius tm - 25 0 [tm) surrounds a transparent waveguiding core (3)0) of a higher refractive index, with a diameter of typically a few to ten Lrn (micrometers). The core is surrounded by a gain 15 medium, (20), which can amplify a signal beam, guided by the core. The gainmedium (20) may be pumped by an optical pump beam, which can amplify a signal beam, guided by the core.

In this example, the gain medium is fon-ned of the same glass as the remainder of the cladding, except that the glass in that region is doped with a dopant providing gain properties. However, different materials could be used for the cladding and the gain medium. The gain 2o region is a generally cylindrical region surrounding the core.

Figure 2 illustrates the normalized modal intensity distribution VJ, the refractive index profile with the core (3)0), and the dopant profile (20), in a transverse cross-section through the center of the fibre. For an optical fibre in glass, the cladding refractive index is typically around 1.5, and the numerical aperture is typically around 0. 1 - 0.3.

Figure 3) shows a waveguiding amplifier or laser. As for the fibre, a transparent cladding (I 10) surrounds a transparent waveguiding core (130) of a higher refractive index. A gain 5 medium (120) (eg formed by doping the cladding glass or as a separate medium) is situated near the core.

Figure 4 is similar to Fig. 1, except that the inner cladding (10) is now surrounded by an outer cladding (2 10), of a lower refractive index. Thus, the inner cladding can guide light, and serves to guide a pump beam launched into the inner cladding. The signal beam is guid6d by the 10 core (30).

Figures 5a and 5b illustrate examples of an erbiurn-doped fibre amplifier and a fibre laser respectively. For the amplifier of Figure 5a, a signal beam in an opticalfibre is launched into a wavelength-selective coupler Q 10). Also an optical pump beam from the pig-tailed pump source (320) is launched into the coupler, which combines the pump and signal beams and launches 15 them both into an erbium-doped fibre (330). In the fibre, the erbium- ions serve to transfer energy from the pump beam to the signal beam, which is thereby amplified. The amplified signal is then, for example, launched into another fibre for ftirther transmission.

For the laser of Figure 5b, a beam from an optical pump source (370) is coupled via a lens (350) into a fibre (340) doped with a gain medium. The ends of the fibre provide some means 20 (360) for reflecting' a signal beam, possibly with wavelength discrimination, thus providing feedback for the laser. The reflector at the pump input end transmits the pump and ieflects the signal, while the out-coupling reflector in the other end transmits a significant fraction of the signal beam. Other components are also often used in the devices of Figures 5a and 5b, e.g., an isolator for the amplifier; however those have been omitted for clarity.

Although it is clear that the ideas and concepts disclosed below apply to many different geometries, the discussion below will for conciseness be focused on ring-doped fibres. Moreover, 5 it will be assumed that the structures are longitudinally uniform, although this is not necessarily SO.

Other waveguiding geometries can also be used. For example, the core may be of a more complicated shape than the traditional ones illustrated in the drawings.

While advantages are described of localizing the active medium in regionswhere the lo normalized modal field is small, the active medium may also extend to regions where it is large.

Principle The disclosed devices provide advantages compared to prior-art, core- doped, ones by suppressing gain and thus radiation losses at undesired wavelengths and/or by reducing the propagation losses in the device. Below follows a description of how these advantagesare

15 obtained. We restrict the discussion to homogeneously broadened gain media; substantial benefits can be realized also in inhomogeneously broadened ones. The description focuses on cladding-pumped devices.

It is known that with a gain medium for which the shape of the gain spectrum depends on the population inversion, the emission wavelength of a fibre can be modified by changing the ?0 strength of the interaction between a signal beam and the gain medium. For instance, thefibre length can be changed. This also changes the absorption of the pump. However, we will demonstrate below that in cladding-pumped devices, the same control can be obtained through ring-doping, while separately controlling the absorption of the pump. In particular, the pump absorption can be kept siufficiently large, as will be rurther described in the rollowing.

C The following relation can be used for evaluating the gain G in a waveguiding device with a homogeneously broadened gain medium [I]:

5 G= 10 [CT, N.(x,y)n,(x,y)1P(x,y)dxdy-c No(x,y)(l - n, (x,y))'V(x,y)&dy] L [dB] (1) Tn 1-0 ff 'Iff where No is the concentration of amplifying centres, n2 is the degree of excitation, V'is the normalized mode intensity, o' and d are the absorption and emission cross- section of the active centres, respectively, and L is the length of the gain region. Equation I can be written in a simplified fon-n:

10 G = (10 / In 10) No Adopd Vd,,pd [n2 d - (I - n2) o] L [dB] (2) whereNo,n2,and Vdp,d have been appropriately averaged over the doped area Adqp,d- (In the literature, the productAdoped Vdp,d is often replaced by the so-called overlap T.) There are two assumptions in Eqs. I and 2, namely, that the gain is homogeneously broadened and that only two levels in the gain medium are significantly populated. However, 15 even for devices that do not meet these assumptions, the problems that we address exist and can generally be countered by designing devices according to our present invention. In the n otation, there is also the implicit, unimportant, assumption that the gain stems from a number of active centres, each of which has been ascribed cross-sections for stimulated emission and absorption.

Other types of gain media also exist, and the results will be valid also for them. To proceed, we 20 will also assume that the degree of inversion is wavelengtl-- independent. This is normally true to a good approximation. If not, this results in a slight inhomogeneity in the gain spectnnn. For simplicity, we have also assumed that other losses are small compared to either the gainG or the bleachable absorption (I O/In I O)N(Ad,,I,,d V_'dr,dcL. Again, this is a non-restrictive assumption, and the equations can be easily modified to include any other loss. For instance, a filter can be used for controlling the gain spectrum and laser output wavelength, both in prior- art devices and C) the devices disclosed here.

5 It follows from Eq. 2 that the gains G1, G2, and G3 at three different wavelengths A,, /12, and A3 are related to each other in the following way:

G3 = G2 ( IF3, dp,dl TS, doped) (q3'1uj' - q3'/aj') / (C2'/Cl' - C2'/Uj') + G, ( '3. dop,dl T1, doped) (C3'/C2' - U3'IU2') / (071'/U2' - Cl"IC2") [dB] (3) Equation 3 makes the important point that for given cross-sections, the only parameters 10 that affect this relation are the normalized mode intensities, averaged over the doped region. Let now A, be the pump wavelength. The pump is then absorbed by an amount apP"'"g =_ -G, in the operating state of the device. In order to operate efficiently, apoperating needs to be sufficiently large, say, at least 5 dB. Also, we assume that we require a certain gain G2 at a wavelength A2. a,,, operating and G2 are then parameters already specified. This also implies a certain gainG3 at 15 other wavelengths A3, but if this gain is too large, prohibitive amounts of power will be lost to ASE. Insofar as the cross-sections cannot be significantly modified, this can only be remedied by designing the device for appropriate values of the normalized modal intensities. The description of such designs is a central part of thepresent invention.

To simplify the further description, we now assume that the pump does not stimulate any

20 emission; hence, cl' = 0. Equation 3 then becomes G3 G2 (.3e T"3, doped / a2e VJ2, doped) + ap operating (C3' '3, doped / Cp' Vpt doped) I(C2'/Cr2') - (CF3'IU3')] [dB] (4) The value of the first term depends on the relative sizes of V2, doped and V-13, doped at A2 and A3. In a fibre, the spot-si-Zes at A2 and A3 may differ. Then, ring-doping implies that the gain at the wavelength with the larger spot-size gets relatively larger than at the other wavelength, compared to a homogeneously doped core. Depending on how close the wavelengths are to each other, this 5 is often not a significant effect, In contrast, in cladding-pumped devices, the second term in Eq. 4 can to a significant extent be controlled by designing the device for an appropriate value of (V-13, dr,d I Vp dqpd)Normally, it is very different in a cladding-pumped device and in a core-pumped device. In the core, the normalized pump intensity V1p is approximately equal to the inverse of the pumped area 10 for both core-pumped and cladding-pumped devices, so the same is true for Vp, doped in a coredoped device. It follows that in a core-doped device, V'p,dopd will be much larger in a corepumped device than in a cladding-pumped device. Thus, the effective area ratio reffective V3, doped / V-' doped) will be much larger. (We will also use "effective area ratio" for the ratio V-'2, d6ped / Vp, doped.) Consequently, a core design which is suitable for the core-pumped device..

15 may be inappropriate for a cladding-pumped device because the effective area ratio becomes too large. In prior-art cladding-pumped devices, rffcj,, is large, typically around 100. Then, the second ten-n in Eq. 4 is potentially large for some undesired %avelength A3, which makes' it difficult to absorb the pump without getting a high gain at the undesired wavelength. Therefore, laser systems with significant reabsorption that work well in a core-doped, core-pumped, 20 geometry will not be efficient core-doped, cladding-pumped lasers. (In a device doped in the core, reffieclive is approximately equal to the area ratio r a Ap,,,,,pd / Adopd, where Ap,,,,pd is the pumped area and Ad,,p,d is the doped area. Hence, for a claddingpumped device homogeneously doped thrOU(YhOUt the core, r = A,jadditig / Acore-) Consider instead a ring-doped, cladding-pumped device. Since n is approximately constant over the inner cladding, Vlp, dap,d Will not change much with the transverse disposition of 5 the gain medium. However, since the light at A3 is confined to the core, V13, dap,d decreases rapidly if the amplifying region is moved away from the core. This obviously reduces the interaction between the gain medium and the signal beam. Hence, the devices disclosed here allows re#e,j, to be substantially reduced, e.g., to values in the range I - 10, whereby the gain at unwanted wavelengths can be suppressed compared to the gain at a desired wavelength.

10 First, we treat the case where the scattering (or absorption) loss of the gain regicn is larger than that of a transparent, passive region. For simplicity, we assume that there is no scattering loss outside the gain region. Starting from Eq. 2, we can then derive the following expression between the scattering loss and the gain G, and G2 at two different wavelengths:

scatter = scatter (a,, +,e) (02, +. e)] e a a2 Cr2 [G2 V2, doed / Pi, dapd) G 1 2 2 _ al 2 15 [dB] (5) In Eq. 5, we have arbitrarily made the nor-restrictive assumption that each active center ,c,lr scatters with a cross-section 2 Also, we have for simplicity assumed that scattering is small compared to the gain. It follows that the scattering losses can become high already at a small value of the ratio between stimulated emission and scattering (CF2"'102e) if reffeczive 100, i.e., in 20 a core-doped, cladding-pumped device. Then, already a value (U2""r/Or2') as low as I /1000 can result in significant losses. In contrast, in ring-doped cladding-pumped devices, acceptable values of (q2""1q,') will be one or two orders of magnitude larger.

Next, we will show how ring-doping also can reduce the sensitivity to quenching.

Very often, some active centres in a gain medium are defect. These quenched centres retain their ground-state absorption (GSA), but, if they absorb a photon, they are not efficiently excited. This leads to a socalled unsaturable absorption, the spectrum of which is approximately 5 proportional to the small-signal ground-state absorption spectrum of the medium. For instance, this type of unsaturable absorption has been observed in the important Yg':glass and Er3+:glass gain media. The smallsignal absorption is given by:

a2 SS = C22 [G2 (UI'7.+ CIO) '2, doOd / T1, d.,,d) G, (q2' + C2'A /(CI'U2' - UI'Cy2a) [dB1 (6) If, for instance, _3 M of the active centres are quenched, we get an unsaturable absorption 10 of 0.03xa2". Equation 6 is very similar to Eq. 5, and the same result holds: A cladding-puniped device with the currently disclosed design will be typically 10- 100 times less sensitive to quenching than are core-doped designs of the prior-art. (This does not apply to four-leve I systems, for which a2" = 0 dB.) Next, we consider the case of excited-state absorption at the signal wavelength A2. Again, 15 a stronger interaction leads to more power lost through ESA, at least for a device with significant small-signal absorption, as the following equations will show. The excited-state absorption can be written as:

a2 ESA = U)EsA [G2 / C'' V2, dopOd / V11, d,,,,,d) G I / ul'] / [(c2' a2E-")/a2' - C1'/UI 4] [dB] (7) For a transition to the ground-state, the total excited-state absorption can be significant 2o already for values of q2ESAI(q2O - o-2s) of I/ 1000. Again, in cladding-pwnped devices, the sensitivity can be reduced one or two order of magnitudes by ring-doping. (For four-level transitions, a,' = 0, so the sensitivity to ESA is independent of any ring-doping, and equal to that of traditional core-doped, core-pumped devices.) Equations I - 7 thus demonstrate how ring-doping makes the disclosed devices less susceptible to absorption loss and scattering losses and to emission losses to ASE at an 5 undesired, high-gain wavelength. The improvements are a direct consequence of the reduction of the effective area ratio reffecnve - r to values around I - 10. In contrast, in prior-art devices, the signal light in the core is confined to an area approximately 100 times smaller than that of the pump, so the area ratios r - rffj, - 100. While the area ratio may well be made larger, a smaller area ratio is troublesome since a smaller area of the inner cladding can make it difficult to launch 10 the pump into the device, and since a larger signal spot-size leads either to a large bend sensitivity or to a multi-mode core.

In addition to the general designs described up to this point, we next describe some particular cladding-pumped fibre lasers and amplifiers with sizable advantages compared to the C> prior art.

15 Ytterbium-doped flbre operating in wavelengths between 975 and 985 nm For Yb 3±doped devices at these wavelengths, the suppression of quasifour-level emission around 10')0 nm can be especially troublesome for cladding- pumped devices designed according to the prior art, For a wavelength of 975 nm (corresponding to the peak of the cross sections) with representative cross-section values (cf. Table A 1), Eq. 3 gives the following 20 relation between the gain at 975 nm, the gain at 1028 run, and the pump absorption of the pumped (i.e., partly bleached) fibre:

G1028 = 0.25 G975 + 0.74 ( Vdopd / P, d.,,,d) apoperafing [dB] (8) Here, we have assumed that V"975, dp,d = T1028, doped, which is a reasonable approximation for guided modes at nedrby wavelengths. Now, assume that we want the laser to work at 975 rim, with 3.5% reflectivity at one end and 100% at the other one. Then, if the background losses are negligible, G975 = 7.28 dB.

5 Consider first.a representative core-doped prior-art design with r rff,,j,, = 100. Then, for every dB of pump absorption we get 74 dB of gain at 1028 rim. Since the gain at unwanted wavelengths must be below approximately 50 dB, we would have to restrict the single-pass pump absorption to below I dB or 20%. This would be a highly inefficient laser. Instead, we propose to use ring-doping. Then, the pump absorption can be 5 dB or more, which allows for a good laser lo efficiency. Note that increasing the end-face reflectivity at 975 rim will not help us much, as the high gain at 1028 rim largely follows from the requirements on pump absorption, while it is comparatively insensitive of the gain at 975 rim. For the same reason, the gain at 1028 nm will not be much higher for a high-gain amplifier at 975 rim than it is for a low-gain laser, so the disclosed design provides benefits for both applications.

15 A high-power laser at 975 rim can be used for pumping Er 3+. Also other wavelengths can be used for this, e.cr., 980 rim and 985 nm. However, also those wavelengths are severely affected by unwanted emission around 1030 rim.

In the following, we will show that lasing at 975 firn will be particularly sensitive to any 3+ unbleachable Yb, the existence of which has been reported in [2]. This sensitivity can be order 20 of magnitudes higher in core-doped designs according to prior art, compared to the devices of the present invention.

From Eq. 6, we get a975' = 1.07 G975 + 6.48 ( Zfdoped / Vlp. doped) apope"""'g [dB] (9) Hence, with a prior-art design for cladding-pumping, we get several thousand decibels of small-signal absorption at 975 nm for a desired pump- absorption of around 5 dB. For r,ff,,,i,, = 100, already an unsaturable fraction of I % of this (the lowest value reported in [2]) 5 leads to an unsaturable absorption of around 30 dB, which is unacceptable. With the new devices, the sensitivity is drastically reduced. Even further reductions are possible by lasing at other wavelengths, e.g., CC9805" = 0.83 Gwo + 1 _34 ( Vjdoped / P, doped) apoperating [dB](l 0) at 980 rim, and 10 a985' = 0. 64 G985 + 0.3 7 ( Vdoped / Vlp. doped) Pope,fing [dB](I 1) at 985 nm. The sensitivity to quenching is much reduced, and can be quite small in a ring-doped device.

While the analytic considerations above clearly demonstrate the advantages ofthe disclosed devices, they do not quantify the advantages in terms of the most important laser 15 characteristics, namely, pump threshold P,h and slope efficiency 7,1,p, . In order to provide a more complete description of the improvements compared to prior art, we next present calculations of P,h and i7,1,p, from simulations with a spectrally and spatially resolved numerical model [3]. The ordy significant simplification in the model is that the pump is always assumed to be uniformly distributed across the inner cladding. Besides that, the gain medium is assumed to be

20 homogeneously broadened, which is reasonable for Yb+:glass systems. With the model, we analyzed fibres of different core-doped and ring-doped designs. In all cases, we kept the doped area constant, equal to the core size, while the outer radius rd""' of the doped area and hence V-'d,,p,d was varied. The area ratio was 80, and Tp, d6ped = (3080 [im 2)- 1. Other parameters are given in Table A2.

A first studied cavity had one laser mirror formed by a bare, cleaved, fibre end, providing a broadband reflectivity of 3.5%, while a narrow-band reflector (typically, a fibre bragg-grating) 5 provided a 99.9% reflectivity in a desired laser wavelength range 975 nm to 977 tim. Outside this range, the reflectivity was zero, as can be achieved with an AR-coated or an angle-cleaved fibre end.

Lasing in the desired wavelength range was prevented by strong ASE at long wavelengths (1028 run - 1035 nm) until rd"""' became 5 Lrn. Then, the diameter of the inner ring is 4.2 [trri lo and rff,,,j, = 5.8 in good agreement with earlier estimates. The results are presented in Table 1.

rdouter /PM rd inner / pm reffective dBIM Ph I W 77,10P,Xloo PP transmitted No lasingfor a pump 3.5-5 0-3.6 64-12 170-31 power of 5 W. ASE 22% around 1030 nm 46% dominates the output 5.5 4.2 5.8 15.3 2,0110.1 69 2 26% 6.0 4.9 2.9 7.47 2.14 --YIO. 1 66 2 29% 6.5 5.5 1.6 4.03 2.3 7--O. 1 62 2 33% 7.0 6.1 1.0 2.58 2.78-+O.l 61 2 34% 7.5 66 0.57 1.43 3.62-+O,l 51 2 44% Table 1. Laser characteristics qf 10 m long unquenchedfibre operating at 9 76 nm, with a high reflectivityfibre grating and a bare, cleaved endproviding the laser cavity reflections. 7he 5 small-signal absorption e applies to a wavelength of 9 77 nm. The transmittedpump power PP transmitted is expressed as afiraction of the launched pump power. rd"r is the inner radius of the gain medium.

Clearly, in contrast to prior-art devices, the device disclosed here can lase at 916 nm with a good efficiency. The range of acceptable effective area ratios is 1- 6. The slope efficiency with lo respect to absorbed pump was approximately 93%- a quite high number which in reality will be lowered by background losses. These were assumed negligible in the caloilations.

A shorter fibre fength favours lasing at shorter wavelengths in a twolevel system like this. However, shortening the fibre to 5 m is not sufficient for lasing at 976 nin in a core-doped design. Moreover, at this length, a significant fraction of the pump is not absorbed. Hence, making the fibre sufficiently short to ensure 976 nm lasing in a core- doped design is not an 5 attractive option, even if the pump is double-passed through the cavity. The conclusion'is that prior-art designs are inadequate for lasing at 976 nm for the considered area ratio.

Above, the smaller ring diameters appear to be better than the larger ones (provided that lasing is obtained). However, if a fraction of the Yb4ons are quenched, this will change, as is evident from Table 2.

rdouter 1,um rd ",,r 1,um reffj,?,? dBlm Pth / W lhlopex,100 PP1"""-it1'= No lasingfor pump 3.5-4.5 0-3.5 64-22 170-57 power of 10 W. ASE 33% around 1030 nm 47% dominates the output 5.0 3.5 12 30.6 1. 69 0. 1 29 2 50% 5.5 4.2 5.8 15.3 1.77 0.1 34 2 53% 6.0 4.9 2.9 7.47 2.04 0.1 33 2 58% 65 5.5 1.6 4.03 2.57 0.1 29 2 64% 7.0 6.1 1.0 2.58 3.58-+O.l 26 2 68% Table 2. Laser characieristics o 5 m longfibre operating at 9 76 nm, with 2 % of the yb3'-ions quenched. A high reflectivityfibre grating and a bare,. cleaved endprovided the laser cavity reflections.

5 At 980 nrn and 985 nrn, the fibre behaved similarly as at 976 nm, except that 985 nm would not lase for an output reflectivity of 3.5%. A grating with 50% reflectivity at the output end allowed for lasing at 985 nm, In contrast, lasing at 976 nm in an unquenched fibre was only marginally improved by a grating also at the output end, and for a partly quenched fibre, results were worse with a grating than with a bare end. Also, as predicted in Eqs. 9-11, the longer lo wavelengths are less sensitive to quenching than are the 976nm lasers.

These and other detailed numerical model calculations have shown:

The earlier analytic considerations are largely accurate in determining whether or not a laser can work efficiently.

The disclosed devices perform much better as lasers at 975 nrn - 985 nm than do prior-art designs.

5 The best value of the effective area ratio is around 3 -10 for this laser.

The sensitivity to quenching is reduced with a smaller effective area ratio.

The susceptibility to quenching is smaller at 980 rim. and especially at 985 nm than it is at 976 n.m.

Neodyndum-doped fibre operating on the 4 F3/2 - 4&2 transition (850 nrn 950 nm) 10 A device designed in a similar way as the Yb 3'-doped cladding-pumped fibre will also improve on prior-art designs for this Nd 3±transition. For Nd 3+ -doped devices at these wavelengths, the suppression of the dominant 4 F312 - 411 1/2 at 1050 nm transition is a problem, especially for cladding-pumped devices. For a wavelength of 870 nm, typical cross-sections (cf Table A 1) gives the following relation between the gains at 870 nin and around 1050 nm and the pump absorption of the pumped fibre:

V, G j o5o 3) G8 70 + 1.5 ( V'dqpd / P. d.P.d) apoperafing [dB](12) The relations will be similar for other wavelengths in this transition. Equation I reveals that the gain at 1050 nm will be at least three times larger than that at 870nm. This limits the 870 nm gain to 15 dB - a comparatively low but still useful number. However, with a prior-art, 20 core-doped device, it will not be possible to absorb the pump properly, since the gain at'1050nm becomes prohibitively high already for a single-pass pump absorption of less than 0.5 dB (. 10%). On the other hand, in a ring-doped device, rff,,,i,e may be reduced by a factor 10 or more, so an absorption apP"" of at least 5 dB (= 68%) is possible, while still allowing for a single-pass gain at 870 nm of 10 dB.

In Eq. 12, we for simplicity assumed that Pdp,d is equal at 1050 nm and 870 n-m.

However, for ring-doping, V-'d,,,,d will be larger at 1050 nm than at 870 nm. This means that the 5 factor "Y in Eq. 12 actually will be larger. For instance, with a numerical aperture of 0. 1 and a core diameter of 6 tm, a doped ring with rdinner = 4 [tm and rd,,,r = 5 Itm gives 1050,doed / T870, doped 1.6. Then, G1050 = 4.8 G870 + 1-5 ( 'doped / 'p. doped) apop"'"ingNevertheless, appropriate designs allow enough gain for efficient lasing at 870 nrn before the gain at 1050 nm becomes unrealistically large, The 870 nm gain can be even higher in modified 10 designs: If the core has a higher cut-off wavelength of, e.g., 950 nm, the core will be multimoded at 870 nrn. Since the higher-order LPI -mode penetrates further into the cladding than the fundamental LPol-mode does, the LPI -mode gain at 870 nm is higher than the gain of the LPOjmode. Hence, higher-order mode lasing at 870 nm becomes relatively easier to achieve compared to the 1050 lasing in thefundamental mode.

15 Erbium-doped fibre operating on the 4J13/2 - 4115/2 transition (1450 nm - 1600 nm) The concerns of this device are similar to those of the cladding-pumped Yb-dope'd fibre described above. For instance, if we want the device to operate at 1531 nm, emission at 1564 nm or longer wavelengths is a potential problem in an aluminosilicate host. From Eq. 3 and Table A I, we get 20 G1564 = 0.6 G1531 + 0.7 ( Vdoped / Vlp. doped) ap operating [dB]( 13) Clustering is a well-known problem in erbium-doped fibres, and results in a saturable absorption. Equation 6 gives 29 operating a153 I' = G1531 + 5 ( Tdoped / 'p. doped) p [dB](14) These numbers are similar to the ones for Yb 31 operating at 976 nm, so ring-doping 3+ allows for similar improvements as for Yb The wavelength range 1550 nm - 1565 nm is technologically important for optical 5 communication systems. In this range, lasing at 1550 nm may be particularly hard to achieve, because the gain at, e.g., 1564 nm may become prohibitively large. From Eq. 3, we get G1564 = 0.79 G1550 + 0.15 ( Vdoped / 'p, doped) Qpop""'i"g [dB](15) Also in this relatively benign case, adequate purnp absorption can be troublesome in a prior-art design for unfavorable values of rff,,,i,, w ( V-'d,,pd / V-p. doped), so a ring-doped fibre will 10 be advantageous, As it comes to the unsaturable absorption, we have that a, 550"" = 0. 6 G 1550 + 2. 0 ( VIdoped / V'p, doped) ap"pe"'t"g [dB](I 6) Core-doped devices may then have an unsaturable absorption of 1000 dB, so a ring-doped fibre is better, Principle 15 The type of high-energy pulse amplifiers and lasers we consider are so- called energy- storage devices in which a pulse extracts significant amounts of energy stored in the gain medium. The energy supplied by the pump during the generation/amplification of a single pulse may be negligible. The amount of energy stored in the device then sets an upper limit on how much energy can be extracted by a pulse. This is a significant difference compared to other laser 2o and amplifiers, for which power extraction is typically limited by the supplied pump power, and in any case not by the stored energy, in order to obtain high-energy pulses from such an energy storage laser or an amplifier, we need both a large stored (and extractable) energy and a sufficiently high gain. While the gain efficiency of waveguiding amplifiers means that it is often easy to meet the second objective, the same gain efficiency can make it difficult to store large amounts of energy in the device: The gain 5 efficiency implies that a comparatively small amount of extractable energy in the gain medium leads to a high gain. However, as already pointed out, since ASE limits the achievable gain of the device, it also limits the energy that can be stored [4].

The gain G in a transverse mode is related to the energy E stored in the gain medium through the following relation:

10 G = (10 In 10) Vldp,dE(o' + c) / h v - aL] = (10 In.10) Vld,,p,dE / U,,, - aL] (10 In 10) Vjd,,r,d Eextractable / Usal = (10 / In 10) E-Iraclable / Ea' [dB](I 7) Here, h v is a photon energy, aL is the unpumped loss of the medium, U,,,, =_ h vl(o' + d) is the saturation energy fluence, Ex,c,ble is the energy over the bleaching level, i.e., the 15 maximum energy that can be extracted from the device, andE,,,,!a U,,,, / V-d,,ped is the saturation energy. The important point is that G is proportional to Tdp.d. Hence, a smaller value Of Vld,,p,d leads to a smaller gain per unit extractable energy. Therefore, for a gain medium located in a region where the normalized modal intensity of the signal beam is small, the extractable energy for a given gain will be high. Then, if the gain is sufficiently large for the device in question, a 20 device with low values of Vld,,pd will be capable of generating or amplifying pulses to high energies.

Here, we disclose the use of devices that, although the light is tightly confined in a single orfiew-moded waveguide, have a small value of V-dpd for high-energy pulse amplifiers and lasers, e.g. Q-switched and gain-switched ones. Note that any effect this may have on the relative gain at different wavelengths can be counteracted by simply making the device longer or 5 increasing the concentration of active centres.

In addition to the general geometries described earlier, we will now describe some specific geometries and devices.

Core-pumped ring-doped pulse fibre amplifier or fibre laser In the important class of core-pumped devices, the pump and the signal are guideid by the 10 same core. For instance, most erbium-doped fibre amplifiers (EDFAs) are of this type. Typically, the gain medium may be a Tm'+, Sm'+, Ho'+, Nd'+, Er'+, or Yb'±doped glass. The desired weakness of the interaction between the signal beam and the gain medium normally then implies that also the interaction with the pump beam is weak, whereby the pumping of the medium becomes weaker and the pump absorption smaller. Nevertheless, the disclosed devices can, show 15 significant improvements.

1. The pump and signal wavelengths are close, so the signal and pump mode profiles are close to each other. In this case, it is just a matter of finding suitable values of Vfor placing the ring. These will depend on the lifetime and cross-sections of the dopant, the pump power 20 and pulse energy, and other parameters. Figure 6 shows how the extractable energy a;nd small-signal gain at 1550 nm depends on the position of the ring for a ring-doped EDF core pumped by 0. 1 W, 0.2 W, and 0.5 W at 1480 nm. Figure 6 illustrates the extractable energy I and small signal gain at 1550nm for a ring-doped erbium-doped fibre (EDF) pumped by 0. 1 W, O.2W, and O.-5W at 1480nm in the core. The ring thickness was sufficiently thin to make variations of the non-nalized intensity of the modal field negligible over its thickness.

Other parameters are listed in Tables A3 and A4 under "normal core amplifier". For 5 comparison, also results for erbium-doped fibre amplifiers (EDFAs) homogeneously doped throughout the core are shown, both for the "normal core" amplifier and a "large core" amplifier. In all cases, a higher pump power gives a higher small-signal gain and a larger extractable energy. Moreover, the fibre length was optimized for maximum small-signal gain in all cases. The advantages compared to the prior-art EDFs (also shown) are 10 substantial. Figure 8 shows model calculation results on how Wdr,d depends on the ring position for the ring-doped EDF. The method used for these and other similar calculations in this specification follows [4].

21. The pump and signal mode profiles are different. In this case, the pump is unlikely to penetrate far into the cladding, so the doped region must be inside the core or immediately 15 outside the core. Unfortunately, for positions for which the signal intensity is suitable, the pump intensity tends to be much too weak. A good design should then aim at reducing this problem as far as possible. Figure 7 is similar to Fig. 6, except that the pump wavelength is now 980 rim. In particular, Figure 7 illustrates the extractable energy and small-signal gain at 1550run for a ring-doped erbium-doped fibre (EDF) pumped by 0. 1 W, 0. 2W and 0.5W at 20 980nm in the core. The ring thickness was sufficiently thin to make variations of the normalized modal intensity negligible over its thickness. Other parameters are listed in Tables A3 and A4 under "normal core amplifier". For comparison, also results for EDFAs homogeneously doped throughout the core are shown, both for the "normal core" amplifier and a "large core" arnp] i Fier. In all cases, a higher pump power gives a higher small,signal gain and a larger exi[ractable energy. Moreover, the fibre length was optimized for maximum small-signal gain in all cases. We see that the results are now worse, and that the benefits of ring-doping are smaller. However, performance is still superior compared to that 5 of prior-art designs.

As an alternative, the core can be single-mode at the signal wavelength, and multi-moded for the pump. It is well-known that pump-light in higher-order modes will penetrate further into the cladding, thereby improving the pumping of the gain medium. Moreover for so-called upeonversion devices, the pump wavelength is shorter than the signal wavelength, with the lo favorable side-effect that the pump extends further into the'ring, even if it is in the same mode as the signal, Figure 9 shows measured results on high-energy pulse amplification for a ring-doped, core-pumped Yb 3±doped Fibre amplifier. Figure 9 illustrates the extractable energy ("pulse energy above cw") vs launched pump power for a core-pumped fibre amplifier with an Yb 3+_ 15 doped ring. The fibre was pumped at 1000 rim, and amplified signal pulses at 1047 nim. The highest recorded extracted pulse energy (above the cvv-level) of more than 60 4.1 can be compared to published 10 [J total pulse energy from large-area core amplifier (albeit at a lower pump power of 160 mW) [5], as used in the prior art for high pulse- energies. The ring-doped fibre had Wd,,pd - 0.02 4m 2. A smaller value can allow for even larger extracted energies, as long 2o as the pump power is large enough to create a significant gain.

The emission cross-section of erbiurn in glass is smaller than for many other gain media, like Nd 3+:glass at 1050 nm and many transition metals. It follows from Eq. 17 that the stbred energy will be smaller in these media. Therefore, the improvements with ring-doping can be relatively larger than for Er':glass.

C Cladding-pumped devices We now describe cladding-pumped ring-doped fibres for high-energy pulse amplification 5 and generation. Because of the typically higher pump powers used with these devices and because of the separately controllable normalized pump and signal mode intensities in the doped region, the disclosed cladding- pumped devices will by far outperform any prior-art core-doped single- or few-moded waveguiding device. A typical device will be a rare -earth- activated glass fibre optically pumped by a pump beam launched into the inner cladding (cf Fig. 3).

10 Figure 10 shows how the extractable energy and small-signal gain at 1550 nm depends on the position of the ring for a ring-doped EDFcladding-pumped by I W and 5 W at 980 nrn. In particular, Figure 10 illustrates the extractable energy and small-signal gain at 1550nm for a ring doped EDF cladding-pumped by I W and 5 W at 980nm. The ring thickness was sufficiently thin to make variations of the normalized modal intensity negligible over its thickness. Other 15 parameters are listed in Tables A3 and A4 under "normal-core amplifier". For comparison, also results for EDFAs homogeneously doped throughout the core are shown, both for the "normal core" amplifier and a "large core" amplifier. InaH cases, a higher pump power gives a higher small-signal gain and a larger extractable energy. Moreover, the fibre length was optimized for maximum small-signal gain in all cases. Other parameters were the same as in Fig. 10, and are 20 listed in Tables A3 and A4. The advantages compared to the prior-art EDFs (also shown) are substantial. The increase of the extractable energy can approach two orders of magnitude in the devices studied here.

In view of these results, we propose a ring-doped cladding-pumped optical fibre where the ring is located at a position where the mode intensity is, e.g., one or two orders of magnitude smaller than it is in its center. In order to get sufficient absorption, sensitization may be used, e.g., as in ytterbium-sensitized erbium-doped fibres.

5 A ring-shaped gain medium outside the core may be better pumped by a beam in the cladding. Thus, while cladding-pumping has normally been considered to facilitate launching of non-diffraction-limited sources like diode bars, we also propose to use cladding-pumped, ring doped fibres even when high-brightness, near-diffraction-limited pumps that could be efficiently launched into the core are available. The high brightness will still be favorable because the area lo of the inner cladding can be small. In these devices, claddingomodes may well see a higher gain than the desired core-mode does, whereby some measure for suppressing cladding-modes would be required.

Even if only a small part of the stored energy is extracted from a ringdoped amplifier, the high stored energy may still be advantageous, since it for instance reduces the distortions of the 15 chirped-pulse amplification with small distortions of the pulse shape.

Passively Q-switched and gain-switched lasers In passively Q-switched lasers, energy and thereby ASE builds up in a region of gain. The ASE then transfers energy to a saturable absorber. The saturable absorber must be so that the absorption change per unit stored energy is smaller than it is in the gain region. h prior-art 2o devices, this is achieved by using a saturable absorber with large absorption and/or stimulated emission cross-sections, compared to those of the gain medium. Ring-doped fibres open up for Q-switched lasers where the gain section and the saturable absorber are made from the same material, e.g. an erbium-doped glass. This is possible since E,,,, =- hvl[ Tdop,d (C' + C')] can be two orders of magnitude higher in the ring- doped fibre than in the core-doped one, even though the material - dependent quantities (c' + d) are equal in the two different fibres.

The gain section may also be a core-doped fibre with a large area core, however, this does 5 not work as well as a properly designed ring-doped fibre.

In a first embodiment, a ring-doped fibre is cascaded with a core-doped fibre, each of which are doped with a similar dopant with a nonnegligible ground-state absorption, to form a laser cavity. A pump beam is launched into a gain section, consisting of the ring-doped fibre, thereby building up a gain and stored energy. A ew pump beam may be used, and thefibre may lo be cladding-pumped. The gain section generates ASE, through which energy is transferred from the gain section to a core-doped fibre constituting a saturable absorber. The pump also acts to bleach the pump- absorption in the ring-doped fibre, whereby the pump penetrates deeper into the cavity, and possibly helps in bleaching the saturable absorber. The transfer of energy from the gain section to the absorber section increases the net gain in the cavity to a point where it exceeds 15 threshold. Then, energ is radiated from the cavity in the forrn of a Qswitched pulse. This Ely substantially reduces the stored energy, and hence the gain, in the cavity, so that the ASE becomes negligible, and the pump power that penetrates to the saturable absorber becomes small. The saturable absorber then relaxes to a state that is at least partly absorbing. Thereby, the absorption in the saturable absorber has increased substantially before the gain section starts to 20 generate ASE again, whereupon the cycle is repeated.

A second embodiment is similar to the first embodiment, except that there is provided a pump-absorber or a pump reflector between the gain-section and the saturable absorber. This substantially reduces the pumping of the saturable absorber.

A third embodiment is similar to the first or second embodiment, except that the active centres in the gain mediu-m and the saturable absorber are different. The pump wavelength may be chosen so that it cannot bleac h the saturable absorber.

Figure I I is a view of a fibre having a saturable absorber (640) in the central part of the 5 core (30), and a ring-shaped gain medium (620) around the absorber. In the illustrated example, the gain medium resides in the core, but the gain medium may be placed partly or wholly in the cladding (10).

Figure 12 illustrates a semiconductor amplifier for signal amplification provides gain for a guided optical signal beam in a region where the normalized modal intensity is small. 4 10:

10 cladding, 420: gain region (active layer), 430: core (index guiding layer), 480: substrate, 490:

contact layer. Also the approximate location of a signal beam is indicated(470). The refractive index of the active layer may be depressed in order to suppress gain guiding.

Figures 13a to c illustrate devices in which unwanted, higher-order modes are suppressed by the inclusion ofan absorber, 13a) A fibre with an amplifying ring 10 further contains an 15 absorbing ring 5 10, suppressing high-order modes. The absorption of the desired fundamental mode is small or even negligible. l3b) Planar waveguide with amplification of the evanescent field by a gain region 120, within an absorbing superstructure 520. Again, the absorption of the desired fundamental mode is small or even negligible. Undesired higher- order modes penetrate further.into the absorber, whereby they are suppressed. 13c) A double clad ring-doped fibre in

20 which a signal-absorbing region 530 has been incorporated into the cladding, thereby preventing any build-up of signal light in the cladding.

Device with a distributed saturable absorber Above, two media with different saturation characteristics were combined in a cascade.

However, the two gain media may also reside side b side in the same fibre. An example of this I y is illustrated in Fig. 11. A fibre having a core (30) and a cladding (10) is doped with a saturable 5 absorber (640) and a gain medium (620). Here, the saturable absorber is located in a region where the normalized modal intensity is larger than it is in the region of the gain medium. Hence, if the absorber and gain media are similar (except that the gain madium is pumped), and the cross-section for stimulated emission of the gain medium is similar to the absorption cross section of the absorber, the small-signal gain of the fibre can be negative or small, even though lo the extractable energy of the gain medium is larger than the energy required to bleach the saturable absorber. Hence, the ASE in the fibre can be suppressed, while the energy that can be extracted from the device, if for instance a signal pulse is launched into it, can be large.

In contrast to the prior art, a ring-shaped gain medium allows the active centres in the absorber and the gain media to be of the same or similar types, as long as it is possible to pump 15 the centre s in the gain medium while leaving those in the absorber medium unpumped. A 3 particular studied embodiment consisted of an E?±doped saturable absorber and an Yb,"' sensitized E?±doped gain medium. The Er+ in the gain medium was excited indirectly (i.e., via the Yb 3+) by an optical pump beam launched into the fibre core. The launched pump power was I W at a wavelength of 1064 tun, which is a wavelength that will not excite the E?+ in the 20 saturable absorber. The fibre had a numerical aperture of 0. 16, and a core diameter of 7 [tm. The diameter of the saturable absorber (640) was I tm, while the inner and outer radii of the ring shaped gain medium (620) were 3.4 [tm and 3.5 tm, respectively. the Er 3+ -concentration was 2.38x 1025 M-3 in both the absorber and gain media, and the Yb± concentration was 2.97x 10" M-3 in the gain medium. The absorption and emission crosssections at the peak (wavelength 1536 run) Were both 6.8x 10-25 M2. Hence, the small-signal absorption at that wavelength was 2.1 dB/m in the saturable absorber, and 1.3 dB in the (unpumped) gain medium.

Moreover, at 1064 nm, the cross-sections for stimulated emission and absorption of the Yb 3+_ 5 ions were at 2x 1 0-26 M2 and 5x] 0-21 M2, respecti vely. The metastable lifetimes of the Er 3+ and the Yb 3+ were 10.2 ms and 1.3 ms, respectively. The energy was transferred from the Yb 3+ to the Er 3+ with a rate coefficient k, of 1.05x 10-" m'/s [6]. The spectral characteristics of the gain and absorber region followed those for E?+ and Yb 3+ in a phosphosilicate glass. Numerical calculations, following those in (4] and [6) and using the parameters above, showed that the lo extractable energy in this device was approximately 0.6mJ at 1536 nm and 1.1 mJ at 1560 run.

In the example above, the ring-shaped gain region was thin and hence the extractable energy per unit length small. This implied that the length of the fibre became so long (several hundred meters) that background losses could become important, and the calculated energy, neglecting background losses, difficult to achieve. By placing a fing- shaped gain medium outside

15 the core (where the non-nalized modal intensity is smaller), its gain can be kept constant while the stored energy in the gain medium is increased (cf. Eq. 17). Hence, thefibre can be shorter. For a cladding-pumped fibre having an inner cladding with a radius of 10 [tm, a saturable absorber (640) with a radius of 0.5 tm (small-signal absorption 2.2 dB/in at 1536 rim), and a gain medium (620) with an inner radius of 4.5 jAm and an outer radius of 5.5 [trn (small-signal absorption 2o 3.3 dB/m at 1536 nm), calculations gave an extractable energy of 0.8 mJ at 1536nm and 1.4 mJ at 1560 nm, for a fibre length of 50 in. The fibre length can be further reduced by using a larger area gain region (e.g., a thicker doped ring). The other parameters of the fibre were the same as above. A problem with this approached is that the preferred host material for a YV±sensitized Er 3±doped gain medium (phosphosilicate glass) has a higher refractive index than the preferred cladding (fused silica). Hence, some extra measure may be needed to level the refractive index of the gain medium with that of cladding.

The calculations have also shown that ASE in the long-wavelength end of the 5 4 113/2 _ 411 5/2 emission spectrum, where the emission cross-sections become relatively larger compared to the absorption cross-section, can build up and partly bleach the absorption and compress the gain. This can be avoided by introducing an unsaturable loss at these long wavelengths. Bending the fibre provides a method for making the fibre lossy at 1600 run, while keeping the unsaturable loss small at 1536 nm. For example, with the fibre parameters above and 10 with a bend radius of 9 mm, the bend-loss is approximately 0.033 dB/m at 1600 rim, 0.012 dB/m at 1560 nm, and 0.0061 dBlm at 1536 nm, i.e., it is five times smaller at 1536 nm than at 1600 mn. Another alternative for an unsaturable loss at longer wavelengths is to use an unsaturable absorber in addition to the saturable absorber. For this particular transition, Tm 3+:glass and Tb3+:glass are suitable systems for an optical fibre, as the absorption of suitable 15 pump wavelengths (e.g., 1064 nm or at least 1047 nm in the case of Tm 3+) is small, as is the absorption for a signal at 1536 nm. Yet another alternative may be to use different host media for the gain and the absorber media. A suitable host medium for the absorber makes its spectrum wider, and can thus prevent the build-up of ASE at long wavelengths.

In the example above, a pump wavelength of 1064 nm was assumed. Other wavelengths 2o are also possible. However, the pump should not pump the E?' directly, since then also the saturable absorber will be excited. Moreover, for pumps on the short- wavelength side of the Yb 3+ absorption peak, emission around 980 nm from the Yb 3+ may build up in the fibre and bleach the Er 3+_ions in the absorber.

Even if the centres providing the gain and the saturable absorption are different, a design according to Fig. I I can improve to prior-art devices in that the gain efficiency of the gain medium is relatively lower than it otherwise would be.

Signal amplifiers for reduced cross-talk 5 In some optical amplifiers, especially semiconductor ones, even the energy of a single signal bit (e.g., 0. 1 - 100 fl) may be non-negl igible comparable to the stored energy. Then, already the amplification of a single bit extracts enough energy to reduce the gain. This leads to four-wave mixing and cross-talk in multi-wavelength amplifiers and inter- symbol interference in single-wave length amplifiers. This can be avoided with the higher stored energy that, for a given 10 gain, accompanies the reduced interaction in the devices disclosed in this invention.

Figure 12 depicts a semiconductor amplifier that provides gain for one or several guided optical signal beams in a region where the normalized modal intensity is small. The device may be electrically pumped. The refractive index of the gain. region may be depressed in order to suppress gain-guiding, since this can otherwise occur in semiconductor optical amplifiers in 15 which the gain per unit length is large. This wo,uld lead to a large normalized modal intensity in the gain-region, thereby preventing substantial reductions of the interaction.

Suppression of unwanted modes Often, lasing on a specific transverse mode is desired, and then normally on the fundamental mode of the core. If so, it may be necessary to suppress other, undesired, modes.

20 Higher-order guided modes of the core extend further into the cladding and thus see a significantly higher gain than does the fundamental mode in a ring-doped device. Although we normally envisage single-moded cores as preferred designs, higher-order modes may also be present due to fabrication errors, etc. However, these modes are less strongly guided and will be more sensitive to bending. Hence, with a fibre, simply bending it can reinstate a net gain advantage for the fundamental mode.

Another alternative is to outside the gain region incorporate a region that absorbs the 5 signal (at desired and possibly also at undesired wavelengths) but has a low loss for the pump. This absorbing region is located so that it preferentially absorbs light in undesired modes. These can be higher- order modes of the core, and also cladding-modes. See Fig. 13.

Several possibilities exist for creating the absorbing region. In the case of a Yb-doped device, Pr+ and Er 3+ can be suitable such absorbers. For Nd3+ at 850 nm - 950 nm, Yb 3+ can be lo used. For Er 3+, TM3 + and Sm 3+ are potential candidates, just to mention some possibilitiies with rare-earth doping. Sm 3+ can also suppress unwanted 1050 rim radiation in Nd3±doped samples. Optionally, some additional measures may be taken to quench the dopant, to prevent it from bleaching.

In the embodiments described above a ring-shaped (generally cylindrical) doped region 15 has been used. However, the doped region does not of course have to be rotationally symmetric, nor evenly distributed along the length of the fibre or waveguide.

Tables

Wavelength Absorption Emission Active nm cross-section cross-section Remark - 2 io-25 M2 medium j(rII in Nd'+:glass 800 20 0 Pump to 4F512.

Nd'+:glass 870 10 10 Na'+:glass 1050 0 30 Unwanted wavelength Yb'+:glass 912 8.25 0.275 Pump Yb'+:glass 975 25.85 25.85 yb3+:g,aSS 980 6.76 8.57 yb3+:g,aSS 985 1.77 2.97 yb3+:glaSS 1030 0.45 63 Unwanted wavelength Er'+:glass 980 2 0 Pump to 411 112 Er3+:g,aSS '1531 5 5 Er'+:glass 1550 2.4 3.8 Er'+:g1ass 1564 1.6 3 Unwanted wavelength Table Al. Cross-sections for absorption and stimulated emission used in some numerical examples.

Quantity Symbol Value Numerical aperture NA 0.1 Core diameter 7 pm Cut-off wavelength 915 nm Doped area Adopd 38.5 'UM2 -3 Yb concentration [Yb'+] 2.7x] (P M Signal overlap with core rcore 0.796 Area of inner cladding APUMP 3080 'UM2 Pump overlap with core I-P. core 1180 Effective area ratiofor core-doped rfftj, 63.7 device Small-signal pump-absorption apss 1.21 dB/m Metastable lifetime 7 0. 76 ms Background loss - 0 dB/m

Reflectivity, pump launch end 99.9% at desired wavelength, 0 elsewhere Reflectivity at other end Either 3.5% broadband, or 50% at desired wavelength and 0% elsewhere Table A2. Values used in detailed Yb-calculations. Other parameters as in Table At.

Quantity Symbol Normal-core Large-core amplifier amplifier Core diameter 5,um I IUm Numerical aperture NA 0.171 0.100 Cut-off wavelength AC 1118 nni 143 7 nm Signal overlap with core 1-core 0,651 0.795 Area of inner cladding,for cladding- Apump 1571 'UM2 1571 pm 2 pumping - Pump overlap with corefor cladding- I-p, c, 1180 11165 pumping Effective area ratiofor core-doped 52.1 13.1 device Background loss 0 aV/m 0 dBlm

Table A3). Geometrical and dopant parameters for energ),-storage EDFAs.

Quantity Symbol Value Metastable lifetime T 10. 9 ms Absorption cross-section at 1480 nm 001480 1.87x] 0-21 M2 Emission cross-section at 1480 nm 01480 0. 75XIO-2-1 M2 Absorption cross-section at 980 nm 0980 2xl 0-25 M2 Absorption cross-section at 1550 nm CeI550 2.45xl 0-21 M2 2 Emission cross-section at 1550 nm C155, 3.83 xl 0-'-' M Pump intensity required at 1480 nm to invert 35.7% of 11w 0.0470 M wl the population UM2 Pump intensity required at 980 nm to invert hafthe 0.0930 M wl population UM2 Table A4. Spectroscopic parameters for energ)-storage EDFAs.

Publication References 1. C. R. Giles and E. besurvire, "Modeling erbiurn-doped fibre amplifiers", J. Lightwave Technol. 9, 271-83, (199 1) 2. R. Paschotta, J. Nilsson, P. R. Barber, A. C. Tropper, and D. C. Hanna, "Lifetime quenching in Yb doped fibres", submitted to Opt. Commun.

3. B. Pedersen, A. Bjarklev, J. H. Povisen, K. Dybdal, and C. C. Larsen, "The design of erbiumdoped fibre amplifiers", J. Lightwave Technol. 9, 1105-1112 (199 1) 4. J. Nilsson and B. Jaskorzynska, "Modeling and optimization of low repetition-rate highenergy pulse amplification in cw-pumped erbium-doped fibre amplifiers", Opt. Lett. 18, 2099-2101 (1993).

5. D. T. Walton, J. Nees, and G. Mourou, "Broad-bandvAdth pulse amplification to the I 0--RJ level in an ytterbium-doped germanosilicate fibre", Opt. Lett. 21, 1061- 1063 (1996) 6 J. Nilsson, P. Scheer, and B. Jaskorzynska, "Modeling and optimization of short Yb 3 sensitized Er3-doped fibre amplifiers", IEEE Photon. Technol. Lett. 6, 383-385 (1994).

Claims

Claims

I An amplifying optical device for amplifying a signal beam having a normalised intensity profile comprising:

a first waveguiding structure which comprises a first core and cladding and which is configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump power; and an amplifying region; wherein the second core is at least partly fon-ned by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly formed by at leastpart of the second core; wherein the pump source is optically coupled to the second core; and wherein in use the normalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value.

2. An amplifying optical device according to claim I wherein substantially all of the first core is formed of a non-amplifying medium.

An amplifying optical device according to claim I or claim 2 wherein the first and second waveguiding structures comprise an optical fibre.

i 4. An amplifying optical device according any one of the preceding claims wherein the first waveguiding structure supports a single transverse mode at the signal wavelength.

5. An amplifying optical device according to any one of the preceding claims wherein the second core is adjacent to at least one region having a lower refractive index than the second core.

6. An amplifying optical device according to any one of the preceding claims wherein the amplifying region comprises at least one rare earth dopant.

7. An amplifying optical device according to any one of the preceding claims wherein the amplifying region is doped with Ytterbium, and the amplifying region is characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are configured such that the amplifying optical device amplifies in a wavelength range from 950 rim to 1050 rim.

8. An amplifying optical device according to claim 7 wherein the amplifying region absorbs at least -3)2% of the optical pump power launched into the second waveguiding structure.

9. An amplifying optical device according to claim 7 or claim 8 wherein the wavelength range is from 975 ran to 985 rim.

10. An amplifying optical device according to any one of claims I to 6 wherein the amplifying region is doped with Erbium, and the amplifying region is characterized by a dopant concentration, a disposition and a length,and wherein the dopant concentration, the disposition and the length of the amplifying region are configured such that the amplifying optical device amplifies in a wavelength range from 1480 nm to 1570 nm.

11. An amplifying optical device according to anyone of claims I to 6 wherein the amplifying region is doped with Erbium co-doped with Ytterbium, and the amplifying region is characterized by a dopant concentration, a disposition and a leng,th, and wherein the dopant concentration, the disposition and the length of the amplifying region are configured such that the amplifying optical device amplifies in the wavelength range from 1480 run to 1570 run.

12. An amplifying optical device according to any one of claims I to 6 wherein the amplifying region is doped with Neodymium, and wherein the amplifying region is characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are arranged such that the amplifying optical device amplifies in a wavelength range from 850 nm to 950 nm.

13. An amplifying optical device according to any one of the preceding claims wherein the first waveguiding structure is defined by a cross-section defining a central area of the first core and wherein the amplifying region surrounds -the central area of the first core.

14. An amplifying optical device according to any one of the preceding claims wherein the first core is circular, and wherein the amplifying region is disposed in a circular ring concentric with the first core.

15. An amplifying optical device according to claim 14 wherein the first core has a diameter, and wherein the circular ring has an inner diameter between one and one point eight times the diameter of the first core.

16. An amplifying optical device according to claim 14 or claim 15 wherein the first core has a diameter and wherein the circular ring has an outer diameter between one and three times the diameter of the first core.

17, An amplifying optical device according to any one of the preceding claims wherein the amplifying region is disposed in a circular ring having an inner radius and a radial thickness.

18. An amplifying optical device according to claim 17 wherein the inner ra8ius of the amplifying region is between 4.2 microns and 6.6 microns.

19. An amplifying optical device according to claim 18 wherein the radial thickness of the amplifying region is between 1. 1 microns and 1.5 microns.

20. An amplifying optical device according to any one of the preceding claims wherein the first waveguiding structure is configured such that in use the optical radiation guided by the first waveguiding structure has a Gaussian equivalent spot size greater than about eight times the wavelength as rn easured in vacuum of the optical radiation guided by the first waveguiding structure 21. An amplifying optical device according to any one of the preceding claims wherein the first and second waveguiding structures are made of glass.

22. An amplifying optical device according to claim 21 wherein the glass is an oxide glass.

23. An amplifying optical device according to claim 22 wherein the oxide glass is selected from the group comprising silica, doped silica, silicate, aluminosilicate and phosphate.

24. An amplifying optical device according to any one of the previous claims and configured such that at least some of the optical pump power propagates along the amplii -ig region at least two times.

25, An ampiifying optical device according to any one of the preceding claims wherein higher-order signal-modes are made lossy by deliberately bending the waveguide.

26. An amplifying optical device according to any one of the preceding claims wherein the first waveguiding structure is configured to guide at least one desired signal mode and unwanted higher-order modes, and wherein the unwanted higher order modes are suppressed by the introduction of a signal-absorbing medium in a region of the cladding where the normalized intensity of the desired signal mode is weak compared to the normalized intensity of the unwanted higher-order modes.

27. A light source comprising an optical amplifying device according to any one of the preceding claims, and including feedback for promoting light generation.

28. A light source according to claim 27 wherein the feedback comprises one or more reflectors.

29. A light source according to claim 28 wherein at least one of the reflectors is a fibre Bragg grating.

30. A light source according to any one of claims 27 to 29 wherein the light source is Z:' a laser. - 31. An amplifying optical device according to any one of claims I to 26 and further comprising an input signal optically coupled to the first waveguiding structure, wherein the amplifying optical device amplifies the input signal to provide high power laser radiation.

32. An amplifying optical device according to any one of claims I to 26 whereinthe amplifying optical device has an intrinsic saturation energy, and further comprising an input signal in the form of an optical pulse optically coupled to the first waveguiding structure, wherein the disposition of the amplifying region is configured such that the amplifying optical device has high energy storage at its intrinsic lasing threshold, and the amplifying optical device is configured to be operated such that the input -signal is amplified so that it has an energy exceeding the intrinsic saturation energy of the amplifying optical device.

An ampl i fying optical device or light source according to any one of the preceding claims further comprising an optical switch, and wherein the amplifying optical device is configured to be operable such that energy is stored in the amplifying region with the optical switch in a blocking state, the energy being released in the form of at least one optical pulse when the optical switch is in a non- blocking state.

34. A method of pumping at least one optical fibre amplifier with a fibre laser, the fibre laser comprising:

a first waveguiding structure which comprises a first core and cladding, and which is configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump power; and an amplifying region; wherein the second core is at least partly formed by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly formed by at least part of the second core; wherein the pump source is optically coupled to the second core; and wherein in use the norTnalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value; the method comprising; configuring the fibre laser to provide optical feedback; pumping the fibre laser with optical pump power to provide a lasing output; and optically pumping the optical fibre amplifier with the lasing output.

35. A method according to claim 34 wherein the amplifying region of the second waveguide is characterized in that it absorbs at least 32% of the optical pump power guided in the second waveguiding structure.

36. A method of amplifying optical pulses to energies exceeding the intrinsic saturation energy of an amplifying optical device, the amplifying. optical device comprising:

a first waveguiding structure which comprises a first core and cladding, and Which is configured to guide optical radiation; at least one pump source configured to supply optical pump power; a second waveguiding structure comprising a second core and configured to guide the optical pump power; and an amplifying region; wherein the second core is at least partly formed by at least part of the cladding of the first waveguiding structure; wherein the amplifying region is at least partly fon-ned by at least part of the second core; wherein the pump source is optically coupled to the second core; and wherein in use the normalised intensity profile of the signal beam is smaller in the amplifying region than its maximum value; the method comprising; guiding optical radiation using the first waveguiding structure; and guiding the optical pump power using the second waveguiding structure such that the amplifying region interacts With the optical radiation guided in the first waveguiding structure and the optical pump power guided in the second waveguiding structure.

Ame, jdments to the claims have been filed as follows I Apparatus for amplifying a signal beam having a normalized intensity distribution comprising: an optical waveguide having a core, and a first cladding surrounding the core; at least one pump source configured to supply an optical pump beam; and a gain medium; wherein the gain medium is configured to be pumped by the optical pump beam, wherein the gain medium is situated in a region of the optical waveguide where the intensity of the signal beam is smaller than its peak intensity, and wherein the first cladding is configured to guide the optical pump beam so that the normalized intensity of the optical pump beam is substantially constant over the inner cladding during use of the apparatus.

2. Apparatus according to claim I whereinthe first cladding is surrounded by a second cladding having a lower refractive index than the first cladding.

3. Apparatus according to any one of the preceding claims wherein at least a portion of the gain medium is located in the core.

4. Apparatus according to any one of the preceding claims wherein at least a portion C> of the gain medium surrounds the core.

5. Apparatus according to any one of the preceding claims wherein the gain medium is a generally cylindrical region surrounding the core.

6. Apparatus according to any one of the preceding claims wherein the core is circular, and wherein the gain medium is disposed in a circular ring concentric with the core.

5-) 7. Apparatus according to claim 6 wherein the core has a diameter, and wherein the circular ring has an inner diameter between one and one point seven times the diameter of the core.

8. Apparatus according to claim 6 or claim 7, wherein the core has a diameter, and wherein the circular ring has an outer diameter between one point four and two point one times the diameter of the core.

9. Apparatus according to any one of the preceding claims wherein the gain medium is disposed in a circular ring having an inner radius and a radial thickness.

10. Apparatus according to claim 9 wherein the inner radius of the gain medium is between 3.5 [trn and 6.6 Rm.

11. Apparatus according to claim 10 wherein the radial thickness of the gain medium is between 1.0 tm and 1.5 Vm.

12. Apparatus according to any one of the preceding claims wherein the core and first cladding are made of glass.

13. Apparatus according to any one of the preceding claims wherein the core supports a single transverse mode at the signal wavelength.

14. Apparatus according to any one of the preceding claims wherein the gain medium comprises at least one rare earth dopant.

15. Apparatus according to any one of the preceding claims, wherein the core and first cladding comprise an optical fibre.

16. Apparatus according to any one of the preceding claims wherein the gain medium is doped with Ytterbium, and the gain medium is characterized by a dopant 5K concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the gain medium are configured such that the apparatus amplifies in a wavelength range from 950 nm to 1050 M.

17. Apparatus according to claim 16 wherein the gain medium absorbs at least 32% of the optical pump beam.

18. Apparatus according to claim 16 or claim 17 wherein the wavelength range is from 975 nin to 985 nrn.

19. Apparatus according to any one of claims I to 15 wherein the gain medium is doped with Erbium, and the gain medium is characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the gain medium are configured such that the apparatus amplifies in a wavelength range from 1450nm to 1600nm.

20. Apparatus according to any one of claims I to 15 wherein the gain medium is doped with Ytterbium and Erbium.

21. Apparatus according to any one of claims I to 15 wherein the gain medium is doped with Neodymium, and wherein the gain medium is characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the gain medium are arranged such that the apparatus amplifies in a wavelength range from 850 nni to 95 0 nrn.

22. Apparatus according to any one of the previous claims and configured such that at least some of the optical pump power propagates along the gain medium at least two times.

S1 23, Apparatus according to any one of the preceding claims wherein higher- order signal-modes are made lossy suppressed by deliberately bending the waveguide.

24. Apparatus according to any one of the preceding claims wherein the core is configured to guide at least one desired signal mode and unwanted higher- order modes, and wherein the unwanted higher-order modes are suppressed by the introduction of a signal-absorbing medium in a region of the first cladding where the normalized intensity of the desired signal mode is weak compared to the normalized intensity of the unwanted higher-order modes.

25. Apparatus according to claim 18, wherein the apparatus comprises feedback means, the apparatus being a laser.

26. Apparatus according to claim 25 wherein the feedback means comprises one or more reflectors.

27. Apparatus according to claim 26 wherein at least one of the reflectors is a fibre Bragg grating.

D 28. Apparatus according to any o-ne of the preceding claims I to 24 wherein, in use, an input signal is optically coupled to the core, and wherein the apparatus amplifies the input signal to provide high-power laser radiation.

29. Apparatus according to claim 28 wherein the input signal comprises a pulse.

30. Apparatus according to any one of the preceding claims, wherein the apparatus is configured to be operable such that a gain and a stored energy ale built up in the gain medium, the stored energy being released in the form of at least one optical pulse when the net gain of the apparatus exceeds threshold.

L c, 31. Apparatus according to any one of claims. I to 15 wherein the apparatus is configured to reduce signal cross-talk.

32. Apparatus according to any one of claims I to 15 wherein the apparatus is configured to reduce inter-symbol interference.

33. Apparatus according to any one of claims I to 14 wherein the waveguide is a planar waveguide.

34. Apparatus substantially as herein described with reference to Figures 2, 3, 4, 8, 10, 13a, l3b and I 3c of the accompanying drawings.