OPTICAL DEVICE

Abstract: An optical device for use as an amplifier or modulator comprises a semiconductor substrate (2) with an elongate waveguide region (1) with a light guiding boundary that extends between an input (3) and an output (4) for optical radiation. An optically active layer of material (6) produces amplification of light travelling inthe waveguide region (1). The width (w) of region (1) tapers outwardly from the input to allow amplification of input optical signals, and tapers towards the output (4) so as to concentrate the amplified light in a single mode to the output. The concentration of the amplified light in the active region may produce non-linear effects which are exploited in order to achieve data modulation or switching. In one embodiment, the waveguide region includes a first active region (30) to produce optical amplification, and a second passive region (31) for concentrating light to the output.

Description: Optical Device Field of the Invention This invention relates to an optical device and has particular but not exclusive . application to integrated structures formed on a substrate, for use as an amplifier or a modulator. Background It is well known that certain materials such as semiconductors are capable of 0 aαing as optical amplifiers. For example, when certain semiconduαors, which exhibit a band gap, are subjeα to an injeαed eleαric current, an incident photon causes an eleαron to traverse the gap with the result that an additional photon is generated, thereby producing optical amplification. Semiconduαor optical amplifiers and lasers which operate in this way are well s known and reference is direαed to "Long Wavelength Semiconduαor Lasers" G.P. Agrawal and N.K. Dutta, Van Nostrand, Chapters 1 to 6. The semiconduαor material which is used as the aαive amplification region of the device suffers from a gain-saturation effeα which imposes a limitation on 0 the maximum power that can be obtained. In order to overcome this problem, proposals have been made in the past to provide the aαive amplification region as a tapered structure on the substrate, which widens along its length so that as amplification proceeds, a greater cross seαion of material is available for amplification, permitting increased amplification s power to be achieved. Reference is direαed to Mehuys et al: "5.25 W, CW Near Diffraαion Limited Tapered Stripe Semiconduαor Optical Amplifier", IEEE Phot Tech. Letts. 5 pp 1179-1182, 1993. In this arrangement, an expanding path through the aαive amplification region is achieved by means of an input lens arrangement and the resulting amplified light is colleαed by O an output lens. Another arrangement with an expanding, tapered aαive region, but with an integrated laser source, is described in Parke et al "2.0W CW Diffraαion Limited Operation of a Monolithically Integrated Master Oscillator Power Amplifier", IEEE Phot Tech. Lens, 5, 297-300 1993. Reference is also direαed to a similar struαure described in Bendelli et al "A New Struαure for High Power TWSLA", IEEE Phot Tech Lets. 1, 1991, pp 42-44. Reference is also direαed to "Q-S witched Bow-Tie Lasers for High Energy Picosecond Pulse Generation," K. A. Williams et al, Eleα Lett, Feb 1994, Vol 30, No. 4, pp 320-21 which illustrates outwardly tapering amplifier regions in a laser to avoid gain saturation. In Koyama et al "Multiple Quantum Well GalnAs/GalnAsP Tapered Broad Area Amplifiers with Monolithically integrated Waveguide Lens for High Power Applications", IEEE Phot Tech Letts, 5 pp 916-919, 1993, there is described the use of an integrated lens to re-focus the output of the expanding, tapered aαive region in order to simplify coupling into a single mode fibre. However, in praαice, difficulties arise in fabricating the integrated lens. In accordance with the present invention there is provided an optical device comprising an elongate waveguide having a boundary that extends along the length thereof to an optical output at one end, and an aαive material for producing amplification of light travelling in the waveguide, the waveguide boundary being configured to concentrate the amplified light laterally within the waveguide towards the output. Thus, by means of the invention, the light can be concentrated within the waveguide region itself, towards the output, without the need for additional lens struαures. The waveguide region may have a width which tapers along the length thereof towards the output to achieve the concentration of the amplified light. Optical amplification can be achieved by applying an eleαric current to the aαive material, and the current density that in use passes through the active material may be imparted with a non-uniform spatial profile in order to avoid optical gain saturation. The eleαric current may be fed into the active material through a coπduαive layer configured in first and second separate s portions so that different values of current can be fed to different regions of the aαive material. Also, in accordance with the invention it has been appreciated that gain saturation may be used advantageously in the device to achieve modulation for 0 data transmission purposes. The device may have a first source of optical data pulses coupled to an input to the waveguide region and a second source of optical radiation e.g. essentially continuous radiation, coupled to the waveguide region. In one embodiment of the invention, the data pulses from the first source and the radiation from the second source interaα so as to s produce a gain saturation in the aαive material, so that the radiation from the second source is amplitude modulated by the data pulses. A filter may be provided to pass preferentially the resulting pulse modulated signal. Also, the sources may interaα so that the radiation from the second source is phase modulated by the data pulses from the first source at a relevant receiver, and a 0 phase deteαor may be used to deteα the modulation. Also, the radiation from the two sources may interaα to produce radiation at the output which is of a different wavelength from the pumping radiation and the data pulses. This output radiation may be a phase conjugate of the radiation from the first source. S The aαive material may extend over the entire extent of the waveguide region, or for a high power amplifier, the waveguide region may include a first aαive portion coupled to the aαive material, and a second passive portion with its boundary configured to concentrate the amplified light, 0 thereby avoiding gain saturation. Refleαive means may be provided at opposite ends of the aαive region in order to induce a laser aαion therein. The device may be fabricated on a substrate, with the waveguide region comprising a buried heterostructure. Alternatively, the waveguide region may s comprise a rib waveguide structure. Brief Description of the Drawings In order that the invention may be more fully understood, embodiments thereof will now be described by way of example with reference to the 10 accompanying drawings in which: Figure 1 is a plan view of an aαive amplification region for a device in accordance with the invention; Figure 2 is a schematic seαional view of the device shown in Figure 1 when is fabricated according to a first embodiment; Figure 3 is a schematic seαional view of a substrate with epitaxial layers used for forming the device as shown in Figure 2; Figure 4 is a schematic seαion of the device of Figure 1 when fabricated according to a second embodiment of the invention; 20 Figure 5 is a schematic diagram of a device in accordance with the invention, which uses gain saturation and/or a wavelength conversion in order to achieve data modulation; Figure 6 illustrates a version of the device which uses a phase shift in its output to achieve phase modulated data pulses; 2. Figure 7 is a plan view of another optical device in accordance with the invention; Figure 8 is a schematic seαional view of the device shown in Figure 7 taken along the line B-B', when fabricated according to a first procedure; and Figure 9 is a corresponding seαional view of a device shown in Figure 7, 0 when fabricated according to a second procedure. Detailed Description Figure 1 illustrates in plan view a semiconduαor optical device A in accordance with the present invention which, in the described embodiments is fabricated in the InGaAsP material system for use in telecommunication systems with wavelengths centred on 1.3 and 1.55 μm. The device consists of an elongate waveguide region 1 which produces aαive amplification, formed on a substrate 2. The elongate waveguide region 1 has an input 3 at one end for optical radiation and an optical output 4 at its other, second end. The input and output 3, 4 are configured to couple onto single mode optical waveguides e.g. optical fibres or alternatively other struαures (not shown) integrated onto the substrate. The width w of the waveguide region 1 increases progressively from the input 3 in the direαion of the length / of the waveguide towards the central or intermediate region 5, from which the width w progressively decreases to the output 4. A typical example of the device has a length / of 500 μm - 1 mm, with the inputs 3 and 4 having a width w of 1 - 2 μm, and the width w in the intermediate region being of the order of 30 - 40 μm. The waveguide region 1 can be fabricated in a number of different ways and two examples will now be described with reference to Figures 2 to 4. Referring now to Figure 2, this shows a schematic cross seαion taken along the line A-A' of Figure 1 for which the waveguide region 1 is fabricated as a buried heterostruαure. Referring to Figure 2, the waveguide region 1 comprises a layer of aαive i-InGaAsP material 6 formed on a n-InP layer 7, itself formed on a n-InP substrate 8. The aαive material 6 is overlaid by a p-InP region 9 formed with a conduαive over-contaα 10 of p-InGaAsP material. The general struαure of the amplifier, the thicknesses of the various layers and the dopant concentrations, are conventional and reference is direαed to A.W. Nelson, W. J. Devlin, R. E. Hobbs, C.G.D. Le on and S. Wong: "High-power, low- threshold BH lasers operating at 1.52 μm grown entirely by MOVPE", Eleαronics Letters, Vol. 21, No. 20, pp 888-889 (26 September 1985). Referring to Figure 3, the struαure is formed from an initial substrate 8 which is formed with layers 6, 7, 9 and 10 over its entire surface in a manner known per se, and material is then seleαively etched from the regions shown in dotted outline, using a mask in a manner known per se, the mask having a shape as shown in Figure 1, so as to define the varying width w of the waveguide region along the length thereof. Thereafter, as shown in Figure 2, p-InP layer 11 and n-InP layer 12 are formed overlying the seleαively etched regions by standard MOVPE eleαric growth techniques. An eleαrically conduαive bottom metallisation layer 13 is formed on the underside of the substrate. Thus, the aαive material 6 is sandwiched between p-InP region 9 and n-InP layer 7 so that when a voltage is applied between the conduαive layer 10 and the bottom layer 13, an eleαric current passes through the aαive material 6. The polarity of the voltage and the arrangement of the n and p doped layers 11 and 12 is such that in use, they form a reverse-biased junαion, with the result that the current is direαed seleαively through the waveguide region 1 rather than to each side. Thus, photons incident on input 3 (Figure 1) cause eleαrons to traverse the band gap of the aαive material 6 so as to generate additional photons, thereby producing optical amplification in the waveguide region. For further details of the struαure of the device and various modifications thereof, reference is direαed to Nelson et al, supra. A different fabrication technique will now be described with reference to Figure 4 which shows an alternative version of the seαion taken on A-A' of Figure 1. In this struαure, the waveguide region has its boundary defined by means of a rib waveguide. The device consists of a substrate 14, formed of n-InP material with an overlayer 15 of optically aαive i-InGaAsP material in a waveguide region 1 shown in dotted outline. The layer 15 is overlaid by an etch-stop layer 16 of p-InGaAsP material which is overlaid by a rib 9 of p-InP material, itself covered by a conduαive contaα region 10 of p-InGaAsP material. For further details of the general struαure of a ridge waveguide struαure, reference is direαed to I.P. Kaminow, R.E. Nahory, M.A. Pollack, I.W. Stulz and J.C. De Winter: "Single-mode c.w. ridge-waveguide laser emitting at 1.55 μm", Eleαronics Letters, Vol. 15, No. 23, pp 763-764 (Nov 1979). The layers 9 and 10 are formed in the curved shape shown in Figure 1 by a lithographic mask and seleαive etching in a similar manner to that described with reference to Figure 2. The resulting struαure is overlaid with a dieleαric material 17, typically silicon dioxide. When a voltage is applied to the layer 10, a current is established through the region of layer 15 which is overlaid by the layer 9, so that amplification occurs therein within the region 1 shown in dotted outline. It will be appreciated that the optical confinement produced by the structure shown in Figure 4 is less well defined than with the configuration of Figure 2. Referring again to Figure 1, the single mode optical signal applied to input 3 is thus amplified within the waveguide region 1. The single mode expands adiabatically towards the central, intermediate region 5, and is thereafter adiabatically contraαed, still in the single mode, and inserted through the output 4 e.g. into an output optical fibre conneαed thereto (not shown). By "adiabatic" we mean no significant coupling into higher order transmission modes. Thus, the boundaries of waveguide region 1 themselves aα to concentrate the amplified light laterally to the output 4, with the advantage that no additional lens struαures are required to focus the amplified light into the output optical fibre, as in the prior art . The conduαive layer 10 is arranged in two portions 10a, 10b that overlie corresponding regions la, lb of the waveguide region 1. The current densities in the regions 10a and 10b are seleαed individually so as to be higher in region 10a than in region 10b, such that amplification primarily occurs in the aαive material 6 in the region la, where the outwardly tapered shape of the region reduces the risk of gain saturation . In the region lb, the waveguide region 1 aαs to concentrate the resulting amplified light into the output 4 without further significant amplification in the underlying aαive material 6, so as to minimise the risk of gain saturation. The region 10a in this example is fed with a higher current than the region 10b in order to achieve the desired current density differential. Furthermore, the conduαive layer 10 could be arranged in more than two portions so as to profile the current density spatially through the aαive material 6 and thereby minimise optical gain saturation. When the device is used as an amplifier, anti-refleαion coatings are provided on facets 18, 19 for input 3 and output 4. In this case, the refleαivity needs to be sufficiently low to avoid laser aαion even at high drive currents, i.e. refleαivity of the order of 0.001 or, preferably, 0.0001. Alternatively, if it is desired to promote a laser aαion within the waveguide region 1, one or more of the facets 18, 19 may be left uncoated or one end may be high-refleαive coated, so as to produce semi-refleαive end regions that promote resonance within the region. The uncoated refleαivity is normally around 0.3, and a high-refleαivity coating would produce refleαivities of around 0.9 or higher. Furthermore, in accordance with the invention, it has been appreciated that when a suitable bias voltage is applied to both of the regions 10a, 10b, non¬ linear effeαs such as gain saturation produced in the region lb, can be used to produce data modulation, as will now be described with reference to Figures 5 and 6. In Figure 5, the device A of Figure 1 is shown with an optical fibre 20 conneαed to its input, the fibre including a coupler 21 which has a first input port 22 that receives a stream of optical data pulses at a wavelength λ, and aαs as a first source for the device, and a second input port 23 that receives essentially continuous wave (cw) radiation at a second different wavelength λ 2> the second port 23 aαing as a second source for the device A. The wavelengths λ„λ 2 are chosen to be within the gain bandwidth of the aαive material 6 in the waveguide region 1. In use, both of the regions 10a,10b are fed with a bias voltage that results in an optical amplification, and the level of amplification in region lb (Figure 1), in the absence of λ, is close to, but does not exceed the level at which gain-saturation occurs. When the pulse stream λ, is applied, no gain-saturation occurs in the absence of a pulse, whereas during the occurrence of a pulse, the increased amplitude raises the optical power level sufficiently to produce gain-saturation so that the gain of the material 6 in waveguide region 1 collapses and a significantly reduced level of the radiation λ, reaches the output 4. For a general discussion of the technique, reference is direαed to B. Mikkelsen et al., ECOC 1993 post deadline paper ThP 12.6 pp. 73-6 "20 Gbit/s Polarisation Independent Wavelength Conversion in a Semiconduαor Laser Amplifier". The device A according to the invention has the significant advantage that the waveguide region la produces sufficient amplification of the data pulse stream λ, and the cross modulation can then occur in region lb, in an integrated device, thereby providing a very compaα and efficient means for producing switching using gain saturation, for use as a modulator. Also, the speed of the cross modulation effeα is enhanced by the concentration of light produced in the region lb, in comparison to a conventional amplifier with an untapered waveguide. The output from the device A in Figure 5 is fed through an optical fibre 24 to a bandpass filter 25 tuned to the radiation wavelength λ ; in order to separate the amplified output signal. The device A according to the invention can also be used as in Figure 6 to exploit another non-linear effeα. As described in B. Mikkelsen et al, supra an input at wavelength λ, can be used to phase shift radiation at a different wavelength λ ; . This effeα is exploited in the configuration shown in Figure 6 to achieve phase modulation. The input data pulses λ, and the cw radiation λ 2 are applied through coupler 26 to the input 3 of the device A through coupler 21, as previously described with reference to Figure 5 and additionally, the cw radiation is fed through the coupler 26 and a variable amplifier or attenuator 27 to be mixed with the output of the device A through a coupler 28. The cw radiation λ 2 is phase modulated in the device A by the data pulses λ,. The resulting phase modulation produced at the output of device A is compared, by means of the coupler 28, with the phase of the cw radiation λ 2 , which aαs as a reference, with the result that the output in fibre 24 is amplitude modulated in accordance with the phase modulation that occurs in device A. A further non-linear effeα is described in M.C. Tatham et al., "20nm Optical Wavelength Conversion Using Nondegenerate Four Wave Mixing", IEEE Phot Tech Lett, 5, pp 1303-6, 1993 and concerns wavelength conversion. When the data pulses at wavelength λ, and a continuous pumping signal at wavelength λ, are fed into an amplifier, a third, different wavelength signal can be produced by a non-linear third order, intensity dependent effect. This effeα can be exploited by means of the apparatus as shown in Figure 5, in which the filter 25 is tuned to the third wavelength in order to separate it from the input wavelengths λ, and λ 2 . The third wavelength signal λ 3 may be a phase conjugate of the cw radiation λ 2 as described in M.C. Tatham et al, "Compensation of fibre Chromatic Dispersion by Optical Phase Conjugation in a Semiconduαor Laser Amplifier", El Lett., 29 pp 1851-2, 1993. Referring again to Figure 1, the specific shape adopted for the waveguide region 1 varies from application to application. In many situations, a symmetrical configuration as shown in Figure 1 is advantageous, with the locus of the boundary of the region 1 conforming to portions of circles, in regions x, y, z close to the inputs and outputs, and in the intermediate region 5. The conneαing regions of the boundary, in regions p conform to portions of a parabola. Many other modifications and variations will be readily s apparent and the rate of increase and decrease of the width w in the waveguide region can vary along the length thereof depending upon the particular application and non-linear effeα that is to be exploited. Also, to increase the non linear effeα, the output 4 of the device may include o a further non-tapered elongate region (not shown) formed integrally on the substrate, which may include a continuation of the layer 6 of optically aαive material. Another embodiment for use as a high power amplifier will now be described with reference to Figures 7 to 9. It can be considered as a modification of the embodiment shown in Figures 1 and 2 and like pans are shown with the same references numerals. In the embodiments described previously, the aαive material 6, which produces optical amplification, extends over the entire extent of the optical waveguide region 1. However, in the embodiments of Figures 7 to 9, rather than have the aαive material 6 disposed over the entire extent of the waveguide region 1, the aαive material 6 is disposed solely in a first aαive portion 30 of the waveguide region 1, and the remaining part of the waveguide 1 constitutes a second, passive portion 31, which is transparent for the relevant wavelength range of light travelling in the waveguide. Thus, light travelling in the waveguide is amplified solely in the aαive portion 30, by the process previously described. Since the width of the waveguide w increases progressively from the input 3 to the intermediate region 5 in the aαive portion 30, the increasing power produced by the amplification does not cause gain saturation. The resulting amplified light then passes into the passive portion 31. The width of the waveguide region 1 in the passive portion thereof, progressively decreases towards the output 4. This causes a lateral concentration of light within the waveguide towards the output. Since the material of the passive portion of the waveguide is non-aαive, gain saturation cannot occur as a result of the concentrating effeα of the tapering boundary of the waveguide region. As a result, the device can be used advantageously as a high power amplifier, with the passive portion 31 providing an integrated struαure for direαing the amplified light into the output 4 e.g. to an optical waveguide, without the problem of gain saturation. The aαive and passive portions 30, 31 of the waveguide can be formed in a number of different ways. Referring now to Figure 8, this shows a seαion taken along B-B' of Figure 7 according to a first struαure. The general struαure is similar to that shown in Figure 2. However, the waveguide region 1 is defined by an additional layer 32 of InGaAsP material, which has a bandgap seleαed so that it is transparent to the amplified optical radiation and is hence passive. The optically aαive material is provided as the layer 6 which, as previously described, is formed of i-InGaAsP material, but in this embodiment, overlies the layer 32 solely in the aαive portion 30. The layer 6 thus produces amplification of light travelling in the waveguide. As previously described, the layer 6 is sandwiched between a p-InP layer 9 and a n-InP layer 7, with a conduαive over-contaα 10 of p-InGaAsP material. In order to fabricate the struαure, the layers 32, 6, 9 and 10 are formed as continuous layers over the entire surface area and then the layers 6, 9 and 10 are seleαively etched from the passive portion, and replaced by a cladding region 33 of i-InP material, which is grown in its place. The resulting configuration is then seleαively masked and etched to provide the curved waveguide boundary, as described with reference to Figures 1 and 2. In a specific example of the device shown in Figures 7 and 8, the aαive layer 6 was formed 0.15 μm thick with a bandgap of about 1.55 μm and the layer 32 had a thickness of 0.4 μm and a bandgap of 1.1 μm. Alternatively, the waveguide may be formed as a ridge struαure, by forming the layer 32 as a continuous layer, and forming a ridge from the layers 6, 9, 10 and 33 according to general techniques disclosed in Sherlock, G., Burton, J., Fiddyment, P., Sully, P., Kelly, A., and Robertson M. "An Integrated 2x2 Optical Switch with Gain", Eleαronics Letters 30 (1994), pp 137-8. s Referring to a second form of the high power amplifier shown in Figure 9, the passive layer 32 is omitted. The layers 6, 9 and 10 are seleαively etched and then a passive layer 34 of InGaAsP is grown so as to form a continuation of the layer 6, as a heterolayer. The layer 34 typically chosen to have a bandgap of 1.3 μm when the device is to operate at 1.5 μm. The layer 34 can o be grown by metal-organic vapour phase epitaxy (MOVPE). By this technique, the layers 6 and 34 can be formed with an optical quality waveguide junαion at which no significant refleαion occurs. The layer 34 is then covered by a cladding region 35 of p-InP material. The resulting struαure is then seleαively masked and etched to provide the curved s waveguide boundary, as previously described. A general example of MOVPE can be found in "1.55 μm Butt-jointed Distributed Bragg Refleαor Lasers Grown Entirely by Low-pressure MOVPE", Y. Tohmori and M. Oishi, Jap. J. App. Phys. 27 (1988), pp. L693-695. 0 Thus, from the foregoing, it will be seen that the embodiments shown in Figure 7 to 9 have the advantage that amplification can occur in the aαive portion without significant gain saturation, due to the increasing width of the aαive portion 30 along the length of the device and that, the resulting amplified light can then be concentrated within the waveguide region 1 by the S aαion of its boundary, so as to be direαed to the output 4 without problems of saturation effeαs due to gain saturation, as a result of the passive nature of portion 31 of the waveguide, and without the need for additional separate lens struαures. 0 As used herein, the term optical radiation includes visible and non-visible radiation such as ultra-violet and infra-red.

Claims: Claims 1. An optical device comprising an elongate waveguide region (1) having a s boundary that extends along the length ( ) thereof to an optical output (4) at one end, and aαive material (6, 15) for producing amplification of light travelling in the waveguide region, the boundary being configured to concentrate the amplified light laterally within the waveguide region (1) towards the output (4). 0 2. A device as claimed in claim 1 wherein the waveguide region has a width (w) which tapers along the length ( ) thereof towards the output (1). 3. A device as claimed in claim 1 or 2 including means (10) for applying s an eleαric current to the aαive material to produce optical amplification therein. 4. A device as claimed in any preceding claim including an optical input (3) at a second end of the waveguide region for introducing optical radiation to be amplified therein. 5. A device as claimed in claim 4 wherein the width (w) of the waveguide region progressively increases along the length (I) thereof from the input (3) to an intermediate region (5), from which the width of the waveguide region progressively decreases along the length thereof to the output (4). 6. A device as claimed in claim 5 wherein the rate of increase and decrease of the width (w) of the waveguide region varies along the length (/) thereof. 7. A device as claimed in claim 5 or 6 wherein the boundary of the waveguide region is parabolic in shape in regions (p) between the intermediate region (5) and the ends (3, 4) thereof, and conforms to portions of circular arcs (x, y, z) at the ends (3, 4) thereof and in the intermediate region (5). 8. A device as claimed in claim 6, 7 or 8 wherein the shape of the boundary is symmetrical on either side of the intermediate region (5). 9. A device as claimed in any one of claims 4 to 8 having a first source (22) of optical data pulses (λ.) coupled to the input of the waveguide region and a second source (23) of optical radiation (λ^ coupled to the waveguide region. 10. A device as claimed in claim 9 wherein the radiation from the second source and the data pulses from the first source interaα so as to produce gain saturation in the aαive material, whereby the radiation from the second source is modulated by the data pulses. 11. A device as claimed in claim 10 wherein the data pulses comprise optical radiation at a first wavelength (λ,) and the radiation (λ^ from the second source comprises essentially continuous wave (cw) optical radiation at a second different wavelength. 12. A device as claimed in claim 11 with a filter (25) coupled to the output of the waveguide region for preferentially passing the modulated radiation at the second wavelength. 13. A device as claimed in claim 11 or 12 and coupling means (21) for coupling both the first source (22) and the second source (23) to the input (3) of the waveguide region (1). 14. A device as claimed in any one of claims 8 to 11 wherein the radiation from the sources interaαs whereby the radiation from the first source is phase modulated by the data pulses from the second source. 15. A device as claimed in claim 14 with phase responsive means (27, 28) coupled to the output (4) of the waveguide region for comparing the phase modulated signals with the phase of signals from said output. 16. A device as claimed in any one of claims 8 to 15 wherein the radiation from the first and second sources interaαs to produce radiation at the output (4) which is of a different wavelength from that of the radiation from the second source and the data pulses from the first source. 17. A device as claimed in claim 15 with filtering means (25) coupled to the output (4) of the waveguide region (1) and seleαively responsive to said different wavelength at the output. 18. A device as claimed in any preceding claim wherein the aαive material (6, 15) extends over the entire extent of the waveguide region (1). 19. A device according to any preceding claim including means (10a, 10b) for profiling the current density that in use passes through the aαive material (6) for controlling optical gain saturation. 20. A device as claimed in claim 19 including a conduαive layer 10 for feeding an eleαric current through the aαive material (6), the conduαive layer being configured in first and second separate portions (10a, 10b) for applying different values of current to different regions of the aαive material. 21. A device according to any one of claims 1 to 17, wherein the waveguide region (1) includes a first aαive portion (30) optically coupled to the aαive material (6) to produce the optical amplification, and a second passive portion (31) which is not coupled to the aαive material and has its boundary configured to produce said concentration of the amplified light within the waveguide region towards the output. 22. A device as claimed in claim 21 wherein the passive portion has a width (w) that tapers along the length (I) thereof towards the output. 23. A device as claimed in claim 21 or 22, wherein the aαive portion (30) of the waveguide region has a width (w) which increases along the length (I) thereof in a direαion towards the output (4). 24. A device as claimed in claim 21, 22 or 23, wherein the waveguide region (1) includes a passive optically transparent layer (32) extending through both of the aαive and passive poπions, and a layer (6) of the aαive optical amplification, said layer (6) and the passive transparent layer (32) overlying one another in the aαive poπion (30). 25. A device as claimed in any one of claims 21 to 23, wherein the waveguide region includes a heterolayer (6, 34) that comprises the aαive amplification material (6) in the aαive poπion (30) and a passive transparent layer (34) in the passive poπion (31). 26. A device as claimed in claim 25, wherein the heterolayer (6, 34) has been formed by seleαively removing a poπion thereof and regrowing the layer (34) in the removed poπion with different optical charaαeristics. 27. A device as claimed in any preceding claim including refleαive means (18, 19) at opposite ends of the waveguide region (1) for promoting a laser aαion therein. 28. A device according to any one of claims 4 to 26 including anti- refleαion coatings on said input (3) and said output (4). 29. A device as claimed in any preceding claim wherein the aαive region comprises a buried heterostruαure (6) on a substrate (2). 30. A device as claimed in claim 29, wherein the aαive material (6) comprises a layer of InGaAsP disposed between a layer (7) of InP of a first conduαivity type and a region (9) of InP of a second conduαivity type. s 31. A device as claimed in any one of claims 1 to 28 wherein the waveguide region comprises a rib waveguide struαure (9, 10). 32. A device as claimed in claim 30 including a layer (14) of InP of a first conduαivity type overlaid by a layer of i-InGaAsP (15) that comprises the o aαive material, itself overlaid by a layer (16) of InGaAsP of a second conduαivity type, and an overlying strip (9) of InP having a configuration that defines the boundary of said waveguide region in said i-InGaAsP layer.

Inventors: DAVIES DAVID ARTHUR OWEN, DAVIES, DAVID, ARTHUR, OWEN
Applicants: BRITISH TELECOMM, DAVIES DAVID ARTHUR OWEN, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, DAVIES, DAVID, ARTHUR, OWEN
Application year: 1995
Main classification: H01S3
IPC codes: G02B6, G02F1, H01S3, H01S5