GB2195822A - Injection lasers - Google Patents

Abstract

A double channel planar buried heterostructure semiconductor injection laser includes a tapered region 17 where the waveguide width is adiabatically reduced to produce a mode spot size closely matched with the model spot of a conventional single mode silica fibre optically coupled with the laser. <IMAGE>

Description

SPECIFICATION Injection lasers This invention relates to injection lasers, and in particular to the design of such lasers for achieving good coupling efficiency to single mode optical fibres in a manner that does not involve particularly tight tolerance in registration of the fibre with the laser.

Injection lasers designed for use with single mode fibre are typically constructed by a method involving the growth of a number of layers defining a heterostructure providing an optical waveguiding effect in the direction normal to the plane of those layers. The laser axis extends along an orthogonal direction lying in the plane of these layers, and an optical waveguiding effect in the remaining orthogonal direction, lateral waveguiding, is provided by some form of stripe geometry. This stripe geometry maybe achieved in a variety of different ways, including, but not limited to, by means of a ridge profile, by means of an inverted rib structure, or by means of a buried heterostructure format.In a Double Channel Planar Buried Heterostructure (DCPBH) laser for instance the stripe geometry is formed in the active region, and is defined by the two channels. in such lasers the mode spot size of the light at the output facet (size defined by the contour at which the intensity is l/e2 of its peak value) measures about 2pm x 1,um, which is quite significantly different from the spot of the mode of a typical single mode fibre, whose model size is typically about 10Am in diameter.The problem that this mismatch presents has been addressed by lensing the end of such a fibre in order to reduce the fibre mode diameter to about 5Am. For a typical DCPBH laser this improves the maximum coupling efficiency, as compared with launching directly into a fibre with a cleaved (planar) end, from less than 15% to about 50%, but this improvement is obtained at the expense of a tightening of the alignment of the fibre with respect to the laser to about + 0.8,um for a 1dB misalignment loss. This is much more stringent than the alignment required in a typical single mode splice for which the alignment requirement is about + 2.4m for a 1dB misalignment loss. With a lensed fibre the launch efficiency is limited to a maximum of about 50% primarily by lens aberration effects.The coupling efficiency can be increasd above the 50% attainable with a lensed fibre by providing the end of the fibre with an overmoded adiabatic taper, that is a taper that sufficiently gradual to perform mode transformation without introducing mode conversion (i.e. it changes mode size and shape, but does not increase the number of modes).

With such a taper it is in principle possible to achieve coupling efficiencies of about 90%, but again the increase in efficiency is attained at the expense of a further tightening of the alignment tolerance. In this instance the 90% efficiency requires an alignment accuracy of about + 0.4m to keep the misalignment loss to 1dB.

The achievement of an alignment to less than 1 um presents some difficulty, and hence cost penalty, but the maintaining of such alignment, once achieved, during the whole in service life of the laser involves considerably more difficulty and expense. The present invention is concerned with a coupling between fibre and laser that will be capable of providing a coupling efficiency greater than that achievable with the lensed fibre end, but at the same time have a less stringent alignment requirement.

According to the present invention there is provided an injection laser having a waveguiding structure which dfines the laser axis which structure is tapered along at least a portion of its length to provide the laser with a mode spot size more closely approximating to the spot size of the mode of a single mode optical fibre optically coupled with that laser.

There follows a description of an InGaAsP DCPBH laser embodying the invention in a preferred form. The description refers to the accompanying drawings in which Figure 1 is a schematic perspective view of the laser, Figure 2 is a diagram illustrating how the laser spot size changes with the width of the quaternary material active region between the two channels of the laser, Figure 3 is a plot of coupling efficiency as a function of modal capacity for coupling light into a typical single mode silica fibre from a circular symmetry waveguide having core cladding refractive indices matching these of a typical DCPBH injection laser, Figure 4 depicts a function of limiting taper semivertical angle cr, also plotted as a function of modal capacity, and Figure 5 is a comparative plot showing different sensitivities of different forms of coupling to loss induced by displacement offset.

The basic structure of the DCPBH laser of Figure 1 is very similar to that of a conventional DCPBH laser. Thus it has an n-type InP substrate 1, with metallic contact layer 2, an n-type epitaxially grown layer 3 of InP, and then an active layer 4 of quaternary material covered with an anti-meltback layer 5 also of quaternary material which is itself covered with a p-type layer 6 of InP. The quaternary layers 4 and 5, and the p-type InP layer 6 are interrupted by two channels 7 which descend into the n-type InP layer 3 and define an intervening stripe 8. These channels 7 are filled by the growth of two further semiconductor layers 9 and 10 and these layers themselves are covered with two more semiconductor layers 11 and 12. Layers 9 and 10 are both layers of InP respectively p-type and n-type.

Layer 11 is also a p-type layer of InP, while layer 12 is a p-type capping layer of quaternary material. Layer 12 is covered with an electrically insulating layer 13 in which is opened up a window 14 conforming in shape with and registering with the stripe 8. Overlying layer 13 is a metallic contact layer 15 which makes electric contact with the underlying semiconductor material in the area defined by the window 14.

Thus far in the description of this device its structure has not been distinguished from that of a conventional DCPBH laser. The structure is distinguished in particular in that the channels 7 are not of uniform cross-section along the whole of their length, but are shaped so that the quaternary material of layers 4 and 5 lying within the bounds defined by the stripe 8 is in the form of a parallel-sided strip 16 over the majority of its length, but has its sides inwardly tapered over a region 17 where it approaches the output facet of the laser.

In a conventional DCPBH laser the channels 7 are of constant cross-section along the whole of their length, and the stripe of quaternary material formed by layers 4 and 5 is typically between 1.2 and 1 .5m in width and between 0.2 and 0.25,um in thickness. These dimensions result in an approximately elliptical modal spot fairly tightly bound to the high index core as depicted schematically by the broken line 21a in Figure 2 for a stripe geometry 22a. If the width of the stripe were progressively narrowed to less than lum as represented by the sequence 22a to 22d the corresponding modal spot 21a to 21d would become enlarged and progressively more nearly circular.

If a conventional DCPBH laser having a strip between 1.2 and 1.5,um in width and between 0.2 and 0.25Am in thickness is directly butted against a conventional single mode silica optical fibre the optimum coupling efficiency is typically less than 15%. This is due to the mismatch in size and shape between the fibre mode and the laser mode.

The field distribution for a small rectangular guide of refractive index n, set in a field of refractive index n2 is difficult to analyse, but in the weakly guiding limit, the field distribution becomes relatively insensitive to the precise details of core shape, and the rectangular guide may be modelled by an equivalent circular guide, which is mathematically more tractable. For step index guides having the same index difference, 'equivalent' guides have equal cross-sectional area (Snyder and Love p.333).

Curve 30 of Fig. 3 shows the calculated coupling efficiency at a wavelength of 1300nm between a typical single mode silica fibre (8 ,um core diameter, .006 index difference) and a model circular section guide (core and cladding indices 3.5 and 3.4-i.e. values typical of a DCPBH laser) as a function of model guide cross-sectional area. The "equal area" comparison should represent a good approximation for areas < 0.25,us2.

The curve shows that a reduction in magnitude of the cross-sectional area produces an increase in coupling efficiency until a maximum value of 90% is reached at area 0.2,us2. At this point the size has been reduced to the point where the mode spot size of the circular cross-section guide has increased to a value giving the best match with the 5.2,um radius Gaussian spot size of the silica fibre. Coupling efficiency is not 100% at this point because the refractive index difference of the fibre produces an intensity distribution which is very close to Gaussian, but the greater refractive index difference of the circular cross-section guide, in relation to its physical size, makes the intensity distribution of this guide a noticeably less close approximation to Gaussian.

In the present instance, in the case of a stripe thickness of 0.2m and a stripe width of 1.5Am the mode is relatively tightly bound to the core and hence the field distribution departs very significantly from circular symmetry. By the time however the stripe width has been reduced to about 1.2Am or smaller, the relevant part of curve 30 provides a good approximation.

In designing a suitable taper 17 it is necessary to take account not only of the size of the core at the small end of the taper but also the rate at which tapering occcurs down the length of the taper. Clearly the rate at which the modal spot size will expand in response to the reduction in stripe width will not be as fast as its expansion in a medium with no waveguiding properties. This means that if the stripe width tapers too quickly it will not be properly effective. An approximate guide to the maximum rate of taper is given by the value a, where a = (1 - n22n21)112. W2/V, and where a is the maximum tolerable halfangle of the taper at any point along its length, n1 and n2 are respectively the core and cladding refractive indices, W is the conventional cladding parameter of the waveguide and V its conventional waveguide parameter.

For V > 1.2 an approximate value for W is given by the expression W = 1.1 428V - 0.996, while for V < 1.2 the corresponding expression is W = 1.122 exp-[Jo(V).(n21/n22- 1) / [J1(V).2V] Given that n, = 3.5 and n2 = 3.4, Figure 4 depicts the variation of a with the cross sectional area.

Taking the a values from Figure 4 for a taper measuring at its large end 1 .3jtm in width and 0.2m in thickness, the accompanying Table shows the calculated minimum length of taper sections as the stripe width is progressively reduced from 1 .3jtm to 0.9,um. Using the data of Figure 3, the Table also lists the calculated coupling efficiency. From this Table it is seen that the first four sections exercise a major beneficial effect, taking the maximum coupling efficiency from a measured value of less than 15% to a calculated maximum of 75%.Extending the total length of the taper by 40% increases the coupling efficiency by a further 5%, but shortly thereafter the rate of improvement falls off sharply with the inclusion of section 8 providing only a 1% improvement.

For this reason it will for many applications not be worthwhile to include the last two or three sections, but to terminate the taper at a width of about 1.05 or 1.00,um.

For the purposes of comparison Figure 5 depicts the additional loss engendered by displacement offset for a single mode lensed fibre coupled to a conventional DCPBH laser, given by trace 50 (experimental), 2) a cleaved end single mode fibre directly butted against a conventional DCPBH laser, given by trace 51, (experimental) and 3) a cleaved end single mode fibre directly butted against a DCPBH laser with a tapered stripe constructed in accordance with the present invention, given by trace 52 (calculated).

Claims (5)

1. An injection laser having a waveguiding structure which defines the laser axis which structure is tapered along at least a portion of its length to provide the laser with a mode spot size more closely approximating to the spot size of the mode of a single mode optical fibre optically coupled with that laser.

2. An injection laser as claimed in claim 1, wherein the waveguiding structure is formed by a stripe which is adiabatically tapered down in width towards the output facet of the laser.

3. An injection laser as claimed in claim 1 or 2, wherein the tapering is to a size providing the laser with a mode spot that is substantially circularly symmetric.

4. An injection laser as claimed in claim 1, 2 or 3, wherein the waveguiding structure is provided by a double channel planar buried heterostructure (DCPBH) laser structure.

5. An injection laser substantially as hereinbefore described with reference to the accompanying drawings.