GB2186115A - Laser array - Google Patents

Abstract

A laser array of n phase-locked injection lasers in which the configuration of the array is such that the (n-1)<th> order supermode of the array is cut-off. In a ridged array laser this is achieved by making the gap between adjacent ridges 1.3 microns or less.

Description

SPECIFICATION Laser array The amount of power that can be generated by a single injection laser is limited. One way of obtaining increased power is to collect the light from an array of lasers, for instance, as described in UKP 1343566 where the light from the individual members of the array is collected by individual fibres whose other ends are collected together into a bundle.

Such an array is an incoherent array for which there is no determined phase relationship between individual emitters. Greater power den- sity should be obtainable with a coherent array whose individual members are phaselocked to each other. Phase-locking is conveniently achieved by adopting a monolithic linear array structure in which the individual members of the array are sufficiently closely spaced for the optical field of the zero order mode of one member to be significantly overlapped with those of both its immediate neighbours. Under these circumstances there is a set of supermodes associated with the array. Usually it is the zero order supermode that is wanted because this has a single-lobed far-field pattern.

In practice it has been found that there is a problem in designing an array which will adequately discriminate against the generation of higher order supermodes over a useful range of power output. In the case of an array of 'n' lasers it is often found that the (n-l)'h order supermode presents the greatest problem. In the zero order supermode each individual member of the array is operating in its individual zero order mode and all such modes are in-phase with each other. The individual members are also operating in zero order mode in the (n- 1)th order supermode, but in this instance each individual member is operating in anti-phase with respect to its immediate neighbours. This supermode is characterised by a double-lobed far-field pattern.

According to the present invention there is provided a laser array of n phase-locked injection lasers in which the configuration of the array is such that the (n- 1)lh order supermode of the array is cut-off.

There follows a further explanation of the context of the invention and a description of an injection laser array embodying the invention in a preferred form. The description refers to the accompanying drawings, in which: Figure 1 depicts a schematic cross-section through part of a uniform real-index guided ridged array laser, Figure 2 depicts a plot of the intensity distribution of the.zero order and fourth order supermodes of a 5-element uniform array.

Figure 3 is a graph depicting the relationship between element width and normalised propagation constant for the array of Fig. 2, and Figure 4 depicts a plot of the intensity distribution of the zero order and fourth order supermodes of a 5-element Vee-chirped array.

In Fig. 1 there is depicted part of a schematic cross-section through a uniform real index guided ridged array laser. An n-type GaAs substrate 1 is covered by epitaxial growth with four layers 2 to 5 comprising an n-type (GaAl)As passive layer 2, a (GaAl)As active layer 3, a p-type (GaAl)As passive layer 4, and a p±type GaAs contact layer 5. Subsequently the semiconductor body is selectively etched to define a set of ridges 7, and the body is also provided with n-side and p-side metallisations 8 and 9. The refractive index of the material of the ridges 7 is higher than that of the medium that separates them, and hence they have an effect upon the waveguiding provided by the sandwich of the active layer 3 between the two lower index passive layers 2 and 4, providing an effective lateral refractive index profile as depicted at 10.Another function of the ridges is to channel the injected current, represented by arrows 11, to produce regions 12 of peak photon density.

Fig. 2 is a plot that shows at 20 the intensity distribution that is provided in the zero order supermode for a uniform 5-element waveguide structure 21 comprising five 2 micron wide stripes of effective refractive index 3.4684 separated by 1.3 microns wide regions of effective refractive index 3.4641.

Also depicted in this figure is a plot 22 of the intensity distribution of the (n- 1)th order supermode, in this case the fourth order. The close similarity between the intensity distributions of the zero order and fourth order supermodes means that in this structure it is difficult to provide a gain profile that will reasonably closely match the intensity profile of the zero order supermode, and hence efficiently excite that mode, without at the same time also providing a fairly close match to the intensity profile of the fourth order supermode.

Hence it is difficult with this structure to discriminate in favour of excitation of the zero order supermode and against excitation of the fourth order supermode. This situation is aggravated by the self-focussing effects of the zero order supermode and the corresponding progressively stronger defocussing effects of the higher order supermodes.

The injection of carriers into semiconductive material has the effect of reducing its refractive index, and therefore, in the case of a single waveguide element ridge laser, the carrier density profile at lasing threshold, being concentrated under the ridge, has the effect of reducing the height of the peak in the lateral refractive index profile. When the laser is driven progressively harder, the stimulated emission associated with the zero order mode tends to deplete the density of carriers in the region under the centre of this peak. This depletion of carriers is accompanied by an in crease in refractive index which strengthens the guiding of the zero order mode, thereby tending to concentrate its power in this central region. Thus the zero order mode is selffocussing.A corollary to this is that the concentration of optical intensity into the central region reduces the amount of stimulated emission in the wings. This, on the one hand reduces the refractive index, which enlarges the proportion of the intensity of higher order modes that resides in the wings, and on the other hand leaves more carriers available for stimulated emission in the higher order modes.

In this way any pre-existing discrimination against the excitation of the higher order modes is progressively eroded. An analogous situation exists also in the way supermodes compete with one another.

In relation to a uniform real index guided ridged array laser of n elements the present invention discloses how the problems associated with unwanted stimulation of the (n--l)'h order supermode are avoided by arranging for this mode to be cut-off. Provided that the carrier injection is arranged to have a profile reasonably similar to the intensity profile of the zero order supermode, the lower order supermodes present a less serious problem because their intensity profiles are more significantly different from that of the zero order supermode, and hence they are less efficiently coupled with the electrical excitation. For the plot of Fig. 2 it was assumed that, for the given refractive indices, the gap between adjacent ridges was 1.3 microns and that the width of each ridges was 2 microns.The value of gap width was chosen to provide a reasonable amount of coupling so that, on the one hand the coupling is not so strong that the array is scarcely distinguishable in behaviour from that of a single wide element, while on the other hand it is not so weak that the device has to is excessively sensitive to perturbation in the refractive indices of the individual emitters. The value of ridge width was selected so that on the one hand each individual element would be narrow enough for sin gle mode operation (first order mode cut-off), while on the other hand it would be as wide as convenient because this lowers the value of peak power density obtaining for any given value of total output for the array. Fig. 3 shows the effect of changing the width of the ridges.These effects are presented in terms of the square of the normalised transverse propagation constant Q. 0 ranges in value from 0, for which a supermode has an effective refractive index numerically equal to that of the high refractive index material of the structure, to- the value 1, for which it has an effective refractive index numerically equal to that of the low refractive index material of the structure and hence is unguided (cut-off).

Fig. 3 shows that, provided the ridge width is kept less than 2.6 microns, fifth order supermodes and higher, that is supermodes associated with 1st order and higher modes of individual ridge elements, are cut off. More particularly it shows that, provided the ridge width is kept less than 1.3 microns, the fourth order supermode, is cut-off. This is the (n - 1 )th order mode of the five element array.

From considerations of power requirements it will not generally be desirable to employ a ridge width much smaller than that indicated by the (n1)th order supermode cut-off, and so a typical value of ridge width for the given values of refractive index and gap width lies in the range from 1 microns to 1.2 microns.

Although the foregoing specific description has related to a uniform array it is to be understood that the invention is applicable also to chirped arrays. In a chirped array the widths of the individual elements are not identical, and hence the waveguiding strength is different in different places across the width of the array.

One form of chirp is the linear chirp in which the width of the individual elements increases in equal amplitude steps from the element on one side of the array to the element on the other side of the array. A primary effect of a linear chirp is to separate the intensity distributions of the zero order and (n--l)'h order supermodes, with that of the zero order supermode being concentrated toward the side with the thicker elements, and that of the (n- 1)h order supermode being concentrated toward the side with the thinner elements.

Another form of chirp is the Vee-chirp in which the elements are thickest at the centre of the array, and are progressively thinner toward both sides of the array. This has the effect of concentrating the zero order supermode intensity toward the centre of the array as depicted in the case of a particular five element array by trace 40 of Fig. 4 and concentrating the (n-l)'h order supermode in the wings of the array as depicted by trace 41.In the particular case of the chirped array 42 of Fig. 4, which is provided for the purposes of illustration only, the chirping can be seen to be rather stronger than optimum insofar as it has concentrated the zero order supermode into the central region registering with the central element almost to the exclusion of elsewhere, and hence the elements other than the central one have little part to play in the generation of the zero-order supermode. This in turn means that the zero-order supermode output is little different from that that would have been obtainable as the zero-order mode output of a simple single element possessing no optical coupling with any other elements.

When self-focussing effects are taken into account in the consideration of the operation of these chirped arrays it is found that the effect is to tend in the direction of cancelling the discrimination afforded in favour of the preferential excitation of the zero order super mode. In the case of the Vee-chirped array for which the (n- 1)lh order supermode is not cutoff (i.e. an array that is not constructed in accordance with the teachings of the present invention), a primary advantage of the chirp is to provide an initial discrimination in favour of the excitation of the zero order supermode and against the excitation of the (n - 1 )th order supermode. This advantage is obtained at the expense of tending to concentrate the zero order supermode intensity toward the central region of the array.It has already been explained how, in a uniform array, the self-focussing phenomenon encountered at high drive levels has the effect of concentrating the zero order supermode intensity at the centre of the array, thereby reducing discrimination against the (n- 1)th order supermode. A similar effect is also observed in the Vee-chirped array, but in this instance it is aggravated by the concentration that results from the chirp profile.

By arranging for the (n- 1)th order supermode to be cut-off, the problem of discriminating against this supermode and in favour of the zero order supermode is eliminated, and hence an array can be constructed with an inverse-Vee-chirp whose elements get progressively wider toward the sides of the array.

This broadens the intensity profile of the zero order supermode at threshold, and this broadening can be made use of to extend the range of single zero order supermode operation to higher output powers that would be achievable with an equivalent uniform or noninverted Vee-chirped array.

Claims (4)

1. A laser array of n phase-locked injection lasers in which the configuration of the array is such that the (n- 1)th order supermode of the array is cut-off.

2. An array as claimed in claim 1, which array is a real-index guided ridged array.

3. An array as claimed in claim 1 or 2, which array is a chirped array.

4. An array as claimed in claim 3, wherein the chirping provides stronger real-index guiding for elements of the array near the edges of the array than that provided for elements near the centre of the array.