GB2377549A

GB2377549A - Tuneable laser

Info

Publication number: GB2377549A
Application number: GB0117228A
Authority: GB
Inventors: Douglas Charles John Reid
Original assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-07-14
Filing date: 2001-07-14
Publication date: 2003-01-15
Also published as: GB0117228D0; AU2002317278A1; WO2003009437A3; WO2003009437A2

Abstract

A tuneable diode laser having a gain region 2, a first mirror section 3 and a second mirror section 4 in which both of the minor sections are formed by distributed Bragg reflectors each having a plurality of repeat units of Bragg reflectors, characterized in that the number of repeat units in the first minor section has oddness or evenness parity with the number of repeat units in the second minor section.

Description

Tuneable Laser This invention relates to tuneable lasers, and has particular, but not exclusive, reference to diode lasers having an active section and front and rear mirror sections and has further particular reference to such lasers which are intended for use in Wavelength Divisional Multiplex telecommunication systems.

Background to the invention Many optical waveguide devices employ Bragg gratings either as a wavelength selective mirror or filter. For example the Distributed Feedback laser (DFB) employs one Bragg grating adjacent to the active pumped region, whereas the Distributed Bragg Reflector laser (DBR) employs a Bragg grating section, gain region and usually a phase control region, with a shared common waveguide. The use of such Bragg gratings with optical components is well known in the art.

Wavelength Divisional Multiplex telecommunication systems (and Dense Wavelength Divisional Multiplex telecommunication systems) rely upon either being fed by a plurality of individual wavelength DBRs, which can be individually selected, or by a wide tuning range tuneable laser that can be electronically driven to provide the wavelength required. Limited tuning range tuneable lasers are available that rely upon thermal effects for tuning. Additionally wider tuning ranges are also available with electronically tuneable lasers that are able to modify the reflective characteristics of the DBRs.

One example of an electronically tuneable laser is the Segmented Grating Distributed Bragg Reflector (SG-DBR) as described in the US Patent to Coldren, USP 4,869, 325. See also"Widely Tuneable Sampled Grating DBR Lasers to address 100 channels over 40nm for WDM applications", Proc. Euro. Conference on Integrated Optics-A M Gulisano, D J Robbins, P J Williams, and P Verhoeve (1997).

Another example is a laser using a Phase Shift Grating Distributed Bragg Reflector (PSG-DBR) as described in UK Patent to Reid, GB 2,337, 135B.

A characteristic of both these tuneable laser designs is that they have mirror sections in the form of Bragg gratings (front and rear) that bound the active or gain section of the diode laser.

The SG-DBR relies upon having a common pitch for the front and rear grating, but with the spacing between the grating segments being different between the front and the rear grating. Both the front and rear mirror sections are made up of a number of segments.

The PSG-DBR works in a different manner again, using a common pitch for the front and rear grating but instead of spaced segmentation relies upon different lengths or segments of the grating being serially combined with substantially 1t phase shift in between each length or segment of grating ; the combined length structure is variously repeated a number of times to make up the front and rear grating.

Both the SG-DBR and PSG-DBR produce comb wavelength responses at both the front and rear gratings characterised in that for a given set of drive currents in the front and rear grating sections there is simultaneous correspondence in reflection peak at only one wavelength; as a consequence the device lases at this wavelength. To change wavelength alternative tuning current drives are applied to the front and rear gratings to select a new singular wavelength of correspondence at which the laser then operates. Thus the front and rear gratings operate in a vernier mode, in which the wavelengths of correspondence determine a supermode wavelength. Any reluctance to lase can be compensated by having a phase control section between the active section and one of the mirror sections (usually the rear section) and changing the phase section current.

There are also variants upon these two generic tuneable lasers, as shown in the embodiment illustrated in Figure 1 of PCT Patent publication No. WO 00/54381 (Application No PCT/SeOO/00294), that utilises gratings which bound the phase region rather than the gain region of the device.

Brief description of the Invention By the present invention there is provided a tuneable diode laser having a gain region a first mirror section and a second mirror section in which both of the mirror sections are formed by distributed Bragg reflectors each having a plurality of repeat units of Bragg reflectors, wherein the invention is characterized in that the number of repeat units in the first mirror section has parity with the number of repeat units in the second mirror section.

Preferably the gain region is bounded on each end by the first and second section, with the first section being a front mirror and the second section being a rear mirror, or the first section being a rear mirror and the second section being a front mirror.

The front mirror preferably reflects 20% to 40% of the wavefront, and the rear mirror preferably reflects 50% to 80% of the wavefront. The front mirror may reflect 30% of the evanescent wavefront, and the rear mirror may reflect 70% of the wavefront.

The mirror sections may both have an even number of repeat units or an odd number of repeat units. The actual numbers of repeat units in the front mirror may be the same as the actual number of repeat units in the rear mirror section, or the number of repeat units in the front mirror section may be more than or less than the actual number in the rear section, and is preferably less than the number in the rear section.

In both the SG-DBR and PSG-DBR designs the selection of the quantity of repeat units in both the front and rear grating, in respect to whether odd or even quantities should be used, appears to have been indeterminate. The essence of the invention has been a determination of an optimal selection of the number of repeat units for a laser mirror structure.

For the avoidance of doubt a grating repeat unit would often be made up of a number of grating segments. A grating or mirror section comprises a number of repeat units. The essential thing about any repeat unit is that it is the same as any

adjacent repeat unit. In itself the repeat unit can and often would be made up of a number of different segments. Usually the segments would differ from one another within a repeat unit, but the invention is concerned with the numbers of repeat units, not the number of segments within the repeat units.

Brief Description of the Drawings The present invention will now be described with reference to the accompanying drawings, of which : Figure 1 is a schematic of a four-section tuneable diode laser Figure 2 is an enlarged portion of Figure I for a SG-DBR, Figure 3 is an enlarged portion of Figure I for a PSG-DBR, Figure 4. is a plot of the intensity reflection spectra of the front and rear mirror of a SG-DBR laser, with correspondence at one wavelength, Figure 5. is a plot of the intensity reflection spectra of the front and rear mirror of a PSG-DBR laser, with correspondence at one wavelength.

Figure 6. is an intensity and an amplitude plot of both an SG-DBR and PSGDBR laser mirror grating in which the number of repeat units is five, that is, an odd number, and Figure 7. is an intensity and an amplitude plot of an SG-DBR and PSG-DBR laser mirror grating in which the number of repeat units is six, that is, an even number.

Figure 8. is an intensity and an amplitude plot of a tuneable laser with two interacting comb mirrors in which both mirrors have an odd number of repeat units.

Figure 9. is an intensity and an amplitude plot of a tuneable laser with two interacting comb mirrors in which both mirrors have an even number of repeat units.

Figure 10. is an intensity and an amplitude plot of a tuneable laser with two interacting comb mirrors wherein the front mirror has an odd number of repeat units and the rear mirror an even number of repeat units.

Figure 11. is an intensity and an amplitude plot of a tuneable laser with two interacting comb mirrors wherein the front mirror has an even number of repeat units and the rear mirror an odd number of repeat units.

Figure 12 is a pictorial representation of the language definitions used in the description.

Description of Embodiments of the Invention Referring to Fig 1, this shows a four section tuneable laser in accordance with the invention. The laser I has an active or gain region 2 for generating light and a pair of mirrors 3,4 bounding the gain region at each end. In this case the mirror 3 is the front mirror and the mirror 4 is the rear mirror. A common waveguide 51 extends the length of the laser through all sections. The front distributed Bragg mirror 3 is the weaker mirror and is designed to reflect around 30% of the wavefront. The rear distributed Bragg mirror 4 is the stronger mirror and is designed to reflect around 70% of the wavefront. With such an arrangement coherent light is emitted from the laser at the front end 3, as at 50, via an antireflective coating 5. The other end of the laser 6 is also antireflective coated to enable access to some of the laser light 52 for analytical

or control purposes e. g. wavelength locking means. Normally a phase change section 7 can be incorporated between the gain region 2 and the mirror 4.

The mirrors 3 and 4 can be either SG-DBRs or PSG-DBRs. The circled portion 8 in Figure I is enlarged in Figs 2 and 3 to show diagrammatically the SGDBR grating and the PSG-DBR gratings respectively.

In the case of the SG-DBR as shown in Fig 2 there is a plurality of grating segments 9, 10 separated by spacings, such as spacing 11. In the case of the PSGDBR as shown in Fig3, the phase shift gratings are not separated but are continuous and an individual segment can be identified by an occurrence of a repeat position such as repeats 12,13 ; which thus define a segment in an PSG-DBR grating.

Fig 4 shows the intensity of reflection, 14, against wavelength, 15, for an SGDBR with the reflective comb for the front mirror shown at 16, above the reflective comb for the rear mirror shown at 17. It can be seen that the spacings of the peaks of the reflections which make up the front comb is slightly greater than that of the spacings of the peaks which go to make up the rear comb, and only peaks 18 and 19 coincide. It will also be seen that the intensity of the reflectance varies over the peaks.

This is a characteristic of the SG-DBR.

Fig 5 is a similar diagram to Fig 4, but this time for a PSG-DBR. Again the intensity is indicated by the vertical axis 20 and the wavelength by the horizontal axis 21. Again the upper line 22 is the reflectance comb for the front grating and the lower line 23 is the reflectance comb for the rear grating. The spacings of the reflectance

peaks which make up the comb of reflections for the front grating is again larger than the comb of reflections for the rear grating. This time only peaks 24 and 25 coincide.

It will be noticed that in the case of the PSG-DBR the reflectance intensity is substantially the same for all peaks.

The reflective peaks in the intensity vs. wavelength graphs shown above will be referred to below as supermode peaks.

The number of minor peaks or lobes between the supermode peaks is commonly known in optics to be two less than the number of repeat units. Referring to Fig 6, this shows in the upper left hand portion the number of minor lobes 27 (three) between the supermode peaks 28,29 for the SG-DBR mirror having an odd number (five) of repeat units. In the upper right hand portion of Fig 6 there is shown at 30 the minor lobes (three) between the supermode peaks 31 and 32 for the PSGDBR mirror again having an odd number (five) of repeat units. It will be appreciated that the upper portions of Fig 6 are no more in principal than repeats of Figs 4 and 5.

The intensity of reflected light energy is proportional to the modulus of the amplitude of the reflected light squared.

In the lower half of Fig 6, below the intensity lines 33 and 34, and shown in correspondence therewith, are the reflectance amplitude lines 35 and 36 for the SGDBR and PSG-DBR mirrors respectively. It can be seen that the minor lobes are both positive and negative about a notional zero line whereas the supermode peaks are mainly positive. This is true for both the SG-DBR and the PSG-DBR.

Fig 7 is a similar set of plots to those shown in Fig 6, except this time the number of repeat units is even (six). The important difference will be seen in the lower traces. In the upper portion of the drawing, the intensity for the supermode peaks and the minor lobes (four) is shown by trace 37 for the SG-DBR and by trace 38 for the PSG-DBR. Again the drawing shows intensity on the vertical axis and wavelength on the horizontal axis. The lower traces 39 and 40 show the amplitude in the vertical axis and wavelength on the horizontal axis with the same correspondence as before. In this case it will be seen that both the minor lobes and the supermode peaks are positive and negative, as at 41 and 42; and 43 and 44.

It can be seen therefore that if both of the mirrors have an odd number of repeat units or both have an even number of repeat units, i. e. there is evenness or oddness parity between the number of repeat units in the two mirrors then there will be constructive reinforcement at coincident supermodes peaks. There would be parity if there were seven repeat units in one mirror and five or seven or any other odd number of repeat units in the other mirror. There would also be parity if there were six repeat units in one mirror and four or six or any other even number of repeat units in the other mirror. However if there is a lack of evenness or oddness parity and one mirror has an even number of repeat units and the other mirror has an odd number then there will be destructive interference at the coincident pair of supermode peaks and as a result a large phase change current will be needed to overcome the consequential reluctance to lase at the required wavelength.

Referring to Fig 8, this shows the grating pattern for a pair of mirrors both having an odd number of repeat units. In the upper portion of the figure there is shown the wavelength pattern of the pair of reflectors. The peaks of the reflectors are shown at 50 to 58 and it can be seen that the major peaks 50,54 and 58 are separated by an odd number of smaller peaks, as would be the case with an odd number of repeat units. Minor peaks 51, 52 and 53 separate major peaks 50 and 54 and minor peaks 55,56 and 57 separate major peaks 54 and 58.

In the lower half of the drawing the amplitudes of the electric vector which produces the light are plotted in synchronism with the wavelengths. It can be seen that the amplitudes vary about a zero line. To determine the light intensity the two amplitudes are combined together and the result is squared to give the intensity.

Because the square of a minus number is positive the resultant intensity is always positive and not negative.

The peaks in the amplitude plot oscillate about the zero line and it can be seen that peaks 50A, 52A and 54A are all positive and peaks 51A and 53A are both negative. The amplitude peaks of 50A to 54A correspond to the wavelength peaks 50 to 54. The drawing in the central portion is of a similar form and peaks 60 to 68 relate to peaks 50 to 58 and peaks 60A to 64A relate to peaks 50A to 54A. There is, however, only one pair of coincident peaks at the wavelength of peaks 54 and 64. The gap between peaks 50 and 54 is slightly greater than the gap between peaks 60 and 64. Similarly the gap between peaks 54 and 58 is slightly greater than the gap between peaks 64 and 68.

In a similar manner, the gap between peaks 50A and 54A is slightly greater than the gap between peaks 60A and 64A. Likewise the gaps between peaks 54 and 58 is greater than the gap between peaks 64 and 68 and the gap between peaks 54A and 58A is slightly greater than the gap between peaks 64A and 68A. Thus distance 59 is greater than distance 69.

The amplitude of the minol peaks can effectively be ignored and it is only the amplitude of the supermode peaks 50A 54A, and 58A and their coincidence or otherwise with the peaks 60A, 64A and 68A which is of significance. As peaks 54A and 64A are at the same wavelength their amplitudes can be combined together and this results in a supermode amplitude peak 74A as shown on the right hand side of the drawing. It can be seen that this is larger than the peaks 71 A and 72A.. Similarly peaks 76A and 77A correspond to peaks 58A and 68A. Squaring the amplitude peaks gives the large supermode peak 74 and this is much larger than the adjacent peaks 71, 72 76 and 77. The peaks 71 and 77 are the same distance apart, 79, as distance 59 and the distance 78 corresponds to distance 69.

The minor amplitude peaks indicated generally by 73A and 75A are shown highly schematically as are intensity peaks generally indicated by 73 and 75.

Thus with oddness parity there is a strong reinforcement of the intensity of the light at the coincident supermode peaks.

An identical effect can be seen in Fig 9, which is a diagram similar to Fig 8, except having an even number of repeat units in each grating. In this case reference

will be made principally to the supermode peaks only. The amplitude graph for the supermode peaks 80,81 and 82 is shown at 80A, 81A and 82A in the lower left of the drawing. Because the amplitude oscillates about the zero line, the peaks 80A, 82A, 83A and 85A have positive amplitudes and the peaks 81A and 84A have negative amplitudes. When these are combined together the amplitude graph shown at the bottom right is produced with peaks 90A, 91A, 93A and 94A all being positive and peak 92A being negative. However when these values are squared to give the intensity plot shown at the top on the right of the drawing, the peak 92 is much greater than peaks 90,91 93 and 94. Again the distance 86 between peaks 80 and 82 is greater than distance 87 between peaks 83 and 85 and this again corresponds to distances 97 and 96 respectively.

Both of the above examples refer to parity of repeat unit evenness or oddness.

The next drawings illustrate the cases where there is a lack of evenness or oddness parity between the front and rear mirrors and the two cases to be illustrated are those where the rear mirror, which has a greater strength, has an even number of repeat units and the weaker front mirror has an odd number of repeat units and then the reverse arrangement is exemplified.

In Fig 10 the description will again focus on the supermode peaks. In the case of the weaker mirror, which has an odd number of repeat units, the amplitude peaks 10OA, 101 A and 102A which correspond to the peaks 100, 101 and 102 are all positive. However in the case of the stronger mirror, which has an even number of repeat units, although the amplitude peaks I I OA and 112A which correspond to peaks 110 and 112 are both positive the peak 1 HA in the amplitude graph and which

corresponds to peak 111 is negative. Thus when amplitudes are combined the amplitude graph shown in the bottom right hand comer is produced. The peaks 120A 121 A 123A and 124A relate to peaks 110A, 100A, 102A and 112A respectively.

However when amplitude 101 A at wavelength 101 is combined with the amplitude I I I A which is at wavelength 111, where wavelength 101 is the same as 111, the smaller positive amplitude IOIA reduces the negative amplitude 11 ira to give the small negative amplitude 122A. When squared to determine the light intensity this gives the lower peak 122. It will be appreciated that because the odd repeat units mirror is weaker than the even repeat units mirror, in this case the peaks 100, 101 and 102 are not as high as peaks 110, 111 and 112. In consequence peaks 120 and 124 are smaller than peaks 121 and 123.

A similar effect is to be obtained if the rear stronger mirror has an odd number of repeat units and the weaker front mirror has an even number of repeat units as shown in Fig 11. In this case the supermode peaks30, 131 and 132 in the front mirror correspond to two positive amplitude peaks 130A and 132 A and one negative peak 131 A. The stronger odd mirror peaks 140, 141, and 142 all correspond to positive amplitude peaks 140A, 141 A, and 142A.

Again the amplitude peaks 150A, 151A 153A and 154A are all positive but in this case the peak 152A is also slightly positive because the stronger positive peak 141A is slightly higher in value than the negative peak 131A. Thus the amplitude peak 152A is also slightly positive, but again the peak 152 in the light intensity is low compared to the values even of the weaker peaks 150 and 154.

As before, but in this case the grating with the even number of peaks is the weaker grating, the peaks 130, 131 and 132 are lower than peaks 140,141 and 142, and the peaks 150 and 154 are lower than the peaks 151 and 153.

For the sake of clarity, and by way of full explanation, Figure 12. illustrates a repeat unit 200 comprising three segments 210,202, and 203. It can be seen that the segments are of different lengths. The mirror section 204 is shown made up of three repeat units 205,206 and 207. It will be noted that each of the repeat units is the same as repeat unit 200, and that repeat unit 206 is identical to its adjacent neighbours 205 and 207. That is how a repeat unit is distinguished from the segments that go to make up the repeat units.

The quantity of segments in a repeat unit is arbitrary as far as the present invention is concerned. It is parity of the number of the repeat units in the two mirror sections which is important. If the mirror sections are SG-DBRs then each repeat unit contains only one segment. If, on the other hand, the mirror sections are formed of PSG-DBRs, then there will be a number of different segments in each repeat unit.

Thus, the inventor has discovered that dual mirror laser tuning systems that use an odd number of repeat units in both the mirrors, or an even number of repeat units in both the mirrors, will straightforwardly form constructive correspondence at supermode peak wavelengths. However, if one mirror has an even quantity of repeat units, whereas the other mirror has an odd quantity of repeat units, then at certain supermode peak wavelengths there will be destructive correspondence that will

require large phase section current changes to overcome the reluctance to lase at desired wavelength.

The inventor has also discovered that for optimum tuneability across all supermode wavelengths, both the front and rear mirrors should have either an odd number, or even number, of repeat units. By this means the supermode peaks will constructively reinforce each other at correspondence. And as a consequence smooth tuning will be obtained without large changes in phase current. However, mixed odd/even numbers of repeat units in dual mirror systems should be avoided else risk having alternate destructive supermode peaks which require large phase current changes to produce lasing, with all its associated problems of thermal changes introducing wavelength perturbations.

It will be appreciated that the principals expounded above may be utilized with any tuneable laser architecture in which two comb spectra created by means of Bragg gratings interact by whatever means. For example comb mirror sections that bound a gain region have a multiplicative effect in the gain region, whilst adjacent comb mirror sections on one side of the gain region have an additive effect in the gain region.

Claims

Claims 1) A tuneable diode laser having a gain region a first mirror section and a second mirror section in which both of the mirror sections are formed by distributed Bragg reflectors each having a plurality of repeat units of-ragg reflectors, characterized in that the number of repeat units in the first mirror section has oddness or evenness parity with the number of repeat units in the second mirror section.
2) A tuneable diode laser as claimed in claim 1 in which the gain region is bounded on each end by the first and second mirrors and the first mirror is a front mirror and the second mirror is a rear mirror.
3) A tuneable diode laser as claimed in claim 1 in which the gain region is bounded on each end by the first and second mirrors and the first mirror is a rear mirror and the second mirror is a front mirror.
4) A tuneable diode laser as claimed in claim 2 or 3 in which the front mirror reflects 20% to 40% of the wavefront, and the rear mirror reflects 50% to 80% of the wavefront.
5) A tuneable diode laser as claimed in claim in which the front mirror reflects 30% of the wavefront, and the rear mirror reflects 70% of the wavefront.

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6) A tuneable laser as claimed in any one of claims I to 5 in which the mirror sections both have an even number of repeat units or an odd number of repeat units.
7) A tuneable laser as claimed in any one of claims I to 6 in which the number of repeat units in the front mirror is more than the number of repeat units in the rear mirror section.
8) A tuneable laser as claimed in which the number of repeat units in the front mirror is less than the number of repeat units in the rear mirror section.
9) A tuneable laser as claimed in any one of claims 1 to 6 in which the number of repeat units in the front mirror is the same as the actual number of segments in the rear mirror section.
10) A tuneable laser as claimed in anyone of claims 1 to 9 in which the laser is a segmented grating distributed Bragg reflector laser.
11) A tuneable laser as claimed in any one of claims 1 to 9 in which the laser is a phase shift grating distributed Bragg reflector laser.
12) A tuneable laser as claimed in any one of the preceding claims in which there is provided a phase shift section between the active section and one or both of the mirror sections.

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13) A tuneable laser substantially as herein described with reference to and as illustrated by the accompanying drawings.