GB2373631A - Tuneable Laser - Google Patents

Abstract

A tuneable laser for example, a three section DBR, III-V semiconductor laser, has an active section 1, a phase section 2 and a Bragg reflector 3 comprising a plurality of discrete grating units. At least two of the gratings have a different pitch. When a current is applied to at least one grating with a longer pitch, the effective wavelength of the grating having a longer pitch can be tuned to the wavelength of a grating having a shorter pitch. The laser may be provided with a simple partial reflecting mirror 20. Each grating unit may have an independently actuable electrode 18. The laser may be manufactured using electron beam grating writing techniques, or manufactured using a holographic laser grating plate.

Description

Tuneable Laser The invention relates to a tuneable laser, in particular but not exclusively to a three section distributed Bragg reflector tuneable laser.

Tuneable lasers are well known in the field of optical communications, particularly in connection with wavelength divisional multiplex telecommunication systems, which rely upon either being fed by stacks of individually wavelength distributed Bragg reflectors (DBR) lasers, 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 that rely upon thermal effects for tuning are also available.

US 4896325 discloses a wavelength tuneable laser having sampled gratings at the front and rear of its gain region. The gratings produce slightly different reflection combs which provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Co-incidence of a maximum from each of the front and rear gratings is referred to as a supermode. To switch the device between super modes requires a small electrical current into one of the gratings to cause a different pair of maxima to co-incide in the manner of a vernier. By applying different electrical currents to the two gratings, continuous tuning within a supermode can be achieved. In practice, the reflection spectra of the known sampled grating structures have a Gaussian type envelope which limits the total optical bandwidth over which the laser can reliably operate as a single mode device.

In contrast to the Segmented Grating Distributed Bragg Reflector (SG-DBR) described above, a Phase Shift Grating Distributed Bragg Reflector (PSG-DBR) is disclosed in GB 2337135. This has a plurality of repeat grating units in which each grating unit comprises a series of adjacent gratings having the same pitch, which gratings are separated by a phase change of 7r radians, wherein the gratings have different lengths to provide a predetermined reflection spectrum.

The known devices have Bragg gratings which bound both ends of the gain and phase regions of a four section tuneable laser, which produces a comb wavelength response.

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 of which the device lases at that wavelength. To change this wavelength a different current is applied to the front and rear gratings. Thus the front and rear gratings operate in a vernier mode, in which the wavelengths of correspondence determine a supermode wavelength. Although the known devices have generally been acceptable, they share a tendency to suffer from short wavelength losses, which in combination with the front grating tuning absorption reduces the output power of the laser.

The present invention seeks to provide a tuneable laser with a higher optical output power whilst having acceptable manufacturing costs.

According to the invention, there is provided a tuneable laser having an active section, a phase section and a Bragg reflector comprising a plurality of discrete grating units, at least two of which gratings have a different pitch, wherein current is applicable to at least the grating having a longer pitch, such that the wavelength of the grating having the longer pitch can be tuned to the wavelength of the grating having a shorter pitch.

In use, current is applied to the grating unit having a longer pitch so that it is optically

equivalent to the adjacent grating having a shorter pitch when the respective reflective i maxima superpose, thereby providing the dominant wavelength at which the device can lase. This lasing wavelength can then also be current tuned in the known manner.

Preferably, the tuneable laser is provided with a simple partial reflecting front mirror. In a preferred embodiment, the reflector comprises a plurality of discrete grating units, each having a constant respective pitch corresponding to respective predetermined wavelengths, the grating pitch increasing with distance from the active section. Preferably each grating unit has an independently actuable electrode. Preferably a conventional switching circuit is provided to switch the current to the electrodes and grating units.

The tuneable laser of the invention has a number of advantages over the known designs, in particular minimising the short wavelength losses inherent in the four section DBR lasers of the prior art, thereby having higher power output. By dispensing with the front Bragg reflector, absorption is minimised as there is no contribution to tuning induced absorption from the front Bragg reflector, which usually dominates absorption. Also the absorption losses due to tuning in the Bragg reflector is reduced than in known SG-DBR lasers as the use of a shorter grating is facilitated.

An exemplary embodiment of the invention will now be described with reference to the drawings which show: Fig. 1 shows a schematic representation of a three section laser; Fig. 2 shows a schematic representation of a Bragg reflector; Fig. 3a shows a wavelength envelope using a known Bragg reflector; Fig. 3b shows a wavelength envelope using the Bragg reflector of the invention.

Figure 1 shows a three section tuneable DBR semiconductor laser having an active or gain section 1, a phase section 2 and a rear mirror section 3. Both the gain section 1 and the'phase section 2 are provided with electrodes 4,5. At the boundary of the gain section 1, a front mirror 20 is provided, which mirror can be a simple partial reflecting mirror or other suitable mirror. The laser further comprises an active region 6, the Bragg reflector layer 7 and substrate layer 8.

The rear mirror section 3 has a plurality of discrete Bragg grating units 10-16 etched into the waveguide. The pitch of the respective grating units increases with distance from the active section, with the grating having the shortest pitch closest to the active section and the grating with the longest pitch furthest from the active section. Each grating unit also has an associated electrode 18, which is actuable independently of the other electrodes 18.

In use in, for example, a C-Band or L-Band 40nm optical communication system, the difference in wavelength between adjacent grating units 10-16 will typically be about 4nm and the pitch of the respective grating unit 10-16 can be determined by the Bragg

condition = 2nef(p\ where is wavelength, nef is the effective refractive index of the waveguide material, A is the pitch for first order gratings, which are preferred as they provide the strongest coupling.

Figure 2 shows a schematic representation of a Bragg reflector forming the rear mirror section 3. The Bragg reflector comprises a plurality, in this case eleven for a 40nm bandwidth of which seven are illustrated, of discrete grating units 10-16, each having a different pitch. The grating unit closest to the phase section 2 has the shortest pitch and the pitch of each successive grating unit remote from the phase section is greater than the pitch of the preceding unit. Each of the grating units 10-16 has an associated electrode 18 which electrodes can be actuated independently of one another. The Bragg grating can be fabricated using electron beam writing techniques or phase mask holographic techniques.

In use. the arrangement of grating units produces a reflection spectrum having a series of comparatively small maxima, which are typically closer together than in the prior art systems. If a current is applied to one of the grating units 10-16 the effective refractive index of the grating and the active material immediately underneath the electrode is decreased and hence the wavelength of the grating can be current tuned. By appropriate tuning it is possible to tune one of the maxima corresponding to one of the grating units until it blends with the adjacent maximum of the grating having a shorter pitch to form a dominant wavelength, at which the device will start to lase. By successively tuning the maxima to blend with the corresponding shorter wavelength adjacent maxima, the laser can be tuned across a wide bandwidth. Typically, the grating pitches will be chosen so that the respective reflective peaks are separated by approximately half the maximum single peak tuning, e. g. 8nm. Therefore, the first wavelength is tuned until it blends with the adjacent wavelength (second), lases and then this wavelength can be current tuned until it reaches the next grating wavelength (third), at which point the current to first grating is cut off, and wavelength tuning continues with current drive in the second and third grating units.

Figure 3a shows part of the typical reflection spectrum obtained when no current is applied to the electrodes. The reflector has a number of discrete reflective peaks, which are of substantially similar intensities. These wavelength peaks are generally separated by about 4nm for a C-Band or L-Band device.

When current is applied an electrode in the manner described in relation to Figure 2 and so that one of the gratings, e. g. 55, is tuned, ultimately a spectrum is obtained as shown in Figure 3b. In this case, the peak 54 has an intensity approximately double that of each of the peaks 51,52, 53 and 56. It is at the wavelength peak 54 that the device will lase.

When the peaks blend together, it is possible that phasing effects generate features on the particular reflection maxima created which could be useful in particular applications such as to avoid mode hopping.

Generally, the phase shift will be controlled using the phase section in a conventional manner. It would, however, be possible to control the phase by selectively applying current to the electrodes located between the lowest wavelength driven reflecting grating and the active section.

Although the front mirror is preferably a simple partial reflecting mirror, it can be any suitable mirror which will have the same reflection spectrum as the rear mirror. Whilst a simple mirror will minimise losses, a Bragg grating could also be used.

The invention design may be suitably applied to solid state lasers manufactured using Group III- V or other semiconductor materials.

The photoluminescent gain curve of semiconductor materials is curved with intensity falloff at the edges of the spectrum. To produce a uniform intensity gain response across the bandwidth of interest, the Bragg grating unit length can be varied to give enhanced reflectivity where required.

Claims (10)

1. Claims 1. A tuneable laser having an active section, a phase section and a Bragg reflector comprising a plurality of discrete grating units, at least two of which gratings have a different pitch, wherein current is applicable to at least the grating having a longer pitch, such that the effective wavelength of the grating having a longer pitch can be tuned to the wavelength of the grating having a shorter pitch.
2. 2. A tuneable laser according to Claim 1, wherein the tuneable laser is provided with a simple partial reflecting front mirror.
3. 3. A tuneable laser according to Claim 1 or Claim 2, wherein the reflector comprises a plurality of discrete grating units, each grating unit having a different pitch, such that the grating unit closest to the phase section has the shortest pitch, the pitch of each successive grating from the phase section is greater than the pitch of the preceding unit.
4. 4. A tuneable laser according to any one of Claims 1 to 3, wherein each grating unit has an independently actuable electrode.
5. 5. A tuneable laser according to any one of Claims 1 to 4, wherein a switching circuit is provided to switch the current to the electrodes and grating units.
6. 6. A tuneable laser according to any one of Claims 1 to 5, manufactured using semiconductor materials.
7. 7. A tuneable laser according to Claim 6, manufactured using a III-V semiconductor material
8. 8. A tuneable laser according to any one of Claims 1 to 7, manufactured electron beam grating writing techniques.
9. 9. A tuneable laser according to any one of Claims 1 to 8, manufactured using holographic phase grating plate.
10. 10. A tuneable laser substantially as described herein, with reference to and as illustrated in the accompanying drawings.