GB2371920A

GB2371920A - Sampled Gating Distribiuted Reflector Laser

Info

Publication number: GB2371920A
Application number: GB0102611A
Authority: GB
Inventors: Peter John Williams; David James Robbins
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: 2001-02-02
Filing date: 2001-02-02
Publication date: 2002-08-07
Also published as: WO2002063731A3; WO2002063731A2; GB0102611D0

Abstract

A sampled grating distributed reflector laser comprises an active section (2), a phase section (8) and at least one sample grating section (4, 6). The or each sample grating section comprises a plurality of grating bursts distributed along the optical axis of the section such that the section exhibits a comb of reflection maxima and is such that the application of a respective electrical current (i<SB>F</SB>, i<SB>R</SB> and i<SB>phase</SB>) to the at least one grating section (4, 6) and phase section (8) shifts the wavelength of the comb and changes the effective cavity length of the laser thereby enabling tuning of the laser. The laser is characterised in that the at least one grating section (4, 6) is provided with an electrode (12, 16) which comprises a plurality of electrical contacts (20a-20d, 22a-22d) which overly a respective grating burst. The dimensions of the contacts (20a-20d, 22a-22d) are selected such that electrical current is substantially localised to the grating bursts. The grating bursts may be of constant period or chirped. The laser may be fabricated as a surface or buried ridge laser.

Description

Sampled Grating Distributed Reflector Laser

This invention relates to a sampled grating distributed reflector laser and more especially, although not exclusively, to such a laser for use in optical wavelength division multiplex (WDM) communications.

WDM optical communications require laser sources which are capable of selectably producing light of wavelengths corresponding to the WDM channels. In early WDM systems having only a few channels, typically eight, it was known to use a separate single wavelength lasers for each channel and to selectively switch between lasers. Each laser was operated continuously to ensure temperature and hence wavelength stability. Although such an arrangement provides an adequate performance for a limited number of channels the demand for ever higher communications capacity has lead to an increase in the number of channels with a consequential decrease in the wavelength spacing of channels making the use of a dedicated single wavelength laser for each wavelength channel impractical.

For example at present in'C'band WDM systems are being proposed which have a

hundred or more channels with a data rate of lOGBs-. It is predicted that data rates will rise and may soon be as high as 40GBs-'or greater. At the same time optical carrier separation has decreased from 1.6nm (this corresponds to a frequency separation of 200GHz) to O. 8nm (lOOGHz) for dense WDM systems and it is expected to further decrease to 0. 4nm (50GHz) for ultra dense WDM systems.

Wavelength tuneable lasers, such as sampled grating distributed reflector lasers, are

emerging as a possible means of meeting future demands in WDM systems. Figure 1 is a schematic longitudinal sectional representation of a known a sampled Bragg grating distributed reflector laser. The laser 1 comprises four sections: an active section 2 in which light is generated; front and rear sample grating reflector sections 4,6 bounding the active section 2 and a phase section 8 located between the active section 2 and the rear reflector section 6. Light is generated within the active section and is guided within an optical guiding layer 10. The phase section 8 is used to tune the effective optical cavity length of the laser by the application of an electrical current phase to an associated electrode 11 to inject carriers into the underlying optical guiding layer and thereby change its refractive index.

The front and rear reflector sections 4,6, each comprise a sampled Bragg grating, that is a plurality of grating bursts 4a-4d, 6a-6d, of constant period that are spaced along the section with a constant period Lp, LR respectively. As a result of the grating being sampled, each grating section acts a reflector having a comb of equally wavelength spaced reflection peaks or maxima. The spacing of reflection peaks is determined by the period of the grating bursts and the period Lp, LR of the sampled grating. Respective electrodes 12,16 are provided on a upper surface off the laser and overlay the length of the associated reflector section. The application of an electrical current iF, iR to the electrodes 12,16 changes the carrier concentration within the guiding layer of the section and thereby changes the refractive index of the reflector section. The change in refractive index displaces the entire comb of reflection peaks in terms of wavelength and is used in combination with the phase section 8 to tune the laser to a selected wavelength of operation.

The periods LF, LR of the sample gratings are selected such that the combs have different wavelength spacing of their reflection peaks and are such that only one reflection peak from each comb can coincide at a given time. The wavelength of operation of the laser corresponds to the wavelength at which the reflection peaks coincide. In order to tune the raser appropriate injection currents iF, iR iphase are applied to the grating and phase sections. A course wavelength tuning is obtained by displacing one comb relative to the other such that a different pair of peaks coincide and fine wavelength tuning achieved by displacing the two combs by an equal amount.

Whilst in principal continuous wavelength tuning of such a laser is possible, its speed of tuning is likely to be too slow for current and future WDM systems. The inventors have appreciated that during tuning, as the injection currents are changed, the temperature of the laser's sections are altered and this in turn affects its wavelength of operation. As a result the speed of tuning of the laser is limited by how quickly it can regain thermal equilibrium and hence a stable wavelength of operation. The present invention has arisen in an endeavour to provide a tuneable sampled grating distributed reflector laser which in part at least overcomes the limitations of the known lasers.

According to the present invention a sampled grating distributed reflector laser comprises: an active section, a phase section and at least one sampled grating section which comprises a plurality of grating bursts distributed along the optical axis of the section such that the section exhibits a comb of reflection maxima and wherein application of electrical current to the at least one grating section and phase section

respectively shifts the wavelength of the comb and changes the effective cavity length of the laser thereby enabling tuning of the laser ; characterised in that the at least one grating section is provided with an electrode which comprises a plurality of electrical contacts which overly a respective grating burst and wherein the contacts are configured such that electrical current is substantially localised to the grating bursts.

By restricting the injection of current to the grating bursts, as opposed to the whole of the grating section, this minimises the total current required to attain a given injected carrier density within the grating bursts and so attain a given change of wavelength of the comb of reflection maxima. This reduction in current minimises the temperature change of the section thereby enabling more rapid tuning. Furthermore, it is found that the reduction in current and associated temperature change reduces the free carrier absorption loss which results in a higher and more stable optical output for a given tuning current.

Preferably each grating burst is a Bragg grating of constant period. Alternatively each grating burst is chirped.

In a preferred embodiment the laser comprises two sampled grating sections bounding the active and phase sections in which each grating section has comb of reflection maxima whose spacing is selected to be different and wherein the laser is tuned by applying a respective electrical current to each grating section.

Preferably the laser is fabricated as a surface ridge structure. Alternatively it can be fabricated as a buried ridge structure.

Advantageously the laser is fabricated in a m-V semiconductor material such as an indium phosphide based material or a gallium arsenide based material.

In order that the invention can be better understood a sampled grating distributed reflector laser in accordance with the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic longitudinal sectional representation of a known sampled grating distributed reflector laser, which has already been described; Figure 2 is a schematic longitudinal sectional representation of a sampled grating distributed reflector laser in accordance with the invention; and Figure 3a to 3f illustrate the steps involved in the manufacture of a laser in accordance with the invention.

Referring to Figure 2 there is shown a schematic representation of a sampled grating distributed Bragg reflector laser in accordance with the invention. In conformity with the known laser described above with reference to Figure 1 like reference numerals are used to denote like parts.

In the laser of the present invention, unlike the known lasers, the electrodes for injecting electrical carriers into the front and rear grating sections 4, 6 are discontinuous (segmented) and do not extend over the entire length of the section. As illustrated in Figure 2 a respective electrode segment 20a-20d and 22a-22d is provided for each grating burst which is configured to overlay only the length of the grating bursts to which it is associated. It is to be noted that the electrode segments 20a-20d, 22a-22d for each grating section 4,6 are electrically connected to together.

In operation of the laser a respective tuning current iF, iR is applied to the electrode segments of the front and rear grating sections in a known manner to tune the laser's wavelength of operation. Due to the segmented electrode arrangement the injection of carriers within the grating sections is substantially confined to the region of the grating bursts. As a result the current iF, iR required to attain a required carrier density within the grating bursts and is reduced. The inventors have appreciated that it is primarily the current density within the grating bursts that determines the tuning capability of the laser. As a result of the reduction in current this reduces heating and the free carrier absorption loss within the guiding layer 10. Furthermore since in general the optical output power of a sampled grating laser reduces with increasing tuning current, the laser of the present invention provides a higher optical output for a given tuning current and is further found to have a reduced output optical power variation with tuning current.

The lengths of the electrode segments 20a-20d, 22a-22d, in a direction along the optical axis of the optical guiding layer, are selected such as to attain a substantially uniform carrier injection within its associated grating burst and will in general therefore be of a

length which substantially corresponds to the length of the grating bursts. However depending on the fabrication of the laser and the fringing of the tuning currents as they pass through the various layers of the structure, the electrodes can for optimum performance be shorter or longer than the grating burst.

Whilst the electrode segments 20a-20d, 22a-22d can be fabricated by selective metalisation or etching, in a preferred embodiment the electrode segments are fabricated by selectively etching windows within a dielectric or other isolation layer on an upper surface of the device and then metalising the entire section.

Referring to Figures 3a-3f these illustrate schematically the steps involved in the fabrication of a surface ridge wavelength tuneable sampled grating reflector laser in accordance with a preferred embodiment of the invention. The tuneable laser is preferably fabricated in indium phosphide (InP)/indium gallium arsenide phosphide (InGaAsP). Advantageously it is configured for"C"band operation (1530-1565nm), or "L"band operation (1570. 42-1603.17nm) as part of a WDM communications system.

Each of the Figures is a schematic longitudinal sectional representation.

Starting with an n+ doped InP substrate 50, layers of InGaAs/InGaAsP 52 is grown on the substrate to form a quantum well or bulk structure as known in the art. A layer of dielectric material 54, typically silicon dioxide (Si02), is deposited on top of the layer 52 and is used as an etch stop layer for selectively removing regions of the InGaAs/InGaAsP layers.

As shown in Figure 3b the InGaAs/InGaAsP layers 52 are selectively etched to leave an island 56 of remaining material which will comprise the active region 2 of the laser.

With the dielectric layer 54 in place the regions around the island of material are infilled with InGaAsP 58,60 by selective area epitaxy (Figure 3c). It will be appreciated that the laser can alternatively be fabricated by a non-selective area growth process in which epitaxial layers for both the tuning (grating/reflector and phase) sections are created in a single epitaxial step. These regions 58,60 respectively comprise the optical guiding layer of the laser with the front grating section 4 being subsequently formed in the region 58 and the rear grating section 6 and phase section 8 being formed in the region 60.

Next a layer of dielectric 62 is deposited over the InGaAsP regions 58,60 and is used to form the grating bursts 64 of the front and rear grating sections (Figure 3d).

An upper optical confinement layer 66 of InP is then overgrown over the surface of the device followed by a contact layer 68 of p+ InGaAs (Figure 3e). The contact and confinement layers 66,68 are etched to form a surface ridge structure to provide lateral (ie in and out of the plane of the paper) confinement of light within the optical guiding layer 10. The contact layer 68 is further selectively etched to leave respective contact areas 68a, 68b, 68c, 68d overlying the front grating, active, phase and rear grating sections. Layer 68 is preferentially also etched to form contact regions substantially overlying only the grating bursts.

As shown in Figure 3f a further dielectric layer 70 is then deposited and selectively etched to form windows through to the contact layer 68. For the front and rear grating sections 4,6 a respective window is defined for each grating burst 64. The windows overly and are substantially the same length as the associated the grating burst. For the active 2 and phase sections 8 the respective window extends over substantially the entire length of the section. Finally the front and rear grating, active and phase sections are selectively metalised to form the respective electrodes 12,16, 14,11.

It will be appreciated that the laser of the present invention is not restricted to the specific embodiments described and that modifications can be made which are within the scope of the invention. For example whilst a surface ridge structure can be used to provide lateral confinement of light within the optical guiding layer the invention is suited to other laser structures such as for example a buried ridge structure. The invention is further suited to lasers fabricated in other material systems such as other m- V semiconductor material systems such as gallium arsenide/gallium aluminium arsenide. Furthermore whilst the use of a sampled grating having grating bursts of a constant period is preferred other forms of sampled grating can be used such as those having grating bursts whose period is chirped. Although the invention has been described in relation to a laser having two sampled grating reflectors it is envisaged to use segmented electrodes for a laser having a single sampled grating reflector.

Claims

CLAIMS 1. A sampled grating distributed reflector laser comprising an active section, a phase section and at least one sampled grating section which comprises a plurality of grating bursts distributed along the optical axis of the section such that the section exhibits a comb of reflection maxima and wherein application of electrical current to the at least one grating section and phase section respectively shifts the wavelength of the comb and changes the effective cavity length of the laser thereby enabling tuning of the laser; characterised in that the at least one grating section is provided with an electrode which comprises a plurality of electrical contacts which overly a respective grating burst and wherein the contacts are configured such that electrical current is substantially localised to the grating bursts.
2. A laser according to Claim I in which each grating burst is a Bragg grating of constant period.
3. A laser according to Claim I in which each grating burst grating burst is chirped.
4. A laser according to any preceding claim and comprising two sampled grating sections bounding the active and phase sections in which each grating section has comb of reflection maxima whose spacing is selected to be different and wherein the laser is tuned by applying a respective electrical current to each grating sections.
5. A laser according to any preceding claim and fabricated as a surface ridge structure.
6. A laser according to any one of Claims 1 to 4 and fabricated as a buried ridge structure.
7. A laser according to any preceding claim and fabricated in a m-V semiconductor material.
8. A laser according to any preceding claim and fabricated in an indium phosphide based material.
9. A laser according to any one of Claims 1 to 7 and fabricated in a gallium arsenide based material.
10. A sample grating distributed reflector tuneable laser substantially as hereinbefore described with reference to or substantially as illustrated in the accompanying drawings.