WO2002065598A2

WO2002065598A2 - Thermally stabilized semiconductor laser

Info

Publication number: WO2002065598A2
Application number: PCT/GB2002/000655
Authority: WO
Inventors: Steven James Borley
Original assignee: Bookham Technology Plc
Priority date: 2001-02-15
Filing date: 2002-02-13
Publication date: 2002-08-22
Also published as: GB0103701D0; GB2372376A; WO2002065598A3; AU2002229980A1

Abstract

A semiconductor laser comprising an optical generation section and a heat balancing region, wherein each region of the optical generation section is paired with associated means in a corresponding region of the heat balancing section and is in thermal communication with such associated means. In operation the total input electrical power applied to said region and its associated means is constant and independent of the electrical power applied to the other regions and their respective associated means. Preferably, each region and its associated means have substantially identical structures.

Description

SEMICONDUCTOR LASER

This invention relates to a semiconductor laser and more especially to a temperature stabilisation arrangement for use with such a laser.

Tuneable wavelength semiconductor lasers are rapidly emerging as a key technology for use in wavelength division multiplex (WDM) telecommunications networks. Tuneable optical sources are required which can provide the many different wavelengths required for the many different channels that make up a WDM network. Generally such networks must include optical amplifiers, typically erbium doped fibre amplifiers, which currently limits the overall operating bandwidth to approximately 35nm. As a consequence work is tending to concentrate on developing optical components that

operate within the erbium bandwidth window.

A laser suitable for optical telecornmunications is a Distributed Feedback (DFB) semiconductor laser. Figures 1 and 2 are schematic representations of a known four section (region) tuneable laser 50 that is a monolithically fabricated structure in a semiconductor Group ILI-N material. The monolithic structure is grown on a substrate 5 and comprises a rear grating region 10, phase region 20, gain region 30 and front grating region 40. Respective electrodes 11, 21, 31, 41 are provided on each region. The laser is operated by current injection into the four different regions 10, 20, 30, 40 in which the output optical power is controlled by the current I_gai_n in the gain region 30 and wavelength tøiing is achieved by the independent control of current I_rear and Ig-ont in the grating regions 10 and 40 respectively. The additional control of current I_Phase n the phase region allows maximisation of the side mode suppression ratio (SMSR) for each wavelength.

As disclosed in US 4 896 325 the grating regions can each comprise a sampled grating which gives rise to a comb of reflection peaks or maxima. The gratings produce slightly different combs of reflection peaks that provide feedback into the device. Supermode tuning is achieved by current tuning the front grating 40 and rear grating 10. Quasi-continuous tuning between the supermodes can be achieved by current timing the grating regions together. Used in combination these two timing methods achieve a total quasi-continuous tuning range that exceeds the 35nm bandwidth of the erbium window.

Summarised in Table 1 are the electrical power profiles for an experimental four section tuneable laser that has been fabricated in an Indium Phosphide based Group πi-N semiconductor material.

TOTAL 600 - 1140mW

Table 1 Experimental values for injection currents and input power for a four section tuneable Indium Phosphide semiconductor laser The power calculations of Table 1 are based on a nominal drive potential of 2 volts. To maximise light output power the gain section of a laser is usually run at a high constant current such as the 300mA. A monolithic tuneable laser when operated with a 300mA gain current, and no timing currents, will typically produce 40mW of usable laser light. Applying taring currents to the device will typically result in a 5-6dB spread in the light output power across the tuning range of the device. As will be appreciated from the table there is a considerable variation in input power for the respective regions of the laser over the range of operating wavelengths of the laser.

A monolithic tuneable laser for telecommunications applications typically has dimensions of 1500μm by 300μm by lOOμm and is a very small, relatively low mass device in which only 1 - 6% of the electrical power input results in the generation of light, the balance of the power constituting heat. For a single wavelength laser based on Indium Phosphide Group HJ-N semiconductor material fabrication the junction

temperature dependence of the laser frequency is typically 10 GHz/°C.

In the 1550nm telecorrimuriications band the wavelength stability criteria set for the International Telecommunications Union (ITU) grid wavelengths is 10pm. At the centre of the band (1550nm) this corresponds to a frequency stability of less than 2 GHz. Therefore to meet this criteria at all times the lasing junction temperature would

need to be stabilised to less than 0.2°C. With such a small device it is not practical to use external temperature stabilising means such as a Peltier cooler to stabilise the lasing junction temperature to this degree of accuracy. This is especially true in the circumstance where the input power to the laser device may be instantaneously varying from 600mW to in excess of lOOOmW as the wavelength is tuned (see table 1), and the majority of the power change results in thermal heating of the laser.

Since the thermal mass of a typical tuneable laser is small, any change in input power will result in a relatively rapid attainment of a new steady state lasing junction temperature, but during this attainment period the laser wavelength will vary significantly. Although it is known to use Peltier cooling devices to temperature stabilise a laser package to a defined operating temperature the response time of such cooling means are typically a few seconds. This method of temperature control is consequently slow compared to the tuning speed of a laser, and can result in imprecise laser wavelength for undesirable periods with the consequential disturbance to the host telecommunication system.

Although any error in a tuneable laser wavelength can be corrected by steering the current drives in the front and rear grating regions, this of necessity involves changing the steady state power status of the laser device with the consequential risk of causing unintended wavelength shifts.

Furthermore tuneable lasers needs to be characterised to relate the control currents, or other control signals, to the wavelength obtained and its line width. In a production environment the characterisation process needs to be automated. If the wavelength control means affects the input power to the laser device then the device temperature will vary and so the lasing wavelength will erroneously vary until the new steady state temperature for the device is achieved. During the settling period the device cannot be accurately characterised.

A need exists therefore for a tuneable semiconductor laser for use in WDM systems, amongst other applications, which overcomes at least in part the limitations imposed by undesired wavelength variation caused by device temperature variation during wavelength tuning.

This invention relates to semiconductor lasers comprising an optical generation section and a heat balancing region, wherein each region of the optical generation section is paired with associated means in a corresponding region of the heat balancing section and is in thermal communication with such associated means. In operation the total input electrical power applied to said region and its associated means is substantially constant and- independent of the electrical power applied to the other regions and their respective associated means. Preferably, each region and its associated means have substantially identical structures.

According to the present invention there is provided a semiconductor laser of a type comprising a plurality of regions to each of which a respective electrical current is applied to control the operation of the laser independently of the electrical current applied to the other regions, characterised by means associated with at least one region which has substantially the same electrical and thermal characteristics as its associated region and which is in thermal communication with its associated region and wherein in operation the total input electrical power applied to said region and its associated means is substantially constant and independent of the electrical power applied to the other regions and their respective associated means. Preferably, each region of the laser has respective associated means.

Preferably the means associated with the or each region has substantially identical structure and is most preferably monolithically fabricated as part of the laser.

In a preferred implementation the laser further comprises a groove between the or each region and its associated means. Conveniently this is etched after fabrication of the laser and provides optical arid electrical isolation between the respective regions of the laser and its associated means.

Preferably the total electrical power applied to each region and its associated means is held substantially constant by applying the same total current to each region and its associated means.

Preferably the laser comprises a four section device in which the one or more regions comprise a gain region, a phase region, a front grating region and a rear grating region.

In one mode of operation the current applied to the gain region is held substantially constant and no current applied to the associated means. Alternatively the current applied to the gain region can modified to maintain a constant optical output power.

To reduce the number of electrical connection to the laser the means associated with the rear grating and phase regions are electrically coupled. Advantageously the front and rear grating regions have a reflection characteristic that comprises a plurality of reflection maxima. Preferably the front and rear grating regions comprise a plurality of repeat grating units in which each grating unit comprises a series of adjacent diffraction gratings having the same pitch; wherein the grating units and adjacent gratings within a grating unit are separated by a phase change of substantially pi radians and wherein at least two of the gratings within a grating unit have different lengths; the lengths being selected so as to provide a predetermined spacing of the reflection maxima.

The laser is preferably fabricated using a buried ridge architecture. Alternatively it can be fabricated using a surface ridge architecture.

In order that the present invention can be better understood four semiconductor lasers 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 representation of a known four section tuneable semiconductor laser;

Figure 2 is an end sectional view of the laser of Figure 1;

Figure 3 is an end sectional view of a semiconductor laser in accordance with the present invention; Figure 4 is a schematic representation of a semiconductor laser in accordance with the present invention;

Figure 5 is a schematic representation of the laser of Figure 4 showing the two sections;

Figure 6 is the laser of Figure 3 showing the two sections driven with current sources;

Figure 7 is schematic representation of drive circuitry for operating the laser of the present invention;

Figure 8 is schematic representation of a second semiconductor laser in accordance with the present invention;

Figure 9 is a schematic representation of a third semiconductor laser in accordance with the invention; and

Figure 10 is a schematic representation of a fourth semiconductor laser in accordance with invention.

Before describing the semiconductor laser of the present invention it is instructive to describe the fabrication of the known four section DBR laser as is illustrated in Figures 1 and 2. A four section tuneable laser can be fabricated using either buried ridge, or surface ridge architectures however, for simplicity the device is described in relation to a buried ridge architecture as shown in Figure 2. In a typical architecture the laser diode is grown on an N doped Indium Phosphide substrate 1 that acts as a lower confinement means for the optical waveguide formed between the active region 2 and the cap 3. The active region 2 is located within an upper confinement region 5 comprising highly resistive material Indium Phosphide. The cap 3 is highly P doped Group m-V semiconductor that gives a low resistance contact with the upper confinement means. Layer 4 is a dielectric to ensure the electrical energy flows through the required regions. Metalisation 6 is provided to make electrical contact with the P doped cap 6. On top of the metalisation layer 6, a pad 7 is laid down to which electrical contacts may be made. Electrical connections for the various regions of the laser diode are isolated by means of dielectric layer 4. The upper confinement layer 5 may also have an implant of further doping to reduce current spreading away from the active area environment. The physical distance between the lower confinement 1 and the cap 3 will typically be 1 to 3

μm. The active region 2 extends the length of the laser device, between the end facets formed when the laser devices are cleaved from the bars of devices grown on the substrate wafer. Any Bragg gratings are located in the active region.

Referring to Figures 3 and 4 these respectively illustrate an end sectional view and a schematic representation of a semiconductor laser in accordance with the present invention. In accordance with the present invention the semiconductor laser comprises two adjacent devices: a first for generating light and hereinafter termed an optical generation section and a second for heat balancing and hereinafter termed a heat balancing section. Both sections are identical and are monolithically fabricated using the steps set out for the device shown in Figure 2. A particular advantage of fabricating the two sections as identical devices is that this simplifies the fabrication process and has the benefit of greater production yield than would be the case if the two section were different and less flexibility was available for cleaving out devices from the wafer. A further particular advantage of having identical structures for the optical generation and heat balancing sections is that corresponding regions of the sections will have the same electrical and thermal characteristics.

Referring to Figure 5 this illustrates the optical generation section and the heat balancing section divided by a virtual plane 16 and the four regions (rear grating, phase, gain and front grating) as identified in Figure 1. For the avoidance of confusion Figure 5 is not annotated with all the elements identified in Figures 3 and 4. Since the optical generation and heat balancing sections are identical there is no intrinsic reason why the sections cannot be swapped over. The choice of which section is driven as the optical generation section will be determined by the placement of the coupling optical fibre 18.

Referring again to Figure 4 respective electrical contacts 11, 21, 31 and 41 are provided for the rear grating, phase, gain and front grating regions of the optical generation section and respective electrical contacts 111, 121, 131, 141 for the corresponding regions of the heat balancing section. As will be described a respective electrical current drive I_rear, Iphase, I ain, and I_front is applied to the regions of the optical generation section and a respective electrical current drive I_rear>, Iphase_', Igain_', and I^onf ^{to me} corresponding regions of the heat balancing section. The relationship between the drive currents for corresponding regions satisfy the following relationships: t-rear ^"■" -irear' ^— -"sr (.1

Iphrase ^"■^" lphrase' - p (2)

tgain P^" -igain' ^— -^-g (?)

Ifront + Ifront' ⁼ Kf (4)

where K_r, K_p, K_g and K_f are constants. The value of the constant K_x for (1), (2), (3) and (4) is set to the maximum current drive that would ever be required in the region of the optical^' generation section. Thus if the maximum current drive I_gai_n was determined to be 300mA then K_g would be set to 300mA.

It will be appreciated that it is actually the total electrical power that needs to be kept constant so that the overall temperature of the laser is constant regardless of its wavelength of operation. However by ensuring I_x + I_x> = K_x this is a first order approximation of keeping the power constant and found to be sufficient in many applications. To cover any second order temperature effects it is envisaged to divide the current drives such that I_x + I_x> = K_x ± δ where δ is a power trim factor to balance the requirement of the input power to be constant.

On the basis that the first order approximation of relations (1) - (4) is sufficient Figure 6 shows diagrammatically the arrangement to drive a pair of corresponding regions (x) with variable currents I_x and I_x> that satisfy the requirement I_x + I_x> = K_x where x can be the rear, phase, gain or front regions.

An implementation of a preferred drive circuit arrangement is shown in Figure 7 in which 200 is a long tailed pair whose collectors feed the optical region load 220, in the optical generation section, and the commensurate heat load 230 in the heat balancing section. Resistor 210 acts as a total current biasing means. The base circuits of the long tailed pair 200, are driven by their own analogue to digital converter (ADC) respectively 242 and 244, which are fed current drive data off a system bus 250 connected to the host system processor. The bit depth of the ADCs 242 and 244 will be determined by the current resolution required in the loads 220 and 230. The drive circuitry of Figure 7. is by way of example and is just one of many ways that equivalent functionality could be designed, as will be known to those of ordinary skill in the art.

Figure 8. shows a further semiconductor laser in accordance with the invention in which the two sections are separated by an etched groove 300 that acts as a barrier to free carrier current flow between the sections. Although such an arrangement provides improved electrical isolation between the corresponding regions of the optical generation and heat balancing sections it will be appreciated that the two sections remain in thermal cornmunication with one another via the substrate. The use of an etched groove also reduces optical interaction between the optical generation section and the heat balancing section that may produce undesired optical power.

Figure 9 shows a variant of the semiconductor laser of Figure 4 in which the I_gai_n current is always operated at a fixed optical output level and consequently there is no need for an Ig_ai_n- current injection and its associated electrical contact. This also saves on requiring an interface pin for the I_gai_n' on the host module. In an optical WDM telecommunication system it is usual to maximise the source wavelength signal level, within safety and lifetime limits, which invariably means that the source laser is operated at a constant high power level compatible with operating life and device stability.

Figure 10 is yet a further variant of the semiconductor laser of Figure 4 in which the heat balancing section rear and phase region electrical contacts are electrically tied together to form a single current injection point 151. This has the advantage of reducing the number of host module pins for driving the semiconductor laser. Clearly, the architectures of the lasers shown in Figures 9 and 10 can be combined.

It will be appreciated that the present invention is not restricted to the specific embodiments described and that variation can be made to the embodiments described which are within the scope of the invention. For example whilst the semiconductor laser has been described as having a buried ridge structure other architectures can be used such as for example a surface ridge structure. As will be appreciated the present invention resides in providing a region corresponding to one or more of the respective regions of the laser which has substantially the same electrical and thermal characteristics and which is in thermal communication with the corresponding region of the laser. By ensuring that the total electrical power applied to the corresponding regions of the optical generating and heat balancing sections is always a constant this ensures that the overall temperature of the corresponding regions remains constant and thereby minimise the effect of thermal effects on the wavelength of operation of the laser. Whilst for ease of fabrication it is convenient to fabricate the heat balancing section in the form of an identical laser structure, a dummy laser, it can equally be fabricated as a separate device provided that it has similar electro/thermal characteristics and there is thermal communication between the corresponding regions.

The principle of the invention is that provided the drive potential of the intended optical generation section regions and heat balancing section regions are the same, then the current "I" into a given active section plus the current into the equivalent heat balancing section" I"¹ can be made a constant. Thus if the current I is increased the current I¹ is decreased by the same amount; and if the current I is decreased the current V is increased by the same amount. As a consequence the power dissipation in the combined monolithic laser structure is maintained constant thereby substantially holding the device at a constant steady temperature. In such an arrangement there is a need for the two sections to be thermally coupled. Achievement of a constant drive potential with current control implies that the impedance of the respective regions of the light generation section and the heat balancing section are substantially equivalent.

The materials used for fabricating monolithic lasers are semiconductors that in their undoped states are highly resistive to current flow and by the same token resistive to heat flow. Thus for temperature equilibrium to be maintained between the two sections of the laser device they need to be in close proximity. For this reason ideally the two sections need to be a monolithic integrated structure at least with a substantial common coupling substrate. The design of the temperature stabilised laser device requires that ideally the regions of the intended optical generation section have the same electrical characteristics as those of the heat balancing section. Through this means the attainment of a constant input power is simplified to ensuring that the sum of the currents into equivalent regions of each section is constant. Making the two sections to be exactly the same ensure that this criteria is met. A consequence of the two sections being exactly the same is that the heat balancing section will also be capable of producing light. This light will not necessarily be laser light, but may be non coherent light similar to that produced by a light emitting diode. Suppression of light production can be achieved by omitting the grating structure within the front and rear grating regions of the heat balancing section.

Any light produced in the heat balancing section needs to be excluded from the host communication system, which is conveniently achieved by close coupling of the output collection optical fibre to the optical generation section.

It will be appreciated that it is not required for the heat balancing section to produce usable optical power, and preferably it should produce no optical power at all. To reduce the number of current drives to the heat balancing section in an alternative embodiment the heat balancing section has its grating and phase regions electrically coupled.

The invention can be applied to any tuneable semiconductor laser device. In particular the invention may be applied to a laser device in which the front and rear gratings of the optical generation regions include a plurality of discontinuities,; or applied to a laser device in which the front and rear gratings of the optical generation region are a diffraction grating structure as disclosed in UK patent GB2337135 which is hereby incorporated by way of reference thereto. Such gratings can also be chirped and of varying lengths, or other means of determining their strength of influence upon the wavelength of operation.

Improved thermal coupling between the intended optical power generation section and the heat balancing section can be achieved through fabricating the two sections closer together, whilst ensuring that optical power coupling between the two sections was minimised. The advantage of this arrangement would be even faster wavelength switch stabilisation for the laser.

Claims

1. A semiconductor laser of a type comprising a plurality of regions to each of which a respective electrical current is applied to control the operation of the laser independently of the electrical current applied to the other regions, characterised by means associated with at least one region which has substantially the same electrical and thermal characteristics as its associated region and which is in thermal communication with its associated region and wherein in operation the total input electrical power applied to said region and its associated means is substantially constant and independent of the electrical power applied to the other regions and their respective associated means.

2. A laser according to Claim 1 in which each region has respective associated means.

3. A laser according to Claims 1 or 2 in which each region and its associated means have substantially identical structures.

4. A laser according to Claim 3 in which the regions and associated means are monolithically fabricated.

5. A laser according to Claim 4 and further comprising a groove between each region and its associated means.

6. A laser according to any preceding claim in which in operation the total electrical power applied to each region and its associated means is held substantially constant by applying the same total current to each region and its associated means.

7. A laser according to any preceding claim in which the regions comprise a gain region, a phase region, a front grating region and a rear grating region.

8. A laser according to Claim 7 in which in operation the current applied to the gain region is held substantially constant and no current applied to its associated means.

9. A laser according to Claim 7 in which in operation the current applied to the gain region is modified to maintain a constant optical output power.

10. A laser according to any one of Claims 7 to 9 in which the means associated with the rear grating and phase regions are electrically coupled.

11. A laser according to any one of Claims 7 to 10 in which the front and rear grating regions have a reflection characteristic which comprises a plurality of reflection maxima.

12. A laser according to Claim 11 in which the front and rear grating regions comprise a plurality of repeat grating units in which each gratmg unit comprises a series of adjacent diffraction gratings having the same pitch; wherein the grating units and adjacent gratings within a grating^' unit are separated by a phase change of substantially pi radians and wherein at least two of the gratings within a grating unit have different lengths; the lengths being selected so as to provide a predetermined spacing of the reflection maxima.

13. A laser according to any preceding claim in which the laser is fabricated using a buried ridge architecture.

14. A laser according to any one of Claims 1 to 12 in which the laser is fabricated using a surface ridge architecture.