WO2021209114A1

WO2021209114A1 - Optical device

Info

Publication number: WO2021209114A1
Application number: PCT/EP2020/060406
Authority: WO
Inventors: Xin Chen
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2020-04-14
Filing date: 2020-04-14
Publication date: 2021-10-21
Also published as: CN115280609A

Abstract

An optical device having a first face, a second face, an optical cavity and an active region, the optical cavity being defined by a semiconductor substrate and having a length extending between the first face and the second face, and the active region being configured for injection of charge into the cavity and having effective bandgap energies at respective distances along the length of the cavity, the device comprising: a modulator extending from a first end located between the first face and the second face and comprising at least part of the active region; and a laser optically coupled to the first end of the modulator; wherein the bandgap energy of said part of the active region adjacent the first end is higher than the bandgap energy of the said part of the active region at a distance from the first end.

Description

OPTICAL DEVICE

FIELD OF THE INVENTION

This invention relates to optical devices, for example electroabsorption modulated lasers.

BACKGROUND

Electroabsorption modulated lasers are widely used in telecommunication systems, for example in high-performance and low-cost optical device modules in applications such as large-capacity and high-speed optical access networks.

As shown in the example of Figures 1(a) and 1(b), a standard high speed electroabsorption modulated laser (EML) comprises a distributed feedback (DFB) laser 10 and an electroabsorption modulator (EAM) 11. The device generally comprises a semiconductor block which has a rear face or facet 12, a front face or facet 13 opposite to the rear facet and an optical cavity formed therebetween. The front and rear facets are normally both cleaved. The cavity traditionally comprises an active layer 14 interposed between layers of p- or n-type semiconductor material, shown at 15 and 16 respectively. One or more coating layers, such as anti-reflection (AR) or high reflection (HR) coatings, may be applied to the front and the rear facets to provide a predetermined reflectivity. In DFB lasers, a Bragg grating acts as the wavelength selective element for at least one of the faces and provides feedback, reflecting light back into the cavity to form the resonator. The rear face of the DFB is generally coated with a HR coating to enhance the power output. In an EAM, the front facet at the emissive face is generally coated with an AR coating to reduce the facet reflection. In some implementations, an EML may alternatively comprise a distributed Bragg reflector (DBR) laser in place of the DFB laser.

In an EML, the isolation between the laser and EAM is conventionally achieved by etching away the top layer of the substrate by a depth of approximately 1.0 to 2.5 pm, shown at 17, or by ion-implantation.

Normally, electroabsorption modulated lasers employ the quantum confined Stark effect (QCSE) to change the absorption of the device. When an external reverse bias (electric field) is applied to the device, the electron states shift to lower energies, while the hole states shift to higher energies, increasing the permitted light absorption at the lasing wavelength. Furthermore, electrons and holes are shifted to opposite sides of the well, which decreases the overlap integral, reducing the recombination efficiency of the system. The QCSE allows optical communication signals to be switched on and off rapidly, such that light can be transmitted through the device as “0” and “1” signals.

The DFB and EAM sections of an EML device are traditionally joined using a butt-couple process (BC), whereby the EAM section is overgrown on the wafer. The DFB and EAM sections are electrically isolated from one another by etching-away the top p-doped or n-doped layers or by ion-implantation. Using a BC process to join the EAM section to the DFB section at interface 18, the active region of the EAM section has the same multiple quantum well (MQW) bandgap energy along the waveguide.

It is also known that the light exiting the DFB section of the device is absorbed exponentially along the EAM waveguide. The absorption is given by:

Absorption = A. exp(TLa) (1) where A is a constant, G is the waveguide MQW confinement, L is the length of the EAM section and a is the absorption coefficient.

Therefore, if the MQW bandgap energy is the same along the waveguide in the EAM section, the absorption exponentially decays along the waveguide. In this case, approximately the first 30 to 50 pm of the EAM section absorbs most of the light. Hence saturation can occur in the first 30 to 50 pm of this section. Furthermore, this can result in a temperature profile that has a strong peak at the EAM input section. This may negatively affect the performance of the EML.

It is desirable to develop a device that is less prone to such problems.

SUMMARY OF THE INVENTION

There is provided an optical device having a first face, a second face, an optical cavity and an active region, the optical cavity being defined by a semiconductor substrate and having a length extending between the first face and the second face, and the active region being configured for injection of charge into the cavity and having effective bandgap energies at respective distances along the length of the cavity, the device comprising: a modulator extending from a first end located between the first face and the second face and comprising at least part of the active region; and a laser optically coupled to the first end of the modulator; wherein the bandgap energy of said part of the active region adjacent the first end is higher than the bandgap energy of the said part of the active region at a distance from the first end. The bandgap energy of the said part of the active region may decrease approximately linearly with distance from the first end. The bandgap energy of the said part of the active region may decrease approximately non-linearly with distance from the first end. This may prevent saturation from occurring in the first part of the EAM section of an electroabsorption modulated laser.

The device may be configured such that the second face is the emissive face of the device. This may allow the device to be integrated with further optically functional structures. For example, a Mach-Zehnder modulator, or an amplifier.

The second face may be coated with an anti-reflection coating. This may reduce facet reflection in the device. This may improve the performance of the device.

The active region may be elongated in a direction extending between the first face and the second face. This may allow emitted light to travel along the cavity.

The optical cavity may comprise a first semiconductor layer of a first doping type, a second semiconductor layer of a second doping type opposite to the first type, and the active region may be located between the first and second semiconductor layers. This is a convenient configuration for manufacturing the device.

The device may further comprise a waveguide extending with the optical cavity for inducing light in the cavity to travel along the length of the cavity. This may efficiently allow emitted light to travel along the cavity.

The waveguide may have substantially constant width. This may be convenient for manufacturing the device.

The waveguide may have a width between 0.5 pm to 3.0 pm. This may allow the effective refractive index of the waveguide to be selected accordingly.

The waveguide may be a ridge waveguide or a buried heterostructure waveguide. This may allow flexibility in manufacturing the laser. The modulator may be an electro-absorption modulator. The laser may be a distributed feedback (DFB) laser. This may allow the device to be used for applications such as telecommunications.

The device may comprise a pair of electrodes disposed on either side of the semiconductor substrate. The laser may comprise a further part of the active region and the laser may be configured such that light emission can be stimulated from the further part of the active region by applying a current across the electrodes. This is a convenient optical device configuration.

A part of each of the pair of electrodes may be disposed on either side of the modulator, and said part of the pair of electrodes may comprise lumped electrodes or travelling wave electrodes. This may allow for versatility in manufacturing the device.

When a bias voltage is applied to the modulator, the electrical field across said part of the active region may vary at distances along the cavity due to variations in doping concentration and/or thickness of the active region.

The waveguide of the modulator section of the device may have a constant width between 1.0 pm to 3.0 pm. Alternatively, the waveguide width may vary along the waveguide.

One or both of the first face and the second face may be constituted by a cleaved facet. This may be convenient for manufacturing the laser.

According to a second aspect there is provided a method of influencing the growth of the active region of an optical device when coupling an optical modulator and a laser, the modulator comprising at least part of the active region of the device, the method comprising growing the part of the active region of the modulator so as to define a variation in bandgap energy in the modulator with distance from an interface between the modulator and the laser, wherein the bandgap energy of the modulator adjacent the interface is higher than the bandgap energy of the modulator at a distance from the interface. The method may be performed during butt coupling of the optical modulator to the laser.

The method may further comprise growing the part of the active region of the modulator past a tapered mask, wherein the width of the mask at the interface is narrower than the width of the mask at a distance from the interface. This may allow the bandgap energy of a part of the active region of a device adjacent a first end of a modulator to be higher than the bandgap energy of that part of the active region at a distance from the first end. The mask may be made from a dielectric material, such as S1O2.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example with reference to the accompanying drawings.

In the drawings:

Figure 1(a) shows a top view of a conventional EML.

Figure 1(b) is a side view along section A-A of Figure 1(a).

Figure 2(a) illustrates a top view of an example of an optical device.

Figure 2(b) is a side view along section B-B of Figure 2(a).

Figure 3 illustrates an example of the variation in bandgap energy of the active region of the EML section of the optical device shown in Figures 2(a) and 2(b).

DETAILED DESCRIPTION OF THE INVENTION

In one exemplary embodiment, as illustrated in Figures 2(a) and 2(b), an EML device comprises a DFB laser 20 and an EAM 21. The DFB laser 20 comprises a semiconductor block which has a first, rear face 22. The second, front face 23 of the EML device is opposite to the rear face and an optical cavity is formed therebetween. The front and/or rear faces may be cleaved facets. It is preferable that the front and rear facets of the device are aligned parallel to one another. A high reflection (HR) coating may be applied to the rear facet. This facet 22 acts as a rear reflector. The front facet 23 at the emissive face of the device is coated with an AR coating to reduce the facet reflection. The EAM section may also be angled relative to the DFB section or curved, for example by an angle of 7-10 degrees, to reduce the AR facet reflection further. The grating of the DFB laser section 20 (not shown) may be a full grating, a l/4 grating or a partial grating.

In the example shown in Figures 2(a) and 2(b), the optical cavity of the EML comprises an active layer 24 interposed between layers of p- and n-type semiconductor material, shown at 25 and 26 respectively in Figure 2(b). In this example, the semiconductor layers are made from InP. However, other semiconductor materials, such as GaAs, may be used. The material forming the cavity may be selectively doped in the region of the p- and n-type layers. The layers are defined in a substrate. The multiple quantum wells MQW1 and MQW2 in the active region of the device are shown at 27 and 28 respectively. MQW1 corresponds to a part of the active region in the laser section of the device and MQW2 corresponds to a part of the active region in the EAM section of the device.

The DFB and EAM are separated by an isolation section, shown at 30 in Figures 2(a) and 2(b). In this isolation section, there is no current injection into the device and the DFB and EAM are electrically isolated from one another. The length of the isolation section may be from approximately 40 to 100 pm and the etch depth in this section from the top of the semiconductor substrate may be approximately 0.8 to 2.0 pm. The top p-lnP layers may be etched away between the DFB and the EAM to achieve electrical isolation between the DFB and the EAM.

A waveguide, the profile of which is shown in the top view of Figure 2(a), extends along the optical cavity for inducing light in the cavity to travel along the length of the cavity. The waveguide comprises a material with a refractive index greater than that of the surrounding substrate. Light is emitted from the end of the waveguide at the front face of the device.

The waveguide may be a ridge waveguide, preferably a shallow ridge waveguide. A ridge waveguide may be created by etching parallel trenches in the material either side of the waveguide to create an isolated projecting strip, typically less than 10 pm wide and several hundred pm long. A material with a lower refractive index than the waveguide material can be deposited at the sides of the ridge to guide injected current into the ridge. Alternatively, the ridge may be surrounded by air on the three sides that are not in contact with the substrate beneath the waveguide. The ridge may also be coated with gold to provide electrical contact and to assist heat removal from the ridge when it is producing light.

Alternatively, the waveguide may be a buried heterostructure waveguide. The waveguide of the device may be straight or curved. The waveguide width for the EAM section is preferably between 0.5 pm to 3.0 pm. The waveguide widths of the DFB and EAM sections can be different, or they may be the same (as is the case for the example shown in Figure 2(a)).

The device comprises a pair of electrodes 29a, 29b disposed on either side of the semiconductor substrate. The device is configured such that light emission can be stimulated from the substrate by applying a current across the electrodes, which are in electrical contact with the substrate. Part of each of the pair of electrodes is disposed on either side of the laser section of the device. In the MQW1 section 27 of the active region 24, light emission is stimulated from the device by applying a current across the part of the pair of electrodes disposed on either side of the laser section.

Part of each of the pair of electrodes is disposed on either side of the modulator section of the device. The part of the pair of electrodes for the EAM section may be, for example, lumped electrodes or travelling wave electrodes. A reverse bias can be applied to the electrodes.

As can be seen in Figure 2(b), the second, front face 23 of the device is the emissive face, where light is output from the device. The optical device may be integrated with further optically functional structures. For example, the device may further comprise a semiconductor optical amplifier adjacent to the second face 23. The semiconductor optical amplifier may be optically coupled to the front face 23.

To mitigate the problem of saturation in the first ~50 pm of the EAM section of the device adjacent to the interface with the DFB section, the bandgap energy of the section 28 of the active region adjacent the first end of the EAM section (at the interface with the DFB section) is higher than the bandgap energy of this section of the active region at a distance from the first end. This variation in bandgap energy in this section of the active region in the EAM section with distance from the interface with the DFB section is schematically illustrated in Figure 3.

Such a variation in bandgap energy of the active region in the EAM section may be achieved by using selective area growth (SAG) to couple the EAM section 21 to the DFB section 20.

The device described above may be manufactured by depositing material onto a substrate to grow and couple the modulator and laser sections of the device. Generally, metal oxide chemical vapour deposition (MOCVD) source material arriving from the gas phase will grow epitaxially in regions which are not masked. When growing the modulator section, a dielectric mask can be deposited on at least one side, and preferably both sides, of the EAM region of the device. Where source material lands on the mask (which may be, for example a S1O2 dielectric mask), it will not readily nucleate.

An example of the profile of a mask used in the SAG process to produce the optical device described herein is shown at 31 in Figure 2(a). In order to achieve a variation in bandgap energy along the EAM section of the device, the shape of the mask may vary with distance along the EAM section of the device. The mask may be tapered. In the example of Figure 2(a), a triangular shape of mask is used. More generally, the width of the mask at the interface between the laser and the modulator may be narrower than the width of the mask at a distance from the interface. Preferably, the mask is widest adjacent the front face 23 of the EML (i.e. at the emissive face of the device). However, other shapes of masks are possible.

Where source material lands on the mask, the source species deposited on the mask may re enter the gas phase and diffuse, due to the local concentration gradient, to find an unmasked region. In some embodiments, this may occur if the growth temperature is sufficiently high, and/or if the mask width is sufficiently narrow. Compared to a completely unmasked substrate, the MQW growth which occurs through a mask for both InGaAs, InGaAsP, InGaAIAs epi- layers may be thicker and richer in Indium, due to the relative diffusion coefficients of In and Ga under typical MOCVD growth conditions. Thus, as a result of both the quantum-size effect and the change in alloy composition, the MQWs in parts of the active region covered by a wider part of the mask are shifted to lower energy band gaps than regions covered by a narrower part of the mask.

Therefore, material forming part of the EML device described herein may be grown so as to define a variation in composition of the active region with distance from an interface between the modulator and the laser using selective area growth. As described above, the bandgap energy of the active region in the EML section of the device can be varied. Furthermore, when a bias voltage is applied to the device, the electrical field across the active region in the EML section varies at distances along the cavity due to variations in doping concentration and thickness.

Therefore, SAG may be used to butt-couple the EAM section to the DFB section to form the EML. A mask made from a dielectric such as Si0₂can be used in the EAM region to enhance the growth of the MQW2 region selectively along the EAM waveguide. As a result, the MQW2 section of the active region in the EAM section of the device will have a higher bandgap energy close to the interface with the DFB section, and a lower bandgap energy close to the EAM facet 23. The mask shape is preferably variable along the waveguide. Consequently, the EAM MQW2 section has a variable bandgap energy along the waveguide, instead of a constant bandgap energy.

Consequently, the EAM absorption along the waveguide is distributed and as result the saturation in the first ~50 pm section of the EAM may be reduced. A method for forming the device described herein may be summarised as follows. This method of manufacture influences the growth of the active region of the optical device when coupling the modulator and the laser, where the modulator comprises at least part of the active region of the device. The method comprises growing the part of the active region of the modulator so as to define a variation in bandgap energy in the modulator with distance from an interface between the modulator and the laser, wherein the bandgap energy of the modulator adjacent the interface is higher than the bandgap energy of the modulator at a distance from the interface. The method may be performed during butt-coupling of the optical modulator to the laser.

As described above, the material forming the modulator (the EAM section 21) is preferably grown by depositing material (for example, MOCVD or dopant material) in between a tapered mask in a selective area growth process. The width of the mask at the interface between the laser and modulator sections of the device is preferably narrower than the width of the mask at a distance from the interface.

The variable bandgap energy along the waveguide of the device described herein disrupts the absorption along the EAM section of the waveguide. This may effectively prevent saturation in the first section of the EAM and help to smooth the temperature profile, preventing a strong absorption peak at the EAM input section which is observed in EMLs that have been manufactured using butt-couple growth. This may improve the performance of the EM L device.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. An optical device having a first face, a second face, an optical cavity and an active region, the optical cavity being defined by a semiconductor substrate and having a length extending between the first face and the second face, and the active region being configured for injection of charge into the cavity and having effective bandgap energies at respective distances along the length of the cavity, the device comprising: a modulator extending from a first end located between the first face and the second face and comprising at least part of the active region; and a laser optically coupled to the first end of the modulator; wherein the bandgap energy of said part of the active region adjacent the first end is higher than the bandgap energy of the said part of the active region at a distance from the first end.

2. The optical device of claim 1 , wherein the bandgap energy of the said part of the active region decreases approximately linearly with distance from the first end.

3. The optical device of any preceding claim, wherein the device is configured such that the second face is the emissive face of the device.

4. The optical device of any preceding claim, wherein the second face is coated with an anti reflection coating.

5. The optical device of any preceding claim, wherein the active region is elongated in a direction extending between the first face and the second face.

6. The optical device of any preceding claim, wherein the optical cavity comprises a first semiconductor layer of a first doping type, a second semiconductor layer of a second doping type opposite to the first type, and wherein the active region is located between the first and second semiconductor layers.

7. The optical device of any preceding claim, wherein the device further comprises a waveguide extending with the optical cavity for inducing light in the cavity to travel along the length of the cavity

8. The optical device of claim 7, wherein the waveguide has substantially constant width.

9. The optical device of claim 7 or claim 8, wherein the waveguide has a width between 0.5 pm to 3.0 pm.

10. The optical device of any of claims 7 to 9, wherein the waveguide is a ridge waveguide or a buried heterostructure waveguide.

11. The optical device of any preceding claim, wherein the modulator is an electro-absorption modulator.

12. The optical device of any preceding claim, wherein the laser is a distributed feedback laser.

13. The optical device of any preceding claim, the device comprising a pair of electrodes disposed on either side of the semiconductor substrate and the laser comprising a further part of the active region, the laser being configured such that light emission can be stimulated from the further part of the active region by applying a current across the electrodes.

14. The optical device of claim 13, wherein a part of each of the pair of electrodes is disposed on either side of the modulator and said part of the pair of electrodes comprises lumped electrodes or travelling wave electrodes.

15. The optical device of any preceding claim, wherein, when a bias voltage is applied to the modulator, the electrical field across said part of the active region varies at distances along the cavity due to variations in doping concentration and/or thickness of the active region.

16. A method of influencing the growth of the active region of an optical device when coupling an optical modulator and a laser, the modulator comprising at least part of the active region of the device, the method comprising growing the part of the active region of the modulator so as to define a variation in bandgap energy in the modulator with distance from an interface between the modulator and the laser, wherein the bandgap energy of the modulator adjacent the interface is higher than the bandgap energy of the modulator at a distance from the interface.

17. The method of claim 16, comprising growing the part of the active region of the modulator past a tapered mask, wherein the width of the mask at the interface is narrower than the width of the mask at a distance from the interface.