CN115280609A

CN115280609A - Optical device

Info

Publication number: CN115280609A
Application number: CN202080097515.1A
Authority: CN
Inventors: 陈欣
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2020-04-14
Filing date: 2020-04-14
Publication date: 2022-11-01
Also published as: WO2021209114A1

Abstract

An optical device having a first face, a second face, an optical cavity defined by a semiconductor substrate and having a length extending between the first face and the second face, and an active region for injecting charge into the cavity and having an effective bandgap energy at various distances along the length of the cavity, the device comprising: a modulator extending from a first end located between the first and second faces and comprising at least part of an active region; and a laser optically coupled to the first end of the modulator; wherein the portion of the active region adjacent the first end has a higher bandgap energy than the portion of the active region remote from the first end.

Description

Optical device

Technical Field

The present invention relates to an optical device, for example, an electroabsorption modulated laser.

Background

Electro-absorption modulated lasers are widely used in telecommunication systems, for example, in high performance low cost optical device modules for high capacity, high speed optical access networks and the like.

As shown in the example of fig. 1 (a) and 1 (b), a standard high-speed electro-absorption modulated laser (EML) includes a Distributed Feedback (DFB) laser 10 and an electro-absorption modulator (EAM) 11. The device generally includes a semiconductor block having a back surface or facet 12, a front surface or facet 13 opposite the back facet, and an optical cavity formed therebetween. Both the front facet and the rear facet are typically cleaved. The cavity conventionally includes an active layer 14 interposed between layers of p-type or n-type semiconductor material, shown at 15 and 16, respectively. One or more coatings, such as anti-reflection (AR) or High Reflection (HR) coatings, may be applied to the front facet and the back facet to provide a predetermined reflectivity. In a DFB laser, a Bragg grating acts as a wavelength selective element for at least one facet and provides feedback, reflecting light back into the cavity to form a resonator. The back side of the DFB is typically coated with an HR coating to enhance power output. In an EAM, the front facet of the emitting surface is typically coated with an AR coating to reduce facet reflection. In some implementations, the EML may instead include a Distributed Bragg Reflector (DBR) laser instead of a DFB laser.

In EML, the laser and EAM are typically isolated by etching the top layer of the substrate to a depth of about 1.0 to 2.5 μm (as shown at 17) or by ion implantation.

Generally, electro-absorption modulated lasers employ the Quantum Confined Stark Effect (QCSE) to alter the absorption of the device. When an external reverse bias (electric field) is applied to the device, the electron state is transferred to a lower energy and the hole state is transferred to a higher energy, increasing the allowed light absorption at the laser wavelength. In addition, electrons and holes move to opposite sides of the well, which reduces overlap integration and thus the recombination efficiency of the system. QCSE allows optical communication signals to be turned on and off quickly so that light can be transmitted through the device as "0" and "1" signals.

The DFB and EAM portions of the EML device are conventionally connected using a butt-coupling process (BC), whereby the EAM portions are overgrown on the wafer. The DFB and EAM portions are electrically isolated from each other by etching away the top p-doped or n-doped layer or by ion implantation. The EAM section is connected to the DFB section at an interface 18 using a BC process, with the active region of the EAM section having the same multi-quantum well (MQW) bandgap energy along the waveguide.

It is well known that light emerging from the DFB portion of the device is exponentially absorbed along the EAM waveguide. The absorption is given by:

Absorption＝A.exp(ΓLα) (1)

where A is a constant, Γ is the waveguide MQW confinement, L is the length of the EAM section, and α is the absorption coefficient.

Thus, if the MQW bandgap energy is the same along the waveguide in the EAM portion, absorption decays exponentially along the waveguide. In this case, the first approximately 30 μm to 50 μm of the EAM portion absorbs most of the light. Therefore, saturation may occur in the first 30 μm to 50 μm of the portion. Furthermore, this may result in a temperature profile with strong peaks at the input section of the EAM. This may negatively impact the performance of the EML.

There is a need to develop a device that is not susceptible to such problems.

Disclosure of Invention

There is provided an optical device having a first face, a second face, an optical cavity defined by a semiconductor substrate and having a length extending between the first face and the second face, and an active region for injecting charge into the cavity and having an effective bandgap energy at various distances along the length of the cavity, the device comprising: a modulator extending from a first end located between the first and second faces and comprising at least part of an active region; and a laser optically coupled to a first end of the modulator; wherein the portion of the active region proximate the first end has a higher bandgap energy than the portion of the active region distal from the first end.

The band gap energy of the partial active region may decrease approximately linearly with a change in distance from the first end. The bandgap energy of the portion of the active region may decrease approximately non-linearly with distance from the first end. This may prevent saturation in the first part of the EAM section of the electroabsorption modulated laser.

The device may be configured such that the second face is an emitting face of the device. In this way, the device can be integrated with other optically functional structures. Such as a Mach-Zehnder modulator or amplifier.

The second face may be coated with an anti-reflection coating. This may reduce facet reflections 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 the emitted light to propagate along the cavity.

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

The device may further include a waveguide extending with the optical cavity for inducing light in the cavity to propagate along a length of the cavity. This may effectively allow the emitted light to propagate along the cavity.

The waveguide may have a substantially constant width. This may facilitate the manufacture of the device.

The width of the waveguide may be between 0.5 μm and 3.0 μm. 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 in applications such as telecommunications.

The device may include a pair of electrodes disposed on either side of the semiconductor substrate. The laser may comprise a further portion of the active region, and the laser may be configured such that light emission may be stimulated from the further portion of the active region by applying a current between the electrodes. This is a convenient arrangement of optics.

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

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

The waveguides of the modulator portion of the device may have a constant width between 1.0 μm and 3.0 μm. 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 face (cleaved face). This may facilitate the manufacture of the laser.

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

The method may further comprise growing a portion of the active region of the modulator beyond a tapered mask, wherein the interfaceThe width of the mask is narrower than the width of the mask away from the interface. Thus, the band gap energy of a portion of the active region of the device adjacent the first end of the modulator is higher than the band gap energy of a portion of the active region at a distance from the first end. The mask may be made of a dielectric material, e.g. SiO₂。

Drawings

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

In the figure:

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

FIG. 1 (b) isbase:Sub>A side view taken along the line A-A in FIG. 1 (base:Sub>A).

Fig. 2 (a) shows a top view of an example of an optical device.

Fig. 2 (B) is a side view taken along the B-B section of fig. 2 (a).

Fig. 3 shows an example of a band gap energy variation of an active region of the EML portion of the optical device shown in fig. 2 (a) and 2 (b).

Detailed Description

In one exemplary embodiment, as shown in fig. 2 (a) and 2 (b), the EML device includes a DFB laser 20 and an EAM 21. The DFB laser 20 includes a semiconductor block having a first back surface 22. The second front surface 23 of the EML device is opposite the back surface and forms an optical cavity between them. The front and/or back surfaces may be cleaved surfaces. Preferably, the front facet and the back facet of the device are aligned parallel to each other. A Highly Reflective (HR) coating may be applied to the back facet. This facet 22 acts as a back reflector. The front facet 23 at the emitting face of the device is AR coated to reduce facet reflection. The EAM section may also be tilted or curved relative to the DFB section, for example, at an angle of 7 to 10 degrees, to further reduce AR facet reflection. The grating (not shown) of the DFB laser section 20 may be an all-grating, a lambda/4 grating or a partial grating.

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

The DFB and EAM are separated by an isolation section, shown at 30 in FIGS. 2 (a) and 2 (b). In this isolated portion, there is no current injection device, and the DFB and EAM are electrically isolated from each other. The length of the isolation portion may be about 40 μm to 100 μm, and the etching depth in the portion from the top of the semiconductor substrate may be about 0.8 μm to 2.0 μm. The top p-InP layer between the DFB and the EAM can be etched away to achieve electrical isolation between the DFB and the EAM.

The profile of the waveguide is shown in the top view of fig. 2 (a), the waveguide extending along the optical cavity for inducing light in the cavity to propagate along the length of the cavity. The waveguide comprises a material having 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. Ridge waveguides can be created by etching parallel trenches into the material on either side of the waveguide to create isolated tabbing strips, typically less than 10 μm wide and hundreds of μm long. A material having a refractive index lower than that of the waveguide material may be deposited on the sides of the ridge to guide the injected current into the ridge. Alternatively, the three sides of the ridge that are not in contact with the substrate below the waveguide may be surrounded by air. The ridges may also be plated with gold to provide electrical contact and to help dissipate heat from the ridges as they generate light.

Alternatively, the waveguide may be a buried heterostructure waveguide. The waveguides of the device may be straight waveguides or curved waveguides. The waveguide width of the EAM portion is preferably between 0.5 μm and 3.0 μm. The waveguide widths of the DFB and EAM portions may be different or the same (as in the case of the example shown in fig. 2 (a)).

The device comprises a pair of

electrodes

29a, 29b disposed on either side of a semiconductor substrate. The device is configured such that light emission can be stimulated from the substrate by applying a current across an electrode in electrical contact with the substrate.

A portion of each of the pair of electrodes is disposed on either side of the lasing portion of the device. In the MQW1 portion 27 of the active region 24, light emission is excited from the device by applying a current across a partial pair of electrodes disposed on either side of the lasing portion.

A portion of each of the pair of electrodes is disposed on either side of the modulator portion of the device. The partial electrode pairs of the EAM section may be, for example, lumped electrodes or traveling wave electrodes. A reverse bias may be applied to the electrodes.

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

To alleviate the first approximately 50 μm saturation problem of the EAM portion of the device adjacent the interface with the DFB portion, the portion 28 of the active region adjacent the first end of the EAM portion (at the interface with the DFB portion) has a higher bandgap energy than the portion of the active region at a distance from the first end. Fig. 3 schematically illustrates the variation of the bandgap energy of this part of the EAM portion active region with distance from the interface of the DFB portion.

This variation of the band gap energy of the active region in the EAM portion may be achieved by coupling the EAM portion 21 to the DFB portion 20 using Selective Area Growth (SAG).

The device may be fabricated by depositing material onto a substrate to grow and couple the modulator and laser portions of the device. Typically, metal Oxide Chemical Vapor Deposition (MOCVD) source material from the vapor phase will grow epitaxially in unmasked regions. When the modulator portion is grown, a dielectric mask may be deposited on at least one side, and preferably both sides, of the EAM region of the device. When the source material falls on the mask (which may be SiO, for example)₂Dielectric mask), it is not easy to nucleate.

An example of a mask profile in an SAG process for producing the optical device described herein is shown at 31 in fig. 2 (a). To achieve a variation in bandgap energy along the EAM portion of the device, the shape of the mask may vary with distance along the EAM portion of the device. The mask may be tapered. In the example of fig. 2 (a), a triangular 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 at the front surface 23 adjacent the EML (i.e., at the emitting surface of the device). However, other shapes of the mask are possible.

Where the source material lands on the mask, the source material deposited on the mask may re-enter the gas phase and diffuse due to the local concentration gradient to find unmasked areas. In some embodiments, this may occur if the growth temperature is high enough, and/or if the mask width is narrow enough. MQW growth of InGaAs, inGaAsP, inGaAlAs epitaxial layers through a mask may be thicker and have a higher indium content than a completely unmasked substrate due to the relative diffusion coefficients of In and Ga under typical MOCVD growth conditions. Thus, as a result of quantum size effects and changes in alloy composition, the MQWs in portions of the active region covered by the wider portions of the mask move to a lower energy bandgap than regions covered by the narrower portions of the mask.

Accordingly, the material forming part of the EML device described herein may be grown to define the variation of the composition of the active region with distance from the interface between the modulator and the laser using selective region growth. As described above, the band gap energy of the active region in the EML portion of the device may vary. Furthermore, when a bias voltage is applied to the device, the electric field across the active region in the EML portion varies with distance along the cavity due to variations in doping concentration and thickness.

Thus, SAG may be used to butt-couple the EAM portion with the DFB portion to form an EML. SiO can be used in the EAM region₂And an isodielectric mask to selectively enhance the growth of the MQW2 region along the EAM waveguide. Thus, near the interface with the DFB portion, the MQW2 portion of the active region in the EAM portion of the device will have a higher MQW2 than the DFB portionA high bandgap energy and a lower bandgap energy near the EAM facet 23. Preferably, the mask shape is variable along the waveguide. Thus, the EAM MQW2 portion has a variable bandgap energy along the waveguide, rather than a constant bandgap energy.

Thus, the EAM absorption along the waveguide is distributed, and therefore, the saturation of the first approximately 50 μm portion of the EAM may be reduced.

The methods for forming the devices described herein can be summarized as follows. This method of fabrication can affect the growth of the active region of an optical device when coupling a modulator, which includes at least part of the active region of the device, to a laser. The method includes growing a portion of an active region of the modulator 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 modulators adjacent the interface is higher than the bandgap energy of modulators further 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 (EAM portion 21) is preferably grown by depositing the material (e.g., MOCVD or dopant material) between tapered masks in a selective area growth process. Preferably, the mask width at the interface between the laser and modulator portions of the device is narrower than the mask width at a distance from the interface.

The variable bandgap energy along the waveguide of the devices described herein disrupts absorption along the EAM portion of the waveguide. This may effectively prevent saturation of the first portion of the EAM and help smooth the temperature distribution, thereby preventing strong absorption peaks at the input portion of the EAM observed in EMLs fabricated using butt-coupled growth. This may improve the performance of the EML 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 defined by a semiconductor substrate and having a length extending between the first face and the second face, and an active region for injecting charge into the cavity and having an effective bandgap energy at various distances along the length of the cavity, the device comprising:

a modulator extending from a first end located between the first and second faces and comprising at least part of an active region; and

a laser optically coupled to a first end of the modulator;

wherein the band gap energy of the portion of the active region proximate to the first end is higher than the band gap energy of the portion of the active region distal to the first end.

2. The optical device of claim 1, wherein the band gap energy of the portion of the active region decreases approximately linearly with distance from the first end.

3. An optical device as claimed in any preceding claim, wherein the device is configured such that the second face is an emitting face of the device.

4. An optical device as claimed in any preceding claim, wherein the second face is coated with an anti-reflection coating.

5. An optical device as claimed in any preceding claim, wherein the active region is elongate in a direction extending between the first and second faces.

6. An optical device according to 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 doping type, wherein the active region is located between the first and second semiconductor layers.

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

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

9. An optical device according to claim 7 or 8, wherein the waveguide has a width of between 0.5 μm and 3.0 μm.

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

11. An optical device as claimed in any preceding claim, wherein the modulator is an electro-absorption modulator.

12. An optical device as claimed in any preceding claim, wherein the laser is a distributed feedback laser.

13. An optical device as claimed in any preceding claim, the device comprising a pair of electrodes disposed on either side of the semiconductor substrate, and the laser comprising a further portion of the active region, the laser being configured such that light emission can be stimulated from the further portion of the active region by application of a current between the electrodes.

14. The optical device of claim 13, wherein a portion of each of the electrode pairs is disposed on either side of the modulator, and the partial electrode pairs comprise lumped or traveling wave electrodes.

15. An optical device according to any preceding claim, wherein the electric field across the portion of the active region may vary at various distances along the cavity due to variations in doping concentration and/or thickness of the active region when a bias voltage is applied to the modulator.

16. A method of affecting growth of an active region of an optical device when coupling an optical modulator and a laser; wherein the modulator comprises at least part of an active region of the device, the method comprising growing part of the active region of the modulator to define a variation in bandgap energy in the modulator with a variation in distance from an interface between the modulator and the laser; wherein the band gap energy of the modulator proximate to the interface is higher than the band gap energy of the modulator distal to the interface.

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