Optical device

Abstract

An optical device having a first face, a second face, an optical cavity and an active region, wherein the optical cavity is defined by a semiconductor substrate and has a length extending between the first and second faces, the active region being 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 including at least part of the active region; and a laser optically coupled to the first end of the modulator; wherein the bandgap energy of the part of the active region adjacent to the first end is higher than that of the part of the active region remote from the first end.

Description

Technical Field

[0001] The present application relates to an optical device, for example, an electroabsorption modulated laser. Background Art

[0002] Electroabsorption modulated lasers are widely used in telecommunication systems, for example, in high-performance low-cost optical device modules for applications such as high-capacity, high-speed optical access networks.

[0003] As Figure 1 (a) and Figure 1 (b) illustrate, a standard high-speed electroabsorption modulated laser (EML) includes a distributed feedback (DFB) laser 10 and an electroabsorption modulator (EAM) 11. The device typically includes a semiconductor block having a back or facet 12, a front or facet 13 opposite the back facet, and an optical cavity formed therebetween. The front and back facets are typically cleaved. The cavity conventionally includes an active layer 14 inserted between p-type or n-type semiconductor material layers, 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 and back facets to provide a predetermined reflectivity. In a DFB laser, a Bragg grating serves as a wavelength-selective element for at least one facet and provides feedback to reflect light back into the cavity to form a resonator. The back of the DFB is typically coated with an HR coating to enhance power output. In an EAM, the front facet of the emission surface is typically coated with an AR coating to reduce facet reflection. In some implementations, the EML may alternatively include a distributed Bragg reflector (DBR) laser instead of a DFB laser.

[0004] In an EML, the laser and the EAM are typically isolated by etching the top layer of the substrate to a depth of approximately 1.0 to 2.5 μm (as shown at 17) or by ion implantation.

[0005] Typically, electro-absorption modulated lasers use 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 transfer to lower energies while the hole states transfer to higher energies, thus increasing the allowed optical absorption at the laser wavelength. Additionally, the electrons and holes move to opposite sides of the well, reducing the overlap integral and thus the recombination efficiency of the system. QCSE allows for the rapid opening and closing of optical communication signals, such that light can be transmitted through the device as "0" and "1" signals.

[0006] The DFB and EAM sections of the EML device are traditionally connected using the butt-couple process (BC), whereby the EAM section is overgrown on the wafer. The DFB and EAM sections are electrically isolated from each other by etching away the top p-doped layer or n-doped layer or by ion implantation. The EAM section is connected to the DFB section at interface 18, and the active region of the EAM section has the same multiple quantum well (MQW) bandgap energy along the waveguide.

[0007] It is well known that the light emitted from the DFB section of the device is exponentially absorbed along the EAM waveguide. The absorption rate is given by:

[0008] Absorption=A.exp(ΓLα) (1)

[0009] 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.

[0010] Therefore, if the MQW bandgap energy is the same along the waveguide in the EAM section, the absorption decays exponentially along the waveguide. In this case, the first approximately 30 μm to 50 μm of the EAM section absorbs most of the light. Thus, saturation may occur in the first 30 μm to 50 μm of this section. Additionally, this may result in a temperature curve with a strong peak at the EAM input section. This may have a negative impact on the performance of the EML.

[0011] A device that is less prone to such problems needs to be developed. Summary of the Invention

[0012] An optical device is provided, having a first face, a second face, an optical cavity, and an active region. The optical cavity is defined by a semiconductor substrate and has a length extending between the first face and the second face. The active region is configured to inject charge into the cavity and has an effective bandgap energy at various distances along the length of the cavity. The device includes: a modulator extending from a first end located between the first face and the second face and including at least a portion of the active region; and a laser optically coupled to the first end of the modulator. The bandgap energy of the portion of the active region adjacent to the first end is higher than the bandgap energy of the portion of the active region farther from the first end.

[0013] The bandgap energy of the portion of the active region may decrease approximately linearly as the distance from the first end increases. The bandgap energy of the portion of the active region may decrease approximately non-linearly as the distance from the first end increases. This can prevent saturation from occurring in a first portion of the EAM portion of an electro-absorption modulated laser.

[0014] The device may be configured such that the second face is the emission face of the device. In this way, the device can be integrated with other optical functional structures. For example, a Mach-Zehnder modulator or an amplifier.

[0015] The second face may be coated with an anti-reflection coating. This can reduce facet reflections in the device. This can improve the performance of the device.

[0016] The active region may be elongated in a direction extending between the first face and the second face. This can allow the emitted light to propagate along the cavity.

[0017] The optical cavity may include a first semiconductor layer of a first doping type and a second semiconductor layer of a second doping type opposite to the first type, and the active region may be located between the first semiconductor layer and the second semiconductor layer. This is a convenient configuration for manufacturing the device.

[0018] The device may further include a waveguide extending together with the optical cavity for inducing the light in the cavity to propagate along the length of the cavity. This can effectively allow the emitted light to propagate along the cavity.

[0019] The waveguide may have a substantially constant width. This may facilitate the manufacture of the device.

[0020] The width of the waveguide may be between 0.5 μm and 3.0 μm. This can allow the effective refractive index of the waveguide to be selected accordingly.

[0021] The waveguide may be a ridge waveguide or a buried heterostructure waveguide. This can achieve flexibility in manufacturing the laser.

[0022] The modulator can be an electro-absorption modulator. The laser can be a distributed feedback (DFB) laser. This can allow the device to be used in applications such as telecommunications.

[0023] The device can include a pair of electrodes disposed on either side of a semiconductor substrate. The laser can include another part of the active region, and the laser can be configured such that by applying a current between the electrodes, light emission can be excited from the other part of the active region. This is a convenient optical device configuration.

[0024] A part of each of the pair of electrodes can be disposed on either side of the modulator, and the part of the pair of electrodes can include lumped electrodes or traveling-wave electrodes. This can achieve the versatility of manufacturing the device.

[0025] When a bias voltage is applied to the modulator, due to variations in the doping concentration and / or thickness of the active region, the electric field on the part of the active region can vary at various distances along the cavity.

[0026] The waveguide of the modulator part of the device can have a constant width between 1.0 μm and 3.0 μm. Alternatively, the waveguide width can vary along the waveguide.

[0027] One or both of the first and second faces can be constituted by a cleaved facet. This may facilitate the manufacture of the laser.

[0028] According to a second aspect, there is provided a method of influencing the growth of the active region of an optical device when coupling a light modulator and a laser; wherein the modulator includes at least a part of the active region of the device, and the method includes growing a part of the active region of the modulator to define a change in the bandgap energy in the modulator as a function of the distance from the interface between the modulator and the laser; wherein the bandgap energy of the modulator adjacent to the interface is higher than the bandgap energy of the modulator away from the interface. The method can be performed during docking of the light modulator to the laser.

[0029] The method can further include growing a part of the active region of the modulator over a tapered mask, wherein the width of the mask at the interface is narrower than the width of the mask away from the interface. Thus, the bandgap energy of a part of the active region of the device adjacent to the first end of the modulator is higher than the bandgap energy of that part of the active region at a certain distance from the first end. The mask can be made of a dielectric material, for example, silicon dioxide (SiO2). Description of the Drawings

[0030] The present application will now be described by way of example with reference to the accompanying drawings.

[0031] In the figures:

[0032] Figure 1 (a) shows a top view of a conventional EML.

[0033] Figure 1 (b) is a side view along the Figure 1 A - A cross - section of (a).

[0034] Figure 2 (a) shows a top view of an example of an optical device.

[0035] Figure 2 (b) is a side view along the Figure 2 B - B cross - section of (a).

[0036] Figure 3 Shows an example of Figure 2 (a) and Figure 2 the change in band - gap energy of the active region of the EML part of the optical device shown in (b). Detailed implementation

[0037] In one exemplary embodiment, as shown in Figure 2 (a) and Figure 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 to the back surface, and an optical cavity is formed therebetween. The front surface and / or the back surface can be cleavage surfaces. Preferably, the front facet and the rear facet of the device are aligned parallel to each other. A high - reflection (HR) coating can be applied to the rear facet. The first back surface 22 serves as a rear reflector. The second front surface 23 at the device emission surface is coated with an AR coating to reduce facet reflection. The EAM part can also be tilted or bent relative to the DFB part, for example, at an angle of 7 to 10 degrees, to further reduce AR facet reflection. The grating (not shown) of the DFB laser part 20 can be a full grating, a λ / 4 grating, or a partial grating.

[0038] In Figure 2 (a) and Figure 2 (b) shown, the optical cavity of the EML includes an active layer 24 inserted between p - type and n - type semiconductor material layers, as shown as 25 and 26 in 2(b). In this example, the semiconductor layer is made of indium phosphide (InP). However, other semiconductor materials can be used, for example, gallium arsenide (GaAs). The material forming the cavity can be selectively doped in regions of the p - type and n - type layers. These 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 of the laser part of the device, and MQW2 corresponds to a part of the active region of the EAM part of the device.

[0039] The DFB and EAM are separated by an isolation part, asFigure 2 (a) and Figure 2 as shown at 30 in (b). In this isolation section, there is no current injection device, and the DFB and EAM are electrically isolated from each other. The length of the isolation section can be about 40 μm to 100 μm, and the etching depth in this part from the top of the semiconductor substrate can be about 0.8 μm to 2.0 μm. The top p-InP layer between the DFB and EAM can be etched away to achieve electrical isolation between the DFB and EAM.

[0040] The profile of the waveguide is as Figure 2 shown in the top view of (a). The waveguide extends along the optical cavity for inducing the light in the cavity to propagate along the length of the cavity. The waveguide includes a material whose refractive index is greater than that of the surrounding substrate. Light is emitted from the end of the waveguide at the front of the device.

[0041] The waveguide can be a ridge waveguide, preferably a shallow ridge waveguide. The ridge waveguide can be produced by etching parallel trenches in the material on either side of the waveguide to produce an isolated protruding strip, usually less than 10 μm wide and hundreds of μm long. A material with a refractive index lower than that of the waveguide material can be deposited on the sides of the ridge to guide the injected current into the ridge. Alternatively, three sides of the ridge that do not contact the substrate under the waveguide may be surrounded by air. The ridge can also be gold-plated to provide electrical contact and help dissipate heat from the ridge when it generates light.

[0042] Optionally, the waveguide can be a buried heterostructure waveguide. The waveguide of the device can be a straight waveguide or a bent waveguide. The waveguide width of the EAM part is preferably between 0.5 μm and 3.0 μm. The waveguide widths of the DFB and EAM parts can be different or the same (as in the case of the example shown in Figure 2 (a)).

[0043] The device includes a pair of electrodes 29a, 29b disposed on either side of the semiconductor substrate. The device is configured such that by applying a current to the electrode in electrical contact with the substrate, light emission can be excited from the substrate.

[0044] A part of each of the pair of electrodes is disposed on either side of the laser part of the device. In the MQW1 part 27 of the active region 24, light emission is excited from the device by applying a current to the partial electrode pair disposed on either side of the laser part.

[0045] A part of each of the pair of electrodes is disposed on either side of the modulator part of the device. The partial electrode pair of the EAM part can be, for example, a lumped electrode or a traveling-wave electrode. A reverse bias can be applied to the electrodes.

[0046] From Figure 2As can be seen in (b), the second front face 23 of the device is the emission face, and light is output from this emission face of the device. The optical device can be integrated with other optical functional structures. For example, the device may further include a semiconductor optical amplifier adjacent to the second front face 23. The semiconductor optical amplifier can be optically coupled to the second front face 23.

[0047] To alleviate the saturation problem of the first approximately 50 μm in front of the EAM part of the device adjacent to the interface of the DFB part, the bandgap energy of the part 28 of the active region adjacent to the first end of the EAM part (at the interface with the DFB part) is higher than the bandgap energy of this part of the active region at a certain distance from the first end. Figure 3 Schematically shows the variation of the bandgap energy of this part of the active region of the EAM part with the distance from the interface of the DFB part.

[0048] This variation in the bandgap energy of the active region in the EAM part can be achieved by coupling the EAM part 21 to the DFB part 20 using selective area growth (SAG).

[0049] The above-described device can be fabricated by depositing materials onto a substrate to grow and couple the modulator and laser parts of the device. Generally, source materials from a metal oxide chemical vapour deposition (MOCVD) source in the gas phase will epitaxially grow in the unmasked regions. When the modulator part is grown, a dielectric mask can be deposited on at least one side, preferably both sides, of the EAM region of the device. When the source material lands on the mask (e.g., it can be a SiO2 dielectric mask), it does not easily nucleate.

[0050] Figure 2 31 in (a) shows an example of the mask profile in the SAG process for producing the optical device described herein. To achieve a variation in the bandgap energy along the EAM part of the device, the shape of the mask can vary with the distance along the EAM part of the device. The mask can be tapered. In Figure 2 the example of (a), a triangular mask is used. More generally, the width of the mask at the interface between the laser and the modulator can be narrower than the width of the mask at a certain distance from the interface. Preferably, the mask is widest adjacent to the second front face 23 of the EML (i.e., at the emission face of the device). However, other shapes of masks are also possible.

[0051] Where the source material lands on the mask, due to local concentration gradients, the source material deposited on the mask may re-enter the gas phase and diffuse to find unmasked areas. In some embodiments, the above may occur if the growth temperature is high enough and / or if the mask width is narrow enough. Compared to a completely unmasked substrate, due to the relative diffusion coefficients of indium (In) and gallium (Ga) under typical MOCVD growth conditions, MQW growth of indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), and indium gallium aluminum arsenide (InGaAlAs) epitaxial layers through a mask may be thicker and have a higher indium content. As a result, due to quantum size effects and alloy composition variations, the MQWs in the active region of the portion covered by the wider part of the mask move to a lower bandgap compared to the region covered by the narrower part of the mask.

[0052] Thus, the growth of the material forming part of the EML device described herein can be made to use selective area growth to define the variation of the composition of the active region with distance from the interface between the modulator and the laser. As described above, the bandgap energy of the active region in the EML portion of the device can be varied. Additionally, when a bias voltage is applied to the device, due to variations in doping concentration and thickness, the electric field across the active region in the EML portion varies with distance along the cavity.

[0053] Thus, SAG can be used to butt-couple the EAM portion to the DFB portion to form an EML. A mask made of a dielectric such as SiO2 can be used in the EAM region 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 bandgap energy, while having a lower bandgap energy near the second front face 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.

[0054] Thus, the EAM absorption along the waveguide is distributed, so the saturation of the first approximately 50 μm portion of the EAM may be reduced.

[0055] The method for forming the device described herein can be summarized as follows. When coupling the modulator and the laser, this manufacturing method affects the growth of the active region of the optical device, where the modulator includes at least a portion of the active region of the device. The method includes growing a portion of the active region of the modulator to define the variation of the bandgap energy in the modulator with distance from the interface between the modulator and the laser, where the bandgap energy of the modulator adjacent to the interface is higher than that of the modulator farther from the interface. The method can be performed during the butt-coupling of the optical modulator to the laser.

[0056] As described above, preferably, the material for forming the modulator (EAM section 21) is grown by depositing a material (e.g., MOCVD or dopant material) between the tapered masks in a selective area growth process. Preferably, the mask width at the interface between the laser and modulator sections of the device is narrower than the mask width at a distance from the interface.

[0057] The variable bandgap energy along the waveguide of the device described herein disrupts the absorption along the EAM section of the waveguide. This can effectively prevent the saturation of the first part of the EAM and contribute to smoothing the temperature distribution, thereby preventing the appearance of strong absorption peaks in the EAM input section observed in EMLs fabricated using butt-coupled growth. This can improve the performance of the EML device.

[0058] The applicant hereby separately discloses each individual feature described herein and any combination of two or more such features, provided that such features or combinations can be implemented on the overall basis of this specification in accordance with the common general knowledge of those skilled in the art, regardless of whether such features or combinations of features solve any of the problems disclosed herein, and are not limited to the scope of the claims. The applicant points out that various aspects of the present application may consist of any such individual features or combinations of features. In view of the foregoing description, it will be apparent to those skilled in the art that various modifications can be made within the scope of the present application.

Claims

1. An optical device having a first surface, a second surface, an optical cavity, and an active region, wherein, The optical cavity is defined by a semiconductor substrate and has a length extending between the first face and the second face. The active region is configured to inject charge into the optical cavity and has an effective bandgap energy at various distances along the length of the optical cavity. The device includes: A modulator extending from a first end located between the first face and the second face and including at least a portion of the active region; and A laser optically coupled to the first end of the modulator; A waveguide extending together with the optical cavity for inducing light in the optical cavity to propagate along the length of the optical cavity, the waveguide being a ridge waveguide or a buried heterostructure waveguide; An isolation portion disposed on the active region of the modulator; An electrode pair disposed on either side of the semiconductor substrate, and the laser includes another portion of the active region. The laser is configured such that by applying a current between the electrodes, light emission is excited from the another portion of the active region. Each electrode of the electrode pair is disposed on either side of the modulator, and the electrode pair includes a lumped electrode or a traveling-wave electrode; Wherein, the bandgap energy of the portion of the active region adjacent to the first end is higher than the bandgap energy of the portion of the active region away from the first end. The first end is at the interface between the modulator and the laser. The change in the bandgap energy is achieved by selectively growing regions to couple the modulator to the laser. The doping concentration and thickness of the active region change. When a bias voltage is applied to the modulator, the electric field on the portion of the active region changes at various distances along the optical cavity. The selective area growth specifically includes: using a triangular mask in the modulator region to selectively enhance the growth of the active region along the waveguide of the modulator, wherein the width of the mask at the interface is narrower than the width of the mask away from the interface.

2. The optical device according to claim 1, wherein The bandgap energy of the portion of the active region linearly decreases as the distance from the first end increases.

3. The optical device according to claim 1 or 2, wherein, The device is configured such that the second face is the emission face of the device.

4. The optical device according to claim 1 or 2, wherein, The second face is coated with an antireflection coating.

5. The optical device according to claim 1 or 2, wherein The active region extends in a direction between the first face and the second face.

6. The optical device according to claim 1 or 2, wherein, The optical cavity includes a first semiconductor layer of a first doping type and a second semiconductor layer of a second doping type opposite to the first doping type, wherein the active region is located between the first semiconductor layer and the second semiconductor layer.

7. The optical device according to claim 1, wherein, The waveguide has a constant width.

8. The optical device according to claim 1 or 7, wherein, The waveguide has a width between 0.5 μm and 3.0 μm.

9. The optical device according to claim 1 or 2, wherein, The modulator is an electro-absorption modulator.

10. The optical device according to claim 1 or 2, wherein, The laser is a distributed feedback laser.

11. A method for manufacturing an optical device, which affects the growth of the active region of the optical device when coupling an optical modulator and a laser; wherein, The modulator includes at least a portion of the active region of the device, the optical device includes an isolation portion disposed on the active region of the modulator, the method includes growing a portion of the active region of the modulator to define a change in the bandgap energy in the modulator as a function of the distance from the interface between the modulator and the laser, the doping concentration and thickness of the active region vary, when a bias voltage is applied to the modulator, the electric field on the portion of the active region varies at each distance along the optical cavity; wherein, the bandgap energy of the modulator adjacent to the interface is higher than that of the modulator far from the interface, the first end is at the interface between the modulator and the laser, the change in the bandgap energy is achieved by coupling the modulator to the laser through selective area growth, and the selective area growth specifically includes: using a triangular mask in the modulator region to selectively grow along the waveguide-enhanced active region of the modulator, wherein the width of the mask at the interface is narrower than the width of the mask far from the interface; Wherein, the waveguide extends together with the optical cavity for inducing light in the optical cavity to propagate along the length of the optical cavity, and the waveguide is a ridge waveguide or a buried heterostructure waveguide; The optical device includes a pair of electrodes disposed on either side of the semiconductor substrate, and the laser includes another portion of the active region, the laser is configured such that by applying a current between the electrodes, light emission is excited from the another portion of the active region, each electrode in the pair of electrodes is disposed on either side of the modulator, and the pair of electrodes includes a lumped electrode or a traveling-wave electrode.

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