Electroabsorption modulated laser

Abstract

An electro-absorption modulated laser has 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 surface and the second surface, the active region is configured to inject charge into the cavity and has an effective bandgap energy at corresponding distances along the length of the cavity, the electro-absorption modulated laser includes a first modulator segment and a second modulator segment, the first modulator segment extending between a first position and a second position and including a first portion of the active region, the second modulator segment extending between the second position and a third position and including a second portion of the active region, wherein the bandgap energy of the first portion of the active region adjacent to the first position is higher than the bandgap energy adjacent to the second position.

Description

Technical Field

[0001] The present application relates to optical devices, such as electro-absorption modulated lasers. Background Art

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

[0003] As shown in the examples of Figures 1(a) and 1(b), a standard high-speed electroabsorption modulated laser (EML) includes a distributed feedback (DFB) laser 10 and an electroabsorption modulator (EAM) 11. The device generally includes a semiconductor block having a rear surface or facet 12, a front surface or facet 13 opposite thereto, and an optical cavity formed therebetween. The front facet and the rear facet are usually cut. The cavity traditionally includes an active layer 14 between p-type and n-type semiconductor material layers (shown as 15 and 16, respectively). One or more coatings, such as anti-reflection (AR) or high reflection (HR) coatings, can be applied to the front facet and the rear facet to provide a predetermined reflectivity. In a distributed feedback (DFB) laser, a Bragg grating acts as a wavelength selection element for at least one of the faces and provides feedback, reflecting light back into the cavity to form a resonator. The rear surface of the DFB is generally coated with an HR coating to enhance power output. In an EAM, the front facet at the emitting surface is generally coated with an AR coating to reduce facet reflection. In some embodiments, the EML may alternatively include a distributed Bragg reflector (DBR) laser instead of a DFB laser.

[0004] In an EML, isolation between the laser and the EAM is typically achieved by etching away the top layer of the substrate to a depth of about 1.0 to 2.5 μm (shown at 17) or by ion implantation.

[0005] Typically, electro-absorption modulated lasers utilize the quantum confined Stark effect (QCSE) to change the absorption of the device. When an external electric field is applied to the device, the electron state shifts to a lower energy level, while the hole state shifts to a higher energy level, increasing the allowed light absorption at the lasing wavelength. In addition, the electrons and holes are transferred to opposite sides of the well, which reduces the overlap integral and thus the recombination efficiency of the system. QCSE allows optical communication signals to be quickly turned on and off so that light can be transmitted through the device as "0" and "1" signals.

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

[0007] Through the Kramers-Kronig relationship, it is known that changes in absorption in an EML can lead to changes in the refractive index and a shift in the frequency of light waves propagating through the device. This is known as frequency chirp. Frequency chirp can cause degradation in the transmission performance of fiber-based communication systems, especially at high baud rates.

[0008] Conventional EML devices can suffer from positive frequency chirp, which results in poor transmission performance. This can be due to enhanced pulse broadening caused by fiber dispersion. Chirp can be improved at high bias levels, but this can result in high insertion loss.

[0009] It has been previously proposed to use a dual-segment EML structure, as shown in Figures 2(a) and 2(b), which can correct EML chirp by data and data-bar modulation. Such a device has a DFB segment 20. The first modulator segment EAM121 with MQW2 is different in length or bias conditions from EAM2 22 with MQW3 or bulk, or EAM2 can have different bandgap energies for MQW3 / bulk. Although a dual-segment electro-absorption modulator EML can improve frequency chirp, the fabrication of such a device generally requires two butt-coupled overgrowths of the two EAM segments when MQW2 and MQW3 have different bandgap energies.

[0010] Segments EAM1 21 and EAM2 22 may have the same active component (MQW) but different bias conditions, but the extinction ratio (ER) may be affected.

[0011] It would be desirable to develop a device that is less susceptible to such problems. Summary of the invention

[0012] According to a first aspect, there is provided an electro-absorption modulated laser 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 surface and the second surface, the active region is configured to inject charge into the cavity and has an effective band gap energy at a corresponding distance along the length of the cavity, the electro-absorption modulated laser comprises a first modulator segment and a second modulator segment, the first modulator segment extending between a first position and a second position and including a first portion of the active region, the second modulator segment extending between the second position and a third position and including a second portion of the active region;

[0013] The bandgap energy of the first portion of the active region adjacent to the first position is higher than the bandgap energy of the first portion adjacent to the second position.

[0014] The electro-absorption modulated laser may include a distributed feedback laser optically coupled to the first modulator segment.

[0015] The bandgap energy of the first portion of the active region may vary at various distances along the cavity due to variations in the composition of the first portion of the active region.

[0016] When current is applied to the laser, the electric field of the first portion of the active region may vary at various distances along the cavity due to variations in doping concentrations of semiconductor layers of the optical cavity and / or thickness of the first portion of the active region.

[0017] The third position may be located at the second surface. The device may be configured so that the second surface is the emitting surface of the device. This may allow the device to be integrated with other optical functional structures. For example, a Mach-Zehnder modulator, or an amplifier

[0018] The first modulator segment and the second modulator segment may be electrically isolated from each other. This may allow the first modulator segment and the second modulator segment to be independently controlled.

[0019] The first modulator segment and the second modulator segment may be configured to be modulated by a common driver.

[0020] The first modulator segment may be data modulated and the second modulator segment may be data stripe modulated.

[0021] A bandgap energy of the second portion of the active region adjacent to the second location may be substantially the same as a bandgap energy at the third location.

[0022] An average bandgap energy of the first portion of the active region may be lower than an average bandgap energy of the second portion of the active region.

[0023] The second surface may be coated with an anti-reflective coating. This may reduce facet reflections in the device. This may improve the performance of the device.

[0024] The active region may be elongated in a direction extending between the first surface and the second surface.

[0025] The device may further comprise a waveguide extending with the optical cavity for inducing light in the cavity to propagate along the length of the cavity. This may effectively allow emitted light to propagate along the cavity.

[0026] The waveguide may have a substantially constant width. This may facilitate device fabrication.

[0027] The second modulator segment may include a portion of a waveguide. This portion of the waveguide may be bent or angled relative to the remainder of the waveguide. This may reduce facet reflections at the second surface of the electro-absorption modulated laser.

[0028] The waveguide may have a width between 0.5 μm and 3.0 μm. This may allow the effective refractive index of the waveguide to be chosen accordingly.

[0029] The waveguide may be a ridge waveguide or a buried heterostructure waveguide. This may allow flexibility in manufacturing the laser.

[0030] 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 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.

[0031] According to a second aspect, there is provided a method of growing an active region of an electro-absorption modulated laser having a first modulator segment and a second modulator segment, the first modulator segment being optically coupled to the laser, the first modulator segment comprising a first portion of the active region, the second modulator segment comprising a second portion of the active region, the method comprising growing the first portion of the active region beyond a mask such that the average bandgap energy of the first portion of the active region is lower than the average bandgap energy of the second portion of the active region. This may allow the device to be manufactured more efficiently.

[0032] The mask may be tapered so that the mask width at the interface between the first modulator segment and the laser is narrower than the mask width at a distance from the interface. This may allow the first portion of the active region to have a higher bandgap energy adjacent to the interface than at a distance from the interface. This may lead to improved device performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present application will now be described by way of example with reference to the accompanying drawings.

[0034] In the attached picture:

[0035] FIG. 1( a ) shows a top view of a conventional EML.

[0036] FIG. 1( b ) is a side view along the AA section in FIG. 1( a ).

[0037] FIG2( a ) shows a top view of an EML with two EAM segments.

[0038] FIG. 2( b ) is a side view along the BB section in FIG. 2( a ).

[0039] FIG3( a ) shows a top view of an example optical device.

[0040] FIG. 3( b ) is a side view along the CC section in FIG. 3( a ).

[0041] FIG. 3( c ) shows an example of the change in band gap energy of the active region of the EAM segment of the optical device shown in FIG. 3( a ) and FIG. 3( b ).

[0042] FIG4( a ) shows a top view of an example optical device.

[0043] FIG. 4( b ) is a side view along the DD section in FIG. 4( a ).

[0044] Figure 4(c) shows Figure 4(a) and 4(b) An example of the variation in bandgap energy in the active region of the EAM segment of the optical device shown. DETAILED DESCRIPTION

[0045] In an exemplary embodiment, as shown in FIGS. 3( a) and 3( b), the EML device includes a DFB laser 30, a first EAM segment 31 and a second EAM segment 32. The DFB laser 30 includes a semiconductor block having a first rear surface 35. The second front surface 36 of the EML device is opposite to the rear surface, and an optical cavity is formed between them. The front surface and / or the rear surface may be a cut facet. Preferably, the front facet and the rear facet of the device are aligned parallel to each other. A high reflective (HR) coating may be applied to the rear facet. The facet 35 serves as a rear reflector. An AR coating is applied to the front facet 36 at the emitting surface of the device to reduce facet reflection. The grating (not shown) of the DFB laser segment 30 may be a full grating, a λ / 4 grating or a partial grating.

[0046] In the example shown in Figures 3(a) and 3(b), the optical cavity of the EML includes an active layer 37 between a p-type semiconductor material layer and an n-type semiconductor material layer (shown as 38 and 39 in Figure 3(b) respectively). In this example, the semiconductor layer is made of InP. However, other semiconductor materials, such as GaAs, can be used. The cavity forming material can be selectively doped in the regions of the p-type layer and the n-type layer. These layers are defined in the substrate. The multi-quantum wells MQW1, MQW2 and MQW3 in the active region of the device are shown as 40, 41 and 42 respectively.

[0047] The DFB, EAM1 and EAM2 segments are separated by isolation segments, as shown in 43 in Figures 3(a) and 3(b). No current is injected into the device in these isolation segments, and the DFB, EAM1 and EAM2 segments are electrically isolated from each other. The length of the isolation segment can be about 40 to 100 μm, and the etching depth from the top of the semiconductor substrate in this segment can be about 0.8 to 2.0 μm. The top p-InP layer between the DFB and the EAM and the first EAM segment and the second EAM segment can be etched away to achieve electrical isolation between the device segments so that they can be controlled independently.

[0048] A waveguide, the profile of which is shown in the top view of Figure 3(a), extends along the optical cavity and is used to guide light in the cavity 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 surface of the device.

[0049] The waveguide may be a ridge waveguide, preferably a shallow ridge waveguide. A ridge waveguide may be created by etching parallel grooves in the material on either side of the waveguide to produce isolated projection strips (typically less than 10 μm wide and a few hundred μm long). A material with a lower refractive index than the waveguide material may be deposited on the sides of the ridge to direct the injected current into the ridge. Alternatively, the ridge may be surrounded by air on three sides that are not in contact with the substrate below the waveguide. The ridge may also be coated with gold to provide electrical contact and to help remove heat from the ridge when it emits light.

[0050] Alternatively, the waveguide may be a buried heterostructure waveguide. The waveguide of the device may be straight or curved. The waveguide width of the EAM segment is preferably between 0.5 μm and 3.0 μm. The waveguide widths of the DFB and the two EAM segments may be different, or they may be the same (this is the case in the example shown in Figure 3(a)). To allow for the effective refractive index of the waveguide to be selected, the width may be selected accordingly. The waveguide of the EML may have a substantially constant width.

[0051] The device includes a pair of electrodes 44a, 44b disposed on either side of a semiconductor substrate. In the active region 37, light emission is stimulated from the active region of the device laser section by applying a current between the electrodes 44a, 44b. The electrodes of the EAM section may be, for example, lumped electrodes or traveling wave electrodes.

[0052] As can be seen from FIG3( b ), the second front surface 36 of the device is an emission surface where light is output from the device. The optical device can be integrated with other optical functional structures. For example, the device can also include a semiconductor optical amplifier adjacent to the second surface 36 .

[0053] The first modulator segment extends between the first position and the second position and includes a first portion of the active region. The second modulator segment extends between the second position and the third position and includes a second portion of the active region. As shown in FIG3(c), the bandgap energy of the first portion of the active region adjacent to the first position is higher than the bandgap energy of the first portion of the active region adjacent to the second position.

[0054] This change in the bandgap energy of the active region in the EAM segment can be achieved by coupling the EAM1 segment 31 with the DFB segment 30 using selective area growth (SAG).

[0055] The above-mentioned EML device can be manufactured by depositing materials on a substrate to grow and couple the modulator segment and laser segment of the device. Typically, metal oxide chemical vapour deposition (MOCVD) source material from the gas phase will be epitaxially grown in the unmasked area. When growing the modulator segment, a dielectric mask can be deposited on at least one side of the EAM region of the device, preferably on both sides. Where the source material is attached on the mask (which can be a SiO2 dielectric mask, for example), it will not be easy to nucleate.

[0056] An example of a mask profile used in a SAG process to produce a dual EAM segment EML as described herein is shown at 34 in FIG. 3( a). In a preferred embodiment, in order to achieve a change in the bandgap energy along the first EAM segment of the device, the shape of the mask varies with distance along the device EAM segment. The mask can taper. In the example of FIG. 3( a), a triangular mask shape is used. More generally, the mask width at the interface between the laser and the first modulator segment can be narrower than the mask width at a distance from the interface. Preferably, the mask is widest near the interface between the first EAM segment and the second EAM segment. However, other mask shapes are possible.

[0057] Where the source material is attached to the mask, source species deposited on the mask may re-enter the gas phase and diffuse due to local concentration gradients 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. Compared to a completely unmasked substrate, the MQW growth that occurs by masking the InGaAs, InGaAsP, InGaAIAs epilayers may be thicker and richer in indium due to the relative diffusion coefficients of In and Ga under typical MOCVD growth conditions. Therefore, due to quantum size effects and changes in alloy composition, the MQW in the portion of the active area covered by the wider portion of the mask shifts to a lower energy bandgap compared to the area covered by the narrower portion of the mask.

[0058] Thus, the material forming a portion of the EML device described herein can be grown so as to define the composition of the active region as a function of distance from the interface between the first modulator segment and the laser using selective area growth. By using a tapered mask, as shown in FIG3(a), the bandgap energy of the active region in the first EAM segment of the device can be varied, as shown in FIG3(c). Furthermore, when a bias voltage is applied to the device, the electric field across the active region within the EAM segment can vary at various distances along the cavity due to variations in doping concentration and thickness.

[0059] In another embodiment, as shown in FIG. 4( a), a rectangular mask 54 can be used to grow an active region in a first EAM segment 51. The EML device includes a DFB laser 50, a first EAM segment 51, and a second EAM segment 52. The segments 50, 51, and 52 are separated by an isolation segment, as shown at 53. The DFB laser includes a semiconductor block having a first rear surface 55. A second front surface 56 of the EML device is opposite to the rear surface, and an optical cavity is formed therebetween. The optical cavity of the EML includes an active layer 57 between a p-type semiconductor material layer and an n-type semiconductor material layer (shown as 58 and 59, respectively, in FIG. 4( b)). The multiple quantum wells MQW1, MQW2, and MQW3 in the active region of the device are shown as 60, 61, and 62, respectively. A pair of electrodes 63a, 63b are disposed on both sides of the semiconductor substrate. In the active region 67, light emission is stimulated from the active region of the laser segment of the device by applying a current to the electrodes 63a, 63b. The electrodes of the EAM segment may be, for example, lumped electrodes or traveling wave electrodes.

[0060] In this example, SAG using a rectangular mask 54 can be used to couple the first EAM segment 51 with the DFB segment 50. The mask 54 (which can be made of a dielectric such as SiO2) is used to enhance the growth of the MQW2 region 61 in the first modulator segment 51. Therefore, the average bandgap energy of the MQW2 segment 61 in the active region will be lower than the MQW3 segment 62 in the active region in the second EAM segment 52 (as shown in FIG. 4(c)), and one SAG growth can be used instead of two butt-coupled growths to form an EML device.

[0061] Thus, as described above, SAG can be used to couple the first EAM segment with the DFB segment and / or the second EAM segment. A mask made of a dielectric such as SiO2 can be used in the EAM region to enhance the growth of the MQW2 region. Thus, the average bandgap energy of the MQW3 region is greater than the average bandgap energy of the MQW2 region.

[0062] In the case of using a tapered mask, growth can be selectively enhanced along the waveguide of the first EAM segment. Thus, the MQW2 segment of the active region in the EAM segment of the device can have a higher bandgap energy near the interface with the DFB segment and a lower bandgap energy near the second EAM segment. The mask shape is preferably variable along the waveguide. Thus, the MQW2 segment in the first part of the EAM can have a variable bandgap energy along the waveguide, rather than a constant bandgap energy.

[0063] The bandgap energy of the MQW3 segment in the second EAM segment of the device is preferably constant, but may vary.

[0064] In any of the above embodiments, a portion of the waveguide in the second modulator segment may be bent or angled relative to the remainder of the waveguide. This may reduce facet reflections at the second surface of the EML.

[0065] A method of growing an active region of an electro-absorption modulated laser having a first modulator segment and a second modulator segment can be summarized as follows. The EML has a first modulator segment that is optically coupled to the laser segment at one end. At its other end, the first modulator segment is optically coupled to the second modulator segment. The first modulator segment includes a first portion of the active region and the second modulator segment includes a second portion of the active region. The method includes growing the first portion of the active region beyond a mask so that the average bandgap energy of the first portion of the active region is lower than the average bandgap energy of the second portion of the active region.

[0066] In the resulting electro-absorption modulated laser (which may be the EML described for any of the above embodiments), the first modulator segment may include a mask for device fabrication.

[0067] As mentioned above Figure 3(a) to Figure 3(c) As described, the mask can be tapered so that the mask width at the interface between the first modulator segment and the laser is narrower than the mask width at a certain distance from the interface. For example, the mask can have a triangular or trapezoidal shape. In the case of using such a mask, for a device in which the electro-absorption modulated laser includes a first modulator segment and a second modulator segment, wherein the first modulator segment extends between a first position and a second position and includes a first portion of an active region, and the second modulator segment extends between the second position and a third position and includes a second portion of the active region, the method can include growing a first portion of the active region of the first modulator segment so as to define a change in the bandgap energy in the modulator as a function of the distance from the first position and the second position in the first modulator segment, wherein the bandgap energy of the first portion of the active region adjacent to the first position is higher than the bandgap energy adjacent to the second position.

[0068] Thus, the material forming the first modulator segment (EAM1 segment 31, 51) can be grown by depositing material (e.g., MOCVD or dopant material) over a mask in a selective area growth process during coupling of the different segments of the EML device in the EAM1 region. Thus, the active region of the second modulator segment will have a higher average bandgap energy than the active region of the first modulator segment. In a preferred embodiment, the mask width at the interface between the laser and the first modulator segment of the device is narrower than the mask width at a certain distance from the interface. Thus, the bandgap of MQW2 can have a higher energy (less absorption) at the beginning of the EAM1 segment, and the absorption will be distributed along the EAM1 segment, which may result in a lower saturation in this segment of the device.

[0069] Therefore, using the above structure, the EAM1 section can perform data modulation, while the EAM2 section can provide chirp compensation without causing much loss in extinction ratio. The chirp compensation depends on the length, voltage bias and peak-to-peak voltage of the EAM2 section, that is, the chirp compensation is adjustable.

[0070] The above method of growing the active region of the device can be performed during the butt coupling of the first optical modulator segment to the laser. The first EAM segment and the second EAM segment can be grown in the same butt coupling growth. Therefore, this selective area growth can reduce two overgrowths to only one overgrowth. This can reduce the cost of chips including EML devices and increase production.

[0071] The first EAM segment and the second EAM segment can be controlled under the same DC bias. As shown in Figures 3(b) and 4(b), the first EAM segment and the second EAM segment can be under a single modulation (data and data stripe, respectively) from the same driver.

[0072] Therefore, the dual-segment EAM waveguide described herein can realize a chirp-controlled EML, where the first EAM segment EAM1 is modulated with data of length L1, and the second EAM segment EAM2 having a short wavelength MQW is modulated with data stripes of length L2. When SAG is used, a tandem EAM structure can be realized using one SAG growth instead of two butt-coupled growths.

[0073] Applicants hereby disclose separately each individual feature described herein and any combination of two or more of these features to the extent that these features or combinations can be implemented as a whole based on this specification and in combination with the common knowledge of those skilled in the art, but without considering whether these features or combinations solve any problems disclosed herein, nor limiting the scope of the claims. Applicants point out that various aspects of this application may include any individual feature or any combination of features. In view of the above description, it is obvious that those skilled in the art can make various modifications within the scope of this application.

Claims

1. An electro-absorption modulated laser, comprising a first surface, a second surface, an optical cavity and an active region, characterized in that: The optical cavity is defined by a semiconductor substrate and has a length extending between the first surface and the second surface, the active region is configured to inject charge into the cavity and has an effective bandgap energy at a corresponding distance along the length of the cavity, the electro-absorption modulated laser comprising: a first electro-absorption modulator segment extending between a first location and a second location and comprising a first portion of the active region; a second electro-absorption modulator segment extending between the second location and a third location and comprising a second portion of the active region; and a distributed feedback laser coupled to the first electro-absorption modulator segment at the first location; The first portion of the active region has a higher bandgap energy adjacent to the first location than adjacent to the second location, and the bandgap energy of the second portion of the active region is substantially constant.

2. The electro-absorption modulated laser according to claim 1, characterized in that: The bandgap energy of the first portion of the active region varies at various distances along the cavity due to variations in the composition of the first portion of the active region.

3. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: When current is applied to the laser, the electric field of the first portion of the active region varies at various distances along the cavity due to variations in doping concentrations of semiconductor layers of the optical cavity and / or thickness of the first portion of the active region.

4. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The third position is located on the second surface.

5. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The electro-absorption modulated laser is configured such that the second surface is an emission surface of the electro-absorption modulated laser.

6. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The first electro-absorption modulator segment and the second electro-absorption modulator segment are electrically isolated from each other.

7. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The first electro-absorption modulator segment and the second electro-absorption modulator segment are configured to be modulated by a common driver.

8. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The first electro-absorption modulator segment is data modulated and the second electro-absorption modulator segment is data stripe modulated.

9. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The second portion of the active region has a bandgap energy adjacent to the second location that is substantially the same as a bandgap energy at the third location.

10. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The first portion of the active region has an average bandgap energy lower than an average bandgap energy of the second portion of the active region.

11. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The second surface is coated with an anti-reflective coating.

12. The electro-absorption modulated laser according to claim 1 or 2, characterized in that: The active region is elongated in a direction extending between the first surface and the second surface.

13. The electro-absorption modulated laser according to claim 1, characterized in that: The electro-absorption modulated laser further includes a waveguide extending with the optical cavity for inducing light in the cavity to propagate along the length of the cavity.

14. The electro-absorption modulated laser according to claim 13, characterized in that: The waveguide has a substantially constant width.

15. The electro-absorption modulated laser according to claim 13 or 14, characterized in that: The waveguide has a width between 0.5 μm and 3.0 μm.

16. The electro-absorption modulated laser according to claim 13 or 14, characterized in that: The waveguide is a ridge waveguide or a buried heterostructure waveguide.

17. A method for growing an active region of an electroabsorption modulated laser, the electroabsorption modulated laser having a first electroabsorption modulator segment and a second electroabsorption modulator segment, the first electroabsorption modulator segment being optically coupled to the laser, the first electroabsorption modulator segment comprising a first portion of the active region, the second electroabsorption modulator segment comprising a second portion of the active region, characterized in that: The method includes growing the first portion of the active region beyond a mask such that an average bandgap energy of the first portion of the active region is lower than an average bandgap energy of the second portion of the active region; The mask is tapered such that a width of the mask at an interface between the first electro-absorption modulator segment and the laser is narrower than a width of the mask at a distance from the interface.

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