CN116235100A - High-speed modulation laser - Google Patents

Abstract

A modulated laser is described comprising a laser (30) optically coupled to a modulator (50), the modulator (50) having an intrinsic region (52), the intrinsic region (52) comprising: a multiple quantum well material (41); a field control layer (42) having a doping density to enhance an electric field in the multiple quantum well material; and a collector layer (43) having a band gap higher than that of the multi-quantum well material (41). The incorporation of a field control layer and a collection layer in the intrinsic region of the modulator of a modulated laser may eliminate the need to trade off between high speed and low external bias of the modulated laser.

Description

High-speed modulated laser

technical field

The present invention relates to lasers, for example to improving the speed of modulating lasers.

Background technique

In externally modulated lasers (ML), there may be a trade-off between high speed and low bias requirements.

A standard approach to this problem is to use a PIN structure, where an intrinsic layer of semiconductor material is sandwiched between a p-type semiconductor layer and an n-type semiconductor layer in the modulator. To meet high speed requirements, the modulator should have low capacitance. This means that, for a standard PIN structure, the intrinsic region should be relatively thick.

However, a larger intrinsic thickness means a weaker electric field across the intrinsic region. Therefore, for a given external bias, the modulator may have a lower extinction ratio.

Figure 1(a) and Figure 1(b) schematically show a standard ML including a distributed feedback (distributed feedback, DFB) laser part 10 and a Mach-Zehnder (Mach-Zehnder, MZ) modulator part 21, respectively. top and side views. The DFB section and the MZ section are usually optically coupled using a butt coupling method with an isolation region between them. The device has a rear surface 13 from which light enters the device and a front surface 14 from which light exits the device. The rear surface 13 may be coated with a high-reflection (HR) coating, and the front surface 14 may be coated with an anti-reflection (AR) coating. An electrical bias may be applied to portions of the device through electrodes 20a, 20b and 20c.

The multiple quantum well (multiple quantum well, MQW) material 19 of the M-Z intrinsic region (MQW2) is grown in butt-coupled manner with the MQW material 16 of the DFB intrinsic region (MQW1). In this example, the MQW1 layer 16 has the p-type semiconductor layer 15 above it and the n-type semiconductor layer 17 below it. MQW2 layer 19 has p-type semiconductor layer 18 thereon and n-type semiconductor layer 17 therebelow. Etching back between the DFB part and the MZ part is used to achieve electrical isolation.

A multi-mode interferometer (MMI) 22 splits the light input to the two arms of the modulators 21a, 21b. The second MMI 24 recombines light from both arms. The MQW material 19 of the Mach-Zehnder modulator (MQW2) is butt-coupled with the MQW material 16 of the DFB (MQW1). The laser can also be a tunable DBR laser. The laser and the Mach-Zehnder modulator can be monolithically integrated or optically coupled. The light-guiding MMI core may have the same composition as the MQW2 region, or may include a separate overgrown material.

Figure 1(c) shows the electric field distribution of the MQW2 region 19 in the modulator. The electric field is higher than that in the n-type and p-type layers on either side of the intrinsic region and is approximately constant. The bandwidth of the modulator is also mainly determined by the capacitance of the PIN structure.

To achieve reliable ML, the preparation and growth conditions of the docking coupling are very critical. In order to increase the output power, DFB is usually coated with HR. The waveguide of a DFB is usually designed with only the fundamental mode, and the waveguide usually has a uniform width, but depending on the laser design, the waveguide width can also be variable. The field over the modulator eigenregion is approximately constant.

In conventional electroabsorption modulated lasers (EML), there is also a trade-off between the bandwidth on the electroabsorption modulator (EAM) and the applied external bias, because the bandwidth in the intrinsic region and the electric field depend on the thickness of the PIN intrinsic region.

It would be desirable to develop a modulated laser with improved properties.

Contents of the invention

According to one aspect, there is provided a modulated laser comprising a laser optically coupled to a modulator having an intrinsic region comprising: a multiple quantum well material; a field control layer having a doping density, to enhance the electric field in the multiple quantum well material; the collection layer has a higher bandgap than the multiple quantum well material.

The modulated laser may be an externally modulated laser. The modulator can modulate the light emitted by the laser. The modulator can have a PIN structure or a NIP structure. The structure includes an intrinsic region between the layer of p-type semiconductor material and the layer of n-type semiconductor material. Such structures can be conveniently fabricated by semiconductor deposition and/or growth techniques.

The intrinsic region may be made of a semiconductor material including but not limited to InP, InGaAsP, InGaAlAs, InGaAlAsSb, GaAs, AlGaAs, GaN, AlGaN Ge or Si. Therefore, the structure is compatible with a range of semiconductor materials, which can be selected according to the application of the modulated laser.

The lasers can be distributed feedback lasers, distributed Bragg reflection lasers or tunable lasers. Thus, the device can be adjusted according to its desired application. A modulated laser may also have a semiconductor optical amplifier (SOA) optically coupled to the modulator. The SOA can receive the light emitted by the modulator. The phase section can also be optically coupled to the laser and modulator.

The modulator can be a Mach-Zehnder modulator or an electroabsorption modulator. Such modulators are commonly used in telecommunication applications.

Voltages can be applied across electrodes or contacts on opposing top and bottom sides of the device sections to control the laser and modulator sections. This allows different parts of the modulated laser to be controlled independently. When the modulator is a Mach-Zehnder modulator, the electrodes may be lumped electrodes or traveling wave electrodes, or capacitively loaded segmented traveling wave electrodes. When the modulator is an electroabsorption modulator, the electrodes may be lumped electrodes or traveling wave electrodes.

The modulator can be a Mach-Zehnder modulator, and the modulated laser can also include a beam splitter. This allows the light leaving the laser to be optically split into two waveguide interferometer arms. If a voltage is applied to one arm in a Mach-Zehnder modulator, light passing through that arm of the modulator causes a phase shift. When the light from each of the two arms is recombined, the phase difference is converted into amplitude modulation.

The doping density of the collection layer may be between 1.0e15/cm ³ and 8e17/cm ³ . This enables control over the behavior of the collection layer. The doping density can depend on the thickness of the collection layer. The introduction of the collection layer can reduce the overall capacitance of the structure. At the same time, it can also make carriers (mainly electrons) move to the n side of the PIN structure.

The collection layer can be p-doped or n-doped. This enables versatility in the modulator structure.

The bandgap of the collection layer can be constant or graded. A constant bandgap allows for easy fabrication of this layer.

The thickness of the collection layer may be between 20 nm and 1000 nm. The thickness of the field control layer may be between 10 nm and 500 nm.

The field control layer may have a bandgap higher than that of the multiple quantum well material. The doping density of the field control layer can be between 1e17/cm ³ and 1e19/cm ³ so that it does not introduce additional absorption and makes the electric field across the MQW material larger.

When the modulator is an electroabsorption modulator, in the collection layer the charge may be carried mainly by electrons. Under the condition that the modulator is an EAM, the EAM absorbs the incident light. Thus, charge carriers are generated in the MQW material in the intrinsic region: electrons in the conduction band and holes in the valence band of the MQW material. Under reverse bias, the hole moves to the p-InP side, similar to a conventional PIN structure. Electrons move to the collection layer and n-InP layer. Therefore, in the collecting layer, electrons are effective charge carriers.

The modulator may include a front surface, which is the emitting surface of the modulating laser, and which is coated with an anti-reflection coating. Anti-reflective coatings can prevent the reflection of light leaving the device at the front surface. This can improve the efficiency of modulating the laser.

The laser may include a back reflector coated with a highly reflective coating. This can increase the power and increase the efficiency of the laser. The rear reflector may be the rear end face and the front surface may be the front face. The front end face and the rear end face may be cleavage faces. This is a convenient way to make modulated lasers. The rear reflector and front surface can also be formed by other convenient methods.

The modulated laser may include a waveguide, which may be a ridge waveguide or a buried heterostructure waveguide, for guiding light along the device. This enables versatility in device construction.

Modulated lasers can be integrated with other optically functional structures and can be used in various applications, such as telecommunication systems.

Description of drawings

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

In the attached picture:

Figures 1(a) and 1(b) show top and side views, respectively, of a standard ML comprising a laser optically coupled to a Mach-Zehnder modulator.

Figure 1(c) shows the structure and electric field distribution (along the B-B section of Figure 1(b)) of the intrinsic region of a standard ML with a Mach-Zehnder modulator.

Figures 2(a) and 2(b) show top and side views, respectively, of an embodiment of an ML optically coupled to a Mach-Zehnder modulator.

Fig. 3(a) schematically shows an example of the structure of the intrinsic region of the Mach-Zehnder modulator.

Fig. 3(b) shows the electric field distribution in the intrinsic region shown in Fig. 3(a).

Detailed ways

One example of the ML described herein includes a DFB laser optically coupled to a Mach-Zehnder modulator, as shown in Figures 2(a) and 2(b). In this example, the device has a PIN structure with the intrinsic region having a p-doped semiconductor layer above it and an n-doped semiconductor layer below it. However, the device can also have a NIP structure with an n-doped semiconductor layer above it and a p-doped semiconductor layer below it.

As shown in FIGS. 2( a ) and 2 ( b ), a DFB laser 30 generally includes a semiconductor block having a rear surface or facet 33 of ML at one end. The DFB laser has an intrinsic region comprising MQW material (MQW3) 35 . The side view along C-C is shown in Fig. 2(b).

The intrinsic region 35 has a p-InP semiconductor layer 36 thereon and an n-InP semiconductor layer 37 therebelow. Therefore, in this example, the laser has a PIN structure. The laser can also have a NIP structure, with the intrinsic region having an n-doped semiconductor layer above it and a p-doped semiconductor layer below it.

The device uses a multi-mode interferometer (MMI) 51 or other beam splitter to split the light exiting the DFB laser 30 into two waveguide interferometer arms 50a, 50b. If a voltage 50 is applied to one arm of a Mach-Zehnder modulator, light passing through that arm of the modulator causes a phase shift. When the light from each of the two arms is recombined, the phase difference is converted into amplitude modulation.

Modulator 50 has an intrinsic region, shown at 52 (MQW FCL). In this example, the intrinsic region of the modulator has a p-InP semiconductor layer 39 above it and an n-InP semiconductor layer 37 below it, ie the modulator has a PIN structure. Alternatively, the modulator can have a NIP structure. The composition of the intrinsic region 52 is shown in Fig. 3(a).

The intrinsic region 52 of the Mach-Zehnder modulator between the p-doped layer 39 and the n-doped layer 37 comprises the MQW (or bulk) material 41, a field control layer (FCL) 42 and a collection layer 43, as shown in Figure 3(a).

The field control layer 42 has a doping density to enhance the electric field in the multiple quantum well material. The field control layer is usually doped so that the electric field is mainly concentrated in the intrinsic MQW (intrinsic MQW, i-MQW) region 41 . Therefore, the electric field in the MQW material is increased relative to the conventional intrinsic region. The doping density can depend on the thickness of the MQW intrinsic region and the required electric field. The thickness of the field control layer can be between 10 nm and 500 nm and the doping density can be between 1e17/cm ³ and 1e19/cm ³ , depending on the structure of the modulator. The field control layer can conveniently perform p-doping on the NIP structure, or n-doping on the PIN structure. The field control layer 42 preferably has a higher bandgap energy than that of the multiple quantum well material 41 . The field control layer generates a higher electric field in the depletion absorbing layer than in conventional depletion layers.

The collection layer 43 preferably has a bandgap higher than that of the multiple quantum well material to avoid additional absorption. The collection layer may be undoped, or may have a low doping concentration (lower than that of the collection layer). The bandgap energy of the collection layer can be constant, for ease of fabrication, or can be graded. The collection layer can be p-doped or n-doped, depending on the structure. The collection layer is formed of a higher bandgap material with little or no doping, making the overall capacitance of the modulator lower to the detriment of additional absorption. In the collecting layer, the charges are mainly carried by electrons. Since only electrons are effective carriers, the fast electron transit time can be exploited. The collection layer is used to reduce the capacitance of the modulator.

Therefore, the introduction of the collection layer can reduce the overall capacitance of the modulated laser structure. At the same time, it can also make carriers (mainly electrons) move to the n side of the PIN structure.

In case the ML comprises a Mach-Zehnder modulator, the collection layer may have the same bandgap energy or a larger bandgap energy than the MQW material of the intrinsic region. Figure 3(b) shows the electric field distribution in the MQW FCL region shown in Figure 3(a) (ie along the D-D section in Figure 2(b)).

Light passing through each arm 50a, 50b of the Mach-Zehnder modulator 50 is recombined at the second MMI 53 before being emitted at the front surface 34 from the modulated laser.

Thus, the modulator generally 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 an intrinsic region. The material forming the device can be selectively doped in regions of the p-type and n-type layers. The first and second semiconductor layers and the intrinsic region are elongated in a direction extending between the rear surface 33 and the front surface 34 of the device.

The length of the ML device is defined between a rear surface or end face 33 and a front surface or front end face 34 . The rear facet with HR coating serves as a rear reflector. Preferably, the front and rear faces are aligned parallel to each other. Preferably, the front face is normal to the length of the device. Preferably, the rear face is normal to the length of the device. One or more front and/or rear surfaces of the device may be formed by cleaving. The width of the waveguides of the laser and modulator sections is preferably perpendicular to the length of the device, and the waveguides preferably direct light in a direction along the length of the ML device. The semiconductor layers 35 , 36 , 37 , 39 and 52 are elongated in a direction extending between the rear surface 33 and the front surface 34 .

Voltages can be applied across electrodes or contacts 40a, 40b and 40c on opposite top and bottom sides of the cavity to control the laser and modulator portions of the device. This enables different parts of ML to be controlled independently.

Light exits the device from the waveguide at the front surface 34, ie the front surface is the emitting surface of the ML.

In the above arrangement the modulator is a Mach-Zehnder modulator. In another embodiment, the modulator of the ML may be an electroabsorption modulator (EAM).

When the modulator is a Mach-Zehnder modulator, the electrodes may be lumped electrodes or traveling wave electrodes, or capacitively loaded segmented traveling wave electrodes. When the modulator is an electroabsorption modulator, the electrodes may be lumped electrodes or traveling wave electrodes.

Due to the field control layer, changes in the electric field in the MQW material of the modulator lead to changes in the refractive index achieved by the M-Z modulators described above, or in absorption changes achieved by modulators including EAMs under reverse bias.

In the above example, the laser is a DFB laser. However, the laser may be any other convenient type of laser, such as a distributed Bragg reflection laser or a tunable laser.

In an embodiment of the present invention, the semiconductor may be InP, InGaAsP, InGaAlAs, InGaAlAsSb, GaAs, AlGaAs, GaN, AlGaN, Ge, Si or other suitable semiconductor materials. The semiconductor material can be selected according to the application of the modulated laser.

As mentioned above, the intrinsic region of the modulator includes the MQW material, the field control layer and the collection layer. Incorporating a field control layer and a collection layer in the intrinsic region of the modulator of a modulated laser eliminates the need for a trade-off between high speed and low external bias of the modulated laser. In such ML, high-speed operation and lower external bias can be achieved simultaneously.

Due to the field control layer, the electric field is stronger in the i-MQW region compared to standard EAM or Mach-Zehnder modulators. Therefore, the modulator requires low external bias. The total capacitance of the modulator is also lower than conventional devices due to the presence of the collection layer. This enables higher speeds, greater bandwidth and better power handling.

Therefore, the modulated lasers described herein may require lower external bias in operation, which may reduce power consumption. The proposed modulated laser can have a larger bandwidth without compromising the bias voltage.

The proposed device is particularly suitable for high-speed operation and may have better chirp and power handling.

Applicants hereby disclose individually each individual feature described herein and any combination of two or more such features. With the common knowledge of those skilled in the art, such features or combinations can be realized as a whole according to the specification, regardless of whether such features or combinations of features can solve any problems disclosed herein; cause restrictions. The applicant indicates that aspects of the invention may comprise any such individual feature or combination of features. In view of the foregoing description it will be apparent to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims (15)

1. A modulated laser comprising a laser (30) optically coupled to a modulator (50), the modulator (50) having an intrinsic region (52), the intrinsic region (52) comprising:

a multiple quantum well material (41);

a field control layer (42) having a doping density to enhance an electric field in the multiple quantum well material;

and a collector layer (43) having a band gap higher than that of the multi-quantum well material (41).

2. The modulated laser according to claim 1, characterized in that the modulator (50) has a PIN or NIP structure.

3. The modulated laser according to any of the preceding claims, characterized in that the intrinsic region (52) is made of at least one of InP, inGaAsP, inGaAlAs, inGaAlAsSb, gaAs, alGaAs, gaN, alGaN, ge and Si.

4. The modulated laser according to any of the preceding claims, characterized in that the laser (30) is a distributed feedback laser, a distributed bragg reflector laser or an adjustable laser.

5. The modulated laser according to any of the preceding claims, characterized in that the modulator is a mach-zehnder modulator (50).

6. A modulated laser according to any of the preceding claims, characterized in that the modulator is an electro-absorption modulator.

7. The modulated laser of claim 5 or 6, wherein in the collecting layer, charge is carried mainly by electrons.

8. The modulated laser according to any of the preceding claims, characterized in that the doping density of the collecting layer (43) is 1.0e15/cm ³ And 8e17/cm ³ Between them.

9. The modulated laser according to any of the preceding claims, characterized in that the collecting layer (43) is p-doped or n-doped.

10. A modulated laser according to any of the preceding claims, characterized in that the field control layer (42) has a higher bandgap than the bandgap of the multiple quantum well material (41).

11. The modulated laser according to any of the preceding claims, characterized in that the bandgap of the collecting layer (43) is constant or graded.

12. The modulated laser according to any of the preceding claims, characterized in that the thickness of the collecting layer (43) is between 20nm and 1000 nm.

13. The modulated laser according to any of the preceding claims, characterized in that the modulator comprises a front surface (34), which is the emitting surface of the modulated laser and which is coated with an anti-reflection coating.

14. The modulated laser according to any of the preceding claims, characterized in that the thickness of the field control layer (42) is between 10nm and 500 nm.

15. The modulated laser according to any of the preceding claims, characterized in that the field control layer (42)Is 1e17/cm ³ And 1e19/cm ³ Between them.

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