CN116235100A

CN116235100A - High-speed modulation laser

Info

Publication number: CN116235100A
Application number: CN202080103897.4A
Authority: CN
Inventors: 陈欣
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2020-08-13
Filing date: 2020-08-13
Publication date: 2023-06-06
Also published as: WO2022033684A1

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 modulation laser

Technical Field

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

Background

In an externally Modulated Laser (ML), a tradeoff between high speed and low bias requirements may be required.

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

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

Fig. 1 (a) and 1 (b) schematically show a top view and a side view of a standard ML comprising a distributed feedback (distributed feedback, DFB) laser section 10 and a Mach-Zehnder (MZ) modulator section 21, respectively. The DFB section and MZ section are typically optically coupled using a butt coupling method with an isolation region between them. The device has a rear surface 13 where light enters the device and a front surface 14 where light is emitted from the device. The rear surface 13 may be coated with a high-reflectance (HR) coating and the front surface 14 may be coated with an anti-reflectance (AR) coating. An electrical bias may be applied to portions of the device through

electrodes

20a, 20b and 20 c.

Multiple quantum well (multiple quantum well, MQW) material 19 of the M-Z intrinsic region (MQW 2) is grown butt-coupled with MQW material 16 of the DFB intrinsic region (MQW 1). In this example, the MQW1 layer 16 has a p-type semiconductor layer 15 above it and an n-type semiconductor layer 17 below it. The MQW2 layer 19 has a p-type semiconductor layer 18 above it and an n-type semiconductor layer 17 below it. The deep etch between the DFB portion and the MZ portion is used to achieve electrical isolation.

The multimode interferometer (multi-mode interferometer, MMI) 22 splits the light input to the two arms of the

modulators

21a, 21 b. The second MMI 24 recombines the light from the two arms. The MQW material 19 of the Mach-Zehnder modulator (MQW 2) is grown butt-coupled with the MQW material 16 of the DFB (MQW 1). The laser may also be a tunable DBR laser. The laser and mach-zehnder modulator may be monolithically integrated or optically coupled. The MMI core that directs the light may have the same composition as the MQW2 region or may include a separate overgrown material.

Fig. 1 (c) shows the electric field distribution of the MQW2 region 19 in the modulator. The electric field is higher than 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 determined primarily by the capacitance of the PIN structure.

The preparation and growth conditions of the butt-coupling are critical in order to achieve reliable ML. To increase output power, DFB is typically coated with HR. The waveguide of the DFB is typically designed to have only a fundamental mode, the waveguide typically having a uniform width, but the waveguide width may also be variable depending on the laser design. The field across the intrinsic region of the modulator is approximately constant.

In conventional electroabsorption modulated lasers (electroabsorption modulated laser, EML) there is also a tradeoff between the bandwidth on the electroabsorption modulator (electroabsorption modulator, EAM) and the applied external bias, since the bandwidth and the electric field in the intrinsic region are both dependent on the thickness of the PIN intrinsic region.

It is desirable to develop a modulated laser with improved performance.

Disclosure of Invention

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

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

The intrinsic region may be made of semiconductor materials including, but not limited to InP, inGaAsP, inGaAlAs, inGaAlAsSb, gaAs, alGaAs, gaN, alGaN Ge or Si. The structure is thus compatible with a range of semiconductor materials, which may be selected according to the application of the modulated laser.

The laser may be a distributed feedback laser, a distributed Bragg reflection laser or an adjustable laser. Thus, the device may be adapted according to its intended application. The modulated laser may also have a semiconductor optical amplifier (semiconductor optical amplifier, SOA) optically coupled to the modulator. The SOA may receive light emitted by the modulator. The phase section may also be optically coupled to the laser and modulator.

The modulator may be a mach-zehnder modulator or an electro-absorption modulator. Such modulators are commonly used in telecommunications applications.

A voltage may be applied to electrodes or contacts on opposite top and bottom sides of the device portion to control the laser and modulator portions. This allows the 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 travelling wave electrodes, or capacitively loaded segmented travelling wave electrodes. When the modulator is an electroabsorption modulator, the electrodes may be lumped electrodes or travelling wave electrodes.

The modulator may be a mach-zehnder modulator and the modulated laser may further include a beam splitter. This may enable the light leaving the laser to be optically split into two waveguide interferometer arms. If a voltage is applied across one arm of a mach-zehnder modulator, the light passing through that arm of the modulator will cause a phase shift. When the light from each of the two arms is recombined, the phase difference is converted to amplitude modulation.

The doping density of the collector layer can be 1.0e15/cm ³ And 8e17/cm ³ Between them. This may enable controlling the behavior of the collection layer. The doping density may depend on the thickness of the collection layer. The introduction of the collection layer may reduce the overall capacitance of the structure. At the same time it also allows carriers (mainly electrons) to move to the n-side of the PIN structure.

The collection layer may be p-doped or n-doped. This may enable versatility of the modulator structure.

The band gap of the collecting layer may be constant or graded. The constant band gap may allow for easy fabrication of the layer.

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

The field control layer may have a band gap higher than that of the multiple quantum well material. The doping density of the field control layer can be 1e17/cm ³ And 1e19/cm ³ So that it does not introduce additional absorption and so that the electric field across the MQW material is greater.

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

The modulator may include a front surface that is the emitting surface of the modulating laser and that is coated with an anti-reflective coating. The anti-reflective coating may prevent reflection of light exiting the device at the front surface. This may increase the efficiency of modulating the laser.

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

The modulated laser may include a waveguide for guiding light along the device, which may be a ridge waveguide or a buried heterostructure waveguide. This may enable versatility of the device architecture.

Modulated lasers can be integrated with other optical functional structures and can be used in a variety of applications, such as telecommunications systems.

Drawings

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

In the drawings:

fig. 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.

Fig. 1 (c) shows the structure and electric field distribution (along section B-B of fig. 1 (B)) of the intrinsic region of a standard ML with a mach-zehnder modulator.

Fig. 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 of the intrinsic region shown in fig. 3 (a).

Detailed Description

As shown in fig. 2 (a) and 2 (b), one example of ML described herein includes a DFB laser optically coupled to a mach-zehnder modulator. In this example, the device has a PIN structure with a p-doped semiconductor layer above the intrinsic region and an n-doped semiconductor layer below it. However, the device may also have an NIP structure with an n-doped semiconductor layer above it and a p-doped semiconductor layer below it.

As shown in fig. 2 (a) and 2 (b), the DFB laser 30 generally includes a semiconductor block having a back surface or back facet 33 of ML at one end. The DFB laser has an intrinsic region that includes MQW material (MQW 3) 35. A side view along C-C is shown in fig. 2 (b).

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

The apparatus uses a multimode interferometer (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 across one arm of a mach-zehnder modulator, the light passing through that arm of the modulator will cause a phase shift. When the light from each of the two arms is recombined, the phase difference is converted to 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, i.e. the modulator has a PIN structure. Alternatively, the modulator may have an NIP structure. The composition of 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 includes MQW (or bulk) material 41, field control layers (field control layer, FCL) 42, and a collector layer 43, as shown in fig. 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 typically doped such that the electric field is concentrated primarily in the intrinsic MQW (i-MQW) region 41. Thus, the electric field in the MQW material is increased relative to the conventional intrinsic region. The doping density may depend on the thickness of the MQW intrinsic region and the required electric field. The thickness of the field control layer can be between 10nm and 500nm, and the doping density can be 1e17/cm ³ And 1e19/cm ³ Depending on the structure of the modulator. The field control layer may conveniently be p-doped to the NIP structure or n-doped to 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 absorption layer than in the conventional depletion layer.

The collector layer 43 preferably has a higher bandgap than the bandgap of the multiple quantum well material to avoid additional absorption. The collector layer may not be doped or may have a low doping concentration (lower than the doping concentration of the collector layer). The band gap energy of the collection layer may be constant for ease of fabrication or may be graded. The collection layer may be p-doped or n-doped, depending on the structure. The collection layer is formed of a higher bandgap material with low doping or no doping, making the overall capacitance of the modulator low, rather than favoring additional absorption. In the collection layer, the charge is carried mainly by electrons. Since only electrons are available carriers, a fast electron transit time can be utilized. The collection layer is used to reduce the capacitance of the modulator.

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

Where the ML includes a mach-zehnder modulator, the collection layer may have the same bandgap energy as the MQW material of the intrinsic region or a larger bandgap energy. Fig. 3 (b) shows the electric field distribution of the MQW FCL region shown in fig. 3 (a) (i.e., along the D-D section in fig. 2 (b)).

The light passing through each

arm

50a, 50b of the mach-zehnder modulator 50 is recombined at the second MMI 53 and then emitted from the modulated laser at the front surface 34.

Thus, the modulator typically includes 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 may be selectively doped in regions of the p-type layer and the n-type layer. The first and second semiconductor layers and the intrinsic region are elongated in a direction extending between the back surface 33 and the front surface 34 of the device.

The length of the ML device is defined between a rear surface or face 33 and a front surface or face 34. The rear face with HR coating acts as a back reflector. Preferably, the front and rear faces are aligned parallel to each other. Preferably, the front face is orthogonal to the length of the device. Preferably, the rear face is orthogonal to the length of the device. One or more of the front and/or back 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 may be applied to 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 allows different parts of the ML to be controlled independently.

Light exits the device from the waveguide at the front surface 34, i.e., 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 ML may be an electroabsorption modulator (electroabsorption modulator, EAM).

When the modulator is a mach-zehnder modulator, the electrodes may be lumped electrodes or travelling wave electrodes, or capacitively loaded segmented travelling wave electrodes. When the modulator is an electroabsorption modulator, the electrodes may be lumped electrodes or travelling wave electrodes.

Due to the field control layer, the electric field variations in the MQW material of the modulator result in refractive index variations realized by the above-described M-Z modulator, or absorption variations realized by the modulator including the EAM 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 an adjustable laser.

In embodiments of the present invention, the semiconductor may be InP, inGaAsP, inGaAlAs, inGaAlAsSb, gaAs, alGaAs, gaN, alGaN, ge, si or other suitable semiconductor material. The semiconductor material may be selected according to the application of the modulated laser.

As described above, the intrinsic region of the modulator includes the MQW material, the field control layer, and the collection layer. 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. In such ML, high speed operation and lower external bias can be achieved simultaneously.

Due to the field control layer, the electric field in the i-MQW region is stronger compared to standard EAM or mach-zehnder modulators. Thus, the modulator requires a lower external bias. The total capacitance of the modulator is also lower than in conventional devices due to the presence of the collecting layer. This may enable higher speeds, greater bandwidth, and better power handling.

Thus, 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.

Applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features. Such features or combinations can be implemented as a whole in accordance with the present specification, irrespective of whether such features or combinations of features solve any problems disclosed herein, or not by means of common knowledge of a person skilled in the art; and do not limit 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. 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.