WO2021259453A1

WO2021259453A1 - Flared dfb laser with partial grating

Info

Publication number: WO2021259453A1
Application number: PCT/EP2020/067412
Authority: WO
Inventors: Xin Chen
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2020-06-23
Filing date: 2020-06-23
Publication date: 2021-12-30
Also published as: CN115702530A

Abstract

A laser having a rear reflector, a front face spaced from the rear reflector and a cavity defined between the rear reflector and the front face, the cavity having a waveguide and comprising: a first cavity section adjacent the front face, the first cavity section comprising a Bragg grating and a first part of the waveguide having a first width, the length of the Bragg grating being in the range from 40% to 60% of the distance from the rear reflector to the front face; and a second cavity section adjacent the rear reflector, the second cavity section comprising a second part of the waveguide having a second width greater than the first width.

Description

FLARED DFB LASER WITH PARTIAL GRATING

FIELD OF THE INVENTION

This invention relates to lasers, for example to improving the operation and manufacturability of distributed feedback (DFB) lasers.

BACKGROUND

As illustrated in Figures 1(a) and 1(b), a standard DFB laser comprises a semiconductor structure which has a rear face or facet 10, a front face or facet 11 opposite to the front face or facet and a laser cavity formed therebetween. The laser cavity comprises an active layer 13 interposed between layers of p- and n-type semiconductor material, shown at 14 and 15 respectively. A voltage can be applied across electrodes or contacts 17a, 17b on opposing top and bottom sides of the cavity. The cavity comprises a waveguide, shown generally at 12, along which light may be guided. Light is emitted from the cavity at the front face 11.

A Bragg grating 16 acts as the wavelength selective element and provides feedback, reflecting light back into the cavity to form the resonator. The grating may be constructed so as to reflect only a narrow band of wavelengths.

In a standard DFB laser, the rear facet it is normally coated with a high-reflection (HR) coating to enhance the output power. The rear facet 10 with the HR coating acts as a rear reflector. The front facet 11 is commonly coated with an anti-reflection (AR) coating to increase output power and reduce reflection. The grating normally spans from the HR facet to the AR facet.

To result in a single emission wavelength (l), the relationship between the grating period, A(x), and the local refractive index profile n_eff (x) should meet the condition l = 2 n_eff(x) ^* A(x)

Single-mode DFB lasers may have straight waveguides or curved waveguides. A standard DFB laser typically has a constant waveguide width and a constant grating pitch, as this allows for efficient mass production.

To result in efficient single mode operation, the waveguide width may be between 1.0 pm to 2.5 pm. However, when narrow waveguide widths are used, the contact resistance is higher, and the current density and optical density along the cavity and at the HR facet are also higher, which can degrade performance. Other approaches use a hi 4 phase shifted grating or use a chirped grating, variable waveguide width, or an asymmetric corrugation-pitch-modulated (ACPM) grating.

However, to further improve the performance of a DFB laser, for example to boost the output power and reliability, is difficult.

It is desirable to develop a laser that has high output power with good reliability and manufacturability.

SUMMARY OF THE INVENTION

According to one aspect there is provided a laser having a rear reflector, a front face spaced from the rear reflector and a cavity defined between the rear reflector and the front face, the cavity having a waveguide and comprising: a first cavity section adjacent the front face, the first cavity section comprising a Bragg grating and a first part of the waveguide having a first width, the length of the Bragg grating being in the range from 40% to 60% of the distance from the rear reflector to the front face (i.e. the total cavity length); and a second cavity section adjacent the rear reflector, the second cavity section comprising a second part of the waveguide having a second width greater than the first width.

A wider waveguide width at the rear facet of the laser can reduce contact resistance and series resistance, which may result in lower resistance and lower current density and may reduce the optical density at the facet, whilst providing a high output power and single wavelength emission. This may result in improved performance. The waveguide width in the non-grating section is wider than the grating section in such a way that no high order optical mode is excited.

The cavity may further comprise a transition section comprising a transition part of the waveguide between and optically coupled to the first cavity section and the second cavity section. This may allow the first and second parts of the waveguide to be optically coupled to guide light along the cavity for output at the front face.

Each of the second cavity section and the transition section may not comprise a Bragg grating. This may result in a more stable threshold current and a higher optical output power, due to more preferential current pumping of the flared non-grating section. The width of the transition part of the waveguide may vary between the first width and the second width. This may allow the first waveguide part to be optically coupled to the second waveguide part. The length of the transition section may be equal to or shorter than the length of the second cavity section.

The first width may be between 1.0 and 3.5 pm. This may allow single mode operation of the laser.

The width of the first part of the waveguide may be constant. This may allow the laser to be more easily manufactured.

The period of the Bragg grating in the first cavity section may be constant. This may further allow the laser to be more easily manufactured.

The second width may be between 1.0 to 5.0 pm. This may allow single mode operation of the laser with a more stable threshold current and higher optical output power.

The laser may be a distributed feedback laser. This may be a convenient operational format for single mode lasing.

The Bragg grating may be elongated along the length of the first cavity section. The elongation of the length may be orthogonal to the front face. This may allow the grating to be disposed between the semiconductor layers of the laser cavity.

The length of the Bragg grating may be in the range from 45% to 55% of the distance from the rear reflector to the front face. A value within this smaller range may result in better performance compared to the broader range described above.

The front face may be coated with an anti-reflection coating and the rear reflector may be coated with a high-reflection coating. This may increase the power and improve the efficiency of the laser.

The waveguide may be a ridge waveguide or a buried heterostructure waveguide. This may allow for versatility of the laser structure.

The front face may be the emissive face of the laser. This may allow the laser to provide an optical output or allow the laser to be integrated with other optically functional structures. The rear reflector may be a back facet and the front face may be a front facet. The front are rear facets may be cleaved facets. This is a convenient method for manufacturing the laser. The rear reflector and the front face may also be formed by other convenient methods.

The grating strength and length product (Kappa*L_g) may be in the range from 0.6 to 1.5. This may result in good performance.

The rear reflector may be planar and the said distance from the rear reflector to the front face may be measured in a direction perpendicular to the rear reflector.

The laser cavity may comprise 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 active region located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the rear reflector and the front face. This is a convenient laser configuration.

The Bragg grating may be located between the first and second semiconductor layers or within one of these layers in the first cavity section. This may be convenient for manufacturing the laser.

BRIEF DESCRIPTION OF THE FIGURES

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

In the drawings:

Figures 1(a) and 1(b) show top and side views respectively of a standard DFB laser.

Figures 2(a) and 2(b) show top and side views respectively of a laser with a first cavity section adjacent to the front facet of the laser cavity having a Bragg grating and a narrower waveguide width than a second section of the cavity adjacent to the rear facet.

Figure 3 shows a side view of an alternative configuration of a DFB laser having separate top contacts for the first and second cavity sections. DETAILED DESCRIPTION

As illustrated in Figures 2(a) and 2(b), one example of the DFB laser described herein comprises three cavity sections 20, 21 , 22 each comprising a part of the waveguide 20a, 21a, 22a which acts to guide light travelling along the cavity.

The laser generally comprises a semiconductor block which has a rear face or facet 23, a front face or facet 24 opposite to the front face or facet. The laser cavity is formed therebetween. The total length of the laser cavity is L. The length is preferably defined between the rear face and the front face. A high-reflection (HR) coating is preferably applied to the rear facet 23 and an anti-reflection (AR) coating is preferably applied to the front facet 24. The back facet with the HR coating acts as a rear reflector. It is preferable that the front and rear facets are aligned parallel to one another. Preferably, the front facet is orthogonal to the length of the cavity and/or to the Bragg grating. Preferably, the rear facet is orthogonal to the length of the cavity. The front and/or rear face(s) of the laser may be formed by cleaving. The width of the waveguide is preferably perpendicular to the length of the cavity.

The laser cavity 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 active region located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the rear reflector and the front face. In the example shown in Figure 2(b), the laser cavity comprises an active layer 25 interposed between layers of p- and n-type semiconductor material, shown at 26 and 27 respectively. In this example, the semiconductor layers are made from InP. However, other semiconductor materials, such as GaAs, may be used. The material forming the cavity may be selectively doped in the region of the p- and n-type layers 26, 27. The active layer 25 of the laser may be a non-intentionally doped multiple quantum well (MQW) structure. The layers 25, 26, 27 are elongated in a direction extending between the rear face 23 and the front face 24.

A voltage can be applied across electrodes or contacts 29a and 29b on opposing top and bottom sides of the cavity.

The waveguide of the laser comprises a material with a refractive index n greater than that of the substrate. The waveguide may be a ridge waveguide or a buried-heterostructure (BH) waveguide for manufacturing versatility. A ridge waveguide may be created by etching parallel trenches in the material either side of the waveguide to create an isolated projecting strip, typically less than 10 pm wide and several hundred pm long. A material with a lower refractive index than the waveguide material can be deposited at the sides of the ridge to guide injected current into the ridge. Alternatively, the ridge may be surrounded by air on the both sides that are not in contact with the substrate beneath the waveguide. A BH waveguide comprises a core made of a longer band-gap wavelength semiconductor material surrounded by cladding made of a shorter band-gap wavelength semiconductor material.

Light exits the laser cavity from the waveguide at the front face 24 (i.e. the front face is the emissive face of the laser).

The cavity comprises a first cavity section 20 adjacent the front face 24. The first cavity section 20 has a length L1 and comprises a Bragg grating 28 and a first part of the waveguide, shown generally at 20a in Figure 2(a). The first part of the waveguide 20a has a first width, d1. d1 is preferably constant. For example, the width of the first part of the waveguide may be constant and may be between 1.0 and 3.5 pm. This helps to ensure the operation of a single optical mode and allows for easier manufacture of the laser.

The Bragg grating 28 has a length L_g. The length of the Bragg grating 28 in the first section of the cavity 20 is in the range from 40% to 60% of the distance from the rear reflector 23 to the front face 24, i.e. from 40% to 60% of L. The period of the Bragg grating in the first section is preferably constant to improve manufacturability. If the waveguide width d1 is variable, the grating period may also be variable to meet the condition l = 2n_eff(x) * A(x). The waveguide may be straight or it may be curved, as long as the condition l = 2n_eff(x) * A(x) is met.

In the first cavity section 20, the grating strength (Kappa) and length product, Kappa*L_g, is preferably between 0.6 - 1.5.

In the example shown in Figure 2(b), the Bragg grating 28 is positioned adjacent (i.e. immediately next to or close to) the front (AR-coated) facet 24 between the active layer 25 and the p-type semiconductor layer 26. The grating may alternatively be positioned between the active layer 25 and the n-type semiconductor layer 27. The Bragg grating is integral with the first cavity section of the laser. The Bragg grating is elongated along the length of the cavity. The elongation of length of the grating is orthogonal to the front facet.

Preferably, the grating extends along the complete length L1 of the first cavity section 20 (i.e. L1 = L_g). The first section of the cavity may therefore have a length L1 in the range from 40% to 60% of the distance from the rear reflector to the front face. Alternatively, the grating may not extend all the way to the front facet of the cavity and there may be a sub-section of the first section of the cavity 20 immediately next to the front face 24 that does not have a Bragg grating. In this case, the length of the Bragg grating is in the range from 40% to 60% of the distance from the rear reflector to the front face. The laser therefore has a partial grating with a length L_g of 40%-60% of the total waveguide length. In both cases, preferably, L_g is in the range from 45% to 55% of the total laser cavity length, L. In certain implementations, L_g may be equal to 40, 45, 50, 55, or 60% of the distance from the rear reflector to the front face.

The cavity comprises a second cavity section 21 adjacent the rear reflector 23. The second cavity section 21 has a length L2 and comprises a second part of the waveguide, shown at 21a in Figure 2(a). The second part of the waveguide 21a has a second width, d2. The second width d2 is greater than the first width d1 , such that no high order optical mode is excited. Preferably, d2 is between 1.0 pm to 5.0 pm. The waveguide width d2 can be constant, or it can alternatively be variable along this part of the waveguide. The laser therefore has a wider waveguide section without a grating adjacent the rear (HR-coated) facet. This second section of the cavity acts as a gain section.

As the second part of the waveguide 21a is wider than the first part 20a, the contact resistance of the laser is lower than in a traditional DFB laser due to the greater surface area of the rear waveguide. This may also result in improved thermal properties, as there may be less heat due to improved heat dissipation. This may also reduce the series resistance, which may result in lower current density and may reduce the optical density at the facet.

Optically coupling the first and second sections of the cavity is a transition section 22 located between the first and second sections 20, 21. The transition section of the cavity 22 has a length L3 and a width d3. The width of the transition section d3 is variable along the length of the transition section between the first width d1 and the second width d2. In the example shown in Figure 2(a), the width of the transition section varies linearly between the first width and the second width. However, in other implementations, the width of the transition section may vary non-linearly between the first width d1 and the second width d2.

As shown in Figures 2(a) and 2(b), each of the second cavity section and the transition section do not comprise a Bragg grating. The non-grating section of the cavity (made up of the second section and the transition section) therefore has total length L2 + L3 adjacent to the rear (HR- coated) face.

The length of the transition section 22 may be equal to or shorter than the length of the second section of the cavity 21. The first and second sections 20, 21 may have a common top contact 29a, as shown in Figure 2(b). Alternatively, as schematically illustrated in Figure 3, the first and second sections of the waveguide may have separate top contacts, 29a and 29c. A voltage can be applied across electrodes or contacts 29a and/or 29c and 29b on opposing top and bottom sides of the cavity. This may allow the different sections of the laser to be controlled independently. This may be useful in DC or modulated laser applications.

The Bragg grating may be fabricated by electron beam lithography. This allows the accuracy of the grating spacing to be controlled very accurately. The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. The layer comprising the Bragg grating may be fabricated from a p-doped or n-doped semiconductor material.

In summary, the DFB laser described herein preferably has an anti-reflection (AR) coated front face and high-reflection (HR) coated rear face to obtain a high output power. The flared partial grating laser has no grating within the rear section and has a grating within the front section. The length of the non-grating section is between 40% - 60% of the total laser cavity length. The waveguide width in the non-grating section can be wider than the grating section in such a way that no high order optical mode is excited.

The DFB described herein therefore has a different waveguide width along the length of the waveguide to improve device reliability and result in a high output power, but may still emit a single wavelength by meeting the single wavelength emission requirement, i.e. l = 2n_eff(x) * A(x).

The flared partial grating DFB laser has a stable same optical longitudinal mode profile. As a result, it is more tolerant to external optical reflection. The front and rear facet output power ratio is more tightly distributed. As a result, the laser may have a higher yield. The electrical field along the cavity is stable for various facet phases, hence there may be less spatial hole burning. The laser also has a more stable threshold current and a higher optical output power, due to more preferential current pumping of the flared non-grating section. The wider waveguide adjacent to the rear face without a grating may also result in lower contact resistance and better thermal properties.

The DFB design described herein can improve both reliability, for example by avoiding HR- COMD, and optical output power whilst emitting light having a single wavelength at the front face of the device. It is suitable for mass production, especially using a stepper to have better waveguide ridge control.

The laser structure may be integrated with another optically functional structure, for example an electroabsorption modulator, a Mach-Zehnder modulator, or an amplifier.

The laser is preferably a single mode laser.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A laser having a rear reflector, a front face spaced from the rear reflector and a cavity defined between the rear reflector and the front face, the cavity having a waveguide and comprising: a first cavity section adjacent the front face, the first cavity section comprising a Bragg grating and a first part of the waveguide having a first width, the length of the Bragg grating being in the range from 40% to 60% of the distance from the rear reflector to the front face; and a second cavity section adjacent the rear reflector, the second cavity section comprising a second part of the waveguide having a second width greater than the first width.

2. The laser of claim 1 , wherein the cavity further comprises a transition section comprising a transition part of the waveguide between and optically coupled to the first cavity section and the second cavity section.

3. The laser of claim 2, wherein each of the second cavity section and the transition section do not comprise a Bragg grating.

4. The laser of claim 2 or claim 3, wherein the width of the transition part of the waveguide varies between the first width and the second width.

5. The laser of any of claims 2 to 4, wherein the length of the transition section is equal to or shorter than the length of the second cavity section.

6. The laser of any preceding claim, wherein the first width is between 1.0 and 3.5 pm.

7. The laser of any preceding claim, wherein the width of the first part of the waveguide is constant.

8. The laser of any preceding claim, wherein the period of the Bragg grating in the first cavity section is constant.

9. The laser of any preceding claim, wherein the second width is between 1.0 to 5.0 pm.

10. The laser of any preceding claim, wherein the laser is a distributed feedback laser.

11. The laser of any preceding claim, wherein the Bragg grating is elongated along the length of the first cavity section.

12. The laser of any preceding claim, wherein the length of the Bragg grating is in the range from 45% to 55% of the distance from the rear reflector to the front face.

13. The laser of any preceding claim, wherein the front face is coated with an anti-reflection coating and the rear reflector is coated with a high-reflection coating.

14. The laser of any preceding claim, wherein the waveguide is a ridge waveguide or a buried heterostructure waveguide.

15. The laser of any preceding claim, wherein the front face is the emissive face of the laser.