WO2012028876A1

WO2012028876A1 - Optoelectronic devices

Info

Publication number: WO2012028876A1
Application number: PCT/GB2011/051630
Authority: WO
Inventors: Lloyd Nicholas Langley
Original assignee: Oclaro Technology Limited
Priority date: 2010-09-03
Filing date: 2011-08-31
Publication date: 2012-03-08
Also published as: GB201014627D0; GB2483283A

Abstract

An optoelectronic device which comprises an opticalwaveguide 100 for modulating light. The device also comprises an electrode102overlying the waveguide100 for electrically biasing the waveguide to effect such modulation. At least a portion 1 of the waveguide underneath the electrode is tapered. This arrangement reduces thermal dissipation density at the front end of the electrode.

Description

Optoelectronic Devices

Field of the Invention

The present invention concerns optoelectronic devices and is more particularly, but not exclusively, concerned with optoelectronic devices in the form of optical modulators comprising single mode waveguides.

Background of the Invention

In the field of optical telecommunications components, monolithically integrated optical circuits are widely used. Such optical circuits commonly comprise a plurality of optical waveguides connected by means of optical modulators, in which light is modulated from different branch waveguides. One example of such an optical modulator is a Mach-Zehnder modulator (MZM). MZMs are used to modulate an optical signal with an electrical data signal by splitting the optical signal into two components, phase modulating one component relative to the other, and recombining the components.

Typically the chirp performance of the MZMs is determined by extinction ratio (ER). Since most transmitter products are tuneable across a wide range of wavelengths, the ER must be held within a tight specification tolerance across the wavelength range. ER is adversely affected by interference in a MZ waveguide between the fundamental optical waveguide mode, and unwanted higher order waveguide modes (HOM). This results in a regular periodic ripple in ER across the wavelength range.

One way of reducing or eliminating ER ripple is to create single mode waveguides. One way of achieving this is to make the waveguides sufficiently narrow to allow propagation of the fundamental mode only while keeping all the other modes cut off.

Indium Phosphate (InP) MZMs usually operate in reverse bias, with the modulation of phase in the MZ arms dominated by, for example, the Quantum Confined Stark Effect (QCSE). When reverse bias is applied, the phase change is accompanied by a corresponding optical absorption. Each photon absorbed contributes a carrier to a generated photocurrent. Electrical dissipation, which is the product of applied voltage and photocurrent (Pdiss = Vapplied x Iphotocurrent), is generated, and this dissipation heats up the waveguide underneath a bias or RF electrode.

The temperature rise in the waveguide is dependent on various factors: waveguide width, optical power, the height of the waveguide ridge, and the thermal resistance properties of the materials within the waveguide. At the upper end of typical MZ operational or test conditions (e.g. Vapplied: 5-15V, optical power: 20-1 OOmW), the photocurrent in each electrode can be more than 10mA, producing an electrical dissipation (Pdiss) of more than 50mW. The resulting temperature rise can be more than 100°C when driven at a high bias and a high optical power.

Because of the reverse bias induced absorption, the electrical power dissipation density (per unit area or per unit length of the electrode) is greatest at the front end of the electrode. The optical power level and hence the Pdiss generally decays exponentially along the electrode. Therefore the hottest part of the electrode will be the front end. The use of narrow single mode waveguides results in a danger of blowing up the electrode under a high reverse bias voltage and a high optical power. This may be particularly applicable where the reverse bias induced optical absorption is very strong as a result of an operation closer to the band-edge for Vpi reduction, and where there is an absence of any optical coupling losses through a lens system between a laser and the MZM.

Blow up may occur when the electrode heats up to a level where the semiconductor in the waveguide ridge melts. The device may then become either electrically open or short circuited, resulting in the failure of the device. In such an arrangement, the waveguide may still work optically with some additional loss, but the electrical modulation function may not operate at all.

Thus there is a need for a waveguide design for optoelectronic devices which is capable of reducing thermal dissipation in the electrode in order to prevent the failure of the device. Statement of Invention

According to one aspect of the present invention there is provided an optoelectronic device comprising an optical waveguide for modulating light. The device further comprises an electrode overlying the waveguide for electrically biasing the waveguide to effect such modulation. At least a portion of the waveguide underneath the electrode is tapered so as to reduce thermal dissipation density proximate a front end of the electrode. The waveguide underneath the electrode includes at least a portion having a sufficiently narrow width to act as a single mode waveguide.

The invention enables the power dissipation density in the RF electrode to be reduced in order to improve the blow-up tolerance of the electrode. Having a wider or tapered waveguide helps in two ways. Firstly, the optical mode spreads out, reducing the generated photocurrent density. Secondly, the waveguide thermal resistance is reduced.

It would be understood that the sufficiently narrow width portion can be a part of the tapered portion to act as a single mode waveguide for suppressing the HOM and the ER ripple.

Conveniently, the portion sufficiently narrow to act as a single mode waveguide is an un-tapered single mode portion distinct from the tapered portion. The single mode waveguide portion may comprise a constant narrow width equal to a narrow width of the tapered portion away from an input end of the waveguide.

The tapered portion of the waveguide may be tapered from a point under the front end of the electrode. Since the electrical power dissipation density is greatest at the front end of the electrode (because of the reverse bias induced absorption), the tapered waveguide structure enables the power dissipation at the front end of the device to be reduced.

Since It is not necessary that the entire waveguide length is in a single mode (to suppress the HOM and the ER ripple), it may be sufficient that the single mode portion (having a narrow width) is of about 0.5 mm or 1.0 mm long or half the length of the entire waveguide underneath the electrode.

Preferably, a relatively wide front end portion of the waveguide starting under the front end of the electrode is un-tapered. In such an arrangement, the tapered portion may be located between the un-tapered front end portion and a narrow single mode waveguide portion. A length of the un-tapered front end portion may be less than or equal to about 300 μηι, preferably less than or equal to about 100 μηι, more preferably about 20 μηη. This arrangement improves the heat transfer from the facet or front end of the waveguide.

An increase in area of the tapered portion, compared to an un-tapered portion, may be controlled so that the overall capacitance of the waveguide is increased no more than 20%.

Preferably a Mach-Zehnder (MZ) modulator incorporates the optoelectronic device.

According to another aspect of the present invention, there is provided an optical waveguide for modulating light in response to an electric field provided by an electrode overlying the waveguide. At least a portion of the waveguide underneath the electrode is tapered so as to reduce thermal dissipation density at a front end of the electrode.

According to another aspect of the present invention, there is provided a method of manufacturing an optoelectronic device. The method comprises forming an optical waveguide on a substrate for modulating light, and forming an electrode on the waveguide for electrically biasing the waveguide to effect such modulation. At least a portion of the waveguide is tapered so as to reduce thermal dissipation density in the electrode. According to another aspect of the present invention, there is provided a method of manufacturing an optical waveguide for modulating light in response to an electric field provided by an electrode overlying the waveguide. The method comprises tapering at least a portion of the waveguide underneath the electrode so as to reduce thermal dissipation density in the electrode. According to another aspect of the present invention, there is provided a method of reducing thermal dissipation density at a front end of an electrode overlying an optical waveguide for modulating light. The method comprises tapering at least a portion of the waveguide underneath the electrode.

Brief Description of the drawings

In order that the invention may be more fully understood, a number of embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a plan view of an optical waveguide underneath an electrode;

Fig, 1 a is a cross sectional view along plane A-A' of the tapered portion of the optical waveguide of Fig. 1 ;

Fig. 1 b is a cross sectional view along plane B-B' of the single mode waveguide portion of the waveguide of Fig. 1 ;

Fig. 1 c is a cross sectional view along a plane of an alternative waveguide underneath an electrode;

Fig. 2 is a plan view of an alternative optical waveguide underneath an electrode in which a front end portion of the waveguide is tapered;

Fig. 3 is a plan view of an optical waveguide underneath an electrode in which the full length of the waveguide is linearly tapered;

Fig. 4 is a plan view of an alternative optical waveguide underneath an electrode in which the full length of the waveguide is exponentially tapered; and

Fig. 5 is a plan view of an alternative optical waveguide underneath an electrode in which a front end of the waveguide is not tapered

Detailed Description of the Drawings

Fig. 1 is a plan view of an optical waveguide 100 underneath an electrode (not shown in Fig. 1 ). The waveguide 100 is located entirely underneath the RF electrode and comprises two portions 104, 105. The first portion 104 is tapered and located at an input or front end or close to the front end of the waveguide, and is sufficiently long and wide to reduce the thermal dissipation density at the front end of the electrode. The second portion 105 is a single mode waveguide portion which is sufficiently long and narrow to suppress or reduce the HOM and ER ripple. The tapered portion 104 comprises a relatively high width at an input end 101 of the waveguide and a relatively narrow width located longitudinally away from the input end 101 for acting as a single mode waveguide. In this embodiment, the tapered portion 104 comprises a linear taper structure and the single mode waveguide portion 105 comprises a constant thickness. The high width at the input end 101 of the waveguide is usually equal to or more than about 1 .5 μηη. The width of the single mode waveguide portion is usually equal to or less than about 1 .2 μηη, and the length of the second portion 105 is generally at least 0.5 mm or at least 50% of electrode length if the total electrode length is less than 1 mm. The length of the tapered portion 104 is generally more than or equal to about 5 μηη and less than or equal to about 500 μηι.

Fig, 1 a is a cross sectional view along plane A-A' of the tapered portion 104 of the arrangement of Fig. 1 , showing an electrode 102 overlying the waveguide 100 is formed on a substrate 103.

Fig. 1 b is a cross sectional view along plane B-B' of the single mode waveguide portion 105 of arrangement of Fig. 1 . As can be seen, the width of the single mode portion of the waveguide 100 along plane B-B' is less than that of the tapered portion (shown in Fig. 1 a). For the arrangements shown in Figs. 1 a and 1 b, the waveguide 100 is buried in the substrate 103. The conducting material 103 around the waveguide 100 improves the temperature control of the waveguide 100. Fig. 1 c is a cross sectional view along a plane of an alternative waveguide 106 underneath an electrode 102. In this embodiment, the waveguide 106 is etched and formed on the substrate 103 so that it extends above the substrate 103. An electrode 102 overlies the etched waveguide 106, which is surrounded by dielectric 104. The waveguide 106 support an optical mode 105.

Fig. 2 is a plan view of an alternative optical waveguide 200 under an electrode (not shown) in which the input or front end of the waveguide is tapered. Many features of the waveguide 200 of Fig. 2 are similar to those of the waveguide 100 of Fig. 1 , except for the configuration of the first portion (the tapered front end portion) 204. In this embodiment, the tapered portion 204 comprises an exponential taper, which in some circumstances may show better performance. This is because the light may be absorbed with an exponential decay along the waveguide 200, resulting in a nearly constant thermal dissipation density (and hence a temperature rise) along the waveguide 200 underneath the electrode. Again, the high width at the input end 201 of the waveguide is generally equal to or more than about 1.5 μηη. As with the arrangement of Fig. 1 , the width of the second portion 205 is generally equal to or less than about 1 .2 μηη, and the length of the second portion 205 is generally at least 0.5 mm or at least 50% of electrode length if the total electrode length is less than 1 mm. The length of the tapered portion 204 is generally more than or equal to about 5 μηη and less than or equal to about 500 μηι.

Fig. 3 is a plan view of an optical waveguide 300 underneath an electrode (not shown) in which the full length of the waveguide is linearly tapered. In this embodiment, the tapered waveguide comprises a relatively high width at the input end 301 of the waveguide and a relatively narrow width located longitudinally away from the input end 301 . The high width at the input end 301 of the waveguide is again generally equal to or more than 1.5 μηη. The relatively narrow end 302 longitudinally distal from the input end 301 has generally a width of less than or equal to about 1 .2 μηη. This arrangement may not always provide the best performance because it does not include the narrow portion of the waveguide to suppress the HOM and the ER ripple. Furthermore the increased area of the entire waveguide may adversely affect electrode capacitance and therefore the MZ Electro Optic (MZ EO) modulation bandwidth. Fig. 4 is an alternative plan view of an optical waveguide 400 underneath an electrode in which the full length of the waveguide is tapered. Many features of the waveguide 400 of Fig. 4 are similar to those of the waveguide 300 of Fig. 3, except that the waveguide 400 is exponentially tapered. As with the arrangement of Fig. 3, the high width at the input end 401 is again equal to or more than 1.5 μηη. The relatively narrow end 402 longitudinally distal from the input end 401 generally has a width of less than or equal to about 1 .2 μηη. This arrangement may be a viable alternative to a shorter taper at the input end (as shown in Figs. 1 and 2). This is because, in this arrangement, most of the waveguide length, or at least a portion of the tapered waveguide, is still sufficiently narrow to be beneficial for HOM suppression and capacitance reduction. It will be appreciated that a step width variation of the taper may also be an alternative to the arrangements shown in Figs. 1 and 2.

Fig. 5 is a plan view of an alternative optical waveguide 500 underneath an electrode in which a front end portion 501 of the waveguide is not tapered. The front end portion 501 is the width of the wider part of a tapered portion 502, which is itself succeeded by a single mode waveguide portion 503. The single mode portion 503 is sufficiently narrow to suppress the HOM and the ER ripple. In some circumstances, the wide un- tapered portion 501 can improve the heat dissipation to the facet or front end of the electrode (not shown) of the waveguide 500. The length of the un-tapered front end portion 501 under the electrode is generally less than or equal to about 300 μηη, preferably less than or equal to about 100 μηη and more preferably about 20 μηη. The length of the tapered portion 502 is generally in a range between 5 μηη and 500 μηη and preferably about 100 μηη. The length of the single mode portion 503 is generally at least 0.5 mm or at least 50% of electrode length. The tapered portion 502 can be tapered linearly, exponentially or step-wise.

It will be appreciated that the electrode metallisation for the arrangements of Figs 1 to 5 can, for example, be of the same shape as the waveguide 100, 200, 300, 400, 500 (i.e. tapered in accordance with the tapered waveguide portion and having a narrow width over the single mode waveguide portion), or may be of a single width usually wider than the waveguide widths. Other suitable arrangements may also be envisaged.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. CLAIMS:

1 . An optoelectronic device comprising:

an optical waveguide for modulating light,

an electrode overlying the waveguide for electrically biasing the waveguide to effect such modulation,

wherein the waveguide underneath the electrode includes a tapered portion for reducing thermal dissipation density proximate a front end of the electrode, and a single mode portion having a sufficiently narrow width to act as a single mode waveguide.

2. An optoelectronic device according to claim 1 , wherein the single mode portion of the waveguide is an un-tapered single mode portion distinct from the tapered portion.

3. An optoelectronic device according to claim 2, wherein the single mode portion of the waveguide comprises a constant narrow width equal to a narrow width of the tapered portion away from an input end of the waveguide. 4. An optoelectronic device according to claim 2 or 3, wherein the width of the single mode portion of the waveguide is less than or equal to about 1 .2 μηη and the length of the single mode portion is more than about 0.5 mm.

5. An optoelectronic device according to any preceding claim, wherein the tapered portion of the waveguide is tapered from a point under the front end of the electrode.

6. An optoelectronic device according to any preceding claim, wherein the tapered portion of the waveguide comprises a relatively high width at a front end of the waveguide and a relatively narrow width longitudinally distal from the front end for acting as the single mode waveguide.

7. An optoelectronic device according to any of claims 1 to 4, wherein a relatively wide front end portion of the waveguide starting under the front end of the electrode is un-tapered.

Claims

8. An optoelectronic device according to claim 7, wherein the tapered portion is located between the un-tapered front end portion and the single mode portion of the waveguide. 9. An optoelectronic device according to claim 7 or 8, wherein a length of the un- tapered front end portion is less than or equal to about 300 μηη, preferably less than or equal to about 100 μηι, more preferably about 20 μηι.

10. An optoelectronic device according to any preceding claim, wherein the tapered portion comprises a relatively high width at its front end which is equal to or more than about 1 .5 μηι.

1 1 . An optoelectronic device according to any preceding claim, wherein the length of the tapered portion is between 5 μηη and 500 μηη.

12. An optoelectronic device according to any preceding claim, wherein the width of the tapered portion is varied linearly, exponentially, or step-wise.

13. A Mach-Zehnder (MZ) modulator incorporating the optoelectronic device according to any preceding claim.

14. An optical waveguide for modulating light in response to an electric field provided by an electrode overlying the waveguide,

15. A method of manufacturing an optoelectronic device comprising:

forming an optical waveguide on a substrate for modulating light, and forming an electrode on the waveguide for electrically biasing the waveguide to effect such modulation,

wherein the waveguide underneath the electrode includes a tapered portion for reducing thermal dissipation in the electrode, and a single mode portion having a sufficiently narrow width to act as a single mode waveguide.

16. A method of manufacturing an optical waveguide for modulating light in response to an electric field provided by an electrode overlying the waveguide, the method comprising:

tapering at least a portion of the waveguide underneath the electrode so as to reduce thermal dissipation density in the electrode, and

forming at least a portion of the waveguide underneath the electrode having a sufficiently narrow width to act as a single mode waveguide

17. A method of reducing thermal dissipation density at a front end of an electrode overlying an optical waveguide for modulating light, the method comprising:

tapering at least a portion of the waveguide underneath the electrode, and providing at least a portion of the waveguide underneath the electrode having a sufficiently narrow width to act as a single mode waveguide.