WO2009054808A1

WO2009054808A1 - Packaged tunable semiconductor laser structure and its fabrication

Info

Publication number: WO2009054808A1
Application number: PCT/SG2007/000364
Authority: WO
Inventors: Venkata Ramana Pamidighantam; Jing Li; Hon-Shing John Lau; Jing Zhang; Yi Yoon Joey Chai; Teck Guan Lim; Jayakrishnan Chandrappan; Dim-Lee Kwong
Original assignee: Agency For Science, Technology And Research
Priority date: 2007-10-26
Filing date: 2007-10-26
Publication date: 2009-04-30

Abstract

A packaged tuneable semiconductor laser (TSL) structure and a method of fabricating a packaged tuneable semiconductor laser (TSL) structure are provided. The TSL structure comprises a laser component, the laser component including a Micro-Electro-Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component; and a modulator component provided separately from the laser component for modulating the laser output signal from the laser component; and an optical bench component for aligning the laser component and the modulator component for optical coupling therebetween.

Description

A Packaged Tuneable Semiconductor Laser Structure And A Method Of Fabricating A Packaged Tuneable Semiconductor Laser Structure

FIELD OF INVENTION

The invention relates broadly to a packaged tuneable semiconductor laser structure and to a method of fabricating a packaged tuneable semiconductor laser structure.

BACKGROUND

Tuneable lasers can be cost-effective for optical networking since the tuneable lasers can replace multiple fixed wavelength lasers in a DWDM (Dense Wavelength- Division Multiplexing) link. Wavelength tuning of the lasers can also allow efficient bandwidth usage and provision of customer services.

Telecom optical applications typically use individual lasers of different wavelengths that are capable of high speed modulation for dynamic networks and wavelength configurable optical modules in WDM (Wavelength Division Multiplexing) networks. However, it is desired that a tuneable laser is developed with high-frequency modulation having performance comparable to existing directly modulated Distributed

Feed Back (DFB) lasers and in a form factor that can be incorporated in existing transceiver modules. Therefore, there is a demand for integration within both optical modules and associated electronics.

A typical single-frequency laser is the DFB laser in which an index grating is formed near an optical waveguide to provide a continuous reflection that provides both mirror functionality and a mode selection filter. A generic tuneable laser typically comprises filter elements, gain elements and cavity modes that are aligned and translated to create a tuneable, single-frequency laser. An example of a tuneable laser is the Fabry-Perot laser that comprises a uniform cleaved semiconductor chip that is structured to provide gain for a guided optical mode with the cleaves functioning as mirrors.

Tuneable lasers are typically wavelength selectable continuous wave optical sources and typically require integration with external modulators for operation in telecom transceivers. Thus, a tuneable modulating coherent wavelength source is desired. Modulation types used for optical telecom include direct modulation which typically can, however, not be used with tuneable semiconductor lasers due to the low bandwidth of the lasers, Electro-Absorption Modulation (EAM) that is typically used up to only 2.5 Gbps and Electro-Optical Modulation (EOM) utilising devices such as a push- pull or Mach-Zhender interferometer.

One existing option for a tuneable laser is to combine an array of DFB lasers of different wavelengths using a multimode interference coupler. The DFB lasers are excited one at a time depending on wavelength requirements. A MEMS mirror is typically used with the array of DFB lasers. However, since a plurality of lasers are used, additional cost is incurred.

Another existing option for a tuneable laser is to use an external cavity laser with a rotating diffraction grating and a translating retro-reflector or with a fixed retroreflector. However, the structure is practically complicated, not easily tuneable due to the use of diffractive elements and has a large form factor. Hence, the structure is not suitable for transmitter applications.

Yet another existing option for a tuneable laser involves mounting a MEMS or electrostatic mirror on top of a Vertical-Cavity Surface-Emitting Laser (VCSEL). The resulting device can be directly modulated up to 10 Gbps. However, the output is a multi mode operation which is typically only applicable for short reach interconnects.

Another existing option for a tuneable laser is to integrate a Distributed Bragg

Reflector (DBR) laser with a wavelength tuning mechanism using refractive index variation electronically on-chip. Further, an EAM modulator is integrated with the tuneable laser on an InP wafer. The EAM modulator can operate up to 10 Gbps and by monolithically integrating the modulator, a smaller footprint and low power dissipation is typically possible. However, the EAM modulator typically suffers from wavelength chirp and can only be used for short distances below 40 kM at 10Gbps.

Hence, there exists a need for a packaged tuneable semiconductor laser structure and a method of fabricating a packaged tuneable semiconductor laser structure which seek to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention, there is provided a packaged tuneable semiconductor laser (TSL) structure, the TSL structure comprising a laser component, the laser component including a Micro-Electro-Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component; a modulator component provided separately from the laser component for modulating the laser output signal; and an optical bench component for aligning the laser component and the modulator component for optical coupling therebetween.

The modulator component may comprise an electro-optical modulator.

The MEMS tuning structure may comprise a reflecting element and an actuator, said reflecting element being operable by the actuator.

The TSL structure may further comprise a waveguide tap provided on the optical bench component; a monitor photo diode provided on the optical bench component; wherein the waveguide tap is coupled to the monitor photo diode for optical power adjustment of the TSL structure.

The optical bench component may be thermally coupled to a cooler layer and the TSL structure may further comprise a thermistor provided on the optical bench component for monitoring temperature. The TSL structure may further comprise a semiconductor optical amplifier optically coupled to the modulator structure, said semiconductor optical amplifier being provided on the optical bench component.

The TSL structure may further comprise a first polymer tapered coupler for optically coupling the modulator component to the laser component, said first polymer tapered coupler being provided on the optical bench component; and a second polymer tapered coupler for optically coupling to an output device, said second polymer tapered coupler being provided on the optical bench component.

The TSL structure may further comprise an optical isolator provided on the optical bench component substantially near the laser component.

The TSL structure may be hermetically packaged.

The laser component may comprise a gain chip separate from the MEMS tuning structure.

. The MEMS structure and the modulator component may be integrated bn the optical bench component via micromachining.

The laser component and the modulator component may be integrated on the optical bench component as individual packages.

In accordance with a second aspect of the present invention, there is provided a method of fabricating a packaged tuneable semiconductor laser (TSL) structure, the method comprising the steps of providing a laser component, the laser component including a Micro-Electro-Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component; providing a modulator component separately from the laser component for modulating the laser output signal from the laser component; and providing an optical bench component for aligning the laser component and the modulator component for optical coupling therebetween. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1(a) is a schematic cross-sectional view diagram illustrating a tuneable modulating Transmitter Optical Sub-Assembly (TOSA) package in an example embodiment.

Figure 1(b) is a schematic three-dimensional view of the TOSA package of Figure 1 (a).

Figure 2(a) is a schematic cross-sectional view diagram illustrating a tuneable modulating TOSA package in another example embodiment.

Figure 2(b) is a schematic three-dimensional view diagram of the TOSA package of Figure 2(a).

Figure 3(a) is a schematic cross-sectional view diagram of a tuneable modulating TOSA package in yet another example embodiment.

Figure 3(b) is a schematic three-dimensional view diagram of the TOSA package of Figure 3(a).

Figure 4 is a schematic diagram illustrating hermetic sealing of a flat top silicon optical bench.

Figure 5 is a schematic flowchart illustrating a method of fabricating a packaged tuneable semiconductor laser (TSL) structure.

Figure 6 is a schematic diagram illustrating a sample wafer arrangement. DETAILED DESCRIPTION

The example embodiments described below may provide a single chip integration of an electro-optical modulator (preferably in silicon) external to a laser component, e.g. off-chip with respect to the laser component, a Micro-Electro- Mechanical Systems (MEMS) tuneable element in silicon and a silicon optical bench for a small form factor Transmitter Optical Sub-Assembly (TOSA) module.

Figure 1(a) is a schematic cross-sectional view diagram illustrating a tuneable modulating TOSA package 100 in an example embodiment. Figure 1 (b) is a schematic three-dimensional view of the TOSA package 100. The TOSA package 100 comprises a Hl-V gain chip 102 having a broad band output, a MEMS structure 104 for tuning the wavelength of a laser output signal of the gain chip 102, a coupling element 106 provided between the gain chip 102 and the MEMS structure 104, a modulator 108, an optical isolator 110 provided substantially near to the gain chip 102, a coupling element 112 provided between the modulator 108 and the gain chip 102, a waveguide tap 114 coupled to a monitor photo diode 116 for power adjustment, an optical fiber 118 for output and a coupling element 120 provided between the modulator 108 and the fiber 118. The gain chip 102 comprises a front facet (not shown) functioning as a partial mirror that provides feedback. The components above are provided on an optical bench component in the form of a silicon substrate 122 mounted on a Thermo-Electrical Cooler (TEC) 124 that is coupled to a thermometer/thermistor 126. The thermometer/thermistor 126 is for monitoring temperature and providing feedback for closed loop operation. It will be appreciated that the waveguide tap 114 and the monitor photo diode 116 are not shown in Figure 1 (a) because these components are cross-sectionally co-located with the modulator 108 and the coupling element 120 respectively. In addition, as illustrated in Figures 1(a) and 1(b), the optical axis (see numeral 136) of the TOSA package 100 is aligned using the substrate 122.

In the example embodiment, the components are integrated on a single wafer arrangement. The MEMS structure 104 comprises a MEMS mirror 128 actuated by a MEMS actuator 130. The gain chip 102 and the MEMS structure 104 forms a tuneable laser component. The modulator 108 is external to and provided separately from the tuneable laser component and is preferably formed on silicon for integration. The modulator component, e.g. modulator 108, is used for modulating the laser output signal from the laser component.

It is to be noted that the optical isolator 110 is an optional component and is preferably included in high frequency optical TOSAs for long haul operations. In the example embodiment, the output of the gain chip 102 is coupled to the silicon optical modulator 108 using a focusing lens as the coupling element 112. The waveguide tap 114 is formed to couple part of the light to the monitor photo diode 116 to control stable optical power output from the TOSA package 100. The output of the modulator 108 is coupled to the fiber 118 directly to output the laser light. The TOSA package 100 is maintained at a stable temperature for reliable operation. This is achieved by using the thermistor 126 for monitoring the temperature and providing the feedback to the TEC 124 that in turn maintains the temperature of the silicon substrate 122. An external TEC Controller (not shown) is used to control the TEC 124 for maintaining the desired temperature.

In the above described example embodiment, an assembly is achieved using silicon micro machining to assemble the components in cavities e.g. 132, 134 formed in the silicon substrate 122 in silicon while maintaining the optical axis (see numeral 136) between the components to a high accuracy. A process is used to integrate a MEMS wafer comprising the MEMS structure 104 with a modulator wafer comprising the modulator 108 into a single wafer arrangement.

Figure 6 is a schematic diagram illustrating a sample wafer arrangement 600.

The wafer arrangement 600 comprises a first wafer 601 and a second wafer 603, which been bonded together through a wafer-to-wafer bonding process.

The first wafer 601 comprises a recess 605, and the second wafer 603 comprises a protrusion 607, where the protrusion 607 of the second wafer 603 is received in the recess 605 of the first wafer 601 , in order to properly align the first wafer 601 with the second wafer 603 before the wafer-to-wafer bonding process is performed. After the wafer-to-wafer bonding process is performed, additional components may then be fabricated. For example, a MEMS component 609 (such as a mirror, for example) may be fabricated on the second wafer 603 on the area on which the protrusion 607 is fabricated. As MEMS components are typically fabricated within thick structured wafers, the combined thickness of the second wafer 603 together with the protrusion 607 makes the area of the second wafer on which the protrusion is fabricated suitable for fabricating MEMS components. Additionally, other components, such as control electronic components 611 and silicon (Si)-photonic components 613, such as a modulator, may also be fabricated on the first wafer 601.

Further, an optical component 615, such as a lens, may also be fabricated on the first wafer 601 in a further recess 617. An optical component may be larger than typical electronic or photonic components. Therefore, in order to prevent the optical component from protruding out unnecessarily from the surface of the wafer arrangement 600, the optical component may be fabricated in a recess instead, such as the further recess 617, for example.

In the example embodiment, the MEMS tuning part can be integrated with the modulator by a micromachining process (compare arrangement 600 of Figure 6) or by packaging the individual MEMS components, e.g. the MEMS mirror 128 and the MEMS actuator 130, and the modulator components, e.g. the modulator 108, on different substrates, before assembling the respective packages into a single wafer arrangement to increase the cavity length.

In the example embodiment, the MEMS mirror and actuator, the silicon optical modulator and the optical trenches are formed at the wafer level followed by a metallization process using e.g. gold for attaching and electrically connecting the active optical dies to multi layer solder structures formed on the top of the silicon wafer. These structures are not shown in the schematic diagrams due to size constraints.

In the described example embodiment, wavelength tuning is not limited to the described MEMS structure 104 and can be realized by using other kinds of MEMS devices, such as, but not limited to, an electro-statically driven or thermally driven micro flat mirror, a curved mirror, or a grating. The coupling elements 102, 112, 120 between optical components may be in the form of, but not limited to, discrete optics such as ball lenses, collimators or tapered polymer couplers. The monitoring coupling can be implemented by either a horizontal or vertical coupling waveguide tap.

Figure 2(a) is a schematic cross-sectional view diagram illustrating a tuneable modulating TOSA package 200 in another example embodiment. Figure 2(b) is a schematic three-dimensional view diagram of the TOSA package 200. In this example embodiment, an on-chip semiconductor optical amplifier 202 is provided between a modulator 204 and an optical fiber 206 for increasing output power and to compensate for losses in the optical path. In addition, as illustrated in Figures 2(a) and 2(b), the optical axis (see numeral 208) of the TOSA package 200 is aligned using the substrate 210.

Figure 3(a) is a schematic cross-sectional view diagram of a tuneable modulating TOSA package 300 in yet another example embodiment. Figure 3(b) is a schematic three-dimensional view diagram of the TOSA package 300. In this example embodiment, the coupling efficiency between a laser component 302 and a silicon modulator 304 can be increased by using a polymer tapered coupler 306. Using the polymer tapered coupler 306 can provide advantages such as reducing the package form factor, improve the coupling between the laser component 302 and the modulator 304 and removing or significantly reducing alignment problems between the laser component 302 and sub- micron waveguides (not shown) in the silicon modulator 304. Furthermore, another tapered polymer coupler 308 is coupled to an optical amplifier 310. The tapered polymer coupler 308 improves the coupling efficiency between the optical amplifier 310 and an optical fiber 312 at the output. In addition, as illustrated in Figures 3(a) and 3(b), the optical axis (see numeral 314) of the TOSA package 300 is aligned using the substrate 316.

The above described example embodiments can be further integrated with laser diode driver electronics and optical modulator driver electronics. The TOSA assemblies in the described example embodiments can be formed in a surface mount package development to carry desired I/O signals. In the described example embodiments, the assembly is provided with at least 10 low frequency pins and two high frequency pins for operations. The low frequency pins may be used for the laser diode anode and cathode, the MEMS plus and minus pins, the TEC plus and minus pins, the thermistor plus and minus pins, and the monitor photo diode plus and minus pins. The high frequency pins may be used for the high and low bias pins for the modulator.

The package of the described example embodiments is hermetically packaged according to the Telcordia standards reliability requirement for optical modules. Figure 4 is a schematic diagram illustrating hermetic sealing of a flat top silicon optical bench 402. A surface mount assembly on the Silicon Optical bench 402 is performed by patterning an edge 404 of the silicon wafer 406 with gold and solder, and sealing the silicon wafer 406 using a silicon cap 408.

Figure 5 is a schematic flowchart 500 illustrating a method of fabricating a packaged tuneable semiconductor laser (TSL). At step 502, a laser component is provided, the laser component including a Micro-Electro-Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component. At step 504, a modulator component is provided separately from the laser component for modulating the laser output signal from the laser component. At step 506, an optical bench component is provided for aligning the laser component and the modulator component for optical coupling therebetween.

In the above described example embodiments, a sample laser diode can be packaged as a transmitter module using hermetic packaging where the module comprises the laser diode, a monitor photo diode, a TEC for maintaining temperature, a temperature monitoring thermistor and optical coupling elements (see e.g. Figure 1(a) and (b)).

The sample laser diode transmitter module can have a schematic electrical interconnection as follows. The transmitter module can be in a 7 pin or a 14 pin butterfly package. The transmitter package size is reduced by packaging the laser diode in a round Transmitter Optical (TO) can package without the temperature control circuitry (compare Figure 3(b)) to form a TOSA package. This sample TOSA package comprises an edge emitting laser or a VCSEL, a monitor photo diode, a sleeve, a lens holder serving as a body tube and having a cylindrical construction, a lens converging input or output optical signals, a stem supporting a lower end of the lens holder, and a cap disposed on the stem.

The above described example embodiments can provide tuneable semiconductor lasers (TSL) that, from a component perspective, can reduce operational costs by reducing laser diode device inventory. In the described example embodiments, the TSL is integrated with an optical modulator such that the TSL high frequency characteristics are optimized over a range of wavelengths. The described example embodiments may provide a laser chip integrated with a MEMS resonator to form a tuneable source and the tuneable source is integrated with an optical modulator. The described example embodiments are suitable for wafer level assembly and for hermetic packaging.

The described example embodiments may provide an integration of an electro- optical modulator with a MEMS tuneable structure and can result in a small form factor tuneable high frequency TOSA package for 10 Gigabit Small Form Factor Pluggable

(XFP) modules or 10-Gigabit Pluggable (XGP) modules. The described example embodiments may achieve high accuracy passive optical components placement on a silicon optical bench and the optical components can couple light on the silicon optical bench.

It has been recognised that by separating a tuneable laser and a high frequency modulator, the individual operating characteristics of the laser and the modulator can be optimized better as compared to a tuneable laser with an integrated EAM modulator.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A packaged tuneable semiconductor laser (TSL) structure, the TSL structure comprising a laser component, the laser component including a Micro-Electro-

Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component; a modulator component provided separately from the laser component for modulating the laser output signal from the laser component; and an optical bench component for aligning the laser component and the modulator component for optical coupling therebetween.

2. The TSL structure as claimed in claim 1 , wherein the modulator component comprises an electro-optical modulator.

3. The TSL structure as claimed in claims 1 or 2, wherein the MEMS tuning structure comprises a reflecting element and an actuator, said reflecting element being operable by the actuator.

4. The TSL structure as claimed in any one of claims 1 to 3, further comprising a waveguide tap provided on the optical bench component; a monitor photo diode provided on the optical bench component; wherein the waveguide tap is coupled to the monitor photo diode for optical power adjustment of the TSL structure.

5. The TSL structure as claimed in any one of claims 1 to 4, wherein the optical bench component is thermally coupled to a cooler layer and the TSL structure further comprises a thermistor provided on the optical bench component for monitoring temperature.

6. The TSL structure as claimed in any one of claims 1 to 5, further comprising a semiconductor optical amplifier optically coupled to the modulator structure, said semiconductor optical amplifier being provided on the optical bench component.

7. The TSL structure as claimed in any one of claims 1 to 6, further comprising a first polymer tapered coupler for optically coupling the modulator component to the laser component, said first polymer tapered coupler being provided on the optical bench component; and a second polymer tapered coupler for optically coupling to an output device, said second polymer tapered coupler being provided on the optical bench component.

8. The TSL structure as claimed in any one of claims 1 to 7, further comprising an optical isolator provided on the optical bench component substantially near the laser component.

9. The TSL structure as claimed in any one of claims 1 to 8, wherein the TSL structure is hermetically packaged.

10. The TSL structure as claimed in any one of claims 1 to 9, wherein the laser component comprises a gain chip separate from the MEMS tuning structure.

11. The TSL structure as claimed in any one of claims 1 to 10, wherein the MEMS structure and the modulator component are integrated on the optical bench component via micromachining.

12. The TSL structure as claimed in any one of claims 1 to 10, wherein the laser component and the modulator component are integrated on the optical bench component as individual packages.

13. A method of fabricating a packaged tuneable semiconductor laser (TSL) structure, the method comprising the steps of:

providing a laser component, the laser component including a Micro-Electro-

Mechanical Systems (MEMS) tuning structure for tuning a wavelength of a laser output signal from the laser component; providing a modulator component separately from the laser component for modulating the laser output signal from the laser component; and providing an optical bench component for aligning the laser component and the modulator component for optical coupling therebetween.