US 7058246 B2
A monolithic photonic integrated circuit (PIC) chip comprises an array of modulated sources providing a plurality of channel signals of different wavelengths and an optical combiner coupled to receive the channel signals and produce a combined output of the channel signals. The arrays of modulated sources are formed as ridge waveguides to enhance the output power from the respective modulated sources so that the average output power from the sources is approximately 2 to 4 times higher than in the case of comparable arrays of modulated sources formed as buried waveguides.
1. A monolithic photonic integrated circuit (PIC) chip comprising:
an array of modulated sources, forming an active waveguide structure comprising a plurality of first waveguides and providing a plurality of channel signals of different wavelengths;
an optical combiner forming a passive waveguide structure comprising a plurality of second waveguides and coupled to receive the channel signals and produce a combined output of the channel signals;
all of said first and second waveguides are formed as ridge waveguides to enhance the output power from the circuit so that the average output power from the circuit is approximately 2 to 4 times higher than in the case where the circuit partially includes buried heterostructure (BH) waveguides.
2. The monolithic photonic integrated circuit (PIC) chip of
3. The monolithic photonic integrated circuit (PIC) chip of
4. The monolithic photonic integrated circuit (PIC) chip of
5. The monolithic photonic integrated circuit (PIC) chip of
6. The monolithic photonic integrated circuit (PIC) of
7. A monolithic photonic integrated circuit (PIC) comprising:
an array of modulated sources, each source being formed of a ridge waveguide and having an output wavelength at a designated wavelength on a wavelength transmission grid; and
an optical combiner coupled to receive and produce a multiplexed output of the modulated output wavelengths from said modulated sources, the optical combiner including a plurality of waveguides that are also ridge waveguides so that the circuit is formed of ridge waveguides.
8. The monolithic photonic integrated circuit (PIC) of
9. The monolithic photonic integrated circuit (PIC) of
10. The monolithic photonic integrated circuit (PIC) of
11. The monolithic photonic integrated circuit (PIC) of
12. The monolithic photonic integrated circuit (PIC) of
13. A monolithic photonic integrated circuit (PIC) comprising:
an array of laser sources, the laser sources each having an active region with a selected bandgap providing a cw output wavelength at a designated wavelength on a wavelength transmission grid;
an array of electro-optic modulators, one for each of the laser source output wavelengths to provide a modulated signal on each respective output wavelength, each of the modulators having an active region having a bandgap larger than the bandgap of a corresponding laser source;
an arrayed waveguide grating coupled to receive and produce a multiplexed output of the modulated output wavelengths, the arrayed waveguide grating including a plurality of waveguides having a bandgap larger than any of the bandgaps of the electro-optic modulators;
the output of the arrayed waveguide grating coupled for transmission on an optical link.
14. A monolithic photonic integrated circuit (PIC) comprising:
an epitaxial growth comprising:
an active region for an array of laser sources with a selected bandgap providing a cw output wavelength at a designated wavelength on a wavelength transmission grid;
an active region for an array of electra-optic modulators, each optically coupled to a laser source and having a bandgap larger than the bandgap of its corresponding laser source;
the arrays grown by employing selective area growth (SAG) within a SAG budget providing operational wavelengths within the wavelength transmission grid;
another epitaxial growth comprising:
a core waveguides for an arrayed waveguide grating having a larger bandgap than the bandgap of the active regions of the arrays which may be either inside or outside the SAG budget for operational wavelengths of the rays.
15. A monolithic photonic integrated circuit (PIC) having a plurality of active and passive elements, comprising:
an array of distributed feedback (DFB) sources, the DFB sources each having an active region with a selected bandgap providing a cw wavelength output at a designated wavelength on a wavelength transmission grid, each DFB source being complex-coupled;
an array of electro-optic modulators, one for each of said DFB sources and providing a modulated signal on each respective output wavelength, each of the modulators having an active region with identical bandgap to the bandgap of its corresponding source;
the distributed feedback (DFB) source/modulator pairs formed in a plurality of waveguides; and
an optical combiner coupled to receive the modulated signals from the distributed feedback (DFB) source/modulator output waveguides and produce a multiplexed output of the modulated signals;
the optical combiner further comprising a plurality of waveguides having a bandgap larger than any of the bandgaps of the electro-optic modulators; and
the output of the arrayed waveguide grating coupled for transmission on an optical link.
16. The monolithic photonic integrated circuit (PIC) of
17. The monolithic photonic integrated circuit (PIC) of
18. The monolithic photonic integrated circuit (PIC) of
19. A monolithic photonic integrated circuit (PIC) chip comprising an array of distributed feedback (DFB) lasers each providing an output at a designated wavelength on a wavelength transmission grid, each of the DFB laser outputs coupled to an input of an arrayed waveguide grating (AWG) and providing an output comprising multiplexed outputs from the DFB lasers, the DFB lasers and the AWG all formed as ridge waveguide structures.
20. The monolithic photonic integrated circuit (PIC) chip of
21. The monolithic photonic integrated circuit (PIC) chip of
22. The monolithic photonic integrated circuit (PIC) chip of
23. The monolithic photonic integrated circuit (PIC) chip of
24. The monolithic photonic integrated circuit (PIC) chip of
25. The monolithic photonic integrated circuit (PIC) chip of
26. The monolithic photonic integrated circuit (PIC) chip of
27. A monolithic photonic integrated circuit (PIC) chip comprising:
an array of distributed feedback (DFB) lasers each providing an output at a designated wavelength and together approximating a wavelength transmission grid;
each of the DFB laser outputs optically coupled to a corresponding electro-absorption (EA) modulator;
outputs of the EA modulators coupled to an input of an arrayed waveguide grating (AWG) which has an output comprising multiplexed, modulated outputs from the EA modulators;
the DFB lasers, EA modulators and the AWG all formed as ridge waveguide to enhance the mode intensity at the outputs of the EA modulators.
28. The monolithic photonic integrated circuit (PIC) chip of
29. The monolithic photonic integrated circuit (PIC) chip of
30. The monolithic photonic integrated circuit (PIC) chip of
31. The monolithic photonic integrated circuit (PIC) chip of
32. The monolithic photonic integrated circuit (PIC) chip of
33. The monolithic photonic integrated circuit (PIC) chip of
34. A monolithic photonic integrated circuit (PIC) chip comprising an array of distributed feedback (DFB) lasers providing output at a designated wavelength on a wavelength transmission grid, each of the outputs optically coupled to a corresponding electra-absorption (EA) modulator, the EA modulators having a quantum well active region with an interstep barrier in the region.
35. The monolithic photonic integrated circuit (PIC) chip of
36. The monolithic photonic integrated circuit (PIC) chip of
37. The monolithic photonic integrated circuit (PIC) chip of
38. The monolithic photonic integrated circuit (PIC) chip of
39. The monolithic photonic integrated circuit (PIC) chip of
40. A monolithic photonic integrated circuit (PIC) chip comprising an array of distributed feedback (DFB) lasers providing output at a designated wavelength on a wavelength transmission grid, each of the outputs optically coupled to a corresponding electro-absorption (EA) modulator, the EA modulators having a quantum well stepped region comprising at least two well steps of different well depth providing negative chirp with high extinction ratio and minimized insertion loss compared to deployment of a single quantum well region having a similar strain.
41. The monolithic photonic integrated circuit (PIC) chip of
42. A monolithic photonic integrated circuit (PIC) chip comprising at least one EA modulator having multiple stepped quantum well active region having at least two different potential wells where the electron-hole exciton splits at a lower applied electric field compared to a single well active region.
43. The monolithic photonic integrated circuit (PIC) chip of
44. The monolithic photonic integrated circuit (PIC) chip of
45. The monolithic photonic integrated circuit (PIC) chip of
46. The monolithic photonic integrated circuit (PIC) chip of
This nonprovisional application claims the benefit of priority of provisional applications, Ser. No. 60/378,010, filed May 10, 2002 and entitled, TRANSMITTER PHOTONIC INTEGRATED CIRCUIT (TXPIC) CHIP WITH ENHANCED POWER AND YIELD WITHOUT ON-CHIP AMPLIFICATION; Ser. No. 60/328,207, filed Oct. 9, 2001 and entitled, PHOTONIC INTEGRATED CIRCUITS FOR DWDM OPTICAL NETWORKS, now U.S. nonprovisional patent application Ser. No. 10/267,331, filed Oct. 8, 2002 and entitled. TRANSMITTER PHOTONIC INTEGRATED CIRCUITS (TxPIC) AND OPTICAL TRANSPORT NETWORKS EMPLOYING TxPICs; Ser. No. 60/370,345, filed Apr. 5, 2002 and entitled, WAVELENGTH STABILIZATION IN TRANSMITTER PHOTONIC INTEGRATED CIRCUITS (TxPICs), now U.S. nonprovisional patent application, Ser. No. 10/267,330, filed Oct. 8, 2002 and entitled, TRANSMITTER PHOTONIC INTEGRATED CIRCUIT (TxPIC) CHIP ARCHITECTURES AND DRIVE SYSTEMS AND WAVELENGTH STABILIZATION FOR TxPICs; Ser. No. 60/392,494 filed Jun. 28, 2002 and entitled DIGITAL OPTICAL NETWORK ARCHITECTURE, now U.S. nonprovisional patent application, Ser. No. 10/267,212, filed Oct. 8, 2002 and entitled, DIGITAL OPTICAL NETWORK ARCHITECTURE all of which are owned by the assignee herein and are incorporated herein by their reference.
1. Field of the Invention
This invention relates generally to photonic integrated circuits (PICs) and more particularly to optical transmitter photonic integrated circuit (TxPIC) chips having on-chip arrays of modulated light sources, not requiring additional on-chip amplification components.
2. Description of the Related Art
As used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
AWG—Arrayed Waveguide Grating.
BCB—benzocyclobutene or derivatives thereof.
DBR—Distributed Bragg Reflection Laser.
DFB—Distributed feedback Laser.
EA or EAM—Electro Absorption or Electro Absorption Modulator.
GC-SOA—Gain Clamped Semiconductor Optical Amplifier.
ITU Grid—Wavelengths and frequencies used in WDM systems that have been standardized on a frequency grid by the International Telecommunications Union (ITU).
MOCVD—Metalorganic Chemical Vapor Deposition or organometallic vapor phase epitaxy.
NID—Not Intentionally Doped.
PIC—Photonic Integrated Circuit.
Q—A Group III-V quaternary comprising InGaAsP or InAlGaAs.
RxPIC—Receiver Photonic Integrated Circuit.
SAG—Selective Area Growth usually performed in MOCVD.
SOA—Semiconductor Optical Amplifier.
SSC—Spot Size Converter—sometimes called also a mode adaptor.
TxPIC—Transmitter Photonic Integrated Circuit.
Wavelength Grid—Wavelengths and frequencies in a periodic or aperiodic frequency grid whether a standardized grid or not.
There exists a great demand at this time that future generations of optical transmitters and optical receivers or optical transceivers for optical telecommunications to be much more cost effective than present optical telecommunication equipment that comprise optical discrete optical components that are separately manufactured, assembled and packaged. It is clear that a solid approach to achieve this goal is a photonic integrated circuit (PIC) that includes, in monolithic form, the integrated arrays of active electro-optic components and optical passive components, i.e., multiple signal channels within a standardized grid where each channel includes a modulated source (which may comprise either a directly modulated laser or a laser and an external modulator, sometimes referred to as a semiconductor modulator/laser (SML), e.g., an EML) coupled to an optical combiner. It has been suggested that transmitter photonic integrated circuits (TxPICs) comprise, in monolithic form, a laser (which may be tunable), and electro absorption modulator (EAM), such as shown in the articles of Thomas L. Koch et al. entitled, “Semiconductor Photonic Integrated Circuits”, IEEE Journal of Quantum Electronics, Vol. 27(3), pp. 641–653, March, 1999 and D. A. Ackerman et al. entitled, “A Practical DBR Laser based Wavelength Selectable DWDM Source”, IEEE LEOS Newsletter, pp. 7–9, October, 2001; DFB laser arrays and EA modulator arrays such as shown in U.S. Pat. Nos. 5,891,748 and 5,784,183; DBR laser arrays, EA modulators, optical combiner and output amplifier on a single chip such as shown in the article of M. G. Young et al. entitled, “A 16X1 Wavelength Division Multiplexer with Integrated Distributed Bragg reflector Lasers and Electroabsorption Modulators”, IEEE Photonics Technology Letters, Vol. 5(8), pp. 908–910, August, 1993. Also, there is the article of Charles H. Joyner et al., entitled, “Low-Threshold Nine-Channel Waveguide Grating Router-Based Continuous Wave Transmitter”, Journal of Lightwave Technology, Vol. 17(4), pp. 647–651, April, 1999 disclosing a single monolithic optical chip, i.e., a photonic integrated circuit (PIC), having a plurality of semiconductor optical amplifiers (SOAs) with their optical outputs coupled via a plurality of passive waveguides to an AWG to form a multiple wavelength laser source having multiple established laser cavities between these coupled optical components. To be noted is that there is an absence in the art, at least to the present knowledge of the inventors herein, of an integrated laser source array, such as in the form of a DFB array, and an optical combiner in the form of an array waveguide grating (AWG). A principal reason is that it is difficult to fabricate, on a repeated basis, an array of DFB lasers with a wavelength grid that matches the wavelength grid of the AWG. Also, as the numbers of electro-optic components are added to a PIC chip, insertion losses increase requiring that some on-chip or off-chip optical signal amplification is included.
It has been suggested that receiver photonic integrated circuits (RxPICs) comprise, in monolithic form, ridge waveguide, arrayed waveguide gratings (AWGs) and an array of photodetectors as shown in the articles of Masaki Kohtoku et al. entitled, “Polarization Independent Semiconductor Arrayed Waveguide Gratings Using a Deep-Ridge Waveguide Structure”, IEICE Trans. Electron., Vol. E81-C, No. 8, pp 1195–1204, August, 1998 and “Packaged Polarization-Insensitive WDM Monitor with Low Loss (7.3 dB) and Wide Tuning Range (4.5), IEEE Photonics Technology Letters, Vol. 16(11), pp. 1614–1616, November, 1998. Another example is the article of M. Zirngibl et al. entitled, “WDM receiver by Monolithic Integration of an Optical Preamplifier, Waveguide Grating router and Photodiode Array”, ELECTRONIC LETTERS, Vol. 31(7), pp. 581–582, Mar. 30, 1995, discloses a 1 cm by 4 mm PIC chip, fabricated in InP, that includes the integrated components comprising an optical amplifier (SOA) optically coupled to an AWG DEMUX having a plurality of different signal channel outputs each coupled to a respective photodiode (PD) in an array of on-chip photodiodes. The SOA boosts the multiplexed input channel signals. The AWG DEMUX demultiplexes the signals into separate channel signals which signals are respectively detected by a PD in the array.
As indicated above, many of the above mentioned PIC devices include an on-chip optical amplifier to boost the power of optical channel signals generated by or received in the PIC, such as a SOA or an optical laser amplifier. These added gain components are useful to enhance the power of the channel signals especially where on-chip insertion loss exceeds the insertion loss budget allowed in the design of such PIC chips. However, the presence of additional active optical components, while solving gain needs, provides additional constraints on the resulting PIC chip thermal budget through the requirement of additional PIC operating power which translates into higher PIC heat generation and required dissipation. Also, the addition of a plurality of SOAs on the TxPIC chip tightens what we term the selective area growth (SAG) budget where the wavelengths of the active/waveguide core of the DFBs, EA modulators and added SOAs, for example, must be monotonically shifted via SAG processing. This results in the bandgap in each consecutive optical component in an optical waveguide formed in the PIC to be optimized for performance. For example, the wavelength of the AWG waveguide region is less than the wavelength of the MOD active region which is less than the wavelength of the DFB active region which is less than the wavelength of the SOA active region (λAWGi<λMODi<λDFBi<λSOAi where λDFBi+1=λDFBi+Δλ, λMODi+1=λMODi+Δλ, λSOAi+1=λSOAi+Δλ and λAWG<<λMODi,). Δλ is the channel spacing. Note that it is possible to vary the wavelength spacing Δλ1 across the array in a proprietary PIC system.
Also, the presence of SOAs on a monolithic PIC chip increases fabrication and test complexity. Their deployment on the TxPIC side (versus the RxPIC side) can add to unwanted dispersive effects on the transmitted waveform or may otherwise degrade the signal transmission properties. An SOA may amplify the optical reflections between integrated components, resulting in increased and undesireableundesirable back reflection. Further, the addition of on-chip SOAs increases the stress on the available SAG budget, albeit it may be only a same percentage of the total budget, such as around 10%. The SAG budget may be defined as the range of attainable operating wavelengths with sufficient wavelength separation to enable the proper wavelength targets for totally all optical components or devices on the chip. It would be preferred to reserve the SAG budget for DFB laser wavelength budget or for the DFB/MOD wavelength budget by reducing the number of optical components on the chip, in particular, eliminating any need for on-chip SOAs making it easier to optimize the DFB array performance/yield or the DFB/MOD performance/yield or DFB/MOD/MUX performance/yield. Further, the elimination of SOAs from the PIC chip renders it also possible to increase the density of DFBs included on a single semiconductor chip, which translates into an increase in the number of signal channels per TxPIC chip, reducing the cost per channel for a PIC transmitter module.
It is an object of the present invention to improve the output performance and quality of PIC chips for employment in optical transport networks.
It is another object of this invention to provide a TxPIC that does not require or need on-chip amplification.
According to this invention, a monolithic PIC chip comprises an array of modulated sources providing a plurality of channel signals of different wavelengths and an optical combiner coupled to receive the channel signals and produce a combined output of the channel signals. The arrays of modulated sources are formed as ridge waveguides to enhance the output power from the respective modulated sources so that the average output power from the sources is approximately 2 to 4 times higher than in the case of comparable arrays of modulated sources formed as buried waveguides.
A TxPIC chip, as disclosed, comprises an array of modulated sources, preferably DFB lasers and optically coupled EA modulators, with the modulated outputs of the coupled to an integrated optical combiner, preferably an arrayed waveguide grating (AWG), from which the multiplexed output is presented at a facet output for optical coupling to an optical transport network or optical telecommunication system. The DFB lasers and EA modulators are optimized for highest output power, with improved chirp and extinction ratio as well as minimized insertion loss at the modulator, so that any need for on-chip signal amplification, such as a semiconductor optical amplifier (SOA), or a semiconductor optical laser amplifier or GC-SOA is eliminated.
A feature of this invention is the elimination of need for on-chip amplifiers in TxPIC chips, such as SOAs or GC-SOAs, which has the advantages of eliminating (1) the need of another integrated optical component on the chip, (2) additional metal contacts needed to operate the on-chip amplifier, (3) additional on-chip heat generated by the on-chip amplifier, (4) the requirement of blueshift of the active region of the on-chip lasers, and (5) waveform distortion induced by the amplifiers. On-chip amplification is accomplished by providing a higher power DFB lasers or a DFB laser plus MOD array which is accomplished by deploying a ridge waveguide structure. On-chip power levels of 5 dBm are obtainable from the chip EMLs which is sufficiently high as not to require an on-chip amplifier such as an SOA. Further note that the DFB lasers may also be preferably replaced with DBR lasers that are either directly modulated or externally modulated.
A further feature of this invention is the employment of SAG growth techniques in forming the core waveguide region of the AWG multiplexer and over the entire area of optical active components on the TxPIC chip, and employing masks to create the active components, such as the fabrication of arrays of semiconductor laser sources and/or modulators and an AWG multiplexer. The deployment of a single SAG mask over the area of the DFB/MOD/AWG region or DFB/.AWG throughout the InP wafer makes it possible to epitaxially grow appropriate bandgaps and corresponding operational wavelengths in the respective core waveguides (Q layers comprising InGaAsP or AlInGaAs or multiple quantum well layers and barriers of such quaternaries) across the DFB or DFB/MOD arrays in a single epitaxial growth step using MOCVD. The deployment SAG is extended to the field of the AWG region of the TxPIC as well as the DFB or DFB/MOD regions. SAG performed in the core waveguide may be accomplished with a quaternary (Q) in the InP regime, such as, InGaAsP or AlInGaAs. Again, note that DBR lasers may be substituted for DFBs in the above description. Another feature of this invention is the provision of an optical butt joint between the laser/modulator regions verses the AWG multiplexer region of the chip to allow even tighter control of the modulator structure by only using the SAG techniques to create the DFB region from the modulator (MOD) QW region.
Another feature of this invention is the provision of a core waveguide comprising AlInGaAs in the employment of SAG. AN extension of this feature is the utility of AlInGaAs at an optical butt joint formed in the core waveguide at the interface between an AWG and MOD of a TxPIC chip.
Another feature of the TxPIC embodiments disclosed is the provision of a ridge waveguide structure employed throughout the entire waveguide network fabricated in the PIC for the optically coupled components such as arrays of modulated sources, (e.g., DFBs and/or MODs) optically coupled to an AWG. The ridge waveguide structure is preferred compared to the buried waveguide structures where for example, the active/waveguide core is buried between current blocking layers, such as InP:Fe. The ridge waveguide structure provides for lower confinement of the optical mode and is roughly three to four times larger in cross-sectional area compared to the mode in buried waveguide structures, which translates into more optical power available in the TxPIC chip as well as provided from the TxPIC chip. The ridge waveguide type PIC is employed throughout the entire PIC optical waveguide structure enabling higher chip yield (for example, as much as 50% or more yield) over buried waveguide type PICs as well as providing lower optical confinement to achieve higher on-chip optical power. The improved yield in the ridge structure results from improved ability to control the modal index in a ridge DFB or DBR source as well as the elimination of a low yield undercut etch that is required for buried structures. Further, by employing different ridge widths for the optical components on the PIC, the waveguide width of the DFB and EAM structures can be optimized for higher output power and the EA modulator structures can be optimized for higher extinction ratios. As an example, the ridge waveguide structures in the laser regions may have, in one approach, a narrower width than the ridge waveguide structures in the MOD regions where both the laser sources and the modulators have the same cross-sectional profile. In another approach, the laser sources may have a shallower ridge waveguide and the modulator sources have a deeper ridge waveguide, reference being made here to a resulting height of the ridge, and with both ridges having the same cross-sectional profile. Furthermore, the AWG or optical combiner may have a ridge height that is the same or different from that of the laser and/or modulator. The ridge height of the AWG is optimized for at least one of low-insertion loss, low back reflection (between the combiner and the modulated source), and center channel alignment of the combiner (if it is wavelength selective) to the modulated source array grid.
Another feature of this invention is an InP-based TxPIC chip comprising an array of DFB lasers coupled to an AWG where the DFB lasers are directly modulated with data signals and are provided with improved transient chirp characteristics through the employment of gain-coupled or gain/index coupled active/waveguide regions. Furthermore, a gain-coupled structure may provide for lower optical mode confinement, and hencedhence improved optical power. The enhanced stability facilitated by the gain coupling allows the lasers to be driven at a higher output power without incurring reduced performance due to chirping of the transmitted signal. Note that any of the gain coupled active regions desribeddescribed above may be substituted with loss-coupled regions or gain+loss coupled regions. Further, optical mode confinement may be further lowered by also providing the DFB lasers with a ridge waveguide structure. The success of integrating an array of DFB lasers having different operational wavelengths with an AWG MUX on a single chip is the employment of controlled bandgap shifting techniques, e.g.,. SAG growth techniques, over the entire area of the TxPIC chip.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.
In the drawings wherein like reference symbols refer to like parts:
Reference is now made to
DFB lasers 12(1) . . . 12(N) of TxPIC chip 10 of
InP-based TxPIC chip 10 may include DFB lasers 12 having an index-coupled active region, such as illustrated in
In order to improve the transient chirp characteristics of directly modulated DFB lasers 12(1) . . . 12(N), a gain coupled active region, shown in
An alternate index/gain coupled structure is shown in
Reference is now made to
An important aspect of the TxPICs of
In yet a further embodiment, the ridge of the AWG may be deeper than the DFB ridge. This facilitates improved mode confinement for decreased bend losses as well as reduced insertion losses of the optical combiner (e.g., AWG). Ridge-waveguides are also a preferred for the laser array as a result of their improved fabrication tolerances for realizing a multi-wavelength DFB array with accurate wavelength spacing. See, for example, U.S. Pat. No. 5,805,755.
It should be noted that the teaching of this invention differs from that of U.S. Pat. No. 5,805,755 which teaches the combination of a directly modulated ridge-waveguide DFB array in combination with a buried ridge star-coupler combiner. In this patent, the ridge-waveguide DFB array is utilized for improved wavelength accuracy wherein a buried heterostructure passive waveguide is utilized for low-bend losses. The buried-ridge was utilized as a result of the desire of the inventors to realize low bend losses in a buried heterostructure passive waveguide structure. Hence, the disclosure of U.S. Pat. No. 5,805,755 combines precise DFB wavelength control (via ridge-waveguides) with low-bend loss buried heterostructure passive structures. However, the structures of patent '755 do not realize a high-performance, high-yield TxPIC. A passive buried heterostructure waveguide has numerous disadvantages. Low-loss combiners require very stringent control of the critical dimension and placement of the waveguides entering and exiting the optical combiner. As disclosed in patent '755, buried heterostructure waveguides do not provide accurate control of the width or etch profile, and hence they exhibit significant variations in control and reproducibility of the critical dimension of the waveguide as well as the placement of the waveguides around the input and output ports of the optical combiner. This results in higher insertion loss and variations as in insertion loss across the combiner channels. In the case of wavelength-selective coribiners, the lack of control of the critical dimension and placement of the waveguides also makes it difficult to control the center wavelength of the combiner and the channel spacing of the grid of wavelengths that the combiner accepts. Thus, the performance as well as the yield (cost) of such structures is significantly compromised. The present invention provides for a low-loss passive ridge waveguide (with acceptable bend losses) that can be integrated with a DFB and/or an EA modulator. Low-loss optical combiners, such as, AWGs, have been fabricated with a total insertional loss of 6 dB for a 10 channel combiner. The utilization of a ridge structure in the optical combiner (or AWG region) in concert with a ridge structure in the DFB (and optional modulator region) facilitates the minimization of back-reflection between these elements, minimizing the chirp of the modulated source.
Furthermore, the ridge-waveguide optical combiner facilitates lower insertion loss, better channel-channel uniformity in the optical combiner as well as better center channel control and channel spacing control for wavelength-selective combiners. Thus, the ridge-waveguide structure is preferred for a high-power, highly accurate (wavelength), modulated sources that can be used in combination with highly accurate (wavelength) low-loss combiners that provide minimal reflection for improved chirp and extended transmission distances.
Reference is now made to
A last epitaxial growth is then performed over AWG region 28, the DFB/MOD regions 24 and 26 being masked to prevent growth on these surfaces, such as a SiOx mask. The growth over AWG region 28 is a NID-InP 46B layer having a thickness such as in the range of about 1 μm to 2 μm. The remaining portion 46A of layer 46 remains in DFB and MOD regions 24 and 26. As previously explained above, the reason for regrowth over AWG region 28 is that p-InP layer 46 in this region is absorbing to propagating channel signals so that the regrowth with an undoped InP layer eliminates or otherwise substantially suppresses this absorption. However, it is possible for NID-InP layer 46B to also be lightly doped, especially n-type, or composite doped, e.g., NID-InP closer to Q waveguide layer 44 and n or p doped further away from the optical mode. Note that the layer 46B may alternatively comprise other transparent, low-index semiconductor materials, including InAlAs, or Q with a refractive index lower than that of layer 44. The surface of the in-wafer PIC may then be passivated by deposition of a layer of SixNy, BCB, SiOx, SOG, or polyimide.
It should be noted that, instead of the removal of a portion of the heavy doped confinement layer 46 at 46B, extending to 50, the epitaxial growth of layer 46 may be deposited as NID-InP. After growth of layer 46, the portion of NID-InP layer 46 over active device regions 24 and 26 may be selectively etched away to the point indicated at dotted line 52, after which a layer 46A of p-InP is deposited followed by contact layer 48, with AWG region 28 being masked, such as with SiO2, during this epitaxial deposition.
As is well known in the art, the conductivity type of the layers comprising the PIC structure may be reversed so that the structure would start with a p-InP or InP:Fe substrate 32.
With reference to
It should be noted that the embodiments herein are not limited to a rib-loaded type or the non-rib-loaded type of ridge waveguides structures as well as any other type of ridge waveguide structure known in the art may also be deployed in the embodiments herein which enhance the intensity of the fundamental mode of the channel signals.
It should be further noted that the width of the ridge waveguides 31 in the AWG region 28 (
As a still further note, the use of the Q comprising InAlGaAs in the active region/waveguide core 40 formed via SAG processing across the TxPIC chip in lieu of InGaAsP provides for better bandgap uniformity across the wafer and in-wafer chips, better DFB laser structures due to better carrier confinement and transport properties and better modulator performance due to reduced hole “pile-up” and reduced valence band offsets as well as potentially better quantum well interfaces for enhanced modulator/DFB performance. In the use of a Q layer comprising InGaAsP, the nonuniformity of growth across the wafer can vary as much as 10 nm to 20 nm in wavelength shift. The reason is that, in the MOCVD reactor, the flow of constituent gases over the wafer, particularly, arsine and phosphine, these gaseous constituents crack at different temperatures relative to the flow of these gases at the center of a wafer compared to their flow at the outer edges of the wafer within the MOCVD reactor. Arsine cracks at a lower temperature compared to phosphine. As a result, the P:As ratio in the deposited Q layers across the wafer will not be uniform. Therefore, the employment of a Q compound comprising InAlGaAs with SAG processing for the active/passive waveguide region for a DFB/MOD/AWG structure provides for improved device performance. Also, for similar reasons, targeting of the optical PIC component wavelengths from run to run is improved.
Thus, in summary, better uniformity of deposited InAlGaAs is achieved principally due to the lack of P in the Q compound. The cracking temperature of PH3 is sufficiently different than AsH3 in the MOCVD process that it is difficult to achieve high compound uniformity of InGaAsP particularly over a large surface area of an InP wafer. Also, the employment of a Q Al-bearing layer provides for potentially improved interface abruptness between the quantum wells in the quantum well stack, leading to improved DFB and modulator performance. Furthermore, InAlGaAs offers better electron confinement for improved DFB performance and reduced hole pile-up and valence band offsets in the quantum wells of the EA modulator core 40 providing for improved EA modulator performance.
Reference is now made to
Monotonic increase, itself, of the mask widths will render the deposited material bandgap to be narrower and, correspondingly, the operating wavelength of formed regions in the mask openings become progressively longer. Monotonic decrease, itself, of the openings between mask pairs will render the bandgap narrower and, correspondingly, the operating wavelength of the formed regions in the mask openings become progressively longer. The decrease in bandgap occurs as a result of both enhanced In concentration and enhance growth rate in the mask openings. Thus, a combination of monotonic increasing of mask widths with a monotonic decrease in mask set openings will provide a monotonic decrease in bandgap and, correspondingly, the operational wavelength of formed regions in the mask openings will monotonically increase along the plurality of mask set openings. It is best that both of these parameters are monotonically changed together to progressively increase the operational wavelengths across the arrays of DFB lasers and MODs because either too large of a mask width or a too narrow width of mask set openings by themselves will not successfully achieve the desired wavelength grid across the arrays. However, with a properly designed simulation program, the progression of designed and set wavelengths of the fabricated DFB array can be achieved to match the operational wavelengths of a standardized wavelength grid, such as the ITU grid. Note that the operational wavelength of the DFBs is governed by a number of other factors in addition to the bandgap and thickness of the SAG region, including the grating pitch, composition and duty cycle as well as the ridge-waveguide width and etch depth. However the SAG bandgap and thickness are two of the larger factors that determine the operational wavelength of the DFB. After TxPIC chip fabrication, any necessary changes to operational wavelengths of any of the respective DFB laser sources in the TxPIC array can be adjusted or tuned by changes in the laser operating current or applied bias and/or changes in the laser operating temperature as described in more detail in U.S. application Ser. No. 10/267,330, filed Oct. 8, 2002, which is incorporated herein by its reference.
It should be noted that other combinations can be utilized relative to the monotonic progression in changes of mask widths and/or mask openings. For example, the mask openings can be made progressively or monotonically larger (X, X+Δ1, X+Δ2, etc., and Y, Y+δ1; X+δ2, etc.) across the DFB/MOD arrays and/or the mask widths can be made monotonically smaller (e.g., W1>W2>W3, etc. and WA>WB>WC, etc.).
The dN, the center pitch of the active region mask pairs, as illustrated in
In summary, then, SAG is a process where different widths of paired mask stripes and different opening widths between the pair of mask stripes change the thickness of the Group III–V alloy composition deposited in the mask openings thereby changing the bandgap of the as-grown compound formed in the array of mask openings. The thicker the as-grown SAG layer, the more In that is deposited into the mask opening which means a longer wavelength material.
Changes in the thickness of the various active regions (wells and barriers) grown for an array of active devices having progressively varying mask widths and/or openings for growth provides for varying thickness and bandgaps forming a series of progressively wavelength-shifted active regions capable of generating different lasing wavelengths which can be selectively provided to be within a standardized wavelength grid. The pairs of mask sets for MOD region 26 have progressively increasing widths as well as openings Y, Y−δ1 . . . , designed to have the layers in region 26 to have smaller thicknesses with larger bandgaps (shorter wavelengths) for transparency to DFB generated light. As a specific example, the core bandgap of the Q core 40 in the DFB region 24 for the longest operational wavelength may correspond to about 1.58 μm whereas, in this case, in the MOD region 26 may be a core bandgap corresponding to about 1.52 μm, and in the case of the field of AWG region 28 may have a core bandgap corresponding to about 1.38 μm. In the field region of the mask where the AWG is fabricated, the bandgaps are further larger for the same region. Thus, a feasible SAG budget is limited to a range of selective bandgaps achievable within the constraints of the widths of the mask surfaces, W1, W2, WA, WB, etc. and their opening spacing X, X−Δ1, etc., Y, Y−δ1, etc as well as those governed by the limits of the strain/thickness that can be accommodated in the crystal. The inclusion of additional active or passive optical components, such as, an array of SOAs, puts further constraints on the SAG budget. Thus, it can be seen that if SAG budget can be reserved for bandwidth in the formation of additional channel wavelengths on a TxPIC chip, this provides for more effective cost savings in the manufacture and sale of a TxPIC module for use by carrier service providers. Also, the process control and yield of a TxPIC may be improved by a design that does not require the maximum attainable SAG budget to be utilized.
To be noted is that the SAG budget is extended into the field of the AWG. This will also limit the SAG budget. In the ideal case, the quantum wells in DFB region 24 are compressively strained. But, at best, in a design that utilizes a single SAG growth to deposit the core active/passive waveguide regions, i.e., core 40, including AWG region 28, the modulator quantum wells in core 40 will likely be unstrained or heavily tensely strained which means, in the context of this description, unstrained, slightly tensile strained or slight compressively strained, such as within a range of about <+2,000 ppm strained from the lattice constant. As a result, it is difficult to achieve, within the SAG budget and process, a tensile strained modulator active region 26 between a tensile strained AWG waveguide region 28 and a DFB compressively strained active region 24. If EA modulator quantum well region 26 is not tensile strained, there is a constraint on the EA modulator performance relative to chirp, extinction ratio and insertion loss. This desired balance in strain between the three different regions can be made easier by the provision of an additional growth step in forming the waveguide core 40 in AWG region 28 from the growth step employed to prepare the SAG growth for the quantum well active regions 24 and 26 of the DFB and modulator arrays. This also enhances the SAG budget for the design of the bandgaps to be utilized in the latter active regions, i.e., the range is extended for additional SAG grown layers of different bandgap.
In connection with the foregoing discussion, reference is made to
The fifth epitaxial growth step includes an optional NID InP stop etch layer 74, an optional Q rib-loading layer 76, a p-InP, thick confinement layer 78 and a p++-InGaAs contact layer 80. The optional stop etch layer may also be NID AlInAs, InAlGaAs, InAlAsP or InAlGaAsP. Q rib-loading layer 76 is optional as the preferred embodiment has a ridge waveguide structure.
As in the ease of the embodiment in
To be noted is the butt joint 84 formed between p-InP layer 78A and NID-InP layer 78B is preferably displaced longitudinally relative to butt joint 71 formed at the active/passive waveguide core 70 at MOD/AWG interface 79, i.e., they are not vertically aligned on top of one another. This separation in vertical alignment prevents double reflections of the propagating mode from the joints as occurs in the case where these butt joints are vertically aligned. As an example, the displacement of the respective butt joints 71 and 84 may be in the range of about 200 μm to about 500 μm.
In addition the butt-joint formed in
Alternatively, a second technique to form a similar structure as described in
The rib loaded waveguides 69 and 71 are then formed by selective etchback of the active and passive components, as seen in
Ridge waveguide performance for this and the previous embodiment for both active and passive optical components are strongly dependent on ridge height. Furthermore, the ridge sidewall angle and angle control are critical as well. The TxPIC embodiments described herein have waveguides which are curved and hence span multiple crystal planes. Consequently, it is desirable to utilize anisotropic dry etching with minimal crystallographic etching that occurs when utilizing wet etches when forming the ridge waveguides. Additionally, it is important to stop, at a precise depth, when defining the ridge waveguides in order to control the mode size, and hence optical confinement in the waveguides. This is typically accomplished via stop etch layer 42 and 74, such as, for example, InGaAsP, that is used in conjunction with a wet etch. However, in the case of the embodiments here, the routing of the waveguides in, out and through the multiplexer, such as in the case of an AWG, requires the waveguides to bend and thus span multiple crystal planes. Virtually all wet etches have some crystallographic dependence, making the employment of such an etching method, as applied to a TxPIC chip described herein, unpractical because the resulting ridge waveguides will vary in width across the chip. Thus, a stop etch layer that facilitates an etch stop with dry etching will significantly improve the control and reproducibility of the process. Potential stop etch layers 42 and 74 for dry etching, e.g., CH4/H2, are InAlAs, InAlGaAs, InAlAsP and InAlGaAsP. The key in all of these stop etch layers is to have as high an Al content as possible. Furthermore, additional P to the Al-bearing stop etch compound should further improve etch selectively (or stopping power). It is not required for the stop etch layer to be lattice matched (e.g., tensile strained InAlAs with higher Al composition than lattice matched InAlAs is preferred).
As previously discussed, the ridge waveguide is desired so the propagating mode in the various waveguide paths formed on the TxPIC chip experience lower confinement which enhances the intensity output from the DFB and MOD, helping to eliminate the need for providing any on-chip amplification, such as SOAs.
Distinctive advantages of employing a waveguide core butt joint 71, as shown in
As a still further note, the use of the Q comprising InAlGaAs in the active region/waveguide core 40 and 70 in TxPICs 30 and 60 formed via SAG processing across the TxPIC chip, in lieu of InGaAsP, provides for better bandgap uniformity, a better DFB laser due to better optical confinement and better modulator performance due to reduced hole “pile-up” in the valence band. The better uniformity of InAlGaAs is achieved principally due to the lack of P in the quaternary compound. The cracking temperature of PH3 is sufficiently different than other constituents for this quaternary in the MOCVD process that it is difficult to achieve high compound uniformity of InGaAsP particularly over a large surface area. Also, such an Al-bearing layer provides for increased interface abruptness of the quantum well interfaces.
Also, it is within the scope of the embodiment of
It should be noted that in connection with EA modulators in MOD regions 26 and 65 in the two discussed embodiments, at least a double quantum well stack (e.g., double quantum well with n number of repeats) is one approach to achieving high performance operation. For long optical transmission spans, a TxPIC chip with EA modulators that possess negative chip, low insertion loss and high extinction ratio is desired. In order to realize this with the full SAG processing of a TxPIC (while maintaining the constraints imposed by the SAG budget), a double-quantum well structure is preferred for the EA modulator. Preferably, the active region of the EA modulators includes at least two different regions where the potential well of a first region is deeper than that of a second region. This preference arises from the fact that it is not possible to achieve the requisite tensile stain in the modulator region for optimal performance in a single well structure for a TxPIC chip with full SAG processing of the active/passive waveguide. The double well structure provides the ability to achieve good negative chirp characteristics, extinction ratio, and insertion loss without the utilization of significant strain (e.g., <+/−2000 ppm strain). Each of these regions can be a composite of several layers, i.e., pairs of quantum wells and barriers, but a key factor is that the average electron potential of the first region should be less than that of the second region. Generally, also the thickness of the first region is greater than or equal to that of the second region, although in all of the embodiments to be discussed, which are shown in
As shown in
The modulator quantum well region 114 in
It should be noted that there are many possible alternative configurations and these illustrations in
It should be noted that additional multiple well steps can be formed in each of the two steps or regions of the embodiments of
A complex-coupled grating structure in the DFB arrays, as previously described, may be used in conjunction with the ridge-waveguide PIC structures described herein. A complex-coupled grating structure is provides more enhanced stability for high-power operation and is more immune to back reflections from within the TxPIC. This may be used advantageously with the TxPIC ridge waveguide structures described herein where different ridge widths or heights are utilized for various elements in the PIC. These different ridge widths and heights create an index step between elements which causes back reflection of the propagating light to the DFB. Similarly, the butt-joint(s) of the devices described in
The utilization of complex-coupled gratings facilitates a high-performance EML structure that utilizes an identical active layer (IAL) approach. The IAL approach may also be deployed with a band-edge Mach-Zehnder modulator structures. Such IAL approaches are known in the art. See, for example the article of R. A. Salvatore et al, “Electroabsorption Modulated Laser For Long Transmission Spans”, IEEE Journal of Quantum Electronics, Vol. 38(5), pp. 464–476, May, 2002,. Such structures may be utilized advantageously in the TxPIC disclosed herein. The IAL EML does not require any bandgap shift between the laser and the modulator. Thus, the SAG budget is effectively improved for the TxPIC structures of either
The complex coupling allows the greatest degree of design freedom for the ridge structures while being the most immune to back reflection. The IAL approach may also be used in conjunction with the full SAG approach. In this approach, the IAL approach reduces the SAG budget by about 50 nm. This facilitates a wider process window for the SAG growth as well as allowing for improved uniformity as the reduced SAG shift may provide better composition and thickness uniformity.
Note that other selective bandgap shifting techniques may also be employed to vary the wavelength across any of the elements in the PIC. These may be substituted or utilized in conjunction with any of the aforementioned SAG processing steps. These selective bandgap shifting techniques include disordering (also known as layer intermixing) or multiple regrowths (forming butt joints across the array or along a single channel). Disordering may be implemented by a variety of methods, including impurity-induced layer disordering, vacancy-enhanced layer disordering, or implantation (defect) enhanced layer disordering. If disordering is employed in the AWG or optical combiner region, it is preferably does not introduce significant impurities into the materials that form optical waveguides. This preference is dictated by the fact that impurities can act as optical absorption centers, increasing the propagation loss in the passive structure. Furthermore, care must be taken to ensure that dislocations are not introduced in the PIC materials during the disordering process, resulting in degraded performance and reliability. Note that any of the aforementioned bandgap shifting techniques may be used solely or in concert with each other throughout this invention. Specifically, these bandgap shifting techniques may be utilized in the devices of
Correctly targeting and maintaining the uniformity of the composition and thickness of the material in the DFB and MOD regions is essential to the performance and yield of the TxPIC. The utilization of any selective bandgap shifting technique, including SAG, introduces further error and uncertainty in the composition and thickness uniformity. Thus, it is most desirable to only shift the bandgap of the least number of elements in a TxPIC. The wavelength of the DFB is significantly affected by the grating pitch and stripe width. Furthermore, the window of acceptable operation of an EA modulator is typically on the order of about 5 to 10 nm. Thus, for a TxPIC, it is not necessary to shift every channel with a bandgap shifting technique. Therefore, an embodiment of this invention is to only shift the composition and/or thickness every ith element in the PIC, wherein i>1. In such cases for the DFB lasers, a shift in the wavelength is still accomplished with a composition and/or thickness shift every ith laser.
While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.
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