Optical signal receiver, an associated photonic integrated circuit (RxPIC), and method improving performance

Photonic integrated circuits (PICs), also referred to as opto-electronic integrated circuits (OEICs), and more particularly to a PIC in the form of an optical receiver PIC or RxPIC chip and an optical transmitter PIC (TxPIC) are employed in an optical transport network. Integrated on the RxPIC chip, starting at the input end which is coupled to receive multiplexed optical data signals from an optical transport network is an optical amplifier, an optical demultiplexer, and a plurality of on-chip photodiodes (PDs) each to receive a demultiplexed data signal from the AWG DEMUX for optical-to-electrical signal conversion. The optical input amplifier may be an on-chip gain clamped semiconductor optical amplifier (GC-SOA) or an off-chip fiber amplifier. The optical input amplifier may be optional if the channel signal demultiplexer provides for minimal insertion loss which is optimum with a properly designed arrayed waveguide grating (AWG) demultiplexer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical telecommunications and more particularly to the deployment of photonic integrated circuits (PICs), in particular, optical receiver photonic integrated circuits (RxPICs) and transmitter photonic integrated circuits (TxPICs) utilized in optical transport networks.

2. Description of the Related Art

The employment of photonic integrated circuits (PICs), also sometimes referred to as planar lightwave circuits (PLCs), are on the rise in optical telecommunication systems. These devices provide the integration of both active and passive optical components on a single substrate and are integrated with other optical components to form a multi-functional optical device for use in such systems. The gravitation to PICs is strong because it leads to utility of providing an entire system function, let alone a component function, in a single chip in a single package. Compared to the deployment of discrete optical components, such monolithic PIC chips can significantly reduce the size of optical components necessary in the optical system, albeit an optical transmitter (TxPIC) or optical receiver (RxPIC), for example, as well as significantly reduce the over cost of the system.

Optical PICs are already known in the art. As related to an optical receiver on a chip, the article to 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 the array of PDs. The optical receiver chip may also be placed on a thermoelectric cooler (TEC) so that the spectral response or wavelength grid of the AWG can be fine tuned. A similar PIC chip configuration is shown in U.S. Pat. No. 5,913,000 to Doerr et al. but relates to a laser structure without an array of photodiodes, but rather an array of second optical amplifiers in their place, and where the PIC chip facets include reflective mirror surfaces to form multiple laser cavities. Further, an article to C. Cremer et al. entitled, “Grating Spectrograph Integrated with Photodiode Array in InGaAsP/InGaAs/InP”,IEEE Photonics Technology Letters,Vol. 4(1), pp. 108110, January 1992, discloses a 4 mm by 7 mm InGaAsP/InP chip comprising a grating demultiplexer integrated with a photodiode array. The grating demultiplexer comprises a slab waveguide having multiple input waveguides and output waveguides to and from the slab. The slab has one end as a reflective mirror and, thus, “mirrors” one half of a full slab waveguide structure. The output waveguides from the slab are respectively coupled to an array of photodiodes integrated on the InP chip. See also the papers of J. B. Soole et al., Integrated Grating demultiplexer and PIN array for High Density Wavelength Division Multiplexed Detection at 1.5 mm”,ELECTRONIC LETTERS,Vol. 29, pp. 558–560, 1993; M. R. Amersfoort et al., “Low-Loss Phased-Array Based 4-Channel Wavelength Demultiplexer Integrated with Photodetectors”,IEEE Photonics Technology Letters,Vol. 6(1), pp. 62–64, January 1994; and S. Chandrasekhar et al., “Monolithic Eight-Wavelength Demultiplexed Receiver for Dense WDM Applications”,IEEE Photonics Technology Letters,Vol. 7(11), pp. 1342–1344, November 1995.

A combination WDM/PD array is shown in the article of F. Tong et al. entitled, “Characterization of a 16-Channel Optical/Electronic Selector for Fast Packet-Switched WDMA Networks”,IEEE Photonics Technology Letters,Vol. 6(8), pp. 971–974, August 1994, except that, in the case here, the InGaAs/GaAs PDs are on a separate chip integrated with electronic transimpedance amplifiers, selectable switches and output limiting amplifier. Light generated from the multiple output waveguides of a separate AWG DEMUX chip is focused through a lens array to the array of photodetectors or photodiodes (PDs).

See also the article of B. Glance et al. entitled, “Applications of the Integrated Waveguide Grating Router”,Journal of Lightwave Technology,Vol. 12(6), pp. 957–962, June 1994, which shows multiple applications for AWG devices with multiple inputs/outputs and their integration with various types of active components.

In some of the foregoing disclosures, optical semiconductor amplifiers (SOAs) are employed to boost the incoming channel signals such as from an optical link. Thus, the first on-chip optical component is an active component comprising an SOA to amplify the channel signals. Since these signals are of different wavelengths, however, the gain of the SOA is not equally distributed to all of the channel signals and, as a result, the signals to be amplified do not receive the same gain. This is a problem because the signals should have substantially equal intensity or power before they are demultiplexed; otherwise, some of the channel signals will have significantly degraded BER due to the dynamic range of the receiver photodiodes and transimpedance amplifiers.

OBJECTS OF THE INVENTION

It is an object of this invention to provide, in combination, photonic integrated circuit (PIC) chips in combination with electronic circuit chips useful in optical transport networks, in particular digital optical networks disclosed in U.S. patent application, Ser. No. 10/267,21, filed Oct. 8, 2002, also published on May 29, 2003 as Publication No. US 2003/0099018 A1. which is incorporated herein by reference.

It is another object of this invention to provide an optical receiver photonic integrated circuit (RxPIC) with improved performance.

It is a further object of this invention to provide an optical transport network utilizing an optical receiver photonic integrated circuit (RxPIC) and or optical transmitter photonic integrated circuit (TxPIC) or a transceiver or transponder in an optical transport network.

It is another object of this invention to RxPIC monolithic chip that comprises at least one gain clamped optical semiconductor amplifier (GC-SOA), an optical demultiplexer, preferably an AWG DEMUX, and an array of photodiodes (PDs), preferably an array of PIN PDs, all integrated on one monolithic PIC chip.

It is further object of this invention to provide an RxPIC chip that provides redundancy to improve chip yield.

SUMMARY OF THE INVENTION

According to this invention, a optical receiver photonic integrated circuit (RxPIC) comprises a single chip casted from an InP wafer and is made from Group III-V elemental materials in the InGaAsP/InP regime with fabrication accomplished through selective metalorganic vapor phase epitaxy (MOVPE) or also known as metalorganic chemical vapor deposition (MOCVD). Integrated on the chip, starting at the input end which is coupled to receive multiplexed optical data signals, may include an on-chip input optical amplifier, an optical demultiplexer (DEMUX), and a plurality of on photodiodes (PDs) each to receive a respective demultiplexed data signal from the DEMUX for optical-to-electrical signal conversion. The RxPIC chip input is optically coupled to receive a multiplexed channel signals from an optical transport network and are optically coupled to a signal demultiplexer providing a plurality of channel signal outputs optically coupled to an array of photodiodes, such as PIN photodiodes (PDs) or avalanche photodiodes (APDs). The PDs each have a contact pad for transfer of the generated electrical signal off the chip or, alternatively, the RxPIC can include on-chip integrated transimpedance amplifiers to receive electrically converted channel signals.

A RxPIC chip may provide for minimal optical loss between the input optical fiber from the optical transport network optical fiber link and a first point of amplification in order to achieve high optical signal to noise ratio (OSNR). This can be accomplished by having an optical amplifier at the chip input such as, for example, on-chip, gain-clamped semiconductor optical amplifier or a GC-SOA or an off-chip EDFA or Raman amplifier. If no amplification is to be provided at the input of the RxPIC chip, then the channel signal demultiplexer will have to provide minimal insertion loss. In this case, the preferred optical demultiplexer is an AWG so that if it is properly designed, it will provide minimum loss as compared to other types of optical demultiplexers, also which are disclosed herein, as well as also provide for a filter function to select one optical channel from the plurality of channels with minimal optical crosstalk and to eliminate undesired noise carried along with the channel signals, such as ASE and the gain clamping lasing signal.

One important feature disclosed is the employment of an on-chip, gain clamped semiconductor optical amplifier (GC-SOA) rather than a semiconductor optical amplifier (SOA). The use of a GC-SOA provides for a saturated SOA that has continuous gain in spite of continuous changes in the incoming optical multiplexed channel signal gain; otherwise, without the gain clamped signal of the GC-SOA, the gain provided by the SOA would gain-starve higher signal wavelengths over time. For purposes of enhancing the yield of optical receiver photonic integrated circuit (RxPIC) chips produced from an InP wafer, a plurality of GC-SOAs are utilized at the input of the RxPIC chip and tested to see which one best matches the optical mode from the GC-SOA to the AWG to ensure polarization insensitivity with low loss and minimal back reflections from the AWG.

It is another feature of this invention to place integrated SOAs in an RxPIC chip between the demultiplexer and the array of photodiodes, with one in each waveguide from a demultiplexer output to a respective photodiode. The SOAs optimize the received demultiplexed channel signals by render them all of equalized intensity or power so that the responsivity of the photodiodes will all be substantially the same.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made toFIG. 1which illustrates one feature of this invention.FIG. 1is a diagrammatic view of integrated optical components comprising the optical receiver photonic integrated circuit (RxPIC)10of this invention. RxPIC chip10comprises a gain clamped semiconductor amplifier (GC-SOA)12having an input at an input facet (not shown) of chip10to receive, such as from an optical transmission link, multiplexed optical data signals λ1. . . λNfor immediate amplification prior to signal demultiplexing. This is an important function in order to insure that the optical signal to noise ratio or OSNR is maintained at a low noise figure. More importantly, GC-SOA12is used instead of a SOA, such as disclosed in the M. Zirngibl et al. paper, supra. A GC-SOA is an amplifier in which feedback is created through an established laser cavity in the amplifier around the amplifying medium so that oscillation is generated inside the amplifier cavity at a predetermined wavelength as defined by a grating formed in the amplifier cavity. This device is, therefore, a semiconductor laser amplifier having a DFB laser cavity, although the lasing cavity could also be a DBR lasing cavity within the scope of this invention. The reasons why a GC-SOA is better than a SOA in this application is to provide a gain clamped signal to eliminate loss of gain to higher wavelength channel signals and also the TE/TM gain ratio is fixed due to the presence of the gain camp signal and, therefore, this ratio does not change due to power variances in the input channel signals.

As shown inFIG. 1, the output from GC-SOA is provided to an optical demultiplexer (DEMUX)14where the signal channels are demultiplexed and placed on DEMUX waveguide outputs as channel signals, λ1. . . λN, to respective photodiodes16PD(1) . . . PD(N), which produce electrical signals which are then initially amplified by low noise figure, transimpedance amplifiers (TIAs)18as is known in the art. The preferred demultiplexer is an arrayed waveguide grating because of its low insertion loss properties. However, it is within the scope of this invention to also include as a demultiplexer, an Echelle grating.

It is within the scope of this invention that, instead of employing an on-chip optical amplifier12as shown inFIG. 1, an off-chip fiber amplifier may be employed, such as EDFA12A illustrated inFIG. 67. As shown inFIG. 67, this monolithic RxPIC chip10would then be comprised of integrated components comprising, for example, a demultiplexer14in the form of an AWG, for example, and photodetectors16(1) . . .16(N).

It is further within the scope of this invention that RxPIC10chip10primarily consists of an AWG and an array of photodetectors which will be explained in more detail later.

It is also with the scope of this invention that the primary components comprising this invention, to wit, a GC-SOA12, demultiplexer14(preferably an AWG) and photodetector array16be of separate discrete optical elements. However, it will be understood by those skilled in the art the impact of their integration on a single InP chip to be a highly desirable, compact, cost effective and easily replaceable component as an optical receiver system.

It is within the scope of this application that photodiodes16PD(1) . . . PD(N) may be comprise of a PIN photodiode as shown inFIG. 25, or an avalanche photodiode as shown inFIG. 26, or a metal-semiconductor-metal (MSM) device comprising inter-digitized contacts as shown inFIG. 27. Examples of an avalanche photodiode are disclosed in pending provisional application, Ser. No. 60/342,984, filed Dec. 21, 2001, and entitled, “InP-BASED PHOTONIC INTEGRATED CIRCUITS WITH Al-CONTAINING WAVEGUIDE CORES AND InP-BASED ARRAY WAVEGUIDE GRATINGS (AWGs) AND AVALANCHE PHOTODIODES (APDs) AND OTHER OPTICAL COMPONENTS WITH AN InAlGaAs WAVEGUIDE CORE”, now U.S. patent application, Ser. No. 10/327,362, filed Dec. 20, 2002, also published on Sep. 4, 2003 as Publication No. US 2003/0099018 A1, which application is incorporated herein by its reference. Examples of MSM photodetectors are disclosed in articles of B. D. Soole, et al., entitled, “Waveguide MSM photodetector on InP”,ELECTRONICS LETTERS, Vol. 24(24), 24 November, 1988; “High-Speed Performance of InAlAs/InGaAs MSM Photodetectors at 1.3 μm and 1.5 μm Wavelengths”,IEEE Photonics Technology Letters, Vol. 2(8), August, 1989; and “InGaAs Metal-Semiconductor-Metal Photodetectors for Long Wavelength Optical Communications”,IEEE Journal of Quantum Electronics, Vol. 27(3), pp. 737–752, March, 1991, which articles are incorporated herein by their reference.

FIGS. 62A and 62Bprovide for alternative type of photodetectors that may be employed on RxPIC10as compared to the types that have been previously explained relative toFIGS. 25–27and comprise high speed velocity-matched distributed photodetectors (VMDPs). VMDPs are optical waveguides upon which are fabricated a plurality of photodetectors that are interconnected with optical and coplanar electrical waveguides and their quantum efficiency is dependent upon the number of photodetectors deployed in an array along the electrical waveguides. “Velocity-matched” refers to matching the velocity of the RF optics and RF signal along the optical waveguides. Each of the photodetectors in the array contribute constructively to an optimum output so that all the photodetectors in the array must operate in phase with one another relative to any signal channel in order for an optimum electrical response to be produced. InFIG. 62A, each output channel on a waveguide39includes an array of photodiodes270which produce an electrical signal proportional to the amplitude of the channel signal in a waveguide39. The electrical signal is collected by a separate microwave transmission line272that is velocity matched to the optical waveguide39and the electrical signals are taken off of chip10at pads274. See, for example, the article of L. Y. Lin et al., entitled “Velocity Matched Distributed Photodetectors With High-Saturation Power and Large Bandwidth”,IEEE Photonics Technology Letters,Vol. 8(10), pp. 1376–1378, October, 1996, which article is incorporated herein by its reference. InFIG. 62B, TIAs276and limiting amplifiers278are integrated on RxPIC chip10employing InP-HBT or InP-HEMT technology.

Reference is now made toFIG. 63which discloses another type of photodetector that may be deployed in this invention comprising traveling-wave photodetectors (TWPDs). InFIG. 63, the arrangement comprises a photodetector280, such as a PIN photodiode, fabricated at the end of waveguide39from AWG30. Photodetector280is central of a parallel plate, co-planar transmission line comprising signal line284and ground lines286providing for a matched electrical termination at the output end. As an example, see the article of Kirk S. Giloney et al., entitled “Traveling-Wave Photodetectors”,IEEE Photonics Technology Letters,Vol. 4(12), pp. 1363–1365, December, 1992, which article is incorporated herein by its reference.

As shown inFIG. 2, additional amplification of the DEMUX′ed channel signals can be provided with integrated semiconductor optical amplifiers (SOAs)20in the optical waveguides formed between DEMUX16and photodiodes (PDs)16.

As illustrated inFIG. 3, there may be more than one GC-SOA12A,12B and12B at the RxPIC input for purposes of redundancy so that the on-chip performance of these respective GC-SOAs12A–12C can be checked relative to the ITU grid of the DEMUX14in order that the best performing GC-SOA can be selected, e.g., the one with the best gain, saturated power, noise figure, etc. characteristics. The number of SOAs12included on chip10is preferably in the range of about 2 to 5 such devices. Three are shown in the illustration here. However, more such devices are preferred, such a sufficient number to cover or extend slightly beyond the spectral range for DEMUX14where the wavelength variation of the spectral grid in the fabrication of DEMUX14may be not be the same for all devices formed in the same wafer or for devices formed from wafer to wafer. In this manner, the yield of RxPIC chips10obtainable from a wafer can be decisively increased. Once the best performing GC-SOA is selected, the input coupling of the channel signals from the fiber link can be aligned and fixed to the selected GC-SOA. The selection of performance is enhanced also by the employment of heaters22placed in close proximity to each GC-SOA12A–12C so that the response of the individual SOAs20can be adjusted to better matched to the ITU grid of optical DEMUX14.

Reference is now made toFIG. 4illustrating the plan view or layout of a more detailed form of RxPIC chip10of this invention which further includes input optical mode adapters (MAs)24A,24band24C for the respective GC-SOAs12A,12B and12C as well as respective output optical mode adapters (MAs)26A,26B and26C. Passive MAs24A–24C permit multi-wavelength beam expansion into the GC-SOAs from the single mode fiber coupled to one of the selected inputs, while passive MAs26A–26C permit beam reduction to a single mode passive waveguide37connecting the respective GC-SOAs12to DEMUX14. It is preferred that MAs24and26adiabatically increase and decrease the input beam, respectively, in order that the beam is gradually expanded and then contracted for lowest optical loss. Also, the use of MAs are critical from the standpoint of forming a composite beam of light that is circular and render it less critical in tolerances relative to fiber alignment of the fiber input to RxPIC10with regard to input MAs24. Also, output MAs26provide for matching the optical mode from SOAs12to DEMUX14to insure polarization insensitivity is preserved with low optical losses and lower optical back reflections such as from downstream optical components. More will be said about these mechanisms later. Also, shown inFIG. 4are the contact pads28at the output end of chip10for receiving the respective electric signals from PDs16for transfer off the chip to an RF submount board for electrical domain amplification and subsequent processing.

It would be best to have the alignment of the array of PDs16on chip10to be out of direct alignment of the axial optical path of GC-SOAs12. Spontaneous emission (ASE generated at the selected GC-SOA12propagating through chip10will provide added noise to photodetectors16. A scheme to spare photoconductors16from this noise is illustrated inFIG. 5, next to be discussed.

Reference is next made toFIG. 5which illustrates in even more detail embodiment of this invention for RxPIC chip10comprising this invention. Chip10is formed in the InGaAsP/InP regime and, for example, may have dimensions of about 1.45 mm by 6.2 mm. Chip10includes a plurality of input MAs24A–24C to expand the input beam at the channel signal input to a selected GC-SOA12A–12C, as previously explained, and the channel signals are reduced to single mode by a respective output MA26A–26C. The input fact to RxPIC chip10may include an AR coating, as may be the case of any of the other embodiments disclosed. The AR coating aids in coupling multiplexed channel signals into the chip as well as prevents internal backward reflections from occurring and interfering with the operation of the chip, particularly the operation of photodiodes16. The signals are then provided, via a passive on-chip waveguide37, to a vernier input of a first slab or free space region32of AWG DEMUX30. It is preferred that the length of the MAs be as small as possible so as not to add to increasing the area real estate required for chip10. The vernier input shown here comprises three different inputs to the input slab32of AWG30so that a best operational match of GC-SOA12to the wavelength grid of AWG30can be selected. Thus, through the selection of the best vernier input in the first order Brillouin zone and the best performing GC-SOA12, the best wavelength grid alignment to AWG30can be selected that provides optimized wavelength matching and lowest coupling loss. This is shown in more detail inFIG. 5A, which is a bit exaggerated in scale to illustrate this invention. InFIG. 5A, there are five GC-SOAs12A–12E shown integrated on chip10with their respective output waveguides37comprising a group of vernier inputs37V in the central portion of the first order Brillouin zone of slab or space region32. Because fabricating techniques may not precisely place the amplifier waveguide input at the exact position desired at the slab32input, the placement of a plurality of waveguide inputs along the center of the first order Brillouin zone forms a vernier permitting the selection through signal testing of the respective GC-SOAs12to determined which first order input provides the optimum performance in handing the multiplexed channel signals, such as in terms of signal separation, low optical noise and narrow signal bandwidth. While the best performing GC-SOA12may be on the wrong waveguide arm to the input of AWG30for best wavelength matching to the grid of the AWG, a lower performance-GC-SOA12may be chosen in combination with temperature tuning of AWG, via an AWG heater30A, to optimize the matching of the wavelength grid of AWG30to the selected GC-SOA.

While other types of optical demultiplexers may be utilized in this invention, such as an echelle grating, a multichannel grating demultiplexer comprising wavelength-select angled or blazed gratings, a reflector stack filter, or multimode interference (MMI) couplers.

A demultiplexer can also be comprised of a series of angled gratings each of which has a grating period designed to remove from the waveguide a selected wavelength channel from the propagating multiplexed channel signal. This type of demultiplexer is illustrated inFIG. 68. RxPIC10comprises a GC-SOA12to receive the incoming channel signals, λ1. . . λN, which provides signal amplification after which the signals propagate through mode adaptor26and onto a single mode waveguide182. Waveguide182contains a series of angled or blazed gratings180(1) . . .180(N), one for each channel signal. Each grating period is designed to have a peak reflection wavelength equal to one of the signal wavelengths, λ1. . . λN, so that each of the signal wavelength, λ1, λ2, λ3, etc., is consecutively reflected out of waveguide182to a corresponding photodetector16(1) . . .16(N) at the same semiconductor layer level in the chip as waveguide182. Photodetectors16may be formed along one edge of chip10to transfer the detected signals off-chip. Each of the gratings180(1) . . .180(N) may be also provided with a heater184in close proximity to a grating so that the gratings180may be individually tuned at the factory or in the field to insure that that their reflection wavelength peaks are at or very close to the respective channel signal wavelength peak for optimum detection at a corresponding output photodetector16. By the same token, if a channel signal or signals are slight off their peak wavelengths, the respective gratings180can be selectively temperature tuned to be made closer to the off-peak wavelength or wavelengths. An advantage of this embodiment as an optical demultiplexer is that the clamping signal and any ASE developed at the GC-SOA12is directed forward along waveguide182and out of chip10providing for high OSNR in signal detection by photodetectors16.

A reflector stack filter functioning as a demultiplexer may be of the type that has plural reflector surfaces that provide for successive reflection of peak wavelengths comprising the channel signal wavelengths spatially along the filter so that the spatial array of demultiplexed channels signals may be directed to an array of corresponding photodetectors (see U.S. Pat. No. 6,111,674 which is incorporated herein by its reference) or narrow band elliptical mirrors or elliptical Bragg reflectors of the type disclosed in the paper of Charles H. Henry et al., entitled “Four-Channel Wavelength Division Multiplexers and Bandpass Filters Based on Elliptical Bragg Reflectors”,Journal of Lightwave Technology,Vol. 8(5), pp. 748–755, May, 1990, which paper is incorporated herein by its reference.

A multichannel grating reflector functioning as a demultiplexer is illustrated in the article of P. A. Kirby, entitled, “Multichannel Wavelength-Switched Transmitters and Receivers—New Component Concepts for Broad-Band Networks and Distributed Switching Systems”,Journal of Lightwave Technology,Vol. 8(2), pp. 202–211, February, 1990, which is incorporated herein by its reference.

An MMI coupler device comprises a multi-mode slab waveguide, which can support several modes, with N inputs (in the case here including demultiplexing only one input is needed) and M outputs and is based upon a self-imaging property wherein an input field profile is reproduced in a single or multiple images at periodic intervals along the propagation direction of the slab waveguide. See, for example, the articles of Lucas B. Soldano et al., entitled, “Optical Multi-Mode Interference devices Based on Self-Imaging: Principles and Applications”,Journal of Lightwave Technology,Vol. 13(4), pp. 615–627, April, 1995, and of K. Okamoto et al., entitled “Fabrication of Coherent Optical Transversal Filter Consisting of MMI Splitter/Combiner and Thermo-Optic Amplitude and Phase Controllers”,ELECTRONIC LETTERS,Vol. 35(16), pp. 1331–1332, Aug. 5, 1999, which articles are incorporated herein by their reference.

With respect to all of these different embodiments for an optical demultiplexer, an AWG device is preferred because of its better routing and filtering characteristics. Reference is now made toFIG. 5where an AWG30is shown in RxPIC chip10. As is well known in the art, the slab32provides for diffraction of a multi-wavelength signal beam into a plurality of outputs to waveguide arms34, each having a different path length. The outputs of waveguide arms34are coupled to a second slab or free space region36where the respective channel wavelengths are place respectively on a plurality of output passive waveguides such as along the first order, central Brillouin zone of slab36. Each of these outputs in passive waveguides39from WAG30is coupled to a respective PD16and the electrical signal output of the PD16is placed on a respective output signal pad28of chip10.

GC-SOAs12A–12C of chip10are provided with segmented electrodes or contacts32, that may take on a countless number of configurations, but only a few examples are shown here with respect toFIGS. 5,6,9and10. In connection with GC-SOA12A, two segmented electrodes or contacts32A and32B are shown approximately equal distance from the ends of the optical amplifier. These respective contacts32A and32B are respectively coupled to receive currents, I1and I2. On the other hand, as shown relative to GC-SOA12B, segmented contacts32A and32B are positioned closer to the output end of the amplifier. Conversely, a multiple number segment contacts32A–32E may be utilized such as shown in connection with GC-SOA12C. Segmented contacts32A–32E are respectively contacted to receive different currents I1–I5. In each of these three different cases of GC-SOA contacts, it can be that I1>I2in order to improve the noise figure (NF) of the amplifier. On the other hand, if I2>I1, then better power saturation, PSAT, can be achieved. By placing the contacts closer to the output of the amplifier, such as shown in connection with GC-SOA12B, improved performance relative to noise figure (NF) as well as saturation gain might be achieved. In this connection, the multiple array of segmented electrodes32A–32E in GC-SOA12C may be provided with a variety of distributions of applied current, for example a monotonically increasing applied current, I1–I5, applied respectively to contacts32A–32E, i.e., I1<I2<I3<I4<I5. or a distribution that is relatively larger at both the input and output ends than in the middle, e.g. I1>I2>I3, I3<I4<I5. The current distribution is optimized to provide the best noise figure and saturation power performance

RxPIC chip10may also have an open trough or layer barrier38formed in the chip body which, for example, extends into the substrate of the chip for the purpose of blocking or otherwise scattering stray light out of the chip, particularly stray light (e.g. ASE) from GC-SOAs12, so that such stray light does not affect PDs16, i.e., does not interfere with the true channel signal sensing to be accomplished by these photodiodes for demultiplexed channel signals received from AWG DEMUX30. If barrier38is an open trough, it is best that its side surface38A be somewhat rough so that stray light propagating to the trough edge from the direction of GC-SOA12will be scattered out by the rough side surface of the trough and out of chip10. On the other hand, if barrier38is to be filed with a light blocking or absorbing material, a number of materials may be used, which may be semiconductor, dielectric, amorphous or polycrystalline in nature.

Lastly, the input surface of chip10is preferably coated at its input surface11with an antireflecting (AR) material, as is known in the art, in order to increase the input coupling efficiency to the chip and also to eliminate spurious reflections from the facet altering the behavior of GC-SOA12.

Reference is now made toFIG. 6where the RxPIC layout provides for less scattered light interference between optical components but the compromise is that this necessitates larger chip real estate. However, the area real estate necessary for chip components may be reduced to some degree by fanning out the bonding pads28on chip10, particularly for PDs16so that they are positioned along more than one edge of chip10as shown inFIG. 6, compared toFIG. 5where they are aligned only along one edge of the chip. Thus, RxPIC chip10inFIG. 6is larger than the RxPIC chip10shown inFIG. 5and may be, for example, 4.3 mm by 4.2 mm. Also, further shown inFIG. 6is a SOA40and an ASE filter42formed in each of the output waveguides39from AWG30. SOAs40of selectively controlled, via an applied positive bias, or even negative bias to function as an absorber, to provide for equalization in gain across the demultiplexed channel signals emanating from AWG30. Filters42have a narrow bandwidth within the channel signal spectrum to filter out other wavelengths, particularly higher wavelengths of spontaneous emission or ASE generated in SOAs40.

It should be noted in this embodiment as well as previous embodiments that GC-SOAs12generate amplified spontaneous emission or ASE. Another reason why AWGs are the preferred choice for demultiplexing channel signals on-chip is because of the narrow wavelength filter quality of AWG30can filter out the clamping signal associated with the GC-SOA which are in the bandwidth of ASE.

It is desirous to keep photodetectors16positioned out of direct axial alignment with the GC-SOAs12so as to prevent spontaneous emission from these devices from being detected by PDs16. This is accomplished by the placement of the outputs of GC-SOAs12not to be in direct optical alignment with PDS16. However, this is not always possible to one hundred percent because the chip size may become too large in the direction44so that it is more apt to break during processing and becomes more difficult to manage for placement in a hermetic package. In the embodiment ofFIG. 6, therefore, a trough (not shown) may be utilized in the manner of trough38illustrated inFIG. 5.

As indicated in the previous embodiment, multi-segment SOA contacts32aid to minimize the total current drawn by these devices, particularly in the upstream portion of the device where the gain should be higher. Moreover, if the saturation power is not required to be too high, the full bias applied to reach saturation need not be applied at the downstream end of these devices. The last electrode segment32C may, alternatively, be employed to measure photocurrent of the GC-SOA12to monitor its gain and correspondingly increase or decrease the gain as necessary to optimize the operation of the device.

Since the optical power increases from the upstream end towards the downstream end of a GC-SOA then the optimum use of pump current in a GC-SOA should increase towards the downstream end if it is desired to avoid saturation effects. This can be accomplished in theFIG. 6embodiment by placing less bias on electrode segments32A compared, for example, to segments32B and32C. Thus, for example, the applied bias may be controlled such that I3>I2>I1. Alternatively, as shown inFIG. 9, the electrode segments44may be monotonically increasing in size toward the downstream end of GC-SOA12so that for equal biases the currents will vary, such that resulting currents drawn via electrode segments44A-44F, due to their area, would be44F>44E>44D>44C>44B>44A. Another alternative is shown inFIG. 10where electrode segment46is tapered so as to be monotonically increasing in segment contact area from the upstream end to the downstream end of the device. Thus, monotonically increasing current will be supplied along the length of GC-SOA12.

A further embodiment relating to current control to GC-SOA12is shown inFIG. 11where a cross-section of GC-SOA12is shown, in particular at the region of the GC-SOA at its downstream end. InFIG. 11, GC-SOA12comprises a buffer layer62of n-InP on an n-InP substrate60followed by a grating layer64of InGaAsP, which is also referred throughout the description in this application as a “Q” layer or “Q” multi-quantum well region, meaning a “InGaAsP quaternary” layer or region. Also, it should be realized that a semi-insulating substrate, such as InP:Fe or InP:O, may be used instead of semiconductor substrate, e.g., n-InP with an appropriate change in metal contacting as is known in the art. Q layer64is followed by an InP layer66that smoothes out the growth surface for the growth of the amplifier active region65which is Q layer or quantum well region. This is followed by the growth of confinement layer68of undoped or p-doped InP. At this point, the initial MOCVD growth, process is completed, a selective etch is made to buffer layer62, for example, and a second growth comprising the growth of current blocking layer74is accomplished which may be, for example, InP:Fe, InP:O or InP:O:Fe, followed by a selective etch though the overgrown blocking layer72to InP confinement layer68. This isotropic etch step is accomplished with a tapered mask so that the width of opening69formed along the length of the GC-SOA monotonically increases from the upstream end to the downstream end of the device, the largest width being as shown inFIG. 11. Then, a third growth is accomplished comprising cladding layer70on p-InP followed by the growth of the contact layer72p+-InGaAs. Thus, the resulting structure is a tapered current channel formed along the optical axis of GC-SOA12so that the gain created through contact layer72monotonic increases from the upstream end to the downstream end of the device.

Reference is now made toFIG. 7which illustrates a plan view of a particular embodiment for GC-SOA12and its optically coupled MAs24and26. The similar structural device is shown in the paper of P. Doussiere et al., entitled “1.55 μm Polarization Independent Semiconductor Optical Amplifier with 25 dB Fiber to Fiber Gain”,IEEE Photonics Technology Letters,Vol. 6(2), pp. 170–172, February, 1994, which paper is incorporated herein by its reference. This figure in particular illustrates the waveguide layer of the device where the optical mode of the multiplexed signal is initially expanded in input MA24, amplified in GC-SOA12(unlike the device in the paper to Doussiere et al.) and then the optical mode is converted back to single mode via output MA26for proper mode entry into waveguide37and AWG30. The cavity of GC-SOA12includes a light feedback mechanism to provide a laser signal at a frequency not within the bandwidth of the channel signals. As will be seen later inFIG. 8A, the feedback mechanism may be a grating. Another such mechanism would be built-in mirrors at the laser signal wavelength at each end of the GC-SOA cavity.

Reference is now made toFIG. 7Ashowing another embodiment for GC-SOA12and MAs24and26. In this embodiment, RxPIC chip10includes an input waveguide formed in the chip for receiving the multiplexed channel signals, λ1. . . λN, from an optical link via optical fiber46. To be noted is that input waveguide48is curved relative to input facet52. Fiber46is angularly disposed relative to input facet52and is aligned to optimize the coupling of the signal into input waveguide48. It then is mode expanded via MA24, amplified by GC-SOA12, mode contracted via MA26and propagates on waveguide37to AWG30. In the InP regime, these waveguiding components48,24,12,26and37may utilize a light propagating waveguide comprised of InGaAsP or Q, which is cladded by n-type and p-type InP layers as shown in the paper to P. Doussiere et al. The angularity of input waveguide48relative to a normal to the input facet52may be several degrees, in the case here shown as 7°. Input facet52also includes an antireflecting (AR) coating54to reduce stray or scattered light from reflecting back into input waveguide48. Also, the angularly disposed input waveguide48at 7° further reduces input facet reflectivity. Curved waveguide48to facet52prevents optical feedback into the cavity of GC-SOA12. Also, undesired reflections are caused from input facet52where optical fiber46is butt coupled to RxPIC10and these reflections can affect the incoming signals, such as, changes in intensity, which is undesirable in DWDM applications.

A further shape for GC-SOA12and its associated MAs24and26may be of the type shown in the paper to Hatakeyama et al., entitled, Uniform and High-Performance Eight-Channel Bent Waveguide SOA Array for Hybrid PICs”,IEEE Photonics Technology Letters,Vol. 13(5), pp. 418–420, may, 2001, which paper is incorporated herein by its reference. The waveguides, comparable to waveguides48and37, as well as coupled MAs, comparable to MAs24and26, are bow-shaped so that the straight SOA section and the MAs/passive waveguides are smoothly coupled by an 820 μm curvature. The SOA in this paper, however, is not gain-clamped and is utilized in a different application (switching) and the SOA active layer and the MAs/passive waveguides are one and the same layers.

InFIG. 7B, GC-SOA12is also curved with expanding mode adaptor24provided at input facet52to receive the incoming multiplexed channel signals. The grating for the laser signal of GC-SOA12may be provided only in the downstream section12G1or, alternatively, could be provided, as well, in the upstream curved section12G2where the gratings in both sections are normal to direction of light propagation and the pitch of the grating in section12G2is greater than that in section12G1. See U.S. Pat. No. 6,008,675, andFIG. 3, which patent is incorporated herein by its reference.

There are different types of gain clamped-semiconductor optical amplifiers that may be utilized in connection with this invention.FIG. 12shows a first and preferred type comprising a DFB type GC-SOA70which basically comprises a plurality of semiconductor layers deposited on an n-InP substrate72. These layers, in sequence, comprise a n-InP confinement layer74, a Q grating layer76within which is formed a periodic grating to cause lasing at a clamp signal frequency, an InP smoothing layer78, an active region80such as a plurality of quantum well layers of InGaAsP, which also functions as the waveguide layer of the device, a p-InP confinement layer82, a p+-InGaAs contact layer84, followed by a metal contact86. Carrier recombination occurs in active region80to provide for multiplexed signal amplification as well as lasing action at the clamping signal wavelength, λS, for providing a clamping gain function, which wavelength is within the gain bandwidth of SOA70but outside the wavelength grid of wavelengths, λ1. . . λN, to be amplified by the SOA. See for example, published patent application EP 0639876A1, published Feb. 22, 1995, which is incorporated herein by its reference.

FIG. 13illustrates another type of GC-SOA comprising a DBR type GC-SOA90illustrated inFIG. 13, having a plurality of layers formed on a n-InP substrate92comprising, in sequence, confinement layer94, grating layer96with distributed feedback grating regions96A and96C and central gain region96B, smoothing layer98of InP, Q region100comprising an InGaAsP layer or quantum well layers of this quaternary, confining layer102of p-InP, contact layer of p+-InGaAs, and segment contacts106A,106B and106C. See the article of D. Wolfson et al., entitled, “Detailed Theoretical Investigation of the Input Power Dynamic Range for Gain-Clamped Semiconductor Optical Amplifier Gates at 10 Gb/s”,IEEE Photonic Technology Letters,Vol. 10(9), pp. 1241–1243, September, 1998, which article is incorporated herein by its reference. With respect to this article, either a DBR GC-SOA with active DBR grating regions or passive DBR grating regions may be utilized. In the case of a passive type, contacts106A and106C to the DBR grating regions would not be pumped. However, in the case of an active type, contacts to the DBR grating regions106A and106C would be pumped. The latter case is preferred because the currents, I1and I3, can be varied to tune the period between these grating regions in order to selectively tune the laser clamping signal. In this manner, the tuning of the clamping signal can be easily accomplished outside of the amplification bandwidth of the incoming signals, λ1. . . λN, thereby enhancing the manufacturing yield of chips10incorporating these types of devices. Current I2adjusts the gain of the SOA. Such an arrangement for adjusting the gain can include the adjustment of all three contacts106A,106B and106C and also by providing a difference in the currents I1and I2with I1remaining constant, for example, will also adjust the gain of the amplifier.

It is within the scope of this invention that grating regions96A and96C inFIG. 11can be chirped so that through the employment of multi-segment contacts106A,106B and106C the tunability range of the clamping signal can be greater over a larger range of tunable wavelengths.

FIG. 14illustrates a still further type of GC-SOA comprising, in this case, a SOA110. SOA110is shown here as a discrete component but is integrated into RxPIC10, and is provided at its input with an injected gain clamping signal along with the multiplexed multi-wavelength signals, I1. . . λN, from the optical link. The gain clamping signal may be optically coupled to the input of chip10from an external source or, alternatively, may be integrated in a separate region of RxPIC chip10and optically coupled into the input waveguide, such as, for example, waveguide48inFIG. 7A, into SOA110. SOA110comprises n-InP substrate112upon which is epitaxially deposited lower confinement layer114of n-InP, waveguide core and active region116of Q, either an InGaAsP layer or multiple quantum wells of this quaternary, upper confinement layer of p-InP118, and contact layer119of p+-InGaAs. In this embodiment, the clamping signal can also be tuned or changed. Also, it is within the scope of this invention to also tune this clamping wavelength to be at a Raman amplification wavelength with counter propagating of the Raman signal so that it can be coupled out of the RxPIC chip10via its input port for coupling into the optical fiber link and counter propagating therein to amplify the incoming signals, λ1. . . λN.

Reference is now made toFIG. 15which is a further embodiment of a GC-SOA110A that may be integrated into RxPIC chip10comprising this invention. GC-SOA110A comprises a vertical cavity surface emitting laser (VCSEL) having a n-InP substrate112A upon which is epitaxially deposited (following the initial deposit of a n-InP buffer layer) a stack of semiconductor DBR layers comprising bottom mirror127, confinement layer116A of n-InP, Q waveguide layer118A, spacer layer120A of InP, active region122comprising a Q layer or a Q-QW region, confinement layer124of p-InP, a stack of semiconductor DBR layers comprising top mirror128and a contact layer123of P++-InGaAs. Bottom and top DBR mirrors127and128may be comprised of 20 to 50 layers of InAlGaAs layers of alternating different mole fractions, or alternating layers of InAlAs/InGaAs or InAlGaAs/InP or InGaAsP/InP. This is followed by the necessary metallization comprising p-side metal contact125and metal contact129. Operation through current and bias of GC-SOA110A provides lasing action producing gain clamping signal117A between mirrors127and128providing gain at active region122for amplification of the channel signals, λ1. . . λN, propagating in waveguide layer118A. An advantage of deploying VCSEL GC-SOA110A is that the portion of the gain of clamping signal117A not utilized can exit the chip via the top or bottom of amplifier110A since DBR mirrors127and128are not 100% reflective. On the other hand, amplifier110A is more difficult to fabricate than many of the other embodiments disclose herein. There are several other ways of eliminating the unutilized gain of the clamping signal which will be described latter.

There is also a Mach-Zehnder (MZ) type of SOA that may be utilized in this invention. In this case, the clamping signal can be coupled into the MZ-SOA.

It should be noted that in all of the forgoing embodiments of this invention, the gain clamping signal can be either on the long wavelength (red) or short wavelength (blue) side of the bandwidth or wavelength spectrum of the multiplexed channel signals.

Reference is now made toFIGS. 8A,8B and8C which respectively disclose representative InP-based embodiments, in cross-section, comprising a DFB GC-SOA120, a photodetector, shown here as a PIN photodiode16, and a grating arm34of AWG30. InFIG. 8A, DFB GC-SOA120comprises a first epitaxial growth process on n-InP substrate112upon which is epitaxially deposited in sequence a n-InP buffer layer124, a lower confinement layer126of n-InP, Q grating/waveguide layer128(bandgap of about 1.3), an InP spacer layer130(bandgap of about 1.4), active/waveguide region of Q-QW layers132(bandgap of about 1.5), and upper confinement layer134of p-InP. Next, a selective etchback is performed with masking of the yet to be defined current confinement region129, followed by a second epitaxial growth process comprising blocking layers which comprise a first layer131of n-InP followed by semi-insulating (SI) layer133of InP:Fe, InP:O or InP:O:Fe, and a third layer141of p-InP. Masking over the current confinement region129of the formed device is removed and a third epitaxial growth process is commenced comprising cladding layer135of p-InP, followed by contact layer136of p+-InP, which may be optional here but is desired elsewhere in the integrated chip, and thence contact layer138of p+-InGaAs. Device120is completed with p and n electrodes138and139. The channel signals propagate along active layer132with evanescent overlap with waveguide layer128and these signals absorb gain and are amplified. Layer128also includes a DFB grating128A for generation of the gain clamping signal.

It should be realized that the fabrication of GC-SOA120is done in conjunction with other optical components included in integrated form on RxPIC10. Thus, the epitaxial fabrication sequence in the description here may be modified or include other steps so that the layers and structures of such components can be also be added or otherwise realized. This same statement also applies relative to the structures shown inFIGS. 8B and 8C.

In conjunction with the GC-SOA120ofFIG. 8A, reference is now made toFIGS. 31 and 32which illustrate input mode adaptors or converters24that may be employed for GC-SOA12or120. In this connection, with reference toFIG. 31, the mode adaptor taper24is in the vertical plane of PIC chip10whereas, inFIG. 32, the mode adapter24is tapered in the horizontal plane of PIC chip10, i.e., in the plane of its as-grown layers. InFIG. 31, generally the same layers are shown as in the case of previously describedFIG. 8A. However, with a selective masking technique, such as selective area growth (SAG), taper24may be formed in Q active region132during its growth to provide for an adiabatic expansion of the input light comprising channel signals, λ1. . . λN. In this connection, see U.S. Pat. No. 6,141,477, which patent is incorporated herein by its reference. Patent '477 illustrates such a taper at the output end of an active region rather then the input end of an active region. Such an output taper can be also employed at the output end of GC-SOA12ofFIG. 31.

InFIG. 32, the input tapered mode adapter24is shown in the horizontal plane. In fact, tapering can be provided in both the vertical and lateral directions to provide an input or output mode adapter24or26that adiabatically transform the input or output light as taught in U.S. Pat. No. 6,174,748 relative to output light, which patent is incorporated herein by its reference. In patent '748, the purpose is to produce an output beam of substantially circular mode profile, which need not be the case here as long as the multiplexed channel signals are adiabatically converted to single mode.

Reference is now made toFIG. 8Bwhich illustrates a cross-section of a photodetector that may be utilized in RxPIC chip10. Photodetector140is a PIN photodiode comprising multiple epitaxially deposited layers, some of which are not necessary to its operation but present because of their necessity for other optical component(s) on the same monolithic chip10. The structure comprises, as is the case of GC-SOA120, a first epitaxial growth process of n-InP substrate122upon which is epitaxially deposited in sequence a n-InP buffer layer124, a lower confinement layer126of n-InP, Q layer128(bandgap of about 1.3 with no grating128A provided in this portion of the chip), an InP spacer layer130(bandgap of about 1.4), active/waveguide region of Q-QW layers132(bandgap of about 1.5), and upper confinement layer134of p-InP. Next, a selective etchback is performed with masking of the yet to be defined current confinement region129, followed by a second epitaxial growth process to form current blocking layers comprising a first layer131of n-InP followed by SI layer133of InP:Fe, InP:O or InP:O:Fe and a third layer of p-InP. Masking over the current confinement region129of the formed device is then removed and a third epitaxial growth process is commenced comprising cladding layer135of p-InP. This is followed by the deposition of contact layer136of p+-InP and dielectric passivation layer137. Contact layer136is defined by dielectric layer137which may be SiO2or other such dielectric. The device is completed with a p-side metal contact138and a n-side metal contact139. It should be noted that GC-SOA120is substantially the same as PIN photodiode140except that waveguide layer128contains no grating at photodiode140. Q layer128at photodiode140functions as a carrier depletion device by being reversed biased and generating a current signal proportional to the light entering the photodiode. Also, as will be realized from the previous description of RxPIC chip10, there are N such photodiodes140(or16) formed on the chip.

Reference is now made toFIG. 8Cwhich illustrates the epitaxially deposited layers for AWG130such as AWG arrayed arms34as well as representative of cross-sections of input and output waveguides37and39.FIG. 8Crepresents an exemplary cross-section of any of these waveguide structures comprising AWG130. The slab or space regions32and36of AWG30would have the same composite structure except the waveguiding region would have a larger extent in the lateral direction. As in the case of the optical components120and140inFIGS. 8A and 8B, a first epitaxial growth process on n-InP substrate122comprises epitaxial deposition, in sequence, a n-InP buffer layer124, a lower confinement layer126of n-InP, Q waveguide AWG layer128(bandgap of about 1.3 with no grating in this portion of the chip), an InP spacer layer130(bandgap of about 1.4), active region of Q-QW layers132(bandgap of about 1.5 but having no direct function in AWG30), and upper confinement layer134of p-InP. Next, a selective etchback is performed with masking to define waveguide structure127, followed by a second epitaxial growth process comprising cladding layers (in previously described structures ofFIGS. 8A and 8Bfunctioning as blocking layers) comprising a first layer131of n-InP followed by SI layer133and a third layer135of p-InP. It is within the scope of this invention that waveguide127be not covered, i.e., it can be an air-exposed waveguide or, alternatively, other layers may be utilized for burying waveguide127, such as, SiO2, glass (silica), BCB, ZnS or ZnSe as examples.

The preceding described embodiments ofFIGS. 8A(GC-SOA),8B (PD) and8C (AWG) are examples of buried types of devices. On the other hand, these devices can be deep ridge waveguide devices as illustrated, for example, inFIGS. 65A,65B and65C. GC-SOA330inFIG. 65A, along with AWG350inFIG. 65B, comprise a n-InP substrate332upon which are epitaxially deposited n-InP confinement layer344, Q-grating/waveguide layer336(with grating layer336A and smoothing layer336B), and undoped InP layer338. This is followed by the growth of active region340in GC-SOA330employing selective area growth (SAG) so as to taper this layer at331as shown inFIG. 65C. This is followed next with the growth of p-InP confinement layer342over both GC-SOA330and AWG350. Then an etchback is performed to form the deep ridge waveguide structures as shown inFIGS. 65A and 65B. Note that the ridge is formed back through the waveguide core336to provide for birefringence at the AWG and ease of manufacturability, i.e., providing for less epitaxial growth steps. These structures may be exposed to air or covered with a high refractive index material such as BCB, ZnS or ZnSe.

To be noted inFIG. 65C, the waveguide layer336is continuous. However, through several epitaxial growth steps, the regrowths provide for lateral guiding with an index step optimized for birefringence at the AWG and single mode guiding at the GC-SOA or the photodetector. In this connection, reference is now made toFIG. 66.FIG. 66shows a longitudinal cross-section of RxPIC10comprising input mode converter400, GC-SOA360, AWG380and PIN photodetectors390(N). A typical process for fabrication of this structure is a first growth process comprising the deposition of a n+-InP buffer layer364followed by the deposition of a Q-grating layer366, followed by an n-InP cap or stop etch layer (not shown due to subsequent removal). Next, a selective dry etch employing a photoresist mask is made in Q layer366to form DFB grating367for GC-SOA360. Next, a second growth process is initiated comprising an undoped-InP planarization layer368to planarize over grating367. This is followed by a third growth process comprising the deposition of Q-waveguide layer370and thence an n-InP cap or stop etch layer (not shown due to subsequent removal). Then a photoresist mask is applied to the area of AWG380and the AWG is defined via selective etching. Then, waveguide layer370over the regions comprising GC-SOA360and PIN photodetectors390are etched away. This is followed by a fourth epitaxial growth process for overgrowing the grating367as well as forming active region374for both GC-SOA360and PIN photodetector390. First, an undoped InP layer372is deposited followed by a Q-active layer or MQW active region374, followed by p-InP layer376and contact layer378of p+-InGaAs. These layers372,374,376and378are then etched over mode converter400and AWG380. Subsequent processing provides for a buried waveguide structure such as shown inFIGS. 8A,8B and8C or a ridge waveguide structure such as shown inFIGS. 65A and 65B. The etched regions382over mode converter400and AWG380may be overlaid, for example, with InP:Fe, InP:O, BCB, ZnS or ZnSe. Arrow lines385inFIG. 66shows the path of the channel signal mode as it propagates through the one illustrated signal channel of RxPIC10.

FIGS. 16–22,37and54relate to various ways of either eliminating the amplified spontaneous emission (ASE) or residual laser gain clamping signal, or both, from RxPIC chip10. The residual gain clamping signal and ASE generated by the amplifying function of GC-SOA12or120is undesirable on chip10as it will interfere with the accurate detection functioning of photodiodes16or120. When current is injected into GC-SOA12or120, ASE is emitted which is optical noise that interferes with the detection response of photodetectors16as well as providing reflected light back into GC-SOA12. Also, the DFB grating generated laser light to maintain the gain of GC-SOA12is not totally utilized and, therefore, propagates out of the amplifier to AWG30. These figures illustrate approaches to eliminate this noise from RxPIC chip10.

InFIG. 16, reliance on AWG30per se to filter out this noise is selected. In the case here, the AWG30must be designed to function as such a filter so that the cone filter function is limited strictly to wavelengths within the wavelength grid of the channel signals. Wavelengths outside this spectrum, such as ASE at higher frequencies or a laser clamping signal at a higher or lower wavelength not within this spectrum, is rejected by the narrow band filtering of AWG30. In this case, AWG30must be designed to have a large free spectral range (FSR) to filter out the GC-SOA clamping wavelength signal.

As illustrated inFIG. 16A, the input waveguide37from GC-SOA12can be provided with a high angular bend at37A along which the signal wavelengths can be guided but not the higher wavelengths of ASE or of a gain clamping signal if of sufficiently higher wavelength than that of the channel signal spectrum. Otherwise, where the laser gain clamping signal is a shorter wavelength than the channel signal spectrum, other means may be necessary to remove this signal from chip10.

The free spectral range (FSR) of AWG30can, thus, be designed so as to filter the gain clamping signal propagating from GC-SOA12to AWG30. If the wavelengths of the modes are very different, then the FSR of AWG30will have to also be large as well. This can therefore become a design constraint for the AWG. Thus other measures will have to be taken to rid the RxPIC chip10of this residual clamping signal, which is to be described in several subsequent embodiments.

FIG. 17illustrates another way of filtering out ASE and the laser gain clamping signal. In this illustration, a bank of on-chip SOAs31(1) . . .31(N) are integrated in each of the output waveguides39of AWG30to provide immediate gain to the demultiplexed channel signals which have experienced some insertion loss. These SOAs31are followed by in-line filter devices33(1) . . .33(N) which may be on-chip angled or blazed gratings with broadband reflective gratings within the bandwidth of the clamping signal and ASE to, not only eject the ASE and clamping signal light from GC-SOA12from chip10, but also eject the ASE generated by SOAs31. Thus, these unwanted wavelengths which are noise are eliminated from proceeding on with the demultiplexed channel signals to photodetectors16(1) . . .16(N).

FIG. 18is a further embodiment for rejection of ASE and the residual gain clamping signal employing architecture similar toFIG. 17except that the input amplification to the multiplied channel signals is a Raman or a rear earth fiber amplifier35, such as an EDFA, as opposed to the employment of GC-SOA12or120. Here, instead of on-chip initial amplification via GC-SOA12of the multiplexed channel signals, an off-chip booster fiber amplifier35is utilized. (This is similar toFIG. 67, previously described). In the case here, SOAs31(1) . . .31(N), shown inFIG. 18, are optional. In-line filters33(l) . . .33(N) each have a filter band that passes the demultiplexed channel signal but ejects the ASE and the residual clamping signal from chip10. As shown in the embodiment ofFIG. 19, no initial booster amplification of the multiplexed channel signals may be necessary so that, after channel signal demultiplexing, the individual channel signals may be amplified via SOAs31(1) . . .31(N) and the ASE and other optical noise removed by in-line filters33(1) . . .33(N) formed on chip10. Alternatively, as shown inFIG. 19A, in a side view of RxPIC chip10, output waveguides39from AWG30include SOAs20(N) which have a design essentially the same as a PIN photodiode, such as the photodiode140inFIG. 8C. SOAs20(N) are inserted in each waveguide39to provide for channel signal amplification due such as to insertion loss. The amplified channel signal then proceeds into a respective angled grating183(N) which functions as a narrow passband filter for reflecting the channel signal upwardly or transversely of PD16. Grating183(N) is transparent to the ASE and other optical noise such as residual clamping lasing signal so that these different wavelengths exit the chip as shown at185.

Reference is now made toFIG. 20illustrating a still further way of rejecting ASE and the residual clamping signal from RxPIC chip10. In this case, compared to the embodiment ofFIG. 19A, the angled or blazed grating filter33A here is deployed on the input side of AWG30rather on its output side where a single grating filter39is designed to reflect the bandwidth spectrum37A of the multiplexed channel signals. The ASE and clamping signal are outside this spectrum and, therefore, are transparent to filter39and, as a result, are transferred through the filter and out of chip10as shown at185. A heater33B may be associated with filter39to tune the bandwidth of the grating to better match the wavelength spectrum of the channel signal grid to the wavelength grid of AWG30.

FIG. 21illustrates another embodiment for extraction of the ASE noise from chip10by employing a Mach-Zehnder interferometer (MZI)41in chip10between GC-SOA12and AWG30. Since spontaneous emission from GC-SOA12is not coherent, such emission cannot be guided through MZI41and, therefore, functions as a filter for receiving only coherent channel signal wavelengths. The residual clamping signal can be filtered by AWG30, as in the embodiment shown inFIG. 16or by the employment of angled grating filters31(1) . . .31(N) ofFIGS. 17–19.

InFIG. 22, an additional AWG43is employed as a filter mechanism for ASE and the residual gain clamping lasing signal. In this embodiment, the channel signals, λ1. . . λN, are demultiplexed via AWG30and passed along output waveguides39through SOAs31(1) . . .31(N), to cover for insertion loss, to NxN AWG43which provides a narrow signal passband rejecting any wavelengths outside the channel signal spectrum. The channel signals are then forwarded via waveguides39A to their respective photodetectors16(1) . . .16(N) for optical to electrical signal conversion.

InFIG. 33, the residual gain clamping signal is removed by forming in the AWG input waveguide37from GC-SOA12a higher order angled or blazed grating170which deflects the higher or lower wavelength gain clamping signal, outside of the wavelength spectrum of the channel signals, out of RxPIC chip10. Grating170can be part of GC-SOA12. The filtering out of the ASE in this embodiment would be accomplished in AWG30as described in the embodiment ofFIG. 16. It is important that waveguide37be single mode so that grating170functions to eject the gain clamping signal from chip10.

FIG. 34is similar to the embodiment ofFIG. 33. In the plan view ofFIG. 34, InP-based RxPIC chip10comprises a GC-SOA12with an output coupled to a mode adapter26and a waveguide182. Waveguide182includes higher order grating180with an integrated heater184and PIN photodiode17positioned in the same planar level to receive light reflected from grating180. In the case here, the grating180has a peak wavelength that is substantially the same as the peak wavelength of the residual gain clamping signal generated by GC-SOA12. The residual gain clamping signal is, therefore, deflected out of waveguide to PIN photodiode17where it is detected and provides an electrical signal off-chip to monitor the optical characteristics of the gain clamping signal, such as, for example, its intensity and wavelength so that adjustments can be made, if necessary, to the applied bias of GC-SOA12.

FIG. 35is similar toFIG. 34except that it is a view perpendicular to the view ofFIG. 34and illustrates, in cross-section, angled grating filter180in waveguide182. A lateral waveguide188is provided in the same as-grown layer as waveguide182to direct the residual gain clamping signal laterally to an etched trough or groove187formed in InP chip10. Trough187has an angled surface at 45° with a deposited reflective surface189formed on the angled surface to reflect, along its length, the gain clamping signal upwardly at 50 (or possibly downwardly depending at what vertical position photodiode17is integrated into chip10) to an optical aligned photodiode17integrated in chip10. Trough187may be etched by employing RIE. The space of trough187may be filled with air or contain some others low refractive index medium.

Instead of the integrated photodiode17being directly vertical (FIG. 35) or directly lateral (FIG. 34) of clamping signal filter180, photodiode17can be offset transversely in chip10from the position of grating filter180as illustrated inFIG. 36. Also, instead of photodiode17being directly above or transversely of waveguide182, photodiode17can be positioned below waveguide182, as shown inFIG. 37, and the reflected light from angled grating filter180is directed downwardly at an angle, employing a second order or higher order integrated grating183to reflect the residual clamping signal downwardly to integrated PIN photodiode17. Such a second order grating183can be in the same semiconductor layer in which filter180is formed or in a different or separate semiconductor layer.

A final approach for removing the residual gain clamping lasing signal is to employ this signal for pre-amplification of the oncoming channel signals, provided that the gain clamping signal is also chosen to be within the absorption spectrum of the channel signals. This illustrated inFIG. 54where the gain clamping is reflected back to mode adapter26and propagates out of the front facet of chip10, indicated as λR, and into the fiber link to counter-propagate in the link. With λRdesigned to be within the absorption bandwidth of the incoming channel signals, these signals will receive gain from its counter-propagation. Rather than a built-in reflector at the downstream end of GC-SOA12, a quarter wavelength shift to the channel signal can be provided in the grating of GC-SOA12or a multiple of that wavelength over 4N, so that most of the power of the gain clamp signal not utilized in the amplifier will be directed out of the back or input port of chip10into the fiber link.

In connection with the foregoing embodiment ofFIG. 54, it is within the scope of this invention to provide on-chip laser pumps to provide for counter-propagation of gain into optical link to provide for initial amplification of the incoming channel signals. This is illustrated in the embodiments ofFIGS. 44 and 45. InFIG. 44, an on-chip semiconductor Raman pump laser230is provided at one of the remaining first order outputs or higher order Brillouin zone outputs of AWG30to provide counter propagating signal, λR, through AWG30and GC-SOA12into the fiber link to provide for pre-amplification of the incoming channel signals. Such a pumping signal is transparent to the operation of GC-SOA12. Alternatively, as shown inFIG. 45, an on-chip semiconductor Raman pump laser232could be coupled into AWG input waveguide37to provide counter propagating signal, λR, through SOA12A into the fiber link to provide for pre-amplification of the incoming channel signals. Note here, that an SOA12A is denoted rather than a GC-SOA since the Raman lasing pump laser signal can provide the on-chip gain clamping. Such Raman pumps could be provided at both such locations if desired. Also, and importantly so, it should be noted that Raman pump laser232, whether an on-chip semiconductor Raman laser or off-chip Raman fiber amplifier coupled in a waveguide39of AWG30or waveguide37, can be deployed instead of having an on-chip GC-SOA12or SOA12A so that RxPIC chip10, in this embodiment, would be comprised of Raman pump laser232, AWG30and photodetectors16(1) . . .16(N).

Reference is now made toFIG. 46, which discloses an on-chip signal monitoring circuit and transmitter laser for providing a service channel signal, λS. Electro-optical circuit234is coupled to one of the first order outputs or higher order Brillouin zone outputs of AWG30and monitors the channel signals, via AWG output233, for their peak wavelength value to determine if the channel signals are on the peak wavelengths and, if not, to provide digitized information in service channel signal, λS, back to a correspondent optical transmitter about the quality of the channel signal wavelengths relative to the standardized wavelength grid at the optical transmitter. As shown alternatively inFIG. 47, such an on-chip electro-optical circuit236may also be provided with its input235into waveguide37to counter-propagate service channel signal, λS, through GC-SOA12and into the optical link.

In connection with circuit234or236ofFIGS. 46 and 47, reference is now made toFIG. 64illustrating an application of these types of circuits in an optical communication system. The system shown inFIG. 64comprises an optical transmitter PIC (TxPIC) chip300optically linked in an optical point-to-point transmission system via optical link288to RxPIC chip10. TxPIC300comprises a plurality of integrated components in plural paths to an AWG multiplexer310where each such path includes a DFB laser source302, an electro-optical modulator306and a SOA308(optional) coupled to an input of AWG310. Each laser source302is operated cw at a designated peak wavelength corresponding to a standardized grid, such as the ITU grid. The output of each laser source302is modulated with an information signal at its respective modulator306. Modulator306may be, for example, a semiconductor electroabsorption (EA) modulator or a Mach-Zehnder (MZ) modulator as known in the art. The modulated signal may then be provided with gain via SOA308. SOAs308are optional and are preferred not to be an on-chip optical component because the overall power consumption of TxPIC chip300will be less without them since most of the on-chip power consumption will come from the operation of SOAs308. In the absence of SOAs308, DFB sources302will have to be operated at higher thresholds and operating currents. The output of AWG multiplexer310is coupled off-chip to optical link288.

In order to operate TxPIC chip300in a stabilized manner, each DFB source302is provided with a corresponding, integrated heater304and AWG310is provided with TEC310A. A small sample of the multiplexed channel signal output from AWG310is provided through a 1% tap, for example, and is provided as an electrical signal input, via optical to electrical domain conversion at on-chip PD312, on line311to programmable logic controller (PLC)316. PLC316discriminates among the different channel signals, λ1. . . λN, to determine if the operating wavelengths of DFB sources are at their desired wavelength peaks as determined by reference to a peak wavelength reference memory. This discrimination process can be carried out by employing dithering signals on the modulated channel signals providing each such signal with an identification tag. As a result, each of the channel signals can be separated and analyzed as to its wavelength to determine if it is at a proper wavelength relative to a standardized grid, such as the ITU grid. Such a discrimination scheme is disclosed in provisional patent application, Ser. No. 328,332, filed Oct. 9, 2001 and entitled APPARATUS AND METHOD OF WAVELENGTH LOCKING IN AN OPTICAL TRANSMITTER SYSTEM, which application is owned by the assignee herein and is incorporated herein by its reference. If the peak wavelength of any particular DFB laser source302is off, its operating wavelength is corrected to the desired peak grid wavelength by a signal provided from PLC316to heater control circuit (HCC)320which provides a temperature control signal to a corresponding laser source heater304for increasing or decreasing the operating temperature of its DFB laser source302by an amount necessary to increase or decrease its operating wavelength to be substantially the same as desired and stored peak wavelength.

The temperatures of DFB laser sources302are not monitored but the temperature of AWG is monitored with a thermistor313which provides PLC316current information of the AWG ambient temperature via input315. PLC316can then provide a control signal to heater control circuit (HCC)318to provide a temperature control signal to TEC310A to increase or decrease the ambient temperature of AWG310. In this manner the wavelength passband grid of AWG310may be shifted and adjusted to optimize it to be as close as possible to the standardized grid and the wavelength grid of DFB laser sources304.

Also, the input side of AWG310includes a port317relative to a higher order Brillouin zone of the input side of AWG310for the purpose of receiving a service signal, λS, from RxPIC10via optical link288, which is explained in further detail below. This service signal is demultiplexed by AWG310and provided on port317as an output signal and thence converted to the electrical domain by integrated, on-chip PD314. The electrical signal from PD314is taken off-chip and provided as an input319to PLC316.

At RxPIC chip10, AWG demultiplexer30includes higher order Brillouin zone outputs289A and289B to receive respective channel signals, such as, for example, λ1and λ2or any other such signal pairs, in order to determine if their grid wavelengths are off the desired peak wavelength and, if so, by how. much. Also, using these two channel signals as a wavelength grid sample, a determination can be made as to whether the AWG wavelength is shifted and, if so, by how much. Photodetectors290A and290B provide an electrical response to optical signals on outputs289A and289B which signals are provided on lines291A and291B to PLC292. These PDs290A and290B are sensitive to the peak optical responses of these signal outputs and can be deployed in the electrical domain to determine if their peak wavelengths are off a desired peak wavelength. Also, if the delta shift,6, of both is approximately same amount and in the same direction (both either a red shift or a blue shift relative to their desired wavelength peak), this delta shift is indicative that a shift in the wavelength grid of either Rx AWG30or possibly Tx AWG310has occurred. In these cases, PLC292can first make adjustment to the RX AWG grid via heater control circuit (HCC)294via line295to Rx TEC30A to either increase or decrease the ambient operating temperature of AWG30to shift its wavelength grid either to the longer or shorter wavelength side based on the determined delta shift. If this adjustment does not resolve the issue, then data relating to either the DFB channel signal wavelengths or the Tx AWG wavelength gird being offset from its desired setting can be forwarded over optical link288as a service channel signal, λS, for correction at the transmitter end. In these circumstances, PLC292can forward such wavelength and grid correction data as a service channel signal, λS, via an electrical correction data signal on output line293to service signal modulator296, which may be comprised of an on-chip integrated DFB laser and EO modulator, to provide this signal through AWG30and counter propagation via optical link288to TxPIC300. This service channel signal, λSis then demultiplexed via AWG310and provided on higher order output317to PD314. The electrically converted service signal data is deciphered by PLC316which makes a correction to the operating wavelength of a DFB laser source302via HCC320and/or makes a correction to the wavelength grid of AWG310via HCC318.

Reference is now made toFIG. 69where the RxPIC chip10and the TxPIC chip300ofFIG. 64are deployed as an optical-to-electrical-to-optical (OEO) converter400for optical signal regeneration in an optical transmission link. As shown inFIG. 69, the incoming multiplexed channel signals, λ1. . . λN, are received by RxPIC chip10from optical link408, demultiplexed and converted into corresponding electrical channel signals and provided through low impedance coupling lines403to electronic regenerator401comprising a plurality of circuits402,404and406, which may be comprised of a chip set for each demultiplexed electrical channel signal received from RxPIC10. Circuit402comprises a transimpedance amplifier and a limiting amplifier. Circuit404comprises retiming and reshaping circuit where the bit clock is extracted from the signal to reclock the signal and regenerate the channel signal. The regenerated electrical channel signal may be further amplified via circuit406and provided as an output on low impedance lines407to the electro-optical modulators306of TxPIC chip300where the corresponding optical signals are again regenerated, the plural channels are multiplexed via AWG310and provided as multiplexed channel signal, λ1. . . λN, on optical link410. OEO converter400has the advantage of being cost effective, compact, easily field-replaceable compared to previous OEO converters and eliminates of the problems of optical-to-optical converters comprising erbium doped fiber amplifiers (EDFAs), functioning as line amplifiers, that need continuous attention relative to saturation where if the input signal power increases or decreases, the amplifier gain drops or increases. Also there is a problem of gain nonflatness across the channel signal wavelength band so that gain equalization techniques need to be provided. Also, these optical line amplifiers, while being the choice today for transmission line optical signal amplification, they are not as compact or readily replaceable as converter400particular when changes are made to increase the number of multiplexed channels and traffic to carried over an optical link. OEO converter400ofFIG. 69eliminates this problems and considerations particularly since the converter can be easily swapped in between optical links408and410with a converter having larger channel capacity.

An important feature of RxPIC chip10is the monolithic incorporation of optical components, in particular GC-SOA12and AWG30, which can provide polarization independent gain to the channel signals and function as a polarization insensitive waveguide grating router or demultiplexer. In GC-SOA12, either the stable lasing in the TE mode or TM mode to provide the DFB clamping signal is preferred. It should be stable over the life of GC-SOA12or chip10, i.e., discrimination between the TE mode and the TM mode should be made large. If both TE and TM modes lase or alternate between lasing in these polarization modes, this will lead to unstable operation of GC-SOA12. This can also manifest itself in gain variation and additional noise in GC-SOA12.

Relative to GC-SOA12, one way of accomplishing polarization independent gain as incorporated in a PIC is to provide for the active region, such as active region132in GC-SOA inFIG. 8A, to have alternately strained tensile and compressive multiple quantum wells of Q (InGaAsP) to balance the polarization dependent gain across the plural wells. Thus, if six such wells are utilized in active region132, three wells are tensile strained and three wells are compressively strained and the former are alternated with the latter. In this regard, see the article of M. A. Newkirk et al., entitled, “1.5 mm Multiquantum-Well Semiconductor Optical Amplifier with Tensile and Compressively Strained Wells for Polarization-Independent Gain”,IEEE Photonics Technology Letters,Vol. 4(4), pp. 406–408, April, 1993, which article is incorporated herein by its reference. Another approach is to potentially utilize the technique suggested in U.S. Pat. No. 5,790,302, which patent is incorporated herein by reference, where a two part grating would be utilized which has a minimum reflection at a first wavelength, which is also minimum for TE portion of the light, and a minimum reflection at a second wavelength, which is also minimum for TM portion of the light, and a product of these reflections is a minimum for both wavelengths and optimized at an intermediate wavelength so that the resulting TE and TM modes will be substantially the same.

A further way of rendering GC-SOA12polarization insensitive, which is shown inFIGS. 38 and 39, is to employ a λ/4 grating, shown at171inFIG. 38, or two λ/8 gratings, shown at173inFIG. 39, in the gain clamping grating of GC-SOA12. These gratings can suppress the stronger of the two TE and TM modes to render them more substantially the same. Another way, illustrated inFIG. 40A, is to employ a loss refractive index grating to enable one of the two modes over the other mode. This is accomplished by making the grating layer128to have a bandgap similar to that of active region132so that the grating will function as a selective loss for one of the modes thereby enhancing the other mode. On the other hand, by making the grating layer128to have a significantly larger bandgap compared to that of active region132, then the grating will function to be index selective of one of the modes while suppressing the other mode. A further approach is to perturb the active region132itself employing a grating128A, as shown inFIG. 40B, to provide selective gain for one of the modes.

A still further way of ridding or otherwise suppressing on of the polarization modes in the GC-SOA12is by employment of an AR coating as illustrated inFIG. 41. The AR coatings applied to the input and output regions of GC-SOA can be designed to favor one polarization mode over the other since these AR coatings190are typically broadband. This embodiment would be best utilized where GC-SOA12and AWG30are separate, discrete optical components.

Another way of ridding or otherwise suppressing one of the polarization modes in the GC-SOA12is through the employment of a grating shape that is tailored to favor one polarization mode over another. As shown inFIG. 42, this is illustrated as a square wave form192but it could be specifically tailored via other grating shapes, such as triangular or a waveform similar to sinusoidal, to accomplish the same result.

Also, as illustrated inFIG. 43, the grating in the grating layer191could be made to be a second or higher order grating where the plane of scattering is relaxed for one of the polarization modes, that is, βTEis not equal to βTM. With the first order grating, the direction of mode scattering is in the plane of the waveguide. However, with a second or higher order grating, one of the modes, shown at194, can be preferentially scattered transversely out of waveguide193while the other mode196remains in waveguide193.

Lastly, a frequency selective feedback optical element that selects between the TE mode and the TM mode can be utilized in the RxPIC chip10as incorporated in waveguide37between GC-SOA12and AWG30.

With respect to polarization insensitivity at AWG30, reference is made toFIG. 23where AWG30comprises at least one input waveguide37and a plurality of output waveguides39between which are space regions32and36and N grating arms34. Also, shown are higher order Brillouin zone input arms37A and higher order Brillouin zone output arms39A. As shown in the cross-section inFIG. 8Cof an AWG waveguide34, the fabrication process generally lends itself to geometrically forming rectilinear shaped cross-sectional waveguide structures. Thus, the TM and TE modes of the signals will be favored one over the other. Only way of solving this problem is illustrated in U.S. Pat. No. 5,623,571, which is incorporated herein by its reference. As is known, the TM mode will propagate faster through grating arms34than the TE mode. What can be done is slow down the propagation of the TM to equal, in phase, the propagation of the TE mode. A patch30E is made in the overlying cladding layer or top glass layer over the waveguide, i.e., some of the overlying layer is removed in patch region30E to increase the birefringence in region30E relative to the birefringence remaining in other overlying regions of the same grating arms34. As a result, a balance can be achieved in the propagation phase between the TE and TM modes so that an in-phase relationship between these modes is maintained dependent on the path lengths of the arms in region30E versus those portions outside of this region for the same arrayed arms. One way of determining the extent and depth of patch30E is providing a separate wavelength, λT, in higher order inputs37A and monitor those wavelengths at higher order outputs39A to look at the polarization characteristics of the these signals to determine what depth must be etch for patch region30E to appropriately change the TE/TM ratio and achieve polarization independence of AWG30.

Another approach to achieve polarization insensitivity in AWG30is to provide a fabrication technique that provides for nearly square cross-sectional arrayed waveguides so that AWG30will have substantially zero birefringence waveguides. This is described and taught in the articles of J. Sarathy et al., entitled, “Polarization Insensitive Waveguide Grating Routers in InP”,IEEE Photonics Technology Letters,Vol. 10(12), pp. 1763–1765, December, 1998, and in J. B. D. Soole et al., entitled, “Polarization-Independent InP Arrayed Waveguide Filter Using Square Cross-Section Waveguides”,ELECTRONIC LETTERS,Vol. 32(4), pp. 323–324, Feb. 15, 1996, both of which are incorporated herein by their reference.

Reference is now made toFIG. 24which illustrates another approach for achieving polarization insensitivity through the incorporation of SOAs45(1) . . .45(N) in each of the arms34of AWG30. As is known, the TM mode will propagate faster through grating arms than the TE mode. What can be done is slow down the propagation of the TM to equal in phase the propagation of the TE mode. This was done inFIG. 23by using patch30E. Here, effectively it is accomplished by using SOAs which are of different lengths, so that the TM modes in arms34will be attenuated. The amount of required attenuation can be calculated through AWG computer simulation of the individual wavelength channels so that the length of SOAs45can be determined and attenuation of the TM mode over the TE mode can be provided for their equalization in arrayed arms34. In this case, see, for example, the article of M. Zingibl et al., entitled “Planarization Independent 8×8 Waveguide Grating Multiplexer on InP”,ELECTRONICS LETTERS,Vol. 29(2), pp. 201–202, Jan. 21, 1993 and published European patent application EP 0731576A2, dated Sep. 11, 1996, both of which are incorporated herein by their reference. Thus, arms34can be provided to polarization insensitive as well as provide for equalization of signal gain across the wavelength grid of AWG30.

Reference is now made to several embodiments relating to architecture for coupling the electrical signal outputs from bonding pads28of the RxPIC chip photodiodes16to a RF submount substrate or a miniature circuit board or a monolithic microwave integrated circuit (MMIC), with particular reference being made toFIGS. 28,29,30,51and55. InFIG. 28, a side view of a particular configuration for RxPIC chip10is shown in a schematic form showing in integrated form GC-SOA12, AWG30and plural photodetectors or PINs16(1) . . .16(N). Output pads28(1) . . .28(N) of PINs16(1) . . .16(N) are solder bumped to output pads154(1) . . .154(N) of respective transimpedance amplifiers (TIAs)152(1) . . .152(N) formed on MMIC150. TIAs152provide for conversion of the current signals developed by the respective PINs28into voltage signals. MMIC150is also shown here to include a portion153of RF submount150which includes other circuit components as known in the art, such as an automatic gain control (AGC) circuit for increasing the signal strength and range, which circuit can apply a gain control signal to TIAs152or provide the signal across the differential input of TIAs152; a power amplifier (PA) to increase the signal gain from TIAs152; and a clock and data recover (CDR) circuit. A CDR circuit (not shown) recovers the embedded clock from a baseband non-return-to-zero (NRZ) or return-to-zero (RZ) data stream and generates a clean data stream (e.g., data that does not have timing jitter due to, for example, the limited bandwidth of the transmission channel). The clock recovery function of a CDR circuit is typically performed with a phase-locked loop (PLL) which requires a tunable clock signal, such as generated by a voltage controlled oscillator (VCO). This arrangement provides for compactness with RF submount150, carrying receiver electronics in overlying relation to PIC chip10and supported at a bonding point of bonding pads28and154. As a result, RF board150is spatially supported above chip10to provide for a space between them for circulation of air and cooling.

FIG. 55is substantially the same asFIG. 28but a more detailed version ofFIG. 28. As shown inFIG. 55, an optical link is coupled to the input end of PIC chip10, where the received multiplexed signals, λ1. . . λN, are provided to GC-SOA12for amplification and thence via waveguide37provided to AWG30where the signals are demultiplexed and provided on output waveguides39to PIN photodetector array16(1) . . .16(N). The electrical signal outputs from these photodetectors is provided to PIN contact28(1) . . .28(N) which are then solder bonded to corresponding TIA contacts154(1) . . .154(N) providing electrical connection to corresponding TIAs160(1) . . .160(N) and thence to power or limiting amplifiers162(1) . . .162(N). The outputs of power amplifiers162may be provided to other circuit components such as CDR circuits or the electrical signals can be taken off of RF submount150via RF transmission lines163at bonding pads163A.

With reference toFIG. 28A, the arrangement is shown where PIC chip10comprises input GC-SOA12and AWG demultiplexer130. The multiplexed signal outputs, λ1. . . λN, are mirrored off of chip10by 45° mirror155. Note that, in this embodiment, the PIN photodiodes16(1) . . .16(N) are formed on RF submount150rather then on chip10. The optical signals reflected from mirror155are directed up to the aligned array of PINs16(1) . . .16(N) where the converted electrical signals are directed to corresponding TIAs, PAs and CDR circuits on RF submount150. Also, a lens array on a separate board can be employed between submount150and chip10to aid in focusing the signals, λ1. . . λN, onto the top detection surface PIN photodiodes16(1) . . .16(N) on RF submount150, such as in a manner illustrated in the article of A. E. Stevens et al., entitled, “Characterization of a 16-Channel Optical/Electronic Selector for Fast Packet-Switched WDMA Networks”,IEEE Photonics Technology Letters,Vol. 6(8), pp. 971–974, August, 1994, which article is incorporated herein by its reference. RF submount150is secured to chip10by means of solder ball bonding via solder balls156.

Reference is now made toFIG. 28Bwhich illustrates a further example of an arrangement of chip10and RF submount150. In this arrangement, additional boards are employed comprising submount166and filler board164. Only one end of InP chip10is shown that includes AWG demultiplexer30. A 45° angled edge157is formed along the output edge of output waveguides39from AWG30handling demultiplexed channel signals, λ1. . . λN. The angled edge157is coated with a mirror surface as is known in the art. As in the case of the embodiment ofFIG. 28A, RF submount includes PIN photodetectors16(1) . . .16(N) which are surface photodetectors aligned with the respective signals, λ1. . . λN, where the signals are then processed via the TIAs, PAs162(1) . . .162(N) and CDR circuits on RF submount150. Filler board164and RF submount150provide support for PIC chip10, as secured via ball bonding156, and both filler board164and RF submount150are supported on submount166.

Reference is now made toFIGS. 29 and 30which disclose wire bonded versions of connecting photodetector pads28to RF submount pads159on one or two RF circuit boards. All the electronic RF circuit components are on microwave submounts150,150A and150B as it is easier to control the circuit impedance on these circuit submounts rather than on PIC chip10. Microwave submounts150,150A and150B may be, for example, ceramic submounts. As shown inFIG. 29, photodetector pads28are wire bonded to corresponding bonding pads159on microwave submount150and the signals are then feed into TIAs160and thence on to other circuit components, as previously described, via high speed transmission lines168.

As shown inFIG. 30, in order to save space and provide for more compactness, photodetector bonding pads28may be placed in two or more staggered rows on the edge of PIC chip10and one or more pad rows of pads28A are wire bonded to TIA bonding pads159A on a first RF submount150A which are correspondingly coupled to TIA circuits160A. The remaining pad row or rows of pads28B are wire bonded to TIA bonding pads159B on a second RF submount150B which are correspondingly coupled to TIA circuits160B. By staggering the pad rows on chip10as well as employing more than one RF submount, short bonding wires can be employed so that the inductance relative to the microwave circuits can be minimized. In this connection, high speed transmission boards150A and150B are mounted in spaced relation to RxPIC chip10by a spacing distance greater than 5 μm. Also, the staggered spacing of pads28still allows for good separation between the accompanying photodetectors16while permitting the shrinkage of the overall PIC dimensions, particularly if the staggered rows are provided along two sides of the PIC chip10, as demonstrated for one row of pads28shown inFIG. 6. For example, both the diagonal spacing and the side-by-side spacing of pads28A and28B on chip10may be a minimum of about 250 μm. This spacing is critical to insure minimal crosstalk between channel signals.

Reference is now made toFIGS. 48–51. InFIG. 48, the simple transimpedance amplifier (TIA)200with a feedback resistor201is shown and is well known in the art. The gain of amplifier200is dependent upon the input signal level and the signal current, IS, can vary over a fairly large range. An automatic gain control (AGC) circuit can be coupled to amplifier200to linearly control its gain.

As shown inFIG. 49, the transimpedance amplifier (TIA)202can have a differential output. The differential output helps to reduce the noise at the output of the amplifier. However, the employment of a truly differential input, as illustrated inFIG. 50, can significantly reduce any cross-talk on RxPIC10between the channel signals output. For this scheme to work, a photodiode reference input is also provided to TIA202. As shown inFIG. 50, TIA204has a differential input as well as differential output. Feedback resistors205are provided for both differential inputs to TIA204as well as DC blocking capacitors203are provided in these inputs too. The differential outputs of TIA204are coupled to limiting amplifier206. As shown inFIG. 50, one of the differential inputs is coupled to a respective signal photodetector16and the other differential input is coupled to a respective reference photodetector206which is also formed in RxPIC chip10as shown inFIG. 51, to be discussed next. In the case here, both photodetectors16and206will substantially detect the same noise environment, i.e., photodetector16will detect the signal plus noise and photodetector206will detect the noise scattered in chip10, which is generally crosstalk noise and optical noise from other optical components integrated in the chip. As a result, the crosstalk and noise can be substantially cancelled out via the differential input to TIA204.

As shown inFIG. 51, photodetectors16(1) . . .16(N), for detecting the channel signals via waveguides39are formed along an edge of chip10and are primary photodetectors PDP1, PDP2, etc. Companion photodetectors206(1) . . .206(N), for detecting optical noise, in particular crosstalk noise, are alternating with photodetectors16(1) . . .16(N) and are secondary photodetectors PDS1, PDS2, etc. Also, ground pads208on chip10are connected via bonding wires to ground pads214on the RF submount150. Photodetectors16and206are separated a sufficient amount to prevent undue crosstalk, e.g., about 250 μm or more.

Alternatively, one or less than all of the entire companion photodetectors206can be deployed on RxPIC chip10to provide an optical noise signal for all or more than one primary photodetector16(N), rather than providing one secondary photodetector206(N) in proximity to each and every primary photodetector16(N). Although this will function to establish a useful noise floor for the detected RxPIC channel signals for use with differential TIAs204, the established signal noise floor will not be as accurate as in the case where there is one companion photodetector206for every primary photodetector16since the amount of optical noise at every primary photodetector location on the chip will not necessarily be the same.

It should be noted that inFIG. 51, bonding wires216should be as short as possible to reduce photodetector inductance. Another way of eliminating this inductance is to eliminate these bonding wires216all together by forming TIAs204directly on chip10. This is accomplished by utilizing InP-HBT or InP-HEMT circuitry on chip10.

It is desirable that certain components be included with chips10or formed in an InP wafer with the chip die configuration to provide for testing capabilities. Some examples are shown in connection withFIGS. 52 and 53to be described now.

InFIG. 52, higher order Brillouin zone arms39A and39B are taken off of output space region36of AWG30and angled facets220are provided in a manner similar to facet225shown inFIG. 53, to be next described, so that the signal light of one or more channels can be taken off chip while the chip is still in the InP wafer. The light input is provided to AWG30in a manner as shown inFIG. 53. Thus, the optical output from the angled on-chip reflectors can be detected employing an optical interrogation probe where the output is collected and analyzed to determine if the signals are being properly demultiplexed through AWG30relative to the AWG wavelength grid, the relative intensity of the signals, their peak frequency, etc. This approach saves a great deal of time and expense by eliminating wafers with poor quality optical components without going to the expense of cleaving the wafer into individual die and testing them separately. Alternatively, instead of an angle facet220, a higher order grating can be formed at this point to deflect the signal out of the chip for detection. Also, instead of either an angled facet220or a higher order grating, photodetector or photodetectors222can be fabricated directly on chip and employed to test the channel signal properties while the chip die remain in-wafer. These same photodetectors222can be employed later, after the remove of chip from the wafer for signal monitoring and feedback indicative of the operating wavelength peaks of the channel signals and the amount, if any, that they are off relative to a predetermined wavelength grid, such as standard ITU grid.

It should be pointed out that, in connection withFIG. 52, photodetectors16(1) . . .16(N) of an as-cleaved chip can be initially employed to examine the total signal and differential signal between detectors to tune the wavelength grid of AWG30via TEC30A. The temperature of AWG30is changed so that its wavelength grid best matches the wavelength grid of the channel signals to be demultiplexed. In order to accomplish this grid tuning, it is preferred that at least two of the channel signals need to be detected. Once the AWG wavelength grid has been optimized to a standardized grid, such as the ITU grid, the factory setting for TEC30A is placed in memory of the RxPIC controller circuitry. Such circuitry is beyond the scope of this disclosure and will be detailed in later applications. More is said about this monitoring and adjustment in connection withFIGS. 73,73A and73B.

Reference is now made toFIG. 53illustrating the in-wafer testing of RxPIC chips10. As mentioned previously, it is advantageous to test the RxPIC chips10in-wafer because if the initial testing of a majority of them results in poor performance, time and expense of dicing the chips from the wafer as well as subsequent individual chip testing has been circumvented. Also, such in-wafer testing can be easily automated since the input to each chip is at a known or predetermined location in the InP wafer. As a result, in-wafer testing can be handled in a matter of seconds to a few minutes where individual die testing would take a period of days and, therefore, lead to higher product costs. InFIG. 53, the in-wafer chips or die10include an in-wafer chip sacrificial spacing or region224. Within region224, there is formed an in-wafer groove223having an angular mirror surface225A formed via selective etching. A mirror coating, which is optional, could be also deposited on these surfaces. The angled surfaces225are preferably angled at 45° so that an interrogation beam, normal to the surface of the wafer, may be moved by an automated mechanism over the wafer to surface225to provide an optical signal input, such as a plurality of test pulse channel optical signals into chip10via its chip input. The testing is accomplished by probe testing the outputs of the respective photodiodes16. Alternatively, it should be noted that that grooves223also served at the opposite end of the in-wafer RxPIC chip10serve as a point to detect the rear end light from photodetectors16through an optical pickup such as an optical fiber coupled to an off-chip photodetector. The testing of an on-chip GC-SOA12and AWG30is conducted as indicated relative to the discussion ofFIG. 52or using the probe card as disclosed in U.S. patent application, Ser. No. 10/267/331, filed Oct. 8, 2002 and published on May 22. 2003 as Publication No. US 2003/0095737 A1, which application has been previously incorporated herein by reference. Characteristics that may be checked, for example, include optical power of the gain clamped-SOA or passband response and insertion loss of the AWG. After testing is complete, the region224can be cleaved away from chip10as indicated by the set of cleave lines inFIG. 53.

Reference is now made toFIGS. 57 and 58. Active region240of GC-SOA12may be a multiple quantum well (MQW) region as illustrated inFIG. 57or may be a single active layer, such as illustrated inFIG. 58. InFIG. 57, the MQWs242are compressively strained and the barriers244may be tensile strained to produce an overall strain-balanced structure in which the electron light hole, mostly TM barrier transition is slightly favored over the larger energy electron heavy hole TE well transition. Alternatively, the MQWs242can be alternately tensile and compressively strained. The TE:TM emission ratio can be adjusted by the amount of strain and the number of compressive versus tensile barriers having identical effective bandgaps. Such treatment provides for a polarization insensitive SOA. See Chapter 5, “Semiconductor Laser Growth and Fabrication Technology”, Section IV, “Polarization Insensitive Amplifiers by Means of Strain”, pp. 177–179, in the book entitled, “Optical Fiber Communication -IIIB” (Vol. 2), edited by Kaminow and Koch, Academic Press, published in 1997, which Section is incorporated herein by its reference. For further background, see the articles of M. A. Newkirk et al., entitled, “1.5 mm Multiquantum-Well Semiconductor Optical Amplifier with tensile and compressively Strained Wells for Polarization-independent gain”,IEEE Photonics Technology Letters,Vol. 4(4), pp. 406–408, April, 1993, and of Young-Sang Cho et al, entitled, “Analysis and Optimization of Polarization-Insensitive Semiconductor Optical Amplifiers with Delta-Strained Quantum Wells”,IEEE Journal of Quantum Electronics,Vol. 37(4), pp. 574–579, April, 2001, both of which are incorporated herein by their reference.

InFIG. 58, if a single active layer is employed, the Q active layer246may be tensile strained and the confinement layers248may be compressively strained (the latter is optional).

AWG30can be made substantially temperature insensitive and its wavelength spectrum stabilized over time so that changes in the ambient do not affect changes in the AWG arm lengths thereby changing its narrow passband characteristics. This T-insensitivity can be achieved in a InGaAsP/InP AWG30by employing a high dn/dT array waveguide1.3Q region260and two low dn/dT arrayed waveguide1.1.Q regions262as illustrated inFIG. 59and described in the article of H. Tanobe et al., entitled, “Temperature Insensitive Arrayed Waveguide Gratings in InP Substrates”,IEEE Photonics Technology Letters,Vol. 10(2), pp. 235–237, February, 1998, which article is incorporated herein by its reference. As taught in this article, the difference in the optical path length of any pairs of waveguide arms34in the array becomes longer when the waveguides are heated. This brings about a red shift to the demultiplexed channel wavelength toward a longer wavelength. This effect is cancelled out by employing1.1Q and1.3Q regions260and262providing different sections with different values for dn/dT in the waveguides dependent on a predetermined relationship between their lengths. Alternatively, as shown inFIG. 60, a heater or electrically pumped region264can be provided over a portion of wavelength arms34, operated via a temperature controller, to stabilize the wavelength spectrum of AWG30by changing the refractive index of the arms to compensate for wavelength changes from desired peak wavelength passbands of arms34. Region264can also include one or more regions that are provided with a material or materials overlying AWG30where the materials have different coefficient of thermal expansions, such as decreasing or increasing in length with increasing temperature, to provide a more athermal structure. This can also be accomplished by using heater strips or current pumping stripes266, as shown inFIG. 61, where the applied bias to strips266can be uniform or non-uniform across the strip array, e.g., applied in a monotonically increasing to monotonically decreasing manner across the array. Thus, the applied bias can be selectively and/or independently varied to compensate for temperature driven index changes of the AWG.

Reference is now made toFIG. 70illustrating a forward error correction (FEC) enhanced optical transport network500shown as single direction transmission in a point-to-point optical transmission link including at least one TxPIC502and at least one RxPIC504optically coupled by optical link506. It should be understood that network500can be bidirectional where TxPIC502can also be a transceiver and including a receiver, such as RxPIC504, which transceivers are also illustrated in provisional application, Ser. No. 60/328,207, filed Oct. 9, 2001, now U.S. patent application. Ser. No. 10/267,331 filed Oct. 8, 2002 and published on May 22, 2003 as Publication No. US 2003/0095737 A1, which is incorporated herein. In such a bidirectional network, optical link506would be deployed for use with the eastbound and westbound traffic on different channel wavelengths. TxPIC502includes a plurality of DFB laser sources508(1) . . .508(N) optically coupled, respectively, to electro-optic modulators (MODs)510(1) . . .510(N), in particular, electro-absorption modulators or Mach-Zehnder modulators. The outputs of the modulators510(1) . . .510(N) are optically coupled to an optical combiner or MUX511, e.g., an arrayed waveguide grating (AWG). As shown inFIG. 70, each DFB laser source508has a driver circuit512(1) . . .512(N). Each modulator510(1) . . .510(N) includes a driver514(1) . . .514(N) for input of the bias point of the modulator and the data stream for modulation. FECl. . . FECNencoders518(1) . . .518(N) are used to reduce errors in transmission of data transmitted over network500. These encoders may also be a joint FEC encoder520to jointly encode bit code representative of transmitted data. As well known in the art, the performance of a received data signal is measured deploying an eye diagram, such as shown inFIG. 71A, which will be discussed in more detail later. Further, FECl. . . FECNencoders518(1) . . .518(N) are deployed to reduce the bit error rate (BER) by transmitting on the laser source light output with additional bits through the employment of error-correcting code containing redundant information of the data bit stream, along with the transmission of the main data bits. The error-correcting code is deployed at the optical receiver for correcting most errors occurring in transmission of the data bits thereby increasing the immunity of system500from noise resulting in reduced channel crosstalk. The encoders518are shown in connection with the transmission of redundant encoded data bits at DFB laser sources508but this redundant code can also be transmitted at modulators510, i.e., the FEC encoders518can be deployed between the modulated data source or modulators510and the multiplexer511. At the optical receiver end, the RxPIC chip504comprises a demux or demultiplexer522and a plurality of photodiodes (PDs)523(1) . . .523(N), one each for each channel signal λ1. . . λNtransmitted from TxPIC chip502. RxPIC chip may also include an optical amplifier at its input, either integrated into the input of the chip, e.g., a gain-clamped SOA (not shown), or an external optical amplifier, e.g., an EDFA (not shown). The electrically converted signals are respectively received in receivers524(1) . . .524(N) and the FEC encoded date is decoded at FEC1. . . FECNdecoders526(1) . . .526(N). The details relative receivers524are shown inFIG. 71and will be discussed in further detail later. It should be noted that FEC1. . . FECNdecoders526(1) . . .526(N) may also be a joint FEC decoder530for all signal channels to be decoded as a group.

As shown inFIG. 70, the FEC decoded data is received on line527for providing the BER data to real-time BER controller528. Controller528discerns such parameters such as output power level of DFB laser sources508, the bias point and chirp of modulators510and decision threshold values, such as the threshold decision voltage at the receiver as well as the phase and threshold offset relative to the eye diagram. Controller528provides feedback service channel information via line531feedback through demultiplexer522, optical link506(as shown at arrow526) to controller516on the transmitter side. The service channel data is then provided to DFB laser sources508via line513drivers512to correct the intensity of a respective laser source. Also, correction signal are provided on line509to modulator drivers514of modulators510to make adjustments on the rise time of modulated data, particularly in cases of lone data pulse “1's”, to adjust the cross-over point to be further discussed in connection withFIG. 71B, to adjust the bias point of the modulator and to enhance the extinsion ratio of the modulator as well as change the modulator chirp (alpha parameter), and to make RF magnitude adjustments on the modulated data stream. At the receiver, controller528also makes adjustments to receivers524in particular to adjust decision threshold values such as phase and threshold offset to respectively to achieve better data recovery within the decision window of the eye and offset the decision threshold from the eye position of most noise on the data stream.

Reference is now made toFIG. 71which is a detail of one signal channel in the optical receiver comprising RxPIC chip504and receiver524. Chip504includes photodiodes523from which an electrical data signal is obtained and pre-amplified by transimpedance amplifier and main amplifier by automatic gain control amplifier shown together at540. The amplified signal is then provided to electronic dispersion equalization (EDE) circuit542followed by clock and data recovery (CDR) circuit544after which the data is passed on SerDes circuit546which is a serializer/deserializer circuit, as know in the art, for converting the serial data into parallel format for faster handing of the data stream. CDR circuit544may be part of the SerDes circuit546. At this point, the trailing overhead, following the client payload, which carries the FEC coded data format is decoded at FEC decoder526and may be provided as feedback on line552to EDE circuit542as well as CDR circuit544to respectively provide information on the amount of eye dispersion distortion and to provide correction information for adjusting for timing errors due to imperfect clock recovery. The parallel data proceeds on at550to a cross-point switch for rerouting, etc.

FIG. 71Apictorially shows a typical eye diagram560for recovered data where dotted line562shows the center of the eye560. The upper portion560A of eye carries more noise then the lower portion560B of eye560. As a result, the threshold decision level is reduced to be below the center562of eye560to a level, for example, at563. In this manner, the threshold decision point or offset voltage566is set within a narrower window margin564for decision threshold since line563is shorter than center line562of eye560. However, as provided by this invention relative to EDE circuit542, a wider window margin574is achieved, as shown inFIG. 71Bthrough the lowering of the eye center572relative to the wrap-around on bit boundaries to lower the point of the vertical opening of the eye to provide for a lower threshold for a better margin for lower bit errors due to noise relative to the higher noise on the upper rail570A of the eye. Also the cross points for the lower rail wrap-around in bit boundaries relative to the horizontal opening of the eye are made lower so that the margin for timing errors due, for example, to imperfect clock recovery is wider as indicated at574inFIG. 71B, for timing determination to read a bit within its respective bit boundary, such as indicated at572. The wider timing margin in eye570as well as a lower threshold below the bit boundary center crossing away from the upper eye rail of greatest noise provides for optimum data recovery. This optimum eye data recovery is provided by the EDE circuit542providing for maximum phase margin at the correct threshold decision voltage.

Reference is now made toFIG. 72which is a flowchart illustrating an example of a process of feedback correction relative toFIG. 70, in the case here correcting for modulator operating parameters, e.g., bias voltage and voltage swing to adjust for modulator chirp and extinsion ratio, and DFB laser source parameters, e.g., DFB laser intensity or channel wavelength, through feedback control signal service channel, λS, from the optical receiver to the optical transmitter to reduce the bit error rate (BER). The chirp parameter of a quantum well electro-absorption modulator, such as may be modulators510inFIG. 70, is a function of the change in absorption characteristics and refractive index of the modulator with bias voltage. Typically, a voltage bias may be selected over a range within which the chirp parameter of the modulator shifts from positive to negative. In a high data rate channel close to the dispersion limit, a positive chirp increases the BER while a negative chirp decreases the BER. Similarly, a high extinction ratio tends to decrease the BER while a low extinction ratio tends to increase the BER. Forward error correction (FEC) decoders526in the optical receiver are employed to determine the BER of each channel. While this information may be forwarded to the optical transmitter in a variety of ways, it is shown here being transmitted through an optical service channel. The modulator operating parameters, e.g., bias voltage and voltage swing of the modulator, relative to a particular signal channel are adjusted using data relative to the channel BER determined at FEC decoder526. As illustrated inFIG. 72, BER data is transmitted from the optical receiver to an optical TxPIC or transceiver via the optical service channel shown inFIG. 70. Electronic controller516of TxPIC502employs this data to tune the bias voltage and/or voltage swing of a corresponding modulator510to adjust the bias and voltage swing of modulation to optimize BER, including optimized BER for the particular fiber type of optical fiber link506.

With reference toFIG. 72, the BER of the respective received channel signals is monitored at580, one channel at a time, but done sequentially at high speed via transmitter and receiver controllers516and528inFIG. 70. As indicated at582, the FEC encoded information is decoded at decoders526and provided to receiver controller528, via lines527where it is sorted and respectively sent via the service channel as signal λS, on line531and received at transmitter controller516for distribution to modulators510. Also, inFIG. 70, information is also sent via line529from receiver controller528to RxPIC chip504to temperature tune its AWG DEMUX522via its TEC521, as well as, for example, TEC30A inFIG. 52or TEC602inFIG. 73, next to be discussed, based upon the transmission of one signal channel to match the AWG wavelength grid passband to the wavelength grid of DFB laser source array512(1) . . .512(N) ofFIG. 70. As shown at583inFIG. 72, the chirp or chirp factor, α, of a respective channel modulator510is adjusted and, again, the BER for each respective channel is checked (584) and determined whether or not the BER has been reduced to a satisfactory level. A satisfactory level is, for example, a BER below 10−12. If yes, other channels at the transmitter TxPIC502are checked (585) until all signal channels have satisfactory chirp or α. If no, then adjustment of the signal channel wavelength is accomplished (586) and, again, the BER monitored at the receiver is checked to determine if it is reduced to an acceptable level. If not, the process is redone, starting with adjustment of the modulator channel chirp (583) followed by channel wavelength adjustment (586), if necessary, until an acceptable channel BER level is achieved. When all of the channels have been checked (588to585) and adjusted relative to both modulator chirp and laser intensity and/or channel wavelength with satisfactory BER (at587inFIG. 72), the process is complete and the monitoring process for these laser source and modulator parameters may begin all over again at580.

To be noted that in connection withFIG. 70, the wavelength adjustment of respective DFB laser sources508is made relative to bias changes to the respective source via drivers512. However, it is within the scope of FEC enhanced system500to also change the channel wavelength via a DFB laser source heater such as with heaters208shown inFIG. 37of provisional application, Ser. No. 60/328,207, incorporated herein by reference and as taught in its corresponding non-provisional application, U.S. patent application. Ser. No. 10/267,331, such as relative toFIGS. 12–16in the non-provisional application, filed concurrently herewith and incorporated by reference.

Reference is now made toFIG. 73depicting RxPIC chip600and its associated TEC602. Chip600may include at its input604from the fiber channel link an optical amplifier606for adding gain to the multiplexed channel signals after which they provided at first order Brillouin zone input to input slab610of the arrayed waveguide grating (AWG)608via on-chip optical waveguide706. AWG708includes a plurality of waveguide gratings coupled between input slab610and output slab614wherein the multiplexed channel signals, as known in the art, are demultiplexed and provided as an output at a first order Brillouin zone of output slab614and the respective demultiplexed channel signals are provided on output waveguides616to respective on-chip PIN photodiodes622(1) . . .622(12).

As shown inFIG. 73, monitoring PIN photodiodes624and626are fabricated in the higher order +/− Brillouin zones (e.g., the −1 and +1 Brillouin zones) of AWG608and are optically coupled to these zones of AWG608via respective waveguides618and620. The two photodiodes624and626are placed there for the purposes of detection on opposite sides of the AWG passband. A DFB laser in a TxPIC, such as TxPIC502inFIG. 70, is aligned to the passband of AWG608when the DFB laser source wavelength, such as wavelength628shown inFIGS. 73A and 73B, is tuned such that photodiodes624and626have a balanced output, i.e., their outputs are of the same magnitude on adjacent sides of laser source wavelength628such as shown at630and632inFIGS. 73A and 73B.