Abstract:
An optoelectronic interconnect which includes optical transmitter and detector having capacitances below 150 femto-Farads each suitable for transmission of optical signals at speeds at and above 20 Gigabit per second at power consumption below 10 milliWatt per Gigabit per second

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims an invention which was disclosed in Provisional Application No. 61/338,677, filed Feb. 23, 2010, entitled “OPTOELECTRONIC INTERCONNECT FOR HIGH FREQUENCY DATA TRANSMISSION AT LOW POWER CONSUMPTION”. The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to the field of optical links suitable for applications in long haul telecommunications, fiber-to-the home, radio over fiber, data communications and optical interconnects 
         [0004]    2. Description of Related Art 
         [0005]    There is a need in optical links operating at speeds above 20 Gigabit per second (Gb/s). Presently such links are realized by using power-consuming and expensive optical systems which include external modulators absorbing a significant power. For matching the needs of ever-growing serial transmission speed per wavelength per channel at low cost a new type of optical links is needed. However, the existing optical links based on standard devices suitable for data transmission rate up to 10-1 3  Gb/s include optical devices with a high capacitance exceeding 150 femto-Farads. It makes it difficult to realize high speed high quality data transmission in the optical link at low power consumption using these devices as the integrated circuits needed to drive this device require significant power. 
         [0006]    A prior art optical interconnect ( 50 ) is shown schematically in  FIG. 1 . A modulator driver ( 1 ) representing an integrated circuit (IC) is electrically connected to the electrooptic modulator ( 2 ). The laser ( 3 ) provides lasing light of constant intensity which is modulated by electrooptic modulator ( 2 ). The electrooptic modulator ( 2 ) can be, for example, electrorefractive or electroabsorption modulator. Due to a high capacitance of the modulator device the power consumption of the driver IC is high, typically above 500 milliWatt (mW) and up to 10 Watt (W). The modulator driver is connected to the control IC ( 4 ) with a certain functionality, for example multiplexing/demultiplexing IC. The control IC ( 4 ) generates current pulses ( 10 ) that control the modulator driver ( 1 ). The modulator driver ( 1 ) generates current pulses ( 15 ) that cause the modulator ( 2 ) to modulate light. Light pulses ( 11 ) coming out from the modulator ( 2 ) are delivered from the modulator ( 2 ) by the optical waveguide ( 5 ). 
         [0007]    The input light is delivered by the waveguide ( 6 ). The light pulses ( 21 ) delivered from the waveguide ( 6 ) come to the detector ( 7 ). The detector ( 7 ) transforms light pulses ( 21 ) into current pulses ( 20 ). Current pulses ( 20 ) are amplified by the amplifier ( 8 ). Amplified pulses of current ( 25 ) come to the control IC ( 4 ). 
         [0008]    Various means can be used to deliver light. Light ( 9 ) from the laser ( 3 ) is delivered to the modulator ( 2 ), for example, through air or via optical system which may include lenses, optical isolators, waveguides and other elements. The current pulses ( 10 ) from the control IC ( 4 ) to the modulator driver ( 1 ) and the current pulses ( 20 ) from the from the detector to the amplifier are delivered via high frequency electrical lines. 
         [0009]    Input ( 12 ) or output ( 13 ) pulses connect the electro-optic circuit ( 50 ) to the external devices for data acquisition.  FIG. 1  shows only a part of the interconnect and a similar part is placed on the other end of the optical waveguide link. The optical waveguide link can be realized by, for example, plastic waveguide, duplex single mode fiber, duplex multimode fiber, fiber ribbon. 
       SUMMARY OF THE INVENTION 
       [0010]    An optoelectronic interconnect suited for high frequency (HF) data transmission, preferably above 20 Gigabit per second (Gbps) and the most preferably above 40 Gbps capable to ultralow power consumption data transmission is disclosed. A unique combination of the parameters provides a possibility for data transmission at ultra-low power consumption (preferably below 10 mW/Gbps and most preferably below 5 mW/Gbps), as the optoelectronic interconnect includes a combination of: (i) a low capacitance optical chip suitable for generation of light pulses with rise and fall times shorter than 20 pico-second (ps) with single or multiple dielectric or air apertures, the chip being driven by either current modulation or electrooptic modulation with a capacitance preferably below 150 femto-Farads (IF) and, most preferably, below 100 IF under operation conditions; (ii) a low capacitance optical chip suitable for detection of light pulses and transforming them to current pulses with rise and fall times shorter than 20 ps with a capacitance preferably below 150 fF and, most preferably, below 100 IF under the operation conditions; (iii) a pad layout integrated on chip representing a high-frequency electric transmission line capable to 20 Gbps and preferably to 40 Gbps transmission at losses preferably below 3 dB; (iv) an electronic chip representing an integrated circuit capable to operation at preferably 20 Gbps and most preferably to 40 Gbps suitable for modulation of the light from the optical chip used for generation of light pulses; (v) an electronic chip representing an integrated circuit capable to operation preferably at or above 20 Gbps and most preferably at or above 40 Gbps suitable for amplification of current pulses of the optical chip suitable for detection of light pulses, (vi) a high-frequency-compatible substrate with HF pads to which all the above components are attached and the means of attachment suitable for HF operation; (vii) an optical system for coupling of the light from the optical chip suited for generation of light pulses to optical fiber; (viii) an optical system for coupling of light pulses from the optical waveguide to the optical chip suited for detection of light pulses, which does not disturb HF signal penetration at or above 20 Gbps and, most preferably, at or above 40 Gbps; (ix) a packaging of the optoelecronic interconnect suitable for the HF operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a schematic representation of the prior art optical interconnect. The device consumes high power. 
           [0012]      FIG. 2  shows a schematic representation of an optical interconnect according to an embodiment of the present invention. 
           [0013]      FIG. 3  shows a schematic representation of an optical link via a bundle consisting of several single mode fibers, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    An optical interconnect ( 100 ) according to an embodiment of the present invention is shown in  FIG. 2 . Opposite to the prior art of  FIG. 1 , the modulator section is integrated with the laser into a monolithic structure. The laser ( 33 ) is driven by the driver ( 31 ). A one skilled in the art will appreciate that the modulator section integrated with a laser can have a very low capacitance. Practically, a capacitance below 150 femto-Farad can be achieved. Further, even a lower capacitance below 100 femto-Farad can be achieved. Means to achieve such a low capacitance include, but are not limited to using dielectric apertures close to the modulator section, using multiple layer apertures, using apertures formed by air gaps, reducing the mesa size. The laser ( 33 ) including the modulator section having such a low capacitance will be capable to generate pulses of light having rise and fall time below 20 picoseconds. 
         [0015]    The capacitance of the photodetector ( 8 ) used in the optical interconnect should be below 150 femto-Farads, and, most preferably, below 100 femto-Farads. A one skilled in the art will appreciate that the photodetector having such a low capacitance will be capable to generate pulses of current having rise and fall times below 20 picoseconds. 
         [0016]    In order to have a pad layout capable to transmit high frequency signals at the transmission rate above 20 Gbps, and, most preferably, above 40 Gbps, with the losses below (−3 dB), the pad layout should be integrated on chip. 
         [0017]    The laser driver ( 31 ) should be capable to operation at the transmission rate 20 Gbps. Most preferably, the laser driver ( 31 ) should be capable to operate at a transmission rate 40 Gbps. 
         [0018]    The amplifier ( 8 ) should be capable to amplify the current pulses at the rate of 20 Gbps. Most preferably, the amplifier should be capable to amplify current pulses at the rate of 40 Gbps. 
         [0019]    To enable the high frequency operation of the optical interconnect ( 100 ), the components ( 31 ), ( 33 ), ( 7 ) and ( 8 ) should be preferably attached to a high frequency substrate. One skilled in the art will appreciate that means of attachment should be compatible with the required high frequency operation. 
         [0020]    All components capable to high frequency operation enable constructing an optical interconnect capable as a whole to transfer data at a high rate 20 Gigabit per second. In the preferred embodiment the optical interconnect enables data transmission at a high rate above 40 Gigabit per second. 
         [0021]    Various types of lasers can be used in the optical interconnect. In one embodiment of the present invention this can be an edge-emitting lasers. 
         [0022]    In another embodiment of the present invention, a vertical cavity surface emitting laser can be used in the optical interconnect system. 
         [0023]    In yet another embodiment of the present invention, a laser used in the optical interconnect is a tilted cavity laser, as disclosed in the patent “TILTED CAVITY SEMICONDUCTOR LASER (TCSL) AND METHOD OF MAKING SAME”, U.S. Pat. No. 7,031,360, filed Feb. 12, 2002, issued Apr. 18, 2006, by the inventors of the present invention, wherein the patent is herein incorporated by reference. 
         [0024]    In a further embodiment of the present invention, a laser used in the optical interconnect is a tilted wave laser, as disclosed in the patent “EXTERNAL CAVITY OPTOELECTRONIC DEVICE ”, U.S. Pat. No. 7,421,001, filed Jun. 16, 2006, issued Sep. 2, 2008, and in the patent “OPTOELECTRONIC DEVICE AND METHOD OF MAKING SAME”, U.S. Pat. No. 7,583,712, filed Jan. 3, 2007, issued Sep. 1, 2009, both by the inventors of the present inventions. Both patents are herein incorporated by reference. 
         [0025]    In another embodiment of the present invention, a passive cavity device can be used in the optical interconnect, wherein the passive cavity device is disclosed in the patent application of Ledentsov “OPTOELECTRONIC DEVICE AND METHOD OF MAKING SAME”, U.S. patent application Ser. No. 12/803,747, filed Jul. 6, 2010, which is herein incorporated by reference. 
         [0026]    Different types of the photodetector can be used in the optical interconnect. In an embodiment of the present invention, the photodetector used in the optical interconnect can be a p-i-n photodetector chip. 
         [0027]    In another embodiment of the present invention, the photodetector used in the optical interconnect, is an avalanche photodetector. 
         [0028]    A one skilled in the art will appreciate that various concepts of modulation can be employed to modulate light coming out from the laser ( 31 ). In one embodiment of the present invention, direct, or current modulation is employed. 
         [0029]    In another embodiment of the present invention, an indirect, or electrooptic modulation is applied to modulate light. In the preferred embodiment of the present invention the electrooptic modulation applied is electrorefraction modulation. In yet another embodiment of the present invention the electooptic modulation applied is the electroabsorption modulation. 
         [0030]    Electrooptic modulation can be realized by using various approaches. In one embodiment of the present invention, a filter cavity approach is used as disclosed in the patent “Electrooptically wavelength-tunable resonant cavity optoelectronic device for high-speed data transfer”, U.S. Pat. No. 7,369,583, filed Jun. 2, 2005, issued May 6, 2008, by the inventors of the present invention, wherein the patent is herein incorporated by reference. 
         [0031]    In another embodiment of the present invention the concept of the electrooptic tuning of the stopband edge of a distributed Bragg reflector is applied, wherein the concept is disclosed in the patent “ELECTROOPTICALLY BRAGG-REFLECTOR STOPBAND TUNABLE OPTOELECTRONIC DEVICE FOR HIGH-SPEED DATA TRANSFER ”, U.S. Pat. No. 7,593,436, filed Jun. 16, 2006, issued Sep. 22, 2009, and the patent application “OPTOELECTRONIC DEVICE FOR HIGH-SPEED DATA TRANSFER WITH ELECTROOPTICALLY TUNABLE STOPBAND EDGE OF A BRAGG REFLECTOR”, U.S. patent application, filed Jul. 27, 2009, both by the inventors of the present invention. Both the patent and the patent application are herein incorporated by reference. 
         [0032]    In yet another embodiment of the present invention, the modulation of the laser light can be achieved by applying a coupled cavity approach as disclosed in the publication of Fischer et al. “Coupled resonator vertical-cavity laser diode”, Applied Physics Letters, Volume 75, Number 19, pages 3020-3022, 8 Nov. 1999, wherein this publication is incorporated herein by reference. 
         [0033]    In a further embodiment of the present invention, triggered self-pulsation approach is used to modulate the laser light as disclosed in the patent by Bimberg et al. “DATA TRANSMISSION OPTOELECTRONIC DEVICE”, U.S. Pat. No. 7,580,595, filed May 9, 2008, issued Aug. 25, 2009, wherein the patent is incorporated herein by reference. 
         [0034]    Various kinds of optical waveguides can be used in the optical interconnect. These include, but are not limited to, silicon optical waveguide, glass optical fiber, plastic optical fiber, a bundle of optical fibers. Both single mode and multimode optical fibers can be used. 
         [0035]      FIG. 3  illustrates schematically the optical link according to an embodiment of the present invention. Using of single mode fibers is preferred to avoid mode dispersion of the optical signal and enabling transfer over a larger distance. However, it is difficult to couple light emitted by a vertical cavity surface emitting laser to a single mode fiber, since the diameter of the single mode fiber is significantly smaller that the output optical aperture of a vertical cavity surface emitting laser. By using only one single mode fiber a significant fraction of the optical power could be lost. In the embodiment of  FIG. 3  the light emitted from a VCSEL ( 43 ) is transferred via a bundle ( 50 ) combining multiple single mode fibers ( 55 ). On the other end of the optical link the light impinges on the photodetector ( 47 ). This approach combines the advantages of using a single mode fiber thus eliminating a danger of signal dispersion and using an optical fiber of a large diameter capable to receive all light from the laser. In one of the further embodiments of the present invention the bundle of single mode fibers can be fabricated within a single fiber comprising multiple single mode channels. 
         [0036]    To couple the light from the laser to an optical waveguide and from an optical waveguide to a photodetector, various means can be used. In one embodiment of the present invention, the coupling can be provided via the near field zone of the optical device without using lenses. 
         [0037]    In another embodiment of the present invention, lenses are used for coupling. Both external lenses and lenses can be used. 
         [0038]    In some embodiments of the present invention Fresnel or plasmonic lenses can be used. 
         [0039]    A one skilled in the art will appreciate that, since a modulator section integrated monolithically with a laser has a very low capacitance, the integrated circuit does not absorb a lot of power. Typical values are below 10 milliWatt per Gigabit per second. Most preferable are optical interconnects having even lower power consumption below 5 milliWatt per Gigabit per second. 
         [0040]    Further advantage is that a very low power consumption allows using very compact device as no need in heat sinking occurs. For example, the laser ( 33 ) and the laser driver integrated circuit ( 31 ) can be matched in a TO-can package. Similarly, a photodetector ( 7 ) can be integrated with the amplifier integrated circuit ( 8 ). 
         [0041]    In another embodiment of the present invention, both integrated circuits ( 31 ) and ( 8 ) can be matched on a single chip. 
         [0042]    In yet another embodiment of the present invention, this single chip can be further matched with the control integrated circuit ( 4 ) to form a single integrated circuit. 
         [0043]    In a further embodiment of the present invention the whole electrooptic circuit can be packaged into a housing having a QSFP form factor. 
         [0044]    This single integrated circuit can be suitable to multiple applications, including but not limited to multiplexing of signals, demultiplexing of signals, electronic compensation of signal dispersion, retiming, and routing. 
         [0045]    In another embodiment of the present invention, multiple optical chips can be used for transmitting of optical signals. 
         [0046]    In yet another embodiment of the present invention, multiple photodetectors can be used for detecting optical signals and their transformation into electric signals. 
         [0047]    In a further embodiment of the present invention, an optical interconnect including multiple lasers and multiple photodetectors still has a very low power consumption below 3.3 Watt. 
         [0048]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. 
         [0049]    All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. 
         [0050]    Although the invention has been illustrated and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiments set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.