Voltage-controlled electro-optical serializer / deserializer (SERD)

Abstract

A voltage controlled electro-optical serializer / deserializer (serdeserializer) is disclosed. An optoelectronic transmitter (10) includes an electro-optical modulator (12), a digital drive circuit (14), and a feedback circuit (30). The electro-optical modulator is configured to modulate an optical signal in response to an electrical drive signal. The digital drive circuit is coupled to the electro-optical modulator and is configured to generate the electrical drive signal. The feedback circuit is configured to measure a quantity indicative of a power level of a modulated optical signal generated by the electro-optical modulator, and to adjust a supply voltage to the digital drive circuit in response to the measured quantity.

Description

Technical Field

[0001] The present invention relates generally to optical communications, and more particularly to an apparatus for transmitting and receiving optical communication signals at a high data rate and a control method thereof. Background Art

[0002] Closed-loop control of power consumption of communication transceivers operating at high data rates has been previously proposed in the patent literature. For example, U.S. Patent 9,467,231 describes various embodiments of providing a communication interface configured to transmit data at high bandwidth using a PAM format over an optical communication network. A feedback mechanism for adjusting transmission power levels is provided. In one embodiment, a data communication system includes a network interface device, the network interface device including a first optical communication interface, the first optical communication interface configured to receive an optical signal transmitted on an optical communication link, the optical signal transmission being characterized by a signal level and a noise level, wherein the optical signal is received from a transmitter device. The system also includes: a photodiode configured to convert the optical signal into an electrical signal; a TIA configured to amplify the electrical signal transmission; an analog-to-digital converter for converting the electrically amplified electrical signal into a digital signal; and a processor configured to calculate a set of threshold levels based at least on the signal level and the noise level, and generate a set of power transmission levels for the transmitter device, wherein the power transmission levels are based on unequally spaced threshold levels. The system includes: an encoder for encoding a power transmission level; and a second optical communication interface for sending the encoded power transmission level to a transmitter device. The system further includes a PAM modulation driver for driving the encoded power transmission level and a laser for driving an output signal. The transmitter device adjusts the transmission power level for sending data to a network interface based on the encoded power transmission power level. The noise level includes a signal independent component, a component proportional to the signal power, and a component proportional to the square of the signal power.

[0003] As another example, U.S. Patent Application Publication 2012 / 0216084 describes a system that includes a first serializer / deserializer (Serdes) link from a first integrated circuit (IC) to a second IC and a second link from the second IC to the first IC. Power consumption settings in the circuits of the first link are adjusted to control power consumption so that the bit error rate of the first link is maintained within a certain range, wherein the lower limit of the range is substantially greater than zero. Power consumption settings in the circuits for the second link are adjusted to control power consumption so that the bit error rate of the second link is maintained within a certain range, wherein the lower limit of the range is substantially greater than zero. In one example, the circuit in the second IC detects errors in the first link and reports back via the second link. The first IC uses the reported information to determine the bit error rate of the first link. Summary of the invention

[0004] One embodiment of the present invention provides an optoelectronic transmitter, including an electro-optic modulator, a digital drive circuit, and a feedback circuit. The electro-optic modulator is configured to modulate an optical signal in response to an electrical drive signal. The digital drive circuit is coupled to the electro-optic modulator and configured to generate the electrical drive signal. The feedback circuit is configured to measure a quantity indicating a power level of the modulated optical signal generated by the electro-optic modulator, and to adjust a supply voltage to the digital drive circuit in response to the measured quantity.

[0005] In some embodiments, the feedback circuit is configured to set a supply voltage to the digital drive circuit to be in an inversely proportional relationship to the measured quantity.

[0006] In some embodiments, the digital driving circuit includes a serializer-deserializer (SerDes).

[0007] In one embodiment, the feedback circuit includes a power detector configured to receive a portion of the modulated optical signal from the optical modulator and measure the quantity by measuring a photocurrent proportional to the square root of the optical power level of the portion.

[0008] According to an embodiment of the present invention, there is also provided a method for manufacturing an optoelectronic transmitter, the method comprising manufacturing an electro-optic modulator for modulating an optical signal in response to an electrical drive signal, manufacturing a digital drive circuit for generating the electrical drive signal, manufacturing a feedback circuit and connecting it between the electro-optic modulator and the drive circuit, for measuring a quantity indicating a power level of a modulated optical signal generated by the electro-optic modulator, and adjusting a supply voltage to the digital drive circuit in response to the measured quantity.

[0009] In some embodiments, fabricating the electro-optic modulator includes implementing the electro-optic modulator in a back-end-of-line (BEOL) layer of a semiconductor chip, wherein fabricating the digital drive circuit includes implementing a serializer-deserializer (SerDes) in a front-end-of-line (FEOL) layer of the semiconductor chip, and includes coupling the SerDes to the electro-optic modulator via a through hole in the semiconductor chip.

[0010] In some embodiments, fabricating the feedback circuit includes implementing coupling of the optical power detector to an analog-to-digital converter (ADC).

[0011] In one embodiment, fabricating the optical power detector includes implementing the optical power detector in a front end of line (FEOL) layer of a semiconductor chip and coupling the optical power detector to an output of an electro-optic modulator using a through-BEOL optical connection.

[0012] In another embodiment, fabricating the feedback circuit includes implementing the ADC in a front end of line (FEOL) layer of a semiconductor chip and implementing the optical power detector external to the semiconductor chip.

[0013] In yet another embodiment, fabricating the feedback circuit includes implementing the ADC and the optical power detector external to the semiconductor chip on which the electro-optic modulator is fabricated.

[0014] According to an embodiment of the present invention, there is also provided a control method for an optoelectronic transmitter, the method comprising generating an electrical drive signal using a digital drive circuit, modulating an optical signal using the electrical drive signal, measuring a quantity indicating a power level of the modulated optical signal, and adjusting a supply voltage to the digital drive circuit in response to the measured quantity.

[0015] The present invention will be more fully understood from the following detailed description of embodiments in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic block diagram of an optoelectronic transmitter according to an embodiment of the present invention, the optoelectronic transmitter including a feedback circuit configured to adjust a drive voltage amplitude of an electro-optic modulator of the transmitter;

[0017] Figure 2 According to an embodiment of the present invention Figure 1 A schematic diagram isometric view of an embodiment of a transmitter;

[0018] Figure 3 is a diagram showing an embodiment of the present invention Figure 2 A schematic cross-sectional view of elements of a transmitter;

[0019] Figure 4 According to another embodiment of the present invention Figure 1 A schematic diagram isometric view of an embodiment of a transmitter;

[0020] Figure 5 According to another embodiment of the present invention Figure 1 A schematic block diagram of an embodiment of a transmitter of; and

[0021] Figure 6 According to another embodiment of the present invention Figure 1 A schematic block diagram of an embodiment of a transmitter. DETAILED DESCRIPTION

[0022] Overview

[0023] Optical data center networks typically utilize multiple optoelectronic transmitters and receivers to communicate between nodes of the data center network using high Gb / sec modulated optical signals transmitted over optical fiber links. In a typical design process, the allowable minimum signal-to-noise ratio (SNR) of the optical communication signal is determined and margins are considered to ensure that the communication signal has sufficient quality, taking into account manufacturing tolerances and worst-case scenarios in the field.

[0024] For a given optical fiber link, the SNR of the modulated optical signal can be varied, for example, by adjusting the input optical power, by adjusting the electrical (e.g., voltage) amplitude of the modulating signal, and / or by adjusting the output optical power via an attenuator at the output of the modulator. A common method of characterizing the SNR is to estimate it from an "eye diagram" of the waveform of the modulated optical signal output at the receiver. Specifically, by modulating the optical signal with a higher voltage amplitude (up to a certain limit, above which increased drive voltage will not affect the shape of the eye diagram, and therefore will not affect the SNR estimate), the integrity of the generated optical waveform can be maintained under conditions of increased optical attenuation. However, checking the quality of the eye diagram during field operation is expensive and generally impractical.

[0025] Therefore, margins for optical power and modulation voltage amplitude must be included in order to ensure that communications between nodes remain intact even under rare worst-case conditions in the field, such as abnormally high optical losses in the system. For example, a manufacturer may provide five times the nominal rated optical power to mitigate worst-case optical losses in the field. However, typically such losses vary depending on field conditions, and in many cases the actual losses are close to nominal. However, a strong optical signal is modulated using an excessively large voltage amplitude as if the optical power was nominal. In this case, the resulting optical SNR is much higher than required, and the electro-optical chip is driven with electrical power much higher than actually required.

[0026] Embodiments of the invention described hereinafter provide techniques for optimizing the magnitude of the drive voltage. The disclosed techniques take into account variations in the RMS optical power output in the field as well as variations in manufacturing to provide a transmitter design that can handle the aforementioned worst-case scenarios while minimizing electrical power consumption under nominal conditions without requiring complex equipment (e.g., such as that required for eye diagram estimation).

[0027] In some embodiments, a photoelectric transmitter is provided, which includes an electro-optic modulator, a drive circuit, and a feedback circuit. The drive circuit is coupled to the electro-optic modulator and provides a drive signal with an adjustable amplitude to drive the electro-optic modulator to output a modulated optical signal. The feedback circuit is calibrated to associate the SNR of the modulated optical signal with a measured quantity indicating the power level of the modulated optical signal for a range of drive signal amplitudes. For example, the measured quantity may include the RMS optical power of the modulated optical signal. The feedback circuit is coupled to the optical output of the electro-optic modulator to measure the quantity (e.g., the RMS output power of the modulated optical signal), and using the calibration, the feedback circuit is coupled to the drive circuit to adjust the amplitude of the drive signal provided by the drive circuit according to the measured RMS output power to maintain a pre-specified SNR of the modulated optical signal. In one embodiment, the feedback circuit is configured to set the supply voltage to the digital drive circuit to be inversely proportional to the measured quantity.

[0028] In some embodiments, the feedback circuit includes a photodiode detector coupled at the output of the optical modulator. Using the photocurrent reading from the photodiode output, the feedback circuit adjusts the amplitude of the drive signal based on the RMS photocurrent output by the photodiode so as to maintain a pre-specified optical SNR of the modulated output signal. The RMS photocurrent output of the photodiode is proportional to the square of the RMS output optical power of the portion of the modulated signal that is tapped and monitored. Specifically, when the measured RMS photocurrent (and therefore the light output) is sufficiently high, the voltage amplitude of the drive signal is reduced. Using the RMS measured photocurrent power in this manner (e.g., rather than examining an eye diagram) is made possible by considering an SNR budget that is factory calibrated relative to the RMS photocurrent of the monitoring photodiode over the linear portion of the optical modulation characteristic.

[0029] The above-mentioned optoelectronic transmitter is usually implemented as a system on a semiconductor chip. The transmitter includes a driving circuit, for example, including a SerDes (serializer / deserializer) component, for example, driving an electro-optical modulator on the chip to output a modulated optical signal. Typically, using SerDes, the transmitter multiplexes external serial data from a system-level input source such as Serial ATA, InfiniBand, Gigabit Ethernet, etc. into a serial fiber channel data stream.

[0030] Less often but still possible is the use of direct laser modulation (e.g. DML, VCSEL), where a laser driver is employed to regulate the drive signal to be fed into the laser. In this case, the SerDes feeds the laser driver module. In principle, this approach would require another mapping of how the SerDes output voltage affects the laser driver output. The optical output power can then be determined and depends somewhat linearly on the SerDes voltage.

[0031] In high Gb / sec applications, SerDes often uses a large portion of the chip's power budget. The electrical power modulation dissipation of the SerDes is proportional to the square of the voltage amplitude supplied to the SerDes (~V 2 ). Therefore, minimizing the voltage rating required in the field (while maintaining an acceptable SNR for the optical signal) minimizes power consumption, which in turn saves the need for expensive multi-chip solutions. As another example, continued overestimation of SerDes power requirements can lead to higher cooling costs.

[0032] Therefore, accurate management of the SerDes power budget is essential to achieve optimal integration levels.By using the disclosed control of real-time RMS power aspects of optical communication signal integrity, the disclosed techniques can enable wider adoption of optical data center networks.

[0033] Voltage-controlled electro-optical SerDes

[0034] Figure 1 is a schematic block diagram of an optoelectronic transmitter 10 according to an embodiment of the present invention, which includes a feedback circuit 30 configured to adjust the driving voltage amplitude of the electro-optic modulator 12 of the transmitter 10. The driving voltage (i.e., the modulation signal) is provided to the SerDes 14, which is coupled to the modulator 12 to drive the modulator 12.

[0035] In the illustrated embodiment, the electro-optic modulator 12 is a Mach-Zehnder interferometry modulator. The modulator 12 is configured to modulate a continuous wave (CW) input optical signal from a continuous wave (CW) laser source 19. The modulated optical signal is output from the modulator 12 via an optical fiber link 13.

[0036] As seen, the optical power detector 16 of the feedback circuit 30 is coupled to the output of the modulator 12 via the link 15 to measure the RMS output power of the output optical signal (using the photocurrent at the output of the power detector), where the measured RMS power indicates the power of the output optical signal in the optical fiber link 13.

[0037] The power detector photocurrent is converted to a digital signal using an analog-to-digital converter (ADC) 18 of a feedback circuit 30. A voltage control circuit 20 receives the digital signal and adjusts the voltage level provided to the SerDes 14 according to a predetermined relationship (eg, as provided below by Equation 1).

[0038] In one embodiment, a calibration procedure is provided where the RMS output power is measured in the factory and correlated with the eye diagram 150 to generate a calibration (e.g., a lookup table) that provides a relationship between the actual RMS measured optical output power of the modulator 12 and the required voltage amplitude provided by the SerDes 14. In the field, the disclosed technology can maintain a given level of SNR (related to the measured RMS optical power) by applying the following relationship between the actual RMS optical power and the required voltage amplitude provided to the SerDes 14:

[0039] Equation 1 V SerDes (P) = α·P0-β·(P-P0),

[0040] Among them, V SerDes is the nominal voltage supplied to the SerDes (e.g., peak-to-peak voltage amplitude), α and β are factory calibrated positive coefficients, P0 is the nominal RMS optical power, and P is the actual RMS optical power in the field.

[0041] Using Equation 1, when the actual optical power in the field is measured to be higher than the factory-determined value required to achieve a given SNR, i.e., P-P0>0, indicating that the actual SNR in the field is higher than required, the voltage control circuit 20 increases the required voltage amplitude V SerDes (P) from the nominal value V SerDes (P0) = α·P0 is reduced to the value provided by Eq. 1.

[0042] On the other hand, if the actual optical power at the site is measured to be lower than the factory-determined value required to achieve a given SNR, indicating that the actual SNR at the site is lower than required, the voltage control circuit 20 increases the rated voltage of the SerDes 14 according to Equation 1 to improve waveform quality.

[0043] Just for the sake of conceptual clarity, choose Figure 1 To simplify the presentation, elements that are not mandatory for understanding the disclosed technology, such as electrical amplification stages, have been omitted from the figure.

[0044] Figure 2 According to an embodiment of the present invention Figure 1 Schematic diagrammatic isometric view of an implementation of the transmitter 10 of FIG. In the illustrated embodiment, the SerDes 141 is developed in the front-end-of-line (FEOL) layer 110 of the system-on-chip along with the ADC 181 and the voltage regulator 201 (i.e., the control voltage circuit 201). These chips are interconnected with low-speed electrical transmission lines. An electro-optic modulator 121 (e.g., a Mach-Zehnder modulator 121) and an optical power detector 161 are developed in the back-end-of-line (BEOL) layer 111. Typically, the power from a CW laser source ( Figure 1The optical signal (shown) is coupled to the modulator through an optical waveguide in the BEOL. The drive signal generated by the SerDes 141 is fed to the modulator 121 using a high-speed electrical via 202 that passes through the metallization layer. The optical output of the modulator is split into two paths using an optical coupler. One path of the coupler is connected to the optical fiber link 131 and output to the network, and the other path of the coupler carries a signal that is tapped to indicate the RMS power in the link 131. The tapped signal is fed to the optical guide 151 and measured by the power detector 161 in the BEOL.

[0045] The resulting electrical output signal of the detector 161 is fed to the FEOL and the ADC 181 through the low speed electrical via 171. The voltage regulator 201 receives the control digital signal output by the ADC 181 and adjusts the voltage level provided to the SerDes 141 according to the relationship of Equation 1.

[0046] Figure 3 is a diagram showing an embodiment of the present invention Figure 2 Schematic cross-sectional view of the elements of a transmitter. As can be seen, the transmitter is realized using processing in multiple FEOL and BEOL layers, for example, to create vias and links using photolithography. Figure 2 and Figure 3 The actual process flow for manufacturing the system on chip shown in the subsequent figures includes dozens of semiconductor technology steps, which will not be described in detail for the sake of simplicity.

[0047] Figure 4 According to another embodiment of the present invention Figure 1 A schematic diagram isometric view of an embodiment of a transmitter. Figure 4 In the illustrated embodiment, the optical power detector 416 resides in the FEOL layer 410 .

[0048] The drive signal generated by the SerDes 441 is fed to the modulator 421 using high-speed electrical vias 402 passing through the metallization layer. The optical output of the modulator is split into two paths using an optical coupler. The optical output of the modulator is split into two paths using an optical coupler. One of the couplers is connected to the optical fiber link 413 and is output to the network, and the other output path (415) of the coupler carries a signal that is tapped to indicate the RMS power in the link 413. The tapped signal in the optical guide 415 is fed to the optical conversion 414, which guides the optical signal to the power detector 416 using an optical connection 417 through the BEOL.

[0049] The electrical signal output by the power detector 416 is directly fed to the ADC 418 through the low speed electrical lines in the FEOL layer 410. The voltage regulator 401 receives the control digital signal output by the ADC 418 and adjusts the voltage level provided to the SerDes 441 according to the relationship in Equation 1.

[0050] Figure 5 According to another embodiment of the present invention Figure 1 530. In the embodiment shown, the power detector is placed outside the chip 530. The modulator output is coupled to the optical fiber 513 at the edge of the chip. The power detector that monitors the output power is a commercially available online optical tap monitor 516, such as produced by OZ Optics (Ottawa, Canada). The monitored signal is interfaced to the chip (and ADC) from the electrical pins of the tap via the low-speed electrical rails on the PCB 529.

[0051] Figure 6 According to another embodiment of the present invention Figure 1 A schematic block diagram of an embodiment of a transmitter is shown in isometric view. Figure 6 In the illustrated embodiment, an in-line optical tap monitor 616 (eg Figure 5 The ADC 618 and the voltage regulator 601 are implemented external to the chip 630, on the PCB 629, and the voltage regulator-SerDes power supply line is implemented as a low-speed PCB track.

[0052] Although the embodiments described herein primarily address the control of high data rate transmissions in optical communication systems, the methods and systems described herein may also be used in other applications, such as for systems with different optical transceiver topologies, including systems with front panel transceivers, on midplane cards or on-board optics and multi-chip modules.

[0053] Therefore, it should be understood that the above embodiments are cited by way of example, and the present invention is not limited to what is specifically shown and described above. On the contrary, the scope of the present invention includes the combination and sub-combination of the various features described above, as well as changes and modifications thereof that would occur to those skilled in the art upon reading the above description and that are not disclosed in the prior art. The documents incorporated by reference in this patent application are considered to be an integral part of this application, except that to the extent that any term is defined in these incorporated documents in a manner that conflicts with the explicit or implicit definition in this specification, only the definition in this specification should be considered.

Claims

1. A photoelectric transmitter, comprising: an electro-optic modulator configured to modulate an optical signal in response to an electrical drive signal; a digital drive circuit coupled to the electro-optical modulator and configured to generate the electrical drive signal, wherein the digital drive circuit comprises a serializer-deserializer (SerDes); as well as a feedback circuit configured to measure a quantity indicative of a power level of a modulated optical signal produced by the electro-optical modulator and to adjust a supply voltage to the digital drive circuit in response to the measured quantity, the feedback circuit comprising a power detector configured to receive a portion of the modulated optical signal from the optical modulator and to measure the quantity by measuring a photocurrent that is proportional to a square root of an optical power level of the portion.

2. The transmitter of claim 1, wherein the feedback circuit is configured to set the supply voltage to the digital drive circuit to be in an inversely proportional relationship to the measured quantity.

3. The transmitter of claim 1, wherein the electro-optic modulator is fabricated in a back-end-of-line (BEOL) layer of a semiconductor chip, and wherein the feedback circuit is fabricated in a front-end-of-line (FEOL) layer of the semiconductor chip.

4. The transmitter of claim 3, wherein the semiconductor chip includes a via to connect the feedback circuit between the electro-optic modulator and the drive circuit. 5 . The transmitter of claim 1 , the feedback circuit comprising an analog-to-digital converter (ADC) coupled to the power detector. 6 . The transmitter of claim 5 , wherein the ADC is implemented in a front-end-of-line (FEOL) layer of a semiconductor chip, and wherein the power detector is implemented outside the semiconductor chip.

7. The transmitter of claim 5, wherein the ADC and the power detector are implemented outside a semiconductor chip, and the electro-optic modulator is fabricated on the semiconductor chip.

8. The transmitter of claim 1, wherein the power detector is implemented in a front-end-of-line (FEOL) layer of a semiconductor chip, and wherein the power detector is coupled to the output of the electro-optic modulator using a through-BEOL optical connection.

9. A method for manufacturing a photoelectric transmitter, the method comprising: An electro-optic modulator is fabricated in a back-end-of-line (BEOL) layer of a semiconductor chip to modulate an optical signal in response to an electrical drive signal; Manufacturing a digital driving circuit including a serializer-deserializer SerDes in the front-end process FEOL layer of the semiconductor chip, for generating the electrical driving signal; Make a feedback circuit; as well as The feedback circuit is connected between the electro-optic modulator and the driving circuit, including coupling the SerDes to the electro-optic modulator through a through hole in the semiconductor chip, for measuring a quantity indicative of a power level of a modulated optical signal generated by the electro-optic modulator, and for adjusting a supply voltage to the digital driving circuit in response to the measured quantity, wherein measuring the quantity indicative of the power level of the modulated optical signal generated by the electro-optic modulator includes measuring the quantity by receiving a portion of the modulated optical signal from the optical modulator and measuring a photocurrent proportional to a square root of an optical power level of the portion.

10. A method for manufacturing a photoelectric transmitter, the method comprising: making an electro-optic modulator for modulating an optical signal in response to an electrical drive signal; Making a digital driving circuit for generating the electrical driving signal, wherein the digital driving circuit comprises a serializer-deserializer SerDes; making a feedback circuit, including implementing an optical power detector coupled to an analog-to-digital converter ADC; and The feedback circuit is connected between the electro-optic modulator and the drive circuit for measuring a quantity indicative of a power level of a modulated optical signal produced by the electro-optic modulator and for adjusting a supply voltage to the digital drive circuit in response to the measured quantity.

11. The method of claim 10, wherein fabricating the optical power detector comprises implementing the optical power detector in a front-end-of-line (FEOL) layer of a semiconductor chip and coupling the optical power detector to an output of the electro-optic modulator using a through-BEOL optical connection. 12 . The method of claim 10 , wherein fabricating the feedback circuit comprises implementing the ADC in a front-end-of-line (FEOL) layer of a semiconductor chip, and implementing the optical power detector outside the semiconductor chip.

13. The method of claim 10, wherein fabricating the feedback circuit comprises implementing the ADC and the optical power detector external to a semiconductor chip on which the electro-optic modulator is fabricated.

14. A method for controlling a photoelectric transmitter, the method comprising: Generating an electrical drive signal using a digital drive circuit, wherein the digital drive circuit includes a serializer-deserializer (SerDes); modulating an optical signal using the electrical drive signal; measuring a quantity indicative of a power level of a modulated optical signal by receiving a portion of the modulated optical signal and measuring a photocurrent proportional to a square root of an optical power level of the portion; as well as A supply voltage to the digital drive circuit is adjusted in response to the measured quantity.

15. The method of claim 14, wherein adjusting the supply voltage comprises setting the supply voltage to be in an inverse relationship with the measured quantity.

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