Optical Transceiver

Abstract

Provided is an integrated optical transmitter / receiver in which transmission / reception crosstalk is suppressed. The optical transmitter / receiver in which an optical transmission unit and an optical reception unit are integrated together with digital signal processing units comprises a storage unit that stores a transmission / reception crosstalk component measured in advance at the optical reception unit, with respect to a digital input signal to be inputted to the digital signal processing unit of the optical transmission unit. The digital signal processing unit of the optical reception unit acquires, from the storage unit, a transmission / reception crosstalk component corresponding to a digital input signal inputted to the digital signal processing unit of the optical transmission unit, and subtracts the component from a digital signal converted from a received optical signal.

Description

[Technical Field]

[0001] The present invention relates to an optical transceiver in which an optical transmitter and an optical receiver are integrated together with a digital signal processing unit. [Background technology]

[0002] With the explosive development and spread of the Internet and its many applications, high-speed, broadband optical communication systems are being laid out all over the world. Optical communication systems are not only used for long-distance intercity communications, but are also widely used for communications between and within data centers, which have seen a remarkable increase in recent years.

[0003] The communication bandwidth of each channel in optical communication systems continues to increase, and the transmission capacity per channel and per wavelength continues to increase. Currently, optical communication systems with a communication capacity of 100 Gb / s to 400 Gb / s or more per wavelength are beginning to be introduced. Accordingly, there is an increasing demand for optical transceivers that relay high-speed, large-capacity optical lines to not only increase their transmission capacity, but also to become more compact and cost-effective.

[0004] As optical transceivers become smaller and cheaper, their internal structures are becoming simpler, with more low-cost components being used, and integration and integration are progressing. Recently, silicon photonics technology has been used to combine optical modulators, which serve as the front end of optical transmitters, with electro-optical conversion elements such as laser diodes on a single chip. Furthermore, optical modules have been developed that integrate the front end of optical receivers, such as photodiodes and preamplifiers, and that combine digital signal processing units to perform both functions as an optical transceiver.

[0005] In an integrated optical transceiver, the optical transmitter section has a driver amplifier required to drive the optical modulator, which outputs a large electrical amplitude. Meanwhile, the optical receiver section has a high-gain preamplifier to receive the small electrical signal from the photodiode. In an integrated optical transceiver, the large electrical signal from the driver amplifier propagates inside the optical transceiver as an electromagnetic wave and is received by the optical receiver section. For this reason, the electromagnetic wave essentially becomes noise for the optical receiver section, degrading the reception characteristics. This leakage signal that degrades the characteristics is called transmit / receive crosstalk.

[0006] The most reliable method for reducing transmission / reception crosstalk in an optical transceiver is to isolate the optical transmitter and receiver. However, as optical transceivers become smaller, the distance between the optical transmitter and receiver becomes shorter, making it difficult to isolate them. For example, this can be improved by inserting a metal wall or forming a metal pattern between the optical transmitter and receiver to provide electromagnetic isolation. However, when an optical transceiver is integrated on a single chip, it is not possible to secure the space for such isolation (see, for example, Non-Patent Document 1). Furthermore, as speeds increase, the frequency of the drive signal for the driver amplifier also increases, making it more difficult to achieve electromagnetic isolation. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Yusuke Nasu and Shogo Yamanaka, "Development of Photonics-Electronic Integrated Optical Transceiver Module Using Silicon Photonics Technology," NTT Technical Journal, August 2020, pp. 10-14 Summary of the Invention

[0008] An object of the present invention is to provide an integrated optical transceiver that suppresses transmission and reception crosstalk.

[0009] In order to achieve the above object, one embodiment of the present invention provides an optical transceiver in which an optical transmitting section and an optical receiving section are integrated together with a digital signal processing section, the optical receiving section measuring a transmission / reception crosstalk component in advance in response to a digital input signal input to the digital signal processing section of the optical transmitting section. , a time step corresponding to the propagation time of the electromagnetic wave propagating through the optical transceiver a digital signal processing unit of the optical receiving unit for storing a transmission / reception crosstalk component corresponding to a digital input signal input to the digital signal processing unit of the optical transmitting unit; In front The signal is obtained from the storage unit and subtracted from the digital signal converted from the received optical signal. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical transceiver according to an embodiment of the present invention; [Figure 2] FIG. 2 is a diagram showing a procedure for compensating for transmission and reception crosstalk in the optical transceiver of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Transmit / receive crosstalk in an optical transceiver is determined by the electrical signal transmitted from the optical transmitter. Therefore, in the optical transceiver of this embodiment, the transmit / receive crosstalk component corresponding to the electrical signal transmitted is subtracted from the received signal, thereby canceling out the transmit / receive crosstalk and suppressing degradation of the reception characteristics of the optical receiver.

[0012] Figure 1 shows the configuration of an optical transceiver according to one embodiment of the present invention. The optical transmitter comprises, in order, a digital signal processor (DSP-Tx) 11 that performs digital modulation such as PAM4 or QPSK on a digital input signal, a digital-to-analog converter (DAC) 12 that converts the digitally modulated signal to an analog signal, and a driver amplifier 13 that drives the optical modulator. The output light from a laser (LD) 14, which serves as a light source, is modulated by an optical modulator in a transceiver chip 31, and the modulated optical signal is output to an optical fiber connected to the optical transceiver.

[0013] Meanwhile, in the optical receiving unit, the signal light input from the optical fiber is received by a photodiode in the transceiver chip 31, converted into an electrical signal, and amplified in the preamplifier 23. At this time, in the case of a coherent optical receiving method rather than an intensity modulation method, the input signal light is mixed with local light and polarization-separated in a polarization multiplexed optical hybrid (DPOH) circuit before being received by the photodiode. The optical receiving unit further includes an analog-to-digital converter (ADC) 22 that converts the output of the preamplifier 23 into a digital signal, and a digital signal processing unit (DSP-Rx) 21 that demodulates the converted digital signal.

[0014] Furthermore, the optical transceiver 10 includes an arithmetic and memory unit 33 for suppressing transmission and reception crosstalk, which will be described later, and a clock circuit (clk) 32 for supplying a common clock signal to the DSP-Tx 11, DAC 12, DSP-Rx 21, ADC 22, and arithmetic and memory unit 33. The DSP-Tx 11, DSP-Rx 21, and arithmetic and memory unit 33 can be implemented as a processor equipped with memory, or as a programmable logic array (PLA).

[0015] The transmission / reception crosstalk mainly occurs when the electrical signal from the driver amplifier 13 propagates as an electromagnetic wave inside the optical transceiver 10 and is received by the analog circuit portion of the optical receiving unit from the transmission / reception chip 31 to the ADC 22. However, in many cases, the main transmission / reception crosstalk occurs through the electromagnetic wave propagation within the transmission / reception chip 31, or in the vicinity of the chip, where the portion through which the transmission electrical signal output from the driver amplifier 13 propagates and the portion through which the reception electrical signal input to the preamplifier 23 propagates are closest.

[0016] Referring to FIG. 2, the procedure for compensating for transmit / receive crosstalk in the optical transceiver of this embodiment will be described. First, before the optical transceiver 10 is put into practical use, the transmit / receive crosstalk components for a given digital input signal are measured and recorded. The extent to which the transmit / receive crosstalk components generated by the optical transmitter affect the optical receiver is limited by the chip size. For example, if the length of the optical modulator through which the transmission electrical signal output from the driver amplifier 13 propagates is 1 mm and the distance to the optical receiver is constant, and the propagation speed of the electromagnetic wave within the optical modulator is 1E8 m / s, the propagation time of the signal within the optical modulator is 1E-3 / 1E8 = 10 ps. This time corresponds to one time step for a 100 Gbaud digital input signal. The signal within this optical modulator propagates sequentially, resulting in transmit / receive crosstalk. If the component of the electromagnetic wave traveling directly from the optical transmitter to the optical receiver is the primary transmit / receive crosstalk, then only the 10 ps time step needs to be considered as transmit / receive crosstalk. Since the reflected components of the electromagnetic waves are attenuated on the path from the optical transmitter to the optical receiver, it is only necessary to consider signals within a range of a certain number of times the main transmission / reception crosstalk.

[0017] That is, the transmit / receive crosstalk components corresponding to the transmit electrical signal can be subtracted from the receive signal, taking into account only the transmit electrical signal for the time step that will have an effect within the chip. Therefore, if only one time step is affected, for example, in the case of the PAM4 modulation method, the transmit / receive crosstalk components can be measured and recorded for four types of signals per time step. In this way, the transmit / receive crosstalk components can be measured in advance for the transmit electrical signal pattern for a predetermined number of time steps, based on the propagation time of the electromagnetic wave within the semiconductor chip and the digital modulation method of the transmit signal.

[0018] In the first embodiment, a 200 Gb / s digital input signal 41 is input to the optical transceiver 10 in a fixed pattern, digitally modulated using the PAM4 modulation method, and a 100 Gbaud, four-value signal is output from the DSP-Tx 11. The optical input to the transceiver chip 31 is set to 0, and the digital signal output from the ADC 22 is recorded in the recording unit 42 as a transmitting / receiving crosstalk component. At this time, because a common clock signal from clk 32 is used, timing information for the timing of the output from the DSP-Tx 11 can be recorded as a crosstalk component.

[0019] The calculation unit 43 calculates the transmit / receive crosstalk component for each four-level signal of the PAM4 modulation format. For example, since thermal noise and the transmit / receive crosstalk component are likely to have similar signal intensities, a certain pattern may be repeatedly input and averaged to mitigate the influence of thermal noise and calculate the transmit / receive crosstalk component. Furthermore, if the DSP-Tx 11 performs equalization processing in addition to digital modulation, the transmit / receive crosstalk component may be calculated for each equalization setting. Furthermore, the transmit / receive crosstalk component may be measured or converted using a theoretical formula depending on the gain of the driver amplifier 13, the amplitude of the electrical signal applied to the optical modulator, the temperatures of the driver amplifier 13 and the preamplifier 23, and the like, and stored as a correction coefficient for the transmit / receive crosstalk component measured under specified conditions.

[0020] Furthermore, when measuring the transmit / receive crosstalk components, if the transmit / receive crosstalk components in a specific frequency range are particularly large, the frequency dependency of the input signal can be calculated by fitting only the amplitude and phase of the specific frequency, thereby improving the accuracy of the calculation of the transmit / receive crosstalk. The calculated transmit / receive crosstalk components and correction coefficients including timing information with respect to the transmission electrical signal are stored in the calculation / storage unit 33 in the optical transceiver 10.

[0021] Next, during actual operation of the optical transceiver 10, corresponding transmit / receive crosstalk components are extracted from the arithmetic and memory 33 according to the pattern of the digital input signal input to the DSP-Tx 11 or the pattern of the modulated digital signal output from the DSP-Tx 11. The corresponding transmit / receive crosstalk components and a correction coefficient are sent to the DSP-Rx 21. Based on the correction coefficient including timing information obtained from the arithmetic and memory 33, the DSP-Rx 21 subtracts the obtained transmit / receive crosstalk components from the digital signal output from the ADC 22, or subtracts the transmit / receive crosstalk components multiplied by the correction coefficient as necessary.

[0022] While the above discussion focuses on a single-channel optical transceiver consisting of a pair of optical transmitter and receiver, i.e., a single-channel optical transceiver, transmission and reception crosstalk compensation can also be performed in optical transceivers with multiple channels integrated. For example, a coherent optical transceiver, such as a DP-QPSK transceiver, includes a four-channel optical transceiver. In this case, the transmission and reception crosstalk components are measured for each optical reception channel affected by the optical transmitter. For the optical transmitter, the transmission and reception crosstalk components and correction coefficients for a given digital input signal can also be calculated, starting with the optical transmission channel closest to the optical reception channel being measured. In actual operation, it is possible to compensate only for the transmission and reception crosstalk components from the optical transmission channel that is expected to have the greatest transmission and reception crosstalk impact, or to compensate for the combined transmission and reception crosstalk components from all optical transmission channels. Furthermore, even in optical transceivers with integrated wavelength-multiplexed transmitters and receivers, transmission and reception crosstalk can be suppressed for each single-channel optical transceiver for each wavelength using the same compensation procedure described above.

Claims

1. In an optical transceiver in which an optical transmitting section and an optical receiving section are integrated together with a digital signal processing section, a storage unit that stores a transmission / reception crosstalk component measured in advance in the optical receiving unit in accordance with a digital input signal input to a digital signal processing unit of the optical transmitting unit, for a time step corresponding to a propagation time of an electromagnetic wave propagating within the optical transceiver; An optical transceiver characterized in that the digital signal processing unit of the optical receiving unit obtains from the storage unit a transmit / receive crosstalk component corresponding to a digital input signal input to the digital signal processing unit of the optical transmitting unit, and subtracts it from a digital signal converted from the received optical signal.

2. 2. The optical transceiver according to claim 1, wherein a common clock signal is supplied to the digital signal processing unit of the optical transmitting unit, the digital signal processing unit of the optical receiving unit, and the memory unit, and the memory unit stores the transmit / receive crosstalk components including timing.

3. 3. The optical transceiver according to claim 1, wherein the storage unit receives the digital input signal a plurality of times and stores averaged transmission / reception crosstalk components.

4. 3. The optical transceiver according to claim 1, wherein the storage unit further stores a correction coefficient for a transmission / reception crosstalk component measured under predetermined conditions.

5. 3. The optical transceiver according to claim 1, wherein the storage unit stores only transmission / reception crosstalk components in a specific frequency range.

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