US-12624989-B2 - Optical detector, optical receiver, and optical transceiver

US12624989B2US 12624989 B2US12624989 B2US 12624989B2US-12624989-B2

Abstract

An optical detector includes a photodetector that converts signal light into an electric signal, and a mode filter. The photodetector includes a first port to which signal light of a first mode is input, and a second port to which signal light of a second mode is input. The mode filter includes a third port connected to the first port, and a fourth port connected to the second port. The mode filter passes the signal light of the first mode towards the first port to be output from the third port, passe the signal light of the second mode towards the second port to be output from the fourth port, removes the signal light of the second mode from the first port input from the third port, and removes the signal light of the first mode from the second port input from the fourth port.

Inventors

Akira Oka

Assignees

FUJITSU OPTICAL COMPONENTS LIMITED

Dates

Publication Date: 20260512
Application Date: 20240626
Priority Date: 20230707

Claims (12)

1 . An optical detector comprising: a photodetector that converts signal light into an electric signal and includes two or more ports; and a mode filter including two or more ports, wherein the photodetector includes: a first port to which signal light of a first mode is input; and a second port to which signal light of a second mode is input, the mode filter includes: a third port connected to the first port; and a fourth port connected to the second port, and the mode filter passes the signal light of the first mode towards the first port to be output from the third port, passes the signal light of the second mode towards the second port to be output from the fourth port, removes the signal light of the second mode to be input from the third port, the signal light of the second mode being output from the first port, and removes the signal light of the first mode to be input from the fourth port, the signal light of the first mode being output from the second port.
2 . The optical detector according to claim 1 , wherein the mode filter passes the signal light of the first mode towards the first port to be output from the third port, passes the signal light of the second mode towards the second port to be output from the fourth port, removes signal light including the signal light of the second mode other than the signal light of the first mode to be input from the third port, the signal light including the signal light of the second mode being output from the first port, and removes signal light including the signal light of the first mode other than the signal light of the second mode to be input from the fourth port, the signal light including the signal light of the first mode being output from the second port.
3 . The optical detector according to claim 1 , wherein the mode filter includes: a first mode remover that is connected to the third port, passes the signal light of the first mode, and removes the signal light of the second mode from the first port; and a second mode remover that is connected to the fourth port, passes the signal light of the second mode, and removes the signal light of the first mode from the second port.
4 . The optical detector according to claim 3 , wherein the mode filter includes a branch coupler that causes the signal light of the first mode to branch, the first mode remover passes the signal light of the first mode from the branch coupler, and removes the signal light of the second mode from the second port, and the second mode remover converts the signal light of the first mode from the branch coupler into the signal light of the second mode, passes the signal light of the second mode after being converted, and removes the signal light of the first mode from the first port.
5 . The optical detector according to claim 4 , wherein the first mode remover includes a first adiabatic converter that passes the signal light of the first mode from the branch coupler and removes the signal light of the second mode from the second port, and the second mode remover includes: a second adiabatic converter that converts the signal light of the first mode from the branch coupler into the signal light of the second mode; and a tapered waveguide that passes the signal light of the second mode after being converted by the second adiabatic converter, and removes the signal light of the first mode from the first port.
6 . The optical detector according to claim 1 , wherein the mode filter includes: a mode converter that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode to be output; and a mode remover including a third port connected to the first port and a fourth port connected to the second port, and the mode remover passes the signal light of the first mode towards the first port to be output from the third port, passes the signal light of the second mode towards the second port to be output from the fourth port, removes the signal light of the second mode from the first port input from the third port, and removes the signal light of the first mode from the second port input from the fourth port.
7 . The optical detector according to claim 6 , wherein the mode converter includes a Y-branch waveguide that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode to be output, and the mode remover includes a directional coupler that passes the signal light of the first mode from the Y-branch waveguide and removes the signal light of the second mode from the first port, and passes the signal light of the second mode from the Y-branch waveguide and removes the signal light of the first mode from the second port.
8 . The optical detector according to claim 1 , wherein the mode filter includes: a mode converter that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode to be output; and a mode remover including a fifth port connected to the first port and a sixth port connected to the second port, and the mode remover passes the signal light of the first mode towards the first port to be output from the fifth port, passes the signal light of the second mode towards the second port to be output from the sixth port, removes the signal light of the second mode from the first port input from the fifth port, and removes the signal light of the first mode from the second port input from the sixth port.
9 . The optical detector according to claim 8 , wherein the mode converter includes a Y-branch waveguide that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode to be output, and the mode remover includes: a tapered waveguide that passes the signal light of the first mode from the Y-branch waveguide, and converts the signal light of the second mode from the Y-branch waveguide into signal light of a third mode; and an adiabatic converter that passes the signal light of the third mode from the tapered waveguide and removes the signal light of the first mode from the second port, and passes the signal light of the first mode from the tapered waveguide and removes the signal light of the third mode from the first port.
10 . The optical detector according to claim 1 , wherein the photodetector includes: a fifth port to which signal light of a third mode is input; and a sixth port to which signal light of a fourth mode is input, the mode filter includes: a seventh port connected to the fifth port; and an eighth port connected to the sixth port, and the mode filter passes the signal light of the third mode towards the fifth port to be output from the seventh port, passes the signal light of the fourth mode towards the sixth port to be output from the eighth port, removes the signal light of the fourth mode from the fifth port input from the seventh port, and removes the signal light of the third mode from the sixth port input from the eighth port.
11 . An optical receiver comprising an optical receiver element that receives an electric signal from signal light using light, wherein the optical receiver element includes an optical detector including: a photodetector that converts the signal light into an electric signal and includes two or more ports; and a mode filter including two or more ports, the photodetector includes: a first port to which signal light of a first mode is input; and a second port to which signal light of a second mode is input, the mode filter includes: a third port connected to the first port; and a fourth port connected to the second port, and the mode filter passes the signal light of the first mode towards the first port to be output from the third port, passes the signal light of the second mode towards the second port to be output from the fourth port, removes the signal light of the second mode to be input from the third port, the signal light of the second mode being output from the first port, and removes the signal light of the first mode to be input from the fourth port, the signal light of the first mode being output from the second port.
12 . An optical transceiver comprising: an optical modulator element that transmits transmission light by optically modulating light using a transmission signal; and an optical receiver element that receives reception signal from received light using light, wherein the optical receiver element includes an optical detector including: a photodetector that converts the received light into a reception signal and includes two or more ports; and a mode filter including two or more ports, the photodetector includes: a first port to which signal light of a first mode is input; and a second port to which signal light of a second mode is input, the mode filter includes: a third port connected to the first port; and a fourth port connected to the second port, and the mode filter passes the signal light of the first mode towards the first port to be output from the third port, passes the signal light of the second mode towards the second port to be output from the fourth port, removes the signal light of the second mode to be input from the third port, the signal light of the second mode being output from the first port, and removes the signal light of the first mode to be input from the fourth port, the signal light of the first mode being output from the second port.

Description

CROSS-REFERENCE TO RELATED APPLICATION This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2023-112319, filed on Jul. 7, 2023, the entire contents of which are incorporated herein by reference. FIELD The embodiments discussed herein are related to an optical detector, an optical receiver, and an optical transceiver. BACKGROUND In recent years, high-speed and large-capacity communication has been implemented in an optical fiber communication network by using coherent optical communication technology. In an optical receiver using the coherent optical communication technology, after mixing received light the optical power of which has been lowered due to long-distance transmission with local light emission generated inside the optical receiver, the received light after the mixing is converted into a current by a Photo Detector (PD). As a result, the optical receiver enables highly-sensitive reception by reducing influence of thermal noise generated after the PD. However, in the optical receiver, reception sensitivity is increased as optical power of local light emission is increased, but a response speed of the PD is limited by a space-charge effect when large optical power is input to the PD, so that high-speed communication is difficult to be performed. In the space-charge effect, when light input to the PD generates electron-hole pairs, an electric field generated in the PD by these carriers works to counteract an applied bias to the PD to reduce the applied bias, and force of carrying the carriers in the PD to the outside of the PD is weakened. As a result, the response speed of the PD is lowered. Additionally, the space-charge effect becomes remarkable in a case in which the number of electron-hole pairs generated in the PD is large, in other words, in a case in which input power is large. Thus, the space-charge effect can be mitigated by reducing the input power. As a method for reducing the input power to mitigate the space-charge effect, for example, there is known a method of dividing optical power before being input to the PD and inputting respective pieces of light to the PD from two directions. In the PD, light is converted into electron-hole pairs while being propagated therein, so that the optical power is reduced as being separated away from a light input unit of the PD. Thus, in a case in which the optical power to be input is divided into two pieces and input to the PD from two directions, light intensity per cross section can be suppressed to be about half, so that influence of the space-charge effect can be reduced. As a result, even if the optical power input to the PD is high, deterioration of the response speed of the PD can be suppressed. As the PD, for example, a waveguide-type PD is employed. The waveguide-type PD includes a PD integrated on the same substrate and an optical waveguide that guides light, and guides light from the waveguide to the PD to perform conversion of light/current by the PD. The waveguide-type PD is constituted of a Silicon-On-Insulator (SOI) layer of an SOI substrate and Ge epitaxially grown thereon. In a communication wavelength band, for example, in a C band, a band gap of Ge is narrower as compared with Si, so that light propagated in Ge is converted into electron-hole pairs in Ge, and photoelectric conversion can be implemented by extracting the electron-hole pairs by doped Si or an electrode. A layer for converting light into electron-hole pairs like Ge is called an absorbing layer. The absorbing layer is a portion that has a band gap different from that of the optical waveguide, and absorbs input light to be converted into electron-hole pairs. As described above, the waveguide-type PD includes the optical waveguide and the PD. The optical waveguide includes an Si substrate, a Buried Oxide (BOX) layer laminated on the Si substrate, an optical waveguide of Si formed on the BOX layer, and a buffer layer covering the BOX layer and the optical waveguide. The optical waveguide of Si is formed on the BOX layer by lithographically etching Si of the SOI layer of the SOI substrate formed of the Si substrate and the BOX layer. The PD includes the Si substrate, the BOX layer, the absorbing layer of Ge, and the buffer layer covering the BOX layer and the absorbing layer. The absorbing layer is connected to the optical waveguide, absorbs signal light from the optical waveguide, and converts the signal light into electron-hole pairs. The related technologies are described, for example, in: U.S. Patent Application Publication No. 2019/0391006; A. Beling, X. Xie and J. C. Campbell, “High-power high-linearity photodiodes”, Optica, vol. 3, no. 3, pp. 328-338, 2016.; Jishi Cui, et al., “The dual-injection Ge-on-Si photodetectors with high saturation power by optimizing light field distribution”, Optics Communications, Vol 480, 1 Feb. 2021, 126467; Xiao Hu, et al., “High-speed and high-power germanium photodetector wit