JP7493992B2 - Optical receiving device - Google Patents

Description

The present invention relates to an optical receiving device for a system applied to optical communications, and in particular to a device that converts high-speed optical signals with a transmission speed exceeding 1 Gbps into electrical signals in a system that uses a large-diameter plastic optical fiber with a core diameter of approximately 980 μm as the transmission medium.

A conventional optical receiver is shown in Figure 6. An optical receiver that achieves a transmission speed of over 1 Gbps is composed of a wideband light receiving element 9 with sufficient margin for the transmission speed, a transimpedance type amplifier circuit 10, and a limiter amplifier 14. The light receiving element 9 mounted on a conventional optical receiver employs a GaAs or InGaAs pin photodiode (Pin-PD) to match the wavelengths of the LD light source used, 850 nm, 1310 nm, and 1550 nm. In either case of the material, the light receiving diameter is reduced from 50 μm to 80 μm to ensure the desired bandwidth in order to keep the total capacitance, which is the combination of the junction capacitance Cj and the parasitic capacitance Cs of the package, below 1 pF.

In more detail, the bandwidth of the pin photodiode (Pin-PD) is limited by the sum of the junction capacitance Cj and the parasitic capacitance Cs caused by the package, and the junction capacitance Cj changes in proportion to the area of the light receiving area. Therefore, if the light receiving diameter is reduced, the junction capacitance Cj will be reduced, resulting in wideband characteristics. On the other hand, the optical fibers that have been used in receivers exceeding 1 Gbps in the past generally use multimode optical fibers with a core diameter of 50 μm and single mode optical fibers with a core diameter of 10 μm. The light emitted from an optical fiber with a core diameter of 50 μm has a beam diameter of approximately 50 μm at the output end, and when this is coupled to a light receiving element, a lens system with an image magnification of 1:1 has been generally used in the past. Therefore, the beam diameter when coupled to the light receiving surface of the light receiving element becomes 50 μm, and all of the incident light fits within the light receiving area. In the case of 80 μm, there is even more room for coupling, but the junction capacitance Cj increases as the light receiving diameter increases, and the bandwidth characteristics are inferior to those of 50 μm.

Furthermore, the transimpedance amplifier circuit 10 installed in the conventional optical receiving device is optimized for the required transimpedance gain, band characteristics, and noise characteristics, taking into account the junction capacitance Cj + parasitic capacitance Cs of the light receiving element 9.

As mentioned above, optical transmission systems exceeding 1 Gbps generally use a multimode optical fiber with a quartz core and a core diameter of 50 μm, or a single mode optical fiber with a core diameter of 10 μm, as the transmission medium. In this case, when the light emitted from each optical fiber is coupled to the light receiving element 9, a simple lens system with a lens image magnification of approximately 1:1 can fit the light within the light receiving diameter of the light receiving element 9, and the optical coupling system is designed so that excessive coupling loss does not occur. In addition, in conventional optical receiving devices, a low-cost ball lens integrated with the sealing glass window of the TO-CAN package is widely used.

Recently, systems that apply light to communication have expanded from optical communication systems that support public networks to applications that require noise resistance and high bandwidth. In particular, for computer numerical controls that control robots, which are essential for factory automation, servo pumps, and programmable controllers, systems that apply light to the transmission of control signals have long been popular, with an emphasis on noise resistance.

Conventionally, a transmission speed of about 150 Mbps was sufficient for this type of system, and so acrylic resin-based step-index plastic optical fiber with a core diameter of 980 μm was widely used. However, while this acrylic resin-based plastic optical fiber is easy to connect due to its large core diameter, its bandwidth is narrow at about 5 MHz/km, so there was a risk that a 150 Mbps signal would not be transmitted at a distance of about 20 m due to the bandwidth limitations.

Therefore, when transmission up to about 100 m was required within a factory, step-index (SI) or graded-index (GI) hard plastic clad optical fiber with a core diameter of 200 μm, consisting of a quartz core and plastic cladding, was used.

Furthermore, conventional optical receiving devices use silicon pin photodiodes as the light receiving element 9 to match the 650 nm wavelength band of the light source, and silicon pin photodiodes with a light receiving diameter of 500 to 600 μm are used as the light receiving element 9 to ensure sufficient bandwidth characteristics at a transmission speed of 150 Mbps.

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Incidentally, in recent years, the progress of IoT in factories has been remarkable, and robot control has also evolved from an independent form for each production process to a form connected via a field network, and various sensors are being installed to monitor the operating status of robots and detect faults in real time.

As the amount of information flowing through field networks increases, there is a demand for faster field networks, and it is hoped that the signal speed, which was previously around 150 Mbps, will be increased to a transmission speed of over 1 Gbps. On the other hand, since it will be necessary to coexist with conventional devices, there is also an essential demand for intercommunication with 150 Mbps systems that use conventional plastic optical fibers with a core diameter of 980 μm.

Optical transmitters and receivers, which are widely used in computer numerical control, servo amplifiers, programmable controllers, etc., are equipped with an LED light source with a wavelength of 650 nm and a silicon pin photodiode light receiving element, and are each composed of an LSI that drives the LED and an LSI for the light receiving circuit.

In addition, conventional optical transmitters and receivers are used in combination with a plastic optical fiber with a core diameter of 980 μm, or a hard plastic clad optical fiber with a core diameter of 200 μm, either step-index type (SI) or graded-index type (GI), as the transmission medium. When a conventional optical receiver of over 1 Gbps is inserted into a field network based on a plastic optical fiber with a core diameter of 980 μm, or a hard plastic clad optical fiber with a core diameter of 200 μm, either step-index type or graded-index type, the optical receiver equipped with an InGaAs-based pin photodiode (PIN-PD) as a light receiving element cannot be used because it has almost no light receiving sensitivity in the 650 nm band. On the other hand, optical receivers equipped with GaAs-based pin photodiodes (Pin-PDs) have inferior light receiving sensitivity in the 650 nm band compared to receivers equipped with Si-based pin photodiodes (Pin-PDs). At the same time, due to a coupling system with a light receiving diameter of 50 μm and an image magnification of approximately 1:1, most of the incident light falls outside the light receiving area, causing excess loss of about 10 to 20 dB in the optical coupling system, resulting in a level below the minimum reception level and preventing the optical receiver from operating normally.

That is, conventionally, optical fibers used in field networks for FA equipment are plastic optical fibers with a core diameter of 980 μm, or hard plastic clad optical fibers with a core diameter of 200 μm, either step index type (SI) or graded index type (GI). When light is input from an optical fiber with a core diameter of 980 μm or 200 μm to a light receiving module with a coupling system of 1:1 image magnification, assuming the use of an optical fiber with a core diameter of 50 μm or 10 μm, excess loss occurs because the outgoing light from the optical fiber cannot be entirely contained within the numerical aperture NA (Numerical Aperture) of the lens at the input end of the lens. At the same time, because the image magnification is 1:1, the beam diameter on the light receiving surface is 980 μm or 200 μm, which is outside the light receiving area, and this results in large excess loss. In other words, a 980 μm or 200 μm circular beam is received by a 50 μm light receiving area, so most of the signal light ends up outside the light receiving area.

The present invention has been devised in consideration of the various circumstances that existed in the past as described above, and aims to provide an optical receiving device that can communicate with an optical transmitting device equipped with a conventional 650 nm band light source at an operating speed of 150 Mbps or less, and that can transmit the optical output signal of the optical transmitting device through a plastic optical fiber with a core diameter of 980 μm, or a step-index type (SI) or graded-index type (GI) hard plastic clad optical fiber with a core diameter of 200 μm.

In order to solve the above-mentioned problems, the present invention provides an optical receiving device that is composed of a photodetector that converts an input optical signal into an electrical signal, a transimpedance amplifier circuit that converts the photocurrent output from the photodetector into a voltage signal with low noise and amplifies it, and a limiter amplifier that amplifies the output of the transimpedance amplifier circuit and has the function of distinguishing between 0 level and 1 level of the signal at a predetermined level, and the photodetector is mounted in a package together with the chip of the transimpedance amplifier circuit, forms an optical receiving module equipped with a lens barrel, and has the function of converting an optical signal input from an optical fiber with a core diameter of about 980 μm into an electrical signal.

The light receiving element is characterized by having an effective light receiving area optimized to ensure the necessary bandwidth even when an optical signal of 1 Gbps or more is input from an optical fiber with a core diameter of approximately 980 μm, and a light blocking mask for coupling leakage light.

The lens barrel that constitutes the optical receiving module comprises a sleeve into which an optical fiber is inserted and an aspherical lens, and the aspherical lens is optimized to tolerate a coupling loss of about 2 to 4 dB for signals input from an optical fiber with a core diameter of about 980 μm, and the lens barrel is adhesively fixed to the TO-CAN package without any alignment.

The lens barrel is characterized by the fact that when an optical signal is input via an optical fiber with a core diameter of 200 μm or less, the coupling loss is nearly zero.

According to the present invention, the optical receiver 18 can achieve stable reception at operating speeds exceeding 1 Gbps, regardless of whether the transmission medium is a plastic optical fiber 7 with a core diameter of 980 μm, or a step-index type (SI) or graded-index type (GI) hard plastic clad optical fiber with a core diameter of 200 μm. At the same time, stable transmission characteristics can be guaranteed even when facing an optical transmitter that uses a 650 nm band LED as a light source, which is widely used in conventional FA equipment, and an effective communication means can be provided even when updating existing FA systems.

1A is an overall explanatory diagram of an optical receiving device showing one embodiment of the present invention, and FIG. FIG. 1 is a diagram showing the relationship between a standardized reception band and a minimum reception sensitivity. 11 is a diagram showing the relationship between the capacitance of a light receiving element and a 3 dB drop band of a receiving circuit. FIG. 13 is a diagram showing the relationship between the light receiving diameter and the capacity. FIG. 11 is a diagram showing the capacitance and minimum receiving sensitivity of a light receiving element. FIG. 1 is an overall explanatory diagram showing a conventional optical receiving device.

An optical receiving device according to an embodiment of the present invention will now be described in detail with reference to the drawings.
In this embodiment, as shown in FIG. 1, in the optical receiving device 18, an optical signal input to the light receiving module 5 via a plastic optical fiber 7 with a core diameter of 980 μm or a step index type (SI) or graded index type (GI) hard plastic clad optical fiber 8 with a core diameter of 200 μm is coupled to the light receiving element 1 via a lens barrel 17. This light receiving element 1 has an optimized effective light receiving area 1A described later, and a light shielding mask 1B made of resin or metal arranged on its outer periphery. The photocurrent photoelectrically converted by the light receiving element 1 is amplified by the transimpedance type amplifier circuit 2 and input to the limiter amplifier 6 connected in the subsequent stage, and the signal amplified inside the limiter amplifier 6 is output by determining the 0 level or 1 level of the input digital signal by the discrimination function of the limiter amplifier 6. In this embodiment, the transimpedance type amplifier circuit 2 and the limiter amplifier 6 are AC-coupled, but it is also possible to integrate both circuits on one chip and use DC-coupling.

Figure 2 is a graph that shows a schematic relationship between the reception band normalized by the signal speed and the minimum reception sensitivity (Pr). Generally, the optimum value for the minimum reception sensitivity (Pr) is determined based on the relationship between noise and the band. Usually, the reception band is set at about 0.75 times the signal speed B, as a point where inter-symbol interference and the signal-to-noise ratio (S/N) are balanced. If the reception band is narrower than this, the signal-to-noise ratio S/N improves, but the inter-symbol interference due to insufficient reception band becomes the dominant factor and the minimum reception sensitivity deteriorates. Conversely, if the reception band is wider than 0.75 x B, the signal-to-noise ratio S/N deteriorates and the minimum reception sensitivity deteriorates.

In other words, if the bandwidth of the receiving circuit is lowered, the response characteristics will be such that the pulse "1" does not rise all the way up, and the pulse "0" does not fall all the way down. Incidentally, the narrowing of the eye pattern opening in the amplitude direction at this time is generally called inter-symbol interference. On the other hand, if the bandwidth of the receiving circuit is extended, noise components over a wide band will be superimposed on the signal, lowering the signal-to-noise ratio (S/N) and causing degradation of receiving sensitivity.
Figure 2 shows a schematic diagram of the degradation of receiver sensitivity due to the above factors. When the receiving band is narrower than 0.75 x B, inter-symbol interference becomes the dominant factor, causing degradation of receiver sensitivity, and conversely, when the receiving band is wider than 0.75 x B, degradation of the signal-to-noise ratio (S/N) becomes the dominant factor, causing degradation of receiver sensitivity. The point with the smallest penalty in Figure 2 is the optimal receiving band where both are balanced, and is generally taken to be 0.75 times the signal speed.

Conventionally, optical receiving circuits that have been applied to optical communication devices of 1 Gbps or more use multimode optical fiber (quartz core) with a core diameter of 50 μm for short distance applications, and single mode optical fiber (quartz core) with a core diameter of 10 μm for long distance applications.

As shown in Figure 6, in the conventional optical receiving device 19, in order to optimize the receiving band to about 0.75 x B as mentioned above, the light receiving diameter of the light receiving element 9 is reduced to 50 μm to 80 μm, and the junction capacitance Cj of the light receiving element 9 and the parasitic capacitance Cs of the light receiving element package are reduced, so that the desired receiving band can be obtained.

The light receiving module 13 mounted on the conventional optical receiving device 19 is connected to a multimode optical fiber with a core diameter of 50 μm in combination with an optical fiber with the largest core diameter, so the lens for focusing the input signal light to the light receiving element 9 generally uses a ball lens 12 with an image magnification set to around 1. Since the light receiving diameter of the light receiving element 9 is 50 μm to 80 μm, it is possible to couple all the input light to the light receiving area, and the configuration is such that the occurrence of excess loss in the optical coupling system can be suppressed to almost zero.

On the other hand, the optical receiving device 18 of this embodiment shown in Figure 1 aims to operate stably at a transmission speed of more than 1 Gbps when a plastic optical fiber 7 with a core diameter of 980 μm or a hard plastic clad optical fiber 8 with a core diameter of 200 μm of step index type (SI) or graded index type (GI) is used as the optical fiber, which is the transmission medium.

The bandwidth of the optical receiving circuit 3, which is a combination of a light receiving element 1 and a transimpedance type amplifier circuit 2 that converts the photocurrent converted by the light receiving element 1 into a voltage signal, is determined by the junction capacitance Cj of the light receiving element 1, the parasitic capacitance Cs parasitic to the package of the light receiving element 1, and the transimpedance Zt, as described above. The transimpedance Zt is selected so as to be an optimal value based on the transmission speed and the target minimum receiving sensitivity (Pr). The bandwidth of the optical receiving circuit 3 at this time is (1/2) x {GBW/π(Cj + Cs) Zt} ^1/2 , where GBW indicates the gain bandwidth product. Therefore, in order to extend the bandwidth of the optical receiving circuit 3, it is effective to lower the transimpedance Zt or to reduce the junction capacitance Cj + parasitic capacitance Cs, but since the transimpedance is uniquely determined by a value required to obtain the desired receiving sensitivity, it is necessary to reduce the junction capacitance Cj.

Figure 3 shows an example of the change in 3 dB drop in bandwidth of an optical receiving circuit designed to operate at 1.25 Gbps when the transimpedance Zt is kept constant and the junction capacitance Cj + parasitic capacitance Cs is changed. The points indicated by black circles in the figure correspond to 0.75 times the signal speed, which is generally considered to be the optimal point for the receiving band. From Figure 3, it can be seen that a junction capacitance Cj + parasitic capacitance Cs of around 0.8 pF is the optimal value for junction capacitance Cj + parasitic capacitance Cs when operating at 1.25 Gbps.

On the other hand, the junction capacitance Cj + parasitic capacitance Cs changes when the light receiving diameter of the light receiving element 1 is changed. Strictly speaking, the parasitic capacitance Cs caused by the package of the light receiving element 1 is a constant value, but the junction capacitance Cj changes depending on the value of the reverse bias applied to the light receiving element 1. Figure 4 shows the change when a sufficient reverse bias is applied to the point where the bias dependence of the junction capacitance Cj is almost saturated.

The minimum receiving sensitivity Pr is highly dependent on the bandwidth characteristics of the optical receiving circuit 3, and the minimum receiving sensitivity Pr deteriorates when the bandwidth is 0.75 times or less than the signal speed B. In other words, as mentioned above, when the receiving bandwidth becomes narrow, the opening of the eye pattern in the amplitude direction becomes narrower, which causes the receiving sensitivity to deteriorate.

Next, Figure 5 shows the results of measuring the minimum receiving sensitivity Pr for various junction capacitances Cj + parasitic capacitances Cs with a constant transimpedance Zt at a signal speed of 1.25 Gbps. When the junction capacitance Cj + parasitic capacitance Cs exceeds 0.8 pF and falls below the required bandwidth of the optical receiving circuit 3 of 0.75 x B, the minimum receiving sensitivity Pr deteriorates. In this embodiment, we aimed to achieve a minimum receiving sensitivity Pr of approximately -22 dBm at an operating speed of 1.25 Gbps.

From Figure 5, it can be seen that the desired minimum receiving sensitivity Pr can be achieved when the junction capacitance Cj + parasitic capacitance Cs is about 1.4 pF to 1.5 pF. This junction capacitance Cj + parasitic capacitance Cs of 1.5 pF corresponds to a light receiving diameter of the light receiving element 1 of about 260 μm, as shown in Figure 4.

To reduce to almost zero the excess loss associated with the optical coupling system of the signal input to the light receiving module 5 via a hard plastic clad optical fiber 8 of step index type (SI) or graded index type (GI) with a core diameter of 200 μm, the image magnification of the lens should be set to approximately 1x so that all of the optical signal input from the hard plastic clad optical fiber 8 to the light receiving module 5 fits within the receiving diameter.

That is, as described in this embodiment, when the light receiving diameter is set to around 260 μm, the beam diameter at the fiber output end of a step index type (SI) or graded index type (GI) hard plastic clad optical fiber with a core diameter of 200 μm is 200 μm, so if all the light is taken into the lens by the lens numerical aperture NA (Numerical Aperture) and the image magnification is set to 1:1, an image will be formed on the light receiving surface as a beam diameter of 200 μm, which is smaller than the light receiving diameter of 260 μm. In this case, all of the optical signals input to the light receiving element are coupled onto the light receiving surface, so there is no excess loss.

On the other hand, when combined with a plastic optical fiber 7 with a core diameter of 980 μm, a large excess loss occurs. In this embodiment, by changing from the ball lens 12 generally used in the conventional example to the aspherical lens 4 of the lens barrel 17, the aspherical lens 4 was optimized so that the excess loss when combined with an optical fiber with a core diameter of 200 μm is almost zero, and the excess loss when combined with an optical fiber with a core diameter of 980 μm is 2 to 4 dB. Incidentally, when connected via a plastic optical fiber 7 with a core diameter of 980 μm, even if the aspherical lens 4 is used, not all of the input light enters the effective light receiving area, so it leaks outside the light receiving area. As a result, when the leaked light is photoelectrically converted outside the effective light receiving area of the light receiving element 1, it is converted into an electrical signal as a component with a very slow response, resulting in noise.

In this invention, in order to eliminate factors that impede high-speed operation, a light-shielding mask 1B made of resin or metal is provided around the periphery of the effective light-receiving area 1A to prevent this. As a result, at an operating speed of 1.25 Gbps, a minimum receiving sensitivity of -22 dBm has been achieved when combined with an optical fiber with a core diameter of 200 μm, and a minimum receiving sensitivity of -18 dBm has been achieved when combined with an optical fiber with a core diameter of 980 μm.

1 Light receiving element 1A Effective light receiving area 1B Light shielding mask 2 Transimpedance type amplifier circuit 3 Light receiving circuit 4 Aspheric lens 5 Light receiving module 6 Limiter amplifier 7 Plastic optical fiber 8 Step index type hard plastic clad optical fiber and graded index type hard plastic clad optical fiber 9 Light receiving element 10 Transimpedance type amplifier circuit 11 Light receiving circuit 12 Ball lens 13 Light receiving module 14 Limiter amplifier 15 Multimode optical fiber 16 Single mode optical fiber 17 Lens barrel 18 Optical receiving device 19 Optical receiving device

Claims (3)

An optical receiving device comprising a photodetector that converts an input optical signal into an electrical signal, a transimpedance amplifier circuit that converts and amplifies a photocurrent output from the photodetector into a voltage signal with low noise, and a limiter amplifier that amplifies the output of the transimpedance amplifier circuit and has a function of distinguishing between 0 level and 1 level of the signal at a predetermined level, wherein the photodetector is mounted in a package together with a chip of the transimpedance amplifier circuit to constitute an optical receiving module equipped with a lens barrel, the lens barrel constituting the optical receiving module comprising a sleeve for inserting an optical fiber and an aspherical lens, the aspherical lens being optimized to tolerate a coupling loss of about 2 to 4 dB for a signal input from an optical fiber having a core diameter of about 980 μm, the lens barrel being adhesively fixed to a TO-CAN package without alignment, and the optical receiving device having a function of converting an optical signal input from an optical fiber having a core diameter of about 980 μm into an electrical signal. The optical receiving device according to claim 1, characterized in that the light receiving element is provided with an effective light receiving area optimized to ensure the necessary bandwidth even when an optical signal of 1 Gbps or more is input from an optical fiber with a core diameter of about 980 μm, and a light shielding mask for coupling leakage light. 2. The optical receiving device according to claim 1 , wherein the lens barrel is configured such that a coupling loss becomes substantially zero when an optical signal is input via an optical fiber having a core diameter of 200 [mu]m or less.

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