Optical transmitter and optical transmission system

Abstract

To provide an optical transmitter capable of appropriately implementing control of transmission power in a simple configuration.SOLUTION: An optical transmitter 100 comprises: an automatic gain controlled amplifier 110 which amplifies an input signal to generate a first signal; a waveform adjustment section 120 which adjusts a waveform of the first signal to generate a second signal; and a driver section 130 which converts a signal of a voltage amplitude of the second signal into a current to generate a third signal. The optical transmitter 100 also comprises: a light-emitting device 140 to which the third signal is applied, thereby generating an optical signal; and a photocoupling section 150 which couples the light-emitting device 140 and an optical fiber 200 and transmits the optical signal to the optical fiber 200. The optical transmitter 100 further comprises: a current detection section 400 which detects a current value flowing to a power supply terminal of the automatic gain controlled amplifier; and a power supply control section 170 which controls an operation voltage supplied to the waveform adjustment section 120 and / or the driver section 130 based on the current value.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter and an optical transmission system. [Background technology]

[0002] Conventionally, techniques have been proposed for achieving low power consumption in transmitters of communication systems. Patent Document 1 discloses a transmission circuit provided in a communication terminal or a base station. The transmission circuit disclosed in Patent Document 1 distributes an output signal obtained by amplifying an input signal using a coupler, detects the input signal, and controls transmission power. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-222889 Summary of the Invention [Problem to be solved by the invention]

[0004] The transmission circuit disclosed in Patent Document 1 uses a coupler to branch a signal from a signal line and detects the signal for power measurement. As a result, this transmission circuit has difficulty detecting weak signals and even high-speed signals. Therefore, a transmission circuit that can appropriately control transmission power is needed in a communication system.

[0005] The present invention has been made in view of the problems inherent in the conventional technology, and an object of the present invention is to provide an optical transmitter that has a simple configuration and is capable of appropriately controlling transmission power. [Means for solving the problem]

[0006] An optical transmitter according to an embodiment of the present invention comprises an automatic gain control amplifier that amplifies an input signal and generates a first signal, a waveform adjustment unit that adjusts the waveform of the first signal and generates a second signal, a driver unit that converts a signal of the voltage amplitude of the second signal into a current and generates a third signal, a light-emitting element that generates an optical signal by applying the third signal, an optical coupling unit that couples the light-emitting element and an optical fiber and transmits an optical signal to the optical fiber, a current detection unit that detects the value of the current flowing in a power supply terminal of the automatic gain control amplifier, and a power control unit that controls the operating voltage supplied to the waveform adjustment unit and / or the driver unit based on the current value.

[0007] An optical transmission system according to another aspect of the present invention comprises an optical transmitter and an optical fiber for transmitting an optical signal output from the optical transmitter, wherein the optical transmitter comprises an automatic gain control amplifier that amplifies an input signal to generate a first signal, a waveform adjustment unit that adjusts the waveform of the first signal to generate a second signal, a driver unit that converts a signal of the voltage amplitude of the second signal into a current and generates a third signal, a light-emitting element that generates an optical signal by applying the third signal, an optical coupling unit that couples the light-emitting element and the optical fiber and transmits the optical signal to the optical fiber, a current detection unit that detects the value of the current flowing in a power supply terminal of the automatic gain control amplifier, and a power supply control unit that controls the operating voltage supplied to the waveform adjustment unit and / or the driver unit based on the current value. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an optical transmitter that can appropriately control transmission power with a simple configuration. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram illustrating a configuration of an optical transmitter. [Figure 2] FIG. 10 is a diagram for explaining a comparative example of power control in an optical transmitter. [Figure 3] FIG. 1 is a diagram illustrating a configuration of an optical transmitter according to an embodiment of the present invention. [Figure 4] 1 is a diagram illustrating a configuration of an automatic gain control amplifier according to an embodiment of the present invention. [Figure 5] FIG. 2 is a diagram illustrating a configuration of a current detection unit according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The optical transmitter 100 and the optical transmission system 10 according to this embodiment will be described in detail below with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for the sake of explanation and may differ from the actual ratios. In addition, in the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] Fig. 1 is a diagram showing the configuration of an optical transmitter. As shown in Fig. 1, the optical transmitter includes an automatic gain control amplifier 110 that amplifies an input signal, a waveform adjustment unit 120, a driver unit 130, a light emitting element 140, and an optical coupling unit 150.

[0012] The automatic gain control amplifier 110 amplifies the input signal. In the configuration shown in Fig. 1, the signal input to the optical transmitter is a differential voltage (D+, D-).

[0013] Waveform adjustment unit 120 adjusts the waveform of the signal amplified by automatic gain control amplifier 110. Waveform adjustment unit 120 may also include a main amplifier 121. For example, since high-speed signals tend to have deteriorated and distorted signal waveforms, waveform adjustment unit 120 amplifies the signal using main amplifier 121 and also includes functions for timing (CDR: Clock Data Recovery), waveform correction, and waveform shaping.

[0014] The driver section 130 has a function of converting an input signal, which is a differential voltage, into a current signal having a voltage amplitude.

[0015] The light emitting element 140 generates an optical signal by applying the current converted by the driver unit 130. For example, the light emitting element 140 is configured as a vertical cavity surface emitting laser (VCSEL), and emits light by applying the current generated by the driver unit 130.

[0016] The light-emitting element 140 is not limited to a configuration using a VCSEL. For example, the light-emitting element 140 may be configured with an LED (Light-Emitting Diode), a DFB (Distributed Feed-Back) laser, or a membrane laser, which can be directly modulated like a VCSEL. The light-emitting element 140 may also be configured with external modulation.

[0017] The optical coupling section 150 couples the light emitting element 140 to the optical fiber 200 and transmits an optical signal to the optical fiber 200 .

[0018] In conventional configurations, the electrical signals input to the optical transmitter are relatively slow, and there is little loss in the connectors and transmission lines on the circuit board. However, high-speed signals, such as 25 Gbps, contain many high-frequency components, and so there is a large loss when the signals are transmitted.

[0019] As one method for reducing power consumption of the transmitter, for example, a method has been proposed in which the power supply to the driver unit 130 is suppressed when there is no input signal and no communication is being performed. In this case, for example, the presence or absence of an input signal is detected from a signal branched off from a signal line using a coupler or the like.

[0020] 2 shows, as a comparative example, an example of an optical transmitter having a configuration that can stop the function of driver unit 130 and the waveform adjustment function. In the example shown in Fig. 2, a signal branched from the signal line by coupler 300 is detected by input signal detection unit 310, and based on the result of detection by input signal detection unit 310, the operating voltage supplied from power supply 160 to waveform adjustment unit 120 and driver unit 130 is controlled to be turned on or off.

[0021] In this configuration, since the input signal for controlling the power supply 160 is detected from the signal line through the coupler 300, it is necessary to use the weak signal after branching at the coupler 300.

[0022] The coupler 300 requires a branching ratio of, for example, -20 dB (1 / 10) or more to branch the signal without affecting the transmission signal. This requires the use of an additional amplifier to amplify the branched signal. Furthermore, as signal speeds increase (25 Gbps or higher), the detection time also becomes shorter, making signal detection more difficult.

[0023] The optical transmitter 100 according to this embodiment does not directly detect the signal input from the transmission line, but detects the current supplied to the automatic gain control amplifier 110 used for signal amplification. As a result, the optical transmitter 100 detects the signal by utilizing the fact that when there is no signal input, the gain is maximized (current consumption: large), and when there is a signal input, the gain is small (current consumption: small), making it easier to detect the presence or absence of a signal compared to direct detection.

[0024] (Configuration of the optical transmitter 100) Next, a description will be given of the configuration of the optical transmitter 100 according to this embodiment. Fig. 3 is a diagram showing the configuration of an optical transmission system 10 including the optical transmitter 100 according to this embodiment.

[0025] Note that the automatic gain control amplifier 110, waveform adjustment section 120, driver section 130, light emitting element 140, and optical coupling section 150 shown in FIG. 3 have the same configuration as those shown in FIG. 1 described above, and therefore detailed description thereof will be omitted here.

[0026] The signal amplified by automatic gain control amplifier 110 corresponds to a first signal, the signal adjusted by waveform adjustment unit 120 corresponds to a second signal, and the signal converted into a current by driver unit 130 corresponds to a third signal.

[0027] The configuration shown in FIG. 3 differs from the comparative example shown in FIG. 2 in that the coupler 300 and the input signal detection unit 310 are not present, and instead a current detection unit 400 is provided.

[0028] 3, the current supplied to the automatic gain control amplifier 110 is detected by the current detection unit 400. Specifically, when no signal is input to the optical transmitter 100, the gain is maximized (current consumption: large), and a high current value is detected by the current detection unit 400. On the other hand, when a signal is input to the optical transmitter 100, the gain is small (current consumption: small), and a low current value is detected by the current detection unit 400.

[0029] The current detection unit 400 of the optical transmitter 100 according to this embodiment detects the presence or absence of an input signal by detecting the current flowing through the power supply terminal of this automatic gain control amplifier 110. Furthermore, the power supply control unit 170 of the optical transmitter 100 according to this embodiment controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the result detected by the current detection unit 400.

[0030] Specifically, when the current value is greater than a predetermined value, power supply control unit 170 turns off the operating voltage supplied to waveform adjustment unit 120 and / or driver unit 130. Here, the predetermined value in this embodiment may be a value that is several mA smaller than the value of the current that flows through the power supply terminal of automatic gain control amplifier 110 when there is no input signal, which is measured in advance.

[0031] For example, the predetermined value may be 5 mA less than the value of the current flowing through the power supply terminal of the automatic gain control amplifier 110 when there is no input signal. In other words, when there is no input signal, the current detection unit 400 will detect a current greater than this predetermined value. Note that the predetermined value in the optical transmitter 100 according to this embodiment is not limited to the above-mentioned value, as long as it is a value that can distinguish between the presence and absence of an input signal.

[0032] Fig. 4 is a diagram showing the configuration of automatic gain control amplifier 110 according to this embodiment. In the example shown in Fig. 4, a configuration for differential signal D+ is shown. By providing a configuration for differential signal D- similar to that in Fig. 4 (not shown), an amplification function for differential signals is realized. In the configuration example shown in Fig. 4, the gain is constant due to the resistance values ​​(R1 to R3), but the gain fluctuates because the output voltage saturates with the power supply voltage as the upper limit.

[0033] 5 is a diagram showing the configurations of the current detection unit 400 and the power supply control unit 170 according to this embodiment. As shown in FIG. 5, the current detection unit 400 is configured to include a shunt resistor 410. Furthermore, the power supply control unit 170 is configured to include a switch unit 171. In this embodiment, the shunt resistor 410 is a resistor having a resistance value of 1 Ω or less.

[0034] 5 shows an example in which the switch unit 171 is configured with a bipolar transistor. The switch unit 171 is not limited to a configuration in which a bipolar transistor is used, and may be a configuration in which, for example, a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is used.

[0035] Current detection unit 400 generates a voltage when a certain current flows through shunt resistor 410 connected to the power supply terminal of automatic gain control amplifier 110. This generated voltage and the voltage between the resistors operate switch unit 171, which applies an operating voltage to operating terminals (Disable / Enable) provided in waveform adjustment unit 120 and driver unit 130. If waveform adjustment unit 120 and driver unit 130 do not have operating terminals, switch unit 171 may be directly connected to the power supply terminal of power supply 160 to control the supply.

[0036] 2, in addition to using coupler 300, when the signal is weak (feeble), an amplifier is used to amplify the signal branched by coupler 300. In contrast, optical transmitter 100 according to this embodiment can capture current changes as voltage changes by connecting shunt resistor 410 with a small resistance value for current detection in series with the power supply line of automatic gain control amplifier 110. Note that the presence or absence of an input signal for the detected current fluctuations may be determined by a comparator configured with an operational amplifier or the like.

[0037] Furthermore, the current consumption of the automatic gain control amplifier 110 according to this embodiment is about 70 mA, and the voltage generated across a shunt resistor of 0.1 Ω is about 7 mV according to Ohm's law (voltage=current×resistance).

[0038] For example, when a coupler is used, the signal voltage is 200 mV, and when the input signal is multi-level PAM4 (Pulse Amplitude Modulation 4-level), the minimum amplitude voltage is 67 mV, which is 1 / 3 of the multi-level signal. After passing through the coupler, the detected voltage is 1 / 10, or approximately 7 mV. Therefore, the detected voltage in the optical transmitter 100 according to this embodiment can be maintained at a voltage equivalent to the conventional detected voltage.

[0039] In addition, in the optical transmitter 100 according to this embodiment, as an example, the power consumption of the 100 Gbps driver unit 130 is 920 mW, and the power consumption of the waveform adjustment unit 120 is 790 mW, and by turning off the power supply when no communication is taking place, a total power consumption reduction of approximately 1.7 W can be achieved.

[0040] Furthermore, the optical transmitter 100 according to this embodiment does not require the coupler 300, thereby enabling a reduction in circuit scale. Furthermore, for example, when an optical transmitter is configured with multiple channels, multiple couplers 300 are required. However, even when configured with multiple channels, the optical transmitter 100 according to this embodiment can be implemented by providing one shunt resistor 410. For example, since the coupler 300 is more expensive than the shunt resistor 410, there is a significant cost advantage when configuring an optical transmitter using the optical transmitter 100 according to this embodiment.

[0041] 2, which uses the coupler 300, it is necessary to suppress signal loss due to the increased signal speed, and this raises concerns about loss and impedance mismatch (adverse effects on waveforms) due to the insertion of the coupler 300. On the other hand, the optical transmitter 100 according to this embodiment does not use the coupler 300, making it possible to prevent effects on signals.

[0042] As described above, the optical transmitter 100 according to this embodiment includes an automatic gain control amplifier 110 that amplifies an input signal to generate a first signal, and a waveform adjustment unit 120 that adjusts the waveform of the first signal to generate a second signal. The optical transmitter 100 also includes a driver unit 130 that converts a signal having the voltage amplitude of the second signal into a current and generates a third signal, and a light emitting element 140 that applies the third signal to generate an optical signal. The optical transmitter 100 also includes an optical coupling unit 150 that couples the light emitting element 140 to an optical fiber 200 and transmits an optical signal to the optical fiber 200. The optical transmitter 100 also includes a current detection unit 400 that detects the value of a current flowing through a power supply terminal of the automatic gain control amplifier 110, and a power supply control unit 170 that controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the current value.

[0043] As a result, the optical transmitter 100 according to this embodiment detects the value of the current flowing through the power supply terminal of the automatic gain control amplifier 110, and based on this current value, controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130. Therefore, the optical transmitter 100 does not need to provide a branching unit such as a coupler, or an amplifier that amplifies the branched signal in order to detect the input signal based on the branched weak signal, and can appropriately control the transmission power with a simple configuration.

[0044] Furthermore, when the current value is greater than a predetermined value, the power supply control unit 170 of the optical transmitter 100 may turn off the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130. This allows the optical transmitter 100 to appropriately reduce power consumption when no input signal is being input by turning off the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 when no input signal is being input.

[0045] Furthermore, the current detection unit 400 of the optical transmitter 100 may be configured with a shunt resistor 410 of 1 Ω or less. This eliminates the need for the optical transmitter 100 to provide a branching unit such as a coupler or an amplifier for amplifying the branched signal in order to detect the input signal based on the branched weak signal, and makes it possible to appropriately control the transmission power with a simple configuration using the shunt resistor 410.

[0046] Furthermore, the power supply control unit 170 of the optical transmitter 100 may be configured with a transistor. With this configuration, the optical transmitter 100 can more appropriately control the transmission power with a simple configuration by configuring the switching process of the power supply control unit 170 with a transistor.

[0047] Furthermore, the power supply control unit 170 of the optical transmitter 100 may be configured with a MOSFET. With this configuration, the optical transmitter 100 can more appropriately control the transmission power with a simple configuration by configuring the switching process of the power supply control unit 170 with a MOSFET.

[0048] (Other embodiments) Although the embodiments have been described in detail with reference to the drawings, the present embodiments are not limited to the contents described in the above embodiments. Furthermore, the components described above include those that can be easily imagined by a person skilled in the art and those that are substantially the same. Furthermore, the configurations described above can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the configurations can be made without departing from the spirit of the embodiments.

[0049] In the above-described embodiment, the input signal is a binary differential signal. The input signal is not limited to binary NRZ (Non Return to Zero) and may be multi-level PAM4 (Pulse Amplitude Modulation 4-level), for example. Since the minimum amplitude of PAM4 is 1 / 3 of that of NRZ, the input signal is weak, and the optical transmitter 100 according to this embodiment can achieve greater benefits than conventional configurations.

[0050] In the above-described embodiment, the shunt resistor 410 is connected to the power supply side, but this configuration does not limit the configuration of the embodiment. For example, the shunt resistor 410 included in the current detection unit 400 may be connected to the ground side.

[0051] Furthermore, a detection method using the Hall effect may be applied as a current detection method by the current detection unit 400. By applying this detection method using the Hall effect, it becomes possible to install the current detection unit 400 above the power line in a non-contact manner.

[0052] The features of the optical transmitter 100 and the optical transmission system 10 are described below.

[0053] An optical transmitter 100 according to a first aspect includes an automatic gain control amplifier 110 that amplifies an input signal to generate a first signal, and a waveform adjustment unit 120 that adjusts the waveform of the first signal to generate a second signal. The optical transmitter 100 also includes a driver unit 130 that converts a signal having the voltage amplitude of the second signal into a current and generates a third signal, and a light emitting element 140 that applies the third signal to generate an optical signal. The optical transmitter 100 also includes an optical coupling unit 150 that couples the light emitting element 140 to an optical fiber 200 and transmits an optical signal to the optical fiber 200. The optical transmitter 100 also includes a current detection unit 400 that detects the value of a current flowing through a power supply terminal of the automatic gain control amplifier 110, and a power supply control unit 170 that controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the current value.

[0054] According to the above configuration, the optical transmitter 100 of this embodiment detects the value of the current flowing through the power supply terminal of the automatic gain control amplifier 110, and controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the current value. This eliminates the need for the optical transmitter 100 to provide a branching unit such as a coupler for current detection, or an amplifier for amplifying the branched signal in order to detect the input signal based on the branched weak signal, making it possible to appropriately control the transmission power with a simple configuration.

[0055] The power supply control section 170 of the optical transmitter 100 according to the second aspect may turn off the operating voltage supplied to the waveform adjustment section 120 and / or the driver section 130 when the current value is greater than a predetermined value.

[0056] According to the above configuration, when no input signal is being input, the optical transmitter 100 turns off the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130, thereby making it possible to appropriately suppress power consumption when no input signal is being input.

[0057] The current detection section 400 of the optical transmitter 100 according to the third embodiment may be configured with a shunt resistor 410 of 1 Ω or less.

[0058] According to the above configuration, the optical transmitter 100 does not need to have a branching section such as a coupler or an amplifier for amplifying the branched signal in order to detect the input signal based on the branched weak signal, and can appropriately control the transmission power with a simple configuration using the shunt resistor 410.

[0059] The current detection section 400 of the optical transmitter 100 according to the fourth embodiment may be configured by a Hall element.

[0060] According to the above configuration, the optical transmitter 100 can also install the current detection unit 400 above the power line in a non-contact manner by applying a detection method using the Hall effect.

[0061] The power supply control section 170 of the optical transmitter 100 according to the fifth embodiment may be configured by a transistor.

[0062] According to the above configuration, the optical transmitter 100 performs the switching process of the power supply control unit 170 using transistors, and thus can more appropriately control the transmission power with a simple configuration.

[0063] The power supply control section 170 of the optical transmitter 100 according to the sixth embodiment may be configured by a MOSFET.

[0064] According to the above configuration, the optical transmitter 100 uses MOSFETs for the switching process of the power supply control unit 170, thereby enabling more appropriate control of transmission power with a simple configuration.

[0065] An optical transmission system 10 according to a seventh aspect includes an optical transmitter 100 and an optical fiber 200 that transmits an optical signal output from the optical transmitter 100. The optical transmitter 100 includes an automatic gain control amplifier 110 that amplifies an input signal to generate a first signal, a waveform adjustment unit 120 that adjusts the waveform of the first signal to generate a second signal, and a driver unit 130 that converts a signal having a voltage amplitude of the second signal into a current to generate a third signal. The optical transmitter 100 also includes a light-emitting element 140 that applies the third signal to generate an optical signal, and an optical coupling unit 150 that couples the light-emitting element 140 and the optical fiber 200 and transmits the optical signal to the optical fiber 200. The optical transmitter 100 also includes a current detection unit 400 that detects the value of a current flowing through a power supply terminal of the automatic gain control amplifier 110, and a power supply control unit 170 that controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the current value.

[0066] According to the above configuration, the optical transmission system 10 of this embodiment detects the value of the current flowing through the power supply terminal of the automatic gain control amplifier 110, and controls the operating voltage supplied to the waveform adjustment unit 120 and / or the driver unit 130 based on the current value. This eliminates the need for the optical transmission system 10 to provide a branching unit such as a coupler for current detection, or an amplifier for amplifying the branched signal in order to detect the input signal based on the branched weak signal, making it possible to appropriately control the transmission power with a simple configuration. [Explanation of symbols]

[0067] 100 Optical Transmitter 110 Automatic Gain Control Amplifier 120 Waveform adjustment section 130 Driver section 140 Light-emitting element 150 Optical coupling section 160 Power supply 170 Power supply control unit 200 Optical Fiber 300 Coupler 310 Input signal detection unit 400 Current detection unit 410 Shunt Resistor R41, R42 resistors

Claims

1. an automatic gain control amplifier that amplifies an input signal to generate a first signal; a waveform adjusting unit that adjusts the waveform of the first signal to generate a second signal; a driver unit that converts a signal having a voltage amplitude of the second signal into a current and generates a third signal; a light emitting element that generates an optical signal by applying the third signal; an optical coupling unit that couples the light emitting element and an optical fiber and transmits the optical signal to the optical fiber; a current detection unit that detects a current value flowing through a power supply terminal of the automatic gain control amplifier; a power supply control unit that controls an operating voltage supplied to the waveform adjustment unit and / or the driver unit based on the current value; An optical transmitter comprising:

2. 2. The optical transmitter according to claim 1, wherein the power supply control unit turns off the operating voltage supplied to the waveform adjustment unit and / or the driver unit when the current value is greater than a predetermined value.

3. 2. The optical transmitter according to claim 1, wherein the current detection unit is configured by a shunt resistor of 1 Ω or less.

4. 2. The optical transmitter according to claim 1, wherein the current detection unit is configured by a Hall element.

5. 2. The optical transmitter according to claim 1, wherein the power supply control unit is configured by a transistor.

6. 2. The optical transmitter according to claim 1, wherein the power supply control unit is configured by a MOSFET.

7. an optical transmitter; an optical fiber for transmitting an optical signal output from the optical transmitter; The optical transmitter comprises: an automatic gain control amplifier that amplifies an input signal to generate a first signal; a waveform adjusting unit that adjusts the waveform of the first signal to generate a second signal; a driver unit that converts a signal having a voltage amplitude of the second signal into a current and generates a third signal; a light-emitting element that generates the optical signal by applying the third signal; an optical coupling unit that couples the light emitting element and the optical fiber and transmits the optical signal to the optical fiber; a current detection unit that detects a current value flowing through a power supply terminal of the automatic gain control amplifier; a power supply control unit that controls an operating voltage supplied to the waveform adjustment unit and / or the driver unit based on the current value; An optical transmission system comprising:

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