JPH08331063A - Optical fiber amplifier and optical transmitter - Google Patents

Abstract

PURPOSE: To conduct fault monitor with respect to an optical fiber amplifier in the standby state without giving adverse effect onto a substantial transmission signal regardless of redundant configuration. CONSTITUTION: A PD module 29 detects a level of an optical signal being an output of an optical fiber amplifier 13 and an error amplifier 31a and an LD drive circuit 32 control a level of a stimulated light generated by the LD module 24 so that the level has a prescribed relation to a prescribed reference voltage. A 1st reference voltage V1 generated from a reference voltage generating circuit 31b in the active state and a 2nd reference voltage V2 generated from a reference voltage generating circuit 31c in the standby state are selected by an output interrupt control circuit 31d.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises an optical fiber amplifier for amplifying an optical signal by exciting an amplifying optical fiber with light emitted from an exciting light emitting means, and a plurality of such optical fiber amplifiers. The present invention relates to an optical transmitter.

[0002]

2. Description of the Related Art In recent years, an optical fiber amplifier capable of directly amplifying an optical signal without converting it into an electric signal has been developed. An optical fiber amplifier is a type of optical fiber that is excited by a pumping light by injecting pumping light together with an optical signal into an amplifying optical fiber doped with a rare earth element such as Er (erbium). It amplifies the signal.

Such an optical fiber amplifier has come to be used as a transmission amplifier in an optical transmitter. In addition, such an optical transmitter is often required to have high reliability, and often has a redundant configuration using two optical fiber amplifiers.

When a redundant configuration is formed by using two optical fiber amplifiers, the output of each optical fiber amplifier is generally selectively output to a transmission line by an optical switch. However,
In the current optical switch, the isolation between two inputs is as bad as about 20 dB, and when the optical fiber amplifier on the standby side is performing optical output, the optical signal output by this optical fiber amplifier on the standby side is transmitted to the transmission line. It will leak and adversely affect the original transmission signal.

Therefore, it is necessary to stop the operation of the optical fiber amplifier on the stand-by side and put it into a state where the optical output is cut off. However, in this case, the failure of the optical fiber amplifier on the standby side cannot be monitored, and even if a failure occurs in the optical fiber amplifier, it cannot be detected.

[0006]

As described above, the prior art is as follows.
When using a redundant configuration using multiple optical fiber amplifiers, in order to prevent the optical signal output by the optical fiber amplifier on the standby side from leaking into the transmission line, stop the operation of the optical fiber amplifier on the standby side, There is a problem that the optical output on the standby side cannot be monitored for troubles because the optical output must be kept off.

The present invention has been made in consideration of such circumstances, and the first object thereof is to monitor a fault concerning an optical fiber amplifier on the standby side even in a redundant configuration. It is an object of the present invention to provide an optical fiber amplifier capable of performing the above-mentioned operation without adversely affecting the original transmission signal.

Secondly, while a redundant configuration is formed by using a plurality of optical fiber amplifiers, it is possible to perform fault monitoring on the optical fiber amplifiers on the standby side without adversely affecting the original transmission signal. It is to provide a possible optical transmitter.

[0009]

In order to achieve the above object, the present invention provides an amplification optical fiber and an excitation light emission such as a laser diode module which emits excitation light for exciting the amplification optical fiber. Means and a predetermined first
A predetermined first reference level according to the output optical signal level and a predetermined minimum level or more and less than or equal to the first output optical signal level and a predetermined difference greater than or equal to the first output optical signal level. Reference level generating means for selectively generating a predetermined second reference level according to the second output optical signal level as a reference level, for example, reference voltage generating circuits 31b and 31c and an output interruption control circuit 31d, and the amplification. Detecting means for detecting the intensity of the output signal light from the optical fiber for use and outputting a detection signal at a level corresponding to the intensity, and a detection signal level output by the detecting means and the reference level The emission intensity of the excitation light emitting means is controlled so that the reference level generated by the generating means has a predetermined relationship, for example, an error amplifier and a laser datum. And configure the optical fiber amplifier and an excitation light intensity control means, such as a diode driving circuit.

Further, according to the present invention, a plurality of the above-mentioned optical fiber amplifiers are used to transmit the signal light output by one of the optical fiber amplifiers whose operation is designated.
Of the plurality of optical fiber amplifiers, the first level is used as a reference level by the reference level generation means of the one designated to operate.
The optical transmitter is provided with a reference level control unit such as an A / B system switching control unit for outputting the reference level and for outputting the second reference level as the reference level to the reference level generation unit of another optical fiber amplifier. Configured.

[0011]

By taking these measures, in the optical fiber amplifier, the output optical signal level is above the predetermined first output optical signal level and above the predetermined minimum level and below the first output optical signal level. Further, it becomes possible to switch to a predetermined second output optical signal level whose difference from the first output optical signal level is a predetermined value or more. Thus, a redundant configuration is formed by using a plurality of the optical fiber amplifiers, and the output optical signal level of any one optical fiber amplifier is set to the first output optical signal level, and the output optical signal levels of the remaining optical fiber amplifiers are set to the first output optical signal level. By controlling each of the two output optical signal levels by the reference level control means, it is possible to operate all the optical fiber amplifiers and perform fault monitoring in each optical fiber amplifier. The output optical signal of the main optical fiber amplifier having one output optical signal level is prevented from being influenced by the output optical signals of the other optical fiber amplifiers.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the optical transmitter according to the present embodiment. In the figure, 1-A and 1-B are both transmitters. The transmitters 1-A and 1-B have the same configuration, and each has a laser diode module (LD module).
11, an external modulator 12, and an optical fiber amplifier 13. The LD module 11 generates modulated light which is a source of an optical signal to be transmitted and gives it to the external modulator 12. The external modulator 12 receives a transmission signal obtained by multiplexing a plurality of signals in the multiplexing unit 2, and externally modulates the modulated light provided from the LD module 11 using this transmission signal as a modulation signal to generate an optical signal. To generate. The optical fiber amplifier 13 amplifies the optical signal generated by the external modulator to a certain level.
The optical fiber amplifier 13 has different operation modes during operation and during standby, as will be described later.

External modulator 12 of transmitter 1-A and transmitter 1-B
The transmission signal output from the multiplexing unit 2 is commonly applied to the external modulator 12 of FIG. Further, one of the optical signal output by the optical fiber amplifier 13 of the transmitter 1-A and the optical signal output by the optical fiber amplifier 13 of the transmitter 1-B is selected by the optical switch 3, and the optical fiber transmission line is selected. Sent to F. Thus, a redundant configuration is adopted in which the transmitter 1-A is the A system and the transmitter 1-B is the B system.

Reference numeral 4 is an A / B system switching controller. The A / B system switching control unit 4 controls the optical switch 3 to switch the optical signal to be output to the optical fiber transmission path F, and at the same time, the transmitting unit on the side where the optical signal is selected by the optical switch 3 is switched. The operating states of the optical fiber amplifiers 13 of the respective systems are controlled so that the optical fiber amplifiers 13 are in the operating state and the optical fiber amplifiers 13 of the other transmitting section are in the standby state.

FIG. 2 is a block diagram showing a detailed structure of the optical fiber amplifier 13. As shown in this figure, the optical fiber amplifier 13 includes an optical coupler 21, an isolator 22, and a W
DM coupler 23, laser diode module (LD module) 24, amplification optical fiber 25, isolator 26, optical bandpass filter (optical BPF) 27, optical coupler 28, photodiode module (PD module) 29, operational amplifier 30, automatic output Level control circuit (ALC circuit) 31, laser diode drive circuit (LD
Drive circuit) 32, automatic temperature control circuit (ATC circuit) 3
3, an input monitoring circuit 34, an output abnormality alarm circuit (output abnormality ALM circuit) 35, and a laser diode monitoring circuit (LD monitoring circuit) 36.

Of these, the optical coupler 21 and the isolator 2
2, the WDM coupler 23, the amplification optical fiber 25, the isolator 26, the optical bandpass filter (optical BPF) 27, and the optical coupler 28 are connected in series in the same order as described here, and the external modulator 12 It forms the main transmission path of the optical signal supplied from the.

The optical coupler 21 branches the optical signal and supplies it to the isolator 22 and the PD module 34, respectively. The isolators 22 and 26 prevent the backflow of the optical signal due to reflection. The WDM coupler 23 is an LD
The excitation light emitted by the module 24 is multiplexed with the optical signal.

The LD module 24 generates light having a predetermined wavelength (for example, 1.48 μm band) as pumping light for pumping the amplification optical fiber 25. Amplifying optical fiber 25
Is a quartz optical fiber doped with a rare earth element such as Er (erbium).

The optical bandpass filter 27 transmits only the optical signal component of the light passing through the amplification optical fiber 25 and removes the excitation light component. The optical coupler 28 branches the optical signal output from the optical bandpass filter 27 and supplies the branched optical signal to the optical switch 3 and the PD module 29.

On the other hand, the PD module 29 and the operational amplifier 3
0, the ALC circuit 31, and the LD drive circuit 32 are connected in series in the same order as described here, and form a feedback system for controlling the output optical signal level of the optical fiber amplifier 13 at a constant level. There is.

The PD module 29 monitors the intensity of the optical signal supplied from the optical coupler 28 and generates a detection voltage of a corresponding level. The operational amplifier 30 amplifies the detection voltage generated by the PD module 29 with a constant gain. A
The LC circuit 31 compares the detection voltage given from the operational amplifier 30 with a predetermined reference voltage, and outputs a control voltage corresponding to the level difference. The LD drive circuit 32 controls the intensity of the excitation light emitted from the LD module 24 so that the control voltage supplied from the ALC circuit 31 falls within a predetermined control target range. The ATC circuit 33 controls the temperature inside the LD module 24 at a constant level by using a Peltier element or the like,
It is prevented that the intensity of the excitation light changes due to the temperature change.

By the way, the ALC circuit 31 includes the error amplifier 3
1a, reference voltage generation circuits 31b and 31c, and output disconnection control circuit 31d, the detection voltage given from the operational amplifier 30 is supplied to the inverting input terminal of the error amplifier 31a, and the first reference voltage V1 generated by the reference voltage generation circuit 31b. And the second reference voltage V2 generated by the reference voltage generating circuit 31c is input to the non-inverting input terminal of the error amplifier 31a, and the output of the error amplifier 31a is supplied to the LD drive circuit 32 as a control voltage. Has become. Further, the reference voltage generating circuits 31b and 31c are the output disconnection control circuit 31d based on the instruction from the A / B system switching control unit 4.
Either of them is operated under the control of. Here, the first reference voltage V1 generated by the reference voltage generation circuit 31b is set to a voltage value that allows the output optical signal level of the optical fiber amplifier 13 to be a level suitable for sending to the optical fiber transmission line F. To be done. The second reference voltage V2 generated by the reference voltage generation circuit 31c is: L supplied from the LD drive circuit 32 to the LD module 24 when the output optical signal level is stable at a predetermined level.
The LD drive circuit 32 continues to supply the LD module 24 with a current value larger than the current value (threshold current value) Ith at which the output optical signal level of the optical fiber amplifier 13 becomes 0. L supplied from the LD drive circuit 32 to the LD module 24 when the output optical signal level is stable at a predetermined level
The D drive current is smaller than the LD drive current supplied to the LD module 24 by the LD drive circuit 32 in the stable operation state when the first reference voltage V1 is applied to the non-inverting input terminal of the error amplifier 31a. The output optical signal level of the optical fiber amplifier 13 when the output optical signal level is stable at a predetermined level,
When the first reference voltage V1 is applied to the non-inverting input terminal of the error amplifier 31a, the ratio to the output optical signal level of the optical fiber amplifier 13 in the stable operation state is a predetermined value (for example, 30d).
B) or more. It is set to satisfy each condition.

Specifically, based on the relationship between the LD drive current and the output optical signal level of the optical fiber amplifier 13 as shown in FIG. 3, the first reference voltage V1 is L in the stable operation state.
The D drive current is 180 mA, and the LD drive current is 30 mA in the stable operation state of the second reference voltage V2.
To be set respectively.

Now, the input monitoring circuit 34, the output abnormality ALM
The circuit 35 and the LD monitoring circuit 36 monitor the operating states of the optical fiber amplifier 13 and the transmitter 1 including the optical fiber amplifier 13.

The input monitoring circuit 34 has a photodiode module (PD module) 34a and an input disconnection alarm circuit (input disconnection ALM circuit) 34b. The PD module 34a generates a detection voltage of a level corresponding to the intensity of the optical signal supplied from the optical coupler 21, and disconnects the input AL.
It is given to the M circuit 34b. The input disconnection ALM circuit 34b detects the input disconnection based on the level of the detection voltage supplied from the PD module 34a, and when the input disconnection occurs, notifies the A / B system switching unit 5 of the fact and performs alarm processing. To do.

The output abnormality ALM circuit 35 is the ALC circuit 3
1 detects the abnormality of the output optical signal level of the optical fiber amplifier 13 based on the control voltage given to the LD drive circuit 32, and when the abnormality occurs, the A / B system switching unit 5
And the alarm process.

The LD monitor circuit 36 includes the LD output detection circuit 3
6a, a comparison circuit 36b, and a laser diode abnormality alarm circuit (LD abnormality alarm circuit) 36c. L
The D output detection circuit 36a photoelectrically converts the excitation light generated by the LD module 24 and supplies it to the comparison circuit 36b. The comparison circuit 36b compares the current level of the electric signal given from the LD output detection circuit 36a with the level of the drive current given to the LD module 24 by the LD drive circuit 32, and outputs a voltage of a level corresponding to the difference between the LD abnormal ALMs. It is applied to the circuit 36c. The LD abnormality ALM circuit 36c is the comparison circuit 36.
Based on the voltage given by b, the LD module 24
Monitors whether or not the excitation light of a level corresponding to the drive current is normally generated, and notifies the A / B system switching unit 5 of that when an abnormality occurs and also performs an alarm process.

Next, the operation of the optical transmitter configured as described above will be described. First, the transmission signal obtained by the multiplexing unit 2 is given to the external modulators 12 of the transmitting units 1-A and 1-B. Then, in the transmitters 1-A and 1-B, the modulated light generated by the LD module 11 and supplied to the external modulator 12 is externally modulated by using the transmission signal supplied from the multiplexer 2 as a modulation signal. A signal is generated and given to the optical fiber amplifier 13.

In each of the optical fiber amplifiers 13 of the transmitters 1-A and 1-B, the optical signal is amplified as follows. That is, the optical signal given from the external modulator 12 is
Optical coupler 21, isolator 22 and WDM coupler 2
It is incident on the amplification optical fiber 25 via the light source 3. On this occasion,
The pumping light emitted from the LD module 24 is multiplexed into the optical signal by the WDM coupler 23, and the optical fiber 25 for amplification is used.
Excitation light is also incident on. When the pumping light enters the amplification optical fiber 25, the doped rare earth element ions are excited and stimulated emission occurs, whereby the optical signal is amplified.

The optical signal thus amplified is input to the optical BPF 27 via the isolator 26. Then, in the optical BPF 27, the pumping light component is removed and only the optical signal component is extracted, and this optical signal is output to the optical switch 3 via the optical coupler 28.

The optical signal output to the optical switch 3 is branched by the optical coupler 28 and given to the PD module 29, and its intensity is monitored. Then, in the PD module 29, a detection voltage of a level corresponding to the output optical signal level is generated, amplified by the operational amplifier 30, and then supplied to the ALC circuit 31. In the ALC circuit 31, the detection voltage supplied from the operational amplifier 30 is input to the inverting input terminal of the differential amplifier 31a, is output from either of the reference voltage generating circuits 31b and 31c, and is input to the non-inverting input terminal of the reference voltage. And the control voltage corresponding to the level difference is generated. This control voltage is given to the LD drive circuit 32, and the intensity of the excitation light emitted from the LD module 24 is controlled by the LD drive circuit 32 so that this control voltage falls within a predetermined control target range. Thus, the detected voltage output from the operational amplifier 30 and the reference voltage output from any one of the reference voltage generation circuits 31b and 31c have a predetermined relationship, that is, the output optical signal level is a predetermined value indicated by the reference voltage. The intensity of the excitation light emitted from the LD module 24 is feedback-controlled so as to be constant at the level.

By the way, the above operation is performed by the transmitter 1-A.
, 1-B at the same time, but the optical fiber amplifier 13 on the side currently designated by the A / B system switching control unit 4 is used.
Then, the output disconnection control circuit 31d changes the reference voltage generation circuit 31c.
Is cut off and the first reference voltage V1 output from the reference voltage generating circuit 31b is applied to the error amplifier 31a as a reference voltage. Since the first reference voltage V1 is set to a voltage value that allows the output optical signal level of the optical fiber amplifier 13 to be a level suitable for sending to the optical fiber transmission line F, it is designated as the current one. Further, the transmitting unit 1 outputs an optical signal controlled to a level suitable for sending to the optical fiber transmission line F.

On the other hand, in the optical fiber amplifier 13 on the standby designated side, the output disconnection control circuit 31d disconnects the output of the reference voltage generation circuit 31b, and the reference voltage generation circuit 31c outputs it to the error amplifier 31a. The second reference voltage V2 is given as a reference voltage. The second reference voltage V2 is
Since the settings are made so as to satisfy the above-mentioned conditions, the level of the transmission unit 1 designated as the standby is sufficiently lower than the optical signal output from the transmission unit 1 designated as the active (for example, the level ratio). (30 dB or more) An optical signal is output, and this optical signal is not interrupted.

Now, the A / B system switching control unit 4 includes the transmitting unit 1-
The optical switch 3 is controlled so that either active or standby is set for each of A and 1-B, and the transmitter 1 on the active side is selected. Thus, the optical signal output from the transmitter 1 on the currently set side, that is, the optical signal controlled to a level suitable for sending to the optical fiber transmission line F, is almost attenuated by the optical switch 3. It is sent out to the optical fiber transmission line F without being transmitted. However, the isolation of the optical switch 3 is as bad as about 20 dB, and the optical signal output from the transmitter 1 on the standby side leaks into the optical fiber transmission line F. However, the optical signal output from the transmitter 1 on the standby side is sufficiently smaller than the optical signal output from the transmitter 1 on the active side, and is greatly attenuated in the optical switch 3. Therefore, the optical signal output from the transmitter 1 on the currently set side is hardly affected.

By the way, the A / B system switching control unit 4 is a transmission unit.
Input disconnection AL depends on which of 1-A and 1-B is to be used
It is determined based on the monitoring result of each of the M circuit 34b, the output abnormality ALM circuit 35, and the LD abnormality ALM circuit 36c, and a switching instruction from another control unit or an operator. That is, when both the transmitters 1-A and 1-B are normal,
Although the side designated by another control unit or operator is set to the working side, from this state, the input disconnection ALM circuit 34b, the output abnormality ALM circuit 35 and the LD of the side set to the working side are set.
If an abnormality is detected in any of the abnormal ALM circuits 36c, the A / B system switching control unit 4 switches between active / standby. Further, the input disconnection ALM circuit 34b, the output abnormality ALM circuit 35, and the LD abnormality ALM circuit 36c on the side set to the standby state.
If an abnormality is detected in any of the above, the A / B system switching control unit 4 prohibits the system from being set to active. Even on the standby side, the LD module 24 continues to emit the excitation light and does not stop the output of the optical signal. Therefore, the output abnormality ALM circuit 35 and the LD abnormality ALM circuit 36c are generated.
Can monitor the abnormality, and the transmitter 1 on the standby side can also monitor the failure.

As described above, according to the present embodiment, the output optical signal level of the optical fiber amplifier 13 is made variable between the active state and the standby state, and the output optical signal level at the standby side is the output optical signal level at the active side. Since the LD module 24 continues to emit the excitation light in the range of -30 dB or less, the output optical signal on the standby side affects the output optical signal on the working side even if the isolation of the optical switch 3 is poor. In addition to the above, it is possible to monitor the failure in the optical fiber amplifier 13 on the standby side.

Further, according to the present embodiment, since the element for switching the system is not required to have isolation property, it is possible to use a more reliable element such as an optical coupler in place of the optical switch 3. Becomes

The present invention is not limited to the above embodiment. For example, in the above-mentioned embodiment, the optical transmission device having a duplex configuration using two optical fiber amplifiers 13 is used, but a more multiplexed optical transmission device using three or more optical fiber amplifiers 13 may be used. In this case,
Only one optical fiber amplifier 13 is set to be in use, and all the others are set to be in standby. In addition, various modifications can be made without departing from the scope of the present invention.

[0039]

According to the present invention, the output optical signal level can be switched between the first output optical signal level and the second output optical signal level, and the second output optical signal level is equal to or higher than a predetermined minimum level. Since the optical fiber amplifier is configured such that the difference between the first output optical signal level and the first output optical signal level is equal to or more than the predetermined value, a plurality of optical fiber amplifiers are used, and the output optical signal of any one optical fiber amplifier is used. By controlling the level to the first output optical signal level and the output optical signal level of the remaining optical fiber amplifiers to the second output optical signal level by the reference level control means, respectively, the redundant configuration can be achieved, and It becomes possible to carry out fault monitoring on the existing optical fiber amplifier without adversely affecting the original transmission signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical transmitter according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a detailed configuration of an optical fiber amplifier 13 in FIG.

FIG. 3 is an LD supplied to an LD module 24 in FIG.
The figure which shows an example of the relationship between a drive current and the output optical signal level of the optical fiber amplifier 13.

[Explanation of symbols]

 1 (1-A, 1-B) ... Transmitter 2 ... Multiplexer 3 ... Optical switch 4 ... A / B system switching controller 11 ... Laser diode module (LD module) 12 ... External modulator 13 ... Optical fiber amplifier 21 ... Optical coupler 22 ... Isolator 23 ... WDM coupler 24 ... Laser diode module (LD module) 25 ... Amplifying optical fiber 26 ... Isolator 27 ... Optical bandpass filter (optical BPF) 28 ... Optical coupler 29 ... Photodiode module (PD) Module) 30 ... Operational amplifier 31 ... Automatic output level control circuit (ALC circuit) 31a ... Error amplifier 31b, 31c ... Reference voltage generation circuit 31d ... Output disconnection control circuit 32 ... Laser diode drive circuit (LD drive circuit) 33 ... Automatic temperature control Circuit (ATC circuit) 34 ... Input monitoring circuit 35 ... Output error alert Circuit (output abnormality ALM circuit) 36 ... laser diode monitoring circuit (LD monitor circuit)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 1/74 3/46 10/08

Claims (2)

[Claims] 1. An amplifying optical fiber, a pumping light emitting means for emitting pumping light for pumping the amplifying optical fiber, a predetermined first reference level and a predetermined first output optical signal level according to a predetermined first output optical signal level. Second reference level according to a predetermined second output optical signal level that is equal to or higher than the lowest level and is equal to or lower than the first output optical signal level, and a difference from the first output optical signal level is equal to or higher than a predetermined value. And a reference level generating means for selectively generating as a reference level, a detecting means for detecting the intensity of the output signal light from the amplification optical fiber and outputting a detection signal of a level corresponding to the intensity, Excitation light intensity control means for controlling the emission intensity of the excitation light emitting means so that the level of the detection signal output by the means and the reference level generated by the reference level generating means have a predetermined relationship. Optical fiber amplifier, characterized in that. 2. An optical transmitter for transmitting a signal light output by a designated one of a plurality of optical fiber amplifiers, wherein each of the plurality of optical fiber amplifiers includes an amplification optical fiber and the amplification optical fiber. Pumping light emitting means for emitting pumping light for pumping the optical fiber for use, and a predetermined first reference level and a predetermined minimum level or more in accordance with a predetermined first output optical signal level and below the first output optical signal level And a reference level generation for selectively generating, as a reference level, a predetermined second reference level corresponding to a predetermined second output optical signal level whose difference from the first output optical signal level is a predetermined value or more. Means, detecting means for detecting the intensity of the output signal light from the amplification optical fiber, and outputting a detection signal of a level corresponding to the intensity, and the level of the detection signal output by this detecting means The excitation light intensity control means for controlling the emission intensity of the excitation light emission means so that the reference level generated by the reference level generation means has a predetermined relationship. Among the fiber amplifiers, the first reference level is output as the reference level to the reference level generating means of the one designated for operation, and the second reference level is output to the reference level generating means of the other optical fiber amplifier.
An optical transmitter comprising a reference level control means for outputting a reference level.