JPS5937877B2 - Laser protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides laser protection for preventing excessive current from flowing in a laser (laser diode or semiconductor laser) when an optical signal is cut off in a repeater or terminal station in a digital section of an optical communication system. It is related to circuits.

Lasers (laser diodes or semiconductor lasers) are generally used in the transmitting section of optical communication systems, and function to convert electrical signals into optical signals.

However, at this stage, the laser element itself is not yet stable, and it is subject to changes in ambient temperature and aging. Since the output level of the laser changes, the output level of the laser is generally controlled using a negative feedback loop that always maintains the output at a constant level. This loop is generally used as a dynamic output level control circuit (Automatic PowerC).
(hereinafter referred to as APC loop). FIG. 1 is a block diagram of an APC loop with peak detection.

In the figure, 1 is a laser, 2 is a modulation signal input terminal, and 3 is a laser.
4 is an amplifier, 5 is a peak detector, 6 is a reference voltage input terminal, 7 is a differential amplifier, and 8 is a bias current drive circuit. When this APC loop is activated, a reference voltage is provided in a part of the APC loop, and the differential amplifier T determines whether the laser output is greater or less than this reference voltage, and the output is determined accordingly. The laser oscillation output is adjusted by increasing or decreasing the excitation current to the laser. On the other hand, when modulating light in optical communication, a certain DC bias current is passed through the bias current drive circuit 8, and a current with the signal from the input terminal 2 added thereto is applied to the laser 1. The optical output of the laser 1 is optically modulated by.

In the APC loop having the above function, if the laser input signal is cut off, the output of the photodetector circuit becomes extremely small. That is, only bias currents with values smaller than the optical threshold can be applied to the laser. Therefore A
The PC loop operates as if the laser output level were decreased, and the bias current of the laser increases due to the action of the differential amplifier T. As this condition progresses, the bias current of the laser becomes saturated, excessive current flows through the laser, and the laser deteriorates. Therefore, there is a need for a laser protection circuit that prevents this phenomenon and prevents the laser from deteriorating even if the input signal is interrupted. Conventionally, this type of circuit is called APC.
There is a method of obtaining a loop reference voltage by performing average value rectification or peak value rectification from a signal that drives a laser.

However, this method has the disadvantage that a rectifier circuit is also required on the reference circuit side, making the circuit complicated. An object of the present invention is to provide a commonly used automatic output level control circuit (APC loop) in an optical repeater, in which the bias current increases when the input signal to the laser is cut off, and the laser is overloaded. The object of the present invention is to provide a new laser protection circuit for preventing the flow of current, which eliminates the above-mentioned drawbacks.

To achieve this objective, the laser protection circuit according to the present invention provides $? The timing signal (clock) of the modulation signal, which is always used in the process, is led to the signal presence/absence determination circuit via the extraction circuit, and only when the timing signal is cut off, the reference voltage of the APC loop is controlled, and the laser It is characterized by blocking the flow of current.

Embodiments of the laser protection circuit according to the present invention will be described in detail below. FIG. 2 is a block diagram of a laser protection circuit according to the present invention.

In the same figure, 1 to 8 are the same as those shown in FIG. 9 is an adder, 10 is a signal presence/absence determination circuit, and 11 is a timing extraction circuit.

3Rs of light (Reshaping9Regeneration)
ng, Retiming) In a repeater, a clock equal to the bit rate of the signal is always present to time the input signal. The output of this timing signal (clock) extraction circuit has a good S/N ratio,
Even if there is some noise, the presence or absence of a timing signal (clock) can be easily determined. Therefore, the presence or absence of a signal can be replaced with the presence or absence of a timing signal (clock). That is, the presence or absence of an input signal is detected based on the presence or absence of a timing signal. The timing signal extracted from the timing extraction circuit 11 is connected to the following signal presence/absence determining circuit 10, which constantly monitors the presence or absence of the timing signal and makes a determination. For example, when there is a timing signal, an output ``11'' is obtained that maintains a certain constant potential, and when there is no timing signal, the output is 0 potential and the output is not output.In other words, the output is 10''. The output of the signal presence/absence determination circuit 10 is connected to an adder 9, which outputs the reference voltage of the APC loop and the two types of output 1.
The reference voltage is controlled by adding the voltage corresponding to 1P0r601), and by adding this controlled reference voltage to a known APC loop, the APC loop goes out of control and an excessive current flows to the laser. protect the laser by preventing Figure 3 shows the timing signal (clock)
This is an example of a circuit for determining the presence or absence of.

In the figure, 12 and 17 are phase detectors, 13 and 18 are low-pass ▲ wavers, 14 is a voltage controlled oscillator, 15 is a hybrid transformer, 16 is a π/2 phase shifter, and 19 is a voltage comparator. , 20 indicate constant voltage input terminals. In this block diagram, the circuit that returns to the phase detector 12 from the phase detector 12 via the resistance-pass P-wave generator 13, the voltage controlled oscillator (VCO) 14, and the hybrid transformer 15 is generally a phase locked loop. (PLL), and this PL
A timing wave extraction circuit is constructed around L.

When this PLL is in synchronization, the voltage controlled oscillator (
The output of CO) 14 is 900 out of phase compared to the input signal. That is, if there is an input signal, the output of CO14 will be accurately shifted in phase by 900 degrees from the input signal. Therefore, it is sufficient to use some method to detect that the phase difference between the input waveform and the output waveform of CO14 is 90 degrees. For this purpose, a synchronization detector 17 is used in this embodiment. That is, the signal obtained by passing the input signal through the 900 (π/2) phase shifter 16 and the signal obtained by separating a part of the output waveform of CO14 by the hybrid transformer 15 are synchronously detected. In this case, if the input signal is completely shifted by π/2, there will be no DC component in the output of the synchronous detector 17, but if there is no DC component, there will be a bead. (This bead is only an alternating current component.) If this is passed through a reduced pass r wave generator 18 and compared with a constant voltage by a voltage comparator 19, the output of the voltage comparator 19 will switch depending on the presence or absence of a signal. This signal is used for determination. FIG. 4 shows an embodiment of the adder 9 shown in FIG.

In the same figure, 21 is the timing signal (clock).
This is an input terminal to which constant voltages corresponding to two types of outputs of the presence/absence determining circuit are applied. 22 is the output terminal of this adder, 2
3 is an operational amplifier, 24 is a reference voltage setting section, and 6 is a reference voltage input terminal.

Here, if the voltage applied to the input terminal 21 is 1., the reference voltage applied to the terminal 6 is 2, and the output voltage of the output terminal 22 is V3, the following relational expression holds true. R3R321X-+2X-
(1) R1 R2 In the above equation, if R1 = R2, then in the (supporting) equation, if R, = R3, then considering the relationship between the voltage value and resistance value shown above, A desired reference voltage v can be set, and the reference voltage of the APC loop can be moved depending on the presence or absence of an input signal.

As explained above, according to the present invention, the circuit is simpler than the conventional example, and since the presence or absence of an input signal is detected based on the presence or absence of a timing signal, the S /N can be extracted in good condition.

In addition, the reference voltage is moved significantly depending on the detection signal, and the AP
It is possible to switch the stable point of the C-Lube when there is no signal.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 shows an automatic output level control circuit, and FIG. 2 is a block diagram showing an embodiment of a laser protection circuit according to the present invention.
3 and 4 show an embodiment of the input signal presence/absence determining circuit used in FIG. 2 and an embodiment of the adder. 1... Laser (laser tie auto, semiconductor laser, etc.), 2... Fast signal input terminal, 3...
・Photodetector (PIN diode, APD, etc.), 4...
...Amplifier, 5...Peak detector, 6...
...Reference voltage input terminal, 7...Differential amplifier, 8...Bias current drive circuit, 9...
・Adder, 10... Input signal presence/absence determination circuit, 1
1...timing signal extraction circuit, 12, 17.
...Phase detector, 13,18...Resistance pass wave generator, 14...Voltage damping oscillator, 15...
...Hybrid transformer, 16 ... Phase shifter, 19 ... Voltage comparator, 20 ......〒
Constant voltage input terminal, 21... Adder input terminal, 2
2... Adder output terminal, 23... Operational amplifier, 24... Reference voltage setting section.

Claims (1)

[Claims] 1. The automatic output level control circuit of an optical repeater includes a signal presence/absence discrimination circuit that extracts a timing signal and determines the presence or absence of the timing signal, and an adder that adds the output information of the signal presence/absence discrimination circuit to a reference voltage. A laser protection circuit comprising: using the output of the adder as a reference voltage in the automatic output level control circuit, and changing the reference voltage to prevent excessive current from flowing through the laser.