JPS60147716A - Optical transmitter of extinction ratio control - Google Patents

Abstract

PURPOSE:To hold an extinction ratio optimum without loss of signal light for variation of the ambient temperature by using the first waveguide output light of a waveguide-type optical modulator is a transmission signal light and detecting variation of the output of the second waveguide output light to control a bias point of the modulator. CONSTITUTION:An output light 2 of a transmission light source 1 is made incident on a waveguide-type optical modulator 3. A waveguide output 4 of the modulator 3 has the amplitude modulated by a modulating signal 5 and is used as the transmission signal. A waveguide output 6 of the modulator 3 is made incident on an optical fiber 7 and is led to a photodetector 8. An average output voltage 9 of the detector 8 is compared with a reference signal 11 by a comparator 10, and an output error signal 12 passes a modulator bias control circuit 13 to become a feedback signal 14, and the bias level of the modulator 3 is set to an optimum value. Thus, an extinction ratio is held optimum without loss of the signal light for variation of the ambient temperature.

Description

TECHNICAL FIELD The present invention relates to an optical communication device, and more particularly to an optical signal transmitting device that maintains the extinction ratio of a transmitted signal at a constant value, particularly at an optimal value.

(Description of Prior Art) Conventionally, semiconductor lasers have been mainly used as light sources for optical communications. Semiconductor lasers have the advantage of being compact and capable of direct intensity modulation, but during direct modulation, the oscillation frequency changes as the injected current changes.
Therefore, an external modulator is usually used for coherent optical transmission using ARK heterodyne detection, P8 heterodyne detection, etc. Generally, an electro-optic modulator capable of high-speed modulation is used as the external modulator.
This electro-optic modulator has the disadvantage that the extinction ratio changes due to pyroelectric effects caused by temperature changes, and when the extinction ratio is particularly poor, unnecessary DC components are generated during signal demodulation, resulting in poor reception sensitivity. .

(Object of the Invention) An object of the present invention is to provide an optical transmitter using an external modulator, which solves the above-mentioned conventional drawbacks and keeps the extinction ratio at a constant value, particularly an optimum value.

(Structure of the Invention) An extinction ratio controlled optical transmitter of the present invention includes a light source, first and second waveguides, and a first waveguide. a waveguide-type optical modulator that modulates light from a light source by controlling coupling of a second waveguide;
An optical transmission device that uses the output light from the first waveguide of this waveguide type optical modulator as the transmission signal light changes the bias point of the first waveguide type optical modulator to the output light from the second waveguide. This can be obtained by adding a variation detection section that detects the output variation and a control section that controls the bias point of the waveguide optical modulator based on the error signal output from the variation detection section.

(Principle of the invention) Detailed explanation of the present invention using the following drawings: Figure 1 is L
This figure shows the transmitted output characteristics of the applied voltage envelope 1 and the second waveguide of a waveguide optical modulator using iNbOs. Moreover, FIG. 2 shows the change in the average output of the second waveguide with respect to the change in the bias point. Let us now consider the case where the bias voltage of the waveguide optical modulator is set at point a in FIG. 1, and binary amplitude modulation is performed between points a and a.

In this case, the extinction ratio becomes almost 0, making it possible to transmit an ideal signal. However, if the bias point changes to 32 points due to temperature fluctuation, the transmission output will be a
, b, and the average output of the signal demodulation waveguide also changes from point e to point e' in FIG. This e
By generating a control signal that returns the bias point of the optical modulator to its original state based on the difference between the outputs at point and point e', and feeding it back to the bias voltage of the waveguide optical modulator, ambient temperature, etc., extinction can be achieved. The ratio can always be kept at the optimum value. Here, a case in which the bias voltage is controlled will be explained in detail by taking as an example. Second
The average output of the waveguide is detected by a photodetector via an optical fiber or the like. The output voltage of this photodetector is compared with the voltage level at which the extinction ratio becomes optimal, and the difference signal is fed back to the bias voltage. At this time, if the bias point shifts in the direction of point e' in Figure 2, if the sign of the feedback signal is determined to lower the captured bias voltage, the bias point will be stabilized at point e, where the extinction ratio is optimal. Ru.

(Example) Next, the present invention will be explained in detail by way of examples.

FIG. 3 is a block diagram for explaining the first embodiment of the present invention.

Output light 2 from the transmitting light source 1 is input to a waveguide type optical modulator 3 . The first waveguide output 4 of the waveguide type optical modulator 3 is amplitude-modulated with a modulation signal 5 and used as a transmission (IT signal).

The second waveguide output eti of the waveguide type optical modulator 3 enters the optical fiber 7 and is guided to the photodetector 8. The average output voltage 9 of the photodetector 8 is compared with a reference signal 11 by a comparator 10, and its output error signal 12 is sent to the modulator bias circuit]
3 becomes a feedback signal 14, and the bias level of the waveguide optical modulator 3 is set to the optimum value, thereby realizing an optical transmitter in which the extinction ratio is always kept at the optimum value.

In this embodiment, a semiconductor laser was used as the transmission light source 1. As the waveguide type optical modulator 3, a LiNb0. 100M using directional coupling type optical modulator
Amplitude modulation of b/8 was performed. A photodiode was used as the photodetector 8, and a comparator O was constructed using a differential amplifier.

This optical transmitter was actually used to transmit a signal at a rate of 1ooMb/s. At this time, the ambient temperature of the optical transmitter is 1σC
Despite the above changes, L f N
b The bias voltage of the Os directionally coupled optical modulator changes as the temperature changes, and the extinction ratio of the output signal is always 0.0.
It was kept below 1. Therefore, even during optical heterodyne detection, it was possible to suppress the deterioration in receiving sensitivity due to extinction ratio deterioration to within 1 dB.

FIG. 4 is a block diagram for explaining a second embodiment of the present invention. In this example, the second waveguide output 6
is guided to the photodetector 8 via the lens 15. Further, the output error signal 12 of the comparator 10 is passed through the temperature control circuit 16 as a temperature control signal 17 to control the temperature control element 58 to control the ambient temperature of the waveguide optical modulator 3. The other configurations are the same as the first embodiment. In this embodiment, a Bertier element is used as the temperature control element 18 to control the bias point depending on the temperature.
As in the example described above, the extinction ratio of the output signal could be kept at or below KO.01.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, when using an interferometric optical modulator, a mode conversion optical modulator, or a diffractive optical modulator in addition to a directional coupling optical modulator as the waveguide optical modulator 3, similar extinction occurs. A ratio control optical transmitter can be configured.

It is also possible to use the AC component of the signal instead of the average output as the detection signal from the second waveguide, and it is possible to use the AC component of the signal as the detection signal from the second waveguide.
It is also possible to monitor the output level only when transmitting 0 level or when transmitting code 00 level.

Note that it is also possible to branch the modulated signal I and use the average value of the branched modulated signals as the reference value number 11 for obtaining the output error signal 12.

If the average value of the modulation signal is used as the reference signal 11, even if there is a signal mark rate fluctuation, the reference signal 11 and the average output voltage 9 of the photodetector@H will change at the same rate according to this mark rate fluctuation. Therefore, the output error signal 12 changes rapidly,
Therefore of the modulated signal! Even when there are fluctuations in the extinction ratio, the extinction ratio can be stably controlled to a constant value, especially to an optimal value.

(Effects of the Invention) As described above, according to the present invention, even if the ambient temperature of the external modulator changes, the extinction ratio of the modulator output can be optimally maintained at a constant ρ, thereby eliminating the problem of extinction ratio deterioration. It is possible to provide an optical transmitter that is less affected by the extinction ratio during coherent transmission and the like.

In addition, this device detects the change in extinction ratio using the output of the second waveguide, so compared to the case where the change in extinction ratio is detected using the output of the first waveguide, that is, the signal light output, extinction is possible without loss of signal light. The advantage is that the ratio can be controlled.

[Brief explanation of the drawing]

FIG. 1 shows Ic, LiNb for detailed explanation of the present invention.
FIG. 2 is a diagram showing the transmitted output characteristics of a nine-waveguide optical modulator using O5 with respect to applied voltage, and FIG. 2 is a diagram showing changes in the average output voltage of the second waveguide with respect to changes in bias voltage. FIG. 3 is a block diagram for explaining the first embodiment of the present invention, and FIG. 4 is a block diagram for explaining the second embodiment of the present invention. In the figure, 1... transmitting light source, 2... output light, 3...
... Waveguide optical modulator, 4, 6... Waveguide output, 5.
- Modulation signal, 7... Optical fiber, 8... Photodetector, 9... Average output voltage, 10... Comparator, 11...
- Reference signal, 12... Output error signal, 13... Modulator bias circuit, 14... Feedback signal, 15
... Lens, 16 ... Temperature control circuit, 17 ... Temperature control signal, 18 ... Temperature control element. Figure 2 Bias voltage support Figure 5

Claims (1)

[Claims] (1) A waveguide-type optical modulator that has a light source and first and second waveguides, and modulates the light from the light source by controlling the coupling between the first and second waveguides. In the extinction ratio control forwarding 4M device □, which uses the output light from the first waveguide of the waveguide optical modulator as the transmission signal light, the fluctuation of the bias point of the waveguide optical modulator is A control section that controls the bias point of the AiI waveguide optical modulator using an error signal output from the fluctuation detection section to a fluctuation detection section that detects the output fluctuation from the second waveguide. An extinction ratio controlled optical transmitter comprising: