JPH06177835A - Optical transmitter - Google Patents

Abstract

PURPOSE:To improve the stability of a modulation factor corresponding to operating temperatures concerning an optical transmitter for fetching an analog signal and emitting an intensity modulated optical signal through a laser diode by the analog signal. CONSTITUTION:This optical transmitter equipped with a laser diode 11 for fetching an analog signal (a) and directly emitting an intensity modulated optical signal (b) corresponding to the analog signal is provided with a modulation factor detecting means 13 for performing optical/electric conversion to the optical signal emitted by the laser diode 11 and extracting the AC component of a signal provided by the conversion, and level varying means 15 for varying the level of the analog signal corresponding to the level of the AC component extracted by the modulation factor detecting means 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for receiving an analog signal and emitting an intensity-modulated optical signal via a laser diode by the analog signal.

[0002]

2. Description of the Related Art In a CATV, a current television, a high-definition television and a video telephone system, an image signal and an audio signal indicating transmission information are analog-modulated by a carrier signal having a high frequency above the VHF band by these signals. Be transmitted. In recent years, in the field of optical transmission systems,
As described above, technical development of a method of directly electro-optically converting a signal generated by the analog modulation method and transmitting the signal is being actively performed. A particularly important point in such development is to suppress the modulation distortion and avoid the deterioration of S / N at the receiving end to perform faithful transmission of the above-mentioned wideband signal, so that the fluctuation of the operating temperature is suppressed. On the other hand, it is to realize an optical transmitter having a characteristic that the degree of modulation is stable.

Conventionally, as a method of realizing such an optical transmitter, there has been a method of suppressing the characteristic variation with respect to the operating temperature of the laser diode by using a Peltier.
However, in such a method, the Peltier device is generally expensive and consumes a large amount of electric power (for example, a driving current is about 1 A in a case where the Peltier device is driven by a ± 5 V DC power source), so that the cost is reduced. It could not be applied to equipment that requires strict reduction of power consumption. Therefore, for such a device, an optical transmitter configured by adding a circuit for compensating for the above-mentioned characteristic fluctuation is used.

FIG. 5 is a diagram showing a configuration example of a conventional optical transmitter. In the figure, an analog-modulated carrier signal is applied to the input of the preamplifier 51, and its output is connected to the input of the differential efficiency compensation circuit 53 and one input of the error amplifier 54 via the gain control circuit 52. . The output of the differential efficiency compensation circuit 53 is an amplifier 55 and a capacitor 56.
Is connected to a modulation input of a laser diode module (hereinafter, referred to as “LD module”) 57 via. L
The detection output of the D module 57 is connected to one input of the error amplifier 58. A predetermined reference voltage V _ref1 is applied to the other input of the error amplifier 58, and its output is connected to the drive input of the LD module 57 via the drive circuit 59. The other input of the error amplifier 54 has a predetermined reference voltage V
_ref2 is provided and its output is connected to the control input of the gain control circuit 52. The output of the temperature detection circuit 60 is connected to the control input of the differential efficiency compensation circuit 53, and the thermistor 61 is connected to the input thereof.

In the optical transmitter having such a structure, the operating point of the laser diode included in the LD module 57 is set by the drive circuit 59 in a linear region on the optical output characteristic. The carrier wave signal input to the preamplifier 51 is given from its output to the above-mentioned laser diode via the gain control circuit 52, the differential efficiency compensation circuit 53, the amplifier 55 and the capacitor 56. Therefore, the LD module 5
As shown in FIG. 6, 7 emits an optical signal whose intensity is modulated according to the above-mentioned carrier signal.

The error amplifier 54 also includes a gain control circuit 5
The difference between the instantaneous value of the carrier signal obtained at the output of 2 and the above-mentioned reference voltage V _ref2 is _calculated . The gain control circuit 52 keeps the level of the carrier wave signal supplied to the circuit in the subsequent stage from the differential efficiency compensation circuit 53 constant by changing the gain according to the difference. The photodiode built in the LD module 57 photoelectrically converts the optical signal described above, and the error amplifier 58 _calculates the difference between the instantaneous value of the electric signal obtained by the conversion and the reference voltage V _ref1 described above. Drive circuit 5
Negative feedback to 9. Therefore, the average intensity of the optical signal emitted from the LD module 57 is maintained at a constant value P ₀ via the drive circuit 59.

Further, the thermistor 61 is arranged so as to be thermally tightly coupled to the LD module 57, and exhibits a resistance value following the operating temperature of the laser diode described above.
The temperature detection circuit 60 constantly outputs a control signal indicating its operating temperature according to such a resistance value. The differential efficiency compensating circuit 53 stores the value of the differential efficiency η _k (k is an integer value corresponding to each value of the operating temperature) previously obtained at each operating temperature of the LD module 57, and these differential values are stored. Among the efficiency values, the reciprocal of the value corresponding to the operating temperature indicated by the control signal is multiplied by the instantaneous value of the carrier signal given through the preamplifier 51 and output. In this way, the differential efficiency compensating circuit 53 compensates for the variation (η ₁ / η ₂ ) of the differential efficiency of the LD module 57 according to the operating temperature, so that the modulation degree becomes constant with respect to the level of the carrier signal described above. To be kept.

[0008]

By the way, in such a conventional optical transmitter, the operating temperature of the LD module 57 detected by the thermistor 61 includes an error due to the variation in the characteristics of the thermistor. Although the variation in the differential efficiency (light output characteristic) with respect to the operating temperature of the module 57 also varies, the differential efficiency compensating circuit 53 compensates for the variation in the modulation with respect to the operating temperature by the feedforward method. , Compensation error was large. Further, such a compensation error may be magnified by variations in the characteristics of other circuit elements, and cannot be easily absorbed even by making full use of circuit technology.

However, such an error in the degree of modulation causes modulation distortion according to the nonlinearity near the threshold on the optical output characteristic in the overmodulation state caused by the error,
On the other hand, when the modulation degree is lowered, the S / N ratio at the receiving end opposed via the optical transmission line may be deteriorated. Therefore, it can be neglected particularly in an optical transmission system that faithfully analog-transmits a wideband signal. could not.

An object of the present invention is to provide an optical transmitter capable of improving the stability of the modulation factor with respect to the operating temperature.

[0011]

FIG. 1 is a block diagram showing the principle of the present invention. The present invention captures an analog signal,
In an optical transmitter provided with a laser diode 11 that directly emits an optical signal whose intensity is modulated according to the signal, the optical signal emitted by the laser diode 11 is optoelectrically converted, and the AC of the signal obtained by the conversion is converted. It is characterized in that it is provided with a modulation degree detecting means 13 for extracting a minute portion, and a level varying means 15 for varying a level of an analog signal according to the level of the alternating current component extracted by the modulation degree detecting means.

[0012]

In the optical transmitter according to the present invention, the differential efficiency of the laser diode 11 changes as the operating temperature changes,
Although the modulation degree also shifts in parallel, the modulation degree detecting means 13 performs optical-electrical conversion on the optical signal emitted from the laser diode 11 to extract an alternating current component, and the level varying means 15 modulates from the level of the alternating current component. And the level of the analog signal given to the laser diode 11 is changed according to the detection result.

That is, even if the above-mentioned differential efficiency fluctuates, an analog signal whose level is changed in a direction of compensating for the fluctuation is given to the laser diode 11 as a modulation signal, and an optical signal emitted according to the signal. The degree of modulation is kept constant over operating temperature. Therefore, the occurrence of distortion at the time of overmodulation due to the non-linearity in the vicinity of the threshold on the optical output characteristics of the laser diode 11 is avoided, and at the receiving ends facing each other via the optical transmission path, S due to the decrease in the modulation degree is caused. /
The deterioration of N is avoided.

Further, the modulation degree detecting means 13 and the level changing means 15 are realized with low power consumption and low cost as compared with the conventional example using the Peltier, and in the process of the variable control of the level of the analog signal described above, the laser diode. Since variations in other characteristics are absorbed, an optical transmitter having high stability while satisfying demands for power saving and size reduction can be realized at low cost.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 2 is a diagram showing a first embodiment of the present invention.

In the figure, parts having the same functions and configurations as those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted here. In the present embodiment, the characteristic feature of the present invention is that the LD module 57 is replaced by a laser diode and two photodiodes optically coupled to the LD.
The module 21 is provided, and in place of the differential efficiency compensation circuit 53, the temperature detection circuit 60, and the thermistor 61, a capacitor 22, an amplifier 23, and a detector 24 that are cascade-connected to one output of a photodiode built in the LD module 21 are provided. The output of the detector 24 is connected to one input of the error amplifier 54 instead of the output of the gain control circuit 52. It should be noted that, of the two photodiodes described above, one of the photodiodes described above is denoted by reference numeral 25 ₁ , and the other photodiode is denoted by reference numeral 25 ₂ .

Regarding the correspondence relationship between this embodiment and the block diagram shown in FIG. 1, the LD module 21 corresponds to the laser diode 11, and the photodiode 25 ₁ , the capacitor 22 and the error amplifier 23 are the modulation degree detecting means 13.
Corresponding to the preamplifier 51, the gain control circuit 52, the amplifier 55, the capacitor 56, the detector 24 and the error amplifier 5.
Reference numeral 4 corresponds to the level changing means 15.

The operation of this embodiment will be described below. The laser diode built in the LD module 21 emits an optical signal directly modulated according to the carrier wave signal given through the preamplifier 51, the gain control circuit 52, the amplifier 55 and the capacitor 56.

The photo diode 25 ₁ photoelectrically converts such an optical signal into an electric signal, and the capacitor 22 and the amplifier 23.
Extracts only the AC component from the electric signal obtained by the conversion, amplifies it, and performs peak detection via the detector 24. Therefore, at the output of the detector 24, a signal indicating the above-mentioned leading value of the intensity of the optical signal by a voltage is obtained, and the error amplifier 54 obtains the difference between the instantaneous value of the signal and the reference voltage V _ref2 . The gain control circuit 52 maintains the level of the carrier signal obtained at its output constant by varying the gain based on such a difference.

That is, as shown in FIG. 6, even if the intensity of the optical signal emitted from the laser diode fluctuates because the operating temperature changes and the differential efficiency of the LD module 21 changes, the intensity of the optical signal also changes. Is kept almost constant.

As described above, according to this embodiment, the LD of the LD module 21 is changed according to the level of the AC component of the emitted light.
Since the level of the carrier signal given to the module is negatively feedback-controlled, a circuit that consumes less power and is less expensive than the conventional method using the Peltier is used, and the operating point of the LD module is controlled by the feedforward control. Compared with the conventional example, it is possible to absorb variations in the characteristics of the LD module 21 and other circuit elements and stabilize the modulation degree with respect to temperature with high accuracy.

In this embodiment, the gain control circuit 52 negatively feeds back the signal indicating the level of the AC component of the optical signal emitted from the LD module 21 to stabilize the modulation degree and suppress the distortion generated during overmodulation. However, in the present invention, the APC performed through the error amplifier 58 and the drive circuit 59.
As long as the total amount of negative feedback including the control loop is set to an appropriate value, for example, as shown by the dotted line in FIG. 2, the output of the photodiode 25 ₁ is negatively fed back to the laser diode via the drive circuit 59. The above method may be used alone or in combination to compensate for the non-linear distortion in the region where the operating point is set on the optical output characteristics of the LD module 21.

FIG. 3 is a diagram showing a second embodiment of the present invention. In the figure, the difference from the embodiment shown in FIG. 2 is that a preamplifier 31 that amplifies the pilot signal, a preamplifier 32 that amplifies the first carrier signal, and a preamplifier that amplifies the second carrier signal are used. An amplifier 33 is provided in place of the preamplifier 51, and the outputs of these amplifiers are directly connected to each other to connect the gain control circuit 5
It is connected to the input of 2 and a low pass filter 34 is arranged between the amplifier 23 and the detector 24.

The operation of this embodiment will be described below. The pilot signal is a single sine wave signal, and its frequency is, as shown in FIG. 4, the first carrier signal (for example,
The VHF band television broadcast wave) and the second carrier signal (for example, a UHF band radio signal assigned to mobile radio) are set to be small and separated on the frequency axis. These signals are amplified through the preamplifiers 31 to 33 to a predetermined level and then combined (hereinafter, the signal obtained by such combination is simply referred to as a “combined signal”), and the gain control circuit 52. , Is supplied to the LD module 21 via the amplifier 55 and the capacitor 56, is electro-optically converted, and is emitted as an optical signal.

In the negative feedback path from the photodiode 25 ₁ to the gain control circuit 52, the low-pass filter 34 has the frequency characteristic shown by the dotted line in FIG. 4, and is obtained by optoelectric conversion of the above optical signal. Only the component of the pilot signal is extracted from the combined signal. That is, the signal negatively fed back to the gain control circuit 52 is obtained according to the level of the pilot signal, but the power ratio of each signal included in the combined signal is constant, so that it is similar to the embodiment shown in FIG. And LD
The modulation degree of the module 21 is stabilized with respect to the operating temperature.

Therefore, according to the present embodiment, the optical transmitter having both the low distortion and the stability which are strictly required in the operating temperature range can be realized at low cost, and the frequency-multiplexed analog signal can be faithfully reproduced. Can be transmitted.

In each of the above-described embodiments, the LD module 21 is formed by integrating a single laser diode and two photodiodes, which are opposed to each other via a prism. The invention is not limited to such an LD module, and when the band limited by the stray capacitance formed on the connection with the error amplifier 23 is negligible, for example, a single photodiode is incorporated. A similar circuit may be formed by using the LD module and electrically dividing the output of the photodiode.

[0028]

As described above, according to the present invention, the modulation degree of a laser diode for intensity modulation is detected by using an inexpensive circuit which consumes less power and is given to the laser diode according to the detection result. The level of the modulation signal is variably controlled.

That is, since the modulation degree is kept constant with respect to the change of the differential efficiency of the laser diode according to the change of the operating temperature, the optical output characteristic of the laser diode is affected by the nonlinearity near the threshold. The occurrence of distortion during modulation and the deterioration of S / N due to the reduction of the modulation degree at the receiving end opposed via the optical transmission path are avoided while absorbing the variations in the characteristics of the circuit elements.

Therefore, in the optical transmission system to which the present invention is applied, the transmission quality can be improved at low cost.

[Brief description of drawings]

FIG. 1 is a principle block diagram of the present invention.

FIG. 2 is a diagram showing a first embodiment of the present invention.

FIG. 3 is a diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing frequency allocation of input signals.

FIG. 5 is a diagram showing a configuration example of a conventional optical transmitter.

FIG. 6 is a diagram showing a change in light output characteristic of a laser diode with respect to temperature.

[Explanation of symbols]

 11 laser diode 13 modulation degree detecting means 15 level changing means 21,57 laser diode module 22,56 capacitor 23,55 amplifier 24 detector 25 photo diode 31, 32, 33, 51 preamplifier 34 low-pass filter 52 gain control circuit 53 Differential efficiency compensation circuit 54,58 Error amplifier 59 Drive circuit 60 Temperature detection circuit 61 Thermistor

Claims (1)

[Claims] 1. An optical transmitter provided with a laser diode (11) for directly capturing an analog signal and intensity-modulating the optical signal according to the analog signal, wherein the optical signal emitted by the laser diode (11) is Modulation degree detecting means (13) for opto-electrically converting and extracting the AC component of the signal obtained by the conversion, and the level of the analog signal according to the level of the AC component extracted by the modulation degree detecting means. An optical transmitter comprising: a level changing means (15) for changing.