Optical Transmitter

Abstract

An object of the present invention is to provide an optical transmitter that can suppress degradation of distortion characteristic and RIN characteristic caused by relaxation oscillation at high temperature by controlling average driving current of a light emitting element so that the average optical output power of the light emitting element is increased as the temperature increases like a case where the environmental temperature around the light emitting element increases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] is a block diagram showing the construction of an optical transmitter according to a first embodiment of the present invention.

[FIG. 2] is a graph showing the difference in temperature dependence of optical power between a laser according to the first embodiment of the present invention and a conventional laser.

[FIG. 3] is a graph showing the difference in temperature dependence of relaxation oscillation frequency between the laser according to the first embodiment of the present invention and the conventional laser.

[FIG. 4] is a graph showing the temperature dependence of a slope efficiency of the laser according to the first embodiment of the present invention.

[FIG. 5] is a graph showing the temperature dependence of a threshold current of the laser according to the first embodiment of the present invention.

[FIG. 6] is a graph showing the difference of the temperature dependence of driving current between the laser according to the first embodiment of the present invention and the conventional laser.

[FIG. 7] is a block diagram showing the construction of an average driving current control circuit according to a third embodiment of the present invention.

[FIG. 8] is a block diagram showing the construction of a conventional optical transmitter.

[FIG. 9] is a graph showing the temperature dependence of the driving current according to a conventional laser modulating method.

[FIG. 10] is a graph showing the temperature dependence of relaxation oscillation frequency according to the conventional laser modulating method.

DESCRIPTION OF REFERENCE NUMERALS

• • 1 data input terminal
  • 2 modulation circuit
  • 3 light emitting element (semiconductor laser; LD)
  • 4 monitor PD
  • 5 temperature sensor
  • 6 temperature detecting circuit
  • 7 average driving current control circuit (control unit)
  • 71 constant current circuit
  • 72 variable resistor
  • 73 variable resistor (voltage control type)

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows an optical transmitter according to an embodiment of the present invention. The optical transmitter is equipped with a data input terminal 1, a modulation circuit 2, a semiconductor laser (LD) 3 serving as a light emitting element, a monitor PD 4, a temperature sensor 5, a temperature detecting circuit 6 and an average driving current control circuit 7.

An end face light emission type is used as the semiconductor laser 3, and specifically a Fabry-Perot type laser (FP-LD) is used. However, a distributed feedback type laser (DFB) for emitting coherent light of stable single mode or the like may be used.

The monitor PD 4 monitors the intensity of emitted light of the semiconductor laser 3. When receiving output backlight of the end face emission type semiconductor laser 3, the monitor PD 4 carries out photoelectrical conversion to generate photocurrent (monitor PD current), and also outputs the monitor PD current to the average driving current control circuit 7. Therefore, the output of the monitor PD4 is connected to the input of the average driving current control circuit 7.

When detecting the environmental temperature (ambient temperature) of the semiconductor laser 3, the temperature sensor 5 outputs the voltage corresponding to the temperature thus detected, and the output thereof is connected to the input of the average driving current control circuit.

The temperature detecting circuit 6 outputs a predetermined detection signal to the modulation circuit 2 and the average driving current control circuit 7 in response to a voltage signal from the temperature sensor 5. The input of the temperature detecting circuit 6 is connected to the output of the temperature sensor 5, and the output thereof is connected to the respective inputs of the modulation circuit 2 and average driving current control circuit 7.

The average driving current control circuit 7 controls the average driving current of the semiconductor laser in accordance with the detection signal of the temperature detecting circuit 6. In this case, current control is carried out so that as the ambient temperature of the semiconductor laser (LD) 3 increases, the monitor PD current output from the monitor PD4 is increased, and thus the average driving current of the semiconductor laser 3 is increased. In accordance with the monitor PD current of the monitor PD4 and the detection signal of the temperature detecting circuit 6, a predetermined average driving current in consideration of the environmental temperature of the periphery of the semiconductor laser 3 is input as bias current I to the semiconductor laser (LD) 3.

Here, the definition, etc. of main terms used in the present invention will be described hereunder for confirmation.

The driving current (I_b) is defined as current (injection current) exceeding a threshold level needed to drive the semiconductor laser 3. When modulation signal current is superposed on laser driving current, an amplitude-modulated optical signal is achieved. A method of superposing the modulation signal on the laser driving current to achieve signal light as described above will be referred to as a direct modulation method.

The driving current output from the average driving current control circuit is referred to as “average driving current” by adding a word “average” to the head of “driving current”. In this invention, attention is paid to variation of environmental temperature which is a relatively moderate time variation (variation of about several-second order at most, and the driving signal is varied while imitating the temperature variation concerned. Accordingly, the average value of the driving current for about several seconds will be referred to as “average driving current”. Furthermore, the same is applied to “average optical output (power)” described later.

Next, the operation of this embodiment will be described.

A data signal which is input to a data input terminal 1 and is to be transmitted, that is, a modulation signal is input to the modulation circuit 2. In the modulation circuit 2, modulation current corresponding to the data signal is generated, and the modulation current thus generated is input to the semiconductor laser 3. The semiconductor laser 3 is also supplied with driving current from the average driving current control circuit 7, whereby the semiconductor laser 3 is excited and emits coherent light having a predetermined wavelength which is subjected to light intensity modulation by the direct modulation system. At this time, the monitor PD current of the monitor PD 4 for monitoring the emitted light intensity of the semiconductor laser 3 and the output voltage of the temperature sensor 5 for measuring the environmental temperature of the periphery of the semiconductor laser (LD) 3 are input to the average driving current control circuit 7.

In the average driving current control circuit 7, the driving current is controlled so that the monitor PD current is increased as the environmental temperature of the periphery of the semiconductor laser 3 increases, and the driving current thus controlled is input as average driving (bias) current to the semiconductor laser 3, whereby the average driving (bias) current is controlled in accordance with the variation of the environmental temperature of the semiconductor laser 3.

In order to compensate for the variation of the slope efficiency (=a variation of the optical output (power) to the driving current of the laser) of the semiconductor laser 3 to the temperature variation described above, in the modulation circuit 2 the modulation degree of the semiconductor laser 3 is also controlled with the detection signal from the temperature detecting circuit 6 which is based on the output voltage of the temperature sensor 5.

Accordingly, for example when the environmental temperature of the semiconductor laser 3 increases, the average driving current control circuit 7 increases the average driving (bias) current so that the monitor PD current of the monitor PD4 is increased, whereby the average optical output (power) is increased. Conversely, when the environmental temperature of the semiconductor laser 3 decreases, the average driving current control circuit 7 reduces the average driving (bias) current so that the monitor PD current of the monitor PD4 is reduced, whereby the average optical output is reduced. The output control is carried out so that the optical output of the semiconductor laser 3 is increased in connection with the increase of the environmental temperature of the semiconductor laser 3 as described above.

Next, with respect to a method of controlling the driving current of the semiconductor laser 3 and a method of controlling the modulation degree, the principles thereof will be described hereunder in detail. FIGS. 2 and 3 show examples of the temperature variation of “average optical output (power)” and “relaxation oscillation frequency f_r” in a case where the conventional APC control for fixing the average optical output (power) is carried out and in a case where the temperature control of the first embodiment of the present invention is carried out.

In the conventional APC control, the driving current is increased under the state that the environmental temperature in the vicinity of the semiconductor laser 3 is high as shown in FIG. 9. However, according to the results of various experiments and analysis carried out by the inventor of this invention, it has been found that even when the “relaxation oscillation frequency f_r” is reduced as shown in FIG. 10 at high temperature even when the APC control is carried out, and this serves as one of predominant factors inducing degradation of “distortion characteristic” and “RIN characteristic” at high temperature.

On the other hand, according to the temperature control of the present invention, the average optical output (power) is increased as the environmental temperature around the semiconductor laser 3 increases as shown in FIG. 2 by increasing the driving current more than that in the conventional APC control. With this construction, it has been confirmed that the reduction of “relaxation oscillation frequency f_r” at high temperature can be excellently controlled, that is, the “relaxation oscillation frequency f_r” can be kept substantially constant from a low temperature (for example, 0° C.) to a high temperature (for example, 80° C.) as shown in FIG. 3.

As described above, the inventor has the following knowledge. That is, if the average optical output (power) of the semiconductor laser 3 is not controlled so as to be fixed irrespective of the peripheral temperature of the semiconductor laser 3, but the average driving current of the semiconductor laser 3 is controlled so that “the average optical output (power) is increased as the environmental temperature of the semiconductor laser 3 is increased (see FIG. 2)”, the degradation of “distortion characteristic” and “RIN characteristic” which are caused by “relaxation oscillation frequency f_r” when the ambient temperature of the semiconductor laser 3 is high can be excellently suppressed. The reason for this will be described hereunder in detail.

First, FIGS. 4 and 5 show examples of estimation results on the temperature dependence of “slope efficiency” (=the variation rate of the optical output power to the driving current of the laser) and the threshold current (the current needed for the laser to excite) of a Fabry-Perot type laser (FP-LD).

As shown in FIG. 4, the slope efficiency (W/A) of the semiconductor laser 3 is reduced as the ambient temperature of the semiconductor laser 3 increases, and thus the current control is carried out under the conventional APC control so that the average driving current is increased under high temperature.

That is, according to the conventional modulation system, as described in the column of “Background Art”, the APC control is carried out so that the average optical output (power) is fixed. FIG. 9 shows a graph showing an aspect of the variation of the average driving current with respect to the temperature, which was achieved when the APC control was actually carried out.

Furthermore, FIG. 10 shows the variation of the “relaxation oscillation frequency f_r” of the semiconductor laser 3 when the current control under the APC control was carried out.

It is apparent that under the conventional APC control, the “relaxation oscillation frequency fr” is gradually reduced as the ambient temperature of the semiconductor laser 3 increases as shown in FIG. 10, and “distortion characteristic” and “RIN characteristic” is degraded at high temperature.

With respect to the actual laser, according to the conventional APC control, the threshold current I_th is more greatly increased than the increase of the driving current I_b at high temperature, and thus the term of the threshold current I_th corresponding to the dominator and the numerator of the equation (1) has a greater effect. Therefore, actually, the “relaxation oscillation frequency f_r” at high temperature is reduced, which makes the “relaxation oscillation frequency f_r” become a graph (function) plotting a monotone-decreasing curve as shown in FIG. 10.

As described above, the conventional APC control system is the control of increasing the average driving current I_b at high temperature, however, it has been found through inventor's experiments that it is insufficient to suppress the degradation of “relaxation oscillation frequency f_r” and it is necessary to further increase the average driving current I_b.

That is, it has been found that the suppression of the degradation of “distortion characteristic” and “RIN characteristic” due to “relaxation oscillation” can be performed by driving current still larger than the average driving current value based on the APC control at high temperature.

Particularly when the frequency of the transmission signal is high, it approaches to the relaxation oscillation frequency and thus it is liable to be influenced by the relaxation oscillation frequency, so that the characteristic improving effect of this embodiment is further enhanced.

Second Embodiment

Next, a second embodiment according to the present invention will be described.

A feed-forward method is known as one method of controlling the driving current so that the optical power is increased in connection with the temperature increase. This method will be described hereunder. In this embodiment, the same circuit construction as the first embodiment may be used, however, the monitor PD is unnecessary.

i) First, the temperature dependence of the semiconductor laser 3 on the threshold current I_th (I_th=I_th(T), T: temperature) is measured, and the temperature dependence function I_b(T) of the driving current I_b is determined so that the “relaxation oscillation frequency fr” of the equation (1) described above does not decrease.

ii) secondly, there may be provided a driving current control circuit for varying the driving current I_b on the basis of the function of the output voltage of the temperature sensor so as to satisfy the driving current I_b=I_b(T), and outputting the driving current I_b to the semiconductor laser.

With this construction, the average driving current is uniquely determined with respect to the temperature T. Therefore, it is unnecessary to input the monitor PD current to the feed-forward type average driving current control circuit, and only an output signal of the temperature sensor 5 may be input thereto. That is, in this embodiment, the optical transmitter can be implemented by the construction excluding the monitor PD in the first embodiment as described above.

When some dispersion occurs in samples of the threshold current I_th and the temperature dependence of the semiconductor laser 3 being used according to the feed-forward system, it is necessary to estimate the temperature dependence every semiconductor laser 3, and thus some degree of time is needed for the process of manufacturing the optical transmitter having the semiconductor laser.

Third Embodiment

Next, an optical transmitter according to a third embodiment of the present invention which uses the other driving current control method will be described with reference to FIG. 7. In this embodiment, the same parts as the first embodiment will be represented by the same reference numerals, and the duplicative description thereof is avoided.

The difference of the optical transmitter of this embodiment from the first embodiment resides in the average driving current control circuit 7 shown in FIG. 7. The average driving current control circuit 7 will be mainly described hereunder.

In the average driving current control circuit 7 of this embodiment, a variable resistor 72 is inserted in parallel between the voltage input terminal of a constant current circuit 71 for controlling current and the output terminal of monitor PD, and also a voltage control type variable resistor 72 is additionally inserted to be connected to the variable resistor 72 in series. Specifically, the average driving current control circuit 7 is designed so that the control voltage input terminal of the constant current circuit 71 for supplying laser driving current is connected to the input of the monitor PD, and the variable resistors 72 and 73 are provided between the control voltage input terminal and the ground.

The output of the temperature detecting circuit 6 is connected to the series connected variable resistor 73, and the resistance value is varied in accordance with the detection signal. The variation of the resistance value of the variable resistor varies the control voltage based on the monitor PD current, and the driving current is controlled.

The optical output power is determined by the sum of the resistance values of the variable resistors 73 and 72. At normal temperature, desired optical power can be set by adjusting the variable resistor 72. On the other hand, the resistance value of the variable resistor 73 is controlled by the output voltage of the temperature sensor 5, and it is controlled so that the resistance value is increased under the state that the temperature of the surrounding of the semiconductor laser is high, and the laser driving current is increased.

Accordingly, the laser average driving current is larger than the laser average driving current under the conventional APC control at high temperature, and the control of increasing the average optical output (power) at high temperature can be easily performed. The increasing degree of the average optical output at high temperature is determined by the resistance value of the variable resistor 73 and the variation rate to the output voltage of the temperature sensor 5.

As compared with the feed-forward system of the second embodiment, according to the control method of the third embodiment, the average optical power control of a higher output can be more easily performed at high temperature without measuring the temperature dependence of the threshold current I_th of the laser in advance, and also the feedback control of the average optical power is carried out, so that the laser optical output can be stabilized.

It has been found by the inventor that the degradation of “distortion characteristic” and “RIN characteristic” due to “relaxation oscillation” is suppressed as the increasing degree of the optical power at high temperature is larger. However, if the increasing degree of the optical power is excessively larger, the optical power at high temperature is increased, so that the lifetime of the laser may be lowered. However, if “relaxation oscillation frequency” is not lowered, “distortion characteristic” and “RIN characteristic” would not be degraded. Therefore, as “relaxation oscillation frequency” increases, large optical power is unnecessary. Therefore, in this embodiment, this can be easily implemented by setting the resistance values of the variable resistors 72 and 73 to the optimal value.

In the driving current control system of the present invention, temperature variation occurs in the average optical output (power) as shown in FIG. 2, and a countermeasure to the temperature variation will be described hereunder.

In the transmission of analog signals, unlike the digital transmission, if the modulation signal intensity of the laser is fixed, the reception signal level is hardly varied even when the average driving current of the laser (=DC component of the signal) is varied. Accordingly, the signal level variation and the variation of the average driving current are irrelevant to each other, and the control for suppressing the signal level variation may be carried out independently of the driving current control described above.

This can be implemented by providing the modulating circuit 2 with a control circuit for measuring the dependence of the slope efficiency on the temperature variation in advance and varying the modulation signal intensity in accordance with the temperature. Furthermore, in the optical transmission system, it is general that in consideration of dispersion of the transmission loss a reception dynamic range is secured at the optical receiver so that the reception level may be varied, and some degree of amplitude variation of the modulation signal is allowed.

As described above, the temperature variation of the average optical power based on the driving current control system of the present invention does not affect the transmission characteristic of analog signals, and the signal level adjustment can be implemented independently of the modulation degree control.

Furthermore, in the driving current control of the above embodiment, the average optical power is controlled so that “relaxation oscillation frequency” is not degraded over the range from low temperature to high temperature. Therefore, in the case of the present invention, the effect of “relaxation oscillation frequency” is small at the low temperature, and thus the control of the average optical power may be performed only in a high temperature area above a predetermined temperature.

Still furthermore, the present invention relates to the optical transmitter which is designed to implement the control concerning the average driving current in order to suppress the characteristic degradation of the light emitting element which is caused by the carrier (carrier wave) signal frequency, and no reference is made to the modulation signal.

In the prior art, the temperature sensor is located adjacently to the semiconductor laser in order to accurately measure the temperature of the semiconductor laser. However, in the present invention, the temperature control of the driving current described above is carried out on the operating environmental temperature, and the temperature sensor is designed so as to measure the temperature at a place which is slightly away from the light emitting element (semiconductor laser).

This is because according to the driving current control method of the present invention, when the temperature of the light emitting element (semiconductor laser) is increased by the increase of the driving current at high temperature and this temperature increase is detected by the temperature sensor, positive feedback is applied to the driving current of the light emitting element (semiconductor laser), and the control circuit may be run away.

The present invention has been described in detail by referring to the specific embodiments, however, it is obvious to persons skilled in the art that various modifications and alterations can be applied without departing from the subject matter of the spirit and scope of the present invention.

This application is based on Japanese Patent Application (Japanese Patent Application No. 2004-007786) filed on Jan. 15, 2004, and the content thereof is taken in here as a reference.

INDUSTRIAL APPLICABILITY

According to the present invention, the average driving current of the light emitting element is controlled so as to be increased as the temperature of the average optical output (power) of the light emitting element increases, whereby it is possible to suppress degradation of the distortion characteristic and the RIN characteristic which is caused by the relaxation oscillation at high temperature, and also the present invention is effective used as an optical transmitter in an optical transmission system for carrying out video picture distribution and mobile wireless signal transmission by using the optical transmission technique.

Claims

1. An optical transmitter, comprising: a light emitting element that outputs signal light subjected to light intensity modulation; anda control unit that increases an average optical output of the light emitting element when the environmental temperature in the vicinity of the light emitting element increases.

2. The optical transmitter according to claim 1, wherein the control unit controls average driving current of the light emitting element.

3. The optical transmitter according to claim 2, wherein the control unit controls the average driving current in accordance with the environmental temperature in the vicinity of the light emitting element and the average optical output of the light emitting element.

4. The optical transmitter according to claim 1, wherein the control unit controls the average driving current based on only the data corresponding to the environmental temperature in the vicinity of the light emitting element.