JP2004140509A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter whose temperature compensation is easy and exact. <P>SOLUTION: The optical transmitter provided with a biasing means 2 for controlling optical power of a light source 1 and a modulating means 3 for controlling the amplitude of an optical signal is provided with a function calculating means 4 for calculating a function expressing the relation between control amounts in the means 2, 3 and an input variable proportional to an atmospheric temperature, and a temperature sensor 5 for measuring the atmospheric temperature and providing the temperature detection electric amount as an input variable to the means 4. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmitter that controls the optical power of a light source by using a bias current and a modulation current, and more particularly to an optical transmitter that can easily and accurately compensate for temperature.
[0002]
[Prior art]
This type of optical transmitter includes a light source such as a laser diode (hereinafter, referred to as an LD) that outputs light with an intensity corresponding to a bias current and a modulation current, and a bias current that applies the optical power (average level) of the light source to the light source. And a modulation means for controlling the amplitude of the optical signal by a modulation current applied to the light source. When a transmission signal provided from an external device is amplified and a bias is applied, and the amplified signal with the bias is applied to an LD serving as a light source, an optical signal whose light output intensity fluctuates above and below an average level is obtained.
[0003]
In an optical transmitter, it is necessary to keep the optical power and the extinction ratio constant according to the standard of optical communication. However, since the characteristics of the light source have temperature dependence, the optical power and the amplitude of the optical signal change due to a change in ambient temperature, and the extinction ratio also changes.
[0004]
Therefore, conventionally, a voltage signal proportional to the ambient temperature is extracted using a temperature sensor such as a thermistor, and is applied as a temperature compensation control voltage to a driver that generates a bias current and a modulation current.
[0005]
As shown in FIG. 2, the conventional optical transmitter includes a light source 21, a driver 22, and a temperature sensor 23, and is configured such that an output voltage of the temperature sensor 23 changes a bias current and a modulation current of the driver 22. Have been.
[0006]
FIG. 3 shows the IL characteristics (applied current vs. light output intensity characteristics) of the light source when the ambient temperature is 25 °, and FIG. 4 shows the IL characteristics of the light source when the ambient temperature is 70 °. Further, these figures, an optical signal S _o of the light source outputs are appended in accordance with the drive signal S _d and I-L characteristics supplied to the light source.
[0007]
Comparing FIGS. 3 and 4, when the ambient temperature is low, as shown in FIG. 3, the IL characteristic 31 of the light source 21 rises sharply in a current range lower than that of FIG. ing. On the other hand, when the ambient temperature is high, as shown in FIG. 4, the IL characteristic 41 of the light source 21 rises more gradually from a current range higher than that in FIG. 3 than in FIG. When Accordingly ambient temperature as shown in FIG. 3 is low, reducing the bias current I _bia, modulation current I _mod also by entering the small control drive signal S _d, the amplitude P ₁ and below the optical power P _av - An optical signal _So having P ₀ can be obtained. When the ambient temperature is high as in Figure 4, increasing the bias current _{I bia,} modulation current _{I mod} also by entering a larger control drive signal _{S d,} the amplitude _P 1 -P ₀ and below the optical power _{P av} it is possible to obtain an optical signal S _o with. By such control of the bias current and modulation current, FIG. 3, an optical power P _av is the same in both FIG. 4, the extinction ratio ER = P 1 _/ P ₀ is the same.
[0008]
Patent Document 1 is a prior art document.
[0009]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-204022
[Problems to be solved by the invention]
However, the temperature-dependent characteristics of the LD vary from one device to another, and even if temperature sensors having the same temperature characteristics are used uniformly, good temperature compensation cannot always be achieved. Therefore, conventionally, after once assembling the optical transmitter, the optical transmitter is tested in various temperature environments and the optical power P _av and the extinction ratio ER are measured while replacing the temperature sensor having a different temperature characteristic. It required a trial and error process of checking whether the temperature compensation was achieved. However, it is troublesome and troublesome to replace the temperature sensor for each optical transmitter many times.
[0011]
Further, since the temperature-dependent characteristics of the LD do not exactly match the temperature-resistance characteristics of the temperature sensor, it is difficult to make the optical power _Pav and the extinction ratio ER completely constant over the entire temperature range. However, it is necessary to keep the extinction ratio ER strictly constant as the modulation speed is increased in order to increase the optical communication speed, and the accuracy of the compensation by the temperature sensor is limited.
[0012]
Then, an object of the present invention is to solve the above-mentioned problems and to provide an optical transmitter that is easy and accurate in temperature compensation.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides an optical transmitter including a bias unit for controlling the optical power of a light source and a modulation unit for controlling the amplitude of an optical signal. Provided with a function calculating means for calculating a function representing a relationship with an input variable proportional to temperature, and a temperature sensor for measuring an ambient temperature and providing the temperature detection electric quantity as an input variable to the function calculating means. It is.
[0014]
The above function may be an n-th order function of the input variable.
[0015]
Storage means may be provided for storing a coefficient relating to the input variable term of the function and a constant of the constant term.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[0017]
As shown in FIG. 1, an optical transmitter according to the present invention includes a light source 1 such as a laser diode (hereinafter, referred to as an LD) that outputs light at an intensity corresponding to a bias current and a modulation current, and an optical power of the light source 1. Bias means 2 for controlling the (average level) by a bias current applied to the light source 1, modulation means 3 for controlling the amplitude of the optical signal by a modulation current applied to the light source, control amounts and atmospheres in the bias means and the modulation means A function calculating means 4 for calculating a function representing a relationship with an input variable proportional to the temperature, and a temperature sensor 5 for measuring the ambient temperature and providing the temperature detection electric quantity to the function calculating means 4 as an input variable. Have. When a thermistor is used as the temperature sensor 5, the temperature detection electric quantity is a voltage.
[0018]
In this embodiment, the bias means 2, the modulation means 3, and the function calculation means 4 are configured by incorporating a control / operation program into a microprocessor (CPU) 6. A D / A converter 7 and an A / D converter 8 are provided as an interface between the microprocessor 6 and analog elements. Further, in this embodiment, the control value of the bias current and the control value of the modulation current are superimposed in the microprocessor 6, and the superimposed control value is input to the driver 9 as a current, for example, by analog conversion. Has become. The driver 9 is to provide a drive signal Sd to the transmission signal Si from the transmitting signal generator (not shown) is amplified in accordance with the control current is the bias current I _bia and the modulation current I _mod in the light source 1.
[0019]
A storage means 10 for storing coefficients and constants used for calculating functions is provided in the microprocessor 6 internally or externally. The storage means 10 is configured by a semiconductor memory element such as a ROM or a non-volatile RAM that can store data without power supply.
[0020]
The operation of temperature compensation in the optical transmitter of FIG. 1 will be described.
[0021]
Here, the function calculated by the function calculating means 4 is:
^{_{f (x) = A n x}} n + A n-1 x n-1 + ... + A 1 x + A 0
And x is an input variable consisting of a temperature detection electric quantity of the temperature sensor 5. A _{n to} A ₁ are coefficients, and A ₀ is a constant, which is stored in the storage unit 10. The function f (x) is represented by an n-th order expression of the input variable x, and the calculation result is a control value obtained by superimposing the bias current and the modulation current. That is, the function f (x) is a function for obtaining a control value according to the temperature. By providing this control value to the D / A converter 7, the driver 9 outputs a bias current and a modulation current whose temperature has been compensated.
[0022]
When the ambient temperature is low as shown in FIG. 3, the microprocessor 6 calculates a function f (x) based on an input variable x obtained by A / D converting the output voltage of the temperature sensor 5 with the A / D converter 8. I do. Given the calculation result of the function f (x) to the D / A converter 7, the bias current _{I bia} is small, the modulation current _{I mod} is also controlled small, and below the optical power _{P av} amplitude _P 1 -P ₀ An optical signal is obtained.
[0023]
When the ambient temperature is high as shown in FIG. 4, the microprocessor 6 calculates a function f (x) based on an input variable x obtained by A / D converting the output voltage of the temperature sensor 5 by the A / D converter 8. I do. Given the calculation result of the function f (x) to the D / A converter 7, the bias current _{I bia} large, the modulation current _{I mod} is also largely controlled, and below the optical power _{P av} amplitude _P 1 -P ₀ An optical signal is obtained.
[0024]
The coefficients A _{n to} A _{1 of the first} to n-order terms of the input variable x and the constant A _{0 of the} constant term are predetermined such that the optical power P _av is constant in a wide temperature range including the temperature in FIGS. The extinction ratio ER = P ₁ / P ₀ may be obtained from experiments at a plurality of temperatures and registered in the storage unit 10 so that the extinction ratio ER = P ₁ / P ₀ is also constant.
[0025]
By such control of the bias current and modulation current, FIG. 3, the optical power P _av in both FIG. 4 is constant, the extinction ratio ER = P 1 _/ P ₀ is also constant.
[0026]
Conventionally, only the output voltage of the temperature sensor 23 is supplied to the driver 22 in an analog manner without any change, so that the temperature dependence of the characteristics of the light source 21 does not exactly match the temperature characteristics of the temperature sensor 23 in a wide temperature range. This limits the accuracy of temperature compensation. In the present invention, a control value is obtained by digitally processing the output voltage of the temperature sensor 5 to obtain a control value, and the driver 9 is controlled by the control value. Therefore, if the coefficients and constants of the function are appropriately determined, a wide range is obtained. Temperature compensation can be accurately performed over the temperature range.
[0027]
Next, how to obtain the coefficients A _n to A ₁ and the constant A ₀ corresponding to variations among individuals of the light source 1 for (registration method) will be described.
[0028]
After assembling the optical transmitter, the optical transmitter is placed in an environment where the temperature is constant and can be arbitrarily controlled, such as a thermostat, and a test transmission signal _Si is input. Factor A _n to A ₁ and the constant A ₀ is the initial value decided as appropriate (e.g., a known value for an average light source 1) to. In this state, the optical signal _So is received by the measuring instrument, and the optical power _Pav and the extinction ratio ER are measured from the received waveform. This measurement is performed at appropriate temperature steps over a wide temperature range.
[0029]
Estimating a desired bias current I _bia and the modulation current I _mod at each temperature from the comparison between the measured value of a desired optical power P _av and the extinction ratio ER and each temperature. Calculating back the control value to be given to the driver 9 at each temperature from the estimated bias current I _bia and the modulation current I _mod, the control value for each temperature (the value of the function f (x)) and temperature (input variable x) the by fitting the function it can be determined whether the coefficient a _n to a ₁ and the constant a ₀ may be changed much from its initial value.
[0030]
Writing a coefficient A _n to A ₁ and the constant A ₀ obtained in this way in the storage means 10, an optical transmitter, regardless of the variation of each individual light source 1, a wide beam over a temperature range power P _av constant and extinction ratio It can be shipped as an optical transmitter with a constant ER.
[0031]
In the past, a trial and error process was required to replace the temperature sensor 23 for each optical transmitter many times until a suitable temperature sensor 23 was found. In the present invention, the temperature sensor 5 does not need to be replaced once without being attached, and only the writing to the storage means 10 is performed, so that the work for temperature compensation becomes easy.
[0032]
In the present embodiment, the function f (x) is an nth-order function, but may be another function, as long as the temperature characteristics of the light source can be compensated as a result. The order of the n-order function is not limited, and may be, for example, a cubic function.
[0033]
Next, a specific embodiment using a cubic function as the function f (x) will be described.
[0034]
Here, the coefficient _A 3 to A ₁ and the constant _{A 0} of three different functions f (x) ▲ 1 ▼ ~ ▲ 3 ▼ prepared in advance. Assuming that the value of the function f (x) is y,
^{▲ 1 ▼ y = 0.00166x 3 -0.0642x} 2 + 3.45x + 703
^{▲ 2 ▼ y = 0.00175x 3 -0.0534x} 2 + 4.42x + 678
^{▲ 3 ▼ y = 0.00131x 3 -0.041x} 2 + 1.505x + 745
It is.
[0035]
FIG. 5 shows the relationship between the ambient temperature (horizontal axis) and the output (vertical axis) of the D / A converter 7 (the relationship between the input variable x and the function value y) for each of the functions (1) to (3). Show. The characteristic curves (1) to (3) correspond to the functions (1) to (3), respectively. For a plurality of samples of the light source 1, for example, assuming that the characteristic curve {circle around (1)} obtains a function value y suitable for compensating for the average temperature dependency, the characteristic curve {circle around (2)} is different from this. The value y is large in the high temperature range and small in the low temperature range. Conversely, in the characteristic curve (3), the function value y is small in the high temperature range and large in the low temperature range. Apply the characteristic curve (1) to the average sample, and apply the characteristic curve (2) or (3) to the sample that is out of the average so that the temperature compensation of all samples can be performed as accurately as possible. It can be carried out.
[0036]
Thus, without individually setting the coefficients A _{n to} A ₁ and the constant A ₀ for each sample, a representative finite number of coefficients A _{n to} A ₁ and the constant A ₀ are set in advance, The temperature compensation of all the samples can be performed as accurately as possible by simply selecting and applying them.
[0037]
【The invention's effect】
The present invention exhibits the following excellent effects.
[0038]
(1) Since the mismatch of the temperature characteristics between the light source and the temperature sensor can be eliminated by the interposition of the function, the temperature can be accurately compensated over a wide temperature range.
[0039]
(2) The temperature compensation is easy because the temperature sensor need not be replaced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of an optical transmitter showing one embodiment of the present invention.
FIG. 2 is a circuit diagram of a conventional optical transmitter.
FIG. 3 is a diagram of the IL characteristic of the light source when the ambient temperature is 25 °.
FIG. 4 is a diagram showing IL characteristics of a light source when an ambient temperature is 70 °.
FIG. 5 is a diagram showing an input / output relationship of functions according to the present invention.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 light source 2 bias means 3 modulation means 4 function calculation means 5 temperature sensor 6 microprocessor 9 driver 10 storage means

Claims (3)

In an optical transmitter including a bias unit for controlling the optical power of a light source and a modulation unit for controlling the amplitude of an optical signal, a relation between a control amount of the bias unit and the modulation unit and an input variable proportional to an ambient temperature is shown. An optical transmitter, comprising: a function calculating means for calculating the calculated function; and a temperature sensor for measuring the ambient temperature and providing the temperature detection electric quantity as an input variable to the function calculating means. 2. The optical transmitter according to claim 1, wherein the function is an n-th order function of an input variable. 3. The optical transmitter according to claim 1, further comprising storage means for storing a coefficient relating to an input variable term of the function and a constant of a constant term.

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