1. Field of the Invention
The present invention relates to an optical transmitting module.
2. Related Prior Art
The optical transmitting module has been applied in the optical communication system, in which the module converts an electrical signal inputted therein into a corresponding optical signal to output in an optical transmitting medium such as an optical fiber. In the wavelength division multiplexing (WDM) system, which is one type of intelligent optical communication systems to send a large capacity of information, a plurality of optical transmitting modules simultaneously outputs a plurality of optical signals each having a specific wavelength, therefore, it is strongly requested for the wavelength of the optical signal to show quite high accuracy and stability even when environment conditions, such as an ambient temperature, are varied.
One solution for solving the above subject has been disclosed in Japanese Patent published as JP-2003-273447A. The optical transmitting module disclosed in this patent document controls in feedback the current supplied to the Peltier element that mounts the laser diode (hereinafter denoted as LD) thereon to adjust the temperature thereof, based on the preset value that corresponds to the desired temperature of the Peltier element. Thus, the temperature of the LD may be kept constant regardless of the ambient temperature.
However, in the WDM system, components or equipments used therein require a performance to be operable in a wide temperature range from −40° C. to +85° C. When the conventional optical transmission module is applied to such WDM system, the maximum range of the ambient temperature, that means the maximum operable range in the temperature, becomes 125° C. The Peltier element has a paired plates, one is cooled down while the other is heated up by supplying a driving current thereto. The direction of the current determines the operational mode of the Peltier element, namely, whether the target plate is cooled down or heated up. In the optical transmitting module, the LD is mounted on one plate of the Peltier element, while the other plate is thermally coupled with the ambient. Therefore, by supplying the driving current to the Peltier element, the LD mounted thereon is cooled down or heated up.
However, the Peltier element generally shows an operating limit of about 50° C. between two plates. That is, when one of plates is exposed to the ambient, the other plate is restricted to be controlled in the temperature thereof within +/−50° C. with respect to the ambient temperature. Therefore, when the temperature of the LD should be kept constant at 40° C., it is barely able to control the temperature of the LD when the ambient temperature is 85° C., the upper limit of the WDM system, while it is unable to control when the ambient temperature is −40° C., the lower limit of the standard of the WDM system.
When no temperature control is performed for the LD, various problems may occur. That is, in the coarse wavelength division multiplexing system (CWDM system), which is one type of the WDM communication system, a wavelength interval between signal channels is set to be 10 nm. On the other hand, the temperature dependence of the emission wavelength from the LD becomes about +0.1 nm/° C. even for a LD with the distributed feedback (DFB) type, which stably oscillates in a single mode and shows a quite sharp emission spectrum. Therefore, the optical transmitting module without any temperature control function for the LD shows a deviation of the emission wavelength of about 12.5 nm within a whole operable range of the ambient temperature, which exceeds the CWDM standard.
One aspect of the present invention relates to an optical transmitting module. The module comprises a laser diode (LD), a Peltier element, first and second temperature sensors, a Peltier driver, and a reference generator. The laser diode emits light with an emission wavelength. The first temperature sensor senses a current temperature of the LD, while the second temperature sensor senses the ambient temperature of the module. The reference generator calculates a reference temperature, to which the temperature of the LD is adjusted, by receiving the ambient temperature from the second sensor. The Peltier driver drivers the Peltier element by (1) receiving the current temperature of the LD and the reference temperature from the reference generator, (2) comparing these temperatures, and (3) outputting a driving current to the Peltier element such that a difference between these temperatures disappear, that is, the current temperature becomes equal to the reference temperature, by adjusting the magnitude of the driving current and its direction.
Since the present optical module senses the ambient temperature and determines the reference temperature of the LD based on this sensed ambient temperature, a range of the reference temperature may be smaller than an operable range of the ambient temperature. That is, even the ambient temperature has a wide operable range of 125° C., from −40° C. to +85° C., the reference temperature of the LD may be set in a smaller range. It is preferable to set the range of the reference temperature is 100° C., because the LD is operated within this temperature range, the shift of the emission wavelength thereof may be kept within 10 nm, which satisfies the course wavelength division multiplexing system. Moreover, it is further preferable to set the difference between the reference temperature and the ambient temperature smaller than 50° C., which can protect the Peltier element from the thermal runway.
Another aspect of the present invention relates to a method to control the temperature of the LD. The method comprises steps of: (a) sensing a current temperature of the LD by the first sensor; (b) sensing an ambient temperature of the module by the second sensor; (c) calculating a reference temperature of the LD such that a range of the reference temperature is smaller than a range of the ambient temperature; (d) comparing the reference temperature with the current temperature; and (e) driving the Peltier element to disappear the difference between the reference temperature and the ambient temperature. In another mode of the method according to the present invention, the step (c) comprises to calculate a reference temperature of the LD such that the difference from the ambient temperature becomes smaller than 50° C.
Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. In the description below, same numerals or symbols will refer to same elements without overlapping explanations.
The driver 11, connected to the LD 5, supplies the bias current IB and the modulation current IM to the LD 5. The driver 11 includes a first section 21 to modulate the modulation current IM and a second section 23, including a constant current source 23a and an inductor 23b, to generate the bias current IB.
On the Peltier element 3 is mounted with the LD 5. By supplying the driving current to the Peltier element 3, the LD may be cooled down or heated up to vary the temperature thereof. The mode whether the LD is cooled down or heated up may be determined by the direction of the driving current.
Immediate to the LD 5 and on the Peltier element 3 is mounted with a thermistor 25 as the first temperature sensor 7. By dividing the constant voltage Vref with the thermistor 25 and a resistor 27, the resistance of the thermistor widely changes, a voltage signal VL that corresponds to the temperature of the Peltier element 3 and nearly equal to that of the LD is output to the Peltier driver 15 as the current temperature signal.
The Peltier driver 15 supplies the driving current to the Peltier element 3 so as to keep the current temperature of the LD 5 constant. This Peltier driver 15 includes the automatic temperature control (hereinafter denoted as ATC) circuit 29 and the current driver 31. The ATC circuit 29 receives the current temperature signal VL from the first temperature sensor 7 and the reference signal VLC from the reference generator 19, and outputs a signal so as to equalize these input signals, VL and VLC, namely, to close the current temperature signal VL to the reference signal VLC. The current driver 31 converts this signal output from the ATC circuit 29 into the driving current and determines the direction of this driving current. The current driver 31 may operate in the PID control, the PI control and the switching control of the current to the Peltier element.
The optical module 1 further provides the second temperature sensor 17 configured to monitor the ambient temperature of the module 1 and to output the reference signal. A thermistor and a junction diode may be available for the second temperature sensor 17. This second temperature sensor 17 is preferable to be installed within the module apart from the LD 5 or the Peltier element 3 so as not to be affected from the Peltier element 3.
The reference generator 19 calculates the reference temperature TLC from the ambient temperature T(amb) sensed by the second sensor 17. For example, the following function may be applicable for the calculation;
TLC=T(ref)+α×(T(amb)−T(ref)),
where T(ref) denotes the temperature at which the reference temperature becomes equal to the ambient temperature T(amb).
A parameter α may be a positive number. The reference temperature TLC may be calculated from the ambient temperature T(amb) so as to narrow the range of the reference temperature TLC for the LD smaller than that of the ambient temperature T(amb). The temperature difference between two plates of the Peltier element should be smaller than 50° C., and one plate is exposed to the ambient while the other plate mounts the LD. Therefore, from the function above, this relation of the temperatures of two plates of the Peltier element 3 becomes;
|T(ref)+α×(T(amb)−T(ref))−T(amb)|<=50° C.,
that is;
1-50/|T(ref)−T(amb)|<=α
Under an extreme condition, namely, T(ref) is set to be the uppermost or lowermost within the range of the ambient temperature and the ambient temperature becomes the lowermost or uppermost temperature within the range, the value |T(ref)−T(amb)| becomes 125° C., then, a condition of α>=3/5 can be obtained. This case reflects the extreme conditions that T(ref) is set to be 125° C. or −40° C. While, under normal conditions that T(ref) is set in a room temperature, typically in a range from 10° C. to 40° C., a preferable range of α>=2/5 may be obtained.
Moreover, it is further preferable that the parameter a becomes smaller than or equal to 4/5, α<=4/5, because the range of the temperature of the LD TLC becomes smaller than 100° C., accordingly, the shift of the emission wavelength of the LD, specifically for the DFB-LD with the temperature coefficient of 0.1 nm/° C. for the emission wavelength, may be compressed smaller than an interval of the CWDM standard, which is 10 nm.
The reference generator 19 may calculate the reference temperature TLC based on the ambient temperature T(amb) by using data stored in the memory 33 such as a read only memory (ROM). For example, the reference generator 19 reads the parameters, α and T(ref), from the memory 33 and calculates the reference temperature TLC by using these parameters according to the above function. Or, the reference temperature TLC may be obtained by reading data configured in a look-up-table within the memory 33 that relates the reference temperature TLC with respect to the ambient temperatures. After obtaining the reference temperature TLC, the reference generator 19 converts it into a voltage value VLC and not only outputs this voltage VLC to the ATC circuit 29 of the Peltier driver 15 but also sends the reference temperature TLC to the APC circuit 13.
The voltage signal VLC is output to the Peltier driver 15 from the reference generator 19, the Peltier driver 15 controls the driving current IP such that the current temperature of the LD sensed by the first sensor 7 becomes equal to the reference temperature TLC. For example, as shown in
On the Peltier element 3 is mounted with a photodiode 9 for monitoring light emitted from the back facet of the LD 5. This photodiode 9 converts the optical signal from the LD 5 into a current signal and outputs it to the APC circuit 13. The APC circuit 13, based on this current signal, adjusts the modulation current IM and the bias current IB to keep the magnitude of the current signal constant.
The APC circuit 13 also adjusts the bias current IB based on the reference temperature TLC sent from the reference generator 19. That is, the APC circuit 13 determines the bias current IB by accessing the memory 35 in which the relation between the bias current IB and the reference temperature TLC of the LD is stored. In this case, the data stored in the memory 35 may be a set of coefficients of a function that gives a relation between the bias current IB and the reference temperature TLC, or may have a configuration of a look-up-table.
Referring to
Thus, the optical transmitting module 1 monitors the temperature of the LD 5 by the first sensor 7 and maintains the temperature of the LD 5 to be the reference temperature TLC by controlling the driving current supplied to the Peltier element 3 such that the difference between the temperature signal output from the first sensor 7 and the reference temperature becomes equal. According to the present control, the reference temperature TLC for the LD 5 is varied based on the ambient temperature sensed by the second sensor 17, the Peltier element 3 may be driven within its operable range even for the wide range of the ambient temperature. Moreover, the APC circuit 13 adjusts the bias current IB supplied to the LD based on the reference temperature TLC, the extinction ratio for the optical signal may be kept constant even the temperature of the LD changes.
Next, the present optical transmitter will be compared with a conventional one.
As shown by the broken line in
As shown in
Moreover, as shown in the sold line in
Although a preferred embodiment of this invention has been described herein, various modifications and variations will be apparent to those skilled in the art without departing from the spirit or scope of the invention. For example, although the reference generator 19 sets the reference temperature so as to linearly depend on the ambient temperature T(amb), various relations with the nonlinear function such as quadratic and logarithmic relations may be applied as long as the range of the reference temperature TLC becomes smaller than that of the ambient temperature T(amb). Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.
Number | Date | Country | Kind |
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2005-013246 | Jan 2005 | JP | national |