JPH07249817A - Wavelength-stabilized light source - Google Patents

Abstract

PURPOSE:To obtain a wavelength-stabilized light source which is low-cost and compact by means of a comparatively low-cost light-emitting element and by means of a simple control constitution by installing a means by which wavelength light extracted by an optical filter is detected and by which a heating/ cooling element is driven so as to make its optical power maximum. CONSTITUTION:A light-emitting-element drive means 6 is driven in such a way that the output optical power of a light-emitting element whose oscillation wavelength is changed with reference to a temperature change becomes definite, and single-wavelength light is output. A heating/cooling-element, drive means 7 is driven in such a way that wavelength light extracted by an optical filter 22 is detected so as to make its optical power maximum. Then, the temperature of the light-emitting element is increased or decreased in such a way that the transmitted-light power of the optical filter 22 becomes maximum, and the oscillation wavelength of the light-emitting element is stabilized to the central wavelength of the optical filter 22. In this case, a comparatively low-cost and ordinary DFB-LD can be used, and the constitution of a light-emitting-element drive part is as it is. As a result, a device as a whole can be made relatively compact.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source used in, for example, an optical wavelength demultiplexing system, and a technique for stabilizing an output light wavelength of a light source so that interference does not occur between a plurality of multiplexed optical wavelengths. Regarding

[0002]

As is well known, optical wavelength demultiplexing (WD).
M: Wavelength Division multiplexing system
On the transmitting side, different wavelengths (λ1
, Λ2, ..., λn) is generated, each wavelength light is modulated by an individual information signal, and then multiplexed by a star coupler or the like to obtain n-wavelength multiplexed light, which is sent to the optical fiber transmission line. .
On the other hand, on the receiving side, the n-wavelength multiplexed light transmitted through the optical fiber transmission line is received, and the received light is passed through an optical filter that transmits only the specific wavelength λk to extract the wavelength light λk containing the necessary information signal. Then, this is demodulated to obtain a desired information signal.

The WDM system is characterized in that the transmission capacity of the optical fiber can be increased and that each wavelength light can be handled independently, so that a flexible system can be constructed. However, it is necessary to stabilize the oscillation wavelength of each light source so that interference does not occur due to the wavelengths of the optical signals approaching or overlapping. In addition, the light-emitting element used for each light source has a narrow wavelength spectrum width and oscillates in a single mode so that the wavelength spacing can be narrowed and the number of wavelengths multiplexed can be increased. Laser (DFB-LD: Distribu
ted Feedback-Laser Diode) is recommended.

The structure of a conventional light source used in the WDM system is shown in FIG. In FIG. 5, 1 is DFB
-LD (light emitting element), 2 is a photodiode (light receiving element), 3 is a Peltier element (heating and cooling element), 4 is a temperature sensor, and these are hermetically sealed in the housing 5.

The LD drive section 6 drives the LD1 to oscillate, has an APC (auto power control) function, and when activated by a light emission command signal, outputs one of the output lights of the LD1 through the photodiode 2. The LD drive signal is increased or decreased so that the output power becomes constant.

The heating / cooling drive unit 7 drives the Peltier element 3 in accordance with the heating / cooling control signal from the temperature control unit 8 to change the temperature of the LD 1. The temperature of the LD 1 is monitored by the temperature sensor 4, and its temperature information is obtained by the temperature control unit 8
Sent to. The temperature control unit 8 generates a heating / cooling control signal so that the temperature of the LD 1 becomes the set temperature.

That is, generally, the light emitting element has a temperature / wavelength characteristic that the oscillation wavelength becomes longer as the temperature rises, but the case of the DFB-LD1 is no exception. Therefore, the temperature of the LD1 is monitored and compared with a set temperature, and the temperature of the LD1 is maintained constant by heating / cooling the LD1 so as to correct the temperature difference, thereby stabilizing the oscillation wavelength of the LD1. I am trying to

However, in the above stabilization method, even if the temperature of the light emitting element is maintained constant, the light emitting element is an active element, so that the temperature / wavelength characteristic changes over time and the oscillation wavelength changes. There is a problem of doing.

As a method for solving this problem, a configuration as shown in FIG. 6 has been conventionally considered. In FIG. 6, the same parts as those in FIG. 5 are designated by the same reference numerals and the description thereof will be omitted.

In this light source, a three-electrode laser whose emission power, oscillation wavelength and phase can be controlled by three currents is used for the LD 11, and the emission power is controlled to be constant by the APC function in the LD drive section 12. Then, the output light of the LD 11 is partly branched by the optical branching device 13, only the set wavelength is extracted by the optical filter 14, and the light is received by the photodiode 15. Further, the wavelength control unit 16 causes the LD 11 to pass through the LD drive unit 12 so that the light receiving level becomes maximum.
The wavelength of is controlled.

That is, the light source having the above-mentioned structure detects the transmitted light of the wavelength-set optical filter 14 and controls the currents of the three electrodes so that the optical power becomes maximum, and the wavelength of the output light is changed to the optical wavelength. It is designed to stabilize at the wavelength of the filter. Since the optical filter is a passive element, its wavelength stability can be made much better than that of the light emitting element alone by hermetically sealing and temperature stabilization.

Therefore, according to the above method, even if the temperature is constant, the fluctuation of the oscillation wavelength generated in the light emitting element can be stabilized by the wavelength monitoring by the stable optical filter. However, this method requires an expensive three-electrode laser, resulting in a significant cost increase. In addition, there is a problem that the control of three electrodes such as the LD drive section becomes complicated and the entire apparatus becomes large.

[0013]

As described above, in the conventional wavelength-stabilized light source, since the light emitting element is an active element, the oscillation wavelength fluctuates with time even if the temperature is constant. Although a method of preventing this fluctuation has been considered, there is a problem that an expensive three-electrode laser and a complicated control structure are required, resulting in an increase in cost and an increase in size.

The present invention has been made to solve the above problems, and an object thereof is to provide a low-cost, compact wavelength-stabilized light source with a relatively inexpensive light emitting element and a simple control structure.

[0015]

In order to achieve the above object, a wavelength-stabilized light source according to the present invention comprises a light-emitting element whose oscillation wavelength fluctuates in response to temperature changes, and a constant output light power of this light-emitting element. Light emitting element driving means for driving the light emitting element to output a single wavelength light, a heating / cooling element for controlling the temperature of the light emitting element, and an optical branching device for extracting a part of the output light of the light emitting element And an optical filter for extracting a specific wavelength light from the light extracted by the optical branching device, and detecting the wavelength light extracted by the optical filter and driving the heating / cooling element so that the optical power becomes maximum. The heating / cooling element driving means is provided.

[0016]

In the wavelength-stabilized light source having the above-mentioned structure, it is noted that the temperature stability of the optical filter, which is a passive element, can be made much better than that of the light-emitting element alone by hermetically sealing and temperature stabilization. The element driving means stabilizes the oscillation wavelength of the light emitting element at the center wavelength of the optical filter by increasing or decreasing the temperature of the light emitting element so that the transmitted light power of the optical filter becomes maximum. In this case, a relatively inexpensive normal DFB-LD can be used, and the configuration of the light emitting element drive unit is unchanged, and it can be realized by adding a few optical components and signal processing circuits, so that the entire device is relatively compact. Can fit in.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described in detail below with reference to FIGS. However, in FIG. 1, the same parts as those in FIG. 5 are designated by the same reference numerals, and different parts will be mainly described here.

FIG. 1 shows the structure of a wavelength-stabilized light source according to the present invention, in which a part of the output light of the LD 1 is taken out by an optical branching device 21 and an optical filter 2 is maintained at a constant temperature.
Only the light of the set wavelength is extracted through the step 2. This wavelength light is photoelectrically converted by the photodiode 23, and the converted output is sent to the wavelength controller 24.

The wavelength controller 24 is specifically constructed as shown in FIG. In FIG. 2, reference numeral 31 is a coupling capacitor, which cuts the DC component from the output of the photodiode 23 and extracts only the AC component. The light receiving signal limited to this AC component is converted into a rectangular wave by the waveform shaper 32, and the AND circuit 33 capable of switching the normal / inverted input.
Inputs are forward input and reverse input.

On the other hand, 34 is a low-frequency signal source, and the low-frequency signal generated here is converted into a rectangular wave by a waveform shaper 35 and then supplied to the control input terminal of the AND circuit 33 as an input switching signal. It Further, the delay circuit 36 delays the time t, the waveform shaper 37 converts the rectangular wave into a rectangular wave, and the AND circuit 33 inputs the normal input and the inverted input.

The AND circuit 33 calculates a logical product of both non-inverted inputs when the switching signal has a positive polarity, and calculates a logical product of both inverted inputs when the switching signal has a negative polarity. The output of the AND circuit 33 is integrated by the integrating circuit 38, then amplified by the amplifier 39, sent to the adder 40, and added with the low-frequency signal from the signal source 34 to generate the second heating / cooling control signal. Is sent to the heating / cooling drive unit 7.

In this case, the heating / cooling driving unit 7 has the first heating / cooling control signal from the temperature control unit 8 and the wavelength control unit 24.
The Peltier element 3 is driven in response to the combined signal with the second heating / cooling control signal from.

The operation of the above configuration will be described below with reference to FIGS. 3 and 4. The output power of the LD 1 is controlled to be constant by the APC function of the LD drive unit 6, and the set temperature is maintained constant by the temperature control unit 8. Here, the low-frequency signal output from the signal source 32 of the wavelength control unit 24 is sent to the heating / cooling driving unit 7 as a heating / cooling control signal and is superimposed on the driving signal of the Peltier element 3. As a result, the temperature of the LD 1 slightly changes according to the low-frequency signal, and the oscillation wavelength also minutely changes accordingly.

The output light of the LD 1 is partially input to the optical filter 22 by the optical branching device 21. As the optical filter 22, for example, a Fabry-Perot resonator or a multilayer filter is used. FIG. 3A shows the wavelength transmission characteristics of the optical filter 22, and FIGS. 3B and 3C show the input / output relationship thereof.

That is, as shown in FIG. 3A, the wavelength transmission characteristic of the optical filter 22 has a central wavelength (setting wavelength) λ.
At 0, the transmittance becomes maximum, and as it deviates from this, it decays exponentially.

Now, assuming that the LD1 outputs a wavelength light equal to the center wavelength λ0 of the optical filter 22 and accompanied by a low frequency minute fluctuation, the optical filter 22 receives L0 in FIG. 3 (b).
, The output light is L in FIG. 3 (c).
As in 0 ', the optical power becomes maximum and the low frequency component is doubled.

When the wavelength of the output light of the LD1 becomes longer than the central wavelength λ0 of the optical filter 22 from this state, the output light is input to the optical filter 22 as shown by L1 in FIG. (C) It becomes like L1 'in the input light L
The optical power is lower than that of 1, and the low frequency components have the same frequency but opposite phase.

Further, when the wavelength becomes shorter than the center wavelength λ0 of the optical filter 22, it is input to the optical filter 22 as shown by L2 in FIG. 3 (b), so that its output light is L2 'in FIG. 3 (c).
The optical power is lower than that of the input light L2, and the low frequency components have the same frequency and the same phase.

The light transmitted through the optical filter 22 is photoelectrically converted by the photodiode 23 and sent to the wavelength controller 24. The wavelength controller 24 utilizes the input / output characteristics of the optical filter. FIG. 3 shows the waveform of each part of the configuration shown in FIG. 2 to explain the operation.

In FIG. 4, (a) shows a low frequency signal output from the signal source 34. Now, assuming that the output wavelength of the LD1 matches the center wavelength λ 0 of the optical filter 22, the AC output of the coupling capacitor 31 has a frequency twice that of the original low frequency signal, as shown in FIG. 4 (b). Becomes This AC signal is processed by the waveform shaper 32 in FIG.
As shown in (f), the AND circuit 33 converts the rectangular wave.
Inputs are forward input and reverse input.

On the other hand, the low-frequency signal output from the signal source 34 is delayed by the delay circuit 36 for t time, and then the waveform shaper 3
It is converted into a rectangular wave by 7 and, as a result, FIG.
A reference signal as shown in is generated. This reference signal is A
The normal input and the inverted input are input to the ND circuit 33.

The AND circuit 33 is supplied with a normal / inverted input switching signal (not shown) obtained by converting the low-frequency signal into a rectangular wave by the waveform shaper 35. When this switching signal has a positive polarity. , AND operation of both non-inverted inputs, and when negative polarity, AND operation of both inversion inputs. Therefore, the AND circuit 33
4 (i), the polarity is inverted every half cycle π of the low-frequency signal, and the output of the integration circuit 38 becomes 0 level. Therefore, the second heating / cooling control signal is only a low frequency signal, the LD1 is maintained at the set temperature, and continues to oscillate and output at the wavelength λ0.

Next, assuming that the output wavelength becomes longer than the center wavelength λ 0 of the optical filter 22 even though the temperature of the LD 1 is maintained at the set temperature, the AC output of the coupling capacitor 31 becomes as shown in FIG. As shown in c), the original low-frequency signal has the same frequency and opposite phase. This AC signal is converted by the waveform shaper 32 into a rectangular wave as shown in FIG.

AN in which this rectangular wave signal is input in the normal / reverse direction
The output of the D circuit 33 has a large proportion in the negative polarity period as shown in FIG. Therefore, the output of the integrating circuit 38 has a negative polarity level, and the second heating / cooling control signal has a negative polarity, which serves to lower the temperature of the LD1. This allows
The oscillation wavelength of LD1 approaches λ0.

On the contrary, if the output wavelength becomes shorter than the central wavelength λ 0 of the optical filter 22 even though the temperature of the LD 1 is maintained at the set temperature, the AC output of the coupling capacitor 31 becomes as shown in FIG. As shown in d), the original low frequency signal has the same frequency and the same phase. This AC signal is converted by the waveform shaper 32 into a rectangular wave as shown in FIG.

AN in which this rectangular wave signal is forward / reversely input
The output of the D circuit 33 has a large proportion of the positive polarity period as shown in FIG. Therefore, the output of the integrating circuit 38 has a positive polarity level, and the second heating / cooling control signal has a positive polarity and acts to raise the temperature of the LD1. This allows
The oscillation wavelength of LD1 approaches λ0.

That is, the wavelength control section 24 increases or decreases the temperature of the LD1 so that the transmitted light power of the optical filter 22 becomes maximum, so that the oscillation wavelength of the LD1 is adjusted by the optical filter 2.
It stabilizes at the center wavelength λ 0 of 2. Here, since the optical filter 22 is a passive element, its wavelength stability can be made much better than that of a single LD by airtight sealing and temperature stabilization.

Therefore, according to the above configuration, the normal D
The problem that the oscillation wavelength fluctuates using the FB-LD is monitored by controlling the temperature of the LD1 by monitoring the wavelength with the stable optical filter 22 which is a passive element.
The oscillation wavelength of can be stabilized.

In this case, a low cost ordinary DFB-L
D can be used, and the configuration of the LD drive unit is the same,
Since it can be realized by adding a few optical components and a signal processing circuit, the entire apparatus can be made compact as compared with the conventional configuration shown in FIG. Of course, by replacing the optical filter 22 with one having an arbitrary center wavelength,
It can be stabilized at other wavelengths.

As a result of the above, it is possible to provide a low-cost and compact wavelength-stabilized light source. Therefore, if it is used for each wavelength light source of a WDM system, it is possible to prevent the wavelengths of the respective optical signals from approaching each other or overlapping, and to prevent interference. Can be prevented and can be realized at low cost. It is needless to say that the present invention is not limited to the above-described embodiments and can be similarly implemented even if various modifications are made without departing from the gist of the present invention.

[0041]

As described above, according to the present invention, it is possible to provide a low-cost, compact wavelength-stabilized light source with a relatively inexpensive light emitting element and a simple control configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a wavelength stabilized light source according to the present invention.

FIG. 2 is a block circuit diagram showing a specific configuration of a wavelength control unit of the embodiment.

FIG. 3 is a diagram showing a wavelength transmission characteristic of the optical filter of the embodiment and its input / output relationship.

FIG. 4 is a waveform diagram showing waveforms of various portions for explaining the operation of the wavelength control unit.

FIG. 5 is a block configuration diagram showing a basic configuration of a conventional wavelength stabilized light source.

FIG. 6 is a block configuration diagram showing another configuration example as a conventional wavelength stabilized light source.

[Explanation of symbols]

1 ... DFB-LD, 2 ... Photodiode, 3 ... Peltier element, 4 ... Temperature sensor, 5 ... Housing, 6 ... LD drive part,
Reference numeral 7 ... Heating / cooling drive unit, 8 ... Temperature control unit, 11 ... Three-electrode LD, 12 ... Three-electrode LD drive unit, 13 ... Optical splitter, 14
... optical filter, 15 ... photodiode, 16 ... wavelength control section, 21 ... optical branching device, 22 ... optical filter, 23 ... photodiode, 24 ... wavelength control section, 31 ... coupling capacitor, 32,35,37 ... waveform Molding machine, 33 ... A
ND circuit, 34 ... Low frequency signal source, 36 ... Delay circuit, 38
... integrator circuit, 39 ... amplifier, 40 ... adder.

Claims (5)

[Claims] 1. A light emitting element whose oscillation wavelength varies with temperature change, and light emitting element driving means for driving the light emitting element so that the output light power is constant and outputting a single wavelength light to the light emitting element. A heating / cooling element for controlling the temperature of the light emitting element, an optical branching device for extracting a part of the output light of the light emitting element, and an optical filter for extracting a specific wavelength light from the light extracted by the optical branching element, A wavelength stabilizing light source comprising: a heating / cooling element driving means for detecting the wavelength light extracted by the optical filter and driving the heating / cooling element so that the optical power becomes maximum. 2. The wavelength stabilized light source according to claim 1, wherein the light emitting element is a distributed feedback semiconductor laser. 3. The wavelength stabilized light source according to claim 1, wherein the heating / cooling element is a Peltier element. 4. The wavelength stabilized light source according to claim 1, further comprising temperature control means for controlling the temperature of the optical filter to be constant. 5. The heating / cooling element driving means applies a minute low-frequency signal to the heating / cooling element, and detects a low-frequency component appearing in a detection output of wavelength light extracted by the optical filter with reference to an original low-frequency signal. 2. The wavelength stabilized light source according to claim 1, wherein changes in phase and frequency are detected, and the heating / cooling element is driven based on the detection results.