JP3940724B2 - Wavelength stabilizer - Google Patents

Description

  The present invention relates to a wavelength stabilization device that stabilizes the wavelength of a laser beam with respect to a semiconductor laser mainly for the purpose of application to wavelength division multiplexing transmission.

  A semiconductor laser is generally used as a light source for wavelength multiplexing transmission. However, this semiconductor laser has relatively poor temperature stability, and the wavelength of the emitted light tends to shift due to environmental changes. Since such a wavelength shift causes interference with adjacent wavelengths, a wavelength stabilizing device that stabilizes the wavelength of laser light with respect to a semiconductor laser is conventionally used in a wavelength multiplexing transmission system.

  As an example of this wavelength stabilizing device, Patent Document 1 discloses a device that uses a quartz etalon resonator. In this type of wavelength stabilizing device, a layer of a light reflection film is formed on the Z-cut surface of a crystal etalon, an electrode is provided on the X-cut surface, and a dither signal is applied, and the crystal etalon is resonantly oscillated by this dither signal. In this state, the laser light is transmitted through the quartz etalon, and the laser light optically modulated by the resonance vibration of the etalon is received by the photodetector and converted into an electrical signal. An error signal is generated by synchronously detecting the detected electrical signal with a dither signal. By controlling the injection current or temperature of the semiconductor laser based on this error signal, the wavelength of light generated by the semiconductor laser is stabilized at the extreme value of the light transmittance of the quartz etalon.

  By the way, in the conventional wavelength stabilizing device, it is necessary to match the frequency of the dither signal with the mechanical resonance frequency of the crystal etalon. When the Q value of the mechanical resonance of the crystal etalon becomes as high as several thousand, the frequency accuracy to be combined needs to be several tens ppm, and the adjustment becomes extremely difficult. In addition, since the crystal etalon gives priority to the optical characteristics, it cannot be cut out with a cut in an orientation in which the mechanical resonance frequency is stable. For this reason, the frequency shifts due to a temperature change or the like, and in the worst case, the wavelength may not be controlled.

Patent Document 1 describes a configuration in which a pair of electrodes of a crystal etalon constitute a feedback loop of a feedback amplifier and oscillate. However, this Patent Document 1 does not describe the conditions of the crystal etalon and the amplifier. For this reason, trial and error are unavoidable in the implementation, and adjustment work is expected to be difficult.
Japanese Patent Laid-Open No. 03-072686.

  As described above, the conventional wavelength stabilization device needs to adjust the frequency of the dither signal to the mechanical resonance frequency of the crystal etalon, but it is extremely difficult to adjust, and in the worst case, wavelength control is impossible. There is a risk of falling. In addition, it is difficult to follow the resonance frequency shift due to temperature change or secular change, and stable operation is lacking.

  The present invention has been made in view of the above problems, and can automatically obtain a dither signal that matches the mechanical resonance frequency of the crystal etalon, thereby making the frequency of the dither signal unadjusted, and temperature changes and secular changes. It is an object of the present invention to provide a wavelength stabilization device that can prevent the resonance frequency from being shifted due to the above.

  In order to achieve the above object, the present invention provides a wavelength stabilizing device for stabilizing the oscillation wavelength of laser light emitted from a semiconductor laser, and a pair of light beams on a Z-cut surface and an X-cut surface of the quartz bulk that face each other. A crystal etalon formed with a reflective film layer and a pair of electrodes, a photodetector that receives laser light transmitted through the crystal etalon and converts it into an electrical signal, and a pair of electrodes of the crystal etalon are respectively connected to an amplifier. A dither signal generation that generates a dither signal having a frequency that matches the mechanical resonance frequency of the crystal etalon by incorporating a terminal of a pair of electrodes of the crystal etalon into the feedback loop by forming a feedback loop connected to the output end Means, an error signal generating means for generating an error signal by comparing an electric signal output from the photodetector with the dither signal, and based on the error signal Control means for controlling the oscillation wavelength of the semiconductor laser, wherein the product of the input impedance and gain of the amplifier is higher than the impedance between the pair of electrodes at the mechanical resonance frequency of the crystal etalon. .

  With the above configuration, since a dither signal that matches the mechanical resonance frequency of the crystal etalon is automatically obtained, the frequency of the dither signal can be made unadjusted. Further, since the resonance frequency does not shift due to temperature change and normal change, the reliability is remarkably improved.

  According to the present invention, it is possible to automatically obtain a dither signal that coincides with the mechanical resonance frequency of the crystal etalon, thereby making the frequency of the dither signal unadjusted, and causing a shift in the resonance frequency due to temperature change or secular change. It is possible to provide a wavelength stabilizing device that can be eliminated.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a perspective view schematically showing a wavelength stabilizing device according to an embodiment of the present invention. In FIG. 1, 11 is a semiconductor laser (LD). The semiconductor laser 11 oscillates by an injection current and emits signal light having a predetermined wavelength toward the front and rear. The signal light emitted from the front of the semiconductor laser 11 is incident on the optical fiber transmission line 10 by an optical system (not shown), and the signal emitted from the rear is incident on the wavelength stabilizing device of this embodiment.

  In the wavelength stabilizing device, signal light (hereinafter referred to as laser light) from the semiconductor laser 11 is converted into parallel light by a collimator 12 disposed on the optical axis, passes through a crystal etalon 13, and is detected by a photodetector (PD). ) 14 and converted into an electrical signal.

  The quartz etalon 13 cuts the quartz bulk into a rectangular parallelepiped shape, forms light reflecting film layers Z1 and Z2 on a pair of Z cut surfaces (surfaces perpendicular to the optical axis), and a pair of X cut surfaces (parallel to the optical axis). The electrode layers X1 and X2 are formed on the surface. The electrode layers X1 and X2 are connected by lead wires 15 and 16 to the input / output terminal of the amplifier 17 having a high input impedance, thereby forming a feedback loop. The amplifier 17 is an inverting amplifier, and capacitors 18 and 19 and a resistor 20 are connected in the feedback loop to form a Colpitts oscillation circuit as a whole. A resistor 21 connected between the input and output terminals of the amplifier 17 is a DC feedback resistor and functions to stabilize the DC component of the output of the amplifier 17. In general, the resistor 21 is a high resistance of 1 MΩ or higher.

  The feedback loop circuit configured as described above oscillates at a frequency that matches the mechanical resonance frequency of the crystal etalon 13. This oscillation signal can be used as a dither signal, and is naturally applied between the electrodes X1 and X2 of the crystal etalon. For this reason, the crystal etalon 13 vibrates at the frequency of the dither signal applied between the electrode layers X1 and X2, that is, the mechanical resonance frequency of the etalon. The laser light incident on the quartz etalon 13 is subjected to light intensity modulation by the resonance vibration of the etalon 13 and is transmitted.

  The photodetector 14 receives the laser light that has undergone light intensity modulation by the quartz etalon 13 and generates an electrical signal S1 corresponding to the light intensity change. This electrical signal (hereinafter, photodetection signal) S1 is supplied to the synchronous detector 22 together with the dither signal S2 output from the amplifier 17. The synchronous detector 22 performs synchronous detection of the light detection signal S1 and the dither signal S2, and generates a phase error signal S3. The phase error signal S3 is sent to the semiconductor laser 11 and used for controlling the injection current or temperature or the wavelength variable electrode. Thereby, the wavelength of the light of the semiconductor laser 11 can be stabilized at the extreme value of the light transmittance of the quartz etalon 13.

  The operation of the wavelength stabilizing device having the above configuration will be described below with reference to FIG. FIG. 2 is a characteristic diagram showing light transmission characteristics and synchronous detection output characteristics of the apparatus shown in FIG. In FIG. 2, a waveform A represents a state in which the light transmittance of the crystal etalon 13 is changed by a dither signal.

  When the dither signal is applied to the electrode layers X1 and X2 on the X-cut surface, the quartz etalon 13 performs expansion / contraction movement due to the piezoelectric effect. Therefore, the light transmission characteristic is also a small amount due to the dither signal, but moves left and right. As a result, the light intensity of the laser light transmitted through the quartz etalon 13 also changes. The sign of the slope of the light transmission characteristic differs between the wavelength region W1 from the minimum value Ab to the maximum value Ap of the light transmission property and the wavelength region W2 from the maximum value Ap to the minimum value Ab. For this reason, the phase of the transmitted light intensity modulated by the quartz etalon 13 differs by 180 degrees in the wavelength regions W1 and W2.

  Therefore, synchronous detection is performed using a signal obtained by photoelectrically converting the light transmitted through the crystal etalon 13 and a dither signal, and a signal of waveform B is obtained as an error signal. The error signal B is used for wavelength control, the injection current or temperature of the semiconductor laser 11 is controlled, and the oscillation wavelength of the semiconductor laser 11 is drawn into the zero cross points B1 and B2 of the error signal B. The wavelengths B1 and B2 are the maximum value Ap and the minimum value Ab of the light transmittance of the quartz etalon 13, respectively. Therefore, by changing the polarity of the feedback control, the wavelength of the laser light can be selectively stabilized at a wavelength that gives the maximum value and the minimum value of the light transmittance in the quartz etalon 13. Since the method using the dither signal is not affected by the DC drift of the amplifier or the dark current change of the photodetector, the wavelength can be stabilized with high accuracy.

  In the present invention, the basic operation principle is the same as in the prior art, but the method of generating a dither signal is greatly different. FIG. 3 shows an electrical equivalent circuit when the crystal etalon 13 is regarded as a crystal resonator. In FIG. 3, the coil L1 and the capacitor C1 connected in series form a series resonance circuit. R1 connected in series to this is a resistance component, and C2 connected in parallel represents the capacitance between the electrodes of the crystal etalon 3. The impedance at the resonance frequency of the series resonance circuit is substantially equal to R1.

  In a normal crystal resonator, the resistance component is about several tens of ohms to several hundreds of ohms. On the other hand, since the crystal etalon 13 allows the light beam to pass between the electrodes, the distance between the electrodes is remarkably large. For this reason, the resistance component is also as large as several tens kΩ to several hundreds kΩ. However, a mechanical resonance Q value of several thousand is obtained by using the midpoint holding type described below, and oscillation is possible if the conditions are met. For example, in our prototype, a resonance frequency of 892 kHz was obtained when C2 = 0.6 pF, C1 = 0.015 pF, L1 = 21 H, and R1 = 62 kΩ.

  The most important condition is that the product of the input impedance and the gain G of the amplifier 17 has a higher value than the resistance component R1. In an amplifier with a low impedance input, the feedback amount becomes small and oscillation is impossible. FIG. 4 shows a feedback amplifier circuit in which the resistors 20 and 21 in FIG. 1 are ignored and only the resistance component R1 due to the crystal etalon 13 is considered.

If the output impedance of the amplifier 17 is small, the capacitor 19 can also be ignored, so that the expression (1) is obtained from the amplitude condition of the oscillation circuit.
Z (G-1)> R (1)
Here, Z represents a parallel connection value of the input impedances of the capacitor 18 and the amplifier 17. As G >> 1,
ZG> R (2)
Is obtained. Since the capacitors 18 and 19 need to have a relatively small capacity, Z is approximately equal to the input impedance of the amplifier 17. Therefore, the product of the input impedance and the gain of the amplifier 17 needs to be higher than the impedance at the mechanical resonance frequency of the crystal etalon 13.

  Strictly speaking, the Colpitts oscillation circuit oscillates at a frequency slightly higher than the series resonance frequency of L1 and C1. Further, since the capacitors 18, 19 and the resistor 20 are ignored, the expression (2) needs to be satisfied with a margin.

  As the amplifier 17 that sufficiently satisfies the expression (2), an operational amplifier using an FET (field effect transistor) in the input stage can be used. The operational amplifier of FET input has a feature that the input impedance is higher than that of a normal operational amplifier. In addition, since the gain of the operational amplifier is high, it is suitable as an amplifier that satisfies the equation (2).

  Depending on the type of amplifier, the phase difference between the input and output may deviate from 180 °. Such an amplifier may not oscillate because the phase conditions are not met. In such a case, a phase adjustment portion may be further provided in the feedback loop. FIG. 5 shows the configuration of the embodiment in such a case. In FIG. 5, the same parts as those in FIG. In FIG. 5, a portion surrounded by a broken line is a phase adjusting unit 23, which is constituted by a resistor 24 and a capacitor 25. Although the phase adjustment unit 23 can delay the phase by a maximum of 90 degrees, it is desirable to design the phase to about 60 degrees or less in consideration of the amplitude reduction.

  Although FIG. 5 shows an example in which the resistor 24 and the capacitor 25 are configured, 24 may be a coil and 25 may be a resistor. If it is better to advance the phase, 25 may be a resistor and 24 may be a capacitor. Alternatively, the resistor 20 and the capacitor 19 of FIG. 1 may be removed, and the high impedance amplifier 17 may be an amplifier having the same input / output phase.

  By the way, in the said embodiment, unless the Q of the mechanical resonance of the crystal etalon 13 is large to some extent, it does not oscillate. In the conventional structure in which the crystal etalon is fixed to the base, since the Q value is about several tens and cannot be taken large, oscillation is extremely difficult. Therefore, when a structure is adopted in which a conductive shaft (wire) is used for the lead wires 15 and 16 connected to the central portions of the electrodes X1 and X2 shown in FIG. 1, and the crystal etalon 13 is held by this shaft, Q The value dramatically increases to several thousand. The reason for this is that in the lowest order mode that expands and contracts in the vertical direction, the central portion of the etalon is a node and both ends are antinodes, and the holding structure supports the node, so the mechanical loss is reduced.

  In the above embodiment, when the phase difference between the two input signals S1 and S2 of the synchronous detector 22 is approximately 90 °, the synchronous detection output is small. In such a case, it is preferable to provide a phase adjustment unit in one of the two inputs S1 and S2 of the synchronous detector 22. Such an example is shown in FIG. Also in FIG. 6, the same parts as those in FIG. In FIG. 6, the phase adjustment unit 26 is inserted on the input side of the dither signal S2 of the synchronous detector 22, but may be inserted on the input side of the light detection signal S1. The phase adjustment unit 26 includes a capacitor 27 and a resistor 28, and can shift the phase by 90 ° at the maximum. Since the purpose of this phase adjustment unit 26 is to shift the phase difference between S1 and S2 from 90 °, the resistor 28 and the capacitor 27 may be interchanged. Moreover, you may comprise by a coil and resistance, and you may comprise by a coil and a capacitor | condenser. If constituted by a coil and a capacitor, a phase difference of 90 ° or more can be obtained.

  As described above, the Colpitts type oscillation circuit has been described as the oscillation circuit constituting the feedback loop circuit, but other types of oscillation circuits such as a Hartley type and a Pierce type can also be applied.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The block diagram which shows schematically the structure of the wavelength stabilization apparatus which concerns on one Embodiment of this invention. In the said embodiment, the characteristic view which shows the optical transmission characteristic and synchronous detection output characteristic of a wavelength stabilizer. The circuit diagram shown in the electrical equivalent circuit at the time of considering a crystal etalon as a crystal oscillator in the feedback loop circuit of the said embodiment. The circuit diagram which shows an example of the feedback amplifier circuit which considered only the resistance component by a crystal etalon in the feedback loop circuit of the said embodiment. In the said embodiment, the block diagram for demonstrating the adjustment method when the phase difference between the input-output of the amplifier used for a feedback loop circuit has shifted | deviated from 180 degrees. In the said embodiment, the block diagram for demonstrating the adjustment method when a synchronous detection output becomes small when the phase difference of two input signals of a synchronous detector is about 90 degrees.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Optical fiber transmission path, 11 ... Semiconductor laser (LD), 12 ... Collimator, 13 ... Quartz etalon, 14 ... Photodetector (PD), 15, 16 ... Lead wire, 17 ... Amplifier, 18, 19 ... Capacitor, 20, 21 ... resistors, 22 ... synchronous detectors, 23, 26 ... phase adjusters.

Claims (4)

In a wavelength stabilization device that stabilizes the oscillation wavelength of laser light emitted from a semiconductor laser,
A crystal etalon in which a light reflecting film layer is formed on each of a pair of Z-cut surfaces facing each other in a quartz crystal bulk, and an electrode is formed on each of a pair of X-cut surfaces ;
A photodetector that receives part of the laser light emitted from the semiconductor laser that has passed through the pair of Z-cut surfaces of the quartz etalon and converts them into an electrical signal;
The electrodes formed on the pair of X-cut surfaces of the crystal etalon are connected to the input and output terminals of the amplifier, respectively , and an oscillation circuit is formed by a feedback loop using the crystal etalon as a vibrator. dither signal generating means for outputting a frequency signal as the dither signal to match the mechanical resonance frequency of the quartz etalon that,
An error signal generating means for generating an error signal by comparing an electric signal output from the photodetector with the dither signal;
Control means for controlling the oscillation wavelength of the semiconductor laser based on the error signal,
The amplifier is higher than the impedance between the electrodes the product of the input impedance and gain are respectively formed on the pair of X-cut surface in the mechanical resonance frequency of the quartz etalon, the dither signal to the input stage or output stage phase A wavelength stabilizing device comprising a phase adjusting unit for adjusting the wavelength.   2. The wavelength stabilizing device according to claim 1, further comprising a phase adjusting unit in at least one of a signal path from the photodetector and a signal path from the amplifier.   2. The wavelength stabilizing device according to claim 1, wherein the amplifier is an operational amplifier using a field effect transistor as an input stage. 2. The wavelength stabilizing device according to claim 1, wherein shafts are respectively attached to central portions of the electrodes formed on the pair of X-cut surfaces, and the crystal etalon is held by the shafts.

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