JP3974255B2 - Frequency tunable laser light source device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser light source device used in the fields of optical communication and optical measurement, and more particularly to a frequency variable laser light source device that has a variable frequency over a wide band and is excellent in frequency accuracy and frequency stability of output light.
[0002]
[Prior art]
Regarding elemental technologies for constructing a laser light source device with excellent frequency accuracy and frequency stability of output light, (1) frequency reference light source, (2) wavelength variable light source, (3) frequency tracking control, (4) broadband Various techniques for generating sidebands are known.
[0003]
First, each of these will be briefly described.
(1) As a frequency reference light source, a semiconductor laser (hereinafter referred to as LD: laser diode) or a solid laser whose oscillation frequency is stabilized using an atomic or molecular absorption line as a frequency reference, a gas laser, etc. are generally used. Is.
(2) As a wavelength variable light source, an external resonant light source combining a wavelength selective element such as an LD or a solid state laser and a diffraction grating, a DFB-LD (distributed feedback-laser diode), a DBR-LD (distributed Bragg reflector-laser diode) In general, the one in which the wavelength selection element is incorporated in the LD. Naturally, such a light source alone does not guarantee frequency accuracy or long-term frequency stability.
[0004]
(3) Frequency follow-up control is to control the output light frequency of another laser so as to follow the output light frequency of one laser. In such control, a laser that is a reference for frequency is called a master laser, and a laser that follows the laser is called a slave laser. Since the LD can directly control the oscillation frequency by the injection current, the LD is used as a slave laser. Usually, the output light of the master laser and the slave laser is combined and received to detect the beat signal, and the offset frequency is set. The injection current of the slave laser is controlled so as to coincide with. By this control, the output optical frequency of the slave laser is separated by the set frequency with respect to the output optical frequency of the master laser, so this control is also called optical frequency offset lock control. Since the information obtained from the beat signal is the absolute value of the frequency difference between the two laser beams, a control malfunction occurs if the offset frequency is brought close to zero. Depending on the control method, the output optical frequency of the slave laser relative to the output optical frequency of the master laser, that is, the sign of the offset frequency can be set to either positive or negative. In addition, the upper limit of the offset frequency is 100 GHz at most because of the band of the light receiving system that generates the beat signal. When using a commercially available light receiving element or the like, the band in which light reception and amplification can be easily performed is up to several GHz.
[0005]
{Circle around (4)} Wideband sideband generation includes optical frequency comb generators, short optical pulse light sources, and the like as means.
An optical frequency comb generator (hereinafter referred to as a comb generator) performs deep modulation on input laser light, generates several hundreds or more sidebands at a modulation frequency interval on the frequency axis, and outputs the generated sideband waves. . The modulation frequency is normally set to about several GHz to 20 GHz, and a sideband having a strength that can be used over several THz or more can be obtained. If the frequency of the input light and the modulation frequency are known, the frequency of each sideband is also known, so that each sideband can be used as the frequency reference light.
[0006]
The short optical pulse light source has a saturable absorber, optical intensity modulator, optical phase modulator, etc., which are means for modulating light inside the optical resonator, and an optical amplification medium, and a pulse time width of 10 ps. It repeatedly outputs less than or equal to light. This output light includes a large number of sidebands having optical pulse repetition frequency intervals. In particular, a general short light pulse light source based on forced mode synchronization has a built-in light intensity modulator, and an optical pulse train synchronized with the microwave is generated by driving the light intensity modulator with the microwave. For this reason, the frequency interval of the sideband has high frequency accuracy and high stability of the driving microwave, and this is the same as the case of using the comb generator.
However, even if the frequency interval of the sidebands is kept constant as in forced mode synchronization, the short optical pulse light source does not have an optical frequency reference inside, so the frequency of each sideband is time. Will fluctuate. Therefore, in order to generate a frequency reference light in a wide band using a short optical pulse light source, light having a known frequency may be prepared and the light injection locking phenomenon or frequency tracking control may be used. The light injection locking phenomenon is a phenomenon in which, when light having a frequency close to the oscillation frequency is injected into the self-excited LD from the outside, the oscillation frequency of the LD is drawn into the frequency of the injected light and coincides. Similarly, when light having a frequency close to a low-order sideband included in the output light is injected into the short light pulse light source, the frequency of the sideband is drawn into the frequency of the injected light. Therefore, in the short light pulse light source by forced mode synchronization, if the frequency of the injected light is known, the frequency of each sideband is also known.
As described above, the frequency tracking control is to make the oscillation frequency of other LDs follow the frequency of the light to be referenced. Therefore, reference is made to light with a known frequency even in the short light pulse light source by forced mode synchronization. If the frequency of a certain sideband is used as light, the frequency of each sideband is also known.
[0007]
Thus, when used in combination with the frequency reference light source, the comb generator and the short optical pulse light source can be said to be equivalent sideband generation means.
Since the output light of the comb generator (hereinafter referred to as the comb signal light) and the output light of the short optical pulse light source are used for heterodyne detection with other light, in order to increase the SN ratio of the beat signal, The narrower the spectral line width of light or reference light, the more advantageous. For this reason, an external resonant LD light source or a self-injection locked LD light source is often used as an input light source.
Note that the means for generating sidebands in a wide band here means that the sidebands can be generated at equal intervals on the frequency axis, so the pulse is output with respect to the output of the comb generator or the output of the short optical pulse light source. Also includes those that have been compressed. Since these can essentially be handled in the same manner, the following description describes the case of a comb generator.
[0008]
Next, an existing frequency variable light source excellent in frequency accuracy and frequency stability combining the above elemental technologies will be described.
First, there is a frequency tunable laser light source apparatus shown in FIG. 9 that combines (1) a frequency reference light source, (2) a wavelength variable light source, and (3) frequency tracking control. As shown in FIG. 9, this is a system in which frequency tracking control is performed using the frequency reference light source 11 as a master light source and the variable wavelength light source 31 as a slave light source. The frequency reference light from the frequency reference light source 11 and the output light from the wavelength variable light source 31 are combined by the multiplexer 331 of the frequency tracking control means 33, received by the light receiver 332, and the beat signal is detected. The detected beat signal and the signal from the offset frequency oscillator 334 are phase-compared by the phase comparator 333, and the result is added to the current from the drive current source 337 via the loop filter 335 and added by the adder 336. 31 is supplied to the LD. By controlling in this way, the output light follows the frequency reference light with an offset frequency set by the offset frequency oscillator 334. In this system, the output light frequency of the slave light source can be continuously changed while maintaining high accuracy and high stability. However, the variable frequency range is limited to the frequency tracking control band including the light receiving system, and is at most 100 GHz. Is the limit. Note that 100 GHz corresponds to a wavelength band of less than 1 nm in the 1.5 μm band.
[0009]
In view of this point, the use of a comb generator or the like has been proposed, and its operation has been confirmed experimentally. Then, (4) the optical frequency comb generation technique, which is one of the wideband sideband generation techniques, is added to the above three techniques, that is, the comb obtained by inputting the frequency reference light to the comb generator. Since a large number of sidebands contained in the signal light can be used as frequency reference light, a system for performing frequency tracking control on a wavelength tunable light source based on an arbitrary order of sidebands has been considered. The configuration is shown in FIG. In this system, the frequency of the output light is changed by changing the offset frequency of the frequency tracking control means 33.
[0010]
[Problems to be solved by the invention]
This system can generate light having high accuracy and high stability over a wide band, but cannot generate an arbitrary frequency within a wide frequency range. This is due to the presence of two types of dead zones. One is that the offset frequency cannot be brought close to 0 when performing frequency tracking control. That is, when the frequency of the sideband of a certain order and the wavelength of the wavelength tunable light source that is the slave light source substantially coincide with each other, proper frequency tracking control cannot be performed (FIG. 11A). This is because normal heterodyne detection acquires information on the absolute value of the frequency difference between the two lights, so that both the positive and negative heterodyne frequency components have the same frequency when the laser light frequency difference is about the oscillation line width or less. It is caused by mixing. The other is that when a comb generator or short optical pulse light source is used, a large number of sidebands grow on the frequency axis. The beat signal of the sideband and the beat signal of the higher-order sideband are mixed at substantially the same frequency, and proper frequency tracking control cannot be performed (FIG. 11B). As a guideline for the frequency width of the two types of dead zones (FIG. 11C), if an appropriate margin is expected, the dead zone near the sideband is about 20 MHz, and the deadzone near the center between the sidebands is about several hundred MHz. .
[0011]
The object of the present invention is to solve the above-mentioned problems of the dead zone, (1) excellent in output light frequency accuracy, frequency stability, and setting resolution, (2) wide frequency variable range, and (3) To provide a frequency variable laser light source device capable of arbitrarily setting or sweeping a frequency within a frequency variable range. The standards for high accuracy, high stability, and high resolution in (1) are 100 MHz, 10 −9, and 1 MHz or less, respectively. In addition, the lowest standard for the broadband in the above (2) is 10 nm or more in the 1.5 μm band. In the 1.5 μm band, a frequency width of 1 THz corresponds to a wavelength width of 8 nm. Hereinafter, the notation regarding the wavelength width is a value in the 1.5 μm band.
[0012]
In recent years, with the expansion of the amount of information to be communicated, WDM (Wavelength Division Multiplexing) optical communication technology has been sought, and a light source that simultaneously outputs light of a plurality of predetermined frequencies is desired. In consideration of this requirement, it is desirable to adopt a configuration in which one comb signal light is generated, and a plurality of wavelength tunable light sources can be independently set with reference to an arbitrary sideband included therein. For this purpose, a means for avoiding a dead zone must be realized while the frequency of the comb signal light is fixed.
[0013]
[Means for Solving the Problems]
As described above, the dead zone is due to the fact that in normal heterodyne detection, only the absolute value of the frequency difference between two lights can be detected, and the relative level of the frequency cannot be known. Therefore, in order to solve this problem, it has been conventional to generate a sideband based on the frequency reference light, that is, the frequency reference light, and use the sideband as a reference for the output light of the wavelength tunable light source. Although it is the same, it is not a simple light receiving method as in the past, but a quadrature heterodyne signal is obtained by using two light receivers for the output light and sideband of the wavelength tunable light source, and the frequency including the code that this signal has Based on the difference information, the frequency of the output light of the wavelength tunable light source is controlled. That is,
[0014]
The frequency tunable laser light source device of the present invention includes a frequency reference light source, sideband wave generating means for receiving the frequency reference light output from the frequency reference light source and generating light having a plurality of sideband waves, and the wavelength of the output light is A variable wavelength light source that is variable, and two combined lights that are 90 ° out of phase with each other in response to the output light output from the variable wavelength light source and the light having a plurality of sidebands output from the sideband generation means. An optical 90 ° hybrid coupler to output, a receiver that receives the two combined lights and outputs heterodyne signals that are 90 ° out of phase with each other, and receives the heterodyne signal, and the frequency of the frequency reference light is the sideband A real port that outputs the heterodyne signal when the frequency is higher than the frequency of the sideband, and the heterodyne signal when the frequency of the frequency reference light is lower than the frequency of the sideband. A 90 ° hybrid coupler having an output image port, and receiving the output of one of the output from the real port and the image port of the 90 ° hybrid coupler, the frequency of the heterodyne signal and the plurality of sidebands Comparing means for comparing a frequency of an offset frequency signal corresponding to one sideband determined in relation to a desired wavelength of the output light and outputting a frequency difference signal, and using the frequency difference signal To control the wavelength of the wavelength tunable light source.
[0015]
Since it is decided to take such means, the details will be described in the column of action, in the variable frequency laser light source device of the present invention, the sign of the difference between the frequency of the reference sideband and the output frequency of the variable wavelength light source, That is, it is detected including the relative high and low frequencies, and the dead zone is essentially eliminated. Further, since the frequency tunable laser light source device of the present invention sets the frequency of the wavelength tunable light source with reference to the sideband having a fixed frequency, the frequencies of the plurality of wavelength tunable light sources can be set independently of each other. Is possible.
[0016]
[Action]
The operation of the present invention will be described from the viewpoint of frequency sweep.
If there is a dead zone, the frequency of the wavelength tunable light source, which is a slave light source, cannot be set arbitrarily. Therefore, this frequency cannot be changed in detail in a range that is equal to or greater than the frequency interval of the sidebands. That is, frequency sweep cannot be performed in a state where frequency tracking control is applied over a wide frequency range.
[0017]
First, FIG. 7A shows an example of the time change of the frequency of the output light of each wavelength variable light source and each sideband. When the frequency of the wavelength tunable light source increases with time as shown in FIG. 7A, the frequency of the wavelength tunable light source is rewritten with reference to each sideband as shown in FIG. 7B. The vertical axis in this figure represents the frequency difference including the sign. At this time, the frequency of the beat signal obtained by receiving the light from one wavelength detector after combining the output light of the wavelength variable light source and the sideband is described within a positive frequency range as shown in FIG. be able to. This is because such a simple light receiving method cannot know the level of the frequency of light contributing to the beat signal. FIG. 7C can be regarded as a negative frequency portion of FIG. 7B folded back to a positive frequency. Here, the dead zone appears in the vicinity of the intersection of the lower right line and the upper right line in FIG. That is, in the vicinity of this intersection, frequency components corresponding to two intersecting lines are mixed in the beat signal, so that proper frequency tracking control becomes difficult.
Note that a balanced receiver is known as a light receiver incorporating two light receiving elements. However, since the balanced receiver is a single output light receiver as a whole, a single normal light receiving element is incorporated here. It can be considered that the receiver is equivalent in function.
[0018]
By the way, quadrature modulation / demodulation techniques are frequently used in the field of telecommunications, particularly microwave communications. Among these, an image rejection mixer (hereinafter referred to as IRM) having the most easily usable form in the present invention will be outlined. This is a 2-input 2-output mixer. When a signal wave is input to one input port and a reference wave is input to the other input port, if the frequency of the signal wave is higher than the frequency of the reference wave, Heterodyne signal is output only to the output port (real port), and when the frequency of the signal wave is lower than the frequency of the reference wave, the heterodyne signal is output only to the other output port (image port). . Inside this IRM, each of the signal wave and the reference wave is branched into two and down-converted (generally mixed) by two (normal) mixers. Then, the respective outputs are combined and output to two output ports. The phase of the signal wave entering the two mixers and the phase of the reference wave are shifted by 90 °, and therefore the two mixer outputs are also 90 ° out of phase. A mixer having such a function is called a quadrature down converter (generally a quadrature mixer). Two heterodyne signals obtained from this quadrature down-converter are combined by a 90 ° hybrid coupler, and due to the interference effect, heterodyne signal outputs appear at different output ports depending on the level of the frequency of the signal wave and the reference wave. When the signal wave has a plurality of frequency components, the frequency component higher and lower than the frequency of the reference wave are separated and appear at the two output ports.
[0019]
It is known that the IRM can be similarly configured in the light region by using two light receivers as two mixers in the IRM. In the field of optical communication, particularly in the field related to coherent optical communication, such a light receiving technique is used for a receiver, so it is called by a name such as “(optical heterodyne) image removal receiver”. Hereinafter, for the sake of simplicity, this will be referred to as “optical IRM”. The optical IRM is described in, for example, “Terumi Koma, Shigeki Watanabe, Takao Naito, Hideo Kuwahara,“ Examination of optical heterodyne image removal reception method ”, 1989 IEICE Spring National Convention, B-757, etc. ing.
[0020]
In the present invention, the dead zone is eliminated by using an optical IRM (more essentially, a quadrature optical down-converter). As described above, when the optical IRM is used, the heterodyne signal separated in accordance with the output light of the wavelength tunable light source and the sideband frequency can be output to the two ports. In the following, the output light of the wavelength tunable light source is considered as a signal wave, and the sideband wave is a reference wave, and when the output optical frequency of the wavelength tunable light source is higher than a certain sideband wave, a heterodyne signal appears in the real port of the optical IRM, It is assumed that a heterodyne signal appears at the image port when the output optical frequency of the wavelength tunable light source is lower than the wave.
The output signal frequency of the optical IRM when the output optical frequency of the wavelength tunable light source changes as shown in FIG. 7A is shown in FIG. In FIGS. 8A to 8C, two outputs are depicted in one diagram, with the positive frequency corresponding to the real port output and the negative frequency corresponding to the image port output. By using the optical IRM in this way, a heterodyne signal including the frequency level (symbol) similar to that shown in FIG. 7B can be obtained. As a result, the return of the negative frequency seen in FIG. 7C is eliminated, and the plurality of lines do not cross each other, so that the dead zone disappears in principle.
[0021]
Next, a method for using the output of the optical IRM for frequency tracking control will be described.
As is apparent from FIG. 7A, in FIGS. 7 and 8, for example, the frequency of the wavelength variable light source is changed by the frequency interval of the sidebands from time t0 to t2. Since sideband waves periodically exist in the frequency domain, if frequency tracking control is always possible from t0 to t2, frequency tracking control is performed over the entire range where sideband waves of sufficient strength exist thereafter. Is possible.
FIGS. 8B and 8C schematically show a frequency tracking control method based on the same optical IRM output as in FIG. 8A. In this figure, the signal used for frequency tracking control is indicated by a thick solid line for the real port output and by a thick broken line for the image port output.
FIG. 8B shows the simplest method that is used while switching between two outputs of the optical IRM. This figure shows that when the frequency interval of the sidebands is 4 GHz, the -4 to -2 GHz component of the optical IRM output derived from the (N + 1) -order sideband, that is, 4 to 2 GHz of the image port output, between t0 and t1. Example of performing frequency tracking control based on the component, and performing frequency tracking control based on the +2 to +4 GHz component of the optical IRM output derived from the Nth order sideband, that is, the 2 to 4 GHz component of the real port output during t1 to t2. Is shown.
FIG. 8C shows an example of a method using only one output of the optical IRM. This figure shows an example in which frequency tracking control is performed based on +2 to +6 GHz components, that is, 2 to 6 GHz components of the real port output. Of course, the same control is possible using only image output.
[0022]
In addition, in order to always realize the frequency tracking control between t0 and t2, it is only necessary to always refer to one of the sidebands within this time, and there are innumerable switching methods. In addition, although frequency tracking control including DC is possible, such as the frequency of the heterodyne signal to be referred to is −2 GHz to DC and DC to +2 GHz, the implementation is somewhat complicated.
As described above, since the problem of the dead zone is caused by the light receiving method, if a light receiving unit that outputs heterodyne signals having different phases included in the optical IRM is used, the processing of the plurality of outputs is performed. Can effectively utilize various techniques known as electrical signal processing methods.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a variable frequency laser light source apparatus according to a first embodiment of the present invention.
As shown in FIG. 1, the frequency variable laser light source apparatus of the present embodiment includes a frequency reference light source 1, a sideband generation unit 2, a wavelength variable light source 3, an optical IRM 4, a comparison unit 5, and a control unit 6.
The frequency reference light source 1 is a frequency-stabilized light source using an atomic or molecular absorption line as a frequency reference. The sideband generation means 2 generates a large number of sidebands of known frequency using the output light of the frequency reference light source 1 as input light, and the aforementioned comb generator or the like can be used. The wavelength tunable light source 3 is a laser light source that is frequency follow-up controlled using the sideband of the comb signal light as a frequency reference light. A part of the output light of the wavelength tunable light source 3 becomes the output light of the frequency variable laser light source apparatus of the present invention.
The optical IRM 4 receives the output of the sideband generation means 2 and part of the output light of the wavelength tunable light source 3, and outputs two heterodyne signals that are 90 ° out of phase with each other. This function is the same as the quadrature phase mixer that has been used for a long time in the field of microwave communication.
Based on the quadrature heterodyne signal from the optical IRM 4, the comparison unit 5 outputs a control signal for the wavelength tunable light source 3 so that the frequency of the heterodyne signal becomes a predetermined offset frequency including the sign.
The control unit 6 outputs a signal for coarse adjustment of the output frequency for the wavelength tunable light source 3, an offset frequency setting signal including a sign for the comparison unit 5, and various switching signals according to a desired optical frequency.
[0024]
FIG. 2 shows a variable frequency laser light source apparatus according to the second embodiment of the present invention.
As shown in FIG. 2, the frequency tunable laser light source device of the present embodiment includes a frequency reference light source 1, a sideband generation means 2, a duplexer 7, and two sets of wavelength tunable light sources 3a and 3b, an optical IRM 4a, 4b, comparison means 5a, 5b, and control units 6a, 6b.
The frequency reference light source 1 and the sideband generation means 2 are exactly the same as those in the first embodiment. There are two sets of the variable wavelength light source, the light IRM, the comparison unit, and the control unit, 3a to 6a and 3b to 6b, each of which is the same as 3 to 6 in the first embodiment. The duplexer 7 is for branching the output light of the sideband generation means 2 and inputting it to the optical IRMs 4a and 4b.
[0025]
In this configuration, not only the dead zone in the frequency tracking control of each wavelength variable light source 3a, 3b is avoided, but also the frequency of each wavelength variable light source 3a, 3b can be set independently of each other, such as WDM optical communication. It can be used as a multi-output variable frequency laser light source device used in the above.
In addition, since a plurality of output lights are stabilized with respect to one frequency reference light through sideband generation and frequency tracking control, the relative frequency fluctuation between the output lights is a plurality of frequency reference light sources. Thus, it has a feature that is smaller by about two orders of magnitude than the relative frequency fluctuation among a plurality of output lights obtained from the above.
[0026]
In the present embodiment, the case where there are two sets of wavelength tunable light sources and the like has been described. However, even in the case of three or more sets, the duplexer 7 can be similarly implemented with three or more branches. Also, in applications where the output optical frequencies of the wavelength variable light sources are not close to each other, if the light from the sideband generation means 2 is not simply branched, but is branched using a wavelength selective branching means, distribution is possible. Loss can be suppressed.
[0027]
【Example】
Since the second embodiment of the present invention can be easily obtained by extending the first embodiment, only the first embodiment will be described as an example.
FIG. 3 shows a first embodiment of the present invention. This embodiment corresponds to the frequency tracking control shown in FIG.
As shown in FIG. 3, the variable frequency laser light source apparatus of this embodiment includes a frequency reference light source 1, a sideband generation unit 2, a variable wavelength light source 3, an optical IRM 4, a comparison unit 5, and a control unit 6.
[0028]
The frequency reference light source 1 is a frequency stabilized light source using an absorption line of atoms or molecules as a frequency reference. For example, a semiconductor laser whose frequency is stabilized with reference to an absorption line near 1.53 μm of acetylene molecules is used to output light with excellent frequency accuracy and stability.
The sideband wave generating means 2 includes a comb generator 21 and a microwave oscillator 22 that inputs a driving microwave to the comb generator 21. The sideband wave generating means 2 uses the output light of the frequency reference light source 1 as input light and has a known frequency. Many sidebands are generated. In this embodiment, the frequency of the microwave is set to 4 GHz. Therefore, the frequency interval of the sideband of the generated comb signal light is 4 GHz. The generation range of the comb signal light is several THz or more, and the oscillation frequency thereof has the same high accuracy and high stability as the output light of the frequency reference light source 1.
For the comb generator 21, it is desirable to provide a resonator length control means in order to obtain optimum conditions for the input optical frequency. The optimum condition here is that the sideband generation range is the widest, that is, that the optical resonator constituting the comb generator satisfies the resonance condition for the input light. This is because it is also a condition that the frequency component of the sideband interval (4 GHz) is minimized in the temporal change in the output light intensity of the generator. At this time, there is ideally no frequency component in the sideband interval. In this way, when the frequency of the heterodyne signal obtained by the optical IRM described later becomes particularly around 4 GHz, the 4 GHz component derived from the change in the output light intensity of the comb generator regarded as noise can be reduced. it can.
The wavelength tunable light source 3 is an external resonator type laser light source that is frequency follow-up controlled using the sideband of the comb signal light as a frequency reference light. A part of the output light of the wavelength tunable light source 3 becomes the output light of the frequency variable laser light source apparatus of the present invention.
Depending on the variable bandwidth of the wavelength tunable light source 3, it is possible to use a single semiconductor laser instead of the external resonator type laser light source.
[0029]
The optical IRM 4 includes an optical 90 ° hybrid coupler 41, light receivers 42a and 42b, and a 90 ° hybrid coupler 43 (for electrical signals).
An example of the configuration of the optical 90 ° hybrid coupler is shown in FIG. This optical 90 ° hybrid coupler is formed of a glass waveguide, and after the two lights incident on the input ports 410a and 410b are branched by the demultiplexers 411a and 411b, they are combined by the multiplexers 412a and 412b. And output to the output ports 413a and 413b. The difference between the lengths of the two optical paths from the input port 410a to the output ports 413a and 413b and the two from the input port 410b to the output ports 413a and 413b. The difference from the difference in length of the optical path is adjusted to be a value obtained by adding or subtracting a quarter wavelength to an integral multiple of the wavelength. In the simplest example, the lengths of the two optical paths from the input port 410a to the output ports 413a and 413b are equal, and the difference between the lengths of the two optical paths from the input port 410b to the output ports 413a and 413b is 1 / There are four wavelengths. By doing in this way, the light into which the two incident lights were combined with a phase different by 90 ° is output to the two output ports. A method of adjusting the above-described difference in optical path length using a thin film heater or the like is also known. In particular, when the wavelength band of incident light is wide, the phase difference may change depending on the wavelength of the incident light, so it is desirable to perform adjustment according to the wavelength.
[0030]
As in this example, the optical 90 ° hybrid coupler 41 receives a part of the output light (signal light) of the wavelength tunable light source 3 and the output light (reference light) of the sideband generation means 2 and splits each of them into two branches. Then, the two sets of the signal light and the reference light are combined at a phase different by 90 ° from each other to obtain two output lights.
The two output lights of the optical 90 ° hybrid coupler 41 are photoelectrically converted by the light receivers 42a and 42b, respectively, and output as detection signals. These two heterodyne signals are equal in frequency to the frequency difference between the two lights incident on the optical 90 ° hybrid coupler 41 and are 90 ° out of phase with each other. That is, it functions as a quadrature phase down converter of light.
The 90 ° hybrid coupler 43 receives the two heterodyne signals output from the light receivers 42a and 42b, demultiplexes and combines the two input signals in the same manner as the optical 90 ° hybrid coupler, and outputs two output ports. Respectively output in-phase component and quadrature component signals. As a result, when the frequency of the signal light is higher than the frequency of the reference light, a heterodyne signal is output from only one of the output ports of the 90 ° hybrid coupler 43. Here, this output port is called a real port, and the other output port is called an image port. Conversely, when the frequency of the signal light is lower than the frequency of the reference light, a heterodyne signal is output only from the image port.
[0031]
The comparison means 5 includes a switch 51, a filter 52, an offset frequency oscillator 53, a phase comparator 54, and terminators 58a and 58b.
The switch 51 receives two outputs of the optical IRM4, outputs one of them to the filter 52, and outputs the other to the terminator 58a or 58b. The switch 51 is switched by a command from the control unit 6.
The filter 52 limits the band of the heterodyne signal selected by the switch 51, and passes the frequency components from half the frequency interval of the sideband to the frequency interval. In the present embodiment, since the frequency interval of the sidebands is 4 GHz, a band pass filter that passes the band of 2 to 4 GHz may be used. Note that the filter 52 may be unnecessary depending on the frequency characteristics of the optical IRM 4 and an amplifier (not particularly shown) placed to amplify the signal after photoelectric conversion.
[0032]
The offset frequency oscillator 53 generates a signal having an offset frequency set by a command from the control unit 6. The setting range of the offset frequency is 2 to 4 GHz as an absolute value.
The phase comparator 54 performs phase comparison between the heterodyne signal from the filter 52 and the output signal of the offset frequency oscillator 53, and outputs a control signal to the wavelength variable light source 3 based on this phase difference. The polarity of the control signal and the intermittentness of the control signal (switching between the output stop state and the output state) are set by a command from the control unit 6. The polarity of the control signal here refers to the case where the voltage is increased (decreased) or decreased (increased) when the phase difference increases (decreases) when the control signal is output as a voltage. , Means either. By switching the polarity, appropriate frequency tracking control can be performed in a negative feedback state both when the output optical frequency of the wavelength tunable light source 3 is higher and lower than the reference sideband frequency.
The terminators 58a and 58b perform non-reflective termination of outputs of the 90 ° hybrid coupler 43 that do not directly contribute to the phase comparison according to the state of the switch 51.
The control unit 6 outputs a signal for coarse adjustment of the output frequency to the variable wavelength light source 3 according to a desired optical frequency. Further, a switch switching signal, an offset frequency setting signal, a control polarity switching signal, and a control stop signal are output to the comparison means 5. Note that since the polarity of the control signal to be output by the phase comparator 54 is determined according to either the real port output or the image port output selected by the switch 51, the switch switching signal and the control polarity switching signal are substantially different. Are identical.
[0033]
A method of setting the output frequency of the frequency variable laser light source apparatus configured as described above will be described.
The output optical frequency of the frequency reference light source 1, that is, the center frequency of the comb signal light is νr, the frequency interval of the sidebands is fm, the reference sideband wave order is N, and the offset frequency of the output light of the tunable light source 3 with respect to the sidebands Is f including the sign, the output optical frequency ν of the wavelength tunable light source 3 is
ν = νr + N · fm + f (1)
And confirm. Hereinafter, symbols and numerical values representing frequencies in the equations are in GHz units.
In the present embodiment, the frequency interval of the sideband is set to 4 GHz, and the frequency setting range of the offset frequency oscillator 53 is set to 2 to 4 GHz. Since the phase comparator 54 can switch the polarity of the control signal, the frequency ranges in which the frequency tracking control with respect to the wavelength variable light source 3 can be performed with reference to the frequency of a certain sideband are −4 to −2 GHz and +2 to +4 GHz.
[0034]
When ν is considered as a desired output optical frequency in the expression (1), if the integer N is defined as the maximum integer of (ν−νr) / fm or less, f becomes a value in the range of 0 to 4 GHz. When f is 0 to 2 GHz, the difference between the output optical frequency of the wavelength tunable light source 3 and the frequency of the (N + 1) -order sideband ν− (νr + (N + 1) · fm) = fimage is −4 to −2 GHz. , (N + 1) frequency tracking control with the offset frequency based on the image port output as fimage may be performed on the basis of the next sideband. On the other hand, when f is 2 to 4 GHz, ν− (νr + N · fm) = freal, and therefore, frequency tracking control may be performed with the offset frequency based on the real port output as freal with the Nth-order sideband as a reference. .
Thus, by determining the order of the sideband wave as a reference and the offset frequency, the output optical frequency of the wavelength tunable light source 3 can be set to an arbitrary frequency within a frequency band of several THz where the sideband wave can be used.
[0035]
An example of the frequency setting procedure is as follows.
Assuming that the frequency reference light source 1 and the sideband wave generating means 2 are operating, first, the output of the comparing means 5 (phase comparator 54) is stopped, and the output frequency of the wavelength tunable light source 3 is set near the desired frequency. Adjust until coarse.
Next, the output signal frequency of the offset frequency oscillator 53 is set to the offset frequency (absolute value) calculated as described above, and the polarity of the control signal output from the phase comparator 54 is set according to the sign of this offset frequency. The switch 51 is also set in accordance with the sign of the offset frequency so as to select the real port output 43a of the 90 ° hybrid coupler 43 if positive and to select the image port output 43b if negative.
Finally, the phase comparator 54 is set in the output state, and the control signal is fed back to the wavelength variable light source 3. The frequency tracking control functions by the above operation, the output frequency of the wavelength tunable light source 3 is controlled to be a desired frequency, and the frequency is maintained.
When adjustment is difficult at the stage of coarse adjustment of the wavelength tunable light source 3 because the frequency accuracy of the light source is low, the frequency (wavelength) of the output light may be controlled while measuring with a wavelength meter. Usually, the frequency interval of the sidebands is several GHz or more, and the measurement accuracy of the wavelength meter is on the order of 100 MHz, so this control is easy.
Further, if the time constant of the frequency tracking control is sufficiently longer than the time required for switching the switch 51, setting the frequency of the offset frequency oscillator 53, and switching the polarity of the phase comparator 54, it is necessary to stop the output of the phase comparator 54. Absent.
[0036]
Since the output frequency of the wavelength tunable light source 3 can be arbitrarily set in this way, the output frequency of the wavelength tunable light source 3 can be swept by sequentially changing the set frequency.
When the absolute value of the heterodyne frequency is exactly 2 GHz or 4 GHz, either the real port output or the image port output may be used for frequency tracking control. Therefore, which one to use may be determined in advance. For example, if the real port output is used in both cases of 2 GHz and 4 GHz, the frequency of the heterodyne signal is +2 to +4 GHz or −3.999 to − when the frequency sweep is performed in the 1 MHz step toward the high frequency side. While changing to 1.999 GHz, switching of the switch 51 and switching of the polarity of the phase comparator 54 are not necessary, and the frequency setting of the offset frequency oscillator 53 and the stop and start of the phase comparator 54 are sequentially performed as necessary. Just do it. When switching the heterodyne frequency used for control from +4 GHz to -3.999 GHz, switching of the switch 51 from the state using the real port output to the state using the image port output, switching the polarity of the phase comparator 54, offset frequency oscillator The frequency setting from 53 4 GHz to 3.999 GHz may be performed, and the phase comparator 54 may be stopped and started as necessary. The same applies when the heterodyne frequency is switched from -1.999 GHz to +2 GHz.
[0037]
In this embodiment, one of the two outputs of the 90 ° hybrid coupler 43 is selected by the switch 51. However, two systems of filters and phase comparators are prepared after the 90 ° hybrid coupler 43, A configuration for switching the output of the phase comparator is also possible.
Further, in the comparison means 5 of the present embodiment, the configuration using the offset frequency oscillator 53 and the phase comparator 54 is shown so that the relationship of the frequency of the signal handled there can be simply shown, but the frequency division of the heterodyne signal is shown. Processing for normal electrical signals such as frequency conversion and frequency conversion may be used effectively. Depending on the accuracy required for frequency tracking control, an fV (frequency-voltage) converter may be used instead of the offset frequency oscillator 53 and the phase comparator 54. This is the same in other embodiments.
[0038]
FIG. 4 shows the comparison means 5 of the second embodiment of the present invention. The other parts are the same as those in the first embodiment, and will be omitted. The present embodiment corresponds to the frequency tracking control shown in FIG.
The comparison means 5 of this embodiment includes switches 51a and 51b, filters 52a and 52b, an offset frequency oscillator 53, a phase comparator 54, and a terminator 58.
[0039]
The switch 51a outputs the real port output of the 90 ° hybrid coupler 43 to one of the filters 52a and 52b. The filter 52a is a band pass filter that passes 2 to 4 GHz, and the filter 52b is a band pass filter that passes 4 to 6 GHz. The switch 51b selects and outputs one of the outputs of the filters 52a and 52b, and is switched in conjunction with the switch 51a. A combination of the switches 51a and 51b and the filters 52a and 52b constitutes a band pass filter capable of switching the pass band.
The offset frequency oscillator 53 generates a signal having an offset frequency set by a command from the control unit 6. The setting range of the offset frequency is 2 to 6 GHz.
The phase comparator 54 compares the phase of the heterodyne signal from the switch 51b and the output signal of the offset frequency oscillator 53, and outputs a control signal to the wavelength variable light source 3 based on this phase difference. This intermittent control signal is set by a command from the control unit 6. The polarity of the control signal is fixed so that the frequency tracking control functions in a negative feedback state corresponding to the port used in the two output ports of the 90 ° hybrid coupler 43.
[0040]
A method of setting the output frequency of the frequency variable laser light source apparatus configured as described above will be described.
In this embodiment, the frequency interval of the sideband is set to 4 GHz, and the frequency setting range of the offset frequency oscillator 53 is set to 2 to 6 GHz. The frequency range in which frequency tracking control with respect to the wavelength tunable light source 3 can be performed with reference to the frequency of a certain sideband is +2 to +6 GHz because the real port output is used.
[0041]
In equation (1), if integer N is the largest integer of (ν-2-νr) / fm or less (denominator, numerator is in GHz), f in equation (1) is a value in the range of 2 to 6 GHz. become. Therefore, it is only necessary to perform frequency tracking control in which the offset frequency based on the real port output is f. The switches 51a and 51b may be switched so as to select the filter 52a or 52b that passes the offset frequency f to be used.
In the present embodiment, the pass band is switched because the component near 2 GHz and the component near 6 GHz are mixed in the output of the optical IRM4 (90 ° hybrid coupler 43) near the times t1 and t3 in FIG. It is. By switching the passband, only the heterodyne signal derived from the desired sideband can be extracted.
[0042]
FIG. 5 shows the comparison means 5 of the third embodiment of the present invention. Other parts are the same as those of the first embodiment. This embodiment corresponds to the frequency tracking control shown in FIG. 8C described in the section of action, and uses the superheterodyne method.
The comparison means 5 of this embodiment includes a frequency variable oscillator 55, a mixer 56, a filter 52, a frequency fixed oscillator 57, and a phase comparator 54.
[0043]
The variable frequency oscillator 55 generates a signal having a frequency set by a command from the control unit 6. This frequency setting range is 1 to 5 GHz. The mixer 56 receives the heterodyne signal output from the real port 43a of the 90 ° hybrid coupler 43 and the output of the frequency variable oscillator 55, and mixes and outputs them. The filter 52 is a band-pass (or low-pass) filter that allows the vicinity of 1 GHz to pass, and the output of the mixer 56 is band-limited and output. The fixed frequency oscillator 57 generates a 1 GHz signal. The phase comparator 54 receives the output of the filter 52 and the output of the fixed frequency oscillator 57, performs phase comparison, and outputs a control signal to the wavelength variable light source 3.
[0044]
A method of setting the output frequency of the frequency variable laser light source apparatus configured as described above will be described.
In this embodiment, the frequency interval of the sidebands is set to 4 GHz, and the offset frequency setting range is set to 2 to 6 GHz. Since the real port output of the 90 ° hybrid coupler 43 is used, the frequency range in which frequency tracking control with respect to the wavelength variable light source 3 can be performed with respect to the frequency of a certain sideband is +2 to +6 GHz.
[0045]
In the formula (1), if the integer N is a maximum integer of (ν-2-νr) / fm or less, f becomes a value in the range of 2 to 6 GHz. This f is defined as an offset frequency. At this time, since the oscillation frequency of the frequency variable oscillator 55 is variable in the range of 1 to 5 GHz, this oscillation frequency can be determined as (f-1) GHz. If the output optical frequency of the wavelength tunable light source 3 is roughly adjusted, the real port output resulting from the beat with the Nth order sideband has a frequency close to the offset frequency f, and this is the (f-1) GHz signal. If it is down-converted with reference to, it becomes about 1 GHz. At this time, the real port output resulting from the beat with the (N-1) secondary sideband and the beat with the (N + 1) secondary sideband is also down-converted and appears at about 3 GHz and about 5 GHz, respectively. Therefore, if only the signal of about 1 GHz is extracted by the filter 52, only the signal resulting from the beat with the Nth order sideband can be extracted. If this signal is phase-compared by the phase comparator 54 with reference to the 1 GHz signal from the frequency fixed oscillator 57, a signal necessary for frequency tracking control is obtained.
[0046]
【The invention's effect】
The sideband is generated based on the frequency reference light, and the sideband is used as the frequency reference for the output light of the wavelength tunable light source. Obtain the quadrature heterodyne signal obtained by using two light receivers for the output light of the light source and the sideband, and based on the information of the frequency difference including the code of this signal, the frequency of the output light of the wavelength tunable light source Because we decided to control
Essentially eliminates the dead zone problem associated with the use of optical frequency comb generators, (1) excellent output light frequency accuracy, frequency stability, and setting resolution, (2) wide frequency variable range, and (3) A frequency variable laser light source device capable of arbitrarily setting or sweeping the frequency within the frequency variable range has been realized.
If a plurality of wavelength tunable light sources are provided as in the second embodiment of the present invention, it is possible to independently set the frequency of each of the plurality of output lights with respect to one frequency reference light. This is a variable frequency laser light source device that can be applied to a light source for communication.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of the present invention.
FIG. 2 is a diagram showing a second embodiment of the present invention.
FIG. 3 is a diagram showing a first embodiment of the present invention.
FIG. 4 is a diagram showing a second embodiment of the present invention.
FIG. 5 is a diagram showing a third embodiment of the present invention.
FIG. 6 is a diagram illustrating a configuration example of an optical 90 ° hybrid coupler.
7A and 7B are diagrams for explaining a dead zone that causes a problem when setting and sweeping the frequency of output light using a sideband as a frequency reference, from the viewpoint of frequency sweeping, and FIG. ) And the frequency sweep condition of the output light that is the premise of FIG. 8, (b) is the frequency of the output light with reference to each sideband, and (c) is the heterodyne signal and dead zone obtained by one light receiver. FIG.
8A and 8B are diagrams for explaining the setting and sweeping of the frequency of output light based on the sideband wave as a frequency reference in the present invention on the premise of FIG. 7A, and FIG. The frequency of a heterodyne signal, (b), (c) is a figure which shows the example of the setting of the frequency of the output light based on the output of (a), and the method of a sweep.
FIG. 9 is a diagram showing a configuration of a conventional variable frequency laser light source device.
FIG. 10 is a diagram showing a configuration of a conventional frequency variable laser light source device.
FIGS. 11A and 11B are diagrams for explaining a dead zone that causes a problem when setting and sweeping the frequency of output light using a sideband as a frequency reference. FIG. 11A shows the vicinity of the sideband, and FIG. Explanatory drawing of the problem in the center vicinity between waves, (c) is a figure which shows a dead zone.
[Explanation of symbols]
1 Frequency reference light source
2 Sideband generation means
3 Tunable light source
4 Hikari IRM
5 comparison means
6 Control unit
21 Com generator
22 Microwave oscillator
41 Optical 90 ° hybrid coupler
42a Receiver
42b Receiver
43 90 ° hybrid coupler (for electrical signals)
51 switch
51a switch
51b switch
52 Filter
52a filter
52b filter
53 Offset Frequency Oscillator
54 Phase comparator
55 Frequency Variable Oscillator
56 mixer
57 fixed frequency oscillator
58 Terminator
58a terminator
58b terminator
331 multiplexer
332 Receiver
333 Phase comparator
334 Offset frequency oscillator
335 Loop filter
336 Adder
337 Drive current source

Claims (1)

A frequency reference light source (1), sideband wave generating means (2) for receiving the frequency reference light output from the frequency reference light source and generating light having a plurality of sidebands, and a wavelength at which the wavelength of the output light is variable A variable light source (3);
An optical 90 ° hybrid coupler that receives the output light output from the wavelength tunable light source and the light having a plurality of sidebands output from the sideband generation means and outputs two combined lights that are 90 ° out of phase with each other ( 41),
A receiver (42a, 42b) that receives the two combined lights and outputs heterodyne signals that are 90 ° out of phase with each other;
A real port (43a) that receives the heterodyne signal and outputs the heterodyne signal when the frequency of the frequency reference light is higher than the frequency of the sideband, and the frequency of the frequency reference light is lower than the frequency of the sideband A 90 ° hybrid coupler (43) having an image port (43b) for outputting the heterodyne signal,
Receiving one of the outputs of the serial port and image port of the 90 ° hybrid coupler, determined in relation to the desired wavelength of the output light of the frequency and the plurality of sidebands of the heterodyne signal Comparing means (5) for comparing the frequency of the offset frequency signal corresponding to one sideband wave to be output and outputting the frequency difference signal, and for controlling the wavelength of the wavelength tunable light source using the frequency difference signal Variable laser light source device.

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