JPH11288014A - Frequency variable laser light source device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the likelihood ratio, stability, set resolution and variable range of frequencies by obtaining the orthogonal phase heterodyne signals from the output light and sidebands of a wavelength variable light source by using two photodetectors and controlling the frequency of the output light. SOLUTION: The frequency variable light source apparatus consists of a frequency reference light source 1, a sideband generating means 2, the wavelength variable light source 3, an optical heterodyne image removing receiver (optical IRM) 4, a comparing means 5 and a control section 6. The sideband generating means 2 is inputted with the output light of the frequency reference light source 1 as input light and generates the many sidebands of the known frequencies. The optical IRM 4 receives the output of the sideband generating means 2 and part of the output light of the wavelength variable light source 3 and outputs the two heterodyne signals varying in phase by 90 deg. from each other. The comparing means 5 outputs the control signal to the wavelength variable light source 3 in such a manner that the frequencies of the heterodyne signals attain the prescribed offset frequencies including codes in accordance with the orthogonal phase heterodyne signals from the optical IRM 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light source device used in the field of optical communication and optical measurement, and more particularly, to a frequency variable having a variable frequency over a wide band, and having excellent frequency accuracy and frequency stability of output light. The present invention relates to a laser light source device.

[0002]

2. Description of the Related Art With respect to elemental technologies for constructing a laser light source device having excellent frequency accuracy and frequency stability of output light, there are various technologies such as a frequency reference light source, a wavelength variable light source, a frequency tracking control, and a broadband sideband wave generation. Are known.

First, each of these will be briefly described. As a frequency reference light source, a semiconductor laser (hereinafter, referred to as LD) is referred to as an atomic or molecular absorption line.
D: laser diode), the one in which the oscillation frequency of a solid-state laser is stabilized, and the gas laser are common. As the wavelength variable light source, an external resonance type light source in which an LD or a solid-state laser is combined with a wavelength selection element such as a diffraction grating, or a DFB-LD (distributed feedback-laser dio)
de), DBR-LD (distributed Bragg reflector-
It is common to incorporate a wavelength selection element inside an LD such as a laser diode). Naturally, such a light source alone does not guarantee frequency accuracy or long-term frequency stability.

The frequency tracking control controls the output light frequency of another laser so as to follow the output light frequency of one laser. In such control, a laser serving as a frequency reference is called a master laser, and a laser following 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. Normally, the output light of the master laser and the slave laser is multiplexed and received, the beat signal is detected, and the frequency is set to the offset frequency. The injection current of the slave laser is controlled so as to coincide with. This control causes the output light frequency of the slave laser to be separated from the output light frequency of the master laser by a set frequency, 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 lights, a control malfunction may be caused if the offset frequency approaches 0. Depending on the control method,
The level of the output light frequency of the slave laser with respect to the output light frequency of the master laser, that is, the sign of the offset frequency can be set to either positive or negative. The upper limit of the offset frequency is at most 100 GHz due to the band of the light receiving system that generates the beat signal. When a commercially available light receiving element or the like is used, the band in which light reception and amplification can be easily performed is up to several GHz.

[0005] As for the wideband sideband generation, there are an optical frequency comb generator, a short optical pulse light source, and the like as its means. Optical frequency comb generator (hereinafter referred to as comb generator)
Applies deep modulation to the input laser light to generate and output several hundred or more sideband waves at a modulation frequency interval on the frequency axis. Modulation frequency is usually several GHz to 20G
Hz, and a sideband having an intensity usable over several THz is obtained. If the frequency and modulation frequency of the input light are known, the frequency of each sideband is also known, so that each sideband can be used as frequency reference light.

The short optical pulse light source has a saturable absorber, an optical intensity modulator, an optical phase modulator, etc., which are means for modulating light inside an optical resonator, and an optical amplifying medium. It repeatedly outputs light with a width of about 10 ps or less.
This output light contains a number of sidebands having the repetition frequency interval of the light pulse. In particular, a short light pulse light source by general forced mode locking has a built-in light intensity modulator,
By driving the light intensity modulator with the microwave, an optical pulse train synchronized with the microwave is generated.
Therefore, the frequency interval of the sideband has a high frequency accuracy and a high stability of the microwave to be driven, and this point is similar to the case where the comb generator is used. However, even if the frequency interval between the sidebands is kept constant as in forced mode locking, the short light pulse light source does not have an optical frequency reference inside, so the frequency of each sideband is time-dependent. Will fluctuate. Therefore, in order to generate the frequency reference light in a wide band using the short light pulse light source, light having a known frequency may be prepared and the light injection locking phenomenon or the 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 from the outside into a self-excited oscillation LD, the oscillation frequency of the LD is drawn to the frequency of the injected light and coincides. Similarly, in the short light pulse light source, when light having a frequency close to the lower-order sideband included in the output light is injected, the frequency of this sideband is drawn into the frequency of the injected light. Therefore, in the short light pulse light source by forced mode locking, if the frequency of the injected light is known, the frequency of each sideband is also known. Further, as described above, the frequency tracking control is to cause the oscillation frequency of another LD or the like to follow the frequency of the light to be referred to. Therefore, a short light pulse light source by forced mode locking also refers to light with a known frequency. If the frequency of a certain sideband is made to follow as light, the frequency of each sideband also becomes known.

When used in combination with the frequency reference light source, the comb generator and the short light pulse light source can be said to be equivalent sideband wave generating means. Since the output light of the comb generator (hereinafter referred to as "comm signal light") and the output light of the short light pulse light source are used for heterodyne detection with other light, the input signal must be increased 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 purpose, an external resonance type LD light source or a self-injection locking LD is used as an input light source.
Light sources are often used. Note that the means for generating sidebands in a wide band here means those capable of generating sidebands at equal intervals on the frequency axis, so that the output of a comb generator or the output of a short optical pulse light source is pulsed. Also includes those subjected to compression. Since they can be handled essentially in the same way, the following description will be made in the case of a comb generator.

Next, a description will be given of an existing frequency-variable light source which is excellent in frequency accuracy and frequency stability by combining the above element technologies. First, there is a frequency variable laser light source device shown in FIG. 9 in which a frequency reference light source, a wavelength variable light source, and frequency tracking control are combined. 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 variable wavelength light source 31 are multiplexed 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 compared in phase by the phase comparator 333, and the result is added to the current from the drive current source 337 via the loop filter 335 by the adder 336, and the wavelength is changed. 31 is supplied to the LD. With this control, the output light follows the frequency reference light with the 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, but the frequency variable width is limited to the frequency tracking control band including the light receiving system,
The limit is at most 100 GHz. Note that 100 GHz corresponds to a wavelength band of less than 1 nm in the 1.5 μm band.

In view of this, use of a comb generator or the like has been proposed, and the operation has been experimentally confirmed. An optical frequency comb generation technique, which is one of the broadband sideband generation techniques, is added to the above three techniques, that is,
Since many sidebands included in the comb signal light obtained by inputting the frequency reference light to the comb generator can be used as the frequency reference light, the frequency tracking is performed on the wavelength tunable light source based on the sideband of any order. A system for controlling was considered. The configuration is shown in FIG. In this system, the frequency tracking control means 33
, The frequency of the output light is changed.

[0010]

Although this system can generate light with high accuracy and high stability over a wide band, it cannot generate an arbitrary frequency within a wide frequency range. This is due to the existence of two types of dead zones. One is that the offset frequency cannot be brought close to 0 when performing the frequency tracking control. That is, when the frequency of the sideband wave of a certain order and the frequency of the variable wavelength light source as the slave light source substantially match, proper frequency tracking control cannot be performed (FIG. 11A). This means that in normal heterodyne detection, information on the absolute value of the frequency difference between the two lights is obtained, so that when the frequency difference between the laser beams becomes less than about the oscillation line width, both the positive and negative heterodyne frequency components become the same frequency. It occurs because they are mixed. The other is that when using a comb generator or short optical pulse light source, many sidebands stand on the frequency axis, so if the frequency of the wavelength-tunable light source almost coincides with the center between the sidebands, the received signal will have a lower order. The beat signal of the sideband and the beat signal of the higher-order sideband are mixed at almost the same frequency, and proper frequency tracking control cannot be performed (FIG. 11B). The two types of dead zones (FIG. 11)
The standard of the frequency width of (c)) is about 20 MHz in the dead zone near the sideband and about several hundred MHz in the dead zone near the center between the sidebands, if an appropriate margin is expected.

An object of the present invention is to solve the above-mentioned problem of the dead zone, and to improve the frequency accuracy, frequency stability, and
Excellent setting resolution, wide frequency variable range, and
An object of the present invention is to provide a frequency variable laser light source device which can set or sweep a frequency arbitrarily within the frequency variable range. Standards of the high accuracy, high stability, and high resolution referred to above are 100 MHz, 10 −9, and 1 MHz or less, respectively. In addition, the minimum standard of the wide band mentioned above is 1.5 μm.
It is 10 nm or more in the m 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.

Further, in recent years, with the increase in 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 generate a single comb signal light and to set the frequency of a plurality of wavelength tunable light sources independently based on an arbitrary sideband included therein.
To do this, with the frequency of the comb signal light fixed,
Means to avoid dead zones must be implemented.

[0013]

As described above, the dead zone is such that ordinary heterodyne detection can detect only the absolute value of the frequency difference between two lights and cannot know the relative height of the frequency. Is attributed to Therefore, in order to solve this problem, a sideband is generated based on light serving as a frequency reference, that is, a frequency reference light,
The use of the sideband as a reference for the output light of the wavelength-tunable light source is the same as that of the related art. To obtain a quadrature-phase heterodyne signal, and control the frequency of the output light of the wavelength-variable light source based on information on the frequency difference including the sign of the signal. That is,

The frequency variable laser light source device according to the present invention comprises: a frequency reference light source; a sideband generating means for receiving the frequency reference light output from the frequency reference light source to generate light having a plurality of sidebands; A real-port which receives a light having a plurality of sidebands output from the wavelength-tunable light source and the sideband generating means, and outputs a heterodyne signal. An optical IRM having an optical port and an image port, and receiving one of the outputs from the real port and the image port of the optical IRM to obtain a desired frequency of the heterodyne signal and the output light of the plurality of sidebands. And comparing means for comparing the frequency of the offset frequency signal corresponding to one sideband determined in relation to the wavelength of the signal, and outputting a frequency difference signal. Controlling the wavelength of the variable wavelength light source using the frequency difference signal.

Since such means are adopted, the details will be described in the section of operation. However, in the frequency variable laser light source device of the present invention, the difference between the reference sideband frequency and the output frequency of the wavelength variable light source is described. , That is, the relative height of the frequency, and the dead zone is essentially eliminated. Further, since the frequency-variable laser light source device of the present invention sets the frequency of the wavelength-variable light source with reference to the sideband having a fixed frequency, the frequencies of the plurality of wavelength-variable light sources can be set independently of each other. It is possible.

[0016]

The operation of the present invention will be described from the viewpoint of frequency sweep. If a dead zone exists, the frequency of the variable wavelength light source, which is the slave light source, cannot be set arbitrarily. Therefore, this frequency cannot be sequentially and finely changed within a range equal to or more than the frequency interval of the sideband. That is, the frequency sweep cannot be performed in a state where the frequency tracking control is performed over a wide frequency range.

First, FIG. 7A shows an example of a time change of the output light of the wavelength variable light source and the frequency of each sideband. FIG.
When the frequency of the wavelength variable light source increases with time as shown in FIG. 7A, the frequency of the wavelength variable light source is rewritten based on each sideband as shown in FIG. 7B. The vertical axis of this figure represents the frequency difference including the sign. At this time, the frequency of the beat signal obtained by combining the output light of the variable wavelength light source and the sideband wave and receiving light by one light receiver is described within a positive frequency range as shown in FIG. be able to. This is because the level of the frequency of the light contributing to the beat signal cannot be known by such a simple light receiving method. In FIG. 7C, it can be seen that the negative frequency portion in FIG. 7B is folded back to a positive frequency. Here, the dead zone appears near the intersection of the downward-sloping line and the upward-sloping line in FIG. In other words, near the intersection, the frequency components corresponding to the two intersecting lines are mixed in the beat signal, making it difficult to perform appropriate frequency tracking control.
A balanced receiver is known as a light receiver incorporating two light receiving elements. However, since the balanced receiver is a light receiver having one output as a whole, a normal single light receiving element is incorporated here. The light receiver and its function can be regarded as equivalent.

Incidentally, quadrature phase modulation / demodulation techniques are frequently used in the field of telecommunications, especially in microwave communications.
Among them, an image rejection mixer (hereinafter, referred to as IRM) having the most usable form according to the present invention will be outlined. This is a two-input, two-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, one of the two is used. Output the heterodyne signal only to the output port (real port), and output the heterodyne signal only to the other output port (image port) when the frequency of the signal wave is lower than the frequency of the reference wave. . Inside this IRM, a signal wave and a reference wave are respectively branched into two, and down-converted (generally mixed) by two (normal) mixers.
Is done. Then, the respective outputs are multiplexed and output to two output ports. The phases of the signal wave and the reference wave entering the two mixers are shifted by 90 °, so that the two mixer outputs also have a 90 ° phase difference. A mixer having such a function is called a quadrature downconverter (generally, a quadrature mixer). Two heterodyne signals obtained from the quadrature downconverter are combined by a 90 ° hybrid coupler, and a heterodyne signal output appears at different output ports depending on the level of the frequency of the signal wave and the reference wave due to the effect of interference. When the signal wave has a plurality of frequency components, the frequency components higher and lower than the frequency of the reference wave are separated and appear at two output ports.

It is known that an IRM can be similarly configured in a light region by using two light receivers as two mixers inside 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, and is therefore called by a name such as "(optical heterodyne) image removal receiver".
Hereinafter, for the sake of simplicity, it will be referred to as “optical IRM”.
The optical IRM is described in, for example, "Terumi Chikama, Shigeki Watanabe, Takao Naito, Hideo Kuwahara," Study of Optical Heterodyne Image Rejection Reception System ", 1989 IEICE Spring National Convention, B-757" and the like. ing.

In the present invention, the dead zone is eliminated by using an optical IRM (more essentially, an optical quadrature down converter). As described above, if the optical IRM is used, the output light of the variable wavelength light source and the heterodyne signal separated according to the level of the sideband frequency can be output to two ports. Hereinafter, the output light of the wavelength tunable light source is considered as a signal wave, and the sideband is considered as a reference wave. When the output light frequency of the wavelength tunable light source is higher than a certain sideband, 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 light frequency of the variable wavelength light source is lower than that of the wave. FIG.
FIG. 8A shows the output signal frequency of the optical IRM when the output light frequency of the variable wavelength light source changes as shown in FIG. In FIGS. 8A to 8C, two outputs are illustrated in one diagram assuming that a positive frequency corresponds to a real port output and a negative frequency corresponds to an image port output. By using the optical IRM in this way, FIG.
A heterodyne signal including the same frequency level (sign) as that of (b) is obtained. As a result, the aliasing of the negative frequency shown in FIG. 7C is eliminated, and a plurality of lines do not intersect, so that the dead zone disappears in principle.

Next, a method of using the output of the optical IRM for frequency tracking control will be described. As is clear from FIG. 7A, in FIGS. 7 and 8, for example, from time t0 to t2
In the drawing, the frequency of the wavelength variable light source is changed by the frequency interval of the sideband. Since the sideband exists periodically in the frequency domain, if the frequency follow-up control is always possible during the period from t0 to t2, the frequency follow-up control is similarly performed over the entire range in which the sideband having sufficient intensity exists. Is possible. FIGS. 8B and 8C show FIG.
FIG. 9 schematically shows a frequency tracking control method based on the optical IRM output similar to FIG. In this figure, the signal used for the frequency tracking control is shown by a thick solid line with respect to the real port output, and with a thick broken line with respect to the image port output. FIG. 8B shows the simplest method of using the two outputs of the optical IRM while switching them. This figure shows that, when the frequency interval of the sideband is 4 GHz, during the period from t0 to t1, the output of the optical IRM derived from the (N + 1) -order sideband is -4.
−２-2 GHz component, that is, 4-2 of the image port output
Frequency tracking control is performed based on the GHz component, and t1 to t
During the period between 2, the optical IRM output derived from the Nth sideband is +2 to +
4 GHz component, that is, 2 to 4 GHz of real port output
An example is shown in which frequency tracking control is performed based on components.
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 can be performed using only the image output.

In addition, in order to always realize the frequency tracking control between t0 and t2, it is only necessary to refer to any of the sidebands during this time, and there are countless switching methods. I do. Note that frequency tracking control including DC is possible, such as when the frequency of the heterodyne signal to be referred to is -2 GHz to DC and DC to +2 GHz, but the implementation is slightly complicated. Further, as described above, since the problem of the dead zone is caused by the light receiving method, the light IR
If a light receiving unit that outputs heterodyne signals having different phases included in M is used, for processing of the plurality of outputs, various techniques known as a method of processing an electric signal can be effectively used. it can.

[0023]

FIG. 1 shows a frequency variable laser light source device according to a first embodiment of the present invention. As shown in FIG. 1, the frequency-variable laser light source device according to the present embodiment includes a frequency reference light source 1, a sideband generation unit 2, a wavelength-variable light source 3,
RM 4, comparing means 5 and control unit 6. The frequency reference light source 1 is a frequency stabilized light source that uses the absorption lines of atoms and molecules as a frequency reference. Sideband wave generating means 2
Uses the output light of the frequency reference light source 1 as input light and generates a number of sidebands of a known frequency, and the above-described comb generator or the like can be used. The variable wavelength light source 3 is a laser light source that is frequency-followed controlled using the sideband of the comb signal light as the frequency reference light. A part of the output light of the wavelength variable light source 3 becomes the output light of the frequency variable laser light source device of the present invention. The optical IRM 4 receives the output of the sideband generating means 2 and a part of the output light of the wavelength tunable light source 3, and outputs two heterodyne signals having phases different from each other by 90 °. This function is the same as that of a quadrature mixer that has been used for a long time in the field of microwave communication. The comparing means 5 outputs a control signal to the wavelength tunable light source 3 based on the quadrature heterodyne signal from the optical IRM 4 so that the frequency of the heterodyne signal becomes a predetermined offset frequency including a sign. The control unit 6 outputs a signal for coarse adjustment of the output frequency to the wavelength tunable light source 3, an offset frequency setting signal including a sign to the comparing unit 5, and various switching signals according to a desired optical frequency.

FIG. 2 shows a frequency variable laser light source device according to a second embodiment of the present invention. As shown in FIG. 2, the frequency tunable laser light source device according to the present embodiment includes a frequency reference light source 1, a sideband wave generating means 2, a duplexer 7, two sets of tunable light sources 3a and 3b, an optical IRM 4a, 4b, comparison means 5a,
5b, and control units 6a and 6b. The frequency reference light source 1 and the sideband generating means 2 are exactly the same as in the first embodiment. The wavelength tunable light source, the optical IRM, the comparing unit, and the control unit include two sets of 3a to 6a and 3b to 6b.
Each of them is the same as 3 to 6 of the first embodiment.
The splitter 7 is for splitting the output light of the sideband generating means 2 and inputting the split light to the optical IRMs 4a and 4b.

In this configuration, the variable wavelength light sources 3a, 3b
In addition to avoiding a dead zone in the frequency tracking control, the frequency of each of the wavelength tunable light sources 3a and 3b can be set independently of each other, and a multi-output frequency tunable laser light source device used for WDM optical communication or the like. Available as Further, since a plurality of output lights are stabilized with respect to one frequency reference light through sideband generation or frequency tracking control, the relative frequency fluctuation between the output lights is
It has a feature that relative frequency fluctuation between a plurality of output lights obtained from a plurality of frequency reference light sources is smaller by about two orders of magnitude.

In this embodiment, the case where there are two sets of wavelength tunable light sources and the like has been described. However, in the case where there are three or more sets, the demultiplexer 7 can be implemented with three or more branches. In applications where the output optical frequencies of the wavelength tunable light sources are not close to each other, the light from the sideband generating means 2 is not simply split, but is split by using a wavelength-selective splitter. Loss can be reduced.

[0027]

EXAMPLES The second embodiment of the present invention can be easily obtained by expanding the first embodiment, and therefore, only examples of the first embodiment will be described. FIG. 3 shows a first embodiment of the present invention. The present embodiment corresponds to the frequency tracking control shown in FIG. As shown in FIG. 3, the frequency tunable laser light source device according to the present embodiment includes a frequency reference light source 1, a sideband generation unit 2, a wavelength tunable light source 3, an optical IRM 4, a comparison unit 5, and a control unit 6.
Consists of

The frequency reference light source 1 is a frequency stabilized light source using the absorption lines of atoms and molecules as a frequency reference. For example,
A semiconductor laser whose frequency is stabilized based on an absorption line near 1.53 μm of acetylene molecules is used,
Outputs light with excellent frequency accuracy and stability. The sideband generating means 2 is composed of a comb generator 21 and a microwave oscillator 22 for inputting a driving microwave to the comb generator 21. The output light of the frequency reference light source 1 is used as input light, and Generates many sideband waves. In this embodiment, the frequency of the microwave is set to 4 GHz. Therefore, the frequency interval between the sidebands of the generated comb signal light is 4G.
Hz. The range in which the comb signal light is generated extends over several THz, and its oscillation frequency has the same high accuracy and high stability as the output light of the frequency reference light source 1. Note that it is desirable to provide resonator length control means for the comb generator 21 in order to obtain optimum conditions for the input optical frequency. The optimum condition here is a condition under which the generation range of the sideband wave is the widest, that is, the optical resonator constituting the comb generator satisfies the resonance condition with respect to the input light. This is because it is also a condition that the frequency component of the sideband interval (4 GHz) becomes minimum in the time change of the output light intensity of the generator. At this time, ideally, there is no frequency component at the sideband interval. By doing so, the light IR described later can be used.
The frequency of the heterodyne signal obtained at M is especially 4 GHz
When it is near, it is possible to reduce a 4 GHz component derived from a change in the output light intensity of the comb generator, which is regarded as noise. The wavelength variable light source 3 is an external resonator type laser light source that is frequency-followed and controlled using the sideband of the comb signal light as frequency reference light. A part of the output light of the wavelength variable light source 3 becomes the output light of the frequency variable laser light source device of the present invention. In addition,
Depending on the variable bandwidth of the wavelength tunable light source 3, a semiconductor laser alone may be used instead of the external cavity laser light source.

The light IRM 4 is made up of the light 90 ° hybrid coupler 41, the light receivers 42 a and 42 b and the 9
It comprises a 0 ° hybrid coupler 43. Light 90 °
FIG. 6 shows an example of the configuration of the hybrid coupler. The optical 90 ° hybrid coupler is formed by a glass waveguide, and after two lights incident on the input ports 410a and 410b are branched by the demultiplexers 411a and 411b, respectively, and then combined by the multiplexers 412a and 412b. Waves are output to the output ports 413a and 413b.
The difference between the length of the two optical paths from the input port 410b to the output ports 413a and 413b is 1/1 / integer times the wavelength.
The value is adjusted to be a value obtained by adding or subtracting four wavelengths. In the simplest example, the length of two optical paths from the input port 410a to the output ports 413a and 413b is 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 /. It has four wavelengths. By doing so, the two lights that have been incident are combined at a phase that differs by 90 ° from each other, and are output to the two output ports. It is to be noted that a method of adjusting the difference in the optical path length using a thin film heater or the like is also known. In particular, when the wavelength band of the incident light is wide, the phase difference may change depending on the wavelength of the incident light. Therefore, it is desirable to perform the adjustment according to the wavelength.

As in this example, the optical 90 ° hybrid coupler 41 is a part of the output light of the wavelength tunable light source 3 (signal light).
And the output light (reference light) of the sideband generating means 2, each of which is split into two, and the two sets of the signal light and the reference light are separated from each other by 9.
The two output lights are obtained by multiplexing at phases different from each other by 0 °. 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. The two heterodyne signals have a frequency of 90 ° hybrid coupler 4
The frequency is equal to the frequency difference between the two lights incident on 1 and their phases are different by 90 °. That is, it functions as an optical quadrature phase down converter. The 90 ° hybrid coupler 43 receives the two heterodyne signals output from the photodetectors 42a and 42b, demultiplexes and combines the two input signals in the same manner as the 90 ° hybrid coupler of light, and provides two output ports. Output signals of the in-phase component and the quadrature-phase component, respectively. 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.

The comparing means 5 includes a switch 51, a filter 5
2. It comprises an offset frequency oscillator 53, a phase comparator 54, and terminators 58a and 58b. The switch 51 is connected to the light I
RM4 receives two outputs, one of which is a filter 5
2 and the other terminal 58a or 58b
Output to Switching of the switch 51 is based on a command from the control unit 6. The filter 52 band-limits the heterodyne signal selected by the switch 51, and passes a frequency component from half the frequency interval of the sideband to the frequency interval.
In this embodiment, since the frequency interval between the sidebands is set to 4 GHz, a bandpass filter that passes a band of 2 to 4 GHz may be used. Note that the filter 52 may not be necessary depending on the frequency characteristics of the optical IRM 4 and an amplifier (not particularly shown) provided for amplifying the signal after photoelectric conversion.

The offset frequency oscillator 53 includes a controller 6
The signal of the offset frequency set by the command of is generated. The setting range of the offset frequency is 2 to 4 GHz as an absolute value. The phase comparator 54 compares the phase of the heterodyne signal from the filter 52 with the output signal of the offset frequency oscillator 53, and outputs a control signal to the wavelength tunable light source 3 based on the phase difference. The polarity of the control signal and the intermittent 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 as used herein means whether the voltage is increased (decreased) or decreased (increased) when the phase difference becomes large (small) 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 light frequency of the wavelength variable light source 3 is higher and lower than the reference sideband frequency. Terminators 58a, 58b
Terminates the output of the 90 ° hybrid coupler 43 which does not directly contribute to the phase comparison according to the state of the switch 51 without reflection. The control unit 6 outputs a signal for coarse adjustment of the output frequency to the wavelength variable light source 3 according to a desired optical frequency. Further, it outputs a switch switching signal, an offset frequency setting signal, a control polarity switching signal, and a control stop signal to the comparing means 5. 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. Is the same as

A method of setting the output frequency of the frequency variable laser light source device configured as described above will be described. The output light 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 sideband is fm, the sideband order used as a reference is N, and the offset frequency of the output light of the wavelength tunable light source 3 with respect to the sideband. Is assumed to be f including the sign, the output optical frequency ν of the wavelength tunable light source 3 is determined as follows: ν = νr + N · fm + f (1) Hereinafter, symbols and numerical values representing frequencies in the formula are
GHz unit. 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 4 GHz. Since the phase comparator 54 can switch the polarity of the control signal, the frequency ranges in which the frequency tracking control of 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.

In Equation (1), when ν is considered to be a desired output light frequency, if the integer N is defined as the largest integer equal to or less than (ν−νr) / fm, f is a value in the range of 0 to 4 GHz. become.
When f is 0 to 2 GHz, the difference between the output light frequency of the wavelength tunable light source 3 and the frequency of the (N + 1) -order sideband ν− (ν
r + (N + 1) .fm) = fimage is -4 to -2GH
Since it is z, it is sufficient to perform frequency tracking control using the offset frequency based on the image port output as fimage with reference to the (N + 1) th sideband. On the other hand, f is 2 to 4G
Hz, ν− (νr + N · fm) = fre
Since it is al, frequency tracking control with the offset frequency based on the real port output as freal may be performed based on the Nth sideband. In this manner, by determining the order and offset frequency of the reference sideband,
The output light frequency of the wavelength tunable light source 3 can be set to an arbitrary frequency within a frequency band of several THz in which a sideband can be used.

An example of a frequency setting procedure is as follows. Assuming that the frequency reference light source 1 and the sideband 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 close to the desired frequency. Roughly adjust until. Next, the output signal frequency of the offset frequency oscillator 53 is set to (the absolute value of) the offset frequency calculated as described above, and the polarity of the control signal output from the phase comparator 54 is set according to the sign of the offset frequency. The switch 51 is also set to select the real port output 43a of the 90 ° hybrid coupler 43 if positive and select the image port output 43b if negative according to the sign of the offset frequency. Finally, the control signal is fed back to the wavelength tunable light source 3 with the phase comparator 54 in the output state. With the above operation, the frequency tracking control functions, the output frequency of the wavelength variable light source 3 is controlled to be a desired frequency, and the frequency is maintained. In addition,
If it is difficult to adjust the wavelength of the variable wavelength light source 3 in the rough adjustment stage 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 sideband is several GHz or more, and the measurement accuracy of the wavelength meter is on the order of 100 MHz, so that this control is easy. 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 not necessary to stop the output of the phase comparator 54. Absent.

As described above, since the output frequency of the tunable light source 3 can be arbitrarily set, the output frequency of the tunable light source 3 can be swept by sequentially changing the set frequency. If 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, in both cases of 2 GHz and 4 GHz, if the real port output is used, when the frequency sweep is performed to the high frequency side in 1 MHz steps, the frequency of the heterodyne signal is +2 to +4 GHz.
During the change to z or -3.999 to -1.9999 GHz, the switching of the switch 51 and the polarity of the phase comparator 54 are not necessary.
The frequency setting of No. 3 and the stop and start of the phase comparator 54 may be sequentially performed as necessary. When the heterodyne frequency used for control is switched from +4 GHz to -3.999 GHz, the switch 51 is switched from the state using the real port output to the state using the image port output, the polarity of the phase comparator 54 is switched, and the offset frequency oscillator is used. What is necessary is just to set the frequency of 53 from 4 GHz to 3.999 GHz, and stop and start the phase comparator 54 as necessary. Heterodyne frequency is -1.999 GHz
The same applies when switching from to +2 GHz.

In the present embodiment, one of the two outputs of the 90 ° hybrid coupler 43 is selected by the switch 51. However, two filters and phase comparators are prepared at the subsequent stage of the 90 ° hybrid coupler 43. A configuration in which the outputs of two phase comparators are switched is also possible.
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 between the frequencies of the signals handled therein can be simply specified. Processing for ordinary electric signals, such as frequency conversion and frequency conversion, may be effectively used. Further, depending on the accuracy required for the frequency tracking control, f−
V (frequency-voltage) converter is connected to an offset frequency oscillator 5
3 and the phase comparator 54 can be used instead of the configuration. This is the same in other embodiments.

FIG. 4 shows comparison means 5 according to the second embodiment of the present invention.
Is shown. The other parts are the same as in the first embodiment, and will not be described. This embodiment corresponds to the frequency tracking control shown in FIG. The comparison means 5 of this embodiment includes switches 51a and 51b, a filter 52a,
52b, offset frequency oscillator 53, phase comparator 54
And a terminator 58.

The switch 51a outputs the real port output of the 90 ° hybrid coupler 43 to the filters 52a and 52b.
Is output to any of. Filter 52a is 2
The filter 52b is a band-pass filter that passes 4 to 6 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.
It is switched in conjunction with 1a. Switches 51a, 51
A combination of the filter b and the filters 52a and 52b constitutes a band-pass filter whose pass band can be switched. The offset frequency oscillator 53 generates a signal of 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 with the output signal of the offset frequency oscillator 53, and outputs a control signal to the wavelength tunable light source 3 based on the 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 among the two output ports of the 90 ° hybrid coupler 43.

A method for setting the output frequency of the frequency variable laser light source device having the above-described configuration will be described. 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 6 GHz. The frequency range in which the frequency tracking control of the wavelength variable light source 3 can be performed based on the frequency of a certain sideband is +2 to +6 GHz because the real port output is used.

In the equation (1), the integer N is expressed as (ν−2−νr) /
Assuming that the maximum integer is fm or less (the denominator and the numerator are in GHz), f is a value in the range of 2 to 6 GHz in the equation (1). Therefore, frequency tracking control with an offset frequency f based on the real port output may be performed. Switch 5
1a 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 around the time t1 or t3 in FIG.
2G for the output of RM4 (90 ° hybrid coupler 43)
This is because components near Hz and components near 6 GHz are mixed. By switching the passband, it is possible to extract only the heterodyne signal derived from the desired sideband wave.

FIG. 5 shows the comparison means 5 of the third embodiment of the present invention.
Is shown. Other parts are the same as in the first embodiment. The present embodiment corresponds to the frequency tracking control shown in FIG. 8C described in the section of operation, and utilizes the superheterodyne method. The comparison means 5 of this embodiment includes a variable frequency oscillator 55, a mixer 56, a filter 52, a fixed frequency oscillator 57, and a phase comparator 54.

The variable frequency oscillator 55 generates a signal having a frequency set by a command from the control unit 6. The frequency setting range is 1 to 5 GHz. Mixer 56 is 9
The heterodyne signal output from the real port 43a of the 0 ° hybrid coupler 43 and the output of the variable frequency oscillator 55 are received, mixed, and output.
The filter 52 is a band-pass (or low-pass) filter that passes near 1 GHz, and band-limits the output of the mixer 56 to output. The fixed frequency oscillator 57
Generate a 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 tunable light source 3.

A method of setting the output frequency of the frequency variable laser light source device configured as described above will be described. In this embodiment, the frequency interval of the sideband 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 the frequency tracking control of the wavelength tunable light source 3 can be performed with respect to the frequency of a certain sideband wave is +2 to +6 GHz.

In the equation (1), the integer N is represented by (ν−2−νr) /
Assuming that it is the largest integer equal to or less than fm, f takes a value in the range of 2 to 6 GHz. This f is defined as the offset frequency.
At this time, the oscillation frequency of the variable frequency oscillator 55 is 1 to 5
Since the frequency is variable in the range of GHz, this oscillation frequency can be determined as (f-1) GHz. If the output light frequency of the wavelength tunable light source 3 is roughly adjusted, the real port output due to the beat with the Nth sideband has a frequency close to the offset frequency f.
Down-converted with reference to the signal of
Becomes At this time, the beat with the (N-1) next sideband,
The real port output due to the beat with the (N + 1) next sideband is also downconverted to about 3 GHz,
Appears at about 5 GHz. Therefore, about 52 GHz
By extracting only the signal of z, it is possible to extract only the signal caused by the beat with the Nth sideband. If this signal is compared with the phase of the 1 GHz signal from the fixed frequency oscillator 57 by the phase comparator 54, a signal necessary for the frequency tracking control can be obtained.

[0046]

According to the present invention, a sideband is generated based on the frequency reference light and the sideband is used as a frequency reference for the output light of the wavelength tunable light source. Instead, a quadrature-phase heterodyne signal obtained by using two photodetectors with the output light of the wavelength-tunable light source and the sideband is obtained, and the wavelength-variable light source is Since the frequency of the output light is controlled, the dead zone problem associated with the use of the optical frequency comb generator is essentially eliminated, and the frequency accuracy, frequency stability, and setting resolution of the output light are excellent.
A frequency variable laser light source device having a wide frequency variable range and capable of arbitrarily setting or sweeping a frequency within the frequency variable range can be realized. Further, if a plurality of wavelength tunable light sources are provided as in the second embodiment of the present invention, it becomes possible to set the frequency of each of the plurality of output lights independently for one frequency reference light, The frequency variable laser light source device can be applied to a light source for communication and the like.

[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.

FIG. 7 is a diagram for explaining a dead zone which is a problem when setting and sweeping the frequency of output light using a sideband wave as a frequency reference from the viewpoint of frequency sweeping. ) And the situation of the frequency sweep of the output light which is the premise of FIG. 8, (b) is the frequency of the output light with reference to each sideband,
(C) is a diagram showing a heterodyne signal and a dead zone obtained by one light receiver.

8A and 8B are diagrams for explaining setting and sweeping of the frequency of the output light with reference to the sideband wave in the present invention on the premise of FIG. 7A, and FIG. (B), (c) is a diagram showing an example of a method of setting and sweeping the frequency of the output light based on the output of (a).

FIG. 9 is a diagram showing a configuration of a conventional frequency variable laser light source device.

FIG. 10 is a diagram showing a configuration of a conventional frequency variable laser light source device.

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, where FIG.
FIG. 4 is an explanatory diagram of a problem near the center between band waves, and FIG. 4C is a diagram illustrating a dead zone.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 frequency reference light source 2 sideband generating means 3 variable wavelength light source 4 optical IRM 5 comparing means 6 control unit 21 comb generator 22 microwave oscillator 41 optical 90 ° hybrid coupler 42a optical receiver 42b optical receiver 43 (of electrical signal) 90 ° Hybrid coupler 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 frequency fixed oscillator 58 terminator 58a terminator 58b terminator 331 multiplexer 332 light receiver 333 phase Comparator 334 Offset frequency oscillator 335 Loop filter 336 Adder 337 Drive current source

Claims (1)

[Claims] 1. A frequency reference light source (1), sideband generating means (2) for receiving a frequency reference light output from the frequency reference light source and generating light having a plurality of sidebands, and a wavelength of the output light. A variable wavelength light source (3), which outputs a heterodyne signal by receiving output light output from the wavelength variable light source and light having a plurality of sidebands output from the sideband generating means. An optical IRM (4) having a port and an image port, and receiving one of the outputs from the real port and the image port of the optical IRM and receiving the one of 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 output light and outputting a frequency difference signal; Frequency tunable laser source apparatus for controlling the wavelength of the wavelength tunable light source with a number difference signal.