JP2008288390A - Wavelength variable optical frequency stabilizing light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable optical frequency stabilizing light source which provides an output light stabilized to an arbitrary optical frequency near many optical frequency references within a wavelength variable band of wavelength variable laser, with a simple configuration which uses only one wavelength variable laser light source. <P>SOLUTION: The output light of a wavelength variable laser light source 1 is split by a beam splitter 2. One of the split outputs is modulated with an optical modulator 3 to generate a modulation side band. Under the control of a control circuit 12, the optical frequency of its side band component is locked with the absorption line of the gas which is a reference in an optical absorption cell 4, so that the output light frequency of the wavelength variable laser light source is controlled to a desired value. The modulation frequency of the optical modulator 3 is set to be a target optical frequency and a difference frequency of reference optical frequency near the optical frequency under the control of a controller 30. The controller 30 generates a table including operation parameters of wavelength variable laser light source and modulation frequencies of the optical modulator or the like, which is stored in a memory. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wavelength tunable optical frequency stabilized light source, and more particularly to a wavelength tunable optical frequency stabilized light source in an optical communication band using a semiconductor laser light source.

  The demand for communication networks such as the Internet and in-house communication networks is increasing, and further increases in capacity and speed of communication networks are desired. To meet this demand, optical technology facilitates multiplexing and demultiplexing optical signals with different wavelengths. In recent optical communication networks, a wavelength division multiplexing (WDM) method called WDM (Wavelength Division Multiplexing) that demultiplexes optical signals with different wavelengths is used. The transmission capacity per optical fiber is expanded to constitute an economical network.

  In this WDM system, the line is switched using the difference in wavelength. However, in order to achieve a higher wavelength density, high accuracy and stability are required for the optical frequency of the signal light source.

  Conventionally, a wavelength tunable laser light source capable of oscillating at a constant optical frequency interval has been put into practical use by using an element having a periodic transmission characteristic with respect to an optical frequency, such as a wavelength locker using an etalon, and DWDM (Dense Wavelength) Division Multiplexing) Used as a light source for optical communication.

However, since the etalon is not an element that determines the absolute value of the optical frequency, the optical frequency accuracy of the light source obtained using the etalon is not as high as about 10 ⁻⁶ , and the deterioration with time is also high. For this reason, this type of tunable laser light source is not satisfactory in terms of performance as a light source for optical communication or a light source for precision measurement using a coherent technology.

  In addition, in order to obtain a high-accuracy optical frequency, there has been provided a method for stabilizing the oscillation wavelength of a wavelength tunable laser to an absorption line wavelength such as a reference atom or molecule, but the optical frequency of the absorption line is generally It is different from the ITU grid frequency, which is the standard for optical communication.

  When realizing a high-accuracy frequency-stabilized light source that matches the optical frequency on the ITU-T grid, a configuration using two laser light sources and one absorption cell has been proposed as a conventional technique (for example, see Patent Document 1). Yes.

  In such a frequency stabilized light source using two laser light sources and one absorption cell, cw (continuous wave) controlled to a specific optical frequency such as an optical frequency on the ITU grid at a single wavelength (optical frequency). ) To obtain light, first lock the optical frequency of the first laser light source to an optical absorption line that serves as the reference for the optical frequency, and then change the optical frequency of the second laser to the optical frequency of the first The desired value is controlled by offset locking.

  In addition, in order to control the wavelength with high accuracy, a method has also been proposed in which the output optical frequency of one tunable laser light source is dither modulated to detect the peak of the absorption line (see, for example, Patent Document 2).

  FIG. 15 is a diagram showing a configuration of a wavelength tunable stabilized light source using a method of dither modulating the output optical frequency of one wavelength tunable laser light source and detecting the peak of the absorption line. The wavelength tunable stabilized light source shown in FIG. 15 includes a wavelength tunable laser light source 1, a beam splitter 2 that divides light incident from the wavelength tunable laser light source 1, and a beam splitter 7 that divides light incident from the beam splitter 2 into two. A wavelength reference unit 8 that transmits one of the lights branched by the beam splitter 7, a light receiver 5a that receives the light from the wavelength reference unit 8, and a light reception that receives one of the lights branched by the beam splitter 7. Switch 5b, differential amplifier 14 for detecting the output difference between the signals output from light receivers 5a and 5b, and a switch for switching the signal from differential amplifier 14 to wavelength stabilizing means 33 and peak detecting circuit 34 9, a current source 31 that supplies a current for controlling a transmission wavelength of the light source 1, a temperature control circuit 32 that controls a transmission temperature of the light source 1, and a control signal to the current source 31 The wavelength stabilization means 33 to be supplied, the peak detection circuit 34 for detecting the peak of the signal from the differential amplifier 14, the temperature control circuit 32, the current source 31 and the switch 9 are supplied with temperature setting signal, injection current setting signal and switching, respectively. And a controller 30 for supplying signals.

  In the above configuration, the output of the wavelength tunable laser light source 1 based on the semiconductor laser light source is branched into two by the beam splitter, one for output to the outside and the other for wavelength control. The control optical signal is split into two by the beam splitter 7, one is directly received by the light receiver 5 a, and the other is transmitted through the wavelength reference device 8 and then received by the light receiver 5 b. The wavelength reference unit 8 serves as a reference for wavelength control. For example, a cell filled with a gas whose wavelength of the absorption line is accurately priced is used. A difference occurs in the optical power detected by the light receiver according to the presence or absence of absorption by the gas. This power fluctuation is detected by the differential amplifier 14, and wavelength control is performed by locking to the absorption peak.

  In order to set a desired control wavelength, first, the temperature control circuit 32 scans the temperature of the light source, counts the output from the peak detection circuit, and determines the wavelength of the absorption line to be used and the operating temperature at that time. Thereafter, the switch 9 is switched, and the wavelength stabilization means 33 is used to perform precise wavelength control. The entire device is controlled by the controller 30.

Japanese Patent Laid-Open No. 2000-357840, FIG. Japanese Patent No. 2520740, FIG. 1 W. C. Swann and S.L.Gilbert, “Pressure-induced shift and broadening of 1560-1630-nm carbon nonoxide wavelength-calibration lines”, J. Opt. Soc. Am. B, vol. 19, pp.2461-2467 (2002)

  However, in the frequency stabilized light source using two laser light sources and one absorption cell, it is necessary to precisely control the optical frequencies of the two light sources, so that the configuration of the control system becomes very complicated. there were.

  In addition, in the wavelength tunable stabilized light source that dither modulates the output light frequency of one wavelength tunable laser light source, the drive current is modulated to modulate the output light frequency, so that the line width of the output light is widened. there were.

  The present invention has been made in view of the above-described circumstances, and has a simple configuration using one wavelength tunable laser light source, and an arbitrary optical frequency in the vicinity of an optical frequency reference that exists in a large number within the wavelength tunable band of the wavelength tunable laser. An object of the present invention is to provide a wavelength tunable optical frequency stabilized light source capable of obtaining highly stabilized output light.

  In order to achieve the above object, according to the present invention, the invention according to claim 1 is a wavelength-tunable optical frequency stabilized light source, which outputs a continuous wave light capable of changing an oscillation optical frequency. A light source, an optical demultiplexer for branching a part of the power of the laser light generated by the laser light source, an optical modulator for modulating the branched laser light to generate a modulation sideband, and the light A signal generating means for generating a frequency-modulated electric signal to be applied to the modulator, a low-frequency oscillator for controlling a frequency fluctuation period of the electric signal, and a laser beam provided with modulation output from the optical modulator. A light absorption cell having a plurality of absorption lines existing in the oscillation light frequency range of the laser light source, and a light receiver that detects the laser light modulated by the light modulator that has passed through the light absorption cell. And before A synchronous detector that performs synchronous detection of an output signal of the light receiver in synchronization with an oscillation frequency of a low-frequency oscillator, control means that controls the optical frequency of the laser light source according to the output of the synchronous detector, and And control means for setting the output light frequency of the laser light source and the carrier frequency of the frequency-modulated electric signal of the signal generating means.

  The invention according to claim 2 is a wavelength tunable optical frequency stabilizing light source, a laser light source that outputs continuous wave light capable of changing an oscillation light frequency, and a power of laser light generated by the laser light source. An optical demultiplexer that branches a part, an optical modulator that modulates the branched laser light to generate a modulation sideband, and an electric signal that is frequency-modulated to be applied to the optical modulator A signal generator, a low-frequency oscillator for controlling a frequency fluctuation period of the electric signal, a second optical demultiplexer for branching the power of the laser light from the optical modulator, and the second optical demultiplexer An optical absorption cell having a plurality of absorption lines in the oscillation light frequency range of the laser light source, wherein one of the laser beams modulated by the optical modulator branched by Transmitted light modulator A first light receiving device that detects a laser beam that has been further modulated and a second light beam that detects the other of the laser light that has been modulated by the light modulator branched by the second optical demultiplexer. A photoreceiver, an operational amplifier for obtaining a difference component between the outputs of the first and second photoreceivers, and a synchronous detector for performing synchronous detection of an output signal of the operational amplifier in synchronization with an oscillation frequency of the low-frequency oscillator; Control means for controlling the optical frequency of the laser light source in accordance with the output of the synchronous detector; and setting the output optical frequency of the laser light source and the carrier frequency of the frequency-modulated electrical signal of the signal generating means And a control means.

  The invention according to claim 3 is the wavelength-tunable optical frequency stabilized light source according to claim 1 or 2, wherein the signal generating means for generating the frequency-modulated electrical signal corresponds to a carrier frequency of the modulated signal. A high-frequency oscillator that generates a sine wave signal, a frequency modulator that operates in synchronization with an output frequency of an external low-frequency oscillator, and an output adjustment unit that adjusts the output amplitude of the frequency-modulated signal. .

  The invention according to claim 4 is the wavelength tunable optical frequency stabilized light source according to claim 1 or 2, wherein the signal generating means for generating the frequency-modulated electric signal corresponds to a carrier frequency of the modulated signal. A first high-frequency oscillator that generates a sine wave signal, a second high-frequency oscillator that oscillates with a constant frequency difference from the oscillation frequency of the first high-frequency oscillator, and an output frequency of the low-frequency oscillator It has a changeover switch which switches the output of the first oscillator and the second oscillator in synchronization, and an output adjustment means for adjusting the output amplitude of the changeover switch.

  The invention according to claim 5 is the wavelength tunable optical frequency stabilized light source according to claim 1 or 2, wherein the control means for controlling the optical frequency of the laser light source is a modulation generated by the optical modulator. Of the sideband components, the optical frequency of the laser light source is controlled so that the optical frequency of a higher-order component matches the center optical frequency of the absorption line.

  The invention described in claim 6 is the wavelength tunable optical frequency stabilized light source according to claim 1 or 2, wherein the optical frequency detector has a periodic detection characteristic with respect to a change in the optical frequency of the laser light source. It further has these.

  As described above, according to the present invention, a simple configuration using only one wavelength tunable laser light source enables stabilization to an arbitrary optical frequency in the vicinity of the optical frequency reference that exists in the wavelength tunable band of the wavelength tunable laser light source. Thus, it is possible to realize a wavelength-tunable optical frequency stabilized light source from which the output light can be obtained.

  In the present invention, a single wavelength tunable laser light source is used as a light source, the output light is branched, and one of the branched outputs is modulated by an external modulator to generate a modulation sideband, and the optical frequency of the sideband component Is locked to a reference gas absorption line, etc., and the output optical frequency of the wavelength tunable laser light source is controlled to a desired value, thereby making the wavelength variable and the optical frequency controlled to the desired value. An optical frequency stabilized light source is realized.

  The modulation frequency of the optical modulator is set to the difference frequency between the optical frequency to be obtained by the tunable optical frequency stabilized light source according to the present invention and the reference optical frequency in the vicinity of the optical frequency. The cw having high optical frequency accuracy at a large number of optical frequencies by tabulating the operation parameters of the wavelength tunable laser light source, the modulation frequency of the optical modulator, etc. for realizing the optical frequency to be obtained by the present invention. A light source can be realized.

  This makes it possible to realize a cw light source having a desired optical frequency (an optical frequency on the ITU grid in many applications in the optical communication field) with a relatively simple configuration in the vicinity of the reference optical frequency. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
FIG. 1 shows an embodiment of a tunable optical frequency stabilized light source according to the present invention. The wavelength tunable optical frequency stabilized light source of the present embodiment is a laser light source that outputs continuous wave light that can change its oscillation light frequency in accordance with an external control input (in this specification, simply a wavelength tunable laser light source). 1), a beam splitter 2 as an optical demultiplexer for branching a part of the laser light generated by the wavelength tunable laser light source 1, and a frequency-modulated electric signal applied from the signal generating means An optical modulator 3 that modulates the laser light branched by the splitter 2 to generate a modulation sideband, and a tunable laser light source 1 that receives the modulated light output from the optical modulator 3 as input. The light absorption cell 4 filled with a substance having a plurality of absorption lines existing in the oscillation light frequency range, the light receiver 5 for detecting the optical signal transmitted through the light absorption cell 4, and the oscillation frequency of the low frequency oscillator 22 A synchronous detector 11 that performs synchronous detection of the output signal of the light receiver 5 in synchronization with the number, a control means that controls the optical frequency of the tunable laser light source 1 according to the output of the synchronous detector 11, and a tunable laser. And a controller 30 as a control means for setting the output optical frequency of the light source 1 and setting the carrier frequency of the frequency-modulated electric signal of the signal generating means. In addition, the wavelength tunable optical frequency stabilized light source of this embodiment includes an amplifier 10 that amplifies the electrical signal detected by the light receiver 5.

  In the present embodiment, the signal generating means a carrier generating oscillator 20 for generating a carrier, an FM modulator 21 for frequency-modulating the carrier by a signal from the low frequency oscillator 22, and a signal from the FM modulator 21 are amplified. The high-frequency amplifier 23 and an RF output adjuster 24 that adjusts the intensity of a signal supplied to the optical modulator 3 are configured.

  In this embodiment, the control means for controlling the optical frequency of the wavelength tunable laser light source 1 is obtained by the control circuit 12 for obtaining the control amount of the optical frequency control loop from the output of the synchronous detector 11 and the control circuit 12. The laser drive circuit 13 adjusts the drive current supplied to the wavelength tunable laser light source 1 based on the control amount.

  In the wavelength tunable optical frequency stabilized light source of the present embodiment, the laser light output from the wavelength tunable laser light source 1 is branched by the output beam splitter 2, one of which is used as output light and the other is used for optical frequency control. The The beam splitter 2 can be replaced with an element such as an optical coupler or a splitter.

The laser beam branched by the beam splitter 2 is detected by the light receiver 5 after sequentially passing through the light modulator 3 and the light absorption cell 4. A signal obtained from the light receiver 5 is amplified by the amplifier 10, and an error signal corresponding to the optical frequency deviation from the reference optical frequency v _cell of the light absorption cell 4 is obtained by the phase detector 11 connected thereto.

  The control circuit 12 obtains the control amount of the optical frequency control from the output of the synchronous detector 11, and controls the laser driving circuit 13 using this, thereby controlling the optical frequency of the wavelength tunable laser light source 1.

As a signal for driving the optical modulator 3, the carrier frequency v _offset, the maximum frequency shift v _FM, the FM modulation signal of the modulation frequency f _m is used. In this embodiment, a carrier generating oscillator 20, an FM modulator 21, a low frequency oscillator 22, a high frequency amplifier 23, and an RF output adjustment circuit 24 are used to generate this signal. The output of the low frequency oscillator 22 is also used for the operation of the phase detector 11.

  A controller 30 for controlling the operation of the entire wavelength-tunable optical frequency stabilized light source controls the operations of the carrier generating oscillator 20 and the laser driving circuit 13.

  The optical modulator 3 is used to generate a modulation sideband around the oscillation optical frequency of the wavelength tunable laser light source 1, and a waveguide type phase modulator, intensity modulator, SSB modulator, etc. can be used. It is. In this embodiment, an LN phase modulator will be described as an example.

The light absorption cell 4 has a plurality of optical frequency references in a wide optical frequency range, and generally uses atomic or molecular absorption lines whose frequency stability has been accurately evaluated by a laser spectroscopy technique or the like. Some of them are certified as reference materials in the Metric Convention. As the light absorption cell 4, for example, in the optical communication band, molecules such as acetylene (C ₂ H ₂ ), hydrogen cyanide (HCN), carbon monoxide ( ¹² C ¹⁶ O, ¹³ C ¹⁶ O), water vapor (H ₂ O), etc. A light absorption cell or the like in which a single or a plurality of are encapsulated is used. In this embodiment, carbon monoxide ( ¹² C ¹⁶ O) will be described as an example.

  As the wavelength tunable laser light source 1, various lasers such as a DFB laser (DFB-LD: distributed feedback laser diode) array, a DBR laser (Distributed Bragg Reflector laser), an external cavity semiconductor laser, and a fiber laser can be used. Here, a DFB laser array will be described as an example. This element has a structure that controls the optical frequency of laser light to be output by arranging DFB laser chips with different oscillation light frequencies in an array, selecting the chip to be operated, and controlling the operating temperature and drive current. ing.

FIGS. 2A and 2B are diagrams showing examples of absorption spectra when a light absorption cell in which carbon monoxide ( ¹² C ¹⁶ O) is sealed is used as the light absorption cell 4. ¹² C ¹⁶ O has a total of 40 or more absorption lines in the L band, and the wavelength of these absorption lines is known to be stable with sub-pm uncertainty (eg, non-patent document). 1).

FIG. 3 is a table in which the difference v _offset between the optical frequency corresponding to the absorption peak of FIG. 2 and the ITU grid frequency at 100 GHz intervals and 50 GHz intervals is arranged. As shown in the table of FIG. 3, v _offset is about several GHz to several tens GHz. If the output optical frequency of the wavelength tunable laser light source is v _out (i),
v _out (i) = v _cell (i) + v _offset (i) (1)
It becomes. Here, v _cell (i) represents the optical frequency of the i-th absorption line. Since the frequency offset v _offset (i) for obtaining the ITU grid frequency v _out (i) in the vicinity of the i-th absorption line of interest is uniquely determined from the table of FIG. 3, it is stored in the memory of the controller 30. I can keep it. The output light frequency v _out (i) of the light source can be set to a value different from that of the ITU grid from the above relationship.

FIG. 4 is a diagram for explaining the relationship among the transmission spectrum of the gas absorption line, the oscillation light frequency of the wavelength tunable laser light source, and the frequency of the modulation sideband (modulation sideband) generated by the optical modulator. The signal component having a frequency difference corresponding to v _offset (i) is one of the components obtained by branching a part of the output (optical frequency v _out (i)) of the wavelength tunable laser light source 1 by the beam splitter 2, and the optical modulator. 3 is obtained as a modulated sand band component. By controlling the sideband component v _SB (i) = v _out (i) −v _offset (i) to coincide with the optical frequency v _cell (i) of the light absorption cell,
v _out (i) = v _SB (i) + v _offset (i) = v _cell (i) + v _offset (i) (2)
Therefore, v _out (i) can be controlled to the ITU grid frequency. The difference between the sideband component v _SB (i) and the optical frequency v _cell (i) of the absorption line corresponds to the control error. This detection is performed as follows.

In the present embodiment, a signal for driving the optical modulator 3, the carrier frequency v _offset (i), the maximum frequency deviation v _FM, an FM modulated signal of the modulation frequency f _m. As FM modulation waveform, but it is also possible to use a sine wave of the maximum frequency shift v _FM, where for simplicity, assume the maximum frequency shift v _FM, the FSK modulation waveform of the switching frequency f _m. Since the spectral intensity of the sideband component after receiving the light modulation depends on the modulation index (modulation intensity) of the light modulation, the high-frequency amplifier 23 and the RF output adjustment circuit 24 adjust the spectral intensity to an optimum intensity. .

  The optical carrier component after passing through the optical modulator 3 and the sideband component on the opposite side across the optical carrier component and the sideband component of interest have an absorption line width of the used gas absorption line of several GHz. Since it is as narrow as possible and the interval between absorption lines is as wide as several tens of GH, it is hardly affected by absorption lines. Accordingly, the light output of the above components after passing through the light absorption cell 4 does not change with time.

On the other hand, when the center value v _SB (i) of the sideband optical frequency of interest and the optical frequency v _cell (i) of the absorption line substantially coincide, v _SB (i) and v _cell (i the positional relationship), the sideband component transmitted through the light absorbing cell 4, the time-varying component appears at the frequency f _m. This is shown in FIG.

As shown in FIG. 5A, when v _SB (i) and v _cell (i) match, two frequency components v _out (i) −v _offset (i) + v that have been FM-modulated. _{Since FM} and _vout (i) _-voffset (i) -v _FM suffer the same optical loss, the transmitted light does not change with time. As shown in FIG. 5B, when v _out (i) −v _offset (i)> v _cell (i), the two frequency components v _out (i) −v _offset (i ) + v to produce a difference in light loss received by the _FM and _{v out (i) -v offset (} i) -v FM, the light intensity transmitted through the light absorbing cell 4 varies at the frequency f _m. As shown in FIG. 5C, in the case of v _out (i) −v _offset (i) <v _cell (i), the light intensity fluctuates in the opposite phase to the case shown in (b).

Therefore, the output of the light absorption cell 4, and received by the photodetector 5, the frequency f by detecting the component of the amplitude and phase _m, v _cell of interest to have sideband component v _SB (i) (i) A deviation amount (error signal) from can be obtained.

Figure 6 is a plot of the amplitude of f _m frequency components detected by the photodetector 5 with respect to the frequency deviation amount from v _cell (i) of the sideband component v _SB (i). v _SB (i) and v _cell (i) is consistent with the case that the amplitude becomes zero, f _m frequency component is detected in response to detuning.

  In order to detect the error signal, the electric signal obtained by the light receiver 5 is amplified by the amplifier 10 and then the phase detector 11 performs phase sensitive detection. In order to perform phase sensitive detection, the signal of the low frequency oscillator 22 is used. The output of the phase detector 11 is smoothed, and a desired control amount is calculated by the control circuit 12. Based on this control amount, the laser drive circuit 13 is controlled to control the optical frequency of the wavelength tunable laser light source 1.

When a DFB laser array is used as the wavelength tunable laser light source 1, when the optical frequency of the i-th carrier to be generated is determined, the chip number of the DFB-LD to be used, the operating temperature of the DFB-LD, the DFB- By uniquely determining the operating current of the LD, v _offset (i), it is possible to set v _SB (i) and v _cell (i) to approximately coincide with accuracy of about several hundred MHz. This enables detection of the error signal f _m frequency components, it is possible to perform optical frequency control.

  When a DFB laser array is used as in this embodiment, the operating temperature of the DFB-LD is kept constant by a temperature control device (not shown), and the drive current is finely adjusted according to the magnitude of the detected error signal. Thus, optical frequency control can be achieved. Since the change in the optical frequency of the laser with respect to the amount of change in the drive current of the DFB laser is about 1 GHz / mA, the optical frequency can be controlled with an accuracy of about 1 MHz by controlling the current on the order of 1 μA.

Further, the carrier generating oscillator 20 that determines v _offset (i) uses a PLL (Phase Locked Loop) synthesizer type oscillator that is commonly used, so that the wide range listed in the table of FIG. It is possible to cope with an oscillation frequency in the frequency range.

  By integrating the elemental technologies described above, a light source having a variable wavelength and high optical frequency accuracy can be realized. Therefore, the operation parameters of the wavelength tunable laser light source (for example, in the case of a DFB laser array, the DFB-LD chip number to be selected, The operating temperature of the LD, the driving current of the LD, etc.) and the oscillation frequency of the carrier generating oscillator 20 are stored.

The table of FIG. 7 shows the control target optical frequency when the DFB laser array is used as the wavelength tunable laser light source 1, the operating parameters of the DFB laser array for realizing it, and the set values of the oscillation frequency of the carrier generating oscillator 20. A part is illustrated. A ¹² C ¹⁶ O P branch is used as the absorption line, and the optical frequency of the absorption line is shown in the second column from the left in the table of FIG. The accuracy of the optical frequency of the reference light absorption line is within several MHz, and the line width of the absorption line is several GHz or less, so that the operating temperature of the DFB laser is 1/100 degrees and the operating current is 1/100 degrees. Note that a setting accuracy of about 100 mA is required.

  FIG. 8 is a diagram for explaining the operation of the wavelength tunable optical frequency stabilized light source according to the present invention. First, in response to an optical frequency setting command sent to the controller 30 from the outside of the apparatus, the controller 30 sets the operating parameters of the wavelength tunable laser light source 1 and the oscillation frequency of the carrier generating oscillator 20 in the built-in memory of the controller 30. (S802), the respective values are sent to the laser drive circuit 13 and the carrier generating oscillator 20, and control of the wavelength tunable laser light source 1 and the carrier generating oscillator 20 is started (S804, S806). Acquisition of an error signal is started at the timing when the operating parameter given to the laser drive circuit 13 converges to a desired value (S808) and the oscillation frequency of the carrier generating oscillator 20 is locked to a desired value (S810) ( S812). The operation of the feedback circuit that feeds back the acquired error signal to the laser drive circuit 13 via the control circuit 12 is started (S814).

The sideband component v _SB (i) generated by the optical modulator 3 can be matched with the optical frequency v _cell (i) of the absorption line of interest of the light absorption cell 4 by the control mechanism described above. As a result, the optical frequency v _out (i) of the wavelength tunable laser light source 1 can be controlled to a desired value. When the sideband component v _SB (i) generated by the optical modulator 3 converges within a desired error range with respect to the optical frequency v _cell (i) of the absorption line of interest of the light absorption cell 4, the light The frequency lock state is determined (S816), and the optical frequency lock alarm is released to the outside (S818).

Further, when the light source is operated at an optical frequency v _cell (i) different from that described above, the operation of the optical frequency control feedback loop is once canceled, and then the wavelength tunable laser light source is operated in the order described above. Perform motion control.

  In the description of the present embodiment, the DFB laser array is taken as an example of the wavelength tunable laser light source 1, but as described above, various lasers such as a DBR laser, an external resonator type semiconductor laser, and a fiber laser can be applied. In this case, it should be noted that the set of operating parameters of the wavelength tunable laser 1 stored in advance in the internal memory of the controller 30 differs depending on the laser used.

In the description of this embodiment, carbon monoxide ( ¹² C ¹⁶ O) is taken as an example of the light absorption cell, but depending on the desired wavelength band of the light source realized by the present invention, It goes without saying that you make a choice.

(Example 2)
FIG. 9 is a diagram showing the configuration of the second embodiment of the wavelength tunable optical frequency stabilized light source according to the present invention. Components that are the same as or similar to those in the first embodiment are indicated by the same or similar numbers. Therefore, repeated description is omitted.

In the first embodiment, the FM-modulated carrier component (center frequency v _offset (i), maximum frequency deviation f _FM , modulation frequency f _m ) is used as a modulation signal by the optical modulator 3 to change the wavelength tunable laser light source 1. In principle, the modulated output should not fluctuate in time even if the cw light output from the light is modulated.

However, in practice, due to causes such as physical characteristics of the optical modulator 3, there are cases where a phenomenon that the strength of the modulated varies at the frequency f _m may occur. For example, such a phenomenon is observed when a Mach-Zehnder type intensity modulator is used as the optical modulator 3. To light absorbing cell 4, the originally light intensity modulated at a frequency f _m is input, the control circuit 12 operates to f _m frequency components detected by the photodetector 5 is zero. As a result, an offset occurs in the optical frequency v _out (i) of the controlled light source. This causes a deterioration in the optical frequency accuracy of the optical frequency stabilized light source of the present invention.

Therefore, in this embodiment, the intensity modulation component of the frequency f _m caused by the light modulation by the optical modulator 3 is branched by the beam splitter 7 provided between the optical modulator 3 and the optical absorption cell 4. The one branched by the beam splitter 7 is detected by the light receiver 5b, and the other branched is transmitted through the light absorption cell 4 and received by the light receiver 5a. By detecting amplifying the difference component of the signal obtained by the two light receivers 5a and 5b in the differential amplifier 14, to accurately detect the change in light absorption that varies at the frequency f _m of the light absorption cell 4 after transmission can do.

  Each of the light receivers 5a and 5b has an amplitude adjustment circuit, and performs adjustment so that the output amplitudes of both are equal. Alternatively, the first stage circuit block of the two inputs of the differential amplifier may have an amplitude adjustment circuit that makes the output amplitudes equal. Further, the above-described adjustment may be performed by installing an attenuator whose attenuation is variable in the light receiver output and adjusting the attenuation of both.

  With the above configuration, for example, it is relatively easy to reduce the offset amount of the output optical frequency, which was 10 MHz at the maximum in the previous configuration, to less than 1 MHz by this configuration.

In the configuration of the present embodiment, the filter 6 is inserted between the differential amplifier 14 and the phase detector 11, which is used to prevent deterioration in accuracy of control error detection due to circuit noise. . The filter 6, in the transmission region, a low-pass filter including a number of modulation fraction f _m of the frequency modulation, band pass filter, and the like can be used. Originally small component amplitude of the frequency f _m which is detected by a photodetector, for sensitive operation of the control circuit of the optical frequency by the circuit noise, the effect of inserting this filter 6 is large. Nature phase detector 11 is high ability to remove frequency components of the frequency f _m components other than the input signal is used as a reference, depending on the circuit configuration, which also detects the odd-order harmonic components of f _m Therefore, the effect of inserting a filter as shown in this embodiment is great. The filter 6 can also be applied to the first embodiment and has a great effect.

(Example 3)
FIG. 10 is a diagram showing the configuration of the third embodiment of the wavelength tunable optical frequency stabilized light source according to the present invention. Constituent elements that are the same as or similar to those in the first embodiment and the second embodiment are indicated by the same or similar numbers. Therefore, the overlapping description is omitted.

  In this embodiment, the configuration of the signal generating means is different from the configuration of the wavelength-tunable optical frequency stabilized light source of the second embodiment (in this specification, for generating carriers in the configuration shown in FIG. 9). A circuit composed of the oscillator 20 and the FM modulator 21 is also referred to as an FM modulation signal generation circuit). The signal generating means in this embodiment includes a carrier generating oscillator 20a whose oscillation frequency is controlled by the controller 30 and the frequency control circuit 25, a carrier generating oscillator 20b whose oscillation frequency is controlled by the frequency control circuit 25, and carrier generation. RF output adjustment circuits 24a and 24b for adjusting the outputs from the oscillators 20a and 20b, the high frequency switch 26 for switching the outputs of the RF output adjustment circuits 24a and 24b based on the output of the low frequency oscillator 22, and the RF output adjustment circuit The high-frequency amplifier 23 amplifies the outputs of 24a and 24b. The configuration of the signal generating means in the present embodiment can also be applied to the configuration of the wavelength tunable optical frequency stabilized light source of the first embodiment.

Next, the operation of the signal generating means in this embodiment will be described. In response to an optical frequency setting command sent to the controller 30 from the outside of the apparatus, the controller 30 generates a carrier generating oscillator from the memory in the controller 30 according to the optical frequency to be generated from the wavelength tunable optical frequency stabilized light source. The oscillation frequency 20a is read and the oscillation frequency is set. Here, the difference from the first embodiment is that the set oscillation frequency is not v _offset (i),
v _OSC1 (i) = v _offset (i) −v _FM (3)
V _OSC1 (i) given by. Therefore, a synthesizer type oscillator or the like can be used as the carrier generating oscillator 20a. Next, using the frequency control circuit 25, the oscillation frequency of the carrier generating oscillator 20b is set to v _OSC2 (i) = v _OSC1 (i) + 2 · v _FM = v _offset (i) + v _FM (4)
Set to. The frequency control circuit 25 can be configured using, for example, a PLL circuit, and a circuit in which the oscillators 20a and 20b always oscillate at a constant frequency difference can be realized. In the case of the present embodiment, the above-described frequency relationship can be realized by setting the frequency difference between the transmission frequencies of the oscillators 20a and 20b to 2v _FM .

  FIG. 11 is a diagram for explaining the relationship among the transmission spectrum of the gas absorption line, the oscillation light frequency of the wavelength tunable laser light source, and the modulation sideband frequency generated by the optical modulator in the configuration of the present embodiment. .

  In the present embodiment, the oscillation frequency of the carrier generating oscillator 20a on the low frequency side is controlled from the outside. However, the oscillation frequency of the carrier generating oscillator 20b on the high frequency side is controlled from the outside to control the frequency. The low frequency oscillation frequency may be controlled by the frequency control circuit 25.

Next, the output intensities from the two carrier generating oscillators 20 a and 20 b are adjusted to be equal values by the RF output adjusting circuits 24 a and 24 b, respectively, and input to the high frequency switch 26. The high frequency switch 26 switches the output of the 1 / f two carriers generating oscillator with a period of _m by the output of the low-frequency oscillator 22. For example, a microwave PIN switch can be used as the high-frequency switch 26 that performs the above operation.

Thus, from the output terminal of the high frequency switch 26, the average frequency v _offset (i), the maximum frequency deviation v _FM, digital modulation signal of the modulation frequency f _m is obtained.

Example 4
In the first embodiment, the sideband component v _SB (i) equal to the modulation frequency of the optical modulator 3 is controlled to coincide with the optical frequency v _cell (i) of the i-th absorption line. In such a control method, when the optical frequency of the light source is controlled in the ITU grid adjacent to the absorption line, it is necessary to generate a frequency offset of up to 25 GHz in the case of 50 GHz grid and up to 50 GHz in the case of 100 GHz grid by the optical modulator. There is.

  In general, when the drive frequency exceeds approximately 20 GHz, operation characteristics of the carrier generating oscillator 20, the FM modulator 21, the high frequency amplifier 23, the RF output adjustment circuit 24, the optical modulator 3, and the like start to deteriorate. Therefore, it becomes difficult to apply the control method described in the first embodiment.

Therefore, in the present embodiment, by performing control so that the higher-order sideband component generated by the optical modulation matches the optical frequency v _cell (i) of the light absorption cell 4 serving as a reference, Try to solve it.

12A to 12C are diagrams showing the spectrum of the modulation sideband obtained when a phase modulator is used as the optical modulator 4. FIG. In FIG. 12, the horizontal axis indicates the relative value from the optical frequency of the wavelength tunable laser light source 1 input to the optical modulator 3. 12A shows a case where the modulation amplitude is small, FIG. 12B shows a case where the modulation amplitude is adjusted by the RF output adjustment circuit 24 so as to maximize the primary sideband component, and FIG. 12C shows a secondary sideband component. The spectra when the modulation amplitude is adjusted so that (2 · v _offset (i)) is maximized are shown. In order to generate a secondary or higher sideband component, it is sufficient that the primary sideband component can be easily generated.

When an optical frequency stabilized light source is configured by applying this embodiment, the optical frequency stabilized light source is operated while maintaining the frequency relationship shown in FIG. When controlling the optical frequency v _cell (i) of the absorption line focusing on the secondary sideband component, v _out (i) = v _cell (i) + 2 · v _offset (i) (5)
This relationship is used. When control is performed using a third-order sideband component, v _out (i) = v _cell (i) + 3 · v _offset (i) (6)
Use the relationship.

In this embodiment, the deviation of the optical frequency of the high-order sideband components and the light absorption line used to control the first embodiment similarly modulated carrier the maximum frequency shift v _FM, frequency modulated at a frequency f _m and it can be detected by detecting the n · f _m component of the light intensity after passing through the optical frequency cell 4. Here, n indicates the order of the modulation sideband used for optical frequency control.

  In configuring the wavelength tunable optical frequency stabilized light source using the present embodiment, a phase modulator has been described as an example of the optical modulator 3, but even when an intensity modulator or an SSB modulator is used, the modulation spectrum of The operation of making the higher-order modulation sideband component coincide with the optical frequency of the absorption line of the light absorption cell 4 is the same, except that the shape is different from that of the phase modulator.

(Example 5)
FIG. 14 is a diagram showing a configuration of a fifth embodiment of a wavelength tunable optical frequency stabilized light source according to the present invention. Constituent elements that are the same as or similar to those in the above embodiment are indicated by the same or similar numbers. Therefore, the overlapping description is omitted.

  The configuration of the present embodiment is different from the configuration of the second embodiment in that a wavelength locker 40 is provided between the wavelength tunable light source 1 and the beam splitter 2. The configuration of this embodiment can also be applied to the first embodiment, the third embodiment, and the fourth embodiment.

  In the first embodiment, a set of operating parameters of the wavelength tunable laser light source 1 for giving a desired output light frequency is read from the controller 30 and set through the laser driving circuit 13, and then the light absorption cell 4 is used. The optical frequency is stabilized. However, in the initial combination of parameter values, the value of the output optical frequency of the wavelength tunable laser light source 1 may exceed the detection range of the optical frequency shift of the light absorption cell 4 due to the secular variation of the wavelength tunable laser light source. there were. This range varies depending on the type of absorbing material to be used and the filling pressure in the light absorption cell 4, but is generally in the order of several hundred MHz to GHz.

  Therefore, in the present embodiment, first, as in the first embodiment, a set of operation parameters of the reflection variable light source 1 for giving a desired output light frequency is read from the controller 30 and passed through the laser drive circuit 13. Set up. Next, the wavelength locker 40 is operated and controlled using the controller 30 and the laser drive circuit 13 so that the value of the optical frequency monitored by the wavelength locker 40 matches the absorption center of the absorption line of interest. . When the wavelength locker 40 is used, the optical frequency of the wavelength tunable laser light source 1 can be controlled with an optical frequency accuracy of the order of 1 GHz or less even when the secular change of the wavelength locker 40 is taken into consideration. It is possible to stably shift to the optical frequency control according to.

  Many of the wavelength tunable laser light sources 1 that are currently manufactured for the purpose of DWDM optical communication have the wavelength locker 40 built in the light source from the beginning. The wavelength-tunable optical frequency stabilized light source shown in the present embodiment can be realized without change.

It is a figure which shows the 1st Example of the wavelength variable stabilized light source which concerns on this invention. (A) is a diagram showing a wavelength characteristic of transmittance of the light absorption cell filled with ¹² C ¹⁶ O molecules, (b) is a diagram transmittance was larger image normalized for one of a number of absorption lines It is. Is a table showing a relationship between a frequency offset (in GHz) from the absorption line wavelength and ITU grid ¹² C ¹⁶ O molecules. It is a figure for demonstrating the relationship between the transmission spectrum of a gas absorption line, the oscillation light frequency of a wavelength variable laser light source, and the frequency of the modulation sideband produced by an optical modulator. The waveform of f _m frequency component after passing through the light absorbing cell, modulation sideband frequency v _offset (i) and for explaining the magnitude relationship Influence of the optical frequency of the gas absorption line v _cell (i) FIG. It is a figure which shows the relationship between the difference of the optical frequency of modulation | _alteration sideband frequency _voffset (i) and gas absorption line _vcell (i), and the magnitude | size of the _fm frequency component detected with a light receiver. FIG. 6 is a table showing a relationship between a control target optical frequency when a DFB laser array is used as the wavelength tunable laser light source 1, an operation parameter of the DFB laser array for realizing it, and a setting value of an oscillation frequency of the carrier generating oscillator 20. is there. It is a figure which shows the operation | movement order of the wavelength variable optical frequency stabilization light source which concerns on this invention. It is a figure which shows the structure of the 2nd Example of the wavelength tunable stabilization light source which concerns on this invention. It is a figure which shows the structure of the 3rd Example of the wavelength variable stabilized light source which concerns on this invention. In the configuration of the third embodiment of the wavelength tunable stabilized light source according to the present invention, the relationship between the transmission spectrum of the gas absorption line, the oscillation light frequency of the wavelength tunable laser light source, and the modulation sideband frequency generated by the optical modulator It is a figure for demonstrating. It is a figure which shows the example of a spectrum of the high-order modulation sideband at the time of using an optical phase modulator. Explains the relationship between the transmission spectrum of the gas absorption line, the oscillation light frequency of the wavelength tunable laser light source, and the modulation sideband frequency generated by the optical modulator in the fourth embodiment of the wavelength tunable stabilized light source according to the present invention. It is a figure for doing. It is a figure which shows the structure of the 5th Example of the wavelength variable optical frequency stabilization light source which concerns on this invention. It is a figure which shows the structure of the conventional wavelength variable optical frequency stabilization light source.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wavelength variable laser light source 2 Beam splitter 3 Optical modulator 4 Optical absorption cell 5,5a, 5b Photoreceiver 6 Filter 7 Beam splitter 8 Wavelength reference device 9 Switch 10 Amplifier 11 Phase detector 12 Control circuit 13 Laser drive circuit 20, 20a 20b Oscillator for carrier generation 21 FM modulator 22 Low frequency oscillator 23 High frequency amplifier 24, 24a, 24b RF output adjustment circuit 25 Frequency control circuit 26 High frequency switch 30 Controller 31 Current source 32 Temperature control circuit 33 Wavelength stabilization means 34 Peak detection Circuit 40 Wavelength Locker

Claims (6)

A laser light source that outputs continuous wave light capable of changing the oscillation light frequency;
An optical demultiplexer for branching a part of the power of the laser light generated by the laser light source;
An optical modulator that modulates the branched laser light to generate a modulation sideband;
Signal generating means for generating a frequency-modulated electrical signal applied to the optical modulator;
A low-frequency oscillator for controlling the frequency variation period of the electrical signal;
A light-absorbing cell having a plurality of absorption lines existing in the oscillation light frequency range of the laser light source, wherein the laser light that has been modulated and output from the light modulator is input;
A light receiver for detecting the laser light modulated by the light modulator transmitted through the light absorption cell;
A synchronous detector that performs synchronous detection of the output signal of the light receiver in synchronization with the oscillation frequency of the low-frequency oscillator;
Control means for controlling the optical frequency of the laser light source according to the output of the synchronous detector;
A tunable optical frequency stabilized light source comprising: control means for setting an output optical frequency of the laser light source and a carrier frequency of a frequency-modulated electric signal of the signal generating means. A laser light source that outputs continuous wave light capable of changing the oscillation light frequency;
An optical demultiplexer for branching a part of the power of the laser light generated by the laser light source;
An optical modulator that modulates the branched laser light to generate a modulation sideband;
Signal generating means for generating a frequency-modulated electrical signal applied to the optical modulator;
A low-frequency oscillator for controlling the frequency variation period of the electrical signal;
A second optical demultiplexer for branching the power of the laser light from the optical modulator;
Light absorption having a plurality of absorption lines existing in the oscillation light frequency range of the laser light source, wherein one of the laser lights modulated by the optical modulator branched by the second optical demultiplexer is input. Cell,
A first light receiver for detecting a laser beam modulated by the light modulator transmitted through the light absorption cell;
A second photodetector for detecting the other of the laser beams modulated by the optical modulator branched by the second optical demultiplexer;
An operational amplifier for obtaining a difference component between the outputs of the first and second light receivers;
A synchronous detector that performs synchronous detection of the output signal of the operational amplifier in synchronization with the oscillation frequency of the low-frequency oscillator;
Control means for controlling the optical frequency of the laser light source according to the output of the synchronous detector;
A tunable optical frequency stabilized light source comprising: control means for setting an output optical frequency of the laser light source and a carrier frequency of a frequency-modulated electric signal of the signal generating means. The signal generating means for generating the frequency-modulated electrical signal includes:
A high-frequency oscillator that generates a sine wave signal corresponding to the carrier frequency of the modulation signal;
A frequency modulator that operates in synchronization with the output frequency of an external low frequency oscillator;
The wavelength tunable optical frequency stabilized light source according to claim 1, further comprising: an output adjusting unit that adjusts an output amplitude of the frequency modulation signal. The signal generating means for generating the frequency-modulated electrical signal includes:
A first high-frequency oscillator that generates a sine wave signal corresponding to the carrier frequency of the modulation signal;
A second high-frequency oscillator that oscillates while maintaining a constant frequency difference with respect to the oscillation frequency of the first high-frequency oscillator;
A changeover switch that switches between the output of the first oscillator and the second oscillator in synchronization with the output frequency of the low-frequency oscillator;
The tunable optical frequency stabilized light source according to claim 1, further comprising: an output adjusting unit that adjusts an output amplitude of the changeover switch.   The control means for controlling the optical frequency of the laser light source, the modulation sideband component generated by the optical modulator, the higher order component of the optical frequency so as to coincide with the center optical frequency of the absorption line 3. The wavelength tunable optical frequency stabilized light source according to claim 1, wherein the optical frequency of the laser light source is controlled.   3. The wavelength tunable optical frequency stabilized light source according to claim 1, further comprising optical frequency detection means having a periodic detection characteristic with respect to a change in optical frequency of the laser light source.

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