JP5717392B2 - Light source device and imaging device using the same - Google PatentsLight source device and imaging device using the same Download PDF
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The present invention relates to a light source device capable of changing an oscillation wavelength and an imaging device using the same.
As light sources, particularly laser light sources, various types having variable oscillation wavelengths have been used in the fields of communication networks and inspection devices.
In the field of communication networks, high-speed wavelength switching is required, and in the field of inspection equipment, high-speed and wide-range wavelength sweeping is desired.
Applications of the wavelength tunable (sweep) light source in the inspection apparatus include a laser spectrometer, a dispersion measuring instrument, a film thickness measuring instrument, and a wavelength sweep optical coherence tomography (SS-OCT) apparatus.
Optical tomography captures tomographic images of specimens using optical interference, and research in the medical field has become popular in recent years because of the ability to obtain micron-order spatial resolution and non-invasiveness. Technology.
In wavelength-swept optical coherence tomography, spectral interference is used to obtain depth information, and no spectroscope is used. Therefore, it is expected to acquire an image with a high S / N ratio with little light loss.
In the case of configuring a medical imaging apparatus to which the SS-OCT technology is applied, the image acquisition time can be shortened as the sweep speed increases, and the spatial resolution of the tomographic image can be increased as the wavelength sweep width increases. These parameters are important.
Specifically, when the wavelength sweep width is Δλ and the oscillation wavelength is λ0, the depth resolution is
It is represented by Therefore, in order to increase the depth resolution, the wavelength sweep width needs to be expanded, and a broadband wavelength sweep light source is required.
Under such circumstances, as the light source that can be used in the SS-OCT apparatus, the wavelength dispersion of the refractive index in the resonator (hereinafter also simply referred to as “dispersion”) that has been studied mainly in the band used in the communication field is used. Non-Patent Document 1 discloses a dispersion tuning method for changing the wavelength.
In this dispersion tuning, the oscillation wavelength in the active mode-locked state is controlled by using the fact that the free spectral range (hereinafter referred to as “FSR”) of the resonator has wavelength dependency. That is, since the wavelength sweep is performed by changing the frequency of the modulation signal that causes the active mode synchronization, the wavelength sweep can be performed at a high speed by changing the frequency of the modulation signal at a high speed.
Here, the free spectral interval indicates the frequency interval of the resonator mode with respect to the light circulating in the resonator. The free spectral interval (FSR) is expressed by the following equation (2), where c is the speed of light in vacuum, n is the refractive index of the resonator, and L is the resonator length.
The dispersion tuning method is a technique for sweeping the center wavelength at the time of mode lock by sweeping the mode lock frequency by utilizing the fact that the FSR has wavelength dependency.
In Non-Patent Document 1, the wavelength sweep range Δλ by dispersion tuning is expressed by the following equation.
Here, n is the refractive index of the resonator, D is the dispersion parameter of the resonator, and N is the order (natural number) of the mode lock.
S. Yamashita, et al. Opt. Exp. Vol. 14 pp. 9299 (2006)
In the wavelength sweep by the dispersion tuning method disclosed in Non-Patent Document 1, it is possible to increase the wavelength sweep speed by changing the frequency of the modulation signal at high speed.
However, since it is a mode-locked laser, multiple modes with a certain phase relationship oscillate simultaneously, so the spectrum width (line width) of the oscillation spectrum is relatively wide, and for applications that require a narrow spectrum width. The situation is not always enough.
An object of the present invention is to provide a light source device capable of simultaneously achieving narrowing of an oscillation spectrum line width and high-speed wavelength sweep.
A light source device provided by the present invention is a light source device including an optical resonator including an optical amplification medium that amplifies light, an optical waveguide, and an optical modulator that modulates the intensity of light. The optical resonator is configured such that an amplification factor for light circulating around the optical resonator has a maximum value and a minimum value in a plurality of frequency ranges, and the frequency of the light is ν, and the optical resonator The frequency fm for modulating the optical modulator is fm = a × c / (n (ν) where n (ν) is the refractive index of the optical resonator, l is the length of the optical resonator, c is the speed of light, and a is the natural number. Xl) is satisfied, the oscillation frequency changes corresponding to the frequency fm, the oscillation frequency is determined corresponding to the frequency having the maximum value in the plurality of frequency ranges, and the bandwidth of the maximum value is It is characterized by being larger than 2 × fm.
In the light source device of the present invention, the optical resonator is configured such that the amplification factor for the light circulating around the optical resonator takes a maximum value and a minimum value in a plurality of frequency ranges, respectively. That is, the amplification factor with respect to the frequency has a plurality of maximum values and minimum values. Since the frequency fm for modulating the optical modulator satisfies fm = a × c / (n (ν) × l) and the oscillation frequency changes corresponding to the frequency fm, the frequency fm is controlled. It becomes a mode-lock type light source device.
In the light source device of the present invention, since the bandwidth of the maximum value is larger than 2 × fm, the oscillation occurs in a state where the sideband in the vicinity of the oscillation frequency is suppressed, so that the oscillation spectrum wavelength is steep and narrowed. It will be a thing. Since the oscillation frequency is determined corresponding to the frequency having the maximum value in a plurality of frequency ranges, the light source device can change the oscillation wavelength at high speed by controlling the frequency fm.
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing an example of a light source device of the present invention.
In the light source device shown in FIG. 1, an optical resonator 104 is configured including an optical amplifier 101 as an optical amplification medium for amplifying light, an optical waveguide 102, and an optical modulator 103 for modulating the intensity of light. Yes.
Reference numeral 105 denotes a light transmission member that constitutes a part of the optical resonator 104, reference numeral 106 denotes a light extraction coupler, and reference numeral 107 denotes a drive control unit that controls driving of the optical amplifier 101. Reference numeral 108 denotes an optical isolator that is provided as necessary when a ring resonator is configured as an optical resonator and circulates light in one direction.
Here, the optical amplifier 101 will be described by taking a semiconductor optical amplifier (SOA) as an example.
The optical waveguide 102 can be constituted by, for example, a single mode optical fiber whose refractive index has wavelength dependency.
The optical modulator 103 provides an optical signal that provides a modulation signal that modulates the intensity of light (temporally) with respect to a change in transmittance in the optical resonator 104 in order to obtain mode synchronization (hereinafter also referred to as mode lock). An element, for example, an electro-optic element.
The light transmission member 105 is prepared so that the transmittance thereof has a maximum value and a minimum value with respect to a plurality of frequencies. For example, it can be composed of a bulk member or an air gap type etalon plate.
When the frequency dependence of the transmittance of optical elements other than the light transmissive member 105 is not so strong as compared with that of the light transmissive member 105, the spectrum of the amplification factor when the photon makes a round in the optical resonator 106 is the light transmissive member. This is the same as the transmittance spectrum of 105.
The drive control unit 107 is a device for inputting energy into the optical amplifier and controlling the gain thereof, and includes a power supply device and a PC for controlling them.
FIG. 2 shows a light amplification factor spectrum 201 when a photon makes a round in the optical resonator 104 of the light source device of FIG.
In the optical resonator 104 shown in FIG. 1, the gain spectrum of light is basically a product of the gain spectrum of the optical amplifier 101 and the transmittance of another optical member constituting the optical resonator. .
In the case of the apparatus of FIG. 1 in which the light transmitting member 105 is inserted in the optical resonator 104, the amplification factor spectrum 201 takes a maximum value 202 and a minimum value 203 at a plurality of frequencies ν as shown in FIG.
This can be paraphrased as taking a maximum value and a minimum value in each of a plurality of frequency ranges. The relationship between the light frequency ν and the light wavelength λ satisfies νλ = c, where c is the speed of light.
In the light source device of the present invention, the optical resonator is configured so that the amplification factor spectrum of light takes a maximum value and a minimum value in each of a plurality of frequency ranges, and this maximum value is used as an oscillation center frequency at the time of mode-synchronous driving. By suppressing sideband waves (hereinafter also referred to as “sidebands”) generated in the vicinity of the center frequency, the line width of the oscillation spectrum is narrowed by making the oscillation spectrum steep.
A plurality of local maximum values of the light amplification factor spectrum are sequentially driven as the oscillation center frequency, and can be used as a wavelength sweep (wavelength variable) light source.
Hereinafter, details of narrowing of the oscillation spectrum and wavelength sweeping by the light source device of the present invention that performs wavelength sweeping operation by an active mode synchronization method called dispersion tuning will be described.
<Active mode synchronization>
Active mode locking is a technique for obtaining a high-frequency pulse oscillation operation of a laser when a plurality of resonator modes are excited simultaneously (longitudinal multimode oscillation) and their phase relationship is made constant.
In order to determine the longitudinal multimode oscillation and the phase relationship between the modes, typically, a nonlinearity is provided in the optical system of the laser, and some optical modulator is introduced. For example, when the optical modulator is a transmittance control type optical modulator, the sidebands on the low frequency side and the high frequency side of the resonator mode excited first by changing the transmittance to a high frequency by the optical modulator. Excited. When the frequency applied from the optical modulator is ω ′, the sideband is excited to a frequency of ω0 ± ω ′ when the frequency of the resonator mode excited first is ω0.
If ω ′ is equal to the mode interval of the resonator or an integral multiple thereof, the sideband excites the resonator mode next to ω0. In this way, the resonator modes are excited with each other through the sidebands, and longitudinal multimode oscillation is possible.
Further, by introducing the nonlinearity of the optical amplifying medium, nonlinear medium, or optical modulator itself into the resonator, an inter-mode interaction occurs, and the phase relationship between the modes is determined. As a result, the laser oscillates and outputs a pulse train.
In this way, the mode-locked state is forcibly generated by applying external modulation to the resonator.
For example, if the length of the optical resonator is 200 m and the refractive index is about 1.5, the optical frequency of the optical resonator is about 300 m as the modulation frequency applied to the resonator from the outside for mode synchronization. For this reason, the light propagating in this circulates in the resonator at about 1 MHz. Therefore, the resonator mode interval (free spectrum interval FSR) of this resonator is also about 1 MHz.
Therefore, mode synchronization can be obtained by setting the drive frequency of the optical modulator to 1 MHz or an integral multiple thereof. In this state, a pulse train whose repetition frequency is an integer multiple of 1 MHz is generated. Actually, it is preferable to use at a repetition frequency of about 100 to 1000 times in order to stabilize the mode synchronization operation. In this case, modulation for mode synchronization is performed at about 100 MHz to 1 GHz.
Dispersion tuning means that when the refractive index of an optical resonator of a laser that obtains the above mode locking has wavelength dependency, active mode locking is performed by utilizing the wavelength dependency of the FSR of the optical resonator as a result. This is an operation method for changing the oscillation wavelength of the laser.
As described above, active mode locking can be realized by applying modulation of an FSR included in the optical resonator or an integral multiple thereof in the oscillation frequency band. In dispersion tuning, since the FSR has wavelength dependency, the mode-locked oscillation wavelength is changed by changing the modulation frequency.
That is, in the dispersion tuning method, the oscillation wavelength in the mode-locked state is changed by changing the amplification factor of the optical amplification medium or the repetition frequency of the transmittance change in the optical modulator.
When the refractive index of the optical resonator is n (ν) and the resonator length is L, FSR is expressed by the following equation (4).
Active mode locking is obtained by applying amplification factor modulation or transmittance modulation at a frequency fm that is a natural number times (× a) times the FSR, and wavelength sweeping by dispersion tuning is realized by changing fm. As understood from this description, the wavelength-tuned light source of the dispersion tuning system is basically a mode-locked laser.
Here, the case where the refractive index of the optical resonator has a wavelength dependency has been described, but in other words, the optical resonator has a wavelength dispersion with a strong refractive index, and further constitutes the optical resonator. It can also be said that the optical waveguide is composed of a member having a strong refractive index wavelength dispersion.
<Narrowing of oscillation spectrum in the present invention>
FIG. 3 shows a graph of the oscillation spectrum in the mode synchronization operation. In FIG. 3, the oscillation line width 301 is the total line width of the longitudinal multimode oscillation in the mode synchronization operation. In FIG. 3, reference numeral 302 denotes each resonator mode, and 303 denotes an oscillation spectrum.
Here, in a laser device that performs mode-locking operation including a dispersion tuning method, it is useful to suppress the sideband excited near the center frequency during oscillation in order to narrow the spectral line width of the oscillation wavelength. The inventor recognized.
As a technique for suppressing the sideband, it is possible to lower the amplification factor at the frequency of the surrounding sideband with respect to the center frequency.
Specifically, as shown in FIG. 4, the total gain spectrum for the light in the optical resonator has a plurality of maximum values 402 and a minimum value 403 (maximum values in each of a plurality of frequency ranges). Have a local minimum). Then, the center frequency 404 is set to any frequency having the maximum value, and oscillation is performed.
By setting the center frequency of oscillation to the maximum value of the amplification factor, the amplification factor for the sidebands around the oscillation frequency is naturally smaller than the oscillation frequency, so that the sideband is less likely to be excited.
For this reason, the oscillation line width of the light source during the mode-locking operation is consequently narrower than that in the case where the frequency dependence of the gain spectrum is small.
For pulse oscillation, longitudinal multimode oscillation is required, and at least the center frequency and the two nearest sidebands, that is, the simultaneous excitation of three longitudinal modes are necessary. Therefore, if the interval between the sidebands is set to f (since f corresponds to the modulation frequency fm), the maximum value bandwidth needs to be 2 × fm or more.
Here, the bandwidth of the maximum value of the amplification factor will be described with reference to FIG. FIG. 9 is a graph showing the amplification factor spectrum.
When attention is paid to one maximum value 901 in the gain spectrum 903, the gain between the minimum value 904 with the higher gain among the minimum values on both sides of the maximum value 901 and the gain between the gains of the maximum value 901 are intermediate. The frequency width of the amplification factor is defined as a bandwidth 902.
For example, assuming image acquisition using an OCT apparatus, when the coherence length necessary for the light source for OCT image acquisition is set as b, the upper limit value of the oscillation line width of the light source is c when the speed of light is set as c. You can write / b.
Therefore, it is preferable to suppress the oscillation line width of the light source to c / b or less. For this purpose, it is also preferable to set the bandwidth of the maximum value of the amplification factor to c / b or less.
For example, considering that the SS-OCT apparatus is applied particularly to fundus measurement, it is preferable that a tomographic image having a depth of 2 mm or more in the depth direction can be acquired. In order to acquire a tomographic image over a depth direction of 2 mm, a coherence length of 4 mm, which is twice as long as the depth direction of 2 mm, is required.
Further, in order to separate the interference signal from various low-frequency noises riding on the interference spectrum, it is preferable to measure by making the difference between the distance to the object to be measured and the distance to the reference mirror of the interference system doubled to 8 mm. . In other words, a coherence length of 8 mm is preferable. The line width corresponding to a coherence length of 8 mm is about 38.5 GHz in frequency. It is preferable to oscillate with a line width less than this value.
Here, when the refractive index of the Fabry-Perot etalon member is n, the thickness is d, the reflectance of both end faces is R, and the light velocity is c, the transmission bandwidth Δf of the etalon is expressed by the following equation (5). It is.
At this time, FSR (free spectral range) is expressed by the following equation (6).
For example, when R = 0.5 and the optical thickness of the etalon is n × d = 4.38 mm, Δf = 7.7 GHz. When the wavelength of light is 840 nm, it is about 0.02 nm in terms of wavelength width. The FSR is 0.08 nm.
Due to its characteristics, the etalon has an equal frequency interval. This is preferable when the light source of the present invention is used for an SS-OCT apparatus.
When a tomographic image of a test object is measured by optical interference using an SS-OCT apparatus, the measured data is a broadband interference spectrum generated by interference between reflected light from the test object and reflected light from another reference mirror. It is. The tomographic structure of the test object is calculated by Fourier transforming this interference spectrum.
Therefore, for the Fourier transform, it is most preferable to measure the interference spectrum at equal frequency intervals and directly Fourier transform the obtained value. Conversely, when the interference spectrum cannot be measured at equal frequency intervals, it is necessary to complement the interference spectrum data at unequal frequency intervals, calculate the interference spectrum data at equal frequency intervals, and then perform the Fourier transform. That is, since an error occurs in the complementing process, it is preferable to acquire interference spectrum data at equal frequency intervals that do not require a complementing operation.
The light source of the present invention is a light source that performs discrete wavelength variable operation at equal frequency intervals and is not a light source that performs continuous wavelength sweep operation. However, as described above, the OCT apparatus performs discrete Fourier transform on interference spectrum data acquired as final data processing. Therefore, since the interference spectrum data may be discrete, the wavelength sweep of the light source for obtaining the spectrum may be performed at discrete wavelength intervals.
The light source of the present invention performs such a discrete wavelength sweeping operation, and in particular, can be configured such that the wavelength sweeping steps are at equal frequency intervals. Such a light source is suitable as a wavelength swept light source for an OCT apparatus.
Here, it is also preferable that the minimum value of the amplification factor spectrum is less than 1 (less than 1 at the frequency at which the amplification factor takes the minimum value). Since the amplification factor is less than 1, no laser oscillation occurs at that frequency. That is, it is impossible to excite the sideband of the mode synchronization operation at a frequency with an amplification factor lower than 1.
When the oscillation frequency is set to a maximum value sandwiched between two minimum values with an amplification factor value less than 1, it is easy to limit the maximum value of the oscillation line width to less than the maximum line width 505 as shown in FIG. It is suitable. In FIG. 5, the amplification factor is 1 or more at the frequency at which the maximum value is obtained.
The optical element that can be used to give a plurality of maximum values to the overall gain spectrum in the optical resonator is not limited to the Fabry-Perot etalon (etalon plate) as described above.
In addition, a polarization beat generated when a polarization maintaining fiber is used may be used.
The polarization beat is a phenomenon in which, when light is incident on a birefringent material and the transmitted light is observed with a specific polarization, the beat is superimposed on the transmittance spectrum.
Here, the polarization maintaining fiber is an optical fiber that can transmit while maintaining the incident polarization state, and causes a propagation constant difference between the two polarization modes, so that each polarization mode is switched to the other polarization mode. This is a fiber that suppresses the coupling and enhances the polarization maintaining ability.
For example, a polarization-maintaining fiber such as a panda type (PANDA: Polarization-Maintenance AND Absorption-Reducing) medium has a slow axis and a fast axis, and the polarization direction of incident linearly polarized light is between the slow axis and the fast axis. If they do not coincide with each other, a phase difference occurs between the polarization component incident on the slow axis and the polarization component incident on the fast axis, according to the distance that light propagates through the polarization maintaining fiber.
If this phase difference is an integral multiple of 2π, the polarization direction of the propagating light is linearly polarized light having the same polarization direction as that of the incident light. If the phase difference is an odd multiple of π, the polarization direction of the propagating light is different from the polarization direction of the incident light, that is, linearly polarized light obtained by inverting the polarization direction of the incident light with respect to a certain main axis.
Here, the length of the polarization-maintaining fiber is set so that a phase difference of 2π is obtained after light propagation with respect to incident light of a certain wavelength, and the transmittance of the propagating light is measured through the polarizer at the output end of the fiber. Then, if the polarizer is arranged so as to transmit only the same polarization component as the polarization component of the incident light, the polarization direction of the outgoing light and the polarization direction through which the polarizer transmits are the same. Therefore, the transmittance at that wavelength shows a value close to 1.
However, when the wavelength of the propagating light is different, the phase difference of 2π is generated, so that the necessary fiber propagation distance is basically proportional to the wavelength.
In other words, even with the same fiber length, the phase difference generated for the slow-axis polarization component and the fast-axis polarization component after propagation differs depending on the wavelength. Therefore, even if there is a phase difference of 2π with respect to a certain wavelength, it may be π at another wavelength. When the phase difference is π, the polarization component of the emitted light is linearly polarized light orthogonal to the incident light, and thus the transmittance is close to 0 in the above system.
Thus, when propagating through a certain length of polarization-maintaining fiber, there are wavelengths with high and low transmittance, so that the transmittance spectrum has beats at substantially equal frequency intervals.
This is a polarization beat.
In order to generate a polarization beat, a member exhibiting birefringence and a polarizer are necessary. The member exhibiting birefringence is not limited to the polarization-maintaining fiber as described above, and an optical crystal having birefringence and other optical elements may be used.
Any polarizer can be used as long as it is an optical element having different transmittance depending on the polarization direction, in addition to a normal polarizer. For example, a semiconductor optical amplifier having a rectangular cross-sectional shape has a different amplification factor with respect to an incident polarization direction, and thus can function as a polarizer for generating the above-described polarization beat.
When the polarization of the light after propagating through the birefringent medium becomes linearly polarized light, the polarization direction is the same polarization direction as the incident light or a polarization direction obtained by inverting the polarization direction of the incident light with respect to a certain principal axis direction.
Setting the angle between these two linearly polarized states to 90 degrees is preferable because it makes it possible to increase the extinction ratio by the polarizer and consequently contribute to deepening the fringe of the polarization beat. In order to make the angle between the two linearly polarized states 90 degrees, the direction of the linearly polarized light incident on the birefringent medium is incident so that the angle formed by the two principal axes of the birefringent medium is just halved. do it.
Here, when the speed of light is c, the length of the birefringent medium is L, and the difference in refractive index between the two main axes is Δneff, the fringe frequency interval Δν on the transmittance spectrum due to the polarization beat is expressed by the following formula ( 7) is satisfied.
As described above, by providing the transmittance spectrum with a plurality of maximum and minimum values using an etalon plate or a birefringent medium, it is possible to narrow the oscillation line width during the mode lock operation.
<Other forms that can be adopted>
In the above, a semiconductor optical amplifier (SOA) has been described as an example of an optical amplifying medium. In addition to this, as an optical amplifying medium, a rare earth-doped (ion-doped) optical fiber containing erbium, neodymium, or the like, or a dye in an optical fiber. Those that are added and amplified with a dye can be employed.
The rare earth-doped optical fiber is suitable for obtaining good noise characteristics with high gain. In the dye-doped optical fiber, the choice of the variable wavelength is increased by appropriately selecting the fluorescent dye material or its host material.
The semiconductor optical amplifier is preferable because it is small and can be controlled at high speed. As the semiconductor optical amplifier, both a resonator type optical amplifier and a traveling waveform optical amplifier can be used. As a material constituting the semiconductor optical amplifier, a compound semiconductor constituting a general semiconductor laser can be used. Specifically, compounds such as InGaAs, InAsP, GaAlSb, GaAsP, AlGaAs, and GaN are used. A semiconductor can be mentioned. The semiconductor optical amplifier can be appropriately selected and employed from among those having a gain center wavelength of, for example, 840 nm, 1060 nm, 1300 nm, and 1550 nm according to the use of the light source.
In the present invention, an optical waveguide can be basically used as long as it has a function of propagating light and wavelength dispersion, but a slab waveguide that confines and propagates light to suppress external influences as much as possible. Alternatively, it is preferable to use an optical fiber. A waveguide for confining and propagating light basically has a portion with a high refractive index (core) and a portion with a low refractive index (cladding). In order to obtain a finely spaced FSR, a relatively long resonator is used. From this point of view, it is preferable to use an optical fiber. This is because, from the principle of the dispersion tuning method, the pitch with which the oscillation wavelength is selected becomes finer when the FSR interval is smaller. Examples of the optical fiber include those using quartz (SiO 2 ) glass, those using plastic, and those using both quartz and plastic.
In the present invention, the dispersion value of chromatic dispersion possessed by the optical waveguide is an optical amplifying medium employed from normal dispersion (dispersion value is negative) to anomalous dispersion (dispersion value is positive), sweep speed, A predetermined dispersion value can be appropriately employed in consideration of the sweep wavelength range and the like.
Examples of the optical modulator include a waveguide type modulator capable of high-speed modulation. Specific examples include an LN intensity modulator (using a LiNbO 3 substrate) using an electro-optic effect (Pockels effect) and an electroabsorption type. An optical modulator (EA modulator) may be mentioned. The LN intensity modulator performs ON / OFF control of light by a change in interference state obtained by changing the refractive index of one optical path with a configuration including an interferometer, and is excellent in high-speed control.
An electroabsorption optical modulator is an intensity modulator that utilizes the fact that the absorption edge of a semiconductor shifts when an electric field is applied, and is small in size and capable of operating at a low voltage.
As the optical resonator that can be employed in the present invention, a linear resonator, a σ-type resonator, or the like can be employed in addition to the above-described ring resonator. As the ring resonator, in addition to a resonator using an optical fiber, a slab waveguide, a mirror using an optical system that propagates light in the air using a mirror, or the like can be adopted.
Examples of the linear resonator include an optical resonator having a pair of parallel planes (a so-called Fabry-Perot resonator), and a resonator having a linear shape with the end face of the optical fiber as a mirror.
Hereinafter, the present invention will be described in detail with specific examples.
FIG. 6 shows a schematic diagram of the light source device of this embodiment.
The light source device shown in FIG. 6 includes an optical resonator including a semiconductor optical amplifier 601, an isolator 608, an LN intensity modulator 603, an optical fiber 603 as an optical waveguide, a coupler 604, and an etalon plate 605 as a light transmission member. ing. A driving power source 607 is connected to the semiconductor optical amplifier 601, and the driving power source 607 is controlled in injection current and amplification factor by a signal sent from the control unit 609.
The refractive index of the optical fiber 603 has a large wavelength dependency (frequency dependency), and the refractive index is smaller as the wavelength is longer. For this reason, the FSR of the entire optical resonator also has a frequency dependency, and a dispersion tuning operation that makes the oscillation wavelength variable by controlling the modulation signal is possible.
The semiconductor optical amplifier has a gain (gain) between wavelengths of 800 nm and 880 nm, and the optical resonator length including the optical waveguide is 100 m. The optical fiber as the optical waveguide is composed of a single mode fiber.
The LN intensity modulator 603 is driven to modulate the transmittance of the modulator at high speed to obtain active mode synchronization. When the average refractive index of the entire optical resonator is 1.46, the effective resonator length is 146 m.
Therefore, the FSR of the entire optical resonator is 2.053 MHz.
The frequency of optical modulation for applying active mode locking is set to an integral multiple of FSR. For example, when the frequency is set to 500 times the FSR, the frequency of optical modulation is 1.027 GHz.
By changing the modulation frequency of the LN intensity modulator 603, the oscillation wavelength is swept from a wavelength of 800 nm to 880 nm. The sweep cycle is 10 kHz, and the pulse rate is 1.027 GHz corresponding to the above-described optical modulation frequency.
The etalon plate used as the light transmitting member has an end face reflectance R = 0.5, a refractive index of 1.46, and a physical thickness d = 3 mm. Here, the transmission band of the etalon plate is 7.7 GHz. Focusing on the oscillation wavelength of the light source of this example, 840 nm, the transmission band is about 0.02 nm in terms of wavelength width. The FSR of the etalon plate itself is 0.08 nm.
The oscillation wavelength changes corresponding to the modulation frequency of the optical modulator. In the light source of the present invention, the center frequency is set by setting the oscillating center frequency to a plurality of maximum values included in the amplification factor spectrum of the entire optical resonator incorporating the light transmitting member such as the etalon plate. The amplification factor for the surrounding sidebands is suppressed, and as a result, the oscillation line width can be narrowed.
Further, since the gain spectrum of the entire optical resonator changes due to changes in the temperature of the semiconductor optical amplifier, the amount of injected current, the temperature of each optical element, etc., the gain spectrum of the light source of the present invention also changes depending on the driving conditions. . Therefore, it is preferable to measure the amplification factor spectrum in advance before the operation and to verify in advance the center frequency of the oscillation for operating the light source.
In the above description, a single mode fiber is used as an optical waveguide and an etalon plate is used as a light transmission member. However, the light source device of the present invention is not limited to this configuration.
A configuration is also adopted in which a polarization maintaining fiber is used for the optical waveguide, and a polarizer having a transmission axis set in a direction that bisects the angle formed by the slow axis and the fast axis of the polarization maintaining fiber is inserted after the polarization maintaining fiber. can do.
Alternatively, a polarizer may not be inserted, and an optical amplifier having an amplification factor that is polarization-dependent can be used instead. In this case, for example, it is also preferable that the cross-sectional shape of the active layer of the optical amplifier is rectangular, and the long side direction is set to divide the angle formed by the two main axes of the fiber into two. With such a configuration, a polarization beat is generated. As a result, a beat having a maximum value of the amplification factor at a plurality of frequencies can be generated in the amplification factor spectrum of the entire optical resonator.
Further, in this embodiment, the transmittance is temporally modulated by the optical modulator, and as a result, the amplification factor in the entire optical resonator is temporally modulated. However, the modulation method in the optical resonator is not limited to this, The function of the optical modulator may be replaced with an optical amplifier. That is, it is possible to adopt a method of obtaining active mode locking by temporally modulating the amplification factor of the entire optical resonator by temporally modulating the amount of current injected into the optical amplifier.
In this embodiment, an example of an optical coherence tomography apparatus using the light source of the present invention is shown.
FIG. 7 is a schematic diagram of the OCT apparatus of this example.
The OCT apparatus of FIG. 7 basically includes a light source unit (701, etc.), a sample measuring unit (707, etc.) that irradiates the sample with light from the light source unit and transmits reflected light from the sample unit, and a light as a reference mirror. A reference unit (702, etc.) that transmits the reflected light from the reference mirror, an interference unit (703) that causes the two reflected lights to interfere, and a light detection unit (709, etc.) that detects the interference light obtained by the interference unit ), And an image processing unit (711) that performs image processing (obtains a tomographic image) based on the light detected by the light detection unit. Hereinafter, each component will be described.
The light source unit includes a tunable light source 701 and a light source control unit 712 that controls the tunable light source. The tunable light source 701 includes a fiber coupler 703 that forms an interference unit via an optical fiber 710 for light irradiation. It is connected to the.
The fiber coupler 703 of the interference unit is configured by a single mode in the wavelength band of the light source, and various fiber couplers are configured by 3 dB couplers.
The reflection mirror 704 is connected to the reference light optical path fiber 702 to form a reference unit, and the fiber 702 is connected to the fiber coupler 703.
The inspection light optical path 705 fiber, the irradiation condensing optical system 706, and the irradiation position scanning mirror 707 constitute a measuring unit, and the inspection light optical path 705 fiber is connected to the fiber coupler 703. In the fiber coupler 703, the backscattered light generated from the inside and the surface of the inspection object 714 interferes with the return light from the reference unit to become interference light.
The light detection unit includes a light receiving fiber 708 and a photodetector 709, and guides interference light generated by the fiber coupler 703 to the photodetector 709.
The light received by the photodetector 709 is converted into a spectrum signal by the signal processing device 711, and further subjected to Fourier transform to acquire depth information of the test object. The acquired depth information is displayed on the image output monitor 713 as a tomographic image.
Here, the signal processing device 711 can be configured by a personal computer or the like, and the image output monitor 713 can be configured by a display screen of a personal computer or the like.
A characteristic of the present embodiment is a light source unit, and the wavelength tunable light source 701 is controlled by a light source control device 712 for its oscillation wavelength and intensity and its temporal change.
The light source control device 712 is connected to a signal processing device 711 that also controls a drive signal and the like of the irradiation position scanning mirror 707, and controls the variable wavelength light source 701 in synchronization with the drive of the scanning mirror 707.
The wavelength tunable light source 701 using the light source device of the present invention has a narrow line width during wavelength sweeping, and can acquire an interference image from a position equidistant to a reference mirror at the time of optical coherence tomography. Become. To explain this somewhat, the narrow spectral width of the oscillation wavelength in the wavelength sweep corresponds to a long coherence length, that is, a long coherence distance. As a result, an interference signal can be obtained even if the optical path length difference between the two optical paths constituting the interference optical system is long. That is, the OCT apparatus using the light source device of the present invention having a narrow line width of the oscillation spectrum has an effect that it can detect a deep structure of the object to be inspected.
In this embodiment, an example of an optical tomographic imaging apparatus having an optical system for differentially detecting interference signals will be described. The optical tomographic imaging apparatus of the present embodiment is schematically shown in FIG. 8, and the same parts as those in the apparatus shown in FIG.
The apparatus shown in FIG. 8 is different from the apparatus shown in FIG. 7 in that a balance photo detector 710 having both a photodetector and a differential amplifier and fiber couplers 703 and 704 are incorporated in place of the photo detector 709 shown in FIG.
The balance photodetector 710 has a signal processing unit 711 connected to one end and two terminals on the other end. One of the terminals is connected to the optical coupler 703 via the fiber 716, and the remaining one terminal is connected to the optical coupler 705 constituting the coupling portion via the fiber 717 and the optical coupler 704.
With this connection, the apparatus of the present embodiment divides the interference signal due to the reflected light from the measurement object 714 and the reference mirror 704 into two parts, and detects the differential between one and the other.
By dividing the light into two before reaching the balance photodetector 710, the phase of the interference signal becomes opposite. Therefore, when the two are subtracted, only the DC component contained in the signal before the division is removed, and only the interference signal is obtained. Is preferable.
In the figure, 702 is an isolator, and 718 and 719 are polarization controllers.
It is also possible to sequentially monitor the intensity of light emitted from the light source 701 and use the data for amplitude correction of the interference signal.
DESCRIPTION OF SYMBOLS 101 Optical amplification medium 102 Optical waveguide 103 Optical modulator 104 Optical resonator 201 Amplification factor spectrum 202 Maximum value 203 Minimum value
- A light source device including an optical resonator including an optical amplification medium that amplifies light, an optical waveguide, and an optical modulator that modulates the intensity of the light, wherein the optical resonator includes the optical resonator The amplification factor for the light circulating around the resonator has a maximum value and a minimum value in a plurality of frequency ranges, respectively, wherein the frequency of the light is ν, the refractive index of the optical resonator is n (ν), The frequency fm for modulating the optical modulator, where the length of the optical resonator is l, the speed of light is c, and the natural number is a, satisfies fm = a × c / (n (ν) × l), and the frequency fm And the oscillation frequency is determined corresponding to the frequency that takes the maximum value in the plurality of frequency ranges,
The amplification factor at the oscillation frequency and the frequency of the light of the two sidebands closest to the oscillation frequency is 1 or more, and the amplification factor of the oscillation frequency is the amplification factor of the frequency of the light of the two sidebands Bigger than
Of the two local minimum values adjacent to the local maximum value, the frequency range of the amplification factor at an intermediate amplification factor between the local minimum amplification factor and the local maximum amplification factor is greater than 2 × fm. light source device it is greater.
- The light source device according to claim 1, wherein the optical waveguide has a refractive index wavelength dispersion.
- The light source device according to claim 1, wherein the optical resonator includes a light transmission member.
- The light source device according to claim 3 , wherein the light transmitting member is a Fabry-Perot etalon.
- A member on which the optical waveguide exhibits birefringence, the light source apparatus according to claim 1 or 2, characterized in that it comprises a polarizer.
- When the light in the resonator is incident on the member exhibiting birefringence, the light has polarization components in two principal axis directions of the member, and the transmission axis of the polarizer is the same as any of the two principal axes of the member. The length L of the member satisfies the following equation, where Δν is the frequency interval of polarization beats generated in the resonator and Δneff is the difference between the refractive indexes of the two principal axes of the member. The light source device according to claim 5 .
- Said member indicating the birefringence, the light source apparatus according to claim 5 or 6, characterized in that a polarization maintaining fiber.
- The light source device according to claim 5 , wherein the optical amplification medium also serves as the polarizer.
- The light source device according to claim 1, wherein the frequency at which the amplification factor takes the maximum value is an equal frequency interval.
- The light source device according to claim 1, wherein the optical amplification medium also serves as the optical modulator.
- 2. The light source device according to claim 1, wherein the amplification factor is less than 1 at a frequency at which the minimum value is taken and is 1 or more at a frequency at which the maximum value is taken.
- A light source unit using the light source device according to any one of claims 1 to 11 ,
A sample measurement unit that irradiates the sample with light from the light source unit and transmits reflected light from the sample;
A reference unit for irradiating a reference mirror with light from the light source unit and transmitting reflected light from the reference mirror;
An interference unit that causes reflected light from the specimen measurement unit and reflected light from the reference unit to interfere with each other;
A light detection unit for detecting interference light from the interference unit;
An image processing unit that obtains a tomographic image of the specimen based on the light detected by the light detection unit;
An optical coherence tomographic imaging apparatus comprising:
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|JP2010225992A JP5717392B2 (en)||2010-10-05||2010-10-05||Light source device and imaging device using the same|
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|JP2010225992A JP5717392B2 (en)||2010-10-05||2010-10-05||Light source device and imaging device using the same|
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|JP2604479B2 (en) *||1990-02-05||1997-04-30||日本電信電話株式会社||Mode-locked laser device|
|JP2772360B2 (en) *||1992-08-10||1998-07-02||日本電信電話株式会社||Mode-locked optical fiber laser device|
|JP3363950B2 (en) *||1993-06-28||2003-01-08||日本電信電話株式会社||Mode-locked laser device|
|JPH08148747A (en) *||1994-11-24||1996-06-07||Fuji Photo Film Co Ltd||Solid-state laser apparatus|
|JP4951762B2 (en) *||2007-03-29||2012-06-13||国立大学法人東北大学||Pulsed laser light source|
|JP4916427B2 (en) *||2007-12-18||2012-04-11||キヤノン株式会社||Laser beam scanning device|
|JP5495506B2 (en) *||2008-05-13||2014-05-21||キヤノン株式会社||Laser apparatus and optical tomographic imaging apparatus|
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