JP2017092070A - Wavelength sweeping light source - Google Patents

Abstract

A wavelength-swept light source that enables high-speed wavelength sweep without shortening the coherence length of the light source. A wavelength swept light source includes a light source including a first optical deflector that temporally changes a transition of an oscillation wavelength, a demultiplexer that demultiplexes light emitted from the light source, and demultiplexing. The delay unit 103 that delays the output of the optical filter and the output of the duplexer or the output of the delay unit are switched at a predetermined timing so that the fluctuation period of the oscillation wavelength is faster than the fluctuation period, and KTN (KTa1-xNbxO3: And an optical switch unit 104 including a second optical deflector using a 0 ≦ x ≦ 1) crystal or a KLTN (K1-yLiyTa1-xNbxO3: 0 ≦ x ≦ 1, 0 <y <1) crystal. [Selection] Figure 1

Description

  The present invention relates to a wavelength swept light source including an optical deflector using an electro-optic crystal that changes a transition of an oscillation wavelength with time.

  The wavelength swept light source is used as a light source for optical coherence tomography (OCT) for observing a cross section of a living body, an electronic device or the like in a non-destructive manner, or an optical sweep ranging. The observation time is shortened by increasing the wavelength sweep speed.

  Conventionally, when a wavelength swept light source is applied to OCT, the observation range in the surface of the test object can be expanded without losing the resolution in the depth direction, or a high-speed phenomenon in the living body can be observed. It is expected to be possible (Non-Patent Document 1).

  Further, the conventional wavelength swept light source performs wavelength selection by changing the direction of light output from the semiconductor optical amplifier (SOA) by a KTN optical deflector and changing the angle at which the light enters the diffraction grating. . In this case, since the operating speed of the KTN optical deflector is high, high-speed wavelength sweep is realized (Non-Patent Document 2).

Shogo Yagi, Tadayuki Imai, Yasuo Shibata, Shigeo Ishibashi, Masahiro Sasaura, Kazutoshi Kato, Kazunori Naganuma, Yuzo Sasaki, and Kazuo Fujiura, “A mechanical-free 150-kHz repetition swept light source incorporated a KTN electro-optic deflector,” Proc SPIE 7889, 78891J (2011) Shogo Yagi, Kazunori Naganuma, Tadayuki Imai, Yasuo Shibata, Jun Miyazu, Masahiro Ueno, Yuichi Okabe, Yuzo Sasaki, Kazuo Fujiura, Masahiro Sasaura, Kazutoshi Kato, Masato Ohmi and Masamitsu Haruna, “Improvement of Coherence Length in a 200-kHz Swept Light Source equipped with a KTN Deflector, ”Proc. SPIE 8213, 821333 (2012)

  In the conventional wavelength swept light source, when a sinusoidal drive voltage is applied to the KTN optical deflector, the temporal change of the oscillation wavelength generally outputs sinusoidal light. When such a wavelength swept light source in which the oscillation wavelength periodically increases and decreases with the passage of time is applied to the OCT, conventionally, a time zone in which the oscillation wavelength increases with the passage of time (or vice versa, Only the output light in the time zone during which the oscillation wavelength decreases was used for observation of the test object. Therefore, in conventional OCT, it is possible to observe only half of the emission time of the wavelength swept light source. As a result, in order to observe the test object at high speed with OCT, the wavelength swept light source It was necessary to sweep the wavelength at a high speed.

  In a conventional wavelength swept light source, the wavelength sweep speed is determined by the operating speed of the KTN optical deflector. However, if the operating speed of the KTN optical deflector is increased, the deflection angle decreases due to the heat generated by the KTN chip, and the swept wavelength range There was also a problem that it narrowed. Furthermore, by increasing the operating speed of the KTN optical deflector, the number of times that light of a specific wavelength circulates within the resonator of the light source is reduced, and as a result, the coherence length, which is an important parameter of light source characteristics, is shortened. There was a problem of becoming.

  The present invention has been made in view of such problems, and an object thereof is to provide a wavelength-swept light source that enables high-speed wavelength sweep without shortening the coherence length of the light source.

In order to solve the above problems, the present invention provides a light source including a first optical deflector that temporally changes a transition of an oscillation wavelength, a demultiplexer that demultiplexes light emitted from the light source, A delay device that delays the output of the demultiplexer, and the output of the demultiplexer or the output of the delay device are switched at a predetermined timing so that the fluctuation period of the oscillation wavelength is faster than the fluctuation period. KTa _1-x Nb _x O ₃ : 0 ≦ x ≦ 1) crystal or KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ : 0 ≦ x ≦ 1, 0 <y <1) crystal was used And an optical switch unit composed of a second optical deflector.

  In the first optical deflector, a voltage driving waveform for high-speed deflection may be a sine wave or a triangular wave.

  A voltage for high-speed deflection may be applied to the first optical deflector so that the time change of the wave number of the emitted light becomes a triangular wave.

  The delay device may be an optical fiber spool.

  According to the present invention, high-speed wavelength sweep can be performed while maintaining the coherence length of the light source.

It is a schematic diagram which shows the structural example of the wavelength sweep light source in embodiment of this invention. It is a figure which shows the structural example of a light source. It is a figure for demonstrating the principle of the switching operation | movement in an optical switch. It is a figure for demonstrating the mode of switching implement | achieved by the wavelength sweep light source of embodiment. It is a figure for demonstrating the mode of the voltage applied to the KTN optical deflector of the optical switch at the time of switching operation | movement.

  Hereinafter, the wavelength swept light source 10 according to an embodiment of the present invention will be described. The wavelength swept light source 10 changes the transition of the oscillation wavelength in a sine wave shape over time.

[Overall configuration of wavelength swept light source]
FIG. 1 is a schematic diagram illustrating a configuration example of the wavelength sweeping apparatus 10.

As shown in FIG. 1, the wavelength swept light source 10 includes a light source 101 and an optical device 201. The optical device 201 includes an optical coupler (demultiplexer) 102, a delay unit 103, and a 1 × 2 optical switch (optical switch unit) 104. In this embodiment, the light source 101 is, for example, a KTN (KTa _1-x Nb _x O ₃ : 0 ≦ x ≦ 1) light deflector or KLTN (K _1-y Li _y Ta _1-x Nb _x ) as an electro-optic crystal. O ₃ : 0 ≦ x ≦ 1, 0 <y <1) An optical deflector is used. The configuration of the light source 101 will be described in detail with reference to FIG.

  The optical coupler 102 demultiplexes the light emitted from the light source 101. The delay device 103 delays the light emitted from the light source 101. The delay device 103 is, for example, an optical fiber spool. As will be described later, in the wavelength swept light source 10 of this embodiment, when the wavelength sweep period (oscillation wavelength) of light is T, the delay unit 103 delays by half that T / 2.

  The optical switch 104 includes an optical deflector 31 to be described later, and the light emitted from the optical coupler 102 via the upper arm 40 and the light delayed by the delay unit 103 and emitted from the delay unit 103 via the lower arm 50. Switch between light. The switched light propagates through the transmission medium 105 as an optical waveguide or an optical fiber. The switching process will be described later in detail.

[Configuration of light source]
FIG. 2 is a diagram illustrating a configuration example of the light source 101.

  As shown in FIG. 2, the light source 101 includes a semiconductor optical amplifier 11, a collimating lens 12, an optical deflector 13 using KTN or KLTN, and a control voltage source 14. Further, the light source 101 includes a diffraction grating 15, an end mirror 16, a condenser lens 17, and an output coupling mirror 18.

  In the light source 101, the direction of light of the output light from the semiconductor optical amplifier 11 is changed, and the angle at which the light is incident on the diffraction grating 15 is changed. Then, only the light having a specific wavelength corresponding to the incident angle is reflected by the end mirror 16, and the light is recombined by the semiconductor optical amplifier 11, laser-oscillated, and given to the optical device 201 as output light 1. Can do. In this case, the wavelength of laser oscillation can be changed at high speed by changing the angle at which the optical deflector 13 is incident on the diffraction grating 15 at high speed.

[Coherence length of light source]
Next, the coherence length of the light source 101 will be described.

  In general, the coherence length lc is known to be represented by the following formula (1) (see, for example, Non-Patent Document 2).

In Equation (1), λ: wavelength, L: resonator length, W _T : wavelength sweep width, F: wavelength sweep frequency, W _C : bandwidth of diffraction grating.

The bandwidth W _C of the diffraction grating is expressed by the following formula (2) from the formula (1).

  In equation (2), Λ: period of the diffraction grating, w: beam radius.

  It can be seen that in the light source 101, when the wavelength sweep frequency is increased, the coherence length lc is shortened according to the above equation (1).

  In the wavelength sweep light source 10 of this embodiment, the wavelength sweep frequency can be increased by the optical device 201 while the wavelength sweep frequency in the light source 101 is fixed (without being changed). That is, the coherence length is not shortened, and only the wavelength sweep frequency can be increased.

  When a wavelength swept light source is used as a light source for optical coherence tomography (OCT), this method is called SS-OCT (Swept-Source OCT). This SS-OCT is based on the coherence length. Is determined.

  For this reason, the wavelength swept light source 10 of the present embodiment is suitable as an OCT light source of the above SS-OCT system because the coherence length is not shortened as the wavelength sweep speed increases.

[Jitter in light source]
In the light source 101, time fluctuation of the output wavelength occurs, which will be referred to as jitter in the following description.

  In the wavelength swept light source 10 of the present embodiment, especially when the number N of additional optical systems is large, the end portion of the swept wavelength region is selected due to the influence of jitter, and the temporal wavelength variation is considered to be small. It is done. Therefore, the light source 101 with small jitter is preferable. However, since the jitter is very small at 100 psec or less in the light source 101, a light source using a KTN optical deflector or a KLTN optical deflector is used to increase the wavelength sweep speed by the additional optical system. Is more preferable.

[Configuration of optical deflector]
Next, a configuration example of the optical deflector 13 will be described.

  The optical deflector 13 is composed of a rectangular parallelepiped electrochemical crystal (KTN or KLTN) and electrodes formed on two opposing surfaces thereof. Light applied to the electrochemical crystal by applying a voltage to the electrodes is To deflect. The deflection angle θ is expressed by the following formula (3) (provided that the applied voltage is alternating current and high speed (mainly several tens of kHz or more)).

In equation (3), n: refractive index, g ₁₁ : secondary electro-optic coefficient, e: elementary electric charge, N: electron density stored in advance in the electrochemical crystal, ε: dielectric constant, L: The length in the crystal where incident light travels in the traveling direction, V: applied voltage, d: distance between electrodes.

  When a sufficiently low frequency (1 kHz or less) voltage is applied to the optical deflector 13, electrons are injected into the crystal and trapped in an electron trap in the crystal. As a result, electrons are accumulated in advance.

The density of the electron trap is about 10 ²⁰ to 10 ²¹ m ⁻³ , and it is considered that the low frequency voltage is applied and the electrons are uniformly captured in the crystal.

  Near the phase transition temperature, since it has a very large dielectric constant (relative dielectric constant tens of thousands), the deflection angle (full angle) reaches about 10 °.

[KTN or KLTN crystal]
The crystal phase of the KTN or KLTN crystal undergoes phase transition from the high-temperature phase to cubic, tetragonal, orthorhombic and rhombohedral. Cubic crystals exhibit paraelectric properties, and the others exhibit ferroelectricity.

  Depending on the Ta / Nb ratio and the Li addition amount (only in the case of KLTN crystal), the temperature at which the phase transition from the cubic crystal to the tetragonal crystal can be changed, but the KTN or KLTN crystal in the paraelectric phase is cubic. It is assumed to be used.

Since the lowest-order electro-optic effect in the paraelectric phase is the second-order electro-optic effect (Kerr effect), the refractive index change amount Δn ₁ of polarized light parallel to the electric field E accompanying the application of the electric field E is It is represented by the following formula (4).

In Equation (4), g ₁₁ is a second-order electro-optic coefficient for polarized light parallel to the electric field E.

On the other hand, the refractive index change amount Δn ₂ of polarized light perpendicular to the electric field E is expressed by the following equation (5).

In Equation (5), g ₁₂ is a second-order electro-optic coefficient for polarized light perpendicular to the electric field E.

In KTN or KLTN crystal, an absolute value of g ₁₂ is about 1/3 of the absolute value of g _11. For this reason, when a KTN or KLTN crystal is used as the optical deflector 13, the polarization of incident light should be parallel to the electric field in order to realize efficient optical deflection with a larger refractive index.

[Operation of optical switch]
Next, the operation of the optical switch 104 will be described with reference to FIG. 1, FIG. 3, FIG. 4, and FIG.

  FIG. 3 is a diagram for explaining the principle of the switching operation in the optical switch 104. FIG. 4 is a diagram for explaining a switching state realized by the optical switch 104. FIG. 5 is a diagram for explaining the state of the voltage applied to the KTN optical deflector 31 of the optical switch 104. Instead of the KTN optical deflector 31, a KLTN optical deflector can be applied.

  As shown in FIG. 3, the optical switch 104 includes a KTN optical deflector 31 and two convex lenses 32 and 33. The KTN optical deflector 31 receives the light from the upper arm 40 and the light from the lower arm 50 via the convex lens 32 and switches to one of the two lights R1 and R2 via the convex lens 32. Then, the light is emitted to an optical waveguide or optical fiber transmission medium 60.

  The light from the upper arm 40 means the light demultiplexed by the optical coupler 102, and the light from the lower arm 50 means the light delayed by T / 2 by the delay unit 103.

  Lights R1 and R2 (see FIG. 3) emitted to the transmission medium 60 are switched every T / 2 as shown in FIG. 4C. As shown in FIG. 4A, the light R1 demultiplexed by the optical coupler 102 is emitted to the transmission medium 60 in the case of T / 2 periods a1 and a2. In this case, the voltage applied to the KTN optical deflector 31 is + V (for example, 200 v). T / 2 is, for example, 2.5 μsec.

  On the other hand, as shown in FIG. 4B, the light R2 delayed by T / 2 by the delay unit 103 is emitted in the T / 2 periods b1 and b2. In this case, the voltage applied to the KTN optical deflector 31 is −V (for example, −200 v).

  In the optical switch 104 of this embodiment, the optical coupler 102 or the delay is obtained at the timing for obtaining the maximum oscillation wavelength and the timing for obtaining the minimum oscillation wavelength in the output light of any one of the output lights of the optical coupler 102. The optical path from the device 103 is switched. Thereby, the period of the oscillation wavelength is T / 2, that is, the wavelength sweep frequency is doubled, and high-speed wavelength sweep can be realized.

[Types of optical switch]
The optical switch is a KTN (KTa _1-x Nb _x O ₃ : 0 ≦ x ≦ 1) crystal or KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ : 0 ≦ x ≦ 1, 0 <y < 1) Realized by spatially switching the optical path by applying a voltage to an optical deflector using a crystal (for example, see Japanese Patent No. 475138).

  As described above, according to the wavelength swept light source 10 of the present embodiment, the output of the optical coupler 102 or the output of the delay unit 103 is switched at a predetermined timing without changing the coherent length of the light source 101. The predetermined timing is set so that the fluctuation period of the oscillation wavelength is faster than the fluctuation period. Therefore, high-speed wavelength sweep can be realized without shortening the coherence length of the light source 101.

  Next, Example 1 of the above embodiment will be described with reference to FIGS. 1, 2, and 5.

Example 1
In the wavelength sweeping apparatus 10 shown in FIG. 1, a KLTN optical deflector is used as the optical deflector 13 used for the light source 101. The crystal of the KLTN optical deflector had a length direction (4.0 mm) × width direction (3.2 mm) × thickness direction (1.0 mm), and electrodes were formed on two sides of 4.0 mm × 3.2 mm. Then, optical polishing was performed on two surfaces of 3.2 mm × 1.0 mm where light was incident.

  The crystal was a paraelectric phase and maintained at a temperature at which the relative dielectric constant was 17500. In order to store electrons in the crystal, a DC voltage of +400 V was applied for 10 seconds and a DC voltage of -400 V was applied for 10 seconds. Thereafter, an AC voltage of ± 400 V (sine wave having a frequency of 200 kHz) was applied to perform light deflection. The alternating voltage to be applied may be a triangular wave. The semiconductor optical amplifier 11 used was a 1.3 um band, and the wavelength sweep realized a center wavelength of 1.32 μm and a sweep width of 100 nm.

  An optical fiber spool was used as the delay device 103. Since the refractive index of the optical fiber is 1.467, by setting the length of the optical fiber to 511 m, the delay device 103 delays 2.5 μsec, which is half of the period of the AC voltage of 5 μsec.

The optical switch 104 has a KTN (KTa _1-x Nb _x O ₃ : 0 ≦ x ≦ 1) crystal or KLTN (K _1-y Li _y Ta ₁ ) every T / 2 (= 2.5 μsec) shown in FIG. _-x Nb _x O ₃ : 0 ≦ x ≦ 1, 0 <y <1) Light having a wavelength sweep frequency of 400 kHz can be output by switching the optical path by applying a voltage to an optical deflector using a crystal. It was. The wavelength sweep width of the output light was 100 nm.

DESCRIPTION OF SYMBOLS 10 Wavelength sweep light source 31 Optical deflector 101 Light source 102 Optical coupler 103 Delay device 104 Optical switch 201 Optical apparatus

Claims (4)

A light source comprising a first optical deflector that temporally changes the oscillation wavelength transition;
A duplexer for demultiplexing the light emitted from the light source;
A delayer for delaying the output of the duplexer;
The output of the duplexer or the output of the delay device is switched at a predetermined timing so that the fluctuation period of the oscillation wavelength is faster than the fluctuation period, and KTN (KTa _1-x Nb _x O ₃ : 0 ≦ x ≦ 1) Optical switch composed of a second optical deflector using crystal or KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ : 0 ≦ x ≦ 1, 0 <y <1) crystal And a wavelength-swept light source.   2. The wavelength swept light source according to claim 1, wherein a driving waveform of a voltage for high-speed deflection in the first optical deflector is a sine wave or a triangular wave.   2. The wavelength swept light source according to claim 1, wherein a voltage for high-speed deflection is applied to the first optical deflector so that the time change of the wave number of the emitted light becomes a triangular wave.   4. The wavelength swept light source according to claim 1, wherein the delay device is an optical fiber spool.