JP5121150B2 - Tunable laser light source - Google Patents

Description

  The present invention relates to a tunable laser light source capable of generating single mode light and changing its emission wavelength.

  In the fields of optical coherent tomography (OCT) and optical fiber sensing, there is an increasing demand for detecting a detection target at high speed and with high sensitivity. As a light source for such frequency domain interferometry and spectroscopic analysis, a wavelength scanning laser light source capable of broadband scanning is required. As a wavelength tunable light source having a narrow spectrum and a wide band, an external resonator type using a complicated variable mechanism is generally used. However, in order to stabilize the output and continuously change the wavelength, it is necessary to mechanically control a complicated mechanism with a high-precision motor or the like. It was.

  Next, the oscillation principle of the external resonance type laser light source will be described. In a general external resonance type laser, a gain medium such as a semiconductor laser and an external mirror are used as external resonators, and an optical bandpass filter is provided therebetween. In such a wavelength tunable light source, there is a type that uses a diffraction grating as a function of a bandpass filter and an external mirror, and changes a selection wavelength by changing an incident angle to the diffraction grating. If the incident angle to the diffraction grating is changed to change the wavelength, and the external resonance mode is also synchronized with this change rate, the wavelength can be changed without causing a mode hop. For this reason, there is also known a wavelength tunable light source that realizes a wavelength sweep without a mode hop by rotating the diffraction grating itself around a specific pivot instead of the center where the optical axes of the diffraction grating intersect. (Non-patent document 1) In this wavelength tunable light source, when the rotation angle is changed, the incident angle of light on the diffraction grating and the external resonator length change in conjunction with each other.

A distributed feedback (DFB) type laser or the like cannot be expected to be as wide-band as an external cavity laser due to limitations on semiconductor materials and structures. Also, an integrated external resonator wavelength that has a gain region that adjusts the output level, a phase adjustment region that configures the wavelength variable region and the resonator length, and allows the wavelength to be varied by adjusting the wavelength and the phase. A variable laser is also known (Non-Patent Document 2).
“Continuously Tuned External Cavity Semiconductor Laser”, WR Trutan et al., Journal of Lightwave technology Vol. 11, No.8 August 1993 PP1279〜1286 “Monolithically Integrated Grating Cavity Tunable Lasers”, Oh Kee Kwon et al., IEEE PHOTONICS TECHNOLOGY LETTERS VOL. 17, NO.9, SEPTEMBER 2005

  However, conventional external cavity lasers require a complex mechanism that avoids mode hops and tunes the phase, and are difficult to sweep or scan at high speeds. In Non-Patent Document 1, it is necessary to increase the distance between the pivot and the diffraction grating, which increases the rotational moment. Since the diffraction grating requires high-precision angle adjustment accuracy, the speed at which the wavelength can be swept continuously is about several seconds, and there is a drawback that wavelength scanning cannot be performed at high speed. In Non-Patent Document 2, it is difficult to adjust the two parameters of wavelength and phase simultaneously and continuously at high speed. That is, a laser light source that realizes narrow line width, wide bandwidth, and high-speed continuous variable has not been realized.

  The present invention has been made in view of the above-described conventional problems. The wavelength can be swept and scanned at high speed over a wide band in a single mode without rotating the diffraction grating, and can be varied at an arbitrary speed. Another object is to provide a wavelength tunable laser light source that can be fixed.

In order to solve this problem, a wavelength tunable laser light source according to the present invention includes a gain medium having a gain with respect to an oscillation wavelength, and a light medium provided adjacent to the gain medium so that light incident from the gain medium is the same light. A first mirror reflecting along a path, a diffraction grating provided at a position symmetrical to the first mirror with the gain medium as a center, and provided between the gain medium and the diffraction grating, An external resonator mode in which the refractive index is changed by an external signal and the angle of incidence on the diffraction grating is deflected by refracting light, and the external resonator length is determined by L and the optical path length Where λ _{m is} a wavelength, λ _{G is} a selected wavelength determined by the diffraction grating , and c is the speed of light.
| F _G −f _m | <c / 4L
By satisfying the above inequality, the change in the optical path length accompanying the change in the refractive index of the light deflector and the change in the wavelength accompanying the change in the optical path length of the external resonator accompanying the deflection of the light beam, and the incidence on the diffraction grating The laser oscillation wavelength is continuously changed in a single mode in synchronization with the change in wavelength accompanying the change in angle.

  Here, the light deflection unit may use an element having any one of an electro-optic effect, a thermo-optic effect, a magneto-optic effect, and an acousto-optic effect.

  Here, the diffraction grating may be formed on an end face of a waveguide element, and the waveguide element may further integrate the light deflection section in an optical waveguide.

  Here, the gain medium may be a semiconductor laser, and the waveguide element may be brought into contact with the emission surface of the semiconductor laser, and the semiconductor laser and the waveguide element may be integrated on the same substrate.

  Here, the diffraction grating may be configured by a Littman arrangement that reflects light reflected by the diffraction grating to the same position.

  According to the present invention having such a feature, the external resonator is configured by the first mirror and the diffraction grating with the gain medium as the center. Light is deflected by the light deflecting unit to change the incidence on the diffraction grating. The diffraction grating is used as a filter whose wavelength changes according to the incident angle, and the oscillation wavelength can be determined by the selected wavelength. The same external resonator mode is used by continuously changing the incident angle to the diffraction grating, continuously changing the selected wavelength of the diffraction grating, and changing the external resonator length in conjunction with this. The oscillation wavelength can be changed instead of the mode hop. By sufficiently increasing the deflection speed of the optical deflector, it is possible to perform wavelength scanning at a high speed over a wide range in a single mode. Further, by using this light source, since the optical frequency scanning range is narrow with a narrow line width, it is possible to realize a high resolution image display in a deep range in OCT or the like.

  FIG. 1 is a diagram showing a configuration of an external resonator type semiconductor laser according to a first embodiment of the present invention. As shown in the figure, a semiconductor laser 11 is disposed on the base 10. A waveguide element 12 is provided on the right emission surface of the semiconductor laser 11. The waveguide element 12 uses an element having an electro-optic effect, and has a spot size conversion unit 13, a light deflection unit 14, and a diffraction grating 15 that convert the spot size of light emitted from the right end of the semiconductor laser 11 as illustrated. ing. The spot size conversion unit 13 achieves a function corresponding to a collimating lens that enlarges the spot diameter of light from the semiconductor laser 11. The waveguide element 12 forms electrodes only above and below the portion indicated by the hatching of the light deflection section 14. The light deflecting unit 14 applies a voltage from the voltage source 16 to a hatched region, thereby changing the refractive index in the region by the electro-optic effect, and deflecting light according to the voltage. The voltage source 16 uses an AC power source such as a triangular sawtooth wave in accordance with a desired wavelength change. The diffraction grating 15 is formed on the end face of the waveguide element 12. The left end face of the semiconductor laser 11 constitutes a first mirror, which transmits a part of the light. A condensing lens 17 and an isolator 18 are provided on the outgoing side of the transmitted light, and an optical fiber 19 is connected thereto. By forming the diffraction grating 15 on the end face of the waveguide element 12 using a deep dry etching (DRIE) apparatus or a molding manufacturing method, a diffraction grating having an arbitrary pitch can be easily manufactured.

  FIG. 2 shows only the main part of the external cavity laser according to the first embodiment of the present invention. In this figure, the left end of the semiconductor laser 11 which is a gain element is a mirror surface, which is a first mirror constituting an external resonator. The other surface is an AR coating (antireflection coating), and guides the emitted light to the light deflecting unit 14. The light deflecting unit 14 is provided in the central portion of the waveguide element 12 and changes the light deflection direction within a certain angle range according to a signal from the outside. As described above, the light deflection unit 14 is provided with electrodes on the upper and lower sides only from the upper part of the trapezoidal portion. In a state where no voltage is applied, the light from the semiconductor laser 11 travels straight because it has the same refractive index as the other regions. When a voltage is applied, the refractive index changes according to the voltage, and the light is deflected in a certain angle range. A diffraction grating 15 is provided at a position for receiving the deflected light. The diffraction grating 15 is an optical element in which a triangular wave grating is continuously formed at the end of the waveguide element 12. In this embodiment, incident light is projected through the same optical path even if the incident direction is changed by the Littrow arrangement. It is configured to return in the direction. The selected wavelength changes depending on the incident angle. The diffraction grating 15 has functions of a second mirror of an external resonator and an optical bandpass filter.

Here, the Littrow arrangement will be described. If the incident angle of the light beam with respect to the diffraction grating is θ and the reflection angle is δ, diffracted light can be obtained by the following equation.
Λ (sinθ + sinδ) = kλ (1)
Here, k is an order and takes values of 0, ± 1, ± 2,. Λ is the reciprocal of the grating pitch (μm) of the diffraction grating, that is, the number of grating lines a (lines / mm) per unit length.

The diffracted light has a Littrow arrangement and a Littman arrangement. In the Littrow arrangement, the angles of the −1st order diffracted light and the incident light are equal. Therefore, if θ = δ in equation (1), the wavelength of diffracted light is determined by the following equation from equation (1).
λ = 2Λsinθ (2)
In the Littman arrangement, the angles of incident light and reflected light do not match.

Here, the light deflecting unit 14 continuously deflects light from a straight traveling state as shown in the figure. Here it is assumed that changes the deflection angle φ to 0~φ _1. As shown in the figure, the incident light to the diffraction grating 15 has an incident angle θ ₀ when the light is not deflected by the light deflector 14, and an incident angle θ ₀ + φ ₁ when the light deflector 14 is deflected to the maximum. To do. The incident angle θ can be expressed as θ ₀ to θ ₀ + φ. In this case, the wavelength λ _{G of the} reflected light selected by the diffraction grating 15 can be expressed by the following equation (3) using φ and θ ₀ from the equation (2).
λ _G (φ) = 2Λsinθ = 2Λsin (θ ₀ + φ) (3)

As shown in FIG. 2, the external resonator length L ₀ when the light is not deflected is equal to the optical path length L 1 from one end of the semiconductor laser 11 that is a gain element to the light deflector 14, and the inside of the light deflector 14. , And the optical path length L2 from the optical deflection unit 14 to the diffraction grating 15 is determined.
L ₀ = L1 + b + L2 (4)
Here, as the external resonator length L, a length is selected such that one external resonator mode is included in the full width at half maximum of the bandpass filter by the diffraction grating 15. In the present embodiment, the external resonator length L itself is changed in synchronism with the change in the incident angle θ of the light with respect to the diffraction grating 15 due to the deflection of the light by the light deflecting unit 14, thereby one external The wavelength can be scanned while changing the wavelength of the resonator mode itself. Assuming that the optical path length of the optical deflector 14 is changed from the original b to b ′ (= b + Δb), Δb is an increase in the optical path length based on the change in the refractive index in the optical deflector 14 and an increase in the geometric optical path length. Is the added value. Then, the phase change amount due to the sum of the increase Δb2 in the optical path length and the increase ΔL2 in the optical path length L2 up to the diffraction grating accompanying the deflection of the light beam is optimized to synchronize with the wavelength change due to the change in the deflection angle. By doing so, it is possible to realize a wavelength scanning type light source having no mode hop in a certain wavelength range by continuously changing the control parameter, for example, the voltage of the light deflecting unit 14.

Here, as shown in FIG. 2, the incident angle from the semiconductor laser 11 to the light deflector 14 is α, the exit angle is β, the incident angle from the optical deflector 14 to the waveguide element 12 is α−β, and the exit angle. Is φ, Snell's law gives the following equation.
sinα = n'sinβ (5)
sinφ = n′sin (α−β) (6)
Here, n ′ is a refractive index obtained by normalizing the refractive index of the light deflection unit 14 with the surrounding refractive index, and varies depending on the voltage applied to the light deflection unit 14. When the voltage is applied, the refractive index changes, the optical path length b increases from the light deflecting unit 14 to b ′, and the length from the light deflecting unit 14 to the diffraction grating also increases from L2 to L2 ′. Here, when the amount of displacement from the original optical axis in the light deflecting unit 14 is H, and the amount of light deflection between the light deflecting unit 14 and the diffraction grating 15 is Y, from the light deflecting unit 14 to the diffraction grating 15. Is represented by the following equation.

従ってこのＬ２’を用いて全光路長Ｌは次式で表される。

Therefore, using this L2 ′, the total optical path length L is expressed by the following equation.

又出射角βは式（５），（６）より次式（９）で表される。

The exit angle β is expressed by the following equation (9) from equations (5) and (6).

Therefore, the wavelength λ _m of the external resonator mode determined by the optical path length is expressed as a function of the variable φ and is determined by the following equation.
λ _m = λ ₀ L / L ₀ (10)
Here, the optical path length when no voltage is applied is L ₀ as described above, and λ ₀ is a wavelength determined by the Littrow arrangement of the diffraction grating when φ = 0.

The selected wavelength λ _G determined by the Littrow arrangement of the diffraction grating 15 is expressed by the above-described equation (3). These wavelengths lambda _m, lambda _G light of the optical frequency f _m, f _G, when the velocity of light is c, represented by respectively the following formulas.
f _m = c / λ _m
f _G = c / λ _G
When the absolute value of the frequency difference is equal to or less than ½ of the external mode interval F (= c / 2L), that is, the following inequality is satisfied, mode-hop free oscillation can be performed.
| F _G −f _m | <c / 4L (11)

Examples of satisfying the above inequality (11) include the following.
L1 = 5.25mm
b = 29mm
L2 = 2mm
α = 35 °
a = 800 / mm
The relationship between lambda _m the diffraction angle and the wavelength lambda _G of the diffraction grating in the condition in FIG. 3, showing mode hop numbers and the relationship between the oscillation wavelength in FIG. As is known from these figures, the range where the mode hop number is less than ± 0.5 is the range where the above inequality (11) holds. In this case, a wavelength range of 19 nm is obtained, and oscillation without mode hops is possible in this angular range. That is, within this range, the change in wavelength and the change in phase can be synchronized, and the oscillation wavelength can be changed without mode hops. This oscillation wavelength can be changed by a voltage applied to the optical deflecting unit 14. Since the response speed of the electro-optic element is fast, the wavelength can be changed at a high speed according to the voltage waveform by applying a voltage having an arbitrary waveform such as a triangular wave or a sine wave.

  In this embodiment, since all the components of the wavelength scanning laser light source are configured as an integrated circuit on the base 10, there is an effect that there is no movable part and the size and weight can be made extremely small.

  Furthermore, in this embodiment, a wavelength scanning light source is used by using an arbitrary AC power source as the voltage source 16, but the wavelength is variable to obtain a light having a wavelength corresponding to the angle by fixing to an arbitrary deflection angle according to the voltage. It can be used as a light source.

  Next, a second embodiment of the present invention will be described. In this embodiment, as shown in FIG. 5, the diffraction grating 15 formed at the end of the waveguide element 12 has a feature. In this embodiment, the number of lattice lines a is changed in accordance with the position where light is reflected in order to expand the oscillatable wavelength range that is mode hop free. Since the diffraction grating itself can be formed at an arbitrary pitch by forming it on the end face of the waveguide using deep dry etching (DRIE apparatus) or a molding manufacturing method, there is no mode hop by appropriately changing this pitch. The wavelength range in which oscillation is possible can be expanded. FIG. 5 is a graph showing changes in the number of grating lines when the length direction of the diffraction grating formed on the end face of the waveguide element is the horizontal axis and the number of grating lines a is the number of lines. Thus, the wavelength variable range can be expanded by changing the number of grating lines continuously or discontinuously so that the number of grating lines becomes the number of grating lines corresponding to the deflection angle from the constant of the first embodiment. .

  FIG. 6 is a diagram showing a wavelength tunable laser light source according to the third embodiment of the present invention. In this figure, the same parts as those of the first embodiment described above are denoted by the same reference numerals, and detailed description thereof is omitted. In this embodiment, only the waveguide element and the diffraction grating at its end face are different from those of the first embodiment described above. In this embodiment, a waveguide element 21 is used. Similar to the first embodiment described above, the spot size conversion unit 13 and the light deflection unit 14 are provided on the waveguide element 21. Further, the diffraction grating 22 is similarly formed at the end portion, and the position facing the diffraction grating 22 is a plane, and the surface is formed as a mirror 23. The diffracted light from the diffraction grating 22 is reflected as it is at the same position. If it carries out like this, it will become the Littman arrangement | positioning which utilizes 1st-order diffracted light. In this case, the optical path length L2 after exiting from the optical deflecting unit 14 is the sum of the reciprocating optical path length from the diffraction grating 22 to the mirror unit 23, but the others are the same as in the first embodiment. It is the same. In this case, the mirror 23 becomes a second mirror constituting one end of the external resonator. In this case as well, a mode-hop-free wavelength tunable laser light source can be realized with the same configuration.

Here, a nonlinear optical material capable of modulating the refractive index with high efficiency, a liquid crystal, a polymer, or the like can be used for the light deflecting unit. Examples of the element having an electro-optic effect include PZT and LiNbO _3. By applying a voltage to this element, the refractive index changes, so that light can be deflected at a refraction angle corresponding to the voltage.

  Note that the refractive index of only a specific region of the region of the light deflecting unit 14 may be changed in advance, and the refractive index of that region may be changed with a voltage. Also, the deflection angle can be expanded by providing a plurality of regions where the refractive index changes.

  Furthermore, in each of the above-described embodiments, a diffraction grating is formed on the end face of the waveguide element. However, the semiconductor laser, which is a gain medium, and the optical deflection unit and the diffraction grating are independent of each other without using the waveguide element. It may be formed as an element. In each of the embodiments, the semiconductor laser can be formed on the substrate at the same time so that the whole can be integrated.

  Since the present invention can sweep and scan a wavelength at an arbitrary speed in a single mode, it can be used as a wavelength scanning light source in various fields such as OCT and optical fiber sensing and other light sources.

It is a figure which shows the wavelength variable laser light source by the 1st Embodiment of this invention. It is a figure which shows the structure of the principal part of the wavelength variable laser light source by this Embodiment. It is a graph which shows the change of the wavelength with respect to the diffraction angle by embodiment of this invention. It is a graph which shows the wavelength change and mode hop number by this Embodiment. It is a figure which shows the change of the number of grating lines with respect to the position of the diffraction grating of the wavelength tunable laser light source by the 2nd Embodiment of this invention. It is a figure which shows the wavelength variable laser light source by the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Base 11 Semiconductor laser 12,21 Waveguide element 13 Spot size conversion part 14 Optical deflection part 15,22 Diffraction grating 16 Voltage source 17 Condensing lens 18 Isolator 19 Optical fiber 23 Mirror

Claims (5)

A gain medium having a gain for the oscillating wavelength;
A first mirror provided adjacent to the gain medium and reflecting light incident from the gain medium along the same optical path;
A diffraction grating centered on the gain medium and provided at a position symmetrical to the first mirror;
An optical deflector provided between the gain medium and the diffraction grating, and having a refractive index changed by an external signal and deflecting an incident angle to the diffraction grating by refracting light; and
When the external resonator length is L, one wavelength of the external resonator mode determined by the optical path length is λ _m , the selected wavelength determined by the diffraction grating is λ _G , and the speed of light is c,
| F _G −f _m | <c / 4L
By satisfying the above inequality, the change in the optical path length accompanying the change in the refractive index of the light deflector and the change in the wavelength accompanying the change in the optical path length of the external resonator accompanying the deflection of the light beam, and the incidence on the diffraction grating A wavelength tunable laser light source that continuously changes the laser oscillation wavelength in a single mode in synchronization with the change in wavelength accompanying the change in angle.   The tunable laser light source according to claim 1, wherein the light deflecting unit uses an element having any one of an electro-optic effect, a thermo-optic effect, a magneto-optic effect, and an acousto-optic effect. The diffraction grating is formed on the end face of the waveguide element,
The wavelength tunable laser light source according to claim 1, wherein the waveguide element further includes the optical deflection unit integrated in an optical waveguide. The gain medium is a semiconductor laser;
The wavelength tunable laser light source according to claim 1, wherein the waveguide element is brought into contact with an emission surface of the semiconductor laser, and the semiconductor laser and the waveguide element are integrated on the same substrate.   The wavelength tunable laser light source according to claim 1, wherein the diffraction grating is configured by a Littman arrangement that reflects light reflected by the diffraction grating to the same position.

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