JP4793311B2 - Laser amplifier - Google Patents

Description

  The present invention relates to a laser amplifying apparatus and a laser apparatus including a tapered gain region.

  In recent years, there has been a demand for high-power laser devices such as laser printer devices and laser display devices. As one type of such a laser device, a laser device is known which includes a semiconductor optical amplifier having a tapered stripe (tapered laser amplifier) and an optical fiber grating whose tip is processed into a lens (for example, Patent Documents). 1).

JP2000-261089

  In the conventional taper type laser amplifier disclosed in Patent Document 1, since the taper stripe is composed of a linear portion having a width of 4 μm and a taper portion at the input end, it is caused by a sudden change in carrier density along both side ends of the taper portion. There has been a problem that a filament phenomenon in which light rays converge due to the lens effect may occur.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a laser amplifying apparatus capable of reducing the filament phenomenon and obtaining a high-output pseudo-diffraction-limited light with a simple configuration. .

A laser amplifying device according to the present invention includes a tapered laser amplifier having a tapered gain region extending in a horizontal direction from an incident end to an emitting end, and a lens disposed opposite to the incident end of the gain region of the tapered laser amplifier. Is an optical coupling means having a polarization-maintaining optical fiber formed at the tip and preserving a horizontal plane of polarization at the tip, wherein the light emitted from the lens is more horizontal than the incident end of the gain region in incident so that the width becomes narrower, the incident light is propagated through the gain region with a large spread angle in the horizontal direction than the spread angle of the tapered shape, the horizontal than the exit end of the light the propagation said gain region And an optical coupling means in which the polarization-maintaining optical fiber is arranged so that the width becomes wider in the direction.

  According to the present invention, since the filament phenomenon along both ends of the tapered gain region can be reduced in the laser amplifying device, a large output pseudo-diffraction limited light can be obtained with a simple configuration.

Embodiment 1 FIG.
In the laser amplifying apparatus according to Embodiment 1 of the present invention, a polarization-maintaining optical fiber with a tip lens is disposed opposite to an incident end of a tapered laser amplifier, and the width of propagating light is narrower than the gain region at the incident end. The width of the propagation light is wider than the width of the gain region at the end, and the boundary of the propagation light and the boundary of the gain region are shifted. As a result, the filament phenomenon along both ends of the tapered gain region can be reduced, so that a large output pseudo-diffraction limited light can be obtained.

  1 is a top view showing a configuration of a laser apparatus according to Embodiment 1 of the present invention, FIG. 2 is a side view thereof, FIG. 3 is an explanatory view showing a state of light incidence, and FIG. 4 is a tapered laser amplifier by a light incidence method. FIG. 5 is an explanatory diagram showing the beam quality of the output of the tapered laser amplifier according to the light incident method. In the figure, 1 is a semiconductor laser diode element, 2 is a polarization-maintaining optical fiber, and 3 is a tapered laser amplifier.

  Next, a detailed configuration and operation will be described. The semiconductor laser diode element 1 is a Fabry-Perot resonator type laser diode element having an optical waveguide 1a, has a maximum gain in the wavelength of 1064 nm band, and has a reflectance of 90% on the back surface in order to increase the light extraction efficiency. The high reflection film 1b is provided with a low reflection film 1c having a reflectance of 0.5% on the front surface, and the semiconductor laser diode element 1 oscillates in a multi-longitudinal mode.

  In the semiconductor laser diode element 1, the optical waveguide 1a is composed of an InGaAs single quantum well active layer, a light guide layer, and a cladding layer in the thickness direction, and oscillates in a single transverse mode. On the other hand, the optical waveguide 1a needs to have a relatively low current density and light density in order to improve reliability in the surface direction, and a wide ridge type optical waveguide is adopted. Therefore, since the confinement as an optical waveguide is loose, excitation is performed in the fundamental mode, that is, the single transverse mode when the injection current value is small, but higher order due to the spatial hole burning effect when the injection current value increases and the light density increases. The mode is easy to be excited, and the width is about 4 to 6 μm so that the fundamental mode is maintained until the light output to be used. The polarization extinction ratio is equal to or greater than the laser oscillation current threshold and is close to 27 dB at the element emission position.

  Next, a polarization-maintaining optical fiber 2 having a mode field diameter of 6.6 μm is adopted, and the Bragg diffraction grating 2b is disposed by irradiating the core portion 2a with ultraviolet rays through a phase mask. Since the polarization-maintaining optical fiber 2 prevents the coupling of both modes by changing the propagation speed of the HE11 even mode and the HE11 odd mode, which are propagation modes, the linearly polarized light is converted into the slow axis of the polarization-maintaining optical fiber 2. Or linearly polarized wave is maintained when it coincides with the first axis. On the other hand, since the equivalent refractive index of the optical waveguide 2a is slightly different between the first axis and the slow axis, the reflection peak wavelength of the Bragg diffraction grating 2b is shifted between the first axis and the slow axis, but here the electric field direction ( (Horizontal direction) is aligned with the slow axis, and the polarization extinction ratio is 20 dB or more at the output end although it deteriorates due to the stress when the polarization-maintaining optical fiber 2 is fixed.

  The outgoing beam of the semiconductor laser diode element 1 has a large aspect ratio (beam ellipticity) because the optical waveguide 1a is flat, and is optically coupled to the polarization maintaining optical fiber 2 including the circular optical waveguide 2a with low loss. For this purpose, the incident end face 2c of the polarization-maintaining optical fiber 2 is processed with a biconical lens, and the aspect ratio is corrected by using an asymmetric lens system.

  Since the gain band of the semiconductor laser diode element 1 has a single quantum well structure, the wavelength dependence is relatively gentle. If the reflection peak wavelength of the Bragg diffraction grating 2b is set in this gain band, the vicinity of the reflection peak wavelength is obtained. Gain is maximized, and the oscillation wavelength of the semiconductor laser diode element 1 is controlled. The longitudinal mode by this composite resonator is a single mode from the relationship between the phase relationship due to the optical length of the semiconductor laser diode element 1 and the Bragg diffraction grating 2b and the amount of return light from the Bragg diffraction grating 2b to the semiconductor laser diode 1. It can oscillate in various states such as multimode and coherent collapse mode. Here, in order to oscillate in a single mode, the back surface reflectance and the front surface reflectance of the semiconductor laser diode element 1 are 90% and 0.5%, respectively, the resonator length is 1.8 mm, and the reflectance of the Bragg diffraction grating 2b. Is 30%, the reflection bandwidth is 0.4 nm, the coupling efficiency is 80%, and the longitudinal mode is stabilized by disposing the Bragg diffraction grating 2b and the semiconductor laser diode element 1 close to 10 cm or less, in particular. .

  Next, the tapered laser amplifier 3 has a planar active layer 3c, and forms a tapered gain region 3a with a current confinement structure. The width of the incident end in the plane direction (horizontal direction) of the gain region 3a is 10 μm, the width of the exit end is 324 μm, the taper-shaped spread full angle of the gain region 3a is 6 degrees (3 degrees with respect to the optical axis), and the gain region 3a The length in the optical axis direction is 3 mm, and the refractive index of the gain region 3a is 3.5. In order to prevent the filament phenomenon, antireflection films 3b are provided on both end surfaces of the chip of the tapered laser amplifier 3. Since the core 2a of the polarization-maintaining optical fiber 2 is circular, the exit end face 2d is processed by a biconical lens, and the optical waveguide 3c composed of a quantum well and an optical guide layer is formed in the thickness direction (vertical direction). The mode is matched with the spot diameter of 0.6 μm in the waveguide mode, and the spot diameter is approximately 1.65 μm in the plane direction (horizontal direction) to propagate the gain region 3 a of the tapered laser amplifier 3 in the Gaussian beam mode. The light was amplified. Note that the curvature radii of the biconical lens 2d in the vertical and horizontal directions are 4 μm and 11 μm, respectively.

In FIG. 3, when the spreading half angle θ of the tapered portion of the gain region 3a is 3 degrees, the wavelength λ is 1064 nm, and the refractive index n is 3.5, the spot diameter ω of the light as the beam waist is expressed by the equation (1). When the thickness is 1.8 μm, a light beam having a light intensity of 1 / e ² propagates along the tapered gain region 3a as indicated by a broken line 4c in FIG.
ω = λ / {π · n · tan (θ)} (1)

Here, since the carrier density changes suddenly at the positions along the both ends of the tapered gain region 3a, a lens effect due to a large carrier-induced refractive index change occurs, and a strong filament phenomenon may occur. FIG. 4A shows a case where light is incident on a polarization-maintaining optical fiber 2 with a tip lens having a spot diameter 4a of about 1.8 μm, which is substantially equal to the spot diameter as a beam waist calculated by the equation (1). It is explanatory drawing which shows the observation result which observed the near-field image at the output terminal of the taper type laser amplifier 3 driven with the electric current 8A. In FIG. 4A, the vertical axis represents (relative) light intensity, and the horizontal axis represents (relative) horizontal position. As shown in FIG. 4A, a light intensity peak 5a corresponding to the above-described filament phenomenon is observed at positions along both ends of the tapered gain region 3a. FIG. 5 shows the dependence of the beam quality (M ² ) on the injection current. A case where light is incident with a spot diameter of about 1.8 μm as a beam waist corresponding to FIG. In FIG. 5, since the light amplification is small at a low current, the beam quality is good with a Gaussian light output distribution, and as the current increases, the beam quality gradually deteriorates as the top hat light output distribution is obtained. When the current is about 8 A, there is a problem that the light output distribution is accompanied by the light intensity peak 5 a corresponding to the strong filament phenomenon at both ends of the gain region as shown in FIG. 4A.

Next, the function and effect of the optical coupling means of the tapered laser amplifier that exhibits the characteristics of the first embodiment of the present invention will be described. In Embodiment 1 of the present invention, in FIG. 3, the spot diameter 4a as the beam waist of the incident light is made smaller than the spot diameter 4b as the beam waist calculated by the equation (1), and the light intensity is 1 / The spread angle 4d of the e ² light beam is made larger than the spread half angle 4c of the tapered gain region 3a, and the light beam having an intensity of 1 / e ² is bounded by the tapered gain region 3a as shown by the solid line 4d in FIG. 4c is propagated so as to cross. This increases the light density of the propagating light on the input side of the taper type gain region 3a. Therefore, the entire taper type gain region 3a is saturated from the time when the injection current density is low, and the light output distribution is in a top hat shape. Thus, optical amplification can be performed efficiently. Further, the propagating light covers all the current injection region on the output side of the tapered gain region 3a, so that the injection current is not wasted and a large output amplified light can be obtained. Further, since the boundary 4d of propagating light as 1 / e ² of the light intensity distribution is shifted inward or outward from the boundary 4c of the gain region 3a so as to reduce overlap, carriers along both side ends of the tapered portion are reduced. Since the lens effect, which is a carrier-induced refractive index change due to a sudden change in density, is reduced, a special effect that the filament phenomenon at both ends is reduced is obtained.

FIG. 4B shows a tapered laser amplifier 3 in which light is incident on the polarization-maintaining optical fiber 2 with a tip lens having a spot diameter 4a as a beam waist of incident light of about 1.7 μm and driven by an injection current 8A. It is explanatory drawing which shows the observation result which observed the near-field image at the output terminal. In FIG. 4B, the vertical axis represents (relative) light intensity, and the horizontal axis represents (relative) horizontal position. As shown in FIG. 4B, a top hat-like light output distribution having a substantially constant luminance is observed over the entire tapered gain region 3a. When this state is observed with the beam quality (M ² ), as indicated by the reference numeral 6b in FIG. 5, the beam shape is a top hat shape in the low current region, so that the beam quality is rapidly deteriorated. Then, since the light intensity peak 5b corresponding to the filament phenomenon is reduced, the beam quality is stabilized at a relatively low position, and pseudo-diffraction limited light can be obtained. That is, when the beam quality is compared in a large current region of 6 A or more, it can be seen that the configuration according to the present invention is superior.

  Note that there is a known single-pass filament phenomenon in which a large amplitude output distribution can be produced at the output end of a tapered laser amplifier if there is a small ripple-like disturbance in the intensity distribution of the input light. For this reason, a lens for converting a single transverse mode of an optical fiber into a beam needs to have extremely small aberration. Since the microlens processed at the tip of the optical fiber according to the present invention is refracted at a position where the ray height from the optical axis of the lens is small, the aberration is very small and the single-pass filament phenomenon can be reduced.

  In addition, since the first embodiment of the present invention is configured as described above, a ridge waveguide (straight line portion) for propagating a single transverse mode to the input portion of the tapered laser amplifier 3 and a residual output end face. There is no need to provide a reflection spoiler for preventing the return light reflected by the reflection from returning to the tapered gain region again, and there is an effect that a simple structure can be obtained.

  As described above, in the laser amplifying apparatus according to the first embodiment of the present invention, the polarization-maintaining optical fiber with the tip lens is disposed opposite to the incident end of the tapered laser amplifier, and the width of the propagation light is set to the gain region at the incident end. The width of propagating light is wider than the width of the gain region at the output end, and the boundary between the propagating light and the boundary of the gain region is shifted. As a result, the filament phenomenon along both ends of the tapered gain region can be reduced, so that a large output pseudo-diffraction limited light can be obtained. Furthermore, since the optical density of the propagation light increases near the input end of the gain region, the entire gain region can be gain saturated and optical amplification can be performed efficiently.

Embodiment 2. FIG.
In the laser device according to the second embodiment of the present invention, a polarization-maintaining optical fiber with a tip lens is disposed opposite to an incident end of a tapered laser amplifier, the distance between them is widened, defocused and coupled, and The optical density is configured to be higher than the saturation gain density at the incident end of the gain region. Thereby, the change of the output light with respect to an axial shift can be made small.

  FIG. 6 is a block diagram showing a laser apparatus according to Embodiment 2 of the present invention, and FIGS. 7 and 8 are explanatory views for explaining the laser apparatus according to Embodiment 2 of the present invention. In FIG. 6, 2 is a polarization-maintaining optical fiber, and 3 is a tapered laser amplifier. The detailed structure including the semiconductor laser diode element 1 is the same as that according to the first embodiment shown in FIGS.

  As shown in FIG. 6, the laser amplifying apparatus according to the second embodiment of the present invention increases the distance between the lens 2d processed at the tip of the polarization-maintaining optical fiber 2 and the waveguide 3a of the tapered laser amplifier 3, One feature is that the spots are defocused and coupled from the optimum coupling position at the spot diameters 4a and 4c as waists.

  Next, the operation will be described. FIG. 7 shows, as a solid line 7a, the input light dependence of the output light under the condition in which the TE polarized incident light and the waveguide 3c of the tapered laser amplifier 3 are mode-matched. From the figure, the input light is almost saturated when the optical density at the input position of the tapered gain region is about 1 mW / μm, that is, 7b of about 10 mW. The obtained distributed feedback laser diode element is used with an optical input of 10 mW saturation gain density 7b. On the other hand, in the second embodiment of the present invention, as shown in FIG. 6, the optical waveguide 3c and the beam waist 4e are defocused and the intensity of the input light is made sufficiently larger than the saturation gain density 7b. It is characterized by. In the configuration according to the second embodiment of the present invention, since the wavelength of the Fabry-Perot semiconductor laser diode element 1 is controlled by the optical fiber diffraction grating 2b, the optical output at the fiber end can be easily obtained at about 100 mW. It has a typical structure.

  With the configuration as described above, the operating point of the taper type laser amplifier 3 is the input light of FIG. 7 with respect to the axis deviation in the vertical direction where the axis deviation characteristics of the polarization-maintaining optical fiber 2 and the taper type laser amplifier 3 are the steepest. Since the output light is within the range of the gradual region 7c, there is a special effect that the change amount of the output light with respect to the axial deviation is approximately 1/5 compared with the change amount of the coupling efficiency with respect to the axial deviation. High quality products can be provided. For example, FIG. 8A shows a change in optical output with respect to a vertical axis deviation when the polarization-maintaining optical fiber 2 and the tapered laser amplifier 3 are moved away from the optimum coupling position by 5 μm. From the figure, it can be seen that the output change amount is about 1 dB even if the axis is shifted by 2 μm in the vertical direction, and the tolerance becomes very gradual.

  In addition, as described in the first embodiment, the tapered laser amplifier 3 is not a ridge waveguide for propagating a single transverse mode, for example, but a planar slab guide having no waveguide function in the horizontal direction. Since the waveguide structure is used, as shown in FIG. 8B, there is a special effect that the output change is very small even if the optical fiber is largely displaced in the horizontal direction.

  As described above, in the laser device according to the second embodiment of the present invention, the polarization-maintaining optical fiber with the tip lens is disposed opposite to the incident end of the tapered laser amplifier, and the distance between them is widened and defocused and coupled. In addition, the light density of the incident light is configured to be higher than the saturation gain density at the incident end of the gain region. Thereby, the change of the output light with respect to an axial shift can be made small.

Embodiment 3 FIG.
In the laser device according to Embodiment 3 of the present invention, a polarization-maintaining optical fiber with a tip lens is disposed opposite to an incident end of a tapered laser amplifier having a high TE polarization gain, and the polarization-maintaining optical fiber is twisted to obtain a TM polarization. It is configured to be incident with a wave. Thereby, a high isolation is obtained with respect to the semiconductor laser diode as the master oscillator.

  FIG. 9 is a block diagram showing a laser apparatus according to Embodiment 3 of the present invention. In FIG. 9, 1 is a semiconductor laser diode element, 2 is a polarization-maintaining optical fiber, and 3 is a tapered laser amplifier. Note that these detailed structures are the same as those according to the first embodiment.

  Next, the operation will be described. In FIG. 9, in the laser apparatus according to the third embodiment of the present invention, the TE polarized wave 8a output from the semiconductor laser diode element 1 is made to coincide with the slow axis of the polarization-maintaining optical fiber 2, and then polarized. The wave-preserving optical fiber 2 is twisted and is incident on the optical waveguide of the tapered laser amplifier 3 with TM polarization 8b (linearly polarized light having a vertical polarization plane). The active layer of the taper type laser amplifier 3 is a compression strained InGaAs single quantum well, and since the gain of TE polarization is high, the output light becomes TE polarization 8c. The input light dependency of the output light when the TM polarized wave 8b is incident on the taper type laser amplifier 3 is shown by a broken line 7d in FIG. As can be seen from FIG. 7, for the same input optical power as a whole, a broken line 7d for entering the TM polarized wave, which is an optical input configuration according to the third embodiment of the present invention, rather than the solid line 7a of the TE polarized input. Although the light output power is smaller, it is possible to obtain a light output with a light intensity that is practically acceptable.

  Here, when reflected return light from the output side to the input side of the taper type laser amplifier 3 is generated, since the gain of TE polarization is high, most of the polarization of the reflected return light becomes a TE polarization component. This coincides with the first axis of the polarization-maintaining optical fiber 2. Since the polarization-maintaining optical fiber 2 is twisted 90 degrees, the reflected return light is incident on the optical waveguide of the semiconductor laser diode 1 with TM polarization. Since the semiconductor laser diode 1 as a master oscillator that oscillates with TE polarization has a single transverse mode optical waveguide, when the reflected return light is incident on the semiconductor laser diode 1 with TM polarization, the mode conversion is usually performed. Therefore, there is an effect that high isolation can be obtained.

  In Embodiment 3 described above, the TE polarized wave is aligned with the slow axis, but the same effect can be obtained even when the TE polarized wave is aligned with the fast axis.

  As described above, in the laser apparatus according to the third embodiment of the present invention, the polarization-maintaining optical fiber with the tip lens is disposed opposite to the incident end of the tapered laser amplifier having a high TE polarization gain, and the polarization-maintaining light The fiber is twisted and configured to be incident with TM polarization. Thereby, high isolation can be obtained for the semiconductor laser diode oscillating with TE polarization.

1 is a block diagram showing a laser amplifying device according to Embodiment 1 of the present invention. 1 is a block diagram showing a laser amplifying device according to Embodiment 1 of the present invention. 1 is a block diagram showing a laser amplifying device according to Embodiment 1 of the present invention. Explanatory drawing for demonstrating the laser amplifier by Embodiment 1 of this invention Explanatory drawing for demonstrating the laser amplifier by Embodiment 1 of this invention Configuration diagram showing a laser apparatus according to Embodiment 2 of the present invention Explanatory drawing for demonstrating the laser apparatus by Embodiment 2 of this invention. Explanatory drawing for demonstrating the laser apparatus by Embodiment 2 of this invention. Configuration diagram showing a laser apparatus according to Embodiment 3 of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser diode element 2 Polarization plane preserving optical fiber 2d Lens 3 Tapered laser amplifier 3a Gain region

Claims (3)

A tapered laser amplifier having a tapered gain region extending horizontally from the incident end to the output end ;
An optical coupling means having a polarization-maintaining optical fiber in which a lens disposed opposite to an incident end of the gain region of the taper type laser amplifier is formed at a tip portion and a polarization plane in a horizontal direction is preserved at the tip portion. The light radiated from the lens is incident such that the width is narrower in the horizontal direction than the incident end of the gain region, and the incident light has a larger spread angle in the horizontal direction than the spread angle of the tapered shape. An optical coupling means in which the polarization-maintaining optical fiber is disposed so that the propagated light is wider in the horizontal direction than the output end of the gain region;
A laser amplifying apparatus comprising: Laser amplification equipment according to claim 1, wherein the horizontal radius of curvature of the lens is a 1 1 [mu] m. Laser amplification equipment according to claim 1, wherein the spread em of the gain region is 6 °.

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