JPH0836201A - Wavelength variable laser device and laser printer device - Google Patents

Abstract

PURPOSE:To stably control the oscillation wavelength of a semiconductor laser and to realize miniaturization and low power consumption by using a wavelength control element for controlling the oscillation wavelength in the external resonator of an external resonator laser. CONSTITUTION:A P type semiconductor laser of AlGaI having a rear end face where a non-reflection coating 12 having 2% reflectance to the oscillation wavelength is formed and having a front end face 13 where a reflection surface is formed by cleavage is used as the semiconductor laser 11. The oscillating beam 17 emitted from the semiconductor laser 11 is condensed by a lens 14 and forms the external resonator between a plane mirror 16 to which total reflection coating having >=99.9% reflectance is applied and the front end face 13. The wavelength is controlled by arranging the wavelength control element 15 between the lens 14 and the plane mirror 16. A double refraction filter consisting of three quartz plates having different thickness is used as the wavelength control element 15, which is arranged at a Brewsters's angle to an optical axis and rotated around the optical axis, thereby controlling the wavelength. The laser beam whose wavelength is controlled is emitted from the front end face 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of optoelectronics, and more particularly to a laser printer device or an optical disk device using a laser light source and a visible laser light source.

[0002]

2. Description of the Related Art With the progress of the advanced information age, there is an increasing demand for a shorter wavelength in order to satisfy the demands for higher recording density and higher speed printing in the optical recording field such as optical disc devices and laser printer devices. However, as a light source that satisfies the blue region, which is a highly demanded wavelength region at the commercial level,
e-Cd (helium-cadmium) laser device and Ar
There is only a gas laser device such as an (argon) laser device, and it is not suitable for mounting on an optical disk device, for example, because of its large size and large power consumption. Further, although the gas laser device is mounted as a light source in some laser printer devices, there is a possibility that it will be an obstacle to further miniaturization and lower power consumption in the future.

On the other hand, optical second harmonic generation (SHG;
A technology for shortening the wavelength by using Second Harmonic Generation) was proposed. The examination of the technology for practical use of the SHG light source has advanced along with the increase in the output of the semiconductor laser. Behind that, there was the possibility of realizing small size and low power consumption without the need for discharge like conventional gas lasers,
Next is the high reliability (output stability, long life) of the SHG light source that depends on the output stability and long life of the pumping semiconductor laser. As an SHG light source having an output wavelength equivalent to that of the gas laser, for example, a near-infrared semiconductor laser output is used as a first oscillating wave, that is, a fundamental wave, and it is resonated by an external resonator, and a nonlinear optical crystal (which will be dealt with below) Since all wavelength conversion is SHG, it is simply referred to as SHG crystal.) A method has been proposed in which KN (KNbO ₃ ; potassium niobate), which is a SHG crystal, is arranged to obtain a blue laser beam that is a second oscillation wave, that is, an SH wave. (WJ Kozlovsky and W.Lenth, "G
eneration of 41mW of blue radiation by frequency d
oubling of a GaAlAsdiode laser ", Appl.Phys.Lett., vo
l.56, no.23, p2291, 1990).

[0004]

The problem to be solved by the present invention is to stably adjust the oscillation wavelength of a semiconductor laser which is easily affected by disturbance in an SHG light source to a wavelength at which the conversion efficiency of a nonlinear crystal is maximized. That is, the resonator length of the external resonator including the nonlinear crystal is tunably controlled in the wavelength order of the fundamental wave.

The wavelength control of the fundamental wave light source required for the SHG light source is an essential technique for stabilizing the SHG output and increasing the output. As a method of controlling the wavelength of a laser oscillator which is a light source of a fundamental wave, there have been conventionally known a dispersion prism method, a grating method and the like. The features and problems are described below.

(1) In the dispersion prism method, the wavelength separation angle is changed by the wavelength dispersion of the refractive index of the prism material,
This is a method of oscillating a specific wavelength in a resonator by placing a reflection mirror at an arbitrary angle. Because the separation angle is small,
This is effective when the oscillation lines are more than 10 nm apart, such as in a gas laser.

(2) The grating method takes advantage of the fact that the periodic grooves formed on the surface of the glass substrate serve as a diffraction grating, and the diffraction efficiency differs depending on a specific wavelength and order. Diffracted light can be extracted. However, since the diffraction efficiency is as low as about 60% at maximum and it cannot be inserted into the laser resonator, the wavelength is controlled by using it as a composite external resonator. FIG. 6 is a configuration diagram in which the oscillation wavelength of the semiconductor laser 11 is controlled by the grating 61. The laser beam 17 incident on the grating 61 needs to be a parallel beam, and if the incident angle shifts, the diffraction efficiency decreases and the oscillation spectrum width widens. Further, there is a problem in that it is difficult to adjust the resonator composed of the grating surface and the end face of the semiconductor laser because it easily enters the unstable region.

[0008]

DISCLOSURE OF THE INVENTION As a result of continuous studies, the inventors of the present invention have performed wavelength control for controlling an oscillation wavelength in an external cavity of an external cavity laser formed by a semiconductor laser and a cavity mirror. It has been conceived that a tunable laser device in which the oscillation wavelength of a semiconductor laser is stably controlled can be obtained by using the element. Further, by using a birefringent crystal tilted at Brewster's angle as the wavelength control element, it is possible to tune the wavelength in a wide band with less loss. The birefringent crystal includes quartz (SiO ₂ ), LiNbO ₃ and LiTa.
Any of O ₃ is used. The semiconductor laser having both end faces or one end face coated with an antireflection film having a reflectance of 2% or less is used.

Generally, the birefringence filter method is a combination of a plurality of wave plates (birefringence crystals) and a polarizer, and it is used that ordinary light and extraordinary light propagate at different phase velocities due to the birefringence of the crystals. Since the polarization characteristics with respect to the wavelength differ depending on the thickness of the crystal, the bandpass filter transmits only an arbitrary wavelength. By inserting the birefringent crystal into the resonator at Brewster's angle, theoretically, the loss of the filter is eliminated and the filter can have a function as a polarizer, so that the polarizer becomes unnecessary.

The external resonator comprises at least two resonator mirrors, at least one of which has a reflectance of 99.9%.
A wavelength tunable laser device is configured by using the above total reflection mirror.

Further, a non-linear crystal is arranged in the external resonator to generate the second harmonic of half the oscillation wavelength. The nonlinear crystal includes KN (KNbO ₃ ), KLN (K ₃ Li _2-X Nb
_{5 + X} O ₁₅₊ ), LBO (LiB ₃ O ₅ ) and at least one mirror forming the resonator structure has a transmittance of the second harmonic of 60% or more for efficient SHG light Can be generated.

By adopting these means, a compact and low power consumption wavelength tunable laser device using a semiconductor laser can be realized. This tunable laser device can be used as a light source of a laser printer device.

[0013]

【Example】

(Embodiment 1) FIG. 1 is a diagram for explaining an embodiment of the present invention. The semiconductor laser 11 (LD) is non-reflective (hereinafter simply referred to as AR;
The Anti-Reflection) coating 12 and the front end face 13 are made of an AlGaInP-based semiconductor laser forming a reflection surface by cleavage, and have an output of 150 mW and an oscillation wavelength of 850 nm. The oscillation beam 17 emitted from the semiconductor laser is condensed by the lens 14 and is provided between the front end face 13 and the plane mirror 16 which is subjected to total reflection (hereinafter simply referred to as HR; High-Reflection) coating having a reflectance of 99.9% or more. To form an external resonator. The wavelength is controlled by disposing the wavelength control element 15 between the lens 14 and the plane mirror 16. The wavelength control element 15 uses a birefringent filter composed of three crystal plates (1, 2, 3 mm) having different thicknesses, and is arranged at Brewster's angle with respect to the optical axis and rotated around the optical axis. Control the wavelength. The wavelength-controlled laser light is emitted from the front end face 13.

FIG. 4 shows wavelength tunable characteristics when the birefringent filter 15 is rotated. Maximum wavelength variable width is about 11 nm
Shows a discontinuous wavelength tunable characteristic with a small width of several nm. This is because the gain width, which is the oscillation characteristic of the LD itself, is 10
Since it is about nm, the birefringent filter oscillates with a wavelength tunable width of 10 nm at the first peak of the transmission wavelength characteristics, and the lasing gain of the LD is high even at the second peak, so that the oscillation characteristics are shown with a width of several nm. FIG. 5 shows the oscillation spectrum (LD mode) of the LD itself and the oscillation spectrum (Control mode) whose wavelength is controlled by the birefringent filter. From the figure, the controlled oscillation spectrum has a peak away from the mode of the LD itself, and shows stable oscillation characteristics in which the side mode is suppressed by 40 dB or more.

(Embodiment 2) FIG. 2 is a diagram for explaining another embodiment of the present invention. The optical system from the rear end face to the reflection mirror when the both end faces of the semiconductor laser 11 are AR-coated 12 is the same as that of the first embodiment. This is a case where a resonator is formed by the concave mirror 21 on the front end face side and the plane mirror 16. The concave mirror 21 has a reflectance of 95 with respect to the oscillation wavelength.
By applying a reflective coating of 5%, 5% of the output is taken out of the resonator.

(Embodiment 3) FIG. 3 is a view for explaining another embodiment of the present invention. The semiconductor laser 11 and the resonator optical system are the same as in the second embodiment. The nonlinear crystal 31 is inserted on the front end face side, and the concave mirror 32 is provided with an HR coating having a reflectance of 99.9% or more for the fundamental wavelength and an AR coating having a transmittance of 85% or more for the SH wavelength. . The non-linear crystal 31 is a 3 × 3 × 5 mm KN crystal and has a reflectance of 0.2% or less with respect to the fundamental wave wavelength on both end surfaces, S
An AR coating having a reflectance of 1% or less for H wavelength was applied. The birefringent filter was rotationally adjusted to a wavelength at which the conversion efficiency of the nonlinear crystal 31 was maximized. Output mirror 32 at this time
From, an SH output 33 of 10 mW was obtained outside the resonator.
The non-linear crystal 31 includes KLN (K ₃ Li _2-X Nb _{5 + X} O ₁₅ ), LBO.
A (LiB ₃ O ₅ ) crystal may be used.

(Embodiment 4) FIG. 7 is a diagram for explaining the configuration of a laser beam printer as an embodiment using the present invention. Tunable laser device 10 described in the third embodiment
The SH output 33 emitted from 0 is an acousto-optic (AO; Acousto-Optical) modulator 201, a folding mirror 206, a beam expander 202, a rotary polygon mirror 20.
3, the light passes through the fθ lens 204 and is condensed on the photosensitive drum 205. The AO modulator 201 modulates the SH output 33 according to the image information, and the rotary polygon mirror 203 scans in the horizontal (inside the paper) direction. With this combination, the two-dimensional information is recorded on the photosensitive drum 205 as a partial potential difference. The photosensitive drum 205 attaches toner according to the potential difference and rotates to reproduce information on a recording sheet.

[0018]

According to the present invention, a stabilized wavelength tunable laser device is realized by controlling the oscillation wavelength of a semiconductor laser with a birefringent filter. Moreover, the efficiency and reliability of the second harmonic generation are improved by using the oscillation wavelength as the fundamental wave. Furthermore, the reliability of a laser printer etc. has been improved as an applied product using this tunable laser device as a light source.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining another embodiment of the present invention.

FIG. 3 is a diagram for explaining another embodiment of the present invention.

FIG. 4 is a diagram showing wavelength tunable characteristics in a wavelength control element for the oscillation wavelength of a semiconductor laser.

FIG. 5 is a diagram showing an oscillation spectrum of a semiconductor laser with and without wavelength control.

FIG. 6 is a diagram for explaining a conventional wavelength control method for a semiconductor laser.

FIG. 7 is a diagram for explaining one embodiment using the present invention.

[Explanation of symbols]

 11 Semiconductor Laser 12 AR Coating 14 Lens 15 Wavelength Control Element 16 Planar Mirror 17 Oscillation Beam 21 Concave Mirror 31 Nonlinear Crystal 32 Concave Mirror 33 SH Wave or SH Output

Claims (10)

[Claims] 1. A wavelength tunable laser device having a wavelength control element for controlling an oscillation wavelength in an external resonator of an external resonator laser formed by a semiconductor laser and a resonator mirror. 2. A birefringent crystal tilted at Brewster's angle is used as the wavelength control element.
The wavelength tunable laser device described in. 3. The birefringent crystal is made of quartz (SiO ₂ ), LiNb.
_{3. The} tunable laser device according to claim 2, wherein either O ₃ or LiTaO ₃ is used. 4. The wavelength tunable laser device according to claim 1, wherein an antireflection film having a reflectance of 2% or less is formed on at least one end face of the semiconductor laser. . 5. The external resonator comprises at least two resonator mirrors.
The tunable laser device according to any one of items 1 to 4. 6. The resonator according to claim 1, wherein at least one of the resonator mirrors is totally reflected with a reflectance of 99.9% or more.
5. The wavelength tunable laser device according to any one of items 5 to 5. 7. The non-linear crystal is arranged in the external resonator, and the non-linear crystal generates a second harmonic having a half of the oscillation wavelength. Tunable laser device. 8. The nonlinear crystal contains KN (KNbO ₃ ), KLN
The tunable laser device according to claim 7, wherein any one of (K ₃ Li _2-X Nb _{5 + X} O ₁₅ ) and LBO (LiB ₃ O ₅ ) is used. 9. At least one forming the external resonator.
9. The wavelength tunable laser device according to claim 7, wherein the second harmonic has a transmittance of 60% or more in one mirror. 10. A laser printer device using the wavelength tunable laser device according to claim 1. Description: