JPH0738480B2 - Laser light source - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a laser light source having a narrow spectrum and variable wavelength using an external resonator.

<< Prior Art >> Coherent communication is being energetically studied as a next-generation optical communication system. However, improvement of the spectral purity of semiconductor lasers (narrowing of the spectrum) has become a technical issue for practical use. The required spectral width is about 1 MHz or less. Similar requirements exist in the field of optical applied measurement. There is also a demand for a laser light source whose oscillation wavelength is variable at the same time as narrowing the spectrum.

Conventionally, such a narrow spectrum semiconductor laser has been realized by providing a resonator outside the semiconductor laser and increasing the resonator length. As an external resonator,
A total reflection mirror and a diffraction grating were used. An example using a diffraction grating is shown in FIG. The light emitted from the semiconductor laser 21 in both directions is collimated by the left and right lenses 22, and the diffraction grating 2
And resonate between the output mirror 24. The resonance wavelength can be changed by rotating the diffraction grating 23. However, in the external resonance type laser with such a configuration, the spectrum width becomes wider when the phase of the return light is shifted, so if the positional relationship between the external mirror and the laser is several tenths of the oscillation wavelength and is not stable, the desired spectrum The width cannot be obtained and it tends to be mechanically extremely unstable.

FIG. 3 is a method proposed by Yariv et al. Of the California Institute of Technology in order to eliminate this drawback, which is a semiconductor laser light source using a phase conjugate mirror as an external resonator. The light emitted from the semiconductor laser 31 is a nonlinear optical crystal.
Pass through the phase conjugate mirror 32 made of BaTiO ₃ single crystal
The light is reflected by 33 and 34 and returned to the inside of the phase conjugate mirror 32 (ray 35), and intersects the laser light at an angle. This ray 35 produces a ray 36 traveling in the opposite direction due to scattering inside the crystal 32. That is, crystal 32 → mirror 33 → mirror 34 → crystal 32 →
A loop of mirror 34 → mirror 33 → crystal 32 forms a ring resonator to generate a standing wave. Therefore, apparently,
The light rays 35 and 36 become pump light and generate a phase conjugate wave for the incident light from the laser 31 (self-phase conjugation) and crystal.
From 32, it returns as reflected light along the same optical path as the incident light.
(Here, the phase conjugate wave refers to its complex conjugate E ^* with respect to the incident light E. In the BaTiO ₃ single crystal, the generation of the phase conjugate wave is performed by the process of degenerate four-photon mixing.) As a result, the crystal 32 acts as an external cavity for laser 31.
Due to the nature of the phase conjugate wave, even if the positions of the laser 31 and the phase conjugate mirror 32 move, the light automatically returns to the laser 31 so as to cancel them, so that a mechanically stable external resonator is provided.

<< Problems to be Solved by the Invention >> However, in the semiconductor laser light source configured as described above, it takes time for the resonator to be formed due to scattering in the crystal 32, etc., so that the laser 31 starts to emit light. It may take several seconds to several hours to generate a phase conjugate wave. In particular, it is known that the longer the wavelength, the longer the time required for stabilization. Moreover, it is difficult to control the oscillation wavelength.

The present invention has been made in order to solve such a practical problem. The spectral characteristics are mechanically and thermally stable, the characteristics can be obtained immediately after laser driving, and the oscillation wavelength can be controlled. The object is to realize a simple external resonance type laser light source.

<< Means for Solving the Problems >> The present invention relates to a laser light source using a phase conjugate mirror as an external resonator, and is characterized by a phase conjugate mirror that makes light emitted from a laser incident and a phase conjugate mirror Optical means for obliquely intersecting the laser light with the transmitted light as pump light in the conjugate mirror through the wavelength selection means, a mirror for reflecting the pump light transmitted through the phase conjugate mirror and returning it to the original state, and this mirror And a blocking means for blocking the reflected light from returning to the laser, and the oscillation wavelength is variable by controlling the wavelength selecting means.

<Operation> Since the pump light is forcibly produced by the mirror, a phase conjugate wave is generated immediately after the laser is driven and operates as a phase conjugate resonator, and the wavelength of the standing wave is controlled by the wavelength selection means to make the phase conjugate wave. The oscillation wavelength can be made variable by changing the wavelength.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration block diagram showing an embodiment of a laser light source according to the present invention. 1 is a semiconductor laser, 11 is an anti-reflection (AR) coated end face of the semiconductor laser 1, and 2 is a nonlinear optical crystal of BaTiO ₃ (barium titanate) single crystal that receives the light emitted from the end face 11. 3 is a phase conjugate mirror (hereinafter referred to as crystal 2), 3 is a collimator lens that makes light transmitted through the crystal 2 incident, 4 is a first mirror that makes light that has passed through the collimator lens 3 incident, and 5 is this mirror 4. Is an optical isolator for injecting the reflected light, and 6 is a diffraction grating for forming the wavelength selecting means to inject the output light of the optical isolator 5, and 7 is an incident light after the diffraction light of the diffraction grating 6 has passed through the crystal 2. A third mirror 8 is a control circuit for controlling the current and temperature of the semiconductor laser 1. The lens 3, the mirror 4 and the diffraction grating 6 constitute an optical means for returning the transmitted light of the phase conjugate mirror 2 as pump light and obliquely intersecting the laser light in the phase conjugate mirror 2. The optical isolator 5 constitutes blocking means for blocking the reflected light of the mirror from returning to the laser light source 1. Here, it is preferable that the crystal 2, the collimator lens 3, and the optical isolator 5 are arranged so as to be inclined by a few degrees with respect to the optical axis so that the Fresnel reflected light on each surface does not return to the laser 1. The optical isolator 5 is preferably one that does not change the plane of polarization of light, and for example, an isolator using the Faraday effect such as YIG 7 connected in the opposite direction can be used.

The light emitted from the end face 11 of the semiconductor laser 1 passes through the crystal 2 and becomes a parallel light by the collimator lens 3, and the mirror 4
Is reflected by and passes through the optical isolator 5. The light that has passed through the optical isolator 5 is diffracted by the diffraction grating 6, and only the diffracted light having a specific wavelength passes through the crystal 2 again, is reflected by the mirror 7, and returns to the original optical path in the opposite direction. The reflected light of the mirror 7 forms a standing wave as the forced pump light with the diffracted light of the diffraction grating 6, and immediately generates a phase conjugate wave of the incident light from the laser 1. This phase conjugate wave becomes light of the same wavelength as the standing wave (the wavelength may be slightly shifted depending on the nonlinear medium, but it is generally extremely small and can be ignored). That is, of the light incident on the crystal 2 from the laser 1, only the light of the wavelength selected by the diffraction grating 6 returns to the laser 1 as a phase conjugate wave, and this becomes the oscillation wavelength of the laser 1. Here, the crystal 2 operates as an external resonator (here, a phase conjugate resonator) of the laser 1. Therefore, if the diffraction grating 6 is rotated to change the angle, the wavelength of the standing wave changes and the oscillation wavelength can be changed. The light reflected by the mirror 7 is blocked by the optical isolator 5 so as not to return to the laser 1 and disturb its resonance state. The laser output to the outside is taken out from the end surface of the semiconductor laser 1 opposite to the end surface 11.

According to the laser light source having such a configuration, a variable wavelength laser light source with a narrow spectrum can be realized with a simple configuration by controlling the wavelength of the standing wave.

Also, because the pump light is forcibly created by the mirror,
As soon as the laser emits light, the phase conjugate mirror is activated. Therefore, the responsiveness is extremely superior to the conventional one. A stable pump light source is not required, so the configuration is simple.

Further, due to the nature of the phase conjugate wave, even if the positions of the laser 1 and the phase conjugate mirror 2 move, the light automatically returns to the laser 1 so as to cancel them, so that a thermally and mechanically stable external resonator is provided. Becomes

In the above embodiment, the direction of light may be changed by an optical fiber or the like instead of the mirror 4.

Also, instead of BaTiO ₃ , a BSO single crystal with an electric field applied or N
Any material that can generate a phase conjugate wave such as a gas can be used.

The collimator lens 3 can also be arranged between the laser 1 and the crystal 2.

Further, the mirror 4 and the diffraction grating 6 may be exchanged.

Although the resolution is lowered, a prism can be used as the wavelength selecting means instead of the diffraction grating.

Further, the diffraction grating 6 is replaced with a mirror, and an acousto-optic modulator (AO) is used as a wavelength selecting means in the optical path between the mirror and the crystal 2.
M) and by changing its ultrasonic frequency,
It is also possible to electrically change the wavelength of the standing wave.

If a polarizing prism and a quarter-wave plate are combined instead of the mirror 4, the blocking means can be constructed without using the optical isolator 5.

<< Effects of the Invention >> As described above, according to the present invention, the external resonance type laser whose spectral characteristics are mechanically and thermally stable, the characteristics are obtained immediately after laser driving, and the oscillation wavelength can be controlled. The light source can be realized with a simple structure.

[Brief description of drawings]

FIG. 1 is a structural block diagram showing an embodiment of a laser light source according to the present invention, and FIGS. 2 and 3 are structural block diagrams showing a conventional laser light source. 1 ... Laser, 2 ... Phase conjugate mirror, 3, 4, 6 ... Optical means, 5 ... Blocking means, 6 ... Wavelength selecting means, 7 ... Mirror.

Claims (4)

[Claims] 1. A laser light source using a phase conjugate mirror as an external resonator, wherein a phase conjugate mirror into which emitted light from a laser is incident and transmitted light in the phase conjugate mirror as pump light via wavelength selecting means. Optical means for obliquely intersecting the laser light, a mirror for reflecting the pump light transmitted through the phase conjugate mirror and returning it to the original state, and a blocking means for blocking the reflected light of the mirror from returning to the laser are provided. A laser light source characterized in that the oscillation wavelength is variable by controlling the wavelength selection means. 2. The laser light source according to claim 1, wherein a mirror is used as the optical means. 3. A laser light source according to claim 1, wherein a diffraction grating is used as the wavelength selecting means. 4. The laser light source according to claim 1, wherein an optical isolator is used as the blocking means.