JPS62285480A - Light pulse train generator - Google Patents

Abstract

PURPOSE:To obtain the pulse output lights of laser lights, in which a TE mode and a TM mode are changed alternately, by using a semiconductor laser and a resonator having a collimation lens function, a function turning the direction of polarized light at 90 deg. and a reflection function of lights. CONSTITUTION:The direction of polarized light is rotated at 90 deg. by an external resonator consisting of a collimation lens 2, a lambda/4 plate 3 and a mirror 4 in a TE mode emitted from a semiconductor laser 1, and beams are fed back to a TM mode in the laser 1. Accordingly, the TE mode and the TM mode are excited alternately at every round trip time of the external resonator.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a direct current driven optical pulse train generator.

(Prior Art) Pulse operation of semiconductor lasers has been widely researched and developed for the purpose of using them as clock signals in the field of optical communications and for instantaneously obtaining large outputs.

The latter high output pulse is used as a light source for second harmonic generation.

Conventionally, such a pulse train is ■Direct current modulation method.

■Q switch method that changes the Q value of an external resonator formed outside the semiconductor laser.

■Passive mode-locking method in which a supersaturated absorber is inserted into an external resonator formed outside the semiconductor laser.

■Active mode rotator method that modulates a semiconductor laser using the round trip frequency of an external resonator formed outside the semiconductor laser.

It was carried out by

(Problems to be Solved by the Invention) Among the conventional methods described above, methods (1) and (3) require current modulation and modulation synchronization, making the device complex.

Method (2) requires an optical switch, making the device complex, and method (2) has problems such as low reliability of the supersaturated absorber and low efficiency due to inherent absorption, and requires a simple configuration. It was difficult to easily generate an optical pulse train.

(Means for Solving the Problems) That is, in order to solve the above-mentioned problems, the present invention provides a structure having at least a semiconductor laser, a collimation lens function, a function of rotating the polarization direction of light by 90 degrees, and a light reflection function. With this resonator, it is possible to obtain an optical pulse train in which the polarization direction of laser light alternates under conditions of direct current injection into the semiconductor laser.

Furthermore, the time for light to travel back and forth along the optical resonator length of the resonator having the above configuration (2L/C: C is the speed of light) is approximately the same as the carrier slowing time (τR) of the semiconductor laser. As a result, a large pulse train in TE mode can be obtained.

(Function) In the present invention, in the above configuration, pulsed output light of a semiconductor laser light in which the TE mode and the TM mode are alternately switched under DC current injection conditions can be obtained.

In other words, the polarized light component emitted from the semiconductor laser is made parallel by a collimation lens, the polarization direction is rotated by 45 m by a λ/4 plate, reflected by a mirror, and then passes through the λ/4 plate again, where the polarization direction is further changed. rotates 45@, passes through the collimation lens, and returns to the semiconductor laser again.

At this time, for example, the TM mode light returns to the light emitted in the TE mode, and the TE mode □ light returns to the light emitted in the TM mode, so the mirror loss for each mode is determined by the time determined by 2L/C. It has a changing effect.

When the round trip time 2L/C of the external cavity and the carrier relaxation time τ□ of the semiconductor laser become equal, in TM mode, the mirror loss of the semiconductor laser on the output side is large, so that the carriers inside the semiconductor laser The density is high, and when it shifts to the TE mode, the mirror loss in the TE mode is small, so the carrier density shifts to a low carrier density, and the excess carriers generate strong light in the TE mode.

This effect becomes more pronounced as the mirror loss difference in the TE mode of the semiconductor laser increases.

(Embodiment) FIG. 1 is a configuration diagram of a laser device according to an embodiment of the present invention, in which 1 is a semiconductor laser, 2 is a collimation lens, 3 is a λ/4 plate, 4 is a mirror, 5 is a lens, and 6 is a Laser output light, 7 is anti-reflection coating.

In FIG. 1, the TE mode emitted from a semiconductor laser 1 is transmitted through a collimation lens 2. The external resonator consisting of the λ/4 plate 3 and the mirror 4 rotates the polarization direction by 90 degrees and returns the light to the TM mode inside the semiconductor laser 1, so that the round trip time of the external resonator (
TE mode and TM mode are alternately excited every 2L/C).

FIG. 2 is an explanatory diagram of an optical pulse train according to an embodiment of the present invention.

In the figure, the horizontal axis shows time, and the vertical axis shows the intensity of the laser output light 6.

Figure 2(a) shows 2L/C>τ9, FIG. 2(b) shows 2L/C to τR.

The case is shown below.

In a normal semiconductor laser, the open face of the crystal is used as a light reflector, and the reflectance is larger in the TE mode than in the TM mode.

Therefore, the threshold current (or threshold carrier density) for the TE mode is smaller than that for the TE mode, so the intensity of the laser output light 6 becomes greater in the TE mode.

This tendency becomes stronger when the external cavity length becomes shorter and the round trip interval (2L/C) becomes comparable to the relaxation time τR of the carriers in the semiconductor laser 1, and a large TE
A modal pulse is obtained.

Note that, as long as the semiconductor laser 1 used in the present invention is of the Fapley-Bello type, the same effect can be obtained regardless of its material and oscillation wavelength.

Further, although the anti-reflection coating 7 shown in the embodiment is formed of a dielectric film, it may be omitted if the intensity ratio of the pulses in the TE mode and the TM mode is to be reduced.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain pulsed output light of a semiconductor laser light in which the TE mode and the TM mode are alternately switched under DC current injection.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a laser device according to an embodiment of the present invention, and FIG.
The figure is an explanatory diagram of an optical pulse train according to an embodiment of the present invention. 1...Semiconductor laser, 2...Collimation lens, 3...λ/4 plate, 4...Mirror, 5
... Lens, 6 ... Laser output light. 7...Non-reflective coating. Figure 1 1 ・Rade 2 Collimation Lens 3... ゝ/4穣4 ・Mirror 5... Lenda 6... Rede Kitsune Rikimaru

Claims (2)

[Claims] (1) A resonator consisting of a semiconductor laser and a mechanism having at least a collimation lens function, a function to rotate the polarization direction of light by 90 degrees, and a mechanism having a light reflection function allows the laser beam to be An optical pulse train generation device characterized in that it obtains an optical pulse train in which the polarization direction alternates. (2) Time for light to travel back and forth along the optical resonator length L of the resonator (2
The optical pulse train generator according to claim 1, wherein L/C (C is the speed of light) is approximately the same as the carrier relaxation time (τ_R) of a semiconductor laser.