JP2651701B2 - Laser module with optical isolator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser module provided with an optical isolator for blocking reflected light returning to a semiconductor laser.

2. Description of the Related Art A laser module including a semiconductor laser and an optical lens system for efficiently transmitting light emitted from the semiconductor laser to an optical fiber is used as a light emitting source of optical communication. As a result of technological progress so far, a method of lowering the coupling efficiency of the laser module itself to about 3 dB or less has been found due to the selection of the lens and the improvement of the end of the optical fiber. On the other hand, the reflected return light from the lens, optical connector, and other optical circuit system returns to the light source, disturbing the stable oscillation of the semiconductor laser,
There is a problem that the noise of the laser modulation signal increases. For this reason, it is common to mount an optical isolator in a semiconductor laser module, and various laser modules with an optical isolator have been proposed. A conventional laser module with an optical isolator generally comprises a semiconductor laser 1, collimator lenses 2 and 3, polarizing elements 4 and 5, a Faraday rotator 6, and an optical fiber 7, as shown in the configuration diagram of FIG. It is a target. The light emitted from the semiconductor laser is mostly TE mode light (the electric vector is parallel to the bonding surface of the semiconductor laser, that is, perpendicular to the paper surface) and slight TM mode light (the magnetic vector is perpendicular to the bonding surface.) ).

Therefore, when inserting the optical isolator between the collimator lenses, it is important to adjust the polarizer 4 so that the TE mode light of the semiconductor laser emission light is maximally coupled. The light linearly polarized by the polarizer 4 has its polarization plane rotated by 45 ° by the Faraday rotator 6 and passes through the analyzer 5 and the lens 3 adjusted so as to transmit the linear light rotated by 45 ° at the maximum. Is combined with On the other hand, the reflected return light transmits only the component that matches the polarization plane of the analyzer 5 and is further rotated by 45 ° by the Faraday rotator 6. Also polarizing element
There is also a method of inserting a polarizing prism using birefringent crystal rutile in 4,5.

[Problems to be Solved by the Invention] Factors required for a semiconductor laser module with an optical isolator include important issues such as small size, low loss, easy manufacturing, and low cost. When a splitter is used, miniaturization of the optical isolator is limited, and the distance between the collimator lenses is required to be 10 mm or more. Therefore, there are disadvantages that the coupling loss is large and the shape is large. Further, in a polarizing prism or the like using rutile, there is a problem in lowering the price because rutile itself is expensive. In addition, in order to obtain the optical axis O of calcite, it is necessary to cut out a hatched portion s as shown in FIG. 3 for various prisms using calcite. In addition, there is a limit to miniaturization as in the case of the polarization beam splitter.

Means for Solving the Problems In order to solve the above-mentioned conventional problems, the present invention relates to an optical system including a semiconductor laser, a pair of collimator lenses, and an optical fiber. A single parallel plate utilizing birefringence is used. In this case, a parallel veneer in which the cleavage planes of the calcite are parallel planes greatly improves the yield and the like and simplifies the manufacturing method, so that the cost can be reduced and the size reduction is effective.

Embodiment FIG. 1 is a principle diagram of an isolation operation using a parallel single-plate birefringent crystal of the present invention. Each of the polarizer 4 and the analyzer 5 is a parallel single plate using a calcite cleavage plane and slightly optically polished. The Faraday rotator 6 is a Bi-substituted garnet single crystal that is liquid phase epitaxially grown on a garnet substrate, and its thickness is polished and adjusted so that its polarization plane rotates 45 ° with respect to light having a wavelength of 1.3 μm. . When unpolarized light enters the polarizer 4 from the forward light source, the ordinary ray goes straight, the extraordinary ray is separated by d, and enters the Faraday rotator 6. Here, the plane of polarization is rotated by 45 °, the polarizer 4 and the analyzer 5 whose optical axis is rotated by 45 ° are incident on the analyzer 5, and the ordinary ray goes straight as it is; (A). Next, when the light that is coincident with the optical axis of the analyzer returns in the reflected light returning in the opposite direction, it is separated into an ordinary ray and an extraordinary ray by the analyzer 5 and rotated by 45 ° by the Faraday rotator 6. At this time, the ordinary ray of the analyzer becomes an extraordinary ray because it is rotated by 90 ° with respect to the optical axis of the polarizer, and the extraordinary ray when passing through the analyzer becomes the ordinary ray. As a result, after passing through the polarizer 4, the light is separated from the initial optical axis by d (b). On the other hand, the coupling loss due to the axial displacement between the lens and the optical fiber is, as shown in the embodiment of FIG.
It has been confirmed that a loss of −40 dB occurs when an axis deviation of ± 80 μm or more occurs. When applying this property to an optical illuminator, by displacing the optical axis by ± 100 μm or more,
Return light returning to the semiconductor laser can be prevented.

The displacement d from the optical axis when the cleavage plane of calcite is used as it is for a single plate for birefringence is calculated from the following equation.

Where l is the thickness of the parallel single plate, θ is the angle between the optical axis and the cleavage plane (up to 44.6 °), no is the refractive index of ordinary light (= 1.658), N
e is the refractive index of the extraordinary ray (= 1.486). Suppose d is 100
When displaced by μm, 1 = 0.916 mm from the above equation. In the experimental example of the coupling loss, the optical fiber is a graded index type (GI type), and if a single mode fiber has a displacement of 100 μm, there is almost no coupling, so that high isolation is obtained. Therefore, if the thickness of the calcite parallel veneer is 1 mm, the displacement d is 109 μm from the above equation, and sufficient isolation can be expected. At a wavelength of 1.3 μm, the thickness of the Faraday rotator is about 0.8 mm including the substrate, so the total length of the optical isolator is 3 mm.
It can be constructed in less than mm. Therefore, when the semiconductor laser-side collimator lens 2 of FIG. 1 uses a spherical lens having a diameter of about 1 mm and the fiber-side collimator lens 3 uses a GRIN lens, the distance between the lenses is 3 to 4 mm, and the coupling efficiency is improved. can do.

[Effects of the Invention] As described above, when the polarizing element of the optical isolator for a module disclosed in the present invention is formed of a single parallel plate using the cleavage plane of calcite as it is, the raw material calcite can be used without waste. The price was lower than when a birefringent crystal such as rutile was used, and the module was downsized and high isolation was realized.

[Brief description of the drawings]

FIG. 1 is a principle diagram of an isolation operation according to an embodiment of the present invention. (A): forward light, (b): backward light FIG. 2 is a configuration diagram of a laser module with an optical isolator. FIG. 3 is a schematic diagram showing an optical axis O of calcite and a cut-out portion s thereof. FIG. 4 is a diagram showing the relationship between the coupling efficiency and the axis deviation between the optical fiber and the lens. 1: Semiconductor laser, 2, 3: Collimator lens 4, 5: Polarizing element, 6: Faraday rotator 7: Optical fiber

Claims (1)

(57) [Claims] A semiconductor laser, a pair of collimator lenses,
An optical system comprising an optical fiber, a pair of polarizing elements formed of calcite whose optical axes are rotated by 45 ° between a pair of collimator lenses, and a Faraday rotation for rotating a polarization plane by 45 ° between the polarizing elements. , An optical isolator composed of permanent magnets for magnetic saturation of the Faraday rotator,
A laser module with an optical isolator, wherein a parallel single plate having a parallel cleavage plane of the calcite is used.