JPS6239838B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coupling device for a semiconductor laser and an optical fiber in optical fiber communication.

Semiconductor lasers are small, capable of low-voltage oscillation, and can be directly modulated, so they are important elements, along with light-emitting diodes, as light sources for optical communications. However, when the output light of the semiconductor laser returned by an external reflector is recombined into the active layer, a change called a self-coupling phenomenon occurs. This causes a direct deterioration of the current-optical output characteristics, an increase in the optical output, and a change in the oscillation spectrum of the semiconductor laser due to the returned light.
This causes deterioration of the S/N ratio and is a very inconvenient phenomenon in optical communications.

By the way, an optical fiber is made of glass fiber consisting of a part with a high refractive index called the core and a part with a low refractive index called the cladding, and the light propagates through total reflection at the boundary between the core layer and the cladding layer. It is used as a transmission line.

Light that is guided through repeated total reflections is called a mode, so an optical fiber in which multiple modes are propagated by being reflected at different angles at the boundary between the core and the cladding is called a multimode optical fiber. . In the case of optical communication, for example, when coherent light from a laser light source such as a semiconductor laser is transmitted using a multimode optical fiber, the phase velocity differs between the modes, so the difference between the various propagation modes of the optical fiber Interference occurs, and when the light emitted from the optical fiber is observed, a speckled pattern appears. The noise caused by this is called speckle intensity noise.

This speckling phenomenon changes spatially and temporally due to vibrations of the optical fiber, fluctuations in the wavelength of the light source, and even when a semiconductor laser oscillates in a single mode, the wavelength changes depending on current and temperature. For this reason, especially when an optical fiber has a connection point, such as a connector connection or splicing, speckle loss occurs at that connection point, and this changes over time, resulting in a significant S/N difference when transmitting an analog signal. resulting in a deterioration of the ratio. When coherent light propagates through a multimode optical fiber, the speckle phenomenon described above is unavoidable, so when using a semiconductor laser for optical fiber communication, it is necessary to take measures to reduce the speckle.

This invention was made in view of the above circumstances, and it is possible to reduce the influence of return light due to reflection and noise due to speckle in optical fiber communication when a semiconductor laser is used as a light source with a relatively simple configuration. The purpose of the present invention is to provide a device for coupling a semiconductor laser and an optical fiber.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 1 is a semiconductor laser, 2 is a coupling lens installed between the semiconductor laser 1 and an optical fiber 5, which will be described later, and 3 is a polarizing plate. It is arranged so that light with an electric field component parallel to the plane passes through. 4
is a quarter wavelength plate, made of a transparent piezoelectric crystal such as quartz, and arranged so that its optical axis forms an angle of 45 degrees with respect to the bonding surface of the semiconductor laser 1. Interdigital electrodes 4a are provided on either the front or front surfaces of the quarter-wave plate 4. 5 is an optical fiber, and 6 is a high frequency oscillation circuit that applies a current to the interdigital electrode 4a so that a surface acoustic wave is generated at the quarter wavelength.

Next, the operation of the above configuration will be explained.

The refractive index ellipsoid of a crystal with optical anisotropy, for example, a uniaxial crystal, is X ² + Y ² /n _p ² +Z ₂ /n _e ² =1 ... (1) n _p = n _x = n _y ... Ordinary ray _ne = n _z ...Represented by extraordinary ray.

The principal axes of the refractive index ellipsoid are X, Y, and Z, and the thickness of this crystal in the Y-axis direction is D. The refractive index for light of the electric field component vibrating in the X-axis direction is n _x , so the time required to pass through a crystal plate of thickness D is t
_x is t _x =D/(c/n _x ). c is the speed of light in vacuum. Similarly, the transit time t _z of light vibrating in the Z-axis direction is t _z =D/(c/n _z ). Therefore, if the angular frequency of light is ω, the phase difference Δ is Δ=
ω(t _x −t _z ). Here ω=2πc/λo
Therefore, the phase difference Δ is given as follows.

Δ=2π/λo(n _x -n _z )D...(2) The vibration plane of the incident linearly polarized light is aligned with the X axis and Z axis of the crystal plate.
If the angle with the axis is θ = 45 degrees, the phase difference Δ is π/2
If the thickness D is such that (called a quarter-wave plate) circularly polarized light is obtained. Therefore, if a mirror 7 is placed on the side opposite to the light source side of the quarter-wave plate 4 as shown in FIG. 2, the reflected light will be perpendicular to the plane of vibration of the linearly polarized light of the incident light. This is equivalent to the phase difference Δ being π in the round trip, that is, passing through a half-wave plate.

The light emitted from the semiconductor laser 1 passes through the polarizing plate 3, which transmits the electric field component parallel to the bonding surface.Only the light having the electric field component parallel to the bonding surface of the semiconductor laser (TE mode light) is transmitted, and the electric field perpendicular to the bonding surface is transmitted. Light with components (TM mode light) is blocked. Therefore, the light emitted from the semiconductor laser 1 passes through the coupling lens 2, the polarizing plate 3, and the quarter-wave plate 4, and is coupled into the optical fiber 5, and the light that is partially reflected at the end face is again quarter-wavelength. It passes through the one-wavelength plate 4 and is converted into polarized light having an electric field component perpendicular to that at the time of incidence, and cannot pass through the polarizing plate 3.

It is generally known that the polarization of the oscillated light of a semiconductor laser is the TE mode, and the polarization ratio TE/TM of the laser light actually oscillated in the TE mode is
It can be seen that =E _y ² /E _x ² is biased at 10 to several tens. Therefore, the transmission loss caused by the polarizing plate 3 is small, and the polarizing plate 3 can function as an isolator for blocking returning light due to reflection.

Furthermore, since the quarter-wave plate 4 is made of a piezoelectric single crystal such as quartz, interdigitated electrodes 4a are formed on the surface of the quarter-wave plate 4, and the wavelength on the quarter-wave plate 4 is When an electric signal of a frequency that matches the pitch of this electrode 4a is applied between both electrodes,
Surface acoustic waves of this frequency are generated in the direction perpendicular to the interdigital electrodes 4a and bidirectionally across the intersecting width of the electrodes. Therefore, by transmitting the light from the semiconductor laser 1 through the portion of the quarter-wave plate 4 where this surface acoustic wave propagates, the incident conditions such as the phase at the incident surface of the optical fiber 1 can be changed periodically. As a result, the interference conditions within the optical fiber 5 will change, and the position and shape of the speckle will change. Therefore, by adding the high frequency oscillation circuit 6 having a frequency higher than the modulation frequency of the signal, the speckle within the optical fiber 5 can be smoothed and the speckle noise can be reduced. Then, in the receiver, the frequency component of the surface acoustic wave may be removed by passing it through a low-pass filter.

By the way, the quarter wavelength plate 4 is (2m+
1) The thickness may be equivalent to λ/4 (m=0, 1, 2..., λ is the wavelength), or a quarter wavelength plate such as a glass substrate may be attached.

Furthermore, the polarizing plate 3 may be a normal polarizing plate or a polarizing prism such as a Glan-Thompson prism or a Rochon prism, and the coupling lens 2 may be a normal optical lens, but the refractive index is 2 times the radius.
A gradient index lens whose refractive index decreases in inverse proportion to the power of the refractive index may be used.

Furthermore, as transparent piezoelectric crystals that can be used as quarter-wave plates, in addition to crystal, LiNbo ₃ ,
Examples include LiTaO ₃ and Bi ₁₂ SiO ₂₀ .

As described above, according to the present invention, since the surface acoustic wave is configured to propagate on the quarter-wave plate made of piezoelectric crystal, it functions as an isolator for returned light and smooths speckles. A coupling device for a semiconductor laser and an optical fiber can be easily realized.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of a coupling device for a semiconductor laser and an optical fiber according to an embodiment of the present invention, and FIG. 2 is a perspective view for explaining a part of the principle of the invention. 1... Semiconductor laser, 2... Coupling lens, 3
. . . polarizing plate, 4 . . . quarter wavelength plate made of transparent piezoelectric crystal, 4 a . In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A coupling lens provided between a semiconductor laser and an optical fiber, a polarizing plate arranged to pass light of an electric field component parallel to the bonded surface of the semiconductor laser, and a quarter-wave plate made of a transparent piezoelectric crystal having interdigital electrodes on one surface and arranged so that the optical axis makes an angle of 45 degrees with respect to the bonded surface; A coupling device for a semiconductor laser and an optical fiber, comprising a high frequency oscillation circuit that applies a current to the electrode so that a surface acoustic wave is generated in the single wavelength plate of the semiconductor laser. 2. Claims in which the frequency of the surface acoustic wave generated in the quarter-wave plate by the application of current from the high-frequency oscillation circuit is set higher than the frequency band of the signal current applied to the semiconductor laser. 2. A device for coupling a semiconductor laser and an optical fiber according to claim 1.