JPS6239837B2 - - 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 that returns due to external reflection is recombined into the active layer, a phenomenon called a self-coupling effect occurs.
This causes the return light to degrade the linearity of the current-optical output characteristics, increase the optical output, and change the oscillation spectrum of the semiconductor laser, causing a deterioration of the S/N ratio, which is a problem in optical communications. This is a very inconvenient phenomenon.

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 while being totally reflected at the boundary between the core layer and the cladding layer. It is used as a transmission line.

Since light guided through repeated total reflections is called a mode, 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 is 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,
An object of the present invention is to provide a coupling device for a semiconductor laser and an optical fiber, which can reduce the influence of return light due to reflection and noise due to speckle, which occur when a semiconductor laser is used as a light source, with a relatively simple configuration in optical fiber communication. The purpose is

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 1 is a semiconductor laser, and 2 is a coupling lens installed between the semiconductor laser 1 and an optical fiber 5, which will be described later. 3 is a polarizing plate that receives light from the semiconductor laser 1 via the coupling lens 2;
It is arranged so that light with an electric field component parallel to the bonding surface passes through. Reference numeral 4 denotes a quarter wavelength plate, which is made of a transparent piezoelectric crystal such as quartz, and is arranged so that its optical axis forms an angle of 45 degrees with respect to the bonding surface of the semiconductor laser 1. Transparent electrodes 4a, 4a are provided on both the front and rear surfaces of the quarter-wave plate 4, and each electrode 4a, 4a has a lead terminal 4.
b, 4b are fixedly installed. 5 is optical fiber, 6
is a high frequency oscillation circuit that applies an electric field to vibrate the quarter wavelength plate 4.

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 ₀ ² +Z ² /n _e ² =1 ... (1) n ₀ = 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 the crystal plate of thickness D is t
_x is expressed as t _x =D/(c/n _x ). Here c is the speed of light in vacuum. Similarly, the transit time t _x of light vibrating in the Z-axis direction is t _z =D/(c/n
_z ). Therefore, when the angular frequency of light is ω, the phase difference Δ is Δ=ω(t _x −t _z ). Here ω
=2πc/ _λ0 , so the phase difference Δ is given as follows.

Δ=2π/λ ₀ (n _x - n _z )D...(2) The vibration plane of the incident linearly polarized light is on 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 (referred to as 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 3 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 is converted into light having an electric field component parallel to the junction surface of the semiconductor laser (TE mode light) by the polarizing plate 3 which passes the electric field component parallel to the junction surface.
Only the light that is transmitted and has a vertical electric field component (TM
mode light) is blocked. Therefore, the light emitted from the semiconductor laser 1 is transmitted through the coupling lens 2, the polarizing plate 3,
The light that is coupled to the optical fiber 5 through the quarter-wave plate 4 and partially reflected at the end face passes through the quarter-wave plate 4 again and is converted into polarized light having an electric field component perpendicular to the incident light. It 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 due to the polarizing plate 3 is small, and the polarizing plate 3 can function as an isolator to prevent return light due to reflection.

Furthermore, since the quarter-wave plate 4 is made of a piezoelectric single crystal such as quartz, applying an electric field causes strain and stress on the quarter-wave plate 4 due to a piezoelectric reverse effect. This effect causes the quarter-wave plate 4 to act as a vibrating element. That is, transparent electrodes 4a, 4a are provided on both the front and rear surfaces of the quarter-wave plate 4, and each electrode 4a,
By attaching lead wires 4b and 4b to 4a and adding a high frequency oscillation circuit 6,
The half-wave plate 4 can be vibrated at a specific frequency. When the quarter-wave plate 4 made of this piezoelectric crystal is vibrated, the incident conditions such as the phase of the light from the semiconductor laser 1 on the incident surface of the optical fiber 5 are periodically changed.
The interference conditions within the speckle will change, and the position and shape of the speckle will change. Therefore, by vibrating at a frequency higher than the modulation frequency of the signal, the speckle in the optical fiber 5 is smoothed,
Speckle noise can be reduced. Then, in the receiver, the frequency component of the vibrator may be removed by passing it through a low-pass filter.

By the way, the quarter wavelength plate 4 is (2m+1)・λ/4 (m=0, 1, 2..., λ is the wavelength), taking into consideration its use as a vibrator and ease of manufacturing. Preferably, the thickness corresponds to .

In addition, the polarizing plate 3 may be a normal polarizing plate or a grand polarizing plate.
The lens 2 may be a polarizing prism such as a Thomson prism or a Roschon prism, and the lens 2 may be a normal optical lens or a gradient index lens whose refractive index decreases in inverse proportion to the square of the radius.

Furthermore, in addition to crystal, LiNbO ₃ and LiTaO ₃ are transparent piezoelectric crystals that can be used as quarter-wave plates.
etc.

In the above embodiment, a case has been described in which light is allowed to pass through the transparent electrode portion of the quarter-wave plate, but a non-transparent electrode having a window formed only in the portion through which light passes may also be used.

As described above, according to the present invention, with a simple configuration using a polarizing plate or a quarter-wave plate made of piezoelectric crystal, it is possible to eliminate the adverse effects of returned light on the semiconductor laser and to smooth the speckle. A device for coupling a semiconductor laser and an optical fiber can be provided.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of a semiconductor laser and optical fiber coupling device according to an embodiment of the present invention;
FIG. 2 is a perspective view for explaining part of the principle of the invention. 1... Semiconductor laser, 2... Coupling lens, 3
...Polarizing plate, 4... Quarter wavelength plate, 4a... Electrode, 5... Optical fiber, 6... High frequency oscillation circuit.
In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. A coupling lens provided between a semiconductor laser and an optical fiber, a polarizing plate arranged to pass light having an electric field component parallel to the bonded surface of the semiconductor laser, and both front and rear surfaces. A quarter-wave plate made of a transparent piezoelectric crystal, which has an electrode on the surface and is arranged so that the optical axis makes an angle of 45 degrees with respect to the bonded surface, and the quarter-wave plate is vibrated. and a high-frequency oscillation circuit that applies an electric field between the electrodes in order to generate a semiconductor laser and an optical fiber. 2. Claim No. 2, wherein the vibration frequency when the quarter-wave plate made of the transparent piezoelectric crystal is operated as a vibrator is set higher than the frequency band of the signal current applied to the semiconductor laser. A device for coupling a semiconductor laser and an optical fiber according to item 1.