JPS6164182A - Optical feedback type semiconductor laser device - Google Patents

Abstract

PURPOSE:To suppress reflecting noises, and to enable single transverse mode oscillation having narrow spectral-line width by feeding beams back to a semiconductor laser. CONSTITUTION:Laser beams 11 projected from a projecting end surface 10 in a semiconductor laser element 9 are coupled with an optical fiber 12, reflected by an optical element having high amplitude reflectance, a reflection increasing film 13 in the figure, and projected to the semiconductor laser element 9 again. Laser beams 15 projected from a projecting end surface in the semiconductor laser element 9 are coupled with an optical fiber 16. When external cavity length, the quantity of beams fed back, phase conditions to projecting beams of feedback beams, etc. are specified, oscillating frequency hardly changes even on the modulation of a semiconductor laser, reflecting noises are suppressed, the narrow single transverse mode oscillation of a spectral-line axis is realized, and extremely excellent characteristics can be obtained. When such a semiconductor laser is modulated regarding modulation, low noises can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improvement in an optical feedback semiconductor laser device that can be used as a light source for optical fiber communications.

Conventional Structure and Problems When using a semiconductor laser as a light source for optical communication, it is desirable that the semiconductor laser oscillate in a single longitudinal mode. However, it is difficult to achieve complete single longitudinal mode oscillation with a single semiconductor laser having a cavity surface formed by normal cleavage.

When coupled with an optical fiber, a small amount of return light from the end face of the optical fiber returns to the semiconductor laser element, increasing noise in the semiconductor laser element and changing the longitudinal mode spectrum, thereby stabilizing the semiconductor laser device.

This is an obstacle to achieving high quality.

One way to solve this problem is to use a semiconductor laser device having a composite resonator structure, which actively utilizes feedback light to the semiconductor laser element, as shown in FIG. Laser light 3 emitted from the resonator end face 2 of the semiconductor laser device 1 passes through the lens 4 and is reflected by the reflector 5, and the reflected light 6 passes through the lens 4 again and enters the semiconductor laser device 1.

With this type of external cavity configuration, the semiconductor laser stably oscillates in a single longitudinal mode with very little noise, allowing for modulation,
Particularly when performing analog modulation, changes in the oscillation frequency of the semiconductor laser due to modulation are suppressed. At this time, the optical path length between the semiconductor laser element and the reflector or the fouling rate r of one fouling body. The larger the oscillation frequency is, the more the oscillation frequency change can be suppressed.

If the oscillation frequency change due to semiconductor laser modulation is large,
This is equivalent to effectively expanding the spectrum width of the semiconductor laser. As the spectral width increases, the error rate of signal transmission in the transmission system increases due to chromatic dispersion, making long-distance transmission difficult, and distortion and the like increase in analog transmission systems, which is a major problem in practice. However, how to suppress the oscillation frequency change due to this modulation becomes an important issue when performing high-quality signal transmission. Therefore, in the configuration shown in Figure 1, the larger L is, the more the oscillation frequency change is suppressed, but in the configuration shown in Figure 1, increasing L causes axis misalignment of the semiconductor laser, lens, and reflector. Easy.

It causes instability of the semiconductor laser. There is also the problem that the device becomes larger, and there is a limit to the length of the device.

Figure 2 shows an external cavity type laser shown in Japanese Patent Application Laid-Open No. 57-28391, where 7 is an optical fiber, 7 is a spherical part,
7B is a semi-transparent film, and 7G is a tapered portion. In such a configuration, since the role is performed by a single optical fiber 7 without using lenses or plane mirrors, the optical path length and reflectance can be arbitrarily selected, and the optical path length can be increased. However, miniaturization is possible. But in this example,
Although semiconductor lasers have an external cavity configuration,
The purpose is to make a semiconductor laser oscillate in multiple longitudinal modes with close longitudinal mode spacing, and when used as a light source for optical communications, it has a higher error rate and higher noise level than a semiconductor laser with single mode oscillation. Become. Further, the laser light output 8 is taken out from the fiber end face 7C which is the resonator end face of the external resonator, and the semiconductor laser cleavage plane 1a is
The length of the optical fiber is not clearly defined. .

OBJECT OF THE INVENTION The present invention solves the above-mentioned conventional problems, performs single longitudinal mode oscillation with a narrow spectral linewidth using an optical fiber,
It is an object of the present invention to provide an optical feedback semiconductor laser device suitable for a light source for optical communication, in which oscillation frequency change due to modulation is suppressed.

Structure of the Invention The present invention is characterized in that an optical fiber with a refractive index of n2 is disposed opposite to one output end face of a semiconductor laser element with an optical length of n1β1 made of a compound semiconductor with an active layer having a refractive index of n1 and a cavity length of 61. , a high amplitude reflectance r at the far end of the optical fiber at a distance e2 from the semiconductor laser element. n2e2/n, g1≧100 + rO≧0
7, the emitted light from the semiconductor laser element is returned to the semiconductor laser element to form a single longitudinal mode oscillation semiconductor laser element, and the speed of light is C, and the modulation frequency is < C.
/2n2t2 or 9=C/2n2β2 and its harmonics are used for modulation, and the emitted light is extracted from the other emitting end facet of the semiconductor laser element. High amplitude reflectance r.

As the optical element, a reflection increasing film, a grating formed at one end of the focusing light transmitter, etc. can be used.Also, a photodetector is placed opposite the optical element to detect the output from the semiconductor laser. A part of the emitted light may be received by the photodetector, and the received light output may be electrically fed back to the drive power circuit of the semiconductor laser element.Also, a polarization maintaining optical fiber may be used as the optical fiber. good.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. Third
The figure shows a first embodiment. In FIG. 3, a laser beam 11 emitted from an output end face 1Q of a semiconductor laser element 9 is coupled to an optical fiber 12.

An optical element with high amplitude reflectance, here a reflection increasing film 13
The light is reflected by the laser beam and enters the semiconductor laser device 9 again. The laser beam 15 emitted from the emission end face 14 of the semiconductor laser element 9 is still coupled to the optical fiber 16.

Even though it is called a semiconductor laser having an external resonator configuration, the characteristics of the semiconductor laser are extremely different depending on the length of the external resonator, the amount of optical feedback, the phase condition of the feedback light with respect to the emitted light, and the like. The purpose of the present invention is to make the oscillation longitudinal mode of a semiconductor laser a complete single mode oscillation, to have low noise,
An object of the present invention is to provide a semiconductor laser whose oscillation frequency does not change even when modulated. From this point of view,
Analyzing the characteristics of a semiconductor laser, it is 02g□/n, β.

>100, ro≧0.7, even if the semiconductor laser is modulated, the oscillation frequency change is extremely small, reflection noise is suppressed, and single longitudinal mode oscillation with a narrow spectral linewidth is realized, which is extremely excellent. We were able to obtain the following characteristics.

Regarding modulation, when such a semiconductor laser is modulated, noise tends to increase as the modulation degree m increases, but by setting the modulation frequency to 9<, C/2n, , 12, m = Even if 0.7, relative noise intensity RIN=
Low noise of 1s5dB/Hz can be achieved. Further, by modulating with f, = C/2n212 and its harmonics, the modulation degree can be set to m':1.

In addition, since an optical fiber is used as an external resonator,
Even if the external resonator length is increased, size reduction is possible.

FIG. 4 shows a second embodiment, in which a grating 17 formed at one end of a convergent light transmitter is used as an optical element having a high amplitude reflectance.

In this way, the oscillation frequency can be selected by changing the angle at which the grating 16 is formed.

FIG. 5 shows a third embodiment, in which a photodetector 18 is arranged opposite to a grating 17, a part of the emitted light from the semiconductor laser element 8 is received by the photodetector 18, and the output of the received light is used to Electrical feedback is provided to the drive power supply circuit 19 of the laser element 9.

In this way, the semiconductor laser device is equipped with a power servo mechanism, and the optical output can be stabilized.

Furthermore, if the optical fiber 12 is a polarization-maintaining optical fiber, the semiconductor laser oscillating in TE mode can be effectively coupled to the optical fiber, and the feedback light to the semiconductor laser can be made only in the same TE mode. Excellent properties can be stably obtained.

Effect of the invention As described above, the present invention uses optical fibers.

By setting n2A2/n+L≧100 and ro≧0.7, by performing optical feedback to the semiconductor laser, reflection noise can be suppressed and single longitudinal mode oscillation with a narrow spectral linewidth can be achieved, resulting in modulation. Even if the modulation frequency is set to 9≦C/2n2g2, low noise can be achieved even if the modulation degree is increased, and 9=C/2n2β2 and its harmonics. By modulating with The present invention provides an optical feedback semiconductor laser device that exhibits unprecedented effects that are extremely advantageous.

[Brief explanation of drawings]

1 and 2 are schematic cross-sectional views of conventional optical feedback semiconductor laser devices, and FIGS. 3 to 5 are configuration diagrams of optical feedback semiconductor laser devices according to embodiments of the present invention. 9... Semiconductor laser element, 10.14...
...Emission end face, 11.15...Laser light, 12.1
6... Optical fiber, 13... Reflection increasing film, 1 Ryo... Grating, 18...
- Photodetector, 19... Drive power supply circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Figure 4 Figure I

Claims (5)

[Claims] (1) An optical fiber with a refractive index of n_2 is arranged opposite to one output end face of a semiconductor laser element with an optical length of n_1l_1 made of a compound semiconductor with an active layer having a refractive index of n_1 and a cavity length of l_1, and the optical fiber has a refractive index of n_2. An optical element having a high amplitude reflectance r_0 is provided at the far end of the distance l_2 from the semiconductor laser element, so that n_2l_2/n_1l_1≧100
, r_0≧0.7, the emitted light from the semiconductor laser device is returned to the semiconductor laser device to form a single longitudinal mode oscillation semiconductor laser device, the speed of light is C, the modulation frequency is 0≦f_m≦C/2n_2l_2, or f_m=C/
An optical feedback semiconductor laser device characterized in that modulation is performed using 2n_2l_2 and its high frequency, and emitted light is extracted from the other emission end facet of the semiconductor laser element. (2) The optical feedback semiconductor laser device according to claim 1, wherein a reflection increasing film is used as the optical element having a high amplitude reflectance r_0. (3) The optical feedback semiconductor laser device according to claim 1, characterized in that a grating formed at one end of the convergent light transmission body is used as the optical element having a high amplitude reflectance r_0. (4) Opposing the optical element with high amplitude reflectance r_0,
A patent claim characterized in that a photodetector is arranged, a part of the light emitted from the semiconductor laser is received by the photodetector, and the received light output is electrically fed back to the drive power circuit of the semiconductor laser element. The optical feedback semiconductor laser device according to item 1. (5) Claim 1, characterized in that a polarization maintaining optical fiber is used as the optical fiber with a refractive index of n_2.
The optical feedback semiconductor laser device described in .