JPS6255983A - External light feedback semiconductor laser using optical fiber - Google Patents

Abstract

PURPOSE:To emit laser light preferably having a narrow spectral width while suppressing multimode oscillation and intensity noise by bonding a ring resonator to an optical fiber feedback system. CONSTITUTION:A laser light from a semiconductor laser 1 is also emitted to a feedback type optical fiber 2, and the laser 1 is controlled to be fed back to increase the narrow spectrum of the emitted light. A ring resonator 7 formed of an optical fiber is bonded to the fiber 1 to compositely feedback it. The mode interval is increased from 333MHz to 1.33GHz to emit the laser light preferably having a narrow width while substantially suppressing multimode oscillation and intensity noise.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to optical communications, particularly coherent optical communications systems such as optical heterodyne detection systems or optical homodyne detection systems.
This invention relates to a semiconductor laser for use in electronic devices.

(Prior Art) Coherent optical communication such as an optical heterodyne detection method or an optical homodyne detection method requires a light source having an oscillation spectrum with a narrow spectrum width and less frequency fluctuation. For this reason, when a semiconductor laser with a wide spectrum width is used as a light source for coherent optical communication, it is necessary to narrow the spectrum width, and one method proposed is to feed the emitted light back into the semiconductor laser using an external optical feedback system. I (Kikuchi, Ohkoshi, Reference ``Methods for improving the spectral purity of semiconductor lasers by optical feedback include configuring the feedback system using a mirror or diffraction grating as a reflector, and configuring the feedback system using an optical fiber.'') Proposed and prototyped
I'm there. Among these, the method using an optical fiber is superior to the method using a mirror or a diffraction grating in terms of stability, system size, etc. The oscillation spectrum width of a semiconductor laser having such an external optical feedback system becomes narrower as the delay time in the feedback system becomes longer, so when using an optical fiber for the feedback system,
By increasing the length of the optical fiber, output light with a narrow spectral width can be obtained.

However, when the delay time of the feedback system becomes longer, while narrowing the spectrum width can be achieved, multimode oscillation and intensity noise are more likely to occur in the eigenmode spacing of the feedback system. however,
Conventionally, there has been no external optical feedback semiconductor laser using an optical fiber that can suppress this multimode oscillation and intensity noise and can obtain output light with a narrow spectral width.

(Object of the Invention) Therefore, an object of the present invention is to provide an external optical feedback semiconductor laser using an optical fiber that can obtain emitted light with a narrow spectral width and simultaneously achieve multimode oscillation and suppression of intensity noise. .

(Structure of the Invention) The present invention uses an optical fiber to narrow the spectrum width of the emitted light by propagating the emitted light from the semiconductor laser through an external optical feedback system composed of an optical fiber and then returning it to the semiconductor laser. This external optical feedback semiconductor laser is characterized in that a ring resonator formed of an optical fiber is connected to the external optical feedback system.

(Principle of the invention) In an external optical feedback semiconductor laser using an optical fiber,
The emitted light from the semiconductor laser is made incident on a feedback system composed of an optical fiber, and the delay time until the incident light returns to the semiconductor laser again in this feedback system is defined as τ(see). A ring resonator using an optical fiber is formed in this feedback system, and the resonant frequency is set to 1/τ (Hz) or more.

Therefore, in order to simplify the explanation, the resonant frequency of a ring resonator (hereinafter referred to as a ring resonator) using an optical fiber is 4/τ
(Hz). As a result, the ratio of the mode spacing of the resonance frequency of the feedback system when the ring resonator is not included to the mode spacing of the ring resonator becomes 1:4. In the entire feedback system including the ring resonator, the intensity of the feedback light is weakened except for the modes every 4/τ (Hz). That is,
The mode spacing of the feedback system including the ring resonator is substantially four times as wide as that of the feedback system not including the ring resonator. However, since the delay time of this feedback system is (sec) regardless of the presence or absence of the ring resonator, the effect of narrowing the spectrum width does not decrease.

By using a feedback system with widened mode spacing in this way, the following two advantages can be obtained. First, when using a feedback system without a ring resonator, there are many modes of the feedback system within the excavated gain band of the semiconductor laser, so multimode oscillation is likely to occur. By using this, the number of modes is substantially reduced, so multimode oscillation can be suppressed. In addition, in external optical feedback semiconductor lasers, intensity noise is likely to occur at each mode interval in the feedback system, but by using a feedback system with a ring resonator, it is possible to widen the frequency interval at which intensity noise occurs. . This effect makes it possible to reduce intensity noise within a certain frequency band.

(Embodiment) FIG. 1 is a block diagram showing a first embodiment of the present invention. a
In the figure t, the semiconductor laser 1 has an oscillation wavelength of 15 μm and is driven by a DC power supply 9. A first emitted light 20 from one end face of the semiconductor laser 1 passes through a lens 4 and enters a feedback fiber 2 constituted by a single mode fiber. Since the end face 7 of the feedback fiber 2 is perpendicular to the optical axis and is formed with a mirror, the incident light is reflected at the end face 7 and returns to the incident side end face 22, and passes through the lens 4 to the semiconductor laser 1. will be returned.

Second emitted light 21 from the opposite end facet of the semiconductor laser 1
is incident on the transmission fiber 8 through the lens 3.

As shown in the figure, a ring resonator 6 formed of an optical fiber is connected to the feedback fiber 2 by fusing the optical fiber. The length of the feedback fiber 2 is 3
0c'IrL, and the loop length of the ring resonator 6 is 15crrL. Here, how the mode spacing of the resonance frequency of the feedback system changes by adding the ring resonator 6 will be explained with reference to FIG. This figure shows the mode of the feedback system within the rooting gain band of the semiconductor laser 1 (FIG. 2(a)). The mode spacing of the feedback system when there is no ring resonator 6 is fo =
333 MHz (the refractive index inside the fiber is 1)
(value set at 5). On the other hand, since the mode spacing of the ring resonator 6 is 4fo=1.33GHz as shown in FIG. 2(c),
When connected to the cholera return system, f! 1.3
A weakened composite feedback system is configured except for the modes every 3 GHz. In other words, by adding the ring resonator 6, the mode spacing of the composite feedback system is substantially increased from 333 MHz to 1.
This means that the frequency has been increased by 33GHzKm.

Next, it will be explained with reference to FIG. 3 that by adding such a ring resonator 6, a semiconductor laser can be obtained in which multimode oscillation is suppressed and intensity noise is reduced. First, when there is no ring resonator 6 in the feedback system, f as shown in FIG. 3(a). = Multimode oscillation is likely to occur at 333MHz intervals,
Therefore, f as shown in Figure 3 (b). "" 333 M
Intensity noise tends to occur at Hz intervals. In contrast, if a ring resonator 6 is added to form a composite feedback system, the 31st f.
The multimode oscillation is suppressed as shown in Figure 3 (d).
), the frequency interval at which intensity noise occurs is 4 fo =
It becomes 1.33GH2. Further, even in the case of W9 in which the ring resonance 56 is added, the delay time of the feedback system remains unchanged, so that excessive broadening of the spectrum width does not occur.

FIG. 4 is a diagram showing a second embodiment of the present invention. The reason why this @2 embodiment is selected from the first embodiment is that in the feedback system, the first embodiment uses the end face reflection of the fiber, whereas the second embodiment uses the fiber loop. It is something that is being used. That is, in FIG. 4, the @1 emitted light 20 from the semiconductor laser 1 enters the loop-shaped feedback fiber II, returns to the input end again by the directional coupler 12, and is fed back to the semiconductor laser 1. The end face 15 of the return fiber 11 is cut obliquely to the optical axis so as to be a non-reflective end face. The length of the feedback system when the ring resonator 6 is not added is 6oα.

By setting the loop length of the ring resonator 6 to 15 boxes, the same change as in the first embodiment occurs in the mode spacing of the composite feedback system. As a result, the oscillation spectrum and intensity noise are improved as shown in FIG. 3, as in the first embodiment.

In both of the above embodiments, the resonant frequency of the ring resonator 6 was set to four times the resonant frequency of the feedback system, but by setting the resonant frequency to an even higher resonant frequency, the effect of multimode oscillation suppression and intensity noise reduction can be improved. Become bigger. Note that if it is necessary to obtain a narrower spectrum, the fiber length of the feedback system may be further increased.

Since a portion of the light that cannot be returned back to the semiconductor laser is emitted from the end faces 7.15 of the feedback fibers 2 and 11, this can also be used as wavelength and spectrum monitoring light. Further, in both embodiments, only one ring resonator is added, but by adding a plurality of ring resonators, unnecessary modes can be suppressed more strongly.

Regarding the present invention, several modifications are possible in addition to the above-described embodiments. For example, single-mode fibers were used for sending fibers 2 and 11, but by using polarization-maintaining fibers, the return light intensity due to polarization fluctuations within the fibers depending on temperature fluctuations, etc. It becomes possible to suppress changes in

(Effects of the Invention) As explained in detail above, the present invention provides an external optical feedback semiconductor laser using an optical fiber that can narrow the spectral width of the emitted light, suppress multimode oscillation, and reduce intensity noise. Obtainable.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention, and FIG. 2 (
a), (b), (c), and (d) are diagrams showing how the number of modes in the feedback system is reduced in the example, and Fig. 3 (
a), (b), (C). (d) is a diagram showing the oscillation spectrum and intensity noise improvement of the embodiment, and FIG. 4 is a configuration diagram showing the second embodiment of the present invention. 1...Semiconductor laser, 2.11...Return system fiber, 3.4...Lens, 6...
...Ring resonator, 8...Transmission fiber, 9
・・・・・・DC power supply, 12. ...Directional coupler, 20...First emitted light, 21...Second emitted light, 22.25...Incidence end surface. Protective Agent Patent Attorney Uchihara Hi; Rokuken α) Bon, 4, Ki Nozu

Claims (1)

[Claims] An external optical feedback semiconductor laser using an optical fiber, which narrows the spectrum width of the emitted light by propagating the emitted light from the semiconductor laser through an external optical feedback system composed of an optical fiber and returning it to the semiconductor laser. In,
An external optical feedback semiconductor laser using an optical fiber, characterized in that a ring resonator formed of an optical fiber is joined to the external optical feedback system.